Visible-Light-Induced Ni-Catalyzed Radical Borylation of Chloroarenes

A highly selective and general photoinduced C–Cl borylation protocol that employs [Ni(IMes) 2 ] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical borylation of chloroarenes is reported. This photoinduced system operates with visible light (400 nm) and achieves borylation of a wide range of chloroarenes with B 2 pin 2 at room temperature in excellent yields and with high selectivity, thereby demonstrating its broad utility and functional group tolerance. Mechanistic investigations suggest that the borylation reactions proceed via a radical process. EPR studies demonstrate that [Ni(IMes) 2 ] undergoes very fast chlorine atom abstraction from aryl chlorides to give [Ni I (IMes) 2 Cl] and aryl radicals. Control experiments indicate that light promotes the reaction of [Ni I (IMes) 2 Cl] with aryl chlorides generating additional aryl radicals and [Ni II (IMes) 2 Cl 2 ]. The aryl radicals react with an anionic sp 2 –sp 3 diborane [B 2 pin 2 (OMe)] − formed from B 2 pin 2 and KOMe to yield the corresponding borylation product and the [Bpin(OMe)] •– radical anion, which reduces [Ni II (IMes) 2 Cl 2 ] under irradiation to regenerate [Ni I (IMes) 2 Cl] and [Ni(IMes) 2 ] for the next catalytic cycle.