Identify the suitable reagent for the following conversion.

Answer

Selective Reduction of Esters to Aldehydes

Reaction Overview:

Reagent Specificity:

• DIBAL-H (Diisobutylaluminium hydride): Selectively reduces esters to aldehydes under controlled conditions. It stops at the aldehyde stage, avoiding further reduction to alcohols.
• LiAlH₄ (Lithium Aluminium Hydride): Too strong; reduces esters all the way to primary alcohols.
• NaBH₄ (Sodium Borohydride): Too mild; ineffective for esters.
• Rosenmund Reduction (H₂/Pd-BaSO₄): Works only on acid chlorides, not esters.

Conclusion:

To reduce esters to aldehydes effectively and selectively:

• Use DIBAL-H at low temperatures (typically –78°C), followed by aqueous work-up.
• Avoid LiAlH₄ (over-reduction) and NaBH₄ (no reduction).
• Rosenmund reduction is suitable for acid chlorides only.