The Cope rearrangement is a type of [3,3]-sigmatropic shift that occurs in molecules containing a 1,5-diene system—a structure where two double bonds are separated by three single bonds. This reaction is a classic example of a pericyclic reaction, where all bond-making and bond-breaking steps occur simultaneously in a single, concerted step through a continuous, cyclic flow of electrons. The Cope rearrangement typically requires heat to proceed and is considered thermally induced. ^[1-4]

The reaction was first reported in the 1940s by Arthur C. Cope and Elizabeth Hardy and has since become a valuable tool in organic synthesis. It enables the rearrangement of molecular frameworks without adding or removing atoms, providing new structural possibilities from existing compounds.

Mechanism

In the Cope rearrangement, the molecule passes through a six-membered cyclic transition state, often resembling a chair-like conformation, similar to cyclohexane. During the rearrangement: ^[1-4]

• One π-bond donates electrons to form a new σ-bond.
• The existing σ-bond breaks and becomes a new π-bond.
• The second π-bond shifts to preserve conjugation in the system.

This concerted mechanism preserves orbital symmetry and follows the Woodward–Hoffmann rules, which predict the allowedness of pericyclic reactions based on electron count and symmetry considerations. Importantly, no atoms are gained or lost—only the positions of bonds are rearranged.

Example

A classic example of the Cope rearrangement is the conversion of 3-methyl-1,5-hexadiene into 1,5-heptadiene. In this reaction: ^[1-4]

• The central σ-bond between C3 and C4 breaks.
• A new σ-bond forms between C1 and C6.
• The π-bonds shift positions to maintain conjugation.

It is important to note that while it may seem like the methyl group moves during the reaction, it does not actually migrate. Instead, the rearrangement occurs by shifting the bonding framework around the fixed methyl group.

Oxy-Cope Rearrangement

A widely used variant is the oxy-Cope rearrangement, where a hydroxyl group (–OH) is attached at the C3 position of the 1,5-diene. This reaction also proceeds via a [3,3]-sigmatropic shift and a six-membered cyclic transition state. The product is typically an enol, which rapidly undergoes keto–enol tautomerism to form a more stable carbonyl compound (such as a ketone or aldehyde). This irreversible tautomerization step drives the reaction forward, making the oxy-Cope rearrangement a powerful method in organic synthesis for forming carbonyl-containing compounds. ^[1-4]

Cope and Claisen Rearrangement

The Cope rearrangement is part of a broader family of pericyclic reactions involving [3,3]-sigmatropic shifts. Another well-known reaction in this family is the Claisen rearrangement. While the Cope rearrangement involves only carbon atoms in a 1,5-diene system, the Claisen rearrangement introduces an oxygen atom into the shifting framework. ^[1-4]

In a typical Claisen rearrangement, an allyl vinyl ether is transformed into a γ,δ-unsaturated carbonyl compound through a cyclic, concerted mechanism. Both rearrangements proceed via a six-membered transition state, but the involvement of oxygen in the Claisen rearrangement opens pathways for synthesizing carbonyl-containing compounds, expanding the synthetic utility of these [3,3]-sigmatropic processes.