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N-Heterocyclic carbene-catalysed transformations : the crossed benzoin condensation and oxidative aldehyde esterifications
Delany, Eoghan
THESIS 11267 The focus of this thesis is the development of a suite of heteroazolium ion-based salts as precursors to N-heterocyclic carbene (NHC) catalysts for a range of transformations involving aldehyde substrates. It has been demonstrated that, under basic conditions, a novel triazolium ion-based precatalyst can promote the chemoselective crossed benzoin condensation between ortho-substituted benzaldehydes and a range of aromatic aldehyde coupling partners in excellent yield. The chemoselectivity of the methodology was found to be reliant on a careful balance of catalyst and substrate control, with an ortho-bromine atom serving as a temporary directing group to aid the catalyst in differentiating between the two aromatic aldehydes. It has then been shown how this ortho-bromine atom can be conveniently and quickly removed via transition metal-catalysed reduction (to allow access to a further range of cross-benzoin adducts) or utilised as a functional handle for further transformations. Extension of this methodology to develop the first asymmetric crossed benzoin condensation between two disparate aromatic aldehydes revealed how the resulting cross-benzoin products are subject to base- and carbene-mediated racemisation under the reaction conditions. It was demonstrated that the optical purity of these adducts is negatively impacted by these pathways and how, through careful optimisation of the reaction conditions and employment of carefully selected ortho-substituted aromatic aldehydes, the first ever highly enantioselective crossed benzoin condensation was subsequently reported. Finally, a highly efficient, broad scope, additive-free mild protocol for the oxidative carbene-catalysed esterification of aldehydes has been developed. Benzoin (and neither the Breslow intermediate nor the NHC?aldehyde tetrahedral adduct) has been unambiguously identified as the oxidised species in aerobic NHC-catalysed aldehyde esterifications. The first organocatalytic aerobic oxidative cleavage of cyclic 1,2-diketones is reported. These reactions proceed in either aqueous or alcoholic media and are promoted by a simple N-heterocyclic carbene catalyst derived from a 1,2,4-triazolium ion.
Keyword(s): Chemistry, Ph.D.; Ph.D. Trinity College Dublin
Publication Date:
2016
Type: Doctoral thesis
Peer-Reviewed: Unknown
Language(s): English
Institution: Trinity College Dublin
Citation(s): Eoghan Delany, 'N-Heterocyclic carbene-catalysed transformations : the crossed benzoin condensation and oxidative aldehyde esterifications', [thesis], Trinity College (Dublin, Ireland). School of Chemistry, 2016, pp.274
Publisher(s): Trinity College (Dublin, Ireland). School of Chemistry
Supervisor(s): Connon, Stephen
First Indexed: 2018-06-22 06:38:16 Last Updated: 2019-11-29 07:09:02