Enol forms of 1,3-indanedione, their stabilization by strong hydrogen bonding, and zwitterion-assisted interconversion

By analyzing NMR spectroscopic data, and supported by IR, UWVis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3-indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all-keto and enol forms, the latter being stabilized by an intramolecular O-H⋯N hydrogen bond. Only the all-keto form was observed in the 5,6-dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6-dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O-H⋯N hydrogen bonds and thus it exists in the all-keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O^-⋯H⋯O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto-enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods.