Wacker ethylene oxidation

However, even with the use of high temperatures and pressures, low productivity and poor selectivity did not make MeOH carbonylation to AcOH attractive commercially compared with processes such as oxidation of butane or naptha fractions or the Pd catalysed oxidation of ethylene (Wacker). 

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. 

One of the earliest uses of palladium(II) salts to activate alkenes towards additions with oxygen nucleophiles is the industrially important Wacker process, wherein ethylene is oxidized to acetaldehyde using a palladium(II) chloride catalyst system in aqueous solution under an oxygen atmosphere with cop-per(II) chloride as a co-oxidant.1,2 The key step in this process is nucleophilic addition of water to the palladium(II)-complexed ethylene. As expected from the regioselectivity of palladium(II)-assisted addition of nucleophiles to alkenes, simple terminal alkenes are efficiently converted to methyl ketones rather than aldehydes under Wacker conditions. 

For long time, Pd has been used mainly as a heterogeneous catalyst for the hydrogenation of unsaturated bonds. A revolution in Pd chemistry occurred with the development of homogeneous Pd catalysts. The first example was the invention of the Wacker process in 1959, by which ethylene is oxidized to acetaldehyde using PdCl2 and CuCl2 as catalysts in aqueous solution (eq. 1.10) [13]. 

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

As shown, osmium tetroxide bearing the chiral ligand interacts with the olefin to give an Os(VI) ester, which upon hydrolysis releases the chiral diol. The actual oxidant is the metal itself that reduces from Os(VIII) to Os(VI). This reaction was known since the 1930 s and in this respeet it resembles the Wacker system where ethylene is oxidized to acetaldehyde with reduction of Pd(II) to... 

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. 

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. 

This development began to reduce steadily the capacities of acetaldehyde which previously had been made by oxidation of ethylene (Wacker-Hoechst process cf. Section 2.4.1) and converted to acetic acid (cf. Section 2.4.4). Moreover, the Monsanto process, the second-generation process for methanol carbonylation is now being followed by the third generation of highly efficient carbonylation processes, enabling acetic anhydride as well as acetic acid to be produced (cf Scheme 2 Tennessee-Eastman [36] and BP [37] processes). The most advanced process (Hoechst [40]) has so far not been implemented industrially because of neglects... 

Route a is the Moiseev reaction-, the industrial Wacker oxidation (route b) is Pd-catalyzed and produces acetaldehyde by ethylene oxidation in aqueous solution [4, 8, 9]. 

Oxidation of ethylene (Wacker) PdCf/CuCf (Section 2.4.1) kCKCpiCf- = 2 [13]... 

The Waclcer oxidation (pronounced vocker ) is used industrially to convert ethylene and O2 into acetaldehyde. The Wacker oxidation is catalyzed by PdCl2 and CUCI2 and requires H2O as solvent. The O atom in the product comes from the water, not the O2. 

The mechanism of the Wacker oxidation is simply another Pd-catalyzed nucleophilic substitution of an alkene, with H2O as the nucleophile. H2O adds to a Pd(II) complex of ethylene, and /3-hydride elimination occurs to give a 77 complex of the enol of acetaldehyde. After rotation about the Pd(II)-alkene a bond, the alkene reinserts into the Pd(II)-H bond to give a new Pd(II)-alkyl. This complex undergoes /3-hydride elimination one more time to give acetaldehyde itself and Pd(II)-H. Deprotonation of the Pd(II) complex converts it to Pd(0), and oxidation of Pd(0) by air (see below) brings it back to Pd(II). 

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). 

Acetaldehyde synthesis by liquid phase oxidation of ethylene (Wacker-Hoechst processes)... 

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... 

The Wacker oxidation is one of the longest known palladium-catalyzed organic reactions. It is the industrial process where ethylene is oxidized to acetaldehyde with oxygen in the presence of a catalytic amount of palladium and copper salt as the redox co-catalyst.1 There are plants that produce thousands of tons of acetaldehyde per year. Many reviews exist on this topic.2-6... 

The first process to be discussed is a traditional one that probably does not occur via a Jt-complex. Nonetheless, it sets the stage for the organopalladium chemistry to be discussed in this section. The Wacker process (or the Wacker oxidation) is used in industry to convert ethylene to acetaldehyde using soluble palladium catalysts.207 in this reaction, ethylene reacts with cupric chloride (CuCl2) and palladium chloride (PdCl2) to... 

Karakhanov s group has also been exploring poly(ethylene oxide)- and poly(alkene oxide)-copolymer-bound catalysts [99-102]. A notable aspect of this work is the design of polyethers like 39 that contain jS-cyclodextrins and calyx[4]- and calyx[6]arenes. Such polyethers couple the molecular recognition associated with these macrocycles with the catalytic activity of acac, phosphine, dipyridyl, and catechol ligands. Metals complexed to such ligands have been used in reactions like hydroformylation, Wacker oxidations, and arene hy-droxylation. 

When lighter and harder X groups are involved (X = NR2 and OR), insertion is less favored (see Section 6.4.2 (a)) and other mechanistic pathways, particularly nucleophilic attack in the case of late transition metals, are prevalent. This is the case of an important catalytic process, the Wacker oxidation of alkenes that transforms ethylene to acetaldehyde or terminal alkenes in ketones. For a long time a controversy was on, regarding the nature of the step that leads to the new... 

Note that, in some studies, poly(ethylene oxide) oligomers, micelle-forming surfactants derived from them, and their complexes were used to perform such reactions in aqueous and alcohol solutions as hydroformylation [31-33], Wacker oxidation [34,35], hydroxylation of aromatic compounds [36-38], carbon dioxide hydrogenation [39], and epoxidation [40]. It was shown that using poly(ethylene oxide)s substantially increases the reaction rate and, in some cases, allows us to separate a metal complex containing oligo(ethylene oxide) [31,40]. 

Water-soluble macromolecular metal complexes based on terminally functionalized ethylene oxides and ethylene oxide-propylene oxide block copolymers have been used as catalysts for hydroformylation, hydrogenation, Wacker oxidation of imsaturated compounds, hydroxylation of aromatic compounds, oxidation of saturated and alkylaromatic hydrocarbons, metathesis, Heck reaction, and some asymmetric reactions. 

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