Keto-enol tautomerism acidity

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... 

Another approach for the synthesis of enantiopure amino acids or amino alcohols is the enantioselective enzyme-catalyzed hydrolysis of hydantoins. As discussed above, hydantoins are very easily racemized in weak alkaline solutions via keto enol tautomerism. Sugai et al. have reported the DKR of the hydantoin prepared from DL-phenylalanine. DKR took place smoothly by the use of D-hydantoinase at a pH of 9 employing a borate buffer (Figure 4.17) [42]. 

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 colour of curcumin varies with the pH of the medium. Under acid conditions a bright yellow is obtained but under alkaline conditions a reddish brown hue is obtained. This colour shift occurs because curcumin undergoes keto-enol tautomerism. 

The reaction of metal ion M"+ with the keto, enol tautomeric mixture of acetylacetone (acacH) in acidic aqueous solution has been treated by a similar approach to that outlined above (see Prob. 16). 

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. 

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 more complex structures are inappropriate for consideration here, but the two compounds orsellinic acid and phloracetophenone exemplify nicely the enolate anion mechanisms we have been considering, as well as the concept of keto-enol tautomerism. 

On the one hand. Rueping s protocol involved a combination of chiral BINOL phosphate (R)-3j (10 mol%, R = 2-naphthyl) bearing 2-naphthyl substituents and achiral acetic acid (20 mol%) [62], While stronger Brpnsted acid 3j is expected to activate electrophile 86, the weaker Brpnsted acid is proposed to facilitate the keto-enol tautomerism of nucleophile 101 (Scheme 40). On the other hand, Gong... 

The C-C cleavage pathway that leads to EG takes place to a smaller extent in the presence of Ni/Re, when keto-enol tautomerization is favored as is shown by the higher sensitivity of the reaction to the amount of base. GO consumption rate closely follows the solution pH, slowing greatly when the reaction medium becomes close to neutral (Figure 1). The acids identified in the reaction mixture, formic and lactic, are not present in high enough quantities to account entirely for the pH decrease (e.g. to... 

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. 

Keto-acids, decarboxylation, 286 Keto-enol tautomerism, 201, 219, 225 carbanions in, 278 catalysis of, 277... 

Problem 17.3 Compare the mechanisms for (a) base-catalyzed and (b) acid-catalyzed keto-enol tautomerism. 

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. 

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). 

S-Diketones, by virtue of their keto-enol tautomerism (Scheme 21), are weak acids and in most cases bind nickel(II) in the deprotonated enol form, acting as uninegative chelating ligands. 

Hydroxamic acid exists in two tautomeric forms, (1) and (2), and such keto-enol tautomerism provides a number of sites for coordination and chelation. The keto form (1) predominates in acid media and the enol form (2) in alkaline media 25 this has been corroborated by the extraction of vanadium benzohydroxamic acid complexes in organic solvents.26... 

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. 

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. 

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. 

Scheme 2 Acid-, base-, and uncatalyzed reaction paths of keto-enol tautomerism.

---