A general and inexpensive protocol for the Cu-catalyzed C-S cross-coupling reaction between aryl halides and thiols

Article abstract of DOI:10.1016/j.tetlet.2015.10.014

A cost effective and easily available CuI/DABCO catalytic system has been developed for C-S cross-coupling reaction. This method is extremely useful for the thioetherification of aryl and heteroaryl halides providing excellent yields and good chemoselectivity.

Full text of DOI:10.1016/j.tetlet.2015.10.014

Accepted Manuscript
A general and inexpensive protocol for the Cu-catalyzed C-S cross-coupling
reaction between aryl halides and thiols
Anns Maria Thomas, Sujatha Asha, K.S. Sindhu, Gopinathan Anilkumar
PII:
S0040-4039(15)30211-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.10.014
TETL 46843
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
24 July 2015
25 September 2015
4 October 2015
Please cite this article as: Thomas, A.M., Asha, S., Sindhu, K.S., Anilkumar, G., A general and inexpensive protocol
for the Cu-catalyzed C-S cross-coupling reaction between aryl halides and thiols, Tetrahedron Letters (2015), doi:
http://dx.doi.org/10.1016/j.tetlet.2015.10.014
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A general and inexpensive protocol for the
Cu- catalyzed C-S cross-coupling reaction
between aryl halides and thiols
Leave this area blank for abstract info.
Anns Maria Thomas,a Sujatha Asha,a K. S Sindhua and Gopinathan Anilkumar a, *

1
Tetrahedron Letters
journal homepage: www.elsevier.com
A general and inexpensive protocol for the Cu-catalyzed C-S cross-coupling reaction
between aryl halides and thiols
∗
Anns Maria Thomas,a Sujatha Asha,a K. S. Sindhu a and Gopinathan Anilkumar a,
aSchool of Chemical Sciences, Mahatma Gandhi University, PD Hills PO, Kottayam 686 560, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A cost effective and easily available CuI/DABCO catalytic system has been
developed for C-S cross-coupling reaction. This method is extremely useful for the
thioetherification of aryl and heteroaryl halides providing excellent yields and good
chemoselectivity
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Catalysis
Copper
C-S coupling
DABCO
Thioetherification
———
∗Corresponding author. Tel.: +91-481-2731036; fax: +91-481-2731036; e-mail: anilgi1@yahoo.com

2
Tetrahedron
3
L3
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
_
DME
DME
DME
DME
CH3CN
Water
Toluene
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
K3PO4
NaOtBu
KOtBu
TEA
80
12
34
98
-
Introduction
4
80
Transition metal catalyzed C-S coupling reactions have gained
5
120
RT
considerable attention in recent years because of the prevalence
of these moieties in many important compounds of biological,
pharmaceutical or material interest. 1 The traditional methods for
C-S bond formation require high temperature, polar solvents or
strong reducing agents.2 However the discovery of Pd-catalyzed
C-S coupling by Migita and co-workers was a breakthrough in
this area and it provided a novel pathway for the preparation of
thioethers.3 Unfortunately the high cost of Pd salts, low turnover
numbers and sensitivity to air limited its application on an
industrial scale unless highly efficient or recyclable catalyst is
used. These constraints prompted the search for other viable tools
for this transformation with wide substrate scope and better
functional group tolerance. In this regard, other transition metal
catalyzed C-S coupling reactions have emerged in recent years.4-
13. As part of our ongoing research in transition metal catalyzed
coupling reactions, we recently reported Zn and Fe-catalyzed C-S
coupling reactions.8a,13 In continuation of our efforts to develop a
more cheaper catalyst with better yield and functional group
tolerance we focused on Cu catalysts. Cu has certain advantages
such as low toxicity, low cost and is compatible with many
readily available and cheap ligands. In addition, Cu has readily
accessible coordination numbers 2, 3 and 4, which allows it to
play a significant role in many catalytic reactions.1f Herein, we
report CuI/DABCO catalytic system as a general tool for the C-S
cross-coupling reaction avoiding the use of expensive Pd-based
catalytic system. The ligand DABCO is very cheap and readily
available and when used in combination with air stable CuI, it
provides an excellent catalytic system that can be used for the
synthesis of electronically diverse alkyl, aryl and heterocyclic
sulfides providing excellent yields in short reaction time.
Furthermore the excellent chemoselectivity of this method
enabled the synthesis of 4-(4-Nitrophenylsulfanyl)phenylamine,
which is otherwise difficult to obtain from metal-catalyzed
coupling reactions. 4-(4-Nitrophenylsulfanyl)phenylamine can be
used as the precursor for the synthesis of Dapsone, a drug for the
treatment of leprosy.14
6
7
120
120
120
120
120
120
120
120
120
120
120
120
120
120
93
31
45
53
93
59
65
85
80
81
96
96
69
Trace
8
9
10
11
12
13
14
15
16
17
18
19
20
THF
DMSO
t-BuOH
DME
DME
DME
DME
DME
DME
DME
_
K2CO3
[a]: Reaction conditions: 1a (0.6 mmol), 2a (0.5 mmol), CuI (10
mol%), Ligand (20 mol%), Base (2 equiv.), 120 C, 12 h, N2, [b]:
isolated yield, [c]: 2.2 equiv. of DABCO was used.
o
The success of cross-coupling greatly depends upon the choice of
ligand, and therefore we first examined the effect of some
inexpensive N,N and N,O bidentate ligands for the C-S cross-
coupling reaction by taking 4-iodoanisole and thiophenol as the
model substrates, and air stable CuI as the metal source in the
presence of K2CO3 and DME. Initially all experiments were
performed using 10 mol% CuI and 20 mol% of ligand for 12 h
o
under N2 atmosphere at 80 C. To our delight, the use of ligand
L4 (DABCO) resulted in 34% isolated yield of the C-S coupled
product in 12 h (table 1, entry 4).. Further increasing the
temperature to 120 oC resulted in a sharp increase in the yield of
the product to 98% (table 1, entry 5). As expected, lowering the
reaction time or temperature showed a negative effect on the
yield of the reaction (supporting information, table 1, entries 4 &
7).).Thus we selected DABCO, which is very cheap and easily
available, as the suitable ligand for this coupling reaction. The
use of DABCO as an efficient ligand for other metal-catalyzed
coupling reactions has already been reported in literature.15 We
further explored the effect of different bases, solvents and
catalyst loading. The screening of different solvents revealed that
DME is superior to other solvents (table 1, entries 7-13).
Choosing DME as the solvent and DABCO as the ligand, we
screened different inorganic and organic bases and found that
K2CO3, KOtBu and triethylamine are excellent bases for this
coupling reaction (table 1, entries 14-18). A blank experiment
revealed that only trace amount of the product could be formed in
the absence of Cu-catalyst or ligand (supporting information,
table 1, entries 21 & 22). We also tested the possibility of using
DABCO as a ligand and a base, but in that case the yield of the
product dropped to 69% (table 1; entry 19). Notably, when the
reaction was carried out in aerobic atmosphere, the yield of the
product was reduced due to the formation of significant amount
of disulfide (supporting information, table 1; entry 23). Next, we
investigated the effect of catalyst loading and found that 5 mol%
CuI and 10 mol% DABCO is the most suitable combination to
afford maximum yield (table 2, entry 1). To understand how the
copper:ligand ratio affects the reaction, we performed the
reaction by taking a 1:1 ratio of Cu salt:ligand and found that the
Results and discussion
In order to explore the efficiency of various copper
based catalysts for C-S cross-coupling reactions, the coupling of
4-iodoanisole with thiophenol was chosen as the model reaction.
The results of the optimization studies are summarized in table 1.
Table 1
Optimization studies for Cu-catalyzed C-S cross-coupling
reaction between 4-iodoanisole and thiophenol.[a]
S
CuI, Ligand
+
I
HS
MeO
Base, Solvent,12h
MeO
1a
2a
3a
HO
OH
NH2
N
N
H2N
OH
O
N
L1
L2
L3
L4
Entry
Ligand
Solvent
Base
Temperature
Yield
(%)[b]
8
(oC)
1
2
L1
L2
DME
DME
K2CO3
K2CO3
80
80
32

3
CuI(5 mol%)
yield of the product was reduced to half (table 2, entry 4). Further
studies revealed that a 1:2 ratio of copper:ligand is needed
presumably to make a 4-coordinated Cu-ligand complex.
DABCO (10 mol%)
S
Ar-X
+
R-SH
Ar
R
K2CO3 (2 equiv),DME
120 oC,12 h
RSH
R = aryl, alkyl
Table 2
Entry
1
ArX
Product
Yield
(%)[b]
Optimization of catalyst loading. [a]
SH
SH
SH
I
S
98
3a
H3CO
H3CO
Entry
CuI
DABCO
(mol%)
Yield
(%)[b]
S
2
3
60[c]
99
B r
(mol%)
3a
H3CO
H 3 C O
1
2
3
4
5
10
20
5
10
20
40
5
98
98
92
57
S
I
3b
H3COC
H3COC
SH
SH
S
Br
4
5
6
78
28
90
3b
H3COC
H3COC
H3COC
H3COC
[a]: Reaction conditions: 4-iodoanisole (0.6 mmol), thiophenol (0.5
mmol), K2CO3 (2 equiv.), DME (3 ml), 120 oC, 12 h, N2, [b]:
isolated yield.
S
Cl
3b
SH
S
I
Thus the optimal condition for C-S cross-coupling reaction is
5 mol% CuI, 10 mol% DABCO and 2 equiv of K2CO3 in DME at
120 oC.
3c
CF3
CF3
SH
I
S
7
96
We then applied the optimized reaction conditions to a broad
range of substrates. Both electron rich and electron deficient
thiophenols effectively coupled with various aryl iodides giving
excellent yields of the product (table 3, entries 1-15). Coupling of
heterocyclic compounds which are prevalent in many
biologically active compounds often seems to be difficult to
synthesize with metal-catalyzed coupling reactions.16 But the
present protocol is successfully applied to the coupling of
heterocyclic aryl iodides as well giving excellent yields of the
sulfides (table 3, entries 12 & 13). Furthermore, the reaction is
not much sensitive to steric effects as evidenced by the reaction
of sterically demanding 2-iodoanisole which also afforded 96%
yield of the product (table 3, entry 7). More importantly, aryl
bromides also worked well for this reaction (table 3, entries 2, 4,
8 & 14) and could be extended to aryl chlorides also (table 3,
entry 5). Unfortunately when 4-bromotoluene was reacted with
thiophenol and 4-methylthiophenol, the required products were
detected in GC-MS, but we could not separate the required
sulphide product from the disulfide by-product formed by
homocoupling during the reaction. With aryl triflate and
thiophenol, only a trace amount of the product was obtained. The
product in entry 15 can be used as a precursor for the preparation
of Dapsone, a drug used in the treatment of leprosy. Next, we
extended this methodology to aliphatic thiols, which also
afforded excellent yields of the product when reacted with aryl
iodides having both electron donating or electron withdrawing
groups (table 3, entries 16-21).
OMe
OMe
S
3d
B r
SH
SH
8
49[c]
97
O M e
OMe
S
3d
I
9
NC
NC
3e
S
OMe
I
10
95
H 3 C O C
O M e
HS
HS
3 f
H3COC
OMe
S
I
11
90
NC
OCH3
NC
3g
SH
S
I
12
13
94
88
3h
N
N
CH3
S
I
N
CH3
HS
HS
HS
3i
N
CH3
S
Br
14
58[d]
H3C
CN
NC
3j
NH2
S
15
16
96[d]
97
I
O2
N
NH2
3k
O2N
The present catalytic system holds several advantages over the
Zn and Fe- based catalytic systems reported by us.8a,13 The CuI/
DABCO catalytic system has better yield, low cost and requires
only less catalyst loading in comparison with the Zn and Fe
catalytic systems. Moreover, sensitive functional groups like NH2
also afforded good yields of the product with the CuI/ DABCO
system.
SH
I
S
3l
SH
I
17
18
99
89
S
CF3
3m
CF3
Table 3
S
I
Substrate scope of the Cu-catalyzed C-S cross-coupling
SH
between aryl iodides and thiols.[a]
H3CO
3n
H3CO

4
Tetrahedron
I
S
19
95
efforts to probe the mechanism using EPR studies are still going
SH
SH
3o
on in our laboratory.
I
I
S
20
21
91
H3C
H3C
O2N
3p
Conclusion
S
86
In summary, we have developed a versatile, simple and
economical C-S cross-coupling reaction which can be applied for
the coupling of aryl or heteroaryl iodides with alkyl and aryl
thiols. The CuI/DABCO catalytic system is very cheap, readily
available and shows excellent chemoselectivity. These features
are highly useful when considering the scaling up of the reaction.
Further efforts to elucidate the mechanism and expand the scope
of the reaction are currently underway.
SH
O2N
3q
[a]: Reaction conditions: ArX (0.6 mmol), thiol (0.5 mmol), CuI (5
mol%), ligand (10 mol%), base (2 equiv), 120 C, N2, [b]: isolated
yield, [c]: 1.6 equiv. of thoiophenol is used and reaction carried out
for 24h. [d]: Reaction was carried out for 17 h.
o
Acknowledgements
Mechanistic Rationale
GA thanks the Kerala State Council for Science, Technology
and Environment (KSCSTE), Trivandrum (Order no.
341/2013/KSCSTE dated 15.03.2013) for financial support.
AMT and SA thank the Council of Scientific and Industrial
Research (CSIR, India) for Junior research fellowships. SKS
thanks the University Grants Commission (UGC, India) for a
junior research fellowship. We thank the Institute for Intensive
Research in Basic Sciences (IIRBS) of Mahatma Gandhi
University for NMR facility.
On the basis of the above observations we propose a possible
reaction pathway for the copper (I) catalyzed C-S cross-coupling
reaction of aryl halides with thiols (Scheme 2).
Scheme 2
Plausible reaction mechanism
CuI
C u I
DABCO
D A B C O
I
A rS -R
References and notes
N
N
ArX
A rX
(I)
Cu
ArS-R
(D A B C O )2 C u (I)
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SR
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cannot coordinate with the same Cu atom since they are 1800
apart from each other. Thus DABCO-Cu complex may exist as
an oligomer or as a dimer, where two Cu atoms are linked
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forms a dimeric Cu (I) complex I which is consistent with the
Cu:ligand ratio in the reaction (table 2). This complex formation
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5
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