Enolate isomer

In the preceding chapter you learned that nucleophilic addition to the carbonyl group IS one of the fundamental reaction types of organic chemistry In addition to its own reactivity a carbonyl group can affect the chemical properties of aldehydes and ketones m other ways Aldehydes and ketones having at least one hydrogen on a carbon next to the carbonyl are m equilibrium with their enol isomers... 

Terreic acid is a naturally occurring antibiotic substance. Its actual structure is an enol isomer of the structure shown. Write the two most stable enol forms of terreic acid, and choose which of those two is more stable. 

Note the difference between tautomers and resonance forms. Tautomers are constitutional isomers—different compounds with different structures—while resonance forms arc different representations of a single structure. Tautomers have their atoms arranged differently, while resonance forms differ only in the position of their electrons. Note also that tautomers are rapidly interconvertible. Thus, keto and enol isomers are tautomers, but aJkene isomers such as 1-butene and 2-butene are not, because they don t interconvert rapidly under normal circumstances. 

In this context, 14 different reaction conditions were investigated in 14 hours. By software-supported process optimization (factorial design), the initial yield of 49% could be improved to 78% with a simultaneous increase in keto/enol isomer ratio (A B) of65 35 to95 5. 

Carbonyl compounds can react with electrophiles via their enol isomers or equivalents, and these reactions result in a-substitution. 

The ionization potentials, using mass spectrometry, for both 2-hydroxy-and 3-hydroxythiophenes have been compared with data for compounds derived from either tautomeric form in order to analyze the tautomeric composition.124 125 In the 2-hydroxy-substituted system the enol isomer could not be detected. Of the two possible unsaturated lactones the oc,/l-unsaturated form was the major isomer. In the 3-hydroxy-substituted case both the oxo form and the enol form are important. The position of the equilibrium was compared with those of the corresponding furan and sele-nophene systems for both isomers. 

As a further illustration of the phenomenon of H-bond resonance coupling let us consider the intramolecular H-bond of (3-hydroxyacrolein (0=CHCH=CH0H), a prototypical enolone (2-en-3-ol-l-one, or enol isomer of (3-diketone).55 This molecule may be envisioned as existing in two distinct isomeric forms, according to the position of the proton in the O- H—O hydrogen bond ... 

In the other mechanism, the catalytic cycle initiates through the insertion of CO into a Pd-alkoxy bond, with formation of a Pd-carboalkoxy intermediate, which inserts the olefin with formation of an alkylcarboalkoxy /i-chelate, which undergoes protonolysis by the alkanol through the intermediacy of its enolate isomer (see Sect. 2.3.1), yielding the ester and the Pd-alkoxy species, which then initiates a new catalytic cycle [122-125]. 

Protonation of DNM might lead to three different tautomers (i) dinitrosomethane (H2C(NO)2) when the protonation occurs at the carbon (ketonic isomer) , (ii) methylnitrosolic acid (10) when one of the oxygen atoms of the nitroso groups forms an oxime (enolic isomer) and (iii) a dioxime (HON=C=NOH) of CO2 if a 1,3-hydrogen shift... 

Yoshino (51) studied the abundance ratios of the gauche and trans forms of dichloroethane and the keto-enol isomers of acetyl acetone when these compounds were adsorbed (less than monolayer quantities) on silica gel. The gauche-trans ratio, which is unity in chloroform solution and 1.4 in the pure liquid, was found to be 1.9 in the adsorbed state. The 1430-cm.-1 gauche band and the 1450-cm.-1 trans band were not affected by the adsorption. The absorption bands of acetyl acetone were measured for a 1.5 % chloroform solution, the pure liquid, liquid saturated with water, and the adsorbed state. The relative intensity of the 1600-cin. 1 enol band and the 700-cm.-1 keto baud were used to determine the isomer ratio. The 1600-... 

Racemic colchiceine was obtained by Corrodi and Hardegger by a base-catalyzed equilibration of the Schiff base obtained by reacting deacetyl-colchiceine with benzaldehyde (34). Aldimine-ketimine isomerization was found to be the mechanism by which the racemization had occurred (35). The optical resolution of deacetylcolchiceine was accomplished with cam-phorsulfonic acids, affording, after O-methylation with diazomethane, separation of the enolate isomers and after N-acetylation, unnatural (+)-and natural (-)- colchicine (Fig. 1). Racemization of colchicine in refluxing acetic anhydride followed by mild hydrolysis of the intermediate triacetate represents a much improved method of preparing ( )-colchicine (36). The Blade-Font procedure was later extended to the preparation of ( )-3-demethylcolchicine and other racemic analogs (5). 

Let us consider a problem that arose in connection with an experimental study. Propanone (acetone) was subjected to ionization followed by neutralization of the radical cation, and the products were frozen in an inert matrix and studied by IR spectroscopy [13]. The spectrum of the mixture suggested the presence of the enol isomer of propanone, 1 -propen-2-ol (Reaction 2.2) ... 

However, carbacepham derivatives 179 and 180 could be generated from 178 by Jones oxidation to furnish a mixture (10 90) of keto-enol isomers 179 and 180, which can be methylated using diazomethane to give 181 in 63% yield (Scheme 24). Evaluation of various carbacepham analogues as well as carbapenam analogues indicated that some are promising potent antibiotics. 

In hydrocarbon solvents, /3-diketones are predominantly (> 90%) enolized and these solutions have been subjected to flash photolysis, which causes photoisomerization to the diketo form9). Reversion of the diketone to the more stable cis enol was then followed by UV spectroscopy and pseudo first order rate constants at room temperature were in the range 14-68 x 10-3 s-1 (e.g. AA = 23 x 10-3 s 1) with half-lives of several hours. The same research also reveals an alternative transformation on irradiation. In this the cis enol form is converted by rotation about a C-C or C=C bond into one of the possible trans enol isomers. These may then go on to the diketo form but mainly they revert very rapidly to the cis enol with rate constants of 0.1 to 70 s 1 (e.g. AA = 0.27 s-l)9). 

Besides the characterisation of the individual G tautomers, CP-MD simulations of the 9H-enol tautomer in the gas phase at various temperatures have been carried out to investigate tautomerisation mechanisms [46], Spontaneous tautomerisation involving proton transfer is not observable in the time window of a few picoseconds permitted by AIMD even at increased temperature as high as 1000 K. However, frequent cis-trans isomerisation events are seen to take place suggesting that the two enol isomers are indistinguishable experimentally. 

Despite the fact that very little of the enol isomer is present at room temperature, enols are very important because they are reactive. 

Finally, oxaloacetate is simultaneously decarboxylated andphosphorylated by phosphoenolpyruvate carboxykinase in the cytosol. The CO2 that vv as added to pyruvate by pyruvate carboxylase comes off in this step. Recall that, in glycolysis, the presence of a phosphoryl group traps the unstable enol isomer of pyruvate as phosphoenolpyruvate (Section 16.1.7). In gluconeogenesis, the formation of the unstable enol is driven by decarboxylation—the oxidation of the carboxylic acid to CO2 —and trapped by the addition of a phosphate to carbon 2 from GTP. The two-step pathway... 

Every type of bond requires a different amount of energy to separate the component parts. This value is called the bond energy and it is measured in kJ mol-1. Using the values given below, calculate the difference in the bond energies of the keto and enol isomer of the carbonyl compound. 

A variety of 2-pyrazolin-5-ones having carboxyl or carboxyl derivatives as substituents and a second functional group are known (Table XXXIII). Such compounds are usually prepared by methods already described, and only those compounds having an arylazo substituent as the second functional group will be discussed. These compounds have been extensively investigated because of their commercial importance as dyes. The important dye tartrazine is of this class. It is the trisodium salt of 4-(4-sulfophenylazo)-l-(4-sulfophenyl)-5-oxo-2-pyrazolin-3-carboxylic acid. These dyes exist largely as enol isomers. The principal means of preparation is coupling of a diazonium salt with a 5-oxo-2-pyrazolin-3-carboxylic acid or its derivatives. Another important preparation of these compounds (eq. 231) is by the reaction of... 

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