Palladium-catalyst oxidants copper® chloride

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Palladium catalysts, 10 42 14 49 16 250 Palladium-catalyzed carbonylation, 13 656 Palladium chloride/copper chloride, supported catalyst, 5 329 Palladium compounds, 19 650-654 synthesis of, 19 652 uses for, 19 653-654 Palladium films, 19 654 Palladium membranes, 15 813 Palladium monoxide, 19 651 Palladium oxide, 19 601... 

The metal-catalysed autoxidation of alkenes to produce ketones (Wacker reaction) is promoted by the presence of quaternary ammonium salts [14]. For example, using copper(II) chloride and palladium(II) chloride in benzene in the presence of cetyltrimethylammonium bromide, 1-decene is converted into 2-decanone (73%), 1,7-octadiene into 2,7-octadione (77%) and vinylcyclohexane into cyclo-hexylethanone (22%). Benzyltriethylammonium chloride and tetra-n-butylammo-nium hydrogen sulphate are ineffective catalysts. It has been suggested that the process is not micellar, although the catalysts have the characteristics of those which produce micelles. The Wacker reaction is also catalysed by rhodium and ruthenium salts in the presence of a quaternary ammonium salt. Generally, however, the yields are lower than those obtained using the palladium catalyst and, frequently, several oxidation products are obtained from each reaction [15]. 

After ARCO patents issued, Stille and coworkers published on butadiene oxycarbonylation(14-16). Palladium was utilized as the oxidative carbonylation catalyst and copper(II) chloride was employed as a stoichiometric reoxidation agent for palladium. Although the desired hex-3 -enedioate is the exclusive product, commercial technology which uses stoichiometric copper is not practical. Once the copper(Il) is consumed, the monoatomic palladium spent catalyst agglomerates affording polymeric palladium which is not easily reoxidized to an active form. 

Dicarboxylation reactions of alkenes can be carried out such that predominately 1,2-addition of the two ester functions occurs (equation 61). The reaction takes place under mild conditions (1-3 bar, 25 C) in alcohol. It is stoichiometric in palladium, since the palladium(II) catalyst is reduced to palladium(O) in the process, but by use of an oxidant (stoichiometric copper chloride or catalytic copper chloride plus oxygen equation 62 and 63) the reaction becomes catalytic in palladium. In the reoxidation process, water is generated and the build-up of water increases the water gas shift reaction at the expense of the carboxylation. Thus a water scavenger such as triethyl orthoformate is necessary for a smooth reaction. 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

A PEG-SCCO2 system has also been used in the aerobic oxidation of styrene (Figure 8.7). In the presence of cuprous chloride co-catalyst the reaction favours acetophenone formation, whereas in the absence of copper benzaldehyde is favoured. The catalyst could be recycled five times and it was suggested that the PEG acts to prevent the palladium catalyst from decomposing and also assists in product separation. 

The oxidation of propylene has been chosen as a probe reaction to study the catalytic activity of Cu Pd -TSM. The olefin oxidation in an acidic solution of Cu(II) and Pd(U) chlorides, well known as the Wacker reaction, is achieved when olefins are selectively oxidized to ketones or aldehydes by hydrated Pd, leaving Pd . The Pd is oxidized back to Pd by 2Cu, and the resulting Cu is reoxidized by dissolved oxygen. Because the corrosive nature of the catalyst solution is a serious disadvantage for practical use, supported copper-palladium catalysts have been proposed to operate the reaction in a gas flow reactor (40). 

Palladium chemistry dominates this area and the main problems are related to the way of reoxidizing Pd° efficiently. In general the reaction could be made catalytic in palladium by the use of an additional oxidant capable of reoxidizing the Pd to Pd . Typically, stoichiometric copper chloride, or catalytic amounts of copper chloride in the presence of air, have been used [28]. Other catalyst systems which have been described for bisalkoxycarbonylation of olefins to succinate derivatives are PdCl2 and butyl nitrite [29], Pd(OAc>2, O2 and benzoqui-none [30], and Pd(acac)2 and di-t-butyl peroxide [31]. So far, low TONs have delayed industrial applications. Because the reoxidation process is generating water, which causes side reactions, it is also necessary to add a water scavenger such as triethyl orthoformate in order to obtain good conversions and selectivities. 

Hydrogenation Copper chromite (Lazier catalyst). Copper chromium oxide (Adkins catalyst). Lindlar catalyst (see also Lithium ethoxyacetylide, Malealdehyde, Nickel boride). Nickel catalysts. Palladium catalysts. Palladium hydroxide on carbon. Perchloric acid (promoter). Platinum catalysts. Raney catalysts, Rhenium catalysts. Rhodium catalysts. Stannous chloride. Tributylborane. Trifluoroicetic acid, Tris (triphenylphosphine)chlororhodium. 

Even with our modified definition of indifferent , we still require that the catalytic material should act indefinitely once introduced. This requirement is also fulfilled by a number of essential materials added to some catalytic processes, and often referred to as co-catalysts or promoters. For example, the copper (I)-copper (II) chloride redox system used in Wacker s palladium-catalysed oxidation of ethylene to acetaldehyde (section 11.7.7.3) behaves in a true catalytic manner in the single-reactor variant of the process (ethylene and O2 introduced into the same reaction vessel). 

Several studies have used palladium catalysis in the arylation of benzoxazoles. A palladium catalyst with a phosphine ligand allows their reaction with aryl mesylates without the requirement for acid or copper additives. In the reaction with arene-sulfonyl chloride, palladium is used in combination with copper. A plausible mechanism involves initial cupration of the benzoxazole followed by copper—palladium exchange and oxidative addition of the sulfonyl chloride to palladium to give (84). This intermediate may lose sulfur dioxide to give an aryl palladium species, which, on reductive elimination, yields 2-arylbenzoxazole. The arylation of benzoxazoles and benzthiazoles with aryl boronic acids is also catalysed by a combination of palladium... 

Also, catalytic systems were studied by early workers. With palladium chloride or palladium acetate as catalyst, and copper(ll) chloride as oxidant, complicated reaction... 

Palladium/graphite in combination with copper(ll) chloride and flthium chloride is a good catalytic system for the oxidative dicarbonylation of alkenes using a 20 1 ratio of CO/O2 (Eq. 16). The ratio of diester to dimethyl carbonate is sensitive to the nature of the palladium catalyst precursor (Pd/graphite or PdCl2). 

A new mild procedure for the conversion of terminal acetylenes directly into acetylene carboxylates using carbon monoxide at atmospheric pressure and a palladium catalyst has been described (e.g. Scheme 11To enable palladium chloride, the oxidizing species, to be used in catalytic quantities, a stoicheiometric amount of copper(ii) chloride is added as a reoxidant. 

The applicability of the decarboxylative biaryl synthesis was rapidly extended to a broad range of aryl electrophiles with the help of new catalyst generations, including not only aryl iodides, bromides, and triflates but also the inexpensive but unreactive aryl chlorides and tosylates [46, 47]. Its preparative utility was demonstrated, e.g., in the synthesis of telmisartan and valsartan [48, 49]. The key factor in these advances was the identification of ligands that strongly activate the palladium catalysts toward oxidative addition steps while not interfering with the decarboxylation activity of the copper cocatalysts (Scheme 14). 

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