[3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement

The Claisen rearrangement is an oxy-variant of the Cope rearrangement where one saturated carbon of a 1,5-diene is replaced by an oxygen atom. It converts allyl vinyl ethers to γ,δ-unsaturated carbonyl compounds through a concerted reorganization of six electrons via a chair-like transition state. The key difference is the oxygen substitution, which creates a stable carbon-oxygen bond favoring product formation.

The Claisen rearrangement proceeds through a chair-like transition state where substituents adopt equatorial positions, minimizing steric strain. This geometry strongly favors specific stereoisomers. For substituted allyl vinyl ethers, the rearrangement preferentially produces products where the double bonds adopt an E configuration, resulting in high stereoselectivity.

Allyl aryl ethers undergo Claisen rearrangement to form an unstable ketone intermediate, which tautomerizes into a stable enol form that restores aromaticity to the ring. This produces ortho-substituted phenols. When both ortho positions are occupied, the reaction proceeds via two sequential Claisen rearrangements to yield the para-substituted product.

The equilibrium strongly favors the Claisen rearrangement product because the reaction yields a compound with a stable carbon-oxygen bond. This thermodynamic driving force, combined with the concerted nature of the pericyclic mechanism, makes the rearrangement essentially irreversible under typical conditions.

The Claisen rearrangement follows a concerted pericyclic mechanism involving reorganization of six electrons. The reaction proceeds through a chair-like transition state where bonds are simultaneously broken and formed. This concerted pathway ensures the reaction occurs in a single step without discrete intermediates, maintaining high stereoselectivity throughout.

When both ortho positions of an allyl aryl ether are occupied by substituents, direct ortho-substitution is blocked. The reaction then proceeds through two sequential Claisen rearrangements, ultimately delivering the allyl group to the para position of the aromatic ring instead.

Thermal isomerization of allyl vinyl ethers produces γ,δ-unsaturated carbonyl compounds through the Claisen rearrangement. These products contain both a carbon-carbon double bond and a carbonyl group in the gamma and delta positions relative to each other, representing a significant structural rearrangement.