Alcohol oxidations in aqueous media

The paUadium(II) -catalyzed aerobic oxidation of alcohols is well documented. A general problem encountered in palladium-catalyzed aerobic oxidations is the sluggish reoxidation of Pd(0) to Pd(II), which results in the agglomeration of the Pd(0) particles to palladium black and accompanying deactivation of the catalyst. In the classical Wacker process for the oxidation of ethylene to acetaldehyde in an aqueous medium, this problem is circumvented 

Based on our previous work with palladium complexes of sulfonated phosphines as catalysts for carbonylations in aqueous media, we reasoned that analogous water-soluble 

We subsequently found that the palladium(II) complex of sulfonated bathophenanthroline and related water-soluble diamine ligands are stable, recyclable catalysts for the aerobic oxidation of alcohols in a two-phase aqueous-organic medium whereby the organic phase consists of the alcohol substrate and the carbonyl product (Fig. 7.14). Reactions were generally complete in 5 h at 100°C/30 bar air with as little as 0.25 mol% catalyst. No organic solvent is required (unless the substrate is a solid) and the product is easily recovered by phase separation. The catalyst is stable and remains in the aqueous phase, facilitating recychng to the next batch. 

A wide range of primary and secondary alcohols were oxidized with turnover frequencies ranging from 10 to 100 h , depending on the structure and the solubility of the alcohol in water (since the reaction occurs in the water phase the alcohol must be at least sparingly 

A catalytic cycle was proposed for the reaction in which, consistent with the observed half-order in palladium, the active catalyst is formed by initial dissociation of a hydroxyl bridged palladium(ll) dimer (Fig. 7.15). This is followed by coordination of the alcohol and -hydrogen elimination affording the carbonyl product and paUadium(O). The latter is reoxidized to paUadium(ll) by dioxygen. 

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