Pericyclic rearrangements of N-heterocyclic carbenes of indazole to substituted 9-aminoacridines.

On deprotonation, 1-arylindazolium salts form 1-arylindazol-3-ylidenes which rearrange spontaneously via ring cleavage, ring closure and subsequent proton transfer to substituted 9-aminoacridines. By contrast, the N-heterocyclic carbene of 2-phenylindazolium cannot rearrange similarly and was trapped by sulfur.

Sydnone anions and abnormal N-heterocyclic carbenes of O-ethylsydnones. Characterizations, calculations and catalyses.

Deprotonated sydnones, which can be represented as anionic N-heterocyclic carbenes, were prepared as Li adducts and compared with deprotonated O-ethylsydnones (5-ethoxy-1,2,3-oxadiazol-4-ylidenes) which belong to the class of abnormal NHCs. The Pd complexes of the sydnone anions (X-ray analysis) as well as of the O-ethylsydnone carbenes proved to be efficient catalysts in aryl couplings of thiophenes.