REACTIONS OF BENZYNE WITH SUBSTITUTED BENZENES

The reactions of benzyne with substituted benzenes (anisole, chlorobenzene, methyl benzoate, benzylidyne trifluoride and toluene) giving the Diels-Alder adducts were investigated, where two positionally isomeric adducts were possible, i.e., a geminal-para adduct (1,4-adduct with respect to the substituent) and an ortho-meta adduct (2,5-adduct). From the competition reactions, relative reactivities of benzyne to substituted benzenes were estimated for the 1,4-addition and 2,5-addition. Markedly different substituent effects were observed. For the 1,4-addition, large negative ρ value (−1.79) was observed, but for the 2,5-addition, substituent effect was small. From benzyne and toluene, o-benzylbiphenyl (13) and 2-benzyl-3-phenylbenzo[5,6]bicyclo[2.2.2]octatriene (14) were obtained as the successive “ene” products together with the normal Diels-Alder adducts.