苯甲醛肟偶极互变反应的理论研究

Theoretical Study of the Tautomerization Between Benzaldoximes and Their Polar Tautomers

研究了苯甲醛肟 Z构型和 E构型两种异构体与相应偶极体的 3条互变反应途径 :( 1 )单分子内质子转移反应 ,质子由肟羟基转移至邻位的氮上 ,过渡态为三角形结构 ,反应能垒较高 ;( 2 )二聚体内的质子互换反应 ,质子分别从一个肟羟基转移到另一个肟的氮上 ,过渡态为六元环结构 ,能垒较低 ,理论反应速度较大 ,但平衡常数较小 ;( 3 )肟羟基与甲醇的质子互换反应 ,过渡态具有五元环结构 ,能垒和反应速度介于上述两者之间 .结果表明 ,在 3条反应途径上 ,Z构型和 E构型均有类似的过渡态 ,Z构型有利于偶极体存在 .在室温下主要通过二聚体内质子交换进行互变反应 ,实际体系中由于偶极体不断被消耗 。

Three tautomerization reactions between benzaldoximes and their polar tautomers have been studied theoretically. All geometries of reactants, products and transition states were optimized by DFT method at B3LYP/6-31G(d) level with Gaussian 98 software package. The species with Z and E configurations have similar transition states in the three reactions. Compared with E isomers, Z isomers are favorable to the polar products. The mechanism of the first reaction involves proton migration from hydroxyl to nitrogen of oxime within single molecule through a triangle transition state . This reaction takes place difficultly at room temperature because of the high energy barrier. The second reaction is related to proton exchange within the dimers. The proton of hydroxyl group in one oxime moves to the nitrogen of another oxime of the dimer through a hexaangular ring transition state with a lower barrier. The reaction rate of this reaction is large,but the thermodynamic balances is not favorable to the products. The third path is a catalyzed reaction by methanol with a pentagonal transition states. The hydroxyl of methanol acts as a bridge of proton transfer. The reaction barrier and rate are moderate. Our results show that the tautomerization between benzadoxime and their polar tautomers can take place at room temperature. The main reaction path is the proton exchange within the dimers duo to its large reaction rate. The reaction will reach the end as the product is consumed continuously.