Claisen rearrangement template

Allyl vinyl ether rearranges to pent-4-enal; allyl phenyl ether gives a cyclohexadienone that tautomerises to 2-allylphenol.

Mechanism step by step

A [3,3]-sigmatropic shift of an allyl vinyl or allyl aryl ether to a C=O compound

1. Aliphatic Claisen – The C–O σ bond breaks as C–C forms: a chair of six atoms
2. Aromatic Claisen – The allyl group moves to the ortho carbon (inverted: C3 of the allyl bonds)
3. Tautomerisation – Loss of the ring C–H restores the aromatic ring: 2-allylphenol

Key points

• Concerted and suprafacial [3,3] shift: six electrons, thermally allowed, chair-like transition state.
• Making a strong C=O bond (and losing C–O) makes the aliphatic version irreversible and exothermic.
• Stereospecific: E or Z allyl ethers give different diastereomers, and a stereocentre on the allyl chain is transferred to the new C–C bond.
• Variants: Ireland–Claisen (silyl ketene acetals), Johnson–Claisen (orthoesters), Eschenmoser–Claisen (amide acetals).

Drawn with 67 atoms, 13 arrows, 5 shapes – every one editable in MolDraw.

Sources

• Woodward & Hoffmann, Angew. Chem. Int. Ed. Engl. 1969, 8, 781 Review
• Fleming, Molecular Orbitals and Organic Chemical Reactions, Wiley 2010 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 35 Textbook