[5,5]-Sigmatropic Rearrangement
Extended Claisen Principles
- Announce: An expansion of the standard [3,3]-shift, manipulating extended conjugated systems to form new carbon-carbon bonds at distant sites on an aromatic ring.
- State: A purely thermal, concerted pericyclic reaction where an initial $\sigma$ bond breaks at position 1, and a new $\sigma$ bond forms precisely between position 5 of the migrating group and position 5 of the stationary framework.
- Define: The reaction passes through a larger, 10-membered cyclic transition state. In an aryl system, this shift occurs through the extended $\pi$ system, directly targeting the para position of the benzene ring.
- Apply: Heating a penta-2,4-dienyl phenyl ether yields a para-substituted phenol via a non-aromatic dienone intermediate.
Overall Transformation
Notice the complete transposition of the extended 5-carbon diene chain directly to the para position, bypassing the ortho position entirely.
Penta-2,4-dienyl Phenyl Ether
(Reactant)
(Reactant)
$\ce{->[\text{Heat} (\Delta)][\text{[5,5] shift}]}$
4-(Penta-2,4-dienyl)phenol
(para-Substituted Product)
(para-Substituted Product)
Complete Mechanism & 10-Membered Transition State
- Phase 1: 5 electron pairs shift simultaneously. The C–O bond breaks, and a new C–C bond forms at the para position, creating a non-aromatic dienone.
- Phase 2: The thermodynamic drive to regain aromaticity forces rapid keto-enol tautomerization, yielding the final stable phenol.
Phenyl Ether
$\ce{->[\Delta]}$
10-Membered Cyclic T.S.
$\ce{->}$
para-Dienone
(Non-Aromatic)
(Non-Aromatic)
$\ce{<=>[\text{Tautomerization}][\text{Fast}]}$
para-Allylphenol
(Rearomatized)
(Rearomatized)
