Enol form, of carbonyl compound

Carbanions derived from carbonyl compounds are often referred to as enolates, a name derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl compounds. The anions of nitro compounds are called nitronates and are also resonance stabilized. The stabilization of anions of sulfones is believed to be derived primarily from polar and polarization effects. 

The keto and enol forms of carbonyl compounds are constitutional isomers, but of a special type. Because they are easily interconverted by proton transfers in the presence of an acid or base, chemists use a special term to describe this type of constitutional isomerism. 

Enohzable carbonyl compounds could also be employed in the gold(III)-catalyzed [4+2] cycloaddition with 2-alkynylbenzaldehydes to give the corresponding naphthyl ketones (Scheme 21.43) [50]. This reaction may proceed via the inverse electron demand Diels-Alder reaction of benzopyrylium intermediates with the enol form of carbonyl compounds followed by dehydration. 

The formation of the intermediate enol-form of carbonyl compounds in aldehyde and ketone photolysis was first suggested by Davis and Noyes (1947) and was invoked by Gruver and Calvert (1958) to explain some unexpected results in the deuterium... 

Draw keto-enol tautomers of carbonyl compounds, identify acidic hydrogens, and draw the resonance forms of enolates. 

Keto-enol tautomerism of carbonyl compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that loses from its a carbon to yield a neutral eno) (Figure 22.1). This proton loss from the cation intermediate is similar to what occurs during an El reaction when a carbocation loses H to form an alkene (Section 11.10). 

Tautomerization of the enol forms a carbonyl compound. The overall result is addition of H2O to a triple bond. 

Kung (1974) mentioned that numerous reports appear in the literature on the study of keto -enol tautomerism of carbonyl compounds, triacylmethanes and cyclodiketones depending on the state as pure liquid, diluted in an organic solvent or in a gas phase. Except for compound D.45, which will be discussed, the aliphatic structures are represented as a-diketones and the cyclic structures under the keto-enolic form. 

The only known Ni(ii)-containing dioxygenase ARD (acireducton dioxygenase), which catalyzes the oxidative decomposition of yS-diketones, operates in the analogous way [40], This applies to the functional enzyme models, namely, Cu(ii)- and Fe(ii)-containing quercetin 2,3-dioxygenases, which catalyze the decomposition of yS-diketones in the enol form to carbonyl compounds with CO evolution [41-42],... 

To be capable of existing in an enol form, a carbonyl compound must have a hydrogen atom attached to the carbon atom adjacent to the carbonyl group. This hydrogen is called an a-hydrogen and is attached to the a-carbon atom (from the first letter of the Greek alphabet, a, or alpha). 

More complex carbonyl compounds can be much more strongly enolized. P-Dicarbonyl compounds (1,3-dicarbonyl compounds) exist largely in their enol forms, for example (Fig. 19.21). In the enol forms of these compounds, it is possible to form an intramolecular hydrogen bond, thus forcing the equilibrium position away from the diketo forms. There is also conjugation between the carbon—carbon... 

Ends are usually identified in the form of carbonyl compounds. As the enol and the keto forms usually exist in rapidly attainable equilibrium, the whole amount of the compound is eventually converted to a derivative of the keto form ... 

The selective intermolecular addition of two different ketones or aldehydes can sometimes be achieved without protection of the enol, because different carbonyl compounds behave differently. For example, attempts to condense acetaldehyde with benzophenone fail. Only self-condensation of acetaldehyde is observed, because the carbonyl group of benzophenone is not sufficiently electrophilic. With acetone instead of benzophenone only fi-hydroxyketones are formed in good yield, if the aldehyde is slowly added to the basic ketone solution. Aldols are not produced. This result can be generalized in the following way aldehydes have more reactive carbonyl groups than ketones, but enolates from ketones have a more nucleophilic carbon atom than enolates from aldehydes (G. Wittig, 1968). 

The ff-oxidation of carbonyl compounds may be performed by addition of molecular oxygen to enolate anions and subsequent reduction of the hydroperoxy group, e.g. with triethyl phosphite (E.J. Bailey, 1962 J.N. Gardner, 1968 A,B). If the initially formed a-hydroperoxide possesses another enolizable a-proton, dehydration to the 1,2-dione occurs spontaneously, and further oxidation to complex product mitctures is usually observed. 

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

Most carbonyl compounds exist almost exclusively in the keto form at equilibrium, and it s usually difficult to isolate the pure enol. For example, cyclohexanone contains only about 0.0001% of its enol tautomer at room temperature, and acetone contains only about 0.000 000 1% enol. The percentage of enol tautomer is even less for carboxylic acids, esters, and amides. Even though enols are difficult to isolate and are present only to a small extent at equilibrium., they are nevertheless responsible for much of the chemistry of carbonyl compounds because they are so reactive. 

The reaction can, however, be made preparative for (91) by a continuous distillation/siphoning process in a Soxhlet apparatus equilibrium is effected in hot propanone over solid Ba(OH)2 (as base catalyst), the equilibrium mixture [containing 2% (91)] is then siphoned off. This mixture is then distilled back on to the Ba(OH)2, but only propanone (b.p. 56°) will distil out, the 2% of 2-methyl-2-hydroxypentan-4-one ( diacetone alcohol , 91, b.p. 164°) being left behind. A second siphoning will add a further 2% equilibrium s worth of (91) to the first 2%, and more or less total conversion of (90) — (91) can thus ultimately be effected. These poor aldol reactions can, however, be accomplished very much more readily under acid catalysis. The acid promotes the formation of an ambient concentration of the enol form (93) of, for example, propanone (90), and this undergoes attack by the protonated form of a second molecule of carbonyl compound, a carbocation (94) ... 

Another attractive domino approach starts with an aldol reaction of preformed enol ethers and carbonyl compounds as the first step. Rychnovsky and coworkers have found that unsaturated enol ethers such as 2-237 react with different aldehydes 2-238 in the presence of TiBr4. The process consists of an aldol and a Prins-type reaction to give 4-bromotetrahydropyrans 2-239 in good yields, and allows the formation of two new C-C-bonds, one ring and three new stereogenic centers (Scheme 2.56) [131]. In the reaction, only two diastereomers out of eight possible isomers were formed whereby the intermediate carbocation is quenched with a bromide. 

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