Wacker oxidation of ethylene to acetaldehyde

Figure 8.2 Catalytic cycle for the Wacker oxidation of ethylene to acetaldehyde.

Despite the utilization of polyoxometallates as Bnansted acid catalysts in organic synthesis [13], the most important application is the palladium-catalyzed Wacker oxidation of ethylene to acetaldehyde in aqueous phase. Under standard conditions (PdCl2, CuCl2, 02, HC1), chlorine ions are corrosive and produce chlorinated by-products (mainly from CuCl2) these conditions are not suitable for the oxidation of higher olefins, such as 1-butene to methyl ethyl ketone. For this reason... 

Prior to 1979, the Wacker oxidation of ethylene to acetaldehyde was generally suggested to proceed via a cis addition of Pd(II) and coordinated hydroxide based upon kinetic evidence.The original rate law is given in equation (14) ... 

The last catalytic reaction we examine in this overview is the Wacker oxidation of ethylene to acetaldehyde with O2, now used to make about 4 million tons a year of aldehydes from alkenes. This reaction shows several new features of great interest. Although the work started with a commonplace observation— the stoichiometric oxidation of alkenes by Pd(II) salts with formation of Pd(0)— the authors were able to make the system catalytic by finding a clean way to reoxidize the Pd(0) to Pd(II) with air. The mechanism was obscure for years because the kinetics gave an incomplete picture and it was only with sophisticated labeling studies that the currently accepted mechanism was discovered. 

Fuel cell is a device to convert Gibbs free energy in chemical reaction into electricity through electrochemical cell reactions. In an H2-O2 fuel cell, electricity is obtained through formation of water from O2 and H2. When an acidic electrolyte is used, electrochemical oxidation of H2 to e and H" occurs at an anode and reduction of O2 with e and to H2O occurs at a cathode. The net reaction is formation of water from H2 and O2. In other words, catalytic reaction of water formation can be decomposed to two electrochemical reactions at an anode and cathode. This principle indicates that catalytic oxidation and reduction in chemical synthesis can convert fuel cell reactions at an anode and cathode. For example, the Wacker oxidation of ethylene to acetaldehyde with O2 would be able to perform using fuel cell reactions. 

To illustrate the inner-sphere characteristics of the CH activation chemistry, an analogy can be made between CH activation by coordination of an alkane CH bond to a metal center and the known catalysis resulting from coordination of olefins via the CC double bond (note that the nature of the orbitals involved in bonding are quite different). It is well known that coordination of olefins to electrophilic metal centers can activate the olefin to nucleophilic attack and conversion to organometallic, M-C, intermediates. The M-C intermediates thus formed can then be more readily converted to functionalized products than the uncoordinated olefin. An important example of this in oxidation catalysis is the Wacker oxidation of ethylene to acetaldehyde. In this reaction, catalyzed by Pd(II) as shown in Fig. 7.14, ethylene is activated by coordination to the inner-sphere of an electrophilic Pd(II) center. This leads to attack by water and facile formation of an organometallic, palladium alkyl intermediate that is subsequently oxidized to acetaldehyde. The reduced catalyst is reoxidized by Cu(II) to complete the catalytic cycle. The Wacker reaction is very rapid and selective and it is possible to carry out the reaction is aqueous solvents. This is largely possible because of the favorable thermodynamics for coordination of olefins to transition metals that can be competitive with coordination to the water solvent. The reaction is very selective presumably because the bonds of the product (po-... 

Ca.ta.lysis, The most important iadustrial use of a palladium catalyst is the Wacker process. The overall reaction, shown ia equations 7—9, iavolves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-cataly2ed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or ia a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chloriaated by-products. 

The Wacker process for the oxidation of ethylene to acetaldehyde with PdCb/CuCb at 100°C (212°F) with 95 percent yield and 95 to 99 percent conversion per pass. 

Palladium-catalyzed oxidation of hydrocarbons has been a matter of intense research for about four decades. The field was initiated by the development of the aerobic oxidation of ethylene to acetaldehyde catalyzed by palladium chloride and co-catalyzed by cupric chloride (the Wacker process, equation l)1. 

The complex ion catalyzes various types of organic reactions including oxidation of ethylene to acetaldehyde in aqueous solution (the Wacker Process) ... 

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-... 

Although the oxidation of ethylene to acetaldehyde was known for a number of years,506 its utility depended on the catalytic regeneration of Pd(0) in situ with cop-per(II) chloride discovered by Smidt and coworkers.507 508 Air oxidation of Cu(I) to Cu(n) makes a complete catalytic cycle. This coupled three-step transformation is known as the Wacker process [Eqs. (9.97)-(9.99)]. The overall reaction [Eq. (9.100)] is the indirect oxidation with oxygen of alkenes to carbonyl compounds ... 

Selective oxidation of ethylene to acetaldehyde was carried out over carbon-supported Pd and Pt membrane catalysts.1322 The concept of supported liquid-phase catalysis was also successfully applied in the Wacker oxidation.1323 The Wacker reaction can be performed in alcohol-supercritical C02.1324 C02 as cosolvent accelerates reaction rates and remarkably affects the selectivity towards methyl ketone in the presence of an alcohol. 

The oxidation of ethylene to acetaldehyde in the gas phase, carried out at rather low temperatures (100—200°C), is very similar to the Wacker liquid phase process. One of the main steps in this process is... 

WACKER REACTION. The oxidation of ethylene to acetaldehyde in the presence of palladium chloride and cupric chloride. 

Heterolytic liquid-phase oxidation processes are more recent than homolytic ones. The two major applications are the Wacker process for oxidation of ethylene to acetaldehyde by air, catalyzed by PdCl2-CuCl2 systems,98 and the Arco oxirane" or Shell process100 for epoxidation of propylene by f-butyl or ethylbenzene hydroperoxide catalyzed by molybdenum or titanium complexes. These heterolytic reactions require less drastic conditions than the homolytic ones... 

Discovered by Phillips in 1894,382 the oxidation of ethylene to acetaldehyde by palladium(ll) salts in an aqueous solution was developed into a commercial process about 60 years later by Smidt and coworkers at Wacker Chemie.383,384 These researchers succeeded in transforming this stoichiometric oxidation by a precious metal (equation 150) into a catalytic reaction through the reoxidation of the resulting Pd° by molecular oxygen in the presence of copper salts (equations 151-152). 

Another example is the palladium-catalyzed oxidation of ethylene to acetaldehyde in the presence of oxygen and cupric salts, the so-called Wacker reaction. This catalytic cycle combines two stoichiometric processes, which involve first the reduction of Pd11 to Pd°, followed by reoxidation with Cu11. The understanding of the first step of this process came from the earlier work of Kharasch et al., who showed that the stoichiometric dinuclear complex shown in Figure 2.14 decomposed in the presence of water to acetaldehyde (ethanal), Pd° and HC1 [38]. 

The aqueous palladium chloride oxidation of ethylene to acetaldehyde has been developed into an important commercial process. The discovery of how to make the reaction catalytic with respect to palladium chloride was, perhaps, as important to the process as the discovery of the oxidation reaction itself. This process known as the Wacker-Process, employs cupric chloride as a catalyst for the oxygen (air) reoxidation of... 

The palladium-catalyzed oxidation of ethylene to acetaldehyde (the Wacker process) was discovered by Smidt and co-workers514-518 in 1959. This process combines the stoichiometric reduction of Pd(II) with reoxidation of metal in situ by molecular oxygen in the presence of copper salts. The overall reaction constitutes a palladium-catalyzed oxidation of ethylene to acetaldehyde by molecular oxygen ... 

Fig Proposed mechanism for the oxidation of ethylene to acetaldehyde in the Wacker process. Chloride ligands have been omitted. The oxidation number ofpalladium is + 2 at all stages of this cycle except the upper left where eductive elimination of acetaldehyde gives Pd (0), which is oxidised by Cu (II). The complete cycle for the reoxidation of Cu (I) is not shown. 

Ever since the initial discovery of the Wacker process [1], i.e. the Pd/Cu-catalyzed oxidation of ethylene to acetaldehyde (1) in water, methods for the palladium (II) - mediated oxidative functionalization of alkenes have found widespread application in the synthesis of complex molecules [2J. 

Figure 28 shows that the chemistry involved in the Wacker process could in principle be extended to other nucleophiles. The modern catalytic manufacturing process making vinyl acetate from ethylene and acetic acid is based on the observation that palladium catalyzed oxidation of ethylene to acetaldehyde can be converted into an acetoxylation reaction if carried out in a solution of acetic acid and in the presence of sodium acetate (Equation 42). 

The oxidation of ethylene to acetaldehyde using PdCb and CuCb as catalysts undo- an oxygm atmosphere is well known as the Wacker process (Scheme 1), and is one of the most important industrial processes employing transition metal catalysts.This industrial oxidation reaction of ethylene involves the following three stoichiometric reactions. These sequential oxidation and reduction reactions constitute a catalytic cycle. 

Copper chloride complexes can be used as catalysts in a number of organic reactions. Examples include the Wacker process, which is the oxidization of ethylene to acetaldehyde by oxygen and aqueous Cu and Pd precatalysts (or, alternatively using iron catalysts) plus the synthesis of acrylonitrile from acetylene and hydrogen cyanide using CuCl. Cuprous chloride has also been used as a desulfiuizmg and... 

Nucleophilic addition to metal-activated alkenes as a synthetic method can be traced to the Wacker Process, the oxidation of ethylene to acetaldehyde with Pd and... 

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