Oxidation of benzene by molecular oxygen catalysed by vanadium

Vanadium compounds can efficiently catalyse the oxidation of benzene to phenol with molecular oxygen. The reaction is truly catalytic only in the presence of a reducing agent able to recycle the oxidised catalyst. Among the reducing agents tested, ascorbate afforded the highest selectivity and reactivity to phenol. With other reductants, such as dithioalcohols, high selectivity (>96%) was obtained toward more oxidised products (i.e. hydroquinone). The reactions were performed in the two-phase system formed by mixing water/acetonitrile/benzene which allows a good separation between reactant and product. The reaction rate and product selectivity depend upon the type of ligand interacting with vanadium as well as upon the ascorbate/V molar ratio. ESR measurements confirmed that V-IV species is rapidly formed in the reaction medium after the initial addition Of (VCl3)-Cl-III. Moreover, the observed initial induction period of benzene oxidation can be related to the formation of olicomeric V-IV species, as assessed by ESR analysis, rather than to the slow formation of V-ascorbate complex. (C) 2003 Elsevier Science B.V. All rights reserved.