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)3SiCl, (CH3)3SnCl, and the 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 chlorodimethoxyborane and HBCl2 · S(CH3)2 were used as electrophiles. In both cases only the imidazabole 14a could be isolated. Imidazole 5, the imidazole borane adduct 3a and the imidazabole 14a were characterized by X-ray structure analyses.