Sigmatropic Rearrangements
By
Shri.
Rajendra Prasad
Cope
rearrangement
When 1,5-dienes are heated they undergo a [3,3]-sigmatropic rearrangement.
This reaction is called Cope rearrangement.
-
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.
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