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Group 16 Elements in Frustrated Lewis Pair Chemistry

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Group 16 Elements in Frustrated Lewis Pair Chemistry Group 16 Elements in Frustrated Lewis Pair Chemistry by Fu An Tsao A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto © Copyright by Fu An Tsao 2018 Group 16 Elements in Frustrated Lewis Pair Chemistry 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. iv 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 v 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
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