Stereospecificity of Ring-closing Reactions
When performing an electrocyclic reaction it is sometimes desirable to predict the cis/trans geometry of the reaction's product. The first step in this process is to determine whether a reaction proceeds through conrotation or disrotation. Although the mechanism of these reactions is complex, only two pieces of information are needed to predict conrotation or disrotation.
| System | Thermally Induced (ground state) | Photochemically Induced (excited state) |
|---|---|---|
| Even # of conjugation | Conrotatory | Disrotatory |
| Odd # of conjugation | Disrotatory | Conrotatory |
For the example given below, the thermal reaction of (trans,cis,trans)-octa-2,4,6-triene will happen through a disrotatory mechanism. After determining the type of rotation, whether the product will be cis or trans can be determined by examining the starting molecule. In the example below, the disrotation causes both methyls to point upwards, causing the product to be cis-dimethylcyclohexadiene.
In addition, the torquoselectivity in an electrocyclic reaction refers to the direction of rotation. For example, a reaction that is conrotatory can still rotate in two directions, producing enantiomeric products. A reaction that is torquoselective restricts one of these directions of rotation (partially or completely) to produce a product in enantiomeric excess.
Read more about this topic: Electrocyclic Reaction
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