Aza-Morita-Baylis-Hillman-Type Products via Enamine Catalysis

Significance

The authors describe an approach for the α-substitution on α,β-unsaturated aldehydes. Unlike the Morita-Baylis-Hillman (MBH) reaction, which gives the same class of products, the catalyst is not assumed to act via a Michael-type 1,4-addition, but via an enamine mechanism in which the carbonyl group is attacked. Enamine A undergoes a Mannich reaction with the imine to yield iminium ion B. If only proline is used as catalyst, isomerization of the double bond occurs spontaneously. However, it is shown that the addition of imidazole enhances the reaction rate, which is reasoned to be due to facilitation of the isomerization step. However, imidazole might also play a role in the catalytic cycle itself (see references in the comment).