Catalyst PD black

The selective catalytic hydrogenation of exocyclic a,/3-unsaturated ketones was successfully utilized in the assymetric synthesis process of the chiral building blocks, a-hydroxy acids. The influence of the catalyst support, solvent and additives in the selective 1,4-reduction was studied in a series of exocyclic enone systems (Scheme The best conditions for the selective reduction of the exocyclic double bond were obtained in an apolar solvent like toluene over a range of Pd-supported catalysts (Pd black, Pd/Ti02, Pd/Al203, Pd/SiOa, Pd/C). The usage of polar solvents like methanol and DMF decreased the selectivity dramatically but addition of triethyl amine, pyridine or potassium acetate to the reaction mixtures revert this result successfully. 

H2/Pd-C. If hydrogenation is carried out in the presence of (BOC)20, the released amine is directly converted to the BOC derivative. H2/Pd-C, NH3, —33°, 3-8 h, quant.When ammonia is used as the solvent, cysteine or methionine units in a peptide do not poison the catalyst. Pd-C or Pd black, hydrogen donor, solvent, 25° or reflux in EtOH, 15 min-2 h, 80-100% yield. Several hydrogen donors, including cyclohex-... 

Pd-Poly(ethylenimine), HC02H. This catalyst system was reported to be better than Pd/C or Pd black for Z removal. 

The ligandless Beletskaya catalysts, PdCl2(CH3CN)2 used in DMF or acetone with tin-substituted alkynes and sp -hybridized iodides and bromides. This system is very reactive and relatively cheap. The disadvantage is that it decomposes quickly under development of catalyticaUy inactive Pd-black particles [20 c]. 

In this study, commercial catalysts were used. As the cathode catalyst Pt black was used. Different amounts of Pt black, Pt-Ru black, 10wt% Pt-Pd/C and 20wt% Pt-Pd/C catalysts were used for the anode. 

The general experimental procedure employed in the study here has been described previously (7), thus only a brief overview is presented here. For all experiments, 45 mL deionized water and catalyst (50 mg Pd-black for 3-buten-2-ol and 25 mg for l,4-pentadien-3-ol) were added to the reaction cell. For ultrasound-assisted, as well as stirred (blank) experiments, the catalyst was reduced with hydrogen (6.8 atm) in water for 5 minutes at an average power of 360 W (electrical 90% amplitude). The reagents (320 mg 3-buten-2-ol or 360 mg l,4-pentadien-3-ol) were added to the reduced catalyst solution to achieve... 

In the enantioselective hydrogenation of isophorone in the presence of (-)-DHVIN modifier the best optical purity was afforded by small dispersion (<0,05) Pd black catalyst (up to 55%) (7). The influence of the preparation method of Pd black on the optical yield was reported (8). A correlation was found between the oxidation state of the metal surface and the enantioselectivity, the catalyst having more oxidised species on its surface giving higher enantiomeric excess, while the Pd black with lower surface area was more enantioselective. 

The object of the present study was to use in the above mentioned hydrogenations improved carbon supported catalysts, which could compete with the Pd black catalyst. Carbon materials are common supports, their surface properties can be modified easily and it is possible to prepare carbons with different proportion of micro-, meso- and macropores, which can be key factors influencing their performances. A highly mesoporous carbon was synthesised and used as support of Pd catalysts in the enantioselective hydrogenations. To our knowledge this is the first report on the use of highly mesoporous carbon for the preparation of Pd catalysts for liquid-phase hydrogenation. 

N,N-Debenzylation was accomplished by Pd/C and HC02NH4.349 Debenzy-lation of an N,N-dibenzyltryptamine derivative was sluggish and incomplete using Pd/C as the catalyst. In contrast, transfer hydrogenolysis using ammonium formate-formic acid in the presence of Pd black efficiently afforded the free tryptamine (27) (Scheme 4.102).350... 

Fig. 5.13 H2 consumption in the hydrogenation of 1-hexene over various Pd catalysts. Conditions 1 atm. H2, 23 0.5°C. (X) Pd black, (A) conventional Pd/Al203, ( ) Pd/Al203 sonochemically prepared in 20 mM methanol, (O) Pd/Al203 sonochemically prepared in 20 mMl-propanol [31]...

So far, no systematic work has been done on the use of recyclable, solid-phase catalysts in cross-coupling reactions. Most of the examples have been obtained for cross-couplings with either arylboronic acids or terminal acetylenes. It should be noted, however, that due care should be exercised when interpreting results on the cross-coupling of arylboronic acids with aryl iodides, as this extremely facile reaction can be catalyzed by practically any palladium-containing material, including trivial Pd black,481 e.g., as a sediment on the reaction vessel. Therefore, this reaction cannot serve as a reliable test for comparison between different catalytic systems. 

The retentions of the catalysts were also measured on a synthetic reaction mixture. The Heck-catalyst showed a retention of 96% while under experimental conditions retentions lower than 90% were obtained. For the PTC the values are both higher than 99%. The authors assume that this big difference for the Heck-catalyst is caused by the formation of smaller Pd species in the catalytic cycle. However, no precipitation or Pd-black formation was observed. 

We have studied the effect of cavitating ultrasound on the heterogeneous aqueous phase hydrogenation of a 5-2-buten-l-ol (C4 olefin) and a5-2-penten-l-ol (C5 olefin) on Pd-black (1.5+0.1 mg catalyst) to form the lra/ 5-olefins trans-2-buten-l-ol and fra/3s-2-penten-l-ol) and saturated alcohols (1-butanol and 1-pentanol, respectively). These chemistries are illustrated in Scheme 2. A full analysis of this study has been recently presented [10]. 

Pd black was found to be the most effective catalyst for such stereoselective hydrogenation methanol or dimethyl formamide was the best solvent. 

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