STRUCTURE OF 3-AMINO-4,5-DIHYDROPYRAZOLES IN ACID-MEDIA - X-RAY STRUCTURE OF 3-AMINO-1-PHENYL-4,5-DIHYDROPYRAZOL-2-IUM PICRATE AND THE ORIGIN OF BROAD SIGNALS IN H-1-NMR SPECTROSCOPY

The origin of the broadening of NMR signals in 3-amino-4,5-dihydropyrazoles has been studied in the 1-phenyl series. It has been proved that the broadening is due to the presence of acid in some solvents, like deuteriochloroform, by reproducing it by addition of trifluoroacetic acid. To determine the protonation site of 3-amino-1-phenyl-4,5-dihydropyrazole 1, the crystal structure of its picrate 5 was determined. Contrary to other dihydropyrazoles, 1 protonates on N-2, like an amidine.