The indirect anodic oxidation of 2-methylnaphthalene Part I. Ruthenium compounds as catalysts

The indirect anodic oxidation of 2-methylnaphthalene and naphthalene to 2-methylnaphthoquinone-1,4 and naphthoquinone, respectively, was carried out with RuCl₃·3H₂O, Ru(acac)₃ and Ru(NH₄)₂Cl₆ as catalysts, in an undivided cell using platinum electrodes. Current-potential analysis of the hydrocarbon shows two oxidation waves in solutions of acetonitrile-H₂SO₄ which collapse into a single wave in presence of water, implying an electrodic-chemical-electrodic (ECE) reaction sequence. RuCl₃·3H₂O or Ru(NH₄)₂-Cl₆ increases the selectivity for quinone formation, whereas Ru(acac)₃ has no such effect. Voltammetry shows that RuCl₃·3H₂O and Ru(acac)₃ oxidize water and not the hydrocarbon. It also identifies an activated complex in which RuCl₃·3H₂O, but not Ru(acac)₃, binds to the oxidised form of the hydrocarbon. This analysis explains the catalytic effect of RuCl₃·3H₂O and the inefficiency of the coordinatively saturated Ru(acac)₃. RuCl₃·3H₂O is also an efficient catalyst in non-acidic solutions containing high concentrations of diisobutylamine and diisopropylamine and in which oxidation of 2-methylnaphthalene to 2-methylnaphthoquinone-1,4 is possible at a very low potential.