钐(II)类卡宾CH3SmCH2I与乙烯环丙烷化反应及溶剂化效应的理论研究

用密度泛函B3LYP方法研究了过渡金属钐类卡宾与乙烯的环丙烷化反应的机理. 对钐类卡宾试剂CH₃SmCH₂I和CH₂CH₂反应的反应物、中间体、过渡态和产物构型的全部结构几何参数进行了优化, 并计算了THF溶液的溶剂化效应, 用内禀反应坐标(IRC)计算和频率分析方法, 对过渡态进行了验证. 结果表明: CH₃SmCH₂I与CH₂CH₂环丙烷化反应按亚甲基转移机理(通道A)和卡宾金属化机理(通道B)都可以进行, 与锂类卡宾的反应机理相同, 通道A比通道B反应的势垒降低了14.65 kJ/mol. 溶剂化效应使通道B比通道A的反应势垒大幅度提高, 更有利于反应沿通道A进行, 而不利于通道B.

Theoretical Study of Samarium(II) Carbenoid Promoted Cyclopropanation Reaction with Ethylene and the Effect of THF on the Reaction Pathways

The cyclopropanation reaction of ethylene with samarium(II) carbenoid was studied by means of the B3LYP hybrid density functional method. The geometries for the reactants, the transition states and the products were completely optimized, and all the transition states were verified by the vibrational analysis and the intrinsic reaction coordinate calculations. The effect of solvent THF was investigated by explicit coordination of the solvent THF molecules to the Sm(II) center of the carbenoid. The results show that all CH₃SmCH₂I/(THF)_n (n＝0, 1, 2) can react with ethylene fast, and both methylene transfer and carbometalation pathways are the same as those of lithium carbenoid, but the reaction barrier to cyclopropanation via methylene transfer pathway is lower than carbometalation pathway. The cyclopropanation reactions are able to occur at low temperatures because of the lower barrier. CH₃SmCH₂I/(THF)_n (n＝0, 1, 2) carbenoid carbometalation barrier to reaction became systematically higher as more solvent THF was added (from 24.36 kJ/mol without THF molecule to 58.44 kJ/mol with two THF molecules). In contrast, the barrier to the methylene transfer pathways remained low.