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Verdazyl Radicals As Substrates for Organic Synthesis Verdazyl Radicals as Substrates for Organic Synthesis by Matthew Bancerz A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Chemistry University of Toronto © Copyright by Matthew Bancerz 2012 Verdazyl Radicals as Substrates for Organic Synthesis Matthew Bancerz Doctor of Philosophy Department of Chemistry University of Toronto 2012 Abstract Verdazyl radicals, discovered in 1963, are a family of exceptionally stable radicals defined by their 6-membered ring containing four nitrogen atoms. Verdazyl radicals are highly modular compounds with a large assortment of substitution patterns reported. Their stability and high degree of structural variability has been exploited in the fields of materials, inorganic, polymer and physical chemistry; however their deliberate use as starting materials towards organic synthesis had only been reported in recent years by the Georges lab. In 2008, the Georges group reported a disproportionation reaction that was observed to a occur with 6-oxoverdazyl radicals resulting in azomethine imines capable of undergoing 1,3-dipolar cycloaddition reactions. With this discovery, the door to using verdazyl radicals as substrates towards organic synthesis had been opened. Their utility in synthesis was soon discovered not to be limited to just the cycloadducts their azomethine imine derivatives could generate but also the increasing number of N-heterocycles that could be generated from these cycloadducts via unique rearrangement reactions, a major theme of this thesis. In addition, triphenyl verdazyl radicals, a distinct class of verdazyl radicals, has been shown to react with alkynes by direct radical addition and rearrangement to afford isoquinolines. ii As part of this thesis, a new synthetic methodology of generating 6-oxoverdazyl radicals is reported that does not rely on the use of phosgene or hydrazines. This new synthesis allows for the expansion of available alkyl substituents possible on N1 and N5 positions of 6-oxoverdazyl radicals, as well as, generation of unsymmetrical examples of 6-oxoverdazyl radicals with non- identical N1 and N5 alkyl substituents. Employing the new 6-oxoverdazyl radicals synthesized via this method, a study on the effects of different alkyl substituents on the disproportionation reaction of 6-oxoverdazyls was undertaken. Lastly, given the assortment of N-heterocyclic molecular scaffolds capable of being synthesised starting from verdazyl radicals as precursors, the applicability of verdazyl radicals in making a diversity oriented synthesis (DOS) based library was explored. In a group effort with other Georges lab members, a small library composed of various classes of verdazyl radical derived compounds was synthesized and non-specifically tested for cytotoxicity against acute myeloid leukemia and multiple myeloma cell lines in collaboration with The Princess Margaret Hospital. One example was shown to effectively kill cancer cells in both these lines in 250 µM concentration pointing out the potential of using verdazyl radical based chemistry in drug discovery. iii Acknowledgments I would like to thank my supervisor Professor Michael Georges for all of your guidance in my research projects as well as the flexibility to take things in my own direction. You’ve taught me a lot about chemistry and about life over the years for which I am very grateful for. Your support and friendship has been invaluable. I am thankful to all the other lab members including Dr. Gord Hamer, my fellow graduate students and undergrads who have passed through the Georges lab as well as members of the Gunning lab for a fulfilling experience in my graduate studies. I also thank my family for their support throughout my studies. Thank you all. iv Table of Contents Acknowledgments .......................................................................................................................... iv Table of Contents ............................................................................................................................ v List of Tables ................................................................................................................................. ix List of Schemes ............................................................................................................................... x List of Figures .............................................................................................................................. xiv List of Abbreviations .................................................................................................................... xv Chapter 1 ......................................................................................................................................... 1 1 Introduction ................................................................................................................................ 1 1.1 Stable Radicals .................................................................................................................... 1 1.2 Verdazyl Radical Overview ................................................................................................ 2 1.3 Verdazyl Radical Synthesis ................................................................................................ 4 1.4 Historical Verdazyl Radical Chemistry ............................................................................ 10 1.5 Application of Verdazyl Radicals ..................................................................................... 12 1.6 Summary ........................................................................................................................... 13 1.7 References ......................................................................................................................... 13 Chapter 2 ....................................................................................................................................... 17 2 1,3-Dipolar Cycloadditions ...................................................................................................... 17 2.1 Introduction to Cycloadditions ......................................................................................... 17 2.2 1,3-Dipolar Cycloadditions ............................................................................................... 18 2.3 Catalysis in 1,3-Dipolar Cycloadditions ........................................................................... 22 2.4 Azomethine Imines in 1.3-Dipolar Cycloadditions .......................................................... 24 2.5 Summary ........................................................................................................................... 30 2.6 References ......................................................................................................................... 30 Chapter 3 ....................................................................................................................................... 33 v 3 Five-Membered N-Heterocyclic Rearrangements ................................................................... 33 3.1 Introduction ....................................................................................................................... 33 3.2 Dimroth Rearrangements .................................................................................................. 33 3.3 Other Rearrangements ...................................................................................................... 38 3.4 Summary ........................................................................................................................... 40 3.5 References ......................................................................................................................... 40 Chapter 4 ....................................................................................................................................... 42 4 Diversity Oriented Synthesis (DOS) ........................................................................................ 42 4.1 Introduction ....................................................................................................................... 42 4.2 Combinatorial Chemistry .................................................................................................. 42 4.3 Principles of Diversity Oriented Synthesis ....................................................................... 43 4.4 Synthetic Strategies ........................................................................................................... 44 4.5 Summary ........................................................................................................................... 47 4.6 References ......................................................................................................................... 48 Chapter 5 ....................................................................................................................................... 50 5 Emergence of Verdazyl Radicals as Substrates for Small Molecule Synthesis ....................... 50 5.1 Beginnings in Living Radical Polymerization (LRP) ....................................................... 50 5.2 Verdazyl Radicals as Precursors to Azomethine Imines .................................................. 52 5.3 Verdazyl Radical-Derived Azomethine Imines in 1,3-Dipolar Cycloadditions ............... 56 5.4 Verdazyl Radicals in the Synthesis
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