REDUCTIVELY ACTIVATED POLAR NUCLEOPHILIC AROMATIC-SUBSTITUTION .4. THERMAL AND PHOTOCHEMICAL BEHAVIOR OF POLYCHLORONITROBENZENE AND POLYFLUORONITROBENZENE IN FRONT OF SOFT NUCLEOPHILES

The nucleophilic aromatic substitution of pentachloronitrobenzene (PCN B) with benzenethiolate and benzeneselenide anions in aqueous solution give rise to fast nitro group substitution in both cases. In the same conditions, pentafluoronitrobenzene (PFNB) and 3,4,5-trichloronitrobe nzene (TCNB) undergo substitution of the halogen atom placed in para p osition with respect to the nitro group. Mechanistic studies suggest t hat the reactions of PCNB and PFNB (substitution of very electronegati ve leaving groups) follow a mechanism that includes a ''polar'' attack of the nucleophile on the aromatic substrate in the first step and a fast ''radical'' evolution of the sigma-complex intermediate. On the o ther hand, the chloride substitution in the TCNB reactions follows the classical SNAr mechanism. The photochemical reactions of PCNB, PFNB a nd TCNB in the presence of nucleophiles (electron donors) lead in all cases to photoreduction products (anilines) probably through the aroma tic radical anion.