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.