N-Heterocyclic Carbenes of 5-Haloindazoles Generated by Decarboxylation of 5-Haloindazolium-3-carboxylates

Syntheses and properties of 5‐fluoro‐, chloro‐ bromo‐, and iodo‐substituted 1,2‐dimethyl indazolium‐3‐carboxylates as new representatives of pseudo‐cross‐conjugated heterocyclic mesomeric betaines are described, and results of an X‐ray single crystal analysis of the nonhalogenated parent compound are presented. These betaines decarboxylate on heating to yield 5‐halo‐1,2‐dimethyl‐indazol‐3‐ylidenes, which can be trapped by protons, sulfur, and 2,4‐dichlorophenylisocyanate, respectively. The decarboxylation is studied by electrospray‐ionization mass spectrometry, NMR spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)