What is Triphenylphosphine?

Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the chemical formula P(C6H5)3, often abbreviated as PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds because it is a potent reducing agent and neutral ligand. At room temperature, PPh3 crystals are relatively stable and colourless in air. PPh3 is insoluble in water; slightly soluble in petroleum ether and alcohol, soluble in xylene, toluene, acetone, carbon tetrachloride and ether.

Convenient Synthesis of Triphenylphosphine Sulfide from Sulfur and Triphenylphosphine

Although the direct reaction of triphenylphosphine with sulfur leading to triphenylphosphine sulfide was known, this reaction was described to proceed slowly at rt or require heating at high temperatures. Very recently, the rotary ball mill technique (at 400 rpm) was applied to this reaction to lower the reaction temperature to rt with shortened reaction time (4 h). However, in this case, although solvents were not used during the reaction, extraction of the product from the apparatus required a significant amount of solvent. It should also be noted that although this technique could be useful in some cases, it is not suitable as a common synthetic method for any synthetic organic laboratory due to the high price of the apparatus (varying from many thousands to tens of thousands of dollars for typical laboratory models).

In this context, the simplification of reaction conditions by lowering the solvent amounts, using standard and inexpensive laboratory glassware and purifying the obtained products by simple filtration and washing should be an excellent approach to valorize and promote the widespread application of the developed synthetic procedure.

Herein we report a very rapid reaction of a stoichiometric mixture of triphenylphosphine with sulfur (reaction time less than 1 min) under very simple conditions of shaking.

During the course of our study on new reactivities of elemental sulfur applied to organic synthesis, we have noticed that some reactions with this element proceeded very efficiently in highly concentrated media or even in the absence of solvent. When the reactions were performed at high temperatures, the reaction media were in general liquid, and the stirring was possible without any difficulty. However, if the reactions were carried out with solid starting materials (including solid sulfur), the addition of an inert or weakly interacting solvent could be performed to facilitate the stirring. Such solvents should be used in sufficient quantity to avoid excessive dilution, which in turn reduces the reactivity significantly.

Experiments

Our reaction is visualized in Figure 1. The reaction could be performed in a test tube, although larger reactors could be used for larger scales (100 mmol). Equimolar amounts (10 mmol) of solid triphenylphosphine (as flakes) 1a and sulfur were added to a test tube (Figure 1a,b). The solvent (5 mL was added) (Figure 1c). We noticed that common laboratory solvents such as CH2Cl2, CHCl3, toluene . . . were all suitable for this reaction. The reaction tube was shaken vigorously with a vortex mixer. The reaction mixture became rapidly homogeneously with a slight increase in temperature (about 40 ?C). The solid triphenylphosphine sulfide product was precipitated out after 40 s of shaking as a white crystalline powder. The reaction mixture cooled down to rt was filtered, washed with methanol (2 mL × 3) and dried to afford the product as a white crystalline solid triphenylphosphine sulfide 2a (yield = 2.59 g, 88%). The filtrate could be concentrated to provide a pure product. The purity of the product could be confirmed by 1H, 31P and 13C NMR spectra as well as by elemental analysis. Compared to 31P NMR of the starting PPh3 (7.32 ppm in CDCl3), the chemical shift of the corresponding PPh3S is significantly different (43.3 ppm). Compared to known methods, ours is highlighted by its high atom-efficiency with short reaction time (<1 min) under room temperature conditions with a very small amount of moderately polar solvents such as chloroform, CH2Cl2, toluene and its easy purification by simple filtration.

Results and Discussion

Mechanistically, the reaction could be initiated by a nucleophilic attack of triphenylphosphine to S8 molecules to provide zwitterion A (Scheme 1), as previously proposed by Bartlett and Meguerian. A cascade of seven-time attacks of the other seven molecules of Ph3P would consume all the polysulfide chain. The first step could be rate determining and depends on the concentration of both starting materials. This suggests that the reaction is best performed under highly concentrated conditions. Since sulfur is not well soluble in common protic polar solvents such as alcohol (methanol or ethanol) or dipolar aprotic solvents (DMF, DMSO), the reaction performed in these solvents did not succeed in the same manner or required longer reaction times.

The reaction conditions could also be extended to other analogs as exemplified by two derivatives of triphenylphosphines bearing a para substituent such as chloro (1b) or methoxy (1c). In both cases tested, the reactions on a smaller scale (1 mmol) could be conveniently performed in a 7-mL test tube with magnetic stirring and provided the corresponding products 2b-2c in quantitative yields (See Supplementary Material).

Conclusions

The salient aspects of our developed synthetic procedure are its high atom-efficiency, its short reaction time (<1 min), its room temperature conditions, the involvement of only a very small amount of moderately polar solvents such as halogenated solvents (chloroform, CH2Cl2) or aromatic hydrocarbon solvents (toluene, xylenes) and its easy purification by simple filtration.

As a matter of fact, the purification step, especially in large-scale production, may be time-consuming, difficult or even impossible using other chromatography techniques than simple washing with solvents. This purification technique is once again demonstrated to be easily achieved. Previously, some organic reactions with elemental sulfur leading to complex scaffolds could be purified by applying the same technique.

References:

[1] NGUYEN T. Convenient Synthesis of Triphenylphosphine Sulfide from Sulfur and Triphenylphosphine[J]. Clean Technologies and Environmental Policy, 2022. DOI:10.3390/cleantechnol4020013.