The journey of C–S bond formation from metal catalysis to electrocatalysis

Abstract

Due to widespread applications of sulfur-containing frameworks, the construction of the C–S bond has emerged as a useful tool for synthetic organic and medicinal chemists in the recent years. Initially, the C–S bond formations were carried out under transition metal catalysis. The last 10–15 years have been a glorified era for the C–S bond constructions during which various sustainable procedures have been developed. This perspective describes the journey of C–S bond construction starting from transition metal catalysis through oxidant catalysis, photocatalysis and the very recently employed electrocatalysis using various sulfur surrogates.

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Ze-Wei Chen

Ze-Wei Chen was born in Kaohsiung, in 1998. He received his BS degree from the Department of Applied Science at National Taitung University (Taiwan) in 2020. He is currently in the second year of his Master's studies under Professor Chin-Fa Lee at the National Chung Hsing University.

Rekha Bai

Rekha Bai was born in 1988 in the village Jonaicha Khurd of Alawar District, Rajasthan, India. She received her MSc degree in 2012 and the PhD degree in 2020 (under Dr Satpal Singh Badsara) from the University of Rajasthan, Jaipur, India. After working as a research associate in Dr Badsara's group, very recently, she joined Prof. Chin-Fa Lee's group as a postdoctoral researcher. Her research interests include electro-organic synthesis, and cross-coupling reactions.

Pratheepkumar Annamalai

Pratheepkumar Annamalai was born in 1990 at Katchamangalam, a village of Thanjavur, Tamilnadu, India and received his MSc degree (2012) from the Department of Applied Chemistry from the National Institute of Technology, Tiruchirappalli, Tamilnadu, India, and his PhD (2019) from the Department of Applied Chemistry from the National Yang-Ming Chiao Tung University (NYCU) Hsinchu, Taiwan. He is currently undertaking postdoctoral research under Professor Chin-Fa Lee's guidance at the Department of Chemistry, of National Chung Hsing University. His research interests are in the areas of sustainable catalysis and optoelectronic materials.

Satpal Singh Badsara

Satpal Singh Badsara was born in 1983 and obtained his PhD degree from the University of Hyderabad, India in 2013 (under Professor Deevi Basavaiah). During 2013–14, he was a Postdoctoral Fellow under Prof Chin-Fa Lee at the National Chung Hsing University, Taiwan. Currently, he is working as an Assistant Professor at the University of Rajasthan, Jaipur India. He was the recipient of the ISCB Young Scientist Award 2019 in Chemical Science. His research interests include electro-organic synthesis, C–H functionalization, cross-coupling reactions and Baylis–Hillman chemistry.

Chin-Fa Lee

Chin-Fa Lee was born in 1975 and obtained his PhD degree from National Taiwan University in 2002 (under Professor T.-Y. Luh). After spending one postdoctoral year each under Professor T.-Y. Luh at NTU and under Professor J. F. Hartwig at Yale University (2003–2004), he was a Postdoctoral Associate under Professor D. A. Leigh at Edinburgh University from 2005 to 2007. He is currently a Chairman of the Chemistry Department and a Lifetime Distinguished Professor at National Chung Hsing University. His research interests include homogeneous catalysis, optoelectronic materials, and self-assembly chemistry.

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1. Introduction

In recent years, due to the versatile applications of organosulfur compounds, the synthesis of these molecules via C–S bond formation has emerged as a useful tool for synthetic organic and medicinal chemists.¹ The organosulfur frameworks show ubiquitous importance in organic synthesis,¹ materials science,^2a–c natural products,^2d,e chemical biology,^2f pharmaceutics,^2g–ketc. Initially, the organosulfur frameworks were synthesized via transition metal-catalyzed C–S cross-coupling reactions between aryl halides and thiols/thiolates.³ Many transition metals, such as palladium, copper, nickel, iron, indium, cobalt, gold, manganese, silver, etc., and their salts or complexes have shown catalytic efficiency in the synthesis of thioethers and thioesters.^1,4 The past decade has witnessed the development of sustainable approaches for the synthesis of organosulfur compounds using a variety of substrates and sulfur surrogates under metal-free conditions via C–H thiolation and cross-coupling reactions.⁵ Many alternative catalytic systems, such as oxidants, photocatalysis, electro-catalysis, etc., have also emerged as sustainable alternatives to traditional transition metal catalysis.⁶ Our research group has been actively working in the area of C–S bond formation and has contributed significantly both to metal catalysis as well as sustainable catalysis. In this perspective, we provide a comprehensive discussion about our contributions along with related works to explain the journey of C–S bond formation, starting from transition metal catalysis to sustainable catalysis using a variety of substrates and sulfur surrogates, that is separated into different sections.

2. Transition metal-catalysed C–S bond construction

2.1. Palladium catalysis in C–S bond construction

In 1978, Migita et al.³ employed Pd(PPh₃)₄ as a catalyst for the first C–S cross-coupling between aryl halides and thiols using NaOtBu as a base in DMSO to provide the corresponding diaryl sulfides or aryl alkyl sulfides in good yields (Scheme 1). As shown in Scheme 2, this reaction proceeds via the oxidative addition of aryl halide on Pd(0) to form intermediate A that reacts with thiolate salts via transmetallation to form the thiolate–Pd complex B. Reductive elimination of B produces diaryl sulfides. In 2004, Itoh and Mase⁷ developed the palladium-catalyzed C–S bond formation between aryl halide/triflates and thiols using Xantphos (1) as a ligand as shown in Path A of Scheme 3. The resulting thioethers were obtained in 70–92% yields. Similarly, palladium-catalyzed syntheses of thioethers were also developed by Murata and Buchwald⁸ using the Pd(OAc)₂/DiPPF (2) catalytic system, as shown in Path B of Scheme 3.

Scheme 1 First C–S cross-coupling between aryl halides and thiols by Migita et al.

Scheme 2 Mechanistic pathway for palladium-catalysed C–S bond formation.

Scheme 3 Palladium-catalysed C–S bond formation between aryl halide/triflates and thiols.

In 2006, Hartwig's research group⁹ contributed to the palladium-catalyzed synthesis of thioethers using aryl chlorides and thiols as coupling partners under the influence of CyPF-t-Bu (3) as a powerful ligand, as shown in Scheme 4. In 2011, Beller and co-workers¹⁰ reported the palladium-catalyzed ligand-free C–H thiolation of arenes using aryl sulfonyl cyanide as a sulfur source. The coupling partners possessing different functional groups were well tolerated under the reaction conditions employed to provide the corresponding diaryl sulfides in good to excellent yield, as shown in Scheme 5.

Scheme 4 Palladium-catalysed synthesis of thioethers by Hartwig's research group.

Scheme 5 Palladium-catalysed ligand-free C–H thiolation of arenes using aryl sulfonyl cyanide as a sulfur source.

In 2014, Nishihara and co-workers¹¹ reported an interesting palladium-catalysed C–H thiolation process on arenes using disulfides as thiolating agents in the presence of copper as a co-catalyst and phosphines as a ligand, as shown in Scheme 6. This method showed ortho selectivity that was attributed to various directing groups.

Scheme 6 Palladium-catalysed C–H thiolation process on arenes using disulfides as thiolating agents.

In 2014, Jiang and co-workers¹² developed a novel palladium-catalyzed synthesis of thioethers using aryl halides/alkyl halides as the aryl/alkyl source, and Na₂S₂O₃·5H₂O as a sulfur source, as shown in Scheme 7. In 2015, Messaoudi, Alami, and co-workers¹³ reported an interesting palladium-catalyzed C–S bond formation between methyl 2-iodobenzoate and peracetylated glycosyl thiols 4 using Xantphos (1) as a ligand to provide the corresponding thioglycosylated products 5 in 31–100% yields. These thioglycosylated products 5 were then further converted into fused thioglycosylated benzoxathiepinones 6via a K₂CO₃-mediated deprotection process (Scheme 8). In 2018, Wang et al.¹⁴ employed KSAc as a source of sulfur for the palladium-catalyzed synthesis of alkyl aryl thioethers via the reaction between aryl/heteroaryl halides and various alkylating reagents such as DMC (dimethyl carbonate), DEC, (diethyl carbonate), DPC (diphenyl carbonate), etc. Representative examples are shown in Scheme 9. The developed strategy was then extended for the synthesis of precursors for various pesticides and for the synthesis of firocoxib (7), a non-steroid anti-inflammatory drug (Scheme 10).

Scheme 7 Palladium-catalysed synthesis of thioethers using Na₂S₂O₃·5H₂O as a sulfur source.

Scheme 8 Palladium-catalysed synthesis of thioglycosylated products.

Scheme 9 Palladium-catalysed synthesis of alkyl aryl thioethers using KSAc as a sulfur source.

Scheme 10 Synthesis of firocoxib.

In 2018, an interesting palladium-catalyzed intramolecular C–H thiolation of masked thiols 8 was developed by Jiang and co-workers.¹⁵ The resulting dihydrobenzothiophene 9 and thiochromans 10 were obtained in good to excellent yields, as shown in Scheme 11. The developed method was further extended to obtain a variety of semiconductor moieties such as BTT, BTBT and BBTT (Scheme 11). Sanford and co-workers¹⁶ performed a comparative catalytic study between palladium and nickel for the decarbonylation process of thioesters using different ligands, as shown in Scheme 12. When this interesting decarbonylation process was carried out using the Pd[p(o-tol)₃]₂/PAd₂Bn catalytic system, the resulting thioethers were obtained in 27–85% yields, whereas the same reaction under the catalytic influence of Ni(cod)₂/PCy₃ provided the thioethers up to 99% yields.

Scheme 11 Synthesis of dihydrobenzothiophene and thiochromans.

Scheme 12 Decarbonylation process of thioesters.

2.2. Nickel catalysis in C–S bond construction

In 1981, Cristau and co-workers¹⁷ for the first time found the catalytic application of Ni complex 11 for the formation of C–S bonds between thiolates and aryl halides to afford the corresponding aryl thioethers in 67–100% yields (Path A, Scheme 13). In 1987, Takagi¹⁸ also reported a similar type of thioether synthesis via the formation of C–S bonds between various aryl halides and thiols using the NiBr₂/dppf catalytic system in the presence of Zn, as shown in Path B of Scheme 13.

Scheme 13 Nickel-catalysed C–S bond formation between thiolates and aryl halides.

Zhang et al.¹⁹ for the first time employed an N-heterocyclic carbene (NHC)-based Ni complex catalyst in the presence of a base for the C–S bond construction between a variety of aryl bromides/iodides and thiols to afford the corresponding thioethers in 78–99% yields (Scheme 14). In 2015, an interesting nickel-catalyzed C-sp³ C–H bond-activation process on a substrate possessing an 8-aminoquinoline as a directing group for thiolation using disulfides as thiolating agents was developed by Shi's group²⁰ and by Zhang's group.²¹ Representative examples²⁰ are presented in Scheme 15.

Scheme 14 Ni(NHC)₂-catalysed C–S bond formation.

Scheme 15 Nickel-catalysed C–S bond formation via sp³ C–H bond activation.

In 2018, Liu and Szostak²² employed the nickel salt/complex [Ni(dppp)Cl₂] for the decarbonylation of thioesters in the presence of a base to obtain thioethers, as shown in Path A of Scheme 16. Almost at the same time, Yamauchi and co-workers²³ reported a similar nickel-catalysed decarbonylation protocol of thioesters as shown in Path B, Scheme 16.

Scheme 16 Nickel-catalysed decarbonylation of thioesters.

2.3. Copper catalysis in C–S bond construction

The first copper-catalyzed synthesis of thioethers was developed in 1984 by Yamamoto and Sekine²⁴ using aryl thiols and aryl/alkyl halides. The authors reported two alternative copper-catalyzed procedures for the preparation of thioethers, as shown in Scheme 17. When thiophenols were reacted with metallic copper, CuSAr (12) was obtained, which on reaction with aryl bromides at 270 °C in an autoclave provided the resulting thioethers in 24–67% yields (Path A). Alternatively, the reaction between thiols and aryl iodides in the presence of Cu powder at 240–300 °C provided the resulting thioethers in 18–88% yields via Path B.

Scheme 17 First copper-catalysed synthesis of thioethers.

Later on, in 2002 Kwong and Buchwald²⁵ used CuI as the catalyst for C–S bond formation between aryl iodide and thiols in the presence of ethyl glycol as a ligand and K₂CO₃ as a base to provide the corresponding thioethers in 71–95% yields (Scheme 18).

Scheme 18 CuI-catalysed C–S bond formation between aryl iodide and thiols.

Similarly, Naus and co-workers²⁶ also used CuI for the arylation of sugar thiols 13 with o-haloaryl triazenes 14 in the presence of K₂CO₃/pyridine, as shown in Path A of Scheme 19. The authors also extended the developed procedure to the diarylation of sugar thiols, i.e., 1-thioglycopyranoses 13 with a variety of 2,6-dihaloaryl triazenes 15 and the results are shown in Path B, Scheme 19.

Scheme 19 CuI-catalysed arylation of sugar thiols.

In 2002, Liebeskind and co-workers²⁷ developed an alternative synthesis of thioethers using N-thioimides 16 as a thiolating agent for the copper-catalyzed C–S bond formation with boronic acids, as shown in Path A of Scheme 20. A variety of aryl and alkyl boronic acids underwent CuMeSal-catalyzed C–S bond formation with aromatic and heteroaromatic thiols to provide the corresponding thioethers in 51–79% yields. Later on, in 2014, Jiang and co-workers²⁸ performed a copper-catalyzed Chan–Lam reaction of S₈ with boronic acids to afford the aryl thioethers shown in Path B, Scheme 20. Similarly, in 2015, Tian and co-workers²⁹ developed a copper-catalyzed synthesis of thioethers via the C–S bond formation between boronic acids and sulfonyl hydrazides as shown in Path C, Scheme 20.

Scheme 20 Copper-catalysed synthesis of thioethers from boronic acids.

In 2006, Yu and co-workers³⁰ for the first time reported an interesting Cu(II)-catalyzed direct C–H thiolation of phenyl pyridine (17) with thiols or disulfides, as shown in Scheme 21. The plausible mechanistic pathway for this protocol is believed to proceed via Cu(II)-mediated single-electron transfer (SET), and ortho selectivity was achieved through intramolecular anion transfer via a nitrogen-bound Cu(I) ate complex.

Scheme 21 Cu-Catalysed direct C–H thiolation of phenyl pyridine.

In 2010, Lee's research group³¹ prepared an efficient catalyst of CuO on mesoporous silica and employed it for C–S bond formation between a variety of aryl iodides and thiols in the presence of K₂CO₃ as a base, which provided the resulting thioethers in 66–97% yields (Path A, Scheme 22). This catalytic system was also found to be suitable for C–S coupling between aryl iodides and heteroaromatic thiols (Path B, Scheme 22). In 2011, Lee's research group³² also employed Cu₂O along with 1,10-phenanthroline as a catalytic system for C–S bond formation between a variety of vinyl halides (with aryl as well as alkyl substitution) and thiols to provide the resulting thioethers in good to excellent yields as shown in Scheme 23. It is worthy of mention here that, for the C–S coupling between vinyl iodides and thiols, only 0.5 mol% of Cu₂O was required (Path A, Scheme 23), whereas for vinyl bromide and chloride, 5 mol% of Cu₂O was required (Path B, Scheme 23). It was also observed that the C–S coupling between vinyl bromide and chloride with thiol can also be performed under catalyst-free conditions to afford the corresponding thioethers in excellent yields (Path C, Scheme 23).

Scheme 22 Synthesis of thioethers catalysed by CuO on mesoporous silica.

Scheme 23 Cu₂O-catalysed C–S bond formation between vinyl halides and thiols.

In 2012, an interesting iridium-catalyzed meta-C–H borylation process followed by copper-catalyzed C–S bond formation was also developed by Lee and co-workers.³³ In a one-pot synthetic procedure, first aryl boronic esters were prepared in situ via an Ir[(cod)OMe]₂-catalyzed meta-borylation of arenes/heteroarenes. The volatile compounds were then removed and the crude mixture upon subsequent treatment with aryl disulfides under the catalytic influence of CuCl provided thioethers in 46–88% yields, as shown in Scheme 24. In 2013, Bolm's research group³⁴ also employed copper as a catalyst for C–S bond formation between β-diketones/esters and disulfides via a decarbonylation process to afford the corresponding α-thioaryl ketone/esters in 30–99% yields, as shown in Scheme 25.

Scheme 24 Iridium-catalysed meta-C–H borylation followed by copper-catalyzed C–S bond formation.

Scheme 25 Copper-catalysed C–S bond formation between β-diketones/esters and disulfides.

Our research group^35,36 then studied the catalytic efficiency of CuO for C–S coupling between aryl iodides and thiols using Cs₂CO₃ as a base to provide the resulting thioethers in 36–99% yields (Path A, Scheme 26). CuO was also found to be a suitable catalyst under microwave conditions for C–S bond formation between aryl iodides and thiols (Path B, Scheme 26).

Scheme 26 CuO-catalysed C–S coupling between aryl iodides and thiols.

A facial copper-catalyzed synthetic protocol for the preparation of benzoisothiazolones via C–S/N–S bond formation through a C–H activation process on arenes possessing a bidentate directing group (pyridine-2-yl)isopropyl amine (PIP-amine) (18) and S₈ as a sulfur source was developed by Chen et al.,³⁷ as shown in Scheme 27 in which representative examples are presented.

Scheme 27 Copper-catalysed synthesis of benzoisothiazolones.

In 2015, Sekar's research group³⁸ developed an efficient copper-catalyzed domino process for the synthesis of thioflavanones via the C–S bond formation between 2′-iodochalcones 19 (Path A) or 2′-bromochalcones 20 (Path B) and xanthate (21) as a sulfur source, as shown in Scheme 28. Later on, Sangeetha and Sekar³⁹ also developed a copper-catalyzed synthesis of 2,3-dihydrobenzo[b]thiophene frameworks 22via a similar type of domino C–S bond formation between 2-iodoketones and commercially available xanthate (21) as a sulfur surrogate, as shown in Path C, Scheme 28. Interestingly, when the reaction was performed under the same catalytic system and conditions in the presence of H₂SO₄, it provided benzothiophene derivatives 23 (Path D, Scheme 28).

Scheme 28 Xanthate as a sulfur source for C–S bond formation.

Messaoudi and co-workers⁴⁰ reported a thioglycosylation process of the C(sp²)–H bonds of benzamide derivatives with the variety of thiosugars 24 using Cu(OAc)₂·H₂O as a catalyst and Ag₂CO₃ as an additive to afford the resulting products in good yields with exclusive β-selectivity. Representative examples are shown in Scheme 29. In 2017, Lee's research group⁴¹ developed a highly regioselective copper-catalyzed synthetic protocol for the preparation of thioethers starting from arenes. In a one-pot operation, first anisoles were injected for p-selective bromination under the catalytic influence of FeCl₃ using NBS as a brominating agent. The reaction mixtures were then filtered and the filtrates were then treated with a variety of thiols using CuI as the catalyst and N,N′-dihexyloxalamide (25) as a ligand to afford the corresponding thioethers in good to excellent yields (Scheme 30). CuI along with ligand 25 was also found to be suitable for C–S bond formation between different haloarenes and thiols, as shown in Scheme 31.⁴²

Scheme 29 Thioglycosylation of the C(sp²)–H bonds of benzamide derivatives.

Scheme 30 Copper-catalysed synthesis of thioethers from arenes.

Scheme 31 CuI-catalysed synthesis of thioethers.

Very recently, Ma and co-workers⁴³ employed sodium sulfinate as a thiolating agent for copper-catalyzed C–S bond formation with aryl iodides, as shown in Scheme 32.

Scheme 32 Sodium sulfinate as a sulfur source for C–S bond formation.

Lee's research group⁴⁴ developed an interesting copper-catalyzed C–H thiolation of aldehydes with a variety of thiols in the presence of TBHP in aqueous media (Scheme 33). Both the alkyl and aryl thiols coupled well with both aromatic and heteroaromatic aldehydes under the reaction conditions employed to provide the resulting thioethers in 31–94% yields (Path A, Scheme 33).^44a It is worthy of mention here that aliphatic aldehydes were not found to be suitable substrates in this method. When the authors performed the same C–H thiolation process of aldehydes with thiols under microwave conditions using the same catalytic system, both the aryl and alkyl aldehydes coupled well with thiols to provide the resulting thioesters in 38–99% yields (Path B, Scheme 33).^44b Patel and co-workers⁴⁵ also developed an interesting copper-catalyzed cross-dehydrogenative S-arylation process of thiols using methyl arenes as aryl surrogates in the presence of TBHP as an oxidant, as shown in Scheme 34. The resulting thioethers were obtained in good to excellent yields without the assistance of any directing group.

Scheme 33 Copper-catalysed C–H thiolation of aldehydes with thiols.

Scheme 34 Copper-catalysed oxidative C–H thiolation of methyl arenes with thiols.

In 2016, Jiang and co-workers⁴⁶ reported an interesting synthesis of unsymmetrical disulfides via the copper-catalyzed oxidative cross-coupling of arylboronic acids with masked disulfides 26, as shown in Scheme 35. The desulfurizing reagents, i.e., masked disulfides 26, were synthesised from sodium 4-methylbenzenesulfonothioate in a two-step process as shown in Scheme 35. Representative examples are presented.

Scheme 35 Synthesis of unsymmetrical disulfides.

2.4. Cobalt catalysis in C–S bond construction

Cobalt was first employed as a catalyst for C–S bond formation by Cheng and co-workers.⁴⁷ This group used a variety of bromo- and iodo-arenes for C–S bond formation with thiols in the presence of the CoI₂(dppe) catalyst using Zn as a co-catalyst to afford the corresponding thioethers as shown in Scheme 36. Interestingly, this catalytic system was also found to be effective for C–S coupling between vinyl iodides and thiols. Later on, Dong et al.⁴⁸ also extended the applicability of cobalt catalysis to C–S bond formation between aryl zinc pivalates and diaryl disulfides, as shown in Scheme 37. Various electron-rich and electron-poor organozinc pivalates were found to be suitable for C–S coupling with different aryl disulfides, and the resulting thioethers were obtained in 60–95% yields.

Scheme 36 Cobalt-catalysed C–S bond formation between aryl halides and thiols.

Scheme 37 Cobalt-catalysed C–S bond formation between organozinc pivalates and disulfides.

2.5. Iron catalysis in C–S bond construction

In 2008, Bolm and co-workers⁴⁹ for the first time reported the catalytic applicability of iron salts for C–S bond formation. After screening different iron salts, they observed that FeCl₃ with DMEDA (27) as a ligand was suitable for the C–S bond construction between a variety of aryl iodides and thiols to afford the resulting thioethers in 32–98% yields (Scheme 38). It is worthy of mention here that only aryl iodides were found to be suitable coupling partners, and aryl chlorides and bromides could not couple with thiols under this catalytic system. Also, FeCl₃ was not suitable for C–S coupling between aryl iodides and alkyl thiols.

Scheme 38 FeCl₃-catalysed C–S bond formation between aryl iodides and thiols.

In 2009, almost at the same time, our research group⁵⁰ and Tsai's research group⁵¹ also developed iron-catalyzed synthetic protocols for thioethers via the coupling between aryl iodides and thiols (Scheme 39). We carried out the C–S bond-forming reaction between aryl iodides and thiols (both alkyl and aryl) in the presence of FeCl₃ using phosphine ligands 1 and 28 and NaOtBu, which affords the corresponding thioethers in 62–99% yields (Paths A and B, Scheme 39). Aryl iodides that possess both electron-rich and electron-poor groups reacted well with both aryl and alkyl thiols under the reaction conditions employed. Tsai's group⁵¹ used ligand 29 with FeCl₃ for C–S bond formation between aryl iodides and thiols (Path C, Scheme 39). However, low yields were obtained in the case of alkyl thiols. Later on in 2012, Lee and co-workers⁵² also found FeCl₃ to be a suitable catalyst with phosphine (1) for the C–S coupling reaction between vinyl halides and thiols (Scheme 40). The catalytic system was found to be suitable for the C–S coupling of vinyl chloride, bromide and iodide with thiols to afford the desired vinyl thioethers in good yields.

Scheme 39 FeCl₃-catalysed synthesis of thioethers.

Scheme 40 FeCl₃-catalysed C–S bond formation between vinyl halides and thiols.

In 2012, Lei and co-workers⁵³ also employed FeCl₃ along with Na₂S₂O₈ as a catalytic system for the preparation of benzothiazole derivatives 30via intramolecular C–H functionalization and C–S bond formation as shown in Scheme 41. Representative examples are presented. Ranu and co-workers⁵⁴ used iron nanoparticles with tBuONO for the coupling between amine and disulfides to provide the corresponding thioethers in 70–81% yields (Scheme 42). In 2014, Lee's research group⁵⁵ developed an interesting FeBr₂-catalyzed C–H thiolation process of aldehydes with thiols in the presence of TBHP in aqueous media, as shown in Scheme 43. Both, the aryl and alkyl aldehydes were coupled with thiols under the influence of this catalytic system.

Scheme 41 FeCl₃/Na₂S₂O₈-catalysed synthesis of benzothiazoles.

Scheme 42 Iron nanoparticle-catalyzed synthesis of thioethers.

Scheme 43 FeBr₂-catalysed C–H thiolation of aldehydes with thiols.

2.6. Miscellaneous metal catalysis in C–S bond construction

Yadavalli and co-workers⁵⁶ developed a manganese-catalyzed C–S bond formation between aryl iodides and thiols (Path A, Scheme 44). This group also extended this method to vinyl iodides with thiols (Path B, Scheme 44). Lee's research group⁵⁷ also reported a manganese-catalyzed cross-coupling reaction of aryl iodides with thiols using 31 or 32 as ligands to afford the corresponding aryl thioethers in good to excellent yields (Path C, Scheme 44).

Scheme 44 Manganese-catalysed synthesis of thioethers.

An interesting rhodium-catalyzed C–S bond formation reaction between aryl fluorides and aryl disulfides was reported by Yamaguchi and co-workers^58a,b (Scheme 45). A variety of aryl fluorides possessing both electron-donating as well as electron-withdrawing functionalities underwent C–S bond formation with disulfides under the influence of RhH(PPh₃)₄ in the presence of excess dppBz to afford the resulting thioethers in good to excellent yields. Interestingly, the fluoro group was found to be more prone towards C–S bond formation compared with the bromo and chloro groups. The authors^58a also studied the polyarylthiolation reaction on polyfluorobenzenes with disulfides, and interestingly observed that the product formation depends on the amount of dppBz and PPh₃, as shown in Scheme 45, i.e., an enhancement in the amounts of dppBz and PPh₃ leads to the formation of a large amounts of polyarylthiolated products. Later on, the same research group^58c also extended this interesting strategy to the synthesis of symmetrical polyfluorinated diaryl sulfides using S₈ or di-tert-butyl tetrasulfides as a sulfur source via the C–S cross-coupling reaction as shown in Scheme 46.

Scheme 45 Rhodium-catalysed C–S bond formation between aryl fluorides and aryl disulfides.

Scheme 46 Synthesis of polyfluorinated diaryl sulfides.

Yamaguchi and co-workers^58d also developed the rhodium-catalyzed synthesis of fluorinated dithioarylbenzenes via an acyl transfer reaction between thioesters and hexafluorobenzene, as shown in Scheme 47. A rhodium-catalysed synthesis of unsymmetrical diaryl sulfides via aryl exchange between symmetrical diaryl sulfides and aromatic fluorides was also reported by Yamaguchi's research group (Scheme 48).^58e

Scheme 47 Rh-Catalysed synthesis of diarylthio tetrafluorobenzenes via acyl cleavage.

Scheme 48 Rh-Catalysed synthesis of unsymmetrical diaryl sulfides via aryl exchange.

Zhou, Li, and co-workers⁵⁹ also reported a rhodium-catalyzed C–H thiolation process on arenes possessing directing groups using disulfides as sulfur surrogates, as shown in Scheme 49. A variety of arenes underwent C–H thiolation with aryl and alkyl disulfides under the catalytic influence of [RhCp*Cl₂]₂ and under the influence of AgOTf/Cu(OAc)₂, providing the resulting monothioethers (Paths A–C, Scheme 49), whereas AgOTf/Ag₂CO₃ resulted in bis-thioethers (Path D, Scheme 49). In 2021, Rui et al.⁶⁰ reported a one-pot atom-economic synthesis of structurally important phenothiazines via the Rh(III)-catalyzed direct C–H thiolation of acetanilide with 2-bromothiophenol followed by an intramolecular C–N bond formation reaction sequence as shown in Path A of Scheme 50. The developed protocol was further extended to the gram-scale synthesis of the drug molecule chlorpromazine (33) (Path B, Scheme 50).

Scheme 49 Rhodium-catalysed C–H thiolation of arenes.

Scheme 50 Rhodium-catalysed C–H thiolation of acetanilide and synthesis of the drug molecule chlorpromazine.

An interesting iridium-catalyzed decarbonylation-C–H bis-arylsulfenylation of indoles at the C-2 and C-4 C–H bonds using disulfides was reported by Kathiravan et al.⁶¹ A variety of indole frameworks possessing C-2, and C-4 C–H bonds underwent C–S bond formation under the influence of the pentamethylcyclopentadienyl iridium dichloride dimer ([Cp*IrCl₂]₂) as a catalyst to afford the resulting bis-thioethers in 32–76% yields (Scheme 51).

Scheme 51 Iridium-catalysed decarbonylation-C–H bis-arylsulfenylation of indoles.

Baidya and co-workers⁶² performed a ruthenium-catalyzed o-C–H thiolation of aromatic and heteroaromatic carboxylic acids using disulfides as a thiolating agent, as shown in Scheme 52. The resulting o-thioethers were then utilized for the synthesis of thioxanthones 34via treatment with TfOH.

Scheme 52 Ruthenium-catalysed o-C–H thiolation of carboxylic acids.

A silver-catalyzed synthetic protocol for diaryl thioethers from arylboronic acids and aryl thiols using KOH as a base was developed in 2012 by Chakraborty and Das,⁶³ as shown in Scheme 53.

Scheme 53 Silver-catalysed C–S bond formation between arylboronic acids and aryl thiols.

Rao and co-workers⁶⁴ employed In(OTf)₃ along with TMEDA (35) as a ligand for C–S bond formation between aryl halides and thiols, as shown in Path A of Scheme 54. Rao's research group⁶⁵ also performed the same C–S coupling reaction using In₂O₃ nanoparticles in the absence of ligands using KOH as a base to afford the resulting thioethers in 5–97% yields (Path B, Scheme 54).

Scheme 54 In(OTf)₃-catalysed C–S bond formation between aryl halides and thiols.

3. Oxidant-mediated C–S bond construction

Various research groups^66–70 have employed iodine as a catalyst for the C–H thiolation process on various substituents using a variety of sulfur surrogates. Representative examples are shown in Schemes 55 and 56. Similarly, other iodide sources, such as NH₄I, PhI(OAc)₂, KI, etc., were also found to be suitable for the C–H thiolation process, as shown in Scheme 57.^71–73

Scheme 55 Iodine-catalysed C–H thiolation of cyclic ketones and indoles.

Scheme 56 Iodine-catalysed C–H thiolation using thiols as thiolating agents.

Scheme 57 C–H thiolation process catalysed by different iodide sources.

An interesting I₂/DTBP-catalyzed C–H thiolation process for the sp³ C–H bond of methyl arenes/alkanes with thiols was reported by Lie and co-workers,⁷⁴ as shown in Scheme 58. A variety of methyl arenes/alkanes underwent C–S bond formation with different thiols to afford the corresponding thioethers in 33–90% yields. In 2013, Guo et al.⁷⁵ reported a facile DTBP-promoted direct oxidative thiolation of the sp³ C–H bond of ethers with disulfides to afford the resulting alkyl aryl thioethers in good to excellent yields (Paths A and B in Scheme 59). This protocol was also extended to the sp³ C–H thiolation of amides (Path C, Scheme 59). Later on, Han and co-workers⁷⁶ also employed DTBP as a catalyst for the sp³ C–H bond thiolation of cycloalkanes with disulfides as shown in Path D of Scheme 59. Almost at the same time, Sun and co-workers⁷⁷ used TBHP for the sp³ C–H bond thiolation of alkanes with disulfides, as shown in Path E, Scheme 59. Lee's research group⁷⁸ developed an interesting iodine-catalyzed oxidative C–S coupling reaction between 1,3-diketones and disulfides using K₂S₂O₈ as an oxidant to afford the α-thio-β-dicarbonyl compounds as shown in Path A of Scheme 60. Almost at the same time, Lie, Hu, and co-workers⁷⁹ also reported similar types of oxidative C–S coupling between 1,3-diketones and thiols using the I₂/DTBP catalytic system as shown in Path B, Scheme 60. Later on, in 2018, Wang and co-workers⁸⁰ performed the oxidative C–S bond formation of 1,3-diketone frameworks with thiols using KI as a catalyst, as shown in Path C of Scheme 60.

Scheme 58 I₂/DTBP-catalysed sp³C–H thiolation process for the C–H bond of methyl arenes.

Scheme 59 I₂/oxidant-catalysed sp³C–H thiolation process.

Scheme 60 Oxidative C–S bond formation between 1,3-diketone frameworks and thiols.

In 2017, Badsara and co-workers^81,82 developed the interesting solvent-dependent regio- and stereo-selective syntheses of allylic thioethers 36–39 under metal-free conditions (Scheme 61). The C–S coupling of allyl halides with disulfides was carried out under the influence of DCP using DMSO/DPSO as a solvent to provide allylic thioethers 36 and 37 with complete stereo- and regio-selectivity (Paths A and B, Scheme 61).⁸¹ However, the same reaction using CH₃CN as the solvent and DTBP as the oxidant provided aryl/alkyl allyl thioethers 38 and 39 in 63–92% yields with complete stereo- and regio-selectivity (Paths C and D, Scheme 61).⁸²

Scheme 61 Regio- and stereo-selective syntheses of allylic thioethers.

Lee's research group⁸³ developed a facile I₂-catalyzed synthetic protocol for o-hydroxybenzyl thioethers via C–S bond formation between o-hydroxybenzyl alcohols 40 and thiols (Scheme 62). The reaction proceeded via the in situ preparation of o-quinone methides that upon thio-Michael reaction with thiols resulted in the corresponding o-hydroxybenzyl thioethers as shown in Path A, Scheme 62. The developed protocol was further extended to functionalized thiols 41 as shown in Path B, Scheme 62.

Scheme 62 I₂-catalysed synthesis of o-hydroxybenzyl thioethers.

Zhu and co-workers⁸⁴ reported the tetraethylammonium bromide (TEAB)-catalyzed oxidative thioesterification of aldehydes and alcohols with thiols using K₂S₂O₈ as an oxidant, as shown in Scheme 63. A variety of aromatic and heteroaromatic alcohols and aldehydes underwent oxidative C–S bond formation with different thiols to provide the corresponding thioesters in good to excellent yields.

Scheme 63 TEAB/K₂S₂O₈-catalysed oxidative thioesterification of aldehydes and alcohols.

Our research group⁸⁵ also developed an interesting DTBP-catalyzed oxidative C–H thiolation of the aldehydic C–H bond with disulfides as shown in Scheme 64. Both aromatic as well as heteroaromatic aldehydes possessing an electron-rich or electron-poor functionality underwent C–H thiolation with both aryl and alkyl disulfides to afford the corresponding thioethers in 40–99% yields.

Scheme 64 DTBP-catalysed oxidative C–H thiolation of aldehydes.

Subsequently, in 2014, our research group⁸⁶ also performed the temperature-dependent metal-free DTBP-catalysed oxidative C–H thiolation process of the sp³ C–H bond of methyl arenes/heteroarenes with disulfides to provide the corresponding thioesters in 47–88% yields (Scheme 65). Methyl arenes and heteroarenes possessing different functionalities underwent oxidative C–H thiolation with both aryl and alkyl disulfides, as shown in Path A, Scheme 65. Interestingly, the reaction of ethyl arenes with disulfides under the influence of DTBP could yield only the thioether via thiolation, as shown in Path B, Scheme 65.

Scheme 65 DTBP-catalysed oxidative C–H thiolation process of the sp³ C–H bond of methyl arenes/heteroarenes with disulfides.

A variety of other catalysts/catalytic systems, such as VA004/CTAB, DMP (Dess–Martin periodinane), DBU, etc., were also employed for the thioesterification of aldehydes with thiol/disulfides by various research groups, as shown in Paths A–D of Scheme 66.^87–90

Scheme 66 Thioesterification of aldehydes with thiols/disulfides.

4. Photocatalyzed C–S bond construction

Fu, Peters, and co-workers⁹¹ developed for the first time a photoinduced copper-catalyzed C–S cross-coupling reaction between aryl halides and thiols following a single-electron-transfer process. Both aromatic and heteroaromatic halides and thiols possessing electron-rich as well as electron-poor functionalities underwent C–S bond formation to afford the corresponding thioethers in 59–90% yields (Path A, Scheme 67). Later on, Fu and co-workers⁹² also reported the room-temperature visible-light-induced photo redox arylation of thiols with aryl halides using 44 as a photocatalyst, as shown in Path B, Scheme 67. This photoinduced protocol was found to be effective on aryl chlorides and both the aryl and alkyl thiols coupled well under the reaction conditions employed. Later on, in 2017, an interesting visible-light-promoted C–S cross-coupling reaction between aryl halides and thiols in the presence of Cs₂CO₃ without using any photocatalyst was developed by Miyake and co-workers⁹³ (Path C, Scheme 67). The authors⁹³ also demonstrated the pharmaceutical utility of the developed protocol for the synthesis of pharmaceutical ingredients such as indomethacin, fenofibrate, moclobemide, hydrochlorothiazide, etc., via late-stage modification. Majek and Wangelin⁹⁴ also developed a visible-light-mediated C–S bond-formation reaction between the arene diazonium salt and dimethyl disulfides using Eosin Y as a photocatalyst to afford the corresponding aryl methyl thioethers in 31–89% yields (Scheme 68). The reaction also proceeded via a single-electron-transfer mechanism.

Scheme 67 Photoinduced C–S cross-coupling reactions between aryl halides and thiols.

Scheme 68 Visible-light-mediated C–S bond formation between the arene diazonium salts and dimethyl disulfides.

In 2018, Hajra's research group⁹⁵ also developed the visible-light-mediated synthesis of aryl thioethers via a cross-coupling reaction between aryl hydrazines and thiols using rose bengal as a photocatalyst, as shown in Scheme 69.

Scheme 69 Visible-light-mediated C–S cross-coupling reaction between aryl hydrazines and thiols.

Lee's research group⁹⁶ also developed an interesting light-induced tBuONO-catalyzed C–S bond-formation reaction between anilines and thiols using a 10 W blue LED as a light source in the absence of photocatalyst. The resulting thioethers were obtained in 33–96% yields as shown in Scheme 70.

Scheme 70 Light-induced C–S bond formation between anilines and thiols.

The first visible-light photo-reduced thioacetalization process of aldehydes with thiols using Eosin Y as a photocatalyst to afford dithioacetals under metal-free and solvent-free conditions was developed by Lee's research group.⁹⁷ A variety of aromatic, heteroaromatic and α,β-unsaturated aldehydes underwent the thioacetalization process with aromatic aliphatic thiols to afford the resulting diacetals in 30–99% yields (Path A, Scheme 71). This thioacetalization proceeded via a radical pathway and the developed protocol was further extended to the preparation of cyclic dithioacetals (Path B, Scheme 71). Lee's research group⁹⁸ also employed the visible-light metal-free protocol for the oxathiacetalization of aldehydes and ketones with 2-mercaptoethanol/3-mercaptopropanol using Eosin Y as the photocatalyst to afford the resulting 1,3-oxathiolanes and 1,3-oxathianes, respectively (Path C, Scheme 71 in which representative examples are presented). Sekhar and co-workers⁹⁹ reported a facile visible-light-induced synthesis of heteroaryl aryl thioethers via I₂/TBHP-catalyzed C–S bond formation between heteroarenes and thiols as shown in Scheme 72. This one-pot synthesis of thioethers involves two sequential reactions, i.e., formation of the iodinated heteroarenes followed by C–S coupling reactions. Representative examples are presented in Scheme 72.

Scheme 71 Visible-light photo-reduced thioacetalization process of aldehydes with thiols.

Scheme 72 Visible-light-induced C–H thiolation of heteroarenes.

In 2017, Jiang and co-workers¹⁰⁰ developed an interesting photochemical-condition-controlled protocol for the sulfenylation and sulfoxidation of diaryl iodonium salts with organic thiosulphate salts using eosin Y as the photocatalyst in the presence of DIPEA and Zn(OAc)₂via tandem electron/energy-transfer and single-electron transfer, respectively (Scheme 73). When the C–S coupling was carried out using a 23 W CFL, it provided the resulting thioethers (Path A), whereas an 18 W green LED provided the sulfoxides (Path B). The developed protocol was further employed for late-stage sulfoxidation to access the pharmaceutically important metronidazole, furanose and pyranose derivatives. Furthermore, mechanistic study of these interesting transformations was also carried out by Jiang and co-workers¹⁰¹ using electron paramagnetic resonance experiments.

Scheme 73 Photochemical-condition-controlled sulfenylation and sulfoxidation of diaryl iodonium salts.

5. Electricity-promoted C–S bond constructions

Long ago in 1974, Tabakovic and co-workers¹⁰² performed the electrochemical synthesis of 2-phenylbenzothiazole via intramolecular C–S bond formation on thiobenzanilide as shown in Scheme 74. In 1979, this group¹⁰³ extended their electrochemical C–S bond-forming protocol to the preparation of 1,3-thiazole derivatives as shown in Scheme 74. In 2017, Qian et al.¹⁰⁴ performed a TEMPO-catalyzed electrochemical intramolecular C–H thiolation process on thioamide derivatives using an RVC/Pt electrode in an undivided cell to obtain benzothiazoles and thiazolopyridines (Scheme 75). This work was an extension of the work of Tabakovic's group.¹⁰² Representative examples are shown.

Scheme 74 Electrochemical synthesis of 2-phenylbenzothiazole and 1,3-thiazole derivatives.

Scheme 75 Electrochemical synthesis of benzothiazoles and thiazolopyridines.

Lei and co-workers¹⁰⁵ reported an electrochemical oxidant-free dehydrogenative C–H/C–S cross-coupling reaction at the C-3 position of indoles using thiols as a sulfur surrogate and Pt as the electrodes in an undivided cell to provide the resulting indole thioethers in 36–99% yields (Scheme 76). This group¹⁰⁵ also extended this electrochemical strategy for the C–H thiolation of the sp² C–H bond of arenes with thiols. Representative examples are presented. Mei and co-workers¹⁰⁶ also developed a nickel-catalyzed electrochemical organic synthesis of aryl thioethers via the thiolation of aryl/heteroaryl halides with thiols, as shown in Scheme 77. In this electrochemical organic synthesis, magnesium was used as the anode and nickel as the cathode. A variety of halides possessing electron-rich and electron-poor groups underwent thiolation with different thiols.

Scheme 76 Electrochemical dehydrogenative C–H/C–S cross-coupling reactions between indoles and thiols.

Scheme 77 Nickel-catalysed electrochemical thiolation of aryl/heteroaryl halides with thiols.

Recently, Wu and co-workers¹⁰⁷ also developed the electro-catalyzed C–H thiolation of the p-sp² C–H bonds of aromatic ethers using thiols as thiolating agents and platinum electrodes in an undivided cell, as shown in Scheme 78.

Scheme 78 Electrochemical thiolation of the p-sp² C–H bonds of aromatic ethers using thiols.

6. Conclusions

In conclusion, we have summarised the recent important developments in C–S bond formation in terms of various catalytic systems and substrates. A variety of metal-catalysed C–S cross-coupling protocols are discussed in detail by emphasising the original contributions along with the contributions of our group. Similarly, the utility of other sustainable catalytic methods such as oxidant-mediated, photo- and electro-catalysis for the construction of C–S bonds between a variety of substrates and sulphur surrogates are also discussed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Ministry of Science and Technology, Taiwan (Most 110-2113-M-005-014-MY3), National Chung Hsing University, i-Center for Advanced Science and Technology (iCAST), the “Innovation and Development Center of Sustainable Agriculture (IDCSA)” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan and Science and Engineering Research Board-Department of Science and Technology, Government of India, New Delhi, India (CRG/2020/003634).

Notes and references

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Footnote

† These authors contributed equally to this work.

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