Cobalt-catalysed unactivated C(sp3)–H amination: two-state reactivity and multi-reference electronic character

Abstract

Theoretical calculations have been performed to gain deeper understanding on the mechanism of Cp*Co(III)-catalysed C(sp³)–H amination of 8-methylquinoline with phenyl-1,4,2-dioxazol-5-one. The result suggests that the additive AgSbF₆ could thermodynamically facilitate the generation of catalytically active species Cp*Co(OPiv)⁺. The subsequent catalytic cycle involves sequential external base assisted C(sp³)–H activation, decarboxylation, nitrene insertion, and protonation. Importantly, a remarkable “two-state reactivity” scenario was disclosed for this reaction. A distinct multi-reference feature was found in the processes for Co-nitrene radical intermediate and C–N bond formations, which thus greatly enriches the cobalt catalysis chemistry. Such a multi-reference character was the outcome of double excitation from the Co–N π-bonding orbital to its antibonding orbital. DFT results indicate that aminating reagents with a rigid structure would be preferred because they form strong orbital interaction with the metal center. In addition, it is theoretically predicted that the Fe(III) species is a promising candidate for efficient C(sp³)–H activation and Tp (Tp = hydridotris(pyrazolyl)borate) can serve as a more effective ligand for Co(III) catalysed C(sp³)–H activation, thanks to its stronger interaction with the Co center which thus stabilizes the transition state.