From Bis(borylene)-Substituted Xanthenes as Reactive Intermediates to Diboraoxirane Complexes.

The first -heterocyclic carbene (NHC)-stabilized diboraoxirane complex [NHC = IPr = C{N(iPr)CMe}] was synthesized through the reduction of the corresponding bis(dichloroboryl-IPr)xanthene with potassium graphite. Intriguingly, its formation stems from a diboron(I)-mediated C-O-C deoxygenation of the xanthene spacer via a bis(borylene)xanthene as a reactive intermediate. Consistent with the proposed pathway, bis(borylene)xanthene with three-coordinate B(I) atoms could be isolated when the sterically less demanding NHC ligand IMe [IMe = C{N(Me)CMe}] was employed. Due to its ring strain, the B-B bond of the BO ring in undergoes versatile ring-expansion reactions with small molecules to engender new boron-containing heterocycles. In fact, oxidation of with trimethylamine -oxide, O, and elemental sulfur afforded the unprecedented 1,3-dioxa-2,4-diboretane , 1,3,4-trioxa-2,5-diborolane , and 1-oxa-3,4-dithio-2,5-diborolane , respectively. Moreover, activates isocyanide to produce 1-oxa-2,4-diborete and readily reacts with the C═O groups of benzophenone and CO to generate the ring-expansion products and , respectively.