Keto-enolates

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

C2H5OOC CH2 COOC2H5 C2H500C CH C(0H)0C2He keto enol... 

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) ... 

Tablb XII. Keto-Enol Equiubbia of Ethyi,. 4cbtoacbtatb... 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

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... 

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)... 

Certain structural features can make the keto-enol equilibrium more favorable by stabi hzmg the enol form Enolization of 2 4 cyclohexadienone is one such example... 

Many nitrogen containing compounds engage in a proton transfer equilibrium that is anal ogous to keto-enol tautomensm... 

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... 

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). 

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... 

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 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). 

Figure 5 A suggested mechanism for the enolization of acetyl-CoA by the enzyme citrate synthase (CS). The keto, enolate, and enol forms of the substrate are shown.

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)]...

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. 

Methylphenylhydrazine and both 1- and 2-naphthylhydrazines are also reported to react similarly. Phenols, in general, do not undergo this reaction, which is favoured by compounds exhibiting keto-enol tautomerism. ... 

---