Enol/keto tautomers

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

The compounds 2- (16) and 4-pyridone (38) undergo chlorination with phosphoms oxychloride however, 3-pyridinol (39) is not chlotinated similarly. The product from (38) is 4-chloropyridine [626-61-9]. The 2- (16) and 4-oxo (38) isomers behave like the keto form of the keto—enol tautomers, whereas the 3-oxo (39) isomer is largely phenolic-like, and fails to be chlotinated (38). 

Nomenclature is based on the keto-enol tautomers. The trihydroxy form is variously designated cyanuric acid, j -triaziae-2,4,6-triol or 2,4,6-trihydroxy-j -triaziQe. The trioxo stmcture, or j -triaziae-2,4,6(lJT,3JT,5JT)-trione is the basis for the isocyanuric acid nomenclature. 

Examples of the remaining potential 3,4-dihydroxy heterocycles are presently restricted to furan and thiophene. Although the parent 3,4-dihydroxyfuran apparently exists as the dioxo tautomer (86), derivatives bearing 2-alkyl or 2,5-dialkyl substituents prefer the keto-enol structure (87) (71T3839, 73HCA1882). The thiophene analogues also prefer the tautomeric structure (87), except in the case of the 2,5-diethoxycarbonyl derivative which has the fully aromatic structure (88) (71T3839). 

The aromaticity of the pyrimidine and purine ring systems and the electron-rich nature of their —OH and —NHg substituents endow them with the capacity to undergo keto-enol tautomeric shifts. That is, pyrimidines and purines exist as tautomeric pairs, as shown in Figure 11.6 for uracil. The keto tautomer is called a lactam, whereas the enol form is a lactim. The lactam form vastly predominates at neutral pH. In other words, pA) values for ring nitrogen atoms 1 and 3 in uracil are greater than 8 (the pAl, value for N-3 is 9.5) (Table 11.1). 

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 equihbrium and that above involving acetone and propen-2-ol. 

Naphtho[2,l-h]furan-2-(3 -one 28 has been described as a keto tautomer (91JA2301). Naphtho[l,2-h]furan-3-(2//)-ones of type 29 (R = H, Me, Et, Pr, pentyl, heptyl) show keto-enol tautomerism with the enol form predominating (88RRC917). 

For 3-acetyl-5-benzylpyrrolin-2-one, the keto-enol tautomer 109 predominates according to spectral data (UV, IR, and NMR) (80HCA121) (for further work on pyrrolin-2-ones see 77JHC681 95MI1). 

The 0,N-dideuterated enol was formed by hydrolysis of the O-trimethylsilyl ether 123 (R = TMS) (in 80% [D6]DMSO/20% D2O with 5. lO " M DCl). N-Methylindoxyl (formed by hydrolysis of its acetate) exists in the solid state as a mixture of the enol and the keto tautomers (34% enol/66% keto). The NMR spectrum of freshly prepared solution in DMSO demonstrated signals of both enol and keto forms. However, at equilibrium (reached in 18 h at RT) the ratio of enol to ketone depends strongly on the polarity of the solvent used thus, in [Dg]DMSO the tautomeric mixture contains 92% enol, while in CDCI3 the keto form predominates (97%). A solution with 100% enol could be generated by hydrolysis of its O-trimethylsilyl ether [conditions 80% [Dfi]DMSO/20% D2O with 5 10" M DCl at 32°C (86TL3275 87PAC1577 88TL250)]. 

Armulated thiophenes of types 195 and 197 (A benzo, naphtho) were studied concerning keto-enol tautomerism. The ring fusion has a remarkable influence upon these equilibria. Whereas for the c-fused thiophenes 197 only keto tautomers were present, for h-fused derivatives 195 also the enol forms 196 were found (the equilibria are solvent dependent) (82JOC705). 

Once such effects had been noted, it became necessary to interpret the observed results and to classify the solvents. The earliest attempts at this were by Stobbe, who reviewed the effects of solvents on keto-enol tautomers [4]. Since then many attempts have been used to explain solvent effects, some based on observations of chemical reactions, others on physical properties of the solvents, and yet others on spectroscopic probes. All of these have their advantages and disadvantages and no one approach can be thought of as exclusively right . This review is organized by type of measurement, and the available information is then summarized at the end. 

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerisni. The individual keto and enol forms are said to be tautomers, a word used to describe constitutional isomers that interconvert rapidly. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely... 

A carbonyl compound with a hydrogen atom on its a carbon rapidly equilibrates with its corresponding enol (Section 8.4). This rapid interconversion between two substances is a special kind of isomerism known as keto-enol tautomerism, from the Greek Canto, meaning "the same," and meros, meaning "part." The individual isomers are called tautomers. 

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... 

Tomic K, Jorg T, Marian CM (2005) Quantum chemical investigation of the electronic spectra of the keto, enol, and keto-imine tautomers of cytosine. J Phys Chem A 109 8410-8418... 

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

If one considers a TMS group to be equivalent to a proton, a recent paper describes the formation of a phosphanyl-substituted allene by a keto-enol tautomer-ization-like silyl migration [346],... 

Tables 5.2 and 5.3 display vertical ionization energies of the two tautomers (keto and enol) with the lowest energies. The keto form is shown in Fig. 5.1. In the enol form, a proton is transferred from nitrogen 1 to the oxygen atom. P3 ionization energies for both isomers are close to the lowest peak in the photoelectron spectrum (PES) [44],...

The aci-form of the nitro group is frequently claimed in pure chemistry. However, only ad-nitromethane appears to have been comprehensively studied as an isolated species15. Ionized keto-enol systems are characterized by reversal of the relative stabilities of the single species compared to their neutral counterparts. Thus, the ionized enols are generally the thermodynamically more favoured tautomers by approximately 15-20 kcalmol-1, the... 

The large temperature range is due to the distillation of a mixture of the keto-enol tautomers. 

The keto/enol equilibrium (15) has been a spur to much research. In the absence of catalysts the equilibrium is established slowly and is very sensitive to a variety of influences, both internal, such as the nature of a- and P-substituents, and external, such as temperature and solvent. The discovery that the equilibrium was established sufficiently slowly to permit both keto and enol tautomers to be observed by H-nmr spectroscopy allowed these several influences to be easily investigated (see Kol tsov and Kheifets, 1971, for a review of the early work, and Emsley, 1984, for later work). 

The carbonyl stretching frequency of both the keto and enol tautomers can be recognized in the vibrational spectrum of pentane-2,4-dione. The enol has v(C=0) at 1618cm" , generally the dominant peak in the spectrum and more intense than the in- and out-of-phase v(C=0) stretching modes of the keto form, which are found at 1727 and 1707 cm" , respectively. These are identified by their Raman counterparts at 1719 cm" (polarized) and 1697 cm" (depolarized) (Ernstbrunner, 1970). The ratio of absorbances of the enol and the out-of-phase keto bands in the ir was used as an early method of analysis of the keto/enol equilibrium in different solvents (Le Fevre and Welsh, 1949). 

Hydroxyaminobenzo-furan and -thiophene (32a X = O, S) are the unstable enam-ine tautomers of the corresponding oximes (32b). Kinetics of the tautomeric interconversions have been measured, yielding tautomeric constants the latter have been compared with the corresponding keto-enol constants. The enamines are ca 40 times less stable, relative to the oximes, than are the enols, relative to the ketones. The minor tautomers are ca 100 times more stable (relative to the major) for the benzothiophene system. 

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... 

This covers the most common tautomer pairs including, for example, imidazole and tetrazole. No other tautomerism is considered, for example, keto-enol. Two additional rules prevent combinatorial explosion ... 

The difference in conjugation between neutral molecules and their ion-radicals can also be traced for keto-enol tautomerism. As a rule, enols are usually less stable than ketones. Under the equilibrium conditions, enols exist only at a very low concentration. However, the situation becomes different in the corresponding cation-radicals, where gas-phase experiments have shown that enol cation-radicals are usually more stable than their keto tautomers. This is because enol cation-radicals profit from allylic resonance stabilization that is not available to ketones (Bednarek et al. 2001, references therein). 

There is a distinct relationship between keto-enol tautomerism and the iminium-enamine interconversion it can be seen from the above scheme that enamines are actually nitrogen analogues of enols. Their chemical properties reflect this relationship. It also leads us to another reason why enamine formation is a property of secondary amines, whereas primary amines give imines with aldehydes and ketones (see Section 7.7.1). Enamines from primary amines would undergo rapid conversion into the more stable imine tautomers (compare enol and keto tautomers) this isomerization cannot occur with enamines from secondary amines, and such enamines are, therefore, stable. 

Myrtaceae used in Australian traditional medicine), exists in a keto-enol tautomer ratio of 3 2. This compound was known previously as a synthetic product but only limited physical (melting point) and no spectroscopic data were available. The (3-hydroxychalcone constituent obtained from Millettia leucantha (26) is a further example of a retrochalcone. ... 

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