The Claisen Rearrangement

Introduction

The Claisen rearrangement converts an allyl vinyl ether into a carbon framework containing a new carbon–carbon bond and a carbonyl group. The change can be surprising because an ether oxygen that once linked two carbon fragments becomes the oxygen of the final carbonyl. Its mechanism is a thermal [3,3] sigmatropic rearrangement followed by tautomerisation.

Core explanation

An allyl vinyl ether has an allylic three-carbon fragment attached through O to a vinyl fragment. During the [3,3] step, the allylic carbon–oxygen σ bond breaks and a new carbon–carbon σ bond joins the terminal carbon of the allyl component to the terminal carbon of the vinyl component. The two π bonds shift, and the oxygen becomes part of an enol in the immediate rearrangement product. The enol then tautomerises to a carbonyl compound. The overall product is often a γ,δ-unsaturated aldehyde or ketone. The OpenStax account of sigmatropic rearrangements shows this distinction between the rearranged enol and the isolated carbonyl.

For a simple unsubstituted substrate, CH₂=CH–CH₂–O–CH=CH₂ , the carbon skeleton of the final product is O=CH–CH₂–CH₂–CH=CH₂ , commonly named pent-4-enal. This formula is an instructive atom map: five carbon atoms remain five, and the oxygen stays in the molecule but changes from ether oxygen to carbonyl oxygen. The double bond ends up between the γ and δ carbons when counting away from the aldehyde carbonyl. A student who merely cleaves the ether and reconnects carbon ends without shifting π bonds will not obtain the correct carbonyl position.

The Claisen [3,3] step is a six-electron pericyclic rearrangement. Two π bonds and one σ bond participate in a six-membered cyclic transition-state array. Chair-like geometries are often useful for predicting stereochemistry, especially with substituted allyl or vinyl groups. The reaction can be driven by formation of a strong carbonyl bond after enol–keto tautomerisation, unlike a simple Cope rearrangement that may leave two similar diene isomers in equilibrium. Yet reaction conditions still matter: a hindered substrate may require heat, and side reactions or competing conformations can affect yield.

There are aromatic Claisen variants in which an allyl aryl ether rearranges to an ortho-allylated phenol under suitable conditions. Aromaticity is restored after the initial pericyclic bond rearrangement and proton shifts. This is related [3,3] logic but gives a different product family from the allyl vinyl ether to unsaturated-carbonyl case. Identify the substrate first, then predict the correct downstream proton transfers or tautomerisation.

Step-by-step reasoning

Circle the allyl three-atom fragment, the vinyl two-carbon fragment and the bridging oxygen as the six-atom array. Mark the old allylic C–O bond. Form a new bond between the distal allyl carbon and distal vinyl carbon, then move the π bonds around the cycle. Draw the immediate enol with oxygen on its original vinyl-side carbon. Only afterward transfer a proton and move the enol π bond to give the carbonyl product. Check that no carbon or oxygen atom disappeared.

Visual explanation

Draw an allyl vinyl ether folded into a six-membered, chair-like transition-state outline. Highlight the breaking C–O bond blue and the forming terminal C–C bond red. Under it draw two distinct product boxes: the first labeled “enol from [3,3]” and the second “carbonyl from tautomerisation,” with a separate arrow between them.

Real-world analogy

A hinged bridge joins two paths through a central connector. During rearrangement, a new link forms between the far ends, while the connector is reassigned to a different structural role. Afterward a small local adjustment locks the new route into a more stable layout. The six-electron rearrangement is the bridge swap; tautomerisation is the stabilising local adjustment.

Real-world example

Claisen rearrangements are used in synthesis to form C–C bonds while positioning a carbonyl group for later aldol, reduction or oxidation chemistry. The reaction can set several atoms and a new unsaturated side chain in one operation, making it attractive for building complex natural-product fragments.

Why?

The allyl, oxygen and vinyl orbitals can overlap in a six-membered cyclic geometry that permits thermal [3,3] redistribution. Formation of a carbonyl after the cyclic step can provide a thermodynamic driving force. The arrangement also preserves a predictable relationship among substituents because they pass through a constrained transition-state geometry.

Common misconception

The oxygen is not expelled as a leaving group in the classic Claisen rearrangement. It remains in the molecule and becomes the carbonyl oxygen after tautomerisation. Another common error is to draw the final aldehyde as the direct single-step pericyclic product; the immediate [3,3] product is an enol.

Worked example

Question: Predict the carbon skeleton from allyl vinyl ether CH₂=CH–CH₂–O–CH=CH₂ . Reasoning: The allyl C–O bond breaks, a new bond joins the remote allyl and vinyl carbons, and π bonds shift through a six-atom array. The enol tautomerises with O becoming the carbonyl oxygen. Answer: The final carbonyl product is pent-4-enal, O=CH–CH₂–CH₂–CH=CH₂ ; the five carbons and one oxygen are all conserved.

Quick check

1. What is the immediate product type of the pericyclic step before a Claisen carbonyl appears? Answer: An enol, which subsequently undergoes enol–keto tautomerisation to a carbonyl compound.

Exam focus

Identify the allyl vinyl ether first. Show the old C–O bond, new C–C bond and two shifted π bonds. Draw enol and carbonyl on separate arrows, and number carbon positions from the carbonyl to locate the γ,δ double bond.

Advanced insight

Substituted Claisen substrates can have chair and boat transition-state families. Comparing steric placement of substituents in the chair often predicts relative stereochemistry, but solvent, ring constraints and substituent electronics can alter the preference. An aromatic Claisen route also needs restoration of aromaticity after C–C bond formation, so its final product pattern should not be copied from an allyl vinyl ether example.

Summary

The Claisen rearrangement is a six-electron [3,3] shift of an allyl vinyl ether. It breaks an allylic C–O bond, forms a new terminal C–C bond and rearranges π bonds to an enol, which tautomerises to a γ,δ-unsaturated carbonyl. Oxygen remains in the product, and stereochemistry follows the constrained cyclic transition state.

Practice questions

1. Which σ bond breaks in a classic allyl vinyl ether Claisen rearrangement? Answer: The bond between the allylic carbon and the bridging oxygen.

2. What new bond forms during the [3,3] step? Answer: A C–C bond between the distal allyl and distal vinyl termini.

3. Where does the ether oxygen end up in the common final product? Answer: As the oxygen of an aldehyde or ketone carbonyl after tautomerisation.

4. Why is the final carbonyl drawn after a second arrow? Answer: The concerted [3,3] step produces an enol, and proton-transfer tautomerisation makes the carbonyl later.