Anthracenes 2.5- dimethyl-2,3-hexadiene

In contrast to the course of reaction with the parent anthracene, 2,S-dimethyl-2,4-hexadiene has been reported to yield chiefly the photochemically forbidden [4 + 2] cycloadduct (105) when exposed to 9-cyanoanthracene. The rem able variability in periselectivity displayed by these systems has been rationalized in terms of the [arene 1,3-diene] exciplex which is presumably involved in these transformations. Polar exciplexes are descried as leading to [4 + 2] products while less polar exci-plexes are believed to be the precursors of [4 + 4] adducts. Of interest is the observation that cyclohexa-diene provides principally the corresponding [4 + 4] adduct when reacted with both anthracene and 9-cyanoanthracene thus it has been proposed Aat a less polar exciplex is involved in both of these reactions. ... 

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