Synthesis and Characterisation of Phosphenium Ions with Aromatic Amido Substituents

Abstract

σ2λ3-Phosphenium cations A are valence isoelectronic and isolobal with silenes B and hence, carbenes C. Notably, the chemistry of these divalent phosphorus cations mirrors that of their carbon-based analogues (e.g. cycloaddition hemistry, C-H insertions, etc.), making them versatile reagents in a variety of transformations and reactions. Just as for carbenes (e.g. amino carbenes D and E), a combination of kinetic and electronic stabilisation has been used to prepare and isolate stable phosphenium ions such as F, with amido substituents. This approach is also followed in this work. In particular, the synthesis, characterisation, and properties of a range of phosphenium ions (4) with alkyl or aryl amido substituents (R2N, R = alkyl or aryl) is described.
The desired phosphenium ions 4 are synthesized in three steps by reacting the secondary amines 1 with nBuLi, which results in the production of the appropriate lithium amide salts (2) followed by subsequent reaction with PCl3, which yields the chlorophosphines 3. Reaction of 3 with a halogen abstracting reagent such as AlCl3 (or TMSOTf) results in the formation of phosphenium ions (4), which were studied by NMR spectroscopy and X-ray diffraction.