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