Theoretical study on the CoII/CoIII and CoII/CoIV catalytic cycles in CoII(salen)-catalyzed radical fluorination with various different NF-type fluorinating reagents

Abstract

Co^II(salen)-catalyzed radical fluorination provides a mild and efficient approach for the construction of carbon–fluorine bonds. However, the mechanistic role of different NF-type fluorinating reagents and the corresponding catalytic cycle pathways (Co^II/Co^III vs. Co^II/Co^IV) in the Co^II(salen)-catalyzed fluorination system remain controversial. Therefore, this work employs density functional theory (DFT) to computationally analyze the reaction mechanisms of radical fluorination involving three classes of NF-type fluorinating reagents: N-tert-butylfluoramide, N-ethyl-N-fluorobenzenesulfonamide, and Me₃NFPy·BF₄. The results indicate that the nature of the NF-type fluorinating reagent exerts a decisive influence on the reaction pathway. N-tert-butylfluoramide and N-ethyl-N-fluorobenzenesulfonamide tend to undergo single-electron oxidation with Co^II(salen), generating Co^III(salen)–F and the corresponding amidyl radical, with the reaction proceeding via a Co^II/Co^III cycle. In contrast, Me₃NFPy·BF₄ favors a two-electron oxidation process, directly forming the [Co^IV(salen)–F]⁺ intermediate, and the reaction proceeds through a Co^II/Co^IV cycle. Furthermore, for the hydrofluorination of unactivated alkenes involving Me₃NFPy·BF₄, the calculations not only clearly elucidate the formation pathway of Co^III(salen)–H but also confirm that the occurrence of Wagner–Meerwein rearrangement in the product is related to the presence of a substituent at the benzylic position of the substrate. Through theoretical calculations, this study provides a unified theoretical analysis of the roles of different NF-type fluorinating reagents in the Co^II(salen)-catalyzed fluorination system, offering an important theoretical basis for the rational design and condition optimization of future cobalt-catalyzed fluorination reactions.