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) yi elds the adduct Ph3P+-CH2-B(C6F5)(3)(-) (3a), which was characterized by X-ray crystal structure analysis. The ylide Ph3P=CHPh (2b) reacts a nalogously with B(C6F5)(3) at 0 degrees C to form Ph3P+-CHPh-B(C6F5)(3 )(-) (3b), but this adduct formation is reversible. Increasing the tem perature 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. C ompound 4 is also cleaved upon prolonged thermolysis to eventually yie ld Ph3P+-CHPh-(p-C6F4)-BF(C6F5)(2)(-) (5), which is the product of the rmodynamic control in this series. Compound 5 arises from a nucleophil ic aromatic substitution reaction of the nonstabilized phosphorus ylid e at a -C6F5 ring of the B(C6F5)(3) reagent. Compound 5 was also chara cterized by an X-ray crystal structure analysis.