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

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journal contribution
posted on 10.12.2014, 00:00 by Noel Nebra, Vladimir V. Grushin
The fluoride [(bpy)­CuF2(H2O)]·2H2O (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 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 1 is used in stoichiometric quantities.

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