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Abstract We report copper(II) and copper(III) trifluoromethyl complexes supported by a pyridinedicarboxamide ligand (L) as a platform for investigating the role of electron transfer in C(sp²)−H trifluoromethylation. While the copper(II) trifluoromethyl complex is unreactive towards (hetero)arenes, the formal copper(III) trifluoromethyl complex performs C(sp²)−H trifluoromethylation of a wide range of (hetero)arenes. Mechanistic studies using the copper(III) trifluoromethyl complex suggest that the mechanism of arene trifluoromethylation is substrate‐dependent. When the thermodynamic driving force for electron transfer is high, the reaction proceeds through a previously unidentified single electron transfer (SET) mechanism, where an initial electron transfer occurs between the substrate and oxidant prior to CF₃group transfer. Otherwise, a CF₃radical release/electrophilic aromatic substitution (S_EAr) mechanism is followed. These studies provide valuable insights into the role of strong oxidants and potential mechanistic dichotomy in Cu‐mediated C(sp²)−H trifluoromethylation. 

High-valent metal oxo complexes are prototypical intermediates for the activation and hydroxylation of alkyl C–H bonds. Substituting the oxo ligand with other functional groups offers the opportunity for additional C–H functionalization beyond C–O bond formation. However, few species aside from metal oxo complexes have been reported to both activate and functionalize alkyl C–H bonds. We herein report the first example of an isolated copper( iii ) cyanide complex (LCu III CN) and its C–H cyanation reactivity. We found that the redox potential ( E ox ) of substrates, instead of C–H bond dissociation energy, is a key determinant of the rate of PCET, suggesting an oxidative asynchronous CPET or ETPT mechanism. Among substrates with the same BDEs, those with low redox potentials transfer H atoms up to a million-fold faster. Capitalizing on this mechanistic insight, we found that LCu III CN is highly selective for cyanation of amines, which is predisposed to oxidative asynchronous or stepwise transfer of H + /e − . Our study demonstrates that the asynchronous effect of PCET is an appealing tool for controlling the selectivity of C–H functionalization.