Molecular Structure, Intramolecular Hydrogen Bonding, Solvent-Induced Isomerization, and Tautomerism in Azolylmethylidene Derivatives of 2-Indanone

Bis adducts of 2-indanone with 1H-pyrrole-2-carbaldehyde, 1H-imidazole-2-carbaldehyde, 1H-imidazole-5-carbaldehyde, and 1H-indazole-3-carbaldehyde 3–6 have been synthesized and the intertwined processes of tautomeric NH proton transfer, intra- and intermolecular H-bond rupture and formation, and Z/Z, Z/E, and E/E-isomerization have been studied by NMR and FTIR spectroscopy and by conducting DFT calculations. Pyrrole derivative 3 both in chloroform and in dimethyl sulfoxide (DMSO) exists exclusively in the form of the Z,Z-isomer stabilized by two intramolecular N–H···O hydrogen bonds. For derivatives 4–6, the isomeric composition depends on the solvent so that in polar media such as DMSO and pyridine the Z,E- and E,E-isomers predominate. The driving force for isomerization of 4–6 is stabilization of the Z,E- and E,E-isomers in basic polar solvents due to formation of intermolecular NH···B and intramolecular C–H···N hydrogen bonds. The suggested mechanism of isomerization includes proton transfer from nitrogen to oxygen in the Z-moiety followed by rotation about the C–C bond in the formed enol and isomerization of the latter to the E-moiety of the ketone tautomer.