Wacker process palladium catalysis

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

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 ... 

Catalysis. The most important industrial use of a palladium catalyst is the Wacker process. The overall reaction, shown in equations 7—9, involves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-catalyzed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or in a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chlorinated by-products. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

The field of homogeneous palladium catalysis traces its origin to the development of the Wacker process in the late 1950s (Eq. 7) [83]. Since this discovery, palladium-catalyzed reactions have evolved into some of the most versatile reactions for the synthesis of organic molecules [84,85]. Palladium-catalyzed Wacker-type oxidation of alkenes continues to be an active field of research [86-88], and several recent applications of NHC-coordinated Pd catalysts have been reported for such reactions. 

Conversion of ethylene to acetaldehyde with a soluble palladium complex was one of the early applications of homogeneous catalysis. Traditionally, acetaldehyde was manufactured either by the hydration of acetylene or by the oxidation of ethanol. As most of the acetic acid manufacturing processes were based on acetaldehyde oxidation, the easy conversion of ethylene to acetaldehyde by the Wacker process was historically a significant discovery. With the... 

The oxidation of ethene by palladium salts in water to give acetaldehyde has been known for 100 years see Oxidation Catalysis by Transition Metal Complexes). It is often called the Wacker Process, after Wacker Chemie GmbH, which first developed the process. The key steps in this oxidation are shown in Scheme 2. Palladium catalyzes the nucleophilic addition of water to ethene, leading to the reduction of Pd to Pd°. Then the palladium is reoxidized back to Pd with Cu salts, giving Cu which in turn is oxidized by oxygen. 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

Homogeneous catalysis by redox metals is also known for nonelectro-chemical processes. Thus, ethylene is oxidized to acetaldehyde in the Wacker process in aqueous solutions containing Pd " (504). Apart from complex formation and insertion (505), ionic oxidation and reduction may take place. It is noteworthy that palladium oxidation to form ions that act as homogeneous catalysts has been suggested as an important step in ethylene electrooxidation on solid palladium electrocatalysts 28, 29). 

The synthesis of 2-oxoalkylphosphonates may also be accomplished in good yields under palladium catalysis conditions (Wacker process). 

The oxidation of olefins to carbonyl compoimds (the Wacker process in technical concerns, also called the Hoechst-Wacker process) was of great importance for the recognition of the usefulness of organometalhc homogeneous catalysis in the bulk chemicals industry [32]. The Wacker ethylene oxidation is one of the key steps in industrial homogeneous catalysis. Palladium catalysts are usually applied and have... 

After the invention of the Wacker process, much attention was devoted to the commercial production of vinyl acetate via the acetoxylation of ethylene. The study originated from the result reported by Moiseev and co-workers that no acetoxylation of ethylene in acetic acid takes place in the absence of NaOAc (Scheme The reaction of Pd(OAc>2 and ethylene gives vinyl acetate. Since the palladium(II) salt employed in these reactions is reduced to Pd(0), co-oxidants are required for the catalysis, similar to the Wacker process. However, a simple combination of PdCl2, CuCl2, and O2 in AcOH results in various products as shown in Scheme 4. 

Only a small minority of organometallic reactions have cleared the hurdle to become catalytic reality in other words, catalyst reactivation under process conditions is a relatively rare case. As a matter of fact, the famous Wacker/Hoechst ethylene oxidation achieved verification as an industrial process only because the problem of palladium reactivation, Pd° Pd", could be solved (cf. Section 2.4.1). Academic research has payed relatively little attention to this pivotal aspect of catalysis. However, a number of useful metal-mediated reactions wind up in thermodynamically stable bonding situations which are difficult to reactivate. Examples are the early transition metals when they extrude oxygen from ketones to form C-C-coupled products and stable metal oxides cf. the McMurry (Ti) and the Kagan (Sm) coupling reactions. Only co-reactants of similar oxophilicity (and price ) are suitable to establish catalytic cycles (cf. Section 3.2.12). In difficult cases, electrochemical procedures should receive more attention because expensive chemicals could thus be avoided. Without going into details here, it is the basic, often inorganic, chemistry of a catalytic metal, its redox and coordination chemistry, that warrant detailed study to help achieve catalytic versions. 

The Wacker oxidation [146], amongst other nucleophilic additions to alkenes, is the most important reaction based on a palladium(II) catalysis. It is also used industrially for the synthesis of acetaldehyde from ethene and water. This oxidative process has been combined with a Mizoroki-Heck reaction by Tietze and coworkers [13] for an enantioselective total synthesis of vitamin E (293) [147] using BOXAX ligand 291 [148]. In this way the chromane ring and parts of the side chain of vitamin E (293) can be introduced in one... 

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). 

Water is so extensively used in catalytic oxidation reactions that usually this fact is regarded as a natural feature and remains unnoticed. Wacker oxidation of olefins by palladium complexes involves water as a nucleophilic reagent, and thus the whole Wacker-type chemistry, which has developed into a powerful and versatile method of organic synthesis, is derived from aqueous catalysis [178]. The role of the nature of the co-oxidant and the mechanism of deactivation of the palladium catalyst due to aggregation and growth of inactive metal particles were recently investigated, and such study may have relevance for other processes catalyzed by phosphine-less palladium catalysts [179]. 

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