Keto–enol tautomerism refers to a chemical equilibrium between a "keto" form (a carbonyl, named for the common ketone case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electrons. The keto and enol forms are tautomers of each other.[2]
In the case of ketones, the conversion is called a keto-enol tautomerism, although this name is often more generally applied to all such tautomerizations. Usually the equilibrium constant is so small that the enol is undetectable spectroscopically.
In some compounds with two (or more) carbonyls, the enol form becomes dominant. The behavior of 2,4-pentanedione illustrates this effect:[3]
The acid-catalyzed conversion of an enol to the keto form proceeds by proton transfer from O to carbon. The process does not occur intramolecularly, but requires participation of solvent or other mediators.
Stereochemistry of ketonization
If R1 and R2 (note equation at top of page) are different substituents, there is a new stereocenter formed at the alpha position when an enol converts to its keto form. Depending on the nature of the three R groups, the resulting products in this situation would be diastereomers or enantiomers.Script error: No such module "Unsubst".
Enediols
Enediols are alkenes with a hydroxyl group on each carbon of the C=C double bond. Normally such compounds are disfavored components in equilibria with acyloins. One special case is catechol, where the C=C subunit is part of an aromatic ring. In some other cases however, enediols are stabilized by flanking carbonyl groups. These stabilized enediols are called reductones. Such species are important in glycochemistry, e.g., the Lobry de Bruyn–Van Ekenstein transformation.[5]
File:Ascorbic acidity3.pngConversion of ascorbic acid (vitamin C) to an enolate. Enediol at left, enolate at right, showing movement of electron pairs resulting in deprotonation of the stable parent enediol. A distinct, more complex chemical system, exhibiting the characteristic of vinylogy.
Phenols represent a kind of enol. For some phenols and related compounds, the keto tautomer plays an important role. Many of the reactions of resorcinol involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione.[6]
Keto–enol tautomerism is important in several areas of biochemistry.Script error: No such module "Unsubst".
The high phosphate-transfer potential of phosphoenolpyruvate results from the fact that the phosphorylated compound is "trapped" in the less thermodynamically favorable enol form, whereas after dephosphorylation it can assume the keto form.Script error: No such module "Unsubst".
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Addition of electrophiles
The terminus of the double bond in enols is nucleophilic. Its reactions with electrophilic organic compounds is important in biochemistry as well as synthetic organic chemistry. In the former area, the fixation of carbon dioxide involves addition of CO2 to an enol.Script error: No such module "Unsubst".
Deprotonation: enolates
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Deprotonation of enolizable ketones, aldehydes, and esters gives enolates.[8][9] Enolates can be trapped by the addition of electrophiles at oxygen. Silylation gives silyl enol ether.[10] Acylation gives
esters such as vinyl acetate.[11]
Stable enols
In general, enols are less stable than their keto equivalents because of the favorability of the C=O double bond over C=C double bond. However, enols can be stabilized kinetically or thermodynamically.Script error: No such module "Unsubst".
Some enols are sufficiently stabilized kinetically so that they can be characterized.Script error: No such module "Unsubst".
Delocalization can stabilize the enol tautomer. Thus, very stable enols are phenols.[13] Another stabilizing factor in 1,3-dicarbonyls is intramolecular hydrogen bonding.[14] Both of these factors influence the enol-dione equilibrium in acetylacetone.
