An expedient and eco-friendly approach for multicomponent synthesis of dihydropyrano[2,3-c]pyrazoles using nano-Al2O3/BF3/Fe3O4 as reusable catalyst

Article abstract of DOI:10.1080/24701556.2019.1661458

The pyranopyrazole derivatives have many biological activities. They were synthesized via a four-component coupling of aromatic aldehyde, malononitrile, ethyl acetoacetate and hydrazine hydrate. In this study, dihydropyrano[2,3-c]pyrazoles have been synth

Full text of DOI:10.1080/24701556.2019.1661458

Inorganic and Nano-Metal Chemistry
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An expedient and eco-friendly approach
for multicomponent synthesis of
dihydropyrano[2,3-c]pyrazoles using nano-Al2O3/
BF3/Fe3O4 as reusable catalyst
Elaheh Babaei & Bi Bi Fatemeh Mirjalili
To cite this article: Elaheh Babaei & Bi Bi Fatemeh Mirjalili (2019): An expedient and eco-
friendly approach for multicomponent synthesis of dihydropyrano[2,3-c]pyrazoles using
nano-Al2O3/BF3/Fe3O4 as reusable catalyst, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2019.1661458
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INORGANIC AND NANO-METAL CHEMISTRY
https://doi.org/10.1080/24701556.2019.1661458
An expedient and eco-friendly approach for multicomponent synthesis of
dihydropyrano[2,3-c]pyrazoles using nano-Al2O3/BF3/Fe3O4 as reusable catalyst
Elaheh Babaei
and Bi Bi Fatemeh Mirjalili
Department of Chemistry, College of Science, Yazd University, Yazd, Iran
ABSTRACT
ARTICLE HISTORY
Received 30 August 2018
Accepted 24 August 2019
The pyranopyrazole derivatives have many biological activities. They were synthesized via a four-
component coupling of aromatic aldehyde, malononitrile, ethyl acetoacetate and hydrazine
hydrate. In this study, dihydropyrano[2,3-c]pyrazoles have been synthesized in the presence of
nano-Al2O3/BF3/Fe3O4 as catalyst in water/ethanol under reflux conditions. The catalyst was
removed from the mixture of reaction by an external magnet and was reusable for several times
without any loss of its activity. The obtained pyranopyrazoles were characterized by various meth-
ods such as FT-IR, 1HNMR and melting point. Easy purification, clean and convenient procedure
are some advantages of this method.
KEYWORDS
Nano-Al2O3/BF3/Fe3O4; pyra-
nopyrazole; magnetic
catalyst; Lewis acid;
multicomponent reaction
GRAPHICAL ABSTRACT
Introduction
Recently, pyranopyrazole synthesis have been promoted by
disulfonic acid imidazolium chloroaluminate
The distinctive aspect of multi-component reactions (MCRs)
is that the products contain all parts of substrates and creat-
ing almost no by-products. Thus, MCRs are an ideal and
eco-friendly reaction module by reducing production of
waste and energy consumption.[1–3] Final compounds can be
obtained with fewer steps, rapidly and in one pot. Therefore,
MCRs have been paid attention in modern organic chemis-
try, such as combinatorial chemistry or medicinal chemistry.
So, applications of MCRs are very popular with only a min-
imum of endeavor and offer a wealth of products.[4,5]
Pyranopyrazole derivatives is getting enormous attention
among chemists due to widespread synthetic utilities and
potential biological activity[6] such as anticancer,[7] antibacter-
ial,[8] vasodilatory,[9] analgestic properties and Chk1 kinase[10]
inhibitory activity and also have acted as insecticidal.[11]
Significant attention has been focused on the improvement of
new methods for the synthesis of these fused heterocyclic com-
pounds. Most of reported synthetic methods to synthesize
f[Dsim]AlCl4g,[12] c-alumina,[13] hexadecyltrimethyl ammo-
nium bromide (HTMAB),[14] isonicotinic acid,[15] cinchona
alkaloid derivatives,[16] per-6-amino-b-cyclodextrin[17] and sili-
cotungsticacid.[18] Some of these protocols have disadvantages,
such as long reaction time, expense and use of toxic organic
solvent. Therefore, an important target is the development of
effectual, simple and eco-friendly catalysts for the synthesis of
pyranopyrazoles.[19–21] Previously, nano-Al2O3/BF3/Fe3O4 has
been synthesized and characterized as a new reusable cata-
lyst.[22] In this study, we wish to report effective and eco-
friendly procedure for the synthesis of dihydropyrano[2,3-
c]pyrazoles in the presence of nano-Al2O3/BF3/Fe3O4.
Experimental
General
The chemicals were used without any additional purifica-
1
pyranopyrazoles implicate the MCR of aldehyde, ethyl acetoa- tion. The products were characterized by FT-IR, H NMR
cetate, malononitrile and hydrazine hydrate.[6]
and a comparison of their physical properties with those
CONTACT Bi Bi Fatemeh Mirjalili
fmirjalili@yazd.ac.ir
Department of Chemistry, College of Science, Yazd University, Yazd, Iran
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsrt.
Supplemental data for this article is available online at at the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
E. BABAEI AND B. B. F. MIRJALILI
reported in the literature. FT-IR spectra were run on a General procedure for synthesis of
Bruker, Equinox 55 spectrometer. A Bruker (DRX-400
Avance) NMR was used to record the 1H NMR spectra.
Melting points were determined by a Buchi melting point B-
540 B.V.CHI apparatus.
dihydropyrano[2,3-c]pyrazoles
A mixture of aromatic aldehyde (1 mmol), malononitrile
(0.07 g, 1 mmol), ethyl acetoacetate (0.13 g, 1 mmol), hydra-
zine hydrate (1.25 mmol) and nano-Al2O3/BF3/Fe3O4
(0.03 g) has been refluxed in 3 mL water/ethanol (1:2). After
completion of the reaction that has been monitored by TLC,
the reaction mixture was cooled to room temperature and
dissolved in hot ethanol. The catalyst was separated by an
external magnet. By adding water to residue, the product
was appeared as a pure solid in high yields.
Hot filtration test
A hot filtration test was done to confirm that the nano-cata-
lyst was recoverable heterogeneous one. In this test, a mix-
ture of hydrazine hydrate (1.25 mmol), benzaldehyde
(1 mmol), ethyl acetoacetate (1 mmol), malononitrile
(1 mmol) and water/ethanol (1:2) (3 mL) was refluxed in the
presence of nano-Al2O3/BF3/Fe3O4 (0.03 g) for 25 min. The
catalyst was filtered off from the hot reaction mixture, and
the reaction in the filtrate was still monitored. No increase
in conversion was observed in the filtrate.
Figure 1. X-ray diffraction (XRD) pattern of (a) fresh and (b) recovered nano-
Al2O3/BF3/Fe3O4.
Figure 2. (a) FESEM and (b) TEM image of nano-Al2O3/BF3/Fe3O4.
Table 1. The condensation reaction of hydrazine hydrate (1.25 mmol), ethyl acetoacetate (1 mmol), malononitrile (1 mmol) and benzaldehyde (1 mmol) in the
presence of nano-Al2O3/BF3/Fe3O4 (0.03 g, 7.3 mol%) under various conditions.
Entry
Solvent
solvent-free
solvent-free
EtOH/H2O
CH2Cl2
solvent-free
EtOH/H2O
EtOH
Conditions
Time (min)
Yielda (%)
1
2
3
4
5
6
7
R.T.
120
70
25
60
15
5
0
40
90
15
0
25
35
60 ꢀC
Reflux
Reflux
Mixer Mill
Microwave
Ultrasonic
60
aIsolated yield.

INORGANIC AND NANO-METAL CHEMISTRY
3
0.5510, 0.3936, 0.62298 and 0.6298, respectively, show the exist-
ence of Fe3O4. Other signal in 2h equal to 15.73 proves the
bonding of flourine to the catalyst backbone. The comparison
between XRD pattern of fresh and recovered catalyst shows the
high stability of catalyst under reaction conditions.
In order to investigate the particle size of nano-Al2O3/
BF3/Fe3O4, FESEM and TEM were measured and presented
in Figure 2. In these results, the dimensions of catalyst par-
ticles were achieved below 20 nm.
Initially, in order to investigate the activity of nano-
Al2O3/BF3/Fe3O4 catalyst, in preparation of 6-amino-3-
methyl-4-phenyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbon-
itrile, the condensation of benzaldehyde, malononitrile, ethyl
acetoacetate and hydrazine hydrate as a model reaction was
carried out in the presence of it under various conditions
and the results are summarized in Table 1. Among the dif-
ferent conditions screened, ethanol/water gave the product
in good yield under reflux condition (Table 1, entry 3).
Results and discussion
The XRD pattern of nano-Al2O3/BF3/Fe3O4 before and after
recycling is shown in Figure 1. The signals at 2h equal to 37, 45
and 67 are shown nano-c-Al2O3 structure and the values of 2h
equal to 30.27, 35.64, 57.31 and 62.88 with FWHM equal to
Table 2. The condensation reaction of hydrazine hydrate (1.25 mmol), ethyl
acetoacetate (1 mmol), malononitrile (1 mmol) and benzaldehyde (1 mmol) in
the presence of various amounts of nano-Al2O3/BF3/Fe3O4.a
Entry
nano-Al2O3/BF3/Fe3O4 (g)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
0.007
0.015
0.030
0.040
0.050
0.100
0.150
None
40
30
25
25
25
25
25
180
37
55
90
90
90
88
85
None
aCondition: reflux in ethanol:H2O (2:1).
bIsolated yield.
Table 3. Synthesis of dihydropyrano[2,3-c]pyrazoles derivatives using nano-Al2O3/BF3/Fe3O4 in ethanol/water under reflux conditions.a,b
aIsolated yield.
bAldehyde (1 mmol), ethyl acetoacetate (1 mmol), malononitrile (1 mmol), hydrazine hydrate (1.25 mmol) and nano-Al2O3/BF3/Fe3O4 (0.03 g).

4
E. BABAEI AND B. B. F. MIRJALILI
Other solvent such as CH2Cl2 gave the desired products in was shown in Scheme 1. BF3 in nano-Al2O3/BF3/Fe3O4 as a
low yields even after elongated reaction times and also Lewis acid activates the C¼O group in b-ketoester and then
increasing the temperature (Table 1, entry 4). But then, the pyrazolone formed from the reaction of ethyl acetoacetate
and hydrazine hydrate as an intermediate (5). In the next step,
between aromatic aldehyde and malononitrile, in the presence
of catalyst produced compound 6 by Knoevenagel condensa-
tion. Ultimately, Michael addition reaction between 5 and 6,
intramolecular cyclization, and aromatization, the desired pyra-
nopyrazole was prepared.
The separated nano-catalyst was reused in the mentioned
reaction five times with only a slight decrease in its catalytic
activity (Figure 3). Partial loss of activity may be due to
blockage of active sites of the catalyst.
As presented in Table 4, the use of nano-Al2O3/BF3/
Fe3O4 resulted in an improved method in terms of reac-
tion time, compatibility with environment, and yield
when compared with other reported catalysts. Our
reported catalyst has TOF equal to 27.4 hꢁ1 and is better
than many others.
when the reaction was performed under solvent-free condi-
tions in 60 ꢀC, the product was formed in 40% (Table 1,
entry 2). Moreover, we have tried to carry out the model
reaction using mixer mill, ultrasonic and microwave, but
these conditions did not give the product in good yield
(Table 1, entries 5–8). To optimize the amount of catalyst
and time of reaction, the model reaction was carried out
using different amount of catalyst (Table 2).
In the absence of the catalyst, no product was obtained
(Table 2, entry 8). On the other hand, by increasing the
amount of catalyst the yields of products improved (Table 2,
entries 3–7). The highest yield was obtained in 25 min with
the use of 0.03–0.05 g of catalyst (Table 2, entries 3–5).
Increasing the amount of catalyst up to 0.05 g did not affect
observably on the yields of products (Table 2, entries 6, 7).
Using nano-Fe3O4 as catalyst for promotion of model reac-
tion has produced only 40% yield.
According to the optimized reaction conditions, a variety
of aldehydes (containing electron-releasing group or with
electron-withdrawing group) have been chosen for synthesis
Spectral data for selected compounds
6-Amino-4-(2,4-dichlorophenyl)-3-methyl-1,4-dihydropyrano
of dihydropyrano[2,3-c]pyrazoles in the presence of nano- [2,3-c]pyrazole-5-carbonitrile (Table 3, P4):
Al2O3/BF3/Fe3O4 and the results are presented in Table 3.
Pale yellow solid. M.p. 200–202 ꢀC. FT-IR (ATR)/
A proposed mechanism for preparation of dihydropyr- ῡ(cmꢁ1): 3482, 3243, 3115, 2186, 1638, 1587, 1491, 1408,
ano[2,3-c]pyrazoles in the presence of nano-Al2O3/BF3/Fe3O4 1100, 1052, 866, 741.; 1H-NMR (400 MHz, DMSO-d6)/d
Scheme 1. Proposed mechanism for preparation of dihydropyrano[2,3-c]pyrazoles in the presence of nano-Al2O3/BF3/Fe3O4.

INORGANIC AND NANO-METAL CHEMISTRY
5
Figure 3. Catalyst recycling experiment.
Table 4. Synthesis of dihydropyrano[2,3-c]pyrazoles in the presence of previously reported catalysts.a
Entry
Catalyst (mol%)
Solvent
H2O
Conditions
Time (h)
Yieldb (%)Ref.
TOF (h–1
)
1
2
3
4
5
6
a
CTACl (20)
Isonicotinic acid (10)
90 ꢀC
4
0.16
9
0.51
1.5
0.45
88[30]
1.1
56.2
1.02
5.9
6.2
27.4
ꢂ
–
85 ꢀC
90[15]
ꢂ
MorT (10)
c-Alumina (30)
ꢂꢂꢂ
EtOH/H2O
H2O
H2O
Reflux
Reflux
Reflux
Reflux
92[31]
ꢂ
90[13]
ꢂꢂ
Ba(OH)2 (10)
Nano-Al2O3/BF3/Fe3O4 (7.3)
ꢂꢂ
93[32]
EtOH/H2O
90[this work]
ꢂ
ꢂꢂꢂ
The used aldehyde: benzaldehyde, 4-chlorobenzaldehyde,
3,4,5-trimethoxy benzaldehyde.
ppm: 1.85 (s, 3 H), 5.13 (s, 1 H), 7.07 (s, 2 H), 7.29
(d, J ¼ 8 Hz, 1 H), 7.47 (dd, J ¼ 8.4 Hz, J ¼ 2 Hz, 1 H), 7.65
(d, J ¼ 2.4 Hz, 1 H), 12.23 (s, 1 H).; 13C NMR (100 MHz,
DMSO-d6)/d ppm: 161.30, 154.88, 140.07, 135.44, 132.81,
132.10, 128.83, 128.02, 120.25, 96.32, 55.21, 33.07, 9.53.
6-Amino-3-methyl-4-(3-nitrophenyl)-1,4-dihydropyrano
[2,3-c]pyrazole-5-carbonitrile (Table 3, P5):
Conclusion
In summary, we have reported an expedient protocol for the
preparation of pyranopyrazole derivatives catalyzed by nano-
Al2O3/BF3/Fe3O4 as reusable catalyst under reflux condition
in ethanol/water. Meanwhile, some advantage of the pre-
sented methodology are low cost, easy work up, simplicity
of operation, efficiency, high yields, environmentally benign
conditions and short reaction time.
White solid. M.p. 218–220 ꢀC. FT-IR (ATR)/ῡ(cmꢁ1): 3484,
3231, 3120, 2190, 1645, 1597, 1519, 1491, 1410, 1351, 733.
1HNMR (400 MHz, DMSO-d6)/d (ppm): 1.82 (s, 3 H), 4.89 (s,
1 H), 7.08 (s, 2 H), 7.64–7.70 (m, 2 H), 8.04 (s, 1 H), 8.13-8.15
(d, J ¼ 8 Hz, 1 H), 12.23 (s, 1 H).; 13C NMR (100 MHz,
DMSO-d6)/d ppm: 161.63, 155.17, 148.36, 147.32, 136.38,
134.88, 130.47, 122.33, 121.01, 97.15, 56.59, 36.11, 10.25.
6-Amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano
[2,3-c]pyrazole-5-carbonitrile (Table 3, P8):
Funding
The Research Council of Yazd University is gratefully acknowledged
for financial support of this work.
ORCID
Elaheh Babaei
http://orcid.org/0000-0002-9197-6716
White solid. M.p. 239–242 ꢀC. FT-IR (ATR)/ῡ(cmꢁ1):
3475, 3227, 3106, 2195, 1646, 1592, 1513, 1399, 1348, 1163,
1109, 810, 744.; 1H-NMR (400 MHz, Acetone-d6)/d ppm: 2 (s, References
3 H), 4.88 (s, 1 H), 6.30 (br s, 2 H), 7.55 (d, J ¼ 8 Hz, 2 H),
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