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Pericyclic Reactions: Electrocyclic Reactions

Electrocyclic Reactions: Stereochemistry

Two possibilities exist for the stereochemical outcome of these reactions.

Fig.1
Disrotatory
Fig.2
Conrotatory

3D Animation of conrotatory ringclosure

3D Animation of disrotatory ringclosure

In a conjugated polyene the terminal substituents a and b are lying in a plane. In order to bring about cyclization the groups have to rotate out of the plane. If both rotate in the same direction, i.e. the movements are both clockwise (or both counterclockwise) the motion is called conrotatory. It is called disrotatory if one substituent moves clockwise and the other counterclockwise. The stereochemical outcome of an electrocyclic reaction depends on the number of participating electrons and wether it is a thermal or photochemical process.

Tab.1
Examples of conrotatory and disrotatory ringclosures
ConrotatoryDisrotatory

4 Electrons Möbius aromatic

6 Electrons Hückel aromatic

As a general rule the following applies:

Stereochemical rules for thermally induced electrocyclic reactions
  • 4n + 2 Electrons: disrotatory
  • 4n Electrons: conrotatory
The reverse is true for photochemically induced reactions.
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