Fluorophosphonium Chemistry: Applying Strategies Learned ... · ii Fluorophosphonium Chemistry:...

Fluorophosphonium Chemistry:

Applying Strategies Learned from Boron to Phosphorus

by

Shawn William Postle

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Department of Chemistry

University of Toronto

ii

Fluorophosphonium Chemistry:

Applying Strategies Learned from Boron to Phosphorus

Shawn William Postle

Doctor of Philosophy

Department of Chemistry

University of Toronto

Abstract

Since the inception of frustrated Lewis pair chemistry, interest in main group catalysts has

undergone a resurgence. Central to the success of many main group systems is the

pentafluorophenyl substituent, which provides both chemical stability and electrophilicity to the

catalyst. Pentafluorophenyl substituents have been used with boranes, alanes, and recently in

fluorophosphonium cations. This thesis investigates a range of related aryl substituents applied to

fluorophosphonium chemistry to elicit new catalyst properties. Nitrene insertion into the bonds of

borane substituents, including perfluorophenyl groups, was used to tune the electrophilicity of

main group systems. Sterically demanding pentachlorophenyl substituents were used to add

protection to sensitive fluorophosphonium catalysts. Perfluorobiphenyl groups were used to

generate more electrophilic fluorophosphonium catalysts. Binaphthyl substituents were employed

to create chiral fluorophosphonium cations.

iii

This work is dedicated in memory of my grandpa,

William Kerr,

for instilling in me a genuine curiosity of the world.

iv

Acknowledgements

First and foremost, I would like to thank Prof. Doug Stephan for providing me with this experience.

By providing me with the freedom to pursue my own interests and offering insightful guidance

whenever it was sought, you have helped me become a better chemist. I would like to thank my

committee members, Prof. Bob Morris and Prof. Ulrich Fekl, for their enthusiasm and

encouragement. I am grateful to Prof. Datong Song and my external examiner Prof. Chuck

Macdonald for providing me with invaluable feedback. I am also extremely thankful to Dr. Barb

Morra for her mentorship and collaboration. Additionally, I would like to thank my undergraduate

supervisor Prof. Peter Legzdins for setting me upon this path.

I have been incredibly fortunate to have worked alongside such a fantastic group of labmates, both

past and present. I am grateful to Dr. Gab Menard for taking the time to get me situated in the lab.

Vitali Podgorny, it was a pleasure to collaborate with you on our perchloroaryl chemistry. James

LaFortune, I couldn’t ask for a better desk or gym partner. Tim Johnstone, you have been

incredibly generous with your time and expertise, for which I am incredibly appreciative. I would

also like to thank Julia Bayne, Louie Fan, James LaFortune, Eliar Mosaferi and Kevin Szkop for

editing chapters of my thesis. I’m also indebted to our group’s fantastic evolving crystallography

team over the years for providing me with some colour to break up the text herein.

I am grateful to Darcy Burns, Sergiy Nokhrin, and Jack Sheng from the NMR department for

fixing our spectrometer, helping me with difficult experiments, and fixing our spectrometer. I

would like to thank Matthew Forbes, Fung Chung Woo, and Michelle Young of the AIMS lab for

all their effort in finding just the right method for my compounds. I would like to thank Rose

Balazs and the rest of the ANALEST staff for their expertise. I am also very appreciative of all the

help John Ford of the Machine Shop has provided me over the years.

I would like to thank Mom, Dad, Chris, and Michelle for all their unwavering love and continuous

support. And finally, to my dearest Samantha, every day with you is a joy and you make me excited

for everything yet to come.

v

List of Abbreviations

α alpha

Å angstrom, 10-10 m

AN Gutmann acceptor number

atm. atmosphere

β beta

BCF B(C6F5), tris(pentafluorophenyl)borane

br broad

Bu butyl

C Celsius

cm centimeter

C6D6 deuterated benzene

C6F5 pentafluorophenyl

C12F9 2-nonafluorobiphenyl

C10H6 naphthyl

C20H12 1,1’-binaphthyl

calcd. calculated

CH2Cl2 dichloromethane

Cls SO2(4-ClC6H4), 4-chlorobenzenesufonyl

COSY correlational spectroscopy

δ delta, chemical shift

vi

Δ Delta

° degrees

d doublet, days

-dn n-deuteron isotopologue

DART direct analysis in real time

DEPT distortionless enhancement by polarization transfer

DFT density functional theory

η eta, hapticity

E energy

eq. equivalent(s)

eV electron volts

ESI electrospray ionization

Et ethyl

Et2O diethyl ether

FIA fluoride ion affinity

FLP frustrated Lewis acid

g gram

GEI global electrophilicity index

h hour

HRMS high resolution mass spectrometry

HMBC heteronuclear multiple bond correlation

vii

HOESY heteronuclear Overhauser effect spectroscopy

HSQC heteronuclear single quantum coherence

Hz Hertz

iPr Isopropyl

IR infrared

nJxy n-bond scalar coupling between nuclides x and y

κ kappa, denticity

K Kelvin

kJ/mol kilojoules per mole

μ mu, bridging, absorption coefficient

m multiplet, meta

Mes mesityl

Me methyl

mg milligram

MHz megahertz

mL milliliter

mmol millimole

MS mass spectrometry

Ms SO2(CH3), methanesulfonyl

NHC N-heterocyclic carbene

NMR nuclear magnetic resonance

viii

Ns SO2(4-NO2C6H4), 4-nitrobenzenesufonyl

ω omega, global electrophilicity index value

o ortho

OTf (CF3SO3)-, triflate anion

π pi

p para

ppm parts-per-million, 10-6

pent. n-pentane

POV-Ray Persistence of Vision Raytracer

Ph phenyl

q quartet

quin. quintet

RT room temperature

σ sigma

s singlet

SCE saturated calomel electrode

t triplet

tBu tert-butyl

THF tetrahydrofuran

tol tolyl

Ts SO2(4-(CH3)C6H4), 4-toluenesulfonyl

ix

Table of Contents Abstract ........................................................................................................................................... ii

Acknowledgements ........................................................................................................................ iv

List of Abbreviations ...................................................................................................................... v

Table of Tables ............................................................................................................................ xiii

Table of Schemes ......................................................................................................................... xiv

Table of Figures .......................................................................................................................... xvii

Chapter 1 Introduction .................................................................................................................... 1

1.1 Elemental Phosphorus ...................................................................................................... 1

1.2 Lewis Acidic Phosphorus Species .................................................................................... 2

1.2.1 Phosphenium Cations................................................................................................ 2

1.2.2 Phosphonium Cations ............................................................................................... 3

1.2.3 Fluorophosphonium Catalysts .................................................................................. 5

1.2.4 Dicationic Phosphorus Catalysts............................................................................... 9

1.3 Electrophilicity and Lewis Acidity Measurement Scales .............................................. 10

1.3.1 Chemical Shift Electrophilicity Scales ................................................................... 11

1.3.2 Computational Electrophilicity Measures............................................................... 14

1.4 Scope of Thesis .............................................................................................................. 16

1.5 References ...................................................................................................................... 18

Chapter 2 Nitrene Insertion into Boranes ..................................................................................... 23

2.1 Introduction .................................................................................................................... 23

2.1.1 Hypervalent Iodine Reagents .................................................................................. 23

2.1.2 Electrophilic Borane Reagents ................................................................................ 25

2.1.3 Frustrated Lewis Pair Chemistry ............................................................................ 27

2.1.4 Post-Synthetic Tuning Strategies ............................................................................ 28

x

2.2 Results and Discussion ................................................................................................... 29

2.2.1 Synthesis and Characterization of Aminoboranes .................................................. 29

2.2.2 Electrophilicity of Aminoboranes ........................................................................... 39

2.2.3 Reactivity of Aminoboranes ................................................................................... 43

2.2.4 Mechanism of Nitrene Insertion ............................................................................. 45

2.2.5 Synthesis and Characterization of Phosphinimines ................................................ 46

2.3 Conclusion ...................................................................................................................... 50

2.4 Experimental .................................................................................................................. 51

2.4.1 General Experimental Methods .............................................................................. 51

2.4.2 X-ray Crystallography ............................................................................................ 55

2.5 References ...................................................................................................................... 58

Chapter 3 Perchloroaryl Fluorophosphonium Cations ................................................................. 63

3.1 Introduction .................................................................................................................... 63

3.1.1 Perchloroaryl Boranes ............................................................................................. 63

3.1.2 Perchloroaryl Phosphines........................................................................................ 67

3.2 Results and Discussion ................................................................................................... 68

3.2.1 Synthesis and Characterization of Phosphines ....................................................... 68

3.2.2 Synthesis of Perchloroaryl Difluorophosphoranes ................................................. 73

3.2.3 Synthesis of Pentachlorophenyl Phosphonium Cations .......................................... 77

3.2.4 Electrophilicity of Fluorophosphonium Cations ..................................................... 82

3.2.5 Reactivity of Pentachlorophenyl Fluorophosphonium Cations .............................. 89

3.2.6 Catalytic Activity of Perchlorophenyl Phosphonium Cations ................................ 91

3.3 Conclusion ...................................................................................................................... 93

3.4 Experimental .................................................................................................................. 95

3.4.1 General Experimental Methods .............................................................................. 95

xi

3.4.2 X-ray Crystallography .......................................................................................... 102

3.5 References .................................................................................................................... 106

Chapter 4 Perfluorobiphenyl Fluorophosphonium Cations ........................................................ 111

4.1 Introduction .................................................................................................................. 111

4.1.1 Perfluorobiphenyl Groups in Transition Metal Applications ............................... 111

4.1.2 Perfluorobiphenyl Groups in Main Group Applications ...................................... 116

4.2 Results and Discussion ................................................................................................. 118

4.2.1 Synthesis of Perfluorobiphenyl Phosphines by Lithiation .................................... 118

4.2.2 Synthesis of Perfluorobiphenyl Phosphines by Zincation .................................... 122

4.2.3 Synthesis of 2-Perfluorobiphenyl Difluorophosphoranes ..................................... 126

4.2.4 Synthesis of Fluorophosphonium cations ............................................................. 128

4.2.5 Measures of electrophilicity .................................................................................. 131

4.2.6 Solubility of Perfluorobiphenyl Salts .................................................................... 138

4.2.7 Stability of Perfluorobiphenyl Phosphonium Cations .......................................... 139

4.2.8 Catalysis ................................................................................................................ 141

4.3 Conclusion .................................................................................................................... 144

4.4 Experimental ................................................................................................................ 144

4.4.1 General Experimental Methods ............................................................................ 144


4.5 References .................................................................................................................... 161

Chapter 5 Binaphthyl Fluorophosphonium Cations ................................................................... 166

5.1 Introduction .................................................................................................................. 166

5.2 Results and Discussion ................................................................................................. 170

5.2.1 Synthesis and Characterization of BINAP Fluorophosphonium Cation............... 170

5.2.2 Electrophilicity of BINAP-derived Fluorophosphonium Cation .......................... 173

xii

5.2.3 Catalytic Activity of BINAP-derived Fluorophosphonium Cation ...................... 173

5.2.4 Synthesis of Pentafluorophenyl Binaphthyl Fluorophosphonium Cation ............ 174

5.2.5 Mechanism for Phosphole Synthesis .................................................................... 178

5.2.6 Electrophilicity of Perfluorophenyl Binaphthyl Fluorophosphonium. ................. 179

5.2.7 Reactivity of a Perfluoroaryl Binaphthyl Fluorophosphonium Cation ................. 181

5.3 Conclusion .................................................................................................................... 182

5.4 Experimental ................................................................................................................ 182

5.4.1 General Experimental Methods ............................................................................ 182


5.5 References .................................................................................................................... 190

Chapter 6 Conclusion .................................................................................................................. 194

6.1 Future Work ................................................................................................................. 194

6.2 Summary ...................................................................................................................... 195

6.3 References .................................................................................................................... 199

xiii

Table of Tables

Table 2.1. Gutmann-Beckett 31P{1H} NMR and Gutmann acceptor numbers ............................. 40

Table 2.2. Computed LUMO and HOMO Energies and Calculated ω Values. ........................... 43

Table 3.1. Calculated LUMO and HOMO energies for cations of 3-10 – 3-13,

[FP(C6Cl5)3][B(C6F5)4], and [FP(C6F5)3][B(C6F5)4]. .................................................................... 85

Table 3.2. Calculated Electrophilic Index, ω, values for the cations of 3-10 – 3-13,

[FP(C6Cl5)3][B(C6F5)4], and [FP(C6F5)3][B(C6F5)4]. .................................................................... 86

Table 3.3. Fluoride Ion Affinities for the Cations of 3-10 – 3-13, [FP(C6Cl5)3][B(C6F5)4], and

[FP(C6F5)3][B(C6F5)4]. .................................................................................................................. 88

Table 3.4. Air-Stability of Fluorophosphonium Salts 3-10 – 3-13 and [FP[C6F5)3][B(C6F5)4] in

PhBr. ............................................................................................................................................. 90

Table 4.1. Selected NMR Chemical Shift Data for Phosphines 4-1 – 4-8. ................................. 120

Table 4.2. Calculated LUMO and HOMO energies for cations of 4-20 – 4-23, and

[PF(C6F5)3][B(C6F5)4]. ................................................................................................................ 135

Table 4.3. Calculated ω values for cations of 4-20 – 4-23, and [PF(C6F5)3][B(C6F5)4]. ............ 136

Table 4.4. Fluoride Ion Affinities for Cations of 4-20 – 4-23 and [PF(C6F5)3][B(C6F5)4]. ........ 137

xiv

Table of Schemes

Scheme 1.1. Synthesis of benzothiazolium phosphenium cation. .................................................. 2

Scheme 1.2. Phosphenium-phosphine adduct formation. ............................................................... 2

Scheme 1.3. Phosphenium cation cycloaddition with 1,3-diene. .. Error! Bookmark not defined.

Scheme 1.4. Hypervalent interaction between a phosphonium cation and Lewis base. ................. 3

Scheme 1.5. The Wittig reaction mechanism. ................................................................................ 4

Scheme 1.6. Catalytic activity of oxo-bridged bis-phosphonium salt. ........................................... 5

Scheme 1.7. Phosphonium ionic liquid carbonyl activation. .......................................................... 5

Scheme 1.8. Catalytic hydrodefluorination activity of fluorophosphonium catalysts. ................... 6

Scheme 1.9. Dehydrocoupling hydrogenation reactivity of fluorophosphonium cations. ............. 7

Scheme 1.10. Ketone hydrosilylation reactivity of chloro- and bromophosphonium cations. ....... 8

Scheme 1.11. Dimerization of 1,1-diphenylethylene reactivity of alkyl-linked bis-phosphonium

cations. ............................................................................................................................................ 9

Scheme 1.12. Oxide-fluoride exchange reactivity of NHC-stabilized dication. ........................... 10

Scheme 1.13. Hydrodefluorination reactivity of pyridinium-phosphonium dications. ................ 10

Scheme 1.14. Childs’ method protocol for BCF. .......................................................................... 12

Scheme 1.15. Gutmann-Beckett protocol for BCF. ...................................................................... 13

Scheme 1.16. Fluoride ion affinity for a fluorophosphonium cation. ........................................... 15

Scheme 2.1. Synthesis of hypervalent iodine reagents from diacetoxyiodobenzene. .................. 24

Scheme 2.2. General reactivity patterns of λ3-iodanes. ................................................................ 24

Scheme 2.3. Reactivity of iodine reagent with triphenylphosphine (top) and dimethylsulfoxide

(bottom)......................................................................................................................................... 25

Scheme 2.4. Synthesis of tris(pentafluorophenyl)borane (BCF). ................................................. 26

Scheme 2.5. Summary of BCF Lewis acid catalyzed reactivity. .................................................. 26

Scheme 2.6. Reactivity of Piers’ borane with THF (top) and styrene (bottom). .......................... 27

xv

Scheme 2.7. Synthesis of phenyliodonium sulfonylimides. ......................................................... 29

Scheme 2.8. Synthesis of tosylaminoborane 2-1. ......................................................................... 30

Scheme 2.9. Synthesis of adduct (C6F5)2BN(Ts)(C6F5)·OPEt3 2-2. ............................................. 31

Scheme 2.10. Reaction of PhI=NTs with (C6F5)2BPh to generate 2-3. ........................................ 33

Scheme 2.11. Synthesis of nitrene inserted products 2-4 – 2-7 from Piers’ borane. .................... 35

Scheme 2.12. Synthesis of nitrene inserted products 2-4 – 2-7 from ClB(C6F5)2. ....................... 38

Scheme 2.13. Summary of reactivity of aminoboranes. ............................................................... 45

Scheme 2.14. Mechanism of Insertion reaction between PhI=NTS with boranes. ...................... 46

Scheme 2.15. Synthesis of phosphinimines 2-8 and 2-9. ............................................................. 47

Scheme 2.16. Hydrolysis of 2-8 and 2-9 to corresponding phosphine oxides. ............................ 47

Scheme 2.17. Proposed mechanism of formation for phosphinimines 2-8 and 2-9. .................... 49

Scheme 3.1. Hydrolysis of BCF (top). Water-adduct formation of B(C6F5)2(C6Cl5) (bottom) .... 65

Scheme 3.2. Proposed mechanism for hydrogenation with B(C6F5)2(C6Cl5). .............................. 66

Scheme 3.3. Hydrogen activation using B(C6Cl5)3/PR3 FLPs (top). Formic acid activation using

B(C6Cl5)3/PR3 FLPs (bottom). ...................................................................................................... 66

Scheme 3.4. Synthesis of pentachlorophenyl-substituted phosphines. ......................................... 67

Scheme 3.5. Functionalization of pentachlorophenyl groups with trimethylchlorosilane. ........... 68

Scheme 3.6. Synthesis of perchlorophenyl substituted phosphines 3-1 – 3-5. ............................. 69

Scheme 3.7. Synthesis of 1,2-diphosphine ((C6Cl5)2P)2. .............................................................. 70

Scheme 3.8. Synthesis of pentachlorophenyl-substituted difluorophosphoranes. ........................ 74

Scheme 3.9. Reactivity of 3-3 with XeF2. ..................................................................................... 76

Scheme 3.10. Synthesis of pentachlorophenyl-substituted phosphonium cations. ....................... 78

Scheme 3.11. Direct fluorophosphonium synthesis from 3-6 using NFSI. .................................. 80

Scheme 3.12. Reaction of 3-3 with fluorinating agent NFSI to generate 3-14. ............................ 81

Scheme 4.1. Synthesis of 2-bromoperfluorobiphenyl, BrC12F9. ................................................. 111

xvi

Scheme 4.2. Mechanism of 2-bromoperfluorobiphenyl synthesis. ............................................. 112

Scheme 4.3. Synthesis of perfluorobiphenyl substituted borane (top) and fluoroaluminate

(bottom)....................................................................................................................................... 113

Scheme 4.4. Proposed decomposition route of cationic zirconocene compounds. .................... 114

Scheme 4.5. Divergent reactivity of KCN with B(C12F9) (top) and BCF (bottom). ................... 115

Scheme 4.6. Decarboxylation of B(C12F9)3 FLP (top) and CO2 activation by BCF FLP (bottom).

..................................................................................................................................................... 117

Scheme 4.7. Divergent reactivity of ethylene towards [Mes3PH][(μ-H)((Al(C12F9)3)2] (top) and

[Mes3PH][(μ-H)(Al(C6F5)3)2] (bottom). ..................................................................................... 118

Scheme 4.8. Synthesis of perfluorobiphenyl phosphines 4-1 – 4-7. ........................................... 119

Scheme 4.9. Synthesis of bis(perfluorobiphenyl) phosphines 4-9 and 4-10. .............................. 121

Scheme 4.10. Synthesis of 1,2-diphosphines 4-11 and 4-12. ..................................................... 125

Scheme 4.11. Synthesis of 2-perfluorobiphenyl substituted difluorophosphoranes 4-13 – 4-19.

..................................................................................................................................................... 126

Scheme 4.12. Synthesis of perfluorophenyl fluorophosphonium cations 4-20 – 4-25. .............. 129

Scheme 4.13. Summary of reactivity for catalysts 4-20 – 4-23. ................................................. 143

Scheme 5.1. Chiral induction in decarboxylation reaction by brucine. ...................................... 167

Scheme 5.2. Chiral BINAP rhodium hydrogenation catalyst ..................................................... 168

Scheme 5.3. Chiral binaphthyl FLP hydrosilylation of 1,2-diketone. ........................................ 169

Scheme 5.4. Asymmetric amination of β-keto ester with phosphonium catalyst. ...................... 169

Scheme 5.5. Oxidation of BINAP by XeF2 to generate bis(difluorophosphorane) 5-1. ............. 171

Scheme 5.6. Synthesis of fluorophosphonium salt 5-2. .............................................................. 171

Scheme 5.7. Synthesis of bis(fluorophosphonium) 5-2b (top) and fluorophosphonium-phosphine

5-2c (bottom) from BINAP using NFSI. .................................................................................... 173

Scheme 5.8. Summary of catalytic activity for 5-2. .................................................................. 174

xvii

Scheme 5.9 Reaction of binaphthyl Grignard reagent with BrP(3,5-(CF3)2C6H3)2 (top, literature

procedure21), and BrP(C6F5)2 to form 5-3 (bottom). .................................................................. 175

Scheme 5.10. Air-oxidation of 5-3 to form phosphine oxide 5-4 ............................................... 176

Scheme 5.11. Oxidation of 5-3 by XeF2 to synthesize difluorophosphorane 5-5. ...................... 177

Scheme 5.12. Synthesis of fluorophosphonium salt 5-6. ............................................................ 177

Scheme 5.13. Proposed mechanism for the formation of 5-3 and P(C6F5)3. ............................. 179

Scheme 5.14. Summary of reactivity for 5-6. ............................................................................. 181

xviii

Table of Figures

Figure 1.1. Lewis-acidic orbitals of a fluorophosphonium cation (left), silylium cation (middle),

and borane (right). ........................................................................................................................... 7

Figure 1.2. Electrophilic index values (ω) and FIA values of a series of phenoxyphosphonium

cations. ............................................................................................................................................ 8

Figure 2.1. Structural types of λ3-iodanes (left, middle) and λ5-iodanes (right). .......................... 23

Figure 2.2. Seminal FLP systems: arene-linked phosphine-borane (left), ethylene-bridged

phosphine-borane (middle), and intermolecular BCF-phosphine (right). .................................... 28

Figure 2.3. POV-ray depiction of 2-2; C: light grey, B: yellow-green, O: red, S: yellow, N: blue,

P: orange, F: pink, hydrogen atoms have been omitted for clarity. .............................................. 32

Figure 2.4. Partial 19F NMR spectrum (-127 – -164 ppm), variable temperature study of 5-3 in

toluene-d8 (bottom – top: 25 °C, 40 °C, 60 °C, 80 °C, 100 °C).................................................... 34


P: orange, F: pink, hydrogen atoms have been omitted for clarity. .............................................. 36

Figure 2.6. POV-ray depiction of 2-5 (top) and 2-6 (bottom); C: light grey, B: yellow-green, O:

red, S: yellow, N: blue, Cl: green, F: pink, hydrogen atoms have been omitted for clarity. ........ 37

Figure 2.7 Surface contour plot of LUMO of 2-4 (top left), 2-5 (top right), 2-6 (bottom left),

and 2-7 (bottom right); C: dark grey, H: light grey, N: dark blue, O: red, F: light blue. .............. 42

Figure 2.8. POV-ray depiction of 2-8; C: light grey, P: orange, O: red, S: yellow, N: blue, F: pink,

hydrogen atoms have been omitted for clarity. ............................................................................. 48

Figure 2.9. Surface contour plots of 2-8 orbitals: HOMO (left) and LUMO (right). ................... 49

Figure 3.1. Electrophilicity and Lewis acidity trends for perchlorophenyl-substituted boranes. . 63

Figure 3.2. Halogen mesomeric stabilization effects of fluorine (left) and chlorine (right). ........ 64

Figure 3.3. POV-ray depiction of 3-1 (top) and 3-4 † (bottom); C: light grey, P: orange, F: pink,

Cl: green, hydrogen atoms have been omitted for clarity. ............................................................ 69

Figure 3.4. Orthogonal POV-ray depictions of ((C6Cl5)2P)2; C: light grey, P: orange, Cl: green,

hydrogen atoms have been omitted for clarity. ............................................................................. 71

xix

Figure 3.5. Stacked partial 31P{1H} NMR spectra (top to bottom: 3-3, 3-2, 3-1, PPh3, P(C6F5)Ph2,

P(C6F5)3). ...................................................................................................................................... 72

Figure 3.6. Sample DART mass spectrum displaying isotopic distribution of 3-3. ..................... 73

Figure 3.7. POV-ray depiction of 3-6 † (top) and 3-9 † (bottom); C: light grey, P: orange, F: pink,

Cl: green, hydrogen atoms have been omitted for clarity. ............................................................ 75

Figure 3.8. POV-ray depiction of (C6Cl5)2; C: light grey, Cl: green. ........................................... 77

Figure 3.9. POV-ray depiction of 3-11; C: light grey, B: yellow-green, P: orange, F: pink, Cl:

green, hydrogen atoms have been omitted for clarity. .................................................................. 79

Figure 3.10. POV-ray depiction of [Ph2PF(C6Cl5)][N(SO2Ph)2], 3-10NFSI †; C: light grey, B:

yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, Cl: green, hydrogen atoms have been

omitted for clarity. ........................................................................................................................ 80

Figure 3.11. Surface contour plots of the LUMO oriented along the P-F bond for cations of 3-10

(top left), 3-11 (top right), 3-12 (bottom left), 3-13 (bottom right); P: orange, C: black, F: blue, Cl:

green, H: light grey. ...................................................................................................................... 84

Figure 3.12. Correlation of LUMO energies to ω for salts 3-10 – 3-13, [FP(C6Cl5)3][B(C6F5)4], and

[FP(C6F5)3][B(C6F5)4] (left). Correlation of chemical shift to ω for salts 3-10 – 3-13, with

[FP(C6F5)3][B(C6F5)4] outlier (right). ........................................................................................... 86

Figure 3.13. Correlation of 31P{1H} NMR chemical shifts and FIA values for the cations of

3-10 – 3-13, and [FP(C6F5)3][B(C6F5)4]. ....................................................................................... 88

Figure 3.14. Catalytic activity of fluorophosphonium salts 3-10 – 3-13. Catalytic screening for

compounds 3-10, 3-12, and 3-13 was completed by Vitali Podgorny. ......................................... 93

Figure 4.1. Steric encapsulation of cyano group by B(C12F9): POV-ray (left, fluorine atoms

omitted), space-filling model (right). .......................................................................................... 116

Figure 4.2. POV-ray depiction of 4-11 (top), Br-aryl interaction of 4-11 (middle), T-shaped π-π

interaction of 4-11 (bottom); C: light grey, B: yellow-green, Br: purple, P: orange, F: pink,

Centroid: red; selected fluorine atoms have been omitted for clarity. ........................................ 124

Figure 4.3. POV-ray depiction of (C12F9)POF2,H2O; C: light grey, P: orange, F: pink, H: white.

..................................................................................................................................................... 128

xx

Figure 4.4. POV-ray depiction of 4-20 (top left, counterion omitted), 4-21 (top right, counterion

omitted), and 4-22 (bottom); C: light grey, B: yellow-green, P: orange, F: pink, hydrogen atoms

have been omitted for clarity. ..................................................................................................... 130

Figure 4.5. POV-ray depiction of 4-23; C: light grey, B: yellow-green, O: red, P: orange, F: pink,

H: white. ...................................................................................................................................... 131

Figure 4.6. Surface contour plot of the LUMO oriented along the P-F bond for cations 4-20 (top

left), 4-21 (top right), 4-22 (bottom left), 4-23 (bottom right); P: orange, C: black, F: blue, H: light

grey. ............................................................................................................................................ 134

Figure 4.7. Partial 19F NMR spectrum (3 – -2 ppm) of competition experiment between 4-23 and

PF2(C6F5)3 in CH2Cl2 after 1 h (left) and 24 h (right). ................................................................ 138

Figure 5.1. POV-ray depiction of 5-4; C: light grey, O: red, P: orange, F: pink, hydrogen atoms

have been omitted for clarity. ..................................................................................................... 176

Figure 5.2. Contour plot for the LUMO of 5-6. .......................................................................... 180

1

Chapter 1 Introduction

1.1 Elemental Phosphorus

Phosphorus is the eleventh most abundant element and the most abundant pnictogen in the Earth’s

crust, found mainly as inorganic mineral phosphates.1 Phosphorus is found ubiquitously in an

incredibly diverse range of roles: organometallic catalysts, detergents, water treatments, fertilizers,

biological systems, pesticides and electronics to name a few.2 The versatility arises from the

structural and electronic diversity accessible to phosphorus containing compounds. Phosphorus

compounds range from one-coordinate to six-coordinate with oxidation states ranging from +5 to

-3 have been observed.3,4

Elemental phosphorus exists as a variety of allotropes, the most common of which are white and

red phosphorus. White phosphorus, or tetraphosphorus, is a molecule containing four phosphorus

atoms arranged in a tetrahedron. White phosphorus exhibits spherical aromaticity, delocalization

of electron density around the outer radius of the tetrahedron.5 White phosphorus is obtained by

vapourizing the phosphorus from phosphate minerals at temperatures exceeding 1200 °C and

condensed under water. Red phosphorus is a polymeric allotrope resulting from the linkage

between P4 fragments found in white phosphorus and is obtained when white phosphorus is heated

or exposed to light.2

The controlled reaction of white phosphorus with elemental chlorine yields phosphorus

trichloride,6 which, along with the other trihalides of phosphorus, are used as starting materials in

the synthesis of a diverse array of phosphorus(III) compounds. Phosphines of the form PR3, where

R is an organic substituent, play an important role in synthetic inorganic and organic chemistry as

versatile Lewis bases. By varying the phosphine substituents, the steric bulk and electronics can

be fine-tuned to achieve desired properties.

2

1.2 Lewis Acidic Phosphorus Species

1.2.1 Phosphenium Cations

While organophosphorus species are commonly associated with the Lewis basic properties of

phosphines, there are reported examples of Lewis acidic phosphorus centres. Phosphenium cations

are two-coordinate phosphorus cations, which are known to exhibit both Lewis acidic and Lewis

basic behaviour. Dimroth and Hoffman reported the first phosphenium cations, which were

obtained by the reaction of reaction of 2-chlorobenzothiazolium salts with

tris(hydroxymethyl)phosphine (Scheme 1.1).4

Scheme 1.1. Synthesis of benzothiazolium phosphenium cation.

Phosphenium cations are stabilized by extensive charge delocalization over the substituents, which

are commonly amido groups due to their high π-donor ability.7 Phosphenium cations derive their

Lewis acidic behavior from the vacant p-orbital and formal positive charge on the phosphorus

centre. Phosphenium cations form adducts with phosphines, as first observed by Parry in the

reaction of tris(dimethylamino)phosphine with various phosphenium cations to form diphosphorus

cations (Scheme 1.2)8

Scheme 1.2. Phosphenium-phosphine adduct formation.

Interestingly, due to the presence of Lewis basic lone-pair of electrons and a Lewis acidic vacant

orbital on the phosphorus centre, phosphenium cations are also very effective reagents in the

reaction with 1,3-dienes. The related McCormack reaction that combines 1,3-dienes with

dihalophosphines proceeds slowly at very high temperatures to produce P-heterocyclic products.9

The reaction of phosphenium reagents as dieneophiles with substituted 1,3-butadienes was

reported by Cowley and Baxter in back-to-back reports (Scheme 1.3).10,11 The reaction of

3

phosphenium with 1,3-butadienes readily generates cyclic phosphonium cations, in contrast to the

slow generation of cyclic phosphines generated by the McCormack reaction.

Scheme 1.2. Phosphenium-phosphine adduct formation.

1.2.2 Phosphonium Cations

Phosphonium cations are another class of phosphorus compounds that exhibits Lewis acidic

behavior. Unlike phosphenium cations, phosphonium cations do not possess a lone-pair of

electrons. Phosphonium cations have a tetrahedral four-coordinate phosphorus centre, and are

related to phosphenium cations by the process of oxidative addition.12 Phosphonium cations

contain neither a lone-pair of electrons nor a vacant p-orbital on the phosphorus centre. Unlike

borane and phosphenium Lewis acids which use a vacant p-orbital, phosphonium cations instead

use a low energy σ* orbital.13 Interaction of the Lewis base with a phosphonium cation forms a

five-coordinated trigonal-bipyramidal phosphorane species with a hypervalent interaction

(Scheme 1.4).

Scheme 1.4. Hypervalent interaction between a phosphonium cation and Lewis base.

The stability of pentavalent phosphorane species increases with the total electronegativity of the

substituents.2 The hypervalent interaction is most stabilized when the two apical positions involved

in the interaction are electron withdrawing.13 The apicophilicity, or tendency of a group to occupy

the apical position, is related to the electronegativity, steric size, and π-acceptor nature. The

following trend has been proposed for apicophilicity:

F > H > CF3 > OPh > Cl > SMe > OMe > NMe2 > Me > Ph

4

Depending on the relative apicophilicity of substituents on a phosphorane, rearrangements of

geometry can occur through either Berry pseudorotation or turnstile rotation.14 In Berry

pseudorotation, the two apical substituents distort to form a square pyramidal intermediate,

followed by rotation of the two previously equatorial substituents into the apical positions. The net

change of a Berry pseudorotation exchanges two equatorial substituents with two apical

substituents. In a turnstile rotation, two pairs of one equatorial and one apical substituent rotate

about an axis perpendicular to the three other substituents. Pseudorotation is significantly inhibited

by the presence of two substituents with low apicophilicity and virtually eliminated when three

such substituents are present.15

Wittig reported the reaction of a triphenylphosphonium ylide with ketones to generate an alkene

and triphenylphosphine oxide.16 Quaternary phosphonium salts are generated through the reaction

of triphenylphosphine with an alkyl halide and are subsequently used to generate the ylide through

deprotonation by a strong organolithium base. The mechanism for the reaction between the ylide

and ketone proceeds through a [2+2] cycloaddition with a four-membered cyclic oxaphosphetane

intermediate. While the ylide α-carbon acts as a nucleophile, the phosphorus centre acts as a Lewis

acid, accepting a lone pair of electrons to form a bond with oxygen (Scheme 1.5).

Scheme 1.5. The Wittig reaction mechanism.

In an extension of this chemistry, Merz reported employing a two-phase system of CH2Cl2 and

aqueous NaOH solution with a variety of phosphonium cations to carry out olefination of

aldehydes.17 Deprotonation of phosphonium with NaOH generated an ylide species which would

react with aldehydes to form the corresponding alkenes. The reaction of the ylide proceeds

sufficiently fast that decomposition of the quaternary salts to phosphine oxide does not occur

appreciably.

Matsu and coworkers have reported the use of bis-phosphonium salts as catalysts in the formation

of β-aminoesters from corresponding imines and ketene silyl acetals (Scheme 1.6).18 The

phosphonium species, an oxo-bridged bis-trialkylphosphonium salt, has also demonstrated

5

catalytic competency in the aldol-type reaction of aldehydes or acetals with various nucleophiles,

and the Michael reaction of ketones or acetals with silyl nucleophiles.19 High yields were obtained

even with catalyst loading as low as 2.5 mol%. The reaction of 4-dimethylaminobenzaldehyde

with tert-butyldimethylsilyl ketene acetal was catalyzed effectively by the phosphonium catalyst,

while the amine was found to quench other Lewis acids. The mechanism is proposed to proceed

through initial activation of the imine by the Lewis-acidic bis-phosphonium and subsequent attack

of the ketene silyl acetal to generate the β-aminoester and regenerate the catalyst.

Scheme 1.6. Catalytic activity of oxo-bridged bis-phosphonium salt.

Considerable utility has been found in the application of phosphonium salts as ionic liquids, which

demonstrate enhanced thermal and chemical stability over more conventional imidazolium and

ammonium ionic liquids.20,21 While most phosphonium based ionic liquids employ very large alkyl

substituents, the phosphonium centres are still able to function as weak Lewis acid catalysts.

McNulty and coworkers employed trihexyl(tetradecyl)phosphonium decanoate to catalyze the

nucleophilic attack of diethylzinc onto the activated benzaldehyde carbonyl (Scheme 1.7).22

Scheme 1.7. Phosphonium ionic liquid carbonyl activation.

1.2.3 Fluorophosphonium Catalysts

In 2013, our group reported the synthesis of two highly electrophilic fluorophosphonium cations

[(C6F5)3-nPFPhn][B(C6F5)4] for n = 0, 1.23 These fluorophosphonium cations were obtained by

6

oxidation of P(C6F5)3 or (C6F5)2PPh using XeF2 to generate the corresponding

difluorophosphorane species and subsequent fluoride-abstraction using [SiEt3][B(C6F5)]·C7H8.

Both fluorophosphonium cations were found to activate alkyl C-F bonds leading to concomitant

generation of a carbocation and the corresponding difluorophosphorane species. Catalytic

hydrodefluorination of alkanes was achieved by introducing HSiEt3 as a hydride source and

fluoride sink (Scheme 1.8).

Scheme 1.8. Catalytic hydrodefluorination activity of fluorophosphonium catalysts.

The mechanism was proposed to begin with the activation of a fluoroalkane by

[PF(C6F5)3][B(C6F5)4] to form difluorophosphorane and carbocation. Subsequent delivery of

hydride by HSiEt3 produces an alkane and silylium cation, which regenerates the

fluorophosphonium catalyst by fluoride abstraction from the difluorophosphorane. This

mechanism is supported by the observation that combination of [PF(C6F5)3][B(C6F5)4] with HSiEt3

results in no reaction, even after several weeks, whereas [PF(C6F5)3][B(C6F5)4] reacts rapidly with

fluoroalkanes. Further, addition of silylium cation [SiEt3][B(C6F5)]·C7H8 to a 1:1 mixture of

PF2(C6F5)3 and perfluorotoluene results in the exclusive reaction with PF2(C6F5)3 to generate

[PF(C6F5)3][B(C6F5)4] and FSiEt3. Computations were carried out that support initial C-F

activation by [PF(C6F5)3][B(C6F5)4] as more favourable than initial H-Si activation. Computations

of the LUMO show a significant lobe on the phosphorus centre, consistent with a low energy P-F

σ*-orbital being the site of Lewis acidity (Figure 1.1). Dissimilar to phosphonium cations, both

silylium cations and boranes use a vacant p-orbital as the site of reactivity.

7

Figure 1.1. Lewis-acidic orbitals of a fluorophosphonium cation (left), silylium cation (middle),

and borane (right).

This family of fluorophosphonium cations represent a very significant improvement in

phosphonium catalyst design. The high combined electron-withdrawing nature of the substituents

stabilizes difluorophosphorane intermediates. Additionally, the drastically different

apicophilicities of the F and C6F5 substituents eliminate rearrangements of corresponding five-

coordinate species. The observed stability of related trifluorophosphorane species, which contain

a highly apicophilic substituent in the equatorial position have demonstrated significantly reduced

stability compared to difluorophosphorane species. Additionally, the high electrophilicity of

fluorophosphonium cations allows for a broad range of accessible reactivity.

A diverse range of catalytic reactivity has been observed for fluorophosphonium cations including

the hydrodefluorination of fluoroalkanes; hydrosilylation of alkenes, alkynes, imines and ketones;

and dimerization of 1,1-diphenylethylene. Fluorophosphonium cations have also been reported to

effect the dehydrocoupling of silanes with amines, thiols, phenols and carboxylic acids to generate

a Si-E bond (E= N, S, O) and molecular hydrogen. The hydrogen generated by dehydrocoupling

can further be used to effect the hydrogenation of olefins.24 Additionally, the dehydrocoupling of

ditolylamine with triethylsilane yields bulky silylamine p-tol2NSiEt3 which could be used with

fluorophosphonium to effect dihydrogen activation and catalytic olefin hydrogenation (Scheme

1.9).25

Scheme 1.9. Dehydrocoupling hydrogenation reactivity of fluorophosphonium cations.

Our group has also investigated the chloro- and bromo- analogues [PCl(C6F5)3][B(C6F5)4] and

[PBr(C6F5)3][B(C6F5)4], generated by phosphine oxidation by sulfuryl chloride and elemental

8

bromine, respectively.26 Interestingly, [PCl(C6F5)3][B(C6F5)4] showed the lowest catalytic activity

of the three halo-analogues, despite the higher electronegativity of Cl compared to Br. The low

activity of [PCl(C6F5)3][B(C6F5)4] was attributed to quenching of electrophilicity by back-donation

of the chlorine substituent onto the phosphorus centre. The three phosphonium cations

[PX(C6F5)3][B(C6F5)4] X=F, Cl, Br, were used to catalyze the hydrosilylation of ketones, nitriles

and imines (Scheme 1.10).

Scheme 1.10. Ketone hydrosilylation reactivity of chloro- and bromophosphonium cations.

The use of fluoro-substituted phenoxy groups has also been investigated in a series of

phosphonium cations.27 The electrophilicity of the series was evaluated computationally using the

general electrophilicity index (ω) and fluoride ion affinity (FIA). As with the chloro- and

bromophosphonium cations, the phenoxyphosphonium cations display lower electrophilicity than

the corresponding fluorophosphonium. The ω and FIA values correlated very well, displaying a

trend of increasing electrophilicity with increasing fluorine substitution on the phenoxy ring

(Figure 1.2).

Figure 1.2. Electrophilic index values (ω) and FIA values of a series of phenoxyphosphonium

cations.

9

1.2.4 Dicationic Phosphorus Catalysts

Developing a phosphonium cation with electrophilicity greater than that of [PF(C6F5)3][B(C6F5)4]

without substituting the stabilizing C6F5 groups for additional reactive halides is a challenging

endeavor. One approach investigated by our group to generate highly Lewis acidic phosphonium

salts is the inclusion of multiple linked phosphonium centres that interact cooperatively with

substrates. A series of bis-diphenylfluorophosphonium cations connected by alkyl chains of varied

length (one to five carbon atoms) were evaluated in a series of Lewis acid catalytic transformations,

including the dimerization of 1,1-diphenylethylene (Scheme 1.11).28 The reactivity of the bis-

phosphonium cations decreased with increasing chain length, decreasing significantly after a

length of three carbon atoms. While the bis-phosphonium cations with methylene and ethylene

linkers display similar activity in most test reactions, the ethylene linked bis-phosphonium cation

displays significantly reduced activity in the hydrodefluorination of fluoropentane. The reactivity

of the related fluorophosphonium [Ph3PF][B(C6F5)4] is comparable to that of the

bis-fluorophosphonium with a five carbon tether.29 The source of enhanced Lewis acidity of the

bis-fluorophosphonium cations was not fully elucidated.

Scheme 1.11. Dimerization of 1,1-diphenylethylene reactivity of alkyl-linked bis-phosphonium

cations.

Another approach in generating more electrophilic phosphonium catalysts involves the

incorporation of additional cationic charge. Our group has previously used this method in

developing electrophilic borenium catalysts, which derive Lewis acidity from cationic charge

instead of inductively electron-withdrawing substituents employed by neutral borane

catalysts.30,31,32 Carbene-stabilized borenium cations were able to catalyze the hydrogenation of

imines with a turn-over frequency significantly higher than neutral B(C6F5)3. Applying this

methodology to generate fluorophosphonium dicationic species has also proved fruitful. N-

10

heterocyclic carbene-stabilized diphenylfluorophosphonium cations were generated using a

similar synthetic strategy to the monocationic species, but instead starting from NHC-stabilized

phosphenium cation.33 Attempts to ascertain the Lewis acidity of the phosphorus dication using

the Gutmann-Beckett (vide infra) method resulted in the fluorodeoxygenation of triethylphosphine

oxide with concomitant formation of the NHC-stabilized phosphenium oxide (Scheme 1.12). The

fluorophosphonium dication is very competent in hydrodefluorination and hydrosilylation

chemistry, indicative of enhanced Lewis acidity.

Scheme 1.12. Oxide-fluoride exchange reactivity of NHC-stabilized dication.

In a related work, phosphorus dications were also generated by incorporation of a

methylpyridinium substituent onto a fluorophosphonium cation.34 Phosphine starting material 2-

pyridyldiphenylphosphine was used to generate both the methyl- and fluorophosphonium dications

after methylation of the pyridyl group. The methylphosphonium dication proved to be significantly

less active than the fluorophosphonium cation in the hydrodefluorination of fluoropentane,

highlighting the importance of the highly electronegative fluorine substituent (Scheme 1.13).

Scheme 1.13. Hydrodefluorination reactivity of pyridinium-phosphonium dications.

1.3 Electrophilicity and Lewis Acidity Measurement Scales

Methods to measure electrophilicity and Lewis acidity are critical to understanding observed

reactivity trends and predicting catalyst suitability in new applications. To be an effective method,

11

the data required to compare and rank electrophiles must be easily obtained, but also consider

sufficient variables related to the steric and electronic structure of the electrophile as to allow

comparison across many systems. The ease of measurement leads to more widespread adoption of

the method, which leads to the generation of more data points for comparison. Two general types

of scales will be discussed throughout the thesis: trends arising from NMR chemical shift data and

those obtained through ab initio calculations of systems.

Lewis acidity scales require that a sufficient interaction occur between the analyte and Lewis base

in order to obtain a meaningful measurement. Excessive steric encumbrance can preclude

formation of adducts rendering a Lewis acid incompatible with that testing method, therefore

methods like Childs’, Gutmann-Beckett and FIA all employ Lewis bases with minimal steric

demand. While the upside of employing small Lewis bases in Lewis acidity tests is more

widespread compatibility, the downside is that the trends observed may be less applicable to

predicting reactivity trends with larger substrates.

As outlined by Drago and Matwiyoff “[a]ny order of donor or acceptor strengths must be

established relative to a given donor or acceptor. Reversals may be expected when orders towards

different donors (or acceptors) are compared”.35 However, obtaining multiple measures of Lewis

acidity and electrophilicity allows for better understanding of a particular and allows for more

accurate prediction of future reactivity trends.

1.3.1 Chemical Shift Electrophilicity Scales

Obtaining electrophilicity measurements from NMR chemical shift data is an attractive method

because the data can be obtained easily using techniques and instrumentation available to most

synthetic chemists. Lewis acid quantification by NMR chemical shift data has been reported for a

wide variety of nuclei, including 1H, 2H, 19F, 23Na and 31P.,36, 37,38 The most widely utilized Lewis

acidity quantification scales are the Childs’ and Gutmann-Beckett methods (vide infra).

Our group has also reported the use of 31P and 19F NMR spectroscopic data to assess the

electrophilicity across a series of fluorophosphonium cations.39 Both the chemical shift of the

phosphorus centre and phosphorus bound fluorine are both responsive to changes in Lewis acidity,

12

displaying complementary trends to the computed FIA trend. The sensitivity of the

fluorophosphonium chemical shift to changes in electrophilicity is not unprecedented as the

Gutmann-Beckett method (vide infra) reliably uses the chemical shift of triethyloxyphosphonium

with various oxygen substituents to assess Lewis acidity.

1.3.1.1 Childs’ Method

Childs reported using trans-crotonaldehyde, among other aldehydes, to measure Lewis acidity by

1H NMR spectroscopy.38 By this method, crotonaldehyde is added 1:1 to a cooled solution of the

desired Lewis acid and a 1H NMR spectrum is obtained (Scheme 1.14).

Scheme 1.14. Childs’ method protocol for BCF.

The proton signal of vinylic proton H3 to the carbonyl oxygen atom exhibits a downfield shift

when an adduct is formed, the magnitude of this downfield shift is proportional to the

electrophilicity of the Lewis acid. This method has seen widespread adoption, though not to the

same degree as the Gutmann-Beckett method, perhaps owing to the comparative difficulty of

preparing air-sensitive samples at low temperatures.40 The Childs’ method has been reported to be

incompatible with a wide range of fluorophosphonium cations, as addition of crotonaldehyde to

solutions of fluorophosphonium cations yields complex mixtures of products. 39,33

1.3.1.2 Gutmann Beckett

Gutmann proposed using the Acceptor Number (AN) as a useful guide for “choosing the most

appropriate solvent for a given reaction.” The AN is determined by comparing the 31P NMR

chemical shift for OPEt3 in the solvent of interest to the chemical shift of OPEt3 in hexane.41 The

AN scale is based off arbitrary fixed points of hexane (AN = 0) and SbCl5 (AN = 100).

Additionally, Gutmann measured the 31P NMR chemical shift at various dilutions and extrapolated

to infinite dilution to correct for concentration effects. Triethylphosphine oxide is ideal for this

13

application because it is a strong base that can only interact through one well-defined site at

oxygen, and the ethyl substituents enhanced solubility and provide electronic shielding without

significant steric encumbrance.

Beckett modified the scale developed by Gutmann to quantify the Lewis acidity boron-containing

Lewis acids.42 The polymerization of epoxy resins can be initiated by boron Lewis acids, with a

rate dependence on the strength of the Lewis acid. Since 11B NMR shifts of boranes are highly

variable and do not show correlation to electrophilicity of the boron centre, the Gutmann scale

proved to be a useful alternative.43 The AN obtained for boranes showed correlation with the

measured reaction rates of epoxide polymerization. The AN is dependent on how well a boron-

bound heteroatom competes with OPEt3 for the boron acceptor orbital. Gutmann provides a

simplified procedure in which an OPEt3 is added to neat borane in a 5mm NMR tube nested in a

10mm NMR tube filled with CDCl3 for use as a lock. The acceptor number was calculated using

AN = (δ(sample) - 41.0) × {100/(86.14 - 41.0)}

where 86.14 is the 31P NMR chemical shift of OPEt3 with SbCl5 and 41.0 is the chemical shift of

OPEt3 in hexane. The simplified measurement of AN proposed Beckett gave results with good

agreement to those obtained by Gutmann without requiring multiple data points for different

dilutions.

Britovsek proposed a further simplified methodology, wherein a 1:1 mixture of borane and OPEt3

were dissolved in C6D6 and the 31P NMR spectrum was obtained (Scheme 1.15).44

Scheme 1.15. Gutmann-Beckett protocol for BCF.

The same change in chemical shift was observed for the borane adducts in C6D6, THF and CDCl3,

indicating that with a significantly strong adduct formed with the Lewis acid, the effect of the

solvent is negated. Britovsek measured the Lewis acidity of the series (C6F5)3-nB(OC6F5)n for

n = 0 – 3 to find that using the Gutmann-Beckett method the Lewis acidity increases as n increases;

14

however, the opposite trend was obtained using the Childs’ method. This reversal was rationalized

as hard Lewis base OPEt3 interacts more strongly as the hardness of the Lewis acid increased with

OC6F5 substitution, but a soft base like crotonaldehyde interacts less strongly as the hardness

increased. This illustrates the limited predictive power of Lewis acidity trends, as the trends only

necessarily hold for closely related systems and trends can reverse when different donor types are

used.

O’Hare and many others have used this modified Gutmann-Beckett method to assess the strength

of Lewis acids in FLP chemistry.45 Additionally, this method has been applied to

fluorophosphonium systems with mixed results. Some systems have been amenable to the

Gutmann-Beckett method,23 while other systems do not form adducts with OPEt3,39 while others

yet react with OPEt3 in a deoxygenative fashion.28

1.3.2 Computational Electrophilicity Measures

The use of ab initio computational methods to assess the electrophilicity of Lewis acidity provides

the significant advantage of studying systems that are not synthesized or have not been suitably

purified. Additionally, computations like fluoride ion affinities and general electrophilicity indices

are compatible with a wider range of Lewis acids when compared to empirical methods like

Childs’ or Gutmann-Beckett.

1.3.2.1 Fluoride Ion Affinity

Christe and coworkers proposed FIA as an ab initio method to quantize Lewis acidity (Scheme

1.16).46 The fluoride ion is an ideal Lewis base for this application as it exhibits very high basicity

with negligible steric encumbrance and forms adducts with “essentially all Lewis acids.” The

reaction enthalpy of fluoride ion-adduct formation can then be used to quantify relative Lewis acid

strength.

15

Scheme 1.16. Fluoride ion affinity for a fluorophosphonium cation.

Due to difficulties in calculating the electron affinity of F, Christe used the experimentally

determined FIA value for difluoroketone, COF2, of 49.9 kcal/mol.47 Using this empirical value

obtained for COF2, the FIA values of Lewis acids can be calculated using the following equations

CF2O + F- CF3O- ΔH1 = 49.9kcal/mol

CF3O- + LA CF2O + F-LA- ΔH2

LA + F- F-LA- ΔH3 = ΔH2 + 49.9kcal/mol

where LA and F-LA represent the Lewis acid and fluoride adduct respectively. The first equation

represents the FIA of CF2O with experimentally determined enthalpy. An estimation of enthalpy

of the second equation can be obtained using the computed internal energies of the Lewis acid, the

fluoride-Lewis acid adduct, CF2O and the CF3O- anion. Using the values from the first two

equations, the enthalpy for the FIA of LA can be obtained as the sum.

Fluoride ion affinity calculations are typically resource intensive, as optimized geometries and

energies for both the Lewis acid and fluoride-Lewis acid adduct need to be obtained. Fluoride ion

affinities allow for comparison of a wide range of Lewis acids, including systems that are not

compatible with empirical tests like the Gutmann-Beckett method. Additionally, FIAs can be used

predictively to ascertain the effectiveness of synthetically challenging systems. Fluoride ion

affinities have been used extensively to evaluate the Lewis acidity of boranes and

fluorophosphonium cations.48,39

1.3.2.2 General Electrophilicity Index

The General Electrophilicity Index is a system proposed by Parr as a quantitative scale for

electrophiles.49 Parr proposed the electrophilicity index of a compound be defined as “the square

of its electronegativity divided by its chemical hardness.” This definition was strongly motivated

16

by an earlier work in which Maynard observed a strong correlation between the value χ2/η and the

reaction rates of electrophiles with a protein they were studying.50 The electrophilicity index, ω,

estimates the ability of a chemical species to “soak up” electrons and is defined as

ω ≡ μ2/2η

where μ is chemical potential and η is the chemical hardness. Both chemical potential and chemical

hardness can be defined in terms of ionization energy (I) and electron affinity (A) as

χ = –μ = (I + A)/2 and η = (I – A)

where χ is the absolute electronegativity. Mulliken electronegativity is defined as being equal to (I

+ A)/2. Per Koopman’s theorem, 51,52 chemical hardness and chemical potential can be related to

frontier orbital energies by

EHOMO = I and ELUMO = A

Using these equations, the electrophilic index of a system can be calculated using the following

equation

The general electrophilicity index is an attractive method, as it requires single point energy

calculations of only the free Lewis acid. Our group has reported the use of ω values to quantify the

electrophilicity of a series of phenoxyphosphonium cations and found high correlation with FIA

values.27 Since steric effects and hybridization energies are not taken into account when calculating

ω values, only similar systems can be accurately compared.

1.4 Scope of Thesis

The objective of this thesis is to develop strategies for main group borane and fluorophosphonium

catalyst enhancement. Chapter 2 investigates the use of hypervalent iodine reagents to effect

nitrene insertion into borane-substituent bonds to tune the Lewis acidity of the resulting

aminoboranes. Chapter 3 explores the use of perchloroaryl substituents on phosphonium cations

17

as a means of imparting air-stability. In chapter 4, 2-nonafluorobiphenyl substituents are

incorporated into a series of fluorophosphonium cations to enhance both electrophilicity and

chemical stability.

In chapter 5, the use of atropisomeric binaphthyl substituents are investigated as a means to

develop chiral fluorophosphonium cations.

Chapter 3 contains work performed collaboratively with Vitali Podgorny. The synthesis of

compounds 3-4, 3-5, 3-8, 3-9, 3-12 and 3-13 and catalytic testing of 3-10, 3-12 and 3-13 was

carried out by Mr. Podgorny, characterization of these compounds are detailed in his MSc.

thesis.53 Additionally, certain crystals obtained for X-ray diffraction studies were grown by Mr.

Podgorny and are indicated as such within Chapter 3. Refinement of all solid-state structures herein

were carried out by the author. All other synthetic work herein was performed by the author.

Elemental analysis and high-resolution mass spectrometry were performed in-house by the

ANALEST and AIMS laboratory respectively.

Portions of chapters 2 and 3 have been published at the time of writing:

Chapter 2: Postle, S.; Stephan, D. W. Reactions of Iodine-Nitrene Reagents with Boranes. Dalton

Trans. 2015, 44, 4436-4439.

Chapter 3: Postle, S.; Podgorny, V.; Stephan, D. W. Electrophilic Phosphonium Cations (EPCs)

with Perchlorinated-Aryl Substituents: Towards Air-Stable Phosphorus-Based Lewis Acid

Catalysis. Dalton Trans. 2016, 45, 14651-14657.

18

1.5 References

1. Holland, H. D.; Turekian, K. K., Treatise on Geochemistry. 1st ed.; Elsevier/Pergamon:

Amsterdam; Boston, 2004.

2. Corbridge, D. E. C., Phosphorus: Chemistry, Biochemistry and Technology. 6th ed.; Taylor

& Francis: Boca Raton, 2013; p 1439

3. Gier, T. E., HCP, a Unique Phosphorus Compound. J. Am. Chem. Soc. 1961, 83, 1769.

4. Dimroth, K.; Hoffmann, P., Phosphacyanines New Class of Compounds Containing

Trivalent Phosphorus. Angew. Chem., Int. Ed. 1964, 3, 384.

5. Hirsch, A.; Chen, Z. F.; Jiao, H. J., Spherical aromaticity of inorganic cage molecules.

Angew. Chem., Int. Ed. 2001, 40, 2834.

6. Forbes, M. C.; Roswell, C. A.; Maxson, R. N., Phosphorus(III) Chloride. Inorg. Synth.

1946, 2, 145.

7. Cowley, A. H.; Kemp, R. A., Synthesis and Reaction Chemistry of Stable 2-Coordinate

Phosphorus Cations (Phosphenium Ions). Chem. Rev. 1985, 85, 367.

8. Schultz, C. W.; Parry, R. W., Structure of [2((CH3)2N)2PCl].AlCl3,

((CH3)2N)3P.((CH3)2N)2PCl.AlCl3, and Related Species Diphosphorus Cations. Inorg. Chem.

1976, 15, 3046.

9. Quin, L. D., The Heterocyclic Chemistry of Phosphorus: Systems Based on the

Phosphorus-Carbon Bond. Wiley: New York, 1981; p 434.

10. Soohoo, C. K.; Baxter, S. G., Phosphenium Ions as Dienophiles. J. Am. Chem. Soc. 1983,

105, 7443.

11. Cowley, A. H.; Kemp, R. A.; Lasch, J. G.; Norman, N. C.; Stewart, C. A., Reaction of

Phosphenium Ions with 1,3-Dienes - a Rapid Synthesis of Phosphorus-Containing 5-Membered

Rings. J. Am. Chem. Soc. 1983, 105, 7444.

12. Lindner, E.; Weiss, G. A., Oxidative Addition of a C-H Bond at a 2-Coordinated

Phosphenium Cation. Chem. Ber. Recl. 1986, 119, 3208.

13. Werner, T., Phosphonium Salt Organocatalysis. Adv. Synth. Catal. 2009, 351, 1469.

14. Ugi, I.; Marquarding, D.; Klusacek, H.; Gillespie, P.; Ramirez, F., Berry Pseudorotation

and Turnstile Rotation. Acc. Chem. Res. 1971, 4, 288.

19

15. Corbridge, D. E. C., Phosphorus: Chemistry, Biochemistry and Technology. 6th ed.; Taylor

& Francis: Boca Raton, 2013; p 1439.

16. Wittig, G.; Schollkopf, U., Uber Triphenyl-Phosphinmethylene Als Olefinbildende

Reagenzien .1. Chem. Ber. Recl. 1954, 87, 1318.

17. Markl, G.; Merz, A., Carbonyl Olefination with Non-Stabilized Phosphine Alkylenes in

Aqueous Systems. Synthesis (Stuttg) 1973, 295.

18. Mukaiyama, T.; Kashiwagi, K.; Matsui, S., A Convenient Synthesis of Beta-Aminoesters

- the Reaction of Imines with Ketene Silyl Acetals Catalyzed by Phosphonium Salts. Chem. Lett.

1989, 1397.

19. Mukaiyama, T.; Matsui, S.; Kashiwagi, K., Effective Activation of Carbonyl and Related-

Compounds with Phosphonium Salts - the Aldol and Michael Reactions of Carbonyl-Compounds

with Silyl Nucleophiles and Alkyl Enol Ethers. Chem. Lett. 1989, 993.

20. Keglevich, G.; Baan, Z.; Hermecz, I.; Novak, T.; Odinets, I. L., The Phosphorus Aspects

of Green Chemistry: the use of Quaternary Phosphonium Salts and 1,3-Dialkylimidazolium

Hexafluorophosphates in Organic Synthesis. Curr. Org. Chem. 2007, 11, 107.

21. Chowdhury, S.; Mohan, R. S.; Scott, J. L., Reactivity of Ionic Liquids. Tetrahedron 2007,

63, 2363.

22. Cheekoori, S. Phosphonium Salt Ionic Liquids in Organic Synthesis. Doctoral Thesis,

McMaster University, Hamilton, ON., 2008.

23. Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W., Lewis Acidity of

Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor. Science 2013,

341, 1374.

24. Pérez, M.; Caputo, C. B.; Dobrovetsky, R.; Stephan, D. W., Metal-Free Transfer

Hydrogenation of Olefins via Dehydrocoupling Catalysis. Proc. Nat. Acad. Sci. 2014, 111, 10917.

25. vom Stein, T.; Pérez, M.; Dobrovetsky, R.; Winkelhaus, D.; Caputo, C. B.; Stephan, D.

W., Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and

Catalytic Hydrogenation of Olefins. Angew. Chem. Int . Ed. 2015, 54, 10178.

26. Perez, M.; Qu, Z. W.; Caputo, C. B.; Podgorny, V.; Hounjet, L. J.; Hansen, A.;

Dobrovetsky, R.; Grimme, S.; Stephan, D. W., Hydrosilylation of Ketones, Imines and Nitriles

Catalysed by Electrophilic Phosphonium Cations: Functional Group Selectivity and Mechanistic

Considerations. Chem. Eur. J. 2015, 21, 6491.

20

27. LaFortune, J. H. W.; Johnstone, T. C.; Perez, M.; Winkelhaus, D.; Podgorny, V.; Stephan,

D. W., Electrophilic Phenoxy-Substituted Phosphonium Cations. Dalton Trans. 2016, 45, 18156.

28. Holthausen, M. H.; Hiranandani, R. R.; Stephan, D. W., Electrophilic Bis-

Fluorophosphonium Dications: Lewis Acid Catalysts from Diphosphines. Chem. Sci. 2015, 6,

2016.

29. Mehta, M.; Holthausen, M. H.; Mallov, I.; Perez, M.; Qu, Z. W.; Grimme, S.; Stephan, D.

W., Catalytic Ketone Hydrodeoxygenation Mediated by Highly Electrophilic Phosphonium

Cations. Angew. Chem., Int. Ed. 2015, 54, 8250.

30. Farrell, J. M.; Hatnean, J. A.; Stephan, D. W., Activation of Hydrogen and Hydrogenation

Catalysis by a Borenium Cation. J. Am. Chem. Soc. 2012, 134, 15728.

31. Farrell, J. M.; Posaratnanathan, R. T.; Stephan, D. W., A Family of N-heterocyclic

Carbene-Stabilized Borenium Ions for Metal-Free Imine Hydrogenation Catalysis. Chem. Sci.

2015, 6, 2010.

32. Welch, G. C.; Stephan, D. W., Facile Heterolytic Cleavage of Dihydrogen by Phosphines

and Boranes. J. Am. Chem. Soc. 2007, 129, 1880.

33. Holthausen, M. H.; Mehta, M.; Stephan, D. W., The Highly Lewis Acidic Dicationic

Phosphonium Salt: [(SIMes)PFPh2][B(C6F5)4]2. Angew. Chem. Int. Ed. 2014, 53, 6538.

34. Bayne, J. M.; Holthausen, M. H.; Stephan, D. W., Pyridinium-Phosphonium Dications:

Highly Electrophilic Phosphorus-based Lewis Acids Catalysts. Dalton Trans. 2016, 45, 5949.

35. Drago, R. S.; Matwiyoff, N. A., Acids and Bases. Heath: Lexington, Mass., 1968; p 121.

36. Hilt, G.; Punner, F.; Mobus, J.; Naseri, V.; Bohn, M. A., A Lewis Acidity Scale in Relation

to Rate Constants of Lewis Acid Catalyzed Organic Reactions. Eur. J. Org. Chem. 2011, 5962.

37. (a) Chu, Y. Y.; Yu, Z. W.; Zheng, A. M.; Fang, H. J.; Zhang, H. L.; Huang, S. J.; Liu, S.

B.; Deng, F., Acidic Strengths of Bronsted and Lewis Acid Sites in Solid Acids Scaled by 31P

NMR Chemical Shifts of Adsorbed Trimethylphosphine. J. Phys. Chem. C 2011, 115, 7660; (b)

Erlich, R. H.; Popov, A. I., Study of Solvation of Sodium Ions in Nonaqueous Solvents by 23Na

Nuclear Magnetic Resonance. J. Am. Chem. Soc. 1971, 93, 5620.

38. Childs, R. F.; Mulholland, D. L.; Nixon, A., Lewis Acid Adducts of α,β-Unsaturated

Carbonyl and Nitrile Compounds .2. A Calorimetric Study. Can. J. Chem. 1982, 60, 809.

39. Caputo, C. B.; Winkelhaus, D.; Dobrovetsky, R.; Hounjet, L. J.; Stephan, D. W., Synthesis

and Lewis Acidity of Fluorophosphonium Cations. Dalton Trans. 2015, 44, 12256.

21

40. Sivaev, I. B.; Bregadze, V. I., Lewis Acidity of Boron Compounds. Coord. Chem. Rev.

2014, 270, 75.

41. Mayer, U.; Gutmann, V.; Gerger, W., Acceptor Number - Quantitative Empirical

Parameter for Electrophilic Properties of Solvents. Monatsh. Chem. 1975, 106, 1235.

42. Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Varma, K. S., A Convenient NMR method

for the Measurement of Lewis Acidity at Boron Centres: Correlation of Reaction Rates of Lewis

Acid Initiated Epoxide Polymerizations with Lewis Acidity. Polymer 1996, 37, 4629.

43. Nöth, H.; Wrackmeyer, B., Nuclear Magnetic Resonance Spectroscopy of Boron

Compounds. In NMR - Basic Principles and Progress, Diehl, P.; Fluck, E.; Kosfeld, R., Eds.

Springer-Verlag: 1978; Vol. 14.

44. Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P., From B(C6F5)3 to B(OC6F5)3: Synthesis of

(C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity. Organometallics 2005, 24,

1685.

45. Binding, S. C.; Zaher, H.; Chadwick, F. M.; O'Hare, D., Heterolytic Activation of

Hydrogen Using Frustrated Lewis Pairs Containing Tris(2,2',2''-Perfluorobiphenyl)Borane. Dalton

Trans. 2012, 41, 9061.

46. Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A.,

On a Quantitative Scale for Lewis Acidity and Recent Progress in Polynitrogen Chemistry. J.

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47. Larson, J. W.; Mcmahon, T. B., Strong Hydrogen-Bonding in Gas-Phase Anions - an Ion-

Cyclotron Resonance Determination of Fluoride Binding Energetics to Bronsted Acids from Gas-

Phase Fluoride Exchange Equilibria Measurements. J. Am. Chem. Soc. 1983, 105, 2944.

48. Bohrer, H.; Trapp, N.; Himmel, D.; Schleep, M.; Krossing, I., From Unsuccessful H2

activation with FLPs Containing B(Ohfip)3 to a Systematic Evaluation of the Lewis Acidity of 33

Lewis Acids Based on Fluoride, Chloride, Hydride and Methyl Ion Affinities. Dalton Trans. 2015,

44, 7489.

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1988, 110, 7684.

53. Podgorny, V. Electrophilic Phosphenium and Phosphonium Cations: Synthesis and

Reactivity of Perfluoro- & Perchloroaryl Phosphorus Systems. University of Toronto, 2016.

23

Chapter 2 Nitrene Insertion into Boranes

2.1 Introduction

2.1.1 Hypervalent Iodine Reagents

Hypervalent iodine reagents are those in which an iodine containing compound has an oxidation

state greater than the commonly found -1 state.1,2 In 1886, Willgerodt synthesized PhICl2, the first

hypervalent iodine compound.3 Since this seminal work, there has been a surge of interest in these

reagents given their unique reactivity and low toxicity of iodine.1 There are two general classes of

hypervalent iodine complexes: trivalent λ3-iodanes and pentavalent λ5-iodanes (Figure 2.1).

Figure 2.1. Structural types of λ3-iodanes (left, middle) and λ5-iodanes (right).

In each of these structural types of hypervalent iodine reagents, there exists one (λ3) or two (λ5) 3-

centre-4-electron (3c-4e) bonds. In the case of λ3-iodanes, the iodine centre contains 10 electrons

in its valence shell, making it a formal decet structure. Conversely, the pentavalent λ5-iodane has

12 electrons in its valence shell, making it a formal dodecet structure. Iodine is diffuse and

polarizable and favours hypervalent bonding in which two ligands interact with an unhybridized

5p orbital. The resulting 3c-4e interaction results in an elongated, weak, and highly polarizable

bond. The presence of this hypervalent interaction dictates both the coordination geometry and the

reactivity of these species. Both λ3-iodane subclasses exhibit T-shaped coordination geometry,

whereas λ5-iodanes have an additional orthogonal hypervalent interaction leading to square-based

pyramidal geometry.4,5,6,7 The stability of the 3c-4e interaction has been investigated by Suresh,

who reports that the stability of the hypervalent interaction depends on the pairing of ligands with

appropriate trans-influence.8

In 1931, Arbuzov synthesized the first diacetoxyaryl-λ3-iodanes by oxidation of iodobenzene with

peracetic. 9 Diacetoxyaryl-λ3-iodanes are convenient starting materials used in the synthesis of

hypervalent iodine reagents due to their high stability. One such compound,

24

diacetoxyiodobenzene, is commercially available and its utility in the synthesis of a wide range of

hypervalent iodine reagents has been demonstrated (Scheme 2.1).10,11,12,13,14

Scheme 2.1. Synthesis of hypervalent iodine reagents from diacetoxyiodobenzene.

The reactivity of hypervalent compounds has been extensively investigated and documented by

Zhdankin in a series of comprehensive reviews.15 Trivalent iodane reagents typically exhibit three

general modes of reactivity: ligand exchange, reductive elimination, and ligand coupling

(Scheme 2.2).16 The ligand exchange mechanism likely occurs through an associative pseudo-

octahedral intermediate, due to the instability of the two-coordinate iodonium ion that would be

involved in a dissociative mechanism.17 An important feature of hypervalent iodine species is their

high propensity to reductively eliminate highly reactive intermediates like carbenes and nitrenes

under mild conditions. The reactivity of hypervalent iodine reagents resembles that of heavy

metals such as Hg(II), Tl(III), and Pb(IV), without the associated high toxicity.1

Scheme 2.2. General reactivity patterns of λ3-iodanes.

Aryliodonium imides, ArI=NR, are a family of λ3-iodanes that act as facile nitrene precursors.

Abramovitch and coworkers synthesized phenyliodonium tosylimide, the first literature report of

an aryliodonium imide, while attempting to find sulfonyl nitrene sources other than azides.18 The

group of Okawara developed a strategy to generate a series of aryliodonium tosylimides a year

25

later.19 Reacting diacetoxy(phenyl)-λ3-iodane with toluenesulfonamide in basic conditions affords

the desired phenyliodonium tosylimide in good yield upon recrystallization from methanol. The

resulting iodonium imide is poorly soluble in most organic solvents due to a polymeric structure

resulting from strong intermolecular N-I interactions.

Phenyliodonium N-tosylimide reacts with triphenylphosphine at 100 °C to afford iodobenzene and

N-tosyliminophosphorane in 69% yield. This reaction is proposed to proceed via a sulfonyl nitrene

intermediate. Alternatively, the iodonium imide reacts with dimethylsulfoxide to give the oxidized

product N-tosyliminodimethylsulfurane oxide quantitatively at room temperature. Given the

stability of the iodane starting material at room temperature, this reaction is proposed to occur

through a substitution reaction at nitrogen (Scheme 2.3).19

Scheme 2.3. Reactivity of iodine reagent with triphenylphosphine (top) and dimethylsulfoxide

(bottom).

2.1.2 Electrophilic Borane Reagents

Pentafluorophenyl-substituted organoboranes have developed very rich and diverse chemistry.

Tris(pentafluorophenyl)borane, the first such compound, was synthesized in moderate yields by

Massey in 1964 by combining three equivalents of pentafluorophenyllithium with boron

trichloride (Scheme 2.4).20

26

Scheme 2.4. Synthesis of tris(pentafluorophenyl)borane (BCF).

There were few reports which used BCF in the decades following its discovery. One such report

used BCF to transfer a perfluorophenyl group onto xenon difluoride to form a

pentafluorophenylxenon(II) fluoroborate species, the first stable compound containing a Xe-C

bond.21 In 1991, Marks employed BCF as an alkyl abstracting cocatalyst in concert with

zirconocene olefin polymerization catalysts. Using highly electrophilic BCF in lieu of the

commonly used aluminum alkyl or methylaluminoxane resulted in the first stoichiometrically

precise and crystallographically characterized system of this type.22,23 Yamamoto and coworkers

thoroughly investigated BCF as a versatile Lewis acid catalyst in allylation reactions, Diels-Alder

reactions, as well as aldol-type and Michael reactions of silyl enol ethers with carbonyls (Scheme

2.5).24

Scheme 2.5. Summary of BCF Lewis acid catalyzed reactivity.

Chivers and Piers produced a comprehensive review on the Lewis acidic chemistry of BCF and

related bis(pentafluorophenyl)borane, or Piers’ borane (HB(C6F5)2).25 Piers’ borane reacts readily

with a variety of unsaturated substrates, forms a dimer in the solid state, and forms a 4:1

27

dimer/monomer ratio in benzene. THF was added to break up the dimer to enhance borane

reactivity, but instead resulted in the formation of stable adduct HB(C6F5)2·THF. At higher

temperatures, the Pier’s borane-THF adduct undergoes reductive ring opening of the THF to

generate bis(pentafluorophenyl)butylborinate (Scheme 2.6, top). In non-coordinating solvents, like

benzene, Pier’s borane reacts with styrene to form the kinetically favoured single hydroboration

product. Over the course of several days, the hydroborated product disproportionates to form BCF

and dialkyl(pentafluorophenyl)borane, the thermodynamic products (Scheme 2.6, bottom).26

Scheme 2.6. Reactivity of Piers’ borane with THF (top) and styrene (bottom).

2.1.3 Frustrated Lewis Pair Chemistry

In 2006, Stephan and coworkers reported a sterically hindered, linked borane-phosphine system

capable of reversible H2 activation. The para-nucleophilic attack of a bulky secondary

dimesitylphosphine on BCF leads to the formation of a linked phosphonium-fluoroborate species,

(C6F5)2BF(C6F4)PHMes2, which was subsequently reacted with ClMe2SiH to yield

(C6F5)2BH(C6F4)PHMes2. Upon heating to 100 °C, the phosphonium-borate species quantitatively

loses H2 (Figure 2.2, left). Excitingly, the linked phosphine-borane species is then able to activate

dihydrogen.27,28 The term “frustrated Lewis pairs” was coined to describe the interaction between

donor and acceptor sites of sterically-congested molecules which are precluded from adduct

formation. The linked species was found to catalyze the hydrogenation of imines in good yield

under an atmosphere of hydrogen.29 Erker and coworkers reported an ethylene-bridged

dimesitylphosphine-bis(pentafluorophenyl)borane system, Mes2PCH2CH2B(C6F5)2, made through

the hydroboration of Piers’ borane onto an alkenyl phosphine (Figure 2.2, middle).30 The ethylene-

linked phosphine-borane forms an internal adduct, yielding a planar four-membered ring. The four

28

membered ring activates dihydrogen at room temperature and can be used to effect the reduction

of imines and enamines.31 In 2007, Stephan and coworkers reported that sufficiently sterically

bulky phosphines could form intermolecular pairings with BCF and still effect similar reactivity

(Figure 2.2, right).32 BCF and tri-tert-butylphosphine do not form an adduct in solution, but readily

activate a variety of small molecules, including 1,2-addition to olefins.32

Figure 2.2. Seminal FLP systems: arene-linked phosphine-borane (left), ethylene-bridged

phosphine-borane (middle), and intermolecular BCF-phosphine (right).

The discovery of intermolecular FLPs has led to intense research into different FLP-type reactions

and, with it, a demand for new electrophilic boranes.33,34 However, the synthesis of electrophilic

boranes is often difficult, requiring the use of hazardous lithium reagents or toxic tin reagents.

2.1.4 Post-Synthetic Tuning Strategies

The concept of tuning the Lewis acidic properties of an electrophilic borane as a means to

circumvent difficult and dangerous synthetic preparations of new boranes is an idea as old as FLP

chemistry. Shortly after the seminal report of FLPs, our group reported using tertiary phosphines

to effect para-nucleophilic attack on BCF to tune the Lewis acidity of the resulting phosphonium-

substituted borane. Phosphonium substitution resulted in increased Lewis acidity, though the effect

of the various substituents on the phosphorus centre played a minor role compared to the cationic

charge afforded by the cationic group. The related phosphine-borane species show slightly

decreased Lewis acidity, while the steric environment of the boron centre is not perturbed due to

the orientation of the phosphorus substituent in both the phosphonium and phosphine substituted

borane.28 Additionally, tuning strategies have been investigated that involved the insertion of

functional groups into the B-C bond. In 2012, Stephan reported the use of diazomethanes to effect

the insertion of pentafluoro-, trimethylsilyl-, or diphenylmethylene fragments into a variety of

boranes. Depending on the borane used, single- or double-insertion of these fragments was

29

observed.35 Braunschweig similarly used azide reagents to effect nitrene insertion into five-

membered borole rings.36 The insertion of the nitrene into the antiaromatic boroles yielded

aromatic 6-membered 1,2-azaborinies. In this chapter, we explore the use of iodonium

sulfonylimines of the form PhI=N(SO2)R to effect nitrene insertion into electrophilic borane B-C

bonds, thereby tuning their electronic character.

2.2 Results and Discussion

2.2.1 Synthesis and Characterization of Aminoboranes

Six iodonium sulfonylimines were synthesized following a modified literature procedure (Scheme

2.7).19 The iodine reagents were selected because of the electron withdrawing ability of the

sulfonyl substituents to potentially enhance the electrophilicity of the resulting insertion products

or, at a minimum, mitigate the quenching effect of the lone pair of electrons on the nitrogen centre.

Scheme 2.7. Synthesis of phenyliodonium sulfonylimides.

Only four phenyliodonium sulfonamide derivatives were synthesized in sufficiently high yield and

purity for further reactivity: toluenesulfonyl (PhI=NTs, Ts = SO2(4-MePh)),

para-chlorobenzenesulfonyl (PhI=NCls, Cls = SO2(4-ClPh)), para-nitroxybenzenesulfonyl

(PhI=NNs, Ns = SO2(4-NO2Ph)) and methanesulfonyl (PhI=NMs, Ms = SO2Me). Suitable

1H NMR spectra could be obtained only in deuterated DMSO due to the insolubility of iodonium

imides in most common organic solvents, though all the isolated iodine reagents reacted rapidly to

oxidize the DMSO solvent.

The trifluoromethanesulfonyl (SO2CF3) derivative was targeted due to the high electron

withdrawing capability of trifluoromethyl groups. Attempts to synthesize the

trifluoromethanesulfonyl-substituted iodine reagent were successful, though yields were

significantly lower than with other derivatives. Investigations using the trifluoromethanesulfonyl

30

derivative were abandoned due to the high cost of the sulfonamide precursor and the low overall

yield of the corresponding iodine reagent.

Addition of PhI=NTs into pentane forms a white slurry to which there is no perceptible change

upon adding BCF. After stirring overnight, the reaction mixture remained a white slurry which

was filtered to obtain a white powder. Recrystallization of this white powder from CH2Cl2

at -35 °C yielded 2-1 as colourless crystals in 74% yield. The 11B{1H} NMR spectrum of 2-1

shows a broad singlet at 43.6 ppm, and 19F NMR displays the nine resonances, which can be

attributed to three inequivalent sets of C6F5 environments. One of ortho-fluorine signals displays

a considerable upfield shift at 142.2 ppm, relative to 128.8 ppm in BCF. This upfield shift is

consistent with migration of a C6F5 substituent away from the boron centre. These data are

consistent with the insertion of the N-tosyl fragment into the B-C bond of BCF and infer the

formulation of 2-1 as (C6F5)2BN(Ts)(C6F5) (Scheme 2.8).

Scheme 2.8. Synthesis of tosylaminoborane 2-1.

Switching the reaction medium to either benzene or toluene resulted in considerably better

solubility of the iodine reagent, though the solution turned dark red-brown over the course of 24 h

and 1H NMR reveals complete consumption of the iodine reagent along with the formation of

multiple side products. This finding is consistent with previous observations of sulfonyl nitrenes

reacting with arenes.37 Addition of PhI=NTs to BCF in polar coordinating solvent resulted in

solvent-borane adduct formation that precluded reactivity between the borane and iodine reagent

and resulted in a complex mixture of products when the mixture was heated to 70 °C.

The presence of three inequivalent groups of C6F5 resonances, rather than two sets integrating in

a 2:1 ratio, is indicative of restricted rotation about the B-N bond. This restricted rotation could

arise from a steric interaction between the substituents on the boron and nitrogen centre.

31

Alternatively, the restriction could be indicative of multiple bond character between boron and

nitrogen. The implication of the latter case would indicate that despite the two electron-

withdrawing substituents on nitrogen, the lone pair of nitrogen still has a significant interaction

with the empty p-orbital of the adjacent boron centre.

Addition of an equimolar amount of OPEt3 to a toluene solution of 2-1 yields crystals of 2-2 which

can be obtained in good yield. The 11B{1H} NMR spectrum of 2-2 shows a sharpened singlet at

1.4 ppm, indicative of a four-coordinate boron centre. Interestingly, the 19F NMR spectrum

exhibits a set of three sharp resonances and a set of three broad resonances attributable to two C6F5

environments, which integrate in a 2:1 ratio. The meta-para gap is significantly reduced, indicative

of a four-coordinate boron centre. These data are consistent with the formulation of 2-2 as the acid-

base adduct (C6F5)2BN(Ts)(C6F5)·OPEt3 (Scheme 2.9).

Scheme 2.9. Synthesis of adduct (C6F5)2BN(Ts)(C6F5)·OPEt3 2-2.

The presence of only two inequivalent C6F5 environments suggests that 2-2 does not exhibit

significant restriction about the B-N bond as is seen in 2-1. Adduct formation significantly

increases steric congestion while coordinatively saturating the boron centre. The loss of restricted

rotation in 2-2 strongly supports that the restriction arises from B-N multiple bond character and

not from steric interaction between the groups on the boron and nitrogen centres. The solid-state

structure of 2 displays a B-O bond length of 1.504(2) Å and a B-N bond length of 1.584(2) Å,

which is consistent with a B-N single bond. The boron centre displays tetrahedral geometry, while

nitrogen is very slightly pyramidalized (sum of angles = 356.2(3)°).

32


P: orange, F: pink, hydrogen atoms have been omitted for clarity.

The reaction of (C6F5)2BPh with the PhI=NTs iodine in pentane yielded a white powder 2-3 upon

workup. Similar to 2-1, the 11B{1H} NMR spectrum of 2-3 shows a singlet at 42.0 ppm. The

19F NMR spectrum shows two sets of three C6F5 inequivalent resonances. The ortho-fluorine

signals appear at -131.4 and -131.6 ppm, suggesting that neither C6F5 ring had migrated from the

boron centre of 2-3, contrary to the large upfield shift observed in 2-1. These data are consistent

with the formulation of 2-3 as (C6F5)2BN(Ts)Ph (Scheme 2.10).

33

Scheme 2.10. Reaction of PhI=NTs with (C6F5)2BPh to generate 2-3.

Unlike in the formation 2-1, the different substituents of (C6F5)2BPh give rise to the possibility of

constitutional isomers depending on which group migrates to the nitrogen centre. Inspection of 19F

NMR spectrum of the crude reaction mixture shows no signals attributable to a product in which

insertion occurred into a B-C6F5 bond. Variable temperature 19F NMR was used to probe the

chemical exchange of C6F5 groups on the boron centre (Figure 2.4). At room temperature, ortho-,

meta-, and para-fluorine signals of the two C6F5 substituents are sharp and inequivalent. At 60 °C

the ortho-fluorine signals have overlapped and the para- and meta-fluorine signals have broadened

significantly. Coalescence of the remaining fluorine signals is observed at 80 °C, with gradual

sharpening when heated to 100 °C. A minor decomposition product, HC6F5, is observed in the

spectra.

34

Figure 2.4. Partial 19F NMR spectrum (-127 – -164 ppm), variable temperature study of 5-3 in

toluene-d8 (bottom – top: 25 °C, 40 °C, 60 °C, 80 °C, 100 °C).

Since insertion was found to occur in (C6F5)2BPh selectively into the B-Ph bond, BPh3 was

selected for reaction with PhI=NTs to exploit the higher migratory aptitude of Ph groups. The

reaction slurry of BPh3 with PhI=NTs in pentane was filtered after 14 h to yield a white powder.

The 1H NMR spectrum of the white powder indicated the presence of mostly unreacted PhI=NTs.

An NMR scale reaction of PhI=NTs and BPh3 in toluene-d8 was monitored by 1H and 11B{1H}

NMR. Two new boron resonances formed at 45.6 ppm and 29.4 ppm at 45 °C, with small amounts

of starting material remaining even after one week. The resonance at 45.6 ppm is consistent with

formation of Ph2BOH. The second product giving rise to the resonance at 29.4 ppm could not be

isolated. The 1H NMR spectrum of the reaction mixture contained many overlapping aryl

resonances, preventing characterization of the products. The slow reaction between BPh3 and

PhI=NTs highlights the need for a sufficiently electrophilic boron centre.

35

Returning to more electron deficient borane starting materials, Piers’ borane HB(C6F5)2 was

reacted with PhI=NTs and similarly yields a white powder 2-4 upon filtration and recrystallization.

Compound 2-4 exhibits a signal at 34.4 ppm in 11B{1H} NMR and, similar to 2-3, exhibits two

sets of C6F5 resonances by 19F NMR with no significant upfield shifted ortho-fluorine signal,

indicative of no C6F5 ring migration. These data are consistent with the assignment of 2-4 as

(C6F5)2BN(Ts)H (Scheme 2.11). As is the case with the formation of 2-3, the crude reaction

mixture of 2-4 shows only minor impurities, none attributable to the C6F5 migrated product.

Scheme 2.11. Synthesis of nitrene inserted products 2-4 – 2-7 from Piers’ borane.

The selective migration of Ph, H, or Cl over C6F5 substituents is consistent with reactivity trends

observed in related diazomethane insertion and carboboration chemistry.35,38 The migratory

aptitude can be rationalized primarily by electron density arguments, with the more electron rich

substituent migrating.

A solid-state structure of 2-4 was obtained by X-ray crystallography (Figure 2.6). The B-N bond

distance is 1.408(3) Å, consistent with a double bond. The boron substituents are coplanar, with

the sulfonyl group on the nitrogen centre, and the S-N-B-C dihedral angles are 176.8(1)°

and -1.4(3)°. The boron centre has a trigonal planar geometry with angles summing to 360.0°. The

centroid-centroid distance of the tolyl ring and closest C6F5 ring is 4.098 Å, and the angle between

the planes of those rings is 9.63°, indicative of a parallel-displaced π-π stacking interaction. The

rings are offset by 2.518 Å, which allows the ortho-fluorine substituent of the C6F5 ring to be

positioned over the centre of the tolyl ring (offset of 0.384 Å). In an ab initio study of the

intermolecular interaction between benzene and hexafluorobenzene, the slipped-parallel complex

was found to be the most stable interaction with a stabilization energy of -5.38 kcal/mol when the

intercentroid distance was 3.64 Å with an offset of 1.0 Å.39 The calculated stabilization energies

36

of the non-offset benzene-hexafluorobenzene complex show a slight stabilization effect occurring

at intercentroid distances less than 6 Å, increasing to a maximum stabilization at 3.6 Å.


P: orange, F: pink, hydrogen atoms have been omitted for clarity.

Piers’ borane was selected to test reactivity with other hypervalent iodine reagents because of the

straightforward reactivity observed with PhI=NTs. Piers’ borane was reacted in a similar fashion

with PhI=NCls, PhI=NMs, and PhI=NNs to yield (C6F5)2BN(Cls)H 2-5, (C6F5)2BN(Ms)H 2-6, and

(C6F5)2BN(Ns)H 2-7 (Scheme 2.11). The 11B{1H} NMR of 2-5 – 2-7 spectra each show one singlet

at 39.3, 39.7, and 39.6 ppm respectively. The 19F NMR spectra of 2-5 – 2-7 all show two

inequivalent sets of C6F5 resonances and no evidence of upfield shifted ortho-fluorine signals

indicative of C6F5 ring transfer to nitrogen. The downfield amine proton signal is visible for 2-4 –

2-7 between 7.3 and 7.8 ppm. Additionally, the solid-state structures of 2-5 and 2-6 were obtained

from X-ray diffraction of single crystals (Figure 2.6).

37

Figure 2.6. POV-ray depiction of 2-5 (top) and 2-6 (bottom); C: light grey, B: yellow-green, O:

red, S: yellow, N: blue, Cl: green, F: pink, hydrogen atoms have been omitted for clarity.

38

Both 2-5 and 2-6 display similar bond metrics to 2-4, with B-N bond distances of 1.407(2) Å and

1.396(4) Å respectively, both consistent with double bond character. Boron centres of 2-5 and 2-6

are both planar with angles around boron summing to 359.9(3)° and 359.8(6)° respectively.

Additionally, the sulfonyl substituents of 2-5 and 2-6 are both coplanar with the substituent plane

of the boron centre. Compound 2-5, in a similar fashion to 2-4, displays features indicative of π-π

stacking. The centroid-centroid distance between the para-chlorophenyl ring and the nearest C6F5

ring is 4.157 Å, and the angle between the planes of the two rings is 9.43°. The ring offset is 2.632

Å, allowing the ortho-fluorine atom to be situated above the centre of the chlorophenyl ring, with

an offset of 0.413 Å. Compound 2-6, which has no possibility for a similar π-π interaction, displays

a C-S-N-B torsion angle of 82.0(3)°, whereas the dihedral angle between those atoms is 59.4(2)°

for 2-4 and 60.9(2)° for 2-5.

Interestingly, the reaction between ClB(C6F5)2 with PhI=NTs, PhI=NCls, PhI=NMs, or PhI=NNs

cleanly yields 2-4, 2-5, 2-6, or 2-7 respectively (Scheme 2.12). Insertion into the B-Cl bond is

believed to occur, transiently generating a B-N-Cl species before subsequent reaction with

adventitious water or solvent. Only the single insertion product is observable by 19F NMR and

11B{1H} NMR of the reaction mixture in pentane. Upon removing pentane from the reaction

mixture and obtaining 1H NMR in C6D6, a single product with diagnostic downshifted N-H

resonance is observed. Attempts to monitor the reaction by 1H NMR in C6D6 and toluene-d8 lead

to decomposition, consistent with earlier observations of reactivity in aromatic solvents.

Scheme 2.12. Synthesis of nitrene inserted products 2-4 – 2-7 from ClB(C6F5)2.

In the related work by our group, multiple bond insertions using diazomethane reagents were

observed.35 The reaction of PhB(C6F5)2 with equimolar Me3SiCH(N2) yielded a mixture of the

39

single- and double-insertion product. Reaction of two or more equivalents of Me3SiCH(N2) with

PhB(C6F5)2 yielded the double-insertion product exclusively.

Unlike the work with diazomethane reagents, all reactions between equimolar hypervalent iodine

reagents and boranes showed no evidence of multiple bond-insertion products. Highly

electrophilic BCF was chosen to test for multiple insertions as it is believed that 2-1 has the highest

residual Lewis acidity. When three equivalents of PhI=NTs were reacted with BCF in pentane, 2-

1 was obtained as the only product, even upon heating the mixture. At temperatures exceeding 80

°C, decomposition of both excess PhI=NTs and 2-1 occurs, but no significant secondary product

is observable by 19F or 11B{1H} NMR attributable to a double-insertion product.

Similarly, 2-6 was also tested for multiple bond-insertion reactivity, as it is the least sterically

crowded around the boron centre. The reaction of either Piers’ borane or ClB(C6F5)2 with three

equivalents of PhI=NMs in pentane yielded 2-6 exclusively. Compound 6 is considerably less

sterically crowded than the intermediate reported (Me3SiCH(Ph)B(C6F5)2 which does react with a

second equivalent of diazomethane.

The mechanism for insertion into the B-C bond likely consists of the formation of an adduct

between the borane and the nitrogen of the iodine reagent. After the initial insertion, the double

bond character of the resulting aminoborane sufficiently quenches the Lewis acidity at the boron

centre to preclude adduct formation with additional equivalents of iodine reagent.

2.2.2 Electrophilicity of Aminoboranes

2.2.2.1 Gutmann-Beckett Method

The Gutmann-Beckett method was used to determine the relative Lewis acidity of compounds 2-

1, 2-3 – 2-7. Reaction of excess 2-1 with OPEt3 in CH2Cl2 yields 2-2 in-situ. The 31P{1H} NMR

chemical shift of the bound phosphorus in 2-2 is 80.7 ppm, which is 2.68 ppm downfield from the

related (C6F5)3B·OPEt3. These data infer that, by the Gutmann-Beckett method, 2-1 is more Lewis-

acidic that BCF. The relative Lewis acidity is contrary to the observation that BCF undergoes

reaction with PhI=NTs while 2-1 does not. However, Lewis acid-base interactions are highly

dependent on the nature of Lewis acid and Lewis base, and are therefore often not directly

40

transferable between unlike systems. The Gutmann-Beckett test depends on how well boron-bound

heteroatoms compete with OPEt3 for the boron acceptor orbital.40

The Gutmann-Beckett protocol was applied to compounds 2-3 – 2-7 and yielded 31P{1H} NMR

chemical shifts of 77.8, 80.2, 80.5, 79.8, and 80.7 ppm respectively. These data infer slightly

increased Lewis-acidity from the parent boranes PhB(C6F5)2 (76.6 ppm) and HB(C6F5)2 (75.6

ppm). The Gutmann acceptor number (AN) for compounds 2-1, 2-3 – 2-7 was calculated to be

88.0, 81.5, 86.8, 87.3, 85.7, and 87.7 (Table 2.1).

Table 2.1. Gutmann-Beckett 31P{1H} NMR and Gutmann acceptor numbers

Compound Δδ 31P (ppm)a ANb

5-1 30.0 88.0

5-3 27.1 81.5

5-4 29.5 86.8

5-5 29.8 87.5

5-6 29.1 85.9

5-7 30.0 87.9

BCF 27.3 82.0

HB(C6F5)2 24.9 76.7

PhB(C6F5)2 25.9 78.9 a Δ31P are calculated by subtracting the chemical shift of the reference OPEt3 in CH2Cl2 from the observed

chemical shift. b AN are calculated using the formula (observed chemical shift – chemical shift of OPEt3 in

hexane) * 2.21.

While the Gutmann-Beckett method doesn’t provide a reliable method to compare insertion

products to the borane starting materials because of the significantly altered steric and bonding

environment due to the nitrogen heteroatom connected to the boron centre, it does allow for the

comparison of related aminoboranes 2-4 – 2-7. The effect of the varied sulfonyl groups on the

Lewis acidity follows the expected trend with para-nitrophenyl-substituted 2-7 being the most

Lewis acidic, then para-chlorophenyl-substituted 2-5, then tolyl-substituted 2-4, and methyl-

substituted 2-6 being the least Lewis acidic.

41

The difference in Lewis acidity between the most (2-7) and least (2-6) Lewis acidic aminoborane

generated from Piers’ borane (ΔAN = 2.0) is less than the difference in Lewis acidity between

BCF and PhB(C6F5)2 (ΔAN = 3.1). This range in Lewis acidity is consistent with the earlier post-

synthetic tuning strategy which appended various phosphines to the para-position of BCF.28 The

authors reported acceptor numbers the para-substitution of phosphines as follows: PiPr3

(AN = 85.6), PCy3 (AN = 85.2), PMes3 (AN = 84.6) and HPtBu2 (AN = 80.2).28


Computations were carried out using Gaussian 0941 to further investigate the electronic structure

of these electrophilic aminoboranes. Geometry optimizations and frequency calculations were

carried out on 2-4 – 2-7 using the B3LYP functional42 with the 6-31+G(d) basis set.43 Single point

energies were calculated on the optimized geometries using the MP244 and def2-TZVPP basis

set.45 The LUMOs of 2-4 – 2-6 display large lobes localized on the boron centre as well as one

aryl substituent. The LUMO of 2-7 displays a smaller lobe on boron with substantial LUMO

distribution on the 4-NO2Ph substituent (Figure 2.7).

42

Figure 2.7 Surface contour plot of LUMO of 2-4 (top left), 2-5 (top right), 2-6 (bottom left),

and 2-7 (bottom right); C: dark grey, H: light grey, N: dark blue, O: red, F: light blue.

Using the HOMO and LUMO energies, the global electrophilicity index (GEI) values, ω, were

calculated for 2-4 – 2-7 using the formula

with all energies in kJ/mol (Table 2.2).

43

Table 2.2. Computed LUMO and HOMO Energies and Calculated ω Values.

Compound ELUMO (eV) EHOMO (eV) ω (eV)

2-4 0.943 -9.96 0.932

2-5 0.764 -10.2 1.01

2-6 0.844 -10.3 1.00

2-7 0.026 -10.4 1.29

The ω values give the trend for increasing electrophilicity 2-4 < 2-6 < 2-5 < 2-7. This gives a

different result than the trend obtained using the Gutmann acceptor numbers of 2-4 – 2-7,

specifically the switched positions of 2-4 and 2-6. Compounds 2-4 – 2-7 should be ideal for

comparison using the GEI as the substituent set on the boron centres are very similar and the

electronic environment is perturbed by a peripheral functional group.

Since 2-4 – 2-7 all readily formed adducts with OPEt3, the Gutmann-Beckett method is the more

reliable method to compare their effective electrophilicities. The ω values are obtained using only

static energy values and, as such, do not take steric effects or energy costs associated with

rearrangements to form an adduct into account. The additional effects that the Gutmann-Beckett

method accounts for make it the more reliable measure, since the aim of these methods is to help

rationalize observed reactivity trends which are similarly affected by steric effects.

2.2.3 Reactivity of Aminoboranes

The reactivity of select nitrene insertion products was assessed in a series of Lewis acid catalysis

applications (Scheme 2.13). The dimerization of 1,1-diphenylethylene is a useful test reaction for

Lewis acids since weak and strong Lewis acids alike catalyze this reaction. As the dimerization of

1,1-diphenylethylene occurs, the reaction mixture becomes strongly coloured, which allows the

reaction to be monitored visually. Additionally, this reaction can be easily monitored by reduction

of the olefinic proton signal and increase of the diastereotopic proton signals in 1H NMR.

Reactions with 1,1-diphenylethylene and 5 mol% of 2-4 – 2-7 each were done in C6D6. Upon

addition of the solution to the solid aminoboranes, a pale yellow colour is observed. The reaction

44

proceeds slowly over the course of days. Significant decomposition is observed in all reactions

after two days by 19F NMR, with no additional conversion observed after three days. Final

conversions for 2-4 – 2-7 as determined by 1H NMR integration were 64%, 67%, 47%, and 78%

respectively. The final conversions are consistent with Lewis acidity as determined by the

Gutmann-Beckett method, though low catalyst stability prevented complete conversion in all

cases.

Related dialkylaminobis(trifluoromethyl)boranes, (CF3)2BNR2, have been reported to react with a

variety of 1,3-dienes to undergo [2+4] cycloadditions.46 Steric constraints were noted to be

significant in determining reactivity: (CF3)2BNEt2 reacted with fewer substrates than

(CF3)2BNMe2, and (CF3)2BN(Me)tBu was found to be inactive. Aminoboranes 2-1 and 2-6 were

each reacted with cyclopentadiene in toluene, in an effort to effect cycloaddition. Aminoborane 2-

6 was selected for test reactivity as it is the least sterically encumbered, and 2-1 was selected as it

is the most electrophilic. The 1H NMR spectra of these reactions revealed only the

cyclodimerization of cyclopentadiene. To slow the dimerization of cyclopentadiene to allow

reaction between the aminoborane and cyclopentadiene to occur, bulkier analogue 1,2,3,4,5-

pentamethylcyclopentadiene, was used. Compounds 2-1 and 2-6 effectively catalyzed the

dimerization of the bulkier analogue after one hour.

To reduce the steric congestion, two acyclic 1,3-dienes were tested for cyclodimerization

reactivity. Compounds 2-1 and 2-6 were unreactive towards 2,3-dimethyl-1,3-butadiene, even

upon heating. Compounds 2-1, 2-3, and 2-6 were each reacted with trans-1-methoxy-3-

trimethylsilyloxy-1,3-butadiene, Danishefsky’s diene, however only adduct formation was

observed in all three cases.

Select aminoboranes were tested for FLP reactivity by combination with a bulky phosphine under

an atmosphere of hydrogen. No adduct is observed when 2-1 is mixed with tBu3P in a solution of

toluene. Upon exposing the reaction mixture to an atmosphere of hydrogen the FLP fails to

generate the hydrogen-split phosphonium-hydridoborate salt [HPtBu3][(C6F5)2BHN(Ts)(C6F5)].

Further, solutions of either 2-1, 2-4, or 2-6 with N-benzylidene-tert-butylamine under an

atmosphere of hydrogen failed to generate the corresponding reduced amine product.

Aminoboranes 2-1, 2-4, and 2-6 were also found to be unreactive towards other small molecules

including CO2, SO2, alkenes, and alkynes. All aminoboranes demonstrated high sensitivity to

45

water, resulting from the electron-withdrawing amide fragment. Hydrolysis lead to the cleavage

of the B-N bond to generate the corresponding tosylamide.

Scheme 2.13. Summary of reactivity of aminoboranes.

2.2.4 Mechanism of Nitrene Insertion

The mechanism of nitrene insertion into boranes likely occurs through initial adduct formation

between the nitrogen of the iodine reagent and the borane. The iodine reagent is stable for long

periods of time in pentane at room temperature, which makes the generation of a free transient

nitrene species as the initial step unlikely. Highly electron-withdrawing substituents on the boron

centre, such as pentafluorophenyl groups, are important to facilitate the initial adduct formation.

46

After initial complexation, the migrating group donates electron density in the σ* orbital leading

to formation of a new N-C/H/Cl bond and cleavage of B-C/H/Cl and N-I bonds (Scheme 2.14).

With the limited scope of migrating substituents observed (H, Cl, and Ph substituents selectively

migrate before C6F5 groups), no definite trend can be defined, though migration of more electron

rich substituents is preferred.

Scheme 2.14. Mechanism of Insertion reaction between PhI=NTS with boranes.

2.2.5 Synthesis and Characterization of Phosphinimines

The use of hypervalent iodine reagent PhI=NTs to oxidize triphenylphosphine, PPh3, to the

corresponding phosphinimine TsN=PPh3 has been reported.19 This methodology provides a

potential route to electron-deficient phosphinimines with potential for Lewis acidic reactivity. To

this end, PhI=NTs was reacted with electron deficient phosphines Ph2P(C6F5) and P(C6F5)3. No

reaction occurs between PhI=NTs and Ph2P(C6F5) at room temperature. At 60 °C, the white slurry

reaction mixture begins to darken and the 19F NMR spectrum shows the growth of two new sets

of C6F5 resonances. The minor set of fluorine resonances at -128.8, -147.1, and -160.0 ppm

matches the spectrum of authentic PO(C6F5)Ph2, while the other major set of resonances are

attributed to the electron-deficient phosphinimine TsN=P(C6F5)Ph2 (2-8). Similarly, reaction of

PhI=NTs with P(C6F5)3 at elevated temperatures yields two new products: PO(C6F5)3 and

TsN=P(C6F5)3 (2-9) with resonances at -135.9, -152.1, and -164.6 ppm (Scheme 2.15). Compounds

2-8 and 2-9 display 31P{1H} NMR resonances at -0.7 and -31.6 ppm respectively. Since the iodine

47

reagent PhI=NTs decomposes at the reaction temperature used, sequential additions of PhI=NTs

are required to consume all the phosphine starting material.

Scheme 2.15. Synthesis of phosphinimines 2-8 and 2-9.

Both P(C6F5)Ph2 and P(C6F5)3 are difficult to oxidize, with P(C6F5)3 slowly oxidizing only under

reflux conditions with H2O2.47 After full consumption of phosphine starting material has occurred,

continued heating results in the slow conversion of 2-8 or 2-9 to the corresponding phosphine

oxides. This conversion likely occurs through reaction with adventitious water (Scheme 2.16).

Despite stringent water exclusion techniques used during successive openings of the reaction

vessel to add additional PhI=NTs, concomitant phosphine oxide formation was always observed

during the synthesis of 2-8 or 2-9.

Scheme 2.16. Hydrolysis of 2-8 and 2-9 to corresponding phosphine oxides.

The solid-state structure of 2-8 was obtained by X-ray diffraction (Figure 2.8). The phosphorus

centre displays tetrahedral geometry and a P-N bond distance of 1.5953(14) Å. As observed in the

solid-state structures of 2-4 and 2-5, there is a π-π interaction between the tosyl ring and the C6F5

substituent. The centroid-centroid distance is 3.654 Å and the ring offset is 1.105 Å, which are

considerably closer than 2-4 and 2-5 to the ideal calculated distances of a 3.64 Å intercentroid

distance with a 1.0 Å offset.48 The tetrahedral geometry of the phosphorus centre likely allows a

more favourable π-π interaction by reducing internal bond strain compared to the trigonal planar

boron centres of 2-4 and 2-5. The angle between the planes of the C6F5 and tolyl rings is 11.19°.

48

The other phenyl substituents not participating in a π-π interaction adopt torsion angles of 31.0(2)°

and 1.1(2)° relative to the P-N bond, which are significantly smaller than the 105.0(2)° dihedral

angle of the C6F5 ring relative to the P-N bond.

Figure 2.8. POV-ray depiction of 2-8; C: light grey, P: orange, O: red, S: yellow, N: blue, F: pink,

hydrogen atoms have been omitted for clarity.

The mechanism for the formation of 2-8 and 2-9 is likely the same as that reported for the formation

of TsN=PPh3.19 Since the reaction only occurs at elevated temperatures at which PhI=NTs

decomposes slowly, it is likely that the first step is generation of free tosylnitrene, which lacks a

full octet and is highly electrophilic. The nitrene is then readily attacked by even weakly

nucleophilic phosphines P(C6F5)Ph2 and P(C6F5)3 to form 2-8 and 2-9 respectively (Scheme 2.17).

49

Scheme 2.17. Proposed mechanism of formation for phosphinimines 2-8 and 2-9.

The optimized geometry of 2-8 was computed with Gaussian 0941 using the functional B3LYP42

with a 6-31+G(d) basis set.43 The single point energy of the optimized geometry of 2-8 was

obtained using the MP244 with the def2-TZVPP basis set.45 The HOMO of 2-8 is largely

delocalized around the tosyl substituent with an energy of -9.125 eV (Figure 2.9, left). The LUMO

of 2-8 displays a significant lobe on the phosphorus centre and delocalization onto the C6F5 ring

with an energy of 1.721 eV (Figure 2.9, right).

Figure 2.9. Surface contour plots of 2-8 orbitals: HOMO (left) and LUMO (right).

50

The large lobe of the LUMO located at phosphorus is consistent with LUMOs of Lewis acidic

phosphonium cations (Chapter 3, pg 82; Chapter 4, pg 133). However, the LUMO energy of 2-8

is considerably higher than diphenyl-substituted phosphonium cations 3-10 (ELUMO = -2.289 eV)

and 4-22 (ELUMO = -2.197 eV). The LUMO energies of all fluorophosphonium cations in later

chapters differ by less than 2 eV, as such, the nearly 4 eV difference between 2-8 and 3-10 is

significant.

2.3 Conclusion

The use of hypervalent iodine reagents of the form PhI=NR, where R is a sulfonyl group, to effect

post-synthetic borane tuning was investigated. Compound PhI=NTs was reacted with BCF,

PhB(C6F5)2, and HB(C6F5)2 to effect tosylnitrene insertion into borane B-C bonds, forming

(C6F5)2BN(Ts)(C6F5) (2-1), (C6F5)2BN(Ts)Ph (2-3), and (C6F5)2BN(Ts)H (2-4). The migratory

aptitude of borane substituents was favoured by Ph and H substituents over C6F5 substituents.

However, C6F5 substitution was beneficial in driving insertion reactivity, as less electrophilic PPh3

failed to cleanly generate the desired insertion product, despite the greater migratory aptitude of

Ph. Subsequently a range of tosyl substituents were investigated reacting PhI=NCls, PhI=NMs,

and PhI=NNs with HB(C6F5)2 to generate insertion products (C6F5)2BN(Cls)H (2-5),

(C6F5)2BN(Ms)H (2-6), and (C6F5)2BN(Ns)H (2-7) respectively. Interestingly, reacting each of the

four iodine reagents with ClB(C6F5)2 similarly generated corresponding 2-4 – 2-7 as the sole

product. Compound 2-1 was subjected to the Gutmann-Beckett method, generating the

triethylphosphine oxide adduct (C6F5)2BN(Ts)(C6F5)·OPEt3 (2-2). The series 2-4 – 2-7 was also

subjected to the Gutmann-Beckett method, yielding the increasing electrophilicity trend of 2-6

(Ms) < 2-4 (Ts) < 2-5 (Cls) < 2-7 (Ns). While the use of iodine reagents was effective at tuning

the Lewis acidity, the resulting aminoboranes proved unreactive in FLP chemistry, only effecting

Lewis acid catalysis. Additionally, the reaction between PhI=NTs with electron-deficient

phosphines Ph2P(C6F5) and P(C6F5)3 slowly generates phosphinimines TsN=P(C6F5)Ph2 (3-8) and

TsN=P(C6F5)3 (3-9). Both 3-8 and 3-9 were found to be very sensitive to decomposition in the

presence of water, yielding the corresponding phosphine oxide and tosylamide.

51

2.4 Experimental

2.4.1 General Experimental Methods

Air-sensitive manipulations were carried out under an atmosphere of dry, O2-free N2 using either

an MBraun MB Unilab Glovebox or a dual-manifold Schlenk line. Hexane, pentane, and Et2O

were all purified using a Grubbs-type column system produced by Innovative Technology and

dispensed into thick-walled Straus flasks equipped with Teflon greaseless stopcock and stored over

4 Å sieves. Tetrahydrofuran and benzene were each dried over sodium metal and benzophenone

before being distilled to Schlenk bombs and stored over 4 Å sieves. Dichloromethane was dried

using calcium hydride before being vacuum transferred to a Schlenk bomb. Deuterated solvents

were degassed using three successive freeze-pump-thaw cycles. Other bulk solvents were degassed

by three successive cycles of headspace-evacuation and sonication. High-resolution mass spectra

data was obtained using a JEOL AccuTOF or an Agilent 6538 Q-TOF mass spectrometer. All

NMR data were collected on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz, or Agilent

DD2 600 MHz spectrometer at 25 °C unless otherwise noted. NMR chemical shift data are given

relative to an external standard (1H, 13C: SiEt4; 11B: 15% BF3·Et2O; 19F: CFCl3; 31P: H3PO4). In

some cases, 2-D NMR techniques were employed to assign individual resonances. Piers’ borane,

HB(C6F5)2, and chloroborane ClB(C6F5)2, were synthesized by literature procedures.49

Tris(pentafluorophenyl)borane, BCF, was purchased from Boulder Scientific Company and

sublimed before use. Reagents obtained from Sigma-Aldrich including PhB(C6F5)2, P(C6F5)3,

Ph2P(C6F5) and PhI(OAc)2 were used without further purification, while BPh3 was recrystallized

from Et2O before use. All sulfonamides were purchased from Apollo Scientific and used without

additional purification. Computational work was performed using resources provided by the

Shared Hierarchical Academic Research Computing Network and Compute Canada.

Synthesis of (C6F5)2BN(Ts)(C6F5) (2-1): A 50 mL Schlenk flask was charged with TsN=IPh (88.6

mg, 0.237 mmol) in pentane (10 mL) to form a white slurry. A solution of B(C6F5)3 (121.5 mg,

0.237 mmol) in pentane (10 mL) was added to the flask. After stirring overnight, a white insoluble

powder was collected via filtration. The crude product was dissolved in a small volume of CH2Cl2

(2 mL) and filtered through a plug of Celite. The filtrate was collected and stored at -35 °C to allow

52

for the formation of small clear, colourless crystals. Yield 120 mg (74%). 1H NMR (400 MHz,

C6D6, 25 °C): δ 7.35 (d, 3JHH = 8 Hz, 2H, tol CH), 6.49 (d, 3JHH = 8 Hz, 2H, tol CH), 1.75 (s,

3H, tol CH3). 11B{1H} NMR (128 MHz, C6D6, 25 °C): δ 43.6 (br). 19F (376 MHz, C6D6, 25 °C):

δ -128.60 (m, 2F, o-F (C6F5)), -131.35 (m, 2F, o-F (C6F5)), -142.19 (m, 2F, o-F N(C6F5)), -146.68

(t, 3JFF = 21 Hz, 1F, p-F (C6F5)), -148.69 (t, 3JFF = 22 Hz, 1F, p-F N(C6F5)), -150.02 (t, 3JFF = 19

Hz, 1F, p-F (C6F5)), -159.04 (m, 2F, m-F (C6F5)), -160.27 (m, 2F, m-F N(C6F5)), -161.06 (m, 2F,

m-F (C6F5)). HRMS (EI-TOF) m/z: [M]+ Calcd for C25H7BF15NO2S 681.0085, Found 681.0068.

Synthesis of (C6F5)2BN(Ts)(C6F5) · OPEt3 (2-2): A 20 mL scintillation vial was charged with 1

(53.3 mg, 0.0782 mmol) in toluene (1 mL). To the solution, OPEt3 (10.5 mg, 0.0783 mmol) was

added. The solution was left at room temperature overnight to afford large rectangular crystals,

suitable for X-ray diffraction. Yield 56.1 mg (88%). 1H NMR (400 MHz, toluene-d8, 25 °C): δ

7.64 (d, 3JHH = 8 Hz, 2H, tol CH), 6.66 (d, 3JHH = 8 Hz, 2H, tol CH), 1.88 (m, 6H, Et CH2), 1.84

(s, 3H, tol CH3), 0.68 (dt, 3JPH = 18 Hz, 3JHH = 8 Hz, 9H, Et CH3). 11B{1H} NMR (128 MHz,

toluene-d8, 25 °C): δ 1.41 (s) ppm. 19F NMR (376 MHz, toluene-d8, 25 °C): δ -133.44 (br, 4F, o-F

B(C6F5)), -137.99 (s, 2F, o-F N(C6F5)), -154.59 (s, 1F, p-F N(C6F5)), -156.70 (br, 2F, p-F

B(C6F5)), -163.86 (br, 4F, m-F B(C6F5)), -164.10 (s, 2F, m-F N(C6F5)) ppm. 31P{1H} NMR (162

MHz, toluene-d8, 25 °C): δ 78.50 (s) ppm.

Synthesis of (C6F5)2BN(Ts)Ph (2-3): A 50 mL Schlenk flask was charged with TsN=IPh (17.4

mg, 0.0466 mmol) in pentane (10 mL). To the flask, a solution of PhB(C6F5)2 (19.7 mg, 0.0467

mmol) in pentane (10 mL) was added. The reaction mixture was stirred overnight. A white solid

was collected via filtration. The solid was redissolved in CH2Cl2 and cooled to -35 °C to afford

clear colourless crystals. Yield 20.9 mg (76%). 1H NMR (400 MHz, toluene-d8, 25 °C): δ 7.36 (d,

3JHH = 8 Hz, 2H, tol CH), 6.94 (d, 3JHH = 7 Hz, 2H, Ph CH), 6.63 (m, 3H, Ph CH), 6.54 (d, 3JHH =

8 Hz, 2H, tol CH), 1.81 (s, 3H, tol CH3) ppm. 11B{1H} NMR (128 MHz, toluene-d8, 25 °C): δ

42.00 (s) ppm. 19F (376 MHz, toluene-d8, 25 °C): δ -131.42 (br, 4F, o-F (C6F5)), -131.62 (br, 4F,

o-F (C6F5)), -150.95 (t, 3JFF = 18 Hz, 2F, p-F (C6F5)), -151.59 (t, 3JFF = 18 Hz, 2F, p-F (C6F5)), -

161.06 (br, 4F, m-F (C6F5)), -131.42 (br, 4F, m-F (C6F5)) ppm. HRMS (EI-TOF) m/z: [M]+ Calcd

for C25H12BF10NO2S 591.0522, Found 591.0529.

53

Aminoboranes of the general formula (C6F5)2BN(R)H, where R = Ts, Ms, Cls, or Ns, can be

generated by reacting equimolar amounts of the appropriate ylide with either ClB(C6F5)2 or

HB(C6F5)2. A sample preparation is provided below.

Synthesis of (C6F5)2BN(Ts)H (2-4): A 50 mL Schlenk flask was charged with TsN=IPh (16.6 mg,

0.044 mmol) in pentane (10 mL) to form a white slurry. A solution of ClB(C6F5)2 (16.9 mg, 0.044

mmol) was added to the Schlenk flask. The resulting slurry was stirred overnight and the white

insoluble solid was subsequently collected via filtration. The powder was then dissolved in CH2Cl2

and stored at -35 °C to allow for the formation of clear, colourless crystals. Crystals suitable for

X-ray diffraction were obtained from a CH2Cl2 solution for compounds 2-4, 2-5, and 2-6. (84%

yield)

1H NMR (400 MHz, C6D6, 25 °C): δ 7.88 (s, 1H, NH), 7.74 (d, 3JHH = 8 Hz, 2H, tol CH), 6.67 (d,

3JHH = 8 Hz, 2H, tol CH), 1.84 (s, 3H, tol CH3) ppm. 11B{1H} NMR (128 MHz, C6D6, 25 °C): δ

34.4 (s) ppm. 19F (376 MHz, C6D6, 25 °C): δ -136.07 (br, 2F, o-F (C6F5)), -136.43 (br, 2F, o-F

(C6F5)), -151.24 (br, 1F, p-F (C6F5)), -156.19 (br, 1F, p-F (C6F5)), -165.07 (br, 2F, m-F

(C6F5)), -165.35 (br, 2F, m-F (C6F5)) ppm. HRMS (EI-TOF) m/z: [M]+ Calcd for C19H8BF10NO2S

515.0209, Found 515.0225.

Synthesis of (C6F5)2BN(Cls)H (2-5): (78% yield) 1H NMR (400 MHz, C6D6, 25 °C): δ 7.46 (s,

1H, NH), 7.20 (d, 3JHH = 9 Hz, 2H, Ph CH), 6.73 (d, 3JHH = 8 Hz, 2H, Ph CH) ppm. 11B{1H} NMR

(128 MHz, C6D6, 25 °C): δ 39.30 (br) ppm. 19F (376 MHz, C6D6, 25 °C): δ -131.32 (s, 4F, o-F

(C6F5)), -145.84 (br, 2F, p-F (C6F5)), -149.90 (br, 2F, p-F (C6F5)), -160.73 (br, 4F, m-F (C6F5))

ppm. HRMS (EITOF) m/z: [M]+ Calcd for C18H5BF10NO2S 534.9663, Found 534.9659.

Synthesis of (C6F5)2BN(Ms)H (2-6): (85% yield) 1H NMR (400 MHz, toluene-d8, 25 °C): δ 7.32

(s, 1H, NH), 2.26 (s, 3H, CH3) ppm. 11B{1H} NMR (128 MHz, toluene-d8, 25 °C): δ 39.74 (br)

ppm. 19F (376 MHz, toluene-d8, 25 °C): δ -131.54 (br, 2F, o-F (C6F5)), -132.19 (br, 2F, o-F

(C6F5)), -146.09 (br, 1F, p-F (C6F5)), -150.01 (br, 1F, p-F (C6F5)), -160.30 (br, 2F, m-F

(C6F5)), -161.38 (br, 2F, m-F (C6F5)) ppm. HRMS (EI-TOF) m/z: [M]+ Calcd for C13H4BF10NO2S

438.9896, Found 438.9897.

Synthesis of (C6F5)2BN(Ns)H (2-7): (73% yield) 1H NMR (400 MHz, C6D6, 25 °C): δ 7.35 (d,

3JH-H = 9 Hz, 2H, Ph CH), 7.23 (s, 1H, NH), 7.09 (d, 3JH-H = 8 Hz, 2H, Ph CH) ppm. 11B{1H}

54

NMR (128 MHz, C6D6, 25 °C): δ 39.55 (br) ppm. 19F (376 MHz, C6D6, 25 °C): δ -131.36 (s, 4F,

o-F (C6F5)), -144.69 (br, p-F (C6F5)), -149.25 (br, p-F (C6F5)), -160.31 (br, 4F, m-F (C6F5)) ppm.

HRMS (EI-TOF) m/z: [M]+ Calcd for C18H5BF10N2O4S 545.9903, Found 545.9904.

Phosphinamines of the form TsN=P(Ar)(Ar’2), where Ar = C6F5, Ar’ = C6F5 2-8 or Ar = Ar’ =

C6F5 2-9, can be generated by reacting excess PhI=NTs with either Ph2P(C6F5) or P(C6F5)3. A

sample preparation is provided below.

Synthesis of TsN=P(C6F5)Ph2 (2-8): A J-Young NMR tube was charged with Ph2P(C6F5) (35.2

mg, 0.100 mmol) and PhI=NTs (37.3 mg, 0.100 mmol) in a toluene suspension (1.0 mL). The

mixture was heated in an oil bath overnight at 40 °C, resulting in decomposition of the PhI=NTs

and partial conversion. Filtration of the decomposed iodine reagent and addition of additional

equivalents of PhI=NTs results in the complete consumption of Ph2P(C6F5). (76% yield)

1H NMR (400 MHz, toluene-d8): δ 7.48 – 7.21 (m) ppm. 31P{1H} NMR (125 MHz, toluene-d8): δ

-0.7 (s, br) ppm. 19F NMR (376 MHz, toluene-d8): -125.3 (d, 3JFF = 20 Hz, 2F, o-F (C6F5)), -144.5

(td, 3JFF = 20 Hz, 4JFF = 6 Hz, 1F, p-F (C6F5)), -159.7 – -159.8 (m, 2F, m-F (C6F5)) ppm.

Synthesis of TsN=P(C6F5)3 (2-9): (45% yield) 31P{1H} NMR (125 MHz, toluene-d8): δ -31.6 (s,

br) ppm. 19F NMR (376 MHz, toluene-d8): -135.9 (t, br, 3JFF = 23 Hz, o-F (C6F5)), -152.1 (t, 3JFF

= 21 Hz, o-F (C6F5), -164.6 (t, 3JFF = 23 Hz, o-F (C6F5) ppm.

Gutmann-Beckett Method: A 5 mm NMR tube was charged with 3:1 analyte Lewis

acid:triethylphosphine oxide, OPEt3, in CH2Cl2. The 31P{1H} NMR chemical shift of the adduct

was recorded, as well as either an internal or external standard of uncoordinated OPEt3 in CH2Cl2.

The relative chemical shift was determined by subtraction of the reference chemical shift from the

adduct chemical shift.

Dimerization of cyclopentadienes: To a vial containing one equivalent of aminoborane was

added a solution of a cyclopentadiene (0.1 mmol) in toluene-d8 (0.7 mL). The solution was

transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Dimerization of 1,1-diphenylethylene: To a vial containing 5 mol% catalyst was added a solution

of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was then transferred

to a 5 mm NMR tube for monitoring by NMR spectroscopy.

55

2.4.2 X-ray Crystallography

2.4.2.1 X-ray Data Collection and Reduction

Crystals were coated in Paratone-N oil in a glovebox before being mounted on a MiTegen

Micromount under an N2 stream to maintain a dry, O2 free environment for each crystal.

Diffraction data were collected on a Bruker Apex II diffractometer using a graphite

monochromator with Mo Kα (λ = 0.71073 Å) radiation. Temperature was maintained at 150(2) K

using an Oxford cryo-stream cooler. Data collection strategies were determined using Bruker

Apex II software. Frame integration was carried out using Bruker SAINT software. Data

absorbance correction was carried out using the empirical multiscan method SADABS. Structure

solutions were obtained by direct methods and refined using SHELXTL or Olex2 software.50,51

Refinement was carried out using full-matrix least squares techniques to convergence of weighting

parameters. When data quality was sufficient, all non-hydrogen atoms were refined

anisotropically.

56

2.4.2.2 X-Ray Tables

2-4 2-5 2-6

Formula C19H8BF10NO2S C18H5BClF10NO2S C13H4BF10NO2S

Weight (g/mol) 512.16 535.57 439.06

Crystal system triclinic triclinic triclinic

Space group P-1 P-1 P-1

a (Å) 8.3826(6) 8.3755(3) 6.0068(7)

b (Å) 11.3424(8) 11.2582(5) 9.6962(12)

c (Å) 11.4015(8) 11.2869(4) 13.5918(15)

α (°) 101.452(4) 76.807(3) 74.330(7)

β (°) 92.380(4) 87.603(2) 81.758(7)

γ (°) 111.680(4) 68.315(2) 85.914(7)

Volume (Å3) 979.42(13) 961.79(7) 753.89(16)

Z 2 2 2

Density (calcd.) (gcm–3) 1.7466 1.8491 1.9339

R(int) 0.0254 0.0280 0.0483

μ, mm–1 0.278 0.421 0.342

F(000) 512 529 432

Index ranges -10 ≤ h ≤ 10 -10 ≤ h ≤ 10 -7 ≤ h ≤ 7

-14 ≤ k ≤ 14 -14 ≤ k ≤ 14 -12 ≤ k ≤ 12

0 ≤ l ≤ 14 -14 ≤ l ≤ 14 -17 ≤ l ≤ 17

Radiation Mo Kα Mo Kα Mo Kα

θ range (min, max) (°) 1.84, 27.44 1.86, 27.48 1.57, 27.42

Total data 14021 15198 12448

Max peak 0.4 0.4 0.6

Min peak -0.4 -0.4 -0.6

>2(FO2) 4394 3618 3427

Parameters 306 306 252

R (>2σ) 0.0383 0.0320 0.0440

Rw 0.0834 0.0849 0.0881

GoF 1.046 1.066 1.031

57

2-2 2-8

Formula C31H22BF15NO3PS, C7H8 C50H34F10N2O4P2S2

Weight (g/mol) 907.51 1042.90

Crystal system triclinic triclinic

Space group P-1 P-1

a (Å) 8.2520(4) 11.2973(10)

b (Å) 13.7632(7) 13.1220(12)

c (Å) 17.0976(8) 17.8139(14)

α (°) 83.047(3) 105.855(4)

β (°) 80.953(3) 103.798(4)

γ (°) 87.098(3) 102.497(5)

Volume (Å3) 1902.59(16) 2352.3(4)

Z 2 2

Density (calcd.) (gcm–3) 1.584 1.472

R(int) 0.0382 0.0267

μ, mm–1 0.241 0.269

F(000) 921 1066

Index ranges -10 ≤ h ≤ 10 -13 ≤ h ≤ 14

-17 ≤ k ≤ 17 -17 ≤ k ≤ 16

0 ≤ l ≤ 22 -23 ≤ l ≤ 23

Radiation Mo Kα Mo Kα

θ range (min, max) (°) 1.69, 27.46

Total data 31528 10658

Max peak 0.8 0.6

Min peak -0.5 -0.5

>2(FO2) 8605 8317

Parameters 541 633

R (>2σ) 0.0382 0.0392

Rw 0.1070 0.1039

GoF 1.057 1.044

58

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48. Tsuzuki, S.; Uchimaru, T.; Mikami, M., Intermolecular Interaction Between

Hexafluorobenzene and Benzene: Ab Initio Calculations Including CCSD(T) Level Electron

Correlation Correction. J. Phys. Chem. A 2006, 110, 2027.

49. Parks, D. J.; Spence, R. E. v. H.; Piers, W. E., Bis(Pentafluorophenyl)Borane - Synthesis,

Properties, and Hydroboration Chemistry of a Highly Electrophilic Borane Reagent. Angew.

Chem. Int. Ed. 1995, 34, 809.

50. Sheldrick, G. M., A Short History of SHELX. Acta Crystallogr. Sect. A 2008, 64, 112.

51. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H.,

OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl.

Crystallogr. 2009, 42, 339.

63

Chapter 3 Perchloroaryl Fluorophosphonium Cations

3.1 Introduction

3.1.1 Perchloroaryl Boranes

Perfluorophenyl substituents are commonly employed electron-withdrawing moieties in many

main group FLP catalysts.1,2,3 Ashley and coworkers have exploited the related perchlorophenyl

substituents on boron, generating a family of mixed C6F5/C6Cl5 substituted boranes.4 They found

that increasing the number of C6Cl5 moieties resulted in a concomitant increase in electrophilicity;

however, the high steric demand of ortho-chlorine atoms resulted in a decrease of Lewis acidity

(Figure 3.1).

Figure 3.1. Electrophilicity and Lewis acidity trends for perchlorophenyl-substituted boranes.

Based on electronegativity, the increase in electrophilicity is counter-intuitive, however

mesomeric stabilization effects in C6F5-substituted boranes allow for partial occupancy of the

vacant p-orbital on boron by donation from the fluorine atom lone-pairs in aromatic systems.4 The

orbital size increase from fluorine to chlorine prevents effective overlap with the vacant p-orbital

on boron, resulting in diminished quenching of Lewis acidity (Figure 3.2). This can be explained

using the Hammett parameter, which is obtained by establishing a free-energy relationship

between reaction rates and equilibrium constants for reactions involving benzoic acid with

systematically varied meta- and para-substituents.5 Hammett parameter values positively correlate

with the electron-withdrawing strength of a substituent at the specified phenyl substitution

64

position. Despite fluorine (χPauling= 3.98) being more electronegative than chlorine (χPauling= 3.16),

the reduced ability of the 3p chlorine lone pairs to interact with the 2p aromatic orbitals result in a

higher para-position Hammett parameter (σp = 0.23) compared to fluorine (σp = 0.06).6,7 The low

para-position Hammett parameter of fluorine is consistent with the observation that substitution

of the para-fluorine substituents of BCF for hydrogen substituents has a negligible impact on

Lewis acidity, as was determined by the Gutmann-Beckett method.8 The meta-position Hammett

parameter of chlorine and fluorine are much more comparable: σm = 0.37 and 0.34 respectively.

Figure 3.2. Halogen mesomeric stabilization effects of fluorine (left) and chlorine (right).

Generally, electrophilic boranes containing C6F5 rings are susceptible to decomposition by

hydrolysis. Initial adduct formation occurs between the borane and water molecule, followed by

elimination of HC6F5 to form bis(pentafluorophenyl)borinic acid (Scheme 3.1).9,10 Ashley and

coworkers exploited the decreased propensity of C6Cl5-substituted boranes to form Lewis acid-

base adducts to make water-stable electrophilic borane catalysts. The sterically demanding ortho-

chlorine atoms prevent close adduct formation required for loss of HC6F5. Instead, the C6Cl5

substituted boranes form stable reversible adducts with water (Scheme 3.1). While

B(C6F5)2(C6Cl5) forms reversible water-adducts, B(C6Cl5)3 is hydrolytically inert, even in

refluxing toluene/H2O for days. The water-adduct of B(C6F5)2(C6Cl5) can be reversed by removing

the water using either vacuum or 3 Å sieves. Additionally, C6Cl5 substituted boranes were also

found to be more thermally robust than the corresponding C6F5 substituted boranes.

65

Scheme 3.1. Hydrolysis of BCF (top). Water-adduct formation of B(C6F5)2(C6Cl5) (bottom)

The Gutmann acceptor number (AN) of B(C6F5)3-n(C6Cl5)n deceases linearly as n increases, while

B(C6Cl5)3 could not form an adduct with OPEt3 to determine an acceptor number. To assess the

electron density at the boron acceptor orbital without adduct formation, the reduction potentials of

the series of boranes were obtained. Since there is minimal rearrangement upon electron transfer,

the potentials were used as a measure of electrophilicity11. The reduction potentials of the boranes

increased as a function of increasing C6Cl5 substitution.

The hydrolytic and thermal stability of perchlorophenyl-substituted electrophilic boranes

represents a significant advance for the applicability of main group catalysts by general chemists.

The rapidly expanding array of transformations afforded by main group catalysts remain

inaccessible for many chemists that are not able to work in stringently water-free conditions. Air-

stability is therefore a critical element for broad-scale adoption of main group catalytic reagents.

In a demonstration of the stability of these boranes, Ashley employed B(C6F5)2(C6Cl5) in FLP-

type reactions.12 When the borane is dissolved in THF, B(C6F5)2(C6Cl5) forms only a weak adduct

with the solvent due to the increased steric encumbrance. In the presence of dihydrogen and

without the use of an auxiliary base, the THF-borane mixture effects the reduction of weakly basic

substrates including imines and nitrogen-containing heterocycles (Scheme 3.2).

66

Scheme 3.2. Proposed mechanism for hydrogenation with B(C6F5)2(C6Cl5).

O’Hare and coworkers have extensively studied the FLP reactivity of the bulky B(C6Cl5)3.13 Due

to the high steric crowding in B(C6Cl5)3, even small phosphines such as PEt3 are unable to form

an adduct and instead forms an FLP, which could be employed in H2 activation (Scheme 3.3). The

authors selected B(C6Cl5)3 to promote catalytic reduction of CO2, rationalizing that the increased

steric repulsion would disfavour strong formatoborate B-O bonds. Activation of formic acid by a

series of B(C6Cl5)3 FLPs could be effected, however no activation of CO2 was observed by the

phosphine-borane pair or the corresponding hydrogen-activated salt (Scheme 3.3). This stands in

contrast to the reactivity observed for BCF systems.14

Scheme 3.3. Hydrogen activation using B(C6Cl5)3/PR3 FLPs (top). Formic acid activation using

B(C6Cl5)3/PR3 FLPs (bottom).

67

3.1.2 Perchloroaryl Phosphines

Fluorophosphonium cations derive their high electrophilicity from C6F5 groups in a similar fashion

to electrophilic boranes used in FLP chemistry.15 The enhanced electrophilicity from decreased

mesomeric stabilization, as well as the chemical and thermal stability imparted by C6Cl5

substituents on borane systems are all desired properties in phosphonium cation catalysts. As such,

the incorporation of C6Cl5 substituents into fluorophosphonium cations was targeted for

investigation.

The series of mixed Ph/C6Cl5 phosphines, analogous to the Ph/C6F5 phosphines used in the seminal

fluorophosphonium work, were first synthesized by Dua in 1970.16 These phosphines were

obtained by first generating C6Cl5Li from hexachlorobenzene before reacting with the appropriate

halophosphine (Scheme 3.4). The target C6Cl5-substituted phosphines were generated in moderate

to poor yields and were only characterized by UV-spectroscopy, melting point and elemental

analysis.

Scheme 3.4. Synthesis of pentachlorophenyl-substituted phosphines.

The authors reference an earlier work for the synthesis of C6Cl5Li in which the formation and

stability of C6Cl5Li was investigated in ethereal solvents17. The stability of C6Cl5Li is noted to be

significantly lower in THF solutions than in Et2O solutions. Despite the lower stability, THF was

employed in the synthesis of the C6Cl5-substituted phosphines, resulting in poor isolated yields

(Scheme 3.4). Additionally, hazardous CCl4 and refluxing benzene were used in the purification

of the target phosphines- techniques which are not congruent with the safety standards of modern

laboratories.

The authors probed the reactivity of the C6Cl5-substituted phosphines, demonstrating the selective

para-functionalization of P(C6Cl5)2Ph with chlorotrimethylsilane (Scheme 3.5). Treatment of

68

P(C6Cl5)2Ph with two equivalents of nBuLi effects the selective two-fold lithiation at the para-

position on the C6Cl5 substituents. This lithiated product reacts with chlorotrimethylsilane to yield

the bis-para-functionalized P(C6Cl4SiMe3)2Ph in 75% yield. The selective functionalization of

C6Cl5 rings represents an effective route for additional fine-tuning of catalysts derived from these

phosphines. This functionalization of C6Cl5 substituents is similar in function to the

para-nucleophilic substitutions observed with C6F5 substituents, but has the potential for a wider

array of functional groups to be applied. While an analogous para-functionalization was carried

out using P(p-HC6F4)2Ph, the final product was only obtained in 13.6% yield.

Scheme 3.5. Functionalization of pentachlorophenyl groups with trimethylchlorosilane.


3.2.1 Synthesis and Characterization of Phosphines

In this chapter, we explore the synthesis and reactivity of a family of Ph/(C6Cl5) and

(C6F5)/(C6Cl5)-substituted phosphonium cations. Following a modified literature procedure, C6Cl6

was reacted with nBuLi in Et2O at -15 °C. Hexachlorobenzene is mostly insoluble in ethereal

solvents, forming a white slurry. As the reaction with nBuLi proceeded, the white slurry turned to

a clear yellow solution. Once all suspended material was consumed, the C6Cl5Li solution was

cooled down to -78 °C. Addition of appropriate equivalents of Ph2PCl, PhPCl2, (C6F5)2PBr or

(C6F5)PBr2 resulted in the formation of phosphines Ph2P(C6Cl5) (3-1), PhP(C6Cl5)2 (3-2),

(C6F5)P(C6Cl5)2 (3-4), or (C6F5)2P(C6Cl5) (3-5) respectively (Scheme 3.6). Compounds 3-1 and 3-

4 were characterized by X-ray diffraction (Figure 3.3).

69

P Cl

3-nn

n eq. LiC6Cl5

pent./Et2O, -78 °CP

3-n

Cl Cl

Cl

ClCln

P Cl

3-nn

pent./Et2O, -78 °CP

Cl Cl

Cl

ClCln

F

FF

F

F

3-n

F

FF

F

F

n = 1 (3-1)n = 2 (3-2)n = 3 (3-3)*

n = 2 (3-4)n = 1 (3-5)

*from PBr3, or w/ CuI

n eq. LiC6Cl5

Scheme 3.6. Synthesis of perchlorophenyl substituted phosphines 3-1 – 3-5.

Figure 3.3. POV-ray depiction of 3-1 (top) and 3-4 † (bottom); C: light grey, P: orange, F: pink,

Cl: green, hydrogen atoms have been omitted for clarity.

70

Both 3-1 and 3-4 display trigonal pyramidal geometry at the phosphorus centre. The geometry of

3-4 (sum of angles = 318.0(3)°) are slightly less pyramidalized than 3-1 (sum of angles =

311.3(3)°), consistent with increased steric bulk of substituents in 3-4.

The reaction of C6Cl5Li with PCl3 failed to yield the desired homoleptic phosphine P(C6Cl5)3 (3-

3) in appreciable yield. However, reaction of PBr3 with three equivalents of C6Cl5Li in the same

reaction conditions cleanly yielded 3-3. Alternatively, addition of equimolar or substoichiometric

amounts of CuI to the reaction between PCl3 and C6Cl5Li generated 3-3 in low yield. The addition

of CuI has been reported in the synthesis of another homoleptic electron-deficient phosphine

P(C6F4H)3.18 The reaction of PCl3 with LiC6Cl5 yielded two products by 31P{1H} NMR. In the

absence of CuI, the major product appears as a singlet at 10 ppm, while the desired species 3-3 is

evidenced by a singlet at 19 ppm. Crystals of the second product suitable for X-ray diffraction

were obtained from such reaction mixtures, which elucidated the structure of the major

side-product as ((C6Cl5)2P)2 (Figure 3.4). Altering the addition rate of PCl3 or employing a large

excess of LiC6Cl5 did not favour the formation of 3-3 over ((C6Cl5)2P)2. Coupled diphosphines

similar to ((C6Cl5)2P)2 have been reported to form through the alkali metal reduction by lithium

(Scheme 3.7).19,20,21 Similar phosphorus coupling reactivity has been observed using zinc reagents

and is discussed in Chapter 4 (page 122). Compound 3-3 is thermally stable and does not

decompose to the corresponding 1,2-diphosphine. In the solid-state, each phosphorus centre adopts

a trigonal pyramidal geometry with pyramidalization in opposite directions, with a P-P bond length

of 2.265(2) Å. One C6Cl6 on each phosphorus centre is canted towards the other with P-P-C bond

angles of 91.9(2)° and 93.2(2)°, which are significantly smaller than the P-P-C bond angles of the

remaining C6Cl6 substituents (108.2(2)° and 109.4(2)°). The centroid-centroid distance for the

canted C6Cl6 rings is 3.760 Å, with an angle of 17.68°between the planes of the rings.

Scheme 3.7. Synthesis of 1,2-diphosphine ((C6Cl5)2P)2.

71

Figure 3.4. Orthogonal POV-ray depictions of ((C6Cl5)2P)2; C: light grey, P: orange, Cl: green,

hydrogen atoms have been omitted for clarity.

72

While 1-1 – 1-3 have previously been synthesized, only 31P{1H} NMR characterization had been

reported for 1-1 in trace amounts.22 Interestingly, the trend for the 31P{1H} NMR chemical shifts

in the Ph3-nP(C6Cl5)n series is opposite to the trend observed in the analogous Ph3-nP(C6F5)n series.

While successive substitutions of Ph groups with C6F5 groups result in a 20 ppm upfield shift,

substitutions with C6Cl5 groups result in downfield shifting of 5 ppm (Figure 3.5). Electronically,

C6Cl5 and C6F5 groups behave similarly, however, the high steric demand imparted by

ortho-chlorine atoms strains the geometry of the phosphines towards planarity. Bond angles of

phosphorus have been reported to be a suitable parameter for rationalizing chemical-shift values.23

Other important parameters in determining 31P NMR chemical shift are coordination number,

electronegativity of substituents, and the amount of π bonding. Ramsey developed a general

expression for rationalizing chemical shift based on the aforementioned parameters.24,25

Figure 3.5. Stacked partial 31P{1H} NMR spectra (top to bottom: 3-3, 3-2, 3-1, PPh3, P(C6F5)Ph2,

and P(C6F5)3).

Phosphines 3-1 – 3-5 were all obtained in moderate yields and high purity after recrystallization

from CH2Cl2 without the use of dangerous purification techniques used in previously reported

syntheses.

One challenge associated with the use of C6Cl5 groups instead of C6F5 is the characterization of

compounds because 19F NMR can no longer be used. This makes elucidation of products within

mixtures of perchloroaryl compounds more difficult. However, an interesting benefit of

incorporating C6Cl5 rings, is their ability to act as mass spectral tags. Chlorine has two principle

73

isotopes, 35Cl and 37Cl, in a 3:1 ratio, so compounds containing chlorine atoms produce a

diagnostic 3:1 mass distribution. Moreover, the more chlorine atoms incorporated into a molecule,

the greater number of possible isotopologues, as is shown in the mass spectrum of 3-3 (Figure 3.6).

Figure 3.6. Sample DART mass spectrum displaying isotopic distribution of 3-3.

3.2.2 Synthesis of Perchloroaryl Difluorophosphoranes

Difluorophosphoranes were synthesized through the oxidation of phosphines with XeF2.

Effervescence was observed after solid XeF2 was added to a stirring solution of 3-1 in CH2Cl2,

wherein the solution rapidly turned pale-yellow. The solution was stirred for two hours before the

solvent was removed in vacuo, yielding 3-6 as a white powder in excellent yield (Scheme 3.8).

The 19F NMR spectrum of 3-6 shows a doublet at -41.8 ppm with 715 Hz coupling, which is

consistent with reported five-coordinate P-F coupling values.15.18 The 31P{1H} NMR spectrum

shows a triplet at -50.7 ppm, with the same P-F coupling constant. These data are consistent with

the formation of 3-6 as Ph2PF2(C6Cl5).

74

Scheme 3.8. Synthesis of pentachlorophenyl-substituted difluorophosphoranes.

Following an analogous procedure with compounds 3-2, 3-4 or 3-5, difluorophosphoranes

PhPF2(C6Cl5)2 (3-7), (C6F5)PF2(C6Cl5)2 (3-8), and (C6F5)2PF2(C6Cl5) (3-9) were isolated in good

yields, respectively (Scheme 3.8). Compounds 3-7 – 3-9 display similar 31P{1H} NMR data as 3-6,

with triplet resonances at -44.9, -41.2, and -44.6 ppm, respectively with similar P–F coupling

constants. The 19F NMR spectra of 3-7 – 3-9 display doublet resonances at -28.9, -10.9 and -6.4

ppm. Difluorophosphorane 3-8 displays an additional set of three fluorine resonances attributable

to a C6F5 substituent.

The formulations of 3-6 and 3-9 were additionally confirmed by single-crystal X-ray diffraction

(Figure 3.7). Both 3-6 and 3-9 display distorted trigonal bipyramidal geometries with fluorine

substituents occupying the apical positions with F-P-F bond angles of 175.9(5)° and 178.23(9)°

respectively. The fluorine atoms are canted slightly towards the C6Cl5 substituent. The plane of

C6Cl5 lies closer to the equatorial plane than the other aryl rings in both structures, with torsion

angles of 31.4(2)° and 31.3(3)° in 3-6 and 3-9. The plane phenyl substituents are rotated 56.3(1)°

and 64.4(2)° away from the equatorial plane, whereas the C6F5 rings are rotated 44.6(3)° and

41.3(3)° away from the equatorial plane. While π-donation has been reported to cause hindered

rotation of substituents in the equatorial position of phosphoranes, the observed orientations appear

to result predominantly from steric effects.26 The P-F bond lengths are 1.6643(9) Å and

1.6552(9) Å in 3-6, while 3-9 displays slightly shorter lengths of 1.641(2) Å and 1.626(2) Å.

75

Figure 3.7. POV-ray depiction of 3-6 † (top) and 3-9 † (bottom); C: light grey, P: orange, F: pink,

Cl: green, hydrogen atoms have been omitted for clarity.

Treatment of 3-3 with XeF2 in a CH2Cl2 solution yields a mixture of two products as observed by

19F and 31P{1H} NMR spectroscopy. The 31P{1H} NMR spectrum shows a triplet resonance

at -38.8 ppm with 818 Hz P-F coupling and a doublet of triplets at -26.0 ppm with 1011 Hz and

76

894 Hz coupling. The triplet signal is consistent with the formation of the desired

difluorophosphorane PF2(C6Cl5)3, while the doublet of triplets is consistent with the formation of

trifluorophosphorane PF3(C6Cl5)2. The 19F NMR spectrum displays a doublet at -15.5 ppm

consistent with PF2(C6Cl5)3; as well as a doublet of doublets at -5.5 ppm and doublet of triplets

at -71.2 ppm that integrate 2:1, which are consistent with the formation of PF3(C6Cl5)2 (Scheme

3.9). The highest ratio of the product generated was 3:1 of PF3(C6Cl5)2 to the desired PF2(C6Cl5)3

species. Adjusting the stoichiometry of the reaction by addition of excess XeF2 to 3-3 forms

PF3(C6Cl5)2 exclusively. Difluorophosphorane PF2(C6Cl5)3 is thermally stable at room temperature

in solution. Repeated attempts to separate these phosphorane species by crystallization and

chromatography were unsuccessful.

Scheme 3.9. Reactivity of 3-3 with XeF2.

The formation of other trifluorophosphoranes by reaction with XeF2 has previously been

observed.27 The reaction of diphenylphosphinoimidazole with XeF2 generated PF3Ph2, which was

also generated directly from PPh2Cl with 1.5 equivalents of XeF2. Similarly, the reaction of

P(C6F5)2Br with XeF2 forms the corresponding trifluorophosphorane PF3(C6F5)2, which displays a

similar diagnostic doublet of triplet phosphorus resonance in the 31P{1H} NMR spectra. For PF3Ph2

and PF3(C6F5)2, subsequent decomposition was observed to OPFPh2 and OPF(C6F5)2 respectively.

Similar decomposition was observed for PF3(C6Cl5)2, albeit at a much slower rate. Interconversion

of the apical substituents in fluorophosphoranes is observed only upon the presence of a third

apicophilic fluorine substituent and is likely tied to the generally lower chemical stability of

trifluorophosphoranes. Concomitant formation of FC6Cl5 is not observed in the reaction of XeF2

with 3-3. Instead, single crystals of decachlorobiphenyl (C6Cl5)2 suitable for X-ray diffraction were

obtained from the reaction mixture (Figure 3.8). The two rings are twisted by 84.9(2)° and joined

77

by a C-C bond distance of 1.501(2), consistent with a C-C single bond. The formation of

decachlorobiphenyl has been observed through reductive elimination from a germanium species.28

However, given the stability of PF2(C6Cl5)3, the mechanism of (C6Cl5)2 likely does not proceed

through direct reductive elimination, but rather a pathway involving two phosphine species.

Figure 3.8. POV-ray depiction of (C6Cl5)2; C: light grey, Cl: green.

Our group has previously reported the synthesis of 1,2-diphosphonium dication on a naphthyl

framework, [(C10H6)(Ph2P)2][B(C6F5)4]2. 29 The reported 1,2-dicationic system can activate

dihydrogen with an auxiliary base and C-H activate cycloheptatriene and 1,4-cyclohexadiene.

Electron-deficient diphosphine starting materials are uncommon and as such oxidation of

diphosphine ((C6Cl5)2P)2 was carried out. The reaction of ((C6Cl5)2P)2 with two equivalents XeF2

cleaves the P-P bond and forms F3P(C6Cl5)2 as the major product, which is also the major product

observed in the oxidation of 3-3 with XeF2. Reactions with various sub-stoichiometric amounts of

XeF2 resulted in unreacted starting material and F3P(C6Cl5)2.

3.2.3 Synthesis of Pentachlorophenyl Phosphonium Cations

Solid 3-6 was added to a white slurry of [SiEt3][B(C6F5)4] in toluene, which rapidly formed a dark-

red oily suspension. The mixture was stirred overnight before the oil was allowed to settle and the

supernatant was decanted. The red oil was then washed and triturated with pentane until 3-10 was

78

obtained as a white powder (Scheme 3.10). Compound 3-10 displays a doublet resonance at 89.5

ppm with 1009 Hz coupling in the 31P{1H} NMR spectrum, while the 19F NMR spectrum displays

a doublet signal at -116.0 ppm with a matching P-F coupling constant and three resonances

attributable to the B(C6F5)4 anion. These data are consistent with the assignment of 3-10 as

[Ph2PF(C6Cl5)][B(C6F5)4].

Scheme 3.10. Synthesis of pentachlorophenyl-substituted phosphonium cations.

In an analogous fashion, reaction of [SiEt3][B(C6F5)4] with compounds 3-7, 3-8 and 3-9 yields the

corresponding fluorophosphonium cations [PhPF(C6Cl5)2][B(C6F5)4] (3-11),

[(C6F5)PF(C6Cl5)2][B(C6F5)4] (3-12), and [(C6F5)2PF(C6Cl5)][B(C6F5)4] (3-13) (Scheme 3.10).

Compounds 3-11 – 3-13 show resonances in the 31P{1H} NMR spectra at 84.4, 71.0 and 66.3 ppm

respectively. The 19F NMR spectra of 3-11 – 3-13 contain resonances at -125.6, -117.0,

and -112.1 ppm as well as three signals corresponding to the B(C6F5)4 anion. The solid-state

structure of 3-11 was obtained by X-ray diffraction (Figure 3.9). Compound 3-11 displays a

distorted tetrahedral geometry at the phosphorus centre. The P-F bond length of 3-11 is 1.539(2)

Å, which is shorter the P-F bond length in difluorophosphoranes 3-6 and 3-9, due to an increase in

P-F bond order.

79

Figure 3.9. POV-ray depiction of 3-11; C: light grey, B: yellow-green, P: orange, F: pink, Cl:

green, hydrogen atoms have been omitted for clarity.

Direct synthesis of fluorophosphonium cations can be effected from the corresponding phosphines

using a variety of fluorinating agents. To this end, compound 3-1 was reacted with one equivalent

of N-fluorobenzenesulfonimide, NFSI, in CH2Cl2. Removing the solvent in vacuo and washing

the resulting powder with pentane yields the corresponding phosphonium salt

[Ph2PF(C6Cl5)][N(SO2Ph)2] in excellent yield (Scheme 3.11). The solid state structure of

[Ph2PF(C6Cl5)][N(SO2Ph)2], 3-10NFSI, was obtained by X-ray diffraction (Figure 3.10). As in 3-

11, the phosphorus centre in 3-10NFSI displays a distorted tetrahedral geometry with a

comparable P-F bond length of 1.486(4) Å. The nitrogen atom of the anion is oriented towards the

phosphonium fluorine at a distance of 2.659 Å, consistent with a non-coordinating anion.

80

1 eq. NFSI

CH2Cl2, RT, 2 h

N

S

SF

P

ClCl

Cl

ClCl F

N(SO2Ph)2

3-63-10NFSI

O

O

OO

P

ClCl

Cl

ClCl

Scheme 3.11. Direct fluorophosphonium synthesis from 3-6 using NFSI.

Figure 3.10. POV-ray depiction of [Ph2PF(C6Cl5)][N(SO2Ph)2], 3-10NFSI †; C: light grey, B:

yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, Cl: green, hydrogen atoms have been

omitted for clarity.

Since 3-3 was not amenable to clean oxidation by XeF2 to form the corresponding

difluorophosphoranes PF2(C6Cl5)3, and instead loss of a C6Cl5 substituent resulted in the formation

of PF3(C6Cl5)2 as the major product, we targeted an alternative route for oxidation.

81

Upon addition of NFSI to a CH2Cl2 solution of 3-3, no reaction was observed even upon heating.

Addition of Lewis acid ZrCl4 has been reported as a catalyst in the fluorination of pyrroles by

NFSI, so we attempted the analogous reaction with 10mol% ZrCl4; however, no formation of the

desired fluorophosphonium cation was observed.30 NFSI, with a reduction potential of -0.78 V (vs.

SCE), appears to be too weak of a fluorinating agent to oxidize 3-3.31

The N-fluoropyridinium salts represent a stronger family of fluorinating agents. The reduction

potential of N-fluoropyridinium salts vary based on substitution; 2,4,6-trimethyl-N-fluoropyridium

has a reduction potential of -0.73 V (vs. SCE), while unsubstituted N-fluoropyridinium has a

reduction potential of -0.47 V (vs. SCE).31 Adding N-fluoropyridinium triflate to 3-3 in CH2Cl2

yields no reaction, even with temperatures elevated to 80°C.

Selectfluor, with a reduction potential of -0.04 V (vs. SCE), is a stronger fluorinating agent than

NFSI and N-fluoropyridinium triflate.31 Due to the poor solubility of Selectfluor in most organic

solvents, MeCN was chosen as the solvent for this reaction. To this end, adding Selectfluor to a

stirring solution of 3 in MeCN resulted in a rapid dark-purple colour change, before slowly turning

green. Solvent was removed in vacuo resulting in isolation of 3-14 as a pale-yellow powder. The

31P{1H} NMR spectrum shows a doublet resonance at -21.8 ppm with a coupling constant of 1065

Hz, which is similar to the values observed in 3-10 – 3-13. The 19F NMR spectrum displays a

doublet signal at -54.3 ppm, however, no resonance was observed that could be attributed to a BF4

counterion. High resolution mass spectral analysis displays an ion consistent with the formulation

of 3-14 as the fluorophosphine oxide O=PF(C6Cl5)2 (Scheme 3.12). The formation of 3-14 likely

arises through reaction of a phosphonium intermediate with residual water.

Scheme 3.12. Reaction of 3-3 with fluorinating agent NFSI to generate 3-14.

82

3.2.4 Electrophilicity of Fluorophosphonium Cations

3.2.4.1 Gutmann Beckett Method

The Gutmann-Beckett method was applied to fluorophosphonium salts 3-10 – 3-13 to measure

electrophilicity and to better rationalize observed reactivity and stability trends. A CH2Cl2 solution

of triethylphosphine oxide OPEt3 was mixed with an excess of fluorophosphonium salts 3-10 –

3-13 and the resulting OPEt3 31P{1H} chemical shifts were measured and referenced against

uncoordinated OPEt3 in CH2Cl2. Mixing 3 equivalents of 3-10 or 3-11 with OPEt3 results in no

interaction between the two compounds, as no broadening or change in chemical shift was

observed for either species by 31P{1H} NMR. Compounds 3-12 and 3-13 also failed to produce

adducts with OPEt3. The Gutmann-Beckett method has been successfully applied to gauge relative

electrophilicity in other fluorophosphonium cations, including [FP(C6F5)3][B(C6F5)4].15,18 The

increased steric bulk imparted by the C6Cl5 substituents seems to effectively prevent adduct

formation with OPEt3, which precludes measurement of electrophilicity using this protocol. The

decreased propensity of salts 3-10 – 3-13 to form Lewis acid-base adducts with OPEt3 is consistent

with previously reported trends observed with the related borane series.4

3.2.4.2 Chemical Shift Trends of Fluorophosphonium Cations

The 31P{1H} NMR chemical shifts of fluorophosphonium phosphorus centres has been used as a

relative measure of electrophilicity.18 More electrophilic phosphonium cations tend to be

correlated with an upfield shift. The 31P{1H} NMR chemical shifts of the fluorophosphonium

cations 3-10 – 3-13 are 89.5, 84.4, 71.0 and 66.3 ppm, respectively. To this end, increasing cation

electrophilicity follows the order 3-10 < 3-11 < 3-12 < 3-13. The upfield chemical shift of 3-11

with respect to 3-10 indicates C6Cl5 is more electron-withdrawing than Ph. Similarly, the upfield

shift of 3-12 compared to 3-13 indicates C6F5 is more electron-withdrawing than C6Cl5. This

observation is consistent with electronegativity arguments, but inconsistent with mesomeric

stabilization trends and observed trends with the analogous borane system.4 The mesomeric

stabilization provided by fluorine substituents to borane requires donation of π-electron density

into the vacant p-orbital on boron. In this case, the Lewis acidic orbital of fluorophosphonium

cations is not oriented perpendicular to the aromatic plane, which reduces possibility for

83

mesomeric stabilization. Without mesomeric effects, the electronegativity effects dominate the

electronic interaction with the phosphonium centre, making C6F5 rings more potent than C6Cl5

rings in fluorophosphonium cations. If the observed 31P{1H} chemical shift trend of phosphonium

salts was a quantitative measure of only electrophilicity, the fluorophosphonium

[FP(C6F5)3][B(C6F5)4] would be expected to have a 31P{1H} NMR chemical shift of approximately

61.6 ppm (based on the difference between 3-12 and 3-13). However, the chemical shift of

[FP(C6F5)3][B(C6F5)4] is reported to be 67.8 ppm, which illustrates the qualitative nature of this

method. As was observed in the synthesis of C6Cl5 substituted phosphines (discussed in 3.2), the

steric demand of the substituents must have a role in determining the chemical shift.

3.2.4.3 General Electrophilicity Index of Fluorophosphonium Cations

Geometry optimization and frequency calculations were performed on phosphonium cations of

3-10 – 3-13 ([3-10]+ – [3-13]+ respectively), [FP(C6Cl5)3]+ and [FP(C6F5)3]+ with Gaussian 0932

using hybrid functional B3LYP33 and split valence double zeta 6-31+G(d) basis set.34 Single point

energy calculations of the cations of 3-10 – 3-13, [FP(C6Cl5)3]+ and [FP(C6F5)3]+ were performed

on the optimized structures using the MP235 and polarized triple-zeta basis set def2-TZVPP.36

When available, solid-state structures were used to generate the input coordinates. The LUMO of

the optimized structures for cations [3-10]+ – [3-13]+, [FP(C6Cl5)3]+ and [FP(C6F5)3]+ all display a

large lobe largely localized on the phosphorus centre opposite the P-F bond, consistent with the

P–F σ*-orbital being the Lewis acidic site (Figure 3.11).

84

Figure 3.11. Surface contour plots of the LUMO oriented along the P-F bond for cations of 3-10

(top left), 3-11 (top right), 3-12 (bottom left), 3-13 (bottom right); P: orange, C: black, F: blue, Cl:

green, H: light grey.

The computed HOMO and LUMO energy values for the cations of 3-10 – 3-13,

[FP(C6Cl5)3][B(C6F5)4], and [FP(C6F5)3][B(C6F5)4] are tabulated below (Table 3.1).

85

Table 3.1. Calculated LUMO and HOMO energies for cations of 3-10 – 3-13,

[FP(C6Cl5)3][B(C6F5)4], and [FP(C6F5)3][B(C6F5)4].

Cation ELUMO (eV) EHOMO (eV) ΔE (ELUMO – EHOMO) (eV)

[3-10]+ -2.289 -12.642 10.353

[3-11]+ -2.471 -12.675 10.204

[3-12]+ -2.979 -12.905 9.926

[3-13]+ -3.108 -13.064 9.956

[FP[C6F5)3]+ -3.455 -14.090 10.635

[FP[C6Cl5)3]+ -2.187 -12.737 10.550

No clear trend emerges from the energy difference between the HOMO and LUMO, which is

associated with “hardness” or “softness” properties: large HOMO-LUMO energy gaps are

indicative of hard Lewis acid or bases, while small energy gaps are indicative of more polarizable

soft acids and bases.37 The fluorophosphonium cations have hardness values comparable to other

soft Lewis acids. The descending order of HOMO and LUMO energy levels, reflecting

stabilization of the cation by the electron withdrawing substituents, builds on the earlier chemical

shift trends to give [FP(C6Cl5)3]+ < [3-10]+ < [3-11]+ < [3-12]+ < [3-13]+ < [FP(C6F5)3]+. While

the stabilizing effect of C6Cl5 substituents is again observed between the pair 3-10 and 3-11, the

relative LUMO energy of [FP(C6Cl5)3]+ is unexpectedly less stabilized than both Ph-substituted

phosphonium salts 3-10 and 3-11. With the noted exception, LUMO energy stabilization correlates

to total electronegativity of phenyl substituents. The electrophilicity indices, ω,38,39 can be

calculated using a formula with HOMO and LUMO energies, wherein energies are in units of

electron volts (eV).

The ω values for the cations of 3-10 – 3-13, [FP(C6Cl5)3]+ and [FP(C6F5)3]+ are tabulated below

(Table 3.2).

86

Table 3.2. Calculated Electrophilic Index, ω, values for the cations of 3-10 – 3-13,

[FP(C6Cl5)3][B(C6F5)4], and [FP(C6F5)3][B(C6F5)4].

Cation ω (eV)

[3-10]+ 2.692

[3-11]+ 2.810

[3-12]+ 3.177

[3-13]+ 3.284

[FP[C6F5)3]+ 3.618

[FP[C6Cl5)3]+ 2.639

The same trend emerges for the electrophilicity indices, ω, as was observed for LUMO energies:

[FP(C6Cl5)3]+ < [3-10]+ < [3-11]+ < [3-12]+ < [3-13]+ < [FP(C6F5)3]+. The calculated ω values

have a strong linear correlation with the LUMO energies, with an R2 value of 0.9934 (Figure 3.12).

However, the calculated ω values have poor linear correlation with the HOMO energies (R2 =

0.7837). The ω values of the cations of 3-10 – 3-13 have remarkably high correlation (R2 = 0.999)

to the experimentally observed chemical shift of the corresponding salts (Figure 3.12).

Fluorophosphonium [FP(C6F5)3][B(C6F5)4] does not follow the trend observed in the series (red

outlier, Figure 3.11).

Figure 3.12. Correlation of LUMO energies to ω for salts 3-10 – 3-13, [FP(C6Cl5)3][B(C6F5)4],

and [FP(C6F5)3][B(C6F5)4] (left). Correlation of chemical shift to ω for salts 3-10 – 3-13, with

[FP(C6F5)3][B(C6F5)4] outlier (right).

y = 1.3069x ‐ 1.2204R² = 0.9934

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4

2.50 2.75 3.00 3.25 3.50 3.75 4.00

LUMO energy (eV

)

ω (eV)

y = ‐38.445x + 192.78R² = 0.999

60

65

70

75

80

85

90

95

2.5 2.7 2.9 3.1 3.3 3.5 3.7

Chem

ical Shift (ppm)

ω (eV)

87

Since GEI calculations only require the free Lewis acid, they do not take steric influences and

geometry rearrangement energies into account. Because of this, ω values are most effectively used

as a quick method to compare systems which differ electronically, but do not differ significantly

in steric encumbrance near the Lewis acidic site. Moreover, the GIE values require half the

computation resources compared to FIA calculations (vide infra) and provide accurate quantization

of electrophilicity in appropriate systems.

3.2.4.4 Fluoride Ion Affinity of Fluorophosphonium Cations

Using the same methodology described in the previous section, the energies for the corresponding

fluoride adducts of species 3-10 – 3-13, [FP(C6Cl5)3]+ and [FP(C6F5)3]+ were calculated.

Computing fluoride ion affinity using the fluoride anion is highly technique dependent. Due to the

difficulty associated with calculating FIA using fluoride anion, COF2 was used as a reference

compound,40 which has an experimentally measured FIA value of 209 kJ/mol. Using this value,

the FIA values of species 3-10 – 3-13, [FP(C6Cl5)3]+ and [FP(C6F5)3]+ were calculated using the

following formula (energies in kJ/mol):

FIA = (Ecation + EF3CO) – (Ephosphorane + EF2CO) + 209

The derivation of the formula used is discussed in more detail in the introduction (Chapter 1, pg

14). The FIA values for the cations of 3-10 – 3-13, [FP(C6Cl5)3]+, [FP(C6F5)3]+, and BCF were

calculated to be 686, 697, 755, 731, 744, 707, 451 kJ/mol, respectively (Table 3.3).

88

Table 3.3. Fluoride Ion Affinities for the Cations of 3-10 – 3-13, [FP(C6Cl5)3][B(C6F5)4], and

[FP(C6F5)3][B(C6F5)4].

Cation FIA (kJ/mol)

[3-10]+ 686

[3-11]+ 697

[3-12]+ 731

[3-13]+ 755

[FP[C6F5)3]+ 744

[FP[C6Cl5)3]+ 707

BCF 451

The FIA values for [FP(C6Cl5)3]+ and BCF are in good agreement with earlier reported

values.18,41,42 Consistent with the other electrophilicity trends, the FIA values increase with C6Cl5

or C6F5 substitution within the series of 3-10 – 3-13. The substitution of a C6Cl5 substituent for

C6F5 increases the FIA value by 24 kJ/mol (seen between 3-13 and 3-12); however, an additional

substitution decreases the FIA by 11 kJ/mol (3-12 to [FP[C6F5)3]+). Overall, the FIA values

correlate well with experimentally-observed 31P{1H} NMR chemical shifts (Figure 3.13).

Figure 3.13. Correlation of 31P{1H} NMR chemical shifts and FIA values for the cations of

3-10 – 3-13, and [FP(C6F5)3][B(C6F5)4].

y = ‐0.3463x + 326R² = 0.9842

60

65

70

75

80

85

90

95

680 700 720 740 760

Chem

ical Shift (ppm)

Fluoride Ion Affinity (kJ/mol)

89

Since fluoride ion affinity calculations consider the energy of both the free Lewis acid and the

fluoride adduct, it accounts for energies associated with rehybridization and rearrangement.

Fluoride ion affinity calculations are more resource intensive, but the resulting values allow for a

more accurate comparison between disparate Lewis acidic systems compared to GIE calculations.

Both computational methods, GIE and FIA, correlate well with experimentally-observed chemical

shift measures of electrophilicity. The advantage of using either computational method is the

potential for them to be used as a predictive tool to study compounds that have not been

synthesized. Additionally, GEI computations are less resource intensive than FIA since they

require only a single structure to be optimized, rather than the two structures required by FIA.

However, since GEI does not use the structure of the corresponding fluoride-adduct species, it

does not take energy costs associated with rearrangements or changing hybridization into account

and is therefore best suited for investigating sterically similar systems. Another advantage of GIE

and FIA is compatibility with systems that aren’t amenable to empirical methods such as Gutmann-

Beckett or Childs’. All measures of electrophilicity have specific drawbacks, from empirical

methods that may be chemically incompatible with certain Lewis acids to ab initio computational

methods that are resource intensive.

3.2.5 Reactivity of Pentachlorophenyl Fluorophosphonium Cations

3.2.5.1 Air- and Moisture-Stability Tests

Increasing the overall air-stability of fluorophosphonium cations was a main target in utilizing

C6Cl5 substituents. To evaluate their stability to atmospheric moisture, solutions of species 3-10 –

3-13 were exposed to air and monitored over time. Solutions of salts 3-10 – 3-13 in 5 mm NMR

tubes were exposed to air for one minute, capped, and agitated before the 19F and 31P{1H} NMR

spectra were recorded. Afterwards, the solutions were exposed to air for successively longer

periods of time to monitor for the onset of decomposition of the phosphonium salts. For

benchmarking purposes, stability tests were also performed with [FP(C6F5)3][B(C6F5)4]. Initial

tests were performed using CH2Cl2 as solvent, however, solvent evaporation occurred before

decomposition of species 3-10 and 3-11 was observed, thus, PhBr was used for the remaining tests.

Compound 3-10 showed signs of decomposition after 21 h, while species 3-11 proved to be

90

significantly more robust, showing no sign of decomposition after 48 h (Table 3.4). Compound 3-

12 proved be significantly less stable than salts 3-9 or 3-10, showing signs of decomposition by

19F NMR after just 4 h. Decomposition of species 3-12 was observed after 15 min of exposure to

air, albeit still significantly longer than the 1 min onset observed for [FP(C6F5)3][B(C6F5)4].

Table 3.4. Air-Stability of Fluorophosphonium Salts 3-10 – 3-13 and [FP[C6F5)3][B(C6F5)4] in

PhBr.

Phosphonium Salt Decomposition Onset

3-10 21 h

3-11 >48 h

3-12 4 h

3-13 15 min

[FP[C6F5)3][B(C6F5)4] 1 min

The solution air-stability trend observed for the fluorophosphonium cations decreases in the order

3-11 > 3-10 > 3-12 > 3-13 > [FP(C6F5)3][B(C6F5)4]. Interestingly, the stability trend is not identical

to the electrophilicity trend observed in section 3.6. The notable deviation from this trend is 3-11,

indicative that C6Cl5 substituents not only increase electrophilicity, but the ortho-chlorine

substituents also provide steric protection relative to Ph substituents. The inclusion of C6F5

substituents seems to drastically decrease the air-stability of fluorophosphonium cations, as

observed in 3-12, 3-13, and [FP(C6F5)3][B(C6F5)4], consistent with a significant increase in

electrophilicity (vide supra). It is noteworthy that the generation of HC6F5 is observed as a

decomposition product for C6F5-substituted phosphonium cations, but the analogous HC6Cl5 was

not observed by 1H NMR for species 3-10 – 3-13.

The air-stability of salts 3-10 and 3-11 was also evaluated as isolated solids. Both compounds were

exposed to air as solids for 4 h hours before being dissolved in benchtop grade PhBr. The resulting

NMR spectra showed no signs of decomposition.

91

3.2.6 Catalytic Activity of Perchlorophenyl Phosphonium Cations

The effectiveness of compounds 3-10 – 3-13 as Lewis acid catalysts was evaluated in an array of

different organic transformations (Figure 3.14). For all test reactions, 5 mol% catalyst was

employed.

Compounds 3-10 – 3-13 all catalyzed the dimerization of 1,1-diphenylethylene at room

temperature, completing the reaction in 18 h, 6 h, 1 h, and 50 min, respectively. The 1H NMR

spectra display a decrease in the intensity of the olefin signal at 5.33 ppm and the growth of two

new distinct diastereotopic resonances at 2.94 and 2.61 ppm as the reaction proceeds, indicative

of 1-methyl-1,3,3-triphenyl-2,3-dihydro-1H-indene formation.

The hydrodefluorination of 1-fluoroadamantane was rapidly effected by all four catalysts in the

presence of HSiEt3. Compound 3-10 took 3.5 h to complete the reaction, while 3-11 – 3-13 had

completed the reaction before NMR spectra could be obtained. The consumption of the

fluoroadamantane C-F resonance and growth of triethylfluorosilane Si–F resonance at -175 ppm

in the 19F NMR spectra allowed the reactions to be easily monitored.

Compound 3-10 shows negligible activity in the deoxygenation of benzophenone with HSiEt3 at

room temperature, completing the transformation only at 140 °C. On the other hand, the

deoxygenation of benzophenone can be completed by salts 3-11 – 3-13 at room temperature after

40 h, 25 min and 2 h, respectively. The C6F5-substituted phosphonium cations 3-12 and 3-13

proved to be significantly more active than the Ph-substituted 3-10 and 3-11. The deoxygenation

reaction can be monitored by 1H NMR, given that the aromatic resonance at 7.66 ppm disappears

and the olefinic signal at 3.77 ppm appears throughout the course of the reaction.

The dehydrocoupling of phenol with HSiEt3 reaction shows similar reactivity trends as the

deoxygenation of benzophenone, wherein only compounds 3-11 – 3-13 are able to effect the

transformation at room temperature and salt 3-10 required significantly higher temperatures. The

growth of 1H NMR product resonance at 6.95 ppm and disappearance of starting material

resonances at 6.72 ppm allowed facile tracking of the reaction progress.

Hydrosilylation of α-methylstyrene with triethylsilane required elevated reaction temperatures for

all catalysts except compound 3-13, which completed the reaction in 5 h at room temperature. The

92

Ph-substituted 3-10 and 3-11 catalyzed the reaction very slowly and require high reaction

temperatures. Reaction progress was monitored by 1H NMR via the decrease of olefinic resonances

at 5.27 ppm and 4.95 ppm. The formation of new signal at 0.95 ppm is concomitant with formation

of the hydrosilylated product.

The benzylation and hydrodefluorination of 4-(trifluoromethyl)bromobenzene in C6D6 with

triethylsilane could not be achieved using the less electrophilic salts 3-10 or 3-11.. However,

species 3-12 and 3-13 completed the reaction at 80 °C in 22 h and 1.5 h respectively. Benzylation

occurs through initial activation of a trifluoromethyl C-F bond to generate a carbocation, which is

attacked by an arene. Subsequent hydrodefluorination of the remaining two fluorine substituents

yields the benzylated product Consumption of starting material was monitored by concomitant

decrease of the C–F resonance at -62.7 ppm and appearance of a Si–F resonance at -175.5 ppm by

19F NMR spectroscopy.

Of all the test reactions performed, the dimerization of 1,1-diphenylethylene provides the best

point of comparison for a relative reactivity trend because all the catalysts could perform this

reaction under the same reaction conditions, with time as the only variable. The order of increasing

catalytic activity is 3-10 < 3-11 < 3-12 < 3-13, which is the same trend obtained from the

computational evaluations of electrophilicity. This reactivity trend is consistent throughout all the

test reactions, though the less electrophilic catalysts 3-10 and 3-11 were incapable of effecting the

benzylation of 4-trifluoromethylbromobenzene.

93

Figure 3.14. Catalytic activity of fluorophosphonium salts 3-10 – 3-13. Catalytic screening for

compounds 3-10, 3-12, and 3-13 was completed by Vitali Podgorny.

3.3 Conclusion

A series of C6Cl5-substituted phosphines were synthesized: Ph2P(C6Cl5) (3-1), PhP(C6Cl5)2 (3-2),

P(C6Cl5)3 (3-3) (C6F5)2P(C6Cl5) (3-4), or (C6F5)P(C6Cl5)2 (3-5). Oxidation of phosphines 3-1, 3-2,

3-4, and 3-5 by XeF2 lead to the clean formation of the corresponding difluorophosphoranes

94

Ph2PF2(C6Cl5) (3-6), PhPF2(C6Cl5) (3-7), (C6F5)2PF2(C6Cl5) (3-8), or (C6F5)PF2(C6Cl5)2 (3-9).

Oxidation of 3-3 yields the desired difluorophosphorane PF2(C6Cl5)3 as a minor product and

PF3(C6Cl5)2 as the major product with concomitant generation of (C6Cl5)2. Fluoride abstraction

from compounds 3-6 – 3-9 yields fluorophosphonium salts [Ph2PF(C6Cl5)][B(C6F5)4] (3-10),

[PhPF(C6Cl5)2][B(C6F5)4] (3-11), [(C6F5)2PF(C6Cl5)][B(C6F5)4] (3-12), and

[(C6F5)PF(C6Cl5)2][B(C6F5)4] (3-13), respectively. Reaction of 3 with Selectfluor yielded

fluorophosphine oxide OPF(C6Cl5)2 (3-14).

Measuring the relative electrophilicity of fluorophosphonium salts 3-10 – 3-13 using the Gutmann-

Beckett method was unsuccessful, due to the inability of OPEt3 to form adducts with the cations.

Evaluation of the 31P NMR chemical shifts indicated increasing electrophilicity in the order 3-10 <

3-11 < 3-12 < 3-13, consistent with electron-withdrawing strength of substituents increasing in the

order Ph < C6Cl5 < C6F5. Cations 3-10 – 3-13, [PF(C6Cl5)3]+, and [PF(C6F5)3]+ were evaluated

computationally. The FIA values of compounds 3-10 – 3-13 supported the chemical shift trend,

yielding FIA values of 686, 697, 731, and 755 kJ/mol, respectively. Similarly, the GEI values, ω,

were calculated which showed high linear correlation with the 31P NMR chemical shifts and

LUMO energies.

The air-stability of salts 3-10 – 3-13 was evaluated by exposing solutions of the salts to air for

certain periods of time. The less electrophilic Ph-substituted 3-10 and 3-11 proved to be quite

robust, while C6F5-substituted 3-12 and 3-13 decomposed rapidly when exposed to air. The

decreasing air-stability trend of 3-11 > 3-10 > 3-13 > 3-12 parallels the increasing electrophilicity

trends observed, with the exception of 3-11. Phosphonium salt 3-11 displays enhanced

electrophilicity and significantly higher air-stability over salt 3-10, presumably resulting from

increased steric protection from a second bulky C6Cl5 group.

The catalytic activity of 3-10 – 3-13 was tested in diphenylethylene dimerization, deoxygenation,

hydrodefluorination, dehydrocoupling, hydrosilylation and benzylation reactions. The reactivity

trend was in good agreement with electrophilicity trends of 3-10 < 3-11 < 2-12 < 3-13. The less

electrophilic salts 3-10 and 3-11 required significant heating to effect full conversion in many

reactions, while both compounds 3-12 and 3-13 were competent catalysts at room temperature.

Further, salts 3-10 and 3-11 were unable to effect benzylation reactions, even at elevated

temperatures. The C6Cl5-substituted phosphonium salts 3-10 – 3-13 demonstrate significantly

95

enhanced air-stability compared to [PF(C6F5)3][B(C6F5)4], However, this air-stability comes at the

cost of catalytic activity.

3.4 Experimental










by three successive cycles of headspace-evacuation, and sonication. All elemental analyses were

performed in house using a Perkin Elmer 2400 Series II CHNS Analyzer. High-resolution mass

spectra data was obtained using a JEOL AccuTOF or an Agilent 6538 Q-TOF mass spectrometer.

All NMR data were collected on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz or Agilent

DD2 600 MHz spectrometer at 25 °C unless otherwise noted. NMR chemical shift data are given

relative to an external standard (1H, 13C: SiEt4; 11B: 15% BF3·Et2O; 19F: CFCl3; 31P: H3PO4). In

some cases, 2-D NMR techniques were employed to assign individual resonances. Both

(C6F5)2PBr and (C6F5)PBr2 were synthesized by literature procedure and obtained in high purity

through successive recrystallizations from Et2O.43 Other reagents, including Ph2PCl, PhPCl2, PBr3,

PCl3, Selectfluor, NFSI, 1-Fluoropyridinium triflate, and hexachlorobenzene were obtained from

Sigma Aldrich and used without further purification. Perfluorophenyl-substituted systems were

prepared by Vitali Podgorny. Full characterization of these compounds are detailed in his thesis.44

Computational work was performed using resources provided by the Shared Hierarchical

Academic Research Computing Network and Compute Canada. Crystals for structures denoted

with † were grown by Vitali Podgorny.

96

Compounds 3-1 – 3-3 were prepared in a similar fashion and thus only one sample preparation is

detailed below.

Synthesis of Ph2P(C6Cl5) (3-1): A 100 mL Schlenk flask was charged with C6Cl6 (318 mg, 1.12

mmol), Et2O (30 mL) and a magnetic stir bar, generating a white slurry. The reaction flask was

cooled to -15 °C in a dry ice/acetone bath. A hexane solution of 2.5 M nBuLi (0.44 mL, 1.12

mmol) was added dropwise to the stirring solution, which gradually turned the slurry to a clear,

light yellow solution. The solution was then cooled to -78 °C, before a solution of Ph2PCl (247

mg, 1.12 mmol) in Et2O (3 mL) was added to the reaction flask in a dropwise fashion. The stirred

solution was warmed to room temperature overnight. The solvent was removed in vacuo leaving

an off-white solid. The product was dissolved in CH2Cl2 (6 mL) and filtered through a Celite plug.

The solvent was reduced and the solution was cooled to -35 °C to produce a white precipitate,

which was collected by filtration. The white filtrate was then washed with pentane (2 x 2 mL) to

yield 3-1 (340 mg, 70%) as a white solid. The recrystallization of 3-1 and 3-4 from vapour

diffusion of n-pentane into a solution of the compound in dichloromethane yielded X-ray quality

crystals. (338 mg, 70% yield).

1H NMR (500 MHz, C6D6): δ 7.33 – 7.39 (m, 4H, m-C6H5), 7.08 – 7.03 (m, 6H, o- & p-C6H5)

ppm. 31P{1H} NMR (162 MHz, C6D6): δ 10.7 (s) ppm. 13C{1H} NMR (125 MHz, C6D6): δ 139.7

(d, 2JPC = 18 Hz, o-C6Cl5), 136.4 (s, p-C6Cl5), 136.0 (d, 1JPC = 24 Hz, i-C6Cl5), 134.3 (d, 1JPC = 15

Hz, i-C6H5), 133.3 (s, m-C6Cl5), 132.9 (d, 2JPC = 21 Hz, o-C6H5), 129.1 (s, p-C6H5), 129.0 (d, 3JPC

= 6 Hz, m-C6H5) ppm. Anal. Calcd. for PC18H10Cl5: C: 49.76, H: 2.32. Found: C: 49.82%, H:

1.99%. HRMS (DART Ionization) m/z: [M+H]+ Calcd. for C18H10Cl5P 432.90410, Found:

432.90360.

97

Synthesis of PhP(C6Cl5)2 (3-2): (pale yellow solid, 36% yield, from Ph2PCl). 1H NMR (500 MHz,

C6D6): δ 7.44 (apparent triplet, 3JPH = 7 Hz, 3JHH = 7 Hz, 2H, m-C6H5), 6.98 – 7.07 (m, 4H, o- &

p-C6H5) ppm. 31P{1H} NMR (243 MHz, C6D6): δ 15.1 (s) ppm. 13C{1H}NMR (125 MHz, C6D6): δ

137.1 (d, 2JPC = 19 Hz, o-C6Cl5), 136.0 (d, 1JPC = 36 Hz, i-C6Cl5), 135.3 (d, 4JPC = 1 Hz, p-C6Cl5),

133.4 (d, 3JPC = 1 Hz, m-C6H5), 131.8 (d, 1JPC = 15 Hz, i-C6H5), 130.8 (s, p-C6H5), 128.9 (d, 2JPC

= 9 Hz, o-C6H5), 128.5 (d, 3JPC = 14 Hz, m-C6H5) ppm. Anal. Calcd. for PC18H5Cl10: C: 35.63, H:

0.83. Found: C: 35.29%, H: 0.92%. HRMS (DART Ionization) m/z: [M+H]+ Calcd. for C18H5Cl10P

602.70924, Found: 602.70831.

Synthesis of P(C6Cl5)3 (3-3): (white solid, 21% yield, from PBr3).

31P{1H} NMR (162 MHz, C6D6): δ 19.1 (s) ppm. Anal. Calcd. for PC18Cl18: C: 27.76. Found: C:

% 25.87. HRMS (EI-TOF) m/z: [M]+ Calcd. for C18HCl15P 778.50553, Found: 778.50594.

98

Compounds 3-6 and 3-7 were prepared in a similar fashion and thus only one preparation is

detailed.

Synthesis of Ph2PF2(C6Cl5) (3-6): A 20 mL vial was charged with 3-1 (257 mg, 0.59 mmol),

CH2Cl2 (4 mL) and a magnetic stir bar, forming a light-yellow solution. To the stirring solution, a

solution of XeF2 (100 mg, 0.59 mmol) in CH2Cl2 (3 mL) was quickly added. The effervescent

solution lightened as it was left to stir for 2 hr. The solvent was reduced and the solution cooled to

-35 °C to produce a white precipitate. The precipitate was collected by filtration and washed with

n-pentane (2 x 2 mL) yielding a white solid. The CH2Cl2 was removed in vacuo yielding 3-6 (230

mg, 83%) as a yellow solid. Recrystallization of 3-6 from vapour diffusion of n-pentane into a

solution of the compound in dichloromethane yielded X-ray quality crystals (321 mg, 98 % yield).

1H NMR (500 MHz, C6D6): δ 8.17 – 8.10 (m, 4H, m-C6H5), 7.08 – 7.01 (m, 6H, o- & p-C6H5) ppm.

31P{1H} NMR (162 MHz, C6D6): δ -50.7 (t, 1JPF = 716 Hz) ppm. 19F{1H} NMR (376 MHz, C6D6):

δ -41.8 (d, 1JPF = 716 Hz, PF2) ppm. 13C{1H} NMR (125 MHz, C6D6): δ 140.1 (dt, 1JPC = 217 Hz,

2JFC = 42 Hz, i-C6Cl5), 137.5 (dt, 1JPC = 181 Hz, 2JFC = 24 Hz, i-C6H5), 135.0 (dt, 2JPC = 13 Hz,

3JFC = 10 Hz, o-C6H5), 134.5 (dt, 2JPC = 3 Hz, 3JFC = 2 Hz, o-C6Cl5), 133.5 (d, 4JPC = 17 Hz, p-

C6Cl5), 132.6 (dt, 3JPC = 3 Hz, 4JFC = 3 Hz, m-C6Cl5), 131.7 (dt, 4JPC = 4 Hz, 5JFC = 1 Hz, p-C6H5),

128.6 (dt, 3JPC = 13 Hz, 4JPC = 10 Hz, m-C6H5) ppm. Anal. Calcd. for PC18H10Cl5F2: C: 45.76, H:

2.13. Found: C: 45.24%, H: 2.00%.

Synthesis of PhPF2(C6Cl5)2 (3-7): (light orange solid, 76% yield).

99

1H NMR (500 MHz, C6D6): δ 8.11 – 8.04 (m, 2H, m-C6H5), 7.16 – 7.08 (m, 3H, o-, p-C6H5) ppm.

31P{1H} NMR (202 MHz, C6D6): δ -44.9 (t, 1JPF = 747 Hz) ppm. 19F{1H} NMR (470 MHz, C6D6):

δ -28.9 (d, 1JPF = 747 Hz, PF2) ppm. 13C{1H} NMR (125 MHz, C6D6): δ 137.7 (dt, 1JPC = 210 Hz,

2JFC = 32 Hz, i-C6Cl5), 136.7 – 136.6 (m, o-C6Cl5), 135.8 (dt, 2JPC = 14 Hz, 3JFC = 10 Hz, o-C6H5),

135.4 (dt, 1JPC = 187 Hz, 2JFC = 23 Hz, i-C6H5), 134.3 (d, 4JPC = 18 Hz, p-C6Cl5), 134.0 – 133.8

(m, m-C6Cl5), 132.98 (br d, 4JPC = 4 Hz, p-C6H5), 129.3 (dm, 3JPC = 18 Hz, m-C6H5) ppm. Anal.

Calcd. for PC18H5Cl10F2: C: 33.53, H: 0.78. Found: C: 34.16%, H: 0.86%.

Compounds 3-10 and 3-11 were prepared in a similar fashion and thus only one preparation is

detailed below

Synthesis of [Ph2PF(C6Cl5)][B(C6F5)4] (3-10): A 20 mL vial was charged with [Et3Si][B(C6F5)4]

(682 mg, 0.77 mmol), toluene (10mL) and a magnetic stir bar, forming a white slurry. To the

stirring slurry, a solution of 3-6 (382 mg, 0.81 mmol) in toluene (5 mL) was added. The solution

was allowed to stir overnight at room temperature, resulting in a dark orange solution. When the

reaction mixture was allowed to settle, a dark orange oil collected at the bottom of the vial leaving

a clear supernatant. After decanting the toluene from the oil, additional toluene (2 x 3 mL) was

added to wash the oil. After decanting the toluene washes, the oil was washed with n-pentane (3 x

4 mL) before removing the solid in vacuo resulting in 3-10 as a fluffy white solid (697 mg, 80 %

yield).

1H NMR (500 MHz, CDCl3): δ 8.09 – 8.04 (m, 1H, p-C6H5), 7.88 – 7.78 (m, 4H, o- & m-C6H5)

ppm. 31P{1H} NMR (162 MHz, C6D5Br): δ 89.5 (d, 1JPF = 1009 Hz) ppm. 19F{1H} NMR (376

MHz, C6D5Br): δ -116.0 (d, 1JPF = 1010 Hz, PF2), -132.0 (m/br, 8F, B(o-C6F5)), -162.3 (t, 3JFF =

21 Hz, 4F, B(p-C6F5)), -166.2 (m/br, 8F, B(m-C6F5)) ppm. 11B{1H} NMR (128 MHz, C6D5Br): -

16.5 (s) ppm. 13C{1H} NMR (125 MHz, CDCl3): δ 148.3 (br d, 1JFC = 241 Hz, B(o-C6F5)), 145.3

(d, 4JPC = 3 Hz, P(p-C6Cl5)), 139.5 (dd, 4JPC = 2 Hz, 5JFC = 2 Hz, P(p-C6H5)), 138.3 (br d, 1JFC =

100

238 Hz, B(p-C6F5)), 137.7 (dd, 2JPC = 6 Hz, 3JFC = 1.0 Hz, P(o-C6Cl5)), 137.1 (d, 3JPC = 12 Hz,

P(m-C6Cl5)), 136.3 (br d, 1JFC = 235 Hz, B(m-C6F5)), 133.5 (dd, 2JPC = 14 Hz, 3JFC = 1 Hz, P(o-

C6H5)), 131.5 (d, 3JPC = 15 Hz, P(m-C6H5)), 123.8 (br s, B(i-C6F5)), 116.1 (dd, 1JPC = 112 Hz, 2JFC

= 14 Hz, P(i-C6H5)), 115.9 (dd, 1JPC = 121 Hz, 2JFC = 11 Hz, P(i-C6Cl5)) ppm. Anal. Calcd. for

PC42H10Cl5F21B: C: 44.54, H: 0.89. Found: C: 45.15%, H: 0.94%. HRMS (DART Ionization) m/z:

[M]+ Calcd. for C18H10Cl5PF 450.89468, Found 450.89445.

Synthesis of [PhPF(C6Cl5)2][B(C6F5)4] (3-11): (320 mg, 90 % yield) as an off white solid.

1H NMR (500 MHz, CDCl3): δ 8.13 – 8.08 (m, 1H, p-C6H5), 7.98 – 7.76 (m, 4H, o-, m-C6H5) ppm.

19F{1H} NMR (470 MHz, C6H5Br): δ -125.64 (d, 1JPF = 1011 Hz, PF2), -138.89 (m/br, 8F, B(o-

C6F5)), -169.24 (m/br, 4F, B(p-C6F5)), -173.12 (m/br, 8F, B(m-C6F5)) ppm. 31P{1H} NMR (162

MHz, CD2Cl2): δ 84.38 (d, 1JPF = 1011 Hz) ppm. 11B{1H} NMR (128 MHz, CD2Cl2): -16.66 (s)

ppm. 13C{1H} NMR (125 MHz, CDCl3): δ 148.28 (br d, 1JFC = 240 Hz, B(o-C6F5)), 145.51 (dm,

4JPC = 3.2 Hz, P(p-C6Cl5)), 140.54 (m, P(p-C6H5)), 138.25 (br d, 1JFC = 239.2 Hz, B(p-C6F5)),

137.20 (d, 3JPC = 13.3 Hz, P(m-C6Cl5)), 136.65 (d, 2JPC = 6.2 Hz, P(o-C6Cl5)), 136.33 (br d, 1JFC =

239.0 Hz, B(m-C6F5)), 133.12 (d, 2JPC = 13.5 Hz, P(o-C6H5)), 132.15 (d, 3JPC = 16.6 Hz, P(m-

C6H5)), 124.12 (br s, B(i-C6F5)), 118.30 (dd, 1JPC = 130.7 Hz, 2JFC = 10.8 Hz, P(i-C6H5)), 115.99

(dd, 1JPC = 113.9 Hz, 2JFC = 12.5 Hz, P(i-C6Cl5)) ppm. Anal. Calcd. for PC42H5Cl10F21B: C: 38.66.

H: 0.39. Found: C: 42.17%, H: 0.87%. HRMS (DART Ionization) m/z: [M]+ Calcd. for

C18H5Cl10PF 620.69982, Found 620.69896.

101

Synthesis of OPF(C6Cl5)2 (3-14): A 20 mL vial was charged with 3-4 (78 mg, 0.10 mmol), MeCN

(3 mL) and a magnetic stir bar. A solution of 1-chloromethyl-4-fluoro-1,4-diazonia-bicyclo-

[2.2.2]octanebis(tetrafluoroborate) in MeCN was added. The solution briefly turned dark-purple

as a black precipitate was formed before returning to a pale green colour. The supernatant was

decanted and the solvent was removed in vacuo yielding 3-14 (24 mg, 43%) as a yellow solid.

31P{1H} NMR (162 MHz, CH2Cl2): δ 21.2 (d, 1JPF = 1065 Hz) ppm. 19F NMR (376 MHz, CH2Cl2):

δ −54.3 (d, 1JPF = 1065 Hz) ppm. HRMS (EI-TOF) m/z: [M]+ Calcd for C12Cl10FPO: 564.65753,

Found: 564.65713.

Hydrodefluorination of 1-fluoroadamantane: To a vial containing 5 mol% catalyst was added

a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7 mL). The solution was added to a vial

containing 1-fluoroadamantane (15.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR

and monitored by NMR spectroscopy.

Dehydrocoupling of triethylsilane with phenol: To a vial containing 5 mol% catalyst was added

a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7mL). The solution was added to a vial

containing phenol (9.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube and

monitored by NMR spectroscopy.

Benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene: To a vial containing

5 mol% catalyst was added a solution of HSiEt3 (41.8 mg, 0.36 mmol) in PhBr (0.7 mL). The

solution was added to a vial containing 4-trifluoromethylbromobenzene (22.5 mg, 0.1 mmol),

which was then transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.

102

Deoxygenation of benzophenone: To a vial containing 5 mol% catalyst was added a solution of

HSiEt3 (23.2 mg, 0.2 mmol) in PhBr (0.7 mL). The solution was added to a vial containing

benzophenone (18.2 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube and

monitored by NMR spectroscopy.

Hydrosilylation of α-methylstyrene with triethylsilane: To a vial containing 5 mol% catalyst

was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7 mL). The solution was added to

a vial containing α-methylstyrene (11.8 mg, 0.1 mmol), which was then transferred to a 5 mm

NMR tube and monitored by NMR spectroscopy.


of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in benzene-d6 (0.7 mL). The solution was then

transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.




Micromount under an N2 stream to maintain a dry, O2-free environment for each crystal.









anisotropically.

103


3-1 3-4* 3-6*

Formula C18H10Cl5P C18Cl10F5P C18H10Cl5F2P

Weight (g/mol) 434.52 696.69 472.51

Crystal system orthorhombic triclinic orthorhombic

Space group Pbca P-1 Pbca

a (Å) 10.8832(6) 8.6247(6) 11.2850(4)

b (Å) 11.0570(5) 13.0708(10) 10.8896(4)

c (Å) 29.4458(15) 13.2510(10) 30.4024(11)

α (°) 90 81.217(4) 90

β (°) 90 71.522(4) 90

γ (°) 90 89.820(4) 90

Volume (Å3) 3543.4(3) 1398.55(18) 3736.2(2)

Z 8 2 8


R(int) 0.1077 0.0697 0.0270

μ, mm–1 0.906 1.098 0.882

F(000) 1751 706 1895

Index ranges -12 ≤ h ≤ 14 -11 ≤ h ≤ 10 -14 ≤ h ≤ 14

-14 ≤ k ≤ 12 -16 ≤ k ≤ 16 -14 ≤ k ≤ 13

-38 ≤ l ≤ 37 -17 ≤ l ≤ 17 -39 ≤ l ≤ 39


θ range (min, max) (°) 2.33, 26.75 1.58, 27.51 2.25, 27.49

Total data 4072 6362 4289

Max peak 0.5 0.5 0.4

Min peak -0.6 -0.5 -0.3

>2(FO2) 2778 3917 3665


R (>2σ) 0.0297 0.0398 0.0255

Rw 0.0630 0.1012 0.0654

GoF 0.841 0.907 1.066 * Crystal for X-ray diffraction obtained by Vitali Podgorny. Structure solution refined by the author.

104

3-8* 3-10NFSI* 3-11

Formula C18Cl5F12P C30H20Cl5FNO4PS2 C42H5BCl10F21P

Weight (g/mol) 652.42 729.71 1304.74

Crystal system orthorhombic triclinic monoclinic

Space group Pbca P-1 P21/n

a (Å) 12.0956(11) 8.619(2) 11.8072(4)

b (Å) 10.4377(11) 12.825(4) 20.1065(8)

c (Å) 33.662(3) 15.422(5) 21.8370(7)

α (°) 90 66.433(13) 90

β (°) 90 85.560(14) 100.528(7)

γ (°) 90 89.858(14) 90

Volume (Å3) 4249.8(7) 1557.2(8) 5096.9(3)

Z 8 2 4


R(int) 0.0699 0.0515 0.1077

μ, mm–1 0.871 0.695 0.688

F(000) 2537 722 2544

Index ranges -15 ≤ h ≤ 15 -11 ≤ h ≤ 11 -15 ≤ h ≤ 12

-7 ≤ k ≤ 13 -16 ≤ k ≤ 16 -26 ≤ k ≤ 26

-43 ≤ l ≤ 43 -19 ≤ l ≤ 20 -27 ≤ l ≤ 28


θ range (min, max) (°) 2.42, 27.58 1.73, 28.01 2.03, 27.50

Total data 4897 7098 11706

Max peak 0.7 1.2 0.8

Min peak -0.6 -1.1 -0.9

>2(FO2) 3294 5940 5178


R (>2σ) 0.0430 0.0750 0.487

Rw 0.0931 0.2290 0.1110

GoF 1.046 1.034 0.995 * Crystal for X-ray diffraction obtained by Vitali Podgorny. Structure solution refined by the author.

105

(C6Cl5)2 ((C6Cl5)2P)2

Formula C6Cl5 C24Cl20P2

Weight (g/mol) 249.31 1059.27

Crystal system orthorhombic triclinic

Space group Pbcn P-1

a (Å) 13.3969(9) 8.7639(8)

b (Å) 10.5584(8) 14.4199(14)

c (Å) 11.6957(9) 18.4538(15)

α (°) 90 103.340(4)

β (°) 90 92.793(4)

γ (°) 90 104.772(4)

Volume (Å3) 1654.4(2) 2179.8(4)

Z 8 2

Density (calcd.) (gcm–3) 2.002 1.6137

R(int) 0.0453 0.0882

μ, mm–1 1.673 1.345

F(000) 968 1034

Index ranges -17 ≤ h ≤ 17 -11 ≤ h ≤ 11

-13 ≤ k ≤ 13 -18 ≤ k ≤ 18

-15 ≤ l ≤ 15 -24 ≤ l ≤ 23

Radiation Mo Kα Mo Kα

θ range (min, max) (°) 2.46, 27.54 2.11, 27.55

Total data 1900 10002

Max peak 0.4 1.4

Min peak -0.3 -1.4

>2(FO2) 1627 5118

Parameters 100 416

R (>2σ) 0.0244 0.0572

Rw 0.0600 0.1303

GoF 0.990 0.947

106

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42. Bohrer, H.; Trapp, N.; Himmel, D.; Schleep, M.; Krossing, I., From Unsuccessful H2

activation with FLPs Containing B(Ohfip)3 to a Systematic Evaluation of the Lewis Acidity of 33

110

Lewis Acids Based on Fluoride, Chloride, Hydride and Methyl Ion Affinities. Dalton Trans. 2015,

44, 7489.

43. Cowley, A. H.; Pinnell, R. P., Pentafluorophenyl-Phosphorus Ring System. J. Am. Chem.

Soc. 1966, 88, 4533.

44. Podgorny, V. Electrophilic Phosphenium and Phosphonium Cations: Synthesis and

Reactivity of Perfluoro- & Perchloroaryl Phosphorus Systems. University of Toronto, 2016.


46. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H., OLEX2:

A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42,

339.

111

Chapter 4 Perfluorobiphenyl Fluorophosphonium Cations

4.1 Introduction

4.1.1 Perfluorobiphenyl Groups in Transition Metal Applications

Massey and coworkers first reported the synthesis of 2-bromononafluorobiphenyl, BrC12F9, in

1964.1 This is the first example of any polyfluorobiphenyl compound containing a bromine

substituent capable of undergoing lithium exchange. This first synthesis was carried out through

the addition of 2-bromopentafluorobenzene, BrC6F5, to an equimolar Et2O solution of

pentafluorophenyllithium, C6F5Li, at room temperature (Scheme 4.1).

Scheme 4.1. Synthesis of 2-bromoperfluorobiphenyl, BrC12F9.

Performing the reaction at room temperature yields bromoperfluorobiphenyl in a low yield of 30%.

The authors proposed a mechanism in which a perfluorobenzyne intermediate reacts further with

an equivalent of BrC6F5. Massey prepared a series of BrC12F9 transfer agents including the lithium,

mercury, and magnesium (Grignard) reagents. An Ullmann reaction using copper to couple

BrC12F9 yielded the corresponding perfluoroquaterphenyl compound.

This interesting compound went largely unnoticed until Marks reported the synthesis of

tris(2,2’,2’’-nonafluorobiphenyl)borane, B(C12F9)3 and the related triphenyl carbenium species

tris(2,2’,2’’-nonafluorobiphenyl)fluoroaluminate, [Ph3C][FAl(C12F9)], in 1997.2 In a modification

of Massey’s original synthesis, Marks reported adding a half molar equivalent to a cooled Et2O

solution of BrC6F5, resulting in BrC12F9 being obtained in high purity and an excellent yield of

83%. Marks proposes a more sophisticated mechanism in which perfluorobenzyne reacts with

C6F5Li to yield the lithiated perfluorobiphenyl, C12F9Li. Next, C12F9Li is proposed to undergo

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metal-halogen exchange with C6F5Br to yield desired BrC12F9 and regenerate C6F5Li (Scheme

4.2). This alternative mechanism of carbolithiation occurring to generate 2-lithioperfluorobiphenyl

and subsequent lithium-halogen exchange with BrC6F5 is consistent with the electrophilic behavior

of benzyne intermediates.3

Scheme 4.2. Mechanism of 2-bromoperfluorobiphenyl synthesis.

Reaction of three equivalents of C12F9Li with boron trichloride in Et2O yields the borane etherate

B(C12F9)3·Et2O. Sublimation of B(C12F9)3·Et2O and subsequent recrystallization from pentane

yields adduct free borane B(C12F9)3 (Scheme 4.3). Similar treatment of three equivalents of

C12F9Li with aluminum trichloride yields lithium

tris(2,2’,2’’-nonafluorobiphenyl)fluoroaluminate, which can be treated with triphenylmethyl

chloride, Ph3CCl, to generate [Ph3C][FAl(C12F9)] (Scheme 4.3).

113

Scheme 4.3. Synthesis of perfluorobiphenyl substituted borane (top) and fluoroaluminate

(bottom).

Marks investigated both B(C12F9)3 and [Ph3C][FAl(C12F9)] for their effectiveness as abstractors

and subsequent counterions in cationic early transition metal-mediated homogenous Ziegler-Natta

type olefin polymerization. Mounting evidence at the time suggested that the anions had a

significant impact on catalytic performance including chain-transfer characteristics and product

tacticity.4,5 The borane B(C12F9)3 was reacted with a wide range of group 4 and Tl metallocene

dimethyl compounds to cleanly abstract a methyl substituent and generate isolable cationic

complexes with [MeB(C12F9)3]- counterions.

The bulky [MeB(C12F9)3]- anion proved to enhance stability of these metallocene cations and

allowed for the isolation of dimeric, μ-Me, cationic metallocene systems. The enhanced stability

is related to a reduced coordinative propensity of [MeB(C12F9)3]- towards cationic metallocenes

compared to the related [MeBCF]- that had previously been used.4 The cationic metallocene

complexes with [MeB(C12F9)3]- anions were all noted to have enhanced thermal stability compared

to the corresponding [MeBCF]- salts. The common decomposition product of dinuclear cationic

methyl-bridged metallocenes is the corresponding fluoro-bridged compound. This is proposed to

occur through either the transfer of a fluoroaryl group from the anion to the cation or through direct

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fluoride abstraction by the metallocene cation from the anion fluoroaryl group (Scheme 4.4). In

both possible decomposition routes, the enhanced thermal stability the metallocene salt is derived

from the increased chemical stability of the anion bearing C12F9 substituents. Cationic

metallocenes generated using [Ph3C][FAl(C12F9)3] also demonstrated enhanced thermal stability.

Scheme 4.4. Proposed decomposition route of cationic zirconocene compounds.

The dinuclear bridged metallocene catalysts that were obtained as a result of the increased stability

of the [MeB(C12F9)3]- demonstrated ethylene polymerization activity that exceeded that of the

monomeric catalysts obtained with [MeBCF]- counterions. The bridged metallocenes also

produced higher molecular weight polyethylenes. Most notable is the 70 – 10,000 times rate

enhancement observed for constrained geometry ethylene polymerization catalysts activated with

B(C12F9)3 instead of BCF. Constrained geometry catalysts activated with B(C12F9)3 were also

found to have comparable comonomer incorporation with narrower polydispersities at higher rates.

Marks used the Childs’ method to gauge the relative electrophilicity of common cocatalysts and

referenced their strength to a BBr3 standard (defined as 1.00).6,7 By this method, C12F9 substituted

boranes display enhanced electrophilicity compared to -C6F5 substituted boranes. Substitution of

a single C6F5 group for a C12F9 group increases the relative electrophilicity from 0.77 in BCF to

0.79 in B(C12F9)(C6F5)2, while B(C12F9)3 displayed a relative electrophilicity of 0.85. The

enhanced electron withdrawing nature of C12F9 and high chemical stability make C12F9

substituents an attractive target for electrophilic catalyst design. Very few fluoroaryl-substituted

boranes have reported electrophilicities higher than BCF.8

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Bochmann and coworkers employed B(C12F9)3 in the synthesis of weakly coordinating

cyanoborate anions. Reaction of potassium cyanide, KCN, with B(C12F9)3 and subsequent workup

with Ph3CCl yields 1:1 cyanoborate adduct [Ph3C][NC-B(C12F9)3] (Scheme 4.5). The analogous

reaction using BCF generates instead the bridging 1:2 cyanoborate adduct [Ph3C][BCF-CN-BCF]

(Scheme 4.5).9 The solid-state structure of [Ph3C][NC-B(C12F9)3] displays the encapsulation of the

cyano group by the ortho-C6F5 substituents of B(C12F9)3, which the authors use to rationalize the

preclusion of further reactivity by the anion (Figure 4.1). The decomposition of

fluorophosphonium cations is believed to occur through reaction of the P-F substituent,

encapsulation or partial encapsulation may enhance the overall stability of the fluorophosphonium

catalysts.

Scheme 4.5. Divergent reactivity of KCN with B(C12F9) (top) and BCF (bottom).

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Figure 4.1. Steric encapsulation of cyano group by B(C12F9): POV-ray (left, fluorine atoms

omitted), space-filling model (right).

4.1.2 Perfluorobiphenyl Groups in Main Group Applications

In 2012, O’Hare reported the first use of B(C12F9)3 as a Lewis acid in FLP chemistry.10 Lewis bases

(1,4-diazabicyclo[2.2.2]octane) (DABCO), 2,6-lutidene, and 2,2,6,6-tetramethylpiperidine (TMP)

each formed an FLP with B(C12F9)3. The increased steric bulk of B(C12F9)3 prevents adduct

formation with 2,6-lutidene, as was observed in the corresponding BCF/2,6-lutidene FLP.11 The

three B(C12F9)3 FLP systems were able to activate dihydrogen, though B(C12F9)3/2,6-lutidene and

B(C12F9)3/TMP systems required heating to 90 °C. Subsequent CO2 insertion chemistry of the

hydrogen-activated salts to form reduced formatoborate species was not observed. The reactivity

is in contrast to the CO2 insertion observed with [HBCF][HTMP] salts (Scheme 4.6).12 The

formatoborate salts, independently synthesized from the reaction of B(C12F9)3 with formic acid

and either 2,6-lutidene or TMP, heated in an atmosphere of CO2 undergo facile decarboxylation

to the corresponding borohydride salts (Scheme 4.6). In a later report by O’Hare, the hydrogen-

activated phosphonium-borate salt [tBu3PH][HB(C12F9)3] was able to form the corresponding

formatoborate product through reaction with CO2 at 145 °C.13 The differing reactivity trends

between most B(C12F9)3 and BCF hydrogen-activated FLP salts towards CO2 results from the

decreased steric accessibility afforded by the ortho-C6F5 rings in B(C12F9)3.

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Scheme 4.6. Decarboxylation of B(C12F9)3 FLP (top) and CO2 activation by BCF FLP (bottom).

O’Hare used the Gutmann-Beckett method to further probe the electrophilic properties of

B(C12F9)3. The acceptor number of B(C12F9)3 was calculated to be 87.9, a 10% increase compared

to BCF (AN 79.8). These results are consistent with the earlier study by Marks using the Childs

method, which also indicated a 10% electrophilicity enhancement of B(C12F9)3 over BCF. Unlike

the enhanced electrophilicity offered by C6Cl5 rings which rely on decreased mesomeric

stabilization (Chapter 3, pg. 63), C12F9 substituents are inductively more electron withdrawing than

analogous C6F5.

In 2013, our group reported the use of 2:1 FLP of Al(C12F9)3 with PMes3 to activate dihydrogen.14

The free alane, Al(C12F9)3, was obtained by first using Li[AlH4] to form the hydrido- analogue

from Marks’ anion, [FAl(C12F9)][Ph3C], followed by hydride-abstraction using [Ph3C][B(C6F5)4].

The activation of hydrogen by Al(C12F9)3 with PMes3 results in the formation the corresponding

salt with hydrido-bridged anion [Mes3PH][(μ-H)(Al(C12F9)3]. The resulting [Mes3PH][(μ-

H)(Al(C12F9)3] salt is unreactive towards ethylene, unlike analogous [Mes3PH][(μ-H)(Al(C6F5)3]

which effects ethylene insertion and subsequent disproportionation to from [Mes3PH][Al(C6F5)4]

and Al(CH2CH3)(C6F5)2 (Scheme 4.8).15 The author attributes the lack of reactivity of

[Mes3PH][(μ-H)(Al(C12F9)3] towards ethylene to the enhanced Lewis acidity of Al(C12F9)3 and to

the steric encapsulation of the hydride.

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Scheme 4.7. Divergent reactivity of ethylene towards [Mes3PH][(μ-H)((Al(C12F9)3)2] (top) and

[Mes3PH][(μ-H)(Al(C6F5)3)2] (bottom).


4.2.1 Synthesis of Perfluorobiphenyl Phosphines by Lithiation

A series of perfluorobiphenyl-substituted phosphines were generated by the 1:1 reaction of C12F9Li

with either iPr2PCl, (o-tol)2PCl, Ph2PCl, (C6F5)2PCl, (3,5-(CF3)2C6H3)2PCl, Cy2PCl, or Me2PCl to

yield iPr2P(C12F9) (4-1), (o-tol)2P(C12F9) (4-2), Ph2P(C12F9) (4-3), (C6F5)2P(C12F9) (4-4), (3,5-

(CF3)2C6H3)2P(C12F9) (4-5), Cy2P(C12F9) (4-6), and Me2P(C12F9) (4-7) respectively in good yields

(Scheme 4.8). Compounds 4-1 – 4-7 phosphines represent the first series of C12F9 substituted

phosphorus species. Best yields were obtained when lithiation of BrC12F9 was carried out at very

low temperatures using a cyclohexane and liquid nitrogen cooling bath for two hours before the

addition of a halophosphine reagent.

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Scheme 4.8. Synthesis of perfluorobiphenyl phosphines 4-1 – 4-7.

The reaction of C12F9Li with either tBu2PCl or tBu(Ph)PCl fail to cleanly yield the desired

tBu2P(C12F9) (4-8) or tBu(Ph)P(C12F9). The standard synthetic methodology yielded 4-8 only as a

minor product, which was purified by successive recrystallizations and isolated in poor yield.

Formation of tBu(Ph)P(C12F9) was not observed, despite several synthetic attempts with varied

reaction conditions. The steric bulk of substituents near the phosphorus centre appears have a

significant impact on how readily C12F9 substitution occurs.

The 19F NMR spectra of 4-1 – 4-8 all display seven resonances attributable to the (C12F9), including

equivalent F-2’/6’ and F-3’/6’ signals. Additionally, 4-4 displays the expected 4:2:4 C6F5

resonances, and 4-5 displays a large singlet resonance, which is attributed to the CF3 group. The

presence of seven C12F9 resonances is indicative that that no isomerism is present. “Up/down”

rotational isomerism could result if the other substituents on the phosphine were sufficiently large

to prevent as to interact with the ortho-C6F5 and prevent rotation about the P-(C12F9) bond, which

would result in a more complicated 19F NMR spectrum. Restricted rotation is not observed in the

free borane B(C12F9)3; however, upon abstracting methyl to form [MeB(C12F9)3]- restriction of the

internal (C6F4)-(C6F5) bond is observed, which results in the inequivalence of the ortho- and meta-

fluorine signals on the C6F5 ring.2 Restricted rotation of the ortho-C6F5 group may be present in 4-

1 – 4-8, but the ortho- and meta-fluorine substituents remain equivalent due to the symmetry of

the other substituents on phosphorus. The 19F NMR shifts on the ortho-C6F5 ring are insulated

from electronic changes at the phosphorus centre and remain at relatively constant chemical shifts

in compounds 4-1 – 4-8.

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The 31P NMR resonances of 4-1 – 4-8 are as follows 15.4, -25.6, -11.0, -63.0, -12.8, 5.9, -39.0,

and -65.6 ppm (Table 4.1). No clear trend emerges relating the chemical shift of these phosphines,

unlike the trends observed in chapter 3, with stepwise substitution of similar aryl groups. Since the

31P NMR chemical shift is sensitive to be the electron-withdrawing or donating nature and steric

demand of the substituents, it is very difficult to ascertain the electronic character. Aromatic ring

current effects likely contribute significantly to the 31P NMR chemical shift values. By varying the

number and type of aromatic substituents, the chemical shift is perturbed in a in a manner that is

difficult to predict.

The 19F NMR spectra of 4-1 – 4-8 give a better indication of the electronic character of the

phosphines. The chemical shift of the para-fluorine with respect to the phosphorus centre on the

C6F4 ring, F-4, is sensitive to electronic changes of the phosphorus centre, but is not affected by

changes in sterics. The chemical shift of F-4 gives the trend (3,5-(CF3)2C6H3) > C6F5 > Ph > Cy >

iPr > tBu > o-tol > Me (Table 4.1). The F-4 chemical shift of (3,5-(CF3)2C6H3)-substituted 4-5 is

significantly downfield shifted beyond even that of C6F5-substituted 4-4. The chemical shift of

o-tol-substituted 4-2 is unexpectedly upfield shifted with respect to the F-4 chemical shift of

alkyl-substituted 4-3, 4-6, 4-8.

Table 4.1. Selected NMR Chemical Shift Data for Phosphines 4-1 – 4-8.

Phosphine 31P{1H} NMR (ppm) F-4 19F NMR (ppm)

4-1 15.4 -150.0

4-2 -25.6 -151.4

4-3 -11.0 -149.0

4-4 -63.0 -147.4

4-5 -12.8 -143.3

4-6 5.9 -149.8

4-7 -39.0 -152.9

4-8 -65.6 -150.4

The synthesis of phosphines bearing multiple C12F9 substituents was carried out using a similar

methodology to that used in the synthesis of 4-1 – 4-8. Dihalophosphines MePCl2, PhPCl2, tBuPCl2

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and (C6F5)PBr2 were each reacted with two molar equivalents of C12F9Li at low temperatures in

pentane/Et2O solutions (Scheme 4.9). Only the less sterically crowded MePCl2 and PhPCl2 reacted

to cleanly generate the desired phosphines MeP(C12F9)2 (4-9) and PhP(C12F9)2 (4-10). The 19F

NMR spectra of 4-9 – 4-10 display 8 and 9 signals respectively. The increase in fluorine signals is

the result of restricted rotation and inequivalence of F-2’/6’ signals in 4-9 and inequivalence in

both F-2’/6’ and F-3’/5’ signals in 4-10. The reaction of tBuPCl2 with C12F9Li yielded

tBuP(C12F9)2 which displays a 31P NMR signal at 101.0 ppm; however, the 19F NMR spectrum

displays several broad peaks in addition to those attributed to the desired product. The reaction of

(C6F5)PBr2 with C12F9Li yields a single product observable in the 31P{1H} NMR spectrum as a

very broad signal. The 19F NMR spectrum is extremely complex, as would be the case with

(C6F5)P(C12F9)2 in which restricted rotation could produce inequivalence of C6F5 signals as well

as rotational isomers. The further purification of tBuP(C12F9)2 or (C6F5)P(C12F9)2 was not pursued.

Scheme 4.9. Synthesis of bis(perfluorobiphenyl) phosphines 4-9 and 4-10.

The synthesis of tris(nonafluorobiphenyl) phosphine, P(C12F9)3, was attempted using a variety of

reagents and conditions. The reaction of three equivalents of C12F9Li with PBr3 in a pentane/Et2O

solution yields an intractable mixture of products. Introducing either substoichiometric or

stoichiometric amounts of CuI to the reaction mixture had no discernable effect. Employing PCl3

or PI3 instead of PBr3 similarly gave a mixture of products. Significant effort was invested into

attempts to obtain P(C12F9)3, as it would maximize any encapsulating effects imparted to the

resulting fluorophosphonium by the large C12F9 groups. The increased steric congestion observed

in 4-9, 4-10, tBuP(C12F9)2 or (C6F5)P(C12F9)2, as exhibited by inequivalence of F-3’/5’ and F-2’/6’

in the 19F NMR spectra, suggest that P(C12F9)3 may be too sterically encumbered to be accessed.

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Additionally, compounds bearing two C12F9 groups exhibited significantly reduced solubility in

common organic solvents compared with single C12F9 substituted phosphines.

4.2.2 Synthesis of Perfluorobiphenyl Phosphines by Zincation

Perfluorobiphenylzinc reagents offer several advantages over the corresponding lithium reagents:

zinc reagents can be isolated and stored long-term and do not require low reaction temperatures

that lithium reagents require, enabling zinc reagents to be easily used in very small test-scale

reactions in a glovebox. Pentafluorophenyllithium in ether solution decomposes rapidly at -10 °C,

with 25% of the initial C6F5Li decomposing within 20 minutes.16 The decomposition occurs

through the elimination of LiF salt to generate transient tetrafluorobenzyne intermediates. The

elimination of LiF from C6F5Li is very exothermic and reportedly explosive above -30 °C.17

A modified literature procedure was used to generate the toluene adduct of

bis(nonafluorobiphenyl)zinc, Zn(C12F9)2·PhMe, in good yield.18 In an effort to synthesize

P(C12F9)3, 1.5 equivalents of Zn(C12F9)2·tol was reacted with PBr3 in toluene at room temperature,

which resulted in the consumption of PBr3 and the production of a single product 4-11 observable

by 31P{1H} NMR. The phosphorus resonance for 4-11 was observed as a triplet at 116.4 ppm, the

upfield shift from the PBr3 starting material at 226 ppm, is consistent with diminished halogen

substitution on the phosphorus centre. The 19F NMR spectrum of the reaction mixture showed

significant resonances attributable to unreacted Zn(C12F9), suggesting that 4-11 was the result of a

substoichiometric reaction of Zn(C12F9)2·PhMe with PBr3. Even upon heating the mixture to 90

°C, the excess Zn(C12F9)2·tol was not consumed. To verify, the reaction was repeated using 0.5

equivalents of Zn(C12F9)2·PhMe with PBr3 and the 31P NMR spectrum similarly showed full

consumption of the phosphorus starting material and generation of 4-11. Full consumption of

resonances attributable to Zn(C12F9)2·PhMe was observed in the 19F NMR spectrum when 0.5

equivalents were used. The formation of 4-11 was very sensitive to changes in concentration of

starting materials, broadening of Zn(C12F9)2·PhMe fluorine resonances indicate adduct formation

between Zn(C12F9)2·tol and 4-11 which prevents reaction with PBr3. No reaction occurs between

PBr3 and Zn(C12F9)2·Et2O, indicating that stronger adducts with zinc drastically reduce the

reactivity. The analogous reaction of Zn(C12F9)2·tol with PCl3 does not occur, as only PCl3 is

visible by 31P NMR after three days of heating the mixture to 90 °C. Single crystal X-ray diffraction

123

elucidated the structure of 4-11 as the 1,2-diphosphine species ((C12F9)BrP)2 (Figure 4.2).

Diphosphine 4-11 displays trigonal pyramidal geometry at the phosphorus centres, with an

inversion centre between the phosphorus atoms. The P-P bond distance is 2.227(1) Å, similar to

that of ((C6Cl5)2P)2 (2.265(2) Å) in Chapter 3. The angle between the planes of the

perfluorobiphenyl rings is 74.29°. The twisted ring orientation allows for each perfluorobiphenyl

ring to take part in a T-shaped π-π interaction, with intercentroid distances of 5.654 Å. In an ab

initio study of the interaction between benzene and hexafluorobenzene, the centroid-centroid

distance of the most favourable T-shaped complex of side-on hexafluorobenzene was 6.0 Å.19

Additionally, the bromine substituents are oriented towards the C6F4 perfluorobiphenyl ring

centroid at a separated by 3.656 Å. Stabilizing interactions between halogens and π-systems have

been evaluated, with the reported bounding distance for such interactions between 2.8 Å to 4.2 Å.20

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Figure 4.2. POV-ray depiction of 4-11 (top), Br-aryl interaction of 4-11 (middle), T-shaped π-π

interaction of 4-11 (bottom); C: light grey, Br: purple, P: orange, F: pink, Centroid: red; selected

fluorine atoms have been omitted for clarity.

125

Alkali and alkaline earth metal reduction has been used as general synthetic route to the formation

of diphosphines.21,22 Similar formation of 1,2-diphosphines was observed in the reaction of

LiC6Cl5 with PCl3 and is discussed in Chapter 3 (pg. 68). To further explore this reactivity of

Zn(C12F9)2·tol was reacted with a series of alkyl- and arylhalophosphines: PhPCl2, Ph2PCl,

(C6F5)2PBr, (C6F5)PBr2, tBuPCl2 and iPrPCl2. Of all the additional halophosphines tested, PhPCl2

was the only reagent that reacted at room temperature to generate a single product 4-12. In a similar

fashion to 4-11, 4-12 forms quantitatively through the reaction of PhPCl2 with 0.5 equivalents of

Zn(C12F9)2·tol. Compound 4-12 displays a 31P NMR resonance at 61.2 ppm, the large upfield shift

is consistent with reduced halogen substitution. The 1H and 19F NMR data of 4-12 are consistent

the formulation of 4-12, by analogy to 4-11, as ((C12F9)PhP)2 (Scheme 4.10). Restricted rotation

of the ortho-C6F5 groups is observed by 19F NMR only for 4-12.

Scheme 4.10. Synthesis of 1,2-diphosphines 4-11 and 4-12.

The only other halophosphine to react with of Zn(C12F9)2·tol at room temperature was tBuPCl2,

which slowly forms two new 31P{1H} NMR resonances at 56.4 and 57.2 ppm, consistent with the

upfield shift observed for 4-11 and 4-12. The two peaks are assumed to correspond to isomers

caused by restricted the bulky tBu groups restricting the rotation of the P-P bond. The reaction of

Zn(C12F9)2·tol with tBuPCl2 could not be driven to completion, as decomposition was observed at

elevated temperatures. All other halophosphines tested for reactivity with Zn(C12F9)2·tol that did

not react at room temperature were subjected to gradually higher reaction temperatures, resulting

in decomposition to multiple products after days at 100 °C. The formation of

bis(nonafluorobiphenyl)-1,2-diphosphines using Zn(C12F9)2·tol does not appear to be a broadly

generalizable synthetic methodology.

126

4.2.3 Synthesis of 2-Perfluorobiphenyl Difluorophosphoranes

Addition of solid XeF2 to stirring solutions of 4-1 – 4-7 in CH2Cl2 yielded the corresponding

difluorophosphorane iPr2PF2(C12F9) (4-13), (o-tol)2PF2(C12F9) (4-14), Ph2PF2(C12F9) (4-15),

(C6F5)2PF2(C12F9) (4-16), (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17), Cy2PF2(C12F9) (4-18), or

Me2PF2(C12F9) (4-19) in excellent yields (Scheme 4.11).

Scheme 4.11. Synthesis of 2-perfluorobiphenyl substituted difluorophosphoranes 4-13 – 4-19.

The 31P{1H} NMR spectra of difluorophosphoranes 4-13 – 4-19 each display a characteristic

triplet, consistent with two equivalent fluorine atoms bound to the phosphorus centre. The 19F

NMR spectra 4-13, 4-15, 4-17 – 4-19 of display seven C12F9 resonances, similar to the

corresponding phosphines, in addition to a downfield shifted doublet corresponding to the

phosphorus-bound fluorine substituent. The compounds with the highest steric congestion at the

phosphorus centre, 4-14 and 4-16, both display restricted rotation of C6F5 rings manifested as very

broad signals in the 19F NMR spectra.

The reaction of 4-8 with XeF2 did not generate the desired difluorophosphorane. The 31P{1H}

NMR spectrum displays the presence of a quartet signal instead of the expected triplet. The 19F

NMR spectrum displays a doublet that integrates 3:1 with respect to F-3 of the C12F9 substituent.

These data are consistent with the formation of tBuPF3(C12F9) as the major product. The loss of

bulky substituents during oxidation by XeF2 is consistent with earlier reactivity observed in the

oxidation of P(C6Cl5)3 (Chapter 3, pg. 73).

127

Oxidation of bis(nonafluorobiphenyl) phosphines 4-9 and 4-10 with XeF2 was carried out in DCM.

In both cases the corresponding difluorophosphoranes MePF2(C12F9)2 from 4-9 and PhPF2(C12F9)2

from 4-10 were obtained in low yield. Both difluorophosphoranes exhibited very low solubility in

common organic solvents including toluene, benzene, and CH2Cl2. Subsequent reaction of

PhPF2(C12F9)2 with fluoride abstracting agent [SiEt3][B(C6F5)4] as a slurry in toluene yielded an

intractable mixture of many products. Further investigation of bis(perfluorobiphenyl) phosphorus

species was not pursued, as the high steric crowding of the phosphorus centre significantly

decreased the chemical stability by promoting ligand dissociation.

There are limited reports of 1,2-diphosphines bearing fluoroaryl substituents, the exploration of

reactivity of these compounds is limited to complexation with metal carbonyls.23,24 To further

investigate the chemistry of this class of compounds, 1,2-diphosphines 4-11 and 4-12 were each

oxidized with XeF2. The reaction of 4-11 with XeF2 yields a mixture of products by 31P{1H} NMR,

the major product displays a quintet at -49.3 ppm and the minor product displays a triplet at 1.3

ppm. The triplet is consistent with the formation of a difluorophosphoranes species, while the

quintet is indicative of a tetrafluorinated phosphorus, PF4(C12F9). The 31F NMR spectrum shows

two downfield doublets with 1:7 relative integration, and many overlapping signals further upfield.

Attempts to resolve this mixture by recrystallization yielded few single crystals suitable for X-ray

diffraction (Figure 4.3). The solid-state structure obtained is of a four coordinate phosphonium

cation with three monoatomic substituents and monoatomic species. The bond lengths of the

phosphorus-bound substituents 1.547(2) Å, 1.541(2) Å and 1.498(2) Å. The two longer are

consistent with P-F or P-O single bonds, while the shorter 1.498(2) Å distance is consistent with a

P-O double bond. Given the neutral charge of the phosphine oxide, the monoatomic species is

assumed to be cocrystallized water. The distance between the water oxygen and nearest fluorine

atom is 2.479(3) Å with a hydrogen oriented towards the fluorine, consistent with hydrogen

bonding.25 This decomposition product likely results from the reaction of PF4(C12F9) with water,

eliminating two equivalents of HF. Insufficient decomposition product was obtained for

exhaustive analysis.

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Figure 4.3. POV-ray depiction of (C12F9)POF2,H2O; C: light grey, P: orange, F: pink, H: white.

The reaction of two equivalents of XeF2 with 4-12 also displays a mixture of products, however,

instead of a triplet and quintet observed for the reaction of related 4-11¸ the 31P{1H} NMR

spectrum displays three signals: a singlet at 67.2 ppm, a triplet at -28.8 ppm and a doublet of triplets

observed -32.4 ppm. The singlet corresponds to starting material 4-12, the triplet likely

corresponds to the difluorophosphoranes species ((C12F9)PhF2P)2, and the doublet of triplets signal

corresponds to a trifluorophosphorane species PF3Ph(C12F9). The reaction of ((C12F9)PhF2P)2 with

an additional equivalent of XeF2 results in the cleavage of the P-P bond to form PF3Ph(C12F9),

which appears as a doublet of triplets in the 31P{1H} NMR spectrum. Reaction of 4-12 with three

equivalents of XeF2 cleanly forms PF3Ph(C12F9). The more robust P-C aryl bond does not appear

to be cleaved as is observed with P-Br bonds in the reaction of 4-11 with XeF2.

4.2.4 Synthesis of Fluorophosphonium cations

Addition of a toluene solution of 4-13 to a stirring white slurry of [SiEt3][B(C6F5)4] in toluene,

rapidly resulted in the formation of a dark red oily suspension. The red suspension was stirred for

two hours before the toluene was decanted and the red oil was triturated in pentane until a white

powder 4-20 was obtained. The 31P{1H} NMR spectrum of 4-20 displays a doublet at 143.8 ppm,

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with a coupling of 1030 Hz, consistent with the abstraction of fluoride from 4-13. The 19F NMR

spectrum of 4-20 displays a set of resonances attributable to a [B(C6F5)4]- anion, a doublet

consistent with phosphorus-bound fluorine, and a set of seven C12F9 signals. These data are

consistent with formulation of 4-20 as fluorophosphonium salt [iPr2PF(C12F9)][B(C6F5)4].

Similarly, reaction of difluorophosphoranes 4-14 – 4-18 with [SiEt3][B(C6F5)4] yield the

corresponding fluorophosphonium salts [(o-tol)2PF(C12F9)][B(C6F5)4] (4-21),

[Ph2PF(C12F9)][B(C6F5)4] (4-22), [(C6F5)2PF(C12F9)][B(C6F5)4] (4-23), [(3,5-

(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24), and [Cy2PF(C12F9)][B(C6F5)4] (4-25) respectively in

good isolated yields (Scheme 4.12).

Scheme 4.12. Synthesis of perfluorophenyl fluorophosphonium cations 4-20 – 4-25.

The 31P{1H} NMR spectra of 4-21 – 4-25 each display a single doublet at 95.3, 90.7, 72.3, 90.2

and 133.8 ppm respectively. The 19F NMR spectra of fluorophosphonium salts 4-21 – 4-25 display

resonances attributable to the [B(C6F5)4]- anion, (C12F9) substituent and phosphorus-bound fluorine

substituent. The 19F NMR spectra of 4-21 – 4-24 all display a P-F signal between -119 and -125

ppm, while the P-F signals for 4-20 and 4-25 appear significantly upfield at -171.5 and -170 ppm

respectively.

The solid-state structures of 4-20, 4-21, and 4-22 were obtained by X-ray diffraction of single

crystals (Figure 4.4). Structures of 4-20, 4-21, and 4-22 display tetrahedral geometry of the

phosphorus centre with P-F bond lengths of 1.541(2), 1.549(1) and 1.547(2) Å.

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Figure 4.4. POV-ray depiction of 4-20 (top left, counterion omitted), 4-21 (top right, counterion

omitted), and 4-22 (bottom); C: light grey, B: yellow-green, P: orange, F: pink, hydrogen atoms

have been omitted for clarity.

The solid-state structure of a decomposition product of 4-23 was obtained (Figure 4.5). The

counterion of 4-23 unexpectedly refined to [HOB(C6F5)3]- instead of the expected [B(C6F5)4]-

anion. Two hydrogen atoms were present in the difference map after assigning the borate oxygen.

Analysis of the mother liquor by 11B and 19F NMR spectroscopy show no indication of degradation

of 4-23 and integration of fluorine signals is consistent with the presence of 1:1

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[(C6F5)2PF(C12F9)]+ cation to [B(C6F5)4]- anion. The anion [FB(C6F5)3]- was also considered,

however, the reaction of BCF with difluorophosphoranes 4-16 does not produce

[(C6F5)2PF(C12F9)][FB(C6F5)3]. The borate oxygen substituent it oriented towards the fluorine of

the fluorophosphonium cation at a distance of 2.554(2) Å with the hydrogen oriented towards the

fluorine, consistent with hydrogen bonding between anion and cation.25

Figure 4.5. POV-ray depiction of 4-23; C: light grey, B: yellow-green, O: red, P: orange, F: pink,

H: white.

4.2.5 Measures of electrophilicity


Compounds 4-20, 4-21, 4-22, and 4-23 were each subjected to the Gutmann-Beckett protocol by

combination of three equivalents of each Lewis acid with one molar equivalent OPEt3 in CH2Cl2

solutions. The 31P{1H} NMR spectra of each was obtained to determine the chemical shift change

of adducted OPEt3 against the reference of free OPEt3 in CH2Cl (50.7 ppm). For 4-20 – 4-22, the

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OPEt3 signal appears as a very broad peak at approximately 50 ppm, indicating only a very weak

adduct interaction. The Gutmann-Beckett method is therefore incompatible with 4-20 – 4-22,

yielding no information on their relative Lewis acidity. The 31P{1H} NMR spectrum of 4-23 with

displays two broad singlets at 91.7 ppm and -65 ppm as well as a doublet attributable to excess

4-23 at 72.7 ppm. The signal at 91.7 ppm is similar to the adducted OPEt3 chemical shift observed

for related system [PF(C6F5)3][B(C6F5)4]·OPEt3.26 The signal at -65 ppm is too broad to observe

expected P-F coupling, but is assigned to be the adducted fluorophosphonium phosphorus. The

shift observed for the 4-23·OPEt3 adduct (Δδ31P = 41.0 ppm) is slightly larger than reported shift

for PF(C6F5)3][B(C6F5)4]·OPEt3 (Δδ31P = 40.4 ppm). This finding is consistent with the enhanced

electron withdrawing power of C12F9 over C6F5 groups.

4.2.5.2 Chemical shift as a measure of electrophilicity

The phosphorus chemical shift of fluorophosphonium cations has been used previously as a simple

method of ascertaining the relative electrophilicity.27 The 31P{1H} NMR chemical shift of the

phosphines 4-1 – 4-8 did not yield any trend that coincided with expected electrophilicity, based

off electron-withdrawing capabilities of the substituents. The effect of ligand size on the geometry

and chemical shift of the phosphorus centre could not be negated. The 19F NMR chemical shift of

para-fluorine with respect to the phosphorus centre, F-4, was found to be sensitive to electronic

changes of the phosphorus centre and is positioned such that steric influences would be minimized.

The electrophilicity trend derived from the 19F NMR chemical shift gave the trend

(3,5-(CF3)2C6H3) > C6F5 > Ph > Cy > iPr > tBu > o-tol > Me.

In fluorophosphonium salts 4-20 – 4-25, the F-4 chemical shifts give a similar trend

(3,5-(CF3)2C6H3) > C6F5 > Ph ≈ o-tol > iPr > Cy. The most notable difference in these trends is the

relative position of o-tol. The 31P{1H} NMR chemical shifts of 4-20 – 4-25 are also congruent with

these observed patterns, giving the trend C6F5 > (3,5-(CF3)2C6H3) > Ph > o-tol > Cy > iPr. The

fourth substituent seems to reduce the effect of ligand steric demand on 31P{1H} NMR chemical

shift. All three trends from NMR metrics are easily obtained and have good overall agreement, but

the relative electrophilicity of similar groups is not well defined using this method. Additionally,

obtaining electrophilicity measurements from NMR data of the free Lewis acid eliminates the

possibility for chemical incompatibility as was observed with the Gutmann-Beckett method. 28

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Geometry optimization and frequency calculations were performed on cations from 4-20 – 4-23

([4-20]+ – [4-23]+ respectively) with Gaussian 0928 using B3LYP functional29 and 6-31+G(d) basis

set.30 Single point energy calculations were performed on the optimized geometries using MP231

and basis set def2-TZVPP.32 When available, solid-state structures were used to generate input

coordinates. The LUMO of cations 4-20 – 4-23 all display a characteristic large lobe on the

phosphorus centre, indicative of the site of Lewis acidity (Figure 4.6).

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Figure 4.6. Surface contour plot of the LUMO oriented along the P-F bond for cations 4-20 (top

left), 4-21 (top right), 4-22 (bottom left), 4-23 (bottom right); P: orange, C: black, F: blue, H: light

grey.

The orientation of the C12F9 group relative to the P-F bond in the optimized geometries is only

consistent with the orientation in solid state-structure in the case of 4-23. The optimized geometry

of 4-21 adopted a similar propeller configuration with the ortho-methyl substituents on the

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opposite side as the ortho-C6F5 group. The HOMO and LUMO energies for 4-20 – 4-23 and

[PF(C6F5)3]+ are provided below (Table 4.2).

Table 4.2. Calculated LUMO and HOMO energies for cations of 4-20 – 4-23, and

[PF(C6F5)3][B(C6F5)4].

Cation ELUMO (eV) EHOMO (eV)

[4-20]+ -2.436 -13.551

[4-21]+ -2.249 -12.625

[4-22]+ -2.197 -12.927

[4-23]+ -3.209 -13.247

[PF(C6F5)3]+ -3.455 -14.090

The LUMO energies for both C6F5 bearing cations [4-23]+ and [PF(C6F5)3]+ are significantly lower

than the remaining cations. Surprisingly, the LUMO energy of the iPr substituted [4-20]+ is lower

than either [4-21]+ or [4-22]+. White the LUMO of [4-21]+ and 4-22]+ appears to be delocalized

onto the respective o-tol and Ph substituents, the LUMO of [4-20]+ extends significantly onto both

rings of the C12F9 substituent. The trend provided by decreasing LUMO energies of cations is thus

[4-22]+ < [4-21]+ < [4-20]+ < [4-23]+ < [PF(C6F5)3]+. The general electrophilicity index (GEI)

values, ω, are calculated using only the HOMO and LUMO energies of the free cations using the

formula

with energies in units of electron volts (eV). The calculated ω values for cations [4-20]+ – [4-23]+,

and [PF(C6F5)3]+ are tabulated below (Table 4.3).

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Table 4.3. Calculated ω values for cations of 4-20 – 4-23, and [PF(C6F5)3][B(C6F5)4].

Cation ω (eV)

[4-20]+ 2.874

[4-21]+ 2.665

[4-22]+ 2.665

[4-23]+ 3.372

[PF(C6F5)3]+ 3.618

Unlike the cations discussed in Chapter 3 (pg 83), significant linear correlation is not observed

between the ω values and either 19F or 31P{1H} NMR data for 4-20 – 4-23. The trend generated by

increasing ω of [4-21]+ = [4-22]+ < [4-20]+ < [4-23]+ < [PF(C6F5)3]+ is similar in order to the related

trend observed in LUMO energies. The notable deviation between the trends is the position of iPr

(4-20), which is lower than Ph (4-22) and o-tol (4-21) in NMR trends and higher in LUMO and ω

trends. Additionally, the ω value for [PF(C6F5)3]+ is significantly higher than [4-23]+, while by the

Gutmann-Beckett method 4-23 is slightly more Lewis-acidic than [PF(C6F5)3][B(C6F5)4].

4.2.5.4 Fluoride ion affinity

Using the same methodology outlined in the previous section, the optimized geometries of the

fluoride-adducts of [4-20]+ – [4-23]+ were used to obtain single point energies using the MP231

with the def2-TZVPP basis set.32 The energies of the fluoride-adducts were then used with cations

[4-20]+ – [4-23]+ to determine the FIA with COF2 as a reference compound using the equation

FIA = (Ecation + EF3CO) – (Ephosphorane + EF2CO) + 209

where all energies are in units of kJ/mol. The FIA values for 4-20 – 4-23 are tabulated below

(Table 4.4).

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Table 4.4. Fluoride Ion Affinities for Cations of 4-20 – 4-23 and [PF(C6F5)3][B(C6F5)4].

Cation FIA (kJ/mol)

[4-20]+ 709

[4-21]+ 702

[4-22]+ 713

[4-23]+ 767

[PF(C6F5)3]+ 744

The trend obtained from increasing FIA values is therefore [4-21]+ < [4-20]+ < [4-22]+ <

[PF(C6F5)3][B(C6F5)3] < [4-23]+. The calculated FIA values show only a small 11 kJ/mol

difference between any of [4-20]+ – [4-22]+, consistent with the relative position of these three

compounds changing often within other electrophilicity scales. The large gap between [4-20]+ –

[4-22]+ and either [PF(C6F5)3]+ or [4-23]+ is also consistent with the differing reactivity observe in

the Gutmann-Beckett test, where only the latter two interacted significantly with OPEt3. The

relative position of [4-23]+ to [PF(C6F5)3]+ is consistent with the results of the Gutmann-Beckett

method, but neither NMR chemical shift nor ω value trends.

4.2.5.5 Competition Reaction

Because of the conflicting results from previous Lewis acidity measurements, competition

experiments were carried out to ascertain the relative empirical FIAs between C12F9

fluorophosphonium salts and their C6F5 analogues. Monitoring a CH2Cl2 solution of 4-23

combined with equimolar PF2(C6F5)3 by 31P{1H} NMR shows the slow appearance of

fluorophosphonium [PF(C6F5)3][B(C6F5)4], while the formation of 4-16 is obscured by

overlapping PF2(C6F5)3 signals. The 19F NMR spectrum is very complicated with five different

C6F5 containing species present, two of which contain C12F9 groups. However, the downfield

shifted P-F doublet resonances of 4-16 and PF2(C6F5)3, which overlap partially, can be used to

track relative difluorophosphoranes amounts. The initial spectrum obtained after one hour after

mixing shows a relative 4-16 to PF2(C6F5)3 concentration of 1:5.3 (Figure 4.7, left). After 24 h, the

ratio of 4-16 to PF2(C6F5)3 increased to 1:0.43 (Figure 4.7, right). This evidence supports the

138

calculated FIA results that 4-23 has a higher FIA than C6F5 analogue [PF(C6F5)3][B(C6F5)4]. The

long timescale required to obtain this equilibrium is indicative of high steric bulk precluding rapid

fluoride transfer.

Figure 4.7. Partial 19F NMR spectrum (3 – -2 ppm) of competition experiment between 4-23 and

PF2(C6F5)3 in CH2Cl2 after 1 h (left) and 24 h (right).

Monitoring the less sterically encumbered pairing of 4-22 with Ph2PF2(C6F5) by 19F NMR shows

only three resonances attributable to the [B(C6F5)4]- anion. The two fluorophosphonium cations

are in rapid equilibrium with the two difluorophosphoranes species, resulting in an absence of

observable signals at room temperature. The rapid equilibrium of 4-22 with Ph2PF2(C6F5) is in

contrast with the slow reaction observed with the slow reaction between 4-23 and PF2(C6F5)3,

resulting from the lower steric demand of the Ph substituents. The 19F NMR spectrum obtained

after cooling the reaction mixture of 4-22 and Ph2PF2(C6F5) to -40 °C shows a sharpening of

resonances attributable PF2(C6F5)3 and [PF(C6F5)3][B(C6F5)4], and a very broad doublet associated

with 4-13. The dynamic equilibrium that exists between 4-23 and PF2(C6F5)3 precludes obtaining

accurate ratio of species in solution to determine relative FIA values.

4.2.6 Solubility of Perfluorobiphenyl Salts

As a general feature of fluorophosphonium salts, the solubility tends to decrease as C6F5

substitution increases. In the series [Ph3-nPF(C6F5)n][B(C6F5)4], when n = 1 solubility is poor in

most common organic solvents and good in CH2Cl2 and PhBr, when n is increased to 3 the

139

solubility decreases even in CH2Cl2 and PhBr. The solubility of bis(nonafluorobiphenyl)

phosphines 4-9 and 4-10 is notably lower in all tested solvents than related

mono(nonafluorobiphenyl) phosphines 4-1 – 4-8. Unexpectedly the solubility of

fluorophosphonium 4-23 was found to be significantly greater than C6F5 analogue

[PF(C6F5)3][B(C6F5)4], showing good solubility in CH2Cl2 and PhBr as well as in C6H6 and

toluene. To illustrate this solubility difference, 0.2 mmol of 4-23 and [PF(C6F5)3][B(C6F5)4] were

each dissolved 1mL of C6H6 in separate vials. After stirring the solutions for 1 h, the solution of

4-23 was cloudy, but stayed evenly suspended, whereas a large amount of solid quickly settled to

the bottom in the mixture of [PF(C6F5)3][B(C6F5)4]. The two mixtures were filtered using a

0.45 μm pore size filter paper, and the solvent was removed in vacuo from both filtrates to obtain

the mass of dissolved fluorophosphonium salt. The recovered yield was 20% (5.6 mg recovered of

27.6 mg initial) for 4-23, whereas only 3% (0.8 mg recovered of 24.6 mg initial) of

[PF(C6F5)3][B(C6F5)4] was recovered. The increased solubility allows NMR-scale reactions to be

carried out in C6D6 instead of CD2Cl2, which allows for monitoring reactions by 1H NMR

spectroscopy at considerably lower cost.33

4.2.7 Stability of Perfluorobiphenyl Phosphonium Cations

Compounds 4-20 – 4-25 are stable as isolated solids stored under an inert atmosphere at -35 °C for

months, and with the exception 4-24, are also stable in CH2Cl2 solutions at room temperature for

weeks. Attempts to obtain single crystals of 4-24 suitable for X-ray diffraction from a CH2Cl2

solution layered with cyclohexane did not produce crystals. The solvent was removed in vacuo

and the residue was dissolved in C6D6 to verify the stability of the phosphonium salt. The 31P{1H}

NMR spectrum revealed that fluorophosphonium 4-24 had cleanly decomposed to the

corresponding difluorophosphoranes 4-17 through fluoride abstraction. The 31P{1H} NMR

spectrum of this reaction mixture shows only one triplet signal at -62.6 ppm with 716 Hz coupling.

There are three fluorine chemical environments present in 4-24: phosphorus bound fluorine,

trifluoromethyl fluorine, and pentafluorophenyl fluorine present in the borate counterion.

Fluorophosphonium cations are reported to activate trifluoromethyl C-F bonds as observed in

hydrodefluorination chemistry.26,34 The decomposition of fluorophosphonium with concomitant

formation of phosphine and difluorophosphoranes species has also been observed as a common

140

decomposition pathway. Had either of these cases occurred, additional phosphorus containing

species would be observed by 31P{1H} NMR spectroscopy. The 19F NMR spectrum of the reaction

mixture displays major signals associated with 4-24. No signals associated with the [B(C6F5)4]-

anion are observed, though several small, broad signals are present. The 11B NMR spectrum shows

very broad signals at 40 ppm and 0 ppm as well as a weak sharp resonance at -16 ppm. The C6D6

solution was homogenous with no insoluble precipitate visible. These data are consistent with the

aryl C-F activation of the [B(C6F5)4]- anion by the [(3,5-(CF3)2C6H3)2PF(C12F9)]+ cation to

generate 4-17. Activation of fluorobenzene aryl C-F bonds by triethylsilylium carborane,

[SiEt3][CHB11Cl11] to generate the fluorosilane has been reported.35 The C-F activation requires

an extremely weak coordinating [CHB11Cl11]- anion. The phenyl cation generated reacts with the

carborane to form neutral CHB11Cl10-(μ-Cl)-Ph. The phenyl cation also reacts with additional

equivalents of fluorobenzene to generate fluorobiphenyl. A carborane anion is not required in the

case of 4-24, as the C-F activation is proposed to occur on the anion itself. The Lewis acidity of

related tris[3,5-bis(trifluoromethyl)phenyl]borane has been evaluated by Ashley.36 The Lewis

acidity was determined to be 6% greater than that of BCF by the Gutmann-Beckett method and

38% less by the Childs’ method. While this broad range is not conclusive, it suggests that

(3,5-(CF3)2C6H3) substituents may be more electron withdrawing than C6F5. In this case, 4-24

would be the most Lewis acidic mono-cationic fluorophosphonium, even stronger than 4-23.

Additionally, 4-24 would have less steric encumbrance at the phosphorus centre than either 4-23

or [PF(CF5)3][B(C6F5)4], which would allow larger substrates to approach and the

difluorophosphorane 4-17 would be favoured.

Air-stability of 4-20 – 4-23 and 4-25 were evaluated by exposing PhBr solutions of each in 5 mm

NMR vials to air for successively longer periods of time. Most electrophilic 4-23 shows

decomposition after one minute of air exposure and complete decomposition after five minutes.

Both 4-20 and 4-22 show significant decomposition after two days of air exposure, while only

minor onset of decomposition is observed by 31P{1H} or 19F NMR for 4-21 after 3 days. While

4-21 is similarly electrophilic to 4-20 and 4-22, it has the most steric bulk at the phosphorus centre.

141

4.2.8 Catalysis

The catalytic competency of 4-20 – 4-23 was evaluated in a series of Lewis acid catalyzed

reactions: dimerization of 1,1-diphenylethylene, hydrodefluorination of fluoroadamantane,

deoxygenation of benzophenone, hydrosilylation of α-methylstyrene, dehydrocoupling of phenol

and triethylsilane, as well as benzylation and hydrodefluorination of

4-trifluoromethylbromobenzene (Scheme 4.13). All catalytic runs were carried out in C6D6, with 2

mol% catalyst at room temperature unless otherwise noted.

In the dimerization of 1,1-diphenylethylene 4-23 completed the reaction in 30 min, 4-22 took 4.5

h, and 4-20 took 40 h to complete the reaction. Interestingly 4-21 showed no reaction at room

temperature over multiple days, however upon gentle heating to 40 °C the reaction slowly

proceeded over 21 d. The FIA of 4-21 was determined to be the lowest of the catalysts in question,

though the small difference is not sufficient to account for dramatically different reactivity. The

significant steric bulk provided by ortho-methyl groups likely hampers reactivity.

Catalysts 4-20, 4-22, and 4-23 were all able to rapidly effect the hydrodefluorination of

1-fluoroadamantane in the presence of one equivalent of HSiEt3 within 10 minutes. Catalyst 4-21

completed the reaction within 22 h upon heating the mixture to 40 °C.

Only 4-23 was able to effect the deoxygenation of benzophenone in the presence of two

equivalents of HSiEt3 at room temperature, taking 2 h to complete. When the reaction mixtures

were heated to 60 °C, 4-20 – 4-22 were able to complete the deoxygenation of benzophenone in

22 h, 24 h, and 20 h respectively. Interestingly the difference in reactivity between 4-20 – 4-22 is

minimized at higher temperatures.

Catalysts 4-24 completed the dehydrocoupling of phenol with triethylsilane within 3 h. Again, less

electrophilic 4-20 – 4-22 required significant heating to 100 °C in toluene solution to complete the

reaction in 24 h, 28 h, and 24 h respectively.

The benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene with 3.6 equivalents

of HSiEt3 was catalyzed by 4-23 at 60 °C after 4.5 h. Even upon heating the reaction mixtures to

130 °C, none of 4-20 – 4-22 were able to catalyze the reaction. Similarly, only 4-23 was able to

142

catalyze the hydrosilylation of α-methylstyrene with HSiEt3 after 1 d at room temperature.

Catalysts 4-20 – 4-22 were all unreactive for hydrosilylation.

The relative competency of catalysts 4-20 – 4-23 is best exemplified in dimerization of

1,1-diphenylethylene, which gives the trend for increasing activity of 4-21 < 4-20 < 4-22 << 4-23.

Interestingly, this is the exact same trend for increasing FIA values of the catalysts. Highly

electrophilic catalyst 4-23 remains significantly more active than the remaining catalysts 4-20 – 4-

22. The notably low activity of 4-21 is likely the result of significant steric encumbrance hindering

interaction with substrate. The reactivity differences between 4-22 and 4-20/4-21 is most

pronounced at low temperatures, as their activities are nearly equal in both deoxygenation and

dehydrocoupling reactions.

143

Scheme 4.13. Summary of reactivity for catalysts 4-20 – 4-23.

144

4.3 Conclusion

A series of 2-perfluorobiphenyl-substituted phosphines iPr2P(C12F9) (4-1), (o-tol)2P(C12F9) (4-2),

Ph2P(C12F9) (4-3), (C6F5)2P(C12F9) (4-4), (3,5-(CF3)2C6H3)2P(C12F9) (4-5), Cy2P(C12F9) (4-6), and

Me2P(C12F9) (4-7), tBu2P(C12F9) (4-8), MeP(C12F9)2 (4-9) and PhP(C12F9)2 (4-10) were generated

using lithium reagent LiC12F9. These represent the first series of perfluorobiphenyl-substituted

phosphines. Additionally, two perfluorobiphenyl-substituted 1,2-diphosphine, ((C12F9)BrP)2 (4-

11) and ((C12F9)PhP)2 (4-12), were synthesized using zinc reagent Zn(C12F9)2·PhMe. The synthesis

of coupled perfluorobiphenyl phosphines was incompatible with a wide range of halophosphine

starting materials used. Phosphines 4-1 – 4-7 were oxidized by XeF2 to generate the corresponding

difluorophosphorane species iPr2PF2(C12F9) (4-13), (o-tol)2PF2(C12F9) (4-14), Ph2PF2(C12F9)

(4-15), (C6F5)2PF2(C12F9) (4-16), (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17), Cy2PF2(C12F9) (4-18), or

Me2PF2(C12F9) (4-19) in excellent yield. The oxidation of Bis(2-nonafluorobiphenyl)phosphines

4-9 and 4-10 with XeF2 did not cleanly generating difluorophosphorane species. Both 4-9 and 4-

10 and their corresponding difluorophosphoranes exhibited limited solubility in common organic

solvents. Oxidation of 1,2-diphosphines 4-11 and 4-12 resulted in P-P bond cleavage and the

formation of corresponding tetrafluorophosphorane in 4-11 and trifluorophosphorane in 4-12.

Difluorophosphoranes 4-13 – 4-18 were each reacted with fluoride abstracting agent

[SiEt3][B(C6F5)4] to form [iPr2PF(C12F9)][B(C6F5)4] (4-20), [(o-tol)2PF(C12F9)][B(C6F5)4] (4-21),


(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24), and [Cy2PF(C12F9)][B(C6F5)4] (4-25) respectively. The

electrophilicity of 4-20 – 4-23 was evaluated by NMR chemical shift trends, general

electrophilicity index ω values, fluoride ion affinity calculations, and competition experiments.

The reactivity of catalysts 4-20 – 4-23 was further investigated in a wide array of transformations.

4.4 Experimental





145






by three successive cycles of headspace-evacuation, and sonication. All elemental analyses were

performed in house using a Perkin Elmer 2400 Series II CHNS Analyzer. All NMR data were

collected on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz or Agilent DD2 600 MHz

spectrometer at 25 °C unless otherwise noted. NMR chemical shift data are given relative to an

external standard (1H, 13C: SiEt4; 11B: 15% BF3·Et2O; 19F: CFCl3; 31P: H3PO4). In some cases, 2-

D NMR techniques were employed to assign individual resonances. Zn(C12F9)2·PhMe was

synthesized using a modified literature procedure.18 A 1:1 mixture of Et2Zn and ZnCl2 in

pentane/Et2O is used in place of the EtZnCl reagent. Marks’ literature procedure was used in the

synthesis of 2-bromoperfluorobiphenyl.2 A literature procedure was followed in the synthesis of

PF2(C6F5)3 and PF2Ph2(C6F5) used in the competition reactions. Halophosphines Br2P(C6F5) and

BrP(C6F5)2 were obtained by literature procedure and successive recrystallizations from Et2O.37

Other halophosphines were obtained from Sigma-Aldrich and used without further purification.

Computational work was performed using resources provided by the Shared Hierarchical

Academic Research Computing Network and Compute Canada.

The syntheses of mono(perfluorobiphenyl)phosphines 4-1 – 4-8 all follow a similar synthetic

methodology; a sample procedure for 4-1 is provided below.

Synthesis of iPr2P(C12F9) (4-1): A 100 mL round-bottom flask was charged with BrC12F9 (200

mg, 0.5 mmol), 1:1 Et2O/pent. (50 mL), a stir bar, and topped with a rubber septum. Once

connected to a Schlenk line, the flask was cooled to -104 °C using a N2 / cyclohexane bath. A

solution of nBuLi in hexane (0.2 mL, 2.5 M) was added dropwise by syringe. The solution was

146

stirred at -104 °C for an addition 2 h. Next, a solution of iPr2PCl (76.3 mg, 0.5 mmol) in Et2O (5

mL) was added dropwise to the reaction flask by syringe. The solution was allowed to warm to

room temperature overnight before the solvent was removed in vacuo yielding a white solid. The

residue was taken up CH2Cl2 (10 mL) and filtered through a Celite plug. The solvent was reduced

in vacuo and cooled to -35 °C to yield colourless crystals of 4-1 in good yield (195.1 mg, 90%

yield)

1H NMR (500 MHz, C6D6): δ 0.83 (d, 7 Hz, 3H, iPrCH3), 0.79 (d, 7 Hz, 6H, iPrCH3), 0.76 (d 7

Hz, 3H, iPrCH3) ppm. 31P {1H} NMR (162 MHz, C6D6): δ 15.4 (s, br) ppm. 19F NMR (377 MHz,

C6D6): δ -125.6 (t, 3JFF = 22 Hz, F-3), -135.4 (s, br, 1F, F-6), -137.9 (td, 3JFF = 22 Hz, 4JFF = 7 Hz,

2F, F-2’/6’), -150.0 (td, 3JFF = 22, 4JFF = 7 Hz, 1F, F-4), -151.6 (t, 3JFF = 22 Hz, 1F, F-5), -151.7 (t,

3JFF = 22 Hz, 1F, F-4’), -161.7 – -162.7 (m, 2F, F-3’/5’) ppm.

Synthesis of (o-tol)2P(C12F9) (4-2): (white solid, 72% yield). 1H NMR (500 MHz, C6D6): δ 7-10

– 7.08 (m, 2H, o-H tol), 6.98 (td, 3JHH= 7 Hz, 4JHH= 1Hz, 2H, p-H tol), 6.89 (t, 3JHH= 7 Hz, 2H,

m-H tol), 6.85 (t, 3JHH= 7 Hz, 4JPH= 7 Hz, 2H, m-H tol), 2.07 (s, 6H, CH3) ppm. 31P{1H} NMR

(162 MHz, C6D6): δ -25.6 (s, br) ppm. 19F NMR (377 MHz, C6D6): δ -122.8 – -123.7 (m, 1F, F-3),

-135.7 (s, br, 1F, F-6), -138.0 (t, 3JFF = 20 Hz, 2F, F-2’/6’), -151.2 (t, 3JFF = 21 Hz, 1F, F-5), -151.4

(td, 3JFF = 20 Hz, 4JFF = 6 Hz, 1F, F-4), -152.5 (t, 3JFF = 20 Hz, 1F, F-4’), -162.3 (td, 3JFF = 22 Hz,

4JFF = 7 Hz, 2F, F-3’/5’) ppm. 13C{1H} NMR (126 MHz, C6D6): δ 150.1 (d, 1JFC = 253 Hz, C12F9),

145.5 (d, 1JFC = 252 Hz, C12F9), 144.3 (d, 1JFC = 250 Hz, C12F9), 141.7 (d, JPC = 27 Hz, tol), 141.4

(d, 1JFC = 258 Hz, C12F9), 141.3 (d, 1JFC = 254 Hz, C12F9), 131.4 (d, 1JFC = 252 Hz, C12F9), 132.6

(s, tol), 130.8 (d, JPC = 9 Hz, tol), 130.3 (d, JPC = 6 Hz, tol), 129.5 (s, tol), 126.2 (s, tol), 121.2 (m,

C12F9), 116.2 (m, C12F9), 107.0 (m, C12F9), 20.6 (s, tol) ppm.

147

Synthesis of Ph2P(C12F9) (4-3): (light yellow solid, 85% yield). 31P{1H} NMR (162 MHz, C6D6):

δ -11.0 (s, br) ppm. 19F NMR (377 MHz, C6D6): δ -120.7 – -120.9 (m, 1F, F-3), -136.1 (s, br, 1F,

F-6), -138.9 (t, 3JFF = 20 Hz, 2F, F-2’/6’), -149.0 (td, 3JFF = 21 Hz, 4JFF = 8 Hz, 1F, F-4), -150.1 (t,

3JFF = 21 Hz, 1F, F-5), -151.0 (t, 3JFF = 21 Hz, 1F, F-4’), -161.7 (td, 3JFF = 21 Hz, 4JFF = 8 Hz, 2F,

F-3’/5’) ppm.

Synthesis of (C6F5)2P(C12F9) (4-4): (white solid, 78% yield). 31P{1H} NMR (162 MHz, C6D6): δ

-63.0 (br) ppm. 19F NMR (377 MHz, C6D6): δ -121.4 – -121.8 (m, 1F, F-3), -131.4 (t, 3JFF = 23

Hz, 4F, o-C6F5), -134.4 (s, br, 1F, F-6), -138.8 (t, 3JFF = 20 Hz, 2F, F-2’/6’), -147.4 (td,

3JFF = 20 Hz, 4JFF = 7 Hz, 1F, F-4), -149.3 (t, 3JFF = 20 Hz, 2F, p-C6F5), -150.3 (s, br, 1F,

F-3), -150.4 (t, 3JFF = 20 Hz, F-4’), -160.46 (t, 3JFF = 20 Hz, 4F, m-C6F5), -161.08 (td, 3JFF = 20 Hz,

4JFF = 7 Hz, 2F, F-3’/5’) ppm. 13C{1H} NMR (126 MHz, C6D6): δ 150.3 (d, 1JFC = 251 Hz, C12F9),

146.9 (d, 1JFC = 245 Hz, C6F5), 145.4 (d, 1JFC = 253 Hz, C12F9), 144.4 (d, 1JFC = 245 Hz, C12F9),

143.0 (d, 1JFC = 260 Hz, C12F9), 142.6 (d, 1JFC = 253 Hz, C6F5), 141.1 (d, 1JFC = 260 Hz, C12F9),

137.6 (d, 1JFC = 262 Hz, C6F5), 115.6 (m, C6F5), 105.5 (m, C12F9), 104.4 (m, C12F9) ppm.

148

Synthesis of (3,5-(CF3)2C6H3)2P(C12F9) (4-5): (white solid, 85% yield).

1H NMR (400 MHz, C6D6): δ 7.71 (s, 2H, o-H), 7.70 (s, 2H, o-H), 7.60 (s, 2H, p-H) ppm. 31P{1H}

NMR (162 MHz, C6D6): δ -12.8 (br) ppm. 19F NMR (377 MHz, C6D6): δ -63.14 (s, 12F,

CF3), -121.35 – -121.56 (m, 1F, F-3), -132.87 (s, br, 1F, F-6), -139.2 (t, 3JFF = 20 Hz, 2F, F-2’/6’),

-143.3 (td, 3JFF = 20 Hz, 4JFF = 8 Hz, 1F, F-4), -147.4 (td, 3JFF = 20 Hz, 4JFF = 7 Hz, 1F,

F-5), -148.8 (t, 3JFF = 20 Hz, 1F, F-4’), -159.87 – -160.11 (m, 2F, F-3’/5’) ppm. 13C{1H} NMR

(126 MHz, C6D6): δ 149.5 (d, 1JFC = 251 Hz, C12F9), 146.0 (d, 1JFC = 252 Hz, C12F9), 144.1 (d, 1JFC

= 247 Hz, C12F9), 142.9 (d, 1JFC = 263 Hz, C12F9), 142.1 (d, 1JFC = 257 Hz, C12F9), 141.6 (d, 1JFC =

263 Hz, C12F9), 137.6 (d, 1JFC = 253 Hz, C12F9), 135.6 (d, 4JPC = 17 Hz, (CF3)2Ph), 132.5 (qd, 2JFC

= 33 Hz, 3JPC = 6 Hz, (CF3)2Ph), 131.8, 2JPC = 22 Hz, (CF3)2Ph), 123.6 (m, (CF3)2Ph),122.7 (q, 1JFC

= 272 Hz, (CF3)2Ph), 118.1 (m, C12F9), 116.9 (m, C12F9), 106.4 (m, C12F9) ppm.

Synthesis of Cy2P(C12F9) (4-6): (white solid, 89% yield). 1H NMR (400 MHz, C6D6): δ 1.77 –

1.40 (m, 6H, C6H11), 1.25 – 0.93 (m, 6H, C6H11). 31P{1H} NMR (162 MHz, C6D6) δ 5.9 (br) ppm.

19F NMR (377 MHz, C6D6): δ -125.0 (t, 3JFF = 22, 1F, F-3), -135.4 (s, br, 1F, F-6), -137.85 (t,

3JFF = 20 Hz, 2F, F-2’/6’), -149.8 (td, 3JFF = 20 Hz, 4JFF = 8 Hz, 1F, F-4), -151.5 (t, 3JFF = 20 Hz,

1F, F-5), -151.8 (t, 3JFF = 20 Hz, 1F, F-4’), -161.87 – -162.24 (m, 2F, F-3’/5’) ppm.

149

Synthesis of Me2P(C12F9) (4-7): (white solid, 80% yield). 1H NMR (500 MHz, C6D6): δ 1.26 (s,

br, 6H, CH3) ppm. 31P{1H} NMR (162 MHz, C6D6): δ -39.0 (br) ppm. 19F NMR (377 MHz, C6D6):

δ -129.4 – -129.7 m, 1F, F-3), -136.7 (s, br, 1F, F-6), -138.95 (t, 3JFF = 20 Hz, 2F, F-2’/6’), -152.7

(t, 3JFF = 20 Hz, 1F, F-5), -152.9 (td, 3JFF = 20 Hz, 4JFF = 8 Hz, 1F, F-4), -153.3 (t, 3JFF = 20 Hz,

1F, F-4’), -162.68 (td, 3JFF = 22 Hz, 4JFF = 7 Hz, 2F, F-3’/5’) ppm. 13C{1H} NMR (126 MHz,

C6D6): δ 147.2 (d, 1JFC = 251 Hz, C12F9), 144.8 (d, 1JFC = 254 Hz, C12F9), 144.0 (d, 1JFC = 250 Hz,

C12F9),143.3 (d, 1JFC = 251 Hz, C12F9), 134.7 (d, 1JFC = 256 Hz, C12F9), 141.3 (d, 1JFC = 253 Hz,

C12F9), 137.3 (d, 1JFC = 256 Hz, C12F9), 109.9 (m, C12F9), 106.5 (m, C12F9). 20.0 (CH3) ppm.

Synthesis of tBu2P(C12F9) (4-8): (light yellow solid, 26% yield). 31P{1H} NMR (162 MHz, C6D6):

δ -65.6(br) ppm. 19F NMR (377 MHz, C6D6): δ -115.7 (dd, 3JFF = 26 Hz, 4JFF = 13.3 Hz, 1F,

F-3), -126.9 (s, br, 1F, F-6), -138.86 – -139.09 (m, 2F, F-2’/6’), -148.60 (ddd, 3JFF = 24 Hz,

3JFF = 17 Hz, 4JFF = 6 Hz, 1F, F-5), -150.4 (dd, 3JFF = 22 Hz, 3JFF = 20 Hz, 1F, F-4), -151.80 (t,

3JFF = 21 Hz), -161.68 – -162.18 (m, 2F, F-3’/5’) ppm.

150

The synthesis of bis(perfluorobiphenyl)phosphines 4-9 and 4-10 both follow a similar

methodology; a sample procedure for 4-9 is provided below.

Synthesis of MeP(C12F9)2 (4-9): Et2O (5 mL) was added dropwise to the reaction flask by syringe

over 1 h using a syringe pump. The solution was maintained at -104 °C for another 2 before being

allowed to warm to room temperature overnight. The solvent was removed in vacuo yielding a

white solid, that was taken up CH2Cl2 (10 mL) and filtered through a Celite plug. The Celite plug

was flushed with additional CH2Cl2 (10 mL) to ensure high recovery. The solvent was reduced in

vacuo and cooled to -35 °C to yield white powder of 4-9 in moderate yield (195.1 mg, 90% yield).

1H NMR (500 MHz, C6D6): δ 1.64 (d, 2JPH = 15 Hz, 3H, CH3) ppm. 31P{1H} NMR (162 MHz,

C6D6): δ -32.4 (br) ppm. 19F NMR (377 MHz, C6D6): δ -126.2 (s, br, 2F, F-3), -135.01 (d, 3JFF =

20 Hz, 2F, F-6), -137.1 (t, 3JFF = 26 Hz, 2F, F-2’), -139.6 (s, br, 2F, F-6’), -150.0 (t, 3JFF = 20 Hz

2F, F-4), -151.10 (t, 3JFF = 20 Hz, 2F, F-5), -151.35 (t, 3JFF = 21Hz, 2F, F-4’), -161.42 – -161.62

(m, 4F, F-3’/5’) ppm.

Synthesis of PhP(C12F9)2 (4-10): 1H NMR (400 MHz, C6D6): δ 7.07 (dd, J = 9.8, 7.3 Hz, 1H),

6.90 – 6.78 (m, 1H) ppm. 31P{1H} NMR (162 MHz, C6D6): δ -23.7 (br) ppm. 19F NMR (377 MHz,

C6D6): δ -124.28 (s, br, 2F, F-4), -135.22 (d, 3JFF = 22 Hz, 2F, F-6), -137.41 (t, 3JFF = 19 Hz, 2F,

F-2’), -138.85 (t, 3JFF = 20 Hz, 2F, F-6’), -148.26 (td, 3JFF = 22 Hz, 4JFF = 6 Hz, 2F, F-4), -149.8

(t, 3JFF = 20 Hz, 2F, F-5), -150.48 (t, 3JFF = 21 Hz, 2F, F-4’), -161.09 (td, 3JFF = 23 Hz, 4JFF = 8

Hz, 2F, F-3’), -161.27 (td, 3JFF = 24 Hz, 4JFF = 8 Hz, 2F, F-5’) ppm.

151

The syntheses of 4-11 and 4-12 were both carried out using the same methodology, a sample

procedure of 4-11 is provided below.

Synthesis of ((C12F9)BrP)2 (4-11): A 4 dram vial is charged with PBr3 (54.1 mg, 0.2 mmol),

pentane (10 mL), and a stir bar. Solid Zn(C12F9)2·PhMe (78.8 mg, 0.1 mmol) was added to the

stirring reaction mixture. The reaction was stirred for 16 h before being removing the solvent in

vacuo, yielding a light yellow solid. The solid was taken up in CH2Cl2 (5 mL) and filtered through

a Celite plug. The plug was flushed with additional CH2Cl2 (5 mL) and the filtrates were collected,

reduced in vacuo, and cooled to -35 °C in the freezer to yield 4-11 in good yield (71.3 mg, 84%

yield)

31P{1H} NMR (162 MHz, C6D6): δ 116.4 (tt 1JPP = 33 Hz, 3JPF = 7 Hz) ppm. 19F NMR (377 MHz,

C6D6): δ -120.5 – 120.7 (m, 1F, F-3), -136.8 – 137.0 (m, 1F, F-6), -138.0 – -138.2 (m, 2F,

F-2’/6’), -144.3 (td, 3JFF = 21 Hz, 4JFF = 11 Hz, 1F, F-4), -146.8 (td, 3JFF = 22 Hz, 4JFF = 7 Hz, 1F,

F-5), -148.1 (t, 3JFF = 22 Hz, 1F, F-4’), -159.2 – -159.4 (m, 2F, F-3’/5’) ppm.

Synthesis of ((C12F9)PhP)2 (4-12): 1H NMR (400 MHz, C6D6): δ 7.48 (t, 3JHH = 8 Hz, 2H, o-H

Ph), 7.06 – 6.97 (m, 3H, o/p-H Ph) ppm. 31P{1H} NMR (162 MHz, C6D6): 66.5 (br) ppm. 19F

NMR (377 MHz, C6D6): δ -122.3 – -122.5 (m, 1F, F-3), -136.4 – -136.6 (m, 1F,

F-6) -137.7 – -137.9 (m, 1F, F-2’), -138.8 – 139.0 (m, 1F, F-6’), -146.6 (td, 3JFF = 21 Hz, 4JFF =

152

10 Hz, 1F, F-4), -148.3 – -148.4 (m, 1F, F-5), -149.8 (t, 3JFF = 21 Hz, 1F, F-4’), -160.0 (td, 3JFF =

21 Hz, 4JFF = 9 Hz, 1F, F-3’), -161.0 (td, 3JFF = 21 Hz, 4JFF = 9 Hz, 1F, F-3’) ppm.

Difluorophosphoranes 4-13 – 4-19 were all prepared following a similar synthetic strategy, a

sample procedure for 4-13 is provided below.

Synthesis of iPr2PF2(C12F9) (4-13): 31P{1H} NMR (162 MHz, C6D6): δ -19.3 (t, 1JPF = 702 Hz)

ppm. 19F NMR (377 MHz, C6D6): δ -40.1 (d, 1JPF = 702 Hz, PF2), -124.7 – -124.9 (m, 1F,

F-3), -135.0 – -135.2 (m, 1F, F-6), -136.7 – -136.9 (m, 2F, F-2’/6’), -151.2 – -151.4 (m, 1F, F-4),

152.4 (td, 3JFF = 21 Hz, 4JFF = 9 Hz, F-5), -152.9 (t, 3JFF = 21 Hz, F-4’), -163.2 – -163.4 (m, 2F,

F-3’/5’) ppm.

Synthesis of (o-tol)2PF2(C12F9) (4-14): 31P{1H} NMR (162 MHz, C6D6): δ -41.6 (t, 1JPF = 670

Hz). 19F NMR (377 MHz, C6D6): δ -126.4 (s, br, 1F, F-3), -133.2 (s, br, 1F, F-2’), -133.9 (s, br,

1F, F-6’), -134.8 (s, 1F, F-6), -151.3 (t, 3JFF = 18 Hz, 1F, F-5), -151.5 (t, 3JFF = 18 Hz, 1F,

F-4), -151.9 (t, 3JFF = 20 Hz, 1F, F-4’), -163.1 (s, br, 1F, F-3’), -164.4 (s, br, 1F, F-5’) ppm. 13C{1H}

NMR (126 MHz, C6D6): 147.6 (d, 1JFC = 250 Hz, C12F9), 146.2 (d, 1JFC = 252 Hz, C12F9), 109.1 (d,

1JFC = 246 Hz, C12F9), 142.2 (d, 1JFC = 251 Hz, C12F9), 140.7 (d, 1JFC = 253 Hz, C12F9), 137.5 (d,

1JFC = 250 Hz, C12F9), 137.5 (s, tol), 132.3 (s, tol), 132.0 (s, tol), 128.9 (s, tol), 128.1 (s, tol), 125.3

(s, tol), 118.6 (m, C12F9), 117.9 (m, C12F9), 105.8 (m, C12F9), 21.1 (s, tol) ppm.

153

Synthesis of Ph2PF2(C12F9) (4-15): 31P{1H} NMR (162 MHz, C6D6): δ -58.5 (t, 1JPF = 700 Hz)

ppm. 19F NMR (377 MHz, C6D6): δ -38.1 (d, 1JPF = 756 Hz, 2F, PF2), -130.2 (s, br, 1F, F-3), -135.8

(s, br, 1F, F-6), -136.6 (d, 3JFF = 22 Hz, 2F, F-2’/6’), -151.5 (tdd, 3JFF = 21 Hz, 4JFF = 9 Hz, 4JFF =

6 Hz, 1F, F-4), -152.9 (t, 3JFF = 22 Hz, 1F, F-5), -153.2 (t, 3JFF = 21 Hz, 1F, F-4’), -162.9 – -163.1

(m, 2F, F-3’/5’) ppm.

Synthesis of (C6F5)2PF2(C12F9) (4-16): 31P{1H} NMR (162 MHz, C6D6): δ -47.8 (t, 1JPF = 707

Hz) ppm. 19F NMR (377 MHz, C6D6): δ -124.4 – 124.7 (m, 1F, F-3), -131.6 – 131.7 (m, 1F, F-6),

-132.5 (s, br, 2F, F-2’/6’), -145.4 (s, br, 4F, o-C6F5), -145.6 (tdd, 3JFF = 22 Hz, 4JFF = 8 Hz, 4JFF =

5 Hz, 1F, F-4), -148.9 (tdd, 3JFF = 21 Hz, 4JFF = 10 Hz, 4JFF = 5 Hz, 1F, F-5), -159.2 (s, br, 4F, m-

C6F5), -162.2 (s, br, 2F, F-2’/6’) ppm.

Synthesis of (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17): 31P{1H} NMR (162 MHz, C6D6): δ -62.7 (t,

1JPF = 707 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -36.6 (d, 1JPF = 707 Hz, 2F, PF2), -36.2 (s, br,

12F, CF3), -129.5 (s, br, 1F, F-3), -134.4 (s, br, 1F, F-6), -136.3 (d, 3JFF = 20 Hz, 2F, F-2’/6’), -147.2

– -147.4 (m, 1F, F-4), -147.9 (t, 3JFF = 22 Hz, 1F, F-5), -148.8 (t, 3JFF = 20 Hz, F1, F-4’), -161.2

(td, 3JFF = 21 Hz, 4JFF = 6 Hz, 2F, F-3’/5’) ppm.

154

Synthesis of Cy2PF2(C12F9) (4-18): 1P{1H} NMR (162 MHz, C6D6): δ -25.5 (t, 1JPF = 694 Hz)

ppm. 19F NMR (377 MHz, C6D6): -42.8 (d, 1JPF = 694 Hz, 2F, PF2), -124.1 – -124.3 (m, 1F, F-3),

-135.1 – -135.3 (m, 1F, F-6), -137.0 – -137.2 (m, 2F, F-2’/6’), -152.1 (t, 3JFF = 20 Hz, 1F,

F-4), -152.5 (td, 3JFF = 20 Hz, 4JFF = 7 Hz, 1F, F-5), -153.3 (t, 3JFF = 20 Hz, 1F, F-4’), -163.2

– -163.3 (m, 2F, F-3’/5’) ppm.

Synthesis of Me2PF2(C12F9) (4-19): 31P{1H} NMR (162 MHz, C6D6): δ -22.8 (tsep., 1JPF = 614

Hz, 2JPH = 9 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -129.7 (s, br, 1F, F-3), -136.0 (s, br, 1F, F-6),

-136.8 (d, 3JFF = 21 Hz, 2F, F-2’/6’), -149.5 (tdd, 3JFF = 21 Hz, 4JFF = 9 Hz, 4JFF = 6 Hz, 1F, F-4),

-150.6 (t, 3JFF = 22 Hz, 1F, F-4’), -151.1 (td, 3JFF = 21 Hz, 4JFF = 6 Hz, 1F, F-5), -161.5 – -161.7

(m, 2F, F-3’/5’) ppm.

Synthesis of [iPr2PF(C12F9)][B(C6F5)4] (4-20): 31P{1H} NMR (162 MHz, C6D6): δ 143.6 (d, 1JPF

= 1020 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -120.2 (s, br, 1F, F-3), -123.2 (s, br, 1F,

F-6), -132.0 – -132.2 (m, 1F, F-4), -133.2 (s, 8F, B(o-C6F5)), -138.1 (d, 3JFF = 20 Hz, 2F, F-2’/6’),

-142.4 – -142.6 (m, 1F, F-5), -145.7 (t, 3JFF = 21 Hz, 1F, F-4’), -158 – -158.2 (m, 2F,

F-3’/5’), -163.8 (t, 3JFF = 20 Hz, 4F, B(p-C6F5)), -167.7 (t, br, 8F, B(m-C6F5)), -171.5 (dt, 1JPF =

1020 Hz, 4JFF = 17 Hz, 1F, PF) ppm. 11B{1H} NMR (128 MHz, C6D6): δ -16.7 (s) ppm.

155

Synthesis of [(o-tol)2PF(C12F9)][B(C6F5)4] (4-21): 31P{1H} NMR (162 MHz, C6D6): δ 95.3 (d,

1JPF = 1012 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -116.0 (s, br, 1F, F-3), -119.8 (d, 1JPF = 1012

Hz, 1F, PF), -125.2 (s, br, 1F, F-6), -131.9 (td, 3JFF = 23 Hz, 4JFF = 11 Hz, 1F, F-4), -133.2 (s, 8F,

B(o-C6F5)), -135.7 (s, br, 2F, F-2’/6’), -142.8 – -143.0 (m, 1F, F-5), -145.5 (t, 3JFF = 20 Hz, 1F,

F-4’), -158.1 (td, 3JFF = 22 Hz, 4JFF = 10 Hz, 2F, F-3’/5’), -163.9 (t, 3JFF = 20 Hz, 4F, B(p-C6F5)),

-167.7 (t, 3JFF = 18 Hz, 8F, B(m-C6F5)) ppm. 11B{1H} NMR (128 MHz, C6D6): δ -16.7 (s) ppm.

Synthesis of [Ph2PF(C12F9)][B(C6F5)4] (4-22): 1H NMR (500 MHz, C6D6): δ 7.04 – 7.00 (m, 2H,

p-CH5), 6.92 – 6.76 (m, 4H, o,m-CH5) ppm. 31P{1H} NMR (162 MHz, C6D6): δ 90.7 (dd, 1JPF =

1012 Hz, 3JPF = 7 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -116.1 (s, 1F, F-3), -120.9 (d, 1JPF =

1012 Hz, 1F, PF), -125.7 (s, 1F, F-6), -131.9 (td, 3JFF = 20 Hz, 4JFF = 13 Hz, 1F, F-4), -133.4 (s,

8F, B(o-C6F5)), -136.7 (d, 3JFF = 18 Hz, 2F, F-2’/6’), -143.6 (t, 3JFF = 22 Hz, 1F, F-5), -146.0 (t,

3JFF = 20 Hz, 1F, F-4’), -158.5 (td, 3JFF = 21 Hz, 4JFF = 7 Hz, 2F, F-3’/5’), -164.0 (t, 3JFF = 20 Hz,

4F, B(p-C6F5)), -167.9 (t, 3JFF = 20 Hz, 8F, B(m-C6F5)) ppm. 11B{1H} NMR (128 MHz, C6D6): δ

-16.7 (s) ppm. Anal. Calcd. for C48H10BF30P: C: 48.11% H: 0.84% Found: C: 48.20%, H: 0.82%.

156

Synthesis of [(C6F5)2PF(C12F9)][B(C6F5)4] (4-23): 31P{1H} NMR (162 MHz, C6D6): δ 72.3 (d,

1JPF = 1034 Hz) ppm. 19F NMR (377 MHz, Benzene-d6): δ -117.5 (s, br, 1F, F-3), -121.0 – -121.2

(m, 2F, P(p-C6F5)), -122.0 (s, br, 1F, F-6), -124.7 (d, 1JPF = 1034 Hz, 1F, PF), -124.8 (s, br, 4F,

P(o-C6F5)), -125.3 – 125.5 (m, 1F, F-4), -133.5 (s, br, 8F, B(o-C6F5)), -138.5 (d, 3JPF = 17 Hz, 2F,

F-2’/6’), -141.2 – -141.4 (m, 1F, F-5), -143.4 (t, 3JPF = 20 Hz, 1F, F-4’), -149.6 (td, 3JPF = 17 Hz,

4JPF = 7 Hz, 4F, P(m-C6F5)), -156.7 (td, 3JPF = 21 Hz, 4JPF = 7 Hz, 2F, F-3’/5’), -164.0 (t, -149.6

(t, 3JPF = 20 Hz, 4F, B(p-C6F5)), -168.0 (t, br, 3JPF = 18 Hz, 8F, B(m-C6F5)) ppm. 11B{1H} NMR

(128 MHz, C6D6): δ -16.7 (s) ppm.

Synthesis of [(3,5-(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24): 31P{1H} NMR (162 MHz, C6D6):

δ 90.2 (d, 1JPF = 1025 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -63.7 (s, 12F, CF3), -133,6 (s, br,

1F, F-3), -121.2 (s, br, 1F, F-6), -124.4 – 124.6 (m, 1F, F-4), -125.2 (d, 1JPF = 1025 Hz, 1F,

PF), -133.2 (s, br, 8F, B(o-C6F5)), -136.2 (d, 3JFF= 20 Hz, 2F, F-2’/6’), -139.6 – -139.8 (m, 1F,

F-5), -142.4 (t, 3JFF= 21 Hz, 1F, F-4’), -156.3 (t, 3JFF= 20 Hz, 2F, F-3’/5’), -163.7 (t, 3JFF= 20 Hz,

4F, B(p-C6F5)), -167.7 (s, br, 8F, B(m-C6F5)) ppm. 11B{1H} NMR (128 MHz, C6D6): δ -16.7 (s)

ppm.

157

Synthesis of [Cy2PF(C12F9)][B(C6F5)4] (4-25): 31P{1H} NMR (162 MHz, C6D6): δ 133.8 (d, 1JPF

= 1005 Hz) ppm. 19F NMR (377 MHz, C6D6): δ -120.3 (s, br, 1F, F-3), -123.7 (s, br, 1F,

F-6), -132.9 – -133.1 (m, 1F, F-4), -133.2 (s, br, 8F, B(o-C6F5)), -137.8 (d, 3JFF= 18 Hz, 2F,

F-2’/6’)), -143.0 (t, br, 3JFF= 21 Hz, 1F, F-5), -146.0 (t, 3JFF= 21 Hz, 1F, F-4’), -158.2 (td, 3JPF =

20 Hz, 4JPF = 6 Hz, 2F, F-3’/5’), -163.8 (t, 3JFF= 20 Hz, 4F, B(p-C6F5)), -167.7 (t, br, 3JPF = 18 Hz,

8F, B(m-C6F5)) -170.0 (d, 1JPF = 1005 Hz, 1F, PF) ppm. 11B{1H} NMR (128 MHz, C6D6): δ -16.7

(s) ppm.


a solution of HSiEt3 (11.6 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was added to a vial


tube for monitoring by NMR spectroscopy.

Dehydrocoupling of triethylsilane with phenol: To a vial containing 2 mol% catalyst was added

a solution of HSiEt3 (11.6 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was added to a vial

containing phenol (9.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for

monitoring by NMR spectroscopy.

Benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene: To a vial containing

2 mol% catalyst was added a solution of HSiEt3 (41.8 mg, 0.36 mmol) in C6D6 (0.7 mL). The

solution was added to a vial containing 4-trifluoromethylbromobenzene (22.5 mg, 0.1 mmol),

which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Deoxygenation of benzophenone: To a vial containing 2 mol% catalyst was added a solution of

HSiEt3 (23.2 mg, 0.2 mmol) in C6D6 (0.7 mL). The solution was added to a vial containing

158

benzophenone (18.2 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for

monitoring by NMR spectroscopy.


was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was added to


NMR tube for monitoring by NMR spectroscopy.


of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was then transferred

to a 5 mm NMR tube for monitoring by NMR spectroscopy.













anisotropically.

159


4-3 4-20 4-21

Formula BrC12F9P1 C43H14BF30P C50H14BF30P

Weight (g/mol) 851.99 1130.33 1226.42

Crystal system triclinic triclinic monoclinic

Space group P-1 P-1 P21/c

a (Å) 6.655(3) 10.783(4) 12.9172(14)

b (Å) 10.255(5) 11.138(5) 12.7837(15)

c (Å) 10.986(6) 18.067(7) 27.407(3)

α (°) 106.692(16) 105.862(12) 90

β (°) 107.193(11) 101.677(11) 90.857(5)

γ (°) 100.037(12) 96.101(11) 90

Volume (Å3) 658.0(6) 2013.6(14) 4525.2(9)

Z 1 2 4


R(int) 0.0356 0.0485 0.0376

μ, mm–1 3.347 0.244 0.255

F(000) 406 1113 2419

Index ranges -7 ≤ h ≤ 8 -13 ≤ h ≤ 14 -16 ≤ h ≤ 16

-12 ≤ k ≤ 12 -14 ≤ k ≤ 14 -13 ≤ k ≤ 13

-13 ≤ l ≤ 13 -23 ≤ l ≤ 23 -35 ≤ l ≤ 35


θ range (min, max) (°) 2.08, 26.49 2.04, 29.52 2.18, 27.51

Total data 2706 9182 10399

Max peak 0.5 0.8 0.5

Min peak -0.6 -0.8 -0.6

>2(FO2) 2114 6164 7260


R (>2σ) 0.0288 0.0582 0.0397

Rw 0.0653 0.1801 0.0891

GoF 1.088 0.969 1.044

160

4-22 4-23 (C12F9)POF2, H2O

Formula C48H10BF30P C42HBF35OP C12F11OP, H2O

Weight (g/mol) 1198.36 1228.22 418.10

Crystal system monoclinic triclinic monoclinic

Space group P21/n P-1 P21/c

a (Å) 12.1923(15) 12.7430(13) 12.6412(9)

b (Å) 13.0218(14) 13.9662(14) 6.1671(4)

c (Å) 27.387(3) 14.9737(16) 18.1472(11)

α (°) 90 67.639(5) 90

β (°) 92.785(6) 80.300(5) 99.477(2)

γ (°) 90 81.745(5) 90

Volume (Å3) 4343.0(9) 2420.0(4) 1395.44(16)

Z 4 2 4


R(int) 0.0542 0.0328 0.0784

μ, mm–1 0.232 0.226 0.338

F(000) 2355 1194 817

Index ranges -14 ≤ h ≤ 14 -16 ≤ h ≤ 16 -16 ≤ h ≤ 16

-15 ≤ k ≤ 15 -18 ≤ k ≤ 18 -8 ≤ k ≤ 7

-32 ≤ l ≤ 27 -19 ≤ l ≤ 19 -23 ≤ l ≤ 23


θ range (min, max) (°) 2.16, 25.25 1.63, 27.71 2.28, 27.61

Total data 7757 11092 3207

Max peak 0.9 0.6 0.8

Min peak -0.6 -0.5 -0.8

>2(FO2) 5511 8047 2336


R (>2σ) 0.0437 0.0464 0.0454

Rw 0.1028 0.1466 0.1257

GoF 1.096 1.026 1.071

161

4.5 References

1. Fenton, D. E.; Park, A. J.; Shaw, D.; Massey, A. G., 2-Substituted Nonafluorodiphenyls.

Tetrahedron Lett. 1964, 949.

2. Chen, Y. X.; Metz, M. V.; Li, L. T.; Stern, C. L.; Marks, T. J., Sterically Encumbered

(Perfluoroaryl) Borane and Aluminate Cocatalysts for Tuning Cation-Anion Ion Pair Structure and

Reactivity in Metallocene Polymerization Processes. A Synthetic, Structural, and Polymerization

Study. J. Am. Chem. Soc. 1998, 120, 6287.

3. Diemer, V.; Begaud, M.; Leroux, F. R.; Colobert, F., Regioselectivity in the Aryne Cross-

Coupling of Aryllithiums with Functionalized 1,2-Dibromobenzenes. Eur. J. Org. Chem. 2011,

341.

4. Wang, Q. Y.; Gillis, D. J.; Quyoum, R.; Jeremic, D.; Tudoret, M. J.; Baird, M. C., The

Formation and Structures of the Methyl-bridged Complexes Cp*TiMe2(μ-Me)B(C6F5)3 and

[Cp*TiMe2(μ-Me)TiMe2Cp*][MeB(C6F5)3]. J. Organomet. Chem. 1997, 527, 7.

5. Sun, Y. M.; Spence, R. E. V. H.; Piers, W. E.; Parvez, M.; Yap, G. P. A., Intramolecular

Ion-Ion Interactions in Zwitterionic Metallocene Olefin Polymerization Catalysts Derived from

''Tucked-In'' Catalyst Precursors and the Highly Electrophilic Boranes XB(C6F5)2 (X=H, C6F5). J.

Am. Chem. Soc. 1997, 119, 5132.

6. Chen, E. Y. X.; Marks, T. J., Cocatalysts for Metal-Catalyzed Olefin Polymerization:

Activators, Activation Processes, and Structure-Activity Relationships. Chem. Rev. 2000, 100,

1391.

7. Childs, R. F.; Mulholland, D. L.; Nixon, A., The Lewis Acid Complexes of α,β-

Unsaturated Carbonyl and Nitrile Compounds .1. A Nuclear Magnetic-Resonance Study. Can. J.

Chem. 1982, 60, 801.

8. Chase, P. A.; Piers, W. E.; Patrick, B. O., New Fluorinated 9-Borafluorene Lewis Acids.

J. Am. Chem. Soc. 2000, 122, 12911.

9. Lancaster, S. J.; Walker, D. A.; Thornton-Pett, M.; Bochmann, M., New Weakly

Coordinating Counter Anions for High Activity Polymerisation Catalysts: [(C6F5)3B-CN-

B(C6F5)3]- and [Ni{CNB(C6F5)3}4]2-. Chem. Commun. 1999, 1533.

162

10. Binding, S. C.; Zaher, H.; Chadwick, F. M.; O'Hare, D., Heterolytic Activation of

Hydrogen Using Frustrated Lewis Pairs Containing Tris(2,2',2''-Perfluorobiphenyl)Borane. Dalton

Trans. 2012, 41, 9061.

11. Geier, S. J.; Stephan, D. W., Lutidine/B(C6F5)3: At the Boundary of Classical and

Frustrated Lewis Pair Reactivity. J. Am. Chem. Soc. 2009, 131, 3476.

12. Ashley, A. E.; Thompson, A. L.; O'Hare, D., Non-Metal-Mediated Homogeneous

Hydrogenation of CO2 to CH3OH. Angew. Chem., Int. Ed. 2009, 48, 9839.

13. Travis, A. L.; Binding, S. C.; Zaher, H.; Arnold, T. A. Q.; Buffet, J. C.; O'Hare, D., Small

Molecule Activation by Frustrated Lewis Pairs. Dalton Trans. 2013, 42, 2431.

14. Menard, G.; Tran, L.; Stephan, D. W., Activation of H2 Using P/Al Based Frustrated Lewis

Pairs and Reactions with Olefins. Dalton Trans. 2013, 42, 13685.

15. Menard, G.; Stephan, D. W., H2 Activation and Hydride Transfer to Olefins by Al(C6F5)3-

Based Frustrated Lewis Pairs. Angew. Chem., Int. Ed. 2012, 51, 8272.

16. Coe, P. L.; Stephens, R.; Tatlow, J. C., Aromatic Polyfluoro-Compounds .11.

Pentafluorophenyl-Lithium and Derived Compounds. J. Chem. Soc. 1962, 3227.

17. Pohlmann, J. L.; Brinckma, F. E., Pentafluorophenyl-Metal Chemistry .2. Preparation and

Characterization of Group 3a Derivatives. Z. Naturforsch., B: Chem. Sci. 1965, B 20, 5.

18. Guerrero, A.; Martin, E.; Hughes, D. L.; Kaltsoyannis, N.; Bochmann, M., Zinc(II) η1- and

η2-Toluene Complexes: Structure and Bonding in Zn(C6F5)2·(toluene) and Zn(C6F4-2-

C6F5)2·(toluene). Organometallics 2006, 25, 3311.

19. Tsuzuki, S.; Uchimaru, T.; Mikami, M., Intermolecular Interaction Between

Hexafluorobenzene and Benzene: Ab Initio Calculations Including CCSD(T) Level Electron

Correlation Correction. J. Phys. Chem. A 2006, 110, 2027.

20. Matter, H.; Nazare, M.; Gussregen, S.; Will, D. W.; Schreuder, H.; Bauer, A.; Urmann, M.;

Ritter, K.; Wagner, M.; Wehner, V., Evidence for C-Cl/C-Br ··· π Interactions as an Important

Contribution to Protein-Ligand Binding Affinity. Angew. Chem., Int. Ed. 2009, 48, 2911.

21. Dumont, W. W.; Kubiniok, S.; Severengiz, T., Properties of Te-Te Bonds .4. Dismutation

Reactions of Di-P-Tolylditelluride with Tetra-tert-Butyldiphosphane and Tetra-

Isopropyldiphosphane. Z. Anorg. Allg. Chem. 1985, 531, 21.

163

22. Alder, R. W.; Canter, C.; Gil, M.; Gleiter, R.; Harris, C. J.; Harris, S. E.; Lange, H.; Orpen,

A. G.; Taylor, P. N., Medium-Ring Diphosphines from Diphosphabicyclo[k.l.0]alkanes:

Stereoselective Syntheses, Structure and Properties. J. Chem. Soc., Perkin Trans. 1 1998, 1643.

23. Fild, M.; Hollenbe, I.; Glemser, O., Reaktionen Der Pentafluorophenyl-

Phosphorhalogenide. Sci. Nat. 1967, 54, 89.

24. Green, M.; Tauntonr, A.; Stone, F. G. A., Chemistry of Metal Carbonyls .56. Reactions of

Bis(Pentafluorophenyl)Phosphine and Tetrakis(Pentafluorophenyl)Diphosphine with Group 6

Metal Hexacarbonyls. J. Chem. Soc. 1969, 1875.

25. Jeffrey, G. A., An Introduction to Hydrogen Bonding. Oxford University Press: New York,

1997; p 303.



341, 1374.



















164













Chem. Phys. Lett. 1988, 153, 503.





33. Sigma-Aldrich best-available-prices (CAD): benzene-d6 = $540.00 / 100g, methylene

chloride-d2 = $407.50 / 25g. Methylene chloride-d2 approximately 4.6x more expensive than

benzene-d6 per unit volume. http://www.sigmaaldrich.com/chemistry/stable-isotopes-

isotec/stable-isotope-products.html, accessed April, 2017.

34. Zhu, J.; Perez, M.; Caputo, C. B.; Stephan, D. W., Use of Trifluoromethyl Groups for

Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination. Angew. Chem., Int.

Ed. 2016, 55, 1417.

35. Duttwyler, S.; Douvris, C.; Fackler, N. L. P.; Tham, F. S.; Reed, C. A.; Baldridge, K. K.;

Siegel, J. S., C-F Activation of Fluorobenzene by Silylium Carboranes: Evidence for Incipient

Phenyl Cation Reactivity. Angew. Chem., Int. Ed. 2010, 49, 7519.

36. Herrington, T. J.; Thom, A. J. W.; White, A. J. P.; Ashley, A. E., Novel H2 Activation by

a Tris[3,5-bis(trifluoromethyl)phenyl]borane Frustrated Lewis Pair. Dalton Trans. 2012, 41, 9019.


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165




339.

166

Chapter 5 Binaphthyl Fluorophosphonium Cations

5.1 Introduction

Chiral molecules represent a crucial facet of synthetic organic chemistry, specifically with respect

to the development of medicinal compounds. Different enantiomers of the same compound can

have vastly different therapeutic outcomes in biological systems. The most infamous example of

a compound with divergent in vivo effects is α-phthalimido glutarimide, thalidomide, a drug

initially marketed as a sedative in the 1950s. The (R)-enantiomer has anti-nausea properties and

thalidomide found off-label use in combating morning sickness in pregnant women. Unfortunately,

thalidomide is capable of crossing the placental barrier and the (S)-enantiomer was soon found to

be a powerful teratogen as thousands of children were born with limb-reduction deformities.1

While chiral separation would be fruitless in the case of thalidomide, given its rapid

interconversion of enantiomers in vivo, it nevertheless highlights the significance of

stereospecificity in synthesis.2

Certain methods of obtaining enantiopure compounds, such as chiral resolution of racemic

mixtures, are inherently inefficient. Crystallization of enantiopure compounds can take many

successive cycles and yield only 50% of a desired enantiomer from a racemic mixture. Catalytic

asymmetric induction offers a much more efficient route to enantioenriched or enantiopure

compounds. Chiral catalysts can be used to generate an enantiomeric excess of one product from

achiral reagents, resolve a chiral mixture by preferential reaction with only one enantiomer, or

approach enantioconvergence by stereoablation of chiral centres and selective reformation of a

single enantiomer.3

The first reported instance of catalytic chiral induction was by Marckwald in 1904.4 The

decarboxylation of 2-ethyl-2-methylmalonic acid was catalyzed in the presence of brucine, a chiral

organic alkaloid (Scheme 5.1). The enantiomeric enrichment of the product was measured by

rotary polarization, which indicated a 16% excess of L-enantiomer.

167

Scheme 5.1. Chiral induction in decarboxylation reaction by brucine.

The first asymmetric hydrogenation catalyst was reported by Knowles and Sabacky in 1968.5 The

chiral phosphines (-)-methylpropylphenylphosphine and bis(2-methylbutyl)phenylphosphine were

used to generate rhodium catalysts that were employed in the hydrogenation of methylenesuccinic

acid and 2-phenylacrylic acid. The phosphine with chirality centre on the phosphorus centre

yielded up to 15% enantiomeric excess, while the phosphine with the chiral alkyl substituents gave

near racemic mixtures. Despite the modest chiral induction observed, the authors were aware of

significance of their work, concluding:

“The inherent generality of this method offers almost unlimited opportunities for matching

substrates with catalysts in a rational manner and we are hopeful that our current effort will result in real progress towards complete stereospecificity”.

This prediction by Knowles and Sabacky has been thoroughly wrought out. Transition-metal

catalysts have since been used extensively to carry out asymmetric transformations including

hydrogenations6, cross-couplings7, and hydrosilylations8 using a vast array of chiral ligands. One

such chiral ligand, developed by Noyori and Takaya, is 2,2’-bis(diphenylphosphino)-1,1’-

binaphthyl or BINAP. Bisphosphine BINAP is atropisomeric, deriving chirality from hindered

rotation about the bond between naphthyl groups.9 A series of BINAP-rhodium catalysts were

tested for activity in the hydrogenation of α-(N-acylamino)-acrylic acids. The BINAP systems

proved to be very effective chiral hydrogenation catalysts, able to generate enantiopure

N-benzoylphenylalanine in high yield (Scheme 5.2).

168

Scheme 5.2. Chiral BINAP rhodium hydrogenation catalyst

The highly modular synthesis of many transition-metal complexes allows for the facile tuning of

chiral environment around the active site for specific transformations. Fine-tuning is enabled by

the availability of a wide library of commercially-available and literature reported chiral ligands.10

However, there are drawbacks associated with the use of metal catalysts, including cost and

toxicity associated with some systems.11 The development of highly active chiral main group

catalysts is an attractive avenue to provide a wider variety of options for any given transformation.

Metal-free catalysts have also seen significant improvement since the first report by Marckwald,

both in range of reactions and in chiral induction. With the advent of FLP catalysis, many different

approaches have been taken towards highly enantioselective transformations. A significant

challenge in chiral main group catalysis is balancing the activity of a catalyst with chiral

inductivity. In most examples, electron-withdrawing groups like C6F6 are replaced with weakly

donating chiral substituents like α-pinene, camphor, or binaphthyl.12.13

The group of Du and coworkers have made significant contributions to the development of chiral

FLP systems. In 2013, Du investigated an extensive range aryl-substituted binaphthyl dienes that

were reacted with Piers’ borane to generate chiral FLP catalysts in situ.14 The generated binaphthyl

diboranes were used to reduce a variety of imines under a hydrogen atmosphere. The aryl

substituents ranged widely in chiral induction, with the larger substituents proving to be the most

effective. This rigorous screening approach to chiral FLP catalyst design yielded an enantiomeric

excess of 89%, the highest that had been observed for FLP systems.

169

In 2016, Du further refined the diborane FLP catalytic system by using binaphthyl diyne starting

materials instead of dienes for reaction with Piers’ borane.15 The chiral diyne derived catalysts

were used in the hydrosilylation of 1,2-diketones and α-keto esters with incredibly high

enantiomeric excess of 99% (scheme 5.3). The chiral diyne systems resulted in much higher yields

and enantiomeric excess than the corresponding binaphthyl diene systems.

Scheme 5.3. Chiral binaphthyl FLP hydrosilylation of 1,2-diketone.

Maruoka and coworkers have also exploited the chirality of the binaphthyl moiety to effect

asymmetric amination of β-keto esters using phosphonium catalysts.16 Dibutylphosphine was

reacted with a binaphthyl dibromide starting material to generate the quaternary

tetraalkylphosphonium bromide catalyst. The phosphonium catalyst was able to act as a chiral

phase transfer catalyst in the amination of β-keto esters in high yield with moderate enantiomeric

excesses (Scheme 5.4).

Scheme 5.4. Asymmetric amination of β-keto ester with phosphonium catalyst.

170

Given the vast array of commercially available chiral phosphines, investigating chiral

fluorophosphonium systems seems to be a natural extension. Use of binaphthyl containing

fluorophosphonium systems is attractive given the success of the moiety to induce asymmetry in

both transition metal and metal-free catalysis (vide supra). Point chiral phosphines have the

disadvantage of racemizing during the synthesis of oxidation by XeF2 and subsequent fluoride

abstraction by [SiEt3][B(C6F5)4], since the chirality of BINAP resides on the ligand, the initial

enantiomer is retained through the synthesis of fluorophosphonium cations.

Additionally, it has been shown that systems containing two closely linked fluorophosphonium

centres exhibit enhanced reactivity.17 A series of commercially available diphosphines with varied

alkyl linkers was converted to the corresponding difluorophosphonium salts and tested in a series

of Lewis acid catalytic applications. The activity of the dications decreased drastically as the linker

length between phosphonium centres exceeded two carbon atoms. While BINAP does not contain

electron-withdrawing C6F5 moieties, the cooperative effect of two phosphorus centres held in

relatively close proximity may overcome the low electrophilicity of each individual

fluorophosphonium centre.


5.2.1 Synthesis and Characterization of BINAP Fluorophosphonium Cation

Two equivalents of solid XeF2 were added to a stirring solution of (±)-BINAP, which resulted in

effervescence and a slight darkening of the solution. The solution was stirred for 2 h before

removing the solvent in vacuo yielding a white crystalline powder 5-1. The 19F NMR spectrum of

5-1 shows a doublet at -35.9 ppm, indicative of clean oxidation of both phosphorus centres. The

31P{1H} NMR spectrum of 5-1 displays a single triplet at -52.9 ppm with 694 Hz coupling,

consistent with two phosphorus-bound fluorine substituents. The 1H NMR displays many

overlapping aryl resonances. All these data are consistent with the formulation of 5-1 as

2,2’-bis(diphenyldifluorophosphorano)-1,1’-binaphthyl, ((Ph2F2P)C10H6)2 (Scheme 5.5).

171

Scheme 5.5. Oxidation of BINAP by XeF2 to generate bis(difluorophosphorane) 5-1.

Unsymmetrically-substituted fluorophosphonium/phosphine binaphthyl could act as a chiral

intramolecular FLP. In an effort to synthesize 2-(diphenylphosphino)-2’-

(diphenyldifluorophosphorano)-1,1’-binaphthl, (±)-BINAP was reacted with a single equivalent

of XeF2 in a CH2Cl2 solution. Upon removing the solvent in vacuo an oily residue remains.

Multiple products were observed by 31P and 19F NMR. Even at low temperatures, the attempted

oxidation of a single BINAP phosphorus centre yielded an intractable mixture of products.

A solution of 5-1 in toluene was added to two molar equivalents of [SiEt3][B(C6F5)4]·PhMe as a

slurry in toluene, resulting a colourless suspension of immiscible oil. The suspension stirred for

2 h before decanting the toluene from the oily product. The colourless oil is washed several times

with toluene, then several times with pentane before being triturated in pentane to yield a while

powder 5-2. The 31P{1H} NMR spectrum of 5-2 displays a doublet at 97.4 ppm with 993 Hz

coupling, consistent with the fluoride abstraction from both difluorophosphoranes centres. The 19F

NMR spectrum displays a doublet at -128.2 ppm and a set of resonances attributable to [B(C6F5)4]-

anion. These NMR data infer the formulation of 5-2 as bis(fluorophosphonium)

[((Ph2FP)2C10H6)2]2[B(C6F5)4] (Scheme 5.6).

Scheme 5.6. Synthesis of fluorophosphonium salt 5-2.

Attempts were made to obtain unsymmetrically substituted fluorophosphonium-

difluorophosphorane binaphthyl from 5-1. This species was targeted to test for an interaction

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between the fluorophosphonium centre and the fluoride of the linked difluorophosphoranes. Such

interactions have been observed for linked borane-phosphonium systems by Gabbai and more

recently by our group.18,19 When 5-1 was added to an equimolar amount of

[SiEt3][B(C6F5)4]·PhMe in toluene, a colourless oil is generated. The solvent was removed in

vacuo and the residue was taken up in CH2Cl2. The resulting 31P NMR and 19F NMR spectra were

consistent with the presence of 5-2 and 5-1, no mixed difluorophosphorane-fluorophosphonium

product was observed.

Reaction of (±)-BINAP with one or two molar equivalents of Selectfluor in CH2Cl2 yields no

reaction due to the very low solubility of Selectfluor in most common organic solvents, aside from

MeCN. If MeCN is employed as solvent, the reaction of 5-1 with one or two molar equivalents

proceeds to generate an intractable mixture of products.

The reaction of (s)-BINAP with two equivalents of NFSI in CH2Cl2 cleanly generates a single

product with similar 31P{1H} and 19F NMR features to those of 5-2, consistent with direct

formation of bis(fluorophosphonium) [((Ph2FP)2C10H6)2]2[N(SO2Ph)2] 5-2b (Scheme 5.7, top).

When a single equivalent of NFSI was added to a CH2Cl2 solution of BINAP a different

fluorophosphonium species is generated. The 31P{1H} NMR displays a doublet 93.6 ppm with

1003 Hz coupling and a doublet at -15.7 ppm with smaller 137 Hz coupling which are consistent

with the formation of mixed fluorophosphonium-phosphine binaphthyl salt

[((Ph2FP)C10H6)(C10H6(PPh2)][N(SO2Ph)2], 5-2c (Scheme 5.7, bottom). A small impurity of

[((Ph2FP)2C10H6)2]2[N(SO2Ph)2] is visible in the 31P{1H} NMR spectrum at 96.3 ppm. Because

the chirality resides on the C2-symmetric binaphthyl linker, only a single enantiomer is formed.

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Scheme 5.7. Synthesis of bis(fluorophosphonium) 5-2b (top) and fluorophosphonium-phosphine

5-2c (bottom) from BINAP using NFSI.

5.2.2 Electrophilicity of BINAP-derived Fluorophosphonium Cation

A CH2Cl2 solution of 5-2 and OPEt3 was made to evaluate the Lewis acidity of 5-2 by the

Gutmann-Beckett method. The 31P{1H} NMR spectrum shows a doublet resonance at 97.4 ppm

and a broad OPEt3 signal at 56 ppm, close to the reference shift of OPEt3 in CH2Cl2 is 50.9 ppm.

The near negligible shift of 5.1 ppm and the broad OPEt3 is indicative that 5-2 is incompatible

with the Gutmann-Beckett method. This finding is consistent with the less electrophilic

phosphonium cations discussed previously (Chapter 3, pg. 82 and Chapter 4, pg. 131). This

observation is also consistent with previously reported incompatibility of closely related

[PFPh3][B(C6F5)4] with the Gutmann-Beckett method.20

5.2.3 Catalytic Activity of BINAP-derived Fluorophosphonium Cation

The catalytic activity of 5-2 was evaluated in the dimerization of 1,1-diphenylethylene,

hydrodefluorination of 1-fluoroadamantane, and hydrosilylation of α-methylstyrene with HSiEt3

(Scheme 5.8). All catalytic reactions were performed using 2 mol% 5-2 (4 mol%

fluorophosphonium centre) at room temperature unless otherwise noted. The hydrodefluorination

of 1-fluoradamantane was complete before an NMR spectrum could be obtained. Catalyst 5-2

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proved to be competent in the dimerization of 1,1-diphenylethylene, completing the reaction in

3 h. Unfortunately, even upon heating the reaction to 130 °C in bromobenzene, no hydrosilylation

of α-methylstyrene with HSiEt3 was brought about by 5-2.

Despite the absence of electron-withdrawing aryl substituents, 5-2 proved to be considerably

active. The two fluorophosphonium centres in 5-2 are separated by four carbon atoms, but 5-2

shows comparable reactivity to alkyl-linked bis(fluorophosphonium) catalysts with lengths of two

or three carbon atoms. The comparable reactivity at a longer linker length is likely due to a

reduction of rotational degrees of freedom by the aryl backbone in 5-2, promoting closer

interaction between fluorophosphonium cations.17 Unfortunately, 5-2 was not sufficiently Lewis

acidic to effect hydrosilylation, which would be an ideal test reaction for a chirality induction by

a fluorophosphonium cation.

Scheme 5.8. Summary of catalytic activity for 5-2.

5.2.4 Synthesis of Pentafluorophenyl Binaphthyl Fluorophosphonium Cation

Since the Lewis acidity of 5-2 was insufficient to effect the hydrosilylation of α-methylstyrene,

which could be influenced by chiral induction, a more electrophilic analogue of 5-2 was targeted.

A 2011 report by the Wu group, outlined the synthesis of two electrophilic

bis(diarylphosphino)binaphthyl species, substituted with 3,5-bis(trifluoromethyl)phenyl or 4-

trifluoromethylphenyl groups.21 The group obtained the binaphthyl bis-Grignard reagent by

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reacting 2,2’-dibromo-1,1’-binaphthyl with Mg turnings in THF and toluene, before adding to it

the desired diarylbromophosphine (Scheme 5.9, top). The use of diarylchlorophosphines resulted

in very poor yields compared to the analogous diarylbromophosphine reagents. This synthetic

route was noted to be particularly effective for synthesis of electron-deficient systems. Using this

approach, two equivalents of bis(pentafluorophenyl)bromophosphine, (C6F5)2PBr, were reacted

with the binaphthyl di-Grignard reagent in a toluene/THF solution, resulting in the immediate

change from a pale-yellow slurry to a clear yellow solution. A 31P{1H} NMR spectrum obtained

of the reaction mixture display displays two major signals at -36.2 and -74.9 ppm and a minor

signal at -54.4 ppm. The major peak at -36.2 ppm is a triplet, consistent with the loss of a C6F5

substituent from the (C6F5)2PBr starting material. The other major product peak at -74.9 ppm is a

septet, coupling with six equivalent ortho-fluorine atoms, consistent with increased C6F5

substitution of (C6F5)2PBr and indeed matches the chemical shift of an authentic sample of

P(C6F5)3. These data suggested the formation of a phosphole (C6F5)P(κ2-C20H12), 5-3 (Scheme 5.9,

bottom).

Scheme 5.9 Reaction of binaphthyl Grignard reagent with BrP(3,5-(CF3)2C6H3)2 (top, literature

procedure21), and BrP(C6F5)2 to form 5-3 (bottom).

The two major products, 5-3 and P(C6F5)3 could both be isolated by chromatography on a silica

column, eluting with 4:1 pentane to Et2O. Phosphine 5-3 slowly oxidizes in air to form

(C6F5)PO(κ2-C20H12) 5-4 (Scheme 5.10). Phosphine oxide 5-4 displays a broad singlet at 19.6 ppm

in the 31P{1H} NMR spectrum and resonances attributable to a C6F5 substituent in the 19F spectrum.

176

P

F F

F

FF

5-3

as solid, 50 °C, 4 h PF

F

F

FF

5-4

O

Scheme 5.10. Air-oxidation of 5-3 to form phosphine oxide 5-4

The solid-state structure was obtained for 5-4, which confirmed the connectivity of 5-4 and 5-3

(Figure 5.1). The solid-state structure of 5-4 displays distorted tetrahedral geometry at the

phosphorus centre. The bite angle of the naphthyl substituent is 92.90(7)°, indicating moderate

distortion away from the ideal angle of 109.5°. The binaphthyl substituent shows significant

twisting, with an angle of 47° between the planes of the two furthest separated aryl rings.

Figure 5.1. POV-ray depiction of 5-4; C: light grey, O: red, P: orange, F: pink, hydrogen atoms

have been omitted for clarity.

Solid XeF2 was added to a colourless solution of 5-3, which resulted in the solution immediately

changing colour to yellow. The solution was stirred for two hours before removing the solvent in

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vacuo to yield a yellow solid. The 31P{1H} NMR spectrum shows a triplet at -33.2 ppm with 874

Hz coupling. The 19F NMR spectrum displays a broad doublet at 43.6 ppm as well as three C6F5

resonances. All NMR data are consistent with the formation of (C6F5)PF2(κ2-C20H12), 5-5 (Scheme

5.11).

Scheme 5.11. Oxidation of 5-3 by XeF2 to synthesize difluorophosphorane 5-5.

A toluene solution of 5-5 was added to a slurry of [SiEt3][B(C6F5)4]·PhMe in toluene, yielding a

red solution. The product was sufficiently soluble in toluene that decanting the supernatant was

not possible, instead the solvent was removed in vacuo and the residue was triturated with pentane

to yield a red powder 5-5. The 19F NMR spectrum of 5-5 displays a doublet at -126.9 ppm and two

sets of C6F5 resonances that integrate 4:1. The 1H NMR spectrum displays a series of overlapping

aryl resonances and the 31P{1H} NMR spectrum displays a doublet at 82.5 ppm. The formulation

of 5-6 as [(C6F5)PF(κ2-C20H12)][B(C6F5)4] was supported by all these NMR data (Scheme 5.12).

Scheme 5.12. Synthesis of fluorophosphonium salt 5-6.

As noted previously with the synthesis of 5-2, the chirality of 5-5 is derived from the C2-symmetric

ligand so the same chirality is maintained from the binaphthyl starting material throughout

Grignard reagent, the oxidation and fluoride abstraction. If chirality was derived from a non-C2-

symmetric bridging ligand or the chirality existed at the phosphorus centre, as in

(-)-methylpropylphenylphosphine, fluorophosphonium cations generated through a

difluorophosphorane intermediate would form enantiomers or diastereomers. These types of chiral

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phosphines would only be amenable to fluorination of phosphines directly to fluorophosphonium

cations by reagents like NFSI, Selectfluor or fluoropyridinium salts. Electron-deficient phosphines

required to make active fluorophosphonium catalysts are typically not amenable to direct

fluorination by weaker reagents.

5.2.5 Mechanism for Phosphole Synthesis

Binaphthylphenylphosphole can be generated in 25% yield through the pericyclic [4+1]

McCormack reaction of neat dichlorophenylphosphine, PhPCl2, with

3,3’,4,4’-tetrahydrobinaphthyl at 220 °C.22 Alternatively, reaction of PhPCl2 with

2,2’-dilithio-1,1’-binaphthyl proceeds at room temperature to yield the cyclized product at room

temperature in good yield. However, the direct loss of C6F5 and Br from (C6F5)2PBr in a pericyclic

reaction and reaction of anionic C6F5 with (C6F5)2PBr is unlikely. Instead, sequential σ-bond

metatheses offer a precise route to the observed products, and is consistent with reported

magnesium reactivity.23

A possible mechanism for the concomitant formation of 5-3 and P(C6F5)3 from (C6F5)2PBr and

di-Grignard-binaphthyl begins with the σ-bond metathesis between the Mg-naphthyl and P-Br to

form an naphthyl-P bond and MgBr2. A second σ-bond metathesis between Mg-naphthyl and

P-C6F5 to generate 5-3 and BrMgC6F5. Finally, reaction of BrMgC6F5 with a second equivalent of

(C6F5)2PBr to generate P(C6F5)3 and MgBr2 (Scheme 5.13).

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Scheme 5.13. Proposed mechanism for the formation of 5-3 and P(C6F5)3.

5.2.6 Electrophilicity of Perfluorophenyl Binaphthyl Fluorophosphonium.


To subject 5-6 to the Gutmann-Beckett protocol, a solution containing three equivalents of the

Lewis acid and one molar equivalent of OPEt3 in CH2Cl2 was prepared. The P{1H} NMR spectrum

displays a doublet attributable to free 5-6 and no downfield shift is observed in the signal of OPEt3.

As with other less electrophilic fluorophosphonium cations, 5-6 does not appear to be amenable to

quantification by the Gutmann-Beckett method.

5.2.6.2 Fluoride Ion Affinity

The optimized geometries for the cation of 5-6 ([5-6]+) and the corresponding fluoride adduct 5-5

were calculated with Gaussian 0924 using the hybrid functional B3LYP25 and split valence basis

set 6-311+G(d).26 The optimized geometries were used to determine the single point energies using

MP227 and def2-TZVPP.28 The LUMO of 5-6 is delocalized across the binaphthyl and C6F5 rings

with a large lobe centralized on the phosphorus centre (Figure 5.2).

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Figure 5.2. Contour plot for the LUMO of 5-6.

The LUMO and HOMO energies of [5-6]+ were determined to be -3.130 and -10.873 eV

respectively. The ω value was determined to be 3.166 eV, though the unique substituent set on

phosphorus hampers meaningful comparison to other fluorophosphonium cations using ω values.

To allow for more broad comparison of electrophilicity, the FIA of [5-6]+ was calculated to be

651 kJ/mol. While the FIA of [5-6]+ is still significantly greater than the calculated FIA of BCF

(451 kJ/mol, by the same computational protocol), it is lower than any of the FIA values of

fluorophosphonium cations studied in Chapter 3 (pg. 87) and Chapter 4 (pg. 136).

Fluorophosphonium bearing two unsubstituted phenyl groups and one perhalophenyl group,

specifically [PF(C6Cl5)Ph2][B(C6F5)4] (3-10) and [PF(C6Cl5)Ph2][B(C6F5)4] (4-22) display higher

fluoride ion affinities than 5-6 at 686 and 713 kJ/mol, respectively. The lower FIA of 5-6 compared

to 3-10 despite the greater electron-withdrawing power of C6F5 compared to C6Cl5 suggests that

the κ2-binaphthyl has a negative impact on electrophilicity. The κ2-binaphthyl bite angle on

phosphorus was measured to be 92.90(7)°, 97.0° and 95.2° in the solid-state structure of 5-4, the

optimized geometry of 5-5, and the optimized geometry of 5-6 respectively. The constrained

binaphthyl angle is much closer to the ideal tetrahedral bond angle (109.5°) than the ideal trigonal

bipyramidal bond angle of 120° for equatorial substituents. The small angle disfavours the five-

coordinate fluoride-adduct 5-5, resulting in a decreased FIA of 5-6. The reverse effect has been

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observed in which a small constrained substituent angle on a boron centre enhanced the Lewis

acidity by disfavouring the trigonal planar geometry.29

5.2.7 Reactivity of a Perfluoroaryl Binaphthyl Fluorophosphonium Cation

Compound 5-6 was tested for catalytic activity in the hydrodefluorination of 1-fluoroadamantane,

the dimerization of 1,1-diphenylethylene, and the hydrosilylation of α-methylstyrene with HSiEt3

(Scheme 5.14). Hydrodefluorination of 1-fluoroadamantane occurred rapidly with 2 mol% of 5-6

in CH2Cl2, the reaction was complete before an NMR spectrum could be obtained. The

dimerization of 1,1-diphenylethylene was catalyzed by 5-6 in 4 h, which is slower than 5-2.

Similarly, 5-6 was insufficiently Lewis acidic to effect the hydrosilylation of α-methylstyrene.

Despite the enhanced electron-withdrawing ability of the C6F5 substituent on 5-6, the overall Lewis

acidic reactivity is marginally worse than 5-2. Catalyst 5-2 benefits from two cooperative

fluorophosphonium centres, while 5-6 is hampered by a small bite angle bidentate binaphthyl

substituent that disfavours the five-coordinate reaction intermediate 5-5.

Scheme 5.14. Summary of reactivity for 5-6.

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5.3 Conclusion

Two chiral binaphthyl fluorophosphonium cations were synthesized and characterized. The first

fluorophosphonium was obtained by oxidation of BINAP with XeF2 to yield the

bis(difluorophosphorane) species (PF2Ph2)2(C20H12) (5-1). Fluoride abstraction of 5-1 with

[SiEt3][B(C6F5)4]·PhMe results in the formation of bis(fluorophosphonium) (5-2). In catalytic

applications, 5-2 was found to be competent in hydrodefluorination and dimerization of

diphenylethylene, but not able to effect hydrosilylation. In an effort to generate a more Lewis

acidic binaphthyl fluorophosphonium, BrP(C6F5)2 was reacted with a binaphthyl Grignard reagent.

Instead of the expected BINAP analogue, cyclized pentafluorophenyl binaphthylphosphole

(C6F5)P(κ2-C20H12) (5-3) was generated with concomitant formation of P(C6F5)3. Oxidation of 5-3

in air led to the formation of phosphine oxide (C6F5)PO(κ2-C20H12) (5-4). Oxidation of 5-3 using

XeF2 resulted in difluorophosphorane (C6F5)PF2(κ2-C20H12) (5-5), which was subsequently

converted to fluorophosphonium salt [(C6F5)PF(κ2-C20H12)][B(C6F5)4] (5-6). The fluoride ion

affinity of 5-6 was evaluated to be lower than similarly substituted compounds and the catalytic

activity was found to be lower than that of bis(fluorophosphonium) cation 5-2.

5.4 Experimental










by three successive cycles of headspace-evacuation, and sonication. All NMR data were collected

on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz or Agilent DD2 600 MHz spectrometer

at 25 °C unless otherwise noted. NMR chemical shift data are given relative to an external standard

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(1H, 13C: SiEt4; 11B: 15% BF3·Et2O; 19F: CFCl3; 31P: H3PO4). In some cases, 2-D NMR techniques

were employed to assign individual resonances. Both [SiEt3][B(C6F5)4]·PhMe and BrP(C6F5)2 was

synthesized by literature procedure.30,31 Binaphthyl starting materials ((±)-2,2’-dibromo-1,1’-

binaphthylene, (S)- and (±)-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP)) and

N-Fluorobenzenesulfonimide (NFSI) were all obtained from Sigma-Aldrich and used without

further purification. Computational work was performed using resources provided by the Shared

Hierarchical Academic Research Computing Network and Compute Canada.

Synthesis of (PF2Ph2)2(C20H12) (5-1): To a solution of racemic 2,2’-(diphenylphosphino)-1,1’-

binaphthyl ((±)-BINAP, 62.3 mg, 0.1 mmol) in a 20 mL vial, was added solid XeF2 (33.8 mg, 0.2

mmol). Effervescence was immediately observed in the clear colourless solution. The reaction was

stirred for 2 h before the solvent was reduced to a minimum in vacuo. Recrystallization at -35 °C

yielded 5-1 as a crystalline white solid in excellent yield (68.3 mg, 98% yield).

1H NMR (400 MHz, C6D6): δ 7.85 – 7.71 (m, 10H), 7.48 – 7.40 (m, 6H), 7.13 – 6.91 (m, 16H).

31P{1H} NMR (162 MHz, C6D6): δ -52.9 (d, 1JPF = 694 Hz). 19F NMR (377 MHz, C6D6): δ -35.9

(d, 1JPF = 694 Hz).

Synthesis of [(PFPh2)2(C20H12)]2[B(C6F5)4] (5-2): To a stirred slurry of [SiEt3][B(C6F5)4]·PhMe

(86.9 mg, 0.1 mmol) in toluene (2 mL) in a 4 dram vial, was added a solution of 5-1 (68.3 mg, 0.1

mmol) in toluene (2 mL). The white slurry rapidly changed to a dark red oily suspension. The

reaction stirred for 2 h before allowing the red oil to settle and decanting the supernatant. The red

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oil was washed with an aliquot of toluene (1 mL) and several washes of pentane (3 x 1 mL), before

being triturated in pentane to yield 5-2 as a white powder in excellent yield (187.4 mg, 95% yield).

1H NMR (400 MHz, CD2Cl2) δ 8.41 (d, 3JHH = 8 Hz, 1H, H-8), 8.40 (d, 3JHH = 8 Hz, 1H, H-8’),

8.11 (t, 3JHH = 8 Hz, 2H, H-4,4’), 8.09 (d, 3JHH = 9 Hz, 2H, H-5,5’), 7.84 – 7.67 (m, 10H), 7.53 –

7.47 (m, 4H, p-C6H5), 7.30 – 7.19 (m, 8H), 7.04 (t, 3JHH = 8 Hz, 2H, H-6,6’), 6.60 (d, 3JHH = 8 Hz,

2H, H-3,3’). 31P{1H} NMR (162 MHz, CD2Cl2) δ 97.4 (d, 1JPF = 998 Hz). 19F NMR (377 MHz,

CD2Cl2) δ -128.2 (d, 1JPF = 998 Hz, 1F, PF), -132.97 (s, 8F, B(o-C6F5), -163.3 (t, 3JFF = 20 Hz,

B(p-C6F5), -167.4 (t, 3JFF = 16 Hz, B(m-C6F5)).

The synthesis of the bis(phosphonium) cation 5-2b and monophosphonium-phosphine binaphthyl

5-2c species are identical aside from the stoichiometry of NFSI added, a sample procedure is

provided below.

Synthesis of [(PFPh2)2(C20H12)]2[N(S(SO2Ph)2] (5-2b): A 1 dram vial was charged with (S)-

BINAP (18.6 mg, 0.03 mmol), CH2Cl2 (1 mL) and a stirbar. To the stirring solution was added a

solution of NFSI (19 mg, 0.06 mmol) in CH2Cl2. The solution immediately turned yellow colour.

The solution was stirred for 1 h before the solvent was removed in vacuo to yield a white yellow

powder 5-2b. The yellow powder was washed with pentane to give 5-2b in excellent yield (33.8

mg, 90%).

31P{1H} NMR (162 MHz, CD2Cl2) δ 96.0 (d, 1JPF = 1000 Hz). 19F NMR (377 MHz, CD2Cl2)

δ -128.9 (d, 1JPF = 1000 Hz).

185

Synthesis of [(PFPh2)(PPh2)(C20H12)][N(S(SO2Ph)2] (5-2c): (yellow powder, 84%)

31P{1H} NMR (162 MHz, CD2Cl2) δ 93.3 ppm (d, 1JPF = 993 Hz, PF), -15.7 (d, JPF = 129 Hz, P).

19F NMR (377 MHz, CD2Cl2) δ -125.2 (dd, 1JPF = 994 Hz, JPF = 18 Hz).

Synthesis of (C6F5)P(κ2-C20H12) (5-3): A 100 mL round-bottom flask was charged with

magnesium turnings (25 mg, 1.0 mmol), THF (20 mL), and a stirbar. To the stirring magnesium

turnings was added 1,2-dibromoethane dropwise until effervescence was observed. The reaction

flask was heated to 60 °C and to it was added dropwise a solution of 2,2’-dibromo-1,1’-

binaphthylene (206.6 mg, 0.5 mmol) in toluene (20 mL) over 2 h. As the 2,2’-dibromo-1,1’-

binaphthylene solution was added, the reaction mixture changed to a light-yellow slurry. The slurry

was stirred at 60 °C for an additional 2 h, before being cooled to room temperature. To the reaction

slurry was added a solution of BrP(C6F5)2 (44.5 mg, 1.0 mmol) which resulted in a homogenous

yellow solution. The product mixture was then chromatographed on silica eluting with 4:1

pentane/Et2O. After P(C6F5)3 had eluted with the solvent front; the following eluent was collected

and the solvent was removed in vacuo to yield 5-3 as a white solid in poor yield (143.0 mg, 32 %

(from halophosphine).

31P{1H} NMR (162 MHz, C6D6): δ -36.2 ppm (t, 3JPF = 31 Hz). 19F NMR (377 MHz, C6D6):

δ -130.1 – -130.3 (m, 2F, o-C6F5), -149.9 (tt, 3JFF = 21 Hz, 4JFF = 4 Hz, 1F, p-C6F5), -160.3 – -160.5

(m, 2F, m-C6F5).

186

Synthesis of (C6F5)PO(κ2-C20H12) (5-4): A 1 dram vial was charged with solid 5-3 (15 mg, 0.03

mmol) and heated to 50 °C with the vial open to air. After 4 h, 5-4 is obtained in quantitative yield.

Initially, 5-4 was obtained from the in-air chromatography of the reaction mixture from 5-3

synthesis (vide supra), which was eluted from the silica column using THF. Slow evaporation of

Et2O from a solution of 5-4 yielded bright yellow block crystals suitable for X-ray diffraction.

31P{1H} NMR (162 MHz, C6D6): δ 19.6 ppm (s). 19F NMR (377 MHz, C6D6): δ -129.7 (d, 3JFF =

21 Hz, 2F, o-C6F5), -147.9 (t, 3JFF = 21 Hz, 1F, p-C6F5), -159.9 – -160.1 (m, 2F, m-C6F5).

Synthesis of (C6F5)PF2(κ2-C20H12) (5-5): To a solution of racemic 5-3 (90 mg, 0.2 mmol) in a

4 dram vial, was added solid XeF2 (33.8 mg, 0.2 mmol). The solution immediately turned bright

orange while effervescence was formed. The reaction was stirred for 2 h before the solvent was

reduced to a minimum in vacuo. Recrystallization at -35 °C yielded 5-5 as a crystalline orange

solid in excellent yield (68.3 mg, 98% yield).

31P{1H} NMR (162 MHz, C6D6): δ -33.2 ppm (t, 1JPF = 874 Hz). 19F NMR (377 MHz, C6D6):

δ -43.6 (d, br, 1JPF = 874 Hz, 1F, PF), -133.9 – -135.4 (m, 2F, o-C6F5), -150.3 – -152.4 (m, 1F, p-

C6F5), -159.2 – -160.9 (m, 2F, m-C6F5).

187

Synthesis of [(C6F5)PF2(κ2-C20H12)][B(C6F5)4] (5-6): To a stirred slurry of

[SiEt3][B(C6F5)4]·PhMe (86.9 mg, 0.1 mmol) in toluene (2 mL) in a 4 dram vial, was added a

solution of 5-5 (48.8 mg, 0.1 mmol) in toluene (2 mL). The white slurry rapidly changed to a dark

red oily solution. The reaction stirred for 2 h before removing the solvent in vacuo to yield a sticky

red residue. The red residue was washed with several washes of cold pentane (3 x 1 mL), before

being triturated in pentane to yield 5-6 as a red powder in excellent yield (108.3 mg, 94% yield).

31P{1H} NMR (162 MHz, CD2Cl2) δ 82.5 ppm (d, 1JPF = 1080 Hz). 19F NMR (377 MHz, CD2Cl2)

δ -123.8 (q, 4JFF = 14 Hz, 2F, P(o-C6F5)), -126.9 (dt, 1JPF = 1080 Hz, 4JFF = 13 Hz, 1F, PF), -130.2

– -130.4 (m, 1F, P(p-C6F5)), -133.01 (s, 8F, B(o-C6F5)), -152.7 (t, 3JFF = 21 Hz, 2F, P(m-C6F5)), -

163.7 (t, 3JFF = 21 Hz, 4F, P(p-C6F5)), -167.5 (t, 3JFF = 14 Hz, 8F, P(m-C6F5)).


a solution of HSiEt3 (11.6 mg, 0.1 mmol) in CH2Cl2 (0.7 mL). The solution was added to a vial


tube for monitoring by NMR spectroscopy.


was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7 mL). The solution was added to


NMR tube for monitoring by NMR spectroscopy.


of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in CH2Cl2 (0.7 mL). The solution was then

transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

188













anisotropically.

189


5-4

Formula C26H12F5OP

Weight (g/mol) 466.35

Crystal system triclinic

Space group P-1

a (Å) 8.1019(19)

b (Å) 10.671(3)

c (Å) 12.378(3)

α (°) 105.067(12)

β (°) 98.590(12)

γ (°) 102.193(11)

Volume (Å3) 986.2(4)

Z 2

Density (calcd.) (gcm–3) 1.570

R(int.) 0.0367

μ, mm–1 0.2040

F(000) 473

Index ranges -10 ≤ h ≤ 10

-13 ≤ k ≤ 13

-15 ≤ l ≤ 15

Radiation Mo Kα

θ range (min, max) (°) 2.25, 27.10

Total data 15198

Max peak 0.4

Min peak -0.3

>2(FO2) 3618

Variables 298

R (>2σ) 0.0338

Rw 0.0882

GoF 1.031

190

5.5 References

1. Lenz, W., A Short History of Thalidomide Embryopathy. Teratology 1988, 38, 203.

2. Eriksson, T.; Bjorkman, S.; Roth, B.; Fyge, A.; Hoglund, P., Stereospecific Determination,

Chiral Inversion in-Vitro and Pharmacokinetics in Humans of the Enantiomers of Thalidomide.

Chirality 1995, 7, 44.

3. Bhat, V.; Welin, E. R.; Guo, X. L.; Stoltz, B. M., Advances in Stereoconvergent Catalysis

from 2005 to 2015: Transition-Metal-Mediated Stereoablative Reactions, Dynamic Kinetic

Resolutions, and Dynamic Kinetic Asymmetric Transformations. Chem. Rev. 2017, 117, 4528.

4. Marckwald, W., Asymmetrical Synthesis. Chem. Ber. 1904, 37, 349.

5. Knowles, W. S.; Sabacky, M. J., Catalytic Asymmetric Hydrogenation Employing a

Soluble Optically Active Rhodium Complex. Chem. Commun. 1968, 1445.

6. Zhang, Z. F.; Butt, N. A.; Zhang, W. B., Asymmetric Hydrogenation of Nonaromatic

Cyclic Substrates. Chem. Rev. 2016, 116, 14769.

7. Cherney, A. H.; Kadunce, N. T.; Reisman, S. E., Enantioselective and Enantiospecific

Transition-Metal-Catalyzed Cross-Coupling Reactions of Organometallic Reagents To Construct

C-C Bonds. Chem. Rev. 2015, 115, 9587.

8. Du, X. Y.; Huang, Z., Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS

Catal, 2017, 7, 1227.

9. Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.; Souchi, T.; Noyori, R.,

Synthesis of 2,2'-Bis(Diphenylphosphino)-1,1'-Binaphthyl (Binap), an Atropisomeric Chiral

Bis(Triaryl)Phosphine, and Its Use in the Rhodium(I)-Catalyzed Asymmetric Hydrogenation of

Alpha-(Acylamino)Acrylic Acids. J. Am. Chem. Soc. 1980, 102, 7932.

10. Ojima, I., Catalytic Asymmetric Synthesis. 2nd ed.; Wiley-VCH: New York, 2000; p 864.

11. Aaseth, J.; Criponi, G.; Andersen, O., Chelation Therapy in the Treatment of Metal

Intoxication. Academic Press is an imprint of Elsevier: London, UK ; San Diego, CA, USA, 2016;

p 371.

12. Chen, D.; Schmitkamp, M.; Franciò, G.; Klankermayer, J.; Leitner, W., Enantioselective

Hydrogenation with Racemic and Enantiopure BINAP in the Rresence of a Chiral Ionic Liquid.

Angew. Chem., Int. Ed. Engl. 2008, 47, 7339.

191

13. Chen, D.; Wang, Y.; Klankermayer, J., Enantioselective Hydrogenation with Chiral

Frustrated Lewis Pairs. Angew. Chem., Int. Ed. Engl. 2010, 49, 9475.

14. Liu, Y.; Du, H., Chiral Dienes as "Ligands" for Borane-Catalyzed Metal-Free Asymmetric

Hydrogenation of Imines. J. Am. Chem. Soc. 2013, 135, 6810.

15. Ren, X. Y.; Du, H. F., Chiral Frustrated Lewis Pairs Catalyzed Highly Enantioselective

Hydrosilylations of 1,2-Dicarbonyl Compounds. J. Am. Chem. Soc. 2016, 138, 810.

16. He, R. J.; Wang, X. S.; Hashimoto, T.; Maruoka, K., Binaphthyl-Modified Quaternary

Phosphonium Salts as Chiral Phase-Transfer Catalysts: Asymmetric Amination of β-Keto Esters.

Angew. Chem., Int. Ed. 2008, 47, 9466.

17. Holthausen, M. H.; Hiranandani, R. R.; Stephan, D. W., Electrophilic Bis-

Fluorophosphonium Dications: Lewis Acid Catalysts from Diphosphines. Chem. Sci. 2015, 6,

2016.

18. Hudnall, T. W.; Kim, Y. M.; Bebbington, M. W. P.; Bourissou, D.; Gabbaï, F. P., Fluoride

Ion Chelation by a Bidentate Phosphonium/Borane Lewis Acid. J. Am. Chem. Soc. 2008, 130,

10890.

19. Mobus, J.; vom Stein, T.; Stephan, D. W., Cooperative Lewis Acidity in Borane-

Substituted Fluorophosphonium Cations. Chem. Commun. 2016, 52, 6387.



21. Liu, L.; Wu, H. C.; Yu, J. Q., Improved Syntheses of Phosphine Ligands by Direct

Coupling of Diarylbromophosphine with Organometallic Reagents. Chem. Eur. J. 2011, 17,

10828.

22. Dore, A.; Fabbri, D.; Gladiali, S.; Delucchi, O., Dinaphtho[2,1-B-1',2'-D]Phospholes - a

New Class of Atropisomeric Phosphorus Ligands. J. Chem. Soc., Chem. Commun. 1993, 1124.

23. Hill, M. S.; Liptrot, D. J.; Weetman, C., Alkaline Earths as Main Group Reagents in

Molecular Catalysis. Chem. Soc. Rev. 2016, 45, 972.






192
























Chem. Phys. Lett. 1988, 153, 503.





29. Chase, P. A.; Piers, W. E.; Patrick, B. O., New Fluorinated 9-Borafluorene Lewis Acids.

J. Am. Chem. Soc. 2000, 122, 12911.

193


Soc. 1966, 88, 4533.

31. Connelly, S. J.; Kaminsky, W.; Heinekey, D. M., Structure and Solution Reactivity of

(Triethylsilylium)triethylsilane Cations. Organometallics 2013, 32, 7478.




339.

194

Chapter 6 Conclusion

6.1 Future Work

While using hypervalent iodine reagents to effect nitrene insertion into electrophilic boranes was

a successful method towards tuning Lewis acidity, the resulting aminoboranes were unreactive in

FLP chemistry. Instead of trying to quench the reactivity of the nitrogen substituent with electron-

withdrawing sulfonyl groups and C6F5 substituents, an interesting avenue of research would be

trying to promote reactivity with a more Lewis basic and less sterically encumbered nitrogen

substituent. This could be achieved by cleavage of the sulfonyl groups, using common sulfonyl

deprotection methods to yield the secondary or primary aminoborane.

Perchloroaryl substituted phosphines have been reported to undergo functionalization at the para-

position when lithiated and treated with chlorotrimethylsilane.1 This functionalization could be

explored as a means to tune the electrophilicity or solubility properties of subsequently generated

phosphonium cations. Additional Lewis acidic or basic sites could be incorporated to investigate

cooperative behavior or to effect intramolecular FLP chemistry.

During the air-stability tests of fluorophosphonium salts in Chapter 3 and 4, decomposition of the

cation was typically observed, while the [B(C6F5)4]- anion was left intact. However, evidence of

decomposition of the [B(C6F5)4]- anion reactivity was observed in the recrystallization of highly

electrophilic 4-23 and 4-24. In the case of 4-23, the solid-state structure contained a

[(OH)B(C6F5)3] anion, consistent with the loss of a C6F5 substituent. Whereas attempts to

recrystallize 4-24 resulted in the generation of corresponding difluorophosphorane 4-17, and

apparent decomposition of the [B(C6F5)4]-. These observations suggest that employing an

alternative, more robust anions is instrumental in developing more stable fluorophosphonium

catalysts. Fluorophosphonium carboranate compounds could be generated from

difluorophosphoranes by fluoride abstraction using silylium carboranate salts. Carborane anions

play a critical role in silylium chemistry, allowing for both aryl C-F bond activation and ring-

opening polymerization chemistry that is otherwise inaccessible.2,3 When 4-24 decomposes to 4-

17, there was no apparent degradation of the CF3 substituents. The observed decomposition of the

[B(C6F5)4] anion of 4-24 suggest that the tris[3,5-bis(trifluoromethyl)phenyl]fluorophosphonium

195

cation could be a stronger Lewis acid than [PF(C6F5)3][B(C6F5)4], without the drastically increased

steric bulk associated with C12F9 groups. Additionally, no degradation of trifluoromethyl groups

in 4-24 was observed, despite reports of [PF(C6F5)3][B(C6F5)4] readily activating trifluoromethyl

groups.4,5

Further, employing 3,5-bis(trifluoromethyl) substituents could also be effective with respect to the

binaphthyl chemistry in Chapter 5. Both the 3,5-bis(trifluoromethyl) and 4-trifluoromethyl

analogues of BINAP have been reported.6 Since attempts to generate the C6F5 substituted BINAP

analogue resulted in the formation of 5-3, the trifluoromethyl analogues be used as starting

materials in the synthesis of a highly active chiral bis(fluorophosphonium) cation. Additionally,

binaphthyl substituted phosphorus compounds 5-4 – 5-6 display interesting fluorescence

properties. Lewis acidic 5-6 could be investigated as a possible chemical recognition fluorescence

sensor.

6.2 Summary

In chapter 2, a series of iodine reagents were used to effect nitrene insertion into the B-C bonds of

electrophilic boranes. Insertion of the nitrene was used as a methodology to tune the Lewis acidity

of a boron centre, while avoiding dangerous tin or lithium reagents required generally used in the

synthesis of new electrophilic boranes. The reaction between PhI=NTs with BCF yielded the single

insertion product (C6F5)2BN(Ts)C6F5 (2-1). The Gutmann-Beckett method was administered by

treating 2-1 with OPEt3 to yield the adduct (C6F5)2BN(Ts)C6F5·OPEt3 (2-2). Similarly, the reaction

between PhI=NTs with PhB(C6F5) or HB(C6F5) yields insertion products (C6F5)2BN(Ts)Ph (2-3)

and (C6F5)2BN(Ts)H (2-4) respectively. The effect of varied nitrene substitutions on the

electrophilicity of HB(C6F5)2 was also investigated. Iodine reagents PhI=NCls, PhI=NMs, and

PhI=NNs were each reacted with HB(C6F5)2 to generate (C6F5)2BN(Cls)H (2-5), (C6F5)2BN(Ms)H

(2-6), and (C6F5)2BN(Ns)H (2-7) respectively. Aminoboranes 2-4 – 2-7 could similarly be

generated from ClB(C6F5)2 with the corresponding iodine reagent. The Gutmann-Beckett acceptor

numbers of 2-1, 2-3 – 2-7 indicate enhanced Lewis acidity of the insertion products with respect

to the respective borane starting materials. Further, the series 2-4 – 2-7 displayed a predictable

trend of increasing Lewis acidity based on nitrogen substitution of 2-6 < 2-4 < 2-5 < 2-7. The range

of electrophilicity of the aminoboranes tuning (ΔAN = 2.0) is less than difference in Lewis-acidity

between BCF and PhB(C6F5)2 (ΔAN = 3.1). The reactivity of iodine reagents with electron-

196

deficient phosphenes was investigated as a means of generating Lewis acidic phosphinimines.

Iodine reagent PhI=NTs was reacted with P(C6F5)Ph2 and P(C6F5)3 to yield TsN=P(C6F5)Ph2 (2-8)

and TsN=P(C6F5)3 (2-9) respectively. Both 2-8 and 2-9 proved to be sensitive to hydrolysis, readily

forming the corresponding oxides. While nitrene insertion proved to be an effective method to tune

the Lewis acidity of electrophilic boranes, the relative inactivity of the resulting aminoboranes

precludes application of this method to FLP chemistry.

Chapter 3 explored the incorporation of C6Cl5 rings into fluorophosphonium systems to increase

air-stability. The reaction of LiC6Cl5 with either Ph2PCl, PhPCl2, PBr3, (C6F5)2PBr or (C6F5)PBr2

in appropriate molar ratios resulted in phosphines Ph2P(C6Cl5) (3-1), PhP(C6Cl5)2 (3-2),

(C6F5)P(C6Cl5)2 (3-4), or (C6F5)2P(C6Cl5) (3-5) respectively. The reaction of PCl3 with three

equivalents of LiC6Cl5 yielded coupled 1,2-diphosphine ((C6Cl5)2P)2 was the major product and

3-3 as a minor product. Perchlorophenyl phosphines 3-1, 3-2, 3-4, and 3-5 were oxidized using

XeF2 to the corresponding difluorophosphoranes Ph2PF(C6Cl5) (3-6), PhPF2(C6Cl5) (3-7),

(C6F5)PF2(C6Cl5)2 (3-8), and (C6F5)2PF2(C6Cl5) (3-9). The oxidation of 3-3 generates a

trifluorophosphorane as the major product, resulting from loss of a C6Cl5 substituent.

Difluorophosphoranes 3-6 – 3-9 underwent fluoride abstraction by [SiEt3][B(C6F5)4] to generate

fluorophosphonium salts [Ph2PF(C6Cl5)][B(C6F5)4] (3-10), [PhPF(C6Cl5)2][B(C6F5)4] (3-11),

[(C6F5)PF(C6Cl5)2][B(C6F5)4] (3-12), and [(C6F5)2PF(C6Cl5)][B(C6F5)4] (3-13) respectively.

Phosphine 3-1 could be reacted with NFSI to directly generate fluorophosphonium

[Ph2PF(C6Cl5)][N(SO2Ph)2] (3-10NFSI). Treatment of 3-3 with Selectfluor failed to generate the

corresponding fluorophosphonium, instead forming OPF(C6Cl5)2 (3-14). Fluorophosphonium salts

3-10 – 3-13 were not amenable to analysis by the Gutmann-Beckett method. Instead,

computational methods GIE and FIA were used to evaluate the electrophilicity of 3-10 – 3-13. The

ω values of the fluorophosphonium cations gave the increasing electrophilicity trend 3-10 < 3-11

< 3-12 < 3-13, consistent with total electronegativity of aryl substituents. The ω values of 3-10 –

3-13 were found to linearly correlate with the experimentally obtained 31P{1H} NMR chemical

shifts. The FIA values were also found to be in good agreement with ω values. Compounds 3-10

– 3-13 were evaluated as catalysts in the dimerization of 1,1-diphenylethylene,

hydrodefluorination of 1-fluoroadamantane, deoxygenation of benzophenone, dehydrocoupling of

phenol and HSiEt3, hydrosilylation of α-methylstyrene with HSiEt3, and benzylation and

hydrodefluorination of 4-trifluoromethylbromobenzene. Reactivity trends were consistent with

197

both computed and experimentally obtained electrophilicity trends, with Ph containing 3-10 and

3-11 performing worse than C6F5 substituted 3-12 and 3-13. Further, 3-10 and 3-11 were unable

to effect benzylation reactivity. The air-stability of the fluorophosphonium solutions was evaluated

and resulted in the following trend 3-11 > 3-10 > 3-12 > 3-13. Interestingly, despite 3-11 being

more electrophilic and a more active catalyst than 3-10, it was also found to be considerably more

air-stable. While perchloroaryl substituents were effective at increasing air-stability through steric

protection, the also resulted in a decrease in electrophilicity compared to the analogous

perfluoroaryl fluorophosphonium cations.

In Chapter 4, C12F9 substituents were incorporated into fluorophosphonium cations as a method of

enhancing electrophilicity. A series of C12F9 phosphines were generated by reaction of the lithium

reagent with the appropriate halophosphine: iPr2P(C12F9) (4-1), otol2P(C12F9) (4-2), Ph2P(C12F9)

(4-3), (C6F5)2P(C12F9) (4-4), (3,5-(CF3)2C6H3)2P(C12F9) (4-5), Cy2P(C12F9) (4-6), Me2P(C12F9)

(4-7), tBu2P(C12F9) (4-8), MeP(C12F9)2 (4-9) and PhP(C12F9)2 (4-10). Reaction with zinc reagent

Zn(C12F9)2·PhMe with PBr3 and PhPCl2 yielded 1,2-diphosphines ((C6Cl5)2P)2 (4-11), and

((C12F9)PhP)2 (4-12). Difluorophosphoranes iPr2PF2(C12F9) (4-13), otol2PF2(C12F9) (4-14),

Ph2PF2(C12F9) (4-15), (C6F5)2PF2(C12F9) (4-16), (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17),

Cy2PF2(C12F9) (4-18), and Me2PF2(C12F9) (4-19) were each generated by reaction of the

corresponding phosphines with XeF2. Fluoride abstraction from 4-13 – 4-18 yielded

fluorophosphonium cations [iPr2PF(C12F9)][B(C6F5)4] (4-20), [otol2PF(C12F9)][B(C6F5)4] (4-21),


(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24), and [Cy2PF(C12F9)][B(C6F5)4] (4-25). Of 4-20 – 4-23,

only 4-23 was compatible with the Gutmann-Beckett protocol, which indicated Lewis acidity

slightly higher than [PF(C6F5)3][B(C6F5)4]. The FIA values for 4-20 – 4-23 showed only slight

difference between 4-20 – 4-22, while 4-23 displayed a higher FIA value than

[PF(C6F5)3][B(C6F5)4]. A competition experiment further confirmed the greater FIA of 4-23

compared to [PF(C6F5)3][B(C6F5)4]. The catalytic activity of 4-20 – 4-23 was evaluated in a series

of test reactions to give the trend 4-21 < 4-20 < 4-22 < 4-23. The catalytic activity of 4-23 was

found to be lower than [PF(C6F5)3][B(C6F5)4] despite the higher electrophilicity, likely a result of

increased steric bulk. Similarly, compound 4-21 displayed considerably lower catalytic activity

than 4-20 and 4-22, despite comparable activity, due to high steric bulk at the phosphorus centre.

The air-stability of 4-20 – 4-23 was evaluated and gave the trend of increasing stability of 4-23 <

198

4-22 < 4-20 < 4-21. Interestingly, inclusion of perfluorobiphenyl substituents was an effective

strategy in the development of monocationic fluorophosphonium species that are more

electrophilic than the corresponding perfluorophenyl species.

Chapter 5 developed chiral fluorophosphonium cations by incorporation of binaphthyl

substituents. Oxidation of BINAP with two molar equivalents of XeF2 results in the formation of

binaphthyl bis(difluorophosphorane) (Ph2F2P)2(C20H12) (5-1). The corresponding

bis(fluorophosphonium) cation [((Ph2FP)2C10H6)2]2[B(C6F5)4] (5-2) was generated by fluoride

abstraction of 5-1 using [SiEt3][B(C6F5)4]. Compound 5-2 was found to be an effective catalyst in

the dimerization of 1,1-diphenylethylene, and the hydrodefluorination of 1-fluoroadamantane, but

was unable to effect hydrosilylation reactivity. In an effort to generate a more electrophilic chiral

catalyst, a binaphthyl Grignard reagent was reacted with two equivalents of halophosphine

BrP(C6F5)2. Instead of the expected bis(phosphine), (C6F5)P(κ2-C20H12) (5-3) was formed with

concomitant formation of P(C6F5)3. Phosphine 5-3 was oxidized in air to yield phosphine oxide

(C6F5)PO(κ2-C20H12) (5-4) and by XeF2 to generate (C6F5)PF2(κ2-C20H12) (5-5). The

corresponding fluorophosphonium [(C6F5)PF(κ2-C20H12)][B(C6F5)4] (5-6) was subsequently

generated from 5-5. Despite the addition of a perfluorophenyl substituent, the measured FIA of

5-6 was considerably lower than comparable fluorophosphonium cations bearing a single

perhaloaryl substituent, likely a result of the constrained geometry.

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6.3 References

1. Dua, S. S.; Edmondson, R. C.; Gilman, H., Polyhaloaryl Compounds Containing

Phosphorus. J. Organomet. Chem. 1970, 24, 703.

2. Duttwyler, S.; Douvris, C.; Fackler, N. L. P.; Tham, F. S.; Reed, C. A.; Baldridge, K. K.;

Siegel, J. S., C-F Activation of Fluorobenzene by Silylium Carboranes: Evidence for Incipient

Phenyl Cation Reactivity. Angew. Chem., Int. Ed. 2010, 49, 7519.

3. Zhang, Y.; Huynh, K.; Manners, I.; Reed, C. A., Ambient Temperature Ring-Opening

Polymerisation (ROP) of Cyclic Chlorophosphazene Trimer [N3P3Cl6] Catalyzed by Silylium

Ions. Chem. Commun. 2008, 494.



341, 1374.

5. Zhu, J.; Perez, M.; Caputo, C. B.; Stephan, D. W., Use of Trifluoromethyl Groups for

Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination. Angew. Chem., Int.

Ed. 2016, 55, 1417.

6. Liu, L.; Wu, H. C.; Yu, J. Q., Improved Syntheses of Phosphine Ligands by Direct

Coupling of Diarylbromophosphine with Organometallic Reagents. Chem. Eur. J. 2011, 17,

10828.

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