Enols equilibria involving

Which tautomer is more stable Would you expect to be able to observe both tautomers at room temperature Rationalize any differences between this keto-enol equilibrium and that above involving acetone and propen-2-d. 

More complex equilibria (Fig. 76), also involving chemical modifications, are shown by the aminomethyl derivatives of a-arylidene-P-ketoesters (207), studied as anticancer drugs - these have been found to be in equilibrium with cyclic quaternary ammonium salts, which in turn are characterized by the presence of kcto-enol equilibrium. 

The tautomeric transformations of phenols can be subdivided into two groups (i) keto-enol tautomerism which is accompanied by loss of the aromatic character of the ring, and (ii) tautomeric equilibrium involving participation of substituents where the aromatic phenol nucleus is conserved. 

For most purposes, organic chemists are satisfied with less rigorous treatments of equilibrium processes than those exemplified by the above. In the remainder of this Section are listed some of the many such treatments, some of which are more rigorous than others. They are classified according to the nature of the equilibrium involved. For convenience, those systems in which an equilibrium process was shown to be absent, and those, such as keto-enol systems, where equilibration is slow, are included here. 

In the Legal reaction, discovered in 1883 by E. Legal, nitroprusside reacts witir tire a-carbon of enolizable ketones and aldehydes. The exact mechanism is not established, but it is clear tirat the first step involves the enolate ion of the substance to be examined. A compound having an unsaturated a-carbon will in solution partly be present as the resonans form called the enol form as opposed to the parent keto form. The kefol-enol equilibrium using acetone as an example is shown here (Figure 3.17.2). 

If P replaces CH in Equation 13.246, there is keto-enol-type equilibrium involving transfer of an H atom between P and O. 

Initial Cu(oxac)k formation is accompanied by the appearance of a transient in the u.v. region of the spectrum. Since the keto-enol equilibrium for Cu(oxac) lies closer to the enol form than for free oxac, this transient disappears at comparable rates by enolization and by decarboxylation to a steady-state concentration. The third phase of the reaction is decarboxylation through this steady-state species. Between these two phases is an intermediate process which is manifest only in pH-stat measurements and involves no decarboxylation. This is assigned to protonation of the copper pyruvate-enolate product Cu(pyr)ei ... 

Notably, the configuration of the silicon enolate 155 is crucial to high enan-tioselectivity. Thus, in the reaction with 1-naphthyl triflate, ( )-154 provided 90% ee, whereas the (Z)-diastereomer gave only 50% ee. Therefore, the authors concluded that neither a lithium enolate is involved, which, they believe, would equilibrate, nor that, after transmetallation, an equilibrium will exist between C- and O-bound palladium enolates (cf. Scheme 5.46). Instead, they hold the opinion that, under their conditions, the C-bound palladium enolates (that are diastereomeric due to the chiral ligand) will undergo fast reductive elimination before a slow epimerization through an O-bound tautomer can occur. It seems not surprising that the t-butyl ester of 2-trimethylsilylpropanoate (a kind of C-bound silicon enolate ) does not react under the conditions applied. 

In spite of the usefulness of the Beirut reaction, mechanistically it is not well understood. It has been suggested that the first step involves the nucleophilic attack by the enolate or the enamine at N-3 of the benzofuroxan to yield an intermediate iV-oxide (Scheme 50) which subsequently undergoes tautomerism to an hydroxylamino derivative. This intermediate then cyclizes to the dihydroquinoxaline 1,4-dioxide. This suggestion has not been proven, and indeed there is evidence that benzofuroxan is in equilibrium with 1,2-dinitrosobenzene... 

In the following the reaction is outlined for an a-bromination. The reaction mechanism involves formation of the corresponding acyl bromide 3 by reaction of carboxylic acid 1 with phosphorus tribromide PBr3. The acyl bromide 3 is in equilibrium with the enol derivative 4, which further reacts with bromine to give the a -bromoacyl bromide 5 ... 

The employment of NMR-active isotopes permits to access experimental parameters which are intrinsically difficult to measure, unless a significant concentration of the sugar is present in the NMR tube. For instance, aqueous solutions of N-acetyIncuraminic acid, labeled with 13C at Cl, C2, and/or C3, were analyzed to detect and quantify the various chemical species present in equilibrium at different pHs. In fact, in addition to the expected a and (3 pyranose forms, acyclic keto, keto hydrate and enol forms were identified on the basis of 13C NMR spectroscopic data. Besides, DFT methods were employed to predict the effect of enol and hydrate structure on the coupling constant values Jc,u and /c c involving C2 and C3, finding that 2/c2,h3 can be safely used to differentiate the cis and tram isomers of the enol forms.9... 

This is another exampie of substrate-level phosphorylation, but differs from the earlier example that involved hydrolysis of a mixed anhydride. Here, we have merely the hydrolysis of an ester, and thus a much lower release of energy. In fact, with 1,3-diphosphoglycerate, we specifically noted the difference in reactivity between the anhydride and ester groups. So how can this reaction lead to ATP synthesis The answer lies in the stability of the hydrolysis product, enolpyruvic acid. Once formed, this enol is rapidly isomerized to its keto tautomer, pyruvic acid, with the equilibrium heavily favouring the keto tautomer (see Section 10.1). The driving force for the substrate-level phosphorylation reaction is actually the position of equilibrium in the subsequent tautomerization. 

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