A direct method for phosphorus atom insertion via phosphorous acid for synthesizing P-doped curved polycyclic π-systems

Abstract

The introduction of heteroatoms into π-conjugated frameworks has emerged as a powerful strategy to modulate their electronic and optical properties. Among these, the incorporation of phosphorus atoms imparts electron-accepting character and distinct photophysical behaviors. However, the direct formation of three C–P bonds from inorganic phosphorus sources remains extremely challenging. While Friedel–Crafts-type C–P bond formation using chlorophosphines offers an effective route to P-doped π-conjugated molecules, the direct synthesis of triarylphosphine from inorganic phosphorous compounds through a triple C–P bond formation has been rarely achieved. Recently, triflic anhydride (Tf₂O)-mediated C–P bond formation using organophosphorus oxides has enabled the synthesis of diverse P-doped π-conjugated molecules, demonstrating that activation of phosphorus centers by triflate significantly enhances their electrophilicity. Nevertheless, the utilization of inorganic phosphorus sources devoid of pre-existing C–P bonds has not yet been realized. Herein, we report a metal-free and direct phosphorus atom insertion into electron-rich π-systems using phosphorous acid (H₃PO₃) as a readily available and easily handled inorganic phosphorus source. Activation of H₃PO₃ with Tf₂O enables the formation of three C–P bonds in a single step, affording P-doped curved π-conjugated molecules. The resulting compounds exhibit pronounced emission arising from π–π* transitions enhanced by a multiple resonance effect, and display chiroptical properties due to their nonplanar geometries. This work establishes a new direct heteroatom insertion strategy, extending the concept previously demonstrated for boron to phosphorus, thereby expanding the chemical space of heteroatom-doped π-systems and providing a basis for future functional molecular design.