Palladium-based catalyst

The most common support materials exploited commercially have been silica and, particularly, low surface area a-alumina which combines high strength and 

In this case high ethene selectivity is maintained up to a maximum support surface area of approximately 50 m (g catalyst) .  

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The most widely used homogeneous palladium catalyst is palladium carboxylate. Table 6 lists the palladium-based catalyst systems developed for NBR hydrogenation [75-87]. They are regarded as homogeneous cata-... 

Figure 3. Transient response of 1,2 C-vinyl acetate (MW=88) obtained from a typical pump-probe experiment with the palladium-based catalysts and 1,2 C-ethylene.

The reaction is typically performed via a gas/liquid process on palladium-based catalysts, e.g. deposited on carbon [17]. Ethanol may be chosen as solvent. 

Telomerisation is an important, atom efficient reaction, which generates functionalised dienes from 1,3-diene feedstocks. The reaction, which typically employs a palladium based catalyst, comprises coupling two molecnles of a conjugated 1,3-diene... 

The hydroamination of allene with morpholine or allylamines has been attempted with palladium-based catalysts. Usually, a mixture of 1 1 telomers (hydroamination products) and 1 2 telomers is obtained, the latter being the major [308, 309] or only... 

Recent developments showed promising results over platinum and palladium-based catalysts after deposition on reducible materials [ 104-106] (see Figures 10.11 and 10.12). Further improvements might be expected on Pd-based catalysts supported on those... 

A Michaelis-Arbusov rearrangement followed by a Wittig-Horner reaction is involved in preparation of the distyrylbenzene derivative 11.37, as shown in Scheme 11.15. Precautions must be taken in the first stage to minimise formation of the carcinogenic by-product bis(chloromethyl) ether 11.16. The stilbene bis-ester 11.38 can be made by a similar procedure, or alternatively by the reaction of ethyl acrylate with 4,4 -dibromostilbene in the presence of a palladium-based catalyst (Scheme 11.16), a synthesis that yields the required trans form of the brightener. 

The rhodium catalyst has several distinct advantages over the cobalt catalyst it is much faster and far more selective. The higher rate is in process terms translated into much lower pressures (the cobalt catalyst is operated at pressures of 700 bar). Nickel- and palladium-based catalysts have also been reported, but no applications have resulted from these. The mechanism for group 10 metals has not been studied (see Section 9.3.2.3). 

Mikami et al X developed a palladium-based catalyst system 22, capable of forming quaternary centers via a carbonyl-ene reaction. This is one of the few recent examples of a carbonyl-ene reaction that uses a ketone rather than an aldehyde and affords 84% yields in the case of the five-membered ring (23, n = 1) to near quantitative yields for the six-membered ring (23, n = 2), with 96% ee or better in both cases (Equation (12)). This catalyst system also shows selectivity when other ene partners are used, giving synl anti ratios equal to or better than the titanium systems and affords linear products with complete ( )-selectivity. 

In recent years the interest in hydroformylating higher alkenes with catalysts other than cobalt has increased. Platinum and palladium based catalysts have been studied and the results of the latter [10] seem very promising. Platinum has been known for many years to have a high preference for the formation of linear products, but ligand decomposition hampers applications [11]. 

Cross-coupling of terminal alkynes with aryl and vinyl halides are usually carried out in organic solvents, such as benzene, dimethylformamide or chloroform with a palladium-based catalyst and a base scavenger for the hydrogen halide. Copper(I) iodide is a particularly effective co-catalyst allowing the reaction to proceed under mild conditions. 

Platinum and Palladium Based Catalysts. Researchers at ANL have also developed an ATR catalyst formulation comprised of a transition metal element supported on an oxide ion-conducting substrate, such as ceria. 

A unique feature of azoles amongst five membered heterocycles is that they can act both as the carbon or the heteroatom donor during the bond formation. This possibility is frequently exploited in synthetic transformations. Pyrrole, for example, coupled effectively with bromoarenes in the presence of palladium based catalysts (6.67.), The use of PBuj as ligand and rubidium carbonate as base allowed for the reduction of catalyst loading to 1% without significant deterioration of the yield"... 

The palladium based catalyst systems were also effective in the construction of carbon-nitrogen bonds on benzannulated five membered heterocycles. The 4-chloroindole derivative shown in 6.78. was coupled with piperazine in excellent yield, using a ferrocene based ligand.109 The analogous 5-bromo-benzimidazole derivative gave similar results (6.79.),110... 

Pyridylzinc bromide, a commercially available reagent, was used in the synthesis of a series of bipyridyl derivatives (7.20.). Activated coupling partners, such as 2-chloro-4-cyanopyridine, reacted readily at room temperature in the presence of a palladium based catalyst, while less electron deficient halopyridines were coupled at elevated temperatures.27 The same approach was also extended to the preparation of other heterobiaryls.28... 

Two selective processes are important in the oxidation of ethylene the production of ethylene oxide and acetaldehyde. The first process is specifically catalyzed by silver, the second one by palladium-based catalysts. Silver catalysts are unique and selective for the oxidation of ethylene. No similar situation exists for higher olefins. The effect of palladium catalysts shows a resemblance to the liquid phase oxidation of ethylene in the Wacker process, in which Pd—C2H4 coordination complexes are involved. The high selectivity of the liquid phase process (95%), however, is not matched by the gas phase route at present. 

The interest in palladium-based catalysts is due to the double bond oxyhydration capacity of palladium, unique among the noble metals, and well known from the Wacker process. Fuyimoto and Kunugi [119] report that palladium salts on active charcoal are excellent catalysts for the oxidation of olefins, particularly ethylene but the higher olefins as well. A selectivity of 89% with respect to acetone beside 10% aldehyde production is obtained at a conversion level of 27%, using excess water and a very low temperature (105°C). Careful analysis of the charcoal does not indicate that metal oxide impurities are of importance. 

The rhodium(II) catalysts and the chelated copper catalysts are considered to coordinate only to the carbenoid, while copper triflate and tetrafluoioborate coordinate to both the carbenoid and alkene and thus enhance cyclopropanation reactions through a template effect.14 Palladium-based catalysts, such as palladium(II) acetate and bis(benzonitrile)palladium(II) chloride,l6e are also believed to be able to coordinate with the alkene. Some chiral complexes based on cobalt have also been developed,21 but these have not been extensively used. 

In the case of ethylene/carbon monoxide copolymerisation with nickel- and palladium-based catalysts, a strictly alternating high molecular weight copolymer is formed (average molecular weight in the range 10 x 103 100 x 103).When more developed catalysts are used, the copolymerisation conditions can be mild a temperature of 25 °C combined with a pressure of ca 20 atm. The obtained copolymer, poly(ethylene-c// -carbon monoxide), poly(l-oxytrimethylene)... 

Catalysts for ethylene/carbon monoxide copolymerisation were initially obtained from Ni(II) derivatives, such as K2Ni(CN)4 and (w-Bu4N)2 Ni(CN)4, and Pd(II) derivatives, such as [(w-Bu3P)PdCl2]2, Pd(CN)2 and HPd(CN)3, often combined with alcohol or protonic acid as a cocatalyst [241]. It must be emphasised that, in contrast to titanium-, zirconium- or vanadium-based catalysts, nickel- and palladium-based catalysts tolerate polar functional groups (including hydroxyl, carboxylic and sulfonic groups)... 

Active sites present in palladium-based catalysts, which promote the alternating insertion of coordinating comonomers, ethylene to the acyl Pd-C(O) bond and carbon monoxide to the alkyl Pd-CH2 bond, appear to be cationic Pd(II) species with a square planar, formally d° 8-electron structure, [L2(M)Pd(II)—P ]+, accompanied with weakly coordinating counter-anions [478 180,484],... 

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