异硫氰酸与甲亚胺环加成反应机理的理论研究

对异硫氰酸与甲亚胺形成四元或六元环产物的环加成反应进行了理论研究,结果表明,一分子异硫氰酸与一分子甲亚胺形成四元环(1:1)产物P1的反应(1)为经过一个两性离子中间体的分步反应,其中第二步为速控步骤,其活化热垒为107.86kJ/mol.此外,反应(1)的中间体还可与另一甲亚胺或异硫氰酸分子继续反应形成两个不同的六元环(1:2或2:1)产物P2或P3;这两个反应均为协同反应,其活化势垒分别为15.88和21.82kJ/mol.这些结果与当异硫氰酸酯与亚胺发生环加成反应时,只有类似于P2和P3的两种六元环产物生成的实验事实一致。

Theoretical Studies on the Mechanism of the Cycloaddition Reaction Between Isothiocyanic Acid and Methylenimine

The cycloaddition reactions of isothiocyanic acid and methylenimine forming four-or six-mem- bered ring products were studied theoretically. Bader type electron density topological analysis was also used to study the electronic structure of stationary points for the equimolar reaction. All the geometries of the stationary points on the reaction path have been optimized by RHF/6-31G*, MP2/6-31G* and B3LYP/6-31G* with energy gradient technique, and the single point energy calculations of the stationary points of equimolar reaction have been performed by MP 4. 6-31G* method at MP2/6-31G* optimized geometries. All the transition states were characterized by vibration frequency analysis. The equimolar reaction is stepwise, proceeding via a zwitterionic intermediate; the second step is the rate controlling one and the activation barrier is 107. 86 kJ/mol. In addition, the above intermediate can react further with another methylenlmine or isothiocyanic acid molecule respectively to form two six-membered ring(1: 2 and 2: 1) products. The 1: 2 reaction is concerted but asynchronous, the 2: 1 reaction is a synchronous concerted process. The barriers of these two reactions are 15. 88 and 21. 82 kJ/mol respectively and they are lower than that of reaction forming four-membered ring product. This is consistment with experimental fact.