Palladium dehydrogenation catalyst

Another appHcation for this type catalyst is ia the purification of styrene. Trace amounts (200—300 ppmw) of phenylacetylene can inhibit styrene polymerization and caimot easily be removed from styrene produced by dehydrogenation of ethylbenzene using the high activity catalysts introduced in the 1980s. Treatment of styrene with hydrogen over an inhibited supported palladium catalyst in a small post reactor lowers phenylacetylene concentrations to a tolerable level of <50 ppmw without significant loss of styrene. 

Hydroxy-5-MC (29a) was itself prepared (87) by a modification of the synthetic sequence employed in the preparation of 2-chrysenol (Figure 13). Reaction of 2-(l-(3-methylnaphthyl) ethyl bromide with the 1-lithio salt of 1,5-dimethoxycyclohexa-1,4-diene furnished the diketone 8a (Figure 18). Cyclization of the latter in polyphos-phoric acid afforded the 1-keto derivative which underwent dehydrogenation over a palladium catalyst to yield 29. 

The electrocyclic reaction of divinylindoles to functionalized carbazole derivatives has emerged as an efficient method for the synthesis of a diverse range of carbazole derivatives. This reaction has been achieved at high temperatures in the presence of palladium on charcoal or DDQ. The palladium catalyst or DDQ act as dehydrogenating agent (534,535) (Scheme 5.19). 

The palladium catalyst is prepared according to Organic Syntheses, Coll. Vol. 3, 686 (1955). It can be reused for the dehydrogenation of 4,5,6,7-tetrahydroindazole to indazole. The yield is somewhat better with used catalyst than with fresh catalyst. 

Palladium - carbon catalysts, 948, 950 Palladium catalysts, for dehydrogenation, 948... 

Among the important reagents for which preparative procedures are given are 2,2 -bipyridine (by nickel directed and catalyzed dehydrogenation of pyridine p. 5), formamidine acetate (p. 39), phenyltrichloromethylmercury (p. 98), and trimethyl- and triethyloxonium fluoroborate (pp. 120, 113). The preparation of palladium catalyst ( Lindlar ) for the selective reduction of acetylenes is described (p. 89), as is the use of di-phenyliodonium-2-carboxylate, as a precursor of benzyne in the synthesis of 1,2,3,4-tetraphenylnaphthalene (p. 107). 

A series of l,2,4-triazino[5,6-c]quinolines have been prepared via a common route. Thus, 4-[fi-(ethoxymethylene)hydrazino]-3-nitroquinoline (244), prepared from 4-hydrazino-3-nitroquinoline (243) by reaction with triethyl orthoformate, was hydrogenated in ethanol over a palladium catalyst to give the dihydro compound (184) (96%). Dehydrogenation yielded the desired compound (185) (90%) (Scheme 19) <73GEP(0)2322394>. 

Harlay (84) has used Raney nickel to dehydrogenate dihydropapaverine to papaverine in 50% yield. He found Raney nickel to be more satisfactory for this purpose than the nickel of Sabatier and Senderens, but not as effective as a palladium catalyst. Mosettig and Duvall (85) used Raney nickel to transform the tetrahydrophenanthrone-1 and -4 into the respective phenanthrols at the boiling point of benzene, but also found this catalyst less advantageous than palladium. 

The unreacted cyclohexane is distilled off and recycled. The ca. 1 1 mixture of cyclohexanol and cyclohexanone is then subjected to dehydrogenation over a palladium catalyst (the same catalyst as is used in phenol hydrogenation) to give pure cyclohexanone. 

Alkylated triazines of the current have also been prepared by heating melamine or melamine derivative and an alcohol in the presence of a hydrogenation-dehydrogenation mixed catalyst such as 5% palladium on carbon (50% hydrated) with 25 mg of Cu0/Cr203/Ni0/Ba0 and is reviewed (4). 

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