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 degrees 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.