Claisen Rearrangement

Claisen Rearrangement: Master Revision Notes

1. The Core Concept

Mechanism Breakdown

• Announce: Foundational example of a thermal [3,3]-sigmatropic rearrangement.
• State: Driven entirely by heat ($\Delta$) → breaks a C-O $\sigma$-bond to form a stronger C=O $\pi$-bond and a new C-C $\sigma$-bond.
• Define: Concerted process; no ionic intermediates; completely intra-molecular. Both substrate types proceed through a highly ordered, chair-like six-membered cyclic transition state.
• Apply: Master the two primary substrates below: Allyl Vinyl Ethers & Allyl Aryl Ethers.

2. Allyl Vinyl Ethers

Mechanism Breakdown

• Announce: Direct synthesis of $\gamma,\delta$-unsaturated carbonyls from ethers.
• State: Heating an allyl vinyl ether yields an aldehyde or ketone.
• Define: Concerted sigmatropic shift via the six-membered cyclic transition state → concurrent bond breaking (C-O) and making (C-C) → directly yields the final unsaturated product.
• Apply: Observe the simultaneous electron flow in the precise TS below:

Allyl Vinyl Ether

$\ce{->[\Delta]}$

Six-Membered Cyclic T.S.

$\ce{->}$

Product
($\gamma,\delta$-Unsaturated Carbonyl)

3. Allyl Aryl Ethers (Allyl Phenyl Ether)

Mechanism Breakdown

• Announce: A functionalization method that temporarily sacrifices aromaticity to form a new C-C bond on a benzene ring.
• State: Heating an allyl aryl ether yields an ortho-allylphenol.
• Define: Occurs in two distinct phases:
  1. Concerted [3,3]-sigmatropic shift → disrupts aromaticity to form a non-aromatic dienone intermediate.
  2. Rapid keto-enol tautomerism → restores aromatic stability.
• Apply: Trace the shared edge between the cyclic TS and the intact portion of the benzene ring below:

Allyl Phenyl Ether

$\ce{->[\Delta]}$

Six-Membered Cyclic T.S.

$\ce{->}$

Dienone
(Non-Aromatic Int.)

$\ce{<=>[\text{Tautomerization}][\text{Fast}]}$

ortho-Allylphenol
(Rearomatized)