Ketones enol content

The amount of enol present at equilibrium the enol content is quite small for sim pie aldehydes and ketones The equilibrium constants for enolization as shown by the following examples are much less than 1... 

Enolization (Sections 18 4 through 18 6) Aldehydes and ke tones having at least one a hydro gen exist in equilibrium with their enol forms The rate at which equilibrium is achieved is in creased by acidic or basic cata lysts The enol content of simple aldehydes and ketones is quite small p diketones however are extensively enolized... 

The enol content of a carboxylic acid is far less than that of an aldehyde or ketone and introduction of a halogen substituent at the a carbon atom requires a different set... 

The chemical properties of cycHc ketones also vary with ring size. Lower members (addition reactions, than corresponding acycHc ketones. The Cg—C 2 ketones are unreactive, reflecting the strain and high enol content of medium-sized ring systems. Lactones are prepared from cycHc ketones by the Bayer-ViUiger oxidation reaction with peracids. S-Caprolactone is manufactured from cyclohexane by this process ... 

Molecules in which the enolic double bond is in conjugation with another double bond. Some of these are shown in Table 2.1. As the table shows, carboxylic esters have a much smaller enolic content than ketones. In molecules... 

Table II indicates those compounds which, on reaction with D-glucose, have not yielded any crystalline product. A large enol content in the ketonic...

The equilibrium position between a simple ketone and its enol usually lies far on the side of the ketone (see Table 17-2). However, there are some interesting and important exceptions to this generalization. For instance, the influence of two carbonyl groups on the enol content is very striking, as we can see from the fact that 85% of 2,4-pentanedione is the enol form at equilibrium ... 

Kinetic and thermodynamic measurements show that 2-phenylacetylthiophene (92a) has a low enol content K = 3.55 x 10 7 (or )K = 6.45).136 The keto and enol tautomers have pKa values of 14.60 and 8.15, respectively. Relative to a phenyl or furanyl substituent at the carbonyl carbon, the thiophene increases the acidity of the enol tautomer, but stabilizes the ketone, probably via the resonance contribution (92b). Thus 2-thiophenyl stabilizes the enolate by electron attraction, but the ketone by donation. Effects of micelles on the equilibria are also reported. 

This technique, called bromine titration method , was extensively used by K.H. Meyer in the early twentieth century.18 It was later extended to determine the enol content of simple ketones using faster flow methods combined with more sensitive potentiometric measurements of bromine uptake, but this technique sometimes produced apparent enol contents that were far too high, such as the enol content of acetone of 2.5 x 10 4% that is frequently quoted in older textbooks of organic chemistry. The excessive values so obtained have been attributed to the presence of small amounts of impurities reacting with bromine. 

Let us have a look at some instructive pH-rate profiles. That for acetophenone was already discussed in the section pH-Rate Profiles (Fig. 3). Its general shape is characteristic for the behavior of the enols of simple ketones and aldehydes. The enolization constants of aldehydes tend to be higher than those of ketones compare, for example, pA h(acetonc) = 8.33 and pA"E(acetaldehyde) 6.23. This is in line with the well-known stabilizing effect of alkyl substitution on double bonds, in particular of the polar C=0 bond, a-Substitution of ketones and aldehydes by alkyl or, better still, by aryl groups further stabilizes the enol, so that the enol content of 2,2-diphenylacetaldehyde reaches 10%.34... 

The enolization constants of carboxylic acids to form enediols are generally still lower than those of ketones. The pAE of acetic acid is about 20.35 Due to the relatively high acidity of 1,1-enediols, the enol content of carboxylate anions is somewhat higher. When the carboxylate is attached to cyclopenta-dienyl, a strong mesomeric electron acceptor, the conjugate acid of the enol, fulvene-l,l-diol, becomes a strong acid, pAa = 1.3, and the pAE of the enol anion is reduced to 5.0 (Almstead JIK and Wirz J, Unpublished data).36,37... 

Fig. 12.1. Substituent dependency of the enol content of aldehydes and ketones. Here, the p/CE values, i.e., the negative of the base-10 logarithm of the equilibrium constants /fE of the respective tau-tomerization carbonyl compound enol, are used as a measure.

The bottom line complements Figure 12.1 by adding the >Ky values of representative ketones. The comparison of E G reveals the same substituent effects that are familiar from the analogous aldehydes A-C the enol content is increased by alkyl substituents in the on-position, and even more so by aryl substituents in the a-position. The ketone H in Figure 12.1, the nonexistent isophenol has by far the highest propensity to enolization of all the carbonyl compounds shown. The reason, of course, is that the tautomeric enol, phenol, is favored because of its aromaticity and thus particularly efficient C=C double bond stabilization. 

This latter thought has an important consequence if compounds with C=0 double bonds are sorted in decreasing order of resonance stabilization of their C=0 group they are at the same time sorted according to their increasing propensity to enolization. So as the resonance stabilization of the C=0 double bond decreases from 22 kcal/mol to somewhere near zero in the order carboxylic acid amide > carboxylic acid ester/carboxylic acid > ketone > aldehyde > carboxylic acid chloride/-bromide, the enol content increases in this same order (Figure 12.2). These circumstances immediately explain why no enol reactions whatsoever are known of carboxylic acid amides, virtually none of normal carboxylic acid esters/carboxylic acids, but are commonly encountered with ketones, aldehydes and carboxylic acid halides. 

The stabilization of ketone-substituted ketone enols that has just been presented suggests that all ketone enols that are acceptor-substituted should have an enhanced enol content. All car-... 

The enol content of simple ketones is much lower than that of /1-ketoesters or /3-diketones. For a number of electrophiles it is often too low. Hence, functionalizations with the respective electrophile via the enol form do not succeed in these cases. This problem can be managed, though, by converting the ketone (Formula A in Figure 12.16) into an enamine D with the aid of a condensation with a secondary amine that is in line with Figure 9.29 and the mechanism given there. Enamines are common synthetic equivalents for ketonic and aldehyde enols. 

Figure 12.24 depicts the oxidation of a silyl enol ether A to give an a,/3-unsaturated ketone B. Mechanistically, three reactions must be distinguished. The first justifies why this reaction is introduced here. The silyl enol ether A is electrophilically substituted by palladium(II) chloride. The a-palladated cyclohexanone E is formed via the intermediary O-silylated oxocarbenium ion C and its parent compound D. The enol content of cyclohexanone, which is the origin of the silyl enol ether A, would have been too low to allow for a reaction with palladium(II) chloride. Once more, the synthetic equivalence of a silyl enol ether and a ketonic enol is the basis for success (Figure 12.24). 

Table 20.1 shows that the amount of enol tautomer that is present at equilibrium in the case of simple aldehydes and ketones is very small. Simple carboxylic acid derivatives, such as esters, have an even smaller enol content. However, 1,3-dicarbonyl compounds... 

Most of these methods are unable to distinguish between enol and another fast-reacting species and, as stressed by Dubois and Barbier, results are highly dependent on the purity of the carbonyl compound. Another procedure which apparently does not suffer the same drawback was more recently suggested by Bell and Smith and was applied to cyclopentanone and cyclohexanone (Bell and Smith, 1966) and to acetophenone (Novak and Loudon, 1977) ketone solutions were pretreated with bromine and the enol content was measured by potentiometry after allowing the enol to form. This method was also used for determining the acidity constants of ends. 

In general, the dehydrogenation of steroidal ketones is carried out in dry benzene or dioxane at reflux with 1.1 to 2 equiv. of the quinone. ) Similar conditions have also been used to prepare flavones, chromones" and spirodienones" in good yields (Table 6). Consistent with the apparent requirement that enolization is a prerequisite to dehyi genation with quinones,reactants such as a-formyl ketones, e.g. (37) and (38), that have a high enol content, dehydrogenate raindly at room temperature (Table6)."5- 6... 

Enols are much more easily oxidized than the corresponding ketone tautomers, however, their tendency to ketonize rapidly has precluded extensive electrochemical and other one-electron oxidation studies so far. Hence, oxidation potentials were only determined of few relatively stable enols of fi-dicarbonyl compounds or a-cyano ketones, where the enol content may be as... 

Fortunately, the low enol content in simple ketone systems does not necessarily impose an obstacle to generating the corresponding enol radical cations in solution. As outlined in Sect. 2 the selective oxidation of the enol tautomer even in the presence of a vast excess of the ketone opens up an indirect, but quantitative access to enol radical cation intermediates for all systems, if an appropriate oxidant has been chosen. The first, albeit indirect evidence for this selective oxidation step stems from kinetic studies by Henry [109] and Littler [110-112] and will be discussed in more detail in Sect. 3.3. Direct evidence for a specific oxidation of enols was provided by Orliac-Le Moing and Simonet [108]. Using voltammetry at a rotating disc electrode they were able to establish a linear correlation between the anodic current and the enol content for various a-cyano ketones 11. In electrolysis experiments the corresponding 1,4-diketones 13 were obtained in high current yield (ca. 90%). 

Quite obviously, direct measurement of the oxidation potentials of the intermediate, highly unstable enols poses a severe experimental problem. From work by Kresge, Capon and others [58-60] it is known that under appropriate conditions even enols of aliphatic ketones can be prepared with lifetimes in the order of minutes. However, no electrochemical data on these systems are available so far. Even for arylacetones, where the enol content is in the order of 10 %, direct measurement of the oxidation potential is not known to date. Two approaches have been applied to estimate their oxidation potentials [183]. First, using a linear correlation [184] between the experimental oxidation potentials of benzyl radicals [185,186] and their AMI calculated ionization potentials (IPs) the Fiyj of the four enols 86a-86d were derived from the calculated IPs. Secondly, oxidation potentials of silyl enol ethers were used to... 

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