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Ketones keto-enol tautomerism

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... [Pg.1287]

There is no simple, commonly accepted method for the preparation of imidazoles, but rather many different approaches. One approach, somewhat related to chemistry seen in previous chapters, involves the reaction of an a-hydroxy-ketone such as 121 with formamide, 122. The -NH2 unit of forma-mide attacks the carbonyl (acyl addition), and loss of water (elimination) gives an enol that tautomerizes to the ketone. (Keto-enol tautomerism was first discussed in Chapter 10, Section 10.4.5.) A second molecule of formamide reacts with this ketone via acyl addition to give a product, which loses water. An intramolecular attack of the nitrogen atom from this product to one -CHO rmit on the carbonyl of the other CHO unit, followed by loss of water under the reaction conditions, gives imidazole, 123. [Pg.1335]

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... [Pg.379]

Enols are related to an aldehyde or a ketone by a proton transfer equilibrium known as keto-enol tautomerism (Tautomensm refers to an mterconversion between two struc tures that differ by the placement of an atom or a group)... [Pg.759]

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... [Pg.264]

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... [Pg.866]

After succeeding in the asymmetric reductive acylation of ketones, we ventured to see if enol acetates can be used as acyl donors and precursors of ketones at the same time through deacylation and keto-enol tautomerization (Scheme 8). The overall reaction thus corresponds to the asymmetric reduction of enol acetate. For example, 1-phenylvinyl acetate was transformed to (f )-l-phenylethyl acetate by CALB and diruthenium complex 1 in the presence of 2,6-dimethyl-4-heptanol with 89% yield and 98% ee. Molecular hydrogen (1 atm) was almost equally effective for the transformation. A broad range of enol acetates were prepared from ketones and were successfully transformed into their corresponding (7 )-acetates under 1 atm H2 (Table 19). From unsymmetrical aliphatic ketones, enol acetates were obtained as the mixtures of regio- and geometrical isomers. Notably, however, the efficiency of the process was little affected by the isomeric composition of the enol acetates. [Pg.75]

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. [Pg.24]

The difference in conjugation between neutral molecules and their ion-radicals can also be traced for keto-enol tautomerism. As a rule, enols are usually less stable than ketones. Under the equilibrium conditions, enols exist only at a very low concentration. However, the situation becomes different in the corresponding cation-radicals, where gas-phase experiments have shown that enol cation-radicals are usually more stable than their keto tautomers. This is because enol cation-radicals profit from allylic resonance stabilization that is not available to ketones (Bednarek et al. 2001, references therein). [Pg.183]

There is a distinct relationship between keto-enol tautomerism and the iminium-enamine interconversion it can be seen from the above scheme that enamines are actually nitrogen analogues of enols. Their chemical properties reflect this relationship. It also leads us to another reason why enamine formation is a property of secondary amines, whereas primary amines give imines with aldehydes and ketones (see Section 7.7.1). Enamines from primary amines would undergo rapid conversion into the more stable imine tautomers (compare enol and keto tautomers) this isomerization cannot occur with enamines from secondary amines, and such enamines are, therefore, stable. [Pg.367]

Keto-enol-tautomerization is not resonance. The ketone and enol forms are different compounds that are in equilibrium. [Pg.164]

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). [Pg.81]

Equilibrium and rate constants for the keto-enol tautomerization of 3-hydroxy-indoles and -pyrroles are collected in Table 32 (86TL3275). The pyrroles ketonize substantially (103-104 times) faster than their sulfur or oxygen analogues, and faster still than the benzo-fused systems, indole, benzofuran, and benzothiophene. The rate of ketonization of the hydroxy-thiophenes and -benzothiophenes in acetonitrile-water (9 1) is as follows 2-hydroxybenzo[b]thiophene > 2,5-dihydroxythiophene > 2-hydroxythiophene > 3-hydroxybenzo[/ Jthiophene > 3-hydroxythiophene. 3-Hydroxythiophene does not ketonize readily in the above solvent system, but in 1 1 acetonitrile-water, it ketonizes 6.5 times slower than 2-hydroxythiophene (87PAC1577). [Pg.88]

Activation energies for unimolecular 1,3-hydrogen shifts connecting ketones and enols are prohibitive, so that thermodynamically unstable enols can survive indefinitely in the gas phase or in dry, aprotic solvents. Ketones are weak carbon acids and oxygen bases enols are oxygen acids and carbon bases. In aqueous solution, keto-enol tautomerization proceeds by proton transfer involving solvent water. In the absence of buffers, three reaction pathways compete, as shown in Scheme 2. [Pg.327]

The addition of water to an alkyne leads to the formation of an unstable vinyl alcohol. These unstable materials undergo keto-enol tautomerization to form ketones. The hydration of propyne forms 2-propanone, as the following figure illustrates. [Pg.112]

When a terminal alkyne is treated with an excess of hydrogen halide the halogens both end up on the more substituted carbon (Fig. F). This is in accordance with the Markovnikov s rule which states that the additional hydrogens end up on the carbon which already has the most hydrogens. The same rule applies for the reaction with acid and mercuric sulphate which means that a ketone is formed after keto-enol tautomerism instead of an aldehyde (Fig. G). [Pg.129]

The initial product has a hydroxy group attached to a carbon-carbon double bond. Compounds such as this are called enols (ene + ol) and are very labile—they cannot usually be isolated. Enols such as this spontaneously rearrange to the more stable ketone isomer. The ketone and the enol are termed tautomers. This reaction, which simply involves the movement of a proton and a double bond, is called a keto—enol tautomerization and is usually very fast. In most cases the ketone is much more stable, and the amount of enol present at equilibrium is not detectable by most methods. The mechanism for this tautomerization in acid is shown in Figure 11.6. The mercury-catalyzed hydration of alkynes is a good method for the preparation of ketones, as shown in the following example ... [Pg.425]

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. [Pg.28]

Enols tend to be unstable and isomerize to the ketone form. As shown next, this isomerization involves the shift of a proton and a double bond. The (boxed) hydroxyl proton is lost, and a proton is regained at the methyl position, while the pi bond shifts from the C = C position to the C=O position. This type of rapid equilibrium is called a tautomerism. The one shown is the keto-enol tautomerism, which is covered in more detail in Chapter 22. The keto form usually predominates. [Pg.411]

In addition to its mechanistic importance, keto-enol tautomerism affects the stereochemistry of ketones and aldehydes. A hydrogen atom on an a carbon may be lost and regained through keto-enol tautomerism such a hydrogen is said to be enolizable. [Pg.1047]

An isomerism involving the migration of a proton and the corresponding movement of a double bond. An example is the keto-enol tautomerism of a ketone or aldehyde with its enol form. (p. 1047)... [Pg.1095]

Equilibrium and rate constants for the keto-enol tautomerization of hydroxy heterocycles are summarized in Table 38 <1986TL3275>. The pyrroles ketonize (i.e., 226 — 230) substantially faster (103-104 times) than their sulfur or oxygen analogues, and still faster than the benzo-fused systems (indole, benzofuran, and benzothiophene). [Pg.135]


See other pages where Ketones keto-enol tautomerism is mentioned: [Pg.36]    [Pg.319]    [Pg.199]    [Pg.29]    [Pg.86]    [Pg.36]    [Pg.338]    [Pg.19]    [Pg.264]    [Pg.62]    [Pg.295]    [Pg.36]   
See also in sourсe #XX -- [ Pg.437 , Pg.438 ]

See also in sourсe #XX -- [ Pg.631 , Pg.632 , Pg.633 , Pg.634 ]




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Enol ketones

Enolization keto-enol

Enols keto-enol tautomerization

Enols ketonization

Enols tautomerism

Keto enol tautomerism

Keto-enol tautomerisms

Keto-enol tautomerization

Keto-enol tautomerization reactions acid-catalyzed ketonization

Keto-enolates

Keto-enols

Ketone enolate

Ketone enolates

Ketones enolization

Ketones tautomerization

Ketones, tautomerism

Ketonization-enolization

Tautomeric enol

Tautomerization enols

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