P-C Bond Activation Chemistry: Evidence for 1,1-Carboboration Reactions Proceeding with Phosphorus-Carbon Bond Cleavage

A series of diarylphosphinyl-substituted acetylenes of the type (aryl)(2)P-C C-R (aryl = phenyl or mesityl, R = Ph or n-propyl) react with the strongly Lewis acid reagent B(C6F5)(3) in toluene at elevated temperatures (70-105 degrees C) to give the 1,1-carboboration products 4. Treatment of bis(diphenylphosphinyl)acetylene with B(C6F5)(3) under analogous conditions proceeded with phosphinyl migration to yield the 1,1-carboboration product 4d, bearing a geminal pair of Ph2P substituents at one former acetylene carbon atom and a C6F5 substituent and the remaining -B(C6F5)(2) group at the other. Prolonged thermolysis of 4d resulted in an intramolecular aromatic substitution reaction by means of Ph2P attack on the adjacent C6F5 ring to yield the zwitterionic phospha-indene derivative 7. The compounds 4a, 4c, 4d, and 7 were characterized by X-ray diffraction.