Simultaneous sonication assistance for the synthesis of tetrahydropyridines and its efficient catalyst ZrP2O7 nanoparticles

Article abstract of DOI:10.1016/j.ultsonch.2013.11.011

In this research, a general synthetic method for the synthesis of tetrahydropyridines were developed using ZrP₂O₇ nanoparticles under ultrasonic irradiations. Firstly by a simple and green process, nano zirconium pyrophosphate was pr

Full text of DOI:10.1016/j.ultsonch.2013.11.011

Ultrasonics Sonochemistry 21 (2014) 1150–1154
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Simultaneous sonication assistance for the synthesis
of tetrahydropyridines and its efficient catalyst ZrP2O7 nanoparticles
b
b,
Abdollah Javidan a, , Abolfazl Ziarati , Javad Safaei-Ghomi
⇑
⇑
a Chemistry Department, Imam Hossein University, P.O. Box 16575-347, Tehran, Iran
b Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan 51167, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2013
Received in revised form 11 November 2013
Accepted 18 November 2013
Available online 27 November 2013
In this research, a general synthetic method for the synthesis of tetrahydropyridines were developed
using ZrP2O7 nanoparticles under ultrasonic irradiations. Firstly by a simple and green process, nano zir-
conium pyrophosphate was prepared via sonication. Subsequently, this nanoparticle was used as an effi-
cient catalyst for the synthesis of highly functionalized tetrahydropyridines via five-component reaction
of aromatic aldehyde, amine and ethyl acetoacetate in ethanol under ultrasound irradiation. The present
approach offers several advantages such as high yields, environmentally benign, simple work-up, excel-
lent yield of products, short reaction times as well as recoverability and reusability of the catalyst.
Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
Keywords:
Ultrasonic irradiation
Tetrahydropyridines
Multicomponent reactions
ZrP2O7 nanoparticles
Green chemistry
1. Introduction
functionalized tetrahydropyridines. Tetrahydropyridine derivatives
have received much attention owing to their biological and pharma-
The effective high-throughput preparation of organic com-
pounds is one of the most important purposes in modern drug dis-
covery. Organic reactions should be facile and the target products
should be simply purified in high to excellent yields. From this
point of view, there is much interest in the implementation of
new methods and new synthetic strategies. In this regard, non-
classical routes such as microwave-assisted synthesis, supercritical
fluids and sonication found applications as attractive methods to
attain these goals [1]. Ultrasonic activation, based on cavitation ef-
fects leading to mass transfer development, is extensively used to
promote nano structures synthesis and also organic reactions [2]. A
survey of literature displays that the synthesis of organic com-
pounds and nano crystalline structures have been accelerated by
ultrasound irradiation. Compared with traditional methods, this
method is more efficient and appropriate in the consideration of
green chemistry concepts [3,4].
cological activities such as anti-hypertensive [7], anti-bacterial [8],
anti-malarial [9] anti-convulsant and anti-inflammatory [10].
In recent years, nano crystalline structures have interested sig-
nificant attention as a convenient and efficient catalysts in many
organic reactions due to their high surface-to-volume ratio and
coordination parts which provides a large number of active sites
per unit area [11,12]. Zirconium pyrophosphate is an important
class of inorganic compounds that is widely studied in various
chemical fields especially in catalysis aspects [13–18].
In continuous to progress the synthetic approach for the pro-
duction of various medicinally compounds using reusable nano
catalysts [19–24], herein we combined the advantages of sonica-
tion method and nanotechnology to design a new and efficient
method for the synthesis of highly functionalized tetrahydropyri-
dines using ZrP2O7 nanoparticles as catalyst under ultrasonic irra-
diation (Scheme 1).
The study of multicomponent reactions is an attractive topic in
organic chemistry because of their advantages in the preparation
of diverse heterocyclic molecules and in drug discovery procedures
[5]. Multicomponent reactions supply an operative method for the
synthesis of various compounds with pharmaceutical and biological
activities [6]. One type of these reactions is the synthesis of highly
2. Experimental
2.1. Materials and apparatus
All the chemicals reagents used in our experiments were ana-
lytical grade and were used as received without further purifica-
tion. A multiwave ultrasonic generator (Sonicator 3200; Bandelin,
MS 73, Germany), equipped with a converter/transducer and tita-
nium oscillator (horn), 12.5 mm in diameter, operating at 20 kHz
⇑
Corresponding authors. Tel./fax: +98 21 77104930.
E-mail addresses: Ajavidan@ihu.ac.ir (A. Javidan), safaei@kashanu.ac.ir
(J. Safaei-Ghomi).
1350-4177/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2013.11.011

A. Javidan et al. / Ultrasonics Sonochemistry 21 (2014) 1150–1154
1151
with a maximum power output of 200 W, was used for the ultra-
sonic irradiation. The ultrasonic generator automatically adjusted
the power level. 1H NMR and 13C NMR spectra were recorded on
Bruker Avance-400 MHz spectrometers in the presence of tetra-
methylsilane as internal standard. The IR spectra were recorded
on FT-IR Magna 550 apparatus using with KBr plates. The elemen-
tal analyses (C, H, N) were obtained from a Carlo ERBA Model EA
1108 analyzer. Melting points were determined on Electro thermal
9200, and are not corrected. Microscopic morphology of products
was visualized by SEM (LEO 1455VP). Powder X-ray diffraction
(XRD) was carried out on a Philips diffractometer of X’pert com-
3.1 Hz, H-5), 2.88 (dd, 1H, J = 15.5, 3.1 Hz, H-5), 4.39–4.46 (m, 2H,
CH3CH2), 5.11 (s, 1H, H-6), 6.18 (m, 2H, J = 8.5 Hz, ArH), 6.40 (s,
1H, H-2), 6.49 (d, 2H, ArH), 6.88 (d, 2H, J = 8.5 Hz, ArH), 7.11 (d,
2H, J = 7.8 Hz, ArH), 7.25 (d, 2H, ArH) 7.32–7.35 (m, 6H, ArH),
7.44 (m, 2H, J = 7.8 Hz, ArH), 7.48 (m, 2H, ArH), 10.29 (s, 1H, NH);
13C NMR (100 MHz,CDCl3) d ppm = 15.1, 33.6, 55.0, 57.7, 59.8,
98.2, 112.6, 116.1, 125.5, 125.7, 126.2, 126.6, 128.5, 128.7, 128.8,
129.1, 136.2, 136.7, 138.3, 139.7, 141.4, 142.6, 144.3, 147.6, 156.4
and 168.0; FT-IR (KBr): 3439, 2985, 1861, 1649, 1590, 1516,
1443, 1366, 1334, 1249, 1170, 1067 and 1027 cmÀ1; Anal. Calcd
for C32H30N2O2: C, 80.98; H, 6.37; N, 5.90. Found: C, 81.02; H,
6.53; N, 5.94.
pany with mono chromatized Cu K
a radiation (k = 1.5406 Å).
Transmission electron microscopy (TEM) images were obtained
on a Philips EM208 transmission electron microscope with an
accelerating voltage of 100 kV.
2.4.2. Ethyl 2,6-bis(4-nitrophenyl)-1-(phenyl)-4-(phenylamino)-
1,2,5,6-tetrahydropyridine-3-carboxylate (4j)
Light yellow solid; mp = 241–243 °C. 1H NMR (400 MHz, CDCl3)
d ppm = 1.47 (t, 3H, J = 7.5 Hz,CH3CH2), 2.84 (d, 2H, J = 16 Hz, H-5),
4.29–4.49 (m, 2H, CH3CH2,) 5.37 (m, 1H, H-6), 6.37 (m, 1H, H-2),
6.48 (d, 2H, ArH and H-2), 6.66 (d, 2H, J = 8.6 Hz, ArH), 6.83 (d,
2H, J = 8.3 Hz, ArH), 6.90 (m, 4H, ArH), 7.06 (d, 2H, J = 8.6 Hz,
ArH), 7.16 (d, 2H, J = 8.3 Hz, ArH), 10.32 (s, 1H, NH); 13C NMR
(100 MHz,CDCl3) d = 14.6, 34.4, 56.8, 57.5, 61.7, 96.6, 114.5,
115.1, 123.9, 124.2, 127.8, 127.9, 131.5, 137.0, 140.4, 146.3,
147.8, 149.2, 152.4, 152.6, 155.8, 158.3 and 168.0; FT-IR (KBr):
3212, 2884, 2321, 1672, 1607, 1522, 1472, 1336,1231, 1180 and
1089 cmÀ1. Anal. Calcd for: C34H32N4O8: C, 65.38; H, 5.16; N,
8.97. Found: C, 65.72; H, 5.31; N, 8.84.
2.2. Preparation of zirconium pyrophosphate nanoparticles under
ultrasound irradiation
The catalyst was prepared via sonochemical method (worked at
20 kHz frequency and 80 W power). ZrOCl2 was used as the zirco-
nium source. Firstly the stoichiometric amount of ZrOCl2/8H2O was
added in 20 mL of distilled water and sonicated to completely dis-
solution. Afterward H3PO4 (85%) was added dropwise in 20 min
and the mixture was sonicated. When the reaction was completed,
disperse white precipitate was obtained. The solid was filtered and
washed with distilled water and ethanol several times. Subse-
quently the catalyst was dried at 100 °C for 8 h and calcined at
500 °C for 1 h to obtain pure nano zirconium pyrophosphate.
3. Results and discussion
2.2.1. Reusability of catalyst
3.1. Structural analysis of ZrP2O7 nanoparticles
The recovered catalyst from the experiment was washed by
chloroform and hot ethanol (3 Â 5 mL). Then, it was dried in
100 °C and used in the synthesis of tetrahydropyridines. The cata-
lyst was reused for five times.
In order to investigate the particle size and morphology of zir-
conium pyrophosphate nanoparticles, SEM and TEM images of
ZrP2O7 nanoparticles were presented in Figs. 1 and 2. By SEM im-
age some data about the morphology and size of catalyst particles
were obtained (Fig. 1). These results show that spherical ZrP2O7
nanoparticles were obtained with an average diameter of 10–
30 nm.
2.3. General procedure for the synthesis of tetrahydropyridines
2.3.1. Typical reflux method (method A)
A solution of amine (2 mmol), ethyl acetoacetate (1 mmol),
nano ZrP2O7 (0.5 mol%) and ethanol (3 mL) was being stirred under
reflux for 30 min. Then, aldehyde (2 mmol) was added and vigor-
ous stirring continued until the solid precipitation was observed
(monitored by TLC). After being cooled to room temperature, the
solid was filtered off and washed with hot ethanol. The residue
was dissolved in chloroform and then filtered until heterogeneous
catalyst was recovered. The filtrate solution was evaporated to gain
pure tetrahydropyridines in 81–87% yield.
Also TEM image of nano ZrP2O7 was presented in Fig. 2, which
confirms small size of zirconium pyrophosphate till about 10 nm.
Moreover the XRD pattern of the ZrP2O7 nanoparticles was
shown in Fig.3. All reflection peaks can be readily indexed to pure
cubic crystal phase of nano crystalline zirconium pyrophosphate.
Also no specific peaks due to any impurities were observed. The
pattern agrees well with the reported pattern for zirconium pyro-
phosphate (JCPDS No. 49–1079). The crystallite size diameter (D) of
the ZrP2O7 nanoparticles has been calculated by Debye–Scherrer
2.3.2. Ultrasound irradiation method (method B)
In a two-necked flask, a solution of amine (2 mmol), ethyl ace-
toacetate (1 mmol), nano ZrP2O7 (0.5 mol%) in ethanol (3 ml) was
sonicated at 20 kHz frequency and 80 W power, for 10 min at room
temperature. Then aldehyde (2 mmol), was added and sonicated
for appropriate times (monitored by TLC). After completed reaction
the solid was filtered off and washed with hot ethanol. The residue
was dissolved in chloroform and then filtered until heterogeneous
catalyst was recovered. The filtrate solution was evaporated so as
to gain pure tetrahydropyridines in 88–93% yield.
2.4. Representative spectral data
2.4.1. Ethyl 1,2,5,6-tetrahydro-1,2,6-triphenyl-4-(phenylamino)-
opyridine-3-carboxylate (4a)
Pale yellow solid; mp = 174–175 °C. 1H NMR (400 MHz, CDCl3)
d ppm = 1.41 (t, 3H, J = 7.2 Hz, CH3CH2), 2.81 (dd, 1H, J = 15.5,
Fig. 1. SEM image of the nano ZrP2O7.

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A. Javidan et al. / Ultrasonics Sonochemistry 21 (2014) 1150–1154
Table 2
Optimization of the model reaction using various catalysts under ultrasonic
irradiation.
Entry
Catalyst (mol%)
Time (min)
Yielda, %
1
2
3
4
5
6
8
ZrO2 (25)
CuO (25)
ZnO (25)
ZrP2O7 (20)
Nano ZrP2O7 (4)
Nano ZrP2O7 (5)
Without catalyst
120
120
120
100
60
81
78
69
86
92
93
0
60
180
a
Isolated yield.
Table 3
Optimization of the model reaction using various solvents.
Fig. 2. TEM image of ZrP2O7 nanoparticles.
Entry
Solvent
Time (min)
Yielda, (%)
1
2
3
4
5
6
CH3CN
Toluene
CHCl3
EtOH
MeOH
120
120
120
60
80
220
86
67
60
93
89
0
Solvent-free
a
Isolated yield.
upward accessibility of the substrate molecules on the catalyst
surface.
3.2. Catalytic behaviors of ZrP2O7 nanoparticles
In order to study the effects of this nano catalyst under ultra-
sonication; catalytic behaviors of four types of catalysts such as
CuO, ZrO2, ZnO and ZrP2O7 were compared in the reaction of ani-
line, ethyl acetoacetate, and benzaldehyde (Table 2). The obtained
results were showed that in presence of nano ZrP2O7 the reaction
was carried out in high yield and in absence of catalyst, the reac-
tion did not progress at all. Also the results in Table 2 displays
the optimum amount of the catalyst was 5 mol% of zirconium
pyrophosphate nanoparticles. Notably, increasing of this amount
did not show any change in yield and time of reaction.
Subsequently to establish the best reaction conditions, we have
focused on the effect of various solvents with varying polarity. It
was observed that ethanol was the best choice for this reaction
over any organic solvents such as chloroform, acetonitrile and tol-
uene. Also the considerable results were not observed in solvent
free conditions (Table 3).
To explain the role of ultrasound, reactions were compared at
reflux conditions (method A) and ultrasonic irradiation (method
B) in ethanol in the presence of 5 mol% of nano ZrP2O7 (Table 4).
When the reaction was carried out under reflux conditions the
reaction was proceed in long times and yields of products gave
comparatively low, while the same reaction was carried under
ultrasonic irradiation gave excellent yields of products in short
reaction times.
As shown in Table 4, in all cases sonication method was more
effective than conventional method. This should be explained
based on adsorption on the surface and the higher mass transfer
of organic molecule between the liquid phase and the nano cata-
lyst surface which are due to the shockwave and microjet formed
Fig. 3. The XRD pattern of nano ZrP2O7.
equation (D = Kk/bcosh), where FWHM (full-width at half-maxi-
mum or half-width) is in radians and h is the position of the max-
imum of diffraction peak, K is the so-called shape factor, which
usually takes a value about 0.9, and k is the X-ray wavelength.
Crystallite size of ZrP2O7 has been found to be 11 nm.
For investigation of the influence of ultrasound irradiation in
this reaction, the synthesis of nano ZrP2O7 was compared with pre-
vious techniques such as hydrothermal, solvothermal and solid
state methods [25–28] (Table 1).
As shown in Table 1, particle sizes of nano ZrP2O7 prepared via
conventional methods were ranging from 200 nm to 20 lm. How-
ever, size of ZrP2O7 nanoparticles prepared in the presence of ultra-
sound was reduced until 10 nm. This observation is described by
the effect of sonication. It is known that size control must be done
within a very short nucleation period and the final particle number
does not change during the particle growth. When the solution
contain precursor is exposed to ultrasound irradiation, extre-
mely-high pressures and temperatures are produced during acous-
tic cavitation, providing energy to generate ZrP2O7 nuclei. Also, the
produced high temperature and the adsorbed bubbling on nuclei
surface reduce the interfacial free energy between nuclei and solu-
tion, consequently inhibiting the growth of particles. Thus, a much
more rapid nucleation followed with a slower grain growth leads
to smaller particle sizes with high specific surface area in this pro-
cess [29,30]. The high surface area due to small particle size in-
creases reactivity of particles. This factor is responsible for the
Table 1
Comparison of ultrasonic method with other methods to prepare ZrP2O7.
Method
Ultrasonic (nm)
10–30
Hydrothermal (nm)
1000
Solvothermal with surfactant (nm)
About 200
Solvothermal without surfactant (
About 20
lm)
Solid state reaction (
About 1
lm)
Particle size

A. Javidan et al. / Ultrasonics Sonochemistry 21 (2014) 1150–1154
1153
Table 4
Synthesis of tetrahydropyridines under reflux conditions and sonication (method A and B).
Entry
R1
R2
Product
Method A
Method B
Time (min)
Yielda, (%)
Time (min)
Yieldb, (%)
1
2
3
4
5
6
7
8
9
C6H5
C4H3S
4-CH3OC6H4
4-ClC6H4
C6H5
C6H5
C6H4
4-ClC6H4
C6H5
4-CH3C6H4
4-BrC6H4
4-ClC6H4
4-ClC6H4
2-NO2C6H4
C6H5
4a
4b
4c
4d
4e
4f
4g
4h
4i
350
360
400
360
370
360
390
380
400
380
390
400
87
84
82
87
84
86
83
82
81
83
83
81
60
65
75
60
65
60
70
70
75
65
70
75
93
91
89
93
91
92
89
89
88
90
89
88
C6H5
4-ClC6H4
4-NO2C6H4
2,4-Cl2C6H3
4-NO2C6H4
C6H5
10
11
12
4j
4k
4l
3-ClC6H4
4-BrC6H4
4-ClC6H4
a
Isolated yields.
Isolated yields.
b
R2
NH
N
O
O
O
nano ZrP2O7
OEt
2 R1-CHO
2
R2-NH2
H3C
OEt
I) reflux conditions
(1)
R1
R1
(2)
or
(3)
II) U.S., rt
R2
(4a-4l)
Scheme 1. Synthesis of tetrahydropyridines in the presence of zirconium pyrophosphate nanoparticles under reflux and sonication conditions.
O
O
OHC
NH
O
OEt
N
OEt
))))))
))))))
(I)
NH2
(II)
N
+
O
N
O
H3C
OEt
CHO
OEt
Inter-molecular
mannich reaction
))))))
I + II
HN
))))))
(IV)
(III)
H
N
O
N
O
NH
N
O
H2C
OEt
Inter-molecular
OEt
OEt
mannich reaction
+
N
N
))))))
))))))
(V)
4
(VI)
: nano ZrP2O7
))))))
: ultrasonic iradiations
Scheme 2. Possible mechanism for the synthesis of tetrahydropyridines in the presence of nano ZrP2O7.

1154
A. Javidan et al. / Ultrasonics Sonochemistry 21 (2014) 1150–1154
ultrasonically approach has several advantages such as mild reac-
100
80
60
40
20
0
tion conditions, short reaction times and produces excellent yields.
Notably, the lowest size and highest surface of nano ZrP2O7 was
prepared as a reusable catalyst by this method.
Acknowledgments
The authors are grateful to University of Kashan for supporting
this work by Grant No. 159196/XXX. Also authors thank the Uni-
versity of Imam Hossein research affairs for their support.
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4. Conclusion
In summary, we have developed an effective route for the syn-
thesis of tetrahydropyridines under ultrasound irradiation via nano
zirconium pyrophosphate that was prepared by sonication. This