Electrocyclic reaction

In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction where the net result is one pi bond being converted into one sigma bond. These reactions are usually unnamed, being categorized by the following criteria:

• electrocyclic reactions are photoinduced or thermal
• the number of pi electrons in the species with more pi bonds determines reaction mode
• electrocyclic reaction can be a ring closure or a ring opening reaction
• the stereospecifity is determined by conrotatory or disrotatory mode of transition state formation.

Additional recommended knowledge

Essential Laboratory Skills Guide

Weighing the right way

What is the Correct Way to Check Repeatability in Balances?

The torquoselectivity in an electrocyclic reaction is the measure of selectivity in the direction of the conrotatory or disrotatory mode.

For example in scheme 1 a [6-pi] electrocyclic ring closure takes all-cis hexatriene to 1,4-cyclohexadiene.

An often studied electrocyclic reaction is the conrotatory thermal ring-opening of benzocyclobutane. The reaction product is a very unstable ortho-quinodimethane but this molecule can be trapped in an endo addition with a strong dienophile such as maleic anhydride to the Diels-Alder adduct. The chemical yield for the ring opening of the benzocyclobutane depicted in scheme 2 is found to depend on the nature of the substituent R ^[1]. With a reaction solvent such as toluene and a reaction temperature of 110°C, the yield increases going from methyl to isobutylmethyl to trimethylsilylmethyl. The increased reaction rate for the trimethylsilyl compound can be explained by silicon hyperconjugation as the βC-Si bond weakens the cyclobutane C-C bond by donating electrons.

The Nazarov cyclization reaction is a named electrocyclic reaction converting divinylketones to cyclopentenones.

References

1. ^  Accelerated Electrocyclic Ring-Opening of Benzocyclobutenes under the Influence of a -Silicon Atom Yuji Matsuya, Noriko Ohsawa, and Hideo Nemoto J. Am. Chem. Soc.; 2006; 128(2) pp 412 - 413; (Communication) DOI: 10.1021/ja055505+ Abstract