(NH 4) 6Mo 7O 34·4H 2O as an efficient, selective, and reusable catalyst for the oxidation of thiols to disulfides using potassium bromate

Article abstract of DOI:10.1080/10426507.2012.745076

Ammonium molybdate, (NH ₄) ₆Mo ₇O ₃₄·4H ₂O, is found to be an efficient and selective catalyst for the oxidation of thiols to corresponding disulfides using potassium bromate in aqueous acetonitrile as the solvent. Among various solvents tested, CH₃CN/H₂O showed better results in terms of the reaction yield and rate. In the absence of (NH₄)₆Mo ₇O₂₄·4H₂O, the oxidation reaction is not selective for the formation of disulfides. The catalyst can be easily recovered after completion of the reaction and reused without any significant loss of its activity. [Supplementary materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfer, and Silicon and the Related Elements for the following free supplemental files: Additional text and figures.]

Full text of DOI:10.1080/10426507.2012.745076

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(NH4)6Mo7O24·4H2O as an Efficient,
Selective, and Reusable Catalyst for the
Oxidation of Thiols to Disulfides Using
Potassium Bromate
Samad Shoghpour Bayraq a , Ahmad Nikseresht b & Iman Khosravi c
a Department of Chemistry , Ferdowsi University of Mashhad ,
Mashhad , Iran
b Department of Chemistry , Payame Noor University , Tehran , Iran
c Department of Chemistry, Qeshm Branch , Islamic Azad University ,
Qeshm , Iran
Accepted author version posted online: 10 Dec 2012.Published
online: 16 Aug 2013.
To cite this article: Samad Shoghpour Bayraq , Ahmad Nikseresht & Iman Khosravi (2013)
(NH4)6Mo7O24·4H2O as an Efficient, Selective, and Reusable Catalyst for the Oxidation of Thiols to
Disulfides Using Potassium Bromate, Phosphorus, Sulfur, and Silicon and the Related Elements, 188:9,
1236-1243, DOI: 10.1080/10426507.2012.745076
To link to this article: http://dx.doi.org/10.1080/10426507.2012.745076
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Phosphorus, Sulfur, and Silicon, 188:1236–1243, 2013
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.745076
(NH4)6Mo7O24·4H2O AS AN EFFICIENT, SELECTIVE, AND
REUSABLE CATALYST FOR THE OXIDATION OF THIOLS
TO DISULFIDES USING POTASSIUM BROMATE
Samad Shoghpour Bayraq,1 Ahmad Nikseresht,2 and Iman
Khosravi3
1Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran
2Department of Chemistry, Payame Noor University, Tehran, Iran
3Department of Chemistry, Qeshm Branch, Islamic Azad University, Qeshm, Iran
GRAPHICAL ABSTRACT
Abstract Ammonium molybdate, (NH4)6Mo7O24·4H2O, is found to be an efficient and selec-
tive catalyst for the oxidation of thiols to corresponding disulfides using potassium bromate
in aqueous acetonitrile as the solvent. Among various solvents tested, CH3CN/H2O showed
better results in terms of the reaction yield and rate. In the absence of (NH4)6Mo7O24·4H2O,
the oxidation reaction is not selective for the formation of disulfides. The catalyst can be easily
recovered after completion of the reaction and reused without any significant loss of its activity.
[Supplementary materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfer, and Silicon and the Related Elements for the following free supplemental
files: Additional text and figures.]
Keywords Ammonium molybdate; oxidation of thiols to disulfides; potassium bromate;
aqueous acetonitrile; reusable catalyst
INTRODUCTION
From the chemical and biological applications points of view,1–3 there are many
reasons to favor the development of the selective oxidation routes of thiols to the desired
disulfides products. In biological systems, disulfides are known to control cellular redox
potentials in which thiols are oxidized to prevent oxidative damage.4 Disulfides are also
used as a protecting group for thiols in synthetic organic chemistry.5 In addition, disulfides
are utilized as vulcanizing agent in rubber and elastomer industry.3 Thus, oxidation of
thiols to disulfides is one of the most studied transformations. A great number of oxidizing
Received 28 April 2012; accepted 27 October 2012.
Support of this investigation by Ferdowsi University of Mashhad is gratefully acknowledged.
Address correspondence to Samad Shoghpour Bayraq, Department of Chemistry, Ferdowsi University of
Mashhad, Mashhad, P.O. Box: 91779 Iran. E-mail: s.shoghpour@gmail.com
1236

AMMONIUM MOLYBDATE CATALYZED OXIDATION OF THIOLS
1237
reagents have been employed to accomplish this transformation which of them bromine,6
nitric oxide,7 sodium perborate,8 potassium permanganate,9 dimethyl sulfoxide,10 molecular
oxygen,11 2,6-dicarboxypyridinium chlorochromate,12 hydrogen peroxide,13 iodine,14 and
iodine monochloride15 are examples.
The bromate anion, BrO3−, is an interesting candidate for the oxidation of organic
compounds due to its capability in multielectron transfer. This ion shows a higher tendency
to reduction in aqueous acidic media (E◦ = 1.52 V) than in aqueous alkaline media (E◦ =
0.61 V). It has also very low solubility in most of organic solvents. So, it is usually used
in aqueous media in the presence of acidic co-reactants such as: NaHSO3,16,17 Na2HPO4,18
NaHCO3,19 (NH4)2Ce(NO3)6,20,21 FeCl3,22 CH3COOH,23 HBr,24 H2SO4,25 and HClO4.26
Although these aqueous acidic reagents facilitate the synthesis of various types of organic
compounds, use of them imposes some restrictions; need to neutralization of acidic media
and low selectivity of oxidation reactions due to more oxidizing power of BrO3− in aqueous
acidic media are among of them.
Therefore, the development of new reagents for the oxidation of organic compounds
with BrO3− salts in the absence of aqueous acids will be valuable from the synthetic aspects.
To the best of our knowledge, there is only one report in the literature included the oxidation
coupling of thiols with sodium and silver bromates in the absence of acid catalysts,27 but
examined only a few substrates and thus lacked generality; Moreover, the reactions were
performed in a flammable solvent (ether) at reflux condition and needed to tedious work-up
procedure.
It has been found that molybdenum compounds having Mo–O bond in the structure,
show important catalytic applications. Ammonium molybdate is one of them which has
been used as catalyst in oil and petroleum refining,28 removing of organic compounds from
water,29 producing high-temperature resistant resins30 and manufacturing of an antiparasitic
drug.31 In this work, we introduce ammonium molybdate, (NH4)6Mo7O24·4H2O, as a cheap,
safe, stable, readily available and non-acidic catalyst for the rapid and selective oxidation
of thiols to disulfides in aqueous acetonitrile using potassium bromate as the oxidant
(Scheme 1). This reaction not only overcomes the above-mentioned disadvantages but
also has other advantages: easy separation of the products from the oxidant and catalyst,
recovery and reusability of catalyst, high yields, and faster reactions.
Scheme 1 (Color figure available online).
RESULTS AND DISCUSSION
To choose the best solvent, we tested various solvents such as THF, H2O, CH2Cl2,
EtOAc, CH3CN, C2H5OH, n-C6H14, CH3OH, DMF, and CH3CN/H2O (7:3) for the ox-
idation of 4-chlorothiophenol in the presence of (NH4)6Mo7O24·4H2O. As indicated in
Table 1, THF needs very long reaction time and gives very low yield of disulfide product.
In the case of EtOAc, no reaction takes place after 24 h. In DMF, although the reaction
is performed in a short time, it is not selective to disulfide. For all other solvents listed

1238
S. SHOGHPOUR BAYRAQ ET AL.
Table 1 Solvent effect on oxidative coupling of 4-chlorothiophenol with KBrO3/(NH4)6Mo7O24·4H2O at room
temperature
Entry
Solvent
Time
Yield of disulfide (%)a
1
2
3
4
5
6
7
8
THF
H2O
CH2Cl2
CH3COOC2H5
CH3CN
C2H5OH
n-C6H14
CH3OH
DMF
CH3CN:H2O (7:3)
48 h
36 h
26 h
24 h
22 h
20
98
98
0
98
98
98
98
0
10 h
6.5 h
30 min
10 min
5 min
9
10
98
Reaction conditions: solvent 5 mL, 4-chlorothiophenol (1 mmol, 0.1 mL), KBrO3 (1 mmol, 0.167 g),
(NH4)6Mo7O24·4H2O (10 mmol%, 0.124 g).
aIsolated yield based on the weight of the pure product obtained.
in Table 1, almost quantitative yields of pure disulfide product are formed. However, the
reaction times are quite long in these solvents with the exception of CH3OH (t = 30 min)
and CH3CN/H2O (7:3) (t = 5 min). Thus, CH3CN/H2O was selected as the most effective
solvent on the basis of the reaction yield and time.
In continuation, the oxidation of various thiols was examined in the presence of
(NH4)6Mo7O24·4H2O catalyst in aqueous acetonitrile using KBrO3 as the oxidant (Table 2).
As can be seen from Table 2, a variety of aromatic (entries 1–9), aliphatic (entries 11–17),
alicyclic (entry 10), and heterocyclic (entries 18–20) thiols were easily converted to cor-
responding disulfides in very short times with excellent yields. Moreover, it is obvious
from the data of column 3 in table 2 that aromatic thiols react faster than aliphatic and
heterocyclic thiols which is in agreement with the experimental results for (NO3)3CeBrO3
catalyzed coupling of thiols.46
Another set of comparative experiments was carried out to examine the efficiency of
the catalyst (Table 2, entry 21–23). It was concluded that potassium bromate in the absence
of (NH4)6Mo7O24·4H2O is not able to oxidize thiols to disulfides selectively and the final
oxidation products are the corresponding sulfonic acids.47 Sanz et al. have shown that diox-
omolybdenum(VI) complexe, MoO2Cl2(DMSO)2, can catalyze selective oxidation of thiols
to disulfides by DMSO.10 In this oxotransfer process, it is supposed that oxidative coupling
of thiols lead to the formation of oxomolybdenum(IV) species, MoOCl2(DMSO)2, which in
turn are readily oxidized to the parent dioxomolybdenum(VI) complexe by DMSO (as co-
oxidant). Probably, the selective oxidation of thiols in the presence of (NH4)6Mo7O24·4H2O
proceeds via a similar catalytic cycle, except that different species of Mo are involved and
KBrO3 acts as co-oxidant.
We also investigated the reusability of the catalyst in the case of some thiols (Table 3).
To do this, after completion of the reaction, the solvent was evaporated in vacuum and then
the resulting solid mixture was washed with CH2Cl2 to remove the disulfide product.
The solid precipitate was filtered off and reused in further oxidation reactions with the
same substrate. Table 3 shows that after four runs of recycling, the catalytic activity of
(NH4)6Mo7O24·4H2O was almost the same as that of fresh catalyst.

AMMONIUM MOLYBDATE CATALYZED OXIDATION OF THIOLS
1239
Table 2 Oxidative coupling of thiols using KBrO3/(NH4)6Mo7O24·4H2O at room temperature in aqueous ace-
tonitrile
mp (◦C)
Time
(min)
Yield of
disulfide (%)a,b
Entry
R
Found
Literature
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
C6H5
2-naphthyl
4-Br-C6H4
4-Cl-C6H4
3-Cl-C6H4
2-Cl-C6H4
4-F-C6H4
4-H3C-C6H4
4-H3CO-C6H4
c-C6H11
<5
5
<5
<5
5
5
<5
5
98
95
98
98
98
95
98
98
95
92
92
95
92
85
90
90
85
61–62
137–139
91–93
72–74
Liquid
85–87
49–51
41–44
42–44
Oil
69–70
Oil
Oil
Oil
Oil
61[32]
135–137[33]
90–93[34]
72[32]
Liquid[35]
84–86[33]
50–52[36]
38–40[37]
44–45[38]
Oil[39]
5
25
12
15
20
20
20
30
55
C6H5CH2
69–71[40]
Oil[41]
2-furylmethyl
CH3(CH2)2CH2
CH3(CH2)3CH2
CH3(CH2)6CH2
CH3(CH2)10CH2
Oil[42]
Oil[43]
Oil[44]
31–33
235–238 dec
32[41]
243–245 dec[14]
18
19
40
45
90
90
51–54
54–56[34]
132–134
133–135[45]
20
50
90
176–178
177–179[15]
21c
22c
23c
C6H5
4-Cl-C6H4
C6H5CH2
—
—
—
0
0
0
—
—
—
—
—
—
Reaction conditions: solvent 5 mL, thiol (1 mmol), KBrO3 (1 mmol, 0.167 g), (NH4)6Mo7O24·4H2O
(10 mmol%, 0.124 g).
aIsolated yield based on the weight of the pure product obtained.
bThe products were identified by comparison of their physical and spectroscopic properties with the respective
authentic compound.
cIn the absence of (NH4)6Mo7O24·4H2O.

1240
S. SHOGHPOUR BAYRAQ ET AL.
Table 3 Recyclability of (NH4)6Mo7O24·4H2O in the oxidation reactions of several thiols vs. the yields of
corresponding disulfides
Yield (%)a
Entry
Thiol
1
2
3
4
1
2
3
4-Cl-C6H4SH
C6H5SH
C6H5CH2SH
98
98
92
98
98
92
96
96
90
95
95
89
The reaction conditions for the first run: solvent 5 mL, thiol (1 mmol), KBrO3 (1 mmol, 0.167 g),
(NH4)6Mo7O24·4H2O (10 mmol%, 0.124 g).
aIsolated yield based on the weight of the pure product obtained.
In order to show the advantages and drawbacks of the presented method, we have
compared the results of the oxidation of some thiols by our method and other known
methods reported in the literature (Table 4).
As can be seen, our method provides similar or higher yields of disulfides in most
cases and the reaction times are shorter than the reported methods. Also, use of lower volume
of organic solvent in comparison with some existing methods, makes our method greener
and more sustainable; however, those methods which were performed under solvent-free
conditions,48,51 used low-vapor-pressure ionic liquids as solvent49 or used environmentally-
friendly oxidants such as H2O2,41 are particularly interesting from a green point of view.
Besides these, there are other advantages to our approach: safety and availability of reagents,
recovery and reusability of catalyst, the easy procedure and cleaner reaction profile.
EXPERIMENTAL
All materials were obtained from Merck and Aldrich chemical companies (ammo-
nium molybdate was purchased from Merck) and used without further purification. Melting
points were measured on an Electrothermal 9100 apparatus and remained uncorrected. IR
spectra were recorded on a Shimadzu IR-4300 spectrometer. 1H NMR spectra were recorded
at room temperature in CDCl3 on a 100 MHz BRUKER or 300.13 MHz BRUKER DRX-
300 AVANCE spectrometer. The chemical shifts were reported relative to tetramethylsilane
(TMS) as an internal standard. Identification of the products was carried out by comparison
1
of physical and spectral (IR and H NMR) data to those reported in the literature. Se-
lected physical characterization data and NMR spectra are contained in the Supplemental
Materials (Figures S1–S8).
Typical Procedure
4-chlorothiophenol (0.145 g, 1 mmol) was added to a heterogeneous mixture of
KBrO3 (0.167 g, 1 mmol), (NH4)6Mo7O24·4H2O (0.124 g, 10 mmol%), and CH3CN/H2O
(7:3) (5 mL) and the mixture was stirred under a hood for 4 min at ambient atmosphere and
room temperature. The progress of the reaction was monitored by TLC (eluent: EtOAc/n-
C6H14, 1/13). After completion, CH2Cl2 (20 mL) was added and the reaction mixture was
filtered. The filtrate was washed with 5% NaOH, water, and dried over anhydrous MgSO4.
Finally, evaporation of the solvent gave the product with sufficient purity for most purposes.

1241

1242
S. SHOGHPOUR BAYRAQ ET AL.
Yield: 0.142 g (%98), colorless solids, mp 72–74 ◦C (Lit.28 mp 72 ◦C); 1H NMR (100 MHz,
CDCl3): δ = 7.38 (d, J = 8.9 Hz, 4H), 7.53 (d, J = 8.8 Hz, 4H).
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