Chiral Mesoporous Silicates Immobilizing Titanium Dioxide for Catalytic Asymmetric Epoxidation of Alkenes

Two kinds of titanium-containing silicates were prepared by doping of titanium salt and chiral additive in a sol-gel process, which were further modified by chiral sulfonyl chloride in order for catalytic asymmetric epoxidation of alkenes. These titanium-containing materials had good porosities, ordered pore size distributions, high titanium incorporation yields, ordered morphologies, as well as internal chiral configurations. Particularly, they contained a lot of titanium dioxide particles according to TEM. In catalysis, the titanium silicates showed good conversion of alkenes, satisfactory yields and e.e. values of epoxides. The chiral inducing synergy appeared between silicate matrix and attached ligand, which was significant for transformations of styrene and α-methylstyrene. Moreover, iodosylbenzene, tert-butyl hydroperoxide and hydrogen peroxide were all promising oxidants. In addition, the present titanium catalysts showed satisfactory recycling behaviors and chemical stabilities. This work would contribute to the design of efficient chiral catalysts.