Theoretical Studies on N‒X (X = Cl, O) Bond Activation Mechanism in Catalytic C‒H Amination
A comprehensive DFT study has been performed on the mechanism of Rh-catalysed C‒H amination of N-arylbenzamide with N-chloromorpholine. The catalytic cycle starting from the N‒H deprotonation of N-arylbenzamide was ruled out due to the inconsistence between the calculated and experimentally observed rate-determining step. Instead, the CsOAc induced pre-deprotonated form of N-arylbenzamide participates in the catalytic cycle. The reaction then goes through C‒H activation via concerted-metalation deprotonation, N‒Cl cleavage, reductive elimination, and protonation to complete the catalytic cycle. I has been found that an SN2-type N‒Cl cleavage fashion is more favourable compared with the conventional oxidative addition manner. Both cleavage manners of N‒Cl bond were manifested by geometry structure and orbital population analyses. More importantly, such an SN2-type mechanism also works for Ru-catalysed N‒Cl cleavage and Rh-/Ru-/Pd-catalysed N‒O cleavage involved in C‒H amination systems. It is found that less geometrical distortion accounts for the priority of SN2-type. It is theoretically demonstrated that the manner of N−O cleavage is affected by the ligand substituents. Modification of N-benzoyloxy morpholine with electron-withdrawing substituent could facilitate SN2-type N‒O cleavage process. Such an SN2-type N‒X (X = N, O) cleavage was established for the first time in the field of C‒H amination, which enriches the C‒H amination chemistry.