Aromaticity transfer in an annulated 1,4,2-diazaborole: facile access to C_s symmetric 1,4,2,5-diazadiborinines

A tricyclic annulated 1,4,2-diazaborole is readily accessed via reaction of a bidentate pyridyl-carbene ligand with MesBBr₂ followed by reduction. Dearomatization of the flanking rings is shown to increase reactivity of this heterocycle in the form of a B-centred alkylation with MeI. Its reaction with hydrido-, fluoro-, and chloro-boranes reveal an unprecedented ring expansion reaction to form a diverse family of B₂C₂N₂ heterocycles, reduction of which allows facile access to the first examples of C_s symmetric 1,4,2,5-diazadiborinines. DFT calculations have shed light on the electronic structures of the reduced species and provide insight into mechanistic aspects of the observed ring-expansion.