Platinum Metals Rev., 2002, 46, (1), 26
Aerobic Oxidation of Alcohols with Palladium-Hydrotalcite
The oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively, plays an important part in organic synthesis. Traditionally, such transformations have been performed with stoichiometric quantities of oxidants, such as chromium salts, oxalyl chloride, and so on. These work well in small-scale reactions, however on an industrial scale, there is a waste of heavy metals and unwanted coproducts, so the search for effective cleaner catalytic systems which use inexpensive primary oxidants such as molecular oxygen, hydrogen peroxide and/or air (a ‘green method’) for oxidising alcohols to aldehydes and ketones, remains an important challenge. Several metal catalysts, including Pt, Pd, Ru, Co, Cu, V, Os, Ce and Ni, have all been reported for this system, with Pd being used to advantage with pressurised air. A system using homogeneous Palladium acetate has also been reported for the aerobic oxidation of alcohols using pure oxygen.
The scientists who developed the latter system, working at the Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Japan, have now prepared and successfully used a Palladium(II) acetate-pyridine hydrotalcite catalyst to oxidise alcohols to aldehydes and ketones using air as sole oxidant at 65°C (N. Kakiuchi, Y. Maeda, T. Nishimura and S. Uemura, J. Org. Chem., 2001, 66, (20), 6620–6625).
Palladium(II) acetate-pyridine was readily supported on hydrotalcite, Mg6Al2(OH)16(CO3.4H2O, a clay mineral, to give a heterogeneous Palladium catalyst. With this catalyst alcohols could be oxidised at a lower temperature using air at atmospheric pressure instead of pure oxygen. A variety of primary and secondary alcohols, such as benzyl alcohol and dodecan-l-ol, in toluene were oxidised to the corresponding aldehydes and ketones in high yield. Allylic alcohols, such as geraniol and nerol, were effectively oxidised without any alkenic isomerisation. The catalyst can be easily recovered and reused several times and leaching of the Pd(II) species could be avoided.