DIBAL-H Reduction of Esters to Aldehydes
DIBAL-H Reduction of Esters to Aldehydes: DIBAL-H can reduce an ester (COOR) to an aldehyde (CHO) when it is used under carefully controlled, cold conditions.
What happens
DIBAL-H can reduce an ester (COOR) to an aldehyde (CHO) when it is used under carefully controlled, cold conditions. The carbonyl carbon is retained and the carbon count of the acyl portion is unchanged. For easy product prediction: OR′ OUT → H IN.
Reaction summary
Ester + DIBAL-H (cold) → aldehyde (COOR → CHO)
Reagents and conditions
- Starting material: Methyl 2-methylpropanoate (ester)
- Reagents: DIBAL-H (diisobutylaluminium hydride)
- Conditions: DIBAL-H, −78 °C; Aqueous workup (H₂O / H₃O⁺)
- Products: 2-Methylpropanal (aldehyde)
Mechanism
- Hydride Attacks: DIBAL-H provides H⁻. The hydride attacks the electrophilic carbonyl carbon of the ester, the C=O opens, and an aluminium-bound tetrahedral intermediate forms.
- Controlled Reduction: At low temperature, this intermediate can be maintained without simply reducing everything all the way to an alcohol. Cold + controlled DIBAL-H = stop early.
- Workup: During aqueous/acidic workup, the aluminium-containing intermediate is hydrolyzed. The final product is R–CHO, an aldehyde.
- Product: The aldehyde: the OR′ group is gone and an H has taken its place.
Selectivity
- Chemoselectivity: The cold temperature (about −78 °C) and the controlled amount of DIBAL-H help stop the reduction at the aldehyde instead of continuing to the primary alcohol.
Common mistakes
- DIBAL-H can over-reduce the aldehyde to a primary alcohol if the conditions are not controlled. That is why you commonly see low temperature (−78 °C) and a controlled amount of DIBAL-H. If the reduction continues: ester → aldehyde → primary alcohol.
Memory trick
DIBAL = “Don’t reduce It Beyond ALdehyde.” Ester + DIBAL-H (cold) → aldehyde.