Etudes par RMN en serie heterocyclique–III. Non-equivalence magnetique et inversion de l'azote dans une serie d'heteocycles pentagonaux diazotes,pyrazolines-2 et 3, pyrazolidines et pyrazolidones

The non-equivalence of ethyl, benzyl and isopropyl groups fixed on nitrogen atoms in 2-and 3-pyrazolines, pyrazolidines and pyrazolidones depends on the assymetry existing in the molecule. The asymmetric centres are either a ring carbon atom, a quaternary nitrogen atom (protonation, quaternarization) or a tertiary nitrogen atom (slow nitrogen inversion). The nitrogen inversion process was observed only in the case of pyrazolidines (possibility of inversion at the two adjacent nitrogens) and of pyrazolidones (inversion of the ‘non-amidic’ nitrogen). The inversion is temperature-dependent and is affected by substituents.