Metal insertion Wacker oxidation

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... 

The mechanism of the Wacker oxidation has been the subject of many mechanistic studies and much discussion for nearly 50 years. At this point, the identity of the elementary steps of this process appears to depend on the reaction conditions. The majority of the mechanistic discussion has focused on whether the C-O bond is formed by nucleophilic attack of water on a coordinated olefin or by insertion of an olefin into a metal-hydroxo complex. These elementary reactions were discussed in Qiapters 11 and 9, respectively. It appears that the mechanism involving nucleophilic attack occurs under conditions of high chloride concentration, and the mechanism involving olefin insertion occurs imder conditions of low chloride concentration. ... 

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). 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44]... 

The applications of palladium in organic syntheses are numerous,this metal possibly being the most important one in the field. Numerous reactions are known oxidations of the Wacker type, C-C coupling by transmetallation and/or insertion of CO or olefin (see following section). A category of reactions that is also very common concerns the catalytic use of palladium to carry out the substitution of a nucleofuge (typically acetate) in allylic position by a carbanion or any other nucleophile. This is the Tsuji-Trost allylic substitution ... 

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