Reaction of the Lewis Acid Tris(pentafluorophenyl)borane with a Phosphorus Ylide:  Competition between Adduct Formation and Electrophilic and Nucleophilic Aromatic Substitution Pathways

Treatment of the phosphorus ylide Ph₃PCH₂ (<b>2a</b>) with B(C₆F₅)₃ (<b>1</b>) yields the adduct Ph₃P⁺−CH₂−B(C₆F₅)₃^- (<b>3a</b>), which was characterized by X-ray crystal structure analysis. The ylide Ph₃PCHPh (<b>2b</b>) reacts analogously with B(C₆F₅)₃ at 0 °C to form Ph₃P⁺−CHPh−B(C₆F₅)₃^- (<b>3b</b>), but this adduct formation is reversible. Increasing the temperature leads to the formation of Ph₃P⁺−CH₂−(<i>p</i>-C₆H₄)−B(C₆F₅)₃^- (<b>4</b>), which is formed by an electrophilic aromatic substitution reaction of the electron-deficient borane reagent at the ylidic phenyl group. Compound <b>4</b> is also cleaved upon prolonged thermolysis to eventually yield Ph₃P⁺−CHPh−(<i>p</i>-C₆F₄)−BF(C₆F₅)₂^- (<b>5</b>), which is the product of thermodynamic control in this series. Compound <b>5</b> arises from a nucleophilic aromatic substitution reaction of the nonstabilized phosphorus ylide at a −C₆F₅ ring of the B(C₆F₅)₃ reagent. Compound <b>5</b> was also characterized by an X-ray crystal structure analysis.