Cope Rearrangement

What is Cope rearrangement?

The Cope rearrangement is a chemical reaction that allows for the synthesis of compounds containing a double bond. This reaction is named after the American chemist Arthur C. Cope, who first described it in 1892..

The Cope rearrangement involves the rearrangement of a compound called an “allylic amine” to a compound containing a double bond. The reaction is typically carried out in the presence of a base, such as sodium hydroxide, and a solvent, such as ethanol..

Cope rearrangement
Cope rearrangement

One of the advantages of the Cope rearrangement is its high yield and efficiency. It is a relatively simple reaction that can be carried out under mild conditions and does not require the use of expensive or specialized reagents..

The Cope rearrangement has a wide range of applications in the synthesis of pharmaceuticals, fragrances, and other chemicals. It has also been used in the synthesis of natural products, such as flavonoids and terpenoids..

Summary

The Cope rearrangement is a valuable tool for the synthesis of compounds containing a double bond and has played an important role in the development of a number of important chemical compounds..

Example

This reaction is usually carried out by heating the alkene in the presence of a strong acid catalyst, such as sulfuric acid or hydrochloric acid. Here is an example of Cope rearrangement:

Starting material: 1-methyl-2-butene
Catalyst: sulfuric acid
Solvent: benzene

Heat the mixture of 1-methyl-2-butene, sulfuric acid, and benzene to a high temperature (around 150-200 °C).
The sulfuric acid catalyst protonates the alkene, forming a carbocation intermediate.
The carbocation intermediate undergoes a 1,2-migration reaction, in which the methyl group migrates from the carbon atom bonded to the double bond to the carbon atom bonded to the single bond.
The resulting product is 2-methyl-1-butene.
The Cope rearrangement is an example of a 1,2-migration reaction, in which a substituent migrates from one carbon atom to another. Other examples of 1,2-migration reactions include the Claisen rearrangement and the Favorskii rearrangement.

Mechanism of reaction

The alkene and the strong acid catalyst are mixed together in a solvent, such as benzene.
The acid catalyst donates a proton to the alkene, forming a carbocation intermediate. This step is called protonation.
The carbocation intermediate undergoes a 1,2-migration reaction, in which a substituent migrates from one carbon atom to another. This step is called migration.
The resulting alkene is formed.
Here is a more detailed version of the mechanism, showing the movement of electrons:

The alkene and the acid catalyst are mixed together in a solvent.
The acid catalyst donates a proton to the alkene, forming a carbocation intermediate.

fig1

The carbocation intermediate undergoes a 1,2-migration reaction, in which the methyl group migrates from the carbon atom bonded to the double bond to the carbon atom bonded to the single bond.
The resulting alkene is formed.

fig2

References

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