Exploring the Synthesis and Reactivity of Electrophilic ......Figure 1-4 Types of Boron Cations 7...

Exploring the Synthesis and Reactivity of Electrophilic Phosphonium Salts

by

Meera Mehta

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

Department of Chemistry University of Toronto

© Copyright by Meera Mehta 2017

ii

Exploring the Synthesis and Reactivity of Electrophilic

Phosphonium Salts

Meera Mehta

Doctor of Philosophy

Department of Chemistry

University of Toronto

2017

Abstract

Phosphorus compounds have a rich chemical history as Lewis donor ligands in transition metal

and organometallic chemistry. In this chemistry, they have typically played auxiliary roles,

permitting critical breakthroughs in catalysis centred at transition metal active sites. Phosphorus

based Lewis acids, which can themselves serve as a primary locus of activity, have been studied

to a lesser extent. Previously, the Stephan Group reported the preparation and Lewis acidity and

consequent reactivity of the fluorophosphonium cations [(C6F5)2PhPF]+ and [(C6F5)3PF]+. This

reactivity has been attributed to their energetically accessible σ*(P-F) acceptor orbitals. This

original system requires strongly electron-withdrawing perfluoroaryl substituents, thus limiting

potential structural variations.

The present work focuses on maintaining potent Lewis acidity at a fluorophosphonium centre

while avoiding perfluoroarenes. In this dissertation, the preparation of several dicationic

phosphonium salts is discussed. In addition, the versatility of this synthetic approach is

investigated. To this end, phosphenium cations supported with triazole, chiral, and cAAC-

carbenes were prepared. These dicationic phosphonium salts exhibit remarkable Lewis acidity in

stoichiometric reactions and act as effective Lewis acid catalysts. These systems effect the

iii

hydrodefluorination of fluoroalkanes, hydrosilylation of olefins, deoxygenation of ketones, and

the reduction of phosphine oxides and amides. Attempts to perform the Michaelis-Arbuzov

rearrangements and subsequent reductions led to the catalytic generation of PH3, as well as

primary and secondary phosphines from air stable phosphoethers and phosphoesters.

Finally, the preparation of tricationic and tetracationic phosphonium salts was investigated.

Three synthetic strategies were explored, viz. preparation of a phosphenium dication, a 4,5-

phosphinoimidazolium cation, as well as two linked carbene stabilized phosphenium cations.

Subsequent oxidation of these species led to unexpected results, which are further discussed in

Chapter 5.

iv

Acknowledgments

First and foremost, I would like to thank my supervisor, Professor Doug Stephan. Thank you for

always having an open door; all of our insightful discussions about chemistry and politics have

made me a better scientist. Thank you for giving me the freedom to explore my curiosities and

venture into new areas of chemistry. You have made the lab an exciting place to be and have

always encouraged a healthy discussion of ideas. I would also like to thank my committee

members, Professor Bob Morris and Professor Datong Song, for their guidance and support.

Finally, I would like to acknowledge Professor David Emslie and Professor Mike Brook, both of

whom gave me the opportunity to work in their respective labs as an undergraduate. Professor

Brook introduced me to the life of a researcher and paired me with a very talented graduate

student, Dr. Amanda Grande. Professor Emslie taught me Schlenk handling of air sensitive

compounds and inspired me to be an inorganic chemist. I got my first X-ray crystal structure

while working with him!

All the Stephan group members, both past and present, have been instrumental in my training.

Their discussions and friendship have fostered a great learning environment. In particular, I

would like to thank Dr. Michael Holthausen for acting as a mentor in the lab and for his

contribution on the phosphonium dication project, which is central to my dissertation. Thank you

to Dr. Manual Pérez, his extensive background in organic chemistry helped me explore new

catalytic applications for my system. He has been a source of laughter and calm in my more

stressful hours. I would also like to thank Dr. Timothy Johnstone, soon to be Professor

Johnstone, for teaching me X-ray crystallography and always making the time to address my

many questions. He has been a source of inspiration, reminding me to branch out and learn as

many different techniques as I can.

My work here would not be possible without the departmental support team. Thanks to Dr. Darcy

Burns, Dmitry Pichugian, Jack Sheng, and Dr. Sergiy Nokhrin for your help with my NMR concerns.

In addition, thanks to Shanna Pritchard, Dr. Alan Lough, Anna-Liza Villavelez, Rose Balazs, Ken

Greaves and John Ford.

Last but not least, I would like to thank my family. Thanks to Vivek, Vishal, Mom, and Dad for your

support and understanding while I underwent this process. Finally, I would like to acknowledge my

v

partner, Matt Rubel, for his thoughtful support, his endurance of chemistry related discussions at

home and at parties, as well as for being my voice of reason.

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Table of Contents

Abstract ii

Acknowledgments iv

Table of Contents vi

List of Tables ix

List of Schemes x

List of Figures xi

List of Symbols and Abbreviations xiv

Chapter 1 Introduction 1

1.1 Phosphorus – the Element 1

1.2 History of Lewis Acid / Lewis Base Chemistry 2

1.2.1 Lewis Base Catalysis 3

1.2.2 Lewis Acid Catalysis 5

1.2.2.1 Boron-Based Lewis Acids 5

1.2.2.2 Aluminium-Based Lewis Acids 9

1.2.2.3 Carbon-Based Lewis Acids 10

1.2.2.4 Silicon-Based Lewis Acids 11

1.2.2.5 Phosphorus-Based Lewis Acids 13

1.2.3 Frustrated Lewis Pair Chemistry and Small Molecule Activation 16

1.3 Dative Bonding in Main Group Compounds 18

1.4 Scope of Thesis 20

1.5 References 23

Chapter 2 Preparation of Lewis Acidic Phosphorus Cations 36

2.1 Introduction 36

2.1.1 Phosphenium Cations 36

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2.2 Results and Discussion 37

2.2.1 Synthesis of Phosphonium Dication 37

2.2.2 Lewis Acidity and Fluorophilicity Tests 42

2.2.3 Catalytic Hydrodefluorination 47

2.2.4 Exploring Phosphonium Cation Derivatives 49

2.3 Conclusion 63

2.4 Experimental Details 65

2.5 References 99

Chapter 3 Hydrosilylation of Olefins, Carbonyls & Amides 103

3.1 Introduction 103

3.1.1 History of Catalytic Hydrosilylation 103


3.2.1 Hydrosilylation of Olefins 104

3.2.2 Deoxygenation of Ketones 106

3.2.3 Reduction of Amides 115

3.3 Conclusion 121


3.5 References 152

Chapter 4 Reduction of Phosphine Oxides and Reactivity with Phosphoethers 157


4.1.1 Synthesis of Phosphines 157


4.2.1 Catalytic Reduction of Phosphine Oxides 158

4.2.2 Reactivity with Phosphoethers 169

4.2.3 Catalytic Generation of PH3, Primary and Secondary Phosphine 173

4.3 Conclusion 176


4.5 References 195

Chapter 5 Towards Tricationic and Tetracationic Electrophilic Phosphonium Salts 199

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5.1.1 Polycationic Phosphorus Cations 199


5.2.1 Two Carbenes One Phosphorus Centre 200

5.2.2 One Carbene Multiple Phosphorus Centres 202

5.2.3 Two Carbenes Two Phosphorus Centres 211

5.3 Conclusion 213


5.5 References 222

Chapter 6 Conclusion 224

6.1 Summary of This Work 224

6.1.1 Preparation of Dicationic Phosphonium Salts 224

6.1.2 Lewis Acid Catalysis 224

6.1.3 Towards Polycationic Phosphonium Salts 225

6.2 Future Work 226

6.3 References 227

ix

List of Tables

Table 2-1 Catalytic Hydrodefluorination of Fluoroalkanes Using Catalyst 2-5 48

Table 3-1 Catalytic Hydrosilylation of Olefins and Alkynes 105

Table 3-2 Deoxygenation / Hydrosilylation of Benzophenone

and 2-methylpentan-3-one 108

Table 3-3 Silane Screening for Deoxygenation of Benzophenone using 2-5 109

Table 3-4 Catalytic Deoxygenation of Aryl-substituted Ketones 111

Table 3-5 Catalytic Deoxygenation of Alkyl-substituted Ketones 112

Table 3-6 Reduction of N,N-dimethylbenzamide with Catalyst 2-1, 2-2, 2-5 116

Table 3-7 Amide Reductions Using Catalysts 2-2 and 2-3 118

Table 4-1 Catalytic Reduction of Triphenylphosphine Oxide to Triphenylphosphine 158

Table 4-2 Catalytic Reduction of Phosphine Oxides to Phosphines 163

Table 4-3 Michaelis-Arbuzov Rearrangement Mediated by 2-2 171

Table 4-4 Catalytic Generation of PH3, PhPH2, and Ph2PH 174

x

List of Schemes

Scheme 2-1 Preparation of 2-3 38


Scheme 2-3 Reaction of 2-5 with OPEt3 43




Scheme 2-7 Attempted Preparation of 2-16 53








Scheme 4-1 Tandem Michaelis-Arbuzov Rearrangement and Reduction of Methyl

Diphenylphosphonite and Ethyl Diphenylphosphonite 172

Scheme 4-2 Synthesis of PH3 Adducts with Lewis Acids 175






xi

List of Figures

Figure 1-1 Lewis Acid/Base Adduct Formation 3

Figure 1-2 General Mechanism for Lewis Base Catalyzed Acylation of Alcohols 4

Figure 1-3 Mechanism for Hydrosilylation of Carbonyls Employing

B(C6F5)3 as the Catalyst 6

Figure 1-4 Types of Boron Cations 7

Figure 1-5 Methods for Preparing Borenium Cations 8

Figure 1-6 Aluminium-Based Catalysts 10

Figure 1-7 Methods for Generating Silicon Cations 11

Figure 1-8 Examples of Organic Transformations Facilitated by Silicon Catalysts 13

Figure 1-9 Gabbaï’s Fluoride Ion Senors 14

Figure 1-10 Examples of Phosphonium Salts Tested in Diels-Alder Catalysis 15

Figure 1-11 Catalytic Transfer Hydrogenation of Diazobenzene 16

Figure 1-12 Hydrogenation of Aromatic Bonds 17

Figure 1-13 FLP Reactivity with Carbon Monoxide 18

Figure 1-14 Bonding Descriptions of [LPPh2]+ 20

Figure 2-1 POV-ray Depiction of 2-3. C: black, N: blue, P: orange.

Hydrogen atoms and anion have been omitted for clarity. 38

Figure 2-2 31P NMR and 19F NMR Spectra for 2-4 40

Figure 2-3 POV-ray Depiction of 2-4. C: black, N: blue, P: orange, F: magenta.




Hydrogen atoms and anions have been omitted for clarity. 42

Figure 2-6 POV-ray Depiction of 2-7. C: black, N: blue, P: orange, O: red.


Figure 2-7 31P and 19F NMR Spectral Data for the Reaction of 2-5 with

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Crotonaldehyde at –20 °C in CD2Cl2 44

Figure 2-8 1H NMR Spectrum from the Catalytic Friedel-Crafts

Dimerization of 1,1-diphenylethylene using 2-5 45

Figure 2-9 31P and 19F NMR Spectral Data for the Reaction of 2-5 with F2P(C6F5) 46

Figure 2-10 31P NMR Spectrum for the Reaction of 2-5 with Ph3CF 47

Figure 2-11 Hydrodefluorination Mechanism 49





Figure 2-14 POV-ray Depiction of 2-14. C: black, N: blue, P: orange, O: red.


Figure 2-15 31P NMR and 19F NMR Spectra from Attempted Synthesis of 2-16 54

Figure 2-16 31P NMR and 19F NMR Spectra Showing Intermediate 2-18 56


Figure 2-18 POV-ray Depiction of the dicationic dimeric core of 2-20.

C: black, N: blue, P: orange, Ag: purple, Cl: green.

Hydrogen atoms and anions have been omitted for clarity. 58



Figure 2-20 31P NMR Spectrum for 2-25 61


Figure 3-1 Metal based Hydrosilyation Catalysts 103

Figure 3-2 1H NMR Spectrum for the Reaction of 2-5 with an

Excess of Triethylsilane 104

Figure 3-3 Electrophilic Phosphonium Cations 107

Figure 3-4 Mechanism for Deoxygenation of Ethylbenzene with 2-2 115

Figure 3-5 Proposed Mechanism for Amide Reduction 121

Figure 4-1 1H NMR, 13C NMR and 31P NMR Spectra for the Attempted Reduction

xiii

of Benzophenone Using 2-5 and Et3SiH in the Presence of OPPh3 161

Figure 4-2 1H NMR, 13C NMR, 31P NMR Spectra for the Reduction of OPPh3

Using 2-5 and Phenylsilane in the Presence of Benzophenone 162

Figure 4-3 Proposed Mechanism for Phosphine Oxide Reduction 166

Figure 4-4 1H NMR Spectrum for H2 Observed During the Catalytic Reduction of

Triphenylphosphine Oxide 167

Figure 4-5 31P NMR Spectrum for Monitoring Catalyst Decomposition during

Catalytic Reduction of Triphenylphosphine Oxide 168

Figure 4-6 31P NMR and 19F NMR Spectra for Reaction of 2-5 with Phenylsilane 169

Figure 4-7 General Mechanism for Lewis Acid Catalyzed Michaelis-Arbuzov

Rearrangement 170

Figure 4-8 Synthetic Strategy for Phosphites as a Viable Route to Phosphines 170

Figure 4-9 1H NMR and 31P NMR Spectra of PH3 in Crude Reaction Mixture 173

Figure 4-10 1H NMR and 31P NMR Spectra for H3P•B(C6F5)3 Adduct 175

Figure 4-11 31P NMR Spectrum for H3P•Ga(C6F5)3 Adduct 176

Figure 4-12 31P NMR Spectrum for H3P•Al(C6F5)3 Adduct 176

Figure 5-1 Family of Phosphonium Salts 199





Figure 5-5 31P NMR and 19F NMR Spectra for Postulated Species 5-5 205


Figure 5-7 31P NMR Spectra for 5-8 209

Figure 5-8 1H NMR spectra for 5-8 210

Figure 5-9 31P NMR Spectrum of 5-9 211

Figure 5-10 31P NMR and 19F NMR Spectra for Reaction Mixture of 5-10 Oxidation 212

Figure 6-1 Carbene-stabilized Pnictide Dications 226

xiv

List of Symbols and Abbreviations

Å angstroms

° degrees

°C degrees Celsius

η eta (bonding mode)

μ bridging

δ chemical shift

ΔG Gibbs free energy

ΔH enthalpy

π pi

σ sigma

μL microliters

μmol micromole

mmol millimole

Ad adamantyl

Ar aromatic

atm atmospheres

br broad

calcd. calculated

C6D5Br deuterated bromobenzene

xv

CD2Cl2 deuterated dichloromethane

CDCl3 deuterated chloroform

d8-toluene deuterated toluene

d8-THF deuterated tetrahydrofuran

MeCN acetonitrile

DCM dichloromethane

CF3 trifluoromethyl

C6F5 pentafluorophenyl

CO carbon monoxide

conv. conversion

d doublet

dd doublet of doublets

DFT density functional theory

EPC electrophilic phosphonium cation

ESI electrospray ionization

DART direct analysis in real time

eq. equivalent

Et ethyl

Et2O diethyl ether

Fc calculated structure factor

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Fo observed structure factor

FLP frustrated Lewis pair

g grams

GOF goodness of fit

h hour

Hz hertz

HOMO highest occupied molecular orbital

iPr isopropyl

SIMes 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene

IBox (3S)-3,7-alkyl-2,3,7,8-tetrahydroimidazo[4,3-b:5,1-b’]bis[1,3]oxazol-4-ylidene

cAAC cyclic alkyl amino carbene

K degrees kelvin

kcal kilocalories

kJ kilojoule

LUMO lowest unoccupied molecular orbital

m multiplet

Me methyl

Mes mesityl

MHz megahertz

mg milligram

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mL millilitre

MS mass spectrometry

m/z mass-to-charge ratio

NHC N-heterocyclic carbene

NHP N-heterocyclic phosphenium cation

NMR Nuclear Magnetic Resonance

o ortho

o-tol ortho-tolyl

OTf trifluoromethanesulfonate

p para

Ph phenyl

POV-Ray Persistence of Vision Raytracer

ppm parts per million

Rw weighted residual

rt room temperature

s singlet

sept septet

T temperature

t triplet

dt doublet of triplets

xviii

THF tetrahydrofuran

TMS trimethylsilyl

tol toluene

py pyridine

tBu tert-butyl

quart quartet

VT variable temperature

TON turn over number

Trip 2,4,6-triisopropylphenyl

Dipp 2,6-diisopropylphenyl

NFSI N-fluorobenzenesulfonimide

Pn pnictogen

1

Chapter 1 Introduction

1.1 Phosphorus – the Element

Elemental phosphorus was first isolated in 1669, by a German physician and alchemist Hennig

Brand, who boiled and processed 60 buckets of urine to obtain it.1,2 Brand had managed to isolate

white phosphorus, which emits a faint glow upon reaction with oxygen, prompting him to give it

the Greek name phōsphoros meaning “light-bearer” or “morning star”. In modern times,

phosphorus is isolated from calcium phosphate rocks.3

There are four naturally occurring allotropes of elemental phosphorus: white, red, violet, and

black.3 The most dangerous of these forms is white phosphorus because it is both poisonous and

pyrophoric.4,5 Heating white phosphorus to 250 °C or prolonged exposure to sunlight will form

red phosphorus.5,6 Red phosphorus is used in matches, fireworks, smoke bombs, and pesticides.5

Continued heating of red phosphorus at 530 °C followed by slow cooling yielded Hittorf’s

allotrope, the violet allotrope.7 This allotrope was found to be a high molecular weight polymer

which could decompose to produce P2 units. Black phosphorus can also be prepared by the heating

of white phosphorus, however a mercury catalyst and seed crystal of black phosphorus is required.8

There are limited commercial uses of black phosphorus and it is the most robust of the allotropes.

Phosphorus is an essential element in all known forms of life, and it is the sixth most common

element, by weight, in the human body.9,10 It can be found in polymeric nucleic acids, such as

DNA and RNA, as well as small molecules like ATP.11 Despite the importance of these molecular

species, 80% of the phosphorus in our bodies can be found in our bones and teeth in extended

solid-state networks.10 Studies of the phosphorus biogeochemical cycle have revealed that

phosphorus gradually becomes less available on land and is slowly lost in runoff.12,13 Although

phosphorus-based nutrients are vital to the survival of living organisms and the maintenance of a

healthy ecosystem, humans have had a great influence on the natural phosphorus cycle.14 By

mining phosphorus, converting it to fertilizer and using it on a global scale, humans have greatly

increased the amount of phosphorus-based nutrients that run off into large bodies of water.14 This

has been detrimental to aquatic life, because it can cause eutrophication. This phenomenon arises

from excessive grow in aquatic life, whereupon putrefaction the water is depleted of oxygen. The

2

lack of oxygen causes plant and animal deaths to surge, which further depletes the level of oxygen

in water. This vicious cycle can take decades to correct.

Organophosphorus compounds can have an oxidation state ranging between –3 and +5 at the

phosphorus centre.15 Trivalent-phosphorus ligands have been heavily employed in conjunction

with precious metals.16 These ligands allow chemists to manipulate the solubility of the active

species, change the steric properties of the catalyst, and impact the enantioselectivity in reactions.

Phosphine ligands are usually thought of as strong σ-donors and weak π-acceptors.17 The σ-

donation effect can be improved by employing electron-donating substituents at the phosphorus,

whereas electron-withdrawing substituents favor the π-backbonding interactions. The steric

demands of the ligands are described using the Tolman cone angle of the phosphine.18 These

concepts have been exploited in homogenous catalysis. For example, smaller ligands that are

electron rich promote oxidative addition at the metal centre, whereas bulky ligands that are weak

donors tend to promote reductive elimination.

Despite the prevalence of phosphorus(V) compounds found naturally in the environment and

human biology, their catalytic utility remains limited. In this dissertation phosphorus(V) salts are

prepared and their activity in Lewis acid catalysis studied.

1.2 History of Lewis Acid / Lewis Base Chemistry

An American physical chemist Gilbert Newton Lewis is widely recognized for discovering

covalent bonds and his discussions on electron pairs.19,20 Lewis’s eponymous dot structures and

his contributions to valence bond theory have shaped the modern understanding of chemical

bonding and are taught in first year general chemistry undergraduate courses worldwide. In 1924,

he defined a Lewis acidic substance as one that “can employ an electron lone pair from another

molecule in completing the stable group of one of its own atoms.”21 Conversely a Lewis base

contributes a lone pair to another molecule. An interaction between a Lewis acid and base is

commonly referred to as a dative bond, and the combined complex is considered a Lewis adduct

(Figure 1-1).22 Lewis never won the Nobel Prize in Chemistry, although he was nominated 41

times.

3

Figure 1-1 Lewis Acid/Base Adduct Formation

Ubiquitous examples of Lewis acids are typically charge neutral group 13 compounds, with the

general formula EX3 (E = B, Al, Ga).23 These compounds have an empty p-orbital, which can

accept electron density from a donor. Likewise, classical Lewis bases are compounds

incorporating group 15 elements, like amines and phosphines.23

In 1942, Brown reported the first reaction between a Lewis acid and base that did not yield an

adduct.24 In this work, 2,6-dimethylpyridine and trimethylborane did not react with one another,

whereas the analogous reactions with BF3 afforded a classical adduct. This result was attributed to

the steric considerations of both the borane and the pyridine. Following this work, Wittig observed

a THF ring-opening reaction in the presence of a borane and trityl anion.25 In a similar vein,

triphenylphosphine and triphenylborane were found to add across the C–C triple bond of benzyne,

giving the phosphonium borate zwitterion.26 In 1966, Tochtermann observed addition of Lewis

acids (BPh3, AlPh3, BePh2, and MgPh2) and trityl anion to olefins, when attempting anionic

polymerization of dienes.27 Unconventional reactions between Lewis acids and bases continued to

be observed and formed the foundation of the Frustrated Lewis Pairs (FLP) systems developed in

the Stephan group.28 This reactivity continues to be exploited for small molecule activation and

catalysis, further discussed in Section 1.2.3.

Despite the high catalytic conversions observed by metal-based catalysts, there is increasing

interest in metal-free catalysis, with a focus on main group catalysis and organocatalysis.

1.2.1 Lewis Base Catalysis

Beyond the extensive use of Lewis bases as ligands in transition metal catalysis, there are examples

of non-metallic Lewis base catalysis. In Lewis base catalysis, the Lewis base enhances rates of

reaction by interaction of its electron pairs with an acceptor site on the organic substrate.29 This

binding enhances the nucleophilicity or electrophilicity of the substrate. Moreover, the Lewis base

4

must not be consumed in the reaction – the hallmark of any catalytic process. This definition

excludes the use of Lewis basic solvents to increase the rate of reaction as well as previously

mentioned ligand-supported metal catalysis.

In the mid-1960s Litvinenko, Kirichenko, and Steglich found that donor-substituted pyridines

showed enhanced rates of acylation of alcohols.30,31 Subsequent Hammett studies revealed that 4-

dimethylaminopyridine (DMAP) was a practical and effective catalyst for this transformation, in

accordance with the mechanism shown in Figure 1-2.30,32-36 This work formed the backdrop for

the development of asymmetric catalysts used in the kinetic resolution of alcohols and amines by

acylation.37-40 Since this work, acylations have been effectively catalyzed by N-heterocyclic

carbenes (NHC) and phosphines.41-47

Figure 1-2 General Mechanism for Lewis Base Catalyzed Acylation of Alcohols

Extensions of this general reaction were found by altering the nucleophile. Analogous reaction

pathways include Lewis base catalyzed sulfinylations48 and phosphorylations.49-51 Although

mechanistically distant, Lewis bases have also been reported as effective catalysts for the

cyanoformylation52,53 and silylcyanation54-56 of carbonyls, cycloadditions of ketenes,57-60 Morita-

Baylis-Hillman reactions,61-63 Rahut-Currier cyclization,64,65 reactions of allenolates66,67 and

alkynoates,68,69 Robinson annulations,70,71 and Mannich reactions.72 This is not a comprehensive

lists of Lewis base catalyzed reactions and this area has seen tremendous growth in the last 10

years. However, it is still difficult to predict the coordination and conformation of the active

catalyst and transition states.

5

1.2.2 Lewis Acid Catalysis

Lewis acid catalysts function by accepting electron density from a nucleophilic substrate,

activating the substrate for subsequent reactivity.73,74 Some of the earliest reports of Lewis acid

catalysts were titanium and zirconium based.73 Early d0 transition metal complexes have

interesting properties, they are unable to access a pathway incorporating oxidative addition, instead

the ionic character of these complexes allows them to polarize substrates and act as Lewis acid

catalysts. After this work, there was a movement to use boron- and aluminium-based compounds

for similar chemistry, given the empty p-orbital of many neutral compounds with Group 13

elements.75 Moving away from transition metal-based catalysts to main group catalysts allows for

lower toxicity and operational cost, however main group catalysts have suffered from low turnover

numbers. Main group Lewis acid catalysts have been expanded to included gallium, carbon,

silicon, phosphorus and sulfur based compounds.76

It is noteworthy that Lewis acids have also been employed in the activation of transition metals to

effect polymerization, C–C bond formation, and metathesis.77,78 However, Lewis acids as co-

catalysts in transition metal catalysis is beyond the scope of this discussion. In the interest of

brevity, the discussion of explicit transition metal based Lewis acids and their activity has been

omitted.

1.2.2.1 Boron-Based Lewis Acids

Boron halides (BX3) are classic examples of Lewis acids. The reactive nature of three-coordinate

boranes has made them difficult to handle. This led to the synthesis of

tris(pentafluorophenyl)borane in the 1960s, as a route to electrophilic boranes that would not be

prone to hydrolytic cleavage.79 The reactivity of B(C6F5)3 remained unexplored until the 1980s,

when it was reported to transfer a perfluoroaryl group to xenon upon reaction with XeF2 to generate

[C6F5Xe][F2B(C6F5)2].80,81 This perfluoroaryl borane was reported by Yamamoto as an effective

catalyst for the addition of silyl enol ethers to aldehydes, alkyl chlorides, and ɑ,β-unsaturated

ketones.82 Other reactions that B(C6F5)3 facilitates include the addition of ketene silyl acetals to N-

benzylimines,82 hydrosilylation of carbonyls,83 transfer hydrogenation of imines,84,85 transfer

hydrosilylation of aldehydes, olefins, and imines,86,87 and Diels-Alder-type reactions.82 Similar

reactivity has also been observed with substituted perfluoroaryl borane HB(C6F5)2.88 For the

electrophilic borane hydrosilylation catalysis, the mechanism was investigated by both Piers and

6

Oestreich.89,90 Although it is possible that the reaction proceeds by the Lewis acid activation of the

carbonyl, kinetic experiments revealed that the most basic substrate (benzaldehyde) was reduced

the slowest, whereas ethyl benzoate was reduced the fastest. Subsequent experiments showed that

increasing the concentration of the carbonyl-bearing reactant inhibited the hydrosilylation reaction.

These results are consistent with a mechanism in which the borane activates the silane rather than

the carbonyl. Piers further supported this work via DFT calculations, whereas Oestreich showed

that with the use of a chiral silane, inversion of stereochemistry can be observed at the

hydrosilylated product, consistent with a SN2-type reaction at the silane. These studies are

consistent with the mechanism shown in Figure 1-3.

Figure 1-3 Mechanism for Hydrosilylation of Carbonyls Employing B(C6F5)3 as the

Catalyst

Furthermore, the use of boron-centred cations as an access point to highly electrophilic boron

compounds has garnered increasing interest in the context of both stoichiometric and catalytic

reactions. These cations can be categorized into three structural groups based on coordination

number (Figure 1-4).91 Borenium and boronium cations can be thought of as ligand-stabilized

derivatives of two-coordinate borinium cations. Whereas borinium cations are two-coordinate with

7

the substituents often N-based, where π-donation can relieve some of the electronic deficiency at

the boron centre.91

Figure 1-4 Types of Boron Cations

The nature of the counterion accompanying these cations has a profound impact on the observed

reactivity. Cations paired with a robust, weakly-coordinating anion like [B(C6F5)4]– tend to be

more active catalysts than those paired with more coordinating anions such as [OTf]–. Of the boron

cations, boreniums are the most interesting for application in Lewis acid catalysis. Ligand

stabilization at the boron centre allows for borenium cations to be readily prepared when compared

to their borinium counterparts, as well as access to the empty p-orbital allows for high catalytic

activity, unlike the boronium salts. Commonly used methods for the preparation of borenium salts

are (Figure 1-5): a) halide abstraction; b) hydride abstraction; c) Lewis acid coordination to a

nitrogen alpha to the boron centre; d) protonation of a nitrogen alpha to the boron centre; e)

nucleophilic displacement of a substituent on boron.91

8

Figure 1-5 Methods for Preparing Borenium Cations

Matsumoto and Gabbaϊ have described the synthesis of the borenium cation shown in Figure 1-5

a from a four-coordinate boron species.92 This compound was prepared by reaction of the carbene

with the fluoroborane, followed by halogen abstraction with a Lewis acid.92,93 The driving force

for these types of reactions is the formation of a strong Si–X bond, when employing TMSOTf, or

Ag–X bond, when employing Krossing’s salt [Ag(CH2Cl2)][Al{OC(CF3)3}4].92,94 In Figure 1-5 b,

[HB(C6F5)3]– is less hydridic than pinacolborane, allowing for the quantitative generation of

[(DABCO)Bpin][HB(C6F5)3]; however the [HB(C6F5)3]– counter ion would not be ideal for

catalysis.95 In the Stephan group, borenium cations have been prepared by hydride abstraction with

[Ph3C][B(C6F5)4] to yield the more robust tetrakis(pentafluorophenyl)borate salt.96,97 Triflic acid

has been reported as a potent Brønsted–Lowry acid to protonate B–H bonds, releasing H2, and

generating a ligand-stabilized borenium cation.98 A family of borenium salts have been prepared

using a number of halogen and hydride acceptors.99 An alternative approach to preparing boron

cations is preparing boron species with an alpha pentavalent nitrogen substituent (Figure 1-5, c

9

and d). This can be done by reacting the alpha nitrogen with an external Lewis acid or via

protonation / methylation.100-103 The final example given in Figure 1-5, is displacement of a leaving

group on the boron centre to generate the borenium salt.104

In 2010, Lindsay published a proof-of-principle study where a chiral NHC-borane was employed

as a hydride donor to activated ketones to yield the reduced carbonyl with high

enantioselectivity.105 However, the catalytic activity of NHC-borenium cations as catalysts

remained underexplored until 2012, when Stephan reported the borenium-catalyzed hydrogenation

of imines.96 Since this report, a number of NHC-borenium derivatives have been prepared and

their reactivity explored.97 In 2015, the Crudden group reported triazole-supported borenium

cations that were also active for hydrogenation.106 Boron cation catalyzed hydrosilylation of

ketones was first reported in 2013 by Denmark and Ueki.104 In related work, Jäkle prepared a

planar-chiral ferrocenyl-borenium ion for the hydrosilylation of acetophenone in 20% ee.94 The

Stephan group has continued to expand the library of chiral borenium cations by using chiral

carbenes as ligands at the boron centre; however these catalysts have suffered from low

enantioinduction.107 The Crudden group reported DABCO-supported boron cations as the active

catalysts for the hydroboration of aldimines and ketimines.95 Seminal work in this area was

reported by Ingelson, where the trans-hydroborated product of olefins could be obtained when

using an NHC-supported borenium salt as the catalyst.108

1.2.2.2 Aluminium-Based Lewis Acids

Aluminium is the most abundant metal in the Earth’s crust.109 Aluminium compounds are widely

used in industrial chemical processes. Perhaps the most well-known examples are the use of

methylaluminoxane (MAO) as an effective co-catalyst for Ziegler-Natta olefin

polymerizations,110,111 and AlCl3 in Friedel-Crafts reactions.112,113 Aluminium(III) chloride can

also introduce aldehyde moieties on an arene by a Gattermann-Koch reaction, which employs

carbon monoxide, hydrogen chloride, and a copper(I) chloride co-catalyst.114,115 In 1960, Eaton

reported accelerated Diels-Alder reactions in the presence of aluminium(III) chloride.116 Following

these early reports, in 1980 Snider studied the Alder-ene reactions of acrylate and propiolate esters

catalyzed by dialkylaluminium halides and trialkylaluminium species,117 the mechanism of which

was later investigated by Singleton in 2000.118 These neutral aluminium species were also found

to catalyze the hydrosilylation of olefins.119,120 Influential work in this area was reported by Reed

10

in 2002 where the Et2Al+ alumenium ion was prepared by reacting triethylaluminium with

[Ph3C][CB11H6X6] (X = Cl, Br) via β-hydride abstraction.121 The crystal structure was reminiscent

of that of [Et3Si][CB11H6X6] where interaction between the Lewis acid and halogens of the

carborane could be observed.122 As a comparative study, the catalytic oligomerization of ethene

was carried out. On this basis, the following order of electrophilicity was determined: Et3Al <

Et2AlCl < EtAlCl2 < Et2Al(carborane) < AlCl3. Wehmschulte later reported [Et2Al][CB11H6I6] as

an effective catalyst for the deoxygenative reduction of carbon dioxide to methane, toluene, and

diphenylmethane via hydrosilylation.123 In addition to this work, salen-stabilized aluminium

complexes have been applied as catalysts for the ring-opening polymerization of cyclic

esters.124,125 The related aluminium complexes stabilized by a bisiminopyridine ligand were

reported by Berben as catalysts for dehydrogenative coupling of benzylamine,126 dehydrogenation

of formic acid,127 and electrocatalytic production of hydrogen.128 (Figure 1-6) Wright focused on

the preparation of dimeric aluminium hydrides [tBuOAlH2]2 and [tBuO2AlH]2 to effect the

dehydrocoupling of amine–borane,129 as well as amines with silanes,130 whereas Graves employed

a α-diimine complex of aluminium to effect epoxidation of olefins.131 More recently, neutral

aluminium hydrides NacNacAlRH were reported by Roesky et al. to catalyze hydroboration of

aldehydes, ketones, and olefins using pinacolborane (Figure 1-6).132,133

Figure 1-6 Aluminium-Based Catalysts

1.2.2.3 Carbon-Based Lewis Acids

Over the last century, carbocations have been extensively spectroscopically and computationally

investigated.134,135 In spite of this volume of work, they are seldom represented in organocatalysis.

Trityl perchlorate was the first reported carbenium cation catalyst; it was found to effect

Mukaiyama aldol-type reactions, Michael additions, and Sakurai allylations.75,136-139 Further

11

mechanistic investigations of the Mukaiyama aldol reaction by Denmark, Chen, and Bosnich gave

conflicting results, and it is still unclear whether these reactions proceed via carbocation catalysis

or in situ generation of silylium cations.140-142 Another degradation pathway observed by Kagan

leads to formation of a Brønsted acid, when a chiral ferrocenyl carbocation was employed as a

catalysts in Diels-Alder reactions.143-146 When Franzén et al. used trityl tetrafluoroborate as a

catalysts for Diels-Alder reactions, however, they were able to exclude the silicon-cation pathways

by avoiding silicon containing substrates.147 More recently, the Stephan group has prepared air

stable trityl cations with para-substituted methoxy substituents and tested their activity toward

hydrothiolation reactions.148

1.2.2.4 Silicon-Based Lewis Acids

Early investigations into silylium cations, [R3Si]+, were motivated by the desire to better

understand heavier group 14 analogues of carbocations.149-151 To date, various methods are known

to generate silicon cations (Figure 1-7, a-e): hydride abstraction,122,152 the allyl-leaving-group

approach,153 ring-opening protonolysis of strained cyclic silanes,154 substituent exchange,155 and

the cyclohexadienyl-leaving-group approach.156

Figure 1-7 Methods for Generating Silicon Cations

12

Hydride abstraction was first successfully described by the Lambert and co-workers, where

triethylsilane was allowed to react with trityl tetrakis(pentafluorophenyl)borate in toluene to

generate a triethylsilylium toluene complex.157,158 (Figure 1-6, a) Initial attempts at this type of

reaction by Corey in 1975 were unsuccessful, due to the use of incompatible counterions and

solvents.149 Following this work, Reed et al. prepared silylium cations with some of the most

chemically robust and least coordinating anions known to date, [HCB9H4Br5]– and [HCB11H5X6]

–

(X = Cl, Br, I).159-161 X-ray crystallography revealed that both of the silylium cations prepared by

Lambert and Reed were not truly free. Lambert’s cation had one molecule of toluene coordinated

and Reed’s cation coordinated one of the halogen atoms of the carborane anion.122,152

Lambert attempted to address this problem of the free silylium cation by increasing the steric bulk

around silicon with mesityl groups. This turned out to be true, but it led to an inaccessible Si–H

bond. Schade and Mayr developed a novel pathway to access these cations, the allyl-leaving group

approach, which was applied by Lambert.162,163 (Figure 1-6, b) The stable β-Si-substituted

carbenium ion reacts with allyltrimesitylsilane, a commercially available compound, to form the

free silylium cation [Mes3Si][B(C6F5)4] as confirmed by X-ray crystallography and a remarkable

downfield shift in the 29Si NMR resonance to δ = 225.5 ppm. A decisive factor dictating the

outcome of this reaction is high steric encumbrance around the silane.

Since this work, continued efforts have been put forth to generate silicon cations. An interesting

example was reported by Manners et al. where strained sila[1]ferrocenophanes underwent ring-

opening protonolysis in the presence of [H(OEt2)2][B(C6F5)4] at low temperature (Figure 1-7, c),164

resulting in the ether adduct of the corresponding silylium cation substituted with a ferrocenyl unit.

A similar ferrocenyl-silylium cation was prepared by the Oestreich group using the Corey method,

hydride abstraction with [CPh3][B(C6F5)4], in the absence of ethereal solvents.165 In this case, an

interaction between the silicon cation and iron centre was proposed given the increased stability

of this complex in halogenated solvents, resulting in the first example of silylium as a Z-type ligand

on a transition metal. More recently, the Oestreich group has taken inspiration from both the Corey

approach and Schade/Mayr approach to generate the ferrocenyl-silylium cation, where trityl

tetrakis(pentafluorophenyl)borate was employed to abstract a hydride from cyclohexa-2,5-dien-1-

yl group from a silane-ferrocene complex,156 resulting in an arene-stabilized ferrocenyl-silylium

cation. In 2011, Müller found a feasible route to triarylsilylium cations via hydride abstraction

followed by substituent exchange (Figure 1-7, d).155 More recently, the Oestreich group has

13

developed a method to prepare silylium cations by hydride abstraction at a cyclohexadienyl unit

at a silicon centre (Figure 1-7, e).156 The generated Wheland complex can be thought of as an

arene-stabilized silylium cation, similar to those mentioned above.

Silicon cations are of continuing interest in main group and organic chemistry but its high

electrophilicity renders difficult the formation of a free tricoordinate silicon cation. In spite of

[Et3Si(tol)][B(C6F5)4] being a adduct stabilized silylium, it was found to be very active catalyst for

a number of organic transformations. In 2005, Ozerov reported the hydrodefluorination of

fluoroalkanes (Figure 1-8, a).166 Trivalent silicon cations, even in the presence of mild donors like

arenes, are found to be very active for hydrosilylation of alkenes and ketones, and Diels Alder

reactions for unreactive dienes (Figure 1-8).149,165,167-170 More recently, Müller has reported that

silylium cations are acceptable Lewis acids for heterolytic activation of H2 gas with phosphines in

an FLP manner.155,171,172 The limitation with silylium/phosphorus FLP systems is the Si–H bond

is not hydridic enough to reversibly activate H2 under ambient conditions.

Figure 1-8 Examples of Organic Transformations Facilitated by Silicon Catalysts

1.2.2.5 Phosphorus-Based Lewis Acids

Perhaps the most well-known example of the use of phosphorus(V) reagents in organic chemistry

is in the Wittig reactions,173 where the electrophilicity at the phosphorus ylide is exploited for the

conversion of carbonyls to olefins. Pentacoordinate phosphoranes were recognized as Lewis acids

in the 1960s and the coordination chemistry of these compounds was widely investigated.174,175

For example, Schmutzler et al. reported the reaction of PF5 with N-trimethylsilylimidazole.176 It

14

was determined that the phosphorus centre was coordinated to the Lewis basic nitrogen in the

heterocycle. Upon heating of this adduct, trimethylsilylfluoride was lost and yielded the

(tetrafluoro)phosphorus substituted imidazole. In 1977, Cavell reported the first example of CO2

insertion into a pentacoordinate amidophosphorane.177 Similar reactivity was reported by a

previous graduate student from the Stephan group, Dr. Chris Caputo, where CO2 was irreversibly

sequestered between a ring-strained amidofluorophosphorane.178 This reaction was interpreted as

FLP activation of CO2 between a nitrogen centre and a fluorophosphonium cation. In related work,

Gabbaï explored the enhanced Lewis acidity of systems with two adjacent Lewis acids.179 This

was revealed when an ortho-substituted phosphonium to a borane showed higher fluoride ion

affinity as compared to the para- derivative (Figure 1-9).

Figure 1-9 Gabbaï’s Fluoride Ion Sensors

Mukaiyama explored phosphonium salts as Lewis acid catalysts in the 1980s, and these simple

tetraalkylphosphonium salts were found to be active for aldol-type reactions of aldehydes or

acetals with nucleophiles, as well as Michael reactions of ɑ,β-unsaturated ketones / acetals.180 An

important compound developed in this group was the [tBu3P–O–PtBu3][OTf]2 dication,181 which

was found to be a stronger Lewis acid when compared to TiCl4 and SnCl4; it could effect the

synthesis of β-aminoesters from the corresponding aniline and ketene silyl acetal. Following this

work, in 2006, Terada and Kouchi investigated the ability of phosphonium salts to catalyze Diels-

Alder reactions (Figure 1-10).182 The salts were prepared from phosphine oxides or phosphinates

and trifluoromethanesulfonic anhydride. Of the salts studied, it was found that a five-membered

ring of a catechol substituent enhanced reactivity. Since this work, phosphonium salts like

methyltriphenylphosphonium iodide and benzyltriphenylphosphonium chloride have been

successfully employed to effect the silylcyanation of aldehydes and ketones using TMSCN.183,184

Related cations are active catalysts for the N,N-dimethylation of primary amines,185

cyclotrimerization of aldehydes,186 and for the protection/deprotection of alcohols with alkyl vinyl

ethers.187,188

15

Figure 1-10 Examples of Phosphonium Salts Tested in Diels-Alder Catalysis

The Lewis acidity of pentavalent tetracoordinate phosphorus species is derived from the σ* orbital

oriented opposite the most electron-withdrawing substituent.189 Exploiting this principle, highly

electrophilic phosphonium cations (EPCs) were prepared by previous graduate student Dr. Chris

Caputo and tested in Lewis acid catalysis.190 These species could be readily accessed by oxidation

of commercially available phosphines using XeF2 then subsequent fluoride abstraction. The most

electrophilic of the prepared EPCs was [(C6F5)3PF][B(C6F5)4], which was active in the

hydrodefluorination of fluoroalkanes. Although preliminary FLP chemistry for these phosphonium

cations was investigated, they were found to have limited stability in the presence of external

bases.191-193

An innovative phosphorus catalyst designed by Radosevich et al. was reported in 2012. This

system was the first to employ redox active P(III)/P(V) catalytic cycles to effect transfer

hydrogenation of azobenzene (Figure 1-11).194 The phosphorus centre possessed an O-N-O

tridentate ligand that imposed a distorted geometry and allowed for cycling of the oxidation states.

16

Figure 1-11 Catalytic Transfer Hydrogenation of Diazobenzene

1.2.3 Frustrated Lewis Pair Chemistry and Small Molecule Activation

In 2006, the Stephan group noted that the sterically encumbered system Mes2P(C6F4)B(C6F5)2

could reversibly activate hydrogen gas to form the Mes2P(H)(C6F4)B(H)(C6F5)2 zwitterion.195 This

was the first example of reversible H2 activation by a metal-free system. Based on this reactivity,

the term “frustrated Lewis pairs” (FLPs) was coined. It was proposed that Lewis adduct formation

was precluded due to the steric demands on the Lewis acid and base, and that this unquenched

reactivity could be exploited for the activation of small molecules. This definition is being

broadened today, where some Lewis acid/base systems that form a reversible adduct have been

reported to show active FLP chemistry.196 Hydrogen activation is not limited to phosphorus/borane

base systems, but has been expanded to encompass both carbenes and nitrogen-based Lewis

bases.197-203 One interesting example of subsequent reactivity observed by a previous graduate

student, Dr. Steve Geier, was activation of H2 using N-heterocyclic base and

tris(pentafluorophenyl)borane (BCF). Under harsh conditions, reduction of the aromatic system

could be achieved (Figure 1-12).202 This reduction was reported in substoichiometric amounts, as

the first metal-free catalytic reduction of N-heterocycles. Following this work, previous graduate

student Dr. Tayseer Mahdi reported the use of anilines in FLP chemistry and in a similar fashion,

at elevated temperatures observed the reduction of the aromatic moiety.204

17

Figure 1-12 Hydrogenation of Aromatic Bonds

A number of FLP systems are active hydrogenation catalysts for unsaturated substrates.205 For

instance, the original linked FLP, Mes2P(C6F4)B(C6F5)2, can effect the hydrogenation of bulky

imines,206,207 whereas 1,8-bis(diphenylphosphino)-naphthalene and B(C6F5)3 can rapidly and

reversibly activate hydrogen and can be further employed in the catalytic reduction of silyl enol

ethers.208 More challenging substrates, like olefins, could also be reduced with substoichiometric

amounts of (C6F5)Ph2P and B(C6F5)3.209 However the mechanism for this reduction differs from

that of imine reduction, where the first step is delivery of the proton from the phosphine to generate

a carbocation to which hydride is subsequently delivered from the hydrioborate.210

Numerous substrates have been investigated with FLP systems. Perhaps the most noteworthy, is

the sequestration of CO2, given its effect on climate change. Both inter- and intramolecular FLP

systems have been reported to capture CO2, sometimes irreversibly.211-216 Seminal work in this

area was reported by Piers et al., where the catalytic reduction of CO2 via hydrosilylation could be

attained by employing B(C6F5)3 and TMP, yielding methane.217 tBu3P and B(C6F5)3 have been

reported to capture the greenhouse gas N2O.218 Upon heating of this system, the phosphine oxide

and free B(C6F5)3 could be observed. More recently, Stephan and Erker have investigated the

reactivity of FLP systems with carbon monoxide. It was found that HB(C6F5)2 could activate CO

in the presence of an in situ generated intramolecular FLP system to yield a side-on bound formyl

group (Figure 1-13; top).219,220 The intermolecular system of tBu3P and two equivalents of

B(C6F5)3 could react with syn gas to form an epoxyborate, which underwent subsequent reaction

with either hydrogen or carbon monoxide to give the mixture of products shown in the bottom

scheme of Figure 1-13.221

18

Figure 1-13 FLP Reactivity with Carbon Monoxide

The reactivity of FLP systems is a growing field, with stoichiometric activations including olefins,

carbonyls, NO, SO2, and R–NSO derivatives.28,222,223 FLP catalysis has grown to incorporate

ketone hydrogenation, hydroamination, cyclization of propargyl amides, and C–H bond

borylation. These systems have also been expanded to include aluminium, carbon, silicon,

phosphorus, zirconium, and hafnium based Lewis acids.

1.3 Dative Bonding in Main Group Compounds

Dative bonding in main group cations like phosphenium and borenium salts is an area of contention

in the inorganic community. In 2014, Himmel, Krossing, and Schnepf published an open letter

presenting the case for fewer dative bonds.224 The three examples contested in this report include

the long-known PPN cation [N(PPh3)2]+,225,226 the [P4]

2+ dication [P4(AsPh3)2]2+,227 and the

nitrogen trication [N(L)33+] (L = cyclo-C3(NMe2)2).

228 These authors reference Haaland when

discussing the characteristics of dative bonds as being weaker, longer, and exhibiting lower charge

transfer when compared to single bonds.229 Haaland claims that one way to differentiate between

dative bonds and single bonds is to calculate the gas phase cleavage of these bonds. For normal

bonds, both polar and non-polar, homolytic cleavage should be preferred, whereas for dative bonds

19

heterolytic cleave should be preferred. For instance, when discussing [H3C–NH3]+ the energies for

homolytic and heterolytic C–N bond cleavage are 466 and 439 kJ/mol, respectively. Suggesting

the favored bonding description to be [H3CNH3]+, an ammonia stabilized methyl cation.

However, this formulation is not consistent with observed reactivity, and it is rightfully an

ammonium cation. By way of this example, it is apparent that that observed reactivity of contested

salts is an important metric to consider when discussing bonding. In the case of the PPN cation,

the authors disagree with the [Ph3PN+PPh3] description because the charge at the nitrogen is

calculated to be –1.38 e. When discussing the reactivity of this compound, Knapp et al. reports the

addition of a fluoride anion to a phosphorus centre. Himmel, Krossing, and Schnepf advocate for

[Ph3P+–N––+PPh3] as a more accurate description of this compound. Similar to the PPN cation, the

partial charge calculations of the [P4(AsPh3)2]2+ dication show +0.12 e at the P4 unit and +1.88 e

at each AsPPh3 unit. However, experimentally [P4(AsPh3)2]2+ can undergo further reactivity with

PPh3 to yield [P4(PPh3)2]2+,227 suggesting at least partial dative bonding, for instance [Ph3AsP4

+–

+AsPh3]. Finally, the authors suggest that a better description for the “ligand-stabilized N3+”

prepared by Alcarazo is [L+3–N], as the NPA analysis reveal a charge of –0.45 e at the nitrogen.

The open letter from Himmel, Krossing, and Schnepf did not go unnoticed and a quick response

was published by Frenking presenting a case for more dative bonds.230 Frenking begins his

criticism of Himmel, Krossing, and Schnepf by discussing the difference between formal charge

from a Lewis structure and partial charge. For instance, when considering carbon monoxide, the

carbon centre has a formal charge of –1 but is also electrophilic.231 In accordance with its Lewis

structure, the nitrogen in NH4+ has a formal charge of +1 but the partial charge is negative. In the

case of BF4– and ammonia borane, each boron centre has a formal charge of –1 but the partial

charge is positive. In a similar fashion, the calculated charge of –1.38 e at the nitrogen for the PNN

cation is the partial charge and cannot be used to dismiss the [Ph3PN+PPh3] description. In

fact, the partial negative charge can be predicted from the strong attraction between N+ and the

phosphine. In addition, when discussing the related species [HB(PPh3)2], the lone pair of the

coordinated phosphine occupies the p(π) orbital at the boron, resulting in a negative partial charge

of –0.97 e and a short and strong B–P bond. Frenking also points out that the criteria published

by Haaland discussing the differences between dative bonding and electron-sharing was limited

by the donor–acceptor complexes known in the 1980s, largely restricted to classical Group 13/15

adducts. Frenking claims that the advances made by his group in 2006 with the development of

20

carbodiphosphorane232 and related [E(L2)]+ species has extended the knowledge about dative

bonding since the publication of Haaland’s work. Showing that dative bonds can be strong and

short, and can accompany a significant amount of charge transfer.

As this continues to be an area of active discussion, Frenking reminds the reader that bonding

models are not right or wrong but they are more or less useful.233 The usefulness of an acceptor–

donor model vs the more classical Lewis structure ultimately lies in the reactivity that is observed.

In particular, the discussion of “carbene–stabilized phosphenium cation” (Figure 1-14; a) vs

“imidazolium-substituted phosphine” (Figure 1-14; b) is relevant to this thesis.

Figure 1-14 Bonding Descriptions of [LPPh2]+

1.4 Scope of Thesis

At the time of my matriculation, the initial chemistry of the highly electrophilic phosphonium

cation [(C6F5)3PF][B(C6F5)4] had been established by Dr. Chris Caputo and further work was

underway to investigate the Lewis acid catalysis of fluorophosphonium cations. The objective of

this graduate work was to expand the family of fluorophosphonium cations by generating

polycationic Lewis acids and to subsequently investigate their catalytic activity.

Chapter 2 focuses on the development of a synthetic route to generate polycationic Lewis acids,

which were ultimately prepared via the oxidation of phosphenium cations to generate

phosphonium dications. Variations at the phosphorus-centre were investigated, as well as changes

to the donor ligand to include traditional carbenes, triazole-based donors, IBox-chiral carbenes,

and one example of a cyclic alkyl(amino) carbene (cAAC) ligand. The parent dication was found

to effect the hydrodefluorination of fluoroalkanes and was selected for further investigation in

Lewis acid catalysis.

Following from the insight garnered in Chapter 2, Chapter 3 explores catalytic hydrosilylation

employing fluorophosphonium cations. These cations were found to be active for the

21

hydrosilylation of olefins and ketones. It was discovered that in the presence of an excess silane,

catalytic deoxygenation of ketones could be attained. In a similar fashion, amides could be

deoxygenated to generate amines.

Building on the oxophilicity observed in Chapter 2, and the propensity of catalytic hydrosilylation,

Chapter 4 investigates the use of these electrophilic phosphonium cations for the catalytic

reduction of phosphine oxides to phosphines. This reaction showed surprisingly high

chemoselectively. Further investigations with phosphites led to complicated results, but catalytic

elimination of –OR (R = alkyl, aryl) from the phosphite could be observed. This allowed for

catalytic generation of PH3, as well as primary and secondary phosphines.

Finally, in an effort to further enhance the Lewis acidity, the synthesis of tricationic and

tetracationic phosphorus Lewis acids was attempted using a number of different frameworks. The

details of these investigations are discussed in Chapter 5.

The synthesis of the parent fluorophosphonium dications was prepared in collaboration with

postdoctoral fellow Dr. Michael Holthausen. The work presented in Chapter 3 was conducted in

collaboration with postdoctoral fellow Dr. Michael Holthausen, Ph.D. student Dr. Ian Mallov and

MSc graduate Ms. Alessandra Augurusa, along with insightful discussions from Dr. Manuel Pérez.

Calculations for Chapter 3 were performed by Dr. Stefan Grimme and his student. Finally, the

work done in Chapter 4 was performed in collaboration with Dr. Isaac Garcia and with insightful

discussions from Dr. Manuel Pérez.

Portions of each chapter have been published or drafted as stated below:

Chapter 2: Holthausen, M. H.; Mehta, M.; Stephan, D. W. Angew. Chem. Int. Ed., 2014, 53,

6538–6541. Mehta, M.; Johnstone, T. C.; Lam, J.; Bagh, B.; Hermannsdorfer, A.; Driess, M.;

Stephan, D. W. in prep.

Chapter 3: Holthausen, M. H.; Mehta, M.; Stephan, D. W. Angew. Chem. Int. Ed., 2014, 53,

6538–6541. Mehta, M.; Holthausen, M. H.; Mallov, I.; Pérez, M.; Qu, Z.-Q.; Grimme, S.;

Stephan, D. W. Angew. Chem. Int. Ed., 2015, 54, 8250–8254. Mehta, M.; Augurusa, A.; Pérez,

M.; Zhu, J.; Stephan, D. W. Chem. Commun., 2016, 52, 12195-12198.

22

Chapter 4: Mehta, M.; Garcia, I.; Pérez, M.; Porwal, D.; Oestreich, M.; Stephan, D. W.

Organometallics, 2016, 35, 1030 – 1035. Phosphite chemistry is unpublished.

Chapter 5: unpublished.

23

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(232) Tonner, R.; Öxler, F.; Neumüller, B.; Petz, W.; Frenking, G. Angewandte Chemie


(233) Primas, H.; Müller-Herold, U. Elementare Quantenchemie; Vieweg+Teubner

Verlag, 2013.

36

Chapter 2 Preparation of Lewis Acidic Phosphorus Cations

2.1 Introduction

2.1.1 Phosphenium Cations

Historically, phosphorus compounds have been widely exploited as Lewis base donor

ligands in transition metal and organometallic chemistry. Such ancillary roles have been

critical to a number of landmark advances in catalysis.1 While studied to a lesser extent,

phosphorus based compounds have also been shown to exhibit Lewis acidity.2-4 This work

has been focused on P(III) tri-coordinate phosphenium ions and tetra-coordinate P(V)-

phosphonium cations. Phosphenium cations are investigated as a route to highly

electrophilic phosphonium dications.

In 1964, Hoffmann reported the first synthesis of a phosphenium cation.5 Continued

interest in such species has resulted in several reviews over the years.4,6-8 Early reactivity

of these compounds included C–H insertion reactions with Cp2Sn, and reactions with 1,3-

and 1,4-dienes to yield the corresponding cyclopentene-phosphenium derivatives.6,9 In

2000, Nieger et al. prepared N-heterocyclic phosphenium (NHP) cations [(C2H2NR)2P]+,

which are isovalent with N-heterocyclic carbenes (NHC).10 Following this work, Ragogna

et al. described the binding of donors to the phosphorus centre of an NHP in the presence

of a pendant intramolecular donor.11 Most recently, Power and Burford examined the

phosphinophosphenium cation [Ph3PPPh2][OTf].2 NHC and pyridine have also been

reported as competent bases to trap phosphorus cations.12-14 A 2012 computational study

by Slattery and Hussein investigated the Lewis acidity of phosphenium cations by

computing their fluoride ion affinities (FIAs).15 There are, however, limited examples of

Lewis acid catalysts employing phosphenium cations.

Investigations into the electrophilicity of phosphorus(V) cations revealed an accessible low-lying

σ* orbital.16 This Lewis acidity has been exploited to effect the catalysis of Diels-Alder reactions

of α,β-unsaturated amides,17 the Mukaiyama-aldol reaction,18 addition of silyl nucleophiles to

acetals,18 the Henry reaction,18 cyclotrimerisation of aldehydes,19 and the cyanosilylation of

aldehydes20 and ketones.21 One of the most well-known stoichiometric examples is the Wittig

reaction, where addition of P-based ylides to ketones is facilitated by the electrophilicity of the

37

phosphorus centre.22 In an analytical application, Gabbaï et al. have used such electrophilic

phosphorus species to develop fluoride ion sensors.23

A previous graduate student in the Stephan research group, Dr. Christopher B. Caputo, reported

the preparation and Lewis acidity of the fluorophosphonium cations [(C6F5)2PhPF][B(C6F5)4] (2-

1) and [(C6F5)3PF][B(C6F5)4] (2-2) and the application of these species in the catalytic

hydrodefluorination of fluoroalkanes, the isomerization of terminal olefins and the hydrosilylation

of alkenes and alkynes.24,25 Further diversifying these electrophilic phosphonium cations (EPCs)

and establishing new Lewis acid catalysis was the stimulus for this work.

2.2 Results and Discussion

2.2.1 Synthesis of Phosphonium Dication

In 2014, Alcarazo reported the facile synthesis of cyclopropenium-substituted phosphines and

investigated their ability to act as a donor on Rh, Pt and Pd metal centres.26 The availability of this

lone pair on phosphorus for coordination to late transition metals inspired us to attempt the

oxidization of ligand-supported phosphenium cations with XeF2. Oxidation of an NHC- stabilized

phosphenium cation provides a cationic difluorophosphorane, while subsequent fluoride

abstraction affords a phosphonium dication salt.27-30 This new strategy precludes the need for

fluoroarene substituents on the phosphorus centre as a means for attaining high Lewis acidity.

The reaction of Ph2PCl, the saturated NHC 1,3-dimesitylimidazolidin-2-ylidene (SIMes) and

[Et3Si(tol)][B(C6F5)4] afforded the cationic phosphenium salt [(SIMes)PPh2][B(C6F5)4] (2-3) in

almost quantitative yield (Scheme 2-1). The 31P NMR spectrum of 2-3 in CD2Cl2 shows a singlet

resonance at −1.5 ppm, consistent with chemical shifts observed for related carbene-phosphenium

salts.31 The molecular structure of 2-3 was confirmed crystallographically (Figure 2-1), revealing

a P−Ccarbene bond length of 1.861(4) Å. This bond length is longer than the P−Ccarbene bond length

reported by Chauvin (1.834(1) Å), however this system employs a carbene with an unsaturated

backbond.32 Alternatively, the Ccarbene–N bond lengths (1.333(5) Å and 1.340(5) Å) are consistent

with those observed by Chauvin. The remaining metric parameters were unexceptional.

38

Scheme 2-1 Preparation of 2-3

Figure 2-1 POV-ray Depiction of 2-3. C: black, N: blue, P: orange. Hydrogen atoms and

anion have been omitted for clarity.

The phosphenium salt 2-3 is cleanly oxidized with XeF2 to the corresponding cationic

difluorophosphorane [(SIMes)PF2Ph2][B(C6F5)4] 2-4 which was isolated in 81% yield (Scheme 2-

2). This species gives rise to a triplet in the 31P{1H} NMR spectrum at −62.9 ppm with a 1JPF

coupling constant of 733 Hz, consistent with the presence of two equivalent fluorine atoms (Figure

2-2). This chemical shift is downfield relative to that observed for the neutral phosphorane

(C6F5)3PF2 at −48.0 ppm (1JPF = 694 Hz).25

The molecular structure of 2-4 shows a distorted trigonal bipyramidal arrangement at the P atom

(Figure 2-3). The fluoro-substituents occupy the axial positions with a F−P−F angle of 168.8(2)°

while the carbene and the phenyl-substituents occupy equatorial positions. While carbenes have

39

been exploited to stabilize low valent phosphorus species,33,34 compound 2-4 is the first cationic

halophosphorane derivative to be structurally characterized.35 When less than 1 equivalent of the

silylium cation is employed during the preparation of 2-3, upon oxidation a minor impurity in the

31P NMR spectrum could be observed as a doublet at −59.0 ppm with a coupling constant of 714

Hz. This species was later characterized as the mixed halophosphorane

[(SIMes)PFClPh2][B(C6F5)4] (Figure 2-2).


40

Figure 2-2 31P NMR and 19F NMR Spectra for 2-4


Hydrogen atoms and anion have been omitted for clarity.

41

One of the fluoro-substituents on the 2-4 cation is removed upon treatment with

[Et3Si(tol)][B(C6F5)4], giving rise to the dication [(SIMes)PFPh2][B(C6F5)4]2 2-5 and the expected

by-product Et3SiF. While Burford and co-workers have recently described dicationic Sb and Bi

species,36 2-5 is a rare example of an isolable dicationic phosphonium salt (Scheme 2-2).37,38

Compound 2-5 exhibits a dramatic downfield shift in the 31P NMR spectrum to 78.1 ppm with a

P−F coupling constant of 1040 Hz (Figure 2-4). The corresponding 19F NMR resonance is

observed at −131.7 ppm. Interestingly, the 31P and 19F NMR chemical shifts are reminiscent of

those observed for the very Lewis acidic phosphonium ions 2-1 (δ(31P) = 78 ppm,

δ(19F) = −122 ppm) and 2-2 (δ(31P) = 68 ppm, δ(19F) = −121 ppm). The formulation of 2-5 was

further confirmed crystallographically (Figure 2-5). The P−F bond length (1.532(2) Å) is

substantially shorter than those observed in phosphorane 2-4 (1.628(4) and 1.660(4) Å). Similarly,

the P−C bonds to the phenyl-substituents are shortened to 1.753(3) and 1.756(3) Å in 2-5 in

comparison to those in 2-3 and 2-4, which range from 1.799(6) Å to 1.866(5) Å).


42


Hydrogen atoms and anions have been omitted for clarity.

2.2.2 Lewis Acidity and Fluorophilicity Tests

The Gutmann-Beckett method was employed to probe the Lewis acidity of 2-5.39 Compound 2-5

was combined with Et3PO in CD2Cl2. 31P NMR spectroscopy revealed the formation of a mixture

containing fluorophosphonium cation 2-6 and phosphine oxide 2-7 (Scheme 2-3). This was

evidenced by the 31P NMR spectrum, which reveals a doublet resonance at low field

(δ(31P) = 147.2 ppm) with a typical 1JPF coupling constant of 989 Hz, attributed to 2-6. A singlet

resonance observed at 14.0 ppm was attributed to 2-7.35 These data indicate simple coordination

of Et3PO to the phosphonium P atom of 2-5 does not occur, but rather an oxide-fluoride exchange

reaction takes place, affording [Et3PF][B(C6F5)4] 2-6 and [(SIMes)POPh2][B(C6F5)4] 2-7. The salts

2-6 or 2-7 could not be separated from the reaction mixture, however, these species were

independently synthesized and fully characterized, see section 2.4 for details. The formulation of

2-7 was further confirmed by X-ray crystallography, rendering it the first structurally characterized

cationic phosphine oxide derivative featuring an imidazolium-based substituent (Figure 2-6).35

Similar to 2-5, the cation of 2-7 shows a distorted tetrahedral environment at the P atom. However,

2-7 exhibits longer P−C bond distances (P−Ccarbene 1.881(2) Å, av. P−Caryl 1.797(2) Å) consistent

with the formal mono-cationic charge. This bond is longer than the P−Ccarbene observed for 2-3,

43

however the P=O bond length (1.480(2) Å) is consistent with the P=O bond length observed for

Ph3PO.32

Scheme 2-3 Reaction of 2-5 with OPEt3

Figure 2-6 POV-ray Depiction of 2-7. C: black, N: blue, P: orange, O: red. Hydrogen

atoms and anion have been omitted for clarity.

Efforts to apply the Childs protocol to assess the Lewis acidity involved combination of 2-5 with

excess crotonaldehyde at −20 °C in CD2Cl2.40 The 31P NMR spectrum revealed a mixture of 2-7

and 2-4 and an inseparable mixture of organic products in the 1H NMR spectrum (Figure 2-7).

While these Lewis acidity tests do not permit comparison to other Lewis acids, the observed

reactivity suggests that the dication 2-5 is highly electrophilic. This notion is supported by the fact

that exposure of 1 mg of 2-5 to d8-THF leads to polymerization, as gel permeation chromatography

44

(GPC) after 2h reveals a molecular weight of the polymer of 8.6 x 104 Da with a polydispersity of

2.6.

Figure 2-7 31P and 19F NMR Spectral Data for the Reaction of 2-5 with Crotonaldehyde

at –20 °C in CD2Cl2

Similar evidence of Lewis acidity is observed during the addition of 2 mol% of 2-5 to a solution

of 1,1-diphenylethylene in CD2Cl2. This effects complete Friedel-Crafts dimerization of the olefin

in less than 30 minutes, yielding the indene-product in 97% isolated yield (Figure 2-8).25,41

45

Figure 2-8 1H NMR Spectrum from the Catalytic Friedel-Crafts Dimerization of 1,1-

diphenylethylene using 2-5

In order to gauge the relative fluorophilicity of this dication, 2-5 was reacted with phosphorane

(C6F5)3PF2 in CD2Cl2. This led to the clean and complete conversion to fluorophosphonium salt 2-

2 and difluorophosphorane 2-4 as revealed by 31P and 19F NMR spectroscopy (Figure 2-9). This

indicates that 2-5 is more fluorophilic than 2-2.

46

Figure 2-9 31P and 19F NMR Spectral Data for the Reaction of 2-5 with F2P(C6F5)

The fluorophilicity of 2-5 was further demonstrated in the 1:1 reaction of 2-5 with Ph3CF. This led

to the conversion of 2-5 to 2-4 and trityl cation as evidenced by 31P, 19F and 13C NMR spectra

(Figure 2-10).

47

Figure 2-10 31P NMR Spectrum for the Reaction of 2-5 with Ph3CF

2.2.3 Catalytic Hydrodefluorination

This evidence of Lewis acidity and fluorophilicity suggested that 2-5 could act as an effective

Lewis acid catalyst. This prompted an investigation of 2-5 as a catalyst for hydrodefluorination

reactions. A series of fluoroalkanes was combined with Et3SiH in the presence of 2-5 as a catalyst

(Table 2-1).

At ambient temperature, almost complete conversion of 1-fluoroadamantane, 1-fluoroheptane and

1-fluorocyclohexane was observed within one hour. The consumption of substrate and growth of

the resonance attributable to Et3SiF was evidenced by the 19F NMR spectra of the reaction

mixtures. Trifluorotoluene (Table 2-1; entry 4) is consumed within 24 h, while higher catalyst

loadings are required (10 mol%) to fully convert 1,4-C6H4(CHF2)2. In the case of perfluorotoluene,

incomplete reaction of the aliphatic C−F bonds is achieved after 24 h. The aryl C−F bonds remain

intact.

48

Table 2-1 Catalytic Hydrodefluorination of Fluoroalkanes Using Catalyst 2-5

Entry Substrate Cat.

(mol%)[a]

Time (h) % Si-F[b][c] % C-F[b][d] TON

1

1 1 >99 100 100

2

1 1 92 99 100

3 5 1 >99 100 20

4

1 24 53 88 88

5

5

10

24

24

31

66

44

72

8.8

7.2

6

5

10

24

24

14

29

20

28

4.0

2.8

[a] Relative to fluoroalkane substrate, [b] conversions were determined by 19F NMR

spectroscopy using fluorobenzene as an internal standard, [c] calculated from the proportion of

C−F bonds originally present relative to Si−F bonds formed, [d] calculated from the proportion

of C−F bonds consumed after time t (h).

Mechanistically, these hydrodefluorination reactions are thought to proceed via a mechanism

analogous to those described for other strong, main-group Lewis acids, including B(C6F5),42,43 2-

2,25 silylium44 and carbenium ions45 and aluminium species.46 This would involve an abstraction

49

of fluoride by 2-5 from the fluoroalkane to give 2-4 and a carbocation. The latter intermediate

reacts with silane to give alkane and generates silylium cation, which abstract fluorine from 2-4 to

regenerate 2-5 (Figure 2-11). This mechanism is supported by the observation that 2-5 does not

react with Et3SiH even after several days, precluding the involvement of a silylium ion as a

hydrodefluorination catalyst.25

Figure 2-11 Hydrodefluorination Mechanism

2.2.4 Exploring Phosphonium Cation Derivatives

The phosphenium cation route to EPCs is particularly attractive in that it allows the extensive

library of previously explored carbene chemistry to be exploited. The potential generality of this

synthetic route was probed. The synthetic protocols to access a variety of phosphenium cationic

precursors in which the carbene donors and the phosphorus bound substituents are varied was

developed. Oxidation of these species with XeF2 was probed and the potential of the resulting

difluorophosphoranes as precursors to dicationic fluorophosphonium salts was considered.

In section 2.2.1 we described the synthesis of the carbene-phosphenium cation adduct

[(SIMes)PPh2][B(C6F5)4] using SIMes, diphenylchlorophosphine, and [Et3Si(tol)][B(C6F5)4].47 In

seeking to make a series of analogs of 2-3, the reaction was performed with a range of

chlorophosphines and carbenes. Initial success came from the reaction of SIMes with

dimethylchlorophosphine and [Et3Si(tol)][B(C6F5)4], which yielded the phosphenium salt

50

[(SIMes)PMe2][B(C6F5)4] 2-8 (Scheme 2-4), which exhibited a 31P NMR shift at −22.7 ppm.

Subsequent oxidation with XeF2 resulted in a triplet in the 31P NMR spectrum with a shift of −17.6

ppm and a coupling constant of 644 Hz. Consistent with this assignment was the observation of a

doublet in the 19F NMR spectrum at −8.9 ppm with the same coupling constant. The formulation

[(SIMes)PF2Me2][B(C6F5)4] (2-9) was confirmed by X-ray crystallography (Figure 2-12).

Successive fluoride ion abstraction with [Et3Si(tol)][B(C6F5)4] yielded phosphonium cation 2-10

in good yield (Scheme 2-4).




Analogous to the preparation of 2-3 and 2-8, diethylchlorophosphine was reacted with

SIMes in the presence of [Et3Si(tol)][B(C6F5)4] to give [(SIMes)PEt2][B(C6F5)4] (2-11) in

51

quantitative yield. [(SIMes)PEt2][B(C6F5)4] (2-11) appears as a singlet in 31P NMR at 0.7

ppm, which could be further oxidized with XeF2 to [(SIMes)PF2Et2][B(C6F5)4] (2-12,

Scheme 2-5), appearing as a triplet in the 31P NMR spectrum at −17.5 ppm with a coupling

constant of 685 Hz. This difluorophosphorane was characterized by X-ray crystallography.

(Figure 2-13) In line with previously reported cationic difluorophosphoranes 2-4 and 2-9,

2-12 shows a doublet in the 19F NMR at −32.5 ppm. The formulation for 2-12 was further

confirmed by X-ray crystallography. Upon abstraction of one fluoride ion from 2-12 using

[Et3Si(tol)][B(C6F5)4], 2-13 could be generated in 80% yield (Scheme 2-5).

[(SIMes)PFEt2][B(C6F5)4] appears as a doublet in the 31P NMR spectrum at 126.6 ppm with

a coupling constant of 1056 Hz and as a doublet in the 19F NMR spectrum at −152.9 ppm.




52

Related compounds [(SIMes)PR2][B(C6F5)4] (R = iPr2, tBu/Ph, o-tol, p-C6H4F, p-C6H4CF3)

and the corresponding difluorophosphoranes, [(SIMes)PF2R2][B(C6F5)4] (R = tBu/Ph, p-

C6H4F) were prepared by post-doctoral fellow Dr. Michael Holthausen and are outside the

scope of this thesis. Likewise, dichlorophosphorane [(SIMes)PCl2Ph2][B(C6F5)4] and the

chlorophosphonium dication [(SIMes)PClPh2][B(C6F5)4]2 were prepared by Ian Mallov,

and are also outside the scope of this thesis.

The carbene can also be modified to include chirality by using IBox-iPr2, shown in scheme

2-6. Reaction of the imidazolium salt [(IBox-iPr2H)][OTf] with KH in the presence of a

catalytic amount of KOtBu and subsequent addition of Ph2PCl afforded [(IBox-

iPr2)PPh2][OTf] 2-14 in 61% isolated yield (Scheme 2-6). The 1H, 19F, and 13C NMR

spectral data were consistent with the formulation and the 31P NMR resonance was

observed at −21.8 ppm. X-ray diffraction data further supported the formulation and

revealed the P−Ccarbene bond distance to be 1.826(2) Å (Figure 2-14), significantly shorter

than that observed for 2-3 and Chauvin’s system.32


53

Figure 2-14 POV-ray Depiction of 2-14. C: black, N: blue, P: orange, O: red. Hydrogen

atoms and anion have been omitted for clarity.

Chiral phosphenium cation 2-14 could be oxidized to the corresponding phosphorane 2-15 at −40

°C with XeF2, however a sample pure enough to attain a successful elemental analysis could not

be obtained. Successful fluoride ion abstraction could be attained with Mg(OTf)2 (Scheme 2-7)

and the chiral phosphonium 2-16 could be observed as a doublet in the 31P NMR spectrum at 40.2

ppm with a coupling constant of 1016.7 Hz and 19F NMR at −75.4 ppm (Figure 2-15). However

the resulting species was found to be too reactive and readily decomposed. [(IBox-iPr2H)]+ could

be observed in the 1H NMR spectrum upon decomposition.

Scheme 2-7 Attempted Preparation of 2-16

54

Figure 2-15 31P NMR and 19F NMR Spectra from Attempted Synthesis of 2-16

In a second effort to generate a chiral phosphonium dication, 2-17 was prepared from the in situ

deprotonation of [(IBox-iPrMe2H)][OTf] with potassium hydride and a catalytic amount of

potassium t-butoxide and subsequent addition of diphenylchlorophosphine. The chiral

phosphenium cation 2-17 was isolated as the triflate salt in a 73% yield and could be observed as

a singlet in the 31P NMR at −24.2 ppm.


55

Although attempts to oxidize 2-17 with XeF2 led to a mixture of products, 2-17 could be oxidized

over several days with an excess of pyridinium triflate. (Scheme 2-9) Although the mechanism for

this oxidation is not well understood, 2-18 could be observed in the 31P NMR spectrum as a triplet

at −61.7 ppm with a coupling constant of 700 Hz and in the 19F NMR spectrum at −39.0 ppm, after

one day (Figure 2-16). After five days at room temperature in acetonitrile (MeCN), only chiral

phosphonium dication 2-19 could be observed in the NMR (Figure 2-17). Although minor

impurities could be observed in the 1H and 13C NMR spectra, preliminary tests proved 2-19 to be

completely inactive for hydrosilylation catalysis. This inactivity with phosphonium dications has

previously been reported to be a consequence of the triflate counterion.48 Attempts to exchange

the [OTf]– counterion with K[B(C6F5)4], Na[B(C6F5)4], Li[B(C6F5)4], and [Et3Si(tol)][B(C6F5)4]

proved to be unsuccessful.


56

Figure 2-16 31P NMR and 19F NMR Spectra Showing Intermediate 2-18

57


Attempts to include triazole-based carbenes into phosphenium cations were less

straightforward, because the free carbenes could not be generated. Instead, a

transmetallation approach was adopted. The known triazolium-silver adduct

[(TripCH2N2(NMe)C2Ph)2Ag][AgCl2]49 was allowed to react with Ph2PCl. Subsequent

work up afforded a new species 2-20 in 97% yield (Scheme 2-10). The 1H and 13C NMR

spectra were consistent with the incorporation of both reagents into the product and the 31P

NMR spectrum exhibited a resonance at −25.1 in solution and −23.6 ppm in the solid state.

This suggests that the triazole-phosphenium cation was coordinated to silver in both the

solution and solid state.

Crystallographic data confirmed the formulation of 2-20 as a dimeric species with the

phosphenium centre bearing the carbene and two phenyl groups, but also coordinated to

silver atom. Both dimers feature a Ag2Cl2 rhomb with each silver further bound to one

58

phosphenium ligand (Figure 2-18). Significant distortion of AgCl2 units from linearity was

observed, with an Cl−Ag−Cl angle of 158.60(5)°. The P−C distance from the triazolium

carbon to the phosphorus was 1.825(4)/1.831(4) Å, shorter then that observed for 2-3 but

consistent with Chauvin’s system.32 The geometry about the phosphorus centre is pseudo-

tetrahedral with a Ag−P distance of 2.430(1)/2.435(1) Å.


Figure 2-18 POV-ray Depiction of the dicationic dimeric core of 2-20. C: black, N: blue,

P: orange, Ag: purple, Cl: green. Hydrogen atoms and anions have been omitted for

clarity.

Compound 2-20 could be oxidized with XeF2 at −78 °C in toluene to yield

[PhN2(NMe)C(CH2Tripp)CPF2Ph2][AgCl2] (2-21) in 64% yield (Scheme 2-11). 2-21 has a 31P

NMR triplet resonance at −65.0 ppm with a coupling constant of 696 Hz, and a 19F NMR doublet

59

resonance at −37.5 ppm. The formulation of the product with a pseudo-trigonal bipyramidal

geometry at phosphorus was further confirmed via X-ray crystallography (Figure 2-19). The P−F

distances were 1.644(2) and 1.663(2) Å and the P−C distance to the triazolium ligand was 1.813(4)

Å. Distances of 1.806(4) and 1.810(4) Å were observed to the two phenyl fragments. All attempts

to abstract a fluoride ion to generate the corresponding triazole-substituted phosphorus dication

proved to be unsuccessful.




60

A similar synthetic strategy was employed using the triazolium salt

[(PhCH2N2(NMe)C(CH2OMe)CH][OTf] 2-22 (Scheme 2-12). Reaction with Ag2O

afforded the silver salt 2-23, which was then allowed to react with Ph2PCl, affording 2-24

in a 91% isolated yield. NMR spectroscopic data were consistent with the formation of a

phosphenium cation, the 31P NMR resonance was at −27.2 ppm. Combustion analysis

supported the elemental composition of [PhCH2N2(NMe)C(CH2OMe)CPPh2][AgCl2] but

the structure of 2-24 could not be confirmed crystallographically.


Upon oxidation of 2-24 with XeF2 at −78 °C in DCM, a triplet in the 31P NMR spectrum at −61.1

ppm with a coupling constant of 685 Hz was observed, consistent with the formulation of 2-25

(Scheme 2-13, Figure 2-20). Subsequent fluoride ion abstraction with TMSOTf in toluene resulted

in observation of a doublet resonance in the 31P NMR spectrum at 81.6 ppm with a coupling

constant of 957 Hz, and a corresponding doublet resonance in the 19F NMR spectrum at −114.4

ppm, presumed to be 2-26 (Scheme 2-13, Figure 2-21). However, the product from this product

had limited stability and would readily decompose even at −40 °C.

61


Figure 2-20 31P NMR Spectrum for 2-25

62


In 2005, Bertrand et al. reported the first synthesis of a cyclic alkyl(amino) carbene

(cAAC), where one of the nitrogen atoms in a traditional NHC is replaced with a carbon

atom.50 This results in a lower HOMO-LUMO energy gap, making cAAC ligands stronger

σ-donors and better π-acceptors. This was experimentally established by the Bertrand group

by generating a number of carbene-phosphinidene adducts and studying the 31P NMR

resonances.51 In an effort to increase the stability and further expand the scope of carbene

substituents for these fluorophosphonium cations, 2-27 shown in scheme 2-14 was prepared

in accordance with the literature.50

By analogy with the synthetic procedures used for the NHC chemistry described

previously, reaction of a cAAC precursor salt [cAACH][BF4] with K[N(SiMe3)2] generated

the free cAAC, which was reacted in situ with Ph2PCl. Subsequent anion exchange with

63

excess TMSOTf allowed for the isolation of [(cAAC)PPh2][OTf] 2-28 in 89% yield

(Scheme 2-14). The 31P NMR spectrum of 2-28 showed a resonance at −0.3 ppm, and 1H

and 13C NMR data were as expected for this formulation. Additionally, the 19F NMR

spectrum showed the expected resonance at −78.9 ppm, corresponding to the triflate anion.

2-28 could be further oxidized to 2-29 with XeF2 in DCM (Scheme 2-14), appearing in the

31P NMR spectrum as a triplet at −54.7 with a coupling constant of 737 Hz. The

corresponding resonance in the 19F NMR spectrum appeared as a doublet at −48.1 ppm.

Subsequent efforts to abstract a fluoride ion with [Et3Si(tol)][B(C6F5)4] only showed 2-29

in the NMR, even at elevated temperatures. This limited reactivity may be the result of

increased steric bulk at the difluorophosphorane centre.


2.3 Conclusion

Cationic phosphenium salts are viable precursors to cationic difluorophosphoranes and thus to the

dicationic fluorophosphonium derivatives. The species 2-5 was shown to be highly Lewis acidic

in stoichiometric reactions with 2-2 and Ph3CF, and to act as a Lewis acid catalyst in

hydrodefluorination reactions. It was also capable of effecting the polymerization of THF and

Friedel-Crafts dimerization of 1,1-diphenylethylene. It is noteworthy that the previously reported

electrophilic boranes or fluorophosphonium monocations exploit highly fluorinated, strongly

electron-withdrawing substituents. In contrast, the phosphonium dication 2-5 achieves similar

levels of Lewis acidity and reactivity without highly electron withdrawing substituents. The

versatility of this synthetic route was investigated by altering the halophosphine to yield both 2-10

and 2-13 in high yields, although employing other carbene donors proved to less successful. Even

though a family of phosphenium cations could be accessed, further oxidation to the

difluorophosphorane and subsequent fluoride abstraction to the fluorophosphonium dication

64

proved challenging. Although, triazole and chiral ligand-based fluorophosphonium dications could

be observed by NMR spectroscopy, but these systems showed limited stability and could not be

investigated further. Nevertheless, further reactivity and Lewis acid catalysis of phosphonium salts

2-5, 2-10, and 2-13 is discussed in Chapter 3.

65

2.4 Experimental Details

General Remarks

All manipulations were performed in a MB Unilab glovebox produced by MBraun or using

standard Schlenk techniques52 under an inert atmosphere of anhydrous N2. Dry, oxygen-free

solvents (CH2Cl2, n-pentane, n-hexane, toluene) were prepared using an Innovative Technologies

solvent purification system. Fluorobenzene (C6H5F) was distilled from CaH2 and stored over

molecular sieves (4 Å) prior to use. Deuterated benzene (C6D6) and D8-THF were purchased from

Sigma-Aldrich, distilled from sodium and stored over molecular sieves (4 Å) for at least two days

prior to use. Deuterated dichloromethane (CD2Cl2) and bromobenzene (C6D5Br) were purchased

from Sigma-Aldrich, distilled from CaH2 and stored over molecular sieves (4 Å) for at least two

days prior to use. Reagents such as R2PCl, Et3PO, Et3SiH, crotonaldehyde, 1-fluoroadamantane,

1-fluoroheptane, fluorocyclohexane, trifluorotoluene, 1,4-bis(difluoromethyl)benzene,

perfluorotoluene, 1,2-diphenylethyne, N-fluorobenzenesulfonimide, trimethylsilyl triflate, 1-

fluoropyridinium triflate, magnesium triflate, silver oxide, potassium hydride, potassium

bis(trimethylsilyl)amide and potassium t-butoxide were purchased either from Sigma-Aldrich,

Strem Chemicals or Alfa Aesar and, if applicable, distilled prior to use. XeF2 was purchased from

Apollo Scientific and used without further purification. Reagents such as

[Et3Si][B(C6F5)4]*2(C7H8),53 1,3-dimesityl-4,5-dihydroimidazol-3-ium-2-ylidene,54 (C6F5)3PF2,

25

Ph3CF,25 triazole (PhCH2N3C(CH2OMe))CH55, and triazolium silver salt

[(TripCH2N2(NMe)C2Ph)2Ag][AgCl2]56 were prepared according to literature known procedures.

Where imidazolium salts 5-(diphenylphosphanyl)-3,7-diisopropyl-2,3,7,8-tetrahydroimid-

azo[4,3-b:5,1-b']bis(oxazole)-4-ium trifluoromethanesulfonate [IBox-iPr2H][OTf], and 5-

(diphenylphosphanyl)-3-diisopropyl-7-dimethyl-2,3,7,8-tetrahydroimid-azo[4,3-b:5,1-

b']bis(oxazole)-4-ium trifluoromethanesulfonate [IBox-iPrMe2H][OTf] were prepared by other

students in the group. All glassware was oven-dried at temperatures above 180°C prior to use.

NMR spectra were obtained on an Agilent DD2-700 MHz, an Agilent DD2-500 MHz, a Bruker

AvanceIII-400 MHz, or a Varian Mercury-300 MHz spectrometer. All 13C NMR spectra were

exclusively recorded with composite pulse decoupling. Assignments of the carbon atoms in the

13C spectra were performed via indirect deduction from the cross-peaks in 2D correlation

experiments (HMBC; HSQC). Chemical shifts were referenced to δTMS = 0.00 ppm (1H, 13C) and

δH3PO4(85%) = 0.00 ppm (31P, externally). Chemical shifts (δ) are reported in ppm, multiplicity is

66

reported as follows (s = singlet, d = doublet, t = triplet, quart. = quartet, m = multiplet) and

coupling constants (J) are reported in Hz. Assignments of individual resonances were done using

2D techniques (HMBC, HSQC, HH-COSY) when necessary. Yields of products in solution were

determined by integration of all resonances observed in the respective NMR spectra if not stated

otherwise. High-resolution mass spectra (HRMS) were obtained on a micro mass 70S-250

spectrometer (EI), an ABI/Sciex QStar Mass Spectrometer (DART), or on a JOEL AccuTOF-

DART (DART). Elemental analyses (C, H, N) were performed at the University of Toronto

employing a Perkin Elmer 2400 Series II CHNS Analyzer.

X-ray Diffraction Studies.

Single crystals were coated with Paratone-N oil, mounted using a glass fibre pin and frozen in the

cold nitrogen stream of the goniometer. Data sets were collected on a Bruker Apex II CCD

diffractometer which was equipped with a rotation anode using graphite-monochromated MoKα

radiation (λ = 0.71073 Å). Data reduction was performed using the SAINT software package.57

Data sets were corrected for absorption effects using SADABS routine (empirical multi-scan

method).58 Structure solutions were found by direct methods using SHELXS or by intrinsic

phasing using SHELXT. Full-matrix least-squares refinement of the initial solutions was carried

out on F2 using SHELXL, following standard practices.59,60 All non-hydrogen atoms were refined

anisotropically.

67

Syntheses and Spectroscopic Data

Preparation of [(SIMes)PPh2][B(C6F5)4] (2-3)

Diphenylchlorophosphine (150 mg, 0.679 mmol, 1.0 eq.) was added dropwise to a solution of 1,3-

dimesityl-4,5-dihydroimidazol-3-ium-2-ylidene (208 mg, 0.679 mmol, 1.0 eq.) in toluene (15

mL). The solution was stirred at ambient temperature for 1 h, then freshly prepared

[Et3Si(tol)][B(C6F5)4] (602 mg, 0.679 mmol, 1.0 eq.) was added. The reaction mixture was stirred

for 30 min at ambient temperature, then all volatiles were removed in vacuo. The residue was

washed with n-pentane (3 x 5 mL) and dried in vacuo giving 2-3 as analytically pure, colourless

solid (787 mg, 99% yield). Single crystals of 2-3, suitable for X-ray single crystal structure

determination, were obtained by slow diffusion of n-hexane into a CH2Cl2 solution.

1H NMR (400 MHz, CD2Cl2): δ = 2.15 (6H, s, p-Me), 2.25 (12H, s, o-Me), 4.25 (4H, d,

4JHP = 2.4 Hz, CH2), 6.64 (4H, s, m-Mes), 7.11 - 7.17 (4H, m, m-Ph), 7.27 - 7.34 (6H, m, o-,p-Ph);

11B{1H}[ (128 MHz, CD2Cl2): δ = −16.6 (s); 13C{1H} (101 MHz, CD2Cl2): δ = 18.6 (d,

5JCP = 5 Hz, o-Me), 20.9 (s, p-Me), 52.8 (d, 3JCP = 2 Hz, CH2), 125.5 (d, 1JCP = 3 Hz, i-Ph), 129.2

(d, 3JCP = 11 Hz, m-Ph), 130.6 (s, m-Mes), 130.9 (s, i-Mes), 131.8 (d, 4JCP = 2 Hz, p-Ph), 135.0 (s,

o-Mes), 135.6 (d, 2JCP = 28 Hz, o-Ph), 136.7 (d(br), 1JCF = 248 Hz, C6F5), 138.7 (d(br),

1JCF = 245 Hz, C6F5), 141.4 (s, p-Mes), 148.5 (d(br), 1JCF = 243 Hz, C6F5), 174.5 (d, C2,

1JCP = 62 Hz, C2); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.6 (8F, m, m-F), −163.7 (4F, m,

p-F), −133.0 (8F, m, o-F); 31P{1H} NMR (162 MHz, CD2Cl2): δ = −1.5 (s) ppm. Elemental

analysis for C57H36BF20N2P: calcd.: C 58.5, H 3.1, N 2.4; found: C 59.1, H 3.7, N 2.5; DART

MS: m/z: 491.26201 (calcd. for [M]+: 491.26161).

68

Preparation of [(SIMes)PPh2F2][B(C6F5)4] (2-4)

XeF2 (93 mg, 0.55 mmol, 1.1 eq.) was suspended in a solution of 2-3 (585 mg, 0.50 mmol, 1.0 eq.)

in CH2Cl2 (5 mL). The reaction mixture was stirred for 12 h at ambient temperature giving a

colourless solution. All volatiles were removed in vacuo. The residue was washed with toluene

(3 x 2 mL) and dried in vacuo yielding 2-4 as colourless, microcrystalline solid (538 mg, 81%

yield). Single crystals of 2-4, suitable for X-ray single crystal structure determination, were

obtained by slow diffusion of n-hexane into a CH2Cl2 solution.

1H NMR (400 MHz, CD2Cl2): δ = 2.07 (6H, s, p-Me), 2.31 (12H, s, o-Me), 4.33 (4H, d,

4JHP = 2.4 Hz, CH2), 6.57 (4H, s, m-H), 7.16 (4H, dt, 3JHH = 7.5 Hz, 4JHP = 6.5 Hz, m-Ph), 7.40

(2H, t, 3JHH = 7.5 Hz, p-Ph), 7.49 (4H, dd, 3JHH = 7.5 Hz, 3JHP = 15.3 Hz, o-Ph); 11B{1H} (128

Mhz, CD2Cl2): δ = −16.7 (s); 13C{1H} (101 MHz, CD2Cl2): δ = 17.6 (t, 6JCF = 2 Hz, o-Me), 20.9

(s, p-Me), 52.1 (d, 3JCP = 7 Hz, CH2), 127.0 (dt, 1JCP = 177 Hz, 2JCF = 20 Hz, i-Ph), 129.5 (d,

3JCP = 2 Hz, i-Mes), 129.5 (dt, 3JCP = 17 Hz, 4JCF = 2.9 Hz, m-Ph), 130.2 (s, m-Mes), 133.6 (dt,

4JCP = 4 Hz, 5JCF = 1 Hz, p-Ph), 135.8 (dt, 2JCP = 14 Hz, 3JCF = 12.6 Hz, o-Ph), 136.2 (s, o-Mes),

136.2 (d(br), 1JCF = 246 Hz, C6F5,), 138.6 (d(br), 1JCF = 247 Hz, C6F5), 142.2 (s, p-Mes), 148.5

(8C, d(br), 1JCF = 240 Hz, C6F5), 168.9 (C2); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.6 (8F,

m, m-F), −163.8 (4F, m, p-F), −133.1, (8F, m, o-F), −52.3 (2F, d, PF2, 1JFP = 733 Hz); 31P{1H}

NMR (162 MHz, CD2Cl2): δ = −62.9 (t, 1JPF = 733 Hz) ppm. Elemental analysis for

C57H36BF22N2P: calcd.: C 56.6, H 3.0, N 2.3; found: C 57.1, H 3.2, N 2.3; ESI MS: m/z: 529.2577

(calcd. for [M]+: 529.2579).

Preparation of [(SIMes)PPh2F][B(C6F5)4]2 (2-5)

69

Freshly prepared [Et3Si(tol)][B(C6F5)4] (372 mg, 0.38 mmol, 1.0 eq.) was added portionwise to a

suspension of 2-4 (460 mg, 0.38 mmol, 1.0 eq.) in toluene / C6H5F (1:1, 2 mL). The reaction

mixture was stirred for 3 h at ambient temperature giving a yellow solid and a yellowish

supernatant. CH2Cl2 was added until a clear, yellow solution was obtained (~8 mL). The addition

of n-hexanes (4 mL) initiated the formation of a yellowish precipitate. The supernatant was

removed and the residue was washed with toluene (3 x 2 mL) and n-hexanes (3 x 2 mL). Removal

of all volatiles gave 2-5 as yellowish solid (426 mg, 60% yield). Single crystals of 2-5, suitable for

X-ray single crystal structure determination, were obtained by slow diffusion of n-pentane into a

CH2Cl2 solution.

1H NMR (400 MHz, CD2Cl2): δ = 2.17 (12H, s, o-Me), 2.32 (6H, s, p-Me), 4.69 (4H, s, CH2),

6.96 (4H, s, m-Mes), 7.57 (4H, dd, 3JHH = 8.2 Hz, 3JHP = 4.8 Hz, o-Ph), 7.82 (4H, td, 3JHH = 8.0 Hz,

4JHP = 6.3 Hz, m-Ph), 8.22 (2H, t, 3JHH = 8.0 Hz, p-Ph); 11B{1H} (128 MHz, CD2Cl2): δ = −16.7

(s); 13C{1H} (101 MHz, CD2Cl2): δ = 18.3 (s, o-Me), 21.2 (s, p-Me), 55.6 (d, 3JCP = 5 Hz, CH2),

108.2 (dd, 1JCP = 108 Hz, 2JCF = 10 Hz, i-Ph), 127.3 (s, i-Mes), 132.0 (s, m-Mes), 132.5 (d,

3JCP = 16 Hz, m-Ph), 133.7 (dd, 2JCP = 14 Hz, 3JCF = 3 Hz, o-Ph), 134.9 (s, o-Mes), 136.7 (d(br),

1JCF = 246 Hz, C6F5), 138.6 (d(br), 1JCF = 247 Hz, C6F5), 142.9 (s, p-Ph), 145.6 (s, p-Mes), 148.5

(d(br), 1JCF = 245 Hz, C6F5), 155.0 (C2); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.4 (8F, m,

m-F), −163.4 (4F, m, p-F), −133.0, (8F, m, o-F), −131.7 (1F, d, 1JFP = 1040 Hz, PF); 31P{1H}

NMR (162 MHz, CD2Cl2): δ = 78.1 (d, 1JPF = 1040 Hz) ppm. Elemental analysis for

C81H36B2F41N2P: calcd.: C 52.1, H 1.9, N 1.5; found: C 52.1, H 2.0, N 1.6; ESI MS: m/z: 529.3

(calcd. for [(SIMes)PPh2F2]+: 529.3), 507.3 (calcd. for [(SIMes)PPh2O]+: 507.3), 491.3 (calcd. for

[(SIMes)PPh2]+: 491.3), 307.2 (calcd. for [(SIMes)H]+: 307.2).

Investigation of the Lewis Acidity of 5

70

Reaction of 5 with Et3PO (Gutmann-Becket Test)

Et3PO (3 mg, 0.02 mmol, 1.0 eq.) was added to a solution of 2-5 (37 mg, 0.02 mmol, 1.0 eq.) in

CD2Cl2 (1 mL). The reaction mixture was investigated by means of 31P and 19F NMR spectroscopy

after 1 h at ambient temperature and the obtained spectra indicate the formation of

fluorophosphonium ion [Et3PF][B(C6F5)4] (2-6) and oxophosphorane derivative

[(SIMes)PPh2O][B(C6F5)4] (2-7). In order to verify the formation of 2-6 and 2-7, both were

independently synthesized, as detailed below.

19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.4 (m, C6F5), −163.4 (m, C6F5), −158.3 (d,

1JFP = 989 Hz, 2-6), −133.1 (m, C6F5), −131.3 (d, 1JFP = 1041 Hz, 2-5), −75.8 (d(br), J = 1011 Hz);

31P{1H} NMR (162 MHz, CD2Cl2): δ = 14.0 (s, 2-7), 41.8 (d(br), J = 1011 Hz), 44.5 (d, J = 8 Hz),

50.3 (s, Et3PO), 54.1 (d, J = 8 Hz), 76.3 (d(br), 2-5, 1JPF = 1041 Hz), 147.2 (d, 1JPF = 989 Hz, 2-6)

ppm.

Preparation of Et3PF2

Et3PF2 was previously synthesized following a distinct synthetic approach,61 however, only

incomplete characterization data was reported. XeF2 (169 mg, 1.00 mmol, 1.0 eq.) was added

portionwise to a solution of Et3P (118 mg, 1.00 mmol, 1.0 eq.) in CH2Cl2 (5 mL).Strong

effervescence was observed upon addition. The reaction mixture was stirred at ambient

temperature for 1 h. All volatiles were removed in vacuo giving Et3PF2 as a colourless liquid

(155 mg, >99% yield).

1H NMR (400 MHz, C6D6): δ = 1.05 (9H, dt, 3JHH = 7.7 Hz, 3JHP = 22.5 Hz, CH2CH3), 1.75 - 1.92

(6H, m, CH2CH3); 13C{1H} NMR (101 MHz, C6D6): δ = 7.95 (d, 2JCP = 7 Hz, CH2CH3), 26.2

(d(br), 1JCP = 125 Hz, CH2CH3); 19F{1H} NMR (377 MHz, C6D6): δ = −40.9 (d, 1JFP = 586.5 Hz);

31P{1H} NMR (162 MHz, C6D6): δ = −14.0 (t, 1JPF = 586.5 Hz) ppm.

71

Preparation of [Et3PF][B(C6F5)4] (2-6)


solution of Et3PF2 (40 mg, 0.26 mmol, 1.0 eq.) in C6H5F (5 mL). The formation of a colourless

precipitate was gradually observed while the reaction mixture was stirred for 1 h at ambient

temperature. The supernatant was decanted and the residue was washed with n-pentane (3 x 3 mL).

Removal of all volatiles in vacuo yielded 2-6 as colourless, microcrystalline solid (197 mg, 93%

yield).

1H NMR (400 MHz, CD2Cl2): δ = 1.43 (9H, dt, 3JHH = 7.7 Hz, 3JHP = 20.3 Hz, CH2CH3), 2.51

(6H, ddquart., 3JHH = 7.7 Hz, 2JHP = 10.1 Hz, 3JHF = 11.8 Hz, CH2CH3); 11B{1H} NMR (128

MHz, CD2Cl2): δ = −16.7 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 4.47 (dd, 2JCP = 6 Hz,

3JCF = 3 Hz, CH2CH3), 26.2 (dd, 1JCP = 53 Hz, 2JCF = 9 Hz, CH2CH3), 136.7 (d(br), 1JCF = 244 Hz,

C6F5), 138.6 (d(br), 1JCF = 245 Hz, C6F5), 148.5 (d(br), 1JCF = 242 Hz, C6F5); 19F{1H} NMR (377

MHz, CD2Cl2): δ = −167.5 (8F, m, m-F), −163.5 (4F, m, p-F), −158.3 (1F, d, 1JFP = 989 Hz, PF),

−133.1 (8F, m, o-F); 31P{1H} NMR (162 MHz, CD2Cl2): δ = 147.2 (d, 1JPF = 989 Hz) ppm.

Elemental analysis for C30H15BF21P: calcd.: C 44.2, H 1.9; found: C 43.9, H 2.2.

Preparation of [(SIMes)POPh2][Cl]

72

1,3-Dimesityl-4,5-dihydroimidazol-3-ium-2-ylidene (31 mg, 0.1 mmol, 1.0 eq.) was added

portionwise to a solution of Ph2P(O)Cl (24 mg, 0.1 mmol, 1.0 eq.) in C6H5F (5 mL). The formation

of a colourless precipitate was gradually observed while the reaction mixture was stirred for 30 min

at ambient temperature. The supernatant was decanted and the residue was washed with n-pentane

(3 x 3 mL). Removal of all volatiles in vacuo yielded [(SIMes)PPh2O][Cl] as colourless,

microcrystalline solid (38 mg, 70% yield). Single crystals of [(SIMes)PPh2O][Cl], suitable for X-

ray single crystal structure determination, were obtained by slow diffusion of n-pentane into a

CH2Cl2 solution at −35 °C.

1H NMR (400 MHz, CD2Cl2): δ = 2.14 (6H, s, p-Me), 2.36 (12H, s, o-Me), 4.70 (4H, s, CH2),

6.58 (4H, s, m-H), 7.23 - 7.29 (4H, m, m-Ph), 7.41 - 7.46 (2H, m, p-Ph), 7.55 - 7.62 (4H, m, o-Ph);

13C{1H} NMR (101 MHz, CD2Cl2): δ = 19.1 (s, o-Me), 20.9 (s, p-Me), 54.2 (d, 3JCP = 3 Hz,

CH2,), 126.4 (d, 1JCP = 111 Hz, i-Ph,), 129.0 (d, 3JCP = 14 Hz, m-Ph), 130.1 (s, i-Mes), 130.8 (s,

m-Mes), 131.3 (d, 2JCP = 11 Hz, o-Ph), 133.6 (d, 4JCP = 3 Hz, p-Ph), 136.1 (s, o-Mes), 141.0 (s, p-

Mes), 164.8 (d, 1JCP = 72 Hz, C2); 31P{1H} NMR (162 MHz, CD2Cl2): δ = 13.4 (s) ppm.

Elemental analysis for C33H36ClN2PO: calcd.: C 73.0, H 6.7, N 5.2; found: C 72.6, H 7.2, N 5.3;

ESI MS: m/z: 507.2545 (calcd. for [M]+: 507.2559).

Preparation of [(SIMes)PPh2F][B(C6F5)4] (2-7)


suspension of [(SIMes)PPh2O][Cl] (109 mg, 0.20 mmol, 1.0 eq) in fluorobenzene (5 mL).

Gradually, [(SIMes)PPh2O][Cl] dissolved and a clear, pale yellow solution was obtained while

stirring the reaction mixture for 30 min at ambient temperature. n-Pentane (5 mL) was added

leading to the formation of a colourless precipitate. The supernatant was removed and the residue

73

was washed with n-pentane (3 x 3 mL). Removal of all volatiles gave 2-7 as colourless,

microcrystalline material (218 mg, 92% yield).

1H NMR (400 MHz, CD2Cl2): δ = 2.16 (6H, s, p-Me), 2.26 (12H, s, o-Me), 4.29 (4H, s, CH2),

6.63 (4H, s, m-H), 7.25 - 7.31 (4H, m, m-Ph), 7.46 - 7.56 (6H, m, o-,p-Ph); 11B{1H} NMR (128

MHz, CD2Cl2): δ = −16.6 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 18.4 (s, o-Me), 20.9 (s, p-

Me), 52.9 (d, 3JCP = 3 Hz, CH2), 125.8 (d, 1JCP = 112 Hz, i-Ph), 129.3 (d, 3JCP = 14 Hz, m-Ph),

130.0 (s, i-Mes), 130.4 (s, m-Mes), 131.0 (d, 2JCP = 11 Hz, o-Ph), 134.1 (d, 4JCP = 3 Hz, p-Ph),

135.4 (s, o-Mes), 136.7 (d(br), 1JCF = 246 Hz, C6F5), 138.6 (d(br), 1JCF = 247 Hz, C6F5), 141.9 (s,

p-Mes), 148.5 (d(br), 1JCF = 243 Hz, C6F5), 166.1 (d, 1JCP = 67.7 Hz, C2); 19F{1H} NMR (377

MHz, CD2Cl2): δ = −167.5 (8F, m, m-F), −163.7 (4F, m, p-F), −133.1 (8F, m, o-F); 31P{1H} NMR

(162 MHz, CD2Cl2): δ = 14.0 (s) ppm. Elemental analysis for C57H36BF20N2PO: calcd.: C 57.9,

H 3.5, N 2.4; found: C 57.7, H 3.1, N 2.4.

Reaction of 2-5 with Crotonaldehyde (Childs Test)

Crotonaldehyde was added in three times excess to a solution of 5 (20 mg, 0.012 mmol) in CD2Cl2

at −30 °C. The reaction mixture was kept at −30 °C and investigated by means of NMR

spectroscopy at −20 °C. The 31P and 19F NMR spectra of the reaction mixture reveal the formation

of 2-4 and 2-7, whereas 1H and 13C NMR spectra indicate the formation of multiple products,

hampering the determination of the fate of the deoxygenated acrylaldehyde.

31P{1H} NMR (162 MHz, CD2Cl2): δ = −63.7 (t, 1JPF = 737 Hz, 2-4), 13.6 (s, 7). 19F{1H} NMR

(377 MHz, CD2Cl2) δ = -53.23 (d, 1JPF = 735 Hz, 2-4), -133.33 (m, C6F5), -163.17 (m, C6F5), -

167.12 (m, C6F5) ppm.

Reaction of 2-5 with (C6F5)3PF2

The phosphorane (C6F5)3PF2 (12 mg, 0.02 mmol, 1.0 eq.) was added to a solution of 2-5 (38 mg,

0.02 mmol, 1.0 eq.) in CD2Cl2. Within ten minutes the formation of a large amount of colourless

74

precipitate of 2-2 was observed. The supernatant was investigated by means of 31P{1H} and 19F

NMR spectroscopy. The precipitate was isolated by decanting the supernatant, washing the residue

with a small amount of CH2Cl2 (2 x 0.5 mL) and removal of all volatiles. The obtained material

was dissolved in CD2Cl2 and investigated by 31P and 19F NMR spectroscopy, confirming the

formation of 2-2.

31P{1H} NMR (162 MHz, CD2Cl2): δ = −62.9 (t, 1JPF = −737 Hz, 2-4), −48.0 (t, 1JPF = 687 Hz,

(C6F5)3PF2), 67.9 (d, 1JPF = −1054 Hz, 2-2); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.7 (m,

BC6F5), −163.8 (m, BC6F5), −146.3 (m, (C6F5)3PF2, PC6F5), −149.8 (m, 2-2, PC6F5), −159.3 (m,

(C6F5)3PF2, PC6F5), −133.3 (m, BC6F5), −132.5 (m, (C6F5)3PF2, PC6F5), −124.5 (m, 2-2, PC6F5),

−121.8 (m, PC6F5), −119.5 (d, 1JPF = 1060 Hz, 2-2), −52.2 (d, 1JPF = 733 Hz, 2-4), 1.6 (d,

1JPF = 695 Hz, (C6F5)3PF2) ppm.

Reaction of 2-5 with Ph3CF

Ph3CF (2.8 mg, 0.012 mmol) was added to a solution of 2-5 (20 mg, 0.012 mmol) in CD2Cl2. After

30 min the reaction mixture was investigated by means of NMR spectroscopy. The 31P and 19F

NMR spectra show the formation of 2-4 whereas the formation of tritylium ion is indicated in the

13C NMR spectruma.

13C{1H} NMR (101 MHz, CD2Cl2): δ = 143.99 (s, C6H5); 143.03 (s, C6H5); 140.31 (s, (C(C6H5)3);

131.02 (s, C6H5); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.6 (m, C6F5), −163.7 (m, C6F5),

−133.1 (m, C6F5), −52.3 (d, 1JFP = 737 Hz, 2-4); 31P{1H} NMR (162 MHz, CD2Cl2): δ = −63.9

(t, 1JPF = 737 Hz, 2-4) ppm.

Catalysis of the Friedel-Crafts Dimerization of 1,1-Diphenylethylene

75

Compound 2-5 (8 mg, 2 mol%) was added to a solution of 1,1-diphenylethylene (36 mg,

0.2 mmol) in CD2Cl2 (0.7 mL) at ambient temperature. The reaction mixture was investigated by

NMR spectroscopy. All volatiles were removed in vacuo and the remaining residue was suspended

in n-pentane. The mixture was filtered through a Celite plug and the solvent was removed in vacuo

giving 1-methyl-1,3,3-triphenyl-2,3-dihydro-1H-indene as a colourless solid (35 mg, 97% yield).

1H NMR (400 MHz, C6D6): δ = 1.49 (3H, s, CH3), 3.02 (1H, d, 3JHH = 13.4 Hz, CH2), 3.42 (1H,

d, 3JHH = 13.4 Hz, CH2), 6.89 - 7.23 (19H, m); 13C{1H} (101 MHz, C6D6): δ = 29.1 (s, CH3), 51.5

(s, CH2), 61.4 (s, CPh), 61.8 (s, CPh), 125.4 (s, Ph), 125.9 (s, Ph), 126.0 (s, Ph), 126.3 (s, Ph), 127.

3 (s, Ph), 127.3 (s, Ph), 127.9 (s, Ph), 128.0 (s, Ph), 128.3 (s, Ph), 128.3 (s, Ph), 129.1 (s, Ph), 129.3

(s, Ph), 147.9 (s, Ph), 149.2 (s, Ph), 149.4 (s, Ph), 149.7 (s, Ph), 151.0 (s, Ph) ppm.

Polymerisation of Tetrahydrofuran

A solution of 2-5 (1 mg) in d8-tetrahydrofuran (1 mL) was kept for 2 h at ambient temperature. A

thickening of the reaction mixture indicated the polymerization of tetrahydrofuran. All volatiles

were removed from the reaction mixture in vacuo. GPC analysis, with THF as an eluent, showed

an average molecular weight of MW = 85,937 Da with a dispersity of 2.57.

Hydrodefluorination Catalysis

General Procedure for Hydrodefluorination Catalysis

All reactions were carried out under identical conditions, with the exception of catalyst loading. A

general procedure using 1-fluoroheptane as an example follows: Compound 2-5 (2 mg, 1 mol%)

was added to a solution of Et3SiH (18.8 µL, 0.1 mmol) in CD2Cl2 (0.7 mL) in a vial. Fluorinated

substrate 1-fluorocyclohexane (11.8 µL, 0.1 mmol) and C6H5F (10 µL, 0.1 mmol) as an internal

standard were added to a J Young NMR tube. The solution of catalyst and silane from the vial was

transferred to the J Young NMR tube. The sample was sealed, agitated and allowed to react at

ambient temperature. The reaction mixture was monitored by NMR spectroscopy at the following

intervals: 1 h, 3 h, 5 h, and 24 h.

76

19F NMR Spectra of Hydrodefluorination Catalysis

After 1 h, 1-fluoroadamantane (δ(19F) = –128 ppm) is completely consumed, and

Et3SiF (δ(19F) = –175 ppm) is produced.

After 1 h, 1-fluorocyclohexane (δ(19F) = –172 ppm) is completely consumed, and

Et3SiF (δ(19F) = –175 ppm) is produced.

77

After 1 h, 1-fluoropentane (δ(19F) = −218 ppm) is completely consumed, and Et3SiF

(δ(19F) = –175 ppm) is produced.

After 24 h, the fluoro-substituents on trifluorotoluene (δ(19F) = –62 ppm) are mostly

consumed, and Et3SiF (δ(19F) = –175 ppm) is produced.

78

After 24 h, the fluoro-substituents on 1,4-bis(difluoromethyl)benzene (δ(19F) =

–111 ppm) are somewhat consumed, and Et3SiF (δ(19F) = –175 ppm) is produced.

After 24 h, the fluoro-groups on 1,4-bis(difluoromethyl)benzene (δ(19F) =

–111 ppm) are mostly consumed, and Et3SiF (δ(19F) = –175 ppm) is produced.

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After 24 h, the aliphatic fluoro-groups on perfluorotoluene (δ(19F) = –55 ppm) are

somewhat consumed, and Et3SiF (δ(19F) = –175 ppm) is produced.

After 24 h, the aliphatic fluoro-groups on perfluorotoluene (δ(19F) = –55 ppm) are

somewhat consumed, and Et3SiF (δ(19F) = –175 ppm) is produced.

Preparation of [(SIMes)PMe2][B(C6F5)4] (2-8)

80


portionwise to a solution of Me2PCl (20 mg, 0.2 mmol, 1.0 eq.) giving a yellowish solution. The

reaction mixture was stirred at ambient temperature for 10 min, then freshly prepared

[Et3Si(tol)][B(C6F5)4] (196 mg, 0.2 mmol, 1.0 eq.) was added. The reaction mixture was stirred for

another 10 min, then n-pentane (3 mL) was added, which led to the formation of a colourless

precipitate. The supernatant was decanted and the residue was washed with n-pentane (3 x 3 mL).

All volatiles were removed in vacuo giving the respective imidazolidinium-substituted phosphine

salt as colourless, microcrystalline solid (96% yield).

1H NMR (400 MHz, CD2Cl2): δ = 0.80 (6H, d, 2JHP = 4.4 Hz, PCH3), 2.35 (18H, m(br), o/p-CH3),

4.24 (4H, d, 4JHP = 2.1 Hz, CH2), 7.07 (4H, s, m-Mes); 11B{1H} (128 MHz, CD2Cl2): δ = −16.7

(s); 13C{1H} (101 MHz, CD2Cl2): δ = 8.6 (d, 1JCP = 13 Hz, PCH3), 17.5 (d, 6JCP = 4 Hz, o-CH3),

20.8 (s, p-CH3), 52.3 (d, 3JCP = 2 Hz, CH2), 124.1 (s(br), i-C6F5), 130.5 (s, m-Mes), 130.8 (s, i-

Mes), 135.0 (s, o-Mes), 136.2 (d(br), 1JCF = 245 Hz, C6F5), 138.2 (d(br), 1JCF = 239 Hz, C6F5),

141.8 (s, p-Mes), 148.1 (d(br), 1JCF = 243 Hz, C6F5), 177.4 (d, 1JCF = 67 Hz, C2); 19F{1H} NMR

(377 MHz, CD2Cl2, 26 °C): δ = −167.6 (8F, m, m-F), −163.7 (4F, m, p-F), −133.1 (8F, m, o-F);

31P{1H} NMR (162 MHz, CD2Cl2): δ = −22.7 (s) ppm. Elemental analysis for C47H32BF20N2P:

calcd.: C 53.9, H 3.1, N 2.7; found: C 53.4, H 3.8, N 2.7; ESI MS: m/z: 367.2285 (calcd. for [M]+:

367.2297).

Preparation of [(SIMes)PF2Me2][B(C6F5)4] (2-9)

81

XeF2 (25 mg, 0.149 mmol, 1.1 eq.) was added portionwise to a solution of 2-8 (141 mg,

0.135 mmol, 1.0 eq.) in C6H5F (4 mL). The reaction mixture turned brownish and was stirred for

1 h at ambient temperature. n-Pentane (3 mL) was added leading to the formation of a colourless

precipitate. The supernatant was removed, the residue was washed with n-pentane (3 x 3 mL) and

dried in vacuo giving 2-9 as colourless, microcrystalline material (99% yield). Single crystals of

2-9, suitable for X-ray single crystal structure determination, were obtained by slow diffusion of

n-pentane into a CH2Cl2 solution at −35 °C.

1H NMR (400 MHz, CD2Cl2): δ = 1.06 (6H, dt, 2JHP = 16.0 Hz, 3JHF = 12.1 Hz, PCH3), 2.35 (6H,

s, p-Me), 2.37 (12H, s, o-Me), 4.36 (4H, s, CH2), 7.10 (4H, s, m-Mes); 11B{1H} (128 MHz,

CD2Cl2): δ = −16.6 (s); 13C{1H} (101 MHz, CD2Cl2): δ = 17.4 (t, 6JCF = 3 Hz, o-Me), 19.0 (dt,

1JCP = 124 Hz, 2JCF = 23 Hz, PCH3), 21.2 (s, p-Me), 52.1 (d, 3JCP = 7 Hz, CH2), 124.4 (s(br), i-

C6F5), 129.5 (d, 3JCP = 2 Hz, i-Mes), 131.0 (s, m-Mes), 136.6 (d(br), 1JCF = 247 Hz, C6F5), 136.7

(s, o-Mes), 138.7 (d(br), 1JCF = 245 Hz, C6F5), 143.2 (s, p-Mes), 148.5 (d(br), 1JCF = 243 Hz, C6F5),

165.9 (dt, 1JCP = 200.4 Hz, 2JCF = 51.6 Hz, C2); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.6

(8F, m, m-F), −163.7 (4F, m, p-F), −133.0 (8F, m, o-F), −8.9 (2F, d, 1JFP = 644 Hz, PF2); 31P{1H}

NMR (162 MHz, CD2Cl2): δ = −17.6 (t, 1JPF = 644 Hz) ppm. Elemental analysis for

C47H32BF22N2P: calcd.: C 52.1, H 3.0, N 2.6; found: C 51.8, H 3.0, N 2.6; ESI MS: m/z: 405.2263

(calcd. for [M]+: 405.2266).

Preparation of [(SIMes)PFMe2][B(C6F5)4]2 (2-10)

82

Freshly prepared [Et3Si(tol)][B(C6F5)4] (56 mg, 0.057 mmol, 1.0 eq.) was added to a solution of

[(SIMesPF2Me2][B(C6F5)4] (62 mg, 0.057 mmol, 1.0 eq.) in C6D5Br (1 mL). The reaction mixture

was stirred for 30 min at ambient temperature accompanied by the formation of a yellowish

precipitate. The supernatant was removed, the residue was washed with CH2Cl2 (3 x 3 mL) and

dried in vacuo giving 2-10 as yellowish powder (80% yield). Compound 2-10 is only sparingly

soluble in non-coordinating, polar solvents (CH2Cl2, C6H5Br, C6H5F).

1H NMR (400 MHz, CD2Cl2): δ = 2.22 (6H, dd, 3/2JHF/P = 13.2 Hz, 3/2JHF/P = 12.0 Hz, PCH3), 2.40

(12H, s, o-Me), 2.43 (6H, s, p-Me), 4.78 (4H, s, CH2N), 7.28 (4H, s, m-Mes); 11B{1H} (128 MHz,

CD2Cl2): δ = −16.7 (s); 13C{1H} (101 MHz, CD2Cl2): due to the low solubility of 2-10 a

sufficiently resolved 13C{1H} NMR spectra could not be obtained; 19F{1H} NMR (377 MHz,

CD2Cl2): δ = −167.4 (8F, m, m-F), −163.4 (4F, m, p-F), −135.0 (1F, d, 1JFP = 1053 Hz, PF),

−133.1 (8F, m, o-F); 31P{1H} NMR (162 MHz, CD2Cl2): δ = 120.7 (d, 1JPF = 1053 Hz) ppm.

Elemental analysis for C71H32BF41N2P: calcd.: C 48.9, H 1.9, N 1.6; found: C 48.6, H 2.0, N 1.7;

ESI MS: m/z: 307.2 (calcd. for SIMesH+: 307.2), 367.2 (calcd. for [(SIMesPMe2][B(C6F5)4]:

367.2), 383.2 (calcd. for SIMesP(O)Me2+: 383.2), 405.2 (calcd. for [(SIMesPF2Me2][B(C6F5)4]:

405.2).

Preparation of [(SIMes)PEt2][B(C6F5)4] (2-11)

83


portion-wise to a solution of Et2PCl (24 mg, 0.2 mmol, 1.0 eq.) in C6H5F (3 mL), giving a

yellowish solution. The reaction mixture was stirred at ambient temperature for ten minutes, then

freshly prepared [Et3Si(tol)][B(C6F5)4] (196 mg, 0.2 mmol, 1.0 eq.) was added. The reaction

mixture was stirred for another 10 min, then n-pentane (3 mL) was added which led to the

formation of a colourless precipitate. The supernatant was decanted and the residue was washed

with n-pentane (3 x 3 mL). All volatiles were removed in vacuo giving the respective

imidazolidinium-substituted phosphine salt as colourless, microcrystalline solid (96% yield).

1H NMR (400 MHz, CD2Cl2): δ = 0.83 (6H, dt, 3JHP = 17.3 Hz, 3JHH = 7.5 Hz, CH2CH3),

1.08 - 1.12 (2H, m, CH2CH3), 1.25 - 1.37 (2H, m, CH2CH3), 2.35 (6H, s, p-CH3), 2.36 (12H, s, o-

Me), 4.23 (4H, d, 4JHP = 1.9 Hz, NCH2), 7.07 (4H, s, m-Mes); 11B{1H} (128 MHz, CD2Cl2):

δ = −16.7 (s); 13C{1H} (101 MHz, CD2Cl2): δ = 10.1 (d, 2JCP = 14 Hz, CH2CH3), 12.9 (d,

1JCP = 15 Hz, CH2CH3), 17.7 (s, o-Mes), 17.7 (s, o-Mes), 20.8 (s, p-Mes), 52.5 (d, 3JCP = 2 Hz,

CH2N), 130.5 (s, m-Mes), 131.0 (s, i-Mes), 134.9 (s, o-Mes), 136.3 (d(br), 1JCF = 244 Hz, C6F5),

138.2 (d(br), 1JCF = 241 Hz, C6F5), 141.7 (s, p-Mes), 148.1 (d(br), 1JCF = 241 Hz, C6F5), 177.0 (d,

1JCP = 70 Hz, C2); 19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.6 (8F, m, m-F), −163.7 (4F, m,

p-F), −133.1 (8F, m, o-F); 31P{1H} NMR (162 MHz, CD2Cl2): δ = 0.67 (s) ppm. Elemental

analysis for C49H36BF20N2P: calcd.: C 54.8, H 3.4, N 2.6; found: C 55.5, H 3.9, N 2.7; ESI MS:

m/z: 395.2615 (calcd. for [M]+: 395.2611).

Preparation of [(SIMes)PF2Et2][B(C6F5)4] (2-12)

XeF2 (37 mg, 0.20 mmol, 1.1 eq.) was added portion-wise to a solution of 2-11 (214 mg,

0.20 mmol, 1.0 eq.) in CH2Cl2 (4 mL). The reaction mixture turned brownish and was stirred for

84

1 h at ambient temperature. n-Pentane (3 mL) was added, leading to the formation of a colourless

precipitate. The supernatant was removed, the residue was washed with n-pentane (3 x 3 mL) and

dried in vacuo giving 2-12 as colourless, microcrystalline material (99% yield). Single crystals of

2-12, suitable for X-ray single crystals structure determination were obtained by slow diffusion of

n-pentane into a CH2Cl2 solution at −35 °C.

1H NMR (400 MHz, CD2Cl2): δ = 0.62 (6H, dtt, 3JHP = 26.5 Hz, 3JHH = 7.7 Hz, 4JHF = 1.5 Hz,

CH2CH3), 1.37 (4H, dtt, 2JHP = 18.1 Hz, 3JHH = 7.7 Hz, 3JHF = 7.7 Hz, CH2CH3), 2.36 (6H, s, p-

Me), 2.37 (12H, s, o-Me), 4.33 (4H, s, CH2N), 7.10 (4H, s, m-Mes); 11B{1H} (128 MHz, CD2Cl2):

δ = −16.6 (s); 13C{1H} (101 MHz, CD2Cl2): δ = 6.9 (td, 3JCF = 9 Hz, 2JCP = 5 Hz, CH2CH3), 17.1

(t, 6JCF = 3 Hz, o-Mes), 20.8 (s, p-Mes), 24.9 (dt, 1JCP = 113 Hz, 2JCF = 20 Hz, CH2CH3), 51.7 (d,

3JCP = 7 Hz, CH2N), 123.8 (s(br), C6F5), 129.3 (d, 3JCP = 1 Hz, i-Mes), 130.5 (s, m-Mes), 136.2

(d(br), 1JCF = 244 Hz, C6F5), 136.3 (s, o-Mes), 138.3 (d(br), 1JCF = 247 Hz, C6F5), 142.6 (s, p-Mes),

148.1 (d(br), 1JCF = 242 Hz, C6F5), 166.7 (dt, 1JCP = 195 Hz, 2JCF = 55 Hz, C2); 19F{1H} NMR

(377 MHz, CD2Cl2): δ = −167.6 (8F, m, m-F), −163.7 (4F, m, p-F), −133.1 (8F, m, o-F), −32.5

(2F, d, 1JFP = 685 Hz, PF2); 31P{1H} NMR (162 MHz, CD2Cl2): δ = −17.5 (t, 1JPF = 685 Hz) ppm.

Elemental analysis for C49H36BF22N2P: calcd.: C 52.9, H 3.3, N 2.5; found: C 52.3, H 3.6, N 2.6;

ESI MS: m/z: 433.2579 (calcd. for [M]+: 433.2579).

Preparation of [(SIMes)PF2Et2][B(C6F5)4] (2-13)

Freshly prepared [Et3Si(tol)][B(C6F5)4] (98 mg, 0.10 mmol, 1.0 eq.) was added to a solution of

[(SIMesPF2Et2][B(C6F5)4] (119 mg, 0.10 mmol, 1.0 eq.) in C6D5Br (2 mL). The reaction mixture

was stirred for 30 min at ambient temperature accompanied by the formation of a yellowish

precipitate. The supernatant was removed, the residue was washed with CH2Cl2 (3 x 3 mL) and

85

dried in vacuo giving 2-13 as yellowish powder (86% yield). Compound 2-13 is only sparingly

soluble in non-coordinating, polar solvents (CH2Cl2, C6H5Br, C6H5F).

1H NMR (400 MHz, CD2Cl2): δ = 1.39 (6H, dtd, 3JHP = 24.4 Hz, 3JHH = 8.1 Hz, 4JHF = 0.9 Hz,

CH2CH3), 2.27 - 2.37 (2H, m, CH2CH3), 2.49 - 2.59 (2H, m., CH2CH3), 2.43 (18H, s, o/p-Me),

4.77 (4H, s, NCH2), 7.27 (4H, s, m-Mes); 13C{1H} (101 MHz, CD2Cl2): due to the low solubility

of [(SIMes)PFEt2][B(C6F5)4]2 a sufficiently resolved 13C{1H} NMR spectra could not be obtained;

19F{1H} NMR (377 MHz, CD2Cl2): δ = −167.4 (8F, m, m-F), −163.3 (4F, m, p-F), −152.9 (1F, d,

1JFP = 1056 Hz, PF), −133.1 (8F, m, o-F); 31P{1H} NMR (162 MHz, CD2Cl2): δ = 126.6 (d,

1JPF = 1056 Hz) ppm. Elemental analysis for C73H36BF41N2P: calcd.: C 49.5, H 2.1, N 1.6; found:

C 49.3, H 2.6, N 1.7; ESI MS: m/z: 307.2173 (calcd. for SIMesH+: 307.2169), 411.2560 (calcd.

for [(SiMes)POMe2]+: 411.2560), 433.2576 (calcd. for [(SiMes)PFMe2]

2++e−: 433.2579).

Preparation of [(IBox-iPr2)PPh2][OTf] (2-14)

To a solution of [(IBox-iPr2H)][OTf] (150 mg, 0.39 mmol, 1.0 eq) in ether (10 mL) KH (20

mg, 0.50 mmol, 1.3 eq) and a catalytic amount of KOtBu was added. The solution was

stirred at ambient temperature for 7 h. The solution was filtered over Celite into 7 mL of

toluene, the ether was then removed under reduced pressure. To the remaining toluene

solution, a solution of Ph2PCl (94 mg, 0.43 mmol, 1.1 eq) in toluene (8 mL) was added

dropwise. The reaction mixture was allowed to stir overnight. The solution was reduced to

3 mL and the crude product precipitated with n-pentane. The resulting white solid was

dissolved in 10 mL of CH2Cl2 and filtered over Celite. The solvent was reduced to 3 mL

and pure product was precipitated with n-pentane (61% yield). X-ray quality crystals where

86

obtained by slow vapor diffusion of n-pentane to a concentrated solution of the 2-14 in

CH2Cl2.

1H NMR (500 MHz, CD2Cl2): δ = 7.54 (6H, m, Ph), 7.46 (4H, m, Ph), 5.05 (2H, dd, OCH2,

3JHH = 9 Hz, 8.0 Hz, 2H), 4.85 (2H, dd, OCH2, 3JHH = 9, 3 Hz), 4.19 (2H, m, CH(iPr)), 1.95

(2H, m, CH(CH3)), 0.80 (6H, d, 3JHH = 7 Hz, CH(CH3)2), 0.71 (6H, d, 3JHH = 7 Hz,

CH(CH3)2);. 13C{1H} NMR (126 MHz, CD2Cl2): δ = 133.4 (d, 1JCP = 21 Hz, i-Ph), 133.2

(d, 1JCP = 21 Hz, i-Ph), 131.3 (d, 2JCP = 11 Hz, o-Ph), 130.0 (t, 3JCP = 8 Hz, m-Ph), 129.3

(s, OCN), 128.0 (d, 4JCP = 7 Hz, o-Ph), 127.7 (d, 4JCP = 7 Hz, o-Ph), 77.7 (s, CH(iPr)),

64.9 (s, OCH2), 30.7 (s, CH(CH3)2), 18.8 (s, CH3), 14.3(s, CH3); 19F{1H} NMR (377 MHz,

CD2Cl2): δ = −78.9; 31P{1H} NMR (162 MHz, CD2Cl2): δ = −21.8 ppm. Elemental

Analysis for C26H30F3N2PO5S: C 54.7, H 5.3, N 4.9 Found: C 54.9, H 5.0, N 4.9.

Preparation of [(IBox-iPr2)PF2Ph2][OTf] (2-15)

To a solution of 2-14 (20 mg, 0.03 mmol, 1.0 eq) in dichloromethane (7 mL) at –40 oC XeF2 (6mg,

0.03 mmol, 1.0 eq) was added portion-wise. The solution was allowed to slowly warm to ambient

temperature overnight. The solvent was reduced to 2 mL and the product recrystallized from n-

pentane to yield a waxy off-white solid (62 % yield).

1H NMR (500 MHz, CD2Cl2): δ = 8.66 (1H, s, Ph) 7.80 (4H, m, Ph), 7.65 (1H, m, Ph), 7.53 (4H,

m, Ph), 5.08 (2H, m, CH(iPr)), 4.85 (4H, m, OCH2), 2.34 (2H, m, CH(CH3)2), 1.05 (6H, d, 3JHH =

6.9 Hz, CH(CH3)2), 1.01 (6H, d, 3JHH = 6.8, CH(CH3)2); 13C{1H} NMR (126 MHz, CD2Cl2):

δ = 133.3 (dd, 3JPC = 3Hz, 4JCF = 1 Hz, m-Ph), 131.3 (dd, 2JPC = 11 Hz, 3JCF = 2 Hz, o-Ph), 128.8

(d, 1JPC = 14 Hz, i-Ph), 125.9 (s, OCN), 115.5 (s, p-Ph), 79.3 (s, CH(iPr)), 64.2 (s, OCH2), 31.1 (s,

87

CH(CH3)2), 17.4 (s, CH(CH3)2), 16.6 (s, CH(CH3)2) ppm. 19F NMR (564 MHz, CD2Cl2): δ =

−46.2 (2F, d, 1JPF = 694 Hz, PF2), −79.0 (3F, OTf); 31P{1H} NMR (162 MHz, CD2Cl2): δ = −63.9

(t, 1JPF = 694 Hz) ppm.

Preparation of [(IBox-iPrMe2)PPh2][OTf] (2-17)

To a solution of 7-isopropyl-3,3-dimethyl-2,3,7,8-tetrahydroimidazo[4,3-b:5,1-b']bis(oxazole)-4-

ium trifluoromethanesulfonate (150 mg, 0.40 mmol, 1.0 eq) in ether (10 mL) KH (20 mg, 0.50

mmol, 1.3 equiv.) and a catalytic amount of potassium t-butoxide was added. The solution was

stirred at ambient temperature for 7 hours. The solution was filtered over celite into 7 mL of

toluene, the ether was then removed under reduced pressure. To the remaining solution of toluene,

diphenylchlorophosphine (98 mg, 0.44 mmol, 1.1 eq) in toluene (8 mL) was added dropwise. The

reaction mixture was allowed to stir overnight. The solution was reduced to 3mL and the crude

product crashed out with n-pentane. The resulting white solid was dissolved in 10 mL of

dichloromethane and filtered over Celite. The solvent was reduced to 3 mL and pure product was

crashed out with n-pentane (73% yield).

1H NMR (500 MHz, CD2Cl2): δ = 7.52 (8H, m, Ph), 7.37 (2H, m, Ph), 4.91 (4H, m, OCH2),

4.70 (1H, m, CHiPr), 3.29 (1H, m, CH(CH3)2), 1.93 (3H, s, C(CH3)2), 1.61 (3H, s, C(CH3)2), 0.82

(3H, d, 3JHH = 7 Hz, CH(CH3)2), 0.45 (3H, d, 3JHH = 7 Hz, CH(CH3)2) ppm. 13C{1H} NMR (126

MHz, CD2Cl2): δ = 133.9 (d, 3JPC = 21 Hz, m-Ph), 133.6 (d, 3JPC = 21 Hz, m-Ph), 131.6 (d, 2JPC

= 28 Hz, o-Ph), 130.4 (d, 4JPC = 6 Hz, p-Ph), 130.4 (d, 4JPC = 5 Hz, p-Ph), 121.38 (d, 1JPC = 55

Hz, i-Ph), 88.8 (s, OCH2CiPr), 77.6 (s, OCH2CMe2), 67.8 (s, CHiPr), 64.4 (s, CMe2), 32.0 (s,

CH(CH3)2), 26.5 (s, CH3), 25.6 (s, CH3), 18.9 (s, CH(CH3)2), 14.4 (s, CH(CH3)2) ppm. 31P NMR

88

(243 MHz, CD2Cl2): δ = −24.2 ppm. 19F NMR (564 MHz, CD2Cl2): δ = −78.9 ppm. DART

MS: m/z 407.18854 (calcd: for [M]+: 407.18884).

Preparation of [(TripCH2N2(NMe)C2Ph)PPh2)2(AgCl)2][AgCl2]2 (2-20)

[(TripCH2N2(NMe)C2Ph)2Ag][AgCl2] (200 mg, 0.25 mmol, 2.0 eq.) was added

portionwise to a solution of Ph2PCl (27 mg, 0.13 mmol, 1.0 eq) in dichloromethane (10 mL)

giving a partially soluble white precipitate. The reaction mixture was stirred at ambient

temperature for 2 h. The reaction mixture was reduced to 3 mL and the white precipitate

isolated by washing with n-pentane (2 x 15 mL). All volatiles were removed in vacuo giving

the pure product. X-ray quality crystals could be isolated by slow vapour diffusion of n-

pentane in a concentrated solution of the phosphenium silver complex in dichloromethane

(97% yield).

1H NMR (500 MHz, CD2Cl2): δ = 7.47 (4H, m, Ar), 7.38 (2H, m, Ar), 7.32 (5H, m, Ar),

7.19 (2H, m, Ar), 7.12 (4H, m, Ar), 5.75 (2H, s, CH2Tripp), 3.92 (3H, s, NCH3), 2.89 (3H,

m, CH(CH3)2), 1.25 (6H, d, CH(CH3)2, 3JHH = 7 Hz, 6H), 1.11 (12H, d, CH(CH3)2,

3JHH =

7 Hz); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 151.4 (s, Ar), 149.5 (s, Ar), 147.3 (s, Ar),

134.2 (d, 2JPC = 21 Hz, o-Ph), 137.2 (d, 1JPC = 32 Hz, i-Ph), 131.0 (s, Ar), 130.9 (s, Ar),

130.1 (s, Ar), 129.4 (d, 3JPC = 9 Hz, m-Ph), 129.0 (s, Ar), 127.2 (s, Ar), 121.9 (s, Ar), 121.8

(d, 4JPC = 2 Hz, p-Ph), 121.7 (s, Ar), 50.4 (s, CH2Trip), 38.5 (s, NCH3), 34.3 (s, CH(CH3)2),

30.2 (s, CH(CH3)2), 23.9 (s, CH(CH3)2), 23.6 (s, CH(CH3)2), 13.8 ppm (s,

CH(CH3)2); 31P{1H} NMR (243 MHz, CD2Cl2): δ = −25.1; 31P NMR (283 MHz, SS):

89

−23.6 ppm. DART MS: m/z: 344.1 (calcd. for [M]H+: 344.1; C21H19N3P fragment), 217.2

(calcd: for [M]+: 217.2; C16H25 fragment).

Preparation of [TripCH2N2(NMe)C2Ph)PF2Ph2][AgCl2] (2-21)

To a solution of 2-20 (50 mg, 0.04 mmol, 1.0 eq) in DCM (15 mL) at –78 oC, a 3 mL

solution of XeF2 (13 mg, 0.08, 2.0 eq) in DCM was added dropwise over 5 min. The

solution was allowed to slowly warm to ambient temperature and then stirred overnight.

The solution was reduced to 3 mL and the product recrystallized from n-pentane. X-ray

quality crystals could be isolated by slow vapour diffusion of n-pentane in a concentrated

solution of the phosphenium silver complex in dichloromethane (64% yield).


7.34 (4H, m, Ar), 7.17 (2H, m, Ar), 7.08 (2H, s, Ar), 5.66 (2H, s, CH2Trip), 3.98 (3H, s,

NCH3), 2.91 (1H, m, CH(CH3)2), 2.82 (2H, m, CH(CH3)2), 1.24 (6H, d, 3JHH = 7 Hz,

CH(CH3)2), 1.10 (12H, d, 3JHH = 7 Hz, CH(CH3)2); 13C{1H} NMR (151 MHz, CD2Cl2): δ

= 149.9 (s, Ar), 136.9 (s, Ar), 132.4 (s, Ar), 130.9 (s, Ar), 130.1 (d, 2JPC = 18 Hz, o-Ph),

129.5 (s, Ar), 122.4 (s, Ar), 39.2 (s, NCH3), 34.7 (s, CH2Trip), 32.3 (s, CH(CH3)2), 30.7 (s,

CH(CH3)2), 24.4 (s, CH(CH3)2), 24.1 (s, CH(CH3)2); 19F NMR (564 MHz, CD2Cl2): δ =

−37.5 (d, 1JPF = 696 Hz); 31P{1H} NMR (243 MHz, CD2Cl2): δ = −65.0 (t, 1JPF = 696 Hz)

ppm. DART MS: m/z: 376.3 (calcd. for M+H+: 376.3; C25H34N3 fragment), 360.1 (calcd:

for [M]+: 360.2; C24H30N3 fragment), 344.1 (calcd. for [M]H+: 344.1; C21H19N3P fragment),

217.2 (calcd: for [M]+: 217.2; C16H25 fragment), 160.1 (calcd: for [M]H+: 160.1; C9H10N3

fragment).

Preparation of [PhCH2N2(NMe)C(CH2OMe)CH][OTf] (2-22)

90

1-Benzyl-4-(methoxymethyl)-1,2,3-triazole (1.00 g, 4.92 mmol) was stirred with excess

MeOTf in 15 mL of CH2Cl2 at ambient temperature overnight. The solution was reduced

to 3 mL and the product isolated from n-pentane. The crude product was washed with n-

pentane (3 x 18 mL) to yield a yellow oil in 96% isolated yield.

1H NMR (500 MHz, CDCl3): δ = 8.67 (1H, s, NCHCPh), 7.48 (2H, m, Ph), 7.41 (3H, s,

Ph), 5.71 (2H, s, CH2Ph), 4.68 (2H, s, CH2OMe), 4.27 (3H, s, NCH3), 3.41 (3H, s, OCH3);

13C{1H} NMR (126 MHz, CDCl3): δ = 140.8 (s, Ph), 131.2 (s, NCCH2), 130.2 (s, Ph),

129.8 (s, Ph), 129.7 (s, Ph), 129.6 (s, CCHN), 61.9 (s, CH2OMe), 59.4 (s, OCH3), 57.7 (s,

NCH3), 38.7 (s, CH2Ph) ppm. DART MS: m/z: 204.1 (calcd. for [M]H+: 204.1; C11H16N3O

fragment).

Preparation of [PhCH2N2(NMe)C(CH2OMe)C]AgCl (2-23)

To a solution of 2-22 (300 mg, 0.82 mmol, 1.0 eq.) in 1:1 MeCN:CH2Cl2 (18 mL), Ag2O

(104 mg, 0.45 mmol, 0.6 eq.) and [Et4N][Cl] (208 mg, 0.90 mmol, 1.1 eq) were added. The

solution was stirred in the dark overnight. The solvent was removed under reduced pressure

and the resulting residue taken up in 15 mL of CH2Cl2. The solution was filtered over Celite

and solvent reduced to 3 mL. The product was precipitated with n-pentane to yield a dark

yellow-brown oil, which was stored in the dark. (84% yield)

1H NMR (500 MHz, CD2Cl2): δ = 7.49 (1H, m, Ph), 7.35 (4H, m, Ph), 5.55 (2H, s, CH2Ph),

4.54 (2H, s, CH2OMe), 4.14 (3H, s, NCH3), 3.36 (3H, s, OCH3); 13C{1H} NMR (126 MHz,

91

CD2Cl2): δ = 143.5 (s, Ph), 135.0 (s, NCCH2), 130.0 (s, Ph), 129.6 (s, Ph), 128.9 (s, Ph),

65.0 (s, CH2OMe), 59.9 (s, OCH3), 58.9 (s, NCH3), 37.4 (s, CH2Ph) ppm.

Preparation of [PhCH2N2(NMe)C(CH2OMe)CPPh2][AgCl2] (2-24)

To 2-23 (150 mg, 0.42 mmol, 1.0 eq.) was added portion-wise a solution of the Ph2PCl (92

mg, 0.42 mmol, 1.0 eq) in CH2Cl2 (10 mL) giving a partially soluble white precipitate. The

reaction mixture was stirred at ambient temperature for 2 h. The reaction mixture was

reduced to 3 mL and the white precipitate isolated by washing with n-pentane (2 x 15 mL).

All volatiles were removed in vacuo giving the pure product (91% yield).


5.96 (2H, s, CH2Ph), 4.39 (3H, s, NCH3), 3.87 (1H, s, CH2OMe), 3.45 (1H, s, CH2OMe),

3.00 (3H, s, OCH3); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 145.0 (s, Ph), 139.9 (s, d, 1JPC

= 33 Hz, i-Ph), 134.5 (d, 2JPC = 21 Hz, o-Ph), 131.6 (s, NCCH2), 130.1 (d, 3JPC = 8 Hz, m-

Ph), 129.9 (s, Ph), 129.9 (s, Ph), 129.5 (s, Ph), 129.3 (d, 4JPC = 2 Hz, p-Ph), 128.3 (s, Ph),

61.2 (s, CH2OMe), 59.5 (s, OCH3), 39.8 (s, NCH3), 22.9 (s, CH2Ph); 31P{1H} NMR (243

MHz, CD2Cl2): δ = −27.2 ppm. Elemental Analysis for C24H25AgN3POCl2: calcd.: C 49.6,

H 4.3, N 7.2; found: C 50.2, H 4.5, N 7.7.

Preparation of [(cAAC)PPh2][OTf] (2-28)

92

To a diethyl ether solution of 2-27 (150 mg, 0.36 mmol, 1.0 eq.) at –40 oC, K[N(SiMe3)2]

(72 mg, 0.36 mmol, 1.0 eq.) was added slowly. The solution was allowed to slowly warm

to room temperature and stirred for an additional 2 h. The solution was filtered over Celite

into 10 mL of toluene and the diethyl ether removed under reduced pressure. To this

mixture, Ph2PCl (80 mg, 0.36 mmol, 1.0 eq.) dissolved in of toluene (7 mL) was added

dropwise to give a yellow solution. The solution was stirred for 3 h before adding an excess

of TMSOTf and allowed to react for 1 h. The toluene was reduced to 3 mL and the product

isolated as a yellow solid after washing with n-pentane (89% yield).


7.35 (1H, s, Ar), 7.33 (1H, s, Ar), 2.54 (3H, s NCH3), 2.48 (2H, m, CH(CH3)2), 2.34 (3H,

s, NCH3), 1.90 (1H, b, NCCH2), 1.75 (1H, b, NCCH2), 1.59 (10H, m, Cy), 1.31 (6H, d, 1JHH

= 7 Hz, CH(CH3)2), 0.88 (6H, d, 1JHH = 7 Hz, CH(CH3)2); 13C{1H} NMR (126 MHz,

CD2Cl2): δ = 144.8 (d, 4JPC = 3 Hz, p-Ph), 136.5 (d, 1JPC = 24 Hz, i-Ph), 132.6 (s, Dipp),

132.5 (s, Dipp), 130.5 (d, 2JPC = 12 Hz, o-Ph), 129.7 (d, 3JPC = 9 Hz, m-Ph), 129.5 (s, Dipp),

128.7 (s, Dipp), 127.3 (s, Dipp), 125.8 (s, Dipp), 83.8 (d, 2JPC = 3 Hz, NC(CH3)2), 62.9 (s,

CCy), 46.9 (s, CH2CCy), 35.4 (s, Cy), 30.4 (s, Cy), 30.1 (s, CH(CH3)2), 26.1 ( s, NC(CH3)2),

25.0 (s, CH(CH3)2), 24.7 (s, Cy), 21.9 (s, CH(CH3)2); 19F{1H} NMR (564 MHz, CD2Cl2):

δ = −78.9; 31P{1H} NMR (243 MHz, CD2Cl2): δ = −0.3 ppm. Elemental Analysis for

C36H45F3NPO3S: calcd.: C 65.5, H 6.9, N 2.1; found: C 66.0, H 7.1, N 1.9.

Preparation of [(cAAC)PF2Ph2][OTf] (2-29)

To a solution of 2-28 (50 mg, 0.76 mmol, 1.0 eq.) in DCM at (15 mL) –40 oC XeF2 (13 mg, 0.76

mmol, 1.0 eq.) was added portionwise. The reaction mixture was allowed to warm to room

temperature and stirred for an additional 4 h. The solution was reduced to 3 mL and the product

isolated as a pale yellow solid after washing with n-pentane.

93

1H NMR (500 MHz, CD2Cl2): δ = 8.16 (2H, b, Ar), 7.62 (2H, b, Ar), 7.39 (2H, m, Ar), 7.32 (2H,

m, Ar), 7.15 (5H, m, Ar), 2.57 (2H, m, CH(CH3)2), 2.57 (2H, s, NCCH2), 1.96 (6H, m, C(CH3)2),

1.31 (10H, m, Cy), 1.31 (6H, d, 3JHH = 6.5 Hz, CH(CH3)2), 1.26 (6H, d, 3JHH = 6.5 Hz, CH(CH3)2);

13C{1H} NMR (126 MHz, CD2Cl2): δ = 146.1 (s, Dipp), 133.3 (s, Dipp), 127.4 (s, Dipp), 46.7 (s,

NC(CH3)2), 35.4 (NCCH2), 39.3 (s, Cy), 30.1 (s, CH(CH3)2), 28.2 (s, Cy), 24.9 (s, CH(CH3)2),

23.9 (s, CH(CH3)2), 22.9 (s, Cy), 21.9 (s, Cy); 19F{1H} NMR (470 MHz, CD2Cl2): δ = −48.1 (2F,

d, 1JPF = 737 Hz, PF2), -78.9 (3F, OTf); 31P{1H} NMR (202 MHz, CD2Cl2): δ = −54.7 (t, 1JPF =

737 Hz) ppm. Elemental Analysis for C36H45F5NPO3S: calcd.: C 62.0, H 6.5, N 2.0; found: C

60.1, H 6.2, N 1.9.

94

Table 2-2 Crystallographic data and details of the structure refinements of compounds

2-3 and 2-4

2-3 2-4

formula C57H36BF20N2P C58H38BCl2F22N2P

Mr [g mol1] 1170.66 1293.58

colour, habit colourless block colourless, block

crystal system tetragonal triclinic

Space group P41212 P-1

a [Å] 18.493(1) 11.189(1)

b [Å] 18.493(1) 16.139(2)

c [Å] 30.308(3) 17.358(2)

[°] 90 116.11(1)

[°] 90 92.65(1)

[°] 90 91.70(1)

V [Å3] 10366(1) 2807.0(4)

Z 8 2

T [K] 150(2) 150(2)

Crystal size [mm] 0.10x0.05x0.05 0.05x0.04x0.03

c [g cm3] 1.500 1.530

F(000) 4736 1304

min [°]

max [°]

1.29

28.30

1.31

27.33

Index range

24 h 24

24 k 22

30 l 40

14 h 14

20 k 20

22 l 21

[mm1] 0.167 0.259

absorption correction SADABS SADABS

reflections collected 60016 36258

reflections unique 12863 12074

Rint 0.1917 0.1231

reflection obs.

[F>3(F)] 5677 4916

residual density

[e Å3]

0.270,

–0.298

0.843,

–0.618

parameters 736 791

GOOF 0.961 0.992

R1 [I>2(I)] 0.0672 0.0872

wR2 (all data) 0.2040 0.2264

95


2-5 and 2-7

2-5 2-7

formula C82.15H38.3B2Cl2.3F41N2

P C35H38Cl5N2OP

Mr [g mol1] 1966.37 710.89

colour, habit yellow, block colourless plate

crystal system orthorhombic triclinic

Space group Pbca P-1

a [Å] 21.564(1) 9.400(1)

b [Å] 17.911(1) 12.470(1)

c [Å] 41.173(2) 16.457(1)

[°] 90 74.725(4)

[°] 90 80.821(4)

[°] 90 73.879(4)

V [Å3] 15902(1) 1779.8(3)

Z 4 2

T [K] 150(2) 150(2)


c [g cm3] 1.641 1.327

F(000) 7818 740

min [°]

max [°]

1.56

27.48

1.29

27.56

Index range

22 h 27

23 k 23

26 l 53

8 h 12

16 k 16

21 l 21

[mm1] 0.255 0.483




Rint 0.0528 0.0318

reflection obs.

[F>3(F)] 8839 6158

residual density

[e Å3]

0.717,

–0.671

0.765,

–0.516

parameters 1234 422

GOOF 1.002 1.033

R1 [I>2(I)] 0.0549 0.0460

wR2 (all data) 0.1358 0.1299

96


2-9 and 2-12

2-9 2-12

formula C47.8H34.5BCl1.3F22N2P C49.6H37.3BCl1.3F22N2P

Mr [g mol1] 1140.37 1165.66

colour, habit colourless, block colourless, block

crystal system monoclinic tetragonal

Space group P21/c P41212

a [Å] 18.064(2) 18.00(1)

b [Å] 17.896(2) 18.00(1)

c [Å] 16.749(2) 30.61(1)

[°] 90 90

[°] 115.463(4) 90

[°] 90 90

V [Å3] 4888(1) 9913(4)

Z 4 8

T [K] 149(2) 149(2)


c [g cm3] 1.549 1.562

F(000) 2297 4706

min [°]

max [°]

1.69

27.2

1.31

27.56

Index range

23 h 23

23 k 20

21 l 21

23 h 23

19 k 23

28 l 39

[mm1] 0.246 0.245




Rint 0.0544 0.0975

reflection obs.

[F>3(F)] 6885 7585

residual density

[e Å3]

0.433

–0.620

0.663

–0.290

parameters 666 692

GOOF 1.031 0.974

R1 [I>2(I)] 0.0616 0.0560

wR2 (all data) 0.1846 0.1329

97


2-14 and 2-20

2-14 2-20

formula C26H30F3N2O5PS C81H103Ag3Cl5N6O0.50

P2

Mr [g mol1] 570.55 1731.49

colour, habit colourless, needle colourless, block

crystal system orthorhombic monoclinic

Space group P212121 C2/c

a [Å] 10.4215(11) 20.2421(7)

b [Å] 10.4310(13) 25.5536(9)

c [Å] 24.812(3) 31.5604(11)

[°] 90 90

[°] 90 94.9190(10)

[°] 90 90

V [Å3] 2697.2(6) 16264.8(10)

Z 4 8

T [K] 150(2) 150(2)

Crystal size [mm] 0.480 x 0.080 x 0.080 0.070 x 0.060 x 0.040

c [g cm3] 1.405 1.414

F(000) 1192 7128

min [°]

max [°]

1.641

27.608

2.57

27.46

Index range

–6 h 13

–13 k 13

–32 l 32

–26 h 25

–32 k 33

–41 l 25

[mm1] 0.240 0.965




Rint 0.0346 0.0340

reflection obs.

[F>2(F)] 5640 14516

residual density

[e Å3] 0.212 - –0.298 5.526 - –1.192

parameters 347 951

GOF 1.030 1.027

R1 [I>2(I)] 0.0297 0.0512

wR2 (all data) 0.0690 0.1465

98

Table 2-6 Crystallographic data and details of the structure refinements of compound

2-21

2-21

formula C39H46AgCl6F2N3P

Mr [g mol1] 946.33

colour, habit colourless, block

crystal system monoclinic

Space group P21/c

a [Å] 22.377(4)

b [Å] 10.6024(18)

c [Å] 18.089(3)

[°] 90

[°] 91.717(7)

[°] 90

V [Å3] 4289.6(12)

Z 4

T [K] 150(2)

Crystal size [mm] 0.270 x 0.120 x 0.060

c [g cm3] 1.465

F(000) 1932

min [°]

max [°]

2.42

27.56

Index range

–29 h 29

–13 k 13

–22 l 23

[mm1] 0.921

absorption correction SADABS

reflections collected 36624

reflections unique 9846

Rint 0.0386

reflection obs.

[F>2(F)] 7811

residual density

[e Å3] 1.734 - –1.253

parameters 486

GOF 1.045

R1 [I>2(I)] 0.0624

wR2 (all data) 0.1637

99

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(61) Bartsch, R.; Stelzer, O.; Schmutzler, R. Journal of Fluorine Chemistry 1982, 20,

85-88.

103

Chapter 3 Hydrosilylation of Olefins, Carbonyls & Amides

3.1 Introduction

3.1.1 History of Catalytic Hydrosilylation

Hydrosilylation is a reaction where a silicon–hydrogen bond is added across a site of unsaturation.

Hydrosilylation represents one of the most important reactions in silicon chemistry, as an efficient

method for the formation of organosilicon compounds. On an industrial scale olefin

hydrosilylation is employed to produce a variety of silicon-containing polymers, which can be

further employed in the preparation of oils, resins, and rubbers.1 The first report of hydrosilylation

was in 1947 by Sommer, wherein trichlorosilane was reacted with 1-octene in the presence of

peroxide.2 This reaction proceeded through a radical mechanism and showed low selectivity for

the hydrosilylated product. Later in 1957, Speier's eponymous catalyst proved to be a very efficient

homogenous hydrosilylation catalyst.3 Another breakthrough in this field came in 1973, when

Karstedt prepared platinum(0) complexes containing vinyl siloxane ligands, which showed

improved activity as well as high solubility in siloxane polymers.4 Despite their high costs, Speier,

Karstedt, and Declercq’s5 catalysts are widely employed in industrial processes due to their

remarkable activity (Figure 3-1). The prevalence of this late transition metal catalysis is apparent,

in effect nearly 6 tons of platinum is consumed on an annual basis.6 Although early research was

focused on platinum based systems, it has been found that earth metals, early transition metals,

and middle transition metals can act as effective catalysts for this reaction.7-12 For example, as a

viable alternative to Pt-based systems, Chirik, in 2012, reported Fe-based complexes that were

active for hydrosilylation catalysis (Figure 3-1).13

Figure 3-1 Metal based Hydrosilylation Catalysts

104

The first metal-free hydrosilylation catalysts were aluminium halides (e.g. Et2AlCl, EtAlCl2, and

AlCl3); however, these systems suffered from limited substrate and silane scope.14 In 1999,

Lambert reacted arene stabilized silylium cations, such as [Et3Si(tol)][B(C6F5)4], with 1,1-

diphenylethylene to generate β-silyl carbocations, which could subsequently react with silane to

give the hydrosilylated product.15 Furthermore, Piers et al. discovered the ability of B(C6F5)3 to

catalytically effect the hydrosilylation of aldehydes and ketones in 1996.16 Despite these efforts,

efficient metal free variants for hydrosilylation are still limited.


3.2.1 Hydrosilylation of Olefins

The fluorophosphonium cation 2-5, outlined in Chapter 2, was found to be stable in the presence

of excess triethylsilane for multiple days. The 1H NMR spectrum for this reaction revealed a

noticeable broadening of the hydride peak at 3.64 ppm, which would otherwise appear as a septet

(Figure 3-2), which suggests reversible coordination of the silane to the fluorophosphonium cation

through the hydride. We suspected that the elongation of this bond would activate the silane toward

addition across unsaturated moieties. This led to the study of 2-5 as a potential hydrosilylation

catalyst.

Figure 3-2 1H NMR Spectrum for the Reaction of 2-5 with an Excess of Triethylsilane

Through this investigation, it was found that within 14 to 24 h at 45 °C complete hydrosilylation

of 1-hexene, cis-2-hexene, cyclohexene, and 1-methylcyclohexene was observed in the presence

of Et3SiH and 2 mol% of 2-5 (Table 3-1) affording the corresponding hydrosilylation products in

105

high isolated yields (80-90%).17 In the case of 1-methylcyclohexene, exclusively anti 1,2-addition

of the Si−H bond to the olefin gives triethyl(cis-2-methylcyclohexyl)silane whereas norbornene is

selectively transformed to the exo-hydrosilylated product (Table 3-1; Entry 5) in a fashion similar

to other Lewis acid catalysts B(C6F5)3 18-20 or 2-2.21 Styrene substrates are also hydrosilylated to

the respective products as per Table 3-1, Entries 7 – 11. It is noteworthy, that silyl ether and chloro

functional groups are tolerated by this catalyst. In the case of p-methoxy-α-methylstyrene

conversion of the methoxy-group to the silyl ether functionalized product was also observed.

Finally, diphenylacetylene was selectively converted to the cis-substituted alkene (Table 3-1;

Entry 12).

Table 3-1 Catalytic Hydrosilylation of Olefins and Alkynes

Entry Substrate Time (h) Conversion

(%)

Product

1

19 >99 (89)

2

22 98 (90)

3

22 >99 (89)

4

22 >99 (80)

5

22 96 (88)

6

24 98 (94)

106

7

19 >99 (91)

8

4 >99

(91)

9

4 >99 (96)

10

24 99 (91)

11

5 99 (77)

12 24 96 (93)

[a] Conversions were determined by means of 1H NMR spectroscopy, isolated yields are given in

parenthesis. [b] Two equivalents Et3SiH were utilized.

3.2.2 Deoxygenation of Ketones

Hydrodeoxygenation of ketones has garnered increasing attention given its many applications in

biofuels and fine-chemical syntheses.22-26 Classic protocols for deoxygenation include the Barton-

McCombie (R3SnH),27 Clemmensen (Zn/Hg, HCl),28,29 or Wolff-Kishner reductions (N2H4,

KOH)30-33 and these methods generally require harsh reaction conditions, use stoichiometric

amounts of toxic reagents, and show poor functional group tolerance. Heterogeneous catalysis

employing PtO234 and Ni/Al2O3

35 and H2 as the reducing agent have been described, while a recent

report has described the use of Pd/C with polymethylhydrosiloxane (PMHS) for the reduction of

aromatic ketones.36

107

In initial efforts, 2-methylpentan-3-one or benzophenone and Et3SiH were reacted in the presence

of a catalytic amount of one of the Lewis acidic EPCs (3-1, 3-2, 2-1, 2-2, 2-5, 2-10, 2-13, 2-7)

(Figure 3-3).

Figure 3-3 Electrophilic Phosphonium Cations

It is noteworthy that hydrosilylation of ketones have been previously reported using the Lewis acid

catalyst B(C6F5)3.16 For comparative purposes, the substrates were treated with silane in the

presence of the catalyst B(C6F5)3. Quantitative hydrosilylation of 2-methylpentan-3-one yielding

exclusively a1 was observed (Table 3-2). In the case of benzophenone, 1.0 mol% of B(C6F5)3

effected hydrosilylation, while 5 mol% borane gave a 91 : 9 mixture of a2 and b2, and 10 mol%

catalyst gave an 80 : 20 mixture of these products (Table 3-2).

Using 1.0 mol% of the EPC catalyst 2-237 and 2.1 equivalents of Et3SiH in CD2Cl2, quantitative

deoxygenation of both substrates was observed yielding 2-methylpentane (b1) and Ph2CH2 (b2),

respectively. The reduction of the dialkyl ketone 2-methylpentan-3-one required heating to 50 °C

for 24 h while benzophenone is reduced after 5 h at room temperature (Table 3-2). The

corresponding reaction using 2-methylpentan-3-one and Et3SiH in 1 : 1 ratio gave selective

formation of the hydrosilylated intermediate a1, whereas a 1: 1 ratio of benzophenone and Et3SiH

gave a 1 : 1 mixture of starting substrate and the deoxygenated product b2.

The impact of reduced Lewis acidity of the EPC was also probed. Whereas 2-1 effects complete

deoxygenation of 2-methylpentan-3-one and benzophenone, the catalyst 3-2 effects only the

hydrosilylation of both substrates.37 Nonetheless, 3-2 does catalyze the reduction of

benzophenone, affording b2 in 85% yield. Further reduction of the Lewis acidity as in the

108

phosphonium ion [Ph3PF]+ (3-1)38 eliminated catalytic activity completely and neither

hydrosilylation nor deoxygenation was observed (Table 3-2).

The dicationic phosphonium ion discussed in Chapter 2, 2-539 was an effective catalyst for the

deoxygenation of 2-methylpentan-3-one and benzophenone in the presence of 2 equivalents of

silane, affording quantitative formation of b1 and b2, respectively. The alkyl-substituted dicationic

analogs [(SIMes)Me2PF]2+ (2-10) and [(SIMes)Et2PF]2+ (2-13), also discussed in Chapter 2,

proved to be effective catalysts for the deoxygenation of substrates 2-methylpentan-3-one and

benzophenone yielding the corresponding products b1 and b2 quantitatively. However, it is

interesting to note that the monocationic species [(SIMes)Ph2PO][B(C6F5)4] (2-7) is completely

inactive in catalysis.

Table 3-2 Deoxygenation / Hydrosilylation of Benzophenone and 2-methylpentan-3-

one

Substrate

Entry Catalyst (X)[a] T [oC] Conv. [%][b] T [oC] Conv. [%][b]

1 B(C6F5)3 (1.0) rt >99 (a1) rt >99 (a2)

2 B(C6F5)3 (5.0) 50 >99 (a1) 50 91 (a2) / 9 (b2)

3 B(C6F5)3 (10) 50 >99 (a1) 50 80 (a2) / 20 (b2)

4 3-1 (1.0) 50 0 rt 0

109

5 3-2 (1.0) 50 >99 (a1) 50 15 (a2) / 85 (b2)

6 2-1 (1.0) 50 >99 (b1) rt >99 (b2)

7 2-2 (1.0) 50 >99 (b1) rt >99 (b2)

8 2-5 (1.0) 50 >99 (b1) rt >99 (b2)

9 2-10 (1.0) 50 >99 (b1) rt >99 (b2)

10 2-13 (1.0) 50 >99 (b1) rt >99 (b2)

11 2-7 (1.0) 50 0 rt 0

[a] Et3SiH (2.1 eq.) was added to a solution of catalyst (1 mol%) in CD2Cl2 (0.7 mL) and then

substrate ( 0.39-0.11 mmol, 1.0 eq.). [b] Determined by 1H NMR spectroscopy.

These results establish that EPCs are highly effective Lewis acid catalysts for the deoxygenation

of ketones. To investigate the versatility of this reaction catalyst 2-5 was employed as a catalyst in

the deoxygenation of benzophenone and different silanes were screened (Table 3-3). It was found

that catalyst 2-5 in combination with triethylsilane, triphenylsilane, polymethylhydrosiloxane

(PMHS), triethoxysilane, 1,1,3,3-tetramethyldisiloxane, or triisopropylsilane effected the

deoxygenation of benzophenone within 5 h. It was noted that when PMHS was employed the

reaction mixture formed a gel, making it difficult to follow the reaction by 1H NMR spectroscopy.

The conversion to b2 could only be determined upon isolation of the product. The low conversion

observed from using triisopropylsilane is thought to be due to the increased steric bulk about the

hydride. Given the ability to follow reactions in situ and the relative low cost triethylsilane was

employed for substrate screening.

Table 3-3 Silane Screening for Deoxygenation of Benzophenone using 2-5

110

Silane Time (h) Conversion (%)[b]

Et3SiH 5 >99

Ph3SiH 5 >99

PMHS 5 98[c]

(EtO)3SiH 5 >99

Me2HSi-O-SiHMe2 5 >99

iPr3SiH 5 11

[a] R3SiH (2.1 eq.) was added to a solution of 2-5 (1 mol%) in CD2Cl2 (0.7 mL) and then

benzophenone ( 0.11 mmol, 1.0 eq.). [b] Determined by 1H NMR spectroscopy. [c] Isolated yield.

p-fluoro, p-bromo-, o-chloro-, and o-methyl-substituted benzophenone derivatives were converted

in high yields to the respective deoxygenated products (Table 3-4; Entry 1: a, b, c, d) by both

catalysts 2-2 and 2-5 within five hours at ambient temperature. The p-methoxy-substituted

benzophenone was selectively deoxygenated in the presence of 2.1 equivalents of Et3SiH yielding

1-benzyl-4-methoxybenzene. Using longer reaction times and 3.2 equivalents of silane, the

reduction proceeds further with methoxy-ether cleavage affording PhCH2(C6H4OSiEt3) (Table 3-

4; Entry 2: b). Acetophenone was quantitatively reduced to ethylbenzene, while the deactivated,

α-CF3-acetophenone derivative is selectively hydrosilylated by EPC catalysts 2-2 and 2-5 with

Et3SiH in 1 : 1 stoichiometry giving the corresponding silyl ether (Table 3-4; Entry 4: a) in high

yields. Interestingly, increasing the catalyst loading to 5 mol% and use of 7.0 equivalents of Et3SiH

effects hydrodefluorination37,39 yielding ethylbenzene in high yields. Finally, the ketone 1,2-bis(4-

methoxyphenyl)ethan-1-one is quantitatively converted to 1,2-bis(4-methoxyphenyl)ethane with

2.1 equivalents Et3SiH (Table 3-4; Entry 5). It is interesting to note that for ketones listed in Table

3-4, excluding α-CF3-acetophenone, reactions with Et3SiH in 1 : 1 stoichiometry gave 1 : 1

mixtures of starting material and deoxygenation products. The respective hydrosilylated

intermediates were not observed.

111

Table 3-4 Catalytic Deoxygenation of Aryl-substituted Ketones

Entry Substrate Cat. T[oC] t[h] Conv.[%] Product

1

2-2

2-5

2-2

2-5

2-2

2-5

2-2

2-5

rt

rt

rt

rt

rt

rt

rt

rt

5

5

5

5

5

5

5

5

84(a)

89(a)

>99(b)

>99(b)

>99(c)

>99(c)

>99(d)

88(d)

2

2-2

2-5

2-5

rt

rt

rt

5

5

12

71(a)

>99(73a)

>99(60b)[b]

3

2-2

2-5

rt

rt

5

5

>99

>99

4

2-2

2-5

2-5

rt

rt

rt

5

5

5

76(b)[c]

>99(74a)

81(b)[c]

112

5

2-2

2-5

rt

rt

5

5

>99

>99(35)

Conditions: Et3SiH (2.1 eq.) was added to a solution of catalyst (1 mol%) in CD2Cl2 (0.7 mL)

and then substrate (0.16-0.11 mmol, 1.0 eq.) was added. [a] Determined by 1H NMR

spectroscopy, isolated yields are given in parenthesis. [b] 3.2 eq. of Et3SiH were used. [c] 7 eq.

of Et3SiH and 5.0 mol% catalyst loading were used.

In contrast to aryl-substituted ketones, the dialkyl ketones, listed in Table 3-5, were selectively

hydrosilylated at room temperature after 1 h employing either 2-2 or 2-5 as the catalyst. Allowing

longer reaction times of 24 h and heating the reactions to 50 °C resulted in quantitative

deoxygenation. Ketones featuring one methyl substituent, such as 1-cyclohexylethan-1-one, 3-

methylbutan-2-one, and 3-methylpentan-2-one, were reduced to the corresponding alkanes. 1-

Acetyladamantane was reduced to 1-ethyladamantane as the minor product and the corresponding

rearranged product (Table 3-5; Entry 4: c) as the major species. The presence of a chloro

substituent on the α-carbon atom, as in chloroacetone, deactivates the carbonyl functionality and

allows only for hydrosilylation (Table 3-5; Entry 5: a), even upon heating and longer reaction

times. After 24 h, instead of deoxygenation formal elimination of Et3SiOH was observed with

substrates cyclohexanone and heptan-4-one, yielding the alkenes cyclohexene and hept-3-ene. For

dicyclohexylketone both the deoxygenated product and the elimination product were observed

(Table 3-5; Entry 8).

Table 3-5 Catalytic Deoxygenation of Alkyl-substituted Ketones

Entry Substrate Cat. Eq. T[oC] t

[h]

Conv.[%][a] Product(s)

1

2-2

2-2

2-5

3.1

3.1

3.1

rt

50

50

1

24

24

>99(84a)

96(b)

>99(b)

113

2

2-2

2-5

2-5

3.1

3.1

3.1

50

rt

rt

24

1

24

91(b)

>99(a)

>99(b)

3

2-2

2-2

2-5

3.1

3.1

3.1

rt

rt

50

1

24

1

>99(77a)

96(b)

>99(b)

4

2-2

2-5

2-5

3.1

3.1

3.1

50

rt

50

24

1

24

20(b)/80(c)

>99(72a)

10(b)/ 90(c)

5

2-2

2-5

3.1

3.1

50

rt

24

24

>99(a)

>99(a)

6

2-2

2-5

2-5

2.1

2.1

2.1

rt

rt

rt

24

1

24

76(a)/20(b)

>99(84a)

78(a)/16(b)

7

2-2

2-2

2-5

1.1

3.1

3.1

rt

rt

rt

24

1

1

>99(83a)

43(a)/57(b)

95(a)/5(b)

8

2-2

2-5

2-5

3.1

3.1

3,1

50

rt

50

24

1

24

81(a)/19(c)

89(a)/11(c)

72(b)/28(c)

114

Conditions: Et3SiH (1.1 to 3.1 eq.) was added to a solution of catalyst (1.0 mol%) in CD2Cl2 (0.7

mL) and then substrate (0.16-0.11 mmol, 1.0 eq.) was added. [a] Determined by 1H NMR

spectroscopy, isolated yields are given in parenthesis.

The hydrosilylation of alkenes, alkynes, imines, and nitriles catalyzed by 2-2 has been previously

reported and computational and experimental data supported a mechanism involving interaction

of the EPC with silane (Figure 3-4, (A)).37,40,41 This hydrosilylation mechanism is mechanistically

analogous to long-established B(C6F5)3 mediated hydrosilylation of carbonyl compounds

established by Piers16,42,43 and Oestreich.18,44,45 To gain insight into the mechanism for the

subsequent deoxygenations, DFT computational were performed by the Grimme group.

Two pathways for deoxygenation were considered (Figure 3-4). One possible mechanism involves

the SN2-like nucleophilic attack by the silyl ether oxygen at the Si-centre of the silane-2-2 adduct

A. This reaction generates (C6F5)3PFH 2-2H and [((Et3Si)2OCHPhMe]+ (C)). Subsequent hydride

delivery from 2-2H prompts elimination of (Et3Si)2O and gives ethylbenzene regenerating 2-2.

This mechanism is exergonic by 5.4 kcal mol-1 with a free energy barrier of 23.8 kcal mol-1.

An alternative pathway involves reaction of the transiently generated [((Et3Si)OCHPhMe]+ (B)

with silane to generate (C) which again abstracts hydride from 2-2H to yield the product alkane,

siloxane, and 2-2. Calculations predict the transformation of (B) to (C) is exergonic by 6.7 kcal

mol-1 with an activation barrier of 17.7 kcal mol-1 (Figure 3-4). By these computations, the latter

pathway is energetically favorable, however, it may well be that variations in the catalyst,

substrate, and silane could reverse the energetic preference and thus either reaction mechanism

may be operative.

115

Figure 3-4 Mechanism for Deoxygenation of Ethylbenzene with 2-2

It is noteworthy that in closely related chemistry, B(C6F5)3 has been shown to catalyze the

deoxygenation of unsaturated cyclic ethers and silyl ethers46-51 affording allylic and homoallylic

alcohols52 and siloxanes, respectively.53 In addition B(C6F5)3 has also been used to dehydrocouple

silanes with alcohols to give silylethers.46-48,54

3.2.3 Reduction of Amides

Amines are a fundamental group of compounds in both chemistry and biology.55,56 They are a key

functional group used by the pharmaceutical industry for pesticides and drug development.57 As

well as their increasing use for the preparation of advanced materials.58-60 Reduction of amides to

amines is an attractive strategy for amine synthesis given the large variety of commercially

available amides.61,62 The most common method for amide reduction requires the use of

stoichiometric aluminium and boron hydrides.63-68 In spite of their wide spread use, such methods

suffer from the production of stoichiometric by-products, over reduction, and challenging

purifications. As a result, there has been increasing interest in catalytic transformations of amides

to amines. The direct hydrogenation of amides has been described by Cole-Hamilton and

coworkers using ruthenium catalysts at temperatures above 160 °C and 40 bar of H2.69,70

116

Subsequent studies by the groups of Breit and Thompson explored mixed metal heterogeneous

catalysts for such hydrogenations.71,72 Using an alternative approach, a series of studies have

explored transition metal catalysts incorporating Mo, Rh, Ru, Pt, Pd, and Ir for reduction of amides

to amines via hydrosilylation.73-84

Metal-free approaches to amide reduction have begun to garner some recent attention. To this end,

the Adronov group used B(C6F5)3 to effect catalytic hydrosilylation of tertiary amides.85 The

Huang group was able to expand this scope to secondary amides with a protocol using a triflic

anhydride pre-treatment of the amides.60 However, both methods suffer from a limited substrate

scope.

Attempts to employ 3 mol% of the EPCs 2-1, 2-2, and 2-5 as catalysts for the hydrosilylative

reduction of N,N-dimethylbenzamide in the presence of 2.1 equivalents of a series of silanes at

100 °C were undertaken (Table 3-6). In general the observed reactivity trends for each silane

parallels the increasing Lewis acidity of the EPCs, 2-1 < 2-2 < 2-5. Conversion to the amine was

enhanced with increasing hydricity of the silane employed. Thus, low conversions were generally

observed with Et3SiH and Ph3SiH, while essentially quantitative conversion was seen for each

catalyst with the use of PhSiH3.

Table 3-6 Reduction of N,N-dimethylbenzamide with Catalyst 2-1, 2-2, 2-5

Entry Catalyst Silane Conv. [%]

1 B(C6F5)3 Et3SiH 39

2 B(C6F5)3 Ph3SiH 4

3 B(C6F5)3 Ph2SiH2 90

4 B(C6F5)3 HSiMe(OEt)2 60

117

5 B(C6F5)3 PhSiH3 100

6 2-1 Et3SiH 0

7 2-1 Ph3SiH 0

8 2-1 Ph2SiH2 9

9 2-1 HSiMe(OEt)2 5

10 2-1 (HSiMe2)2O 65

11 2-1 PhSiH3 98

12 2-2 Et3SiH 9

13 2-2 Ph3SiH 0

14 2-2 Ph2SiH2 26

15 2-2 HSiMe(OEt)2 55

16 2-2 (HSiMe2)2O 89

17 2-2 PhSiH3 99

18 2-5 Et3SiH 11

19 2-5 Ph3SiH 6

20 2-5 Ph2SiH2 45

21 2-5 HSiMe(OEt)2 12

22 2-5 (HSiMe2)2O 92

23 2-5 PhSiH3 98

118

Conditions: R3SiH (2.1 eq.) was added to a solution of catalyst (3.0 mol%) in C6D5Br (0.7 mL)

and then substrate (0.39-0.11 mmol, 1.0 eq.). [a] Determined by 1H NMR spectroscopy.

With the optimal silane established, the substrate scope of the reduction of tertiary amides to

amines was probed using 2-2 and 2-5 as the catalysts (Table 3-7). High conversions of the amide

to the amine were observed for the N,N-dimethyl p-Cl, p-Br, and o-methoxy-benzamides and

pyrrole-p-methyl-benzamide with no significant difference in activity between catalyst 2-2 and 2-

5. N,N-dimethyl-p-nitro- benzamide was reduced to the corresponding amine in 78% and 74%

conversions using 2-2 and 2-5 as the catalyst, respectively. Similarly, N-benzyl-N-ethyl-4-

methoxybenzamide was reduced in 88% and 87% to the corresponding amine using 2-2 and 2-3

as the catalyst, respectively.

In a similar fashion, the aliphatic amide N,N-diethylpropionamide as well as N-phenyl-benzamide,

N-p- bromophenylbenzamide and N-p-nitrophenyl-benzamide, were reduced by both catalyst 2-2

and 2-5 in high conversions. No conversion was observed for N,N-dimethyl-p-aminobenzamide,

benzamide, N-methylbenzamide or N-benzylbenzamide. These latter cases suggest that more basic

amides may inhibit the reduction. This view is consistent with stoichiometric reactions of 2-5 with

N-phenylbenzamide which show a shift in the 31P NMR spectrum from 78.1 ppm to 68.7 ppm.

Similarly, addition of N-benzylbenzamide to 2-5 showed a 31P resonance at 36.0 ppm. In both

cases the P–F coupling constant is decreased, consistent with coordination of the amide to the

Lewis acid P centre in 2-5.

Table 3-7 Amide Reductions Using Catalysts 2-2 and 2-3

Entry Substrate Catalyst Conv. [%] Product

1

2-2

2-5

>99

>99 (72)

119

2

2-2

2-5

>99

98 (96)

3

2-2

2-5

>99

>99 (46)

4

2-2

2-5

78

74

5

2-2

2-5

>99

>99 (74)

6

2-2

2-5

88

87

7

2-2

2-5

>99

87

8

2-2

2-5

>99

94

9

2-2

2-5

>99

>99

10

2-2

2-5

>99

>99

120

11

2-2

2-5

0

0

12

2-2

2-5

0

0

13

2-2

2-5

0

0

14

2-2

2-5

0

0

Conditions: PhSiH3 (2.1 eq.) was added to a solution of catalyst (3.0 mol%) in C6D5Br (0.7 mL)

and then substrate (0.16-0.11 mmol, 1.0 eq.) was added. [a] Determined by 1H NMR

spectroscopy, isolated yields are given in parenthesis.

The scope of this reaction was increased to include the reduction of 2,2,2-

trifluoroacetamides, as a facile route to 2,2,2-trifluoroethyl substituted amines and anilines.

However, this work was carried out by MSc graduate Alessandra Augurusa and is outside

the scope of this thesis.

This amide reduction is thought to proceed via an FLP-type mechanism involving the action

of the Lewis acid and the amide on the Si–H fragment. Initial interaction of silane by the

Lewis acidic P centre is thought to weaken the Si–H bond and facilitate nucleophilic attack

at the Si centre by the amide. The transient R2NCH(OSiR’3)R” is thought to react further

with silane and liberate silyl ether and the amine, regenerating the phosphonium salt for

further catalysis (Figure 3-5). This mechanism is directly analogous to that discussed for

the EPC mediated reduction of ketones discussed in Section 3.2.2.86

121

Figure 3-5 Proposed Mechanism for Amide Reduction

3.3 Conclusion

In summary, the highly electrophilic phosphonium dication 2-5, discussed in Chapter 2, was found

to be an effective catalyst for the hydrosilylation of alkenes and alkynes. Furthermore, dicationic

species 2-10 and 2-13 were investigated for the deoxygenation of ketones via hydrosilylation,

alongside catalyst 2-5 and previously established 2-2. In general, it was found that high

electrophilicity was a prerequisite for catalyst activity toward deoxygenation, noted by the

significant decrease in reactivity observed for [Ph3PF][B(C6F5)4] (3-1) and B(C6F5)3. While

establishing a broad substrate scope, different reactivity was observed aliphatic and aryl-

substituted ketones. Calculations and further mechanistic investigation allowed for the conception

of two possible catalytic pathways.

Finally the catalytic reduction of amides was undertaken. Catalysts 2-5 and 2-2 under high

temperatures and in the presence of PhSiH3 were shown to be effective catalyst for this

transformation. Reductions were limited to tertiary amides or secondary anilines. Moving toward

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less sterically-encumbered or more basic amides inhibited the catalytic cycle by competing with

silane for donation to the σ* orbital on phosphorus.

The reduction of ketones and amides are thought to undergo a similar mechanistic pathway.

Notably, no significant difference in Lewis acidity was observed between salts 2-5, 2-10, and 2-

13.

123


General Remarks

All manipulations were performed in a Glove box MB Unilab produced by MBraun or using

standard Schlenk techniques87 under an inert atmosphere of anhydrous N2. Dry, oxygen-free



molecular sieves (4 Å) prior to use. Deuterated benzene (C6D6) and D8-thf were purchased from

Sigma-Aldrich, distilled from sodium and stored over molecular sieves (4 Å) for at least two days

prior to use. Deuterated dichloromethane (CD2Cl2) and bromobenzene (C6D5Br) were purchased

from Sigma-Aldrich, distilled from CaH2 and stored over molecular sieves (4 Å) for at least two

days prior to use. Reagents such as silanes, 1,4-bis(difluoromethyl)benzene, perfluorotoluene, hex-

1-ene, (E)-hex-2-ene, cyclohexene, 1-methylcyclohex-1-ene, norbornene, 1,1-diphenylethylene,

prop-1-en-2-ylbenzene, 1-methyl-4-(prop-1-en-2-yl)benzene, 1-chloro-4-(prop-1-en-2-

yl)benzene, 1-methoxy-4-(prop-1-en-2-yl)benzene, 1,2-diphenylethyne, and all ketones were

purchased either from Sigma-Aldrich, Strem Chemicals or Alfa Aesar and, if applicable, distilled

prior to use. Reagents such as tertbutyl(dec-9-enyloxy)diphenylsilane,21 and amides88,89 not

commercially available were prepared according to literature known procedures. All glassware

was oven-dried at temperatures above 180°C prior to use. NMR spectra were obtained on an

Agilent DD2-700 MHz, an Agilent DD2-500 MHz, a Bruker AvanceIII-400 MHz, or a Varian

Mercury-300 MHz spectrometer. All 13C NMR spectra were exclusively recorded with composite

pulse decoupling. Assignments of the carbon atoms in the 13C spectra were performed via indirect

deduction from the cross-peaks in 2D correlation experiments (HMBC; HSQC). Chemical shifts

were referenced to δTMS = 0.00 ppm (1H, 13C) and δH3PO4(85%) = 0.00 ppm (31P, externally).

Chemical shifts (δ) are reported in ppm, multiplicity is reported as follows (s = singlet, d = doublet,

t = triplet, quart. = quartet, m = multiplet) and coupling constants (J) are reported in Hz.

Assignments of individual resonances were done using 2D techniques (HMBC, HSQC, HH-

COSY) when necessary. Yields of products in solution were determined by integration of all

resonances observed in the respective NMR spectra if not stated otherwise. High-resolution mass

spectra (HRMS) were obtained on a micro mass 70S-250 spectrometer (EI), an ABI/Sciex QStar

Mass Spectrometer (DART), or on a JOEL AccuTOF-DART (DART). Elemental analyses (C, H,

124

N) were performed at the University of Toronto employing a Perkin Elmer 2400 Series II CHNS

Analyzer.

General Procedure for Olefin Hydrosilylation Catalysis

All reactions were carried out under identical conditions on a 0.1 - 0.2 mmol scale. In a glove box,

compound 2-5 (2 mol%) was added to a solution of Et3SiH in CD2Cl2 (0.7 mL) followed by the

respective substrate. The reaction mixture was transferred to a J-young NMR tube, sealed and

investigated by 1H NMR spectroscopy. The J-young NMR tube was kept in an oil bath at 45 °C

for the respective reaction time and the reaction progress was monitored by 1H NMR spectroscopy.

In selected cases the reaction progress was also monitored by 31P and 19F NMR spectroscopy. In

some cases, toluene (10.6 μL, 0.1 mmol) was added as an internal standard to allow for the

determination of substrate conversion. After the reaction was completed, all volatiles were

carefully removed in vacuo. The residue was suspended in n-pentane (3 mL) and remaining solid

was removed by filtration. Evaporation of all volatiles gave the respective products as colorless

oils. The reported isolated yields are not optimized. The formation of the respective products was

confirmed by 1H and 13C{1H}x NMR spectroscopic investigations (C6D6).

Triethyl(hexyl)silane

Isolated Yield = 89 %. 1H NMR (400 MHz, C6D6): δ = 0.51 - 0.57 (8H, m., SiCH2), 0.98 (3H, t,

3JHH = 6.9 Hz, CH3), 0.99 (9H, t, 3JHH = 8.1 Hz, SiCH2CH3), 1.27 - 1.38 (8H, m, CH2); 13C{1H}

(101 MHz, C6D6): δ = 3.6 (s, SiCH2CH3), 7.7 (s, SiCH2CH3), 11.6 (s, C6H13), 14.3 (s, C6H13), 23.0

(s, C6H13), 24.2 (s, C6H13), 31.9 (s, C6H13), 33.9 (s, C6H13) ppm.

Triethyl(hexan-2-yl)silane

125

Isolated Yield = 90 %. 1H NMR (400 MHz, C6D6): δ = 0.51 - 0.62 (6H, m, SiCH2CH3),

0.73 - 0.85 (1H, m, SiCH), 0.86 - 1.04 (3H, m, CH3), 0.93 (3H, t, 3JHH = 7.1 Hz, CH3), 1.00 (9H,

t, 3JHH = 8.1 Hz, SiCH2CH3), 1.12 - 1.58 (6H, m, CH2); 13C{1H} (101 MHz, C6D6): δ = 2.7 (s,

SiCH2CH3), 8.0 (s, SiCH2CH3), 14.4 (s, hexan-2-yl), 14.5 (s, hexan-2-yl), 17.0 (s, hexan-2-yl),

23.2 (s, hexan-2-yl), 31.4 (s, hexan-2-yl), 32.1 (s, hexan-2-yl) ppm.

Triethyl(cyclohexyl)silane

Isolated Yield = 89 %. 1H NMR (400 MHz, C6D6): δ = 0.53 (6H, m, SiCH2), 0.68 - 0.77 (1H, m,

SiCH), 0.99 (9H, t, 3JHH = 8.0 Hz, CH3), 1.11 - 1.27 (5H, m, CH2), 1.63 - 1.76 (5H, m, CH2);

13C{1H} (101 MHz, C6D6): δ = 2.4 (s, SiCH2), 8.0 (s, CH3), 24.0 (s, C6H11), 27.5 (s, C6H11), 28.3

(s, C6H11), 28.7 (s, C6H11) ppm..

Triethyl(cis-2-methylcyclohexyl)silane

Isolated Yield = 80 %. 1H NMR (400 MHz, C6D6): δ = 0.52 - 0.60 (6H, m, SiCH2), 0.95 - 1.03

(1H, m, C6H10), 0.97 (3H, d, 3JHH = 7.2 Hz, Me), 0.99 (9H, t, 3JHH = 8.0 Hz, SiCH2CH3),

1.16 - 1.28 (1H, m, C6H10), 1.39 - 1.55 (6H, m, C6H10), 1.66 - 1.74 (1H, m, C6H10), 1.95 - 2.03

(1H, m, C6H10); 13C{1H} (101 MHz, C6D6): δ = 3.4 (s, SiCH2), 8.0 (s, SiCH2CH3), 16.6 (s, C7H13),

21.4 (s, C7H13), 22.6 (s, C7H13), 28.3 (s, C7H13), 28.9 (s, C7H13), 29.5 (s, C7H13), 35.5 (s, C7H13)

ppm.

The conformation was confirmed by comparing to previous reports of this compound and related

compounds.21,90

Bicyclo[2.2.1]heptan-2-yltriethylsilane

126


0.60 - 0.66 (1H, m, SiCH), 0.99 (9H, t, 3JHH = 8.0 Hz, SiCH2CH3), 1.10 - 1.14 (1H, m, CH2),

1.17 - 1.25 (3H, m, CH2), 1.32 - 1.39 (1H, m, CH2), 1.41 - 1.49 (1H, m, CH2), 1.50 - 1.55 (2H, m,

CH2), 2.15 - 2.25 (2H, m, CH); 13C{1H} (101 MHz, C6D6): δ = 3.2 (s, SiCH2CH3), 8.1 (s,

SiCH2CH3), 26.6 (s, SiCH), 29.4 (s, CH2), 33.1 (s, CH2), 34.7 (s, CH2), 37.7 (s, CH), 38.4 (s, CH),

38.6 (s, CH2) ppm.

Tert-butyldiphenyl(10-(triethylsilyl)decyloxy)silane

Isolated Yield = 94 %. 1H NMR (400 MHz, C6D6): δ = 0.55 (6H, quart, 3JHH = 8.0 Hz,

SiCH2CH3), 0.54 - 0.65 (2H, m, SiCH2CH2), 1.00 (9H, t, 3JHH = 8.0 Hz, SiCH2CH3), 1.19 (9H, s,

t-Bu), 1.22 - 1.42 (14H, m, CH2), 0.53 - 0.65 (2H, m, OCH2CH2), 3.69 (2H, t, 3JHH = 6.6 Hz,

OCH2CH2), 7.22 - 7.27 (6H, m, Ph), 7.76 - 7.81 (4H, m, Ph); 13C{1H} (101 MHz, C6D6): δ = 3.8

(s, SiCH2CH3), 7.8 (s, SiCH2CH3), 11.8 (s, SiCH2CH2), 19.6 (s, SiCCH3), 24.2 (s, CH2), 26.3 (s,

CH2), 27.2 (s, SiCCH3), 29.9 (s, CH2), 29.9 (s, CH2), 30.1 (s, CH2), 30.2 (s, CH2), 33.1 (s,

OCH2CH2), 34.5 (s, CH2), 64.4 (s, OCH2CH2), 128.1 (s, Ph), 129.9 (s, Ph), 134.6 (s, Ph), 136.1 (s,

Ph) ppm.

(2,2-Diphenylethyl)triethylsilane

Isolated Yield = 91 %. 1H NMR (400 MHz, C6D6): δ = 0.38 (6H, m, SiCH2CH3), 0.87 (9H, t,

3JHH = 8.0 Hz, SiCH2CH3), 1.39 (2H, d, 3JHH = 7.9 Hz, CH2), 4.07 (1H, t, 3JHH = 7.9 Hz, CH),

6.94 - 7.04 (2H, m, Ph), 7.09 - 7.15 (4H, m, Ph), 7.21 - 7.25 (4H, m, Ph); 13C{1H} (101 MHz,

C6D6): δ = 3.9 (s, SiCH2CH3), 7.7 (s, SiCH2CH3), 19.4 (s, CH2), 47.6 (s, CH), 126.3 (s, Ph), 127.9

(s, Ph), 128.7 (s, Ph), 147.7 (2s, Ph) ppm.

127

Triethyl(2-phenylpropyl)silane

Isolated Yield = 91 %. 1H NMR (400 MHz, C6D6): δ = 0.30 - 0.46 (6H, m, SiCH2CH3), 0.82 (1H,

dd, 2JHH = 14.8 Hz, 3JHH = 8.4 Hz, CH2), 0.87 (9H, t, 3JHH = 7.9 Hz, SiCH2CH3), 0.95 (1H, dd,

2JHH = 14.8 Hz, 3JHH = 8.4 Hz, CH2), 1.22 (3H, d, 3JHH = 6.8 Hz, CH3), 2.74 - 2.83 (1H, m, CH),

7.01 - 7.06 (1H, m, Ph), 7.08 - 7.17 (4H, m, Ph); 13C{1H} (101 MHz, C6D6): δ = 3.9 (s,

SiCH2CH3), 7.6 (s, SiCH2CH3), 21.7 (s, CH2), 26.9 (s, CH3), 36.5 (s, CH), 126.1 (s, Ph), 126.8 (s,

Ph), 128.5 (s, Ph), 149.9 (s, Ph) ppm.

Triethyl(2-p-tolylpropyl)silane


0.86 - 1.04 (2H, m, CH2), 0.92 (9H, t, 3JHH = 8.0 Hz, SiCH2CH3), 1.28 (3H, d, 3JHH = 6.9 Hz, CH3),

2.16 (3H, s, p-Me), 2.79 - 2.89 (1H, m, CH), 6.97 - 7.10 (4H, m, ArH); 13C{1H} (101 MHz, C6D6):

δ = 4.0 (s, SiCH2CH3), 7.6 (s, SiCH2CH3), 20.9 (s, p-Me) 21.7 (s, CH2) 27.2 (s, CH3), 36.1 (s, CH),

126.7 (s, p-tol), 129.2 (s, p-tol), 135.2 (s, p-tol), 146.9 (s, p-tol) ppm.

(2-(4-chlorophenyl)propyl)triethylsilane


0.71 - 0.85 (2H, m, CH2), 0.88 (9H, t, 3JHH = 7.9 Hz, SiCH2CH3), 1.11 (3H, d, 3JHH = 6.9 Hz, CH3),

2.61 - 2.71 (1H, m, CH), 6.79 - 6.84 (2H, m, Ar), 7.11 - 7.15 (2H, m, Ar); 13C{1H} (101 MHz,

128

C6D6, 26 °C): δ = 4.1 (s, SiCH2CH3), 7.7 (s, SiCH2CH3), 21.7 (s, CH2) 26.9 (s, CH3), 36.0 (s, CH),

128.4 (s, p-tol), 128.8 (s, p-tol), 131.8 (s, p-tol), 148.6 (s, p-tol) ppm.

Triethyl(4-(1-(triethylsilyl)propan-2-yl)phenoxy)silane

Isolated Yield = 77 %. 1H NMR (400 MHz, C6D6): δ = 0.34 - 0.49 (6H, m, SiCH2CH3), 0.70 (6H,

quart., 3JHH = 7.6 Hz, p-SiCH2CH3), 0.83 - 0.94 (2H, m, CH2), 0.91 (9H, t, 3JHH = 8.0 Hz,

SiCH2CH3), 1.00 (9H, t, 3JHH = 7.6 Hz, p-SiCH2CH3), 1.26 (3H, d, 3JHH = 6.9 Hz, CH3),

2.76 - 2.85 (1H, m, CH), 6.86 - 6.92 (2H, m, Ar), 7.00 - 7.06 (2H, m, Ar); 13C{1H} (101 MHz,

C6D6): δ = 4.1 (s, SiCH2CH3), 5.5 (s, SiCH2CH3), 7.0 (s, SiCH2CH3), 7.8 (s, SiCH2CH3), 22.2 (s,

CH2) 27.5 (s, CH3), 36.0 (s, CH), 120.1 (s, Ar), 128.0 (s, Ar), 142.8 (s, Ar), 154.3 (s, Ar) ppm.

(Z)-(1,2-Diphenylvinyl)triethylsilane

Isolated Yield = 93 %. 1H NMR (400 MHz, C6D6): δ = 0.34 (6H, quart., 3JHH = 8.0 Hz,

SiCH2CH3), 0.68 (9H, t, 3JHH = 8.0 Hz, SiCH2CH3), 6.84 - 7.24 (11H, m, Ph, CH); 13C{1H} (101

MHz, C6D6): δ = 5.2 (s, SiCH2CH3), 7.9 (s, SiCH2CH3), 126.1 (s, Ph), 127.6 (s, Ph), 127.8 (s, Ph),

128.1 (s, Ph), 128.3 (s, Ph), 128.7 (s, Ph), 140.3 (s, CHCSi), 145.6 (s, Ph), 146.9 (s, CHCSi), 147.9

(s, Ph) ppm.

General Procedure for Ketone Hydrosilylation


the respective catalyst (1 mol%, 2-2: 2 mg, 2-5: 2 mg, 2-10: 2 mg, 2-13: 2 mg) was added to a

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solution of Et3SiH in CD2Cl2 (0.7 mL). The respective substrate (2-2: 0.16 mmol , 2-5: 0.11 mmol,

2-10: 0.11 mmol, 2-13: 0.11 mmol) was then added. The reaction mixture was transferred to a

NMR tube, sealed and monitored by 1H NMR, 13C{1H} NMR and 29Si NMR spectroscopy. For

aliphatic ketone substrates the hydrosilylated intermediates were obtained by using 3 equivalents

of Et3SiH and leaving the reaction at room temperature for 1 h. The hydrosilylated products were

isolated by removing the excess Et3SiH and solvent under reduced pressure, dissolving in n-

pentane and filtration over a silica plug.

General Procedure for Ketone Deoxygenation


the respective catalyst (1 mol%, 2-2: 2 mg, 2-5: 2 mg, 2-10: 2 mg, 2-13: 2 mg) was added to a

solution of Et3SiH (0.23 – 0.48 mmol, 2.1 – 3.0 eq.) in CD2Cl2 (0.7 mL). The respective substrate

(2-2: 0.16 mmol , 2-5: 0.11 mmol, 2-10: 0.11 mmol, 2-13: 0.11 mmol) was then added. The

reaction mixture was transferred to a NMR tube, sealed and monitored by 1H NMR and 13C NMR

spectroscopy. The generation of bis(triethylsilyl) ether was identified by a 29Si NMR resonance at

8.88 ppm. When the deoxygenation product had a low boiling point, an internal standard of toluene

was added to determine degree of conversion. For less volatile products, the desired products were

isolated by removing the di(triethylsilyl) ether by-product in vacuo. To avoid loss of product under

reduced pressure the solution was occasionally monitored by 1H NMR. Upon completion of

catalysis, 31P NMR spectroscopy reveals that the [(SIMes)PFPh2][B(C6F5)4]2 (2-5) catalyst

decomposes to [(SIMes)POPh2][B(C6F5)4] (2-7), observed as a peak at 14.1 ppm, and

[(SIMes)PF2Ph2][B(C6F5)] (2-4), observed as a peak as a triplet at –62.9 ppm (1JPF = 735 Hz).

Catalyst [(C6F5)3PF][B(C6F5)4] (2-2) decomposes to [OPC6F5)3], and [C6F5)3PF2] observed in the

31P NMR spectrum as a singlet at –8.29 and a triplet –47.99 ppm consecutively. Routinely,

catalytic runs were monitored by 11B and 19F NMR spectroscopy. According to the NMR data,

decomposition of borate anion does not occur for any catalysts used.

Ketone Deoxygenation Test with B(C6F5)3

Catalyst B(C6F5)3 (1 mol%, 2 mg) was dissolved in CD2Cl2 (0.7 mL). Triethylsilane (125 µL,

0.82 mmol, 2.1 eq.) was added with a micro-syringe. Benzophenone (71 mg, 0.39 mmol, 1.0 eq.)

130

was added and the reaction mixture was monitored by 1H NMR spectroscopy for 24 h. For higher

catalyst loadings of B(C6F5)3 the same protocol was used.

Ketone Deoxygenation Test with [FPPh3][B(C6F5)4] (3-1)

Catalyst [Ph3PF][B(C6F5)4] (3-1) (1 mol%, 2 mg) was dissolved in CD2Cl2 (0.7 mL).

Triethylsilane (67 µL, 0.44 mmol, 2.0 eq.) was added with a micro-syringe. Benzophenone

(38 mg, 0.21 mmol, 1.0 eq.) was added and the reaction mixture was monitored by 1H NMR

spectroscopy for 24 h.

Ketone Deoxygenation Test with [(C6F5)PFPh2][B(C6F5)4] (3-2)

Catalyst [(SIMes)P(O)Ph2][B(C6F5)4] (3-2) (1 mol%, 2 mg) was dissolved in CD2Cl2 (0.7 mL).




Ketone Deoxygenation Test with [(C6F5)2PFPh][B(C6F5)4] (2-1)

Catalyst [(C6F5)2PPh][B(C6F5)4] (2-1) (1 mol%, 2 mg) was dissolved in CD2Cl2 (0.7 mL).




Ketone Deoxygenation Test with [(SIMes)P(O)Ph2][B(C6F5)4] (2-7)

Catalyst [(SIMes)P(O)Ph2][B(C6F5)4] (2-7) (1mol%, 2 mg) was dissolved in CD2Cl2 (0.7 mL).




General Procedure for Silane Screening

To a solution of catalyst 2-5 (2 mg) and 2.1 equivalents of silane (0.23 mmol) in CD2Cl2

benzophenone (0.11 mmol) was added and the reaction monitored by 1H NMR and 13C{1H} NMR

spectroscopy.

131

Characterization Data for Hydrosilylation and deoxygenation Products of Aromatic

Ketones

Triethyl((2-methylpentan-3-yl)oxy)silane

1H NMR (400 MHz, CD2Cl2): δ = 3.41 (1H, quart, 3JHH = 4Hz, SiOCHC), 1.71 ( 1H, m,

(CH3)2CH), 1.42 (2H, m, CH3CH2CHO), 0.97 (9H, m, 3JHH = 8.1 Hz, CH3), 0.86 (9H, m,

SiCH2CH3), 0.61 (6H, quart, 3JHH = 8.1 Hz, SiCH2); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 78.7

(s, CHO), 33.0 (s, CH(CH3)2), 26.7 (s, CH2), 18.6 (s, CH3), 18.0 (s, CH3), 10.2 (s, CH3), 7.4 (s,

SiCH2), 5.8 (s, SiCH2CH3); 29Si NMR (119 MHz, CD2Cl2): δ = 15.2 ppm.

2-Methylpentane

1H NMR (400 MHz, CD2Cl2): δ = 1.55 (1H, m, CH(CH3)2); 1.30 (2H, m, CH2); 1.17 (2H, m,

CH2), 0.88 (6H, d, 3JHH = 6.8 Hz, CH(CH3)2); 0.87 (3H, m, CH3); 13C{1H} NMR (101 MHz,

CD2Cl2): δ = 42.0 (s, CH(CH3)2), 28.3 (s, CH2), 23.0 (s, CH2), 21.8 (s, CH3), 14.7 (s, CH3) ppm.

(Diphenylmethoxy)triethylsilane

1H NMR (400 MHz, CD2Cl2): δ = 7.45 (4H, m, Ph), 7.35 (4H, m, Ph), 7.26 (2H, m, Ph), 5.85

(1H, s, PhCHPh), 0.96 (9H, t, 3JHH = 8.3 Hz, SiCH2CH3), 0.66 (6H, quart, 3JHH = 8.3 Hz, SiCH2);

13C{1H} NMR (101 MHz, CD2Cl2): δ = 146.1 (s, Ph), 128.8 (s, Ph), 127.6 (s, Ph), 126.8 (s, Ph),

132

77.0 (s, PhCHPh), 7.2 (s, SiCH2), 5.5 (s, SiCH2CH3); 29Si NMR (126 MHz, CD2Cl2): δ = 20.1 (s)

ppm. DART MS: m/z: 316.21017 (calcd. for [M]NH4+: 316.20967).

Diphenylmethane

1H NMR (500 MHz, CD2Cl2): δ = 7.29 (4H, m, Ph), 7.20 (6H, m, Ph), 3.98 (2H, s, PhCH2Ph);

13C{1H} NMR (126 MHz, CD2Cl2): δ = 141.4 (s, Ph), 128.8 (s, Ph), 128.4 (s, Ph), 126.0 (s, Ph),

41.9 (s, PhCH2Ph) ppm. DART MS: m/z: 186.12872 (calcd. for [M]NH4+: 186.12827).

1-Benzyl-4-fluorobenzene


(2H, s, PhCH2Ph); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 162.0 (d, 1JCF = 243 Hz, Ph), 141.8 (s,

Ph), 137.8 (s, Ph), 130.9 (s, Ph), 129.3 (d, 2JCF = 34 Hz, Ph), 126.8 (s, Ph), 115.7 (d, 3JCF = 21.1

Hz, Ph), 41.6 (s, PhCH2Ph); 19F NMR (376 MHz, CD2Cl2): δ = −118.1 (s) ppm.

1-Benzyl-4-bromobenzene


(2H, m, Ph), 7.10 (2H, m, Ph), 3.94 (2H, s, PhCH2Ph); 13C{1H} NMR (126 MHz, CD2Cl2):

133

δ = 141.2 (s, Ph), 141.1 (s, Ph), 132.0 (s, Ph), 131.2 (s, Ph), 129.4 (s, Ph), 129.1 (s, Ph), 126.8 (s,

Ph), 120.3 (s, Ph), 41.8 (s, PhCH2Ph) ppm.

1-Benzyl-2-chlorobenzene

1H NMR (500 MHz, CD2Cl2): δ = 7.40 (1H, s, Ph), 7.30 (2H, m, Ph), 7.22 (6H, m, Ph), 4.13 (2H,

s, PhCH2Ph); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 140.3 (s, Ph), 139.4 (s, Ph), 134.7 (s, Ph),

131.7 (s, Ph), 130.1 (s, Ph), 129.5 (s, Ph), 129.0 (s, Ph), 128.3 (s, Ph), 127.5 (s Ph), 126.8 (s, Ph),

39.7 (s, PhCH2Ph) ppm. DART MS: m/z: 220.08995 (calcd. for [M]NH4+: 220.08930).

1-Benzyl-2-methylbenzene

1H NMR (500 MHz, CD2Cl2): δ = 7.30 (2H, m, Ph), 7.17 (7H, m, Ph), 4.02 (2H, s, PhCH2Ph),

2.28 (3H, s, CH3); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 141.3 (s, Ph), 139.8 (s, Ph), 137.2 (s,

Ph), 130.8 (s, Ph), 130.5 (s, Ph), 129.3 (s, Ph), 129.0 (s, Ph), 127.0 (s, Ph), 126.6 (s, Ph), 126.5 (s,

Ph), 40.0 (s, PhCH2Ph), 20.0 (s, CH3) ppm. DART MS: m/z: 200.14314 (calcd. for [M]NH4+:

200.14392).

1-Benzyl-4-methoxybenzene

134

Isolated Yield = 73%. 1H NMR (500 MHz, CD2Cl2): δ = 7.28 (2H, m, Ph), 7.19 (3H, m, Ph),

7.12 (2H, m, Ph), 6.84 (2H, m, Ph), 3.92 (2H, s, PhCH2Ph), 3.77 (3H, s, CH3); 13C{1H} NMR (126

MHz, CD2Cl2): δ = 158.9 (s, Ph), 142.7 (s Ph), 134.2 (s, Ph), 130.5 (s, Ph), 129.5 (s, Ph), 129.2

(s, Ph), 126.7 (s, Ph), 114.6 (s, Ph), 55.7 (s, CH3), 41.5 (s, PhCH2Ph) ppm. DART MS: m/z:

216.13914 (calcd. for [M]NH4+: 216.13884).

(4-Benzylphenoxy)triethylsilane

Isolated Yield = 60%. 1H NMR (500 MHz, CD2Cl2): δ = 7.27 (2H, s, Ph), 7.18 (3H, m, Ph), 7.06

(2H, m, Ph), 6.75 (2H, m, Ph), 3.90 (2H, s, PhCH2Ph), 0.94 (9H, t, 3JHH = 7.9 Hz, SiCH2CH3),

0.54 (6H, quart, 3JHH = 7.9 Hz, SiCH2); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 154.6 (s, Ph),

142.4 (s, Ph), 134.1 (s, Ph), 130.5 (s, Ph), 129.3 (s, Ph), 129.0 (s, Ph), 126.5 (s, Ph), 115.7 (s, Ph),

41.5 (s, PhCH2Ph), 7.2 (s, SiCH2), 6.9 (s, SiCH2CH3); 29Si NMR (119 MHz, CD2Cl2): δ = −37.0

(s) ppm. DART MS: m/z: 184.1 (calcd. for [M]NH4+: 184.1).

Ethylbenzene

1H NMR (400 MHz, CD2Cl2): δ = 7.26 (5H, m, Ph), 2.70 (2H, quart, 3JHH = 8 Hz, PhCH2), 1.29

(3H, t, 3JHH = 8 Hz, CH3); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 145.1 (s, Ph), 129.7 (s, Ph),

128.9 (s, Ph), 126.2 (s, Ph), 29.6 (s, PhCH2), 16.2 (s, CH3) ppm.

Triethyl(2,2,2-trifluoro-1-phenylethoxy)silane

135

Isolated yield: 74%; 1H NMR (500 MHz, CD2Cl2): δ = 7.47 (2H, m, Ph), 7.37 (3H, m, Ph), 4.97

(1H, quart, 2JHF = 6.7 Hz, PhCH), 0.90 (9H, t, 3JHH = 7.9 Hz, SiCH2CH3), 0.61 (6H, quart, 3JHH =

7.9 Hz, SiCH2); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 136.1 (s, Ph), 129.7 (s, Ph), 128.8 (s, Ph),

128.2 (s, Ph), 125.0 (quart, 1JCF = 282.3 Hz, CF3), 73.9 (quart, 2JCF = 31.8 Hz, PhCH2), 6.7 (s,

SiCH2), 5.0 (s, SiCH2CH3); 19F NMR (564 MHz, CD2Cl2): δ = −78.8 (d, 3JHF = 7.2 Hz); 29Si

NMR (119 MHz, CD2Cl2): δ = 24.6 (s) ppm. DART MS: m/z: 308.16629 (calcd. for [M]NH4+:

308.165751).

1,2-Bis(4-methoxyphenyl)ethane


3.76 (6H, s, OCH3), 2.82 (4H, s, CH2); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 158.4 (s, Ph),

134.5 (s, Ph), 129.9 (s, Ph), 114.1 (s, Ph), 55.7 (s, OCH3), 37.7 (s, CH2) ppm. DART MS: m/z:

260.16538 (calcd. for [M]NH4+: 260.16505).

In order to obtain pure 1,2-bis(4-methoxyphenyl)ethane, it was necessary to leave the reaction

mixture under reduced pressure for an extended amount of time. Despite the high molecular weight

of the product, these conditions caused significant loss of yield. However the crude NMR spectra

of the reaction mixture shows quantitative conversion to the single 1,2-bis(4-

methoxyphenyl)ethane product.

Investigation into the Mechanism of Deoxygenation

Hydrodeoxygenation of Benzophenone with HSiiPr3

136

Catalyst (2-5) (1 mol%, 2 mg) was dissolved in CD2Cl2 (0.7 mL). Triisopropylsilane (44 µL,

0.76 mmol, 2.1 eq.) was added with a micro-syringe. Benzophenone (20 mg, 0.36 mmol, 1.0 eq.)

was added and the reaction mixture was monitored by 1H NMR spectroscopy for 48 h.

Reaction of Catalyst 2-2 with Acetophenone

Catalyst (2-2) (10 mg) was dissolved in CD2Cl2 (0.7 mL), and an excess of acetophenone was

added. The solution was allowed to sit for 1 h, before 31P NMR spectrum was obtained. The 1H

NMR spectrum revealed a mixture of products and the organic decomposition product could not

be identified.

137

Reaction of Catalyst 2-5 with Acetophenone

Catalyst (2-5) (10 mg) was dissolved in CD2Cl2 (0.7 mL), and an excess of acetophenone was

added. The solution was allowed to sit for 1h, before 31P NMR spectrum was obtained. The 1H

NMR spectrum revealed a mixture of products and the organic decomposition product could not

be identified.

138

Characterization Data for Hydrosilylation and Deoxygenation Products of Aliphatic

Ketones

(1-cyclohexylethoxy)triethylsilane

Isolated Yield: 84%; 1H NMR (500 MHz, CD2Cl2): δ = 3.56 (1H, p, J = 5 Hz, CH3CH2); 1.84

(1H, m, Cy), 1.76 (2H, m, Cy), 1.66 (2H, m, Cy), 1.32-0.96 (9H, m, Cy, CH3), 0.92 (9H, t, 3JHH =

7.9 Hz, SiCH2CH3), 0.51 (6H, quart, 3JHH = 7.9 Hz, SiCH2CH3); 13C{1H} NMR (126 MHz,

CD2Cl2): δ = 73.07 (s, CHO), 46.37 (s, Cy), 29.37 (s, Cy), 29.28 (s, Cy), 27.38 (s, Cy), 27.08 (s,

Cy), 27.06 (s, Cy), 21.17 (s, CH3), 7.33 (s, SiCH2), 5.62 (s, SiCH2CH3); 29Si NMR (79.5 MHz,

CD2Cl2) δ: 15.4 (s) ppm. DART MS: m/z: 243.21399 (calcd. for [M]H+: 243.214418).

Ethylcyclohexane

139

1H NMR (400 MHz, CD2Cl2): δ = 1.73 (4H, m, Cy); 1.21 (5H, m, Cy, CH2); 0.89 (3H, t, 3JHH =

8 Hz, CH3); 0.63 (4H, m, Cy). 13C{1H} NMR (101 MHz, CD2Cl2): δ = 40.22 (s, Cy); 33.72 (s,

Cy); 30.78 (s, CH2); 27.48 (s, Cy); 27.16 (s, Cy); 11.84 (s, CH3) ppm.

Triethylsilyl((3-methylbutan-2-yl)oxy)silane

1H NMR (500 MHz, CD2Cl2): δ = 3.57 (1H, m, CHO), 1.58 (1H, m, CH(CH3)2), 1.09 (3H, d,

3JHH = 6.2 Hz, CH3), 0.93 (15H, m, SiCH2CH3, CH3), 0.53 (6H, m, SiCH2); 13C{1H} NMR (126

MHz, CD2Cl2): δ = 73.5 (s, CHO), 36.1 (s, CH(CH3)2), 35.6 (s, CH(CH3)2), 20.6 (s, CH3), 20.4

(s, CH3), 18.5 (s, CH3), 18.3 (s, CH3), 18.2 (s, CH3), 7.3 (s, SiCH2), 5.6 (3C, s, SiCH2CH3); 29Si

NMR (79.5 MHz, CD2Cl2): δ = 14.9 ppm.

Isopentane

1H NMR (400 MHz, CD2Cl2): δ = 1.48 (1H, m, CH(CH3)2), 1.24 (2H, m, CH3CH2), (9H, d, 3JHH

= 8Hz, CH3); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 32.29 (s, CH3CH2), 30.47 (s, CH(CH3)2),

22.64 (s, CH3), 12.11 (s, CH3) ppm.

Triethyl((3-methylpentan-2-yl)oxysilane

Isolated Yield: 77%; 1H NMR (500 MHz, CD2Cl2): δ = 3.71 (1H, m, CHO), 1.52 (2H, m,

CH3CH2), 1.11 (1H, m, CH3CH), 1.05 (3H, dd, 3JHH = 17.1, 6.2 Hz, CH3CHO), 0.96 (9H,

140

m, SiCH2CH3), 0.89 (3H, t, 3JHH = 7.4 Hz, CH3CH2), 0.84 (3H, dd, 3JHH = 6.8, 2.5 Hz,

CH3), 0.56 (6H, m, SiCH2); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 72.2 (s, CHO), 43.0 (s,

CHCH3), 42.9 (CHCH3), 26.2 (s, CH3CH2), 25.6 (s, CH3CH2), 20.9 (s, CH3CHO), 19.7 (s,

CH3CHO), 14.7 (s, CH3), 14.1 (s, CH3), 12.4 (s, CH3CH2), 12.3 (s, CH3CH2), 7.31 (s, SiCH2), 6.99

(s, SiCH2), 5.64 (s, SiCH2CH3), 5.60 (s, SiCH2CH3); 29Si NMR (79.5 MHz, CD2Cl2) δ: 15.4 (s),

15.2(s) ppm. DART MS: m/z: 217.19867 (calcd. for [M]H+: 217.198768).

Mixture of Diastereomers

3-methylpentane

1H NMR (400 MHz, CD2Cl2): δ = 1.37 (1H, m, CH3CH), 1.16 (4H, m, CH3CH2), 1.02 (3H, d,

3JHH = 8.2 Hz, CHCH3), 0.89 (6H, t, 3JHH = 8Hz, CH3CH2). 13C{1H} NMR (101 MHz, CD2Cl2):

δ = 36.8 (s, CH3CH), 29.8 (s, CH3CH2), 21.8 (s, CH3CH2), 11.9 (s, CH3CH2) ppm.

(1-(Adamantan-1-yl)ethoxy)triethylsilane

Isolated Yield = 72%. 1H NMR (500 MHz, CD2Cl2): δ = 3.28 (1H, m, CHO), 1.96 (3H, m, Ad),

1.57 (12H, m, Ad), 1.03 (3H, d, 3JHH = 4 Hz, CH3CHO,), 0.96 (9H, m, SiCH2CH3), 0.60 (6H, m,

SiCH2); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 76.7 (s, CHO), 38.7 (s, Ad), 38.3 (s, Ad), 29.3

(s, Ad), 17.5 (s, CH3), 7.4 (s, SiCH2), 5.8 (s, SiCH2CH3); 29Si NMR (79.5 MHz, CD2Cl2): δ = 15.5

ppm.

Hydrodeoxygenation of 1-adamanyl methyl ketone

141

((1-chloropropan-2-yl)oxy)triethylsilane

142

1H NMR (400 MHz, CD2Cl2): δ = 3.99 (1H, m, CHOSi), 3.45 (2H, m, CH2Cl), 1.24 (3H, d, 3JHH

= 4 Hz, CH3), 0.99 (9H, m, SiCH2CH3), 0.62 (6H, m, SiCH2CH3); 13C{1H} NMR (100 MHz,

CD2Cl2): δ = 69.27 (s, CHO), 51.01 (s, CH2Cl), 21.99 (s, CH3), 8.55 (s, SiCH2), 7.15 (s,

SiCH2CH3) ppm.

Triethyl(cyclohexyloxy)silane

Isolated Yield = 84 %. 1H NMR (400 MHz, C6D6): δ = 0.60 (6H, m, SiCH2), 1.02 (9H, m,

SiCH2CH3), 1.09 - 1.83 (10H, m, Cy), 3.62 (1H, m, CHO); 13C{1H} (126 MHz, C6D6): δ = 5.5 (s,

SiCH2CH3), 7.3 (s, SiCH2CH3), 24.4 (s, Cy), 26.1 (s, Cy), 36.5 (s, Cy), 70.7 (s, CHO), 29Si{1H}

(76.5 MHz, C6D6): δ = 15.1 (s) ppm. DART MS m/z: 215.1838 (calcd. for [M]H+: 215.1826).

Hydrodeoxygenation of Cyclohexanone

143

(Heptan-4-yloxy)triethylsilane

Isolated Yield = 83 %. 1H NMR (400 MHz, CD2Cl2) δ: 3.67 (1H, m, CHO), 1.39 (8H, m, CH2),

0.93 (15H, t, 3JHH = 8.0 Hz, CH3, SiCH2CH3), 0.52 (6H, quart, 3JHH = 8.0 Hz, SiCH2); 13C{1H}

NMR (100 MHz, CD2Cl2) δ: 72.7 (s, CHO), 40.2 (s, CHCH2), 19.2 (s, CH2CH2CH3), 14.7 (s,

CH2CH3), 7.3 (s, SiCH2), 5.6 (s, SiCH2CH3); 29Si NMR (79.5 MHz, CD2Cl2) δ: 14.9 (s) ppm.

DART MS m/z: 231.21496 (calcd. for [M]H+: 231.21442).

144

Olefin Polymerization of 4-heptanone

145

(Dicyclohexylmethoxy)triethylsilane

1H NMR (500 MHz, CD2Cl2) δ: 3.19 (1H, t, 3JHH = 5.1 Hz, CHO), 1.75 (6H, m, Cy), 1.61 (5H,

m, Cy), 1.43 (2H, m, Cy), 1.17 (9H, m, Cy), 0.99 (9H, m, SiCH2CH3), 0.64 (6H, quart, 3JHH = 8

Hz, SiCH2); 13C{1H} NMR (126 MHz, CD2Cl2) δ:82.6 (s, CHO), 41.9 (s, Cy), 31.4 (s, Cy), 28.9

(s, Cy), 27.2 (s, Cy), 7.7 (s, SiCH2), 6.3 (s, SiCH2CH3); 29Si NMR (79.5 MHz, CD2Cl2) δ: 4.3

(s) ppm.

Hydrodeoxygenation of Dicyclohexylketone

147

General Procedure of Hydrodeoxygenation of Amides

Catalyst (3 mol%, 2-1: 6 mg; 2-2: 6 mg; 2-5: 6 mg) was added to a small vial and dissolved in

C6D5Br (1 mL). PhSiH3 (0.23 - 0.34 mmol, 2.1 eq) was added with a microliter syringe into the

vial, along the desired amide substrate (0.11 – 0.16 mmol, 1 eq). The mixture was stirred in an oil

bath for 24h at 100°C. The crude product was quenched with water and NaOH (1 mL), and stirred

for 3h at ambient temperature. The mixture was extracted with diethyl ether and dried over MgSO4

to give pure product. The resulting amines were characterized using 1H NMR, 13C{1H} NMR, and

DART MS.

Characterization Data from the Reduction of Amides

N,N-Dimethylbenzamine

1H NMR (500 MHz, C6D5Br) δ = 2.18 (6H, s, NCH3), 3.41 (2H, s, PhCH2N), 7.10-7.37 (5H, m,

Ph) ppm. 13C{1H} NMR (125 MHz, CDCl3) δ = 44.7 (s, NCH3), 63.9 (s, PhCH2N), 127.6 (s, Ph),

128.3 (s, Ph), 129.3 (s, Ph), 134.1 (s, Ph) ppm. DART MS m/z: 136.1121 (calcd. for [M]H+

136.11262).

4-Chlorobenzamine

Isolated Yield = 72 %. 1H NMR (500 MHz, CDCl3) δ = 2.20 (6H, s, NCH3), 3,37 (2H, s,

PhCH2N), 7.20-7.30 (4H, m, Ph) ppm. 13C{1H} NMR (125 MHz, C6D5Br) δ = 45.4 (s, NCH3),

148

63.5 (s, PhCH2N), 128.2 (s, Ph), 129.8 (s, Ph), 130.1 (s, Ph), 135.9 (s, Ph) ppm. DART MS m/z:

170.07365 (calcd. for [M]H+ 170.07365).

4-Bromobenzamine

Isolated Yield = 96 %. 1H NMR (500 MHz, CDCl3) δ = 1.82 (6H, s, NCH3), 2.93 (2H, s,

PhCH2N), 7.16 (2H, d, 3JHH = 8.3 Hz, Ph), 7.43 (2H, d, 3JHH = 8.3 Hz, Ph) ppm. 13C{1H} NMR

(125 MHz, C6D5Br) δ = 45.3 (s, NCH3), 63.2 (s, PhCH2N), 128.1 (s, Ph), 129.8 (s, Ph), 130.5 (s,

Ph), 135.9 (s, Ph) ppm. DART MS m/z: 214.02314 (calcd. for [M]H+ 214.02314)

N-(4-methylbenzyl)pyrrolidine

Isolated Yield = 46 %. 1H NMR (500 MHz, C6D5Br) δ = 1.38 (4H, m, NCH2CH2), 1.97 (3H, s,

PhCH3), 2.17 (4H, m, NCH2CH2), 3.28 (2H, s, PhCH2N), 6.78 (2H, d, 3JHH = 7.6 Hz, Ph), 7.18

(2H, d, 3JHH = 7.6 Hz, Ph) ppm. 13C{1H} NMR (125 MHz, C6D5Br) δ = 23.76 (s, PhCH3), 30.38

(s, NCH2CH2), 38.75 (s, NCH2CH2), 53.91 (s, NCH2CH2), 60.15 (s, NCH2CH2), 68.19 (s,

PhCH2N), 122.27 (s, Ph), 126.38 (s, Ph), 129.33 (s, Ph), 135.43 (s, Ph), 136.68 (s, Ph), 167.74 (s,

Ph) ppm. DART MS m/z: 176.14392 (calcd. [M]H+ 176.14392).

N,N-Dimethyl-4-nitrobenzamine

149

1H NMR (500 MHz, CDCl3) δ = 2.20 (6H, s, NCH3), 3.37 (2H, s, PhCH2N), 7.22 (2H, d, 3JHH =

8.0 Hz, Ph), 7.27 (2H, d, 3JHH = 8.0 Hz, Ph) ppm. 13C{1H} NMR (125 MHz, C6D5Br) δ = 45.3 (s,

NCH3), 64.5 (s, PhCH2N), 128.2 (s, Ph), 129.8 (s, Ph), 130.8 (s, Ph), 135.9 (s, Ph) ppm. DART

MS m/z: 181.09770 (calcd. for [M]H+ 181.09770).

N,N-Dimethyl-3-methoxybenzamine

Isolated Yield = 74 %. 1H NMR (500 MHz, C6D5Br) δ = 1.94 (6H, s, NCH3), 3.12 (2H, s,

PhCH2N), 3.34 (3H, s, OCH3), 6.70 (2H, d, 3JHH = 7.5 Hz, Ph), 7.25 (2H, d, 3JHH = 7.5 Hz, Ph)

ppm. 13C{1H} NMR (125 MHz, C6D5Br) δ = 43.36 (s, NCH3), 54.71 (s, OCH3), 64.21 (s,

PhCH2N), 112.63 (s, Ph), 114.29 (s, Ph), 121.98 (s, Ph), 126.55 (s, Ph), 129.83 (s, Ph), 130.80 (s,

Ph), 135.85 (s, Ph), 159.85 (s, Ph) ppm. DART MS m/z: 166.12319 (calcd. for [M]H+ 166.12319).

N-Benzyl-N-ethyl-4-methoxybenzamide

1H NMR (500 MHz, C6D5Br) δ = 0.78 (3H, t, 3JHH = 7.1 Hz, NCH2CH3), 2.23 (2H, quart, 3JHH =

7.1 Hz, NCH2CH3), 3.26 (2H, s, PhCH2N), 3.28 (2H, s, PhCH2N), 3.33 (3H, s, OCH3), 6.62 (2H,

d, 3JHH = 8.6 Hz, Ph), 6.95-7.10 (7H, m, Ph) ppm. 13C{1H} NMR (125 MHz, C6D5Br) δ = 12.1

(s, NCH2CH3), 47.0 (s, NCH2CH3), 54.8 (s, OCH3), 57.5 (s, NCH2Ph), 113.7 (s, Ph), 126.8 (s, Ph),

128.2 (s, Ph), 128.8 (s, Ph), 129.8 (s, Ph), 129.93 (s, Ph), 135.9 (s, Ph), 158.7 (s, Ph) ppm. DART

MS m/z: 256.17014 (calcd. for [M]H+ 256.17014)

N,N-Diethylpropionamine

150

1H NMR (500 MHz, C6D5Br) δ = 0.66 (3H, t, 3JHH = 7.3 Hz, CH2CH3), 0.76 (6H, t, 3JHH = 7.1

Hz, NCH2CH3), 1.20 (2H, sept, 3JHH = 7.3 Hz, CH2CH3), 2.10 (2H, t, 3JHH = 7.3 Hz, CH2CH2N),

2.22 (4H, quart, 3JHH = 7.1 Hz, NCH2CH3) ppm. 13C{1H} NMR (125 MHz, C6D5Br) δ = 12.1 (s,

CH3CH2), 20.7 (s, NCH2CH3), 47.1 (s, NCH2CH3), 55.1 (s, NCH2CH2) ppm. DART MS m/z:

116.14392 (calcd. for [M]H+ 116.14392).

N-Benzylaniline

1H NMR (500 MHz, CDCl3) δ = 4.00 (1H, br, NH), 4.24 (2H, s, CH2), 6.57 (2H, d, 3JHH = 7.6

Hz, Ph), 6.64 (1H, t, 3JHH = 7.3 Hz, Ph), 7.07 (2H, dd, 3JHH = 8.6 Hz, 7.2 Hz, Ph), 7.32-7.14 (5H,

m, Ph) ppm; 13C{1H} NMR (125 MHz, CDCl3) δ = 48.5 (s, CH2), 113.1 (s, Ph), 117.8 (s, Ph),

127.3 (s, Ph), 127.6 (s, Ph), 128.7 (s, Ph), 129.3 (s, Ph), 139.3 (s, Ph), 147.9 (s, Ph) ppm. DART

MS m/z: 184.11211 (calcd. for [M]H+ 184.11262).

4-Bromobenzaniline

1H NMR (500 MHz, C6D5Br) δ = 3.99 (1H, br, NH), 4.22 (2H, s, PhCH2N), 6.43 (2H, d, 3JHH =

8.8 Hz, Ph), 7.14-7.33 (5H, m, Ph), 7.40-7.52 (2H, m, Ph) ppm. 13C{1H NMR (125 MHz, CDCl3)

δ = 47.9 (s, PhCH2N), 108.9 (s, Ph), 114.1 (s, Ph), 122.0 (s, Ph), 127.1 (s, Ph), 128.9 (s, Ph), 131.7

(s, Ph), 138.6 (s, Ph), 147.8 (s, Ph) ppm. DART MS m/z: 262.02258 (calcd. for [M]H+ 262.02314).

151

4-Nitrobenzaniline

1H NMR (500 MHz, C6D5Br) δ = 4.71 (2H, s, PhCH2N), 7.07-7.21 (5H, m, Ph), 7.52 (2H, d, 3JHH

= 9.2 Hz, Ph), 7.78 (1H, s, Ph), 7.90 (2H, d, 3JHH = 9.2 Hz, Ph) ppm. 13C{1H} NMR (125 MHz,

C6D5Br) δ = 53.2 (PhCH2N), 114.6 (s, Ph), 188.8 (s, Ph), 124.5 (s, Ph), 127.9 (s, Ph), 128.4 (s,

Ph), 131.8 (s, Ph), 133.7 (s, Ph), 143.5 (s, Ph) ppm. DART MS m/z: 229.09770 (calcd. for [M]H+

229.0977).

152

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157

Chapter 4 Reduction of Phosphine Oxides and Reactivity with

Phosphoethers

4.1 Introduction

4.1.1 Synthesis of Phosphines

Phosphines are central reagents in many classical organic transformations, such as the

Mitsunobu, Wittig, Rauhut‒Currier, and Appel reactions.1-6 Furthermore, phosphines

represent a large proportion of classic donor ligands employed in organometallic chemistry.

While classic synthetic routes to phosphines include the use of phosphides as nucleophiles

or chlorophosphines as electrophiles7 more recent developments have included metal-

catalyzed phosphination of aryl halides, alkenes, and alkynes.8-12 An alternative and

important approach involves the reduction of phosphine oxides to phosphines.13 This latter

approach is particularly attractive to industry as a number of large-scale processes generate

phosphine oxides as byproducts, making their recycling an avenue for the monetization of

an otherwise waste material. Known methods for the reduction of phosphine oxides

typically require the use of LiAlH4,14 DiBAL‒H,15 or Cl3SiH.16,17 While hydrosilane

reduction of phosphine oxides has been employed commercially, these reactions require

high temperatures and long reaction times.16,17 Electrochemical reductions of phosphine

oxides have also been reported.16-18 Meanwhile, the Buchwald,19 Lawrence,20 Lin,21

Lemaire,22,23 and Beller24 groups and many others25 have reported the catalytic

hydrosilylation of phosphine oxides mediated by Ti(Oi-Pr)4 and Cu(OTf)2. More recently,

Beller and co-workers have also reported an example of a metal-free system for phosphine-

oxide reduction using Brønsted acids.26 In addition, Pietrusiewicz and co-workers have

reported the stoichiometric use of BH3 to effect the reduction of hydroxyalkylphosphines.27

Given the high activity observed for catalyst 2-2 and 2-5 with respect to affecting

hydrosilylation of olefins, carbonyls, and amides, the reduction of phosphine oxides was

investigated.

158


4.2.1 Catalytic Reduction of Phosphine Oxides

In Chapter 2 the high oxophilicity of 2-5 was discussed, noted by the Gutmann-Beckett test

where cationic phosphenium oxide 2-7 was observed. Subsequently, deoxygenation of

ketones and amides was investigated via catalytic hydrosilylation. Following this work,

EPCs 2-2 and 2-5 were used as catalysts for the deoxygenation of Ph3PO. Employing

various hydrosilanes at different temperatures, reaction conditions were optimized (Table

4-1).

To a solution of 2-5 (2.0 mol%) in C6D5Br, an excess of the indicated hydrosilane (3.0

equiv.) and Ph3PO were added sequentially. At 100 oC, representative triorganosilanes such

as Ph3SiH, Et3SiH, and i-Pr3SiH lead to low or no conversion. While Ph2SiH2 gave

moderate conversion to triphenylphosphine, the more reactive alkoxy-substituted

hydrosilanes (EtO)2MeSiH and (Me2SiH)2O did not react at 60 °C but afforded high

conversion at 100 °C; (MeSiHO)n and PHMS performed poorly. Finally EPC 2-2 was tested

with those hydrosilanes that had shown the highest reactivity in combination with Lewis

acid 2-5: PhSiH3, (EtO)3SiH, (EtO)2MeSiH, and (Me2SiH)2O. Interestingly, 2-2 would only

catalyse the triphenylphosphine oxide reduction with PhSiH3 as reducing agent. Low

loadings of both 2-5 or 2-2 (2.0 mol%) mediated this deoxygenation at 45 °C. Negligible

conversion was seen in the absence of any catalyst. Alongside our work, the Oestreich

group investigated B(C6F5)3 to effect the hydrosilylative reduction of phosphine oxides

with phenylsilane.28 Although this work is reported in collaboration, the reactions involving

B(C6F5)3 have been omitted and are outside the scope of this thesis.

Table 4-1 Catalytic Reduction of Triphenylphosphine Oxide to Triphenylphosphine

Catalyst mol% Silane[a] t (h) T (°C) Conv. (%) [c]

2-5 2.0 PhSiH3 24 45 95

2-5 2.0 PhSiH3 24 60 >99

159

2-5 2.0 PhSiH3 24 100 >99

2-5 2.0 Ph2SiH2 24 100 50

2-5 2.0 Ph3SiH 24 100 5

2-5 2.0 Et3SiH 24 100 0

2-5 2.0 i-Pr3SiH 24 100 0

2-5 2.0 (EtO)2MeSiH 24 60 5

2-5 2.0 (EtO)2MeSiH 24 100 95

2-5 2.0 (Me2SiH)2O 24 60 5

2-5 2.0 (Me2SiH)2O 24 100 99

2-5 2.0 (MeSiHO)n 24 100 10

2-5 2.0 PHMS 24 45 8

2-2 2.0 PhSiH3 24 45 >99 (87)

2-2 2.0 PhSiH3 24 100 >99

2-2 2.0 (EtO)3SiH 24 100 15

2-2 2.0 (EtO)2MeSiH 24 100 10

2-2 2.0 (Me2SiH)2O 24 100 10

‒ ‒ PhSiH3 24 45 0

‒ ‒ (EtO)2MeSiH 24 100 <5

‒ ‒ (Me2SiH)2O 24 100 <5

[a] R3SiH (3.5 or 3.0 equiv.) was added to a solution of the indicated catalyst in d8-toluene or

C6D5Br (0.7 mL) followed by triphenylphosphine oxide (0.39‒0.11 mmol). [b] Determined by 1H NMR and 31P NMR spectroscopy, isolated yield in parentheses.

The fluorophosphonium salts 2-5 and 2-2 in combination with PhSiH3 proved to be efficient

catalysts for the reduction of a series of phosphine oxides (Table 4-2). Et3PO was

quantitatively reduced to Et3P at 60 oC using 2-2 while 2-5 afforded just 70% conversion

under the same conditions. The secondary phosphine oxide Ph2P(O)H was efficiently

160

reduced by both catalysts at 45 °C to afford Ph2PH. Ph2P(O)Cl was reduced and

dehalogenated, also yielding Ph2PH. This was particularly effective with catalyst 2-2 at 100

°C. Ph2(C6F5)PO and (C6F5)3PO were both reduced to the corresponding phosphine at 45

°C using either catalysts. Increased loadings of 2-5 or 2-2 (5.0 instead of 2.0 mol%) along

with higher temperatures were required to generate dppm [from dppm(O)2] and binap [from

binap(O)2] more effectively. Only traces of the monooxides were seen with these

bisphosphine dioxides. Conversely, reduction of dppb(O)2 with 2-5 furnished a 45:55

mixture of dppb(O) and desired dppb while the use of catalyst 2-2 resulted in quantitative

reduction to dppb. While the fluorophosphonium cations 2-5 and 2-2 have been reported to

be active for hydrodefluorination of C(sp3)‒F bonds,29 (4-CF3C6H4)3PO was

chemoselectively reduced in the presence of excess PhSiH3 to (4-CF3C6H4)3P, leaving the

CF3 group untouched. Similarly, (MeOCH2)Ph2PO was fully reduced to (MeOCH2)Ph2P,

despite the previous reports of the ability of catalysts 2-5 and 2-2 to activate the C‒O bonds

of ethers.30 Moreover, [4-t-BuC(O)C6H4]Ph2PO and [4-HO(O)CC6H4]Ph2PO were

converted into the corresponding phosphines, neither reducing the carbonyl nor the free

carboxyl group. These observations are consistent with the greater Lewis basicity of the

phosphine oxides, prompting their preferential reduction. It also suggests that the generated

phosphines inhibit both C(sp3)‒F activation and C‒O bond cleavage. This view was tested

in a competition experiment where an equimolar mixture of Ph3PO and Ph2CO were

exposed to 2 mol% of 2-5 and Et3SiH. This experiment showed no reaction (Figure 4-1).

Changing to PhSiH3 then resulted in chemoselective reduction of the phosphine oxide

(Figure 4-2), thus supporting the proposition concerning chemoselectivity.

161

Figure 4-1 1H NMR, 13C NMR and 31P NMR Spectra for the Attempted Reduction of

Benzophenone Using 2-5 and Et3SiH in the Presence of OPPh3

162

Figure 4-2 1H NMR, 13C NMR, 31P NMR Spectra for the Reduction of OPPh3 Using 2-5

and Phenylsilane in the Presence of Benzophenone

The protocol for catalytic phosphine oxide reduction was also tolerant of an amino group

as [2-Me2NC6H4]Ph2PO was reduced quantitatively by PhSiH3 in the presence of 2.0 mol%

163

of 2-5 at 100 °C or 2.0 mol% of 2-2 at 50 °C. Similarly, 2.0 mol% of 2-2 at 100 °C were

effective for the reduction of Ph2(2-py)PO. In the case of the bidentate amido-linked

bisphosphine dioxide tolN(Ph2PO)2, clean reduction to tolN(Ph2P)2 was found using 2-2 as

catalyst at 100 oC; with 2-5, a mixture of the starting dioxide, the monooxide, and the free

bisphosphine was obtained. Interestingly, neither catalyst was effective for the reduction of

tris-morphilinidophosphine-oxide N(CH2CH2O)3PO. Both the five membered cyclic

phosphinic acid, S,S-[(CH2CHPh)2P](O)OH and the acyclic acid Ph2P(O)OH were reduced

to S,S-[(CH2(CHPh)2PH] and Ph2PH, respectively using either 2-5 or 2-2 as catalyst at 45

oC. In contrast, the acylphosphine oxide [MesC(O)]Ph2PO was reluctant to deoxygenate,

yielding Ph2PH in 20% yield with 2-5 at 100 °C and 30% yield with 2-2 at 45 °C. Finally,

an effort to extend this protocol to Ph3PS proved challenging but the desulfurization did

indeed work in acceptable yield. The poorer efficiency of the phosphine sulfide in

comparison to the phosphine oxide reduction is consistent with the lesser polarization of

the P=S bond and the formation of the weaker Si‒S compared to the Si‒O bond.31,32

However, 5.0 mol% of 2-5 at 100 °C promoted the desulfurization in 77% yield; catalyst

2-2 was not competent in this transformation. Unfortunately, analogous efforts to reduce

the phosphinimine Ph3PNSiMe3 at 100 °C and 5.0 mol% catalyst loading were not met with

the same success.

Table 4-2 Catalytic Reduction of Phosphine Oxides to Phosphines

Entry Substrate Cat.

(mol%)

T

(°C)

Conv.

(%)[b]

Product

1

2-5 (2.0)

2-2 (2.0)

60

60

70

>99

2

2-5 (2.0)

2-2 (2.0)

45

45

>99

>99

3

2-5 (2.0)

2-2 (2.0)

60

100

60

>97

4

2-5 (2.0)

2-2 (2.0)

45

45

50

>99 (83)

164

5

2-5 (2.0)

2-2 (2.0)

45

45

>99

>99 (85)

6

2-5 (5.0)

2-2 (5.0)

2-2 (5.0)

100

100

50

>99 (35a)

4 (a)/96(b)

84(a)/16(b)

7

2-5 (5.0)

2-2 (5.0)

100

100

50(a)/4(b)

>99 (48b)

8

2-5 (2.0)

2-2 (2.0)

50

50

45(a)/55(b)

>99 (24b)

9

2-5 (2.0)

2-2 (2.0)

45

45

80

80

10

2-5 (2.0)

2-2 (2.0)

45

45

>99

>99

11

2-5 (2.0)

2-2 (2.0)

50

50

30

>99

12

2-5 (2.0)

2-2 (2.0)

50

50

77

>99 (71)

13

2-5 (2.0)

2-2 (2.0)

2-2 (2.0)

50

100

50

17

>99

>99 (83)

165

14

2-5 (2.0)

2-2 (2.0)

100

100

51

>99

15

2-5 (2.0)

2-2 (2.0)

100

100

19(a)/23(b)

95(b)

16

2-5 (5.0)

2-2 (5.0)

100

100

0

0

17

2-5 (2.0)

2-2 (2.0)

45

45

>99 (28)

>99

18

2-5 (2.0)

2-2 (2.0)

45

45

>99

>99

19

2-5 (2.0)

2-2 (2.0)

2-2 (2.0)

45

100

45

15

20

30

20

2-5 (5.0)

2-2 (5.0)

100

100

77

20

21

2-5 (5.0)

2-2 (5.0)

100

100

17

25

[a] All reactions were performed on a 0.11‒0.16-mmol scale in C6D5Br at the indicated

temperature for 24 h with PhSiH3 (3.1 equiv.) as reducing agent. [b] Determined by 31P

NMR spectroscopy; isolated yields given in parentheses.

The common mechanism of these phosphine-oxide reductions is thought to be analogous

to those proposed for hydrosilylations mediated by B(C6F5)333-35,36 and EPCs 2-5 and 2-2

(Figure 4-3; a).37,38 As an example, a general catalytic cycle is depicted for monocationic

2-2 (Figure 4-3; b). Initial interaction of the hydrosilane R’3SiH with the phosphonium

cation prompts weakening of the Si‒H bond (2-2…H….SiR’3, Figure 4-3; b), thereby

166

facilitating nucleophilic attack at the silicon atom by phosphine oxide R3PO. Hydride

transfer yields transient R3P(H)OSiR’3 together with 2-2, and both react further with

another equivalent of R3’SiH to afford [R3P(H)(OSiR’3)2]+. This is believed to dissociate

to give the weakly Lewis-basic disiloxane (R’3Si)2O and Brønsted-acidic phosphonium

cation [R3PH]+.39 Subsequent protonation of the 2-2H by the phosphonium liberates free

phosphine and H2 and regenerates 2-2.

Figure 4-3 Proposed Mechanism for Phosphine Oxide Reduction

It is noteworthy that a weak resonance attributable to the phosphonium cation 2-5 is seen

throughout. The corresponding 19F NMR spectral data confirmed the persistent presence of 2-5

167

over the course of the reaction. Also, H2 could be observed by 1H NMR during the reduction of

triphenylphosphine oxide (Figure 4-4). Nonetheless, upon completion of the reaction, in addition

to 2-5, minor amounts of the difluorophosphorane 2-4 and the phosphine 2-3 were observed. All

the decomposition products generated from catalyst decomposition where independently

synthesized and tested in catalysis (Figure 4-5), in which none were found to be active. In an

independent reaction, 2-5 in the presence of PhSiH3 alone was shown to generate both 2-4 and 2-

3 (Figure 4-6). It should be noted that when the reaction is repeated in DCM, 2-5 can be clearly

observed by 31P and 19F NMR spectroscopy due to increased solubility. This suggests that

PhSiH3 also acts to partially reduce the EPC in the absence of phosphine oxide. In contrast, the

stability of 2-5 in the presence of Et3SiH, forming an equilibrium mixture of 2-5 and the

corresponding hydrosilane adduct (Figure 4-3; a) was previously reported.

Figure 4-4 1H NMR Spectrum for H2 Observed During the Catalytic Reduction of

Triphenylphosphine Oxide

168

Figure 4-5 31P NMR Spectrum for Monitoring Catalyst Decomposition during Catalytic

Reduction of Triphenylphosphine Oxide

169

Figure 4-6 31P NMR and 19F NMR Spectra for Reaction of 2-5 with Phenylsilane

4.2.2 Reactivity with Phosphoethers

Since the beginning of last century, the Michaelis-Arbuzov rearrangement has been widely

exploited in organophosphorous chemistry to prepare phosphonates, phosphinates, and phosphine

oxides.40-42 In its simplest form, this transformation begins with a phosphorus(III) ester (R2P-OR)

which affords the corresponding tetracoordinate phosphorous(V) species [R2P(O)R]. In some

170

cases these reactions can be driven thermally43 or catalyzed by alkyl halides.44 More recently,

Renard and co-workers have demonstrated the ability of Lewis acids such as trimethylsilyl halides

to facilitate this reaction under mild conditions.45 Subsequently their group showed that BF3·OEt2

and Me3Si(O3SCF3) were also effective catalysts for this rearrangement.45 While this study

reported the formation of phosphine-oxides in good yields, this reaction was effective only for the

migration of primary alkyl- or activated secondary alkyl groups. These Lewis acid catalyzed

reactions were proposed to proceed via a Lewis acid activation, by a mechanism that is bimolecular

in phosphite (Figure 4-7).

Figure 4-7 General Mechanism for Lewis Acid Catalyzed Michaelis-Arbuzov

Rearrangement

With these previous reports of Lewis acid catalyzed Michaelis-Arbuzov reactions and given the

catalytic reduction of phosphine oxides discussed in Section 4.2.1, the reactivity of EPCs 2-2 and

2-5 in cascade reactions, where the phosphite was first rearranged to the phosphine oxide and

subsequently reduced to the phosphine, was investigated (Figure 4-8). This method would prove

to be a new synthetic route to air sensitive phosphines starting from air stable phosphites.

Figure 4-8 Synthetic Strategy for Phosphites as a Viable Route to Phosphines

Treatment of P(OMe)3 or P(OEt)3 with 5 mol% of 2-2 at 100 °C for 24 h gave quantitative

conversion to OP(OMe)2Me and OP(OEt)2Et, respectively (Table 4-3). Application of these

conditions to P(OiPr)3 afforded OP(OiPr)2H which was observed as the major product generated

171

in 93% yield. This is thought to result from the β-hydride elimination of the Michaelis-Arbuzov

rearrangement product resulting in the loss of propylene. Similar eliminations of butene and

cyclohexene from tertiary phosphines has been previously observed.46 In the case of Ph2P(OR) (R

= Me Et), catalytic conversions to the corresponding phosphine oxides R2P(O)R were observed in

95 and 80 % yields, respectively. In these cases, the known products of rearrangement were

characterized unambiguously by 31P and 1H NMR spectroscopy, demonstrating that 2-2 effectively

mediates Michaelis-Arbuzov rearrangement. Interestingly, attempts to effect similar Michaelis-

Arbuzov rearrangements of the aryl phosphinites, Ph2P(OMes) and P(OPh)3 led to no reaction.

However, analogous treatment of the quinolate-derivative tBu2P(OC9H6N) did not lead to the

Michaelis-Arbuzov rearrangement, rather elimination of butene from one of the t-butyl groups

generating the secondary phosphinite, tBuPH(OC9H6N). Comparatively it was found that 2-5 was

completely ineffective at mediating the Michaelis-Arbuzov rearrangement and was not further

investigated for this reactivity.

Table 4-3 Michaelis-Arbuzov Rearrangement Mediated by 2-2

Entry Substrate Product Conv. (%)[b]

1 (MeO)3P (MeO)2P(O)Me 99

2 (EtO)3P (EtO)2P(O)Et 99

3 (iPrO)3P (iPrO)2P(O)H 93

4 Ph2P(OMe) Ph2P(O)Me 95

5 Ph2P(OEt) Ph2P(O)Et 80

6 tBu2P(OC9H6N) tBuPH(OC9H6N) 60

7 Ph2P(OMes) - 0

172

8 (PhO)3P - 0

[a] All reactions were performed on a 0.16-mmol scale in d8-toluene at 100 °C for 24 h. [b]

Determined by 31P NMR spectroscopy.

Tandem Michaelis-Arbuzov rearrangement and reduction were performed using 2-2 as the

catalyst with the subsequent addition of PhSiH3 to the reaction mixture (Scheme 4-1). In the case

of precursors Ph2P(OR) (R = Me, Et), the corresponding phosphines Ph2P(R) (R = Me Et) were

formed in high yields again as evidenced by 31P and 1H NMR data, confirming the ability of 2-2

to mediate sequential rearrangement and reduction reactions.

Scheme 4-1 Tandem Michaelis-Arbuzov Rearrangement and Reduction of Methyl

Diphenylphosphonite and Ethyl Diphenylphosphonite

In contrast, one-pot treatment of (MeO)3P and (OEt)3P with 2-2 and PhSiH3 did not lead to

the formation of (MeO)2PMe and (EtO)2PEt as might have been expected. Instead, very

low yields of MePH2 and EtPH2 were observed presumably as a result of elimination of the

silylethers. In further contrast, treatment of (iPrO)3P, first with a catalytic amount of 2-2

followed by addition of PhSiH3, yielded phosphine (PH3) in high yield as evidenced by the

observation of a quartet observed in 31P NMR spectrum at δ = –242.1 ppm (q, 1JHP = 189

Hz) and the corresponding doublet at δ = 1.33 ppm in 1H NMR spectrum (Figure 4-9).

173

Figure 4-9 1H NMR and 31P NMR Spectra of PH3 in Crude Reaction Mixture

4.2.3 Catalytic Generation of PH3, Primary and Secondary Phosphine

The gaseous phosphine (PH3) is known to be a versatile precursor for the synthesis of

organophosphorus compounds with high atom efficiency;47-51 however its main drawbacks are its

high toxicity and spontaneous flammability in air.52 The more general method to synthesize PH3

incorporates the use of white phosphorous (P4) as a starting material. Chemical hydrogenation of

P4 requires very harsh conditions.53 Some alternatives use transition metal complexes to keep the

resulting phosphine coordinated.54-56 However, this can lead to unpredictable reactivity.57-62

Recently, Yakhvarov and coworkers investigated the electrochemical transformation of white

phosphorus, considered to be a more appropriate method for in situ generation of PH3 for industrial

purposes.63

Extending on the observation of PH3 when attempting a Michael-Arbuzov rearrangement

of (iPrO)3P, (PhO)3P was treated with PhSiH3 in the presence of 5 mol% of 2-2. After 48 h

at 25 °C or 24 h at 50 °C resulted in the quantitative conversion to PH3 as evidenced by

174

NMR data (Table 4-4). In a similar fashion, OP(OPh)3 was also employed as a substrate to

yield phosphine. In addition, both phenyl (OPh)PPh2 and (OPh)P(O)Ph2 were reduced to

diphenylphosphine at 50 °C. However, the scope of phosphoethers that would yield the

corresponding secondary phosphine was found to be very limited. R2P(OPh) (R = o-tol, p-

FC6H4, Cy, o-MeOC6H5, tBu/Ph) where all prepared from the corresponding R2PCl,

chlorophosphine and phenol analogous to the synthetic method reported for the preparation

of (OPh)2PPh and (OPh)PPh2.64-66 None of these substrates generated the secondary

phosphine. Finally, (OPh)2PPh and (PhO)2P(O)Ph both reacted to successfully generate

phenylphosphine in high yields.

Table 4-4 Catalytic Generation of PH3, PhPH2, and Ph2PH

Entry Substrate Temp. Product Conv. (%)[b]

1 (PhO)3P 50 PH3 99

2 (PhO)3P(O) 50 PH3 11

3 (PhO)3P(O) 100 PH3 98

4 Ph2P(OPh) 50 Ph2PH 99

5 Ph2P(O)(OPh) 50 Ph2PH 99

6 PhP(OPh)2 50 PhPH2 99

7 PhP(O)(OPh2) 50 PhPH2 99

8 (iPrO)3P 50 PH3 40

9 (iPrO)3P 100 PH3 99

[a] All reactions were performed on a 0.16-mmol scale in d8-toluene or C6D5Br at the

indicated temperature with 3 eq. of PhSiH3 for 24 h. [b] Determined by 31P NMR

spectroscopy.

To further confirm the generation of phosphine (PH3), B(C6F5)3 was added to the above

reactions mixtures to generate the Lewis acid-base adduct (H3P)B(C6F5)3 (Scheme 4-2).

This resulted in the observation of a 31P NMR signal at δ = –100.0 ppm as a quartet with

P–H coupling of 409 Hz. The corresponding 1H NMR doublet resonance was seen at 3.16

ppm (Figure 4-10). The 11B NMR signal for the Lewis adduct was seen at –16.0 ppm,

175

typical of four coordinate boron. While these observations are consistent with the

formulation of the adduct, this was further confirmed by the independent synthesis of this

species in a direct reaction of PH3 and B(C6F5)3. This data were consistent with previously

published data.67 10 mol% B(C6F5)3 was also independently tested in the presence of

phenylsilane and triphenyl phosphite at 50 °C and 100 °C and showed no conversion to

phosphine.

Scheme 4-2 Synthesis of PH3 Adducts with Lewis Acids

Figure 4-10 1H NMR and 31P NMR Spectra for H3P•B(C6F5)3 Adduct

In analogous fashion, both a Ga(C6F5)3 and Al(C6F5)3 adducts of PH3 were observed in the 31P

NMR spectra by the quartet at –157.5 ppm with a coupling constant of 364 Hz (Figure 4-11) and

176

–183 ppm with a coupling constant of 317 Hz (Figure 4-12), respectively. However none of

these adducts could be cleanly isolated from the siloxane by-product, making it difficult to

pursue further chemistry.

Figure 4-11 31P NMR Spectrum for H3P•Ga(C6F5)3 Adduct

Figure 4-12 31P NMR Spectrum for H3P•Al(C6F5)3 Adduct

4.3 Conclusion

In conclusion, the presented work demonstrates that EPCs 2-2 and 2-5 are efficient catalysts for

the reduction of a broad range of phosphine oxides in the presence of silanes under mild

conditions. This reactivity further affirms that EPCs are highly effective Lewis acid catalysts.

177

Notably, chemoselective reduction of phosphine oxides was observed in the presence of other

reducible functional groups such as halogens, ethers, ketones, and carboxylic acids. With these

results, investigations are now underway to test if transformations that incorporate phosphines as

stoichiometric reagents and result in phosphine oxide as a by-product can be made catalytic.

Furthermore, the ability of 2-2 and 2-5 to affect Michaelis-Arbuzov rearrangements of

phosphoethers was investigated. It was found that 2-5 is completely inactive for these

transformations, whereas 2-2 led to the rearranged product in high yields. However, the substrate

scope for this transformation was limited to alkyl-substituents at the ether. Subsequent addition

of silane allowed for the reduction of the corresponding phosphine oxide. Attempts toward one-

pot synthesis of these phosphines revealed cleavage of the P–OR functional group. Moreover, it

was found that phosphites with both alkyl and aryl substituents could be employed to

catalytically produce phosphine (PH3). Subsequent capture of PH3 with B(C6F5)3, Al(C6F5)3 and

Ga(C6F5)3 could be observed by 31P NMR spectroscopy. In a similar fashion, phenyl- substituted

phosphoethers and phosphoesters could be used to as a route to more reactive primary and

secondary phosphines.

178



standard Schlenk techniques under an inert atmosphere of anhydrous N2. Dry, oxygen-free



molecular sieves (4 Å) prior to use. Deuterated benzene (C6D6) and d8-toluene were purchased

from Sigma-Aldrich, distilled from sodium and stored over molecular sieves (4 Å) for at least two

days prior to use. Deuterated dichloromethane (CD2Cl2) and bromobenzene (C6D5Br) were

purchased from Sigma-Aldrich, distilled from CaH2 and stored over molecular sieves (4 Å) for at

least two days prior to use. Reagents such as silanes, phosphines, phosphine oxides, and

phosphinites, phosphinates, and phosphites, phosphonites, phosphinate, and B(C6F5)3 were

purchased either from Sigma-Aldrich, Strem Chemicals or Alfa Aesar and, if applicable, distilled

prior to use. Reagents such as Al(C6F5)3,68 Ga(C6F5)3,

69 as well as not commercial available

phosphine oxides70 were prepared according to literature known procedures. P(OPh)3, Ph2P(OPh)

and PhP(OPh)2 were oxidized to the corresponding phosphoesters in a procedure analogous to the

oxidation of phosphines to phosphine oxides. All glassware were oven-dried at temperatures above

180°C prior to use. NMR spectra were obtained on an Agilent DD2-700 MHz, an Agilent DD2-

500 MHz, a Bruker AvanceIII-400 MHz, or a Varian Mercury-300 MHz spectrometer. All 13C

NMR spectra were exclusively recorded with composite pulse decoupling. Assignments of the

carbon atoms in the 13C spectra were performed via indirect deduction from the cross-peaks in 2D

correlation experiments (HMBC; HSQC). Chemical shifts were referenced to δTMS = 0.00 ppm

(1H, 13C) and δH3PO4(85%) = 0.00 ppm (31P, externally). Chemical shifts (δ) are reported in ppm,

multiplicity is reported as follows (s = singlet, d = doublet, t = triplet, quart. = quartet,

m = multiplet) and coupling constants (J) are reported in Hz. Assignments of individual resonances

were done using 2D techniques (HMBC, HSQC, HH-COSY) when necessary. Yields of products

in solution were determined by integration of all resonances observed in the respective NMR

spectra if not stated otherwise. High-resolution mass spectra (HRMS) were obtained on a micro

mass 70S-250 spectrometer (EI), an ABI/Sciex QStar Mass Spectrometer (DART), or on a JOEL

AccuTOF-DART (DART). Elemental analyses (C, H, N) were performed at the University of

Toronto employing a Perkin Elmer 2400 Series II CHNS Analyzer.

179

General Procedure A: Reduction of Phosphine Oxides to Phosphines


the respective catalyst (2 mol%, 2-2: 4 mg, 2-5: 4 mg) was added to a solution of PhSiH3 in C6D5Br

(0.7 mL). The respective substrate (2-2: 0.11 mmol, 2-5: 0.16 mmol) was then added in one

equivalence. The reaction mixture was transferred to a NMR tube, sealed and heat at the reported

conditions. Often the evolution of H2 gas could be observed. The reaction could be monitored and

characterized by 1H NMR, 13C NMR and 31P NMR spectroscopy. Upon completion the mixture

was cooled to room temperature. Solvent was removed, residue was washed with hexanes and the

result was eluted with dichloromethane through silica affording the final product.

Characterization Data for Reduction of Phosphine Oxides

Triphenylphosphine

Isolated Yield = 87%. 1H NMR (600 MHz, C6D5Br): δ = 7.30 (6H, m, Ph), 7.20 (3H, m, Ph),

7.14 (6H, s, Ph)); 31P{1H} NMR (243 MHz, C6D5Br): δ = –5.4 (s); 13C{1H} NMR (151 MHz,

C6D5Br): δ = 137.6 (d, 1JCP = 12 Hz, Ph), 128.7 (s, Ph), 128.5 (s, Ph), 128.4 (s, Ph) ppm. DART

MS: m/z: 263.09919 (calcd. for [M]H+: 263.098962).

Triethylphosphine

1H NMR (600 MHz, C6D5Br): δ = 1.21 (6H, dq, 2JPH = 7, 3JHH = 1 Hz, PCH2CH3), 1.94 (9H, m,

PCH2CH3); 31P{1H} NMR (243 MHz, C6D5Br): δ = –19.9 (s); 13C{1H} NMR (151 MHz,

C6D5Br): δ = 20.1 (d, 1JCP = 65 Hz, PCH2), 9.8 (d, 2JCP = 13 Hz, PCH2CH3) ppm. DART MS:

m/z: 119.09877 (calcd. for [M]H+: 119.098963).

180

Diphenylphosphine

1H NMR (400 MHz, C6D5Br): δ = 7.02 (4H, m, Ph), 6.88 (2H, m, Ph), 6.74 (4H, m, Ph), 4.80

(1H, d, 1JPH = 216 Hz, PH); 31P{1H} NMR (162 MHz, C6D5Br): δ = –40.5 (s); 31C{1H} NMR

(101 MHz, C6D5Br): δ = 134.9 (d, 1JCP = 11 Hz, Ph), 128.6 (s, Ph), 128.5 (s, Ph), 128.4 (s, Ph)

ppm.

(Pentafluorophenyl)bisphenylphosphine

Isolated Yield = 83%. 1H NMR (400 MHz, CD2Cl2): δ = 7.41 (10H, m, Ph); 31P NMR{1H} (162

MHz, CD2Cl2): δ = –24.9 (t, 3JPF = 37 Hz); 19F{1H} NMR (377 MHz, CD2Cl2): δ = –128.0 (2F,

m, m-F), –151.2 (1F, t, 3JFF = 22, 4JFF = 4 Hz, p-F), –161.4 (2F, m, o-F); 31C{1H} NMR (101 MHz,

CD2Cl2): δ = 148.7 (d, 1JCF = 247 Hz, C6F5), 142.9 (d, 1JCF = 253 Hz, C6F5), 138.1 (d, 1JCF = 254

Hz, C6F5), 133.8 (d, 1JCP = 11 Hz, Ph), 133.2 (d, 2JCP = 21 Hz, Ph), 129.1 (d, 3JCP = 7 Hz, Ph),

129.8 (s, Ph), 115.5 (m, C6F5) ppm. DART MS: m/z: 353.051853 (calcd. for [M]H+: 353.05277).

Tris(pentafluorophenyl)phosphine

Isolated Yield = 85%. 31P{1H} NMR (162 MHz, CD2Cl2): δ = –74.5 (sept, 3JPF = 36 Hz); 19F{1H}

NMR (377 MHz, CD2Cl2): δ = –130.4 (6F, m, m-F), –148.7 (3F, tt, 3JFF = 20 Hz, 4JFF = 5 Hz, p-

F), –160.5 (6F, m, o-F); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 148.2 (d, 1JCF = 248 Hz, C6F5),

143.6 (d, 1JCF = 259 Hz, C6F5), 138.2 (d, 1JCF = 253 Hz, C6F5), 104.7 (m, C6F5) ppm.

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Bis(diphenylphosphanyl)methane

1H NMR (600 MHz, CD2Cl2): δ = 7.54 (8H, m, Ph), 7.39 (12H, m, Ph), 2.92 (2H, t, 2JHP = 2 Hz,

PCH2P); 31P{1H} NMR (243 MHz, CD2Cl2): δ = –22.4 (s); 13C{1H} NMR (151 MHz, CD2Cl2):

δ = 138.9 (t, 1JCP = 4 Hz, Ph), 133.0 (t, 2JCP = 10 Hz, Ph), 128.8 (s, Ph), 128.5 (t, 3JCP = 4 Hz, Ph),

28.2 (t, 1JCP = 23 Hz, PCH2P) ppm. DART MS: m/z: 385.12786 (calcd. for [M]H+: 385.127500).

Bis(diphenylphosphanyl)methane monoxide


7.28 (6H, m , Ph), 3.13 (2H, d, 2JHP = 13 Hz, PCH2P); 31P{1H} NMR (243 MHz, CD2Cl2):

δ = 28.2 (1P, d, 2JPP = 50 Hz, Ph2PO), –28.0 (1P, d, 2JPP = 50 Hz, Ph2P); 13C{1H} NMR (151 MHz,

CD2Cl2): δ = 139.0 (d, 1JCP = 15 Hz, 3JCP = 7.3 Hz, Ph), 134.6 (dd, 1JCP = 100 Hz, 3JCP = 2 Hz,

Ph), 133.3 (d, 2JCP = 28 Hz, Ph), 132.2 (d, 3JCP = 2.9 Hz, Ph) 131.4 (dd, 2JCP = 9 Hz, 4JCP = 2 Hz,

Ph) 129.4 (s, Ph), 129.1 (d, 3JCP = 7 Hz, Ph), 129.0 (d, 4JCP = 2 Hz, Ph) 31.4 (dd, 1JCP = 68 Hz, 1JCP

= 33 Hz, PCH2P) ppm.

2,2’-bis(diphenylphosphanyl)-1,1’-binaphthalene

182

Isolated Yield = 48%. 1H NMR (600 MHz, CDCl3): δ = 7.77 (2H, d, 3JHH = 8 Hz, Ar), 7.71 (2H,

d, 3JHH = 8 Hz, Ar), 7.64 (4H, m, Ar), 7.33-6.95 (18H, m, Ar), 6.79 (2H, m, Ar), 6.70 (2H, d, 3JHH

= 8 Hz, Ar), 6.53 (2H, d, 3JHH = 8 Hz, Ar); 31P{1H} NMR (243 MHz, CDCl3): δ = –15.5 (s);

13C{1H} NMR (151 MHz, CDCl3): δ = 145.4 (d, 1JPC = 4 Hz, Ar), 138.4 (m, Ar), 137.8 (m, Ar),

135.9 (m, Ar), 134.5 (m, Ar), 133.4 (m, Ar), 130.2 (s, Ar), 130.0 (s, Ar), 129.8 (s, Ar), 128.9 (s,

Ar), 127.8 (m, Ar), 127.3 (s, Ar), 127.1 (s, Ar), 126.3 (s, Ar) ppm. DART MS: m/z: 623.20513

(calcd. for [M]H+: 623.205751).

This reaction was done for the enantiopure (R)-BINAP dioxide as starting materials. Both HPLC

chromatograms shown below were run in a chiral column (ChiralPak iA), confirming that retention

of the configuration is achieved during the reduction. Racemic mixture of the BINAP dioxide was

used as a reference. Upon completion of catalysis the BINAP product was characterized by

multinuclear NMR and then oxidized with H2O2 to regenerate the (R)-BINAP dioxide. The

enantiopure product was characterized by chiral HPLC, no separation could be obtained for the

reduced phosphine product.

Racemic mixture: 2 peaks observed. tr = 17.68 and 20.99 min.

Enantiopure (R) BINAP dioxide as starting material. 1 peak observed. tr = 19.72 min.

183

Reaction Mixture for Reduction of 2,2'-bis(diphenylphosphine oxide)-1,1'-binaphthyl with

Catalyst 2-5

184

Bis(diphenylphosphanyl)butane


2.20 (4H, m, PCH2), 1.84 (4H, m, PCH2CH2); 31P{1H} NMR (243 MHz, CD2Cl2): δ = –16.1 (s);

13C{1H} NMR (151 MHz, CD2Cl2): δ = 139.5 (d, 1JCP = 14 Hz, Ph), 133.0 (d, 2JCP = 19 Hz, Ph),

128.8 (s, Ph), 128.7 (d, 3JCP = 7 Hz, Ph), 28.0 (PCH2), 27.9 (s, PCH2CH2) ppm. DART MS: m/z:

427.17348 (calcd. for [M]H+ 427.174451).

Reaction Mixture for Reduction of 2,2'-bis(diphenylphosphine oxide)-1,1'-binaphthyl with

Catalyst 2-5

185

Tris(4-(trifluoromethyl)phenyl)phosphine

1H NMR (400 MHz, C6D5Br): δ = 7.23 (6H, m, Ph), 7.00 (6H, m, Ph); 31P{1H} NMR (162 MHz,

C6D5Br): δ = –6.2 (s); 19F{1H} NMR (377 MHz, CD2Cl2): δ = –63.2; 13C{1H} NMR (101 MHz,

CD2Cl2): δ = 141.0 (d, 1JPC = 14 Hz, Ph), 134.4 (d, 2JPC = 20 Hz, Ph), 132.9 (d, 3JPC = 11 Hz, Ph)

125.9 (m, CF3), 125.7 (s, Ph) ppm.

(Methoxymethyl)diphenylphosphine

1H NMR (400 MHz, CD2Cl2): δ = 7.50 (4H, m, Ph), 7.40 (6H, m, Ph), 4.23 (2H, d, 2JPH = 6 Hz,

PCH2), 3.49 (3H, s, OCH3); 31P{1H} NMR (162 MHz, CD2Cl2): δ = –20.8 (s); 13C{1H} NMR

(101 MHz, CD2Cl2): δ = 137.2 (d, 1JCP = 13 Hz, Ph), 128.7 (s, Ph), 128.5 (s, Ph), 128.4 (s, Ph),

74.1 (d, 1JCP = 7 Hz, PCH2), 60.14 (d, 3JCP = 8 Hz, OCH3) ppm. DART MS: m/z: 231.09386

(calcd. for [M]H+: 231.093878).

1-(4-(Diphenylphosphanyl)phenyl)-2,2-dimethylpropan-1-one

1H NMR (400 MHz, C6D5Br): δ = 7.53 – 7.17 (14H, m, Ph), 1.23 (9H, s, tBu); 31P{1H} NMR

(162 MHz, CD2Cl2): δ = –5.5 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 209.0 (s, PhCO), 141.6

(d, 1JCP = 14 Hz, Ph), 139.0 (s, Ph), 137.0 (d, 1JCP = 11 Hz, Ph), 134.2 (d, 2JCP = 20 Hz, Ph), 133.3

186

(d, 2JCP = 17 Hz, Ph), 129.4 (s, Ph), 129.0 (d, 3JCP = 8 Hz, Ph), 128.1 (d, 3JCP = 7 Hz, Ph), 44.5 (s,

C(CH3)3), 28.1 (s, C(CH3)3) ppm.

4-(Diphenylphosphanyl)benzoic acid

Isolated Yield = 71%. 1H NMR (600 MHz, CD2Cl2): δ = 11.87 (1H, b, COOH), 8.07 (2H, m,

Ph), 7.41 (12H, m, Ph); 31P{1H} NMR (243 MHz, C6D5Br): δ = –5.3 (s); 13C{1H} NMR (151

MHz, CD2Cl2): δ = 171.6 (s, COOH), 146.0 (d, 1JCP = 15 Hz, Ph), 136.6 (d, 1JCP = 11 Hz, Ph),

134.4 (d, 2JCP = 20 Hz, Ph), 133.5 (d, 2JCP = 9 Hz, Ph), 130.1 (d, 3JCP = 6 Hz, Ph), 129.6 (s, Ph),

129.4 (s, Ph), 129.1 (d, 3JCP = 7 Hz, Ph) ppm. DART MS: m/z: 307.08901 (calcd. for [M]H+:

307.088793).

2-(Diphenylphosphanyl)-N,N-dimethylaniline


6.90 (1H, m, Ph), 6.67 (1H, m, Ph), 2.51 (6H, s, NCH3); 31P{1H} NMR (162 MHz, CD2Cl2): δ = –

13.6 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 158.5 (d, 2JCP = 19 Hz, Ph), 138.9 (d, 1JCP = 13

Hz, Ph), 135.2 (d, 1JCP = 11 Hz, Ph), 134.1 (d, 2JCP = 20 Hz, Ph), 131.8 (d, 3JCP = 10 Hz, Ph), 131.3

(d, 3JCP =3 Hz, Ph), 128.6 (s, Ph), 128.4 (d, 2JCP = 12 Hz, Ph), 124.8 (s, Ph), 121.1 (d, 3JCP = 3 Hz,

Ph), 45.6 (d, 4JCP = 4 Hz, NCH3) ppm. DART MS: m/z: 306.14168 (calcd. for [M]H+:

306.141162).

2-(Diphenylphosphanyl)pyridine

187

1H NMR (400 MHz, CD2Cl2): δ = 8.58 (1H, d, 3JHH = 4 Hz, Ar), 7.48 (1H, m, Ar), 7.28 (10H, m,

Ar), 7.10 (1H, m, Ar), 7.03 (1H, d, 3JHH = 8 Hz, Ar); 31P{1H} NMR (162 MHz, C6D5Br): δ = –

4.2 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 150.6 (d, 1JCP = 12 Hz, Ar), 136.8 (d, 1JCP = 11

Hz, Ar), 136.0 (d, 3JCP = 3 Hz, Ar), 134.6 (d, 2JCP = 20 Hz, Ar), 132.4 (d, 3JCP = 9 Hz, Ar), 129.4

(s, Ar), 128.9 (d, 3JCP = 7 Hz, Ar), 128.3 (d, 2JCP = 19 Hz, Ar), 122.6 (s, Ar) ppm. DART MS:

m/z: 264.09382 (calcd. for [M]H+: 264.094211).

N-(Diphenylphosphanyl)-1,1-diphenyl-N-(p-tolyl)phosphanamine

1H NMR (400 MHz, CD2Cl2): δ = 7.24 (10H, m, Ph), 6.67 (2H, d, 3JHH = 8 Hz, Ph), 6.44 (2H, d,

3JHH = 8 Hz, Ph), 2.09 (3H, s, PhCH3); 31P{1H} NMR (243 MHz, CD2Cl2): δ = –40.2 (s); 13C{1H}

NMR (101 MHz, CD2Cl2): δ = 139.8 (t, 2JCP = 7 Hz, Ph), 135.7 (dd, 1JCP = 20 Hz, 3JCP = 7 Hz,

Ph), 133.6 (d, 2JCP = 12 Hz, Ph), 131.6 (s, Ph), 130.1 (s, Ph), 129.4 (s, Ph), 129.1 (s, Ph), 128.4 (d,

3JCP = 3 Hz, Ph), 116.4 (s, Ph), 20.9 (s, PhCH3) ppm.

Reaction Mixture for Reduction of N-(diphenylphosphoryl)-P,P-diphenyl-N-(p-

tolyl)phosphinic amide with Catalyst 2

188

(2S, 5S)-2,5-Diphenylphospholane


6.83 (3H, m, Ph), 3.78 (1H, m, PhCH), 3.35 (1H, dt, 1JHP = 186 Hz, 3JHH = 12 Hz, PH), 3.28 (1H,

m, PhCH), 2.43 (1H, m, CH2CHPh), 2.33 (1H, m, CH2CHPh), 1.92 (1H, m, CH2CHPh), 1.57 (1H,

m, CH2CHPh); 31P{1H} NMR (243 MHz, C6D5Br): δ = –19.1 (s); 13C{1H} NMR (101 MHz,

C6D5Br): δ = 41.2 (d, 2JPC = 11 Hz, CH2CHPh), 44.9 (d, 1JPC = 13 Hz, PhCHPH), 127.2 (s, Ph),

128.6 (s, Ph), 141.6 (s, Ph), 145.3 (d, 2JPC = 17 Hz, Ph) ppm. DART MS: m/z: 241.11457 (calcd.

for [M]H+: 241.114613).

189

General Procedure for Rearrangement of Phosphorous Esters to Phosphine Oxides


catalyst 2-2 (5 mol%, 5 mg) was added to a solution of the respective substrate in C6D6 (0.7 mL). The

reaction mixture was transferred to a NMR tube, sealed and heat at the reported conditions. The reaction

could be monitored and characterized by 31P NMR, 1H NMR and 13C NMR spectroscopy.

General Procedure for Reduction of Phosphine Oxides to Phosphines

In a glove box, to the reaction mixture of the respective phosphine oxide formed in the rearrangement,

3 equivalents of PhSiH3 were added (0.3 mmol, 32 mg). The reaction mixture was again sealed and heat

190

at the reported conditions. The reaction could be monitored and characterized by 31P NMR, 1H NMR

and 13C NMR spectroscopy.

General Procedure for Synthesis for Phosphite

The general procedure for phosphites was adapted from the literature.64-66 In a Schlenk tube under

an atmosphere of nitrogen phenol (34 mmol), chlorophosphine (34 mmol) and 50 mL of dry

toluene was added. To the solution triethylamine (7.0 mL, 50 mmol) was add dropwise. The

resulting solution was heated at 100 oC overnight. The Et3NHCl precipitate was removed by

filtration over celite and the solvent removed under reduced pressure to yield either an oil or solid.

For phenyl di(o-tolyl)phosphite, the crude product was dissolved in n-pentane and filter over a

silica plug. Organic solvent was removed under reduced pressure the product was isolated.

General Procedure for Synthesis of PH3 and its PH3•B(C6F5)

In a glove box, catalyst 2-2 (5 mol%, 5 mg) was added to a solution of phosphite (0.1 mmol) and

PhSiH3 (0.3 mmol, 32 mg) in C6D6 (0.7 mL). The reaction mixture was transferred to a NMR tube,

sealed and heat at the reported conditions. The reaction could be monitored and characterized by

31P NMR and 1H NMR spectroscopy. The same reaction was carried out in the presence of

B(C6F5)3 (0.1 mmol, 51 mg) to afford the corresponding adduct with the PH3 formed.

General Procedure for Synthesis of PH3•Ga(C6F5)3

In a glove box, catalyst 2-2 (5 mol%, 5 mg) was added to a solution of P(OPh)3 (0.1 mmol, 29 mg)

and PhSiH3 (0.3 mmol, 32 mg) in C6D6 (0.7 mL). The reaction mixture was transferred to a NMR

tube, sealed and heat at the reported conditions. The reaction could be monitored and characterized

by 31P NMR and 1H NMR spectroscopy. Upon completion one equivalence of Ga(C6F5)3 (0.1

mmol, 54 mg) was added to the solution, adduct formation was detected by 31P NMR spectroscopy.

191

General Procedure for Synthesis of PH3•Al(C6F5)3

In a glove box, catalyst 2-2 (5 mol%, 5 mg) was added to a solution of P(OPh)3 (0.1 mmol, 29 mg)

and PhSiH3 (0.3 mmol, 32 mg) in C6D6 (0.7 mL). The reaction mixture was transferred to a NMR

tube, sealed and heat at the reported conditions. The reaction could be monitored and characterized

by 31P NMR and 1H NMR spectroscopy. Upon completion one equivalence of Al(C6F5)3 (0.1

mmol, 50 mg) was added to the solution, adduct formation was detected by 31P NMR spectroscopy.

Characterization Data for Products

Dimethoxymethylphosphine oxide

1H NMR (400 MHz, C6D6): δ = 3.16 (6H, d, 3JHP = 11 Hz, OCH3), 0.87 (3H, d, 2JHP = 17 Hz,

PCH3); 31P{1H} NMR (162 MHz, C6D6): δ = 31.8 (s); 13C{1H} NMR (101 MHz, C6D6): δ = 51.4

(d, 2JCP = 6 Hz, OCH3), 9.8 (d, 1JCP = 144 Hz, PCH3) ppm.

Diethoxyethylphosphine oxide

1H NMR (400 MHz, C6D6): δ = 3.90 (4H, m, OCH2), 1.48 (2H, dquart, 3JHH = 8 Hz, 2JHP = 18

Hz, PCH2), 1.04 (6H, t, 3JHH = 7 Hz, OCH2CH3), 0.99 (3H, dt, 3JHH = 8 Hz, 3JHP = 13 Hz,

PCH2CH3); 31P{1H} NMR (162 MHz, C6D6): δ = 32.3 (s); 13C{1H} NMR (101 MHz, C6D6):

δ = 60.9 (d, 2JCP = 6 Hz, OCH2), 19.3 (d, 1JCP = 143 Hz, PCH2), 16.4 (d, 3JCP = 6 Hz, OCH2CH3),

6.7 (d, 2JCP = 7 Hz, PCH2CH3) ppm.

192

Diisopropoxyphosphine oxide

1H NMR (400 MHz, C6D6): δ = 6.81 (1H, d, 1JHP = 681 Hz, PH), 4.64 (2H, m, CH(CH3)2), 1.17

(6H, t, 3JHH = 6 Hz, CH(CH3)2), 1.14 (6H, d, 3JHH = 6 Hz, CH(CH3)2); 31P{1H} NMR (162 MHz,

C6D6): δ = 4.0 (dt, 1JPH = 681 Hz, 3JPH = 7 Hz); 13C{1H} NMR (101 MHz, C6D6): δ = 70.1 (d,

2JCP = 6 Hz, OCH(CH3)2), 23.8 (d, 3JCP = 4 Hz, OCH(CH3)2), 23.6 (d, 3JCP = 4 Hz, OCH(CH3)2)

ppm.

Methyldiphenylphosphine Oxide

1H NMR (400 MHz, C6D6): δ = 7.61 (4H, m, Ph), 7.17 (6H, m, Ph), 1.66 (3H, d, 2JHP = 13 Hz,

PCH3); 31P{1H} NMR (162 MHz, C6D6): δ = 25.9 (s); 13C{1H} NMR (101 MHz, C6D6):

δ = 136.0 (d, 1JCP = 99 Hz, Ph), 131.3 (d, 4JCP = 3 Hz, Ph), 130.7 (d, 3JCP = 10 Hz, Ph), 128.6 (d,

2JCP = 12 Hz, Ph), 16.5 (d, 1JCP = 73 Hz, PCH3) ppm.

Ethyldiphenylphosphine Oxide

1H NMR (400 MHz, CD2Cl2): δ = 7.77 (4H, m, Ph), 7.14 (6H, m, Ph), 1.93 (2H, dquart, 2JHP =

13 Hz, 3JHH = 8 Hz, PCH2), 1.06 (3H, dt, 3JHP = 17 Hz, 3JHH = 8 Hz, PCH2CH3); 31P{1H} NMR

(162 MHz, CD2Cl2): δ = 29.1 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 140.2 (d, 1JCP = 19

193

Hz, Ph), 134.5 (s, Ph), 131.1 (d, 3JCP = 9 Hz, Ph), 128.6 (d, 2JCP = 12 Hz, Ph), 23.0 (d, 1JCP = 73

Hz, PCH2), 5.61 (d, 2JCP = 5 Hz, PCH2CH3) ppm.

Methyldiphenylphosphine

1H NMR (400 MHz, CD2Cl2): δ = 7.31 (4H, m, Ph), 7.24 (6H, m, Ph), 1.52 (3H, d, 2JHP = 4 Hz,

PCH3); 31P{1H} NMR (162 MHz, CD2Cl2): δ = –27.1 (s); 13C{1H} NMR (101 MHz, CD2Cl2):

δ = 140.9 (d, 1JCP = 13 Hz, Ph), 132.4 (d, 2JCP = 19 Hz, Ph), 128.8 (d, 3JCP = 6 Hz, Ph), 128.7 (s,

Ph), 12.5 (d, 1JCP = 14 Hz, PCH3) ppm.

Methyldiphenylphosphine

1H NMR (400 MHz, CD2Cl2): δ = 7.39 (4H, m, Ph), 7.10 (6H, m, Ph), 1.87 (2H, d, 3JHH = 8 Hz,

PCH2), 1.03 (3H, d, 2JHP = 13 Hz, 3JHH = 8 Hz, PCH2CH3); 31P{1H} NMR (162 MHz, CD2Cl2):

δ = –11.8 (s); 13C{1H} NMR (101 MHz, CD2Cl2): δ = 139.7 (d, 1JCP = 15 Hz, Ph), 133.1 (d, 2JCP

= 18 Hz, Ph), 128.6 (s, Ph), 128.5 (d, 3JCP = 2 Hz, Ph), 21.0 (d, 1JCP = 12 Hz, PCH2), 10.3 (d, 2JCP

= 17 Hz, PCH2CH3) ppm.

Phenylphosphine

1H NMR (400 MHz, C6D6): 3.87 (d, 1JHP = 200 Hz, PH2), 6.87 (m, Ph), 7.08 (m, Ph), 7.21 (m,

Ph); 31P NMR (162 MHz, C6D6): –122.43 (t, 1JHP = 200 Hz) ppm.

Phosphine-Tris(pentafluorophenyl)borane adduct

194

1H NMR (400 MHz, C6D6): δ = 3.16 (3H, d, 1JHP = 409 Hz); 31P NMR (162 MHz, C6D6): δ = –

100.0 (quart, 1JHP = 409 Hz); 11B NMR (128 MHz, C6D6): δ = –16.0 ppm.

195

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199

Chapter 5 Towards Tricationic and Tetracationic Electrophilic Phosphonium

Salts

5.1 Introduction

5.1.1 Polycationic Phosphorus Cations

Since the publication of 2-5, 2-10, and 2-13, it has been demonstrated that dicationic phosphonium

salts are a new avenue to highly electrophilic phosphonium cations that can effect the

hydrodefluorination of fluoroalkanes, hydrosilylation of olefins and alkynes, deoxygenation of

ketones, and the reduction of phosphine oxides and amides.1-4 The Stephan group has expanded

the library of dicationic fluorophosphonium salts (Figure 5-1) derived from diphosphines5-7 and

pyridylphosphines.8 These species offer the advantage of not requiring the electron-withdrawing

fluorinated aryl substituents. More recently, the Stephan group reported the readily accessible PNP

pincer ligand as a new scaffold for both dicationic and tricationic phosphonium salts.9

Figure 5-1 Family of Phosphonium Salts

In related work, Burford has developed tricationic group 15 salts and demonstrated their reactivity

with 4-dimethylaminopyridine.10 Most routes to polycationic phosphenium cations employ

chlorophosphine with a Lewis base, sometimes promoted by halide abstraction. Lewis bases that

have been used to trap these cationic phosphines include pyridines, tertiary phosphines, and

NHC.11-14 In essence, this is the synthetic route employed in Chapter 2 to prepare 2-3.1

200

Imidazolium carboxylates and 2-silylimidazolium salts can be employed as convenient starting

materials to trap these phosphenium cations when the free carbene cannot be isolated or generated

in situ.15,16 An alternative route to dicationic phosphorus(III) and tricationic phosphorus(III)

compounds have been described by reacting cyclopropenium salts with RP(TMS)2 (R = alkyl/aryl)

or P(TMS)3.17-19 Finally, the selective alkylation of 2-phosphinoimidazole has also been reported

as a convenient route to the corresponding cation.20-22

Despite these reports, polycationic Lewis acids remain underexplored in the literature. Drawing

on the precedence established by previous research on cationic phosphorus(III) centres and the

dicationic phosphonium ion work done by the Stephan group, the synthesis of tricationic and

tetracationic phosphonium salts was embarked on.


Three synthetic approaches were undertaken for the preparation of polycationic phosphonium

Lewis acids: multiple carbenes at one phosphorus centre, multiple phosphorus centres on one

imidazolium ring, and linked carbene-stabilized phosphenium centres.

5.2.1 Two Carbenes One Phosphorus Centre

The first synthetic approach was inspired by Andrieu, where an imidazolium carboxylate

zwitterion was reacted with dichlorophosphine to yield the phosphenium dication.23 Initial

attempts to oxidize this chloride salt with XeF2 and electrophilic fluorinating agents like NFSI or

N-fluoropyridinium salts resulted in inseparable mixtures. However, exchanging the chloride

counterion for a [OTf]– using TMSOTf had an enormous impact. There was no observable change

in the 1H NMR and 31P NMR spectra for 5-1 when compared to the chloride salt, however

phosphenium dication 5-1 could be oxidized to the corresponding difluorophosphorane 5-2 using

XeF2 at –40 °C (Scheme 5-1). This species was observed in the 31P NMR spectrum as a triplet at

–64.5 ppm with 1JPF = 726 Hz, as well as a doublet in the 19F NMR spectrum at –29.2 ppm (Figure

5-2).

201



Subsequent attempts to abstract a fluoride from 5-2 using TMSOTf, B(C6F5)3, Al(C6F5)3,

[SiEt3(tol)][B(C6F5)4], Ag(OTf), Mg(OTf)2, and SbF5 at room temperature and 50 °C proved to be

unsuccessful. Only the starting materials could be observed for these reactions, with the exception

of [SiEt3(tol)][B(C6F5)4], where the [OTf]– counterions were exchanged for the borate anions.

202

5.2.2 One Carbene Multiple Phosphorus Centres

The second synthetic approach was inspired by the 4,5-diphosphino-substituted imidazolium salt

prepared from methylimidazole and chlorophosphine, by the Ruiz group22 (Scheme 5-2). This

compound was later applied by the Ruiz group as both carbene and phosphine donors to generate

mixed heterometallic complexes.22,24


The 4,5-diphosphino-substituted imidazolium salt could be readily oxidized using XeF2 to the

bis(difluorophosphorane) 5-4 shown in Figure 5-3. This bis(difluorophosphorane) could be

observed in the 31P NMR spectrum at –58.4 ppm as a triplet with a coupling constant of 699 Hz.

The corresponding 19F NMR spectrum showed a doublet at –32.4 ppm with the same coupling

constant, in addition to the [OTf]– signal observed at –79.1 ppm. The structure of 5-4 was

confirmed using X-ray crystallography (Figure 5-4).

203


The molecular structure of 5-4 shows an average P–C bond length of 1.825(6) Å between the

phosphorus centre and the imidazolium ring. This bond length is shorter than the P–C bond length

observed from the cationic difluorophosphorane 2-4 (1.866(5) Å) discussed in Chapter 2, where

the phosphorus is substituted in the normal position on the carbene. As expected, each phosphorus

centre is in a distorted trigonal bipyramidal arrangement, with an average P–F bond length of

1.656(3) Å and average F–P–F bond angle of 173.8(3)°, both of which are consistent with the bond

metric data observed for 2-4.

204



Attempts to abstract the two fluoride anions from 5-4 failed to give clean product. Abstraction

with [SiEt3(tol)][B(C6F5)4], Ag(OTf), TMSOTf and Mg(OTf)2 resulted in a mixture of products.

However, in fluorobenzene (FC6H5) when using 3 eq. of [SiEt3(tol)][B(C6F5)4] with 5-4, although

not cleanly, a quartet in the 31P NMR spectrum could be observed at 14.5 ppm with a coupling

constant of 380 Hz (Figure 5-4). This splitting pattern is consistent with 1JPF coupling of each P

atom with two inequivalent fluorine centres. Furthermore, in the 19F NMR spectrum a broad triplet

could be observed around –52.9 ppm, suggestive of fluxional behavior. However, variable

temperature NMR (VT NMR) spectroscopy of this reaction mixture was inconclusive. In the crude

reaction mixture the triflate anion, fluorobenzene, borate anion and Et3SiF could all be observed.

This species was found to have limited stability, with complete decomposition observed at room

temperature within a few hours, and significant decomposition observed over 48 hours at –40 °C.

Although complete NMR data, EA, and X-ray crystallographic data for this compound could not

be attained, it is thought to be 5-5 (Figure 5-5). Attempts to attain the bis(fluorophosphonium)

trication via oxidation with electrophilic fluorinating agents, such as NFSI and N-fluoropyridinium

salts, also proved to be unsuccessful.

205

Figure 5-5 31P NMR and 19F NMR Spectra for Postulated Species 5-5

Given the difficulties associated with isolating 5-5 and the preparation of the

bis(fluorophosphonium) trication using a 4,5-diphosphino-substituted imidazolium salt, the 2-

position on the imidazolium ring was substituted. This 4,5-diphosphino-substituted imidazolium

salt was deprotonated with LiHMDS in toluene, the subsequent addition of

diphenylchlorophosphine gave 5-6 in high yield (Scheme 5-3). This compound appears in the 31P

NMR spectrum as two singlets at –17.0 and –28.4 ppm in a 1:2 ratio.

206


The oxidation of 5-6 with XeF2 at room temperature gave an inseparable mixture of products.

However the counterion exchange with [SiEt3(tol)][B(C6F5)] yielded the borate salt. No significant

difference was observed in the 31P NMR spectra between the [OTf]– or [B(C6F5)4]– salts. Repeating

the oxidation at –40 °C yielded the tri(difluorophosphorane) substituted imidazolium salt 5-7

shown in Figure 5-6. The 31P NMR spectrum revealed two overlapping triplets in a 2:1 ratio at –

57.9 and –62.3 ppm with coupling constants of 652 and 655 Hz, respectively. In the 19F NMR

spectrum two doublets could be detected at –31.8 and –41.6 ppm in a 2:1 ratio, as shown in Figure

5-6. These chemical shifts and coupling constants are consistent with the difluorophosphoranes

discussed in Chapter 2 as well as 5-4.

207


Similar to the reactivity of 5-4, attempts to abstract the fluoride anion were unsuccessful.

Furthermore, the use of NFSI as an electrophilic fluorinating agent with 5-6 resulted in the

detection of two doublets in the 31P NMR spectrum, one at 41.8 ppm with a coupling constant of

1013 Hz and the other at 39.5 ppm with a coupling constant of 1045 Hz in a 1:2 ratio. Both the

coupling constant and chemical shift of these signals are consistent with previously reported

fluorophosphonium species. As expected, two doublets in the 19F NMR spectra at –75.2 and –88.2

ppm were observed, again in a 1:2 ratio. The subsequent washing of this reaction mixture revealed

that this product was soluble in n-pentane, where changing intensities between the two signals in

both the 31P NMR and 19F NMR spectra were also observed. This observation suggests that the

two doublets in the 31P NMR and 19F NMR spectra are not structurally connected. The 1H NMR

208

spectra revealed a mixture of products as well as a downfield imidazolium proton signal, indicating

degradation of the phosphine substituent on the imidazolium ring.

Given the difficulties with generating fluorophosphonium substituted derivatives of this scaffold,

both 4,5-diphosphino-substituted imidazolium salts 5-3 and 5-6 were reacted with methyl triflate

in order to isolate the corresponding methylphosphonium trication and tetracation, respectively.

When 5-3 was allowed to react with an excess of methyl triflate, the oxidation of only one

phosphorus centre could be observed, even at elevated temperatures (Scheme 5-4). The resulting

phosphonium dication 5-8, appears in the 31P NMR spectrum as two doublets at 13.1 and –30.5

ppm with a 3JPP = 13 Hz. The phosphorus-phosphorus coupling could be confirmed by a homo-

decoupled 31P NMR experiments (Figure 5-7). Furthermore, in the 1H NMR spectrum a doublet at

3.51 ppm with a coupling constant of 14 Hz could be detected; this signal is the methyl substituent

on a phosphorus centre, confirmed by a 1H{31P} NMR experiment (Figure 5-8). It is also

noteworthy that from the 1H NMR spectrum, the two methyl substituents at the nitrogen centres

of the imidazolium ring are inequivalent. This linked phosphonium-phosphine was tested in FLP

activation of Et3SiH, CO2, CS2, H2, benzaldehyde, benzylazide, and t-butylisocyanate. For these

reactions, no small molecule activation could be observed, since the 31P NMR spectrum only

showed unreacted starting materials.


209

Figure 5-7 31P NMR Spectra for 5-8

210

Figure 5-8 1H NMR spectra for 5-8

The oxidation of 5-6 using methyl triflate resulted in bis(phosphonium) phosphine trication 5-9

shown in Figure 5-9 as the major product. This species appears in the 31P NMR spectrum as a

singlet at 18.9 ppm and two doublets at 15.7 and –26.6 ppm with a coupling constant of 17 Hz.

However the 1H NMR spectrum revealed inseparable minor impurities. At the time of this

dissertation attempts to further purify 5-9 were still underway.

211

Figure 5-9 31P NMR Spectrum of 5-9

5.2.3 Two Carbenes Two Phosphorus Centres

The final synthetic route employed a linked bis(carbene) ligand first reported by the Ingleson group

in 2011 (Scheme 5-5).25 Bis(phosphenium) dication 5-10 was prepared by deprotonating the linked

imidazolium salt with t-BuOK in ether at –40 °C, allowing the reaction mixture to slowly warm to

room temperature overnight, followed by filtration of the free carbene into a stirring solution of

diphenylchlorophosphine in toluene. The volatile solvents were then removed under reduced

pressure and the dicationic chloride salt could be isolated as a white solid. Prior to oxidation,

similar to the reactivity of 5-1, the counterions were exchanged with TMSOTf to yield 5-10.


212

Following the isolation of 5-10, the oxidation was attempted in DCM at –40 °C by portion-wise

adding 2 equivalents of XeF2. Close inspection of the 31P NMR spectrum reveals two triplets at –

64.0 and –64.1 ppm with coupling constants of 720 and 718 Hz, respectively. These signals are

consistent with two distinct phosphorane moieties with one bond P–F coupling. The corresponding

19F NMR signals can be observed at –39.8 and –38.3 ppm. Although the product from this reaction

mixture is not fully understood, it is postulated to be 5-11 (Figure 5-10). At the time for this

dissertation variable temperature experiments, clean isolation, and full characterization from this

this reaction mixture were underway.

Figure 5-10 31P NMR and 19F NMR Spectra for Reaction Mixture of 5-10 Oxidation

Preliminary attempts to abstract a fluoride from the product of the above reaction mixture proved

to be unsuccessful, giving multiple decomposition products, some of which are consistent with the

213

cleavage of the phosphorus unit from the carbene. Attempts to oxidize 5-10 with electrophilic

fluorinating agent NFSI also led to a mixture of products.

5.3 Conclusion

In summary, the synthesis of highly oxidized tricationic and tetracationic fluorophosphonium salts

was undertaken. To this end, three synthetic routes were investigated: oxidation of a dicationic

phosphenium centre, oxidation of a 4,5-phosphino-subsituted imidazolium cation, as well as a

linked phosphenium system. The reaction of these phosphenium salts with XeF2 often led to

isolable difluorophosphorane derivatives. However, the subsequent abstraction of fluoride gave

complicated results. The common decomposition pathway seemed to involve dissociation of the

phosphorus centre from the carbene framework. For 5-3, monoxidation with methyltriflate was

observed, giving the linked phosphonium-phosphenium dication 5-8. To date, this species has not

shown any FLP reactivity, however investigations are still underway. In a similar vein, 5-6 could

be bismethylated to yield a polycationic phosphonium-phosphenium salt (5-9), observed by 31P

NMR spectroscopy.

214



standard Schlenk techniques under an inert atmosphere of anhydrous N2. Dry, oxygen-free



molecular sieves (4 Å) prior to use. Deuterated benzene (C6D6) and d8-toluene were purchased

from Sigma-Aldrich, distilled from sodium and stored over molecular sieves (4 Å) for at least two

days prior to use. Deuterated dichloromethane (CD2Cl2) and bromobenzene (C6D5Br) were

purchased from Sigma-Aldrich, distilled from CaH2 and stored over molecular sieves (4 Å) for at

least two days prior to use. Reagents such as Cl2PPh, ClPPh2, TMSOTf, Ag(OTf), SbF5, Mg(OTf)2,

LiHMDS, XeF2, B(C6F5)3, and potassium t-butoxide were purchased either from Sigma-Aldrich,

Strem Chemicals or Alfa Aesar. Reagents such as Al(C6F5)3,26 [Et3Si(tol)][B(C6F5)4],

27

imidazolium carboxylate23, and 5-322 were prepared according to literature known procedures. All

glassware was oven-dried at temperatures above 180°C prior to use. NMR spectra were obtained

on an Agilent DD2-700 MHz, an Agilent DD2-500 MHz, a Bruker AvanceIII-400 MHz, or a

Varian Mercury-300 MHz spectrometer. All 13C NMR spectra were exclusively recorded with

composite pulse decoupling. Assignments of the carbon atoms in the 13C spectra were performed

via indirect deduction from the cross-peaks in 2D correlation experiments (HMBC; HSQC).

Chemical shifts were referenced to δTMS = 0.00 ppm (1H, 13C) and δH3PO4(85%) = 0.00 ppm (31P,

externally). Chemical shifts (δ) are reported in ppm, multiplicity is reported as follows (s = singlet,

d = doublet, t = triplet, quart. = quartet, m = multiplet) and coupling constants (J) are reported in

Hz. Assignments of individual resonances were done using 2D techniques (HMBC, HSQC, HH-

COSY) when necessary. Yields of products in solution were determined by integration of all

resonances observed in the respective NMR spectra if not stated otherwise. High-resolution mass

spectra (HRMS) were obtained on a micro mass 70S-250 spectrometer (EI), an ABI/Sciex QStar

Mass Spectrometer (DART), or on a JOEL AccuTOF-DART (DART). Elemental analyses (C, H,

N) were performed at the University of Toronto employing a Perkin Elmer 2400 Series II CHNS

Analyzer.

215

X-ray Diffraction Studies.

Single crystals were coated with Paratone-N oil, mounted using a glass fibre pin and frozen in the

cold nitrogen stream of the goniometer. Data sets were collected on a Bruker Apex II CCD

diffractometer which was equipped with a rotation anode using graphite-monochromated MoKα

radiation (λ = 0.71073 Å). Data reduction was performed using the SAINT software package.28

Data sets were corrected for absorption effects using SADABS routine (empirical multi-scan

method).29 Structure solutions were found by direct methods using SHELXS or by intrinsic

phasing using SHELXT. Full-matrix least-squares refinement of the initial solutions was carried

out on F2 using SHELXL, following standard practices.30,31 All non-hydrogen atoms were refined

anisotropically.

216

Preparation of [(IMe)2PPh][OTf] (5-1)

In 5 mL of acetonitrile a slurry of [(IMe)2PPh][Cl]2 (50 mg, 0.17 mmol) was prepared. To this

solution an excess of TMSOTf was added dropwise. After a few minutes, the solution became

colourless. The mixture was filtered over celite and subsequently washed with pentane (3 x 12

mL). 5-1 was isolated as a white solid and used without further purification (98% yield).

1H NMR (400 MHz, CD2Cl2): δ = 7.76 (2H, m, Ph), 7.59 (3H, m, Ph), 4.71 (4H, s, NCH), 3.65

(12H, s, CH3); 31P{1H} NMR (162 MHz, CD2Cl2): δ = –50.3 (s); 19F{1H} NMR (377 MHz,

CD2Cl2): δ = –78.8 ppm.

Preparation of [(IMe)2PF2Ph][OTf] (5-2)

A solution of 5-1 (30 mg, 0.05 mmol, 1.0 eq) in 5 mL of DCM was cooled to –40 °C. To a stirring

solution of this mixture XeF2 (9 mg, 0.05 mmol, 1.0 eq) was added portion wise. The solution was

left at –40 °C overnight. Subsequent washing with of pentane (3 x 12 mL) yielded 5-2 as a white

solid (93% yield).

1H NMR (500 MHz, CD2Cl2): δ = 8.26 (2H, m, Ph), 7.97 (1H, m, Ph), 7.81 (1H, m, Ph), 7.69 (1H

s, Ph), 7.68 (4H, s, NCH), 3.92 (12H, s, CH3); 13C{1H} NMR (126 MHz, CD2Cl2) could not be

obtained due to relaxation for the imidazole rings. 31P{1H} NMR (202 MHz, CD2Cl2): δ = –64.5

217

(t, 1JPF = 726 Hz) ppm. 19F{1H} NMR (470 MHz, CD2Cl2): δ = –29.2 (2F, d, 1JPF = 726 Hz, PF2),

–78.8 (6F, s, OTf) ppm.

Preparation of [IMe(F2PPh2)2][OTf] (5-4)

XeF2 (55 mg, 0.32 mmol, 2.0 eq.) was suspended in a solution of [IMe(PPh2)2][OTf] (100 mg,

0.16 mmol, 1.0 eq) in CH2Cl2 (5 mL). The reaction mixture was stirred for 12 h at ambient

temperature giving a colourless solution. All volatiles were removed in vacuo. The residue was

washed with pentane (3 x 10 mL) and dried in vacuo yielding 5-4 as a colourless, microcrystalline

solid (96% yield). Single crystals suitable for X-ray diffraction structure determination were

obtained by slow diffusion of n-pentane into a CH2Cl2 solution.

1H NMR (500 MHz, CD2Cl2): δ = 9.40 (1H, s, N2CH), 7.73 (8H, m, Ph), 7.58 (4H, m, Ph), 7.54

(8H, m, Ph), 3.65 (6H, s, CH3) ppm. 13C{1H} NMR (126 MHz, CD2Cl2): δ = 142.6 (bt, 3JPC =

9Hz, m-Ph), 135.4 (dt, 2JPC = 15 Hz, 3JFC = 9 Hz, o-Ph) , 133.7 (dt, 1JPC = 184 Hz, 2JFC = 25 Hz,

i-Ph), 133.5 (d, 4JPC = 4 Hz, p-Ph), 129.3 (d, 1JPC = 18Hz, CPPh2), 121.3 (q, 1JFC = 320 Hz, OTf),

38.9 (s, CH3); 31P{1H} NMR (202 MHz, CD2Cl2): δ = –58.4 (t, 1JPF = 699 Hz) ppm. 19F{1H}

NMR (470 MHz, CD2Cl2): δ = –32.4 (d, 1JPF = 699 Hz), –79.1 ppm. Elemental Analysis for

C30H27F7N2P2O3S: calcd.: C 52.2, H 3.9, N 4.1; found: C 52.3, H 3.9, N 4.0.

Preparation of [IMe(PPh2)3][OTf] (5-6)

218

LiHMDS (27mg, 0.16 mmol, 1.0 eq) was added portion wise to a suspension of [IMe(PPh2)2][OTf]

(100 mg, 0.16 mmol, 1.0 eq) in toluene (12 mL). This solution was allowed to stir under ambient

conditions for 4 h giving a dark red solution. To this mixture Ph2PCl (36 mg, 0.16 mmol, 1.0 eq)

dissolved in 5 mL of toluene was added dropwise. After 10 minutes the solution precipitated an

off-white solid. The solution was allowed to stir under ambient conditions overnight. All volatiles

were removed in vacuo. The residue was dissolved in 5 mL of CH2Cl2 and filtered over celite. 5-6

was precipitated from pentane (17 mL x 3) in 84% isolated yield.

1H NMR (400 MHz, CD2Cl2): δ = 7.52 (9H, m, Ph), 7.38 (21H, m, Ph), 3.27 (6H, s, CH3); 13C{1H}

NMR (126 MHz, CD2Cl2): δ = 133.9 (d, 1JPC = 21 Hz, C-2), 132.5 (t, JPC = 8 Hz, CPPh2), 131.7

(s, Ph), 130.5 (s, Ph), 130.4 (s, Ph), 130.1 (s, Ph), 129.7 (t, JPC = 4 Hz, Ph), 37.9 (s, CH3); 31P{1H}

NMR (162 MHz, CD2Cl2): δ = –17.0 (1P, s, PCN2), –28.4 (2P, s, PCCN); 19F{1H} NMR (377

MHz, CD2Cl2): δ = –78.6 ppm. DART MS: m/z 649.20874 (calcd. for [M]+: 649.20914).

Preparation of [IMe(PF2Ph2)3][B(C6F5)4] (5-7)

To a solution of 5-6 (50 mg, 0.06 mmol, 1.0 eq) in 10 mL of toluene, freshly prepared

[Et3Si][B(C6F5)4]*2(C7H8) (56 mg, 0.06 mmol, 1.0 eq.) was added and the solution was allowed

to stir for 2 h under ambient conditions. All volatiles were removed under reduced pressure and

the residue washed with 20 mL of n-pentane. The resulting waxy white solid was dissolved in 10

mL of DCM and cooled to –40 °C. To this solution, XeF2 (32 mg, 0.18 mmol, 3 eq) was added

219

portion wise. The solution was allowed to warm to ambient conditions overnight, giving a clear

solution. The solution was reduced to 2 mL in vacuo and subsequently washed with n-pentane (3

x 15 mL). 5-7 was isolated as a white solid in 79 % yield.


(7H, m, Ph), 3.50 (6H, s, CH3); 11B{1H} (128 MHz, CD2Cl2): δ = −16.7 (s); 13C{1H} NMR (500

MHz, CD2Cl2): could not be obtained; 31P{1H} NMR (162 MHz, CD2Cl2): δ = –57.3 (2P, t, 1JPF

= 652 Hz, F2PCCN), –62.3 (1P, t, 1JPF = 655 Hz, F2PCN2); 19F{1H} NMR (377 MHz, CD2Cl2):

δ = –31.8 (4F, d, 1JPF = 652 Hz, F2PCCN), –41.6 (2F, d, 1JPF = 655 Hz, F2PCN2), –133.1 (8H, m,

o-C6F5), –163.6 (4H, m, p-C6F5), –167.6 (8H, m, m-C6F5) ppm.

Preparation of [IMe(PPh2)(PMePh2)][OTf]2 (5-8)

An excess of MeOTf was added to a solution of [IMe(PPh2)2][OTf] (100 mg, 0.16 mmol) in

CH2Cl2 (15 mL) and allowed to stir under ambient conditions. The solution was reduced to 2 mL

in vacuo and 5-8 precipitated from n-pentane in 91% yield.

1H NMR (400 MHz, CD2Cl2): δ = 8.73 (1H, s, HCN2), 8.00 (4H, m, Ph), 7.77 (4H, m, Ph), 7.62

(4H, m, Ph), 7.48 (5H, m, Ph), 7.29 (3H, s, Ph), 3.51 (3H, d, 2JPH = 14 Hz, PCH3), 3.41 (3H, s,

CH3), 3.25 (3H, s, CH3); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 133.6 (dd, JPC = 12, 1.4 Hz,

CCP), 132.0 (d, 1JPC = 19 Hz, i-Ph), 130.7 (d, 2JPC = 14 Hz, o-Ph), 130.2 (s, N2CH), 129.6 (d, 3JPC

= 7 Hz, m-Ph), 127.3 (d, 4JPC = 5 Hz, p-Ph), 37.9 (d, 1JPC = 153 Hz, PCH3), 34.1 (s, CH3); 31P{1H}

NMR (162 MHz, CD2Cl2): δ = 13.1 (1P, d, 3JPP = 13 Hz, PCH3), –30.5 (1P d, 3JPP = 13 Hz, PPh2);

19F{1H} NMR (470 MHz, CD2Cl2): δ = –78.8 ppm.

Preparation of [{(IDipp)2(PPh2)2}CH2][OTf]2 (5-10)

220

To a precooled solution of [(IDipp)2CH2][Br]2 (50 mg, 0.08 mmol, 1.0 eq) in ether at –40 °C, a

precooled solution of t-BuOK (18 mg, 0.16 mmol, 2.0 eq) also in ether was added dropwise. The

solution was allowed to warm to ambient conditions overnight, giving a dark red solution. This

solution was filtered over celite and dropwise added to a stirring solution of Ph2PCl (35 mg, 0.16

mmol, 2.0 eq) in toluene (7 mL). The resulting solution was allowed to stir overnight, giving an

off-white precipitate. All volatiles were removed in vacuo and the resulting residue was washed

with pentane (2 x 15 mL). The resulting white solid was dissolved in 10 mL of CH2Cl2 and an

excess of TMSOTf added. The solution was allowed to stir for 4 h under ambient conditions. The

CH2Cl2 solution was reduced to 2 mL under reduced pressure and 5-10 was precipitated from n-

pentane (85% yield).

1H NMR (400 MHz, CD2Cl2): δ = 9.03 (2H, d, 3JHH = 2.2 Hz, CH2NCH), 7.82 (2H, t, 3JHH = 2.2

Hz, DippNCH), 7.58 (4H, m, Ar), 7.46 (19H, m, Ar), 7.29 (5H, m, Ar), 6.56 (2H, s, NCH2N), 2.32

(4H, m, CH(CH3)2), 1.19 (12H, d, 3JHH = 7 Hz, CH(CH3)2), 0.94 (12H, d, 3JHH = 6.7 Hz,

CH(CH3)2); 13C{1H} NMR (126 MHz, CD2Cl2): δ = 147.1 (d, 1JPC = 63 Hz, i-Ph), 145.6 (s, Dipp),

134.2 (d, 2JPC = 22.1 Hz, o-Ph), 132.1 (s, Dipp), 131.0 (s, Dipp), 130.2 (d, 3JPC = 8.1 Hz, m-Ph),

128.3 (s, NCH), 126.3 (d, 4JPC = 7.7 Hz, p-Ph), 124.7 (s, Dipp), 62.1 (s, NCH2N), 28.9 (s,

CH(CH3)2), 26.0 (s, CH(CH3)2), 21.6 (s, CH(CH3)2); 31P{1H} NMR (162 MHz, CD2Cl2): δ = –

22.7 (s); 19F{1H} NMR (470 MHz, CD2Cl2): δ = –79.1 ppm. DART MS: m/z 419.2174 (calcd.

for [M]2+: 419.21465).

221

Table 5-1 Crystallographic data and details of the structure refinements of compound

5-4

5-4

formula C30H27F7N2O3P2S

Mr [g mol1] 690.53

colour, habit colourless, block

crystal system orthorhombic

Space group Pca21

a [Å] 20.0888(10)

b [Å] 12.1259(7)

c [Å] 25.0665(13)

[°] 90

[°] 90

[°] 90

V [Å3] 6106.1(6)

Z 8

T [K] 150(2)

Crystal size [mm] 0.600 x 0.400 x 0.400

c [g cm3] 1.502

F(000) 2832

min [°]

max [°]

1.625

27.645

Index range

–26 h 26

–15 k 15

–32 l 32

[mm1] 0.290

absorption correction SADABS

reflections collected 111626

reflections unique 14112

Rint 0.1111

reflection obs.

[F>2(F)] 9071

residual density

[e Å3] 0.655 - –0.452

parameters 816

GOF 1.027

R1 [I>2(I)] 0.0573

wR2 (all data) 0.1561

222

5.5 References


Edition 2014, 53, 6538-6541.





(4) Augurusa, A.; Mehta, M.; Perez, M.; Zhu, J. T.; Stephan, D. W. Chemical

Communications 2016, 52, 12195-12198.

(5) Holthausen, M. H.; Bayne, J. M.; Mallov, I.; Dobrovetsky, R.; Stephan, D. W.


(6) Holthausen, M. H.; Hiranandani, R. R.; Stephan, D. W. Chemical Science 2015,

6, 2016-2021.

(7) Mallov, I.; Stephan, D. W. Dalton Transactions 2016, 45, 5568-5574.

(8) Bayne, J. M.; Holthausen, M. H.; Stephan, D. W. Dalton Transactions 2016, 45,

5949-5957.

(9) Szkop, K. M.; Stephan, D. W. Dalton Transactions 2017, 46, 3921-3928.

(10) Chitnis, S. S.; Robertson, A. P. M.; Burford, N.; Patrick, B. O.; McDonald, R.;

Ferguson, M. J. Chemical Science 2015, 6, 6545-6555.

(11) Burford, N.; Losier, P.; Phillips, A. D.; Ragogna, P. J.; Cameron, T. S. Inorganic

Chemistry 2003, 42, 1087-1091.

(12) Burford, N.; Ragogna, P. J.; McDonald, R.; Ferguson, M. J. Journal of the


(13) Kuhn, N.; Fahl, J.; Bläser, D.; Boese, R. Zeitschrift für anorganische und

allgemeine Chemie 1999, 625, 729-734.

(14) Kuhn, N.; Henkel, G.; Göhner, M. Zeitschrift für anorganische und allgemeine

Chemie 1999, 625, 1415-1416.

(15) Azouri, M.; Andrieu, J.; Picquet, M.; Richard, P.; Hanquet, B.; Tkatchenko, I.

European Journal of Inorganic Chemistry 2007, 2007, 4877-4883.

(16) Weigand, J. J.; Feldmann, K.-O.; Henne, F. D. Journal of the American Chemical

Society 2010, 132, 16321-16323.

223

(17) Carreras, J.; Gopakumar, G.; Gu, L.; Gimeno, A.; Linowski, P.; Petuskova, J.;

Thiel, W.; Alcarazo, M. Journal of the American Chemical Society 2013, 135, 18815-18823.

(18) Carreras, J.; Patil, M.; Thiel, W.; Alcarazo, M. Journal of the American Chemical

Society 2012, 134, 16753-16758.

(19) Alcarazo, M. Chemistry - A European Journal 2014, 20, 7868-7877.

(20) Tolmachev, A. A.; Yurchenko, A. A.; Merculov, A. S.; Semenova, M. G.;

Zarudnitskii, E. V.; Ivanov, V. V.; Pinchuk, A. M. Heteroatom Chemistry 1999, 10, 585-597.

(21) Debono, N.; Canac, Y.; Duhayon, C.; Chauvin, R. European Journal of Inorganic

Chemistry 2008, 2008, 2991-2999.

(22) Ruiz, J.; Mesa, A. F. Chemistry – A European Journal 2012, 18, 4485-4488.

(23) Azouri, M.; Andrieu, J.; Picquet, M.; Cattey, H. Inorganic Chemistry 2009, 48,

1236-1242.

(24) Ruiz, J.; Mesa, A. F. Chemistry – A European Journal 2014, 20, 102-105.

(25) Zlatogorsky, S.; Muryn, C. A.; Tuna, F.; Evans, D. J.; Ingleson, M. J.

Organometallics 2011, 30, 4974-4982.

(26) Lee, C. H.; Lee, S. J.; Park, J. W.; Kim, K. H.; Lee, B. Y.; Oh, J. S. Journal of

Molecular Catalysis A: Chemical 1998, 132, 231-239.

(27) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430-2443.



(30) Müller, P. Crystal Structure Refinement: A Crystallographer's Guide to SHELXL;

Oxford University Press: Oxford ; New York, 2006.

(31) Müller, P. Crystallography Reviews 2009, 15, 57-83.

224

Chapter 6 Conclusion

6.1 Summary of This Work

6.1.1 Preparation of Dicationic Phosphonium Salts

Electrophilic phosphonium cation [(C6F5)3PF][B(C6F5)4] (2-2) was first reported by the Stephan

group in 2013.1 This species was found to be an active catalyst for the hydrodefluorination of

fluoroalkanes. However, this reactivity was dependent on the electron-withdrawing perfluorinated

arene substituents, as well as the P–F bond of this compound. A significant decrease in reactivity

was observed when substituting one of the C6F5 substituents for C6H5. The motivation for this

work was to establish a family of electrophilic phosphonium salts and study their activity toward

Lewis acid catalysis.

This was accomplished by reacting 1,3-dimesitylimidazolidin-2-ylidene (SIMes) with

diphenylchlorophosphine to produce the SIMes stabilized phosphenium cation. The subsequent

oxidation with XeF2, followed by fluoride abstract with [SiEt3(tol)][B(C6F5)4] yielded the isolable

dicationic phosphonium salt (2-5).2 The stoichiometric reaction with F2P(C6F5)3 revealed that this

dicationic salt was more fluorophilic when compared to the parent fluorophosphonium salt

discovered in 2013. Consequently, this fluorophosphonium cation was found to be active for the

hydrodefluorination of fluoroalkanes at room temperature.2 Following this work, the SIMes was

reacted with a series of chlorophosphines as a route to fluorophosphonium cations with varying

substituents at the phosphorus centre.3 Furthermore, a number of carbene-based ligands were

reacted with diphenylchlorophosphine to continue to expand this family of dicationic salts. To this

end, a variety of phosphenium cations with cAAC, triazole, and chiral ligands were discussed in

Chapter 2, as well as their corresponding difluorophosphoranes. Attempts to generate the

corresponding fluorophosphonium salts gave complicated results.

6.1.2 Lewis Acid Catalysis

More extensively discussed in Chapter 2, it was noted that 2-5 in the presence of an excess of

triethylsilane appeared to form a reversible adduct observed by 1H NMR spectroscopy. This led to

investigating 2-5 as the catalyst for hydrosilylation reactions. It was found that 2-5 could effect the

hydrosilylation of alkenes and alkynes at 45 °C,2 whereas the hydrosilylation of ketones with

225

aliphatic substituents could be observed under ambient conditions.3 Furthermore, employing 2.1

equivalents of silane at 50 °C led to the deoxygenation of ketones, yielding the alkanes and silyl

ethers in high yields. The deoxygenation of ketones with an aromatic substituent could be attained

at room temperature under short reaction times. This catalytic reduction was extended further to

amides; in this case tertiary amides and N-aryl-substituted secondary amides could be reduced to

the corresponding amine or aniline.4 Compared to the hydrosilylation of olefins and ketones, the

reduction of amides required the use of phenylsilane at elevated temperatures.

Following this observation, the reductive deoxygenation of phosphine oxides was undertaken. It

was found the phosphonium salt 2-5 could effect the reduction of phosphine oxides to phosphines

under mild conditions, with low catalyst loadings.5 It is noteworthy that this reduction showed

unexpected chemoselectivity. The reduction of phosphine oxides could be carried out in the

presence of ethers, carboxylic acids, ketones, and –CF3 functional groups.

Inspired by this work and other reports in the literature of Lewis acid catalyzed Michaelis-Arbuzov

rearrangements of phosphoethers, the rearrangement, followed by the subsequent reduction of

phosphites was attempted as a route to more reactive phosphines. It was found that catalyst 2-2

was an active catalyst for the Michaelis-Arbuzov rearrangement, however the examples of

phosphoethers that could be successfully transformed was limited. The attempts toward one-pot

rearrangement and reduction led to the discovery that 2-2 could catalyze the cleavage of P–OR

bonds, to yield the primary or secondary phosphines. It was found that air stable phosphites,

P(OR)3, could be employed as a feedstock for the highly reactive and explosive phosphine (PH3).

The subsequent Lewis acid adducts of PH3 could be observed by 31P NMR spectroscopy.

6.1.3 Towards Polycationic Phosphonium Salts

Given the high Lewis acidity observed with the dicationic fluorophosphonium 2-5, strides were

made to access polycationic phosphorus containing molecules. There were three synthetic

approaches taken. First and foremost, a dicationic phosphenium was prepared with two carbene

ligands. The subsequent oxidation of the phosphorus center could be attained to yield the

difluorophosphorane dication. However, the fluoride abstraction from this species proved to be

unsuccessful. Even in the presence of harsh fluoride abstracting agents like the silylium cation and

antimony pentafluoride only the starting material could be observed in the 31P NMR spectrum.

226

After this work, a 4,5-diphosphino-substituted imidazolium salt and a 2,4,5-triphosphino-

substituted imidazolium salt were prepared and could be successfully oxidized to their

corresponding fluorophosphorane-substituted derivatives. However, the fluoride abstraction from

this species often led to complicated decomposition products. In the case of the 4,5-diphosphino-

substituted imidazolium salt, the reaction with methyl triflate yielded the phosphonium-

phosphenium dication, which was tested in FLP reactivity. For the 2,4,5-triphosphino-substituted

imidazolium salt bisoxidation with methyl triflate was observed in the 31P NMR spectra,

suggesting a tricationic phosphonium-phosphenium salt. Investigations with these scaffolds are

still underway. Finally, a linked phosphenium dication was prepared. Results from the oxidation

from this species are under investigation.

6.2 Future Work

Generating isolable phosphonium trications or tetracations is still an area of investigation and

interest, as well as further establishing phosphonium-phosphine linked FLP systems for the

activation of small molecules. Alternatively, Gabbaï has reported the synthesis of a bis(antimony)-

substituted naphthalene dication and studied its ability to capture fluoride anions.6 In addition, the

carbene adducts of antimony and arsenic have been reported by Hudnall, Arduengo, and

Bertrand.7-9 These species have been investigated as the Pn(III) and Pn(I) (Pn = As, Sb) adducts.

Given this precedence, it would be interesting to prepare carbene-stabilized Pn(V) tetra-coordinate

dications and compare the Lewis acidity and stability to 2-5. (Figure 6-1)

Figure 6-1 Carbene-stabilized Pnictide Dications

227

6.3 References


341, 1374-1377.


Edition 2014, 53, 6538-6541.



(4) Augurusa, A.; Mehta, M.; Perez, M.; Zhu, J. T.; Stephan, D. W. Chemical

Communications 2016, 52, 12195-12198.



(6) Hirai, M.; Gabbaï, F. P. Angewandte Chemie International Edition 2015, 54,

1205-1209.

(7) Dorsey, C. L.; Mushinski, R. M.; Hudnall, T. W. Chemistry – A European

Journal 2014, 20, 8914-8917.

(8) Arduengo, A. J.; Calabrese, J. C.; Cowley, A. H.; Dias, H. V. R.; Goerlich, J. R.;

Marshall, W. J.; Riegel, B. Inorganic Chemistry 1997, 36, 2151-2158.

(9) Kretschmer, R.; Ruiz, D. A.; Moore, C. E.; Rheingold, A. L.; Bertrand, G.


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