Cleavage of the Inert C(sp2)–Ar σ‑Bond of Alkenes by a Spatial Constrained Interaction with Phosphinidene

[1 + 2] cycloaddition is a classical reaction between the electrophilic phosphinidene and an alkene. However, a spatial constraint blocks this well-known reaction and enables an unprecedented chemoselective C­(sp2)–Ar σ-bond insertion of the alkene. The theoretical calculations demonstrate that this C–C bond cleavage is energetically feasible and thermodynamically favored through an electrophilic rearrangement and concomitant 1,9-aryl migration without involving any strained intermediate.