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Bifunctional Systems in the Chemistry of Frustrated Lewis Pairs

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Bifunctional Systems in the Chemistry of Frustrated Lewis Pairs Bifunctional Systems in the Chemistry of Frustrated Lewis Pairs by Xiaoxi Zhao A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto © Copyright by Xiaoxi Zhao 2012 Bifunctional Systems in the Chemistry of Frustrated Lewis Pairs Xiaoxi Zhao Doctor of Philosophy Department of Chemistry University of Toronto 2012 Abstract Three classes of bifunctional compounds related to frustrated Lewis pair chemistry were studied. The first class, alkynyl-linked phosphonium borates, was strategically synthesized and the corresponding neutral alkynyl-linked phosphine boranes generated in solution. They were reacted with THF, alkenes and alkynes to undergo either ring- opening or multiple bond addition reactions, giving rise to zwitterionic macrocycles. In two select alkynyl-linked phosphonium borates, thermolysis resulted in unique rearrangements transforming the phosphino- and boryl-substituted alkynyl moieties into C4 chains. The alkynyl-linked phosphine boranes were further demonstrated to coordinate as η3-BCC ligands in Ni(0) complexes. The rigid nature of the coordination was confirmed by dimerization without cleavage of the Ni–B interaction upon the addition of acetonitrile or carbon monoxide. Moreover, reactions with Al-, Zn- and B- based Lewis acids prompted hydride transfer within the alkynyl-linked phosphonium borate and interesting functional group transfer reactions. The second class of the bifunctional systems, a series of gem-substituted bis-boranes, was subjected to reactions with tBu3P and CO2. The O-linked bis-borane was shown to ii coordinate the phosphino-carboxylate moiety with one B, while the methylene-linked bis-boranes were demonstrated to chelate the carboxyl group. The third bifunctional system class, vinyl-group tethered boranes, was examined to elucidate the mechanism of the frustrated Lewis pair addition reaction to olefins. Using a bis(pentafluorophenyl)alkylborane, the close proximity of the olefinic protons and the ortho-fluorine nuclei were evident by both NOE measurements and DFT calculations. Moreover, its reactions with phosphine bases suggested that an initial interaction between the highly electrophilic borane and the olefinic fragment precedes such frustrated Lewis pair addition reaction. Furthermore, a bis(pentafluorophenyl)alkoxyborane was synthesized and reacted with P-, N-, C- and H- based nucleophiles, demonstrating the wide range of Lewis bases that can be applied in olefin addition reactions with complementary regioselectivity. iii Acknowledgements I would like to express my sincere gratitude first and foremost to Professor Doug Stephan for letting me work in his laboratory, and giving me invaluable suggestions, encouragement and guidance over the past four years. I would also like to take this opportunity to thank all of the past and present members of the Stephan group who made my graduate years a wonderful time. Without the great amount of knowledge in chemistry, laboratory, instrumental and writing skills that they kindly shared with me, I would not have been able to complete this thesis. I would especially like to thank Dr. Alberto Ramos and Dr. Edwin Otten for their valuable advice that they gave me when I was a new student with limited ideas on the chemistry I was doing. I would also like to express my appreciation to Professor Bob Morris and Professor Datong Song for serving on my supervisory committee. Lastly, I would like to thank my beloved family and friends for the great amount of support they provided me throughout the course of the years. iv Table of Contents Acknowledgements ......................................................................................................... iv Table of Contents............................................................................................................. v List of Schemes ............................................................................................................... x List of Figures ............................................................................................................... xiii List of Tables.................................................................... Error! Bookmark not defined. List of Abbreviations, Nomenclature and Symbols ....................................................... xvii 1 Introduction .................................................................................................................. 1 1.1 Frustrated Lewis Pairs with Dihydrogen ................................................................ 1 1.1.1 Heterolytic Splitting of Hydrogen and Hydrogenation Reactions ................ 1 1.1.2 Transition Metal Centers and Frustrated Lewis Pairs in H2 Activation ........ 2 1.2 Reactions of Frustrated Lewis Pairs with Multiple Bonds and Cyclic Compounds ........................................................................................................... 7 1.2.1 Alkenes and Cycloalkanes ......................................................................... 7 1.2.2 Carbon Dioxide, Nitrous Oxide, Nitric Oxide and Carbon monoxide........... 7 1.3 Reactions of Alkynes with Frustrated Lewis Pairs ................................................. 9 1.3.1 Reactions of Trialkylalkynylborates with Electrophiles ................................ 9 1.3.2 1,1-Carboboration Reactions - Revisited .................................................. 10 1.3.3 Reactions of Lewis Acid-Base Pairs with Alkynes .................................... 10 1.4 Objectives of This Thesis .................................................................................... 12 2 Synthesis and Frustrated Lewis Pair Reactivity of Alkynyl-Linked Phosphine Boranes ..................................................................................................................... 14 2.1 Introduction ......................................................................................................... 14 2.1.1 Intramolecular Frustrated Lewis Pair Systems ......................................... 14 2.1.2 Previously Reported Alkynyl-Linked P/B Compounds .............................. 14 2.2 Results and Discussion ....................................................................................... 15 v 2.2.1 Synthesis of Ethynylphosphines ............................................................... 15 2.2.2 Synthesis of Alkynyl-Linked Phosphonium Borates .................................. 16 2.2.3 Attempted Use of the Alkynyl-Linked P/B Compounds in Hydrogen Activation and Hydrogenation Catalysis ................................................... 24 2.2.4 Zwitterionic Macrocycles of Alkynyl-Linked Phosphonium Borates .......... 33 2.2.5 Synthesis of Borataallene Compounds ..................................................... 45 2.3 Conclusions ......................................................................................................... 47 2.4 Experimental Section .......................................................................................... 48 2.4.1 General Considerations ............................................................................ 48 2.4.2 Syntheses ................................................................................................. 48 2.4.3 X-ray Crystallography ............................................................................... 64 3 Reactivity of Alkynyl-Linked Phosphonium Borates Toward Main-Group and Transition Metal Species ........................................................................................... 73 3.1 Introduction ......................................................................................................... 73 3.1.1 Alkyne and Phosphine Complexes of Transition Metals ........................... 73 3.1.2 Coordination Chemistry of Borane Ligands .............................................. 73 3.2 Results and Discussion ....................................................................................... 75 3.2.1 Syntheses of Ni(0) Complexes of Alkynyl-Linked Phosphine Boranes ..... 75 3.2.2 Computational Analysis of a Ni(0) Complex of Alkynyl-Linked Phosphine Boranes .................................................................................. 82 3.2.3 Reactions of an Alkynyl-Linked Phosphonium Borate with Al-, B-, and Zn-Based Lewis Acids .............................................................................. 85 3.3 Conclusions ......................................................................................................... 95 3.4 Experimental Section .......................................................................................... 95 3.4.1 General Considerations ............................................................................ 95 3.4.2 Syntheses ................................................................................................. 96 3.4.3 X-ray Crystallography ............................................................................. 102 vi 4 Bis-Boranes in Frustrated Lewis Pair Chemistry ..................................................... 107 4.1 Introduction ....................................................................................................... 107 4.1.1 Activation of Carbon Dioxide by Frustrated Lewis Pairs ......................... 107 4.1.2 Chelating Bis-boranes ............................................................................ 107 4.2 Results and Discussion ....................................................................................
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