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

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journal contribution
posted on 2004-07-19, 00:00 authored by Pierre M. Esteves, Kenneth K. Laali
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.