Development of dehydrogenation reaction using palladium catalyst and ethylene system

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Recently reported oxidation process of alcohols using molecular oxygen as an oxidizing agent in the presence of catalytic amount of transition metals such as copper, ruthenium, and palladium compounds will be reviewed. Oxidation of alcohols based on hydrogen transfer reaction between alcohols and simple olefins such as ethylene is also described. That is, some benzylic and allylic alcohols can be converted into the corresponding carbonyl compounds using a catalytic amount of palladium on activated carbon (Pd-C) under ethylene atmosphere. The combination of Pd(OAc)2 and vinyl acetate also works efficiently. Selective oxidation of hydroxy groups on carbohydrates (D-glycals) and steroids as well as simpler systems has been achieved. Oxidative aromatization reaction has been also realized by Pd-C/acetic acid/02 system. 1,3,5-Trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines were converted into the corresponding pyrazoles and pyridines effectively by the treatment of a catalytic amount of Pd/C in acetic acid under oxygen atmosphere. Direct synthesis of 2-arylbenzoxazoles from 2-aminophenols and aldehydes was achieved in the presence of activated carbon (Darco KB) under oxygen atmosphere. These environmentally benign oxidation methods have potentialities to replace the conventional stoichiometric chromium and other metal oxidizing agents.