Simple, efficient and recyclable catalytic system for performing copper-catalyzed C-S coupling of thiols with aryl iodides in PEG and PEG-H2O

Article abstract of DOI:10.1016/j.tetlet.2008.11.082

A new protocol for the coupling of aryl iodides with thiophenols or alkanethiols is reported. The reaction is catalyzed by CuI-PEG or CuI-PEG-H₂O system in the absence of ligands and volatile organic solvents. A variety of functionalized aryl sulfides are prepared in excellent yields. The isolation of the products is readily performed by the extraction with diethyl ether or petroleum ether, and the CuI-PEG catalyst can be reused without significant loss in activity. The simple catalytic system is economically competitive and environmentally friendly.

Full text of DOI:10.1016/j.tetlet.2008.11.082

Tetrahedron Letters 50 (2009) 593–596
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Simple, efficient and recyclable catalytic system for performing
copper-catalyzed C–S coupling of thiols with aryl iodides in PEG and PEG–H O
2
Jin She, Zheng Jiang, Yanguang Wang *
Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2008
Revised 18 November 2008
Accepted 21 November 2008
Available online 25 November 2008
A new protocol for the coupling of aryl iodides with thiophenols or alkanethiols is reported. The reaction
is catalyzed by CuI–PEG or CuI–PEG–H O system in the absence of ligands and volatile organic solvents. A
2
variety of functionalized aryl sulfides are prepared in excellent yields. The isolation of the products is
readily performed by the extraction with diethyl ether or petroleum ether, and the CuI–PEG catalyst
can be reused without significant loss in activity. The simple catalytic system is economically competitive
and environmentally friendly.
Ó 2008 Elsevier Ltd. All rights reserved.
8
The formation of C–S bonds is one of the most important reac-
tions in numerous synthesis of intermediates and targets with bio-
logical and pharmaceutical impact, and in molecular precursors for
of solvents and catalysts. Unlike several of the ‘neoteric solvents’
such as ionic liquids whose toxicity and environmental burden
data are for most part unknown, complete toxicity profiles are
available for a range of PEG molecular weights, and a number of re-
views have also covered PEG chemistry and its application in bio-
1
the development of materials. The classical methods for the syn-
thesis of aryl sulfides involving condensation of thiols with aryl ha-
lides often require harsh reaction conditions such as strong bases
and elevated temperatures. However, these methods are not suit-
able for molecules containing sensitive functional groups. To over-
come these difficulties, Migita and co-workers reported palladium-
9
technology and medicine.
Table 1
catalyzed C–S coupling of iodo and bromo arenes with thiols using
a
Coupling of thiophenol with iodobenzene in different conditions
2
Pd(PPh
3
)
4
as the catalyst under mild conditions, and many ligands
[
Cu]
3
have now been developed for this reaction. Other transition me-
tal-based catalytic systems have also been studied, such as nickel,
PhSH
+
PhI
PhSPh
4
o
Base, PEG, 12 h,110 C
[Cu] (mol)
Cu (10%)
5
6
7
cobalt, copper, and iron.
Yieldb (%)
Entry
PEG
Base
From an industrial view point, the low cost of copper and the
use of readily accessible and stable ligands provide an indisputable
advantage than the other catalytic systems. The general approach
for the copper-mediated C–S coupling requires a catalytic amount
of copper (5–10 mol %) and the presence of a ligand (10–20 mol %).
1
2
3
4
5
6
7
8
9
PEG1000
PEG1000
PEG1000
PEG1000
PEG1000
PEG1000
PEG1000
PEG1000
PEG1000
K
K
K
K
K
K
K
3
3
3
3
3
3
2
PO
PO
PO
PO
PO
PO
4
4
4
4
4
4
94
96
93
97
84
97
95
97
21
96
97
97
96
93
19
2
Cu O (10%)
CuO (10%)
CuI (10%)
CuI (2%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
CuI (5%)
—
Some ligand-free copper-catalyzed C–S coupling reactions have
CO
3
also been developed.6
l–n,s
However, many of these ligand-free
NaOH
c
methods suffer from certain drawbacks, such as the use of nano-
K
3
PO
4
4
4
4
4
4
4
4
6
l,m
6m
10
11
PEG600
K3PO
copper particles,
the requirement of microwave irradiation,
6m,s
PEG2000
K
3
K
3
K
3
K
3
K
3
K
3
PO
PO
PO
PO
PO
PO
and poor efficiency with aliphatic thiols
or electron-rich aryl io-
1
1
2
3
PEG1000
PEG1000
—
Á3H
Á3H
Á3H
Á3H
Á3H
2
O
2
O
2
O
2
O
2
O
6
l
dides. In addition, of limited number of methods available
employing copper catalysts, most involve the use of non-recyclable
catalytic system and toxic organic solvents. Hence, the develop-
ment of simple and green chemical methods for the C–S coupling
reaction is of particular value.
d
e
f
14
1
1
5
6
—
PEG1000
N.D.
a
Reaction conditions: Iodobenzene (2.4 mmol), benzenethiol (2 mmol), Cu salt,
base (4 mmol), and PEG (2 g) were stirred for 12 h at 110 °C.
The preliminary studies have revealed that PEG could be used as
a reaction medium for selective reactions with easy recycle ability
b
Yield of the isolated product.
c
At 80 °C.
d
H
2
O (0.5 mL) was added.
2% (w/w) PEG1000–H O (2 mL) was used instead of PEG1000
Pure water (2 mL) was used.
e
f
*
Corresponding author. Tel./fax: +86 571 87951512.
2
.
E-mail address: orgwyg@zju.edu.cn (Y. Wang).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.082

5
94
J. She et al. / Tetrahedron Letters 50 (2009) 593–596
Table 2
Considering our previous work in the field with the application
a
CuI–PEG-catalyzed S-arylation of thiols
8b,10
of PEG to the generation of C–C bond,
we tried the C–S cou-
pling which would allow the construction of aryl thioether deriva-
tives in PEG. The reactions are effective at 110 °C in pure PEG or
5
3
mol %CuI
K PO 3H O
4
2
RSH
+
ArI
ArSR
2
even in PEG–H O solution. To the best of our knowledge, there
PEG1000 or PEG
o
-H O
2
1000
have been only two examples which allowed the C–S cross-cou-
6
p,s
1
2 h, 110 C
pling in aqueous medium.
As a model study, we chose thiophenol and iodobenzene as the
coupling partners, and the efficiency of several copper(0), (I) and
Entry Thiol
Aryl-I
Product
Yieldb (%)
A
B
(
II) sources, base, and commercially available PEGs was tested.
SH
I
SPh
The results are summarized in Table 1.
1
2
97
93
0
I
II
Exhilaratingly, all the Cu , Cu , and Cu sources tested resulted
in satisfactory yields (Table 1, entries 1–3), which are in agreement
with the previously reported results that copper compounds in
various oxidation states are catalytically active. As CuI turned
out to give the best result, it was chosen as the standard Cu source
for subsequent experiments. Various bases were found to be highly
effective (Table 1, entries 5–7). All the PEGs tested had similar re-
sults (Table 1, entries 6 and 10–11). When the reaction was per-
SH
SH
I
SPh
93
93
81
86
1
1
I
SPh
3
MeO
Ac
MeO
Ac
I
SPh
SH
4
5
85
94
78
93
3 4
formed with 5 mol % CuI in the presence of K PO as a base in
PEG1000 at 80 °C for 12 h, the expected coupling product of diphe-
nyl sulfide was afforded in only 21% yield (Table 1, entry 9). We
also investigated the reaction in the presence of water (Table 1, en-
tries 12–14). Interestingly, the aqueous medium containing only
SH
I
SPh
SPh
I
2
% PEG1000 without other additives or ligands furnished target
SH
compound in excellent yield (Table 1, entry 14).
6
7
8
9
97
95
98
97
95
99
99
99
Thus, the optimized reaction conditions utilized 5 mol % of CuI,
Br
Br
K
1
3
PO
10 °C for 12 h. Since K
NaOH and is easier to be maintained than K
as the standard base. To minimize the chemical waste, we investi-
gated the reaction in aqueous medium containing 2% PEG1000
4
or K
3
PO
4
Á3H
2
O or NaOH (2 equiv) in PEG1000 as a solvent at
PO O provides milder condition than
Á3H
PO , it was chosen
SH
SH
I
SPh
SPh
3
4
2
3
4
I
I
.
Next, the scope of this novel protocol in coupling reactions of
various thiols and iodobenzenes was evaluated in CuI–PEG and
CuI–PEG–H O systems (Table 2). In general, all reactions were very
2
SH
SPh
clean in CuI–PEG system, and the thioethers were obtained in good
to excellent yields under the previously optimized conditions. The
coupling reactions tolerated a wide scope of functional groups,
including free hydroxyl moieties (Table 2, entries 10 and 17, meth-
SH
SPh
I
1
0
96c 24c
1
3
HO
HO
od A ) which readily couple with aryl halides in the presence of a
6
i,q,14
copper catalyst.
The CuI–PEG system efficiently coupled thiols
SH
SPh
SPh
I
with electron-rich, electron-neutral, and electron-deficient aryl io-
dides (85–98% yield, Table 2, method A). The steric hindrance of
ortho-substituents on both partners of the reaction did not affect
the outcome (Table 2, entries 5 and 7, method A).
1
1
1
1
2
3
95
87
94
94
81
82
Br
Br
SH
I
Most of the thiophenols could couple with aryl iodides in CuI–
Ac
Ac
2
PEG–H O system, furnishing target diaryl thioethers in good to
SH
I
I
SPh
excellent yield except for 4-mercaptophenol (Table 2, entry 10,
1
3
method B ). Unfortunately, compared to CuI–PEG system, all at-
tempts to couple aliphatic thiols with aryl iodides led to sharply
SH
SPh
degressive coupling efficiency in CuI–PEG–H
entries 14–17, method B). The different coupling efficiency be-
tween thiophenols and aliphatic thiols in CuI–PEG–H O system
2
O system (Table 2,
1
1
1
4
5
6
96
97
88
83
79
28
I
SC12H25
2
C
C
12
H
H
25SH
25SH
may be due to the different nucleophilic abilities between thiolates
and aliphatic thiols. Minimization of chemical waste, of which 80%
1
2
I
SC12H25
is estimated to be solvents, is a constant challenge as environ-
mental concerns are increasingly brought into focus. With this pur-
12
2
pose, PEG–H O may be a seemly choice.
OH
I
Extension of this C–S coupling process was carried out using
aryl bromides. The results are shown in Table 3. The catalytic sys-
tem efficiently coupled thiophenol with electron-deficient aryl
bromide (62% yield, Table 3, entry 3), but it did not work well with
electron-rich and electron-neutral aryl bromides.
The recycle experiment was carried out on the coupling of iodo-
benzene with benzenethiol in the CuI–PEG1000 system (Table 4). In
order to reduce the amount of salt generated in the reaction, NaOH
S
95c 15c
1
7
HS
OH
a
Method A: aryl iodide (2.4 mmol), thiol (2 mmol), CuI (0.1 mmol), K
4 mmol), and PEG1000 (2 g) were stirred for 12 h at 110 °C. Method B: iodobenzene
2.4 mmol), thiol (2 mmol), CuI (0.1 mmol), K PO O (4 mmol), and 2 mL 2% (w/
O solution were stirred for 12 h at 110 °C.
3
PO
4
Á3H
2
O
(
(
3
4
Á3H
2
w) PEG1000–H
2
b
Yield of the isolated product.
c
Aryl iodide (2 mmol)and thiol (2 mmol) were used.

J. She et al. / Tetrahedron Letters 50 (2009) 593–596
595
Table 3
was used as the base. After the completion of the reaction, the
product was isolated from the reaction mixture by extracting with
petroleum ether. The insoluble CuI–PEG system was reused with-
out additional CuI and PEG in the first six recycles to afford excel-
lent yields.
CuI–PEG1000-catalyzed C–S coupling between aryl bromides and thiophenola
5
mol %CuI
PhSH
+
ArBr
ArSPh
2
eq K PO 3H O
3 4 2
PEG1000, 12 h, 110
o
C
When we were preparing this manuscript, Sperotto et al. re-
ported a ligand-free CuI-catalyzed C–S coupling of aryl iodides and
Entry
ArBr
Product
Yieldb (%)
15
thiols, Compared to this literature method, our procedure brings
Br
two significant improvements: (1) the coupling for aliphatic thiols
is more efficient and (2) the catalytic system can be recovered and
reused.
SPh
1
2
3
K
Trace
Trace
62
We propose a possible mechanism for the copper-catalyzed S-
arylation of thiols (Scheme 1). It is presumed that PEG works not
only as the reaction medium or phase transfer catalyst but also
Br
SPh
MeO
Ac
MeO
Ac
8
as a ligand. Cu(I) or Cu(II) was reduced by PEG to form Cu(0),
Br
SPh
16
which conjugates with PEG to form a reactive species A. The sub-
sequent oxidative addition of the A with aryl iodides leads to the
intermediate B. In the presence of base, thiols react with B readily
to afford complex C, which undergoes a reductive elimination to
provide the target product and to regenerate the reactive species A.
In summary, we have demonstrated a selective, efficient, and
general protocol for the synthesis of aryl sulfides via a CuI–PEG cat-
alyzed coupling of aryl iodides with thiophenols or alkanethiols.
Compared with the published methods, our procedure possesses
several advantages: (1) the volatile organic solvent-free and li-
gand-free reactions are economically competitive and environ-
mental friendly; (2) the catalytic system is applicable for both
aryl thiols and aliphatic thiols; (3) the catalyst could be recovered
and reused; and (4) the workup of the reaction mixture and isola-
tion of the desired product are simple and convenient. Because of
these issues, we believe that our protocol could find its applica-
tions in organic synthesis.
a
Reaction conditions: Aryl bromide (2.4 mmol), thiol (2 mmol), CuI (0.1 mmol),
PO O (4 mmol), and PEG1000 (2 g) were stirred for 12 h at 110 °C.
Á3H
Isolated yield.
3
b
4
2
Table 4
a
Recycle experiment of CuI–PEG1000
5
mol %CuI
eq NaOH
PEG1000, 12 h, 110 C
PhSH + PhI
PhSPh
2
o
Product (yield, %)b
Run
1
2
3
4
5
6
97
98
98
97
95
95
Acknowledgment
We thank the National Natural Science Foundation of China
a
Reaction conditions: Iodobenzene (2.4 mmol), benzenethiol (2 mmol), CuI
0.1 mmol), NaOH (4 mmol), and PEG1000 (2 g) were stirred for 12 h at 110 °C.
Isolated yield.
(
No. 20672093).
(
b
Supplementary data
Supplementary data (detailed experimental procedures, charac-
1
13
OH
OH
terization data, copies of H and C NMR spectra for all products.)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.11.082.
HO(CH CH O) H
2
2
n
PEG
Cu(0)
H
References and notes
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3
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Á3H
2
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