Bifunctional Systems in the Chemistry of Frustrated Lewis ... · Bifunctional Systems in the...

Bifunctional Systems in the Chemistry of Frustrated Lewis Pairs

by

Xiaoxi Zhao

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Chemistry University of Toronto

© Copyright by Xiaoxi Zhao 2012

ii

Bifunctional Systems in the Chemistry of Frustrated Lewis Pairs

Xiaoxi Zhao

Doctor of Philosophy

Department of Chemistry University of Toronto

2012

Abstract

Three classes of bifunctional compounds related to frustrated Lewis pair chemistry were

studied. The first class, alkynyl-linked phosphonium borates, was strategically

synthesized and the corresponding neutral alkynyl-linked phosphine boranes generated

in solution. They were reacted with THF, alkenes and alkynes to undergo either ring-

opening or multiple bond addition reactions, giving rise to zwitterionic macrocycles. In

two select alkynyl-linked phosphonium borates, thermolysis resulted in unique

rearrangements transforming the phosphino- and boryl-substituted alkynyl moieties into

C4 chains. The alkynyl-linked phosphine boranes were further demonstrated to

coordinate as η3-BCC ligands in Ni(0) complexes. The rigid nature of the coordination

was confirmed by dimerization without cleavage of the Ni–B interaction upon the

addition of acetonitrile or carbon monoxide. Moreover, reactions with Al-, Zn- and B-

based Lewis acids prompted hydride transfer within the alkynyl-linked phosphonium

borate and interesting functional group transfer reactions.

The second class of the bifunctional systems, a series of gem-substituted bis-boranes,

was subjected to reactions with tBu3P and CO2. The O-linked bis-borane was shown to

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coordinate the phosphino-carboxylate moiety with one B, while the methylene-linked

bis-boranes were demonstrated to chelate the carboxyl group.

The third bifunctional system class, vinyl-group tethered boranes, was examined to

elucidate the mechanism of the frustrated Lewis pair addition reaction to olefins. Using a

bis(pentafluorophenyl)alkylborane, the close proximity of the olefinic protons and the

ortho-fluorine nuclei were evident by both NOE measurements and DFT calculations.

Moreover, its reactions with phosphine bases suggested that an initial interaction

between the highly electrophilic borane and the olefinic fragment precedes such

frustrated Lewis pair addition reaction. Furthermore, a

bis(pentafluorophenyl)alkoxyborane was synthesized and reacted with P-, N-, C- and H-

based nucleophiles, demonstrating the wide range of Lewis bases that can be applied in

olefin addition reactions with complementary regioselectivity.

iv

Acknowledgements

I would like to express my sincere gratitude first and foremost to Professor Doug

Stephan for letting me work in his laboratory, and giving me invaluable suggestions,

encouragement and guidance over the past four years.

I would also like to take this opportunity to thank all of the past and present members of

the Stephan group who made my graduate years a wonderful time. Without the great

amount of knowledge in chemistry, laboratory, instrumental and writing skills that they

kindly shared with me, I would not have been able to complete this thesis. I would

especially like to thank Dr. Alberto Ramos and Dr. Edwin Otten for their valuable advice

that they gave me when I was a new student with limited ideas on the chemistry I was

doing.

I would also like to express my appreciation to Professor Bob Morris and Professor

Datong Song for serving on my supervisory committee. Lastly, I would like to thank my

beloved family and friends for the great amount of support they provided me throughout

the course of the years.

v

Table of Contents

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

Table of Contents............................................................................................................. v

List of Schemes ............................................................................................................... x

List of Figures ............................................................................................................... xiii

List of Tables.................................................................... Error! Bookmark not defined.

List of Abbreviations, Nomenclature and Symbols ....................................................... xvii

1 Introduction .................................................................................................................. 1

1.1 Frustrated Lewis Pairs with Dihydrogen ................................................................ 1

1.1.1 Heterolytic Splitting of Hydrogen and Hydrogenation Reactions ................ 1

1.1.2 Transition Metal Centers and Frustrated Lewis Pairs in H2 Activation ........ 2

1.2 Reactions of Frustrated Lewis Pairs with Multiple Bonds and Cyclic Compounds ........................................................................................................... 7

1.2.1 Alkenes and Cycloalkanes ......................................................................... 7

1.2.2 Carbon Dioxide, Nitrous Oxide, Nitric Oxide and Carbon monoxide........... 7

1.3 Reactions of Alkynes with Frustrated Lewis Pairs ................................................. 9

1.3.1 Reactions of Trialkylalkynylborates with Electrophiles ................................ 9

1.3.2 1,1-Carboboration Reactions - Revisited .................................................. 10

1.3.3 Reactions of Lewis Acid-Base Pairs with Alkynes .................................... 10

1.4 Objectives of This Thesis .................................................................................... 12

2 Synthesis and Frustrated Lewis Pair Reactivity of Alkynyl-Linked Phosphine Boranes ..................................................................................................................... 14

2.1 Introduction ......................................................................................................... 14

2.1.1 Intramolecular Frustrated Lewis Pair Systems ......................................... 14

2.1.2 Previously Reported Alkynyl-Linked P/B Compounds .............................. 14

2.2 Results and Discussion ....................................................................................... 15

vi

2.2.1 Synthesis of Ethynylphosphines ............................................................... 15

2.2.2 Synthesis of Alkynyl-Linked Phosphonium Borates .................................. 16

2.2.3 Attempted Use of the Alkynyl-Linked P/B Compounds in Hydrogen Activation and Hydrogenation Catalysis ................................................... 24

2.2.4 Zwitterionic Macrocycles of Alkynyl-Linked Phosphonium Borates .......... 33

2.2.5 Synthesis of Borataallene Compounds ..................................................... 45

2.3 Conclusions ......................................................................................................... 47

2.4 Experimental Section .......................................................................................... 48

2.4.1 General Considerations ............................................................................ 48

2.4.2 Syntheses ................................................................................................. 48

2.4.3 X-ray Crystallography ............................................................................... 64

3 Reactivity of Alkynyl-Linked Phosphonium Borates Toward Main-Group and Transition Metal Species ........................................................................................... 73

3.1 Introduction ......................................................................................................... 73

3.1.1 Alkyne and Phosphine Complexes of Transition Metals ........................... 73

3.1.2 Coordination Chemistry of Borane Ligands .............................................. 73

3.2 Results and Discussion ....................................................................................... 75

3.2.1 Syntheses of Ni(0) Complexes of Alkynyl-Linked Phosphine Boranes ..... 75

3.2.2 Computational Analysis of a Ni(0) Complex of Alkynyl-Linked Phosphine Boranes .................................................................................. 82

3.2.3 Reactions of an Alkynyl-Linked Phosphonium Borate with Al-, B-, and Zn-Based Lewis Acids .............................................................................. 85

3.3 Conclusions ......................................................................................................... 95

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

3.4.1 General Considerations ............................................................................ 95

3.4.2 Syntheses ................................................................................................. 96

3.4.3 X-ray Crystallography ............................................................................. 102

vii

4 Bis-Boranes in Frustrated Lewis Pair Chemistry ..................................................... 107

4.1 Introduction ....................................................................................................... 107

4.1.1 Activation of Carbon Dioxide by Frustrated Lewis Pairs ......................... 107

4.1.2 Chelating Bis-boranes ............................................................................ 107

4.2 Results and Discussion ..................................................................................... 109

4.2.1 Examination of a Borinic Anhydride in FLP Reactions ............................ 109

4.2.2 Diborylmethylene Compounds in FLP Carbon Dioxide Activation .......... 119

4.3 Conclusions ....................................................................................................... 123

4.4 Experimental Section ........................................................................................ 124

4.4.1 General Considerations .......................................................................... 124

4.4.2 Syntheses ............................................................................................... 124

4.4.3 X-ray Crystallography ............................................................................. 128

5 Tethered Olefin-Borane “van der Waals Complexes” and Frustrated Lewis Pair Addition Reactions Using Various Nucleophiles ...................................................... 133

5.1 Introduction ....................................................................................................... 133

5.1.1 Olefin Addition Reactions by Frustrated Lewis Pairs .............................. 133

5.1.2 Interactions and Reactions of Olefins and Boranes ................................ 134

5.1.3 Intramolecular Stabilization of d0 Metal-Olefin Interactions..................... 135

5.2 Results and Discussion ..................................................................................... 136

5.2.1 The Bis(pentafluorophenyl)alkylborane System – Computational Approach ................................................................................................ 136

5.2.2 The Bis(pentafluorophenyl)alkylborane System – NMR Spectroscopic Investigations ......................................................................................... 138

5.2.3 The Bis(pentafluorophenyl)alkylborane System – Addition Reactions.... 141

5.2.4 The Bis(pentafluorophenyl)alkoxyborane System –Synthesis, DFT Calculations and NMR Spectroscopic Analysis ...................................... 145

5.2.5 The Bis(pentafluorophenyl)alkoxyborane System – Addition Reactions Employing Various Nucleophiles ............................................................ 147

viii

5.3 Conclusions ....................................................................................................... 159

5.4 Experimental Section ........................................................................................ 160

5.4.1 General Considerations .......................................................................... 160

5.4.2 NMR measurements ............................................................................... 161

5.4.3 Syntheses ............................................................................................... 163

5.4.4 DFT calculations ..................................................................................... 172

5.4.5 X-ray crystallography .............................................................................. 173

6 Conclusions ............................................................................................................. 180

6.1 Summary of the Thesis ..................................................................................... 180

6.2 Future Work ...................................................................................................... 180

References .................................................................................................................. 182

ix

List of Tables Table 2.1 – Selected crystallographic data for (2-2), (2-3), and (2-4). ....................................... 66

Table 2.2 – Selected crystallographic data for (2-5), (2-6) and (2-9). ........................................ 67

Table 2.3 – Selected crystallographic data for (2-10), (2-11) and (2-12). ................................... 68



Table 2.6 – Selected crystallographic data for (2-20), (2-21a) and (2-21b). ............................... 71

Table 2.7 – Selected crystallographic data for (2-22) and (2-23). .............................................. 72

Table 3.1 – Comparison of bond parameters and IR stretches among [tBu2PC≡CB(C6F5)2]calc, (3-

1), (3-1calcd), (3-3) and (3-4). ............................................................................................. 85

Table 3.2 – Selected crystallographic data for (3-1), (3-3) and (3-4). ...................................... 104


Table 3.4 – Selected crystallographic data for (3-8) and (3-9). ................................................ 106

Table 4.1 – Comparison of salient bond parameters and IR stretching modes among CO2

complexes of FLPs. ........................................................................................................ 123



Table 4.4 – Selected crystallographic data for (4-8) and (4-9). ................................................ 132

Table 5.1 – Calculated differences in Gibbs free energies and total electronic energies between

(5-1a) and (5-1b)............................................................................................................. 138

Table 5.2 – Selected crystallographic data for (5-4), (5-5), and (5-6). ..................................... 175

Table 5.3 – Selected crystallographic data for (5-7), (5-10), and (5-11)................................... 176

Table 5.4 – Selected crystallographic data for (5-12), (5-13), and (5-14). ................................ 177

Table 5.5 – Selected crystallographic data for (5-15) and (5-16). ............................................ 178

Table 5.6 – Selected crystallographic data for (5-17) and (5-18). ............................................ 179

x

List of Schemes Scheme 1.1 – Heterolytic splitting of H2 by a frustrated Lewis pair. ........................................... 1

Scheme 1.2 – Catalytic cycle of hydrogenation of bulky imines using B(C6F5)3. ......................... 2

Scheme 1.3 – Stoichiometric hydrogenation of anilines with B(C6F5)3. ....................................... 2

Scheme 1.4 – H2 cleavage by transition metal centers. ................................................................ 3

Scheme 1.5 – Reactions of pentaarylboroles with H2. ................................................................. 6

Scheme 1.6 – Reactions of FLPs with alkenes and cycloalkanes. ................................................ 7

Scheme 1.7 – Reactions of FLPs with carbon dioxide, nitrous oxide, nitric oxide and carbon

monoxide. ........................................................................................................................... 8

Scheme 1.8 – 1,1-Carboboration reaction of alkynylmetallic compounds by trialkylboranes. ...... 9

Scheme 1.9 – 1,1-Carboboration reaction of bis(alkynyl)lead compounds by trialkylboranes. ..... 9

Scheme 1.10 – 1,1-Carboboration reactions of terminal and internal alkynes. ........................... 10

Scheme 1.11 – Reactions of terminal alkynes with frustrated Lewis pairs. ................................ 11

Scheme 1.12 – Synthesis of zwitterionic ladder stilbene compounds. ........................................ 12

Scheme 2.1 – Examples of intramolecular frustrated Lewis pair systems that heterolytically

cleave H2. ......................................................................................................................... 14

Scheme 2.2 – Preparation of alkynyl-linked P/B compounds in literature. ................................ 15

Scheme 2.3 – Syntheses of ethynylphosphines (2-1) and (2-2). ................................................. 16

Scheme 2.4 – Syntheses of (2-3) and (2-4). ............................................................................... 17



Scheme 2.7 – Synthesis of (2-9). .............................................................................................. 22

Scheme 2.8 – Generation of Mes2PC≡CB(C6F5)2 and synthesis of (2-10). ................................ 24

Scheme 2.9 – Proton and hydride transfer from (2-6) to an imine and generation of (2-11). ...... 25

Scheme 2.10 – Thermal rearrangement reaction of (2-6) to (2-12). ........................................... 26

Scheme 2.11 – Proposed mechanism of thermal formation of (2-12) from (2-6). ...................... 28

Scheme 2.12 – Formation of (2-13a) and (2-13b)...................................................................... 30

Scheme 2.13 – Reaction of Mes2PC≡CB(C6F5)2 to form (2-14). .............................................. 31

Scheme 2.14 – Proposed mechanism for the formation of (2-14). ............................................. 32

xi

Scheme 2.15 – Removal of proton and hydride from (2-6) and synthesis of (2-15). .................. 33

Scheme 2.16 – Synthesis of (2-16)............................................................................................ 35

Scheme 2.17 – Synthesis of (2-17) and (2-18). ......................................................................... 36

Scheme 2.18 – Reaction of (2-17) with THF to form (2-19). ..................................................... 38

Scheme 2.19 – Reaction of (2-17) with an excess of 1-hexene at 80 °C. ................................... 40

Scheme 2.20 – Thermolysis of (2-17) to generate (2-21)........................................................... 42



Scheme 3.1 – Activation of H2 and hydrogenation of alkenes by a Ni complex with a

diphosphine-borane ligand. ............................................................................................... 75


Scheme 3.3 – Alternative synthesis of (3-1). ............................................................................. 78


Scheme 3.5 – Generation of (3-3) and (3-4) from (3-1). ............................................................ 80

Scheme 3.6 – Reaction of (2-6) with aluminum halides to form (3-5) and (3-6). ....................... 86

Scheme 3.7 – Reaction of (2-6) with Zn(C6F5)2 and synthesis of (3-7). ..................................... 88

Scheme 3.8 – Reaction of (2-6) with Al(C6F5)3 and synthesis of (3-8). ..................................... 89

Scheme 3.9 – Reaction of (2-6) with HB(C6F5)2 and synthesis of (3-9). .................................... 91

Scheme 3.10 – Synthesis of (3-11)............................................................................................ 94


Scheme 4.1 – Synthesis of (4-1). ............................................................................................ 110

Scheme 4.2 – Generation of (4-2) by reaction of (4-1) and tBu3P with CO2. ........................... 112


Scheme 4.4 – Generation of (4-4) and its reaction with tBu3P. ................................................ 115

Scheme 4.5 – Reactions of (4-1) with a nitrile and bases......................................................... 116

Scheme 4.6 – Catalytic hydrogenation of N-benzylidene-tert-butylamine by (4-1). ................. 118

Scheme 4.7 – Reaction of (4-1) and tBu3P with N2O. ............................................................. 118

Scheme 4.8 – Reaction of (4-7) and tBu3P with CO2 to form (4-8).......................................... 120

Scheme 4.9 – Generation of (4-9) and its subsequent reaction with tBu3P and CO2. ................ 121

xii

Scheme 5.1 – Examples of addition reactions of frustrated Lewis pairs to olefins. .................. 133

Scheme 5.2 – Proposed pathways of addition to ethylene by tBu3P and B(C6F5)3. ................... 134

Scheme 5.3 – Allyl group transfer in the reaction of B(C6F5)3 and Sn(C3H5)Bu3. .................... 135

Scheme 5.4 – Equilibria governing the formation of (5-1a) and (5-1b). ................................... 139

Scheme 5.5 – Syntheses of (5-4), (5-5) and (5-6). ................................................................... 142



Scheme 5.8 – Reactions of (5-9) with tBu3P and Me3P. .......................................................... 148


Scheme 5.10 – Synthesis of (5-14a) and (5-15a) and 2,1-proton migration in DMSO-d6. ........ 152

Scheme 5.11 – Synthesis of (5-16) and (5-17). ....................................................................... 153

Scheme 5.12 – Reaction of (5-9) with [tBu3PH][HB(C6F5)3] and synthesis of (5-18). ............. 156

Scheme 5.13 – Proposed reaction pathways for the hydride transfer from HB(C6F5)3 anion to (5-

9). ................................................................................................................................... 157

Scheme 6.1 – Proposed synthesis of an alkynyl-amino-borane. ............................................... 181

Scheme 6.2 – Proposed polymerization of a vinyl boryl ether. ................................................ 181

xiii

List of Figures Figure 1.1 – Electric field gradient generated in an encounter complex polarizing a H2 molecule.

........................................................................................................................................... 4

Figure 1.2 – Qualitative depiction of the frontier molecular orbitals in a classical Lewis adduct

and a frustrated Lewis pair. ................................................................................................. 5

Figure 1.3 – Qualitative depiction of the frontier molecular orbitals involved in the H2 splitting

reaction by a frustrated Lewis pair. ..................................................................................... 5

Figure 2.1 – POV-Ray depiction of (2-2). ................................................................................. 16

Figure 2.2 – POV-Ray depiction of (a) (2-3) and (b) (2-4). ....................................................... 18


Figure 2.4 – 1H NMR spectrum of (2-6) in CD2Cl2. .................................................................. 20



Figure 2.7 – POV-Ray depiction of (2-10). ............................................................................... 24



Figure 2.10 – Energy diagram of proposed mechanism of formation of (2-12) based on DFT

calculations. ...................................................................................................................... 29

Figure 2.11 – POV-Ray depiction of (2-13a). ........................................................................... 30

Figure 2.12 – POV-Ray depiction of (2-14). ............................................................................. 32

Figure 2.13 – 31P and 19F NMR spectra of tBu2PC≡CB(C6F5)2 generated in situ in C6D6. ......... 34



Figure 2.16 – 19F NMR of (2-17) at 25 oC (top) and -50 oC (bottom) in CD2Cl2. ....................... 37




Figure 2.20 – 19F NMR spectra of (2-21a) alone (top) and a mixture of (2-21a) and (2-21b)

(bottom) in CD2Cl2. .......................................................................................................... 42

xiv

Figure 2.21 – 31P-31P COSY spectrum of a mixture of (2-21a) and (2-21b) in CD2Cl2............... 43


Figure 2.23 - POV-Ray Depiction of (2-21b). ........................................................................... 44

Figure 2.24 – POV-Ray depictions of (a) (2-22) and (b) (2-23). ................................................ 47

Figure 3.1 – Examples of alkyne and alkynylphosphine complexes of Ni(0). ............................ 73

Figure 3.2 – Early examples of Lewis acids as σ-acceptors for transition metal centers............. 74


Figure 3.4 – 31P/31P{1H} spectra showing the reaction progress of (2-6) with Ni(COD)2 in

toluene-d8. ........................................................................................................................ 78

Figure 3.5 – 11B/11B{1H} spectra showing the reaction progress of (2-6) with Ni(COD)2 in

toluene-d8. ........................................................................................................................ 79



Figure 3.8 – GaussView depiction of pertinent molecular orbitals of (3-1calcd). ......................... 84

Figure 3.9 – (a) Molekel depiction of the NBO for the Ni→B interaction in (3-1calcd) (cutoff:

0.05), and NPA atomic charges and NAO Wiberg bond indices of (b) free ligand

[tBu2PC≡CB(C6F5)2]calcd and (c) (3-1calcd). ........................................................................ 84

Figure 3.10 – POV-Ray depiction of (a) (3-5) and (b) (3-6). ..................................................... 87




Figure 3.14 – Variable temperature 19F NMR spectra of (3-9) in bromobenzene-d5................... 92


Figure 4.1 – CO2 complexation by frustrated Lewis pairs. ...................................................... 107

Figure 4.2 – Examples of chelating bis-boranes. ..................................................................... 108

Figure 4.3 – Catalytic hydrogenation of imines with C10H6(B(C6F5)2)2. .................................. 109


Figure 4.5 – 19F NMR spectra obtained in boromobenzene-d5 showing the reaction of (4-1) and

tBu3P with CO2 to reversibly form (4-2). ........................................................................ 112

xv


Figure 4.7 – POV-Ray depiction of the anion of (4-3). ........................................................... 114




Figure 4.11 – 11B NMR spectra obtained in CD2Cl2 showing the reaction of (4-7) and tBu3P with

CO2 to form (4-8). .......................................................................................................... 120

Figure 4.12 – POV-Ray depiction of (a) (4-8) and (b) (4-10). ................................................. 122

Figure 5.1 – Examples of intramolecularly stabilized of d0 metal-olefin complexes. ............... 135

Figure 5.2 – Optimized structures of (5-1a) and (5-1b) with pertinent distances and NPA charges.

....................................................................................................................................... 137

Figure 5.3 – Partial 1H{19F} HOESY spectra of (5-1) (top) and (5-2) (bottom) measured at -50

°C as 0.17 M CD2Cl2 solutions. ...................................................................................... 140

Figure 5.4 – Partial 1H{19F} HOESY spectra of (5-3) measured at -50 °C as 0.17 M CD2Cl2

solution. .......................................................................................................................... 140

Figure 5.5 – POV-Ray drawing of (5-4).................................................................................. 141

Figure 5.6 – POV-ray depiction of (5-5). ................................................................................ 142



Figure 5.9 – Calculated transition state of (a) tBu3P addition to (5-1b) and (b) calculated reaction

profile to (5-5) (H atoms are omitted for clarity). ............................................................ 145

Figure 5.10 – Partial 1H{19F} HOESY NMR spectrum of (5-9) measured at -50 °C as 0.17 M

CD2Cl2 solution. ............................................................................................................. 146

Figure 5.11 – Optimized structures of (5-9a) and (5-9b) with pertinent distances. ................... 147

Figure 5.12 – POV-Ray depiction of (5-10). ........................................................................... 148

Figure 5.13 – POV-ray depiction of (5-11). ............................................................................ 149

Figure 5.14 – POV-Ray depiction of (a) (5-12) and (b) (5-13). ............................................... 151

Figure 5.15 – POV-Ray depiction of (a) (5-14a) and (b) (5-15a). ............................................ 152



xvi


Figure 5.19 – Reaction coordinate leading to the formation of (5-18) anion and the calculated

transition state structure. ................................................................................................. 159

xvii

List of Abbreviations, Nomenclature and Symbols

Å Angstrom

AIBN 2,2’-azobisisobutyronitrile

atm atmosphere

avg average

br broad

nBu n-butyl (C4H9)

calcd calculated

COD 1,5-cyclooctadiene (C8H12)

°C degrees Celsius

d doublet

DMSO dimethyl sulfoxide

equiv. equivalents

Et ethyl (C2H5)

FLP frustrated Lewis pair

g grams

GOF goodness of fit

HOMO highest occupied molecular orbital

h hour

xviii

Hz Hertz

i ipso

iPr iso-propyl (CH(CH3)2)

IR infrared

ItBu 1,3-di-tert-butylimidazol-2-ylidene, C3H2(NtBu)2

J scalar coupling constant

K Kelvin

kcal kilocalories

LUMO lowest unoccupied molecular orbital

m multiplet

m meta

m multiplet

Me methyl (CH3)

Mes mesityl (2,4,6-(CH3)3C6H2)

MesF 2,4,6-tris(trifluoromethyl)phenyl (2,4,6-(CF3)3C6H2)

mg milligram

MHz megahertz

min minute

mL milliliter

mmol millimole

xix

NMR nuclear magnetic resonance

nPr n-propyl (CH(CH3)2)

o ortho

p para

Ph phenyl (C6H5)

POV-Ray Persistence of Vision Raytracer

ppm parts per million

q quartet

quat quaternary

RT room temperature

s singlet

t triplet

tBu tert-butyl (C(CH3)3)

TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl radical

THF tetrahydrofuran

TMS trimethylsilyl (Si(CH3)3)

tol tolyl (C6H4(CH3))

xs. excess

1

1 Introduction 1.1 Frustrated Lewis Pairs with Dihydrogen 1.1.1 Heterolytic Splitting of Hydrogen and Hydrogenation Reactions

The chemistry of frustrated Lewis pairs (FLPs) has been an exciting field of study since its first

appearance in 2006.1 Frustrated Lewis pair is a term that describes a pair of a Lewis acid and a

Lewis base that reacts with a third molecule even though each of them alone does not show

reactivity towards it. Such anomalous reactivity is achieved by the “unquenched reactivity”

provided by large substituent groups around the acidic and basic centers prohibiting the

formation of a Lewis adduct.

The activation of kinetically stable, inert small molecules had long been the domain of transition

metal-containing species. An example of such molecule is H2, the activation of which is a useful

reaction from the viewpoint of hydrogenation catalysis. Heterolytic cleavage of H2 has been

achieved by various FLPs, the prototypical example of which is the pair of B(C6F5)3 as the Lewis

acid and PR3 (R = tBu, Mes etc.) as the Lewis base (Scheme 1.1).2

Scheme 1.1 – Heterolytic splitting of H2 by a frustrated Lewis pair.

In some FLP systems, the separated proton and hydride derived from H2 can be subsequently

transferred to unsaturated substrates, effecting hydrogenation of multiple bonds using H2 as the

reductant and a FLP as the hydrogenation catalyst.3 A wide variety of substrates, including bulky

imines, protected nitriles, aziridines, enamines and silyl enol ethers,4 have been reported to be

hydrogenated using FLP catalysts. In the simplest case, the substrate itself acts as the Lewis base

partner in the activation of H2, thus requiring only a catalytic amount of a Lewis acid in the

hydrogenation reaction (Scheme 1.2).5

2

Scheme 1.2 – Catalytic cycle of hydrogenation of bulky imines using B(C6F5)3.

Recently a particularly interesting stoichiometric hydrogenation of anilines with B(C6F5)3 has

been reported (Scheme 1.3).6 Computational evidence suggests that, in the initial H2 activation

step in this cascade of reactions, the proton from the heterolytically cleaved H2 binds to the para-

carbon of the aryl ring rather than the nitrogen. This study implies the possibility of a wide range

of substrate groups that still need to be uncovered and potential applications in commercial

syntheses of fine chemicals and pharmaceutical ingredients.

Scheme 1.3 – Stoichiometric hydrogenation of anilines with B(C6F5)3.

1.1.2 Transition Metal Centers and Frustrated Lewis Pairs in H2 Activation

Dihydrogen is the most atom-economical reductant for hydrogenation of unsaturated substrates.

Because of its kinetically inert nature, H2 does not spontaneously add across most multiple

bonds. Nevertheless, scientists have discovered efficient transition metal-based catalysts of both

heterogeneous and homogeneous nature that facilitate such reactions. The σ-H2 complexes are

believed to be important intermediates in such catalytic mechanisms, and the first proof of the

3

independent existence of such complex was reported by Kubas et al., who characterized

W(CO2)3(PR3)2(H2).7 Since then various studies on σ-complexes support that the coordination of

molecular hydrogen can be described as donation of electron density from the σ-bond of H2 to

the vacant metal d-orbital and back-donation from the filled metal d-orbital to σ* of the

dihydrogen bond.8 Dihydrogen-metal interaction, in this sense, is analogous to the Dewar-Chatt-

Duncanson description of ethylene-metal interaction, which infers that dihydrogen is a good π-

acceptor ligand. Either by incorporation of a basic site in the ligand framework or by addition of

an external base, the coordinated H2 can be heterolytically cleaved. Alternatively, oxidative

addition to the metal center gives rise to homolytic scission of the coordinated H2. In either

scenario, a metal-hydride species may be generated and further react with substrates in a

catalytic fashion (Scheme 1.4).

Scheme 1.4 – H2 cleavage by transition metal centers.

In contrast to versatile bonding modes available in transition metal complexes, reactions of main

group species or compounds that lack accessible d-orbitals with H2 are unexpected and their

mechanisms are not obvious. In particular, H2 splitting reactions by pairs of boranes and

phosphines are especially interesting as they are nominally termolecular reactions. There are a

few pathways proposed with different initial interactions in the mixture of three components.

Such proposed mechanisms include initial one-electron transfer from phosphine to borane

followed by homolytic cleavage of H2, initial end-on interaction of H2 with phosphine and initial

side-on interaction with borane.9 A number of theoretical chemists, such as the groups of Papai10-

11 and Grimme,12 have addressed this tough question with the tools of the state-of-the-art

quantum chemical calculations. One interesting proposal on the prototypical FLP, the

combination of B(C6F5)3 and tBu3P, involves initial formation of a phosphine-borane “encounter

complex” held together primarily by substituent secondary interactions, namely multiple H…F

4

hydrogen bonds and dispersion forces. The separation of the boron and phosphorus centers in

this species is approximately 4.2 Å, which is an insufficient proximity for orbital overlap.10

Furthermore, the electric field gradient generated in the small cavity created by the reactive

centers can polarize H2, that acts as an electron transfer bridge, and effect its heterolytic scission

(Figure 1.1).12

Figure 1.1 – Electric field gradient generated in an encounter complex polarizing a H2

molecule.

In terms of molecular orbitals, the formation of a classical Lewis adduct is a result of the mixing

of the base HOMO and acid LUMO giving rise to a σ-bond. In contrast, in a sterically crowded

pair of Lewis acid and base, even though the components are in close proximity, the orbitals of

the reactive centers do not significantly overlap, providing a complex that bears a small HOMO-

LUMO gap (Figure 1.2).13 This species may be seen to be analogous to a transition metal center

containing an empty d-orbital and an occupied d-orbital available for H2 binding and subsequent

homolytic or heterolytic cleavage. The organization of the two orbitals is essential for H2

splitting with FLPs, but it is the electric field gradient generated between the reactive centers in a

frustrated complex that polarizes and energizes the H2 molecule to break the H–H bond and form

new H–B and H–P bonds (Figure 1.3).

5

Figure 1.2 – Qualitative depiction of the frontier molecular orbitals in a classical Lewis

adduct and a frustrated Lewis pair.

Figure 1.3 – Qualitative depiction of the frontier molecular orbitals involved in the H2

splitting reaction by a frustrated Lewis pair.

6

More recently quantum chemical calculations by the group of Rhee14 have pointed out a non-

negligible amount of overlap between the lone pair orbital of tBu3P and the unoccupied orbital of

B(C6F5)3 at the most stable conformation. They also demonstrate very soft interactions by

dispersion forces that provide entropic stabilization to the FLP in comparison with a classical

Lewis adduct. This proposition has further been supported by the recent sophisticated solid-state

NMR techniques and DFT calculations that suggest the presence of small degree of covalent

bond character in the reactive centers of frustrated Lewis pairs.15

Although probed a number of times computationally, mechanistic investigation of hydrogen

activation as well as other FLP reactions lack experimental evidence. Recently Piers et al.

reported reactions of H2 with boroles substituted with either phenyl or pentafluorophenyl groups

(Scheme 1.5).9, 16 These compounds exhibit extremely strong electrophilicity due to their

antiaromatic nature. Their results suggest the possibility of H2 activation by FLP also occurring

with initial interaction of H2 with the Lewis acid. In support of this proposition is the matrix

isolation studies of η2-H2 complex of BH3 by Tague and Andrews17 and subsequent

computational work18-19 although attempts to observe the interaction of B(C6F5)3 with H2

spectroscopically has not been successful.2

Scheme 1.5 – Reactions of pentaarylboroles with H2.

More recently, the group of Li and Wang20 reported the unprecedented H/D exchange reaction

on a single borane of DBMesF2 under an atmosphere of H2 at 50 oC, and their DFT calculations

found a reasonable reaction intermediate of H2∙DBMesF2 adduct for this reaction. Furthermore,

7

they concluded that H2 activation by the pair of DBMesF2 and Et3N is likely to go through the

H2.borane adduct as an intermediate. Shortly after this, Nikonov et al.21 also reported that their

experimental and computational evidence support H2 activation by HB(C6F5)2 by a direct σ-bond

metathesis mechanism.

1.2 Reactions of Frustrated Lewis Pairs with Multiple Bonds and Cyclic Compounds

1.2.1 Alkenes and Cycloalkanes

Reactions of frustrated Lewis pairs with hydrocarbons are of particular interest as they imply

synthetic utility. Unactivated alkenes are not prone to nucleophilic attack by mild nucleophiles,

such as phosphines and amines. Nevertheless, it has been reported that ethylene, propylene and

1-hexene undergo addition reactions by the pair of tBu3P and B(C6F5)3 to yield the zwitterionic

products (C6F5)3BCH2CH(R)PtBu3 (Scheme 1.6).22 In a similar fashion, substituted

cyclopropanes have also been demonstrated to react with tBu3P/B(C6F5)3 to ring-open and give

(C6F5)3BCH2CH2CH(R)PtBu3 as the products (Scheme 1.6).23

Scheme 1.6 – Reactions of FLPs with alkenes and cycloalkanes.

1.2.2 Carbon Dioxide, Nitrous Oxide, Nitric Oxide and Carbon monoxide

FLPs do not only add to hydrocarbons, but have also been demonstrated to undergo addition

reactions across polar multiple bonds in gaseous small molecules. As carbon dioxide and nitrous

oxide are potent greenhouse gases,24 their sequestration and subsequent chemical transformation

into useful molecules are of great interest. A number of FLP systems have been shown to

8

activate these molecules (Scheme 1.7),25-28 and in the case of carbon dioxide, further

transformation of the activated molecule, including the stoichiometric reduction to methanol29-30

and carbon monoxide,31 and the catalytic deoxygenative hydrosilylation to methane,32 have been

achieved. Moreover an ethylene-bridged intramoelcular FLP has been reported to capture NO to

give a persistent N-oxyl radical33 while boron imidinates also act as FLPs to insert small

molecules, including carbon monoxide.34

Scheme 1.7 – Reactions of FLPs with carbon dioxide, nitrous oxide, nitric oxide and carbon

monoxide.

9

1.3 Reactions of Alkynes with Frustrated Lewis Pairs 1.3.1 Reactions of Trialkylalkynylborates with Electrophiles

Some years ago, Wrackmeyer and coworkers35-36 described the 1,1-organoboration of

alkynylmetallic compounds of the form LnM–C≡C–R by trialkylboranes (Scheme 1.8) as a

part of their excursion to generate important building blocks in both organic and

organometallic chemistry. Such reactions, proceeding with varying rates, produce alkenes

where the boryl group is placed with either cis or trans disposition with respect to the MLn

group. Since the reaction involves a B–C bond addition to a single carbon, this is termed 1,1-

carboboration. The reaction is believed to proceed via an initial cleavage of a B–C bond and

generation of an alkynylborate bound to the MLn cation forming a zwitterionic intermediate.

The proposed intermediate was further investigated by the 1,1-carboboration of

bis(alkynyl)lead species to give an isolable product in which intramolecular coordination of

the alkynyl group to Pb was observed (Scheme 1.9).37

Scheme 1.8 – 1,1-Carboboration reaction of alkynylmetallic compounds by trialkylboranes.

Scheme 1.9 – 1,1-Carboboration reaction of bis(alkynyl)lead compounds by

trialkylboranes.

10

1.3.2 1,1-Carboboration Reactions - Revisited

Reactions of electrophilic boranes with alkynes have been known for a long time. For instance,

haloboranes react with both terminal and internal alkynes, affording a strategy for the facile

synthesis of variously substituted vinylboranes.38 By substituting alkynes with organostannyl

groups, they can be activated easily enough to react even with trialkylboranes.39-40 Recently such

chemistry, termed carboboration, has been revisited by the Erker group to exploit its synthetic

utility.41 With the use of readily available B(C6F5)3 and its derivative MeB(C6F5)2, the scission of

the B–C bond that adds to the same carbon of the alkyne has been demonstrated to proceed at RT

in the case of terminal alkynes, and at 110 to 125 °C in the case of internal alkynes (Scheme

1.10). With respect to terminal alkynes and asymmetrically substituted alkynes, the

thermodynamically stable stereoisomer is preferentially formed using the light-promoted

isomerization process.42 Subsequent treatment of the vinylboranes under the Suzuki coupling

conditions successfully converted them into trisubstituted alkenes and tetrasubstituted alkenes for

terminal alkynes and internal alkynes, respectively.43

Scheme 1.10 – 1,1-Carboboration reactions of terminal and internal alkynes.

1.3.3 Reactions of Lewis Acid-Base Pairs with Alkynes

Related to the 1,1-carboboration reactions is the FLP reactivity of terminal alkynes reported by

Stephan et al.44-45 While a terminal alkyne itself reacts with B(C6F5)3 to give the 1,1-

11

carboboration products, addition of a Lewis base, such as a phosphine, to this reaction can

provoke an alternative reactivity. With the use of a highly bulky and basic phosphine, such as

tBu3P, the deprotonation of the alkyne occurs with the concurrent formation of an alkynylborate

anion (Scheme 1.11). In contrast, less bulky and basic phosphines, such as (o-tol)3P and Ph3P,

add to the alkyne as a nucleophile with addition of B(C6F5)3 as an electrophile to the other

carbon of the triple bond to yield zwitterionic phosphonium borates linked by an alkenyl group.

Other Lewis acids, including BPh3 and Al(C6F5)3, as well as Lewis bases, including

diphosphines, sulphides, amines and imines, have been shown to undergo such alkyne addition

reactions, indicating the broad range of compounds acting as FLPs.

Scheme 1.11 – Reactions of terminal alkynes with frustrated Lewis pairs.

A similar idea is utilized in efforts to develop novel π-conjugated materials with unique

photophysical and electrochemical properties by the group of Yamaguchi.46-47 They synthesized

boryl and phosphino group substituted diphenyl acetylenes that converted through intramolecular

cyclization to the zwitterionic ladder stilbene compounds (Scheme 1.12). In contrast to the

alkyne addition reactions by FLPs where the use of highly electrophilic Lewis acids is essential,

these cascade cyclization reactions are promoted by the incorporation of highly nucleophilic

phosphines. Quantum chemical calculations revealed the addition reaction to go through the

transition state in which the acetylene moiety is activated by the nucleophile.

12

Scheme 1.12 – Synthesis of zwitterionic ladder stilbene compounds.

1.4 Objectives of This Thesis While some of the original work on the FLP chemistry had already been published or performed

by the time the candidate joined the Stephan group,4, 48-50 the objectives of this Ph.D. work

involve the exploitation of more complex systems in aiding further understanding and future

development of the FLP chemistry. In particular, the following bifunctional systems were

examined: (1) a phosphino-borane system linked by an alkynyl fragment for FLP reactivity and

reactions with metal species; (2) bis-boranes in which the two boryl groups are linked by one-

atom spacers for the activation of carbon dioxide and related chemistry; (3) vinyl group tethered

boranes for gaining insight into olefin addition reactions by FLPs.

All syntheses, characterization and analyses were performed by the candidate with the exception

of the DFT calculations for the thermal rearrangement reaction leading to (2-12) by Professor Dr.

Thomas M. Gilbert and those for the Ni-alkynylborane complex (3-1) by Professor Dr. Datong

Song. The help of Dr. Alan J. Lough was appreciated in solving some of the crystal structures in

this thesis.

Most of these chapters have already been published. (1) Zhao, X.; Otten, E.; Song, D.; Stephan,

D. W. “NiB Interactions in Nickel Phosphino-Alkynyl-Borane Complexes”. Chemistry – A

European Journal 2010, 16, 2040-2044. (2) Zhao, X.; Gilbert, T. M.; Stephan, D. W. “C-C

13

Coupling by Thermolysis of Alkynyl Phosphonium Borates”. Chemistry – A European Journal

2010, 16, 10304-10308. (3) Zhao, X.; Stephan, D. W. “Bis-boranes in the frustrated Lewis pair

activation of carbon dioxide”. Chemical Communications 2011, 47, 1833-1835. (4) Zhao, X.;

Lough, A. J.; Stephan, D. W. “Synthesis and Reactivity of Alkynyl-Linked Phosphonium

Borates”. Chemistry – A European Journal 2011, 17, 6731-6743. (5) Zhao, X.; Stephan, D. W.

“Olefin-Borane “van der Waals Complexes”: Intermediates in Frustrated Lewis Pair Addition

Reactions”. Journal of the American Chemical Society 2011, 133, 12448-12450. (6) Zhao, X.;

Stephan, D. W. “Frustrated Lewis pair olefin addition reactions: P-, N-, C- and H-based

nucleophilic additions to an olefin-tethered borane”. Chemical Science 2012, in press.

14

2 Synthesis and Frustrated Lewis Pair Reactivity of Alkynyl-Linked Phosphine Boranes

2.1 Introduction 2.1.1 Intramolecular Frustrated Lewis Pair Systems Since the report on the first metal-free reversible dihydrogen activation by

(C6F5)2B(C6F4)PMes2,1 a number of intramolecular FLP systems with Lewis acidic and basic

centers linked by spacer groups have been developed for applications in hydrogen activation and

hydrogenation of polar unsaturated substrates (Scheme 2.1). The nature of the organic linkers

between the Lewis basic and acidic centers in such systems are frequently arene-based3 although

two-carbon alkyl51-52 or alkenyl53 as well as benzyl54-55 chains have also been examined. The

proximity of the reactive centers in these ansa-phosphine-borane or amine-borane species seems

essential to efficient hydrogenation catalysis. Envisioning to facilitate hydrogenation of certain

substrates, we set about to examine the alkynyl-linked P/B system that will presumably bear

more defined distances between PH and BH.

Scheme 2.1 – Examples of intramolecular frustrated Lewis pair systems that heterolytically

cleave H2.

2.1.2 Previously Reported Alkynyl-Linked P/B Compounds The syntheses of compounds that contain the P–C≡C–B fragment were reported as early as 1989

by Bestmann and coworkers.56 In this publication they described the zwitterionic species

15

Ph2MeP–C≡C–BR3 (R = Ph, CH2Ph) as a naked C2 unit stabilized by complexation of a

phosphine and a borane. While the central C2 unit did not react with 1,3-dipoles, the methyl

group on P was demonstrated to undergo deprotonation and methylation, with no further

exploration of the reactivity of these compounds. An alkynyl-linked system bearing neutral P and

B centers, Ph2P–C≡C–BMes2, was prepared some years ago by Marder and colleagues as a π-

conjugated compound, and its nonlinear optical properties were examined.57-58 It is noted that, in

both of the above reports, the P–C≡C–B unit was constructed by reacting the lithiated

ethynylphosphine with the corresponding borane (Scheme 2.2).

Scheme 2.2 – Preparation of alkynyl-linked P/B compounds in literature.

2.2 Results and Discussion 2.2.1 Synthesis of Ethynylphosphines

In an attempt to develop a system that exhibits FLP reactivity, we set about to explore alkynyl-

linked P/B compounds baring electron-withdrawing pentafluorophenyl groups on B. Since the

phosphino group needs to have sufficient Lewis basicity and yet not undergo irreversible adduct

formation with the strongly Lewis acidic boryl group to retain reactivity as a FLP, tBu and Mes

groups were examined as substituents of the phosphine moiety in this study.

Reaction of the commercially available ethynylmagnesium bromide with either ClPtBu2 or

ClPMes2 did not proceed in contrast to the analogous reactions of ethynylmagnesium bromide

with ClPR2 (R = Et, nPr, nBu, n-pentyl, Ph),59 presumably due to the prevention of nucleophilic

attack by the bulky substituents. Since the more reactive ethynyllithium is not readily available,60

we employed a modification of the literature procedure by Oshima et al.61 Lithiation of

trimethylsilylacetylene followed by reaction with ClPR2 (R = tBu, Mes) gave R2PC≡CTMS,

which was then converted to R2PC≡CH by protodesilylation with overall yields of 71% for R =

16

tBu (2-1) and 45% for R = Mes (2-2) (Scheme 2.3). tBu2PC≡CH is a colourless liquid at RT with

an acetylenic proton signal at 2.55 ppm in 1H NMR and 31P{1H} NMR resonance at 12.5 ppm,

while Mes2PC≡CH is a colourless crystalline solid with an acetylenic proton signal at 2.65 ppm

in 1H NMR and 31P{1H} NMR resonance at -54.5 ppm. X-ray crystallographic studies of (2-2)

further confirmed its identity (Figure 2.1).

Scheme 2.3 – Syntheses of ethynylphosphines (2-1) and (2-2).

Figure 2.1 – POV-Ray depiction of (2-2). C: gray, H: gray, P: orange. Hydrogen atoms except

the acetylenic H are omitted for clarity. Selected bond distances (Å) and angles (°): C(1)–C(2),

1.191(3); P(1)–C(1), 1.774(2); P(1)–C(1)–C(2), 164.81(18).

.

2.2.2 Synthesis of Alkynyl-Linked Phosphonium Borates

The most obvious route to the P–C≡C–B motif is to react ClB(C6F5)2 with the organometallic

reagent of the corresponding ethynylphosphine. However, neither tBu2PC≡CLi nor

tBu2PC≡CMgBr generated in situ gave the desired product, R2P–C≡C–B(C6F5)2, upon reaction

with ClB(C6F5)2. Having been inspired by the three component reaction of tBu3P, B(C6F5)3 and

17

terminal alkyne,44 we then examined the reaction of tBu2PC≡CH with B(C6F5)3. This reaction,

when carried out at -35 °C, indeed gave a new product (2-3) in 88% yield (Scheme 2.4). The 1H

and 31P spectral data for (2-3) revealed a resonance attributable to a PH fragment with a P-H

coupling constant of 466 Hz, and the 31P{1H} chemical shift at 24.5 ppm was consistent with a

phosphonium cation. The 11B{1H} NMR chemical shift of (2-3) at -21.4 ppm is consistent with

the presence of a borate fragment.62 The 19F NMR spectrum of (2-3) showed resonances at -

133.1, -161.5 and -166.6 ppm, consistent with equivalent C6F5 rings on a 4-coordinate borate

center.63-65 These data, together with mass spectral data and elemental analyses support the

formulation of (2-3) as the zwitterionic species tBu2P(H)C≡CB(C6F5)3. An X-ray

crystallographic study of (2-3) confirmed the structure with a central C-C bond of 1.217(4) Å

and PC≡C and C≡CB angles of 171.2(2) and 174.9(3)° (Figure 2.2 (a)). Subsequent reaction of

(2-3) with MeLi followed by treatment with MeI afforded (2-4) in 58% yield (Scheme 2.4). The 31P{1H} NMR chemical shift at 32.8 ppm and the doublet 1H NMR signal at 1.71 ppm for PCH3

are consistent with the formulation of (2-4) as tBu2P(Me)C≡CB(C6F5)3. This was also confirmed

crystallographically (Figure 2.2 (b)).

Scheme 2.4 – Syntheses of (2-3) and (2-4).

18

Figure 2.2 – POV-Ray depiction of (a) (2-3) and (b) (2-4). C: gray, H: gray, B: yellow-green,

F: pink, P: orange. Hydrogen atoms except PH are omitted for clarity. Selected bond distances

(Å) and angles (°): (a) C(9)–C(10), 1.217(4); P(1)–C(9), 1.708(3); B(1)–C(10), 1.596(4); P(1)–

C(9)–C(10), 171.2(2); C(9)–C(10)–B(1), 174.9(3). (b) C(1)–C(2), 1.2113(14); P(1)–C(1),

1.7160(10); B(1)–C(2), 1.5995(14); P(1)–C(1)–C(2), 175.18 (9); C(1)–C(2)–B(1), 175.79(10).

In an analogous fashion, the reaction of (2-1) with ClB(C6F5)2 proceeded in a facile manner at -

35 °C to give a new off-white product (2-5) in 72% isolated yield (Scheme 2.5). As in (2-3),

compound (2-5) also exhibits a 1H NMR doublet resonance at 5.80 ppm with a P-H coupling

constant of 469 Hz, indicative of the presence of a PH phosphonium fragment. Compound (2-5)

also shows a 11B{1H} NMR signal at -12.8 ppm, consistent with the presence of a borate unit.

The 31P{1H} NMR spectrum also gives a signal at 25.5 ppm, shifted downfield from the

resonance of the alkynylphosphine. Collectively these data infer the formulation of (2-5) as

tBu2P(H)C≡CB(Cl)(C6F5)2. Subsequently X-ray crystallographic studies showed that the metric

parameters of (2-5) were similar to those in (2-3) and (2-4), with a B–Cl bond distance of

1.911(4) Å (Figure 2.3).

19


Figure 2.3 – POV-Ray depiction of (2-5). C: gray, H: gray, B: yellow-green, Cl: green, F: pink,

P: orange. Hydrogen atoms except PH are omitted for clarity. Selected bond distances (Å) and

angles (°): C(18)–C(19), 1.208(5); P(1)–C(20), 1.727(3); B(1)–C(18), 1.585(5); B(1)–Cl(1),

1.911(4); P(1)–C(20)–C(18), 169.8(3); C(20)–C(18)–B(1), 173.1(4).

Treatment of (2-5) with excess Me2SiHCl resulted in the exchange of the B-bound chloride for

hydride generating a B–H unit and Me2SiCl2. The resulting off-white product (2-6) was isolated

in 79% yield (Scheme 2.5). NMR data confirm the replacement of the chloride by hydride as

evidenced by the observation of a 1:1:1:1 quartet at 3.25 ppm with a B-H coupling constant of 91

Hz (Figure 2.4), while the 11B{1H} resonance shifts to -29.2 ppm. One of the acetylenic carbons

is observed in the 13C{1H} NMR spectrum at 64.4 ppm with a C-P coupling constant of 158 Hz.

The resonance for the B-bound acetylenic carbon was not observed, presumably due to

quadrupolar broadening arising from the adjacent B center. Nonetheless, infrared data revealed

an absorption at 2125 cm-1 in accord with the presence of the acetylenic unit. The proposed

connectivity for (2-6), tBu2P(H)C≡CB(H)(C6F5)2, was confirmed via X-ray crystallography

which showed the metric parameters and geometry to be unexceptional (Figure 2.5).

Interestingly, viewing the molecule along the P–C≡C–B vector in the solid state, it is noted that

20

the substituents on B and P are eclipsed, with the BH and PH occupying almost the same plane

and the hydride-proton separation of approximately 5.32 Å. This observation contrasts that of the

arene-based Mes2P(H)(C6F4)B(H)(C6F5)2 in which the B–H and P–H bonds are pointing in

opposite directions with a hydride-proton separation of approximately 6.94 Å.1 Viewing the

packing of two molecules of (2-6), the B–H bond from one of the molecules and the P–H bond

from another are facing toward each other with a H…H distance of approximately 2.21 Å, which

is slightly too long to be considered a dihydrogen bond.66

Figure 2.4 – 1H NMR spectrum of (2-6) in CD2Cl2.

21

Figure 2.5 – POV-Ray depiction of (2-6). C: gray, H: gray, B: yellow-green, F: pink, P: orange.

Hydrogen atoms except PH and BH are omitted for clarity. Selected bond distances (Å) and

angles (°): C(9)–C(10), 1.208(4); P(1)–C(9), 1.716(3); B(1)–C(10), 1.598(4); P(1)–C(9)–C(10),

175.7 (3); C(9)–C(10)–B(1), 177.9(3).

In a procedure similar to that used to prepare (2-4), deprotonation of (2-6) with MeLi and

subsequent treatment with MeI afforded tBu2P(Me)C≡CB(H)(C6F5)2 (2-7) in 84% yield (Scheme

2.6). This formulation was consistent with the 1H NMR resonances at 1.72 ppm and 3.20 ppm,

arising from the P-bound methyl group and the preserved B-H fragment, respectively. In a

related reaction of (2-6), treatment with [Ph3C][B(C6F5)4] resulted in the generation of

[tBu2P(H)C≡CB(C6F5)2][B(C6F5)4] which was not stable enough to be isolated in its analytically

pure form. Instead it was isolated as a THF adduct (2-8) in 77% yield (Scheme 2.6). The two 11B{1H} signals at -3.2 and -16.6 ppm correspond to the B bound to THF and the [B(C6F5)4]

anion, respectively, inferring abstraction of the hydride to form the alkynyl-phosphonium borane

cation. These data, together with other NMR spectra, were consistent with the formulation of (2-

8) as [tBu2P(H)C≡CB(C6F5)2(THF)][B(C6F5)4]. The generation of (2-7) and (2-8) demonstrate

that the proton and the hydride of (2-6) can be readily removed, suggesting that (2-6) is

potentially a good proton and hydride mediator for hydrogenation reactions.

22


To probe the impact of substituent variation, the phosphine Mes2PC≡CH (2-2) was also

examined for an analogous reaction, but (2-2) by itself did not react with B(C6F5)3 for its lower

basicity in comparison with (2-1). On that account, a more basic phosphine, PtBu3, was added to

this mixture of (2-2) and B(C6F5)3 to effect deprotonation of the ethynyl group of (2-2) and gave

a crystalline product (2-9) in 87% yield (Scheme 2.7). This species gave rise to 31P{1H}

resonances at 61.4 and -52.9 ppm, with a 11B{1H} signal at -20.9 ppm consistent with the

presence of a borate unit, supporting the formulation of (2-9) as the salt

[tBu3PH][Mes2PC≡CB(C6F5)3]. This was unambiguously confirmed crystallographically (Figure

2.6).

Scheme 2.7 – Synthesis of (2-9).

23

Figure 2.6 – POV-Ray depiction of (2-9). C: gray, H: gray, B: yellow-green, F: pink, P: orange.

Hydrogen atoms except PH are omitted for clarity. Selected bond distances (Å) and angles (°):

C(1)–C(2), 1.211(3); P(1)–C(1), 1.767(2); B(1)–C(2), 1.600(3); P(1)–C(1)–C(2), 166.7(2); C(1)–

C(2)–B(1), 170.8(2).

Efforts to form the analogous salt [tBu3PH][Mes2PC≡CBCl(C6F5)2] employing ClB(C6F5)2 in

place of B(C6F5)3 resulted in facile loss of the chloride. This is due to the greater Lewis acidity of

ClB(C6F5)2 in comparison with Mes2PC≡CB(C6F5)2. Thus two equivalents of ClB(C6F5)2 were

added to (2-2) pre-mixed with PtBu3 to generate Mes2PC≡CB(C6F5)2 and

[tBu3PH][Cl2B(C6F5)2]. Subsequent precipitation of the byproduct salt left the alkynyl-phosphine

borane in a red-orange solution. NMR of the solution in toluene-d8 revealed 19F resonances at -

128.9, -147.5 and -162.0 ppm, indicating the presence of a 3-coordinate B center, while giving a 31P{1H} signal at -53.4 ppm attributable to the formation of Mes2PC≡CB(C6F5)2. However, all

efforts to isolate Mes2PC≡CB(C6F5)2 in its analytically pure form was unsuccessful as the

compound seemed to decompose over time. Nonetheless addition of acetonitrile allowed the

isolation of the yellow compound Mes2PC≡CB(NCMe)(C6F5)2 (2-10) in 53% yield (Scheme

2.8). The 11B{1H} signal was observed at -12.6 ppm and the 19F NMR signals at -133.9, -157.9

and -164.5 ppm were consistent with the quaternized B center while the 31P{1H} resonance at -

53.0 ppm was consistent with a phosphine fragment. Crystallographic data confirmed this

formulation and revealed a B-N bond length of 1.596(2) Å (Figure 2.7).

24

Scheme 2.8 – Generation of Mes2PC≡CB(C6F5)2 and synthesis of (2-10).

Figure 2.7 – POV-Ray depiction of (2-10). C: gray, B: yellow-green, F: pink, N: aquamarine,

P: orange. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°):

C(1)–C(2), 1.2125(17); P(1)–C(1), 1.7605(13); B(1)–C(2), 1.5728(18); B(1)–N(1), 1.5959(18);

P(1)–C(1)–C(2), 164.18(11); C(1)–C(2)–B(1), 172.33(13).

2.2.3 Attempted Use of the Alkynyl-Linked P/B Compounds in Hydrogen Activation and Hydrogenation Catalysis

In order to examine the suitability of species (2-6) as a hydrogenation catalyst, it was reacted

with the imine tBuN=CHPh and was found to afford colorless crystals of (2-11) in 2 hours at 25

°C in 57% yield (Scheme 2.9). The 1H NMR data inferred the loss of both signals arising from

PH and BH and the generation of new resonances at 5.10 and 3.75 ppm attributable to NH and

CH2 of the amine tBuN(H)CH2Ph. The 31P{1H} resonance at 18.6 ppm suggests a 3-coordinate

phosphine fragment, while the 11B{1H} NMR spectrum shows a broad signal at -9.6 ppm.

Together with the 19F spectrum that shows diastereomeric C6F5 groups attached to a 4-coordinate

25

B center, these data infer the formation of the borane-amine adduct,

tBu2PC≡CB(tBuN(H)CH2Ph)(C6F5)2 (2-11). This was subsequently confirmed by an X-ray

crystal structure (Figure 2.8). The formation of (2-11) results from proton and hydride transfer to

the imine. While imine reductions have previously been shown to be catalytic with aryl and

alkyl-linked phosphonium borates,53, 67 efforts to utilize (2-6) as a catalyst to effect the

hydrogenation of the imine tBuN=CHPh under an H2 atmosphere at 80 °C were unsuccessful.

This was attributed not only to the strength of the B–N bond, but also to the inherent reactivity of

the polarized alkyne unit in (2-6) since these reactions afforded a complex mixture of

unidentified degradation products. It is noteworthy that alkynylboranes with strongly electron-

withdrawing substituents on B have been reported to be unstable at RT and/or in their solid

states.68-69

Scheme 2.9 – Proton and hydride transfer from (2-6) to an imine and generation of (2-11).

Figure 2.8 – POV-Ray depiction of (2-11). C: gray, H: gray, B: yellow-green, F: pink, N:

aquamarine, P: orange. Hydrogen atoms except NH are omitted for clarity. Selected bond

distances (Å) and angles (°): C(1)–C(2), 1.2139(15); P(1)–C(1), 1.7742(11); B(1)–C(2),

1.5850(16); B(1)–N(1), 1.6669(15); N(1)–C(23), 1.5152(14); P(1)–C(1)–C(2), 172.18 (10);

C(1)–C(2)–B(1), 177.37(11).

26

In order to examine thermal liberation of H2 from (2-6), the compound was heated in

bromobenzene-d5 at 150 °C for 12 h. The solution turned orange within 5 minutes. Following

completion of the thermolysis, the solution was allowed to cool and a product (2-12) crystallized

from the solution. The crystals were isolated and washed with benzene affording a yellow

product in 40% yield (Scheme 2.10). The 1H NMR spectrum of (2-12) showed broad multiplets

at 5.90 and 3.01 ppm as well as doublets at 1.46 and 1.18 ppm. The latter resonances correspond

to diastereomeric tBu groups. A broad 11B{1H} signal was observed at 3.1 ppm while the 19F

NMR spectrum exhibited a broad resonance at -185.99 ppm consistent with the presence of BF

fragments. In addition, the 19F NMR resonance patterns were consistent with the presence of

C6F5 and ortho-substituted C6F4 fragments. A singlet was observed in the 31P NMR spectrum at

75.2 ppm. EI-mass spectral data revealed a molecular ion with an m/z ratio of 1032.2, which

corresponds to dimerization of the phosphonium borate (2-6). Unambiguous confirmation of the

nature of (2-12) was obtained via X-ray crystallography (Figure 2.9). This species is a trans-

disubstituted olefin in which the two equivalent substituents are five-membered rings derived

from boron and phosphorus atoms linked by a fluoroarene ring and a methine carbon. The

methine carbon atoms are bound to the C=C linking fragment. Each of the P centers are cationic

as they are also bound to two tBu groups, while the B centers are anionic being quaternized by

an additional C6F5 ring and a fluoride. The metric parameters are unexceptional, with B–F

distance of 1.758(6) Å and the olefinic C=C bond length of 1.331(4) Å. It is noteworthy that the

molecule sits on a crystallographically imposed center of symmetry and thus is the meso-isomer

for the chiral carbon and boron centers.

Scheme 2.10 – Thermal rearrangement reaction of (2-6) to (2-12).

27

Figure 2.9 – POV-Ray depiction of (2-12). C: gray, B: yellow-green, F: pink, P: orange.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): C(22)–

C(22a), 1.331(4); C(1)–C(22), 1.500(3); P(1)–C(1), 1.823(2); B(1)–C(1), 1.687(3); B(1)–F(1),

1.436(3); C(1)–C(22)–C(22a), 125.2(3).

As the thermolysis reaction yields significant amounts of unidentified side products, probing the

reaction mechanism by kinetic measurements proved difficult. Nonetheless it is noteworthy that

radical pathways are not likely since both a radical scavenger (TEMPO) and a radical initiator

(AIBN) did not substantially affect the reaction. A process involving rapid H2 evolution and re-

uptake is also dismissed since there is no deuterium incorporation when the thermolysis reaction

is performed in the presence of D2. Meanwhile, no intermediate species was observed in the

reaction mixture of the thermolysis, suggesting that the first step of the process may be slow. It is

noted that irradiating a solution of (2-6) (λmax = 258 nm, ε = 350 M-1cm-1) with a mercury vapour

lamp that emits at 254 nm did not give any conversion to (2-12). From these observations, we

envisaged a reaction mechanism that involves a first rearrangement step to a three-membered

cyclic structure followed by a head-to-head concerted dimerization, a proton migration and the

final ortho-substitution of the C6F5 group by the nucleophilic phosphine (Scheme 2.11).

28

Scheme 2.11 – Proposed mechanism of thermal formation of (2-12) from (2-6).

To probe the feasibility of this proposed mechanism for the formation of (2-12), particularly the

C-C coupling reaction that leads to the C4 chain, DFT calculations were performed by Professor

Dr. Thomas M. Gilbert. The results from calculations suggest that the rate-determining step

involves nearly isoenergetic rearrangement of (2-6) to boratacyclopropene (2-6a) through a

transition state (2-6aTS) similar to that for hydroboration of the alkyne fragment, with a barrier

of ca. 40 kcal/mol (Figure 2.10). Cyclic structures related to boratacyclopropene (2-6a) have

been reported in literature,70 and are described to be powerful reductants. The presence of a

reducible moiety within the molecule presumably prompts the facile dimerization of (2-6a) via

C–C coupling that passes through transition state (2-6bTS) with a barrier of ca. 30 kcal/mol. The

product of bond rearrangement and dimerization is a symmetrical trans-alkene with a

borataalkene substituent on each side (2-6b) that lies ca. 65 kcal/mol below (2-6). (2-6b)

undergoes a slightly endothermic proton migration from P to the adjacent C to form monoene (2-

29

6c). Subsequent P attacks at ortho-positions of the fluoroarene rings with concurrent fluoride

transfer to B are collectively exothermic by nearly 190 kcal/mol. These latter steps are not rate-

determining, and so were not studied in detail.

Figure 2.10 – Energy diagram of proposed mechanism of formation of (2-12) based on DFT

calculations. [Relative energies in kcal/mol]

In the generation of (2-12), a major driving force is believed to be the formation of 5-membered

cycles via ortho-F activation of C6F5 groups by the gem-substituted “frustrated” phosphino and

boryl groups. As indicated in Figure 2.10, one cyclization provides ca. 65 kcal/mol of

thermodynamic stabilization especially for the formation of the strong B–F bond. In agreement

with the facile nature of this cyclization process, an attempt to synthesize a CH2-linked

phosphine-borane from ClB(C6F5)2 and tBu2PCH2Li led to the isolation of the cyclic product (2-

13) (Scheme 2.12). (2-13) was isolated as a mixture of the fluoro- and chloroborate compounds,

and X-ray data confirmed the structure to be (C6F5)(C6F4)XBCH2PtBu2 (X = F 2-13a/Cl 2-13b)

(Figure 2.11). It is noteworthy that similar nucleophilic aromatic substitution reactions of

fluoroarylboranes at the para-position by phosphines have been reported.71

30

Scheme 2.12 – Formation of (2-13a) and (2-13b).

Figure 2.11 – POV-Ray depiction of (2-13a). C: gray, B: yellow-green, F: pink, P: orange.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): P(1)–C(1),

1.782(3); B(1)–C(1), 1.651(5); B(1)–F(1), 1.486(8); P(1)–C(1)–B(1), 108.6(2).

Exposure of a solution of the phosphine-borane Mes2PC≡CB(C6F5)2 generated in situ to H2

resulted in the isolation of the product (2-14) in 37% yield (Scheme 2.13). The 1H and 19F NMR

spectra of (2-14) obtained at RT contained substantially broadened signals, presumably due to

high degrees of rotational inhibition of bulky substituents. The 19F NMR spectrum measured at

100 °C revealed resonances indicative of two distinct B(C6F5)2 groups. Similarly the 11B{1H}

NMR data showed two signals at -16.9 (doublet with coupling to P) and -20.3 (broad) ppm.

Meanwhile the 1H NMR spectrum obtained at 100 °C showed the presence of an olefinic H

31

coupled to P, two distinct Mes2PH groups and a BH signal. The 31P{1H} resonances also

indicated two environments, with signals at -23.3 and -48.7 ppm. The precise structure of (2-14)

was unambiguously determined by crystallography to be

(C6F5)2(H)BC(H)=C(P(H)Mes2)((C6F5)2BC≡CP(H)Mes2) (Figure 2.12). These data reveal that

the phosphine-borane activates H2 although the mechanism for the formation of (2-14) is unclear.

Presumably the transiently formed intermolecularly H2 activated ion pair

[Mes2P(H)C≡CB(C6F5)2][Mes2PC≡CB(H)(C6F5)2] undergoes rearrangement and hydride

migration from B to its adjacent C (similar reactivity shown in Chapter 3), followed by addition

of another H2 to generate dissymmetric di-zwitterion (2-14) (Scheme 2.14). It is also possible

that the activation of the first equivalent of H2 occurs intramolecularly to form

Mes2P(H)C≡CB(H)(C6F5)2 that reacts with another molecule of Mes2PC≡CB(C6F5)2 to effect

Lewis acid-mediated hydride migration followed by the activation of another H2 to yield (2-14).

This reactivity stands in stark contrast to the reversible H2 activation that allows the facile

interconversion of Mes2PC6F4B(C6F5)2 and Mes2P(H)C6F4B(H)(C6F5)2.1 Indeed, despite the fact

that the present alkynyl-linked system demonstrates reactivity typical of an FLP, this reactivity

precludes its use as a hydrogenation catalyst.

Scheme 2.13 – Reaction of Mes2PC≡CB(C6F5)2 to form (2-14).

32

Figure 2.12 – POV-Ray depiction of (2-14). C: gray, H: gray, B: yellow-green, F: pink, P:

orange. Hydrogen atoms except PH, BH and olefinic H are omitted for clarity. Selected bond

distances (Å) and angles (°): C(1)–C(2), 1.201(4); P(1)–C(1), 1.711(3); B(1)–C(2), 1.599(4);

C(33)-C(34), 1.342(3); B(1)-C(33), 1.647(4); P(2)-C(33), 1.799(3); B(2)-C(34), 1.613(4); P(1)–

C(1)–C(2), 171.5(3); C(1)–C(2)–B(1), 173.9(3); B(1)–C(33)–C(34), 125.4(2); B(2)–C(34)–

C(33), 131.5(2).

Scheme 2.14 – Proposed mechanism for the formation of (2-14).

33

2.2.4 Zwitterionic Macrocycles of Alkynyl-Linked Phosphonium Borates

In probing the reactivity of the alkynyl-linked phosphine-borane species, we sought to exploit

these compounds to form macrocyclic products via FLP reactivity. The corresponding neutral

phosphine-borane to (2-6), tBu2PC≡CB(C6F5)2, was generated in situ by the reaction of (2-6)

with tBu3P and B(C6F5)3. This reaction results from the greater basicity and acidity of tBu3P and

B(C6F5)3, respectively, and generates the known salt [tBu3PH][HB(C6F5)3] as a by-product

(Scheme 2.15).2 Attempts to isolate the neutral species tBu2PC≡CB(C6F5)2 resulted in

unidentified decomposition products. Nonetheless the formation of this phosphine-borane was

confirmed by NMR analysis of the reaction mixture that showed 19F NMR resonances at -128.85,

-147.53, -162.36 ppm, and a 31P signal at 17.7 ppm without a characteristic P-H coupling (Figure

2.13). This species is believed to exist in the monomeric form in solution despite the highly

acidic B and basic P centers. In other work, we have demonstrated that FLPs are capable of

effecting ring opening reactions of THF72 as well as dioxane and thioxane.73 Here we exploit this

reactivity to prepare a symmetric macrocyclic species via reaction of tBu2PC≡CB(C6F5)2,

generated in situ, with THF. The ring opening of THF produced

[(tBu2PC≡CB(C6F5)2)(O(CH2)4)]2 (2-15) in 39% isolated yield (Scheme 2.15). The low yield is

presumably due to both the low stability of tBu2PC≡CB(C6F5)2 and the formation of larger

oligomers with ring-opened THF. While the spectroscopic data are as expected, X-ray

crystallography (Figure 2.14) reveals that the symmetry of the macrocycle (2-15) orients the

PC≡CB fragments approximately parallel to each other with the intramolecular P…P and B…B

distances of 6.67 and 7.96 Å, respectively.

Scheme 2.15 – Removal of proton and hydride from (2-6) and synthesis of (2-15).

34

Figure 2.13 – 31P and 19F NMR spectra of tBu2PC≡CB(C6F5)2 generated in situ in C6D6.

Figure 2.14 – POV-Ray depiction of (2-15). C: gray, B: yellow-green, F: pink, O: red, P:

orange. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°):

C(1)–C(2), 1.202(4); P(1)–C(1), 1.729(3); B(1)–C(2), 1.626(5); B(1)-O(1), 1.459(4); P(1)-C(26),

1.819(3); P(1)–C(1)–C(2), 172.9(3); C(1)–C(2)–B(1), 174.3(3).

Another example of using FLP reactivity to generate macrocycles is the reaction of the

phosphine-borane Mes2PC≡CB(C6F5)2, generated in situ, with an excess amount of PhC≡CH.

We have previously shown that FLPs which include less basic donors will effect donor and

acceptor addition to the alkyne.44 In the present case, this results in the formation of the

35

macrocyclic product [(Mes2PC≡CB(C6F5)2)(CH=CPh)]2 (2-16) (Scheme 2.16). In this

compound, the rigidity of the linkages enforces an approximately planar [PC≡CBC=C]2 core,

with the maximum deviation from the mean-square plane of 0.181 Å and the intramolecular

P…P and B…B distances of 6.85 and 5.59 Å, respectively (Figure 2.15).



Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): C(1)–C(2),

1.204(4); P(1)–C(1), 1.730(3); B(1)–C(2), 1.604(4); P(1)-C(33), 1.833(3); C(33)-C(34),

1.349(4); C(34)-B(1a), 1.641(4); P(1)–C(1)–C(2), 174.3(3); C(1)–C(2)–B(1), 171.3(3); P(1)–

C(33)–C(34), 117.8(2); C(33)–C(34)–B(1a), 130.7(3).

36

In attempts to carry out analogous additions to an olefin, tBu2PC≡CB(C6F5)2, generated in situ,

was treated with an excess amount of 1-hexene. A fluffy white solid (2-17) was isolated in 74%

yield, and its 1H and 13C{1H} NMR spectra were consistent with the incorporation of hexene and

the alkyne-linked phosphine-borane in a 1:2 ratio. The 31P{1H} and 11B data are consistent with

inequivalent quaternary P and B centers. The 19F spectrum obtained at RT showed broad signals

which split into 20 resonances at -50 °C (Figure 2.16), inferring that the B–C6F5 bond rotations

are in the intermediate exchange regime at RT. These data collectively are consistent with

intermolecular addition of P and B to hexene to generate phosphonium and borate centers. The

addition reaction enhances the Lewis acidity and basicity of the pendant B and P centers,

respectively, to promote dative bond formation on the other end. Thus the product is believed to

be the cyclic compound (tBu2PC≡CB(C6F5)2)2(BuCH2CH) (Scheme 2.17). The activation

parameters for the P–B dative bond cleavage could not be determined due to an alternative

reactivity that it shows concurrently with bond breakage (vide infra). This reaction is analogous

to the addition of simpler FLPs to olefins that we have previously reported.22

Scheme 2.17 – Synthesis of (2-17) and (2-18).

37

Figure 2.16 – 19F NMR of (2-17) at 25 oC (top) and -50 oC (bottom) in CD2Cl2.

Although (2-17) could not be structurally characterized, simple treatment of (2-17) with

methanol resulted in the protonation of the phosphine center of (2-18), cleaving the P–B dative

bond. With the methoxide binding to the borane center, the reaction afforded

(tBu2P(H)C≡CB(C6F5)2)(nBuCH2CH)(tBu2PC≡CB(C6F5)2)(OMe) (2-18) in 74% yield (Scheme

2.17). This species was crystallized and the structural data confirmed the formulation (Figure

2.17).

38

Figure 2.17 – POV-Ray depiction of (2-18). C: gray, H: gray, B: yellow-green, F: pink, O: red,

P: orange. Hydrogen atoms except PH are omitted for clarity. Selected bond distances (Å) and

angles (°): C(1)–C(2), 1.204(3); P(1)–C(1), 1.730(2); B(1)–C(2), 1.617(3); B(1)–O(1), 1.462(3);

C(23)–C(24), 1.207(3); P(2)–C(23), 1.721(3); B(2)–C(24), 1.601(3); B(2)–C(45), 1.651(3);

P(1)–C(46), 1.846(2); P(1)–C(1)–C(2), 175.6(2); C(1)–C(2)–B(1), 176.4(2); P(2)–C(23)–C(24),

170.9(2); B(2)–C(24)–C(23), 175.8(2); P(1)–C(46)–C(45), 113.81(15); B(2)–C(45)–C(46),

116.28(18).

To access a dissymmetric macrocycle, (2-17) was further reacted with THF, affording the new

macrocycle (tBu2PC≡CB(C6F5)2)(nBuCH2CH)(tBu2PC≡CB(C6F5)2)(O(CH2)4) (2-19) in 81%

yield (Scheme 2.18). The dissymmetry of the macrocycle (2-19) (Figure 2.18) results in PC≡CB

vectors oriented at an angle of 23.3o with respect to each other.

Scheme 2.18 – Reaction of (2-17) with THF to form (2-19).

39



C(1)–C(2), 1.206(3); P(1)–C(1), 1.732(2); B(1)–C(2), 1.619(3); C(23)-C(24), 1.206(3); P(2)–

C(23), 1.731(2); B(2)–C(24), 1.601(3); P(1)–C(1)–C(2), 173.7(2); C(1)–C(2)–B(1), 173.6(3);

P(2)–C(23)–C(24), 163.0(2); B(2)–C(24)–C(23), 169.0(2).

Species (2-17) precipitated out of the solution without complexation of second equivalent of 1-

hexene because of the formation of an intramolecular P–B adduct. Nonetheless (2-17) was

heated in the presence of a large excess of 1-hexene in an attempt to labilize the presumably

weak P–B dative bond and insert another molecule of 1-hexene (Scheme 2.19). Upon heating at

80 °C for 24 hours, a new species (2-20) was successfully isolated despite the formation of major

amounts of side products that gave rise to an yield of 28% to (2-20) (vide infra). The 11B{1H} of

(2-20) shows a broad peak at -18.1 ppm while the 31P{1H} NMR signals at 43.0 and 41.3 ppm

are consistent with the formation of a mixture of diastereomers (2-20a) and (2-20b) arising from

P/B addition to a second equivalent of olefin. 1H and 19F NMR data were also in agreement with

the generation of two diastereomers. It was speculated that these data were consistent with the

dimeric formulation [(tBu2PC≡CB(C6F5)2)(nBuCH2CH2)]2, a proposition that was confirmed

crystallographically on a single crystal of the RR/SS diastereomer (2-20a) (Figure 2.19).

40

Interestingly the P/B-alkynyl fragment adopts a quasi-parallel geometry. The macrocyclic nature

of (2-20a) results in intramolecular P…P and B…B distances of 5.82 and 5.40 Å, respectively.

Scheme 2.19 – Reaction of (2-17) with an excess of 1-hexene at 80 °C.

Figure 2.19 – POV-Ray depiction of (2-20a). C: gray, B: yellow-green, F: pink, P: orange.


1.213(4); P(1)–C(1), 1.718(3); B(1)–C(2), 1.605(5); C(23)–C(24), 1.206(4); P(2)–C(23),

1.729(3); B(2)–C(24), 1.593(4); C(45)–C(46), 1.553(4); C(51)–C(52), 1.551(4); P(1)–C(1)–C(2),

171.5(3); C(1)–C(2)–B(1), 170.8(3); P(2)–C(23)–C(24), 173.1(3); B(2)–C(24)–C(23), 172.8(3).

Since major amounts of side products resulting from competing reactions were observed in the

formation of (2-20) causing its low isolated yield, compound (2-17) was subjected to a

41

thermolysis experiment to uncover the identities of these undesired products. Heating a sample

of (2-17) in toluene-d8 at 80 °C for 10 hours converted the initially white suspension into a

yellow solution. Following workup, a new air- and moisture-stable compound (2-21), consisting

of two diastereomers (2-21a) and (2-21b) in a 1.6:1 ratio, was isolated in 52% yield (Scheme

2.20). Although it was impossible to resolve the diastereomers in quantitative yields, single

crystals of both diastereomers (2-21a) and (2-21b) were obtained on a few crystallization

attempts. The 31P{1H} and 11B{1H} NMR data for (2-21a) were consistent with the presence of

two types of phosphonium and borate fragments while the 19F NMR spectrum revealed the

presence of a B–F unit as well as an ortho-substituted C6F4 fragment and 3 inequivalent C6F5

rings (Figure 2.20). These NMR data suggest nucleophilic displacement of an ortho-F by the

thermally labilized phosphino group with simultaneous B–F bond formation. However, the

presence of significant degrees of 31P-31P coupling (32 and 29 Hz for (2-21a) and (2-21b),

respectively) was obscure for a macrocyclic compound (Figure 2.21). Although it was not

possible to unambiguously determine the backbone structure of (2-21a) from the spectroscopic

data, X-ray methods confirmed its formulation as

(tBu2P)C6F4BF(C6F5)C4B(C6F5)2(BuCH2CH)(PtBu2) (Figure 2.22) which is a di-zwitterion with

two discrete phosphonium-borate fragments linked via a chain of four carbons forming a

butatriene fragment. It is noted that the overall geometry is one in which the two B centers and

the two P centers adopt trans dispositions. The molecule contains two chirality centers, one on

the carbon derived from hexene and the second on the B center which is chiral as a result of

ortho-substitution of one of the fluoroarene rings. The crystal structure of the racemic mixture of

RS and SR enantiomers (2-21a) shows the cumulene linkage to be essentially linear with C–C–C

angles of 175.37(16)° and 178.22(16)° and C–C distances of 1.325(2), 1.267(2) and 1.328(2) Å,

consistent with the long-short-long bond distances commonly seen in butatrienes.74-76 1H, 11B{1H}, 19F and 31P{1H} NMR and IR data of the mixture of (2-21a) and (2-21b) revealed these

species to be structurally similar (Figure 2.20). Indeed, X-ray data confirmed (2-21b) is then the

RR/SS diastereomer of (2-21) (Figure 2.23). Butatrienes have attracted attention for their

potential applications in molecular wires,77 non-linear optic materials,78-79 and conjugated

polymers.80-81

42

Scheme 2.20 – Thermolysis of (2-17) to generate (2-21).

Figure 2.20 – 19F NMR spectra of (2-21a) alone (top) and a mixture of (2-21a) and (2-21b)

(bottom) in CD2Cl2.

43

Figure 2.21 – 31P-31P COSY spectrum of a mixture of (2-21a) and (2-21b) in CD2Cl2.

Figure 2.22 – POV-Ray depiction of (2-21a). C: gray, H: gray, B: yellow-green, F: pink, P:

orange. Hydrogen atoms except the chiral CH are omitted for clarity. Selected bond distances

(Å) and angles (°): C(1)–C(2), 1.325(2); C(2)–C(24), 1.267(2); C(24)–C(23), 1.328(2); P(1)–

C(1), 1.8018(15); B(1)–C(1), 1.667(2); B(1)–F(1), 1.418(2); P(2)–C(23), 1.8039(15); B(2)–

C(23), 1.666(2); P(2)–C(46), 1.8324(18); B(2)–C(45), 1.644(2); C(45)–C(46), 1.546(2); C(1)–

C(2)–C(24), 175.37(16); C(2)–C(24)–C(23), 178.22(16); B(1)–C(1)–P(1), 109.79(10); P(2)–

C(23)–B(2), 109.55(10).

44

Figure 2.23 - POV-Ray Depiction of (2-21b). C: gray, H: gray, B: yellow-green, F: pink, P:

orange. Hydrogen atoms except the chiral CH are omitted for clarity. Selected bond distances

(Å) and angles (°): C(1)–C(2), 1.323(3); C(2)–C(24), 1.265(3); C(24)–C(23), 1.330(3); P(1)–

C(1), 1.798(2); B(1)–C(1), 1.658(4); B(1)–F(1), 1.424(3); P(2)–C(23), 1.808(2); B(2)–C(23),

1.663(4); P(2)–C(46), 1.830(2); B(2)–C(45), 1.632(4); C(45)–C(46), 1.538(3); C(1)–C(2)–

C(24), 172.0(2); C(2)–C(24)–C(23), 178.3(3); B(1)–C(1)–P(1), 111.58(16); P(2)–C(23)–B(2),

109.33(16).

In the present case of the thermolysis of (2-17), the absence of B–H fragments precludes

boracyclopropene intermediates suggested to be involved in the thermal rearrangement of (2-6)

(vide supra). However, the strong polarization of the alkyne fragments resulting from the

phosphonium, phosphine, borane and borate groups in (2-17) suggests the possibility of

thermally induced intramolecular nucleophilic attack by phosphine82 and subsequent

rearrangements to give the resulting cumulene unit. Similar P attack at the ortho-positions of one

of the fluoroarene rings with concurrent F transfer to B completes the transformation to (2-21)

(Scheme 2.21).

45


2.2.5 Synthesis of Borataallene Compounds

In relation to the synthesis of the unique cumulene (2-21), we envisioned the preparation of a

boratacumulene. Boron-stabilized carboanions had long been known in organic synthesis,83-84

and the first stable species of such anionic borataalkene [CH2BMes2]- was isolated as a lithium

salt, and structurally characterized in 1987 by Power et al.85 In a fashion related to the synthesis

of this stable borataalkene, we examined the deprotonation of CH3C≡CBMes. Treatments with

tBuLi, LDA, or the combination of LDA and benzo-12-crown-4 did not give the target

boratacumulene in an isolable form. This is presumably due to the insufficient steric protection

of the deprotonated methylene site that retains a significant amount of negative charge.

We then examined the synthesis of a stable borataallene by reacting PhC≡CBMes2 with 1,3-di-

tert-butylimidazol-2-ylidene (ItBu) (Scheme 2.22). The two reagents were mixed together in

benzene to yield the yellow product (2-22) which was isolated in 77% yield. Its NMR data

46

revealed the presence of equivalent Mes groups and tBu groups along with 5 inequivalent phenyl

protons. The broad 11B NMR resonance at 21.0 ppm suggested a B center that is likely to be

planar. X-ray crystallographic analysis of (2-22) confirmed its structure to be

Mes2BCC(Ph)(ItBu), the target borataallene (Figure 2.24 (a)). (2-22) could be further protoneted

with lutidine hydrochloride at the C adjacent to B, showing that is a masked carbanion site. The

scope of such borataallene was expanded by employing the perfluorinated equivalent of the boryl

group, BMesF2, to synthesize MesF

2BCC(Ph)(ItBu) (2-23) (Scheme 2.22, Figure 2.24 (b)). The

solid-state structures of (2-22) and (2-23) both showed an approximately linear B–C–C fragment

and orthogonal substituent planes, consistent with the borataallenic formulation. The B–C

distances were determined to be 1.418(3) and 1.429(3) Å, respectively. These values are

somewhat shorter than those of previously reported such B–C linkages (1.444(8) Å,85 1.522 (10)

Å,86 1.475(6) Å87). This is likely to be caused by the sp-hybridized C(1). Interestingly, (2-23)

was stable under air, and even in wet solvents. This may be a result of both enhanced steric

protection by replacing H with F on the Mes groups, and increased Lewis acidity at B which

reduces the carbanionic character of the adjacent C.


47

Figure 2.24 – POV-Ray depictions of (a) (2-22) and (b) (2-23). C: gray, B: yellow-green, F:

pink, N: aquamarine. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and

angles (°): (a) B(1)–C(1), 1.418(3); C(1)–C(2), 1.327(3); C(2)–C(27), 1.490(3); B(1)–C(1)–C(2),

176.1(2). (b) B(1)–C(1), 1.429(3); C(1)–C(2), 1.327(3); C(2)–C(27), 1.493(3); B(1)–C(1)–C(2),

171.99(19).

2.3 Conclusions The present results demonstrate synthetic routes to a variety of alkynyl-linked phosphonium

borate derivatives. The neutral phosphine-boranes with an acetylenic spacer were generated in

situ from these derivatives. These species react as FLPs to afford such reactions as the

stoichiometric reduction of imine, activation of H2, addition to olefins and alkynes and ring

opening of THF. Exploiting this reactivity, synthetic routes to novel macrocyclic products have

been developed. Moreover alkynyl-phosphonium borates undergo unusual thermolysis reactions

to afford C4 derivatives, providing a new strategy to extended and conjugated systems with

electron rich and deficient centers.

48

2.4 Experimental Section 2.4.1 General Considerations

All manipulations were carried out under an atmosphere of dry, O2-free N2 employing an

MBraun glove box and a Schlenk vacuum-line. Solvents were purified with a Grubbs-type

column system manufactured by Innovative Technology and dispensed into thick-walled Schlenk

glass bombs equipped with Young-type Teflon valve stopcocks (hexanes, pentane, toluene,

CH2Cl2, THF, diethyl ether), or were dried over the appropriate agents and distilled into the same

kind of Young bombs (C6H5Br, methanol). All solvents were thoroughly degassed after

purification (repeated freeze-pump-thaw cycles). Deuterated solvents were dried over the

appropriate agents, vacuum-transferred into Young bombs and degassed accordingly (C6D5Br,

CD2Cl2, C6D6, toluene-d8). NMR spectra were recorded at 25 °C on Varian 300 and 400 MHz

and Bruker 400 MHz spectrometers unless otherwise noted. Chemical shifts are given relative to

SiMe4 and referenced to the residual solvent signal (1H, 13C) or relative to an external standard

(11B: (Et2O)BF3; 19F: CFCl3; 31P: 85% H3PO4). In some instances, signal and/or coupling

assignment was derived from two-dimensional NMR experiments. Chemical shifts are reported

in ppm and coupling constants as scalar values in Hz. Combustion analyses were performed in

house employing a Perkin-Elmer CHN Analyzer. Mes2PCl,88 ClB(C6F5)2,89 tBu2PCH2Li90 and

Mes2BCCPh91 were synthesized employing literature procedures. Tris(pentafluorophenyl)borane

was purchased from Boulder Scientific Company, and tri-tert-butylphosphine and chloro-di-tert-

butylphosphine were purchased from Strem Chemicals Inc. All other reagents were purchased

from Sigma-Aldrich, Alfa Aesar or AcrosOrganics. Trimethylsilylacetylene was dried over

CaCl2, distilled and stored over 4Å molecular sieves.

2.4.2 Syntheses

Synthesis of tBu2PC≡CH (2-1) This was prepared using a modification of the

literature method.61 Method 1: To a THF (20 mL) solution of HC≡CTMS (1.567 g,

15.95 mmol) was added nBuLi solution in hexane (1.6 M, 10.0 mL, 16.0 mmol) at 0

°C. After stirring at 0 °C for 30 min, ClPtBu2 (3.00 mL, 15.79 mmol) in THF (15 mL) was added

to the reaction mixture at -78 °C and stirred at that temperature for 30 min. The reaction was then

warmed to room temperature and stirred overnight. THF was pumped off completely while the

49

flask was kept in an ice bath. The product mixture was redissolved in pentane (15 mL) and

filtered through Celite. The solvent was evaporated under vacuum with the flask kept in an ice

bath to give tBu2PC≡CTMS. This product was stirred with potassium carbonate (4.42 g. 32.0

mmol) in methanol (17 mL) overnight followed by filtration through Celite. Pentane (10 mL)

was then added to the filtrate and the mixture separated using a separation funnel under a N2

atmosphere. The methanol layer was washed with pentane (10 mL) twice. The combined pentane

phase was passed through a plug of neutral alumina to eliminate dissolved methanol. Pentane

was pumped off while the flask was kept in an ice bath. The lightly colored liquid product was

used in subsequent reactions without further purification. The compound was stored at -35 °C to

avoid decomposition. Yield: 1.915 g, 71%. Method 2: Same as above for the generation of

tBu2PC≡CTMS. This product was stirred with potassium carbonate in methanol (8 mL)

overnight followed by filtration through Celite and washing with pentane (20 mL). The filtrate

was further passed through a column of neutral alumina and washed with pentane. Pentane was

pumped off while the flask was kept in an ice bath. 1H NMR (C6D6): 2.55 (s, 1H, C≡CH), 1.21

(d, 18H, 3JHP = 12.3 Hz, tBu). 13C{1H} NMR (C6D6): 95.45 (d, 2JCP = 5.0 Hz, PC≡C), 84.89 (d, 1JCP = 27.9 Hz, PC≡C), 33.26 (d, 1JCP = 17.4 Hz, quat-tBu), 30.59 (d, 2JCP = 14.7 Hz, tBu).

31P{1H} NMR (C6D6): 12.5 (s). EI-MS: calculated [HC≡CPtBu2]+: 170.1224, found: 170.1222.

Synthesis of Mes2PC≡CH (2-2) The procedure for synthesis of (2-1) was

followed in general except for the work up. TMSC≡CPMes2 was synthesized

using TMSC≡CH (1.2 g, 12.2 mmol), nBuLi (1.6 M hexane solution, 6.7 mL,

10.7 mmol) and Mes2PCl (3.00 g, 9.84 mmol). After Mes2PC≡CH formed in

methanol solution with potassium carbonate (3.0 g, 22 mmol), methanol was

completely pumped off from the reaction mixture. The product was extracted

with pentane, filtered through Celite, further passed through a plug of neutral alumina, and

recrystallized from pentane to yield colourless crystals, that were further dried in vacuo. This

crystalline product was suitable for X-ray diffraction. Yield: 1.293 g, 45%. 1H NMR (C6D6):

6.66 (d, 4H, 4JHP = 2.8 Hz, Ar-H), 2.65 (s, 1H, C≡CH), 2.48 (s, 12H, o-CH3), 2.04 (s, 6H, p-

CH3). 13C {1H} NMR (C6D6): 142.04 (d, 2JCP = 16.1 Hz, 2,6-C6H2), 138.32 (s, 4-C6H2), 130.21

(d, 3JCP = 3.9 Hz, 3,5-C6H2), 129.73 (d, 1JCP = 11.6 Hz, P–Cmes), 96.21 (d, 2JCP = 6.3 Hz,

PC≡CH), 83.25 (d, 1JCP = 13.1 Hz, PC≡CH), 23.10 (d, 2JCP = 14.1 Hz, o-CH3), 20.72 (s, p-CH3).

50

31P {1H} (C6D6): -54.5 (s). Anal. Calcd. for C20H23P: C, 81.60; H, 7.87. Found: C, 81.57; H, 7.82

%.

Synthesis of tBu2P(H)C≡CB(C6F5)3 (2-3) To a solution of B(C6F5)3

(150 mg, 0.29 mmol) in toluene (2 mL) was added a toluene (3 mL)

solution of (1) (50 mg, 0.29 mmol) at -35 °C. The reaction was stirred at room temperature for 1

h, and toluene was pumped down. Pentane was then added, and the mixture was filtered and

washed with pentane to isolate an off-white product. The product was further dried in vacuo.

Yield: 175 mg, 88%. Single crystals suitable for X-ray diffraction were grown by slow diffusion

of pentane into a CH2Cl2 solution of the product at 25 °C. 1H NMR (CD2Cl2): 5.71 (d, 1H, 1JHP =

466 Hz, PH), 1.45 (d, 18H, 3JHP = 18.8 Hz, tBu). 11B{1H} NMR (CD2Cl2): -21.4 (s).13C{1H}

NMR (CD2Cl2): δ 148.65 (dm, 1JCF = 242 Hz, o-C6F5), 139.66 (dm, 1JCF = 234 Hz, p-C6F5),

137.32 (dm, 1JCF = 240 Hz, m-C6F5), 120.51 (br, i-C6F5), 65.82 (d, 1JCP = 173 Hz, C≡CP), 35.28

(d, 1JCP = 43 Hz, quat-tBu), 26.98 (d, 2JCP = 2.6 Hz, tBu). C≡CB carbon was not observed. 19F

NMR (CD2Cl2): δ -133.11 (d, 6F, 3JFF = 23 Hz, o-C6F5), -161.46 (t, 3F, 3JFF = 20 Hz, p-C6F5), -

166.56 (m, 6F, m-C6F5). 31P{1H} NMR (CD2Cl2): 24.5 (s). EI-MS (m/z) in CH2Cl2: 682.1

[tBu2P(H)C≡CB(C6F5)3]+. Anal. Calcd. for C28H19BF15P: C, 49.30; H, 2.81. Found: C, 49.36; H,

3.02 %.

Synthesis of tBu2P(Me)C≡CB(C6F5)3 (2-4) CH3Li (150 μL, 1.6 M in

ether) was added to (2-3) (163 mL) in toluene (7 mL) solution. The

reaction was stirred overnight. The solvents were completely pumped off, and the white solid

was washed with pentane. To a suspension of the solid in CH2Cl2 (3 mL) was added CH3I (0.5

mL). The reaction was stirred overnight, filtered through Celite, and all volatiles were pumped

off from the filtrate. The solid was washed with pentane and recrystallized from CH2Cl2/pentane.

Yield: 96 mg, 58%. Single crystals suitable for X-ray diffraction were grown by layering a

CH2Cl2 solution of the product with pentane at RT. 1H NMR (CD2Cl2): 1.71 (d, 3H, 2JHP = 12.1

Hz, PCH3), 1.39 (d, 18H, 3JHP = 16.9 Hz, tBu). 11B{1H} NMR (CD2Cl2): -21.5 (s). 13C{1H}

NMR (CD2Cl2): 148.67 (dm, 1JCF = 237 Hz, o-C6F5), 139.57 (dm, 1JCF = 247 Hz, p-C6F5), 137.33

(dm, 1JCF = 228 Hz, m-C6F5), 121.07 (br, i-C6F5), 70.48 (d, 1JCP = 145 Hz, C≡CP), 35.35 (d, 1JCP

= 47 Hz, quat-tBu), 26.71 (d, 2JCP = 1.5 Hz, tBu), 4.44 (d, 1JCP = 57 Hz, PCH3). C≡CB carbon

was not observed. 19F NMR (CD2Cl2): -132.82 (m, 4F, o-C6F5), -161.34 (t, 2F, 3JFF = 21 Hz, p-

51

C6F5), -166.30 (m, 4F, m-C6F5). 31P {1H} NMR (CD2Cl2): 32.8 (s). Anal. Calcd. for

C29H21BF15P: C, 50.03; H, 3.04. Found: C, 49.49; H, 3.08 %.

Synthesis of tBu2P(H)C≡CB(Cl)(C6F5)2 (2-5) To a solution of

ClB(C6F5)2 (733 mg, 1.93 mmol) in toluene (2 mL) was added a solution

of (2-1) (328 mg, 1.93 mmol) in toluene (8 mL) at -35 °C. The reaction was stirred for 1 h,

during which time the solution color turned yellow. After the solvent was completely pumped

off, pentane (12 mL) was added and the mixture was stirred until an off-white solid formed. The

supernatant liquid was decanted and the solid product was washed with pentane. Drying in vacuo

afforded an off-white powder. This product was used in the subsequent reactions without further

purification. Yield: 761 mg, 72%. Single crystals suitable for X-ray diffraction were grown by

layering a CH2Cl2 solution of the product with hexane at 25oC. 1H NMR (CD2Cl2): 5.80 (d, 1H, 1JHP = 469 Hz, PH), 1.51 (d, 18H, 3JHP = 18.8 Hz, tBu). 11B{1H} NMR (CD2Cl2): -12.8 (s). 13C{1H} NMR (CD2Cl2): 148.26 (dm, 1JCF = 243 Hz, o-C6F5), 140.07 (dm, 1JCF = 251 Hz, p-

C6F5), 137.57 (dm, 1JCF = 248 Hz, m-C6F5), 119.81 (br, i-C6F5), 67.43 (d, 1JCP = 155 Hz, C≡CP),

35.41 (d, 1JCP = 42 Hz, quat-tBu), 26.99 (d, 2JCP = 2.5 Hz, tBu). C≡CB carbon was not observed. 19F NMR (CD2Cl2): -133.09 (m, 4F, o-C6F5), -160.50 (t, 2F, 3JFF = 20 Hz, p-C6F5), -166.08 (m,

4F, m-C6F5). 31P{1H} NMR (CD2Cl2): 25.5 (s). Anal. Calcd. for C22H19BClF10P: C, 47.99; H,

3.48. Found: C, 47.87; H, 3.80 %.

Synthesis of tBu2P(H)C≡CB(H)(C6F5)2 (2-6) Compound (2-5) (368 mg,

0.668 mmol) was stirred with Me2SiHCl (1.0 g, 5.83 mmol) overnight in

CH2Cl2. After confirming full conversion to the product by NMR of the reaction mixture, all the

volatiles were completely pumped off. The residue was dissolved in toluene and passed through

neutral alumina to eliminate high molecular weight impurities. The filtrate was dried and the

product was recrystallized by layering CH2Cl2 solution with pentane at RT overnight. The

product colorless crystals were separated from the supernatant, from which more product was

recrystallized using CH2Cl2/pentane. The combined product was dried in vacuo. Yield: 273 mg,

79%. Single crystals suitable for X-ray diffraction were grown by layering a CH2Cl2 solution of

the product with pentane at RT. 1H NMR (CD2Cl2): 5.70 (dd, 1H, 1JHP = 464 Hz, 5JHH = 2.0 Hz,

PH), 3.25 (q, 1H, 1JHB = 91 Hz, BH), 1.48 (d, 18H, 3JHP = 18.8 Hz, tBu). 11B{1H}: -29.2 (s). 13C{1H} NMR (CD2Cl2): 147.98 (dm, 1JCF = 235 Hz, o-C6F5), 138.77 (dm, 1JCF = 245 Hz, p-

C6F5), 136.93 (dm, 1JCF = 261 Hz, m-C6F5), 120.90 (br, i-C6F5), 64.37 (d, 1JCP = 158 Hz, C≡CP),

52

34.72 (d, 1JCP = 44 Hz, quat-tBu), 26.70 (d, 2JCP = 2.6 Hz, tBu). C≡CB carbon was not observed. 19F NMR (CD2Cl2): -133.23 (dm, 4F, 3JFF = 21 Hz, o-C6F5), -162.79 (t, 2F, 3JFF = 20 Hz, p-C6F5),

-166.67 (m, 4F, m-C6F5). 31P{1H} NMR (CD2Cl2): 24.2 (s). IR (CH2Cl2, cm-1): 2125 (υ(C≡C)).

UV-Vis (acetonitrile): C = 1.2 x 10-5 M; λmax = 258 nm, ε = 350 M-1cm-1; λmax = 229 nm, ε =

4600 M-1cm-1; λmax = 208 nm, ε = 12000 M-1cm-1. Anal. Calcd. for C22H20BF10P: C, 51.19; H,

3.91. Found: C, 51.35; H, 4.02 %.

Synthesis of tBu2P(Me)C≡CB(H)(C6F5)2 (2-7) This compounds was

prepared in a fashion analogous to (2-4). Yield: 43 mg, 84%. 1H NMR

(CD2Cl2): 3.20 (q, 1H, 1JHB = 91 Hz, BH), 1.72 (d, 3H, 2JHP = 12.1 Hz, PCH3), 1.42 (d, 18H, 3JHP

= 16.9 Hz, tBu). 11B{1H} NMR (CD2Cl2): -29.2 (s). 13C{1H} NMR (CD2Cl2): 148.42 (dm, 1JCF =

238 Hz, o-C6F5), 139.14 (dm, 1JCF = 239 Hz, p-C6F5), 137.30 (dm, 1JCF = 246 Hz, m-C6F5),

121.65 (br, i-C6F5), 69.44 (d, 1JCP = 165 Hz, C≡CP), 35.29 (d, 1JCP = 48 Hz, quat-tBu), 26.83 (d, 2JCP = 1.3 Hz, tBu), 4.70 (d, 1JCP = 57 Hz, CH3). The C≡CB carbon was not observed. 19F NMR

(CD2Cl2): -132.91 (d, 4F, 3JFF = 20 Hz, o-C6F5), -162.65 (t, 2F, 3JFF = 20 Hz, p-C6F5), -166.40

(m, 4F, m-C6F5). 31P{1H} NMR (CD2Cl2): 31.9 (s). Anal. Calcd. for C23H22BF10P: C, 52.10; H,

4.18. Found: C, 51.74; H, 3.97 %.

Synthesis of [tBu2P(H)C≡CB(C6F5)2(THF)][B(C6F5)4]

(2-8) To a stirring solution of [CPh3][B(C6F5)4] (66 mg,

0.072 mmol) in CH2Cl2 (1 mL) was drop-wise added a

solution of (2-6) (50 mg, 0.097 mmol) in CH2Cl2 (1 mL)

until the reaction mixture reached a point where the intense yellow colour of [CPh3][B(C6F5)4]

faded. (Not all (2-6) solution was used.) The reaction mixture was stirred for a further 5 min,

followed by addition of THF (30 μL, 0.37 mmol) and stirring for another 5 min. The colorless

solution was pumped down to ca. 0.5 mL. Addition of pentane (5 mL) precipitated a sticky solid

product, which was left stirring for 3 h. The mother liquor was removed, and the sticky solid

dried in vacuo. The product was further recrystallized from CH2Cl2/pentane at RT. Yield: 70 mg,

77%. 1H NMR (CD2Cl2): 5.83 (d, 1H, 1JHP = 475 Hz, PH), 4.37 (m, 4H, O(CH2CH2)2), 2.30 (m,

4H, O(CH2CH2)2), 1.51 (d, 18H, 3JHP = 19.2 Hz, tBu). 11B{1H} NMR (CD2Cl2): -3.2 (br,

B(THF)), -16.6 (s, B(C6F5)4). 13C{1H} NMR (CD2Cl2): 150.3-135.5 (broad signals of o-, p- and

m-C6F5 overlapping), 124.62 (br, i-C6F5 , B(C6F5)4), 113.19 (br, i-C6F5 , B(C6F5)2), 78.05 (s,

O(CH2CH2)2), 73.66 (d, 1JCP = 143 Hz, C≡CP), 35.70 (d, 1JCP = 39 Hz, quat-tBu), 27.04 (d, 2JCP

53

= 3.0 Hz, tBu), 25.65 (s, O(CH2CH2)2). C≡CB was not observed. 13C{19F} NMR (CD2Cl2):

147.18 (s, o-C6F5 , B(C6F5)4), 146.74 (s, o-C6F5 , B(C6F5)2), 140.50 (s, p-C6F5 , B(C6F5)2), 137.44

(s, p-C6F5 , B(C6F5)4), 136.87 (s, m-C6F5 , B(C6F5)2), 135.54 (q, 3JCB = 3.0 Hz, m-C6F5 ,

B(C6F5)4), 123.38 (q, 1JCB = 51 Hz, i-C6F5 , B(C6F5)4). 19F NMR (CD2Cl2): -133.51 (m, 8F, o-

C6F5, B(C6F5)4), -133.81 (m, 4F, o-C6F5, B(C6F5)2), -153.92 (t, 2F, 3JFF = 20 Hz, p-C6F5,

B(C6F5)2), -162.44 (m, 4F, m-C6F5, B(C6F5)2), -164.12 (t, 4F, 3JFF = 20 Hz, p-C6F5, B(C6F5)4), -

167.97 (m, 8F, m-C6F5, B(C6F5)4). 31P{1H} NMR (CD2Cl2): 26.8 (s). Anal. Calcd. for

C50H27B2F30OP: C, 47.43; H, 2.15. Found: C, 47.83; H. 2.58 %.

Synthesis of [HPtBu3][Mes2PC≡CB(C6F5)3] (2-9) To a

toluene (3 mL) solution of B(C6F5)3 (83 mg, 1.6 mmol) and

PtBu3 (33 mg, 1.6 mmol) was added a toluene (3 mL) solution of (2-2) (48 mg, 1.6 mmol) at -35

°C. After 3 h of stirring, the solvent was pumped off completely. Pentane (6 mL) was then added

and the mixture was stirred overnight. The white solid precipitated. The supernatant liquid was

removed, and the product was further washed with pentane and dried. The product was

recrystallized from benzene/heptane at RT. Yield: 142 mg, 87%. The crystalline product was

suitable for X-ray diffraction. 1H NMR (CD2Cl2): 6.70 (s, 4H, Ar-H), 5.04 (d, 1H, 1JHP = 428 Hz,

HP), 2.26 (s, 12H, o-CH3), 2.19 (s, 6H, p-CH3), 1.61 (d, 27H, 3JHP = 15.6 Hz, tBu). ). 11B{1H}

NMR (CD2Cl2): -20.9 (s). 13C{1H} NMR (CD2Cl2): 148.62 (dm, 1JCF = 240 Hz, o-C6F5), 142.30

(d, 2JCP = 15 Hz, o-C6H2), 138.76 (dm, 1JCF = 246 Hz, p-C6F5), 137.92 (s, p-C6H2), 137.06 (dm, 1JCF = 245 Hz, m-C6F5), 132.39 (d, 1JCP = 12 Hz, i-C6H2), 129.91 (d, 3JCP = 4 Hz, m-C6H2), 38.19

(d, 1JCP = 26 Hz, quat-tBu), 30.52 (s, tBu), 22.72 (d, 3JCP = 14 Hz, o-CH3, Mes), 21.10 (s, p-CH3,

Mes). C≡CP, C≡CB and i-C6F5 carbons were not observed. 19F NMR (CD2Cl2): -132.25 (dm, 6F, 3JFF = 23 Hz, o-C6F5), -164.04 (t, 3F, 3JFF = 20 Hz, p-C6F5), -167.46 (tm, 6F, 3JFF = 20 Hz, m-

C6F5). 31P{1H} (CD2Cl2): 61.4 (s, HPtBu3), -52.9 (s, PMes2). Anal. Calcd. for C50H50BF15P2: C,

59.54; H, 5.00. Found: C, 59.27; H, 5.02 %.

Synthesis of Mes2PC≡CB(NCMe)(C6F5)2 (2-10) Compound (2-2) (37

mg, 0.13 mmol) and PtBu3 (25 mg, 0.12 mmol) were dissolved in

toluene (1 mL) together, and pre-cooled at -35 °C. The phosphine

mixture was added to a pre-cooled solution of ClB(C6F5)2 (95 mg, 0.25

mmol) in toluene (1 mL) at -35 °C, giving a red solution. After the reaction was stirred for 1 h, it

was pumped down to ca. 0.5 mL. Hexanes (6 mL) were added to precipitate a yellow solid

54

([HPtBu3][Cl2B(C6F5)2]), and the mixture was kept at -35 °C for 2 h. This was filtered through

Celite to give a red-orange solution of Mes2PC≡CB(C6F5)2. After acetonitrile (20 μL) was added,

the solution was kept at -35 °C to afford yellowish crystals of the product. Yield: 43 mg, 53%.

The product crystals were suitable for X-ray diffraction. 1H NMR (CD2Cl2): 6.77 (d, 4H, 4JHP =

3.0 Hz, Ar-H), 2.45 (s, 3H, CH3, NCMe), 2.29 (s, 12H, o-CH3), 2.22 (s, 6H, p-CH3). 11B{1H}

NMR (CD2Cl2): -12.6 (br). 13C{1H} NMR (CD2Cl2): 148.19 (dm, 1JCF = 243 Hz, o-C6F5), 142.47

(d, 2JCP = 15 Hz, o-C6H2), 140.67 (dm, 1JCF = 255 Hz, p-C6F5), 138.83 (s, p-C6H2), 137.12 (dm, 1JCF = 248 Hz, m-C6F5), 130.39 (d, 1JCP = 10 Hz, i-C6H2), 130.21 (d, 3JCP = 4 Hz, m-C6H2),

114.95 (s, C≡N), 22.93 (d, 3JCP = 14 Hz, o-CH3, Mes), 21.12 (s, p-CH3, Mes), 3.53 (s, CH3,

NCMe). The i-C6F5 and acetylenic carbons were not observed. 19F NMR (CD2Cl2): -133.93 (m,

4F, o-C6F5), -157.88 (t, 2F, 3JFF = 20 Hz, p-C6F5), -164.54 (m, 4F, m-C6F5). 31P{1H} NMR

(CD2Cl2): -53.0 (s). Anal. Calcd. for C34H25BF10NP: C, 60.11; H, 3.71; N, 2.06. Found: C, 59.43;

H, 3.74; N, 2.11 %.

Synthesis of tBu2PC≡CB(tBuN(H)CH2Ph)(C6F5)2 (2-11) Compound

(2-6) (32 mg, 0.06 mmol) and tBuN=CHPh (10 mg, 0.06 mmol) were

stirred in CH2Cl2 (1-2 mL) for 2 h at RT. Colorless crystals formed from

concentrate at -35 °C over 2 days. The supernatant liquid was quickly decanted and the crystals

quickly washed with cold pentane and dried. More crystalline product was obtained from the

combined and concentrated washings at -35 °C. Yield: 24 mg, 57%. The crystalline product was

suitable for X-ray diffraction. 1H NMR (C6D6): 6.82 (m, 3H, Ph), 6.52 (br, 2H, Ph), 5.10 (br, 1H,

NH), 3.75 (br, 2H, CH2), 1.41 (br, 18H, PtBu2), 1.26 (s, 9H, NtBu). 11B{1H} NMR (C6D6): -9.6

(br). 13C{1H} NMR (C6D6): 148.55 (dm, 1JCF = 243 Hz, o-C6F5), 140.77 (dm, 1JCF = 243 Hz, p-

C6F5), 137.95 (dm, 1JCF = 249 Hz, m-C6F5), 135.87 (s, i-Ph), 129.16 (s, Ph), 126.10 (s, Ph),

119.74 (br, i-C6F5), 65.31 (br, C≡CP), 53.66 (s, quat-NtBu), 53.27 (s, CH2), 33.49 (d, 1JCP = 17

Hz, quat-PtBu2), 30.51 (d, 2JCP = 14 Hz, PtBu2), 28.42 (s, NtBu). C≡CB carbon was not

observed. 19F NMR (C6D6): -128.46 (br, 2F, o-C6F5), -128.93 (br, 2F, o-C6F5), -156.86 (br, 2F, p-

C6F5), -163.89 (br, 2F, m-C6F5), -164.21 (br, 2F, m-C6F5). 31P{1H} (C6D6): 18.6 (s). Anal. Calcd.

for C33H35BF10NP: C, 58.43; H, 5.20; N, 2.21. Found: C, 58.05; H, 5.60; N, 2.30 %.

55

Synthesis of (tBu2PCHBF(C6F4)(C6F5)CH)2 (2-12) A J-Young

NMR tube charged with (2-6) (84 mg, 0.163 mmol) dissolved in

bromobenzene-d5 (1 mL) was heated at 150 °C for 12 h. The colour

of the solution started turning yellow after a few minutes, and then

turned orange in ca. 5 minutes. After stopping the heat, the product

was allowed to precipitate out of the orange solution by layering

benzene (1 mL) overnight. The crystalline solid was filtered out and

washed with benzene, and was dried in vacuo to give a yellow

powder. Yield: 34 mg, 40%. Single crystals suitable for X-ray

diffraction were obtained from a reaction mixture. 1H NMR (DMSO-d6): 5.90 (m, 2H, =CH),

3.01 (m, 2H, P(B)CH), 1.46 (d, 18H, 3JHP = 16.0 Hz, tBu), 1.18 (d, 18H, 3JHP = 15.6 Hz, tBu). 11B {1H} NMR (DMSO-d6): 3.1 (br). 13C{1H} NMR (DMSO-d6): 28.46 (s, tBu), 27.92 (s, tBu).

No other carbon resonances were observed due to the low solubility of the compound. 19F NMR

(DMSO-d6) -122.24 (m, 2F, C6F4), -130.59 (br, 2F, o-C6F5), -133.04 (m, 2F, C6F4), -135.99 (br,

2F, o-C6F5), -147.81 (m, 2F, C6F4), -154.10 (m, 2F, C6F4), -161.06 (m, 2F, p-C6F5), -165.29 (br,

4F, m-C6F5), -185.99 (br, 2F, BF). 31P {1H} (DMSO-d6): 75.2 (s). EI-MS (m/z) in acetone:

1032.2 [(tBu2PCHBF(C6F4)(C6F5)CH)2]+. Anal. Calcd. for C44H40B2F20P2: C, 51.19; H, 3.91.

Found: C, 51.05; H, 4.27 %.

Synthesis of (C6F5)(C6F4)XBCH2PtBu2 (X = F (2-13a)/Cl (2-13b) = 3.2/1)

A solution of ClB(C6F5)2 (400 mg, 1.05 mmol) in hexanes (10 mL) was

added to a suspension of tBu2PCH2Li (182 mg, 1.10 mmol) in hexanes (8

mL) at RT. The reaction was stirred overnight and then the solvents were

pumped off completely. The resulting solid residue was redissolved in benzene (10 mL) and the

insoluble lithium salts were filtered out using Celite. The filtrate was passed through a column of

neutral alumina to eliminate the brown-colored impurities. Benzene was pumped down from the

filtrate, and the concentrated solution (0.5 mL) was layered with hexanes (10 mL). After 2 days

of standing at 25 °C, a colorless microcrystalline solid was isolated, washed with pentane, and

dried in vacuo. Yield: 196 mg, 38%. Single crystals suitable for X-ray diffraction were grown by

layering a CH2Cl2 solution of the product with pentane at 25 °C. 5a: 1H NMR (CD2Cl2): 1.53 (m,

1H, CH2), 1.44 (d, 9H, 3JHP = 16.0 Hz, tBu), 1.38 (d, 9H, 3JHP = 15.6 Hz, tBu), 1.29 (m, 1H,

CH2). 11B (CD2Cl2): 4.6 (d, 2JBF = 74 Hz). 13C{1H} NMR (CD2Cl2): 35.21 (d, 1JCP = 34.0 Hz,

56

quat-tBu), 34.77 (d, 1JCP = 34.9 Hz, quat-tBu), 27.25 (s, tBu), 11.70 (br, CH2). 19F NMR

(CD2Cl2): -123.28 (m, 1F, C6F4), -130.67 (t, 1F, 3JFF = 21 Hz, C6F4), -135.17 (m, 2F, o-C6F5), -

146.95 (m, 1F, C6F4), -154.98 (m, 1F, C6F4), -160.78 (t, 1F, 3JFF = 20 Hz, p-C6F5), -165.39 (m,

2F, m-C6F5), -177.28 (br, 1F, B-F). 31P{1H} NMR (CD2Cl2): 87.1 (m). 5b: 1H NMR (CD2Cl2):

1.99 (dd, 1H, 2JHH = 16.5 Hz, 2JHP = 12.8 Hz, CH2), 1.65 (dd, 1H, 2JHH = 16.5 Hz, 2JHP = 8.8 Hz,

CH2), 1.50 (d, 9H, 3JHP = 16.0 Hz, tBu), 1.25 (d, 9H, 3JHP = 15.6 Hz, tBu). 11B (CD2Cl2): -2.0

(br). 13C{1H} NMR (CD2Cl2): 35.34 (d, 1JCP = 34.3 Hz, quat-tBu), 35.27 (d, 1JCP = 33.1 Hz,

quat-tBu), 27.52 (d, 2JCP = 2.7 Hz, tBu), 26.57 (s, tBu), 14.64 (br, CH2). 19F NMR (CD2Cl2): -

123.90 (m, 1F, C6F4), -125.94 (m, 1F, C6F4), -132.55 (m, 2F, o-C6F5), -146.88 (m, 1F, C6F4), -

154.84 (m, 1F, C6F4), -159.79 (t, 1F, 3JFF = 20 Hz, p-C6F5), -165.02 (m, 2F, m-C6F5). 31P{1H}

NMR (CD2Cl2): 88.8 (m). Anal. Calcd. for C21H20BF9PX (X = F/Cl = 3.2/1): C, 49.64; H, 3.87.

Found: C, 49.67; H, 3.65 %.

Synthesis of

(C6F5)2B(H)HC=C(P(H)Mes2)((C6F5)2BC≡CP(H)Mes2) (2-14) The

procedure described in the synthesis of (2-10) was followed to

generate Mes2PC≡CB(C6F5)2 in situ from compound (2-2) (75 mg,

0.25 mmol), ClB(C6F5)2 (190 mg, 0.50 mmol) and PtBu3 (50 mg, 0.25 mmol) in toluene/hexanes

mixture. The red-orange solution was transferred to a 50 mL Schlenk bomb which was

subsequently charged with ca. 4 atm of H2 through three freeze-pump-thaw cycles. The reaction

was kept under H2 pressure for 2 days, during which time a light yellow precipitate formed. All

the contents of the bomb were transferred to a vial, and the yellow solid was isolated and washed

with hexanes. The product was further recrystallized from CH2Cl2/pentane at -35 °C and

extensively washed with pentane, yielding a white powder. Yield: 59 mg, 37%. 1H NMR

(C6D5Br, 373 K): 8.90 (br d, 1H, 3JHP = 78.1 Hz, =CH), 8.14 (d, 1H, 1JHP = 511 Hz, PH, ≡CPH),

7.72 (br d, 1H, 1JHP = 457 Hz, PH, =CPH), 6.73 (d, 4H, 4JHP = 5.2 Hz, Ar-H, ≡CPHMes2), 6.54

(br, 4H, Ar-H, =CPHMes2), 3.9-3.0 (br, 1H, BH), 2.5-2.0 (br, 12H, o-CH3, =CPHMes2), 2.27 (s,

12H, o-CH3, ≡CPHMes2), 2.09 (s, 6H, p-CH3), 2.05 (s, 6H, p-CH3). 11B{1H} NMR (C6D5Br, 373

K): -16.9 (d, 2JBP = 17 Hz, B(C6F5)2), -20.3 (br, BH(C6F5)2). 13C{1H} NMR (C6D5Br, 273 K):

147.68 (dm, 1JCF = 240 Hz, overlapped signals of o-C6F5), 145.18, 142.78, 142.30, 142.18,

142.07, 141.86, 140.79, 136.28 (dm, 1JCF = 248 Hz, overlapped signals of m-C6F5), 111.84 (br,

ArMes), 21.45 (br, CH3), 20.69, 20.61, 20.53, 20.44, 20.14. A number of carbon signals were

57

hidden underneath the solvent peaks or could not be unambiguously assigned for a spectrum

obtained at 0 °C. 19F NMR (C6D5Br, 373 K): -127.02 (br, 4F, o-C6F5, B(C6F5)2), -130.53 (br, 4F,

o-C6F5, BH(C6F5)2), -159.44 (br, 2F, p-C6F5, B(C6F5)2), -163.77 (br, 2F, p-C6F5, BH(C6F5)2), -

164.22 (br, 4F, m-C6F5, B(C6F5)2), -166.32 (br, 4F, m-C6F5, BH(C6F5)2). 31P{1H} NMR (C6D5Br,

373 K): -23.3 (br, =CP), -48.7 (s, ≡CP). Anal. Calcd. for C64H48B2F20P2: C, 60.03; H, 3.78.

Found: C, 59.99; H, 3.66 %.

Synthesis of [(tBu2PC≡CB(C6F5)2)(O(CH2)4)]2 (2-15) To a toluene

solution of (2-6) was added a mixture of B(C6F5)3 and PtBu3 dissolved

in toluene. The reaction mixture immediately turned orange-yellow,

and became cloudy upon stirring for 10 min. Hexanes were added, and

the reaction was left standing at -35 °C for 30 min. The mixture was

then filtered through Celite and was washed with cold hexanes. More

hexanes were added to the combined filtrate to a total amount of 18 mL. (ca. 2mM of the

(C6F5)2BC≡CPtBu2 species) Upon addition of THF (40 μL), the solution immediately turned

colorless. The reaction was kept at 25 °C without stirring until a colorless crystalline material

precipitated out (typically 2 days). The product was washed with pentane and dried in vacuo.

Yield: 8 mg, 39%. Single crystals suitable for X-ray diffraction were obtained from a reaction

mixture. 1H NMR (CD2Cl2): 3.55 (t, 4H, 3JHH = 5.93 Hz, CH2OB), 2.22 (m, 4H, PCH2), 1.96 (m,

4H, PCH2CH2CH2), 1.75 (m, 4H, PCH2CH2), 1.38 (d, 36H, 3JHP = 16.4 Hz, tBu). 11B{1H} NMR

(CD2Cl2): -7.3 (s). 13C{1H} NMR (CD2Cl2): 148.36 (dm, 1JCF = 240 Hz, o-C6F5), 139.44 (dm, 1JCF = 246 Hz, p-C6F5), 137.32 (dm, 1JCF = 245 Hz, m-C6F5), 69.91 (d, 1JCP = 149 Hz, C≡CP),

65.47 (s, BOCH2), 36.39 (d, 1JCP = 44.7 Hz, quat-tBu), 33.62 (d, 3JCP = 13.7 Hz, PCH2CH2CH2),

26.96 (s, tBu), 25.06 (d, 2JCP = 6.4 Hz, PCH2CH2), 18.24 (d, 1JCP = 48.7 Hz, PCH2). The i-C6F5

and C≡CB carbons were not observed. 19F NMR (CD2Cl2): -133.98 (dm, 16F, 3JFF = 24 Hz, o-

C6F5), -161.63 (t, 8F, 3JFF = 20 Hz, p-C6F5), -166.20 (m, 16F, m-C6F5). 31P{1H} NMR (CD2Cl2):

38.1 (s). Anal. Calcd. for C52H52B2F20O2P2: C, 53.27; H, 4.47. Found: C, 53.36; H, 3.95 %.

Synthesis of [(Mes2PC≡CB(C6F5)2)(CH=CPh)]2 (2-16) The

procedure described in the synthesis of (2-10) was followed to generate

Mes2PC≡CB(C6F5)2 in situ from compound (2-2) (26 mg, 0.088

mmol), ClB(C6F5)2 (68 mg, 0.18 mmol) and PtBu3 (18 mg, 0.089

mmol) in toluene/hexanes mixture. Phenylacetylene (100 μL, 0.91 mmol) was syringed into the

58

reaction in one portion. After stirring at RT for 30 min, the reaction was pumped down and

pentane was added to precipitate the product. The yellow product was further recrystallized from

CH2Cl2/pentane at RT. Yield: 18 mg, 28%. Single crystals suitable for X-ray diffraction were

grown by layering a CH2Cl2 solution of the product with pentane at 25 °C. 1H NMR (CD2Cl2): δ

9.07 (d, 2H, 3JHP = 40.3 Hz, CH), 6.91-6.74 (m, 18H, Ph, Mes), 2.35 (s, 24H, ortho-CH3), 2.27

(s, 12H, para-CH3). 11B{1H} NMR (CD2Cl2): δ -19.4 (s). 13C{1H} NMR (CD2Cl2): 177.5 (=CB,

observed in HSQC), 144.00 (d, 4JCP = 2.8 Hz, p-C6H2), 143.10 (d, 2JCP = 10.3 Hz, o-C6H2),

132.04 (d, 3JCP = 11.5 Hz, m-C6H2), 130.34 (d, JCP = 5.3 Hz, Ph), 127.95 (d, JCP = 2.9 Hz, Ph),

127.17 (d, JCP = 1.5 Hz, Ph), 120.53 (d, 1JCP = 91 Hz, i-C6H2), 23.84 (d, 3JCP = 6.2 Hz, o-CH3,

Mes), 21.25 (s, p-CH3, Mes). 13C{19F} NMR (CD2Cl2): 147.74 (s, o-C6F5), 139.04 (s, p-C6F5),

136.93 (s, m-C6F5). 19F NMR (CD2Cl2): -132.53 (dm, 8F, 3JFF = 20 Hz, o-C6F5), -162.26 (t, 4F, 3JFF = 20 Hz, p-C6F5), -166.53 (m, 8F, m-C6F5). 31P{1H} NMR (CD2Cl2): -4.7 (s). Anal. Calcd.

for C80H56B2F20P2: C, 64.89; H, 3.81. Found: C, 62.85; H, 4.08 %.

Synthesis of (tBu2PC≡CB(C6F5)2)(BuCH2CH)(tBu2PC≡CB(C6F5)2)

(2-17) To a toluene (1.5 mL) solution of (2-6) (88 mg, 0.17 mmol) was

added a mixture of B(C6F5)3 (86 mg, 0.17 mmol) and tBu3P (34 mg,

0.17 mmol) dissolved in toluene (1.0 mL). The reaction mixture

immediately turned orange-yellow, and became cloudy upon stirring for 1 h. Hexanes (8 mL)

were added, and the reaction was left standing at -35 °C for 2 h. The mixture was then filtered

through Celite and washed with cold hexanes. After addition of 1-hexene (2.3 mL) to the

combined filtrate, the yellow solution was left standing at 25 °C without stirring until a white

fluffy solid precipitated out (typically 1-2 days). The product was washed with pentane and dried

in vacuo. Yield: 70 mg, 74%. 1H NMR (CD2Cl2, 298 K): 2.71 (m, 1H, PCH), 2.61 (m, 2H,

PCHCH2), 1.83 (m, 2H, BCH2), 1.60 (d, 9H, 3JHP = 15.4 Hz, tBu), 1.56 (m, 2H, PCHCH2CH2)

1.49 (d, 9H, 3JHP = 14.9 Hz, tBu), 1.43 (d, 9H, 3JHP = 14.7 Hz, tBu), 1.07 (d, 9H, 3JHP = 14.9 Hz,

tBu), 1.02 (m, 2H, CH2CH3), 0.68 (t, 3H, 3JHH = 7.3 Hz, CH2CH3). 11B{1H} NMR (CD2Cl2, 298

K): -16.50 (br s), -18.02 (s). 13C{1H} NMR (CD2Cl2, 223 K): 147.67 (dm, 1JCF = 232 Hz, o-

C6F5), 140.10(dm, 1JCF = 250 Hz, p-C6F5), 136.76 (dm, 1JCF = 238 Hz, m-C6F5), 80.58 (d, 1JCP =

156 Hz, C≡CP), 78.26 (d, 1JCP = 122 Hz, C≡CP), 40.99 (d, 1JCP = 22 Hz, quat-tBu), 39.56 (d, 1JCP = 36 Hz, quat-tBu), 39.02 (d, 1JCP = 22 Hz, quat-tBu), 37.52 (d, 1JCP = 33 Hz, quat-tBu),

37.12 (d, 1JCP = 38 Hz, PCH), 34.35 (s, BCH2), 31.31 (d, 3JCP = 16 Hz, CH2CH2CH2), 29.79 (s,

59

tBu), 29.34 (s, tBu), 29.01 (s, tBu), 28.64 (s, tBu), 26.46 (br s, PCHCH2), 23.32 (s, CH2CH3),

14.02 (s, CH2CH3). C≡CB and i-C6F5 carbons were not observed. 19F NMR (CD2Cl2, 223 K): -

120.95 (dm, 1F, 3JFF = 23 Hz, o-C6F5, PB), -122.73 (m, 1F, o-C6F5, PB), -128.12 (dm, 1F, 3JFF =

24 Hz, o-C6F5, PB), -130. 40 (dm, 1F, 3JFF = 21 Hz, o-C6F5, PCHCH2B), -131.95 (dd, 1F, 3JFF =

25 Hz, 4JFF = 7.4 Hz, o-C6F5, PCHCH2B), -132.31 (m, 1F, o-C6F5, PB), -133,17 (dm, 1F, 3JFF =

25 Hz, o-C6F5, PCHCH2B), -135,12 (m, 1F, o-C6F5, PCHCH2B), -155.35 (t, 1F, 3JFF = 21 Hz, p-

C6F5, PB), -157.04 (t, 1F, 3JFF = 21 Hz, p-C6F5, PB), -162.19 (t, 1F, 3JFF = 21 Hz, p-C6F5,

PCHCH2B), -162, 60 (t, 1F, 3JFF = 21 Hz, p-C6F5, PCHCH2B), -163.04 (tm, 1F, 3JFF = 22 Hz, m-

C6F5, PB), -163.73 (tm, 1F, 3JFF = 22 Hz, m-C6F5, PB), -164.51 (m, 1F, m-C6F5, PB), -164.79 (m,

1F, m-C6F5, PB), -165.13 (m, 1F, m-C6F5, PCHCH2B), -165.68 (m, 1F, m-C6F5, PCHCH2B), -

166.34 (m, 1F, m-C6F5, PCHCH2B), -166.89 (m, 1F, m-C6F5, PCHCH2B).31P{1H} (CD2Cl2, 295

K): 41.47 (s), 25.45 (br s). IR (thin film from CH2Cl2, cm-1): 2124 (υ(C≡C)). Anal. Calcd. for

C50H48B2F20P2: C, 53.98; H, 4.35. Found: C, 54.09; H, 4.40 %.

Synthesis of

(tBu2P(H)C≡CB(C6F5)2)(nBuCH2CH)(tBu2PC≡CB(C6F5)2)

(OMe) (2-18) To a solution of compound (2-17) (55 mg,

0.049 mmol) in CH2Cl2 (8 mL) in a well-dried Schlenk flask

was added pre-dried CH3OH (250 μL). After stirring at RT for 2days, all volatiles were removed

in vacuo. The remaining was redissolved in CH2Cl2 (5 mL), solvent pumped down, and the crude

product precipitated with addition of pentane (12 mL). Recrystallization from toluene/pentane

gave a colourless crystalline product. Yield: 42 mg, 74%. The product crystals were suitable for

X-ray diffraction. 1H NMR (CD2Cl2) : 5.71 (d, 1H, 1JHP = 465 Hz, PH), 3.26 (s, 3H, OCH3), 2.42

(br m, 1H, PCH), 1.95 (m, 1H, PCHCH2), 1.82 (br m, 2H, BCH2), 1.59 (d, 9H, 3JHP = 15.6 Hz,

tBu), 1.50 (d, 9H, 3JHP = 18.8 Hz, tBu), 1.46 (d, 9H, 3JHP = 18.7 Hz, tBu), 1.45 (d, 9H, 3JHP =

15.1 Hz, tBu), 1.43 (m, 1H, PCHCH2), 1.41 (m, 1H, PCHCH2CH2), 1.04 (m, 1H, PCHCH2CH2),

0.82 (m, 1H, PCHCH2CH2CH2), 0.65 (m, 1H, PCHCH2CH2CH2), 0.65-0.60 (br m, 3H,

PCHCH2CH2CH2CH3). 11B NMR (CD2Cl2): -6.9 (s, BOCH3), -18.6 (br s, BCH2CHP). 13C{1H}

NMR (CD2Cl2): 148.35 (dm, 1JCF = 245 Hz, overlapped signals of o-C6F5), 139.33 (dm, 1JCF =

253 Hz, overlapped signals of p-C6F5), 137.45 (dm, 1JCF = 248 Hz, overlapped signals of m-

C6F5), 124.7-121.8 (br, overlapped signals of i-C6F5), 52.97 (s, OCH3), 39.10 (d, 1JCP = 38.3 Hz,

quart-tBu), 37.59 (d, 1JCP = 40.1 Hz, quart-tBu), 37.30 (d, 1JCP = 34.7 Hz, PCH), 35.59 (d, 1JCP =

60

42.5 Hz, quart-tBu), 35.51 (d, 1JCP = 42.5 Hz, quart-tBu), 34.71 (d, 3JCP = 3.8 Hz, PCHCH2CH2),

33.26 (d, 2JCP = 1.9 Hz, PCHCH2), 29.02 (s, tBu), 28.41 (s, tBu), 27.14 (d, 2JCP = 1.4 Hz, tBu),

27.09 (d, 2JCP = 1.7 Hz, tBu), 23.9 (br, BCH2), 23.20 (s, PCHCH2CH2CH2), 13.92 (s,

PCHCH2CH2CH2CH3). The acetylenic carbons were not observed. 19F NMR (CD2Cl2): -131.85

(m, 2F, o-C6F5), -132.24 (m, 2F, o-C6F5), -134.16 (m, 4F, o-C6F5), -160.39 (t, 1F, 3JFF = 20 Hz,

p-C6F5), -160.87 (t, 1F, 3JFF = 20 Hz, p-C6F5), -162.10 (t, 1F, 3JFF = 20 Hz, p-C6F5), -162.14 (t,

1F, 3JFF = 20 Hz, p-C6F5), -165.13 (m, 4F, m-C6F5), -166.40 (m, 4F, m-C6F5). 31P{1H} NMR

(CD2Cl2): 39.3 (br s, PCHCH2B), 24.4 (s, PH). Anal. Calcd. for C51H51B2F20OP2: C, 53.57; H,

4.50. Found: C, 53.83; H, 5.00 %.

Synthesis of (tBu2PC≡CB(C6F5)2)(nBuCH2CH)-

(tBu2PC≡CB(C6F5)2) (O(CH2)4) (2-19) Compound (2-17) (30 mg)

was dissolved in CH2Cl2 (10 mL) and THF (0.5 mL). The resulting

mixture was left standing at 25 °C without stirring for 1 day. The

solvent was completely pumped off. Following the addition of a

small amount of pentane, the solid product was further dried in vacuo. Recrystallization from

CH2Cl2/pentane was performed when further purification was necessary. Yield: 26 mg, 81%.

Single crystals suitable for X-ray diffraction were grown by layering a CH2Cl2 solution of the

product with pentane at 25 °C. 1H NMR (CD2Cl2): 3.55-3.41 (m, 2H, OCH2), 2.89-2.77 (m, 1H,

PCH), 2.30-2.09 (m, 2H, BCH2), 2.05-1.89 (m, 4H, PCHCH2CH2, PCH2), 1 70-1.61 (m, 2H,

OCH2CH2), 1.58 (d, 9H, 3JHP = 15.5 Hz, tBu), 1.52 (d, 9H, 3JHP = 15.3 Hz, tBu), 1.48 (d, 9H, 3JHP = 16.5 Hz, tBu), 1.41 (d, 9H, 3JHP = 16.5 Hz, tBu), 1.19-1.08 (m, 2H, PCH2CH2), 0.86-0.77

(m, 2H, PCHCH2CH2), 0.71 (t, 3H, 3JHH = 7.0 Hz, CH3), 0.55-0.44 (m, 2H, CH2CH3). 11B{1H}

NMR (CD2Cl2): -7.1 (s), -18.3 (br s). 13C{1H} NMR (CD2Cl2): 148.40 (dm, 1JCF = 236 Hz, o-

C6F5), 139.33 (dm, 1JCF = 249 Hz, p-C6F5), 137.44 (dm, 1JCF = 254 Hz, m-C6F5), 123.52 (br, i-

C6F5), 73.29 (dm, 1JCP = 144 Hz, C≡CP), 71.37 (dm, 1JCP = 146 Hz, C≡CP), 64.22 (s, OCH2),

41.23 (m, PCH), 39.14 (d, 1JCP = 38 Hz, quat-tBu), 38.39 (d, 1JCP = 38 Hz, quat-tBu), 37.10 (d, 1JCP = 44 Hz, quat-tBu), 36.38 (d, 1JCP = 45 Hz, quat-tBu), 34.34 (s, OCH2CH2), 34.20 (d, 2JCP =

10 Hz, PCH2CH2), 31.69 (s, PCHCH2CH2), 29.67 (s, tBu), 29.30 (s, tBu), 27.22 (s, tBu), 26.86

(s, tBu), 24.71 (d, 2JCP = 6 Hz, BCH2), 23.56 (s, CH2CH2CH3), 18.45 (d, 1JCP = 49 Hz, PCH2),

13.81 (s, CH3). 19F NMR (CD2Cl2): -131.63 (m, 2F, o-C6F5), -132.12 (d, 2F, 3JFF = 21 Hz, o-

C6F5), -134.15 (d, 4F, 3JFF = 22 Hz, o-C6F5), -161.58 (t, 2F, 3JFF = 21 Hz, p-C6F5), -161.87 (t, 1F,

61

3JFF = 20 Hz, p-C6F5), -162.07 (t, 1F, 3JFF = 21 Hz, p-C6F5), -165.48 (m, 2F, m-C6F5), -165.84

(m, 2F, m-C6F5), -166.45 (m, 2F, m-C6F5), -166.73 (m, 2F, m-C6F5). 31P{1H} NMR (CD2Cl2):

40.6 (br s), 38.2 (s). IR (thin film from CH2Cl2, cm-1): 2126 (υ(C≡C)). Anal. Calcd. for

C54H56B2F20OP2: C, 54.75; H, 4.76. Found: C, 54.69; H, 4.92 %.

Synthesis of [(tBu2PC≡CB(C6F5)2)(nBuCH2CH2)]2 (2-20) A 50 mL

Schlenk bomb was charged with compound (2-17) (23 mg, 0.021

mmol) in toluene (1.5 mL) and a stir bar. 1-Hexene (1.5mL, 12.0

mmol) was syringed into the bomb, which was then tightly closed and

heated at 80 °C for 24 h. The light yellow solution was poured out into a vial and hexanes (8 mL)

was added to the solution. After standing in a freezer at -35 °C for 3 days, colourless crystals

precipitated out. They were isolated, washed with hexanes and dried in vacuo to give a white

product as a mixture of two diastereomers. Yield: 7 mg, 28%. The crystalline product was

suitable for X-ray diffraction studies. Mixture of two diastereomers: 1H NMR (CD2Cl2): 2.59

(m), 2.42 (m), 2.09 (m), 1.81 (m), 1.57 (d, 18H, 3JHP = 14.9 Hz, tBu), 1.51 (d, 18H, 3JHP = 15.3

Hz, tBu), 1.46 (d, 18H, 3JHP = 15.4 Hz, tBu), 1.41 (m), 1.24 (m), 1.21 (d, 18H, 3JHP = 15.0 Hz,

tBu), 0.99 (m), 0.88 (t, 3JHH = 7.2 Hz), 0.73 (t, 3JHH = 7.2 Hz), 0.67 (m). 11B{1H} NMR

(CD2Cl2): -18.1 (m). 13C{1H} NMR (CD2Cl2): 40.33 (d, 1JCP = 38 Hz, quart-tBu), 39.21 (d, 1JCP

= 38 Hz, quart-tBu), 38.54 (d, 1JCP = 40 Hz, quart-tBu), 37.41 (d, 1JCP = 39 Hz, quart-tBu),

34.71, 34.41, 33.13 (s), 32.84 (d, JCP = 10 Hz), 31.56 (d, JCP = 13 Hz), 29.66 (s, tBu), 29.56 (s,

tBu), 29.43 (s, tBu), 28.18 (s, tBu), 23.73, 23.53 (s), 22.91(s), 14.39 (s, CH3), 13.97 (s, CH3),

13.88 (s, CH3). 13C{19F} NMR (CD2Cl2): 148.48 (s, o-C6F5), 148.40 (s, o-C6F5), 148.09 (s, o-

C6F5), 139.09 (s, p-C6F5), 137.49 (s, m-C6F5), 137.41 (s, m-C6F5). 19F NMR (CD2Cl2): -129.34

(dm, 4F, 3JFF = 20 Hz, o-C6F5), -131.23 (dm, 4F, 3JFF = 20 Hz, o-C6F5), -132.30 (m, 4F, o-C6F5),

-132.68 (dm, 4F, 3JFF = 20 Hz, o-C6F5), -161.68 (t, 2F, 3JFF = 20 Hz, p-C6F5), -162.00 (t, 2F, 3JFF

= 20 Hz, p-C6F5), -162.16 (t, 2F, 3JFF = 20 Hz, p-C6F5), -162.52 (t, 2F, 3JFF = 20 Hz, p-C6F5), -

165.77 (m, 8F, m-C6F5), -166.18 (m, 8F, m-C6F5). 31P{1H} NMR (CD2Cl2): 43.0 (m), 41.3 (s).

Anal. Calcd. for C56H60B2F20P2: C, 56.21; H, 5.05. Found: C, 55.68; H, 5.25 %.

Synthesis of (tBu2P)C6F4BF(C6F5)C4B

(C6F5)2(BuCH2CH)(PtBu2) (2-21) A suspension of (2-17) (21

mg, 0.02 mmol) in toluene-d8 (0.5 mL) was heated at 80 °C in

a J-Young NMR tube for 10 h, during which time the reaction

62

mixture turned yellow. After confirming the full conversion of the product by NMR, the solvent

was pumped off completely from the mixture. The product was then purified on a flash column

of neutral alumina (initial wash with hexane followed by elution of the yellow band with

hexane:ethyl acetate = 9:1). The eluent was dried under vacuum to afford a yellow product.

Yield: 11 mg, 52%. Slow evaporation of toluene from a product solution gave single crystals of

(2-21a), and another setup afforded single crystals of (2-21b), both of which were suitable for X-

ray diffraction. (2-21a): 1H NMR (CD2Cl2): 3.08 (m, 1H, PCH), 2.42 (m, 2H, BCH2), 2.07 (m,

1H, PCHCH2), 1.84 (m, 1H, PCHCH2), 1.80 (m, 2H, PCHCH2CH2), 1.58 (d, 9H, 3JHP = 16.7

Hz, tBu), 1.46 (m, 2H, CH2CH3), 1.31 (d, 9H, 3JHP = 16.5 Hz, tBu), 1.29 (d, 9H, 3JHP = 14.7 Hz,

tBu), 1.17 (d, 9H, 3JHP = 14.5 Hz, tBu), 0.99 (t, 3H, 3JHH = 7 Hz, CH3). 11B{1H} NMR (CD2Cl2):

3.50 (br), -9.76 (br). 19F NMR (CD2Cl2): -122.42 (m, 1F, C6F4), -130.59 (dm, 2F, 3JFF = 24 Hz,

o-C6F5), -131.41 (m, 2F, o-C6F5), -131.86 (m, 1F, C6F4), -132.15 (m, 2F, o-C6F5), -146.27 (tm,

1F, 3JFF = 21 Hz, C6F4), -154.19 (tm, 1F, 3JFF = 21 Hz, C6F4), -160.00 (t, 1F, 3JFF = 20 Hz, p-

C6F5), -161.13 (t, 1F, 3JFF = 20 Hz, p-C6F5), -162.74 (t, 1F, 3JFF = 20 Hz, p-C6F5), -164.66 (m,

2F, m-C6F5), -165.72 (m, 4F, two overlapping signals of m-C6F5), -173.46 (br, 1F, BF). 31P{1H}

(CD2Cl2): 59.1 (m), 56.7 (dm, 5JPP = 32 Hz). (2-21b): 1H NMR (CD2Cl2): 3.01 (m, 1H, PCH),

2.42 (m, 2H, BCH2), 2.07 (m, 1H, PCHCH2), 1.86 (m, 1H, PCHCH2), 1.80 (m, 2H,

PCHCH2CH2), 1.51 (d, 9H, 3JHP = 14.6 Hz, tBu), 1.48 (d, 9H, 3JHP = 15.0 Hz, tBu), 1.46 (m, 2H,

CH2CH3), 1.44 (d, 9H, 3JHP = 14.6 Hz, tBu), 0.97 (d, 9H, 3JHP = 14.4 Hz, tBu), 0.96 (t, 3H, 3JHH

= 7 Hz, CH3). 11B{1H} NMR (CD2Cl2): 3.50 (br), -9.76 (br). 19F NMR (CD2Cl2): -122.48 (m, 1F,

C6F4), -129.75 (dm, 2F, 3JFF = 24 Hz, o-C6F5), -131.19 (m, 1F, C6F4), -131.44 (m, 2F, o-C6F5), -

131.73 (m, 2F, o-C6F5), -146.39 (tm, 1F, 3JFF = 22 Hz, C6F4), -154.34 (tm, 1F, 3JFF = 20 Hz,

C6F4), -159.89 (t, 1F, 3JFF = 20 Hz, p-C6F5), -161.12 (t, 1F, 3JFF = 20 Hz, p-C6F5), -162.64 (t, 1F, 3JFF = 20 Hz, p-C6F5), -164.93 (m, 2F, m-C6F5), -165.50 (m, 2F, m-C6F5), -165.73 (m, 2F, m-

C6F5), -171.20 (br, 1F, BF). 31P{1H} (CD2Cl2): 59.1 (m), 55.6 (dm, 5JPP = 29 Hz). (2-21a)/(2-

21b) mixture: 13C{1H} NMR (CD2Cl2): 194.46. (m, C=C), 191.45 (m, C=C), 190.87 (m, C=C),

148.52 (dm, 1JCF = 241 Hz, C6F5), 147.52 (dm, 1JCF = 235 Hz, C6F5), 137.54 (dm, 1JCF = 250 Hz,

C6F5), 151-109 (broad multiplets corresponding to cumulenic, C6F5 and C6F4 carbons), 40.19 (d, 1JCP = 26 Hz, quat-tBu), 39.70 (d, 1JCP = 29 Hz, quat-tBu), 39.41 (d, 1JCP = 30 Hz, quat-tBu),

38.84 (d, 1JCP = 27 Hz, quat-tBu), 38.83 (m, PCH), 38.72 (d, 1JCP = 26 Hz, quat-tBu), 38.38 (m,

PCH), 37.85 (d, 1JCP = 28 Hz, quat-tBu), 37.50 (d, 1JCP = 29 Hz, quat-tBu), 36.24 (d, 1JCP = 30

Hz, quat-tBu), 33.11 (s, PCHCH2), 33.00 (s, PCHCH2), 31.97 (s, PCHCH2CH2), 31.84 (s,

63

CH2CH3), 30.24 (m, BCH2), 28.88 (s, tBu), 28.67 (s, tBu), 28.07 (s, tBu), 27.51 (s, tBu), 27.22

(s, tBu), 23.16 (s, tBu), 14.38 (s, CH2CH3). IR (thin film from CH2Cl2, cm-1): 1734 (υ(=C=C=)).

Anal. Calcd. for C50H48B2F20P2: C, 53.98; H, 4.35. Found: C, 53.95; H, 4.53 %.

Synthesis of Mes2BCC(Ph)(ItBu) (2-22) A solution of 1,3-di-tert-

butylimidazol-2-ylidene (60mg, 0.33 mmol) in benzene (3 mL) was added

to a solution of Mes2BCCPh (117 mg, 0.33 mmol) in benzene (2 mL), and

the mixture was stirred at RT for 5 h. The solvent was pumped off and the yellow remaining

solid was washed with pentane (3 x 5 mL). The product was then dried in vacuo. Yield: 134 mg,

77%. Single crystals suitable for crystallographic analysis were obtained by layering a

dichloromethane solution with pentane at RT. 1H NMR (CD2Cl2): 7.96 (d, 1H, J = 7.0 Hz, Ph),

7.36 (m, 1H, Ph), 7.23 (s, 2H, =CH, NHC), 7.16 (m, 1H, Ph), 7.07 (t, 1H, J = 7.0 Hz, Ph), 6.67

(s, 4H, Mes), 6.39 (d, 1H, J = 7.0 Hz, Ph), 2.21 (s, 12H, o-CH3, Mes), 2.18 (s, 6H, p-CH3, Mes),

1.43 (s, 18H, C(CH3)3, NHC). 11B NMR (CD2Cl2): 21.0 (br). 13C{1H} NMR (CD2Cl2): 147.29 (s,

NCN), 145.03 (br, B-Cmes), 141.66 (s, 4o), 141.55 (s, 4o, Mes), 133.54 (s, 4o, Mes), 128.95 (s,

CH, Ph), 127.87 (s, CH, Ph), 127.55 (s, CH, Mes), 127.28 (s, CH, Ph), 125.65 (s, CH, Ph),

124.98 (s, CH, Ph), 117.70 (s, =CH, NHC), 103.47 (s, 4o), 63.11 (s, C(CH3)3, NHC), 30.31 (s,

C(CH3)3, NHC), 24.43 (s, o-CH3, Mes), 21.25 (s, p-CH3, Mes). UV-Vis (THF): C = 3.0 x 10-5 M,

6.0 x 10-5 M, 1.2 x 10-4 M; λmax = 359 nm, ε = 6400 M-1cm-1; λmax = 277 nm, ε = 25000 M-1cm-1.

Anal. Calcd. for C37H47BN2: C, 83.76; H, 8.93; N, 5.28. Found: C, 83.58; H, 8.82; N, 5.22 %.

Synthesis of MesF2BCC(Ph)(ItBu) (2-23) At RT, a solution of 1,3-di-

tert-butylimidazol-2-ylidene (37 mg, 0.21 mmol) in toluene (3 mL) was

added to a solution of alkynylborane MesF2BCCPh (138 mg, 0.20 mmol)

on toluene (2 mL). The reaction was left stirring for 12 hrs during which time the solution turned

yellow and a solid precipitated out. Pentane (5 mL) was added to facilitate precipitation, and the

solid product was isolated. It was further washed with pentane (5 mL) and dried well under

vacuum. Yield: 167 mg, 98 %. Single crystals of the compound were obtained by layering a

benzene solution with cyclohexane at RT. 1H NMR (CD2Cl2): 7.98 (s, 4H, Ar-H, MesF), 7.90 (d,

1H, J = 7.9 Hz, Ph), 7.38 (m, 1H, Ph), 7.22 (s, 2H, =CH, ItBu), 7.15 (m, 2H, Ph), 6.32 (m, 1H,

Ph), 1.23 (s, 18H, C(CH3)3, ItBu). 11B NMR (CD2Cl2): 17.3 (br). 13C{1H} NMR (CD2Cl2)

:151.07 (br, B-CAr), 146.93 (s, NCN), 140.74 (s, 4o), 136.39 (q, 2JCF = 30 Hz, CAr(CF3)), 129.44

(s, CH, Ph), 128.93 (s, CH, Ph), 127.84 (q, 2JCF = 33 Hz, CAr(CF3)), 127.57 (s, 4o), 126.75 (s,

64

CH, Ph), 126.71 (s, CH, Ph), 126.17 (br s, Ar-H, MesF), 124.88 (q, 1JCF = 275 Hz, CF3), 124.23

(q, 1JCF = 275 Hz, CF3), 118.29 (s, =CH, ItBu), 108.32 (s, 4o), 63.75 (s, C(CH3)3, ItBu), 30.04 (s,

C(CH3)3, ItBu). 19F NMR (CD2Cl2): -55.92 (br s, 12F, o-CF3), -63.19 (s, 6F, p-CF3). UV-Vis

(acetonitrile): C = 1.4 x 10-5 M, 2.8 x 10-5 M, 5.6 x 10-5 M; λmax = 384 nm, ε = 11000 M-1cm-1;

λmax = 289 nm, ε = 15000 M-1cm-1. Anal. Calcd. for C37H29BF18N2: C, 52.01; H, 3.42; N, 3.28.

Found: C, 51.99; H, 3.52; N, 3.31 %.

2.4.3 X-ray Crystallography 2.4.3.1 X-ray Data Collection and Reduction

Crystals were coated in paratone-N oil in the glovebox, mounted on a MiTegen Micromount and

placed under an N2 stream, thus maintaining a dry, O2-free environment for each crystal. The

data were collected on a Bruker Apex II diffractometer employing Mo Kα radiation (λ = 0.71073

Å). Data collection strategies were determined using Bruker Apex software and optimized to

provide >99.5% complete data to a 2θ value of at least 55°. The frames were integrated with the

Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for

absorption effects using the empirical multi-scan method (SADABS).

2.4.3.2 X-ray Data Solution and Refinement

Non-hydrogen atomic scattering factors were taken from the literature tabulations.92 The heavy

atom positions were determined using direct methods employing the SHELXTL direct methods

routine. The remaining non-hydrogen atoms were located from successive difference Fourier

map calculations. The refinements were carried out by using full-matrix least squares techniques

on F, minimizing the function (Fo-Fc)2 where the weight is defined as 4Fo2/2 (Fo

2) and Fo

and Fc are the observed and calculated structure factor amplitudes, respectively. In the final

cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors

in the absence of disorder or insufficient data. In the latter cases atoms were treated isotropically.

C-H atom positions were calculated and allowed to ride on the carbon to which they are bonded

assuming a C-H bond length of 0.95 Å. H-atom temperature factors were fixed at 1.20 times the

isotropic temperature factor of the C-atom to which they are bonded. The H-atom contributions

65

were calculated, but not refined. The locations of the largest peaks in the final difference Fourier

map calculation as well as the magnitude of the residual electron densities in each case were of

no chemical significance.

66

2.4.3.3 Select Crystallographic Data

Table 2.1 – Selected crystallographic data for (2-2), (2-3), and (2-4).

(2-2) (2-3) (2-4)

Formula C20H23P C28H19BF15P C29H21BF15P

Formula wt 294.35 682.21 696.24

Crystal system triclinic triclinic triclinic

Space group P-1 P-1 P-1

a(Å) 8.4239(2) 11.324(2) 11.4977(5)

b(Å) 8.5161(2) 11.779(2) 12.0518(9)

c(Å) 13.2078(4) 12.130(2) 12.1979(5)

α(deg) 89.5780(10) 97.80(3) 94.603(2)

β(deg) 81.6420(10) 116.71(3) 117.814(2)

γ(deg) 63.2320(10) 99.71(3) 104.561(2)

V(Å3) 835.18(4) 1382.9(5) 1407.88(13)

Z 2 2 2

T (K) 296(2) 293(2) 293(2)

d(calc) gcm-3 1.170 1.638 1.642

Abs coeff,μ, mm-1 0.157 0.220 0.218

Data collected 10827 13306 32523

Rint 0.0225 0.0388 0.0239

Data used 2922 4853 9541

Variables 194 410 415

R (>2σ) 0.0404 0.0431 0.0339

wR2 0.1169 0.1295 0.0979

GOF 1.070 0.969 1.030

Data collected Mo Kα radiation (λ = 0.71073 Å).

67

Table 2.2 – Selected crystallographic data for (2-5), (2-6) and (2-9).

(2-5).1/2 hexane (2-6) (2-9)

Formula C25H26BClF10P

·½C6H14 C22H20BF10P C50H50BF15P2

Formula wt 593.69 516.16 1008.65

Crystal system triclinic triclinic monoclinic

Space group P-1 P-1 P21/c

a(Å) 10.994(2) 10.627(2) 11.8014(6)

b(Å) 11.115(2) 11.169(2) 11.8058(5)

c(Å) 12.530(3) 11.373(2) 34.0730(17)

α(deg) 63.76(3) 111.60(3) 90

β(deg) 87.74(3) 94.38(3) 99.949(2)

γ(deg) 72.91(3) 108.67(3) 90

V(Å3) 1305.3(5) 1159.7(4) 4675.8(4)

Z 2 2 4

T (K) 293(2) 293(2) 150(2)

d(calc) gcm-3 1.511 1.478 1.433

Abs coeff,μ, mm-1 0.293 0.206 0.189


Rint 0.0476 0.0510 0.0372

Data used 5908 5243 10638


R (>2σ) 0.0608 0.0562 0.0515

wR2 0.2143 0.2039 0.1298

GOF 0.874 0.796 1.037


68


(2-10) (2-11).dichloromethane (2-12)

Formula C34H25BF10NP C33H35BF10NP.CH2Cl2 C44H40B2F20P2

Formula wt 679.33 762.33 1032.32


Space group P-1 P-1 P21/n

a(Å) 10.5253(12) 12.4714(4) 8.8534(18)

b(Å) 11.1465(13) 12.5975(4) 16.981(3)

c(Å) 13.5988(15) 12.8627(4) 15.482(3)

α(deg) 87.034(6) 115.3710(10) 90

β(deg) 73.772(6) 91.599(2) 105.76(3)

γ(deg) 89.045(6) 103.967(2) 90

V(Å3) 1529.8(3) 1751.96(10) 2240.2(8)

Z 2 2 2

T (K) 150(2) 296(2) 293(2)

d(calc) gcm-3 1.475 1.445 1.530

Abs coeff,μ, mm-1 0.178 0.311 0.214


Rint 0.0325 0.0275 0.0440

Data used 14491 11462 5087


R (>2σ) 0.0536 0.0393 0.0501

wR2 0.1767 0.1092 0.1395

GOF 1.051 1.026 1.033


69


(2-13) (2-13a/2-13b =

1/1) (2-14) (2-15)

Formula C21H20BCl0.5F9.5P C64H48B2F20P2 C52H52B2F20O2P2

Formula wt 512.38 1280.59 1172.50

Crystal system monoclinic monoclinic monoclinic

Space group P21/n P21/n P21/n

a(Å) 12.3226(7) 15.1889(3) 12.1107(8)

b(Å) 11.8199(6) 18.5653(3) 12.1556(9)

c(Å) 15.8360(10) 20.6828(5) 18.3862(14)

α(deg) 90 90 90

β(deg) 109.850(2) 92.7150(10) 107.725(4)

γ(deg) 90 90 90

V(Å3) 2169.5(2) 5825.7(2) 2578.2(3)

Z 4 4 2

T (K) 296 (2) 296(2) 296(2)

d(calc) gcm-3 1.569 1.460 1.510

Abs coeff,μ, mm-1 0.276 0.181 0.198


Rint 0.0428 0.0414 0.1237

Data used 3804 10082 5925


R (>2σ) 0.0460 0.0479 0.0573

wR2 0.1019 0.1334 0.1268

GOF 1.085 1.022 0.998


70


(2-16).2 pentane (2-18).1/2 dichloromethane (2-19)

Formula C80H56B2F20P2

.2(C5H12)

C51H52B2F20OP2

.1/2CH2Cl2 C54H56B2F20OP2

Formula wt 1625.15 1186.96 1184.55

Crystal system monoclinic triclinic triclinic

Space group P21/n P-1 P-1

a(Å) 15.3523(5) 11.4003(5) 9.8925(5)

b(Å) 17.5876(5) 14.3264(6) 15.2017(7)

c(Å) 16.0607(6) 18.8399(8) 18.9044(9)

α(deg) 90 69.583(2) 85.827(2)

β(deg) 103.4400(10) 81.810(2) 83.141(2)

γ(deg) 90 75.939(2) 74.115(2)

V(Å3) 4217.8(2) 2791.7(2) 2712.4(2)

Z 2 2 2

T (K) 296(2) 150(2) 296(2)

d(calc) gcm-3 1.336 1.410 1.450

Abs coeff,μ, mm-1 0.143 0.229 0.188


Rint 0.0374 0.0424 0.0442

Data used 7260 15519 10392


R (>2σ) 0.0574 0.0579 0.0422

wR2 0.1758 0.1749 0.1242

GOF 1.047 1.001 1.045


71

Table 2.6 – Selected crystallographic data for (2-20), (2-21a) and (2-21b).

(2-20) (2-21a) (2-21b)

Formula C56H60B2F20P2 C50H48B2F20P2 C50H48B2F20P2

Formula wt 1196.60 1112.44 1112.44

Crystal system monoclinic monoclinic triclinic

Space group P21/n P21/c P-1

a(Å) 11.6457(3) 18.6390(9) 12.3275(6)

b(Å) 26.1762(8) 13.2382(7) 12.9875(7)

c(Å) 22.0144(7) 20.6464(11) 17.7581(10)

α(deg) 90 90 83.416(3)

β(deg) 91.7130(10) 108.277(2) 79.967(3)

γ(deg) 90 90 65.581(2)

V(Å3) 6707.9(3) 4837.4(4) 2546.3(2)

Z 4 4 2

T (K) 296(2) 296(2) 150(2)

d(calc) gcm-3 1.185 1.527 1.451

Abs coeff,μ, mm-1 0.152 0.204 0.194


Rint 0.0417 0.0434 0.0491

Data used 11556 6908 14737


R (>2σ) 0.0582 0.0334 0.0439

wR2 0.1780 0.0795 0.1151

GOF 1.118 1.012 1.018


72

Table 2.7 – Selected crystallographic data for (2-22) and (2-23).

(2-22) (2-23)

Formula C37H47BN2 C37H29BF18N2

Formula wt 530.58 854.43

Crystal system orthorhobmic orthorhobmic

Space group Pbca P212121

a(Å) 14.3783(6) 11.7597(7)

b(Å) 20.5288(9) 11.8232(7)

c(Å) 21.8293(9) 25.9106(16)

α(deg) 90 90

β(deg) 90 90

γ(deg) 90 90

V(Å3) 6443.3(5) 3602.5(4)

Z 8 4

T (K) 150(2) 223(2)

d(calc) gcm-3 1.094 1.575

Abs coeff,μ, mm-1 0.062 0.157

Data collected 100796 59566

Rint 0.1284 0.0463

Data used 7433 8304

Variables 373 529

R (>2σ) 0.0579 0.0379

wR2 0.1546 0.0836

GOF 1.007 1.013


73

3 Reactivity of Alkynyl-Linked Phosphonium Borates Toward Main-Group and Transition Metal Species

3.1 Introduction 3.1.1 Alkyne and Phosphine Complexes of Transition Metals

Both alkynes and phosphines, as well as alkynylphosphines, a few Ni(0) complexes of which are

presented as examples in Figure 3.1,93-96 are ubiquitous ligands in transition metal coordination

compounds. It is well-documented that metal complexation of alkynes results in the “bend-back”

of the substituents, consistent with a π-backbonding model involving donation of metal electron

density to the π*-orbital of the alkyne and reduction of the C≡C bond according to the classical

Dewar-Chatt-Duncanson model.97 In the meantime, phosphines and polydentate phosphines are a

class of ligands very important in transition metal catalysis,98-99 and for this reason, their

coordination behaviour with varying electronic and steric properties has been thoroughly

studied.97, 100-101

Figure 3.1 – Examples of alkyne and alkynylphosphine complexes of Ni(0).

3.1.2 Coordination Chemistry of Borane Ligands

The classical acid-base theory described by Lewis,102 accounts for much of the chemistry of the

main group elements. In addition, the interactions of Lewis bases with formally Lewis acidic

transition metals is a concept critical to coordination chemistry. However, it is the inverse

situation, that is the ability of transition metals to act as Lewis bases and form Lewis acid-base

adducts with Lewis acidic species that has garnered much interest in the last 10 years.103 Despite

the recent flurry of activity in this area, it was indeed some 30 years ago that Hughes and

coworkers104 first described the species [CpFe(CO)2AlPh3][NEt4] containing an Fe→Al dative

74

bond. More recent work on such interactions began in 1999 with the report by Hill et al.105 of a

Ru complex of tris-thioimidazolylborane. The chelating nature of the ligand in this

ruthenaboratrane provided a borane center in close proximity to Ru, affording a Ru→B dative

bond. Since then boryl-bridged heterobimetallic complexes106-108 have also been shown to

incorporate M→B dative interactions. In addition, Piers and coworkers have proposed possible

contributions from M→B dative interaction in their metal-borataalkene complexes.109-111 The

groups of Hill,105, 112-117 Bourissou,118-123 Parkin,124-126 and Emslie127-128 among others129-130 have

employed ambiphilic ligands to probe the nature and impact of these unconventional donor-

acceptor interactions, which have been reviewed a number of times.131-133 Using such ligands, an

intramolecular M→B dative interaction can be thermodynamically facilitated by the chelate

effect and without ligand strain or distortion.134 Motivated by such literature precedence, we

explored Ni complexation of a phosphino-alkynyl-borane.

Figure 3.2 – Early examples of Lewis acids as σ-acceptors for transition metal centers.

It is notable that recently a highly electron-rich Ni(0) center bound by a diphosphine-borane

ligand has been demonstrated to undergo reversible addition of H2 and subsequently catalyze

hydrogenation of alkene substrates (Scheme 3.1),135 implying unique strategies that such metal-

borane complexes provide in small molecule activation and catalysis.

75

Scheme 3.1 – Activation of H2 and hydrogenation of alkenes by a Ni complex with a

diphosphine-borane ligand.

3.2 Results and Discussion

3.2.1 Syntheses of Ni(0) Complexes of Alkynyl-Linked Phosphine Boranes

Treatment of tBu2P(H)C≡CB(H)(C6F5)2 (2-6) with tBu3P and B(C6F5)3 generated the neutral

phosphino-alkynyl-borane tBu2PC≡CB(C6F5)2 and the known salt [tBu3PH][HB(C6F5)3]2 in the

same fashion as described in Section 2.2.4 for the syntheses of zwitterionic macrocycles.

Although attempts to isolate this species from solution resulted in unidentified decomposition

products, reaction of tBu2PC≡CB(C6F5)2 generated in situ with Ni(COD)2 in the presence of

excess 1,5-cyclooctadiene led to the formation of a new species (3-1) (Scheme 3.2). The broad 11B signal observed at 7.0 ppm along with the 19F signals at -129.32, -154.85 and -163.13 ppm

indicate the presence of a B center that is not 3-coordinate. A signal in the 13C NMR spectrum at

120.7 ppm exhibiting C-P coupling of 57 Hz was attributed to the alkynyl carbon alpha to P.

One of the olefinic 13C{1H} signals from COD also shows a coupling to P, demonstrating the

coordination of both COD and phosphino-alkynyl-borane ligands on the metal. 1H NMR data

revealed a COD:PCCB-fragment ratio of 1:1, prompting the formulation of (3-1) as

(tBu2PC≡CB(C6F5)2)Ni(COD). Crystallographic data obtained from a single crystal confirmed

that Ni adopts a pseudo square planar coordination geometry comprised of a bidentate COD

ligand, an alkynyl fragment and a coordinated B center (Figure 3.3). The elongated alkynyl C–C

bond length of 1.254(4) Å is consistent with a weakened triple bond and is comparable to typical

76

alkyne (1.278(7) Å, 1.268(3) Å, 1.251(9) Å, 1.260(4) Å)96 and phosphinoalkyne (1.280(4) Å93,

1.269(15) Å95) fragments coordinated to Ni(0). This is also evident from the decrease in the

triple bond stretching frequency from 2125 cm-1 in (2-6) to 1881 cm-1 in (3-1).


Figure 3.3 – POV-Ray depiction of (3-1). C: gray, B: yellow-green, F: pink, P: orange, Ni:

blue. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): C(1)–

C(2), 1.254(4); C(1)–P(1), 1.792(3); B(1)–C(2), 1.486(4); Ni(1)–C(1), 2.005(3); Ni(1)–C(2),

1.987(3); Ni(1)–B(1), 2.358(3); Ni(1)–C(23), 2.144(3); Ni(1)–C(30), 2.132(3); Ni(1)–C(26),

2.092(3); Ni(1)–C(27), 2.085(3); C(23)–C(30), 1.357(5); C(26)–C(27), 1.370(4); P(1)–C(1)–

C(2), 155.5(3); C(1)–C(2)–B(1), 156.3(3).

77

The salient structural feature in (3-1) is the short Ni–B distance of 2.358(3) Å and concomitant

unusual trans disposition of the P- and B-groups on the (partially reduced) alkynyl unit. The Ni–

B distance is somewhat longer than those previously reported for Ni(triphosphine-borane)

(2.1677(16) Å)123 and NiX(tris-thioimidazolylborane) (X=Cl, N3, NCS, OAc) (2.079(13)-

2.112(3) Å).126 This may result from the constraints imposed by the metal-alkyne interaction and

the required bending of the B toward Ni (C–C–B angle: 156.3(3)°), as is observed in Emslie’s

M(η3-(BCC)-triarylborane) complexes (2.294(4) Å).127 The Ni–CCOD bonds trans to B are

significantly longer (Ni-Cavg 2.138(3) Å) than those trans to the alkynyl fragment (Ni–Cavg

2.088(3) Å), consistent with a dative Ni→B interaction that is known to give rise to strong trans

influence.136 Despite this interaction, the sum of the C–B–C angles about B is 357.6°, that is, the

dative Ni to B interaction does not lead to significant pyramidalization at boron. It is also

noteworthy that the B–C bond length of 1.486(4) Å in (3-1) is significantly shorter than in (2-6)

and somewhat shorter than in those alkynylboranes that have been structurally characterized

(1.504(6)-1.529(6) Å).58, 137-139 This suggests some degree of “borato-allene” character in the

BCC fragment as in its all-carbon analogues that adopt the η3-propargyl/allenyl coordination

mode.140-143

Species (3-1) was also successfully generated in the reaction of (2-6) with Ni(COD)2 in the

presence of an excess amount of 1,5-cyclooctadiene (Scheme 3.3). Monitoring the reaction by

NMR revealed the initial formation of a species exhibiting 31P and 11B NMR signals at 28.2 and -

24.7 ppm, both of which are doublets with diagnostic 1JPH and 1JBH, respectively. Over the

course of one day these signals are replaced by those attributable to (3-1) (Figure 3.4, Figure

3.5). The intermediate species is proposed to result from a classical metal-alkyne complex

(Scheme 3.3). The fate of the proton and hydride on P and B is not entirely clear. Evidence of

cyclooctene is observed in the reaction mixture although it does not seem to be the only by-

product from proton and hydride transfer. Regardless of the mechanism of loss of H2, it appears

that the Lewis acidity of the resulting free borane drives the rearrangement of the metal-bound

fragment to permit the Ni→B dative interaction.

78

Scheme 3.3 – Alternative synthesis of (3-1).

Figure 3.4 – 31P/31P{1H} spectra showing the reaction progress of (2-6) with Ni(COD)2 in

toluene-d8.

79

Figure 3.5 – 11B/11B{1H} spectra showing the reaction progress of (2-6) with Ni(COD)2 in

toluene-d8.

Likewise the neutral phosphino-alkynyl-borane Mes2PC≡CB(C6F5)2 was generated in situ by

reacting Mes2PC≡CH with tBu3P and two equivalents of ClB(C6F5)2 in the same fashion as that

described in Section 2.2.2, and then reacted with Ni(COD)2 in the presence of excess 1,5-

cyclooctadiene to give a red solid product (3-2) in 42% overall yield (Scheme 3.4). 1H NMR

resonances corresponding to the coordinated COD and a PMes2 group that integrate to a 1:1 ratio

were observed. Moreover, the chemical shifts of 11B and 19F NMR signals are very similar to

those observed in (3-1), suggesting the identity of (3-2) to be (Mes2PC≡CB(C6F5)2)Ni(COD)

with the phosphino-alkynyl-borane ligand coordinated to Ni in a η3-BCC fashion.


Compound (3-1) was further examined for reactions with some common ligands in used

organometallic chemistry. Treatment of (3-1) with MeCN resulted in the formation and

precipitation of a new product (3-3) (Scheme 3.5). NMR spectra obtained in THF-d8 showed a 31P resonance at 53.2 ppm, notably shifted downfield from that of (3-1). Unfortunately, the 1H

80

NMR spectrum is broad and uninformative, likely a result of acetonitrile exchange, however this

could not be confirmed due to its poor solubility at low temperature. No 11B signal was observed

for (3-3) although the resemblance of its 19F NMR spectrum to that of (3-1) suggests a similar B

environment. The IR spectrum of (3-3) in the solid state exhibited a coordinated C≡N stretch at

2269 cm-1 and a C≡C stretch at 1838 cm-1, suggesting a more reduced alkynyl group than in (3-1)

(Table 3.1). The X-ray structure of (3-3) (Figure 3.6) reveals its centrosymmetric dimeric nature

in which the B–C–C interaction with Ni is retained. In addition, the formally pendant P is

coordinated to a second Ni center. The coordination spheres of the two Ni centers are completed

by coordination of NCMe to give (3-3) as [(tBu2PC≡CB(C6F5)2)Ni(NCMe)]2. The Ni–Calkyne

bonds were found to be 1.9536(12) and 1.9714(13) Å while the Ni–B approach is 2.3243(15) Å,

slightly shorter than that seen in (3-1) (Table 3.1). This is presumably a result of a more electron

rich Ni center due to coordination of a nitrile and a phosphine. The six-membered ring formed by

the Ni2P2C2 core of the dimer is approximately planar with a maximum deviation from the least-

squares plane of 0.0805 Å.

Scheme 3.5 – Generation of (3-3) and (3-4) from (3-1).

81

Figure 3.6 – POV-Ray depiction of (3-3). C: gray, B: yellow-green, F: pink, N: aquamarine, P:

orange, Ni: blue. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles

(°): C(1)–C(2), 1.2681(18); C(1)–P(1), 1.7877(13); B(1)–C(2), 1.486(2); Ni(1)–C(1),

1.9536(12); Ni(1)–C(2), 1.9714(13); Ni(1)–B(1), 2.3243(15); Ni(1)–P(1a), 2.1982(4); Ni(1)–

N(1), 1.8674(11); N(1)–C(23), 1.1399(18); P(1)–C(1)–C(2), 145.44(11); C(1)–C(2)–B(1),

153.50(13).

In a related reaction, a sample of (3-1) was exposed to an atmosphere of CO to form the yellow

product (3-4) in 57% yield (Scheme 3.5). The broad 11B{1H} resonance at 10.9 ppm and the 19F

signals with ∆m-p of ca. 9 ppm are similar to those observed in (3-1), suggesting that the η3-BCC

coordination of the phosphino-alkynyl-borane ligand is retained in (3-4). An IR measurement of

the compound exhibited a stretching frequency at 2022 cm-1, indicating the presence of

coordinated carbon monoxide. The higher stretching frequency in comparison to those in

[(Ph2PC≡CtBu)Ni(CO)]2 (1980, 1938 cm-1)95 is consistent with the coordination of the borane as

an electron-accepting ligand. The structure of (3-4) was confirmed to be

[(tBu2PC≡CB(C6F5)2)Ni(CO)]2, analogous to that of (3-3) by crystallographic studies (Figure

3.7). The Ni(1)–C(2) bond was found to be 1.949(4) Å, somewhat shorter than that in (3-3)

while the Ni–B approach is 2.364(4) Å, slightly longer than that seen in (3-3) (Table 3.1), as a

result of competition against coordinated CO for electron density from the metal. These

observations are anticipated results of substituting σ-donating nitrile ligands for π-accepting

carbonyl ligands. The six-membered ring formed by the Ni2P2C2 core of the dimer is

approximately planar with a maximum deviation from the least-squares plane of 0.0112 Å.

82

Figure 3.7 – POV-Ray depiction of (3-4). C: gray, B: yellow-green, F: pink, O: red, P: orange,

Ni: blue. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°):

C(1)–C(2), 1.270(5); C(1)–P(1), 1.794(4); B(1)–C(2), 1.497(5); Ni(1)–C(1), 1.956(3); Ni(1)–

C(2), 1.949(4); Ni(1)–B(1), 2.364(4); Ni(1)–P(1a), 2.2099(14); Ni(1)–C(23), 1.751(4); C(23)–

O(1), 1.145(4); P(1)–C(1)–C(2), 144.5(3); C(1)–C(2)–B(1), 155.7(4).

Unfortunately, reactions of these metal-borane complexes with small molecules, such as H2, CO2

and N2O, did not proceed to give insertion products. This is presumably due to the strong Lewis

acidity at B with insufficient steric demands provided by C6F5 groups to dissociate from the

metal center.

3.2.2 Computational Analysis of a Ni(0) Complex of Alkynyl-Linked Phosphine Boranes

To probe the nature of the Ni→B interaction DFT calculations144 were undertaken with the help

of Professor Dr. Datong Song and Dr. Edwin Otten. The geometry of (3-1) was optimized using

the B3PW91 functional and 6-311G(d,p) basis set affording (3-1calcd), which was found to be

very similar to the crystallographically determined structure. The calculated Ni→B separation

was longer than the experimental value by 0.06 Å while all other pertinent bond lengths differed

by less than 0.03 Å (Table 3.1). Importantly, (3-1calcd) showed B bending towards Ni with an

approximately coplanar Ni(BC≡CP) fragment and a B–C≡C angle of 156.5°, almost identical to

the experimentally determined value. The HOMO of (3-1calcd) not only involves the interaction

83

between the filled Ni dxy-orbital and vacant B px-orbital, but also demonstrates significant

contributions from the interaction of Ni with the acetylenic carbon on P (C(P)), as well as π-

delocalization over the BC≡C fragment (Figure 3.8). The HOMO-1 of (3-1calcd) also shows some

contribution to the Ni-(BC≡C) interaction, while the HOMO-2, HOMO-13 and HOMO-20

exhibit classical metal-alkyne π-antibonding, π-donating and σ-donating MOs, respectively.

Interestingly, a NBO analysis found a natural bond orbital corresponding to the Ni→B

interaction (Figure 3.9 (a)). This NBO, with an occupancy of 1.63, is highly polarized towards Ni

with approximately 80.2% contribution from the Ni d-orbital, signifying the dative nature of the

bond. The NPA atomic charges on B and C(P) show moderate decrease by 0.30 and 0.25,

respectively, from the free ligand [tBu2PC≡CB(C6F5)2]calcd to the complex (3-1calcd), while

charges on the P and C(B) only exhibit minimal changes (Figure 3.9 (b) and (c)). The NAO

Wiberg bond index for (3-1calcd) suggests Ni-C(P), Ni-C(B) and Ni-B bond orders of 0.40, 0.17 and

0.31, respectively. These data support the notion that there is a moderate degree of Ni→B dative

interaction as the electron density is delocalized over the BC≡C moiety affording a

hyperconjugation-like stabilization. This is also evidenced by significant delocalization energies

provided by second-order NBO interactions between Ni-B σ-orbitals and C≡C π-orbitals (64

kcal/mol for σ→π*; 33 kcal/mol for π→σ*). Such delocalization is believed to be responsible for

the shortening of the B-C distance in (3-1calcd) in comparison to that in [tBu2PC≡CB(C6F5)2]calcd,

and the retained planarity at the B center of (3-1).

84

Figure 3.8 – GaussView depiction of pertinent molecular orbitals of (3-1calcd). (cutoff: 0.04)

(a) LUMO (b) HOMO (c) HOMO-1 (d) HOMO-2 (e) HOMO-13 (f) HOMO-20.

Figure 3.9 – (a) Molekel depiction of the NBO for the Ni→B interaction in (3-1calcd) (cutoff:

0.05), and NPA atomic charges and NAO Wiberg bond indices of (b) free ligand

[tBu2PC≡CB(C6F5)2]calcd and (c) (3-1calcd). C: gray, B: yellow-green, F: pink, P: orange, Ni: red.

85

[tBu2PC≡CB

(C6F5)2]calcd (3-1) (3-1calcd) (3-3) (3-4)

Ni–B (Å) - 2.358(3) 2.421 2.3243(15) 2.364(4)

Ni-C(B) (Å) - 1.987(3) 1.991 1.9714(13) 1.949(4)

Ni-C(P) (Å) - 2.005(3) 1.960 1.9536(12) 1.956(3)

Ni–P (Å) - - - 2.1982(4) 2.2099(14)

C≡C (Å) 1.228 1.254(4) 1.267 1.2681(18) 1.270(5)

C–B (Å) 1.491 1.486(4) 1.479 1.486(2) 1.497(5)

C–P (Å) 1.742 1.792(3) 1.799 1.7877(13) 1.794(4)

C–C–B (o) 179.4 156.3(3) 156.5 153.50(13) 155.7(4)

C–C–P (o) 172.9 155.5(3) 153.5 145.44(11) 144.5(3)

∑ C–B–C (o) 360.0 357.6(5) 358.0 359.3(2) 356.9(5)

υ(C≡C) (cm-1) - 1881 - 1838 1858

Table 3.1 – Comparison of bond parameters and IR stretches among

[tBu2PC≡CB(C6F5)2]calc, (3-1), (3-1calcd), (3-3) and (3-4).

3.2.3 Reactions of an Alkynyl-Linked Phosphonium Borate with Al-, B-, and Zn-Based Lewis Acids

In exploring a final aspect of the reactivity of alkynyl-phosphonium borates, the reactivity with

several Lewis acids was probed. The phosphonium borate (2-6) was reacted with AlCl3 in

toluene (Scheme 3.6). Following stirring overnight at 25 °C and subsequent workup, a new white

solid (3-5) was isolated in 74% yield. The 31P{1H} NMR spectrum of (19) showed a broad signal

at 76.9 ppm, while a 27Al NMR resonance was seen at 103.8 ppm. The 11B{1H} NMR signal at

86

12.6 ppm and the 19F NMR resonances at -130.14, -155.27 and -163.90 ppm, suggest a borane

center weakly coordinated by a fourth ligand. The 1H NMR data were consistent with the

retention of the P–H fragment as a doublet was observed at 5.42 ppm with a 1JHP of 429 Hz. In

addition, a doublet at 8.23 ppm with P–H coupling of 45.9 Hz suggested the reduction of the

alkynyl moiety yielding a =CH fragment. This view was corroborated by the observation of a 13C{1H} signal at 186.90 ppm which corresponds to the olefinic carbon. In a similar fashion, the

corresponding reaction with AlBr3 led to the formation and isolation of (3-6) in 56% yield

(Scheme 3.6). The NMR data for (3-6) were similar to those observed for (3-5). Both products

were structurally characterized by X-ray crystallography and were determined to be

(C6F5)2BC(H)=C(P(H)tBu2)(AlX3) (X = Cl (3-5), Br (3-6)) (Figure 3.10). The two products

contain similar pseudo-five membered rings in which the alkynyl fragment is formally reduced

to an alkenyl group. The hydride has migrated from boron to the adjacent carbon, while AlX3

binds to the carbon adjacent to the phosphonium fragment with a halide atom bridging to boron.

The Al–C distances in (3-5) and (3-6) are 1.958(3) and 1.965(3) Å, respectively, while the

resulting C=C bond distances are identical at 1.344(4) Å. The corresponding B–X distances were

found to be 2.198(3) and 2.343(4) Å, respectively. These comparatively long B–X distances are

consistent with Al exhibiting higher Lewis acidity toward and better orbital overlap with chloride

and bromide than B.

Scheme 3.6 – Reaction of (2-6) with aluminum halides to form (3-5) and (3-6).

87

Figure 3.10 – POV-Ray depiction of (a) (3-5) and (b) (3-6). C: gray, H: gray, Al: violet, B:

yellow-green, Cl: green, F: pink, P: orange. Hydrogen atoms except olefinic H and PH are

omitted for clarity. Selected bond distances (Å) and angles (°): (a) C(9)–C(10), 1.346(5); P(1)–

C(9), 1.789(3); C(10)–B(1), 1.597(5); Al(1)–C(9), 1.955(3); Al(1)–Cl(1), 2.1983(14); Al(1)–

Cl(2), 2.1308(13); Al(1)–Cl(3), 2.1298(14); B(1)–Cl(1), 2.200(4); P(1)–C(9)–C(10), 122.2(3);

C(9)–C(10)–B(1), 127.4(3); C(9)–Al(1)–Cl(1), 97.02(11); Al(1)–Cl(1)–B(1), 94.45(11); C(10)–

B(1)–Cl(1), 103.3(2). (b) C(9)–C(10), 1.349(4); P(1)–C(9), 1.792(3); C(10)–B(1), 1.597(5);

Al(1)–C(9), 1.958(3); Al(1)–Br(1), 2.3335(11); Al(1)–Br(2), 2.2879(11); Al(1)–Br(3),

2.2816(11); B(1)–Br(1), 2.343(4); P(1)–C(9)–C(10), 121.4(3); C(9)–C(10)–B(1), 129.1(3); C(9)–

Al(1)–Br(1), 97.88(10); Al(1)–Br(1)–B(1), 90.21(10); C(10)–B(1)–Br(1), 103.5(2).

In a similar fashion, reaction of (2-6) with the Lewis acid Zn(C6F5)2.C7H8 afforded a new white

solid (3-7) in 70% yield (Scheme 3.7). This new species exhibited a 31P NMR resonance at 73.6

ppm with P-H coupling of 425 Hz and 19F NMR signals at -130.40, -158.84, and -163.64 ppm.

These resonances together with the 11B signal at -12.9 ppm are consistent with the presence of a

4-coordinate borate fragment derived from B(C6F5)3. Additionally integration of the 19F NMR

signals at -117.48, -151.65 and -160.48 ppm are consistent with the presence of an additional

C6F5 ring, presumably on Zn. Compound (3-7) also gives rise to 1H signals at 8.92 and 5.28 ppm

attributable to an olefinic CH and PH fragments respectively. These data supported the

formulation of (3-7) to be a vinyl-borate derivative (C6F5)3BC(H)=C(P(H)tBu2)(Zn(C6F5)), the

precise structure of which was determined in a crystallographic study (Figure 3.11). The metric

parameters within the borate and phosphonium fragments are as expected. Zn is bound to the

88

olefinic fragment adopting a linear two coordinate geometry with a C-Zn-C angle of 172.66(7)°

and Zn-C distances of 1.928(2) and 1.935(2) Å, to the olefinic and fluoroaryl fragments

respectively.

Scheme 3.7 – Reaction of (2-6) with Zn(C6F5)2 and synthesis of (3-7).


orange, Zn: brown. Hydrogen atoms except olefinic H and PH are omitted for clarity. Selected

bond distances (Å) and angles (°): C(1)–C(2), 1.348(2); P(1)–C(1), 1.7636(16); C(2)–B(1),

1.619(2); Zn(1)–C(1), 1.9290(16); Zn(1)–C(11), 1.9352(17); P(1)–C(1)–C(2), 117.45(12); C(1)–

C(2)–B(1), 127.33(14); C(1)–Zn(1)–C(11), 172.66(7).

The analogous reaction of (2-6) with Al(C6F5)3∙C7H8 proceeded in a similarly straightforward

fashion to give the product (3-8) in 61% yield (Scheme 3.8). The 19F NMR spectrum reveals

89

broad signals attributable to two types of C6F5 groups that integrate to a 3:2 ratio while the sharp 11B resonance at -13.8 ppm is consistent with a 4-coordinate B-center. The solid state structure of

(3-8) was unambiguously confirmed by X-ray crystallography to be

(C6F5)3BC(H)=C(P(H)tBu2)(Al(C6F5)2) (3-8) (Figure 3.12). In this species the Al has added to

the C attached to the phosphonium center. The hydride from B has added to the adjacent C,

resulting in a central olefinic fragment. Interestingly a C6F5 group has migrated to B affording an

anionic B(C6F5)3 borate unit. One of the ortho-fluorine atoms on the aryl rings on B coordinates

to the formally unsaturated Al center. The Al…F separation is 1.926 Å, that is comparable to the

previously reported bonds of such kind (2.156(3) Å,145 1.864(2) Å, 2.770(8) Å146). It is

interesting to note that transfer of a C6F5 group normally proceeds from B to Al.147 However, in

(3-8), the presence of the phosphonium cation presumably elevates the Lewis acidity of B as well

as the steric congestion about Al.

Scheme 3.8 – Reaction of (2-6) with Al(C6F5)3 and synthesis of (3-8).

90

Figure 3.12 – POV-Ray depiction of (3-8). C: gray, H: gray, Al: violet, B: yellow-green, F:

pink, P: orange. Hydrogen atoms except olefinic H and PH are omitted for clarity. Selected bond

distances (Å) and angles (°): C(1)–C(2), 1.358(5); P(1)–C(1), 1.792(4); C(2)–B(1), 1.641(6);

Al(1)–C(1), 1.944(4); Al(1)–F(1), 1.929(2); C(4)–F(1), 1.425(4); P(1)–C(1)–C(2), 112.0(3);

C(1)–C(2)–B(1), 136.9(4); C(2)–C(1)–Al(1), 118.5(3); C(1)–Al(1)–F(1), 91.70(14).

Compound (2-6) also reacts with the Lewis acid HB(C6F5)2, yielding a new species (3-9) in 68%

isolated yield (Scheme 3.9). The 31P{1H} NMR spectrum of (3-9) shows a resonance at 55.6

ppm, along with the 1H doublet resonance at 4.61 ppm with a P-H coupling of 433 Hz, indicating

the retention of the phosphonium fragment. The 11B{1H} NMR spectrum of (3-9) shows two

signals, one broad peak at 60.5 ppm and a second sharp signal at -15.9 ppm. These data infer the

presence of both 3-coordinate borane and 4-coordinate borate fragments. The corresponding 19F

NMR spectrum showed 12 resonances at room temperature, suggesting that steric congestion in

(3-9) restricts rotation about the B-C bonds. The 1H NMR signals at 3.39 and 2.44 ppm are

attributable to methine protons, suggesting full reduction of the alkynyl fragment. The nature of

(3-9) was unambiguously confirmed by X-ray crystallography to be

((C6F5)2B)2HC=CH(P(H)tBu2) (Figure 3.13). This species contains a central boratirane ring, with

phosphonium and borane substituents on the C atoms. Within the BC2 ring the Bcyclic–C(P) and

Bcyclic–C(B) bonds are 1.609(2) and 1.712(2) Å, respectively, while the C–C bond is 1.525(2) Å

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(average values of the two crystallographically independent molecules in each asymmetric unit).

The exocyclic phosphonium and planar borane substituents give rise to P–C and B–C bond

distances to the boratirane ring of 1.774(1) and 1.483(2) Å, respectively. In addition the Bcyclic–

C–P and Bcyclic–C–B angles are 123.25(11) and 112.18(12)°, respectively. These metric

parameters, the long Bcyclic–C(B) distance, short exocyclic B–C length and small Bcyclic–C–B

angle suggest some degree of hyperconjugation in the Bcyclic–C–B moiety,68 stabilizing the

structure of (3-9). This reaction is believed to proceed with initial hydroboration of (2-6) with

HB(C6F5)2 followed by intramolecular hydride transfer from B to its adjacent C, the net effect of

which gives the complete reduction of the alkynyl group of (2-6).

Scheme 3.9 – Reaction of (2-6) with HB(C6F5)2 and synthesis of (3-9).


orange. Hydrogen atoms except methine CH and PH are omitted for clarity. Selected bond

distances (Å) and angles (°): C(1)–C(2), 1.520(3)/1.530(3); P(1)–C(1), 1.776(2)/1.772(2); C(2)–

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B(1), 1.481(3)/1.485(3); C(1)–B(2), 1.612(3)/1.605(3); C(2)–B(2), 1.714(3)/1.710(3); P(1)–

C(1)–C(2), 123.83(16)/124.64(15); C(1)–C(2)–B(1), 125.7(2)/122.78(19); B(2)–C(2)–C(1),

59.44(13)/59.09(13); C(2)–C(1)–B(2), 66.26(14)/66.03(14); C(1)–B(2)–C(2),

54.29(12)/54.88(12); B(1)–C(2)–B(2), 109.75(17)/114.61(17); P(1)–C(1)–B(2),

121.58(15)/124.92(15).

Interestingly, 19F NMR measurements of (3-9) at elevated temperatures revealed coalescence to

only three resonances corresponding to ortho-, para- and meta-F (Figure 3.14). Such observation

infers the equivalence of the two B(C6F5)2 groups, indicating that the breakage of C(1)–B(2)

bond allows the rotation of C(1)–C(2) bond at high temperatures. Determination of the activation

parameters for C(1)–C(2) bond rotation from the variable temperature NMR spectra proved

difficult due to its coupling with B(2)–C(2) bond rotation. Nevertheless the ∆G‡ for C(1)–C(2)

bond rotation could be roughly estimated to be 16 kcal/mol.148

Figure 3.14 – Variable temperature 19F NMR spectra of (3-9) in bromobenzene-d5.

93

Besides transition metal and main group element centers, organic electrophiles also react with

electron-rich alkynes. In an attempt to generate asymmetrically tetrasubstituted alkenes, the

known phosphonium alkynylborate salt [HPtBu3][PhC≡CB(C6F5)3]45 was treated with the

diazonium salt [p-Cl(C6H4)N2][B(C6F5)4] (3-10). Working up the reaction, a yellow crystalline

product (3-11) was obtained, and showed a 11B NMR signal at -16.7 ppm, indicating a 3-

coordinate B center, and 19F NMR resonances that correspond to two sets of C6F5 groups in a 1:2

ratio. It is notable that the o-C6F5 signal that corresponds to 1 equivalent of the ring is observed

at –139.03 ppm, suggesting that this group is not bound to a B atom. The acetylenic signal

disappeared and a vinyl resonance at 167.67 ppm was instead observed in 13C{1H} NMR. X-ray

structural analysis confirmed the formulation of (3-11) to be the vinylborane (Ph)(p-

Cl(C6H4))C=C(B(C6F5)2)(C6F5) (Scheme 3.10, Figure 3.15). The configuration of the structure

was the (E)-isomer, and this was also confirmed to be the major isomer in the bulk sample by

NOE experiments. The ratio of (E)-/(Z)-isomer in the crude product was as least 80/20. This

compound is reminiscent of the series of products from 1,1-carboboration chemistry with

B(C6F5)3 reported by Erker and co-workers,41 although their 1,1-carboboration reactions with

internal alkynes requires significantly more forcing conditions (110 °C/7 days in case of

diphenylacetylene and B(C6F5)3). The present study, on the other hand, shows that the formation

of similar compounds can be achieved under milder conditions by the use of strong electrophiles.

In a similar fashion, (p-CF3(C6H4))(p-Cl(C6H4))C=C(B(C6F5)2)(C6F5) and (p-CF3(C6H4))(p-

MeO(C6H4))C=C(B(C6F5)2)(C6F5) were synthesized by Dr. Liyuan Liang by using [p-

Cl(C6H4)N2][B(C6F5)4] and [p-MeO(C6H4)N2][B(C6F5)4] as the sources electrophiles,

respectively, to react with [HPtBu3][p-CF3(C6H4)C≡CB(C6F5)3]. The stereochemistry of the

major isomers was both shown to be the (E)-isomers. The information collectively suggests that

the intermediate species to the vinylborane formation may be the vinylidene phenonium cation

(Scheme 3.11). Such vinylidene phenonium ions have been suggested to be important

intermediates and stereochemistry determinants in solvolysis of vinyl triflates149-150 and

acetolysis of vinyl iodonium salts.151 Quantum chemical calculations have shown that the LUMO

of the vinylidene phenonium ion has σ* character rather than π*,152 making the intramolecular

SN2 type displacement reaction at the sp2 carbon possible and result in an inverted product.

94


Figure 3.15 – POV-Ray depiction of (3-11). C: gray, B: yellow-green, F: pink, Cl: green.


1.369(5); B(1)–C(1), 1.552(6); B(1)–C(1)–C(2), 121.7(3).

95


3.3 Conclusions The crystallographic and computational data herein illustrate that complex (3-1) contains a dative

Ni→B interaction which prompts an unconventional trans metal-alkyne binding mode. This

complex can be further reacted with a common ligands acetonitrile and carbon monoxide to yield

dimeric compounds (3-3) and (3-4), respectively, where the η3-BCC coordination to Ni is

retained. In addition alkynyl-phosphonium borates are shown to react with a variety of Lewis

acids, prompting hydride transfer from the borohydride fragment to the alkyne unit, affording

reduction to the corresponding olefin. In the same vein of chemistry, an asymmetrically

tetrasubstituted alkene (vinylborane) was synthesized with a good stereoselectivity using a strong

organic electrophile and an alkynyl borate salt.

3.4 Experimental Section

3.4.1 General Considerations




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kind of Young bombs (C6H5Br, acetonitrile). All solvents were thoroughly degassed after









house employing a Perkin-Elmer CHN Analyzer. ClB(C6F5)2,89 HB(C6F5)2,89 Zn(C6F5)2.C7H8,153

Al(C6F5)3.C7H8,147 [p-Cl(C6H4)N2][PF6]154 [HPtBu3][PhCCB(C6F5)3]45 were synthesized

employing literature procedures. Tris(pentafluorophenyl)borane was purchased from Boulder

Scientific Company, and tri-tert-butylphosphine and bis(cyclooctadiene)nickel(0) were

purchased from Strem Chemicals Inc. Carbon monoxide was purchased from Sigma-Aldrich. All

other reagents were purchased from Sigma-Aldrich, Alfa Aesar or AcrosOrganics.

3.4.2 Syntheses

Synthesis of (tBu2PC≡CB(C6F5)2)Ni(COD).0.5C7H8 (3-1) This

compound was obtained in via two synthetic routes. Method 1: To a

toluene (1.0 mL) solution of (2-6) (100 mg, 0.19 mmol) was added a

mixture of B(C6F5)3 (99 mg, 0.19 mmol) and PtBu3 (39 mg, 0.19 mmol)

dissolved in toluene (2.0 mL) at RT. The reaction mixture immediately turned orange-yellow,

and became cloudy upon stirring for 1 h. Hexanes (10 mL) were added, and the reaction was left

standing at -35 °C for 3 h. The mixture was then filtered through Celite and washed with cold

hexanes. Upon addition of the combined filtrate to a mixture of Ni(COD)2 (50 mg, 0.18 mmol)

and 1,5-cyclooctadiene (190 mg, 1.8 mmol), the reaction immediately started turning orange, and

became red in 15 min. After 6 h of stirring, the solution was pumped down and some toluene was

added to dissolve all the red precipitate. The mixture was then filtered through Celite to eliminate

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small amount of Ni(0) metal that precipitated out of the reaction. The filtrate was further pumped

down until a red precipitate started to form. Upon addition of pentane (8 mL) and cooling at -35

°C overnight, the supernatant liquid was removed to afford an orange-red fine crystalline

product, which was washed with pentane and dried in vacuo. Yield: 86 mg, 65%. Method 2: (2-

6) (100 mg, 0.19 mmol), Ni(COD)2 (53 mg, 0.19 mmol) and 1,5-cyclooctadiene (210 mg, 1.9

mmol) were mixed together in toluene (5 mL). The initially yellow solution turned deep red upon

overnight stirring at RT. The reaction work up was the same as method 1. Yield: 101 mg, 72 %.

The isolated product was stored at -35 °C to avoid decomposition. The crystalline product was

suitable for X-ray diffraction. 1H NMR (C6D6): 7.13 (m, 1.5H, o/p-Ph, toluene), 7.02 (m, 1H, m-

Ph, toluene), 5.50 (br, 2H, =CH, COD), 4.88 (br, 2H, =CH, COD), 2.11 (s, 1.5H, CH3, toluene),

1.90 (m, 4H, CH2, COD), 1.67 (m, 4H, CH2, COD), 1.22 (d, 18H, 3JHP = 12.3 Hz, tBu). 11B{1H}

NMR (C6D6): 7.0 (br). 13C{1H} NMR (C6D6): 147.89 (dm, 1JCF = 241 Hz, o-C6F5), 140.49 (dm, 1JCF = 247 Hz, p-C6F5), 137.96 (dm, 1JCF = 250 Hz, m-C6F5), 137.90 (s, i-Ph, toluene), 129.34 (s,

o-Ph, toluene), 128.69 (s, m-Ph, toluene), 125.68 (s, p-Ph, toluene), 120.70 (d, 1JCP = 57 Hz,

C≡CP), 108.62 (d, 3JCP = 11 Hz, =CH, COD), 98.70 (s, =CH, COD), 33.03 (d, 1JCP = 25 Hz,

quat-tBu), 31.03 (s, CH2, COD), 29.60 (d, 2JCP = 14 Hz, tBu), 28.31 (s, CH2, COD), 21.42 (s,

CH3, toluene). C≡CB and i-C6F5 carbons were not observed. 19F NMR (C6D6): -129.32 (m, 4F,

o-C6F5), -154.85 (t, 2F, 3JFF = 21Hz, p-C6F5), 163.13 (m, 4F, m-C6F5). 31P{1H} NMR (C6D6): 6.9

(s). IR (thin film from CH2Cl2, cm-1): 1881 (υ(C≡C)). Anal. Calcd. for C30H30BF10PNi.C3.5H4: C,

55.34; H, 4.71. Found: C, 55.35; H, 5.00 %.

Synthesis of (Mes2PC≡CB(C6F5)2)Ni(COD) (3-2) Mes2PC≡CB(C6F5)2

was generated in situ from (2-2) (73 mg, 0.25 mmol), ClB(C6F5)2 (190

mg, 0.50 mmol) and PtBu3 (50 mg, 0.25 mmol) in a toluene/hexane

solution, following the procedure described to synthesize (2-10) (Chapter

2). This solution was added to a mixture of Ni(COD)2 (55 mg, 0.20 mmol) and 1,5-

cyclooctadiene (216 mg, 2.0 mmol), and stirred at RT for 6 h. The mixture was filtered through

Celite to eliminate a small amount of Ni(0) metal that fell out of the solution, and the red filtrate

was pumped down. Hexane (5 mL) was added to further facilitate the precipitation of the

product, and the red solid was isolated and dried in vacuo. The isolated product was stored at -35

°C to avoid decomposition. Yield: 67 mg, 42%. 1H NMR (C6D6): 6.68 (d, 4H, 4JHP = 2.7 Hz, Ar-

H, Mes), 5.48 (br, 2H, =CH, cod), 4.94 (br, 2H, =CH, cod), 2.58 (s, 12H, o-Me, Mes), 1.97 (s,

98

6H, p-Me, Mes), 1.89 (m, 4H, CH2, cod), 1.57 (m, 4H, CH2, cod). 11B NMR (C6D6): 9.8 (br). 19F

NMR (C6D6): -129.66 (dd, 4F, 3JFF = 24 Hz, 4JFF = 8 Hz, o-C6F5), -154.76 (t, 2F, 3JFF = 21 Hz, p-

C6F5), 163.38 (m, 4F, m-C6F5). 31P{1H} NMR (C6D6): -61.3 (s). Anal. Calcd. for C40H34BF10PNi:

C, 59.67; H, 4.26. Found: C, 58.75; H, 4.45 %. (Low C content is likely due to incomplete

combustion.)

Synthesis of [(tBu2PC≡CB(C6F5)2)Ni(NCMe)]2 (3-3) Compound

(3-1) (24 mg, 0.033mmol) and acetonitrile (50 μL, 0.96 mmol)

were mixed in benzene (2 mL). Dark brown crystals formed out of

the reaction mixture overnight at RT. The crystals were isolated and washed with benzene (2 x 1

mL) and pentane (2 mL). Yield: 9 mg, 45%. The crystalline product was suitable for X-ray

diffraction. 1H NMR (THF-d8): 2.52 (br), 2.18 (s), 1.28 (br,m). 11B{1H} NMR (THF-d8): no

signal was observed. 19F NMR (THF-d8): -130.33 (m, 8F, o-C6F5), -159.68 (t, 4F, 3JFF = 20Hz, p-

C6F5), -165.97 (m, 8F, m-C6F5). 31P{1H} NMR (THF-d8): 53.2 (s). IR (KBr, cm-1): 2269

(υ(N≡C)), 1838 (υ(C≡C)). Anal. Calcd. for C48H42B2F20N2P2Ni2: C, 46.96; H, 3.45; N, 2.28.

Found: C, 47.22; H, 3.51; N, 2.82 %.

Synthesis of [(tBu2PC≡CB(C6F5)2)Ni(CO)]2 (3-4) In a 25 mL

Schlenk bomb, (3-1) (40mg, 0.055 mmol) was dissolved in

benzene (6 mL). The vessel was charged with 1 atm of CO, and

left standing at RT overnight, during which time a yellow precipitate formed. The reaction was

then degassed with N2 through three freeze-pump-thaw cycles. The reaction mixture was poured

into a vial, and the yellow solid was isolated and washed further with pentane followed by drying

under vacuum. Yield: 19 mg, 57%. Single crystals suitable for x-ray diffraction studies were

obtained from a reaction mixture. 1H NMR (THF-d8): 1.19 (m). 11B{1H} NMR (THF-d8): 10.9

(br). 19F NMR (THF-d8): -129.70 (dm, 8F, 3JFF = 23 Hz, o-C6F5), -155.80 (t, 4F, 3JFF = 20 Hz, p-

C6F5), -164.28 (m, 8F, m-C6F5). 31P{1H} NMR (THF-d8): 70.7 (s). IR (thin film from CH2Cl2,

cm-1): 2022 (υ(C≡O)), 1858 (υ(C≡C)) Anal. Calcd. for C46H36B2F20O2P2Ni2: C, 45.98; H, 3.02.

Found: C, 46.52; H, 3.39 %.

Synthesis of (C6F5)2BC(H)=C(P(H)tBu2)(AlCl3) (3-5) To a stirring

suspension of AlCl3 (19 mg, 0.14 mmol) in toluene (4 mL) was added a

solution of (2-6) (75 mg, 0.15 mmol) in toluene (4 mL). The reaction was

99

stirred overnight at RT. After passing the reaction mixture through a plug of Celite, the solvent

was pumped off from the filtrate. Recrystallization from toluene/hexanes gave a white solid that

was further washed with hexanes and dried in vacuo. Yield: 67 mg, 74%. Single crystals suitable

for X-ray diffraction were grown by layering a CH2Cl2 solution of the product with hexanes at 25

°C. 1H NMR (CD2Cl2): 8.23 (d, 1H, 3JHP = 45.9 Hz, =CH), 5.42 (d, 1H, 1JHP = 429 Hz, PH), 1.51

(d, 18H, 3JHP = 16.5 Hz, tBu2). 11B{1H} NMR (CD2Cl2): 12.6 (br).13C{1H} NMR (CD2Cl2):

186.90 (br, =CH), 148.40 (dm, 1JCF = 245 Hz, o-C6F5), 141.69 (dm, 1JCF = 253 Hz, p-C6F5),

137.84 (dm, 1JCF = 250 Hz, m-C6F5), 34.72 (d, 1JCP = 35 Hz, quat-tBu), 28.47 (s, tBu). The i-

C6F5 and C=CAl carbon resonances were not observed. 19F NMR (CD2Cl2): -130.14 (dm, 4F, 3JFF = 20 Hz, o-C6F5), -155.27 (t, 2F, 3JFF = 20 Hz, p-C6F5), -163.90 (m, 4F, m-C6F5). 27Al NMR

(CD2Cl2): 103.8 (s). 31P{1H} NMR (CD2Cl2): 76.9 (br). Anal. Calcd. for C22H20AlBCl3F10P: C,

41.37; H, 3.16. Found: C, 41.05; H, 3.74 %.

Synthesis of (C6F5)2BC(H)=C(P(H)tBu2)(AlBr3) (3-6) To compound (2-6)

(58 mg, 0.11 mmol) dissolved in toluene (2.0 mL) was added AlBr3 (30 mg,

0.11 mmol) in toluene (1.0 mL) at -35 °C, and the mixture was stirred at that

temperature for 5 min. The reaction was warmed up to RT and stirred for a

further 30 min. The precipitated product was separated on a filter frit, and washed with toluene

followed by pentane. The product was then dried in vacuo. Yield: 42 mg, 48%. Single crystals

suitable for X-ray diffraction were grown from a bromobenzene solution of the compound at 25

°C. 1H NMR (C6D5Br): 7.87 (d, 1H, 3JHP = 45.0 Hz, =CH), 4.94 (d, 1H, 1JHP = 432 Hz, PH), 1.12

(d, 18H, 3JHP = 16.2 Hz, tBu2).11B NMR (C6D5Br): no signal observed. 13C{1H} NMR (C6D5Br):

147.43 (dm, 1JCF = 248 Hz, o-C6F5), 140.82 (dm, 1JCF = 260 Hz, p-C6F5), 136.72 (dm, 1JCF = 247

Hz, m-C6F5), 114.21 (br, i-C6F5), 33.61 (d, 1JCP = 33 Hz, quat-tBu), 27.30 (s, tBu). The olefinic

carbon resonances were not observed. 19F NMR (C6D5Br): -127.72 (dm, 4F, 3JFF = 23 Hz, o-

C6F5), -152.50 (br s, 2F, p-C6F5), -162.06 (br m, 4F, m-C6F5). 27Al NMR (C6D5Br): 81.0 (s). 31P{1H} NMR (C6D5Br): 84.5 (br). Anal. Calcd. for C22H20AlBBr3F10P: C, 33.75; H, 2.57.

Found: C, 34.10; H, 2.68 %.

Synthesis of (C6F5)3BC(H)=C(P(H)tBu2)(Zn(C6F5)) (3-7) Compound (2-

6) (29 mg, 0.056 mmol) and Zn(C6F5)2.C7H8 were stirred together in

CH2Cl2 overnight at RT. Pumping off the solvent afforded a white solid

product. Yield: 36 mg, 70%. Single crystals suitable for X-ray diffraction

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were grown by layering a CH2Cl2 solution of the product with pentane at 25 °C. 1H NMR

(CD2Cl2): 8.92 (d, 1H, 3JHP = 46 Hz, C=CH), 5.28 (d, 1H, 1JHP = 425 Hz, PH), 1.36 (d, 18H, 3JHP

= 15.7 Hz, tBu). 11B{1H} NMR (CD2Cl2): -12.9 (d, 3JBP = 20 Hz). 13C{1H} NMR (CD2Cl2):

149.21 (dm, 1JCF = 234 Hz, o-C6F5, Zn(C6F5)), 148.79 (dm, 1JCF = 241 Hz, o-C6F5, B(C6F5)3),

142.20 (dm, 1JCF = 260 Hz, p-C6F5, Zn(C6F5)), 140.38 (dm, 1JCF = 241 Hz, p-C6F5, B(C6F5)3),

138.07 (dm, 1JCF = 246 Hz, m-C6F5, B(C6F5)3), 137.45 (dm, 1JCF = 254 Hz, m-C6F5, Zn(C6F5)),

33.68 (d, 1JCP = 38 Hz, quart-tBu), 27.99 (s, tBu). The i-C6F5 and olefinic carbons were not

observed. 19F NMR (CD2Cl2): -117.48 (d, 2F, 3JFF = 21 Hz, o-C6F5, Zn(C6F5)), -130.40 (d, 6F, 3JFF = 21 Hz, o-C6F5, B(C6F5)3), -151.65 (t, 1F, 3JFF = 20 Hz, p-C6F5, Zn(C6F5)), -158.84 (t, 3F, 3JFF = 21 Hz, p-C6F5, B(C6F5)3), -160.48 (m, 2F, m-C6F5, Zn(C6F5)), -163.64 (m, 6F, m-C6F5,

B(C6F5)3). 31P{1H} (CD2Cl2): 73.6 (br). Anal. Calcd. for C34H20BF20PZn: C, 44.60; H, 2.20.

Found: C, 44.90; H, 2.42 %.

Synthesis of (C6F5)3BC(H)=C(P(H)tBu2)(Al(C6F5)2) (3-8) In a small

vial, compound (2-6) (30 mg, 0.058 mmol) was dissolved in

bromobenzne (1.0 mL), and the solution was layered with a benzene (2.0

mL) solution of Al(C6F5)3.toluene (36 mg, 0.068 mmol). After 4 days of

standing at RT without stirring, a microcrystalline solid product

precipitated out. The mother liquor was pipetted out, and the product was washed with benzene

and pentane, followed by drying in vacuo. Yield: 37 mg, 61%. Single crystals suitable for X-ray

diffraction were obtained from a reaction mixture. 1H NMR (C6D5Br): 9.43 (br d, 1H, 3JHP = 51.2

Hz, =CH), 4.75 (d, 1H, 1JHP = 423 Hz, PH), 0.94 (d, 18H, 3JHP = 16.1 Hz, tBu2).11B NMR

(C6D5Br): -13.8 (s). 13C{1H} NMR (C6D5Br): 27.10 (br s, tBu). All other carbon resonances were

not observed due to poor solubility of the compound in C6D5Br. More polar solvents were not

suitable as they react with the compound. 19F NMR (C6D5Br): -121.34 (br, 4F, o-C6F5), -131.01

(br, 6F, o-C6F5), -147.79 (br, 2F, p-C6F5), -156.87 (br, 3F, p-C6F5), -158.26 (br, 4F, m-C6F5), -

161.83 (br, 6F, m-C6F5). 27Al NMR (C6D5Br): no signal observed. 31P{1H} NMR (C6D5Br): no

signal observed. Anal. Calcd. for C44H29AlBF25P: C, 47.98; H, 2.65. Found: C, 46.28; H, 3.01 %.

Synthesis of ((C6F5)2B)2HC≡CH(P(H)tBu2) (3-9) Compound (2-6) (100

mg, 0.19 mmol) and HB(C6F5)2 (67 mg, 0.19 mmol) were stirred in

toluene (5 mL) at RT overnight. The solvent was then completely

pumped off, and the residue was stirred in hexanes (6 mL) overnight. The white solid product

101

was isolated, washed with haxanes, and dried in vacuo. Yield: 113 mg, 68%. Single crystals

suitable for X-ray diffraction were grown by layering a CH2Cl2 solution of the product with

hexanes at RT. 1H NMR (CD2Cl2, 298 K): 4.61 (dd, 1H, 1JHP = 433 Hz, 3JHH = 12.7 Hz, PH),

3.39 (dd, 1H, 3JHP = 16.5 Hz, 3JHH = 11.7 Hz, CH(B(C6F5)2)2), 2.44 (t, 1H, 3JHH = 12.0 Hz,

CHPHtBu2), 1.52 (d, 9H, 3JHP = 15.9 Hz, tBu), 1.43 (d, 9H, 3JHP = 15.6 Hz, tBu). 11B{1H} NMR

(CD2Cl2, 298 K): 60.5 (br), -15.9 (s).13C{1H} NMR (CD2Cl2, 298 K): 151.0-135.1 (m, o-, p-, m-

C6F5), 41.64 (s, CHPHtBu2), 36.15 (d, 1JCP = 35 Hz, quat-tBu), 33.90 (d, 1JCP = 35 Hz, quat-tBu),

28.68 (s, tBu), 27.76 (s, tBu), 13.28 (br, CH(B(C6F5)2)2). The i-C6F5 carbons were not observed. 19F NMR (CD2Cl2, 193 K): -126.94 (dm, 1F, 3JFF = 23 Hz, o-C6F5), -128.86 (m, 1F, o-C6F5), -

129.84 (m, 2F, o-C6F5), -130.21 (br, 1F, o-C6F5), -132.31 (br, 1F, o-C6F5), -132.85 (m, 1F, o-

C6F5), -133.63 (dm, 1F, 3JFF = 23 Hz, o-C6F5), -152.45 (t, 1F, 3JFF = 20 Hz, p-C6F5), -152.84 (t,

1F, 3JFF = 20 Hz, p-C6F5), -156.87 (t, 1F, 3JFF = 20 Hz, p-C6F5), -159.29 (t, 1F, 3JFF = 20 Hz, p-

C6F5), -161.12 (br, 1F, m-C6F5), -162.26 (br, 1F, m-C6F5), -162.61 (m, 1F, m-C6F5), -162.97 (m,

2F, m-C6F5), -163.15 (m, 1F, m-C6F5), -164.12 (m, 1F, m-C6F5), -166.42 (m, 1F, m-C6F5). 31P{1H} NMR (CD2Cl2, 298 K): 55.6 (s). Anal. Calcd. for C34H21B2F20P: C, 47.37; H, 2.46.

Found: C, 46.77; H, 2.36 %.

Synthesis of [p-Cl(C6H4)N2][B(C6F5)4] (3-10) [p-

Cl(C6H4)N2][PF6] (159 mg, 0.56 mmol) and KB(C6F5)4 (402 mg,

0.56 mmol) were combined and stirred in CH2Cl2 (8 mL) at RT overnight. The resultant purple-

brown mixture was filtered through Celite, and the filtrated was pumped down to ca. 5 mL.

Pentane (10 mL) was added to facilitate precipitation of the product. A white solid was isolated

and further washed with pentane followed by drying in vacuo. Yield: 272 mg, 59%. 1H NMR

(CD2Cl2): 8.30 (dm, 2H, 3JHH = 9.2 Hz, Ar-H), 8.00 (dm, 2H, 3JHH = 9.2 Hz, Ar-H). 11B NMR

(CD2Cl2): -16.7 (s). 13C{1H} NMR (CD2Cl2): 154.51 (s, Ar), 148.70 (dm, 1JCF = 240 Hz, o-

C6F5), 138.93 (dm, 1JCF = 252 Hz, p-C6F5), 136.95 (dm, 1JCF = 246 Hz, m-C6F5), 134.75 (s, Ar),

133.40 (s, Ar), 109.00 (s, Ar). The i-C6F5 carbon signal was not observed. 19F NMR (CD2Cl2): -

133.17 (m, 4F, o-C6F5), -163.11 (t, 2F, 3JFF = 20 Hz, p-C6F5), -167.18 (m, 4F, m-C6F5).

Synthesis of (Ph)(p-Cl(C6H4))C=C(B(C6F5)2)(C6F5) (3-11) To a

suspension of (3-10) (60 mg, 0.073 mmol) in CH2Cl2 (2.0 mL) was added

a solution of [HPtBu3][PhCCB(C6F5)3] (60 mg, 0.073 mmol) in CH2Cl2

(3.0 mL) at RT. The reaction mixture was stirred for 10 min, during which

102

time all the components dissolved and turned brownish. All volatiles were pumped off, pentane

(10 mL) added, and the mixture stirred for 3 h. The yellow precipitate was filtered off through a

plug of Celite. The green-brown filtrate was pumped down to 2 mL and kept at -35 °C to give

yellow crystalline precipitate as the product. Yield: 31 mg, 58%. Single crystals suitable for X-

ray diffraction studies were obtained from a pentane solution. The major isomer was determined

to be the (E)-isomer using NOE experiments. Ratio of (E)-isomer : (Z)-isomer = ca. 80:20 by 19F

NMR. 1H NMR (C6D6): 6.97-6.86 (m, 5H, Ar), 6.71 (br, 4H, Ar). 11B NMR (C6D6): 57.9 (br). 13C{1H} NMR (C6D6): 167.67 (s, =C(Ph)(C6H4Cl)), 147.95 (dm, 1JCF = 249 Hz, o-C6F5,

B(C6F5)2), 144.66 (dm, 1JCF = 247 Hz, o-C6F5, C=C(C6F5)), 143.75 (dm, 1JCF = 258 Hz, p-C6F5,

B(C6F5)2), 143.49 (s, Ar), 140.94 (dm, 1JCF = 248 Hz, p-C6F5, C=C(C6F5)), 140.40 (s, Ar), 138.33

(s, Ar), 137.96 (dm, 1JCF = 252 Hz, m-C6F5, C=C(C6F5)), 137.55 (dm, 1JCF = 254 Hz, m-C6F5,

B(C6F5)2), 132.27 (s, Ar), 130.76 (s, Ar), 130.57 (s, Ar), 128.71 (s, Ar), 128.68 (s, Ar). The

signals for carbon atoms adjacent to B were not observed. 19F NMR (C6D6): -129.16 (dm, 4F, 3JFF = 21 Hz, o-C6F5, B(C6F5)2), -139.03 (m, 2F, o-C6F5, C=C(C6F5)), -145.80 (br, 2F, p-C6F5,

B(C6F5)2), -153.51 (t, 1F, 3JFF = 22 Hz, p-C6F5, C=C(C6F5)), -160.84 (m, 4F, m-C6F5, B(C6F5)2), -

161.46 (m, 2F, m-C6F5, C=C(C6F5)). Anal. Calcd. for C32H9BClF15: C, 53.04; H, 1.25. Found: C,

53.59; H, 1.95 %.

3.4.3 X-ray Crystallography 3.4.3.1 X-ray Data Collection and Reduction








103







2) and Fo










104



(3-1).½ toluene (3-3) (3-4).benzene

Formula C30H30BF10NiP

.1/2C7H8 C48H42BF10NNiP

C46H36B2F20Ni2O2P2

.C6H6

Formula wt 727.10 1227.79 1279.80

Crystal system triclinic monoclinic triclinic

Space group P-1 P21/n P-1

a(Å) 8.8528(3) 12.7942(5) 10.328(3)

b(Å) 10.9372(4) 13.7819(6) 11.662(4)

c(Å) 18.9071(6) 14.2113(6) 12.756(4)

α(deg) 96.343(2) 90 65.091(19)

β(deg) 103.295(2) 96.964(2) 68.819(19)

γ(deg) 112.247(2) 90 88.41(2)

V(Å3) 1608.84(10) 2487.37(18) 1285.2(7)

Z 2 2 1

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.500 1.639 1.654

Abs coeff,μ, mm-1 0.734 0.934 0.910


Rint 0.0906 0.0363 0.0812

Data used 8171 8659 5802


R (>2σ) 0.0522 0.0321 0.0515

wR2 0.1204 0.0854 0.0994

GOF 1.012 1.030 1.004

105


(3-5) (3-6) (3-7).1/2 benzene

Formula C22H20AlBCl3F10P C22H20AlBBr3F10P C34H20BF20PZn

·½C6H6

Formula wt 649.50 782.87 954.71

Crystal system monoclinic monoclinic triclinic

Space group P21/c P21/c P-1

a(Å) 18.4053(8) 18.6110(10) 11.9368(4)

b(Å) 9.5652(5) 9.6776(5) 12.3247(4)

c(Å) 15.4275(7) 15.5460(8) 14.4141(5)

α(deg) 90 90 90.285(2)

β(deg) 94.904(2) 96.013(3) 99.075(2)

γ(deg) 90 90 116.5950(10)

V(Å3) 2706.1(2) 2784.6(3) 1865.59(11)

Z 4 4 2

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.592 1.867 1.626

Abs coeff,μ, mm-1 0.511 4.514 0.826


Rint 0.0617 0.0359 0.0454

Data used 6187 6288 13547


R (>2σ) 0.0519 0.0367 0.0425

wR2 0.1516 0.1139 0.1045

GOF 1.041 0.927 1.033


106


(3-8) (3-9) (3-10)

Formula C40H20AlBF25P C34H21B2F20P C32H9BClF15

Formula wt 1044.32 862.10 724.65



a(Å) 11.035(4) 11.0475(4) 7.6340(5)

b(Å) 11.311(4) 15.9300(6) 33.9056(19)

c(Å) 19.748(6) 20.5050(7) 11.2130(7)

α(deg) 86.170(17) 87.693(2) 90

β(deg) 76.144(17) 84.512(2) 99.018(3)

γ(deg) 75.034(17) 78.314(2) 90

V(Å3) 2312.1(13) 3516.8(2) 2866.4(3)

Z 2 4 4

T (K) 150(2) 150(2) 223(2)

d(calc) gcm-3 1.500 1.628 1.679

Abs coeff,μ, mm-1 0.208 0.211 0.255

Data collected 25349 101912 45866

Rint 0.0794 0.0421 0.0666

Data used 7983 17028 6577


R (>2σ) 0.0557 0.0489 0.0687

wR2 0.1469 0.1337 0.1927

GOF 0.855 1.028 1.039


107

4 Bis-Boranes in Frustrated Lewis Pair Chemistry 4.1 Introduction 4.1.1 Activation of Carbon Dioxide by Frustrated Lewis Pairs

The negative impact on global warming and the potential applications as a C1 feedstock have

prompted much interest in the activation and subsequent utilization of thermodynamically

and kinetically stable CO2.155-157 Numerous approaches, including microporous materials

have been developed for capturing CO2,158 while a variety of homogeneous and

heterogeneous catalytic processes have been developed to convert CO2 into alternative fuels,

such as methanol,159-161 or useful organic building blocks.162 In seeking new strategies to

such ends, the Stephan group recently applied the concept of FLPs to probe metal-free

activation of CO2 in collaboration with Erker and coworkers. In these studies, sterically

encumbered phosphines and boranes were demonstrated to reversibly bind CO2 (Figure 4.1

(a), (b)).25 Subsequently O’Hare and coworkers demonstrated the stoichiometric reduction of

amine-borane FLP-CO2 complexes to methanol under rather forcing conditions (6 days, 160

°C).29 Shortly after, our group demonstrated that P/Al based FLPs (Figure 4.1 (c)) could

effect a similar reduction to methanol at 25 °C in 5 min,30 as well as reduction to carbon

monoxide.31 More recently Piers et al. demonstrated that FLP systems could effect the

catalytic deoxygenative hydrosilylation of CO2 to methane.32

Figure 4.1 – CO2 complexation by frustrated Lewis pairs.

4.1.2 Chelating Bis-boranes

Despite the often laborious synthesis of boranes, a number of chelating bis-boranes with various

linker groups and boryl substituents have been reported in literature. Exploration of chelate

adducts of bidentate boranes dates back to 1966 when Biallas and Shriver163 described the ability

108

of 1,2-bis(difluoroboryl)ethane to abstract alkoxy anions from triphenylmethyl ethers and to

coordinate the oxygen atom by both boron centers, establishing the first examples of bis-borane

chelate structures. Their subsequent report on competition experiments showed that the chelate

effect played a role in stabilizing 1,2-bis(difluoroboryl)ethane adduct of the methoxy anion.164

Since then chelating bis-boranes have found a wide range of applications. Fluoride ion

complexation is one of the applications of intrinsically Lewis acidic boranes. Since it is essential

to have a certain degree of steric protection around the Lewis acidic center to maintain high

selectivity for the fluoride anion over other bases, chelating 1,8-diborylnaphthalene structures,

which provide binding pockets with the appropriate size, have been a major focus in the field

(Figure 4.2).165-166 Complex reagents that hold multiple Lewis acidic sites in well-defined

geometries are of great interest in organic catalysis for their enhanced catalytic activity due to

efficient activation of electrophiles as well as increased degree of chiral induction. While the use

of aluminum,167-170 titanium171-172 and other electrophilic elements173-174 are common, chelating

Lewis acid catalysts are also known with boranes, especially in stereoselective Diels-Alder

reactions involving carbonyl substrates (Figure 4.2).175-176 Diboranes have also been popularly

used as co-activators in metallocene and “constrained geometry” catalysts for olefin

polymerization since they produce counter anions that are weaker coordinating than the

monoborane analogues (Figure 4.2).177-178

Figure 4.2 – Examples of chelating bis-boranes.

109

In relation to frustrated Lewis pair chemistry, we note that Berke et al.179 employed

C10H6(B(C6F5)2)2 to effect the heterolytic cleavage of H2, affording a pseudo-six membered ring

species in which the hydride is bridged by the bis-borane. This species was subsequently shown

to catalyze the direct hydrogenation of imines (Figure 4.3). Interestingly, it is suggested that

hydride transfer in hydrogenation catalysis is available only from the “externally” H2 activated

species, but not from the cyclic molecule with a bridged hydride.

Figure 4.3 – Catalytic hydrogenation of imines with C10H6(B(C6F5)2)2.

4.2 Results and Discussion 4.2.1 Examination of a Borinic Anhydride in FLP Reactions

While a variety of Lewis bases2, 180-185 have been used in FLP chemistry, the range of Lewis

acids is more limited. Indeed the majority of work to date has employed B(C6F5)3 or closely

related species. Examples of the use of alkylborane186 and Al-based Lewis acids in FLP

chemistry have been described,30-31, 44 while more recently the creative use of an electron-poor

allene has been described by Alcazaro et al.187 We envisioned the use of bis-boranes to be a

promising direction in order to broaden the scope of Lewis acids in the realm of FLP chemistry.

From the reversible nature of CO2 complexation by phosphine-borane FLP pairs as opposed to

phosphine-aluminum halide pairs, we suspected that the double activation of CO2 with two

Lewis acidic centers may be important in stabilizing the adduct. To probe this, we targeted to

react 2 equivalents of borane with an equivalent each of CO2 and phosphine. However, this

would not be achieved in the same way as aluminum halides since B(C6F5)3 and its derivative

boranes are so bulky that two such molecules cannot be accommodated about single CO2

fragment. On the other hand, smaller boranes will simply form adducts with phosphines. Hence

in targeting new FLP systems designed for complexing CO2, we speculated that bis-boranes

110

might provide for B–O interactions with both O-atoms of one CO2. One-atom-linked bis-boranes

were herein explored as the 6-membered ring formed is believed to be a reasonable size for

stabilizing the chelate.

Our initial efforts involved the use of O(B(C6F5)2)2 (4-1) since this borinic anhydride, unlike

most other electrophilic bis-boranes, is relatively simple to synthesize. This species was

generated by hydrolysis of ClB(C6F5)2 with a half equivalent of H2O (Scheme 4.1).

Alternatively, this compound could also be synthesized using HB(C6F5)2 in place of ClB(C6F5)2.

Compound (4-1) gives rise to three resonances in its 19F NMR spectrum at -131.68, -144.20 and -

159.53 ppm, consistent with the monomeric nature. This was confirmed by an X-ray crystal

structure (Figure 4.4). The planes of the two boryl groups are approximately orthogonal to each

other with a B–O–B bond angle of 153.88(14)° and B…B distance of 2.62 Å. It is noted that

although we were not aware at the onset of this chemistry as the synthesis and structure had not

been published, the groups of Piers and Collins have accessed species (4-1) in their efforts to

develop potentially useful activators for olefin polymerization pre-catalysts.188 However, their

investigations did not involve binding of anions or any other bases to (4-1).


111

Figure 4.4 – POV-Ray depiction of (4-1). C: black, F: pink, O: red, B: yellow-green. Selected

bond distances (Å) and angles (°): O(1)–B(1), 1.3383(19); O(1)–B(2), 1.3517(18); B(1)–C(1),

1.573(2); B(1)–C(7), 1.583(2); B(2)–C(13), 1.578(2); B(2)–C(19), 1.570(2); B(1)–O(1)–B(2),

153.88(14).

NMR data showed no evidence of interaction between (4-1) and added PtBu3. Thus the mixture

was exposed to 1 atm CO2 in a sealed J-Young NMR tube at room temperature. The reaction

gave rise to an upfield shift in 31P{1H} NMR signal to 45.5 ppm, consistent with an adduct

formation with CO2.25-26 In the meantime the 19F NMR spectrum displayed three resonances at -

132.69, -154.64 and -163.10 ppm, and the 11B NMR spectrum had a broad signal at 21 ppm,

suggesting the possibility of the desired bidentate coordination of (4-1) to the phosphino-

carboxylate moiety yielding (4-2a) (Scheme 4.2). However, release of the CO2 pressure from the

solution of (4-2a), evacuating the solution and purging with N2 at room temperature resulted in

regeneration of the original FLP mixture (4-1) and PtBu3 (Figure 4.5). All efforts to isolate

analytically pure bulk samples of (4-2a) proved unsuccessful. Nonetheless, X-ray quality crystals

could be obtained at low temperature under an atmosphere of CO2. These data indicate the

inequivalence of the two boryl groups in the compound with complexation of CO2 by PtBu3 and

one B center, thus giving the structure (4-2) (Figure 4.6). Despite the solid-state structure, NMR

experiments at low temperature (-80 °C) failed to reveal the slow-exchange regime inferring that

the exchange of the O2C–PtBu3 moiety between the two B centers is facile (Scheme 4.2). It is

believed that the relatively large B–O–B bond angle of 139.5(2)° and B…B distance of 2.59 Å in

112

(4-2) infer both additional steric crowding as well as a significant π-character retained in the B–

O bonds. These features combine to inhibit chelation of CO2 by the two B centers.

Scheme 4.2 – Generation of (4-2) by reaction of (4-1) and tBu3P with CO2.

Figure 4.5 – 19F NMR spectra obtained in boromobenzene-d5 showing the reaction of (4-1)

and tBu3P with CO2 to reversibly form (4-2).

113

Figure 4.6 – POV-Ray depiction of (4-2). C: gray, B: yellow-green, F: pink, O: red, P: orange.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): O(1)–B(1),

1.453(3); O(1)–B(2), 1.307(3); B(1)–O(2), 1.550(3); O(2)–C(37), 1.296(3); O(3)–C(37),

1.194(3); C(37)–P(1), 1.886(3); B(1)–O(1)–B(2), 139.5(2); B(1)–O(2)–C(37), 120.88(19); O(2)–

C(37)–O(3), 128.4(2).

Even though (4-2) failed to show the anticipated bidentate coordination of the bis-borane to

phosphino-carboxylate, we demonstrated the ability of (4-1) to act as a chelating Lewis acid by

reacting (4-1) with an equivalent of tetrabutylammonium acetate (Scheme 4.3). The two reagents

reacted immediately in dichloromethane to give a viscous oil (4-3) in a 69% yield. Both the 11B

NMR shift at 3.22 ppm and ∆m-p of 5.0 ppm in the 19F spectrum signify the greater degree of (4-

1) complexation to the base in comparison to (4-2). The solid-state structure confirmed the

anticipated bidentate coordination with a B–O–B angle of 119.26(19)° and a B…B distance of

2.43 Å (Figure 4.7). The 6-membered cycle forms a plane, but O(1) is deviated from the least-

square plane of the other 5 atoms by 0.4842 Å, consistent with the significantly reduced amount

of O–B π-interactions.

114


Figure 4.7 – POV-Ray depiction of the anion of (4-3). C: gray, B: yellow-green, F: pink, O:

red. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): O(1)–

B(1), 1.409(3); O(1)–B(2), 1.407(3); B(1)–O(2), 1.562(3); B(2)–O(3), 1.572(3); O(2)–C(25),

1.270(3); O(3)–C(25), 1.265(3); B(1)–O(1)–B(2), 119.26(19); O(2)–C(25)–O(3), 125.4(2).

Carbodiimides are compounds that are isoelectronic to CO2, but have an increased basicity.

When one equivalent of diisopropylcarbodiimide was reacted with (4-1), the acid-base adduct

was isolated, and the carbodiimide was found to be bound to the diborane through one N

(Scheme 4.4, Figure 4.8). Attempt to react (4-4) with a base tBu3P for nucleophilic addition to

the central C at the carbodiimide failed (Scheme 4.4). This may be a result of enhanced π-

character in the other O–B bond once one B is coordinated by N, inferred by the slightly shorter

O(1)–B(2) bond in (4-4) than in (4-1).

115

Scheme 4.4 – Generation of (4-4) and its reaction with tBu3P.

Figure 4.8 – POV-Ray depiction of (4-4). C: gray, B: yellow-green, F: pink, N: aquamarine, O:


B(1), 1.453(2); O(1)–B(2), 1.322(2); B(1)–N(1), 1.622(3); N(1)–C(25), 1.261(3); C(25)–N(2),

1.171(3); B(1)–O(1)–B(2), 143.26(16); N(1)–C(25)–N(2), 174.1(2).

It is well-known that electrophilic boranes bind nitriles tightly to form isolable adducts. 189-191 Despite this, when (4-1) was mixed with one equivalent of 4-(trifluoromethyl)benzonitrile,

interestingly, no reaction was observed by NMR spectroscopy (Scheme 4.5). In an attempt to

generate a compound that contains both Lewis acidic and basic centers in a single molecule,

potentially interesting as an FLP, the mixture of (4-1) and the nitrile was subjected to reactions

with bulky bases, namely tBu3P and 1,3-di-tert-butylimidazol-2-ylidene. While tBu3P did not

show any reaction, 1,3-di-tert-butylimidazol-2-ylidene reacted with the former mixture to give

an off-white product (4-5), which crystallized to give pale yellow crystals (Scheme 4.5). The 11B

NMR spectrum of (4-5) gives a signal at 1.1 ppm, that is indicative of the formation of a

116

quaternized B center, and a second broad resonance at 37.6 ppm, which is attributed to a 3-

coordinate B center that receives a significant amount of π-electron density from the adjacent O.

Its 19F NMR spectrum is in accordance with this proposition, demonstrating two sets of

inequivalent pentafluorophenyl groups, one of which shows a ∆m-p of 5.7 ppm and another with

10.3 ppm. X-ray crystallographic studies of (4-5) confirmed its structure to be the (Z)-isomer of

the N-boryl substituted imine (Figure 4.9). The 1H NMR spectrum of (4-5) reveals the

equivalence of the imidazolium fragment, but the aromatic protons from the nitrile substituent

show a higher degree of inequivalence, consistent with the X-ray structure, in which the aryl

group is in the plane of the imine C=N bond while the imidazolium plane is orthogonal to the

imine bond.

Scheme 4.5 – Reactions of (4-1) with a nitrile and bases.

117

Figure 4.9 – POV-Ray depiction of (4-5). C: gray, B: yellow-green, F: pink, N: aquamarine, O:


B(1), 1.497(3); O(1)–B(2), 1.310(3); B(1)–N(1), 1.514(3); N(1)–C(1), 1.250(3); C(1)–C(9),

1.527(3); C(9)–N(2), 1.359(3); C(9)–N(3), 1.350(3); B(1)–O(1)–B(2), 141.88(18); B(1)–N(1)–

C(1), 138.11(19); N(1)–C(1)–C(9), 126.31(19).

Although the bis-borane (4-1) failed to give the desired bidentate coordination to phosphino-

carboxylate for enhanced stability of FLP-CO2 adduct, we further examined (4-1) to assess

whether having two boryl groups affects small molecule activation. The FLP combination of (4-

1) and tBu3P does not show any reactivity toward H2 at room temperature. Nevertheless, heating

at 100 °C under ca. 2 atm of H2 for 15 hours gave full conversion of N-benzylidene-tert-

butylamine to N-benzyl-tert-butylamine (Scheme 4.6). Considering that similar boronic esters

and borate esters are not reported to catalyze such hydrogenation reaction,192 it is reasonable to

believe that one of the boryl groups acts as a strongly electron withdrawing substituent to

enhance the Lewis acidity at the other B center to heterolytically split H2 in combination with the

substrate imine as the base.

118

Scheme 4.6 – Catalytic hydrogenation of N-benzylidene-tert-butylamine by (4-1).

Our group has previously reported that various borane/phosphines combinations are capable of

complexing nitrous oxide (N2O),27-28 a greenhouse gas 300 times more potent than CO2.24 Upon

reacting the FLP mixture of (4-1)/tBu3P with an atmosphere of N2O, slow conversion to a new

species (4-6) was observed (Scheme 4.7). The H-P coupling constant between the tBu protons of

the phosphine increased to 14 Hz from that of the free phosphine, indicative of quaternization of

the phosphine. Two sets of signals were observed for the perfluorophenyl groups in both 11B and 19F NMR spectra. The 11B NMR signal of 5.6 ppm and 19F resonances with ∆m-p of 6.46 ppm

infer that one of the B centers is quarternized, but it was not obvious from the NMR data whether

the other B center with 11B NMR signal at 37.7 ppm and 19F NMR resonances with ∆m-p of 9.85

ppm was quaternized or not. Crystallographic analysis gave a definitive solid-state molecular

structure of (4-6) to be O(B(C6F5)2)2(ON2)(PtBu3), in which N2O formed a new O–B bond with

the diborane and N–P bond with the phosphine at the terminal N (Figure 4.10). No interaction of

either N with the other B center of the diborane was detected. Addition of a second equivalent of

tBu3P and charging the reaction vessel with more N2O also failed to yield further reaction,

inferring diminished acidity at the second B center.

Scheme 4.7 – Reaction of (4-1) and tBu3P with N2O.

119

Figure 4.10 – POV-Ray depiction of (4-6). C: gray, B: yellow-green, F: pink, N: aquamarine,

O: red, P: orange. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and

angles (°): O(1)–B(1), 1.460(4); O(1)–B(2), 1.320(4); B(1)–O(2), 1.517(3); O(2)–N(1), 1.322(3);

N(1)–N(2), 1.251(3); N(2)–P(1), 1.705(2); B(1)–O(1)–B(2), 130.3(2); B(1)–O(2)–N(1),

115.2(2); O(2)–N(1)–N(2), 110.4(2); N(1)–N(2)–P(1), 115.3(2).

4.2.2 Diborylmethylene Compounds in FLP Carbon Dioxide Activation

In an attempt to enhance stability and promote a bidentate interaction between the bis-borane and

CO2, a species with an sp2-carbon linker was targeted. This is expected to reduce the bite angle

and facilitate chelation. In addition, β-hydrogen atoms were deliberately excluded to preclude

BH elimination via retrohydroboration. With these considerations, we initially prepared the

known species Me2C=C(BCl2)2 (4-7), employing the synthetic route established by Siebert et

al.193 Monitoring the reaction of (4-7) with tBu3P by NMR spectroscopy, a significant

broadening of the 31P{1H} NMR signal was revealed, and an upfield shift in the 11B NMR

resonance was observed (Figure 4.11). In addition, the 3JHP coupling observed in the 1H spectrum

for the tBu protons was seen to depend on the exact stoichiometry of the two reactants. These

observations indicate a weak interaction between the bis-borane and the phosphine. Introduction

120

of 1 atm CO2 to this FLP mixture at room temperature resulted in the immediate formation of a

white precipitate. This new product (4-8) was subsequently isolated in 68% yield. The 31P{1H}

NMR signal for (4-8) was observed at 52.7 ppm. Further confirmation was provided by the

appearance of a doublet for the CO2 carbon at 169.7 ppm in 13C{1H} spectrum with a 1JCP of

85.7 Hz. 1H NMR measurements for (4-8) showed a singlet for the methyl substituents on the

olefin unit, and one 11B{1H} NMR resonance at 10.3 ppm, indicating that both B centers are

bound to the O2C-PtBu3 moiety. Both the 31P{1H} and 13C{1H} NMR resonances are shifted

downfield in comparison to Figure 4.1(a) and (b),25 consistent with enhanced deshielding of

electron density due to the formation of two B–O bonds. Moreover a strong IR absorption band

at 1608 cm-1 suggests a delocalized carboxylate fragment. This asymmetric υ(C–O) frequency is

also significantly lower than those in NHC-CO2 (1629-1683 cm-1)194 and FLP-CO2 adducts

(Figure 4.1(a): 1695, (b): 1694 cm-1), and is comparable to those in some metal-(μ2-carboxylate)

complexes.195-197 This suggests the formulation of (4-8) is Me2C=C(BCl2)2O2CPtBu3 (Scheme

4.8).

Figure 4.11 – 11B NMR spectra obtained in CD2Cl2 showing the reaction of (4-7) and tBu3P

with CO2 to form (4-8).

Scheme 4.8 – Reaction of (4-7) and tBu3P with CO2 to form (4-8).

121

Bis-borane (4-7) was subsequently derivatized by the reaction with Zn(C6F5)2, affording the

species (4-9) formulated as Me2C=C(B(C6F5)2)2 in situ along with side products. The

spectroscopic data for (4-9) were consistent with this formulation with a singlet 1H NMR signal

at 1.66 ppm for the backbone CH3 groups, and 19F NMR signals at -129.7, -146.0 and -160.7

ppm. Subsequent treatment of (4-9) with PtBu3 and CO2 afforded the corresponding CO2

complex Me2C=C(B(C6F5)2)2O2CPtBu3 (4-10) in 35% overall yield from (4-7) (Scheme 4.9).

The spectroscopic data for (4-10) obtained at -30 °C were consistent with this formulation,

displaying a 31P{1H} NMR signal at 55.0 ppm, a doublet 13C{1H} NMR resonance at 170.9 ppm

with a 1JCP of 92 Hz for the CO2 group, and 19F NMR signals at -132.28, -159.53 and -165.88

ppm. The compound was found to give rise to a IR absorption at 1617 cm-1 for the υ(C–O) mode.

Scheme 4.9 – Generation of (4-9) and its subsequent reaction with tBu3P and CO2.

X-ray diffraction studies of (4-8) (Figure 4.12 (a)) and (4-10) (Figure 4.12 (b)) confirmed

that these species contain phosphine-bound CO2 chelated by the two B centers from the bis-

borane. The six-membered rings formed by the BCB-CO2 fragment are approximately planar

with BCB angles of 117.3(2)° and 121.2(2)° (average of the two crystallographically

independent molecules in each asymmetric unit) and B…B distances of 2.70 and 2.79 Å in

(4-8) and (4-10), respectively. The C–O bonds are 1.259(3) and 1.245(3) Å in (4-8), and

1.257(2) and 1.250(2) Å in (4-10). These are longer than a double bond, but shorter than a

single bond. The P–C bond of (4-8) was determined to be 1.874(3) Å, which is shorter than

those of Figure 4.1 (a) (1.8931(12) Å), (b) (1.900(3) Å) and (4-2) (1.886(3) Å), showing the

greater degree of activation of the CO2 molecule (Table 4.1). Nevertheless, (4-8) gives B–O

bond lengths of 1.577(4) and 1.585(3) Å, that are significantly longer than those in Figure

4.1 (a) (1.5474(15) Å), (b) (1.550(4) Å) and (4-2) (1.550(3) Å), consistent with the less

Lewis acidic nature of individual B centers. Interestingly, (4-10) shows a P–C bond distance

of 1.896(2) Å, similar to those of Figure 4.1 (a), (b) and (4-2), and B–O distances of 1.647(3)

122

and 1.672(3) Å, that are even longer than those in (4-8). This is presumably due to the

enhanced steric hindrance by the introduction of bulky C6F5 groups.

Figure 4.12 – POV-Ray depiction of (a) (4-8) and (b) (4-10). C: gray, B: yellow-green, Cl:

green, F: pink, O: red, P: orange. Hydrogen atoms are omitted for clarity. Selected bond

distances (Å) and angles (°): (a) C(2)–B(1), 1.577(4); C(2)–B(2), 1.585(4); C(2)–C(3), 1.352(4);

B(1)–O(2), 1.577(3); B(2)–O(1), 1.585(3); C(1)–O(1), 1.245(3); C(1)–O(2), 1.259(3); C(1)–P(1),

1.874(3); B(1)–C(2)–B(2), 117.3(2); C(2)–B(1)–O(2), 112.2(2); B(1)–O(2)–C(1), 125.5(2);

O(2)–C(1)–O(1), 126.3(2); C(1)–O(1)–B(2), 125.04(19); O(1)–B(2)–C(2), 112.5(2). (b) C(2)–

B(1), 1.597(5)/1.603(5); C(2)–B(2), 1.605(5)/1.596(5); C(2)–C(3), 1.344(4)/1.342(4); B(1)–

O(1), 1.668(4)/1.650(4); B(2)–O(2), 1.643(4)/1.675(4); C(1)–O(1), 1.250(3)/1.257(3); C(1)–

O(2), 1.256(3)/1.249(3); C(1)–P(1), 1.894(3)/1.897(3); B(2)–C(2)–B(1), 121.0(3)/121.4(3);

C(2)–B(1)–O(1), 109.4(2)/121.4(3); B(1)–O(1)–C(1), 125.5(2)/125.2(2); O(1)–C(1)–O(2),

128.0(3)/128.2(3); C(1)–O(2)–B(2), 124.7(2)/124.6(2); O(2)–B(2)–C(2), 110.9(2)/109.8(2).

Surprisingly, despite this bidentate interaction, variable temperature NMR measurements for

both (4-8) and (4-10) showed loss of CO2 at ca. 15 °C in dichloromethane-d2. Attempts to reduce

the bound CO2 molecule using ammonia borane resulted, instead, in loss of CO2 and formation

123

of the phosphonium hydrideborate salt. Nonetheless, it is noteworthy that combinations of the

mono-borane analogues of (4-7), such as BCl3 and BCl2Ph, with PtBu3 and CO2 do not yield

isolable products, inferring that chelation enhances the strength of CO2 binding.

P–C (Å) B–O (Å) C=O (Å) C–O (Å) C–O stretch

(cm-1)

Figure

4.1(a) 1.8931(12) 1.5474(15) 1.2081(15) 1.2988(15) 1695

Figure

4.1(b) 1.900(3) 1.550(4) 1.209(4) 1.284(4) 1694

Figure

4.1(c) 1.927(8) - 1.251(8)/1.233(8) -

(4-2) 1.886(3) 1.550(3) 1.194(3) 1.296(3) -

(4-8) 1.874(3) 1.577(4)/1.585(3) 1.259(3)/1.245(3) 1608

(4-10) 1.896(2) 1.647(3) 1.257(2)/1.250(2) 1617

Table 4.1 – Comparison of salient bond parameters and IR stretching modes among CO2

complexes of FLPs.

4.3 Conclusions In summary, we showed that judicious design of a bis-borane/phosphine based FLPs can provide

an effective approach to the chelate binding of CO2 into six-membered heterocycles. While the

borinic anhydride with perfluorophenyl groups (4-1), in combination with tBu3P, only complexes

CO2 in a monodentate manner, methylene diboranes (4-7) and (4-9), in combination with tBu3P,

complexes CO2 in a bidentate fashion. The demonstration of a borane without any aryl group to

exhibit FLP reactivity greatly broadens the scope of FLP systems.

124







kind of Young bombs (C6H5Br, acetonitrile). All solvents were thoroughly degassed after









house employing a Perkin-Elmer CHN Analyzer. ClB(C6F5)289 and Me2C=C(BCl2)2

193 were

synthesized employing literature procedures. Tri-tert-butylphosphine was purchased from Strem

Chemicals Inc. Carbon dioxide was purchased from Sigma-Aldrich. All other reagents were

purchased from Sigma-Aldrich, Alfa Aesar or AcrosOrganics.

4.4.2 Syntheses

Synthesis of O(B(C6F5)2)2 (4-1) In a Schlenk tube, ClB(C6F5)2

(1.774 g, 4.66 mmol) was dissolved in dichloromethane (20 mL), and

cool at -78 °C. Distilled H2O (40.0 μL, 2.22 mmol) was added in two

portions, and the reaction stirred at -78 °C for 1 h. Particular attention

was paid to venting of the reaction vessel. The reaction was then

warmed up to room temperature, and volatiles pumped off to dryness.

The residue solid was thoroughly washed with pentane (30 mL) to yield a white microcrystalline

product. A second crop was recrystallized from the pentane washings. Yield: 0.779 g, 50%.

Single crystals suitable for X-ray diffraction were grown by recrystallization from pentane at -35

125

°C. 11B NMR (C6D5Br): no resonance was observed. 13C{1H} NMR (C6D5Br): 148.78 (dm, 1JCF

= 249 Hz, o-C6F5), 143.70 (dm, 1JCF = 262 Hz, p-C6F5), 136.84 (dm, 1JCF = 254 Hz, m-C6F5),

107.43 (br, i-C6F5). 19F NMR (C6D5Br): -131.68 (d, 8F, 3JFF = 21 Hz, o-C6F5), -144.20 (tt, 4F, 3JFF = 21 Hz, 4JFF = 5 Hz, p-C6F5), -159.53 (m, 8F, m-C6F5). Anal. Calcd. for C24B2F20O: C,

40.84; H, 0; N, 0. Found: C, 40.48; H, 0.18; N, 0.43 %.

Synthesis of [N(C4H9)4][O(B(C6F5)2)2(O2CCH3)] (4-3) To a

solution of (4-1) (88 mg, 0.12 mmol) in CH2Cl2 (1 mL) was

added a tetrabutylammonium acetate (38 mg, 0.13 mmol)

solution in CH2Cl2. The homogeneous mixture was shaken well without stirring, followed by

addition of pentane (5 mL) to give two layers. The top layer was decanted and the bottom layer

was washed thoroughly with pentane. Drying in vacuo gave a clear colourless viscous oil. Yield:

83 mg, 69%. Single crystals suitable for X-ray diffraction were grown by slow evaporation of

solvent from a solution in CH2Cl2. 1H NMR (CD2Cl2): 3.07 (m, 8H, N(CH2CH2CH2CH3)4), 2.27

(s, 3H, O2CCH3), 1.60 (m, 8H, N(CH2CH2CH2CH3)4), 1.41 (sextet, 8H, 3JHH = 7.3 Hz,

N(CH2CH2CH2CH3)4), 1.00 (t, 12H, 3JHH = 7.3 Hz, N(CH2CH2CH2CH3)4). 11B NMR (CD2Cl2):

3.22 (br s). 13C{1H} NMR (CD2Cl2): 182.83 (s, O2CCH3), 148.36 (dm, 1JCF = 239 Hz, o-C6F5),

139.46 (dm, 1JCF = 239 Hz, p-C6F5), 137.00 (dm, 1JCF = 242 Hz, m-C6F5), 122.07 (br, i-C6F5),

59.51 (t, 1JCN = 3 Hz, N(CH2CH2CH2CH3)4), 24.31 (s, N(CH2CH2CH2CH3)4), 24.11 (s,

O2CCH3), 20.16 (s, N(CH2CH2CH2CH3)4), 13.75 (s, N(CH2CH2CH2CH3)4). 19F NMR (CD2Cl2):

-135.79 (d, 8F, 3JFF = 24 Hz, o-C6F5), -161.81 (t, 4F, 3JFF = 20 Hz, p-C6F5), -166.83 (m, 8F, m-

C6F5). Anal. Calcd. for C42H39B2F20NO3: C, 50.08; H, 3.90; N, 1.39. Found: C, 49.80; H, 4.14;

N, 1.55 %.

Synthesis of O(B(C6F5)2)2(iPrNCNiPr) (4-4) (4-1) (70 mg,

0.099mmol) and N,N′-Diisopropylcarbodiimide (13 mg, 0.10 mmol)

were combined in CH2Cl2 (3 mL), and the mixture was shaken until

all components dissolved and left standing at RT overnight. The

solution was then pumped down and pentane (2 mL) was added to facilitate precipitation of the

product. The white solid was isolated and further washed with pentane (3 x 2 mL). The product

was dried in vacuo. Yield: 74 mg, 90%. Single crystals suitable for X-ray diffraction were grown

by layering a CH2Cl2 solution with pentane. 1H NMR (CD2Cl2): 4.00 (septet, 2H, 3JHH = 6.6 Hz,

CH, iPr), 1.25 (d, 12H, 3JHH = 6.6 Hz, CH3, iPr). 11B NMR (CD2Cl2): 38.0 (br), 1.2 (br). 13C{1H}

126


(dm, 1JCF = 251 Hz, m-C6F5), 128.99 (s, NCN), 114.49 (br, i-C6F5), 52.64 (s, CH, iPr), 23.40 (s,

CH3, iPr). 19F NMR (CD2Cl2): -133.14 (br, 4F, o-C6F5), -133.72 (br, 4F, o-C6F5), -152.17 (br, 2F,

p-C6F5), -155.73 (br, 2F, p-C6F5), -162.90 (br, 4F, m-C6F5), -163.85 (br, 4F, m-C6F5). Anal.

Calcd. for C31H14B2F20N2O: C, 44.75; H, 1.70; N, 3.37. Found: C, 44.57; H, 1.91; N, 3.41 %.

Synthesis of O(B(C6F5)2)2(NCC6H4CF3)(ItBu) (4-5) To a suspension of (4-

1) (74 mg, 0.10 mmol) and 4-(trifluoromethyl)benzonitrile (18 mg, 0.11

mmol) in toluene (2 mL) was added 1,3-di-tert-butylimidazol-2-ylidene (19

mg, 0.11 mmol) dissolved in toluene (1 mL). The reaction was stirred at RT

for 5 h, during which time a precipitate formed. Pentane (2 mL) was added

to further facilitate precipitation of the product. The supernatant liquid was

removed, and the solid was further washed with pentane. Drying in vacuo gave an off-white

product. Yield: 97 mg, 92%. Single crystals suitable for X-ray diffraction were grown by

layering a CH2Cl2 solution of the product with pentane at RT. 1H NMR (CD2Cl2): 7.70 (br, 1H,

Ar), 7.68 (br, 1H, Ar), 7.64 (br, 2H, Ar), 7.24 (s, 2H, =CH, ItBu), 1.39 (s, 18H, tBu). 11B NMR

(CD2Cl2): 37.6 (br), 1.1 (s). 13C{1H} NMR (CD2Cl2): 148.46 (dm, 1JCF = 241 Hz, o-C6F5),

147.48 (dm, 1JCF = 245 Hz, o-C6F5), 144.19 (s), 144.08 (s), 142.43 (dm, 1JCF = 252 Hz, p-C6F5,

two signals overlapped), 140.27 (s), 137.33 (dm, 1JCF = 249 Hz, m-C6F5, two signals

overlapped), 132.46 (q, 2JCF = 32 Hz, C(CF3), Ar), 128.56 (s, Ar), 126.38 (m, Ar), 124.51 (q, 1JCF

= 272 Hz, CF3), 120.09 (s, =CH, ItBu), 113.75 (br, i-C6F5), 65.28 (s, quart-tBu), 30.74 (s, tBu). 19F NMR (CD2Cl2): -63.15 (s, 3F, CF3), -131.67 (m, 8F, o-C6F5, two signals overlapped), -

153.31 (t, 2F, 3JFF = 20 Hz, p-C6F5), -160.01 (t, 2F, 3JFF = 20 Hz, p-C6F5), -163.62 (m, 4F, m-

C6F5), -165.68 (m, 4F, m-C6F5). Anal. Calcd. for C43H24B2F23N3O: C, 48.85; H, 2.29; N, 3.97.

Found: C, 48.47; H, 2.50; N, 4.02 %.

Synthesis of O(B(C6F5)2)2(ON2)(PtBu3) (4-6) A solution of (4-1)

(84 mg, 0.12 mmol) and PtBu3 (24 mg, 0.12 mmol) in CD2Cl2 (0.7

mL) was transferred into a NMR tube with a J-Young tap. The

sample was exposed to a N2O atmosphere for a few minutes and the tap was closed. After

leaving the sample under 1 atm of N2O for 1 day, the sample was recharged with 1 atm of N2O.

Upon leaving the sample for another day, complete conversion of the starting materials was

confirmed by NMR. The solution was poured into a vial, pumped down, and pentane (3 mL) was

127

added to precipitate a white solid. The product was further recrystallized from CH2Cl2 and

pentane, and dried in vacuo. Yield: 81 mg, 71%. Single crystals suitable for X-ray diffraction

were obtained by layering of a CH2Cl2 solution of the compound with pentane. 1H NMR

(CD2Cl2): 1.46 (d, 27H, 3JHP = 14 Hz, tBu). 11B NMR (CD2Cl2): 37.7 (br), 5.6 (br). 13C{1H}

NMR (CD2Cl2): 148.60 (dm, 1JCF = 241 Hz, o-C6F5), 147.72 (dm, 1JCF = 245 Hz, o-C6F5), 142.34

(dm, 1JCF = 254 Hz, p-C6F5), 140.19 (dm, 1JCF = 250 Hz, p-C6F5), 137.50 (dm, 1JCF = 235 Hz, m-

C6F5), 120.01 (br, i-C6F5), 113.49 (br, i-C6F5), 41.80 (d, 1JCP = 28 Hz, PC(CH3)3), 29.57 (s,

PC(CH3)3). 19F NMR (CD2Cl2): -133.18 (d, 4F, 3JFF = 22 Hz, o-C6F5), -134.28 (d, 4F, 3JFF = 22

Hz, o-C6F5), -153.82 (t, 2F, 3JFF = 20 Hz, p-C6F5), -159.22 (t, 2F, 3JFF = 20 Hz, p-C6F5), -163.67

(m, 4F, m-C6F5), -165.68 (m, 4F, m-C6F5). 31P {1H} (CD2Cl2): 67.55 (s). Anal. Calcd. for

C36H27B2F20N2O2P: C, 45.41; H, 2.86; N, 2.94. Found: C, 45.30; H, 3.16; N, 2.92 %.

Synthesis of Me2CC(BCl2)2(CO2)PtBu3 (4-8) Me2CC(BCl2)2 (4-7) was prepared

according to a literature procedure. It was not possible to completely separate the

Me3SnCl byproduct from Me2CC(BCl2)2 (27% contamination), but the sample

was used as is since Me3SnCl does not affect the reaction in reasonable lengths of

time and can be easily separated from the product. In a well-dried Schlenk flask, (4-7) (200 mg,

0.69 mmol of the diborane, 27% Me3SnCl) and tBu3P (186 mg, 0.92 mmol) were dissolved in

cold bromobenzene (5 mL). 1 atm of CO2 was introduced to the reaction while keeping the flask

in an ice bath. A white solid immediately started to precipitate out. The reaction was quickly

filtered while kept cold. The product was further washed with cold bromobenzene and cold

pentane. Yield: 219 mg, 68%. Single crystals suitable for X-ray diffraction were grown by

layering a CH2Cl2 solution of the product with pentane at -35oC. 1H NMR (CD2Cl2, 253 K): 1.96

(s, 6H, CH3), 1.79 (d, 3JHP = 15.7 Hz, 27H, tBu3). 11B{1H} NMR (CD2Cl2, 253 K): 10.3 (br). 13C{1H} NMR (CD2Cl2, 253 K): 169.72 (d, 1JCP = 85.7 Hz, PCO), 152.95 (s, =CMe2), 43.59 (d, 1JCP = 15.6 Hz, quat-tBu), 30.44 (s, tBu), 26.45 (s, CH3). The olefinic carbon bound to B atoms

was not observed. 31P{1H} (CD2Cl2, 253 K): 52.7 (s). IR (KBr, cm-1): 1608 (C–O asym. stretch).

Anal. Calcd. for C17H33B2Cl4O2P: C, 44.02; H, 7.17. Found: C, 43.92; H, 7.27 %.

Synthesis of Me2CC(B(C6F5)2)2(CO2)PtBu3 (4-10) To Me2CC(BCl2)2

(113 mg, 0.27 mmol of the diborane, 50% Me2SnCl2) stirring in toluene

(1 mL) was added Zn(C6F5)2.C6H8 (266 mg, 0.54 mmol) dissolved in

toluene (2 mL), and the mixture was stirred at RT overnight. A white

128

precipitate formed in a yellow solution. The entire reaction mixture was then transferred to a

well-dried Schlenk bomb, and was heated at 60 °C for 2 days. The reaction mixture was passed

though a plug of Celite in order to eliminate ZnCl2 byproduct. Volatiles were pumped off from

the filtrate, and the resulting yellow oil was redissolved in toluene-d8 (1.0 mL) along with PtBu3

(20 mg, 0.10 mmol). After 30 min the mixture separated into a yellow solution on the top layer

and an orange oil at the bottom layer. The top layer was transferred to a NMR tube equipped

with a J-Young tap, and the sample was charged with 1 atm of CO2. Leaving the sample at RT

under a CO2 atmosphere over 1 day resulted in the precipitation of a colourless crystalline

product. The product was quickly filtered on a frit, and washed with cold toluene and pentane,

and lightly dried in vacuo. Yield: 34 mg, 35%. Single crystals suitable for X-ray diffraction were

grown by layering a CH2Cl2 solution of the product with pentane at -35oC. 1H NMR (CD2Cl2,

243 K): 1.51 (d, 3JHP = 16.0 Hz, 27H, tBu3), 1.30 (s, 6H, CH3). 11B{1H} NMR (CD2Cl2, 243 K):

no signal was observed. 13C{1H} NMR (CD2Cl2, 243 K): 170.85 (d, 1JCP = 92 Hz, PCO), 148.00

(dm, 1JCF = 236 Hz, o-C6F5), 140.50 (s, =CMe2), 139.20 (dm, 1JCF = 263 Hz, p-C6F5), 136.53

(dm, 1JCF = 238 Hz, m-C6F5), 136.27 (br s, =C(B(C6F5)2)2), 119.59 (br s, i-C6F5), 42.36 (d, 1JCP =

17 Hz, quat-tBu), 30.02 (s, tBu), 24.75 (s, CH3). 19F NMR (CD2Cl2, 243 K): -132.28 (br s, 8F, o-

C6F5), -159.53 (br s, 4F, p-C6F5), -165.88 (br s, 8F, m-C6F5). 31P {1H} (CD2Cl2, 243 K): 55.0 (s).

IR (KBr): 1617 cm-1 (C–O asym. stretch). Anal. Calcd. for C41H33B2F20O2P: C, 49.73; H, 3.36.

Found: C, 49.34; H, 3.21 %.

4.4.3 X-ray Crystallography

4.4.3.1 X-ray Data Collection and Reduction








129







2) and Fo










130



(4-1) (4-2) (4-3).dichloromethane

Formula C24B2F20O C37H27B2F20O3P C42H39B2F20NO3·CH2Cl2

Formula wt 705.86 952.18 1092.29

Crystal system triclinic monoclinic monoclinic

Space group P-1 P21/n P21/c

a(Å) 11.1407(6) 14.5904(11) 10.3707(3)

b(Å) 11.3624(6) 17.0600(15) 19.8848(6)

c(Å) 11.4867(6) 16.9806(13) 24.2211(8)

α(deg) 107.322(3) 90 90

β(deg) 94.603(2) 111.554(4) 94.4040(10)

γ(deg) 117.977(2) 90 90

V(Å3) 1183.43(11) 3931.1(5) 4980.1(3)

Z 2 4 4

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.981 1.609 1.457

Abs coeff,μ, mm-1 0.228 0.203 0.244


Rint 0.0231 0.0460 0.0276

Data used 5153 9004 10416


R (>2σ) 0.0328 0.0520 0.0568

wR2 0.0880 0.1506 0.1726

GOF 1.028 1.029 1.031


131


(4-4).dichloromethane (4-5) (4-6)

Formula C31H14B2F20N2O·CH2Cl2 C43H24B2F23N3O C36H27B2F20N2O2P

Formula wt 916.99 1057.27 952.19

Crystal system triclinic orthorhombic triclinic

Space group P-1 Pbca P-1

a(Å) 10.4506(5) 17.9512(8) 10.998(2)

b(Å) 10.5618(5) 20.2161(9) 11.438(2)

c(Å) 17.1332(8) 23.5596(11) 16.248(3)

α(deg) 72.422(2) 90 85.413(9)

β(deg) 79.478(2) 90 83.880(9)

γ(deg) 84.739(2) 90 76.087(10)

V(Å3) 1771.15(15) 8549.9(7) 1969.6(7)

Z 2 8 2

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.719 1.643 1.606

Abs coeff,μ, mm-1 0.322 0.168 0.202


Rint 0.0348 0.0437 0.0661

Data used 11373 10388 8997


R (>2σ) 0.0567 0.0546 0.0656

wR2 0.1793 0.1479 0.2115

GOF 1.028 1.027 1.106


132


(4-8) (4-9).dichloromethane

Formula C17H33B2Cl4O2P C41H33B2F20O2P.CH2Cl2

Formula wt 463.82 1075.19

Crystal system orthorhombic triclinic

Space group Pbca P-1

a(Å) 11.9094(6) 11.3566(4)

b(Å) 17.2458(8) 11.5364(5)

c(Å) 22.7590(11) 33.8526(13)

α(deg) 90 86.446(2)

β(deg) 90 83.838(2)

γ(deg) 90 80.780(2)

V(Å3) 4674.4(4) 4348.0(3)

Z 8 4

T (K) 150(2) 150(2)

d(calc) gcm-3 1.318 1.578

Abs coeff,μ, mm-1 0.585 0.248


Rint 0.0669 0.0591

Data used 5345 22022

Variables 246 1255

R (>2σ) 0.0430 0.0687

wR2 0.1078 0.1984

GOF 1.043 1.029


133

5 Tethered Olefin-Borane “van der Waals Complexes” and Frustrated Lewis Pair Addition Reactions Using Various Nucleophiles

5.1 Introduction 5.1.1 Olefin Addition Reactions by Frustrated Lewis Pairs

Besides mediating catalytic reductions of a range of polar substrates using H2,3, 5, 53-54, 198-200 FLPs

have also been shown to undergo intriguing addition reactions to a variety of substrates with

multiple bonds including olefins (Scheme 5.1).22, 185, 201 Unlike many transition metal-based

systems where d-orbitals and back-bonding stabilize η2-olefin complexes,97 a combination of a

Lewis acidic borane with an olefin does not show a detectable coordination complex by NMR

spectroscopy obtained even at -90 °C.22 Thus the nominally termolecular reaction of an FLP

addition to an olefin raises interesting and important mechanistic questions.

Scheme 5.1 – Examples of addition reactions of frustrated Lewis pairs to olefins.

Computational studies by Papai and coworkers examined the reaction of tBu3P and B(C6F5)3

with ethylene.13 These authors proposed an “encounter complex” in which steric congestion

limits the P...B approach, affording a shallow energy minimum at a P...B distance of 4.2 Å. It is

this intermediate that is proposed to subsequently react with ethylene (Scheme 5.2). In contrast,

134

calculations of Guo and Li pointed out the possibility of the three-component reaction initiated

by the formation of a weak association complex of B(C6F5)3 with ethylene (Scheme 5.2).202

Paucity of direct experimental evidence that might illuminate the mechanism of action of FLPs

motivated us to investigate such addition reactions in further detail.

Scheme 5.2 – Proposed pathways of addition to ethylene by tBu3P and B(C6F5)3.

5.1.2 Interactions and Reactions of Olefins and Boranes

Unlike those species that possess electron pairs to form donor-acceptor adducts with boranes,

interactions of unsaturated bonds with boranes have only been observed once to our

knowledge in an early matrix (argon or nitrogen) isolation study that described IR data

suggesting the formation of BF3–ethylene and BF3–propylene van der Waals complexes at

93–125 K.203 These interactions are weak presumably because of the absence of back-

donation and the small size of boron. Borane-olefin π-complexes have also been pointed out

to be an important intermediate leading to the subsequent rate determining concerted

conversion in the non-catalyzed hydroboration reaction,204-205 one of the most useful organic

transformations. However, the nature of the intermediates has been overlooked due in part to

rapid product formation. Identification of such intermediates may also provide mechanistic

insights into allyl group abstraction reactions by boron-based Lewis acids (Scheme 5.3).206

135

Scheme 5.3 – Allyl group transfer in the reaction of B(C6F5)3 and Sn(C3H5)Bu3.

5.1.3 Intramolecular Stabilization of d0 Metal-Olefin Interactions

While Ziegler-Natta catalysts remain as the most active class of olefin polymerization catalysts,

the key intermediate d0 metal-alkyl-alkene complex has not been observed in a catalytic system.

This is due to weak coordination of olefin for the lack of back-donation from the d0 metal as well

as the rapidity of olefin insertion into the metal-alkyl bond. Circumventing this problem, the

research groups of Casey207-210 and Jordan211-213 employed a strategy of tethering a vinyl group to

an alkoxy or alkyl substituent coordinated to the metal center to facilitate vinyl group

coordination and disfavor insertion (Figure 5.1). It is also noteworthy that Dolzine and Oliver

demonstrated the intramolecular metal-double bond interactions in trialkenylaluminum and

–gallium derivatives by substantial perturbations in NMR parameters.214

Figure 5.1 – Examples of intramolecularly stabilized of d0 metal-olefin complexes.

136

5.2 Results and Discussion 5.2.1 The Bis(pentafluorophenyl)alkylborane System – Computational

Approach

In approaching the question of the mechanism of FLP-olefin addition reactions, we proposed to

explore a tethered olefin-borane species to minimize entropic destabilization of complex

formation by chelate effect. We began by using DFT calculations to assess the feasibility of this

idea. To this end, calculations were performed at the M06 level of theory215 (6-311++G(d,p) for

B, C’s on B, olefinic C and H; 6-31G(d) for all other atoms), for two conformers of

B(C6F5)2(CH2CH2CH2CH=CH2). The “open” form (5-1a) with the least steric congestion and the

“closed” form (5-1b) in which the alkenyl group is oriented towards the B center, were both

found to be energy minima, with the “closed” form being lower in free energy by 3.3 kcal/mol

(with thermal energy corrections at STP) (Figure 5.2, Table 5.1). In contrast with the classical

view of a coordination complex bearing a dative bond, (5-1b) does not show any sign of a

chemical bond between B and the vinyl group. This includes minimum elongation of the C=C

bond and no pyramidalization of the B center. Atomic charges and molecular orbital

considerations also do not indicate notable charge transfer or p-π orbital overlap. Nonetheless the

B-Cvinyl distances (3.04 and 3.26 Å) are within the sum of the van der Waals radii of B and C

(3.62 Å),216 consistent with the existence of a weakly interacted borane-olefin van der Waals

complex, held in close proximity by noncovalent interactions. The use of the popular density

functional B3LYP also displayed an energetic preference for the “closed” form, but to a lesser

extent (∆Gb-a = -1.9 kcal/mol). This is consistent with the well-documented underestimation of

medium-range exchange-correlation energies, such as van der Waals forces, by the B3LYP

method.215 Single point energy calculations at the B3LYP/def2-TZVP level of theory on the

optimized structures gave a ∆Eb-a of 1.4 kcal/mol, while such calculations employing Zhao and

Truhlar’s M06-2X (improved performance for noncovalent interactions)217 or Grimme’s B97-D

(inclusion of empirical dispersion correction)218 method with the same AO basis set gave ∆Eb-a =

-5.0 kcal/mol and -4.6 kcal/mol, respectively (Table 5.1). This is an indication of van der Waals

interactions that affords stabilization to (5-1b). Despite the small free energy difference of 3.3

kcal/mol, simple calculations show that the equilibrium should lie heavily towards (5-1b) (K298 =

2.6 x 102; K223 = 1.7 x 103), encouraging us to attempt to examine such van der Waals complex

spectroscopically. It is noted that using the IEF-PCM solvent model for dichloromethane, no

significant deviation in the geometries and energies was identified

137

(∆Gb-a = -3.0 kcal/mol). Employment of a method designed especially for treating dispersion

forces may be necessary for appropriate solvent modeling in the present system.

Figure 5.2 – Optimized structures of (5-1a) and (5-1b) with pertinent distances and NPA

charges.

138

Level of theory ∆Go or ∆Eo (kcal/mol)

M06/6-311++G(d,p) for B, C on B, olefinic C and H

6-31G(d) for all other atoms ΔGb-a = -3.3

M06/6-311++G(d,p) for B, C on B, olefinic C and H

6-31G(d) for all other atoms

IEF-PCM for dichloromethane

ΔGb-a = -3.0

B3LYP/6-311++G(d,p) for B, C on B, olefinic C and H

6-31G(d) for all other atoms ΔGb-a = -1.9

B3LYP/def2-TZVP ΔEb-a = 1.4

M06-2X/def2-TZVP ΔEb-a = -5.0

B97-D/def2-TZVP ΔEb-a = -4.6

Table 5.1 – Calculated differences in Gibbs free energies and total electronic energies

between (5-1a) and (5-1b).

5.2.2 The Bis(pentafluorophenyl)alkylborane System – NMR Spectroscopic Investigations

The diarylalkylborane species with a pendant vinyl group (5-1) was generated on an NMR

sample scale in CD2Cl2 by hydroboration of 1,4-pentadiene with one equivalent of HB(C6F5)2,

commonly known as Piers borane.89 Isolation of the target species

B(C6F5)2(CH2CH2CH2CH=CH2) (5-1) was not possible due to the facility of retrohydroboration.

Nonetheless species (5-1) was indeed detected as the major species existing in equilibrium with

the starting materials as well as some of the double hydroboration product

(CH2)((CH2)2B(C6F5)2)2 as assessed by the 1H and 19F NMR spectra (Scheme 5.4).

139

Scheme 5.4 – Equilibria governing the formation of (5-1a) and (5-1b).

First described by Yu and Levy219 and by Rinaldi220 independently in 1983, the 2D heteronuclear

NOE (HOESY) technique has provided valuable information in structural and conformational

analysis of organic and biologically relevant molecules221-222 and detection of ion pairing.223 The 1H{19F} HOESY spectrum obtained on a sample of (5-1) in CD2Cl2 at -50 °C showed significant

cross-peaks between each of the three olefinic protons, especially those on the terminal carbon,

and the ortho-F atoms of the C6F5 groups, inferring that the olefinic protons undergo a significant

amount of cross-relaxation with the ortho-F atoms (Figure 5.3). It is noted that the mixing time

was kept relatively short (400 ms) to minimize contribution from spin diffusion, and the

temperature was maintained high enough so that the system stays in the positive NOE regime to

avoid overestimation of enhancements.224 It is noteworthy that no significant change in 1H, 19F

and 11B NMR chemical shifts was observed at -50 °C. Interestingly, these cross-peaks were not

observed as clearly at 25 °C, consistent with the small dissociation energy of the van der Waals

complex. Such NOE cross-peaks were also not observed for the saturated analog

B(C6F5)2(CH2)4(CH3) (5-2), generated in situ from hydroboration of 1-pentene with HB(C6F5)2,

demonstrating that the presence of a vinyl group is essential to stabilizing the “closed” form

(Figure 5.3). Collectively these data provide spectroscopic evidence for the close proximity of

the olefinic fragment in (5-1) to the B center, and thus the van der Waals olefin-borane complex,

or the “closed” form (5-1b).

140

Figure 5.3 – Partial 1H{19F} HOESY spectra of (5-1) (top) and (5-2) (bottom) measured at -

50 °C as 0.17 M CD2Cl2 solutions. X- axis: 1H spectrum; Y-axis: 19F spectrum, 32 equally-

spaced contour levels.

Furthermore, the addition of one equivalent of acetonitrile to a sample of (5-1) diminished the

NOE cross-peaks for olefinic protons and ortho-F atoms (Figure 5.4). This was attributed to the

facile formation of the stable borane-nitrile adduct (5-3).

Figure 5.4 – Partial 1H{19F} HOESY spectra of (5-3) measured at -50 °C as 0.17 M CD2Cl2

solution. X- axis: 1H spectrum; Y-axis: 19F spectrum, 32 equally-spaced contour levels.

141

5.2.3 The Bis(pentafluorophenyl)alkylborane System – Addition Reactions

Since the isolation of (5-1) was not possible, it was reacted with phosphine bases in an

attempt to form stable adducts. (C6F5)2HB.PtBu3 (5-4) is a donor-acceptor adduct that is

isolable and gives NMR parameters characteristic of a 4-coordinate borate. The species also

crystallizes and exhibits a B–P distance of 2.038(3) Å (Figure 5.5), which is significantly

longer than those in known compounds, (C6F5)2HB.PHtBu2 (1.966(9) Å)225 and

(C6F5)2HB.PPh3 (1.986(2) Å),226 suggesting that the B and P centers may be available for

further reactivity. Addition of an excess amount of 1,4-pentadiene to isolated (5-4) in toluene

gave a colorless mixture that upon standing at 25 °C for 2 days gave a crystalline product (5-

5) in 85% yield (Scheme 5.5). Resonances attributable to the vinyl group of (5-1) were not

observed in the 1H NMR spectrum of (5-5), while new 11B and 31P{1H} resonances appeared

at -12.8 ppm and 50.1 ppm respectively. 19F signals corresponding to two inequivalent C6F5

groups suggest the formation of a substituted cyclic borate. X-ray crystallography confirmed

compound (5-5) to be the six-membered borate ring with a phosphonium substituent

(C6F5)2B(CH2CH2CH2CHCH2)(PtBu3) rather than the simple Lewis acid-base adduct (Figure

5.6). It is noteworthy that the P adds to the substituted carbon of the olefin fragment,

affording a substituent that adopts an equatorial position on the chair confirmation of the six-

membered ring. The metric parameters of this zwitterion are unexceptional.

Figure 5.5 – POV-Ray drawing of (5-4). C: gray, H: gray, B: yellow-green, F: pink, P: orange.

Hydrogen atoms except BH are omitted for clarity. Selected bond distances (Å) and angles (°):

142

B(1)–P(1), 2.038(3); B(1)–C(1), 1.638(4); B(1)–C(7), 1.639(4); P(1)–B(1)–C(1), 120.4(2); C(1)–

B(1)–C(7), 108.3(2); C(7)–B(1)–P(1), 120.6(2).

Scheme 5.5 – Syntheses of (5-4), (5-5) and (5-6).

Figure 5.6 – POV-ray depiction of (5-5). C: gray, B: yellow-green, F: pink, P: orange.

Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): B(1)–C(13),

1.626(3); C(13)–C(14), 1.531(3); C(14)–C(15), 1.525(3); C(15)–C(16), 1.544(3); C(16)–C(17),

1.549(3); C(17)–B(1), 1.644(3); C(16)–P(1), 1.874(2); C(13)–B(1)–C(17), 107.37(19).

In a related reaction, a mixture of HB(C6F5)2 and tBu3P in toluene was exposed to an atmosphere

of 1,3-butadiene to afford a microcrystalline solid (5-6) in 62% yield (Scheme 5.5). NMR

spectroscopic and crystallographic studies affirmed the nature of (5-6) as the zwitterion

(C6F5)2B(CH2CH2CHCH2)(PtBu3). This cyclic borate product is analogous to (5-5) with a

phosphonium fragment also adopting a pseudo-equatorial position on the puckered five-

membered borate-ring.

143



1.644(4); C(13)–C(14), 1.534(4); C(14)–C(15), 1.533(4); C(15)–C(16), 1.532(4); C(16)–B(1),

1.653(4); C(15)–P(1), 1.850(3); C(13)–B(1)–C(16), 99.8(2).

In a similar fashion, the significantly less sterically encumbered and less nucleophilic PPh3

was used to give the analogous species (C6F5)2B(CH2CH2CH2CHCH2)(PPh3) (5-7) in 90%

yield (Scheme 5.6, Figure 5.8).


144



1.6507(16); C(13)–C(14), 1.5436(15); C(14)–C(15), 1.5404(15); C(15)–C(16), 1.5330(16);

C(16)–C(17), 1.5376(17); C(17)–B(1), 1.6406(17); C(14)–P(1), 1.8101(11); C(13)–B(1)–C(17),

107.13(9).

DFT calculations (B3LYP/6-31G(d))227 of the reaction profile confirmed (5-1b) as an

intermediate en route to (5-5) (Figure 5.9). The estimates of the energetic values are consistent

with the observed reversibility of (o-tol)3P addition to (5-1). This reaction gives a product

analogous to (5-5) and (5-7) as a precipitate from the reaction mixture, but evidence of reverse

reaction is observed when the product is redissolved.

145

Figure 5.9 – Calculated transition state of (a) tBu3P addition to (5-1b) and (b) calculated

reaction profile to (5-5) (H atoms are omitted for clarity).

It is noteworthy that Erker, Grimme and coworkers228 have described the addition of a linked P/B

system to norbornene, providing experimental and computational evidence for an

asynchronously concerted process. These authors concluded that B–C bond formation occurs to a

larger extent in the transition state, although the evidence for B-olefin or P-olefin interactions

was necessarily indirect.

5.2.4 The Bis(pentafluorophenyl)alkoxyborane System –Synthesis, DFT Calculations and NMR Spectroscopic Analysis

In order to study a system that does not suffer from retrohydroboration, the synthesis of a highly

electrophilic alkoxyborane baring a tethered vinyl group was targeted. Furthermore, gem-bis-CF3

groups229 were exploited to facilitate the orientation of the pendant vinyl group towards the B

center. The lithium alkoxide, LiOC(CF3)2CH2CH=CH2 (5-8), was generated by treatment of 2-

allylhexafluoroisopropanol with nBuLi, and was further treated with ClB(C6F5)2 to afford a clear

colorless oil in 95% yield. The 1H NMR multiplets at 5.66, 4.95 and 4.89 ppm correspond to

three inequivalent olefinic signals while the 11B NMR resonance at 42.6 ppm and 19F NMR

signals at -131.16, -146.49 and -160.27 ppm infer the formation of a 3-coordinate alkoxyborane

B(C6F5)2(OC(CF3)2CH2CH=CH2) (5-9) (Scheme 5.7). Alternatively (5-9) was also obtained by

146

reaction of ClB(C6F5)2 and 2-allylhexafluoroisopropanol with elimination of HCl in 61 % yield.

This species remains an oil even at -35 °C.


In order to examine the possibility of an olefin-borane interaction in solution, 1H-19F HOESY

experiments were performed on (5-9) at -50 °C (Figure 5.10). In this case cross peaks were

observed between the methylene proton resonances and the signals derived from the ortho-

fluorine atoms on the fluoroarene rings. Similarly the olefinic CH-proton shows similar

correlation peaks. In contrast to the species (5-1), the terminal olefinic protons of (5-9) do not

show strong correlation with the ortho-fluorine atoms on the fluoraryl rings.

Figure 5.10 – Partial 1H{19F} HOESY NMR spectrum of (5-9) measured at -50 °C as 0.17 M

CD2Cl2 solution. X- axis: 1H spectrum; Y-axis: 19F spectrum, 32 equally-spaced contour levels.

DFT computations (M06/6-311++G(d,p) for B, C on B, olefinic C and H and 6-31G(d) for all

other atoms) were performed to compare the energetics of the orientations of (5-9). The so-called

“open” form refers to the orientation in which the olefinic fragment is oriented away from B

while the “closed” form refers to the orientation where the olefinic fragment is towards B (Figure

5.11). The calculations reveal the open and closed forms are energetically degenerate (ΔGo = 0.0

147

kcal/mol) while the ΔHo value of -2.0 kcal/mol favors the closed conformation. In contrast to

CH2=CH(CH2)3B(C6F5)2, the B…C distances in this conformer of (5-9) are calculated to be 3.22

and 3.25 Å, with a trigonal planar geometry at B. The presence of some degree of π-character in

the B–O bond presumably impacts the Lewis acidity at B resulting in the longer B…C distances

and presumably accounting for the absence of the 1H-19F correlations for the terminal olefinic

protons.

Figure 5.11 – Optimized structures of (5-9a) and (5-9b) with pertinent distances.

5.2.5 The Bis(pentafluorophenyl)alkoxyborane System – Addition Reactions Employing Various Nucleophiles

Since (5-9) does not undergo retrohydroboration, it is a system suitable for examining various

different nucleophiles for FLP olefin addition reactions. The reaction of (5-9) with tBu3P resulted

in the formation of a colorless precipitate in 95% yield (Scheme 5.8). 1H NMR data of the

isolated product (5-10) showed loss of the olefinic signals and emergence of new methylene

proton signals. 11B NMR resonance at -1.5 ppm indicated the formation of a four-coordinate

borate, while 19F NMR data showed the presence of diastereotopic perfluorophenyl and

trifluoromethyl groups. X-ray crystallographic studies confirmed the formulation of this product

148

to be B(C6F5)2(OC(CF3)2CH2CHCH2)(PtBu3) (5-10) (Figure 5.12). In (5-10), the phosphine adds

to the internal carbon of the vinyl group while B forms a new B–C bond at the terminal vinyl

position.

Scheme 5.8 – Reactions of (5-9) with tBu3P and Me3P.



B(1)–O(1), 1.502(4); O(1)–C(13), 1.368(3); C(13)–C(14), 1.567(4); C(14)–C(15), 1.549(4);

C(15)–C(16), 1.549(4); C(16)–B(1), 1.634(4); C(15)–P(1), 1.877(3); O(1)–B(1)–C(16), 110.5(2);

B(1)–O(1)–C(13), 124.5(2); O(1)–C(13)–C(14), 118.2(2).

In a similar fashion, the much less bulky phosphine, Me3P, also reacts with (5-9). The NMR data

obtained shortly after mixing of the reagents indicated the presence of the adduct

149

Me3P.B(C6F5)2(OC(CF3)2CH2CH=CH2) as equivalency of the two perfluorophenyl groups was

evident in the 19F spectrum. In addition the 11B and 31P{1H} resonances at -1.6 and -11.6 ppm,

respectively, as well as the presence of vinyl protons, were consistent with this initial adduct

formation. However, upon further stirring of the mixture for 3 days, a white precipitate formed

and was isolated in 70% yield (Scheme 5.8). The 1H NMR spectrum showed loss of olefinic

proton signals. X-ray crystallographic analysis showed the structure to be

B(C6F5)2(OC(CF3)2CH2CHCH2)(PMe3) (5-11) (Figure 5.13), the analogue of (5-10). The

addition of Me3P to the olefinic fragment is somewhat surprising as Me3P typically irreversibly

forms adducts with fluoroarylboranes.230-231

Figure 5.13 – POV-ray depiction of (5-11). C: gray, B: yellow-green, F: pink, O: red, P:


B(1)–O(1), 1.4913(19); O(1)–C(13), 1.3810(19); C(13)–C(14), 1.551(2); C(14)–C(15), 1.536(2);

C(15)–C(16), 1.542(2); C(16)–B(1), 1.620(2); C(15)–P(1), 1.8060(17); O(1)–B(1)–C(16),

109.91(12); B(1)–O(1)–C(13), 125.63(13); O(1)–C(13)–C(14), 117.29(13).

The bulky nitrogen-based nucleophile 2,6-lutidine also reacts with (5-9) overnight to afford a

colourless crystalline product (5-12) in 84% yield (Scheme 5.9). The 1H NMR spectrum of (5-

12) showed absence of olefinic signals and the emergence of new inequivalent methylene proton

signals while the 11B resonance suggests the presence of a four-coordinate borate. X-ray crystal

structural data were required to unequivocally determine the nature of (5-12) as the 5-membered

cyclic product, B(C6F5)2(OC(CF3)2CH2CHCH2)(NC5H3Me2) (5-12) (Figure 5.14 (a)). In this

150

species lutidine attacks at the terminal carbon of the vinyl group and a new B–C bond forms at

the internal position of the vinyl group. In an analogous manner, reaction of 2,2,6,6-

tetramethylpiperidine with (5-9) also afforded the 5-membered cyclic product,

B(C6F5)2(OC(CF3)2CH2CHCH2)(NHC5H6Me4) (5-13) (Scheme 5.9, Figure 5.14 (b)). The

observation of (5-12) and (5-13) stands in contrast to previously reported FLP additions to

olefins in which the nucleophilic attack occurs at the more substituted carbon due to the lower

activation barrier to the transition state bearing a partial positive charge on the internal carbon. In

the present cases, it appears that steric hindrance precludes such attack and results in these first

examples of the alternate regioselectivity. It is also noted that, when (5-13) is redissolved in a

solvent, evidence of the regeneration of a vinyl group was observed in its 1H NMR spectrum,

further supporting the reversible nature of such olefin addition reactions by FLPs.


151

Figure 5.14 – POV-Ray depiction of (a) (5-12) and (b) (5-13). C: gray, B: yellow-green, F:

pink, O: red, N: aquamarine. Hydrogen atoms are omitted for clarity. Selected bond distances

(Å) and angles (°): (a) B(1)–O(1), 1.509(4); O(1)–C(13), 1.378(4); C(13)–C(14),

1.585(8)/1.565(7); C(14)–C(15), 1.378(9)/1.463(7); C(15)–B(1), 1.642(5); C(15)–C(16),

1.480(5); C(16)–N(1), 1.504(4); O(1)–B(1)–C(15), 100.5(2); B(1)–O(1)–C(13), 111.8(2); O(1)–

C(13)–C(14), 104.8(4)/109.0(3). (b) B(1)–O(1), 1.494(2); O(1)–C(13), 1.397(2); C(13)–C(14),

1.549(2); C(14)–C(15), 1.539(2); C(15)–B(1), 1.650(3); C(15)–C(16), 1.518(2); C(16)–N(1),

1.535(2); O(1)–B(1)–C(15), 101.03(13); B(1)–O(1)–C(13), 112.34(13); O(1)–C(13)–C(14),

109.51(14).

Nucleophiles in such FLP additions to olefins are not limited to group 5 donors. Reaction of

1,2,5-trimethylpyrrole with (5-9) at -35 °C gave a colorless crystalline compound which was

confirmed via crystallographic analysis to be B(C6F5)2(OC(CF3)2CH2CHCH2)(C4H2Me2NMe)

(5-14a) (Scheme 5.10, Figure 5.15 (a)). In this case, the pyrrole added to the vinyl group of (5-9)

at its β-carbon, in a manner similar to alkyne addition reactions by B(C6F5)3 and pyrroles.232

NMR characterization of this compound performed in DMSO-d6 was consistent with 2,1-proton

migration within the pyrrole moiety, giving (5-14b), as evidenced by the characteristic triplet NH

proton signal at 10.97 ppm although rapid proton exchange at N on the NMR timescale precludes

the observation of the two diastereomers. In a similar manner, reaction of (5-9) with N-tert-

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butylpyrrole proceeded to give the 6-membered cyclic product

B(C6F5)2(OC(CF3)2CH2CHCH2)(C4H4NtBu) (5-15a/b) (Scheme 5.10, Figure 5.15 (b)). Again,

the pyrrole acts as a nucleophile at its β-carbon, presumably a reflection of the steric demands of

the tBu substituent on N.

Scheme 5.10 – Synthesis of (5-14a) and (5-15a) and 2,1-proton migration in DMSO-d6.

Figure 5.15 – POV-Ray depiction of (a) (5-14a) and (b) (5-15a). C: gray, B: yellow-green, F:

pink, O: red, N: aquamarine. Hydrogen atoms are omitted for clarity. Selected bond distances

(Å) and angles (°): (a) B(1)–O(1), 1.490(7)/1.502(6); O(1)–C(13), 1.386(6)/1.383(6); C(13)–

C(14), 1.545(7)/1.545(7); C(14)–C(15), 1.533(7)/1.530(7); C(15)–C(16), 1.534(7)/1.536(7);

C(16)–B(1), 1.618(7)/1.618(7); C(15)–C(20), 1.491(7)/1.490(7); N(1)–C(19), 1.485(7)/1.457(7);

C(19)–C(20), 1.484(8)/1.493(7); C(20)–C(21), 1.332(7)/1.321(7); C(21)–C(22),

1.442(7)/1.431(7); C(22)–N(1), 1.286(7)/1.283(7); O(1)–B(1)–C(16), 111.3(4)/110.1(4); B(1)–

O(1)–C(13), 125.7(4)/126.0(4); O(1)–C(13)–C(14), 116.7(4)/116.8(4). (b) B(1)–O(1), 1.499(3);

O(1)–C(13), 1.383(2); C(13)–C(14), 1.548(3); C(14)–C(15), 1.530(3); C(15)–C(16), 1.529(3);

153

C(16)–B(1), 1.612(3); C(15)–C(20), 1.495(3); N(1)–C(19), 1.463(3); C(19)–C(20), 1.496(3);

C(20)–C(21), 1.340(3); C(21)–C(22), 1.422(3); C(22)–N(1), 1.295(3); O(1)–B(1)–C(16),

110.23(17); B(1)–O(1)–C(13), 124.04(16); O(1)–C(13)–C(14), 116.58(17).

Similarly, the NHC-carbene, 1,3-di-tert-butylimidazol-2-ylidene (ItBu), also reacts as a C-based

nucleophile with (5-9) to give B(C6F5)2(OC(CF3)2CH2CHCH2)(ItBu) (5-16) (Scheme 5.11,

Figure 5.16). Analogous to (5-14) and (5-15), the C-nucleophile adds to the internal vinyl

carbon. A similar reaction was also performed with benzylidene triphenylphosphorane,

PhCHPPh3, a reagent popularly used in organic synthesis to build carbon backbones, and (5-9) to

give an off-white product (5-17). NMR data were consistent with (5-17) being a 1:1 mixture of

two diastereomers of B(C6F5)2(OC(CF3)2CH2CHCH2)(P(CHPh)Ph3) (5-17) (Scheme 5.11, Figure

5.17). It is noteworthy that benzylidene triphenylphosphorane has been shown to react with

B(C6F5)3 to yield the zwitterionic compound Ph3PCH(Ph)(p-C6F4)BF(C6F5)2.233 Nonetheless the

use of this species in FLP addition reactions affords a strategy to C–C bond formation.


154

Figure 5.16 – POV-Ray depiction of (5-16). C: gray, B: yellow-green, F: pink, O: red, N:

aquamarine. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°):

B(1)–O(1), 1.497(6)/1.496(6); O(1)–C(13), 1.397(5)/1.372(5); C(13)–C(14), 1.533(6)/1.544(6);

C(14)–C(15), 1.513(6)/1.518(6); C(15)–C(16), 1.495(7)/1.501(7); C(16)–B(1),

1.666(8)/1.672(7); C(15)–C(19), 1.538(6)/ 1.523(6); O(1)–B(1)–C(16), 111.1(4)/110.2(4); B(1)–

O(1)–C(13), 125.0(4)/125.9(3); O(1)–C(13)–C(14), 116.9(4)/117.0(3).

155



B(1)–O(1), 1.497(2); O(1)–C(13), 1.378(2); C(13)–C(14), 1.548(3); C(14)–C(15), 1.535(2);

C(15)–C(16), 1.523(2); C(16)–B(1), 1.630(3); C(15)–C(19), 1.556(2); C(19)–P(1), 1.8605(17);

O(1)–B(1)–C(16), 110.99(14); B(1)–O(1)–C(13), 125.88(14); O(1)–C(13)–C(14), 117.42(14).

Borohydrides are not known to reduce unactivated olefins without metal catalysts,234-237

therefore, the potential use of hydride from a borohydride as a nucleophile seemed unlikely.

However, combination of (5-9) with [tBu3PH][HB(C6F5)3] resulted in the clean formation of a

new species. As loss of both the BH hydride signal and the olefinic protons was observed by 1H

NMR spectroscopy, it was speculated that the hydride was transferred from B(C6F5)3 to (5-9)

yielding the cyclic borate salt [tBu3PH][B(C6F5)2(OC(CF3)2CH2CH2CH2)]. However, all efforts

to isolate this product from trace [HPtBu3][HB(C6F5)3] failed due to slight reversibility of the

reaction (Scheme 5.12). This issue could be overcome by generating the borohydride in a

catalytic fashion. Thus combination of an equivalent of (5-9) with 1,2,2,6,6-

pentamethylpiperidine (PMP) under an atmosphere of H2 and in the presence of 5 mol%

B(C6F5)3 gave full conversion to the ammonium salt of the cyclic borate [PMPH]

[B(C6F5)2(OC(CF3)2CH2CH2CH2)] (5-18). This species was obtained in pure form and

subsequently crystallographically characterized (Scheme 5.12, Figure 5.18). Formation of (5-18)

156

affirms that transient generation of the [PMPH][HB(C6F5)3] leads to nucleophilic delivery of

hydride from the borohydride anion to (5-9), affording the cycle borate anion of (5-18).

Scheme 5.12 – Reaction of (5-9) with [tBu3PH][HB(C6F5)3] and synthesis of (5-18).

Figure 5.18 – POV-Ray depiction of (5-18). C: gray, B: yellow-green, F: pink, O: red, N:

aquamarine. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°):

B(1)–O(1), 1.4971(16); O(1)–C(13), 1.3807(14); C(13)–C(14), 1.5454(18); C(14)–C(15),

1.519(2); C(15)–C(16), 1.5288(19); C(16)–B(1), 1.6139(19); O(1)–B(1)–C(16), 110.05(10);

B(1)–O(1)–C(13), 125.11(10); O(1)–C(13)–C(14), 117.35(11).

This hydride transfer reaction bears an interesting mechanistic question. One possible

mechanism is the direct hydride transfer from B(C6F5)3 (Scheme 5.13 (a)). The second

possibility is hydride transfer to the boron of (5-9), followed by intramolecular insertion of the

157

vinyl group into the boron-hydride bond (Scheme 5.13 (b)). Another possible mechanism takes

initial scrambling of ligands to transiently generate hydroborane HB(C6F5)2, which subsequently

undergoes hydroboration of the vinyl group, and then cyclization to give the cyclic borate

(Scheme 5.13 (c)).

Scheme 5.13 – Proposed reaction pathways for the hydride transfer from HB(C6F5)3 anion

to (5-9).

In order to examine the feasibility of intramolecular vinyl group insertion to B–H bond, (5-9)

was reacted with LiHBEt3 to generate Li[HB(C6F5)2(OC(CF3)2CH2CHCH2)]. Although the

compound could not be obtained in the analytically pure form, its generation was unambiguously

observed by NMR (in THF-d8) by the olefinic protons at 5.85, 4.97 and 4.85 ppm, and the

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characteristic BH doublet at -9.1 ppm (J = 96 Hz) in 11B spectrum that supports the presence of a

4-coordinate B center. This species does not convert to the cyclic borate upon prolonged

standing, which effectively eliminates the possibility of intramolecular vinyl group insertion to

B–H bond. In an attempt to examine the likelihood of ligand scrambling, (5-8) was mixed with

either B(C6F5)3 or (5-9). However, these reactions did not proceed presumably because the small

B center cannot accommodate four of these large groups, suggesting that ligand scrambling is

not likely to occur in this system.

To further examine to validity of the direct hydride transfer mechanism, this reaction was probed

using DFT calculations performed at the B3LYP/6-31G(d) level of theory in gas phase and with

a IEF-PCM model for bromobenzene. Two conformational isomers of (5-9) in which the olefinic

fragment is either oriented towards or away from the B center were computed to be

approximately equal in free energy. Nonetheless, the formation of the cyclic borate anion of (5-

18) from (5-9) and [HB(C6F5)3] anion was found to be favourable, going through an early

transition state with an activation barrier of 16.6 kcal/mol in the gas phase calculation (Figure

5.19). In the transition state the terminal olefinic C–B distance is 1.87 Å, while the C=C is

elongated slightly to 1.40 Å. The approach of the B–H fragment to the β-carbon results in a C–

H–B vector in which the C–H distance is 1.87 Å while the H–B distance is 1.26 Å. This is best

described as the concerted formation of both the new B–C and C–H bonds. The activation barrier

of 24.7 kcal/mol for the back reaction is also reasonable for the observed slight reversibility of

the reaction at RT. It should also be noted that the computed barrier to hydride transfer from

[HB(C6F5)3] anion to the B center of (5-9) was determined to be energetically uphill, presumably

due to the greater Lewis acidity towards hydride of B(C6F5)3 over (5-9).

159

Figure 5.19 – Reaction coordinate leading to the formation of (5-18) anion and the

calculated transition state structure. Energetic values and bond parameters in brackets are

results of treatment with a solvent model.

5.3 Conclusions In the present study, using a tethering approach, we have detected an interaction of a Lewis

acidic B center with a pendant alkenyl group by 2D heteronuclear NOE techniques. Examination

of the (pentafluorophenyl)alkylborane provided a compelling piece of evidence that the addition

reaction of a sterically encumbered Lewis pair to a pendant vinyl group first forms a borane-

olefin van der Waals complex. Subsequent nucleophilic attack by a Lewis base was used to

afford the novel zwitterionic cyclic borate species. Moreover the

(pentafluorophenyl)alkoxyborane was used to demonstrate tethered olefin addition reactions by a

variety of P-, N-, C-based nucleophiles. In an analogous reaction, the borate anion [HB(C6F5)3]

160

was shown to provide a strategy for hydride delivery. In general, nucleophilic addition occurs at

the β-carbon of the olefinic unit, although in the case of highly hindered nucleophiles, such as

lutidine or tetramethylpiperidine, the regiochemistry of addition is reversed. The ability to form

new C–P, C–N, C–C and C–H bonds in the chemistry augurs well for the applications of this FLP

approach to synthetic targets.







kind of Young bombs (C6H5Br, methanol). All solvents were thoroughly degassed after









house employing a Perkin-Elmer CHN Analyzer. ClB(C6F5)2,89 HB(C6F5)2,89 PhCH2PPh3Br238

and [tBu3PH][HB(C6F5)3]2 were synthesized employing literature procedures.

Tris(pentafluorophenyl)borane was purchased from Boulder Scientific Company, tri-tert-

butylphosphine from Strem Chemicals Inc., 1,4-pentadiene from Tokyo Chemical Industry Co.

and 2-allylhexafluoroisopropanol from Apollo Scientific Ltd. All other reagents were purchased

from Sigma-Aldrich, Alfa Aesar or AcrosOrganics.

161

5.4.2 NMR measurements

All the NMR experiments were performed using Bruker Avance 400 MHz spectrometer

equipped with a 5 mm PABBO BB-1H/D Z-GRD Z108618 probe. The 2D heteronuclear NOE

measurements were performed using the original four-pulse sequence with a 16-step phase-

cycling described by Yu and Levy.239 1H was acquired in the direct dimension with 19F in the

indirectly detected dimension. A relaxation delay of 2.0 s and a mixing time of 400 ms were

applied in all measurements. The t1 and t2 domains were digitized by 256 and 2048 data points,

respectively, giving approximately 3-hour-experiments. HOESY measurements were performed

on sample of the following in situ generated compounds as well as (5-9).

Generation of B(C6F5)2(CH2CH2CH2CH=CH2) (5-1) A 5 mm NMR

tube equipped with a J-Young tap was charged with HB(C6F5)2 (30 mg,

0.087 mmol) and CD2Cl2 (0.50 mL). 1,4-pentadiene (10 μL, 0.097 mmol) cooled at -35 °C was

quickly syringed into the sample and the tap was tightly closed. The sample was shaken until all

was dissolved. 1H, 19F and 11B NMR spectra were obtained at 25 °C and at -50 °C to confirm that

the major species in the sample is B(C6F5)2(CH2CH2CH2CH=CH2) at both temperatures.

Assignments to 1H resonances were made using COSY. 1H-NOESY and 19F-NOESY measured

at -50 °C confirmed that the sample only gives positive NOE cross-peaks. 25 °C: 1H NMR

(CD2Cl2, 298 K): 5.71 (m, 1H, BCH2CH2CH2CH=CH2), 5.13-4.97 (m, 2H,

BCH2CH2CH2CH=CH2), 2.17 (m, 2H, BCH2CH2CH2CH=CH2), 2.11 (m, 2H,

BCH2CH2CH2CH=CH2), 1.74 (m, 2H, BCH2CH2CH2CH=CH2). 11B NMR (CD2Cl2, 298 K):

68.8 (br, ν1/2 = 442 Hz ). 19F NMR (CD2Cl2, 298 K): -130.18 (m, 4F, o-C6F5), -148.95 (tt, 2F, 3JFF = 20 Hz, 4JFF = 3 Hz, p-C6F5), -161.99 (m, 4F, m-C6F5). -50 °C: 1H NMR (CD2Cl2, 223 K):

5.39 (m, 1H, BCH2CH2CH2CH=CH2), 5.11-4.92 (m, 2H, BCH2CH2CH2CH=CH2), 2.11 (m, 2H,

BCH2CH2CH2CH=CH2), 1.97 (m, 2H, BCH2CH2CH2CH=CH2), 1.82 (m, 2H,

BCH2CH2CH2CH=CH2). 11B NMR (CD2Cl2, 223 K): 69.2 (br, ν1/2 = 7328 Hz). 19F NMR

(CD2Cl2, 223 K): -129.15(m, 4F, o-C6F5), -148.47(t, 2F, 3JFF = 21 Hz, p-C6F5), -161.76 (m, 4F,

m-C6F5).

B(C6F5)2(CH2CH2CH2CH2CH3) (5-2) A 5 mm NMR tube equipped with

a J-Young tap was charged with HB(C6F5)2 (30 mg, 0.087 mmol) and

CD2Cl2 (0.50 mL). 1-pentene (10 μL, 0.091 mmol) cooled at -35 °C was quickly syringed into

162

the sample and the tap was tightly closed. The sample was shaken until all was dissolved. 1H, 19F

and 11B NMR spectra were obtained at 25 °C and at -50 °C to confirm that the major species in

the sample is B(C6F5)2(CH2CH2CH2CH2CH3) at both temperatures. Assignments to 1H

resonances were made using COSY. 1H-NOESY and 19F-NOESY measured at -50 °C confirmed

that the sample only gives positive NOE cross-peaks. 25 °C: 1H NMR (CD2Cl2, 298 K): 2.12 (m,

2H, BCH2CH2CH2CH2CH3), 1.59 (m, 2H, BCH2CH2CH2CH2CH3), 1.40 (m, 2H,

BCH2CH2CH2CH2CH3), 1.32 (m, 2H, BCH2CH2CH2CH2CH3), 0.89 (t, 2H, 3JHH = 7 Hz,

BCH2CH2CH2CH2CH3). 11B NMR (CD2Cl2, 298 K): 72.8 (br, ν1/2 = 515 Hz). 19F NMR (CD2Cl2,

298 K): -130.53 (m, 4F, o-C6F5), -148.99 (t, 2F, 3JFF = 19 Hz, p-C6F5), -162.00 (m, 4F, m-C6F5).

-50 °C: 1H NMR (CD2Cl2, 223 K): 2.04 (m, 2H, BCH2CH2CH2CH2CH3), 1.52 (m, 2H,

BCH2CH2CH2CH2CH3), 1.31 (m, 2H, BCH2CH2CH2CH2CH3), 1.19 (m, 2H,

BCH2CH2CH2CH2CH3), 0.82 (t, 2H, 3JHH = 7 Hz, BCH2CH2CH2CH2CH3). 11B NMR (CD2Cl2,

223 K): 75.4 (br, ν1/2 = 6330 Hz). 19F NMR (CD2Cl2, 223 K): -129.94 (m, 4F, o-C6F5), -148.05

(t, 2F, 3JFF = 20 Hz, p-C6F5), -161.53 (m, 4F, m-C6F5).

B(C6F5)2(NCCH3)(CH2CH2CH2CH=CH2) (5-3) A 5 mm NMR tube

equipped with a J-Young tap was charged with HB(C6F5)2 (30 mg, 0.087

mmol) and CD2Cl2 (0.50 mL). 1,4-pentadiene (10 μL, 0.097 mmol) cooled

at -35 °C was quickly syringed into the sample and it was shaken until all

was dissolved. Acetonitrile (4.5 μL, 0.086 mmol) was then syringed into the sample, which was

thoroughly shaken. 1H, 19F and 11B NMR spectra were obtained at 25 °C and at -50 °C to

confirm that the major species in the sample is B(C6F5)2(NCCH3)(CH2CH2CH2CH=CH2) at both

temperatures. Assignments to 1H resonances were made using COSY. 1H-NOESY and 19F-

NOESY measured at -50 °C confirmed that the sample only gives positive NOE cross-peaks. 25

°C: 1H NMR (CD2Cl2, 298 K): 5.87 (m, 1H, BCH2CH2CH2CH=CH2), 5.05-4.92 (m, 2H,

BCH2CH2CH2CH=CH2), 2.66 (s, 3H, NCCH3), 2.14 (m, 2H, BCH2CH2CH2CH=CH2), 1.33 (m,

2H, BCH2CH2CH2CH=CH2), 1.19 (m, 2H, BCH2CH2CH2CH=CH2). 11B NMR (CD2Cl2, 298 K):

-5.8 (br, ν1/2 = 312 Hz). 19F NMR (CD2Cl2, 298 K): -134.99 (m, 4F, o-C6F5), -159.53 (t, 2F, 3JFF

= 20 Hz, p-C6F5), -165.04 (m, 4F, m-C6F5). -50 °C: 1H NMR (CD2Cl2, 223 K): 5.85 (m, 1H,

BCH2CH2CH2CH=CH2), 5.08-4.97 (m, 2H, BCH2CH2CH2CH=CH2), 2.67 (s, 3H, NCCH3), 2.07

(m, 2H, BCH2CH2CH2CH=CH2), 1.20 (m, 2H, BCH2CH2CH2CH=CH2), 1.07 (m, 2H,

BCH2CH2CH2CH=CH2). 11B NMR (CD2Cl2, 223 K): -5.4 (br, ν1/2 = 1948 Hz). 19F NMR

163

(CD2Cl2, 223 K): -134.99 (m, 4F, o-C6F5), -159.12 (t, 2F, 3JFF = 21 Hz, p-C6F5), -164.62 (m, 4F,

m-C6F5).

5.4.3 Syntheses

Synthesis of (C6F5)2HB.PtBu3 (5-4) HB(C6F5)2 (205 mg, 0.59 mmol) and

tBu3P (120 mg, 0.59 mmol) were stirred together in CH2Cl2 (5 mL) for 1 h.

The solvent was then pumped down, and pentane was added to facilitate

product precipitation. The mother liquor was discarded and the solid product washed with

pentane and dried in vacuo. Yield: 225 mg, 70%. Single crystals suitable for X-ray diffraction

were obtained from a solution in CH2Cl2 layered by pentane at 25 °C. 1H NMR (CD2Cl2): 4.10

(br, 1H, BH), 1.11 (d, 3JHP = 11.9 Hz, 27H, tBu). 11B {1H} NMR (CD2Cl2): -26.2 (d, 1JBP = 56

Hz). 13C {1H} NMR (CD2Cl2): 148.93 (dm, 1JCF = 241 Hz, o-C6F5), 140.39 (dm, 1JCF = 247 Hz,

p-C6F5), 138.27 (dm, 1JCF = 250 Hz, m-C6F5), 120.43 (br, i-C6F5), 39.84 (d, 1JCP = 18.4 Hz, quat-

tBu), 31.53 (s, tBu). 19F NMR (CD2Cl2): -123.90 (br, 4F, o-C6F5), -157.71 (t, 3JFF = 21 Hz, 2F, p-

C6F5), -163.12 (br, 4F, m-C6F5). 31P {1H} (CD2Cl2): 47.1 (m). Anal. Calcd. for C24H28BF10P: C,

52.58; H, 5.15. Found: C, 52.56; H, 5.34 %.

Synthesis of (C6F5)2B(CH2CH2CH2CHCH2)(PtBu3) (5-5) (5-4) (60 mg,

0.11 mmol) was dissolved in toluene (3 mL), and 1,4-pentadiene (150

mg, 2.2 mmol) cooled at -35 °C was quickly pipetted into the solution. Leaving the reaction

mixture at RT for 2 days without stirring resulted in precipitation of colourless crystals. Since the

crystals contain toluene molecules, the isolated crystals were redissolved in dichloromethane,

and the product was precipitated by addition of pentane. The white solid was further washed with

pentane and dried in vacuo. Yield: 57 mg, 85%. Single crystals suitable for X-ray diffraction

were obtained from a reaction mixture in toluene at 25 °C. 1H NMR (CD2Cl2): 2.46-2.32 (m, 2H,

PCH, PCHCH2CH2), 2.16 (dd, 1H, 2JHH = 12 Hz, 3JHP = 11 Hz, BCH2CHP), 2.03 (br, 1H,

PCHCH2CH2), 1.85-1.54 (m, 3H, PCHCH2CH2, PCHCH2CH2, BCH2CH2), 1.60 (d, 27H, 3JHP =

12.8 Hz, tBu), 1.41 (m, 1H, BCH2CHP), 0.36 (m, 1H, BCH2CH2). 11B NMR (CD2Cl2): -12.8 (s). 13C{1H} NMR (CD2Cl2): 148.55 (dm, 1JCF = 236 Hz, o-C6F5), 148.00 (dm, 1JCF = 233 Hz, o-

C6F5), 137.69 (dm, 1JCF = 245 Hz, p-C6F5), 137.19 (dm, 1JCF = 245 Hz, p-C6F5), 136.52 (dm, 1JCF

= 246 Hz, m-C6F5), 40.51 (d, 1JCP = 26 Hz, quat-tBu), 39.44 (d, 1JCP = 16 Hz, PCH), 35.00 (d,

164

2JCP = 4.6 Hz, PCHCH2CH2), 30.88 (s, tBu), 28.71 (dm, 3JCP = 12 Hz, PCHCH2CH2), 27.33 (br,

BCH2CHP), 22.78 (br, BCH2CH2). The i-C6F5 carbons were not observed. 19F NMR (CD2Cl2): -

132.43 (dd, 2F, 3JFF = 25 Hz, 4JFF = 8 Hz, o-C6F5), -134.23 (dd, 2F, 3JFF = 25 Hz, 4JFF = 8 Hz, o-

C6F5), -163.62 (t, 1F, 3JFF = 20 Hz, p-C6F5), -165.23 (t, 1F, 3JFF = 20 Hz, p-C6F5), -166.65 (m,

2F, m-C6F5), -167.28 (m, 2F, m-C6F5). 31P{1H} NMR (CD2Cl2): 50.14 (s). Elemental analysis

was performed on the crystals of the compound that contains a toluene molecule for one

molecule of (5-5). Anal. Calcd. For C36H44BF10P: C, 61.03; H, 6.26. Found: C, 61.64; H, 6.44 %.

Synthesis of (C6F5)2B(CH2CH2CHCH2)(PtBu3) (5-6) Pre-isolated (5-

4) (146 mg, 0.27 mmol) was dissolved in toluene (6 mL), and

transferred to a well-dried Schlenk bomb. As 1 atm of 1,3-butadiene was introduced, the reaction

mixture became warm. The reaction was heated at 60 °C for 18.5 h, giving a cloudy mixture.

Excess butadiene was removed by vacuum, and the content was poured into a vial. Pentane was

added to precipitate the product, which was isolated and washed with more pentane and dried in

vacuo. Pure product was recrystallized from CH2Cl2/pentane. Yield: 99 mg, 62%. Single crystals

suitable for X-ray diffraction were obtained from layering a solution of the product in CH2Cl2

with pentane at 25 °C. 1H NMR (CD2Cl2): 2.50 (m, 1H, PCH), 2.38 (br, 1H, PCHCH2CH2),

2.18-2.03 (m, 2H, BCH2CHP, PCHCH2CH2), 1.63 (d, 27H, 3JHP = 12.9 Hz, tBu), 1.30-1.11 (m,

2H, PCHCH2CH2, BCH2CHP), 1.02 (m, 1H, PCHCH2CH2). 11B NMR (CD2Cl2): -12.8 (s). 13C{1H} NMR (CD2Cl2): 147.20 (dm, 1JCF = 235 Hz, o-C6F5), 147.02 (dm, 1JCF = 235 Hz, o-

C6F5), 136.69 (dm, 1JCF = 244 Hz, p-C6F5), 135.80 (dm, 1JCF = 245 Hz, m-C6F5), 135.78 (dm, 1JCF = 245 Hz, m-C6F5), 130.39 (br, i-C6F5), 41.18 (d, 1JCP = 27 Hz, quat-tBu), 37.53 (d, 1JCP =

18 Hz, PCH), 33.77 (s, PCHCH2CH2), 31.41 (s, tBu), 30.42 (br, BCH2CHP), 22.68 (br,

PCHCH2CH2). 19F NMR (CD2Cl2): -133.70 (d, 2F, 3JFF = 25 Hz, o-C6F5), -134.76 (d, 2F, 3JFF =

25 Hz, o-C6F5), -164.59 (t, 1F, 3JFF = 20 Hz, p-C6F5), -165.03 (t, 1F, 3JFF = 20 Hz, p-C6F5), -

166.81 (m, 2F, m-C6F5), -167.18 (m, 2F, m-C6F5). 31P{1H} NMR (CD2Cl2): 53.3 (br). Anal.

Calcd. for C28H34BF10P: C, 55.83; H, 5.69. Found: C, 55.82; H, 5.73 %.

Synthesis of (C6F5)2B(CH2CH2CH2CHCH2)(PPh3) (5-7) To a mixture

suspension of HB(C6F5)2 (100 mg, 0.29 mmol) and PPh3 (76 mg, 0.29

mmol) in toluene (2.0 mL) was added 1,4-pentadiene (100 µL, 0.97 mmol). The reaction was

shaken until all was dissolved. The mixture was then let sit at RT for 2 days, during which time a

crystalline precipitate formed. The mother liquor was decanted off, and the product was washed

165

with toluene (2.0 mL) followed by pentane (3 x 3.0 mL). The solid was dried in vacuo. Yield:

177 mg, 90%. The crystalline product was suitable for X-ray diffraction studies. 1H NMR

(CD2Cl2): 7.75 (m, 3H, p-Ph), 7.66-7.59 (m, 12H, o-Ph, m-Ph), 3.16 (m, 1H, PCH), 2.05 (m, 1H,

PCHCH2), 1.98 (m, 1H, PCHCH2CH2), 1.83 (m, 1H, BCH2CHP), 1.76 (m, 1H, PCHCH2CH2),

1.56 (d, 1H, 2JHH = 13.6 Hz, BCH2CH2), 1.20 (m, 1H, PCHCH2), 0.71 (dt, 1H, 2JHH = 12.6 Hz, 3JHH = 12.6 Hz, 3JHP = 3.8, BCH2CHP), 0.27 (dt, 1H, 2JHH = 13.1 Hz, 3JHH = 13.1 Hz, 3JHH = 3.7,

BCH2CH2). 11B NMR (CD2Cl2): -12.9 (s). 13C{1H} NMR (CD2Cl2): 149.41 (dm, 1JCF = 238 Hz,

o-C6F5), 148.63 (dm, 1JCF = 236 Hz, o-C6F5), 138.49 (dm, 1JCF = 246 Hz, p-C6F5), 137.89 (dm, 1JCF = 244 Hz, p-C6F5), 137.42 (dm, 1JCF = 246 Hz, m-C6F5), 137.01 (dm, 1JCF = 240 Hz, m-

C6F5), 134.78 (d, 4JCP = 2.9 Hz, p-Ph), 134.25 (d, 2JCP = 8.4 Hz, o-Ph), 130.48 (d, 3JCP = 11.7 Hz,

m-Ph), 119.93 (d, 1JCP = 81.0 Hz, i-Ph), 33.75 (d, 1JCP = 34.6 Hz, PCH), 30.62 (d, 2JCP = 2.4 Hz,

PCHCH2), 27.90 (d, 3JCP = 16.5 Hz, PCHCH2CH2), 23.24 (br, BCH2CHP, BCH2CH2). 19F NMR

(CD2Cl2): -132.68 (dd, 2F, 3JFF = 25 Hz, 4JFF = 8 Hz, o-C6F5), -134.31 (d, 2F, 3JFF = 25 Hz, 4JFF =

8 Hz, o-C6F5), -163.46 (t, 1F, 3JFF = 20 Hz, p-C6F5), -164.83 (t, 1F, 3JFF = 20 Hz, p-C6F5), -

166.49 (m, 2F, m-C6F5), -167.18 (m, 2F, m-C6F5). 31P{1H} NMR (CD2Cl2): 27.1 (s). Anal.

Calcd. for C35H24BF10P: C, 62.16; H, 3.58. Found: C, 61.90; H, 4.04 %.

Synthesis of LiOC(CF3)2CH2CHCH2 (5-8) To a solution of 2-

allylhexafluoroisopropanol (0.43 mL, 2.8 mmol) in pentane (10 mL) was

slowly syringed nBuLi (1.7 mL of 1.6 M hexane solution, 2.7 mmol) at 25 °C. After the reaction

mixture was stirred for 1h, all volatiles were removed in vacuo. The resulting white solid was

recrystallized from pentane at -35 °C. Yield: 447 mg, 77%. 1H NMR (400 MHz, C6D6, 298 K): δ

5.84 (m, 1H, =CH), 5.06 (d, 1H, 3JHH = 9.6 Hz, =CH2), 5.05 (d, 1H, 3JHH = 17.2 Hz, =CH2), 2.34

(d, 2H, 3JHH = 7.2 Hz, CH2). 13C{1H} NMR (101 MHz, C6D6, 298 K): δ 133.04 (s, H2C=CH),

126.60 (q, 1JCF = 292 Hz, CF3), 122.33 (s, H2C=CH), 79.63 (septet, 2JCF = 27 Hz, C(CF3)2),

38.42 (s, CH2). 19F NMR (376 MHz, C6D6, 298 K): δ -78.58 (s). Anal. Calcd. for C6H5F6LiO: C,

33.67; H, 2.35. Found: C, 33.93; H, 2.62 %.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2) (5-9) This compound was

prepared using two different methods. Method 1: ClB(C6F5)2 (243 mg,

0.64 mmol) and (5-8) (137 mg, 0.64 mmol) were stirred together in pentane (10 mL) at 25 °C for

1 h, during which time the mixture became cloudy with a fine white precipitate. The solid was

filtered out using a plug of Celite. The filtrate was dried under vacuum to give a colourless oil.

166

Yield: 334 mg, 95%. Method 2: A Schlenk flask charged with ClB(C6F5)2 (496 mg, 1.30 mmol)

in CH2Cl2 (5.0 mL) was cooled at -78 °C. 2-allylhexafluoroisopropanol (0.20 mL, 1.31 mmol)

was syringed into this solution. After stirring the mixture for 30 min at -78 °C, the cooling bath

was removed and the reaction was further stirred for 2 h at 25 °C. All volatiles were removed

under vacuum to afford a colourless oil. Yield: 437 mg, 61%. 1H NMR (C6D6, 298 K): 5.66 (m,

1H, =CH), 4.95 (d, 1H, 3JHH = 10.3 Hz, =CH2), 4.89 (d, 1H, 3JHH = 16.8 Hz, =CH2), 2.59 (d, 2H, 3JHH = 7.0 Hz, CH2). 11B NMR (C6D6, 298 K): 42.6 (br). 13C{1H} NMR (C6D6, 298 K): 148.26

(dm, 1JCF = 249 Hz, o-C6F5), 144.20 (dm, 1JCF = 259 Hz, o-C6F5), 137.93 (dm, 1JCF = 254 Hz, p-

C6F5), 126.73 (s, =CH), 122.40 (s, =CH2), 122.50 (q, 1JCF = 289 Hz, CF3), 109.25 (br, i-C6F5),

84.09 (septet, 2JCF = 30 Hz, C(CF3)2), 35.65 (s, CH2). 19F NMR (C6D6, 298 K): -73.66 (s, 6F,

CF3), -131.16 (d, 4F, 3JFF = 21 Hz, o-C6F5), -146.49 (t, 2F, 3JFF = 21 Hz, p-C6F5), -160.27 (m, 4F,

m-C6F5). 1H-NOESY and 19F-NOESY measured at -50 °C confirmed that the sample only gives

positive NOE cross-peaks. NMR obtained at -50 °C: 1H NMR (CD2Cl2, 223 K): 5.75 (m, 1H,

=CH), 5.24 (m, 2H, =CH2), 2.81 (d, 2H, 3JHH = 6.9 Hz, CH2). 19F NMR (CD2Cl2, 223 K): -74.17

(s, 6F, CF3), -130.87 (d, 4F, 3JFF = 20 Hz, o-C6F5), -147.16 (t, 2F, 3JFF = 19 Hz, p-C6F5), -160.60

(m, 4F, m-C6F5). EI-MS: calculated for [B(C6F5)2(OC(CF3)2CH2CHCH2)]+: 552.0178, found:

552.0187.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2)(PtBu3) (5-10) (5-9) (28

mg, 0.051 mmol) and PtBu3 (10 mg, 0.049 mmol) were mixed in benzene

(1.0 mL), and the reaction was shaken well. After letting the mixture sit

at RT for 10 min, pentane (3 mL) was added to precipitate the product. The product was further

washed with pentane (2 x 3 mL), and dried in vacuo. Yield: 35 mg, 95%. Single crystals suitable

for X-ray diffraction studies were grown from a solution of the product in CH2Cl2 layered with

pentane at 25 °C. 1H NMR (CD2Cl2): 2.88 (m, 1H, PCH), 2.72 (d, 1H, 2JHH = 14 Hz,

BOC(CF3)2CH2), 2.29 (t, 1H, 2JHH = 13 Hz, 3JHH = 13 Hz, BOC(CF3)2CH2), 1.92 (m, 1H, BCH2),

1.67 (m, 1H, BCH2), 1.62 (d, 27H, 3JHP = 13 Hz, tBu). 11B NMR (CD2Cl2): -1.5 (s). 13C{1H}


(dm, 1JCF = 245 Hz, m-C6F5), 124.94 (q, 1JCF = 288 Hz, CF3), 124.59 (q, 1JCF = 291 Hz, CF3),

76.56 (m, C(CF3)2), 41.72 (d, 1JCP = 25 Hz, PC(CH3)3), 31.58 (s, BOC(CF3)2CH2), 31.29 (s,

PC(CH3)3), 30.39 (d, 1JCP = 24 Hz, PCH), 24.21 (br, BCH2). The i-C6F5 signal was not observed. 19F NMR (CD2Cl2): -77.83 (q, 3F, 4JFF = 11 Hz, CF3), -78.52 (m, 3F, CF3), -135.06 (dm, 2F, 3JFF

167

= 23 Hz, o-C6F5), -135.73 (m, 2F, o-C6F5), -161.81 (t, 1F, 3JFF = 20 Hz, p-C6F5), -162.84 (t, 1F, 3JFF = 20 Hz, p-C6F5), -166.13 (m, 2F, m-C6F5), -167.05 (m, 2F, m-C6F5). 31P{1H} NMR

(CD2Cl2): 49.8 (s). Anal. Calcd. for C30H32BF16OP: C, 47.77; H, 4.28. Found: C, 47.72; H, 4.30

%.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2)(PMe3) (5-11) To a

stirring solution of (5-9) (50 mg, 0.091 mmol) in CH2Cl2 (2.0 mL) was

added PMe3 (0.20 mL of 1.0 M toluene solution, 0.2 mmol). The reaction

was stirred at 25 °C for 3 days during which time a white precipitate formed. The mother liquor

was removed and the white solid was further washed with CH2Cl2 (2 x 5 mL), and then dried in

vacuo. Yield: 40 mg, 70%. Single crystals suitable for X-ray diffraction studies were obtained by

layering a solution of (5-9) (42 mg) in CH2Cl2 (3.0 mL) by a solution of PMe3 (0.1 mL of 1.0 M

toluene solution) in toluene (1.0 mL) at 25 °C. 1H NMR (DMSO-d6): 2.62 (m, 1H, PCH), 2.06

(d, 1H, 2JHH = 14 Hz, BOC(CF3)2CH2), 1.81 (d, 9H, 2JHP = 14 Hz, P(CH3)3), 1.76 (m, 1H,

BOC(CF3)2CH2), 1.49 (m, 1H, BCH2), 0.30 (m, 1H, BCH2). 11B NMR (DMSO-d6): -1.7 (br). 13C{1H} NMR (DMSO-d6): 147.33 (dm, 1JCF = 237 Hz, o-C6F5), 146.63 (dm, 1JCF = 238 Hz, o-

C6F5), 137.39 (dm, 1JCF = 247 Hz, p-C6F5), 135.77 (dm, 1JCF = 244 Hz, m-C6F5), 124.28 (q, 1JCF

= 294 Hz, CF3), 123.76 (q, 1JCF = 289 Hz, CF3), 75.19 (m, C(CF3)2), 25.51 (d, 1JCP = 50 Hz,

PCH), 24.00 (s, BOC(CF3)2CH2), 16.41 (br, BCH2), 4.53 (d, 1JCP = 54 Hz, P(CH3)3). The i-C6F5

signal was not observed. 19F NMR (DMSO-d6): -74.29 (q, 3F, 4JFF = 10 Hz, CF3), -77.83 (m, 3F,

CF3), -133.64 (m, 2F, o-C6F5), -134.69 (m, 2F, o-C6F5), -161.56 (t, 1F, 3JFF = 22 Hz, p-C6F5), -

162.00 (t, 1F, 3JFF = 22Hz, p-C6F5), -165.38 (m, 2F, m-C6F5), -165.63 (m, 2F, m-C6F5). 31P{1H}

NMR (DMSO-d6): 33.0 (s). Anal. Calcd. for C21H14BF16OP: C, 40.16; H, 2.25. Found: C, 39.44;

H, 2.25 %.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2)(NC5H3Me2) (5-12) In a

vial, (5-9) (110 mg, 0.20 mmol) and 2,6-lutidine (21 mg, 0.20 mmol)

were dissolved in benzene (3.0 mL), and mixed well by shaking without

stirring. Upon overnight standing, a colourless crystalline precipitate

formed out of the solution. The yellow mother liquor was removed, and the solid product washed

with benzene (3 x 3mL) and dried in vacuo. Yield: 110 mg, 84%. Single crystals suitable for X-

ray diffraction studies were grown from a solution of the product in CH2Cl2 layered with pentane

at 25 °C. 1H NMR (DMSO-d6): 8.30 (t, 1H, 3JHH = 8.0 Hz, p-Ar, lutidine), 7.86 (d, 2H, 3JHH = 8.0

168

Hz, m-Ar, lutidine), 4.59 (dd, 1H, 2JHH = 15 Hz, 3JHH = 4.9 Hz, NCH2), 4.38 (t, 1H, 2JHH = 14 Hz, 3JHH = 14 Hz, NCH2), 2.76 (s, 6H, CH3, lutidine), 2.61 (m, 1H, BCH), 1.82 (m, 2H, BCHCH2). 11B NMR (DMSO-d6): 3.0 (br). 13C{1H} NMR (DMSO-d6): 155.20 (s, o-Ar, lutidine), 146.90

(dm, 1JCF = 237 Hz, o-C6F5, two inequivalent C6F5 signals overlapped), 143.93 (s, p-Ar,

lutidine), 137.74 (dm, 1JCF = 241 Hz, p-C6F5, two inequivalent C6F5 signals overlapped), 136.00

(dm, 1JCF = 248 Hz, m-C6F5, two inequivalent C6F5 signals overlapped), 128.34 (s, m-Ar,

lutidine), 124.36 (q, 1JCF = 288 Hz, CF3), 124.13 (q, 1JCF = 288 Hz, CF3), 83.62 (septet, 2JCF = 29

Hz, C(CF3)2), 56.25 (s, NCH2), 30.92 (s, BCHCH2), 30.02 (br, BCH), 21.04 (s, CH3, lutidine).

The i-C6F5 signal was not observed. 19F NMR (DMSO-d6): -75.45 (m, 3F, CF3), -75.70 (m, 3F,

CF3), -132.50 (m, 2F, o-C6F5), -133.99 (m, 2F, o-C6F5), -160.03 (t, 1F, 3JFF = 22 Hz, p-C6F5), -

161.10 (t, 1F, 3JFF = 21 Hz, p-C6F5), -164.45 (m, 2F, m-C6F5), -165.18 (m, 2F, m-C6F5). Anal.

Calcd. for C25H14BF16NO: C, 45.55; H, 2.14; N, 2.12. Found: C, 45.17; H, 2.27; N, 2.27 %.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2)(NHC5H6Me4) (5-13)

(5-9) (78 mg 0.14 mmol) and 2,2,6,6-tetramethylpiperidine (20 mg,

0.14 mmol) were stirred together in benzene (3 mL) for 2 days at RT,

during which time a white precipitate formed. Pentane (5 mL) was

added to facilitate precipitation. Mother liquor was removed from the reaction, and the solid

product was washed with 1:1 mixture of benzene and pentane (3 x 3 mL). Yield: 80 mg, 82%.

Single crystals suitable for X-ray diffraction studies were grown from dichloromethane.

Although the compound is stable in its solid state, the reaction is reversible when the product is

redissolved. Following the reverse reaction, the components slowly decomposes in solution due

to back reaction, prohibiting the acquisition of an appropriate 13C NMR spectrum. 1H NMR

(CD2Cl2): 3.64 (ddd, 1H, 2JHH = 15 Hz, 3JHH = 5.3 Hz, 3JHH = 2.3 Hz, NCH2), 3.56 (t, 1H, 1JHN =

50 Hz, NH), 3.40 (t, 1H, 2JHH = 14 Hz, 3JHH = 14 Hz, NCH2), 2.29 (m, 2H), 2.13 (m, 1H), 2.00-

1.60 (m, 6H), 1.50 (s, 3H, CH3), 1.42 (s, 3H, CH3), 1.38 (s, 3H, CH3), 1.34 (s, 3H, CH3). 11B

NMR (CD2Cl2): 2.5 (s). 19F NMR (CD2Cl2): -76.83 (m, 3F, CF3), -76.97 (m, 3F, CF3), -132.80

(br, 2F, o-C6F5), -135.75 (m, 2F, o-C6F5), -160.76 (br m, 1F, p-C6F5), -161.94 (br m, 1F, p-C6F5),

-165.28 (br m, 2F, m-C6F5), -165.76 (br m, 2F, m-C6F5). Anal. Calcd. for C27H24BF16NO: C,

46.78; H, 3.49; N, 2.02. Found: C, 46.58; H, 3.73; N, 2.00 %.

169

Synthesis of

B(C6F5)2(OC(CF3)2CH2CHCH2)(C4H2

Me2NMe) (5-14) Compound (5-9) (51

mg, 0.098 mmol) and 1,2,5-

trimethylpyrrole (10 mg, 0.092 mmol) were dissolved separately in CH2Cl2 (3 mL) and cooled at

-35 °C. The solution of (5-9) was added to the solution of pyrrole at -35 °C, and the reaction was

stirred for 1 h. The resultant yellow solution was layered with pentane (5 mL) at -35 °C. This

gave colourless crystalline product, which was then isolated and dried in vacuo. Yield: 50 mg,

86%. Single crystals suitable for X-ray diffraction studies were grown from a solution of the

product in CH2Cl2 layered with pentane at 25 °C, and the X-ray structure was determined to be

the (5-14a) form. The pure product when isolated is not very soluble in CD2Cl2. When the

product is dissolved in DMSO-d6, 2,1-proton migration occurs, therefore the following NMR

characterization shows the (5-14b) form. 1H NMR (DMSO-d6): 10.97 (br, 1H, NH, pyrrole), 5.53

(s, 1H, CH, pyrrole), 3.27 (s, 3H, N-CH3, pyrrole), 2.85 (t, 1H, 3JHH = 12 Hz, BCH2CH), 2.08 (s,

3H, CH3, pyrrole), 2.02 (s, 3H, CH3, pyrrole), 1.77 (d, 1H, 2JHH = 14 Hz, BOC(CF3)2CH2), 1.56

(t, 1H, 2JHH = 3JHH = 13 Hz, BOC(CF3)2CH2), 1.36 (d, 1H, 2JHH = 14 Hz, BCH2), 0.50 (t, 1H, 2JHH = 3JHH = 13 Hz, BCH2). 11B NMR (DMSO-d6): -1.0 (br). 13C{1H} NMR (DMSO-d6):

147.63 (dm, 1JCF = 232 Hz, o-C6F5), 146.72 (dm, 1JCF = 239 Hz, o-C6F5), 136.99 (dm, 1JCF = 242

Hz, p-C6F5), 135.74 (dm, 1JCF = 249 Hz, m-C6F5), 129.03 (br, i-C6F5), 127.26 (br, i-C6F5), 125.61

(s, quaternary, pyrrole), 125.37 (s, quaternary, pyrrole), 125.01 (q, 1JCF = 293 Hz, CF3), 124.51

(q, 1JCF = 288 Hz, CF3), 120.98 (s, quaternary, pyrrole), 102.62 (s, CH, pyrrole), 76.56 (septet, 2JCF = 27 Hz, C(CF3)2), 34.31 (s, BOC(CF3)2CH2), 29.66 (s, CH3), 28.06 (br, BCH2), 26.49 (s,

BCH2CH), 12.26 (s, CH3), 9.48 (s, CH3). 19F NMR (DMSO-d6): -74.76 (m, 3F, CF3), -77.97 (m,

3F, CF3), -133.35 (m, 2F, o-C6F5), -134.50 (m, 2F, o-C6F5), -163.08 (t, 1F, 3JFF = 21 Hz, p-C6F5),

-163.59 (t, 1F, 3JFF = 21 Hz, p-C6F5), -166.37 (m, 4F, m-C6F5). Anal. Calcd. for C25H16BF16NO:

C, 45.41; H, 2.44; N, 2.12. Found: C, 45.51; H, 2.64; N, 2.01 %.

Synthesis of

B(C6F5)2(OC(CF3)2CH2CHCH2)(C

4H4NtBu) (5-15) A N-tert-

butylpyrrole (15 mg, 0.12 mmol) solution in CH2Cl2 (1.5 mL) cooled at -35 °C was added to a

(5-9) (67 mg, 0.12 mmol) solution in CH2Cl2 (1.5 mL) at -35 °C. The reaction was stirred for 2

170

hours at RT. The reaction mixture was layered by pentane, and kept at -35 °C overnight, after

which a white microcrystalline solid precipitated out. The product was further washed with

pentane (3 x 5 mL), and dried thoroughly in vacuo. Yield: 45 mg, 56%. Single crystals suitable

for X-ray diffraction studies were obtained from a reaction mixture, and the X-ray structure was

determined to be the (5-15a) form. The pure product when isolated is not very soluble in CD2Cl2.

When the product is dissolved in DMSO-d6, 2,1-proton migration occurs; therefore, the

following NMR characterization shows the (5-15b) form. 1H NMR (DMSO-d6): 13.84 (s, 1H,

NH, pyrrole), 6.74 (m, 1H, CH, pyrrole), 6.61 (m, 1H, CH, pyrrole), 5.85 (m, 1H, CH, pyrrole),

2.84 (t, 1H, 3JHH = 12 Hz, BCH2CH), 2.02 (d, 1H, 2JHH = 14 Hz, BOC(CF3)2CH2), 1.57 (m, 2H,

two signals overlapped, BOC(CF3)2CH2, BCH2), 1.42 (s, 9H, tBu), 0.45 (t, 1H, 2JHH = 3JHH = 13

Hz, BCH2). 11B NMR (DMSO-d6): -0.9 (br). 13C{1H} NMR (DMSO-d6): 147.56 (dm, 1JCF = 234

Hz, o-C6F5), 146.74 (dm, 1JCF = 238 Hz, o-C6F5), 136.90 (dm, 1JCF = 243 Hz, p-C6F5), 135.67

(dm, 1JCF = 247 Hz, m-C6F5), 131.43 (s, 3-C, pyrrole), 129.01 (br, i-C6F5), 126.89 (br, i-C6F5),

124.93 (q, 1JCF = 292 Hz, CF3), 124.44 (q, 1JCF = 289 Hz, CF3), 116.97 (s, pyrrole), 112.72 (s,

pyrrole), 105.34 (s, pyrrole), 76.47 (septet, 2JCF = 27 Hz, C(CF3)2), 53.37 (s, C(CH3)3), 34.21 (s,

BOC(CF3)2CH2), 30.37 (s, C(CH3)3), 28.05 (br, BCH2), 27.85 (s, BCH2CH). 19F NMR (DMSO-

d6): -74.68 (m, 3F, CF3), -77.89 (m, 3F, CF3), -133.32 (m, 2F, o-C6F5), -134.46 (m, 2F, o-C6F5), -

163.03 (t, 1F, 3JFF = 22 Hz, p-C6F5), -163.52 (t, 1F, 3JFF = 21 Hz, p-C6F5), -166.31 (m, 4F, m-

C6F5). Anal. Calcd. for C26H18BF16NO: C, 46.25; H, 2.69; N, 2.07. Found: C, 45.51; H, 2.58; N,

2.13 %.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2)(ItBu) (5-16) In a 50 mL

Schlenk flask, a toluene (4 mL) solution of (5-9) (116 mg, 0.21 mmol)

was cooled at -78 °C, and solution of 1,3-di-tert-butylimidazol-2-ylidene

(38 mg, 0.21 mmol) in toluene (3 mL) was syringed in. The reaction was

then stirred at -15 °C for 8 h, during which time a precipitate formed. The product was isolated

and washed with pentane (3 x 3 mL) and dried in vacuo. Yield: 115 mg, 75%. 1H NMR (DMSO-

d6): 7.89 (s, 1H, =CH, ItBu), 7.72 (s, 1H, =CH, ItBu), 3.93 (m, 1H, BCH2CH), 2.53 (m, 1H,

BOC(CF3)2CH2), 2.26 (m, 1H, BOC(CF3)2CH2), 2.06 (m, 1H, BCH2), 1.81 (s, 9H, tBu), 1.45 (s,

9H, tBu), 1.15 (m, 1H, BCH2). 11B NMR (DMSO-d6): -1.3 (br). 13C{1H} NMR (DMSO-d6):

150.30 (s, NCN), 147.29 (dm, 1JCF = 240 Hz, o-C6F5), 137.51 (dm, 1JCF = 240 Hz, p-C6F5),

135.84 (dm, 1JCF = 240 Hz, m-C6F5), 124.00 (q, 1JCF = 286 Hz, CF3), 121.68 (s, =CH, ItBu),

171

119.21 (s, =CH, ItBu), 75.07 (septet, 2JCF = 27 Hz, C(CF3)2), 65.01 (s, C(CH3)3), 2.32 (s,

C(CH3)3), 31.39 (s, C(CH3)3), 29.88 (s, BCH2CH), 29.35 (s, C(CH3)3), 27.03 (s,

BOC(CF3)2CH2), 21.65 (br, BCH2). The i-C6F5 carbon signals were not observed. 19F NMR

(DMSO-d6): -75.62 (m, 3F, CF3), -78.11 (m, 3F, CF3), -133.69 (m, 2F, o-C6F5), -134.47 (m, 2F,

o-C6F5), -161.46 (t, 1F, 3JFF = 21 Hz, p-C6F5), -161.67 (t, 1F, 3JFF = 21 Hz, p-C6F5), -165.51 (m,

2F, m-C6F5), -165.84 (m, 2F, m-C6F5). Anal. Calcd. for C29H25BF16N2O: C, 47.56; H, 3.44; N,

3.83. Found: C, 47.81; H, 3.71; N, 3.80 %.

Synthesis of B(C6F5)2(OC(CF3)2CH2CHCH2)(P(CHPh)Ph3) (5-17)

To a suspension of PhCH2PPh3Br (69 mg, 0.16 mmol) in benzene (3

mL) was syringed in nBuLi solution (1.6 M in hexane, 100 μL, 0.16

mmol). The vivid orange reaction mixture was stirred at RT for 2 h, followed by filtration

through a pad of Celite. The filtrate solution was added to a benzene (2 mL) solution of (5-9) (87

mg, 0.16 mmol) with stirring. Upon overnight reaction a solid precipitate formed. All volatiles

were pumped off from the reaction, and the remaining solid was washed with pentane (3 x 5

mL). Drying under vacuum afforded an off-white product. Yield: 94 mg, 65%. Single crystals

suitable for X-ray diffraction studies were grown from a solution of the product in CH2Cl2

layered with pentane at 25 °C. Diastereomer A (the one that selectively crystallized): 1H NMR

(CD2Cl2): 7.78 (m, 3H, Ph), 7.52 (m, 6H, Ph), 7.39 (m, 1H, Ph), 7.35-7.23 (m, 8H, Ph), 6.91 (m,

2H, Ph), 4.14 (dd, 1H, 2JHP = 16 Hz, 3JHH = 8 Hz, PCH), 3.01 (m, 1H, BCH2CH), 1.77 (dm, 1H, 2JHH = 14 Hz, BOC(CF3)2CH2), 1.34 (t, 1H, 2JHH = 13 Hz, 3JHH = 13 Hz, BOC(CF3)2CH2), 1.22

(dm, 1H, 2JHH = 14 Hz, BCH2), 0.85 (t, 1H, 2JHH = 14 Hz, 3JHH = 14 Hz, BCH2). 11B NMR

(CD2Cl2): -1.6 (s). 13C{1H} NMR (CD2Cl2): 148.25 (dm, 1JCF = 240 Hz, o-C6F5), 138.67 (dm, 1JCF = 244 Hz, p-C6F5), 137.08 (dm, 1JCF = 244 Hz, m-C6F5), 135.58 (d, JCP = 3 Hz), 134.91 (d,

JCP = 9 Hz), 132.013 (d, 2JCP = 4 Hz, i-Ph), 131.31 (br), 130.50 (d, JCP = 12 Hz), 130.09 (d, JCP =

2 Hz), 125.12 (q, 1JCF = 289 Hz, CF3), 124.78 (q, 1JCF = 289 Hz, CF3), 118.81 (d, 1JCP = 83 Hz, i-

Ph, PPh3), 77.26 (m, C(CF3)2), 55.01 (d, 1JCP = 42 Hz, PCH), 35.29 (s, BCH2CH), 33.35 (d, 3JCP

= 10 Hz, BOC(CF3)2CH2), 29.81 (br, BCH2). The i-C6F5 signal was not observed. 19F NMR

(CD2Cl2): -77.98 (m, 3F, CF3), -78.81 (q, 3F, 4JFF = 10 Hz, CF3), -134.78 (dm, 2F, 3JFF = 21 Hz,

o-C6F5), -134.99 (dm, 2F, 3JFF = 23 Hz, o-C6F5), -163.38 (t, 1F, 3JFF = 20 Hz, p-C6F5), -163.99 (t,

1F, 3JFF = 20 Hz, p-C6F5), -166.70 (m, 2F, m-C6F5), -167.48 (m, 2F, m-C6F5). 31P{1H} NMR

(CD2Cl2): 23.6 (s). Diastereomer B (assigned by spectrum of both diastereomers – Diastereomer

172

A): 1H NMR (CD2Cl2): 7.83 (m, 3H, Ph), 7.60 (m, 6H, Ph), 7.43 (m, 1H, Ph), 7.35-7.23 (m, 8H,

Ph), 6.91 (m, 2H, Ph), 4.04 (dd, 1H, 2JHP = 15 Hz, 3JHH = 6 Hz, PCH), 3.14 (m, 1H, BCH2CH),

1.82 (dm, 1H, 2JHH = 14 Hz, BOC(CF3)2CH2), 1.45 (t, 1H, 2JHH = 13 Hz, 3JHH = 13 Hz,

BOC(CF3)2CH2), 1.26 (m, 1H, BCH2), 0.88 (m, 1H, BCH2). 11B NMR (CD2Cl2): -1.6 (s). 31P{1H} NMR (CD2Cl2): 24.2 (s). Anal. Calcd. for C43H26BF16OP: C, 57.10; H, 2.90. Found: C,

57.73; H, 3.46 %.

Synthesis of [HPMP][B(C6F5)2(OC(CF3)2CH2CH2CH2)]

(5-18) A 100 mL Schlenk bomb was charged with (5-9) (107

mg, 0.19 mmol), 1,2,2,6,6-pentamethypiperidine (30 mg,

0.19 mmol) and B(C6F5)3 (5 mg, 0.01 mmol) dissolved in bromobenzene (3.0 mL). The bomb

was charged with ca. 2 atm of H2 through three freeze-pump-thaw cycles, after which the

reaction was stirred for 3 days at RT. The H2 atmosphere was then removed, and the product

precipitated by addition of pentane (10 mL). The mother liquor was pipetted out, and the white

solid was further washed with pentane (2 x 10 mL). Extensive drying in vacuo gave the product.

Yield: 119 mg, 88%. Single crystals suitable for X-ray diffraction studies were obtained by

laying a CH2Cl2 solution with pentane (1:2). 1H NMR (CD2Cl2): 4.35 (t, 1H, 1JHN = 50 Hz, NH),

2.92 (d, 3H, 3JHH = 3.7 Hz, NCH3), 1.98 (m, 2H, CH2), 1.88-1.71 (m, 8H, CH2), 1.46 (s,

NC(CH3)2), 1.44 (s, NC(CH3)2), 0.90 (m, BCH2). 11B NMR (CD2Cl2): -1.5 (s). 13C{1H} NMR

(CD2Cl2): 148.33 (dm, 1JCF = 236 Hz, o-C6F5), 138.22 (dm, 1JCF = 239 Hz, p-C6F5), 137.04 (dm, 1JCF = 246 Hz, m-C6F5), 129.00 (br, i-C6F5), 125.74 (q, 1JCF = 291 Hz, CF3), 76.61 (septet, 2JCF =

27 Hz, C(CF3)2), 67.75 (m, NC(CH3)2), 39.23 (s, NC(CH3)2CH2), 31.24 (br, NC(CH3)2), 31.14 (s,

NCH3), 26.88 (s, CH2), 20.19 (br, NC(CH3)2), 19.20 (br, BCH2), 18.77 (s, CH2), 16.08 (s, CH2). 19F NMR (CD2Cl2): -77.32 (s, 6F, CF3), -135.32 (m, 4F, o-C6F5), -163.03 (t, 2F, 3JFF = 20 Hz, p-

C6F5), -167.34 (m, 4F, m-C6F5). Anal. Calcd. for C28H28BF16NO: C, 47.41; H, 3.98; N, 1.97.

Found: C, 47.50; H, 4.27; N, 1.91 %.

5.4.4 DFT calculations

DFT calculations were performed using Gaussian 09.227 Geometry optimization of (5-1a), (5-

1b), (5-9a) and (5-9b) was carried out using the M06 method and 6-311++G(d,p) (for B, C’s on

B, olefinic C and H) and 6-31G(d) (for all other atoms) basis sets implemented in the Gaussian

173

09 software. Thermal energy corrections were extracted from the results of frequency analysis

performed at the same level of theory. The optimized structures of (5-1a), (5-1b), (5-9a) and (5-

9b) were further subjected to single point energy calculations using a large AO basis set (def2-

TZVP) employing B3LYP, M06-2X217 and B97-D218 functionals implemented in the Gaussian

09 software. For the reaction profile leading to (5-5), geometry optimization of (5-1a), (5-1b),

tBu3P, (5-5) and (5-1a)…tBu3P‡ was conducted at the B3LYP/6-31G(d) level of theory.

Frequency analysis of (5-1a), (5-1b), tBu3P and (5-5) contained no imaginary frequency showing

that these are energy minima while that of (5-1a)…tBu3P‡ gave one imaginary frequency

confirming that it is a first-order saddle point. For the reaction profile leading to (5-18),

geometry optimization of (5-9a), (5-9b), B(C6F5)3, (5-9).hydride anion, (5-18) anion, HB(C6F5)3

anion, and (5-9b)…HB(C6F5)3 anion‡ was conducted at the B3LYP/6-31G(d) level of theory.

Frequency analysis of (5-9a), (5-9b), B(C6F5)3, (5-9).hydride anion, (5-18) anion and HB(C6F5)3

anion contained no imaginary frequency showing that these are energy minima while that of (5-

9b)…HB(C6F5)3 anion‡ gave one imaginary frequency confirming that it is a first-order saddle

point.

5.4.5 X-ray crystallography 5.4.5.1 X-ray data collection and reduction







absorption effects using the empirical multi-scan method (SADABS). 4

5.4.5.2 X-ray data solution and refinement



174




2) and Fo










175

5.4.5.3 Selected crystallographic data


(5-4) (5-5).toluene (5-6)

Formula C24H28BF10P C36H44BF10P C28H34BF10P

Formula wt 548.24 708.49 602.33

Crystal system orthorhombic monoclinic monoclinic

Space group Pbca P21/c P21/n

a(Å) 9.1259(3) 13.0483(16) 11.4645(4)

b(Å) 15.5640(6) 15.951(2) 14.2814(5)

c(Å) 34.8441(14) 17.5001(19) 17.6884(6)

α(deg) 90 90 90

β(deg) 90 110.878(5) 105.199(2)

γ(deg) 90 90 90

V(Å3) 4949.1(3) 3403.3(7) 2794.80(17)

Z 8 4 4

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.472 1.383 1.432

Abs coeff,μ, mm-1 0.198 0.161 0.182


Rint 0.0623 0.0739 0.0434

Data used 5676 7751 6389


R (>2σ) 0.0607 0.0509 0.0632

wR2 0.1741 0.1167 0.1845

GOF 1.028 0.993 1.047


176


(5-7) (5-10) (5-11)

Formula C35H23BF10P C30H32BF16OP C21H14BF16OP

Formula wt 675.31 754.34 628.10



a(Å) 11.2176(9) 11.177(2) 16.1479(8)

b(Å) 11.7971(9) 11.228(2) 9.9549(5)

c(Å) 12.2310(9) 13.582(3) 15.5394(7)

α(deg) 68.922(3) 69.214(10) 90

β(deg) 89.060(3) 82.733(11) 109.713(2)

γ(deg) 87.535(3) 82.327(10) 90

V(Å3) 1508.9(2) 1573.6(5) 1751.96(10)

Z 2 2 4

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.486 1.592 1.774

Abs coeff,μ, mm-1 0.179 0.208 0.259


Rint 0.0271 0.0791 0.0332

Data used 9902 7056 6659


R (>2σ) 0.0413 0.0569 0.0428

wR2 0.1229 0.1213 0.1133

GOF 1.032 0.982 1.028


177


(5-12) (5-13) (5-14).1/2

dichloromethane

Formula C25H14BF16NO C27H24BF16NO C25H16BF10NO.0.5CH2Cl2

Formula wt 659.18 693.28 703.66

Crystal system triclinic monoclinic triclinic

Space group P-1 P21/n P-1

a(Å) 9.1801(13) 11.1705(5) 11.9380(9)

b(Å) 10.3330(14) 16.0049(7) 15.0103(12)

c(Å) 13.984(2) 15.9286(8) 16.2877(13)

α(deg) 82.073(8) 90 108.689(4)

β(deg) 74.808(7) 95.113(2) 100.946(4)

γ(deg) 80.634(8) 90 90.150(4)

V(Å3) 1256.7(3) 2836.4(2) 2708.1(4)

Z 2 4 4

T (K) 150(2) 150(2) 150(2)

d(calc) gcm-3 1.742 1.623 1.726

Abs coeff,μ, mm-1 0.188 0.170 0.275


Rint 0.0350 0.0371 0.0634

Data used 5755 6445 12425


R (>2σ) 0.0694 0.0402 0.0839

wR2 0.1995 0.0947 0.2899

GOF 1.047 0.998 1.056


178


(5-15) (5-16)

Formula C26H18BF16NO C29H22BF16N2O

Formula wt 675.22 729.30

Crystal system monoclinic triclinic

Space group P21/n P-1

a(Å) 9.4047(7) 10.7955(4)

b(Å) 11.0955(8) 17.5647(6)

c(Å) 25.2965(19) 20.0048(7)

α(deg) 90 67.7060(10)

β(deg) 95.391(4) 77.8810(10)

γ(deg) 90 89.5610(10)

V(Å3) 2628.0(3) 3420.7(2)

Z 4 4

T (K) 150(2) 150(2)

d(calc) gcm-3 1.707 1.416

Abs coeff,μ, mm-1 0.182 0.146


Rint 0.0576 0.0425


Variables 409 883

R (>2σ) 0.0458 0.0856

wR2 0.1089 0.2489

GOF 1.029 1.072


179


(5-17).dichloromethane (5-18)

Formula C43H26BF16OP.CH2Cl2 C28H28BF16NO

Formula wt 989.35 709.32

Crystal system monoclinic monoclinic

Space group P21/c P21/n

a(Å) 17.2019(6) 11.1411(7)

b(Å) 14.7018(6) 17.8797(11)

c(Å) 16.5689(6) 14.7969(10)

α(deg) 90 90

β(deg) 96.1350(10) 98.591(4)

γ(deg) 90 90

V(Å3) 4166.3(3) 2914.5(3)

Z 4 4

T (K) 150(2) 150(2)

d(calc) gcm-3 1.576 1.617

Abs coeff,μ, mm-1 0.301 0.168


Rint 0.0539 0.0541


Variables 643 433

R (>2σ) 0.0429 0.0461

wR2 0.1155 0.1177

GOF 1.019 0.998


180

6 Conclusions 6.1 Summary of the Thesis In this thesis, the candidate examined bifunctional systems of frustrated Lewis pairs with the

goal of establishing novel intramolecular FLPs, to broaden the range of Lewis acids and bases

useful for small molecule activations, and to gain mechanistic insights on FLP reactions.

In the first part of the thesis, alkynyl-linked phosphonium borates were synthesized. The

corresponding neutral intramolecular phosphine boranes were generated in situ and reacted with

substrates typically activated by FLPs to form various macrocycles. Reactions with H2 and

attempts to liberate H2 resulted in unexpected products, including the formation of new C–C

bonds.

In the second part of the thesis, alkynyl-linked phosphine boranes were exploited to examine the

novel coordination mode of η3-BC≡C in Ni(0) complexes. The corresponding phosphonium

borate was also examined for its reactivity with common Lewis acids to afford new compounds.

In the third part of the thesis, one-atom-linked bis-boranes were shown to act as Lewis acids in

FLP activation reactions. The sp2-C-linked bis-boranes were shown to chelate the phosphine-

bound CO2 fragment.

In the last part of the thesis, vinyl groups tethered to highly electrophilic boranes were observed

to exist in close proximity with the B centers, suggesting that such olefin-borane van der Waals

complexes precedes vinyl group addition reactions by an electrophile and nucleophile.

Moreover, P-, N-, C- and H-based nucleophiles were successfully used in these addition

reactions.

6.2 Future Work Even though the systems described in this thesis themselves are not industrially relevant, they are

instructive on the directions in which research in FLP chemistry may take in the future. Despite

the flurry of boranes baring fluorophenyl substituents within the context of FLP chemistry,

181

Lewis acids outside of this group of compounds should also be studied to find more industrially

applicable systems.

It may be recommended to make use of highly sterically demanding substituents on B to favor

the release of the product in catalytic reactions. As illustrated in Section 2.2.5, Mes and MesF

substituents on B allows for the isolation of borataallene species, which can be described as

masked carbanions. Utilizing the bulky, electron-withdrawing MesF groups may enable the

isolation of phosphino-alkynyl-borane and amino-alkynyl-borane species that may be effective

hydrogenation catalysts (Scheme 6.1).

Scheme 6.1 – Proposed synthesis of an alkynyl-amino-borane.

Vinyl-group tethered boranes that the candidate worked with may serve as monomers for new

polymeric materials baring Lewis acidic sites useful in catalysis and hydrogen storage.

Monomers such as vinyl boryl ethers may be polymerized using cationic polymerization methods

(Scheme 6.2).

Scheme 6.2 – Proposed polymerization of a vinyl boryl ether.

182

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