Ab initio Study of the Keto-Enol Equilibriumof Malonaldehyde

Delchev, Vassil B.; Nikolov, Georgi St.

doi:10.1007/pl00010307

Cited by 21 publications

(7 citation statements)

References 4 publications

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“…Research efforts have been put forward to design systems to improve the enol content. Keto–enol tautomerization has been observed in many biological processes that directly or indirectly proceed through keto–enol or amino–imino tautomerism, e.g., pyranose to furanose ring conversion of cyclic carbohydrate, glucose to fructose conversion, etc. − Keto–enol tautomerism also plays an important role in the formation of kynurenic acid, an antiexcitotoxic and anticonvulsant compound, which is generated due to the metabolism of l -tryptophan. − In the keto–enol tautomerization equilibrium process, one proton transfers from the α-carbon center to carbonyl oxygen through space via bond formation. − This proton transfer process is accelerated in the presence of solvent molecules or by suitable substituents. − In the case of acyclic ketone, reports reveal that the keto form is more stable in a polar solvent compared with the enol forms …”

Section: Introductionmentioning

confidence: 99%

DFT Study To Explore the Importance of Ring Size and Effect of Solvents on the Keto–Enol Tautomerization Process of α- and β-Cyclodiones

Jana

Ganguly

2018

ACS Omega

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We have explored the effect of ring size on keto–enol tautomerization of α- and β-cyclodiones using the M062X-SMD aq /6-31+G(d,p)//M062X/6-31+G(d,p) level of theory. The calculated results show that the activation free energy barrier for the keto–enol tautomerization process of α-cyclopropanedione ( 1 ) is 54.9 kcal/mol, which is lower compared to that of the other cyclic diketo systems studied here. The four-membered α- and β-cyclobutanedione ( 2 and 6 ) do not favor keto–enol tautomerization unlike other studied cyclic systems because of the ring strain developed in the transition-state geometries and their corresponding products. Water-assisted keto–enol tautomerization with one molecule reveals that the free energy activation barriers reduce almost half compared to those for the uncatalyzed systems. The two-water-assisted process is favorable as the activation free energy barriers lowered by ∼10 kcal/mol compared to those of the one-water-assisted process. The ion-pair formation seems to govern the lowering of activation barriers of α- and β-cyclodiones with two water molecules during the keto–enol tautomerization process, which however also overcomes the favorable aromatization in the three-membered ring system. The free energy activation barriers calculated with the M062X-SMD aq /6-31+G(d,p) level predicted that the keto–enol tautomerization process for the α-cyclodiones follows the following trend: 2 > 3 > 4 > 5 > 1 . Water-assisted tautomerization of α-cyclodiones also predicted 1-W and 1-2W as the most favored processes; however, 5-W and 5-2W were found to be disfavored in this case. The β-cyclodione systems also showed similar trends as obtained with α-diketone systems. The influence of bulk solvent on the keto–enol tautomerization process favors the formation of the enol form in a more polar solvent medium even under mixed solvent conditions in acetonitrile and hexane at M062X-SMD acetonitrile /6-31+G(d,p) and M062X-SMD hexane /6-31+G(d,p) levels of theory.

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Section: Introductionmentioning

confidence: 99%

DFT Study To Explore the Importance of Ring Size and Effect of Solvents on the Keto–Enol Tautomerization Process of α- and β-Cyclodiones

Jana

Ganguly

2018

ACS Omega

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“…It is known − that self-tautomerization of β-diketones to keto–enols via intramolecular proton transfer from carbon to oxygen is unfavorable due to a very high energetic barrier (up to 65 kcal/mol). Recently, it was shown that for 1,3-cyclo-hexanedione the formation of C–H···O hydrogen-bonded dimers of a diketo tautomer facilitates intermolecular proton transfer, thereby lowering the tautomerization barrier to 35–45 kcal/mol.…”

Section: Resultsmentioning

confidence: 99%

Keto–Enol Tautomerism of Phenindione and Its Derivatives: An NMR and Density Functional Theory (DFT) Reinvestigation

Sigalov

2015

J. Phys. Chem. A

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Keto-enol tautomerism of phenindione (2-phenyl-1,3-indandione) and of its 4-phenyl-substituted derivatives was reinvestigated by NMR, supported by density functional theory (DFT) quantum-mechanical calculations. The calculated data quantitatively confirmed the stabilization in DMSO solution of the enol form by a strong hydrogen bond. The symmetry of the NMR spectra of the enol forms was explained by a fast proton transfer between carbonyl oxygen atoms, which is facilitated by the formation of a strong ionic complex of the enol form and an anion. It was shown that keto-enol tautomerization also proceeds with the participation of a similar complex between an anion and the diketo form of 2-phenyl-1,3-indandione.

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“…Rotamer G is interesting in view of providing a probability for H(6) proton transfer to C(3), resulting in the keto form [12]. The transition state shows a negative frequency; the corresponding vibrations are shown in Fig.…”

Section: A G Conversionmentioning

confidence: 99%

Ab initio Study of Malonaldehyde Rotamers

Delchev

Nikolov

2000

Monatshefte fuer Chemie

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Malonaldehyde rotamer geometries were optimized using ab initio calculations at the HF level with STO-3G ÃÃ and 6-21G ÃÃ basis sets. The most stable rotamer is the 3-shaped one with cyclic structure and intramolecular hydrogen bond. The most unstable rotamer is that obtained by rotation of the 3-rotamer around the CO single bond by 180 due to the loss of the additional stabilization contributed by the intramolecular H-bond. The energy barriers separating the different rotamers vary between 13 and 233 kJ Á mol À1 . The structure of the transition states is non-planar with rotation angles varying between 72 and 98 .

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Ab initio Study of the Keto-Enol Equilibriumof Malonaldehyde

Cited by 21 publications

References 4 publications

DFT Study To Explore the Importance of Ring Size and Effect of Solvents on the Keto–Enol Tautomerization Process of α- and β-Cyclodiones

DFT Study To Explore the Importance of Ring Size and Effect of Solvents on the Keto–Enol Tautomerization Process of α- and β-Cyclodiones

Keto–Enol Tautomerism of Phenindione and Its Derivatives: An NMR and Density Functional Theory (DFT) Reinvestigation

Ab initio Study of Malonaldehyde Rotamers

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