Alkene–Carbene Isomerization induced by Borane: Access to an Asymmetrical Diborene

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 11B NMR spectrum, X-ray diffraction
analysis and computational studies indicate that π-electrons
on the central B2 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.