Mukaiyama Aldol Reactions Catalyzed by N-Heterocyclic Carbenes

Significance

A highly efficient organocatalytic protocol for Mukaiyama aldol reactions of trimethyl­silyl ketene acetals as well as trimethylsilyl enol ethers with various, mostly aromatic aldehydes is described. In the presence of only 0.5 mol% of bis(adamantyl)-substituted imidazol-2-ylidene, aldol products are obtained in moderate to high yields (25-91%). In addition, the activated ketone 2,2,2-trifluoroacetophenone was also found to undergo aldol reactions in high yields. The authors propose that the N-heterocyclic carbene (NHC) acts as a carbon-centered nucleophilic catalyst activating the Si-O bond of enoxysilanes through the formation of a pentavalent silicon complex. Furthermore, it was shown that NHCs show a better performance compared to commonly employed P- or N-centered Lewis base catalysts.