Development of Silica-Supported Chiral Transition Metal Catalysts and Their Application to Asymmetric Catalysis

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We found that treatment of (allyl) organosilanes with silica gel in refluxing toluene brought about deallylation forming Si-O-Si bond with the silicon on the silica gel.This Si-O-Si bond formation provides us with a new reliable method for the functionalization of silica gel surface. Contrary to the conventional organosilicon compounds used for the modification, which are mostly those containing an alkoxy leaving group (R_nSiX_<4-n>: X=OR'), the allylsilanes can be handled under hydrolytic conditions and can be purified by silica gel chromatography if necessary. Regardless of their such stability, deallylation on the allylsilane takes place at the high temperature. This allylsilane method was applied to the immobilization of chiral phosphine ligands on silica gel surface and they were used for rhodium-catalyzed hydrogenation and palladium-catalyzed asymmetric allylic alkylation to show high enantioselectivity. The silica-supported chiral catalysts were recovered and reused for the next run. We are now in a position to immobilize various types of organic molecules on the silica support.We have also succeeded in developing new catalytic asymmetric reactions including rhodium-catalyzed asymmetric 1,4-addition and palladium-catalyzed asymmetric hydrosilylation of olefins.