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

The first <i>N</i>-heterocyclic carbene (NHC)-stabilized diboraoxirane complex <b>4</b> [NHC = IPr = C{N(iPr)CMe}₂] was synthesized through the reduction of the corresponding bis(dichloroboryl-IPr)xanthene <b>3</b> 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 <b>6</b> with three-coordinate B(I) atoms could be isolated when the sterically less demanding NHC ligand IMe [IMe = C{N(Me)CMe}₂] was employed. Due to its ring strain, the B-B bond of the B₂O ring in <b>4</b> undergoes versatile ring-expansion reactions with small molecules to engender new boron-containing heterocycles. In fact, oxidation of <b>4</b> with trimethylamine <i>N</i>-oxide, O₂, and elemental sulfur afforded the unprecedented 1,3-dioxa-2,4-diboretane <b>7</b>, 1,3,4-trioxa-2,5-diborolane <b>8</b>, and 1-oxa-3,4-dithio-2,5-diborolane <b>9</b>, respectively. Moreover, <b>4</b> activates isocyanide to produce 1-oxa-2,4-diborete <b>10</b> and readily reacts with the C═O groups of benzophenone and CO₂ to generate the ring-expansion products <b>11</b> and <b>12</b>, respectively.