Oppenauer Oxidation: Selective Oxidation of Alcohols
The Oppenauer Oxidation is a highly selective method for oxidizing secondary alcohols to ketones using a metal alkoxide catalyst (usually Aluminium Isopropoxide) in the presence of excess Acetone (or another ketone) which acts as the oxidant.
1. General Reaction
A secondary alcohol reacts with excess acetone in the presence of Aluminium Isopropoxide. The alcohol is oxidized to a ketone, while acetone is reduced to Isopropyl alcohol.
Reagents:
- Catalyst: Aluminium Isopropoxide ($Al(O-iPr)_3$) or Aluminium tert-butoxide.
- Oxidant/Solvent: Acetone (used in large excess to drive equilibrium to the right).
- Substrate: Secondary Alcohol.
2. Detailed Mechanism
The reaction proceeds via a cyclic transition state involving hydride transfer.
Step 1: Ligand Exchange
The alcohol substrate displaces one isopropoxide group from the aluminium catalyst to form a new aluminium alkoxide complex.
Step 2: Coordination and Transition State
A molecule of Acetone coordinates to the aluminium center. A six-membered cyclic transition state is formed.
Step 3: Hydride Transfer
A hydride ion ($H^-$) is transferred from the $\alpha$-carbon of the alcohol to the carbonyl carbon of the acetone. This converts the alcohol part into a ketone and the acetone part into an alkoxide.
Step 4: Product Release
Exchange with another molecule of alcohol releases the product ketone and regenerates the active catalyst species.
3. Relation to MPV Reduction
A Reversible Reaction
The Oppenauer Oxidation is the exact reverse of the Meerwein-Ponndorf-Verley (MPV) Reduction.
MPV: Ketone + Isopropyl Alcohol $\rightarrow$ Secondary Alcohol + Acetone.
Oppenauer: Secondary Alcohol + Acetone $\rightarrow$ Ketone + Isopropyl Alcohol.
The direction is controlled by using an excess of one reactant (Acetone for oxidation, Isopropyl Alcohol for reduction).
4. Key Advantage: Selectivity
The Oppenauer oxidation is extremely mild and highly selective.
- Carbon-Carbon Double Bonds ($C=C$): It does not affect $C=C$ bonds, unlike oxidants like $KMnO_4$ or Jones Reagent.
- Other Functional Groups: It tolerates sensitive groups like amines, sulfides, and acetals.
- Example: Cholesterol is oxidized to Cholest-4-en-3-one (The double bond migrates due to conjugation but is not oxidized).
5. Summary
| Feature | Oppenauer Oxidation | Jones Oxidation |
|---|---|---|
| Conditions | Basic/Neutral ($Al(OR)_3$) | Strongly Acidic ($CrO_3/H_2SO_4$) |
| C=C Bonds | Preserved | May isomerize or react |
| By-product | Isopropyl Alcohol | Chromium(III) salts |
Oppenauer Quiz
Test your knowledge on mild oxidation. 10 MCQs with explanations.
No comments:
Post a Comment