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-dihydropyra zoles 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 a cid. To determine the protonation site of 3-amino-1-phenyl-4,5-dihydro pyrazole 1, the crystal structure of its picrate 5 was determined. Con trary to other dihydropyrazoles, 1 protonates on N-2, like an amidine.