PHOTOCHEMICAL NITRATION BY TETRANITROMETHANE .3. ON THE ANOMALOUS NITRATION AND OXIDATIVE SUBSTITUTION OF DIBENZOFURAN AND A NOVEL TEST OF THE SHAIK-PROSS MODEL OF RADICAL CATION REACTIVITY

The photochemical reaction of dibenzofuran and tetranitromethane in dichloromethane gives predominantly nitro/trinitromethyl adducts and only a small amount of nitro-substitution products. The adducts originate from the reaction between photochemically produced trinitromethanide ion and (dibenzofuran).+, followed by reaction of the trinitromethyl-substituted cyclohexadienyl radical with NO2. The main sites of attack of trinitromethanide ion are the 1- and the 3-positions. The adducts rapidly eliminate nitroform in the presence of a hindered base. Thus photolysis with such a base present gives directly a mixture containing predominantly 2- and 4-nitrodibenzofuran. The unexpected attack of trinitromethanide at the 3-position of (dibenzofuran).+ is also found in more typical oxidative substitution reactions, such as acetoxylation and cyanation. This isomer distribution is distinctly different from that of electrophilic substitution in dibenzofuran (attack at the 2-position), a non-alternant conjugated system. The Shaik-Pross treatment of the radical cation/nucleophile elementary step predicts that the spin density of the corresponding triplet should govern the isomer distribution, and CASSCF calculations show that (dibenzofuran).+ then should react predominantly in the 3-position. Thus the electrophilic reactivity of the radical cations of non-alternant systems constitutes a novel test of the configuration mixing model. It is also concluded that the unusual substitution pattern of dibenzofuran with respect to electrophilic nitration is due to incursion of the nitrous acid catalyzed NAC) reaction.