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Enantioselective N-Heterocyclic Carbene Catalysis Exploiting β Azolium Ylides
thesis
posted on 2017-01-11, 01:42 authored by Yuji NakanoThe 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
