Efficient synthesis of benzophosphole oxides via Ag-promoted radical cycloisomerization

Abstract

Cycloisomerization reactions involving C–P bond formation have been overlooked for the synthesis of P-heterocycles. In this work, we developed a simple, efficient and versatile route to synthesize benzophosphole oxides by reacting ortho-alkynyl secondary phosphine oxides with 5 mol% AgSbF₆. Mechanistic investigations revealed a radical-chain mechanism involving phosphinoyl radicals as key intermediates and rare 5-endo-dig cyclization as a key step, rather than the π-activation of the CC triple bond. The transformation is both efficient and versatile. It effectively complements alternative intermolecular approaches. It works with a wide diversity of substitution patterns (alkynyl, benzo and phosphorus moieties) and enables the exquisite control of regioselectivity. Post-functionalization via direct C–H vinylation of the C2 position is also substantiated.