Keto-enol tautomeric forms

One keto-enol tautomerism forms an enediol a second then forms the ketone carbonyl group in fructose 6-phosphate. (See Section 12.8A and Problems 12.34 and 12.35.) The conversion of the aldose to a ketose is necessary to facilitate the chemistry in Reaction 4. 

Space does not permit any further detailed discussion except for a brief account of two interesting subjects. The first is concerned with keto-enol tautomerism. The classical example is ethyl acetoacetate, which can exist in the keto form (I) and the enol form (II) ... 

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

The keto-enol tautomerism of 1,2-benzisoxazoles has been examined and the existence of either form can be postulated on the basis of reactivity. IR analysis on the solid indicates the exclusive existence of the enol form, while in CHCI3 solution both appear to be present (71DIS(B)4483). 

The aromaticity of the pyrimidine and purine ring systems and the electron-rich nature of their —OH and —NHg substituents endow them with the capacity to undergo keto-enol tautomeric shifts. That is, pyrimidines and purines exist as tautomeric pairs, as shown in Figure 11.6 for uracil. The keto tautomer is called a lactam, whereas the enol form is a lactim. The lactam form vastly predominates at neutral pH. In other words, pA) values for ring nitrogen atoms 1 and 3 in uracil are greater than 8 (the pAl, value for N-3 is 9.5) (Table 11.1). 

Naphtho[2,l-h]furan-2-(3 -one 28 has been described as a keto tautomer (91JA2301). Naphtho[l,2-h]furan-3-(2//)-ones of type 29 (R = H, Me, Et, Pr, pentyl, heptyl) show keto-enol tautomerism with the enol form predominating (88RRC917). 

Armulated thiophenes of types 195 and 197 (A benzo, naphtho) were studied concerning keto-enol tautomerism. The ring fusion has a remarkable influence upon these equilibria. Whereas for the c-fused thiophenes 197 only keto tautomers were present, for h-fused derivatives 195 also the enol forms 196 were found (the equilibria are solvent dependent) (82JOC705). 

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerisni. The individual keto and enol forms are said to be tautomers, a word used to describe constitutional isomers that interconvert rapidly. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely... 

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). 

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

The possibility of keto-enol tautomerism exists. In basic medium, the enolic form is favored. Furthermore, the presence of the CONH group in the trans form could be proved from the ultraviolet absorption spectrum of niclosamide in methanol. 

At the end of the war, he returned to Munich as Associate Professor and Director of the Laboratory of Organic Chemistry. The Department was then under Richard Willstatter s direction, and Meyer occupied the same position as the one held for many years by Willstatter under Baeyer s leadership. He resumed his investigation on keto-enol tautomerism, and, in collaboration with H. Hopff, isolated by distillation the pure enol form of ethyl acetoacetate. Other work from the same period included an examination of the action of nitric acid on phenol and phenol ether (with W. Reppe and W. E. Elbers), and a study of narcosis (with H. G. Billroth)— an extension of his father s work. 

The last step of the reaction is the keto-enol tautomerization from T 4-cyclohexadienone intermediates (15) to aromatic products (16). Such a step is accompanied with a considerable gain in energy about 80 kJ mol 1 for vinylcarbenes [29], (where a phenol system is formed by the tautomerization step), and about 175 kJ mol 1 for phenylcarbenes [25] (where a naphtol system is produced). The energy barrier for such step should be lower than 40 kJ mol 1 according to previous calculations on similar systems [42],... 

Like reaction rates, the effect of solvent polarity on equilibria may be rationalized by consideration of the relative polarities of the species on each side of the equilibrium. A polar solvent will therefore favour polar species. A good example is the keto-enol tautomerization of ethyl acetoacetate, in which the 1,3-dicarbonyl, or keto, form is more polar than the enol form, which is stabilized by an intramolecular H-bond. The equilibrium is shown in Scheme 1.3. In cyclohexane, the enol form is slightly more abundant. Increasing the polarity of the solvent moves the equilibrium towards the keto form [28], In this example, H-bonding solvents will compete with the intramolecular H-bond, destabilizing the enol form of the compound. 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

The mixture of quinone methides initially formed by combination of the coniferyl radicals in their various mcsomeric forms, i.e. (I), (III), (V), (IX) and others, can be detected by means of their characteristic spectrum with a maximum at about 312 mp (52) the haU-hfe of the mixture in 70 % aqueous dioxan is 1 hour. Those quinone methides that can rearomatize by keto-enol tautomerism, e.g. (IX), or intramolecular additions, e.g. (I) or (III) may become stabilized faster than those of type (V) which rely on addition of a foreign molecule. The quinone methides that rearomatize intramolecularly appear to react exclusively in this way, probably by a concerted mechanism that represents collapse of the activated transition state. 

Pulsed radiolysis in NO-saturated aqueous solution at a variety of wavelengths has been used to generate hydroxyl radicals and measure the rate of addition to 1,4-benzoquinones. Mechanistically, the kinetic data indicated that the first-formed adduct undergoes a rapid keto-enol tautomerism to give (56). ... 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

The interconversion of keto and enol forms is termed enolization, or keto-enol tautomerism. The... 

Keto-enol-tautomerization is not resonance. The ketone and enol forms are different compounds that are in equilibrium. 

All the isomers 27-31 of benzoxazinotropone are subject to keto-enol tautomerism (Figure 4), which was proven by their O-acetylation. However, the keto forms were predicted to be favored energetically by calculations using the... 

An investigation of keto-enol tautomerism for perfluorinated keto-enol systems was undertaken. N-methylpyrrolidone (NMP) catalyzes equilibration of the keto and enol forms, but if used in more than trace amounts, it drives the equilibrium strongly toward enol because of hydrogen bonding to the amide. The enol is much more thermodynamically stable than its ketone, and it was found that in mildly Lewis basic solvents, such as ether, THE, acetonitrile, and NMP, the enohzation equilibrium lies too far right to allow detection of ketone (Correa et al., 1994). 

Internal alkynes undergo acid-catalysed addition of water in the same way as alkenes, except that the product is an enol. Enols are unstable, and tautomer-ize readily to the more stable keto form. Thus, enols are always in equilibrium with their keto forms. This is an example of keto-enol tautomerism. 

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