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.