Hydrogenation and Hydrogenolysis. XIV. The Hydrogenation of Ethyl p-Tolyl Ether over Platinum Metal Catalysts

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Abstract The hydrogenation of ethyl p-tolyl ether (1) in ethanol has been studied with five unsupported platinum metals as catalysts, mainly with the purpose of estimating the yields of ethyl 4-methyl-1-cyclohexenyl ether (2) formed as intermediate. Except for ruthenium catalyst, the hydrogenation of 1 in ethanol was accompanied by the formation of acetal 3, ketone 4 and alcohol 5 which seem to be related to the formation of 2. The amounts of 2 and the related compounds which were desorbed from the catalyst surface have been estimated to be 32–33% for Ru, 32–34% for Rh, 72–76% for Pd, 3.7% for Ir and 4.7% for Pt, by application of both kinetic and extrapolation methods. A good agreement was obtained between the results from the two methods. The proportion of hydrogenolysis increased for the catalyst metals in the order Pd<Ru<<Rh<Ir<Pt. With platinum catalyst a noticeable amount of toluene was formed along with methylcyclohexane. The hydrogenation of ethyl 4-methyl-3-cyclohexenyl ether (6), one of the other intermediates of the hydrogenation of 1, has also been studied in ethanol. Hydrogenolysis occurred extensively with palladium and rhodium catalysts, but much less with ruthenium, iridium and platinum catalysts. The hydrogenation route of 1 and its relation to hydrogenolysis and stereochemistry of hydrogenation of 1 are discussed with reference to the formation and hydrogenation of its intermediates.