Wacker mechanism

In this context it is also worth mentioning that Showa Denko has developed a new process for the direct oxidation of ethene to acetic acid using a combination of palladium(II) and a heteropoly acid [104]. However, the reaction probably involves heteropoly acid-catalyzed hydration followed by palladium-catalyzed aerobic oxidation of ethanol to acetic acid rather than a classical Wacker mechanism. 

An important issue in the Wacker mechanism concerns the nature of the cata-lytically active Pd species. Ligand exchange at [PdCLj] was studied by Kovacs et al. [90] and Nair and coworkers [91, 92] by means of AIMD simulations in explicit water. Similar protocols were used, applying periodic boundary conditions to a cell of 10 A edge including the reactants and about 35 water molecules. The reactive events were explored by means of metadynamics. 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

Under the conditions of the Wacker oxidation, 4-trimethylsilyl-3-alkyn-l-ols give 7 -lactones. Similarly, A-carbamoyl or A-acetyl 4-trimethylsilyl-3-alkynamines cyclize to y -lactams. Formulate a mechanism for these reactions. (Hint In DzO,... 

It is of particular value from a synthetic viewpoint that, under Wacker-type conditions the cyclization is catalytic [Pd(OAc)2/Cu(OAc)2/MeOH,H20/ air/55°C]121 albeit not by a Wacker-type mechanism.123 Addition of the chiral alkene, (- )-/l-pinene, to the catalyzed cyclization of 2-(but-2-enyl)phenol causes asymmetric induction with an optical yield of 12%... 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Since nucleophilic addition to a metal-coordinated alkene generates a cr-metal species bonded to an -hybridized carbon, facile 3-H elimination may then ensue. An important example of pertinence to this mechanism is the Wacker reaction, in which alkenes are converted into carbonyl compounds by the oxidative addition of water (Equation (108)), typically in the presence of a Pd(n) catalyst and a stoichiometric reoxidant.399 When an alcohol is employed as the nucleophile instead, the reaction produces a vinyl or allylic ether as the product, thus accomplishing an etherification process. 

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. 

Hegedus proposed the probable course of the cyclization reaction, which follows a Wacker-type reaction mechanism. Coordination of the olefin to Pd(II) results in precipitate 110, which upon treatment with Et3N undergoes intramolecular amination to afford intermediate 111. As expected, the nitrogen atom attack occurs in a 5-exo-trig fashion to afford 112. Hydride... 

The reaction is highly exothermic as one might expect for an oxidation reaction. The mechanism is shown in Figure 15.1. Palladium chloride is the catalyst, which occurs as the tetrachloropalladate in solution, the resting state of the catalyst. Two chloride ions are replaced by water and ethene. Then the key-step occurs, the attack of a second water molecule (or hydroxide) to the ethene molecule activated towards a nucleophilic attack by co-ordination to the electrophilic palladium ion. The nucleophilic attack of a nucleophile on an alkene coordinated to palladium is typical of Wacker type reactions. 

The mechanism is similar to that of the Wacker oxidation (page 610). 

Figure 9.2 Mechanism of the Wacker reaction. Source White)...

In addition to the constitutive production of food supplements, some plants can actively adjust their food provision in response to their biotic environment (Table 2.2). Unlike other defense mechanisms, this induction can be elicited by two distinct mechanisms. Food provision can be raised both by food removal (Risch and Rickson, 1981 Koptur, 1992 Heil el al., 2000) and by tissue damage (Koptur, 1989 Wackers and Wunderlin, 1999 Heil et al, 2001 Wackers el al., 2001). These mechanisms represent active responses by the plants to both ant attendance and herbivore feeding. This receptiveness toward the presence of both the second and the third trophic level represents a unique and highly dynamic type of plant response. 

Gossypium herbaceum Herbivory (caterpillar) and mechanical 10-fold increase No No Wackers and Wunderlin, 1999 Wackers ef a/., 2001... 

The mechanism of vinyl acetate formation is closely related to that of the Wacker oxidation (Scheme 9.11) that is, acetoxypalladation-palladium hydride elimination takes place.498,503 The coordinated alkene is attacked by the external nucleophile acetate ion, or the attack may occur within the coordination sphere. p-Hydride elimination followed by dissociation of the coordinated molecule yields directly the vinyl acetate end product. 

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. 

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