Alcohols Wacker oxidation mechanisms

The Pd(Quinox)Cl2, (Quinox = 2-(2-quinolyl)-4,5-dihydrooxazole) catalysed Wacker-type oxidation of olefins bearing homoallylic alcohols by TBHP led to the corresponding /3-hydroxy ketones in good yields. Since the oxidation was catalyst controlled, it was significantly faster than the substrate-controlled Tsuji-Wacker oxidation. The bis- and fra-homoallylic alcohols were oxidized to cyclic peroxyketals, presumably via nucleophilic attack of the methyl ketone. Kinetics of the Wacker-type oxidation of olefins by TBHP in the presence of Quinox (ligand), and (54) as the catalyst reveal first-order dependence on ligand and olefin, and rate saturation in TBHP, as expected of the proposed mechanism (Scheme 9)... 

The oxidation obeyed the same rate expression (Eq. 1) as ethene and other acyclic olefins under Wacker conditions. This indicates aUyl alcohol is oxidized by the same mechanism as other acyclic olefins. ... 

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. 

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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