Pericyclic — [3,3]-sigmatropic
Cope rearrangement of 1,5-hexadiene
How the Cope rearrangement of 1,5-hexadiene 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: 1,5-Hexadiene (and substituted analogues). Conditions: Heat, ≈ 150–260 °C; or Pd(II) catalysis. Bonds formed: C1–C6 σ bond; two new π bonds; bonds broken: C3–C4 σ bond; two old π bonds. Stereochemistry: Chair TS; suprafacial; E/Z-selective. Rate and selectivity: Degenerate here; ΔG° = 0, driven by substituents in general.
At a glance
Step 1: Six-atom chair array
1,5-Hexadiene's two double bonds and the central σ bond form a six-atom array, again in a chair-like transition state.
Step 2: Closed loop of three arrows
The same closed-loop, three-arrow pattern as the Claisen: the central C–C σ bond breaks as a new one forms three atoms over, with both π bonds shifting in step.
Step 3: Identical 1,5-hexadiene
Because both ends of this symmetric substrate are equivalent, the Cope product here is structurally identical to the starting material — a textbook degenerate rearrangement.
Abbreviations and terms
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Cope rearrangement of 1,5-hexadiene | Claisen rearrangement | Mislow-Evans rearrangement | |
|---|---|---|---|
| Reagents | 1,5-Hexadiene (and substituted analogues) | Allyl vinyl ether (e.g. from allyl alcohol + enol ether, or Johnson–Claisen) | Allylic sulfoxide; thiophile (P(OMe)₃, PhSH, or Et₂NH) |
| Conditions | Heat, ≈ 150–260 °C; or Pd(II) catalysis | Heat, 100–200 °C, no catalyst | THF or MeOH, –20 to 25 °C |
| Stereochemistry | Chair TS; suprafacial; E/Z-selective | Chair TS; suprafacial; chirality transfer | Suprafacial [2,3]; chirality transfer from S to C; E-alkene favored |
| Rate / selectivity | Degenerate here; ΔG° = 0, driven by substituents in general | Concerted [3,3]; accelerated by polar solvents; driven by strong C=O | 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 Cope rearrangement of 1,5-hexadiene?
1,5-Hexadiene (and substituted analogues). Conditions: Heat, ≈ 150–260 °C; or Pd(II) catalysis.
Which bonds are formed and which are broken?
Formed: C1–C6 σ bond; two new π bonds. Broken: C3–C4 σ bond; two old π bonds.
What is the stereochemical outcome?
Chair TS; suprafacial; E/Z-selective
What controls the rate or the selectivity?
Degenerate here; ΔG° = 0, driven by substituents in general
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti