Mesoporous ethane-silicas functionalized with trans-(1R,2R)-diaminocyclohexane as heterogeneous chiral catalysts

Mesoporous ethane-silicas functionalized with trans-(1R,2R)-diaminocyclohexane were synthesized by one-step co-condensation of 1,2-bis(trimethoxysilyl)ethane and N-[(triethoxysilyl)propyl]-(-)-(1R,2R)-diaminocyclohexane using octadecyltrimethylammonium chloride as the template under basic conditions. For comparison, the mesoporous silica containing trans-(1R,2R)-diaminocyclohexane was also synthesized. The structural characterizations (X-ray powder diffraction, transmission electron microscopy, and N-2 sorption isotherms) show that the two materials have ordered two-dimensional-hexagonal mesostructure and wormhole-like structure, respectively. The integrity of the organic groups in the mesoporous materials is confirmed by Fourier transform infrared and solid-state C-13 and Si-29 cross-polarization magic-angle-spinning NMR spectroscopies. Mesoporous materials are used as catalysts for the asymmetric transfer hydrogenation of acetophenone after complexing [Rh(cod)Cl](2) to the chiral ligands, trans-(1R,2R)-diaminocyclohexane, in the mesopores. The catalyst with the ethane-bridged framework exhibits 82-96% conversion with 19-23% enantiomeric excess (ee), while the catalyst with pure-silica framework shows 48% conversion with 14% ee under identical reaction conditions. The enhanced catalytic activity of the mesoporous organosilicas is mainly caused by the specific adsorption and physical properties of the mesoporous network bridged with ethane groups, particularly the hydrophobic properties. The enantioselectivity, albeit not yet high enough, demonstrates the possibility for synthesizing a new kind of chiral solid catalysts for potential applications in asymmetric reactions.