[7440-06-4] · Pt · Platinum on Carbon · (MW 195.08)
Solubility: insol all organic solvents and aqueous acidic and basic media.
Form Supplied in: black powder and pellets containing 0.5-10 wt % of Pt (typically 5 wt %); can be either dry or moist (50 wt % of H2O).
Analysis of Reagent Purity: atomic absorption.
Handling, Storage, and Precautions: can be stored safely in a closed container under air but away from solvents and potential poisons such as sulfur- and phosphorus-containing compounds; pyrophoric in the presence of solvents; general precautions for handling hydrogenation catalysts should be followed; the catalyst must be suspended in the organic solvent under an atmosphere of N2; during filtration the filter cake must not be allowed to go dry; if a filter aid is necessary, a cellulose-based material should be used if catalyst recovery is desired.
In many instances where Platinum(IV) Oxide is used, Pt/C can be a less expensive substitute. Pt/C is a widely used catalyst for many applications. The reduction of alkenes and alkynes can be carried out under mild conditions. Alkenes can be hydrogenated in the presence of terminal alkynes if the latter are protected with a trialkylsilyl group. Without protection the selectivity is not satisfactory (eq 1).1
The diastereoselective reduction of an enamino ketone with Pt/C under acidic conditions is shown in eq 2. This reaction provided the (SSR-R) diastereomer in 63% yield plus 15-18% of the (RRS-R) diastereomer and other byproducts. After acid-catalyzed lactonization, the desired lactone was conveniently isolated in 57% yield. This intermediate was used in a total synthesis of (+)-thienamycin (eq 2).2
Hydrogenolysis of cyclopropanes has been accomplished with this catalyst at high temperatures. The example in eq 3 shows the resilience of a cyclobutane to these reaction conditions.3
The reduction of a nitro group in the presence of an aryl bromide (or chloride) can be accomplished using this catalyst. Aryl fluorides are generally stable to all catalysts. In many instances the amine products undergo further cyclization reactions to form a variety of heterocyclic compounds. The bromide was hydrogenolyzed when Pd/C was used under similar conditions (eq 4).4
Varying the pH of the media has been shown to affect the product outcome in a reduction of a nitro amino acid. A 1-hydroxycarbostyril derivative was obtained under acidic conditions, whereas a lactam was produced under basic conditions (eq 5).5
Reduction of aliphatic nitriles under neutral conditions in the presence of Pt/C gave trialkylamines with high selectivity.6 With aromatic nitriles, dibenzylamines were obtained.7 This can be used to advantage in the preparation of unsymmetrical amines. Thus the reduction of a nitrile, in the presence of a different amine provided the unsymmetrical dialkylamine with greater than 95% selectivity.8 If the desired product is the primary amine, acidic conditions should be used.
Imines can be reduced selectively in the presence of aryl bromide and chloride and hydrazine functions using Pt/C.9 Reductive amination between a ketone and an amine or amine precursor proceeds to give a good yield of dialkylamine.10 Amines can also be formed from the reduction of azides and hydrazines.9b,11 Hydrogenolysis of C-N s-bonds with Pt/C is uncommon, but not unknown. For example, hydrogenolysis of decahydro-1,8-naphthyridines in the presence of this catalyst gave 3-(g-aminopropyl)piperidines (eq 6).12
Pt catalysts are used for the reduction of ketones to minimize hydrogenolysis, especially in the case of aromatic ketones. The solvents used for ketone reduction are important. In the case of Ethyl Acetoacetate, the Pt/C-catalyzed hydrogenation in water gave 12% hydrogenolyzed product, but no hydrogenolysis was observed in THF. The hydrogenolysis side reaction in water can be completely suppressed by the addition of a small amount of zinc acetate.13 Under acidic conditions, dialkyl ketones have been hydrogenolyzed to the hydrocarbon products, but these conditions are impractical for acid-sensitive substrates. The conversion of ketones to the corresponding enol phosphates followed by hydrogenolysis over Pt/C is a mild and efficient route for the overall removal of an oxygen (eq 7).14
O-Acetyl sugar lactones have been cleanly converted to the corresponding 2,3-dideoxy sugar derivatives under mild conditions with Pt/C. A trialkylamine base was necessary for the hydrogenolysis to proceed. The mechanism may involve the alkenic intermediates (1) and (2) (eq 8). This procedure is only applicable to lactones since no deacetylated product was observed with aliphatic and cyclohexyl enol acetates (3) and (4).15
The Pt/C-catalyzed hydrogenation of 4,4-difluorocyclohexadienone provided b-fluorophenol in 90% yield. The dienone had been prepared from phenol via an oxidative fluorination method.16
Although Pt/C has been used for the hydrogenation of aromatic rings, PtO2 is usually the catalyst of choice. High selectivity (>95% cis) was observed in the hydrogenation of pyrroles at atmospheric pressure and rt. Under these conditions, both the phenyl ring and the benzyl ether remained intact (eq 9).17
Pt/C is an effective catalyst for dehydrogenation of reducing sugars under basic conditions. For example, glucose was oxidized to the carboxylic acid in excellent yield (eq 10).18
The mild reaction conditions under which reducing sugars are dehydrogenated with Pt/C also makes them good transfer hydrogenation agents. For example, fructose has been reduced to a mixture of glucitol and mannitol (0.77:1) in the presence of glucose and Pt/C.
The selective oxidation of benzylic alcohols to aldehydes by oxygen and Pt/C in the presence of Cerium(III) Chloride and Bi2(SO4)3 cocatalysts has been reported. The selectivity to the aldehyde was excellent. Without the cocatalysts, carboxylic acids were obtained (eq 11).19
Anthony O. King & Ichiro Shinkai
Merck & Co., Rahway, NJ, USA