Transient Phosphenium and Arsenium Ions versus Stable Stibenium and Bismuthenium Ions

Fluoride abstraction from bis‐m‐terphenylelement fluorides (2,6‐Mes(2)C(6)H(3))(2)EF (E=P, As) generated the highly reactive phosphenium ion [(2,6‐Mes(2)C(6)H(3))(2)P](+) and the arsenium ion [(2,6‐Mes(2)C(6)H(3))(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‐Mes(2)C(6)H(3))(2)E](+) (E=P, As) is in sharp contrast to the related stibenium ion [(2,6‐Mes(2)C(6)H(3))(2)Sb](+) and bismuthenium ion [(2,6‐Mes(2)C(6)H(3))(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.