Chiral ruthenium catalyst immobilized within magnetically retrievable mesoporous silica microcapsules for aqueous asymmetric transfer hydrogenations

The preparation of magnetically separable silica microcapsules that incorporate in their inner shell a chiral catalyst and their application in asymmetric transfer hydrogenation reactions are described. The preparation method is based on the emulsification of an oil phase containing chloroform, a modified Noyori Ru-TsDPEN catalyst, tetraethoxysilane (TEOS), and hydrophobic magnetic nanoparticles in water in the presence of an appropriate surfactant, followed by an interfacial polycondensation process under basic conditions to generate a silica shell around the oil droplets. The resulting catalytic microreactors can be considered a "quasi-homogeneous" system because the immobilized chiral catalyst reacts in a homogeneous zone, the microcapsule core filled with an organic solvent. The catalytic activity was tested in the asymmetric transfer hydrogenation of ketones in an aqueous medium. The catalytic reactions took place only in the presence of surfactants. In addition, the judicious selection of the surfactant plays a crucial role in enhancing the reaction progress through the emulsion-solid transfer (EST) approach. The catalytic activity of the Ru-TsDPEN catalyst immobilized within the silica microcapsules was superior to the same catalyst supported on silica microspheres or linked to the backbone of a silica sol-gel matrix, which indicates the importance of the homogeneous zones for the reactions. We report the design of a "quasi-homogeneous" catalytic system, based on a modified Noyori Ru-TsDPEN catalyst confined within mesoporous silica-based microreactors, for the efficient asymmetric transfer hydrogenation of ketones in water. The new system demonstrates excellent reactivity and enantioselectivity characteristic of homogeneous catalysts, but can be easily recovered.