Mechanism of the Meerwein−Ponndorf−Verley−Oppenauer (MPVO) Redox Equilibrium on Sn− and Zr−Beta Zeolite Catalysts

The mechanism of the Meerwein−Ponndorf−Verley (MPV) reduction of cyclohexanone with 2-butanol catalyzed by Sn−beta and Zr−beta zeolites has been theoretically investigated using density functional theory (DFT) and the cluster approach. An experimental catalytic study has shown that the active sites in the MPV reaction catalyzed by Sn−beta are the same partially hydrolyzed Sn−OH groups that were found to be active for the Baeyer−Villiger (BV) reaction. The computational study indicates that the mechanism of Sn−beta and Zr−beta catalysis is similar, and involves the following steps: adsorption of both the ketone and the alcohol on the Lewis acid center, deprotonation of the alcohol, carbon-to-carbon hydride transfer, proton transfer from the catalyst, and products exchange. As in the aluminum alkoxide catalyzed reaction, the hydride shift occurs through a six-membered transition state, and the role of the hydrolyzed and therefore more flexible M−OH bond is just to facilitate the initial deprotonation of the alcohol.