1.3- diketones, keto-enol equilibrium

An additional example of cycloamylose-induced catalysis which can probably be attributed to a microsolvent effect is the oxidation of a-hy-droxyketones to a-diketones (Scheme VIII). The rate of this oxidation is accelerated by factors ranging from 2.1 to 8.3 as the structure of the substrate is varied. As noted by Cramer (1953), these accelerations may be attributed to a cycloamylose-induced shift of the keto-enol equilibrium to the more reactive enol form. 

The bifluoride ion, HFj 297 Strong hydrogen bonding in p-diketones 309 Keto/enol equilibrium 310 Structures 312 Hydrogen-bond energies 314 Vibrational modes 315 Nmr spectroscopy 317... 

NMR studies have been mainly applied to elucidating the structural features of Schilf bases in solution. These studies are mainly concerned with SchiiT bases derived from benzaldehyde and its substituents, / -diketones, o-hydroxyacetophenones and o-hydroxyacetonaphthones. They were devoted to obtaining an insight into the keto-enol equilibrium (Scheme 1), syn and anti isomerism and steric distortions in different kinds of solvents. Relevant data and results are described in the following sections. 

The keto-enol equilibrium of the 1,3-diketones has been the subject of intensive studies using various physical techniques and theoretical calculations [78-80], Recently, X-ray crystal analysis of acetylacetone (83) was carried out at 110 K, and it was found that it exists as an equilibrium mixture of the two enol forms 83b and 83c [81]. Room-temperature studies show an acetylacetone molecule with the enolic H-atom centrally positioned, which can be attributed to the dynamically averaged structure 83d. Application of a crystal engineering technique showed that a 1 1 inclusion complex of83 can be formed with l,l/-binaphthyl-2,2/-dicarboxylic acid in which the enol form is stabilized by a notably short intramolecular hydrogen bond [82],... 

Tautomerism is an extremely solvent-dependent chemical process which affects the chemical properties of molecules. A well known example is the keto-enol equilibrium of (3-diketones, in which the enol form is the most populated species in apolar solvents, whereas the keto species is the most stable tautomer in aqueous solution [70], Another classical example is the solvent influence on the keto-enol tautomerism of 4-pyridone, where the population ratio between the keto and enol tautomers changes by a factor of 104 upon its transfer from the gas phase to an aqueous solution [71]. 

The hydrogen atom on the a-carbon is activated by the C=0 groups, and a conjugate system is formed. Under appropriate conditions the enolic hydrogen atom of /3-diketone can be replaced by a metal ion (M"+) to produce a six membered chelate ring (1) thereby shifting the keto-enol equilibrium favoring the enol form. 

In recent years a large number of jS-keto esters, and some /5-diketones have been investigated in an attempt to discover some of the smaller effects of the substituents, especially alkyl groups on the keto-enol equilibrium. In Table 34 are given the values for the equilibrium constant... 

Alcohols do not normally behave as acids in water, but the presence of an double bond adjacent to the OH group can substantially decrease the pATa by the mechanism of keto-enol tautomerism. Ascorbic acid is an example of this effect. The diketone 2,4-pentanedione (acetylacetone) is also a weak acid because of the keto-enol equilibrium. In... 

A polar solvent like water is known to have a relevant influence on the covalent structure of polar molecules. This is clearly illustrated by the effect of hydration on the tautomeric equilibria of molecules. A prototypical example is the keto/enol equilibrium of P-diketones whereas the enol form is the most populated species in the gas phase and in apolar solvents, the keto form is the most stable tautomer in aqueous solution [106,107]. Inspection of Figure 5 allows us to rationalize the solvent-induced change in the topology of this molecule. 

In this experiment proton NMR spectroscopy is nsed in evalnating the equilibrium composition of various keto-enol mixtures. Chemical shifts and spin-spin splitting patterns are employed to assign the spectral features to specific protons, and the integrated intensities are used to yield a quantitative measure of the relative amounts of the keto and enol forms. Solvent effects on the chemical shifts and on the equilibrium constant are investigated for one or more j8-diketones and j8-ketoesters. 

For acetone and the majority of cases in which this keto-enol tautomerism is possible, the keto form is far more stable and little if any enol can be detected. However, with j8-diketones and j8-ketoesters, such factors as intramolecular hydrogen bonding and conjugation increase the stability of the enol form and the equilibrium can be shifted significantly to the right. 

The presence of S-carbonyl groups with at least one proton on the carbon between them allows a keto/enol tautomerism to occur and, under appropriate conditions, the eno-lic proton can be removed. The S-5-tricarbonyl compounds are the higher analogues of the / -diketonates and can take triketone, monoenol and dienol forms in their tautomeric equilibrium (equation 86) accordingly, they can behave as bidentate or tridentate ligands to form metal chelate complexes. ... 

Unsymmetrical /i-diketones can form two )S-keto-enol tautomers, (90a), (90b). The corresponding N//-pyrazoles— readily synthesized from the diketones—exhibit annular tautomerism, (91a), (91b). These tautomerisms have been probed via AMI semiempirical calculations that show that the two phenomena are related in each case the position of equilibrium is strongly influenced by whether or not the CC double bond is part of (another) ring system (the Mills-Nixon effect). 

The keto-enol tautomerization of acetylacetone (CH3-CO-CH2-CO-CH3), a prototype /3-diketone, has been extensively studied experimentally, and attention has been paid to its solvent effect. Although the enol form is more stable than the keto in the gas phase owing to the intramolecular hydrogen bonding, the equilibrium is known to shift toward the keto in solution as the solvent polarity increases. The tautomerization in various types of solution, which includes H2O, dimethyl sulfoxide (DMSO), and carbon tetrachloride (CCI4), was examined by means of RISM-SCF method. [18]... 

Diketones usually occur as prototropic tau-tomerisms, so called keto-enol tautomerisms, in the solutions and the solids. Figure 1 shows the keto-enol tautomerism equilibria among three species of j8-diketo, /3-keto-enol, and j8-enol-keto as the substitutes Ri andR2 are different. The positions of the keto-enol tautomerism equilibria are determined by the solvent polarities and substituents, and the presence of bulkier substituents seems to be the driving force able to shift the tautomeric equilibrium toward the less-stabilized /8-diketo form. /3-Diketones occur as only a few percent of the /8-diketo tautomer in the solution, while otherwise almost exclusively as /3-keto-enol form in the solid. 

The tautomerism of slow keto-enol and fast enol-enol tautomeric equilibria of a number of l-(2-hydroxyphenyl)-3-naphthylpropane-l,3-diones can be easily monitored also by NMR spectroscopy, making this nucleus as suitable as and for this kind of studies [diketo form 5( 0) 469 and 548ppm, respectively enol forms 5( 0) 332 to 313 ppm (higher double bond character) and i5( 0) 156 to 135 ppm (lower double bond character)] [34]. Also computed chemical shifts were included in the analysis of the extremely fast tautomeric equilibrium of the two enol forms in asymmetric 1,3-diketones [35], for example, acetylacetone [5(C=0) 473.8 ppm 5(0-H) 191.2 ppm]. The equilibrium constants fCp thus obtained were compared with earlier experimental results based on 5( 0) in model or blocked tautomeric structures. The theoretical methodology could complement some inadequacies in experimental NMR techniques in evaluating equilibrium constants of compounds with rapid dynamic exchange [35]. 

Even though ketones have the potential to react with themselves by aldol addition recall that the position of equilibrium for such reactions lies to the side of the starting materials (Section 18 9) On the other hand acylation of ketone enolates gives products (p keto esters or p diketones) that are converted to stabilized anions under the reaction conditions Consequently ketone acylation is observed to the exclusion of aldol addition when ketones are treated with base m the presence of esters... 

The constant for reaction (31) for /3-diketones in tautomeric keto (HK) and enol (HE) forms, can be partitioned between Kk and KE (reactions 32 and 33). At equilibrium, equations (34) and (35) hold, and values obtained are given in Table 19. Kinetics showed that the reaction occurs through the enol form by parallel acid-independent and inverse-acid paths (Scheme 15).514 K... 

Table 8.6 shows that the equilibrium mixture consists of almost entirely keto form in the case of simple aliphatic and aromatic ketones, whereas significant amounts of enol tautomer are present in /J-diketones and /J-ketoesters. In these latter cases, the enol contains a conjugated tt electron system and an intramolecular hydrogen bond (30). Phenol exists entirely in the enol form, as the alter-... 

It was reported that the structure of the product obtained (68% yield) by acid-catalysed (8% HC1, reflux, 18 hr) hydrolysis and decarboxylation of the (3-keto ester 1 was not the expected diketone, but the stable enol 2 (mp 128-130°C), and that the equilibrium could not be reversed from enol to ketone, even on treatment of 2 with dilute alkali for 10 days. The evidence cited in support of 2 was entirely spectroscopic, as follows "C14H16O3 (232) M+ 232, Xmax (ethanol) 266 nm (log e, 4.09). The IR spectrum of 7 (= 2) showed a broad band between 3000-3500 cm 1 (enolic OH) and a sharp peak at 1705 cm-1 (ring -C=0) in the NMR spectrum (CDCI3) of 7 (s 2) a broad peak was present at 85.95 (1H, olefmic) and a downfield signal (exchangeable with D2O) at 811.1 (1H) indicating the presence of an enolic OH group."... 

Diketones exist as an equilibrium mixture of keto and enol tautomeric forms (see scheme below). Generally the enol tautomer is more stable than the keto tautomer, due to intramolecular H-bonding and simultaneous conjugation39,40 (Scheme 1). 

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