Oxidation of a cyclometalated Pd(II) dimer with "CF3+": formation and reactivity of a catalytically competent monomeric Pd(IV) aquo complex.

The reaction of [(bzq)Pd(OAc)](2) (bzq = benzo[h]quinoline) with "CF(3)(+)" reagents to afford the monomeric Pd(IV) aquo complex (bzq)Pd(CF(3))(OAc)(2)(OH(2)) is demonstrated. Heating this Pd(IV) adduct at 60 °C for 12 h leads to highly chemoselective aryl-CF(3) bond-forming reductive elimination. The rate and yield of this transformation are both significantly enhanced by the addition of Brønsted and Lewis acidic additives. The mechanism of C-CF(3) bond formation from (bzq)Pd(CF(3))(OAc)(2)(OH(2)) has been studied, and the major pathway is proposed to involve pre-equilibrium acetate dissociation followed by C-CF(3) coupling. Finally, this monomeric Pd(IV) complex is shown to be a kinetically competent intermediate for C-H trifluoromethylation with "CF(3)(+)" reagents. Collectively, these studies provide valuable insights about the speciation, nuclearity, and reactivity of Pd intermediates in catalytic C-H trifluoromethylation reactions.