Metal-Mediated Oxidation of Alcohols - Mechanism

Noble metal salts, for example, of Pd(II) or Pt(II), undergo reduction by primary and secondary alcohols inhomogeneous solution. Indeed, the ability of alcohols to reduce Pd(II) was described as early as 1828 by Berzelius, who showed that K2PdCl4 was reduced to palladium metal in an aqueous ethanolic solution [25]. The reaction involves a [3-hydride elimination from an alkoxymetal intermediate and is a commonly used method for the preparation of noble metal hydrides (Eq. (5.1)). In the presence of dioxygen this leads to catalytic oxidative dehydrogenation of the alcohol, for example, with palladium salts [26-30]. 

Peroxometal pathways are typically observed with early transition metal ions with a d configuration, for example, Mo(VI), W(VI), Ti(IV), and Re(VII), which are relatively weak oxidants. Oxometal pathways are characteristic of late transition elements and first row transition elements, for example, Cr(VI), Mn(V), Os(VIII), Ru(VI), and Ru(VIII), which are strong oxidants in high oxidation states. Some metals can operate via both pathways depending, among other things, on the substrate for example, vanadium(V) operates via a peroxometal pathway in alkene epoxidations, but an oxometal pathway is involved in alcohol oxidations [31]. 

---