Hydroxypalladation, Wacker oxidation mechanisms

The point has been made that the conditions of jS-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 hydroxypalladation. When c7s-l,2-dideuteroethene is oxidized in water with PdCl2 under a CO atmosphere, the product is tran5-2,3-dideutero-/3-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 CU 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. 

The oxidation of nondeuteriated allyl alcohol (3) under Wacker conditions gave a complicated reaction mixture because of side reactions. The main products are shown in Scheme 12. The 2-hydroxypropanal (4) and hydroxyacetone (5) products arise from Wacker-type oxidation, while acrolein arises from direct hydride extraction from the alcohol carbon. As would be expected from the mechanism of the Wacker reaction, allyl-l,l-d2 (3a) and allyl-3,3-d2 (3b) alcohols gave the same hydroxypalladation adduct and thus the same distribution of denteriated 2-hydroxypropanals, 4a and 4b (Scheme 12). The deuterium isotope effect, A h/A d = 4a/4b = 1.9, has the same value found previously for oxidation of l,2-ethene-r/2- ... 

Now we are in a position to determine if hydroxypalladation is an equilibrium process as is required by the anti mechanism shown at the top of Scheme 3. The hydroxypalladation intermediate, 6, can eliminate hydroxyl from either end of the molecule and, if hydroxypalladation is an equilibrium process, oxidation of either 3a or 3b should give a 1 1 mixture of the two after the reaction has proceeded to a small extent. The deuterated allyl alcohol, 3a, was oxidized under Wacker conditions and the reaction stopped after one half-life and the unreacted allyl alcohol was analyzed for isomerization. The reaction scheme is shown in Scheme 13. After one half-life the isomerization prodnct 3b was... 

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