Polar and stereochemical aspects of 1,2-photoaddition of ethylenes to benzene

Two observations concerning photochemical 1,2-cycloadditions of mono-olefins to benzene may be of wider significance: the 1,2-process tends to occur most readily with olefins having either marked donor or acceptor properties, otherwise 1,3-addition tends to be preferred;and 1,2-additions of the donor olefins cis-cyclooctene and cis-but-2-ene are stereospecifically endo, whereas 1,2-addition of the acceptor maleimide is stereospecifically exo.