Photodimerization and Photocycloaddition Reactions of Aromatic Compounds

Benzene undergoes photocycloaddition with simple olefins to produce 1,3, 1,2 and 1,4 adducts as shown below for tetramethylethylene  

The products consistently exhibit retention of the original olefin stereochemistry and are probably formed in a concerted manner. An exciplex formed from singlet excited benzene and the ground state olefin (allowing relaxation of the orbital symmetry requirements for concerted 1,3- and 1,4-cycloaddition) has been proposed to account for these products. Srinivasan and Hill reported an unusual photochemical addition to benzene to form cydoadduct (52)  

More recent investigations have revealed the product to be a mixture of (52a) and (52b)  

Irradiation of a 1 1 mixture of benzonitrile and 2-methyl-2-butene for an extended period yields the 1 1 adduct (S3) in a 63% yield  

Irradiation of (53) causes decomposition to starting materials, whereas pyrolysis yields the tetraene (54)/  

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