Nucleophilic Carbenes and Pseudo‐Cross‐Conjugated Mesomeric Betaines of Indazole Starting from Analogues of the Alkaloid‐Betaine Nigellicine

Abstract

Abstract The alkaloid Nigellicine possesses the indazolium‐3‐carboxylate ring system as electronically relevant partial structure which represents a member of the class of pseudo‐cross‐conjugated mesomeric betaines. Indazolium‐3‐carboxylate, prepared starting from indazole‐3‐carboxylic acid by an esterification–methylation–saponification sequence, can be converted into the isoconjugated phenyl‐ and 4‐(nitrophenyl)amidates and the thiocarboxylate as additional examples of pseudo‐cross‐conjugated systems. In accordance with results of ab initio calculations decarboxylation of indazolium‐3‐carboxylate with formation of the nucleophilic carbene indazol‐3‐ylidene begins at approximately 40 °C as evidenced by temperature‐dependent NMR spectroscopy. The carbene can be trapped with protons as indazolium salts, and carbon dioxide which reconstitutes the pseudo‐cross‐conjugated mesomeric betaine. According to the calculations, the carbene adopts a singlet ground state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)