NMR studies in the heterocyclic series. 27—Carbon‐13 NMR determination of the protonation site of aminopyrazoles in trifluoroacetic acid

Abstract The 13C NMR data for 3‐aminoindazole and 14 aminopyrazoles were obtained in neutral and acidic media (trifluoroacetic acid) and compared with those for a series of pyrazoles, aminothiophenes and anilines as models for protonation. From the chemical shifts and coupling constants it is possible to conclude that all 3‐aminopyrazoles, 3‐aminoindazole and 5‐aminopyrazoles protonate on the heterocyclic ring nitrogen atom. This first protonation strongly deactivates the exocyclic nitrogen atom, so that no double protonation is observed, even in sulphuric acid. On the other hand, 4‐aminopyrazoles are stronger bases, which are protonated first at the amino group and then on both nitrogen atoms.