Mechanism and Chemoselectivity of Mn-Catalyzed Intramolecular Nitrene Transfer Reaction: C-H Amination vs. C=C Aziridination

The reactivity, mechanism and chemoselectivity of the Mn-catalyzed intramolecular C-H amination versus C=C aziridination of allylic substrate cis-4-hexenylsulfamate are investigated by BP86 density functional theory computations. Emphasis is placed on the origins of high reactivity and high chemoselectivity of Mn catalysis. The N p orbital character of frontier orbitals, a strong electron-withdrawing porphyrazine ligand and a poor pi backbonding of high-valent Mn-III metal to N atom lead to high electrophilic reactivity of Mn-nitrene. The calculated energy barrier of C-H amination is 9.9 kcal/mol lower than that of C=C aziridination, which indicates that Mn-based catalysis has an excellent level of chemoselectivity towards C-H amination, well consistent with the experimental the product ratio of amintion-to-aziridination I:A (i.e., (Insertion):(Aziridination)) >20:1. This extraordinary chemoselectivity towards C-H amination originates from the structural features of porphyrazine: a rigid ligand with the big pi-conjugated bond. Electron-donating substituents can further increase Mn-catalyzed C-H amination reactivity. The controlling factors found in this work may be considered as design elements for an economical and environmentally friendly C-H amination system with high reactivity and high chemoselectivity.