Phosphenium‐Cation‐Mediated Formal Cycloaddition Approach to Benzophospholes

Abstract A phosphenium‐cation‐mediated formal [3+2]‐cycloaddition reaction of internal alkynes is reported. The reaction proceeds under metal‐free conditions to give the corresponding C−P rearranged benzophospholes regioselectively, even when ortho ‐ and para ‐substituted arylphosphine oxides are starting substrates. Mechanistic studies by 31 P{ 1 H} NMR analysis suggest an involvement of three‐membered phosphirenium cation species and C−P rearrangement prior to a ring closure for benzophosphole skeleton formation.