E-Selective N-heterocyclic carbene-catalyzed reaction of aldehydes and butadienoates: effect of water and chloroform as the proton shuttle

Abstract

As we know, there are many reports on the reactions of the Breslow intermediate with electron-deficient alkenes or alkynes. However, the reactions with allenes are much less studied. In this study we observed that the N-heterocyclic carbene-catalyzed reaction of aldehydes and butadienoates afforded (E)-4-oxo-2-butenoates highly stereoselectively. It was observed that addition of water is critical for the observed high stereoselectivity. Mechanistic studies showed that the solvent chloroform acted as the proton shuttle involved in the formation of the Breslow intermediate, protonation, and deprotonation for this carbene-catalyzed reaction. Some key intermediates such as the Breslow intermediate and its monohydrate have been detected and characterized by MS studies, supporting the proposed mechanism.