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.
十二种环
烯烃(6–17)与M
CPBA的环
氧化反应速率在四个温度(298、303、308和313 K)下被测定。所有反应均遵循二级动力学。
硅基化的环
烯烃(10–13)反应速度快于相应的未取代环
烯烃(6–9),但慢于相应的
甲基环
烯烃(14–17)。因此,对于环
氧化反应来说,
硅基取代基相对于烷基是一种去活化基团,而相对于
氢则是一种活化基团。当考虑同 homologous 系列 6–9、10–13 和 14–17 时,各系列的环
氧化速率似乎遵循其成员的应变能的顺序,即速率按以下顺序降低:5元环 > 8元环 > 7元环 > 6元环。在四个温度下获得的速率数据用于计算 ΔH‡、ΔS‡ 和 ΔG‡,基于此提出了一个共同的反应机制。