Sigmatropic Rearrangements

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Klein 16.9. Cope and Claisen rearrangements. This Claisen is a rearrangement, not the Claisen condensation.

What a sigmatropic shift is: a σ bond moves across a π system in a concerted step.
The [3,3] Cope rearrangement of a 1,5-diene.
The [3,3] Claisen rearrangement of an allyl vinyl ether, and the allyl phenyl ether variant that gives an ortho-allyl phenol.
How to number the chain and predict which bond forms.
A one-line contrast with the Claisen condensation so the names are not confused.

Sigmatropic Rearrangements: The Cope and Claisen Reactions

Continuing our exploration of pericyclic reactions, we turn from electrocyclic ring closures to sigmatropic rearrangements. A sigmatropic shift is a concerted, intramolecular process in which a $\sigma$-bond is migrated across an interconnected $\pi$-electron system to a new position, with the simultaneous reorganization of the $\pi$-bonds. Among these, the [3,3]-sigmatropic rearrangements—exemplified by the Cope and Claisen rearrangements—are among the most powerful carbon-carbon bond-forming tools in synthesis.

[ Insert Image: Overview of [3,3]-Sigmatropic Rearrangements ]

Figure 1. Concerted migration of a $\sigma$-bond across a conjugated $\pi$-system.

1. What Is a Sigmatropic Shift and How to Number It

A sigmatropic shift does not involve ionic intermediates or radicals; it occurs through a cyclic, concerted transition state.

Defining Notation `[i,j]`: The numbers in brackets refer to the positions of the atoms where the bonds are breaking and forming, numbered 1 starting from the original $\sigma$-bond terminus in each fragment. A [3,3]-sigmatropic shift means the migrating $\sigma$-bond connects atom 1 of one fragment to atom 1 of another, and after the shift, it lands on atoms 3 and 3.
Predicting Bond Formation: To determine the outcome, number the carbon chain from the migrating $\sigma$-bond outward in both directions. The $\sigma$-bond breaks between C1 and C1', and a new $\sigma$-bond forms between C3 and C3', while the $\pi$-bonds shift inward to C2=C3 and C2'=C3'.

[ Insert Mechanism: Numbering Convention and Cyclic Transition State for [3,3] Shifts ]

Figure 2. Tracking carbon numbering and chair-like transition state geometry during a [3,3] shift.

2. The Cope Rearrangement ([3,3]-Diene Rearrangement)

The Cope rearrangement is the thermal [3,3]-sigmatropic rearrangement of a 1,5-diene.

Reaction: Heating a 1,5-diene (such as 1,5-hexadiene) causes it to undergo a concerted pericyclic reorganization, isomerizing into a structurally rearranged 1,5-diene isomer.
Equilibrium: Because the starting and product materials are both dienes with similar thermodynamic stabilities, the Cope rearrangement is typically a reversible equilibrium. The reaction favors the more substituted, stable alkene isomer.

[ Insert Reaction: Thermal Cope Rearrangement of a 1,5-Diene ]

Figure 3. Reversible thermal isomerization of a 1,5-diene via a chair-like transition state.

3. The Claisen Rearrangement ([3,3]-Allyl Vinyl Ether Rearrangement)

The Claisen rearrangement is the oxygen analog of the Cope rearrangement, converting an allyl vinyl ether into a $\gamma,\delta$-unsaturated carbonyl compound.

Allyl Vinyl Ether Claisen: When an allyl vinyl ether is heated, it undergoes a [3,3]-sigmatropic shift to yield an enol, which tautomerizes irreversibly into a $\gamma,\delta$-unsaturated carbonyl (aldehyde or ketone). The formation of the strong carbon-oxygen $\pi$-bond ($\text{C}=\text{O}$) drives this reaction entirely to completion.
Allyl Phenyl Ether Variant: When an allyl phenyl ether is heated, it undergoes a [3,3]-shift to form an ortho-dienone intermediate, which quickly rearomatizes via enol tautomerization to yield an *ortho*-allyl phenol.

[ Insert Reaction: Claisen Rearrangements of Allyl Vinyl Ethers and Allyl Phenyl Ethers ]

Figure 4. Thermal Claisen rearrangements driving irreversible carbonyl and phenol formation.

4. Clarification Note

Claisen Rearrangement vs. Claisen Condensation: The Claisen rearrangement is a concerted, pericyclic thermal isomerization of an allyl ether, whereas the Claisen condensation is a polar, base-catalyzed nucleophilic acyl substitution coupling two ester molecules.

--- Would you like to explore cycloadditions beyond the Diels-Alder reaction, such as 1,3-dipolar cycloadditions, next?