Mechanistic investigation of dirhodium-catalyzed intramolecular allylic C-H amination versus alkene aziridination.

The reaction mechanisms and chemoselectivity on the intramolecular allylic C-H amination versus alkene aziridination of 4-pentenylsulfamate promoted by four elaborately selected dirhodium paddlewheel complexes are investigated by a DFT approach. A predominant singlet concerted, highly asynchronous pathway and an alternative triplet stepwise pathway are obtained in either C-H amination or alkene aziridination reactions when mediated by weak electron-donating catalysts. A singlet stepwise C-H amination pathway is obtained under strongly donating catalysts. The rate-determining step in the C-H amination is the H-abstraction process. The subsequent diradical-rebound C-N formation in the triplet pathway or the combination of the allylic carbocation and the negative changed N center in the singlet pathway require an identical energy barrier. A mixed singlet-triplet pathway is preferred in either the C-H insertion or alkene aziridination in the Rh2(NCH3CHO)4 entry that the triplet pathway is initially favorable in the rate-determining steps, and the resultant triplet intermediates would convert to a singlet reaction coordinate. The nature of C-H amination or alkene aziridination is estimated to be a stepwise process. The theoretical observations presented in the paper are consistent with the experimental results and, more importantly, provide a thorough understanding of the nature of the reaction mechanisms and the minimum-energy crossing points.