Visible light-driven borylation of aryl halides by a B12 derivative based on a dual photoredox strategy

Reductive dehalogenation of an aryl halide (Ar-X) subsequent coupling with bis(pinacolato)diboron (B(2)pin2) to form an arylboronate (Ar-Bpin) by the photo-excited B12 complex of the CoI oxidation state with an iridium photosensitizer (PS), [Ir(dtbbpy)(ppy)(2)]PF6, under anaerobic conditions was reported. The robust B12 complex and iridium PS provided the highest turnover numbers of over 3,800 for the borylation reaction at room temperature. The CoI state of the B-12 complex was formed via the reductive quenching pathway of [Ir(dtbbpy)(ppy)(2)]PF6 by visible light irradiation in the presence of N, N-diisopropylethylamine (i-Pr2NEt) as a sacrificial electron donor (SED), which was confirmed by a photoluminescence lifetime analysis. A picosecond time-resolved spectroscopic analysis showed that the electron transfer from the excited CoI state of the B-12 complex (*CoI) to the Ar-X occurred in the order of the rate constants for electron transfer (keT'), Ar-I > Ar-Br > Ar-Cl. Based on the mechanistic studies, dual excitation of PS and the B12 complex is involved in the catalytic reaction and single electron transfer (SET) from *CoI to Ar-X could provide the aryl radical by fragmentation of the Ar-X radical anion. The aryl radical should react with the radical trapping reagent, B(2)pin(2), to form the Ar-Bpin product.