N-heterocyclic carbene catalyzed carba-, sulfa-, and phospha-Michael additions with NHC·CO₂ adducts as precatalysts

J Org Chem. 2014 Mar 21;79(6):2758-64. doi: 10.1021/jo500108a. Epub 2014 Mar 10.

Abstract

N-heterocyclic carbene catalyzed Michael additions have been revisited with 1,3-dialkyl- or 1,3-diarylimidazol(in)ium-2-carboxylates, that is, NHC·CO2 adducts, as the source of the free NHC catalysts in solution. Using these precatalysts, a number of efficient carba-, sulfa-, and phospha-Michael additions were achieved very conveniently, without the need for an external strong base to generate the NHC by deprotonation of an azolium salt. To further expand the scope of the procedure, some NHC-catalyzed sulfa-Michael/aldol organocascades were also investigated.