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

Abstract

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)₃Bi^V(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)₃Bi^V(TFA)₂, the reductive functionalization pathway is potentially competitive with protonation. In contrast, for (Ph)₂Bi^V(TFA)₃ oxyfunctionalization is significantly lower in energy than protonation. For Bi^III-phenyl intermediates, redox neutral protonation is significantly lower in energy than a second functionalization. We also examined the oxyfunctionalization versus protonation of Bi^V-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.