Photochemistry of Bicyclic Compounds. Di-π-Methane Rearrangement of 5-Methylene-2-norbornenes

The direct and sensitized photolysis of three substituted methylenenorbornenes, 2-4, has Wen explored. 5-Benzylidene-2-norbornene (2) undergoes an efficient E-Z photoisorherization from the triplet excited state and a moderate (Φ_E= 0.043, Φ_Z= 0.032) regiospecific di-π-methane rearrangement from the singlet excited state to give fi.Triplet 2 does not rearrange. In contrast, the aliphatic dienes 3 and 4 both give di-π-methane rearrangement products. 8 and 9, respectively, from the triplet [Φ₈(acetone) = 0.32, Φ₉(acetone) = 0.02]. 3 and 4 undergo rearrangement also upon direct irradiation (λ< 254nm). The results indicate a distinct substituent effect on the triplet rearrangement efficiency. The role of rotational deactivation processes in governing rearrangement multiplicity and aspects of regiospecificity are discussed.