Hydrogenation and Hydrogenolysis. XIII. The Hydrogenation of Ethyl 4-Methyl-1-cyclohexenyl Ether over Platinum Metal Catalysts

The hydrogenation and hydrogenolysis of ethyl 4-methyl-1-cyclohexenyl ether (I) have been investigated with six unsupported platinum metals as catalysts at 25°C under atmospheric pressure of hydrogen. The hydrogenation of I in ethanol was accompanied by the formation of 4-methylcyclohexanone diethyl acetal (II), the extent of which depended on the nature of the catalyst metals. The ratio of the acetal formation to hydrogenation increased with respect to the catalyst metals in the sequence Os<Ru<Ir<<Rh<Pd<Pt. The amount of the hydrogenolysis to give methylcyclohexane increased in the order Pd≅Ru<<Os<Rh<Ir<<Pt at an initial stage of the hydrogenation in ethanol. This order in hydrogenolysis was explained in terms of the action of the ionized hydrogen on the catalyst surface and the olefin isomerization activity of the catalyst metals. The stereochemistry of the hydrogenation of I was also studied and the results were interpreted on the basis of the proposed mechanism. I was also hydro-genated in t-butyl alcohol and isopropyl ether as solvents and, except in a few cases, the proportion of hydrogenolysis decreased with solvent in the order ethanol>t-butyl alcohol>isopropyl ether. The stereoselectivity in ether formation did not differ much among the solvents except over palladium.