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

Dobosz R, Gawinecki R - J Mol Model (2013)

Bottom Line: 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.

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Affiliation: Department of Chemistry, University of Technology and Life Sciences, Seminaryjna 3, 85-326 Bydgoszcz, Poland. robertd@utp.edu.pl

ABSTRACT
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.

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The most stable tautomeric forms of 1–3. Following abbreviations were used for the respective moieties: P – pyridine, Q – quinoline, E and E' – 1,2-dihydropyridine (enamine), K − ketone, O – enol
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Sch2: The most stable tautomeric forms of 1–3. Following abbreviations were used for the respective moieties: P – pyridine, Q – quinoline, E and E' – 1,2-dihydropyridine (enamine), K − ketone, O – enol

Mentions: The compounds studied contain three acidic hydrogen atoms (H3/3′ and H6) and four basic centers (N1/1′ and O5/5′). Due to numerous potential proton transfers in their molecules, 4,6-di(pyridin-2-yl)cyclohexane-1,3-dione (1) and its dibezo derivative 3 may equilibrate with 12 different tautomers. Loss of symmetry by the molecule of the respective mono benzo derivative 2 is responsible for increase of this number to 21. The formulas of some of these tautomers are presented in Scheme 2 (the complete set of possible tautomeric forms, as well as their absolute and relative energies can be found in Supplementary materials). As this can be seen in Table 1, the relative free Gibbs energies of these forms, with respect to the most stable form, are less than 10 kcal mol-1.


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

Dobosz R, Gawinecki R - J Mol Model (2013)

The most stable tautomeric forms of 1–3. Following abbreviations were used for the respective moieties: P – pyridine, Q – quinoline, E and E' – 1,2-dihydropyridine (enamine), K − ketone, O – enol
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3713269&req=5

Sch2: The most stable tautomeric forms of 1–3. Following abbreviations were used for the respective moieties: P – pyridine, Q – quinoline, E and E' – 1,2-dihydropyridine (enamine), K − ketone, O – enol
Mentions: The compounds studied contain three acidic hydrogen atoms (H3/3′ and H6) and four basic centers (N1/1′ and O5/5′). Due to numerous potential proton transfers in their molecules, 4,6-di(pyridin-2-yl)cyclohexane-1,3-dione (1) and its dibezo derivative 3 may equilibrate with 12 different tautomers. Loss of symmetry by the molecule of the respective mono benzo derivative 2 is responsible for increase of this number to 21. The formulas of some of these tautomers are presented in Scheme 2 (the complete set of possible tautomeric forms, as well as their absolute and relative energies can be found in Supplementary materials). As this can be seen in Table 1, the relative free Gibbs energies of these forms, with respect to the most stable form, are less than 10 kcal mol-1.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, University of Technology and Life Sciences, Seminaryjna 3, 85-326 Bydgoszcz, Poland. robertd@utp.edu.pl

ABSTRACT
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.

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