Identity Double‐Proton Transfer in (3Z)‐3‐Hydroxy‐1,4‐di(quinolin‐2‐yl)but‐3‐en‐2‐one

Abstract

Although there is a very fast (on the NMR timescale) double-proton transfer in (1Z,3Z)-3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one (the product of the condensation of ethyl oxalate with 2lithiomethylquinoline), it is the only species present in chloroform solution. Comparison of the product of condensation of ethyl oxalate with 2lithiomethyl derivatives of pyridine (recent studies) and quinoline (present studies) shows that benzoannulation considerably affects the tautomeric equilibrium. The observed changes are not only quantitative but also qualitative. Moreover, contrary to the proton transfer in the pyridine tautomers, this process is fast in the quinoline tautomers. Comparison of the experimental and ab initio/DFT GIAO-calculated (13)C and (15)N chemical shifts for the transition states in the proton-transfer reactions between (1Z,3Z)- 3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one and its tautomers support the theory that a concerted identity reaction takes place between the enolimine-enaminone and enaminone-enolimine tautomeric forms. As a consequence, the most stable tautomeric form, (1Z,3Z)-1,4-di(quinolin-2-yl)buta-1,3-diene-2,3-diol, is not present in the tautomeric mixture.

DFT studies on tautomeric preferences of 1-(pyridin-2-yl)-4-(quinolin-2-yl)butane-2,3-dione in the gas phase and in solution

“…Comparison of their energies (Table 1) shows that only OEa and OOa are expected to be present in the tautomeric mixture in vacuum. These results are in agreement with our earlier studies that show that the respective dienol and enol/ enaminone forms are more stable than 1,4-bis(pyridin-2-yl)butane-2,3-dione and 1,4-bis(quinolin-2-yl)butane-2,3-dione, respectively [6,7]. Solvent, especially this more polar and of more basic properties (DMSO) disfavors the enolimine tautomers.…”

“…Their formulas as well as numbering of heavy atoms in the molecule can be seen in Scheme 1. Except some exceptionally unstable rotamers [6,7], other species of this type are also included there.…”

“…Such tautomeric equilibria are not unique [7]. Calculations show that in vacuum the energies of 14.76 and 18.52 kJ/mol are required for OEa to be transformed into OOa and EEa, respectively.…”

“…NMR (chloroform solution), X-ray (crystal) and ab initio studies (vacuum and chloroform solution) show that (1Z,3Z)-1,4-di(pyridin-2-yl)but-1,3-diene-2,3-diol and (3Z)-3-hydroxy-1,4-di(quinolin-2-yl)-but-3-en-2-one are always more stable tautomeric forms than 1,4-di(pyridin-2-yl)butane-2,3-dione and 1,4-di(quinolin-2-yl)butane-2,3-dione molecule, respectively [6,7]. 1-(Pyridin-2-yl)-4-(quinolin-2-yl) butane-2,3-dione, the relative unsymmetrical a-diketone, is not known but its susceptibility to the prototropic rearrangement seems also very interesting.…”

Proton transfer reaction and intermolecular interactions in associates of 2,5-dihydroxy-1,8-naphthyridine

“…Thus, the conjugation in H-N-C=O moiety may influence the stability of the oxo form. Also, the benzoannulation affects the tautomeric equilibria [66][67][68][69][70][71][72][73]. In the studied structures, the aromatic character of the ring fused (let say A) to the one that loses aromaticity (B) is increased and that in turn compensates loss of the aromaticity in ring B and, to some extent, stabilizes the structure.…”