Sigmatropic Rearrangements

By
Shri. Rajendra Prasad

• R = Ph-, RCO-, -CN and others. (see the examples in the early part of this article)
• If R is a group in conjugation with the double bond in the product the reaction takes place with ease at lower temperatures. (Example R = -CN)
• Cope rearrangement takes places usually through a chair like six membered cyclic transition state over the boat like possibility. This difference is due to favourable transition state energy.

• Rearrangement involving a meso diene will result in cis-trans isomer, while the racemic mixture will yield trans-trans isomer as the major product.
• Some cope rearrangements (ex: 1,2-divinylcyclopropane) will go through boat like transition state due to favourable transition state energies.
• Most 1,5-dienes react reversibly and result in a mixture of products and the thermodynamically more stable would predominate.
• Some Cope rearrangements are not reversible because of possibility of tautomerisation.