Synthesis Of Periodic Mesoporous Organosilicas Functionalized With Trans-(1R, 2R)-Diaminocyclohexane For Asymmetric Catalysis

Chiral periodic mesoporous organosilicas (PMOs) represent a novel class of chiral solids. The various weak interactions existing in the nanopores of mesoporous materials may enhance the catalytic activity and enantioselectivity in the chiral catalysis. The investigation of chiral catalysis in mesopores provides new opportunities for the design and synthesis of novel kinds of chiral solids for heterogeneous asymmetric catalysis and chiral separation.The work is focused on the synthesis of PMOs functionalized with trans-(1R,2R)-diaminocyclohexane (DACH). Using the asymmetric transfer hydrogenation (ATH) of acetophenone as a model reaction, the influences of the surface properties, the mesostructures and the linkers of DACH on the catalytic performance of the materials were studied. The post-modification method was also employed to increase the chiral inductivity of the chiral materials. Periodic mesoporous ethanesilicas with DACH protruding in the channel were synthesized using ionic surfactant as template under basic conditions. These novel materials exhibit enhanced catalytic activity compared to the material with pure-silica framework, which is mainly attributed to the enhanced surface hydrophobicity derived from the ethane-bridged mesoporous network. The mesoporous material with cellular foam mesostructure exhibits faster reaction rate than that with MCM-41 type mesostructure, probably due to the faster diffusion rate of the reactants through the three-dimensionally connected cellular foam structure.Large-pore mesoporous ethanesilicas with DACH protruding in the channel were synthesized in acidic medium using triblock copolymer as template. Thepost-modification of DACH in the mesopores by reacting with p-toluenesulfonyl chloride results in the material with higher ee (68%) than the original material (38%) in the ATH of acetophenone. The highest enantioselectivity of 81% was observed in the ATH of 2-acetylnaphthalene.Mesoporous silicas with DACH bridged in the framework were synthesized using cationic surfactant as template under basic conditions. The material with benzyl group as the linker shows higher catalytic activity and enantioselectivity than the material with propyl group as the linker, which is mainly due to the spacial rigidity and electron-withdrawing ability of the benzyl group.