Experimental and theoretical studies on prototropic tautomerism of 1-tetralone-2-carbothioamides and synthesis of regioselectively designed fused 2H-indazole derivatives

• Synthesis and characterization of 1-tetralone-2-carbothioamides • Spectral and DFT studies on prototropic tautomerism of carbothioamides • Regioselectively designed synthesis of 2 H -indazole derivatives • Correlation of X-ray diffraction and theoretical studies of 2 H -indazole Condensation of 1-tetralones with phenylisothiocyanate in presence of sodium hydride produced 1-oxo- N -phenyl-1,2,3,4-tetrahydronaphthalene-2-carbothioamides 2a-b. The structure of carbothioamides 2 was established on the basis of spectral data. Prototropic tautomerism studies of carbothioamide 2a were investigated in solution by NMR ( 1 H and 13 C NMR), electronic spectroscopy and gas chromatography techniques. The proton transfer behaviour in solid state and gaseous phase was studied utilizing FT-IR and Mass spectral studies, respectively. DFT approach was used to accomplish the theoretical computations. The ground state geometries of different tautomers were optimized using B3LYP method and 6-31G(d) basis set. The vibrational and NMR spectra of different tautomers were calculated and correlated with the experimental results. HOMO-LUMO studies of different probable tautomers of carbothioamide 2a were employed in determination of chemical reactivity descriptors.The dynamic photoisomerization experiments were performed in chloroform under UV irradiation (UV lamp, 15W) and spectra were recorded to investigate the photo dynamic keto ⇌ enol ⇌ thioenol tautomerization. Further, condensation of carbothioamides 2 with hydrazine and its aryl derivatives regioselectively furnished 4,5-dihydro-2 H -benzo[g]indazole derivatives 3. Spectrochemical characterizations and single crystal X-ray diffraction analysis of representative indazole derivative 3b resolved the structural ambiguity between the possible 2 H -indazole 3 and 1 H -indazole 4 regioisomers. Also, the optimized geometric bond lengths and bond angles, obtained by using DFT studies, were compared with the values obtained from X-ray diffraction. The regioselectivity of 3 was justified by root mean square deviations (RMSD) calculations which shows the minimum deviation for 2 H -indazole derivative 3 .