Copper immobilized on nano-silica triazine dendrimer (Cu(ii)-TD@nSiO 2) catalyzed synthesis of symmetrical and unsymmetrical 1,3-diynes under aerobic conditions at ambient temperature

Article abstract of DOI:10.1039/c3ra47184e

A highly efficient route for the synthesis of symmetrical and unsymmetrical 1,3-diynes has been developed by Cu(ii)-TD@nSiO₂/DBU catalyzed homocoupling/heterocoupling of aromatic as well as aliphatic terminal alkynes under aerobic conditions at ambient temperature. The catalyst could be easily recovered and reused several times without significant loss of its activity.

Full text of DOI:10.1039/c3ra47184e

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Copper immobilized on nano-silica triazine
dendrimer (Cu(II)-TD@nSiO2) catalyzed synthesis of
symmetrical and unsymmetrical 1,3-diynes under
aerobic conditions at ambient temperature†
Cite this: RSC Adv., 2014, 4, 14291
Received 1st December 2013
Accepted 4th March 2014
a
a
*
Mahboobeh Nasr-Esfahani, Iraj Mohammadpoor-Baltork,
DOI: 10.1039/c3ra47184e
a
Ahmad Reza Khosropour, Majid Moghadam,a Valiollah Mirkhani,a
Shahram Tangestaninejad,a Vladislav Agabekovb and Hadi Amiri Rudbaria
*
www.rsc.org/advances
A highly efficient route for the synthesis of symmetrical and unsym- need oxidants such as O214c,d,16 and iodine17 for the reoxidation
metrical 1,3-diynes has been developed by Cu(II)-TD@nSiO2/DBU of Pd(0) to Pd(II).
catalyzed homocoupling/heterocoupling of aromatic as well as
In order to circumvent the use of expensive palladium cata-
aliphatic terminal alkynes under aerobic conditions at ambient lysts, and due to easy availability as well as economical, and
temperature. The catalyst could be easily recovered and reused environmental concerns, several copper-based catalysts have
several times without significant loss of its activity.
been introduced in recent years for the homocoupling of
terminal alkynes.18,19 Although these copper catalysts are highly
efficient for the homocoupling of terminal alkynes to symmet-
rical 1,3-diynes, the use of stoichiometric amounts of copper
salts,19c high temperature,18d–f,19a long reaction times,19a,b and
low to moderate yields for aliphatic alkynes13b,c,19a are limita-
tions of some of these catalytic systems. Another disadvantage
of some of these methods is that the catalysts cannot be
recovered and reused18e,f,19a which limit their usefulness,
specially in large scale operations. Consequently, there is
enormous interest in the further development of copper-based
catalysts with high activity and excellent reusability for the
homocoupling of terminal alkynes.
Since the discovery of the Glaser homocoupling reaction of
terminal alkynes in 1869,1 1,3-diynes have found wide applica-
tion in the synthesis of natural products and analogues,2
construction of linear p-conjugated acetylenic oligomers and
polymers,3 molecular electronic devices,4 optical materials,5
supramolecular switches,6 as well as in the synthesis of a variety
of heterocyclic compounds.7 In addition, 1,3-diynes are
common structural motifs found in pharmaceutically and bio-
logically active compounds, which have shown antifungal,8 anti-
HIV,9 antibacterial10 and anticancer properties.11,12
The homocoupling of terminal alkynes leading to
symmetrical 1,3-diynes are commonly catalyzed by Pd12 and
Cu13 salts, and/or Pd in combination with Cu(I) salts as co-
catalyst.14 It is well known that the Pd-catalyzed homocoupling
of terminal alkynes is a very important and powerful strategy
for the formation of symmetrical 1,3-diynes owing to its
mildness, selectivity and efficiency. However, their major
drawbacks are that palladium catalysts are expensive and
oen require phosphine ligands which are air- and moisture-
sensitive, expensive and toxic.15 Also, some of these protocols
The most imperative route for synthesis of unsymmetrical
1,3-diynes is Cu-catalyzed Cadiot–Chodkiewicz coupling
between 1-haloalkynes and terminal alkynes.20 Palladium-cata-
lyzed heterocoupling of 1-haloalkynes and terminal alkynes
have been also reported for the preparation of unsymmetrical
1,3-diynes.21 Since, 1-haloalkynes are generally prepared by
halogenation of terminal alkynes, so, coupling of two different
terminal alkynes is a more straightforward approach to gain
unsymmetrical 1,3-diynes. Recently, different Cu-based cata-
lysts have been examined for heterocoupling of terminal alky-
nes.13b,18h–k,22 However, most of these reported protocols suffered
from prolonged reaction times and low yields. Therefore, there
is still crucially needed to develop a more convenient and effi-
cient procedure for the synthesis of these compounds.
aDepartment of Chemistry, Catalysis Division, University of Isfahan, Isfahan,
81746-73441, Iran. E-mail: imbaltork@sci.ui.ac.ir; arkhosropour@sci.ui.ac.ir; Fax:
+98-0311-6689732
b
Nowadays, dendrimers have been widely used not only in
nanomedicine and material science, but also as ligands in
modern organometallic chemistry.23 Dendrimers, due to their
highly branched and three-dimensional structures with many
cavities, have been used for encapsulation of a variety of ions
and molecules.24 Among the known applications of metal
¨
¨
Institut fur Organische Chemie, Albert-Ludwigs-Universitat Freiburg, Albertstrasse 21,
79104 Freiburg, Germany
† Electronic supplementary information (ESI) available: Experimental procedures,
spectroscopic data and copies of 1H NMR and 13C NMR spectra and X-ray
crystallographic data for compound 3ad. CCDC 972614. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ra47184e
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under aerobic conditions at room temperature (Scheme 1). As
far as we know, this is the rst report on the use of heteroge-
neous Cu-based dendritic catalyst for the synthesis 1,3-diynes.
We initiated our investigations by using phenylacetylene (1a)
as a model for screening the reaction conditions (Table 1). At
rst, 20 mol% of a variety of bases such as K2CO3, tBuOK, NEt3,
DABCO (1,4-diazabicyclo[2.2.2]octane) and DBU (1,8-dia-
zabicyclo[5.4.0]undec-7-ene) were tested (Table 1, entries 1–5).
Of these, DBU was found to be superior in terms of conversion
(Table 1, entry 5). NEt3 and DABCO gave only low to moderate
yields, while no reaction was observed in the presence of K2CO3
t
and BuOK. The function of different solvents such as EtOH,
DMF, dioxane, toluene, and CH3CN was also checked (Table 1,
entries 5–9). Among these, only CH3CN was found to be the best
medium for attaining optimum yield (Table 1, entry 5). The
variation in the catalyst loading also affected the yield of 2a.
When 0.4 and 0.6 mol% of Cu(II)-TD@nSiO2 were used for the
model reaction under the identical reaction conditions, it
resulted in 70%, and 99% yields of 2a (Table 1, entries 5 and 10),
respectively. It is worthy to mention that only 15% of 2a was
obtained in the presence of CuCl2$2H2O (10 mol%) as a catalyst
(Table 1, entry 12), and no conversion was observed when the
reaction was performed in the absence Cu(II)-TD@nSiO2 catalyst
(Table 1, entry 11). These results clearly exhibited the crucial
role of this catalyst during the coupling reaction.
Encouraged by the efficiency of the above mentioned reac-
tion protocol, we examined the substrate scope to generalize the
versatility of this methodology. As shown in Table 2, the
homocoupling of aromatic terminal alkynes with electron-
donating as well as electron-withdrawing groups (1a–i) was
performed efficiently in the presence of Cu(II)-TD@nSiO2/DBU
catalytic system to afford the corresponding 1,3-diyne deriva-
tives (2a–i) in excellent yields (92–99%) under aerobic condi-
tions at room temperature. 4-Ethenylphenylacetylene (1j), as an
example of enyne, was found to be efficient and provided
excellent yield of the desired 1,3-diyne (2j). Heteroaromatic
alkyne such as 2-ethynylthiophene (1k) also provided the cor-
responding 1,3-diyne (2k) in excellent yield. Moreover, the
present catalytic system could be applied successfully for
the homocoupling of aliphatic terminal alkynes. Accordingly,
the homocoupling of 1-heptyne (1l), 1-hexyne (1m) and ethyl
propiolate (2n) proceeded effectively to afford the correspond-
ing 1,3-diyne derivatives (2l–n) in 86–87% yields.
In order to further widen the application of this protocol, the
Cu(II)-TD@nSiO2/DBU catalytic system was applied to the
synthesis of unsymmetrical 1,3-diynes via heterocoupling of two
different terminal alkynes. First, the reaction between phenyl-
acetylene 1a and ethyl propiolate 1n was selected as a model.
The reaction was carried out in the presence Cu(II)-TD@nSiO2
(0.6 mol%) and DBU (20 mol%) in acetonitrile under aerobic
conditions at room temperature for 2 h (Table 3). When the
reaction was performed using 1 : 1 molar ratio of 1a to 1n, 43%
heterocoupled product 3an was obtained along with 50% and
15% homocoupled products 2a and 2n, respectively (Table 3,
entry 1). Using a molar ratio of 2 : 1 of 1n to 1a or increasing the
amount of base up to 30 mol%, did not improve the yield of the
heterocoupled product signicantly (Table 3, entries 2 and 3).
Scheme 1 Synthesis of symmetrical and unsymmetrical 1,3-diynes
catalyzed by Cu(II)-TD@nSiO2.
encapsulated-dendritic compounds, catalysis is an important
eld.25 In addition, the immobilization of metal encapsulated-
dendrimers on different insoluble solid supports, with the
advantages of easy separation, recovery and reusability, is of
great importance from economical and environmental point of
views.26,27
Recently, we reported the Cu(II) immobilized on nano-silica
triazine dendrimer, Cu(II)-TD@nSiO2 as an efficient and reus-
able catalyst for the synthesis of benzimidazoles, benzothia-
zoles, bis-benzimidazoles, and bis-benzothiazoles.28 For the
preparation of the catalyst, CuCl2$2H2O was used as the source
of copper(II) species. In continuation of our efforts on the new
application of Cu(II)-TD@nSiO2 catalytic system, herein we
report a Pd-free expedient protocol for the synthesis of
symmetrical and unsymmetrical conjugated 1,3-diynes in
excellent yields through a Glaser oxidative coupling of terminal
alkynes using Cu(II)-TD@nSiO2 as a highly recoverable catalyst
Table 1 Optimization of alkyne homocoupling catalyzed by Cu(II)-
a
TD@nSiO2
Cat.
(mol%)
Base
(20 mol%)
Time
(h)
Yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
K2CO3
tBuOK
NEt3
DABCO
DBU
DBU
DBU
DBU
DBU
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
EtOH
6
6
6
6
1.5
6
6
6
6
—
—
15
60
99
—
78
45
—
70
—
15
DMF
Dioxane
Toluene
CH3CN
CH3CN
CH3CN
9
0.6
0.4
—
10
11c
12
DBU
DBU
DBU
6
6
6
CuCl2 (10 mol%)
a
Containing 0.34 mmol Cu(II) per gram Cu(II)-TD@nSiO2 determined by
ICP analysis. Isolated yield. c Without Cu(II)-TD@nSiO2 catalyst.
b
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a
Table 2 Synthesis of symmetrical 1,3-diynes catalyzed by Cu(II)-TD@nSiO2
a
Isolated yield.
Table 3 Optimization of heterocoupling of alkynes catalyzed by
Cu(II)-TD@nSiO2
78% yields, respectively (Table 4, entries 3 and 4). In addition,
the coupling between 4-ethynylanisole 1d and 2-ethynylth-
iophene 1k furnished the desired product 3dk in 88% yield
(Table 4, entry 5). It is also noteworthy that in comparison to the
reported methods for the preparation of unsymmetrical
1,3-diynes,13b,18h–k,22 the yields are higher, the reaction times are
remarkably shorter, and the required amount of the catalyst for
successful reaction is lesser using this method.
A plausible mechanism for the synthesis of 1,3-diynes is
shown in Scheme 2. The Cu(II) catalyst is rst reduced by DBU29
to give the Cu(I) species which upon reaction with a terminal
alkyne in the presence of DBU generates Cu-acetylide. The
oxidative coupling of the resulting Cu-acetylide in the presence
of oxygen affords the desired 1,3-diyne and releases the catalyst
for the next catalytic cycle. To verify the proposed mechanism,
the model reaction was carried out under argon atmosphere; no
desired product was obtained under these conditions. On the
other hand, bubbling of oxygen into the mixture accelerated
the reaction considerably. All These observations indicated that
1a
1n
DBU
3an
2a
2n
Entry (mmol) (mmol) (mol%) Yielda (%) Yieldb (%) Yieldc (%)
1
2
3
4
1
1
1
2
1
1
2
1
20
30
20
20
43
45
61
89
50
51
25
55
15
15
20
5
Isolated yield based on 1.0 mmol of the reactant. b Yield based on 1a.
Yield based on 1n.
a
c
Whereas, the yield of the heterocoupled product was increased the presence of oxygen is essential for the reaction and the
to 89%, when the molar ratio of 1a to 1n changed to 2 : 1 (Table proposed mechanism is reasonable.
3, entry 4). According to the results presented in Table 3, it
The structures of the products were deduced from their IR,
seems that the best yield of the heterocoupled product could be mass, 1H NMR and 13C NMR spectra, and from their elemental
achieved using excess amount of electron-rich alkyne. Under analysis data. The structure 3ad was also conrmed by X-ray
these conditions, the heterocoupling of different terminal crystallographic analysis (Fig. 1).
alkynes was investigated and the results are summarized in
Finally, we turned our attention to the Cu(II)-TD@nSiO2
Table 4. The heterocoupling of phenylacetylene 1a with 4-ethy- catalyst reusability. The recovery of the catalyst was checked in
nylanisole 1d proceeded efficiently to provide the the homocoupling of phenylacetylene 1a. Aer completion of
corresponding unsymmetrical 1,3-diyne 3ad in 95% (Table 4, the reaction, the mixture was diluted with acetonitrile and the
entry 2). 3-Ethynylanisole 1c underwent smooth coupling with catalyst was separated by centrifugation. The catalyst was
1-ethynyl-4-chlorobenzene 1i and 1-hexyne 1m to furnish the washed with acetonitrile, dried and reused for subsequent
corresponding heterocoupled products 3ci and 3cm in 93% and reactions. As shown in Table 5, the Cu(II)-TD@nSiO2 could be
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Table 4 Synthesis of unsymmetrical 1,3-diynes catalyzed by Cu(II)-TD@nSiO2
Entry
Time (h)
Yielda (%)
1
2
2
89
95
1.5
3
1.5
93
4
5
2
78
88
1.5
a
Isolated yield.
Table 5 Reusability of Cu(II)-TD@nSiO2 catalyst in the synthesis of 2aa
Run
1
99
2
99
3
97
4
98
5
97
6
96
7
95
8
92
Yieldb (%)
a
Reaction conditions: phenylacetylene (1 mmol) Cu(II)-TD@nSiO2 (0.6
b
mol%), rt, 1.5 h. Isolated yield.
Conclusions
In summary, we have developed a highly efficient method for the
synthesis of a variety of symmetrical and unsymmetrical
1,3-diynes via coupling of aromatic as well as aliphatic terminal
alkynes in the presence of Cu(II)-TD@nSiO2/DBU catalytic system
underverymildreactionconditions;aerobicconditionsandroom
temperature,withoutanyoxidant.Theoutstandingfeaturesofthe
present method are excellent yields, very short reaction times,
operational simplicity and reusability of the catalyst which make
it a useful and attractive process for the synthesis of symmetrical
and unsymmetrical 1,4-disubstituted 1,3-diynes.
Scheme 2 Proposed mechanism.
Acknowledgements
Fig. 1 X-ray crystal structure of compound 3ad. Thermal ellipsoids are
drawn at the 30% probability level, while the hydrogen size is arbitrary.
The authors are grateful to the Research Council of the
University of Isfahan for nancial support of this work.
recycled and reused at least eight times without any
obvious loss of activity. Moreover, copper leaching in Cu(II)-
TD@nSiO2 catalyst, measured by inductively coupled plasma
analysis (ICP-OES), indicated that very restricted leaching of
copper (less than 0.1 ppm) has taken place throughout the
reaction.
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