Singlet photosensitization of simple alkenes. Part 2. Photochemical transformation of cyclo-octa-1,5-dienes sensitized by aromatic ester

Abstract

Irradiation of an oxygen-free pentane solution of cis,cis- or cis,trans-cyclo-octa-1,5-diene, (1 cc) or (1 ct), in the presence of methyl benzoate as sensitizer gave the cis,trans-isomer and tricyclo[3.3.0.0^2,6]octane. However, the tricyclo-octane was not an immediate product from (1 cc), but was formed via(1 ct). Kinetic evidence and energetic considerations indicate a non-vertical singlet sensitization mechanism involving a singlet exciplex, which in turn falls apart leaving a twisted, excited singlet of the cyclo-octadiene. Two exciplexes, Ex₁ and Ex₂, and therefore two twisted singlet cyclo-octadienes, c,¹p and t,¹p, are postulated in order to rationalize the different reactivity of (1 cc) and (1 ct). No evidence for transannular interaction between the two double bonds was found from the quenching rate constants. Correlations between the quenching rate constants and the oxidation potentials of the cyclo-octadienes and cyclo-octenes suggest charge-transfer character for the exciplex. Asymmetric cis–trans photoisomerization sensitized by a chiral aromatic ester also supports the involvement of exciplex with a fairly rigid structure.