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.