Palladium hydrogen reduction

In past years, metals in dilute sulfuric acid were used to produce the nascent hydrogen reductant (42). Today, the reducing agent is hydrogen in the presence of a catalyst. Nickel, preferably Raney nickel (34), chromium or molybdenum promoted nickel (43), or supported precious metals such as platinum or palladium (35,44) on activated carbon, or the oxides of these metals (36,45), are used as catalysts. Other catalysts have been suggested such as molybdenum and platinum sulfide (46,47), or a platinum—nithenium mixture (48). 

The hydrogenation of 5a-cholestanone (58) in methanolic hydrobromic acid over platinum gives 3j5-methoxycholestane ° (61). This compound is also obtained from the palladium oxide reduction of (58) in methanol in the absence of acid. Hydrogenation of 5 -cholestanone also gives the 3j5-methoxy product under these conditions. Reduced palladium oxides are quite effective for the conversion of ketones to ethers. The use of aqueous ethanol as the solvent reduces the yield of ether. Ketals are formed on attempted homogeneous hydrogenation of a 3-keto group in methanol. ... 

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

An unusual by-product was obtained in small yield in palladium-catalyzed reduction of 2-amino-4,5-dimethoxyindanone hydrochloride, The reduction was done in two stages first, a rapid absorption of 1 mol of hydrogen at 38 C to give the amino alcohol, and then a much slower reduction in the presence of HCIO4 at 70 "C. The rearranged by-product was shown to arise from attack of acid on the amino alcohol (50), Resistance to hydrogenolysis is characteristic of / -amino aromatic alcohols (56), a fact that makes reduction of aromatic oximino ketones to amino benzyl alcohols a useful synthetic reaction. 

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). 

The above generalities apply particularly to palladium. Hydrogenation over platinum or rhodium are far less sensitive to the influence of steric crowding. Reduction of 1-t-butylnaphthalene over platinum, rhodium, and palladium resulted in values of /ci//c2 of 0.42, 0.71, and 0.024, respectively. Also, unlike mononuclear aromatics, palladium reduces substituted naphthalenes at substantially higher rates than does either platinum or rhodium. For example, the rate constants, k x 10 in mol sec" g catalyst", in acetic acid at 50 C and 1 atm, were (for 1,8-diisopropylnaphthalene) Pd (142), Pt(l8.4), and Rh(7.1)(25). 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

The hydrogen reduction of the metal halides, described in Sec. 1.2, is generally the favored reaction for metal deposition but is not suitable for the platinum-group metals since the volatilization and decomposition temperatures of their halides are too close to provide efficient vapor transport. 1 1 For that reason, the decomposition of the carbonyl halide is preferred. The exception is palladium which is much more readily deposited by hydrogen reduction than by the carbonyl-halide decomposition. 

Other metals on silica supports have been investigated less extensively than platinum and nickel, and average particle diameters have only been estimated by gas adsorption methods, supported in a few cases by X-ray line broadening data. Thus, rhodium, iridium, osmium, and ruthenium (44, 45) and palladium (46) have all been prepared with average metal particle diameters <40 A or so, after hydrogen reduction at 400°-500°C. 

Quaternary ammonium salts aid the transfer of the hypophosphite anion in the palladium-catalysed reduction of, for example, alkynes to alkenes, nitroarenes to aminoarenes, and in the hydrogenolysis of tetrazolyl aryl ethers to phenols [12-14], It has been demonstrated that the hydrogenolysis is ineffective when preformed tetra-n-butylammonium hypophosphite is employed in a dry homogenous organic solvent [13, 14], For optimum hydrogen transfer, the concentration of hypophosphite relative to the substrate must be controlled at a low level and this is most effectively accomplished with a two-phase system. 

The porous wall hollow glass microspheres were filled with palladium by a soak-and-dry process followed by hydrogen reduction. Saturated solutions of palladium salt were prepared at room... 

Hydrogenation reduction of acid chloride to aldehyde using BaS04-poisoned palladium catalyst. Without poison, the resulting aldehyde may be further reduced to alcohol. 

Other methods have appeared, but have been less frequently used, e.g. chromium chloride in acetone 140- 143 (y/yfe infra) and dissolving alkali-metal reductions.144,145 Hydrogenolysis with palladium/hydrogen has also given the monoreduced cyclobutanone.99,100... 

Location of Palladium Atoms before and after Hydrogen Reduction... 

The crystal structure of Pd. h Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd2+ ions are mainly found to occupy SI sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd2+from SI sites. This displacement out of cation sites is attributed to the reduction Pd2+ — Pd(0) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 800° C, Pd 0) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 

The crystal structure analysis of palladium-exchanged zeolite allows the determination of initial cation positions in the dehydrated porous framework. Similar studies after reduction by hydrogen at various temperatures should permit the observation of palladium removal from the cation sites and thus the estimation of the reduction level. Moreover, the presence of metal on the external surface is easily detected. Hence, x-ray diffraction techniques should give a good picture of hydrogen reduction of palladium in Y zeolites. 

The frequency of the linear CO is higher than that found for Pd films (j co = 2085 cm-1) (27) or for supported palladium (Pd/Si02), vco = 2060 cm-1) (28). The increase in frequency reported in this study is the result of the decrease of the backdonation from the d metal orbitals to the 7T orbital of CO. Y zeolites have very strong Lewis acid sites these sites should be able to decrease the electronic density of the palladium atoms bonded to CO. The decrease of the intensity of the band at 2100 cm-1 by increasing the hydrogen reduction temperature could be explained by the formation of agglomerates of palladium still in interaction with a Lewis acid site. 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

In research at the Institute of Radiochemistry, Karlsruhe. West Germany during the early 1970s, investigators prepared alloys of americium with platinum, palladium, and indium. These alloys were prepared by hydrogen reduction of the americium oxide in the presence of finely divided noble metals according to ... 

McNab, W. W. J., and Ruiz, R. (1998). Palladium-Catalyzed Reductive Dehalogenation of Dissolved Chlorinated Aliphatics Using Electrolytically-Generated Hydrogen. Chemosphere, 37(5), 925-936. 

Hardin 3 employed three compounds, diphenylpaUadodiammonium chloride and bromide, and palladium ammonium bromide. Each compound was analysed for palladium by reduction at a red beat in hydrogen, with the following results ... 

Heteroatom-substituted carbohydrates are efficiently assembled by the enzymatic aldol condensation of DHAP with an appropriately appended aldehyde. Iminocyclitols that are inhibitors of glycosidases, such as deoxynojirimycin and deoxymannojirimycin, are simply prepared by condensation of azo-substituted aldehydes under the FDP protocol followed by dephosphorylation and palladium mediated reductive animation (Scheme 5.18a).39 In addition a number of polyhydroxylated pyrrolidines that are efficient glucosidase inhibitors have been synthesized by this chemo-enzymatic strategy (Scheme 5.18 ).1" 30,40 If the palladium mediated hydrogenation is done in the presence of hydrochloric acid, an amino-sugar intermediate is formed as its hydrochloride salt. Treatment with base then forms polyhydroxylated imines, instead of iminocyclitols (Scheme 5.19).41... 

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