A Theroetical Model for the Orientation of 16-Electron [CpML] Insertion into the C-H Bond of Propane and Cyclopropane and Its Regio- and Stereoselectivity

摘要

Complete geometry optimizations were performed with density functional theory (DFT)in order to stdy the potential energy surfaces of [CpM(PH_3)](M=Rh, Ir) complexes inserted into C-H bonds of propane and cyclopropane. The agreement between DFT and experimental results indicates that the B3LYP/LANL2DZ methaod can be a powerful tool for the investigaiton of these oxdative addition reactions. A fragment molecular orbital model suggesting the mechanistic pathway fo the oxidative addition of saturated alkanes to [CpML]is described. It is shown that these oxidative addition reactions all proceed in a concerted fashion via a three-center transition state, and all lead to exothermic reactions. In particular, we show that both electronic and steric effects play a major role in the preference for a #sigma#_(CH_2)-type of approach, from which one may predict the formation and stabilities of the regio-and stereoselective insertion products. Our theoretical findings suggest that highlyreactive [CpIr(PH_3)]tends to be nonless discriminating and reacts randomly,while the less reactive [CpRh(PH_3)]complex is higly selective. We also found that, for both [CpRh(PH_3)]and [CpIr(PH_3)],the ease of oxidative addition is in the order:secondary cyclopropane>primary propane>secondary propane. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. It is demonstrated that both the singlet-triplet energy gap of 16-electron[CpML] and the #sigma#~(C-H)->#sigma#~*(C-H) triplet excitation energy of hydrocarbons play a decisive role in the determination ofthe reactivity as well as the selectivity of [CpML]insertion.