Distinct Mechanism of Oxidative Trifluoromethylation with a Well-Defined Cu(II) Fluoride Promoter: Hidden Catalysis

The fluoride [(bpy)CuF2(H2O)].2H(2)O (1) reacts with CF3SiMe3 and PhB(OH)(2) in DMF at rt to give PhCF3 in >95% yield within 15 min. Although 1 is a Cu(II) complex, this reaction occurs only in air; no Ph-CF3 coupling takes place under anaerobic conditions. A distinct mechanism is operational in this transformation. First, 1 is trifluoromethylated with TMSCF3 to give [(bpy)Cu(CF3)(2)] that spontaneously disproportionates to two Cu-III ([Cu(CF3)(4)](-) and [(bpy)Cu(CF3)(3)]) and two Cu-I ([(bpy)Cu(CF3)] and [Cu(CF3)(2)](-) complexes. In contrast with the Chan-Evans-Lam reaction, where the Cu(III) products of the Cu-II disproprotionation effect the coupling, those formed in the 1-TMSCF3 system, [Cu(CF3)(4)](-) and [(bpy)Cu(CF3)(3)], are stable and unreactive, remaining dead-end spectators throughout the coupling process. Consequently, the trifluoromethylation of PhB(OH)(2) with 1-CF3SiMe3 does not and cannot occur in the absence of O. Only by air oxidation of the Cu(I) disproportionation product, [(bpy)Cu(CF3)] in equilibrium with [Cu(CF3)(2)](-), is the reactive species generated, serving as a catalyst for the Ph-CF3 bond formation even if 1 is used in stoichiometric quantities.