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)(PdMe)-Me-II(OH)] (5) and (dpms)(PdMe)-Me-II(OH2) (8) (dpms = di(2-pyridyl)methanesulfonate) in water in the pH range of 6-14 at 21 degrees C was studied and found to produce, in combined high yield, a mixture of Me OH, C2H6, and MeOOH along with watersoluble 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)(PdMe2)-Me-IV(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 O-2 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.