A heterogeneous copper(I)-catalyzed aerobic oxidative cross-dehydrogenative coupling of terminal alkynes with thiols leading to alkynyl sulfides

Article abstract of DOI:10.1016/j.tetlet.2015.09.153

A heterogeneous copper-catalyzed direct oxidative cross-dehydrogenative coupling of terminal alkynes with thiols was achieved in DMSO at 70 °C in the presence of an MCM-41-supported bidentate nitrogen copper(I) complex [MCM-41-2N-CuCl] and K₂CO

Full text of DOI:10.1016/j.tetlet.2015.09.153

Accepted Manuscript
A heterogeneous copper(I)-catalyzed aerobic oxidative cross- dehydrogenative
coupling of terminal alkynes with thiols leading to alkynyl sulfides
Zhiqiang Fang, Wen He, Mingzhong Cai, Yang Lin, Hong Zhao
PII:
S0040-4039(15)30194-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.09.153
TETL 46825
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 September 2015
27 September 2015
30 September 2015
Please cite this article as: Fang, Z., He, W., Cai, M., Lin, Y., Zhao, H., A heterogeneous copper(I)-catalyzed aerobic
oxidative cross- dehydrogenative coupling of terminal alkynes with thiols leading to alkynyl sulfides, Tetrahedron
Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.09.153
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A heterogeneous copper(I)-catalyzed aerobic oxidative cross-
dehydrogenative coupling of terminal alkynes with thiols
leading to alkynyl sulfides
Zhiqiang Fang,a Wen He,b Mingzhong Cai,b* Yang Lin,a Hong Zhaoa*
a School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University, Guangzhou 510006, P. R. China
bDepartment of Chemistry, Jiangxi Normal University, Nanchang 330022, P. R. China
E-mail: caimzhong@163.com; zhaohong1001@sina.com
Abstract
A heterogeneous copper-catalyzed direct oxidative cross-dehydrogenative coupling
of terminal alkynes with thiols was achieved in DMSO at 70 ºC in the presence of an
MCM-41-supported bidentate nitrogen copper(I) complex [MCM-41-2N-CuCl] and
K2CO3 under an atmosphere of O2, yielding selectively a variety of alkynyl sulfides in
good to excellent yields. This heterogeneous copper catalyst can be easily recovered
by a simple filtration and recycled for 10 times without any decreases in activity.
Keywords: Copper; Cross-dehydrogenative coupling; Alkynyl sulfide; MCM-41;
Heterogeneous catalysis
Introduction
Alkynyl sulfides are extremely useful intermediates in organic synthesis and have
been used as versatile building blocks in a variety of organic transformations, such as
hydrostannylation,1 cycloaddition2 and cross-coupling reactions.3 Generally, the most
precedent methodologies are based on the transition metal-catalyzed cross-coupling
reactions, such as copper-catalyzed coupling of alkynyl bromides with disulfides4 or
1

coupling of terminal alkynes with phenylthio halides,5 copper or rhodium-catalyzed
coupling of terminal alkynes with disulfides.6 Recently, the transition-metal-free
synthesis of alkynyl sulfides has also been reported.7 However, these methods usually
require a prefunctionalization process of terminal alkyne or sulfur nucleophile coup-
ling partners, resulting in the inevitable production of large amounts of byproducts
during the preactivation steps. In addition, these protocols lack generality, and suffer
from some drawbacks such as strict conditions, low chemoselectivity, and/or limited
substrate scope, making them less desirable and impractical.
Metal-catalyzed oxidative cross-dehydrogenative coupling reaction through the
direct activation of C–H and/or X–H bonds (X = heteroatom) has received much
attention as a powerful and straightforward method for the construction of new C–X
bonds.8 However, the activation of sp C–H bonds of terminal alkynes for their
incorporation with heteroatoms into organic molecules remains a challenge because of
the inevitable formation of the diyne as a byproduct by the homocoupling of terminal
alkyne.9 In addition, the coupling reaction usually requires the use of stoichiometric
oxidants such as Cu(II) or Ag(I) salts, organohalides, PhI(OAc)2, and benzoquinone,
which are both environmental and economic concerns. It is therefore of considerable
importance to develop aerobic oxidative cross-dehydrogenative coupling reactions
from the standpoint of environmentally benign organic synthesis. Recently, a few
examples of the direct oxidative cross-dehydrogenative couplings of terminal alkynes
with nitrogen nucleophiles and H-phosphonates using air or O2 as the sole oxidant to
produce C–N and C–P acetylenes have been reported.10
2

Although considerable advances made recently in the copper-catalyzed oxidative
C–H bond functionalizations highlight the possibility for the construction of sulfur-
incorporated functional acetylenes,11 the problem with homogeneous catalysis is the
difficulty to separate the catalyst from the reaction mixture and the impossibility to
reuse it in consecutive reactions. It is well known that homogeneous catalysis might
result in heavy metal contamination of the isolated product. These problems are of
particular environmental and economic concerns in large-scale syntheses and in
industry. In contrast, heterogeneous catalysts have received more and more attention
because of the advantages of high catalytic efficiency and easy recycling.12 Hetero-
geneous catalysis also helps to minimize wastes derived from reaction workup,
contributing to the development of green chemical processes. From the standpoint of
green chemistry, the development of highly active and recyclable supported copper
catalysts is challenging and important. In spite of tremendous effort dedicated to the
immobilization of homogeneous palladium complexes over the last two decades,13 the
supported copper complexes-catalyzed carbon-heteroatom coupling reactions have
received less attention.14 To the best of our knowledge, no heterogeneous copper-
catalyzed oxidative cross-dehydrogenative coupling reaction has been reported until
now. Herein, we wish to report the successful application of MCM-41-supported
bidentate nitrogen copper(I) complex [MCM-41-2N-CuCl] as a highly efficient and
recyclable copper catalyst for the direct oxidative cross-dehydrogenative coupling of
terminal alkynes with thiols using O2 as the sole oxidant under mild conditions to
selectively afford a variety of alkynyl sulfides in good to excellent yields.
3

Results and discussion
The discovery of the hexagonally-ordered mesoporous material MCM-41 has given
an enormous stimulus to research in heterogeneous catalysis and provided a new
possible candidate for a solid support for immobilizing homogeneous catalysts.15
MCM-41 has extremely high surface areas, large and uniform pore size, big pore
volume and rich silanol groups in the inner walls, which provide the opportunity to
support or anchor various functional groups by different postsynthetic modification
methods.16 A series of MCM-41-supported bidentate nitrogen copper complexes
[MCM-41-2N–CuX(CuX2)] were conveniently prepared starting from commercially
available and inexpensive 3-(2-aminoethylamino)propyltrimethoxysilane and simple
copper salts such as CuX or CuX2 (X = Cl, Br, I, NO3, SO4) according to our previous
procedure (Scheme 1).17
OH
OSiMe
3
1. (MeO) Si(CH ) NH(CH ) NH
3 2 3 2 2
2
OH
O
o
Toluene, 100 C, 24 h
Si
N
NH
2
OH
H
O
2. Me SiCl, rt, 24 h
3
OMe
MCM-41
MCM-41-2N
A
MCM-41-2N-CuI [ ], CuX = CuI
OSiMe
3
B
MCM-41-2N-CuBr [ ], CuX = CuBr
O
H
Si
N
CuX or CuX
2
NH
C
MCM-41-2N-CuCl [ ], CuX = CuCl
2
O
D
MCM-41-2N-CuCl [ ], CuX = CuCl
2
DMF, r.t., 7 h
2
2
OMe
CuX
or
E
MCM-41-2N-Cu(NO ) [ ], CuX =Cu(NO )
3 2
2
3 2
CuX
F
MCM-41-2N-CuSO [ ], CuX = CuSO
4
2
4
Scheme 1.
Preparation of MCM-41-2N-CuX(CuX ) complexes
2
In our initial screening experiments, the oxidative cross-dehydrogenative coupling
4

of 4-methylphenylacetylene with thiophenol was selected as a model reaction to
optimize the reaction conditions, and the results are summarized in Table 1. We first
screened various MCM-41-supported bidentate nitrogen copper complexes catalysts
with K2CO3 as base in DMSO at 70 °C under 1 atm. of oxygen. It was found that all
the MCM-41-supported bidentate nitrogen copper complexes [A, B, C, D, E and F]
can effectively promote the coupling reaction to afford desired phenyl p-tolylethynyl
sulfide in good to excellent yields with high selectivity, but MCM-41-2N-CuCl [C]
gave the best result (Table 1, entries 1-6). Our next studies focused on the effect of
solvent on the model reaction and a significant solvent effect was observed. DMF was
also suitable solvent and 88% yield was obtained (entry 7). While other solvents such
as toluene, THF, 1,2-DCE, and hexane were inefficient and a mixture of (E)- and
(Z)-phenyl styryl sulfides was formed as major products (entries 8-11). When Cs2CO3
or Na2CO3 was used as an inorganic base, slightly lower yield was obtained (entries
12 and 13). No formation of desired product was observed when Et3N or pyridine was
used as the base, but diphenyldisulfide was produced in good yield due to the
oxidation of thiophenol (entries 14 and 15). DBU gave only 43% yield, and diphenyl-
disulfide was also formed in 48% yield (entry 16). So, K2CO3 was finally selected as
the base for this coupling reaction. Reducing the amount of K2CO3 to 5 mol% resulted
in lower activity and selectivity, and no desired product was observed without a base
(entries 17 and 18). Lowering the reaction temperature to 30 ºC led to a decreased
yield (entry 19). Finally, the amount of supported copper catalyst was also screened,
and 5 mol% loading of copper was found to be optimal. A lower yield was observed
5

and a longer reaction time was required when the amount of the catalyst was
decreased (entries 20 and 21). Thus, the optimized reaction conditions for this
transformation are the MCM-41-2N-CuCl (5 mol%) in DMSO using K2CO3 (10
mol%) as base at 70 °C under 1 atm. of O2 for 1 h (Table 1, entry 3).
Table 1 Cross-dehydrogenative coupling reaction of 4-methylphenylacetylene with
thiophenol in different conditions.a
Entry Cu catalyst (mol%) Solvent
Base
Time (h) Yield b (%)
1
2
A (5)
B (5)
C (5)
D (5)
E (5)
F (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (2.5)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
Et3N
1
92
91
95
90
89
85
88
0
1
3
1
4
1
5
1
6
1
7
1
8
Toluene
THF
12
12
12
12
1
9
0
10
11
12
13
14
15
16
17c
18
19d
20
1,2-DCE
Hexane
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
0
0
89
86
0
1
12
12
2
Pyridine
DBU
0
43
54
0
K2CO3
–
2
12
3
K2CO3
K2CO3
81
78
3
6

21
C (1)
DMSO
K2CO3
12
61
a
All reactions were performed using 4-methylphenylacetylene (0.5 mmol), thiophenol (0.55
mmol), base (0.05 mmol), and copper catalyst (0.025 mmol) in solvent (2 mL) at 70 ºC under 1
b
c
d
atm O2. Isolated yield based on 4-methylphenylacetylene used. 5 mol% K2CO3 was used. At
30 ºC.
With the optimized conditions in hand, we subsequently investigated the scope of
this heterogeneous copper-catalyzed direct oxidative cross-dehydrogenative coupling
reaction using a wide range of 1-alkynes and various thiols as the substrates and the
results are outlined in Table 2. As shown in Table 2, phenylacetylene and a variety of
substituted phenylacetylenes could effectively couple with thiophenol to selectively
afford desired alkynyl sulfides 3a-3d in excellent yields. The electronic effect of
substituents on the benzene ring has limited influence on the coupling reaction and
both electron-donating groups and electron-withdrawing groups were tolerated well. It
is noteworthy that, in the case of 4-fluorophenylacetylene (1d), excellent chemo-
selectivity was observed and the desired product 3d were obtained in 91% yield
without the occurrence of defluorination. Aliphatic alkynes and propiolates were also
suitable coupling partners in addition to aromatic alkynes. For instance, 1-hexyne (1e),
1-octyne (1f), 5-chloro-1-pentyne (1g), 5-phenyl-1-pentyne (1h), methyl propiolate
(1i) and ethyl propiolate (1j) were effectively coupled with thiophenol to provide the
corresponding alkynyl sulfides 3e-3j in good to high yields.
Substituted thiophenols bearing both electron-donating and electron-withdrawing
groups could also efficiently couple with terminal alkynes to afford the corresponding
alkynyl sulfides 3k-3n in good to high yields. Reactions of p-, m-, and o-methyl-
7

thiophenols (2k, 2l and 2m) with phenylacetylene proceeded well, and almost equal
yields were obtained, indicating that the steric effect of substituents on the benzene
ring is negligible. The electron-deficient thiophenol (2n) displayed superior reaction
efficiency to the electron-rich ones (2k-2m) under the same reaction conditions. The
coupling reaction between benzene-1,3-dithiol and 2 equivalents of 1-octyne also
proceeded smoothly to give the desired product 3o in 88% yield. The result highlights
the power of this protocol for the synthesis of sulfur-rich π-conjugated polymers.
Table 2 Synthesis of alkynyl sulfides by heterogeneous copper-catalyzed aerobic
oxidative cross-dehydrogenative coupling reaction.a,b
a All reactions were performed using terminal alkyne (0.5 mmol), thiol (0.55 mmol), K2CO3 (0.05
mmol), and MCM-41-2N-CuCl (0.025 mmol) in DMSO (2 mL) at 70 ºC under 1 atm O2 for 1 h. b
Isolated yield based on terminal alkyne used. c 1.0 mmol 1-octyne, 0.55 mmol benzene-1,3-dithiol,
8

2 h.
To verify whether the observed catalysis was due to the heterogeneous catalyst
MCM-41-2N-CuCl or to a leached copper species in solution, we performed the hot
filtration test.18 We focused on the coupling reaction of phenylacetylene with
thiophenol. We filtered off the MCM-41-2N-CuCl complex at 70 °C after 30 min of
reaction time and allowed the filtrate to react further. We found that, after this hot
filtration, no further reaction was observed, indicating that leached copper species
from the catalyst (if any) are not responsible for the observed activity. It was
confirmed by ICP-AES analysis that no copper could be detected in the hot filtered
solution.
9

A plausible mechanism for this heterogeneous copper-catalyzed aerobic oxidative
cross-dehydrogenative coupling reaction is proposed as shown in Scheme 2. Firstly,
the MCM-41-2N-CuCl complex reacts with thiol in the presence of a base to provide
an MCM-41-supported bidentate nitrogen copper(I) thiolate (A). Subsequent trans-
metalation of alkyne into the MCM-41-supported bidentate nitrogen copper(I) thiolate
(A) via activation of terminal alkyne with the assistance of O2 produces an MCM-41-
supported bidentate nitrogen copper(II) intermediate (B). Then the newly generated
intermediate (B) undergoes the reductive elimination in excess of thiol to afford the
desired alkynyl sulfide and regenerate the MCM-41-supported bidentate nitrogen
copper(I) thiolate (A). Molecular oxygen plays a key role in facilitating the formation
of the copper(II) intermediate (B) via activation of sp C–H bond of terminal alkyne.
For a heterogeneous catalyst, it is important to know its ease of separation and
possible reuse. We next investigated the recyclability of the MCM-41-2N-CuCl by
using the coupling reaction of 4-methylphenylacetylene (0.5 mmol) with thiophenol
(0.55 mmol) in the presence of MCM-41-2N-CuCl (0.025 mmol) and K2CO3 (0.05
mmol) in DMSO (2 mL) at 70 °C under 1 atm. of oxygen for 1 h. After carrying out
the reaction, the catalyst was separated by simple filtration and washed with distilled
water, DMSO and ethanol. After being air-dried, it can be reused directly without any
further treatment. The recovered copper catalyst was used in the next run, and almost
consistent activity was observed for 10 consecutive cycles (Fig. 1). The high stability
and excellent reusability of the catalyst should result from the chelating action of
bidentate nitrogen ligand on copper and the mesoporous structure of the MCM-41
10

support.
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Reused times
Figure 1. Recycle of the MCM-41-2N-CuCl catalyst
Conclusions
In summary, we have developed a simple, straightforward and atom-economical
route to alkynyl sulfides through the direct oxidative cross-dehydrogenative coupling
of terminal alkynes with thiols by using a recyclable MCM-41-supported bidentate
nitrogen copper(I) complex [MCM-41-2N-CuCl] as catalyst under an atmosphere of
oxygen. The reactions generated selectively a variety of alkynyl sulfides in good to
excellent yields and were applicable to a wide range of terminal alkynes and various
thiols. This heterogeneous copper catalyst can be conveniently prepared by a simple
procedure from commercially available and inexpensive reagents and recycled for 10
times without any decreases in activity, thus making this procedure environmentally
more acceptable.
11

Acknowledgements
We thank the National Natural Science Foundation of China (No. 21272044) and
Scientific Research Fund of Education Department of Jiangxi Province (KJLD13022 )
for financial support.
Supplementary data
Supplementary data (experimental procedures for the heterogeneous copper-
1
catalyzed aerobic oxidative cross-dehydrogenative coupling reactions as well as H,
13C NMR spectra of the products are described) associated with this article can be
found in the Supporting Information.
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Graphical Abstract
A heterogeneous copper(I)-catalyzed aerobic oxidative cross-
dehydrogenative coupling of terminal alkynes with thiols
leading to alkynyl sulfides
Zhiqiang Fang, Wen He, Mingzhong Cai,* Yang Lin, Hong Zhao*
A simple and practical method for selective synthesis of alkynyl sulfides has been
developed through the direct oxidative cross-dehydrogenative coupling of terminal
alkynes with thiols in DMSO by using a recyclable MCM-41-supported bidentate
nitrogen copper(I) complex [MCM-41-2N-CuCl] as catalyst under an atmosphere of
oxygen.
15

Research Highlights
► The MCM-41-2N-CuCl complex can be easily prepared by a simple two-step
procedure.
► Heterogeneous Cu-catalyzed CDC reaction of 1-alkynes with thiols is first
reported.
► The reaction generates selectively a variety of alkynyl sulfides in high yields.
► The Cu catalyst can be recycled for 10 times without any decrease in activity.
► Our catalytic system provides a new, simple and practical route to alkynyl sulfides.
16