Allenic Phosphonium Borate Zwitterions Via a Phosphonium Allenylidene Intermediate

We describe the synthesis of alkynyl phosphanes of the type RP-C[triple bond, length as m-dash]C-C(OCH)Ph (R = Ph, Cy) and investigate their transformation to geminally substituted phosphonium borato-allene zwitterions upon their reaction with B(CF). 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.