- Electrocyclic reaction
In
organic chemistry , an electrocyclic reaction is a type ofpericyclic rearrangement reaction where the net result is onepi bond being converted into onesigma bond . These reactions are usually unnamed, being categorized by the following criteria:* electrocyclic reactions are photoinduced or
thermal
* the number ofpi 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 byconrotatory ordisrotatory mode oftransition state formation.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 asmaleic anhydride to theDiels-Alder adduct. Thechemical yield for the ring opening of the benzocyclobutane depicted in "scheme 2" is found to depend on the nature of thesubstituent R Ref|Matsuya. With a reactionsolvent such astoluene and a reaction temperature of 110°C, the yield increases going frommethyl to isobutylmethyl to trimethylsilylmethyl. The increasedreaction rate for thetrimethylsilyl compound can be explained bysilicon 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.External links
* [http://www.iupac.org/goldbook/E01948.pdf further reading]
* [http://www.chem.ucsb.edu/~kalju/chem109C/Electrocyclic.html further reading]References
# "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+ [http://dr.doi.org/10.1021/ja055505+ Abstract]
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