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

Abstract

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)