Fluorophosphonium Chemistry: Applying Strategies Learned from Boron to Phosphorus

Shawn William Postle

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

Department of Chemistry University of Toronto

Shawn William Postle

Doctor of Philosophy

Department of Chemistry University of Toronto

Abstract

Since the inception of frustrated Lewis pair chemistry, interest in main group catalysts has undergone a resurgence. Central to the success of many main group systems is the pentafluorophenyl substituent, which provides both chemical stability and electrophilicity to the catalyst. Pentafluorophenyl substituents have been used with boranes, alanes, and recently in fluorophosphonium cations. This thesis investigates a range of related aryl substituents applied to fluorophosphonium chemistry to elicit new catalyst properties. Nitrene insertion into the bonds of borane substituents, including perfluorophenyl groups, was used to tune the electrophilicity of main group systems. Sterically demanding pentachlorophenyl substituents were used to add protection to sensitive fluorophosphonium catalysts. Perfluorobiphenyl groups were used to generate more electrophilic fluorophosphonium catalysts. Binaphthyl substituents were employed to create chiral fluorophosphonium cations.

This work is dedicated in memory of my grandpa,

William Kerr, for instilling in me a genuine curiosity of the world.

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Acknowledgements

First and foremost, I would like to thank Prof. Doug Stephan for providing me with this experience. By providing me with the freedom to pursue my own interests and offering insightful guidance whenever it was sought, you have helped me become a better chemist. I would like to thank my committee members, Prof. Bob Morris and Prof. Ulrich Fekl, for their enthusiasm and encouragement. I am grateful to Prof. Datong Song and my external examiner Prof. Chuck Macdonald for providing me with invaluable feedback. I am also extremely thankful to Dr. Barb Morra for her mentorship and collaboration. Additionally, I would like to thank my undergraduate supervisor Prof. Peter Legzdins for setting me upon this path.

I have been incredibly fortunate to have worked alongside such a fantastic group of labmates, both past and present. I am grateful to Dr. Gab Menard for taking the time to get me situated in the lab. Vitali Podgorny, it was a pleasure to collaborate with you on our perchloroaryl chemistry. James LaFortune, I couldn’t ask for a better desk or gym partner. Tim Johnstone, you have been incredibly generous with your time and expertise, for which I am incredibly appreciative. I would also like to thank Julia Bayne, Louie Fan, James LaFortune, Eliar Mosaferi and Kevin Szkop for editing chapters of my thesis. I’m also indebted to our group’s fantastic evolving crystallography team over the years for providing me with some colour to break up the text herein.

I am grateful to Darcy Burns, Sergiy Nokhrin, and Jack Sheng from the NMR department for fixing our spectrometer, helping me with difficult experiments, and fixing our spectrometer. I would like to thank Matthew Forbes, Fung Chung Woo, and Michelle Young of the AIMS lab for all their effort in finding just the right method for my compounds. I would like to thank Rose Balazs and the rest of the ANALEST staff for their expertise. I am also very appreciative of all the help John Ford of the Machine Shop has provided me over the years.

I would like to thank Mom, Dad, Chris, and Michelle for all their unwavering love and continuous support. And finally, to my dearest Samantha, every day with you is a joy and you make me excited for everything yet to come.

List of Abbreviations

α alpha

Å angstrom, 10-10 m

AN Gutmann acceptor number atm. atmosphere

β beta

BCF B(C6F5), tris(pentafluorophenyl)borane br broad

Bu butyl

C Celsius cm centimeter

C6D6 deuterated benzene

C6F5 pentafluorophenyl

C12F9 2-nonafluorobiphenyl

C10H6 naphthyl

C20H12 1,1’-binaphthyl calcd. calculated

CH2Cl2 dichloromethane

Cls SO2(4-ClC6H4), 4-chlorobenzenesufonyl

COSY correlational spectroscopy

δ delta, chemical shift

Δ Delta

° degrees

d doublet, days

-dn n-deuteron isotopologue

DART direct analysis in real time

DEPT distortionless enhancement by polarization transfer

DFT density functional theory

η eta, hapticity

E energy

eq. equivalent(s)

eV electron volts

ESI electrospray ionization

Et ethyl

Et2O diethyl ether

FIA fluoride ion affinity

FLP frustrated Lewis acid g gram

GEI global electrophilicity index h hour

HRMS high resolution mass spectrometry

HMBC heteronuclear multiple bond correlation

HOESY heteronuclear Overhauser effect spectroscopy

HSQC heteronuclear single quantum coherence

Hz Hertz

iPr Isopropyl

IR infrared

n Jxy n-bond scalar coupling between nuclides x and y

κ kappa, denticity

K Kelvin

kJ/mol kilojoules per mole

μ mu, bridging, absorption coefficient

m multiplet, meta

Mes mesityl

Me methyl

mg milligram

MHz megahertz

mL milliliter

mmol millimole

MS mass spectrometry

Ms SO2(CH3), methanesulfonyl

NHC N-heterocyclic carbene

NMR nuclear magnetic resonance

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Ns SO2(4-NO2C6H4), 4-nitrobenzenesufonyl

ω omega, global electrophilicity index value o ortho

- OTf (CF3SO3) , triflate anion

π pi p para ppm parts-per-million, 10-6

pent. n-pentane

POV-Ray Persistence of Vision Raytracer

Ph phenyl

q quartet

quin. quintet

RT room temperature

σ sigma

s singlet

SCE saturated calomel electrode

t triplet

tBu tert-butyl

THF tetrahydrofuran

tol tolyl

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

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Table of Contents Abstract ...... ii

Acknowledgements ...... iv

List of Abbreviations ...... v

Table of Tables ...... xiii

Table of Schemes ...... xiv

Table of Figures ...... xvii

Chapter 1 Introduction ...... 1

1.1 Elemental Phosphorus ...... 1

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

1.2.1 Phosphenium Cations...... 2

1.2.2 Phosphonium Cations ...... 3

1.2.3 Fluorophosphonium Catalysts ...... 5

1.2.4 Dicationic Phosphorus Catalysts...... 9

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

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

1.3.2 Computational Electrophilicity Measures...... 14

1.4 Scope of Thesis ...... 16

1.5 References ...... 18

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

2.1 Introduction ...... 23

2.1.1 Hypervalent Iodine Reagents ...... 23

2.1.2 Electrophilic Borane Reagents ...... 25

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

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

2.2 Results and Discussion ...... 29

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

2.2.2 Electrophilicity of Aminoboranes ...... 39

2.2.3 Reactivity of Aminoboranes ...... 43

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

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

2.3 Conclusion ...... 50

2.4 Experimental ...... 51

2.4.1 General Experimental Methods ...... 51

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

2.5 References ...... 58

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

3.1 Introduction ...... 63

3.1.1 Perchloroaryl Boranes ...... 63

3.1.2 Perchloroaryl Phosphines...... 67

3.2 Results and Discussion ...... 68

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

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

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

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

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

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

3.3 Conclusion ...... 93

3.4 Experimental ...... 95

3.4.1 General Experimental Methods ...... 95

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

3.5 References ...... 106

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

4.1 Introduction ...... 111

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

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

4.2 Results and Discussion ...... 118

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

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

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

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

4.2.5 Measures of electrophilicity ...... 131

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

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

4.2.8 Catalysis ...... 141

4.3 Conclusion ...... 144

4.4 Experimental ...... 144

4.4.1 General Experimental Methods ...... 144

4.4.2 X-ray Crystallography ...... 158

4.5 References ...... 161

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

5.1 Introduction ...... 166

5.2 Results and Discussion ...... 170

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

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

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

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

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

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

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

5.3 Conclusion ...... 182

5.4 Experimental ...... 182

5.4.1 General Experimental Methods ...... 182

5.4.2 X-ray Crystallography ...... 188

5.5 References ...... 190

Chapter 6 Conclusion ...... 194

6.1 Future Work ...... 194

6.2 Summary ...... 195

6.3 References ...... 199

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Table of Tables

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

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

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

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

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

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

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

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

Table 3.4. Air-Stability of Fluorophosphonium Salts 3-10 – 3-13 and [FP[C6F5)3][B(C6F5)4] in PhBr...... 90

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

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

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

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

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

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Table of Schemes

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

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

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

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

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

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

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

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

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

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

Scheme 1.11. Dimerization of 1,1-diphenylethylene reactivity of alkyl-linked bis-phosphonium cations...... 9

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

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

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

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

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

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

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

Scheme 2.3. Reactivity of iodine reagent with triphenylphosphine (top) and dimethylsulfoxide (bottom)...... 25

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Scheme 4.3. Synthesis of perfluorobiphenyl substituted borane (top) and fluoroaluminate (bottom)...... 113

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

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

Scheme 4.6. Decarboxylation of B(C12F9)3 FLP (top) and CO2 activation by BCF FLP (bottom)...... 117

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

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

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

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

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

Scheme 4.11. Synthesis of 2-perfluorobiphenyl substituted difluorophosphoranes 4-13 – 4-19...... 126

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

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

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

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

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

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

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

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

Scheme 5.7. Synthesis of bis(fluorophosphonium) 5-2b (top) and fluorophosphonium-phosphine 5-2c (bottom) from BINAP using NFSI...... 173

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

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Scheme 5.9 Reaction of binaphthyl Grignard reagent with BrP(3,5-(CF3)2C6H3)2 (top, literature 21 procedure ), and BrP(C6F5)2 to form 5-3 (bottom)...... 175

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

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

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

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

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

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Table of Figures

Figure 1.1. Lewis-acidic orbitals of a fluorophosphonium cation (left), silylium cation (middle), and borane (right)...... 7

Figure 1.2. Electrophilic index values (ω) and FIA values of a series of phenoxyphosphonium cations...... 8

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

Figure 2.2. Seminal FLP systems: arene-linked phosphine-borane (left), ethylene-bridged phosphine-borane (middle), and intermolecular BCF-phosphine (right)...... 28

Figure 2.3. POV-ray depiction of 2-2; C: light grey, B: yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, hydrogen atoms have been omitted for clarity...... 32

Figure 2.4. Partial 19F NMR spectrum (-127 – -164 ppm), variable temperature study of 5-3 in toluene-d8 (bottom – top: 25 °C, 40 °C, 60 °C, 80 °C, 100 °C)...... 34

Figure 2.5. POV-ray depiction of 2-4; C: light grey, B: yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, hydrogen atoms have been omitted for clarity...... 36

Figure 2.6. POV-ray depiction of 2-5 (top) and 2-6 (bottom); C: light grey, B: yellow-green, O: red, S: yellow, N: blue, Cl: green, F: pink, hydrogen atoms have been omitted for clarity...... 37

Figure 2.7 Surface contour plot of LUMO of 2-4 (top left), 2-5 (top right), 2-6 (bottom left), and 2-7 (bottom right); C: dark grey, H: light grey, N: dark blue, O: red, F: light blue...... 42

Figure 2.8. POV-ray depiction of 2-8; C: light grey, P: orange, O: red, S: yellow, N: blue, F: pink, hydrogen atoms have been omitted for clarity...... 48

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

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

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

Figure 3.3. POV-ray depiction of 3-1 (top) and 3-4 † (bottom); C: light grey, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity...... 69

Figure 3.4. Orthogonal POV-ray depictions of ((C6Cl5)2P)2; C: light grey, P: orange, Cl: green, hydrogen atoms have been omitted for clarity...... 71

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

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

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

Figure 3.7. POV-ray depiction of 3-6 † (top) and 3-9 † (bottom); C: light grey, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity...... 75

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

Figure 3.9. POV-ray depiction of 3-11; C: light grey, B: yellow-green, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity...... 79

Figure 3.10. POV-ray depiction of [Ph2PF(C6Cl5)][N(SO2Ph)2], 3-10NFSI †; C: light grey, B: yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity...... 80

Figure 3.11. Surface contour plots of the LUMO oriented along the P-F bond for cations of 3-10 (top left), 3-11 (top right), 3-12 (bottom left), 3-13 (bottom right); P: orange, C: black, F: blue, Cl: green, H: light grey...... 84

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

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

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

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

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

Figure 3.14. Catalytic activity of fluorophosphonium salts 3-10 – 3-13. Catalytic screening for compounds 3-10, 3-12, and 3-13 was completed by Vitali Podgorny...... 93

Figure 4.1. Steric encapsulation of cyano group by B(C12F9): POV-ray (left, fluorine atoms omitted), space-filling model (right)...... 116

Figure 4.2. POV-ray depiction of 4-11 (top), Br-aryl interaction of 4-11 (middle), T-shaped π-π interaction of 4-11 (bottom); C: light grey, B: yellow-green, Br: purple, P: orange, F: pink, Centroid: red; selected fluorine atoms have been omitted for clarity...... 124

Figure 4.3. POV-ray depiction of (C12F9)POF2,H2O; C: light grey, P: orange, F: pink, H: white...... 128

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Figure 4.4. POV-ray depiction of 4-20 (top left, counterion omitted), 4-21 (top right, counterion omitted), and 4-22 (bottom); C: light grey, B: yellow-green, P: orange, F: pink, hydrogen atoms have been omitted for clarity...... 130

Figure 4.5. POV-ray depiction of 4-23; C: light grey, B: yellow-green, O: red, P: orange, F: pink, H: white...... 131

Figure 4.6. Surface contour plot of the LUMO oriented along the P-F bond for cations 4-20 (top left), 4-21 (top right), 4-22 (bottom left), 4-23 (bottom right); P: orange, C: black, F: blue, H: light grey...... 134

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

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

Figure 5.1. POV-ray depiction of 5-4; C: light grey, O: red, P: orange, F: pink, hydrogen atoms have been omitted for clarity...... 176

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

Chapter 1 Introduction 1.1 Elemental Phosphorus Phosphorus is the eleventh most abundant element and the most abundant pnictogen in the Earth’s crust, found mainly as inorganic mineral phosphates.1 Phosphorus is found ubiquitously in an incredibly diverse range of roles: organometallic catalysts, detergents, water treatments, fertilizers, biological systems, pesticides and electronics to name a few.2 The versatility arises from the structural and electronic diversity accessible to phosphorus containing compounds. Phosphorus compounds range from one-coordinate to six-coordinate with oxidation states ranging from +5 to -3 have been observed.3,4

Elemental phosphorus exists as a variety of allotropes, the most common of which are white and red phosphorus. White phosphorus, or tetraphosphorus, is a molecule containing four phosphorus atoms arranged in a tetrahedron. White phosphorus exhibits spherical aromaticity, delocalization of electron density around the outer radius of the tetrahedron.5 White phosphorus is obtained by vapourizing the phosphorus from phosphate minerals at temperatures exceeding 1200 °C and condensed under water. Red phosphorus is a polymeric allotrope resulting from the linkage

between P4 fragments found in white phosphorus and is obtained when white phosphorus is heated or exposed to light.2

The controlled reaction of white phosphorus with elemental chlorine yields phosphorus trichloride,6 which, along with the other trihalides of phosphorus, are used as starting materials in

the synthesis of a diverse array of phosphorus(III) compounds. Phosphines of the form PR3, where R is an organic substituent, play an important role in synthetic inorganic and organic chemistry as versatile Lewis bases. By varying the phosphine substituents, the steric bulk and electronics can be fine-tuned to achieve desired properties.

1.2 Lewis Acidic Phosphorus Species 1.2.1 Phosphenium Cations

While organophosphorus species are commonly associated with the Lewis basic properties of phosphines, there are reported examples of Lewis acidic phosphorus centres. Phosphenium cations are two-coordinate phosphorus cations, which are known to exhibit both Lewis acidic and Lewis basic behaviour. Dimroth and Hoffman reported the first phosphenium cations, which were obtained by the reaction of reaction of 2-chlorobenzothiazolium salts with tris(hydroxymethyl)phosphine (Scheme 1.1).4

Scheme 1.1. Synthesis of benzothiazolium phosphenium cation.

Phosphenium cations are stabilized by extensive charge delocalization over the substituents, which are commonly amido groups due to their high π-donor ability.7 Phosphenium cations derive their Lewis acidic behavior from the vacant p-orbital and formal positive charge on the phosphorus centre. Phosphenium cations form adducts with phosphines, as first observed by Parry in the reaction of tris(dimethylamino)phosphine with various phosphenium cations to form diphosphorus cations (Scheme 1.2)8

Scheme 1.2. Phosphenium-phosphine adduct formation.

Interestingly, due to the presence of Lewis basic lone-pair of electrons and a Lewis acidic vacant orbital on the phosphorus centre, phosphenium cations are also very effective reagents in the reaction with 1,3-dienes. The related McCormack reaction that combines 1,3-dienes with dihalophosphines proceeds slowly at very high temperatures to produce P-heterocyclic products.9 The reaction of phosphenium reagents as dieneophiles with substituted 1,3-butadienes was reported by Cowley and Baxter in back-to-back reports (Scheme 1.3).10,11 The reaction of

phosphenium with 1,3-butadienes readily generates cyclic phosphonium cations, in contrast to the slow generation of cyclic phosphines generated by the McCormack reaction.

Scheme 1.2. Phosphenium-phosphine adduct formation.

1.2.2 Phosphonium Cations

Phosphonium cations are another class of phosphorus compounds that exhibits Lewis acidic behavior. Unlike phosphenium cations, phosphonium cations do not possess a lone-pair of electrons. Phosphonium cations have a tetrahedral four-coordinate phosphorus centre, and are related to phosphenium cations by the process of oxidative addition.12 Phosphonium cations contain neither a lone-pair of electrons nor a vacant p-orbital on the phosphorus centre. Unlike borane and phosphenium Lewis acids which use a vacant p-orbital, phosphonium cations instead use a low energy σ* orbital.13 Interaction of the Lewis base with a phosphonium cation forms a five-coordinated trigonal-bipyramidal phosphorane species with a hypervalent interaction (Scheme 1.4).

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

The stability of pentavalent phosphorane species increases with the total electronegativity of the substituents.2 The hypervalent interaction is most stabilized when the two apical positions involved in the interaction are electron withdrawing.13 The apicophilicity, or tendency of a group to occupy the apical position, is related to the electronegativity, steric size, and π-acceptor nature. The following trend has been proposed for apicophilicity:

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

Depending on the relative apicophilicity of substituents on a phosphorane, rearrangements of geometry can occur through either Berry pseudorotation or turnstile rotation.14 In Berry pseudorotation, the two apical substituents distort to form a square pyramidal intermediate, followed by rotation of the two previously equatorial substituents into the apical positions. The net change of a Berry pseudorotation exchanges two equatorial substituents with two apical substituents. In a turnstile rotation, two pairs of one equatorial and one apical substituent rotate about an axis perpendicular to the three other substituents. Pseudorotation is significantly inhibited by the presence of two substituents with low apicophilicity and virtually eliminated when three such substituents are present.15

Wittig reported the reaction of a triphenylphosphonium ylide with ketones to generate an alkene and triphenylphosphine oxide.16 Quaternary phosphonium salts are generated through the reaction of triphenylphosphine with an alkyl halide and are subsequently used to generate the ylide through deprotonation by a strong organolithium base. The mechanism for the reaction between the ylide and ketone proceeds through a [2+2] cycloaddition with a four-membered cyclic oxaphosphetane intermediate. While the ylide α-carbon acts as a nucleophile, the phosphorus centre acts as a Lewis acid, accepting a lone pair of electrons to form a bond with oxygen (Scheme 1.5).

Scheme 1.5. The Wittig reaction mechanism.

In an extension of this chemistry, Merz reported employing a two-phase system of CH2Cl2 and aqueous NaOH solution with a variety of phosphonium cations to carry out olefination of aldehydes.17 Deprotonation of phosphonium with NaOH generated an ylide species which would react with aldehydes to form the corresponding alkenes. The reaction of the ylide proceeds sufficiently fast that decomposition of the quaternary salts to phosphine oxide does not occur appreciably.

Matsu and coworkers have reported the use of bis-phosphonium salts as catalysts in the formation of β-aminoesters from corresponding imines and ketene silyl acetals (Scheme 1.6).18 The phosphonium species, an oxo-bridged bis-trialkylphosphonium salt, has also demonstrated

4 catalytic competency in the aldol-type reaction of aldehydes or acetals with various nucleophiles, and the Michael reaction of ketones or acetals with silyl nucleophiles.19 High yields were obtained even with catalyst loading as low as 2.5 mol%. The reaction of 4-dimethylaminobenzaldehyde with tert-butyldimethylsilyl ketene acetal was catalyzed effectively by the phosphonium catalyst, while the amine was found to quench other Lewis acids. The mechanism is proposed to proceed through initial activation of the imine by the Lewis-acidic bis-phosphonium and subsequent attack of the ketene silyl acetal to generate the β-aminoester and regenerate the catalyst.

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

Considerable utility has been found in the application of phosphonium salts as ionic liquids, which demonstrate enhanced thermal and chemical stability over more conventional imidazolium and ammonium ionic liquids.20,21 While most phosphonium based ionic liquids employ very large alkyl substituents, the phosphonium centres are still able to function as weak Lewis acid catalysts. McNulty and coworkers employed trihexyl(tetradecyl)phosphonium decanoate to catalyze the nucleophilic attack of diethylzinc onto the activated benzaldehyde carbonyl (Scheme 1.7).22

Scheme 1.7. Phosphonium ionic liquid carbonyl activation.

1.2.3 Fluorophosphonium Catalysts

In 2013, our group reported the synthesis of two highly electrophilic fluorophosphonium cations 23 [(C6F5)3-nPFPhn][B(C6F5)4] for n = 0, 1. These fluorophosphonium cations were obtained by

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

difluorophosphorane species and subsequent fluoride-abstraction using [SiEt3][B(C6F5)]·C7H8. Both fluorophosphonium cations were found to activate alkyl C-F bonds leading to concomitant generation of a carbocation and the corresponding difluorophosphorane species. Catalytic

hydrodefluorination of alkanes was achieved by introducing HSiEt3 as a hydride source and fluoride sink (Scheme 1.8).

Scheme 1.8. Catalytic hydrodefluorination activity of fluorophosphonium catalysts.

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

[PF(C6F5)3][B(C6F5)4] to form difluorophosphorane and carbocation. Subsequent delivery of hydride by HSiEt3 produces an alkane and silylium cation, which regenerates the fluorophosphonium catalyst by fluoride abstraction from the difluorophosphorane. This mechanism is supported by the observation that combination of [PF(C6F5)3][B(C6F5)4] with HSiEt3

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

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

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

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

activation by [PF(C6F5)3][B(C6F5)4] as more favourable than initial H-Si activation. Computations of the LUMO show a significant lobe on the phosphorus centre, consistent with a low energy P-F σ*-orbital being the site of Lewis acidity (Figure 1.1). Dissimilar to phosphonium cations, both silylium cations and boranes use a vacant p-orbital as the site of reactivity.

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

This family of fluorophosphonium cations represent a very significant improvement in phosphonium catalyst design. The high combined electron-withdrawing nature of the substituents stabilizes difluorophosphorane intermediates. Additionally, the drastically different

apicophilicities of the F and C6F5 substituents eliminate rearrangements of corresponding five- coordinate species. The observed stability of related trifluorophosphorane species, which contain a highly apicophilic substituent in the equatorial position have demonstrated significantly reduced stability compared to difluorophosphorane species. Additionally, the high electrophilicity of fluorophosphonium cations allows for a broad range of accessible reactivity.

A diverse range of catalytic reactivity has been observed for fluorophosphonium cations including the hydrodefluorination of fluoroalkanes; hydrosilylation of alkenes, alkynes, imines and ketones; and dimerization of 1,1-diphenylethylene. Fluorophosphonium cations have also been reported to effect the dehydrocoupling of silanes with amines, thiols, phenols and carboxylic acids to generate a Si-E bond (E= N, S, O) and molecular hydrogen. The hydrogen generated by dehydrocoupling can further be used to effect the hydrogenation of olefins.24 Additionally, the dehydrocoupling of

ditolylamine with triethylsilane yields bulky silylamine p-tol2NSiEt3 which could be used with fluorophosphonium to effect dihydrogen activation and catalytic olefin hydrogenation (Scheme 1.9).25

Scheme 1.9. Dehydrocoupling hydrogenation reactivity of fluorophosphonium cations.

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

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

26 bromine, respectively. Interestingly, [PCl(C6F5)3][B(C6F5)4] showed the lowest catalytic activity of the three halo-analogues, despite the higher electronegativity of Cl compared to Br. The low activity of [PCl(C6F5)3][B(C6F5)4] was attributed to quenching of electrophilicity by back-donation of the chlorine substituent onto the phosphorus centre. The three phosphonium cations

[PX(C6F5)3][B(C6F5)4] X=F, Cl, Br, were used to catalyze the hydrosilylation of ketones, nitriles and imines (Scheme 1.10).

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

The use of fluoro-substituted phenoxy groups has also been investigated in a series of phosphonium cations.27 The electrophilicity of the series was evaluated computationally using the general electrophilicity index (ω) and fluoride ion affinity (FIA). As with the chloro- and bromophosphonium cations, the phenoxyphosphonium cations display lower electrophilicity than the corresponding fluorophosphonium. The ω and FIA values correlated very well, displaying a trend of increasing electrophilicity with increasing fluorine substitution on the phenoxy ring (Figure 1.2).

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

1.2.4 Dicationic Phosphorus Catalysts

Developing a phosphonium cation with electrophilicity greater than that of [PF(C6F5)3][B(C6F5)4] without substituting the stabilizing C6F5 groups for additional reactive halides is a challenging endeavor. One approach investigated by our group to generate highly Lewis acidic phosphonium salts is the inclusion of multiple linked phosphonium centres that interact cooperatively with substrates. A series of bis-diphenylfluorophosphonium cations connected by alkyl chains of varied length (one to five carbon atoms) were evaluated in a series of Lewis acid catalytic transformations, including the dimerization of 1,1-diphenylethylene (Scheme 1.11).28 The reactivity of the bis- phosphonium cations decreased with increasing chain length, decreasing significantly after a length of three carbon atoms. While the bis-phosphonium cations with methylene and ethylene linkers display similar activity in most test reactions, the ethylene linked bis-phosphonium cation displays significantly reduced activity in the hydrodefluorination of fluoropentane. The reactivity of the related fluorophosphonium [Ph3PF][B(C6F5)4] is comparable to that of the bis-fluorophosphonium with a five carbon tether.29 The source of enhanced Lewis acidity of the bis-fluorophosphonium cations was not fully elucidated.

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

Another approach in generating more electrophilic phosphonium catalysts involves the incorporation of additional cationic charge. Our group has previously used this method in developing electrophilic borenium catalysts, which derive Lewis acidity from cationic charge instead of inductively electron-withdrawing substituents employed by neutral borane catalysts.30,31,32 Carbene-stabilized borenium cations were able to catalyze the hydrogenation of imines with a turn-over frequency significantly higher than neutral B(C6F5)3. Applying this methodology to generate fluorophosphonium dicationic species has also proved fruitful. N-

heterocyclic carbene-stabilized diphenylfluorophosphonium cations were generated using a similar synthetic strategy to the monocationic species, but instead starting from NHC-stabilized phosphenium cation.33 Attempts to ascertain the Lewis acidity of the phosphorus dication using the Gutmann-Beckett (vide infra) method resulted in the fluorodeoxygenation of triethylphosphine oxide with concomitant formation of the NHC-stabilized phosphenium oxide (Scheme 1.12). The fluorophosphonium dication is very competent in hydrodefluorination and hydrosilylation chemistry, indicative of enhanced Lewis acidity.

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

In a related work, phosphorus dications were also generated by incorporation of a methylpyridinium substituent onto a fluorophosphonium cation.34 Phosphine starting material 2- pyridyldiphenylphosphine was used to generate both the methyl- and fluorophosphonium dications after methylation of the pyridyl group. The methylphosphonium dication proved to be significantly less active than the fluorophosphonium cation in the hydrodefluorination of fluoropentane, highlighting the importance of the highly electronegative fluorine substituent (Scheme 1.13).

Scheme 1.13. Hydrodefluorination reactivity of pyridinium-phosphonium dications.

1.3 Electrophilicity and Lewis Acidity Measurement Scales Methods to measure electrophilicity and Lewis acidity are critical to understanding observed reactivity trends and predicting catalyst suitability in new applications. To be an effective method,

the data required to compare and rank electrophiles must be easily obtained, but also consider sufficient variables related to the steric and electronic structure of the electrophile as to allow comparison across many systems. The ease of measurement leads to more widespread adoption of the method, which leads to the generation of more data points for comparison. Two general types of scales will be discussed throughout the thesis: trends arising from NMR chemical shift data and those obtained through ab initio calculations of systems.

Lewis acidity scales require that a sufficient interaction occur between the analyte and Lewis base in order to obtain a meaningful measurement. Excessive steric encumbrance can preclude formation of adducts rendering a Lewis acid incompatible with that testing method, therefore methods like Childs’, Gutmann-Beckett and FIA all employ Lewis bases with minimal steric demand. While the upside of employing small Lewis bases in Lewis acidity tests is more widespread compatibility, the downside is that the trends observed may be less applicable to predicting reactivity trends with larger substrates.

As outlined by Drago and Matwiyoff “[a]ny order of donor or acceptor strengths must be established relative to a given donor or acceptor. Reversals may be expected when orders towards different donors (or acceptors) are compared”.35 However, obtaining multiple measures of Lewis acidity and electrophilicity allows for better understanding of a particular and allows for more accurate prediction of future reactivity trends.

1.3.1 Chemical Shift Electrophilicity Scales

Obtaining electrophilicity measurements from NMR chemical shift data is an attractive method because the data can be obtained easily using techniques and instrumentation available to most synthetic chemists. Lewis acid quantification by NMR chemical shift data has been reported for a wide variety of nuclei, including 1H, 2H, 19F, 23Na and 31P.,36, 37,38 The most widely utilized Lewis acidity quantification scales are the Childs’ and Gutmann-Beckett methods (vide infra).

Our group has also reported the use of 31P and 19F NMR spectroscopic data to assess the electrophilicity across a series of fluorophosphonium cations.39 Both the chemical shift of the phosphorus centre and phosphorus bound fluorine are both responsive to changes in Lewis acidity,

displaying complementary trends to the computed FIA trend. The sensitivity of the fluorophosphonium chemical shift to changes in electrophilicity is not unprecedented as the Gutmann-Beckett method (vide infra) reliably uses the chemical shift of triethyloxyphosphonium with various oxygen substituents to assess Lewis acidity.

1.3.1.1 Childs’ Method Childs reported using trans-crotonaldehyde, among other aldehydes, to measure Lewis acidity by 1H NMR spectroscopy.38 By this method, crotonaldehyde is added 1:1 to a cooled solution of the desired Lewis acid and a 1H NMR spectrum is obtained (Scheme 1.14).

Scheme 1.14. Childs’ method protocol for BCF.

The proton signal of vinylic proton H3 to the carbonyl oxygen atom exhibits a downfield shift when an adduct is formed, the magnitude of this downfield shift is proportional to the electrophilicity of the Lewis acid. This method has seen widespread adoption, though not to the same degree as the Gutmann-Beckett method, perhaps owing to the comparative difficulty of preparing air-sensitive samples at low temperatures.40 The Childs’ method has been reported to be incompatible with a wide range of fluorophosphonium cations, as addition of crotonaldehyde to solutions of fluorophosphonium cations yields complex mixtures of products. 39,33

1.3.1.2 Gutmann Beckett Gutmann proposed using the Acceptor Number (AN) as a useful guide for “choosing the most appropriate solvent for a given reaction.” The AN is determined by comparing the 31P NMR 41 chemical shift for OPEt3 in the solvent of interest to the chemical shift of OPEt3 in hexane. The

AN scale is based off arbitrary fixed points of hexane (AN = 0) and SbCl5 (AN = 100). Additionally, Gutmann measured the 31P NMR chemical shift at various dilutions and extrapolated to infinite dilution to correct for concentration effects. Triethylphosphine oxide is ideal for this

application because it is a strong base that can only interact through one well-defined site at oxygen, and the ethyl substituents enhanced solubility and provide electronic shielding without significant steric encumbrance.

Beckett modified the scale developed by Gutmann to quantify the Lewis acidity boron-containing Lewis acids.42 The polymerization of epoxy resins can be initiated by boron Lewis acids, with a rate dependence on the strength of the Lewis acid. Since 11B NMR shifts of boranes are highly variable and do not show correlation to electrophilicity of the boron centre, the Gutmann scale proved to be a useful alternative.43 The AN obtained for boranes showed correlation with the measured reaction rates of epoxide polymerization. The AN is dependent on how well a boron-

bound heteroatom competes with OPEt3 for the boron acceptor orbital. Gutmann provides a simplified procedure in which an OPEt3 is added to neat borane in a 5mm NMR tube nested in a

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

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

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

OPEt3 in hexane. The simplified measurement of AN proposed Beckett gave results with good agreement to those obtained by Gutmann without requiring multiple data points for different dilutions.

Britovsek proposed a further simplified methodology, wherein a 1:1 mixture of borane and OPEt3 31 44 were dissolved in C6D6 and the P NMR spectrum was obtained (Scheme 1.15).

Scheme 1.15. Gutmann-Beckett protocol for BCF.

The same change in chemical shift was observed for the borane adducts in C6D6, THF and CDCl3, indicating that with a significantly strong adduct formed with the Lewis acid, the effect of the

solvent is negated. Britovsek measured the Lewis acidity of the series (C6F5)3-nB(OC6F5)n for n = 0 – 3 to find that using the Gutmann-Beckett method the Lewis acidity increases as n increases;

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

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

OC6F5 substitution, but a soft base like crotonaldehyde interacts less strongly as the hardness increased. This illustrates the limited predictive power of Lewis acidity trends, as the trends only necessarily hold for closely related systems and trends can reverse when different donor types are used. O’Hare and many others have used this modified Gutmann-Beckett method to assess the strength of Lewis acids in FLP chemistry.45 Additionally, this method has been applied to fluorophosphonium systems with mixed results. Some systems have been amenable to the 23 39 Gutmann-Beckett method, while other systems do not form adducts with OPEt3, while others 28 yet react with OPEt3 in a deoxygenative fashion.

1.3.2 Computational Electrophilicity Measures

The use of ab initio computational methods to assess the electrophilicity of Lewis acidity provides the significant advantage of studying systems that are not synthesized or have not been suitably purified. Additionally, computations like fluoride ion affinities and general electrophilicity indices are compatible with a wider range of Lewis acids when compared to empirical methods like Childs’ or Gutmann-Beckett.

1.3.2.1 Fluoride Ion Affinity Christe and coworkers proposed FIA as an ab initio method to quantize Lewis acidity (Scheme 1.16).46 The fluoride ion is an ideal Lewis base for this application as it exhibits very high basicity with negligible steric encumbrance and forms adducts with “essentially all Lewis acids.” The reaction enthalpy of fluoride ion-adduct formation can then be used to quantify relative Lewis acid strength.

Scheme 1.16. Fluoride ion affinity for a fluorophosphonium cation.

Due to difficulties in calculating the electron affinity of F, Christe used the experimentally 47 determined FIA value for difluoroketone, COF2, of 49.9 kcal/mol. Using this empirical value

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

- - CF2O + F  CF3O ΔH1 = 49.9kcal/mol

- - CF3O + LA  CF2O + F-LA ΔH2

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

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

represents the FIA of CF2O with experimentally determined enthalpy. An estimation of enthalpy of the second equation can be obtained using the computed internal energies of the Lewis acid, the - fluoride-Lewis acid adduct, CF2O and the CF3O anion. Using the values from the first two equations, the enthalpy for the FIA of LA can be obtained as the sum.

Fluoride ion affinity calculations are typically resource intensive, as optimized geometries and energies for both the Lewis acid and fluoride-Lewis acid adduct need to be obtained. Fluoride ion affinities allow for comparison of a wide range of Lewis acids, including systems that are not compatible with empirical tests like the Gutmann-Beckett method. Additionally, FIAs can be used predictively to ascertain the effectiveness of synthetically challenging systems. Fluoride ion affinities have been used extensively to evaluate the Lewis acidity of boranes and fluorophosphonium cations.48,39

1.3.2.2 General Electrophilicity Index The General Electrophilicity Index is a system proposed by Parr as a quantitative scale for electrophiles.49 Parr proposed the electrophilicity index of a compound be defined as “the square of its electronegativity divided by its chemical hardness.” This definition was strongly motivated

by an earlier work in which Maynard observed a strong correlation between the value χ2/η and the reaction rates of electrophiles with a protein they were studying.50 The electrophilicity index, ω, estimates the ability of a chemical species to “soak up” electrons and is defined as

ω ≡ μ2/2η

where μ is chemical potential and η is the chemical hardness. Both chemical potential and chemical hardness can be defined in terms of ionization energy (I) and electron affinity (A) as

χ = –μ = (I + A)/2 and η = (I – A)

where χ is the absolute electronegativity. Mulliken electronegativity is defined as being equal to (I + A)/2. Per Koopman’s theorem, 51,52 chemical hardness and chemical potential can be related to frontier orbital energies by

EHOMO = I and ELUMO = A

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

The general electrophilicity index is an attractive method, as it requires single point energy calculations of only the free Lewis acid. Our group has reported the use of ω values to quantify the electrophilicity of a series of phenoxyphosphonium cations and found high correlation with FIA values.27 Since steric effects and hybridization energies are not taken into account when calculating ω values, only similar systems can be accurately compared.

1.4 Scope of Thesis The objective of this thesis is to develop strategies for main group borane and fluorophosphonium catalyst enhancement. Chapter 2 investigates the use of hypervalent iodine reagents to effect nitrene insertion into borane-substituent bonds to tune the Lewis acidity of the resulting aminoboranes. Chapter 3 explores the use of perchloroaryl substituents on phosphonium cations

as a means of imparting air-stability. In chapter 4, 2-nonafluorobiphenyl substituents are incorporated into a series of fluorophosphonium cations to enhance both electrophilicity and chemical stability.

In chapter 5, the use of atropisomeric binaphthyl substituents are investigated as a means to develop chiral fluorophosphonium cations.

Chapter 3 contains work performed collaboratively with Vitali Podgorny. The synthesis of compounds 3-4, 3-5, 3-8, 3-9, 3-12 and 3-13 and catalytic testing of 3-10, 3-12 and 3-13 was carried out by Mr. Podgorny, characterization of these compounds are detailed in his MSc. thesis.53 Additionally, certain crystals obtained for X-ray diffraction studies were grown by Mr. Podgorny and are indicated as such within Chapter 3. Refinement of all solid-state structures herein were carried out by the author. All other synthetic work herein was performed by the author. Elemental analysis and high-resolution mass spectrometry were performed in-house by the ANALEST and AIMS laboratory respectively.

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

Chapter 2: Postle, S.; Stephan, D. W. Reactions of Iodine-Nitrene Reagents with Boranes. Dalton Trans. 2015, 44, 4436-4439.

Chapter 3: Postle, S.; Podgorny, V.; Stephan, D. W. Electrophilic Phosphonium Cations (EPCs) with Perchlorinated-Aryl Substituents: Towards Air-Stable Phosphorus-Based Lewis Acid Catalysis. Dalton Trans. 2016, 45, 14651-14657.

1.5 References

1. Holland, H. D.; Turekian, K. K., Treatise on Geochemistry. 1st ed.; Elsevier/Pergamon: Amsterdam; Boston, 2004. 2. Corbridge, D. E. C., Phosphorus: Chemistry, Biochemistry and Technology. 6th ed.; Taylor & Francis: Boca Raton, 2013; p 1439 3. Gier, T. E., HCP, a Unique Phosphorus Compound. J. Am. Chem. Soc. 1961, 83, 1769. 4. Dimroth, K.; Hoffmann, P., Phosphacyanines New Class of Compounds Containing Trivalent Phosphorus. Angew. Chem., Int. Ed. 1964, 3, 384. 5. Hirsch, A.; Chen, Z. F.; Jiao, H. J., Spherical aromaticity of inorganic cage molecules. Angew. Chem., Int. Ed. 2001, 40, 2834. 6. Forbes, M. C.; Roswell, C. A.; Maxson, R. N., Phosphorus(III) Chloride. Inorg. Synth. 1946, 2, 145. 7. Cowley, A. H.; Kemp, R. A., Synthesis and Reaction Chemistry of Stable 2-Coordinate Phosphorus Cations (Phosphenium Ions). Chem. Rev. 1985, 85, 367.

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

((CH3)2N)3P.((CH3)2N)2PCl.AlCl3, and Related Species Diphosphorus Cations. Inorg. Chem. 1976, 15, 3046. 9. Quin, L. D., The Heterocyclic Chemistry of Phosphorus: Systems Based on the Phosphorus-Carbon Bond. Wiley: New York, 1981; p 434. 10. Soohoo, C. K.; Baxter, S. G., Phosphenium Ions as Dienophiles. J. Am. Chem. Soc. 1983, 105, 7443. 11. Cowley, A. H.; Kemp, R. A.; Lasch, J. G.; Norman, N. C.; Stewart, C. A., Reaction of Phosphenium Ions with 1,3-Dienes - a Rapid Synthesis of Phosphorus-Containing 5-Membered Rings. J. Am. Chem. Soc. 1983, 105, 7444. 12. Lindner, E.; Weiss, G. A., Oxidative Addition of a C-H Bond at a 2-Coordinated Phosphenium Cation. Chem. Ber. Recl. 1986, 119, 3208. 13. Werner, T., Phosphonium Salt Organocatalysis. Adv. Synth. Catal. 2009, 351, 1469. 14. Ugi, I.; Marquarding, D.; Klusacek, H.; Gillespie, P.; Ramirez, F., Berry Pseudorotation and Turnstile Rotation. Acc. Chem. Res. 1971, 4, 288.

15. Corbridge, D. E. C., Phosphorus: Chemistry, Biochemistry and Technology. 6th ed.; Taylor & Francis: Boca Raton, 2013; p 1439. 16. Wittig, G.; Schollkopf, U., Uber Triphenyl-Phosphinmethylene Als Olefinbildende Reagenzien .1. Chem. Ber. Recl. 1954, 87, 1318. 17. Markl, G.; Merz, A., Carbonyl Olefination with Non-Stabilized Phosphine Alkylenes in Aqueous Systems. Synthesis (Stuttg) 1973, 295. 18. Mukaiyama, T.; Kashiwagi, K.; Matsui, S., A Convenient Synthesis of Beta-Aminoesters - the Reaction of Imines with Ketene Silyl Acetals Catalyzed by Phosphonium Salts. Chem. Lett. 1989, 1397. 19. Mukaiyama, T.; Matsui, S.; Kashiwagi, K., Effective Activation of Carbonyl and Related- Compounds with Phosphonium Salts - the Aldol and Michael Reactions of Carbonyl-Compounds with Silyl Nucleophiles and Alkyl Enol Ethers. Chem. Lett. 1989, 993. 20. Keglevich, G.; Baan, Z.; Hermecz, I.; Novak, T.; Odinets, I. L., The Phosphorus Aspects of Green Chemistry: the use of Quaternary Phosphonium Salts and 1,3-Dialkylimidazolium Hexafluorophosphates in Organic Synthesis. Curr. Org. Chem. 2007, 11, 107. 21. Chowdhury, S.; Mohan, R. S.; Scott, J. L., Reactivity of Ionic Liquids. Tetrahedron 2007, 63, 2363. 22. Cheekoori, S. Phosphonium Salt Ionic Liquids in Organic Synthesis. Doctoral Thesis, McMaster University, Hamilton, ON., 2008. 23. Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W., Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor. Science 2013, 341, 1374. 24. Pérez, M.; Caputo, C. B.; Dobrovetsky, R.; Stephan, D. W., Metal-Free Transfer Hydrogenation of Olefins via Dehydrocoupling Catalysis. Proc. Nat. Acad. Sci. 2014, 111, 10917. 25. vom Stein, T.; Pérez, M.; Dobrovetsky, R.; Winkelhaus, D.; Caputo, C. B.; Stephan, D. W., Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins. Angew. Chem. Int . Ed. 2015, 54, 10178. 26. Perez, M.; Qu, Z. W.; Caputo, C. B.; Podgorny, V.; Hounjet, L. J.; Hansen, A.; Dobrovetsky, R.; Grimme, S.; Stephan, D. W., Hydrosilylation of Ketones, Imines and Nitriles Catalysed by Electrophilic Phosphonium Cations: Functional Group Selectivity and Mechanistic Considerations. Chem. Eur. J. 2015, 21, 6491.

27. LaFortune, J. H. W.; Johnstone, T. C.; Perez, M.; Winkelhaus, D.; Podgorny, V.; Stephan, D. W., Electrophilic Phenoxy-Substituted Phosphonium Cations. Dalton Trans. 2016, 45, 18156. 28. Holthausen, M. H.; Hiranandani, R. R.; Stephan, D. W., Electrophilic Bis- Fluorophosphonium Dications: Lewis Acid Catalysts from Diphosphines. Chem. Sci. 2015, 6, 2016. 29. Mehta, M.; Holthausen, M. H.; Mallov, I.; Perez, M.; Qu, Z. W.; Grimme, S.; Stephan, D. W., Catalytic Ketone Hydrodeoxygenation Mediated by Highly Electrophilic Phosphonium Cations. Angew. Chem., Int. Ed. 2015, 54, 8250. 30. Farrell, J. M.; Hatnean, J. A.; Stephan, D. W., Activation of Hydrogen and Hydrogenation Catalysis by a Borenium Cation. J. Am. Chem. Soc. 2012, 134, 15728. 31. Farrell, J. M.; Posaratnanathan, R. T.; Stephan, D. W., A Family of N-heterocyclic Carbene-Stabilized Borenium Ions for Metal-Free Imine Hydrogenation Catalysis. Chem. Sci. 2015, 6, 2010. 32. Welch, G. C.; Stephan, D. W., Facile Heterolytic Cleavage of Dihydrogen by Phosphines and Boranes. J. Am. Chem. Soc. 2007, 129, 1880. 33. Holthausen, M. H.; Mehta, M.; Stephan, D. W., The Highly Lewis Acidic Dicationic

Phosphonium Salt: [(SIMes)PFPh2][B(C6F5)4]2. Angew. Chem. Int. Ed. 2014, 53, 6538. 34. Bayne, J. M.; Holthausen, M. H.; Stephan, D. W., Pyridinium-Phosphonium Dications: Highly Electrophilic Phosphorus-based Lewis Acids Catalysts. Dalton Trans. 2016, 45, 5949. 35. Drago, R. S.; Matwiyoff, N. A., Acids and Bases. Heath: Lexington, Mass., 1968; p 121. 36. Hilt, G.; Punner, F.; Mobus, J.; Naseri, V.; Bohn, M. A., A Lewis Acidity Scale in Relation to Rate Constants of Lewis Acid Catalyzed Organic Reactions. Eur. J. Org. Chem. 2011, 5962. 37. (a) Chu, Y. Y.; Yu, Z. W.; Zheng, A. M.; Fang, H. J.; Zhang, H. L.; Huang, S. J.; Liu, S. B.; Deng, F., Acidic Strengths of Bronsted and Lewis Acid Sites in Solid Acids Scaled by 31P NMR Chemical Shifts of Adsorbed Trimethylphosphine. J. Phys. Chem. C 2011, 115, 7660; (b) Erlich, R. H.; Popov, A. I., Study of Solvation of Sodium Ions in Nonaqueous Solvents by 23Na Nuclear Magnetic Resonance. J. Am. Chem. Soc. 1971, 93, 5620. 38. Childs, R. F.; Mulholland, D. L.; Nixon, A., Lewis Acid Adducts of α,β-Unsaturated Carbonyl and Nitrile Compounds .2. A Calorimetric Study. Can. J. Chem. 1982, 60, 809. 39. Caputo, C. B.; Winkelhaus, D.; Dobrovetsky, R.; Hounjet, L. J.; Stephan, D. W., Synthesis and Lewis Acidity of Fluorophosphonium Cations. Dalton Trans. 2015, 44, 12256.

40. Sivaev, I. B.; Bregadze, V. I., Lewis Acidity of Boron Compounds. Coord. Chem. Rev. 2014, 270, 75. 41. Mayer, U.; Gutmann, V.; Gerger, W., Acceptor Number - Quantitative Empirical Parameter for Electrophilic Properties of Solvents. Monatsh. Chem. 1975, 106, 1235. 42. Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Varma, K. S., A Convenient NMR method for the Measurement of Lewis Acidity at Boron Centres: Correlation of Reaction Rates of Lewis Acid Initiated Epoxide Polymerizations with Lewis Acidity. Polymer 1996, 37, 4629. 43. Nöth, H.; Wrackmeyer, B., Nuclear Magnetic Resonance Spectroscopy of Boron Compounds. In NMR - Basic Principles and Progress, Diehl, P.; Fluck, E.; Kosfeld, R., Eds. Springer-Verlag: 1978; Vol. 14.

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

(C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity. Organometallics 2005, 24, 1685. 45. Binding, S. C.; Zaher, H.; Chadwick, F. M.; O'Hare, D., Heterolytic Activation of Hydrogen Using Frustrated Lewis Pairs Containing Tris(2,2',2''-Perfluorobiphenyl)Borane. Dalton Trans. 2012, 41, 9061. 46. Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A., On a Quantitative Scale for Lewis Acidity and Recent Progress in Polynitrogen Chemistry. J. Fluorine Chem. 2000, 101, 151. 47. Larson, J. W.; Mcmahon, T. B., Strong Hydrogen-Bonding in Gas-Phase Anions - an Ion- Cyclotron Resonance Determination of Fluoride Binding Energetics to Bronsted Acids from Gas- Phase Fluoride Exchange Equilibria Measurements. J. Am. Chem. Soc. 1983, 105, 2944.

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

activation with FLPs Containing B(Ohfip)3 to a Systematic Evaluation of the Lewis Acidity of 33 Lewis Acids Based on Fluoride, Chloride, Hydride and Methyl Ion Affinities. Dalton Trans. 2015, 44, 7489. 49. Parr, R. G.; Von Szentpaly, L.; Liu, S. B., Electrophilicity Index. J. Am. Chem. Soc. 1999, 121, 1922. 50. Maynard, A. T.; Huang, M.; Rice, W. G.; Covell, D. G., Reactivity of the HIV-1 Nucleocapsid Protein p7 Zinc Finger Domains from the Perspective of Density-Functional Theory. Proc. Natl. Acad. Sci. U. S. A. 1998, 95, 11578.

51. Chattaraj, P. K.; Sarkar, U.; Roy, D. R., Electrophilicity Index. Chem. Rev. 2006, 106, 2065. 52. Pearson, R. G., Chemical Hardness and Bond-Dissociation Energies. J. Am. Chem. Soc. 1988, 110, 7684. 53. Podgorny, V. Electrophilic Phosphenium and Phosphonium Cations: Synthesis and Reactivity of Perfluoro- & Perchloroaryl Phosphorus Systems. University of Toronto, 2016.

Chapter 2 Nitrene Insertion into Boranes 2.1 Introduction 2.1.1 Hypervalent Iodine Reagents

Hypervalent iodine reagents are those in which an iodine containing compound has an oxidation 1,2 state greater than the commonly found -1 state. In 1886, Willgerodt synthesized PhICl2, the first hypervalent iodine compound.3 Since this seminal work, there has been a surge of interest in these reagents given their unique reactivity and low toxicity of iodine.1 There are two general classes of hypervalent iodine complexes: trivalent λ3-iodanes and pentavalent λ5-iodanes (Figure 2.1).

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

In each of these structural types of hypervalent iodine reagents, there exists one (λ3) or two (λ5) 3- centre-4-electron (3c-4e) bonds. In the case of λ3-iodanes, the iodine centre contains 10 electrons in its valence shell, making it a formal decet structure. Conversely, the pentavalent λ5-iodane has 12 electrons in its valence shell, making it a formal dodecet structure. Iodine is diffuse and polarizable and favours hypervalent bonding in which two ligands interact with an unhybridized 5p orbital. The resulting 3c-4e interaction results in an elongated, weak, and highly polarizable bond. The presence of this hypervalent interaction dictates both the coordination geometry and the reactivity of these species. Both λ3-iodane subclasses exhibit T-shaped coordination geometry, whereas λ5-iodanes have an additional orthogonal hypervalent interaction leading to square-based pyramidal geometry.4,5,6,7 The stability of the 3c-4e interaction has been investigated by Suresh, who reports that the stability of the hypervalent interaction depends on the pairing of ligands with appropriate trans-influence.8

In 1931, Arbuzov synthesized the first diacetoxyaryl-λ3-iodanes by oxidation of iodobenzene with peracetic. 9 Diacetoxyaryl-λ3-iodanes are convenient starting materials used in the synthesis of hypervalent iodine reagents due to their high stability. One such compound,

diacetoxyiodobenzene, is commercially available and its utility in the synthesis of a wide range of hypervalent iodine reagents has been demonstrated (Scheme 2.1).10,11,12,13,14

Scheme 2.1. Synthesis of hypervalent iodine reagents from diacetoxyiodobenzene.

The reactivity of hypervalent compounds has been extensively investigated and documented by Zhdankin in a series of comprehensive reviews.15 Trivalent iodane reagents typically exhibit three general modes of reactivity: ligand exchange, reductive elimination, and ligand coupling (Scheme 2.2).16 The ligand exchange mechanism likely occurs through an associative pseudo- octahedral intermediate, due to the instability of the two-coordinate iodonium ion that would be involved in a dissociative mechanism.17 An important feature of hypervalent iodine species is their high propensity to reductively eliminate highly reactive intermediates like carbenes and nitrenes under mild conditions. The reactivity of hypervalent iodine reagents resembles that of heavy metals such as Hg(II), Tl(III), and Pb(IV), without the associated high toxicity.1

Scheme 2.2. General reactivity patterns of λ3-iodanes.

Aryliodonium imides, ArI=NR, are a family of λ3-iodanes that act as facile nitrene precursors. Abramovitch and coworkers synthesized phenyliodonium tosylimide, the first literature report of an aryliodonium imide, while attempting to find sulfonyl nitrene sources other than azides.18 The group of Okawara developed a strategy to generate a series of aryliodonium tosylimides a year

later.19 Reacting diacetoxy(phenyl)-λ3-iodane with toluenesulfonamide in basic conditions affords the desired phenyliodonium tosylimide in good yield upon recrystallization from methanol. The resulting iodonium imide is poorly soluble in most organic solvents due to a polymeric structure resulting from strong intermolecular N-I interactions.

Phenyliodonium N-tosylimide reacts with triphenylphosphine at 100 °C to afford iodobenzene and N-tosyliminophosphorane in 69% yield. This reaction is proposed to proceed via a sulfonyl nitrene intermediate. Alternatively, the iodonium imide reacts with dimethylsulfoxide to give the oxidized product N-tosyliminodimethylsulfurane oxide quantitatively at room temperature. Given the stability of the iodane starting material at room temperature, this reaction is proposed to occur through a substitution reaction at nitrogen (Scheme 2.3).19

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

2.1.2 Electrophilic Borane Reagents

Pentafluorophenyl-substituted organoboranes have developed very rich and diverse chemistry. Tris(pentafluorophenyl)borane, the first such compound, was synthesized in moderate yields by Massey in 1964 by combining three equivalents of pentafluorophenyllithium with boron trichloride (Scheme 2.4).20

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

There were few reports which used BCF in the decades following its discovery. One such report used BCF to transfer a perfluorophenyl group onto xenon difluoride to form a pentafluorophenylxenon(II) fluoroborate species, the first stable compound containing a Xe-C bond.21 In 1991, Marks employed BCF as an alkyl abstracting cocatalyst in concert with zirconocene olefin polymerization catalysts. Using highly electrophilic BCF in lieu of the commonly used aluminum alkyl or methylaluminoxane resulted in the first stoichiometrically precise and crystallographically characterized system of this type.22,23 Yamamoto and coworkers thoroughly investigated BCF as a versatile Lewis acid catalyst in allylation reactions, Diels-Alder reactions, as well as aldol-type and Michael reactions of silyl enol ethers with carbonyls (Scheme 2.5).24

Scheme 2.5. Summary of BCF Lewis acid catalyzed reactivity.

Chivers and Piers produced a comprehensive review on the Lewis acidic chemistry of BCF and 25 related bis(pentafluorophenyl)borane, or Piers’ borane (HB(C6F5)2). Piers’ borane reacts readily with a variety of unsaturated substrates, forms a dimer in the solid state, and forms a 4:1

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

reactivity, but instead resulted in the formation of stable adduct HB(C6F5)2·THF. At higher temperatures, the Pier’s borane-THF adduct undergoes reductive ring opening of the THF to generate bis(pentafluorophenyl)butylborinate (Scheme 2.6, top). In non-coordinating solvents, like benzene, Pier’s borane reacts with styrene to form the kinetically favoured single hydroboration product. Over the course of several days, the hydroborated product disproportionates to form BCF and dialkyl(pentafluorophenyl)borane, the thermodynamic products (Scheme 2.6, bottom).26

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

2.1.3 Frustrated Lewis Pair Chemistry

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

capable of reversible H2 activation. The para-nucleophilic attack of a bulky secondary dimesitylphosphine on BCF leads to the formation of a linked phosphonium-fluoroborate species,

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

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

loses H2 (Figure 2.2, left). Excitingly, the linked phosphine-borane species is then able to activate dihydrogen.27,28 The term “frustrated Lewis pairs” was coined to describe the interaction between donor and acceptor sites of sterically-congested molecules which are precluded from adduct formation. The linked species was found to catalyze the hydrogenation of imines in good yield under an atmosphere of hydrogen.29 Erker and coworkers reported an ethylene-bridged dimesitylphosphine-bis(pentafluorophenyl)borane system, Mes2PCH2CH2B(C6F5)2, made through the hydroboration of Piers’ borane onto an alkenyl phosphine (Figure 2.2, middle).30 The ethylene- linked phosphine-borane forms an internal adduct, yielding a planar four-membered ring. The four

membered ring activates dihydrogen at room temperature and can be used to effect the reduction of imines and enamines.31 In 2007, Stephan and coworkers reported that sufficiently sterically bulky phosphines could form intermolecular pairings with BCF and still effect similar reactivity (Figure 2.2, right).32 BCF and tri-tert-butylphosphine do not form an adduct in solution, but readily activate a variety of small molecules, including 1,2-addition to olefins.32

Figure 2.2. Seminal FLP systems: arene-linked phosphine-borane (left), ethylene-bridged phosphine-borane (middle), and intermolecular BCF-phosphine (right).

The discovery of intermolecular FLPs has led to intense research into different FLP-type reactions and, with it, a demand for new electrophilic boranes.33,34 However, the synthesis of electrophilic boranes is often difficult, requiring the use of hazardous lithium reagents or toxic tin reagents.

2.1.4 Post-Synthetic Tuning Strategies

The concept of tuning the Lewis acidic properties of an electrophilic borane as a means to circumvent difficult and dangerous synthetic preparations of new boranes is an idea as old as FLP chemistry. Shortly after the seminal report of FLPs, our group reported using tertiary phosphines to effect para-nucleophilic attack on BCF to tune the Lewis acidity of the resulting phosphonium- substituted borane. Phosphonium substitution resulted in increased Lewis acidity, though the effect of the various substituents on the phosphorus centre played a minor role compared to the cationic charge afforded by the cationic group. The related phosphine-borane species show slightly decreased Lewis acidity, while the steric environment of the boron centre is not perturbed due to the orientation of the phosphorus substituent in both the phosphonium and phosphine substituted borane.28 Additionally, tuning strategies have been investigated that involved the insertion of functional groups into the B-C bond. In 2012, Stephan reported the use of diazomethanes to effect the insertion of pentafluoro-, trimethylsilyl-, or diphenylmethylene fragments into a variety of boranes. Depending on the borane used, single- or double-insertion of these fragments was

observed.35 Braunschweig similarly used azide reagents to effect nitrene insertion into five- membered borole rings.36 The insertion of the nitrene into the antiaromatic boroles yielded aromatic 6-membered 1,2-azaborinies. In this chapter, we explore the use of iodonium sulfonylimines of the form PhI=N(SO2)R to effect nitrene insertion into electrophilic borane B-C bonds, thereby tuning their electronic character.

2.2 Results and Discussion 2.2.1 Synthesis and Characterization of Aminoboranes

Six iodonium sulfonylimines were synthesized following a modified literature procedure (Scheme 2.7).19 The iodine reagents were selected because of the electron withdrawing ability of the sulfonyl substituents to potentially enhance the electrophilicity of the resulting insertion products or, at a minimum, mitigate the quenching effect of the lone pair of electrons on the nitrogen centre.

Scheme 2.7. Synthesis of phenyliodonium sulfonylimides.

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

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

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

(PhI=NNs, Ns = SO2(4-NO2Ph)) and methanesulfonyl (PhI=NMs, Ms = SO2Me). Suitable 1H NMR spectra could be obtained only in deuterated DMSO due to the insolubility of iodonium imides in most common organic solvents, though all the isolated iodine reagents reacted rapidly to oxidize the DMSO solvent.

The trifluoromethanesulfonyl (SO2CF3) derivative was targeted due to the high electron withdrawing capability of trifluoromethyl groups. Attempts to synthesize the trifluoromethanesulfonyl-substituted iodine reagent were successful, though yields were significantly lower than with other derivatives. Investigations using the trifluoromethanesulfonyl

derivative were abandoned due to the high cost of the sulfonamide precursor and the low overall yield of the corresponding iodine reagent.

Addition of PhI=NTs into pentane forms a white slurry to which there is no perceptible change upon adding BCF. After stirring overnight, the reaction mixture remained a white slurry which

was filtered to obtain a white powder. Recrystallization of this white powder from CH2Cl2 at -35 °C yielded 2-1 as colourless crystals in 74% yield. The 11B{1H} NMR spectrum of 2-1 shows a broad singlet at 43.6 ppm, and 19F NMR displays the nine resonances, which can be

attributed to three inequivalent sets of C6F5 environments. One of ortho-fluorine signals displays a considerable upfield shift at 142.2 ppm, relative to 128.8 ppm in BCF. This upfield shift is

consistent with migration of a C6F5 substituent away from the boron centre. These data are consistent with the insertion of the N-tosyl fragment into the B-C bond of BCF and infer the formulation of 2-1 as (C6F5)2BN(Ts)(C6F5) (Scheme 2.8).

Scheme 2.8. Synthesis of tosylaminoborane 2-1.

Switching the reaction medium to either benzene or toluene resulted in considerably better solubility of the iodine reagent, though the solution turned dark red-brown over the course of 24 h and 1H NMR reveals complete consumption of the iodine reagent along with the formation of multiple side products. This finding is consistent with previous observations of sulfonyl nitrenes reacting with arenes.37 Addition of PhI=NTs to BCF in polar coordinating solvent resulted in solvent-borane adduct formation that precluded reactivity between the borane and iodine reagent and resulted in a complex mixture of products when the mixture was heated to 70 °C.

The presence of three inequivalent groups of C6F5 resonances, rather than two sets integrating in a 2:1 ratio, is indicative of restricted rotation about the B-N bond. This restricted rotation could arise from a steric interaction between the substituents on the boron and nitrogen centre.

Alternatively, the restriction could be indicative of multiple bond character between boron and nitrogen. The implication of the latter case would indicate that despite the two electron- withdrawing substituents on nitrogen, the lone pair of nitrogen still has a significant interaction with the empty p-orbital of the adjacent boron centre.

Addition of an equimolar amount of OPEt3 to a toluene solution of 2-1 yields crystals of 2-2 which can be obtained in good yield. The 11B{1H} NMR spectrum of 2-2 shows a sharpened singlet at 1.4 ppm, indicative of a four-coordinate boron centre. Interestingly, the 19F NMR spectrum exhibits a set of three sharp resonances and a set of three broad resonances attributable to two C6F5 environments, which integrate in a 2:1 ratio. The meta-para gap is significantly reduced, indicative of a four-coordinate boron centre. These data are consistent with the formulation of 2-2 as the acid- base adduct (C6F5)2BN(Ts)(C6F5)·OPEt3 (Scheme 2.9).

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

The presence of only two inequivalent C6F5 environments suggests that 2-2 does not exhibit significant restriction about the B-N bond as is seen in 2-1. Adduct formation significantly increases steric congestion while coordinatively saturating the boron centre. The loss of restricted rotation in 2-2 strongly supports that the restriction arises from B-N multiple bond character and not from steric interaction between the groups on the boron and nitrogen centres. The solid-state structure of 2 displays a B-O bond length of 1.504(2) Å and a B-N bond length of 1.584(2) Å, which is consistent with a B-N single bond. The boron centre displays tetrahedral geometry, while nitrogen is very slightly pyramidalized (sum of angles = 356.2(3)°).

Figure 2.3. POV-ray depiction of 2-2; C: light grey, B: yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, hydrogen atoms have been omitted for clarity.

The reaction of (C6F5)2BPh with the PhI=NTs iodine in pentane yielded a white powder 2-3 upon workup. Similar to 2-1, the 11B{1H} NMR spectrum of 2-3 shows a singlet at 42.0 ppm. The 19 F NMR spectrum shows two sets of three C6F5 inequivalent resonances. The ortho-fluorine signals appear at -131.4 and -131.6 ppm, suggesting that neither C6F5 ring had migrated from the boron centre of 2-3, contrary to the large upfield shift observed in 2-1. These data are consistent

with the formulation of 2-3 as (C6F5)2BN(Ts)Ph (Scheme 2.10).

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

Unlike in the formation 2-1, the different substituents of (C6F5)2BPh give rise to the possibility of constitutional isomers depending on which group migrates to the nitrogen centre. Inspection of 19F NMR spectrum of the crude reaction mixture shows no signals attributable to a product in which 19 insertion occurred into a B-C6F5 bond. Variable temperature F NMR was used to probe the

chemical exchange of C6F5 groups on the boron centre (Figure 2.4). At room temperature, ortho-, meta-, and para-fluorine signals of the two C6F5 substituents are sharp and inequivalent. At 60 °C the ortho-fluorine signals have overlapped and the para- and meta-fluorine signals have broadened significantly. Coalescence of the remaining fluorine signals is observed at 80 °C, with gradual sharpening when heated to 100 °C. A minor decomposition product, HC6F5, is observed in the spectra.

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

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

Since insertion was found to occur in (C6F5)2BPh selectively into the B-Ph bond, BPh3 was selected for reaction with PhI=NTs to exploit the higher migratory aptitude of Ph groups. The reaction slurry of BPh3 with PhI=NTs in pentane was filtered after 14 h to yield a white powder. The 1H NMR spectrum of the white powder indicated the presence of mostly unreacted PhI=NTs. 1 11 1 An NMR scale reaction of PhI=NTs and BPh3 in toluene-d8 was monitored by H and B{ H} NMR. Two new boron resonances formed at 45.6 ppm and 29.4 ppm at 45 °C, with small amounts of starting material remaining even after one week. The resonance at 45.6 ppm is consistent with formation of Ph2BOH. The second product giving rise to the resonance at 29.4 ppm could not be isolated. The 1H NMR spectrum of the reaction mixture contained many overlapping aryl resonances, preventing characterization of the products. The slow reaction between BPh3 and PhI=NTs highlights the need for a sufficiently electrophilic boron centre.

Returning to more electron deficient borane starting materials, Piers’ borane HB(C6F5)2 was reacted with PhI=NTs and similarly yields a white powder 2-4 upon filtration and recrystallization. Compound 2-4 exhibits a signal at 34.4 ppm in 11B{1H} NMR and, similar to 2-3, exhibits two 19 sets of C6F5 resonances by F NMR with no significant upfield shifted ortho-fluorine signal,

indicative of no C6F5 ring migration. These data are consistent with the assignment of 2-4 as

(C6F5)2BN(Ts)H (Scheme 2.11). As is the case with the formation of 2-3, the crude reaction mixture of 2-4 shows only minor impurities, none attributable to the C6F5 migrated product.

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

The selective migration of Ph, H, or Cl over C6F5 substituents is consistent with reactivity trends observed in related diazomethane insertion and carboboration chemistry.35,38 The migratory aptitude can be rationalized primarily by electron density arguments, with the more electron rich substituent migrating.

A solid-state structure of 2-4 was obtained by X-ray crystallography (Figure 2.6). The B-N bond distance is 1.408(3) Å, consistent with a double bond. The boron substituents are coplanar, with the sulfonyl group on the nitrogen centre, and the S-N-B-C dihedral angles are 176.8(1)° and -1.4(3)°. The boron centre has a trigonal planar geometry with angles summing to 360.0°. The centroid-centroid distance of the tolyl ring and closest C6F5 ring is 4.098 Å, and the angle between the planes of those rings is 9.63°, indicative of a parallel-displaced π-π stacking interaction. The

rings are offset by 2.518 Å, which allows the ortho-fluorine substituent of the C6F5 ring to be positioned over the centre of the tolyl ring (offset of 0.384 Å). In an ab initio study of the intermolecular interaction between benzene and hexafluorobenzene, the slipped-parallel complex was found to be the most stable interaction with a stabilization energy of -5.38 kcal/mol when the intercentroid distance was 3.64 Å with an offset of 1.0 Å.39 The calculated stabilization energies

of the non-offset benzene-hexafluorobenzene complex show a slight stabilization effect occurring at intercentroid distances less than 6 Å, increasing to a maximum stabilization at 3.6 Å.

Figure 2.5. POV-ray depiction of 2-4; C: light grey, B: yellow-green, O: red, S: yellow, N: blue, P: orange, F: pink, hydrogen atoms have been omitted for clarity.

Piers’ borane was selected to test reactivity with other hypervalent iodine reagents because of the straightforward reactivity observed with PhI=NTs. Piers’ borane was reacted in a similar fashion with PhI=NCls, PhI=NMs, and PhI=NNs to yield (C6F5)2BN(Cls)H 2-5, (C6F5)2BN(Ms)H 2-6, and 11 1 (C6F5)2BN(Ns)H 2-7 (Scheme 2.11). The B{ H} NMR of 2-5 – 2-7 spectra each show one singlet at 39.3, 39.7, and 39.6 ppm respectively. The 19F NMR spectra of 2-5 – 2-7 all show two

inequivalent sets of C6F5 resonances and no evidence of upfield shifted ortho-fluorine signals indicative of C6F5 ring transfer to nitrogen. The downfield amine proton signal is visible for 2-4 – 2-7 between 7.3 and 7.8 ppm. Additionally, the solid-state structures of 2-5 and 2-6 were obtained from X-ray diffraction of single crystals (Figure 2.6).

Figure 2.6. POV-ray depiction of 2-5 (top) and 2-6 (bottom); C: light grey, B: yellow-green, O: red, S: yellow, N: blue, Cl: green, F: pink, hydrogen atoms have been omitted for clarity.

Both 2-5 and 2-6 display similar bond metrics to 2-4, with B-N bond distances of 1.407(2) Å and 1.396(4) Å respectively, both consistent with double bond character. Boron centres of 2-5 and 2-6 are both planar with angles around boron summing to 359.9(3)° and 359.8(6)° respectively. Additionally, the sulfonyl substituents of 2-5 and 2-6 are both coplanar with the substituent plane of the boron centre. Compound 2-5, in a similar fashion to 2-4, displays features indicative of π-π stacking. The centroid-centroid distance between the para-chlorophenyl ring and the nearest C6F5 ring is 4.157 Å, and the angle between the planes of the two rings is 9.43°. The ring offset is 2.632 Å, allowing the ortho-fluorine atom to be situated above the centre of the chlorophenyl ring, with an offset of 0.413 Å. Compound 2-6, which has no possibility for a similar π-π interaction, displays a C-S-N-B torsion angle of 82.0(3)°, whereas the dihedral angle between those atoms is 59.4(2)° for 2-4 and 60.9(2)° for 2-5.

Interestingly, the reaction between ClB(C6F5)2 with PhI=NTs, PhI=NCls, PhI=NMs, or PhI=NNs cleanly yields 2-4, 2-5, 2-6, or 2-7 respectively (Scheme 2.12). Insertion into the B-Cl bond is believed to occur, transiently generating a B-N-Cl species before subsequent reaction with adventitious water or solvent. Only the single insertion product is observable by 19F NMR and 11B{1H} NMR of the reaction mixture in pentane. Upon removing pentane from the reaction 1 mixture and obtaining H NMR in C6D6, a single product with diagnostic downshifted N-H 1 resonance is observed. Attempts to monitor the reaction by H NMR in C6D6 and toluene-d8 lead to decomposition, consistent with earlier observations of reactivity in aromatic solvents.

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

In the related work by our group, multiple bond insertions using diazomethane reagents were 35 observed. The reaction of PhB(C6F5)2 with equimolar Me3SiCH(N2) yielded a mixture of the

single- and double-insertion product. Reaction of two or more equivalents of Me3SiCH(N2) with

PhB(C6F5)2 yielded the double-insertion product exclusively.

Unlike the work with diazomethane reagents, all reactions between equimolar hypervalent iodine reagents and boranes showed no evidence of multiple bond-insertion products. Highly electrophilic BCF was chosen to test for multiple insertions as it is believed that 2-1 has the highest residual Lewis acidity. When three equivalents of PhI=NTs were reacted with BCF in pentane, 2- 1 was obtained as the only product, even upon heating the mixture. At temperatures exceeding 80 °C, decomposition of both excess PhI=NTs and 2-1 occurs, but no significant secondary product is observable by 19F or 11B{1H} NMR attributable to a double-insertion product.

Similarly, 2-6 was also tested for multiple bond-insertion reactivity, as it is the least sterically crowded around the boron centre. The reaction of either Piers’ borane or ClB(C6F5)2 with three equivalents of PhI=NMs in pentane yielded 2-6 exclusively. Compound 6 is considerably less

sterically crowded than the intermediate reported (Me3SiCH(Ph)B(C6F5)2 which does react with a second equivalent of diazomethane.

The mechanism for insertion into the B-C bond likely consists of the formation of an adduct between the borane and the nitrogen of the iodine reagent. After the initial insertion, the double bond character of the resulting aminoborane sufficiently quenches the Lewis acidity at the boron centre to preclude adduct formation with additional equivalents of iodine reagent.

2.2.2 Electrophilicity of Aminoboranes

2.2.2.1 Gutmann-Beckett Method The Gutmann-Beckett method was used to determine the relative Lewis acidity of compounds 2- 31 1 1, 2-3 – 2-7. Reaction of excess 2-1 with OPEt3 in CH2Cl2 yields 2-2 in-situ. The P{ H} NMR chemical shift of the bound phosphorus in 2-2 is 80.7 ppm, which is 2.68 ppm downfield from the related (C6F5)3B·OPEt3. These data infer that, by the Gutmann-Beckett method, 2-1 is more Lewis- acidic that BCF. The relative Lewis acidity is contrary to the observation that BCF undergoes reaction with PhI=NTs while 2-1 does not. However, Lewis acid-base interactions are highly dependent on the nature of Lewis acid and Lewis base, and are therefore often not directly

transferable between unlike systems. The Gutmann-Beckett test depends on how well boron-bound 40 heteroatoms compete with OPEt3 for the boron acceptor orbital.

The Gutmann-Beckett protocol was applied to compounds 2-3 – 2-7 and yielded 31P{1H} NMR chemical shifts of 77.8, 80.2, 80.5, 79.8, and 80.7 ppm respectively. These data infer slightly

increased Lewis-acidity from the parent boranes PhB(C6F5)2 (76.6 ppm) and HB(C6F5)2 (75.6 ppm). The Gutmann acceptor number (AN) for compounds 2-1, 2-3 – 2-7 was calculated to be 88.0, 81.5, 86.8, 87.3, 85.7, and 87.7 (Table 2.1).

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

Compound Δδ 31P (ppm)a ANb 5-1 30.0 88.0 5-3 27.1 81.5 5-4 29.5 86.8 5-5 29.8 87.5 5-6 29.1 85.9 5-7 30.0 87.9 BCF 27.3 82.0

HB(C6F5)2 24.9 76.7

PhB(C6F5)2 25.9 78.9

a 31 Δ P are calculated by subtracting the chemical shift of the reference OPEt3 in CH2Cl2 from the observed b chemical shift. AN are calculated using the formula (observed chemical shift – chemical shift of OPEt3 in hexane) * 2.21.

While the Gutmann-Beckett method doesn’t provide a reliable method to compare insertion products to the borane starting materials because of the significantly altered steric and bonding environment due to the nitrogen heteroatom connected to the boron centre, it does allow for the comparison of related aminoboranes 2-4 – 2-7. The effect of the varied sulfonyl groups on the Lewis acidity follows the expected trend with para-nitrophenyl-substituted 2-7 being the most Lewis acidic, then para-chlorophenyl-substituted 2-5, then tolyl-substituted 2-4, and methyl- substituted 2-6 being the least Lewis acidic.

The difference in Lewis acidity between the most (2-7) and least (2-6) Lewis acidic aminoborane generated from Piers’ borane (ΔAN = 2.0) is less than the difference in Lewis acidity between

BCF and PhB(C6F5)2 (ΔAN = 3.1). This range in Lewis acidity is consistent with the earlier post- synthetic tuning strategy which appended various phosphines to the para-position of BCF.28 The authors reported acceptor numbers the para-substitution of phosphines as follows: PiPr3 28 (AN = 85.6), PCy3 (AN = 85.2), PMes3 (AN = 84.6) and HPtBu2 (AN = 80.2).

2.2.2.2 General Electrophilicity Index Computations were carried out using Gaussian 0941 to further investigate the electronic structure of these electrophilic aminoboranes. Geometry optimizations and frequency calculations were carried out on 2-4 – 2-7 using the B3LYP functional42 with the 6-31+G(d) basis set.43 Single point energies were calculated on the optimized geometries using the MP244 and def2-TZVPP basis set.45 The LUMOs of 2-4 – 2-6 display large lobes localized on the boron centre as well as one aryl substituent. The LUMO of 2-7 displays a smaller lobe on boron with substantial LUMO distribution on the 4-NO2Ph substituent (Figure 2.7).

Figure 2.7 Surface contour plot of LUMO of 2-4 (top left), 2-5 (top right), 2-6 (bottom left), and 2-7 (bottom right); C: dark grey, H: light grey, N: dark blue, O: red, F: light blue.

Using the HOMO and LUMO energies, the global electrophilicity index (GEI) values, ω, were calculated for 2-4 – 2-7 using the formula

with all energies in kJ/mol (Table 2.2).

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

Compound ELUMO (eV) EHOMO (eV) ω (eV) 2-4 0.943 -9.96 0.932 2-5 0.764 -10.2 1.01 2-6 0.844 -10.3 1.00 2-7 0.026 -10.4 1.29

The ω values give the trend for increasing electrophilicity 2-4 < 2-6 < 2-5 < 2-7. This gives a different result than the trend obtained using the Gutmann acceptor numbers of 2-4 – 2-7, specifically the switched positions of 2-4 and 2-6. Compounds 2-4 – 2-7 should be ideal for comparison using the GEI as the substituent set on the boron centres are very similar and the electronic environment is perturbed by a peripheral functional group.

Since 2-4 – 2-7 all readily formed adducts with OPEt3, the Gutmann-Beckett method is the more reliable method to compare their effective electrophilicities. The ω values are obtained using only static energy values and, as such, do not take steric effects or energy costs associated with rearrangements to form an adduct into account. The additional effects that the Gutmann-Beckett method accounts for make it the more reliable measure, since the aim of these methods is to help rationalize observed reactivity trends which are similarly affected by steric effects.

2.2.3 Reactivity of Aminoboranes

The reactivity of select nitrene insertion products was assessed in a series of Lewis acid catalysis applications (Scheme 2.13). The dimerization of 1,1-diphenylethylene is a useful test reaction for Lewis acids since weak and strong Lewis acids alike catalyze this reaction. As the dimerization of 1,1-diphenylethylene occurs, the reaction mixture becomes strongly coloured, which allows the reaction to be monitored visually. Additionally, this reaction can be easily monitored by reduction of the olefinic proton signal and increase of the diastereotopic proton signals in 1H NMR.

Reactions with 1,1-diphenylethylene and 5 mol% of 2-4 – 2-7 each were done in C6D6. Upon addition of the solution to the solid aminoboranes, a pale yellow colour is observed. The reaction

proceeds slowly over the course of days. Significant decomposition is observed in all reactions after two days by 19F NMR, with no additional conversion observed after three days. Final conversions for 2-4 – 2-7 as determined by 1H NMR integration were 64%, 67%, 47%, and 78% respectively. The final conversions are consistent with Lewis acidity as determined by the Gutmann-Beckett method, though low catalyst stability prevented complete conversion in all cases.

Related dialkylaminobis(trifluoromethyl)boranes, (CF3)2BNR2, have been reported to react with a variety of 1,3-dienes to undergo [2+4] cycloadditions.46 Steric constraints were noted to be

significant in determining reactivity: (CF3)2BNEt2 reacted with fewer substrates than

(CF3)2BNMe2, and (CF3)2BN(Me)tBu was found to be inactive. Aminoboranes 2-1 and 2-6 were each reacted with cyclopentadiene in toluene, in an effort to effect cycloaddition. Aminoborane 2- 6 was selected for test reactivity as it is the least sterically encumbered, and 2-1 was selected as it is the most electrophilic. The 1H NMR spectra of these reactions revealed only the cyclodimerization of cyclopentadiene. To slow the dimerization of cyclopentadiene to allow reaction between the aminoborane and cyclopentadiene to occur, bulkier analogue 1,2,3,4,5- pentamethylcyclopentadiene, was used. Compounds 2-1 and 2-6 effectively catalyzed the dimerization of the bulkier analogue after one hour.

To reduce the steric congestion, two acyclic 1,3-dienes were tested for cyclodimerization reactivity. Compounds 2-1 and 2-6 were unreactive towards 2,3-dimethyl-1,3-butadiene, even upon heating. Compounds 2-1, 2-3, and 2-6 were each reacted with trans-1-methoxy-3- trimethylsilyloxy-1,3-butadiene, Danishefsky’s diene, however only adduct formation was observed in all three cases.

Select aminoboranes were tested for FLP reactivity by combination with a bulky phosphine under an atmosphere of hydrogen. No adduct is observed when 2-1 is mixed with tBu3P in a solution of toluene. Upon exposing the reaction mixture to an atmosphere of hydrogen the FLP fails to

generate the hydrogen-split phosphonium-hydridoborate salt [HPtBu3][(C6F5)2BHN(Ts)(C6F5)]. Further, solutions of either 2-1, 2-4, or 2-6 with N-benzylidene-tert-butylamine under an atmosphere of hydrogen failed to generate the corresponding reduced amine product. Aminoboranes 2-1, 2-4, and 2-6 were also found to be unreactive towards other small molecules including CO2, SO2, alkenes, and alkynes. All aminoboranes demonstrated high sensitivity to

44 water, resulting from the electron-withdrawing amide fragment. Hydrolysis lead to the cleavage of the B-N bond to generate the corresponding tosylamide.

Scheme 2.13. Summary of reactivity of aminoboranes.

2.2.4 Mechanism of Nitrene Insertion

The mechanism of nitrene insertion into boranes likely occurs through initial adduct formation between the nitrogen of the iodine reagent and the borane. The iodine reagent is stable for long periods of time in pentane at room temperature, which makes the generation of a free transient nitrene species as the initial step unlikely. Highly electron-withdrawing substituents on the boron centre, such as pentafluorophenyl groups, are important to facilitate the initial adduct formation.

After initial complexation, the migrating group donates electron density in the σ* orbital leading to formation of a new N-C/H/Cl bond and cleavage of B-C/H/Cl and N-I bonds (Scheme 2.14). With the limited scope of migrating substituents observed (H, Cl, and Ph substituents selectively

migrate before C6F5 groups), no definite trend can be defined, though migration of more electron rich substituents is preferred.

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

2.2.5 Synthesis and Characterization of Phosphinimines

The use of hypervalent iodine reagent PhI=NTs to oxidize triphenylphosphine, PPh3, to the 19 corresponding phosphinimine TsN=PPh3 has been reported. This methodology provides a potential route to electron-deficient phosphinimines with potential for Lewis acidic reactivity. To

this end, PhI=NTs was reacted with electron deficient phosphines Ph2P(C6F5) and P(C6F5)3. No

reaction occurs between PhI=NTs and Ph2P(C6F5) at room temperature. At 60 °C, the white slurry reaction mixture begins to darken and the 19F NMR spectrum shows the growth of two new sets

of C6F5 resonances. The minor set of fluorine resonances at -128.8, -147.1, and -160.0 ppm matches the spectrum of authentic PO(C6F5)Ph2, while the other major set of resonances are

attributed to the electron-deficient phosphinimine TsN=P(C6F5)Ph2 (2-8). Similarly, reaction of

PhI=NTs with P(C6F5)3 at elevated temperatures yields two new products: PO(C6F5)3 and

TsN=P(C6F5)3 (2-9) with resonances at -135.9, -152.1, and -164.6 ppm (Scheme 2.15). Compounds 2-8 and 2-9 display 31P{1H} NMR resonances at -0.7 and -31.6 ppm respectively. Since the iodine

reagent PhI=NTs decomposes at the reaction temperature used, sequential additions of PhI=NTs are required to consume all the phosphine starting material.

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

Both P(C6F5)Ph2 and P(C6F5)3 are difficult to oxidize, with P(C6F5)3 slowly oxidizing only under 47 reflux conditions with H2O2. After full consumption of phosphine starting material has occurred, continued heating results in the slow conversion of 2-8 or 2-9 to the corresponding phosphine oxides. This conversion likely occurs through reaction with adventitious water (Scheme 2.16). Despite stringent water exclusion techniques used during successive openings of the reaction vessel to add additional PhI=NTs, concomitant phosphine oxide formation was always observed during the synthesis of 2-8 or 2-9.

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

The solid-state structure of 2-8 was obtained by X-ray diffraction (Figure 2.8). The phosphorus centre displays tetrahedral geometry and a P-N bond distance of 1.5953(14) Å. As observed in the solid-state structures of 2-4 and 2-5, there is a π-π interaction between the tosyl ring and the C6F5 substituent. The centroid-centroid distance is 3.654 Å and the ring offset is 1.105 Å, which are considerably closer than 2-4 and 2-5 to the ideal calculated distances of a 3.64 Å intercentroid distance with a 1.0 Å offset.48 The tetrahedral geometry of the phosphorus centre likely allows a more favourable π-π interaction by reducing internal bond strain compared to the trigonal planar

boron centres of 2-4 and 2-5. The angle between the planes of the C6F5 and tolyl rings is 11.19°.

The other phenyl substituents not participating in a π-π interaction adopt torsion angles of 31.0(2)° and 1.1(2)° relative to the P-N bond, which are significantly smaller than the 105.0(2)° dihedral angle of the C6F5 ring relative to the P-N bond.

Figure 2.8. POV-ray depiction of 2-8; C: light grey, P: orange, O: red, S: yellow, N: blue, F: pink, hydrogen atoms have been omitted for clarity.

The mechanism for the formation of 2-8 and 2-9 is likely the same as that reported for the formation 19 of TsN=PPh3. Since the reaction only occurs at elevated temperatures at which PhI=NTs decomposes slowly, it is likely that the first step is generation of free tosylnitrene, which lacks a full octet and is highly electrophilic. The nitrene is then readily attacked by even weakly

nucleophilic phosphines P(C6F5)Ph2 and P(C6F5)3 to form 2-8 and 2-9 respectively (Scheme 2.17).

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

The optimized geometry of 2-8 was computed with Gaussian 0941 using the functional B3LYP42 with a 6-31+G(d) basis set.43 The single point energy of the optimized geometry of 2-8 was obtained using the MP244 with the def2-TZVPP basis set.45 The HOMO of 2-8 is largely delocalized around the tosyl substituent with an energy of -9.125 eV (Figure 2.9, left). The LUMO of 2-8 displays a significant lobe on the phosphorus centre and delocalization onto the C6F5 ring with an energy of 1.721 eV (Figure 2.9, right).

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

The large lobe of the LUMO located at phosphorus is consistent with LUMOs of Lewis acidic phosphonium cations (Chapter 3, pg 82; Chapter 4, pg 133). However, the LUMO energy of 2-8

is considerably higher than diphenyl-substituted phosphonium cations 3-10 (ELUMO = -2.289 eV)

and 4-22 (ELUMO = -2.197 eV). The LUMO energies of all fluorophosphonium cations in later chapters differ by less than 2 eV, as such, the nearly 4 eV difference between 2-8 and 3-10 is significant.

2.3 Conclusion The use of hypervalent iodine reagents of the form PhI=NR, where R is a sulfonyl group, to effect post-synthetic borane tuning was investigated. Compound PhI=NTs was reacted with BCF,

PhB(C6F5)2, and HB(C6F5)2 to effect tosylnitrene insertion into borane B-C bonds, forming

(C6F5)2BN(Ts)(C6F5) (2-1), (C6F5)2BN(Ts)Ph (2-3), and (C6F5)2BN(Ts)H (2-4). The migratory

aptitude of borane substituents was favoured by Ph and H substituents over C6F5 substituents.

However, C6F5 substitution was beneficial in driving insertion reactivity, as less electrophilic PPh3 failed to cleanly generate the desired insertion product, despite the greater migratory aptitude of Ph. Subsequently a range of tosyl substituents were investigated reacting PhI=NCls, PhI=NMs,

and PhI=NNs with HB(C6F5)2 to generate insertion products (C6F5)2BN(Cls)H (2-5),

(C6F5)2BN(Ms)H (2-6), and (C6F5)2BN(Ns)H (2-7) respectively. Interestingly, reacting each of the

four iodine reagents with ClB(C6F5)2 similarly generated corresponding 2-4 – 2-7 as the sole product. Compound 2-1 was subjected to the Gutmann-Beckett method, generating the triethylphosphine oxide adduct (C6F5)2BN(Ts)(C6F5)·OPEt3 (2-2). The series 2-4 – 2-7 was also subjected to the Gutmann-Beckett method, yielding the increasing electrophilicity trend of 2-6 (Ms) < 2-4 (Ts) < 2-5 (Cls) < 2-7 (Ns). While the use of iodine reagents was effective at tuning the Lewis acidity, the resulting aminoboranes proved unreactive in FLP chemistry, only effecting Lewis acid catalysis. Additionally, the reaction between PhI=NTs with electron-deficient phosphines Ph2P(C6F5) and P(C6F5)3 slowly generates phosphinimines TsN=P(C6F5)Ph2 (3-8) and

TsN=P(C6F5)3 (3-9). Both 3-8 and 3-9 were found to be very sensitive to decomposition in the presence of water, yielding the corresponding phosphine oxide and tosylamide.

2.4 Experimental 2.4.1 General Experimental Methods

Air-sensitive manipulations were carried out under an atmosphere of dry, O2-free N2 using either

an MBraun MB Unilab Glovebox or a dual-manifold Schlenk line. Hexane, pentane, and Et2O were all purified using a Grubbs-type column system produced by Innovative Technology and dispensed into thick-walled Straus flasks equipped with Teflon greaseless stopcock and stored over 4 Å sieves. Tetrahydrofuran and benzene were each dried over sodium metal and benzophenone before being distilled to Schlenk bombs and stored over 4 Å sieves. Dichloromethane was dried using calcium hydride before being vacuum transferred to a Schlenk bomb. Deuterated solvents were degassed using three successive freeze-pump-thaw cycles. Other bulk solvents were degassed by three successive cycles of headspace-evacuation and sonication. High-resolution mass spectra data was obtained using a JEOL AccuTOF or an Agilent 6538 Q-TOF mass spectrometer. All NMR data were collected on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz, or Agilent DD2 600 MHz spectrometer at 25 °C unless otherwise noted. NMR chemical shift data are given 1 13 11 19 31 relative to an external standard ( H, C: SiEt4; B: 15% BF3·Et2O; F: CFCl3; P: H3PO4). In some cases, 2-D NMR techniques were employed to assign individual resonances. Piers’ borane, 49 HB(C6F5)2, and chloroborane ClB(C6F5)2, were synthesized by literature procedures. Tris(pentafluorophenyl)borane, BCF, was purchased from Boulder Scientific Company and

sublimed before use. Reagents obtained from Sigma-Aldrich including PhB(C6F5)2, P(C6F5)3,

Ph2P(C6F5) and PhI(OAc)2 were used without further purification, while BPh3 was recrystallized from Et2O before use. All sulfonamides were purchased from Apollo Scientific and used without additional purification. Computational work was performed using resources provided by the Shared Hierarchical Academic Research Computing Network and Compute Canada.

Synthesis of (C6F5)2BN(Ts)(C6F5) (2-1): A 50 mL Schlenk flask was charged with TsN=IPh (88.6

mg, 0.237 mmol) in pentane (10 mL) to form a white slurry. A solution of B(C6F5)3 (121.5 mg, 0.237 mmol) in pentane (10 mL) was added to the flask. After stirring overnight, a white insoluble

powder was collected via filtration. The crude product was dissolved in a small volume of CH2Cl2 (2 mL) and filtered through a plug of Celite. The filtrate was collected and stored at -35 °C to allow

for the formation of small clear, colourless crystals. Yield 120 mg (74%). 1H NMR (400 MHz, 3 3 C6D6, 25 °C): δ 7.35 (d, JHH = 8 Hz, 2H, tol CH), 6.49 (d, JHH = 8 Hz, 2H, tol CH), 1.75 (s, 11 1 19 3H, tol CH3). B{ H} NMR (128 MHz, C6D6, 25 °C): δ 43.6 (br). F (376 MHz, C6D6, 25 °C):

δ -128.60 (m, 2F, o-F (C6F5)), -131.35 (m, 2F, o-F (C6F5)), -142.19 (m, 2F, o-F N(C6F5)), -146.68 3 3 3 (t, JFF = 21 Hz, 1F, p-F (C6F5)), -148.69 (t, JFF = 22 Hz, 1F, p-F N(C6F5)), -150.02 (t, JFF = 19

Hz, 1F, p-F (C6F5)), -159.04 (m, 2F, m-F (C6F5)), -160.27 (m, 2F, m-F N(C6F5)), -161.06 (m, 2F, + m-F (C6F5)). HRMS (EI-TOF) m/z: [M] Calcd for C25H7BF15NO2S 681.0085, Found 681.0068.

Synthesis of (C6F5)2BN(Ts)(C6F5) · OPEt3 (2-2): A 20 mL scintillation vial was charged with 1

(53.3 mg, 0.0782 mmol) in toluene (1 mL). To the solution, OPEt3 (10.5 mg, 0.0783 mmol) was added. The solution was left at room temperature overnight to afford large rectangular crystals,

suitable for X-ray diffraction. Yield 56.1 mg (88%). 1H NMR (400 MHz, toluene-d8, 25 °C): δ 3 3 7.64 (d, JHH = 8 Hz, 2H, tol CH), 6.66 (d, JHH = 8 Hz, 2H, tol CH), 1.88 (m, 6H, Et CH2), 1.84 3 3 11 1 (s, 3H, tol CH3), 0.68 (dt, JPH = 18 Hz, JHH = 8 Hz, 9H, Et CH3). B{ H} NMR (128 MHz, 19 toluene-d8, 25 °C): δ 1.41 (s) ppm. F NMR (376 MHz, toluene-d8, 25 °C): δ -133.44 (br, 4F, o-F

B(C6F5)), -137.99 (s, 2F, o-F N(C6F5)), -154.59 (s, 1F, p-F N(C6F5)), -156.70 (br, 2F, p-F 31 1 B(C6F5)), -163.86 (br, 4F, m-F B(C6F5)), -164.10 (s, 2F, m-F N(C6F5)) ppm. P{ H} NMR (162 MHz, toluene-d8, 25 °C): δ 78.50 (s) ppm.

Synthesis of (C6F5)2BN(Ts)Ph (2-3): A 50 mL Schlenk flask was charged with TsN=IPh (17.4 mg, 0.0466 mmol) in pentane (10 mL). To the flask, a solution of PhB(C6F5)2 (19.7 mg, 0.0467 mmol) in pentane (10 mL) was added. The reaction mixture was stirred overnight. A white solid was collected via filtration. The solid was redissolved in CH2Cl2 and cooled to -35 °C to afford 1 clear colourless crystals. Yield 20.9 mg (76%). H NMR (400 MHz, toluene-d8, 25 °C): δ 7.36 (d, 3 3 3 JHH = 8 Hz, 2H, tol CH), 6.94 (d, JHH = 7 Hz, 2H, Ph CH), 6.63 (m, 3H, Ph CH), 6.54 (d, JHH = 11 1 8 Hz, 2H, tol CH), 1.81 (s, 3H, tol CH3) ppm. B{ H} NMR (128 MHz, toluene-d8, 25 °C): δ 19 42.00 (s) ppm. F (376 MHz, toluene-d8, 25 °C): δ -131.42 (br, 4F, o-F (C6F5)), -131.62 (br, 4F, 3 3 o-F (C6F5)), -150.95 (t, JFF = 18 Hz, 2F, p-F (C6F5)), -151.59 (t, JFF = 18 Hz, 2F, p-F (C6F5)), - + 161.06 (br, 4F, m-F (C6F5)), -131.42 (br, 4F, m-F (C6F5)) ppm. HRMS (EI-TOF) m/z: [M] Calcd

for C25H12BF10NO2S 591.0522, Found 591.0529.

Aminoboranes of the general formula (C6F5)2BN(R)H, where R = Ts, Ms, Cls, or Ns, can be

generated by reacting equimolar amounts of the appropriate ylide with either ClB(C6F5)2 or

HB(C6F5)2. A sample preparation is provided below.

Synthesis of (C6F5)2BN(Ts)H (2-4): A 50 mL Schlenk flask was charged with TsN=IPh (16.6 mg,

0.044 mmol) in pentane (10 mL) to form a white slurry. A solution of ClB(C6F5)2 (16.9 mg, 0.044 mmol) was added to the Schlenk flask. The resulting slurry was stirred overnight and the white insoluble solid was subsequently collected via filtration. The powder was then dissolved in CH2Cl2 and stored at -35 °C to allow for the formation of clear, colourless crystals. Crystals suitable for

X-ray diffraction were obtained from a CH2Cl2 solution for compounds 2-4, 2-5, and 2-6. (84% yield)

1 3 H NMR (400 MHz, C6D6, 25 °C): δ 7.88 (s, 1H, NH), 7.74 (d, JHH = 8 Hz, 2H, tol CH), 6.67 (d, 3 11 1 JHH = 8 Hz, 2H, tol CH), 1.84 (s, 3H, tol CH3) ppm. B{ H} NMR (128 MHz, C6D6, 25 °C): δ 19 34.4 (s) ppm. F (376 MHz, C6D6, 25 °C): δ -136.07 (br, 2F, o-F (C6F5)), -136.43 (br, 2F, o-F

(C6F5)), -151.24 (br, 1F, p-F (C6F5)), -156.19 (br, 1F, p-F (C6F5)), -165.07 (br, 2F, m-F + (C6F5)), -165.35 (br, 2F, m-F (C6F5)) ppm. HRMS (EI-TOF) m/z: [M] Calcd for C19H8BF10NO2S 515.0209, Found 515.0225.

1 Synthesis of (C6F5)2BN(Cls)H (2-5): (78% yield) H NMR (400 MHz, C6D6, 25 °C): δ 7.46 (s, 3 3 11 1 1H, NH), 7.20 (d, JHH = 9 Hz, 2H, Ph CH), 6.73 (d, JHH = 8 Hz, 2H, Ph CH) ppm. B{ H} NMR 19 (128 MHz, C6D6, 25 °C): δ 39.30 (br) ppm. F (376 MHz, C6D6, 25 °C): δ -131.32 (s, 4F, o-F

(C6F5)), -145.84 (br, 2F, p-F (C6F5)), -149.90 (br, 2F, p-F (C6F5)), -160.73 (br, 4F, m-F (C6F5)) + ppm. HRMS (EITOF) m/z: [M] Calcd for C18H5BF10NO2S 534.9663, Found 534.9659.

1 Synthesis of (C6F5)2BN(Ms)H (2-6): (85% yield) H NMR (400 MHz, toluene-d8, 25 °C): δ 7.32 11 1 (s, 1H, NH), 2.26 (s, 3H, CH3) ppm. B{ H} NMR (128 MHz, toluene-d8, 25 °C): δ 39.74 (br) 19 ppm. F (376 MHz, toluene-d8, 25 °C): δ -131.54 (br, 2F, o-F (C6F5)), -132.19 (br, 2F, o-F

(C6F5)), -146.09 (br, 1F, p-F (C6F5)), -150.01 (br, 1F, p-F (C6F5)), -160.30 (br, 2F, m-F + (C6F5)), -161.38 (br, 2F, m-F (C6F5)) ppm. HRMS (EI-TOF) m/z: [M] Calcd for C13H4BF10NO2S 438.9896, Found 438.9897.

1 Synthesis of (C6F5)2BN(Ns)H (2-7): (73% yield) H NMR (400 MHz, C6D6, 25 °C): δ 7.35 (d, 11 1 3JH-H = 9 Hz, 2H, Ph CH), 7.23 (s, 1H, NH), 7.09 (d, 3JH-H = 8 Hz, 2H, Ph CH) ppm. B{ H}

19 NMR (128 MHz, C6D6, 25 °C): δ 39.55 (br) ppm. F (376 MHz, C6D6, 25 °C): δ -131.36 (s, 4F,

o-F (C6F5)), -144.69 (br, p-F (C6F5)), -149.25 (br, p-F (C6F5)), -160.31 (br, 4F, m-F (C6F5)) ppm.

HRMS (EI-TOF) m/z: [M]+ Calcd for C18H5BF10N2O4S 545.9903, Found 545.9904.

Phosphinamines of the form TsN=P(Ar)(Ar’2), where Ar = C6F5, Ar’ = C6F5 2-8 or Ar = Ar’ =

C6F5 2-9, can be generated by reacting excess PhI=NTs with either Ph2P(C6F5) or P(C6F5)3. A sample preparation is provided below.

Synthesis of TsN=P(C6F5)Ph2 (2-8): A J-Young NMR tube was charged with Ph2P(C6F5) (35.2 mg, 0.100 mmol) and PhI=NTs (37.3 mg, 0.100 mmol) in a toluene suspension (1.0 mL). The mixture was heated in an oil bath overnight at 40 °C, resulting in decomposition of the PhI=NTs and partial conversion. Filtration of the decomposed iodine reagent and addition of additional

equivalents of PhI=NTs results in the complete consumption of Ph2P(C6F5). (76% yield)

1 31 1 H NMR (400 MHz, toluene-d8): δ 7.48 – 7.21 (m) ppm. P{ H} NMR (125 MHz, toluene-d8): δ 19 3 -0.7 (s, br) ppm. F NMR (376 MHz, toluene-d8): -125.3 (d, JFF = 20 Hz, 2F, o-F (C6F5)), -144.5 3 4 (td, JFF = 20 Hz, JFF = 6 Hz, 1F, p-F (C6F5)), -159.7 – -159.8 (m, 2F, m-F (C6F5)) ppm.

31 1 Synthesis of TsN=P(C6F5)3 (2-9): (45% yield) P{ H} NMR (125 MHz, toluene-d8): δ -31.6 (s, 19 3 3 br) ppm. F NMR (376 MHz, toluene-d8): -135.9 (t, br, JFF = 23 Hz, o-F (C6F5)), -152.1 (t, JFF 3 = 21 Hz, o-F (C6F5), -164.6 (t, JFF = 23 Hz, o-F (C6F5) ppm.

Gutmann-Beckett Method: A 5 mm NMR tube was charged with 3:1 analyte Lewis 31 1 acid:triethylphosphine oxide, OPEt3, in CH2Cl2. The P{ H} NMR chemical shift of the adduct

was recorded, as well as either an internal or external standard of uncoordinated OPEt3 in CH2Cl2. The relative chemical shift was determined by subtraction of the reference chemical shift from the adduct chemical shift.

Dimerization of cyclopentadienes: To a vial containing one equivalent of aminoborane was

added a solution of a cyclopentadiene (0.1 mmol) in toluene-d8 (0.7 mL). The solution was transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Dimerization of 1,1-diphenylethylene: To a vial containing 5 mol% catalyst was added a solution

of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

2.4.2 X-ray Crystallography

2.4.2.1 X-ray Data Collection and Reduction Crystals were coated in Paratone-N oil in a glovebox before being mounted on a MiTegen

Micromount under an N2 stream to maintain a dry, O2 free environment for each crystal. Diffraction data were collected on a Bruker Apex II diffractometer using a graphite monochromator with Mo Kα (λ = 0.71073 Å) radiation. Temperature was maintained at 150(2) K using an Oxford cryo-stream cooler. Data collection strategies were determined using Bruker Apex II software. Frame integration was carried out using Bruker SAINT software. Data absorbance correction was carried out using the empirical multiscan method SADABS. Structure solutions were obtained by direct methods and refined using SHELXTL or Olex2 software.50,51 Refinement was carried out using full-matrix least squares techniques to convergence of weighting parameters. When data quality was sufficient, all non-hydrogen atoms were refined anisotropically.

2.4.2.2 X-Ray Tables

2-4 2-5 2-6

Formula C19H8BF10NO2S C18H5BClF10NO2S C13H4BF10NO2S Weight (g/mol) 512.16 535.57 439.06 Crystal system triclinic triclinic triclinic Space group P-1 P-1 P-1 a (Å) 8.3826(6) 8.3755(3) 6.0068(7) b (Å) 11.3424(8) 11.2582(5) 9.6962(12) c (Å) 11.4015(8) 11.2869(4) 13.5918(15) α (°) 101.452(4) 76.807(3) 74.330(7) β (°) 92.380(4) 87.603(2) 81.758(7) γ (°) 111.680(4) 68.315(2) 85.914(7) Volume (Å3) 979.42(13) 961.79(7) 753.89(16) Z 2 2 2 Density (calcd.) (gcm–3) 1.7466 1.8491 1.9339 R(int) 0.0254 0.0280 0.0483 μ, mm–1 0.278 0.421 0.342 F(000) 512 529 432 Index ranges -10 ≤ h ≤ 10 -10 ≤ h ≤ 10 -7 ≤ h ≤ 7 -14 ≤ k ≤ 14 -14 ≤ k ≤ 14 -12 ≤ k ≤ 12 0 ≤ l ≤ 14 -14 ≤ l ≤ 14 -17 ≤ l ≤ 17 Radiation Mo Kα Mo Kα Mo Kα θ range (min, max) (°) 1.84, 27.44 1.86, 27.48 1.57, 27.42 Total data 14021 15198 12448 Max peak 0.4 0.4 0.6 Min peak -0.4 -0.4 -0.6 2 >2(FO ) 4394 3618 3427 Parameters 306 306 252 R (>2σ) 0.0383 0.0320 0.0440

Rw 0.0834 0.0849 0.0881 GoF 1.046 1.066 1.031

2-2 2-8

Formula C31H22BF15NO3PS, C7H8 C50H34F10N2O4P2S2 Weight (g/mol) 907.51 1042.90 Crystal system triclinic triclinic Space group P-1 P-1 a (Å) 8.2520(4) 11.2973(10) b (Å) 13.7632(7) 13.1220(12) c (Å) 17.0976(8) 17.8139(14) α (°) 83.047(3) 105.855(4) β (°) 80.953(3) 103.798(4) γ (°) 87.098(3) 102.497(5) Volume (Å3) 1902.59(16) 2352.3(4) Z 2 2 Density (calcd.) (gcm–3) 1.584 1.472 R(int) 0.0382 0.0267 μ, mm–1 0.241 0.269 F(000) 921 1066 Index ranges -10 ≤ h ≤ 10 -13 ≤ h ≤ 14 -17 ≤ k ≤ 17 -17 ≤ k ≤ 16 0 ≤ l ≤ 22 -23 ≤ l ≤ 23 Radiation Mo Kα Mo Kα θ range (min, max) (°) 1.69, 27.46 Total data 31528 10658 Max peak 0.8 0.6 Min peak -0.5 -0.5 2 >2(FO ) 8605 8317 Parameters 541 633 R (>2σ) 0.0382 0.0392

Rw 0.1070 0.1039 GoF 1.057 1.044

2.5 References

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39. Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M., Intermolecular Interactions of Nitrobenzene-Benzene Complex and Nitrobenzene Dimer: Significant Stabilization of Slipped- Parallel Orientation by Dispersion Interaction. J. Chem. Phys. 2006, 125. 40. NMR. Springer-Verlag: Berlin ; New York, 1969; p 35. 41. Gaussian 09, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox; Gaussian Inc.: Wallingford CT, 2016. 42. (a) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J., Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force- Fields. J. Phys. Chem. 1994, 98, 11623; (b) Kim, K.; Jordan, K. D., Comparison of Density- Functional and MP2 Calculations on the Water Monomer and Dimer. J. Phys. Chem. 1994, 98, 10089; (c) Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior. Phys. Rev. A 1988, 38, 3098; (d) Lee, C. T.; Yang, W. T.; Parr, R. G., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron- Density. Physical Review B 1988, 37, 785. 43. (a) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L., 6-31G* Basis Set for Atoms K Through Zn. J. Chem. Phys. 1998, 109, 1223; (b) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A., 6-31G* Basis Set for Third-Row Atoms. J. Comput. Chem. 2001, 22, 976; (c) Ditchfield, R.; Hehre, W. J.; Pople, J. A., Self-Consistent Molecular- Orbital Methods .9. Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724; (d) Hehre, W. J.; Ditchfield, R.; Pople, J. A., Self- Consistent Molecular-Orbital Methods .12. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular-Orbital Studies of Organic-Molecules. J. Chem. Phys. 1972, 56, 2257.

44. Headgordon, M.; Pople, J. A.; Frisch, M. J., MP2 Energy Evaluation by Direct Methods. Chem. Phys. Lett. 1988, 153, 503. 45. (a) Weigend, F., Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057; (b) Weigend, F.; Ahlrichs, R., Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297. 46. Pawelke, G.; Burger, H., Trifluoromethyl-Substituted Aminoboranes and Amine Boranes Revealing Alkene and Alkane Chemistry. Appl. Organomet. Chem. 1996, 10, 147. 47. Song, L. M.; Hu, J.; Wang, J. S.; Liu, X. H.; Zhen, Z., Novel Perfluorodiphenylphosphinic Acid Lanthanide (Er Or Er-Yb) Complex with High NIR Photoluminescence Quantum Yield. Photochem. Photobiol. Sci. 2008, 7, 689. 48. Tsuzuki, S.; Uchimaru, T.; Mikami, M., Intermolecular Interaction Between Hexafluorobenzene and Benzene: Ab Initio Calculations Including CCSD(T) Level Electron Correlation Correction. J. Phys. Chem. A 2006, 110, 2027. 49. Parks, D. J.; Spence, R. E. v. H.; Piers, W. E., Bis(Pentafluorophenyl)Borane - Synthesis, Properties, and Hydroboration Chemistry of a Highly Electrophilic Borane Reagent. Angew. Chem. Int. Ed. 1995, 34, 809. 50. Sheldrick, G. M., A Short History of SHELX. Acta Crystallogr. Sect. A 2008, 64, 112. 51. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H., OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339.

Chapter 3 Perchloroaryl Fluorophosphonium Cations 3.1 Introduction 3.1.1 Perchloroaryl Boranes

Perfluorophenyl substituents are commonly employed electron-withdrawing moieties in many main group FLP catalysts.1,2,3 Ashley and coworkers have exploited the related perchlorophenyl 4 substituents on boron, generating a family of mixed C6F5/C6Cl5 substituted boranes. They found that increasing the number of C6Cl5 moieties resulted in a concomitant increase in electrophilicity; however, the high steric demand of ortho-chlorine atoms resulted in a decrease of Lewis acidity (Figure 3.1).

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

Based on electronegativity, the increase in electrophilicity is counter-intuitive, however

mesomeric stabilization effects in C6F5-substituted boranes allow for partial occupancy of the vacant p-orbital on boron by donation from the fluorine atom lone-pairs in aromatic systems.4 The orbital size increase from fluorine to chlorine prevents effective overlap with the vacant p-orbital on boron, resulting in diminished quenching of Lewis acidity (Figure 3.2). This can be explained using the Hammett parameter, which is obtained by establishing a free-energy relationship between reaction rates and equilibrium constants for reactions involving benzoic acid with systematically varied meta- and para-substituents.5 Hammett parameter values positively correlate with the electron-withdrawing strength of a substituent at the specified phenyl substitution

position. Despite fluorine (χPauling= 3.98) being more electronegative than chlorine (χPauling= 3.16), the reduced ability of the 3p chlorine lone pairs to interact with the 2p aromatic orbitals result in a 6,7 higher para-position Hammett parameter (σp = 0.23) compared to fluorine (σp = 0.06). The low para-position Hammett parameter of fluorine is consistent with the observation that substitution of the para-fluorine substituents of BCF for hydrogen substituents has a negligible impact on Lewis acidity, as was determined by the Gutmann-Beckett method.8 The meta-position Hammett

parameter of chlorine and fluorine are much more comparable: σm = 0.37 and 0.34 respectively.

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

Generally, electrophilic boranes containing C6F5 rings are susceptible to decomposition by hydrolysis. Initial adduct formation occurs between the borane and water molecule, followed by 9,10 elimination of HC6F5 to form bis(pentafluorophenyl)borinic acid (Scheme 3.1). Ashley and coworkers exploited the decreased propensity of C6Cl5-substituted boranes to form Lewis acid- base adducts to make water-stable electrophilic borane catalysts. The sterically demanding ortho-

chlorine atoms prevent close adduct formation required for loss of HC6F5. Instead, the C6Cl5 substituted boranes form stable reversible adducts with water (Scheme 3.1). While

B(C6F5)2(C6Cl5) forms reversible water-adducts, B(C6Cl5)3 is hydrolytically inert, even in

refluxing toluene/H2O for days. The water-adduct of B(C6F5)2(C6Cl5) can be reversed by removing

the water using either vacuum or 3 Å sieves. Additionally, C6Cl5 substituted boranes were also

found to be more thermally robust than the corresponding C6F5 substituted boranes.

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

The Gutmann acceptor number (AN) of B(C6F5)3-n(C6Cl5)n deceases linearly as n increases, while

B(C6Cl5)3 could not form an adduct with OPEt3 to determine an acceptor number. To assess the electron density at the boron acceptor orbital without adduct formation, the reduction potentials of the series of boranes were obtained. Since there is minimal rearrangement upon electron transfer, the potentials were used as a measure of electrophilicity11. The reduction potentials of the boranes increased as a function of increasing C6Cl5 substitution.

The hydrolytic and thermal stability of perchlorophenyl-substituted electrophilic boranes represents a significant advance for the applicability of main group catalysts by general chemists. The rapidly expanding array of transformations afforded by main group catalysts remain inaccessible for many chemists that are not able to work in stringently water-free conditions. Air- stability is therefore a critical element for broad-scale adoption of main group catalytic reagents.

In a demonstration of the stability of these boranes, Ashley employed B(C6F5)2(C6Cl5) in FLP- 12 type reactions. When the borane is dissolved in THF, B(C6F5)2(C6Cl5) forms only a weak adduct with the solvent due to the increased steric encumbrance. In the presence of dihydrogen and without the use of an auxiliary base, the THF-borane mixture effects the reduction of weakly basic substrates including imines and nitrogen-containing heterocycles (Scheme 3.2).

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

13 O’Hare and coworkers have extensively studied the FLP reactivity of the bulky B(C6Cl5)3. Due

to the high steric crowding in B(C6Cl5)3, even small phosphines such as PEt3 are unable to form

an adduct and instead forms an FLP, which could be employed in H2 activation (Scheme 3.3). The authors selected B(C6Cl5)3 to promote catalytic reduction of CO2, rationalizing that the increased steric repulsion would disfavour strong formatoborate B-O bonds. Activation of formic acid by a series of B(C6Cl5)3 FLPs could be effected, however no activation of CO2 was observed by the phosphine-borane pair or the corresponding hydrogen-activated salt (Scheme 3.3). This stands in contrast to the reactivity observed for BCF systems.14

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

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

3.1.2 Perchloroaryl Phosphines

Fluorophosphonium cations derive their high electrophilicity from C6F5 groups in a similar fashion to electrophilic boranes used in FLP chemistry.15 The enhanced electrophilicity from decreased mesomeric stabilization, as well as the chemical and thermal stability imparted by C6Cl5 substituents on borane systems are all desired properties in phosphonium cation catalysts. As such,

the incorporation of C6Cl5 substituents into fluorophosphonium cations was targeted for investigation.

The series of mixed Ph/C6Cl5 phosphines, analogous to the Ph/C6F5 phosphines used in the seminal fluorophosphonium work, were first synthesized by Dua in 1970.16 These phosphines were

obtained by first generating C6Cl5Li from hexachlorobenzene before reacting with the appropriate

halophosphine (Scheme 3.4). The target C6Cl5-substituted phosphines were generated in moderate to poor yields and were only characterized by UV-spectroscopy, melting point and elemental analysis.

Scheme 3.4. Synthesis of pentachlorophenyl-substituted phosphines.

The authors reference an earlier work for the synthesis of C6Cl5Li in which the formation and 17 stability of C6Cl5Li was investigated in ethereal solvents . The stability of C6Cl5Li is noted to be

significantly lower in THF solutions than in Et2O solutions. Despite the lower stability, THF was

employed in the synthesis of the C6Cl5-substituted phosphines, resulting in poor isolated yields

(Scheme 3.4). Additionally, hazardous CCl4 and refluxing benzene were used in the purification of the target phosphines- techniques which are not congruent with the safety standards of modern laboratories.

The authors probed the reactivity of the C6Cl5-substituted phosphines, demonstrating the selective

para-functionalization of P(C6Cl5)2Ph with chlorotrimethylsilane (Scheme 3.5). Treatment of

P(C6Cl5)2Ph with two equivalents of nBuLi effects the selective two-fold lithiation at the para- position on the C6Cl5 substituents. This lithiated product reacts with chlorotrimethylsilane to yield the bis-para-functionalized P(C6Cl4SiMe3)2Ph in 75% yield. The selective functionalization of

C6Cl5 rings represents an effective route for additional fine-tuning of catalysts derived from these

phosphines. This functionalization of C6Cl5 substituents is similar in function to the

para-nucleophilic substitutions observed with C6F5 substituents, but has the potential for a wider array of functional groups to be applied. While an analogous para-functionalization was carried out using P(p-HC6F4)2Ph, the final product was only obtained in 13.6% yield.

Scheme 3.5. Functionalization of pentachlorophenyl groups with trimethylchlorosilane.

3.2 Results and Discussion 3.2.1 Synthesis and Characterization of Phosphines

In this chapter, we explore the synthesis and reactivity of a family of Ph/(C6Cl5) and

(C6F5)/(C6Cl5)-substituted phosphonium cations. Following a modified literature procedure, C6Cl6

was reacted with nBuLi in Et2O at -15 °C. Hexachlorobenzene is mostly insoluble in ethereal solvents, forming a white slurry. As the reaction with nBuLi proceeded, the white slurry turned to

a clear yellow solution. Once all suspended material was consumed, the C6Cl5Li solution was cooled down to -78 °C. Addition of appropriate equivalents of Ph2PCl, PhPCl2, (C6F5)2PBr or

(C6F5)PBr2 resulted in the formation of phosphines Ph2P(C6Cl5) (3-1), PhP(C6Cl5)2 (3-2),

(C6F5)P(C6Cl5)2 (3-4), or (C6F5)2P(C6Cl5) (3-5) respectively (Scheme 3.6). Compounds 3-1 and 3- 4 were characterized by X-ray diffraction (Figure 3.3).

Cl Cl

n eq. LiC6Cl5 P Cl n = 1 (3-1) n P Cl pent./Et O, -78 °C n = 2 (3-2) 3-n 2 3-n n Cl Cl n = 3 (3-3)*

*from PBr3, F F F F Cl Cl or w/ CuI

n eq. LiC6Cl5 n = 2 (3-4) F P Cl F P Cl n n = 1 (3-5) pent./Et O, -78 °C 3-n 2 3-n n F F F F Cl Cl Scheme 3.6. Synthesis of perchlorophenyl substituted phosphines 3-1 – 3-5.

Figure 3.3. POV-ray depiction of 3-1 (top) and 3-4 † (bottom); C: light grey, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity.

Both 3-1 and 3-4 display trigonal pyramidal geometry at the phosphorus centre. The geometry of 3-4 (sum of angles = 318.0(3)°) are slightly less pyramidalized than 3-1 (sum of angles = 311.3(3)°), consistent with increased steric bulk of substituents in 3-4.

The reaction of C6Cl5Li with PCl3 failed to yield the desired homoleptic phosphine P(C6Cl5)3 (3-

3) in appreciable yield. However, reaction of PBr3 with three equivalents of C6Cl5Li in the same reaction conditions cleanly yielded 3-3. Alternatively, addition of equimolar or substoichiometric amounts of CuI to the reaction between PCl3 and C6Cl5Li generated 3-3 in low yield. The addition of CuI has been reported in the synthesis of another homoleptic electron-deficient phosphine 18 31 1 P(C6F4H)3. The reaction of PCl3 with LiC6Cl5 yielded two products by P{ H} NMR. In the absence of CuI, the major product appears as a singlet at 10 ppm, while the desired species 3-3 is evidenced by a singlet at 19 ppm. Crystals of the second product suitable for X-ray diffraction were obtained from such reaction mixtures, which elucidated the structure of the major side-product as ((C6Cl5)2P)2 (Figure 3.4). Altering the addition rate of PCl3 or employing a large

excess of LiC6Cl5 did not favour the formation of 3-3 over ((C6Cl5)2P)2. Coupled diphosphines

similar to ((C6Cl5)2P)2 have been reported to form through the alkali metal reduction by lithium (Scheme 3.7).19,20,21 Similar phosphorus coupling reactivity has been observed using zinc reagents and is discussed in Chapter 4 (page 122). Compound 3-3 is thermally stable and does not decompose to the corresponding 1,2-diphosphine. In the solid-state, each phosphorus centre adopts a trigonal pyramidal geometry with pyramidalization in opposite directions, with a P-P bond length

of 2.265(2) Å. One C6Cl6 on each phosphorus centre is canted towards the other with P-P-C bond angles of 91.9(2)° and 93.2(2)°, which are significantly smaller than the P-P-C bond angles of the

remaining C6Cl6 substituents (108.2(2)° and 109.4(2)°). The centroid-centroid distance for the

canted C6Cl6 rings is 3.760 Å, with an angle of 17.68°between the planes of the rings.

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

Figure 3.4. Orthogonal POV-ray depictions of ((C6Cl5)2P)2; C: light grey, P: orange, Cl: green, hydrogen atoms have been omitted for clarity.

While 1-1 – 1-3 have previously been synthesized, only 31P{1H} NMR characterization had been reported for 1-1 in trace amounts.22 Interestingly, the trend for the 31P{1H} NMR chemical shifts in the Ph3-nP(C6Cl5)n series is opposite to the trend observed in the analogous Ph3-nP(C6F5)n series.

While successive substitutions of Ph groups with C6F5 groups result in a 20 ppm upfield shift, substitutions with C6Cl5 groups result in downfield shifting of 5 ppm (Figure 3.5). Electronically,

C6Cl5 and C6F5 groups behave similarly, however, the high steric demand imparted by ortho-chlorine atoms strains the geometry of the phosphines towards planarity. Bond angles of phosphorus have been reported to be a suitable parameter for rationalizing chemical-shift values.23 Other important parameters in determining 31P NMR chemical shift are coordination number, electronegativity of substituents, and the amount of π bonding. Ramsey developed a general expression for rationalizing chemical shift based on the aforementioned parameters.24,25

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

Phosphines 3-1 – 3-5 were all obtained in moderate yields and high purity after recrystallization from CH2Cl2 without the use of dangerous purification techniques used in previously reported syntheses.

One challenge associated with the use of C6Cl5 groups instead of C6F5 is the characterization of compounds because 19F NMR can no longer be used. This makes elucidation of products within mixtures of perchloroaryl compounds more difficult. However, an interesting benefit of

incorporating C6Cl5 rings, is their ability to act as mass spectral tags. Chlorine has two principle

isotopes, 35Cl and 37Cl, in a 3:1 ratio, so compounds containing chlorine atoms produce a diagnostic 3:1 mass distribution. Moreover, the more chlorine atoms incorporated into a molecule, the greater number of possible isotopologues, as is shown in the mass spectrum of 3-3 (Figure 3.6).

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

3.2.2 Synthesis of Perchloroaryl Difluorophosphoranes

Difluorophosphoranes were synthesized through the oxidation of phosphines with XeF2.

Effervescence was observed after solid XeF2 was added to a stirring solution of 3-1 in CH2Cl2, wherein the solution rapidly turned pale-yellow. The solution was stirred for two hours before the solvent was removed in vacuo, yielding 3-6 as a white powder in excellent yield (Scheme 3.8). The 19F NMR spectrum of 3-6 shows a doublet at -41.8 ppm with 715 Hz coupling, which is consistent with reported five-coordinate P-F coupling values.15.18 The 31P{1H} NMR spectrum shows a triplet at -50.7 ppm, with the same P-F coupling constant. These data are consistent with

the formation of 3-6 as Ph2PF2(C6Cl5).

Scheme 3.8. Synthesis of pentachlorophenyl-substituted difluorophosphoranes.

Following an analogous procedure with compounds 3-2, 3-4 or 3-5, difluorophosphoranes

PhPF2(C6Cl5)2 (3-7), (C6F5)PF2(C6Cl5)2 (3-8), and (C6F5)2PF2(C6Cl5) (3-9) were isolated in good yields, respectively (Scheme 3.8). Compounds 3-7 – 3-9 display similar 31P{1H} NMR data as 3-6, with triplet resonances at -44.9, -41.2, and -44.6 ppm, respectively with similar P–F coupling constants. The 19F NMR spectra of 3-7 – 3-9 display doublet resonances at -28.9, -10.9 and -6.4 ppm. Difluorophosphorane 3-8 displays an additional set of three fluorine resonances attributable to a C6F5 substituent.

The formulations of 3-6 and 3-9 were additionally confirmed by single-crystal X-ray diffraction (Figure 3.7). Both 3-6 and 3-9 display distorted trigonal bipyramidal geometries with fluorine substituents occupying the apical positions with F-P-F bond angles of 175.9(5)° and 178.23(9)° respectively. The fluorine atoms are canted slightly towards the C6Cl5 substituent. The plane of

C6Cl5 lies closer to the equatorial plane than the other aryl rings in both structures, with torsion angles of 31.4(2)° and 31.3(3)° in 3-6 and 3-9. The plane phenyl substituents are rotated 56.3(1)° and 64.4(2)° away from the equatorial plane, whereas the C6F5 rings are rotated 44.6(3)° and 41.3(3)° away from the equatorial plane. While π-donation has been reported to cause hindered rotation of substituents in the equatorial position of phosphoranes, the observed orientations appear to result predominantly from steric effects.26 The P-F bond lengths are 1.6643(9) Å and 1.6552(9) Å in 3-6, while 3-9 displays slightly shorter lengths of 1.641(2) Å and 1.626(2) Å.

Figure 3.7. POV-ray depiction of 3-6 † (top) and 3-9 † (bottom); C: light grey, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity.

Treatment of 3-3 with XeF2 in a CH2Cl2 solution yields a mixture of two products as observed by 19F and 31P{1H} NMR spectroscopy. The 31P{1H} NMR spectrum shows a triplet resonance at -38.8 ppm with 818 Hz P-F coupling and a doublet of triplets at -26.0 ppm with 1011 Hz and

894 Hz coupling. The triplet signal is consistent with the formation of the desired

difluorophosphorane PF2(C6Cl5)3, while the doublet of triplets is consistent with the formation of 19 trifluorophosphorane PF3(C6Cl5)2. The F NMR spectrum displays a doublet at -15.5 ppm consistent with PF2(C6Cl5)3; as well as a doublet of doublets at -5.5 ppm and doublet of triplets

at -71.2 ppm that integrate 2:1, which are consistent with the formation of PF3(C6Cl5)2 (Scheme

3.9). The highest ratio of the product generated was 3:1 of PF3(C6Cl5)2 to the desired PF2(C6Cl5)3

species. Adjusting the stoichiometry of the reaction by addition of excess XeF2 to 3-3 forms

PF3(C6Cl5)2 exclusively. Difluorophosphorane PF2(C6Cl5)3 is thermally stable at room temperature in solution. Repeated attempts to separate these phosphorane species by crystallization and chromatography were unsuccessful.

Scheme 3.9. Reactivity of 3-3 with XeF2.

The formation of other trifluorophosphoranes by reaction with XeF2 has previously been 27 observed. The reaction of diphenylphosphinoimidazole with XeF2 generated PF3Ph2, which was also generated directly from PPh2Cl with 1.5 equivalents of XeF2. Similarly, the reaction of

P(C6F5)2Br with XeF2 forms the corresponding trifluorophosphorane PF3(C6F5)2, which displays a 31 1 similar diagnostic doublet of triplet phosphorus resonance in the P{ H} NMR spectra. For PF3Ph2

and PF3(C6F5)2, subsequent decomposition was observed to OPFPh2 and OPF(C6F5)2 respectively.

Similar decomposition was observed for PF3(C6Cl5)2, albeit at a much slower rate. Interconversion of the apical substituents in fluorophosphoranes is observed only upon the presence of a third apicophilic fluorine substituent and is likely tied to the generally lower chemical stability of trifluorophosphoranes. Concomitant formation of FC6Cl5 is not observed in the reaction of XeF2

with 3-3. Instead, single crystals of decachlorobiphenyl (C6Cl5)2 suitable for X-ray diffraction were obtained from the reaction mixture (Figure 3.8). The two rings are twisted by 84.9(2)° and joined

by a C-C bond distance of 1.501(2), consistent with a C-C single bond. The formation of decachlorobiphenyl has been observed through reductive elimination from a germanium species.28

However, given the stability of PF2(C6Cl5)3, the mechanism of (C6Cl5)2 likely does not proceed through direct reductive elimination, but rather a pathway involving two phosphine species.

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

Our group has previously reported the synthesis of 1,2-diphosphonium dication on a naphthyl 29 framework, [(C10H6)(Ph2P)2][B(C6F5)4]2. The reported 1,2-dicationic system can activate dihydrogen with an auxiliary base and C-H activate cycloheptatriene and 1,4-cyclohexadiene. Electron-deficient diphosphine starting materials are uncommon and as such oxidation of diphosphine ((C6Cl5)2P)2 was carried out. The reaction of ((C6Cl5)2P)2 with two equivalents XeF2 cleaves the P-P bond and forms F3P(C6Cl5)2 as the major product, which is also the major product observed in the oxidation of 3-3 with XeF2. Reactions with various sub-stoichiometric amounts of

XeF2 resulted in unreacted starting material and F3P(C6Cl5)2.

3.2.3 Synthesis of Pentachlorophenyl Phosphonium Cations

Solid 3-6 was added to a white slurry of [SiEt3][B(C6F5)4] in toluene, which rapidly formed a dark- red oily suspension. The mixture was stirred overnight before the oil was allowed to settle and the supernatant was decanted. The red oil was then washed and triturated with pentane until 3-10 was

obtained as a white powder (Scheme 3.10). Compound 3-10 displays a doublet resonance at 89.5 ppm with 1009 Hz coupling in the 31P{1H} NMR spectrum, while the 19F NMR spectrum displays a doublet signal at -116.0 ppm with a matching P-F coupling constant and three resonances

attributable to the B(C6F5)4 anion. These data are consistent with the assignment of 3-10 as

[Ph2PF(C6Cl5)][B(C6F5)4].

Scheme 3.10. Synthesis of pentachlorophenyl-substituted phosphonium cations.

In an analogous fashion, reaction of [SiEt3][B(C6F5)4] with compounds 3-7, 3-8 and 3-9 yields the corresponding fluorophosphonium cations [PhPF(C6Cl5)2][B(C6F5)4] (3-11),

[(C6F5)PF(C6Cl5)2][B(C6F5)4] (3-12), and [(C6F5)2PF(C6Cl5)][B(C6F5)4] (3-13) (Scheme 3.10). Compounds 3-11 – 3-13 show resonances in the 31P{1H} NMR spectra at 84.4, 71.0 and 66.3 ppm respectively. The 19F NMR spectra of 3-11 – 3-13 contain resonances at -125.6, -117.0, and -112.1 ppm as well as three signals corresponding to the B(C6F5)4 anion. The solid-state structure of 3-11 was obtained by X-ray diffraction (Figure 3.9). Compound 3-11 displays a distorted tetrahedral geometry at the phosphorus centre. The P-F bond length of 3-11 is 1.539(2) Å, which is shorter the P-F bond length in difluorophosphoranes 3-6 and 3-9, due to an increase in P-F bond order.

Figure 3.9. POV-ray depiction of 3-11; C: light grey, B: yellow-green, P: orange, F: pink, Cl: green, hydrogen atoms have been omitted for clarity.

Direct synthesis of fluorophosphonium cations can be effected from the corresponding phosphines using a variety of fluorinating agents. To this end, compound 3-1 was reacted with one equivalent

of N-fluorobenzenesulfonimide, NFSI, in CH2Cl2. Removing the solvent in vacuo and washing the resulting powder with pentane yields the corresponding phosphonium salt

[Ph2PF(C6Cl5)][N(SO2Ph)2] in excellent yield (Scheme 3.11). The solid state structure of

[Ph2PF(C6Cl5)][N(SO2Ph)2], 3-10NFSI, was obtained by X-ray diffraction (Figure 3.10). As in 3- 11, the phosphorus centre in 3-10NFSI displays a distorted tetrahedral geometry with a comparable P-F bond length of 1.486(4) Å. The nitrogen atom of the anion is oriented towards the phosphonium fluorine at a distance of 2.659 Å, consistent with a non-coordinating anion.

O S O O Cl N F S Cl F Cl P N(SO2Ph)2 O Cl P 1 eq. NFSI Cl Cl Cl CH Cl , RT, 2 h Cl Cl 2 2 3-6 Cl 3-10NFSI Scheme 3.11. Direct fluorophosphonium synthesis from 3-6 using NFSI.

Since 3-3 was not amenable to clean oxidation by XeF2 to form the corresponding difluorophosphoranes PF2(C6Cl5)3, and instead loss of a C6Cl5 substituent resulted in the formation of PF3(C6Cl5)2 as the major product, we targeted an alternative route for oxidation.

Upon addition of NFSI to a CH2Cl2 solution of 3-3, no reaction was observed even upon heating.

Addition of Lewis acid ZrCl4 has been reported as a catalyst in the fluorination of pyrroles by

NFSI, so we attempted the analogous reaction with 10mol% ZrCl4; however, no formation of the desired fluorophosphonium cation was observed.30 NFSI, with a reduction potential of -0.78 V (vs. SCE), appears to be too weak of a fluorinating agent to oxidize 3-3.31

The N-fluoropyridinium salts represent a stronger family of fluorinating agents. The reduction potential of N-fluoropyridinium salts vary based on substitution; 2,4,6-trimethyl-N-fluoropyridium has a reduction potential of -0.73 V (vs. SCE), while unsubstituted N-fluoropyridinium has a 31 reduction potential of -0.47 V (vs. SCE). Adding N-fluoropyridinium triflate to 3-3 in CH2Cl2 yields no reaction, even with temperatures elevated to 80°C.

Selectfluor, with a reduction potential of -0.04 V (vs. SCE), is a stronger fluorinating agent than NFSI and N-fluoropyridinium triflate.31 Due to the poor solubility of Selectfluor in most organic solvents, MeCN was chosen as the solvent for this reaction. To this end, adding Selectfluor to a stirring solution of 3 in MeCN resulted in a rapid dark-purple colour change, before slowly turning green. Solvent was removed in vacuo resulting in isolation of 3-14 as a pale-yellow powder. The 31P{1H} NMR spectrum shows a doublet resonance at -21.8 ppm with a coupling constant of 1065 Hz, which is similar to the values observed in 3-10 – 3-13. The 19F NMR spectrum displays a doublet signal at -54.3 ppm, however, no resonance was observed that could be attributed to a BF4 counterion. High resolution mass spectral analysis displays an ion consistent with the formulation

of 3-14 as the fluorophosphine oxide O=PF(C6Cl5)2 (Scheme 3.12). The formation of 3-14 likely arises through reaction of a phosphonium intermediate with residual water.

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

3.2.4 Electrophilicity of Fluorophosphonium Cations

3.2.4.1 Gutmann Beckett Method The Gutmann-Beckett method was applied to fluorophosphonium salts 3-10 – 3-13 to measure

electrophilicity and to better rationalize observed reactivity and stability trends. A CH2Cl2 solution

of triethylphosphine oxide OPEt3 was mixed with an excess of fluorophosphonium salts 3-10 – 31 1 3-13 and the resulting OPEt3 P{ H} chemical shifts were measured and referenced against

uncoordinated OPEt3 in CH2Cl2. Mixing 3 equivalents of 3-10 or 3-11 with OPEt3 results in no interaction between the two compounds, as no broadening or change in chemical shift was observed for either species by 31P{1H} NMR. Compounds 3-12 and 3-13 also failed to produce adducts with OPEt3. The Gutmann-Beckett method has been successfully applied to gauge relative 15,18 electrophilicity in other fluorophosphonium cations, including [FP(C6F5)3][B(C6F5)4]. The

increased steric bulk imparted by the C6Cl5 substituents seems to effectively prevent adduct

formation with OPEt3, which precludes measurement of electrophilicity using this protocol. The decreased propensity of salts 3-10 – 3-13 to form Lewis acid-base adducts with OPEt3 is consistent with previously reported trends observed with the related borane series.4

3.2.4.2 Chemical Shift Trends of Fluorophosphonium Cations The 31P{1H} NMR chemical shifts of fluorophosphonium phosphorus centres has been used as a relative measure of electrophilicity.18 More electrophilic phosphonium cations tend to be correlated with an upfield shift. The 31P{1H} NMR chemical shifts of the fluorophosphonium cations 3-10 – 3-13 are 89.5, 84.4, 71.0 and 66.3 ppm, respectively. To this end, increasing cation electrophilicity follows the order 3-10 < 3-11 < 3-12 < 3-13. The upfield chemical shift of 3-11 with respect to 3-10 indicates C6Cl5 is more electron-withdrawing than Ph. Similarly, the upfield shift of 3-12 compared to 3-13 indicates C6F5 is more electron-withdrawing than C6Cl5. This observation is consistent with electronegativity arguments, but inconsistent with mesomeric stabilization trends and observed trends with the analogous borane system.4 The mesomeric stabilization provided by fluorine substituents to borane requires donation of π-electron density into the vacant p-orbital on boron. In this case, the Lewis acidic orbital of fluorophosphonium cations is not oriented perpendicular to the aromatic plane, which reduces possibility for

mesomeric stabilization. Without mesomeric effects, the electronegativity effects dominate the

electronic interaction with the phosphonium centre, making C6F5 rings more potent than C6Cl5 rings in fluorophosphonium cations. If the observed 31P{1H} chemical shift trend of phosphonium salts was a quantitative measure of only electrophilicity, the fluorophosphonium 31 1 [FP(C6F5)3][B(C6F5)4] would be expected to have a P{ H} NMR chemical shift of approximately 61.6 ppm (based on the difference between 3-12 and 3-13). However, the chemical shift of

[FP(C6F5)3][B(C6F5)4] is reported to be 67.8 ppm, which illustrates the qualitative nature of this method. As was observed in the synthesis of C6Cl5 substituted phosphines (discussed in 3.2), the steric demand of the substituents must have a role in determining the chemical shift.

3.2.4.3 General Electrophilicity Index of Fluorophosphonium Cations Geometry optimization and frequency calculations were performed on phosphonium cations of + + + + 32 3-10 – 3-13 ([3-10] – [3-13] respectively), [FP(C6Cl5)3] and [FP(C6F5)3] with Gaussian 09 using hybrid functional B3LYP33 and split valence double zeta 6-31+G(d) basis set.34 Single point + + energy calculations of the cations of 3-10 – 3-13, [FP(C6Cl5)3] and [FP(C6F5)3] were performed on the optimized structures using the MP235 and polarized triple-zeta basis set def2-TZVPP.36 When available, solid-state structures were used to generate the input coordinates. The LUMO of + + + + the optimized structures for cations [3-10] – [3-13] , [FP(C6Cl5)3] and [FP(C6F5)3] all display a large lobe largely localized on the phosphorus centre opposite the P-F bond, consistent with the P–F σ*-orbital being the Lewis acidic site (Figure 3.11).

The computed HOMO and LUMO energy values for the cations of 3-10 – 3-13,

[FP(C6Cl5)3][B(C6F5)4], and [FP(C6F5)3][B(C6F5)4] are tabulated below (Table 3.1).

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

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

Cation ELUMO (eV) EHOMO (eV) ΔE (ELUMO – EHOMO) (eV) [3-10]+ -2.289 -12.642 10.353 [3-11]+ -2.471 -12.675 10.204 [3-12]+ -2.979 -12.905 9.926 [3-13]+ -3.108 -13.064 9.956

+ [FP[C6F5)3] -3.455 -14.090 10.635

+ [FP[C6Cl5)3] -2.187 -12.737 10.550

No clear trend emerges from the energy difference between the HOMO and LUMO, which is associated with “hardness” or “softness” properties: large HOMO-LUMO energy gaps are indicative of hard Lewis acid or bases, while small energy gaps are indicative of more polarizable soft acids and bases.37 The fluorophosphonium cations have hardness values comparable to other soft Lewis acids. The descending order of HOMO and LUMO energy levels, reflecting stabilization of the cation by the electron withdrawing substituents, builds on the earlier chemical + + + + + + shift trends to give [FP(C6Cl5)3] < [3-10] < [3-11] < [3-12] < [3-13] < [FP(C6F5)3] . While the stabilizing effect of C6Cl5 substituents is again observed between the pair 3-10 and 3-11, the + relative LUMO energy of [FP(C6Cl5)3] is unexpectedly less stabilized than both Ph-substituted phosphonium salts 3-10 and 3-11. With the noted exception, LUMO energy stabilization correlates to total electronegativity of phenyl substituents. The electrophilicity indices, ω,38,39 can be calculated using a formula with HOMO and LUMO energies, wherein energies are in units of electron volts (eV).

+ + The ω values for the cations of 3-10 – 3-13, [FP(C6Cl5)3] and [FP(C6F5)3] are tabulated below (Table 3.2).

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

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

Cation ω (eV) [3-10]+ 2.692 [3-11]+ 2.810 [3-12]+ 3.177 [3-13]+ 3.284

+ [FP[C6F5)3] 3.618

+ [FP[C6Cl5)3] 2.639

The same trend emerges for the electrophilicity indices, ω, as was observed for LUMO energies: + + + + + + [FP(C6Cl5)3] < [3-10] < [3-11] < [3-12] < [3-13] < [FP(C6F5)3] . The calculated ω values have a strong linear correlation with the LUMO energies, with an R2 value of 0.9934 (Figure 3.12). However, the calculated ω values have poor linear correlation with the HOMO energies (R2 = 0.7837). The ω values of the cations of 3-10 – 3-13 have remarkably high correlation (R2 = 0.999) to the experimentally observed chemical shift of the corresponding salts (Figure 3.12).

Fluorophosphonium [FP(C6F5)3][B(C6F5)4] does not follow the trend observed in the series (red outlier, Figure 3.11).

4 95 3.8 90 3.6 85 3.4 y = ‐38.445x + 192.78

(eV) 3.2 80 R² = 0.999 (ppm) 3 75

2.8 Shift energy

y = 1.3069x ‐ 1.2204 2.6 R² = 0.9934 70 2.4 LUMO 65

2.2 Chemical 2 60 2.50 2.75 3.00 3.25 3.50 3.75 4.00 2.5 2.7 2.9 3.1 3.3 3.5 3.7 ω (eV) ω (eV)

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

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

Since GEI calculations only require the free Lewis acid, they do not take steric influences and geometry rearrangement energies into account. Because of this, ω values are most effectively used as a quick method to compare systems which differ electronically, but do not differ significantly in steric encumbrance near the Lewis acidic site. Moreover, the GIE values require half the computation resources compared to FIA calculations (vide infra) and provide accurate quantization of electrophilicity in appropriate systems.

3.2.4.4 Fluoride Ion Affinity of Fluorophosphonium Cations Using the same methodology described in the previous section, the energies for the corresponding + + fluoride adducts of species 3-10 – 3-13, [FP(C6Cl5)3] and [FP(C6F5)3] were calculated. Computing fluoride ion affinity using the fluoride anion is highly technique dependent. Due to the

difficulty associated with calculating FIA using fluoride anion, COF2 was used as a reference compound,40 which has an experimentally measured FIA value of 209 kJ/mol. Using this value, + + the FIA values of species 3-10 – 3-13, [FP(C6Cl5)3] and [FP(C6F5)3] were calculated using the following formula (energies in kJ/mol):

FIA = (Ecation + EF3CO) – (Ephosphorane + EF2CO) + 209

The derivation of the formula used is discussed in more detail in the introduction (Chapter 1, pg + + 14). The FIA values for the cations of 3-10 – 3-13, [FP(C6Cl5)3] , [FP(C6F5)3] , and BCF were calculated to be 686, 697, 755, 731, 744, 707, 451 kJ/mol, respectively (Table 3.3).

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

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

Cation FIA (kJ/mol) [3-10]+ 686 [3-11]+ 697 [3-12]+ 731 [3-13]+ 755

+ [FP[C6F5)3] 744

+ [FP[C6Cl5)3] 707 BCF 451

+ The FIA values for [FP(C6Cl5)3] and BCF are in good agreement with earlier reported 18,41,42 values. Consistent with the other electrophilicity trends, the FIA values increase with C6Cl5

or C6F5 substitution within the series of 3-10 – 3-13. The substitution of a C6Cl5 substituent for

C6F5 increases the FIA value by 24 kJ/mol (seen between 3-13 and 3-12); however, an additional + substitution decreases the FIA by 11 kJ/mol (3-12 to [FP[C6F5)3] ). Overall, the FIA values correlate well with experimentally-observed 31P{1H} NMR chemical shifts (Figure 3.13).

95 90 85 (ppm)

80 Shift 75 70 y = ‐0.3463x + 326 R² = 0.9842

Chemical 65 60 680 700 720 740 760 Fluoride Ion Affinity (kJ/mol)

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

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

Since fluoride ion affinity calculations consider the energy of both the free Lewis acid and the fluoride adduct, it accounts for energies associated with rehybridization and rearrangement. Fluoride ion affinity calculations are more resource intensive, but the resulting values allow for a more accurate comparison between disparate Lewis acidic systems compared to GIE calculations.

Both computational methods, GIE and FIA, correlate well with experimentally-observed chemical shift measures of electrophilicity. The advantage of using either computational method is the potential for them to be used as a predictive tool to study compounds that have not been synthesized. Additionally, GEI computations are less resource intensive than FIA since they require only a single structure to be optimized, rather than the two structures required by FIA. However, since GEI does not use the structure of the corresponding fluoride-adduct species, it does not take energy costs associated with rearrangements or changing hybridization into account and is therefore best suited for investigating sterically similar systems. Another advantage of GIE and FIA is compatibility with systems that aren’t amenable to empirical methods such as Gutmann- Beckett or Childs’. All measures of electrophilicity have specific drawbacks, from empirical methods that may be chemically incompatible with certain Lewis acids to ab initio computational methods that are resource intensive.

3.2.5 Reactivity of Pentachlorophenyl Fluorophosphonium Cations

3.2.5.1 Air- and Moisture-Stability Tests Increasing the overall air-stability of fluorophosphonium cations was a main target in utilizing

C6Cl5 substituents. To evaluate their stability to atmospheric moisture, solutions of species 3-10 – 3-13 were exposed to air and monitored over time. Solutions of salts 3-10 – 3-13 in 5 mm NMR tubes were exposed to air for one minute, capped, and agitated before the 19F and 31P{1H} NMR spectra were recorded. Afterwards, the solutions were exposed to air for successively longer periods of time to monitor for the onset of decomposition of the phosphonium salts. For

benchmarking purposes, stability tests were also performed with [FP(C6F5)3][B(C6F5)4]. Initial

tests were performed using CH2Cl2 as solvent, however, solvent evaporation occurred before decomposition of species 3-10 and 3-11 was observed, thus, PhBr was used for the remaining tests. Compound 3-10 showed signs of decomposition after 21 h, while species 3-11 proved to be

significantly more robust, showing no sign of decomposition after 48 h (Table 3.4). Compound 3- 12 proved be significantly less stable than salts 3-9 or 3-10, showing signs of decomposition by 19F NMR after just 4 h. Decomposition of species 3-12 was observed after 15 min of exposure to air, albeit still significantly longer than the 1 min onset observed for [FP(C6F5)3][B(C6F5)4].

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

Phosphonium Salt Decomposition Onset 3-10 21 h 3-11 >48 h 3-12 4 h 3-13 15 min

[FP[C6F5)3][B(C6F5)4] 1 min

The solution air-stability trend observed for the fluorophosphonium cations decreases in the order

3-11 > 3-10 > 3-12 > 3-13 > [FP(C6F5)3][B(C6F5)4]. Interestingly, the stability trend is not identical to the electrophilicity trend observed in section 3.6. The notable deviation from this trend is 3-11,

indicative that C6Cl5 substituents not only increase electrophilicity, but the ortho-chlorine

substituents also provide steric protection relative to Ph substituents. The inclusion of C6F5 substituents seems to drastically decrease the air-stability of fluorophosphonium cations, as

observed in 3-12, 3-13, and [FP(C6F5)3][B(C6F5)4], consistent with a significant increase in

electrophilicity (vide supra). It is noteworthy that the generation of HC6F5 is observed as a

decomposition product for C6F5-substituted phosphonium cations, but the analogous HC6Cl5 was not observed by 1H NMR for species 3-10 – 3-13.

The air-stability of salts 3-10 and 3-11 was also evaluated as isolated solids. Both compounds were exposed to air as solids for 4 h hours before being dissolved in benchtop grade PhBr. The resulting NMR spectra showed no signs of decomposition.

3.2.6 Catalytic Activity of Perchlorophenyl Phosphonium Cations

The effectiveness of compounds 3-10 – 3-13 as Lewis acid catalysts was evaluated in an array of different organic transformations (Figure 3.14). For all test reactions, 5 mol% catalyst was employed.

Compounds 3-10 – 3-13 all catalyzed the dimerization of 1,1-diphenylethylene at room temperature, completing the reaction in 18 h, 6 h, 1 h, and 50 min, respectively. The 1H NMR spectra display a decrease in the intensity of the olefin signal at 5.33 ppm and the growth of two new distinct diastereotopic resonances at 2.94 and 2.61 ppm as the reaction proceeds, indicative of 1-methyl-1,3,3-triphenyl-2,3-dihydro-1H-indene formation.

The hydrodefluorination of 1-fluoroadamantane was rapidly effected by all four catalysts in the presence of HSiEt3. Compound 3-10 took 3.5 h to complete the reaction, while 3-11 – 3-13 had completed the reaction before NMR spectra could be obtained. The consumption of the fluoroadamantane C-F resonance and growth of triethylfluorosilane Si–F resonance at -175 ppm in the 19F NMR spectra allowed the reactions to be easily monitored.

Compound 3-10 shows negligible activity in the deoxygenation of benzophenone with HSiEt3 at room temperature, completing the transformation only at 140 °C. On the other hand, the deoxygenation of benzophenone can be completed by salts 3-11 – 3-13 at room temperature after

40 h, 25 min and 2 h, respectively. The C6F5-substituted phosphonium cations 3-12 and 3-13 proved to be significantly more active than the Ph-substituted 3-10 and 3-11. The deoxygenation reaction can be monitored by 1H NMR, given that the aromatic resonance at 7.66 ppm disappears and the olefinic signal at 3.77 ppm appears throughout the course of the reaction.

The dehydrocoupling of phenol with HSiEt3 reaction shows similar reactivity trends as the deoxygenation of benzophenone, wherein only compounds 3-11 – 3-13 are able to effect the transformation at room temperature and salt 3-10 required significantly higher temperatures. The growth of 1H NMR product resonance at 6.95 ppm and disappearance of starting material resonances at 6.72 ppm allowed facile tracking of the reaction progress.

Hydrosilylation of α-methylstyrene with triethylsilane required elevated reaction temperatures for all catalysts except compound 3-13, which completed the reaction in 5 h at room temperature. The

Ph-substituted 3-10 and 3-11 catalyzed the reaction very slowly and require high reaction temperatures. Reaction progress was monitored by 1H NMR via the decrease of olefinic resonances at 5.27 ppm and 4.95 ppm. The formation of new signal at 0.95 ppm is concomitant with formation of the hydrosilylated product.

The benzylation and hydrodefluorination of 4-(trifluoromethyl)bromobenzene in C6D6 with triethylsilane could not be achieved using the less electrophilic salts 3-10 or 3-11.. However, species 3-12 and 3-13 completed the reaction at 80 °C in 22 h and 1.5 h respectively. Benzylation occurs through initial activation of a trifluoromethyl C-F bond to generate a carbocation, which is attacked by an arene. Subsequent hydrodefluorination of the remaining two fluorine substituents yields the benzylated product Consumption of starting material was monitored by concomitant decrease of the C–F resonance at -62.7 ppm and appearance of a Si–F resonance at -175.5 ppm by 19F NMR spectroscopy.

Of all the test reactions performed, the dimerization of 1,1-diphenylethylene provides the best point of comparison for a relative reactivity trend because all the catalysts could perform this reaction under the same reaction conditions, with time as the only variable. The order of increasing catalytic activity is 3-10 < 3-11 < 3-12 < 3-13, which is the same trend obtained from the computational evaluations of electrophilicity. This reactivity trend is consistent throughout all the test reactions, though the less electrophilic catalysts 3-10 and 3-11 were incapable of effecting the benzylation of 4-trifluoromethylbromobenzene.

Figure 3.14. Catalytic activity of fluorophosphonium salts 3-10 – 3-13. Catalytic screening for compounds 3-10, 3-12, and 3-13 was completed by Vitali Podgorny.

3.3 Conclusion

A series of C6Cl5-substituted phosphines were synthesized: Ph2P(C6Cl5) (3-1), PhP(C6Cl5)2 (3-2),

P(C6Cl5)3 (3-3) (C6F5)2P(C6Cl5) (3-4), or (C6F5)P(C6Cl5)2 (3-5). Oxidation of phosphines 3-1, 3-2,

3-4, and 3-5 by XeF2 lead to the clean formation of the corresponding difluorophosphoranes

Ph2PF2(C6Cl5) (3-6), PhPF2(C6Cl5) (3-7), (C6F5)2PF2(C6Cl5) (3-8), or (C6F5)PF2(C6Cl5)2 (3-9).

Oxidation of 3-3 yields the desired difluorophosphorane PF2(C6Cl5)3 as a minor product and

PF3(C6Cl5)2 as the major product with concomitant generation of (C6Cl5)2. Fluoride abstraction

from compounds 3-6 – 3-9 yields fluorophosphonium salts [Ph2PF(C6Cl5)][B(C6F5)4] (3-10),

[PhPF(C6Cl5)2][B(C6F5)4] (3-11), [(C6F5)2PF(C6Cl5)][B(C6F5)4] (3-12), and

[(C6F5)PF(C6Cl5)2][B(C6F5)4] (3-13), respectively. Reaction of 3 with Selectfluor yielded fluorophosphine oxide OPF(C6Cl5)2 (3-14).

Measuring the relative electrophilicity of fluorophosphonium salts 3-10 – 3-13 using the Gutmann-

Beckett method was unsuccessful, due to the inability of OPEt3 to form adducts with the cations. Evaluation of the 31P NMR chemical shifts indicated increasing electrophilicity in the order 3-10 < 3-11 < 3-12 < 3-13, consistent with electron-withdrawing strength of substituents increasing in the + + order Ph < C6Cl5 < C6F5. Cations 3-10 – 3-13, [PF(C6Cl5)3] , and [PF(C6F5)3] were evaluated computationally. The FIA values of compounds 3-10 – 3-13 supported the chemical shift trend, yielding FIA values of 686, 697, 731, and 755 kJ/mol, respectively. Similarly, the GEI values, ω, were calculated which showed high linear correlation with the 31P NMR chemical shifts and LUMO energies.

The air-stability of salts 3-10 – 3-13 was evaluated by exposing solutions of the salts to air for certain periods of time. The less electrophilic Ph-substituted 3-10 and 3-11 proved to be quite robust, while C6F5-substituted 3-12 and 3-13 decomposed rapidly when exposed to air. The decreasing air-stability trend of 3-11 > 3-10 > 3-13 > 3-12 parallels the increasing electrophilicity trends observed, with the exception of 3-11. Phosphonium salt 3-11 displays enhanced electrophilicity and significantly higher air-stability over salt 3-10, presumably resulting from increased steric protection from a second bulky C6Cl5 group.

The catalytic activity of 3-10 – 3-13 was tested in diphenylethylene dimerization, deoxygenation, hydrodefluorination, dehydrocoupling, hydrosilylation and benzylation reactions. The reactivity trend was in good agreement with electrophilicity trends of 3-10 < 3-11 < 2-12 < 3-13. The less electrophilic salts 3-10 and 3-11 required significant heating to effect full conversion in many reactions, while both compounds 3-12 and 3-13 were competent catalysts at room temperature. Further, salts 3-10 and 3-11 were unable to effect benzylation reactions, even at elevated temperatures. The C6Cl5-substituted phosphonium salts 3-10 – 3-13 demonstrate significantly

enhanced air-stability compared to [PF(C6F5)3][B(C6F5)4], However, this air-stability comes at the cost of catalytic activity.

3.4 Experimental 3.4.1 General Experimental Methods

Air-sensitive manipulations were carried out under an atmosphere of dry, O2-free N2 using either

an MBraun MB Unilab Glovebox or a dual-manifold Schlenk line. Hexane, pentane, and Et2O were all purified using a Grubbs-type column system produced by Innovative Technology and dispensed into thick-walled Straus flasks equipped with Teflon greaseless stopcock and stored over 4 Å sieves. Tetrahydrofuran and benzene were each dried over sodium metal and benzophenone before being distilled to Schlenk bombs and stored over 4 Å sieves. Dichloromethane was dried using calcium hydride before being vacuum transferred to a Schlenk bomb. Deuterated solvents were degassed using three successive freeze-pump-thaw cycles. Other bulk solvents were degassed by three successive cycles of headspace-evacuation, and sonication. All elemental analyses were performed in house using a Perkin Elmer 2400 Series II CHNS Analyzer. High-resolution mass spectra data was obtained using a JEOL AccuTOF or an Agilent 6538 Q-TOF mass spectrometer. All NMR data were collected on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz or Agilent DD2 600 MHz spectrometer at 25 °C unless otherwise noted. NMR chemical shift data are given 1 13 11 19 31 relative to an external standard ( H, C: SiEt4; B: 15% BF3·Et2O; F: CFCl3; P: H3PO4). In some cases, 2-D NMR techniques were employed to assign individual resonances. Both

(C6F5)2PBr and (C6F5)PBr2 were synthesized by literature procedure and obtained in high purity 43 through successive recrystallizations from Et2O. Other reagents, including Ph2PCl, PhPCl2, PBr3,

PCl3, Selectfluor, NFSI, 1-Fluoropyridinium triflate, and hexachlorobenzene were obtained from Sigma Aldrich and used without further purification. Perfluorophenyl-substituted systems were prepared by Vitali Podgorny. Full characterization of these compounds are detailed in his thesis.44 Computational work was performed using resources provided by the Shared Hierarchical Academic Research Computing Network and Compute Canada. Crystals for structures denoted with † were grown by Vitali Podgorny.

Compounds 3-1 – 3-3 were prepared in a similar fashion and thus only one sample preparation is detailed below.

Synthesis of Ph2P(C6Cl5) (3-1): A 100 mL Schlenk flask was charged with C6Cl6 (318 mg, 1.12

mmol), Et2O (30 mL) and a magnetic stir bar, generating a white slurry. The reaction flask was cooled to -15 °C in a dry ice/acetone bath. A hexane solution of 2.5 M nBuLi (0.44 mL, 1.12 mmol) was added dropwise to the stirring solution, which gradually turned the slurry to a clear,

light yellow solution. The solution was then cooled to -78 °C, before a solution of Ph2PCl (247

mg, 1.12 mmol) in Et2O (3 mL) was added to the reaction flask in a dropwise fashion. The stirred solution was warmed to room temperature overnight. The solvent was removed in vacuo leaving

an off-white solid. The product was dissolved in CH2Cl2 (6 mL) and filtered through a Celite plug. The solvent was reduced and the solution was cooled to -35 °C to produce a white precipitate, which was collected by filtration. The white filtrate was then washed with pentane (2 x 2 mL) to yield 3-1 (340 mg, 70%) as a white solid. The recrystallization of 3-1 and 3-4 from vapour diffusion of n-pentane into a solution of the compound in dichloromethane yielded X-ray quality crystals. (338 mg, 70% yield).

1 H NMR (500 MHz, C6D6): δ 7.33 – 7.39 (m, 4H, m-C6H5), 7.08 – 7.03 (m, 6H, o- & p-C6H5) 31 1 13 1 ppm. P{ H} NMR (162 MHz, C6D6): δ 10.7 (s) ppm. C{ H} NMR (125 MHz, C6D6): δ 139.7 2 1 1 (d, JPC = 18 Hz, o-C6Cl5), 136.4 (s, p-C6Cl5), 136.0 (d, JPC = 24 Hz, i-C6Cl5), 134.3 (d, JPC = 15 2 3 Hz, i-C6H5), 133.3 (s, m-C6Cl5), 132.9 (d, JPC = 21 Hz, o-C6H5), 129.1 (s, p-C6H5), 129.0 (d, JPC

= 6 Hz, m-C6H5) ppm. Anal. Calcd. for PC18H10Cl5: C: 49.76, H: 2.32. Found: C: 49.82%, H: + 1.99%. HRMS (DART Ionization) m/z: [M+H] Calcd. for C18H10Cl5P 432.90410, Found: 432.90360.

1 Synthesis of PhP(C6Cl5)2 (3-2): (pale yellow solid, 36% yield, from Ph2PCl). H NMR (500 MHz, 3 3 C6D6): δ 7.44 (apparent triplet, JPH = 7 Hz, JHH = 7 Hz, 2H, m-C6H5), 6.98 – 7.07 (m, 4H, o- &

31 1 13 1 p-C6H5) ppm. P{ H} NMR (243 MHz, C6D6): δ 15.1 (s) ppm. C{ H}NMR (125 MHz, C6D6): δ 2 1 4 137.1 (d, JPC = 19 Hz, o-C6Cl5), 136.0 (d, JPC = 36 Hz, i-C6Cl5), 135.3 (d, JPC = 1 Hz, p-C6Cl5), 3 1 2 133.4 (d, JPC = 1 Hz, m-C6H5), 131.8 (d, JPC = 15 Hz, i-C6H5), 130.8 (s, p-C6H5), 128.9 (d, JPC 3 = 9 Hz, o-C6H5), 128.5 (d, JPC = 14 Hz, m-C6H5) ppm. Anal. Calcd. for PC18H5Cl10: C: 35.63, H: + 0.83. Found: C: 35.29%, H: 0.92%. HRMS (DART Ionization) m/z: [M+H] Calcd. for C18H5Cl10P 602.70924, Found: 602.70831.

Synthesis of P(C6Cl5)3 (3-3): (white solid, 21% yield, from PBr3).

31 1 P{ H} NMR (162 MHz, C6D6): δ 19.1 (s) ppm. Anal. Calcd. for PC18Cl18: C: 27.76. Found: C: + % 25.87. HRMS (EI-TOF) m/z: [M] Calcd. for C18HCl15P 778.50553, Found: 778.50594.

Compounds 3-6 and 3-7 were prepared in a similar fashion and thus only one preparation is detailed.

Synthesis of Ph2PF2(C6Cl5) (3-6): A 20 mL vial was charged with 3-1 (257 mg, 0.59 mmol),

CH2Cl2 (4 mL) and a magnetic stir bar, forming a light-yellow solution. To the stirring solution, a

solution of XeF2 (100 mg, 0.59 mmol) in CH2Cl2 (3 mL) was quickly added. The effervescent solution lightened as it was left to stir for 2 hr. The solvent was reduced and the solution cooled to -35 °C to produce a white precipitate. The precipitate was collected by filtration and washed with

n-pentane (2 x 2 mL) yielding a white solid. The CH2Cl2 was removed in vacuo yielding 3-6 (230 mg, 83%) as a yellow solid. Recrystallization of 3-6 from vapour diffusion of n-pentane into a solution of the compound in dichloromethane yielded X-ray quality crystals (321 mg, 98 % yield).

1 H NMR (500 MHz, C6D6): δ 8.17 – 8.10 (m, 4H, m-C6H5), 7.08 – 7.01 (m, 6H, o- & p-C6H5) ppm. 31 1 1 19 1 P{ H} NMR (162 MHz, C6D6): δ -50.7 (t, JPF = 716 Hz) ppm. F{ H} NMR (376 MHz, C6D6):

1 13 1 1 δ -41.8 (d, JPF = 716 Hz, PF2) ppm. C{ H} NMR (125 MHz, C6D6): δ 140.1 (dt, JPC = 217 Hz, 2 1 2 2 JFC = 42 Hz, i-C6Cl5), 137.5 (dt, JPC = 181 Hz, JFC = 24 Hz, i-C6H5), 135.0 (dt, JPC = 13 Hz, 3 2 3 4 JFC = 10 Hz, o-C6H5), 134.5 (dt, JPC = 3 Hz, JFC = 2 Hz, o-C6Cl5), 133.5 (d, JPC = 17 Hz, p- 3 4 4 5 C6Cl5), 132.6 (dt, JPC = 3 Hz, JFC = 3 Hz, m-C6Cl5), 131.7 (dt, JPC = 4 Hz, JFC = 1 Hz, p-C6H5), 3 4 128.6 (dt, JPC = 13 Hz, JPC = 10 Hz, m-C6H5) ppm. Anal. Calcd. for PC18H10Cl5F2: C: 45.76, H: 2.13. Found: C: 45.24%, H: 2.00%.

Synthesis of PhPF2(C6Cl5)2 (3-7): (light orange solid, 76% yield).

1 H NMR (500 MHz, C6D6): δ 8.11 – 8.04 (m, 2H, m-C6H5), 7.16 – 7.08 (m, 3H, o-, p-C6H5) ppm. 31 1 1 19 1 P{ H} NMR (202 MHz, C6D6): δ -44.9 (t, JPF = 747 Hz) ppm. F{ H} NMR (470 MHz, C6D6):

1 13 1 1 δ -28.9 (d, JPF = 747 Hz, PF2) ppm. C{ H} NMR (125 MHz, C6D6): δ 137.7 (dt, JPC = 210 Hz, 2 2 3 JFC = 32 Hz, i-C6Cl5), 136.7 – 136.6 (m, o-C6Cl5), 135.8 (dt, JPC = 14 Hz, JFC = 10 Hz, o-C6H5), 1 2 4 135.4 (dt, JPC = 187 Hz, JFC = 23 Hz, i-C6H5), 134.3 (d, JPC = 18 Hz, p-C6Cl5), 134.0 – 133.8 4 3 (m, m-C6Cl5), 132.98 (br d, JPC = 4 Hz, p-C6H5), 129.3 (dm, JPC = 18 Hz, m-C6H5) ppm. Anal.

Calcd. for PC18H5Cl10F2: C: 33.53, H: 0.78. Found: C: 34.16%, H: 0.86%.

Compounds 3-10 and 3-11 were prepared in a similar fashion and thus only one preparation is detailed below

Synthesis of [Ph2PF(C6Cl5)][B(C6F5)4] (3-10): A 20 mL vial was charged with [Et3Si][B(C6F5)4] (682 mg, 0.77 mmol), toluene (10mL) and a magnetic stir bar, forming a white slurry. To the stirring slurry, a solution of 3-6 (382 mg, 0.81 mmol) in toluene (5 mL) was added. The solution was allowed to stir overnight at room temperature, resulting in a dark orange solution. When the reaction mixture was allowed to settle, a dark orange oil collected at the bottom of the vial leaving a clear supernatant. After decanting the toluene from the oil, additional toluene (2 x 3 mL) was added to wash the oil. After decanting the toluene washes, the oil was washed with n-pentane (3 x 4 mL) before removing the solid in vacuo resulting in 3-10 as a fluffy white solid (697 mg, 80 % yield).

1 H NMR (500 MHz, CDCl3): δ 8.09 – 8.04 (m, 1H, p-C6H5), 7.88 – 7.78 (m, 4H, o- & m-C6H5) 31 1 1 19 1 ppm. P{ H} NMR (162 MHz, C6D5Br): δ 89.5 (d, JPF = 1009 Hz) ppm. F{ H} NMR (376 1 3 MHz, C6D5Br): δ -116.0 (d, JPF = 1010 Hz, PF2), -132.0 (m/br, 8F, B(o-C6F5)), -162.3 (t, JFF = 11 1 21 Hz, 4F, B(p-C6F5)), -166.2 (m/br, 8F, B(m-C6F5)) ppm. B{ H} NMR (128 MHz, C6D5Br): -

13 1 1 16.5 (s) ppm. C{ H} NMR (125 MHz, CDCl3): δ 148.3 (br d, JFC = 241 Hz, B(o-C6F5)), 145.3 4 4 5 1 (d, JPC = 3 Hz, P(p-C6Cl5)), 139.5 (dd, JPC = 2 Hz, JFC = 2 Hz, P(p-C6H5)), 138.3 (br d, JFC =

2 3 3 238 Hz, B(p-C6F5)), 137.7 (dd, JPC = 6 Hz, JFC = 1.0 Hz, P(o-C6Cl5)), 137.1 (d, JPC = 12 Hz, 1 2 3 P(m-C6Cl5)), 136.3 (br d, JFC = 235 Hz, B(m-C6F5)), 133.5 (dd, JPC = 14 Hz, JFC = 1 Hz, P(o- 3 1 2 C6H5)), 131.5 (d, JPC = 15 Hz, P(m-C6H5)), 123.8 (br s, B(i-C6F5)), 116.1 (dd, JPC = 112 Hz, JFC 1 2 = 14 Hz, P(i-C6H5)), 115.9 (dd, JPC = 121 Hz, JFC = 11 Hz, P(i-C6Cl5)) ppm. Anal. Calcd. for

PC42H10Cl5F21B: C: 44.54, H: 0.89. Found: C: 45.15%, H: 0.94%. HRMS (DART Ionization) m/z: + [M] Calcd. for C18H10Cl5PF 450.89468, Found 450.89445.

Synthesis of [PhPF(C6Cl5)2][B(C6F5)4] (3-11): (320 mg, 90 % yield) as an off white solid.

1 H NMR (500 MHz, CDCl3): δ 8.13 – 8.08 (m, 1H, p-C6H5), 7.98 – 7.76 (m, 4H, o-, m-C6H5) ppm. 19 1 1 F{ H} NMR (470 MHz, C6H5Br): δ -125.64 (d, JPF = 1011 Hz, PF2), -138.89 (m/br, 8F, B(o- 31 1 C6F5)), -169.24 (m/br, 4F, B(p-C6F5)), -173.12 (m/br, 8F, B(m-C6F5)) ppm. P{ H} NMR (162 1 11 1 MHz, CD2Cl2): δ 84.38 (d, JPF = 1011 Hz) ppm. B{ H} NMR (128 MHz, CD2Cl2): -16.66 (s)

13 1 1 ppm. C{ H} NMR (125 MHz, CDCl3): δ 148.28 (br d, JFC = 240 Hz, B(o-C6F5)), 145.51 (dm, 4 1 JPC = 3.2 Hz, P(p-C6Cl5)), 140.54 (m, P(p-C6H5)), 138.25 (br d, JFC = 239.2 Hz, B(p-C6F5)), 3 2 1 137.20 (d, JPC = 13.3 Hz, P(m-C6Cl5)), 136.65 (d, JPC = 6.2 Hz, P(o-C6Cl5)), 136.33 (br d, JFC = 2 3 239.0 Hz, B(m-C6F5)), 133.12 (d, JPC = 13.5 Hz, P(o-C6H5)), 132.15 (d, JPC = 16.6 Hz, P(m- 1 2 C6H5)), 124.12 (br s, B(i-C6F5)), 118.30 (dd, JPC = 130.7 Hz, JFC = 10.8 Hz, P(i-C6H5)), 115.99 1 2 (dd, JPC = 113.9 Hz, JFC = 12.5 Hz, P(i-C6Cl5)) ppm. Anal. Calcd. for PC42H5Cl10F21B: C: 38.66. H: 0.39. Found: C: 42.17%, H: 0.87%. HRMS (DART Ionization) m/z: [M]+ Calcd. for

C18H5Cl10PF 620.69982, Found 620.69896.

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Synthesis of OPF(C6Cl5)2 (3-14): A 20 mL vial was charged with 3-4 (78 mg, 0.10 mmol), MeCN (3 mL) and a magnetic stir bar. A solution of 1-chloromethyl-4-fluoro-1,4-diazonia-bicyclo- [2.2.2]octanebis(tetrafluoroborate) in MeCN was added. The solution briefly turned dark-purple as a black precipitate was formed before returning to a pale green colour. The supernatant was decanted and the solvent was removed in vacuo yielding 3-14 (24 mg, 43%) as a yellow solid.

31 1 1 19 P{ H} NMR (162 MHz, CH2Cl2): δ 21.2 (d, JPF = 1065 Hz) ppm. F NMR (376 MHz, CH2Cl2): 1 + δ −54.3 (d, JPF = 1065 Hz) ppm. HRMS (EI-TOF) m/z: [M] Calcd for C12Cl10FPO: 564.65753,

Found: 564.65713.

Hydrodefluorination of 1-fluoroadamantane: To a vial containing 5 mol% catalyst was added

a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7 mL). The solution was added to a vial containing 1-fluoroadamantane (15.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR and monitored by NMR spectroscopy.

Dehydrocoupling of triethylsilane with phenol: To a vial containing 5 mol% catalyst was added

a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7mL). The solution was added to a vial containing phenol (9.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.

Benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene: To a vial containing

5 mol% catalyst was added a solution of HSiEt3 (41.8 mg, 0.36 mmol) in PhBr (0.7 mL). The solution was added to a vial containing 4-trifluoromethylbromobenzene (22.5 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.

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Deoxygenation of benzophenone: To a vial containing 5 mol% catalyst was added a solution of

HSiEt3 (23.2 mg, 0.2 mmol) in PhBr (0.7 mL). The solution was added to a vial containing benzophenone (18.2 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.

Hydrosilylation of α-methylstyrene with triethylsilane: To a vial containing 5 mol% catalyst was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7 mL). The solution was added to a vial containing α-methylstyrene (11.8 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.

Dimerization of 1,1-diphenylethylene: To a vial containing 5 mol% catalyst was added a solution

of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in benzene-d6 (0.7 mL). The solution was then transferred to a 5 mm NMR tube and monitored by NMR spectroscopy.

3.4.2 X-ray Crystallography

3.4.2.1 X-ray Data Collection and Reduction Crystals were coated in Paratone-N oil in a glovebox before being mounted on a MiTegen

Micromount under an N2 stream to maintain a dry, O2-free environment for each crystal. Diffraction data were collected on a Bruker Apex II diffractometer using a graphite monochromator with Mo Kα (λ = 0.71073 Å) radiation. Temperature was maintained at 150(2) K using an Oxford cryo-stream cooler. Data collection strategies were determined using Bruker Apex II software. Frame integration was carried out using Bruker SAINT software. Data absorbance correction was carried out using the empirical multiscan method SADABS. Structure solutions were obtained by direct methods and refined using SHELXTL or Olex2 software.45,46 Refinement was carried out using full-matrix least squares techniques to convergence of weighting parameters. When data quality was sufficient, all non-hydrogen atoms were refined anisotropically.

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3.4.2.2 X-Ray Tables

3-1 3-4* 3-6*

Formula C18H10Cl5P C18Cl10F5P C18H10Cl5F2P Weight (g/mol) 434.52 696.69 472.51 Crystal system orthorhombic triclinic orthorhombic Space group Pbca P-1 Pbca a (Å) 10.8832(6) 8.6247(6) 11.2850(4) b (Å) 11.0570(5) 13.0708(10) 10.8896(4) c (Å) 29.4458(15) 13.2510(10) 30.4024(11) α (°) 90 81.217(4) 90 β (°) 90 71.522(4) 90 γ (°) 90 89.820(4) 90 Volume (Å3) 3543.4(3) 1398.55(18) 3736.2(2) Z 8 2 8 Density (calcd.) (gcm–3) 1.6289 1.7180 1.6799 R(int) 0.1077 0.0697 0.0270 μ, mm–1 0.906 1.098 0.882 F(000) 1751 706 1895 Index ranges -12 ≤ h ≤ 14 -11 ≤ h ≤ 10 -14 ≤ h ≤ 14 -14 ≤ k ≤ 12 -16 ≤ k ≤ 16 -14 ≤ k ≤ 13 -38 ≤ l ≤ 37 -17 ≤ l ≤ 17 -39 ≤ l ≤ 39 Radiation Mo Kα Mo Kα Mo Kα θ range (min, max) (°) 2.33, 26.75 1.58, 27.51 2.25, 27.49 Total data 4072 6362 4289 Max peak 0.5 0.5 0.4 Min peak -0.6 -0.5 -0.3 2 >2(FO ) 2778 3917 3665 Parameters 217 307 234 R (>2σ) 0.0297 0.0398 0.0255

Rw 0.0630 0.1012 0.0654 GoF 0.841 0.907 1.066 * Crystal for X-ray diffraction obtained by Vitali Podgorny. Structure solution refined by the author.

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3-8* 3-10NFSI* 3-11

Formula C18Cl5F12P C30H20Cl5FNO4PS2 C42H5BCl10F21P Weight (g/mol) 652.42 729.71 1304.74 Crystal system orthorhombic triclinic monoclinic

Space group Pbca P-1 P21/n a (Å) 12.0956(11) 8.619(2) 11.8072(4) b (Å) 10.4377(11) 12.825(4) 20.1065(8) c (Å) 33.662(3) 15.422(5) 21.8370(7) α (°) 90 66.433(13) 90 β (°) 90 85.560(14) 100.528(7) γ (°) 90 89.858(14) 90 Volume (Å3) 4249.8(7) 1557.2(8) 5096.9(3) Z 8 2 4 Density (calcd.) (gcm–3) 2.0392 1.5562 1.700 R(int) 0.0699 0.0515 0.1077 μ, mm–1 0.871 0.695 0.688 F(000) 2537 722 2544 Index ranges -15 ≤ h ≤ 15 -11 ≤ h ≤ 11 -15 ≤ h ≤ 12 -7 ≤ k ≤ 13 -16 ≤ k ≤ 16 -26 ≤ k ≤ 26 -43 ≤ l ≤ 43 -19 ≤ l ≤ 20 -27 ≤ l ≤ 28 Radiation Mo Kα Mo Kα Mo Kα θ range (min, max) (°) 2.42, 27.58 1.73, 28.01 2.03, 27.50 Total data 4897 7098 11706 Max peak 0.7 1.2 0.8 Min peak -0.6 -1.1 -0.9 2 >2(FO ) 3294 5940 5178 Parameters 325 280 676 R (>2σ) 0.0430 0.0750 0.487

Rw 0.0931 0.2290 0.1110 GoF 1.046 1.034 0.995 * Crystal for X-ray diffraction obtained by Vitali Podgorny. Structure solution refined by the author.

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(C6Cl5)2 ((C6Cl5)2P)2

Formula C6Cl5 C24Cl20P2 Weight (g/mol) 249.31 1059.27 Crystal system orthorhombic triclinic Space group Pbcn P-1 a (Å) 13.3969(9) 8.7639(8) b (Å) 10.5584(8) 14.4199(14) c (Å) 11.6957(9) 18.4538(15) α (°) 90 103.340(4) β (°) 90 92.793(4) γ (°) 90 104.772(4) Volume (Å3) 1654.4(2) 2179.8(4) Z 8 2 Density (calcd.) (gcm–3) 2.002 1.6137 R(int) 0.0453 0.0882 μ, mm–1 1.673 1.345 F(000) 968 1034 Index ranges -17 ≤ h ≤ 17 -11 ≤ h ≤ 11 -13 ≤ k ≤ 13 -18 ≤ k ≤ 18 -15 ≤ l ≤ 15 -24 ≤ l ≤ 23 Radiation Mo Kα Mo Kα θ range (min, max) (°) 2.46, 27.54 2.11, 27.55 Total data 1900 10002 Max peak 0.4 1.4 Min peak -0.3 -1.4 2 >2(FO ) 1627 5118 Parameters 100 416 R (>2σ) 0.0244 0.0572

Rw 0.0600 0.1303 GoF 0.990 0.947

105

3.5 References

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13. Ashley, A. E.; O'Hare, D., FLP-Mediated Activations and Reductions of CO2 and CO. In Frustrated Lewis Pairs II: Expanding the Scope, 2013; Vol. 334, pp 191. 14. Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.; Stephan, D. W.; Erker, G., Reversible Metal-Free Carbon Dioxide Binding by Frustrated Lewis Pairs. Angew. Chem., Int. Ed. 2009, 48, 6643. 15. Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W., Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor. Science 2013, 341, 1374. 16. Dua, S. S.; Edmondson, R. C.; Gilman, H., Polyhaloaryl Compounds Containing Phosphorus. J. Organomet. Chem. 1970, 24, 703. 17. Rausch, M. D.; Tibbetts, F. E.; Gordon, H. B., Perhaloaryl-Metal Chemistry .2. Pentachlorophenyllithium. J. Organomet. Chem. 1966, 5, 493. 18. Caputo, C. B.; Winkelhaus, D.; Dobrovetsky, R.; Hounjet, L. J.; Stephan, D. W., Synthesis and Lewis Acidity of Fluorophosphonium Cations. Dalton Trans. 2015, 44, 12256. 19. Dumont, W. W.; Kubiniok, S.; Severengiz, T., Properties of Te-Te Bonds .4. Dismutation Reactions of Di-P-Tolylditelluride with Tetra-tert-Butyldiphosphane and Tetra- Isopropyldiphosphane. Z. Anorg. Allg. Chem. 1985, 531, 21. 20. Cowley, A. H.; Dennis, S. M.; Kamepalli, S.; Carrano, C. J.; Bond, M. R., A Triphospholyl- Substituted Phosphine. J. Organomet. Chem. 1997, 529, 75. 21. Alder, R. W.; Canter, C.; Gil, M.; Gleiter, R.; Harris, C. J.; Harris, S. E.; Lange, H.; Orpen, A. G.; Taylor, P. N., Medium-Ring Diphosphines from Diphosphabicyclo[k.l.0]alkanes: Stereoselective Syntheses, Structure and Properties. J. Chem. Soc., Perkin Trans. 1 1998, 1643. 22. Nycz, J. E., New Look into the Synthesis of Polyhalogenoarylphosphanes. Phosphorus, Sulfur Silicon Relat. Elem. 2009, 184, 2605. 23. Purdela, D., Theory of 31P NMR Chemical Shifts .2. Bond-Angle Dependence. J. Magn. Reson. 1971, 5, 23. 24. Ramsey, N. F., Magnetic Shielding of Nuclei in Molecules. Phys Rev 1950, 78, 699. 25. Ramsey, N. F., Chemical Effects in Nuclear Magnetic Resonance and in Diamagnetic Susceptibility. Phys Rev 1952, 86, 243.

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26. Muetterties, E. L.; Hoffmann, R.; Meakin, P., Barrier to Rotation About Phosphorus- Nitrogen Bond in Aminofluorophosphoranes - Relevance to Rearrangements in 5-Coordinate Compounds. J. Am. Chem. Soc. 1972, 94, 5674. 27. Waked, A., Private Communication. 2017. 28. Fajari, L.; Julia, L.; Riera, J.; Molins, E.; Miravitlles, C., An Unusual Cyclization Reaction in the Chemistry of Perchloroorganic Compounds of Silicon and Germanium - Synthesis and Crystal-Structure of Perchloro(2,2'-Biphenylene)Diphenyl-Silane and Diphenyl-Germane. J. Organomet. Chem. 1990, 381, 321. 29. Holthausen, M. H.; Bayne, J. M.; Mallov, I.; Dobrovetsky, R.; Stephan, D. W., 1,2- Diphosphonium Dication: A Strong P-Based Lewis Acid in Frustrated Lewis Pair (FLP)- Activations of B-H, Si-H, C-H, and H-H Bonds. J. Am. Chem. Soc. 2015, 137, 7298. 30. Zhang, Y. H.; Shibatomi, K.; Yamamoto, H., Lewis Acid Catalyzed Highly Selective Halogenation of Aromatic Compounds. Synlett 2005, 2837. 31. Liang, T.; Neumann, C. N.; Ritter, T., Introduction of Fluorine and Fluorine-Containing Functional Groups. Angew. Chem., Int. Ed. 2013, 52, 8214. 32. Gaussian 09, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox; Gaussian Inc.: Wallingford CT, 2016. 33. (a) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J., Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force- Fields. J. Phys. Chem. 1994, 98, 11623; (b) Kim, K.; Jordan, K. D., Comparison of Density- Functional and MP2 Calculations on the Water Monomer and Dimer. J. Phys. Chem. 1994, 98, 10089; (c) Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct

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Asymptotic-Behavior. Phys. Rev. A 1988, 38, 3098; (d) Lee, C. T.; Yang, W. T.; Parr, R. G., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron- Density. Physical Review B 1988, 37, 785. 34. (a) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L., 6-31G* Basis Set for Atoms K Through Zn. J. Chem. Phys. 1998, 109, 1223; (b) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A., 6-31G* Basis Set for Third-Row Atoms. J. Comput. Chem. 2001, 22, 976; (c) Ditchfield, R.; Hehre, W. J.; Pople, J. A., Self-Consistent Molecular-Orbital Methods .9. Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724; (d) Hehre, W. J.; Ditchfield, R.; Pople, J. A., Self-Consistent Molecular- Orbital Methods .12. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular-Orbital Studies of Organic-Molecules. J. Chem. Phys. 1972, 56, 2257. 35. Headgordon, M.; Pople, J. A.; Frisch, M. J., MP2 Energy Evaluation by Direct Methods. Chem. Phys. Lett. 1988, 153, 503. 36. (a) Weigend, F., Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057; (b) Weigend, F.; Ahlrichs, R., Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297. 37. Pearson, R. G., Absolute Electronegativity and Hardness - Application to Inorganic- Chemistry. Inorg. Chem. 1988, 27, 734. 38. Parr, R. G.; Von Szentpaly, L.; Liu, S. B., Electrophilicity Index. J. Am. Chem. Soc. 1999, 121, 1922. 39. Chattaraj, P. K.; Sarkar, U.; Roy, D. R., Electrophilicity Index. Chem. Rev. 2006, 106, 2065. 40. Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A., On a Quantitative Scale for Lewis Acidity and Recent Progress in Polynitrogen Chemistry. J. Fluorine Chem. 2000, 101, 151. 41. LaFortune, J. H. W.; Johnstone, T. C.; Perez, M.; Winkelhaus, D.; Podgorny, V.; Stephan, D. W., Electrophilic Phenoxy-Substituted Phosphonium Cations. Dalton Trans. 2016, 45, 18156.

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

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

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Chapter 4 Perfluorobiphenyl Fluorophosphonium Cations 4.1 Introduction 4.1.1 Perfluorobiphenyl Groups in Transition Metal Applications

Massey and coworkers first reported the synthesis of 2-bromononafluorobiphenyl, BrC12F9, in 1964.1 This is the first example of any polyfluorobiphenyl compound containing a bromine substituent capable of undergoing lithium exchange. This first synthesis was carried out through

the addition of 2-bromopentafluorobenzene, BrC6F5, to an equimolar Et2O solution of

pentafluorophenyllithium, C6F5Li, at room temperature (Scheme 4.1).

Scheme 4.1. Synthesis of 2-bromoperfluorobiphenyl, BrC12F9.

Performing the reaction at room temperature yields bromoperfluorobiphenyl in a low yield of 30%. The authors proposed a mechanism in which a perfluorobenzyne intermediate reacts further with

an equivalent of BrC6F5. Massey prepared a series of BrC12F9 transfer agents including the lithium, mercury, and magnesium (Grignard) reagents. An Ullmann reaction using copper to couple

BrC12F9 yielded the corresponding perfluoroquaterphenyl compound.

This interesting compound went largely unnoticed until Marks reported the synthesis of

tris(2,2’,2’’-nonafluorobiphenyl)borane, B(C12F9)3 and the related triphenyl carbenium species 2 tris(2,2’,2’’-nonafluorobiphenyl)fluoroaluminate, [Ph3C][FAl(C12F9)], in 1997. In a modification

of Massey’s original synthesis, Marks reported adding a half molar equivalent to a cooled Et2O

solution of BrC6F5, resulting in BrC12F9 being obtained in high purity and an excellent yield of 83%. Marks proposes a more sophisticated mechanism in which perfluorobenzyne reacts with

C6F5Li to yield the lithiated perfluorobiphenyl, C12F9Li. Next, C12F9Li is proposed to undergo

111 metal-halogen exchange with C6F5Br to yield desired BrC12F9 and regenerate C6F5Li (Scheme 4.2). This alternative mechanism of carbolithiation occurring to generate 2-lithioperfluorobiphenyl and subsequent lithium-halogen exchange with BrC6F5 is consistent with the electrophilic behavior of benzyne intermediates.3

Scheme 4.2. Mechanism of 2-bromoperfluorobiphenyl synthesis.

Reaction of three equivalents of C12F9Li with boron trichloride in Et2O yields the borane etherate

B(C12F9)3·Et2O. Sublimation of B(C12F9)3·Et2O and subsequent recrystallization from pentane yields adduct free borane B(C12F9)3 (Scheme 4.3). Similar treatment of three equivalents of

C12F9Li with aluminum trichloride yields lithium tris(2,2’,2’’-nonafluorobiphenyl)fluoroaluminate, which can be treated with triphenylmethyl chloride, Ph3CCl, to generate [Ph3C][FAl(C12F9)] (Scheme 4.3).

112

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

Marks investigated both B(C12F9)3 and [Ph3C][FAl(C12F9)] for their effectiveness as abstractors and subsequent counterions in cationic early transition metal-mediated homogenous Ziegler-Natta type olefin polymerization. Mounting evidence at the time suggested that the anions had a significant impact on catalytic performance including chain-transfer characteristics and product 4,5 tacticity. The borane B(C12F9)3 was reacted with a wide range of group 4 and Tl metallocene dimethyl compounds to cleanly abstract a methyl substituent and generate isolable cationic - complexes with [MeB(C12F9)3] counterions.

- The bulky [MeB(C12F9)3] anion proved to enhance stability of these metallocene cations and allowed for the isolation of dimeric, μ-Me, cationic metallocene systems. The enhanced stability - is related to a reduced coordinative propensity of [MeB(C12F9)3] towards cationic metallocenes compared to the related [MeBCF]- that had previously been used.4 The cationic metallocene - complexes with [MeB(C12F9)3] anions were all noted to have enhanced thermal stability compared to the corresponding [MeBCF]- salts. The common decomposition product of dinuclear cationic methyl-bridged metallocenes is the corresponding fluoro-bridged compound. This is proposed to occur through either the transfer of a fluoroaryl group from the anion to the cation or through direct

113

fluoride abstraction by the metallocene cation from the anion fluoroaryl group (Scheme 4.4). In both possible decomposition routes, the enhanced thermal stability the metallocene salt is derived

from the increased chemical stability of the anion bearing C12F9 substituents. Cationic

metallocenes generated using [Ph3C][FAl(C12F9)3] also demonstrated enhanced thermal stability.

Scheme 4.4. Proposed decomposition route of cationic zirconocene compounds.

The dinuclear bridged metallocene catalysts that were obtained as a result of the increased stability - of the [MeB(C12F9)3] demonstrated ethylene polymerization activity that exceeded that of the monomeric catalysts obtained with [MeBCF]- counterions. The bridged metallocenes also produced higher molecular weight polyethylenes. Most notable is the 70 – 10,000 times rate enhancement observed for constrained geometry ethylene polymerization catalysts activated with

B(C12F9)3 instead of BCF. Constrained geometry catalysts activated with B(C12F9)3 were also found to have comparable comonomer incorporation with narrower polydispersities at higher rates.

Marks used the Childs’ method to gauge the relative electrophilicity of common cocatalysts and 6,7 referenced their strength to a BBr3 standard (defined as 1.00). By this method, C12F9 substituted boranes display enhanced electrophilicity compared to -C6F5 substituted boranes. Substitution of a single C6F5 group for a C12F9 group increases the relative electrophilicity from 0.77 in BCF to

0.79 in B(C12F9)(C6F5)2, while B(C12F9)3 displayed a relative electrophilicity of 0.85. The

enhanced electron withdrawing nature of C12F9 and high chemical stability make C12F9 substituents an attractive target for electrophilic catalyst design. Very few fluoroaryl-substituted boranes have reported electrophilicities higher than BCF.8

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Bochmann and coworkers employed B(C12F9)3 in the synthesis of weakly coordinating cyanoborate anions. Reaction of potassium cyanide, KCN, with B(C12F9)3 and subsequent workup

with Ph3CCl yields 1:1 cyanoborate adduct [Ph3C][NC-B(C12F9)3] (Scheme 4.5). The analogous

reaction using BCF generates instead the bridging 1:2 cyanoborate adduct [Ph3C][BCF-CN-BCF] 9 (Scheme 4.5). The solid-state structure of [Ph3C][NC-B(C12F9)3] displays the encapsulation of the

cyano group by the ortho-C6F5 substituents of B(C12F9)3, which the authors use to rationalize the preclusion of further reactivity by the anion (Figure 4.1). The decomposition of fluorophosphonium cations is believed to occur through reaction of the P-F substituent, encapsulation or partial encapsulation may enhance the overall stability of the fluorophosphonium catalysts.

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

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

4.1.2 Perfluorobiphenyl Groups in Main Group Applications

10 In 2012, O’Hare reported the first use of B(C12F9)3 as a Lewis acid in FLP chemistry. Lewis bases (1,4-diazabicyclo[2.2.2]octane) (DABCO), 2,6-lutidene, and 2,2,6,6-tetramethylpiperidine (TMP) each formed an FLP with B(C12F9)3. The increased steric bulk of B(C12F9)3 prevents adduct formation with 2,6-lutidene, as was observed in the corresponding BCF/2,6-lutidene FLP.11 The three B(C12F9)3 FLP systems were able to activate dihydrogen, though B(C12F9)3/2,6-lutidene and

B(C12F9)3/TMP systems required heating to 90 °C. Subsequent CO2 insertion chemistry of the hydrogen-activated salts to form reduced formatoborate species was not observed. The reactivity 12 is in contrast to the CO2 insertion observed with [HBCF][HTMP] salts (Scheme 4.6). The formatoborate salts, independently synthesized from the reaction of B(C12F9)3 with formic acid and either 2,6-lutidene or TMP, heated in an atmosphere of CO2 undergo facile decarboxylation to the corresponding borohydride salts (Scheme 4.6). In a later report by O’Hare, the hydrogen- activated phosphonium-borate salt [tBu3PH][HB(C12F9)3] was able to form the corresponding 13 formatoborate product through reaction with CO2 at 145 °C. The differing reactivity trends between most B(C12F9)3 and BCF hydrogen-activated FLP salts towards CO2 results from the decreased steric accessibility afforded by the ortho-C6F5 rings in B(C12F9)3.

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

O’Hare used the Gutmann-Beckett method to further probe the electrophilic properties of

B(C12F9)3. The acceptor number of B(C12F9)3 was calculated to be 87.9, a 10% increase compared to BCF (AN 79.8). These results are consistent with the earlier study by Marks using the Childs

method, which also indicated a 10% electrophilicity enhancement of B(C12F9)3 over BCF. Unlike

the enhanced electrophilicity offered by C6Cl5 rings which rely on decreased mesomeric

stabilization (Chapter 3, pg. 63), C12F9 substituents are inductively more electron withdrawing than analogous C6F5.

14 In 2013, our group reported the use of 2:1 FLP of Al(C12F9)3 with PMes3 to activate dihydrogen.

The free alane, Al(C12F9)3, was obtained by first using Li[AlH4] to form the hydrido- analogue

from Marks’ anion, [FAl(C12F9)][Ph3C], followed by hydride-abstraction using [Ph3C][B(C6F5)4].

The activation of hydrogen by Al(C12F9)3 with PMes3 results in the formation the corresponding salt with hydrido-bridged anion [Mes3PH][(μ-H)(Al(C12F9)3]. The resulting [Mes3PH][(μ-

H)(Al(C12F9)3] salt is unreactive towards ethylene, unlike analogous [Mes3PH][(μ-H)(Al(C6F5)3]

which effects ethylene insertion and subsequent disproportionation to from [Mes3PH][Al(C6F5)4] 15 and Al(CH2CH3)(C6F5)2 (Scheme 4.8). The author attributes the lack of reactivity of

[Mes3PH][(μ-H)(Al(C12F9)3] towards ethylene to the enhanced Lewis acidity of Al(C12F9)3 and to the steric encapsulation of the hydride.

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

[Mes3PH][(μ-H)(Al(C6F5)3)2] (bottom).

4.2 Results and Discussion 4.2.1 Synthesis of Perfluorobiphenyl Phosphines by Lithiation

A series of perfluorobiphenyl-substituted phosphines were generated by the 1:1 reaction of C12F9Li

with either iPr2PCl, (o-tol)2PCl, Ph2PCl, (C6F5)2PCl, (3,5-(CF3)2C6H3)2PCl, Cy2PCl, or Me2PCl to

yield iPr2P(C12F9) (4-1), (o-tol)2P(C12F9) (4-2), Ph2P(C12F9) (4-3), (C6F5)2P(C12F9) (4-4), (3,5-

(CF3)2C6H3)2P(C12F9) (4-5), Cy2P(C12F9) (4-6), and Me2P(C12F9) (4-7) respectively in good yields

(Scheme 4.8). Compounds 4-1 – 4-7 phosphines represent the first series of C12F9 substituted phosphorus species. Best yields were obtained when lithiation of BrC12F9 was carried out at very low temperatures using a cyclohexane and liquid nitrogen cooling bath for two hours before the addition of a halophosphine reagent.

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

The reaction of C12F9Li with either tBu2PCl or tBu(Ph)PCl fail to cleanly yield the desired

tBu2P(C12F9) (4-8) or tBu(Ph)P(C12F9). The standard synthetic methodology yielded 4-8 only as a minor product, which was purified by successive recrystallizations and isolated in poor yield.

Formation of tBu(Ph)P(C12F9) was not observed, despite several synthetic attempts with varied reaction conditions. The steric bulk of substituents near the phosphorus centre appears have a significant impact on how readily C12F9 substitution occurs.

19 The F NMR spectra of 4-1 – 4-8 all display seven resonances attributable to the (C12F9), including equivalent F-2’/6’ and F-3’/6’ signals. Additionally, 4-4 displays the expected 4:2:4 C6F5

resonances, and 4-5 displays a large singlet resonance, which is attributed to the CF3 group. The

presence of seven C12F9 resonances is indicative that that no isomerism is present. “Up/down” rotational isomerism could result if the other substituents on the phosphine were sufficiently large to prevent as to interact with the ortho-C6F5 and prevent rotation about the P-(C12F9) bond, which would result in a more complicated 19F NMR spectrum. Restricted rotation is not observed in the - free borane B(C12F9)3; however, upon abstracting methyl to form [MeB(C12F9)3] restriction of the internal (C6F4)-(C6F5) bond is observed, which results in the inequivalence of the ortho- and meta- 2 fluorine signals on the C6F5 ring. Restricted rotation of the ortho-C6F5 group may be present in 4- 1 – 4-8, but the ortho- and meta-fluorine substituents remain equivalent due to the symmetry of 19 the other substituents on phosphorus. The F NMR shifts on the ortho-C6F5 ring are insulated from electronic changes at the phosphorus centre and remain at relatively constant chemical shifts in compounds 4-1 – 4-8.

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The 31P NMR resonances of 4-1 – 4-8 are as follows 15.4, -25.6, -11.0, -63.0, -12.8, 5.9, -39.0, and -65.6 ppm (Table 4.1). No clear trend emerges relating the chemical shift of these phosphines, unlike the trends observed in chapter 3, with stepwise substitution of similar aryl groups. Since the 31P NMR chemical shift is sensitive to be the electron-withdrawing or donating nature and steric demand of the substituents, it is very difficult to ascertain the electronic character. Aromatic ring current effects likely contribute significantly to the 31P NMR chemical shift values. By varying the number and type of aromatic substituents, the chemical shift is perturbed in a in a manner that is difficult to predict.

The 19F NMR spectra of 4-1 – 4-8 give a better indication of the electronic character of the phosphines. The chemical shift of the para-fluorine with respect to the phosphorus centre on the

C6F4 ring, F-4, is sensitive to electronic changes of the phosphorus centre, but is not affected by

changes in sterics. The chemical shift of F-4 gives the trend (3,5-(CF3)2C6H3) > C6F5 > Ph > Cy > iPr > tBu > o-tol > Me (Table 4.1). The F-4 chemical shift of (3,5-(CF3)2C6H3)-substituted 4-5 is

significantly downfield shifted beyond even that of C6F5-substituted 4-4. The chemical shift of o-tol-substituted 4-2 is unexpectedly upfield shifted with respect to the F-4 chemical shift of alkyl-substituted 4-3, 4-6, 4-8.

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

Phosphine 31P{1H} NMR (ppm) F-4 19F NMR (ppm) 4-1 15.4 -150.0 4-2 -25.6 -151.4 4-3 -11.0 -149.0 4-4 -63.0 -147.4 4-5 -12.8 -143.3 4-6 5.9 -149.8 4-7 -39.0 -152.9 4-8 -65.6 -150.4

The synthesis of phosphines bearing multiple C12F9 substituents was carried out using a similar methodology to that used in the synthesis of 4-1 – 4-8. Dihalophosphines MePCl2, PhPCl2, tBuPCl2

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and (C6F5)PBr2 were each reacted with two molar equivalents of C12F9Li at low temperatures in

pentane/Et2O solutions (Scheme 4.9). Only the less sterically crowded MePCl2 and PhPCl2 reacted 19 to cleanly generate the desired phosphines MeP(C12F9)2 (4-9) and PhP(C12F9)2 (4-10). The F NMR spectra of 4-9 – 4-10 display 8 and 9 signals respectively. The increase in fluorine signals is the result of restricted rotation and inequivalence of F-2’/6’ signals in 4-9 and inequivalence in both F-2’/6’ and F-3’/5’ signals in 4-10. The reaction of tBuPCl2 with C12F9Li yielded 31 19 tBuP(C12F9)2 which displays a P NMR signal at 101.0 ppm; however, the F NMR spectrum displays several broad peaks in addition to those attributed to the desired product. The reaction of 31 1 (C6F5)PBr2 with C12F9Li yields a single product observable in the P{ H} NMR spectrum as a very broad signal. The 19F NMR spectrum is extremely complex, as would be the case with

(C6F5)P(C12F9)2 in which restricted rotation could produce inequivalence of C6F5 signals as well as rotational isomers. The further purification of tBuP(C12F9)2 or (C6F5)P(C12F9)2 was not pursued.

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

The synthesis of tris(nonafluorobiphenyl) phosphine, P(C12F9)3, was attempted using a variety of reagents and conditions. The reaction of three equivalents of C12F9Li with PBr3 in a pentane/Et2O solution yields an intractable mixture of products. Introducing either substoichiometric or

stoichiometric amounts of CuI to the reaction mixture had no discernable effect. Employing PCl3

or PI3 instead of PBr3 similarly gave a mixture of products. Significant effort was invested into

attempts to obtain P(C12F9)3, as it would maximize any encapsulating effects imparted to the

resulting fluorophosphonium by the large C12F9 groups. The increased steric congestion observed in 4-9, 4-10, tBuP(C12F9)2 or (C6F5)P(C12F9)2, as exhibited by inequivalence of F-3’/5’ and F-2’/6’ 19 in the F NMR spectra, suggest that P(C12F9)3 may be too sterically encumbered to be accessed.

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Additionally, compounds bearing two C12F9 groups exhibited significantly reduced solubility in common organic solvents compared with single C12F9 substituted phosphines.

4.2.2 Synthesis of Perfluorobiphenyl Phosphines by Zincation

Perfluorobiphenylzinc reagents offer several advantages over the corresponding lithium reagents: zinc reagents can be isolated and stored long-term and do not require low reaction temperatures that lithium reagents require, enabling zinc reagents to be easily used in very small test-scale reactions in a glovebox. Pentafluorophenyllithium in ether solution decomposes rapidly at -10 °C, 16 with 25% of the initial C6F5Li decomposing within 20 minutes. The decomposition occurs through the elimination of LiF salt to generate transient tetrafluorobenzyne intermediates. The 17 elimination of LiF from C6F5Li is very exothermic and reportedly explosive above -30 °C.

A modified literature procedure was used to generate the toluene adduct of 18 bis(nonafluorobiphenyl)zinc, Zn(C12F9)2·PhMe, in good yield. In an effort to synthesize

P(C12F9)3, 1.5 equivalents of Zn(C12F9)2·tol was reacted with PBr3 in toluene at room temperature,

which resulted in the consumption of PBr3 and the production of a single product 4-11 observable by 31P{1H} NMR. The phosphorus resonance for 4-11 was observed as a triplet at 116.4 ppm, the upfield shift from the PBr3 starting material at 226 ppm, is consistent with diminished halogen substitution on the phosphorus centre. The 19F NMR spectrum of the reaction mixture showed significant resonances attributable to unreacted Zn(C12F9), suggesting that 4-11 was the result of a substoichiometric reaction of Zn(C12F9)2·PhMe with PBr3. Even upon heating the mixture to 90

°C, the excess Zn(C12F9)2·tol was not consumed. To verify, the reaction was repeated using 0.5 31 equivalents of Zn(C12F9)2·PhMe with PBr3 and the P NMR spectrum similarly showed full consumption of the phosphorus starting material and generation of 4-11. Full consumption of 19 resonances attributable to Zn(C12F9)2·PhMe was observed in the F NMR spectrum when 0.5 equivalents were used. The formation of 4-11 was very sensitive to changes in concentration of starting materials, broadening of Zn(C12F9)2·PhMe fluorine resonances indicate adduct formation

between Zn(C12F9)2·tol and 4-11 which prevents reaction with PBr3. No reaction occurs between

PBr3 and Zn(C12F9)2·Et2O, indicating that stronger adducts with zinc drastically reduce the reactivity. The analogous reaction of Zn(C12F9)2·tol with PCl3 does not occur, as only PCl3 is visible by 31P NMR after three days of heating the mixture to 90 °C. Single crystal X-ray diffraction

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elucidated the structure of 4-11 as the 1,2-diphosphine species ((C12F9)BrP)2 (Figure 4.2). Diphosphine 4-11 displays trigonal pyramidal geometry at the phosphorus centres, with an inversion centre between the phosphorus atoms. The P-P bond distance is 2.227(1) Å, similar to that of ((C6Cl5)2P)2 (2.265(2) Å) in Chapter 3. The angle between the planes of the perfluorobiphenyl rings is 74.29°. The twisted ring orientation allows for each perfluorobiphenyl ring to take part in a T-shaped π-π interaction, with intercentroid distances of 5.654 Å. In an ab initio study of the interaction between benzene and hexafluorobenzene, the centroid-centroid distance of the most favourable T-shaped complex of side-on hexafluorobenzene was 6.0 Å.19

Additionally, the bromine substituents are oriented towards the C6F4 perfluorobiphenyl ring centroid at a separated by 3.656 Å. Stabilizing interactions between halogens and π-systems have been evaluated, with the reported bounding distance for such interactions between 2.8 Å to 4.2 Å.20

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Figure 4.2. POV-ray depiction of 4-11 (top), Br-aryl interaction of 4-11 (middle), T-shaped π-π interaction of 4-11 (bottom); C: light grey, Br: purple, P: orange, F: pink, Centroid: red; selected fluorine atoms have been omitted for clarity.

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Alkali and alkaline earth metal reduction has been used as general synthetic route to the formation of diphosphines.21,22 Similar formation of 1,2-diphosphines was observed in the reaction of

LiC6Cl5 with PCl3 and is discussed in Chapter 3 (pg. 68). To further explore this reactivity of

Zn(C12F9)2·tol was reacted with a series of alkyl- and arylhalophosphines: PhPCl2, Ph2PCl,

(C6F5)2PBr, (C6F5)PBr2, tBuPCl2 and iPrPCl2. Of all the additional halophosphines tested, PhPCl2 was the only reagent that reacted at room temperature to generate a single product 4-12. In a similar fashion to 4-11, 4-12 forms quantitatively through the reaction of PhPCl2 with 0.5 equivalents of 31 Zn(C12F9)2·tol. Compound 4-12 displays a P NMR resonance at 61.2 ppm, the large upfield shift is consistent with reduced halogen substitution. The 1H and 19F NMR data of 4-12 are consistent the formulation of 4-12, by analogy to 4-11, as ((C12F9)PhP)2 (Scheme 4.10). Restricted rotation 19 of the ortho-C6F5 groups is observed by F NMR only for 4-12.

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

The only other halophosphine to react with of Zn(C12F9)2·tol at room temperature was tBuPCl2, which slowly forms two new 31P{1H} NMR resonances at 56.4 and 57.2 ppm, consistent with the upfield shift observed for 4-11 and 4-12. The two peaks are assumed to correspond to isomers caused by restricted the bulky tBu groups restricting the rotation of the P-P bond. The reaction of

Zn(C12F9)2·tol with tBuPCl2 could not be driven to completion, as decomposition was observed at elevated temperatures. All other halophosphines tested for reactivity with Zn(C12F9)2·tol that did not react at room temperature were subjected to gradually higher reaction temperatures, resulting in decomposition to multiple products after days at 100 °C. The formation of bis(nonafluorobiphenyl)-1,2-diphosphines using Zn(C12F9)2·tol does not appear to be a broadly generalizable synthetic methodology.

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4.2.3 Synthesis of 2-Perfluorobiphenyl Difluorophosphoranes

Addition of solid XeF2 to stirring solutions of 4-1 – 4-7 in CH2Cl2 yielded the corresponding

difluorophosphorane iPr2PF2(C12F9) (4-13), (o-tol)2PF2(C12F9) (4-14), Ph2PF2(C12F9) (4-15),

(C6F5)2PF2(C12F9) (4-16), (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17), Cy2PF2(C12F9) (4-18), or

Me2PF2(C12F9) (4-19) in excellent yields (Scheme 4.11).

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

The 31P{1H} NMR spectra of difluorophosphoranes 4-13 – 4-19 each display a characteristic triplet, consistent with two equivalent fluorine atoms bound to the phosphorus centre. The 19F

NMR spectra 4-13, 4-15, 4-17 – 4-19 of display seven C12F9 resonances, similar to the corresponding phosphines, in addition to a downfield shifted doublet corresponding to the phosphorus-bound fluorine substituent. The compounds with the highest steric congestion at the

phosphorus centre, 4-14 and 4-16, both display restricted rotation of C6F5 rings manifested as very broad signals in the 19F NMR spectra.

31 1 The reaction of 4-8 with XeF2 did not generate the desired difluorophosphorane. The P{ H} NMR spectrum displays the presence of a quartet signal instead of the expected triplet. The 19F

NMR spectrum displays a doublet that integrates 3:1 with respect to F-3 of the C12F9 substituent.

These data are consistent with the formation of tBuPF3(C12F9) as the major product. The loss of

bulky substituents during oxidation by XeF2 is consistent with earlier reactivity observed in the oxidation of P(C6Cl5)3 (Chapter 3, pg. 73).

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Oxidation of bis(nonafluorobiphenyl) phosphines 4-9 and 4-10 with XeF2 was carried out in DCM.

In both cases the corresponding difluorophosphoranes MePF2(C12F9)2 from 4-9 and PhPF2(C12F9)2 from 4-10 were obtained in low yield. Both difluorophosphoranes exhibited very low solubility in common organic solvents including toluene, benzene, and CH2Cl2. Subsequent reaction of

PhPF2(C12F9)2 with fluoride abstracting agent [SiEt3][B(C6F5)4] as a slurry in toluene yielded an intractable mixture of many products. Further investigation of bis(perfluorobiphenyl) phosphorus species was not pursued, as the high steric crowding of the phosphorus centre significantly decreased the chemical stability by promoting ligand dissociation.

There are limited reports of 1,2-diphosphines bearing fluoroaryl substituents, the exploration of reactivity of these compounds is limited to complexation with metal carbonyls.23,24 To further investigate the chemistry of this class of compounds, 1,2-diphosphines 4-11 and 4-12 were each 31 1 oxidized with XeF2. The reaction of 4-11 with XeF2 yields a mixture of products by P{ H} NMR, the major product displays a quintet at -49.3 ppm and the minor product displays a triplet at 1.3 ppm. The triplet is consistent with the formation of a difluorophosphoranes species, while the 31 quintet is indicative of a tetrafluorinated phosphorus, PF4(C12F9). The F NMR spectrum shows two downfield doublets with 1:7 relative integration, and many overlapping signals further upfield. Attempts to resolve this mixture by recrystallization yielded few single crystals suitable for X-ray diffraction (Figure 4.3). The solid-state structure obtained is of a four coordinate phosphonium cation with three monoatomic substituents and monoatomic species. The bond lengths of the phosphorus-bound substituents 1.547(2) Å, 1.541(2) Å and 1.498(2) Å. The two longer are consistent with P-F or P-O single bonds, while the shorter 1.498(2) Å distance is consistent with a P-O double bond. Given the neutral charge of the phosphine oxide, the monoatomic species is assumed to be cocrystallized water. The distance between the water oxygen and nearest fluorine atom is 2.479(3) Å with a hydrogen oriented towards the fluorine, consistent with hydrogen 25 bonding. This decomposition product likely results from the reaction of PF4(C12F9) with water, eliminating two equivalents of HF. Insufficient decomposition product was obtained for exhaustive analysis.

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

The reaction of two equivalents of XeF2 with 4-12 also displays a mixture of products, however, instead of a triplet and quintet observed for the reaction of related 4-11¸ the 31P{1H} NMR spectrum displays three signals: a singlet at 67.2 ppm, a triplet at -28.8 ppm and a doublet of triplets observed -32.4 ppm. The singlet corresponds to starting material 4-12, the triplet likely corresponds to the difluorophosphoranes species ((C12F9)PhF2P)2, and the doublet of triplets signal corresponds to a trifluorophosphorane species PF3Ph(C12F9). The reaction of ((C12F9)PhF2P)2 with an additional equivalent of XeF2 results in the cleavage of the P-P bond to form PF3Ph(C12F9), which appears as a doublet of triplets in the 31P{1H} NMR spectrum. Reaction of 4-12 with three equivalents of XeF2 cleanly forms PF3Ph(C12F9). The more robust P-C aryl bond does not appear to be cleaved as is observed with P-Br bonds in the reaction of 4-11 with XeF2.

4.2.4 Synthesis of Fluorophosphonium cations

Addition of a toluene solution of 4-13 to a stirring white slurry of [SiEt3][B(C6F5)4] in toluene, rapidly resulted in the formation of a dark red oily suspension. The red suspension was stirred for two hours before the toluene was decanted and the red oil was triturated in pentane until a white powder 4-20 was obtained. The 31P{1H} NMR spectrum of 4-20 displays a doublet at 143.8 ppm,

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with a coupling of 1030 Hz, consistent with the abstraction of fluoride from 4-13. The 19F NMR - spectrum of 4-20 displays a set of resonances attributable to a [B(C6F5)4] anion, a doublet consistent with phosphorus-bound fluorine, and a set of seven C12F9 signals. These data are

consistent with formulation of 4-20 as fluorophosphonium salt [iPr2PF(C12F9)][B(C6F5)4].

Similarly, reaction of difluorophosphoranes 4-14 – 4-18 with [SiEt3][B(C6F5)4] yield the

corresponding fluorophosphonium salts [(o-tol)2PF(C12F9)][B(C6F5)4] (4-21),

[Ph2PF(C12F9)][B(C6F5)4] (4-22), [(C6F5)2PF(C12F9)][B(C6F5)4] (4-23), [(3,5-

(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24), and [Cy2PF(C12F9)][B(C6F5)4] (4-25) respectively in good isolated yields (Scheme 4.12).

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

The 31P{1H} NMR spectra of 4-21 – 4-25 each display a single doublet at 95.3, 90.7, 72.3, 90.2 and 133.8 ppm respectively. The 19F NMR spectra of fluorophosphonium salts 4-21 – 4-25 display - resonances attributable to the [B(C6F5)4] anion, (C12F9) substituent and phosphorus-bound fluorine substituent. The 19F NMR spectra of 4-21 – 4-24 all display a P-F signal between -119 and -125 ppm, while the P-F signals for 4-20 and 4-25 appear significantly upfield at -171.5 and -170 ppm respectively.

The solid-state structures of 4-20, 4-21, and 4-22 were obtained by X-ray diffraction of single crystals (Figure 4.4). Structures of 4-20, 4-21, and 4-22 display tetrahedral geometry of the phosphorus centre with P-F bond lengths of 1.541(2), 1.549(1) and 1.547(2) Å.

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The solid-state structure of a decomposition product of 4-23 was obtained (Figure 4.5). The - - counterion of 4-23 unexpectedly refined to [HOB(C6F5)3] instead of the expected [B(C6F5)4] anion. Two hydrogen atoms were present in the difference map after assigning the borate oxygen. Analysis of the mother liquor by 11B and 19F NMR spectroscopy show no indication of degradation of 4-23 and integration of fluorine signals is consistent with the presence of 1:1

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+ - - [(C6F5)2PF(C12F9)] cation to [B(C6F5)4] anion. The anion [FB(C6F5)3] was also considered, however, the reaction of BCF with difluorophosphoranes 4-16 does not produce

[(C6F5)2PF(C12F9)][FB(C6F5)3]. The borate oxygen substituent it oriented towards the fluorine of the fluorophosphonium cation at a distance of 2.554(2) Å with the hydrogen oriented towards the fluorine, consistent with hydrogen bonding between anion and cation.25

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

4.2.5 Measures of electrophilicity

4.2.5.1 Gutmann-Beckett Method Compounds 4-20, 4-21, 4-22, and 4-23 were each subjected to the Gutmann-Beckett protocol by combination of three equivalents of each Lewis acid with one molar equivalent OPEt3 in CH2Cl2 solutions. The 31P{1H} NMR spectra of each was obtained to determine the chemical shift change of adducted OPEt3 against the reference of free OPEt3 in CH2Cl (50.7 ppm). For 4-20 – 4-22, the

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OPEt3 signal appears as a very broad peak at approximately 50 ppm, indicating only a very weak adduct interaction. The Gutmann-Beckett method is therefore incompatible with 4-20 – 4-22, yielding no information on their relative Lewis acidity. The 31P{1H} NMR spectrum of 4-23 with displays two broad singlets at 91.7 ppm and -65 ppm as well as a doublet attributable to excess

4-23 at 72.7 ppm. The signal at 91.7 ppm is similar to the adducted OPEt3 chemical shift observed 26 for related system [PF(C6F5)3][B(C6F5)4]·OPEt3. The signal at -65 ppm is too broad to observe expected P-F coupling, but is assigned to be the adducted fluorophosphonium phosphorus. The 31 shift observed for the 4-23·OPEt3 adduct (Δδ P = 41.0 ppm) is slightly larger than reported shift 31 for PF(C6F5)3][B(C6F5)4]·OPEt3 (Δδ P = 40.4 ppm). This finding is consistent with the enhanced electron withdrawing power of C12F9 over C6F5 groups.

4.2.5.2 Chemical shift as a measure of electrophilicity The phosphorus chemical shift of fluorophosphonium cations has been used previously as a simple method of ascertaining the relative electrophilicity.27 The 31P{1H} NMR chemical shift of the phosphines 4-1 – 4-8 did not yield any trend that coincided with expected electrophilicity, based off electron-withdrawing capabilities of the substituents. The effect of ligand size on the geometry and chemical shift of the phosphorus centre could not be negated. The 19F NMR chemical shift of para-fluorine with respect to the phosphorus centre, F-4, was found to be sensitive to electronic changes of the phosphorus centre and is positioned such that steric influences would be minimized. The electrophilicity trend derived from the 19F NMR chemical shift gave the trend

(3,5-(CF3)2C6H3) > C6F5 > Ph > Cy > iPr > tBu > o-tol > Me.

In fluorophosphonium salts 4-20 – 4-25, the F-4 chemical shifts give a similar trend

(3,5-(CF3)2C6H3) > C6F5 > Ph ≈ o-tol > iPr > Cy. The most notable difference in these trends is the relative position of o-tol. The 31P{1H} NMR chemical shifts of 4-20 – 4-25 are also congruent with

these observed patterns, giving the trend C6F5 > (3,5-(CF3)2C6H3) > Ph > o-tol > Cy > iPr. The fourth substituent seems to reduce the effect of ligand steric demand on 31P{1H} NMR chemical shift. All three trends from NMR metrics are easily obtained and have good overall agreement, but the relative electrophilicity of similar groups is not well defined using this method. Additionally, obtaining electrophilicity measurements from NMR data of the free Lewis acid eliminates the possibility for chemical incompatibility as was observed with the Gutmann-Beckett method. 28

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4.2.5.3 General Electrophilicity Index

Geometry optimization and frequency calculations were performed on cations from 4-20 – 4-23 ([4-20]+ – [4-23]+ respectively) with Gaussian 0928 using B3LYP functional29 and 6-31+G(d) basis set.30 Single point energy calculations were performed on the optimized geometries using MP231 and basis set def2-TZVPP.32 When available, solid-state structures were used to generate input coordinates. The LUMO of cations 4-20 – 4-23 all display a characteristic large lobe on the phosphorus centre, indicative of the site of Lewis acidity (Figure 4.6).

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The orientation of the C12F9 group relative to the P-F bond in the optimized geometries is only consistent with the orientation in solid state-structure in the case of 4-23. The optimized geometry of 4-21 adopted a similar propeller configuration with the ortho-methyl substituents on the

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opposite side as the ortho-C6F5 group. The HOMO and LUMO energies for 4-20 – 4-23 and + [PF(C6F5)3] are provided below (Table 4.2).

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

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

Cation ELUMO (eV) EHOMO (eV) [4-20]+ -2.436 -13.551 [4-21]+ -2.249 -12.625 [4-22]+ -2.197 -12.927 [4-23]+ -3.209 -13.247

+ [PF(C6F5)3] -3.455 -14.090

+ + The LUMO energies for both C6F5 bearing cations [4-23] and [PF(C6F5)3] are significantly lower than the remaining cations. Surprisingly, the LUMO energy of the iPr substituted [4-20]+ is lower than either [4-21]+ or [4-22]+. White the LUMO of [4-21]+ and 4-22]+ appears to be delocalized onto the respective o-tol and Ph substituents, the LUMO of [4-20]+ extends significantly onto both

rings of the C12F9 substituent. The trend provided by decreasing LUMO energies of cations is thus + + + + + [4-22] < [4-21] < [4-20] < [4-23] < [PF(C6F5)3] . The general electrophilicity index (GEI) values, ω, are calculated using only the HOMO and LUMO energies of the free cations using the formula

with energies in units of electron volts (eV). The calculated ω values for cations [4-20]+ – [4-23]+, + and [PF(C6F5)3] are tabulated below (Table 4.3).

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

Cation ω (eV) [4-20]+ 2.874 [4-21]+ 2.665 [4-22]+ 2.665 [4-23]+ 3.372

+ [PF(C6F5)3] 3.618

Unlike the cations discussed in Chapter 3 (pg 83), significant linear correlation is not observed between the ω values and either 19F or 31P{1H} NMR data for 4-20 – 4-23. The trend generated by + + + + + increasing ω of [4-21] = [4-22] < [4-20] < [4-23] < [PF(C6F5)3] is similar in order to the related trend observed in LUMO energies. The notable deviation between the trends is the position of iPr (4-20), which is lower than Ph (4-22) and o-tol (4-21) in NMR trends and higher in LUMO and ω + + trends. Additionally, the ω value for [PF(C6F5)3] is significantly higher than [4-23] , while by the

Gutmann-Beckett method 4-23 is slightly more Lewis-acidic than [PF(C6F5)3][B(C6F5)4].

4.2.5.4 Fluoride ion affinity Using the same methodology outlined in the previous section, the optimized geometries of the fluoride-adducts of [4-20]+ – [4-23]+ were used to obtain single point energies using the MP231 with the def2-TZVPP basis set.32 The energies of the fluoride-adducts were then used with cations + + [4-20] – [4-23] to determine the FIA with COF2 as a reference compound using the equation

FIA = (Ecation + EF3CO) – (Ephosphorane + EF2CO) + 209 where all energies are in units of kJ/mol. The FIA values for 4-20 – 4-23 are tabulated below (Table 4.4).

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

Cation FIA (kJ/mol) [4-20]+ 709 [4-21]+ 702 [4-22]+ 713 [4-23]+ 767

+ [PF(C6F5)3] 744

The trend obtained from increasing FIA values is therefore [4-21]+ < [4-20]+ < [4-22]+ < + [PF(C6F5)3][B(C6F5)3] < [4-23] . The calculated FIA values show only a small 11 kJ/mol difference between any of [4-20]+ – [4-22]+, consistent with the relative position of these three compounds changing often within other electrophilicity scales. The large gap between [4-20]+ – + + + [4-22] and either [PF(C6F5)3] or [4-23] is also consistent with the differing reactivity observe in

the Gutmann-Beckett test, where only the latter two interacted significantly with OPEt3. The + + relative position of [4-23] to [PF(C6F5)3] is consistent with the results of the Gutmann-Beckett method, but neither NMR chemical shift nor ω value trends.

4.2.5.5 Competition Reaction Because of the conflicting results from previous Lewis acidity measurements, competition

experiments were carried out to ascertain the relative empirical FIAs between C12F9

fluorophosphonium salts and their C6F5 analogues. Monitoring a CH2Cl2 solution of 4-23 31 1 combined with equimolar PF2(C6F5)3 by P{ H} NMR shows the slow appearance of

fluorophosphonium [PF(C6F5)3][B(C6F5)4], while the formation of 4-16 is obscured by 19 overlapping PF2(C6F5)3 signals. The F NMR spectrum is very complicated with five different

C6F5 containing species present, two of which contain C12F9 groups. However, the downfield

shifted P-F doublet resonances of 4-16 and PF2(C6F5)3, which overlap partially, can be used to track relative difluorophosphoranes amounts. The initial spectrum obtained after one hour after

mixing shows a relative 4-16 to PF2(C6F5)3 concentration of 1:5.3 (Figure 4.7, left). After 24 h, the

ratio of 4-16 to PF2(C6F5)3 increased to 1:0.43 (Figure 4.7, right). This evidence supports the

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calculated FIA results that 4-23 has a higher FIA than C6F5 analogue [PF(C6F5)3][B(C6F5)4]. The long timescale required to obtain this equilibrium is indicative of high steric bulk precluding rapid fluoride transfer.

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

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

19 Monitoring the less sterically encumbered pairing of 4-22 with Ph2PF2(C6F5) by F NMR shows - only three resonances attributable to the [B(C6F5)4] anion. The two fluorophosphonium cations are in rapid equilibrium with the two difluorophosphoranes species, resulting in an absence of observable signals at room temperature. The rapid equilibrium of 4-22 with Ph2PF2(C6F5) is in

contrast with the slow reaction observed with the slow reaction between 4-23 and PF2(C6F5)3, resulting from the lower steric demand of the Ph substituents. The 19F NMR spectrum obtained

after cooling the reaction mixture of 4-22 and Ph2PF2(C6F5) to -40 °C shows a sharpening of

resonances attributable PF2(C6F5)3 and [PF(C6F5)3][B(C6F5)4], and a very broad doublet associated

with 4-13. The dynamic equilibrium that exists between 4-23 and PF2(C6F5)3 precludes obtaining accurate ratio of species in solution to determine relative FIA values.

4.2.6 Solubility of Perfluorobiphenyl Salts

As a general feature of fluorophosphonium salts, the solubility tends to decrease as C6F5

substitution increases. In the series [Ph3-nPF(C6F5)n][B(C6F5)4], when n = 1 solubility is poor in most common organic solvents and good in CH2Cl2 and PhBr, when n is increased to 3 the

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solubility decreases even in CH2Cl2 and PhBr. The solubility of bis(nonafluorobiphenyl) phosphines 4-9 and 4-10 is notably lower in all tested solvents than related mono(nonafluorobiphenyl) phosphines 4-1 – 4-8. Unexpectedly the solubility of fluorophosphonium 4-23 was found to be significantly greater than C6F5 analogue

[PF(C6F5)3][B(C6F5)4], showing good solubility in CH2Cl2 and PhBr as well as in C6H6 and toluene. To illustrate this solubility difference, 0.2 mmol of 4-23 and [PF(C6F5)3][B(C6F5)4] were

each dissolved 1mL of C6H6 in separate vials. After stirring the solutions for 1 h, the solution of 4-23 was cloudy, but stayed evenly suspended, whereas a large amount of solid quickly settled to the bottom in the mixture of [PF(C6F5)3][B(C6F5)4]. The two mixtures were filtered using a 0.45 μm pore size filter paper, and the solvent was removed in vacuo from both filtrates to obtain the mass of dissolved fluorophosphonium salt. The recovered yield was 20% (5.6 mg recovered of 27.6 mg initial) for 4-23, whereas only 3% (0.8 mg recovered of 24.6 mg initial) of

[PF(C6F5)3][B(C6F5)4] was recovered. The increased solubility allows NMR-scale reactions to be 1 carried out in C6D6 instead of CD2Cl2, which allows for monitoring reactions by H NMR spectroscopy at considerably lower cost.33

4.2.7 Stability of Perfluorobiphenyl Phosphonium Cations

Compounds 4-20 – 4-25 are stable as isolated solids stored under an inert atmosphere at -35 °C for months, and with the exception 4-24, are also stable in CH2Cl2 solutions at room temperature for weeks. Attempts to obtain single crystals of 4-24 suitable for X-ray diffraction from a CH2Cl2 solution layered with cyclohexane did not produce crystals. The solvent was removed in vacuo 31 1 and the residue was dissolved in C6D6 to verify the stability of the phosphonium salt. The P{ H} NMR spectrum revealed that fluorophosphonium 4-24 had cleanly decomposed to the corresponding difluorophosphoranes 4-17 through fluoride abstraction. The 31P{1H} NMR spectrum of this reaction mixture shows only one triplet signal at -62.6 ppm with 716 Hz coupling. There are three fluorine chemical environments present in 4-24: phosphorus bound fluorine, trifluoromethyl fluorine, and pentafluorophenyl fluorine present in the borate counterion. Fluorophosphonium cations are reported to activate trifluoromethyl C-F bonds as observed in hydrodefluorination chemistry.26,34 The decomposition of fluorophosphonium with concomitant formation of phosphine and difluorophosphoranes species has also been observed as a common

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decomposition pathway. Had either of these cases occurred, additional phosphorus containing species would be observed by 31P{1H} NMR spectroscopy. The 19F NMR spectrum of the reaction - mixture displays major signals associated with 4-24. No signals associated with the [B(C6F5)4] anion are observed, though several small, broad signals are present. The 11B NMR spectrum shows

very broad signals at 40 ppm and 0 ppm as well as a weak sharp resonance at -16 ppm. The C6D6 solution was homogenous with no insoluble precipitate visible. These data are consistent with the - + aryl C-F activation of the [B(C6F5)4] anion by the [(3,5-(CF3)2C6H3)2PF(C12F9)] cation to generate 4-17. Activation of fluorobenzene aryl C-F bonds by triethylsilylium carborane, 35 [SiEt3][CHB11Cl11] to generate the fluorosilane has been reported. The C-F activation requires - an extremely weak coordinating [CHB11Cl11] anion. The phenyl cation generated reacts with the carborane to form neutral CHB11Cl10-(μ-Cl)-Ph. The phenyl cation also reacts with additional equivalents of fluorobenzene to generate fluorobiphenyl. A carborane anion is not required in the case of 4-24, as the C-F activation is proposed to occur on the anion itself. The Lewis acidity of related tris[3,5-bis(trifluoromethyl)phenyl]borane has been evaluated by Ashley.36 The Lewis acidity was determined to be 6% greater than that of BCF by the Gutmann-Beckett method and 38% less by the Childs’ method. While this broad range is not conclusive, it suggests that

(3,5-(CF3)2C6H3) substituents may be more electron withdrawing than C6F5. In this case, 4-24 would be the most Lewis acidic mono-cationic fluorophosphonium, even stronger than 4-23. Additionally, 4-24 would have less steric encumbrance at the phosphorus centre than either 4-23 or [PF(CF5)3][B(C6F5)4], which would allow larger substrates to approach and the difluorophosphorane 4-17 would be favoured.

Air-stability of 4-20 – 4-23 and 4-25 were evaluated by exposing PhBr solutions of each in 5 mm NMR vials to air for successively longer periods of time. Most electrophilic 4-23 shows decomposition after one minute of air exposure and complete decomposition after five minutes. Both 4-20 and 4-22 show significant decomposition after two days of air exposure, while only minor onset of decomposition is observed by 31P{1H} or 19F NMR for 4-21 after 3 days. While 4-21 is similarly electrophilic to 4-20 and 4-22, it has the most steric bulk at the phosphorus centre.

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4.2.8 Catalysis

The catalytic competency of 4-20 – 4-23 was evaluated in a series of Lewis acid catalyzed reactions: dimerization of 1,1-diphenylethylene, hydrodefluorination of fluoroadamantane, deoxygenation of benzophenone, hydrosilylation of α-methylstyrene, dehydrocoupling of phenol and triethylsilane, as well as benzylation and hydrodefluorination of

4-trifluoromethylbromobenzene (Scheme 4.13). All catalytic runs were carried out in C6D6, with 2 mol% catalyst at room temperature unless otherwise noted.

In the dimerization of 1,1-diphenylethylene 4-23 completed the reaction in 30 min, 4-22 took 4.5 h, and 4-20 took 40 h to complete the reaction. Interestingly 4-21 showed no reaction at room temperature over multiple days, however upon gentle heating to 40 °C the reaction slowly proceeded over 21 d. The FIA of 4-21 was determined to be the lowest of the catalysts in question, though the small difference is not sufficient to account for dramatically different reactivity. The significant steric bulk provided by ortho-methyl groups likely hampers reactivity.

Catalysts 4-20, 4-22, and 4-23 were all able to rapidly effect the hydrodefluorination of

1-fluoroadamantane in the presence of one equivalent of HSiEt3 within 10 minutes. Catalyst 4-21 completed the reaction within 22 h upon heating the mixture to 40 °C.

Only 4-23 was able to effect the deoxygenation of benzophenone in the presence of two equivalents of HSiEt3 at room temperature, taking 2 h to complete. When the reaction mixtures were heated to 60 °C, 4-20 – 4-22 were able to complete the deoxygenation of benzophenone in 22 h, 24 h, and 20 h respectively. Interestingly the difference in reactivity between 4-20 – 4-22 is minimized at higher temperatures.

Catalysts 4-24 completed the dehydrocoupling of phenol with triethylsilane within 3 h. Again, less electrophilic 4-20 – 4-22 required significant heating to 100 °C in toluene solution to complete the reaction in 24 h, 28 h, and 24 h respectively.

The benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene with 3.6 equivalents of HSiEt3 was catalyzed by 4-23 at 60 °C after 4.5 h. Even upon heating the reaction mixtures to 130 °C, none of 4-20 – 4-22 were able to catalyze the reaction. Similarly, only 4-23 was able to

141 catalyze the hydrosilylation of α-methylstyrene with HSiEt3 after 1 d at room temperature. Catalysts 4-20 – 4-22 were all unreactive for hydrosilylation.

The relative competency of catalysts 4-20 – 4-23 is best exemplified in dimerization of 1,1-diphenylethylene, which gives the trend for increasing activity of 4-21 < 4-20 < 4-22 << 4-23. Interestingly, this is the exact same trend for increasing FIA values of the catalysts. Highly electrophilic catalyst 4-23 remains significantly more active than the remaining catalysts 4-20 – 4- 22. The notably low activity of 4-21 is likely the result of significant steric encumbrance hindering interaction with substrate. The reactivity differences between 4-22 and 4-20/4-21 is most pronounced at low temperatures, as their activities are nearly equal in both deoxygenation and dehydrocoupling reactions.

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

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4.3 Conclusion

A series of 2-perfluorobiphenyl-substituted phosphines iPr2P(C12F9) (4-1), (o-tol)2P(C12F9) (4-2),

Ph2P(C12F9) (4-3), (C6F5)2P(C12F9) (4-4), (3,5-(CF3)2C6H3)2P(C12F9) (4-5), Cy2P(C12F9) (4-6), and

Me2P(C12F9) (4-7), tBu2P(C12F9) (4-8), MeP(C12F9)2 (4-9) and PhP(C12F9)2 (4-10) were generated using lithium reagent LiC12F9. These represent the first series of perfluorobiphenyl-substituted

phosphines. Additionally, two perfluorobiphenyl-substituted 1,2-diphosphine, ((C12F9)BrP)2 (4-

11) and ((C12F9)PhP)2 (4-12), were synthesized using zinc reagent Zn(C12F9)2·PhMe. The synthesis of coupled perfluorobiphenyl phosphines was incompatible with a wide range of halophosphine

starting materials used. Phosphines 4-1 – 4-7 were oxidized by XeF2 to generate the corresponding difluorophosphorane species iPr2PF2(C12F9) (4-13), (o-tol)2PF2(C12F9) (4-14), Ph2PF2(C12F9)

(4-15), (C6F5)2PF2(C12F9) (4-16), (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17), Cy2PF2(C12F9) (4-18), or

Me2PF2(C12F9) (4-19) in excellent yield. The oxidation of Bis(2-nonafluorobiphenyl)phosphines

4-9 and 4-10 with XeF2 did not cleanly generating difluorophosphorane species. Both 4-9 and 4- 10 and their corresponding difluorophosphoranes exhibited limited solubility in common organic solvents. Oxidation of 1,2-diphosphines 4-11 and 4-12 resulted in P-P bond cleavage and the formation of corresponding tetrafluorophosphorane in 4-11 and trifluorophosphorane in 4-12. Difluorophosphoranes 4-13 – 4-18 were each reacted with fluoride abstracting agent

[SiEt3][B(C6F5)4] to form [iPr2PF(C12F9)][B(C6F5)4] (4-20), [(o-tol)2PF(C12F9)][B(C6F5)4] (4-21),

[Ph2PF(C12F9)][B(C6F5)4] (4-22), [(C6F5)2PF(C12F9)][B(C6F5)4] (4-23), [(3,5-

(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24), and [Cy2PF(C12F9)][B(C6F5)4] (4-25) respectively. The electrophilicity of 4-20 – 4-23 was evaluated by NMR chemical shift trends, general electrophilicity index ω values, fluoride ion affinity calculations, and competition experiments. The reactivity of catalysts 4-20 – 4-23 was further investigated in a wide array of transformations.

4.4 Experimental 4.4.1 General Experimental Methods

Air-sensitive manipulations were carried out under an atmosphere of dry, O2-free N2 using either

an MBraun MB Unilab Glovebox or a dual-manifold Schlenk line. Hexane, pentane, and Et2O were all purified using a Grubbs-type column system produced by Innovative Technology and

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dispensed into thick-walled Straus flasks equipped with Teflon greaseless stopcock and stored over 4 Å sieves. Tetrahydrofuran and benzene were each dried over sodium metal and benzophenone before being distilled to Schlenk bombs and stored over 4 Å sieves. Dichloromethane was dried using calcium hydride before being vacuum transferred to a Schlenk bomb. Deuterated solvents were degassed using three successive freeze-pump-thaw cycles. Other bulk solvents were degassed by three successive cycles of headspace-evacuation, and sonication. All elemental analyses were performed in house using a Perkin Elmer 2400 Series II CHNS Analyzer. All NMR data were collected on a Bruker Advance III 400 MHz, Agilent DD2 500 MHz or Agilent DD2 600 MHz spectrometer at 25 °C unless otherwise noted. NMR chemical shift data are given relative to an 1 13 11 19 31 external standard ( H, C: SiEt4; B: 15% BF3·Et2O; F: CFCl3; P: H3PO4). In some cases, 2-

D NMR techniques were employed to assign individual resonances. Zn(C12F9)2·PhMe was 18 synthesized using a modified literature procedure. A 1:1 mixture of Et2Zn and ZnCl2 in

pentane/Et2O is used in place of the EtZnCl reagent. Marks’ literature procedure was used in the synthesis of 2-bromoperfluorobiphenyl.2 A literature procedure was followed in the synthesis of

PF2(C6F5)3 and PF2Ph2(C6F5) used in the competition reactions. Halophosphines Br2P(C6F5) and 37 BrP(C6F5)2 were obtained by literature procedure and successive recrystallizations from Et2O. Other halophosphines were obtained from Sigma-Aldrich and used without further purification. Computational work was performed using resources provided by the Shared Hierarchical Academic Research Computing Network and Compute Canada.

The syntheses of mono(perfluorobiphenyl)phosphines 4-1 – 4-8 all follow a similar synthetic methodology; a sample procedure for 4-1 is provided below.

Synthesis of iPr2P(C12F9) (4-1): A 100 mL round-bottom flask was charged with BrC12F9 (200

mg, 0.5 mmol), 1:1 Et2O/pent. (50 mL), a stir bar, and topped with a rubber septum. Once

connected to a Schlenk line, the flask was cooled to -104 °C using a N2 / cyclohexane bath. A solution of nBuLi in hexane (0.2 mL, 2.5 M) was added dropwise by syringe. The solution was

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stirred at -104 °C for an addition 2 h. Next, a solution of iPr2PCl (76.3 mg, 0.5 mmol) in Et2O (5 mL) was added dropwise to the reaction flask by syringe. The solution was allowed to warm to room temperature overnight before the solvent was removed in vacuo yielding a white solid. The residue was taken up CH2Cl2 (10 mL) and filtered through a Celite plug. The solvent was reduced in vacuo and cooled to -35 °C to yield colourless crystals of 4-1 in good yield (195.1 mg, 90% yield)

1 H NMR (500 MHz, C6D6): δ 0.83 (d, 7 Hz, 3H, iPrCH3), 0.79 (d, 7 Hz, 6H, iPrCH3), 0.76 (d 7 31 1 19 Hz, 3H, iPrCH3) ppm. P { H} NMR (162 MHz, C6D6): δ 15.4 (s, br) ppm. F NMR (377 MHz, 3 3 4 C6D6): δ -125.6 (t, JFF = 22 Hz, F-3), -135.4 (s, br, 1F, F-6), -137.9 (td, JFF = 22 Hz, JFF = 7 Hz, 3 4 3 2F, F-2’/6’), -150.0 (td, JFF = 22, JFF = 7 Hz, 1F, F-4), -151.6 (t, JFF = 22 Hz, 1F, F-5), -151.7 (t, 3 JFF = 22 Hz, 1F, F-4’), -161.7 – -162.7 (m, 2F, F-3’/5’) ppm.

1 Synthesis of (o-tol)2P(C12F9) (4-2): (white solid, 72% yield). H NMR (500 MHz, C6D6): δ 7-10 3 4 3 – 7.08 (m, 2H, o-H tol), 6.98 (td, JHH= 7 Hz, JHH= 1Hz, 2H, p-H tol), 6.89 (t, JHH= 7 Hz, 2H, 3 4 31 1 m-H tol), 6.85 (t, JHH= 7 Hz, JPH= 7 Hz, 2H, m-H tol), 2.07 (s, 6H, CH3) ppm. P{ H} NMR 19 (162 MHz, C6D6): δ -25.6 (s, br) ppm. F NMR (377 MHz, C6D6): δ -122.8 – -123.7 (m, 1F, F-3), 3 3 -135.7 (s, br, 1F, F-6), -138.0 (t, JFF = 20 Hz, 2F, F-2’/6’), -151.2 (t, JFF = 21 Hz, 1F, F-5), -151.4 3 4 3 3 (td, JFF = 20 Hz, JFF = 6 Hz, 1F, F-4), -152.5 (t, JFF = 20 Hz, 1F, F-4’), -162.3 (td, JFF = 22 Hz, 4 13 1 1 JFF = 7 Hz, 2F, F-3’/5’) ppm. C{ H} NMR (126 MHz, C6D6): δ 150.1 (d, JFC = 253 Hz, C12F9), 1 1 145.5 (d, JFC = 252 Hz, C12F9), 144.3 (d, JFC = 250 Hz, C12F9), 141.7 (d, JPC = 27 Hz, tol), 141.4 1 1 1 (d, JFC = 258 Hz, C12F9), 141.3 (d, JFC = 254 Hz, C12F9), 131.4 (d, JFC = 252 Hz, C12F9), 132.6

(s, tol), 130.8 (d, JPC = 9 Hz, tol), 130.3 (d, JPC = 6 Hz, tol), 129.5 (s, tol), 126.2 (s, tol), 121.2 (m,

C12F9), 116.2 (m, C12F9), 107.0 (m, C12F9), 20.6 (s, tol) ppm.

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31 1 Synthesis of Ph2P(C12F9) (4-3): (light yellow solid, 85% yield). P{ H} NMR (162 MHz, C6D6): 19 δ -11.0 (s, br) ppm. F NMR (377 MHz, C6D6): δ -120.7 – -120.9 (m, 1F, F-3), -136.1 (s, br, 1F, 3 3 4 F-6), -138.9 (t, JFF = 20 Hz, 2F, F-2’/6’), -149.0 (td, JFF = 21 Hz, JFF = 8 Hz, 1F, F-4), -150.1 (t, 3 3 3 4 JFF = 21 Hz, 1F, F-5), -151.0 (t, JFF = 21 Hz, 1F, F-4’), -161.7 (td, JFF = 21 Hz, JFF = 8 Hz, 2F, F-3’/5’) ppm.

31 1 Synthesis of (C6F5)2P(C12F9) (4-4): (white solid, 78% yield). P{ H} NMR (162 MHz, C6D6): δ 19 3 -63.0 (br) ppm. F NMR (377 MHz, C6D6): δ -121.4 – -121.8 (m, 1F, F-3), -131.4 (t, JFF = 23 3 Hz, 4F, o-C6F5), -134.4 (s, br, 1F, F-6), -138.8 (t, JFF = 20 Hz, 2F, F-2’/6’), -147.4 (td, 3 4 3 JFF = 20 Hz, JFF = 7 Hz, 1F, F-4), -149.3 (t, JFF = 20 Hz, 2F, p-C6F5), -150.3 (s, br, 1F, 3 3 3 F-3), -150.4 (t, JFF = 20 Hz, F-4’), -160.46 (t, JFF = 20 Hz, 4F, m-C6F5), -161.08 (td, JFF = 20 Hz, 4 13 1 1 JFF = 7 Hz, 2F, F-3’/5’) ppm. C{ H} NMR (126 MHz, C6D6): δ 150.3 (d, JFC = 251 Hz, C12F9), 1 1 1 146.9 (d, JFC = 245 Hz, C6F5), 145.4 (d, JFC = 253 Hz, C12F9), 144.4 (d, JFC = 245 Hz, C12F9), 1 1 1 143.0 (d, JFC = 260 Hz, C12F9), 142.6 (d, JFC = 253 Hz, C6F5), 141.1 (d, JFC = 260 Hz, C12F9), 1 137.6 (d, JFC = 262 Hz, C6F5), 115.6 (m, C6F5), 105.5 (m, C12F9), 104.4 (m, C12F9) ppm.

147

Synthesis of (3,5-(CF3)2C6H3)2P(C12F9) (4-5): (white solid, 85% yield).

1 31 1 H NMR (400 MHz, C6D6): δ 7.71 (s, 2H, o-H), 7.70 (s, 2H, o-H), 7.60 (s, 2H, p-H) ppm. P{ H} 19 NMR (162 MHz, C6D6): δ -12.8 (br) ppm. F NMR (377 MHz, C6D6): δ -63.14 (s, 12F, 3 CF3), -121.35 – -121.56 (m, 1F, F-3), -132.87 (s, br, 1F, F-6), -139.2 (t, JFF = 20 Hz, 2F, F-2’/6’), 3 4 3 4 -143.3 (td, JFF = 20 Hz, JFF = 8 Hz, 1F, F-4), -147.4 (td, JFF = 20 Hz, JFF = 7 Hz, 1F, 3 13 1 F-5), -148.8 (t, JFF = 20 Hz, 1F, F-4’), -159.87 – -160.11 (m, 2F, F-3’/5’) ppm. C{ H} NMR 1 1 1 (126 MHz, C6D6): δ 149.5 (d, JFC = 251 Hz, C12F9), 146.0 (d, JFC = 252 Hz, C12F9), 144.1 (d, JFC 1 1 1 = 247 Hz, C12F9), 142.9 (d, JFC = 263 Hz, C12F9), 142.1 (d, JFC = 257 Hz, C12F9), 141.6 (d, JFC = 1 4 2 263 Hz, C12F9), 137.6 (d, JFC = 253 Hz, C12F9), 135.6 (d, JPC = 17 Hz, (CF3)2Ph), 132.5 (qd, JFC 3 2 1 = 33 Hz, JPC = 6 Hz, (CF3)2Ph), 131.8, JPC = 22 Hz, (CF3)2Ph), 123.6 (m, (CF3)2Ph),122.7 (q, JFC

= 272 Hz, (CF3)2Ph), 118.1 (m, C12F9), 116.9 (m, C12F9), 106.4 (m, C12F9) ppm.

1 Synthesis of Cy2P(C12F9) (4-6): (white solid, 89% yield). H NMR (400 MHz, C6D6): δ 1.77 – 31 1 1.40 (m, 6H, C6H11), 1.25 – 0.93 (m, 6H, C6H11). P{ H} NMR (162 MHz, C6D6) δ 5.9 (br) ppm. 19 3 F NMR (377 MHz, C6D6): δ -125.0 (t, JFF = 22, 1F, F-3), -135.4 (s, br, 1F, F-6), -137.85 (t, 3 3 4 3 JFF = 20 Hz, 2F, F-2’/6’), -149.8 (td, JFF = 20 Hz, JFF = 8 Hz, 1F, F-4), -151.5 (t, JFF = 20 Hz, 3 1F, F-5), -151.8 (t, JFF = 20 Hz, 1F, F-4’), -161.87 – -162.24 (m, 2F, F-3’/5’) ppm.

148

1 Synthesis of Me2P(C12F9) (4-7): (white solid, 80% yield). H NMR (500 MHz, C6D6): δ 1.26 (s, 31 1 19 br, 6H, CH3) ppm. P{ H} NMR (162 MHz, C6D6): δ -39.0 (br) ppm. F NMR (377 MHz, C6D6): 3 δ -129.4 – -129.7 m, 1F, F-3), -136.7 (s, br, 1F, F-6), -138.95 (t, JFF = 20 Hz, 2F, F-2’/6’), -152.7 3 3 4 3 (t, JFF = 20 Hz, 1F, F-5), -152.9 (td, JFF = 20 Hz, JFF = 8 Hz, 1F, F-4), -153.3 (t, JFF = 20 Hz, 3 4 13 1 1F, F-4’), -162.68 (td, JFF = 22 Hz, JFF = 7 Hz, 2F, F-3’/5’) ppm. C{ H} NMR (126 MHz, 1 1 1 C6D6): δ 147.2 (d, JFC = 251 Hz, C12F9), 144.8 (d, JFC = 254 Hz, C12F9), 144.0 (d, JFC = 250 Hz, 1 1 1 C12F9),143.3 (d, JFC = 251 Hz, C12F9), 134.7 (d, JFC = 256 Hz, C12F9), 141.3 (d, JFC = 253 Hz, 1 C12F9), 137.3 (d, JFC = 256 Hz, C12F9), 109.9 (m, C12F9), 106.5 (m, C12F9). 20.0 (CH3) ppm.

31 1 Synthesis of tBu2P(C12F9) (4-8): (light yellow solid, 26% yield). P{ H} NMR (162 MHz, C6D6): 19 3 4 δ -65.6(br) ppm. F NMR (377 MHz, C6D6): δ -115.7 (dd, JFF = 26 Hz, JFF = 13.3 Hz, 1F, 3 F-3), -126.9 (s, br, 1F, F-6), -138.86 – -139.09 (m, 2F, F-2’/6’), -148.60 (ddd, JFF = 24 Hz, 3 4 3 3 JFF = 17 Hz, JFF = 6 Hz, 1F, F-5), -150.4 (dd, JFF = 22 Hz, JFF = 20 Hz, 1F, F-4), -151.80 (t, 3 JFF = 21 Hz), -161.68 – -162.18 (m, 2F, F-3’/5’) ppm.

149

The synthesis of bis(perfluorobiphenyl)phosphines 4-9 and 4-10 both follow a similar methodology; a sample procedure for 4-9 is provided below.

Synthesis of MeP(C12F9)2 (4-9): Et2O (5 mL) was added dropwise to the reaction flask by syringe over 1 h using a syringe pump. The solution was maintained at -104 °C for another 2 before being allowed to warm to room temperature overnight. The solvent was removed in vacuo yielding a

white solid, that was taken up CH2Cl2 (10 mL) and filtered through a Celite plug. The Celite plug

was flushed with additional CH2Cl2 (10 mL) to ensure high recovery. The solvent was reduced in vacuo and cooled to -35 °C to yield white powder of 4-9 in moderate yield (195.1 mg, 90% yield).

1 2 31 1 H NMR (500 MHz, C6D6): δ 1.64 (d, JPH = 15 Hz, 3H, CH3) ppm. P{ H} NMR (162 MHz, 19 3 C6D6): δ -32.4 (br) ppm. F NMR (377 MHz, C6D6): δ -126.2 (s, br, 2F, F-3), -135.01 (d, JFF = 3 3 20 Hz, 2F, F-6), -137.1 (t, JFF = 26 Hz, 2F, F-2’), -139.6 (s, br, 2F, F-6’), -150.0 (t, JFF = 20 Hz 3 3 2F, F-4), -151.10 (t, JFF = 20 Hz, 2F, F-5), -151.35 (t, JFF = 21Hz, 2F, F-4’), -161.42 – -161.62 (m, 4F, F-3’/5’) ppm.

1 Synthesis of PhP(C12F9)2 (4-10): H NMR (400 MHz, C6D6): δ 7.07 (dd, J = 9.8, 7.3 Hz, 1H), 31 1 19 6.90 – 6.78 (m, 1H) ppm. P{ H} NMR (162 MHz, C6D6): δ -23.7 (br) ppm. F NMR (377 MHz, 3 3 C6D6): δ -124.28 (s, br, 2F, F-4), -135.22 (d, JFF = 22 Hz, 2F, F-6), -137.41 (t, JFF = 19 Hz, 2F, 3 3 4 F-2’), -138.85 (t, JFF = 20 Hz, 2F, F-6’), -148.26 (td, JFF = 22 Hz, JFF = 6 Hz, 2F, F-4), -149.8 3 3 3 4 (t, JFF = 20 Hz, 2F, F-5), -150.48 (t, JFF = 21 Hz, 2F, F-4’), -161.09 (td, JFF = 23 Hz, JFF = 8 3 4 Hz, 2F, F-3’), -161.27 (td, JFF = 24 Hz, JFF = 8 Hz, 2F, F-5’) ppm.

150

The syntheses of 4-11 and 4-12 were both carried out using the same methodology, a sample procedure of 4-11 is provided below.

Synthesis of ((C12F9)BrP)2 (4-11): A 4 dram vial is charged with PBr3 (54.1 mg, 0.2 mmol), pentane (10 mL), and a stir bar. Solid Zn(C12F9)2·PhMe (78.8 mg, 0.1 mmol) was added to the stirring reaction mixture. The reaction was stirred for 16 h before being removing the solvent in vacuo, yielding a light yellow solid. The solid was taken up in CH2Cl2 (5 mL) and filtered through

a Celite plug. The plug was flushed with additional CH2Cl2 (5 mL) and the filtrates were collected, reduced in vacuo, and cooled to -35 °C in the freezer to yield 4-11 in good yield (71.3 mg, 84% yield)

31 1 1 3 19 P{ H} NMR (162 MHz, C6D6): δ 116.4 (tt JPP = 33 Hz, JPF = 7 Hz) ppm. F NMR (377 MHz,

C6D6): δ -120.5 – 120.7 (m, 1F, F-3), -136.8 – 137.0 (m, 1F, F-6), -138.0 – -138.2 (m, 2F, 3 4 3 4 F-2’/6’), -144.3 (td, JFF = 21 Hz, JFF = 11 Hz, 1F, F-4), -146.8 (td, JFF = 22 Hz, JFF = 7 Hz, 1F, 3 F-5), -148.1 (t, JFF = 22 Hz, 1F, F-4’), -159.2 – -159.4 (m, 2F, F-3’/5’) ppm.

1 3 Synthesis of ((C12F9)PhP)2 (4-12): H NMR (400 MHz, C6D6): δ 7.48 (t, JHH = 8 Hz, 2H, o-H 31 1 19 Ph), 7.06 – 6.97 (m, 3H, o/p-H Ph) ppm. P{ H} NMR (162 MHz, C6D6): 66.5 (br) ppm. F

NMR (377 MHz, C6D6): δ -122.3 – -122.5 (m, 1F, F-3), -136.4 – -136.6 (m, 1F, 3 4 F-6) -137.7 – -137.9 (m, 1F, F-2’), -138.8 – 139.0 (m, 1F, F-6’), -146.6 (td, JFF = 21 Hz, JFF =

151

3 3 10 Hz, 1F, F-4), -148.3 – -148.4 (m, 1F, F-5), -149.8 (t, JFF = 21 Hz, 1F, F-4’), -160.0 (td, JFF = 4 3 4 21 Hz, JFF = 9 Hz, 1F, F-3’), -161.0 (td, JFF = 21 Hz, JFF = 9 Hz, 1F, F-3’) ppm.

Difluorophosphoranes 4-13 – 4-19 were all prepared following a similar synthetic strategy, a sample procedure for 4-13 is provided below.

31 1 1 Synthesis of iPr2PF2(C12F9) (4-13): P{ H} NMR (162 MHz, C6D6): δ -19.3 (t, JPF = 702 Hz) 19 1 ppm. F NMR (377 MHz, C6D6): δ -40.1 (d, JPF = 702 Hz, PF2), -124.7 – -124.9 (m, 1F, F-3), -135.0 – -135.2 (m, 1F, F-6), -136.7 – -136.9 (m, 2F, F-2’/6’), -151.2 – -151.4 (m, 1F, F-4), 3 4 3 152.4 (td, JFF = 21 Hz, JFF = 9 Hz, F-5), -152.9 (t, JFF = 21 Hz, F-4’), -163.2 – -163.4 (m, 2F, F-3’/5’) ppm.

31 1 1 Synthesis of (o-tol)2PF2(C12F9) (4-14): P{ H} NMR (162 MHz, C6D6): δ -41.6 (t, JPF = 670 19 Hz). F NMR (377 MHz, C6D6): δ -126.4 (s, br, 1F, F-3), -133.2 (s, br, 1F, F-2’), -133.9 (s, br, 3 3 1F, F-6’), -134.8 (s, 1F, F-6), -151.3 (t, JFF = 18 Hz, 1F, F-5), -151.5 (t, JFF = 18 Hz, 1F, 3 13 1 F-4), -151.9 (t, JFF = 20 Hz, 1F, F-4’), -163.1 (s, br, 1F, F-3’), -164.4 (s, br, 1F, F-5’) ppm. C{ H} 1 1 NMR (126 MHz, C6D6): 147.6 (d, JFC = 250 Hz, C12F9), 146.2 (d, JFC = 252 Hz, C12F9), 109.1 (d, 1 1 1 JFC = 246 Hz, C12F9), 142.2 (d, JFC = 251 Hz, C12F9), 140.7 (d, JFC = 253 Hz, C12F9), 137.5 (d, 1 JFC = 250 Hz, C12F9), 137.5 (s, tol), 132.3 (s, tol), 132.0 (s, tol), 128.9 (s, tol), 128.1 (s, tol), 125.3

(s, tol), 118.6 (m, C12F9), 117.9 (m, C12F9), 105.8 (m, C12F9), 21.1 (s, tol) ppm.

152

31 1 1 Synthesis of Ph2PF2(C12F9) (4-15): P{ H} NMR (162 MHz, C6D6): δ -58.5 (t, JPF = 700 Hz) 19 1 ppm. F NMR (377 MHz, C6D6): δ -38.1 (d, JPF = 756 Hz, 2F, PF2), -130.2 (s, br, 1F, F-3), -135.8 3 3 4 4 (s, br, 1F, F-6), -136.6 (d, JFF = 22 Hz, 2F, F-2’/6’), -151.5 (tdd, JFF = 21 Hz, JFF = 9 Hz, JFF = 3 3 6 Hz, 1F, F-4), -152.9 (t, JFF = 22 Hz, 1F, F-5), -153.2 (t, JFF = 21 Hz, 1F, F-4’), -162.9 – -163.1 (m, 2F, F-3’/5’) ppm.

31 1 1 Synthesis of (C6F5)2PF2(C12F9) (4-16): P{ H} NMR (162 MHz, C6D6): δ -47.8 (t, JPF = 707 19 Hz) ppm. F NMR (377 MHz, C6D6): δ -124.4 – 124.7 (m, 1F, F-3), -131.6 – 131.7 (m, 1F, F-6), 3 4 4 -132.5 (s, br, 2F, F-2’/6’), -145.4 (s, br, 4F, o-C6F5), -145.6 (tdd, JFF = 22 Hz, JFF = 8 Hz, JFF = 3 4 4 5 Hz, 1F, F-4), -148.9 (tdd, JFF = 21 Hz, JFF = 10 Hz, JFF = 5 Hz, 1F, F-5), -159.2 (s, br, 4F, m-

C6F5), -162.2 (s, br, 2F, F-2’/6’) ppm.

31 1 Synthesis of (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17): P{ H} NMR (162 MHz, C6D6): δ -62.7 (t, 1 19 1 JPF = 707 Hz) ppm. F NMR (377 MHz, C6D6): δ -36.6 (d, JPF = 707 Hz, 2F, PF2), -36.2 (s, br, 3 12F, CF3), -129.5 (s, br, 1F, F-3), -134.4 (s, br, 1F, F-6), -136.3 (d, JFF = 20 Hz, 2F, F-2’/6’), -147.2 3 3 – -147.4 (m, 1F, F-4), -147.9 (t, JFF = 22 Hz, 1F, F-5), -148.8 (t, JFF = 20 Hz, F1, F-4’), -161.2 3 4 (td, JFF = 21 Hz, JFF = 6 Hz, 2F, F-3’/5’) ppm.

153

1 1 1 Synthesis of Cy2PF2(C12F9) (4-18): P{ H} NMR (162 MHz, C6D6): δ -25.5 (t, JPF = 694 Hz) 19 1 ppm. F NMR (377 MHz, C6D6): -42.8 (d, JPF = 694 Hz, 2F, PF2), -124.1 – -124.3 (m, 1F, F-3), 3 -135.1 – -135.3 (m, 1F, F-6), -137.0 – -137.2 (m, 2F, F-2’/6’), -152.1 (t, JFF = 20 Hz, 1F, 3 4 3 F-4), -152.5 (td, JFF = 20 Hz, JFF = 7 Hz, 1F, F-5), -153.3 (t, JFF = 20 Hz, 1F, F-4’), -163.2 – -163.3 (m, 2F, F-3’/5’) ppm.

31 1 1 Synthesis of Me2PF2(C12F9) (4-19): P{ H} NMR (162 MHz, C6D6): δ -22.8 (tsep., JPF = 614 2 19 Hz, JPH = 9 Hz) ppm. F NMR (377 MHz, C6D6): δ -129.7 (s, br, 1F, F-3), -136.0 (s, br, 1F, F-6), 3 3 4 4 -136.8 (d, JFF = 21 Hz, 2F, F-2’/6’), -149.5 (tdd, JFF = 21 Hz, JFF = 9 Hz, JFF = 6 Hz, 1F, F-4), 3 3 4 -150.6 (t, JFF = 22 Hz, 1F, F-4’), -151.1 (td, JFF = 21 Hz, JFF = 6 Hz, 1F, F-5), -161.5 – -161.7 (m, 2F, F-3’/5’) ppm.

31 1 1 Synthesis of [iPr2PF(C12F9)][B(C6F5)4] (4-20): P{ H} NMR (162 MHz, C6D6): δ 143.6 (d, JPF 19 = 1020 Hz) ppm. F NMR (377 MHz, C6D6): δ -120.2 (s, br, 1F, F-3), -123.2 (s, br, 1F, 3 F-6), -132.0 – -132.2 (m, 1F, F-4), -133.2 (s, 8F, B(o-C6F5)), -138.1 (d, JFF = 20 Hz, 2F, F-2’/6’), 3 -142.4 – -142.6 (m, 1F, F-5), -145.7 (t, JFF = 21 Hz, 1F, F-4’), -158 – -158.2 (m, 2F, 3 1 F-3’/5’), -163.8 (t, JFF = 20 Hz, 4F, B(p-C6F5)), -167.7 (t, br, 8F, B(m-C6F5)), -171.5 (dt, JPF = 4 11 1 1020 Hz, JFF = 17 Hz, 1F, PF) ppm. B{ H} NMR (128 MHz, C6D6): δ -16.7 (s) ppm.

154

31 1 Synthesis of [(o-tol)2PF(C12F9)][B(C6F5)4] (4-21): P{ H} NMR (162 MHz, C6D6): δ 95.3 (d, 1 19 1 JPF = 1012 Hz) ppm. F NMR (377 MHz, C6D6): δ -116.0 (s, br, 1F, F-3), -119.8 (d, JPF = 1012 3 4 Hz, 1F, PF), -125.2 (s, br, 1F, F-6), -131.9 (td, JFF = 23 Hz, JFF = 11 Hz, 1F, F-4), -133.2 (s, 8F, 3 B(o-C6F5)), -135.7 (s, br, 2F, F-2’/6’), -142.8 – -143.0 (m, 1F, F-5), -145.5 (t, JFF = 20 Hz, 1F, 3 4 3 F-4’), -158.1 (td, JFF = 22 Hz, JFF = 10 Hz, 2F, F-3’/5’), -163.9 (t, JFF = 20 Hz, 4F, B(p-C6F5)), 3 11 1 -167.7 (t, JFF = 18 Hz, 8F, B(m-C6F5)) ppm. B{ H} NMR (128 MHz, C6D6): δ -16.7 (s) ppm.

1 Synthesis of [Ph2PF(C12F9)][B(C6F5)4] (4-22): H NMR (500 MHz, C6D6): δ 7.04 – 7.00 (m, 2H, 31 1 1 p-CH5), 6.92 – 6.76 (m, 4H, o,m-CH5) ppm. P{ H} NMR (162 MHz, C6D6): δ 90.7 (dd, JPF = 3 19 1 1012 Hz, JPF = 7 Hz) ppm. F NMR (377 MHz, C6D6): δ -116.1 (s, 1F, F-3), -120.9 (d, JPF = 3 4 1012 Hz, 1F, PF), -125.7 (s, 1F, F-6), -131.9 (td, JFF = 20 Hz, JFF = 13 Hz, 1F, F-4), -133.4 (s, 3 3 8F, B(o-C6F5)), -136.7 (d, JFF = 18 Hz, 2F, F-2’/6’), -143.6 (t, JFF = 22 Hz, 1F, F-5), -146.0 (t, 3 3 4 3 JFF = 20 Hz, 1F, F-4’), -158.5 (td, JFF = 21 Hz, JFF = 7 Hz, 2F, F-3’/5’), -164.0 (t, JFF = 20 Hz, 3 11 1 4F, B(p-C6F5)), -167.9 (t, JFF = 20 Hz, 8F, B(m-C6F5)) ppm. B{ H} NMR (128 MHz, C6D6): δ

-16.7 (s) ppm. Anal. Calcd. for C48H10BF30P: C: 48.11% H: 0.84% Found: C: 48.20%, H: 0.82%.

155

31 1 Synthesis of [(C6F5)2PF(C12F9)][B(C6F5)4] (4-23): P{ H} NMR (162 MHz, C6D6): δ 72.3 (d, 1 19 JPF = 1034 Hz) ppm. F NMR (377 MHz, Benzene-d6): δ -117.5 (s, br, 1F, F-3), -121.0 – -121.2 1 (m, 2F, P(p-C6F5)), -122.0 (s, br, 1F, F-6), -124.7 (d, JPF = 1034 Hz, 1F, PF), -124.8 (s, br, 4F, 3 P(o-C6F5)), -125.3 – 125.5 (m, 1F, F-4), -133.5 (s, br, 8F, B(o-C6F5)), -138.5 (d, JPF = 17 Hz, 2F, 3 3 F-2’/6’), -141.2 – -141.4 (m, 1F, F-5), -143.4 (t, JPF = 20 Hz, 1F, F-4’), -149.6 (td, JPF = 17 Hz, 4 3 4 JPF = 7 Hz, 4F, P(m-C6F5)), -156.7 (td, JPF = 21 Hz, JPF = 7 Hz, 2F, F-3’/5’), -164.0 (t, -149.6 3 3 11 1 (t, JPF = 20 Hz, 4F, B(p-C6F5)), -168.0 (t, br, JPF = 18 Hz, 8F, B(m-C6F5)) ppm. B{ H} NMR

(128 MHz, C6D6): δ -16.7 (s) ppm.

31 1 Synthesis of [(3,5-(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24): P{ H} NMR (162 MHz, C6D6): 1 19 δ 90.2 (d, JPF = 1025 Hz) ppm. F NMR (377 MHz, C6D6): δ -63.7 (s, 12F, CF3), -133,6 (s, br, 1 1F, F-3), -121.2 (s, br, 1F, F-6), -124.4 – 124.6 (m, 1F, F-4), -125.2 (d, JPF = 1025 Hz, 1F, 3 PF), -133.2 (s, br, 8F, B(o-C6F5)), -136.2 (d, JFF= 20 Hz, 2F, F-2’/6’), -139.6 – -139.8 (m, 1F, 3 3 3 F-5), -142.4 (t, JFF= 21 Hz, 1F, F-4’), -156.3 (t, JFF= 20 Hz, 2F, F-3’/5’), -163.7 (t, JFF= 20 Hz, 11 1 4F, B(p-C6F5)), -167.7 (s, br, 8F, B(m-C6F5)) ppm. B{ H} NMR (128 MHz, C6D6): δ -16.7 (s) ppm.

156

31 1 1 Synthesis of [Cy2PF(C12F9)][B(C6F5)4] (4-25): P{ H} NMR (162 MHz, C6D6): δ 133.8 (d, JPF 19 = 1005 Hz) ppm. F NMR (377 MHz, C6D6): δ -120.3 (s, br, 1F, F-3), -123.7 (s, br, 1F, 3 F-6), -132.9 – -133.1 (m, 1F, F-4), -133.2 (s, br, 8F, B(o-C6F5)), -137.8 (d, JFF= 18 Hz, 2F, 3 3 3 F-2’/6’)), -143.0 (t, br, JFF= 21 Hz, 1F, F-5), -146.0 (t, JFF= 21 Hz, 1F, F-4’), -158.2 (td, JPF = 4 3 3 20 Hz, JPF = 6 Hz, 2F, F-3’/5’), -163.8 (t, JFF= 20 Hz, 4F, B(p-C6F5)), -167.7 (t, br, JPF = 18 Hz, 1 11 1 8F, B(m-C6F5)) -170.0 (d, JPF = 1005 Hz, 1F, PF) ppm. B{ H} NMR (128 MHz, C6D6): δ -16.7 (s) ppm.

Hydrodefluorination of 1-fluoroadamantane: To a vial containing 2 mol% catalyst was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was added to a vial containing 1-fluoroadamantane (15.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Dehydrocoupling of triethylsilane with phenol: To a vial containing 2 mol% catalyst was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was added to a vial containing phenol (9.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene: To a vial containing

2 mol% catalyst was added a solution of HSiEt3 (41.8 mg, 0.36 mmol) in C6D6 (0.7 mL). The solution was added to a vial containing 4-trifluoromethylbromobenzene (22.5 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Deoxygenation of benzophenone: To a vial containing 2 mol% catalyst was added a solution of

HSiEt3 (23.2 mg, 0.2 mmol) in C6D6 (0.7 mL). The solution was added to a vial containing

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benzophenone (18.2 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Hydrosilylation of α-methylstyrene with triethylsilane: To a vial containing 2 mol% catalyst was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was added to a vial containing α-methylstyrene (11.8 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Dimerization of 1,1-diphenylethylene: To a vial containing 2 mol% catalyst was added a solution

of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in C6D6 (0.7 mL). The solution was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

4.4.2 X-ray Crystallography

4.4.2.1 X-ray Data Collection and Reduction Crystals were coated in Paratone-N oil in a glovebox before being mounted on a MiTegen

Micromount under an N2 stream to maintain a dry, O2 free environment for each crystal. Diffraction data were collected on a Bruker Apex II diffractometer using a graphite monochromator with Mo Kα (λ = 0.71073 Å) radiation. Temperature was maintained at 150(2) K using an Oxford cryo-stream cooler. Data collection strategies were determined using Bruker Apex II software. Frame integration was carried out using Bruker SAINT software. Data absorbance correction was carried out using the empirical multiscan method SADABS. Structure solutions were obtained by direct methods and refined using SHELXTL or Olex2 software.38,39 Refinement was carried out using full-matrix least squares techniques to convergence of weighting parameters. When data quality was sufficient, all non-hydrogen atoms were refined anisotropically.

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4.4.2.2 X-Ray Tables

4-3 4-20 4-21

Formula BrC12F9P1 C43H14BF30P C50H14BF30P Weight (g/mol) 851.99 1130.33 1226.42 Crystal system triclinic triclinic monoclinic

Space group P-1 P-1 P21/c a (Å) 6.655(3) 10.783(4) 12.9172(14) b (Å) 10.255(5) 11.138(5) 12.7837(15) c (Å) 10.986(6) 18.067(7) 27.407(3) α (°) 106.692(16) 105.862(12) 90 β (°) 107.193(11) 101.677(11) 90.857(5) γ (°) 100.037(12) 96.101(11) 90 Volume (Å3) 658.0(6) 2013.6(14) 4525.2(9) Z 1 2 4 Density (calcd.) (gcm–3) 2.1499 1.8641 1.8000 R(int) 0.0356 0.0485 0.0376 μ, mm–1 3.347 0.244 0.255 F(000) 406 1113 2419 Index ranges -7 ≤ h ≤ 8 -13 ≤ h ≤ 14 -16 ≤ h ≤ 16 -12 ≤ k ≤ 12 -14 ≤ k ≤ 14 -13 ≤ k ≤ 13 -13 ≤ l ≤ 13 -23 ≤ l ≤ 23 -35 ≤ l ≤ 35 Radiation Mo Kα Mo Kα Mo Kα θ range (min, max) (°) 2.08, 26.49 2.04, 29.52 2.18, 27.51 Total data 2706 9182 10399 Max peak 0.5 0.8 0.5 Min peak -0.6 -0.8 -0.6 2 >2(FO ) 2114 6164 7260 Parameters 208 672 741 R (>2σ) 0.0288 0.0582 0.0397

Rw 0.0653 0.1801 0.0891 GoF 1.088 0.969 1.044

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4-22 4-23 (C12F9)POF2, H2O

Formula C48H10BF30P C42HBF35OP C12F11OP, H2O Weight (g/mol) 1198.36 1228.22 418.10 Crystal system monoclinic triclinic monoclinic

Space group P21/n P-1 P21/c a (Å) 12.1923(15) 12.7430(13) 12.6412(9) b (Å) 13.0218(14) 13.9662(14) 6.1671(4) c (Å) 27.387(3) 14.9737(16) 18.1472(11) α (°) 90 67.639(5) 90 β (°) 92.785(6) 80.300(5) 99.477(2) γ (°) 90 81.745(5) 90 Volume (Å3) 4343.0(9) 2420.0(4) 1395.44(16) Z 4 2 4 Density (calcd.) (gcm–3) 1.8326 1.6854 1.9900 R(int) 0.0542 0.0328 0.0784 μ, mm–1 0.232 0.226 0.338 F(000) 2355 1194 817 Index ranges -14 ≤ h ≤ 14 -16 ≤ h ≤ 16 -16 ≤ h ≤ 16 -15 ≤ k ≤ 15 -18 ≤ k ≤ 18 -8 ≤ k ≤ 7 -32 ≤ l ≤ 27 -19 ≤ l ≤ 19 -23 ≤ l ≤ 23 Radiation Mo Kα Mo Kα Mo Kα θ range (min, max) (°) 2.16, 25.25 1.63, 27.71 2.28, 27.61 Total data 7757 11092 3207 Max peak 0.9 0.6 0.8 Min peak -0.6 -0.5 -0.8 2 >2(FO ) 5511 8047 2336 Parameters 721 726 239 R (>2σ) 0.0437 0.0464 0.0454

Rw 0.1028 0.1466 0.1257 GoF 1.096 1.026 1.071

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4.5 References

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Formation and Structures of the Methyl-bridged Complexes Cp*TiMe2(μ-Me)B(C6F5)3 and

[Cp*TiMe2(μ-Me)TiMe2Cp*][MeB(C6F5)3]. J. Organomet. Chem. 1997, 527, 7. 5. Sun, Y. M.; Spence, R. E. V. H.; Piers, W. E.; Parvez, M.; Yap, G. P. A., Intramolecular Ion-Ion Interactions in Zwitterionic Metallocene Olefin Polymerization Catalysts Derived from

''Tucked-In'' Catalyst Precursors and the Highly Electrophilic Boranes XB(C6F5)2 (X=H, C6F5). J. Am. Chem. Soc. 1997, 119, 5132. 6. Chen, E. Y. X.; Marks, T. J., Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure-Activity Relationships. Chem. Rev. 2000, 100, 1391. 7. Childs, R. F.; Mulholland, D. L.; Nixon, A., The Lewis Acid Complexes of α,β- Unsaturated Carbonyl and Nitrile Compounds .1. A Nuclear Magnetic-Resonance Study. Can. J. Chem. 1982, 60, 801. 8. Chase, P. A.; Piers, W. E.; Patrick, B. O., New Fluorinated 9-Borafluorene Lewis Acids. J. Am. Chem. Soc. 2000, 122, 12911. 9. Lancaster, S. J.; Walker, D. A.; Thornton-Pett, M.; Bochmann, M., New Weakly

Coordinating Counter Anions for High Activity Polymerisation Catalysts: [(C6F5)3B-CN- - 2- B(C6F5)3] and [Ni{CNB(C6F5)3}4] . Chem. Commun. 1999, 1533.

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10. Binding, S. C.; Zaher, H.; Chadwick, F. M.; O'Hare, D., Heterolytic Activation of Hydrogen Using Frustrated Lewis Pairs Containing Tris(2,2',2''-Perfluorobiphenyl)Borane. Dalton Trans. 2012, 41, 9061.

11. Geier, S. J.; Stephan, D. W., Lutidine/B(C6F5)3: At the Boundary of Classical and Frustrated Lewis Pair Reactivity. J. Am. Chem. Soc. 2009, 131, 3476. 12. Ashley, A. E.; Thompson, A. L.; O'Hare, D., Non-Metal-Mediated Homogeneous

Hydrogenation of CO2 to CH3OH. Angew. Chem., Int. Ed. 2009, 48, 9839. 13. Travis, A. L.; Binding, S. C.; Zaher, H.; Arnold, T. A. Q.; Buffet, J. C.; O'Hare, D., Small Molecule Activation by Frustrated Lewis Pairs. Dalton Trans. 2013, 42, 2431.

14. Menard, G.; Tran, L.; Stephan, D. W., Activation of H2 Using P/Al Based Frustrated Lewis Pairs and Reactions with Olefins. Dalton Trans. 2013, 42, 13685.

15. Menard, G.; Stephan, D. W., H2 Activation and Hydride Transfer to Olefins by Al(C6F5)3- Based Frustrated Lewis Pairs. Angew. Chem., Int. Ed. 2012, 51, 8272. 16. Coe, P. L.; Stephens, R.; Tatlow, J. C., Aromatic Polyfluoro-Compounds .11. Pentafluorophenyl-Lithium and Derived Compounds. J. Chem. Soc. 1962, 3227. 17. Pohlmann, J. L.; Brinckma, F. E., Pentafluorophenyl-Metal Chemistry .2. Preparation and Characterization of Group 3a Derivatives. Z. Naturforsch., B: Chem. Sci. 1965, B 20, 5. 18. Guerrero, A.; Martin, E.; Hughes, D. L.; Kaltsoyannis, N.; Bochmann, M., Zinc(II) η1- and 2 η -Toluene Complexes: Structure and Bonding in Zn(C6F5)2·(toluene) and Zn(C6F4-2-

C6F5)2·(toluene). Organometallics 2006, 25, 3311. 19. Tsuzuki, S.; Uchimaru, T.; Mikami, M., Intermolecular Interaction Between Hexafluorobenzene and Benzene: Ab Initio Calculations Including CCSD(T) Level Electron Correlation Correction. J. Phys. Chem. A 2006, 110, 2027. 20. Matter, H.; Nazare, M.; Gussregen, S.; Will, D. W.; Schreuder, H.; Bauer, A.; Urmann, M.; Ritter, K.; Wagner, M.; Wehner, V., Evidence for C-Cl/C-Br ··· π Interactions as an Important Contribution to Protein-Ligand Binding Affinity. Angew. Chem., Int. Ed. 2009, 48, 2911. 21. Dumont, W. W.; Kubiniok, S.; Severengiz, T., Properties of Te-Te Bonds .4. Dismutation Reactions of Di-P-Tolylditelluride with Tetra-tert-Butyldiphosphane and Tetra- Isopropyldiphosphane. Z. Anorg. Allg. Chem. 1985, 531, 21.

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22. Alder, R. W.; Canter, C.; Gil, M.; Gleiter, R.; Harris, C. J.; Harris, S. E.; Lange, H.; Orpen, A. G.; Taylor, P. N., Medium-Ring Diphosphines from Diphosphabicyclo[k.l.0]alkanes: Stereoselective Syntheses, Structure and Properties. J. Chem. Soc., Perkin Trans. 1 1998, 1643. 23. Fild, M.; Hollenbe, I.; Glemser, O., Reaktionen Der Pentafluorophenyl- Phosphorhalogenide. Sci. Nat. 1967, 54, 89. 24. Green, M.; Tauntonr, A.; Stone, F. G. A., Chemistry of Metal Carbonyls .56. Reactions of Bis(Pentafluorophenyl)Phosphine and Tetrakis(Pentafluorophenyl)Diphosphine with Group 6 Metal Hexacarbonyls. J. Chem. Soc. 1969, 1875. 25. Jeffrey, G. A., An Introduction to Hydrogen Bonding. Oxford University Press: New York, 1997; p 303. 26. Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W., Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor. Science 2013, 341, 1374. 27. Caputo, C. B.; Winkelhaus, D.; Dobrovetsky, R.; Hounjet, L. J.; Stephan, D. W., Synthesis and Lewis Acidity of Fluorophosphonium Cations. Dalton Trans. 2015, 44, 12256. 28. Gaussian 09, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox; Gaussian Inc.: Wallingford CT, 2016. 29. (a) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J., Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force- Fields. J. Phys. Chem. 1994, 98, 11623; (b) Kim, K.; Jordan, K. D., Comparison of Density- Functional and MP2 Calculations on the Water Monomer and Dimer. J. Phys. Chem. 1994, 98, 10089; (c) Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct

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Asymptotic-Behavior. Phys. Rev. A 1988, 38, 3098; (d) Lee, C. T.; Yang, W. T.; Parr, R. G., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron- Density. Physical Review B 1988, 37, 785. 30. (a) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L., 6-31G* Basis Set for Atoms K Through Zn. J. Chem. Phys. 1998, 109, 1223; (b) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A., 6-31G* Basis Set for Third-Row Atoms. J. Comput. Chem. 2001, 22, 976; (c) Ditchfield, R.; Hehre, W. J.; Pople, J. A., Self-Consistent Molecular-Orbital Methods .9. Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724; (d) Hehre, W. J.; Ditchfield, R.; Pople, J. A., Self-Consistent Molecular- Orbital Methods .12. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular-Orbital Studies of Organic-Molecules. J. Chem. Phys. 1972, 56, 2257. 31. Headgordon, M.; Pople, J. A.; Frisch, M. J., MP2 Energy Evaluation by Direct Methods. Chem. Phys. Lett. 1988, 153, 503. 32. (a) Weigend, F., Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057; (b) Weigend, F.; Ahlrichs, R., Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297. 33. Sigma-Aldrich best-available-prices (CAD): benzene-d6 = $540.00 / 100g, methylene chloride-d2 = $407.50 / 25g. Methylene chloride-d2 approximately 4.6x more expensive than benzene-d6 per unit volume. http://www.sigmaaldrich.com/chemistry/stable-isotopes- isotec/stable-isotope-products.html, accessed April, 2017. 34. Zhu, J.; Perez, M.; Caputo, C. B.; Stephan, D. W., Use of Trifluoromethyl Groups for Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination. Angew. Chem., Int. Ed. 2016, 55, 1417. 35. Duttwyler, S.; Douvris, C.; Fackler, N. L. P.; Tham, F. S.; Reed, C. A.; Baldridge, K. K.; Siegel, J. S., C-F Activation of Fluorobenzene by Silylium Carboranes: Evidence for Incipient Phenyl Cation Reactivity. Angew. Chem., Int. Ed. 2010, 49, 7519.

36. Herrington, T. J.; Thom, A. J. W.; White, A. J. P.; Ashley, A. E., Novel H2 Activation by a Tris[3,5-bis(trifluoromethyl)phenyl]borane Frustrated Lewis Pair. Dalton Trans. 2012, 41, 9019. 37. Cowley, A. H.; Pinnell, R. P., Pentafluorophenyl-Phosphorus Ring System. J. Am. Chem. Soc. 1966, 88, 4533.

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Chapter 5 Binaphthyl Fluorophosphonium Cations 5.1 Introduction Chiral molecules represent a crucial facet of synthetic organic chemistry, specifically with respect to the development of medicinal compounds. Different enantiomers of the same compound can have vastly different therapeutic outcomes in biological systems. The most infamous example of a compound with divergent in vivo effects is α-phthalimido glutarimide, thalidomide, a drug initially marketed as a sedative in the 1950s. The (R)-enantiomer has anti-nausea properties and thalidomide found off-label use in combating morning sickness in pregnant women. Unfortunately, thalidomide is capable of crossing the placental barrier and the (S)-enantiomer was soon found to be a powerful teratogen as thousands of children were born with limb-reduction deformities.1 While chiral separation would be fruitless in the case of thalidomide, given its rapid interconversion of enantiomers in vivo, it nevertheless highlights the significance of stereospecificity in synthesis.2

Certain methods of obtaining enantiopure compounds, such as chiral resolution of racemic mixtures, are inherently inefficient. Crystallization of enantiopure compounds can take many successive cycles and yield only 50% of a desired enantiomer from a racemic mixture. Catalytic asymmetric induction offers a much more efficient route to enantioenriched or enantiopure compounds. Chiral catalysts can be used to generate an enantiomeric excess of one product from achiral reagents, resolve a chiral mixture by preferential reaction with only one enantiomer, or approach enantioconvergence by stereoablation of chiral centres and selective reformation of a single enantiomer.3

The first reported instance of catalytic chiral induction was by Marckwald in 1904.4 The decarboxylation of 2-ethyl-2-methylmalonic acid was catalyzed in the presence of brucine, a chiral organic alkaloid (Scheme 5.1). The enantiomeric enrichment of the product was measured by rotary polarization, which indicated a 16% excess of L-enantiomer.

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Scheme 5.1. Chiral induction in decarboxylation reaction by brucine.

The first asymmetric hydrogenation catalyst was reported by Knowles and Sabacky in 1968.5 The chiral phosphines (-)-methylpropylphenylphosphine and bis(2-methylbutyl)phenylphosphine were used to generate rhodium catalysts that were employed in the hydrogenation of methylenesuccinic acid and 2-phenylacrylic acid. The phosphine with chirality centre on the phosphorus centre yielded up to 15% enantiomeric excess, while the phosphine with the chiral alkyl substituents gave near racemic mixtures. Despite the modest chiral induction observed, the authors were aware of significance of their work, concluding:

“The inherent generality of this method offers almost unlimited opportunities for matching substrates with catalysts in a rational manner and we are hopeful that our current effort will result in real progress towards complete stereospecificity”.

This prediction by Knowles and Sabacky has been thoroughly wrought out. Transition-metal catalysts have since been used extensively to carry out asymmetric transformations including hydrogenations6, cross-couplings7, and hydrosilylations8 using a vast array of chiral ligands. One such chiral ligand, developed by Noyori and Takaya, is 2,2’-bis(diphenylphosphino)-1,1’- binaphthyl or BINAP. Bisphosphine BINAP is atropisomeric, deriving chirality from hindered rotation about the bond between naphthyl groups.9 A series of BINAP-rhodium catalysts were tested for activity in the hydrogenation of α-(N-acylamino)-acrylic acids. The BINAP systems proved to be very effective chiral hydrogenation catalysts, able to generate enantiopure N-benzoylphenylalanine in high yield (Scheme 5.2).

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Scheme 5.2. Chiral BINAP rhodium hydrogenation catalyst

The highly modular synthesis of many transition-metal complexes allows for the facile tuning of chiral environment around the active site for specific transformations. Fine-tuning is enabled by the availability of a wide library of commercially-available and literature reported chiral ligands.10 However, there are drawbacks associated with the use of metal catalysts, including cost and toxicity associated with some systems.11 The development of highly active chiral main group catalysts is an attractive avenue to provide a wider variety of options for any given transformation.

Metal-free catalysts have also seen significant improvement since the first report by Marckwald, both in range of reactions and in chiral induction. With the advent of FLP catalysis, many different approaches have been taken towards highly enantioselective transformations. A significant challenge in chiral main group catalysis is balancing the activity of a catalyst with chiral

inductivity. In most examples, electron-withdrawing groups like C6F6 are replaced with weakly donating chiral substituents like α-pinene, camphor, or binaphthyl.12.13

The group of Du and coworkers have made significant contributions to the development of chiral FLP systems. In 2013, Du investigated an extensive range aryl-substituted binaphthyl dienes that were reacted with Piers’ borane to generate chiral FLP catalysts in situ.14 The generated binaphthyl diboranes were used to reduce a variety of imines under a hydrogen atmosphere. The aryl substituents ranged widely in chiral induction, with the larger substituents proving to be the most effective. This rigorous screening approach to chiral FLP catalyst design yielded an enantiomeric excess of 89%, the highest that had been observed for FLP systems.

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In 2016, Du further refined the diborane FLP catalytic system by using binaphthyl diyne starting materials instead of dienes for reaction with Piers’ borane.15 The chiral diyne derived catalysts were used in the hydrosilylation of 1,2-diketones and α-keto esters with incredibly high enantiomeric excess of 99% (scheme 5.3). The chiral diyne systems resulted in much higher yields and enantiomeric excess than the corresponding binaphthyl diene systems.

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

Maruoka and coworkers have also exploited the chirality of the binaphthyl moiety to effect asymmetric amination of β-keto esters using phosphonium catalysts.16 Dibutylphosphine was reacted with a binaphthyl dibromide starting material to generate the quaternary tetraalkylphosphonium bromide catalyst. The phosphonium catalyst was able to act as a chiral phase transfer catalyst in the amination of β-keto esters in high yield with moderate enantiomeric excesses (Scheme 5.4).

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

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Given the vast array of commercially available chiral phosphines, investigating chiral fluorophosphonium systems seems to be a natural extension. Use of binaphthyl containing fluorophosphonium systems is attractive given the success of the moiety to induce asymmetry in both transition metal and metal-free catalysis (vide supra). Point chiral phosphines have the

disadvantage of racemizing during the synthesis of oxidation by XeF2 and subsequent fluoride

abstraction by [SiEt3][B(C6F5)4], since the chirality of BINAP resides on the ligand, the initial enantiomer is retained through the synthesis of fluorophosphonium cations.

Additionally, it has been shown that systems containing two closely linked fluorophosphonium centres exhibit enhanced reactivity.17 A series of commercially available diphosphines with varied alkyl linkers was converted to the corresponding difluorophosphonium salts and tested in a series of Lewis acid catalytic applications. The activity of the dications decreased drastically as the linker length between phosphonium centres exceeded two carbon atoms. While BINAP does not contain

electron-withdrawing C6F5 moieties, the cooperative effect of two phosphorus centres held in relatively close proximity may overcome the low electrophilicity of each individual fluorophosphonium centre.

5.2 Results and Discussion 5.2.1 Synthesis and Characterization of BINAP Fluorophosphonium Cation

Two equivalents of solid XeF2 were added to a stirring solution of (±)-BINAP, which resulted in effervescence and a slight darkening of the solution. The solution was stirred for 2 h before removing the solvent in vacuo yielding a white crystalline powder 5-1. The 19F NMR spectrum of 5-1 shows a doublet at -35.9 ppm, indicative of clean oxidation of both phosphorus centres. The 31P{1H} NMR spectrum of 5-1 displays a single triplet at -52.9 ppm with 694 Hz coupling, consistent with two phosphorus-bound fluorine substituents. The 1H NMR displays many overlapping aryl resonances. All these data are consistent with the formulation of 5-1 as

2,2’-bis(diphenyldifluorophosphorano)-1,1’-binaphthyl, ((Ph2F2P)C10H6)2 (Scheme 5.5).

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

Unsymmetrically-substituted fluorophosphonium/phosphine binaphthyl could act as a chiral intramolecular FLP. In an effort to synthesize 2-(diphenylphosphino)-2’- (diphenyldifluorophosphorano)-1,1’-binaphthl, (±)-BINAP was reacted with a single equivalent of XeF2 in a CH2Cl2 solution. Upon removing the solvent in vacuo an oily residue remains. Multiple products were observed by 31P and 19F NMR. Even at low temperatures, the attempted oxidation of a single BINAP phosphorus centre yielded an intractable mixture of products.

A solution of 5-1 in toluene was added to two molar equivalents of [SiEt3][B(C6F5)4]·PhMe as a slurry in toluene, resulting a colourless suspension of immiscible oil. The suspension stirred for 2 h before decanting the toluene from the oily product. The colourless oil is washed several times with toluene, then several times with pentane before being triturated in pentane to yield a while powder 5-2. The 31P{1H} NMR spectrum of 5-2 displays a doublet at 97.4 ppm with 993 Hz coupling, consistent with the fluoride abstraction from both difluorophosphoranes centres. The 19F - NMR spectrum displays a doublet at -128.2 ppm and a set of resonances attributable to [B(C6F5)4] anion. These NMR data infer the formulation of 5-2 as bis(fluorophosphonium)

[((Ph2FP)2C10H6)2]2[B(C6F5)4] (Scheme 5.6).

Scheme 5.6. Synthesis of fluorophosphonium salt 5-2.

Attempts were made to obtain unsymmetrically substituted fluorophosphonium- difluorophosphorane binaphthyl from 5-1. This species was targeted to test for an interaction

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between the fluorophosphonium centre and the fluoride of the linked difluorophosphoranes. Such interactions have been observed for linked borane-phosphonium systems by Gabbai and more recently by our group.18,19 When 5-1 was added to an equimolar amount of

[SiEt3][B(C6F5)4]·PhMe in toluene, a colourless oil is generated. The solvent was removed in 31 19 vacuo and the residue was taken up in CH2Cl2. The resulting P NMR and F NMR spectra were consistent with the presence of 5-2 and 5-1, no mixed difluorophosphorane-fluorophosphonium product was observed.

Reaction of (±)-BINAP with one or two molar equivalents of Selectfluor in CH2Cl2 yields no reaction due to the very low solubility of Selectfluor in most common organic solvents, aside from MeCN. If MeCN is employed as solvent, the reaction of 5-1 with one or two molar equivalents proceeds to generate an intractable mixture of products.

The reaction of (s)-BINAP with two equivalents of NFSI in CH2Cl2 cleanly generates a single product with similar 31P{1H} and 19F NMR features to those of 5-2, consistent with direct formation of bis(fluorophosphonium) [((Ph2FP)2C10H6)2]2[N(SO2Ph)2] 5-2b (Scheme 5.7, top).

When a single equivalent of NFSI was added to a CH2Cl2 solution of BINAP a different fluorophosphonium species is generated. The 31P{1H} NMR displays a doublet 93.6 ppm with 1003 Hz coupling and a doublet at -15.7 ppm with smaller 137 Hz coupling which are consistent with the formation of mixed fluorophosphonium-phosphine binaphthyl salt

[((Ph2FP)C10H6)(C10H6(PPh2)][N(SO2Ph)2], 5-2c (Scheme 5.7, bottom). A small impurity of 31 1 [((Ph2FP)2C10H6)2]2[N(SO2Ph)2] is visible in the P{ H} NMR spectrum at 96.3 ppm. Because

the chirality resides on the C2-symmetric binaphthyl linker, only a single enantiomer is formed.

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Scheme 5.7. Synthesis of bis(fluorophosphonium) 5-2b (top) and fluorophosphonium-phosphine 5-2c (bottom) from BINAP using NFSI.

5.2.2 Electrophilicity of BINAP-derived Fluorophosphonium Cation

A CH2Cl2 solution of 5-2 and OPEt3 was made to evaluate the Lewis acidity of 5-2 by the Gutmann-Beckett method. The 31P{1H} NMR spectrum shows a doublet resonance at 97.4 ppm

and a broad OPEt3 signal at 56 ppm, close to the reference shift of OPEt3 in CH2Cl2 is 50.9 ppm.

The near negligible shift of 5.1 ppm and the broad OPEt3 is indicative that 5-2 is incompatible with the Gutmann-Beckett method. This finding is consistent with the less electrophilic phosphonium cations discussed previously (Chapter 3, pg. 82 and Chapter 4, pg. 131). This observation is also consistent with previously reported incompatibility of closely related 20 [PFPh3][B(C6F5)4] with the Gutmann-Beckett method.

5.2.3 Catalytic Activity of BINAP-derived Fluorophosphonium Cation

The catalytic activity of 5-2 was evaluated in the dimerization of 1,1-diphenylethylene,

hydrodefluorination of 1-fluoroadamantane, and hydrosilylation of α-methylstyrene with HSiEt3 (Scheme 5.8). All catalytic reactions were performed using 2 mol% 5-2 (4 mol% fluorophosphonium centre) at room temperature unless otherwise noted. The hydrodefluorination of 1-fluoradamantane was complete before an NMR spectrum could be obtained. Catalyst 5-2

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proved to be competent in the dimerization of 1,1-diphenylethylene, completing the reaction in 3 h. Unfortunately, even upon heating the reaction to 130 °C in bromobenzene, no hydrosilylation

of α-methylstyrene with HSiEt3 was brought about by 5-2.

Despite the absence of electron-withdrawing aryl substituents, 5-2 proved to be considerably active. The two fluorophosphonium centres in 5-2 are separated by four carbon atoms, but 5-2 shows comparable reactivity to alkyl-linked bis(fluorophosphonium) catalysts with lengths of two or three carbon atoms. The comparable reactivity at a longer linker length is likely due to a reduction of rotational degrees of freedom by the aryl backbone in 5-2, promoting closer interaction between fluorophosphonium cations.17 Unfortunately, 5-2 was not sufficiently Lewis acidic to effect hydrosilylation, which would be an ideal test reaction for a chirality induction by a fluorophosphonium cation.

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

5.2.4 Synthesis of Pentafluorophenyl Binaphthyl Fluorophosphonium Cation

Since the Lewis acidity of 5-2 was insufficient to effect the hydrosilylation of α-methylstyrene, which could be influenced by chiral induction, a more electrophilic analogue of 5-2 was targeted. A 2011 report by the Wu group, outlined the synthesis of two electrophilic bis(diarylphosphino)binaphthyl species, substituted with 3,5-bis(trifluoromethyl)phenyl or 4- trifluoromethylphenyl groups.21 The group obtained the binaphthyl bis-Grignard reagent by

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reacting 2,2’-dibromo-1,1’-binaphthyl with Mg turnings in THF and toluene, before adding to it the desired diarylbromophosphine (Scheme 5.9, top). The use of diarylchlorophosphines resulted in very poor yields compared to the analogous diarylbromophosphine reagents. This synthetic route was noted to be particularly effective for synthesis of electron-deficient systems. Using this

approach, two equivalents of bis(pentafluorophenyl)bromophosphine, (C6F5)2PBr, were reacted with the binaphthyl di-Grignard reagent in a toluene/THF solution, resulting in the immediate change from a pale-yellow slurry to a clear yellow solution. A 31P{1H} NMR spectrum obtained of the reaction mixture display displays two major signals at -36.2 and -74.9 ppm and a minor

signal at -54.4 ppm. The major peak at -36.2 ppm is a triplet, consistent with the loss of a C6F5

substituent from the (C6F5)2PBr starting material. The other major product peak at -74.9 ppm is a septet, coupling with six equivalent ortho-fluorine atoms, consistent with increased C6F5

substitution of (C6F5)2PBr and indeed matches the chemical shift of an authentic sample of 2 P(C6F5)3. These data suggested the formation of a phosphole (C6F5)P(κ -C20H12), 5-3 (Scheme 5.9, bottom).

Scheme 5.9 Reaction of binaphthyl Grignard reagent with BrP(3,5-(CF3)2C6H3)2 (top, literature 21 procedure ), and BrP(C6F5)2 to form 5-3 (bottom).

The two major products, 5-3 and P(C6F5)3 could both be isolated by chromatography on a silica column, eluting with 4:1 pentane to Et2O. Phosphine 5-3 slowly oxidizes in air to form 2 (C6F5)PO(κ -C20H12) 5-4 (Scheme 5.10). Phosphine oxide 5-4 displays a broad singlet at 19.6 ppm 31 1 19 in the P{ H} NMR spectrum and resonances attributable to a C6F5 substituent in the F spectrum.

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F F O F P F P F as solid, 50 °C, 4 h F F F F 5-3 F 5-4 Scheme 5.10. Air-oxidation of 5-3 to form phosphine oxide 5-4

The solid-state structure was obtained for 5-4, which confirmed the connectivity of 5-4 and 5-3 (Figure 5.1). The solid-state structure of 5-4 displays distorted tetrahedral geometry at the phosphorus centre. The bite angle of the naphthyl substituent is 92.90(7)°, indicating moderate distortion away from the ideal angle of 109.5°. The binaphthyl substituent shows significant twisting, with an angle of 47° between the planes of the two furthest separated aryl rings.

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

Solid XeF2 was added to a colourless solution of 5-3, which resulted in the solution immediately changing colour to yellow. The solution was stirred for two hours before removing the solvent in

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vacuo to yield a yellow solid. The 31P{1H} NMR spectrum shows a triplet at -33.2 ppm with 874 19 Hz coupling. The F NMR spectrum displays a broad doublet at 43.6 ppm as well as three C6F5 2 resonances. All NMR data are consistent with the formation of (C6F5)PF2(κ -C20H12), 5-5 (Scheme 5.11).

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

A toluene solution of 5-5 was added to a slurry of [SiEt3][B(C6F5)4]·PhMe in toluene, yielding a red solution. The product was sufficiently soluble in toluene that decanting the supernatant was not possible, instead the solvent was removed in vacuo and the residue was triturated with pentane to yield a red powder 5-5. The 19F NMR spectrum of 5-5 displays a doublet at -126.9 ppm and two 1 sets of C6F5 resonances that integrate 4:1. The H NMR spectrum displays a series of overlapping aryl resonances and the 31P{1H} NMR spectrum displays a doublet at 82.5 ppm. The formulation 2 of 5-6 as [(C6F5)PF(κ -C20H12)][B(C6F5)4] was supported by all these NMR data (Scheme 5.12).

Scheme 5.12. Synthesis of fluorophosphonium salt 5-6.

As noted previously with the synthesis of 5-2, the chirality of 5-5 is derived from the C2-symmetric ligand so the same chirality is maintained from the binaphthyl starting material throughout

Grignard reagent, the oxidation and fluoride abstraction. If chirality was derived from a non-C2- symmetric bridging ligand or the chirality existed at the phosphorus centre, as in (-)-methylpropylphenylphosphine, fluorophosphonium cations generated through a difluorophosphorane intermediate would form enantiomers or diastereomers. These types of chiral

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phosphines would only be amenable to fluorination of phosphines directly to fluorophosphonium cations by reagents like NFSI, Selectfluor or fluoropyridinium salts. Electron-deficient phosphines required to make active fluorophosphonium catalysts are typically not amenable to direct fluorination by weaker reagents.

5.2.5 Mechanism for Phosphole Synthesis

Binaphthylphenylphosphole can be generated in 25% yield through the pericyclic [4+1]

McCormack reaction of neat dichlorophenylphosphine, PhPCl2, with 22 3,3’,4,4’-tetrahydrobinaphthyl at 220 °C. Alternatively, reaction of PhPCl2 with 2,2’-dilithio-1,1’-binaphthyl proceeds at room temperature to yield the cyclized product at room

temperature in good yield. However, the direct loss of C6F5 and Br from (C6F5)2PBr in a pericyclic

reaction and reaction of anionic C6F5 with (C6F5)2PBr is unlikely. Instead, sequential σ-bond metatheses offer a precise route to the observed products, and is consistent with reported magnesium reactivity.23

A possible mechanism for the concomitant formation of 5-3 and P(C6F5)3 from (C6F5)2PBr and di-Grignard-binaphthyl begins with the σ-bond metathesis between the Mg-naphthyl and P-Br to form an naphthyl-P bond and MgBr2. A second σ-bond metathesis between Mg-naphthyl and

P-C6F5 to generate 5-3 and BrMgC6F5. Finally, reaction of BrMgC6F5 with a second equivalent of

(C6F5)2PBr to generate P(C6F5)3 and MgBr2 (Scheme 5.13).

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

5.2.6 Electrophilicity of Perfluorophenyl Binaphthyl Fluorophosphonium.

5.2.6.1 Gutmann-Beckett Method To subject 5-6 to the Gutmann-Beckett protocol, a solution containing three equivalents of the 1 Lewis acid and one molar equivalent of OPEt3 in CH2Cl2 was prepared. The P{ H} NMR spectrum

displays a doublet attributable to free 5-6 and no downfield shift is observed in the signal of OPEt3. As with other less electrophilic fluorophosphonium cations, 5-6 does not appear to be amenable to quantification by the Gutmann-Beckett method.

5.2.6.2 Fluoride Ion Affinity The optimized geometries for the cation of 5-6 ([5-6]+) and the corresponding fluoride adduct 5-5 were calculated with Gaussian 0924 using the hybrid functional B3LYP25 and split valence basis set 6-311+G(d).26 The optimized geometries were used to determine the single point energies using 27 28 MP2 and def2-TZVPP. The LUMO of 5-6 is delocalized across the binaphthyl and C6F5 rings with a large lobe centralized on the phosphorus centre (Figure 5.2).

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

The LUMO and HOMO energies of [5-6]+ were determined to be -3.130 and -10.873 eV respectively. The ω value was determined to be 3.166 eV, though the unique substituent set on phosphorus hampers meaningful comparison to other fluorophosphonium cations using ω values. To allow for more broad comparison of electrophilicity, the FIA of [5-6]+ was calculated to be 651 kJ/mol. While the FIA of [5-6]+ is still significantly greater than the calculated FIA of BCF (451 kJ/mol, by the same computational protocol), it is lower than any of the FIA values of fluorophosphonium cations studied in Chapter 3 (pg. 87) and Chapter 4 (pg. 136). Fluorophosphonium bearing two unsubstituted phenyl groups and one perhalophenyl group,

specifically [PF(C6Cl5)Ph2][B(C6F5)4] (3-10) and [PF(C6Cl5)Ph2][B(C6F5)4] (4-22) display higher fluoride ion affinities than 5-6 at 686 and 713 kJ/mol, respectively. The lower FIA of 5-6 compared to 3-10 despite the greater electron-withdrawing power of C6F5 compared to C6Cl5 suggests that the κ2-binaphthyl has a negative impact on electrophilicity. The κ2-binaphthyl bite angle on phosphorus was measured to be 92.90(7)°, 97.0° and 95.2° in the solid-state structure of 5-4, the optimized geometry of 5-5, and the optimized geometry of 5-6 respectively. The constrained binaphthyl angle is much closer to the ideal tetrahedral bond angle (109.5°) than the ideal trigonal bipyramidal bond angle of 120° for equatorial substituents. The small angle disfavours the five- coordinate fluoride-adduct 5-5, resulting in a decreased FIA of 5-6. The reverse effect has been

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observed in which a small constrained substituent angle on a boron centre enhanced the Lewis acidity by disfavouring the trigonal planar geometry.29

5.2.7 Reactivity of a Perfluoroaryl Binaphthyl Fluorophosphonium Cation

Compound 5-6 was tested for catalytic activity in the hydrodefluorination of 1-fluoroadamantane,

the dimerization of 1,1-diphenylethylene, and the hydrosilylation of α-methylstyrene with HSiEt3 (Scheme 5.14). Hydrodefluorination of 1-fluoroadamantane occurred rapidly with 2 mol% of 5-6

in CH2Cl2, the reaction was complete before an NMR spectrum could be obtained. The dimerization of 1,1-diphenylethylene was catalyzed by 5-6 in 4 h, which is slower than 5-2. Similarly, 5-6 was insufficiently Lewis acidic to effect the hydrosilylation of α-methylstyrene.

Despite the enhanced electron-withdrawing ability of the C6F5 substituent on 5-6, the overall Lewis acidic reactivity is marginally worse than 5-2. Catalyst 5-2 benefits from two cooperative fluorophosphonium centres, while 5-6 is hampered by a small bite angle bidentate binaphthyl substituent that disfavours the five-coordinate reaction intermediate 5-5.

Scheme 5.14. Summary of reactivity for 5-6.

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5.3 Conclusion Two chiral binaphthyl fluorophosphonium cations were synthesized and characterized. The first

fluorophosphonium was obtained by oxidation of BINAP with XeF2 to yield the

bis(difluorophosphorane) species (PF2Ph2)2(C20H12) (5-1). Fluoride abstraction of 5-1 with

[SiEt3][B(C6F5)4]·PhMe results in the formation of bis(fluorophosphonium) (5-2). In catalytic applications, 5-2 was found to be competent in hydrodefluorination and dimerization of diphenylethylene, but not able to effect hydrosilylation. In an effort to generate a more Lewis acidic binaphthyl fluorophosphonium, BrP(C6F5)2 was reacted with a binaphthyl Grignard reagent. Instead of the expected BINAP analogue, cyclized pentafluorophenyl binaphthylphosphole 2 (C6F5)P(κ -C20H12) (5-3) was generated with concomitant formation of P(C6F5)3. Oxidation of 5-3 2 in air led to the formation of phosphine oxide (C6F5)PO(κ -C20H12) (5-4). Oxidation of 5-3 using 2 XeF2 resulted in difluorophosphorane (C6F5)PF2(κ -C20H12) (5-5), which was subsequently 2 converted to fluorophosphonium salt [(C6F5)PF(κ -C20H12)][B(C6F5)4] (5-6). The fluoride ion affinity of 5-6 was evaluated to be lower than similarly substituted compounds and the catalytic activity was found to be lower than that of bis(fluorophosphonium) cation 5-2.

5.4 Experimental 5.4.1 General Experimental Methods

Air-sensitive manipulations were carried out under an atmosphere of dry, O2-free N2 using either

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1 13 11 19 31 ( H, C: SiEt4; B: 15% BF3·Et2O; F: CFCl3; P: H3PO4). In some cases, 2-D NMR techniques

were employed to assign individual resonances. Both [SiEt3][B(C6F5)4]·PhMe and BrP(C6F5)2 was synthesized by literature procedure.30,31 Binaphthyl starting materials ((±)-2,2’-dibromo-1,1’- binaphthylene, (S)- and (±)-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP)) and N-Fluorobenzenesulfonimide (NFSI) were all obtained from Sigma-Aldrich and used without further purification. Computational work was performed using resources provided by the Shared Hierarchical Academic Research Computing Network and Compute Canada.

Synthesis of (PF2Ph2)2(C20H12) (5-1): To a solution of racemic 2,2’-(diphenylphosphino)-1,1’- binaphthyl ((±)-BINAP, 62.3 mg, 0.1 mmol) in a 20 mL vial, was added solid XeF2 (33.8 mg, 0.2 mmol). Effervescence was immediately observed in the clear colourless solution. The reaction was stirred for 2 h before the solvent was reduced to a minimum in vacuo. Recrystallization at -35 °C yielded 5-1 as a crystalline white solid in excellent yield (68.3 mg, 98% yield).

1 H NMR (400 MHz, C6D6): δ 7.85 – 7.71 (m, 10H), 7.48 – 7.40 (m, 6H), 7.13 – 6.91 (m, 16H). 31 1 1 19 P{ H} NMR (162 MHz, C6D6): δ -52.9 (d, JPF = 694 Hz). F NMR (377 MHz, C6D6): δ -35.9 1 (d, JPF = 694 Hz).

Synthesis of [(PFPh2)2(C20H12)]2[B(C6F5)4] (5-2): To a stirred slurry of [SiEt3][B(C6F5)4]·PhMe (86.9 mg, 0.1 mmol) in toluene (2 mL) in a 4 dram vial, was added a solution of 5-1 (68.3 mg, 0.1 mmol) in toluene (2 mL). The white slurry rapidly changed to a dark red oily suspension. The reaction stirred for 2 h before allowing the red oil to settle and decanting the supernatant. The red

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oil was washed with an aliquot of toluene (1 mL) and several washes of pentane (3 x 1 mL), before being triturated in pentane to yield 5-2 as a white powder in excellent yield (187.4 mg, 95% yield).

1 3 3 H NMR (400 MHz, CD2Cl2) δ 8.41 (d, JHH = 8 Hz, 1H, H-8), 8.40 (d, JHH = 8 Hz, 1H, H-8’), 3 3 8.11 (t, JHH = 8 Hz, 2H, H-4,4’), 8.09 (d, JHH = 9 Hz, 2H, H-5,5’), 7.84 – 7.67 (m, 10H), 7.53 – 3 3 7.47 (m, 4H, p-C6H5), 7.30 – 7.19 (m, 8H), 7.04 (t, JHH = 8 Hz, 2H, H-6,6’), 6.60 (d, JHH = 8 Hz, 31 1 1 19 2H, H-3,3’). P{ H} NMR (162 MHz, CD2Cl2) δ 97.4 (d, JPF = 998 Hz). F NMR (377 MHz, 1 3 CD2Cl2) δ -128.2 (d, JPF = 998 Hz, 1F, PF), -132.97 (s, 8F, B(o-C6F5), -163.3 (t, JFF = 20 Hz, 3 B(p-C6F5), -167.4 (t, JFF = 16 Hz, B(m-C6F5)).

The synthesis of the bis(phosphonium) cation 5-2b and monophosphonium-phosphine binaphthyl 5-2c species are identical aside from the stoichiometry of NFSI added, a sample procedure is provided below.

Synthesis of [(PFPh2)2(C20H12)]2[N(S(SO2Ph)2] (5-2b): A 1 dram vial was charged with (S)-

BINAP (18.6 mg, 0.03 mmol), CH2Cl2 (1 mL) and a stirbar. To the stirring solution was added a

solution of NFSI (19 mg, 0.06 mmol) in CH2Cl2. The solution immediately turned yellow colour. The solution was stirred for 1 h before the solvent was removed in vacuo to yield a white yellow powder 5-2b. The yellow powder was washed with pentane to give 5-2b in excellent yield (33.8 mg, 90%).

31 1 1 19 P{ H} NMR (162 MHz, CD2Cl2) δ 96.0 (d, JPF = 1000 Hz). F NMR (377 MHz, CD2Cl2) 1 δ -128.9 (d, JPF = 1000 Hz).

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Synthesis of [(PFPh2)(PPh2)(C20H12)][N(S(SO2Ph)2] (5-2c): (yellow powder, 84%)

31 1 1 P{ H} NMR (162 MHz, CD2Cl2) δ 93.3 ppm (d, JPF = 993 Hz, PF), -15.7 (d, JPF = 129 Hz, P). 19 1 F NMR (377 MHz, CD2Cl2) δ -125.2 (dd, JPF = 994 Hz, JPF = 18 Hz).

2 Synthesis of (C6F5)P(κ -C20H12) (5-3): A 100 mL round-bottom flask was charged with magnesium turnings (25 mg, 1.0 mmol), THF (20 mL), and a stirbar. To the stirring magnesium turnings was added 1,2-dibromoethane dropwise until effervescence was observed. The reaction flask was heated to 60 °C and to it was added dropwise a solution of 2,2’-dibromo-1,1’- binaphthylene (206.6 mg, 0.5 mmol) in toluene (20 mL) over 2 h. As the 2,2’-dibromo-1,1’- binaphthylene solution was added, the reaction mixture changed to a light-yellow slurry. The slurry was stirred at 60 °C for an additional 2 h, before being cooled to room temperature. To the reaction

slurry was added a solution of BrP(C6F5)2 (44.5 mg, 1.0 mmol) which resulted in a homogenous yellow solution. The product mixture was then chromatographed on silica eluting with 4:1

pentane/Et2O. After P(C6F5)3 had eluted with the solvent front; the following eluent was collected and the solvent was removed in vacuo to yield 5-3 as a white solid in poor yield (143.0 mg, 32 % (from halophosphine).

31 1 3 19 P{ H} NMR (162 MHz, C6D6): δ -36.2 ppm (t, JPF = 31 Hz). F NMR (377 MHz, C6D6): 3 4 δ -130.1 – -130.3 (m, 2F, o-C6F5), -149.9 (tt, JFF = 21 Hz, JFF = 4 Hz, 1F, p-C6F5), -160.3 – -160.5

(m, 2F, m-C6F5).

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2 Synthesis of (C6F5)PO(κ -C20H12) (5-4): A 1 dram vial was charged with solid 5-3 (15 mg, 0.03 mmol) and heated to 50 °C with the vial open to air. After 4 h, 5-4 is obtained in quantitative yield. Initially, 5-4 was obtained from the in-air chromatography of the reaction mixture from 5-3 synthesis (vide supra), which was eluted from the silica column using THF. Slow evaporation of

Et2O from a solution of 5-4 yielded bright yellow block crystals suitable for X-ray diffraction.

31 1 19 3 P{ H} NMR (162 MHz, C6D6): δ 19.6 ppm (s). F NMR (377 MHz, C6D6): δ -129.7 (d, JFF = 3 21 Hz, 2F, o-C6F5), -147.9 (t, JFF = 21 Hz, 1F, p-C6F5), -159.9 – -160.1 (m, 2F, m-C6F5).

2 Synthesis of (C6F5)PF2(κ -C20H12) (5-5): To a solution of racemic 5-3 (90 mg, 0.2 mmol) in a

4 dram vial, was added solid XeF2 (33.8 mg, 0.2 mmol). The solution immediately turned bright orange while effervescence was formed. The reaction was stirred for 2 h before the solvent was reduced to a minimum in vacuo. Recrystallization at -35 °C yielded 5-5 as a crystalline orange solid in excellent yield (68.3 mg, 98% yield).

31 1 1 19 P{ H} NMR (162 MHz, C6D6): δ -33.2 ppm (t, JPF = 874 Hz). F NMR (377 MHz, C6D6): 1 δ -43.6 (d, br, JPF = 874 Hz, 1F, PF), -133.9 – -135.4 (m, 2F, o-C6F5), -150.3 – -152.4 (m, 1F, p-

C6F5), -159.2 – -160.9 (m, 2F, m-C6F5).

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2 Synthesis of [(C6F5)PF2(κ -C20H12)][B(C6F5)4] (5-6): To a stirred slurry of

[SiEt3][B(C6F5)4]·PhMe (86.9 mg, 0.1 mmol) in toluene (2 mL) in a 4 dram vial, was added a solution of 5-5 (48.8 mg, 0.1 mmol) in toluene (2 mL). The white slurry rapidly changed to a dark red oily solution. The reaction stirred for 2 h before removing the solvent in vacuo to yield a sticky red residue. The red residue was washed with several washes of cold pentane (3 x 1 mL), before being triturated in pentane to yield 5-6 as a red powder in excellent yield (108.3 mg, 94% yield).

31 1 1 19 P{ H} NMR (162 MHz, CD2Cl2) δ 82.5 ppm (d, JPF = 1080 Hz). F NMR (377 MHz, CD2Cl2) 4 1 4 δ -123.8 (q, JFF = 14 Hz, 2F, P(o-C6F5)), -126.9 (dt, JPF = 1080 Hz, JFF = 13 Hz, 1F, PF), -130.2 3 – -130.4 (m, 1F, P(p-C6F5)), -133.01 (s, 8F, B(o-C6F5)), -152.7 (t, JFF = 21 Hz, 2F, P(m-C6F5)), - 3 3 163.7 (t, JFF = 21 Hz, 4F, P(p-C6F5)), -167.5 (t, JFF = 14 Hz, 8F, P(m-C6F5)).

Hydrodefluorination of 1-fluoroadamantane: To a vial containing 2 mol% catalyst was added

a solution of HSiEt3 (11.6 mg, 0.1 mmol) in CH2Cl2 (0.7 mL). The solution was added to a vial containing 1-fluoroadamantane (15.4 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Hydrosilylation of α-methylstyrene with triethylsilane: To a vial containing 2 mol% catalyst was added a solution of HSiEt3 (11.6 mg, 0.1 mmol) in PhBr (0.7 mL). The solution was added to a vial containing α-methylstyrene (11.8 mg, 0.1 mmol), which was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

Dimerization of 1,1-diphenylethylene: To a vial containing 2 mol% catalyst was added a solution

of 1,1-diphenylethylene (18.0 mg, 0.1 mmol) in CH2Cl2 (0.7 mL). The solution was then transferred to a 5 mm NMR tube for monitoring by NMR spectroscopy.

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5.4.2 X-ray Crystallography

5.4.2.1 X-ray Data Collection and Reduction Crystals were coated in Paratone-N oil in a glovebox before being mounted on a MiTegen

Micromount under an N2 stream to maintain a dry, O2 free environment for each crystal. Diffraction data were collected on a Bruker Apex II diffractometer using a graphite monochromator with Mo Kα (λ = 0.71073 Å) radiation. Temperature was maintained at 150(2) K using an Oxford cryo-stream cooler. Data collection strategies were determined using Bruker Apex II software. Frame integration was carried out using Bruker SAINT software. Data absorbance correction was carried out using the empirical multiscan method SADABS. Structure solutions were obtained by direct methods and refined using SHELXTL or Olex2 software.32,33 Refinement was carried out using full-matrix least squares techniques to convergence of weighting parameters. When data quality was sufficient, all non-hydrogen atoms were refined anisotropically.

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5.4.2.2 X-Ray Tables 5-4

Formula C26H12F5OP Weight (g/mol) 466.35 Crystal system triclinic Space group P-1 a (Å) 8.1019(19) b (Å) 10.671(3) c (Å) 12.378(3) α (°) 105.067(12) β (°) 98.590(12) γ (°) 102.193(11) Volume (Å3) 986.2(4) Z 2 Density (calcd.) (gcm–3) 1.570 R(int.) 0.0367 μ, mm–1 0.2040 F(000) 473 Index ranges -10 ≤ h ≤ 10 -13 ≤ k ≤ 13 -15 ≤ l ≤ 15 Radiation Mo Kα θ range (min, max) (°) 2.25, 27.10 Total data 15198 Max peak 0.4 Min peak -0.3 2 >2(FO ) 3618 Variables 298 R (>2σ) 0.0338

Rw 0.0882 GoF 1.031

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5.5 References

1. Lenz, W., A Short History of Thalidomide Embryopathy. Teratology 1988, 38, 203. 2. Eriksson, T.; Bjorkman, S.; Roth, B.; Fyge, A.; Hoglund, P., Stereospecific Determination, Chiral Inversion in-Vitro and Pharmacokinetics in Humans of the Enantiomers of Thalidomide. Chirality 1995, 7, 44. 3. Bhat, V.; Welin, E. R.; Guo, X. L.; Stoltz, B. M., Advances in Stereoconvergent Catalysis from 2005 to 2015: Transition-Metal-Mediated Stereoablative Reactions, Dynamic Kinetic Resolutions, and Dynamic Kinetic Asymmetric Transformations. Chem. Rev. 2017, 117, 4528. 4. Marckwald, W., Asymmetrical Synthesis. Chem. Ber. 1904, 37, 349. 5. Knowles, W. S.; Sabacky, M. J., Catalytic Asymmetric Hydrogenation Employing a Soluble Optically Active Rhodium Complex. Chem. Commun. 1968, 1445. 6. Zhang, Z. F.; Butt, N. A.; Zhang, W. B., Asymmetric Hydrogenation of Nonaromatic Cyclic Substrates. Chem. Rev. 2016, 116, 14769. 7. Cherney, A. H.; Kadunce, N. T.; Reisman, S. E., Enantioselective and Enantiospecific Transition-Metal-Catalyzed Cross-Coupling Reactions of Organometallic Reagents To Construct C-C Bonds. Chem. Rev. 2015, 115, 9587. 8. Du, X. Y.; Huang, Z., Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS Catal, 2017, 7, 1227. 9. Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.; Souchi, T.; Noyori, R., Synthesis of 2,2'-Bis(Diphenylphosphino)-1,1'-Binaphthyl (Binap), an Atropisomeric Chiral Bis(Triaryl)Phosphine, and Its Use in the Rhodium(I)-Catalyzed Asymmetric Hydrogenation of Alpha-(Acylamino)Acrylic Acids. J. Am. Chem. Soc. 1980, 102, 7932. 10. Ojima, I., Catalytic Asymmetric Synthesis. 2nd ed.; Wiley-VCH: New York, 2000; p 864. 11. Aaseth, J.; Criponi, G.; Andersen, O., Chelation Therapy in the Treatment of Metal Intoxication. Academic Press is an imprint of Elsevier: London, UK ; San Diego, CA, USA, 2016; p 371. 12. Chen, D.; Schmitkamp, M.; Franciò, G.; Klankermayer, J.; Leitner, W., Enantioselective Hydrogenation with Racemic and Enantiopure BINAP in the Rresence of a Chiral Ionic Liquid. Angew. Chem., Int. Ed. Engl. 2008, 47, 7339.

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13. Chen, D.; Wang, Y.; Klankermayer, J., Enantioselective Hydrogenation with Chiral Frustrated Lewis Pairs. Angew. Chem., Int. Ed. Engl. 2010, 49, 9475. 14. Liu, Y.; Du, H., Chiral Dienes as "Ligands" for Borane-Catalyzed Metal-Free Asymmetric Hydrogenation of Imines. J. Am. Chem. Soc. 2013, 135, 6810. 15. Ren, X. Y.; Du, H. F., Chiral Frustrated Lewis Pairs Catalyzed Highly Enantioselective Hydrosilylations of 1,2-Dicarbonyl Compounds. J. Am. Chem. Soc. 2016, 138, 810. 16. He, R. J.; Wang, X. S.; Hashimoto, T.; Maruoka, K., Binaphthyl-Modified Quaternary Phosphonium Salts as Chiral Phase-Transfer Catalysts: Asymmetric Amination of β-Keto Esters. Angew. Chem., Int. Ed. 2008, 47, 9466. 17. Holthausen, M. H.; Hiranandani, R. R.; Stephan, D. W., Electrophilic Bis- Fluorophosphonium Dications: Lewis Acid Catalysts from Diphosphines. Chem. Sci. 2015, 6, 2016. 18. Hudnall, T. W.; Kim, Y. M.; Bebbington, M. W. P.; Bourissou, D.; Gabbaï, F. P., Fluoride Ion Chelation by a Bidentate Phosphonium/Borane Lewis Acid. J. Am. Chem. Soc. 2008, 130, 10890. 19. Mobus, J.; vom Stein, T.; Stephan, D. W., Cooperative Lewis Acidity in Borane- Substituted Fluorophosphonium Cations. Chem. Commun. 2016, 52, 6387. 20. Caputo, C. B.; Winkelhaus, D.; Dobrovetsky, R.; Hounjet, L. J.; Stephan, D. W., Synthesis and Lewis Acidity of Fluorophosphonium Cations. Dalton Trans. 2015, 44, 12256. 21. Liu, L.; Wu, H. C.; Yu, J. Q., Improved Syntheses of Phosphine Ligands by Direct Coupling of Diarylbromophosphine with Organometallic Reagents. Chem. Eur. J. 2011, 17, 10828. 22. Dore, A.; Fabbri, D.; Gladiali, S.; Delucchi, O., Dinaphtho[2,1-B-1',2'-D]Phospholes - a New Class of Atropisomeric Phosphorus Ligands. J. Chem. Soc., Chem. Commun. 1993, 1124. 23. Hill, M. S.; Liptrot, D. J.; Weetman, C., Alkaline Earths as Main Group Reagents in Molecular Catalysis. Chem. Soc. Rev. 2016, 45, 972. 24. Gaussian 09, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega,

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G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox; Gaussian Inc.: Wallingford CT, 2016. 25. (a) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J., Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force- Fields. J. Phys. Chem. 1994, 98, 11623; (b) Kim, K.; Jordan, K. D., Comparison of Density- Functional and MP2 Calculations on the Water Monomer and Dimer. J. Phys. Chem. 1994, 98, 10089; (c) Becke, A. D., Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior. Phys. Rev. A 1988, 38, 3098; (d) Lee, C. T.; Yang, W. T.; Parr, R. G., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron- Density. Physical Review B 1988, 37, 785. 26. (a) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L., 6-31G* Basis Set for Atoms K Through Zn. J. Chem. Phys. 1998, 109, 1223; (b) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A., 6-31G* Basis Set for Third-Row Atoms. J. Comput. Chem. 2001, 22, 976; (c) Ditchfield, R.; Hehre, W. J.; Pople, J. A., Self-Consistent Molecular-Orbital Methods .9. Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724; (d) Hehre, W. J.; Ditchfield, R.; Pople, J. A., Self-Consistent Molecular- Orbital Methods .12. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular-Orbital Studies of Organic-Molecules. J. Chem. Phys. 1972, 56, 2257. 27. Headgordon, M.; Pople, J. A.; Frisch, M. J., MP2 Energy Evaluation by Direct Methods. Chem. Phys. Lett. 1988, 153, 503. 28. (a) Weigend, F., Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057; (b) Weigend, F.; Ahlrichs, R., Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297. 29. Chase, P. A.; Piers, W. E.; Patrick, B. O., New Fluorinated 9-Borafluorene Lewis Acids. J. Am. Chem. Soc. 2000, 122, 12911.

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30. Cowley, A. H.; Pinnell, R. P., Pentafluorophenyl-Phosphorus Ring System. J. Am. Chem. Soc. 1966, 88, 4533. 31. Connelly, S. J.; Kaminsky, W.; Heinekey, D. M., Structure and Solution Reactivity of (Triethylsilylium)triethylsilane Cations. Organometallics 2013, 32, 7478. 32. Sheldrick, G. M., A Short History of SHELX. Acta Crystallogr. Sect. A 2008, 64, 112. 33. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H., OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339.

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Chapter 6 Conclusion 6.1 Future Work While using hypervalent iodine reagents to effect nitrene insertion into electrophilic boranes was a successful method towards tuning Lewis acidity, the resulting aminoboranes were unreactive in FLP chemistry. Instead of trying to quench the reactivity of the nitrogen substituent with electron-

withdrawing sulfonyl groups and C6F5 substituents, an interesting avenue of research would be trying to promote reactivity with a more Lewis basic and less sterically encumbered nitrogen substituent. This could be achieved by cleavage of the sulfonyl groups, using common sulfonyl deprotection methods to yield the secondary or primary aminoborane.

Perchloroaryl substituted phosphines have been reported to undergo functionalization at the para- position when lithiated and treated with chlorotrimethylsilane.1 This functionalization could be explored as a means to tune the electrophilicity or solubility properties of subsequently generated phosphonium cations. Additional Lewis acidic or basic sites could be incorporated to investigate cooperative behavior or to effect intramolecular FLP chemistry.

During the air-stability tests of fluorophosphonium salts in Chapter 3 and 4, decomposition of the - cation was typically observed, while the [B(C6F5)4] anion was left intact. However, evidence of - decomposition of the [B(C6F5)4] anion reactivity was observed in the recrystallization of highly electrophilic 4-23 and 4-24. In the case of 4-23, the solid-state structure contained a

[(OH)B(C6F5)3] anion, consistent with the loss of a C6F5 substituent. Whereas attempts to recrystallize 4-24 resulted in the generation of corresponding difluorophosphorane 4-17, and - apparent decomposition of the [B(C6F5)4] . These observations suggest that employing an alternative, more robust anions is instrumental in developing more stable fluorophosphonium catalysts. Fluorophosphonium carboranate compounds could be generated from difluorophosphoranes by fluoride abstraction using silylium carboranate salts. Carborane anions play a critical role in silylium chemistry, allowing for both aryl C-F bond activation and ring- opening polymerization chemistry that is otherwise inaccessible.2,3 When 4-24 decomposes to 4-

17, there was no apparent degradation of the CF3 substituents. The observed decomposition of the

[B(C6F5)4] anion of 4-24 suggest that the tris[3,5-bis(trifluoromethyl)phenyl]fluorophosphonium

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cation could be a stronger Lewis acid than [PF(C6F5)3][B(C6F5)4], without the drastically increased steric bulk associated with C12F9 groups. Additionally, no degradation of trifluoromethyl groups in 4-24 was observed, despite reports of [PF(C6F5)3][B(C6F5)4] readily activating trifluoromethyl groups.4,5

Further, employing 3,5-bis(trifluoromethyl) substituents could also be effective with respect to the binaphthyl chemistry in Chapter 5. Both the 3,5-bis(trifluoromethyl) and 4-trifluoromethyl 6 analogues of BINAP have been reported. Since attempts to generate the C6F5 substituted BINAP analogue resulted in the formation of 5-3, the trifluoromethyl analogues be used as starting materials in the synthesis of a highly active chiral bis(fluorophosphonium) cation. Additionally, binaphthyl substituted phosphorus compounds 5-4 – 5-6 display interesting fluorescence properties. Lewis acidic 5-6 could be investigated as a possible chemical recognition fluorescence sensor.

6.2 Summary In chapter 2, a series of iodine reagents were used to effect nitrene insertion into the B-C bonds of electrophilic boranes. Insertion of the nitrene was used as a methodology to tune the Lewis acidity of a boron centre, while avoiding dangerous tin or lithium reagents required generally used in the synthesis of new electrophilic boranes. The reaction between PhI=NTs with BCF yielded the single

insertion product (C6F5)2BN(Ts)C6F5 (2-1). The Gutmann-Beckett method was administered by treating 2-1 with OPEt3 to yield the adduct (C6F5)2BN(Ts)C6F5·OPEt3 (2-2). Similarly, the reaction

between PhI=NTs with PhB(C6F5) or HB(C6F5) yields insertion products (C6F5)2BN(Ts)Ph (2-3)

and (C6F5)2BN(Ts)H (2-4) respectively. The effect of varied nitrene substitutions on the

electrophilicity of HB(C6F5)2 was also investigated. Iodine reagents PhI=NCls, PhI=NMs, and

PhI=NNs were each reacted with HB(C6F5)2 to generate (C6F5)2BN(Cls)H (2-5), (C6F5)2BN(Ms)H

(2-6), and (C6F5)2BN(Ns)H (2-7) respectively. Aminoboranes 2-4 – 2-7 could similarly be generated from ClB(C6F5)2 with the corresponding iodine reagent. The Gutmann-Beckett acceptor numbers of 2-1, 2-3 – 2-7 indicate enhanced Lewis acidity of the insertion products with respect to the respective borane starting materials. Further, the series 2-4 – 2-7 displayed a predictable trend of increasing Lewis acidity based on nitrogen substitution of 2-6 < 2-4 < 2-5 < 2-7. The range of electrophilicity of the aminoboranes tuning (ΔAN = 2.0) is less than difference in Lewis-acidity between BCF and PhB(C6F5)2 (ΔAN = 3.1). The reactivity of iodine reagents with electron-

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deficient phosphenes was investigated as a means of generating Lewis acidic phosphinimines.

Iodine reagent PhI=NTs was reacted with P(C6F5)Ph2 and P(C6F5)3 to yield TsN=P(C6F5)Ph2 (2-8) and TsN=P(C6F5)3 (2-9) respectively. Both 2-8 and 2-9 proved to be sensitive to hydrolysis, readily forming the corresponding oxides. While nitrene insertion proved to be an effective method to tune the Lewis acidity of electrophilic boranes, the relative inactivity of the resulting aminoboranes precludes application of this method to FLP chemistry.

Chapter 3 explored the incorporation of C6Cl5 rings into fluorophosphonium systems to increase air-stability. The reaction of LiC6Cl5 with either Ph2PCl, PhPCl2, PBr3, (C6F5)2PBr or (C6F5)PBr2

in appropriate molar ratios resulted in phosphines Ph2P(C6Cl5) (3-1), PhP(C6Cl5)2 (3-2),

(C6F5)P(C6Cl5)2 (3-4), or (C6F5)2P(C6Cl5) (3-5) respectively. The reaction of PCl3 with three

equivalents of LiC6Cl5 yielded coupled 1,2-diphosphine ((C6Cl5)2P)2 was the major product and 3-3 as a minor product. Perchlorophenyl phosphines 3-1, 3-2, 3-4, and 3-5 were oxidized using

XeF2 to the corresponding difluorophosphoranes Ph2PF(C6Cl5) (3-6), PhPF2(C6Cl5) (3-7),

(C6F5)PF2(C6Cl5)2 (3-8), and (C6F5)2PF2(C6Cl5) (3-9). The oxidation of 3-3 generates a trifluorophosphorane as the major product, resulting from loss of a C6Cl5 substituent.

Difluorophosphoranes 3-6 – 3-9 underwent fluoride abstraction by [SiEt3][B(C6F5)4] to generate fluorophosphonium salts [Ph2PF(C6Cl5)][B(C6F5)4] (3-10), [PhPF(C6Cl5)2][B(C6F5)4] (3-11),

[(C6F5)PF(C6Cl5)2][B(C6F5)4] (3-12), and [(C6F5)2PF(C6Cl5)][B(C6F5)4] (3-13) respectively. Phosphine 3-1 could be reacted with NFSI to directly generate fluorophosphonium

[Ph2PF(C6Cl5)][N(SO2Ph)2] (3-10NFSI). Treatment of 3-3 with Selectfluor failed to generate the corresponding fluorophosphonium, instead forming OPF(C6Cl5)2 (3-14). Fluorophosphonium salts 3-10 – 3-13 were not amenable to analysis by the Gutmann-Beckett method. Instead, computational methods GIE and FIA were used to evaluate the electrophilicity of 3-10 – 3-13. The ω values of the fluorophosphonium cations gave the increasing electrophilicity trend 3-10 < 3-11 < 3-12 < 3-13, consistent with total electronegativity of aryl substituents. The ω values of 3-10 – 3-13 were found to linearly correlate with the experimentally obtained 31P{1H} NMR chemical shifts. The FIA values were also found to be in good agreement with ω values. Compounds 3-10 – 3-13 were evaluated as catalysts in the dimerization of 1,1-diphenylethylene, hydrodefluorination of 1-fluoroadamantane, deoxygenation of benzophenone, dehydrocoupling of

phenol and HSiEt3, hydrosilylation of α-methylstyrene with HSiEt3, and benzylation and hydrodefluorination of 4-trifluoromethylbromobenzene. Reactivity trends were consistent with

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both computed and experimentally obtained electrophilicity trends, with Ph containing 3-10 and

3-11 performing worse than C6F5 substituted 3-12 and 3-13. Further, 3-10 and 3-11 were unable to effect benzylation reactivity. The air-stability of the fluorophosphonium solutions was evaluated and resulted in the following trend 3-11 > 3-10 > 3-12 > 3-13. Interestingly, despite 3-11 being more electrophilic and a more active catalyst than 3-10, it was also found to be considerably more air-stable. While perchloroaryl substituents were effective at increasing air-stability through steric protection, the also resulted in a decrease in electrophilicity compared to the analogous perfluoroaryl fluorophosphonium cations.

In Chapter 4, C12F9 substituents were incorporated into fluorophosphonium cations as a method of

enhancing electrophilicity. A series of C12F9 phosphines were generated by reaction of the lithium reagent with the appropriate halophosphine: iPr2P(C12F9) (4-1), otol2P(C12F9) (4-2), Ph2P(C12F9)

(4-3), (C6F5)2P(C12F9) (4-4), (3,5-(CF3)2C6H3)2P(C12F9) (4-5), Cy2P(C12F9) (4-6), Me2P(C12F9)

(4-7), tBu2P(C12F9) (4-8), MeP(C12F9)2 (4-9) and PhP(C12F9)2 (4-10). Reaction with zinc reagent

Zn(C12F9)2·PhMe with PBr3 and PhPCl2 yielded 1,2-diphosphines ((C6Cl5)2P)2 (4-11), and

((C12F9)PhP)2 (4-12). Difluorophosphoranes iPr2PF2(C12F9) (4-13), otol2PF2(C12F9) (4-14),

Ph2PF2(C12F9) (4-15), (C6F5)2PF2(C12F9) (4-16), (3,5-(CF3)2C6H3)2PF2(C12F9) (4-17),

Cy2PF2(C12F9) (4-18), and Me2PF2(C12F9) (4-19) were each generated by reaction of the

corresponding phosphines with XeF2. Fluoride abstraction from 4-13 – 4-18 yielded

fluorophosphonium cations [iPr2PF(C12F9)][B(C6F5)4] (4-20), [otol2PF(C12F9)][B(C6F5)4] (4-21),

[Ph2PF(C12F9)][B(C6F5)4] (4-22), [(C6F5)2PF(C12F9)][B(C6F5)4] (4-23), [(3,5-

(CF3)2C6H3)2PF(C12F9)][B(C6F5)4] (4-24), and [Cy2PF(C12F9)][B(C6F5)4] (4-25). Of 4-20 – 4-23, only 4-23 was compatible with the Gutmann-Beckett protocol, which indicated Lewis acidity slightly higher than [PF(C6F5)3][B(C6F5)4]. The FIA values for 4-20 – 4-23 showed only slight difference between 4-20 – 4-22, while 4-23 displayed a higher FIA value than

[PF(C6F5)3][B(C6F5)4]. A competition experiment further confirmed the greater FIA of 4-23 compared to [PF(C6F5)3][B(C6F5)4]. The catalytic activity of 4-20 – 4-23 was evaluated in a series of test reactions to give the trend 4-21 < 4-20 < 4-22 < 4-23. The catalytic activity of 4-23 was found to be lower than [PF(C6F5)3][B(C6F5)4] despite the higher electrophilicity, likely a result of increased steric bulk. Similarly, compound 4-21 displayed considerably lower catalytic activity than 4-20 and 4-22, despite comparable activity, due to high steric bulk at the phosphorus centre. The air-stability of 4-20 – 4-23 was evaluated and gave the trend of increasing stability of 4-23 <

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4-22 < 4-20 < 4-21. Interestingly, inclusion of perfluorobiphenyl substituents was an effective strategy in the development of monocationic fluorophosphonium species that are more electrophilic than the corresponding perfluorophenyl species.

Chapter 5 developed chiral fluorophosphonium cations by incorporation of binaphthyl substituents. Oxidation of BINAP with two molar equivalents of XeF2 results in the formation of

binaphthyl bis(difluorophosphorane) (Ph2F2P)2(C20H12) (5-1). The corresponding bis(fluorophosphonium) cation [((Ph2FP)2C10H6)2]2[B(C6F5)4] (5-2) was generated by fluoride abstraction of 5-1 using [SiEt3][B(C6F5)4]. Compound 5-2 was found to be an effective catalyst in the dimerization of 1,1-diphenylethylene, and the hydrodefluorination of 1-fluoroadamantane, but was unable to effect hydrosilylation reactivity. In an effort to generate a more electrophilic chiral catalyst, a binaphthyl Grignard reagent was reacted with two equivalents of halophosphine 2 BrP(C6F5)2. Instead of the expected bis(phosphine), (C6F5)P(κ -C20H12) (5-3) was formed with concomitant formation of P(C6F5)3. Phosphine 5-3 was oxidized in air to yield phosphine oxide 2 2 (C6F5)PO(κ -C20H12) (5-4) and by XeF2 to generate (C6F5)PF2(κ -C20H12) (5-5). The 2 corresponding fluorophosphonium [(C6F5)PF(κ -C20H12)][B(C6F5)4] (5-6) was subsequently generated from 5-5. Despite the addition of a perfluorophenyl substituent, the measured FIA of 5-6 was considerably lower than comparable fluorophosphonium cations bearing a single perhaloaryl substituent, likely a result of the constrained geometry.

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6.3 References

1. Dua, S. S.; Edmondson, R. C.; Gilman, H., Polyhaloaryl Compounds Containing Phosphorus. J. Organomet. Chem. 1970, 24, 703. 2. Duttwyler, S.; Douvris, C.; Fackler, N. L. P.; Tham, F. S.; Reed, C. A.; Baldridge, K. K.; Siegel, J. S., C-F Activation of Fluorobenzene by Silylium Carboranes: Evidence for Incipient Phenyl Cation Reactivity. Angew. Chem., Int. Ed. 2010, 49, 7519. 3. Zhang, Y.; Huynh, K.; Manners, I.; Reed, C. A., Ambient Temperature Ring-Opening

Polymerisation (ROP) of Cyclic Chlorophosphazene Trimer [N3P3Cl6] Catalyzed by Silylium Ions. Chem. Commun. 2008, 494. 4. Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W., Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor. Science 2013, 341, 1374. 5. Zhu, J.; Perez, M.; Caputo, C. B.; Stephan, D. W., Use of Trifluoromethyl Groups for Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination. Angew. Chem., Int. Ed. 2016, 55, 1417. 6. Liu, L.; Wu, H. C.; Yu, J. Q., Improved Syntheses of Phosphine Ligands by Direct Coupling of Diarylbromophosphine with Organometallic Reagents. Chem. Eur. J. 2011, 17, 10828.

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