Mesoporous Silica Nanosphere-Supported Chiral Ruthenium Catalysts: Synthesis, Characterization, and Asymmetric Hydrogenation Studies

Chiral RuCl2-diphosphine-diamine complexes with a pendant siloxy group were synthesized and characterized by NMR spectroscopy and mass spectrometry. The Ru complexes were grafted onto three different types of mesoporous silica nanospheres (MSNs) through the siloxane linkage, and the resulting MSN-supported Ru catalysts were highly active for the asymmetric hydrogenation of aromatic ketones, to afford chiral secondary alcohols, and racemic arylaldehydes, to give chiral primary alcohols. Excellent activities and enantioselectivities were observed for these MSN-supported Ru catalysts owing to readily accessible and uniform catalytic sites within the large channels of MSNs (diameters of > 2 nm) and short diffusion lengths for the organic compounds as a result of small nanoparticle sizes of less than 1 mu m. This catalyst immobilization strategy with MSN supports should be amenable to the design of many highly active and enantioselective heterogeneous asymmetric catalysts.