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Enol and enolates

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This document discusses enols and enolates. Enols are formed when a carbonyl compound loses its alpha proton, forming an anion called an enolate that is stabilized by delocalization. Keto-enol tautomerism allows easy interconversion between keto and enol forms using acid or base. Enolates such as lithium enolates and silyl enolates are stable equivalents that can undergo reactions like halogenation, the haloform reaction, malonic ester synthesis, and alkylation. Thermodynamic enolates predominate under equilibrium while kinetic enolates form fastest with hindered bases. Reactions of enolates include acid/base-catalyzed

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Enol and Enolates Presented by: Rizwana Sarwar Presented to: Dr. Imran Ali Hashmi Dr. Firdos Dr. Samreen
Contents 1. Enol and Enolates 2. Keto Enol Toutomarism 3. Stable Enol Equivelents 4. Synthsis of Enolates 5. Thermodynamic and Kinetic Enolates 6. Reaction of Enolates
What are Enol and Enolates? The carbonyl group is strongly electron withdrawing. When a carbonyl compound loses 留 proton, the anion that is produced, called an enolate. It is stabilized by delocalization. The Term Enol is used because of C=C ene and -OH ol.
Keto Enol Tautomers The keto and enol forms of carbonyl compounds are constitutional isomers. They are easily interconverted by proton transfers in the presence of an acid or base. Interconvertible keto and enol forms are called tautomers, and their interconversion is called Tautomerization.
Keto Enol Tautomers In compounds whose molecules have two carbonyl groups. The amount of enol present at equilibrium is far higher. The greater stability of the enol form of 硫-dicarbonyl compounds is due to hydrogen bonding.
Acid catalyzed Enolization
Base catalyzed Enolization
Some Enol Equivelents Lithium Enolates Silyl Enolates Aza Enolates
Thermodynamic and Kinetic Enolates Thermodynamic Enolates: The Thermodynamic enolate predominates under conditions of thermodynamic control. A deprotonationprotonation equilibrium allows interconversion among the possible enolates. Thermodynamic enolates are more highly substituted enolate and more stable one.
Thermodynamic and Kinetic Enolates Kinetic Enolates: The Kinetic Enolate is that which is formed fastest. It is usually formed by removal of the least sterically hindered 留 hydrogen. Kinetic enolates formed with sterically hindered base in an aprotic solvent. Such as LDA in tetrahydrofuran (THF) or dimethoxyethane (DME)
Example of Thermodynamic and Kinetic Enolates
Lithium Enolates It is a special type of Kinetic Enolate, means reaction occur at low temperature and fast. It always give less substituted Enolates. An strong base for converting carbonyl compounds to enolates is lithium diisopropylamide (LDA) or LiN(i-Pr)2.
Silyl Enolates Silicon is less electropositive than lithium. The silyl enol ethers are more stable, and less reactive, than lithium enolates. They are made by treating an enolate or 留 carbonyl with a silicon electrophile in presence of base. Silyl enol ethers also made from lithium enolates by treating Lithum enolate with trimethylsilyl chloride.
Aza-Enolate The enamines are the nitrogen analogues of enols. Aza-enolates are the nitrogen analogues of enolates. They are made by deprotonating enamines with strong base.
Reaction of Enolates Acid and base catalyzed halogenation. Haloform reaction Malonic easter synthesis Alkylation and halogenation via Lithium and Silyl enolates.
Acid and base catalyzed halogenation Carbonyl compounds bearing an 留 hydrogen can undergo halogen substitution at the 留 carbon in the presence of acid or base.
Acid and base catalyzed halogenation Carbonyl compounds bearing an 留 hydrogen can undergo halogen substitution at the a carbon in the presence of acid.
Haloform Reaction Methyl ketones react with halogens in the presence of excess base. Multiple halogenations always occur at the carbon of the methyl group. Multiple halogenations occur because addition of the first halogen makes the remaining 留 hydrogens more acidic.
Enol and enolates
The Malonic Ester Synthesis (Alkylation of Ester) A useful counterpart of the acetoacetic ester synthesis. That allows the synthesis of mono- and disubstituted acetic acid is called the malonic ester synthesis.
The Malonic Ester Synthesis (Alkylation of Ester)
Halogenation via Silyl enolates. Electrophilic attack occurs at the 留 carbon atom. The halide ion released in this step. Then attacks the silicon atom to release the product and a molecule of Me3SiX.
Alkylation via Lithium Enolates. The formation of lithium enolates using lithium diisopropylamide a useful way to alkylating ketones in a regioselective way.
References Organic Chemistry, T.W. Graham Solomons, Craig B. Fryhle and Scott A. Snyder 11th Edition. Organic Chemistry 2nd Edition. Jonathan Clayden Nick Greeves Stuart Warren
Enol and enolates

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Enol and enolates

  • 1. Enol and Enolates Presented by: Rizwana Sarwar Presented to: Dr. Imran Ali Hashmi Dr. Firdos Dr. Samreen
  • 2. Contents 1. Enol and Enolates 2. Keto Enol Toutomarism 3. Stable Enol Equivelents 4. Synthsis of Enolates 5. Thermodynamic and Kinetic Enolates 6. Reaction of Enolates
  • 3. What are Enol and Enolates? The carbonyl group is strongly electron withdrawing. When a carbonyl compound loses 留 proton, the anion that is produced, called an enolate. It is stabilized by delocalization. The Term Enol is used because of C=C ene and -OH ol.
  • 4. Keto Enol Tautomers The keto and enol forms of carbonyl compounds are constitutional isomers. They are easily interconverted by proton transfers in the presence of an acid or base. Interconvertible keto and enol forms are called tautomers, and their interconversion is called Tautomerization.
  • 5. Keto Enol Tautomers In compounds whose molecules have two carbonyl groups. The amount of enol present at equilibrium is far higher. The greater stability of the enol form of 硫-dicarbonyl compounds is due to hydrogen bonding.
  • 8. Some Enol Equivelents Lithium Enolates Silyl Enolates Aza Enolates
  • 9. Thermodynamic and Kinetic Enolates Thermodynamic Enolates: The Thermodynamic enolate predominates under conditions of thermodynamic control. A deprotonationprotonation equilibrium allows interconversion among the possible enolates. Thermodynamic enolates are more highly substituted enolate and more stable one.
  • 10. Thermodynamic and Kinetic Enolates Kinetic Enolates: The Kinetic Enolate is that which is formed fastest. It is usually formed by removal of the least sterically hindered 留 hydrogen. Kinetic enolates formed with sterically hindered base in an aprotic solvent. Such as LDA in tetrahydrofuran (THF) or dimethoxyethane (DME)
  • 11. Example of Thermodynamic and Kinetic Enolates
  • 12. Lithium Enolates It is a special type of Kinetic Enolate, means reaction occur at low temperature and fast. It always give less substituted Enolates. An strong base for converting carbonyl compounds to enolates is lithium diisopropylamide (LDA) or LiN(i-Pr)2.
  • 13. Silyl Enolates Silicon is less electropositive than lithium. The silyl enol ethers are more stable, and less reactive, than lithium enolates. They are made by treating an enolate or 留 carbonyl with a silicon electrophile in presence of base. Silyl enol ethers also made from lithium enolates by treating Lithum enolate with trimethylsilyl chloride.
  • 14. Aza-Enolate The enamines are the nitrogen analogues of enols. Aza-enolates are the nitrogen analogues of enolates. They are made by deprotonating enamines with strong base.
  • 15. Reaction of Enolates Acid and base catalyzed halogenation. Haloform reaction Malonic easter synthesis Alkylation and halogenation via Lithium and Silyl enolates.
  • 16. Acid and base catalyzed halogenation Carbonyl compounds bearing an 留 hydrogen can undergo halogen substitution at the 留 carbon in the presence of acid or base.
  • 17. Acid and base catalyzed halogenation Carbonyl compounds bearing an 留 hydrogen can undergo halogen substitution at the a carbon in the presence of acid.
  • 18. Haloform Reaction Methyl ketones react with halogens in the presence of excess base. Multiple halogenations always occur at the carbon of the methyl group. Multiple halogenations occur because addition of the first halogen makes the remaining 留 hydrogens more acidic.
  • 20. The Malonic Ester Synthesis (Alkylation of Ester) A useful counterpart of the acetoacetic ester synthesis. That allows the synthesis of mono- and disubstituted acetic acid is called the malonic ester synthesis.
  • 21. The Malonic Ester Synthesis (Alkylation of Ester)
  • 22. Halogenation via Silyl enolates. Electrophilic attack occurs at the 留 carbon atom. The halide ion released in this step. Then attacks the silicon atom to release the product and a molecule of Me3SiX.
  • 23. Alkylation via Lithium Enolates. The formation of lithium enolates using lithium diisopropylamide a useful way to alkylating ketones in a regioselective way.
  • 24. References Organic Chemistry, T.W. Graham Solomons, Craig B. Fryhle and Scott A. Snyder 11th Edition. Organic Chemistry 2nd Edition. Jonathan Clayden Nick Greeves Stuart Warren