Stereoselective Meerwein-Ponndorf-Verley and Oppenauer reactions catalysed by zeolite BEA

Zeolite BEA has been studied as catalyst in the Meerwein-Ponndorf-Verley and Oppenauer (MPVO) reactions of substituted cyclohexanones and cyclohexanols. In the MPV reduction of 4-tert-butylcyclohexanone to 4-tert-butylcyclohexanol a high stereoselectivity (> 95%) to the thermodynamically less stable cis-isomer was obtained while in the complementary Oppenauer oxidation the cis-alcohol was preferentially converted. This stereoselectivity is explained by transition-state selectivity imposed by the zeolite structure. The catalytic activity is related to Lewis-acid aluminium sites which are located in the micropores. FT-IR results indicate that these sites are related to aluminium atoms which are only partially bonded to the framework. The mechanism is proposed to involve a six membered transition state in which both the alcohol and the carbonyl are coordinated to the same aluminium.