Butadienes cyclobutene electrocyclic equilibrium

In contrast to cycloadditions, which almost invariably take place with a total of (4 2) electrons, there are many examples of electrocyclic reactions taking place when the total number of electrons is a (An) number. However, those electrocyclic reactions with (An) electrons, like the butadiene-cyclobutene equilibrium, 6.50 6.51, differ strikingly in their stereochemistry from those reactions mobilising (An+2) electrons, like the hexatriene-cyclohexadiene equilibrium, 6.52 —> 6.53. This is only revealed when the parent systems are... 

Cyclobutenes are in electrocyclic equilibrium with 1,3-butadienes. 1,3-Butadienes are lower in energy, because cyclobutenes are very strained, and it is therefore possible to convert a cyclobutene to a butadiene thermally. The reverse reaction, the conversion of a 1,3-butadiene to a cyclobutene, does not usually proceed under thermal conditions, as it involves an uphill climb in energy. However, the more conjugated 1,3-butadienes absorb light at longer wavelengths than cyclobutenes, so it is possible to convert a 1,3-butadiene to a cyclobutene... 

Electrocyclic ring closures are reversible, and only those which yield significant amounts of cyclic products at equilibrium have been studied. Most butadiene-cyclobutene equilibria greatly favor the butadienes (see below). An exception is the cyclization of irans, a.s-l,2,3,4-tetraphenyl-l-bromo-l,3-butadiene, which undergoes the predicted conrotatory cyclization to a significant extent. ... 

The equilibrium between the cyclobutene and the 1,3-butadiene is shifted in favor of the cyclobutene in benzocyclobutenes. The electrocyclic ring opening of a benzocyclobutene, an aromatic compound, leads to an o-xylylene, a nonaromatic and reactive compound. o-Xylylenes are useful intermediates in Diels-Alder reactions. 

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