Phosphenium-Cation-Mediated Formal Cycloaddition Approach to Benzophospholes.

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.