Tautomerism in o-hydroxyanilino-1,4-naphthoquinone derivatives: Structure, NMR, HPLC and density functional theoretic investigations

• Keto-enol tautomerism of o-hydroxyanilino-1,4-naphthoquinone derivatives is discussed.
• ‘Ortho – para’ tautomeric forms observed in DMSO solution.
• Only ‘para’ tautomer is predominant in CD3CN solvent.
• NMR, HPLC, cyclic voltammetry techniques support the tautomers in solution.
• Vibrational frequencies and NMR chemical shift are determined by DFT studies.

Structure and spectral characteristics of ‘Ortho’ ((E)-4-hydroxy-2-(2′-(4′-R)-hydroxyphenyl)-imino)-naphthalen-1(2H)-one) and ‘para’ (2-(2′-(4′-R)-hydroxyphenyl)-amino)-1,4-naphthoquinone) tautomers of o-hydroxyanilino-1,4-naphthoquinone derivatives (RH, 1A; CH3, 2A; and –Cl, 3A) are investigated using the 1H, 13C, DEPT, gDQCOSY, gHSQCAD NMR, HPLC, cyclic voltammetry techniques combined with the density functional theory. The compound 2A crystallizes in monoclinic space group P21/c. wherein the polymer chain is facilitated via OH⋯O and CH⋯O intermolecular hydrogen bonding. Marginal variations in bond distances in quinonoid and aminophenol moieties render structural flexibility to these compounds those in solution exist as exist in ‘ortho – para’ tautomers. 1H and 13C NMR spectra in DMSO-d6 showed two sets of peaks in all compounds; whereas only the para tautomer of for 1A and 2A, the para tautomer is predominant in CD3CN solution. Further the ortho-para interconversion is accompanied by a large up-field signals for C(3)H(3) in their 1H and 13C NMR spectra. These inferences are corroborated by the density functional theoretic calculations.

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