Wacker oxidation of ethylene

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

These oxidation reactions provide a powerful strategy for the synthesis of cyclohexanone by a combination of Wacker oxidation of ethylene with the present metal-catalyzed oxidation of cyclohexane (Scheme 3.11). 

Among the several types of homogeneously catalyzed reactions, oxidation is perhaps the most relevant and applicable to chemical industry. The well-known Wacker oxidation of ethylene to ethylene oxide is the classic example, although this is not a true catalytic process since the palladium (II) ion becomes reduced to metallic palladium unless an oxygen carrier is present. Related to this is the commercial reaction of ethylene and acetic acid to form vinyl acetate, although the mechanism of this reaction does not seem to have yet been discussed publicly. Attempts to achieve selective oxidation of olefins or hydrocarbons heterogeneously do not seem very successful. 

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) ... 

AUcenes, alkynes, and rr-compounds including even arenes can reduce Pd(II) species to Pd(0) species. Recall that reduction of PdCl2 with ethylene and water is a crucial part of the Wacker oxidation of ethylene (Scheme 2 of Sect. LI). Many other related processes of this type are discussed in Sect V.3. 

First World War, were based on the oxidation of acetaldehyde derived from either acetylene or fermentation ethanol. The latter could well return to favour in countries such as Brazil (section 12.7.1.). After the Second World War, fermentation ethanol gave way to synthetic ethanol, via the direct hydration of ethylene. (Synthetic ethanol made by the sulphuric acid process had already made some inroads in the U.S.A.). From 1960 onwards, the Wacker oxidation of ethylene added a further option for acetaldehyde manufacture. 

Many types of palladium-catalyzed oxidative fimctionalizations of olefins related to the Wacker process have been developed, and these reactions are presented later in this chapter. To imderstand the relationship between these reactions and the basic Wacker oxidation of ethylene to form acetaldehye, the mechanism of the Wacker process is discussed before the related oxidation processes. 

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-... 

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