Tautomeric Equilibrium (tautomeric + equilibrium)

Distribution by Scientific Domains

Selected Abstracts

Long-range substituent and temperature effect on prototropic tautomerism in 2-(acylmethyl)quinolines

Ryszard Gawinecki
Abstract Tautomeric equilibria between 2-(cinnamoylmethyl)quinoline, (Z)-1,2-dihydro-2-(cinnamoylmethylene)quinoline and (Z)-4-phenyl-1-(2-quinolyl)-1,3-butadien-2-ol were studied by 1H, 13C and 15N NMR methods. The ,CHCH, fragment conjugated with phenyl and a strong electron donor p -(1-pyrrolidine) substituent were found to favour the enolimine tautomer. This undergoes fast exchange (on the NMR time-scale) with the enaminone form. The amount of the latter tautomer was found to increase at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd. [source]

NMR Quantification of Tautomeric Populations in Biogenic Purine Bases

Abstract Purine bases such as purine, adenine, hypoxanthine, and mercaptopurine are known to exist in several tautomeric forms. Characterization of their tautomeric equilibria is important not only for predicting the regioselectivity of their N -alkylation reactions, but also for gaining knowledge of the patterns with which these compounds of significant biological activity form hydrogen bonds with their biological targets. The tautomeric equilibria of purine and some purine derivatives in methanol and N,N -dimethylformamide solutions were investigated by low-temperature 1H and 13C NMR spectroscopy. The N(7)H and N(9)H tautomeric forms were quantified by integrating the individual 1H NMR signals at low temperatures. The Gibbs free energy differences were calculated and the effects of substitution on the N(7)H/N(9)H ratio discussed. A previously published theoretically predicted mechanism of the tautomeric exchange is compared with our measurements in deuteriated solvents. The influence of concentration on the temperature of coalescence indicates that supramolecular clusters play a significant role in this proton transfer process. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Synthesis of 2,4-Diaryl-3,4-dihydro-2H - naphth[2,1- e][1,3]oxazines and Study of the Effects of the Substituents on Their Ring - Chain Tautomerism

István Szatmári
Abstract A number of 2-(,-amino-Y-substituted-benzyl)-1-naphthol hydrochlorides were prepared by a convenient Mannich-type aminoalkylation. 2,4-Diaryl-3,4-dihydro-2H -naphth[2,1- e][1,3]oxazines were prepared through the ring-closure reactions of the starting aminonaphthols with aromatic aldehydes, which proved to furnish three-component (ring1,open,ring2) tautomeric mixtures in CDCl3 at 300 K. The electronic effects of the 2-aryl groups on the ratios of the ring - chain tautomeric forms at equilibrium could be described by Equation (1). Study of the effects of substituents X and Y on the tautomeric equilibria [by the aid of the multiple linear regression analysis of Equations (2) and (3)] revealed that the trans -chain equilibrium constants are significantly influenced by the inductive effect (,F) of substituent Y on the 4-phenyl ring. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Predicting the tautomeric equilibrium of acetylacetone in solution.


Abstract This study investigates how the various components (method, basis set, and treatment of solvent effects) of a theoretical approach influence the relative energies between keto and enol forms of acetylacetone, which is an important model system to study the solvent effects on chemical equilibria from experiment and theory. The computations show that the most popular density functional theory (DFT) approaches, such as B3LYP overestimate the stability of the enol form with respect to the keto form by ,10 kJ mol,1, whereas the very promising SCS-MP2 approach is underestimating it. MP2 calculations indicate that in particular the basis set size is crucial. The Dunning Huzinaga double , basis (D95z(d,p)) used in previous studies overestimates the stability of the keto form considerably as does the popular split-valence plus polarization (SVP) basis. Bulk properties of the solvent included by continuum approaches strongly stabilize the keto form, but they are not sufficient to reproduce the reversal in stabilities measured by low-temperature nuclear magnetic resonance experiments in freonic solvents. Enthalpic and entropic effects further stabilize the keto form, however, the reversal is only obtained if also molecular effects are taken into account. Such molecular effects seem to influence only the energy difference between the keto and the enol forms. Trends arising due to variation in the dielectric constant of the solvent result from bulk properties of the solvent, i.e., are already nicely described by continuum approaches. As such this study delivers a deep insight into the abilities of various approaches to describe solvent effects on chemical equilibria. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

Complete assignment of the 1H and 13C NMR spectra of garciniaphenone and keto-enol equilibrium statements for prenylated benzophenones

Priscilla B. M. C. Derogis
Abstract This article reports the structural elucidation by IR, UV and MS spectroscopic data along with 1H and 13C NMR chemical shift assignments of two benzophenones isolated from the fruit pericarp of Garcinia brasiliensis Mart. (Clusiaceae): garciniaphenone, (1R,5S,7S)-3-benzoyl-4-hydroxy-6,6-dimethyl-5,7-di(3-methyl-2-butenyl)bicyclo[3.3.1]non-3-ene-2,9-dione, a novel triprenylated benzophenone; and 7- epi -clusianone, a tetraprenylated benzophenone that has already been extracted from another species of the same family. Furthermore, the keto-enol tautomeric equilibrium at solution-state was described for these compounds by 1D and 2D NMR spectral methods and one attempt to rationalize the different ratios between the noted tautomers was based on stereochemical features. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Chiral recognition of the Schiff bases by NMR spectroscopy in the presence of a chiral dirhodium complex.

Deuterium isotope effect on 13C chemical shift of the optically active Schiff bases, their dirhodium adducts
Abstract The dirhodium method has been successfully applied in chiral recognition of the optically active Schiff bases, derivatives of ortho -hydroxyaldehydes existing in the NH-form. or at tautomeric equilibrium. The position of the equilibrium of Schiff bases as well as their adducts has been established on the basis of measurements of deuterium isotope effects on 13C chemical shifts. The presence of the proton transfer equilibrium or NH-tautomer has promoted the adduct formation. At the equilibrium state, formation of the adducts has shifted the proton transfer equilibrium towards the NH-form. The binding site was the oxygen atom of the proton donor group. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Conformational and tautomeric eccentricities of 2-acetyl-1,8-dihydroxynaphthalenes

Poul Erik Hansen
Abstract Tautomerism in aromatic systems with oxygen substitutents is rare. This is investigated in 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (1) and in 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (2). The tautomeric nature of 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene is supported by long-range hydrogen,hydrogen coupling between the OH-1 and the OH-8 and by the isotope effects on 13C caused by deuteration at the CH3CO methyl group. Compound 2 participates in a degenerate equilibrium between two equivalent nonsymmetrical rotamers (2A and 2B), each having two intramolecular O···HO hydrogen bonds: one involving an acetyl oxygen and the neighboring hydroxyl group, and the other between the oxygen centers at positions 1 and 8. In addition, each rotamer is involved in a tautomeric equilibrium, with a structure having an OH-substituted exocyclic double bond (2AT or 2BT). DFT calculations for a large set of compounds highlight the factors controlling the unusual rotational and tautomeric behaviors. A very important factor seems to be the repulsive interaction between the O -1 and O -8 centers, which is modulated by formation of an OH-1···O-8 or OH-8···O1 hydrogen bond. Steric interactions, mesomeric release of electrons from the oxygen at position 8, and a strong OH···O···C hydrogen bond are other factors. Solid-state 13C NMR spectra of 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene at different temperatures demonstrated no averaging in the solid, whereas partially deuterated 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene showed an isotope effect at C-1 of 1.5 ppm, indicating tautomerism in the solid state. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Hydrogen bonding in 2-(2-oxothiazolidin-3-yl)-4,5-dihydrothiazolium hydrogen sulfate monohydrate

Rodrigo S. Corrêa
The asymmetric unit of the title compound, C6H9N2OS2+·HSO4,·H2O, contains a heterocyclic cation, a hydrogen sulfate anion and a water molecule. There are strong hydrogen bonds between the hydrogen sulfate anions and water molecules, forming an infinite chain along the [010] direction, from which the cations are pendent. The steric, electronic and geometric features are compared with those of similar compounds. In this way, structural relationships are stated in terms of the influence of the sulfate group on the protonation of the heterocycle and on the tautomeric equilibrium in the solid state. [source]