The cope rearrangement is the carbon version of the claisen rearrangement, and usually proceeds through a chair transition state. Although the emphasis has been placed on the claisen rearrangement, there are related reactions that are also synthetically useful.
The azo cope reaction incorporates a nitrogen atom into the diene substrate.
The reaction is reversible, but the introduction of an oxygen atom results in the formation of an enolate whose stability drives the reaction to completion. As the enolate is formed regioselectivity it can be used for further transformations.
Cope Elimination MechanismBack to Top
The mechanism of cope rearrangement is shown below.
It has been suggested that another type of diradical two step mechanism may be preferred by some substrates. Indeed, a non concerted cope rearrangement has been reported. In this pathway, the 1,6-bond is formed before the 3,4-bond breaks.
Oxy Cope RearrangementBack to Top
After the discovery of the thermal Cope rearrangement of 1,5-dienes in 1940, it was shown that the introduction of a hydroxyl group at carbons C3 or C4 allowed the clean reorganization of the functionalized diene unit to a unsaturated carbonyl compound. This observation launched the new thermal oxy cope rearrangement which rapidly became a transformation of high synthetic value in modern organic synthesis.
An example for anionic oxy cope rearrangement is given below.
The mildest conditions for the rearrangements are achieved by removing the counter ion from the alkoxide using a crown ether. As in the case of the normal Cope rearrangement the oxy-cope rearrangement proceeds preferentially a chair like transition state.