Evidence and Governing Factors of the Radical-Ion Photoredox Catalysis

Photoinduced electron transfer of a radical species provides a promising strategy for expanding the scope of photoredox catalysis. However, the mechanism underlying the radical-ion photoredox catalysis has yet been fully established. In the present research, we investigated the generation and annihilation of the key catalytic intermediate in the borylation reaction of aryl halides. To disentangle the intricate catalysis cycle, we conducted experiments using electrochemical, transient absorption and photoluminescence spectroscopy and quantum chemical calculation techniques. Our mechanistic studies provided convincing evidence that the active catalyst species is the excited-state radical anion. We also found that the catalyst activation is retarded by multiple annihilative processes, among which the intrinsic relaxation of the excited-state radical anion of the catalyst is the most destructive. We expect these mechanistic findings will guide the future development of photoredox catalysis of radical species.