Organorhodium-Functionalized Periodic Mesoporous Organosilica: High Hydrophobicity Promotes Asymmetric Transfer Hydrogenation in Aqueous Medium

Three organosilica-bridged periodic mesoporous organosilicas were prepared by the immobilization of a chiral N-sulfonylated diamine-based organorhodium complex within their silicate network. Structural analysis and characterization confirmed their well-defined single-site active rhodium centers, whilst electron microscopy revealed their highly ordered hexagonal mesostructures. Among these three different organosilica-bridged periodic mesoporous organosilicas, the ethylene-bridged periodic mesoporous organosilica catalyst exhibited excellent heterogeneous catalytic activity and high enantioselectivity in the aqueous asymmetric transfer hydrogenation of aromatic ketones. This superior catalytic performance was attributed to its salient hydrophobicity, whilst its comparable enantioselectivity relative to the homogeneous catalyst was derived from the confined nature of the chiral organorhodium catalytic sites. Furthermore, this ethylene-bridged periodic mesoporous organosilica could be conveniently recovered and reused at least 12times without the loss of its catalytic activity. This feature makes this catalyst attractive for practical organic synthesis in an environmentally friendly manner. This study offers a general way of optimizing the bridged organosilica moiety in periodic mesoporous organosilicas, thereby enhancing its catalytic activity in heterogeneous catalysis.