Platinum(IV) Oxide

PtO2

[1314-15-4]  · O2Pt  · Platinum(IV) Oxide  · (MW 227.08)

(catalyst for hydrogenation of various functional groups with minimal hydrogenolysis problems; used as dehydrogenation catalyst; used for selective oxidation of primary alcohols)

Alternate Name: Adams' catalyst.

Solubility: insol most organic solvents.

Form Supplied in: dark brown solid, usually hydrated with 2 or more mol of H2O.

Analysis of Reagent Purity: elemental analysis.

Handling, Storage, and Precautions: no precaution is necessary with the oxide, but after exposure to H2 the Pt black must be handled as a pyrophoric reagent; the general precautions for handling hydrogenation catalysts should be followed; 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.

Hydrogenations and Hydrogenolysis.

Platinum oxide has been used to catalyze the hydrogenation and hydrogenolysis of many functional groups. It has also found use in dehydrogenation and oxidation reactions. PtO2 is not an active catalyst, but in the presence of H2 the oxide is reduced to Pt black which is the active form. Pt black can also be made via other procedures.1 A more recent method for the preparation of Pt black via the reduction of chloroplatinic acid or PtO2 with Sodium Borohydride gives a catalyst with slightly increased activity.2 Unlike the reaction between NaBH4 and Nickel(II) Acetate, which generates Nickel Boride (Ni2B),3 the reaction with Pt gives only pure Pt black. This catalyst, which is generally prepared in situ and used without isolation, was shown to be slightly more active than the Pt black generated from PtO2 and H2 in the reduction of 1-octene.

The reduction of alkenes, catalyzed by PtO2, has been carried out in acidic, neutral, and basic conditions and, in some cases, different results are observed. In the example shown in eqs 1 and 2, stereoselective hydrogenation of a double bond to give the cis- or the trans-fused product was effected with PtO2 in either acidic or neutral media.4 With certain alkenic compounds, PtO2 acted as an isomerization catalyst and no reduction occurred (eq 3).5 Isomerization followed by hydrogenation to give a cis-trans mixture of dimethylcyclopentanes has also been observed (eq 4).6 The normally expected product is the cis isomer.

PtO2 is generally not a satisfactory catalyst for the reduction of alkynes to alkenes. Even when the reaction is stopped after 1 equiv of H2 is absorbed, a substantial amount of alkane has already been formed,7 but a number of exceptions have been reported.8 Lindlar catalyst (Palladium on Calcium Carbonate (Lead Poisoned)), Palladium on Barium Sulfate, nickel boride (P-2Ni), and Palladium on Poly(ethylenimine) are better catalysts for this task.

Amines can be obtained by reduction of several functional groups. Nitro compounds are readily hydrogenated to amines under PtO2 catalysis. The reduction of a benzylated nitro compound gave the aniline without loss of the benzyl ether functions (eq 5). Use of Pd/C gave the hydrogenolyzed product (eq 6).9 In general, Pt catalysts are used in place of Pd catalysts when hydrogenolysis is to be minimized.

Hydrogenation of nitrobenzene in the presence of Hydrogen Fluoride gave p-fluoroaniline.10 Hydrogenation of oximes is best carried out in acidic media to minimize secondary amine formation.11 A quantitative yield of the amine was obtained when 3-hydroxy-5-hydroxymethyl-2-methylpyridine-4-carbaldoxime was reduced in alcoholic Hydrogen Chloride with this catalyst.12 Amines can also be generated easily from azides,13 imines,14 and nitriles.15 Similar to oximes, hydrogenation of nitriles should be conducted in acidic solvents to minimize di- or trialkylamine formation.

Hydrogenation of ketones in the presence of this catalyst can be carried out in acidic, neutral, or basic media. In general, with cyclic ketones axial alcohols predominate in acidic media while equatorial alcohols are favored in neutral or basic media. Thus the stereochemical outcome of ketone reduction may be controlled by varying the pH of the solvent.16 In strong acids the reduction of dialkyl ketones in the presence of alcoholic solvents at rt with PtO2 leads to the formation of ethers instead of alcohols.17 It was suggested that acetals and enol ethers are generated under the reaction conditions before hydrogenation or hydrogenolysis. Although PtO2 is often used to minimize hydrogenolysis of functional groups such as alcohols and halides, the hydrogenolysis of ketones under acidic conditions can give methylenic products (eq 7).18

By converting enolizable ketones to the enol triflates, neutral conditions can be used to effect the overall hydrogenolysis of ketones (eq 8).19

Certain a-diketones can be selectively hydrogenolyzed to monoketones by conversion to cyclic unsaturated oxyphosphoranes followed by treatment with hydrogen in the presence of the catalyst (eq 9).20 The benzylic position, which is the more reactive site, was hydrogenolyzed selectively. Amide carbonyls are very difficult to reduce in general, but reduction has been observed in special cases (eq 10).21 In one report, an anhydride was reduced to an ether at 5000 psi H2 in HF.22

Benzyl groups attached to a heteroatom, such as oxygen and nitrogen, are removed using Pd catalysts, but the removal of phenyl groups of diphenyl phosphonates with these catalysts is ineffective. These phenyl groups can be hydrogenolyzed using PtO2, but high catalyst loading is necessary.23 p,p-Dinitrobenzhydryl has been used for the protection of alcohols. This protecting group can be selectively removed in the presence of other hydrogenolyzable protecting groups, such as benzyl and trityl, by the use of platinum oxide followed by mild acid hydrolysis (eq 11).24

Hydrogenolysis of the N-N bond of hydrazines,25 the N-O bond of nitrones,26 and the C-C bond in cyclopropanes27 are readily accomplished using this catalyst.

Aromatic ring reduction can be carried out with PtO2 in acidic solvents such as acetic acid and alcoholic or aqueous HCl.28 Complete ring reduction without the concomitant hydrogenolysis of benzylic alcohols has been accomplished.29 Reduction of phenols provides cyclohexanols30 and pyridines are reduced to piperidines.31 Imidazole is difficult to reduce, but this has been accomplished in the presence of Acetic Anhydride to give the diamide.32 Oxazoles are cleaved to amides (eq 12),33 and isoxazoles give enamino ketones (eq 13).34

Dehydrogenation.

PtO2 has also been used as a dehydrogenation catalyst. An example to obtain a pyridazine from a 1,4-diketone and hydrazine is shown below (eq 14).35

Oxidation.

The Pt black generated from prereduction of PtO2 with hydrogen, also catalyzes the oxidation of alcohols to carbonyl compounds with oxygen. Primary alcohols are preferentially oxidized to carboxylic acids in the presence of secondary alcohols. Amides, sulfonamides, and azides are stable under the oxidizing reaction conditions.36 Benzyldimethylamine can be oxidized to N-benzyl-N-methylformamide in 85% yield using benzene as the solvent.37 When water was used as the solvent, N-demethylation was observed instead.38

Pt black was also used in the electrolytic N-ethylation of benzylamine in ethanol to give N-ethylbenzylamine in 96% yield. The Pt black anode first oxidized the ethanol to acetaldehyde which in turn formed a Schiff base with the amine. H2 was generated at the Pt coil cathode and was picked up by the suspended Pt black for the hydrogenation of the imine.39

Irradiation of primary amines in the presence of a mixture of Pt black and TiO2 gave low yields of secondary amines (8-67%). For example, diethylamine was formed in 33% yield from ethylamine and 1,4-diaminobutane gave pyrrole in 67% yield.40


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