Trapping of a Borirane Intermediate in the Reductive Coupling of an Arylborane to a Diborene

The reductive coupling of a N-heterocyclic carbene (NHC)-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane–borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B–B bond formation.