Copper chloride reoxidant

The previous examples involve reduction (hydrogenation) of organic molecules, but transition metal complexes can also catalyze oxidation. For example, the Wacker process, which has been widely used to convert ethylene to acetaldehyde, depends on catalysis by palladium(II) in the presence of copper(II) in aqueous HC1. The role of the copper chloride is to provide a means of using air to reoxidize the palladium to palladium(II). Once again, Zeise-type coordination of the ethylene to the metal center is believed to be involved ... 

A very few examples of the use of aryl-tin,21 -lead,21 -lithium43 and -magnesium21143,44 derivatives as the source of the aryl group in vinyl substitutions have been reported. Tin derivatives have been used with palladium dichloride bis(benzonitrile) and a copper(II) chloride reoxidant in a regioselective synthesis of oxygen heterocycles from unsaturated alcohols.43... 

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. 

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. 

While these initial examples were performed in the presence of stoichiometric amounts of palladium, the first catalytic dialkoxycarbonylation of olefins was independently described by Fenton [5] and Medema [6] in 1969 and 1970. More specifically, a catalytic amount of palladium was used together with an equivalent of CuCL, and the reactions were run at high pressure of CO and comparatively high reaction temperatures (140-150 °C). Heck demonstrated that CuCL is not able to efficiently reoxidize Pd(0) at low temperatures [7-9]. In 1972 Fenton and Steinwand reported on the oxidative carbonylation of olefins to succinates [10]. For the reoxidation of palladium, iron and copper chlorides were used, but oxygen should also have been present—otherwise only low yields of succinates were obtained. A related study of the hydroxycarbonylation of olefins was described by the same group [11]. Nowadays, this type of reaction is efficiently performed in the presence of protic acids. ... 

The acetoxylation of ethylene to form the vinyl acetate monomer (VAM) can be catalyzed by homogeneous catalysts comprised of PdCl/CuCl salts and carried out in glacial acetic acidl . The reaction is catalyzed by the Pd + species which are reduced by the adsorption of ethylene and subsequently reoxidized by oxygen and copper chloride. The speculated mechanism is as follows... 

In the presence of water, Wacker cheudstiy tends to predominate, whereby ethylene reacts with water rather than acetic acid and forms acetaldehyde as the primary product. Heniy and Pandeyl l showed that the addition of alkali metal acetates can help to shift the product spectrum in order to form vinyl acetate in higher yields. The reaction is thought to involve the nucleophillic attack of ethylene by acetate to form a C2H4-OAC-Pd complex which subsequently undergoes /3-C-H activation to form VAM. Acetic acid or HCl can desorb from the complex to form Pd ° which is reoxidized back to copper chloride to regenerate Pd2+. 

The process of choice for acetaldehyde production is ethylene oxidation according to the so-called Wacker-Hoechst process [route (c) in Topic 5.3.2]. The reaction proceeds by homogeneous catalysis in an aqueous solution of HQ in the presence of palladium and copper chloride complexes. The oxidation of ethylene occurs in a stoichiometric reaction of PdQ2 with ethylene and water that affords acetaldehyde, metallic palladium (oxidation state 0), and HQ [step (a) in Scheme 5.3.5). The elemental Pd is reoxidized in the process by Cu(II) chloride that converts in this step into Cu(I) chloride [step (b) in Scheme 5.3.5). The Cu(II) chloride is regenerated by oxidation with air to finally close the catalytic cycle [step (c) in Scheme 5.3.5). 

The addition of carbon nucleophiles to nonactivated alkenes has been discovered by Hegedus in the 1980s using stoichiometric amounts of palladium salts. In the last decade, an oxidative eatalytic version has been elaborated mainly by the groups of Pei et and Stoltz. An example is shown in Scheme 5-188. The reoxidation of the palladium(0) generated is promoted by either stoichiometric amount of copper chloride or under aerobic conditions with catal3dic quantities of copper salts. 

Catalyst regeneration occurs by the reaction of thallium(I) chloride with copper(II) chloride in the presence of oxygen or air. The formed Cu(I) chloride is reoxidized by the action of oxygen in the presence of HCI ... 

Nitroso compounds are usually not obtained directly but rather by reoxidation of hydroxylamino compounds or amines. Hydroxylamino compounds are prepared by electrolytic reduction using a lead anode and a copper cathode [573], by zinc in an aqueous solution of ammonium chloride [574 or by aluminum amalgam [147], generally in good yields. 

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. 

The vinyl substitution reaction often may be achieved with catalytic amounts of palladium. Catalytic reactions are carried out in different ways depending on how the organopalladium compound is generated. Usually copper(II) chloride or p-benzoquinone is employed to reoxidize palladium(0) to palla-dium(II) in catalytic reactions when methods (i) or (ii) are used for making the organopalladium derivative. The procedures developed for making these reactions catalytic are not completely satisfactory, however. The best catalytic reactions are achieved when the organopalladium intermediates are obtained by the oxidative addition procedures (method iii), where the halide is both the reoxidant and a reactant. Reviews of some aspects of these reactions have been published.u-le... 

All of the organomercury reactions may be carried out catalytically in palladium by use of copper(II) chloride as a reoxidant for the palladium. This modification, however, often reduces the yield of product and lowers the stereospecificity of the arylation.21,23... 

The cuprous chloride then reoxidizes rapidly with oxygen reforming cupric ion. Thus, the oxidation can be carried out completely catalytically in palladium and copper. 

Fortunately, most of the palladium addition reactions with olefins can be carried out catalytically in the palladium compound so that large amounts of the expensive palladium compounds are not needed. As in the inorganic palladium salt additions, cupric chloride is a useful reoxidant. This, of course, limits the catalytic reaction to cases where olefin isomerization is not a problem. The cupric chloride is reduced to cuprous chloride during the reaction. As in the acetaldehyde synthesis, the reaction may be made catalytic in copper as well as palladium by adding oxygen and, in this case, hydrogen chloride also. 

Both reactions were at the origin of the boom in palladium chemistry, scientifically as well as industrially. To render the previously cited reactions catalytic, the reduced form of Pd is reoxidized with the Cu2+/Cu+ redox couple, with the reduced form of copper finally being reoxidized with dioxygen. This chemistry is performed industrially with the chloride salts of Pd2+ and Cu2+ with ethylene as a feed, either in an aqueous or acetic acid medium. Products are acetaldehyde and vinyl acetate, respectively. 

The invention of the Wacker process was a triumph of common sense. It had been known since 1894 that ethylene is oxidized to acetaldehyde by palladium chloride in a stoichiometric reaction (Figure 27). However, it was not until 1956 that this reaction was combined with the known reoxidation reactions of palladium by copper and, in turn of copper by oxygen. The total process developed by Wacker and Hoechst between 1957 and 1959 can be depicted as an exothermic catalytic direct oxidation to yield acetaldehyde. 

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). 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

The oxidation of ethylene by palladium(ii)-copper(n) chloride solution is essentially quantitative and only low Pd concentrations are required the process can proceed either in one stage or in two stages in the latter the reoxidation by Oz is done separately.82... 

Again, difficulties sometimes arise in chloride media because chlorine is generated at the anode and some platinum may dissolve. Both cause difficulties at the cathode, the first by reoxidizing copper and the second by depositing platinum so hydrazine is often added as an anodic depolarizer since it is easily oxidized to nitrogen ... 

However, in the presence of copper (II) chloride, the palladium is reoxidized without significant metal precipitation, possibily via a chloride bridged Cu-Pd species. The copper (I) chloride is reoxidized to copper (II) chloride in situ, or in a separate stage. 

Moreover, in systems containing higher water concentrations, (5-10 % by weight) such as those resulting in technical conditions from direct O2 reoxidation when the reaction is performed at significant methanol conversion, exhaustive hydrolysis of copper methoxychloride leads to a number of soluble and insoluble cupric species containing chloride and/or hydroxide counterions in different ratios, particularly CuClj and Cu4Cl2(OH)g (atacamite). This reaction system results in DMC and cuprous chloride formation by reduction under CO. 

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