Group 16 Elements in Frustrated Lewis Pair Chemistry

Fu An Tsao

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto

Fu An Tsao

Doctor of Philosophy

Department of Chemistry University of Toronto

2018 Abstract

Frustrated Lewis Pairs (FLPs) describe combinations of sterically encumbered Lewis acids and bases that do not form classical adducts. This unquenched reactivity has been shown to activate a wide plethora of small molecules, including H2, CO2, alkynes and ketones. While many different main group compounds have been applied in this context, studies of group 16 elements in FLP chemistry remain scarce. The main objective of this dissertation is to expand the scope of FLP chemistry to include group 16 elements, as they can function both as Lewis bases and as Lewis acids.

The first portion of this dissertation focuses on the syntheses and reactivity studies of tellurium-boron heterocycles, wherein tellurium acts as a Lewis base. 1,1-carboboration of tellurium-substituted acetylides was shown to proceed smoothly at room temperature, leading to the formation of Te/B intramolecular FLP and 1,4-telluraborine. Several 1,4- telluraborines were found to have mild to high aromaticity as manifested by their unusual stability against oxygen and moisture. They can also undergo a number of unique reactions, including FLP-type alkyne exchange reactions and selective protonolysis of the exocyclic B–C bond. These findings allowed for the facile derivatization of this new class of compounds.

Next, we explored the use of hypervalent sulfur and tellurium compounds as Lewis acids. Fluorosulfoxonium cations were successfully synthesized and shown to be Lewis acidic by both computational and experimental methods. On the other hand, although the syntheses of tellurium(IV)-based Lewis acids were achieved, their instability and

ii fluxional solution state behaviour prevented detailed studies on their utility as Lewis acid catalysts.

Lastly, in chapter 5, we re-visited the known 1,1-carboboration reaction of terminal alkynes by B(C6F5)3 and report a new method of E/Z isomerization using bulky Lewis bases. The mechanism of isomerization was shown computationally to proceed via a zwitterionic borataalkene intermediate.

iii Acknowledgments

First and foremost, I would like to thank my supervisor, Prof. Doug Stephan, for his guidance throughout my graduate degree. Thank you for giving me the freedom to explore any idea that I find interesting, and for providing me with the necessary suggestions to carry out and complete different projects. Thank you for all the traveling and conference opportunities that you have given me to enrich my graduate experience.

I would like to thank my committee members, Prof. Datong Song and Prof. Dwight Seferos, for their feedback during my seminars and committee meetings. I’m grateful for Prof. Bob Morris for his continual interest in my research during the seminar series and for providing me feedback for my dissertation. Additionally, I truly appreciate the time that Prof. Paul Ragogna is taking to serve as the external examiner for my final defense. I would also like to extend my gratitude to Prof. Gerhard Erker for welcoming me into his group at the University of Münster and for his guidance throughout my stay there.

There is not enough room here, but I have to thank all of the Stephan group members, both past and present, for your advice and friendship throughout my time here. In particular, I would like to thank Prof. Chris Caputo, Dr. Fatme Dahcheh and Dr. Conor Pranckevicius for helping me out when I first joined the group. I would like to thank all of the students I’ve supervised in the past four years for their help. A big thank you goes out to all the thesis editors – thank you all very much for your time and your help.

The works in this dissertation would not be complete without the amazing support staff in the department. I’m especially grateful for the help from Dr. Darcy Burns, Dr. Jack Sheng and Dmitry Pichugin for all of my NMR problems. I’m also grateful for the help from Shanna Pritchard, Dr. Alan Lough, Anna-Liza Villavelez, Rose Balazs and Dr. Matthew Forbes.

Last and certainly not least, I would like to thank my family for their continual love and trust in me throughout these years. I would also like to thank all of my friends from the rock climbing crew and from the Newman Centre for keeping me sane throughout the most stressful times. Thank you all – I really couldn’t have done it alone. AMDG.

Table of Contents

Acknowledgments ...... iv

Table of Contents ...... v

List of Figures ...... ix

List of Schemes ...... xi

List of Tables ...... xiv

List of Symbols and Abbreviations ...... xv

Chapter 1. Introduction ...... 1

1.1 Main Group Heterocycles ...... 1

1.1.1 Introduction ...... 1

1.1.2 Aromaticity of main group heterocycles ...... 1

1.1.3 Synthesis and applications of boron-containing heterocycles ...... 3

1.1.4 Synthesis and applications of tellurium-containing heterocycles ...... 5

1.2 Frustrated Lewis Pairs ...... 8

1.2.1 Discovery ...... 8

1.2.2 Small molecule activation and catalysis ...... 9

1.3 Main Group Lewis Acids beyond Boron ...... 12

1.3.1 Group 13 Lewis acids ...... 12

1.3.2 Hypervalency in main group compounds ...... 14

1.3.3 Carbon and silicon Lewis acids ...... 15

1.3.4 Group 15 Lewis acids ...... 17

1.4 Scope of Thesis ...... 20

1.5 References ...... 21

Chapter 2. 1,1-Carboboration of Tellurium Acetylides ...... 29

2.1 Introduction ...... 29

2.1.1 1,1-carboboration of activated alkynes ...... 29

2.1.2 1,1-carboboration using highly electrophilic boranes – FLP applications . 31

2.1.3 Synthesis of heterocyclic compounds using 1,1-carboboration ...... 33

2.2 Results and Discussion ...... 34

2.2.1 Reactions of a tellurium monoacetylide with boranes ...... 34

2.2.2 Exploration of Te/B reactivities ...... 36

2.2.3 Oxidation of tellurium in Te-B FLPs ...... 39

2.2.4 Reactions of a tellurium diacetylide with boranes ...... 41

2.2.5 Conclusion ...... 47

2.3 Experimental Section ...... 48

2.3.1 General considerations ...... 48

2.3.2 Synthetic procedures and spectroscopic characterization ...... 49

2.3.3 X-ray crystallography ...... 64

2.4 References ...... 67

Chapter 3. Reactivity of 1,4-Telluraborines ...... 70

3.1 Introduction ...... 70

3.1.1 Aromatic compounds containing boron and other heteroatoms ...... 70

3.2 Results and Discussion ...... 72

3.2.1 Reactions of 1,4-telluraborines with common nucleophiles ...... 72

3.2.2 Reactions of 1,4-telluraborines with terminal alkynes ...... 74

3.2.3 Reactions of 1,4-telluraborines with alcohols ...... 82

3.2.4 Further derivatization of 1,4-telluraborines ...... 88

3.2.5 Additional comments on 1,4-telluraborines ...... 92

3.2.6 Conclusion ...... 95

3.3 Experimental Section ...... 96

3.3.1 General considerations ...... 96

3.3.2 Synthetic procedures and spectroscopic characterization ...... 97

3.3.3 X-ray crystallography ...... 123

3.3.4 Computational details ...... 127

3.4 References ...... 129

Chapter 4. Exploring Chalcogen-Based Lewis Acids ...... 133

4.1 Introduction ...... 133

4.1.1 Lewis acidity in sulfur compounds ...... 133

4.1.2 Lewis acidity in tellurium compounds ...... 134

4.2 Results and Discussion ...... 135

4.2.1 Reaction of PhSF5 with a fluoride-abstracting agent ...... 135

4.2.2 Synthesis of fluorosulfoxonium cation ...... 137

4.2.3 Reactivities of fluorosulfoxonium cations ...... 141

4.2.4 Synthesis of perfluorinated Te(II) and Te(IV) compounds ...... 142

4.2.5 Conclusion ...... 150

4.3 Experimental Section ...... 151

4.3.1 General considerations ...... 151

4.3.2 Synthetic procedures and spectroscopic characterization ...... 152

4.3.3 X-ray crystallography ...... 157

4.3.4 Computational details ...... 160

4.4 References ...... 161

Chapter 5. Isomerization of Alkenylboranes Using Bulky Lewis Bases ...... 166

5.1 Introduction ...... 166

5.1.1 Alkenylboranes in synthetic chemistry ...... 166

vii

5.1.2 1,1-carboboration of unactivated alkynes ...... 167

5.2 Results and Discussion ...... 169

5.2.1 Z-specific isomerization of alkenylboranes ...... 169

5.2.2 Substrate scope of E- or Z-specific isomerization of alkenylboranes...... 171

5.2.3 Mechanism of E- or Z-specific isomerization of alkenylboranes ...... 175

5.2.4 Catalytic isomerization of alkenylboranes and the use of other Lewis bases ...... 176

5.2.5 Conclusion ...... 178

5.3 Experimental Section ...... 179

5.3.1 General considerations ...... 179

5.3.2 Synthetic procedures and spectroscopic characterization ...... 180

5.3.3 X-ray crystallography ...... 184

5.3.4 Computational details ...... 186

5.4 References ...... 187

Chapter 6. Conclusion ...... 189

6.1 Summary ...... 189

6.2 Future Work ...... 190

6.3 References ...... 192

viii

List of Figures

Figure 1-1. Simplified MO diagram showing the three-centre-four-electron bonding orbitals of XeF2...... 14

Figure 1-2. Representative examples of phosphorus-based Lewis acids...... 19

Figure 1-3. Selected examples of group 15 Lewis acids based on N, As, Sb and Bi. ... 20

Figure 2-1. Solid-state structure of 2-6 ...... 38

Figure 2-2 Solid-state structure of 2-7 ...... 39

Figure 2-3. Solid-state structure of 2-8 ...... 40

Figure 2-4. Solid-state structures of 2-10 and 2-12 ...... 42

Figure 2-5. 19F{1H} NMR spectra of pure 2-11 and the same sample after heating at 80 oC for 8 hours, which gives 2-12...... 44

Figure 2-6. 19F{1H} NMR spectra of pure 2-13 and the same sample after heating at 80 oC for 8 hours, which gives 2-14 ...... 45

Figure 2-7. Solid-state structures of 2-14 and 2-13...... 46

Figure 3-1. Representative examples of oxa- and thiaborines in the literature...... 72

Figure 3-2. Solid-state structures of 3-4 (top) and 3-6 (bottom)...... 77

Figure 3-3. Solid-state structures of the Py–O adduct of compound 3-4...... 78

Figure 3-4. Computed reaction profile of the FLP-alkyne exchange reaction of 3-2 with PhC≡CH ...... 81

Figure 3-5. Solid-state structures of compounds 3-16, 3-19, and 3-24 ...... 85

Figure 3-6. Solid-state structures of compounds 3-26 and 3-27 ...... 87

Figure 3-7. Solid-state structures of 3-39...... 91

Figure 3-8. Solid-state structures of compound 3-41 ...... 92

Figure 3-9. Donor-acceptor interaction between phenyl substituent and boron centre in 3-2 (isovalue = 0.05)...... 95

Figure 4-1. Solid-state structure of 4-1 ...... 136

Figure 4-2. Solid-state structure of 4-3 ...... 138

Figure 4-3. Solid-state structure of 4-4 ...... 140

Figure 4-4. 19F{1H} NMR spectrum of 4-11 at 298 K and 218 K...... 145

Figure 4-5. Solid-state structure of 4-12 ...... 146

Figure 4-6. 19F{1H} NMR spectrum of 4-12 at 298 K and 218 K...... 147

Figure 4-7. Solid-state structure of 4-13 ...... 148

Figure 4-8. 19F{1H} NMR spectrum of 4-14 at 298 K and 218 K...... 149

Figure 4-9. Solid-state structure of 4-15 ...... 150

Figure 5-1. In situ 19F{1H} NMR spectrum of the isomerization of E/Z isomers of 5-1 to exclusively the Z-isomer ...... 170

Figure 5-2. Solid-state structure of Z-5-1 ...... 170

Figure 5-3. Solid-state structure of Z-5-4 ...... 173

Figure 5-4. Solid-state structure of E-5-7...... 174

Figure 5-5. Calculated reaction pathway of the Z-isomerization of alkenylborane E-5-1...... 176

List of Schemes

Scheme 1-1. General synthetic route to boroles ...... 3

Scheme 1-2. Common synthetic strategies of substituted tellurophenes...... 7

Scheme 1-3. Representative examples of tellurium-containing dyes that have been used in biological applications...... 8

Scheme 1-4. Earliest examples of FLPs...... 9

Scheme 1-5. Selected examples of hydrogenation of polar substrates by FLP mechanism...... 10

Scheme 1-6. Selected examples of FLP activation of NO and CO2...... 11

Scheme 1-7. Selected examples of FLP addition reactions to unsaturated organic bonds...... 12

Scheme 1-8. Selected examples of FLP reactions involving heavy group 13 elements...... 13

Scheme 1-9. Selected examples of carbon-based Lewis acids involved in FLP-type activities ...... 16

Scheme 1-10. Selected examples of silicon-based Lewis acids involved in FLP-type reactions...... 17

Scheme 2-1. Discovery of 1,1-carboboration reaction and its mechanism...... 30

Scheme 2-2. Synthesis of various heterocycles via 1,1-carboboration...... 31

Scheme 2-3. Examples of FLPs synthesized via 1,1-carboboration using B(C6F5)3. .... 32

Scheme 2-4. Synthesis and reactivities of S/B FLPs...... 33

Scheme 2-5. Synthesis of phospholes and thiophenes by 1,1-carboboration ...... 34

Scheme 2-6. Reaction between tellurium-substituted acetylene and boranes...... 35

Scheme 2-7. Reactions of intramolecular Te/B FLP 2-2 with organic substrates...... 37

Scheme 2-8. Oxidation of 2-2 by I2 to give 2-8...... 40

Scheme 2-9. Reaction between tellurium acetylide 2-9 and various boranes to form compounds 2-10 to 2-15. (a) 1,1-carboboration; (b) FLP addition...... 42

Scheme 3-1. Different synthetic strategies of monomeric 1,4-azaborines...... 71

Scheme 3-2. Synthesis of compounds 3-3 to 3-14, Spectroscopic conversions are shown with isolated yields presented in brackets...... 75

Scheme 3-3. Synthesis of compound 3-15...... 76

Scheme 3-4. Proposed reaction mechanism for the FLP-alkyne exchange synthesis of compounds 3-3 to 3-14...... 79

Scheme 3-5. Synthesis of compounds 3-16 to 3-25 from 3-1...... 83

Scheme 3-6. Synthesis of compounds 3-26 and 3-27 via terminal alkyne exchange reaction with 3-16...... 87

Scheme 3-7. Synthesis of compound 3-28 from 3-16...... 88

Scheme 3-8. Synthesis of compounds 3-40 and 3-41 from 3-28...... 91

Scheme 4-1. Selected reports of S(II) or S(IV) compounds demonstrating Lewis acidic behaviour...... 133

Scheme 4-2. Selected examples of Te(II) or Te(IV) species displaying Lewis acidic behaviour...... 135

Scheme 4-3. Synthesis of fluorosulfoxonium cation from PhSF5...... 136

Scheme 4-4. Synthesis of compound 4-3 from diphenyl sulfoxide...... 137

xii

Scheme 4-5. Synthesis of compound 4-4 from 4-2 or diphenyl sulfoxide...... 139

Scheme 4-6. Reactivities of fluorosulfoxonium cation 4-3...... 142

Scheme 4-7. Synthetic strategy to access compounds 4-6 and 4-8...... 143

Scheme 4-8. Synthesis of compounds 4-9 and 4-10 from 4-7...... 144

Scheme 4-9. Reactions of 4-9 with halide-abstracting agents to give compounds 4-11 to 4-13...... 145

Scheme 4-10. Proposed mechanism of the formation of compound 4-13 from the reaction between 4-9 and B(C6F5)3...... 148

Scheme 4-11. Reactions of 4-10 with halide-abstracting agents to give compounds 4-14 and 4-15...... 149

Scheme 5-1. Comparison of the mechanism of 1,1-carboboration of (top) alkynes with heavy atom substitution and (bottom) alkynes with only organic substituents...... 168

Scheme 5-2. 1,1-carboboration of 1-pentyne by B(C6F5)3 followed by isomerization t using Bu2PH...... 169

Scheme 6-1. Synthesis of azomycin-linked 1,4-telluraborine by selective protonolysis of the exocyclic B–C bond...... 190

Scheme 6-2. Synthesis of fluorosulfoxonium dication...... 191

xiii

List of Tables

Table 2-1. Selected crystallographic data of compounds 2-6, 2-7 and 2-8...... 65

Table 2-2. Selected crystallographic data of compounds 2-10, 2-12, 2-13 and 2-14 .... 66

Table 3-1. Reactions of compounds 3-1 and 3-2 with various Lewis bases ...... 73

Table 3-2. Selected bond length comparison between 3-4 and Py–O·3-4 based on the solid-state structure...... 78

Table 3-3. Hydroboration of unsaturated organic substrates by telluraborine 3-28...... 89

Table 3-4. NICS(1)ZZ indices of representative examples of 1,4-telluraborines (Ar = 4- bromophenyl)...... 93

Table 3-5. Selected crystallographic data of compounds 3-4, 3-6, and Py–O·3-4...... 124

Table 3-6. Selected crystallographic data of compounds 3-16, 3-19, and 3-24...... 125

Table 3-7. Selected crystallographic data of compounds 3-26 and 3-27 ...... 125

Table 3-8. Selected crystallographic data of compounds 3-39 and 3-41...... 126

Table 4-1. Selected crystallographic data of compounds 4-1, 4-3 and 4-8...... 158

Table 4-2. Selected crystallographic data of compounds 4-12, 4-13 and 4-15...... 160

t Table 5-1. Stoichiometric isomerization of alkenylboranes using Bu2PH...... 171

Table 5-2. Z-isomerization of alkenylborane 5-1 using different Lewis bases ...... 177

t Table 5-3. E/Z-isomerization of alkenylboranes using catalytic amounts of Bu2PH. .. 178

Table 5-4. Selected crystallographic data of compounds Z-5-1, Z-5-4 and E-5-7...... 185

xiv

List of Symbols and Abbreviations

9-BBN 9-borabicyclo[3.3.1]nonane

Å Ångström, 10-10 m

Ar aryl atm atmosphere

BCat catecholboryl

Bpin pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) br broad

C Celsius ca. circa calc. calculated

Cp cyclopentadienyl

Cy cyclohexyl d doublet

DART direct analysis in real time

DCM dichloromethane

Dipp 2,6-diisopropylphenyl

DMAP 4-dimethylaminopyridine

Eq equivalent

ESI electrospray ionization

Et ethyl

Et2O diethyl ether

Fc ferrocenyl g gram

GOF goodness of fit

HOESY heteronuclear multiple bond correlation

HOMO highest occupied molecular orbital

Hz Hertz i ipso iPr iso-propyl

K Kelvin kcal kilocalories

LUMO lowest unoccupied molecular orbital m meta m multiplet

Me methyl

Mes mesityl, 2,4,6-trimethylphenyl

MHz megahertz

MO molecular orbital

xvi

MS mass spectroscopy

Nap naphthyl nBu n-butyl n Jxy n-scalar coupling constant between X and Y atoms

NMR Nuclear magnetic resonance n.o. not observed o ortho

O(TMS)2 hexamethyldisiloxane

OTf trifluoromethanesulfonate p para

Ph phenyl

Py pyridine q quartet r.t. room temperature s singlet t triplet tBu tert-butyl

THF tetrahydrofuran

TMP 2,2,6,6,-tetramethylpiperidine

TMS trimethylsilyl

xvii

Tol tolyl

G Gibbs free energy

xviii

Chapter 1. Introduction 1.1 Main Group Heterocycles

1.1.1 Introduction

Heterocyclic compounds incorporating p-block elements have long attracted the attention of inorganic chemists. Much of the initial interest in these compounds was fundamental in nature, with the primary focus of understanding their electronic structures. In recent years, however, a large number of these heterocycles have progressed from being simple novelties to useful molecules in medicinal and material applications. The value of developing reliable and high-yielding routes to heterocyclic compounds thus cannot be overstated. As many reviews have been written to summarize the major advances in the area of main group heterocyclic chemistry, this introductory chapter will only highlight 5- and 6-membered heterocycles that are the most relevant to the contents of this dissertation.

1.1.2 Aromaticity of main group heterocycles

Electron delocalization in closed systems often gives rise to unique properties that increase a molecule’s stability along with its potential utility in material sciences and ultimately, everyday consumer products. The concept of aromaticity is important in evaluating this electron delocalization. Aromatic compounds are generally characterized by its enhanced stability, bond length equalization and special magnetic properties. On the other hand, antiaromatic compounds are characterized by alternating bond lengths and destabilized structures.1 Ironically, aromaticity is not a directly measurable quantity, nor is there a generally accepted definition, especially regarding cyclic compounds with non-carbon atoms. Discussions concerning the quantification of aromaticity are often confusing, as there are many different methods of aromaticity evaluation based on structural, energetic, or magnetic criteria.

To catch a glimpse of the complications associated with the quantitative assessment of aromaticity, it is helpful to consider the historical development of the term “aromatic”. This term was first used in the late nineteenth century2 by Kekulé to describe

1 compounds such as benzene that were isolated from volatile vegetable oils and had a pleasant, or aromatic, scent. The descriptor “aromatic compound” thus carried no structural or electronic meaning until much later, when more derivatives of benzene were discovered and isolated. Very quickly after Kekulé’s report, Erlenmeyer pointed out that the concept of aromaticity should be based on properties of compounds rather than on common structural features.3 Indeed, in 1925, Armit and Robinson formulated the idea that aromatic compounds were characterized by “reduced unsaturation and the tendency to retain the type”4 as exemplified by their tendency to undergo substitution rather than addition reactions. Nevertheless, it was not until the 1930’s that Hückel provided the definition of aromatic compounds as we know it today, where he stated that “fully conjugated, planar, monocyclic polyolefins possessing (4n+2) π-electrons, where n is an integer, will have special electronic stability.”5

Hückel’s rule certainly has provided a quick guide for organic and inorganic chemists alike to assess and predict a cyclic molecule’s stability as conferred by aromaticity. This description became much more complicated when compounds were discovered to have similar reactivities with benzene, but have either different ring sizes or contain non- carbon elements. Many different ways of evaluating aromaticity have thus been developed to address these challenges. Each of these methods focus on different aspects of the term aromaticity, including energetic, geometric, magnetic, and reactivity properties.6 It is important to note that, while all these criteria are capable of distinguishing between aromatic and non-aromatic systems, different indices do not correlate with one another within each category of compound.7 As a result, aromaticity is considered by theoretical chemists to be a statistically multidimensional property and its use should be carefully evaluated.

Despite all the aforementioned controversy over aromaticity, this term continues to dominate much of the existing literature. In the field of heterocyclic main group compounds, the nucleus-independent chemical shift (NICS) index has become the conventional method of evaluating aromaticity. NICS was first introduced in 19968 and soon gained popularity amongst both experimental and computational chemists as it could be easily computed. NICS indicates the magnetic shielding at different points of a

2 molecule. Significantly negative (shielded) NICS values within heterocyclic rings denote aromaticity due to the electron-induced distropic ring current. On the other hand, positive NICS values represent paratropic ring currents and thus, antiaromatic systems. Over the past two decades, NICS method has been further refined and it is now generally accepted that NICS values computed at 1 Å above the centre of heterocycles provide the most accurate depiction of aromaticity.9 These values are often denoted as

NICS(1)zz and will be the main method of evaluation for the aromaticity of all the heterocycles reported in this dissertation.

1.1.3 Synthesis and applications of boron-containing heterocycles

As the concept of aromaticity developed, the destabilization caused by antiaromaticity also attracted fundamental research attention. Cyclic and conjugated compounds with 4n π-electrons are destabilized and hence are usually highly reactive. One class of antiaromatic compounds that has been extensively studied in the past decade is boroles. Historically, boroles were first prepared in 1969 by Eisch et al.,10 when computational methods of evaluating antiaromaticity were still in their infancy. This compound received very little attention for close to four decades before Braunschweig et al. re-examined the original report and subsequently provided the first elucidation of the solid-state structures of compound 1-1(Scheme 1-1).11 Since then, the field of borole chemistry has expanded quickly and their antiaromaticity has now been supported by both experimental and computational methods.12

Scheme 1-1. General synthetic route to boroles.

Since Braunschweig’s report of the solid-state structure of 1-1 in 2008, many analogous boroles have been synthesized. In almost all cases, these species were prepared by tin-

3 boron exchange reactions (Scheme 1-1). The isolation of these species were often not trivial, especially in the ones containing perfluorinated aryl groups such as compound 1- 2,13 as they are highly Lewis acidic and thus prone to undesired side reactions. This enhanced Lewis acidity of the boron centre is a direct result of antiaromaticity since the destabilization can be alleviated upon its coordination to a Lewis base. Further support of the antiaromatic character of boroles comes from the solid-state structures – almost all structurally characterized boroles show significant bond length alternations within the ring.12

The unique Lewis acidity and antiaromaticity of boroles have led to many interesting reactivities. The highly reactive carbon backbone readily undergoes in Diels-Alder 14 15 16 reactions, insertion reactions into the B-C bond and metal-free H2 activation.

Interestingly, boroles feature much narrower HOMO-LUMO gaps (λmax = 560 nm for 1) than analogous carbon-based five-membered rings such as furan. Moreover, filling the vacant LUMO with two electrons significantly increases the HOMO-LUMO gap, leading to a dramatic blue shifting of the corresponding UV-Vis spectra.10 It is conceivable that this facile tuning of photophysical properties could be applied to organic vapour- detecting technology. Nevertheless, the high air- and moisture-sensitivity of these compounds usually makes boroles synthetically challenging and improvement in their stability is necessary before they can be applied in optoelectronic materials. The stabilization of boroles by steric congestion has indeed been noted and achieved by both Yamaguchi17 and Rupar,18 which pose promising opportunities for future applications.

In addition to boroles, the incorporation of three-coordinate boron into heterocycles has seen a surge of research in recent years.19 The empty p-orbital at boron leads to electron-deficient compounds and, as in the case of boroles, behave as excellent π- acceptors in conjugated organic chromophores. Applications such as anion-selective sensing and biological imaging are rapidly emerging, which highlight the value of developing facile synthetic routes to similar heterocycles.

1.1.4 Synthesis and applications of tellurium-containing heterocycles

Another element that has drawn much recent research attention in the material sciences is tellurium. Tellurium is the heaviest non-radioactive member of the group 16 elements. Elemental tellurium makes up less than 1 ppb of the earth’s crust, and consists of a mixture of several isotopes, the four major ones being 125Te (7.0%) 126Te (18.7%), 128Te (31.8%), and 130Te (34.5%). 125Te has nuclear spin I = ½, which allows for NMR spectroscopic characterization of tellurium compounds. The range of 125Te NMR spans from ca. -1800 ppm for Te2- anions to ca. +3100 for Te2+ cations, thus providing valuable information on the oxidation state of tellurium.20 Tellurium is most often used as an additive to steel or copper to produce more malleable alloys. More recently, tellurium have been used in the electronic industry as components of memory chips and CDs. Semi-conducting metal tellurides have also been used as thermoelectric materials and in solar panels.20

In most chemistry textbooks, tellurium is usually the overlooked member of the chalcogen family. It is often implied that the structures and reactivity of tellurium compounds directly parallel those of sulfur and selenium. Some recent breakthrough discoveries in tellurium chemistry, however, contradict this assumption. The dramatic differences between tellurium compounds and other organochalcogen compounds can be understood from a few key aspects of tellurium. Firstly, the high polarizability and relatively weak Te–C bonds make organotellurium compounds susceptible to three- centre-four-electron bonding. This leads to Lewis acidic behaviour of tellurium, which is manifested in the many reports of tellurium compounds engaged in hypervalent and secondary bonding interactions. Secondly, the large energy gap between 5s and 5p orbitals in tellurium leads to often poor sp hybridization and unusual geometry compared to analogous selenium or sulfur compounds. Lastly, unlike the other light elements in group 16, tellurium is less electronegative (EN = 2.1) than carbon (EN = 2.55).21 This last factor makes many synthetic strategies that are applicable to organosulfur and organoselenium compounds ineffective when applied to the preparation of organotellurium compounds.

Despite these challenges, tellurium-containing heterocycles have recently been synthesized and studied in detail. This rapid increase in interest is driven by a number of application-oriented incentives. The anticancer activity imparted by porphyrins containing tellurophenes was well-established by early 2000’s.22 Furthermore, the incorporation of tellurophenes into conjugated polymers was shown to significantly narrow the HOMO-LUMO gap of the material.23-25 Most recently, certain tellurophenes were reported to exhibit phosphorescence,26, 27 while others were shown to undergo photoreductive elimination28 or reversible oxidation in water.29 All these reports hint at tellurophene’s rich potential in future applications.

Despite all these promising reports, the development of tellurium-containing heterocycles has always been hampered by the limited and often strenuous synthetic routes to the compounds in question (Scheme 1-2). The first report of the isolation of tellurophene involved the direct reaction of 1,4-dilithiotetraphenylbutadiene and tellurium tetrachloride. The reaction gave all-phenyl-substituted tellurophenes (1-3) in 53% yield.30 Currently, mono- or di-substituted tellurophenes are mostly prepared by the reaction between substituted 1,3-butadiynes and sodium telluride.31 Vinylic tellurides have also been used in a number of reports to prepare tri-substituted tellurophenes.32 Most recently, Rivard et al. utilized transmetalation to convert zirconocene precursors to fully substituted tellurophenes.33 It is worth noting, however, that all of these synthetic strategies require that ring closure occurs in the last step. Direct derivatization of tellurophenes can be achieved, but often leading to low yields and requiring very specific reaction conditions.34 This is again a result of the weak Te-C bonds that can be easily cleaved by common nucleophiles.

Scheme 1-2. Common synthetic strategies of substituted tellurophenes.

In addition to tellurophenes, tellurium analogues of common dyes have found promising uses in medicinal applications. Detty et al. made significant contributions to the field by examining the efficacy of telluropyrylium dyes as photosensitizers in photodynamic therapy (PDT).35 They demonstrated that telluropyrylium derivatives have higher absorption maxima and singlet oxygen quantum yields than selenium analogues. This observation is not surprising due to the increased spin-orbit coupling in heavy atoms, allowing for more efficient singlet-to-triplet crossing. In vivo studies thus showed promising tumour-targeting capabilities of the telluropyrylium dye 1-4 (Scheme 1-3).36 Interesting, in the in vitro studies, Detty et al. further noticed fluorescence from the oxidized telluropyrylium dyes.37 This fluorescence turn-on of tellurium centres from reversible oxidation has recently been exploited as a probe for redox cycles in living cells (Scheme 1-3).38, 39 These examples attest to the unique properties of organotellurium chemistry that are not found in lighter organochalcogen species, thus underlining the importance for furthering developing our understanding of organotellurium chemistry.

Scheme 1-3. Representative examples of tellurium-containing dyes that have been used in biological applications.

1.2 Frustrated Lewis Pairs

1.2.1 Discovery

Lewis acids and bases typically form strong adducts that exhibit reactivities different from the constituent compounds.40 In 1942, however, H. C. Brown showed that 2,6- 41 lutidine forms an adduct with BF3 but not with the bulkier Lewis acid BMe3. Wittig also later showed that the combination PPh3, BPh3 and benzyne led to the zwitterionic 1,2- addition product.42 These earlier reports all allude to the possibility of harnessing unquenched reactivity of bulky Lewis acids and bases for the activation of small molecules. This concept was realized in 2006, when Stephan and co-workers reported 43 the activation of H2 by compound 1-5 (Scheme 1-4). Shortly following this initial report, it was demonstrated that combinations of commercially available B(C6F5)3 with bulky t 44 bases such as Bu3P or Mes3P could also effect the same activation. The 1,2-addition of these phosphine/borane mixtures with olefins followed shortly thereafter.45 In the same year, Erker and co-workers reported the H2 activation using ethylene-bridged phosphine/borane 1-6.46 These reports set the foundation for what is now called

“frustrated Lewis pair” (FLP) chemistry, wherein sterically encumbered electron donors and acceptors are combined to act cooperatively in the activation of different substrates. FLPs can often perform reactions that are normally observed only for transition metal systems, and have since grown into a rapidly evolving field of research.

Scheme 1-4. Earliest examples of FLPs.

1.2.2 Small molecule activation and catalysis

Since the reports in 2006 and 2007, FLP chemistry has been applied in a wide variety of contexts. The logical extension from the activation of H2 was metal-free hydrogenation of organic compounds, which has indeed been demonstrated in a variety of systems. The first reports of FLPs used as hydrogenation catalysts involved the reduction of polar substrates such as imines, nitriles, aziridine, and anilines (Scheme 1-5).47 In many cases, the H2 molecule is activated between B(C6F5)3 and the nitrogen donor of the substrate, thus eliminating the need to add an external Lewis base. The hydrogenation of non-polar substrates such as olefins, alkynes, and aryl compounds were also achieved in the past few years. Previously it was believed that ketone or aldehyde reduction could not be achieved due to the protonolysis of B(C6F5)3 by the acidic proton

9 in the alcohol final product. This challenge was elegantly resolved when the use of ethereal solvents or bulky Lewis acids were shown to prevent this unfavourable protonolysis (Scheme 1-5).48-51

Scheme 1-5. Selected examples of hydrogenation of polar substrates by FLP mechanism.

Beyond the reaction with H2, a wide plethora of small molecules have been captured by 52-54 FLPs, including CO2, SO2, N2O, NO and CO. In these reactions, the FLP can either undergo 1,1- or 1,2-addition to the substrate, as shown in two different cases in Scheme 1-6. While the stoichiometric fixation of these small molecules are often facile and proceed quantitatively under mild conditions, the catalytic reduction of CO2 by FLPs remains a challenge.55 One notable exception is a recent report by Fontaine and co- workers, where they demonstrated the hydroboration of CO2 to produce methoxyboranes using the intramolecular FLP 1-7 (Scheme 1-6).56

Scheme 1-6. Selected examples of FLP activation of NO and CO2.

The reactions between FLPs and organic molecules have also been extensively studied. In addition to olefins, FLPs can also undergo 1,1-, 1,2-, or 1,3-addition with a variety of unsaturated organic molecules, including alkynes, cyclopropanes, ketones, disulfides, carbodiimides, and N-sulfonyl amines (Scheme 1-7).57-59 Several of these addition reactions lead to further cyclization, giving rise to exotic zwitterionic heterocycles. For example, anilines with pendant olefins can be cyclized with the 60 addition of B(C6F5)3 to afford zwitterionic N-heterocycles (Scheme 1-7). These cyclization reactions have since been adapted to install boron in a wide range of synthetic building blocks.61 The range of these fundamental transformations using FLPs are especially encouraging as supplies for transition metal-based reagents decrease, and metal-free alternatives become more attractive reagents to access synthetically useful compounds.

Scheme 1-7. Selected examples of FLP addition reactions to unsaturated organic bonds.

1.3 Main Group Lewis Acids beyond Boron

1.3.1 Group 13 Lewis acids

While a wide range of Lewis bases have been shown to participate in FLP-type reactions, commercially available B(C6F5)3 is by far the most commonly used Lewis acid due to its high acidity and steric encumbrance. Moreover, reactions using B(C6F5)3 can be easily monitored by both 19F and 11B NMR spectroscopy.

More recently, both our group and others have investigated the FLP chemistry involving heavier group 13 Lewis acids. In the first few reports, AlX3 (X = Cl, Br, I) were 62 demonstrated to activate CO2 in combination with PMes3, while mixtures of Al(C6F5)3 t 63 and Bu3P were found to undergo 1,2-addition reactions with alkynes. It was soon observed that using heavy-atom Lewis acids in FLP chemistry could lead to unexpected products. For example, the activation of CO2 by of AlX3 and PMes3 gave bridging 62 species (Scheme 1-8), which contrasts the simple 1,2-addition of CO2 by classical B/P

FLPs. This Al/P activation of CO2 was further shown to lead to the reduction of CO2 to

CO and methanol by the addition of ammonia-borane.62, 64 Another interesting example of the unique reactivities of aluminum involves the Al(C6F5)3 activation of N2O with phosphines, which led to a radical reaction that activated C-H bonds (Scheme 1-8).65 During the past decade, Uhl and co-workers have also conducted extensive studies on the reactivities of intramolecular P/Al and N/Al FLPs. They have demonstrated the versatility of these FLPs in reactions with a wide variety of organic substrates and as hydrogenation or phase-transfer catalysts.66-69

Scheme 1-8. Selected examples of FLP reactions involving heavy group 13 elements.

The success of aluminum-based FLPs logically led to the investigation of gallium- and indium-based FLPs. For example, Uhl and co-workers successfully prepared a series of intra-molecular Ga/P and In/P FLPs using a phosphinylvinyl Grignard reagent. Perhaps not surprisingly, the generally less polarizing Ga centre prevented Ga/P FLP from 70 activating classic FLP substrates such as CO2 and aldehydes. These Ga/P FLPs can, however, activate alkynes, HX (F, Cl, Br, I) bonds, as well as heterocumulenes R– 71 NCY(Y = O, S). Furthermore, using the more Lewis acidic Ga(C6F5)3 and In(C6F5)3 acceptors allowed for the activation of H2 with phosphines in a manner similar to Al/P FLPs (Scheme 1-8).72, 73

1.3.2 Hypervalency in main group compounds

Outside of group 13 elements, the synthesis of non-traditional Lewis acids has been a popular research topic in the past few years, with many examples emerging from p- block elements that are not typically considered to be Lewis acidic. An important concept involved in the syntheses and reactivities of group 14 and 15 Lewis acids is hypervalency. A hypervalent compound is generally defined as a species whose “Lewis structure demands the presence of more than an octet of electrons around at least one 21 atom.” Typical examples include XeF2 and SF6, and this phenomenon is often explained by invoking the use of low-lying unfilled d-orbitals for hybridization. The seminal work by Magnusson in 1990, however, refuted this explanation,74 though many textbooks today continue to explain hypervalent compounds in this fashion. The confusion seems to arise from the fact that the use of d-functions is necessary to improve computational basis sets, but the use of d-orbitals is not necessary to increase electron population around the hypervalent atom.

Figure 1-1. Simplified MO diagram showing the three-centre-four-electron bonding

orbitals of XeF2.

The controversy over hypervalent compounds can be understood from a historical perspective.75 The term “hypervalency” was first introduced by Musher in 1969,76 but the debate over the electronic structures of hypervalent species began already in the 1920s and continue even to this day. Today, the dominant explanation of hypervalent compounds is the Rundle-Pimentel model,77, 78 which promotes the concept of three- centre-four-electron bonds, wherein only two of the four electrons in question are actually bonding in a compound with three collinear atoms (Figure 1-1). Later

14 quantitative MO calculations largely confirmed this picture of bonding. However, there is still criticism against using the term “hypervalent”,79 most notably from Ronald Gillespie80, 81 and Paul von Ragué Schleyer.82 Gillespie commented that the term “hypervalency” is misleading as “there is no fundamental difference between the bonds in hypervalent and non-hypervalent molecules”,81 while Schleyer suggested using the term “hypercoordinate” in 1984 to mitigate this debate.82

Despite the debate, the term “hypervalency” has become the norm today in describing compounds whose Lewis structures do no obey the octet rule. It is nevertheless important to keep in mind that the bonds in these species are often highly polarized and thus there are often less than 8 electrons attributed to the “hypervalent” element.83

1.3.3 Carbon and silicon Lewis acids

+ Isoelectronic to the ubiquitous FLP Lewis acid B(C6F5)3 is the trityl cation [Ph3C] . This cation is highly Lewis acidic due to its positive charge and the empty p-orbital at the central carbon. The use of trityl as a Lewis acid catalyst has had a long history since its first report in 1887, including cyclization reactions84, 85 and hydrothiolation of olefins.86 The use of trityl cation in FLP chemistry, however, has been limited due to its susceptibility to nucleophilic aromatic substitution by strong bases, which inhibits all subsequent reactions.87 This limitation has been overcome recently by our group in demonstrating that the combination of trityl cation ([Ph3C][BF4]) and bulky (o-tolyl)3P can activate terminal alkynes and disulfides (Scheme 1-9).88

Scheme 1-9. Selected examples of carbon-based Lewis acids involved in FLP-type activities (Ar = tetra(3, 5-dichlorophenyl)borate).

In addition to trityl cations, there have been a few reports on the FLP activities of carbon-based Lewis acids. The Alcarazo group demonstrated that S-S bonds can be cleaved by electron-poor allenes in conjunction with carbene Lewis bases (Scheme 89, 90 1-9). Shortly after, Arduengo and co-workers demonstrated the activation of H2 by an all-carbon FLP.91 Our group then demonstrated that a Ru-η6-arene complex can act as a carbon-based Lewis acid to effectively hydrogenate imines.92 Most recently, Ingleson and co-workers reported an impressive acridine-stablized borenium cation that can act either as a borenium- or carbon-based Lewis acid to activate H2 (Scheme 1-9).93, 94

Moving down group 14, Manners and co-works explored FLP chemistry using TMSOTf and found that it is effective in the dehydrogenation of amine-boranes in combination of 95 sterically hindered amines. Our group also recently reported the activation of CO2 by silyl triflates (Scheme 1-10).96 Theoretically, silylium cations with more weakly- coordinating anions should be even better Lewis acids,97-100 but the application of such silylium cations to FLP chemistry has always been limited by their high sensitivity to moisture and chlorinated solvents. Pioneering work by the Müller group showed that the bulky silylium cation [(C6Me5)3Si][B(C6F5)4] in combination with PMes3 can indeed

101, 102 cleave H2 and CO2 (Scheme 1-10). Not surprisingly, they noted the necessity of steric bulk to facilitate this reactivity. Lastly, Ashley and co-workers recently demonstrated that silylium cation-phosphine adducts can also activate H2 at elevated temperatures.103

Scheme 1-10. Selected examples of silicon-based Lewis acids involved in FLP-type reactions.

1.3.4 Group 15 Lewis acids

Group 15 elements are typically considered to be Lewis bases – in the most common oxidation state of 3+, they possess one lone pair of electrons. There are many indications in the literature, however, of phosphorus exhibiting Lewis acidic behaviour. One of the earliest classes of such compounds is phosphenium cations, which were discovered in 1964104 and reviewed by Cowley and Kemp in 1985.105 During the next two decades, many donor-stabilized cationic P(III) complexes were reported, as summarized in two reviews by Burford and Ragogna in 2002 and 2006 (Figure 1-2).106, 107 More recently, the groups of Burford,108-110 Weigand,111, 112 and Alcarazo113, 114 have also made significant contributions to this field by using oxidatively resistant donors that allow for the isolation and complete characterization of these highly reactive species. Another notable example came from Nieger and co-workers, who developed N- heterocyclic phosphenium cations (NHPs) and found them to be more electrophilic than the well-known N-heterocyclic carbenes (NHCs).115, 116 A recent breakthrough in the application of P(III) Lewis acids was reported by the Radosevich group in 2012 (Figure 1-2), where they used a planar P(III) redox active catalyst to effect the reduction of azobenzene. At around the same time, Slattery and Hussein developed a scale for

17 fluoride ion affinities and showed that P(III) cations could be more Lewis acidic than 117 traditional main group Lewis acids such as BF3 and AlCl3.

As for P(III) species, P(V) compounds were recognized as Lewis acidic as early as in the 1960s,118, 119 though many examples of P(V) species were already known even earlier, such as phosphates and the Wittig reagent.120 In their 1996 review, Cavell and co-workers noted that the Lewis acidity of P(V) compounds had received little attention beyond their fundamental coordination chemistry.121 In the 2000s, however, phosphonium cations began to gain attention as Lewis acid catalysts that can facilitate additions to unsaturated polar substrates and Diels-Alder reactions.122, 123 In 2013, Stephan and co-workers successfully employed electrophilic phosphonium cations (EPCs) in the activation of strong C-F bonds (Figure 1-2).124 In the case of triarylfluorophosphonium cations, the high acidity is derived from the presence of a σ* orbital oriented trans to the fluoride substituent and sterically sheltered by the peripheral arene rings.125 Since then, EPCs have been applied to a wide number of reactions, including hydrodefluorination of fluoroalkanes,124 hydrosilylation of olefins and alkynes,126, C-C coupling of benzyl fluorides,127 as well as dehydrogenative coupling of silanes with amines, phenols, carboxylic acids, or thiols with concurrent olefin hydrogenation.128 Another class of EPCs developed by the Stephan group is based on phosphenium cations (Figure 1-2),129 which are even more Lewis acidic than triarylfluorophosphonium cations due to the presence of a second positive charge. In addition to the known reactivities of EPCs, these dications can also catalyze ketone deoxygenation130 and phosphine oxide reduction.131

Figure 1-2. Representative examples of phosphorus-based Lewis acids.

Phosphorus is not the only group 15 element known to exhibit Lewis acidic behaviour. Neutral antimony and bismuth compounds can be used as Z-type ligands to induce transition metal-to-ligand electron flow.132, 133 In a similar fashion, many analogous compounds with pendant donor arms show coordination between Sb or Bi and the donor atoms.134 in addition, arsenium and stibenium cations analogous to NHPs have been known for over a decade.135 Sb(III) and Sb(V) cations bearing various charges have also been recently synthesized and systematically studied by the Burford group (Figure 1-3). 109, 110, 136, 137 In terms of applications of these Lewis acids, Gabbaï and co- workers have notably exploited the high fluoride ion affinity of stibonium cations as fluorescent anion-sensing probes.138, 139 Most recently, they have also applied these highly Lewis acidic stibonium cations to the activation of C-F bonds (Figure 1-3)140 and the hydrosilylation of benzaldehyde.141 Some of these reactions can be performed under ambient conditions, which is in stark contrast to their phosphonium cation counterparts. Another important breakthrough in the field of group 15 Lewis acids is the report of nitrogen-based Lewis acid by Gandelman and co-workers this year. This new

19 class of compounds is based on triazolium/nitrenium salts, which have been shown to form stable adducts with common Lewis bases.142

Figure 1-3. Selected examples of group 15 Lewis acids based on N, As, Sb and Bi.

1.4 Scope of Thesis

The main objective of this dissertation is to expand the scope of FLP chemistry to include group 16-based compounds, as they can function both as Lewis bases and as Lewis acids. The first portion of this dissertation (Chapters 2 and 3) is focused on the syntheses and reactivities of tellurium-boron heterocycles, wherein tellurium acts as a Lewis base. Chapter 2 includes discussions of 1,1-carbobobration reactions of tellurium- substituted acetylides, which produce a number of tellurium-boron heterocycles, including the first reports of 1,4-telluraborine. Chapter 3 expands on this work and describes the various reactivities of 1,4-telluraborines. Many of these reactions can be used to derivatize 1,4-tellurborines in high-yielding and selective fashion. Chapter 4 explores the possibility of using hypervalent sulfur and tellurium compounds as Lewis acids both in catalysis and in FLP chemistry. As these two elements exhibit distinct chemistry, the design of sulfur and tellurium Lewis acids differs significantly. Lastly, in chapter 5 we revisit the known 1,1-carboboration reaction of terminal alkyne by B(C6F5)3 and report a new method of E/Z isomerization using commercially available reagents

The computational work detailing reaction profiles presented in Chapters 3 and 5 and fluoride ion affinity calculations in Chapter 4 were performed by the research group of Professor Stefan Grimme at the Mulliken Center for Theoretical Chemistry (University of Bonn). A number of catalytic reactions involving fluorosulfoxonium cations presented in

Chapter 4 were performed by Mr. Alexander Waked, while computational studies of these compounds was conducted by Mr. Levy Cao. The synthesis and isolation of compound 4-1 were first achieved by Ms. Jordan Hofmann. Although not explicitly included in this dissertation, some initial screening of substrates in Chapter 5 was done by Mr. Anson Sathaseevan. The remaining computational and synthetic work was conducted by the author, with the exception of elemental analysis and high resolution mass spectroscopy.

At the time of the writing of this dissertation, portions of each chapter have been published:

Chapter 2: (a) F. A. Tsao and D. W. Stephan, Dalton Trans., 2015, 44, 71. (b) F. A. Tsao, A. J. Lough and D. W. Stephan, Chem. Commun., 2015, 51, 4287.

Chapter 3: (a) F. A. Tsao, L. Cao, S. Grimme and D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 13264. (b) F. A. Tsao and D. W. Stephan. Chem. Commun., 2017, 53, 6311.

Chapter 4: F. A. Tsao, A. E. Waked, L. Cao, J. Hofmann, L. Liu, S. Grimme and D. W. Stephan, Chem. Commun., 2016, 52, 12418.

Chapter 5: F. A. Tsao, A. Sathaseevan, H. Zhu, S. Grimme, G. Erker and D. W. Stephan, Chem. Commun., 2017, 53, 9458.

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Chapter 2. 1,1-Carboboration of Tellurium Acetylides 2.1 Introduction

2.1.1 1,1-carboboration of activated alkynes

The 1,1-carboboration of heteroatom-substituted alkynes by triorganoboranes has been known for decades. As early as 1965, it was demonstrated that the addition of one equivalent of trimethylsilyl chloride to sodium 1-alkynyltriorganoborates generates the corresponding alkenylborane with the elimination of sodium chloride (Scheme 2-1).1 This reaction was soon developed by the Wrackmeyer group into a one-pot procedure that involved the direct reaction between various triorganoboranes and an “activated” alkyne (Scheme 2-1) to generate tetra-substituted alkenylboranes.2, 3 These activated 1 2 alkynes were substrates where R or ER 3 is silyl, germyl, stannyl, or plumbyl group, though some examples of transition metal substituents were also investigated.4-6 Mechanistically, the 1,1-carboboration reaction involves two steps. Firstly, the triorganoborane abstracts the alkynyl moiety to generate the zwitterionic intermediate 2- A. Then, in the second step, the R substituent rearranges to give either the E- or the Z- isomer as the final product 2-B. Indeed, Wrackmeyer et al. were able to isolate and crystallographically characterize examples of the crucial intermediate 2-A, where the 2 7-9 ER 3 group is either a stannyl or plumbyl substituent.

Scheme 2-1. Discovery of 1,1-carboboration reaction and its mechanism.

1,1-carboboration was also applied to reactions between triorganoboranes and bis(alkynyl) substrates. The reaction sequences following the first intermolecular 1,1- carboboration varies (Scheme 2-2). Intermediate 2-C could theoretically undergo one of three competing reactions: 1,2-carboboration (2-D), 1,1-alkylboration (2-E), or 1,1- alkynylboration (2-F).2, 3 In the majority of the cases investigated by the Wrackmeyer group, 2-F was observed as the main product, though products 2-D and 2-E were often observed in the in situ NMR spectra. These reactions were thus shown to be powerful tools in the otherwise labourious syntheses of group 14 heterocycles such as siloles and stannoles.

Scheme 2-2. Synthesis of various heterocycles via 1,1-carboboration.

It is worth noting that the reactions involving heavy atom-substituted alkynes proceed under much milder conditions compared to those involving lighter congeners. Examples involving silyl substituents especially required more forcing conditions. For example, the reaction between triethylborane and (CH3)2Si(C≡CCH3)2 was reported to take place only with the use of excess borane under high temperatures.10 In contrast, the analogous reaction using (CH3)2Sn(C≡CCH3)2 proceeded over several hours at room temperature.8 This differentiation in reaction kinetics is likely a result of the highly polarized E-C bonds as the atomic weight of the heteroatom increases, which allows for the facile abstraction of the acetylenic moiety by boranes.

2.1.2 1,1-carboboration using highly electrophilic boranes – FLP applications

As the crucial step of 1,1-carboboration is the abstraction of the acetylenic fragment by borane, increasing the borane’s Lewis acidity should improve the ease of these reactions. Keeping in line with this reasoning, there has been a recent resurgent interest in applying the strongly electrophilic B(C6F5)3 to 1,1-carboboration reactions. Utilizing this strategy, the Erker group has investigated the reactions between B(C6F5)3 and

31 various phosphorus-, silicon-, boron-, and sulfur-substituted alkynes.11-17 Indeed, with 18-20 few exceptions, reactions between these alkynes and B(C6F5)3 or RB(C6F5)2 (R = alkyl or aryl) usually proceeded to completion within minutes at room temperature. Moreover, these reactions all demonstrated a high stereoselectivity favouring the E- isomer.

Many of products from the aforementioned examples of 1,1-carboboration were employed to synthesize new intramolecular FLPs such as 2-G and 2-H (Scheme 2-3). The reaction between 2-G and isocyanide gave the 1,1-addition product 2-I14 while the

2-H was shown to activate H2 at room temperature.

Scheme 2-3. Examples of FLPs synthesized via 1,1-carboboration using B(C6F5)3.

In addition to the synthesis of P/B FLPs, Erker et al. have also used 1,1-carboboration reaction to access S/B FLPs.21, 22 The reaction between acetylenic thioether

PhC≡CSCH2Ph and B(C6F5)3 readily gave product 2-J (Scheme 2-4), which showed significant intramolecular S-B interaction both in solution and in the solid state. Nevertheless, it was demonstrated the S/B FLP could activate terminal alkynes under mild conditions. It is worth noting that compound 2-J is one of the first examples of an FLP using a group 16 element as the Lewis base. The only other example prior to the synthesis of 2-J was reported in 2011 by our group, and we also were able to show the activation of terminal alkynes by a thioether-methyleneborane.23 In general, sulfur

32 appears to be too weak of a Lewis base to activate classic substrates in FLP chemistry, such as H2 and CO2.

Scheme 2-4. Synthesis and reactivities of S/B FLPs.

2.1.3 Synthesis of heterocyclic compounds using 1,1-carboboration

Encouraged by the ease of reaction between B(C6F5)3 and activated alkynes, the Erker group proceeded to investigate the possibility to synthesize novel heterocycles using the same strategy. Most notably, the syntheses of borane-appended phospholes,11, 18, 20, 24 boroles25-28 and thiophenes29 have been achieved in this fashion (Scheme 2-5). There are many advantages to this synthetic strategy compared to other traditional methods (See chapter 1) of accessing these compounds. For example, the appended borane can be further subjected to Pd-catalyzed cross-coupling reactions to prepare penta-substituted phospholes and thiophenes (Scheme 2-5). Additionally, the synthesis of the acetylenic starting material is relatively simple and the cyclization can be accomplished in one simple and high-yielding step.

Scheme 2-5. Synthesis of phospholes and thiophenes by 1,1-carboboration (tipp = 2, 4, 6-triisopropylphenyl; Thi = 2-thiophenyl).

At the start of this thesis no examples of 1,1-carboboration reactions involving tellurium- substituted alkynes were reported in the literature. Considering the rising interest in tellurium heterocycles in recent years (see chapter 1), we decided to investigate the possibility of synthesizing new tellurium heterocycles via 1,1-carboboration using the highly electrophilic B(C6F5)3. Additionally, as tellurium possesses two lone pairs of electrons that could act as Lewis bases, we were also interested in investigating the FLP chemistry of intramolecular Te/B systems prepared in this fashion.

2.2 Results and Discussion

2.2.1 Reactions of a tellurium monoacetylide with boranes

We began our investigation by synthesizing the tellurium acetylide, Ph(CH2)2TeCCPh (2-1),30 a known compound that could be prepared at 73% yield. Treating 2-1 with one equivalent of B(C6F5)3 in pentane at room temperature led to an immediate colour change from yellow to bright orange (Scheme 2-6). Removal of the volatiles from the reaction mixture in vacuo afforded Ph(CH2)2TeC(Ph)C(C6F5)B(C6F5)2 (2-2) as a bright

34 orange solid at 94% yield. The 1,1-carboboration of 2-1 is evident in the 19F{1H} NMR spectrum of 2-2, which displays two sets of C6F5 resonances in 2:1 ratio of integration, corresponding to the B(C6F5)2 group (-130.4, -154.7, -163.2 ppm) and the vinyl C6F5 group (-138.6, -155.3, -162.8 ppm). These data support the exclusive formation of the E-isomer of 2-2. The presence of the Te-vinyl moiety is supported by the corresponding resonance at 100.6 ppm in the 13C{1H} NMR spectrum. There is also a dramatic shift of the tellurium resonance in the 125Te NMR spectrum from 298.3 ppm in 2-1 to 467.9 ppm in 2-2. The 11B{1H} NMR signal of 2-2 at 1.61 ppm suggests only a weak interaction between the tellurium and the boron centres. This stands in contrast to the analogous S/B systems.21-23

Scheme 2-6. Reaction between tellurium-substituted acetylene and boranes.

Similar 1,1-carboborations of 2-1 using the boranes PhB(C6F5)2 and MeB(C6F5)2 (Scheme 2-6) proceeded in a similar fashion at room temperature, giving exclusively the E-isomers of 2-3 and 2-4, respectively. Monitoring the reactions by 19F{1H} NMR spectroscopy showed complete consumption of the starting material within minutes of reaction. In these cases, carboboration proceeded with migration of the more electron- donating substituents on the borane (phenyl in 2-3; methyl in 2-4) to the alkynyl carbon. This is evident in the 19F{1H} NMR spectra of isolated 2-3 and 2-4, which both show only one set of C6F5 resonances (2-3: -130.1, -155.5, -163.4 ppm. 2-4: -130.7, -156.1, -163.6 ppm), corresponding to the B(C6F5)2 substituent. These observations are consistent with the known mechanism for 1,1-carboborations of activated alkynes2 and the formulations of 2-3 and 2-4 as shown in Scheme 2-6.

Encouraged by the ease of 1,1-carboboration of tellurium-substituted acetylide, we further applied the protocol to the reaction of 2-1 with triphenylborane (BPh3), which is a

35 relatively weak Lewis acid and thus rarely employed in 1,1-carboboration reactions. Surprisingly, the reaction proceeded smoothly at room temperature and was completed within 16 h, giving compound 2-5 as a bright yellow viscous oil that solidified upon standing at room temperature. The 11B{1H} NMR spectrum of isolated 2-5 shows a single 11B{1H} resonance at 42.9 ppm, which is shifted significantly more upfield in comparison to free BPh3 (67.8 ppm) but still corresponds to a three-coordinate boron centre. This observation contrasts the 11B{1H} shift of 2-2, which is consistent with the lower Lewis acidity of the boron centre in 2-5. The 13C{1H} NMR spectrum of 2-5 also shows a resonance at 116.4 ppm, which is characteristic of a Te-bound vinyl moiety. Unfortunately, efforts to crystallographically characterize compounds 2-1 to 2-5 were fraught with difficulty owing to the extremely high solubility of these compounds in organic solvents.

2.2.2 Exploration of Te/B reactivities

As compound 2-2 can be viewed as an intramolecular FLP, we decided to investigate its reactivity towards small molecules and organic substrates. Pressurizing a solution of 2-2 in C6D6 with 1 atm of CO2, H2 or CO all resulted in no change in its spectroscopic characterization. Similar to the case of S/B FLPs, it is likely that tellurium is too weak of a Lewis base to activate these small molecules. On the other hand, a pentane solution containing 2-2 immediately turned from orange to bright red when treated with benzaldehyde (PhCHO). After all volatiles were removed in vacuo, compound 2-6 could be isolated by recrystallization or precipitation with cold pentane (Scheme 2-7). The 11B{1H} NMR spectrum of 2-6 shows a single resonance at 2.1 ppm, which is very close to that of compound 2-2 (1.6 ppm). However, the 19F{1H} NMR resonances attributed to 1 the B(C6F5)2 group of 2-6 are now broad and shifted upfield. Additionally, the H NMR spectrum of 2-6 shows a 2:1 integration between the resonances corresponding to the

TeCH2 fragment and the PhCHO group. This is consistent with the generation of adduct 2-6 (Scheme 2-7). The 13C{1H} and 1H NMR signals assigned to the carbonyl group are both broad, suggesting that compound 2-6 is fluxional in solution at room temperature. 19 1 Indeed, upon cooling a solution of 2-6 in CD2Cl2 to -80 °C, fifteen F{ H} NMR

36 resonances were observed, which is consistent with hindered rotation around the C6F5 groups due to the coordination of PhCHO to the boron center (See 2.3.2).

Scheme 2-7. Reactions of intramolecular Te/B FLP 2-2 with organic substrates.

The formation of 2-6 in the solid state was confirmed by a single-crystal X-ray diffraction study (Figure 2-1). The B–O and C–O bond lengths of 1.537(3) and 1.271(3) Å, respectively, which are typical for such adducts.31 In contrast to the benzaldehyde adduct of closely related P/B FLP,31 there is no interaction between the Lewis basic tellurium and the carbonyl carbon. This is seen in the long Te–C(41) distance of 2.863(2) Å, which is ca. 0.7 Å longer than a typical Te–C single bond.32 These data suggest that that FLP 2-2 cannot activate the C=O bond in PhCHO, which again demonstrates the low Lewis basicity of the telluroether donor.

Figure 2-1. Solid-state structure of 2-6 (50% thermal ellipsoids). C: black, B: yellow green, F: pink, O: red, Te: green. Hydrogen atoms are omitted for clarity.

The reaction of 2-2 with phenylacetylene at room temperature likewise afforded a new species 2-7 in excellent yields (87%) (Scheme 2-7). The 11B{1H} NMR resonance of 2-7 at -10.8 ppm is diagnostic of a 4-coordinate borate species. The two C6F5 groups in the 19 1 B(C6F5)2 fragment are inequivalent in the F{ H} NMR spectrum, and the broad 1:1:1:1 quartet in the 13C{1H} NMR spectrum at 161.5 ppm is assigned to the terminal alkyne carbon coordinated to the boron centre. These data parallel previous work on related S/B FLPs, which were also shown to undergo 1,2-addition with terminal alkynes to yield zwitterionic heterocyclic products.21-23

Figure 2-2 Solid-state structure of 2-7 (50% thermal ellipsoids). C: black, H: white, B: yellow green, F: pink, Te: green. Most hydrogen atoms are omitted for clarity.

The six-membered ring structure of 2-7 was confirmed by single crystal X-ray diffraction analysis. The alkyne bridges between tellurium and boron and give pseudo-trigonal pyramidal coordination geometry for tellurium and pseudo-tetrahedral coordination geometry for boron. The Te–C(3) and B–C(4) distances are 2.146(3) and 1.628(4), respectively, which both fall well within the typical Te–C and B–C single bond lengths. The C(3)–C(4) bond distance of 1.326(4) Å is significantly longer than the typical C–C triple bond and closer to a C–C double bond, thus demonstrating the activation of C–C triple bond by compound 2-2.32

2.2.3 Oxidation of tellurium in Te-B FLPs

As diaryltelluroethers are known to undergo facile oxidation by halogen sources to give the corresponding Te(IV) dihalide species,33, 34 we also became interested in the possibility of oxidizing the tellurium centre in 2-2. Trial oxidations of compound 2-2 with

XeF2, SO2Cl2 and Br2, however, led to the formation of multiple compounds that could not be separated or isolated. Coordinating the boron centre in 2-2 to 2,4,6- trimethylbenzonitrile (MesCN) or tert-butyl isocyanide (tBuNC) prior to oxidation led to similar results. Nevertheless, the reaction between 2-2 and I2 proceeded relatively smoothly and product 2-8 could be isolated at 30% yield (Scheme 2-8).

Scheme 2-8. Oxidation of 2-2 by I2 to give 2-8.

Figure 2-3. Solid-state structure of 2-8 (50% thermal ellipsoids). C: black, B: yellow green, F: pink, Te: green, I: purple. Hydrogen atoms are omitted for clarity.

The oxidation state of tellurium in 2-8 is evident in its 125Te NMR spectrum, which shows a downfield singlet at 955.1 ppm that is typical of a Te(IV) species. The solid-state structure of 2-8 (Figure 2-3) shows seesaw coordination geometry around tellurium with one iodide bridging between tellurium and boron. This affords a unique 5-membered Te–I–B–C–C ring. The B–I(1) distance is 2.462(4) Å, which is only ca. 0.2 Å longer than B–I single bond distances seen in crystallographically characterized iodoborate adducts 35 36 Me3P–BI3 (2.24–2.27 Å) and 1,3,5-triiodo-1,3,5-triborocyclohexane (2.14–2.16 Å). The B–I(2) bond length is also much shorter than the B–I bond length reported for a related Pt–I–B bridging complex (2.75(2) Å). 37 The B–I interaction is further evidenced by pyramidalization at the boron center (Σ(C–B–C) = 346.4(3)°) and the fact that the bridging Te–I(2) bond length of 3.0499(4) Å is much longer than the terminal Te–I(1)

40 bond length of 2.7798(4) Å. This trapping of iodide between tellurium and boron is reminiscent of the work by Gabbaï and co-workers on fluoride sensing using polyfunctional Lewis acids featuring main-group onium ions.38-40

2.2.4 Reactions of a tellurium diacetylide with boranes

We next proceeded to investigate the reaction between a tellurium diacetylide and various boranes, with the hopes of synthesizing tellurium-containing heterocycles in one step. The bis(phenylethynyl)telluroether Te(C≡CPh)2 (2-9) was prepared by a modified 30 literature procedure. The addition of 2-9 in pentane to one equivalent of BPh3 at room temperature did not yield an immediate colour change. After stirring this mixture for 18 hours, however, an orange precipitate separated out of solution and afforded product 2- 10 at 93% yield (Scheme 2-9). The 11B{1H} NMR spectrum of 2-10 shows a single peak at 53.3 ppm. Single crystals of 2-10 suitable for X-ray analysis were grown from a saturated pentane solution and showed that the structure of 2-10 is a planar six- membered 1,4-telluraborine heterocycle (Figure 2-4). Phenyl substituents are found on boron and each of the linking olefinic carbon atoms. The formation of 2-10 is likely a result of an initial intermolecular 1,1-carboboration of 2-9, followed by an intramolecular 1,1-carboboration of the remaining acetylenic group on tellurium, thus affording the six- membered ring (Scheme 2-9). Even though 1,1-carboboration reactions have been documented extensively, there is only one example of analogous 6-membered heterocycle that has been crystallographically characterized.41 Compound 2-10 is also the first example of a 1,4-telluraborine. Analogous 1,4-thiaborine has been reported, albeit via a very different route of tin-boron exchange.42

Scheme 2-9. Reaction between tellurium acetylide 2-9 and various boranes to form compounds 2-10 to 2-15. (a) 1,1-carboboration; (b) FLP addition.

Figure 2-4. Solid-state structures of 2-10 (left) and 2-12 (right) (50% thermal ellipsoids). C: black, B: yellow green, F: pink, Te: green. Hydrogen atoms are omitted for clarity.

In a similar fashion, the addition of 2-9 in pentane to one equivalent of B(C6F5)3 at room temperature resulted in the immediate color change from yellow to dark red. The in situ 11B{1H} NMR spectrum showed two distinct singlets at -4.1 ppm (sharp) and 49.5 ppm (broad) at about a 3:7 ratio. Upon work-up, the product 2-12 was isolated in 57% yield.

An X-ray diffraction study revealed that 2-12 is directly analogous to 2-10, but with C6F5 rings on C2, C3 and B (Scheme 2-9, Figure 2-4). It is worth noting that the concentration and polarity of the solvent used in this reaction is critical in the successful isolation of 2-12. It is important that compound 2-9 must be added as a slurry in pentane or as a solid to a slurry of B(C6F5)3 in pentane in order to prevent undesired side reactions. This is due to the fact that B(C6F5)3 reacts within seconds with compound 2-9 even at -78 oC, which can easily lead to the polymerization of 2-9 if there is too high of a concentration of 2-9 in the solution. The sharp singlet at -4.1 ppm in the crude mixture was assigned to be 2-11 (Scheme 2-9, vide infra), which results from the dimerization of the linked Te/B intermediate. Heating isolated 2-11 at 80 oC for 8 hours in benzene or toluene resulted in the complete conversion of it to 2-12 as evidenced by the 19F{1H} NMR (Figure 2-5). Interestingly, any other polymeric byproducts could not be converted to 2-12 in the same way.

19 1 Figure 2-5. F{ H} (376.4 MHz, tol-d8, 298 K) NMR spectra of (top) pure 2-11 and (bottom) the same sample after heating at 80 oC for 8 hours, which gives 2-12.

In order to shed light on the difference in reactivities of 2-9 with boranes of different 11 1 Lewis acidities, we treated PhB(C6F5)2 with one equivalent of 2-9. The crude B{ H} NMR spectrum shows about a 1:1 mixture of two species, 2-13 (11B{1H}: -3.5 ppm) and 2-14 (11B{1H}: 50.6 ppm). Again, heating isolated 2-13 to 80oC for 8 hours resulted in its conversion to 2-14, albeit now with the formation of a few other side products as evidenced in the in situ 19F{1H} NMR spectrum (Figure 2-6).

19 1 Figure 2-6. F{ H} (376.4 MHz, tol-d8, 298 K) NMR spectra of (top) pure 2-13 and (bottom) the same sample after heating at 80 oC for 8 hours, which gives 2-14 (shaded peaks).

The species 2-14 was confirmed to be the 6-membered ring analogous to 2-10 and 2-12 (Figure 2-7). The solid-state structures of 2-10, 2-12 and 2-14 are very similar, featuring a central 1,4-telluraborine with various aryl substituents. The B–C and C=C bond lengths of the ring in 2-14 are 1.559(3) Å and 1.552(3) Å and 1.360(3) Å and 1.363(3) Å, respectively, suggesting some delocalization over the C–C–B–C–C fragment. The Te–C bond distances in 2-10, 2-12 and 2-14 all fall in the range of 2.063(3)-2.087(2) Å, which is significantly shorter than those seen in other Te–C single bonds,32 thus suggesting some degree of delocalization within the 6-membered heterocycle.

Figure 2-7. Solid-state structures of 2-14 (left) and 2-13 (right) (50% thermal ellipsoids). C: Black, B: yellow green, F: pink, Te: green. Hydrogen atoms are omitted for clarity.

Both 2-11 and 2-13 could be isolated by adjusting the solvent polarity of the reaction.

Indeed, the reaction of 2-9 with B(C6F5)3 or PhB(C6F5)2 in 1:4 mixtures of DCM:pentane directly resulted in the precipitation of 2-11 and 2-13, respectively, after stirring for 7 hours at room temperature. Single crystals of 2-13 could be grown from the slow evaporation of a saturated pentane solution of it at room temperature. The X-ray crystallographic analysis of 2-13 revealed it to be a species consisting of three fused rings featuring a central 1,4-ditellurocyclohexa-2,5-diene heterocycle. Both of the tellurium centres of compound 2-13 adopt distorted trigonal pyramidal geometry and are separated by 3.2896(7) Å which is within the range of previously reported Te–Te interactions in Te(IV)-based heterocycles (3.15–3.28 Å).43, 44 Fused to the Te–C bonds of the central rings are 5-membered TeCBC2 rings which incorporate four-coordinate boron centres (Figure 2-7). It is also noteworthy that the reaction between MeB(C6F5)2 and 2-9 afforded exclusively the product 2-15, which exhibits similar spectral parameters to 2-11 and 2-13, consistent with the formulation of 2-15 as

[(C6F5)2BC(Me)=C(Ph)TeC(CPh)]2.

The formation of compounds 2-10 to 2-15 from reactions of 2-9 with boranes are consistent with two reaction pathways following an initial 1,1-carboboration of one of the alkynyl fragments on tellurium. A second intramolecular 1,1-carboboration accounts for the formation of 1,4-telluraborines 2-10, 2-12 and 2-14 (Scheme 2-9). Alternatively, the intermediate Te/B species can undergo an FLP-type 1,2-alkyne addition45-47 with another molecule of the same intermediate to afford the tricyclic species 2-11 and 2-13. It is important to note that the thermal conversion of 2-11 and 2-13 to 2-12 and 2-14, respectively, illustrate that this Te/B FLP addition to alkynes is reversible, presumably as a result of the destabilizing steric congestion around 2-11 and 2-13. Interestingly, FLP addition products were only observed in cases where the transient Te/B species contains electrophilic B(C6F5)2 fragments, which suggests that a certain level of Lewis acidity is required to effect FLP additions. In the reaction between 2-9 and MeB(C6F5)2, the reduced steric congestion in 2-15 and the increased basicity of tellurium apparently confers stability and precludes dissociation to form 1,4-telluraborines. Efforts to convert 2-15 to 1,4-telluraborine were unsuccessful, leading only to thermal degradation and a mixture of unidentifiable products. Nonetheless, to the best of our knowledge, the reactions described herein are the first examples in which intramolecular 1,1- carboboration is in competition with intermolecular FLP addition.

2.2.5 Conclusion

Despite the well-established literature on 1,1-carboboration reaction of activated alkynes by electrophilic boranes, the 1,1-carboboration of tellurium acetylides were not reported until this present study. In line with previous studies, we found that 1,1- carboboration of tellurium acetylides proceed smoothly at room temperature, even with the weakly Lewis acidic BPh3.

The 1,1-carboboration of tellurium monoacetylide 2-1 provides a facile route to Te/B intramolecular FLP. The low nucleophilicity of tellurium allows for the FLP addition reactions with phenylacetylene but precludes reactions with other typical FLP substrates such as CO2 and H2. The 1,1-carboboration of tellurium diacetylide 2-9 leads to two distinct products depending on the borane employed in the reaction. Following initial

47 intermolecular 1,1-carboboration, the Te/B intermediate can either undergo intramolecular 1,1-carboboration or intermolecular FLP addition. Compounds 2-10, 2-12 and 2-14 are of particular interest as they are the first examples of 1,4-telluraborines. This facile and high-yielding synthesis is especially attractive for future applications in optoelectronic materials. Further derivatization of 1,4-telluraborines can be achieved via equally facile strategies, as will be detailed in Chapter 3 of this dissertation.

2.3 Experimental Section

2.3.1 General considerations

All experimental manipulations were conducted using standard Schlenk techniques or in o an O2-free, N2-filled MBraun LABmaster SP dry box equipped with a -35 C freezer, in either 4-dram glass vials with screw caps or in flame-dried Schlenk flasks. All protio solvents (Caledon Laboratories) were purified using a Grubbs-type column system (Innovative Technologies) and stored over 4 Å sieves or sodium wire in Straus flasks. Deuterated solvents (Cambridge Isotopes) were dried using appropriate drying agent

(CaH2 for CD2Cl2; Na/benzophenone for C6D6 and tol-d8) and distilled under reduced pressure prior to use. All solvents were degassed by repeated freeze-pump-thaw cycles prior to use.

All chemicals were used as received unless otherwise noted. nBuLi (1.6 M in hexanes), diethylzinc (ZnEt2), phenylacetylene, and benzaldehyde were purchased from Sigma- Aldrich. 2-bromoethylbenzene was purchased from TCI America Research Chemicals.

B(C6F5)3 was purchased from Boulder Scientific. Elemental tellurium was purchased from Alfa Aesar. AgNO3 was purchased from Apollo Chemicals. Resublimed iodine was purchased from ACP Chemicals. All liquid reagents were de-gassed by repeated freeze-pump-thaw cycles and stored over 4 Å sieves or CaH2 prior to use. BPh3 was purchased from Strem Chemicals and recrystallized from Et2O prior to use. 48 MeB(C6F5)2 was prepared using standard literature procedure.

NMR spectroscopy was performed on either a Bruker Advance III 400 MHz, an Agilent DD2 500 MHz, or an Agilent DD2 600 MHz. Unless otherwise stated, all spectra were obtained at room temperature. All NMR spectra were referenced to residual protio 1 13 19 solvent peaks of CD2Cl2 ( H = 5.32 ppm; C = 53.84 ppm) or an external standard ( F: 11 125 49 CFCl3 (δ 0.00), B: (Et2O)BF3 (δ 0.00), Te: Ph2Te2 (δ 420.8 ) ).

Combustion elemental analyses were performed on a PerkinElmer CHN Analyzer. HR- MS was performed on a JEOL AccuTOF equipped with a Direct Analysis in Real Time (DART) ion source. Repeated attempts to obtain elemental analysis for compounds 2- 11, 2-13 and 2-15 were all unsuccessful and gave extremely low carbon content. In addition, no molecular ion of these compounds could be detected using the same ionization method as those used for compounds 2-10, 2-12 and 2-14. This is likely a result of the unstable and zwitterionic nature of compounds 2-11, 2-13 and 2-15.

2.3.2 Synthetic procedures and spectroscopic characterization

Preparation and spectroscopic data of Zn(C6F5)2 • toluene

2.0358 g of B(C6F5)3 (3.9763 mmol) was dissolved in ca. 10 mL toluene and 744 mg of

ZnEt2 (6.02 mmol) was dissolved in ca. 3 mL toluene. The ZnEt2 solution was added to the B(C6F5)3 solution at room temperature and stirred for 16 hours. All volatiles were removed in vacuo to give a white solid, which was re-dissolved in minimal amount of toluene, and stored at -35 oC for 6 hours to get 1.7766 g (3.6136 mmol, 90.9% yield) of

Zn(C6F5)2 • toluene as large and colourless crystals. 19 1 3 F{ H} (376.4 MHz, C6D6): δ -118.1 (m, 4F, o-C6F5), -152.6 (t, 2F, JF-F = 19.8 Hz, p-

C6F5), -160.6 (m, 4F, m-C6F5)

Preparation and spectroscopic data of PhB(C6F5)2

302.5 mg of Zn(C6F5)2 • toluene (0.6153 mmol) was dissolved in ca. 5 mL pentane and

98.3 mg of PhBCl2 (0.619 mmol) was dissolved in ca. 2 mL pentane. The PhBCl2 solution was added to the other at room temperature and large quantities of a white

49 powder immediately precipitated. This slurry was stirred for 9 hours, filtered through Celite to give a clear, slightly yellow solution, and all volatiles were removed under vacuum to give 228.9 mg of PhB(C6F5)2 as a white solid (0.5424 mmol, 88.1% yield). 1 3 3 H (400.0 MHz, CD2Cl2): δ 7.72 (d, 2H, JH-H = 7.9 Hz, o-Ph), 7.70 (t, 1H, JH-H = 7.4 Hz, p-Ph), 7.48 (app t, 2H, m-Ph) 19 1 3 F{ H} (376.4 MHz, C6D6): δ -129.3 (m, 4F, o-C6F5), -149.4 (t, 2F, JF-F = 20.8 Hz, p-

C6F5), -161.7 (m, 4F, m-C6F5) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 63.8 (s, ν1/2 ≈ 520 Hz)

Preparation and spectroscopic data of 2-1

1.9 mL of nBuLi (1.6 M in hexanes) was added dropwise to a solution of 315.9 mg of phenylacetylene (3.093 mmol) dissolved in ca. 15 mL THF and cooled to -78 oC. The solution was stirred and kept at this temperature for 30 min, upon which point 391.3 mg of tellurium powder (3.067 mmol) was added. The resulting slurry transferred to a 100- mL flask equipped with a Teflon-valve air-free stopcock seal and heated to 55 oC for 2 h in the dark, giving a clear yellow solution. The solution was then cooled down to room temperature and 563.0 mg of Ph(CH2)2Br (3.042 mmol) dissolved in ca. 5mL THF was added to the cooled solution. No immediate colour change was observed. The mixture was stirred at room temperature for 18 h in the dark to give a dark yellow solution. All volatiles were then removed in vacuo, giving a viscous dark red oil. The product was extracted twice with 10 mL of 1:1 mixture of Et2O:hexanes and passed through a short plug of silica to give a clear yellow solution. All volatiles were then removed in vacuo to give a dark orange oil, which was then washed with pentane (5 mL × 3) to precipitate out 753.9 mg of 2-1 (2.258 mmol, 73.6% yield) as a pale yellow solid. 1 H (400.0 MHz, CD2Cl2): δ 7.43-7.41 (m, 2H, Ar-H), 7.30-7.34 (m, 5H, Ar-H), 7.26-7.23 3 3 (m, 3H, Ar-H), 3.27 (t, 2H, JH-H = 8.0 Hz, TeCH2CH2) 3.13 (t, 2H, JH-H = 8.0 Hz,

TeCH2CH2)

13 1 Te C{ H} (100.6 MHz, CD2Cl2): δ 142.8 (s, i-Ph ), 132.2 (s, o-Ph), 129.1 (s, Ar-C), 128.86 (s, Ar-C), 128.85(s, Ar-C), 128.83(s, Ar-C), 127.0 (s, Ar-C), 124.3 (s, i-Ph),

112.27 (s, ≡CPh), 45.3 (s, TeC≡), 38.3 (s, TeCH2CH2), 11.4 (s, TeCH2CH2) 125 Te (189.3 MHz, CD2Cl2): δ 298.3 (s) Te [Note: Ph denotes Ph(CH2)2Te]

Anal. Calc. for C16H14Te : C 57.56%, H 4.23%. Found: C 57.27%, H 3.81%.

Preparation and spectroscopic data of 2-2

A solution of 157.9 mg of B(C6F5)3 (0.3084 mmol) dissolved in ca. 5 mL pentane was added dropwise at room temperature to 103.9 mg of 2-1 (0.3112 mmol) dissolved in ca. 5 mL pentane. The mixture immediately turned orange. The solution was stirred for an additional 30 minutes at room temperature, then filtered through a short plug of Celite before all volatiles were removed in vacuo, giving 245.5 mg of 2-2 (0.2902 mmol, 94.1% yield) as a bright orange powder. 1 H (400.0 MHz, CD2Cl2): δ 7.34-7.32 (m, 3H, Ar-H), 7.27-7.23 (m, 5H, Ar-H), 6.90 (m, Te 3 3 2H, o-Ph ), 2.75 (t, 2H, JH-H = 7.2 Hz, TeCH2CH2) 2.35 (t, 2H, JH-H = 7.2 Hz,

TeCH2CH2) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 1.6 (s, ν1/2 ≈ 720 Hz) 13 1 1 B C{ H} (100.6 MHz, CD2Cl2): δ = n.o. (=CB), 148.9 (dm, JC-F ≈ 243 Hz, o-C6F5 ), 144.9 1 1 B (dm, JC-F ≈ 252 Hz, o-C6F5), 141.4 ( JC-F ≈ 272 Hz, p-C6F5 ), n.o. (p-C6F5), 140.8 (s, i- Te 1 1 Ph ), 138.4 (dm, JC-F ≈ 232 Hz, m-C6F5), 137.4(s, i-Ph), 136.2 (dm, JC-F ≈ 250 Hz, m- B Te C6F5 ), 130.0 (s, Ar-C), 129.58 (s, Ar-C), 129.56 (s, Ar-C), 128.3 (s, o-Ph ), 128.1 (s, Te B Ar-C), 127.1 (s, m-Ph ), n.o. (i-C6F5 and i-C6F5 ), 100.6 (s, TeC=), 34.5 (s, TeCH2CH2),

23.2 (s, TeCH2CH2) 19 1 B F{ H} (376.4 MHz, CD2Cl2): δ -130.4 (m, 4F, o-C6F5 ), -138.6 (m, 2F, o-C6F5), -154.7 3 B 3 (t, 2F, JF-F = 20.3 Hz, p-C6F5 ), -155.3 (t, 1F, JF-F = 21.1 Hz, p-C6F5), -162.8 (m, 2F, m- B C6F5), -163.2 (m, 4F, m-C6F5 ) 125 2 Te (157.8 MHz, CD2Cl2): δ 467.9 (t, JTe-H = 103.5 Hz)

Te B [Note: Ph denotes Ph(CH2)2Te, C6F5 denotes =CB(C6F5)2, and C6F5 denotes

=C(C6F5)]

Anal. Calc. for C34H14BF15Te: C 48.28%, H 1.67%. Found: C 47.76%. H 1.82 %.

Preparation and spectroscopic data of 2-3

A solution of 79.2 mg of PhB(C6F5)2 (0.188 mmol) dissolved in ca. 5 mL pentane was added dropwise to 62.7 mg of 2-1 (0.188 mmol) in ca. 5 mL pentane at room temperature. The mixture immediately turned light yellow. The solution was stirred for an additional 16 hours at room temperature, then filtered through a short plug of Celite before all volatiles were removed in vacuo, giving 104.0 mg of 2-3 as an orange oil (0.138 mmol, 73.2% yield). 1 Te H (500.0 MHz, CD2Cl2): δ 7.37-7.30 (m, 5H, Ar-H), 7.23-7.20 (m, 3H, m, p-Ph ), 7.11- Te 3 7.05 (m, 5H, Ar-H), 6.78 (m, 2H, o-Ph ), 2.68 (t, 2H, JH-H = 7.6 Hz, TeCH2CH2) 2.28 (t, 3 2H, JH-H = 8.0 Hz, TeCH2CH2) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ -0.4 (s, ν1/2 ≈ 980 Hz) 13 1 trans 1 C{ H} (125.7 MHz, CD2Cl2): δ 166.9 (=CB), 142.3 (s, i-Ph ), 148.9 (dm, JC-F ≈ 243 1 Te Hz, o-C6F5), 141.1 (dm, JC-F ≈ 230 Hz, p-C6F5), 141.0 (s, i-Ph ), 137.7 (s, i-Ph), 137.6 1 (dm, JC-F ≈ 246 Hz, m-C6F5), 129.42 (s, Ar-C), 129.39 (s, Ar-C), 129.2 (s, Ar-C), 128.9 (s, Ar-C), 128.60 (s, Ar-C), 128.58 (s, Ar-C), 128.56 (s, Ar-C), 128.3 (s, o-PhTe), 127.5 (s, Te 2 m-Ph ), 117.4 (s, TeC=), 115.4 (br s, i-C6F5), 34.5 (s, , JTe-C = 19.7 Hz, TeCH2CH2), 1 22.0 (s, , JTe-C = 166.4 Hz, TeCH2CH2, ) 19 1 3 4 F{ H} (376.4 MHz, CD2Cl2): δ -130.1 (dd, 4F, JF-F = 22.6 Hz, JF-F = 6.4 Hz, o-C6F5), - 3 155.5 (t, 2F, JF-F = 20.3 Hz, p-C6F5), -163.4 (m, 4F, m-C6F5) 125 3 Te (189.4 MHz, CD2Cl2): δ 426.0 (t, JTe-H = 117.4Hz) [Note: Phtrans denotes the phenyl group trans to the alkyltellurium substituent and PhTe denotes Ph(CH2)2Te]

MS (DART+): calcd for C34H20BF10Te: 759.05606 amu, found: 759.05426 amu.

Preparation and spectroscopic data of 2-4

A solution of 68.8 mg of MeB(C6F5)2 (0.191 mmol) dissolved in ca. 5 mL pentane was added dropwise to 64.0 mg of 2-1 (0.192 mmol) in ca. 5 mL pentane at room temperature. The mixture immediately turned bright yellow. Stirring was continued for an additional 16 hours, then the solution was filtered through a short plug of Celite before all volatiles were removed in vacuo, giving 114.6 mg of 2-4 (0.1650 mmol, 86.4% yield) as a thick, orange-yellow oil. 1 Te H (400.0 MHz, CD2Cl2): δ 7.40-7.28 (m, 5H, Ph), 7.23-7.18 (m, 3H, m, p-Ph ), 6.88- Te 3 3 6.87 (m, 2H, o-Ph ), 2.72 (t, 2H, JH-H = 7.8 Hz, TeCH2CH2) 2.40 (t, 2H, JH-H = 7.8 Hz,

TeCH2CH2), 1.98 (s, 3H, CH3) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ -4.5 (ν1/2 ≈ 840 Hz) 13 1 1 C{ H} (125.7 MHz, CD2Cl2): δ 170.2 (br s, =CB), 148.7 (dm, JC-F ≈ 246 Hz, o-C6F5), 1 1 Te 141.1 (dm, JC-F ≈ 252 Hz, p-C6F5), 137.8 (dm, JC-F ≈ 255 Hz, m-C6F5), 141.1 (s, i-Ph ), 136.4 (s, i-Ph), 129.37 (s, Ar-C), 129.35 (s, Ar-C), 129.2 (s, Ar-C), 128.5 (s, Ar-C), 128.3 Te Te 2 (s, o-Ph ), 127.5 (s, m-Ph ), 115.8 (br s, i-C6F5), 113.8 (s, TeC=), 34.7 (s, JTe-C = 15.7 1 Hz, TeCH2CH2), 23.9 (s, CH3), 21.9 (s, , JTe-C = 165.7 Hz, TeCH2CH2) 19 1 3 4 F{ H} (376.4 MHz, CD2Cl2): δ -130.7 (dd, 4F, JF-F = 23.7 Hz, JF-F = 9.0 Hz, o-C6F5), - 3 156.1 (t, 2F, JF-F = 20.3 Hz, p-C6F5), -163.6 (m, 4F, m-C6F5) 125 3 Te (157.8 MHz, CD2Cl2): δ 446.4 (t, JTe-H = 131.0 Hz) Te [Note: Ph denotes Ph(CH2)2Te]

MS (DART+): calcd for C29H18BF10Te: 697.03911 amu, found: 697.04041 amu.

Preparation and spectroscopic data of 2-5

A solution of 66.1 mg of BPh3 (0.273 mmol) dissolved in ca. 5 mL pentane was added dropwise to 91.8 mg of compound 2-1 (0.275 mmol) in ca. 5 mL pentane at room

53 temperature. The mixture gradually turned lemon yellow and remained clear. The solution was stirred for an additional 16 hours at room temperature, then filtered through a short plug of Celite before all volatiles were removed in vacuo, giving 105.1 mg of 2-5 (0.182 mmol, 66.8% yield) as a viscous bright yellow oil. 1 B H (400.0 MHz, CD2Cl2): δ 7.90-7.87 (m, 4H, o-Ph ) 7.44-7.41 (m, 8H, Ar-H), 7.29-7.21 (m, 3H, Ar-H), 7.20-7.17 (m, 3H, Ar-H, m, p-PhTe), 7.06-7.02 (m, 5H, Ar-H), 6.78-6.76 (d, 3 Te 3 3 JH-H = 6.4 Hz, 2H, o-Ph ), 2.49 (t, 2H, JH-H = 7.0 Hz, TeCH2CH2) 2.41 (t, 2H, JH-H = 2 7.0 Hz, TeCH2CH2, JTe-H = 199.9 Hz), 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 42.9 (s, ν1/2 ≈ 1400 Hz) 13 1 trans Te C{ H} (125.7 MHz, CD2Cl2): δ n.o. (=CB), 143.9 (s, i-Ph ), 142.9 (s, i-Ph ), 141.8 (s, i-Ph), n.o. (i-PhB), 137.9 (s, o-PhB), 130.2 (s, Ar-C), 130.1 (s, m-PhB), 129.9 (s, Ar-C), 128.89 (s, Ar-C), 128.87 (s, Ar-C), 128.6 (s, Ar-C), 128.4 (s, Ar-C), 128.3 (s, o-PhTe), Te 127.7 (s, Ar-C), 127.3 (s, Ar-C), 126.8 (s, m-Ph ), 116.4 (s, TeC=), n.o. (i-C6F5), 36.2

(TeCH2CH2), 15.8 (TeCH2CH2) 125 3 Te (157.8 MHz, CD2Cl2): δ 426.7 (t, JTe-H = 116.8 Hz) [Note: Phtrans denotes the phenyl group trans to the alkyltellurium substituent, PhTe B denotes Ph(CH2)2Te and Ph denotes BPh2]

MS (DART+): calcd for C34H29BTe: 578.14245 amu, found: 578.14515 amu

Preparation and spectroscopic data of 2-6

A solution of 9.5 mg of benzaldehyde (0.090 mmol) dissolved in ca. 3 mL of DCM was added to a stirring solution of 75.5 mg of compound 2-2 (0.0893 mmol) dissolved in ca. 5 mL DCM. The solution was allowed to stir for 3 hours before all volatiles were removed in vacuo, leaving a dark red oil. The oil was dissolved in ca. 3 mL pentane and stored at -35 oC for 16 hours, giving a clear orange supernatant and dark orange crystals. The supernatant was decanted and the crystals were dried under vacuum to give 53.8 mg of 2-6 (0.0565 mmol, 63.4% yield) as an orange powder. Single crystals

54 suitable for X-ray diffraction studies were grown from the slow evaporation of a saturated solution of 2-6 in DCM. 1 CO H (400.0 MHz, CD2Cl2): δ 9.80 (br s, 1H, C=OH), 7.94 (m, 2H, o-Ph , ), 7.69 (tt, 1H, 3 4 CO 3 CO JH-H = 7.4 Hz, JH-H = 2 Hz, p-Ph ), 7.593 (t, 2H, JH-H = 7.4 Hz, m-Ph ), 7.31-7.30 (m, Te 3 3H, Ar-H), 7.24-7.20 (m, 5H, Ar-H), 6.84 (m, 2H, o-Ph ), 2.60 (t, 2H, JH-H = 7.6 Hz, 3 TeCH2CH2), 2.22 (t, 2H, JH-H = 7.6 Hz, TeCH2CH2) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 2.1 (s, br, ν1/2 ≈ 2700 Hz) 13 1 13 C{ H} (100.6 MHz, CD2Cl2): δ ( C resonances of C6F5 rings not listed) n.o. (=CB), n.o.(COH), 141.2 (s, i-PhTe), 136.0 (s, p-PhCO), 135.8 (s, br, i-Ph), 131.1 (s, br, Ar-C), 130.0 (s, o-PhCO), 129.5 (s, m-PhCO), 129.4 (s, br, Ar-C), 129.3 (s, br, Ar-C), 128.3 (s, br, Te Te Ar-C), 128.2 (s, o-Ph ), 127.5 (s, m-Ph ), 116.0 (s, br, TeC=), n.o. (i-C6F5), 34.7

(s,TeCH2CH2), 22.4 (s, TeCH2CH2) 19 1 B F{ H} (376.4 MHz, CD2Cl2, 298 K): δ -130.9 (br s, 4F, o-C6F5 ), -138.6 (m, 2F, o-C6F5), B B -155.3 (br s, 2F, p-C6F5 ), -156.2 (br s, 1F, p-C6F5), -163.4 (m, 6F, m-C6F5 + m-C6F5) 19 1 B B F{ H} (376.4 MHz, CD2Cl2, 193 K): δ -129.2 (s, 1F, o-C6F5 ), -132.7 (s, 1F, o-C6F5 ), - B B B 132.9 (s, 1F, o-C6F5 ), -136.3 (s, 1F, o-C6F5 ), -138.7 (s, 1F, o-C6F5 ), -138.6 (s, 1F, o- B C6F5), -140.2 (s, 1F, o-C6F5), -155.1 (s, 1F, p-C6F5), -157.1 (s, 1F, p-C6F5 ), -157.7 (s, B 1F, p-C6F5 ), -161.3 (s, 1F, m-C6F5), -162.5 (s, 1F, m-C6F5), -163.0 (s, 1F, m-C6F5), - B B 163.5 (s, 1F, m-C6F5), -164.1 (s, 1F, m-C6F5 ), -164.8 (s, 1F, m-C6F5 ) 125 Te (157.8 MHz, CD2Cl2, 233 K): δ 400.9 (br s) Te CO [Note: Ph denotes Ph(CH2)2Te and Ph denotes PhCOH,] [Note: 19F-19F coupling was lost when sample was cooled to 193 K]

Anal. Calc. for C41H20BF15OTe: C 51.73%, H 2.12%. Found: C 51.53%, H 1.58 %.

Preparation and spectroscopic data of 2-7

0.050 mL of phenylacetylene (0.46 mmol, excess) was added to a solution of 100.3 mg of compound 2-2 (0.1186 mmol) dissolved in ca. 5 mL pentane. The mixture immediately turned orange yellow but remained clear. The solution was stirred for 10

55 minutes and then was allowed to stand at room temperature for 16 hours, giving a clear yellow supernatant and bright orange crystals. The supernatant was removed and the orange crystals dried under vacuum to 98.9 mg of 2-7 (0.104 mmol, 87.7% yield) as an orange powder. Single crystals suitable for X-ray diffraction studies were grown from letting a pentane solution of 2-7 stand at room temperature for one day. 1 ac H (400.0 MHz, CD2Cl2): δ 7.68 (s, 1H, HC=), 7.47 (m, 2H, m-Ph ), 7.42-2.44 (m, 6H, Te Ar-H), 7.26-7.22 (m, 5H, Ar-H), 6.74-6.72 (m, 2H, o-Ph ), 2.84 (m, 1H, TeCH2CH2) 2.68

(m, 3H, TeCH2CH2 + TeCH2’CH2) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ -10.8 (s, ν1/2 ≈ 25 Hz) 13 1 1 C{ H} (125.7 MHz, CD2Cl2): δ 161.5 (1:1:1:1 q, JC-B = 53.4 Hz, TeC=CH), 153.9 1 1 B 1 (1:1:1:1 q, JC-B ≈ 43 Hz, =CB), 148.7 (dm, JC-F ≈ 240 Hz, o-C6F5 ), 142.9 (dm, JC-F ≈ 1 B 1 247 Hz, o-C6F5), 142.5 (dm, JC-F ≈ 243 Hz, p-C6F5 ), 139.5 (dm, JC-F ≈ 228 Hz, p-C6F5), 1 B 1 137.5 (dm, JC-F ≈ 230 Hz, m-C6F5 ), 137.0 (dm, JC-F ≈ 241 Hz, m-C6F5), 139.8 (s, o- Phac), 139.7 (s, i-PhTe), 138.3 (s, i-Ph), 130.7 (s, i-Phac), 130.2 (s, Ar-C), 130.03 (s, Ar- C), 130.00 (s, Ar-C), 129.6 (s, Ar-C), 129.4 (s, m-PhTe), 128.6 (s, o-PhTe), 128.03 (s, Ar-

C), 127.97 (s, Ar-C),127.5 (s, Ar-C), 124.0 (s, TeC=CH), 117.5, (s, TeC=), n.o. (i-C6F5 B and i-C6F5 ), 32.1 (s, TeCH2CH2), 30.7 (s, TeCH2CH2) 19 1 B B’ F{ H} (376.4 MHz, CD2Cl2): δ -130.4 (m, 2F, o-C6F5 ), -131.7 (m, 2F, o-C6F5 ), -138.6 3 (m, 1F, o-C6F5), -139.6 (m, 1F, o’-C6F5), -157.5 (t, JF-F = 21.1 Hz, 1F, p-C6F5), -159.9 (t, 3 B 3 B’ JF-F = 20.3 Hz, 1F, p-C6F5 ), -160.6 (t, JF-F = 20.3 Hz, 1F, p-C6F5 ), -163.6 (m, 1F, m- B B’ C6F5), -164.8 (m, 3F, m’-C6F5 + m-C6F5 ), -165.5 (m, 2F, m-C6F5 ) 125 Te (157.8 MHz, CD2Cl2, 233 K): δ 472.8 (br s) Te ac B [Note: Ph denotes Ph(CH2)2Te and Ph denotes PhC=CH; C6F5 denotes =CB(C6F5)2 and C6F5 denotes =C(C6F5)]

Anal. Calc. for C42H20BF15Te: C 53.21%, H 2.13%. Found: C 52.97%, H 1.93%.

Preparation and spectroscopic data of 2-8

To a solution of 50.9 mg of compound 2-1 (0.152 mmol) in ca. 5 mL of toluene was added a slurry of 79.2 mg of B(C6F5)3 (0.155 mmol) in ca. 5 mL of pentane at room temperature. The resulting clear yellow solution was stirred at room temperature for 10 minutes before a solution of 40.1 mg of I2 (0.158 mmol) in ca. 5 mL toluene was added to it. The solution was allowed to stir for 15 h before all volatiles were removed in vacuo, leaving a dark red oil. The oil was re-dissolved in minimal amount of pentane and stored at -35 oC for 18 hours to give 49.8 mg (0.0453 mmol, 29.7% yield) of 2-8 as small orange crystals, which were then dried under vacuum. Single crystals suitable for X-ray diffraction studies were grown from the slow evaporation of a saturated pentane solution of 2-8 at room temperature. 1 H (400.0 MHz, CD2Cl2): δ 7.37-7.32 (m, 5H, Ar-H), 7.28-7.24 (m, 3H, Ar-H), 7.07 (d, 2H, 3 3 3 JH-H = 7.6 Hz, o-Ph), 4.11 (t, 2H, JH-H = 7.2 Hz, TeCH2CH2), 3.41 (t, 2H, JH-H = 7.2 Hz,

TeCH2CH2) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 5.94 (s, ν1/2 ≈ 720 Hz) 13 1 1 B C{ H} (100.6 MHz, CD2Cl2): δ =154.6 (br s, =CB), 148.4 (dm, JC-F ≈ 245 Hz, o-C6F5 ), 1 1 B 142.3 (dm, JC-F ≈ 252 Hz, o-C6F5), 142.0 (dm, JC-F ≈ 258 Hz, p-C6F5 ), n.o. (p-C6F5), Te 1 143.4 (s, TeC=), 138.8 (s, i-Ph), 138.5 (s, i-Ph ), 138.3 (s, Ar-C), 137.8 (dm, JC-F ≈ 232 1 B Hz, m-C6F5), 137.3 (dm, JC-F ≈ 250 Hz, m-C6F5 ), 130.7 (s, o-Ph), 130.6 (s, m-Ph), B 130.0 (s, Ar-C), 129.3 (s, Ar-C), 128.7 (s, Ar-C), 118.0 (br s, i-C6F5 ), 116.8 (br s, i-C6F5),

44.8 (s, TeCH2CH2), 32.6 (s, TeCH2CH2) 19 1 B F{ H} (376.4 MHz, CD2Cl2): δ -124.9 (br s, 4F, o-C6F5 ), -136.4 (br s, 2F, o-C6F5), - 3 B 3 153.3 (app t, JF-F = 20.3 Hz, 2F, p-C6F5 ), -154.8 (app t, JF-F = 21.1 Hz, 1F, p-C6F5), - B 162.9 (m, 2F, m-C6F5), -163. 1 (m, 4F, p-C6F5 ) 125 3 Te (157.8 MHz, CD2Cl2): δ 955.1 (t, JTe-H = 25.5 Hz) Te B [Note: Ph denotes Ph(CH2)2Te, C6F5 denotes =CB(C6F5)2 and C6F5 denotes

=C(C6F5)]

Anal. Calc. for C34H14BF15I2Te: C 37.14%, H 1.28%. Found: C 37.46%, H 1.47%.

Preparation and spectroscopic data of 2-9

5.3 mL of nBuLi (1.6 M solution in hexanes) was added dropwise to a solution of 0.8704 g of phenylacetylene (8.522 mmol) dissolved in ca. 15 mL THF and cooled to -35 oC. The solution was stirred and kept at this temperature of 30 minutes, at which point 1.087 g of elemental tellurium (8.648 mmol) was added and the resulting slurry transferred to a 100-mL Schlenk bomb and heated to 60 oC for 2 hours in the dark, giving a clear yellow solution. The solution was then cooled to room temperature and quenched with 1.540 g of freshly prepared 1-bromo-2-phenylacetylene (8.507 mmol) dissolved in ca. 10 mL THF. The solution was allowed to warm to room temperature in the dark over 16 hours. All volatiles were then removed in vacuo, giving a dark red oil.

The product was extracted with 15 mL Et2O and passed through a short plug of silica to give a clear red solution. All volatiles were removed again, giving a thick red oil that solidifies, which was then washed with DCM (5 mL × 2) and hexanes (5 mL × 2) to precipitate out 1.684 g of 2-9 (5.105 mmol, 60% yield) as a bright yellow solid. 1 H (400.0 MHz, CD2Cl2): δ 7.49-7.47 (m, 4H, o-Ph) 7.36-35 (m, 6H, m-Ph + p-Ph) 13 1 C{ H} (100.6 MHz, CD2Cl2): δ 132.6 (s, m-Ph), 129.7 (s, p-Ph), 128.9 (s, o-Ph), 123.3 (s, i-Ph), 113.0 (s, ≡CPh), 43.8(s, TeC≡) 125 Te (189.3 MHz, CD2Cl2): δ 360.0 (s)

Anal. Calc. for C16H10Te: C 58.26%, H 3.06%. Found: C 57.51%, H 3.01%.

Preparation and spectroscopic data of 2-10

275.5 mg of compound 2-9 (0.8352 mmol) was added to 200.2 mg of BPh3 (0.8269 mmol) in ca. 40 mL pentane at room temperature. The mixture did not change colour immediately, but over the course of 18 hours produced large amounts of orange precipitate. The precipitate was collected on a glass filter-frit and washed with pentane (5 mL × 3) to give 385.6 mg of compound 2-10 (0.6742 mmol, 81.5% yield) as an

58 orange powder. Single crystals suitable for X-ray diffraction studies were grown from letting a saturated pentane solution of 2-10 stand at room temperature over 12 hours. Repeated attempts of obtaining accurate elemental analysis always gave exceptionally low carbon contents. 1 H (500.0 MHz, CD2Cl2): δ 7.20-7.12 (m, 10H, Ar-H) 6.88-6.78 (m, 7H, Ar-H), 6.77-6.70 (m, 8H, Ar-H) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 53.3 (br s, ν1/2 ≈ 820 Hz) 13 1 B C{ H} (125.7 MHz, CD2Cl2): δ 162.2 (s, TeC=), 155.6 (br s, =CB), 148.8 (br s, i-Ph ), 145.6 (s, i-Ph), 143.9 (s, i-PhTe), 131.9 (s, Ar-C), 131.1 (s, Ar-C), 128.7(s, PhTe), 128.3 (s, PhTe), 127.8 (s, PhTe), 127.2 (s, Ph), 126.1 (s, Ar-C), 125.8 (s, Ar-C), 125.0 (s, Ar-C) 125 Te (189.3 MHz, CD2Cl2): δ 772.8 (s) [Note: PhTe denotes the Ph(Te)C=C phenyl ring, Ph denotes the Ph(B)C=C phenyl ring and PhB denotes the BPh phenyl ring]

MS (DART+): calcd for C34H29BNTe [M+NH4]: 592.14553 amu. Found: 592.14850 amu

Preparation and spectroscopic data of 2-11

A solution of 51.6 mg of B(C6F5)3 (0.101 mmol) dissolved in 2 mL of 1:4 solution of DCM: pentane was cooled to -35 oC, and to it was added dropwise a solution of 33.8 mg of compound 2-9 (0.102 mmol) also cooled to -35 oC in 3 mL of the same solvent. The solution immediately turned orange and gets darker over time. After stirring at this temperature for 10 minutes, the cold-well was removed and this solution was stirred at room temperature for an additional 7 hours. Large amounts of a light yellow powder precipitated out of solution over this time. The dark orange supernatant was decanted, and the yellow powder was then washed with a 1:10 solution of DCM: pentane (3 mL ×

3) and dried under vacuum to give 40.5 mg of compound 2-11 (0.0241 mmol, 47.6% yield) as a pale-yellow solid. 1 3 H (400.0 MHz, CD2Cl2, 193 K): δ 6.89 (m, 16H Ar-H), 6.63 (t, 2H, JH-H = 7.2 Hz, o-Ph), 6.57 (m, 2H, o-PhTe) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ -4.1 (s, ν1/2 ≈ 60 Hz) 13 1 C{ H} (100.6 MHz, CD2Cl2, 193 K): (partial, C6F5 signals not listed) δ 169.8 (br s, 1 (Te)PhC=C(B)(C6F5)), 149.3 (1:1:1:1 q, JC-B = 128 Hz, (Te)(B)C=C(Te)(Ph)), 142.6 (s, Te (Te)PhC=C(B)(C6F5)), 139.4 (s, (Te)(B)C=C(Te)(Ph)), 135.1 (s, i-Ph ), 133.4 (s, i-Ph), 128.6 (s, Ar-C), 128.5 (s, Ar-C), 128.4 (s, Ar-C), 128.2 (s, Ar-C), 126.7 (s, Ar-C), 124.4 (s, Ar-C) 19 1 B’ B’ F{ H} (376.4 MHz, CD2Cl2, 193 K): δ -126.6 (s, 1F, o-C6F5 ), -128.7 (s, 1F, o-C6F5 ), B B -129.5 (s, 1F, o-C6F5 ), -129.9 (s, 1F, o-C6F5 ), -139.4 (s, 1F, o-C6F5), -143.3 (s, 1F, o- B’ B C6F5), -154.4 (s, 1F, p-C6F5), -157.1 (s, 1F, p-C6F5 ), -158.8 (s, 1F, p-C6F5 ), -161.6 (s, B’ B’ 1F, m-C6F5), -162.0 (s, 1F, m-C6F5), -162.6 (s, 1F, m-C6F5 ), -163.7 (s, 1F, m-C6F5 ), - B B 164.3 (s, 1F, m-C6F5 ), -167.3 (s, 1F, m-C6F5 ) 125 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 947.2 (s), 945.9 (s) [Note: no Te resonance could be observed at room temperature] B B’ [C6F5 and C6F5 denotes different =CB(C6F5)2 aryl rings and C6F5 denotes =C(C6F5)] [Note: Ph denotes the Ph(Te)C=C phenyl ring, PhTe denotes the Ph(Te)C=C(Te)(B) phenyl ring in the central 6-membered ring]

Preparation and spectroscopic data of 2-12

145.3 mg of compound 2-9 (0.4405 mmol) in ca. 5 mL pentane was added to a 224.7 mg of B(C6F5)3 (0.4389 mmol) in ca. 10 mL pentane. The mixture immediately turned dark red and over time became opaque with an orange precipitate. All volatiles of this mixture were removed in vacuo after 1 hour, leaving a dark red oil. The residue was then re-dissolved in ca. 10 mL toluene and heated at 80 oC for 8 h. All volatiles were removed again in vacuo, and the deep red residue was triturated with 15 mL of pentane,

60 precipitating out a pale yellow powder with dark red supernatant. The pale yellow powder was collected on a filter frit and washed with minimal amounts of cold pentane to give the 210.5 mg of 2-12 (0.2500 mmol, 56.8% yield). Single crystals suitable for X- ray diffraction studies were grown from a saturated solution of 2-12 in DCM at -35 oC. 1 Te H (400.0 MHz, CD2Cl2): δ 7.34-7.33 (m, 6H, m-, p-Ph), 7.27-7.25 (m, 4H, o-Ph ) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 49.5 (br s, ν1/2 ≈ 1160 Hz) 13 1 1 C{ H} (100.6 MHz, CD2Cl2): δ n.o (BC=), 176.3 (s, TeC=), 146.8 (dm, JC-F ≈ 229 Hz, 1 1 o-C6F5), 144.6 (dm, JC-F ≈ 240 Hz, o-C6F5), 141.2 (dm, JC-F ≈ 233 Hz, p-C6F5), 140.6 1 1 1 (dm, JC-F ≈ 257 Hz, p-C6F5), 137.6 (dm, JC-F ≈ 252 Hz, m-C6F5), 137.2 (dm, JC-F ≈ 248 2 Te Te 3 Hz, m-C6F5), 141.4 (s, JC-Te ≈ 252 Hz, i-Ph ), 129.9 (s, p-Ph ), 129.0 (s, JC-Te ≈ 82 Hz, o-PhTe), 126.8 (s, m-PhTe) 19 1 B F{ H} (376.4 MHz, CD2Cl2): δ -132.9 (m, 2F, o-C6F5 ), -139.7 (m, 4F, o-C6F5), -153.4 3 B 3 (t, 1F, JF-F = 19.9 Hz, p-C6F5 ), -155.9 (t, 2F, JF-F = 19.6 Hz, p-C6F5), -161.9 (m, 2F, m- B C6F5 ), -163.2 (m, 4F, m-C6F5) 125 Te (189.3 MHz, CD2Cl2): δ 938.3 (s) [PhTe denotes the Ph(Te)C=C phenyl ring]

Anal. Calc. for C34H10BF15Te: C 48.51%, H 1.20%. Found: C 48.70%, H 1.10%.

Preparation and spectroscopic data of 2-13

38.9 mg of PhB(C6F5)2 (0.0922 mmol) dissolved in 2 mL 1:4 solution of DCM:pentane was cooled to -35 oC, and to it was added dropwise 30.3 mg of cold 2-9 (0.0919 mmol) in 3 mL of the same solvent. The solution immediately turned orange. After stirring at this temperature for 10 minutes, the cold-well was removed and this solution was stirred at room temperature for an additional 7 hours. Lots of light yellow powder precipitated out of solution over this time. The clear orange supernatant was decanted, and the light

61 yellow powder was then washed with a 1:10 solution of DCM:pentane (3 mL × 3) and dried under vacuum to give 36.6 mg of compound 2-13 (0.0243 mmol, 52.9% yield) as an off-white solid. 1 3 H (400.0 MHz, CD2Cl2, 193K): δ 7.15 (t, JH-H = 7.2 Hz, 2H, Ar-H), 7.09 (br m, 2H, Ar-H), 3 7.04 (t, 2H, JH-H = 7.8 Hz, Ar-H), 6.91 (br m, 2H, Ar-H), 6.81 (m, 4H, Ar-H), 6.73 (br s, 3 12H, Ar-H), 6.55 (br m, 4H, Ar-H), 5.75 (br d, 2H, JH-H = 7.8 Hz, Ar-H) 11 1 B{ H} (128.3 MHz, CD2Cl2, 193 K): δ -3.5 (s, ν1/2 ≈ 55 Hz) 13 1 C{ H} (100.6 MHz, CD2Cl2): (partial, C6F5 signals not listed) δ 171.6 (br s, 1 (Te)PhC=C(B)(C6F5)), 148.2 (1:1:1:1 q, JC-B = 104 Hz, (Te)(B)C=C(Te)(Ph)), 143.6 (s, (Te)PhC=C(B)(Ph)), 141.7 (br s, i-PhB), 137.9 (s, (Te)(B)C=C(Te)(Ph)), 136.9 (s, i-PhTe), 135.9 (s. i-Ph), 130.7 (s, o-Ph), 129.5 (s, p-Ph), 128.8 129.4 (s, Ar-C), 128.7 (s, Ar-C), 128.3 (s, o-PhB), 127.8 (s, Ar-C), 127.7 (s, PhTe), 127.1 (s, Ar-C), 127.1 (s, PhTe) 19 1 B’ B’ F{ H} (376.4 MHz, CD2Cl2): -125.1 (m, 1F, o-C6F5 ), -127.9 (m, 1F, o-C6F5 ), -128.4 B B 3 B’ (br m, 1F, o-C6F5 ), -131.1 (br m, 1F, o-C6F5 ), -159.6 (t, 1F, JF-F = 20.3 Hz, p-C6F5 ), - B B’ B 160.0 (br m, 1F, p-C6F5 ), -164.7 to -164. 9 (m, 3F, m-C6F5 + m-C6F5 ), -166.0 (m, 1F, B’ m-C6F5 ) 125Te: –no 125Te resonance could be observed even at -80 oC [Note: Ph denotes the Ph(Te)C=C(B)Ph phenyl ring, PhTe denotes the Ph(Te)C=C(Te)(B) phenyl ring in the central 6-membered ring, and PhB denotes the B B Ph(Te)C=C(B)Ph ring, and C6F5 and C6F5 ’denotes different =CB(C6F5)2 aryl rings]

Preparation and spectroscopic data of 2-14

100.8 mg of PhB(C6F5)2 (0.2388 mmol) dissolved in ca. 10 mL pentane was cooled to - 35 oC, and to it was added dropwise 79.9 mg of cold 2-9 (0.242 mmol). The mixture gradually turned dark orange and overtime became opaque with lots of orange precipitate. The reaction mixture was allowed to warm to room temperature over 16 hours, upon which point the dark orange supernatant was decanted and filtered through a plug of Celite. All volatiles were then removed in vacuo, leaving a sticky dark orange

62 solid. This solid was washed with minimal amounts of O(TMS)2, giving 79.2 mg of compound 2-14 (0.105 mmol, 44.3% yield) as a dark orange powder. Single crystals suitable for X-ray diffraction studies were grown from the slow evaporation of a saturated pentane solution 2-14 at room temperature. 1 Te Te H (400.0 MHz, CD2Cl2): δ 7.32-7.31 (m, 3H, Ph ), 7.28-7.26 (m, 2H, Ph ), 7.24-7.20 Te 3 (m, 5H, Ph ), 7.00-6.93 (m, 3H, m, p-Ph), 6.87-6.85 (d, 2H, JH-H = 7.2 Hz, o-Ph) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 50.6 (br s, ν1/2 ≈ 1070 Hz) 13 1 C{ H} (100.6 MHz, CD2Cl2): δ n.o (BC=), 175.9 (s, TeC=), 169.5 (s, TeC=), 144.7 (dm, 1 1 JC-F ≈ 239 Hz, o-C6F5), 144.0 (s, i-Ph), 143.5 (dm, JC-F ≈ 247 Hz, o-C6F5), 142.4 (s, i- Te Te 1 1 Ph ), 142.0 (s, i-Ph ), 143.5 (dm, JC-F ≈ 245 Hz, p-C6F5), 140.2 (dm, JC-F ≈ 252 Hz, p- 1 1 C6F5), 137.5 (dm, JC-F ≈ 250 Hz, m-C6F5), 137.0 (dm, JC-F ≈ 244 Hz, m-C6F5), 129.8 (s, p-Ph), 129.6 (s, m-Ph), 129.0 (s, PhTe), 128.8 (s, PhTe), 128.68 (s, PhTe), 128.62 (s, Te Te Te 2 Ph ), 127.8 (s, o-Ph), 127.0 (s, Ph ), 126.3 (s, Ph ), 119.0 (br t, JC-F = 21.6 Hz, i-

C6F5) 19 1 B F{ H} (376.4 MHz, CD2Cl2): δ -132.3 (m, 2F, o-C6F5 ), -139.9 (m, 2F, o-C6F5), -155.9 3 B 3 (t, 1F, JF-F = 20.0 Hz, p-C6F5 ), -156.7 (t, 1F, JF-F = 20.7 Hz, p-C6F5), -163.5 (m, 4F, m-

C6F5 and m-C6F5) 125 Te (189.3 MHz, CD2Cl2): δ 892.7 (s) B Te [C6F5 denotes =CB(C6F5)2 aryl ring and C6F5 denotes =C(C6F5). Ph denotes the Ph(Te)C=C phenyl rings while Ph denotes the Ph(B)C=C phenyl ring. 13C resonances for C6F5 rings are not unambiguously assigned due to many overlap of close signals]

Anal. Calc. for C34H15BF10Te: C 54.31%, H 2.01%. Found: C 54.73%, H 2.15%.

Preparation and spectroscopic data of 2-15

32.6 mg of MeB(C6F5)2 (0.0906 mmol) dissolved in 2 mL 1:4 solution of DCM:pentane was cooled to -35 oC, and to it was added dropwise 29.8 mg of cold 2-9 (0.0903 mmol) in 3 mL of the same solvent. The solution immediately turned orange. After stirring at this temperature for 10 minutes, the cold-well was removed and this solution was stirred at room temperature for an additional 7 hours. Lots of light yellow powder precipitated out of solution over this time. The clear orange supernatant was decanted, and the light yellow powder was then washed with a 1:10 solution of DCM:pentane (3 mL × 3) and dried under vacuum to give 40.5 mg of 2-15 (0.0294 mmol, 65.0% yield) as a white powder. 1 H (400.0 MHz, CD2Cl2): δ6.93-6.86 (m, 2H, Ar-H) 6.86-6.77 (m, 10H, Ar-H), 6.6 (br s,

4H, Ar-H), 6.51-6.49 (m, 4H, Ar-H), 1.60 (br s, 6H, -CH3) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ -3.8 (s) 13 1 C{ H} (100.6 MHz, CD2Cl2): (partial, C6F5 signals not listed) δ 144.3 (s, Te (Te)PhC=C(B)(CH3)), 138.5 (s, (Te)(B)C=C(Te)(Ph)), 137.1 (s, i-Ph ), 135.3 (s, i-Ph), 130.6 (s, o-Ph), 129.8 (s, p-Ph), 129.4 (s, m-Ph), 129.2 (s, PhTe), 128.4 (s, PhTe), 20.6 (br s, Me) 19 1 B B F{ H} (376.4 MHz, CD2Cl2): δ -126.3 (br m, 1F, o-C6F5 ), -126.8 (br m, 1F, o-C6F5 ), - B’ B’ 3 127.8 (br, 1F, o-C6F5 ), -134.4 (br m, 1F, o-C6F5 ), -161.4 (br t, 1F, JF-F = 20.3 Hz, p- B 3 B’ B C6F5 ), -161.8 (br t, 1F, JF-F = 18.8 Hz, p-C6F5 ), -165.6 (br m, 2F, m-C6F5 ), -166.6 (br B’ m, 2F, m-C6F5 ) 125Te – no 125Te resonance could be observed even at -80 oC – compound 2-15 is not very soluble in CD2Cl2 and precipitated out at low temperature.

[Note: C6F5B and C6F5B’denotes different =CB(C6F5)2 aryl rings]

2.3.3 X-ray crystallography

64 software package50 in order to provide 99.5-100% completion to a 2θ value of >55o. The data integration was performed using the Bruker SAINT software package, and the resulting raw data were scaled and absorption corrected using an empirical multi-scan method (SADABS).50

Structure solution and refinement were conducted using the SHELXTL-2016 program suite.51, 52 The heavy atom positions were determined using direct methods, while lighter, non-hydrogen atoms were located by successive difference Fourier map calculations. In the final cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors except in the case of disorder. The H-atom contributions were calculated using the riding model. The locations and magnitudes of the largest peaks in the final difference Fourier maps indicate that, in each case, they were of no chemical significance.

An unidentified disordered solvent molecule was found in the asymmetric unit of 2-13 and could not be satisfactorily refined. The SQUEEZE function of Platon was thus used to mathematically remove the effect of the solvent.53

Table 2-1. Selected crystallographic data of compounds 2-6, 2-7 and 2-8. 2-6 2-7 2-8

Formula C41H20BF15Te C42H20BF15Te C34H14BF15I2Te Formula weight 951.98 947.99 1099.66 Crystal system Monoclinic Monoclinic Monoclinic

Space group P21/n P21/c P21/c a (Å) 10.8865(7) 10.8721(5) 14.3384(15) b (Å) 20.8896(17) 21.4823(12) 19.7349(18) c (Å) 16.4939(13) 16.2937(8) 12.5751(13) α (°) 90 90.00 90.00 β (°) 104.061(3) 109.318(3) 96.781(4) γ (°) 90 90.00 90.00 V (Å3) 3638.6(5) 3591.3(3) 3533.4(6) Z 4 4 4

Temp. (K) 149(2) 150(2) 149(2)

d(calc) (g·cm-1) 1.738 1.753 2.074 Abs. coeff. μ (mm-1) 0.930 0.939 2.695 Reflections Collected 29204 27443 49755 2 2 Data Fo >3σ(Fo ) 6404 6326 6211 Variables 532 532 478

R1 00247 0.0272 0.0268

wR2 0.0518 0.0508 0.0547 GOF 1.001 1.015 1.169

Table 2-2. Selected crystallographic data of compounds 2-10, 2-12, 2-13 and 2-14 2-10 2-12 2-13 2-14

Formula C34H25BTe C34H10BF15Te C68H30B2F20Te2 C34H15BF10Te Formula weight 571.95 841.83 1503.74 751.87 Crystal system Triclinic Triclinic Monoclinic Triclinic Space group P1¯ P1¯ C2/c P1¯ a (Å) 11.3756(11) 12.4545(5) 39.238(15) 11.3615(7) b (Å) 12.5651(12) 13.2182(6) 21.195(6) 11.5042(7) c (Å) 19.3584(18) 20.2994(8) 20.080(5) 13.1803(8) α (°) 82.926(3) 74.416(2) 90 77.723(4) β (°) 75.150(3) 73.514(2) 108.82(2) 66.878(3) γ (°) 77.945(3) 81.701(2) 90 69.212(3) V (Å3) 2608.5(4) 3078.2(2) 15807(8) 1475.82(16) Z 4 4 8 2 Temp. (K) 147(2) 150(2) 150(2) 150(2) d(calc) (g·cm-1) 1.456 1.816 1.264 1.692 Abs. coeff. μ (mm-1) 1.161 1.083 0.818 1.095 Reflections 71454 51387 79086 21613 Collected 2 2 Data Fo >3σ(Fo ) 12013 14024 13920 5152

Variables 649 919 829 415

R1 0.0290 0.0408 0.0608 0.0461

wR2 0.0536 0.0589 0.0903 0.1197 GOF 0.993 0.999 0.879 1.033

2.4 References

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Chapter 3. Reactivity of 1,4-Telluraborines 3.1 Introduction

3.1.1 Aromatic compounds containing boron and other heteroatoms

In chapter 2, we described the first preparation of 1,4-telluraborines via 1,1- carboboration of tellurium acetylides. Interestingly, the synthesis of analogous 6- membered heterocycles with boron and Lewis basic elements in the 1 and 4 positions is an underdeveloped field. This section aims to provide a brief summary of some recent related findings.

While 1,2-azaborines and benzo-fused 1,4-azaborines have been studied extensively in the literature,1, 2 monocyclic 1,4-azaborines were only recently reported by the Braunschweig group (Scheme 3-1, top). They achieved this by a [2+2+2] i 3 cyclotrimerization reaction catalyzed by [{( Pr3P)2RhCl}]2. Compound 3-A is surprisingly stable to water, air, and even strong acids, thus demonstrating its high aromaticity

(NIC(1)ZZ = -17.40). This protocol can be further applied to prepare a wide range of functionalized 1,4-azaborines, though the use of a bulky iminoborane (tBuB≡NtBu) is necessary.4 A different general protocol was developed by the Liu group in 2016 (Scheme 3-1, bottom), where they were able to access a wide variety of monocyclic 1,4- azaborines in only three synthetic steps (compound 3-B).5 This series of compounds again demonstrate unusual air- and moisture-stability. Liu et al. were also able to show that, in contrast to their all-carbon analogues, the substituents on the boron and amine have dramatic impact on their optical properties.

Scheme 3-1. Different synthetic strategies of monomeric 1,4-azaborines.

A close analogue to 1,4-azaborines is 1,4-phosphaborines, which display much less aromaticity and feature a pyramidal phosphorus atom in the solid state. The first example of this class of compounds was reported by Nöth et al. in 1983.6 These species have since been used as 6π-electron ligands in manganese and ruthenium complexes.7 Just as in the case of azaborines, the synthesis of benzo-fused phosphaborines has been explored much more extensively8, 9 than monomeric 1,4- phosphaborines.

Beyond heterocycles containing boron and group 15 elements, oxaborines and thiaborines are two other classes of compounds relevant to the present study of 1,4- telluraborines. There are a few pathways to access 1,2-oxaborines, including carbenoid ring expansion,10 direct insertion of oxygen into boroles,11 and catalytic boron insertion 12 into benzofurans. Nonetheless, to the best of our knowledge, there has been no report of 1,4-oxaborines due to the lack of an available synthetic strategy. Likewise, there are limited examples of 1,2-thiaborines in the literature13-15 and of 1,4- thiaborines.13, 16-18, most of which were reported only very recently (Figure 3-1). Nevertheless, oxaborines and thiaborines are interesting due to their high aromaticity as determined by NICS calculations. Indeed, Ashe noted that the bonds within 1,4- thiaborines are essentially of the same length in the solid state. In terms of applicability

71 in future optoelectronic devices, the absorption maxima of oxaborines and thiaborines are typically significantly more red-shifted compared to analogous benzene derivatives. Agou et al. have also observed that thiaborines have lower LUMO energy levels than those of azaborines.

Figure 3-1. Representative examples of oxa- and thiaborines in the literature.

3.2 Results and Discussion

3.2.1 Reactions of 1,4-telluraborines with common nucleophiles

1,4-telluraborines 3-1 and 3-2 both include a 3-coordinate boron centre that could potentially serve as a Lewis acid. Surprisingly, neither 3-1 nor 3-2 coordinates to common Lewis bases such as PPh3, nor do they seem to exhibit different spectroscopic behaviour in coordinating solvents such as MeCN, THF or Et2O. The results of these preliminary tests are summarized in Table 3-1. Overall, it appears that only small, hard donors such as O–NMe3 and DMAP can form classical Lewis acid-base adducts with 1,4-telluraborines.

Table 3-1. Reactions of compounds 3-1 and 3-2 with various Lewis bases (n.r. indicates no reaction).

Lewis base R = C6F5 (3-1) R = Ph (3-2)

t P Bu3 n.r n.r.

t Bu2PH n.r n.r.

O=PEt3 n.r n.r.

PPh3 n.r n.r.

Coordinates Coordinates

O-NMe3 (11B{1H}: 10.0 ppm) (11B{1H}: 14.7 ppm)

Coordinates Coordinates DMAP (11B{1H}: 0.19 ppm) (11B{1H}: 4.2 ppm)

Et2O n.r. n.r.

THF n.r. n.r.

MeCN n.r. n.r.

Compounds 3-1 and 3-2 are stable at room temperature under an inert atmosphere, but start to decompose into various products upon contact with ambient environment. Because these compounds can be seen as intramolecular FLPs, where tellurium acts as the Lewis base and boron as the Lewis acid, we tested them against many substrates typical of FLP reactions. These tests include reactions with aldehydes,

73 ketones, olefins, CO2, and H2. Unfortunately, none of the above-mentioned substrates reacted with 3-1 or 3-2, which could be due to a number of reasons. Firstly, tellurium is too weak of a Lewis base to act as an effective base for FLP-type reactivity. Furthermore, there is significant electron delocalization within the heterocycle populating the p-orbital of the boron centre, which would significantly attenuate its Lewis acidity. Lastly, the boron centres in 3-1 and 3-2 are sterically hindered by the aryl substituents, which would preclude the approach of any large organic substrates. Indeed, the mixing 3-1 or 3-2 with a number of strong nucleophiles or electrophiles

(R2NH, R2PH, MeOTf, [OMe3][BF4]) even at high temperatures all led to no observable reaction.

3.2.2 Reactions of 1,4-telluraborines with terminal alkynes

Even though the boron centres of 3-1 and 3-2 do not coordinate to most Lewis bases, we noted that terminal alkynes can react with Te/B FLP systems (see chapter 2) and thus proceeded to examine the reaction between 3-2 with phenylacetylene. At room temperature, there was no reaction as determined by NMR spectroscopy. Upon heating above 80oC, however, the liberation of diphenylacetylene was observed to be concurrent with the formation of a clean new species 3-3 (Scheme 3-2). The 1H NMR of this new species exhibits a characteristic downfield resonance at 8.28 ppm. Two equivalents of phenylacetylene were also required to completely convert 3-2 into 3-3. Compound 3-3 exhibits similar solubility to that of PhCCPh, but can be isolated as a yellow solid from the reaction mixture by repeated sonication of the crude mixture in 1 hexamethyldisiloxane (O(TMS)2), giving a final isolated yield of 63%. The simple H and 13C{1H} NMR spectra of the isolated 3-3 suggests a symmetrical product. The 11B{1H} NMR spectrum of 3-3 shows one broad resonance at 49.9 ppm, which is shifted slightly upfield in comparison to the parent compound 3-2 (53.3 ppm), but still consistent with the presence of a three-coordinate boron centre. The downfield 1H resonance at 8.28 ppm is attributed to the (B)C–H proton, while a 125Te resonance at 659.7 ppm is consistent with the presence of the Te(II) centre. This latter signal is significantly shielded relative to the signal for 3-2 (772.8 ppm). Collectively these data support the formulation of 3-3 as the cyclic product PhB(HC=CPh)2Te (Scheme 3-2).

Scheme 3-2. Synthesis of compounds 3-3 to 3-14, Spectroscopic conversions are shown with isolated yields presented in brackets.

The facile and clean reaction between 3-2 and phenylacetylene prompted an investigation of the scope of this unique double alkyne exchange reaction. Indeed, 3-2 could be reacted with a variety of terminal alkynes (R = 4-C6H4Br 3-4, 4-C6H4OMe 3-5, t 4-C6H4 Bu 3-6, 4-C6H4CF3 3-7, 3-C6H4Cl 3-8, 3,5-C6H4F2 3-9, 4-C6H4Ph 3-10,

C6H3CH2C6H4 3-11, 3-C4H3S 3-12, (C5H4)FeCp 3-13, 4-(NMe2)C6H4 3-14) in nearly

75 quantitative yields as determined by 1H NMR spectroscopy in most cases, affording compounds formulated as PhB(HC=CR)2Te (compounds 3-3 to 3-14,Scheme 3-2). These compounds could all be isolated in moderate to high yields (46−73%) as yellow, red or green powders. The reactions all proceeded with high regioselectivity yielding only the product in which the terminal C–H carbon attaches to the Lewis acidic boron centre. This is consistent with the appearance of the characteristic 1H resonances of (B)C=H at 8.17-8.36 ppm and the 13C{1H} resonances of the TeCR fragment at 154-164 ppm. The 125Te resonances of compounds 3-3 to 3-13 are in the 626-684 ppm range, providing a facile spectroscopic handle on the electronic structure of the central six- membered ring as upfield 125Te shifts are observed in species with more donating R groups. Alkyl-substituted terminal alkynes (1-hexyne, cyclopropylacetylene, cyclohexylacetylene) also underwent the same exchange reaction with 3-2, but these products all were all extremely soluble in organic solvents which precluded isolation. It should be noted that internal alkynes and alkynes that are sterically crowded (tert- butylacetylene, 2,6-dimethylphenylacetylene) did not react with 3-2 at all. Lastly, alkynes containing donor moiety close to the acetylenic fragment (2-ethynylpyridine, 2- ethynylbenzaldehyde, 2-ethynylthiophene) did not react cleanly with 3-2, but instead gave rise to complicated mixtures of products that could not be separated. The same alkyne exchange reaction can be applied to 3-1 (Scheme 3-3), giving rise to the C6F5- substitued analogue of 3-4 quantitatively as seen in the in situ 19F{1H} NMR spectrum. The isolated yield of 3-15, however, is only 31% due to its high solubility in organic solvents.

Scheme 3-3. Synthesis of compound 3-15.

Single crystals of 3-4 and 3-6 suitable for X-ray diffraction studies were grown at room temperature from saturated solutions of these compounds in pentane. The structural data (Figure 3-2) are consistent with the above formulation and further confirm the formation of these unique Te/B heterocycles. Like the parent compound 3-2, species 3- 4 and 3-6 feature planar central six-membered rings with the aryl substituents slightly twisted out of the plane of the 1,4-telluraborine.

Figure 3-2. Solid-state structures of 3-4 (top) and 3-6 (bottom) (50% thermal ellipsoids). C: Black, B: yellow green, Br: brown red, Te: green. Hydrogen atoms are omitted for clarity.

The Te−C distances are 2.085(7) and 2.092(2) Å, while the B−C bonds are 1.577(7) and 1.534(3) in 3-4 and 3-6, respectively. The corresponding C−Te−C angles are 96.8(2)° and 96.66(7)°, while the C−B−C angles are 120.1(5)° and 121.8(2)° in 3-4 and 3-6, respectively. Although the C−Te−C angles are similar to that in 3-2 (97.08(9)°), the

C−B−C angle is significantly smaller (124.3(2)°), presumably due to less steric crowding at the boron centres in 3-4 and 3-6 compared to 3-2. These Te−C distances are shorter than typical Te–C single bonds (>2.1 Å),19 which suggests extensive electron delocalization within the 1,4-telluraborine heterocycle. Additional experimental support for electron delocalization in 1,4-telluraborines comes from the solid-state structure of the Py–O adduct of 3-4 (Figure 3-3), the bond distances of which are summarized in Table 3-2. The population of the boron p-orbital leads to a shortening of both the Te–C and B–C bond lengths. Additionally, the C–C bond distances within the 1,4-telluraborine shorten upon Py–O coordination, which is consistent with reduced C=Cπ to Bp donation in 3-4.

Figure 3-3. Solid-state structures of the Py–O adduct of compound 3-4 (50% thermal ellipsoids). C: Black, B: yellow green, N: blue, O: red, Br: brown red, Te: green. Hydrogen atoms are omitted for clarity.

Table 3-2. Selected bond length comparison between 3-4 and Py–O·3-4 based on the solid-state structure.

3-4 Py–O·3-4

C1-C2 1.344(8) Å 1.351(8) Å

C3-C4 1.354(8) Å 1.344(8) Å

Te-C1 2.085(7) Å 2.120(5) Å

Te-C4 2.081(7) Å 2.116(5) Å

B-C2 1.53(1) Å 1.592(8) Å

B-C3 1.55(1) Å 1.594(7) Å

The mechanism of the reaction between compound 3-2 and alkynes was postulated to proceed by a [4+2] cycloaddition reaction, followed by the extrusion of PhCCPh. Repetition of this process would afford the observed heterocycles (Scheme 3-4). In this proposition, the tellurium and boron centres in 3-2 act as an FLP, giving cis-addition to the incoming alkyne to afford the intermediate. Consistent with previous observation, the boron centre attaches exclusively to the less hindered carbon centre of the alkyne, providing an explanation for the observed regioselectivity. Efforts to intercept or isolate this intermediate were unsuccessful, suggesting that the second alkyne exchange has similar energetic barrier as the first.

Scheme 3-4. Proposed reaction mechanism for the FLP-alkyne exchange synthesis of compounds 3-3 to 3-14.

In order to provide further support for this proposed mechanism, computational studies were undertaken at the dispersion-corrected density functional level of theory (PW6B95- D3/def2-TZCP//PBEh-3c)20, 21 in collaboration with Prof. Stefan Grimme. Consistent with the proposition of FLP-type reactivity, initiated from a van der Waals complex of 3-2 with PhC≡CH, a transition state (TS1) was found as shown in Figure 3-4. The free energy ΔG of this species is computed to be 17.1 kcal/ mol above the reagents in toluene. Formation of new B–C and Te–C bonds affords the pseudoborato-telluronium zwitterionic intermediate species I in an overall reaction that is slightly exergonic (ΔG of -1.1 kcal/mol). In a subsequent step the intermediate I loses an equivalent of PhCCPh via an analogous transition state (TS2) to give the product P in an overall exergonic reaction (ΔG = -10.3 kcal/mol). Due to the lower energy barrier in the forward reaction (17.1 kcal/mol) compared to the backward reaction (26.3 kcal/mol), this reaction does not appear to be reversible, which is consistent with experimental observation. The reaction occurs in an asynchronous process, with the C–B bond forming slightly faster than the Te–C bond. The ejection of PhC≡CPh is thermodynamically favoured as this diminishes the steric crowding around the central heterocycle.

Figure 3-4. Computed reaction profile of the FLP-alkyne exchange reaction of 3-2 with PhC≡CH. Gibbs free energies are given in toluene at 298 K. C: Black, B: yellow green, Te: green. Hydrogen atoms are omitted for clarity.

The formation of 3-3 to 3-14 through a [4+2] cycloaddition-elimination sequence is unusual. Related examples include the reversible binding of an alkyne to a Ga-diamide chelate complex22 and the [4+2] cycloaddition of a terminal alkyne and ethylene to ambiphilic boron heterocycles.23-25 The most closely related example is reported by Mathey and Le Floch over 20 years ago.26, 27 They found that diazaphosphinines could undergo clean [4+2] cycloaddition-elimination reactions with terminal and internal alkynes to produce polyfunctional phosphinines. They reasoned that the driving force of this reaction is the increased aromaticity and stability of the product (NICS(1)zz = -10.2) 28 compared to the highly reactive diazaphosphinine starting material (NICS(1)zz = -7.5).

In the present study, however, the central 1,4-telluraborine heterocycle remains unchanged throughout the reaction. Additionally, the solid-state structural data reveal that variations within the C–C, Te–C, and B–C bond distances in 3-2, 3-4, and 3-6 are all within 0.02 Å. We thus reasoned that the main factor contributing to the exergonic reaction could be the relief of steric congestion around the heterocycle, which allows for better conjugation of the aryl substituents with the central 1,4-telluraborine (for details see section 3.2.5).

3.2.3 Reactions of 1,4-telluraborines with alcohols

The ease with which 1,4-telluraborines undergoes alkyne-exchange reactions opened the way to derivatize this new class of heterocycles. Surprisingly, we also found that 1,4-telluraborines can react with alcohols in a selective fashion to access another series of stable alkoxy-derivatives of 1,4-telluraborine.

Subjecting 3-1 to one equivalent of benzyl alcohol at 110 °C in MeCN led to the 19 1 elimination of C6F5H as evidenced by the in situ F{ H} NMR resonances at -139.7, - 155.3 and -163.2 ppm in addition to a new set of resonances at -139.9. -157.6, -163.8 ppm. Following removal of the volatiles in vacuo, the residue was washed repeatedly with pentane to give an off-white solid (3-16) in 85% isolated yield. The 11B{1H} NMR spectrum shows one broad resonance at 36.9 ppm, which is consistent with a 3- coordinate boron environment. Single crystals of 3-16 were grown by layering a solution of it in DCM with cyclopentane, which were subjected to X-ray diffraction study and confirmed that the 1,4-telluraborine remained intact after the reaction, but with the replacement of the C6F5 ring on boron with a benzylate group (Scheme 3-5, Figure 3-5). The B–O bond distance is 1.352(5) Å, while the metric parameters of 1,4-telluraborine remain similar to those observed in 3-1.

Scheme 3-5. Synthesis of compounds 3-16 to 3-25 from 3-1 (*requires 3 days for completion. **requires 7 days for completion). All yields are given as isolated yields.

The reaction of 3-1 with benzyl alcohol results in the exclusive protonolysis of the exocyclic B–C bond, even when 3-1 was treated with excess alcohol (> 5 eq.). It is also interesting to note that while protonolysis of three-coordinate boranes typically proceeds readily at room temperature, the present reaction requires heating at 110 °C for hours or days to proceed to completion. It is also important that the reaction is performed in MeCN; reactions conducted using less polar solvents such as toluene or benzene led to minimal to no product formation. These observations highlight the unusual stability of 1,4-telluraborine. In an analogous fashion, a number of other alcohols were employed in protonolysis reactions (Scheme 3-5), leading to the synthesis of compounds

Te((Ph)C=C(C6F5))2BOR (R = 4-BrC6H4CH2 3-17, PhCH=CHCH2 3-18, Ph2CH 3-19,

C7H9CH2 3-20, PhOCH2CH2 3-21, 4-MeOC6H4 3-22, C6H11 3-23, 2-C6H4CH2NH2 3-24, and Me3Si 3-25). Most of these reactions proceed to completion after 24 hours of

83 heating at 110 °C in MeCN and the products were isolated in moderate to high yields of 50-85%, although in the case of the more sterically hindered alcohol, a longer reaction time was required. Various functional groups can be tolerated, including arylbromides, olefins, ethers and amines. The reactions between 3-1 and phenols also required longer reaction times for the protonolysis to proceed. In fact, the reaction between 3-1 and phenol itself is so sluggish that even after 7 days of heating, only a minimal amount of 19 1 C6F5H could be detected in the in situ F{ H} NMR spectrum. On the other hand, phenols with electron-donating substituents react more smoothly to effect protonolysis (compounds 3-22 and 3-24). This observation suggests that the basicity of the OH group has a strong influence on reactivity, which is consistent with the mechanism that initial coordination of the alcohol to boron is necessary to prompt the protonolysis of B–

C bond. The silanol, Me3SiOH also reacts with compound 3-1 under similar conditions, giving the siloxy-substituted product (compound 3-25). Interestingly, all the products (compounds 3-26 to 3-26) are air- and moisture-stable both in solution and in the solid state for at least one week.

Figure 3-5. Solid-state structures of compounds 3-16 (top left), 3-19 (top right) and 3-24 (bottom left, zoom-in on core structure on bottom right) (50% thermal ellipsoids). C: black, B: yellow green, O: red, F: pink, N: blue, Te: green. Hydrogen atoms are omitted for clarity.

Comparing the parent heterocycle 3-1 to compounds 3-16 to 3-25, the substitution of alkoxy group on boron leads to an upfield shift of the 11B{1H} resonance by ca. 12 ppm, reflecting some π-electron donation from oxygen to the empty p-orbital on boron. X-ray crystallographic analysis for 3-19 (Figure 3-5) confirmed that the Te–B and Te–C bond lengths alter little before and after the protonolysis. In contrast, in the case of compound 3-24, the corresponding 11B{1H} NMR resonance is a sharp singlet at 1.3 ppm, while the

19F{1H} NMR spectrum shows two sets of resonances at -139.4, -158.8, -163.4 ppm and

-141.5, -158.8, -165.1 ppm, which are attributable to inequivalent C6F5 rings. These data suggest that the amine group is coordinated to the boron center in solution – a conclusion that was subsequently supported by single crystal X-ray diffraction analysis (Figure 3-5). In compound 3-24, the N–B distance is 1.620(7) Å, which typical of similar B–N adducts.29-31 Pyramidalization of the boron center is further evidenced by the sum of the angles around boron of 334.6°.

The coordination of the amine group to boron in compound 3-24 supports the notion of residual Lewis acidity at boron in the alkoxy-substituted heterocycles 3-16 to 3-25. This prompted an examination of the reactivity of compound 3-16 with terminal alkynes as it may still function as an FLP as described in section 3.2.2. Heating compound 3-16 with a number of terminal alkynes (4-bromophenylacetylene, 4- trifluoromethylphenylacetylene, 4-ethynylbiphenyl, cyclohexylacetylene, 3- ethynylthiophene, ethynylferrocene) all led to clean alkyne exchange reactions, but only two were isolated due to the high solubility of these products (Scheme 3-6). The in situ 19F{1H} NMR spectra revealed a 1:1 ratio of two sets of resonances attributable to the

C6F5 groups, one of which (-136.1, -152.8 and -161.9 ppm) corresponds to the byproduct C6F5C≡CPh. The structures of compound 3-26 and 3-27 were unambiguously confirmed by single crystal X-ray diffraction studies (Figure 3-6). The formation of 3-26 and 3-27 is thought to proceed in a fashion similar to the reaction of 3-1 and 3-2 with alkynes. However, in contrast to the reactivities of compound 3-1 and 3-2, the treatment of compound 3-16 with excess (>5 eq) alkyne resulted in the selective exchange of only one equivalent of alkyne, affording the unsymmetrically substituted 1,4-telluraborine. This difference in reactivity can be attributed to the reduced Lewis acidity of compound 3-2 due to the alkoxy-substituent on boron.

Scheme 3-6. Synthesis of compounds 3-26 and 3-27 via terminal alkyne exchange reaction with 3-16.

Figure 3-6. Solid-state structures of compounds 3-26 (left), 3-27 (right) (50% thermal ellipsoids). C: Black, B: yellow green, O: red, F: pink, S: yellow, Br: brown red, Te: green. Hydrogen atoms are omitted for clarity.

3-26 and 3-27 are the first examples of unsymmetrically substituted 1,4-telluraborines. It is also noteworthy that, in contrast to other known 3-coordinate boron heterocycles, compounds 3-26 and 3-27 are sufficiently stable on silica and thus can be purified via chromatography, although they decompose slowly over time if left under ambient conditions for over 1 day.

3.2.4 Further derivatization of 1,4-telluraborines

Encouraged by the unique stability and reactivities of 1,4-telluraborines 3-1 and 3-2, we proceeded to explore the possibility of synthesizing 1,4-telluraborine with simple substituents on the boron centre. The installation of a hydride or chloride on boron could lead to more derivatization pathways such as hydroboration reactions. To this end, we initially treated either 3-1 or 3-16 directly with a number of different silanes (e.g. HSiEt3, i HSi Pr3 H2SiPh2, HSiPh3) but surprisingly observed no reaction even after days of o heating at 130 C. Gratifyingly, the addition of B(C6F5)3 as catalyst (10 mol%) to the reaction between 3-16 and H3SiPh led to the successful synthesis of compound 3-28 in 66% yield (Scheme 3-7). The generation of 3-28 is evident in the broad resonance at 7.27 ppm in the 1H NMR. Interestingly, the 11B{1H} NMR of 3-28 shows only one broad signal at δ 53.3, suggesting that it is monomeric in solution. It appears that the steric crowd of the C6F5 substituents nearby is sufficient to prevent dimer formation.

Scheme 3-7. Synthesis of compound 3-28 from 3-16.

The monomeric solution state behaviour of 3-28 is indeed intriguing as many computational and experimental data have shown that the hydroboration of unsaturated

C–C bonds by diorganoboranes depends on the dissociation of the R2B(μ-H)2BR2 dimer. For example, the monomer-dimer equilibrium of 9-BBN strongly favours the dimer. The dissociation of this dimer is estimated to have a barrier of 22 kcal/mol at room temperature,32, 33 but the monomeric borane reacts with alkenes with little to no activation energy barrier.34, 35 We thus proceeded to test 3-28 in a series of hydroboration reactions, the results of which are summarized in Table 3-3.

Table 3-3. Hydroboration of unsaturated organic substrates by telluraborine 3-28.

Conversion Entry Substrate Temp. (oC) Time (h) Product (isolated yield)

1 25 <1 >99% (78%) 3-29

2 25 <1 >99% (65%) 3-30

3 80 24 75% (36%) 3-31

4 25 3 >99% (67%) 3-32

5 25 24 >99% 3-33

~66% (rest 6 25 <1 3-34 polymer)

7 25 <1 >99% 3-35

8 25 <1 >99% 3-36

9 25 <1 >99% 3-37

10 25 24 >99% (92%) 3-38

11 110 24 >99% (31%) 3-39

12 25 96 >99% 3-19

13 25 24 >99% 3-16

The efficiency of 3-28 as a hydroboration reagent can be seen in its reactions with simple aryl- and alkyl-substituted alkenes and alkynes. Most substrates with only one substituent reacted with 3-28 completely within minutes of mixing in CDCl3 at room temperature. Disubstituted olefins such as 1,1-diphenylethylene could also undergo hydroboration reaction with 3-28 although higher temperature was required (entry 3). The reactions of 3-28 with other 1,2- or 1,1-substituted olefins or alkynes all proceeded at room temperature (entries 5-9), though the reaction with diphenylacetylene required longer time (entry 10). These results suggest that this hydroboration reaction is quite sensitive to steric effect, which is unsurprising considering the large C6F5 groups in 3- 28. Interestingly, 3-28 could also undergo hydroboration with ketones and aldehydes (entries 11-13) under relatively mild conditions. Single crystals of 3-39 were be grown from a saturated solution of it in cyclopentane (Figure 3-7), which indirectly confirms the successful synthesis of 3-28.

Figure 3-7. Solid-state structures of 3-39 (50% thermal ellipsoids). C: black, B: yellow green, O: red, F: pink, Te: green. Hydrogen atoms are omitted for clarity.

In addition to its hydroboration capabilities, 3-28 can also be used to synthesize other 1,4-telluraborine derivatives. The reaction of 3-28 with HCl (4.0 M in dioxanes) proceeded smoothly at room temperature to give 3-40 as a white powder. The addition of excess of HCl did not lead to the protonolysis of B–C bonds, thus supporting again the unusual stability of 1,4-telluraborines. 3-40 can be further reacted with AgOTf to give 3-41, the single crystals of which were grown from a saturated solution in THF (Figure 3-8).

Scheme 3-8. Synthesis of compounds 3-40 and 3-41 from 3-28.

Figure 3-8. Solid-state structures of compound 3-41 (50% thermal ellipsoids) (left) with all atoms but hydrogen shown and (right) with only the core 1,4-telluraborine shown. C: black, B: yellow green, O: red, F: pink, S: yellow Te: green.

The Te–C bond lengths are 2.2131(3) Å and 2.120(2) Å in 3-41, which are lengthened compared to those found in 3-1. The C1–C2 and C3–C4 distances are 1.351(3) Å and 1.344(3) Å, which correspond well to those found in C–C double bonds.19 All the angles around the boron centre fall within the ranges of 106.2(2)o and 109.5(2)o, thus providing evidence for the pyramidal 4-coordinate boron.

3.2.5 Additional comments on 1,4-telluraborines

Many experimental observations showed that 1,4-telluraborines are unusually stable for a heterocycle containing a three-coordinate boron centre. We thus decided to evaluate the aromaticity of these compounds by computing their NICS indices (M062X/SDD), the results of which are summarized in Table 3-4. Only NICS(1)ZZ values are tabulated here as they have been shown to be the most accurate description of aromaticity.36, 37

Table 3-4. NICS(1)ZZ indices of representative examples of 1,4-telluraborines (Ar = 4- bromophenyl).

3-1 3-3CF 3-16

NIC(1)ZZ -6.93 -7.02 -1.67

3-2 3-3 3-16Ph

NIC(1)ZZ -5.17 -4.68 +0.40

3-26 3-28 3-40

NIC(1)ZZ -3.16 -7.43 -5.59

A number of interesting trends emerge upon examining the values listed in Table 3-4.

1,4-telluraborines with C6F5 substituents (3-1, 3-3CF, 3-16) are generally more aromatic than phenyl-substituted ones (3-2, 3-3, 3-16Ph), which suggests that the inductive effect of boron has a significant influence on the participation of the tellurium lone pairs in the electron delocalization within the ring. This trend indeed helps to explain some experimental observations. For example, the deliberate protonolysis of 3-1 by HCl in

93 dioxanes at room temperature proceeded slowly and took up to 24 hours to complete. By contrast, 3-2 reacted instantly with HCl at room temperature, giving a complicated mixture of decomposition products.

The aromaticity of 1,4-telluraborines, however, does not seem to be the only factor influencing the stability of these species. For example, 1,4-telluraborines with alkoxy substituents on the boron (3-16, 3-16Ph, 3-26) are the least aromatic out of all the compounds examined based on NICS values. The alkoxy-substituted 1,4-telluraborines can nonetheless be kept under ambient conditions for months without signs of decomposition. The stability of these species is likely a result of the lone pair of electrons on oxygen populating the empty p-orbital on boron. This interaction would preclude effective delocalization of electrons within the central heterocycle but make the boron centre less susceptible to nucleophilic attack. In a similar manner, 3-40 is much less aromatic than 3-28 due to the lone pairs of electrons on the chloride substituent.

Another interesting trend from the NICS(1)ZZ values is that there is no significant difference in aromaticity between 3-1 and 3-3CF, or between 3-2 and 3-3. Experimentally, we noted that while compound 3-2 decomposes within hours upon exposure to air and moisture, compound 3-3 can be stored under ambient conditions for up to a month without evidence of degradation as confirmed by 1H NMR spectroscopy. In addition, analogous compound 3-4 proved to be stable in refluxing water, a feature that augurs well for further derivatization. The unusual stability of 3-3 prompted us to conduct NBO analyses on these compounds. The NBO analysis (B3LYP/SDD) of 3-3 includes one strong (32.36 kcal/mol) donor-acceptor interaction between the phenyl substituent and the empty p-orbital on boron (Figure 3-9), which is not present in the NBO analysis of 3-2. This donor-acceptor interaction appears to be crucial in increasing the stability of 3-3 compared to the parent compound 3-2.

Figure 3-9. Donor-acceptor interaction between phenyl substituent and boron centre in 3-2 (isovalue = 0.05).

3.2.6 Conclusion

Despite the promising potential of tellurium-containing heterocycles in material chemistry, the labourious and low-yielding syntheses of these compounds have always hampered the development of the field. We have now uncovered a number of facile routes of derivatizing 1,4-telluraborines, including a double alkyne exchange reaction and selective protonolysis of the exocyclic B-C bond. Both synthetic strategies can be applied to prepare unsymmetrically substituted 1,4-telluraborines, which are unprecedented up until this study. These heterocycles are unusually stable to moisture and NICS(1)zz indices support their mild to high aromaticity. Through the B(C6F5)3- catalyzed hydrosilylation of B–O bond in the alkoxy-substituted 1,4-telluraborine, we could also access a novel hydroboration reagent, 3-28, which provides a third venue of derivatizing these heterocycles. The selectivity and ease of synthesis of these compounds certainly can open doors for future applications in biological and material sciences.

3.3 Experimental Section

3.3.1 General considerations

(CaH2 for CD2Cl2, Na/benzophenone for C6D6 and THF-d8) and distilled under reduced pressure prior to use. All solvents were also degassed by repeated freeze-pump-thaw cycles prior to use.

All chemicals were used as received unless otherwise noted. Phenylacetylene, 1- pentyne, 3,3-dimethyl-1-butyne, 1-ethynylcyclohexene, 1-hexene, 4-ethynylanisole, 4- tert-butylphenylacetylene, 4-trifluoromethylphenylacetylene, 3-chlorophenylacetylene, 3,5-difluorophenylacetylene, 4-ethynylbiphenyl, ethynylferrocene, benzyl alcohol, 4- bromobenzyl alcohol, cinnamyl alcohol, 5-norbornene-2-methanol (exo + endo), 4- methoxyphenol, diphenylacetylene, 4-heptanone, benzaldehyde, benzophenone, and trimethylsilanol were purchased from Sigma-Aldrich. 1,1-diphenylmethanol, cyclohexene, ɑ-methylstyrene, 2-vinylpyridine, and cyclohexanol were purchased from Alfa Aesar. 4-bromophenylacetylene, 2-phenoxyethanol, 1,1-diphenylethylene, and 3- ethynylthiophene were purchased from TCI Chemicals. All liquid reagents were degassed by repeated freeze-pump-thaw cycles and stored over 4 Å sieves prior to use. All solid reagents were placed under vacuum for 1 h prior to use. 2-ethynylfluorene38 and 2,4-diethynyl-9,9’-dioctylfluorene39 were prepared using standard literature procedure.

NMR spectroscopy was performed on either a Bruker Advance III 400 MHz, an Agilent DD2 500 MHz, or an Agilent DD2 600 MHz spectrometer. Unless otherwise stated, all spectra were obtained at room temperature. All NMR spectra were referenced to 1 13 1 residual protio solvent peaks ( H = 5.32 ppm and C = 53.84 ppm for CD2Cl2; H = 7.16

13 19 ppm and C = 128.06 ppm for C6D6) or an external standard ( F: CFCl3 (0.00 ppm), 11 125 40 B: (Et2O)BF3 (0.00 ppm), Te: Ph2Te2 (420.8 ppm) ). The NMR spectra of compound 3-6 could not be obtained cleanly as it could only be isolated as a mixture of 3-6 and diphenylacetylene.

Combustion elemental analyses were performed on a PerkinElmer CHN Analyzer. HR- MS was performed on a JEOL AccuTOF equipped with a Direct Analysis in Real Time (DART) ion source or a Waters GC Premier TOF-MS equipped with an EI/CI source.

3.3.2 Synthetic procedures and spectroscopic characterization

General procedure the synthesis of compounds 3-3 to 3-15

Compound 3-1 or 3-2 and 2.2 eq. of alkyne were dissolved in 5 mL of toluene. The orange or yellow solution was then transferred into a 50-mL Schlenk flask equipped with a Teflon tab seal. The solution was heated to 110 oC for 16 h. The orange or red solution was then filtered through a plug of Celite before all volatiles were removed. The resulting dark orange or red oil was triturated or sonicated in a variety of solvent to precipitate out the desired product.

Preparation and spectroscopic data of 3-3

60.8 mg of compound 3-2 (0.106 mmol) was reacted with 22.5 mg of phenylacetylene (0.220 mmol) to give 29.2 mg of 3-3 (0.0696 mmol, 65.6%), which precipitated out as a dark yellow powder when sonicated in minimal amounts of O(TMS)2. 1 B H (600.0 MHz, CD2Cl2): δ 8.28 (s, 2H, H(B)C=), 8.22-8.20 (m, 2H, o-Ph ), 7.68-7.65 (m, 4H, o-Ph), 7.50-7.46 (m, 9H, m- p-PhB + m- p-Ph) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 49.9 (s, ν1/2 ≈ 790 Hz)

13 1 B C{ H} (125.1 MHz, CD2Cl2): 164.6 (s, TeC=), 145.9 (s, i-Ph), 145.2 (br s, i-Ph ), 139.3 (br s, =CB), 134.9 (s, o-PhB), 131.0 (s, p-Ph), 129.9 (s, p-PhB), 129.7 (s, m-Ph), 128.7 (s, m-PhB), 127.7 (s, o-Ph) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 659.7 (s)

Anal. Calc. for C22H17TeB : C 62.95%, H 4.08%. Found: C 63.23%, H 4.05%.

Preparation and spectroscopic data of 3-4

101.9 mg of compound 3-2 (0.1782 mmol) was reacted with 68.8 mg of 4- bromophenylacetylene (0.380 mmol) to give 60.3 mg of 3-4 (0.104 mmol, 58.6% yield), which precipitated out of solution as a bright yellow powder when sonicated in pentane. 1 H (600.0 MHz, C6D6): δ 8.21-8.19 (m, 4H, o-PhB + H(B)C=), 7.42-7.38 (m, 3H, m-, p- B 3 2 3 3 Ph ), 7.24 (d, 4H, JH-H = 8.4 Hz, H ), 7.06 (d, 4H, JH-H = 8.4 Hz, H ) 11 1 B{ H} (128.3 MHz, C6D6): δ 50.3 (s, ν1/2 ≈ 1040 Hz) 13 1 B 1 C{ H} (100.6 MHz, C6D6): δ 162.3 (s, TeC=), 144.8 (br s, i-Ph ), 144.4 (s, C ), 139.8 (br s, =CB), 134.9 (s, o-PhB), 132.5 (s, C2), 131.1 (s, p-PhB), 128.9 (s, C3), 128.6 (s, m- PhB), 123.9 (s, C4) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 670.3 (s) [Note: PhB denotes the phenyl ring directly bonded to boron]

Anal. Calc. for C22H15TeBBr2 : C 45.75%, H 2.62%. Found: C 45.89%, H 2.39%.

Preparation and spectroscopic data of 3-5

82.5 mg of compound 3-2 (0.144 mmol) was reacted with 42.8 mg of 4-ethynylanisole (0.324 mmol) to give 40.8 mg of 3-5 (0.0850 mmol, 59.0% yield), which precipitated out of solution as a dark yellow powder when sonicated in pentane.

1 B H (500.0 MHz, CD2Cl2): δ 8.21-8.18 (m, 2H, o-Ph ), 8.17 (s, 2H, H(B)C=), 7.62 (d, 4H, 3 2 B 3 3 JH-H = 11 Hz, H ), 7.50-7.48 (m, 3H, m-, p-Ph ), 7.00 (d, 4H, JH-H = 11 Hz, H ), 3.86 (s,

6H, OCH3) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 49.5 (s, ν1/2 ≈ 720 Hz) 13 1 4 B C{ H} (100.6 MHz, CD2Cl2): δ 163.7 (s, TeC=), 161.4 (s, C ), 145.6 (br s, i-Ph ), 138.4 (s, C2), 137.8 (br s, =CB), 134.7 (s, o-PhB), 130.7 (s, p-PhB), 129.0 (s, C3), 128.6 (s, m- B 1 Ph ), 115.0 (s, C ), 56.0 (s, OCH3) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 626.3 (s) [Note: PhB denotes the phenyl ring directly bonded to boron]

MS (EI+): calcd for C24H21O2BTe [M+]: 482.0697 amu. Found: 482.0694 amu.

Preparation and spectroscopic data of 3-6

84.5 mg of compound 3-2 (0.148 mmol) was reacted with 50.2 mg of 4-tert- butylphenylacetylene (0.137 mmol) to give 42.8 mg of 3-6 (0.0805 mmol, 50.3% yield) and 20% PhCCPh. Compound 3-6 could only be purified by recrystallization in O(TMS)2, and always co-crystallized with PhCCPh at various ratios as orange crystals. 1 B H (500.0 MHz, CD2Cl2): δ 8.25 (s, 2H, H(B)C=), 8.21-8.19 (m, 2H, o-Ph ), 7.61 (d, 4H, 3 2 3 3 B JH-H = 8.5 Hz, H ), 7.52 (ad, 4H, JH-H = 8.5 Hz, H ), 7.50-7.47 (m, 3H, m-, p-Ph ), 1.38

(s, 18H, C(CH3)3) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 49.6 (s, ν1/2 ≈ 940 Hz) 13 1 4 B C{ H} (100.6 MHz, CD2Cl2): δ 164.4 (s, TeC=), 153.4 (s, C ), 145.4 (br s, i-Ph ), 143.1 (s, C1), 138.8 (br s, =CB), 134.8 (s, o-PhB), 132.1 (s, p-PhB), 130.8 (s, m-PhB), 127.4, (s, 2 3 C ), 126.7 (s, C ), 35.3 (s, C(CH3)3), 31.6 (s, C(CH3)3) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 649.5 (s) [Note: PhB denotes the phenyl ring directly bonded to boron]

MS (EI+): calcd for C30H33BTe [M+]: 534.1738 amu. Found: 534.1722 amu.

Preparation and spectroscopic data of 3-7

75.9 mg of compound 3-2 (0.133 mmol) was reacted with 50.5 mg of 4- trifluoromethylphenylacetylene (0.297 mmol) to give 41.6 mg of 3-7 (0.0748 mmol, 57.2% yield), which precipitated out as a light yellow powder when sonicated in minimal amounts of pentane. 1 B H (500.0 MHz, CD2Cl2): δ 8.32 (s, 2H, H(B)C=), 8.22-8.19 (m, 2H, o-Ph ), 7.75 (app s, 8H, H2 + H3), 7.55-7.48 (m, 3H, m-, p-PhB) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 49.8 (s, ν1/2 ≈ 800 Hz) 13 1 4 C{ H} (125.1 MHz, CD2Cl2): δ 162.1 (s, TeC=), 149.0 (br q, JC-F = 1.2 Hz, C ), 144.4 (br s, i-PhB), 140.8 (br s, =CB), 135.1 (s, o-PhB), 132.1 (s, C1), 131.51 (s, p-PhB), 131.17 1 B 2 3 (q, JC-F = 31.3 Hz, -CF3), 128.8 (s, m-Ph ), 128.2 (s, C ), 126.6 (q, JC-F = 3.6 Hz, C -Ph) 19 1 F{ H} (376.4 MHz, CD2Cl2): -62.9 (s) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 681.1 (s) [Note: PhB denotes the phenyl ring directly bonded to boron]

MS (EI+): calcd for C24H15BTeF6 [M+]: 558.0233 amu. Found: 558.0234 amu

Preparation and spectroscopic data of 3-8

38.5 mg of compound 3-2 (0.0673 mmol) was reacted with 24.3 mg of 3- chlorophenylacetylene (0.178 mmol) to give 21.6 mg of 3-8 (0.0442 mmol, 65.7% yield), which precipitated out as a yellow powder when sonicated in minimal amounts of

O(TMS)2.

100

1 B H (500.0 MHz, CD2Cl2): δ 8.26 (s, 2H, H(B)C=), 8.20-8.19 (m, 2H, o-Ph ), 7.66-7.65 (m, 1H, p-PhB), 7.54-5.52 (m, 2H, Ar-H), 7.44-7.51 (m, 2H, m-PhB), 7.37-7.36 (m, 2H, Ar-H) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 50.6 (s, ν1/2 ≈ 560 Hz) 13 1 1 B C{ H} (100.6 MHz, CD2Cl2): δ 162.2 (s, TeC=), 147.5 (s, C -Ph), 144.6 (br s, i-Ph ), 140.1 (br s, =CB), 135.5 (s, C5-Ph), 135.0 (s, o-PhB), 131.3 (s, Ar-C), 131.0 (s, Ar-C), 129.7 (s, Ar-C), 128.7 (s, m-PhB), 127.5 (s, p-PhB), 126.2 (s, Ar-C) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 675.3 (s) [Note: PhB denotes the phenyl ring directly bonded to boron]

MS (EI+): calcd for C22H15BCl2Te [M+]: 489.9706 amu. Found: 489.9700 amu

Preparation and spectroscopic data of 3-9

86.4 mg of compound 3-2 (0.151 mmol) was reacted with 43.9 mg of 3, 5- difluorophenylacetylene (0.318 mmol) to give 34.0 mg (0.0691 mmol, 45.8% yield) of 3- 9, which precipitated out as a bright yellow powder when sonicated in minimal amounts of O(TMS)2. 1 B H (400.0 MHz, CD2Cl2): δ 8.27 (s, 2H, H(B)C=), 8.20-8.17 (m, 2H, o-Ph ), 7.55-7.47 (m, B 2 3 4 4 3H, m-, p-Ph ), 7.19 (m, 4H, H ), 6.94 (tt, 2H, JH-F = 8.8 Hz, JH-H = 2.0 Hz, H ) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 50.5 (s, ν1/2 ≈ 650 Hz) 13 1 1 3 3 C{ H} (125.1 MHz, CD2Cl2): δ 163.8 (dd, JC-F = 248.6 Hz, JC-F = 12.9 Hz, C ), 160.6 4 3 1 B (t, JC-F = 2.5 Hz, TeC=), 148.8 (t, JC-F = 9.1 Hz, C ), 144.2 (br s, i-Ph ), 140.6 (br s, B B B 2 =CB), 135.1 (s, o-Ph ), 131.6 (s, p-Ph ), 128.8 (s, m-Ph ), 110.9 (dd, JC-F = 19.6 Hz, 4 2 4 JC-F = 6.4 Hz), 104.8 (t, JC-F = 25.4 Hz, C ) 19 1 F{ H} (376.4 MHz, CD2Cl2): -109.1 (s) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 684.3 (s) [Note: PhB denotes the phenyl ring directly bonded to boron] + MS (DART+): calcd for C22H13BF4Te [M ]: 494.01087 amu. Found: 494.01114 amu

101

Preparation and spectroscopic data of 3-10

82.5 mg of compound 3-2 (0.144 mmol) was reacted with 54.1 mg of 4-ethnylbiphenyl (0.303 mmol) to give 58.9 mg of 3-10 (0.13 mmol, 71.5% yield), which precipitated out as a deep yellow powder when triturated with pentane. 1 B H (400.0 MHz, THF-d8): δ 8.36 (s, 2H, H(B)C=),8.24-8.22 (m, 2H, o-Ph ), 7.80-7.75 (m, 6 7 3 2 3 B 8H, H + H ), 7.70 (dm, 4H, JH-H = 8.4 Hz, H ), 7.47-7.44 (m, 7H, H + m- + p-Ph ), 7.35 3 8 (app d, 2H, JH-H = 2.0 Hz, H ) 11 1 B{ H} (128.3 MHz, THF-d8): (couldn’t be observed) 13 1 B 1 C{ H} (125.1 MHz, THF-d8): δ 164.1 (br s, TeC=) 145.8 (br s, i-Ph ), 145.3 (s, C ), 143.3 (s, C5), 141.3 (s, C4), 139.5 (br s, =CB), 135.3 (s, o-PhB), 131.3 (s, p-PhB), 129.9 (s, C3), 129.0 (s, m-PhB), 128.7 (s, C6 + C7-), 128.6 (s, C8), 127.9 (s, C2) 125 Te (157.8 MHz, THF-d8, 193 K): δ 649.6 (s) [Note: PhB denotes the phenyl ring directly bonded to boron]

Preparation and spectroscopic data of 3-11

89.5 mg of compound 3-2 (0.156 mmol) was reacted with 65.4 mg of 2-ethynylfluorene (0.343 mmol) to give 67.6 mg of 3-11 (0.113 mmol, 72.7% yield), which precipitated out of solution as a yellow-orange solid when triturated with pentane. 1 B H (400.0 MHz, CD2Cl2): δ 8.34 (s, 2H, H(B)C=), 8.26-8.23 (m, 2H, o-Ph ), 7.90-7.85 (m, 2 9 13 3 12 3 6 6H, H + H + H ), 7.72 (d, 2H, JH-H = 9.0 Hz, H ), 7.61 (d, 2H, JH-H = 7.2 Hz, H ),

102

B 3 8 3 7.51-7.49 (m, 3H, m- + p-Ph ), 7.43 (t, 2H, JH-H = 7.5 Hz, H ), 7.37 (t, 2H, JH-H = 6.6 Hz, H7), 4.02 (s, 4H, H4) 11 1 B{ H} (128.3 MHz, CD2Cl2): (couldn’t be observed) 13 1 B C{ H} (125.1 MHz, CD2Cl2): 164.9 (s, TeC=), 145.4 (br s, i-Ph ), 144.9 (s, Ar-C), 144.5 (s, C1), 144.4 (s, Ar-C), 143.6 (s, Ar-C), 141.4 (s, Ar-C), 139.0 (br s, =CB), 134.9 (s, o-PhB), 130.9 (s, p-PhB), 128.7 (s, m-PhB), 127.9 (s, C7), 127.5 (s, C8), 126.8 (s, C12), 125.7 (s, C6), 124.4 (s, C9), 120.9 (s, C2 or C13), 120.8 (s, C2 or C13), 37.5 (s, C4) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 652.4 (s)

Preparation and spectroscopic data of 3-12

84.6 mg (0.148 mmol) of compound 3-2 was reacted with 34.1 mg of 3- ethynylthiophene (0.315 mmol) to give 32.1 mg of 3-12 (0.0743 mmol, 50.2% yield), which precipitated out of solution as a bright yellow powder when sonicated in O(TMS)2. 1 B H (400.0 MHz, CD2Cl2): δ 8.21 (s, 2H, H(B)C=), 8.17-8.15 (m, 2H, o-Ph ), 7.62 (t, 1H, 3 JH-H = 2.2 Hz, Ar-H), 7.49-7.48 (m, 8H, Ar-H) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 50.3 (s, ν1/2 ≈ 590 Hz) 13 1 B C{ H} (100.6 MHz, CD2Cl2): δ 154.8 (s, TeC=), 147.2 (s, Ar-C), 145.2 (br s, i-Ph ), 137.8 (br s, =CB), 134.7 (s, o-PhB), 131.0 (s, C1), 128.7 (s, m-PhB), 127.7 (s, p-PhB), 126.5 (s, Ar-C), 123.7 (s, Ar-C) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 642.0 (s)

Preparation and spectroscopic data of 3-13

103

100.5 mg of compound 3-2 (0.176 mmol) was reacted with 76.8 mg of ethynylferrocene (0.366 mmol) to give 58.7 mg of 3-13 (0.0923 mmol, 52.5% yield), which precipitated out as a bright red solid when sonicated in minimal amounts of O(TMS)2. 1 B H (500.0 MHz, CD2Cl2): δ 8.11-8.09 (m, 2H, o-Ph ), 7.93 (s, 2H, H(B)C=), 7.48-7.45 (m, B 3 3 3 2 3H, m- + p-Ph ), 4.79 (t, 4H, JH-H = 2.0 Hz, H ), 4.48 (t, 4H, JH-H = 2.0 Hz, H ), 4.20 (s, 10H, Cp) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 48.5 (s, ν1/2 ≈ 950 Hz) 13 1 B C{ H} (125.7 MHz, CD2Cl2): δ 160.6 (s, TeC=), 146.0 (br s, i-Ph ), 134.9 (br s, =CB), 134.3 (s, o-PhB), 130.4 (s, p-PhB), 128.5 (s, m-PhB), 90.4 (s, C1), 71.5 (s, Cp), 70.8 (C2), 68.5 (C3). 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 648.0 (s) [Note: Cp refers to the cyclopentadiene ring not directly bonded to the central Te-B heterocycle. PhB denotes the phenyl ring directly bonded to boron] + MS (DART+): calcd for C30H26BFe2Te [M+H ]: 638.98886 amu. Found: 638.98751 amu

Preparation and spectroscopic data of 3-14

106.0 mg of compound 3-2 (0.185 mmol) was reacted with 57.0 mg of 4-ethynyl-N,N- dimethylaniline (0.393 mmol) to give 58.7 mg of 3-14 (0.116 mmol, 62.7% yield), which precipitated out as a deep green solid when sonicated in minimal amounts of O(TMS)2. 1 B 3 H (500.0 MHz, CD2Cl2): δ 8.49 (s, 2H, H(B)C=), 8.37 (m, 2H, o-Ph ), 7.67 (m, 4H, H ), B 2 7.45-7.37 (m, 3H, m- + p-Ph ), 6.49 (m, 4H, H ), 2.46 (s, 12H, NMe2) 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 51.3 (s, ν1/2 ≈ 1310 Hz), 2.18 (s, ν1/2 ≈ 57 Hz) 13 1 4 B C{ H} (125.7 MHz, CD2Cl2): δ 164.1 (s, TeC=), 151.5 (s, C ), 146.6 (br s, i-Ph ), 136.5 (br s, =CB), 134.7 (s, o-PhB), 134.2 (s, C1), 132.0 (s, p-PhB), 130.0 (s, m-PhB), 128.7 (s, 3 2 C ), 112.8 (s, C ), 39.9 (s, NMe2) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 611.2 (s) [PhB denotes the phenyl ring directly bonded to boron]

104

Preparation and spectroscopic data of 3-15

83.0 mg of compound 3-1 (0.0986 mmol) was reacted with 38.4 mg of 4- bromophenylacetylene (0.212 mmol) to give 20.5 mg of 3-15 (0.0311 mmol, 31% yield), which precipitated out as a yellow solid when triturated with minimal amounts of pentane. 1 3 3 H (500.0 MHz, CD2Cl2): δ 8.05 (s, 2H, H(B)C=), 7.61 (d, 4H, JH-H = 8.3 Hz, H ), 7.45 (d, 3 2 2 4H, JH-H = 8.3 Hz, H ), 6.49 (m, 4H, H ), 11 1 B{ H} (128.3 MHz, CD2Cl2): δ 46.2 (s, ν1/2 ≈ 1240 Hz) 13 1 C{ H} (125.7 MHz, CD2Cl2): δ (partial, C6F5 signals not listed) 167.0 (s, TeC=), 143.5 (s, C4), 140.5, (br s, =CB), 132.6 (s, C3), 128.8 (s, C2), 124.5 (s, s, C1) 125 Te (157.8 MHz, CD2Cl2, 193 K): δ 713.9 (s)

General procedure the synthesis of compounds 3-16 to 3-25

1 eq. of compound 3-1 and 1.1 eq. of alkyne were dissolved in 5 mL of acetonitrile. The yellow slurry was then transferred into a 50-mL Schlenk flask equipped with a Teflon tab seal. The solution was heated to 110 oC for 16 h, upon which point a crude 19F{1H} NMR spectrum was taken to ensure reaction completion. The reaction times of compounds 3- 16 to 3-25 vary between 1 to 7 days (Scheme 3-5). Once the reaction is completed as determined by in situ 19F{1H} NMR spectra, all volatiles were removed under reduced pressure, and the product was extracted into 5 mL of DCM and filtered through a short plug of Celite. The resulting clear yellow or orange solution was then put under vacuum to remove all volatiles again before pentane was used to either precipitate out or recrystallize the product.

105

Preparation and spectroscopic data of 3-16

612.2 mg of compound 3-1 (0.7272 mmol) was reacted with 104.0 mg of benzyl alcohol (0.9617 mmol) to give 478.1 mg of 3-16 (0.6114 mmol, 85.1%), which precipitated out as an off-white powder when triturated with pentane. 1 Te H (500.0 MHz, CDCl3): δ 7.26 (m, 6H, m- + p-Ph ), 7.20 (m, 3H, m- + p-OCH2Ph), Te 7.17 (m, 4H, o-Ph ), 6.81 (m, 2H, o-OCH2Ph), 4.58 (s, 2H, OCH2Ph). 11 1 B{ H} (128 MHz, CDCl3): δ 36.9 (br s, ν1/2 ≈ 817 Hz) 13 1 Te C{ H} (125 MHz, CDCl3): δ 159.6 (s, TeC=), 141.4 (s, i-Ph ), 138.3 (s, i-OCH2Ph), Te Te 129.9 (br s, BC=), 128.8 (s, p-Ph ), 128.5 (s, m-Ph ), 128.2 (s, m-OCH2Ph), 127.6 (s, Te p-OCH2Ph), 126.2 (s, o-Ph ), 125.1 (s, o-OCH2Ph), 68.0 (s, OCH2Ph). 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.0 (m, 4F, o-C6F5), -156.7 (t, 2F, JF-F = 21.5 Hz, p-C6F5), -163.0 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 773.6 (s) + MS (DART+): calcd for C35H18OBF10Te [M+H ]: 785.03533 amu. Found: 785.03695 amu

Preparation and spectroscopic data of 3-17

77.0 mg of compound 3-1 (0.0915 mmol) was reacted with 20.3 mg of 4-bromobenzyl alcohol (0.109 mmol) to give 48.8 mg of 3-17 (0.0567 mmol, 61.9%), which precipitated out as a white powder when triturated with pentane.

106

1 3 H (400.0 MHz, CDCl3): δ 7.34 (d, 2H, JH-H = 8.4 Hz, m-OCH2Ar), 7.28-7.25 (m, 6H, m- Te Te 3 + p-Ph ), 7.16 (m, 4H, o-Ph ), 6.70 (d, 2H, JH-H = 8.4 Hz, o-OCH2Ar), 4.50 (s, 2H,

OCH2Ar) 11 1 B{ H} (128 MHz, CDCl3): δ 36.3 (br s, ν1/2 ≈ 1222 Hz) 13 1 Te C{ H} (125 MHz, CDCl3): δ n.o. (BC=), 160.2 (s, TeC=), 141.4 (s, i-Ph ), 137.3 (s, i- Te Te OCH2Ar), 131.4 (s, m-OCH2Ar), 128.9 (s, p-Ph ), 128.6 (s, m-Ph ), 126.9 (s, o- Te OCH2Ar) , 126.2 (s, o-Ph ), 121.5 (s, o-OCH2Ar), 67.4 (s, OCH2Ar). 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -138.9 (m, 4F, o-C6F5), -156.2 (t, 2F, JF-F = 21.0 Hz, p-C6F5), -162.8 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 777.7 (s) + MS (DART+): calcd for C35H17BBrF10OTe [M+H ]: 862.94584 amu. Found: 862.94394 amu

Preparation and spectroscopic data of 3-18

114.0 mg of compound 3-1 (0.1354 mmol) was reacted with 55.1 mg of cinnamyl alcohol (0.411 mmol) to give 83.2 mg of 3-18 (0.103 mmol, 76.1%), which precipitated out as a yellow powder when triturated with pentane. 1 H (400.0 MHz, CDCl3): δ 7.31-7.22 (m, 9H, Ar-H), 7.20-7.15 (m, 6H, Ar-H), 6.08 (d, 1H, 3 3 3 JH-H = 15.8 Hz, OCH2CHCHPh), 5.87 (dt, 1H, JH-H = 15.8 Hz, JH-H = 5.4 Hz,

OCH2CHPh), 4.15 (m, OCH2CHPh) 11 1 B{ H} (128 MHz, CDCl3): δ 37.2 (br s, ν1/2 ≈ 997 Hz) 13 1 C{ H} (125 MHz, CDCl3): (C6F5 signals not listed, Ar signals tentatively assigned due to closeness in peaks) δ 159.4 (s, TeC=), 141.5 (s, i-PhTe), 136.5 (s, i-Ph), 129.9 (br s, Te Te BC=),129.7 (s, OCH2CHPh), 128.8, (s, p-Ph), 128.7 (s, p-Ph ), 128.5 (s, m-Ph ), Te 127.8 (s, m-Ph), 126.3 (s, o-Ph), 126.2 (s, o-Ph ), 125.7 (s, OCH2CHCHPh), 66.4 (s,

OCH2CHPh)

107

19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -138.8 (m, 4F, o-C6F5), -156.3 (t, 2F, JF-F = 21.4 Hz, p-C6F5), -162.8 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 773.3 (s)

Anal. Calc. for C37H19BF10OTe: C 55.00%, H 2.37%. Found: C 54.67% H 2.24%

Preparation and spectroscopic data of 3-19

84.9 mg of compound 3-1 (0.101 mmol) was reacted with 23.6 mg of 1,1- diphenylmethanol (0.128 mmol) to give 42.8 mg of 3-19 (0.0498 mmol, 49.4%), which precipitated out as a white powder when triturated with pentane. 1 Te H (400.0 MHz, CDCl3): 7.38-7.33 (m, 6H, m- + p-Ph), 7.28-7.24 (m, 10H, m- + p-Ph + Te o-Ph), 6.95 (m, 4H, o-Ph ), 5.84 (s, OCHPh2) 11 1 B{ H} (128 MHz, CDCl3): δ 36.2 (br s, ν1/2 ≈ 1380 Hz) 13 1 Te C{ H} (125 MHz, CDCl3): (C6F5 signals not listed) δ 160.2 (s, TeC=), 142.2 (s, i-Ph ), 141.4 (s, i-Ph), 130.4 (br s, BC=), 128.8 (s, p-PhTe), 128.5 (s, m-PhTe), 128.3 (s, m-Ph), Te 127.7 (s, p-Ph), 126.2 (s, o-Ph ), 125.0 (s, o-Ph), 80.3 (s, OCHPh2) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.1 (m, 4F, o-C6F5), -157.1 (t, 2F, JF-F = 21.1 Hz, p-C6F5), -163.2 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 777.7 (s)

Preparation and spectroscopic data of 3-20

221.1 mg of compound 3-1 (0.2626 mmol) was reacted with 33.3 mg of 5-norbornene-2- methanol (endo:exo ≈ 7:3) (0.268 mmol) to give 129.3 mg of 3-20 (0.1620 mmol, 61.7%), which precipitated out as an off-white powder when triturated with pentane. The

108 ratio of endo and exo isomers in 3-20 is about the same as the starting mixture as confirmed by integration in 1H NMR spectrum. 1 Te Te H (500.0 MHz, CDCl3): 7.35-7.34 (m, 2H, p-Ph ), 7.29-7.23 (m, 4H, m-Ph ), 7.17 (m, 4H, o-PhTe), 6.01 (m, 1H, endo =CH), 5.96 (m, 1H, exo =CH), 5.92 (m, 1H, exo =CH),

5.61 (m, 1H, endo =CH), 3.41 (m, 1H, exo OCH2), 3.21 (app t, 1H, exo OCH2), 3.04 (m, 1 1H, endo OCH2), 2.95 (app t, 1H, endo OCH2), 2.69 (br s, 1H, H ), 2.36 (br s, 1H, endo H4), 2.22 (br s, 1H, exo H4), 1.96 (m, 1H, endo H2), 1.50 (m, 1H, endo H3), 1.33 (m, 1H, endo H7), 1.29 (m, 1H, exo C2), 1.20 (m, 1H, exo H7), 1.11 (m, 1H, endo H7), 0.96 (m, 1H, exo H3), 0.91 (m, 1H, exo H7), 0.71 (m, 1H, exo H3), 0.12 (m, 1H, endo H3) 11 1 B{ H} (128 MHz, CDCl3): δ 34.9 (br s, ν1/2 ≈ 922 Hz) 13 1 C{ H} (125 MHz, CDCl3): δ 158.8 (s, exo TeC=), 158.4 (s, endo TeC=), 141.54 (s, Ar- C), 141.49 (s, Ar-C), 141.2 (s, Ar-C), 137.5 (s, endo C5), 136.8 (s, exo C5), 136.2 (s, exo C6), 131.8 (s, endo C6), 129.9 (s, Ar-C), 129.7(s, Ar-C), 128.8 (s, Ar-C), 128.7 (s, Ar-C), 128.7 (s, Ar-C), 128.5 (s, Ar-C), 128.5 (s, Ar-C), 126.5 (s, Ar-C), 126.3 (s, Ar-C), 70.3 (s, 7 7 4 exo OCH2), 69.7 (s, endo OCH2), 49.4 (s, endo C ), 44.8 (s, exo C ), 43.5 (s, endo C ), 43.26 (s, CNor), 43.28 (s, CNor), 42.2 (s, C1), 41.5 (s, CNor), 41.0 (s, CNor), 40.5 (s, CNor), 28.9 (s, exo C3), 28.5 (s, endo C3) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.1 (m, 4F, o-C6F5), -156.8 (t, 2F, JF-F = 20.5 Hz, 3 exo p-C6F5 ), -156.7 (t, 2F, JF-F = 21.8 Hz, endo p-C6F5), -163.2 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 763.5 (s) Note: when not specified, the endo- and exo-resonances are too close in chemical shifts to be unambiguously assigned by 2D experiments

Preparation and spectroscopic data of 3-21

103.7 mg of compound 3-1 (0.1232 mmol) was reacted with 51.8 mg of 2- phenoxyethanol (0.374 mmol) to give 62.9 mg of 3-21 (0.0775 mmol, 62.9%), which precipitated out as an off-white powder when triturated with pentane.

109

1 Te Te H (500.0 MHz, CDCl3): 7.27-7.22 (m, 8H, m- + p-Ph + m-Ph), 7.15 (m, 4H, o-Ph ), 3 4 O O 6.93 (tt, 1H, JH-H = 7.3 Hz, JH-H = 1.0 Hz, p-Ph ), 6.69 (m, 2H, p-Ph ), 3.73 (m, 2H,

CH2), 3.69 (m, 2H, CH2) 11 1 B{ H} (128 MHz, CDCl3): δ 36.4 (br s, ν1/2 ≈ 1210 Hz) 13 1 O Te C{ H} (125 MHz, CDCl3): δ 159.7 (s, TeC=), 158.6 (s, i-Ph ), 141.4 (s, i-Ph ), 130.1 (br s, BC=), 129.5 (s, m-PhO), 128.8 (s, p-PhTe), 128.5 (s, m-PhTe), 126.3 (s, o-PhTe), O O 121.2 (s, p-Ph ), 114.3 (s, o-Ph ), 67.9 (s, CH2), 64.6 (s, CH2) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.0 (m, 4F, o-C6F5), -156.2 (t, 2F, JF-F = 20.9 Hz, p-C6F5), -162.9 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 771.4 (s)

Anal. Calc. for C36H19BF10O2Te: C 53.25%, H 2.36%. Found: C 52.48%, H 2.24%.

Preparation and spectroscopic data of 3-22

118.5 mg of compound 3-1 (0.1408 mmol) was reacted with 18.2 mg of 4- methoxyphenol (0.147 mmol) to give 72.8 mg of 3-22 (0.0912 mmol, 64.7%) after 7 days of heating at 110 oC, which precipitated out as a light yellow powder when triturated with pentane. 1 Te Te H (400.0 MHz, CDCl3): 7.29-7.25 (m, 6H, m- + p-Ph ), 7.17 (m, 4H, o-Ph ), 6.55 (app 2 3 d, 4H, H + H ), 3.67 (s, 3H, OCH3) 11 1 B{ H} (128 MHz, CDCl3): δ 35.9 (br s, ν1/2 ≈ 1200 Hz) 13 1 4 C{ H} (125 MHz, CDCl3): (C6F5 signals not listed) δ 162.4 (s, TeC=), 155.7 (s, C ), 148.1 (s, C1), 141.3 (s, i-PhTe), 130.3 (br s, BC=), 128.9 (s, p-PhTe), 128.5 (s, m-PhTe), Te 2 3 126.3 (s, o-Ph ), 119.6 (s, C ), 114.1 (s, C ), 55.9 (s, OCH3) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -133.9 (m, 4F, o-C6F5), -157.1 (t, 2F, JF-F = 20.8 Hz, p-C6F5), -163.6 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 781.6 (s) + MS (DART+): calcd for C35H17O2BF10Te [M ]: 800.02241 amu. Found: 800.02322 amu

110

Preparation and spectroscopic data of 3-23

123.5 mg of compound 3-1 (0.1467 mmol) was reacted with 20.3 mg of cyclohexanol (0.203 mmol) to give 70.5 mg (0.0911 mmol, 62.1%) of 3-23, which precipitated out as a yellow powder when triturated with pentane. 1 Te Te H (500.0 MHz, CDCl3): 7.35 (m, 4H, o-Ph ), 7.30-7.24 (m, 6H, m- + p-Ph ), 7.16 (m, 3H, o-PhTe), 3.46 (br m, 1H, H1), 1.36 (br m, 5H, H3 + H2), 1.08 (br m, 3H, H2), 0.92 (br m, 2H, H4) 11 1 B{ H} (128 MHz, CDCl3): δ 35.8 (br s, ν1/2 ≈ 640 Hz) 13 1 Te C{ H} (125 MHz, CDCl3): (C6F5 signals not listed) δ 157.7 (s, TeC=), 141.6 (s, i-Ph ), 130.3 (br s, BC=), 128.6 (s, p-PhTe), 128.5 (s, m-PhTe), 126.2 (s, o-PhTe), 73.6 (s, C1), 34.4 (s, C2), 25.2 (s, C3), 23.2 (s, C4) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.1 (m, 4F, o-C6F5), -156.9 (t, 2F, JF-F = 20.5 Hz, p-C6F5), -163.4 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 754.5 (s) + MS (DART+): calcd for C34H22OBF10Te [M+H ]: 777.06663 amu. Found: 777.06606 amu

Preparation and spectroscopic data of 3-24

139.0 mg of compound 3-1 (0.1651 mmol) was reacted with 22.4 mg of 2- hydroxybenzylamine (0.1819 mmol) to give 86.6 mg of 3-24 (0.109 mmol, 65.8 %), which precipitated out as a white powder when triturated with pentane.

111

1 Te Te H (600.0 MHz, CDCl3): 7.31-7.29 (m, 4H, o-Ph ), 7.27-7.25 (m, 6H, m- + p-Ph ), 6.98 3 3 5 4 3 (app t, 1H, H ), 6.94 (d, 1H, JH-H = 7.6 Hz, H ), 6.78 (app t, 1H, H ), 6.34 (d, 1H, JH-H = 2 3 7 8.1 Hz, H ), 4.92 (br s, 2H, NH2), 4.41 (t, 2H, JH-H = 5.7 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 1.3 (s, ν1/2 ≈ 290 Hz) 13 1 C{ H} (125 MHz, CDCl3): (C6F5 signals not listed) δ 154.8 (s, TeC=), 143.2 (1:1:1:1 q, 1 Te 1 3 Te JC-B = 150 Hz, BC=), 142.9 (s, i-Ph ), 135.5 (s, C ), 129.2 (s, C ), 128.4 (s, m-Ph ), 127.9 (s, p-PhTe), 127.4 (s, o-PhTe), 126.6 (s, C5), 119.4 (s, C4), 119.0 (s, C2), 116.4 (s, C6), 42.0 (s, C7) 19 1 F{ H} NMR (377 MHz, CDCl3): δ -139.4 (m, 2F, o-C6F5), -141.5 (m, 2F, o-C6F5), -

158.8 (m, 2F, p-C6F5), -163.4 (m, 2F, m-C6F5), -165.1 (m, 2F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 691.7 (s)

Anal. Calc. for C34H16BF10NOTe: C 52.16%, H 2.06%, N 1.79%. Found: C 52.22%, H 2.39%, N 1.70%.

Preparation and spectroscopic data of 3-25

163.1 mg of compound 3-1 (0.1937 mmol) was reacted with 21.0 mg of trimethylsilanol (0.2328 mmol) to give 78.3 mg of 3-25 (0.102 mmol, 52.9%), which was purified by recrystallization from pentane. 1 3 H (600.0 MHz, C6D6): 7.11 (d, 4H, JH-H = 6.8 Hz, o-Ph), 6.85-6.77 (m, 6H, m-, p-Ph), -

0.42 (s, 9H, Si(CH3)3) 11 1 B{ H} (128 MHz, C6D6): δ 35.2 (s, ν1/2 ≈ 820 Hz) 13 1 Te C{ H} (125 MHz, C6D6): (C6F5 signals not listed) δ 161.7 (s, TeC=),142.0 (s, i-Ph ), 132.2 (br s, BC=), 129.0 (s, p-PhTe), 128.7 (s, m-PhTe), 126.3 (o-PhTe) 19 1 3 4 F{ H} NMR (377 MHz, C6D6): δ -139.1 (dd, 4F, JF-F = 24 Hz, JF-F = 8 Hz, o-C6F5), - 3 157.1 (t, 2F, JF-F = 21 Hz, p-C6F5), -163.5 (m, 4F, m-C6F5) 125 Te (158 MHz, C6D6): δ 768.4 (s)

Anal. Calc. for C31H19BF10SiOTe: C 48.74%, H 2.51%. Found: C 49.25%, H 2.49%.

Preparation and spectroscopic data of 3-26

112

135.1 mg of compound 3-16 (0.1728 mmol) was reacted with 51.0 mg of 4- ethynylbromobenzene (0.282 mmol) in ca. 4 mL of toluene at 110 oC for 16 h, giving a yellow and clear solution. All volatiles were removed under vacuum and the residue was triturated with pentane to give 84.7 mg of 3-26 (0.122 mmol, 70.6%) as a yellow powder. 1 6 7 H (500.0 MHz, CDCl3): δ 7.57 (m, 2H, H ), 7.43 (s, 1H, H(B)C=), 7.35-7.33 (m, 4H, H

+ Ar-H), 7.29-7.25 (m, 4H, Ar-H), 7.21-7.19 (br m, 4H, Ar-H), 5.24 (s, 2H, BOCH2) 11 1 B{ H} (128 MHz, CDCl3): δ 38.6 (s, ν1/2 ≈ 920 Hz) 13 1 C{ H} (125 MHz, CDCl3): (C6F5 signals not listed) δ 159.0 (s, TeC=), 156.7 (s, TeC=), 143.2 (s, Ar-C), 141.9 (s, Ar-C), 139.2 (s, Ar-C), 132.2 (s, C6), 130.9 (br s, =CB), 129.6 (br s, =CB), 128.5 (s, Ar-C), 128.4 (s, Ar-C), 128.3 (s, Ar-C), 128.2 (s, Ar-C), 127.3 (s,

Ar-C), 126.6 (s, Ar-C), 125.8 (s, Ar-C), 123.7 (s, Ar-C), 67.9 (s, OCH2Ph) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.8 (m, 2F, o-C6F5), -157.8 (t, 1F, JF-F = 19.7 Hz, p-C6F5), -164.0 (m, 2F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 681.7 (s)

Anal. Calc. for C29H17BBrF5OTe: C 50.14%, H 2.47%. Found: C 50.32%, H 2.40%.

Preparation and spectroscopic data of 3-27

116.3 mg of compound 3-16 (0.1487 mmol) was reacted with 51.0 mg of 3- ethynylthiophene (0.472 mmol) in ca. 4 mL of toluene at 110 oC for 16 h, giving a

113 brownish-yellow and clear solution. After all volatiles were removed in vacuo, the residue was dissolved in 3 mL of pentane and passed through a plug of silica. 5 mL more of pentane was used to rinse out the C6F5CCPh from the silica plug. 5 mL of DCM was then used to extract 13 from the silica, giving again a clear and brownish yellow solution. All volatiles were removed again to give 73.0 mg of 3-27 (0.117 mmol, 78.9 %) as a light yellow sticky solid. 1 S S H (600.0 MHz, CDCl3): δ 7.46 (s, 1H, H(B)C=), 7.43 (m, 1H, Ar -H), 7.41 (m, 1H, Ar - H), 7.32 (app d, 2H, Ar-H), 7.28-7.23 (m, 6H, Ar-H), 7.20-7.17 (m, 4H, Ar-H), 5.22 (s, 2H,

BOCH2) 11 1 B{ H} (128 MHz, CDCl3): δ 38.4 (s, ν1/2 ≈ 600 Hz) 13 1 C{ H} (125 MHz, CDCl3): (C6F5 signals not listed) δ 156.0 (s, TeC=), 152.5 (s, TeC=), 145.8 (s, Ar-C), 142.2 (s, Ar-C), 139.5 (s, Ar-C), 128.6 (s, Ar-C), 128.6 (s, Ar-C), 128.5 (s, Ar-C), 127.4 (s, Ar-C), 127.3 (s, Ar-C), 126.8 (s, Ar-C), 125.9 (s, Ar-C), 125.8 (s, Ar-

C), 123.0 (s, Ar-C), 68.0 (s, OCH2Ph) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.8 (m, 2F, o-C6F5), -158.1 (t, 1F, JF-F = 20.3 Hz, p-C6F5), -164.2 (m, 2F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 633.6 (s) + MS (DART+): calcd for C27H17OBF5STe [M+H ]: 625.00755 amu. Found: 625.00837 amu

Preparation and spectroscopic data of 3-28

836.0 mg of compound 3-16 (1.069 mmol), 249.4 mg of H3SiPh (2.305 mmol) and 57.6 mg of B(C6F5)3 (0.113 mmol) were dissolved in 10 mL of toluene, sealed in a Schlenk flask equipped with a Teflon tab. The clear yellow solution was heated to 110 °C for 16 h and turned gradually brighter yellow. All volatiles were removed under vacuum to leave a yellow/orange residue, which was then washed three times with pentane to give 474.5 mg of 3-28 as a bright yellow powder (0.7021 mmol, 65.7%) 1 H (400.0 MHz, CDCl3): δ 7.27 (m, 6H, m- + p-Ph), 7.49 (m, 4H, o-Ph), 5.77 (br s, B-H)

114

11 1 B{ H} (128 MHz, CDCl3): δ 53.3 (br s, ν1/2 ≈ 94 Hz) 13 1 C{ H} (125 MHz, CDCl3): δ 171.6 (s, TeC=), 141.4 (s, i-Ph),137.9 (m, BC=), 129.5 (s, p-Ph), 128.8 (s, m-Ph), 126.9 (s, o-Ph) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -140.4 (m, 4F, o-C6F5), -157.1 (t, 2F, JF-F = 18.2 Hz, p-C6F5), -163.0 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): (could not observe a signal even at 230 K)

Anal. Calc. for C28H11BF10Te: C 49.77%, H 1.64%. Found: C 49.71%, H 1.75%.

General procedure the synthesis of compounds 3-29 to 3-39

1 eq. of compound 3-28 and 1.1 eq. of substrate were dissolved in 1 mL of CDCl3 at room temperature. The reaction mixture was transferred into a 5-mm NMR tube and multinuclear spectra were recorded immediately. The sample was monitored periodically as shown in Table 3-3. Most reactions proceeded cleanly to one single product, which allowed for full NMR characterization without isolation. The isolation of a few of the products were, however, achieved in a few cases to demonstrate the utility of this hydroboration reaction.

Preparation and spectroscopic data of 3-29

99.7 mg of compound 3-28 (0.175 mmol) and 25.1 mg of 1-hexene (0.298 mmol) were dissolved in 3 mL of toluene and allowed to stir at room temperature for 3 hours. The yellow solution very quickly became clear and turned a brighter yellow color. All volatiles were removed in vacuo, leaving an off-white solid, which was washed pentane (3 mL × 3), giving 103.3 mg of compound 3-29 (0.1360 mmol, 77.7% yield) as a white powder.

115

1 H (500.0 MHz, CDCl3): δ 7.03-6.99 (m, 6H, m- + p-Ph), 6.94-6.92 (br m, 4H, o-Ph), 4 2 3 1 5 3 0.78 (br m, 2H, C ), 0.63 (br m, 4H, C + C ), 0.54 (br s, 4H, C + C ), 0.48 (t, 3H, JH-H = 7.3 Hz, C6) 11 1 B{ H} (128 MHz, CDCl3): δ 57.0 (br s, ν1/2 ≈ 1480 Hz) 13 1 C{ H} (125 MHz, CDCl3): δ 166.0 (s, TeC=), 141.9 (s, i-Ph), 137.9 (br s, BC=), 129.0 (s, p-Ph), 128.5 (s, m-Ph), 126.3 (s, o-Ph), 32.7 (s, C2), 31.2 (s, C3), 25.3 (s, C1), 23.6 (s, C5), 22.3 (s, C4), 14.0 (s, C6) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.0 (m, 4F, o-C6F5), -156.7 (t, 2F, JF-F = 20.5 Hz, p-C6F5), -162.9 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 844.2 (s)

Preparation and spectroscopic data of 3-30

99.7 mg of compound 3-28 (0.147 mmol) and 0.05 mL pyridylethylene (excess) were dissolved in ca. 5 mL of pentane stirred at room temperature for 3 hours. Lots of white precipitate separated out of solution over this time. After the supernatant has been decanted, the white powder was dried under vacuum to give 75.3 mg of 3-30 (0.0961 mmol, 65.3%) as a white powder that is only soluble in polar solvents such as acetonitrile and THF. 1 N N H (500.0 MHz, THF-d8): δ 8.75 (br s, 1H, Ar -H), 8.11 (br s, 1H, Ar -H), 7.87 (br s, 1H, ArN-H), 7.55 (br s, 2H, ArN-H), 7.16-7.15 (m, 8H, o- + m-Ph), 7.07 (m, 2H, p-Ph), 5.90 (br s, 1H, H2), 5.68 (br s, 1H, H2’), 4.02 (br s, 1H, H1) 11 1 B{ H} (128 MHz, THF-d8): δ 0.99 (br s, ν1/2 ≈ 46.8 Hz) 19 1 F{ H} NMR (377 MHz, THF-d8): δ -139.8 (br s, 2F, o-C6F5), -142.9 (br s, 2F, o-C6F5’), 3 -161.6 (t, 2F, JF-F = 22.5 Hz, p-C6F5), -165.5 (m, 2F, m-C6F5), -166.6 (m, 2F, m-C6F5’) 125 Te (158 MHz, THF-d8): δ 634.6 (br s)

Anal. Calc. for C35H18BF10NTe: C 53.83%, H 2.32%, N 1.79%. Found: C 53.52%, H 2.38%, N: 1.76 %.

116

Preparation and spectroscopic data of 3-31

68.1 mg of compound 3-28 (0.103 mmol) and 18.8 mg of 1,1-diphenylethylene (0.104 mmol) were dissolved in ca. 3 mL of toluene and heated to 80 oC for 24 hours. The solution turned from a yellow slurry into an almost clear solution. After all volatiles were removed in vacuo, the colourless residue was triturated with 2 mL of pentane to precipitate a white powder. This powder was dried under vacuum to give 26.1 mg of 3- 31 (0.0371 mmol, 36.1%) as a white powder. 1 H (500.0 MHz, CDCl3): δ 7.29-7.26 (m, 6H, m- + p-Ph), 7.19-7.17 (br m, 4H, o-Ph),

3 3 7.09-7.00 (m, 6H, m-P + p-Ph’), 6.91 (d, 4H, JH-H = 7.5 Hz, o-Ph’), 3.55 (t, 1H, JH-H = 6.3

2 3 1 Hz, H ), 1.94 (d, 2H, JH-H = 6.3 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 57.8 (br s, ν1/2 ≈ 1880 Hz) 13 1 C{ H} (125 MHz, CDCl3): δ 167.3 (s. TeC=). 147.1 (s, i-Ph’), 141.9 (s, i-Ph), 138.1 (br s, BC=), 129.0 (s, p-Ph), 128.5 (s, m-Ph), 128.2 (s, p-Ph’), 126.6 (s, o-Ph’), 126.4 (s, o- Ph), 126.0 (s, m-Ph’), 47.4 (s, C2), 32.3 (br s, C1) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -138.7 (m, 4F, o-C6F5), -156.8 (t, 2F, JF-F = 21.4 Hz, p-C6F5), -162.4 (m, 2F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 851.7 (s)

Preparation and spectroscopic data of 3-32

117

49.3 mg of compound 3-28 (0.0730 mmol) and 20.6 mg of ɑ-methylstyrene (0.174 mmol) were dissolved in ca. 5 mL of pentane stirred at room temperature for 24 hours. The solution turned from a yellow slurry into an almost clear solution. After all volatiles were removed in vacuo, the colourless residue was triturated with 2 mL to precipitate out a white powder. This powder was dried under vacuum to give 38.9 mg of 3-32 (0.0490 mmol, 67%) as a white powder. 1 H (500.0 MHz, CDCl3): δ 7.27-7.25 (br m, 6H, m- + p-Ph), 7.17-7.15 (br m, 4H, o-Ph),

6 7 3 5 3 7.11-7.04 (br m, 3H, H + H ), 6.69 (d, 2H, JH-H = 7.2 Hz, H ), 2.38 (app q, 1H, JH-H = 7.0

2 1 2 3 1’ 2 Hz, H ), 1.43 (dd, 1H, H , JH-H = 13.1 Hz, JH-H = 6.8 Hz), 1.28 (dd, 1H, H , JH-H = 13.1 Hz,

3 3 3 JH-H = 7.0 Hz), 0.83 (d, 3H, JH-H = 7.4 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 55.7 (br s, ν1/2 ≈ 1520 Hz) 13 1 4 C{ H} (125 MHz, CDCl3): δ 166.5 (s, TeC=), 149.0 (s, C ), 141.9 (s, i-Ph), 138.2 (br s, BC=), 129.0 (s, p-Ph), 128.5 (s, m-Ph), 128.3 (s, C6), 126.3 (s, o-Ph), 126.0 (s, C7), 125.7 (s, C5), 37.1 (s, C2), 35.5 (s, C1), 25.3 (s, C3) 19 1 F{ H} NMR (377 MHz, CDCl3): δ -138.0 (m, 2F, o-C6F5), -138.7 (m, 2F, o-C6F5), - 3 156.5 (t, 2F, JF-F = 19.6 Hz, p-C6F5), -162.2 (m, 2F, m-C6F5), -162.2 (m, 2F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 849.6 (s)

Spectroscopic data of 3-33

1 H (500.0 MHz, CDCl3): δ 7.30-7.25 (m, 6H, m- + p-Ph), 7.21-7.17 (m, 4H, o-Ph), 1.55- 2 4 3 3 3 1 1.50 (m, 3H, H + H ), 1.44 (d, 2H, JH-H = 12.4 Hz, H ), 1.14 (t, 1H, JH-H = 12.1 Hz, H ), 0.90-0.76 (m, 3H, H3 + H4), 0.61-0.54 (m, 2H, H2) 11 1 B{ H} (128 MHz, CDCl3): δ 54.2 (br s, ν1/2 ≈ 1210 Hz) 13 1 C{ H} (125 MHz, CDCl3): δ 165.4 (s, TeC=), 142.0 (s, i-Ph), 137.6 (br s, BC=), 128.8 (s, p-Ph), 128.4 (s, m-Ph), 126.0 (s, o-Ph)38.5 (s, C1), 28.5 (s, C3), 27.9 (s, C2), 27.0 (s, C4)

118

19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.0 (m, 4F, o-C6F5), -156.5 (t, 2F, JF-F = 22.1 Hz, p-C6F5), -163.0 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 842.1 (s)

Spectroscopic data of 3-35

1 H (500.0 MHz, CDCl3): δ 7.32-7.29 (m, 6H, m- p-Ph), 7.25-7.21 (m, 4H, o-Ph), 5.62 (d, 3 1 3 3 2 1H, JH-H = 18.4 Hz, H ), 5.49 (dt, 1H, JH-H = 18.4 Hz, JH-H = 6.3 Hz, H ), 1.78 (app q, 4 4 3 5 2H, H ), 1.09 (m, 2H, H ), 0.69 (t, 3H, JH-H = 6.9 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 49.6 (br s, ν1/2 ≈ 1170 Hz) 13 1 1 2 C{ H} (125 MHz, CDCl3): δ 165.9 (s, TeC=), n.o. (s, C ), 148.5 (s, C ), 142.1 (s, i-Ph), 137.4 (br s, BC=), 129.2 (s, p-Ph), 128.8 (s, m-Ph), 126.7 (s, o-Ph), 39.0 (s, C3), 22.1 (s, C4), 13.5 (s, C5) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.2 (m, 4F, o-C6F5), -157.2 (t, 2F, JF-F = 21.5 Hz, p-C6F5), -163.4 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 844.4 (s)

Spectroscopic data of 3-36

1 H (500.0 MHz, CDCl3): δ 7.28-7.25 (m, 6H, m- p-Ph), 7.21-7.19 (m, 4H, o-Ph), 5.39 3 1 2 4 (app q, 2H, JH-H = 18.6 Hz, H + H ), 0.62 (s, 9H, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 50.8 (br s, ν1/2 ≈ 892 Hz) 13 1 1 C{ H} (125 MHz, CDCl3): δ 165.9 (s, TeC=), 156.1 (s, C ), 141.6 (s, i-Ph), 137.1 (br s, BC=), 128.8 (s, p-Ph), 128.6 (s, C2), 128.4 (s, m-Ph), 126.3 (s, o-Ph), 34.4 (s, C3), 28.4 (s, C4)

119

19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.0 (m, 4F, o-C6F5), -157.2 (t, 2F, JF-F = 20.6 Hz, p-C6F5), -163.7 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 845.5 (s)

Spectroscopic data of 3-37

1 H (500.0 MHz, CDCl3): δ δ 7.01-6.99 (m, 6H, m- p-Ph), 6.93-6.91 (m, 4H, o-Ph), 5.70 3 1 3 3 2 (d, 1H, JH-H = 18.9 Hz, C ), 5.34 (dt, 1H, JH-H = 18.9 Hz, JH-H = 6.3 Hz, C ), 5.24 (br t, 3 4 5 8 7 8 1H, JH-H = 3.9 Hz, C ), 1.76 (br m, 2H, C ), 1.44 (br m, 2H, C ), 1.25 (m, 4H, C + C ) 11 1 B{ H} (128 MHz, CDCl3): δ 47.4 (br s, ν1/2 ≈ 959 Hz) 13 1 1 2 C{ H} (125 MHz, CDCl3): δ 164.3 (s, TeC=), n.o. (m, C ), 149.6 (s, C ), 141.9 (s, i-Ph), 137.5 (s, C3), 137.1 (br s, BC=), 135.2 (s, C4), 128.9 (s, p-Ph), 128.5 (s, m-Ph), 126.4 (s, o-Ph), 26.4 (s, C5), 23.6 (s, C8), 22.33 (s, C7 or C8), 22.26 (s, C8 or C8) 19 1 3 4 F{ H} NMR (377 MHz, CDCl3): δ -139.1 (dd, 4F, JF-F = 21.5 Hz, JF-F = 8.1 Hz o- 3 C6F5), -157.0 (t, 2F, JF-F = 20.8 Hz, p-C6F5), -163.2 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 839.8 (s)

Preparation and spectroscopic data of 3-38

52.9 mg of compound 3-28 (0.0800 mmol) and 14.1 mg of diphenylacetylene (0.0791 mmol) were dissolved in ca. 5 mL of pentane and stirred at room temperature for 24 hours. The solution turned from a yellow slurry into a white slurry. After the mother liquor was decanted, the colourless residue washed twice more with ca. 3 mL pentane to give 52.7 mg of 3-38 (0.0729 mmol, 92%) as a white powder.

120

1 A A H (500.0 MHz, CDCl3): δ 7.27-7.25 (m, 6H, m- + p-Ph ), 7.23-7.22 (m, 4H, o-Ph ), 7.07-7.06 (m, C 3 B B B 3H, p- m-Ph ), 6.97 (t, 1H, JH-H = 7.2 Hz, p-Ph ), 6.91 (app t, 2H, m-Ph ), 6.83 (m, 2H, o-Ph ), 3 C 2 6.54 (d, 2H, JH-H = 7.3 Hz, o-Ph ), 6.43 (s, 1H, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 52.6 (br s, ν1/2 ≈ 1630 Hz) 13 1 A B C{ H} (125 MHz, CDCl3): δ 169.2 (s, TeC=), 141.8 (s, i-Ph ), 139.9 (s, i-Ph ), 139.0 (br s, BC=), 137.7 (s, i-PhC), 134.7 (s, C2), 130.4 (s, Ar-C), 129.1 (s, Ar-C), 128.8 (s, p- PhA), 128.7 (s, Ar-C ), 128.5 (s, m-PhA), 128.4 (s, Ar-C ), 128.2 (s, Ar-C), 127.8 (s, Ar-C), 126.4 (s, o-PhA) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -138.5 (m, 4F, o-C6F5), -157.3 (t, 2F, JF-F = 22.1 Hz, p-C6F5), -163.9 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 862.4 (s)

Preparation and spectroscopic data of 3-39

75.2 mg of compound 3-27 (0.111 mmol) and 14.1 mg of 4-heptanone (0.123 mmol) were dissolved in 3 mL of toluene and heated to 110 °C for 16 hours in a sealed Schlenk tube with equipped with a Teflon tap. The bright yellow and clear solution was then cooled to room temperature and all volatiles were removed in vacuo, leaving a pale yellow solid. All of the solid was dissolved in 1 mL of pentane and 1 mL of cyclopentane and stored in a -35°C freezer for 18 hours to obtain 26.9 mg of compound 3-39 (0.0341 mmol, 30.7%) as pale yellow crystals. 1 H (500.0 MHz, CDCl3): δ 7.28-7.26 (br m, 6H, m- + p-Ph), 7.18-7.16 (br d, 4H, o-Ph),

3.40 (br m, 1H, BOCH), 1.17 (br m, 2H, OCHCH2), 1.02 (br m, 2H, OCHCH2), 0.95-0.94 3 (br m, 4H, OCHCH2CH2CH3), 0.75 (br t, 6H, JH-H = 6.9 Hz, OCHCH2CH2CH3) 11 1 B{ H} (128 MHz, CDCl3): δ 34.7 (br s, ν1/2 ≈ 1150 Hz)

121

13 1 C{ H} (125 MHz, CDCl3): δ (partial, C6F5 signals not listed) 157.6 (s, TeC=), 141.6 (s, i-Ph), 130.3 (br s, BC=), 128.6 (s, p-Ph), 128.5 (s, m-Ph), 126.3 (s, o-Ph), 75.9 (s,

OCHCH2), 38.6 (s, OCHCH2), 18.4 (s, OCHCH2CH2CH3), 14.0 (s, OCHCH2CH2CH3) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.1 (m, 4F, o-C6F5), -157.0 (t, 2F, JF-F = 19.2 Hz, p-C6F5), -163.5 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): δ 775.16 (s) + MS (DART+): calcd for C35H26BF10OTe [M+H ]: 793.09793 amu. Found: 793.09747 amu

Preparation and spectroscopic data of 3-40

406.2 mg of compound 3-28 (0.601 mmol) was taken up in ca. 10 mL of pentane, forming a yellow slurry. 2.0 mL of HCl in hexanes (4.0 M, excess) was added to the slurry at -35 oC. The slurry was allowed to stir at this temperature for 5 minutes, then at room temperature for 3 hours, during which time lots of grey precipitate separated out of solution. The precipitate was collected on a frit and washed pentane (3 mL × 2) to give 310.8 mg of compound 3-40 (0.438 mmol, 72.8% yield) as a white powder. 1 H (400.0 MHz, CDCl3): δ 7.32-7.31 (m, 6H, m- + p-Ph), 7.23-7.20 (m, 4H, o-Ph) 11 1 B{ H} (128 MHz, CDCl3): δ 48.2 (br s, ν1/2 ≈ 1170 Hz) 13 1 C{ H} (125 MHz, CDCl3): δ 169.8 (s, TeC=), 141.0 (s, i-Ph), n.o. (m, BC=), 129.6 (s, p- Ph), 128.8 (s, m-Ph), 126.4 (s, o-Ph) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -139.2 (m, 4F, o-C6F5), -155.8 (t, 2F, JF-F = 21.3 Hz, p-C6F5), -162.8 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): 868.2 (s)

Anal. Calc. for C28H10BClF10Te: C 47.35%, H 1.42%. Found: C 47.30%, H 1.42%.

Preparation and spectroscopic data of 3-41

122

199.7 mg of compound 3-39 (0.2812 mmol) was taken up in ca. 5 mL of DCM, forming a slightly yellow solution, 80.2 mg of AgOTf (0.312 mmol) in 2 mL DCM was added to the solution at room temperature. This mixture was stirred for 16 hours, forming a clear and bright yellow solution with lots of gray precipitate, which was filtered through a frit, leaving only the yellow solution. All volatiles were removed in vacuo, giving a solid, dim yellow residue, which was washed with pentane (3 mL × 3) to give 200.5 mg of compound 3-40 (0.2434 mmol, 86.5% yield) as a light yellow powder. 1 H (400.0 MHz, CDCl3): δ 7.36-7.32 (m, 6H, m-Ph + p-Ph), 7.18-7.17 (m, 4H, o-Ph) 11 1 B{ H} (128 MHz, CDCl3): δ 41.2 (br s, ν1/2 ≈ 1490 Hz) 13 1 1 C{ H} (125 MHz, CDCl3): δ 175.8 (s, TeC=), 143.7 (dm, JC-F ≈ 249 Hz, o-C6F5), 140.5 1 1 (s, i-Ph), 140.5 (dm, JC-F ≈ 250 Hz, p-C6F5), 137.6 (dm, JC-F ≈ 253 Hz, m-C6F5), 130.9 1 (br s, BC=), 130.0 (s, p-Ph), 128.9 (s, m-Ph), 126.2 (s, o-Ph), 118.0 (q, JC-F ≈ 249 Hz, 2 2 SO3CF3), 114.1 (tt, JC-F ≈ 20.2 Hz, JC-F ≈ 3.6 Hz, i-C6F5) 19 1 F{ H} NMR (377 MHz, CDCl3): δ -77.4 (s, 3F, SO3CF3), -138.5 (m, 4F, o-C6F5), -154.2 3 (t, 2F, JF-F = 21.3 Hz, p-C6F5), -162.3 (m, 4F, m-C6F5) 125 Te (158 MHz, CDCl3): 892.7 (s)

3.3.3 X-ray crystallography

Single crystals were coated in Paratone-N oil inside an O2-free, N2-filled glovebox, transferred out of the glovebox in a vial, quickly mounted on a MiTeGen Micromount and placed under an N2 stream for data collection on a Bruker Kappa Apex II diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073). All collections were conducted at 150 K using an Oxford Cryostream 700 series low- temperature system. Data collection strategies were optimized using the Bruker Apex 2 software package41 in order to provide 99.5-100% completion to a 2θ value of >55o. The data integration was performed using the Bruker SAINT software package, and the resulting raw data were scaled and absorption corrected using an empirical multi-scan method (SADABS).41

Structure solution and refinement were conducted using the SHELXTL-2016 program suite.42, 43 The heavy atom positions were determined using direct methods, while

123 lighter, non-hydrogen atoms were located by successive difference Fourier map calculations. In the final cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors except in the case of disorder. The H-atom contributions were calculated using the riding model. The locations and magnitudes of the largest peaks in the final difference Fourier maps indicate that, in each case, they were of no chemical significance.

Table 3-5. Selected crystallographic data of compounds 3-4, 3-6, and Py–O·3-4. 3-4 3-6 Py–O·3-4

Formula C22H15BBr2Te C37H38BTe C27.5H21BBr2ClNOTe Formula weight 577.57 621.08 715.13 Crystal system Monoclinic Triclinic Triclinic

Space group P21/n P1¯ P1¯ a (Å) 4.0401(6) 9.695(2) 8.0612(14) b (Å) 23.361(4) 10.677(2) 12.873(2) c (Å) 20.748(3) 16.034(3) 13.189(2) α (°) 90.00 107.424(10) 96.186(7) β (°) 95.071(8) 91.189(10) 95.999(7) γ (°) 90.00 96.810(10) 103.819(7) V (Å3) 1950.6(5) 1569.6(5) 1309.2(4) Z 4 2 2 Temp. (K) 150(2) 150(2) 149(2) d(calc) (g·cm-1) 1.967 1.314 1.814 Abs. coeff. μ (mm-1) 5.625 0.970 4.312 Reflections 17153 26525 26895 Collected 2 2 Data Fo >3σ(Fo ) 4448 7167 5991 Variables 235 352 325

R1 0.0514 0.0222 0.0460

wR2 0.0750 0.0500 0.1209 GOF 1.023 1.052 1.102

124

Table 3-6. Selected crystallographic data of compounds 3-16, 3-19, and 3-24. 3-16 3-19 3-24

Formula C35H17BF10OTe C41H21BF10OTe C36.67H21.33BF10N3OTe Formula weight 781.89 857.99 820.29 Crystal system Triclinic Triclinic Monoclinic

Space group P1¯ P1¯ P21/c a (Å) 9.0288(8) 11.0701(10) 31.652(5) b (Å) 11.8661(8) 13.0666(11) 13.842(3) c (Å) 15.9458(12) 13.7393(13) 23.093(4) α (°) 106.139(4) 111.314(4) 90 β (°) 98.474(4) 102.688(4) 108.953(6) γ (°) 108.700(3) 98.616(4) 90 V (Å3) 1501.0(2) 1747.6(3) 9569(3) Z 2 2 12 Temp. (K) 150(2) 150(2) 150(2) d(calc) (g·cm-1) 1.730 1.630 1.708 Abs. coeff. μ (mm-1) 1.083 0.938 1.024 Reflections 25422 29454 90094 Collected 2 2 Data Fo >3σ(Fo ) 6891 8063 22289 Variables 433 487 1369

R1 0.0376 0.0311 0.0491

wR2 0.0842 0.0691 0.1001 GOF 1.078 1.051 1.019

Table 3-7. Selected crystallographic data of compounds 3-26 and 3-27 3-26 3-27

Formula C29H17BBrF5OTe C27H16BF5OSTe Formula weight 694.74 621.87

125

Crystal system Triclinic Triclinic Space group P1¯ P1¯ a (Å) 6.606(2) 7.7832(12) b (Å) 11.962(4) 11.1020(18) c (Å) 16.824(5) 14.344(2) α (°) 88.855(14) 95.903(5) β (°) 84.261(14) 97.575(5) γ (°) 76.347(14) 92.147(6) V (Å3) 1285.5(7) 1220.5(3) Z 2 2 Temp. (K) 150(2) 150(2) d(calc) (g·cm-1) 1.795 1.692 Abs. coeff. μ (mm-1) 2.770 1.362 Reflections 21931 19754 Collected 2 2 Data Fo >3σ(Fo ) 5949 5615 Variables 343 325

R1 0.0254 0.0238

wR2 0.0631 0.0647 GOF 1.125 1.043

Table 3-8. Selected crystallographic data of compounds 3-39 and 3-41. 3-39 3-41

Formula C35H25BF10OTe C38H30BF13O4STe Formula weight 789.96 968.09 Crystal system Triclinic Triclinic Space group P1¯ P1¯ a (Å) 9.092(3) 12.1990(17) b (Å) 13.245(4) 13.1459(16)

126

c (Å) 14.772(4) 13.2577(18) α (°) 72.110(11) 70.801(6) β (°) 79.179(12) 79.133(6) γ (°) 87.274(12) 76.956(6) V (Å3) 1662.6(8) 1940.9(4) Z 2 2 Temp. (K) 150(2) 150(2) d(calc) (g·cm-1) 1.578 1.657 Abs. coeff. μ (mm-1) 0.978 0.923 Reflections 27130 32354 Collected 2 2 Data Fo >3σ(Fo ) 7694 8774 Variables 462 523

R1 0.0456 0.0291

wR2 0.1074 0.0761 GOF 1.047 1.119

3.3.4 Computational details

All computed structures used for NICS calculations were minimized using the Gaussian 09 program at the M062X/SDD level of theory.44 All minimized structures were found to contain no imaginary frequencies. NICS values were obtained from adding ghost atoms (Bq) in the center of the compounds listed in Table 3-4, and GIAO method was chosen to perform the NMR calculation

All structures used for the reaction profile discussed in section 3.2.2 were carried out with the TURBOMOLE suite of programs.21 All structures were fully optimized at the dispersion-corrected DFT level using the PBEh-3c density functional.45. Single-point energies were computed with the larger polarized triple-zeta (def2-TZVP) sets46 in combination with the PW6B95 hybrid functional.47 The atom pairwise D3 correction with BJ-damping is included to account for intra- and intermolecular London dispersion

127 interactions.48, 49 In order to speed up the computations, the resolution-of-the-identity approximation has been used50 for the Coulomb integrals. The numerical quadrature grid m4 has been employed for the integration of the exchange-correlation contribution. In the main text, we report pure electronic gas phase energies (termed ΔE) as well as Gibbs free energies at 298.15 K in toluene as a solvent (termed ΔG). The ro-vibrational corrections to the free energy are obtained from a modified rigid rotor, harmonic oscillator statistical treatment51 based on the harmonic frequencies obtained at the PBEh-3c level. For the entropy, all frequencies with wavenumbers below 100 cm-1 were treated as mixed rigid rotors and harmonic oscillators. In two cases very small imaginary modes of about -7 cm-1 were obtained, which were treated as numerical artifacts and used with their absolute value. The harmonic frequencies in PBEh-3c were scaled by a factor of 0.95.

Solvent effects on the thermochemical properties have been obtained by the COSMO- RS method13 (COSMOtherm software package14) based on BP86/TZVP15 single- point calculations (parametrization from 2014). Solvation contributions to free energies at 298.15 K in toluene solution are computed from the gas phase structures obtained at the above-mentioned levels of theory. The computed free energies are then obtained by ΔG = ΔE + ΔGRRHO + ΔδGCOSMO-RS, where the last two terms refer to the above mentioned ro-vibrational and solvation contributions, respectively, to the free energy.

Wiberg bond orders are reported to analyse the bonding situation in the transition states and these are based on the PW6B95-D3/def2-TZVP orbitals. The results are the following:

TS1 (C37 and C38 are alkyne carbon atoms)

Te C 5 1.102 C 2 1.081 C38 0.156 C36 0.122 C65 0.117

B C 3 1.020 C 4 1.012 C6 0.960 C37 0.422 Te1 0.122 C38 0.112

TS2 (C32 and C33 are the alkyne carbon atoms)

Te C5 1.112 C2 1.069 C33 0.290 C32 0.137 B31 0.085

128

B C3 1.076 C4 1.025 C6 0.986 C32 0.339 C33 0.098 Te 1 0.085

Intermediate (C37 and C38 are alkyne carbon atoms)

Te C2 1.006 C5 0.996 C38 0.958

B C6 0.894 C4 0.873 C37 0.864 C3 0.852

3.4 References

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44. M. J. T. Frisch, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J, 2010, p. Gaussian Inc. Wallingford CT.

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Chapter 4. Exploring Chalcogen-Based Lewis Acids 4.1 Introduction

4.1.1 Lewis acidity in sulfur compounds

Organosulfur(II) compounds are usually classified as Lewis bases due to their two lone pairs. Hypervalent sulfur compounds, however, have a long-standing history of acting as Lewis acids. Between 2009 and 2010, the Ragogna group reported a series of unusually stable S(II) cations supported by mono-, bi-, or tridentate nitrogen-based ligands.1-3 The sulfur centres in these compounds often show coordination with the triflate counter anions, thus alluding to the Lewis acidity of the formally S(II) species.

Organosulfur(IV) compounds have also displayed their Lewis acidity in a number of reports (Scheme 4-1). For example, transannular interactions between nitrogen and sulfur have been noted in many heterocyclic compounds,4-8 while the intramolecular coordination of sulfide (RS-) or amine to S(IV) centres have also been investigated.9 In the field of inorganic sulfur compounds, Gerken et al. have reported the low- 10-14 temperature coordination of SF4 to a wide variety of organic bases. Additionally, 9 SO2 has been shown to act as a Z-type ligand upon reaction with electron-rich metals.

Scheme 4-1. Selected reports of S(II) or S(IV) compounds demonstrating Lewis acidic behaviour.

More recently, Gabbaï developed a S(IV) cation for fluoride ion sensor applications.15-17 In principle, S(VI) persulfurane cations18-22 are expected to exhibit even higher Lewis acidity, we thus attempted to prepare sulfur-based Lewis acids featuring S(VI) centres.

133

4.1.2 Lewis acidity in tellurium compounds

In addition to developing S(VI) Lewis acids, we were also interested in exploring the Lewis acidity of other chalcogen-based systems. Being the heaviest, non-radioactive element in group 16, tellurium has many properties different from those of its lighter 23 congeners. One notable difference is the increasing tendency for EX2 (E = S, Se, Te;

X = Cl, Br, I) species to disproportionate into E2X2 and EX4 as the atomic weight of E increases. As such, the stabilization of Te2+ and RTe+ cations has been the subject of recent research interest. Many complexes of Te2+ cations have been successfully isolated by the coordination of strong σ-donor ligands such as N-heterocyclic carbenes 24-32 (NHCs) and 2,2′-bipyridyl (bipy). Indeed, bipy·TeCl2 is a useful reagent in the preparation of simple tellurides30 and tellurophenes.33 Aryl- and parent tellurenyl (HTe+) cations have also been successfully isolated using either highly bulky groups or strong electron donors.34-38 In the cases for which crystallographic data were obtained, close contacts between the counter anions and the cationic tellurium centre were commonly observed. It is also sometimes possible to form stable and classical Lewis acid-base adducts between tellurium centres and phosphines or amines.

Due to its high polarizability and tendency to form three-centre-four-electron bonding bonds, the coordination chemistry of neutral tellurium species has been well documented (Scheme 4-2). McWhinnie et al.39-41 reported the first crystallographically characterized examples of organotellurium (II) and (IV) species that feature intramolecular coordination of nitrogen to tellurium. Since then, many tellurium complexes with intra-42-45 and inter-molecular46-51 Te–L (L = N, O, P, S, Se) interactions have been reported, most of which contain electron-withdrawing halide substituents on the tellurium atom to facilitate the formation of three-centre-four-electron bonding bonds.

134

Scheme 4-2. Selected examples of Te(II) or Te(IV) species displaying Lewis acidic behaviour.

Despite the extensive studies on the coordination chemistry of tellurium compounds, there has been little research on the utility of tellurium’s Lewis acidity. Two notable deviations from this trend were reported by Gabbaï and co-workers. They were able to show that telluronium centres could act as σ-acceptor ligands52 or as part of a bidentate Lewis acidic complex for fluoride binding.53 These studies serve as inspiration for our present investigation into chalcogen-based Lewis acids and the use of telluronium cations in FLP-type chemistry and in Lewis acid catalysis.

4.2 Results and Discussion

4.2.1 Reaction of PhSF5 with a fluoride-abstracting agent

In the beginning of our investigation, we sought a facile route to S(VI) cations using commercially available reagents. To this end, we treated PhSF5 with the strong fluoride o abstracting agent, [SiEt3][B(C6F5)4] at -35 C (Scheme 4-3), which led to the immediate colour change from colourless to light yellow. Removal of all volatiles in vacuo gave a viscous and brownish yellow oil. Re-dissolving this oil in DCM and layering it with cyclohexane gave large, colourless crystals that were suitable for single crystal X-ray diffraction analysis. Much to our surprise, the isolated compound was shown to be 4-1 – a fluorosulfoxonium cation (Figure 4-1). The geometry of the sulfur atom in the cation is pseudo-tetrahedral with S–C bond distances of 1.725(4) Å and 1.733(4) Å, while the S– F and S–O distances are 1.533(3) Å and 1.404(3) Å, respectively. The reaction leading + to 4-1 is thought to begin with the generation of the cation [PhSF4] , a powerful Lewis

135 acid that can undergo Friedel-Crafts type arylation of the solvent toluene, giving the + intermediate [(p-tol)PhSF3] with the elimination of HF. The reaction of this species with residual moisture or glass could then lead to the formation of 4-1.

Scheme 4-3. Synthesis of fluorosulfoxonium cation from PhSF5.

Figure 4-1. Solid-state structure of 4-1 (50% thermal ellipsoids). C: black, O: red, F: pink, S: yellow. Hydrogen atoms and counter anion are omitted for clarity.

Due to the complex mixture of products obtained from the reaction between PhSF5 and

[SiEt3][B(C6F5)4], detailed mechanistic studies on the formation of 4-1 could not be carried out. This surprising discovery of 4-1 inspired us to pursue a more rational route to fluorosulfoxonium cations as they appeared to be stable and isolable examples of sulfur-based Lewis acids.

136

4.2.2 Synthesis of fluorosulfoxonium cation

Seeking a more rational synthetic strategy to similar cations, we noted the previous work of Ruppert54, 55 and Janzen.56, 57 Following a similar protocol, diphenyl sulfoxide

(Ph2SO) was oxidized with one equivalent of XeF2 at room temperature, cleanly 19 1 affording the difluorosulfur(VI) species Ph2SOF2, 4-2 (Scheme 4-4). The F{ H} NMR spectrum of 4-2 shows a singlet resonances at 103.2 ppm, corresponding to the axial fluoride substituents on the sulfur centre. Attempts to isolate 4-2 proved problematic as it is unstable on prolonged contact with glass. As a result, subsequent reactions were always carried out using freshly prepared 4-2 in Teflon vials.

Scheme 4-4. Synthesis of compound 4-3 from diphenyl sulfoxide.

Addition of one equivalent of [SiEt3][B(C6F5)4] to a toluene solution of 4-2 cooled to - 35 ºC led to an immediate colour change from colourless to dark red. After the solution was warmed to room temperature over 4 hours, all volatiles were removed and trituration of the remaining brown solid residue with pentane afforded the fluorosulfoxonium cation 4-3 in excellent yields (>80%) (Scheme 4-4). 1H and 13C{1H} NMR data of 4-3 show the expected aromatic resonances, while the 11B{1H} signal at -16.7 ppm and the 19F{1H} NMR resonances at -133.1, -163.6 and -167.5 ppm are - 19 1 consistent with the presence of the borate anion, [B(C6F5)4] . In addition, a F{ H} NMR resonance at 32.2 ppm confirms the presence of an S–F moiety. Single crystals of 4-3 were grown from a saturated DCM solution layered with cyclohexane. An X-ray diffraction study of these crystals confirmed the structure of 4-3 (Figure 4-2). The sulfur centre adopts a pseudo-tetrahedral geometry. The S–F bond distance is 1.468(2) Å, while the S–O distance is 1.454(2) Å. Interestingly, 4-3 exhibits high stability in the solid state towards glass once isolated from the reaction mixture. In particular, compound 4-3

137 can be isolated at >450 mg scale and kept under an inert atmosphere at room temperature for over 6 months.

Figure 4-2. Solid-state structure of 4-3 (50% thermal ellipsoids). C: black, O: red, F: pink, S: yellow. Hydrogen atoms and counter anion are omitted for clarity.

The Lewis acidity of 4-3 was probed experimentally by the Gutmann–Beckett method.58 31 1 Upon mixing 4-3 with one equivalent of triethylphosphine oxide (O=PEt3), the P{ H}

NMR resonance of the Et3PO shifted downfield to 84.5 ppm. This shift (Δ = 33.8) demonstrates the ability of 4-3 to function as an electron-pair acceptor. The Δ value of

4-3 is significantly higher than that reported for B(C6F5)3 (Δ = 26.6), suggesting 4-3 to be a stronger Lewis acid.

In order to further evaluate the Lewis acidity of 4-3, density functional theory (DFT) calculations were performed in collaboration with the research group of Prof. Grimme (for details see section 4.3). The LUMO of 4-3 contains a major component localized on the sulfur atom with significant S–F σ* character. In addition, the fluoride ion affinity (FIA) was computed for 4-3 in the gas phase (-132.5 kcal·mol-1) and in toluene (-140.0 kcal·mol-1). These numbers suggest that the Lewis acidity of 4-3 is comparable to -1 common Lewis acids such as BF3 (FIA = -82.7 kcal·mol ) and AlCl3 (FIA = -120.0 kcal·mol-1).59

138

The successful synthesis and demonstration of Lewis acidity of 4-3 prompted us to investigate the possibility of preparing analogous fluorosulfoxonium cations with different counter-ions. The stoichiometric reaction of 4-2 and TMSOTf at -35 oC, however, unexpectedly led to the formation of 4-4 as the only isolable major product (Scheme 4-5). While the exact pathway of this reaction is unclear, this result led us to investigate the direct synthesis of S(IV) cations directly from sulfur oxides. Treating diphenyl sulfoxide with one equivalent of TMSOTf at -35 oC led to no observable colour change. Removal of all volatiles in vacuo afforded a colourless, viscous oil that could be triturated with pentane to give 4-4 as a white powder. Single crystals of 4-4 from this reaction could also be grown and analyzed by X-ray crystallography (Figure 4-3). The triflate anion is not bound in the solid state and the sulfur centre adopts the trigonal pyramidal geometry, as would be expected of a S(IV) cation. The S–O distance is 1.581(2), corresponding to a S–O single bond.

Scheme 4-5. Synthesis of compound 4-4 from 4-2 or diphenyl sulfoxide.

139

Figure 4-3. Solid-state structure of 4-4 (50% thermal ellipsoids). C: black, O: red, F: pink, S: yellow, Si: cyan. Hydrogen atoms and counter anion are omitted for clarity.

In spite of the promising results from the solid-state structure of 4-4, the NMR spectra of 4-4 revealed that compound 4-4 may not be a suitable compound for further studies in Lewis acid catalysis. For example, the integration of the TMS resonance in the 1H NMR never matched what would be expected of compound 4-4. Indeed, the reaction between TMSOTf and diphenyl sulfoxide is reversible at room temperature. Leaving isolated 4-4 under dynamic vacuum for 16 hours at room temperature led to the completion conversion of 4-4 back to diphenyl sulfoxide. Moreover, mixing a solution of isolated 4-4 31 1 with O=PEt3 gave one resonance in P{ H} NMR spectrum at 92.2 ppm, which corresponds to the direct coordination of O=PEt3 to TMSOTf. The reversibility of this reaction is hypothesized to be a result of the similar nucleophilicity of the oxygen atoms on the triflate anion and on diphenyl sulfoxide.

In order to improve the stability of S(IV) cations analogous to 4-4, two approaches were attempted. Firstly, 4-4 was allowed to react with one equivalent of K[B(C6F5)4] in order - to exchange the triflate anion with a more weakly coordinating [B(C6F5)4] anion. This approach was, however, unsuccessful as only 4-4 could be isolated from the reaction mixture even after days of reaction. Secondly, diphenyl sulfoxide was treated with methyl triflate (MeOTf), which also proved to be unsuccessful. Even heating a solution

140 of MeOTf and diphenyl sulfoxide to 130 oC for 48 hours produced no NMR spectroscopic changes.

4.2.3 Reactivities of fluorosulfoxonium cations

Given the more promising preliminary reactivity profile of S(VI) fluorosulfoxonium species as compared to S(IV) cations, we decided to investigate the reactivity of the former in further detail. We began by combining 4-3 with a variety of Lewis bases, including tetramethylpiperidine (TMP), pyridine N-oxide (Py-O), tri-tert-butylphosphine t (P Bu3), tri-ortho-tolylphosphine (P(o-tol)3), triphenylphosphine (PPh3) and phenol (PhOH). In all cases, 4-3 slowly decomposed to diphenyl sulfoxide over 16 hours at room temperature and the in situ 19F{1H} NMR spectra of the product mixtures all showed the disappearance of the S–F signal. All together, these data suggest that the coordination of a Lewis base to the sulfur centre induces reductive elimination of the substituents and reversion of 4-3 to diphenyl sulfoxide.

As the rate of decomposition of 4-3 significantly decreases with increasing steric bulk of t the Lewis base, we selected P Bu3 for all subsequent FLP reactions. The addition of one t equivalent of diphenyldisulfide (PhSSPh) to a stoichiometric mixture of 4-3 and P Bu3 led to an immediate colour change from light orange to colourless. The in situ 31P{1H} NMR showed only one singlet resonance at 85.7 ppm, corresponding to the t + 60 19 1 phosphonium cation [ Bu3P(SPh)] . The in situ F{ H} NMR spectrum, however, - 1 showed only resonances corresponding to the [B(C6F5)4] anion and the H NMR spectrum contained only resonances corresponding to diphenyl sulfoxide. All these data t suggest that, even though 4-3 and P Bu3 can act as an FLP to activate S–S bonds, the resulting hypervalent compound 4-5 is too unstable to isolate (Scheme 4-6). All efforts to isolate and characterize 4-5 proved unsuccessful even with the use of low temperature and Teflon vials and pipettes. Similar observations were made when the t combination of 4-3 and P Bu3 were reacted with classical FLP substrates such as alcohols, thiols and alkynes.

141

Scheme 4-6. Reactivity of fluorosulfoxonium cation 4-3.

Although 4-3 cannot be used in FLP due to product instability, the Lewis acidity of compound 4-3 could be confirmed by its ability to effect the polymerization of THF even in dilute solution (Scheme 4-6). In addition, using 5 mol% of compound 4-3 as a Lewis acid catalyst, the hydroarylation of diphenylamine and pyrrole with 1,1-diphenylethylene were shown to proceed smoothly at room temperature over the course of 24 hours. In addition, the hydrothiolation of 1,1-diphenylethylene with thiophenol proceeded in a similar fashion. In each case, the identities of the products were confirmed unambiguously by 1H NMR spectroscopy (Scheme 4-6).

4.2.4 Synthesis of perfluorinated Te(II) and Te(IV) compounds

Encouraged by the results of fluorosulfoxonium cations, we sought to expand the concept of group 16 cationic Lewis acids to tellurium. We drew inspiration from both the well-known B(C6F5)3 and our group’s recent discovery of highly Lewis acidic 61 fluorophosphonium cation, [FP(C6F5)3][B(C6F5)4]. We thus began our investigation with

142 the synthesis and reactivity studies of the structurally similar Te(C6F5)4 (4-6), which is a known compound62, 63 but has never been tested for its Lewis acidic behaviour.

The synthesis of 4-6 proved to be problematic and attempts to reproduce literature procedures involving the reaction of TeCl4 and C6F5MgBr gave little-to-no product

(Scheme 4-7). The main problem is that the direct reaction between TeCl4 and

C6F5MgBr appeared to lead to a significant production of Te(C6F5)2 (4-7), which could be separated from 4-6 by repeated washing with pentane, but significantly decreased the yield of the desired product. In contrast, we found that the reaction between TeCl4 and

Zn(C6F5)2 led to the formation of 4-6 as the major product and allowed for its isolation at a moderate yield (58%). The telluronium cation 4-8 could then be synthesized in quantitative yields from 4-6 by treating it with excess TMSOTf.62 Unfortunately, 4-8 appears to undergo reductive elimination to give 4-7 and C6F5–C6F5 when combined t with common Lewis bases such as P Bu3 and TMP. We thus concluded that this cation would not be suitable for FLP-type reactions.

Scheme 4-7. Synthetic strategy to access compounds 4-6 and 4-8.

The ready disproportionation of 4-8 in the presence of Lewis bases is thought to be a result from the weak Te–C bonds. We hypothesized that stability could be improved by using halide substituents in place of C6F5 groups. The synthesis of 4-7 was cleanly 64, 65 achieved by the reaction between Na2Te and C6F5Br. 4-7 can be further oxidized to the corresponding difluoro- (4-9) and dichloro-species (4-10) in excellent yields (>90%) 64-66 by XeF2 and SO2Cl2, respectively (Scheme 4-8).

143

Scheme 4-8. Synthesis of compounds 4-9 and 4-10 from 4-7.

We next investigated the possibility of abstracting one of the halide substituents on 4-9 or 4-10 to prepare Lewis acidic telluronium centres. Treating 4-9 with one equivalent of

BF3·Et2O led to the isolation of a white powder 4-11 (Scheme 4-9). In solution, 4-11 11 1 shows one B{ H} NMR resonance at -1.8 ppm, which suggests that BF3 forms an adduct with the fluoride substituents on 4-9 instead of abstracting a fluoride anion from 4-9. Indeed, the 19F{1H} NMR spectrum of 4-11 at room temperature shows only three resonances corresponding to the C6F5 groups on tellurium, but no resonance corresponding to BF3 or Te–F fragment (Figure 4-4). Gratifyingly, these signals became evident in the 19F{1H} NMR spectrum upon cooling a solution of 4-11 in toluene to -80 o C. There are two resonances attributable to o-C6F5 and m-C6F5 groups, which suggests hindered rotation around these groups. A broad resonance at -94.7 ppm is attributed to the Te–F group. There are two resonances at -151.6 and -151.7 ppm with a - 1:3 ratio of integration corresponding to the [BF4] counter anion, which suggests that - one of the fluorine atoms in [BF4] is bound to the tellurium centre. These data demonstrate the high fluxional behaviour of compound 4-11 and the saturation of the coordination sites around tellurium.

144

Scheme 4-9. Reactions of 4-9 with halide-abstracting agents to give compounds 4-11 to 4-13.

19 1 Figure 4-4. F{ H} (376.4 MHz, tol-d8) NMR spectrum of 4-11 at (top) 298 K and (bottom) 218 K.

The analogous treatment of 4-9 with one equivalent of TMSOTf gave 4-12 as a white solid (Scheme 4-9), which seems a more promising candidate for Lewis acid catalysis for a number of reasons. The 19F{1H} NMR spectrum of 4-12 shows one set of resonances that correspond to the C6F5 substituents (-126.4, -140.2 and -156.3 ppm), one singlet at -120.2 ppm that corresponds to the Te–F substituents, and one singlet at -77.8 ppm that is attributed to the triflate counter anion. Single crystals of 4-12 could be grown by slow evaporation of a saturated toluene solution of the compound. The solid- state structure shows a dimeric structure of 4-12, with two distinct telluronium centres

145 connected by a bridging fluoride (Figure 4-5). Both tellurium centres adopt a see-saw geometry, with one of the triflate anions coordinated to the dimer. This is somewhat surprising as the room temperature NMR spectra of 4-12 do not suggest any inequivalency of the C6F5 resonances. While the poor data quality precludes any bond distance analysis, this solid-state structure compelled us to investigate the solution-state o behaviour of 4-12 in further detail. Indeed, cooling a CD2Cl2 solution of 4-12 to -80 C led to the splitting of all resonances in the 19F{1H} NMR spectrum, though no exact assignments could be made due to the large linewidths of all resonances (Figure 4-6).

Figure 4-5. Solid-state structure of 4-12. (50% thermal ellipsoids). C: black, O: red, Te: green, F: pink, S: yellow. Aryl fluoride atoms and one triflate counter anion are omitted for clarity. (Note: due to the poor quality of the crystals, F1 could not be refined anisotropically)

146

19 1 Figure 4-6. F{ H} (377 MHz, CD2Cl2) NMR spectrum of 4-12 at (top) 298 K and (bottom) 218 K.

In addition to BF3·Et2O and TMSOTf, 4-9 was also treated with a number of other fluoride-abstracting agents such as [SiEt3][B(C6F5)4] and B(C6F5)3. In the former case, the reaction turned from colourless to green immediately. This solution is stable up to 0 oC, upon which point the solution becomes dark brown and no discernable product could be isolated from the mixture. The reaction between 4-9 and B(C6F5)3 likewise gave rise to a number of products (Scheme 4-9), including 4-7, 4-9 and 4-13 (Figure 4-7). Single crystals of 4-13 readily formed from the reaction mixture and its solid-state structure suggests that both fluoride and C6F5 substituents are scrambled between the Lewis acidic centres of boron and tellurium. Scheme 4-10 depicts one possible route of the formation of 4-13, though detailed mechanistic studies should be performed in the future to further confirm this hypothesis.

147

Figure 4-7. Solid-state structure of 4-13 (50% thermal ellipsoids). C: black, O: red, B: yellow green Te: green, F: pink. Aryl fluoride atoms are omitted for clarity.

Scheme 4-10. Proposed reaction pathway leading to the formation of compound 4-13

from the reaction between 4-9 and B(C6F5)3.

In the pursuit of the synthesis of chlorotelluronium cations, we began by treating 4-10 with one equivalent of AgOTf in DCM, which led to the immediate precipitation of AgCl and 4-14 could be isolated in excellent yield (88%) upon workup. The 19F{1H} spectrum of 4-14 shows 3 resonances corresponding to the C6F5 (-124.6, -139.2 and -155.6 ppm)

148 substituents and one resonance that is attributable to the triflate counter anion (-77.8 ppm). The resonance corresponding to the o-C6F5 (-124.6 ppm) is broad at room temperature, which suggests dynamic solution-state behaviour as in the case of 4-12.

Cooling a solution of 4-14 in CD2Cl2 indeed led to the splitting of the o-C6F5 resonance in the 19F{1H} NMR spectrum, though the signal remained broad even at -80 oC.

Scheme 4-11. Reactions of 4-10 with halide-abstracting agents to give compounds 4-14 and 4-15.

19 1 Figure 4-8. F{ H} (377 MHz, CD2Cl2, 298K) NMR spectrum of 4-14 at (top) 298 K and (bottom) 218 K.

In a similar fashion, compound 4-10 was treated with one equivalent of GaCl3. Upon workup, compound 4-15 could be isolated as a light yellow powder in a moderate yield

149

(68%). The 19F{1H} NMR spectrum of 4-15 shows only three resonances that are attributed to the C6F5 substituents (-120.9, -136.5 and -154.1 ppm). The solid-state structure of 4-15, however, shows the unambiguous coordination of the GaCl4 counterion to the tellurium centre. The bond distance between Te1 and Cl2 is 2.8377(5) Å, which falls well within their sum of van der Waals radii of 3.83 Å.67

Figure 4-9. Solid-state structure of 4-15 (50% thermal ellipsoids). C: black, Cl: yellow, Te: green, Ga: green-blue. Aryl fluoride atoms are omitted for clarity.

Due to the coordination of the anion to tellurium centres in 4-11 and 4-15, the only telluronium cations suitable for further reactivities studies are 4-12 and 4-14. Treating these compounds with typical Lewis bases such as amines and phosphines, however, led to the rapid decomposition to a number of unidentified compounds. It is conceivable that these species may simply be too reactive to be suitable Lewis acids for catalysis and small molecule activation.

4.2.5 Conclusion

Compounds featuring group 16 elements are typically used as Lewis bases, but herein we report our studies on hypervalent sulfur- and tellurium-based Lewis acids. Fluorosulfoxonium cations were successfully synthesized from commercially available diaryl sulfoxides. Their Lewis acidity was demonstrated by both computational and experimental methods. Indeed, fluorosulfoxonium cations were shown catalyze the

150 hydroarylation and hydrothiolation of olefins. On the other hand, the synthesis of tellurium-based Lewis acids featuring C6F5 substituents proved to be problematic. Even though fluoro- and chlorotelluronium cations could be accessed, their low stability and fluxional solution behaviour led us to conclude that they would not be suitable Lewis acids for further applications.

4.3 Experimental Section

4.3.1 General considerations

(CaH2 for CD2Cl2 and CDCl3, Na/benzophenone for C6D6 and tol-d8) and distilled under reduced pressure prior to use. All solvents were degassed by repeated freeze-pump- thaw cycles prior to use.

All chemicals were used as received unless otherwise noted. Diphenyl disulfide, triphenylphosphine, tri(tert-butyl)phosphine, pyridine N-oxide, tri(o-tolyl)phosphine, pentafluorosulfanylbenzene, cyclohexane, pyrrole, sodium, and sulfuryl chloride solution (1.0 M in DCM) were purchased from Sigma-Aldrich. Boron trifluoride/diethyl ether complex (BF3·OEt2), trimethylsilyl trifluoromethanesulfonate (TMSOTf), diphenyl sulfoxide, 1,1-diphenylethylene, diphenylamine, and phenol were purchased from TCI

America Research Chemicals. B(C6F5)3 was purchased from Boulder Scientific.

Elemental tellurium, thiophenol, trimethylphosphine oxide and gallium trichloride (GaCl3) were purchased from Alfa Aesar. GaCl3 was sublimed under reduced pressured at 50 oC before use. Pentafluorobromobenzene and xenon difluoride were purchased from

Apollo Chemicals. Silver triflate (AgOTf) and tellurium tetrachloride (TeCl4) were

151 purchased from Strem Chemicals. All liquid reagents were degassed by repeated 68 freeze-pump-thaw cycles and dried over 4 Å sieves prior to use. [SiEt3][B(C6F5)4], 4-9, and 4-1065 were prepared according to standard literature procedures, and the preparation of Zn(C6F5)2 is detailed in Section 2.3 of this dissertation.

NMR spectroscopy was performed on a Bruker Advance III 400 MHz, an Agilent DD2 500 MHz, or an Agilent DD2 600 MHz NMR spectrometer. Unless otherwise stated, all spectra were obtained at room temperature. All NMR spectra were referenced to 1 13 residual protio solvent peaks of CD2Cl2 ( H = 5.32 ppm; C = 53.84 ppm) or an external 19 11 125 41 standard ( F: CFCl3 (δ 0.00), B: (Et2O)·BF3 (δ 0.00), Te: Ph2Te2 (δ 420.8 ) ). Combustion elemental analyses were performed on a on a PerkinElmer CHN Analyzer.

4.3.2 Synthetic procedures and spectroscopic characterization

Preparation and spectroscopic data of 4-1

143.0 mg of pentafluorosulfanylbenzene (0.7004 mmol) in ca. 10 mL toluene was o cooled to -35 C, then 619.3 mg of [SiEt3·tol][B(C6F5)4] (0.6986 mmol) in ca. 10 mL toluene was added to the former solution dropwise. The mixture turned red-orange immediately, and was warmed to room temperature over 3 hours. All volatiles were then removed in vacuo, leaving a brown residue, which was first washed with pentane (5 mL × 2) then re-dissolved in DCM and layered with cyclohexane. Letting the solution stand at room temperature for over 24 hours yielded a small crop of colorless crystals, which was confirmed to be 4-1 by both NMR spectroscopy and X-ray diffraction analysis. 1 3 3 H (400 MHz, C6D6): δ 8.24 (tm, 2H, JH-H = 7 Hz, p-Ph), 8.16 (d, 4H, JH-H = 8 Hz, o-Ph), 3 7.98 (app t, 4H, JH-H = 8 Hz, m-Ph). 19 1 3 F{ H} (376 MHz, C6D6): δ 33.4 (s, 1F, S–F), -132.0 (m, 8F, o-C6F5), -162.5 (t, 4F, JF-F

= 26.0 Hz, p-C6F5), -166.5 (m, 8F, m-C6F5).

152

Preparation and spectroscopic data of 4-3

At room temperature, 168.3 mg of XeF2 (0.9942 mmol) in ca. 5 mL DCM was added dropwise to a solution of 196.1 mg of diphenyl sulfoxide (0.9695 mmol) in ca. 5 mL DCM in a Teflon vial charged with a stir bar. The solution remained clear and colourless and was allowed to stir for 24 hours at room temperature. All volatiles were then removed in vacuo, leaving behind a crystalline, colourless solid. The solid was then re- dissolved in ca. 5 mL toluene (5 mL) and cooled to -35 °C. 840.0 mg of freshly prepared

[SiEt3·tol][B(C6F5)4] (0.9476 mmol) in ca. 5 mL toluene was added to the above solution dropwise, leading to an immediate change of color of the solution to dark red/brown. The solution was allowed to warm to room temperature over 4 hours and then transferred to a glass scintillation vial. When the stirring was stopped, the solution separated into two layers (colourless top layer and red bottom layer), the top layer was decanted using a pipette and the bottom layer was triturated 3 times using pentane to give 684.4 mg of 4-3 (0.7602 mmol, 80.2% yield) as a white powder. Single crystals suitable for X-ray diffraction analysis were grown from a solution of 4-3 in dichloromethane layered with cyclohexane. 1 3 H NMR (600 MHz, CD2Cl2, 298 K): δ 8.24 (app t, 2H, JH-H = 7 Hz, p-Ph), 8.16 (d, 4H, 3 3 JH-H = 8 Hz, o-Ph), 7.98 (app t, 4H, JH-H = 8 Hz, m-Ph). 13 1 C{ H} NMR (125 MHz, CD2Cl2, 298 K): δ (B(C6F5)4 peaks not included) 142.4 (s, p- 3 Ph), 132.8 (s, m-Ph), 129.9 (s, o-Ph), 126.1 (d, JC-F = 11 Hz, i-Ph). 19 1 F{ H} NMR (376 MHz, CD2Cl2, 298 K): δ 32.2 (s, 1F, S–F), -133.1 (m, 8F, o-C6F5), - 3 163.6 (t, 4F, JF-F = 19.8 Hz, p-C6F5), -167.5 (m, 8F, m-C6F5). 11 1 B{ H} NMR (128 MHz, CD2Cl2, 298 K): δ -16.7 (s).

Anal. Calc. for C36H10BF21OS: C 48.03%, H 1.12%. Found: C 48.72%, H 1.47%.

153

Lewis acid catalysis using 4-3 – General procedure (example: hydroarylation of 1,1-diphenylethylene)

To a solution of 4.4 mg of catalyst 4-3 (0.0049 mmol, 5 mol%) in CDCl3 (1 mL), 19.0 mg of diphenylamine (0.112 mmol) and 20.4 mg of 1,1-diphenylethylene (0.113 mmol) were added. The solution was transferred to an NMR tube and the reaction was monitored by 1H NMR. Complete conversion to the desired product was observed after 24 hours. The 1H NMR resonances match those previously reported.69

Preparation of Na2Te

Sodium (0.4962 g, 21.6 mmol) was cut into small chunks in the glovebox, then transferred into a slurry of tellurium powder (1.3797 g, 11.0 mmol) and naphthalene (0.1381 g, 1.1 mmol) in ca. 24 mL THF. The slurry turned green immediately, and the solution was heated to 60 oC for 4 days in a sealed Schlenk tube equipped with a Teflon valve, after which time the grey-white slurry was filtered through a frit. The grey powder isolated was washed twice with 10 mL of THF, dried under vacuum and used in the next step with no further purification (0.9624 g, 5.5 mmol, 50% yield).

Modified Preparation and spectroscopic data of 4-7

2.5750 g of C6F5Br (10.437 mmol) was added to a slurry of 0.9071 g of Na2Te (5.226 mmol) in ca. 15 mL THF at room temperature and heated to 60 oC for 16 hours. The grey slurry slowly turned into a light yellow solution with some black precipitate. All volatiles were then removed in vacuo and the black residue was extracted with DCM (10 mL × 3). All volatiles were removed in vacuo again, and the yellow residue washed with 5 mL × 2 pentane to give 1.3886 g of 4-7 (3.0075 mmol, 57.5% yield) as a light yellow powder. 19 1 3 4 F{ H} (376 MHz, C6D6): δ -115.4 (m, 4F, o-C6F5), -149.5 (t, 2F, JF-F = 20.7 Hz, JF-F =

3.5 Hz, p-C6F5), -159.2 (m, 4F, m-C6F5).

Preparation and spectroscopic data of 4-11

154

97.1 mg of 4-9 (0.194 mmol) in ca. 2 mL toluene was cooled to -35 oC, then a solution of 27.1 mg of BF3·OEt2 (0.191 mmol) in ca. 1 mL toluene was added to it. The colourless solution was warmed to room temperature over 16 hours, over which time the solution became somewhat yellow. All volatiles were removed in vacuo, leaving a colourless, viscous residue, which was triturated with pentane (2 mL × 3) to give 52.2 mg of 4-11 (0.0920 mmol, 48.2%) as a white solid. 19 1 F{ H} (376 MHz, C6D6): δ -115.4 (br s, 2F, Te–F), -127.4 (app s, 4F, o-C6F5), -143.5

(app s, 2F, p-C6F5), -157.4 (app s, 4F, m-C6F5). 19 1 F{ H} (376 MHz, tol-d8, 193 K): δ -94.7 (br s, 1F, Te–F), -128.1 (app s, 2F, o-C6F5), -

129.6 (app s, 2F, o-C6F5), -144.6 (app s, 2F, p-C6F5), -151.6, (s, 1F, BF4), -151.7 (s, 3F,

BF4), -156.8 (app s, 2F, m-C6F5), -158.3 (app s, 2F, m-C6F5).

Preparation and spectroscopic data of 4-12

146.0 mg of 4-9 (0.2922 mmol) in ca. 3 mL toluene was cooled to -35 oC, then a solution of 65.4 mg of TMSOTf (0.294 mmol) in ca. 2 mL toluene was added to it. The colourless solution was warmed to room temperature over 16 hours. All volatiles were removed in vacuo, leaving a white solid, which was triturated with pentane (2 mL × 3) to give 149.8 mg (0.2379 mmol, 81.4%) of 4-12 as a white solid. 19 1 F{ H} (376 MHz, C6D6): δ -77.8 (s, 3F, SO3CF3). -120.2 (br s, 1F, Te–F), -126.4 (m, 3 4F, o-C6F5), -140.2 (t, 2F, JF-F = 20.7 Hz, p-C6F5), -156.3 (m, 4F, m-C6F5). 125 Te (189.3 MHz, C6D6): (no shift could be observed even at 193 K)

Anal. Calc. for C13F14O3STe: C 24.79%. Found: C 24.67%.

155

Preparation and spectroscopic data of 4-14

203.8 mg of AgOTf (0.7932 mmol) in ca. 5 mL toluene was added dropwise to a solution of 422.0 mg of 4-10 (0.7923 mmol) in ca. 10 mL toluene cooled to -35 oC, which resulted in the immediate precipitation of AgCl as a white-gray powder. After stirring at this temperature for 5 minutes, the mixture was warmed to room temperature and stirred for 3 hours. The solution was then filtered through a glass frit, leaving a clear light yellow solution. All volatiles were removed in vacuo to give solid residue that is almost black in colour. Pentane (5 mL × 2) was used to triturate this residue to precipitate out 449.3 mg of 4-14 (0.6953 mmol, 87.8%) as an off-white solid. 19 1 F{ H} (376 MHz, C6D6): δ -77.8 (s, 3F, SO3CF3), -122.69 (br s, 4F, o-C6F5), -139.2 (br s, 2F, p-C6F5), -154.5 (br s, 4F, m-C6F5). 125 Te (189.3 MHz, C6D6): δ 886.4 (m) 125 Te (189.3 MHz, tol-d8, 193 K): δ 910.1 (br s)

Anal. Calc. for C13ClF13O3STe: C 24.16%. Found: C 24.22%.

Preparation and spectroscopic data of 4-15

51.1 mg of GaCl3 (0.290 mmol) in ca. 2 mL toluene was added dropwise to a solution of 154.4 mg of 4-10 (0.2899 mmol) in ca. 3 mL toluene cooled to -35 oC. After stirring at this temperature for 5 minutes, the cooling well was removed and the mixture was stirred at room temperature for 3 hours. All volatiles were removed in vacuo to give viscous yellow oil. Cyclopentane (5 mL × 2) was used to triturate this residue to precipitate out 140.0 mg of 4-15 as a yellow powder (0.1975 mmol, 68.1%).

156

19 1 F{ H} (376 MHz, C6D6): δ -120.9 (m, 4F, o-C6F5), -136.5 (m, 2F, p-C6F5), -154.1 (m,

4F, m-C6F5). 125 Te (189.3 MHz, C6D6): δ 900.1 (br s) 125 Te (189.3 MHz, tol-d8, 193 K): δ 955.3 (br s)

Anal. Calc. for C12Cl5F10GaTe: C 20.34% Found: C 20.68%

Reaction between 4-9 and B(C6F5)3

To a solution of 48.4 mg of 4-9 (0.0969 mmol) in ca. 2 mL of toluene, 49.5 mg of

B(C6F5)3 (0.0967 mmol) in 2 mL toluene was added. The solution was stirred for 16 h at room temperature. Then all volatiles were removed to leave a white precipitate. Some of this precipitate was dissolved in C6D6, transferred to an NMR tube and examined by 19F{1H} NMR spectroscopy. It contains compounds 4-7, 4-9 and 4-15 in a ratio of approximately 1:1.6:1.8.

19 1 F{ H} NMR spectrum of the reaction between 4-9 and B(C6F5)3 (red: 4-7, blue: 4-9, green: 4-15)

4.3.3 X-ray crystallography

157 temperature system. Data collection strategies were optimized using the Bruker Apex 2 software package70 in order to provide 99.5-100% completion to a 2θ value of >55o. The data integration was performed using the Bruker SAINT software package, and the resulting raw data were scaled and absorption corrected using an empirical multi-scan method (SADABS).70

Structure solution and refinement were conducted using the SHELXTL-2016 program suite.71, 72 The heavy atom positions were determined using direct methods, while lighter, non-hydrogen atoms were located by successive difference Fourier map calculations. In the final cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors except in the case of disorder. The H-atom contributions were calculated using the riding model. The locations and magnitudes of the largest peaks in the final difference Fourier maps indicate that, in each case, they were of no chemical significance.

The initial solution of 4-1 was obtained in space group Pna21. Since compound 4-1 is chiral and there is no chemical reason for it to have been synthesized with any enantioselectivity, the molecular structure was refined as an inversion twin. The possibility of the crystal belonging to a centrosymmetric space group was explored by attempting to solve it in space group Pnma, but no reasonable solution could be found this way. Additionally, it is important to note that for 4-3, the possibility of substitutional disorder between the fluorine and the oxygen atoms was considered. Attempts to model potential disorder by splitting the site occupancies of the fluorine and oxygen atoms and allowing the structure to refine, resulted in non-positive definite Uij matrices for one of the sulfur-fluorine sets in all cases. Even with restraints, the bond lengths and angles within the molecules became inconsistent with each other with large uncertainties. Based on these findings, the disorder between the sulfur and fluorine atoms was dismissed.

Table 4-1. Selected crystallographic data of compounds 4-1, 4-3 and 4-8. 4-1 4-3 4-8

Formula C37H12BF21OS C36H10BF21OS C16H19F3O4S2Si

158

Formula weight 914.34 900.31 424.52 Crystal system Orthorhombic Orthorhombic Monoclinic

Space group Pna21 Pbca P21/n a (Å) 14.635(2) 15.0990(3) 9.8802(4) b (Å) 9.785(1) 18.8792(5) 12.4373(5) c (Å) 23.547(3) 23.1430(7) 15.9080(5) α (°) 90 90 90 β (°) 90 90 101.9370(12) γ (°) 90 90 90 V (Å3) 3371.9(7) 6597.1(3) 1912.55(12) Z 4 8 4 Temp. (K) 150(2) 150(2) 150(2) d(calc) (g·cm-1) 1.801 1.813 1.474 Abs. coeff. μ (mm-1) 0.247 0.251 0.388 Reflections 13441 33498 17631 Collected 2 2 Data Fo >3σ(Fo ) 6310 7575 4384 Variables 552 547 235

R1 0.0463 0.0416 0.0430

wR2 0.1153 0.1002 0.0998 GOF 1.010 1.0348 1.026

159

Table 4-2. Selected crystallographic data of compounds 4-12, 4-13 and 4-15. 4-12 4-13 4-15

Formula C33H8F28O6S2Te2 C36BF31Te C12Cl5F10GaTe Formula weight 1351.71 1171.78 708.69 Crystal system Triclinic Monoclinic Monoclinic

Space group P1¯ P21/n P21/n a (Å) 12.3281(12) 11.7182(18) 12.7861(10) b (Å) 12.4439(13) 20.292(3) 10.0559(7) c (Å) 16.1340(15) 15.738(2) 14.8993(12) α (°) 74.738(5) 90 90 β (°) 67.935(5) 106.182(4) 91.414(3) γ (°) 71.507(6) 90 90 V (Å3) 2146.0(4) 3594.1(9) 1915.1(3) Z 2 4 4 Temp. (K) 150(2) 150(2) 150(2) d(calc) (g·cm-1) 2.092 2.143 2.458 Abs. coeff. μ (mm-1) 1.622 1.021 3.718 Reflections 35274 23160 16919 Collected 2 2 Data Fo >3σ(Fo ) 9724 8374 4398 Variables 637 622 262

R1 0.0722 0.0339 0.0158

wR2 0.1866 0.0752 0.0357 GOF 1.097 1.035 1.049

4.3.4 Computational details

All density functional theory (DFT) calculations were performed with the Turbomole 7.0 software.73, 74 The structure of 4-3 was optimized at the TPSS level of theory,75 with the

160

BJ-damped variant of the DFT-D3 dispersion correction76, 77 in conjunction with the def2-TZVP basis set.78, 79 The values of FIA were computed using the PW6B95- D3/def2-TZVP// TPSS-D3/def2-TZVP level of theory.80

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Chapter 5. Isomerization of Alkenylboranes Using Bulky Lewis Bases 5.1 Introduction

5.1.1 Alkenylboranes in synthetic chemistry

Alkenylboranes are ubiquitous in modern synthetic organic chemistry due to their use as carbon nucleophiles in the formation of many C–C, C–N and C–O bonds.1, 2 Various alkenylboranes have thus become commercially available, and synthetic strategies towards these species have been extensively studied. Although these compounds can be prepared by simple salt metathesis reactions between organolithium or Grignard reagents and boron electrophiles,1 this route is not ideal due to the highly moisture- sensitive nature of the nucleophiles. The direct borylation of alkynes using simple boranes such as BH3·THF or 9-BBN is also challenging as these reactions are often slow or lead to a mixture of products. The control of stereochemistry is nonetheless often crucial in the synthesis of natural products or pharmaceutical compounds. For these reasons, metal-catalyzed borylation reactions of alkynes offer a more synthetically viable way of accessing alkenylboranes, since they require milder conditions and allow for high stereoselectivity. These reactions can be further divided into hydroboration, carboboration and diboration reactions. Many transition metal catalysts have been applied in this context3, 4 ranging from Schwartz’s reagent for the E-selective synthesis of vinylboranes to earth-abundant Cu catalysts for the Z-selective preparation of tri- substituted alkenylboranes.

In contrast to the rich literature of metal-catalyzed borylation reactions of alkynes, metal- free routes to alkenylboranes remain scarce. One notable exception is the uncatalyzed 5 use of HB(C6F5)2 (Piers’ borane) in the hydroboration of alkynes. Hydroboration of alkynes using Piers’ borane takes place significantly faster than other conventional boranes such as 9-BBN and HBCat. This is a result of both the high electrophilicity of

HB(C6F5)2 and its rapid dimer dissociation in aromatic solvent due to the steric constraints of the C6F5 rings. The high reactivity of Piers’ borane, however, also makes its reaction with alkynes fall under kinetic control. As such, the reaction of Piers’ borane

166 with unsymmetrical alkynes proved to be non-regioselective.6 Metal-free and highly selective borylation of alkynes has only been reported recently by Ingleson and co- workers.7 They demonstrated that trans-hydroboration of alkynes could be achieved through the careful selection of a borenium ion catalyst that polarizes alkynes but does not engage in syn-1,2 or 1,1-elementoboration.

5.1.2 1,1-carboboration of unactivated alkynes

In the field of metal-free borylation of alkynes, one emerging strategy in recent years is the 1,1-carboboration of unactivated alkynes using the highly electrophilic borane,

B(C6F5)3. For decades, it was assumed that 1,1-carboboration of alkynes requires particular substituents at the alkyne, such as transition metals or heavy main group elements.8 This is because the mechanism of such reactions requires the abstraction of the acetylenic fragment, followed by a rearrangement sequence (see section 2.1.1). In 2011, the Erker group reported the surprising finding that terminal alkynes react with 9 B(C6F5)3 readily at room temperature to give an E/Z mixture of alkenylborane products. The mechanism of this transformation was later examined by DFT studies, which showed that this reaction likely does not require an abstraction step, but rather proceeds by a 1,2-hydride shift that gives rise to a mixture of isomers in the product (Scheme 5-1).10

167

Scheme 5-1. Comparison of the mechanism of 1,1-carboboration of (top) alkynes with heavy atom substitution and (bottom) alkynes with only organic substituents.

The Erker group also investigated the isomerization of the E/Z mixture of alkenylboranes using UV radiation. Although both E- and Z-isomers could be isolated from the product mixture in moderate to high yields (56-84%),9 this protocol requires prolonged reaction times (24 hours of irradiation) and also gives only partial isomerization. In order to facilitate future applications in more complicated organic molecules, there is a need for a facile and more efficient way of converting E/Z mixtures of alkenylboranes into one single isomer.

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5.2 Results and Discussion

5.2.1 Z-specific isomerization of alkenylboranes

To begin our investigation, 1-pentyne was treated with one equivalent of B(C6F5)3 at room temperature in toluene-d8, which quantitatively led to an E/Z-mixture of the 1,1- carboboration product 5-1 within minutes (Scheme 5-2). The in situ 19F{1H} NMR spectrum showed four sets of C6F5 resonances, corresponding to the four different perfluorinated aromatic rings in the two different isomers in about 0.84:1 (E:Z) ratio. Initially, we intended to explore the possibility of direct hydrophosphination of these alkenylboranes and thus subjected 5-1 to one equivalent of di-tert-butylphosphine t ( Bu2PH). Quite unexpectedly, we observed the quantitative conversion of the E/Z mixture to the Z-isomer within minutes. This conversion was especially evident in the 19 1 F{ H} NMR spectrum, which showed only two sets of C6F5 resonances: one set at -

129.5, -147.4 and -160.3 ppm are attributable to the B(C6F5)2 moiety and one set at -

139.2, -154.3 and -161.5 ppm are assigned to the HC=C(C6F5) moiety (Figure 5-1). The 11B{1H} NMR spectrum of the product showed some coordination of the phosphine to the B(C6F5)2 moiety. Nevertheless, Z-5-1 can be isolated base-free by simply placing it under vacuum for 2-3 hours followed by pentane wash. After product isolation, single crystals suitable for X-ray diffraction analysis were grown from the slow evaporation of a saturated solution of Z-5-1 in pentane. The stereochemistry of the isomerized product was confirmed unambiguously to be the Z-isomer (Figure 5-2).

Scheme 5-2. 1,1-carboboration of 1-pentyne by B(C6F5)3 followed by isomerization t using Bu2PH.

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Figure 5-1. In situ 19F{1H} NMR spectrum of the isomerization of E/Z isomers of 5-1 to t t exclusively the Z-isomer. (bottom) Before Bu2PH addition. (top) 5 minutes after Bu2PH addition at room temperature.

Figure 5-2. Solid-state structure of Z-5-1 (50% thermal ellipsoids). C: Black, B: yellow green, F: Pink, H: white. Most hydrogen atoms are omitted for clarity.

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t While the isomerization of alkenylboranes by Bu2PH indeed was unexpected, it should t be noted that the combination of B(C6F5)3 and Bu2PH alone at room temperature has been reported to rapidly lead to the nucleophilic attack by the phosphine at the para- 11 t position of the fluorinated arene. In the present study, however, Bu2PH is unreactive towards the alkenylborane product as evidenced by NMR spectroscopy. This observation further highlights an often crucial aspect of FLP chemistry: the order of addition of reagents has significant impact on the reaction outcome.

5.2.2 Substrate scope of E- or Z-specific isomerization of alkenylboranes

Encouraged by the rapid isomerization demonstrated in the synthesis of Z-5-1, we sought to probe the scope of this reaction. We subjected various terminal alkynes to the t 1,1-carboboration by B(C6F5)3, then added one equivalent of Bu2PH to this mixture to observe any isomerization that may take place. 19F{1H} NMR spectroscopy was used to monitor the completion of the 1,1-carboboration reaction, which always resulted in E/Z- mixtures of the corresponding alkenyl-boranes (Table 5-1). The initial ratios of the two isomers of the products R’CH=C(C6F5)B(C6F5)2 (R’ = (CH2)3Me 5-2, (CH2)2Ph 5-3,

CH2CHMe2 5-4, CHMe2 5-5, CMe3 5-6, CH2OMe 5-7, CH2N(CO)2C6H4 5-8) varied but fell within the range of 1:0.6 to 1:1.8 (E:Z)

t Table 5-1. Stoichiometric isomerization of alkenylboranes using Bu2PH.

Initial Time Temp. Prod. Entry R R’ Prod. E:Z (hours) (oC) E:Z

1 C6F5 (CH2)2Me 1:1.2 1 25 >1:99 5-1

2 C6F5 (CH2)3Me 1:1.2 1 25 >1:99 5-2

3 C6F5 (CH2)2Ph 1:1.1 1 25 >1:99 5-3

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4 C6F5 CH2CHMe2 1:1.5 1 25 >1:99 5-4

5 C6F5 CHMe2 1:1.2 24 25 1:1.7 5-5

6 C6F5 CMe3 1:1.8 24 25 1:1.7 5-6

7 C6F5 CH2OMe 1:0.6 1 25 >99:1 5-7

8 C6F5 CH2N(CO)2C6H4 1:1 3 25 5.5:1 5-8

9 (CH2)2Ph (CH2)2Me 1:1.3 20 60 >1:99 5-9

10 (CH2)2Ph (CH2)3Me 1:1.2 20 60 1:5.8 5-10

11 (CH2)2Ph (CH2)3Ph 1:1.2 20 60 1:20 5-11

12 (CH2)2Ph CH2CHMe2 1:1.2 20 60 1:3.2 5-12

t The presence of one equivalent of Bu2PH prompted the isomerization of the alkenylboranes at room temperature within one hour in most cases (Table 5-1, entries 1-8). The alkenylboranes with simple alkyl substituents (5-1–5-4) were isomerized to the Z-isomer in essentially quantitative yields. 2D-1H{19F}HOESY NMR experiments of these products show a clear correlation between the HC=C 1H resonance and the 19F resonances corresponding to o-B(C6F5)2, thus confirming the stereochemistry. In the case of Z-5-4, the stereochemistry was also confirmed crystallographically (Figure 5-3). It is noteworthy that the preferred stereochemistry obtained in this process is consistent with analogous isomerization by UV light.9 For species 5-5 and 5-6, although longer reaction times (24 hours) were employed, the ratio of E/Z-isomers did not change t substantially after the addition of Bu2PH. This observation suggests that sterically demanding substituents hinders the isomerization process and is in line with the mechanism we propose (vide infra).

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Figure 5-3. Solid-state structure of Z-5-4 (50% thermal ellipsoids). C: Black, B: yellow green, F: Pink, H: white. Most hydrogen atoms are omitted for clarity.

Interestingly, for the cases where the alkenylborane contained pendent donor moieties t (5-7 and 5-8), the addition of Bu2PH to their E/Z mixtures afforded the preferential formation of the E-isomer. Again, this stereochemistry was confirmed by 2D- 1H{19F}HOESY NMR experiments, which show a clear correlation between the HC=C 1H 19 resonance and the o-C6F5 F resonances. In the case of E-5-7, the stereochemistry of the product was also confirmed by single-crystal X-ray diffraction studies (Figure 5-4). The preference for E-isomers in the isomerization of 5-7 and 5-8 is presumably a result of the thermodynamically favoured coordination of the oxygen substituents to the boron centres.

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Figure 5-4. Solid-state structure of E-5-7 (50% thermal ellipsoids). C: Black, B: yellow green, F: Pink, H: white. Most hydrogen atoms are omitted for clarity.

The analogous treatment of alkynes with the borane Ph(CH2)3B(C6F5)2 also led to 1,1- carboboration that gave E/Z-isomers of R’CH=C((CH2)3Ph)B(C6F5)2 (R’ = (CH2)2Me 5-9,

(CH2)3Me 5-10, (CH2)2Ph 5-11, CH2CHMe2 5-12), though these reactions required heating to 60 oC to complete within 24 hours. Likewise, the isomerization of alkenylboranes 5-9–5-12 required heating at 60 oC to proceed to completion. The initial ratios of E/Z-mixtures ranged from 1:1.2 to 1:1.3. The subsequent isomerization t 19 1 mediated by Bu2PH was evident from the in situ F{ H} NMR spectra, though no products could be isolated due to the high solubility of these products in all organic solvents. In the case of 5-9, nearly quantitative isomerization to the Z-isomer was observed, whereas for 5-10–5-12, the selectivity was less effective and afforded E:Z ratios of 1:5.8, 1:20 and 1:3.2, respectively. These observations highlight the importance of having the highly electron-withdrawing C6F5 substituent on the C1 carbon (as labelled in Figures 5-2, 5-3 and 5-4) for isomerization to take place effectively.

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5.2.3 Mechanism of E- or Z-specific isomerization of alkenylboranes

The observed isomerization of alkenylboranes it thought to occur via the transient generation of a zwitterionic borataalkene species (Figure 5-5). Free rotation about the carbon center β- to the borane affords the thermodynamically more stable isomer as the final product. Efforts to observe the borataalkene zwitterion intermediate at low temperatures by in situ NMR experiments were not successful. Nonetheless, the proposition of a borataalkene intermediate is consistent with the sluggish isomerization of alkenylboranes with bulky substituents or which lack a C6F5 group on the carbon α to the boron centre. Indeed, it has been shown in the literature that electron-withdrawing substituents on this carbon centre are crucial to the stabilization of borataalkenes.12-16

With the collaboration of Prof. Grimme’s research group, DFT computational studies were undertaken to verify the mechanism of the Z-isomerization of 5-1 in toluene t solution (see section 5.3 for details). The nucleophilic addition of Bu2PH to the β-carbon centre in E-5-1 is 8.8 kcal/mol endergonic over an energy barrier of 19.4 kcal/mol to form the zwitterionic phosphine-borataalkene E-5-1-P. This is evident in the long C–C bond of 1.5275 Å that supports the loss of the olefinic moiety. Additionally, the short C– B bond length of 1.4647 Å corroborates well with past examples of isolated borataalkenes (1.44–1.46 Å).14, 15 Subsequent rotation along the C–C bond is only 1.8 kcal/mol endergonic over a low barrier of 7.4 kcal/mol to form Z-5-1-P. Further phosphine elimination from this species is -21.0 kcal/mol exergonic over a low barrier of 6.8 kcal/mol, leading to the final product Z-5-1. The overall phosphine-catalyzed E to Z conversion is thus -3.6 kcal/mol exergonic with an overall free energy barrier of 19.4 kcal/mol.

175

Figure 5-5. Calculated reaction pathway of the Z-isomerization of alkenylborane E-5-1. C: black, F: pink, H: wheat, P: orange, B: yellow green.

5.2.4 Catalytic isomerization of alkenylboranes and the use of other Lewis bases

The mechanism (Figure 5-5) suggests that other Lewis bases with a suitable steric bulk and nucleophilicity could also effect E/Z-isomerization. More specifically, the Lewis base should not bind strongly to the boron centre and should not be too sterically hindered to prevent the nucleophilic attack at the β-carbon. We thus screened a number of common Lewis bases and found that many could indeed lead to this transformation (Table 5-2). Notably, the composition of the E/Z-mixtures of 5-1 remained essentially unchanged upon the addition of pyridine, Et2O or MeCN (entries 7-9), presumably due to the weak nucleophilicity of these bases. As expected, Lewis bases with too much steric bulk such as tri(cyclohexyl)phosphine (PCy3) did not lead to isomerization either (entry 3). Heating to 80 oC is sometimes required when the base forms too strong of an adduct with the

176

t borane at room temperature (entry 2). It is worth noting, however, that Bu2PH still appears to be the most suitable base for this transformation as all other Lewis bases studied formed relatively stable adduct with the boron centre after isomerization (entries 1, 2, 5 and 6).

Table 5-2. Z-isomerization of alkenylborane 5-1 using different Lewis bases (initial E:Z = 1:1.2).

Equiv. Temp. Time Prod. Entry Base Base (oC) (hours) E:Z

1 Ph2HPO 1 25 1 >1:99

2 Me3N-O 1 80 24 >1:99

3 Cy3P 1 25 24 1:1.2

4 2,6-diisopropylamine 1 25 1 1:4.5

N,N’- 5 1 25 1 >1:99 diisopropylcarbodiimide

6 NEt3 Excess 25 1 >1:99

7 pyridine Excess 25 24 1:1.3

8 Et2O Excess 25 24 1:1.3

9 MeCN Excess 25 24 1:1.2

177

Another important conclusion to draw from the proposed mechanism is that the Lewis base remains intact at the end of the reaction. We thus also investigated the possibility t of performing the E/Z-isomerization using catalytic amounts of Bu2PH (Table 5-3). As t predicted, the addition of 10% of Bu2PH to E/Z-mixtures of alkenylboranes was sufficient to induce isomerization, albeit taking longer reaction times than the stoichiometric reactions. In the majority of the cases, the ratio of E:Z isomers is comparable to that observed in stoichiometric reactions (entries 1-5) The isomerization t o of E/Z-mixtures of 5-8, however, required 20% of Bu2PH and took 24 hours at 60 C to take effect. This result further highlights the influence of steric bulk on the efficiency of isomerization.

t Table 5-3. E/Z-isomerization of alkenylboranes using catalytic amounts of Bu2PH.

Initial Equiv. Temp. Time Prod. Entry R t o Product E:Z Bu2PH ( C) (hours) E:Z

1 (CH2)2Me 1:1.2 0.1 25 5 >1:99 5-1

2 (CH2)3Me 1:1.2 0.1 25 5 >1:99 5-2

3 (CH2)2Ph 1:1.2 0.1 25 5 1:20 5-3

4 CH2CHMe2 1:1.5 0.1 25 5 1:10 5-4

5 CH2OMe 1:0.6 0.1 25 5 >99:1 5-7

6 CH2N(CO)2C6H4 1:1 0.2 60 24 3.7:1 5-8

5.2.5 Conclusion

In summary, we have uncovered a facile method of isomerizing E/Z-mixtures of t alkenylboranes bearing electron-withdrawing groups using Bu2PH. This isomerization

178 favours the Z-isomer in the majority of the substrates investigated, but favours the E- isomer when there are Lewis basic substituents on the alkenylborane. This method is also effective with other Lewis bases of suitable steric bulk and can proceed under both stoichiometric and catalytic conditions. The mechanism of isomerization was shown computationally to proceed via a zwitterionic borataalkene intermediate. This protocol thus is favoured by electron-withdrawing substituents on the α-CH-boryl moiety as this would stabilize the reactive borataalkene intermediate.

5.3 Experimental Section

5.3.1 General considerations

C6D6 and tol-d8 was dried over Na/benzophenone and distilled under reduced pressure. All solvents were degassed by repeated freeze-pump-thaw cycles prior to use.

All chemicals were used as received unless otherwise noted. B(C6F5)3 was purchased t from Boulder Scientific. Di(tert-butyl)phosphine ( Bu2PH), diphenylphophine oxide, trimethylamine N-oxide, N,N’-diisopropylcarbodiimide, trimethylamine, pyridine, 1- pentyne, 1-hexyne, 3-methoxy-1-propyne, N-propargylphthalimide, and 3,3-dimethyl-1- butyne were purchased from Sigma-Aldrich. Tri(cyclohexyl)phosphine was purchased from Strem Chemicals. 2,4-diisopropylamine, 4-phenyl-1-butyne, and 3-methyl-1-butyne were purchased from Alfa Aesar. 4-methyl-1-pentyne was purchased from TCI America Research Chemicals. All liquid reagents were degassed by repeated freeze-pump-thaw cycles and dried over 4 Å sieves prior to use.

NMR spectroscopy was performed on either a Bruker Advance III 400 MHz, an Agilent DD2 500 MHz, or an Agilent DD2 600 MHz. Unless otherwise stated, all spectra were

179 obtained at room temperature. All NMR spectra were referenced to residual protio 1 13 19 solvent peaks of CD2Cl2 ( H = 5.32 ppm; C = 53.84 ppm) or an external standard ( F: 11 CFCl3 (δ 0.00), B: (Et2O)BF3 (δ 0.00)).

Combustion elemental analyses were performed on a on a PerkinElmer CHN Analyzer. HR-MS was performed on a JEOL AccuTOF equipped with a Direct Analysis in Real Time (DART) ion source.

5.3.2 Synthetic procedures and spectroscopic characterization

General procedure for NMR-scale reactions of substrates in Tables 5-1, 5-2, and 5-3

15.0 mg of B(C6F5)3 (0.0293 mmol) was dissolved in 1 mL of tol-d8 and mixed with 1 equivalent of alkyne at room temperature in a 1-dram screwcap vial in an N2-filled glovebox. This solution was mixed several times using a glass pipette and then transferred to a 5-mm NMR tube. NMR spectra were recorded to ensure the completion t of 1,1-carboboration reaction. Once this has been confirmed, 1 or 0.1 equiv. of Bu2PH (or other Lewis bases, see Table 5-2) was added to the sample and NMR spectra were recorded at multiple time intervals afterwards. Reaction progress and E/Z ratios were monitored and determined by in situ 19F{1H} NMR spectra.

General procedure for the isolation of 5-1, 5-2, 5-3, 5-4, 5-7, 5-8

B(C6F5)3 was dissolved in 3 mL of dichloromethane (DCM) and added to 1 equivalent of alkyne at room temperature. This mixture was stirred for 1 hour (5-1, 5-2, 5-3, 5-4) or 24 hours (5-7, 5-8) to ensure the completion of 1,1-carboboration reaction. 1 equivalent of t Bu2PH was then added as a solution in 1 mL of DCM and the mixture was allowed to stir for another 3 h before all volatiles were removed under reduced pressure. The isomerized products were then isolated by either repeated trituration in pentane or by recrystallization from a saturated solution in pentane at -35 oC.

180

Preparation and spectroscopic data of Z-5-1

161.6 mg of B(C6F5)3 (0.3156 mmol) was reacted with 24.0 mg of 1-pentyne (0.352 mmol). After isomerization, it was purified by recrystallization from a 4:1 solvent mixture of pentane:DCM to give 97.8 mg of Z-5-1 (0.169 mmol, 53.5%) as a white powder. 1 3 1 H (400.0 MHz, CDCl3): δ 7.21 (t, 1H, JH-H = 7.3 Hz, HC=CB), 2.28 (m, 2H, H ), 1.55 (m, 2 3 3 2H, H ), 0.91 (t, 3H, JH-H = 7.3 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 62.9 (br s, ν1/2 ≈ 940 Hz) 13 1 C{ H} (100 MHz, CDCl3): δ (C6F5 signals not listed) n.o. (HC=CB), 172.0 (s, HC=CB), 35.4 (s, C1), 21.4 (s, C2), 13.9 (s, C3) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -129.5 (d, 4F, JF-F = 17.7 Hz, o-B(C6F5)2), -139.2 (d, 3 3 3 2F, JF-F = 19.6 Hz, o-C6F5), -147.4 (t, 2F, JF-F = 20.0 Hz, p-B(C6F5)2), -154.3 (t, 1F, JF-

F = 22.2 Hz, p-C6F5), -160.3 (m, 4F, m-B(C6F5)2), -161.5 (m, 2F, m-C6F5)

Anal. Calc. for C23H8BF15: C 47.62%, H 1.39%. Found: C 47.54%, H 1.65%.

Preparation and spectroscopic data of Z-5-2

157.7 mg of B(C6F5)3 (0.3080 mmol) was reacted with 26.3 mg of 1-hexyne (0.320 mmol). After isomerization, it was purified by recrystallization from pentane to give 141.9 mg of Z-5-2 (0.2388 mmol, 77.5%) as a white powder. 1 3 1 H (400.0 MHz, CDCl3): δ 7.20 (t, 1H, JH-H = 7.3 Hz, HC=CB), 2.29 (m, 2H, H ), 1.49 (m, 2 3 3 4 2H, H ), 1.30 (m, 2H, H ), 0.87 (t, 3H, JH-H = 7.3 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 61.3 (br s, ν1/2 ≈ 960 Hz) 13 1 C{ H} (100 MHz, CDCl3): δ (C6F5 signals not listed) n.o. (HC=CB), 172.3 (s, HC=CB), 33.2 (s, C1), 30.0 (s, C2), 22.6 (s, C3), 13.8 (s, C4)

181

19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -129.5 (d, 4F, JF-F = 17.1 Hz, o-B(C6F5)2), -139.1 (d, 3 3 2F, JF-F = 19.6 Hz, o-C6F5), -147.3 (t, 2F, JF-F = 21.0 Hz, p-B(C6F5)2), -154.2 (t, 1F, 3 JF-F = 22.1 Hz, p-C6F5), -160.2 (m, 4F, m-B(C6F5)2), -161.3 (m, 2F, m-C6F5)

Anal. Calc. for C24H10BF15: C 48.52%, H 1.70%. Found: C 48.69%, H 1.63%.

Preparation and spectroscopic data of Z-5-3

153.4 mg of B(C6F5)3 (0.2996 mmol) was reacted with 42.4 mg of 4-phenyl-1-butyne (0.326 mmol). After isomerization, it was purified by repeated washing with pentane to give 188.2 mg of Z-5-3 (0.2930 mmol, 97.8%) as a white powder. 1 3 H (400.0 MHz, CDCl3): δ 7.26 (m, 2H, Ph), 7.20 (m, 1H, Ph), 7.13 (t, JH-H = 7.2 Hz, 3 HC=CB), 7.09 (m, 2H, Ph), 2.8 (t, 2H, JH-H = 7.5 Hz, PhCH2CH2), 2.62 (m, 2H,

PhCH2CH2) 11 1 B{ H} (128 MHz, CDCl3): δ 61.8 (br s, ν1/2 ≈ 1030 Hz) 13 1 C{ H} (100 MHz, CDCl3): δ (C6F5 signals not listed) n.o. (HC=CB), 169.8 (s, HC=CB),

139.7 (s, i-Ph), 128.7 (s, m-Ph), 128.4 (s, o-Ph), 126.7 (s, p-Ph), 34.8 (s, PhCH2CH2),

33.9 (s, PhCH2CH2) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -129.1 (d, 4F, JF-F = 18 Hz, o-B(C6F5)2), -138.9 (d, 3 3 3 2F, JF-F = 19 Hz, o-C6F5), -147.1 (t, 2F, JF-F = 20 Hz, p-B(C6F5)2), -154.0 (t, 1F, JF-F =

20 Hz, p-C6F5), -160.1 (m, 4F, m-B(C6F5)2), -161.2 (m, 2F, m-C6F5)

Anal. Calc. for C28H10BF15: C 52.37%, H 1.57%. Found: C 52.86%, H 1.71%.

Preparation and spectroscopic data of Z-5-4

182

94.4 mg of B(C6F5)3 (0.184 mmol) was reacted with 16.1 mg (0.196 mmol) of 3-methyl- 1-butyne. After isomerization, it was purified by recrystallization from a 4:1 solvent mixture of pentane:DCM to give 71.4 mg of Z-5-4 (0.120 mmol, 65.3%) as a white powder. 1 3 3 H (400.0 MHz, CDCl3): δ 7.25 (t, 1H, JH-H = 6.9 Hz, HC=CB), 2.2 (app t, 2H, JH-H = 1 2 3 3 6.8 Hz, H ), 1.86 (m, 1H, H ), 0.90 (d, 6H, JH-H = 6.8 Hz, H ) 11 1 B{ H} (128 MHz, CDCl3): δ 61.9 (br s, ν1/2 ≈ 960 Hz) 13 1 C{ H} (100 MHz, CDCl3): δ (C6F5 signals not listed) n.o. (HC=CB), 171.6 (s, HC=CB), 42.2 (s, C1), 28.4 (s, C2), 22.5 (s, C3) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -129.4 (d, 4F, JF-F = 18 Hz, o-B(C6F5)2), -139.0 (d, 3 3 3 2F, JF-F = 21 Hz, o-C6F5), -147.3 (t, 2F, JF-F = 18 Hz, p-B(C6F5)2), -154.1 (t, 1F, JF-F =

20 Hz, p-C6F5), -160.2 (m, 4F, m-B(C6F5)2), -161.3 (m, 2F, m-C6F5)

Anal. Calc. for C24H10BF15: C 48.52%, H 1.70%. Found: C 48.52%, H 1.73%.

Preparation and spectroscopic data of E-5-7

187.9 mg of B(C6F5)3 (0.3670 mmol) was reacted with 26.2 mg of 3-methoxy-1-propyne (0.369 mmol). After isomerization, it was purified by recrystallization from pentane to give 116.8 mg of E-5-7 (0.2007 mmol, 54.7%) as a white powder. 1 H (500.0 MHz, CDCl3): δ 6.30 (s, 1H, HC=CB), 5.13 (s, 2H, CH2OCH3), 3.86 (m, 3H,

CH2OCH3) 11 1 B{ H} (128 MHz, CDCl3): δ 7.3 (br s, ν1/2 ≈ 1 Hz) 13 1 C{ H} (100 MHz, CDCl3): δ (C6F5 signals not listed) n.o. (HC=CB), 128.2 (s, HC=CB),

84.2 (s, CH2OCH3), 61.7 (s, CH2OCH3) 19 1 3 F{ H} NMR (377 MHz, CDCl3): δ -134.3 (d, 4F, JF-F = 22.0 Hz, o-B(C6F5)2), -140.0 (m, 3 3 2F, o-C6F5), -155.1 (t, 2F, JF-F = 19.3 Hz, p-B(C6F5)2), -156.4 (t, 1F, JF-F = 21.8 Hz, p-

C6F5), -162.9 - -163.2 (m, 6F, m-B(C6F5)2 + m-C6F5)

Anal. Calc. for C22H6BF15O: C 45.40%, H 1.04%. Found: C 45.40%, H 1.16%.

183

Preparation and spectroscopic data of E-5-8

164.6 mg of B(C6F5)3 (0.3215 mmol) was reacted with 60.0 mg of N- propargylphthalimide (0.324 mmol). After isomerization completes over 24 hours, it was purified by first triturating with pentane, then recrystallized from a 4:1 solvent mixture of pentane:DCM to give 182.7 mg of E-5-8 (0.2621 mmol, 81.5%) as a white powder. 1 3 H (400.0 MHz, CDCl3): δ 7.95 (m, 2H, Ar-H), 7.85 (m, 2H, Ar-H), 6.37 (t, 1H, JH-H = 6.8 3 Hz, HC=CB), 4.48 (d, 2H, JH-H = 7.0 Hz, CH2N) 11 1 B{ H} (128 MHz, CDCl3): δ 3.5 (br s, ν1/2 ≈ 600 Hz) 13 1 C{ H} (100 MHz, CDCl3): δ (C6F5 signals not listed) n.o. (HC=CB), 136.6 (s, Ar-C),

132.8 (s, HC=CB), 125.7 (s, Ar-C), 38.3 (s, CH2N) 19 1 F{ H} NMR (377 MHz, CDCl3): δ -132.6 (m, 4F, o-B(C6F5)2), -140.6 (m, 2F, o-C6F5), - 3 3 155.8 (t, 2F, JF-F = 20.0 Hz, p-B(C6F5)2), -156.8 (t, 1F, JF-F = 20.2 Hz, p-C6F5), -162.9 -

-163.1 (m, 6F, m-B(C6F5)2 + m-C6F5)

Anal. Calc. for C29H7BF15NO2: C 49.96%, H 1.01%, N: 2.01%. Found: C 49.82%, H 1.30%, N: 1.84%.

5.3.3 X-ray crystallography

184 software package17 in order to provide 99.5-100% completion to a 2θ value of >55o. The data integration was performed using the Bruker SAINT software package, and the resulting raw data were scaled and absorption corrected using an empirical multi-scan method (SADABS).17

Structure solution and refinement were conducted using the SHELXTL-2016 program suite.18, 19 The heavy atom positions were determined using direct methods, while lighter, non-hydrogen atoms were located by successive difference Fourier map calculations. In the final cycles of each refinement, all non-hydrogen atoms were assigned anisotropic temperature factors except in the case of disorder. The H-atom contributions were calculated using the riding model. The locations and magnitudes of the largest peaks in the final difference Fourier maps indicate that, in each case, they were of no chemical significance.

Table 5-4. Selected crystallographic data of compounds Z-5-1, Z-5-4 and E-5-7. Z-5-1 Z-5-4 E-5-7

Formula C23H10BF15 C23H10BF15 C22H6BF15O Formula weight 580.10 594.13 582.08 Crystal System Orthorhombic Monoclinic Orthorhombic

Space group Pca21 P21/n Pbca a (Å) 19.592(8) 6.1323(6) 14.1455(13) b (Å) 9.749(3) 20.344(2) 16.6200(17) c (Å) 11.697(4) 18.6285(17) 18.0982(17) α (°) 90 90 90 β (°) 90 91.478(5) 90 γ (°) 90 90 90 V (Å3) 2234.2(14) 2323.2(4) 4254.9(7) Z 4 4 8 Temp. (K) 150(2) 150(2) 150(2) d(calc) (g·cm-1) 1.725 1.699 1.817 Abs. coeff. μ (mm-1) 0.188 0.183 0.201

185

Reflections 10170 20719 21381 Collected

Data F 2>3σ(F 2) 4549 5331 4891 o o Variables 352 361 352

R1 0.0506 0.0445 0.0567

wR2 0.1134 0.0994 0.1339 GOF 1.030 1.045 1.046

5.3.4 Computational details

All DFT calculations were performed using the TURBOMOL V7.0 programs.20, 21 All structures were initially fully optimized at the TPSS-D3/def2-TZVP + COSMO (toluene) level of theory, which combines the TPSS meta-GGA density functional22 with the BJ- damped DFT-D3 dispersion correction23, 24 and the large def2-TZVP AO basis set,25-27 together with the COSMO (for toluene solvent: dielectric constant Ɛr=2.38 , Rsol= 3.48 Å) solvation model.28 The density-fitting RI-J approach was used to accelerate the geometry optimization and harmonic frequency calculations.29, 30 Vibrational frequency analysis was used to identify the nature of located stationary points and to provide thermal and free-energy corrections according to the modified ideal gasrigid rotorharmonic oscillator model.31 The structures were characterized as true minima of reaction intermediates if there was no imaginary frequency, or as transition states when there was only one imaginary frequency. Once structures were optimized, better free energy calculations in toluene solution were obtained from the sum of more reliable PW6B95-D3/def2-QZVP single-point energies,25, 32 COSMO-RS33-35 solvation free energies (default Gsolv using the BP_TZVP_C30_1301.ctd parameter file), and TPSS- D3/def2-TZVP thermal corrections at 298.15 K related to ideal gas under 1 atm (i.e., 0.04 mol/L concentration).

186

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9. C. Chen, T. Voss, R. Froehlich, G. Kehr and G. Erker, Org. Lett., 2011, 13, 62.

10. C. Chen, F. Eweiner, B. Wibbeling, R. Froehlich, S. Senda, Y. Ohki, K. Tatsumi, S. Grimme, G. Kehr and G. Erker, Chem. - Asian J., 2010, 5, 2199.

11. G. C. Welch, R. Prieto, M. A. Dureen, A. J. Lough, O. A. Labeodan, T. Holtrichter-Rossmann and D. W. Stephan, Dalton Trans., 2009, 1559.

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14. J. Moebus, G. Kehr, C. G. Daniliuc, R. Froehlich and G. Erker, Dalton Trans., 2014, 43, 632.

15. P. Moquist, G.-Q. Chen, C. Mueck-Lichtenfeld, K. Bussmann, C. G. Daniliuc, G. Kehr and G. Erker, Chem. Sci., 2015, 6, 816.

16. J. Yu, G. Kehr, C. G. Daniliuc and G. Erker, Eur. J. Inorg. Chem., 2013, 2013, 3312.

17. Bruker AXS Inc., Madison, WI, 2013, vol. 6.

18. G. M. Sheldrick, Acta Crystallogr., 2015, A71.

19. http://shelx.uni-ac.gwdg.de/SHELX/index.php.

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21. R. Ahlrichs, M. Baer, M. Haeser, H. Horn and C. Koelmel, Chem. Phys. Lett., 1989, 162, 165.

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28. A. Klamt and G. Schueuermann, J. Chem. Soc., Perkin Trans. 2, 1993, 799.

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188

Chapter 6. Conclusion 6.1 Summary

The objective of this dissertation is to explore the use of organochalcogen in FLP-type chemistry both as Lewis bases and as Lewis acids. As a Lewis base, tellurium acetylides were found to undergo facile 1,1-carboboration reactions with a number of triorganoboranes, leading to the synthesis and isolation of the first tellurium-boron FLP. The weak Lewis basicity of tellurium precluded typical FLP reactivities such as the activation of CO2 and H2. Nonetheless, we found that 1,1-carboboration of tellurium diacetylides provided a facile route to 1,4-telluraborines, which was an unknown class of compounds prior to this study. These species were found to have mild to high aromaticity, which gave them unusual stability against oxygen and moisture. Treating 1,4-telluraborines with organic substrates led us to discover their unique reactivities, including alkyne exchange reactions and selective protonolysis of the exocyclic B–C bond. We were also able to make a powerful hydroboration reagent based on 1,4- telluraborine. These findings allowed for the facile derivatization of this new class of compounds, which is unusual in the field of tellurium-containing heterocycles.

The next portion of this dissertation involves the exploration of sulfur- and tellurium- centred Lewis acids. Fluorosulfoxonium cations were successfully synthesized and fully characterized from commercially available diaryl sulfoxides. These sulfur(VI) species were computationally and experimentally shown to be Lewis acidic. They were also capable of catalyzing the hydroarylation and hydrothiolation of olefins. On the other hand, the synthesis of tellurium(IV)-based Lewis acids were more challenging. We were able to successfully synthesize a series of fluoro- and chlorotelluronium cations, but their high instability and fluxional solution-state behaviour prevented detailed studies on their utility as Lewis acid catalysts or in FLP-type chemistry.

Lastly, we describe a new method of isomerizing E/Z-mixtures of alkenylboranes prepared by 1,1-carboboration of terminal alkynes. Prior to this study, this reaction could only be achieved by UV irradiation. We now report a more effective method of t isomerization by using commercially available bulky Lewis bases such as Bu2PH. The 189 mechanism of isomerization was shown computationally to proceed via a zwitterionic borataalkene intermediate.

6.2 Future Work

The use of tellurium-containing heterocycles in optoelectronic devices has garnered much research attention in the past decade, but has always been hampered by the lack of facile synthetic routes to these compounds. To this end, the syntheses and reactivities of 1,4-telluraborines described in this dissertation establishes the ground work needed for future applications. More specifically, the unusual stability of these heterocycles towards water could open doors for biological applications. For example, organotellurium compounds have been used in mass cytometry for the analysis of heterogeneous cell populations.1-3 Many of the current tellurium species used are prone to oxidation in aqueous solutions, with the mildly aromatic tellurophene being the most resilient. The high stability and aromaticity of 1,4-telluraborines could potentially out- perform tellurophenes. Additionally, they can easily be attached to biologically relevant molecules (azomycin in the case of mass cytometry) containing hydroxyl or olefinic groups using the protocols developed in this dissertation (Scheme 6-1).

Scheme 6-1. Synthesis of azomycin-linked 1,4-telluraborine by selective protonolysis of the exocyclic B–C bond.

Many of the 1,4-telluraborines reported in Chapter 3 can be used as monomers for the preparation of conjugated polymers. For example, 3-4 can be polymerized via Suzuki cross coupling and 3-20 can be polymerized by Grubbs catalyst (Scheme 6-2).

190

Scheme 6-2. Different routes of preparing conjugated polymers containing 1,4- telluraborine.

The work described in Chapter 4 involving sulfur- and tellurium-based Lewis acids can be further developed in a number of ways. Firstly, works on electrophilic phosphonium cations have shown that dicationic species allow for higher catalytic activities than monocationic ones.4-6 We envision the synthesis of dicationic fluorosulfoxonium cations through a similar protocol described in Chapter 4 using commercially available bis- sulfoxides (Scheme 6-3). Secondly, though the instability of C6F5-substituted telluronium cations preclude FLP activities, it might be possible to stabilize these cations by the addition of crown ethers, as was the case involving several reactive main group cations7, 8 The successful isolation and crystallographic characterization of these species would contribute significantly to the limited literature of telluronium compounds.

Scheme 6-3. Synthesis of fluorosulfoxonium dication.

191

6.3 References

1. L. J. Edgar, R. N. Vellanki, A. Halupa, D. Hedley, B. G. Wouters and M. Nitz, Angew. Chem., Int. Ed., 2014, 53, 11473.

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3. H. Park, L. J. Edgar, M. A. Lumba, L. M. Willis and M. Nitz, Org. Biomol. Chem., 2015, 13, 7027.

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