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

Structure and spectral characteristics of 'Ortho' ((E)-4-hydroxy-2-(2'-(4'-R)-hydroxyphenyI)-imino)-naphthalen-1(2H)-one) and 'para' (2-(2'-(4'-R)-hydroxyphenyl)-amino)-1,4-naphthoquinone) tautomers of o-hydroxyanilino-1,4-naphthoquinone derivatives (R=H =, 1A; -CH3, 2A; and -Cl, 3A) are investigated using the H-1, C-13, DEPT, gDQCOSY, gHSQCAD NMR, HPLC, cyclic voltammetry techniques combined with the density functional theory. The compound 2A crystallizes in monoclinic space group P2(1)/c. wherein the polymer chain is facilitated via O-H center dot center dot center dot O and C-H center dot center dot center dot 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. H-1 and C-13 NMR spectra in DMSOd-d(6) 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 H-1 and C-13 NMR spectra. These inferences are corroborated by the density functional theoretic calculations. (C) 2016 Elsevier B.V. All rights reserved.