Palladium on activated carbon

In another process variant, only 88% of the nitrobenzene is reduced, and the reaction mixture then consists of two phases the precious metal catalyst (palladium on activated carbon) remains in the unreacted nitrobenzene phase. Therefore, phase separation is sufficient as work-up, and the nitrobenzene phase can be recycled direcdy to the next batch. The aqueous sulfuric acid phase contains 4-aminophenol and by-product aniline. After neutralization, the aniline is stripped, and the aminophenol is obtained by crystallization after the aqueous phase is purified with activated carbon (53). 

For more selective hydrogenations, supported 5—10 wt % palladium on activated carbon is preferred for reductions in which ring hydrogenation is not wanted. Mild conditions, a neutral solvent, and a stoichiometric amount of hydrogen are used to avoid ring hydrogenation. There are also appHcations for 35—40 wt % cobalt on kieselguhr, copper chromite (nonpromoted or promoted with barium), 5—10 wt % platinum on activated carbon, platinum (IV) oxide (Adams catalyst), and rhenium heptasulfide. Alcohol yields can sometimes be increased by the use of nonpolar (nonacidic) solvents and small amounts of bases, such as tertiary amines, which act as catalyst inhibitors. 

This reaction is favored by moderate temperatures (100—150°C), low pressures, and acidic solvents. High activity catalysts such as 5—10 wt % palladium on activated carbon or barium sulfate, high activity Raney nickel, or copper chromite (nonpromoted or promoted with barium) can be used. Palladium catalysts are recommended for the reduction of aromatic aldehydes, such as that of benzaldehyde to toluene. 

Complex 5 was more active than the well-known precious-metal catalysts (palladium on activated carbon Pd/C, the Wilkinson catalyst RhCl(PPh3)3, and Crabtree s catalyst [lr(cod)(PCy3)py]PFg) and the analogous Ai-coordinated Fe complexes 6-8 [29] for the hydrogenation of 1-hexene (Table 2). In mechanistic studies, the NMR data revealed that 5 was converted into the dihydrogen complex 9 via the monodinitrogen complex under hydrogen atmosphere (Scheme 4). 

The removal of carbobenzyloxy (Cbz or Z) groups from amines or alcohols is of high interest in the fine chemicals, agricultural and pharmaceutical industry. Palladium on activated carbon is the catalyst of choice for these deprotection reactions. Nitrogen containing modifiers are known to influence the selectivity for certain deprotection reactions. In this paper we show the rate accelerating effect of certain N-containing modifiers on the deprotection of carbobenzyloxy protected amino acids in the presence of palladium on activated carbon catalysts. The experiments show that certain modifiers like pyridine and ethylenediamine increase the reaction rate and therefore shorten the reaction times compared to non-modified palladium catalysts. Triethylamine does not have an influence on the rate of deprotection. 

A commercially available 5% palladium on activated carbon catalyst from Degussa was used for the investigation. Commercially supplied N-(Carbo-benzyloxy)-L-phenylalanine (99%) was purchased from Aldrich. Modifiers such as pyridine, triethylamine, ethylenediamine and DABCO (Diazabicyclooctane) with a purity >99 % are also available commercially and were used as received. 

Pyrazoles were synthesized in the authors laboratory by Le Blanc et al. from the epoxy-ketone as already stated in Sect. 3.1.1a, Scheme 35 [80]. The synthetic strategy employed by Le Blanc et al. [80] was based upon that the strategy published by Bhat et al. [81] who also described the synthesis of pyrazoles but did not report cytotoxic evaluation on the synthesized compounds. Scheme 48 shows the synthesis of the most active compound (178). Dissolution of the epoxide (179) with a xylenes followed by treatment with p-toluenesulfonic acid and hydrazine hydrate produced the pure nitro-pyrazole 180 in good yield (60%). Catalytic hydrogenation with palladium on activated carbon allowed the amino-pyrazole (178) to be obtained in a pure form. This synthesis allowed relatively large numbers of compounds to be produced as the crude product was sufficiently pure. Yield, reaction time, and purification compared to reported approaches were improved [50, 61, and 81]. Cytotoxicity of these pyrazole analogs was disappointing. The planarity of these compounds may account for this, as CA-4, 7 is a twisted molecule. 

Palladium on activated carbon (10%) was purchased from Aldrich Chemical Company, Inc., and used as received. 

Sodium methoxide, 3-methyl-4-nitroanisole, diethyl oxalate, 30% hydrogen peroxide, 97% sodium hydride, methyl acetoacetate, sodium sulfate, 10% palladium on activated carbon, ammonium formate, and 2-nitrophenylacetic acid were purchased from Aldrich Chemical Company, Inc., and were used without further purification. 

The arylamine 780b required for the total synthesis of carbazomycin B (261) was obtained by catalytic hydrogenation, using 10% palladium on activated carbon, of the nitroaryl derivative 784 which was obtained in six steps and 33% overall yield starting from 2,3-dimethylphenol 781 (see Scheme 5.85). Electrophilic substitution of the arylamine 780b with the iron-complex salt 602 provided the iron complex 787 in quantitative yield. The direct, one-pot transformation of the iron complex 787 to carbazomycin B 261 by an iron-mediated arylamine cyclization was unsuccessful, probably because the unprotected hydroxyarylamine moiety is too sensitive towards the oxidizing reaction conditions. However, the corresponding 0-acetyl derivative... 

After dimerization and separation of the product mixture from the palladium catalyst complex, the reaction mixture is hydrogenated over a 1% palladium on activated carbon catalyst. A 50 psig hydrogen pressure and a 100-125°C reaction temperature are... 

It has been discovered that the performances of platinum and palladium catalysts may be improved by promotion with heavy metal salts. However, there is little information available about the role and chemical state of the promoter 8,9). We have recently found that a geometric blocking of active sites on a palladium-on-activated carbon catalyst, by lead or bismuth, suppresses the by-product formation in the oxidation of l-methoxy-2-propanol to methoxy-acetone 10). 

Another part of our investigation deals with the effect of heat treatment on the leaching behavior of palladium on activated carbon catalysts. Heat treatment is a known technique to increase the performance of catalysts. (3) Therefore, standard carbon supported palladium catalysts were exposed to different temperatures ranging from 100 to 400 °C under nitrogen. The catalysts were characterized by metal leaching, hydrogenation activity and CO-chemisorption. 

Palladium on activated carbon was purchased from Alpha Division. 

Engel, D. C., Versteeg, G. F., and Van Swaaij, W. P. M. (1995). Reaction Kinetics of Hydrogen and Aqueous Sodium and Potassium Bicarbonate Catalysed by Palladium on Activated Carbon. Chemical Engineering Research and Design, 73(A6), 701-706. 

The hydrogenated pulp, sample 33, was obtained by hydrogenating sample 7 in the presence of a solid catalyst, palladium-on-activated carbon (5%) [16]. 

Palladium on activated carbon has turned out to be a highly versatile, simple heterogeneous catalyst for one-pot multistep syntheses. Recently, Djakovitch and coworkers [42] have demonstrated that low catalyst loadings of Pd on activated carbon efficiently catalyze the Heck reaction of bromo benzene and styrene giving rise to T-stilbene (1) (92%), Z-slilbcnc (1%), and 1,1-diphenylethene (7%). If the Heck products are not isolated but an atmosphere of 20 bar of hydrogen is imposed onto the reaction vessel the sequence furnishes 1,2-diphenylethane (2) in 93% yield (Scheme 1). 

The Step 4 product (20 mmol) dissolved in 400 ml ethyl alcohol was treated with 10% palladium on activated carbon (500 mg) and hydrogenated 18 hours at 50psi hydrogen. The mixture was filtered, concentrated, and the product quantitatively isolated. 

Scheme 3.14 Reduction of cinnamaldehyde with palladium on activated carbon...

Compounds 330 were prepared by saturation of the C=N bond of the tricyclic compounds 329 by hydrogenation over palladium on activated carbon, or by reduction with sodium borohydride or sodium in alco-hoi. . 98.3 6 Reduction of the tetrahydropyrrolo[2,l-i>]quinazolin-l-one 310 with sodium in amyl alcohol yielded the hexahydropyrrolo[2,l-i>]-quinazoline 330 (R = H, n = 0). Reduction of the 3-hydroxypyr-... 

The reaction was initially tested by the use of PdCliCPPhs) . Although 6 equiv of iodide 80 was required to complete the reaction, the desired product 65 was obtained in 80% yield (Table 11, Entry 1) [97]. The catalyst is, however, inadequate especially in terms of cost. Studies were undertaken to search for a better protocol. Nickel system was resorted in this connection. The use of inexpensive nickel(IT) acetylacetonate [Ni(acac)2] was tested to reduce the cost of raw material, which led to a moderate yield of 65 (78%, Table 11, Entry 2) [98]. Easily recoverable heterogeneous palladium on activated carbon (Pd/C) catalyst was then examined. While the use of the standard conditions using THF and toluene as the solvent resulted in a moderate yield (50%, Table 11, Entry 3), addition of DMF to the reaction mixture considerably improved the yield, providing 65 in 94% yield (Table 11, Entry 4) [99]. Much less pyrophoric Pearlman s catalyst [Pd(OH)2/C] was found to give 65 in an excellent yield with such a tiny catalyst loading as 0.65 mol% (Table 11, Entry 5) [100]. 

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