Carbonyl-enol tautomerizations

Reprotonation occurs on the nitrogen to produce the amide. As was the case with the carbonyl—enol tautomerization, the stability of the carbon—oxygen double bond causes the amide tautomer to be favored at equilibrium. 

Table 20.1 Equilibrium Constants for Carbonyl-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... 

Following hydrolysis, keto-enol tautomerization of the carbonyl group from C2 to Cl gives glucose 6-phosphate. The isomerization is the reverse of step 2 in glycolysis. 

Keto-enol tautomerism (Sections 8.4, 22.1) The rapid equilibration between a carbonyl form and vinylic alcohol form of a molecule. 

Both the aldol and reverse aldol reactions are encountered in carbohydrate metabolic pathways in biochemistry (see Chapter 15). In fact, one reversible transformation can be utilized in either carbohydrate biosynthesis or carbohydrate degradation, according to a cell s particular requirement. o-Fructose 1,6-diphosphate is produced during carbohydrate biosynthesis by an aldol reaction between dihydroxyacetone phosphate, which acts as the enolate anion nucleophile, and o-glyceraldehyde 3-phosphate, which acts as the carbonyl electrophile these two starting materials are also interconvertible through keto-enol tautomerism, as seen earlier (see Section 10.1). The biosynthetic reaction may be simplihed mechanistically as a standard mixed aldol reaction, where the nature of the substrates and their mode of coupling are dictated by the enzyme. The enzyme is actually called aldolase. 

A commonly used nucleophile has been water. Although initial attack affords a hydroxy-carbene derivative, ready cleavage of the Ca—Cp bond resulting from formal keto-enol tautomerism occurs to give either the acyl or the metal carbonyl (usually cationic) and the corresponding organic fragment (Equation 1.13) ... 

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). 

The infrared absorption bands of the carbonyl and the N-H stretching regions have been reported for several other hydroxy-naphthyridines, although no conclusion was offered as to the position of a possible keto-enol tautomerism.38,84... 

Pron et al.569) looked at polyacetylene treated from the gas phase with H2S04 which leads to HS04 counter-ions. They found that the conductivity drops in air with the appearance of C=O bands in the ir, although the rate of decay is much lower than would be expected for undoped samples. The polymer was more rapidly degraded by exposure to water but could be redoped with further acid treatment. Pron et al.570) have also reported hydrolytic instability in polyacetylene with A1C14 as the counterion. In both cases the proposed mechanism involves addition of OH" to the chain and keto-enol tautomerism to form carbonyl groups. 

The treatment of 1,2-diols with Dess-Martin periodinane may lead either to a 1,2-dicarbonyl compound,14 or to an oxidative breakage of a C-C bond14,72 depending on stereoelectronic factors. When a 1,2-dicarbonyl compound is obtained, very often, one of the carbonyl groups tautomerizes to the enol form. Under controlled conditions, very often, it is possible to selectively oxidize one of the alcohols in a 1,2-diol, particularly when this alcohol is an allylic one.73... 

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. 

Scheme 3 Keto-enol tautomerism at carbonyl moieties in cellulose...

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

Aldol condensations also take place under acidic conditions. The enol serves as a weak nucleophile to attack an activated (protonated) carbonyl group. As an example, consider the acid-catalyzed aldol condensation of acetaldehyde. The first step is formation of the enol by the acid-catalyzed keto-enol tautomerism, as discussed earlier. The enol attacks the protonated carbonyl of another acetaldehyde molecule. Loss of the enol proton gives the aldol product. 

The alkene of the dehydroenzyme is conjugated with a carbonyl group—-it s electrophilic and the amino group of Phe can add to it in conjugate fashion. When the enol tautomerizes back to a carbonyl compound, it can be protonated on the inline carbon because the imine is conjugated to the enol. This might remind you of pyridoxaPs chemistry (p. 1384). 

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... 

Two transformations require special note because of their very high potential in organic synthesis. First, the use of allylic alcohols as substrates has been shown to result in /3-hydride elimination toward the OH group, and the resulting enol tautomerizes to the aldehyde. This overall conversion, illustrated for 2-methaUyl alcohol and 1-bromobutene, opens a new strategy for the synthesis of carbonyl-containing compounds, and is equivalent (with very different selectivities)... 

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