Polymer-supported Al and Ti species as catalysts for Diels-Alder reactions

Abstract

Several aminoalcohols and tartaric acid derivatives have been grafted to polystyrene resins. These polymers have been transformed into supported chiral Lewis acids by treatment with EtAlCl2 or, in the case of the diols derived from tartaric acid, with TiCl2(OPri)2 and Et2AlCl. All these solids, as well as the non-chiral ones prepared from simple hydromethyl resins, are efficient catalysts in the Diels-Alder reaction of cyclopentadiene and dienophiles like methacrolein or acryloyl and crotonyl-1,3-oxazolidin-3-ones. In some cases the supported catalysts are much more active that the homogeneous analogues. Some of the catalysts can be recovered and reused without metal leaching, keeping an important catalytic activity and without changes in selectivity. The enantioselectivities obtained are minor than 20% ee, being lower than the ones attained with the homogeneous analogues, which shows that the polymer does not behave as a simple inert support.