Pericyclic chemistry · 8 min read

Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts

Pericyclic reactions have no intermediates: electrons move around a ring in one smooth, concerted step. They look strange until you realise a single idea governs them: orbital symmetry must match throughout the transition state.

The four families

Cycloadditions: two π systems join to make a ring, for example [4+2] in the Diels–Alder reaction and [3+2] in 1,3-dipolar cycloaddition.
Electrocyclic reactions: a conjugated polyene closes to a ring (or a ring opens), converting one π bond into one σ bond. See Thermal 6π electrocyclic ring closure and the Nazarov cyclisation.
Sigmatropic rearrangements: a σ bond migrates across a π system, such as [3,3] in the Claisen and Cope rearrangements, and [2,3] in [[wittigrearr23|the [2,3]-Wittig]].
Group transfer and ene reactions: an alkene with an allylic H reacts with an enophile in the ene reaction.

Counting electrons: the Woodward–Hoffmann rules

You do not need to draw orbitals to predict whether a pericyclic reaction is allowed. Count the electrons in the cyclic transition state and use heat or light accordingly.

Electrons in the transition state	Thermal (heat)	Photochemical (light)
4n + 2 (6, 10, …)	Allowed (suprafacial, disrotatory)	Forbidden
4n (4, 8, …)	Forbidden (suprafacial); conrotatory for electrocyclic	Allowed

Diels–Alder: 4 + 2 = 6 electrons, a 4n+2 system, so it is thermally allowed.
[2+2] cycloaddition: 4 electrons, a 4n system, so it is thermally forbidden but works under light.
Cope and Claisen [3,3]: 6 electrons, thermally allowed through a chair transition state.

Electrocyclic ring closure: conrotatory or disrotatory?

The termini of the polyene must rotate to form the new σ bond. Whether they turn the same way (conrotatory) or opposite ways (disrotatory) depends on the electron count and on heat or light:

4n electrons, heat: conrotatory. (Butadiene to cyclobutene.)
4n electrons, light: disrotatory.
4n+2 electrons, heat: disrotatory. (Hexatriene to cyclohexadiene.)
4n+2 electrons, light: conrotatory.

That is why (E,Z,E)-octatriene closes thermally to give cis-dimethylcyclohexadiene, and (E,Z,Z)-octatriene gives the trans product.

Why the Diels–Alder reaction works so well

The diene must be able to adopt the s-cis conformation. Cyclic dienes such as cyclopentadiene are permanently locked in it and are very reactive.
The best pairings put an electron-rich diene (with donating groups) with an electron-poor dienophile (with carbonyl, nitrile or nitro groups): a small HOMO–LUMO gap.
Endo rule: with cyclic dienes and dienophiles bearing π substituents, the endo product forms faster because of secondary orbital interactions, though it can be less stable.
The reaction is stereospecific: cis substituents on the dienophile stay cis in the product.
The reverse reaction, the retro-Diels–Alder, is favoured at high temperature because of entropy.

Sigmatropic shifts: reading the numbers

A [i,j] shift means a σ bond moves so that it ends up between atom i of one fragment and atom j of the other. In a [3,3] shift, the bond between atoms 1 and 1′ breaks and a new bond forms between atoms 3 and 3′, with the π bonds shifting to match. The Cope rearrangement of 1,5-hexadiene is the simplest case. Claisen is the same idea with an oxygen in the chain, which makes the reaction irreversible because it creates a strong C=O bond.

Where to practise

Step through Diels-Alder, Claisen rearrangement, Thermal 6π electrocyclic ring closure and Retro-Diels-Alder reaction in the interactive app to see each bond-forming and bond-breaking arrow in order.

Practise in the interactive app Browse all 345 reactions