(1Z,3Z)-3-[Quinolin-2(1H)-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol: An unexpected most stable tautomer of 1,3-bis(quinolin-2-yl)acetone

1H, 13C and 15N NMR spectra reveal that CDCl3 solution of 1,3-bis(quinolin-2-yl)acetone contains only (1Z,3Z)-3-[quinolin-2(1H)-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol. The proton transfer takes place between two basic centers of the molecule, which means that the process is an identity reaction by character. The situation is completely different from that detected in chloroform solution of 1,3-bis(pyridin-2-yl)acetone where three different tautomers are in equilibrium with each other. Although the proton transfers in both (1Z,3Z)-3-[quinolin-2(1H)-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol and (1Z,3Z)-3-hydroxy-1-[quinolin-2(1H)-ylidene-4-quinolin-2-yl]but-3-en-2-one have the identity reaction character and are fast on the NMR time-scale, this process takes place in two six-membered rings condensed to each other in the former and in two six-membered rings conjugated with each other in the latter compound. In consequence, NMR spectra of solutions of these compounds differ not only qualitatively but also quantitatively. The character of the double proton transfer in the molecule of (1Z,3Z)-3-[quinolin-2(1H)-ylidene]-1-(quinolin-2-yl)prop-1-en-2-ol is also supported by comparison of the experimental and calculated (GIAO) 13C and 15N chemical shifts.