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利用半经验分子轨道理论AM1方法,研究了烯酮及取代烯酮与环戊二烯环加成反应机理。采用Berny梯度法优化得到各反应的过渡态和中间体,并进行了振动分析确认。计算结果表明,该环加成反应是按照协同的非同步途径进行的,经过一个四元环发生扭曲的过渡态,并有部分电荷从环戊二烯迁移到烯酮或取代烯酮上,前线轨道分析表明反应机理为“2×[1+1]”机理;而氯甲基取代的烯酮与环戊二烯的环加成反应是按照分步途径发生的。计算结果可以很好地说明实验所观察到的立体选择性,并根据烯酮上取代基的电子效应和位阻效应对反应机理的影响进行了分析。
The semi-empirical molecular orbital theory (AM1) method was used to study the cycloaddition reaction of enones and substituted enones with cyclopentadienyl rings. The transition states and intermediates of each reaction were optimized by Berny gradient method and confirmed by vibration analysis. The calculated results show that the cycloaddition reaction proceeds in a synergistic asynchronous manner. After a four-membered ring undergoes a twisted transition state and some charges migrate from cyclopentadiene to enone or substituted enone, The orbital analysis shows that the reaction mechanism is “2 × [1 + 1]”. The cycloaddition reaction of chloromethyl substituted ketene with cyclopentadiene takes place in a stepwise manner. The calculated results can well explain the stereoselectivity observed in the experiment and the influence of the reaction mechanism by the electron effect and the steric effect of the substituents on the ketene.