Topochemistry. Part XXXI. Formation of cyclo-octa-1,5-cis,cis-dienes from 1,4-disubstituted s-trans-butadienes in the solid state. A contribution to the problem of C₄- versus C₈-cyclodimerisation

Solid penta-1,3-diene-1-carboxylic acid (4), penta-1,3-diene-1-carboxamide (5), buta-1,3-diene-1,4-dicarbonitrile (6), styrylacrylic acid (10), its methyl ester (46), and amide (49) (all trans,trans-configurated), all photodimerise on irradiation (λ > 290 nm), to divinylcyclobutane (C₄) derivatives. The structures of the fully characterised photoproducts from (4), (5), (6), and (49) and the light-stability of (N-phenyl)styrylacrylamide (53), are predictable from the known or postulated packing arrangements of their monomers. Cyclo-octa-1,5-trans,trans-dienes, although topochemically and symmetry-allowed from monomers which crystallise with parallel butadiene chains [(4), (5), (6), and possibly (10)], are not observed. Although the formation of C₈-cyclodimers cannot be rigorously excluded our results neither support nor require their presence. The (all-ax)-cyclo-octa-1,5-cis,cis-diene (C₈) derivatives formed during irradiation of (4), (5), and (10) are not primary photoproducts but arise from thermal (Cope) rearrangements of photochemically produced cis-1,2-divinylcyclobutanes.