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Reactions of Enols and Enolates

Reactions of Enols and Enolates. Ketones and aldehydes are in equilibrium with a small amount of the corresponding enol . This can be problematic, if one desires to produce a stereogenic center a to a C=O.

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Reactions of Enols and Enolates

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  1. Reactions of Enols and Enolates

  2. Ketones and aldehydes are in equilibrium with a small amount of the corresponding enol. This can be problematic, if one desires to produce a stereogenic center a to a C=O.

  3. But the enol form is also useful since the electron-rich double bond can serve as a mild nucleophile toward reactive electrophiles.

  4. Alternatively, the proton on the a-carbon can be abstracted by a strong base to generate a (resonance-stabilized) enolate anion. This enolate anion is a much stronger nucleophile than the corresponding enol.

  5. The Aldol Condensation Reaction

  6. KetoneEnolate + Aldehyde

  7. Notice that, in this case, the product alcohol dehydrated to form an a,b-unsaturated carbonyl as a product.

  8. IntramolecularAldol

  9. Ester Enolate + Aldehyde

  10. Ester Enolate + Ester = Claisen Condensation

  11. IntramolecularClaisen Condensation = Dieckmann Condensation

  12. 1,4-Conjugate Addition: The Michael Reaction (Notice that the presence of the conjugated C=C-C=O system is crucial, since this both polarizes the C=C and also leads to a resonance-stabilized carbanion, after addition of the nucleophile)

  13. Conjugate additions are promoted by the addition of copper salts, which are believed to function by coordination with the C=C, prior to addition of the nucleophile.

  14. Decarboxylation Reactions

  15. Decarboxylation produce CO2, and are thus thermodynamically favorable. These reactions proceed most readily when the COOH moiety is attached to an electronegative atom, like oxygen or nitrogen. UnstabilizedCarbanion is NOT formed

  16. However, even in the case of simple carboxylic acid, decarboxylation may proceed readily, if the resultant carbanion is stabilized by resonance, or other electronegative substituent, as shown for acetoacetic acid below. Stabilized (enolate) carbanion is formed The reaction probably proceeds through a cyclic transition state as shown below.

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