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.

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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