Addition reactions Wacker oxidation mechanisms

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. 

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. 

Hegedus proposed that the mechanism of this transformation proceeds through a Wacker-type reaction mechanism that is promoted by Pd(II). As shown below, coordination of the olefin to Pd(II) results in precipitate 121, which upon treatment with Et,N undergoes intramolecular trany-aminopalladation to afford intermediate 122. As expected, the nitrogen atom attack occurs in a 5-exo-trig fashion to afford 123. [l-Hydride elimination of 123 gives rise to exocyclic olefin 124, which rearranges to indole 120. The final step of this mechanism leads to the formation of catalytically inactive Pd(0). However, addition of oxidants such as benzoquinone allows for catalytic turnover. 

Additions at Monoenes.—Results obtained recently have led to a reappraisal of the mechanism of the Wacker process (the [PdCl4] -CuCl2-cataIysed oxidation of ethylene to acetaldehyde). A key feature of the originally proposed mechanism, largely deduced from the kinetic form of the reaction (below) is the formation of... 

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