1,3-Dicarbonyl compounds, keto/enol tautomerism

Like reaction rates, the effect of solvent polarity on equilibria may be rationalized by consideration of the relative polarities of the species on each side of the equilibrium. A polar solvent will therefore favour polar species. A good example is the keto-enol tautomerization of ethyl acetoacetate, in which the 1,3-dicarbonyl, or keto, form is more polar than the enol form, which is stabilized by an intramolecular H-bond. The equilibrium is shown in Scheme 1.3. In cyclohexane, the enol form is slightly more abundant. Increasing the polarity of the solvent moves the equilibrium towards the keto form [28], In this example, H-bonding solvents will compete with the intramolecular H-bond, destabilizing the enol form of the compound. 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

The influence of solvents on chemical equilibria was discovered in 1896, simultaneously with the discovery of keto-enol tautomerism in 1,3-dicarbonyl compounds (Claisen [14] acetyldibenzoylmethane and tribenzoylmethane Wislicenus [15] methyl and ethyl formylphenylacetate Knorr [16] ethyl dibenzoylsuccinate and ethyl diacetylsuccinate) and the nitro-isonitro tautomerism of primary and secondary nitro compounds (Hantzsch [17] phenylnitromethane). Thus, Claisen wrote Es gibt... 

The first attempt to introduce an empirical relationship between an equilibrium constant and solvent polarity was made in 1914 by K. H. Meyer [24]. Studying the solvent-dependent keto-enol tautomerism of 1,3-dicarbonyl compounds, he found a proportionality between the equilibrium constants of various tautomeric compounds in the same set of different solvents cf. Table 4-2 in Section 4.3.1). He therefore split the tautomeric equilibrium constant Ky into two independent factors according to Eq. (7-9). 

Moriyasu M, Kato A, Hashimoto YJ. Kinetic studies of fast equilibrium by means of high-performance liquid chromatography. Part 11. keto-enol tautomerism of some P-dicarbonyl compounds. Chem Soc Perkin Trans 1986 11 515-520. 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl... 

Dicarbonyl compounds are capable of exhibiting keto-enol tautomerism this is illustrated for the case of acetylacetone. 

Perez and Perez [414] have discussed the special case of carbon acids where proton dissociation occurs from the central carbon in 1,3-dicarbonyl compounds. In these cases complications arise not only from keto-enol tautomerisms, but also because of geometric flexibility or lack thereof. Notable differences occur between rigid cyclic P-diketones and more flexible acyclic forms, as in the cases of Meldrum s acid (cyclic, pK = 7.32 in DMSO)... 

Some of the chelate compounds can act as dienophilic reagents, decomposing polyene structures formed as a result of dehydrochlorination of the polymer [24]. Compounds of calcium with 1,3-dicarbonyl derivatives, capable of keto-enol tautomerism, for example, with the ethyl ester of acetoacetic acid [199], have been proposed for the stabilization of polyvinyl chloride ... 

In general, the keto form is much more stable than the enol form, and the equilibrium lies strongly in favor of the keto form. Keto-enol tautomerism is generally considered an intermolecu-lar process. 1,3-Dicarbonyl compounds are capable of exhibiting keto-enol tautomerism this is... 

NMR spectroscopy of enols has been a key tool in determining the relative proportions of keto and enol forms in highly enolized species. For example, in pentane-2,4-dione, the keto and enol forms (Figure 17.7) can be readily identified in the NMR spectrum of the material the keto-enol tautomerism is slow compared with the timescale of the NMR spectroscopy experiment. This experiment has been extended by changing the temperature, which allows the calculation of thermodynamic parameters for the equilibrium, and the use of a range of solvents to study the effect of polarity. The process has also been studied in the gas phase, where enols predominate for almost all p-dicarbonyl compounds. In solution, the keto forms are generally some 8-9 kj moH more stable than in the gas phase. 

Pentanedione (acac) and related jS-dicarbonyl compounds are an extremely important class of hgands that have been studied widely for many years. In general, jS-dicarbonyl compounds exist as mixtures of tautomeric keto and enol forms (equation 15). These compounds are usually easily deprotonated to form monoanions, which form the basis for a large class of coordination compounds, encompassing vir-tuaUy every element. In addition, coordination compounds of the dianions and trianions of jS-diketones have been observed, as well as complexes of the neutral molecules. ... 

Since the preparation of acetylacetone and similar /J-dicarbonyl compounds in the latter half of the nineteenth century, organic chemists have had considerable interest in their properties and reactions. The best known phenomenon of -dicarbonyls is their participation in tautomeric equilibria where an apparently pure compound is actually a mixture of two substances having keto and enol structures. Early studies with / -dicarbonyls revealed that these compounds exhibited chemical properties indicative of both the dicarbonyl and unsaturated hydroxy ketone structures. In an unsuccessful attempt to explain this anomalous behavior Laar (7) in 1885 coined the word tautomerism" which has survived to the present. By the turn of the century it was generally held that the dichotomous properties of -dicarbonyls resulted from the pure compound existing as a mixture of both keto and enol tautomers. Since... 

The )S-dicarbonyl compound (/S-dikH) generally exists as an equilibrium mixture of the tautomeric keto and enol forms. The rate of spontaneous interconversion between these forms is rather slow at room temperature, and their simultaneous NMR spectroscopic observation is possible. For instance, the NMR spectrum of neat acetylacetone (acacH) is composed of OH, CH, CHj, and CH3 signals in accordance with the following equilibrium in ppm from internal Me4Si) ... 

The tautomeric properties of monothio-j8-dicarbonyls are of especial interest because, in principle, three tautomeric forms are possible for these compounds the keto-thioketo-form (340), the enethiol-keto-form (339), and the enol-thioketo-form (341). However, Bayer and Muller have... 

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