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

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.