Photochemical Addition of Benzene to Cyclobutene

The major products of the photochemical addition at 253.7 nm of benzene to cyclobutene in solution were found to be tetracyclo[5.3.0.0
^2,10
0
^3,6
]-decene-8 and tricyclo[4.2.2.0
^2,5
]deca-7,9-diene, which arise from 1,3- and 1,4-additions. The quantum yields for both reactions were independent of benzene concentration below 2M but decreased at higher concentrations. The maximum quantum yields that were obtained are 0.81 and 0.09 for the 1,3- and 1,4-reactions, respectively, indicating that the former reaction is a very efficient process. The 1,3-addition may occur from the singlet (
¹
B
2u
) state of benzene, as has been suggested in the literature. The origin of the 1,4-addition is not clear, but it seems unlikely that it occurs from the triplet state of benzene.