Carbonyl compounds keto-enol tautomerization

A carbonyl compound with a hydrogen atom on its a carbon rapidly equilibrates with its corresponding enol (Section 8.4). This rapid interconversion between two substances is a special kind of isomerism known as keto-enol tautomerism, from the Greek Canto, meaning "the same," and meros, meaning "part." The individual isomers are called tautomers. 

Keto-enol tautomerism of carbon) ] compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that Joses H+ from its a carbon to yield a neutral enol (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). 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

Cyanuric acid exists in two tautomeric forms corresponding to keto-enol tautomerism in carbonyl compounds. The keto form predominates, and most of the reactions of cyanuric acid have their counterparts in the chemistry of the cyclic imides. Many of the reactions involve the replacement of all three imido hydrogens (Scheme 31). Usually, the reaction cannot be controlled to produce the mono- or di-substituted isocyanurates specificially, but there are exceptions, e.g. the reaction between cyanuric acid and aziridine (Scheme 31) (B-79MI22001, 63JOC85, 63AHC(2)245). 

Isomer stabilities and activation energies have been calculated for keto-enol tautomerization of simple carbonyl compounds, MeC(R)=X (X = O R = H, Me) 129 both specific and bulk solvent effects have been analysed. Related isomerizations of acid derivatives (R = F, CN) and other related structures (R = H X = CF12, NH, S) are compared. 

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

However, the lack of analytical methods to distinguish clearly diastereoisomers such as 7a and 7b casts some doubt on the above assumptions. Specifically, it cannot be ensured with ultimate certainty that the reactions have really occurred in the desired manner or whether epimerization has taken place to a certain extent. In the case of 7a/Fe+ and 7b/Fe+, for example, the experimental data clearly reveal the occurrence of diastereoselective dehydrogenation, but the intrinsic SE might even be larger if the samples were not the pure diastereoisomers 7a and 7b but only enriched samples. Epimerization at C(3) is in fact facile via keto/enol tautomerism of the carbonyl compound occurring upon work-up. Therefore, the isotopolog 7c was prepared via an independent synthetic route (N2D2 reduction of 2-methyl-but- -2-enoic acid). As the results obtained with... 

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. ... 

Aliphatic nitro compounds show a number of reactions which parallel those of carbonyl chemistry. Primary and secondary nitro compounds exhibit tautomerism paralleling keto-enol tautomerism (Scheme 3.94a). Aliphatic nitro compounds dissolve in aqueous sodium hydroxide with the formation of sodium salts. The resultant anions behave as carban-ions and will condense with aldehydes. An example involves the formation of m-nitrostyrene from nitromethane and benzaldehyde (Scheme 3.94b). 

We shall take up first the behavior of ketones toward the halogens, and see evidence that carbanions do indeed exist at the same time, we shall see an elegant example of the application of kinetics, stereochemistry, and isotopic tracers to the understanding of reaction mechanisms. And while we are at it, we shall see something of the role that keto-enol tautomerism plays in the chemistry of carbonyl compounds. 

This type of compound is called an enol, and the reorganisation of the carbon/ oxygen double bond in the carbonyl group to the carbon/carbon double bond in the enol compound is called the keto/enol tautomerism. Notice, also, that this is accompanied by a hydrogen atom changing place, from the carbon to the oxygen. Tautomers are different from canonical structures in that the former... 

Alpha hydrogens are hydrogens on carbons directly attached to a carbonyl group. They are weakly acidic and can be abstracted by base to form a carbanion. The carbanion is called an enolate ion and is resonance stabilized. Neutralization of the enolate ion results in an enol, a compound in which an alcohol group is directly bonded to a carbon involved in a carbon-carbon double bond. The enol is in equilibrium with the original aldehyde or ketone in an equilibrium referred to as keto-enol tautomerism. The equilibrium usually favors the keto form. 

Carbonyl (or keto) compounds are interconvertible with their corresponding enols. This rapid interconversion of structural isomers under ordinary conditions is known as tautomerism. Keto-enol tautomerism is catalysed by acids or bases. 

Hydroxy-substituted triazines148,149 are normally transformed into better leaving groups before they are replaced by another nucleophile. As such compounds often exhibit keto-enol tautomerism (see also p 670), nucleophiles may add to the carbonyl group and subsequently lead to the elimination of water, e.g. as in the formation of triamine 2. 

Whenever two or more readily interconvertible isomers of a substance are in (dynamic) equilibrium, there will generally be migration of double bonds. The most often encountered tautomerism is between the keto and tire enol forms of an oxygen-containing compound. Ethyl acetoacetate is probably one of the earliest known cases of keto-enol tautomerism. While the keto-form, XXH, shows a low intensity (e 20) band around 275 mp characteristic of an isolated keto-carbonyl group, the enol form, XXIII, shows a high intensity band (< 18,000) around 245 mp due to the conjugated double... 

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. 

Electrocyclic reactions are not limited to neutral polyenes. The cyclization of a pentadienyl cation to a cyclopentenyl cation offers a useful entry to five-membered carbocycUc compounds. One such reaction is the Nazarov cyclization of divinyl ketones. Protonation or Lewis acid complexation of the oxygen atom of the carbonyl group of a divinyl ketone generates a pentadienyl cation. This cation undergoes electrocyclization to give an allyl cation within a cyclopentane ring. The allyl cation can lose a proton or be trapped, for example by a nucleophile. Proton loss occurs to give the thermodynamically more stable alkene and subsequent keto-enol tautomerism leads to the typical Nazarov product, a cyclopentenone (3.220). 

Some carbonyl compounds exist predominantly in the enol form due to favorable interactions between the tautomeric end s OH group and an adjacent lone pair via a intermolecular hydrogen bond. For example, the equilibrium constant for 2,4-pentanedione in water lies in favor of the enol form and is about 4. Often, the equilibrium constant for the keto-enol tautomerism is very solvent dependent, and the keto form is more favored in less polar solvents, such as cyclohexane. 

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