Syn elimination — pericyclic
Chugaev elimination
How the Chugaev elimination 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) NaH, CS₂, CH₃I 2) heat. Conditions: Neat or inert solvent, 100–250 °C. Bonds formed: C=C; C=O (COS part); S–H; bonds broken: β C–H; C–O. Stereochemistry: Syn elimination via cyclic 6-membered TS. Rate and selectivity: Intramolecular, first order; no base needed.
At a glance
Step 1: Alcohol → xanthate
An alcohol is converted to a xanthate ester (same intermediate used in Barton-McCombie), which is then simply heated.
Step 2: Six-membered cyclic transition state
A cyclic, six-membered transition state lets the C=S sulfur pull off a β-hydrogen while the C–O bond breaks and a new alkene π bond forms — all in one concerted, intramolecular step, needing no external base.
Step 3: Alkene + COS + CH₃SH
Because the mechanism is syn-periplanar (unlike E2's anti requirement), Chugaev elimination gives predictable stereochemistry useful for kinetically controlled alkene synthesis.
Abbreviations and terms
- NaH
- Sodium hydride. Insoluble, irreversible base: removes H⁺ and bubbles off H₂.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- E2
- Bimolecular elimination. Concerted base removal of β-H and loss of the leaving group (anti-periplanar).
How it compares with related reactions
| Chugaev elimination | Cope elimination | 1,3-dipolar cycloaddition | |
|---|---|---|---|
| Reagents | 1) NaH, CS₂, CH₃I 2) heat | H₂O₂ or mCPBA (oxidation); then heat | Nitrile oxide (from hydroximoyl chloride + Et₃N, or ArCH=NOH + NCS/base), alkene |
| Conditions | Neat or inert solvent, 100–250 °C | Oxidation 0–25 °C; elimination 80–160 °C (milder in DMSO/THF) | CH₂Cl₂ or toluene, 25–80 °C; no catalyst |
| Stereochemistry | Syn elimination via cyclic 6-membered TS | Syn elimination via planar 5-membered TS | Suprafacial, stereospecific (alkene geometry retained); 5-substituted regioisomer favored |
| Rate / selectivity | Intramolecular, first order; no base needed | Intramolecular, first order; faster in polar aprotic solvents | Concerted, thermal; HOMO/LUMO of dipole and alkene both matter |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Chugaev elimination?
1) NaH, CS₂, CH₃I 2) heat. Conditions: Neat or inert solvent, 100–250 °C.
Which bonds are formed and which are broken?
Formed: C=C; C=O (COS part); S–H. Broken: β C–H; C–O.
What is the stereochemical outcome?
Syn elimination via cyclic 6-membered TS
What controls the rate or the selectivity?
Intramolecular, first order; no base needed
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti