DIBAL-H: The Selective Reducing Agent
DIBAL-H (Diisobutylaluminium hydride) is a specialized reducing agent widely used in organic synthesis. Unlike strong reducing agents like $LiAlH_4$, DIBAL-H is electrophilic and can perform partial reductions, specifically converting Esters and Nitriles directly into Aldehydes under controlled low-temperature conditions.
1. Structure and Properties
DIBAL-H is an organoaluminium compound existing as a dimer or trimer.
Formula: $[(CH_3)_2CHCH_2]_2AlH$ or $(i-Bu)_2AlH$.
- It acts as a Lewis Acid due to the vacant p-orbital on Aluminum.
- It is typically used in non-polar solvents like Toluene, Hexane, or DCM.
- It is pyrophoric and reacts violently with air/water.
2. Reduction of Esters to Aldehydes
The most famous application of DIBAL-H is the partial reduction of esters to aldehydes. This reaction must be carried out at very low temperatures ($-78^\circ C$).
Mechanism (Why it stops at Aldehyde)
- Complexation: The carbonyl oxygen donates electron density to the electrophilic Aluminum atom.
- Hydride Transfer: A hydride ion ($H^-$) is transferred from Al to the carbonyl carbon, forming a stable Tetrahedral Intermediate.
- Stability at Low Temp: At $-78^\circ C$, this aluminum-alkoxide intermediate is stable and does not collapse to eliminate the alkoxide group (which would form an aldehyde that could be further reduced).
- Hydrolysis: Upon adding water (quenching), the intermediate breaks down to release the Aldehyde.
Temperature is Key!
If the reaction is performed at room temperature, the intermediate collapses, and DIBAL-H (if in excess) will reduce the aldehyde further to a Primary Alcohol.
3. Reduction of Nitriles to Aldehydes
DIBAL-H reduces nitriles ($R-CN$) to Imines, which upon hydrolysis yield Aldehydes.
4. Reduction of Lactones to Lactols
Cyclic esters (Lactones) are reduced to cyclic hemiacetals (Lactols).
Lactols are useful because they are equivalent to hydroxy-aldehydes.
5. Comparison of Reducing Agents
| Substrate | Product with $LiAlH_4$ | Product with $NaBH_4$ | Product with DIBAL-H ($-78^\circ C$) |
|---|---|---|---|
| Ester ($RCOOR'$) | Alcohol ($RCH_2OH$) | No Reaction | Aldehyde ($RCHO$) |
| Nitrile ($RCN$) | Amine ($RCH_2NH_2$) | No Reaction | Aldehyde ($RCHO$) |
| Alkene ($C=C$) | No Reaction | No Reaction | No Reaction |
6. Chemoselectivity
DIBAL-H does not reduce carbon-carbon double ($C=C$) or triple ($C \equiv C$) bonds within the molecule (unless conjugated to the carbonyl, but even then, conditions can be tuned). This allows selective reduction of the ester group in unsaturated esters.
Example: $CH_3-CH=CH-COOEt \xrightarrow{DIBAL-H} CH_3-CH=CH-CHO$
DIBAL-H Quiz
Test your concepts on Selective Reduction. 10 MCQs with explanations.
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