Tautomers 1,2-enolization

The study of the chemistry of carbonyl compounds has shown that they can act as carbon nucleophiles in the presence of acid catalysts as well as bases. The nucleophilic reactivity of carbonyl compounds in acidic solution is due to the presence of the enol tautomer. Enolization in acidic solution is catalyzed by O-protonation. Subsequent deprotonation at carbon gives the enol ... 

The insertion reaction chemistry of the well-known germylene Ge [CH(Si M03)3)2 50 is very diverse.154 323-328 This species inserts readily into the C-H bonds of mono- and dinitrile substrates (Scheme 47),154 the C-H bonds of ethers and alkanes in the presence of Phi (Scheme 48),323 the C-H bonds of ketones in the presence of MgCE (Scheme 49) and the O-H bonds of the tautomer enols in the absence of MgCl2 (Scheme 50),324 and the C-H bonds of diketones resulting in cyclic products (Scheme 51). 

Keto tautomer Enol tautomer Dienol tautomer... 

The higher rate of O-protonation than that of C-protonation at temperatures when the thermodynamic stability of the oxonium cation is lower than or comparable with that of the C-protonated form has analogies. Thus, mesomeric anions formed by the heterolysis of a-C—H bonds of aliphatic ketones and nitro compounds are usually more readily protonated at O than at C though a thermodynamically less stable tautomer (enol, an acinitro form) is formed. TTiese deviations from the Bronsted principle are explained 34i. 342> O-protonation requiring a smaller rearrangement... 

Five-membered cyclic p-ketoesters (p-ketolactones 1) and cyclic p-ketothioesters (p-ketothiolactones 2) differ from their acyclic counterparts in at least two regards (1) by being mostly enolized and (2) by being so acidic that they are called tetronic acids (1) and thiotetronic acids (2), respectively (Fig. 1). Even 5-membered cyclic p-ketoamides, i.e., p-ketolactams 3, are sufficiently acidic to be known as tetramic acids. The parent compound 3 (Fig. 1 R t = H), for instance, exhibits a pKa of 6.4." Standard tetramic acids prefer the keto (keto-3) over the enol tautomer enol-3). 

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 keto and enol forms are constitutional isomers Using older terminology they are referred to as tautomers of each other... 

Write a structural formula for the enol tautomer of cytosine... 

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. 

Only the potentially 2,4-dihydroxy derivatives of furan and thiophene are known and these exist in the solid state and in polar solvents as the monoenols (82) (71T3839). However, in non-polar solvents the furan derivatives exist predominantly in the dioxo form (83). The 2,5-dioxo structure (84) is well established for X=0, NR, S and Se (71BSF3547) and there is no evidence for intervention of any enolic species. The formal tautomer (85) of succinimide has been prepared and is reasonably stable (62CI(L)1576). 

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). 

Benzisoxazole and alicyclic ketones undergo a mercury sulfate-catalyzed cycloaddition in boiling xylene. The enol tautomers (141) are apparently involved and the yields of... 

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

Although nitrosation of (l-dicarbonyl compounds becomes increasingly more facile upon successive replacement of the a alkyl groups with perfluoroalkyl groups because of the increased ionization of the perfluorinated enolate (equation 7), the stability of the nitrosodiketone tautomers decreases Thus, 1,1,1-trifluoro pentane-2,4-dione and 1,1,1,5,5,5-hexafluoropentane 2,4 dione mtrosate much faster than penta-2,4 dione but yield ketoximes, which decompose upon workup... 

Spectroscopic investigation of enamines conjugated with ketone, ester and nitrile groups established the prevalence of enamine rather than imine-enol tautomers in examples of secondary amines (206-212). Similar studies have been made with enamines of acylpyridines and acetophenones (213,214). 

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). 

Aldehydes and ketones ( keto forms) normally exist in equilibrium with their enol tautomers. 

Which tautomer is lower in energy, acetone or propen-2-oll Use equation (1) to calculate the equilibrium distribution of the two at room temperature. If an experiment is capable of detecting concentrations as low as 1 % of the total, would you expect to observe both keto and enol forms of acetone at room temperature ... 

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