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

Abstract

Reductive dehalogenation of an aryl halide (Ar-X) subsequent coupling with bis(pinacolato)diboron (B2pin[Formula: see text] to form an arylboronate (Ar-Bpin) by the photo-excited B[Formula: see text] complex of the Co[Formula: see text] oxidation state with an iridium photosensitizer (PS), [Ir(dtbbpy)(ppy)2]PF6, under anaerobic conditions was reported. The robust B[Formula: see text] 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[Formula: see text] complex was formed via the reductive quenching pathway of [Ir(dtbbpy)(ppy)2]PF6 by visible light irradiation in the presence of [Formula: see text], [Formula: see text]-diisopropylethylamine ([Formula: see text]-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 Co[Formula: see text] state of the B[Formula: see text] complex ([Formula: see text]Co[Formula: see text] to the Ar-X occurred in the order of the rate constants for electron transfer ([Formula: see text], Ar-I [Formula: see text] Ar-Br [Formula: see text] Ar-Cl. Based on the mechanistic studies, dual excitation of PS and the B[Formula: see text] complex is involved in the catalytic reaction and single electron transfer (SET) from [Formula: see text]Co[Formula: see text] 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, B2pin2, to form the Ar-Bpin product.