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Exploring Low-Coordinate Aluminum-Based Lewis Acids for Small Molecule Activation

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Exploring Low-Coordinate Aluminum-Based Lewis Acids for Small Molecule Activation Exploring Low-Coordinate Aluminum-Based Lewis Acids for Small Molecule Activation by Erika Nicole Daley A thesis submitted in conformity with the requirements for the degree of Master of Science Department of Chemistry University of Toronto © Copyright by Erika N. Daley 2016 Exploring Low-Coordinate Aluminum-Based Lewis Acids for Small Molecule Activation Erika Nicole Daley Master of Science Department of Chemistry University of Toronto 2016 Abstract While there have been a significant number of publications in the field of frustrated Lewis pair chemistry, only a fraction employ aluminum-based Lewis acids. The research presented herein expands the scope of Lewis acids to include novel aluminum-based compounds. N-heterocyclic carbene-stabilized three-coordinate aluminum cations were synthesized from the NHC-AlR3 adduct by abstraction of an R substituent from aluminum. These compounds were crystallographically characterized and exhibited solvent-dependent reactivity. Hydrogenation catalysis and radical reactivity were also explored experimentally and computationally. ii Acknowledgments First and foremost, I would like to extend my greatest thanks to my supervisor Professor Doug Stephan, for his patience, support, inspiration, and for giving me the opportunity to carry out my degree in such a wonderful group. Your enthusiasm for chemistry is contagious. Thank you for fostering a productive and rewarding research environment to work in. Thank you to all of my colleagues in the Stephan group for helpful science-related discussions and for making the lab a fun and exciting place to be. I am extremely grateful to Dr. Alex Pulis for your guidance when I started in the group, and throughout the first year of my degree. Thank you to Lauren Longobardi for your helpful conversations and suggestions in every aspect of academics and research, and for being a wonderful friend. I would also like to thank Louie Fan for being such a delightful cubicle-mate and great friend. A special thanks to the crystallographers Lauren, Conor, Eliar, Ian, and Judy for running samples but also for all of your help and advice with solving structures. I am indebted to the staff of the NMR facility, particularly Dr. Darcy Burns, Dmitry Pichugin, and Dr. Sergiy Nokhri and sincerely thank you for your assistance and expertise. I would also like to thank Dr. Timothy Burrow and Levy Cao for assistance with collecting and modeling EPR data. A huge thanks is also extended to the Women in Chemistry Initiative, the Green Chemistry Initiative, and my Nanowizard friends, especially Laura Reyes, who completed my life in the department outside of the lab and made my time at U of T significantly more enjoyable. Thank you to those who edited this thesis, namely Dr. Timothy Johnstone, Dr. Peter Mirtchev, Ian Mallov, and Levy Cao. Moreover, thank you Prof. Dr. Douglas Stephan and Prof. Dr. Datong Song for final review and edits. I owe a huge thanks to my parents, David and CarolAnn, and sisters, Breanna and Kristena, for all of their support, encouragement, and reassurance throughout this degree. I especially enjoyed the visits you paid and all of our adventures in Toronto, and appreciate the laughs and (mostly virtual) hugs that I know I can always count on you for when needed the most. I also need to acknowledge Jake, the best dog in the world, for being my #1 fan and a loyal group mascot. iii Last but not least, thank you to Bradley Joseph for being there for me through all of the laughs and tears, the good times and the bad of this degree, our time at Mount A, and life in general. You’re my best friend and never fail to make me smile, even when you’re half a world away. Sincerely, Erika Daley iv Table of Contents Acknowledgments ......................................................................................................................... iii Table of Contents ............................................................................................................................ v List of Tables .................................................................................................................................. vi List of Figures ............................................................................................................................... vii List of Schemes .............................................................................................................................. ix List of Abbreviations and Symbols ................................................................................................ xi Chapter 1 ......................................................................................................................................... 1 1 Introduction ............................................................................................................................... 1 1.1 Overview of Frustrated Lewis Pair Chemistry ................................................................... 2 1.1.1 Early advances in Lewis acid/base chemistry .............................................................. 2 1.1.2 Development and early work in frustrated Lewis pair chemistry ................................ 4 1.2 Group 13 Lewis Acids ........................................................................................................ 8 1.3 Scope of Thesis .................................................................................................................. 9 Chapter 2 ....................................................................................................................................... 10 2 Synthesis and Reactivity of Low-coordinate Aluminum Species ........................................... 10 2.1 Introduction ...................................................................................................................... 10 2.1.1 Aluminum-Hydride and Low-Coordinate Aluminum Cations .................................. 13 2.2 Results and Discussion ..................................................................................................... 16 2.2.1 Synthesis and Reactivity of Aluminum Cation .......................................................... 16 2.2.2 Synthesis, Reactivity, and Computational Study of Aluminum Radical ................... 27 2.3 Conclusion ........................................................................................................................ 31 2.4 Future Direction ............................................................................................................... 32 2.5 Experimental .................................................................................................................... 32 2.5.1 General Considerations .............................................................................................. 32 2.5.2 Syntheses and Spectroscopic Characterization .......................................................... 33 2.5.3 Hydrogenations using [(IDipp)AlH2][B(C6F5)4] 2-5 ................................................. 43 2.5.4 X-Ray Crystallography .............................................................................................. 46 References ..................................................................................................................................... 58 v List of Tables Table 2.1 Summary of attempted syntheses of intermediate 3-coordinate aluminum species for employment in further reaction with NHC to ultimately generate Al+...................................................................................................................................18 Table 2.2 Summary of attempted syntheses of intermediate 4-coordinate NHC-aluminum diolate species for employment in further reaction to ultimately generate Al+..................................................................................................................19 Table 2.3 Lewis acid mediated catalysis by aluminum cation 2-5; conversions determined by 1H NMR spectroscopy...........................................................................................26 Table 2.4 Hyperfine coupling parameters for proposed radical [(IDipp)AlH2(THF)]•................42 Table 2.5 Selected crystallographic data.......................................................................................47 Table 2.6 Cartesian coordinates of compounds used for calculation study..................................48 vi List of Figures Figure 1.1 Classic Lewis acid-base reactivity resulting in adduct formation.................................2 Figure 1.2 Frustrated Lewis pair that do not react to form a strong, irreversible adduct, owing to steric requirements of the bulky Lewis acid and base..........................................4 Figure 1.3 Examples of various classes of substrates that undergo FLP-mediated hydrogenation...........................................................................................................6 Figure 1.4 General depiction of the formation of an encounter complex and subsequent heterolytic cleavage of H2 by a frustrated Lewis pair...................................................8 Figure 2.1 Select examples of crystallographically characterized cationic aluminum hydride compounds.......................................................................................................14 Figure 2.2 Aluminum-hydride cations reported by Wright and coworkers..................................14 Figure 2.3 The first isolable 3-coordinate base-free cationic aluminum alkyl compounds............................................................................................................................15
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