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

The fluoride [(bpy)­CuF₂(H₂O)]·2H₂O (<b>1</b>) reacts with CF₃SiMe₃ and PhB­(OH)₂ in DMF at rt to give PhCF₃ in >95% yield within 15 min. Although <b>1</b> 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, <b>1</b> 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 <b>1</b>-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 <b>1</b>-CF₃SiMe₃ does not and cannot occur in the absence of O₂. 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 <b>1</b> is used in stoichiometric quantities