Thiazolium salts carbonyl condensation reactions

Studies on thiamine (vitamin Bi) catalyzed formation of acyloins from aliphatic aldehydes and on thiamine or thiamine diphosphate catalyzed decarboxylation of pyruvate have established the mechanism for the catalytic activity of 1,3-thiazolium salts in carbonyl condensation reactions. In the presence of bases, quaternary thiazolium salts are transformed into the ylide structure (2), the ylide being able to exert a cat ytic effect resembling that of the cyanide ion in the benzoin condensation (Scheme 2). Like cyanide, the zwitterion (2), formed by the reaction of thiazolium salts with base, is nucleophilic and reacts at the carbonyl group of aldehy s. The resultant intermediate can undergo base-catalyzed proton... 

In early studies of this chemistry we had examined the ability of such thiazolium salts to catalyse the benzoin condensation, a process which also formally involves an acyl anion but which is really of course the anion in which the thiazolium salt has been added to the carbonyl group (Figure 2.10). In this sense the thiazolium anion is very much like cyanide anion, the normal catalyst for simple benzoin condensations. Benzaldehyde would be expected to bind into a j8-cyclodextrin cavity, so we attached a thiazolium salt to a primary carbon of j8-cyclodextrin and examined it as a catalyst. We found that this was not a better catalyst for the benzoin condensation, apparently because there was no room in the j8-cyclodextrin cavity for the binding of two benzaldehyde molecules. However it was clear that at least the reaction intermediate was being formed we got very rapid tritium exchange from the aldehyde by formation of the thiazolium adduct, and as well a very rapid oxidation of para-f-butyl benzaldehyde by ferricyanide ion since it was able to oxidize the reaction intermediate formed when the bound t-butyl benzaldehyde underwent addition of the thiazolium ring. 

N-Heterocychc carbenes (NHCs) derived from thiazolium salts (e.g., X) have been utihzed as catalysts in benzoin condensations, in additions of aldehydes to enones (Stetter reaction [17]) and recently in the reaction of acylsilanes with a, p-unsaturated carbonyl compounds (sila-SrEiTER reaction [18]), as exemplified in the following sequence of reactions ... 

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