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 a molecular 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 precatalyst via hydrolysis to give the corresponding 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 electrophilic O-transfer, accounts for an overall free-energy barrier of 23.0 kcal mol^-1 for 2-methyl-2-buten-1-ol, in line with the experimental value of 22.3. Conversely, the outer-sphere O-transfer - also accessible to nonfunctionalized alkenes - occurs through a more strained transition state that lays above in energy (by ∼4 kcal mol^-1), giving a clue to explain the low yields reported experimentally for nonfunctionalized olefins. We also found that reducing the bulkiness of the substituents in the silanol functions of the catalyst has a positive influence on the catalytic activity, decreasing the overall free-energy barriers for the outer-sphere path. With this knowledge, we developed other catalytic species with tailored steric properties based on [SbW₉O₃₃(RSiOH)₃]^3- structure (R =ⁱPr and ⁿPr), which were synthesized, characterized, and successfully applied to the catalytic epoxidation of unfunctionalized alkenes. Present results clearly show that the detailed knowledge of the reaction mechanisms, even for complex processes, is possible nowadays and that the acquired information allows designing catalysts with desired activities.