Immobilization of rhodium-based transfer hydrogenation catalysts on mesoporous silica materials

A series of chiral heterogeneous rhodium catalysts obtained via immobilization of chiral N-sulfonylated diamine-based organorhodium complexes within mesoporous silicate networks have been obtained through the postgrafting, postmodification, and co-condensation strategies. Structural analyses and characterizations disclose their well-defined single-site rhodium species within materials, while electron microscopy images reveal their highly ordered dimensional-hexagonal mesostructures. By systemically comparing these prepared strategies, it is found that they exhibit obviously different catalytic activities and enantioselectivities in aqueous asymmetric transfer hydrogenation of aromatic ketones. The direct anchoring of chiral organorhodium complexes onto the outside surface of mesoporous silica can maintain high enantioselectivity, while the co-condensation of chiral organorhodium complexes into the inside surface of mesoporous silica can form a uniform distribution of active rhodium centers. Both strategies show higher catalytic efficiency than the postmodification strategy in enantioselective performance. This outcome from the study clearly demonstrates the nature of these heterogeneous catalysts based on different immobilization strategies and offers a general way to optimize the prepared strategy to adjust the catalytic efficiency of heterogeneous catalysts. (c) 2012 Elsevier Inc. All rights reserved.