Selectivities of Diels−Alder Reactions Catalyzed by Highly Acidic Boronated Aluminas

Abstract

A series of boronated aluminas was prepared by the reaction of BX3 (X = F, Cl, Br) with alumina or by a two-step synthesis involving the reaction of BH3·Me2S with alumina and then with I2. The modified aluminas, BXn/Al2O3, were found to have n = 3 (F), n = 1.5 (Cl), n = 1 (Br), and n = 1 (I). The acidity of the solids (indicator dyes) was large and decreased in the order I ∼ Br > Cl > F. The solids catalyzed the Diels−Alder reactions of methyl acrylate with cyclopentadiene and isoprene with methyl acrylate, both of which occurred in high yield and with high selectivity. The regioselectivity of the isoprene/methyl acrylate reaction in fact is higher than seen previously. The results of the heterogeneous reactions were compared to those catayzed by B-bromo- and B-chlorocatecholborane, which serve as models for the boronated aluminas.