Chiral lewis acids supported on silica gel and alumina, and their use as catalysts in Diels-Alder reactions of methacrolein and bromoacrolein

Abstract

Several derivatives of ( S)-tyrosine are supported on silica gel through the phenolic oxygen atom. The Lewis acids obtained by treatment of these solids with BH 3 are able to promote the reactions of methacrolein and bromoacrolein with cyclopentadiene, but they do not provide asymmetric induction. ( S)-Prolinol is supported on silica gel and alumina through the nitrogen atom, and the solids obtained by treatment with AlEtCl 2 catalyse the reaction between methacrolein and cyclopentadiene, leading to 8% enantiomeric excess (ee). The best results in these reactions (until 31% ee) are obtained by using (−)-menthol-aluminium Lewis acids supported on silica gel and alumina through the aluminium atom. The supported catalysts are compared with the analogous homogeneous Lewis acids.