Pd-catalyzed transfer hydrogenolysis of benzylic alcohol in acidic 2-propanol

The hydrogenolysis of 1-phenylethanol over supported palladium catalyst (Pd/SiO2, Pd/C, Pd/Al2O3, Pd/MgO, or Pd/hydroxyapatite) in 2-propanol under reflux conditions (82 °C) was investigated. The effects of nine diffrent acid additives (formic acid, acetic acid, lactic acid, pyruvic acid, oxalic acid, malonic acid, succinic acid, malic acid, or p-toluenesulfonic acid) were also examined. Ethylbenzene and acetophenone were both produced in the presence of the acid additives; however, only acetophenone was produced in the absence of acid additive. Selectivity for ethylbenzene tended to increase with increasing acid strength of the additive, although there were some exceptions. The highest ethylbenzene selectivity (73.6%) was achieved over Pd/C catalyst in the presence of formic acid, and moderate selectivity (43–46%) was achieved over all of the Pd catalysts in the presence of oxalic acid. Deuterium-labeling experiments revealed a reaction pathway involving a hydrogen-transfer process, in which 2-propanol, formic acid, and 1-phenylethanol function as hydrogen donors.