Phosphomolybdic acid as a reoxidant in the palladium(II)-catalysed oxidation of but-1-ene to butan-2-one

Abstract

Phosphomolybdic and a variety of phosphomolybdovanadic acids were examined as reoxidants for the palladium sulphate-catalysed oxidation of but-1-ene to butan-2-one both in the absence and the presence of oxygen. All of these co-oxidants were approximately equally effective in reoxidising Pd⁰ to Pd^II but they varied substantially in their ability to be reoxidised themselves by air under the optimum reaction conditions in aqueous acid. Phosphomolybdovanadate systems were the most effective at a pH > 0, but V^IV by itself could not be reoxidised by air under these conditions and therefore the molybdenum must play a vital role. Phosphomolybdic acid, H₃[PMo₁₂O₄₀], itself was quite a good co-oxidant under more acid conditions (1 mol dm^–3 sulphuric), but ³¹P n.m.r. spectroscopy showed that in dilute solution it was largely dissociated into phosphoric acid; evidence for the presence under some conditions of other phosphomolybdic acids, which may be related to the active species, is presented.