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 β-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 β-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.
Graphical abstractl-Proline-anchored hydrotalcite catalyst was tested for the asymmetric Michael additions between nitroalkene and acetone and between α,β-unsaturated ketones and nitroalkanes. This chiral amino catalyst acts as a better catalyst for both enamine types as well as for iminium-type Michael additions. Suitable mechanisms are proposed. Figure optionsDownload full-size imageDownload as PowerPoint slide
Journal: Applied Catalysis A: General - Volume 340, Issue 1, 15 May 2008, Pages 25–32