Transient Phosphenium and Arsenium Ions versus Stable Stibenium and Bismuthenium Ions

Fluoride abstraction from bis-m-terphenylelement fluorides (2,6-Mes₂ C₆ H₃ )₂ EF (E=P, As) generated the highly reactive phosphenium ion [(2,6-Mes₂ C₆ H₃ )₂ P]⁺ and the arsenium ion [(2,6-Mes₂ C₆ H₃ )₂ As]⁺ , which immediately underwent intramolecular electrophilic substitution and formation of an 1,2,4-trimethyl-6-mesityl-5-m-terphenyl-benzo[b]phospholium ion and an 1,2,4-trimethyl-6-mesityl-5-m-terphenyl-benzo[b]arsolium ion, respectively. The formation of the latter involved a methyl group migration from the ortho-position of a flanking mesityl group to the meta-position. This reactivity of [(2,6-Mes₂ C₆ H₃ )₂ E]⁺ (E=P, As) is in sharp contrast to the related stibenium ion [(2,6-Mes₂ C₆ H₃ )₂ Sb]⁺ and bismuthenium ion [(2,6-Mes₂ C₆ H₃ )₂ Bi]⁺ , which have been recently isolated and fully characterized (Angew. Chem. Int. Ed. 2018, 57, 10080-10084). On the basis of DFT calculations, a mechanism for the rearrangement of the phosphenium and arsenium ions into the phospholium and arsolium ions is proposed, which is not feasible for the stibenium and bismuthenium ions.