Formation of Isomeric BAr3 Adducts of 2-Lithio-N-methylimidazole

N-Methylimidazole added to the strong organometallic Lewis acid B(C6F5)3 at the donor nitrogen atom to form the adduct 3 (characterized by X-ray diffraction). Deprotonation at the imidazole carbon atom C-2 was achieved by treatment with methyllithium to generate the reactive “Arduengo carbene anion” intermediate 4, which underwent a rapid subsequent intramolecular nucleophilic aromatic substitution reaction at one of the adjacent C6F5 groups to form the tricyclic betaine-type product 5. Rearrangement of 4 by 1,2-boron migration was not observed. The respective isomer 7 was prepared independently by deprotonation of N-methylimidazole, followed by C-2 addition of B(C6F5)3. The corresponding N-methylimidazolide-2−B(C6H5)3 adduct 8 was characterized by X-ray diffraction and shown to have a cyclodimeric structure made up of intermolecular bis(π-arene)(imidazole)lithium units. A DFT study revealed that the N-borated Arduengo carbene anions are in general very effectively kinetically protected from undergoing intramolecular rearrangement to their thermodynamically favored 2-boratoimidazole anion isomers by 1,2-BR3 migration. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)