Nucleophilic substitution reactions with the 3-borane-1,4,5-trimethylimidazol-2-ylidene anion. - Unexpected formation of an imidazabole isomer

The nucleophilic carbene 3-borane-1,4,5-trimethylimidazol-2-ylidene anion ( 1(-)) reacts with the electrophiles CH3I, (CH3)(3)SiCl, (CH3)(3)SnCl, and t he bromodiazaboroline 7 to form the 2-substituted imidazoles 4, 5, 6, and 8 . With triethylborane, the anionic carbene borane adduct 9(-) is obtained. An unexpected result was achieved when chloro-dimethoxyborane and HBCL2 . S (CH3)(2) were used as be isolated. Imidazole 5, the imidazole borane adduct 3a andelectrophiles. In both cases only the imidazabole 14a could be isola ted. Imidazole 5, the imidazole borane adduct 3a and the imidazabole 14a we re characterized by X-ray structure analyses.