[7740-06-4] · Pt · Oxygen-Platinum Catalyst · (MW 195.08)
(selective oxidant; highly regioselective oxidant for cyclic polyols; lactones obtained from diols; primary alcohols oxidized to carboxylic acids)
Solubility: reaction usually heterogeneous.
Form Supplied in: fine powder; widely available.
Preparative Methods: various methods1 are employed, depending on the catalyst: for Pt/C, charcoal, concentrated HCl, and aqueous chloroplatinic acid are reduced with hydrogen or formaldehyde; Adams catalyst (PtO2) is hydrogenated in the solvent to be used for the oxidation; after purging hydrogen, store under chosen solvent.
Purification: dry at 50 °C under reduced pressure.
Handling, Storage, and Precautions: avoid sources of ignition; take precautions against static discharge.
Oxygen (often as air) and a platinum catalyst are used in the oxidation of primary alcohols to yield usually carboxylic acids (eq 1)2 and for oxidation of secondary alcohols. It has been widely used in the selective oxidation of carbohydrates and related systems, and some general guidelines have been developed. Primary alcohols are oxidized more rapidly than secondary alcohols; for secondary cyclic alcohols, axial hydroxyl groups are oxidized more rapidly than equatorial hydroxyl groups.
Aldonic acids are produced by oxidation of aldoses (eq 2)3 and ketoses undergo reaction at the primary position (eq 3).4 Oxidation of reducing sugars at positions other than the
aldehyde position is simply achieved by protection of this position as the corresponding acetal or acetonide, and, as expected, oxidation of the next most reactive position takes place (eq 4).5 For deoxyamino sugars, the amino function is usually protected as a suitable acyl derivative (eq 5).6
A considerable amount of work has been carried out on the oxidation of cyclitols with oxygen-platinum catalysts1 and a set of general rules has been developed: (a) only axial hydroxyl groups are oxidized; (b) oxidation stops at the monoketone even when more than one axial hydroxyl is present; (c) if several nonequivalent axial hydroxyl groups are present, then usually oxidation of only one of these will take place. These general rules are exemplified by eqs 6 and 7.7
The oxidation of 1,4-diols in which one of the hydroxyls is primary gives the corresponding lactones, a reaction which has found use in natural product synthesis,8 including hydroazulenes (eq 8)9 and prostaglandins (eq 9).10
It has been shown that visible light and a catalyst of H2PtCl6-CuCl2 (1:2) can be used to oxidize a series of alcohols,1c and that the oxidation of reducing sugars does not need oxygen if platinum catalysts are used under basic conditions.11 12a-Hydroxylation of a deoxytetracycline has been carried out using oxygen and a platinum catalyst,12 although the usefulness this reaction appears to depend strongly on the structure of the substrate.13
University of Salford, UK