A 2,3-dihydro-1H-1,2-azaborole derivative 2 was converted to a cyclic (alkyl) (amino)carbene (cAAC) via 1,2-hydrogen migration triggered by boranes to afford cAAC-borane adducts. This procedure allowed us to develop an asymmetrical diborene cAAC·(Br)B=B(Br)·IDip 6, which was isolated and fully characterized. The 11 B NMR spectrum, X-ray diffraction analysis and computational studies indicate that π-electrons on the central B 2 moiety in 6 are unequivalently distributed, and thus polarized. A complete scission of the B=B double bond in 6 was achieved by the treatment with an isonitrile, which led to the formation of a base-stabilized B,N-containing methylenecyclopropane 7. © 2017 American Chemical Society.