铁卟啉催化氧化邻、对硝基取代芳烃α-C-H键的密度泛函理论研究

Density Functional Theory Study on α-C-H Bond Oxidation of o-Nitro-Substituted Arenes and p-Nitro-Substituted Arenes Catalyzed by Iron Porphyrin

采用密度泛函理论方法对铁卟啉催化氧化邻、对硝基取代芳烃(甲苯、乙苯、异丙苯)α-C-H键的反应进行研究,并将其与无硝基取代芳烃α-C-H键的氧化反应进行比较,重点考察铁卟啉催化氧化邻硝基取代芳烃和对硝基取代芳烃α-C-H键反应活性的差异,并探究影响反应活性的本质因素。通过计算取代芳烃α-C-H键的键解离能及铁卟啉催化氧化芳烃α-C-H键的反应活化能发现,由于硝基与苯环的共轭效应,对硝基取代芳烃α-C-H键的键解离能减小,铁卟啉高价金属氧化物氧化对硝基取代芳烃α-C-H键反应的活化能较低。邻硝基取代芳烃除共轭效应外还存在空间位阻效应,且空间位阻效应占主导作用。空间位阻效应使得邻硝基取代芳烃α-C-H键的键解离能增大,铁卟啉高价金属氧化物氧化邻硝基取代芳烃α-C-H键反应的活化能较高。对芳烃α-C-H键氧化反应的活化能进一步分析发现,过渡态结构中芳烃与铁卟啉高价金属氧化物之间的相互作用能相对较小,形变能是影响反应活化能的主要因素,且形变能主要来自于芳烃分子的形变。

The catalytic oxidations of α-C-H bond in o-nitro-substituted arenes and p-nitro-substituted arenes(toluene,ethylbenzene and cumene)by iron porphyrin were studied using density functional theory.The calculation results were compared with those ofα-C-H bond oxidation in arenes without nitro substituent.The reactivity difference ofα-C-H bond oxidation between o-nitro-substituted arenes and p-nitro-substituted arenes catalyzed by iron porphyrin were focused on and the essential factors affecting the reactivity were explored.By calculating the bond dissociation energy of α-C-H bond and the activation energy ofα-C-H bond oxidation in arenes by iron porphyrin,it is found that the conjugation effect between nitro and benzene ring in p-nitro-substituted arenes reduce the bond dissociation energy of α-C-H bond and the activation energy of α-C-H bond oxidation by high-valent iron-oxo porphyrin.In o-nitro-substituted arenes,there are not only conjugation effect,but also steric hindrance effect.The steric hindrance effect is dominant in o-nitro-substituted arenes.The bond dissociation energy of α-C-H bond and the activation energy ofα-C-H bond oxidation by high-valent iron-oxo porphyrin increase due to the steric hindrance effect.Further decomposition of the activation energy ofα-C-H bond oxidation reveals that the interaction energy between the arene and high-valent iron-oxo porphyrin in the transition structure is relatively weak and the distortion energy is the main factor affecting the activation energy.The distortion energy mainly comes from the distortion of the arene molecule.