No abstract

Site-selective transformation of benzylic C−H bonds into diverse functional groups is achieved via Cu-catalyzed C−H fluorination with N-fluorobenzenesulfonimide (NFSI), followed by substitution of the resulting fluoride with various nucleophiles. The benzyl fluorides generated in these reactions are reactive electrophiles in the presence of hydrogen-bond donors or Lewis acids, allowing them to be used without isolation in C−O,

A copper catalyst in combination with N-fluorobenzenesulfonimide (NFSI) has been reported to functionalize benzylic C–H bonds to the corresponding benzylic sulfonimides via C–N coupling. Here, we reported a closely related Cu-catalyzed method with NFSI that instead leads to C–F coupling. This switch in selectivity arises from changes to the reaction conditions (Cu/ligand ratio, temperature, addition of base) and further benefits from inclusion of MeB­(OH)2 in the reaction. MeB­(OH)2 is shown to serve as a “redox buffer” in the reaction, responsible for rescuing inactive Cu­(II) for continued promotion of fluorination reactivity.

Azoles are important motifs in medicinal chemistry, and elaboration of their structures via direct N−H/C−H coupling could have broad utility in drug discovery. The ambident reactivity of many azoles, however, presents significant selectivity challenges. Here, we report a copper-catalyzed method that achieves site-selective cross-coupling of pyrazoles and other N−H heterocycles with substrates bearing (hetero)benzylic C−H bonds. Excellent N-site selectivity is achieved, with the preferred site controlled by the identity of co-catalytic additives. This cross-coupling strategy features broad scope for both the N−H heterocycle and benzylic C−H coupling partners, enabling application of this method to complex molecule synthesis and medicinal chemistry.

Copper-catalyzed radical-relay reactions provide a versatile strategy for selective C–H functionalization; however, reactions with peroxide-based oxidants often require excess C–H substrate. Here, we report a photochemical strategy to overcome this limitation by using a Cu/2,2′-biquinoline catalyst that supports benzylic C–H esterification with limiting C–H substrate. Mechanistic studies indicate that blue-light irradiation promotes carboxylate-to-copper charge transfer, reducing resting-state CuII to CuI, which activates the peroxide to generate an alkoxyl radical hydrogen-atom-transfer species. This “photochemical redox buffering” introduces a unique strategy to sustain the activity of Cu catalysts in radical-relay reactions.