Ab Initio Study on the Keto−Enol Tautomerism of the α-Substituted Acetaldehydes XH2CCHO (X = H, BH2, CH3, NH2, OH, F, CN, NC, and Cl):  Comparison with the Tautomerism in α-Substituted Acetaldimines and Acetyl Derivatives

Su, Chien-Chih; Lin, Chao-Kuo; Wu, Chen-Chang; Lien, M. H.

doi:10.1021/jp9839910

Cited by 37 publications

(34 citation statements)

References 37 publications

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“…[29] It has been shown that thio-enethiol tautomerization is greatly favored relative to the enolization of carbonyl compounds. The calculated relative (to the 1 t thio form) energy of the 3 t enethiol form (16.61 kcal mol -1 ) and the free energy barrier for 1 t Ǟ 3 t tautomerization (52.83 kcal mol -1 ) are not significantly different from those calculated at the MP2(full)/6-31G* level for thioacetamide [26] (17.20 kcal mol -1 and 56.66 kcal mol -1 ). The values of the free energies of the 1 t Ǟ 4 t and 1 o Ǟ 4 o tautomerizations are comparable, whereas in the assisted mechanisms the free energy barriers for 4-methyl-1,3-dihydro-2H-1,5-benzodiazepine-2-thione tautomerizations are smaller than those for 4-methyl-1,3-dihydro-2H-1,5-benzodiazepin-2-one.…”

Section: Tautomerism Of 4-methyl-13-dihydro-2h-15-benzodiazepine-2-mentioning

confidence: 68%

“…The high value of free energy for the 1 o Ǟ 4 o tautomerization is consistent with those for imineenamine tautomerization in α-substituted acetaldimines. [26] The effect of a dielectric solvent on the free energies of TS(1 o Ǟ4 o ) and TS(1 o Ǟ3 o ) transition states is negligible (changes are smaller than 1 kcal mol -1 ). However, the free energy barrier for 1 o Ǟ 2 o transformation in solution is higher than in the gas phase, by 5.11 kcal mol -1 .…”

Section: Intramolecular Proton Transfermentioning

confidence: 99%

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DFT Study on Tautomerism of Dihydro‐2H‐1,5‐benzodiazepin‐2‐ones and Dihydro‐2H‐1,5‐benzodiazepine‐2‐thiones

et al. 2009

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DFT calculations were performed to study the tautomeric rearrangements in the isolated, monoethanol-and diethanolsolvated, and dimeric forms of 4-methyl-1,3-dihydro-2H-1,5-benzodiazepin-2-one and 4-methyl-1,3-dihydro-2H-1,5-benzodiazepine-2-thione. Molecular geometries of the tautomers and transition state structures were optimized at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels of theory. The long-range solvent effects on the geometrical and energy parameters of tautomerization were also analyzed by use of the PCM model. The calculated relative Gibbs free energies were used to estimate the equilibrium constants. It

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Section: Tautomerism Of 4-methyl-13-dihydro-2h-15-benzodiazepine-2-mentioning

confidence: 68%

Section: Intramolecular Proton Transfermentioning

confidence: 99%

DFT Study on Tautomerism of Dihydro‐2H‐1,5‐benzodiazepin‐2‐ones and Dihydro‐2H‐1,5‐benzodiazepine‐2‐thiones

et al. 2009

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“…The bond lengthening was accompanied with a decrease in the electron density (Tables 2A and 2B). The BH 2 substituent has no lone pairs but the B atom has an empty π orbital [13] which can accept the electron density from C=C in imine and C=N in enamine.…”

Section: Effect Of Substituentsmentioning

confidence: 99%

Semiempirical Method MNDO for the Evaluation of the Effect of Different Substituents at the Imine-Carbon Position on the Acetaldemine-Vinylamine Tauotomerization and Comparison to the Substitution at α-Position

Al-Omari

2004

Zeitschrift Für Naturforschung A

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MNDO calculations have been employed to study the effect of some substituents of the acetaldemine-vinylamine tautomeric system at the imine-carbon position of CH 3 CXNH, where X = H, F, CN, NH 2 , NO 2 , BH 2 , CH 3 and CF 3 . It is found that the substitutents F, NH 2 and NO 2 encourage the formation of the enamine tautomer. The substitutents CN, CH 3 , CF 3 and BH 2 encourage the formation of the imine tautomer. Isodesmic reactions, free energy change (∆G), charge distribution and energy gap (E g ) between HOMO and LUMO were used to prove these findings. Resonance stabilization was a major factor in the determination of the most stable tautomer. These results were compared with previous work on the substitution at the σ -carbon position.

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“…Enolizations have, of course, been studied extensively, both theoreticallyand experimentally, ever since Lapworth's classic paper on acetone enolization over a century ago. It was R. P. Bell who originally proved that ketone enolization can be catalyzed jointly by a general base and general acid .…”

Section: Introductionmentioning

confidence: 99%

Proton shuffling in acid/base‐catalyzed enolizations: a computational study

Menger

2012

J of Physical Organic Chem

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Enolization of acetaldehyde catalyzed by the combined action of a general base (ammonia) and a general acid (formic acid) was examined by density functional theory at the B3LYP/6-311 + G(3df,2p) level while manipulating distance relationships among the reactants. Computations were carried out in the gas phase, in the presence of four water molecules, and with a dielectric constant of 78.4. Enolization involves an early transition state where general-base catalysis is more developed than general-acid catalysis. Although formic acid does not promote enolization by itself, it does facilitate a-proton transfer from acetaldehyde to the general base by several orders of magnitude. Formic acid accomplishes this feat via a hydrogen bond at a van der Waals distance to the carbonyl oxygen as opposed to forming a low-barrier hydrogen bond. A low-barrier hydrogen bond would indeed be capable of accelerating the enolization were it not for the energy cost of generating it. Formic acid may also facilitate enolization by internal solvation of the ammonium ion that is partially formed in the transition state via carbon-to-nitrogen proton transfer. General-base catalysis by trimethylamine, which is out of position to coordinate with the formic acid carboxyl, actually has lower activation energy than that of ammonia catalysis, possibly owing to basicity/shielding effects. Computations also demonstrate that the proton removed by the ammonia nitrogen remains on the nitrogen throughout rather than being transferred via low-energy rotation processes and secondary proton transfers to an oxygen atom of formic acid or the enol itself. Finally, stepwise and concerted mechanisms for enolizations have been proposed in the literature, with experimental evidence being provided for both. The concerted/non-concerted disagreement seems to stem from the continuum of organic mechanisms that Nature bestows onto organic chemistry. Thus, acid/base catalysis varies from stepwise at one extreme to synchronous at the other extreme with an infinite number of concerted mechanisms in between. Since the degree of concertedness undoubtedly depends upon the particular acid, base, substrate, and solvent, disparate enolization models are to be expected.

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Ab Initio Study on the Keto−Enol Tautomerism of the α-Substituted Acetaldehydes XH₂CCHO (X = H, BH₂, CH₃, NH₂, OH, F, CN, NC, and Cl): Comparison with the Tautomerism in α-Substituted Acetaldimines and Acetyl Derivatives

Cited by 37 publications

References 37 publications

DFT Study on Tautomerism of Dihydro‐2H‐1,5‐benzodiazepin‐2‐ones and Dihydro‐2H‐1,5‐benzodiazepine‐2‐thiones

DFT Study on Tautomerism of Dihydro‐2H‐1,5‐benzodiazepin‐2‐ones and Dihydro‐2H‐1,5‐benzodiazepine‐2‐thiones

Semiempirical Method MNDO for the Evaluation of the Effect of Different Substituents at the Imine-Carbon Position on the Acetaldemine-Vinylamine Tauotomerization and Comparison to the Substitution at α-Position

Proton shuffling in acid/base‐catalyzed enolizations: a computational study

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