亚甲基烯酮与5-亚甲基-1,3-二噁烷-4,6-二酮反应机理的研究

Theoretical Study on the Mechanism of the Cycloaddition Reactions Between Methyleneketene and 5-methylene-1,3-dioxan-4,6-dione

利用半经验分子轨道理论AM1方法，研究了5-亚甲基-1，3-二噁烷-4，6-二酮与亚甲基烯酮的2，3-位C＝C，3，4-位C＝O和1，2-位C＝O三种双键位置上的环加成反应的反应机理．采用Berny梯度法优化得到反应的过渡态，并进行了振动分析确认．计算结果表明，环加成反应是按照协同的非同步途径进行的，经过一个扭曲的六员环过渡态，前线轨道分析表明反应机理为［4＋2］机理．根据AM1优化得到的产物反应物及过渡态的生成热可知三个反应的活化焓分别为27．07kJ·mol-1，32．41kJ·mol-1和137．96kJ·mol-1，2，3-位C＝C双键上的环加成反应的活化焓最低，这与实验中只观察到2，3－位C＝C双键上环加成产物的结论是一致的．

Three different reaction schemes for the cycloaddition reactions betweenmethyleneketene and 5-methylene-1,3-dioxan-4,6-dione have been studied by means ofsemiempirical AM1 method. All the geometries of the stationary points on the reactionspath have been optimized by energy gradient technique, and all transition states were characterized by vibration frequency analysis. The results can be summed up as follows: thecycloaddition of different double bond of methyleneketene with 5-methylene- 1,3-dioxan-4,6-dione are all concerted but nonsynchrorous, taking place through twisted transitionstates. An analysis based on frontier molecular orbital theory shows that the reactionmechanisms correspond to [4＋2] description. The activation barriers were calculated tobe 24.07k J ·mol- 1, 32.41 k J·mol^(-1) and 137.96 k J·mol^(- 1) respectively. The regio-selectivityof the reactions observed by experiments was correctly predicted by the calculations.