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.