Synthesis of indolizines through aldehyde-amine-alkyne couplings using metal-organic framework Cu-MOF-74 as an efficient heterogeneous catalyst

Article abstract of DOI:10.1016/j.jcat.2016.02.013

A crystalline copper-based metal-organic framework Cu-MOF-74 was synthesized, and was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier tr

Full text of DOI:10.1016/j.jcat.2016.02.013

Journal of Catalysis 337 (2016) 167–176
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Synthesis of indolizines through aldehyde–amine–alkyne couplings
using metal-organic framework Cu-MOF-74 as an efficient
heterogeneous catalyst
⇑
⇑
Giao H. Dang, Huy Q. Lam, Anh T. Nguyen, Dung T. Le, Thanh Truong , Nam T.S. Phan
Department of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
A crystalline copper-based metal-organic framework Cu-MOF-74 was synthesized, and was characterized
by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron micro-
scopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption
spectrophotometry (AAS), and nitrogen physisorption measurements. The Cu-MOF-74 exhibited high cat-
alytic activity in the synthesis of indolizines through aldehyde–amine–alkyne three-component coupling
transformation and higher than those of other Cu-MOFs such as Cu2(BDC)2(DABCO), Cu3(BTC)2, Cu(BDC),
Cu2(NDC)2(DABCO), and Cu4I4(DABCO)2. The reactions could only proceed to produce indolizines in the
presence of the solid Cu-MOF catalyst, and contribution from active copper species leached from the solid
Cu-MOF-74, if any, was negligible. The Cu-MOF catalyst be recovered and reused several times for the
synthesis of indolizines without a significant degradation in catalytic activity. To the best of our knowl-
edge, the synthesis of indolizines using a recyclable heterogeneous catalyst was not previously men-
tioned in the literature.
Received 1 December 2015
Revised 9 February 2016
Accepted 9 February 2016
Keywords:
Metal-organic framework
Indolizines
Coupling
Cu-MOF-74
Heterogeneous catalyst
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
co-workers developed the Cu(MeCN)4PF6-catalyzed denitrogena-
tive transannulation reaction of pyridotriazoles with terminal alky-
Indolizines have attracted significant attention as important
scaffolds found in many bioactive natural products, pharmaceuti-
cals, and agrochemicals, and as versatile intermediates in a variety
of organic transformations [1–5]. As a result, several synthesis
approaches have been developed for the construction of these
N-fused heterocycles [6]. One of the most popular methods should
be the 1,3-dipolar cycloaddition of electron deficient alkenes or
alkynes with pyridinium ylides in the presence of a base and an
oxidant [5,7,8]. However, the problem of using the excess amounts
of the base and the oxidant for the transformation still remains to
be solved [5]. Huang and co-workers previously reported the syn-
thesis of substituted indolizines by means of I2-mediated oxidative
tandem cyclization between various substituted aromatic/aliphatic
enolizable aldehydes and 2-pyridylacetates/acetonitrile/acetone
[2]. Catalytic transformations with different transition metals have
recently been investigated for the synthesis of indolizines. Zhang
and co-workers pointed out that arylated indolizines could be pro-
duced by the Cu(OAc)2-catalyzed annulation of 2-alkylazaarenes
nes en route to indolizines [10]. Moreover, these N-fused
heterocycles could also be formed via homogeneous palladium-
[11–13], silver- [14,15], gold- [16], and samarium-catalyzed [17]
transformations. Recently, González-Soria and co-workers
reported the first example of heterogeneous catalytic synthesis of
indolizines through aldehyde–amine–alkyne coupling using cop-
per nanoparticles supported on activated carbon as catalyst [18].
Unfortunately, this solid catalyst could not be recycled and reused
in the synthesis of indolizines, though the same catalyst could be
reutilized in the synthesis of chalcones [18].
Metal-organic frameworks (MOFs), also known as porous coor-
dination polymers, have emerged as a new family of crystalline
materials composed of organic linkers that connect metal ions or
metal clusters to produce one-, two-, or three-dimensional net-
works [19–24]. A large number of MOFs could be formed by linking
numerous organic bridging ligands with several metal cations of
diverse oxidation states and coordination geometries [25], offering
several advantages such as structural diversity, well-defined struc-
tures, high surface areas, high porosity, and the ability to modify the
surface hydrophobicity/hydrophilicity [19–24,26–29]. With these
special properties, potential applications of MOFs in many
fields have been extensively explored during the last decade
[19,20,30–35]. MOFs as heterogeneous catalysts have attracted
with
a,b-unsaturated carboxylic acids [9]. Gevorgyan and
⇑
Corresponding authors. Fax: +84 8 38637504.
E-mail addresses: tvthanh@hcmut.edu.vn (T. Truong), ptsnam@hcmut.edu.vn
(N.T.S. Phan).
http://dx.doi.org/10.1016/j.jcat.2016.02.013
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

168
G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
immense worldwide interests from both industry and academia
[36–38]. All metal cations or functional groups on the organic
bridging ligands in MOF structure could be useful for catalytic reac-
tions; therefore, the dispersion and the loading of active sites on the
solid framework could be maximized [37,39–45]. During the last
few years, many MOF-catalyzed organic reactions have been
reported in the literature [36–38,46,47], ranging from both car-
bon–carbon [48–58] to carbon-heteroatom forming [59–67] trans-
formations. Among a variety of popular MOFs, copper-based
frameworks have been explored as catalysts for numerous organic
transformations due to their unsaturated open copper metal sites
[58,62,68–74]. In this work, we wish to report the synthesis of indo-
lizines through aldehyde–amine–alkyne three-component cou-
pling using a copper-based metal-organic framework Cu-MOF-74
as an efficient heterogeneous catalyst. The solid catalyst could be
recycled and reused many times for the transformation without a
significant degradation in catalytic activity. To the best of our
knowledge, the synthesis of indolizines using a recyclable heteroge-
neous catalyst was not previously mentioned in the literature.
Cu(NO3)2ꢀ3H2O (1.21 g, 5 mmol) was dissolved in a mixture of
DMF (DMF = N,N0-dimethylformamide; 47 mL), and 2-propanol
(3 mL). The resulting solution was then distributed to seven
10 mL vials. The vials were heated at 85 °C in an isothermal oven
for 18 h. After cooling the vials to room temperature, the solid pro-
duct was removed by decanting with mother liquor and washed in
DMF (3 ꢁ 20 mL). Solvent exchange was carried out with
2-propanol (3 ꢁ 20 mL) at room temperature. The material was
then evacuated under vacuum at 150 °C for 5 h, yielding 0.50 g of
Cu-MOF-74, Cu2(DOBDC), in the form of reddish black crystals
(62% yield based on H2dhtp).
2.3. Catalytic studies
In a typical experiment, a mixture of 2-pyridincarboxaldehyde
(0.190 mL, 2.0 mmol), piperidine (0.100 mL, 1.0 mmol), pheny-
lacetylene (0.165 mL, 1.5 mmol) and diphenyl ether (0.156 mL,
1 mmol) as an internal standard in n-butanol (2 mL) was added
into a 10 mL vial containing the pre-determining amount of Cu-
MOF-74 catalyst. The catalyst amount (in mol% and wt.%) was cal-
culated with respect to the copper/piperidine molar ratio (weight
was corrected by adsorbed solvent as seen by TGA). The reaction
mixture was stirred at 100 °C under argon for 5 h. GC yield was
monitored by withdrawing aliquots from the reaction mixture at
different time intervals, quenching with water (1 mL). The organic
components were then extracted into ethyl acetate (4 mL), dried
over anhydrous Na2SO4, and analyzed by GC with reference to
diphenyl ether. The combined organic layers were concentrated
under reduced pressure. The resulting residue was purified by col-
umn chromatography (ethyl acetate/hexane = 1:9) to afford 3-phe
nyl-1-(piperidin-1-yl)indolizine. The product identity was further
confirmed by GC–MS, 1H NMR and 13C NMR. To investigate the
recyclability of Cu-MOF-74, the catalyst was separated from the
reaction mixture by simple centrifugation, washed with copious
amounts of DMF and 2-propanol, dried 150 °C under vacuum in
2 h, and reused if necessary. For the leaching test, a catalytic reac-
tion was stopped after 1 h, analyzed by GC, and centrifuged to
remove the solid catalyst. The reaction solution was then stirred
for a further 5 h. Reaction progress, if any, was monitored by GC
as previously described.
2. Experimental
2.1. Materials and instrumentation
All reagents and starting materials were obtained commercially
from Sigma–Aldrich and Merck, and were used as received without
any further purification unless otherwise noted. Nitrogen
physisorption measurements were conducted using a Micromeritics
2020 volumetric adsorption analyzer system. Samples were pre-
treated by heating under vacuum at 150 °C for 3 h. A Netzsch Ther-
moanalyzer STA 409 was used for thermogravimetric analysis (TGA)
with a heating rate of 10 °C/min under a nitrogen atmosphere. X-ray
powder diffraction (XRD) patterns were recorded using a Cu Karadi-
ation source on a D8 Advance Bruker powder diffractometer. Scan-
ning electron microscopy studies were conducted on a S4800
Scanning Electron Microscope (SEM). Transmission electron micro-
scopy studies were performed using a JEOL JEM 1400 Transmission
Electron Microscope (TEM) at 80 kV. The Cu-MOF-74 sample was
dispersed on holey carbon grids for TEM observation. Elemental
analysis with atomic absorption spectrophotometry (AAS) was per-
formed on an AA-6800 Shimadzu. Fourier transform infrared (FT-IR)
spectra were obtained on a Nicolet 6700 instrument, with samples
being dispersed on potassium bromide pallets.
3. Results and discussion
Gas chromatographic (GC) analyses were performed using a Shi-
madzu GC 2010-Plus equipped with a flame ionization detector
(FID) and an SPB-5 column (length = 30 m, inner diame-
The metal-organic framework Cu-MOF-74 was synthesized in a
yield of 62% by solvothermal method according to a slightly mod-
ified literature procedure [75], and was characterized by a variety
of different techniques, including XRD, SEM, TEM, TGA, FT-IR, AAS,
and nitrogen physisorption measurements (Figs. S1–S7). The anal-
ysis results were in agreement with previous studies [75]. These
confirmed that the synthesized Cu-MOF-74 possesses paddle
wheel structure, open metal sites, high coordinate metal ion, and
exceptionally large pore apertures. In particular, powder X-ray
diffraction pattern showed the typical reflections of MOF-74 phase.
The permanent micro-porosity with Brunauer–Emmett–Teller
specific surface area of 1101 m2/g, a pore volume of 0.50 cm3/g
and an average pore diameter of about 13 Å was confirmed by
the basically type-1 adsorption/desorption isotherm. Scanning
electron microscopy analysis showed the homogeneity with
respect to needle-shaped crystals. Thermal gravimetric analysis
(TGA) of activated Cu-MOF-74 shows high thermal stability
(>300 °C) and the measured mass percent of residue CuO is consis-
tent with the EA data. AAS provided 36.51% copper content which
is close to the calculated value of 37.72%. Finally, FT-IR spectra of
Cu-MOF-74 indicated the presence of bonded carboxylate organic
linkers.
ter = 0.25 mm, and film thickness = 0.25 lm). The temperature pro-
gram for GC analysis held samples at 100 °C for 1 min, heated them
from 100 to 280 °C at 40 °C/min, and held them at 280 °C for
6.5 min. Inlet and detector temperatures were set constant at
280 °C. Diphenyl ether was used as an internal standard to calculate
GC yield. GC–MS analyses were performed using a Shimadzu
GCMS-QP2010Ultra with a ZB-5MS column (length = 30 m, inner
diameter = 0.25 mm, and film thickness = 0.25 lm). The tempera-
ture program for GC–MS analysis held samples at 50 °C for 2 min,
heated samples from 50 to 280 °C at 10 °C/min and held them at
280 °C for 10 min. Inlet temperature was set constant at 280 °C.
MS spectra were compared with the spectra gathered in the NIST
library. The 1H NMR and 13C NMR were recorded on Bruker AV
500 spectrometers using residual solvent peak as a reference.
2.2. Synthesis of the metal-organic framework Cu-MOF-74
In
a
typical preparation,
a
solid mixture of H2dhtp
(H2dhtp = 2,5-dihydroxyterephthalic acid; 0.495 g, 2.5 mmol), and

G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
169
The Cu-MOF-74 was used as a heterogeneous catalyst for the
100
80
60
40
20
0
three-component coupling reaction of 2-pyridincarboxaldehyde,
piperidine, and phenylacetylene to form 3-phenyl-1-(piperidin-1-
yl)indolizine as the principal product (Scheme 1). The reaction
selectivity was calculated with regard to detectable by-products
including homo-coupling of phenyl acetylene and the addition of
acetylide to aldehyde forming chalcone-type products. Initial stud-
ies addressed the effect of temperature (Fig. 1). The three-
component coupling reaction was carried out in n-butanol under
argon for 5 h, using 2-pyridincarboxaldehyde:piperidine:phenyla
cetylene molar ratio of 2:1:1.5, at piperidine concentration of
0.5 M, in the presence of 5 mol% (3.74 wt.%) of Cu-MOF-74 catalyst.
It was observed that no desired product was detected for the reac-
tion carried out at room temperature. The transformation pro-
ceeded with difficulty at 80 °C, though 41% yield was observed at
80 °C. This value could be improved to 45% at 90 °C. Increasing
the reaction temperature to 100 °C led to a significant enhance-
ment in the reaction yield. Indeed, up to 74% yield was achieved
for the reaction carried out at 100 °C. Performing the three-
component coupling reaction at higher temperature than 100 °C
was found to be unnecessary as the formation of (A) was not
improved significantly.
When organic transformations were carried out in the presence
of solid catalysts, the solvent could exhibit a significant effect on
the reaction rate, depending on the nature of the catalyst [76,77].
In the first example of the heterogeneous catalytic aldehyde–ami
ne–alkyne coupling using copper nanoparticles supported on acti-
vated carbon as catalyst, González-Soria and co-workers reported
that indolizines were produced in dichloromethane as solvent,
while chalcones were formed under solvent-free condition [18].
It was therefore decided to investigate the impact of different sol-
vents (Fig. 2). The three-component coupling reaction was carried
out at 100 °C under argon for 5 h, using 2-pyridincarboxaldehyde:
piperidine:phenylacetylene molar ratio of 2:1:1.5, at piperidine
concentration of 0.5 M, in the presence of 5 mol% Cu-MOF-74 cat-
alyst, in n-butanol, dichloroethane, dimethyl sulfoxide (DMSO),
acetonitrile, N-methyl-2-pyrrolidone (NMP), dimethylformamide
(DMF), dimethylacetamide (DMA), diethylformamide (DEF),
toluene, p-xylene, 1,4-dioxane, and diglyme as solvent, respec-
tively. In contrast to previous studies in solvent-dependence on
the same reaction, neither indolizines nor chalcones were pro-
duced in the absence of solvent, indicating the necessity of solvent
for the formation of desired product (A) [18]. DMA, DEF, toluene, p-
xylene, 1,4-dioxane, and diglyme were found to be unsuitable as
solvent for the three-component coupling reaction using Cu-
MOF-74 catalyst. Acetonitrile, NMP, and DMF exhibited better per-
formance, affording 22% yield. This value could be improved to 29%
for the case of DMSO. Dichloroethane was more suitable for the
transformation than DMSO. n-Butanol was found to be the solvent
of choice, with 74% yield of 3-phenyl-1-(piperidin-1-yl)indolizine
being achieved.
RT
80°C
90°C
100°C 110°C
Temperature
Fig. 1. Effect of temperature on the reaction yield.
100
80
60
40
20
0
Solvents
Fig. 2. Effect of solvent on the reaction yield.
phenylacetylene. The three-component coupling reaction was car-
ried out in n-butanol at 100 °C under argon for 5 h, at piperidine
concentration of 0.5 M, in the presence of 5 mol% Cu-MOF-74 cat-
alyst, at various ratios (Fig. 3). It was found that the reactant molar
ratio exhibited a significant impact on reaction efficiency. Using 1
equivalent of pyridincarboxaldehyde and 1 equivalent of pheny-
lacetylene, the reaction proceeded with difficulty, affording only
21% yield. Experimental results indicated that when less than 2
equivalents of pyridincarboxaldehyde was employed, the yield of
(A) was not improved significantly by increasing the amount of
phenylacetylene. Indeed, only 36% yield was detected for the reac-
tion using 2-pyridincarboxaldehyde:piperidine:phenylacetylene
molar ratio of 1.5:1:1.5. However, when more than 2 equivalents
Another factor should be addressed for the three-component
coupling reaction of 2-pyridincarboxaldehyde, piperidine, and
Scheme 1. The three-component coupling reaction of 2-pyridincarboxaldehyde, piperidine, and phenylacetylene using Cu-MOF-74 catalyst.

170
G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
firm that the three-component coupling reaction could only pro-
100
80
60
40
20
0
ceed in the presence of the solid Cu-MOF-74 catalyst, and there
was no contribution from leached active copper species in the
solution.
To further address the issue of where the catalysis occurs,
experiments using grinded Cu-MOF-74 were conducted under
identical conditions (Fig. 5). It was observed that similar reaction
conversions were obtained during the reaction course in both
cases. In addition, the cavity size according to the Cu-MOF-74
structure is about 12–14 Å while the kinetic diameters of aromatic
reactant and product are calculated to be 6 Å and 12 Å, respectively
[78,79]. Furthermore, pore flexibility in MOFs upon temperature
and guest molecules has been previously reported [80]. Though it
is likely that reactions take place inside the catalyst pores, further
spectroscopic studies are still needed.
To gain insights into the reaction pathway, several mechanistic
studies were carried out. In the first experiment, the reaction was
carried out in n-butanol at 100 °C under argon, using 2-pyridincar
boxaldehyde:piperidine:phenylacetylene molar ratio of 2.5:1:1.5,
at piperidine concentration of 0.5 M, in the presence of 5 mol%
Cu-MOF-74 catalyst. After 1 h reaction time with 52% yield of (A)
being obtained, argon was removed and replaced by air, and the
reaction mixture was heated under air at 100 °C for further 5 h. It
was found that 65% yield was detected under this condition. It
should be noted that the transformation carried out under argon
could proceed to 86% yield. Moreover, 11% of by-product (B) and
(C) was observed in the product mixture for the reaction under
air, while only 4% of (B) and (C) was detected for that under argon.
In the second experiment, the reaction was carried out under argon
in the presence of water (5% by volume) and only 54% yield of (A)
in conjunction with 12% of alkyne dimerization product (C) was
detected in the product mixture. These observations suggested
that the formation of (A) was not favored by air or by water residue
in the solvent. In the third experiment, the Cu-MOF-74 catalyst
was soaked in pyridine as catalyst poison prior to use. It was also
found that the Cu-MOF catalyst was significantly deactivated by
pyridine under this condition, with only 18% yield of principal pro-
duct being detected. It could be proposed that the strong adsorp-
tion of pyridine on the copper sites in the Cu-MOF-74
deactivated the catalyst, confirming the role of copper sites in
the transformation. However, further mechanistic studies would
be necessary to elucidate the reaction pathway of the three-
component coupling reaction using Cu-MOF-74 catalyst.
Reactant molar ratio
Fig. 3. Effect of 2-pyridincarboxaldehyde:piperidine:phenylacetylene molar ratio
on the reaction yield.
of pyridincarboxaldehyde were used, the formation of (A) was dra-
matically enhanced by increasing the amount of phenylacetylene.
The reaction yield could be improved to 86% for the reaction using
2-pyridincarboxaldehyde:piperidine:phenylacetylene molar ratio
of 2.5:1:1.5. It was observed that changing this reactant molar ratio
led to a drop in the reaction yield.
The possibility that some of catalytically active sites on the solid
catalyst could dissolve into the liquid phase during the course of
the reaction should be examined for liquid phase organic transfor-
mations [77]. In order to determine whether active copper species
dissolved from the solid Cu-MOF-74 catalyst contributed to the
formation of desired product (A), a control experiment was carried
out (Fig. 4). Apparently, if (A) was still formed after the solid Cu-
MOF-74 catalyst was removed from the mixture, this behavior
would indicate that the three-component coupling reaction would
not proceed under real heterogeneous catalysis conditions. The liq-
uid phase was separated from the solid Cu-MOF-74 after 1 h reac-
tion time by centrifugation. The reaction solution was then
transferred to a new reactor vessel, and stirred for an additional
5 h at 100 °C under argon with aliquots being sampled at different
time intervals, and analyzed by GC. It was found that no further
product (A) was produced in the absence of the solid Cu-MOF-74
catalyst. Additionally, the content of Cu species in reaction filtrate
was also measured using AAS. The results indicated that amount of
Cu in reaction filtrate and pure reaction solvents were similar with
less than 1 ppm of Cu was detected. These observations would con-
With these results in mind, we then decided to investigate the
effect of catalyst concentration (Fig. 6). The three-component cou-
pling reaction was carried out in n-butanol at 100 °C under argon
for 5 h, using 2-pyridincarboxaldehyde:piperidine:phenylacety
100
80
100
80
60
60
40
40
5 mol% catalyst
non-grinded Cu-MOF-74
20
Leaching test
20
0
grinded Cu-MOF-74
0
0
1
2
3
4
5
6
0
1
2
3
4
Time (h)
Time (h)
Fig. 4. Leaching test indicated no contribution from homogeneous catalysis of
leached species.
Fig. 5. Reactions with grinded Cu-MOF-74.

G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
171
lene molar ratio of 2.5:1:1.5, at piperidine concentration of 0.5 M,
100
80
60
40
20
0
in the presence of 0.5 mol%, 1 mol%, 3 mol%, 5 mol%, and 7 mol%
Cu-MOF-74 catalyst, respectively. It should be noted that no pro-
duct was detected in the absence of the Cu-MOF-74 catalyst. As
mentioned earlier, the reaction using 5 mol% catalyst afforded
86% yield. Interestingly, it was found that increasing the catalyst
concentration to 7 mol% resulted in a significant drop in the reac-
tion yield. Decreasing the catalyst concentration from 5 mol% to
3 mol% led to 88% yield, while up to 99% yield of this product
was achieved for the reaction using 1 mol% catalyst. Using lower
than 1 mol% catalyst led to a slight decrease in the reaction yield.
Repeated experiments gave similar results. The explanation for this
phenomenon might come from the cooperative and synergistic
effects of poly-Cu nuclear in reaction mixture facilitating the by-
product formation routes at high copper concentrations [81].
For liquid phase organic reactions using solid catalysts, the
reactant concentration would exhibit a significant effect on the
reaction rate due to the mass transfer phenomenon. It was there-
fore decided to explore the impact of reactant concentration on
the reaction yield (Fig. 7). The coupling reaction was carried out
in n-butanol at 100 °C under argon for 5 h, using 2-pyridincarboxal
dehyde:piperidine:phenylacetylene molar ratio of 2.5:1:1.5, in the
presence of 5 mol% Cu-MOF-74 catalyst, at different piperidine
concentrations. It was found that the reactant concentration also
exhibited a significant impact on the three-component coupling
reaction, and optimal GC yield was observed at piperidine concen-
tration of 0.67 M.
The catalytic activity of Cu-MOF-74 in the three-component
coupling reaction of 2-pyridincarboxaldehyde, piperidine, and
phenylacetylene was also compared with that of other homoge-
neous catalysts (Table 1). It was observed that the reaction using
CuF2 catalyst proceeded with difficulty, affording only 39% yield.
Cu(CH3COO)2 was found to be more active than CuF2, with 66%
yield being detected. Other homogeneous catalysts, including
CuBr, CuCl, CuI, CuBr2, CuCl2, Cu(NO3)2, and Cu(CH3COCH2COO)2
exhibited high activity in the three-component coupling reaction.
In particular, (A) was produced in a yield of 96% for the reaction
using CuI catalyst, while 98% yield was obtained for that using
CuBr, CuBr2, Cu(NO3)2, and Cu(CH3COCH2COO)2. The reaction yield
could be improved to 99% when CuCl2 was used as catalyst. As
mentioned earlier, 99% yield was also achieved in the presence of
Cu-MOF-74 catalyst. It should be noted that (C) was also detected
for the three-component coupling reaction using these catalysts.
Indeed, a selectivity of 96% to (A) was observed for the CuCl2-
catalyzed transformation, while 97% selectivity was obtained for
the case of Cu-MOF-74. It is worth mentioning that as compared
to homogeneous counterparts, traditional heterogeneous catalysts
often provide lower activity. However, in this report, heteroge-
0.33
0.50
0.67
1.00
2.00
Piperidine Concentration (M)
Fig. 7. Effect of piperidine concentration on the reaction yield.
Table 1
Different homogeneous catalysts for the three-component coupling reaction.
Entry
Type
Catalyst
GC yield (%)
1
2
3
3
4
5
6
7
8
Cu-MOF-74
CuBr
CuI
CuBr2
CuCl2
99
98
96
98
99
39
98
66
98
2
Homogeneous copper salts
CuF2
Cu(NO3)
Cu(OAc)2
Cu(CH3COCH2COO)2
MnSO4
Fe(NO3)3
Ni(NO3)2
CoCl2
ZrCl4
AlCl3
Zn(NO3)2
9
Other homogeneous catalysts
10
11
12
13
14
15
2
4
2
2
2
3
Coupling reaction was carried out in n-butanol at 100 °C under argon for 5 h, using
2-pyridincarboxaldehyde:piperidine:phenylacetylene molar ratio of 2.5:1:1.5, at
piperidine concentration of 0.67 M, in the presence of 1 mol% catalyst.
neous Cu-MOF-74 possesses similar or better activity than other
tested Cu salts which were frequently used as homogeneous cata-
lysts in typical cross-coupling reactions. To further elucidate the
kind of active centers, reactions using other first-row transition
metals (entries 9–13) and strong Lewis acids (entries 14, 15) were
performed. The catalysis results confirmed the Cu active site for
tested reactions with <5% of desired products was observed in
other cases.
To highlight the significant point of using Cu-MOF-74 as cata-
lyst, reactions using other heterogeneous catalysts were carried
out (Table 2). These catalysts were synthesized by solvothermal
method, and characterized according to literature procedures
[74,75,90–94]. It was observed that several Cu-MOFs were found
to be active for the three-component coupling reaction. Among
these Cu-MOFs, Cu-MOF-74 offered the best performance.
Cu4I4(DABCO)2 could also be used as catalyst for the three-
component coupling reaction, producing 90% yield. The reaction
using Cu2(BDC)2(DABCO) or Cu3(BTC)2 as catalyst could proceed
to 88% yield, while 84% and 67% yields were detected for the case
of Cu(BDC) and Cu2(NDC)2(DABCO), respectively. All tested
Cu-MOFs contain open metal sites and possess similar surface
areas. High reactivity of Cu-MOF-74 could be rationalized by the
aperture size and the additional coordination of –OH group in Cu
center. In addition, the use of Cu-MF-74 is technically preferred
for the synthesis of other product derivatives which often have bulk
size. Besides, traditional Cu–ZnO, Cu Y Zeolite, and Cu-ZSM-5 were
showed to be inefficient (entries 8–10). As expected from results
100
80
60
40
20
0
0
0.5 0.75
1
3
5
7
Catalyst concentration (%)
Fig. 6. Effect of catalyst concentration on the reaction yield.

172
G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
Table 2
75
Reaction using other heterogeneous catalysts.
(a)
(b)
Entry Type
Catalyst
Reported
aperture
size (Å)
BET
GC
yield
(%)
65
55
45
35
25
15
5
Surface
area
(m2/g)
1
2
3
MOFs
Cu-MOF-74
Cu4I4(DABCO)2
Cu2(BDC)2(DABCO) 7.5
10
–
1101
–
99
90
88
1240
[82]
1250
[83]
625
[84,85]
–
4
5
Cu3(BTC)2
Cu(BDC)
8.0
5.6
–
88
84
6
7
Cu2(NDC)2(DABCO)
CuFe2O4
67
83
4000 3500 3000 2500 2000 1500 1000 500
Other
Wavenumber (cm-1)
heterogeneous
catalysts
8
9
10
11
Cu–ZnO
25
2
2
Fig. 10. FT-IR spectra of the fresh (a) and reused (b) Cu-MOF-74 catalyst.
Cu–Y–Zeolite
Cu-ZSM-5
Ni-MOF-74
from Table 1, Zn-MOF, Ni-MOF, Fe-MOF, In-MOF, and Zr-MOF
exhibited poor activity for the transformation. Mechanistically,
studies using Cu-based homogeneous catalysts proposed the neces-
sity of Cu redox center under organometallic viewpoints. These are
in good agreement with previous investigation reporting that the
saturation of MOFs structure and their oxidation state can be chan-
ged during reaction course and maintain the same after completion
of reactions [95–97]. In addition, the paddle wheel structure of Cu-
MOF-74 is well known to retarding the homocoupling of terminal
alkynes which is one of the prominent side reactions in this study
[98,99,64].
10.3
–
1126
[86]
–
1117
[87]
(1187)
[88]
496
3
12
13
Fe(BDC)
In3O(obb)3(HCO2)
(H2O)
3
3
14
15
Zr4O4(OH)4(BDC)6
–
–
–
Zn-MOF-74
11
[89]
Coupling reaction was carried out in n-butanol at 100 °C under argon for 5 h, using
2-pyridincarboxaldehyde:piperidine:phenylacetylene molar ratio of 2.5:1:1.5, at
piperidine concentration of 0.67 M, in the presence of 1 mol% catalyst.
Although several homogeneous catalysts exhibited high activ-
ity, these catalysts could not be recovered and reused. To empha-
size the significant point of the Cu-MOF-74 catalyst in this
transformation, one issue that should be addressed is the ease of
separation as well as the deactivation and reusability of this Cu-
MOF catalyst. The Cu-MOF-74 was therefore investigated for
recoverability and reusability over 7 successive runs, by repeatedly
separating the Cu-MOF catalyst from the reaction mixture, wash-
ing it and then reusing it (Fig. 8). The three-component coupling
reaction was carried out in n-butanol at 100 °C under argon for
5 h, using 2-pyridincarboxaldehyde:piperidine:phenylacetylene
molar ratio of 2.5:1:1.5, at piperidine concentration of 0.67 M, in
the presence of 1 mol% Cu-MOF-74 catalyst. Upon completion of
the first run, the Cu-MOF catalyst was separated from the reaction
mixture by simple centrifugation, washed with copious amounts of
DMF and methanol, and dried 150 °C under vacuum in 2 h. The
recovered Cu-MOF-74 catalyst was then reused as catalyst in fur-
ther transformation under identical conditions to those of the first
run. Experimental results indicated that the Cu-MOF-74 catalyst
could be recovered and reused many times without a significant
degradation in catalytic activity. Indeed, the reaction still afforded
96% yield of (A) in the 7th run. Furthermore, the structure of the
Cu-MOF-74 catalyst could be maintained during the course of the
transformation, as confirmed by XRD (Fig. 9) and FT-IR (Fig. 10)
results of the recovered Cu-MOF-74.
The study was then extended to the synthesis of several indoli-
zines using different aldehydes, amines, and alkynes. The reaction
was carried out under optimized conditions. The desired indolizine
was isolated and purified by column chromatography. It was found
that (A) was achieved with an isolated yield of 91%. The presence of a
substituent on the benzene ring in phenylaceylene led to a drop in
the reaction yield. 3-(4-Methoxyphenyl)-1-(piperidin-1-yl)indoli
zine was produced in an isolated yield of 78%, while 82% yield of
1-(piperidin-1-yl)-3-p-tolylindolizine was obtained for the case of
4-methylphenylacetylene. Changing the alkyne to 4-trifluorome
thylphenylacetylene, the reaction afforded 61% yield of
100
80
60
40
20
0
1
2
3
4
5
6
7
Run time
Fig. 8. Catalyst recycling studies.
5000
4000
3000
2000
1000
0
(b)
(a)
5
10
15
20
25
30
2-Theta scale
Fig. 9. X-ray powder diffractograms of the fresh (a) and reused (b) Cu-MOF-74
1-(piperidin-1-yl)-3-(4-(trifluoromethyl)phenyl)indolizine.
The
catalyst.

G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
173
Table 3
Three-component synthesis of indolizines using Cu-MOF-74 catalyst.
Entry
1
Aldehydes
Amines
Alkynes
Indolizines
Isolated yields (%)
91
2
78
3
82
4
61
5
88
(continued on next page)

174
G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
Table 3 (continued)
Entry
6
Aldehydes
Amines
Alkynes
Indolizines
Isolated yields (%)
89
7
8
9
90
90
86
three-component coupling reaction using 4-bromophenylacetylene
produced 3-(4-bromophenyl)-1-(piperidin-1-yl)indolizine in
several techniques including XRD, SEM, TEM, FT-IR, TGA, AAS,
and nitrogen physisorption measurements. The Cu-MOF could be
used as an efficient heterogeneous catalyst for the synthesis of
indolizines through aldehyde-amine-alkyne three-component cou-
pling transformation. High yields to indolizines were achieved in
the presence of a catalytic amount of the Cu-MOF (1 mol%). The
formation of indolizines was not favored by air or by water residue
in the solvent. The Cu-MOF-74 offered higher catalytic activity in
the three-component coupling reaction than those of other Cu-
MOFs such as Cu2(BDC)2(DABCO), Cu3(BTC)2, Cu(BDC), Cu2(NDC)2
(DABCO), and Cu4I4(DABCO)2. MOFs containing other metal sites
such as Ni-MOF-74, Zn-MOF-74, Fe3O(BDC)3, In3O(obb)3(HCO2)
(H2O), and Zr6O4(OH)4(BDC)6 exhibited almost no catalytic activity
for the synthesis of indolizines. The aldehyde–amine–alkyne
three-component coupling reaction could only proceed to produce
a
yield of 88%. Using 1-octyne instead of phenylacetylene, the three-
component coupling reaction of this alkyne with 2-pyridincarbo
xaldehyde and piperidine could proceed to 89% yield of 3-hexyl-1-
(piperidin-1-yl)indolizine. Furthermore, using the same protocol, 5
-methyl-3-phenyl-1-(piperidin-1-yl)indolizine, 3-cyclohexyl-1-(pi
peridin-1-yl)indolizine, and 1-(4-methylpiperidin-1-yl)-3-phenylin
dolizine were produced in a yield of 90%, 90%, and 86%, respectively
(Table 3).
4. Conclusions
In summary, the metal-organic framework Cu-MOF-74 was
synthesized by a solvothermal method, and was characterized by

G.H. Dang et al. / Journal of Catalysis 337 (2016) 167–176
175
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6011–6061.
indolizines in the presence of the solid Cu-MOF catalyst, and no
contribution from active copper species leached from the solid
Cu-MOF-74, if any, was detected. The Cu-MOF catalyst could be
separated from the reaction mixture by centrifugation, and could
be recovered and reused several times for the synthesis of indoli-
zines without a significant degradation in catalytic activity. The
fact that indolizines could be produced via aldehyde–amine–alky
ne three-component coupling transformation using a recyclable
heterogeneous catalyst should be of significant advantages.
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