METHYLBENZENE CATION RADICAL ALPHA-FRAGMENTATION SELECTIVITIES REVEALED IN SET-PHOTOADDITIONS OF P-XYLENE DERIVATIVES TO 1,4-DICYANONAPHTHALENE

SET-promoted photoreactions of selected p-xylene derivatives, including p-xylene (3a), p-phenylenediacetic acid (3b), p-bis[(trimethylsilyl)methyl]benzene (3c), p-[(trim ethylsilyl)methyl]toluene (3d), p-tolylacetic acid (3e), and p-[(trimethylsilyl)methyl]phenylacetic acid (3f), to 1,4-dicyanonaphthalene have been investigated. These processes lead to clean and selective formation of 1:1 adducts whose nature is controlled by alpha-heterolytic fragmentation reactions occurring at benzylic positions of arene cation radicals which serve as key reactive intermediates. The results indicate that the relative rates of cation radical fragmentations of the type ([ArCH2E].+ + B: --> ArCH2.+ BE) depend on the nature of the electrofugal group, E, in the following order: E = SiMe3 > CO2H > H. The experimental basis for these conclusions is discussed.