Role of aggregates in Claisen acylation reactions of imidazole, pyrazole, and thioesters with lithium enolates in THF.

[formula: see text] Although phenyl esters react with both monomers and dimers or tetramers of two lithium enolates in THF, the reactions of phenyl thiobenzoates are relatively much faster with the monomers. Similarly, imidazole esters react primarily with the monomers but pyrazole esters react with monomers and aggregates. The results are rationalized by a mechanism in which coordination with two lithium cations within an enolate aggregate is required for the reaction of aggregates to compete with monomers.