Palladium distribution ratios

The observed distribution can be readily explained upon assuming that the only part of polymer framework accessible to the metal precursor was the layer of swollen polymer beneath the pore surface. UCP 118 was meta-lated with a solution of [Pd(AcO)2] in THF/water (2/1) and palladium(II) was subsequently reduced with a solution of NaBH4 in ethanol. In the chemisorption experiment, saturation of the metal surface was achieved at a CO/Pd molar ratio as low as 0.02. For sake of comparison, a Pd/Si02 material (1.2% w/w) was exposed to CO under the same conditions and saturation was achieved at a CO/Pd molar ratio around 0.5. These observations clearly demonstrate that whereas palladium(II) is accessible to the reactant under solid-liquid conditions, when a swollen polymer layer forms beneath the pore surface, this is not true for palladium metal under gas-solid conditions, when swelling of the pore walls does not occur. In spite of this, it was reported that the treatment of dry resins containing immobilized metal precursors [92,85] with dihydrogen gas is an effective way to produce pol-5mer-supported metal nanoclusters. This could be the consequence of the small size of H2 molecules, which... 

Figure 11. Pore size distribution of palladium(II) EnCat 30 and palladium(II) EnCat 40 swollen in THF the materials differ in the isocyanate/solvent ratio in the microencapsulation mixture (see text), which was 30/70 and 40/60 (w/w), respectively. (Reprinted from Ref. [38], 2005, with permission from Reaxa Ltd.)...

Initial studies showed that the encapsulated palladium catalyst based on the assembly outperformed its non-encapsulated analogue by far in the Heck coupling of iodobenzene with styrene [7]. This was attributed to the fact that the active species consist of a monophosphine-palladium complex. The product distribution was not changed by encapsulation of the catalyst. A similar rate enhancement was observed in the rhodium-catalyzed hydroformylation of 1-octene (Scheme 8.1). At room temperature, the catalyst was 10 times more active. For this reaction a completely different product distribution was observed. The encapsulated rhodium catalyst formed preferentially the branched aldehyde (L/B ratio 0.6), whereas usually the linear aldehyde is formed as the main product (L/B > 2 in control experiments). These effects are partly attributed to geometry around the metal complex monophosphine coordinated rhodium complexes are the active species, which was also confirmed by high-pressure IR and NMR techniques. 

Lower aliphatic amines are widely used as intermediates for the synthesis of herbicides, insecticides and drugs or can be applied as rubber accelerators, corrosion inhibitors, surface active agents etc. [l]. The most widespread method for the preparation of lower aliphatic amines involves the reaction of ammonia with an alcohol or a carbonyl compound in the presence of hydrogen. The most common catalysts used for reductive amination of alcohols, aldehydes and ketones contain nickel, platinum, palladium or copper as active component [ I — 3 ]. One of the most important issues in the reductive amination is the selectivity control as the product distribution, i.e. the ratio of primary to secondary or tertiary amines, is strongly affected by thermodynamics. 

The reactions and product distributions thus far reported have been exclusively concerned with hexene. It was of interest to see whether the high specificity of positional substitution could be maintained with the other hexene isomers. By positional substitution specificity is meant ester attachment on ether of the carbons involved in the original carbon-carbon double bond. Table VII shows the results of these studies. The internal olefins reacted more slowly than the a-olefin, and with both palladium chloride-cupric chloride and 7r-hexenylpalladium chloride-cupric chloride systems high substitutional specificity (> 95% ) was also maintained with 2-hexene (Table VII). However, with 3-hexene the specificity is considerably lower (80%). Whether this is caused by 3-hexene isomerization prior to vinylation or by allylic ester isomerization is not known. A surprisingly high ratio of 2-substitution to 3-substitution is found ( 7 1) in the products from 2-hexene. An effect this large... 

Because the possibility of olefinic isomerization still loomed important in considering product distributions, we decided to add the powerful olefin isomerization catalyst (17), rhodium trichloride, to the system. No change in product distribution from that of palladium chloride alone was found with either hexene or 2-hexene when a 1 1 molar ratio of rhodium trichloride/palladium chloride was used (Table VII). This is further evidence that the relative rate of vinylation is greater than that of isomerization. When rhodium chloride was used with hexene without any added palladium chloride at 115°C., only slight reaction occurred, and the product contained 85.7% 2-acetate, 10.2% 1-acetate, and 4.1% 3-acetate. Apparently, vinylation had occurred with rhodium trichloride in a manner analogous to oxymercuration and the low-temperature palladium vinylation reaction. 

This yields products with extremely low glass temperatures. An evaluation of the distribution of the branches indicates that in the polymerization of ethene with the 2,3-bis(2,6-dibromophenylimine)butane palladium catalyst the ratio of chain walking and subsequent insertion into the secondary palladium alkyl bond is higher and olefin elimination and reinsertion is faster than in the diisopropyl h-gand-substituted palladium catalyst (Tab. 3.6). The latter thus results in the production of 1-olefins which - after inserting - give more even numbered branches. 

The mechanism proposed (84, 95) is as follows. The formation of cis-2-butene is adequately described by steps (1), (2), and (3) proposed above for the palladium-catalyzed reaction. Since the initial cisitrana ratio in the butenes is almost independent of initial reactant pressures and temperature, and since the distribution of deuterium in these two isomers is similar, it has been concluded that produced directly from 2-butyne, and not by the subsequent isomerization of cis-2-butene. Steps (6), (7), (8), and (9) describe the simplest route which satisfies the experimental observations and involves the addition of hydrogen to a free radical form of the half-hydrogenated state, which is envisaged to be in equilibrium with the normal form. An analogous equilibrium was postulated in the mechanism for acetylene... 

The interaction of cyclopropane with deuterium was examined over rhodium-pumice catalyst at 50° intervals from 0 to 200°, and the propane distributions ramble those observed with palladium, being unaffected by changes in temperature or the initial reactant ratio. However, A is somewhat lower than with palladium, being close to 0.78 throughout the entire... 

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