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OxidationIntermediate
Swern Oxidation
Also known as: DMSO-Oxalyl Chloride Oxidation
Discovered by Daniel Swern (1978)
General Equation
RCH₂OH → RCHO (chromium-free!)
Description
A mild, chromium-free oxidation that converts alcohols to aldehydes/ketones. Uses activated DMSO as the oxidant. Must be performed at -78°C to avoid Pummerer rearrangement side products.
Reaction Scheme
Reactants:
- • Primary or Secondary alcohol
Products:
- • Primary alcohol → Aldehyde
- • Secondary alcohol → Ketone
Reagents:
- • (COCl)₂ (oxalyl chloride)
- • DMSO
- • Et₃N
Conditions:
- • -78°C (dry ice/acetone), CH₂Cl₂
Mechanism (3 steps)
1
DMSO attacks oxalyl chloride to form the activated DMSO species (chlorosulfonium ion), releasing CO, CO₂, and HCl.
Arrow: curved(CH₃)₂S⁺-Cl (activated DMSO)
Must be at -78°C
2
The alcohol attacks the activated DMSO, displacing chloride to form an alkoxysulfonium ion.
Arrow: curvedR-O-S⁺(CH₃)₂ (alkoxysulfonium ion)
3
Triethylamine (Et₃N) deprotonates the α-carbon of the sulfonium, causing intramolecular elimination to give the carbonyl product and dimethyl sulfide (DMS, smelly!).
Arrow: curved
DMS byproduct has strong odor
Key Points to Remember
- ✓Chromium-free — environmentally preferred
- ✓Must be done at -78°C (side reactions at higher temperatures)
- ✓Stops at aldehyde for primary alcohols
- ✓Very mild — compatible with many sensitive functional groups
- ✓Produces dimethyl sulfide (DMS) — horrible smell
- ✓Alternatives: Dess-Martin (mild), IBX, TPAP
Examples
Reactant:
(S)-Citronellol
Reagent:
(COCl)₂, DMSO, Et₃N, -78°C
Product:
(S)-Citronellal
Yield: 92%
Synthetic Utility
Gold standard for mild oxidation in total synthesis. Compatible with most functional groups due to mild conditions.
Limitations
- ⚠Must be at -78°C (inconvenient)
- ⚠DMS byproduct has terrible smell
- ⚠Somewhat complex procedure (3 reagents, low temperature)
#oxidation#Swern#DMSO#mild#chromium-free#aldehyde