REGIO- AND STEREO-SELECTIVITIES OF THE ORTHO AND META PHOTOCYCLOADDITION REACTIONS OF ETHYLENES TO BENZENE AND ITS SIMPLE DERIVATIVES

Article abstract of DOI:10.1016/S0040-4020(01)92375-0

The photoreactions of benzene, toluene, anisole, and benzonitrile with acrylonitrile, methacrylonitrile, and vinyl acetate, and of toluene and o- and p-xylene with maleic anhydride are described.The acrylonitriles do not react with benzonitrile but yield mixtures of ortho photocycloadducts with the other arenes.Contrary to previous findings both exo and endo stereoisomers of the ortho cycloadducts of benzene and acrylonitrile are formed: the reaction is selective towards the exo isomer but the stereoisomers from methacrylonitrile and benzene are formed with approximately equal efficiencies.Complex mixtures of regio- and stereoisomers of the ortho cycloadducts are formed between toluene and the acrylonitriles but their addition to anisole is more selective and in acetonitrile essentially only 1,2-attack of the ethylene on the arene is observed.The 2:1 photoadducts of maleic anhydride with toluene and o- and p-xylene reflect formation of two regio ortho photocycloadducts in each case.The variation in the ratios of these isomers with temperature and light intensity is interpreted in terms of the differing photolabilities of the 1:1 adducts and their reactivities towards the thermal addition of the second molecule of maleic anhydride.Vinyl acetate undergoes 1,2-cycloaddition to benzonitrile but with the other arenes, meta cycloadducts are favoured.These latter additions are specifically 2,6- with respect to toluene and anisole but there is little regioselectivity with respect to the ethylene although the 7-endo acetate of the meta cycloadduct with benzene does constitute 60percent of the reaction mixture.