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

研究发现，在甲苯溶剂中用硅胶处理烯丙基有机硅烷，可以使其脱烯丙基化，与硅胶表面的硅形成Si-O-Si键，这种Si-O-Si键的形成为硅胶表面的功能化提供了一种新的可靠的方法。与用于改性的常规有机硅化合物（其主要是含有烷氧基离去基团（R_nSiX_<4-n>：X= OR '）的那些）相反，烯丙基硅烷可以在水解条件下处理，并且如果需要，可以通过硅胶色谱法纯化。不管它们的这种稳定性如何，烯丙基硅烷上的脱烯丙基化在高温下发生。将该方法应用于手性膦配体在硅胶表面的固定，并将其用于铑催化的氢化反应和钯催化的不对称烯丙基烷基化反应，显示出很高的对映选择性。回收二氧化硅负载的手性催化剂并重新用于下一次运行。我们已经能够将各种类型的有机分子固定在二氧化硅载体上，并成功地开发了新的催化不对称反应，包括铑催化的不对称1，4-加成反应和钯催化的烯烃不对称硅氢化反应。

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.