Spectroscopic, X-ray and DFT studies of prototropic tautomerism and photophysical properties of regioselectively designed new 3-, 2,3-substituted tetrahydro[4H]indazol-4-ones

Abstract

• Regioselective synthesis of new 3-, 2,3-substituted tetrahydro[4 H ]indazol-4-one derivatives • Prototropic tautomerism studies by spectral, LC-MS and DFT studies • Computational studies on tautomers and regioisomers for structure validation • X-ray diffraction studies of representative compound • Photophysical studies of new obtained products have been reported Reaction of dimedone and arylisothiocyanate in triethyl amine produced carbothioamide 2 which on condensation with hydrazine or phenyl hydrazine generated 3-substitued-2 H -indazole 3 or 2,3-disubstituted-2 H -indazole 5, respectively instead of the expected regioisomers 3-substitued-1 H -indazole 4 or 1,3-disubstituted-1 H -indazoles 6. Spectral data (IR, NMR and Mass) were of little help for structural differentiation. The prototropic tautomerism studies of 3-substituted-2 H -indazol-4-ones have been described on the basis of spectral, LC-MS and DFT studies. The structural ambiguity was resolved by X-ray diffraction studies of compound 5 and validated by theoretical studies on the structure 5a and its regioisomer 6a . Photophysical studies (UV and fluorescence) of compounds 3 and 5 in different solvents are also reported.