Pd black formation

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. 

Insertion of an allene molecule into the Pd-H bond, with regeneration of the starting allene complex 8 which then re-enters the catalytic cycle. In the absence of allene, extensive Pd black formation was observed. 

Iwasawa T, Tokunaga M, Obora Y et al (2004) Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols. J Am Chem Soc 126(21) 6554-6555... 

Recently, Iwasawa et al. presented novel pyridine ligands having 2,3,4,5-tetra-phenylphenyl moieties, which efficiently suppress Pd black formation in Pd-mediated oxidation of alcohols however, the use of these catalyst systems for aryl-aryl coupling has not yet been reported [86]. 

Initial experiments were done in water and resulted in low cyclohexene conversions, low product selectivities, and extensive palladium deactivation by Pd black formation. The low cyclohexanone yield originated from overoxidation of cyclohexanone to 2-cyclohexenone, which undergoes further oxidation to a plethora of by-products. The low cyclohexene conversion can be attributed to the aforementioned low reactivity of the internal double bond as well as the low solubility of cyclohexene in water. Several reaction media have been described in which higher alkenes are oxidized to ketones in organic solvent-based systems. Some typical examples are DMF [4], water mixtures with chlorobenzene, dodecane, sulfolane [5], 3-methylsulfolane andM-methylpyrrolidone [6], or alcohols [7]. These solvent systems indeed lead to increased cyclohexene conversions but still suffer from overoxidation and catalyst deactivation by Pd black formation. Hence, the goal of our research was to find a variation to the Wacker oxidation without over-oxidation of the product and deactivation of the palladium catalyst. 

Apparently, the reoxidation of Pd(0) by CUCI2 alone is not sufficient enough to prevent Pd black formation. Reoxidation of Pd(0) by FeClj is more efficient, thus increasing the reaction rate. Furthermore, when the CuCl2/FeCl3 combination is... 

The activity of the Pd(CH3CN)2Cl2/CuCl2/FeCl3 combination could easily be increased by either increasing the reaction temperature or by increasing the oxygen pressure, without any deactivation of the catalyst by Pd black formation [9]. 

Thus, low loadings of palladium without added ligand can be used in cross-coupling reactions such as the Mizoroki-Heck reaction of atyl bromides, but only when the palladium-substrate ratio is kept particularly low, typically from 0.01-0.1 mol%. Quenching of ionic species (into metal colloids) is less likely at lower concentrations due to a lower collision frequency. Pd black formation is a catalytic dead end. At too low a concentration, the reaction will proceed slowly, if at all. - ... 

The benzyl and p-methoxyphenylmethyl protecting groups could be easily removed by refluxing the amine, Pd black, and ammonium formate in MeOH for 5 hours (Scheme 4.100).345... 

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... 

In line with the above mechanism, catalyst deactivation by formation of palladium black can be retarded by increasing the [P]/[Pd] ratio, however, only on the expense of the reaction rate. Bidentate phosphines form stronger chelate complexes than TPPMS which may allow at working with lower phosphine to palladium ratios. Indeed, the palladium complex of sulfonated XANTPHOS (51) proved to be an effective and selective catalyst for hydroxycarbonylation of propene, although at [51]/[Pd] < 2 formation of palladium black was still observed. The catalyst was selective towards the formation of butyric acid, with 1/b = 65/35 [41]. 

To a soln of 12 (R = Bzl 942 mg, 3.17 mmol) in dry DMF (45 mL), ammonium formate (1.0 g, 15.85 mmol) was added followed by Pd black (942 mg). The mixture was stirred at 110 °C for 1.5 h and then filtered through a medium-sintered glass funnel. The solids were washed several times with fresh DMF and the filtrate was concentrated, affording pure (3S)-methyl 3-amino-4-phenylbutyrate yield ... 

Niu(Pc), Pd (Pc) and Pt"(Pc) are prepared by the condensation method with metallic Ni or NiCl2 6H20, Pd black and PtCl2 respectively.197 PdCl2 causes Pc ring chlorination and metallic Pt catalyzes H2(Pc) formation. 

It has been reported [45] that supported Pd is substantially less thermally stable than other noble metals in reducing environments. Data of Baker et al. [46] for titania-supported Pd and Pt in hydrogen show Pd to be Jess thermally stable over a wide range of temperatures. These results are consistent with those of Sermon [47] showing that neck formation starts at about 60°C in Pd black compared to >200°C in Pt black [39). 

The reason why a large excess of H apparently adsorbs on Pd black at 273 K may be as follows. When the sorption data are plotted as against the H/M atomic ratio (Figure 4) sigmoid-shaped curves are obtained both at 273 and at 298 K. The first part of the curve probably corresponds to readsorption into a monolayer, the middle part to simultaneous completion of the adsorbed monolayer and absorption into the a phase of Pd, and the final part to formation of the / -phase hydride. This presupposes that in small, highly distorted Pd particles the -phase hydride begins to nucleate at lower pressures than in the case of bulk Pd. This is a reasonable assumption because a lowering of the absorption plateau pressure in Pd powders has been observed.Only a small proportion of the (smallest ) Pd particles would need to form the hydride phase to account for the excess sorption. 

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