Question

Propose a mechanism for the following Cope rearrangement.

Short answer

Answer: The Cope rearrangement mechanism involves a [3,3]-sigmatropic shift in a cyclic transition state with six atoms in the same plane. The allyl and vinyl groups trade places, resulting in a rearranged allyl vinyl ether with a different arrangement of atoms.

Step-by-step solution

Identification of the substrate

Begin by identifying the main functional group present in the given compound: an allyl vinyl ether. Locate the allyl group (a carbon-carbon double bond next to a carbon with a single bond to another carbon) and the vinyl group (a carbon-carbon double bond next to a carbon with a single bond to an oxygen).

Identify the transition state

In the Cope rearrangement, the reaction proceeds via a cyclic transition state involving a [3,3]-sigmatropic shift. This means that the six atoms involved in the rearrangement are all in the same plane. The transition state allows the sigma bonds to shift, resulting in a new connectivity of atoms that leads to the product.

Predict the product

In the Cope rearrangement, the allyl and vinyl groups trade places. The allyl group migrates to the carbon atom bonded to the oxygen, while the vinyl group migrates to the initial carbon atom of the allyl group. As a result, a new allyl vinyl ether is formed, but with a different arrangement of atoms.

Complete the mechanism

To complete the mechanism, show the formation of the transition state by breaking and reforming the relevant bonds. Indicate the movement of electrons by drawing curved arrows from the bonds that are being broken towards the new locations of the bonds being formed. This will result in the formation of the final rearranged product. Overall, the Cope rearrangement proceeds through a [3,3]-sigmatropic shift via a cyclic transition state involving six atoms. By identifying the key features of the substrate, determining the transition state geometry, and predicting the product, you can propose a mechanism for this pericyclic reaction.