🧫Organic Chemistry II Unit 6 – Enolates and enols

What are Enolates and Enols?

• Enolates are anions formed by the deprotonation of a carbon atom adjacent to a carbonyl group (α-carbon)
  • Deprotonation occurs due to the acidic nature of the α-hydrogen atoms
  • The resulting negative charge is delocalized between the α-carbon and the oxygen atom of the carbonyl group
• Enols are neutral molecules with a carbon-carbon double bond adjacent to a hydroxyl group
  • Enols are tautomers of ketones or aldehydes, where the carbonyl group is converted to a carbon-carbon double bond and a hydroxyl group
• Enolates and enols are important reactive intermediates in various organic reactions, such as the aldol condensation and Claisen condensation
• The formation of enolates and enols is crucial for the synthesis of complex organic molecules, including natural products and pharmaceuticals
• Enolates and enols exhibit enhanced nucleophilicity compared to their corresponding carbonyl compounds, making them valuable for carbon-carbon bond formation

Formation of Enolates and Enols

• Enolates are typically formed by treating a carbonyl compound with a strong base, such as lithium diisopropylamide (LDA) or sodium hydride (NaH)
  • The base abstracts the acidic α-hydrogen, generating the enolate anion
• Enols can be formed through acid- or base-catalyzed tautomerization of carbonyl compounds
  • In acid-catalyzed tautomerization, the carbonyl oxygen is protonated, followed by the loss of a proton from the α-carbon, forming the enol
  • In base-catalyzed tautomerization, the α-hydrogen is abstracted by the base, followed by protonation of the resulting enolate anion
• The equilibrium between the keto and enol forms is influenced by factors such as solvent polarity, temperature, and the presence of catalysts
• In most cases, the keto form is more stable than the enol form due to the greater stability of the carbon-oxygen double bond compared to the carbon-carbon double bond
• However, in certain cases, such as 1,3-dicarbonyl compounds (e.g., acetylacetone), the enol form can be more stable due to intramolecular hydrogen bonding

Structure and Stability

• Enolates have a planar structure with the negative charge delocalized between the α-carbon and the oxygen atom
  • The delocalization of the negative charge contributes to the stability of enolates
• The stability of enolates is influenced by the substituents attached to the α-carbon
  • Electron-withdrawing groups (e.g., -CN, -NO2) stabilize enolates by delocalizing the negative charge
  • Electron-donating groups (e.g., -CH3, -OR) destabilize enolates by increasing the electron density on the α-carbon
• Enols have a planar structure with a carbon-carbon double bond and a hydroxyl group attached to one of the carbon atoms
• The stability of enols is influenced by the degree of substitution on the carbon-carbon double bond
  • Trisubstituted enols are more stable than disubstituted enols, which are more stable than monosubstituted enols
• Intramolecular hydrogen bonding can stabilize enols, particularly in the case of 1,3-dicarbonyl compounds
• The stability of enolates and enols plays a crucial role in their reactivity and selectivity in organic reactions

Reactions of Enolates

• Enolates are versatile nucleophiles that can participate in a wide range of organic reactions
• Alkylation reactions: Enolates can react with alkyl halides or tosylates to form α-alkylated carbonyl compounds
  • The alkylation occurs through an SN2 mechanism, with the enolate acting as the nucleophile
• Acylation reactions: Enolates can react with acyl halides or anhydrides to form β-keto esters or 1,3-diketones
  • The acylation occurs through a nucleophilic acyl substitution mechanism
• Aldol reactions: Enolates can react with aldehydes or ketones to form β-hydroxy carbonyl compounds (aldols)
  • The aldol reaction is a powerful method for forming carbon-carbon bonds and will be discussed in more detail in a separate section
• Conjugate addition reactions: Enolates can act as nucleophiles in Michael additions, reacting with α,β-unsaturated carbonyl compounds to form 1,5-dicarbonyl compounds
• The regioselectivity and stereoselectivity of enolate reactions can be controlled by the choice of base, solvent, and reaction conditions

Aldol Reactions

• The aldol reaction is a powerful method for forming carbon-carbon bonds between two carbonyl compounds
• In the aldol reaction, an enolate (or enol) of one carbonyl compound reacts with another carbonyl compound (aldehyde or ketone) to form a β-hydroxy carbonyl compound (aldol product)
• The aldol reaction can be carried out under basic or acidic conditions
  • Under basic conditions, the enolate is formed by deprotonation with a strong base (e.g., LDA, NaH)
  • Under acidic conditions, the enol is formed by acid-catalyzed tautomerization
• The aldol reaction can be either intermolecular (between two different carbonyl compounds) or intramolecular (within the same molecule)
• The stereochemistry of the aldol product can be controlled by the choice of enolate geometry (E or Z) and the facial selectivity of the carbonyl electrophile
  • Zimmerman-Traxler transition state model can be used to predict the stereochemical outcome of aldol reactions
• Aldol reactions can be followed by dehydration to form α,β-unsaturated carbonyl compounds (aldol condensation products)
• The aldol reaction has numerous applications in the synthesis of complex organic molecules, such as carbohydrates, antibiotics, and natural products

Claisen Condensation

• The Claisen condensation is a carbon-carbon bond-forming reaction between two esters (or an ester and another carbonyl compound) to form a β-keto ester
• The reaction involves the nucleophilic addition of an enolate of one ester to the carbonyl group of another ester, followed by the elimination of an alkoxide ion
• The Claisen condensation is typically carried out under basic conditions, using strong bases such as sodium ethoxide or sodium hydride
• The mechanism of the Claisen condensation involves the formation of an enolate intermediate, which then attacks the carbonyl group of the other ester
  • The resulting tetrahedral intermediate collapses, eliminating an alkoxide ion and forming the β-keto ester product
• The Claisen condensation can be intramolecular (Dieckmann condensation) or intermolecular
• Intramolecular Claisen condensations are particularly useful for the synthesis of cyclic β-keto esters and are often used in the preparation of five- and six-membered rings
• The Claisen condensation is a valuable tool in organic synthesis, allowing for the formation of carbon-carbon bonds and the introduction of ketone and ester functional groups

Other Important Reactions

• Mannich reaction: A three-component reaction involving an enolizable carbonyl compound, a non-enolizable aldehyde (usually formaldehyde), and an amine to form β-amino carbonyl compounds
  • The Mannich reaction is useful for the introduction of nitrogen-containing functional groups and the synthesis of alkaloids and other biologically active compounds
• Robinson annulation: A two-step reaction sequence involving a Michael addition followed by an intramolecular aldol condensation to form cyclic α,β-unsaturated ketones
  • The Robinson annulation is a powerful method for the synthesis of six-membered rings and is often used in the preparation of steroids and other polycyclic compounds
• Knoevenagel condensation: A condensation reaction between an aldehyde or ketone and an active methylene compound (e.g., malonic acid derivatives, cyanoacetic acid) to form α,β-unsaturated carbonyl compounds
  • The Knoevenagel condensation is useful for the synthesis of substituted alkenes and is often used in the preparation of cinnamic acid derivatives and other aromatic compounds
• Horner-Wadsworth-Emmons reaction: A variation of the Wittig reaction that uses phosphonate esters instead of phosphonium ylides to form α,β-unsaturated carbonyl compounds
  • The Horner-Wadsworth-Emmons reaction offers greater control over the stereochemistry of the product and is often used in the synthesis of polyene natural products

Applications in Organic Synthesis

• Enolates and enols are versatile intermediates in the synthesis of complex organic molecules, including natural products, pharmaceuticals, and materials
• Aldol reactions are widely used in the synthesis of carbohydrates, such as monosaccharides and higher sugars
  • For example, the Kiliani-Fischer synthesis uses an aldol reaction to extend the carbon chain of a sugar by one carbon unit
• The Claisen condensation is a key step in the synthesis of various heterocycles, such as pyridines, quinolines, and indoles
  • The Hantzsch pyridine synthesis involves a Claisen condensation between an aldehyde and two equivalents of a β-keto ester to form a dihydropyridine intermediate
• The Robinson annulation is a powerful tool for the synthesis of steroid hormones and other polycyclic compounds
  • The Wieland-Miescher ketone, a key intermediate in the synthesis of many steroids, is prepared using a Robinson annulation
• Enolate chemistry is central to the synthesis of β-lactam antibiotics, such as penicillins and cephalosporins
  • The formation of the β-lactam ring often involves an intramolecular aldol reaction or a related condensation reaction
• The Knoevenagel condensation is used in the synthesis of various aromatic compounds, such as cinnamic acids and coumarins
  • Cinnamic acids are important precursors for the synthesis of flavonoids and other plant-derived natural products