Pericyclic — [3,3]-sigmatropic
Claisen rearrangement: allyl vinyl ether → γ,δ-unsaturated carbonyl
How the Claisen rearrangement 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: Allyl vinyl ether (e.g. from allyl alcohol + enol ether, or Johnson–Claisen). Conditions: Heat, 100–200 °C, no catalyst. Bonds formed: C–C σ bond; C=O; bonds broken: C–O σ bond; C=C (shifted). Stereochemistry: Chair TS; suprafacial; chirality transfer. Rate and selectivity: Concerted [3,3]; accelerated by polar solvents; driven by strong C=O.
At a glance
Step 1: Six-atom array in a chair-like TS
The allyl vinyl ether adopts a chair-like transition state where the two π systems and the σ bond that will break all line up in a six-atom array.
Step 2: Three arrows in a closed loop
Three curved arrows move in a closed loop — concerted, like the Diels-Alder — breaking the C–O σ bond while forming a new C–C σ bond three atoms away.
Step 3: γ,δ-Unsaturated aldehyde
The product is a γ,δ-unsaturated carbonyl. No catalyst is needed; heat alone drives this [3,3]-sigmatropic shift.
Abbreviations and terms
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Claisen rearrangement | Cope rearrangement of 1,5-hexadiene | Mislow-Evans rearrangement | |
|---|---|---|---|
| Reagents | Allyl vinyl ether (e.g. from allyl alcohol + enol ether, or Johnson–Claisen) | 1,5-Hexadiene (and substituted analogues) | Allylic sulfoxide; thiophile (P(OMe)₃, PhSH, or Et₂NH) |
| Conditions | Heat, 100–200 °C, no catalyst | Heat, ≈ 150–260 °C; or Pd(II) catalysis | THF or MeOH, –20 to 25 °C |
| Stereochemistry | Chair TS; suprafacial; chirality transfer | Chair TS; suprafacial; E/Z-selective | Suprafacial [2,3]; chirality transfer from S to C; E-alkene favored |
| Rate / selectivity | Concerted [3,3]; accelerated by polar solvents; driven by strong C=O | Degenerate here; ΔG° = 0, driven by substituents in general | Equilibrium lies on sulfoxide; thiophile trapping pulls it forward |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Claisen rearrangement?
Allyl vinyl ether (e.g. from allyl alcohol + enol ether, or Johnson–Claisen). Conditions: Heat, 100–200 °C, no catalyst.
Which bonds are formed and which are broken?
Formed: C–C σ bond; C=O. Broken: C–O σ bond; C=C (shifted).
What is the stereochemical outcome?
Chair TS; suprafacial; chirality transfer
What controls the rate or the selectivity?
Concerted [3,3]; accelerated by polar solvents; driven by strong C=O
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti