On the Mechanism of the Nickel-Catalyzed Boron Insertion into the C−O Bond of Benzofuran

The reaction of benzofurans with diboron reagents and Cs2CO3catalyzed by Ni(0) complexes results in the insertion of boron into the C-O bond. The reaction conditions mimic those reported by Martin and Hosoya for borylating aryl C-F bonds, but neither the role of the base nor the sequence of elementary steps is clear. Herein we report on the mechanism of such transformation on the basis of DFT studies, which suggest that the base activates the diboron reagent and generates a reactive boryl group, that Ni(II) is reduced back to Ni(0) during the boryl insertion into the Ni-O bond, and that the classical reductive elimination step is best viewed as a ring-contracting nucleophilic attack.