Unusual Electronic Effects of Ancillary Ligands on the Perfluoroalkylation of Aryl Iodides and Bromides Mediated by Copper(I) Pentafluoroethyl Complexes of Substituted Bipyridines
Several perfluoroalkylcopper compounds have been reported previously that serve as reagents or catalysts for the perfluoroalkylation of aryl halides. However, the relationships between the reactivity of such complexes and the electronic properties of the ancillary ligands are unknown, and such relationships are not well-known in general for copper complexes that mediate or catalyze cross coupling. We report the synthesis and characterization of a series of pentafluoroethylcopper(I) complexes ligated by bipyridine ligands possessing varied electronic properties. In contrast to the limited existing data on the reactivity of L2Cu(I)–X complexes bearing amine and pyridine-type ligands in Ullmann-type aminations with aryl halides, the reactions of aryl halides with pentafluoroethylcopper(I) complexes bearing systematically varied bipyridine ligands were faster for complexes bearing less electron-donating bipyridines than for complexes bearing more electron-donating bipyridines. Analysis of the rates of reaction and the relative populations of the neutral complexes [(R2bpy)CuC2F5] and ionic complexes [(R2bpy)2Cu][Cu(C2F5)2] formed by these reagents in solution suggests that this effect of electronics on the reaction rate results from an unusual trend of faster oxidative addition of aryl halides to [(R2bpy)CuC2F5] complexes containing less electron-donating R2bpy ligands than to those containing more electron-donating R2bpy ligands.
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