Effect of a trimethylsilyl moiety on the nucleophilic character of the CC bond: a comparative kinetic investigation of the epoxidation of substituted and unsubstituted cycloalkenes

Abstract

The rates of epoxidation of twelve cycloalkenes (6–17) with MCPBA were determined at four temperatures (298, 303, 308 and 313 K). All of them were found to follow second-order kinetics. The silylated cycloalkenes (10–13) react faster than the corresponding unsubstituted cycloalkenes (6–9), but slower than the corresponding methyl cycloalkenes (14–17). Thus, for epoxidation, the silyl moiety is a deactivating group relative to an alkyl group, while it is activating in comparison with hydrogen. When the homologous series 6–9, 10–13 and 14–17 are considered, the order of the rates of epoxidation in each series seems to follow the order of strain energy of its members, i.e., the rate decreases in the sequence: 5-membered > 8-membered > 7-membered > 6-membered. The rate data obtained at four temperatures were employed to calculate ΔH^‡, ΔS^‡ and ΔG^‡, based on which a common reaction mechanism is proposed.