Visible-light-mediated facile synthesis of disulfides using reusable TiO2/MoS2nanocomposite photocatalyst

Article abstract of DOI:10.1080/00397911.2016.1199808

A nontoxic and inexpensive TiO₂/MoS₂nanocomposite was prepared and employed as an efficient photocatalyst for the synthesis of symmetrical disulfides under visible light irradiation at room temperature. Both aryl and alkyl thiols survived the reaction conditions, affording the corresponding disulfides in good to excellent yields. The photocatalyst could be easily recovered by simple centrifugation and filtration and reused several times without significant loss in its activity.

Full text of DOI:10.1080/00397911.2016.1199808

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Visible-Light-mediated the Facile Synthesis
of Disulfides Using Reusable TiO2/MoS2
Nanocomposite Photocatalyst
Wenjie Liu, Cancan Wang, Yibo Huang, Qun Chen, Liang Wang & Mingyang
He
To cite this article: Wenjie Liu, Cancan Wang, Yibo Huang, Qun Chen, Liang Wang &
Mingyang He (2016): Visible-Light-mediated the Facile Synthesis of Disulfides Using
Reusable TiO2/MoS2 Nanocomposite Photocatalyst, Synthetic Communications, DOI:
10.1080/00397911.2016.1199808
To link to this article: http://dx.doi.org/10.1080/00397911.2016.1199808
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Date: 21 June 2016, At: 17:38

Visible-light-mediated the facile synthesis of disulfides using reusable TiO2/MoS2
nanocomposite photocatalyst
Wenjie Liu1, Cancan Wang1, Yibo Huang1,2, Qun Chen1, Liang Wang1,, Mingyang He1
1School of Petrochemical Engineering, Changzhou University, Changzhou, P. R. China
2Department of Pharmacy, Changzhou Institute of Engineering and Technology,
Changzhou, P.R. China
Corresponding author E-mail: lwcczu@126.com; hemingyangjpu@yahoo.com
Abstract
A nontoxic and inexpensive TiO2/MoS2 nanocomposite was prepared and employed as an
efficient photocatalyst for the synthesis of symmetrical disulfides under visible light
irradiation at room temperature. Both aryl and alkyl thiols survived the reaction
conditions, affording the corresponding disulfides in good to excellent yields. The
photocatalyst could be easily recovered by simple centrifugation and filtration, and
reused several times without significant loss in its activity.
Graphical Abstract
1

KEYWORDS: Photocatalysis, semiconductor, TiO2/MoS2 nanocomposite, disulfides
1 Introduction
The direct oxidative coupling of thiols to disulfides is of significant importance in both
biological and chemical process.[1,2] Various reagents including metal salts[3-8] and
non-metal oxidating reagents[9-17] have been employed for the synthesis of disulfides up
to date. However, the toxicity and high cost of some metal salts or non-metal oxidating
reagents severly limit their practical use. Recently, much effort in green thiol surrogate,[18]
green reaction medium[19] as well as new reaction technology[20] has been realized.
However, some drawbacks such as over-oxidation, excess and/or expensive oxidating
reagents, high reaction temperatures, long reaction times, and tedious work-up
procedures are still issues to be addressed. It should also be pointed out that although
thiols are malodorous and air-sensitive, they are commercially available or easily
prepared. More importantly, the direct coupling of thiols affords less waste compared
with other thiol surrogates.
Oxidation of thiols into disulfide mediated by visible light in the presence of diaryl
tellurides,[21] iron phthalocyanine immobilized on graphene,[22] CdSe quantum dots,[23]
Eosin Y[24] and Rose Bengal[25] have also been reported. However, these photocatalysts
are either expensive or can not be recovered and reused. We envisioned that an ideal
method for synthesis of disulfide would use air or oxygen as a green and cheap oxidant, a
2

cheap and reusable catalyst to initiate the reaction, and should operate at room
temperature without any additives. Recently, semiconductor composites as photocatalysts
have received much attention.[26-29] Among them, the modified TiO2 photocatalysts with
another semiconductor such as MoS2 have shown good activities in hydrogen evolution
and photo-degradation processes. Moreover, these catalysts are cheap, readily accessible,
stable and less-toxic.[30-33] Our group also reported a visible-light-induced aerobic
thiocyanation of indoles using reusable TiO2/MoS2 nanocomposite as the photocatalyst.[34]
This kind of semiconductor photocatalyst is easily prepared, cheap and effective. Most
importantly, it can be easily recycled after reaction by simple centrifugation and filtration.
Herein, we wish to report a facile and green procedure for the synthesis of disulfides
catalyzed by a recyclable TiO2/MoS2 nanocomposite under visible light irradiation at
room temperature (Scheme 1).
2. RESULTS AND DISCUSSION
The TiO2/MoS2 nanocomposite was prepared according to our previous literature [35] via a
simple hydrothermal process using cheap and readily available TiCl4, sodium molybdate
and thioacetamide as starting materials. The as-prepared TiO2/MoS2 nanocomposite was
well characterized by XRD, UV-vis and TEM (See Supplementary Material). With the
TiO2/MoS2 nanocomposite in hand, 4-chlorothiophenol (1a) was selected as the model
substrate to explore its photocatalytic activity. The results are summarized in Table 1. No
reaction occurred in the presence of either Anatase TiO2 or Rutile TiO2. A 48% yield of
3

desired product 2a was obtained in ethanol by using pure MoS2 as the catalyst (Table 1,
entry 3). Further studies clearly showed that TiO2/MoS2 (10: 1, molar ratio of Ti to Mo,
10 mg) was the most effective catalyst, providing 2a in almost quantitative yield (Table 1,
entries 4~8 and 14), indicating that the heterojunction of TiO2/MoS2 is vital for its
photoactivity. Reactions in other solvents such as CH3CN, DMF, DCM, toluene and
water led to decreased yields (Table 1, entries 9~13). Moreover, no desired product was
detected in the absence of either light irradiation or TiO2/MoS2, highlighting the
importance of the photocatalytic system. Further studies also revealed that air or oxygen
had a pronounced effect on the reaction rate (Table 1, entry 15).
To evaluate the scope and limitations of the current procedure, a series of thiols were
tested under the optimized reaction conditions. As shown from Table 2, aryl thiols
bearing with either electron-withdrawing or electron-donating substituents such as halide
(Cl, Br and F), methyl, methoxy, nitrile group underwent the reaction smoothly to afford
the corresponding products in good to excellent yields (85~99%). Sterically hindered
thiophenol, such as 2-methoxythiophenol could also be efficiently converted to the
corresponding disulfide (2i, 85%). The scope of this protocol was further extended to
heteroaromatic and aliphatic thiols. As shown from the results, reaction of
nitrogen-containing heteroaromatic thiol 1k and benzylic thiol 1l proceeded smoothly to
deliver the desired products 2k and 2l in 97% and 93% yield, respectively. In
4

comparison, longer reaction time was required for other aliphatic thiols to achieve good
yields, which was attributed to the less stability of the alkyl thiol radical intermediates.
To demonstrate the scalability of the present protocol, a gram-scale reaction (10 mmol)
was carried out under the optimized conditions (Scheme 2). To our delight, this
transformation afforded the desired product 2a in 87% yield (2.49 g) within 4 h,
suggesting a high potential for industry scale-up.
The reusability of the catalytic system is also an advantage of this protocol. After
reaction, the catalyst was recovered by centrifugation and filtration, while the product
was extracted with ethyl acetate. Using substrate 1a as example, it was found that the
catalytic system could be readily recovered and reused with slight decrease in its activity
after eight consecutive runs (99%, 99%, 99%, 96%, 95%, 91%, 90% and 86% yield,
respectively). The recovered catalyst was examined by TEM. Aggregation of TiO2/MoS2
was observed and the size of TiO2/MoS2 nanoparticles increased, which might accounted
for the decreased activity.
A plausible mechanism is also outlined in Scheme 3 based on the reported literatures.[33,34]
Because of the quantum confinement effects of nanoscale MoS2, the CB of nanoscale
MoS2 is more positive than that of TiO2, thus under visible-light irradiation, the
photogenerated electrons from MoS2 can be transferred into CB of TiO2, thereby the
5

photoinduced electrons and holes are efficiently separated, and the recombination of
electron-holes can be reduced. The produced holes oxidize thiol 1 into the corresponding
radical cation, which can lose a proton to deliver thiyl radical. Subsequently,
homocoupling of thiyl radical affords the disulfide 2. In another aspect, oxygen acts as a
sacrificial electron acceptor, thereby reducing the hole-electron recombination.
3. CONCLUSION
In summary, we have developed a facile and environmentally benign photocatalytic
method for oxidative coupling of thiols into disulfides using TiO2/MoS2 nanocomposite
under visible light. This protocol affords excellent yields, high functional group
compatibility, as well as the reusability of the catalyst. Moreover, the reactions are
operationally simple and can be performed at room temperature in an open air system
without any additives or bases.
4. EXPERIMENTAL
4.1. General Remarks
All reagents were obtained from local commercial suppliers and used without further
purification. Column chromatography was generally performed on silica gel (300-400
mesh). Thin-layer chromatography (TLC) was visualized using UV light. 1H and 13C
NMR spectra were recorded on a Bruker Advance 400 analyzer in Chloroform-d (CDCl3)
using TMS as an internal standard. Chemical shifts are reported in δ units, parts per
6

million (ppm). The coupling constants J are given in Hz. Low-resolution mass spectra
was obtained from triple quadrupole GC/MS Agilent technology. High-resolution mass
spectrometry (HRMS) was performed on an Agilent 6540 Q-TOF MS instrument with an
ESI source. X-ray diffraction (XRD) patterns were collected on a SHIMADZU
XRD-6000 diffractometer with Cu Ka radiation at 40 kV and 30 mA. HRTEM
(JEM-2010, JEOL) was used to study the microstructures. UV-vis absorption spectra
were obtained by a UV-vis spectrophotometer (SOLID 3700, Shimadzu). The specific
surface areas were obtained by the Brunauer–Emmett–Teller (BET) method based on
nitrogen adsorption/desorption measurements (Micromeritics ASAP 2010C). All the
products except 2o are known compounds and were identified by comparing of their
physical and spectra data with those reported in the literature.
4.2. General Procedure For The Synthesis Of Disulfides
To a solution of thiol 1 (1 mmol) in ethanol (1 mL) was added TiO2/MoS2 (10: 1, molar
ratio, 10 mg). The reaction mixture was stirred under a 14 W CFL irradiation at a distance
of 10 cm at room temperature for a certain time. After reaction (monitored by TLC), ethyl
acetate was added, and the solid catalyst was recovered by centrifugation. The reaction
mixture was extracted with ethyl acetate and washed with water. The combined organic
phase was then dried over Na2SO4 and concentrated under reduced pressure to give the
crude residue, which was purified by column chromatography with petroleum ether/ethyl
7

acetate to afford the pure product 2. The recovered catalyst was then washed with ethanol
and deionized water, dried under vacuum, and reused for the next run.
1,2-Bis(4-chlorophenyl)disulfane (2a): Light yellow solid;[25] 1H NMR (400 MHz,
CDCl3): δ 7.39 (d, J = 8.5 Hz, 4H), 7.26 (d, J = 8.5 Hz, 4H); 13C NMR (100 MHz,
CDCl3): δ 129.5, 129.5, 133.8, 135.3; GC-MS: m/z 286 [M+].
Dimethyl 2,2'-disulfanediyldiacetate (2o): Light yellow solid; 1H NMR (400 MHz,
CDCl3): δ 3.76 (s, 6H), 3.58 (s, 4H); 13C NMR (100 MHz, CDCl3): δ 169.8, 52.6, 41.1;
HRMS (ESI): Calcd. for C6H11O4S2 (M+H)+ 211.0093, found 211.0085.
ACKNOWLEDGMENTS
This project was financially supported by the National Natural Science Foundation of
China (NO. 21302014), the open project “KF1304” from the Jiangsu Province Key
Laboratory of Fine Petrochemical Engineering, and the Natural Science Foundation of
Jiangsu Province (BK20130247).
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Table 1. Optimized the reaction conditions[a]
Entry Catalyst
Solvent Yield(%)[b]
1
TiO2 (Anatase)
EtOH
EtOH
EtOH
n.r.
2
TiO2 (Rutile)
MoS2
n.r.
3
48
4
TiO2/MoS2 (100: 1) EtOH
TiO2/MoS2 (50: 1) EtOH
TiO2/MoS2 (20: 1) EtOH
TiO2/MoS2 (10: 1) EtOH
53
5
64
6
77
7
>99, n.r.[c,d]
94
8
TiO2/MoS2 (5: 1)
EtOH
9
TiO2/MoS2 (10: 1) CH3CN 83
10
11
12
13
14
15
TiO2/MoS2 (10: 1) DMF
TiO2/MoS2 (10: 1) DCM
71
77
TiO2/MoS2 (10: 1) toluene 75
TiO2/MoS2 (10: 1) water
TiO2/MoS2 (10: 1) EtOH
TiO2/MoS2 (10: 1) EtOH
36
68[e], >99[f]
<5[g], >99[h]
12

[a] Reaction conditions: 1a (1 mmol), catalyst (10 mg, the ratio in parenthesis is the molar
ratio of Ti to Mo), solvent (1 mL), air, irradiation under a 14 W CFL at room temperature
for 4 h.
[b] Isolated yields.
[c] Reaction without catalyst.
[d ]Reaction in the dark.
[e]5 mg catalyst was used.
[f] 20 mg catalyst was used.
[g]N2 atmosphere.
[h]O2 atmosphere.
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Table 2 Photocatalytic oxidation of various thiols
[a] Reaction conditions: 1 (1 mmol), TiO2/MoS2 (10: 1 molar ratio) (10 mg), EtOH (1 mL),
air, irradiation under a 14 W CFL at room temperature, isolated yields.
[b] Reaction time: 8 h.
14

Scheme 1. Oxidative coupling of thiols in the presence of TiO2/MoS2
15

Scheme 2. Gram-scale synthesis of disulfide 2a
16

Scheme 3. Proposed mechanism
17