Allenic Phosphonium Borate Zwitterions via a Transient Phosphonium Allenylidene

We describe the synthesis of alkynyl phosphanes of the type R2P-C≡C-C(OCH3)Ph2, (R = Ph, Cy) and investigate their transformation to geminally substituted phosphonium borato-allene zwitterions upon their reaction with B(C6F5)3. The mechanism for this transformation was studied experimentally and by density functional theory computations (DFT), suggesting the intermediacy of an unsaturated 3-coordinate phosphonium electrophile akin to a methylene phosphonium cation.