Tautomerization of carbonyl compounds

Keto-enol tautomerism of carbonyl compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that loses from its a carbon to yield a neutral eno) (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). 

Kung (1974) mentioned that numerous reports appear in the literature on the study of keto -enol tautomerism of carbonyl compounds, triacylmethanes and cyclodiketones depending on the state as pure liquid, diluted in an organic solvent or in a gas phase. Except for compound D.45, which will be discussed, the aliphatic structures are represented as a-diketones and the cyclic structures under the keto-enolic form. 

Primary and secondary nitroso compounds tautomerize to isonitroso compounds - oximes of aldehydes and ketones, respectively. Their reductions are dealt with in the sections on derivatives of carbonyl compounds (pp. 106,132). 

The Tautomerism of Heterocycles J. Elguero etal., Adv. Heterocycl. Chetn., Suppl. 1,1976. Ring-Chain Isomeric Transformations of Hydroxy-, Amino-, and Mercapto-Derivatives of Carbonyl Compounds and Their Heteroanalogues R. Valters, Russ. Chem. Rev. Engl. Transl.), 1974, 43, 665-678. 

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). 

The tautomerism of these compounds has been discussed in detail in the chapters on structure (Sections 4.01.1.1 and 4.06.5.2), and general reactivities have been considered in Chapter 4.02.3.7 wherein the relative reactivities and interconversions of the hydroxy and carbonyl forms are summarized. Some of the reactions have also been covered in the section dealing with non-aromatic derivatives of imidazoles (Section 4.07.2). Discussion here will be limited to reactions which do not lead to ring fission. 

The tautomerism and geometrical isomerism of several nitrogenous derivatives of carbonyl compounds have been examined by means of polarography . For example the phenylhydrazones of acetaldehyde and isobutyraldehyde isomerized in alcoholic solution to the ene-hydrazine form (RNHNHCH-=CHR). On the basis... 

We shall take up first the behavior of ketones toward the halogens, and see evidence that carbanions do indeed exist at the same time, we shall see an elegant example of the application of kinetics, stereochemistry, and isotopic tracers to the understanding of reaction mechanisms. And while we are at it, we shall see something of the role that keto-enol tautomerism plays in the chemistry of carbonyl compounds. 

In Norrish type II cleavage, the O radical abstracts H from the y-carbon in a six-membered TS, and the 1,4-diradical then fragments to give an alkene and an enol, the latter of which tautomerizes to the ketone. Sometimes, the 1,4-diradical undergoes radical-radical combination to give a cyclobutane, instead. The Norrish type II cleavage is closely related to the McLafferty rearrangement that is often seen in the mass spectra of carbonyl compounds. 

The autoxidation of carbonyl compounds can be performed under a variety of conditions, the choice of which depends on the substrate and the desired product. Metal-catalyzed autoxidation proceeds via intermediate enols to generate (x-hydroperoxides or other products resulting from hydroperoxide decomposition [21-23], Autoxidation under basic conditions also requires generation of an enol or enolate [3] under photochemical or protic conditions, tautomerism of the carbonyl compound to an enol may be observed prior to autoxidation [24-25]. Both on the basis of detection of enol radicals as discrete intermediates in certain cases [24] and on kinetic studies... 

Over the years, spectrophotometry (UV-VIS) has been used to study such physicochemical phenomena as heats of formation of molecular addition compounds and complexes in solution, determination of empirical formulas, formation constants of complexes in solution, hydration equilibria of carbonyl compounds, association constants of weak acids and bases in organic solvents, tautomeric equilibria involving acid base systems, protein-dye interactions, chlorophyll-protein complexes, vitamin A aldehyde-protein complex, association of cyanine-dyes, determination of reaction rates, determination of labile intermediates, and dissociation constants of acids and bases. 

Notice that the steps in the enol — acetaldehyde reaction are simply the reverse of the acetaldehyde — enol reaction (Fig. 19.15). Note also that in acid, as in base, aldehydes and ketones that have a hydrogens are in equilibrium with their enol forms. We will soon see that although enols are in equilibrium with the related keto forms, it is usually the keto forms that are favored. This equilibrium is called the keto-enol tautomerization.The carbonyl compound and its associated enol are called tautomers. 

The chapter begins with keto-enol tautomerism and examines stmctural effects on the degree of enolization, its mechanism, and the role of enols in the reactions of carbonyl compounds. We then proceed to the major emphasis of the chapter—enolates and their synthetic value in making carbon-carbon bonds. The applications are numerous, but most share the common feature of enolates acting as nucleophiles. 

In a process analogous to the hydration of alkenes, water can be added to alkynes in a Markovnikov sense to give alcohols—in this case enols, in which the hydroxy group is attached to a double-bond carbon. As mentioned in Section 12-16, enols spontaneously rearrange to the isomeric carbonyl compounds. This process, called tautomerism, interconverts two isomers by simultaneous proton and double-bond shifts. The enol is said to tautomerize to the carbonyl compound, and the two species are called tautomers (tauto, Greek, the same mews, Greek, part). We shall look at tautomerism more closely in Chapter 18 when we investigate the behavior of carbonyl compounds. Hydration followed by tautomerism converts alkynes into ketones. The reaction is catalyzed by Hg(II) ions. 

Third, hydration of the carbon-carbon triple bond yields enols that tautomerize to carbonyl compounds (Sections 13-7 and 13-8). In the presence of mercuric ion, addition of water follows Markovnikov s rule to furnish ketones. 

P-ketoenediynes 3.475 proceeds via tautomerization to give the more reactive enyne-allenes 3.476, which then undergo Myers-Saito cycliza-tion via diradical 3.477 to give anthracene derivatives 3.478. The rate determining step of this reaction is the enolization of the ketone. Since enolization of carbonyl compounds depends on the pH, solvent, and other factors, this opens diverse approaches to the design of reactive enediynes [244]. 

In mordant dyes, phenols, naphthols, and enolizable carbonyl compounds, such as pyrazolones, are generally the couplers. As a rule, 2 1 metal complexes are formed ia the afterchroming process. A typical example of a mordant dye is Eriochrome Black T (18b) which is made from the important dyestuff iatermediate nitro-l,2,4-acid, 4-amiQO-3-hydroxy-7-nitro-l-naphthalenesulfonic acid [6259-63-8]. Eriochrome Red B [3618-63-1] (49) (Cl Mordant Red 7 Cl 18760) (1, 2,4-acid — l-phenyl-3-methyl-5-pyrazolone) is another example. The equiUbrium of the two tautomeric forms depends on the nature of the solvent. 

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