Jairo Quiroga Puello, Braulio Insuasty Obando, C. Foces-Foces
Aug 4, 1997
Citations
1
Influential Citations
40
Citations
Journal
Tetrahedron
Abstract
Abstract The crystal and molecular structures of five 3(5)-amino-5(3)-arylpyrazoles differing in the nature of the substituent at the para position of 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 NH… N O hydrogen bonds and also by OH…N interactions in the monohydrate of 3. It is worth noting that in 1, 3, 4 and 5 there are N-H…π(arene) contacts that might play a role in stabilizing the packing. Solid state 13C NMR results are consistent with the above crystallographic conclusions, 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 (1H and 13C) allow to determine the 3-amino/5-amino tautomeric equilibrium constant, KT, which obeys a Hammett relationship with σp. 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 relatively lower energy by approximately 2 kcal mol−1. The potential energy surface as a function of the hybridization of the amino group and its conformation have also been analyzed.