Enol content of

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 ... 

The liquid ester is an equilibrium mixture, the enol content of which is increased by distillation and falls slowly, on standing, to 30 per cent.1 The boiling point of i45-i47°/n mm., recorded in the literature, is higher than any noted in the present work. 

TABLE 2.1 The Enol Content of Some Carbonyl Compounds... 

The enol content of the product at this point is less than 2%. If the unenolized enedione is desired, the following distillation should be omitted and the product used without purification to avoid further isomerization. 

The product is contaminated by 5-15% of 2-acetylcyclo-hexanone, which was present in the crude selenide. This imputity may be avoided by purifying the selenide as described in Note 5. The enol content of the product obtained by the submitters varied... 

The earliest estimates of the enol content of various p-diketones was made by careful distillation (Meyer and Schoeller, 1920 Meyer and Hopflf, 1921). Although, as a method of studying the equilibrium, this approach has been replaced by non-intrusive methods, work on the physical separation of the tautomers has continued (e.g. Regitzand Schafer, 1981 Vogt and Gompper, 1981). 

Rate and equilibrium constant measurements for the enolization of 3-phenylcoumaran-2-one (82) in aqueous dioxane indicate an enol content of ca 0.1%, a pKg, of 8.9 (6.0 for the enol tautomer), and a fairly symmetrical transition state for enolate anion formation the Brpnsted Pb = 0.52 Below pH 5, enolization is independent of pH, occurring via O-protonation of the enolate. 

Ness, A. B., and S. M. McElvain The Enol Content of Some /S-Keto Esters. 

Enol Contents of Some Esters 1 Yields of 2-Pyrrolones V, Prepared from I"... 

TABLE 2.1 The enol content of some carbonyl compounds... 

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... 

Although the enol content of 2-methylpropanal is quite small, the compound is nevertheless capable of enolization, whereas the other compound, 2,2-dimethylpropanal, cannot enolize— it has no a hydrogens. 

Keto-Enol Tautomerism Enol Content of Carbonyl and Carboxyl Compounds... 

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.

Fig. 12.2. (Mono-)carboxyl-and (mono-)carbonyl compounds, in the order of increasing enolcontent (for the enol contents of active-methylene compounds see Table 12.1.)...

Fig. 12. 3. Enol content of three representative /t-diketones (B-D) ten-million-fold increase as compared to cyclohexanone (A).

Tab. 12.1 Substituent Effects on the Enol Content of Active-Methylene Compounds (for the enol content of monocarboxyl and monocarbonyl compounds cf. Figure 12.2)...

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). 

---