Zeolite titanium beta: A selective catalyst in the meerwein-ponndorf-verley-oppenauer reactions

Abstract

Publisher Summary In recent times, the use of zeolite beta in the Meerwein–Ponndorf–Verley–Oppenauer (MPVO) reduction of 4- t -butylcyclohexanone has been under consideration. The high selectivity toward the thermodynamically less favored cis-alcohol is explained by a restricted transition-state around a Lewis-acidic aluminum in the zeolite pores. When using an aluminum-free zeolite titanium beta in the epoxidation of olefins, it has been shown that Ti-beta has acidic properties when alcoholic solvents were employed. Ti-beta is found to be an excellent catalyst in MPVO reactions with a tolerance for water. In this chapter, results are presented on the high selectivity, stability, and low by-product formation of the catalyst, Ti-beta, in both the liquid-phase and gas-phase MPVO reactions. Under liquid-phase conditions, a very high selectivity in the reduction of 4-substituted cyclohexanones toward the thermodynamically unfavorable cis-alcohols was observed. By-products were observed only during the oxidation of alcohols, using ketone solvents, and consisted primarily of aldol condensation products. Remarkable differences exist between the liquid-phase and gas-phase reactions under otherwise similar conditions. The selectivity toward the cis-alcohol is still above the thermodynamically expected value, but significantly lower than under liquid-phase conditions. In contrast to the liquid-phase reactions, dehydration of the alcohols to the corresponding alkene is an important side-reaction. The oxidation of both the cis- and the trans-alcohol clearly showed that the olefin is exclusively formed from the cis-alcohol. Dehydration of the trans-alcohol is assumed to proceed by isomerization via a MPVO mechanism to the corresponding cis-alcohol.