Mesoporous ethane-silicas functionalized with trans-(1R,2R)-diaminocyclohexane: Relation between structure and catalytic properties in asymmetric transfer hydrogenation

Abstract

trans-(1R,2R)-Diaminocyclohexane (DACH) functionalized mesoporous ethane-silicas with different porous structures were synthesized by addition of 1,3,5-trimethylbenzene (TMB) during co-condensation of 1,2-bis(trimethoxysilyl)ethane and N-[(triethoxysilyl)propyl]-(–)-(1R,2R)-diaminocyclohexane in the presence of octadecyltrimethylammonium chloride under basic conditions. The addition of TMB can not only expand the pore diameter, but also induce the structural transformation from the ordered 2D-hexagonal mesostructure to the mesocellular foam-like (MCF-like) structure and the mixture of vesicle and worm-like structure. The DACH functionalized materials with different porous structures (after complexing with [Rh(cod)Cl]2) were found to be active catalysts for the asymmetric transfer hydrogenation of acetophenone. The material with MCF-like structure exhibits the highest initial rate among all the materials tested. The catalytic activity of the solid catalysts could be greatly enhanced through adjusting the porous structure of the materials.