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Concept
Electrocyclic reaction
Summary
In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa. These reactions are usually categorized by the following criteria:
Reactions can be either photochemical or thermal.
Reactions can be either ring-opening or ring-closing (electrocyclization).
Depending on the type of reaction (photochemical or thermal) and the number of pi electrons, the reaction can happen through either a conrotatory or disrotatory mechanism.
The type of rotation determines whether the cis or trans isomer of the product will be formed.
The Nazarov cyclization reaction is a named electrocyclic reaction converting divinylketones to cyclopentenones.
A classic example is the thermal ring-opening reaction of 3,4-dimethylcyclobutene. The cis isomer exclusively yields whereas the trans isomer gives the trans,trans diene:
This reaction course can be explained in a simple analysis through the frontier-orbital method: the sigma bond in the reactant will open in such a way that the resulting p-orbitals will have the same symmetry as the HOMO of the product (a hexadiene). The only way to accomplish this is through a conrotatory ring-opening which results in opposite signs for the terminal lobes.
When performing an electrocyclic reaction, it is often 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. The table below shows the selectivity rules for thermal and photochemical electrocyclic reactions.
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.
Official source
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