Wacker oxidations mechanistic studies

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. 

The Wacker oxidation has been the focus of numerous recent theoretical studies.58 The most definitive of these are the work of Goddard and co-workers. They have studied the entire catalytic cycle using high-level DFT calculations, and the results of this work have provided some interesting insights into key mechanistic... 

Jiro Tsuji carried out many mechanistic and synthetic studies on the initial Wacker oxidation process.7-" It is now known as the Wacker-Tsuji oxidation for the oxidation of terminal olefin 1 to the corresponding methyl ketone 2 with oxygen in the presence of a catalytic amount of palladium and one equivalent of copper salt.12-" Nowadays, the Wacker-Tsuji oxidation is a standard methodology for transforming the terminal olefin to the corresponding methyl ketone.17 The reaction is so widely used that Tsuji declared that a terminal olefin could be viewed as a masked methyl ketone."... 

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. ... 

Mechanistic Studies on Wacker Oxidations with Aicohoi, Phenoi, and Amide Nucieophiies... 

Uemura s discovery also opened the door to direct 02-coupled asymmetric catalytic oxidative kinetic resolution (OKR) of alcohols with catalytic amounts of chiral ligands. Simultaneously both Stoltz and Sigman reported the aerobic OKR of secondary alcohols using catalytic Pd[(-)-sparteine]Q2 with the addition of catalytic (-)sparteine, 36 (Scheme 5.18E) [66]. Mechanistic studies on this system performed by Sigman and coworkers have found the chiral amine to have a dual role as a hgand and an exogenous base [66e,hj. This system has been further apphed to asymmetric Wacker-type cyclizations [2 c, 22a,bj. 

Extensive studies on the Wacker process have been carried out in industrial laboratories. Also, many papers on mechanistic and kinetic studies have been published[17-22]. Several interesting observations have been made in the oxidation of ethylene. Most important, it has been established that no incorporation of deuterium takes place by the reaction carried out in D2O, indicating that the hydride shift takes place and vinyl alcohol is not an intermediate[l,17]. The reaction is explained by oxypailadation of ethylene, / -elimination to give the vinyl alcohol 6, which complexes to H-PdCl, reinsertion of the coordinated vinyl alcohol with opposite regiochemistry to give 7, and aldehyde formation by the elimination of Pd—H. 

The stereochemical or regiochemical outcome of a reaction may have mechanistic significance, as seen in the study of the Wacker process, for example Reference (44). Here, the inversion of configuration at one carbon of ethylene in its oxidation was interpreted in terms of an exo-attack of the ethylene complex by solvent water, a point that had escaped notice in previous kinetic studies because the concentration of water, as a solvent, cannot be varied and so the appropriate [H2O] term did not appear in the experimental rate equation. 

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