Hydrophobically directed aldol reactions: polystyrene-supported L-proline as a recyclable catalyst for direct asymmetric aldol reactions in the presence of water

Abstract

A simple synthetic methodology for the preparation of a polystyrene- supported L-proline material is reported, and this material has been used as catalyst in direct asymmetric aldol reactions between several ketones and arylaldehydes to furnish aldol products in high yields and stereoselectivities. Screening of solvents showed that these reactions take place only in the presence of water or methanol, at lower levels of conversion in the latter case. This solvent effect, coupled with the observed high stereoselectivities, has been ex- Introduction In the last decade organocatalysis has became a field of great interest.[1] Organocatalysts are metal-free small organic molecules that are able to function as efficient and selective catalysts for a large variety of enantioselective transformations. In this context, -proline and its derivatives have emerged as powerful organocatalysts.[2] -Proline can be regarded as the simplest “enzyme” and it has been successfully applied in many reactions, such as Robinson annulations,[3] aldol reactions,[4] Mannich reactions,[5] Michael reactions,[6] direct electrophilic α-aminations,[7] Diels– Alder reactions,[8] Baylis–Hillman reactions,[9] aza-Morita- Baylis–Hillman reactions,[10] α-selenenylation,[11] oxidation,[ 12] chlorination,[13] and others.[14] Among all these processes, -proline-mediated aldol reactions affording β-hydroxy ketones have been investigated in great depth. Indeed, the aldol reaction is one of the most important C–C bond-formation methods in organic synthesis.[ 15] Proline and its derivatives operate by bifunctional catalysis and play the role of a simplified version of the type I [a] Dipartimento di Chimica Organica “E.Paternò”, Università di Palermo, Viale delle Scienze, Pad. 17, 90128 Palermo, Italy Fax: +39-091-596825 E-mail: mgrutt@unipa.it © 2007 Wiley-VCH Verlag GmbH 4688 & Co. KGaA, Weinheim Eur. J. Org. Chem. 2007, 4688–4698 plained in terms of the formation of a hydrophobic core in the inner surface of the resin, whereas the hydrophilic proline moiety lies at the resin/water interface. Such a microenvironment both promotes the aldol reaction and increases the stereoselectivity. Recycling investigations have shown that this material can be reused, without loss in levels of conversion and stereoselectivity, for at least five cycles.