Insight into the mechanism of the Michael addition of malononitrile to α,β-unsaturated imides catalyzed by bifunctional thiourea catalysts

Michael addition of malononitrile to an alpha,beta- unsaturated imide catalyzed by a bifunctional-thiourea catalyst, which represents a prototype of catalytic asymmetric C-C forming reactions, has been investigated by performing density functional theory calculations. Two enantioselective reaction channels, denoted as (R)- and (S)-channels, which refer to the (R)- and (S)-configurations of the Michael adducts, have been explored in detail. It was shown that the formation of the Michael adduct involves three elementary steps: the catalyst protonation, the C-C bond coupling, and the catalyst deprotonation. The C-C bond coupling step has been identified as the energetic bottleneck along each channel. The calculated barrier for each of the three elementary steps along the (R)-channel is considerably less energy demanding than the corresponding those along the (S)-channel. Factors contributing to the high enantioselectivity have been clarified. The present results provided by our calculations are in agreement with all experimental findings and allow a detailed and consistent view of the mechanistic details of this important reaction sequence. (C) 2008 Elsevier Ltd. All rights reserved.