Visible Light‐Driven Photocatalytic Transformation of Thiols to Disulfides in Water Catalyzed by Bi2S3

Selective coupling of thiols to produce disulfides is one very important and classic type of coupling reactions. Herein, an environmentally friendly and practical photocatalytic system for oxidative coupling of thiols to disulfides is developed. With no sacrificial agent, no additional additive, just sole water as solvent, performed in air at room temperature, excellent conversions of thiols and excellent selectivities of disulfides are obtained using a cheap and simple semiconductor material of bismuth sulfide (Bi2S3) under blue LED illumination. Excellent conversions and selectivities for coupling of thiols to produce disulfides are obtained, and the system is reusable. The as‐prepared photocatalyst is adequately characterized and the mechanism of photocatalytic reaction is discussed deeply.


Introduction
Disulfides are useful compounds to prevent oxidative damage in biological systems, play roles in protein folding and microtubule assembly in vitro, and investigate as drug delivery systems. [1,2]ubbers and elastomers can be bestowed excellent tensile strength by disulfides. [3,4]Some recent study reveals that disulfides have fungicidal properties and can go against cyanide intoxication. [5,6]What is more, disulfides are also used widely in chemical yield because of valuable organic intermediates for synthetic transformations and its protection property of thiols. [7]Therefore, it is of great significance to develop an economical and environmentally friendly synthesis method of disulfide.
In the field of catalysis, photocatalysis has been taken as a promising technology as it uses sustainable sunlight as input energy and ordinary semiconductor materials as catalyst.Significantly, the efficient fusion of free radical chemistry and photocatalysis strategies for the preparation of disulfide has been a breakthrough. [25]Recently, some photocatalytic systems have been developed to produce disulfides by oxidation of thiols, such as diaryl telluride-catalyzed oxidation by a 500 W halogen lamp in dichloromethane, [26] CpMn(CO) 3 -catalyzed system by 355 nm lighting in cyclohexane, [27] Mn(CO) 5 Br-and enzyme-catalyzed reaction by 355 nm lighting in cyclohexane, [28] CsPbBr 3 perovskite-catalyzed formation by white LED in dichloromethane, [29] graphene oxide-immobilized iron phthalocyanine by LED light (>400 nm), [30] and CdSe quantum dots by 500 W high-pressure mercury lamp with a 400 nm cut-off filter in a pH value near the pKa value aqueous solution. [31]Despite many developed systems available, some less desirable problems exist: 1) the low conversion and selectivity; 2) the use of toxic organic solvents; 3) the complicated and costly synthesis of the catalyst; and 4) the uneconomical light source.Therefore, it is of great significance to develop efficient photocatalysts, which can be excited by visible light and exhibits photocatalytic activity in nontoxic solvents.
Bismuth sulfide (Bi 2 S 3 ), as an important semiconductor material with a narrow bandgap, low cost, high biocompatibility, and high photon-to-electron conversion efficiency, has attracted much attention in recent years. [32]Many applications have been reported based on Bi 2 S 3 materials, such as high-performance photodetectors, [33] photothermal performance, [34] enhanced radiation therapy, [35] photocatalytic reduction, [36] thermoelectric properties, [37] high-capacity anodes for batteries, and [38,39] biosensors for micro-RNA detection. [40]Herein, we first used Bi 2 S 3 as a photocatalyst for the coupling oxidation of thiols to produce disulfides.The narrow bandgap of Bi 2 S 3 allows it absorb more visible light, enhancing the photocatalytic performance, and the blue light with long wavelength could be used as exciting light for it.In addition, the blue light with low energy could inhibit side reaction caused by peroxidation of the thiols.With no sacrificial agent, no additional additive, just sole water as solvent, performed in air at room temperature, excellent conversions of thiols and excellent selectivities of disulfides were obtained under blue LED illumination (Figure 1), and the catalyst can be used several times with activity holding.

Results and Discussion
The crystal structure of obtained catalyst of Bi 2 S 3 was investigated by powder X-ray diffraction (XRD).As shown in Figure 2a, the diffraction peaks of catalyst sample agree well with the crystalline data of the standard orthorhombic lattice of Bi 2 S 3 (JCPDF card No. 17-0320).X-ray photoelectron spectroscopy (XPS) was also used to further prove the chemical structure.Figure 2b shows the survey spectrum of the prepared catalyst, clearly indicating that the sample is mainly composed of Bi and S elements.The peaks at 225.6, 442.5, and 465.0 eV are indexed to S 2s, Bi 4d 5/2 , and Bi 4d 3/2 , [41] respectively.As shown   in the high-resolution XPS spectrum of Figure 2c, the peaks at 158.3 and 163.6 eV were assigned to Bi 4f 7/2 and Bi 4f 5/2 , respectively.The two weak peaks at 161.0 and 162.2 eV corresponded to S 2p 3/2 and S 2p 1/2 . [32]e surface morphologies and microstructures of prepared catalyst were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).Figure 3a shows that rod-like Bi 2 S 3 were obtained.Figure 3b depicts a typical TEM image of individual Bi 2 S 3 nanorods with diameters of about nm.The high-resolution transmission electron microscopy (HRTEM) image (Figure 3c) shows a set of parallel fringes with the space of 0.35 nm, responding to the (130) plane of Bi 2 S 3 .Figure 3d depicts a typical TEM image of individual Bi 2 S 3 nanorod with the diameter of about 4 nm.In addition, the spatial distribution of the catalyst is also investigated by element mapping (Figure 3e,f ), which further demonstrates the element dispersion of the Bi 2 S 3 catalyst.
To manifest the special property of the photocatalytic system based on Bi 2 S 3 , different kinds of classic photocatalysts were used for the transformation of 4-chlorobenzenemethanethiol to corresponding disulfide.The reactions were all carried out in the same condition.From Table 1, we could see that low conversions were obtained, catalyzed by TiO 2 and C 3 N 4 (entries 2 and 3), and it is easy to understand that these catalysts need higher energy to be activated and were inactive under blue lighting.Higher conversions were achieved under the same conditions catalyzed by CdS and Bi 4 O 5 Br 2 .However, some undesired byproducts were produced, leading to declining selectivity (entries 4 and 5).In stark contrast, an excellent transformation was successfully achieved for 4-chlorobenzenemethanethiol to the corresponding disulfide using Bi 2 S 3 as catalyst (entry 1).The conversion of thiol and the selectivity of disulfide Reaction conditions: catalyst (10 mg), thiol (0.2 mmol), and water (1 mL) at 298 K, air, blue light, 6 h.Conversions and selectivities were determined by gas chromatography using an internal standard technique.The products were identified by gas chromatography-mass spectrometer (GC-MS).b) Without catalyst.c) 318 K and without lighting.d) Without catalyst and lighting.were all as much as 99%.The control experiment was also investigated to prove that the reaction could not take place by itself without catalyst and lighting (entry 8).Disulfide production was also not observed with only lighting (entry 6).As per earlier descriptions, some nonphotocatalytic systems were also used to synthesize disulfides in previous study.As shown in the table, only Bi 2 S 3 without lighting cannot lead to a satisfied result, increasing the reaction temperature from 298 to 318 K (entry 7), which proves that the effect of photocatalysis plays an important role in transformation.
As shown in Table 2, the protocol can be used to convert a broad range of thiols into their corresponding disulfides.Benzyl mercaptan and its derivatives were all successfully transferred with excellent conversions and selectivities, including 4-chlorobenzenemethanethiol and 4-fluorobenzyl mercaptan (entries 1-3).2-phenylethanethiol and furfuryl mercaptan were also converted into the corresponding disulfides with excellent yields (entries 4-5).For aliphatic thiols, containing 3-methyl-1butanethiol, 1-butanethiol, 1-pentanethiol, and cyclohexanethiol, varying length of the alkyl chains on thiols did not perceptibly affect the conversions, but the selectivities of disulfides from coupling reactions were found to be slightly less than those of aromatic mercaptans (entries 6-9).In general, the green photocatalytic system described in this manuscript for the coulpling oxidation of various aliphatic and aromatic thiols to corresponding disulfides in water under visible light at room temperature is highly effective and practical.
The catalyst used in the reactions is reusable, which could be recovered by centrifugation after the photocatalytic reaction.After simply washing and drying treatments, the recovered catalyst was subsequently used for the next cycle.Figure 4a shows the catalytic results for the recycled catalyst for oxidation of benzyl mercaptan in water with the same reaction conditions, demonstrating that the catalytic system could be used for five continuous runs without obvious loss in activity.In addition, the structure of used catalyst was also detected by XRD (Figure 4b), which shows that the structure of Bi 2 S 3 used in this aqueous photocatalytic system for thiol coupling reaction is stable and robust.
To manifest the possible mechanism of this photocatalytic reaction over Bi 2 S 3 under blue lighting in water, electron paramagnetic resonance (EPR) spectroscopy was deserved.As shown in Figure 5a, an obvious specific signal identified to the superoxide radical anion (•O 2 À ) was found when the catalytic system exposed under blue lighting in water with 5,5-dimethyl-1pyrroline-N-oxide (DMPO).In contrast, no spectral signal was detected for the catalyst without irradiation.In addition, the representative signal of singlet oxygen ( 1 O 2 ) also existed, which was detected using 2,2,6,6-tetramethylpiperidine (TEMP) as the trapping agent (Figure 5b).It was well known that the aerobic photo-oxidation was achieved through the reactive oxygen species, such as •O 2 À and 1 O 2 . [42,43]Based on the above results and experimental data, a simply proposed mechanism was described in Figure 6.When Bi 2 S 3 was excited by blue lighting  in water, O 2 could be activated through electron and energy transfer, producing •O 2 À and 1 O 2 species, respectively. [38]The substrate of thiols could be due to oxidative coupling by •O 2 À and 1 O 2 to produce the products of disulfides.

Conclusion
In summary, we developed a simple and green catalytic system for oxidative coupling of thiols to disulfides, which was driven by visible light in water under mild conditions.Without other additional solvent, without adjusting the PH value, only water was used as dispersed solvent.Without other metal doping, a cheap and simple semiconductor material of bismuth sulfide (Bi 2 S 3 ) was used as photocatalyst.Excellent conversions and selectivity for coupling of thiols to produce disulfides were obtained, and the system was reusable.It is an environmentally friendly and practical photocatalytic system for organic coupling reaction.

Figure 1 .
Figure 1.Schematic for photocatalytic transformation of thiols to disulfides in water catalyzed by Bi 2 S 3 .

Figure 2 .
Figure 2. a) XRD patterns of the as-obtained catalyst of Bi 2 S 3 .b) XPS spectrum of the catalyst.c) High-resolution scan of Bi 4f and S 2p.

Figure 3 .
Figure 3. a) SEM, b) TEM image, and c) HRTEM of Bi 2 S 3 .d) TEM image of an individual Bi 2 S 3 nanorod.e) Bi and f ) S elemental mapping image of Bi 2 S 3 .

Figure 4 .
Figure 4. a) Catalyst recycling for oxidation of benzyl mercaptan in water.b) XRD patterns of the fresh and used catalyst.

Figure 5 .
Figure 5. EPR signals of the catalyst with air in water after mixing DMPO a) and TEMP b) solution under blue light irradiation or in the dark.

Figure 6 .
Figure 6.Proposed mechanism for the photocatalytic coupling oxidation of thiols to produce disulfides catalyzed Bi 2 S 3 in water.

Table 1 .
Transformation of 4-chlorobenzenemethanethiol to disulfide in water.

Table 2 .
Photocatalytic transformation of thiols into disulfides over Bi 2 S 3 .
Reaction conditions: catalyst (10 mg), thiol (0.2 mmol), and water (1 mL) at 298 K, air, blue light, 6 h.Conversions and selectivities were determined by gas chromatography using an internal standard technique.The products were identified by GC-MS.