Palladium-mediated aerobic oxidation of organic substrates: The role of metal versus hydrogen peroxide

Palladium species catalyze the aerobic oxidation of a variety of organic substrates. The reoxidation of palladium(0) to palladium(II) by dioxygen, a necessary step in these catalytic oxidations, results in the simultaneous formation of free peroxide. However, the continued catalytic generation of peroxide requires the rereduction of palladium(II) to palladium(0). This can be achieved by alkenes and alcohols which are then converted to carbonyl compounds. In these cases, no sacrificial coreductant is necessary and the peroxide appears to play no significant part in the oxidation of these substrates. On the other hand, palladium(II) does not readily oxidize primary or aromatic C-H bonds thereby preventing its reduction by them. Thus, a sacrificial coreductant is required to reduce palladium(II), and carbon monoxide and dihydrogen can fulfill this role. The peroxide generated in these cases is subsequently used to effect the oxidation of the alkane and arene either by palladium(II) or by a second catalyst.