Enol-keto tautomeric

Particularly well-studied tautomeric keto/enol equilibria are those of 3-pyridinyl 2-picolyl ketone [138] and t-butyl 2-picolyl ketone [139]. 

In the care of reaction (51) it is the o-complex which is undergoing deprotonation. The pKj, values of these ketone-like o-coraplexes can be derived from the constant Kt for the tautomeric keto-enol equilibrium of the phenol and the acid dissociation constant K, for the phenol. One gets pK pteto) 1-... 

Phenol can be considered as the enol of cyclohexadienone. While the tautomeric keto-enol equilibrium lies far to the ketone side in the case of aliphatic ketones, for phenol it is shifted almost completely to the enol side. The reason of such stabilization is the formation of the aromatic system. The resonance stabilization is very high due to the contribution of the ortho- and / ara-quinonoid resonance structures. In the formation of the phenolate anion, the contribution of quinonoid resonance structures can stabilize the negative charge. 

Carbonyl (or keto) compounds are interconvertible with their corresponding enols. This rapid interconversion of structural isomers under ordinary conditions is known as tautomerism. Keto-enol tautomerism is catalysed by acids or bases. 

Any reaction that simply involves the intramolecular transfer of a proton is called a tautomerism. Keto-enol interconversion may happen in basic as well as acidic solution. The steps are reversed in the base-catalyzed and acid-catalyzed reactions. In the base-catalyzed reaction, the first step is removal of an a-proton and the... 

The aldehyde or ketone is called the keto form, and the keto enol equilibration is referred to as keto-enol isomerism or keto-enol tautomerism. Tautomers are constitutional isomers that equilibrate by migration of an atom or group, and their equilibration is called tautomerism. Keto-enol isomerism involves the sequence of proton transfers shown in Mechanism 9.2. 

Step 2 Keto-enol tautomerism. Keto-enol tautomerism of the cyclohexadienone intermediate gives the product salicylate anion. Note that in this case, the enol, owing to its aromatic character, is the more stable of the two tautomers. 

The more common types of structure showing tautomerism are the keto-enol type. 

Space does not permit any further detailed discussion except for a brief account of two interesting subjects. The first is concerned with keto-enol tautomerism. The classical example is ethyl acetoacetate, which can exist in the keto form (I) and the enol form (II) ... 

Enols are related to an aldehyde or a ketone by a proton transfer equilibrium known as keto-enol tautomerism (Tautomensm refers to an mterconversion between two struc tures that differ by the placement of an atom or a group)... 

A single Kekule structure does not completely descnbe the actual bonding in the molecule Ketal (Section 17 8) An acetal denved from a ketone Keto-enol tautomerism (Section 18 4) Process by which an aldehyde or a ketone and its enol equilibrate... 

Hydroxyquinolines (Quinolinols). A number of methods have been employed for their preparation. A modified Chichibabia reaction of quinoline ia fused KOH—NaOH at 240°C produces 70% of 2-hydroxyquiQoline [59-31-4] (121). Alternative names based on the facile keto—enol tautomerism of two of these compounds are 2(1H) and 4(lJd)-quiQolinone none of the other quinolinols show this property. The treatment of... 

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 keto-enol tautomerism of 1,2-benzisoxazoles has been examined and the existence of either form can be postulated on the basis of reactivity. IR analysis on the solid indicates the exclusive existence of the enol form, while in CHCI3 solution both appear to be present (71DIS(B)4483). 

The keto-enol tautomerization in the excited triplet state of 2-methylacetophenone involves the transfer of an H atom in the CHO fragment... 

Figure 1.11. NMR analysis of the keto-enol tautomerism of 2,4-pentanedione [CDCIa, 50% v/v, 25 °C, 60 MHz for H, 20 MHz for C]. (a) H NMR spectrum with integrais [resuit keto enoi = 13 87] (b) H broadband de-coupied C NMR spectrum (c) C NMR spectrum obtained by inverse gated H decoupiing with integrals [result keto enol = 15 85 ( 1)]...

Many nitrogen-containing compounds engage in a proton-transfer equilibrium that is analogous to keto-enol tautomerism ... 

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). 

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