[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.

The activation of CS 2 by the 2H-phosphindole complex with a low-coordinate phosphadiene moiety is reported. The successive hetero-Diels− Alder reaction between 2H-phosphindoles and CS 2 constructs two bridged rings and one spirocycle simultaneously, affording structurally complex P,Spolycyclic products. The two 2H-phosphindoles approach the C�S bond in a head-to-head disposition to minimize steric hindrance. This work reveals the unique reactivity of low-coordinate organophosphorus species and their potential applications in small molecule activation.

Dearomatization reactions have recently emerged as a powerful tool for the rapid buildup of molecular complexity. Here, an unparalleled thermal dearomatization [4+2] cycloaddition reaction between benzene derivatives and a 2H-phosphindole tungsten complex was reported. The unique reactivity of the in situ-generated 2H-phosphindole complex toward benzene was revealed by density functional theory calculations. We thus provide new insights into the dearomatization of nonactivated arenes and pave the way for the manipulation of the dearomatization for further applications.

The heavier main group multiple bonds offer an effective tool for small molecule activation. Transient 2H‐phosphinidole working as a reactive phosphadiene system undergoes phospha‐Diels–Alder reaction with a wide range of non‐activated aromatic carbocycles and heterocycles, including naphthalene, anthracene, phenanthrene, furan, thiophene, pyrrole, pyridine, and benzo‐fused heterocycles, affording concise access to a range of polycyclic fused rings feature with phosphorus at the bridgehead. These results demonstrate that non‐activated (hetero)arenes are capable of acting as 2π systems in [4+2] cycloaddition with highly reactive 2H‐phosphindole complex.