In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction. The reaction is electrocyclic if the result is one pi bond becoming one sigma bond or one sigma bond becoming a pi bond. Electrocyclic reactions share the following properties:
- electrocyclic reactions are driven by light (photoinduced) or heat (thermal)
- the reaction mode is determined by the number of pi electrons in the part with more pi bonds
- an electrocyclic reaction can close a ring (electrocyclization) or open a ring
- the stereospecifity is determined by a conrotatory or a disrotatory transition state formation as predicted by the Woodward–Hoffmann rules.
The torquoselectivity in an electrocyclic reaction refers to the direction that the substituents rotate. For example, the substituents in a reaction that is conrotatory can still rotate in two directions. It produces a mixture of two products that are the mirror image of each other (enantiomeric products). A reaction that is torquoselective restricts one of these directions of rotation (partially or completely) to produce a product in enantiomeric excess (where one stereoisomer is produced much more than the other).
Chemists are interested in electrocyclic reactions because the geometry of the molecules confirm a number of predictions made by theoretical chemists. They confirm the conservation of molecular orbital symmetry.
The Nazarov cyclization reaction is an electrocyclic reaction that closes a ring. It converts divinylketones to cyclopentenones. (It was discovered by Ivan Nikolaevich Nazarov (1906–1957).)
An example is the thermal ring-opening reaction of 3,4-dimethylcyclobutene. The cis isomer only yields cis,trans-2,4-hexadiene. But the trans isomer gives the trans,trans diene:
The frontier-orbital method explains how this reaction works. The sigma bond in the reactant will open in a way that the resulting p-orbitals will have the same symmetry as the highest occupied molecular orbital (HOMO) of the product (a butadiene). This can only happen with a conrotatory ring-opening that results in opposite signs for the two lobes at the broken ends of the ring. (A disrotatory ring-opening would form an anti-bond.) The following diagram shows this:
The stereospecificity of the result depends on whether the reaction proceeds through a conrotatory or disrotatory process.