Oxidation of a Monomethylpalladium(II) Complex with O2 in Water: Tuning Reaction Selectivity to Form Ethane, Methanol, or Methylhydroperoxide.

Photochemical aerobic oxidation of n-Pr4N[(dpms)Pd(II)Me(OH)] (5) and (dpms)Pd(II)Me(OH2) (8) (dpms = di(2-pyridyl)methanesulfonate) in water in the pH range of 6-14 at 21 °C was studied and found to produce, in combined high yield, a mixture of MeOH, C2H6, and MeOOH along with water-soluble n-Pr4N[(dpms)Pd(II)(OH)2] (9). By changing the reaction pH and concentration of the substrate, the oxidation reaction can be directed toward selective production of ethane (up to 94% selectivity) or methanol (up to 54% selective); the yield of MeOOH can be varied in the range of 0-40%. The source of ethane was found to be an unstable dimethyl Pd(IV) complex (dpms)Pd(IV)Me2(OH) (7), which could be generated from 5 and MeI. For shedding light on the role of MeOOH in the aerobic reaction, oxidation of 5 and 8 with a range of hydroperoxo compounds, including MeOOH, t-BuOOH, and H2O2, was carried out. The proposed mechanism of aerobic oxidation of 5 or 8 involves predominant direct reaction of excited methylpalladium(II) species with O2 to produce a highly electrophilic monomethyl Pd(IV) transient that is involved in subsequent transfer of its methyl group to 5 or 8, H2O, and other nucleophilic components of the reaction mixture.