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<INF>3</INF>P&dbd;CH<INF>2</INF> (<BO>2a</BO>) with B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF> (<BO>1</BO>) yields the adduct Ph<INF>3</INF>P⁺−CH<INF>2</INF>−B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF>^- (<BO>3a</BO>), which was characterized by X-ray crystal structure analysis. The ylide Ph<INF>3</INF>P&dbd;CHPh (<BO>2b</BO>) reacts analogously with B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF> at 0 °C to form Ph<INF>3</INF>P⁺−CHPh−B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF>^- (<BO>3b</BO>), but this adduct formation is reversible. Increasing the temperature leads to the formation of Ph<INF>3</INF>P⁺−CH<INF>2</INF>−(p-C<INF>6</INF>H<INF>4</INF>)−B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF>^- (<BO>4</BO>), which is formed by an electrophilic aromatic substitution reaction of the electron-deficient borane reagent at the ylidic phenyl group. Compound <BO>4</BO> is also cleaved upon prolonged thermolysis to eventually yield Ph<INF>3</INF>P⁺−CHPh−(p-C<INF>6</INF>F<INF>4</INF>)−BF(C<INF>6</INF>F<INF>5</INF>)<INF>2</INF>^- (<BO>5</BO>), which is the product of thermodynamic control in this series. Compound <BO>5</BO> arises from a nucleophilic aromatic substitution reaction of the nonstabilized phosphorus ylide at a −C<INF>6</INF>F<INF>5</INF> ring of the B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF> reagent. Compound <BO>5</BO> was also characterized by an X-ray crystal structure analysis.