Ethylene Wacker-type oxidation

Supported liquid-phase catalysts (SLPCs) combine the salient features of both homogeneous and heterogeneous catalysis for enhanced catalytic and/or process efficiency (337). SLPC catalysts, in which a liquid-phase (homogeneous) catalyst is dispersed within a porous support, have been used in Wacker-type ethylene oxidation for acetaldehyde and vinyl acetate production (337, 338). In the former case, a traditional homogeneous Wacker catalyst (vide supra) consisting of a chlorinated solution of Pd and Cu chlorides retained on a support with monomodal pore size distribution... 

An interesting approach to overcome these limits and thus combine the advantages of homogeneous and heterogeneous catalysis is that of supported liquid phase catalysts (SLPC or SLP). In SLPC the organometallic complex active components are dissolved in a small quantity of liquid phase dispersed in the form of an isle or film on the surface of supports. A SLPC has been applied successfully for several chemical transformations [113], particularly in the Wacker-type ethylene oxidation to acetaldehyde and vinyl acetate production by ethylene acetoxylation [114], and in other reactions catalyzed by Pd-complexes such as the Heck reaction [115]. 

In 1960, Moiseev and coworkers reported that benzoquinone (BQ) serves as an effective stoichiometric oxidant in the Pd-catalyzed acetoxylation of ethylene (Eq. 2) [19,20]. This result coincided with the independent development of the Wacker process (Eq. 1, Scheme 1) [Ij. Subsequently, BQ was found to be effective in a wide range of Pd-catalyzed oxidation reactions. Eor example, BQ was used to achieve Wacker-type oxidation of terminal alkenes to methyl ketones in aqueous DMF (Eq. 3 [21]), dehydrogenation of cyclohexanone (Eq. 4 [22]), and alcohol oxidation (Eq. 5 [23]). In the final example, 1,4-naphthoquinone (NQ) was used as the stoichiometric oxidant. 

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. 

With oxo synthesis, Wacker-type oxidations of alkenes is one of the older homogeneous transition-metal-catalyzed reactions [1], The most prominent example of this type of reaction is the manufacture of acetaldehyde from ethylene. This well-known reaction, which has been successfully developed on an industrial scale (Wacker process), combines the stoichiometric oxidation of ethylene by palladium ) in aqueous solution with the in situ reoxidation of palladium(O) by molecular oxygen in the presence of a copper salt (Eqs. 1 -4) [2]. 

When media other than water are used, different but related processes operate. Thus, the oxidation of ethylene in acetic acid can be directed to give vinyl acetate, ethylene glycol acetate, or 2-chloroethyl acetate [9]. Similarly, the synthesis of acetals or ketals can be achieved in an alcoholic medium [10]. Although the oxidation of alkenes in such a medium is closely parallel to the Wacker process, the chemistry of these reactions is far beyond the scope of this section, which is limited to Wacker-type reactions in aqueous media, and will not be discussed here. 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

The Wacker-type oxidation of the olefins is one of the oldest homogeneous transition metal-catalyzed reactions. The mechanism of the oxidation of ethylene to acetaldehyde by a PdCl2/CuCl2/02 system is shown in Figure 23. Interestingly, the selectivity of the oxidation of olefins with longer alkyl chains is dependent on their solubility in water. Furthermore, the production of chlorinated side-products and isomerized olefins has also occurred for olefins with low water solubility. In order to avoid the solubility issues, co-solvents such as DMSO, acetone, THF, dioxane, acetonitrile, DMF, and ethanol were used and DMF seemed to be the best. ... 

Wacker-type oxidations. Ethylene is oxidized to acetaldehyde in the presence of an aqueous solution of palladium (II) chloride and copper (II) chloride . The initial reaction is believed to follow a sequence of the type... 

The oxidative vinylation reaction of ethylene with alcohols in the presence of oxygen has been reported [14,15] to give vinyl ethers in high yields. Like many Wacker-type reactions, this reaction is typically catalyzed by heterogeneous and homogeneous catalysts containing palladium. 

What is the oxidation product of ethylene by O2 in the presence of CO in a Wacker-type catalytic process Write the mechanism. 

On the basis of the mechanism of Wacker-type oxidation reaction, what are the expected products if (a) but-l-ene is the substrate (b) but-2-ene is the substrate and (c) ethylene is oxidized in ROH rather than in water ... 

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

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 ... 

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. 

The use of liquid membranes for controlling chemical reactions such as that just discussed has been proposed for a number of other systems. This type of application, in which liquid membranes are used as heterogeneous catalysts or as reaction moderators, is an area that deserves more study. Ollis et al. and Wolytdc and Ollis studied liquid membranes as heterogeneous catalyst systems using the catalytic oxidation of ethylene to acetaldehyde (Wacker process) as a model. This process entails the following three... 

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