Tetraphenylporphine-sensitized photooxygenation of (E,E)- and (E,Z)-1-aryl-1,3-pentadienes generating cis-endoperoxides

Photooxygenation of either (E,E)- or (E,Z)-1-aryl-1,3-pentadienes (la-c) se nsitized with tetraphenylporphine (TPP) in benzene gave almost all cis-endo peroxides (2a-c) (cis-3-aryl-6-methyl-1,2-dioxacyclohex-4-enes) in good yie lds. A time course study of photooxygenation of(E,Z)-rich dienes measured b y IH NMR showed that singlet oxygen added exclusively to (E,E)-dienes conve rted from (E,Z)-dienes by photoinduced isomerization, and both rates increa sed when electron-donating groups were attached to the aryl group. A concer ted [4 + 2] cycloaddition mechanism is suggested by the exclusive formation of cis-endoperoxides from (E,E)-dienes, despite the small energy differenc e between cis- and trans-endoperoxides calculated by ab initio methods. Som e experiments were made to explore the observed isomerization.