1.3.5- Hexatrienes cyclohexadiene equilibrium with

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... 

Irradiation at 254 nm transforms 2,5-di-fert-butylhexa-l,3,5-triene (14) (a hexatriene with a dominant cZc equilibrium conformation) into the corresponding cyclohexadiene (15) with a 0.54 quantum yield. The reverse reaction transforms 1,4-di-terr-butylcyclohexa-l,3-diene (15) into the corresponding hexatriene (14) with a 0.46 quantum yield.83-85 Consistently, the computed... 

Cyclohexadienes are in electrocyclic equilibrium with 1,3,5-hexatrienes. Neither compound is strained, and the cyclohexadiene has one more cr bond than the hexatriene, so the cyclohexadiene is lower in energy. The hexatriene is more conjugated than the cyclohexadiene, so it is more reactive photochemically. Under both thermal and photochemical conditions, then, the cyclohexadiene is favored over the hexatriene. 

The first step is a disrotatory cyclohexadiene-hexatriene isomerization. Its product, cf5-dihydrobenzocyclooctatetraene, is less stable than the trans dimer and is known to isomerize to it, [27] the isomerization presumably taking place via an a" displacement that reduces symmetry to Ci, in which no reaction is forbidden. At the higher temperatures at which fragmentation occurs, the first product should be in equilibrium with the reactant, and its eight-membered ring is sufficiently flexible that a similar desymmetrization would allow it to serve as an unstable intermediate. The activation parameters cited above, which - for the postulated mechanism - measure the enthalpy and entropy differences between the reactant and the transition state of the second step, are not inconsistent with concerted electrocyclic rupture of both bonds via a relatively unconstrained transition state. 

Equilibrium between monomer and aggregates with different absorption characteristics is hkely to result in wavelength-dependent chemistry. For example, 1,3-cyclohexadiene upon irradiation with 254-nm light gives 1,3,5-hexatriene as the only product but irradiation at 313 nm produces dimers (Scheme 10)." Upon irradiation at 254 nm, mainly the monomer is expected, while at 313 nm the dimer is excited with the two yielding different products. 

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