Chirally functionalized hollow nanospheres containing L-prolinamide: synthesis and asymmetric catalysis.

Chirally functionalized hollow nanospheres with different surface properties were successfully synthesized by co-condensation of (2S,1'R,2'R)-N-tert-butyloxycarbonylpyrrolidine-2-carboxylic acid [2'-(4-trimethoxysilylbenzylamide)cyclohexyl] amide with 1,2-bis(trimethoxysilyl)ethane or tetramethoxysilane using F127 (EO(106)PO(70)EO(106)) as surfactant in water. The TEM and N(2) sorption characterizations show that the particle size of the hollow nanosphere is 15-21 nm with a core diameter of 10-16 nm. These L-prolinamide-functionalized hollow nanospheres are highly efficient solid catalysts for the direct asymmetric aldol reaction between cyclohexanone and aromatic aldehydes. It was found that the addition of water in the reaction system not only enhanced the catalytic activity but also increased the enantioselectivity, which is probably due to the enhanced hydrogen bond between the amide oxygen atom and the hydroxyl group of water. Moreover, the catalytic activity increases sharply as the surface hydrophobicity of the hollow nanospheres increases. These hollow nanospheres are quite stable and can be reused with almost the same enantioselectivity and only a slight decrease in catalytic activity.