锰、钴卟啉高价氧化物氧化环己烷制备己二酸反应机理的理论研究

Theoretical Study on the Reaction Mechanism of Cyclohexane Oxidation to Adipic Acid by High-valent Mn-Oxo and Co-Oxo Porphyrin

采用密度泛函方法研究了锰、钴卟啉的高价氧化物种(PMnIVO,PCoIVO)氧化环己烷制备己二酸反应的机理。计算通过计算环己烷第一步羟基化、第二步羟基化、C—C键断裂生成1,6己二醛以及1,6己二醛氧化生成己二酸四步反应历程,得到了各基元反应的过渡态,分析了其几何结构、反应的活化能以及反应焓。发现第二步羟基化的夺氢基元反应的活化能最高,对于PMnIVO,PCoIVO分别为31.31 kcal·mol-1、29.00 kcal·mol-1,是整个反应的速控步骤。1,6环己二醇失去两个氢原子C—C键发生断裂的活化能分别为8.67和8.69 kcal·mol-1,比环己烷羟基化的活化能较低,而且反应焓为负值,说明环己烷C—C键断裂容易发生。研究还发现钴卟啉高价氧化物种能使环己烷的C—C键自行发生断裂,更有利于目标产物的生成。

The reaction mechanism of cyclohexane oxidized by high-valent metal-oxo porphyrin （PMn^ⅣO and PCo^ⅣO） was calculated using the density functional method （DFT）.The stable transition state structure,activation energy and reaction energy have been obtained.From the reaction process of the first step hydroxylation of cyclohexane,second step hydroxylation,C—C bond cleavage and finally generating adipic acid.According to the reaction barrier it can be found that hydrogen abstraction（HAT） in the second step of cyclohexane hydroxylation needed the highest energy barrier,respectively 31.31 kcal · mol^-1,29.00 kcal · mol^-1,which was the reaction rate-limiting step.The two hydrogen abstractions of 1,6 cyclohexanol led to the cleavage of C—C bond,and the activation energy barrier is relatively low,8.67 and 8.69 kcal · mol^-1 respectively.Furthermore the reaction enthalpy was negative,which suggested that C—C bond cleavage was favorable in thermodynamic and kinetic.In addition it was also found that PCo^ⅣO paly different roles in the C—C bond breakage of 1,6-cyclohexonol from PMn^ⅣO.PCo^ⅣO make C—C bond breakage broke spontaneously,and it was advantageous to the formation of target products.