Enol

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(Redirected from
Stereochemistry of ketonization of enols and enolates
)
Examples of keto-enol
tautomerism
TBD
Ketone
enediol-type of enol.[citation needed
]

In

anion termed an enolate (see images at right). The enolate structures shown are schematic; a more modern representation considers the molecular orbitals that are formed and occupied by electrons in the enolate. Similarly, generation of the enol often is accompanied by "trapping" or masking of the hydroxy group as an ether, such as a silyl enol ether.[1]

Keto–enol tautomerism refers to a

tautomers of each other.[2]

Enolization

α-hydrogen (C−H bond adjacent to the carbonyl group) often form enols. The reaction involves migration of a proton (H) from carbon to oxygen:[1]

RC(=O)CHR′R′′ ⇌ RC(OH)=CR′R′′

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]

Selected enolization constants[4]
carbonyl enol Kenolization
Acetaldehyde
CH3CHO 		CH2=CHOH 5.8×10−7
Acetone
CH3C(O)CH3 		CH3C(OH)=CH2 5.12×10−7
Methyl acetate
CH3CO2CH3 		CH2=CH(OH)OCH3 4×10−20
Acetophenone
C6H5C(O)CH3 		C6H5C(OH)=CH2 1×10−8
Acetylacetone
CH3C(O)CH2C(O)CH3 		CH3C(O)CH=C(OH)CH3 0.27
Trifluoroacetylacetone
CH3C(O)CH2C(O)CF3 		CH3C(O)CH=C(OH)CF3 32
Hexafluoroacetylacetone
CF3C(O)CH2C(O)CF3 		CF3C(O)CH=C(OH)CF3 ~104
Cyclohexa-2,4-dienone Phenol
C6H5OH 		>1012

Enols are derivatives of

1,3-diketones, such as acetylacetone
(2,4-pentanedione), the enol form is favored.

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.[citation needed]

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

Lobry de Bruyn-van Ekenstein transformation.[5]

Keto-enediol tautomerizations. Enediol in the center; acyloin isomers at left and right. Ex. is hydroxyacetone, shown at right.
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
.

Ribulose-1,5-bisphosphate is a key substrate in the Calvin cycle of photosynthesis. In the Calvin cycle, the ribulose equilibrates with the enediol, which then binds carbon dioxide. The same enediol is also susceptible to attack by oxygen (O2) in the (undesirable) process called photorespiration
.

ribulose-1,5-bisphosphate
.

Phenols

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]

Biochemistry

Keto–enol tautomerism is important in several areas of biochemistry.[citation needed]

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.[citation needed
]

The

2-phosphoglyceric acid to the enol phosphate ester. Metabolism of PEP to pyruvic acid by pyruvate kinase (PK) generates adenosine triphosphate (ATP) via substrate-level phosphorylation.[7]

H2O ADP ATP
H2O

Reactivity

See also: Carbonyl α-substitution reactions

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.[citation needed]

Deprotonation: enolates

Main article: enolate

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.[citation needed]

Some enols are sufficiently stabilized kinetically so that they can be characterized.[citation needed]

Diaryl-substitution stabilizes some enols.[12]

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.

See also

References

  1. ^
    ISBN 0-471-58589-0
    .
  2. ISBN 978-0-19-927029-3
    .
  3. doi:10.1021/ed1010932
    .
  4. doi:10.1002/poc.3168
    .
  5. S2CID 260331550
    .
  6. PMID 16304647
    .
  7. ISBN 0-7167-3051-0
    .
  8. ISBN 978-0-471-72091-1
  9. ISBN 9783527671069
    .
  10. doi:10.1002/0471264180.or046.01
  11. S2CID 241676899
    .
  12. doi:10.1021/ja00203a019
    .
  13. ^ Clayden, Jonathan (2012). Organic Chemistry. Oxford University Press. pp. 456–459.
  14. PMID 23320139
    .

External links

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