Keto-enol tautomerism product

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... 

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],... 

Note that the final product in Figure 15-2 is stabilized as illustrated in Figure 15-3. This is an example of stabilization by keto-enol tautomerization (see Chapter 11 to review). The driving force is the result of the stability of the carbanion as shown in Figure 15-1. In this example of condensation, two ester molecules join, and alcohol, a smaller molecule, splits out. 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

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. 

The resulting enol undergoes keto-enol tautomerism to give the final acid product. 

In this reaction, the electron deficient carbon atom in carbon dioxide is attracted to the electron rich n system of the phenol. The resulting compound undergoes keto-enol tautomerization to create the product. 

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 ... 

Michael addition of tin enolates to a,/3-unsaturated esters is accomplished in the presence of catalytic amount of Bu4NBr. Other typical system using lithium enolates or silyl enolates with catalysts (Lewis acid or Bu4NF) fails to give the Michael products. An ab initio calculation reveals that higher reactivity is caused by high coordination of the tin enolate and the keto enol tautomerization for Michael adducts contributes to thermodynamical stabilization (Equation (77)).231 232... 

Pyridine is an important cofactor in the reaction system that leads to cleaner reactions and better yields of products than in its absence.15 It can act either as a u-donor for Pb(rv) or as a base catalyzing the keto-enol tautomerism. The u-donor effect was evidenced spectroscopically by the formation of adducts of pyridine with lead tetraacetate.45,4511 Moreover, pyridine catalyzed the ligand redistribution of twcfc-methoxyphenyllead triacetate to bis( r/ -mcthoxy-phenyl)lead diacetate. Other u-donor catalysts can be used and their nature is highly important for the success of the reaction. NaOMe and HOBT showed a modest effect, but a thousand-fold increase in rate over the uncatalyzed reaction was observed when 1,10-phenanthroline was employed and near quantitative yields of arylation products were obtained (Equation (16)).44... 

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. 

Photoelectron spectroscopy is an efficient tool for the gas-phase characterization of various elusive compounds <1989CSR317>. It has also been used to investigate the products formed on flash vacuum pyrolysis of alkylthio derivatives of Meldrum s acid <1991JOC3445>. The PESs of the thiophen-3(2//)-ones formed were similar to authentic samples from which it is apparent that no keto-enol tautomerism occurred in the gas phase and that only the keto tautomers are formed in the gas phase. 

The effect of solvent polarity on chemical systems including reaction rates and equilibria can be quite significant. In general, it is necessary to consider the relative polarities of the reactants and products. In equilibria, a polar solvent will favour the more polar species. A good example is the keto-enol tautomerization of ethyl acetoacetate shown in Figure 1.9. The keto tautomer is more polar than the enol tautomer and therefore the equilibrium lies to the left in polar media such as water Table 1.11. 

Raman effect was to keto-enol tautomerism. Thus in the case of acetoacetic ester, a characteristic frequency corresponding to that of the G=G bond is observed. In the keto form, CH3COCH2COOG2H5, such a bond does not occur, nor does such a bond exist in the hydrolysis products (acetone, CH3GOGH3 and ethyl acetate, GH3GOOG2H6) presence of this frequency shows that in... 

Carbonyl compounds are in a rapid equilibrium with their enols, a process called keto-enol tautomerism. Although enol tautomers are normally present to only a small extent at equilibrium and can t usually be isolated in pure form, they nevertheless contain a highly nucleophilic double bond and react with electrophiles, for example, aldehydes and ketones are rapidly halogenated at the a position by reaction with CI2 Br2, or I2 in acetic acid solution. Alpha bromination of carboxylic acids can be similarly accomplished by the Hell-Volhard-Zelinskii (HVZ) reaction, in which an acid is treated with Bi and PHr3. The a-halogenated products can then undergo base-induced E2 elimination to yield ,jS-unsaturated carbonyl compounds. 

Usually, monomers without methyl groups in the position a to the double bond are several times less active that those with an a-methyl group. For instance, the polymerization of EHMA above is a good example. MBL was expected to be active in CCT, but the activity observed was unexpected. Its Cc was 8 x 104, while Cc for MMA is 4 x 104. The keto—enol tautomerization (eq 53) is responsible for the high CCT rates because the product of the tautomerism is further stabilized by aromaticity. 

Compounds presenting the possibility of keto-enol tautomerism react easily with lead tetraacetate to afford products of a-acetoxylation. This reaction is found in the case of ketones and phenols, although it is frequently accompanied by other products derived from alternate mechanistic pathways. 

Butlerov found out that in alkaline medium (calcium hydroxide), formaldehyde HCHO polymerizes to form about 20 different sugars as racemic mixtures, Butlerov 1861. The reaction requires a divalent metal ion. Breslow found a detailed mechanism of reaction that explains the reaction products, (Breslow 1959). He found that glycol-aldehyde is the first product that is subsequently converted into glyceral-dehyde (a triose), di-hydroxy-acetone, and then into various other sugars, tetrose, pentose, and hexose. The formose reaction advances in an autocatalytic way in which the reaction product is itself the catalyst for that reaction with a long induction period. The intermediary steps proceed via aldol and retro-aldol condensations and, in addition, keto-enol tautomerizations. It remains unexplained how the phosphorylation of 3-glyceraldehyde leads to glycral-3-phosphate (Fig. 3.6). Future work should study whether or not ribozymes exist that can carry out this reaction in a stereo-specific way. 

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