Claisen Rearrangement

Claisen Rearrangement

1. Core Concept: Thermal Sigmatropic Rearrangement

• Announce: Foundational pericyclic reaction; powerful method for C-C bond formation.
• State: A [3,3]-sigmatropic rearrangement driven by heat ($\Delta$).
• Define: Proceeds via a concerted six-membered cyclic transition state.
• Thermodynamics: Driven by the formation of a strong C=O bond at the expense of a weaker C-O ether bond.

2. Pathway A: Allyl Vinyl Ethers

• Reactant: Allyl vinyl ether.
• Trigger: Heat ($\Delta$).
• Mechanism: Single-step sigmatropic shift via cyclic transition state.
• Product: $\gamma,\delta$-unsaturated carbonyl compound (aldehyde or ketone).

Allyl Vinyl Ether

$\ce{->[\text{Heat} (\Delta)][\text{[3,3] shift}]}$

4-Pentenal
($\gamma,\delta$-unsaturated carbonyl)

3. Pathway B: Allyl Aryl Ethers

• Reactant: Allyl aryl ether (e.g., Allyl phenyl ether).
• Step 1 (Migration): Initial [3,3]-sigmatropic shift → disrupts benzene aromaticity → yields a high-energy non-aromatic ketone intermediate (dienone).
• Step 2 (Rearomatization): Rapid keto-enol tautomerism restores the aromatic ring stability.
• Final Product: ortho-Allylphenol.

Allyl Phenyl Ether

$\ce{->[\Delta][\text{[3,3] shift}]}$

Non-aromatic Ketone
(Dienone Intermediate)

$\ce{->[\text{Fast}][\text{Tautomerization}]}$

ortho-Allylphenol