Palladium and Platinum Catalysts

Broadly speaking, the differences in effectiveness of palladium and platinum catalysts are very small the choice will generally be made on the basis of availability and current price of the two metals. Charcoal is a somewhat more efficient carrier than asbestos. 

Babcock et al. (Bl) examined the hydrogenation of a-methylstyrene catalyzed by palladium and platinum catalysts in a reactor of 1 -in. diameter under countercurrent flow. Flow rates were above 1500 kg/m2-hr for the liquid phase and above 15 kg/m2-hr for the gas, and it was concluded from the experimental results that mass transfer was not of rate-determining influence under these conditions. 

However, cleavage of the sterically less hindered bond was the major reaction pathway in the hydrogenolysis of propylcyclopropane on palladium and platinum catalysts (Scheme 4.134).511... 

Yields of cis-isomer in the hydrogenation of 1,4-dialkylcyclohexenes over rhodium, palladium and platinum catalysts at atmospheric pressure and 25°C in acetic acid solution... 

EMDE DEGRADATION. Modification of the Hofmann degradation method for reductive cleavage of the carbon-nitrogen bond by treatment of an alcoholic or aqueous solution of a quaternary ammonium halide with sodium amalgam. Also used as a catalytic method with palladium and platinum catalysts. The method succeeds with ring compounds not degraded by the Hofmann procedure. 

The benzyl and phenyl groups were removed by hydro-genolysis over palladium and platinum catalysts, respectively, and the acetyl groups by treatment with ammonia in methanol. In this case too, the monoammonium salt of the phosphorylated disaccharide 2 was isolated physical and chemical parameters of the compounds obtained by both methods were identical. 

Recently, Rooney and co-workers (23,58,59) have questioned the view that triadsorption by loss of 3 hydrogen atoms from the alkane is the minimum requirement for bond-shift reactions. They studied the isomerization of a series of caged hydrocarbons in excess hydrogen on palladium and platinum catalysts. The compounds were chosen in order to render difficult or totally exclude a mechanism involving aory-triadsorbed species. Thus, l,7,7-trimethyl[2,2,l]-heptane interconverts with its endo- and exo-2,3,3-trimethyl isomers, bicyclo-[3,2,2] octane changes to bicyclo[3,3,l] nonane, and protoadamantane to... 

The electron-releasing 4-substituents are considered to stabilize the 3,4 double bond toward hydrogenation. An electron-releasing 7-substituent (OMe, NH2, or OH) may also contribute to the stabilization of the 3,4 double bond. Thus the hydrogenation of 7-methoxy-4-methylquinazoline to the 3,4-dihydro derivative was successful only in aqueous hydrochloride or aqueous acetic acid, using palladium and platinum catalysts (eq. 12.84).158 No reduction took place at room temperature in nonaqueous solvents such as ethanol, ethanol-HCl, and acetic acid. 

Scheme 13.21 Hydrogenation pathways of 3-carene over palladium and platinum catalysts.

Measurements of competitive hydrogenation of butadiene into butenes on palladium and platinum catalysts supported on silica [30] allowed the determination of the relative adsorption coefficients of dienes and olefins. 

Quadricyclene, although stable at higher temperatures (ti/2 > 14 hours at 140°) (23), undergoes facile ring opening at —26° ([Pg.304]

Fio. 12. The reaction paths postulated by Smith and Swoop for oyclohexene deuterium exchange over charcoal-supported palladium and platinum catalysts 42). 

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