Sigmatropic Rearrangements: Cope, Claisen and the Numbering

How the numbering works

Label the atoms of the breaking sigma bond as position one on each side. Then count outward along each fragment to the atoms that form the new bond. The two numbers, written in brackets, name the shift.

The two families

The pattern is the same as for other pericyclic reactions: six electrons are thermally allowed suprafacially, four are not. So a [1,5] hydrogen shift happens readily on heating while a [1,3] does not — a comparison asked frequently.

The Cope rearrangement

A 1,5-diene rearranges to an isomeric 1,5-diene through a [3,3] shift. Since both starting material and product are 1,5-dienes, the reaction is reversible and the position of equilibrium is set by which isomer is more stable.

That is a useful point: a Cope rearrangement drives forward only when the product is significantly more stable, typically because it is more substituted or because it relieves strain. Where the two are similar, an equilibrium mixture results.

The Claisen rearrangement

The same [3,3] shift with an oxygen in the chain. An allyl vinyl ether rearranges to a carbonyl compound, and the aromatic version converts an allyl aryl ether to an ortho-allyl phenol.

Unlike the Cope, the Claisen is essentially irreversible, because the product contains a strong carbon–oxygen double bond. That thermodynamic driving force is why it works so reliably, and naming it is the expected explanation for the difference between the two reactions.

The aromatic Claisen

The initial product of the aromatic version is a cyclohexadienone, which then tautomerises to the aromatic phenol. Rearomatisation supplies additional driving force.

Where both ortho positions are blocked, the allyl group migrates further to the para position through a second [3,3] shift. Predicting para migration for a blocked substrate is a standard question, and it demonstrates that the mechanism is genuinely sigmatropic rather than a simple migration.

The transition state fixes the stereochemistry

A thermal [3,3] shift proceeds through a six-membered transition state that adopts a chair-like geometry, exactly as cyclohexane does and for the same reason. Substituents prefer pseudo-equatorial positions in that arrangement.

Because the geometry is fixed, the stereochemistry of the product follows from that of the starting material in a predictable way. Drawing the chair transition state is therefore the practical method for answering stereochemistry questions on these reactions, and it is far more reliable than trying to reason about the product directly.

Recognising them in a question

1. Identify the sigma bond that breaks and the pi system it migrates across.
2. Number outward from that bond along both fragments.
3. Count the electrons involved — two from the sigma bond plus two per pi bond in the array.
4. Check the conditions and apply the rule.
5. Draw the chair transition state to obtain the stereochemistry.

Frequently asked questions

Why is the Claisen irreversible but the Cope not?

Because the Claisen forms a strong carbon–oxygen double bond, which provides a substantial thermodynamic driving force. The Cope converts one diene to another with no such gain.

Why does a [1,5] hydrogen shift occur readily but not a [1,3]?

Because the [1,5] shift involves six electrons and is thermally allowed suprafacially, while the [1,3] involves four and is not.

What happens if both ortho positions are blocked in an aromatic Claisen?

The allyl group migrates to the para position through a second sigmatropic shift, which is strong evidence for the concerted mechanism.

Why does the transition state adopt a chair conformation?

For the same reason cyclohexane does — it minimises torsional and steric strain in a six-membered arrangement. That preference is what makes the stereochemical outcome predictable.