[7440-06-4] · Pt · Platinum on Alumina · (MW 195.08)
Solubility: insol all organic solvents.
Form Supplied in: typically supplied as 0.5-5 wt % of Pt on alumina, but higher metal content may be custom made; dark brown to black powder or pellets.
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.
The use of Pt/Al2O3 is not as widespread as Platinum on Carbon or Platinum(IV) Oxide, but like the latter two reagents, Pt/Al2O3 has been used for certain selective reactions, including hydrogenation and dehydrogenation reactions. The use of Pt/Al2O3 for the reduction of a diene to a trans-decalin was shown to give the highest selectivity (eq 1).1
The formation of residual monoenes and the cis product was minimized with this catalyst. Pt/Al2O3 was also used in the reduction of an a,b-unsaturated enone to an alcohol (eq 2).2 The stereochemical course of this reduction was attributed to an alumina-alcohol interaction.
A tetrasubstituted enamine has been hydrogenated to the amine in good yield with no hydrogenolysis of the N-benzyl group using Pt/Al2O3 (eq 3).3 Similar to other Pt catalysts, hydrogenolysis is not common with this catalyst.
A cis-1,2-diamine has been obtained using Pt/Al2O3 as the hydrogenation catalyst (eq 4).4 The allylic pyrrolidine directed the hydrogenation to give the cis product.
Heterocyclic ring reductions have been reported and quinoline is reduced selectively at the pyridine ring.5 Even with a 2,4-disubstituted quinoline, only reduction of the substituted pyridine ring in the quinoline system occurred when Carbon Disulfide was used as a poison. Without the poison the selectivity was only 88% (eq 5). Acridine and indole can also be hydrogenated exclusively in the heterocyclic ring.
Pt/Al2O3 has been used to catalyze the formation of halogenated compounds. Chlorination of methane with chlorine has been catalyzed by Pt/Al2O3 to give Chloromethane with 99% selectivity. The minor product (1%) was dichloromethane.6 Another halogenation reaction involving Pt/Al2O3 is the decarboxylation of aryl fluoroformates at high catalyst loading and temperature to give acyl fluorides (eq 6).7
Dehydrogenation in the presence of Pt/Al2O3 has given various heterocyclic aromatics and hydrocarbon derivatives. Cyclohexane, cyclohexene, cyclohexylamine, and cyclohexanol have been aromatized to give benzene in 100% yield. Toluene has been obtained from methylcyclohexane, 1-methylcyclohexene, 2-methylcyclohexanone, and 3-methylcyclohexanone. Decalin and tetralin produced naphthalene in quantitative yield. N-Methylpyrrolidine gave N-methylpyrrole in 81% yield.8 Pyridine derivatives have been obtained from dihydropyrans with NH3 in the presence of Pt/Al2O3 (eq 7).9
The last C-C bond coupling in the preparation of dodecahedrane was effected with Pt/Al2O3 mixed with a reactive Ti0 metal.10 Other substituted dodecahedranes were also prepared via this method.
Anthony O. King & Ichiro Shinkai
Merck & Co., Rahway, NJ, USA