Oxidative homocoupling of terminal alkynes under palladium-, ligand- and base-free conditions using Cu(II)-clay as a heterogeneous catalyst

Article abstract of DOI:10.1016/j.crci.2013.07.012

Homocoupling of terminal alkynes to 1,3-diynes has been investigated with Cu(II)-modified clay under mild and operationally simple conditions without the use of any ligand or a stabilizing agent. The catalyst is robust, ecofriendly, efficient, furnishes g

Full text of DOI:10.1016/j.crci.2013.07.012

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CRAS2C-3801; No. of Pages 8
C. R. Chimie xxx (2013) xxx–xxx
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Full paper/Memoire
Oxidative homocoupling of terminal alkynes under
palladium-, ligand- and base-free conditions using Cu(II)-clay
as a heterogeneous catalyst
Bashir Ahmad Dar a, Dushyant Vyas a, Varsha Shrivastava a, Saleem Farooq a,
*
Amit Sharma a,b, Sadhana Sharma a, Parduman R. Sharma a, Meena Sharma b,
,
Baldev Singh a,
*
a Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu 180001, India
b Department of Chemistry, University of Jammu (J & K), Jammu 180004, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Homocoupling of terminal alkynes to 1,3-diynes has been investigated with Cu(II)-
modified clay under mild and operationally simple conditions without the use of any
ligand or a stabilizing agent. The catalyst is robust, ecofriendly, efficient, furnishes good to
excellent yields of the desired products and can be reused several times with negligible
loss in catalytic activity.
Received 11 April 2013
Accepted after revision 15 July 2013
Available online xxx
Keywords:
ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
C–C coupling
Copper
Heterogeneous catalysis
Clay
Alkynes
1. Introduction
symmetrical diynes. Despite of the several routes available
for the synthesis of conjugated 1,3-diynes, the homocou-
The construction of C–C bonds is of immense sig-
nificance for the synthesis of organic compounds and there
has been significant interest in the development of novel
and highly active catalysts that can be used in C–C bond
formation reactions [1]. Glaser’s copper-mediated oxida-
tive homocoupling of terminal alkynes for the synthesis of
conjugated 1,3-diynes, a representative example of C–C
bond formation, is under intense attention for the reason
that diynes are precious components in organic synthesis
and conjugated 1,3-diynes are recurring building blocks in
natural products, pharmaceutical intermediates, electronic
and optical materials, and conjugated polymers [2].
Oxidative homocoupling of alkyne is one of the
most widely utilized procedures for the synthesis of
pling of terminal alkynes is favored due to its simple
procedure [3]. The most attractive routes for the homo-
coupling of terminal alkynes are the catalytic systems
containing a combination of palladium (Pd) and copper
salts, because of their efficiency and mildness, but
palladium reagents are costly and often require expensive
ligands and inert conditions. Some palladium-free Cu(I)
catalysts have also been utilized for homocoupling, but the
susceptibility of Cu(I) salts to redox process desires the
utilization of foul smelling nitrogen bases, ligands and/or
other additives, etc., to protect and stabilize the active Cu(I)
catalyst [4]. Some palladium-free and ligand-free catalytic
systems using Cu(I) salts in the presence of non-
nitrogenous inorganic bases have also been developed [5].
Jia et al. have described the CuI-mediated alkyne
homocoupling reaction, employing inorganic Na2CO3
instead of amines as the base [6]. However, stoichiometric
amounts of CuI, 200 mol% base and 100 mol% iodine (as the
*
Corresponding author.
E-mail address: drbaldev1@gmail.com (B. Singh).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.07.012
Please cite this article in press as: Dar BA, et al. Oxidative homocoupling of terminal alkynes under palladium-, ligand-
and base-free conditions using Cu(II)-clay as a heterogeneous catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.07.012

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B.A. Dar et al. / C. R. Chimie xxx (2013) xxx–xxx
oxidant) were required. The same group proposed a Cu(I)-
catalyzed green oxidative alkyne homocoupling route
without palladium, ligands, and bases [7].
110 8C, powdered and calcined at 425 8C for 3 h to produce
the Cu(II)-clay catalyst. The catalyst so obtained was
characterized by powder X-ray diffraction using a D-8
ADVANCE (Bruker AXS, Germany), an X-ray diffractometer
using an Ni filter and a Gobel Mirror parallel-beam
˚
geometry (Cu Ka: l = 1.5418 A) in the 2u range between
5 and 708 in step-scan mode (step size: 0.028, scan speed: 2
s/step). The phases were identified by search match
procedure with the help of DIFFRACPLUS software using
JCPDS databank. Temperature programmed reduction
(TPR) and BET surface area were determined by the
CHEMBET-3000 TPR/TPD/TPO instrument. XPS analysis
was performed on a KRATOS-AXIS 165 instrument. SEM
observation of the catalyst was carried out using a Jeol
Model JEM-100CXII electron microscope with an ASID
accelerating voltage of 40.0 kV.
Jiang et al. reported the Cu(II)-promoted oxidative
homocoupling reaction of terminal alkynes in supercritical
carbon dioxide [8], but this protocol requires special
equipment, high CO2 pressure and elevated reaction
temperatures. Dong Wang et al. reported the homocou-
pling reactions of terminal alkynes based on catalytic
amounts of CuCl2 and triethylamine at 60 8C in air [9]. A
copper(II)-chloride-dehydrate-catalyzed oxidative cou-
pling reaction in polyethylene glycol using NaOAc as a
base has also been reported [10]. Very recently, a more
stable compound, Cu(OAc)2ÁH2O, has been utilized without
any ligand, additive, base or palladium for oxidative alkyne
homocoupling [11].
Homogeneous catalysts, though widely used for this
purpose, are generally thought to have shortcomings, such
as difficulties in catalyst separation and catalyst recycling
and product contamination caused by the residual
components of the catalysts. So, developing catalytic
systems that are stable, easy to handle, inexpensive,
environment friendly, and easy to separate and recyclable
are highly demanded. Based on these criteria, hetero-
geneous catalysts are advantageous over their homoge-
neous counterparts, offering easy recovery, enhanced
stability even after recycling them many times [12].
The heterogeneous copper-catalyzed homocoupling of
terminal alkynes has also been a subject of recent interest,
but the reported catalysts still exhibited some drawbacks.
Copper(I) zeolites were tested for this reaction in N,N-
dimethylformamide (DMF) at 110 8C and 30 mol% copper
loading, with no apparent possibility of catalyst recycling
[13]. CuAl hydrotalcite exhibited excellent recyclability at
room temperature in acetonitrile, although stoichiometric
amounts of N,N,N’,N’-tetramethylethylenediamine (TMEDA)
and catalyst (110 mol%) were required [14]. Thus, simpler
but effective and environmentally benign novel heteroge-
neous systems for homocoupling approaches are still
demanded. In continuation of our work on heterogeneous
catalysis [15], we herein report the efficient heterogeneously
catalyzed oxidative homocoupling of alkynes by Cu(II)-clay
without any additive (Scheme 1).
2.2. General procedure for alkyne homocoupling
To a solution of terminal alkyne (0.3 mmol) in DMSO
(3 mL) was added 5 mol% Cu(II)-clay in a two-neck round-
bottom flask. A condenser was attached to one neck of this
flask, whereas the other neck was closed with a rubber
septum. Oxygen was bubbled through a needle inserted
through the septum. The resulting mixture was refluxed
for the required duration. The reaction was cooled at room
temperature and the products were extracted with
dichloromethane. The combined organic layers were dried
with anhydrous MgSO4, filtered off, and concentrated in
vacuo to obtain the crude products, which were purified by
silica-gel chromatography to afford the corresponding
product. The prepared compounds were characterized by
comparing the observed spectral data and physical
properties with those of the authentic samples. NMR
spectra were recorded on a Bruker Avance DPX FT-NMR
400 MHz instrument. ESI–MS and HRMS spectra were
recorded on Agilent 1100 LC and HRMS-6540-UHD
machines. The melting points were recorded on a digital
melting point apparatus. IR spectra were recorded on a
PerkinElmer IR spectrophotometer.
3. Results and discussion
3.1. Morphological analysis of the materials
The XRD patterns of the fresh Cu(II)-clay catalyst are
2. Experimental
2.1. Catalyst preparation
shown in Fig. 1 (left). The peak at 2u = 8.48 is commonly
assigned to the basal spacing (d 001) (2:1 TOT) of
montmorillonite-KSF; the basal reflection represents the
distance between two clay layers, including the thickness
of one of them [16]. The very low intensity diffraction
Our Cu(II)-clay catalyst was prepared by introducing a
calculated amount of aqueous Cu(II) oligomer to form a
10 wt % copper loading (pre-optimized loading) onto a
montmorillonite-KSF clay; the system was stirred for 15 h,
then filtered off and washed several times with distilled
water to remove the chlorides. The cake so formed was
dried at room temperature, kept overnight in an air oven at
peaks at 2u = 36.1, 37.7, 38.9, 48.8, 53.60, and 58.7 are
attributed to hkl values 110, 002, 111, 202 of CuO,
indicating that there are no sharp crystalline phases; thus,
CuO is supported on montmorillonite-KSF in the form of
Scheme 1.
Please cite this article in press as: Dar BA, et al. Oxidative homocoupling of terminal alkynes under palladium-, ligand-
and base-free conditions using Cu(II)-clay as a heterogeneous catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.07.012

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3
Fig. 1. Left: XRD of fresh Cu(II)-clay. Right: XRD of used Cu(II)-clay.
highly dispersed fine particles [17], which is in agreement
with the TPR studies mentioned below. The peaks at
(EDAX) elemental analysis, it was found that the prepared
catalyst contained 9.21% of copper loaded.
2
u
= 20.9, 26.9, 36.5, 39.2, 48.6, 50.0 and 59.6 are due to the
reflection of the quartz [SiO2] impurities [18]. Other peaks
for montmorillonite appear at 2 = 20.0, 24.3, 29.8, and
35.5. A sharp peak observed at 2 = 17.8 along with some
TPR is an important technique to characterize the
dispersion of metal ions in catalysts for understanding the
reduction behavior and thermal stability of the active
metal oxide particles. The reduction profile provides
information about the dispersion of the metal species on
the support and about the interaction between these metal
species and the support. The H2–TPR curve for the
unmodified montmorillonite-KSF and that modified with
copper is depicted in Fig. 3. The unmodified montmor-
illonite-KSF support showed a broad peak at a higher
reduction temperature (Fig. 3, left), which can probably be
attributed to the reduction of some iron species in the
montmorillonite-KSF material [21]. It has been reported
that CuO reduces directly to Cu0 [22] and isolated Cu2+ ions
can be reduced to Cu0 by hydrogen in two steps: first, the
reduction of Cu2+ to Cu+; and second, the reduction of the
Cu+ to Cu0 [23]. The reduction of CuO to Cu0 and isolated
Cu2+ to Cu+ occur at lower temperatures than the reduction
of Cu+ to Cu0. The major reduction peak observed for the
catalyst, centered at 220 8C, was attributed to the reduction
of copper oxide located on the surface of the montmor-
illonite-KSF, probably in the entrance of the pores. The
second broad peak at about 370 8C, which is less
pronounced, may be attributed to the overlapping of
reduction of the isolated Cu2+ to Cu+ ions and the copper
oxide species located in the interlamellar space of
montmorillonite-KSF [23a,24]. The peak for the
Cu+ reduction at 460 8C seems to be less intense and
u
u
small peaks at 30.1, 36.0 and 47.2 correspond to the
presence of melanothallite [Cu2OCl2]. The presence of
kaolinite is proved by a sharp peak at 12.0, which
corresponds to hkl 001, whereas other peaks characteristic
of this material appear at 24.1, 32.3, 38.0, and 42.7. The
remaining peaks at 29.5, 32.3, 41.5, 45.5, 55.0, and 59.6 are
due to the formation of CuAl2O4 [19]. There are two
additional humps, the first one at 55.0 and the other one at
61.9, which are due to montmorillonite and CuO in bulk,
respectively [20]. By comparison of the XRD patterns of the
fresh and of the used catalysts (Fig. 1, right), it is clear that
there is almost no variation.
To study the morphology, SEM observations of the
catalyst were carried out using the Jeol Model JEM-100CXII
Electron Microscope with an ASID accelerating voltage
40.0 kV. The SEM images of the montmorillonite-KSF and
Cu(II)-clay with 20 000 magnifications are displayed in
Fig. 2. From this study, it is clear that after loading copper
on montmorillonite-KSF, the smooth surface of the clay
turns patchy, so, it is confirmed that the CuO particles are
highly dispersed over the support surface in the form of
nano-aggregates. The nano-sized CuO distributed in the
form of a mosaic on the surface of the clay leads to a coarse
surface (thus, an elevated surface area). From the SEM
Fig. 2. Left: SEM image of montmorillonite-KSF. Right: SEM image of Cu(II)-clay.
Please cite this article in press as: Dar BA, et al. Oxidative homocoupling of terminal alkynes under palladium-, ligand-
and base-free conditions using Cu(II)-clay as a heterogeneous catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.07.012

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Fig. 3. Left: TPR image of montmorillonite-KSF. Right: TPR image of Cu(II)-clay.
overlapped with the reduction peak of iron impurities in
montmorillonite-KSF [25]. The results indicate that the
copper ions are located mostly in extra-framework
positions in the exchanged form. The different shoulders
observed for these solids indicate that they contain Cu2+
species present in small clusters in different environments.
The TPR profile of the catalyst suggests that it contains very
high exchanged copper sites and that all of the isolated
Cu2+ sites observed for Cu(II)-clay catalyst are due to the
exchange with the Brønsted acid sites in montmorillonite-
KSF. Since the intensity of the peak representing the
reduction of CuO particles located on the surface of the
montmorillonite-KSF is the highest, this suggests that the
highly dispersed CuO species are dominating. This
indicates that the main species detected is copper oxide,
as CuO aggregates with different particle sizes and
dispersion. In this respect, the obtained TPR profile clearly
reveals a higher copper heterogeneity, in agreement with
other characterization results described above.
showing > 99% conversion and almost 98% product yield
within 3 h in the absence of any base, palladium, ligand,
reducing agent or external oxygen supply (Table 1, entries
1–8). In contrast to the situation when other terminal
alkyne substrates were subjected to the same reaction
conditions, only traces of the desired products were
obtained even after 24 h, indicating that these conditions
are appropriate for phenylacetylene homocoupling only.
Thus, in search of appropriate reaction conditions applic-
able for all the substrates used in our present study, we
changed the model reactant from phenyacetylene to 4-
tolylacetylene (Table 1, entry 9). Increasing the catalyst
amount or catalyst loading could not improve the product
yield of this model reaction (Table 1, entries 10, 11). Upon
investigating the effect of temperature, we found that
there is only little increase in the product yield, even at
100 8C (Table 1, entries 12–14). When molecular oxygen
was bubbled through the reaction mixture, stirred at
100 8C, the product yield increased abruptly (Table 1, entry
15). Similar results were obtained at 80 8C (Table 1, entry
16), but at lower temperatures, supplying molecular
oxygen proved unproductive (Table 1, entries 16–18).
Thus, both O2 as well as high temperature are necessary for
the Cu(II)-clay mediated homocoupling of terminal
alkynes, except for phenylacetylene homocoupling, for
which the catalyst alone does the work. Since the control
experiments showed that no reaction occurred with
unmodified montmorillonite-KSF, these results demon-
strated that Cu(II)-clay acts as a true catalyst for the
homocoupling of terminal alkynes.
3.2. Homocoupling of terminal alkynes
At the onset of this work, we focused on the
optimization of the reaction conditions for the homocou-
pling of phenylacetylene as a model reaction in tetra-
hydrofuran medium (Table 1) using Cu(II)-clay as the
catalyst under atmospheric conditions. Screening was
conducted to optimize copper loading on montmorillo-
nite-KSF support. The catalyst with 10 wt% loading was
found to be an ideal catalyst for the model reaction
Please cite this article in press as: Dar BA, et al. Oxidative homocoupling of terminal alkynes under palladium-, ligand-
and base-free conditions using Cu(II)-clay as a heterogeneous catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.07.012

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5
Table 1
Optimization of reaction conditions.
Cu(II)-Clay
R
R
R
Substrate
Product
Entry
1
Substrate
Catalyst amount (mg)
20
Catalyst loading (wt %)
5
Temperature
RT
Atmosphere
Air
Time
3 h
Yielda
67%
2
20
20
20
10
15
25
30
20
30
50
20
20
20
20
20
20
10
15
20
10
10
10
10
10
10
10
10
10
10
10
10
10
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
50
80
100
100
80
80
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2
3 h
98%
3
3 h
98%
4
3 h
98%
5
3 h
73%
6
3 h
79%
7
3 h
98%
8
3 h
98%
9
24 h
24 h
24 h
24 h
24 h
24 h
25 min
25 min
25 min
Traces
Traces
Traces
Traces
11%
10
11
12
13
14
15
16
17
11%
97%
O2
97%
O2
98%
18
20
10
RT
O2
24 h
Traces
RT: room temperature.
a
Isolated yield.
A range of solvents were screened to determine the best
fit for this method, we observed that dimethylsulfoxide
solvent-free conditions led only to traces of the desired
product (Table 2, entries 1 and 9).
(DMSO) shows excellent behavior as
a
medium for
With optimized conditions in hands, the substrate
scope of the catalytic oxidative coupling was studied, as
summarized in Table 3. Various terminal alkynes, includ-
ing aliphatic and aromatic ones, were examined under the
optimal conditions; the results are summarized in Table 3.
Arylacetylenes showed excellent reactivity to produce the
coupling products, that is, aromatic diynes (Table 3, entries
terminal alkyne homocoupling conducted at 80 8C in the
presence of molecular oxygen (Table 2, entry 7). It is
evident that solvents, like DMSO, DMF, toluene, and
acetonitrile (Table 2, entries 5–8) exhibited significant
superiorities over other solvents, like methanol, ethanol
and THF (Table 2, entries 2–4). The use of water and
Please cite this article in press as: Dar BA, et al. Oxidative homocoupling of terminal alkynes under palladium-, ligand-
and base-free conditions using Cu(II)-clay as a heterogeneous catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.07.012

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Table 2
Solvent optimization.
Cu(II)-Clay, O2
80 o
C
Entry
Solvent
Temperature (8C)
Time (min)
Yielda
1
2
3
4
5
6
7
8
9
Water
80
80
80
80
80
80
80
80
80
320
30
30
30
30
30
30
30
30
Traces
13%
Ethanol
Methanol
THF
13%
11%
Toluene
CH3CN
DMSO
DMF
67%
63%
97%
80%
Neat
Traces
a
Isolated yield.
Table 3
Homocoupling of terminal alkynes catalyzed by Cu(II)-clay.
Cu(II)-Clay
R
R
R
DMSO
Substrate
Product
Entry
Substrate
Temperature
80
Time (min)
25
Atmosphere
Yield (%)a
98
1
2
3
4
5
6
7
O2
O2
O2
O2
O2
O2
O2
80
80
80
80
80
80
90
120
120
60
67
58
53
89
97
96
F3C
F
O2N
Ph
25
25
H3CO
8
9
80
RT
35
O2
83
98
4
180
Air
10
11
12
80
80
80
30
30
30
O2
O2
O2
88
89
93
n-C4H9
n-C5H11
n-C8H17
13
14
80
80
25
45
O2
O2
88
78
OH
15
80
120
O2
65
HO
RT: room temperature.
a
Isolated yield
Please cite this article in press as: Dar BA, et al. Oxidative homocoupling of terminal alkynes under palladium-, ligand-
and base-free conditions using Cu(II)-clay as a heterogeneous catalyst. C. R. Chimie (2013), http://dx.doi.org/10.1016/
j.crci.2013.07.012

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7
Table 4
and 9). This might be due to the high dispersion and to the
nano-sized character of Cu(II) on clay support [26,27]
(Table 4).
Comparison of catalytic activity of Cu doped clay catalyst with other Cu
catalysts.
Entry
Catalyst
Mol% of
catalyst
Solvent
Time
% Yielda
Recyclability of the catalyst was investigated for the
homocoupling of 4-tolylacetylene as a model reaction. This
experiment proved excellent recycling capability of the
catalyst without significant loss of activity. The catalyst
could be easily recovered by simple filtration and reused
without any pre-treatment. Conversions were recorded in
the homocoupling of 4-tolylacetylene for at least six
consecutive runs. These results point to a process of
heterogeneous nature (Fig. 4).
(min)
1
2
3
4
5
6
7
8
9
CuNi/clay
CuMn/clay
CuZn/clay
CuAl/clay
CuFe/clay
CuSn/clay
CuCo/clay
CuO
10
10
10
10
10
10
10
10
10
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
30
30
30
30
30
30
30
30
30
53
53
61
58
39
62
65
34
43
Cu2O
a
Isolated yield.
4. Conclusions
In conclusion, we have developed an easy-to-prepare,
robust, heterogeneous, and inexpensive catalyst for the
synthesis of 1,3-butadiynes from terminal alkynes. A full
characterization of the catalyst allowed concluding that
the copper nanoparticles are mainly oxidized in the form of
CuO. This catalyst was found to be much more efficient
than other commercially available catalysts based on
copper. The catalyst is environmentally friendly and can be
recycled several times without significant loss of activity.
This method is an efficient and simple process for the
synthesis of 1,3-butadiynes without requiring palladium,
base, ligand, or additive. Although phenylacetylene homo-
coupling does not need high temperature or oxygen
supply, the other congeners react under reflux under O2
atmosphere. Overall, the present procedure is fast,
practical, interesting and inexpensive for alkyne homo-
coupling. These features make this methodology econom-
ical and attractive for use in combinatorial chemistry and
industrial preparation.
1–9). For aromatic alkynes, the homocoupling processes
were affected dramatically by the substituents on the
aromatic ring. Electron-donating substituents, such as
alkyl and methoxy groups facilitate the reaction, allowing
better yields and shorter reaction times (Table 3, entries 6–
8), while electron-withdrawing groups, like–F, –CF3, –NO2,
etc., were found to decrease the transformation rate very
much (Table 3, entries 2–5). Linear aliphatic alkynes of 1-
hexyne, 1-heptyne, and 1-decyne were also converted into
the corresponding aliphatic diynes in high yields (Table 3,
entries 10–12). Ethynylcyclopropane was also found to
react smoothly to afford the desired product with good
yields (Table 3, entry 13). Propargyl alcohol was found to
react very slowly to give the corresponding diyne with low
yields (Table 3, entry 15).
Comparing catalytic activity of the catalyst with some
clay-supported copper bimetallic catalysts (prepared for
this study) and commercially available CuO and Cu2O for
the model reaction under optimized conditions, we found
that combining other metals with Cu(II) does not improve
the activity of the catalyst; no improvement in the results
was observed; some decreased yields were obtained
instead (Table 3, entries 1–7). The Cu(II)-clay catalyst
was also found to show far better results than the
commercially available CuO and Cu2O (Table 3, entries 8
Acknowledgement
We are grateful to the Director of the Indian Institute of
Integrated Medicine (IIIM), Canal Road, Jammu for
providing necessary facilities and good environment to
carry out the research work. We are also thankful to CSIR–
Delhi for financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.crci.2013.07.012.
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