Syn elimination via thionocarbonate
Corey-Winter olefination
How the Corey-Winter olefination works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: 1) TCDI (thiocarbonyldiimidazole) 2) P(OMe)₃. Conditions: Toluene or neat, 60–110 °C. Bonds formed: C=C; CO₂; (MeO)₃P=S; bonds broken: two C–O bonds; C=S π. Stereochemistry: Stereospecific syn elimination: cis-diol gives cis-alkene (suprafacial). Rate and selectivity: Driven by strong P=S and C=O (CO₂) bonds.
At a glance
Step 1: Diol → cyclic thionocarbonate
A 1,2-diol condenses with thiocarbonyldiimidazole to form a cyclic thionocarbonate, locking both oxygens into a five-membered ring bridging the two carbons.
Step 2: Phosphite attacks sulfur
Treatment with a phosphine (like P(OMe)₃) attacks the thionocarbonate's sulfur, and a cascade of electron flow — sulfur transferring to phosphorus, the ring fragmenting, and CO₂ being extruded — happens in a single concerted retro-cheletropic step.
Step 3: Fragmentation: C=C forms, CO₂ leaves
Both C–O bonds break simultaneously as the new C=C π bond forms, giving the alkene with clean syn stereospecificity and no carbocation intermediate — a gentle way to convert a 1,2-diol directly into an alkene under conditions mild enough for sensitive substrates.
Abbreviations and terms
- OMe
- Methoxy. –OCH₃.
How it compares with related reactions
| Corey-Winter olefination | Aldol condensation | Cope elimination | |
|---|---|---|---|
| Reagents | 1) TCDI (thiocarbonyldiimidazole) 2) P(OMe)₃ | NaOH or KOH (cat.) | H₂O₂ or mCPBA (oxidation); then heat |
| Conditions | Toluene or neat, 60–110 °C | H₂O/EtOH, heat 50–100 °C | Oxidation 0–25 °C; elimination 80–160 °C (milder in DMSO/THF) |
| Stereochemistry | Stereospecific syn elimination: cis-diol gives cis-alkene (suprafacial) | Mostly E-enal (thermodynamic) | Syn elimination via planar 5-membered TS |
| Rate / selectivity | Driven by strong P=S and C=O (CO₂) bonds | E1cB; loss of HO⁻ from the enolate is rate-determining | Intramolecular, first order; faster in polar aprotic solvents |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Corey-Winter olefination?
1) TCDI (thiocarbonyldiimidazole) 2) P(OMe)₃. Conditions: Toluene or neat, 60–110 °C.
Which bonds are formed and which are broken?
Formed: C=C; CO₂; (MeO)₃P=S. Broken: two C–O bonds; C=S π.
What is the stereochemical outcome?
Stereospecific syn elimination: cis-diol gives cis-alkene (suprafacial)
What controls the rate or the selectivity?
Driven by strong P=S and C=O (CO₂) bonds
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go