A sulfonate-based Cu(I) metal-organic framework as a highly efficient and reusable catalyst for the synthesis of propargylamines under solvent-free conditions

Article abstract of DOI:10.1016/j.cclet.2014.10.022

A new highly efficient and reusable Cu(I)-MOF has been developed for the synthesis of propargylamine compounds via the three-component reaction of secondary amines, alkynes, and aromatic aldehydes under solvent-free conditions. The desired propargylamines were obtained in good to excellent yields with a low catalyst loading. The catalyst may be recovered and reused for up to 5 cycles without major loss of activity. This protocol has the advantages of excellent yields, low catalyst loading, and catalyst recyclability.

Full text of DOI:10.1016/j.cclet.2014.10.022

G Model
CCLET 3128 1–6
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
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A sulfonate-based Cu(I) metal-organic framework as a highly efficient
and reusable catalyst for the synthesis of propargylamines under
solvent-free conditions
1
1
*
Q1 Peng Li , Sridhar Regati , Hui-Cai Huang, Hadi D. Arman, Bang-Lin Chen ,
6
7
*
John C.-G. Zhao
8
Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249-0698, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
A new highly efficient and reusable Cu(I)-MOF has been developed for the synthesis of propargylamine
compounds via the three-components reaction of secondary amines, alkynes, and aromatic aldehydes
under solvent-free conditions. The desired propargylamines were obtained in good to excellent yields
with a low catalyst loading. The catalyst may be recovered and reused for up to 5 cycles without major
loss of activity. This protocol has the advantages of excellent yields, low catalyst loading, and catalyst
recyclability.
Received 17 September 2014
Received in revised form 9 October 2014
Accepted 15 October 2014
Available online xxx
Keywords:
ß 2014 Bang-Lin Chen and John C.-G. Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical
Society. All rights reserved.
Metal-organic framework
Heterogeneous catalysis
Propargylamine
Solvent-free
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1. Introduction
sought out of environmental concerns. In recent years, micro/ 27
mesoporous materials have been extensively studied for their high 28
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Propargylamine is an important motif that can be found in
many natural products and therapeutic drugs [1–4]. They are also
key intermediates for the preparation of many nitrogen-containing
internal surface area where the hybrid solids composed of organic 29
struts and inorganic pore/channel catalytic sites lead to improved 30
efficiency [12,13]. Metal–organic frameworks (MOFs) are crystal- 31
line nodes and have emerged as a novel class of porous materials 32
[14,15]. Thanks to their catalysis friendly features such as large 33
surface areas, porosity, and tunable and functionalizable pore 34
walls, MOFs have proven to be more advantageous than traditional 35
organic and inorganic porous materials in heterogeneous catalysis. 36
Not surprisingly, several MOF-based catalysts, such as Pd/Zn-MOF 37
[16], Cu(2-pymo)2 [17], and post-modified MOF-supported Cu(I) 38
catalysts [18], have been used as the heterogeneous catalysts in the 39
A3 coupling reaction for the synthesis of propargylamines and 40
demonstrated high stability and catalytic activity. Nonetheless, 41
organic solvents are required for these catalytic systems, which is 42
not green according to the green chemistry principles [19]. Recent 43
studies indicate MOFs could be used as green heterogeneous 44
catalysts in water or under solvent-free condition [20–22]. How- 45
ever, to our knowledge, there is no report on a MOF-catalyzed 46
system that operates under solvent-free conditions for this A3 47
natural products and biologically active compounds, such as
b-
lactams [5] and oxotremorine [6] analogues. Because of their
importance, many synthetic methods have been developed [7];
however, the most convenient preparation method for propargy-
lamines is the catalytic three-component reaction involving the
coupling of an aldehyde, an amine, and a terminal alkyne, which is
known as an A3 coupling reaction in the literature [8,9]. Neverthe-
less, most of the reported A3 coupling catalysts used for the
synthesis of propargylamines are homogeneous and have some
serious drawbacks, such as the difficulty in the separation and
subsequent reuse of the expensive catalysts and the indispensable
use of organic solvents [10,11]. New heterogeneous catalysts with
greater reusability, efficiency, and ease of synthesis are constantly
*
Corresponding authors.
coupling reaction.
48
E-mail addresses: banglin.chen@utsa.edu (B.-L. Chen),
As a small subgroup of MOFs, cationic MOFs occur when the 49
positive charges on the metal ions outnumber the negative charges 50
cong.zhao@utsa.edu (John C.-G. Zhao).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.cclet.2014.10.022
1001-8417/ß 2014 Bang-Lin Chen and John C.-G. Zhao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: P. Li, et al., A sulfonate-based Cu(I) metal-organic framework as a highly efficient and reusable catalyst
for the synthesis of propargylamines under solvent-free conditions, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.10.022

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P. Li et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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on the organic linkers [23]. Extra anions are necessary to balance
the charge due to the net positive charge on the frameworks.
Among cationic MOFs, sulfonate-based MOFs have been proven to
be efficient heterogeneous catalysts due to their strong Lewis
acidity as a result of the weak interactions between the sulfonate
templates and the cationic metal-organic frameworks. According-
ly, Oliver and coworkers reported the first application of sulfonate-
based MOFs as the catalyst for the ketal formation reaction
[24]. Our groups are interested in developing novel MOFs for the
applications in chiral separations and organic reactions
[25,26]. Previously, we reported that sulfonate-based Zn- and
Cd-MOFs are highly efficient catalysts for the Biginelli reaction [27]
and for the synthesis of multi-functional pyridine derivatives [28]
under mild conditions. Deng group has also reported that a
sulfonate cadmium coordination polymer can be used to promote
the epoxide ring-opening reaction of epoxides and amines
[29]. Recently, a copper-exchanged USY zeolite has been reported
as the catalyst for the production of propargylamine under solvent-
free conditions [30] and a Ag(I)-exchanged K10 montmorillonite
clay has been used as heterogeneous catalyst for the synthesis of
propargylamine in water [31]. These two materials are both ion-
exchangeable and show high Lewis acidity. On the basis of these
findings, we speculated that Cu(I)-MOFs containing cationic
framework and flexible anionic sulfonate template may also be
highly efficient catalysts for the A3 coupling reaction used for the
synthesis of propargylamines due to their strong Lewis acidity and
structural flexibility and robustness. Herein we report the
synthesis, characterization of a new sulfonate-based Cu(I)-MOF
and its application as a heterogeneous catalyst in the synthesis of
propargylamines under solvent-free conditions.
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2. Experimental
81
82
2.1. General
The crystallographic measurement of the Cu(I)-MOF was
collected at 98(2) K on a Rigaku X-ray diffractometer system
˚
equipped with a Mo-target X-ray tube (l = 0.71073 A) with Saturn
724 CCD detector. Powder X-ray diffraction (PXRD) patterns were
recorded by a RigakuUltima IV diffractometer operated at 40 kV
and 44 mA with a scan rate of 1.0 deg/min. 1H NMR and 13C NMR
spectra were obtained using a Varian Mercury 300 MHz spec-
trometer at room temperature. Tetramethylsilane (TMS) and
83
84
85
86
87
88
89
90
91
92
93
deuterated solvents (CDCl3,
d 77.0; DMSO-d6, d 39.5) were used
as internal standards in 1H NMR and 13C NMR experiments,
respectively.
Fig. 1. (a) Coordination environment of Cu(I) ion in Cu(I)-MOF and (b) crystallographic a-projection of Cu(I)-MOF (copper: cyan; carbon: gray; sulfur: yellow; oxygen: red;
nitrogen: blue, hydrogen atoms are omitted for clarity purpose). (For interpretation of reference to color in this figure legend, the reader is referred to the web version of this
article.)
Please cite this article in press as: P. Li, et al., A sulfonate-based Cu(I) metal-organic framework as a highly efficient and reusable catalyst
for the synthesis of propargylamines under solvent-free conditions, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.10.022

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P. Li et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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Table 1
Optimization of the reaction conditions.a
Entry
Catalyst (mol %)
Temp (8C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
10.0
0
80
80
60
40
rt
14
14
20
28
24
18
24
48
90
0
10.0
10.0
10.0
5.0
85
49
0
80
80
80
91
90
64
2.5
1.0
a
Unless otherwise specified, all reactions were carried out with 1a (0.50 mmol), 2a (0.50 mmol), 3a (0.50 mmol), and the Cu(I)-MOF catalyst (amount specified in the table)
under neat conditions.
b
Yield of the isolated product after column chromatography.
94
2.2. Synthesis of the Cu(I)-MOF
purified by column chromatography. Before reuse, the recovered 112
catalyst was washed with CH2Cl2 (5 mL) and activated by heating 113
95
96
97
98
Cu(OAc)2Á2H2O (0.036 g, 0.2 mmol), 2,4,6-tri(4-pyridyl)-1,3,
5-triazine (TPT) (0.062 g, 0.2 mmol), and 2,6-naphthalenendisul-
fonic acid disodium salt (NSA) (0.032 g, 0.1 mmol) were dissolved
in 10 mL deionized water and were heated at 170 8C for 4 days
under autogenous pressure, followed by slow cooling at a rate of
10 8C/min to room temperature. Dark red block crystals were
isolated after filtration and washed by D.I. water three times (yield:
0.072 g, 74% based on Cu(OAc)2Á2H2O).
at 50 8C under vacuum for 2 h.
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3. Results and discussion
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116
99
100
101
102
3.1. Crystal structure of the Cu(I)-MOF
Single crystal structure reveals that the Cu(I)-MOF crystallizes in 117
triclinic space group P-1 and every Cu center is coordinated by with 118
three nitrogen atoms from TPT ligands and one oxygen from a NSA 119
ligand in a tetrahedral coordination geometry (Fig. 1a). There exists a 120
guest water molecule in the lattice. The interactions between the 121
103
2.3. General procedure for the three-component reaction
104
105
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107
108
109
110
111
In a sample vial, to a mixture of piperidine (42.5 mg,
˚
0.50 mmol), benzaldehyde (53.1 mg, 0.50 mmol), and phenylace-
tylene (51.1 mg, 0.50 mmol) was added the Cu(I)-MOF catalyst
(6.7 mg, 0.0125 mmol, 2.5 mol%). The mixture was then stirred at
80 8C for the time given in the tables. After completion of the
reaction (monitored by TLC), CH2Cl2 (2 mL) was added and the
mixture was centrifuged to recover the catalyst. The crude product
obtained after removing the solvent under reduced pressure was
sulfonate ligand and Cu(I) ion [Cu–O distance is 2.370(2)A] is 122
relatively weak, which renders the Cu(I) site readily accessible as 123
Lewis acid for potential heterogeneous catalysis. The Cu(I) ions are 124
linked by TPT and NSA ligands into 2D frameworks, which are 125
further stabilized by
due to condensed packing, Cu(I)-MOF does not show any pores in 3D 127
structure, and no permanent porosity has been established. 128
p–p stacking along a axis (Fig. 1b). However, 126
Table 2
Three-component synthesis of propargylamines using piperidine catalyzed by the Cu(I)-MOF.a
Entry
R1
R2
Time (h)
Yield (%)b
1
2
3
4
5
6
7
H
Ph
24
24
26
27
24
24
24
90
83
87
85
89
88
88
H
2-BrC6H4
4-MeOC6H4
2-CF3C6H4
Ph
H
H
Cl
Br
Me
Ph
Ph
a
All reactions were carried out with 1a (0.50 mmol), 2 (0.50 mmol), and 3 (0.50 mmol) and the MOF catalyst (0.0125 mmol, 2.5 mol %) under neat conditions at 80 8C.
Yield of the isolated product after column chromatography.
b
Please cite this article in press as: P. Li, et al., A sulfonate-based Cu(I) metal-organic framework as a highly efficient and reusable catalyst
for the synthesis of propargylamines under solvent-free conditions, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.10.022

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P. Li et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Table 3
Three-component synthesis of propargylamines using pyrrolidine catalyzed by the Cu(I)-MOF.a
Entry
R1
R2
Time (h)
Yield (%)b
1
2c
3
H
Ph
24
26
26
24
24
89
58
86
62
59
H
4-MeOC6H4
CH3
Cl
Ph
Ph
Ph
4
5
MeO
a
Unless otherwise indicated, all reactions were carried out with 1b (0.50 mmol), 2 (0.50 mmol), and 3 (0.50 mmol) and the MOF catalyst (0.0125 mmol, 2.5 mol%) under
neat conditions at 80 8C.
b
Yield of the isolated product after column chromatography.
c
With a catalyst loading of 10 mol %.
129
3.2. Optimization of model reaction
3.3. Scope of the A3 coupling reaction
155
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132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
Using piperidine (1a), benzaldehyde (2a), and phenylacetylene
(3a) as the model substrates, we first examined the catalytic
activity of the Cu(I)-MOF in the three-component reaction. The
reaction was attempted under neat conditions in order to avoid the
use of any organic solvent [18]. The results are summarized in
Table 1. As the data in Table 1 show, when 10 mol % of the Cu(I)-
MOF was applied at 80 8C under neat conditions, the desired
propargylamine 4a was formed in a high yield (90%) after 14 h
(entry 1). The catalytic activity of the Cu(I)-MOF for this
reactions was unequivocally established by conducting a control
reaction in the absence of the catalyst: No formation product 4a
was observed without the MOF catalyst under otherwise
identical reaction conditions (Table 1, entry 2). When the
reaction temperature was decreased, a significant loss of the
catalytic activity was observed, as is evident from the longer
reaction times and lower product yields (entries 3 and 4). In
addition, when the reaction was performed at room tempera-
ture, there was no observable conversion of the substrates (entry
5). Nevertheless, the catalyst loading can be reduced (entries 6
and 7) and the reaction proceeds well even with a catalyst
loading of only 2.5 mol %, without affect the product yield,
although the reaction time is slightly longer (entry 7). However,
further reducing the catalyst loading to 1.0 mol% led to a much
slower reaction and the product yield dropped significantly to
64% (entry 8).
With the optimized reaction conditions at hand, we then
evaluated the scope of this reaction. The results are summarized in
Table 2. As shown by the data in Table 2, besides phenylacetylene,
2-bromophenylacetylene also underwent the coupling reaction
smoothly to yield the corresponding propargylamine in good yield
(83%, entry 2). Similarly, high yields were obtained for phenyla-
cetylenes with an electron-donating group (4-MeO) or an electron-
withdrawing group (2-CF3) on the phenyl ring (entries 3 and 4).
Substituted benzaldehydes are also good substrates for this
coupling reaction. High product yields were obtained when 4-
halo-substituted benzaldehydes were reacted with phenylacety-
lene and piperidine (entries 5 and 6). When 4-methylbenzalde-
hyde was used, the corresponding propargylamine was obtained in
88% yield (entry 7).
Next pyrrolidine was used in the three-component coupling
reaction under identical conditions. The results are summarized in
Table 3. When benzaldehyde and phenylacetylene were used as
the starting materials, the corresponding propargylamine was
obtained in 89% yield (entry 1), which is similar to that of
piperidine (Table 2, entry 1). However, unlike that of piperidine
(Table 2, entry 3), 4-methoxyphenylacetylene is much less reactive
and gave a much lower yield of the propargylamine (58%) even
with a much higher catalyst loading (10 mol %, entry 2). On the
other hand, 4-methylbenzaldehyde produced a good yield of the
expected product (entry 3). However, 4-chloro- and 4-methoxy
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161
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163
164
165
166
167
168
169
170
171
172
173
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175
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177
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179
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Table 4
Catalyst recyclability study.a
Entry
Cycle
Catalyst recovery (%)
Time (h)
Yield (%)b
1
2
3
4
5
I
96
96
96
95
95
14
22
15
15
20
91
82
88
88
87
II
III
IV
V
a
Unless otherwise specified, all reactions were carried out with 1 (0.50 mmol), 2a (0.50 mmol), and 3a (0.50 mmol) and the MOF catalyst (0.0125 mmol, 2.5 mol%) under
neat conditions at 80 8C.
b
Yield of the isolated product after column chromatography.
Please cite this article in press as: P. Li, et al., A sulfonate-based Cu(I) metal-organic framework as a highly efficient and reusable catalyst
for the synthesis of propargylamines under solvent-free conditions, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.10.022

G Model
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P. Li et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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Cu(I)-MOFs might lead to its practical applications on the 219
production of propargylamine in an environmentally benign 220
manner.
221
Acknowledgment
222
The authors thank the generous financial support of this 223
research from the Welch Foundation (No. AX-1730 for B. C. and. No.Q2224
AX-1593 for J.C.-G. Z.).
225
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Fig. 2. PXRD of simulated Cu(I)-MOF (black), fresh Cu(I)-MOF (red) and Cu(I)-MOF
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To verify the stability of the catalyst under the experimental
conditions, leaching experiment was performed by examining the
1H NMR of the supernatant of reaction mixture. No leaching of TPT
or NSA ligand was observed in the supernatant. To further confirm
that the catalysis is indeed heterogeneous, we filtered the reaction
mixture after 5 h, and continued to monitor the progress of the
reactions in the filtrate. No further catalytic reactions has been
observed in the filtrated solution.
We also investigated the recyclability of this new Cu(I)-MOF
using piperidine (1a), benzaldehyde (2a) and phenylacetylene (3a).
The results are summarized in Table 4. In fact, the MOF catalyst
could be easily recovered by centrifuging the reaction mixture
after the reaction. The PXRD pattern indicates the crystal
structures of Cu(I)-MOF maintained after the catalytic reactions
(Fig. 2).
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In summary, we have developed a new Cu(I)-MOF that is a
highly efficient catalyst for the three-component synthesis of
propargylamines under solvent-free conditions. The correspond-
ing propargylamines were obtained in good to excellent yields,
especially when piperidine was used as the amine component.
Heterogeneity tests demonstrate the absence of leaching under the
examined experimental conditions. The MOF catalyst can be
readily recycled and reused without major loss of their activity
for up to five cycles. This heterogeneous catalyst based on the
frameworks as solid Lewis acids for the isomerization of a-pinene oxide, Catal. 286
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