Sterically hindered nitrogen inversion in five-membered cyclic hydrazines

We found that (Pr iNH) 2 with MeCHO form only pyrazoline 1 because of the competitive crotonization of the latter followed by reaction with hydrazine; pyrazoline 2, pyrazolidinones 6 and 9, pyrazolidinol 8 and pyrazolidine 7, in which the inversion of nitrogen atoms is hindered by bulky Pr iN substituents, were prepared for the first time, and the inversion barriers were determined.