Palladium blacks

Pd-C, 1,4-cyclohexadiene, 25°, 1.5 h, 95-100% yield.Palladium black, a more reactive catalyst than Pd-C, must be used to cleave the more stable aliphatic benzyl ethers. 

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

The vinyl-free analogues of quinine and quinidine, viz., the 6 -methoxy-ruban-9-ols were also synthesised in 1941 by Rabe and Hagen and further details added by Rabe, Schuler and Voss in 1943. 6 -Methoxy-ruban-9-one, m.p. 89° (Prelog et found m.p. 90-1° and picrate m.p. 211-211-5° the latter was confirmed by Kleiman and Weinhouse on hydrogenation in dilute hydrochloric acid in presence of palladium black gave... 

Hi) Dehydrogenation. j3-Carboline derivatives may be obtained from tetrahydro-)3-carbohnes by zinc dust distillation or high temperatmre dehydrogenation with selenium or palladium black. Many of the complex indole alkaloids may be degraded, with bond cleavage, to yield simple )3-carbolines under these conditions and this approach has become a standard method in structural elucidations. Examples are numerous but outside the scope of this review. 

Simple l,2,3,4-tetrahydro-)3-carbolines have been aromatized in this manner. Palladium black at 160-170° or at higher temperature, palladium-maleic acid in aqueous solution, and platinum/oxygen have been used for the purpose. Palladium-on-charcoal in a high-boiling solvent has been used also to aromatize 5,6,7,8-tetrahydro-j3-carbohnes and 6,7,8,9-tetrahydro-3-carbo-hne. ... 

Recently Swan has employed tetrachloro-o-benzoquinone in the oxidation of the 3,4-dihydro-j8-carbolinium cation 124 to the j8-carbo-linium cation 252. Dehydrogenation with palladium black at 175° or at a higher temperature and with selenium has also been successfully used for the purpose. 

WojQo 1-4652) in absolute ethereal solution was allowed to react with palladium black. After a few hours hydrogen was passed into the liquid at ordinary temperature, giving rise to a terpene possessing the following... 

The behaviour of a-pinene towards palladium black is very different when it is exposed for a period of four weeks to the action of hydrogen under very low pressure. In this case it yields hydropinene (boiling... 

Palladium-reihe, /. palladium series, -salz, n. palladium salt, -schwamm, m. palladium sponge. -Schwarz, n. palladium black, -was-serstoff, m. palladium hydride. 

The more halogens on a single carbon, the greater the ease of hydrogenol ysis, a fact that makes stepwise dehydrohalogenation an easy process, since each loss increases the resistance to further loss (4,13,21,24,29,71). Reduction of the dichloro compound 17 over palladium black gave 18 in 98% yield (8). 

To a solution of 180 parts of -benzyl N-benzyloxycarbonyl-L-aspartvI-L-phenylalanine methyl ester in 3,000 parts by volume of 75% acetic acid is added 18 parts of palladium black metal catalyst, and the resulting mixture is shaken with hydrogen at atmospheric pressure and room temperature for about 12 hours. The catalyst is removed by filtration, and the solvent is distilled under reduced pressure to afford a solid residue, which is purified by re-crystallization from aqueous ethanol to yield L-aspartyl-L-phenylalanine methyl ester. It displays a double melting point at about 190°C and 245°-247°C. 

Ej Ethyl 2-(n)-Propyl-lsonicotinate The product just obtained is dechlorinated by catalyti-cally hydrogenating it in an alcoholic medium in the presence of palladium black and potassium acetate. 30 grams of ester, having a boiling point of 121°-125°C/7 mm, are obtained. 

The results in the ionic liquid were compared with those obtained in four conventional organic solvents. Interestingly, the reaction in the ionic liquid proceeded with very high selectivity to give the a-arylated compound, whereas variable mixtures of the a- and (3-isomers were obtained in the organic solvents DMF, DMSO, toluene, and acetonitrile. Furthermore, no formation of palladium black was observed in the ionic liquid, while this was always the case with the organic solvents. 

Fig. 2. Example of a hysteresis loop on the isotherm pHj = /(H/Pd) obtained during absorption (upper curve) and desorption (lower curve) of hydrogen from palladium black at 100°C. After Sieverts and Danz (19).

A reaction that involves dehydrogenation to an imine, which then reacts further is the reaction of primary or secondary amines with palladium black. The imine initially formed by the dehydrogenation reacts with another molecule of the same or a different amine to give an aminal, which loses NH3 or RNH2 to give a secondary or tertiary amine. An example is the reaction between V-methylbenzylamine and... 

Caution All operations described in these procedures must be carried out in a well-ventilated hood, since ammonia is highly toxic, hydrogen is extremely flammable, and palladium black is pyrophoric. 

To minimize the danger in handling palladium black, the submitters recommend that the filtration, washing, and transfer of the... 

Yamamoto and co-workers reported abase- and ligand-free palladium (II) catalysed method, in methanol at room temperature and under air [49], While the conversion of arylboronic acids bearing an electro-donating group was very efficient, the presence of an electro-withdrawing substituent led to lower conversions. To solve this problem and also prevent the fast formation of palladium black, Yamamoto and co-workers described a new approach where the reaction was catalysed by NHC-bearing complexes 21 and 22 in the presence of an oxidant (Fig. 7.4) [50]. The best results were obtained when complex 21b was used in methanol at room temperature, in the presence of a small excess of p-benzoquinone. 

Hence, P-C bond-cleavage followed by isomerization is responsible for the formation of side products. Furthermore, due to destabilization of the catalyst complex, deactivation occurs and palladium black is formed, which is a notorious disadvantage of Pd-phosphine catalysts in general. Catalyst decomposition and the formation of side products causes additional separation and catalyst recovery problems. These problems have been solved by the discovery of novel catalyst complexes, which are active and stable at temperatures of over 250 °C (Cornils and Herrmann, 1996). 

Substrate reactivity was as expected (Arl > ArBr ArCl). In contrast to the Suzuki cross-coupling, however, Cu and Ru clusters were not active in the Heck reactions, and the activity of Cu/Pd clusters was lower than that of pure Pd clusters. Note the higher activity of Pd clusters prepared in situ (row F) compared to pre-prepared clusters (rows B and G). This increased activity tallies with our findings for Suzuki cross-coupling (7). After reaction, palladium black was observed in all the vials in rows B and G, but not in row F. 

Finely divided palladium (palladium-black) used as a hydrogenation catalyst is usually pyrophoric and needs appropriate handling precautions. 

Dehydrogenation has been used as a method for azomethine ylide formation. Treatment of compound 206 with iV-methylmaleimide in the presence of palladium black gives a 1 1 mixture of the endo- and f .tf-diastereomcrs 207 and 208, in 65% combined yield <1989J(P1)198> (Equation 24). 

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