Structural and computational features of four highly polar quinolin-2(1H)-ylidene derivatives: Equilibrium preference for enaminothione, enamine, and enaminone tautomeric structures

• Syntheses and X-ray diffraction structures of new 2(1H)-quinolylidene compounds.
• DFT computed ground-state energy differences in 2(1H)-quinolylidene compounds.
• Tautomeric preference for enaminothione, enamine, and enaminone structures.

The coupling of the active methylene compounds 2-cyanothioacetamide, malononitrile, and ethyl cyanoacetate with quinoline-N-oxide and 6-methoxyquinoline-N-oxide has been investigated, and the compounds (2Z)-2-cyano-2-quinolin-2(1H)-ylidene-ethanethioamide (1a), 6-methoxyquinolin-2(1H)-ylidene-propanedinitrile (1b), ethyl (2Z)-cyano-6-methoxyquinolin-2(1H)-ylidene-ethanoate (1c), and (2Z)-2-cyano-2-(6-methoxyquinolin)-2(1H)-ylidene-ethanethioamide (1d) have been isolated. Compounds 1a–d have been characterized by a combination of IR and NMR spectroscopies, combustion and X-ray diffraction analyses. The solid-state structure of each molecule is found to be almost planar and contains a hydrogen atom attached to the quinoline nitrogen. The observed pattern of exocyclic bond lengths signifies the existence of strong conjugation between quinoline fragment and the electron-withdrawing substituent situated at C-1. The CS bond distance of 1.708(1) and 1.704(1) Å in 1a and 1d, respectively, is intermediate in length between CS and CS bond distances. The CO bond distance in 1c displays a similar elongation [1.237(2) Å] relative to the CO bond distance in non-conjugated organic carbonyl compounds. All four structures exhibit an exocyclic CC bond between the heterocycle and the electron-withdrawing substituent that is elongated to 1.400 Å, a feature consistent with significant CC bond contribution. Strong intramolecular H-bonds in 1a, 1c, and 1d help promote molecular conjugation of the heterocycle and exocyclic appendage defined by the atoms HNCCCS/O into flat six-membered rings. In the crystals of 1a and 1b, the molecules are aligned in head-to-head stacks, while in the crystalline lattice of 1c and 1d the molecules exhibit a motif involving head-to-tail stacks. Intermolecular H-bonds link molecules 1a, 1b, and 1d into centrosymmetric dimers, while in the crystal of 1c weak intermolecular CH⋯N H-bonds orient molecules into zigzag chains. The bonding in 1a–d has been examined by electronic structure calculations, and these data are discussed with respect to the X-ray diffraction structures and the equilibrium preference for the enaminothione (1a and 1d), enamine (1b), and enaminone (1c) tautomers.

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