Erbium-Organic Framework as Heterogeneous Lewis Acid Catalysis for Hantzsch Coupling and Tetrahydro-4H-Chromene Synthesis

Article abstract of DOI:10.1007/s10562-016-1913-4

Abstract: An Erbium-organic framework was prepared by hydrothermal reaction. The prepared framework was characterized by Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), and X-ray powder diffraction (XRD). The framework has open metal sites at Er(III) centers, thus providing an accessible Lewis acid center for electrophile activation. Accordingly, the synthesized framework was used as Lewis acid heterogeneous catalyst for Hantzsch coupling reaction and tetrahydro-4H-chromene synthesis. The reaction condition has been optimized by variation of the reaction time, temperature, solvent and catalyst concentration. A variety of tetrahydro-4H-chromenes was synthesized and characterized by FT-IR and¹H NMR spectroscopy. Er-MOF, as a Lewis acid heterogeneous catalyst, showed excellent selectivity and high yield for these transformations. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1913-4

Catal Lett
DOI 10.1007/s10562-016-1913-4
Erbium-Organic Framework as Heterogeneous Lewis Acid
Catalysis for Hantzsch Coupling and Tetrahydro-4H-Chromene
Synthesis
Taraneh Hajiashrai1 · Meghdad Karimi2 · Akbar Heydari2 ·
Alireza Azhdari Tehrani2
Received: 1 September 2016 / Accepted: 7 November 2016
© Springer Science+Business Media New York 2016
Abstract An Erbium-organic framework was prepared
by hydrothermal reaction. The prepared framework was
characterized by Fourier transform infrared spectroscopy
(FT-IR), Scanning electron microscopy (SEM), and X-ray
powder difraction (XRD). The framework has open metal
sites at Er(III) centers, thus providing an accessible Lewis
acid center for electrophile activation. Accordingly, the
synthesized framework was used as Lewis acid heterogene-
ous catalyst for Hantzsch coupling reaction and tetrahydro-
4H-chromene synthesis. The reaction condition has been
optimized by variation of the reaction time, temperature,
solvent and catalyst concentration. A variety of tetrahydro-
4H-chromenes was synthesized and characterized by FT-IR
Keywords Heterogeneous catalysis · Catalysis ·
Metal–organic framework · Erbium · Hantzsch coupling ·
Chromene
1 Introduction
Metal–organic frameworks (MOFs) are a class of porous
hybrid inorganic–organic materials whose structure is
made up of extended three-dimensional networks of inor-
ganic metal centers connected via bridging organic link-
ers [1]. The major advantage of MOFs over other porous
materials is the ability to tune their topology, pore size
and functionality by judicious selection of the organic
linkers and inorganic metal centers [2]. Due to these fea-
tures, these materials are considered to be potential can-
didates for a wide range of applications in many areas,
including gas storage, molecular sensing and catalysis
[3–7]. The use of MOFs as heterogeneous catalysts in
the liquid phase is under intense investigation, the tar-
get being to develop industrial processes based on these
materials [8, 9]. According to the catalytically active
sites, these frameworks may be categorized into four
distinct groups, namely metal–organic frameworks with
coordinatively unsaturated metal sites (group I), MOFs
with metalloligands (group II), MOFs with functional
organic sites (group III) and metal nanoparticles embed-
ded in the MOF cavities (group IV) [9]. Among these,
particularly potent candidates for catalysis are MOFs with
coordinatively unsaturated metal sites that may be created
by evacuation of frameworks that have metal-bound sol-
vent molecules. This series of MOFs display heterogene-
ous Lewis acidity, due to the coordinatively unsaturated
metal sites which can activate the organic molecules via
metal–organic interactions in catalysis. So far, numbers
1
and H NMR spectroscopy. Er-MOF, as a Lewis acid het-
erogeneous catalyst, showed excellent selectivity and high
yield for these transformations.
Graphical Abstract
* Taraneh Hajiashrai
t.hajiashrai@alzahra.ac.ir
1
Department of Chemistry, Faculty of Physics and Chemistry,
Alzahra University, PO Box 1993891176, Tehran, Iran
2
Chemistry Department, Tarbiat Modares University, PO
Box 14155-4838, Tehran, Iran
Vol.:(0123456789)
1 3

T. Hajiashrai et al.
of stable porous MOFs with open metal sites (such as
Cu-HKUST-1, Cr-MIL-101, Mg-MOF-74 etc.) have been
reported, which demonstrated the potential for catalytic
activities [10]. Among these, MOFs with open lanthanide
(Ln) sites have attracted less attention compared to the
other open metal site MOFs, especially those with cobalt,
copper, zinc and cadmium [11]. Although Ln-MOFs are
expected to be potential heterogeneous Lewis acid cata-
lysts, only a few reactions catalyzed by Ln-MOFs were
reported [12–16].
ZnO-Zeolite [28], organocatalysts [29] and ionic liquids
[30] through the three-component condensation of an
aldehyde, dimedone and malononitrile have been devel-
oped for the synthesis of tetrahydro-4H-chromenes.
However, many of these methods sufered from certain
drawbacks such as low yields of products, long reaction
times, high temperature, and sometimes tedious work-up
with using of toxic reagents or solvents [27, 28, 30]. So it
is still necessary to develop and explore clean, safe, appli-
cable, high-yielding and mild protocols for the synthesis of
tetrahydro-4H-chromenes. In this regard, we are interested
in exploring the potential of the synthesized Er-MOF as
catalysts for these organic transformations. The framework
has open metal sites at Er(III) centers, thus providing an
accessible Lewis acid center for electrophile activation.
1,4-Dihydropyridine compounds are important
because of their diverse pharmacological and therapeu-
tic properties, such as vasodilatory, bronchodilatory,
geroprotective, hepatoprotective, antitumor, antiathero-
sclerotic, antidiabetic, neuroprotective, and neuropeptide
YY1 antagonist activities, Fig. 1.
Benzopyran and its tetrahydro-4H-chromene deriva-
tives are also important scafolds in medicinal chemistry
[17–20], Fig. 2, due to their wide range of pharmaco-
logical efects including anti-cancer [21], anti-microbial
[22, 23], anti-oxidant [24, 25] and for the treatment of
neurodegenerative diseases, including Huntington’s dis-
ease, Alzheimer’s disease [26], etc. Thus, the synthesis
of tetrahydro-4H-chromenes has attracted the attention
of chemists, and diferent synthetic methods have been
developed to prepare tetrahydro-4H-chromenes. The
typical procedure to synthesize chromene derivatives is
usually a two-step reaction which takes place between
a Michael acceptor and β-dicarbonyl compounds in the
presence of a catalyst. Several catalytic systems includ-
ing Amberlite IRA-40 [27], H6P2W12O62·H2O [27],
2 Experimental Section
2.1 Apparatus and Reagents
Chemicals were purchased from Merck and Aldrich chemi-
cal companies. IR spectra were recorded using Thermo
Nicolet IR 100 FT-IR. The thermal behavior was meas-
ured with a PL-STA 1500 apparatus with the rate of 10°C
min−1 in a static atmosphere of argon. X-ray powder dif-
fraction (XRD) measurements were performed using a
Philips X’pert difractometer with mono chromated Cu-Kα
radiation. The samples were characterized by ield emission
scanning electron microscopy (FE-SEM) KYKY-EM 3200
with gold coating. NMR spectra were taken with a Bruker
400 MHz Ultrashield spectrometer at 400 MHz (1H) and
125 MHz (13C) using CDCl3 or DMSO-d6 as the solvent
with TMS as the internal standard.
NO2
CO2Et
Table 1 Optimization of the three-component Hantzsch coupling
EtO2C
CO2Et
EtO2C
CO2Et
Entry
(mg)
Solvent
Temp. (°C)
Yield (%)
N
H
N
H
1
10
10
10
10
10
10
10
10
20
40
80
–
CH3CN
H2O
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
50
70
70
70
70
70
70
70
35
25
40
35
N.D.
40
45
55
75
92
93
30
80
25
2
Nifedipine
Lacidipine
3
CHCl3
Toluene
n-Hexane
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
–
4
Fig. 1 Examples of biologically active 1,4-dihydropyridyl com-
pounds
5
6
7
O
O
8
N
N
N
9
10
11
12
13
14
O
NH2
O
NH2
O
UCPH-102F
UCPH-102
O
40
10
Fig. 2 Examples of biologically active tetrahydro-4H-chromene
derivatives
Optimized reaction condition is shown in bold
1 3

Erbium-Organic Framework as Heterogeneous Lewis Acid Catalysis for Hantzsch Coupling and…
Table 2 The Er-MOF-catalyzed three-component Hantzsch coupling
R
O
O
O
O
O
Cat. ( ML4)
+
+
NH4OAc
O
O
H
O
Solvent, temp.
4h
N
H
R
NO2
O
EtOOC
COOEt
MeOOC
COOMe
MeOOC
COOMe EtOOC
COOEt
N
N
N
H
N
H
4a
H
H
2a
3a
95 %
m.p. = 168 °C
1a
85 %
m.p. = 159 °C
83%
m.p. = 164 °C
78 %
m.p. = 175 °C
Cl
Cl
O
NO2
Cl
COOMe MeOOC
MeOOC
COOMe
MeOOC
COOMe
MeOOC
COOMe
N
H
N
H
N
H
N
H
8a
70 %
m.p. = 188 °C
7a
90 %
m.p. = 210 °C
5a
80 %
m.p. = 198 °C
6a
75 %
m.p. = 191°C
Br
MeOOC
COOMe
MeOOC
COOMe
N
H
N
H
10a
85 %
m.p. = 185 °C
9a
90 %
m.p. = 202 °C
2.2 Preparation of Er-MOF
H, 2.71; N, 3.26. Found: C, 31.22; H, 2.62; N, 3.30. IR
(cm−1): 1611 m, 1572 w, 1536 w, 1433 m, 1372 s, 1103 w,
939 w, 770 s, 714 m, 702 vs. 666 m, 563 s, 455 s, 434 m,
417 w.
A mixture of Er(NO3)3·6H2O (0.5 mmol), benzene-1,3,5-
tricarboxylic acid (0.25 mmol), DMF (4 mL), and H2O
(4 mL) was sealed in a 20 mL of Telon-lined reactor. The
pure colorless needle crystals of Er(BTC)(H2O).(DMF)1.1
with yield of 45% were obtained after 24 h of heating at
105°C. Anal. Calcd for Er(BTC)(H2O).(DMF)1.1: C, 31.25;
For the purpose of activation, according to the TGA
pattern analysis, when the Er-MOF was heated to 330°C
for 4 h, the coordinating solvent molecules began to be
removed, and the compound retained its stability up to
1 3

T. Hajiashrai et al.
Table 3 Optimization of the synthesis of tetrahydro-4H-chromenes
(s, 6H), 4.15 (q, 4H), 5.03 (s, 1H), 5.72 (s, 1H), 7.26 (d,
2H), 7.32 (d, 2H). 13C NMR: 16.1, 22.1, 41.2, 59.1, 106.1,
132.1, 129.1, 132.0, 140.2, 147.1, 166.2.
Entry
(mg)
Solvent
Temp. (°C)
Yield (%)
1
10
10
10
10
10
10
10
10
20
40
80
–
CH3CN
H2O
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
50
70
70
70
70
70
70
70
30
20
20
<5
N.D.
30
40
50
70
95
95
15
75
45
2
2.3.2 Dimethyl 4-Cyclohexyl-2,6-Dimethyl-1,4-Dihydro-
pyridine-3,5-Dicarboxylate (10a)
3
CHCl3
Toluene
n-Hexane
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
–
4
IR (KBr, νmax/cm−1): 3306, 2944, 1656, 1495, 1435,
1216, 1098, 667; 1H NMR (300 MHz, DMSO-d6): δ
0.90–0.97(m, 2H), 1.07–1.25 (m, 3H), 1.53–1.70 (m,
6H), 2.33 (s, 6H), 3.75 (s, 6H), 3.92 (d, 1H), 5.71 (s, 1H);
13C NMR: 19.5, 26.6, 26.6, 28.6, 38.3, 45.6, 50.9, 101.6,
144.9, 169.2.
5
6
7
8
9
10
11
12
13
14
2.4 General Procedure for the Preparation
of Tetrahydro-4H-Chromenes
40
10
Optimized reaction condition is shown in bold
To a mixture of arylaldehyde (1 mmol) and malononi-
trile (1 mmol) and dimedone (1 mmol) was added a cata-
lytic amount of Er-MOF (40 mg) at 70 °C in ethanol,
Table 3. After completion of the reaction (8 h, monitored
by TLC), the product was dissolved in CH2Cl2 and the
catalyst was removed by centrifugation for 10 min at
6000 rpm. After drying, the precipitate was re-crystal-
lized in ethanol, Table 4. The catalyst was washed with
CH2Cl2 and dried to reuse in the next run. The catalyst
could be recycled four times without measurable loss of
its activity.
temperatures of 450°C. Anal. Calcd for Er(BTC): C, 28.87;
H, 0.81. Found: C, 28.78; H, 0.93. IR (cm−1): 1611 m,
1572 w, 1536 w, 1433 m, 1372 s, 1103 w, 939 w, 770 s,
714 m, 702 vs. 666 m, 563 s, 455 s, 434 m, 417 w.
2.3 General Procedure for the Preparation
of 1,4-Dihydropyridines
Ammonium acetate (1 mmol), was added to the mixture of
benzaldehyde (1 equiv), and ethyl acetoacetate (1.1 equiv)
in Ethanol (1.5 mL) along with an appropriate amount of
Er-MOF as catalyst (20 mg). The mixture was stirred at
70°C for 4 h. After completion of the reaction, the cata-
lyst was removed by centrifuge. Then brine (5 mL) and
EtOAc (5 mL) was added. The mixture was extracted with
EtOAc and the combined organic phase were washed with
saturated aqueous solution of NaHCO3 (10 mL), and brine
(10 mL), dried over anhydrous Na2SO4 and concentrated
under reduced pressure. After removal of the solvent
under vacuum, the crude product was puriied by column
chromatography to aford the desired product, Table 1. It
is found that the Er-MOF catalyst can be recovered and
reused ive times without considerable loss of catalytic
activity.
1
Selected FT-IR and H NMR for relative compounds
are summarized below. Melting points of the products are
listed in Table 4.
2.4.1 2-Amino-3-Cyano-7,7-Dimethyl-4-Phe-
nyl-5-Oxo-4H-5,6,7,8-Tetrahydrobenzopyran (2a)
FT-IR (KBr, νmax, cm−1): 3393, 3322, 3206, 2961,
1
2197, 1668, 1602, 1368, 1213, 1034, 697, 499; H NMR
(300 MHz, DMSOd6): δ 7.25 (2H, ArH), 7.21–7.17 (m,
1H, ArH), 7.15 (2H, ArH), 7.01 (s, 2H, NH2), 4.18 (s, 1H),
2.48 (s, 2H), 2.22–2.09 (d, 2H), 1.02 (s, 3H), 0.95 (s, 3H).
2.4.2 2-Amino-4-(4-Chlorophenyl)-3-Cyano-7,7-Dime-
thyl-5-Oxo-4H-5,6,7,8-Tetrahydrobenzopyran (2b)
1
Selected FT-IR, H NMR and 13C NMR for relative
compounds are summarized below. Melting points of the
products are listed in Table 2.
FT-IR (KBr, νmax, cm−1): 3539, 3467, 3203, 2961,
1
2194, 1670, 1601, 1367, 1213, 1037, 710, 501; H NMR
(300 MHz, DMSOd6): δ 7.36 (d, 2H, ArH), 7.12 (d, 2H,
ArH), 7.00 (s, 2H, NH2), 4.21 (s, 1H), 2.50 (s, 2H), 2.20 (d,
1H), 1.00 (s, 3H), 0.91 (s, 3H).
2.3.1 Diethyl 4-(4-Chlorophenyl)-2,6-Dimethyl-1,4-Dihy-
dropyridine-3,5-Dicarboxylate (5a)
IR (KBr, νmax/cm−1): 3423, 1718, 1606, 1527, 1267, 1094,
1
669. H NMR (300 MHz, DMSO-d6): δ 1.20 (d, 6H), 2.35
1 3

Erbium-Organic Framework as Heterogeneous Lewis Acid Catalysis for Hantzsch Coupling and…
Table 4 The Er-MOF-catalyzed three-component synthesis of tetrahydro-4H-chromenes
R
O
O
CN
O
O
Cat. ( La- MOF)
+
CN
+
H
NC
EtOH, 50°C
6h
O
NH2
R
N
Cl
Cl
O
O
O
Cl
O
CN
CN
CN
CN
O
NH2
O
NH2
O
NH2
O
NH2
2c
75%
2a
95%
2b
85%
2d
75%
m.p. = 214 °C
m.p. = 229 °C
m.p. = 218 °C
m.p. = 234 °C
OH
NO2
NO2
O
O
O
O
O
CN
CN
CN
CN
O
NH2
O
NH2
O
NH2
O
NH2
2g
70%
m.p. = 215 °C
2h
70%
yellow oil
2e
90%
m.p. = 178 °C
2f
80%
m.p. = 210 °C
CN
O
CN
O
NH2
2i
90%
m.p. = 225 °C
1 3

T. Hajiashrai et al.
1H, ArH), 7.10 (s, 2H, NH2), 4.25 (s, 1H), 2.47 (d, 2H),
2.22–2.12 (d, 2H), 1.03 (s, 3H), 0.96 (s, 3H).
3 Result and Discussion
3.1 Synthesis and Characterization
According to the procedure described by Xu and his co-
workers [15], reaction between Erbium(III) nitrate hexa-
hydrate and 1,3,5-benzenetricarboxylate (BTC) in mixed
solvents of DMF and aqua inside a Telon-lined autoclave
at 105°C for 24 h yielded Er(BTC)(H2O).(DMF)1.1, Fig. 3,
which has been characterized by powder X-ray difrac-
tion (PXRD), Fig. 4, and FT-IR spectroscopy. The Er(III)
coordination environment can be considered as a distorted
pentagonal-bipyramidal geometry formed by six oxygen
atoms from six carboxylate groups of six BTC ligands
and one oxygen atom from the terminal water molecule.
The open erbium sites are generated during the activation
process (330°C for 4 h) as result of the removal of coor-
dinated water and N,N-dimethyl formamide molecules,
trapped in pores, that remain after the MOF synthesis.
The activation of this framework was conirmed by FT-IR
spectroscopy, elemental analysis, and powder X-ray dif-
fraction (PXRD) analysis. Also, the BET surface area of
the activated Er-MOF is determined to 775 m2/g, which
is comparable to the reported surface area. The high ther-
mal stability, accessible open metal sites and nanosized
Fig. 3 Representation of the pores of erbium metal–organic frame-
work
2.4.3 2-Amino-3-Cyano-4-(3,4-Dichlorophenyl)-7,7-Dime-
thyl-5-Oxo-4H-5,6,7,8-Tetrahydrobenzopyran (2c)
FT-IR (KBr, νmax, cm−1): 3335, 3165, 2960, 2189, 1658,
1
1602, 1368, 1247, 1031, 697, 574; H NMR (300 MHz,
DMSO-d6): δ 7.57 (1H, ArH), 7.39 (1H, ArH), 7.12 (m,
Fig. 4 PXRD patterns of simu-
lated, as-synthesized and after
catalytic reaction of Er-MOF
1 3

Erbium-Organic Framework as Heterogeneous Lewis Acid Catalysis for Hantzsch Coupling and…
Fig. 5 SEM Analysis of the Er-MOF particles before (a) and after the DHQ (b) and DHP (c) reaction
aperture for the Er-MOF could endow it with a very high
potential in catalysis.
necessary for performing reaction in the presence of a het-
erogeneous catalyst, Fig. 5.
The morphology and size of Er-MOF prepared by the
solvotermal method were characterized by ield emission
scanning electron microscopy (FE-SEM) before and after
reactions. The retention of rodlike morphology of the as-
synthesized crystals of catalysts during the two conden-
sation reactions suggests considerable stability, which is
3.2 Catalytic Performances
The reaction of the formation of dihydropyridines is
afected by the amount of the catalyst and also the reac-
tion temperature. Our results show that the temperature can
1 3

T. Hajiashrai et al.
Fig. 6 Proposed mechanisms
for the synthesis of dihydro-
pyridine derivatives (a) and
tetrahydro-4H-chromenes (b)
catalyzed by Er-MOF
afect the yield of the formation of dihydropyridines. Nota-
bly, the yield of the formation of dihydropyridine increases
from 40 to 55%, when the reaction temperature is increased
to 70°C from room temperature. We found that 70°C is
the optimum reaction temperature (Table 1). Increasing
the reaction temperature does not improve the yield of the
formation, so temperatures higher than 70°C are inefec-
tive for this transformation. The reaction was carried out
in the presence of 10, 20, 40 and 80 mg of catalyst at the
same conditions, separately, in which resulted a serious
improvement of the capitulate (Table 1). Diferent solvents
were also checked for this reaction. Ethanol improved the
conversion yield of the substrate compared to CH3OH,
CH3CN, H2O, CHCl3, Toluene and n-Hexane. It can be
deduced from the results that higher yields were achieved
in polar protic solvents, whereas the reaction occurred
with diiculty in solvents with lower polarity. The reac-
tions under solvent-free condition were slow. It should be
pointed out that in the absence of catalyst, the reaction was
slow and even after a prolonged reaction time, considerable
amounts of starting materials remained unreacted, Table 1.
In order to optimize the reaction condition for the syn-
thesis of chromenes, diferent solvents were tested, Table 3.
Ethanol selected as the suitable solvent for reaction
1 3

Erbium-Organic Framework as Heterogeneous Lewis Acid Catalysis for Hantzsch Coupling and…
100
catalyst could be reused for ive consecutive cycles with-
DHP
PHQ
out noticeable loss in activity, Fig. 7. Further investigation
of the heterogeneous character of the catalytic systems as
well as stability of the structure was carried out using a hot
iltration test. After 4 h of the reaction, the reaction mix-
ture was centrifuged and the catalyst was iltered of. Then,
the supernatant was left stirring at 60°C. Interestingly,
within 8 h of further reaction time, no distinguishable
changes were recognized in the reaction conversion. Also,
the PXRD pattern of Er-MOF indicates that the framework
remains intact after the catalytic reaction, as supported
by the good agreement between the FT-IR measurements
before and after the reaction.
95
90
85
80
75
70
1
2
3
4
5
Run
Fig. 7 Activity lost as a function of the number of reused times of
the Er-MOF for the synthesis of DHP and tetrahydro-4H-chromenes
according to the yield of the reaction in room tempera-
ture (30%). Same as previous, polarity of the solvent has
a profound efect on the yield of formation of chromenes.
In the absence of catalyst, 15% of product was found and
if Er-MOF is used as catalyst, the formation is accelerated,
Table 3. The efect of temperature was also checked on the
4 Conclusion
Metal–organic frameworks (MOFs) can act as heterogene-
ous catalysts for a variety of organic reactions, especially
for liquid-phase reactions. The catalytic applications of
MOFs are often limited by their chemical, hydrolytic and
thermal stabilities. Metal–organic frameworks based on
lanthanides are a very promising class of materials for
addressing the challenges in heterogeneous Lewis acid
catalysis. In summary, herein, Er-MOF with high chemi-
cal and thermal stability was successfully prepared by the
reported procedure and was used as heterogeneous catalyst
for Hantzsch coupling and tetrahydro-4H-chromene synthe-
sis. The framework has open metal sites at Er(III) centers,
thus providing an accessible Lewis acid center for electro-
phile activation. Accordingly, the synthesized framework
was used as Lewis acid heterogeneous catalyst for Hantzsch
coupling reaction and tetrahydro-4H-chromene synthesis.
We have disclosed a new catalyst system which proves to
be very efective not only in reducing the reaction time,
but also in increasing the yield of the product. It involves
simple procedure, is environment friendly and also incor-
porates special features like reagent economy, easy workup
and easy handling.
reaction. With increasing temperature to 70 ̊C, the yield of
the reaction improved to 50%. In the next step, efects of
varying the amount of catalyst were studied. It was found
that 40 mg of Er-MOF was enough to proceed the reac-
tion signiicantly and negligible changes in the yield of the
products were observed with higher catalyst concentra-
tions. The results show that the reaction hardly proceeded
in the absence of Er-MOF, suggesting the potential cata-
lytic activity of the Er-MOF in this reaction (Table 3). Thus
the optimal reaction conditions were considered to include
the arylaldehyde (1 mmol), malononitrile (1 mmol) and
dimedone (1 mmol) at 70°C for 8 h in ethanol (Table 1).
Proposed mechanisms for the synthesis of dihydropyridine
derivatives and tetrahydro-4H-chromenes, catalyzed by Er-
MOF, are shown in Fig. 6. It is assumed that Er(III) species
acts as a Lewis acid to activate the carbonyl group of ethyl
acetoacetate in Hantzsch coupling and dimedone in tetrahy-
dro-4H-chromene synthesis by accepting the lone electron
pair on the oxygen atom [31, 32].
Acknowledgements We gratefully acknowledge inancial support
from the Research Council of Alzahra University and from the Iran
National Science Foundation (INSF). Dedicated to the memory of
Fatemeh Ghazvini.
3.3 Catalyst Reusability
From an industrial point of view, the reusability of the cata-
lyst is important for the large-scale operation. Therefore,
the reusability of the catalyst was examined in the reac-
tion of benzaldehyde, dimedone and/or ethylacetoacetate
and ammonium acetate. The catalyst can be separated from
the reaction medium by centrifuging; therefore it could be
recovered simply by centrifugation followed by washing
with ethanol, drying at 50°C. Afterwards, the catalyst can
be weighed and used directly in the next cycle of the reac-
tion using the fresh substrates. The results showed that the
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