(1Z,3Z)-1,4-Di(pyridin-2-yl)buta-1,3-diene-2,3-diol:  The Planar Highly Conjugated Symmetrical Enediol with Multiple Intramolecular Hydrogen Bonds

Abstract

1H, (13)C, and (15)N NMR spectral data show that in chloroform solution (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-diene-2,3-diol, OO, is in ca. 9:1 equilibrium with (3Z)-3-hydroxy-1,4-di(pyridin-2-yl)but-3-en-2-one, OK, while no 1,4-di(pyridin-2-yl)-2,3-butanedione, KK, was detected. The species present in the tautomeric mixture were identified by comparing their experimental chemical shifts with those known for similar compounds as well as with the theoretically calculated (GIAO-HF/DFT) values. Ab initio calculations show that OK and especially the highly conjugated OO forms are preferred in the tautomeric mixtures both in vacuo and in chloroform solution. Comparison of experimental (Arrhenius) and calculated (ab initio) energies of OK shows that the MP2/6-31G//RHF/6-31G method gives the most precise results. There are one and two strong O-H.N hydrogen bonds present in the molecules of the former and latter compound, respectively. Other tautomeric forms, e.g., dienaminedione [(1Z,4Z)-1,4-di[pyridin-2(1H)-ylidene]butane-2,3-dione], and their rotamers were found to have higher energies. The single-crystal X-ray diffraction studies show that dienediol OO is the only tautomeric form present in the crystal at 173 K. Its almost perfectly planar molecule is stabilized by two strong intramolecular O-H.N hydrogen bonds.

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.…”

“…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.…”

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

“…These two compounds are isoelectronic. Recently [7] we found that proton transfer takes place in 1,4-di(pyridin-2-yl)butane-2,3-dione and that the highly conjugated dienol, (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-diene-2,3-diol, predominates over other tautomeric forms in chloroform. This tautomer is additionally stabilized by two strong intramolecular hydrogen bonds.…”

“…It is the only form present in the crystal. [7] Knowing that benzoannulation may affect the tautomeric equilibrium both qualitatively and quantitatively, [8±14] we were very much interested to discover how it affects the tautomeric preferences of 1,4-di(pyridin-2-yl)butane-2,3-dione. Its tautomers and rotamers are shown in Scheme 1.…”