From bis(borylene)-substituted xanthenes as reactive intermediates to diboraoxirane complexes

The first N-heterocyclic carbene (NHC)-stabilized diboraoxirane complex 4 [NHC = IPr = C{N(iPr)CMe}2] was synthesized through the reduction of the corresponding bis(dichloroboryl-IPr)xanthene 3 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 6 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 4 undergoes versatile ring-expansion reactions with small molecules to engender new boron-containing heterocycles. In fact, oxidation of 4 with trimethylamine N-oxide, O2, and elemental sulfur afforded the unprecedented 1,3-dioxa-2,4-diboretane 7, 1,3,4-trioxa-2,5-diborolane 8, and 1-oxa-3,4-dithio-2,5-diborolane 9, respectively. Moreover, 4 activates isocyanide to produce 1-oxa-2,4-diborete 10 and readily reacts with the C═O groups of benzophenone and CO2 to generate the ring-expansion products 11 and 12, respectively.