Electroreductive thiocarboxylation of alkenes with cyclosulfonium salts and CO₂: access to thioether acids

Abstract

Electroreductive carboxylation has emerged as a prominent strategy in modern organic synthesis, demonstrating well-established applications in two-component coupling reactions. However, multi-component reactions for selective mono-carboxylation remain underdeveloped with only a limited number of examples reported. We herein report a novel electroreductive intermolecular 1,2-thiocarboxylation of alkenes with cyclosulfonium salts and carbon dioxide (CO₂) via selective C–S bond cleavage for the construction of thioether acids. This transformation enables the single-step installation of both distal aryl thioether-functionalized alkyl chains and electrophiles into C=C bonds of alkenes via a radical relay cascade mechanism, achieving sequential covalent bond formation through controlled radical insertion. With demonstrated broad substrate adaptability and excellent functional group tolerance, this method paves a straightforward and selective pathway for the modular synthesis of thiocarboxylation derivatives.

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Introduction

Thioether acid is an essential structural unit in many biologically active compounds, drugs, and natural products, such as glutamyl-S-allyl-mercapto-L-cysteine, γ-L-glutamyl-S-trans-1-propenyl-L-cysteine, N-acetyl-S-allylcysteine, and so on (Fig. 1),¹ which have the ability to significantly inhibit atherosclerosis through antioxidant, lipid-lowering, anti-inflammatory, and antithrombotic mechanisms.^2,3 Due to the significant importance, synthesizing innovative and efficient molecules containing thioether acids has an important place in the toolbox strategies of researchers. The strategic utilization of CO₂ as a versatile C1 building block for carboxylic acid synthesis has emerged as a frontier research area, driven by its inherent advantages of sustainability, abundance, cost-effectiveness, and environmental friendliness.^4–10 In the past decade, the difunctional carboxylation strategy employing CO₂ has emerged as an effective approach in synthetic chemistry, enabling concurrent incorporation of carboxyl and other functional moieties through single-step protocols.^11–24 For example, Martin and co-workers, Wu and co-workers, Yu and co-workers, Han and Jing reported carbocarboxylation, aminocarboxylation, arylcarboxylation, silacarboxylation, phosphonocarboxylation, and alkynylcarboxylation of alkenes with CO₂via visible-light photoredox or electroreduction. Given that there are only a limited number of examples of synthesis of thioether-containing carboxylic acids, we seek to develop the first late-stage thiocarboxylation reaction employing organosulfur compounds with CO₂ and alkenes by using environmentally benign and sustainable strategies.

Fig. 1 Representative bioactive molecules containing thioether acid units.

Sulfonium salts, as a representative class of organosulfur compounds, demonstrate significant potential in organic synthesis owing to their distinctive structural architecture and tunable reactivity profiles.^25–37 Considerable progress has been achieved in selective difunctionalization for the synthesis of both C–C and C–X (X = Br, S, N, etc.) bonds by using sulfonium salts as the radical precursors with alkenes and nucleophiles over the past few decades (Scheme 1a).^38–41 Unfortunately, to date, only a few successful strategies have been reported for the use of sulfonium salts to react with C=C bonds and electrophiles, for example, Xie and co-workers reported a visible-light-driven protocol for achieving the elegant arylcarboxylation of alkenes with CO₂ and aryl thianthrenium salts (Scheme 1b).⁴² Although novel, in conventional sulfonium salt-based methods, sulfur-containing moieties are always discarded as sacrificial leaving groups, leading to low atom economy and severely limiting further applications.

Scheme 1 Difunctionalization of alkenes and sulfonium salts.

To address such a problem, our aim was to further develop the electroreductive thiocarboxylation of alkenes with various cyclothionium salts and CO₂ as an electrophile to synthesize highly valuable thioether acids (Scheme 1c). However, several challenges may impede the success. (1) How to avoid the C(sp²)–S bond cleavage of cyclothionium salts and selectively enable the radical ring-opening pathway; (2) how to preferentially reduce cyclothionium salts compared to alkenes without affecting any sensitive functional groups, as well as avoiding overreduction of thioether-containing alkyl radicals; and (3) how to achieve late-stage functionalization^43–46 with benign and selective installation of a thioether-containing alkyl group on the desired structural motifs of bio-relevant compounds. Once successfully implemented, this strategy will provide a more sustainable and environmentally friendly alternative to previous methods and generate valuable thioether acids, which serve as crucial structural motifs in various naturally occurring complex compounds and drug derivatives.

Results and discussion

We first employed 1,1-diphenyl ethylene (1a) and a cyclosulfonium salt (2a) as model substrates and examined various reaction conditions with CO₂ (1 atm) for the electroreductive alkylcarboxylation of alkenes (Table 1). After systematic screening of various reaction parameters, 1a and 2a treated with CO₂, Me₄NPF₆, dimethyl terephthalate (DMTP) and 10 mA constant current in an undivided cell with a magnesium flake anode and a C felt cathode generated the desired thioether acid 3aa in 72% yield (entry 1). Controlled experiments showed that constant current is essential for the present transformation (entry 2). Electrode systems, such as Zn(+)/C felt(−), Fe(+)/C felt(−), Ni(+)/C felt(−), C(+)/C felt(−), Mg(+)/Nb(−) and Mg(+)/Pt(−), were investigated, while Mg(+)/C felt(−) was proven to be the best electrode system (entries 3–8). It is a remarkable fact that no reaction was observed without the Me₄NPF₆ electrolyte (entry 9). When the Me₄NPF₆ electrolyte was replaced with tetrabutylammonium iodide (TBAI), ⁿBu₄NBF₄, ⁿBu₄NOAc or LiClO₄, the yield was not improved (entries 10–13). However, employing N,N-dimethylformamide (DMF) and MeCN as solvent led to lower conversion of 1a (entries 14 and 15). The desired product 3aa was obtained in 47% yield without the additive DMTP, providing strong support for its ability to facilitate the reaction (entry 16). To our delight, the optimal conditions are compatible to a scale up to 5.0 mmol of 1a, giving 3aa in good yield (entry 17).

Table 1 Screening of optimal reaction conditions^a

Entry	Variation from the standard conditions	Yield^b (%)
a Standard reaction conditions: undivided cell, magnesium flake anode (10 × 15 × 0.5, mm), C felt cathode (10 × 15 × 3, mm), constant current = 10 mA, 1a (0.2 mmol), 2a (0.3 mmol), CO2 (1 atm; balloon), Me4NPF6 (0.1 M), DMSO (2.0 mL), room temperature and 3.5 h. b  1a (5.0 mmol).
1	None	72
2	Without current	0
3	Zn(+)/C felt(−) instead of Mg(+)/C felt(−)	Trace
4	Fe(+)/C felt(−) instead of Mg(+)/C felt(−)	37
5	Ni(+)/C felt(−) instead of Mg(+)/C felt(−)	42
6	C rod(+)/C felt(−) instead of Mg(+)/C felt(−)	26
7	Mg(+)/Nb(−) instead of Mg(+)/C felt(−)	53
8	Mg(+)/Pt(−) instead of Mg(+)/C felt(−)	19
9	Without Me4NI	Trace
10	^nBu4NI instead of Me4NPF6	35
11	^nBu4NBF4 instead of Me4NPF6	43
12	^nBu4NOAc instead of Me4NPF6	<10
13	LiClO4 instead of Me4NPF6	Trace
14	DMF instead of DMSO	43
15	MeCN instead of DMSO	Trace
16	Without DMTP	47
17^b	None	58

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After identifying optimal reaction parameters, we set out to explore the scope of alkenes and cyclosulfonium salts (Table 2). In the presence of CO₂, Me₄NPF₆, DMTP, DMSO and 10 mA constant current, the scope of alkenes 1b–aa was competent for the electroreductive thiocarboxylation reaction with cyclosulfonium salt 2a. To our delight, a wide range of functional groups, such as ^tBu, Me, MeO, MeS, Me₂N, PhO, Ph, CF₃O, CF₃, F, Cl, Br and CO₂Me, on the aryl ring were well tolerated. Substituted substrates at different positions on the aryl ring of the alkene exhibit different yields. For example, 1,1-diaryl ethylene 1c bearing a Me group at the para position was efficiently converted to 3ca in 76% yield. In contrast, the 1,1-diaryl ethylene compound with meta and ortho substituents produced 3oa and 3qa in decreasing yields (63% and 46%, respectively), likely due to the higher steric hindrance leading to lower yield. 1,1Diaryl ethylenes 1i andj and 1n with a strongly electron-withdrawing CF₃O, CF₃ or CO₂Me group could also undergo thiocarboxylation with CO₂ to generate 3ia andja and 3na in good yields. To our regret, α-methylstyrene 1s and styrene 1t were not suitable for the three-component coupling reaction. The desired product 3ua was isolated in a 65% yield using 2-(1-phenylvinyl)naphthalene 1u. Heteroarylethylenes, such as 2-(1-phenylvinyl)furan 1v and 2-(1-phenylvinyl)thiophene 1w, were competent substrates (3va andwa). Moreover, 1,1-diaryl ethylenes bearing multiple substituents, such as perfluorinated groups, a Me group and a Cl group, and an ethyl dioxygen group, were also suitable substrates and could be converted smoothly to 3xa–za. Furthermore, 3-phenyl-1H-indene 1aa was used to enable facile access to the corresponding thiocarboxylation product 3aaa in a 32% yield.

Table 2 Variation of the alkenes (1) and cyclosulfonium salts (2)^a

a Standard reaction conditions: undivided cell, magnesium flake anode (10 × 15 × 0.5, mm), C felt cathode (10 × 15 × 3, mm), constant current = 10 mA, 1 (0.2 mmol), 2 (0.3 mmol), CO₂ (1 atm; balloon), Me₄NPF₆ (0.1 M), DMSO (2 mL), room temperature and 3.5 h.

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Next, using 1,1-diphenyl ethylene (1a) as a model substrate, we investigated a variety of cyclosulfonium salts (2b–g). A number of substituents, including methyl, methoxyl, fluoro, chloro, and bromo, were compatible with the optimized conditions (3ab–ag). Encouraged by the wide scope of the alkenes, we then applied substrates derived from several natural products and drug molecules in the procedure. Various natural product-based or drug-based 1,1-diaryl ethylenes, such as adamantane, borneol, galactose, fructose, ribofuranose, menthol, and naproxen derivatives, underwent the late-stage modification to deliver the highly valuable natural-product-based or drug-based thioether acids 3aba–aha, which highlights the applicability of the protocol in synthesis.

As shown in Scheme 2A, synthetic utilizations of the resulting thioether acid 3aa were examined. In the presence of LiAlH₄, reduction of 3aa occurred to afford 2,2-diphenyl-7-(p-tolylthio)heptan-1-ol 4 in 81% yield. Interestingly, treatment of 3aa with 2 equiv. of TMSCHN₂ was successful and delivered the highly valuable methyl 2,2-diphenyl-7-(p-tolylthio)heptanoate 5. The reaction of 3aa with methyl glycylglycinate hydrochloride in the presence of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI), 1-hydroxybenzotriazole (HOBt), and N,N-diisopropylethylamine (DIPEA) proceeded to furnish product 6 in 61% yield.

Scheme 2 Synthetic applications and control experiments.

We performed several mechanistic experiments to gain insight into the mechanism of this transformation. First, when the model reaction was conducted using 3.0 equiv. of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or butylated hydroxytoluene (BHT) as a radical trapping reagent, only 18% yield of product 3aa was isolated (Scheme 2B, 1 and 2). Next, the deuteration experiment of 1a with 2a and 10.0 equiv. of D₂O under the optimal conditions gave the hydrogenation product (3aa′) in 29% yield with 59%-D (Scheme 2B, 3), suggesting that the carbanion intermediate might be involved in the transformation. Electricity on/off experiments indicated that electrocatalysis is crucial for the reaction, as no reaction is observed in the absence of electricity (Fig. S2; SI). Thereafter, cyclic voltammetry (CV) analysis of 1a and 2a under an argon (Ar) atmosphere revealed onset reduction potentials of −2.43 V and −1.65 V versus SCE, respectively, and under a CO₂ atmosphere, the reductive potential was retained (Fig. S3; SI). These observations imply that 2a is more readily reduced than 1a and CO₂. Furthermore, when CV experiments of DMTP were performed under the condition of increasing equivalents of 2a, a reduction in the oxidation current of the radical anion of DMTP was observed (Fig. S4; SI), indicating that DMTP potentially served as a mediator for the single-electron reduction of 2a.

Based on the above experiments and previous reports, a plausible reaction mechanism is proposed for this electroreductive thiocarboxylation protocol (Scheme 3).^47–49 Initially, the cyclosulfonium salt 2a is reduced at the cathode to generate the radical anion intermediate I. Simultaneously, DMTP undergoes reduction at the cathode to form DMTP˙⁻, which subsequently mediates SET reduction of the remaining portion of cyclosulfonium salt 2a, enabling the formation of key intermediate I. This intermediate subsequently undergoes ring-opening processes, resulting in the generation of the alkyl radical II, which then undergoes sequential addition across the C=C bond of alkene 1a and goes through SET reduction at the cathode, producing the alkyl anion IV. The carboxylation of IV culminates in the formation of the desired product 3aa.

Scheme 3 Possible reaction mechanisms.

Conclusions

In summary, we have developed a novel transition-metal-free electroreductive thiocarboxylation of alkenes with CO₂ and cyclosulfonium salts to afford valuable thioether acids. This protocol enables the direct incorporation of the thioether group and the carboxyl group to achieve 1,2-dicarbonyl functionalization, and its mild conditions, good selectivity and exceptional compatibility toward functional groups are highlighted. Most importantly, the successful thiocarboxylation of structurally complex natural products and pharmaceutically relevant derivatives conclusively demonstrates the operational versatility and drug discovery applicability of the method. Further applications of the electroreductive ring-opening protocols of cyclosulfonium salts are currently underway in our laboratory.

Author contributions

Q. Xiao designed the experiments and directed the project. Y. F. Huang performed the chemical experiments. C. Z. H. Cheng wrote the manuscript. M. J. Luo and L. J. Wang analysed the data. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5qo01313e.

Acknowledgements

We thank the National Natural Science Foundation of China (no. 22261019), the PhD start-up fund of Jiangxi Science & Technology Normal University (no. 2023BSQD14) and the Jiangxi Provincial Key Laboratory of Organic Functional Molecules (no: 2024SSY05141) for financial support.

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Footnote

† These authors contributed equally to this work.

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