1,2,4-Dithiazole-5-ones and 5-thiones as efficient sulfurizing agents of phosphorus(iii) compounds – a kinetic comparative study

Abstract

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.

Mechanism of sulfur transfer from 1,2,4-dithiazolidine-3,5-diones to triphenylphosphines

“…From the data in Table , it is obvious that the second‐order rate constants show a significant dependence on solvent polarity which indicates development of substantial charges in the transition state. Such observations are completely different to what was observed in the case of structurally similar 1,2,4‐dithiazole‐5‐ones and 5‐thiones where the influence of solvent polarity on the rate of sulfurization was only negligible. From the data in Table , it is also possible to plot corresponding Hammett correlations (Fig.…”

“…Using 31 P NMR, we have found that the reaction of all 1,2,4‐dithiazolidines 1a , 1b , 1c , 1d , 1e , 1f , 1g with triphenylphosphines in CDCl 3 and CD 3 CN proceeds smoothly and the only products are the corresponding triphenylphosphinsulphides together with phenylisocyanates and carbonylsulfide which is consistent with previous results. Similar experiment with the least reactive triphenyl phosphite used in our previous study surprisingly showed that the parallel oxidation of trivalent phosphorus occurs and the product therefore contains a mixture of the desired triphenyl phosphorothioate (δ(CDCl 3 ) = 53.7 ppm) and triphenyl phosphate (δ(CDCl 3 ) = −17.3 ppm) in the ratios 1:3 – 1:5 depending on the water content (based on integration of peaks at 53.7 ppm and −17.3 ppm in 31 P NMR spectrum). This result could not be caused only by dissolved oxygen in deuterated solvent or by the presence of oxygen in the air above the sample in the NMR tube during measurement because the triphenyl phosphite itself does not undergo oxidation under these conditions.…”

“…These Hammett ρ‐values are only slightly dependent on a solvent polarity, but their absolute values (ρ ACN = 2.49 ± 0.17, ρ DCM = 2.33 ± 0.11, ρ THF = 2.16 ± 0.11, ρ TOL = 1.90 ± 0.16) are higher than those found for the reaction of related 3‐(3‐ and 4‐subst. phenyl)‐1,2,4‐dithiazole‐5‐ones and 5‐thiones with triphenyl phosphite (ρ = 1.1–1.6) or triphenylphosphine (ρ = 1.7; for source data see Table 10 in Supporting Information). This confirms the proposal that the transition state structures existing on the reaction coordinate are more polarized.…”

“…, it is clear that the slopes of Hammett correlations in individual solvents are virtually the same (ρ ACN = −2.66 ± 0.17, ρ DCM = −2.69 ± 0.15, ρ THF = −2.55 ± 0.15, ρ TOL = −2.63 ± 0.27) but their absolute values are again much higher than those obtained for reaction of related 3‐(3‐ and 4‐subst. phenyl)‐1,2,4‐dithiazole‐5‐ones and 5‐thiones with triphenyl phosphites (ρ = −1.07 and −1.28) or triphenylphosphines (ρ = −1.63). Even smaller values were found for the reaction of 3‐amino‐1,2,4‐dithiazole‐5‐thione and 3‐dimethylamino‐1,2,4‐dithiazole‐5‐thione with triphenylphosphines (ρ = −0.86 and −0.92) 23 .…”

“…The 1,2,4‐dithiazolidine‐3,5‐diones can therefore be considered as a suitable sulfurization agent of various P(III) compounds. The sulfurization efficiency of 4‐phenyl‐1,2,4‐dithiazolidine‐3,5‐dione towards various P(III) compounds was recently evaluated and compared with other known sulfurization agents, but the detailed mechanistic study of this reaction has not been published until now. Therefore, we decided to explore the kinetics and detailed mechanism for the reaction of 4‐(3‐ and 4‐subst.…”