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Inthe context of our longstanding interest in subvalent boroncompounds, we targeted the liberation of a carbene-stabilized borylenefrom a suitable organoboron precursor. For this purpose, we tested7-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 leadingto undesired ring expansion and achieve a cheletropic eliminationof the borylene species instead. Our results presented herein indicatethat coordination of the NHC to 7-boranorbornadienes indeed weakensthe bridgehead boron–carbon bonds. However, ring expansionto form borepine-NHC Lewis adducts remains the favorable reactionpathway. This process is independent of excessive NHC in the reactionmixture, which precludes participation of a free borylene species.As an alternative, an intramolecular mechanism driven mainly by molecularstrain and steric factors is most plausible. Our investigations arebased on spectroscopic measurements and single-crystal X-ray diffractionanalyses. [ABSTRACT FROM AUTHOR]

Preparation of a novel conjugated tetraorganoborate ispresentedin a facile two-step procedure. Successful utilization as a halideabstraction reagent is demonstrated in a metathesis reaction withthe platinum(II) boryl complex [(Cy3P)2Pt(Br)(BC4Ph4)], which was obtained by oxidative additionof 1-bromo-2,3,4,5-tetraphenylborole to a platinum(0) species. Thenovel compounds were investigated by means of spectroscopic and X-raydiffraction techniques. [ABSTRACT FROM AUTHOR]