Ring Expansion of 7‑Boranorbornadienes by Coordination with an N‑Heterocyclic Carbene

In
the context of our longstanding interest in subvalent boron
compounds, we targeted the liberation of a carbene-stabilized borylene
from a suitable organoboron precursor. For this purpose, we tested
7-borabicyclo[2.2.1]­hepta-2,5-dienes (or 7-boranorbornadienes in short)
obtained from facile [4 + 2] cycloaddition of boroles and alkynes.
By formation of a Lewis adduct with an N-heterocyclic carbene (NHC),
we intended to block previously reported pericyclic reactions leading
to undesired ring expansion and achieve a cheletropic elimination
of the borylene species instead. Our results presented herein indicate
that coordination of the NHC to 7-boranorbornadienes indeed weakens
the bridgehead boron–carbon bonds. However, ring expansion
to form borepine-NHC Lewis adducts remains the favorable reaction
pathway. This process is independent of excessive NHC in the reaction
mixture, which precludes participation of a free borylene species.
As an alternative, an intramolecular mechanism driven mainly by molecular
strain and steric factors is most plausible. Our investigations are
based on spectroscopic measurements and single-crystal X-ray diffraction
analyses.