Surface‐Improved Asymmetric Michael Addition Catalyzed by Amino Acids Adsorbed on Laponite

Abstract

AbstractAn unprecedented enantioselectivity increase was observed in the presence of inorganic oxides in the asymmetric Michael addition of aldehydes to trans‐β‐nitrostyrene derivatives catalyzed by amino acids. The best results were reached in the l‐proline catalyzed addition of linear aldehydes using laponite RD as additive, approaching those obtained using synthetic chiral organocatalysts. In our study we have shown that the asymmetric Michael addition is catalyzed by the organic‐inorganic hybrid material formed in situ by adsorption of the amino acid on the solid surface. The chiral solid catalyst could be pre‐prepared and recycled several times with maintained stereoselectivities, showing a slow decrease in the activity, which could be restored by adding proline during reuse. Characterization of the chiral hybrid material by FT‐IR spectroscopy and XRD measurements indicated anchoring of the proline on the surface of the laponite particles. Effect of the chiral organocatalyst structure was used to evidence the involvement of both the carboxylic acid and the amino group in anchoring the organic compound on the solid surface. Linear natural amino acids became also active in the asymmetric Michael addition following adsorption on laponite and provided the opposite enantiomer in excess as compared with l‐proline. The heterogeneous catalyst was applied in reactions of various carbonyl compounds and substituted nitrostyrene derivatives. Based on results obtained in this study combined with the previously reported amino acid catalyzed reactions on solid surfaces and the recently revisited reaction mechanism of this asymmetric addition a plausible reaction pathway occurring on the surface was proposed.magnified image