Organic Photochemistry with 6.7-eV Photons: Photoisomerization of Tricyclo2,4>oct-6-ene (Endo and Exo) and Tricyclo2,4>non-6-ene (Endo and Exo)

Article abstract of DOI:10.1021/ja00536a030

Photolysis of the title compounds at 185 nm in solution leads to internal addition of the olefinic group to the cyclopropane ring and cleavage of the cyclopropane to a bicyclic 1,4-diene.A theoretical analysis of the interactions between the ?-orbitals of the double bond and the ? orbitals of the cyclopropane in each of these compounds has been carried out.The effect of through-bond interactions superimposed upon the through-space effects when extended to the valence excited states gives rise to three low-lying excited states which are (in order to decreasing energy) ?_A -> ?_A^* + ?^* (forbidden), ?_S + ? -> ?^* - ?_A^* (allowed), and ? - ?_S -> ?^* + ?_A^* (allowed).The internal addition reaction is identified with the ? - ?_S -> ?^* + ?_A^* state and the cleavage of the cyclopropane to yield a 1,4-diene with the ?_S + ? -> ?^* + ?_A^* state.The low reactivity of the endo-tricyclo<3.2.2.0^2,4>non-6-ene is believed to relate to a departure from the ordering of the excited states as described above.