Synthesis and Ring Strain of a Benzoborirene‐N‐Heterocyclic Carbene Adduct

: The reduction of an N-heterocyclic carbene (1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene, IiPr Me2 ) adduct of dichloro(ortho-bromophenyl)borane by tert-butyl lithium at low temperature yields the IiPr Me2 adduct A of parent benzoborirene, a highly strained boron-containing bicyclic compound. A is unstable at room temperature and dimerizes at low temperature to the bis-IiPr Me2 adduct of 9,10-dihydro-9,10-diboraanthracene, characterized by single-crystal X-ray crystallography.