Pericyclic — orbital-symmetry ring closure
Thermal 6π electrocyclic ring closure
How the Thermal 6π electrocyclic ring closure 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: (2E,4Z,6E)-octa-2,4,6-triene. Conditions: Heat (thermal), ~130 °C. Bonds formed: C–C σ bond; ring; bonds broken: Two C=C π bonds (become new π bonds). Stereochemistry: Disrotatory: E,Z,E triene gives cis-5,6-dimethylcyclohexa-1,3-diene. Rate and selectivity: Thermally allowed, 6π (4n+2); photochemically conrotatory.
At a glance
Step 1: Triene curls up
A conjugated triene (like (2E,4Z,6E)-octatriene) has its terminal p-orbitals positioned to overlap directly if the chain simply curls up — no external reagent is needed, only heat to populate the reactive conformation.
Step 2: Disrotatory ring closure
Under thermal conditions, a 6π system (4n+2 electrons) closes in a disrotatory fashion — both termini rotate in opposite directions as the new σ bond forms — dictated purely by orbital symmetry (the Woodward-Hoffmann rules), not by any nucleophile or electrophile.
Step 3: cis-Cyclohexadiene
The product is a cyclohexadiene, with stereochemistry at the new sp³ centers fully predictable from the disrotatory motion — a photochemical version of the identical substrate instead closes conrotatory, since exciting an electron to a higher orbital flips which rotation is symmetry-allowed. This thermal/photochemical selectivity switch is the same orbital-symmetry logic already seen in the Nazarov cyclization's conrotatory 4π closure.
Abbreviations and terms
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Thermal 6π electrocyclic ring closure | 1,3-dipolar cycloaddition | 2-Aza-Cope rearrangement | |
|---|---|---|---|
| Reagents | (2E,4Z,6E)-octa-2,4,6-triene | Nitrile oxide (from hydroximoyl chloride + Et₃N, or ArCH=NOH + NCS/base), alkene | Homoallylic amine, aldehyde (to give the iminium), acid |
| Conditions | Heat (thermal), ~130 °C | CH₂Cl₂ or toluene, 25–80 °C; no catalyst | Mild heat or RT (cationic variant), CH₂Cl₂ or MeCN |
| Stereochemistry | Disrotatory: E,Z,E triene gives cis-5,6-dimethylcyclohexa-1,3-diene | Suprafacial, stereospecific (alkene geometry retained); 5-substituted regioisomer favored | Chair TS; stereo transferred |
| Rate / selectivity | Thermally allowed, 6π (4n+2); photochemically conrotatory | Concerted, thermal; HOMO/LUMO of dipole and alkene both matter | Faster than all-carbon Cope (cationic N) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Thermal 6π electrocyclic ring closure?
(2E,4Z,6E)-octa-2,4,6-triene. Conditions: Heat (thermal), ~130 °C.
Which bonds are formed and which are broken?
Formed: C–C σ bond; ring. Broken: Two C=C π bonds (become new π bonds).
What is the stereochemical outcome?
Disrotatory: E,Z,E triene gives cis-5,6-dimethylcyclohexa-1,3-diene
What controls the rate or the selectivity?
Thermally allowed, 6π (4n+2); photochemically conrotatory
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti