Electrochemically driven metal-free synthesis of benzylic thioethers via C–S cross-coupling

Abstract

Organic sulfides play a crucial role in drug discovery and organic synthesis. The construction of C–S bonds usually requires strong basic conditions, transition metal catalysis, or stoichiometric oxidizing/reducing agents. Herein, we report an electrochemically driven C–S radical–radical cross coupling for the synthesis of benzylic thioethers. This approach is applicable to various benzyl halides and disulfides, providing corresponding benzylic thioethers in moderate to excellent yields. The reaction can be conducted at room temperature in air without the need for transition metals, external reducing agents, or sacrificial anodes, making it an environmentally friendly, cost-effective, and mild synthetic strategy.

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Introduction

Organosulfur compounds are widely found in nature. The sulfur-containing moiety is a key component that determines the activity and function of these compounds.¹ Thioethers, as a particularly important class of organosulfur compounds, play crucial roles in various fields, including organic synthesis,² pharmaceuticals,³ agrochemicals,⁴ and materials science.⁵ Several bioactive compounds containing a benzylic thioether moiety are shown in Fig. 1. Therefore, the synthesis of thioethers has received widespread attention.

Fig. 1 Bioactive compounds containing benzylic thioether moiety.

In general, the C–S coupling reaction is considered a reliable and direct method for constructing thioethers. The conventional approach to synthesizing thioethers entails reacting alkyl or aryl thiols with alkyl halides under highly basic conditions, leading to the generation of substantial quantities of salt byproducts.⁶ Additionally, the harsh conditions of these reactions result in severe intolerance towards functional groups. Furthermore, thiols usually have an unpleasant odor. As a result, various catalytic strategies, including transition metal-catalyzed and photocatalytic C–S cross-coupling, have emerged for thioether synthesis.⁷ For example, in 2022, both the Li⁸ and Wang⁹ groups independently utilized nickel-catalyzed C–S reductive cross-coupling reactions to synthesize thioethers from different starting materials. The Li group employed alkyl halides and arylthiosilanes (Scheme 1a), while the Wang group used oxalates and thiosulfonates (Scheme 1b). Both methods utilized stoichiometric Mn as the reductant under argon conditions. In 2023, the Maiti group¹⁰ employed thioxanthone as a photocatalyst to synthesize thioethers through C(sp³)–H thioarylation, using 1-(phenylthio)pyrrolidine-2,5-diones as arylthio radical precursors. The reaction was conducted under a nitrogen atmosphere, in the presence of 2 equivalents of K₃PO₄, and irradiated with 390 nm purple LEDs (Scheme 1c). In 2024, the Picazo group¹¹ reported an iron-catalyzed method for synthesizing benzylic thioethers from benzyl halides and disulfides. The method utilized Fe(CO)₅ as a catalyst, with pinacolone as the solvent, and was conducted at 107 °C under a nitrogen atmosphere for 24 hours (Scheme 1d). These elegant works have greatly contributed to the synthesis of thioethers. However, there are still certain limitations, such as the need for transition metal catalysts, stoichiometric reductants, high temperatures, complex substrates, or an inert gas atmosphere. These factors limit their practical applications. Therefore, there is still a need to develop more environmentally friendly, economical, and easily operable strategies for the synthesis of benzylic thioethers.

Scheme 1 Synthesis of thioethers via C–S cross coupling.

In recent years, electrochemical organic synthesis has become a favored synthetic method.¹² This method utilizes clean electrons instead of potentially polluting or harmful chemical reagents as oxidants or reducing agents,¹³ making it a green, economical, and sustainable synthetic approach.¹⁴ Electrochemical methods have been used to prepare organosulfur compounds such as, sulfones,¹⁵ sulfoxides,¹⁶ sulfonamides,¹⁷ sulfinic esters,¹⁸ thiosulfonates,¹⁹ sulfoximines,²⁰ and sulfonyl fluorides.²¹ Several methods for the electrochemical synthesis of thioethers have also been developed. For example, the reactions of alkenes or alkynes with ArSSAr were employed to produce the corresponding thioethers via diarylthiolation or thio-nucleophile addition to carbon–carbon multiple bonds.²² Electrochemical oxysulfuration of olefins with thiols and nucleophilic oxygen sources (water, alcohols, acids) has been used to prepare thioethers.²³ Additionally, electrochemically oxidative α-C–H sulfenylation of ketones with heteroaromatic thiols has been investigated.²⁴ The electrochemical trifluoromethylation of thiophenols with sodium trifluoromethanesulfinate has also been documented.²⁵ However, to the best of our knowledge, the electrochemical synthesis of benzylic thioethers from benzylic halides and disulfides has not yet been reported.

Here, we present an electrochemical synthesis strategy for benzylic thioethers via C–S cross-coupling of benzyl halides and disulfides (Scheme 1e). This reaction can be carried out in an undivided electrolytic cell under ambient conditions, without the need for transition metal catalysts, external reducing agents, exogenous bases, or sacrificial anodes. The reaction conditions are simple and mild, exhibiting excellent functional group tolerance and economic feasibility.

Results and discussion

Initially, benzyl bromide (1a) and 1,2-diphenyldisulfane (2a) were used as model substrates for the reaction. After a series of optimizations of reaction conditions, standard conditions were established [for details on the optimization of reaction conditions, please refer to the ESI, Tables S1–S5†]. Under the standard conditions, involving the reaction being conducted in an undivided cell with a constant current of 10 mA, Pt sheet as the anode, Ni foam as the cathode, 1a (0.5 mmol, 1.0 equiv.) and 2a (0.75 mmol, 1.5 equiv.) as substrates, ⁿBu₄NI (0.5 mmol, 1.0 equiv.) as the electrolyte, and N,N-dimethylacetamide (DMA) (5 mL) as the solvent, the desired product benzyl(phenyl)sulfane 3a was successfully obtained in 95% isolated yield.

To investigate the effects of different electrodes, solvents, electrolytes and electric current on the reaction, a series of experiments with varying parameters were conducted (Table 1). In the absence of current, the reaction cannot proceed (Table 1, entry 2), highlighting the essential role of electricity in this reaction. Without adding an electrolyte, the reaction system experienced a voltage overload (Table 1, entry 3). When ⁿBu₄NBr and ⁿBu₄NBF₄ were used as electrolytes instead of ⁿBu₄NI, no product 3a was obtained (Table 1, entries 4 and 5). Altering the current intensity, whether increasing or decreasing it, resulted in a decrease in yield (Table 1, entries 6 and 7). When DCM was used as the solvent instead of DMA (Table 1, entry 8), no target product was detected. Replacing DMA with MeCN and DMF as solvents provided product 3a with yields of 29% and 70%, respectively (Table 1, entries 9 and 10). Employing a Pt sheet or Cu foam as cathodes instead of Ni foam yielded product 3a in 83% and 34% yields, respectively (Table 1, entries 11 and 12).

Table 1 Selected optimization of the reaction conditions^a

Entry	Variation from standard conditions	Yield^b (%)
a Standard conditions: 1a (0.5 mmol, 1 equiv.), 2a (0.75 mmol, 1.5 equiv.), ^nBu4NI (0.5 mmol, 1 equiv.), DMA (5 mL), constant current = 10 mA, 6–12 h, room temperature (RT), open air, undivided cell, reactions performed using Standard ElectraSyn 2.0 vessel (10 mL). b Isolated yield.
1	—	95
2	Without current	N.R.
3	Without electrolyte	Overload
4	^nBu4NBr instead of ^nBu4NI	N.D.
5	^nBu4NBF4 instead of ^nBu4NI	Trace
6	5 mA instead of 10 mA	69
7	15 mA instead of 10 mA	78
8	DCM instead of DMA	N.D.
9	MeCN instead of DMA	29
10	DMF instead of DMA	70
11	Pt sheet as cathode	34
12	Cu foam as cathode	83

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Under standard conditions (Table 1, entry 1), the substrate scope of the reaction was investigated (Fig. 2). Initially, the scope of benzyl bromides was studied with 1,2-diphenyldisulfane (2a) as the coupling partner. It is encouraging that benzyl bromides bearing various functional groups were tolerated by the reaction, yielding corresponding benzylic thioethers in good to excellent yields (3a–3x). For example, benzyl bromides with methyl substitution at different positions on the benzene ring gave products 3b–3d in 80–92% yields. The 4-tert-butyl and 3,5-dimethyl substituted benzyl bromides produced products 3e and 3f in 85% and 80% yields, respectively. It is worth noting that different halogen-substituted benzylic thioethers (3g–3l) could be obtained in 81–92% yields. This indicates a satisfactory tolerance of different halogen-substituted substrates in this electrochemical reaction, and demonstrates sufficient selectivity for the benzyl bromide groups. Based on the yields of products 3b–3d and 3j–3l, it is evident that the position of substituents on the benzene ring affects the reaction outcomes. Steric hindrance likely contributes to the slightly lower yields observed for ortho-substituted products (3b and 3j) compared to their meta- and para-substituted counterparts. The 4-methoxy-substituted and 3,5-dimethoxy-substituted benzylic thioethers were successfully obtained with yields of 99% and 80%, respectively (3m and 3n). The biphenyl benzylic thioether 3o was obtained in an excellent yield of 92%. Additionally, benzyl bromides bearing electron withdrawing groups, such as trifluoromethoxy, trifluoromethyl, cyano, and nitro, were also applicable to this reaction, yielding corresponding products 3p–3s successfully, with yields ranging from 86% to 95%. Apart from benzyl bromides with various substituents on the benzene ring, 2-(bromomethyl)naphthalene and (bromomethylene)dibenzene were proven as qualified substrates, yielding products 3t and 3u with yields of 82% and 71%, respectively. Benzyl bromides substituted with methyl, ethyl, and cyclohexyl groups at the benzyl position were also suitable for this reaction, yielding corresponding products 3v–3x in 58–93% yields. The moderate yield of product 3x may be due to steric hindrance caused by the substituent. The higher yield of 3u compared to 3x may be attributed to the stabilizing effect of a larger conjugated system on the free radical intermediate.

Fig. 2 Substrate scope for the synthesis of benzylic thioethers. Reaction conditions: 1 (0.5 mmol, 1 equiv.), 2 (0.75 mmol, 1.5 equiv.), ⁿBu₄NI (0.5 mmol, 1 equiv.), DMA (5 mL), constant current = 10 mA, undivided cell, open air, RT, 6–12 h. Reactions performed using Standard ElectraSyn 2.0 vessel (10 mL). All yields refer to isolated yields.

Next, the substrate scope of disulfide compounds was investigated with benzyl bromide as a coupling partner. Both aryl and alkyl disulfides proved to be applicable in this reaction, yielding the corresponding benzylic thioether products (3y–3ak) in good to excellent yields. The reactions of several halogenated diaryl disulfides yielded products with excellent yields of 90–96% (3y–3aa), highlighting the selectivity of this electrochemical protocol, where only the benzyl bromide was selectively reduced, leaving the aryl halides remained unchanged. The electronic effects of substituents had no significant effect on the reaction. Diphenyl disulfides with electron-donating groups (such as methyl and methoxy) and electron-withdrawing groups (such as trifluoromethyl and nitro) could all be successfully converted into the corresponding products (3ab–3ag) with good to excellent yields. 2-Nitrophenyl disulfide provided product 3ae in relatively low yield, possibly due to steric hindrance caused by the substituent. Heteroaromatic disulfides, such as 1,2-di(pyridin-4-yl)disulfane and 1,2-di(thiophen-2-yl)disulfane, also smoothly participated in the reaction, providing the corresponding products (3ah and 3ai) with yields of 74% and 90%, respectively. Notably, alkane disulfides like dimethyl disulfide and diethyl disulfide could also be successfully converted into the corresponding products 3aj and 3ak, 82% and 88% yields, respectively. Additionally, we attempted the reactions of 2-(bromomethyl)furan, 4-(bromomethyl)pyridine, and 2-(bromomethyl)thiophene with 1,2-diphenyldisulfane 2a, respectively. Unfortunately, none of them could participate in the reaction and remained as unreacted starting materials.

The practicality of this reaction in synthesis was investigated (see reaction details in the ESI, section 7†). When the model reaction was scaled up to the gram level, product 3a could be smoothly obtained in 87% isolated yield (Scheme 2a). The Faraday efficiency was 8.6% for the gram-scale reaction and 21.2% for the 0.5 mmol scale reaction. Sulfones and sulfoxides are important structures widely present in natural products. Product 3a couldbe readily converted to the corresponding sulfoxide 4 and sulfone 5 in good isolated yields using meta-chloroperoxybenzoic acid (mCPBA) (Scheme 2b). In addition, the applicability of the reaction to different benzyl halides and pseudohalides was explored (Scheme 2c). When benzyl chloride 6 and benzyl iodide 7 were used instead of benzyl bromide, product 3a was obtained with yields of 90% and 92%, respectively. However, when benzyl alcohol 8 was used as the substrate, no product was observed under standard conditions. It is worth mentioning that diphenyl phosphate substrate 9 also exhibited activity in the reaction, providing product 3a in 60% isolated yield.

Scheme 2 Practicality investigation.

To understand the mechanism of this electrochemical reaction, radical trapping experiments were conducted using the radical scavengers 2,2,6,6-tetramethylpiperidinooxy (TEMPO) and 1,1-diphenylethylene (DE) (Scheme 3). When TEMPO (4.0 equiv.) was added to the model reaction under standard conditions, only trace amounts of product 3a were generated, and the adduct (10) of TEMPO and benzyl radical was detected by high-resolution mass spectrometry (HRMS). Upon addition of 1,1-diphenylethylene (4.0 equiv.), the yield of product 3a decreased to 30%, and the adduct (11) of 1,1-diphenylethylene and phenylthio radical was detected by HRMS (for reaction details, see the ESI, section 6.2†). These experimental results confirm the formation of radical intermediates B and C during the reaction, suggesting that this electrochemically driven C–S cross-coupling may proceed via a radical pathway. In addition, a control experiment was conducted in which only 1a was added under standard conditions, without the addition of 2a. This successfully resulted in the formation of the benzyl radical self-coupling product 12 in 11% isolated yield (Scheme 3c). This finding provides evidence for the presence of benzyl radicals (for details, please see ESI, section 6.3†).

Scheme 3 Control experiments.

The cyclic voltammetry (CV) experiments (for details, see ESI, section 6.1;† all potentials in this paper are reported vs. Ag/AgCl in DMA) revealed that the onset potential for the oxidation of ⁿBu₄NI is around +0.74 V. For benzyl bromide (1a), there is no significant oxidation peak in the range 0 V to 2 V. For 1,2-diphenyldisulfane (2a), the onset potential for oxidation is around +1.10 V (Fig. S4†). For product benzyl(phenyl)sulfane (3a), the onset potential for oxidation is around +1.76 V (Fig. S5†). These results suggest that ⁿBu₄NI is preferentially oxidized at the anode. For ⁿBu₄NI, there is no significant reduction peak in the range of 0 V to −3 V. The onset potential for the reduction of benzyl bromide (1a) is around −1.01 V. In comparison, 1,2-diphenyldisulfane (2a) has an onset reduction potential of about −1.50 V (Fig. S6†). These results suggest that both 1a and 2a can be reduced at the cathode, but 1a is reduced more readily than 2a.

Considering that the reaction also shows a good yield when using benzyl chloride as a substrate (Scheme 2c), we measured the reduction potential of 1a + ⁿBu₄NI to investigate whether a halogen exchange between benzyl bromide (or chloride) and iodide anions occurs, forming benzyl iodide, which is then reduced at the cathode. Using ⁿBu₄NBF₄ as the electrolyte, an equivalent amount of ⁿBu₄NI was added to the cyclic voltammetry experiment of 1a, and the reduction potential was measured after different stirring durations (10, 20, and 30 minutes). The results indicated that the addition of ⁿBu₄NI slightly increased the current intensity of the reduction peak of 1a (we speculate that this may be due to the increase in electrolyte concentration caused by the addition of ⁿBu₄NI), but it had no effect on the reduction potential of 1a. These results rule out the pathway where benzyl bromide undergoes halogen exchange with iodide anions to form benzyl iodide, which is then reduced at the cathode (for cyclic voltammogram, see ESI, Fig. S7†).

A divided-cell electrolysis experiment was conducted (performed in an H-type divided cell, with identical reaction substrates, solvents, and electrolytes in both the anodic and cathodic chambers). Through TLC detection, no target product 3a was detected at either the anode or the cathode. This result indicates that the reaction involves a paired electrolysis process. Additionally, radical trapping experiments of the divided cell were performed. The radical scavengers DE and TEMPO were added to the cathode and anode chambers of the divided cell, respectively. Adduct 11 was detected at the anode chamber, and adduct 10 at the cathode chamber, indicating that phenylthio radicals were present at the anode chamber and benzyl radicals were present at the cathode chamber. No phenylthio radicals were captured in the cathode chamber, which ruled out the possibility that diphenyl disulfide (2a) could be reduced at the cathode to produce sulfur radicals and negative sulfur intermediates (for reaction details of divided-cell experiments, see ESI, section 6.4†).

Based on the experimental results and literature reports,²⁶ we propose a possible mechanism to describe this electrochemical reaction (Fig. 3). Firstly, the iodine anion from ⁿBu₄NI is oxidized at the anode to generate I₂, which subsequently reacts with disulfide 2a to form intermediate A. The unstable intermediate A undergoes homolytic dissociation to give an iodine radical and phenylthio radical B.²⁷ Meanwhile, at the cathode, benzyl bromide (1a) undergoes reduction, leading to the formation of benzyl radical C and bromine anion.²⁸ The desired product 3a is formed through the cross-coupling reaction between phenylthio radical B and benzyl radical C (as depicted in Scheme 3, where radicals B and C were captured by DE and TEMPO, respectively).

Fig. 3 Proposed mechanism.

Conclusion

In summary, we have developed a novel, mild, and efficient electrochemical method for the coupling of benzyl halides and disulfides to synthesize benzylic thioethers. The reaction is performed at room temperature, in an air atmosphere, without the need for transition metals, catalysts, or external reductants. The practicality of this approach is demonstrated through successful gram-scale synthesis, its applicability to a wide range of halides and disulfides, and the preparation of other valuable sulfur-containing compounds. The study also suggests a possible reaction mechanism, which may involve C–S radical–radical cross-coupling. This method provides a simple, economical, environmentally friendly, and efficient strategy for the synthesis of benzylic thioethers.

Author contributions

Ming-Qiu-Hao Fu: investigation, methodology, writing – original draft; Yan-Hong He: conceptualization, supervision, writing – review & editing; Zhi Guan: conceptualization, funding acquisition, supervision, writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China (no. 22078268 and 22378334), and Innovation Research 2035 Pilot Plan of Southwest University (SWU-XDZD22011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01803f

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