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 (1a- c) sensitized with tetraphenylporphine (TPP) in benzene gave almost all cis- endoperoxides (2a-c) (cis-3-aryl-6-methyl-1,2-dioxacyclohex-4-enes) in good yields. A time course study of photooxygenation of (E,Z)-rich dienes measured by ¹H NMR showed that singlet oxygen added exclusively to (E,E)-dienes converted from (E,Z)-dienes by photoinduced isomerization, and both rates increased when electron-donating groups were attached to the aryl group. A concerted [4 + 2] cycloaddition mechanism is suggested by the exclusive formation of cis-endoperoxides from (E,E)-dienes, despite the small energy difference between cis- and trans-endoperoxides calculated by ab initio methods. Some experiments were made to explore the observed isomerization.