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 31P{1H} NMR analysis suggest an involvement of three‐membered phosphirenium cation species and C−P rearrangement prior to a ring closure for benzophosphole skeleton formation. Positive phosphine: A Tf2O/DMAP‐promoted formal [3+2]‐cycloaddition of secondary diarylphosphine oxides and internal alkynes has been developed. The reaction proceeds under metal‐free conditions to form the corresponding C−P rearranged benzophospholes regioselectively, even when the substituted arylphosphine oxides are employed. The mechanistic studies by 31P{1H} NMR spectroscopy suggest involvement of phosphirenium cation intermediates and unique aryl‐group migration from phosphorus to carbon (see scheme).