The synergistic use
of (organo)photoredox catalysts with
hydrogen-atom
transfer (HAT) cocatalysts has emerged as a powerful strategy for
innate C(sp3)–H bond functionalization, 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 S1excited 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 infrared
and UV–visible spectroscopic measurements implicate rapid association
of N3•with N3–(a favorable 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 that regulates the concentration of N3•.