Mechanistic Investigations of Cu-Catalyzed Fluorination of Diaryliodonium Salts: Elaborating the CuI/CuIII Manifold in Copper Catalysis

A combination of experimental and density functional theory (DFT) investigations suggests that the Cu-catalyzed fluorination of unsymmetrical diaryliodonium salts with general structure [Mes(Ar)I]+ in N,N'-dimethylformamide proceeds through a CuI/CuIII catalytic cycle. A low concentration of fluoride relative to combined iodonium reagent plus copper ensures that [Mes(Ar)I]+ is available as the reactive species for oxidative "Ar+" transfer to a CuI center containing one or two fluoride ligands. A series of different possible CuI active catalysts (containing fluoride, triflate, and DMF ligands) have been evaluated computationally, and all show low-energy pathways to fluorinated products. The oxidation of these CuI species by [Mes(Ar)I]+ to form cis-Ar(F)CuIII intermediates is proposed to be rate-limiting in all cases. Ar-F bond-forming reductive elimination from CuIII is computed to be very facile in all of the systems examined. The conclusions of the DFT experiments are supported by several experimental studies, including tests showing that CuI is formed rapidly under the reaction conditions and that the fluoride concentration strongly impacts the reaction yields/selectivities.