16.25 : [3,3] Sigmatrope Umlagerung von 1,5-Dienen: Cope-Umlagerung

Thermal isomerization of 1,5-dienes via a [3,3] sigmatropic shift is known as the Cope rearrangement.

This reaction is reversible, and the direction of equilibrium depends on the nature of the diene.

If the diene is symmetrical, the product is identical to the reactant. However, with asymmetrical dienes, the equilibrium favors the more substituted product.

These are concerted pericyclic reactions usually proceeding via a chair-like transition state.

Thermal rearrangement of 1,5-dienes can be viewed as the cycloaddition of two allyl systems; the HOMO of the allyl anion and the LUMO of the allyl cation.

Here, the terminal lobes of the interacting π systems should be in phase to maintain a bonding overlap throughout the transition state. To achieve this, the HOMO and LUMO must interact suprafacially.

The preference for a chair-like transition state is reflected in the product distribution accompanying the rearrangement of 3,4-dimethyl-1,5-hexadiene.

In summary, Cope rearrangements are [3,3] sigmatropic shifts that follow a suprafacial pathway under thermal conditions.