N-heterocyclic carbenes of indazole: Ring enlargement reactions by a-halo ketones and dehalogenations of vicinal dihalides

On decarboxylation, 1,2-dimethylindazolium-3-carboxylate forms the N-heterocyclic carbene, 1,2-dimethylindazol-3-ylidene, which deprotonates alpha-halo ketones. The resulting indazolium salt and the corresponding enolate form 1: 1 adducts which undergo a ring enlargement to cinnolines. Reaction with 2-bromo-2.3-dihydro-1H-inden-1-one gives a 4-hydroxyspiro[cinnoline-3,2'-inden]-1'-one by ring enlargement reaction (X-ray crystal Structure analysis). Vicinal dibromides undergo debromination under these conditions to give alkenes, and Substrates with 1,2-dibromoethene partial structure give acetylenes. As 3-bromoindazole is found as the second product of this reaction, an E(1cb) mechanism, initiated by Br(+) abstraction by the N-heterocyclic carbene of indazole, is suggested.