L-proline anchored hydrotalcite clays: An efficient catalyst for asymmetric Michael addition

L-Proline has been immobilized onto the inter layers of hydrotalcite clay. This L-proline-anchored hydrotalcite (HTLP) has good catalytic activity in the asymmetric Michael addition reaction between P-nitrostyrene and acetone. An inversion in the asymmetric induction is observed when compared to the reaction using pure L-proline catalysis. The same Michael adduct has been prepared by two methods: first, by addition of acetone to P-nitrostyrene (an enamine-type addition) and secondly, by addition of nitromethane to benzylideneacetone (an iminium-type addition). This chiral amino catalyst (HTLP) acts as a better catalyst for both enamine-type and iminium-type additions, indicating clearly that hydrotalcite is a better support for L-proline. It also acts as a cocatalyst in the title reaction. Suitable mechanisms are proposed for both enamine- and iminium-type additions. (C) 2008 Elsevier B.V. All rights reserved.