Bromoanthracenes and metal co-catalysts for the autoxidation of para-xylene

The autoxidation of para-xylene (pX) was carried out with the usual Co(OAc)(2) catalyst in HOAc, but with promoters other than inorganic bromide. 9,10-Dibromoanthracene and 9-bromoanthracene were used. They gave acceptable rates and yields of terephthalic acid, particularly with low concentrations of metal co-catalysts. The ones studied and their optimal concentrations are 0.3 mM Mn(OAc)(2), 0.2 mM Ce(OAc)(2) and 20 mM ZrOCl2. The mechanism involves formation of an intermediate radical such as DBA(.+), which oxidizes pX to pX(.+), a strong acid that rapidly releases H+. Under oxygen, peroxyl radicals are formed, which sustain the free radical branching chains. The redox-active metals Mn(II) and Ce(III) accelerate the reaction because they are oxidized by ArCH2OO. ca. 10(3)-times more rapidly than Co(II) is. A different mechanism operates for Zr(IV). By prior addition of t-BUOOH, evidence has been obtained for the intervention of peroxo-zirconium(IV) complexes of this d(0) metal. (C) 2003 Elsevier B.V. All rights reserved.