Thiophosphonothioates from oxiranes

Heating oxiranes with Lawesson's reagent in aromatic solvents results in the formation of thiophosphonothioates. The reaction proceeds most efficiently with substrates where elimination is hindered by their geometry. The observed regioselectivity can be explained by cation stabilization in bicyclic systems. In particular, electron-donating groups significantly enhance the stabilization of carbocations at the homobenzylic position in rigid bicyclic frameworks. • Detailed exploration of reactivity: Heating oxiranes with Lawesson's reagent (LR) in aromatic solvents forms thiophosphonothioates (TPTs) through a ring-expansion mechanism — a new transformation for this reagent class. • Mechanistic Insight: The reaction proceeds via coordination of LR to the oxirane oxygen , leading to carbocation formation and intramolecular trapping by sulfur , producing a five-membered heterocycle with a stereogenic phosphorus center. • Regioselectivity and Stereochemistry: Cation stabilization governs regioselectivity — electron-donating substituents favor carbocation formation and sulfur insertion at the more stabilized (benzylic or homobenzylic) site. The cis isomer of TPTs is typically more stable than the trans form, as supported by DFT and NMR data. • Substrate Scope: The transformation is most efficient in rigid bicyclic and benzo-fused oxiranes , where elimination pathways are geometrically hindered. Flexible oxiranes (e.g., dihydronaphthalene oxide) undergo competing elimination reactions. • Structural and Computational Validation: Single-crystal X-ray diffraction, NMR ( 1 H, 31 P, NOESY), and DFT calculations confirm the molecular structures and energy profiles of the products, providing a predictive framework for designing new TPTs from oxiranes.