Organic Photochemistry with 6.7-eV Photons: Photoisomerization of Tricyclo[3.2.1.0^2,4]oct-6-ene (Endo and Exo) and Tricyclo[3.2.2.0^2,4]non-6-ene (Endo and Exo)

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 of decreasing energy) σA → σA* + π* (forbidden), Ns + π → π* + σA* (allowed), and π - σs → π* + σs (allowed). The internal addition reaction is identified with the π - σs → π* + σA* state and the cleavage of the cyclopropane to yield a 1,4-diene with the σs + π → π* + σA* state. The low reactivity of the endo-tricyclo[3.2.2.02,4]non-6-ene is believed to relate to a departure from the ordering of the excited states as described above. © 1980, American Chemical Society. All rights reserved.