Water-resistant solid Lewis acid catalysts: Meerwein–Ponndorf–Verley and Oppenauer reactions catalyzed by tin-beta zeolite

Abstract

The catalytic activity of Sn-beta zeolite in the Meerwein–Pondorf–Verley (MPV) reduction of carbonyl compounds with secondary alcohols as reductants and Oppenauer oxidation of alcohols were performed with quantitative yields to the corresponding product. This catalyst exhibits a behavior not observed with Ti- and Al-beta zeolites, with excellent activity and selectivity even after four catalytic recycles, and good stereoselectivities to the thermodynamic less favorable cis-alcohol isomer when alkyl-cylohexanones were used as substrates. A prochiral ketone has been reduced within an enantiomeric excess close to 50% when using a chiral alcohol as the reducing reactant. Other transition metals incorporated in the beta zeolite framework (V or B) led to poor catalytic activities for this reaction. IR studies using cyclohexanone as probe molecule over beta zeolites demonstrated that the more specific Lewis acid sites in the framework of Sn-beta are responsibles for its better catalytic activity with respect to Ti- or Al-beta. The influence of the structure of the alcohol and ketone shows that reaction transition states that by size can be fitted within the pore of beta may have problems forming on the Lewis acid sites due to the shielding effect of the neighbor framework oxygen atoms. Finally, Sn-beta containing organic groups attached to the structure is able to perform the MPV in the presence of water, offering a real alternative to conventional aluminum iso-propoxide catalysts.