Claisen rearrangement: practice and mechanism
Allyl vinyl ethers or allyl aryl ethers rearrange on heating through a six-membered transition state ([3,3]-sigmatropic). A new C–C bond forms and the ether becomes a carbonyl compound or an ortho-allylphenol.
Example
A [3,3]-sigmatropic rearrangement through a chair-like transition state: the C–O bond breaks and a new C–C bond forms between the two ends. Allyl vinyl ethers give γ,δ-unsaturated carbonyl compounds, allyl phenyl ethers a 2-allylphenol.
Mechanism step by step
- 1
On heating the six atoms C=C–C–O–C=C arrange themselves in a chair-like ring.
- 2
In one step the O–allyl bond breaks, a new C–C bond forms and the double bonds shift ([3,3]-sigmatropic rearrangement).
- 3
An allyl vinyl ether gives a γ,δ-unsaturated carbonyl compound directly. With an allyl aryl ether the dienone tautomerises to the ortho-allylphenol (the ring becomes aromatic again).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which reagent achieves this transformation?
Typical mistakes
- Heat alone does not simply split the ether into two pieces. The bonds are reorganised in a ring: the O–allyl bond breaks while a C–C bond forms at the same time.
- The allyl group moves to the ortho position, not para, and it is "flipped" in the process: its terminal carbon bonds to the ring.
Frequently asked questions
What drives the reaction?
A C=O bond is considerably more stable than a C=C bond. With aryl ethers, regaining aromaticity adds to this.
What if both ortho positions are blocked?
Then a second rearrangement (Cope) follows and the allyl group ends up para.