Competing Enamine/Iminium Pathways in Michael Additions

Significance

Sunoj and Patil have investigated the mechanism of the pyrrolidine-catalyzed Michael addition of propanal to methyl vinyl ­ketone (MVK) using density functional theory (B3LYP and mPW1PW91) as well as ab initio (MP2) methods. Four different activation modes have been taken into account (see Scheme). The direct Michael addition of the enamine derived from propanal and pyrrolidine to MVK assisted by a hydrogen bond donor co-catalyst appears to be the most preferred pathway. The hydrogen bond donor co-catalyst (protic solvent) seems to facilitate both enamine formation and its addition to MVK due to improved transition state stabilization through hydrogen bonding.