Acid-base equilibrium tautomers

One remarkable feature of these results (Baird and Thomas, 1961) is that the a-protons in the radicals PhNHO and Me3C. NHO give no detectable hyperfine splitting, although the components are quite narrow. It is postulated that either the oxime tautomer is more stable, or that there is a rapid tautomeric equilibrium. In view of the internal consistency of these results (Table 8) it seems probable that the radicals are all of the same form, in which case all that is needed is an acid-base equilibrium sufficiently fast that the lines are not appreciably broadened. 

Despite dramatic differences in coordination environment, neutral iridium(i) allyl complex 158 reacts with acetone to form iridacyclobutane complex 159 (Equation 79) <19940M1592>. This transformation may proceed by nucleophilic addition of the enol tautomer, as the authors suggest, or, plausibly, an acid/base equilibrium induced by the basic Ir(l) center the cationic hydridoiridium intermediate thus formed is alkylated at the central allyl carbon by the enolate. 

Any enolizable carbonyl compound is, at least in principle, in equilibrium with its enol tautomer, though equilibrium constants vary widely. The enol is in an acid/base equilibrium with the enolate anion, and the enolate anion has two important resonance forms, with charge residing either on oxygen or on the a-carbon atom (Figure 17.4). 

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 motive for the present investigation is the question whether high fields promote proton jiomping in acid-base adducts and whether the reaction partners can be regarded as tautomers. According to conventional dielectric measurements by Polish authors such an equilibrium would be well-poised for the title compounds. Malecki analyzed the nonlinear dielectric effects in this system without concern for relaxation. ... 

Nudeic acid bases exist as mbctures of two or more rapidty interconvertible isomers of tautomeric forms. Tautomers, at least in prin< le, can be separated at low temperatures where the rate of interconversion is low. The keto-enol equilibrium is the classic example of tautomerism. Hie enol is present in small amount, since it is usually less stable than the keto form. Uracil (1) is one of the nucleic add bases, that exists as mixture of the tautomeric keto and enol forms (Scheme 1). The ratio of the enol tautomer to the keto tautomer of uradl, however, is small. The existence of the enol form of Sdieme 1 is the basis for referring to uracil as dihydroitypyrimidine. 

Cycloalkenone-2-carboxylates tautomerize to conjugated dienols in the presence of either acids or bases. Iron(III) catalysts have also been found to promote enone-dienol equilibration, and, at room temperature, dimerization64. Thus, treating 87 with 1 mol% iron(III) chloride hexahydrate in methylene chloride at room temperature affords 88 in 81% yield (equation 46). The cyclohexadiene-cyclohexanone is in a rapid equilibrium with its triendiol tautomer, 89 (equation 47). 

Pyridylcarbonyl)- and 2-(2-quinolylcarbonyl)benzoic acids possess the open-chain structure in the solid state (84KGS1231). In dioxane solution, the ring-chain equilibrium is observed. Protonation of the pyridine or quinoline ring nitrogen atom leads to the formation of the protonated cyclic forms 11. Evidently, protonation stabilizes the tautomer that is the stronger base, i.e., the cyclic form. 

The tautomerism of 4-aminopyrido[2,3- ]-l>2,6-thiadiazine-2,2-dioxide, an acid with a of 5.85, has been investigated by PSez et al. (Scheme 1). The 8/f-tautomer 10a is characterized by an ultraviolet (UV) maximum at 365 nm and the 1/f-tautomer 10b by a band at 320 nm, thus allowing the determination of the tautomeric equilibrium by UV spectroscopy. This conclusion that the aminopyridothiadiazinedioxide 10 exists as the 8f/-tautomer in aqueous and dimethyl sulfoxide (DMSO) solutions was based upon a comparison with the UV spectra of the 8-methyl derivative 11 (- max 365 nm) and the 1-methyl derivative 12 322 nm) <2004JST83>. 

IEs were used to verify a tautomeric equilibrium in 2-phenylpropenal-c/ (47 + 48) and to distinguish this mixture from a symmetric structure with a hydrogen centered within the hydrogen bond.120 According to 13C and H chemical shifts, the equilibrium favors 48. From the temperature dependence AH° was estimated as -27 cm-1, or -0.08 kcal mol-1. Because both tautomers have the same conjugate base, 47, which is less stable, must be the stronger acid, owing to a secondary deuterium IE. 

Aldehydes or ketones with an a-hydrogen exist as an equilibrium mixture of keto (H-Ca-C=0) and enol (Ca=C-OH) tautomers. The keto form usually predominates. An a-hydrogen is weakly acidic and can be removed by a base to produce a resonance-stabilized enolate anion. Deuterium exchange of a-hydrogens provides experimental evidence for ends as reaction intermediates. 

Enols are isomers of molecules containing C=0 double bonds. Both isomers equilibrate, and even small amounts of acid or base serve as a catalyst. The mechanism of acid-catalyzed iso-merizations will be presented Figure 12.4. Isomers existing in a dynamic equilibrium of this kind are referred to as tautomers the respective isomerism is termed tautomerism, and the isomerization of tautomers is called tautomerization. 

Carbonyl compounds are in a rapid equilibrium with their cnols, a process called keto-enol tautomerism. Although enol tautomers arc 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. P or 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 Bt2 and PBrThe a--halogenated inoducts can then undergo base-induced E2 elimination to yield a,/3-un.salurated carbonyl compounds. 

Equilibrium favors the keto form largely because a C=0 is much stronger than a C=C. Tautomerization, the process of converting one tautomer into another, is catalyzed by both acid and base. Under the strongly acidic conditions of hydration, tautomerization of the enol to the keto form occurs rapidly by a two-step process protonation, followed by deprotonation as shown in Mechanism 11.3. 

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