Nucleophilic assistance in methane activation by superelectrophiles with halogen-centered cationic sites

Simulation of fragments of potential energy surface for systems CH4 + CBr 3 + , CH4 + CBr 3 + AlBr 4 − , CH4 + CCl 3 + AlCl 4 − , and CH4 + CCl 3 + Al2Cl 7 − was performed by DFT-B3LYP and DFT-PBE methods. The important role of nucleophilic assistance in methane halogenation by these superelectrophiles was confirmed. These reactions occur with a synchronous hydride transfer from methane to the electrophile within the cyclic transition states in linear C-H-C fragment of the rings and a generation of a C-Hlg bond between the carbon atom of the arising methyl group and the halogen atom of the electrophile. The nucleophilic assistance from the unshared electron pair of this halogen atom provides the lowering of the potential barriers to methane halogenation by complexes CBr 3 + AlBr 4 − , CCl 3 + AlCl 4 − , and CCl 3 + Al2Cl 7 − to the values of the order of 20 kcal mol−1. These essential features of the mechanism of methane halogenation are independent of the halogen nature and are retained on going from the model electrophiles to the real ones.