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

The synergistic useof (organo)-photoredox catalysts withhydrogen-atomtransfer (HAT) cocatalysts has emerged as a powerful strategy forinnate C-(sp(3))-H bond functionalization, particularlyfor C-H bonds alpha- to nitrogen. Azide ion (N-3 (-)) was recently identified as an effective HATcatalyst for the challenging alpha-C-H alkylation of unprotected,primary alkylamines, in combination with dicyanoarene photocatalystssuch as 1,2,3,5-tetrakis-(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN).Here, time-resolved transient absorption spectroscopy over sub-picosecondto microsecond timescales provides kinetic and mechanistic detailsof the photoredox catalytic cycle in acetonitrile solution. Directobservation of the electron transfer from N-3 (-) to photoexcited 4CzIPN reveals the participation of the S-1 excited electronic state of the organic photocatalyst as an electronacceptor, but the N-3 (center dot) radical productof this reaction is not observed. Instead, both time-resolved infraredand UV-visible spectroscopic measurements implicate rapid associationof N-3 (center dot) with N-3 (-) (a favorable process in acetonitrile) to form the N-6 (center dot-) radical anion. Electronic structure calculationsindicate that N-3 (center dot) is the active participantin the HAT reaction, suggesting a role for N-6 (center dot-) as a reservoir that regulates the concentration of N-3 (center dot).