N-Heterocyclic Carbenes
of Indazole: Ring Enlargement Reactions by α-Halo
Ketones and Dehalogenations of Vicinal Dihalides

Andreas Schmidt; Bohdan Snovydovych; Mimoza Gjikaj

doi:10.1055/s-2008-1067215

SPECIALTOPIC

N-Heterocyclic Carbenes of Indazole: Ring Enlargement Reactions by α-Halo Ketones and Dehalogenations of Vicinal Dihalides

Andreas Schmidt*^a, Bohdan Snovydovych^a, Mimoza Gjikaj^b
^a Institute of Organic Chemistry, Clausthal University of Technology, Leibnizstraße 6, 38678 Clausthal-Zellerfeld, Germany
Fax: +49(5323)723861; e-Mail: schmidt@ioc.tu-clausthal.de;
^b Institute of Inorganic Chemistry, Clausthal University of Technology, Paul-Ernst-Straße 4, 38678 Clausthal-Zellerfeld, Germany

Abstract

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Abstract

On decarboxylation, 1,2-dimethylindazolium-3-carboxylate forms the N-heterocyclic carbene, 1,2-dimethylindazol-3-ylidene, which deprotonates α-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.

Key words

N-heterocyclic carbene - vicinal dihalides - α-halo ketones - dehalogenation - cinnoline

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Details of the crystal structure have been deposited with the Cambridge Crystallographic Data Center under the number CCDC 689535. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK; Fax: +44(1223)336033; e-mail: fileserv@ccdc.ac.uk or http://www.ccdc.cam.ac.uk.