Oxidation of nitrobenzene, chlorobenzenes and chlorophenols using liquid-phase ruthenium catalysts

Ruthenium trichloride and various ruthenium(II) complexes, such as [Ru(H2O)(2) (L)(4)](BF4)(2) and [RuCl2(L)(4)] (L=DMSO), are effective catalysts for the complete oxidation of substituted aromatics at ambient temperature and in the liquid phase (double phase or water with suitable amounts of a surfactant agent) in the presence of an oxidizing agent, preferably monopersulfate. The oxidation of benzoic acid, chloro-, bromo-, iodo- and nitrobenzene and a number of polychlorobenzenes and polychlorophenols was followed by monitoring the nature and the relative amounts of the final products; chlorinated substrates are mainly converted into hydrochloric acid and carbon dioxide. Factors such as solvent, substrate, oxidant and concentration affect the reactions. The most favorable reaction conditions require the presence of polar media, like nitromethane or water. The fate of the catalyst during the various stages of the reactions was followed, which gave indications of the nature of the active species involved. The overall results imply that a strongly oxidizing ruthenium derivative is responsible for the effective oxidation with monopersulfate, tentatively a highly reactive peroxoruthenium species [Ru-O-O-SO3H]. Oxidation of chlorobenzene is first order in both the substrate and the catalysts and an n (or inverse) kinetic isotope effect is measured. The rates of the oxidation of mono-substituted benzenes depend upon the degree of deactivation of the aromatic ring, thus suggesting an initial electrophilic attack followed by a series of faster steps. Phenol and polychlorophenols are more sensitive to oxidation than substituted benzenes.