Dienes-cyclobutene

Both reactions are reversible, and the position of the equilibrium depends on the specific case. In general, the triene cyclohexadiene equilibrium favors the cyclic product, whereas the diene cyclobutene equilibrium favors the unstrained open-chain product. 

The 1,3-diene—cyclobutene interconversion can even be applied to benzene rings. For example,379 photolysis of l,2,4-tri-/-butylbenzene (92) gives l,2,5-tri-/-butyl[2.2.0]hexadiene (93, a Dewar benzene).380 The reaction owes its success to the fact that once 93 is formed,... 

Fig. 18.4. The 1,3-diene-cyclobutene interconversion. The orbitals shown are not molecular orbitals, but a basis set of p-atomic orbitals, (a) Disrotatory ring closure gives zero sign inversion, (b) Conrotatory ring closure gives one sign inversion. We could have chosen to show any other basis set (e.g., another basis set would have two plus lobes above the plane and two below, etc.). This would change the number of sign inversion, but the disrotatory mode would stiU have an even number of sign inversions, and the conrotatory mode an odd number, whichever basis set was chosen.

The 1,3-diene-cyclobutene interconversion can even be applied to benzene... 

The interconversions of the corresponding dimethylcyclohexadienes and the 2,4,6-octatrienes are also stereospecific (Fig. 29.13). Here, too, thermal and photochemical reactions differ in stereochemistry. If we examine the structures closely, we see something else the stereochemistry of the triene-cyclohexadiene interconversions is opposite to that of the diene-cyclobutene inlerconversions. For the thermal reactions, for example, cis methyl groups in the cyclobutene become cis and trans in the diene cis methyl groups in the cyclohexadiene are trans and trans in the related triene. 

Electrocyclization of conjugated dienes occurs in competition with cis-trans isomerization. The cyclization occurs from the s-cis conformation of the diene. Cyclobutene formation is favored in cyclic dienes and for other dienes where the s-cis diene conformation is dominant. For several dienes, the quantum yield in nonpolar solvents at 257 nm is about 0.1. As the cyclized alkenes do not absorb at this wavelength, the reaction can give substantial preparative yields, despite the competing cis-trans isomerization. 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

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