Enantioselective N-Heterocyclic Carbene Catalysis Exploiting β Azolium Ylides

The 1,4-addition of N-heterocyclic carbenes (NHCs) onto α,β-unsaturated ketones and esters provides an enolate that, upon tautomerisation, reveals a β-azolium ylide intermediate whereby the formerly electrophilic site is rendered a newly nucleophilic center in an umpolung event. Previous studies had demonstrated that the β-azolium ylide could undergo cyclisation or dimerisation events. This thesis extends this underdeveloped area of NHC catalysis through the development of novel reactivity exploiting the β-azolium ylide, with an emphasis on transformations that can be achieved enantioselectively. <br> The first chapter provides an overview of the reactions which NHCs facilitate by interactions of the carbene with carbon-carbon unsaturated bonds. An emphasis is placed on catalysis; with two reactive intermediates commanding the focus of this chapter. <br> In the second chapter the development of an NHC-catalysed cycloisomerisation reaction is detailed. This discovered transformation provides the first entry into enantioselective catalysis exploiting the β-azolium ylide intermediate. Alcoholic additives proved to be integral to the stereoselectivity of the transformation, while in their absence an autocatalytic pathway was implied. <br> Chapter three details the discovery of an enantioselective (5 + 1) annulation reaction under NHC catalysis. The serendipitous discovery of this transformation, and subsequent attempts at intercepting the implicated β-azolium ylide intermediate, are presented. The reaction represents the second enantioselective transformation proceeding by the β-azolium ylide. <br> The fourth chapter in this thesis presents the efforts made into expanding the substrates able to be utilised in accessing the β-azolium ylide intermediate. Through these studies it was uncovered that an α-substituted acrylate acceptor was a viable substrate in providing the β-azolium ylide, and could subsequently be exploited in cyclisations to furnish cyclopentene and indene structures. <br> Finally, the fifth chapter provides details on experimental procedures utilised in the presented studies, and the spectroscopic data obtained from the compounds derived from the preceding chapters