An approach to the synthesis and characterization of HMS/Pr-Rh-Zr as efficient catalyst for synthesis of tetrahydrobenzo[b]pyran and 1,4-dihydropyrano[2,3-c]pyrazole derivatives

Article abstract of DOI:10.1007/s11164-021-04411-z

The aim of present investigation is the synthesis of mesoporous catalyst based on hexagonal mesoporous silica. HMS support has exceptional properties such as easy synthesis, high surface area, large pore volume and wormhole pores. These distinctive charac

Full text of DOI:10.1007/s11164-021-04411-z

Research on Chemical Intermediates (2021) 47:1883–1904
https://doi.org/10.1007/s11164-021-04411-z
An approach to the synthesis and characterization
of HMS/Pr‑Rh‑Zr as efcient catalyst for synthesis
of tetrahydrobenzo[b]pyran and 1,4‑dihydropyrano[2,3‑c]
pyrazole derivatives
Sahar Abdolahi1 · Maryam Hajjami2 · Fatemeh Gholamian1
Received: 14 October 2020 / Accepted: 30 January 2021 / Published online: 19 February 2021
© The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature 2021
Abstract
The aim of present investigation is the synthesis of mesoporous catalyst based on
hexagonal mesoporous silica. HMS support has exceptional properties such as easy
synthesis, high surface area, large pore volume and wormhole pores. These dis-
tinctive characteristics beseem use of it as a support for synthesis of heterogene-
ous catalyst. With functionalization of HMS and immobilization of Zr-rhodanine
complex into functionalized HMS, HMS/Pr-Rh-Zr was obtained. New synthesized
catalyst (HMS/Pr-Rh-Zr) characterized by FT-IR, XRD, TGA, SEM, EDX, adsorp-
tion–desorption of nitrogen at 77 K and ICP techniques. These techniques con-
frmed that the heterogeneous catalyst of HMS/Pr-Rh-Zr was successfully synthe-
sized. Also the catalytic activity of HMS/Pr-Rh-Zr is investigated for the synthesis
of tetrahydrobenzo[b]pyran derivatives from reaction between aldehyde, malononi-
trile, dimedone in PEG at 80 °C and synthesis of 1,4-dihydropyrano[2,3-c]pyrazole
derivatives from reaction between aldehyde, ethyl acetoacetate, malononitrile and
hydrazine hydrate in H2O:EtOH at 35 °C. These results confrmed the HMS/Pr-Rh-
Zr exhibited good catalytic performance. Furthermore, the synthesized heterogene-
ous catalyst could be recovered after the reaction and reused several times without
any noticeable loss in activity. Also zirconium leaching of HMS/Pr-Rh-Zr was stud-
ied whereupon metal leaching of the catalyst was very low.
* Maryam Hajjami
mhajjami@yahoo.com; m.hajjami@basu.ac.ir
1
Department of Chemistry, Faculty of Science, Ilam University, Ilam, PO Box 69315516, Iran
2
Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University,
Hamedan 6517838683, Iran
Vol.:(0123456789)
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Graphical Abstract
Keywords Mesoporous catalyst · Hexagonal mesoporous silica · HMS/pr-rh-zr ·
Tetrahydrobenzo[b]pyran · 1,4-dihydropyrano[2,3-c]pyrazole
Introduction
The most challenging subject in the contemporary chemistry and chemical indus-
try is the extension and advancement of eco-friendly technologies. In this light, the
removal or reduction of wastes by using of eco-friendly solvents and reagents and
efciently reusable catalysts are signifcant parameters to achieve more sustainable
approaches in agreement with green chemistry principles [1].
In porous solids the greater number of surface atoms can be lead to specifc sur-
face area whereupon trepan higher material’s reactivity consequently improved ef-
cacy in relevant applications. The porous material classifed into three groups such
as microporous (less than 2 nm), mesoporous (between 2 and 50 nm) and macropo-
rous (greater than 50 nm) [2]. Among various mesoporous materials, hexagonal
mesoporous silica (HMS) are a promising group of porous materials with distinctive
characteristics such as wormlike mesoporosity, large wall thickness [3], short chan-
nel, thermal stability, easily synthesis and functionalization [3, 4]. Owing to unique
structural property of mesoporous materials like uniform pore size distribution, high
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An approach to the synthesis and characterization of…
1885
surface area and large pore volume [3], these porous structures have been employed
and studied in a wide variety of diferent felds from support for heterogeneous cata-
lysts [3], adsorbents, drug delivery systems, biosensors [5], host for guest molecules
and advanced engineered materials [6]. The post-synthetic grafting strategy dem-
onstrates a simple route for immobilization of functional chelating ligands into the
pores, to obtain the organo-functionalized mesoporous silicas that could be acted as
catalyst [7].
Various research groups around the world are used multi-component reactions for
the formation heterocyclic compounds [8]. Many uses of heterocyclic compounds
such as benzopyran and pyrano[2,3-c]pyrazole have been recognized and are widely
used in medicinal chemistry and biosciences. With a general look at drugs including
antimicrobial [9, 10], anticancer [11, 12], anti-infammatory [13, 14] and antioxidant
[15, 16], it can be easily understood that these heterocyclic compounds display sig-
nifcant roles in medicinal.
In the continuation of our earlier work [4, 17–20] and based on the above descrip-
tions, we sought to synthesized new catalyst-based HMS. For this light, the support
of HMS was synthesized and functionalized with (3-chloropropyl)trimethoxysilane
(CPTMS) to aford HMS/CPTMS. Then, reaction of HMS/CPTMS with rhoda-
nine and ZrOCl2.8H2O leads to synthesis of HMS/Pr-Rh-Zr. The infuences of new
mesoporous catalyst were investigated for the synthesis of tetrahydrobenzo[b]pyran
by one-pot multi-component reactions of aldehyde, malononitrile, dimedone in PEG
at 80 °C and synthesis of 1,4-dihydropyrano[2,3-c]pyrazole from reaction between
aldehyde, ethyl acetoacetate, malononitrile and hydrazine hydrate in H2O:EtOH at
35 °C.
Experimental
Materials and physical measurements
All reagents and solvents were purchased from Aldrich and Merck chemical com-
panies. The FT-IR spectra were conducted as KBr pellets by FT-IR, VERTEX 70,
Bruker, Germany, spectroscopy. The analysis of X-ray difraction was performed
using XRD, X’Pert PRO MPD, PANalytical, Netherland, Kα=1.54. The instru-
ments’ thermogravimetric analysis (TGA) applied was performed from room tem-
perature to 800 °C by TGA, NETZSCH, Germany. The used gas for the thermo-
gravimetric analysis was Argon. The content of Zr was determined by inductively
coupled plasma optical emission spectrometry (ICP-OES, Arcos EOP, company of
Spectro, Germany). Scanning electron microscopy (SEM) images were recorded
on FE-SEM, TESCAN MIRA III, Czech. Elemental analysis was carried out on
EDX, FE-SEM, TESCAN MIRA, SAMX, Czech. The nitrogen adsorption–desorp-
tion isotherm was performed with a BET, Micromeritics, Asap2020, USA at 77 K.
The degassing temperature and treatment time in this analysis were 120 °C and 2 h,
respectively. 1H-NMR spectra of the DMSO-d6 solutions were obtained at 300 MHz
using TMS as an internal standard.
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S. Abdolahi et al.
Synthesis of HMS/Pr‑Rh‑Zr
Synthesis of HMS
HMS was prepared according to previous report in the literature [4]. In a typical
experiment, dodecylamine (5 g) was dissolved in 70% w/w ethanol aqueous solu-
tion. Then, tetraethyl orthosilicate (TEOS, 20.8 g) was added dropwise, stirred
for 5 h at room temperature under vigorous stirring. The mixture was aged for
18 h at room temperature and fltered and dried at room temperature. Finally, the
resultant solid was soxhelt extraction at 80 °C for 24 h. In order to the removing
the template, synthesized support was calcined at 500 °C in air for 5 h. Finally,
HMS was obtained.
Synthesis of HMS/CPTMS
Following a general procedure, HMS have been functionalized with 3‐chloropro-
pyltrimethoxysilane, in toluene, as follows: HMS (0.5 g) in toluene and 3‐chlo-
ropropyltrimethoxysilane (1.5 mL) was added dropwise, refuxed for 24 h, under
nitrogen atmosphere. Then, the resulting silica (HMS/CPTMS) was collected by
fltration, then washed with toluene and dried at room temperature.
Synthesis of HMS/Pr‑Rh
In this step, HMS/CPTMS (1 g) was dispersed in DMF for 30 min and reacted
with rhodanine (2 mmol) at 100 °C for 28 h to aford HMS/Pr-Rh. After that, the
resulting solid was fltered and washed with water and ethanol.
Synthesis of HMS/Pr‑Rh‑Zr
Finally, slightly catalyst (HMS/Pr-Rh-Zr) was synthesized by adding of 2.5 mmol
ZrOCl2·8H2O to 1 g of HMS/Pr-Rh in CH3CN at room temperature for 24 h. The
reaction mixture was fltered, washed several times with H2O and dried at room
temperature to overnight.
General procedure for the synthesis of tetrahydrobenzo[b]pyran
0.05 g of HMS/Pr-Rh-Zr was added to a mixture of aldehyde (1 mmol), dimedone
(1 mmol) and malononitrile (1 mmol) in PEG at 80 °C. Completion of the reac-
tion was monitored by TLC. After the completion of the reaction, ethyl acetate
was added and HMS/Pr-Rh-Zr was separated by fltration. Then, ethyl acetate
and H2O were added and extracted. The organic layer was dried with anhydrous
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1887
Na2SO4. After evaporation of solvent, desired compound was obtained then puri-
fed through recrystallization in EtOH.
General procedure for the synthesis of 1,4‑dihydropyrano[2,3‑c]pyrazole
A molar ratio mixture of aldehyde (1 mmol), hydrazine hydrate (1 mmol), ethyl
acetoacetate (1 mmol) and malononitrile (1 mmol) in the presence HMS/Pr-Rh-Zr
(0.01 g) as catalyst was reacted in H2O:EtOH (3:1 mL) at 35 °C. Completion of the
reaction was continuously observed by TLC. After the consumption of the starting
material, the catalyst was separated with fltration then hot EtOH was added. Finally,
recrystallization with EtOH was applied to aford the pure products.
Characterization data of selected compounds
2–Amino–3–cyano–7,7–dimethyl–4-(2-nitrophenyl)–5–oxo-4H–5,6,7,8-tetrahy-
1
dro benzopyran (Table 3, entry 5): H NMR (300 MHz, DMSO-d6): δ=0.87 (s,
3H), 1.00 (s, 3H), 2.00 (d, J=15 Hz, 1H), 2.19 (d, J=18 Hz, 1H), 2.41–2.56 (m,
2H), 4.93 (s, 1H), 7.17 (s, 2H), 7.33–7.44 (m, 2H), 7.65 (t, J=9 Hz, 1H), 7.80 (d,
J=9 Hz, 1H) ppm.
2–Amino–3–cyano–7,7–dimethyl–4-(4-methylphenyl)–5–oxo-4H–5,6,7,8-tetrahy-
1
dro benzopyran (Table 3, entry 7): H NMR (300 MHz, DMSO-d6): δ=0.94 (s,
3H), 1.02 (s, 3H), 2.07 (d, J=15 Hz, 1H), 2.21–2.26 (m, 3H), 2.43–2.55 (m, 3H),
4.11 (s, 1H), 6.94 (s, 2H), 7.00 (d, J=9 Hz, 2H), 7.07 (d, J=9 Hz, 2H) ppm.
4,4′-(1,4-phenylene)bis(2-amino-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-
4H-chromene-3-carbonitrile) (Table 3, entry 12): 1H NMR (300 MHz, DMSO-d6):
δ=0.98 (s, 6H), 1.02 (s, 6H), 2.03–2.25 (m, 4H), 2.56 (s, 4H), 4.13 (s, 2H), 6.94 (s,
4H), 7.02 (d, J=6 Hz, 4H) ppm.
6-Amino-4-(4-chlorophenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-car-
bonitrile (Table 5, entry 1): 1H NMR (300 MHz, DMSO-d6): δ=1.78 (s, 3H), 4.62
(s, 1H), 6.92 (s, 2H), 7.19 (d, J=9 Hz, 2H), 7.36 (d, J=9 Hz, 2H), 12.13 (s, 1H)
ppm.
6-Amino-3-methyl-4-(3-nitrophenyl)- 1,4-dihydropyrano[2,3-c]pyrazole-5-car-
bonitrile (Table 5, entry 3): 1H NMR (300 MHz, DMSO-d6): δ=1.80 (s, 3H), 4.87
(s, 1H), 7.04 (s, 2H), 7.61–7.68 (m, 2H), 8.01–8.13 (m, 2H), 12.20 (s, 1H) ppm.
4,4′-(1,4-Phenylene)bis(6-amino-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-
1
5-carbonitrile) (Table 5, entry 12): H NMR (300 MHz, DMSO-d6): δ=1.73 (s,
6H), 4.56 (s, 2H), 6.84 (s, 4H), 7.10 (d, J=3 Hz, 4H), 12.07 (s, 2H) ppm.
Results and discussion
Preparation of HMS/Pr‑Rh‑Zr
The synthetic route for the HMS/Pr-Rh-Zr is shown in Scheme 1. First, the silylat-
ing agent of (3-chloropropyl)trimethoxysilane (CPTMS) reacted with the hydroxyl
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S. Abdolahi et al.
Scheme 1 General procedure for synthesis of HMS/Pr-Rh-Zr
groups of HMS to obtain HMS/CPTMS. Then, rhodanine was applied to aford
HMS/Pr-Rh. Finally, ZrOCl2·8H2O was applied to obtain the HMS/Pr-Rh-Zr.
After fabrication of catalyst, for its characterization, diferent techniques such as
FT-IR spectroscopy, N2 adsorption–desorption, inductively coupled plasma (ICP-
OES), X‐ray difraction (XRD), thermogravimetric analysis (TGA), scanning elec-
tron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were
applied.
Characterization of HMS/Pr‑Rh‑Zr
For confrmation of functional groups of the synthesized compounds, the FT-IR
spectroscopy was applied. In Fig. 1, the FT-IR spectroscopy of HMS (a), HMS/
CPTMS (b), HMS/Pr-Rh (c) and HMS/Pr-Rh-Zr (d) indicated. FT-IR spectra of
HMS (a) demonstrated the peaks at 3435 cm−1 contributed to silanol group and
the peaks at 804 cm−1 and 1073 cm−1 are contributed to Si–O–Si symmetric and
asymmetric stretching vibration, respectively [4]. In spectrum of HMS/Pr (b), the
C–H stretching vibrations are appearing in the 2928 cm−1. As shown in spectrum
of HMS/Pr-Rh (c), the peak at 3429 cm−1 can be ascribed to the O–H stretching
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An approach to the synthesis and characterization of…
1889
Fig. 1 FT-IR spectrum of HMS (a), HMS/CPTMS (b), HMS/Pr-Rh (c) and HMS/Pr-Rh-Zr (d)
vibration, also stretching vibration of C=N appeared at 1659 cm−1. In additional,
the C–H stretching vibrations observed in 2926–2975 cm−1. In the spectrum of
HMS/Pr-Rh-Zr (d), due to the coordination with the zirconium, the bond of C=N,
shifts from 1659 cm−1 to 1636 cm−1 and the O–H stretching vibration shifts from
3429 cm−1 to 3435 cm−1.
Low angle X-ray difraction (XRD) patterns of HMS and HMS/Pr-Rh-Zr are
demonstrated in Fig. 2. These patterns exhibit one sharp refection at 2θ angles
about of 2. The location of peak in XRD pattern of HMS/Pr-Rh-Zr was consistent
with the standard difraction pattern of HMS that reported in the previous literature
[21]. As it can be seen, the decrease in intensity of the characteristic difraction peak
contributed to HMS/Pr-Rh-Zr in comparison with HMS was due to immobilization
of organic moieties on the pore wall of HMS. Also, this result indicated that immo-
bilization of organic moieties on the pore wall of HMS for synthesis of catalyst does
not change the phase of HMS Scheme 2.
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S. Abdolahi et al.
4000
3500
3000
2500
2000
1500
1000
500
HMS
Catalyst
0
0
2
4
6
8
10
2theta
Fig. 2 XRD pattern of HMS and HMS/Pr-Rh-Zr
Scheme 2 General procedure for the synthesis of tetrahydrobenzo[b]pyran
The thermogravimetric analysis (TGA) of HMS and HMS/Pr-Rh-Zr is indicated
in Fig. 3. The thermal behavior of HMS/Pr-Rh-Zr was evaluated that demonstrated
the mass loss peaks: the weight loss below 100 °C contributed to volatilization of
the physically adsorbed water and organic solvent, the weight loss about 34% at
100–600 °C attributed to decomposition of organic fragments immobilized in the
surface of pores and at above 600 °C related to densifcation of the silica matrix [7].
Also, in order to determine Zr content loaded in modifed HMS in synthesized cata-
lyst, ICP-AES analysis was applied whereupon 0.16 mmol g−1 resulted.
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An approach to the synthesis and characterization of…
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Fig. 3 Thermogravimetric curves of HMS (green) and HMS/Pr-Rh-Zr (red). (Color fgure online)
The morphology, sizes and the particle sizes distribution of the synthesized cata-
lyst were performed by a scanning electron microscopy (SEM). These images are
shown in Fig. 4. As shown in images, the prepared catalyst has regular and ordered
structure with particle sizes of less than 40 nm.
EDX spectrum carried out on HMS/Pr-Rh-Zr demonstrated that synthesized cata-
lyst has constitutive elements including: Si, O, N, C, S and Zr (Fig. 5).
To assess physicochemical and structural parameters of HMS/Pr-Rh-Zr, N2
adsorption–desorption technique was applied (Fig. 6). According to the IUPAC clas-
sifcation, N2 adsorption/desorption isotherm of HMS/Pr-Rh-Zr revealed typically
reversible type IV isotherms [4]. BET surface area (SBET), total pore volumes (Vtotal
)
and pore diameters (DBJH) were evaluated using the adsorption–desorption of nitro-
gen at 77 K (Table 1). This analysis revealed a mesoporous synthesized catalyst has
high surface area.
Investigation of the catalytic activity of HMS/Pr‑Rh‑Zr for synthesis
of tetrahydrobenzo[b]pyran and 1,4‑dihydropyrano[2,3‑c]pyrazole
After the synthesis, evaluation and afrmation of catalyst, the catalytic efcacy
of HMS/Pr-Rh-Zr was investigated for synthesis of tetrahydrobenzo[b]pyran and
1,4-dihydropyrano[2,3-c]pyrazole.
For optimization of the reaction conditions, initially, the catalytic amount was
studied, and then, various solvents and diferent temperatures were optimized
(Table 2). At the outset, we checked a diferent amount of catalyst such as 0, 0.03 g,
0.04 g, 0.05 g and 0.06 g for the reaction of 4-chlorobenzaldehyde (1 mmol), malo-
nonitrile (1 mmol), dimedone (1 mmol) and HMS/Pr-Rh-Zr as catalyst, in which
0.05 g of catalyst was found to be the best amount catalyst (90%, Table 2, entry 4).
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S. Abdolahi et al.
Fig. 4 SEM images of HMS/Pr-Rh-Zr
Signifcant increase in the yield of product was not observed when the amount of
catalyst was increased from 0.05 g to 0.06 g. Also, to investigate the efect of metal
in synthesized catalyst in promote of the reaction, the reaction was investigated in
the presence of HMS/Pr-Rh. That result indicated that the efect of metal in catalytic
activity is very important (Table 2, entry 12). Next, various solvents including PEG,
H2O, EtOH, solvent-free and mixture of H2O:EtOH (3:1 mL) were checked. We also
evaluated diferent temperatures for the model reaction; the results suggest that the
80 °C is a better temperature for the reaction.
The result clearly reveals that 0.05 g of HMS/Pr-Rh-Zr in PEG at 80 °C was
found to be ideal reaction condition for producing an excellent yield of the desired
product (Table 2, entry 4).
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An approach to the synthesis and characterization of…
1893
Fig. 5 EDX spectrum of HMS/Pr-Rh-Zr
18
16
14
12
10
8
Adsorpꢀon- catalyst
Desorpꢀon-catalyst
6
4
2
0
0
0.2
0.4
0.6
0.8
1
1.2
Relaꢀve pressure (P/P°)
Fig. 6 N2 adsorption–desorption isotherm of HMS/Pr-Rh-Zr
Table 1 Textural properties of
Sample name
Catalyst
SBET (m2 g−1
1043
)
DBJH (nm)
2.0
VTotal (cm3 g–1)
0.54
catalyst obtained by nitrogen
adsorption/desorption analysis
With the optimized reaction conditions, we next assessed the reaction of vari-
ous aldehydes including electron-withdrawing and electron-donating substituted
aldehyde. Twelve derivatives of tetrahydrobenzo[b]pyran were synthesized in
optimal conditions, as reported in Table 3.
In this part a plausible reaction mechanism has been described for the forma-
tion of tetrahydrobenzo[b]pyran in the presence of HMS/Pr-Rh-Zr (Scheme 3).
The reaction begins with the generation of arylidenemalononitrile intermedi-
ate (I) from the reaction of malononitrile with activated aldehyde. In the next step
enolized dimedone (II) reacts with the arylidenemalononitrile intermediate (I)
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S. Abdolahi et al.
Table 2 Optimization
Entry
Catalyst (g)
Solvent
Tempera-
Yield%a
for the synthesis of
ture (°C)
tetrahydrobenzo[b]pyran
with 4-chlorobenzaldehyde,
malononitrile, dimedone and
HMS/Pr-Rh-Zr as catalyst
1
2
3
4
5
6
7
8
0
PEG
PEG
PEG
PEG
80
80
80
80
80
80
80
80
80
60
100
80
37
54
63
90
90
51
65
63
69
78
69
38
0.03
0.04
0.05
0.06
0.05
0.05
0.05
0.05
0.05
0.05
0.05c
PEG
EtOH
Solvent-free
H2O
9
H2O:EtOHb
PEG
10
11
12
PEG
PEG
4-Chlorobenzaldehyde (1 mmol), malononitrile (1 mmol), dimedone
(1 mmol), 20 min
aAfter purifcation
bRatio: 3:1 mL
cThe reaction catalyzed by HMS/Pr-Rh
generated from the previous step. Ultimately, intramolecular cyclization and rear-
rangement occurs leading to the formation of tetrahydrobenzo[b]pyran.
In this part we optimized the reaction by screening various efective fac-
tors including: catalyst dosing, type of solvent and temperature for synthesis of
1,4-dihydropyrano[2,3-c]pyrazole. There for, the reaction of 4-chlorobenzalde-
hyde (1 mmol), malononitrile (1 mmol), ethyl acetoacetate (1 mmol) and hydrazine
hydrate (1 mmol) in the presence of HMS/Pr-Rh-Zr as catalyst was chosen as the
model reaction. With the evaluation of amount of catalyst (catalyst free, 0.008 g,
0.01 g, 0.015 g and 0.02 g), solvents (H2O, EtOH, PEG, H2O:EtOH (3:1 mL) and
solvent-free) and the reaction temperature (25, 35 and 60 °C), we found that 0.01 g
of catalyst, mixture of H2O:EtOH (3:1 mL) at 35 °C were the most efective con-
dition for synthesis of 1,4-dihydropyrano[2,3-c]pyrazole (Table 4, entry 3). For
indicating necessity of the presence of zirconium in the catalyst, the model reac-
tion undertaken in the presence of HMS/Pr-Rh instead of HMS/Pr-Rh-Zr (Table 4,
entry12). The yield of this reaction was obtained 42%.
After optimization of the reaction conditions, a range of functionalized alde-
hydes (substitute of electron-withdrawing and electron-donating) are used and
1,4-dihydropyrano[2,3-c]pyrazole derivatives were obtained in good yields as sum-
marized in Table 5 (Scheme 4).
A plausible reaction mechanism for the synthesis of 1,4-dihydropyrano[2,3-c]
pyrazole using HMS/Pr-Rh-Zr as catalyst is demonstrated in Scheme 5.
The Knoevenagel condensation occurs between malononitrile and activated alde-
hyde with catalyst, whereupon the intermediate of arylidenemalononitrile (interme-
diate I) was generated. The condensation reaction between hydrazine and activated
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An approach to the synthesis and characterization of…
1895
Table 3 One-pot synthesis of tetrahydrobenzo[b]pyran with aldehyde, malononitrile and dimedone cata-
lyzed by HMS/Pr-Rh-Zr
Entry
Product
Time (min)
20
Yield (%)a
90
M.p (°C)
207–208
Reference
[18]
1
2
3
35
83
73
198
[22]
[23]
215
178–180
4
5
40
30
86
89
210–212
231
[24]
[25]
6
7
8
165
215
30
78
71
87
200
[26]
[24]
[27]
215
227–229
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S. Abdolahi et al.
Table 3 (continued)
Entry
Product
Time (min)
150
Yield (%)a
76
M.p (°C)
206–208
Reference
[26]
9
10
11
70
90
77
72
218–220
222–224
[18]
[28]
12b
15
97
256–260
[29]
Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), dimedone (1 mmol), HMS/Pr-Rh-Zr (
0.05 g) in PEG at 80 °C
aAfter purifcation
bReaction conditions: aldehyde (1 mmol), malononitrile (2 mmol), dimedone (2 mmol), HMS/Pr-Rh-Zr (
0.1 g) in PEG at 80 °C
ethyl acetoacetate carried out that consequently pyrazolone (intermediate II) was
obtained. Finally, the enolized pyrazolone reacted to the arylidenmalononitrile by
Michael addition reaction. Then, tautomerization of the intermediate leads to pro-
duce 1,4-dihydropyrano[2,3-c]pyrazole [38].
Reusability of the HMS/Pr‑Rh‑Zr
In the synthesis of catalyst, its recyclability is the most crucial aspects of organic
synthesis. In this light, recyclability of HMS/Pr-Rh-Zr toward the synthesis of
1,4-dihydropyrano[2,3-c]pyrazole was studied to establish the reusability of the syn-
thesized catalyst.
After completion of the reaction, hot ethanol was added and the catalyst was
separated from the reaction mixture by centrifuge instrument followed drying for
overnight. Then, the dried separated catalyst was used for next catalytic run. By this
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An approach to the synthesis and characterization of…
1897
Scheme 3 Possible mechanism for the synthesis of tetrahydrobenzo[b]pyran
Table 4 Optimization of
Entry
Catalyst (g)
Solvent
Tempera-
ture (°C)
Yield%a
reaction condition for synthesis
of 1,4-dihydropyrano[2,3-c]
pyrazole under diferent
conditions
1
2
3
4
5
6
7
8
0
H2O:EtOHb
H2O:EtOHb
H2O:EtOHb
H2O:EtOHb
H2O:EtOHb
H2O:EtOHb
H2O:EtOHb
H2O
35
35
35
35
35
25
60
35
35
35
35
35
40
93
98
96
95
66
85
72
80
52
69
42
0.008
0.01
0.015
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01c
9
PEG
10
11
12
Solvent-free
EtOH
H2O:EtOHb
4-Chlorobenzaldehyde (1 mmol), malononitrile (1 mmol), ethyl ace-
toacetate (1 mmol), hydrazine hydrate (1 mmol), 20 min
aAfter purifcation
bRatio: 3:1 mL
cThe reaction catalyzed by HMS/Pr-Rh
1 3

1898
S. Abdolahi et al.
Table 5 One-pot synthesis of 1,4-dihydropyrano[2,3-c]pyrazole with aldehyde, malononitrile, ethyl ace-
toacetate and hydrazine hydrate catalyzed by HMS/Pr-Rh-Zr
Entry
Product
Time (min)
Yield (%)a
M.p (°C)
230–232
Reference
[30]
1
20
98
2
3
20
20
94
93
234–236
236–238
[31]
[32]
4
5
6
20
30
50
91
90
88
218–220
242–244
208–211
[33]
[34]
[33]
7
8
60
70
87
83
220–222
202–206
[35]
[36]
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An approach to the synthesis and characterization of…
1899
Table 5 (continued)
Entry
Product
Time (min)
65
Yield (%)a
85
M.p (°C)
226–230
Reference
[32]
9
10
70
80
78
99
162–164
206–209
300>
[37]
[30]
–
11
120
16
12b
Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), ethyl acetoacetate (1 mmol), hydrazine
hydrate (1 mmol), HMS/Pr-Rh-Zr ( 0.01 g) in H2O:EtOH (3:1 mL) at 35 °C
aAfter purifcation
bReaction conditions: aldehyde (1 mmol), malononitrile (2 mmol), ethyl acetoacetate (2 mmol), hydra-
zine hydrate (2 mmol), HMS/Pr-Rh-Zr ( 0.02 g) in H2O: EtOH (3:1 mL) at 35 °C
experimental, the results were defned that HMS/Pr-Rh-Zr can be reused for fve
consecutive runs with minimal decrease in the yield of product (Fig. 7).
Characterization of recycled catalyst
Characterization of the catalyst after the reuse process was done by XRD technique
in order to study its stability. The XRD pattern of reused and fresh catalyst is indi-
cated in Fig. 8. As shown in this fgure, there is not any change in XRD pattern of
the recovered catalyst with fresh catalyst. In addition, the XRD pattern of recovered
catalyst showed a good agreement with standard XRD pattern of HMS. This analy-
sis confrmed that the crystalline structure of recovered catalyst has not changed and
was strong evidence for the stability of recovered HMS/Pr-Rh-Zr.
1 3

1900
S. Abdolahi et al.
Scheme 4 General procedure for the synthesis of 1,4-dihydropyrano[2,3-c]pyrazole
Scheme 5 Possible mechanism for the synthesis of 1,4-dihydropyrano[2,3-c]pyrazole
1 3

An approach to the synthesis and characterization of…
1901
100
95
90
85
80
75
70
65
60
55
50
45
40
1
2
3
4
5
Run number
Fig. 7 Recyclability of HMS/Pr-Rh-Zr in the synthesis of 1,4-dihydropyrano[2,3-c]pyrazole
3000
Catalyst
2500
Recovered catalyst
2000
1500
1000
500
0
0
2
4
6
8
10
2theta
Fig. 8 XRD pattern of reused and fresh catalyst
Hot fltration test
The hot fltration test was applied to investigate leaching of zirconium in the reaction
mixture and to show the nature heterogeneous of catalyst. In this light, the reaction
between 4-chlorobenzaldehyde, malononitrile, ethyl acetoacetate, hydrazine hydrate
and HMS/Pr-Rh-Zr in H2O: EtOH (3:1 mL) at 35 °C was investigated. In this exper-
iment we found the yield of correspond product in the half time of the reaction
(10 min) was 58%. Then, the same reaction was repeated meanwhile after half‐time
of reaction (after 10 min), the catalyst was separated and the reaction mixture was
permitted to react for another 10 min. In this stage the product was obtained in 63%
yield. These results confrmed that leaching of zirconium during the reaction did not
occur.
1 3

1902
S. Abdolahi et al.
Comparison results of HMS/Pr‑Rh‑Zr
On the basis of earlier reports, the comparison results of HMS/Pr-Rh-Zr with other
catalysts have been collected for synthesis of 2-amino–4-(4-chlorophenyl)-3–cyano-
7,7–dimethyl-5-oxo-4H-5,6,7,8-tetrahydrobenzo[b]pyran (Table 6, entry 1–9) and
6-amino-4-(4-chlorophenyl)-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carboni-
trile (Table 6, entry 10–15). As shown in Table 6, we fnd that in the present work,
our catalytic system has beneft such as: mild reaction conditions, good to high
yields and low reaction times.
Conclusion
In conclusion, in this work we synthesized new complex of zirconium supported
in the pores of functionalized HMS which showed that HMS/Pr-Rh-Zr catalyst
signifcantly improved activity. In the next step for confrmation and characteriza-
tion of synthesized catalyst, several techniques were applied. Finally, the catalytic
activity of prepared catalyst assessed for the synthesis of tetrahydrobenzo[b]pyran
and 1,4-dihydropyrano[2,3-c]pyrazole derivatives. Additionally, the infuences of
metal of Zr were investigated on the catalytic activity of HMS/Pr-Rh-Zr that result
Table 6 Comparison results of HMS/Pr-Rh-Zr with other catalysts
Entry Conditions
Time (min) Yield % Reference
1
2
CaHPO4 (10 wt%),H2O/EtOH (4:1), 80 °Ca
120
60
92
95
[25]
[39]
Fe3-xTixO4@SO3HNPs (0.03 g), EtOH (3 mL)/H2O (3 mL),
refuxa
3
4
5
6
Fe3O4@SiO2/DABCO (0.05 g), H2O, 80 °Ca
β -Cyclodextrin (2.0 mol %), H2O, r. ta
NH4Al(SO4)2 12H2O (0.2 g), EtOH, 80 °Ca
25
90
93
94
92
[40]
[23]
[41]
[42]
300
120
60
PFPA (pentafuoropropionic acid, 35 mol%), EtOH: H2O
(1:1), r.ta
7
8
Fe3O4@GO-N-(pyridin- 4-amine) (10 mg), H2O, refuxa
30
4
92
95
[43]
[44]
Scolecite (2% weight), EtOH (5 mL), H2O (5 mL), micro-
wave, 90 °Ca
9
HMS/Pr-Rh-Zr (0.05 g), PEG, 80 °Ca
20
40
80
25
15
12 h
20
90
92
80
93
92
84
98
This work
[38]
[37]
[31]
[33]
[45]
This work
10
11
12
13
14
15
CoCuFe2O4 (25 mg), r. t, solvent-freeb
Lemon peel powder (10 wt%), EtOH, refuxb
Ag/TiO2 nano-thin flms, H2O: EtOH (1:2 mL), 70 °Cb
β-cyclodextrin (10 mol %), H2O-EtOH (9:1), 80 °Cb
urea (10 mol %) H2O: EtOH (1:1 v/v), r. tb
HMS/Pr-Rh-Zr (0.01 g), H2O: EtOH (3:1 mL), 35 °Cb
aReaction of 4-chlorobenzaldehyde, dimedone and malononitrile for the synthesis of tetrahydrobenzo[b]
pyran
bReaction of 4-chlorobenzaldehyde, malononitrile, ethyl acetoacetate and hydrazine hydrate for the syn-
thesis of 1,4-dihydropyrano[2,3- c]pyrazole
1 3

An approach to the synthesis and characterization of…
1903
indicated that in the absence of metal, the reaction underdeveloped. Ultimately, the
result of recovery test of synthesized catalyst defned that catalyst could be recov-
ered after the reaction and reused without any noticeable loss in activity.
Supplementary Information The online version contains supplementary material available at (https://
doi.org/10.1007/s11164-021-04411-z).
Acknowledgements Authors thank Ilam University and Bu-Ali Sina University for fnancial support of
this research project.
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