In organic synthesis, carbonyl groups can be transformed to thiocarbonyls using sulfur atom transfer reagents. Generally, P₄S₁₀ or the dimeric Lawesson’s reagent (H₃C–C₆H₄–PS₂)₂ are used for this type of reaction. These reagents are cleaved into reactive dithiophosphoranes of the type RPS₂, which then react with the carbonyl compounds. More reactive monomeric species could be useful for thionation reactions.

Fabian Dielmann, University of Münster, Germany, and University of Innsbruck, Austria, and colleagues have synthesized a Lewis base-free, monomeric thiophosphonium species of the type [R₂P=S]⁺ (pictured, Dipp = 2,6-diisopropylphenyl) that can transfer sulfur atoms to both carbonyl and phosphorus compounds. The compound can be considered a more reactive monomeric alternative to Lawesson’s reagent, stabilized by bulky N-heterocyclic imines. The team started from a SiMe₃-substituted 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine, which was reacted in a stepwise manner with PSCl₃ to replace two of the Cl atoms with imine units. In a final step, a chloride anion was removed by treatment with AgOTf or NaBArF₂₄ (OTf⁻ = trifluoromethanesulfonate, BArF₂₄⁻ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) to give the corresponding thiophosphonium salts and AgCl or NaCl.

The thiophosphonium ions can react with carbonyl compounds to give thiocarbonyls at room temperature, i.e., under milder conditions than Lawesson’s reagent. They can also undergo chalcogen metathesis reactions with phosphine chalcogenides: Compounds of the type R₃P=X (X = O,Se,Te) are converted to R₃P=S.

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• Lewis base-free thiophosphonium ion: a cationic sulfur atom transfer reagent,
  Pawel Löwe, Tim Witteler, Fabian Dielmann,
  Chem. Commun. 2021.
  https://doi.org/10.1039/d1cc01273h