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.

将芳香族sp²杂化碳氢键（aromatic sp² C–H bonds）氧化为官能化碳氧键（C–O bonds）是一类颇具挑战性的转化过程。对于主族金属（main-group metals）而言，金属-芳基键（metal-aryl bond）的氧化官能化步骤通常速率较慢，若在较强酸性溶剂中进行则存在潜在问题：此时质子化（protonation）过程会阻碍氧化官能化的发生。
本研究通过实验与计算手段，对三氟乙酸（trifluoroacetic acid, TFAH）溶剂中(Ph)₃BiV(TFA)₂（其中TFA为三氟乙酸根trifluoroacetate）的氧化官能化反应展开了系统分析。实验结果显示，反应仅生成一种氧化官能化产物三氟乙酸苯酯（phenyl TFA, PhTFA）与两当量的苯。显式/连续介质溶剂化密度泛函理论（explicit/continuum solvent density functional theory）计算表明，分子内直接还原官能化路径的能垒低于自由基路径与离子路径；尤为值得注意的是，针对(Ph)₃BiV(TFA)₂体系，其还原官能化路径与质子化过程具备潜在的竞争可行性。
与之相对，(Ph)₂BiV(TFA)₃的氧化官能化反应能垒显著低于质子化过程。针对三价铋-芳基（BiIII-phenyl）中间体，氧化还原中性的质子化步骤能量远低于第二次官能化步骤。
本研究还针对带有配位联苯配体（biphenyl ligand）与配位联苯砜配体（biphenyl sulfone ligand）的五价铋-芳基配合物，考察了氧化官能化与质子化的竞争路径，两类配体体系均仅生成氧化官能化产物。其中联苯配体配合物的反应被提议遵循"质子化优先"机制，而联苯砜配体配合物的"氧化官能化优先"机制则同时与计算结果和实验数据相符。