Enol Forms of 1,3‐Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion‐Assisted Interconversion

Abstract By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, 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 intramolecularO–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.