23.5 Mixed Aldol Reactions

Mixed Aldol Reactions

Mixed aldol reactions combine two different carbonyl compounds to form a new carbon-carbon bond. One compound donates the enolate (nucleophile), while the other accepts it at its carbonyl carbon (electrophile). The challenge is controlling which compound plays which role, because mixing two enolizable carbonyls can produce up to four different aldol products. That's why careful choice of reactants, bases, and conditions matters so much here.

Conditions for Mixed Aldol Reactions

Two different carbonyl compounds are required:

• The nucleophilic partner must have at least one $\alpha$-hydrogen so it can form an enolate. This is typically an aldehyde (e.g., propanal) or a ketone (e.g., acetone).
• The electrophilic partner is the carbonyl that gets attacked. Ideally, it should not have $\alpha$-hydrogens, so it can't form its own enolate and compete. Benzaldehyde is a classic choice because it has no $\alpha$-hydrogens.

A strong, non-nucleophilic base is needed to deprotonate the nucleophilic partner and generate the enolate cleanly before the electrophile is added:

• LDA (lithium diisopropylamide): the most common choice for generating kinetic enolates
• NaH (sodium hydride): strong, irreversible deprotonation
• NaOEt (sodium ethoxide): useful but can give equilibrating (thermodynamic) enolate mixtures

Anhydrous conditions are essential. Water reacts with both the base and the enolate, killing your nucleophile and lowering yield. Use dry solvents like THF or diethyl ether under an inert atmosphere ($\text{N}_2$ or Ar).

Low temperatures (often $-78\,^{\circ}\mathrm{C}$, achieved with a dry ice/acetone bath) slow the reaction down, improving selectivity by favoring the kinetic enolate and reducing unwanted side reactions like self-condensation.

Products of Mixed Aldol Reactions

The enolate attacks the carbonyl carbon of the electrophilic partner, forming a new C–C bond between the $\alpha$-carbon of the nucleophile and the carbonyl carbon of the electrophile. The resulting alkoxide is protonated during aqueous workup to give a $\beta$-hydroxy carbonyl compound (the aldol product).

For example, the enolate of acetone attacking formaldehyde gives 4-hydroxybutan-2-one.

Regioselectivity becomes an issue when the nucleophilic partner has more than one type of $\alpha$-hydrogen:

• With LDA at $-78\,^{\circ}\mathrm{C}$, deprotonation occurs at the less substituted $\alpha$-carbon (kinetic enolate). For 2-butanone, this means deprotonation at the methyl side.
• Under thermodynamic conditions (e.g., NaOEt at room temperature with equilibration time), the more substituted enolate is favored because it's more stable.
• A ketone like 2-pentanone has two different $\alpha$-positions, so without careful control, you can get a mixture of regioisomeric enolates and therefore a mixture of aldol products.

Stereochemistry arises because the new C–C bond creates up to two new stereocenters. The enolate can attack the re or si face of the electrophilic carbonyl, producing syn and anti diastereomers. The Zimmerman-Traxler model predicts the preferred diastereomer by proposing a chair-like, six-membered transition state where the metal coordinates both oxygens. In general:

• Z-enolates (from LDA, kinetic control) tend to give syn aldol products.
• E-enolates (from thermodynamic conditions) tend to give anti aldol products.

Enolate Formation and Reactivity

Enolization is the key first step. The base removes an $\alpha$-hydrogen, generating a resonance-stabilized enolate ion with nucleophilic character at the $\alpha$-carbon.

Kinetic vs. thermodynamic enolates:

• Kinetic enolate: forms at the less hindered $\alpha$-carbon. Favored by strong, bulky bases (LDA), low temperature ($-78\,^{\circ}\mathrm{C}$), and short reaction times. The proton that's most accessible gets removed first.
• Thermodynamic enolate: the more substituted enolate, which is more stable due to greater substitution of the double bond. Favored by smaller bases (NaOEt), higher temperatures, and longer reaction times that allow equilibration.

After the aldol addition, the $\beta$-hydroxy carbonyl product can undergo E1cB elimination (loss of water) to form an $\alpha,\beta$-unsaturated carbonyl compound (the aldol condensation product). This is especially favorable when the product gains extended conjugation, such as with an aromatic ring.

Synthesis via Mixed Aldol Reactions

Planning a mixed aldol synthesis works best through retrosynthetic analysis:

1. Identify your target molecule (e.g., 4-hydroxy-4-phenylbutan-2-one).
2. Locate the $\beta$-hydroxy carbonyl pattern in the target. The C–C bond between the $\alpha$-carbon and the $\beta$-carbon is the bond that was formed in the aldol reaction.
3. Disconnect that bond to reveal the two carbonyl precursors. In this case: acetone (nucleophile/enolate donor) and benzaldehyde (electrophile, no $\alpha$-hydrogens).

Choosing roles for each partner:

• Pick the compound without $\alpha$-hydrogens as the electrophile whenever possible. This avoids self-condensation of the electrophile.
• If both compounds have $\alpha$-hydrogens, you'll need to preform the enolate of one (using LDA at low temperature) and then add the electrophile to the reaction mixture. This "directed" aldol approach prevents scrambling.

Protecting groups may be needed if other functional groups would interfere:

• Alcohols → silyl ethers (TBS, removed with TBAF) or acetates
• Amines → Boc carbamates or amides

After the aldol step, additional transformations may be required to reach the final target:

• Elimination (dehydration with acid, e.g., $\text{H}_2\text{SO}_4$) to form the $\alpha,\beta$-unsaturated product
• Reduction ($\text{NaBH}_4$) of the carbonyl
• Oxidation (Swern or PCC) of the alcohol
• Deprotection to unmask any protected functional groups