Annular Tautomerism of 3(5)-Disubstituted-1H-pyrazoles with Ester and Amide Groups

A series of disubstituted 1H-pyrazoles with methyl (1), amino (2), and nitro (3) groups, as well as ester (a) or amide (b) groups in positions 3 and 5 was synthesized, and annular tautomerism was investigated using X-ray, theoretical calculations, NMR, and FT-IR methods. The X-ray experiment in the crystal state showed for the compounds with methyl (1a, 1b) and amino (2b) groups the tautomer with ester or amide groups at position 3 (tautomer 3), but for those with a nitro group (3b, 4), tautomer 5. Similar results were obtained in solution by NMR NOE experiments in CDCl3, DMSO-d6, and CD3OD solvents. However, tautomer equilibrium was observed for 2b in DMSO. The FT-IR spectra in chloroform and acetonitrile showed equilibria, which can be ascribed to conformational changes of the cis/trans arrangement of the ester/amide group and pyrazole ring. Theoretical analysis using the M06-2X/6-311++G(d,p) method (in vacuo, chloroform, acetonitrile, and water) and measurement of aromaticity (NICS) showed dependence on internal hydrogen bonds, the influence of the environment, and the effect of the substituent. These factors, pyrazole aromaticity and intra- and inter-molecular interactions, seem to have a considerable influence on the choice of tautomer.

Nitro and aminobenzimidazoles

A summary of the preparation methods of 2 tautomeric and 4N-methylated benzimidazoles with a nitro group on the benzene ring (1–6) and with an amino group in the same positions (7–12) were summarized. Annular tautomerism of the title compounds1–12has been studied using1H,13C and15N NMR spectra in liquid and solid state (CPMAS), UV spectra and quantum chemical calculations.

The annular tautomerism of lithium 1,2,3-triazolate

The tautomeric equilibrium of 1-lithium-1,2,3-triazolate (1Li-TR) and 2-lithium-1,2,3-triazolate (2Li-TR) is studied by X-ray diffraction, NMR spectroscopy and molecular dynamics simulations.