Instability of 2,2-di(pyridin-2-yl)acetic acid. Tautomerization versus decarboxylation

The DFT calculations at the B3LYP level with 6-311G** basis set were carried out in order to reveal whether tautomerization or decarboxylation is responsible for the instability of 2,2-di(pyridin-2-yl)acetic (DPA) and 1,8-diazafluorene-9-carboxylic (DAF) acids. The carboxyl protons in both compounds are involved in the intramolecular hydrogen bonds (the pyridine nitrogen atoms are the hydrogen bond acceptors). Although formation of two intramolecular OH···N hydrogen bonds in the enols of both carboxylic acids enables effective electron delocalization within the quasi rings (···HO − C = C − C = N), only ene-1,1-diol of DAF has somewhat lower energy than DAF itself (ΔE is ca. 7 kcal mol^-1). DPA and its enediol have comparable energies. Migration of the methine proton toward the carbonyl oxygen atom (to form enediols) requires overstepping the energy barriers of 55-57 kcal mol^-1 for both DPA and DAF. The enaminone tautomers of the acids, formed by migration of this proton toward the pyridine nitrogen atom, are thermodynamically somewhat more stable than the respective enediols. The energy barriers of these processes are equal to ca. 44 and 62 kcal mol^-1 for DPA and DAF, respectively. Thus, such tautomerization of the acids is not likely to proceed. On the other hand, the distinct energetic effects (ca. 15 kcal mol^-1) favor decarboxylation. This process involves formation of (E)-2-(pyridin-2(1H)-ylidenemethyl)pyridine and its cyclic analogue followed by their tautomerization to (dipyridin-2-yl)methane and 1,8-diazafluorene, respectively. Although the later compound was found to be somewhat thermodynamically more stable, kinetic control of tautomerization of the former is more distinct.     

Effect of benzoannulation on tautomeric preferences of 4,6-di(pyridin-2-yl)cyclohexane-1,3-dione

Density functional theory (DFT) calculations at the B3LYP/6-311+G(d,p) level show that 4,6-di(pyridin-2-yl)cyclohexane-1,3-dione is a labile compound. On the other hand, its dienolimine tautomer (4,6-di(pyridin-2-yl)cyclohaxa-1,3-diene-1,3-diol) seems stable enough to be present in vacuum. Alternatively the equilibriated species are (i) dienolimine and enolimine-enaminone ((6Z)-3-hydroxy-6-(pyridin-2(1H)-ylidene)-4-(pyridine-2-yl)cyclohex-3-enone) or (ii) dienolimine, enolimine-enaminone and dienaminone ((4Z,6Z)-4,6-di(pyridin-2(1H)-ylidene)cyclohexane-1,3-dione). Benzoannulation of the pyridine ring at position 5,6 was found to increase the contribution of the tautomers which contain the enaminone moiety. Energies of the transition states between the stable tautomers were also calculated in order to estimate activation energy of the proton transfer. Values of the geometry based harmonic oscillator model of aromaticity (HOMA) index and Laplacian of the electron density in the hydrogen bond critical point (based on quantum theory of atom in molecules) shows that the enaminone moiety in the tautomers studied are stabilized by stronger intramolecular hydrogen bond than this present in the enolimine moiety.