Synthesis of Tribenzo[<i>b</i>,<i>e</i>,<i>g</i>]phosphindole Oxides via Radical Bicyclization Cascades of <i>o</i>‑Arylalkynylanilines

A new DTBP/Mg­(NO3)2-mediated bicyclization cascade of o-arylalkynylanilines with secondary arylphosphine oxides has been developed, enabling dual C­(sp2)–H functionalization along with the cleavage of the C–N bond. The combination between regioselective P-centered radical-triggered [3 + 2] cyclization and C-centered radical-induced cross-coupling in a one-pot manner delivered 27 examples of tribenzo­[b,e,g]­phosphindole oxides with generally high regioselectivity. A reasonable mechanism for forming such products involving radical addition–cyclization cascade is proposed.