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) cocatalysts has emerged as a powerful strategy for innate C(sp)-H bond functionalization, particularly for C-H bonds α- to nitrogen. Azide ion (N) 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 N to photoexcited 4CzIPN reveals the participation of the S excited electronic state of the organic photocatalyst as an electron acceptor, but the N radical product of this reaction is not observed. Instead, both time-resolved infrared and UV-visible spectroscopic measurements implicate rapid association of N with N (a favorable process in acetonitrile) to form the N radical anion. Electronic structure calculations indicate that N is the active participant in the HAT reaction, suggesting a role for N as a reservoir that regulates the concentration of N.