Enantioselective epoxidation of β-methylstyrene catalyzed by immobilized Mn(salen) catalysts in different mesoporous silica supports

Mesoporous silica-supported chiral Mn(salen) catalysts were prepared and evaluated in the heterogeneous asymmetric epoxidation of 15-methylstyrene with NaClO as an oxidant. Homogeneous and immobilized Mn(salen) catalysts exhibit similar cis/trans ratios and ee values when trans-substrate is used but different cis/trans ratios and ee values when cis-substrate is used. The production of trans-epoxides is favored in the heterogeneous asymmetric epoxidation of cis-beta-methylstyrene, whereas the production of cis-epoxides is favored in homogeneous reaction. In addition, the cis/trans ratio and ee value of transepoxides produced from cis-beta-methylstyrene change sequentially with changes in support materials, but the ee value of cis-epoxides does not. The textural properties of immobilized Mn(salen) catalysts can be sequentially adjusted by changing pore dimension and channel length, after which the motion restriction and confinement effect in nanochannels of immobilized Mn(salen) catalysts can be adjusted sequentially. Our results reflect that the collapse step of trans-intermediates is considerably affected by the confinement effect. whereas the rotation step of cis-intermediates is greatly influenced by motion restriction. The motion restriction in nanochannels increases the likelihood of rotation for the C-C single bond of cis-intermediates and favors the production of trans-epoxides. (c) 2008 Published by Elsevier Inc.