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 Ph 3 P CH 2 ( 2a ) with B(C 6 F 5 ) 3 ( 1 ) yields the adduct Ph 3 P + −CH 2 −B(C 6 F 5 ) 3 - ( 3a ), which was characterized by X-ray crystal structure analysis. The ylide Ph 3 P CHPh ( 2b ) reacts analogously with B(C 6 F 5 ) 3 at 0 °C to form Ph 3 P + −CHPh−B(C 6 F 5 ) 3 - ( 3b ), but this adduct formation is reversible. Increasing the temperature leads to the formation of Ph 3 P + −CH 2 −( p -C 6 H 4 )−B(C 6 F 5 ) 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 Ph 3 P + −CHPh−( p -C 6 F 4 )−BF(C 6 F 5 ) 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 −C 6 F 5 ring of the B(C 6 F 5 ) 3 reagent. Compound 5 was also characterized by an X-ray crystal structure analysis.