The Ru-Cu-Al-Hydrotalcite-Catalysed Oxidation of Alcohols to Aldehydes or Ketones

Abstract

A new heterogeneous ruthenium-copper-hydrotalcite catalyst for the efficient and selective conversion of a variety of aliphatic, allylic and aromatic alcohols to either aldehydes or ketones is described. Suitable co-oxidants include iodosylbenzene, tetrabutyl ammonium periodate and, depending on substrate, oxygen.Heteronuclear catalysts have often been found to be more active than their individual mononuclear components. 1 A classical example is the Pd/Cu/O 2 -based Wacker process for the industrial preparation of acetaldehyde. 2 To our knowledge there has been a single prior report of a bimetallic Ru/Cu system for the partial oxidation of alcohols. The report describes a mixture of mononuclear ruthenium and copper containing compounds which shows fairly low rates of conversion and selectivity -possibly because the Ru and Cu are not in close proximity. 3 This prompted us to investigate a new Ru/Cu-containing hydrotalcite system as a catalyst for the oxidation of alcohols -a process of key importance in synthetic organic chemistry. The hydrotalcite permits the bringing together of different metals within the same compound, whilst it also contains interlayer spaces which are reactive environments. Some previous applications of hydrotalcite-type anionic clays include uses as catalysts for the polymerisation of alkene oxides, aldol condensation, reforming, alcohol synthesis, methanation, Baeyer-Villiger oxidations and the photoxidation of isopropyl alcohol to acetone. 4,5 An advantage of the hydrotalcite catalysts is that they are heterogeneous catalytic systems, thus permitting the easy removal of the catalysts from the reaction medium. Whilst a number of efficient homogeneous ruthenium catalysts for the mild oxidation of alcohols are known, 6,7 difficulties with product separation remain and often the expensive catalyst cannot be re-used after workup. The Ru-Cu-Al-hydrotalcite (Ru-Cu-HT) was prepared by adding a solution of RuCl 3 .3H 2 O (0.40 g, 1.53 mmol), CuCl 2 ·6H 2 O (2.61 g, 15.3 mmol) and AlCl 3 ·6H 2 O (1.23 g, 5.1 mmol) in water (10 mL) to a solution of Na 2 CO 3 (1.10 g) in 46.4 mL 1 M NaOH. The mixture was stirred at 65 °C for 18 h, after which the green product was filtered off, washed with water and dried at 110 °C for 12 h. The product was ground into a powder before use. EDS confirmed the presence of Ru, Cu and Al and flame emission spectroscopy showed the ruthenium content to be 6.4%.Typical oxidation procedures involved adding activated 4 Å molecular sieves and toluene (6 mL) to a Schlenk tube, after which the substrate (0.478 mmol) and isobutyl-methacrylate (80 mL) as internal standard were added. For the reactions using O 2 as co-oxidant, the reactions were carried out at 60 °C in O 2 saturated toluene under an atmosphere of oxygen. Alternatively, either iodosylbenzene (PhIO) or tetrabutylammonium periodate (TBAP) (0.717 mmol) was added as co-oxidant and the reactions carried out in N 2 saturated toluene at 60 °C under nitrogen. The hydrotalcite (250 mg) was added last in all cases. The reactio...

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Oxidation of Alcohols with Molecular Oxygen

“…Cu, Mn, Co, Cr, Zn, Fe) into hydrotalcites [179][180][181]. One promising new catalyst is Ru − Mn/HT prepared by the consecutive immobilization of Ru and Mn cations on hydrotalcite [182].…”