Comparison of Ta-MCM-41 and Ti-MCM-41 as catalysts for the enantioselective epoxidation of styrene with TBHP

Chiral Ti-MCM-41 and Ta-MCM-41 catalysts have been prepared by grafting of Ti(OiPr)4 and Ta(OEt)5 and the modification with R-(+)-diethyl l-tartrate or R-(+)-diisopropyl l-tartrate. In general, the solid catalysts are more active and selective than their homogeneous counterparts in the epoxidation of styrene with tert-butyl hydroperoxide. The enantioselectivities depend on both the nature of the chiral ligand and the calcination temperature of the support, as it is supposed this controls the type of surface species that are formed. The best result of 71% ee is obtained with DIPT-Ta-MCM550 and is the first example of the use of a Ta catalyst for the enantioselective epoxidation of unfunctionalized alkenes. Nonetheless, the recovered Ta catalysts are less active and selective.