Nickel-Catalyzed Boron Insertion into the C2–O Bond of Benzofurans

Treatment of benzofurans with bis­(pinacolato)­diboron
and Cs2CO3 under nickel-NHC catalysis resulted
in the
insertion of a boron atom into the C2–O bond of benzofurans
to afford the corresponding oxaborins. The scope of benzofuran substrates
is wide, and the reactions proceeded without loss of functional groups
such as fluoro, methoxy, and ester that are potentially reactive under
nickel catalysis. The boron-inserted products proved to be useful
building blocks and subsequently underwent a series of transformations,
one of which led to the synthesis of fluorescent π-expanded
oxaborins.