In Search of Phosphavinyl Cations:  A DFT Study of Electrophilic Attack on Phosphaacetylenes

Electrophilic attack on phosphaacetylenes RCP was studied by DFT at the B3LYP/ 6-311++G** level. The influence of the RCP structure (with R = H, Me, tBu, SiMe₃, F, CF₃, NO₂ as well as NH₂, NMe₂, N(iPr)₂, and N(t-Bu)₂) on the outcome of protonation, nitration, and acetylation reactions (H⁺, NO₂⁺, MeCO⁺ as representative electrophiles) was examined. Changes in the computed multinuclear GIAO NMR chemical shifts (GIAO/B3LYP/ 6-311++G**) and CHelpG charges were employed to derive comparative charge delocalization modes in the resulting cations. Whereas parent HCP is C-protonated to give an open phosphavinyl cation as minimum, protonation of MeCP, tBuCP, and FCP leads to the formation of hydrogen-bridged cations. With Me₃SiCP a bridged cation results that exhibits considerable siliconium ion character. The amino-phosphaacetylenes H₂NCP, Me₂NCP, (iPr)₂NCP, and (tBu)₂NCP are P-protonated to generate delocalized cations with positive charge mainly residing on phosphorus and nitrogen. Interaction of RCP with NO₂⁺ leads in most cases to the formation of oriented π-complexes whose structures depend on the nature of R. With Me₃SiCP, C-nitration gives a phosphavinyl cation involving silicon participation. In selected cases, the interaction between the optimized phosphavinyl cations and SO₂ was examined to explore structural and NMR chemical shift changes in the resulting complexes as models for stable ion study. Quenching of tBuCHP⁺ (with fluorosulfate anion) and its [2+2] cycloaddition reaction with tBuCP were also studied.