Partial Hydrogenation of phenols to Cyclohexanones

An important industrial synthesis of cyclohexanone is by partial hydrogenation of phenol over palladium, carried out in either liquid or vapor phase. 

Because of the industrial magnitude of these processes, many catalysts have been examined with variations in metal distribution, pore size, and alkalinity. In most synthetic work where catalyst life and small variations in yield are not of great importance, most palladium-on-carbon or -on-alumina powder catalysts will be found satisfactory for conversion of phenols to cyclohexanones. Palladium has a relatively low tendency to reduce aliphatic ketones, and a sharp decrease in the rate of absorption occurs at about 2 mol of consumed hydrogen. Nickel may also be used but overhydrogenation is more apt to occur. 

Examples of palladium-catalyzed reduction are 4-chloro-2,6-di-r-butyl-phenol to 2,6-di-t-butylcyclohexanone (750 psig, 25 C) with loss of halogen 24), 1,8-dihydroxynaphthalene to 8-hydroxy-1-tetralone 30), and 2,4-dimethylphenol to 2,4-dimethylcyclohexanone (27). 

Palladium may not be preferred for dihydric aromatics. Dihydroresorcinol was obtained in 87% yield by alkaline hydrogenation of resorcinol over 5% Rh-on-C (65) and in only 50-60% yield over Pd-on-C 15). A 77% yield of the dione was obtained by alkaline hydrogenation of 3,5-dihydroxyphenylacetic acid over a 10% loading of 5% Rh-on-AljOj at 55 C and 55 psig (39a). 

Hydrogenolysis, without ring reduction, of the carbon-oxygen bond in phenols cannot be depended on, but by conversion of the phenol to a better leaving group, such as is formed by interaction of the phenol with 2-chlorobenzoxazole, l-phenyl-5-chlorotetrazole, phenylisocyanate, 

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