1,3,5-hexatriene derivatives photochemical

The case of butadiene-cyclobutene interconversion, which one might expect to provide a straightforward example illustrating the stereoselectivity of photochemical electrocyclization, is actually quite complex, especially when substituted systems are involved. We first consider experimental outcomes from the photolysis of butadiene and substituted derivatives, as well as the reverse reaction, the photochemical ringopening reactions of cyclobutenes. We then examine the 1,3,5-hexatriene system in the same way. 

The cyclohexadiene-hexatriene photochemical interconversion is predicted by orbital symmetry considerations to involve conrotatory motion. Cyclohexadiene derivatives undergo photochemical electrocyclic ring opening. The photostationary state... 

Work by Schmid s group has shown that the nitrile ylide derived from diphenylazirine (68) also undergoes quantitative dimerization to 1,3,5,6-tetraphenyl-2,5-diaza-l,3,5-hexatriene (99) at — 160°C. This result indicates that 99 is not only formed by the indirect route (98 -)- 68- 100 101 ->99) but also by dimerization of 98 by a direct head to head coupling. The intermediacy of azomethine ylide 101 in the photochemical conversion of azabicyclohexene... 

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