Wacker process mechanism

Early mechanistic studies have indicated that the oxypalladation step in the Wacker process proceeds through an <37z/z-pathway,399 although recent deuterium-labeling experiments have shown the viability of a yy/z-mechanism involving insertion of a metal-coordinated oxygen into the alkene.400,401 For example, with excess chloride ion present, the Wacker-type cyclization of a deuterated phenol system occurred in a primarily //-pathway, whereas the oxypalladation step favored a yy/z-mode in the absence of excess chloride ion (Scheme 16). Thus, either mechanism may be operative under a given set of experimental conditions. 

The coordinative polymerization with soluble transition metal systems is part of the growing field of homogeneous catalysis on transition metal centers (Oxo-Process, Wacker-Process, Isomerization, Cyclooligomerization of olefis, etc.). The mechanisms of these reactions have not yet been completely elucidated. Any new knowledge could perhaps contribute to the detection of common trends and parallels, and would thus facilitate prediction and development of new processes. 

Show how these mechanisms in combination with others described in this chapter can explain how PdCI2 can convert CH2=CH2 to CH3CHO (Wacker process). Your mechanism must be in accord with the fact that, when the reaction is carried out in D20, there is no deuterium in the ethanal formed. 

The oxidation of olefins to aldehydes using a palladium chloride-copper(II) chloride catalyst, the Wacker Process, is a well-established industrial reaction. The mechanism of this reaction has not been established in detail, but it most probably involves a cr-7r rearrangement... 

In order to support the proposed mechanism for the Wacker Process, there has recently been growing interest in the preparation of stable n-vinyl alcohol complexes, preferably containing palladium or one of the other metals of the platinum group which show activity as Wacker catalysts. Until recently, the only well-characterized n complexes of vinyl alcohol have contained iron. One of the first reports of the formation of a stable 7r-vinyl alcohol complex by Ariyaratne and Green described the preparation of 7r-cyclopentadienyldicarbonyl(/3 oxoethyl)iron (39)... 

Fig. 9.3. Proposed mechanism for Wacker process for oxidation of ethylene to acetaldehyde.

Catalytic Oxidation of Ethene to Acetaldehyde and Acetic Acid. -Evnin et al120 studied Pd-doped V2 Os catalysts for the vapor-phase oxidation of ethene to acetaldehyde in a heterogeneous type of Wacker process. From a mechanistic study they establish a redox mechanism with Pd both as the site of the ethene oxidation and of the reoxidation of the catalyst. On the basis of the role of the V4+ ions proposed by these authors, Forni and Gilardi121 substantiated this mechanism by adding tetra- and hexa-valent dopants to the V2 05 and studying the effects on the catalytic reaction. 

Historically the homolytic type of catalysis has been known and studied for a long time. The heterolytic catalysts represent a relatively recent innovation but, nevertheless, include important developments such as the Wacker process for the oxidation of olefins. Regardless of the mechanism involved, the most important characteristics of metal catalysts for effecting oxidation are the accessibility of several oxidation states as well as the accommodation of various coordination numbers, both of which are properties of transition metal complexes. 

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. 

Baeckvall JE, Akermark B, Ljunggren SO (1979) Stereochemistry and mechanism for the palladium (Il)-catalyzed oxidation of ethenc in water (the WACKER-process), J Am Chem Soc 101 2411... 

The Wacker process was a major landmark and a great push towards the development of homogeneous catalysis. The mechanism of acetaldehyde formation differs fundamentally from the other oxidation processes as O2 itself is not directly involved. As is clear from Figure 28 the actual oxidant is Pd(II) which is reduced to Pd(0). The intimate pathway of the reaction involves nucleophilic attack and was the subject of much debate. 

Several important nomadical catalytic oxidations go via organometalhc mechanisms. The commercially useful Wacker process converts ethylene to acetaldehyde with air as oxidant, using Pd(II) and Cu(II) catalysts. The Pd(II) binds to the ethylene to give an organometalhc intermediate, the alkene complex. This complex subsequently uses water as the O source to oxidize the ethylene to acetaldehyde, the Pd being reduced in the process. The resulting Pd(0) is reoxidized to Pd(II) with two equivalents of Cu(n) and the Cu(I) so formed is then reoxidized by air to close the cycle. 

A long-standing controversy in the mechanism of the Wacker process has been the addition of water to the... 

The point has been made that the conditions of p-chloroethanol formation are not the same as used for the Wacker oxidation. Cu Pd chlorine-bridged dimers are likely reactants under higher [Cl ] reaction conditions, which may lead to a different reaction mechanism. However, a second stereochemical study also obtained results consistent with trans hydroxypaUadation. When cfr-l,2-dideuteroethene is oxidized in water with PdCl2 under a CO atmosphere, the product is tran5 -2,3-dideutero-jS-propiolactone (Scheme 37). The reaction conditions were, once again, not identical with standard Wacker process conditions, since the solvent was acetonitrile water, the temperature was —25°C, the bis-ethene PdCl2 complex was used, and there was no excess Cl present. Nevertheless, it is clear that, under many reaction conditions, a trans addition of water onto ethene coordinated to Pd is the favored reaction stereochemistry. 

Much of what is currently known about substitution reactions of square planar complexes came from a lar e number of careful studies executed in the I960s and I970S.3 You should not conclude, however, that details of the mechanisms of these I eactions are of historical intei est only. Work in this area continues unabated as studies focus on chelation, steric effects, biological i eactions. and homogeneous catalysts. For example, the mechanism for the Wacker process (Chapter 15), which utilizes squai e planar [PdCl ] as a homogeneous catalyst for the industrial conversion of ethylene to acetaldehyde, is still a subject of investigation. The overall reaction for the process is ... 

This article will be concerned with the mechanisms of some of these reactions and with some of the general principles that underly this relatively new and rapidly developing field of chemistry. The subject in question has attracted much interest in recent years both because of the novelty of much of the chemistry it reveals and because of its potential practical applications, exemplified by at least two processes (the Oxo and Wacker processes) which have already achieved considerable industrial importance. The possible relevance of many of the catalytic reactions in this field as model systems for related heterogeneous and enzymic process also lends interest to the subject although attempts to exploit this theme have thus far met with only limited success. 

Akermark, B., Soederberg, B. C., Hall, S. S. The mechanism of the Wacker process. Corroborative evidence for distal addition of water and palladium. Organometallics, 6, 2608-2610. 

---