Design of mesoporous aluminosilicates supported (1 R,2 S)-(−)-ephedrine: evidence for the main factors influencing catalytic activity in the enantioselective alkylation of benzaldehyde with diethylzinc

Abstract

(−)-Ephedrine, used as a model β-amino alcohol, was covalently anchored on mesoporous micelle templated aluminosilicates (Al-MTS) through nucleophilic substitution of halogenoalkyl(aryl)silane chains previously grafted on the surface. The covalent grafting was performed either by silylation (method a) or by surface sol–gel (method b). The latter method provided higher loading. However, the higher loading lowers the resulting mesoporous volume. The coupling alkyl halide moiety was then substituted with (1 R,2 S)-(−)-ephedrine. Used as chiral auxiliaries in the heterogeneous enantioselective catalysis of the alkylation of benzaldehyde by diethylzinc, these materials showed properties which depend mainly on the grafting method. The best results (activity, enantioselectivity) were obtained with catalysts prepared from supports featuring high initial pore diameter. The effect of the regular porosity on the efficiency and enantioselectivity was shown. Dilution of catalytic sites by alkyl groups and rigidification of the linker were also studied.