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Title: Imidazolylidenes in N-heterocyclic carbene organo-catalysis
Author: Williamson, Craig
ISNI:       0000 0001 3569 6026
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2006
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A library of chiral and achiral imidazolium salts was prepared, and evaluated as potential pre-catalysts of the benzoin condensation.  It was found that although the carbenes derived from these compounds possess catalytic activity, this was not as pronounced as that observed for alternative heterocycle classes.  Asymmetric induction, although observed from imidazolylidenes for the first time in this work, is not of an order that could be considered useful, as has been seen for other NHC catalysts.  This is attributed to the use of harsh reaction conditions, required to promote catalytic activity in this system. Two modifications were thus made to the imidazolium salt pre-catalysts in an attempt to make the reaction conditions less extreme.  Imidazolium carboxylates were found to be suitable pre-catalysts, thus allowing the reaction to be performed under neutral conditions.  Purinium salts were also investigated, based on the theory that these more electron deficient fused heterocycles would allow access to carbenes under milder conditions.  Although problems were encountered in the synthesis of these salts, one example was prepared, however the available data indicate this exists as the incorrect regioisomer. Two strategies for catalyst immobilisation have also been investigated.  It was found that polymer supported imidazolylidenes are capable of catalysing the benzoin condensation of a range of aromatic aldehydes, under a range of conditions, and can undergo a limited number of recycles.  Imidazolium salts supported on silica were found to be unsuitable for the catalysis of this reaction.  This was blamed on incompatibility of the persistently acidic matrix with the basic conditions required by the carbene. Application of imidazolylidenes to the intra-molecular Stetter reaction was also attempted, however this revealed NHC have the ability to act catalytically as bases.  Further investigation of this observation suggested the effect is limited to α,β-unsaturated carbonyl compounds.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available