Substituent effects on the intramolecular photochemical reactions of phenyl-ethenyl non-conjugated bichromophoric systems

The effects of substitution on the photochemistry of phenyl-ethenyl bichromophoric systems are reported. Methyl substitution at the 2-, 3-, 5-, and 1,1,2-positions in the pentene moiety of 5-phenylpent-1-ene reduces both reaction efficiency and selectivity but in contrast to intermolecular analogues the photoreaction of 3-phenethylcyclohexene is comparable with that of the corresponding cyclopentene. Incorporation of ester units in the connecting unit between the chromophores or on the ethene inhibits intramolecular cyclisation as does the presence of para OMe, CN, or COMe groups in 5-phenylpent-1-ene. In contrast reaction selectivity and efficiency are greatly promoted by ortho Me or OMe groups and the products reflect exclusive 1,3-cycloaddition. The presence of a para Me group in 5-phenylpent-1-ene leads to specific 2,6-intramolecular cyclisation but the reaction of the meta-Me derivative leads to four products derived from 1,3- and 1,5-intramolecular cycloaddition. The observations are discussed in terms of mechanisms of arene-ethene photoreactions and preferred conformations of the bichromophores.