Molybdenum‐catalyzed epoxidations of oct‐1‐ene and cyclohexene with organic hydroperoxides: Steric effects of the alkyl substituents of the hydroperoxide on the reaction rate

Abstract

AbstractA kinetic study of the epoxidation of oct‐1‐ene and cyclohexene with alkyl hydroperoxides is reported. The alkyl hydroperoxides were obtained in a moderate to high purity from the corresponding alcohols by acid‐catalyzed exchange with hydrogen peroxide. The reaction rates in pseudo first‐order experiments of these olefins with various alkyl hydroperoxides strongly depend on the structure of the alkyl group of the alkyl hydroperoxide. When one of the methyl groups in tert‐butyl hydroperoxide (TBHP, 4a) is substituted by an alkyl group, R, the reaction rate decreases in the order Et > Pr > Bu >1 BuCH2 > tBu. Substitution of two methyl groups of TBHP as in 1‐ethyl‐1‐methylpropyl hydroperoxide (5a) and 1‐ethyl‐1‐methylbutyl hydroperoxide (5b) showed a further decrease in reaction rate of epoxidation. When all three methyl groups are substituted by, for example, three ethyl groups as in 1,1‐diethylpropyl hydroperoxide (6a) a decrease of approximately 99% in reaction rate is observed. Introduction of a ring system in the hydroperoxide such as in cyclohexyl hydroperoxide (3), 1‐methyl‐cyclohexyl hydroperoxide (2) and pinane hydroperoxide (1) also showed a dramatic decrease in reaction rate of epoxidation.An investigation of relative rates of epoxidation in competition experiments of cyclohexene and hex‐1‐ene with 1‐tert‐butylcyclohexene with different alkyl hydroperoxides also showed them to depend on the structure of the alkyl group of the alkyl hydroperoxide.These results are rationalized on the basis of a mechanism involving nucleophilic attack of the olefin on an alkylperoxomolybdenum(VI) intermediate. Bulky substituents at the α‐position in the alkyl hydroperoxide can seriously impede the approach of the olefin to the O‐O bond.