The sulfurization efficiency of 25 3-substituted-1,2,4-dithiazole-5-ones and 5-thiones towards triphenyl phosphite in acetonitrile, DCM, THF and toluene at 25 °C was evaluated. All the 1,2,4-dithiazoles are much better sulfurizing reagents than commercially available agents (PADS, TETD, Beaucage's reagent). The most efficient sulfurizing agents in all solvents are 3-phenoxy (4), 3-phenylthio (5) and 3-ethoxy-1,2,4-dithiazole-5-one (1) whose reactivity is at least two orders of magnitude higher than that of other 1,2,4-dithiazoles. Contrary to a previous report, the sulfurization with 1 does not yield carbonylsulfide and ethyl cyanate as the additional reaction products but unstable ethoxythiocarbonyl isocyanate which has been trapped with 4-methoxyaniline. Similar trapping experiments have proven that the site of attack is at the sulfur adjacent to the C=O group for compounds 4 and 5. The reaction pathway involves rate-limiting initial nucleophilic attack of the phosphorus at sulfur followed by decomposition of the phosphonium intermediate to the corresponding phosphorothioate and isocyanate/isothiocyanate species. The existence of the phosphonium intermediate during sulfurization of triphenyl phosphine with 3-phenyl-1,2,4-dithiazole-5-thione (7a) was proven using kinetic studies. From the Hammett and Brønsted correlations and from other kinetic measurements it was concluded that the transition-state structure is almost apolar for the most reactive 1,2,4-dithiazoles whereas a polar structure resembling a zwitter-ionic intermediate may be more appropriate for the least reactive 1,2,4-dithiazoles. The extent of P–S bond formation and S–S bond cleavage is very similar in all reaction series but it gradually decreases with the reactivity of the 1,2,4-dithiazole derivatives.

The mechanism of sulfurization of substituted triphenylphosphines with 4-(3-and 4-substituted)-1,2,4dithiazolidine-3,5-diones in acetonitrile, dichloromethane, tetrahydrofuran and toluene at 25 C was studied. The reaction pathway involves rate-limiting initial nucleophilic attack of the phosphorus at sulfur followed by fast decomposition of the phosphonium intermediate to the corresponding phosphine sulfide, phenylisocyanate and carbonylsulfide. From the Hammett correlations and from the solvent dependency, it was concluded that the transition-state structure is very polar and resembles the zwitter-ionic intermediate. The extent of P-S bond formation and S-S bond cleavage is very similar in the solvents series, but the latter gradually decreases with the decreasing polarity of the solvent.

in Wiley Online Library (wileyonlinelibrary.com).Seven new substituted 5-phenoxy-1,2,4-dithiazole-3-ones were prepared in modest yield (53-76%) from corresponding O-phenyl thiocarbamates and chlorocarbonylsulfenyl chloride in dry ether at À10 C. All of the compounds were characterized by NMR and elemental analysis and some of them by X-ray diffraction. Preliminary kinetic measurements showed that the parent 5-phenoxy-1,2,4-dithiazole-3-one is a very efficient sulfurizing agent toward triphenyl phosphite.

Synthesis and Structure of Substituted 5-Phenoxy-1,2,4-dithiazole-3-ones (V). -(PONOMAROV, O.; PADELKOVA, Z.; HANUSEK*, J.; J. Heterocycl. Chem. 48 (2011) 6, 1225-1228, http://dx.doi.org/10.1002/jhet.717 ; Dep. Org. Chem., Fac. Chem. Technol., Univ. Pardubice, CZ-532 10 Pardubice, Czech Republic; Eng.) -A. Forchert 13-153