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.