Photoredox-HAT Catalysis for Primary Amine α-C–H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

The synergistic use of (organo)photoredox catalysts with hydrogen atom transfer (HAT) co-catalysts has emerged as a powerful strategy for innate C(sp³)–H bond functionalization, and particularly for C–H bonds α- to nitrogen. Azide ion (N₃^–) was recently identified as an effective HAT catalyst for the challenging α-C–H alkylation of unprotected, primary alkylamines, in combination with dicyanoarene photocatalysts such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Here, time-resolved transient absorption spectroscopy over sub-picosecond to microsecond timescales provides kinetic and mechanistic details of the photoredox catalytic cycle in acetonitrile solution. Direct observation of the electron transfer from N₃^– to photoexcited 4CzIPN reveals the participation of the S₁ excited electronic state of the organic photocatalyst as an electron acceptor, but the N₃• radical product of this reaction is not observed. Instead, both time-resolved infra-red and UV-visible spectroscopic measurements implicate rapid association of N₃• with N₃^– (a favourable process in acetonitrile) to form the N₆•^– radical anion. Electronic structure calculations indicate that N₃• is the active participant in the HAT reaction, suggesting a role for N₆•^– as a reservoir which regulates the concentration of N₃•.