Keto—enol tautomerism catalysis

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). 

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... 

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

The addition of buffers is required to maintain constant pH during the reaction when experiments are to be carried out in the range 3 < PH <11. However, keto-enol tautomerization reactions usually exhibit so-called general acid and base catalysis. 26 The observed rate acceleration with increasing buffer concentration implies that the components of the buffer participate in some rate-determining step of the reaction. In most cases, the rate of reaction increases linearly with increasing buffer concentration at constant buffer ratio, chb/cb3 = const (Fig. 4a). 

Using these relations, the rate coefficients for specific and general acid catalysis, Ah and Aha, of any keto-enol tautomeric reaction can be predicted from the appropriate free energy of reaction ArG°. The required... 

The effects of cationic and zwitterionic micelles on the keto-enol tautomerism of 2-phenylacetyl-furan and -thiophene (73, X = O, S) have been studied in aqueous media.285 While the micelles perturb the equilibrium only slightly, the apparent acidity of one or other tautomer is increased, as the micelles have an affinity for the enolate. The systems also show lowered water rates at the minima of their pH-rate profiles, allowing an otherwise undetectable metal ion catalysis to be observed. 

We must now consider the kinetic analysis of catalysed reactions involving two proton transfers. These include the so-called prototropic isomerizations, notably keto-enol tautomerism. The simplest mechanisms for catalysis by acids and bases are as follows ... 

The importance of catalysis of the keto-enol tautomerization is brought home by calculations of the energy of the hypothetical transition states for direct intramolecular proton shift between carbon and oxygen. For acetone, it is 53 kcal mol This would require temperatures of 500°C for the reaction to occur without a catalyst. 

Since keto-enol tautomerism is accelerated by both acids and bases (the case of general acid-base catalysis), the reaction rate increases with both pH increase and decrease (V-shaped dependence). For example, the rate of acetone enolization in an acetate buffer is related to concentrations of die reactants by the following expression (H20,298 K) ... 

Tautomerization is the shift of an H from a carbon adjacent to a carbon-heteroatom double bond to the heteroatom itself (and the reverse). It is an acid- or base-catalyzed equilibrium. Two examples are the keto/enol pair (Z = oxygen) and the imine/enamine pair (Z = nitrogen). Base catalysis goes via the enolate anion. 

Enol-keto tautomerism proceeds by either acid or base catalysis. Base simply removes the proton from the enol oxygen, reversing the initial protonation. Subsequent (and slower) C-protonation furnishes the thermodynamically more stable keto form. 

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