Jean Toullec

Enolisation of Simple Carbonyl Compounds and Related Reactions

Publisher Summary This chapter focuses on the enolization of simple carbonyl compounds and related reactions. It presents an overview of the data on the tautomerism of simple carbonyl compounds. “Simple” designates compounds are those that lack enol-stabilizing functions accounting for a significant or great amount of enol. The chapter discusses the mechanism of keto-enol tautomerization and presents the results on stereochemical aspects of enolization (or enolate formation) and on regioselectivity when two enolization sites are in competition. Because the keto-enol equilibrium is so displaced toward the keto form that the enol concentration cannot usually be measured as a function of time, the keto–enol tautomerization cannot be studied directly for simple carbonyl compounds. Most kinetic studies have made use of the property that enolization is the rate-limiting stage of some stepwise reactions that have a rate corresponding to enolization or ionization itself. Enolization and ionization are also the first stages of some elimination reactions, such as aldolization and alkylation, but in most cases, the enolization (or ionization) process is not rate-limiting. Thermodynamic data on enol and enolate formation are needed to understand the kinetic behavior of these reactions; a complete picture must include data on the subsequent steps.

Rate–equilibrium relationships based on the CH-acidity constants of oxocarbenium ions, for proton transfer from hydronium ion to α-methoxystyrenes: evidence for perfect synchronization between bond cleavage, bond formation, and positive-charge delocalization

The CH-acidity constants in water for the oxocarbenium ions derived from ring-substituted α-methoxystyrenes have been calculated from the following literature data. (i) Equilibrium constants for oxocarbenium-ion formation from the corresponding acetals; (ii) equilibrium constants for the acetalto-enol ether process in methanol; (iii) Gibbs free energies of transfer of acetals and enol ethers from methanol to water. The plot of the logarithms of hydronium-ion-catalytic rate constants against the pKa values of the intermediate ions exhibits a slightly curved relationship, with a mean slope βs= 0.58, which can be accounted for by the Marcus equation with an intrinsic barrier of ca. 15 kJ mol–1 and work terms Wr= 44 kJ mol–1 and Wp= 52 kJ mol–1. The agreement of all these parameters with those derived when the catalyst is changed indicates that C–H bond formation and O–H bond cleavage are synchronous concerted primitive changes. Separation of substituted-ring polar effects and direct resonance interactions from the overall substituent effects on rates and equilibria also shows that there is perfect synchronization between proton transfer and positive-charge delocalization.

Acidity constants of protonated simple carbonyl compounds: Comments on literature data and indirect estimates

Abstract The pK a values for OH-acidity of protonated simple carbonyl compounds (oxocarbenium ions) are estimated from keto-enol equilibrium constants combined within CH-acidity constants of the ions, calculated by application of the Marcus equation to the ketonisation process.

Entropic evidence for the formation of carbonyl compound-dependent intermediate complexes in the ivanov reaction

Abstract Enthalpies and entropies of activation for the Ivanov reaction in THF are reported. On the basis of the positive and carbonyl compound-dependent entropies of activation, it is concluded that the reaction occurs via differently solvated complexes between the enolate and the carbonyl compound.

Thermodynamics of the isodesmic enol-to-methyl enol ether equilibrium in water

Abstract Δ G ° values for the isodesmic equilibrium between substituted acetophenone enols and the corresponding methyl enol ethers are calculated from previously reported data on equilibrium constants for keto-enol tautomerism and for enol ether formation from ketones. These values agree with those expected from literature data on analogous alcohol-ether isodesmic systems.

Hydrolyse des α-methoxystyrenes. Correlation de bonsted reliant les constantes de vitese aux constantes d'acidite estimees des ions oxocarbenium intermediares

Resume Acidities (C-H) of the oxocarbneium ions derived from α-methoxystyrenes are estimated from data on ketal-enol ether and ketal-ion equilibrium constants. By plotting hydrolysis hydronium catalytic rate constants vs . these pK values, the Bronsted equation is verified with a slope equal to 0.62.

An accurate spectrophotometric method for determination of small amounts of water in acidic methanol

Abstract The method is based on u.v. measurement of the keto—ketal equilibrium of substituted acetophenones; 4-methoxyacetophenone proved most satisfactory, the standard deviation for 67 ppm of water in hydrobromic acid-containing methanol being 3 ppm.

Determination cinetique des constantes d'equilibre entre les α-methoxystyrenes et les acetals dimethyliques des acetophenones

Abstract Ketal-enol ether equilibrium constants (methanol, 25°C) were determined from kinetic data on bromination of equilibrated ketone-ketal systems and on “methanolysis” of the enol ethers.