Keto-enol tautomerism of phenindione and its derivatives: An NMR and density functional theory (DFT) reinvestigation

Keto-enol tautomerism of phenindione (2-phenyl-1,3-indandione) and of its 4-phenyl-substituted derivatives was reinvestigated by NMR, supported by density functional theory (DFT) quantum-mechanical calculations. The calculated data quantitatively confirmed the stabilization in DMSO solution of the enol form by a strong hydrogen bond. The symmetry of the NMR spectra of the enol forms was explained by a fast proton transfer between carbonyl oxygen atoms, which is facilitated by the formation of a strong ionic complex of the enol form and an anion. It was shown that keto-enol tautomerization also proceeds with the participation of a similar complex between an anion and the diketo form of 2-phenyl-1,3-indandione.