Synthesis of diaryl thioethers from aryl halides and potassium thiocyanate

Article abstract of DOI:10.1002/aoc.3230

An efficient palladium catalyst was synthesized using nicotine, benzyl chloride and palladium chloride. The structure of this catalyst was characterized and it was then used for the synthesis of diaryl sufides. A variety of diaryl thioethers were synthesized under relatively mild reaction conditions. This protocol avoids foul-smelling thiols via cross-coupling of aryl halides with potassium thiocyanate and all substrates give the corresponding products in good to excellent yields in the presence of low amounts of the catalyst.

Full text of DOI:10.1002/aoc.3230

Full Paper
Received: 29 April 2014
Revised: 19 August 2014
Accepted: 15 September 2014
Published online in Wiley Online Library: 20 October 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3230
Synthesis of diaryl thioethers from aryl halides
and potassium thiocyanate
Abdol R. Hajipoura,b*, Raheleh Pourkaveha and Hirbod Karimia
An efficient palladium catalyst was synthesized using nicotine, benzyl chloride and palladium chloride. The structure of this
catalyst was characterized and it was then used for the synthesis of diaryl sufides. A variety of diaryl thioethers were synthesized
under relatively mild reaction conditions. This protocol avoids foul-smelling thiols via cross-coupling of aryl halides with
potassium thiocyanate and all substrates give the corresponding products in good to excellent yields in the presence of low
amounts of the catalyst. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: palladium; potassium thiocyanate; C–S coupling; diaryl sufide; aryl halide
Lee and co-workers developed palladium-catalyzed cross-
Introduction
coupling of aryl halides and thioacetates.[41] Wager and Daniels
Diaryl thioethers and their oxidized forms, sulfones and sulfoxides,
are present in a great number of pharmaceuticals (such as in
antidiabetes, anti-inflammatory, anti-Alzheimer’s, anti-Parkinson’s,
anticancer and anti-HIV formulations), biologically active molecules
and polymeric materials. Hence, the formation of C–S bonds has
received great attention and constitutes a key step in the synthesis
of many biological and pharmaceutical molecules.[1–12] Moreover,
diaryl sulfides are usually the starting materials for the construction
of sulfone and sulfoxide compounds which are critical synthetic
intermediates for the construction of various chemically and
biologically significant compounds.[13–17] The traditional
methods for the formation of C–S bonds require drastic condi-
tions such as polar solvents, hexamethylphosphoramide and
elevated temperatures of around 200°C via aromatic nucleo-
philic substitutions of activated arenes with thiolates. One
alternative method for the synthesis of sulfides is the reduction
of aryl sulfones or aryl sulfoxides with strong reducing agents
(DIBAL-H or LiAlH4).[18–21]
Migita and co-workers reported one of the first examples of aryl
sulfide synthesis by coupling of aryl halides and thiols using Pd
(PPh3)4 as catalyst.[22] Previous studies have shown that milder
conditions are possible when using transition metals such as nickel,
cobalt, indium, lanthanum, copper, iron and palladium as catalysts
for the formation of diaryl sulfides by couplings of aryl halides with
aryl thiols.[23–34] The use of volatile, expensive and foul-smelling
thiols has been a main drawback, which leads to environmental
and safety problems. In addition, sulfur-containing compounds
may act as poisons for metal-based catalysts because of their
strong coordinative properties, often making catalytic reactions
ineffective.[35] To overcome these problems, other sulfur sources
such as potassium thiocyanate[36] and thioacetamide[37] can be
used instead of thiols. It is well known that palladium-catalyzed
synthesis is one of the most efficient procedures for the preparation
of diaryl sulfides.[38–41] The most significant advance in palladium-
catalyzed C–S bond formation was made by Hartwig and co-
workers in 2006[26,42] by employing the strongly coordinating
bidentate Josiphos ligand in the presence of a palladium salt.
established palladium-catalyzed cross-coupling of benzyl
thioacetates and aryl halides.[43]
In continuation of our research to produce efficient and new
palladium catalysts,[44–50] we present here a simple and efficient
route for the synthesis of diaryl sulfides using potassium thiocyanate
as the sulfur source and using a new air- and moisture-stable Pd(II)
complex containing 1-benzyl-3-(1-benzyl-1-methylpyrrolidin-1-ium-
2-yl)pyridin-1-ium with general formula [DBNT][PdCl4]. This quater-
nary nicotinium cation inhibits the aggregation of Pd(0) non-stable
species. The effect of tetraalkylammonium salts on the activity and
stability of palladium catalysts has been described before.[51]
Results and Discussion
1-Benzyl-3-(1-benzyl-1-methylpyrrolidin-1-ium-2-yl)pyridin-1-ium was
prepared according to our previous work.[50] The structure of the
produced catalyst was characterized using 1H NMR, 13C NMR and
infrared spectroscopy and elemental analysis. Based on these data,
the structure of the synthesized catalyst is suggested as shown in
Scheme 1.
UV absorption spectra of (À)-nicotine, 1-benzyl-3-(1-benzyl-1-
methylpyrrolidin-1-ium-2-yl)pyridin-1-ium chloride (benzylated
nicotine), PdCl2 and mono(1-benzyl-3-(1-benzyl-1-methylpyrrolidin-
1-ium-2-yl)pyridin-1-ium)monopalladium(II) tetrachloride (catalyst)
in dimethylsulfoxide (DMSO) were recorded (Fig. 1). The 276 nm
band corresponds to the π →π* transition of the pyridine ring.[52]
*
Correspondence to: Abdol R. Hajipour, Pharmaceutical Research Laboratory,
Department of Chemistry, Isfahan University of Technology, Isfahan 84156, IR
Iran. E-mail: haji@cc.iut.ac.ir
a Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan
University of Technology, Isfahan 84156, IR, Iran
b Department of Pharmacology, University of Wisconsin, Medical School, 1300
University Avenue, Madison, WI 53706-1532, USA
Appl. Organometal. Chem. 2014, 28, 879–883
Copyright © 2014 John Wiley & Sons, Ltd.

A. R. Hajipour et al.
Table 1. Optimization of [DBNT][PdCl4]-catalyzed coupling of iodo-
benzene with KSCN under various conditionsa
Entry Solvent Temperature Base (eq.) Catalyst Time Yield
(°C)
(mol%) (h)
(%)b
Scheme 1. Preparation of catalyst [DBNT][PdCl4].
1
2
3
4
5
6
7
8
9
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
120
120
120
120
120
120
120
120
120
120
120
120
120
120
NaHCO3 (2)
Na2CO3 (2)
K2CO3 (2)
Cs2CO3 (2)
KOH (2)
1
5
5
5
5
2
3
5
3
5
5
3
2
2
5
5
5
5
5
5
5
5
5
5
3
2
2
2
1
3
3
13
37
1
1
43
1
58
1
78
NaOH (2)
K2HPO4 (2)
t-BuOK (2)
DABCO (2)
NaOAc (2)
Et3N (2)
1
60
1
29
1
53
1
Trace
Trace
48
10 DMSO
11 DMSO
12 DMSO
13 DMSO
14 DMF
1
1
KOH (3)
1
85
KOH (4)
1
92
KOH (4)
1
32
15 1,4-Dioxane Reflux
KOH (4)
1
20
16 Toluene
17 NMP
Reflux
120
KOH (4)
1
12
KOH (4)
1
21
Figure 1. Absorption spectra of catalyst and starting materials for its synthesis.
18 THF
Reflux
Reflux
Reflux
Reflux
KOH (4)
1
42
19 CH3CN
20 H2O
KOH (4)
1
Trace
Trace
Trace
Trace
38
KOH (4)
1
Benzylated nicotine shows a 270 nm band which has a hypsochromic
shift relative to (À)-nicotine due to it having positive nitrogen atoms.
In the electronic spectrum of PdCl2 solution, intense bands at 297 nm
and 330 nm having a value of CTB (Charge transfer band) from
chloride ions to the metal.[53] The electronic spectrum of the catalyst
shows two bands at about 267nm (associated with the π →π* transi-
tion) and 309 nm.
Initially to optimize the reaction conditions such as base, solvent
and reaction temperature, iodobenzene and potassium thiocya-
nate were selected as model substrates. The results are summarized
in Table 1.
21 EtOH
22 DMSO
23 DMSO
24 DMSO
25 DMSO
26 DMSO
27 DMSO
28 DMSO
29 DMSO
30 DMSO
KOH (4)
1
Room temp. KOH (4)
1
80
100
110
130
120
120
120
120
KOH (4)
KOH (4)
KOH (4)
KOH (4)
KOH (4)
KOH (4)
KOH (4)
—
1
1
67
1
81
1
92
0.5
1.5
—
1
78
94
—
—
aReaction conditions: iodobenzene (2 mmol), KSCN (1 mmol), [DBNT]
[PdCl4], base, solvent (2 ml).
Our initial choice of solvent was DMSO. At first, various bases
(NaHCO3, Na2CO3, K2CO3, Cs2CO3, KOH, NaOH, K2HPO4, t-BuOK,
1,4-diazabicyclo[2.2.2]octane (DABCO), NaOAc and Et3N) were
examined, and a significant base effect was observed. Among inor-
ganic bases, it is evident that KOH and NaOH give quantitative
results for the coupling reaction to diphenyl thioethers (Table 1,
entries 5 and 6), while, among organic bases, Et3N and t-BuOK give
better results (Table 1, entries 8 and 11) in a homogeneous reaction
mixture. In the presence of other bases, low to moderate yields are
obtained. The reaction does not occur in the absence of the base
(Table 1, entry 30). So KOH (4eq.) was finally selected as the base
for the reaction. Similarly, a series of solvents were screened.
Among the several solvents tested, DMF, 1,4-dioxane, toluene,
N-methyl-2-pyrrolidone (NMP), THF and CH3CN are less effective
compared to DMSO (Table 1, entries 13–19). In solvents such as
H2O and EtOH, trace amounts of product are obtained (Table 1,
entries 20 and 21). The high polarity and dielectric constant of
DMSO could be a reason for its excellent performance (Table 1,
entry 13). Further studies reveal the optimal reaction temperature
to be 120°C (Table 1, entries 13, 22–26). The reaction fails to give
the sulfide product when it is carried out at room temperature
and starting materials are completely recovered (Table 1, entry
22). Finally, the amount of palladium catalyst was also screened. A
1.0mol% loading of palladium is found to be optimal, since a lower
yield is observed when the amount of catalyst is decreased (Table 1,
entry 27). As evident from Table 1, increasing the amount of
palladium catalyst to 1.5mol% can shorten the reaction time, but
bIsolated yield.
does not increase effectively the yield of diphenyl sulfide (Table 1,
entry 28). The coupling reaction does not occur in the absence of
the catalyst (Table 1, entry 29) or base (Table 1, entry 30). It is
significant to observe that the choice of DMSO as the solvent
with KOH (4eq.) as base at 120°C is essential for the present C–S
coupling reaction.
To explore the generality and scope of the proposed catalytic
system, coupling of various aryl halides with KSCN was examined.
The results are summarized in Table 2. In general, all the reactions
are very clean and successful and C–S coupling reactions proceed
smoothly to afford the corresponding diaryl sulfides in good to
excellent yields ranging from 62 to 95%. The electronic nature of
the substituents seems to affect the results. In fact, electron-
withdrawing substituents on aryl iodide (COCH3, CHO, CN and
NO2) (Table 2, entries 2–6) give the corresponding thioethers in
higher yields and shorter reaction times compared to electron-
donating substituents (CH3, OCH3) (Table 2, entries 7 and 8).
Steric hindrance also seems to have an effect on the results. For
example, the reactions of the sterically hindered aryl halides
4-nitroiodobenzene, 2-nitroiodobenzene, 4-bromoacetophenone,
2-bromoacetophenone and 3-bromoacetophenone (Table 2,
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 879–883

Cross-coupling of aryl halides with potassium thiocyanate
Table 2. Synthesis of diaryl sulfides from aryl halides and KSCN,
catalyzed by [DBNT][PdCl4]a
Entry
R
X
Time (h)
Yield (%)b
1
H
I
2
92
93
92
94
95
83
76
78
84
74
83
82
89
77
90
78
88
76
91
77
82
72
88
89
72
83
81
82
73
62
2
4-MeCO
4-CHO
I
2
3
I
2
4
4-CN
I
2
5
4-NO2
I
2
6
2-NO2
I
2.5
3
7
4-MeO
4-CH3
I
8
I
3
9
H
Br
Cl
I
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
H
4
Br
2
Cl
I
2
4-MeCO
4-MeCO
4-NO2
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Br
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
5
4-NO2
4-CHO
4-CHO
4-CN
4-CN
4-MeO
4-MeO
Benzyl
Scheme 2. Proposed mechanism for thioetherification of aryl halides
with KSCN.
5
6
According to previous reports,[3,23,29,54] we propose a mechanism
for thioetherification of aryl halides with KSCN as shown in
Scheme 2. First, an aryl halide is added to Pd(0) species through
an oxidative addition step which causes Pd(II) formation. In the
next step, SCNÀ displaces the halide on the catalyst. The product
of this cycle (aryl thiocyanate), which is gained through a
reductive elimination step, enters the next cycle. It is worth noting
that this intermediate was characterized using FT-IR spectroscopy,
with a signal seen at about 2200 cmÀ1. Then, the secondary aryl
halide displaces CNÀ according to hard–soft acid–base theory.
The final step is reductive elimination which yields the desired
diaryl sulfides.
Benzyl
6
2-MeCO
3-MeCO
2-Iodopyridine
3-Iodopyridine
3-Bromopyridine
3-Chloropyridine
3.5
3.5
3
3
5
5
aReaction conditions: aryl halides (2 mmol), KSCN (1 mmol), [DBNT]
[PdCl4] (1 mol%), DMSO (2 ml), KOH (4 eq.), 120°C.
bIsolated yield.
entries 5, 6, 13, 25 and 26) with potassium thiocyanate provide 95,
83, 89, 72 and 83% of the desired diaryl sulfides, respectively.
Iodobenzene is found to be a more reactive substrate than
bromobenzene and chlorobenzene (Table 2, entries 1, 9 and 10),
which implies that there is good chemoselectivity between
iodide, bromide and chloride (Table 2, entries 11 and 12). Aryl
bromides can also react with KSCN under the same reaction
conditions. Although in order to obtain reasonable yields of diaryl
sulfides, longer reaction times than for aryl iodides are required
(Table 2, entries 9, 13, 15, 17, 19 and 21). It is noteworthy that aryl
chlorides successfully react under our conditions (Table 2, entries
10, 14, 16, 18, 20 and 22). Moreover, this method can well tolerate
a variety of functional groups, including ketone, aldehyde and
cyanide (Table 2, entries 2–4). This procedure is also good enough
for the preparation of dibenzylsulfane from benzyl bromide and
benzyl chloride (Table 2, entries 23 and 24). Some heteroaryl com-
pounds, such as 2-iodopyridine, 3-iodopyridine, 3-bromopyridine
and 3-chloropyridine, can also afford the desired diaryl sulfides
(Table 2, entries 27–30).
Conclusions
In summary, we have reported a new palladium-catalyzed synthesis
of symmetric diaryl sulfides from aryl halides and potassium
thiocyanate in moderate to high yields. The efficiency and
functional-group tolerance of this procedure have been demon-
strated by synthesizing
a number of functionalized diaryl
thioethers. Importantly, the iodo group of an aryl halide was
selectively coupled with a thiol in the presence of a bromo group.
In addition, the use of air-sensitive or costly catalysts and ligands
can be avoided by utilizing this method.
Experimental
General
All chemical reagents were purchased from Merck and were used
without further purification. All products were characterized using
Appl. Organometal. Chem. 2014, 28, 879–883
Copyright © 2014 John Wiley & Sons, Ltd.
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A. R. Hajipour et al.
1
NMR spectroscopy. H NMR spectra were recorded at 400 MHz
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A mixture of aryl halide (2 mmol), potassium thiocyanate
(1 mmol), [DBNT][PdCl4] (1 mol%) and KOH (4 eq.) in DMSO
(2 ml) was stirred at 120°C in an oil bath for an appropriate
amount of time. The progress of the reaction was monitored
using TLC. After completion of the reaction, the reaction
mixture was allowed to cool to room temperature. Then,
20 ml of water was added to the mixture and the product
(reaction mixture), resulting diaryl sulfide, was extracted with
ethyl acetate (3 × 10 ml). The organic phase was dried over
CaCl2, concentrated under vacuum and purified by column
chromatography on silica gel (n-hexane–EtOAc, 9:1). All
products were known compounds and were identified by
comparison of their 1H NMR spectra with those of authentic
samples.[28,30,36,37,41,43]
Acknowledgements
We gratefully acknowledge the funding support received for this
project from Isfahan University of Technology (IUT), IR Iran, and
Isfahan Science and Technology Town (ISTT), IR Iran. Further
financial support from the Center of Excellence in Sensor and Green
Chemistry Research (IUT) is gratefully acknowledged.
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Appl. Organometal. Chem. 2014, 28, 879–883

Cross-coupling of aryl halides with potassium thiocyanate
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[52] F. M. Albertí, J. J. Fiol, A. García-Raso, M. Torres, A. Terrón, M. Barceló-Oliver,
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M. J. Prieto, V. Moreno, E. Molins, Polyhedron 2010, 29, 34.
[53] K. B. Yatsimirskii, I. I. Volchenskova, Theor. Exp. Chem. 1977, 13, 146.
[54] C. C. Eichman, J. P. Stambuli, Molecules 2011, 16, 590.
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web-site.
Appl. Organometal. Chem. 2014, 28, 879–883
Copyright © 2014 John Wiley & Sons, Ltd.
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