Catalyst Design for Alkene Epoxidation by Molecular Analogues of Heterogeneous Titanium-Silicalite Catalysts

The epoxidation of allylic alcohols with H2O2 catalyzed by the hybrid [α-B-SbW9O33(tBuSiO)3Ti(OiPr)]3- (1) anion as molec-ular model of heterogeneous Ti-silicalite TS-1 catalyst was analyzed by means of DFT to determine the main factors that control the catalytic process and, finally, to improve the value of the available catalysts. Our calculations revealed that unlike other alkenes, allylic alcohols can bind the Ti center after activation of the pre-catalyst via hydrolysis to give the correspond-ing Ti-alcoholate, which is the catalyst resting state. Next, the dissociative addition of hydrogen peroxide to Ti causes the cleavage of a Ti—OSi junction to form a Ti(η2-OOH) moiety. The partial detachment of the Ti from the catalyst structure yields an intermediate with a flexible Ti center from which the Ti-OOH group can transfer an electrophilic oxygen to the alkene substrate in an inner-sphere fashion. The rate determining process, which involves the heterolytic activation of H2O2 over the Ti(IV) and the e...