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

A simple synthetic methodology for the preparation of a polystyrene-supported L-proline material is reported, and thismaterial has been used as catalyst in direct asymmetric aldolreactions between several ketones and arylaldehydes to furnishaldol products in high yields and stereoselectivities.Screening of solvents showed that these reactions take placeonly in the presence of water or methanol, at lower levels ofconversion in the latter case. This solvent effect, coupledwith the observed high stereoselectivities, has been ex-IntroductionIn the last decade organocatalysis has became a field ofgreat interest.[1] Organocatalysts are metal-free small organicmolecules that are able to function as efficient andselective catalysts for a large variety of enantioselectivetransformations. In this context, -proline and its derivativeshave emerged as powerful organocatalysts.[2] -Prolinecan be regarded as the simplest “enzyme” and it has beensuccessfully applied in many reactions, such as Robinsonannulations,[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 reactionsaffording β-hydroxy ketones have been investigatedin great depth. Indeed, the aldol reaction is one of the mostimportant C–C bond-formation methods in organic synthesis.[15] Proline and its derivatives operate by bifunctional catalysisand play the role of a simplified version of the type I[a] Dipartimento di Chimica Organica “E.Paternò”, Università diPalermo,Viale delle Scienze, Pad. 17, 90128 Palermo, ItalyFax: +39-091-596825E-mail: mgrutt@unipa.it© 2007 Wiley-VCH Verlag GmbH 4688 & Co. KGaA, Weinheim Eur. J. Org. Chem. 2007, 4688–4698plained in terms of the formation of a hydrophobic core inthe inner surface of the resin, whereas the hydrophilic prolinemoiety lies at the resin/water interface. Such a microenvironmentboth promotes the aldol reaction and increases thestereoselectivity. Recycling investigations have shown thatthis material can be reused, without loss in levels of conversionand stereoselectivity, for at least five cycles.