THE PHOTOCHEMICAL REACTIVITIES OF BENZENES TETHERED TO HALOARENE

The syntheses and photoreactions of haloarenes, in which the aryl and haloaryl moieties are tethered by a simple alkyl group, were studied. For 2-benzyl-1-halobenzene, in which two aryl moieties were connected by methylene group, photoreduced product, diphenylmethane, was obtaine d along with the minor formation of the photocyclized product, fluoren e, in acetonitrile solvent. For 1-halo-2-phenethylbenzene, in which tw o aryl moieties were connected by ethylene group, photocyclized produc ts, 9,10-dihydrophenanthrene and phenanthrene, were obtained along wit h the minor formation of photoreduced product, bibenzyl, in acetonitri le solvent. The photoreaction selectivities in several solvent systems were studied: In cyclohexane, 2-benzyl-1-chlorobenzene was photoreduc ed more effective than 2-benzyl-1-bromobenzene; In the presence of NaO H, 1-halo-2-phenethylbenzenes gave 9,10-dihydrophenanthrene and, in th e presence of toluene, they gave phenanthrene. A radical reaction mech anism is proposed for the explanation of the reactions. This study sho ws that the photoreaction of haloarene, in which haloaryl and aryl moi eties are tethered by ethylene group, can be used for ring formation o f 9,10-dihydrophenanthrene otherwise difficultly accessible.