Enols and Enolates of Carboxylic Acid Derivatives. A G2(MP2) ab Initio Study

Abstract

The effect of substituents on the stability of enols and enolates relative to their oxo (CO) forms is explored using the G2(MP2) ab initio method for CH3COX (14 substituents). Calculated enthalpies of reaction for enolate formation agree well with experimental gas-phase data. Relative to acetaldehyde, enolate formation from a carbonyl compound is favored by the presence of both σ and π electron-withdrawing groups and disfavored by π-donating groups. Isodesmic reactions show that electron withdrawal stabilizes the enolate more than the oxo form. Donation from the π system stabilizes the oxo form but has little or no effect on the enolate form. Destabilization due to electrostatic repulsion between the oxygen bound carbon and the attached atom for X = {CF3, CHO, C⋮N} is more severe in the oxo than enolate form. The effect of substituents on the stability of both enolates, H2CC(O-)X, and ethoxide derivatives, CH3CH(O-)X, are shown to be very similar. Relative to acetaldehyde, the heats of reaction of CH3COX for both enolate formation and hydride addition are also similar. For enols, both σ electron-withdrawing and π-donating substituents favor the oxo over the enol form. Isodesmic reactions show that substituents have a parallel effect on stability in the oxo and enol forms, as well as in ethene derivatives. The magnitude of these effects increases with the polarity of the double bond, i.e., carbonyls > enols > ethenes. To summarize substituent effects, enols can be thought of as ethene derivatives which are slightly polarized by the hydroxy group, while enolates can be thought of as ethoxide derivatives which are perturbed by the double bond.

Superacidity of P(OH)3 and SO(OH)2 derivatives of cyclopentadiene and vinylcyclopentadiene in the gas phase: A computational DFT analysis

“…This is in agreement with earlier studies reporting that the enolic isomers become more stable in the presence of strong EWGs. [44,45] This is beneficial for the corresponding acidity, since the tautomerism in neutral acids leads to system with higher, thus less acidic ΔH acid values. [17][18][19][20]28,29] For example, even in pentacyanocyclopentadiene, the ketenimine tautomer (C C NH) is by 7 kcal mol −1 more stable than the HC(sp 3 ) CN tautomer, which reduces the corresponding acidity from 256.5 to 263.5 kcal mol −1 .…”

Barrier Height Titration by Tunable Photoionization Combined with Chemical Monitoring: Unimolecular Keto/Enol Tautomerization of the Acetamide Cation Radical

“…Furthermore, for simple, nonconjugated carbonyl compounds, the keto forms are thermochemically more stable (Table 1). 3, 4…”

1,1‐Ethenediol: The Long Elusive Enol of Acetic Acid

“…[12,16,17] Enols of carboxylic acids have been intensively studied computationally and found to be substantially less stable than their acid tautomers. [18][19][20] Spectroscopic data for the parent aliphatic acid enol, 1,1ethenediol (1), the enol of acetic acid (3) have not been reported (Scheme 1). As enol formation through decarboxylation reactions are key synthetic (as often found in the posthydrolysis steps of Knoevenagel-type reactions) [21] and a variety of biologically relevant reactions (such as enzymatic decarboxylation), [22] direct spectroscopic information of such species is highly valuable for their identification in such processes and for understanding their stabilities and reactivities.…”