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 B2pin2 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 [NiI(IMes)2Cl] and aryl radicals. Control
experiments indicate that light promotes the reaction of [NiI(IMes)2Cl] with aryl chlorides generating additional aryl
radicals and [NiII(IMes)2Cl2]. The
aryl radicals react with an anionic sp2–sp3 diborane [B2pin2(OMe)]− formed
from B2pin2 and KOMe to yield the corresponding
borylation product and the [Bpin­(OMe)]•– radical
anion, which reduces [NiII(IMes)2Cl2] under irradiation to regenerate [NiI(IMes)2Cl] and [Ni­(IMes)2] for the next catalytic cycle.