Mechanism and Selectivity of Bi(V)-Aryl Oxyfunctionalization in Trifluoroacetic Acid Solvents

The oxidative functionalization of aromatic sp2 C–H bonds to C–O bonds is a difficult transformation. For main-group metals, the oxyfunctionalization step of a metal-aryl bond is generally slow and potentially problematic if carried out in a relatively strong acid solvent where protonation could prevent oxyfunctionalization. In this work, we experimentally and computationally analyzed the oxyfunctionalization reaction of (Ph)3BiV(TFA)2 (TFA = trifluoroacetate) in a trifluoroacetic acid (TFAH) solvent. Experiments showed a single oxyfunctionalization product phenyl TFA (PhTFA) and two equivalents of benzene. Explicit/continuum solvent density functional theory calculations revealed that a direct intramolecular reductive functionalization pathway is lower in energy than radical or ionic pathways, and surprisingly from (Ph)3BiV(TFA)2, the reductive functionalization pathway is potentially competitive with protonation. In contrast, for (Ph)2BiV(TFA)3 oxyfunctionalization is significantly lower in energy than protonation. For BiIII-phenyl intermediates, redox neutral protonation is significantly lower in energy than a second functionalization. We also examined the oxyfunctionalization versus protonation of BiV-phenyl complexes with a coordinated biphenyl ligand and a coordinated biphenyl sulfone ligand, which both resulted in oxyfunctionalization. For the biphenyl ligand complex, a protonation-first mechanism is proposed, while for the biphenyl sulfone ligand, an oxyfunctionalization first mechanism is consistent with both calculations and experiments.