Amination of benzene and toluene with hydroxylamine in the presence of transition metal redox catalysts

The amination of benzene and toluene to aniline and toluidines with hydroxy lamine sulfate has been investigated in water-acetic acid and water-acetic acid-sulfuric acid media in the presence of transition metal compounds as c atalysts. The process yields are strongly dependent on the temperature, add ed sulfuric acid and the composition of catalyst. For the amination of benz ene, the soluble catalysts, NaVO3 and Fe(III) salts produce high yields of aniline without addition of H2SO4, whereas Na2MoO4 and FeSO4 exhibit substa ntial activity only in 5 M H2SO4. Amination is accompanied by a disproporti onation of hydroxylamine catalyzed by the redox active transition metal ion s. The favorable effect of H2SO4 on the amination is due mostly to the grea ter stability of hydpoxylamine in the strongly acidic medium. Mixed oxides containing V(V) and Mo(VI) are active amination catalysts when suspended. i n S M solution of H2SO4 in acetic acid. Introduction of metallic Pd into th ese oxide catalysts improves performance increasing the yield and selectivi ty of amination with respect to the aromatic substrate. Toluene exhibited a close to benzene reactivity in amination giving approximately equal yields of o-, m-, p-toluidines. Mechanistic considerations based on literature da ta and results of ab initio quantum mechanics calculations suggest that the aminating species is the protonated amino radical (NH3+)-N-., which in the rate-determining step reacts with benzene and toluene to yield the corresp onding aminocyclohexadienyl and aminomethylcyclohexadienyl radical intermed iates. These are then oxidatively aromatized to give, respectively, aniline and a non-regiospecific mixture of toluidines. (C) 2000 Elsevier Science B .V. All rights reserved.