STRUCTURE AND TAUTOMERISM OF 3(5)-AMINO-5(3)-ARYLPYRAZOLES IN THE SOLID-STATE AND IN SOLUTION - AN X-RAY AND NMR-STUDY

The crystal and molecular structures of five 3(5)-amino-5(3)-arylpyraz oles differing in the nature of the substituent at the para position o f the phenyl ring (1: X = H; 3.H2O: X = OCH3; 4: X = Cl; 5: X = Br and 6: X = NO2) have been determined by X-ray analysis. Three situations were detected in the crystal structures: the 3-tautomer is present in 1, 3 and 4: the 5-tautomer is only found in 6 and both tautomers (1:1) are observed in 5. The crystal packings are governed by N-H ... N/O h ydrogen bonds and also by O-H ... N interactions in the monohydrate of 3. It is worth noting that in 1, 3, 4 and 5 there are N-H ...pi(arene ) contacts that might play a role in stabilizing the packing. Solid st ate C-13 NMR results are consistent with the above crystallographic co nclusions, thus allowing to determine that the only compound for which no good crystals have been obtained, the p-methyl derivative 2 should be a 3-amino tautomer. NMR solution studies (H-1 and C-13) allow to d etermine the 3-amino/5-amino tautomeric equilibrium constant, K-T, whi ch obeys a Hammett relationship with sigma(p). Geometry optimizations of the 3 and 5-tautomers at semi-empirical level (AM1) were performed. In all compounds, the 3-tautomer has been found to possess a relative ly lower energy by approximately 2 kcal mol(-1) The potential energy s urface as a function of the hybridization of the amino group and its c onformation have also been analyzed. (C) 1997 Elsevier Science Ltd.