Phosphamide-Mediated C═N Condensation through the Horner–Wadsworth–Emmons-Type Reaction: A Sustainable Route for Polyphosphamide Upcycling

Considering the significant importance of the phosphonate-mediated Horner–Wadsworth–Emmons (HWE) olefination reaction in organic synthesis, phosphamides are explored here as a mediator to perform P−N bond scission and simultaneous C═N condensation with a variety of substituted benzaldehydes through a HWE-type pathway under mild reaction conditions (60 °C and 12 h). Reaction kinetics, D-labeling (KIE: 1.23), Hammett study and characterization of reaction intermediates through in situ spectroscopy highlight that the deprotonation of “P(O)-NH” unit occurs in the very first step, followed by the C═N bond formation, and is preferred by electron-deficient substituents. Computationally optimized geometry of the phosphamides ( PN-1 to PN-5 ) and their deprotonated form showed that N–H deprotonation shortens the P–N bond and elongates the P–O bond, indicating charge delocalization with partial P═N and P–O – character. Electron-deficient substituents stabilizes the monoanionic species, and PN-4 (−NH–Ph–Cl) exhibits the highest HOMO energy, suggesting enhanced nucleophilicity and reactivity toward benzaldehyde. The P–N bond dissociation of phosphamides via HWE-type reactions further enables a clean and selective approach for upcycling polyphosphamides into valuable imines. Notably, this method has been depicted as a sustainable path in selectively recovering cotton fabric from polyphosphamides, commonly used as flame-retardant materials, demonstrating its feasible depolymerization applications.