Rhodium platinum oxide, reductions

Reduction of aromatic nitro group takes preference to the reduction of the aromatic ring. Under certain conditions, however, even the benzene ring was reduced. Hydrogenation of nitrobenzene over platinum oxide or rhodium-platinum oxide in ethanol yielded aniline while in acetic acid cyclohexylamine was produced [55]. Heating of nitrobenzene with formic acid in the presence of copper at 200° gave a 100% yield of aniline, whereas similar treatment in the presence of nickel afforded 67% of cyclohexylamine [71]. 

Reduction of carbocyclic rings in aromatic ketones can be accomplished by catalytic hydrogenation over platinum oxide or rhodium-platinum oxide and takes place only after the reduction of the carbonyl group, either to the alcoholic group, or to a methylene group [5S]. 

Nowadays, rhodium or ruthenium are often the preferred catalysts. Rhodium can be used under mild conditions, whereas ruthenium needs elevated pressures. If pressure is available, it might as well be used even with rhodium, for increased pressure makes more efficient use of the catalyst, as well as decreases whatever hydrogenolysis might occur at lower pressure. Rhodium 7,8,12 20,21,38,39,45,65,66,68,69,75) and ruthenium 18,26 8,52,68,69,72,74) are especially advantageous in reductions of sensitive phenols and phenyl ethers that undergo extensive hydrogenolysis over catalysts such as platinum oxide. 

Hydrogenation of 3-pyridinecarboxylic acids is apt to be accompanied by extensive decarboxylation (2S), but this unwanted reaction can be prevented by carrying out the reaction in the presence of one equivalent of base (33,79). Ruthenium (33), rhodium (29), platinum oxide (2S,59), and palladium (30) have all proved effective catalysts for reduction of pyridinecarboxylic acids to the saturated acid. 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

Vinylogs of benzylic alcohols, e.g. cinnamyl alcohol, undergo easy saturation of the double bond by catalytic hydrogenation over platinum, rhodium-platinum and palladium oxides [39] or by reduction with lithium aluminum hydride [609]. In the presence of acids, catalytic hydrogenolysis of the allylic hydroxyl takes place, especially over platinum oxide in acetic acid and hydrochloric acid [39]. 

Figure 5.20 Cyclic voltammograms for platinum, palladium, rhodium, and gold electrodes in 1 M H2S04 f , cathodic current due to oxide reduction ( , anodic current due to oxide formation g , cathodic current due to H2 formation Q, anodic current due to H2 oxidation.

In general, carboxylic acids are hydrogenated with difficulty under mild conditions over usual metallic catalysts. However, it has been recognized that acetic acid may be reduced over platinum oxide under very mild conditions in the presence of perchloric acid.1 Kaplan observed that acetic acid was reduced rapidly over rhodium catalysts at room temperature and pressure, but the reduction stopped abruptly after a very small conversion.2 The occurrence of these reductions may lead to appreciable errors in the amounts of absorbed hydrogen when hydrogenations are carried out in acetic acid as solvent, although the reductions usually proceed only to limited extents, probably due to formation of some poisonous products. These undesired reductions of acetic acid solvent may be depressed in the presence of a substrate that may be adsorbed more strongly than acetic acid. 

Corey et al have also synthesized 3 from 1. Their route involves reduction of 1 with a rhodium oxide-platinum oxide catalyst (this volume) to the corresponding amine, formylation, and dehydration to give 3. 

The treatment of solutions of platinum metals with aqueous borohydride results in the formation of finely divided black precipitates that are active catalysts for alkene hydrogenations. The platinum black obtained in this way was twice as active as that obtained by the hydrogenation of platinum oxide. The borohydride reduced rhodium black is even more active. While the borohydride reduction of base metals gives the corresponding metal borides, there is little, if any, boron incorporated into these platinum metal blacks. Analysis of the borohydride reduced palladium found that while the palladium boron ratio in the bulk was 10 1, less than 1% of the surface was boron.59 7, 5 small amount of boron, however, can impart a significant difference in catalytic activity to this catalyst as compared with other, more common, palladium catalysts. The most striking difference is the inability of the borohydride reduced palladium to promote the hydrogenolysis of activated C-0 and C-N bonds, a reaction that takes place readily over standard palladium catalysts. 

Clearly, the best catalyst for the reduction reactions may not be the best for the oxidation reactions, so two catalysts are combined. The noble metals, although expensive, are particularly useful. Typically, platinum and rhodium are deposited on a fine honeycomb mesh of alumina (AI2O3) to give a large surface area that increases the contact time of the exhaust gas with the catalysts. The platinum serves primarily as an oxidation catalyst and the rhodium as a reduction catalyst. Catalytic converters can be poisoned with certain metals that block their active sites and reduce their effectiveness. Because lead is one of the most serious such poisons, automobiles with catalytic converters must use unleaded fuel. 

The maximum of the peaks are, in both cases, located around 110°C. However the TPR diagram of the bimetallic reveals two different reduction peaks. The first one with a maximum at about 100°C corresponds to the simultaneous reduction of platinum and rhodium oxides, while the second, around 210°C and weaker in intensity, is probably representative of further metallic oxide reduction, including perhaps an alloy. 

Hydrogenation of methylated uridylic acids with platinum oxide is rather sluggish. However, satisfactory reduction of the 2 (3 )-phosphates of uridine and cytidine was accomplished by Cohn and Doherty, who introduced the use of 5% rhodium-on-alumina in these hydrogenations it was from such studies that chemical proof of the position of the phosphoric group in the a and 6 isomers of uridylic and cytidylic acid (see... 

Catalytic hydrogenation of oximes to amines requires conditions resembling those for catalytic hydrogenation of nitro compounds and nitriles.20d The catalyst should be as active as possible, e.g., Raney nickel101 (if necessary, platinized), platinum oxide,102 palladium-charcoal,103 palladium-barium sulfate,104 or rhodium-alumina.105 This rhodium catalyst also serves for reduction of an amidoxime to the amidine.106 Hydrogenation may be effected under pressure, but the temperature should be kept as low as possible to avoid formation of secondary amines. 

While Wheland (91) suggests that 4-aminopyridine is an aromatic amine little success has been achieved in hydrogenation. Orthner (92) cites failure with platinum catalysts under a variety of conditions. Very low yield (16.5%) resulted from reduction of the hydrochloride salt with platinum catalyst under 80 atm pressure (93). In the reduction of A7-(4-pyridyl)morpholine where the 4-amino nitrogen atom is tertiary, hydrogenation in alcohol was successful with ruthenium (5), but unsuccessful with rhodium on alumina or platinum oxide in acetic acid. It is possible that the pressure conditions used for reduction with ruthenium catalysts may be conducive to conversion of 4-aminopyridine, since these catalysts are less inhibited by strong nitrogen bases. 

The simplest and most successful method for the hydrogenation of the isomeric dipyridyls has been reported by Smith (100). Reaction is carried out under low pressure conditions in water or alcohol with a moderate excess of hydrochloric acid in the presence of platinum oxide. Rhodium on a carrier has been tried by this author in the conversion of 2,2 -dipyridyl but the excess acid has an inhibiting effect on the catalyst. When acetic acid was used in place of hydrochloric acid reduction with rhodium was very slow. In the absence of acid no uptake of hydrogen took place. 

The hydrogenation of alkyl, aryl, or aralkylpyridinium salts is a moderately uncomplicated means of obtaining A-substituted piperidines. The quaternary salts reduce more readily than the salts of the parent pyridines (10) and yield of -substituted piperidines is usually better than obtained by reaction of the piperidine with the appropriate halide. The reduction may be carried out under a variety of conditions of pressure and temperature, in the presence or absence of base, in water, alcohol, or acetic acid. Raney nickel, palladium on a carrier, rhodium on a carrier and platinum oxide have all been used, with platinum oxide enjoying the widest use. 

The reduction of phenylpyridyl carbinols in acetic acid or as hydrochloride salts with platinum oxide is widely reported. Rhodium in acetic acid has given good results in this laboratory. Walker (18) reports considerable success with palladium on carbon when reducing the acetate salts of such compounds. The only disadvantage of this method is possible accompanying hydrogenolysis of the OH group. 

Typical commercial three-way catalysts contain both platinum for CO and hydrocarbon oxidation, and rhodium for NO reduction. However, an intimate... 

Selective hydrogenation of quinolines and isoquinolines. Catalytic hydrogenation of quinolines and isoquinolines usually occurs preferentially in the pyridine ring. However, if the hydrogenation is conducted in trifluoroacetic acid, the reverse situation obtains and the benzene ring is reduced more rapidly. The same result can be obtained with mineral acids, but such hydrogenations are much slower. Both 2- and 4-phenylpyiidine can also be reduced preferentially in the benzene ring. Platinum oxide or palladium or rhodium catalysts can be used. Further reduction of 5,6,7,8-tetrahydroquinolines with sodium and ethanol provides a convenient route to rrans-decahydroquinolines. 

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