Platinum-palladium hydrogenolysis

Platinum, palladium, and rhodium will function well under milder conditions and are especially useful when other reducible functions are present. Freifelder (23) considers rhodium-ammonia the system of choice when reducing -amino nitriles and certain )5-cyano ethers, compounds that undergo extensive hydrogenolysis under conditions necessary for base-metal catalysis. 

A generalization derived from many studies is that the tendency toward hydrogenolysis of vinylic and allylic oxygen, nitrogen, and halogen increases in the order ruthenium rhodium palladium < platinum < irridium. In vinylic systems especially, platinum favors hydrogenolysis more than palladium in allylic systems, the trend is not so clear but appears to be the reverse (26). Iridium is placed in this sequence on the basis of limited data. 

Partial reduction of phenols affords mixtures of allylic and vinylic alcohols. From the generality derived for aliphatic systems, the most hydrogenolysis of this mixture is expected with platinum, palladium, and iridium catalysts, and much less with rhodium and ruthenium, an expectation substantiated in practice. For example, hydrogenation of resorcinol in neutral medium affords 20, 19, and 70% cyclohexanediol over palladium-, platinum-, and rhodium-on-carbon, respectively (29). Many examples attest to the value of rhodium and ruthenium at elevated pressure in avoiding hydrogenolysis. 

Rylander and Himelstein studied the hydrogenation of resorcinol over supported platinum, palladium, rhodium, and palladium-rhodium as catalysts in ethanol at 65°C and an initial hydrogen pressure of 0.34 MPa. Extensive hydrogenolysis to give cyclohex -anol took place over 5% Pt-C, 5% Pd-C, and 2.5% Pd-2.5% Rh-C, and the yields of cyclohexane-1,3-diol were only 18.7-26.2%. Rhodium catalysts caused hydro-... 

Platinum, palladium, and nickel catalysts cleave the oxolanes in the sterically less hindered position Cu is inactive toward these molecules. On supported Pt the rupture of the secondary C-0 bond was also observed [24]. Ring-opening is much easier than the hydrogenolysis of open chain ethers, presumably because of the presence of specially bonded aySy-intermediates [25], Neither on Pt nor on Ni is any difference observed between the rates of transformation of the isomers among the dimethyl derivatives of oxolanes [26]. 

Catalytic hydrogenation of alkynes takes place in a stepwise manner, and both the alkene and the alkane can he isolated. Complete reduction of alkynes to the saturated compound is easily accomplished over platinum, palladium or Raney nickel. A complication which sometimes arises, particularly with platinum catalysts, is the hydrogenolysis of hydroxyl groups a- to the alkyne (propargylic hydroxyl groups) (7.15). 

To confirm this reasoning, hydrogenations of humulone have been carried out with platinum, palladium and rhodium catalysts at several pH values. A linear relationship between the pH and the logarithm of the percentage hydrogenolysis has been determined experimentally. At a given pH value, palladium catalysts are more active than platinum or rhodium catalysts. This is in accordance with the theoretical requirements for the intervention of a hydride ion (12). 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

Over palladium this cleavage occurs in preference to the saturation of a 5,6-double bond. " The use of platinum in acetic acid allows saturation of the A -olefin in (36) without hydrogenolysis of the 22,23-dibromides. 

Another example is the hydrogenation of the homoallylic eompound 4-methyl-3-cyclohexenyl ethyl ether to a mixture of 4-methylcyclohexyl ethyl ether and methylcyclohexane. The extent of hydrogenolysis depends on both the isomerizing and the hydrogenolyzing tendencies of the catalysts. With unsupported metals in ethanol, the percent hydrogenolysis decreased in the order palladium (62.6%), rhodium (23 6%), platinum (7.1%), iridium (3.9%), ruthenium (3.0%) (S3). 

Hydrogenation of carbonyls, or incipient carbonyls such as phenols (86), in lower alcohol solvents may result in the formation of ethers. The ether arises through formation of acetals or ketals with subsequent hydrogenolysis. The reaction has been made the basis of certain ether syntheses (45,97). Reaction of alcohols with carbonyls may be promoted by trace contamination, such as iron in platinum oxide (22,53), but it is also a property of the hydrogenation catalyst itself. So strong is the tendency of palladium-hydrogen to promote acetal formation that acetals may form even in basic media (61). 

Platinum may be more useful than palladium in reduction of nitro compounds containing functions easily reduced by palladium. Hydrogenation of I over 5% Pd-on-C was nonselective with hydrogenolysis of the benzyl ethers competing with nitro hydrog ation, but over PtO in ethanol 2 was obtained in 96% yield (4). 

Anilines have been reduced successfully over a variety of supported and unsupported metals, including palladium, platinum, rhodium, ruthenium, iridium, (54), cobalt, and nickel. Base metals require high temperatures and pressures (7d), whereas noble metals can be used under much milder conditions. Currently, preferred catalysts in both laboratory or industrial practice are rhodium at lower pressures and ruthenium at higher pressures, for both display high activity and relatively little tendency toward either coupling or hydrogenolysis,... 

Aziridines, like oxiranes, undergo hydrogenolysis easily with or without inversion of configuration, depending on the catalyst, reaction parameters, and various additives 65aJ08). For example, hydrogenolysis of 2-methyl-2-phenylaziridine in ethanol occurs mainly with inversion over palladium but with retention over platinum, Raney nickel, or Raney cobalt. Benzene solvent or alkali favor retention over palladium as well. 

Palladium is usually used in the hydrogenolysis of isoxazoles, but platinum and nickel have been used successfully. The rate of hydrogenolysis may be affected markedly by the pH (104). Neutral or alkaline media are frequently used. 

Extreme differences between 5% palladium-on-carbon and platinum oxide were found on reduction of the 5-aryl substituted oxazole 14. Over palladium, 15 was formed in quantitative yield by hydrogenolysis of the benzyl hydroxyl, whereas over Pt, scission of the oxazole occurred to give 13 quantitatively (48). Hydrogenation of 15 over platinum oxide gave the phenethylamide 16. 

A more expected difference between platinum oxide and palladium-on-carbon was found in the hydrogenolysis of 5-phenyI-2-(3,4-dimethoxybenzyI)-2-oxazoline. Cleavage occurred at the benzyl-oxygen bond over both catalysts, but over platinum, the less substituted phenyl group was saturated as well (78). 

If saturation occurs first, the product will be relatively stable toward further reduction but if hydrogenolysis occurs first, the resulting olefin is readily reduced. This ratio depends greatly on substrate structure, the catalyst, and environment. Hydrogenolysis is best achieved over platinum, whereas palladium (77a,82a,122bJ62a), rhodium (I09a), or ruthenium (I0a,I09a) tend to favor olefin saturation. 

The catalyst exerts some influence on the bonds broken in hydrogenolysis of saturated cyclopropanes (775), but in vinyl and alkylidene cyclopropanes the effect is pronounced. Platinum or palladium are used frequently. In one case, Nishimura s [124a) catalyst, rhodium-platinum oxide (7 3), worked well where platinum oxide failed (.75). An impressive example of the marked influence of catalyst is the hydrogenation of the spirooctane 42, which,... 

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

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