Nano-ZrO2 sulfuric acid: A heterogeneous solid acid nano catalyst for Biginelli reaction under solvent free conditions

Article abstract of DOI:10.1039/c5ra19350h

Nano-ZrO₂ sulfuric acid [n-ZrO₂-SO₃H (n-ZrSA)] has been synthesized from the reaction of nano-ZrO₂ with chlorosulfonic acid as sulfonating agent. This catalyst was characterized via FT-IR, XRD, TGA, FESEM, TEM,

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Nano–ZrO2 sulfuric acid: A heterogeneous solid acid nano catalyst for
Biginelli reaction under solvent free conditions
Eskandar Kolvari,* Nadiya Koukabi, Maliheh M. Hosseini, Mitra Vahidian, Elham Ghobadi
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
NanoꢀZrO2 sulfuric acid [nꢀZrO2ꢀSO3H (nꢀZrSA)] has been synthesized from the reaction of nanoꢀZrO2
with chlorosulfonic acid as sulfonating agent. This catalyst was characterized via FTꢀIR, XRD, TGA,
FESEM, TEM, EDX, BET, BJH, ICP and pH analysis. We show that nꢀZrSA catalyzed Biginelli reaction
for the synthesis of 3,4ꢀdihydropyrimidinꢀ2(1H) one derivatives in high yields under solventꢀfree
10 conditions by the reaction of aldehyde, βꢀdiketoester and urea (or thiourea). Main advantages of the
catalyst are its nonꢀtoxic nature, high stability and reusability.
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Based on the above fundamental understandings and in continuing
our efforts towards the development of efficient and environmentally
1. Introduction
36ꢀ38
benign heterogeneous catalysts,
we prepared a green solid acid
Introduction of acid catalysts is one of the most frequently applied
processes in chemical industry, which has been a major area of
research interest.1ꢀ5 The need for development of heterogeneous
catalysts has arisen from the fact that homogeneous catalysts pose
several drawbacks. These flaws include equipment corrosion,
production waste, difficulty separation and recycling, high cost and
low efficiency. In this regard, heterogeneous catalysts compensates
the disadvantages mentioned of homogeneous catalysts, because
they have the benefit of decreasing corrosion, nonꢀtoxic, ease of
handling, high selectivity, easy separation and reusability to render
the process as green.6ꢀ11
The commonly used support for the more facile heterogeneous
catalytic synthesis are polymers, mesoporous materials,12 metal
oxides13 and nanoparticles.14 In the last decade, the synthesis and
application of metal oxide nanoparticles as heterogeneous catalysts
with different shapes and sizes have been developed. Solid acid nano
catalysts that are bridge between homogeneous and heterogeneous
catalysts, exhibit higher activity and selectivity than their
corresponding bulk materials. They are expected to be suitable
candidates for the design of highly active and selective
heterogeneous catalysts for organic synthesis due to their unique
physical, surface chemical and catalytic properties and large surfaceꢀ
toꢀvolume ratio. In addition, transport pathways of reactants and
products in the nanomaterialꢀbased catalysts could be significantly
shortened unlike many heterogeneous support matrices where a great
portion of catalyst sites are present deep inside the matrix backbones
and reactants have only limited access to the catalytic sites.15, 16
Sulfonation with chlorosulfonic acid17, 18 is an efficient procedure for
heterogenization of homogeneous catalysts that has been much
attention after Zolfigol’s report on the preparation of silica sulfuric
acid.19
nanoꢀcatalyst ZrO2–SO3H (nꢀZrSA) that has been applied for the
synthesis of 3,4ꢀdihydropyrimidinꢀ2(1H)ꢀones.
2. Results and Discussion
Due to the considerations needs to clean and green recovery of the
heterogeneous solid acid catalyst, we prepared nanoꢀZrO2 sulfuric
acid (nꢀZrSA) as a new heterogeneous solid acid nano catalyst.
Firstly, nano zirconia was prepared. Then, the reaction of nꢀZrSA
with chlorosulfonic acid in dry CH2Cl2 at 0 ˚C gave NanoꢀZrO2
sulfuric acid (nꢀZrSA) (Scheme 1). The synthesized catalyst was
characterized by different methods such as IR, FESEM, TEM, XRD,
TGA, EDX, BET, BJH, ICP and acid–base titration.
O
ClSO3H
n-ZrO2
n-ZrO2
OH
O
S
OH
HCl
O
Scheme 1. Preparation of nanoꢀZrO2ꢀSO3H (nꢀZrSA)
2.1. Catalyst characterization
2.1.1. FT-IR spectra
Fig. 1 shows the FTꢀIR results of the synthesized nanoꢀZrO2 powder
with and without SO3H loading, respectively. It revealed the
presence of major bands at 757 and 584 cmꢀ1 which are attributed to
the strong stretching vibrations of the Zr–O group. The vibration
bands at around 1649 cmꢀ1 is due to Zr–OH vibration and the
absorbance band at around 3434 cmꢀ1 was certified to the adsorbed
water (Fig. 1, graph a and b) which is consistent with the reported IR
spectra for nanoꢀZrO2.39 In the sulfuric acidꢀfunctionalized nanoꢀ
ZrO2 (Fig. 1b), some new bands appeared at 825–881 and 1059–
1172 cmꢀ1, which are attributed to the O=S=O asymmetric and
symmetric stretching vibration and SꢀO stretching vibration of the
sulfonic groups (ꢀSO3H), respectively. The spectrum also shows a
Zirconia (ZrO2) is one of the most important ceramic materials that
are widely used in chemical industry due to their unique electrical,
mechanical, optical and thermal properties.20ꢀ22 In this regard, relatively broad band around 2700–3600 cmꢀ1 due to OH stretching
absorption of the SO3H group. All these observations confirm that
the sulfonic groups have functionalized the surface of the nanoꢀZrO2.
significant progress in the synthesis of nanoꢀZirconia has been made
over the last decades and most applications of these novel materials
have been directed toward acidꢀcatalyzed reactions. In addition
zirconia has attracted considerable interest on account of its potential
application as a catalyst support.23ꢀ26
Synthesis of 3,4ꢀdihydropyrimidinꢀ2(1H)ꢀones (DHPMs) have
attracted great attention recently in synthetic organic chemistry due
to their pharmacological and therapeutic properties.27ꢀ30 The reaction
is accomplished from oneꢀpot condensation of βꢀdicarbonyl
compound, aldehyde and urea or (thiourea) in the presence of
various catalyst. A wide range of sulfunated solid acid catalysts such
as silica sulphuric acid,31 cellulose sulfuric acid,32 alumina sulfuric
acid,33 tungstate sulfuric acid,34 molybdate sulfuric acid35 and perlite
sulfonic acid36 have recently been used for this reaction.
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Fig. 2. Powder XRD patterns of (a) nano ZrO2 (b) nꢀZrSA
2.1.3. Thermo gravimetric analysis (TGA)
Fig. 1. FTꢀIR spectra of (a) nano ZrO2 (b) nꢀZrSA
The thermal behaviour of nꢀZrSA and pure nanoꢀZrO2 is shown in
Fig. 3. The results indicate that nanoꢀZrO2 (Fig. 3a) showed a weight
loss (3 wt.%) below 120 ˚C which corresponds to the loss of the
physically adsorbed water and then had a steady weight loss (3
wt.%) at the temperature lower than 800 ˚C, which possibly
corresponds to the dehydroxylation of ZrO2. The TGA analysis of nꢀ
ZrSA (Fig. 3b) shows twoꢀstages decomposition, completely
different from nanoꢀZrO2. A significant decrease in the weight
percentage (4 wt.%) of the nꢀZrSA at about 154 ˚C to is due to the
slow decomposition of covalently bounded SO3H groups. Finally, a
mass loss of approximately 8% weight occurred between 590 and
660 ˚C that is related to the sudden mass loss of SO3H groups.40ꢀ42
Also, from the TGA, it can be understood that ZrSA has a great
thermal stability (up 150 ˚C) confirming that it could be safely used
in organic reactions at temperatures in the range of 80ꢀ140 ˚C.
2.1.2. X-ray diffraction (XRD) analysis
Fig.2a is the Xꢀray powder diffraction pattern for the ZrO2
nanoparticles before modification. Combination of both phases’
tetragonal ZrO2 and monoclinic ZrO2 morphologies are clearly
shown according to the XRD. As can be seen, the broad diffraction
peaks are assigned to (111), (002), (200), (220), (113), (311) and
(222) reflections of tetragonal ZrO2 which coincides with JCPDS
card no 14ꢀ0534. The following peak signals with miler indices
(011), (110), (111), (ꢀ111), (002),(200), (021), (211), (ꢀ102), (121),
(ꢀ112), (202), (220), (ꢀ202), (013), (113), (311), (222), (ꢀ222) and (ꢀ
132) in Fig. 2a confirm the formation of monoclinic ZrO2, which
coincides well with the standard data for monoclinic ZrO2 (JCPDS
No. 07ꢀ0343).
The crystal size of the nano ZrO2 powder was also determined from
Xꢀray pattern using the DebyeꢀScherrer formula given as
t=0.9λ/B1/2cosθ, that t is the average crystal size, λ the Xꢀray
wavelength used (1.54 Å), B1/2 the angular line width at half
maximum intensity and θ the Bragg′s angle. The average crystal size
of the nanoꢀZrO2 powder for 2θ= 52.18º is calculated to be around
10 nm. Fig.2b illustrates XRD patterns of the samples of modified
nanoꢀZrO2. As shown in Fig.2b, the peak intensities of nꢀZrSA are
almost the same as those of nanoꢀZrO2 (Fig. 2a) and sulfonate
modification does not change the phase of nanoꢀZrO2.
Fig. 3. TGA spectra of (a) nano ZrO2 (b) nꢀZrSA
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2.1.4. FE-SEM and TEM analysis
are summarized in table 1. Also, Table 2 shows the textural
properties of nꢀZrSA.
Field emission scanning electron microscopy (FESEM) images of
the obtained materials were recorded to show the surface
morphological changes occurred on the catalyst due to the
modification of nanoꢀZrO2. It is evident that the morphology size
and distribution of the nanoparticles are almost homogeneous; and
the particle size has increased after the modification (Fig. S1, ESI†).
The TEM images as well as the particle size distribution profile are
shown in Fig. 4aꢀe, respectively. As could be seen from Fig. 4, the
material is mainly consisted of spherical particles. According to the
Fig. 4c the particle diameter size is 20 nm. It also indicates that the
maximum particle size distribution was in the range of 20ꢀ30 nm.
Table 1. BET data showing the textural properties of nꢀZrSA
BET surface
area (m2/g)
15.5
Pore volume Pore width
Sample
n-ZrSA
(cm3/g)
(nm)
0.0125
1.7
Table 2. BJH data showing the textural properties of nꢀZrSA
Sample
n-ZrSA
8.6 nm
Desorption Pore Diameter
Cumulative Desorption Pore
Volume
0.062 cm3/g
21.6 m2/g
Desorption Surface Area
2.1.7. PH analysis of catalyst
The optimum concentration of H+ of ZrSA was determined by acidꢀ
base potentiometric titration of the aqueous suspension of the
weighed amount of thoroughly washed catalyst with standard NaOH
solution. At first 100 mg ZrSA was dispersed in 20 ml H2O by
ultrasonic bath for 60 min at room temperature after which the pH of
solution was 1.62. The amount of the acid was neutralized by
addition of standard NaOH solution (0.085 N) to the equivalence
point of titration. The required volume of NaOH to this point was
3.41 ml. This is equal to a loading of 2.9 mmol H+/g.
2.2. Catalytic performances in multicomponent one-pot synthesis
of dihydropyrimidinones
After characterization, catalytic activity of the catalyst was examined
in the synthesis of dihydropyrimidinone derivatives. In a model
reaction and in the reaction between ethyl acetoacetate, urea and
benzaldehyde, the effect of the catalyst amount was investigated
(Table 3). To minimize the formation of byproducts and to achieve
good yield of the desired product, the reaction is optimized by
varying the amount of catalyst (5, 10, 15 and 20 mol%). The
percentage of the product formation using nꢀZrSA as the catalyst
was found 86, 98, 90 and 83%, respectively (Table 3, entries 2ꢀ5).
Therefore, it was found that the use of 10 mol% of the catalyst was
sufficient to promote the reaction. Larger amounts of the
nanocatalyst were found to have an inhibitory effect on the
formation of the product (entries 4 and 5). The catalytic activity of nꢀ
ZrSA was evident when trace product was obtained in the absence of
the catalyst (entry 1). The same reaction in the presence of 10 mol%
of nꢀZrSA was carried out at different temperatures (60, 90, 100, 120
˚C) under solventꢀfree conditions to assess the effect of temperature
on the reaction yield.
Fig. 4. TEM images of the synthesized nꢀZrSA nanomaterial and
particle size distribution
2.1.5. EDX analysis
Energyꢀdispersive Xꢀray spectroscopy (EDX) analysis of nꢀZrSA
indicated the presence of Zr, O, S as the components of nꢀZrSA
structure that confirm the formation of SO3H groups after
modification of nꢀZrO2 by sulfunation agent (Fig. S2, ESI†).
The yield increased as the reaction temperature was raised. At 90 ˚C,
product was obtained in high yield within 30 min. A further increase
in temperature and time did not improve the product yield (entries 7
and 8). To investigate the effect of the solvent on the catalytic
reaction, the model reaction in the presence of 10 mol% of nꢀZrSA
was carried out in various solvents under reflux conditions (Table 3,
entries 9ꢀ11). The result shows that the MeOH is the best solvent in
terms of the time and yield (Table 3, entry 11). There are also more
efficient, as well as solventꢀfree conditions. Because of the toxicity
of organic solvents, we considered green media. When the reactions
were conducted in solventꢀfree conditions, the expected products
2.1.6. BET and BJH texture analysis
The surface area and pore volume and pore size of the catalyst were
calculated using the BrunauerꢀEmmettꢀTeller (BET) and BJH
equations. BET surface area was acquired on a Beckman Coulter
SA3100 Surface Area Analyzer. Prior to N2ꢀphysical adsorption
measurements, catalyst was degassed at 150 ˚C for 120 min in the
nitrogen atmosphere. The specific surface area (SBET) of the nꢀ
ZrSA was determined with adsorptionꢀdesorption isotherms of N2 at
77 K. The surface area, pore volumes and pore width of the catalyst
4
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were obtained with excellent yields in low reaction times (Table 3,
entry 3).
Scheme 2. Synthesis of DHPMs catalyzed by nꢀZrSA
According to the optimization results, several substituted aromatic
aldehydes were reacted with ethyl acetoacetate (or methyl
acetoacetate) and urea (or thiourea) in the presence of 10 mol% nꢀ
ZrSA at 90 ˚C under solventꢀfree condition (Scheme 2). In all cases,
the threeꢀcomponent reaction proceeded smoothly to give the
corresponding 3,4ꢀdihydropyrimidinꢀ2(1H)ꢀones/thiones in moderate
to good yields. The reaction with aromatic aldehydes carrying
electronꢀdonating or electronwithdrawing groups gave the
corresponding product in good yields and high purity. Thiourea also
provided the Biginelli products in moderate yields (Table 4, entries
1–21).
Table 3. Optimization of synthesis of dihydropyrimidinonesa
Amount of Catalyst
Time
(min)
50
50
30
50
50
50
50
50
50
50
50
Yield
(%)
Trace
86
98
78
83
83
85
89
Entry
Solvent
Condition
(mol%)
-
5
1
2
3
4
5
6
7
8
9
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
EtOH
90 ˚C
90 ˚C
90 ˚C
90 ˚C
90 ˚C
10
15
20
10
10
10
10
10
10
60 ˚C
110 ˚C
120 ˚C
Reflux
Reflux
Reflux
45
91
Trace
10
11
[a]
MeOH
H2O
Benzaldehyde 1 (1 mmol), ethylacetoacetate 2 (1 mmol), urea 3 (1.2
mmol).
[b] Refers to isolated yield.
Table 4. nꢀZrSAꢀcatalyzed oneꢀpot synthesis of dihydropyrimidinonesa
Time
(min)
Yieldb
(%)
mp (˚C)
Entry
Ar
R
X
Product
Found
Reported
202ꢀ20443
231ꢀ23344
222ꢀ22443
214ꢀ21643
167ꢀ17045
20145
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Ph
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Me
Me
Me
Me
Me
Et
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
30
20
70
70
55
40
50
35
55
55
60
80
45
50
40
40
40
40
40
42
45
98
97
96
91
70
76
93
97
75
98
86
72
92
97
96
99
95
94
98
90
91
202ꢀ204
233ꢀ235
222ꢀ224
214ꢀ216
169ꢀ171
201ꢀ203
262ꢀ264
204ꢀ206
177ꢀ179
194ꢀ196
215ꢀ217
225ꢀ227
208ꢀ211
210ꢀ212
253ꢀ254
204ꢀ206
213ꢀ215
225ꢀ226
212ꢀ214
204ꢀ206
193ꢀ195
4ꢀHOC6H4
2ꢀClC6H4
4ꢀClC6H4
3ꢀHOC6H4
2ꢀHOC6H4
2ꢀMeOC6H4
4ꢀMeOC6H4
3,4ꢀMeOC6H3
3ꢀEtOꢀ4ꢀHOC6H3
4ꢀMeC6H4
3ꢀO2NC6H4
4ꢀO2NC6H4
4ꢀMeC6H4
3ꢀEtOꢀ4ꢀHOC6H3
4ꢀClC6H4
4ꢀ(CH3)2NC6H4
C6H5
C6H5
4ꢀHOC6H4
4ꢀO2NC6H4
26044
202ꢀ20444
17843
193ꢀ19543
216ꢀ21843
227ꢀ22844
208ꢀ21046
210ꢀ21344
253ꢀ25447
203ꢀ20544
213ꢀ21547
221ꢀ22244
212ꢀ21444
202ꢀ20348
193ꢀ19449
S
S
S
4s
4t
4u
Et
Et
a All products were characterized by IR, 1H NMR spectroscopic data, and melting points.
b Yields refer to isolated products.
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Table 5. Comparison study of the efficiency of the nꢀZrSA with some different reported catalysts in Biginelli reaction
Entry
Catalyst
Condition
Time (h)
Yield (%)
Ref.
50
51
52
53
54
1
2
3
4
5
MCMꢀ41ꢀRꢀSO3Ha
12ꢀTungstophosphoric acid
HClO4ꢀSiO2
MeCN /Reflux
AcOH /Reflux
Solventꢀfree/100 ˚C
Solventꢀfree/120 ˚C
H2O/100 ˚C
3
6
1.5
0.5
5
94
98
90
89
80
Bentonite/PSꢀSO3H
CSAb
55
56
57
6
7
8
DBSAc
Nafion NRꢀ50
TCCAd
Solventꢀfree/80 ˚C
MeCN /Reflux
EtOH/Reflux
3
94
95
94
3.5
12
Peresent
work
9
nꢀZrSA
Solventꢀfree/90 ˚C
0.5
98
a MCMꢀ41 anchored sulfuric acid
b Cellulose sulfuric acid
c pꢀdodecylbenzenesulfuric acid
d trichloroisocyanuric acid
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To show the merit of the present work in comparison with
was done by using a thermogravimetric analyzer on a Du Pont
reported results in the literature, we compared the results of nꢀ 40 2000 thermal analysis apparatus at a heating rateof 5 ˚C min−1
ZrSA catalyst with reported catalysts in the Biginelli reaction
(Table 5).
under air atmosphere. Field emission scanning electron
microscope (FESEM) images were acquired with a Philips XL30
field emission scanning electron microscope (Royal Philips
Electronics, Amsterdam, The Netherlands instrument operating at
45 10 kV. ZrO2 nanoparticles were dispersed in water and cast onto
a copper grid to study the sizes and morphology of the particles
by TEM (Transmission Electron Microscopy) using a Philips –
CM300 ꢀ 150 KV microscope. The average particle size
distribution was carried out using Image software. The EDX
50 characterization of the catalyst was performed using a Mira 3ꢀ
XMU scanning electron microscope equipped with an energy
dispersive Xꢀray spectrometer operating. BET surface area was
acquired on a Beckman Coulter SA3100 surface area analyzer.
The loading value of sulfur in the catalyst was estimated by
55 inductively coupled plasma ICP analysis with a ICP MS ELAN
DRCꢀe. The NMR spectra were recorded with a Bruker Avance
300 MHz instruments (1H NMR 300 MHz) in pure deuteriated
chloroform and dimethyl sulfoxide with tetramethylsilane (TMS)
as the internal standard. The purity of products was checked by
60 thin layer chromatography (TLC) on glass plates coated with
silica gel 60 F254 using nꢀhexane/ethyl acetate mixture as mobile
phase.
5
2.3. Plausible mechanism
A plausible mechanism for the Biginelli reaction in the presence
of nꢀZrSA is depicted in (Fig. S3, ESI†). Probably, this
transformation is triggered by an initial activation of aldehyde 1
by protonation which then undergoes Knoevenagel condensation
10 with βꢀketoester 2 forming intermediate I and after dehydration
gives the olefin II. Nucleophilic attack of the amino group in urea
or thiourea 3 at the βꢀcarbon of the α,βꢀunsaturated carbonyl
group followed by cyclization yields the product 4.
2.4. Reusability of the catalyst
15 One of the most important advantages of heterogeneous catalysis
is the possibility of reusing the catalyst by simple filtration,
without loss of activity. The recovery and reusability of the
catalyst was investigated in the Biginelli reaction. After
completion of the reaction, the catalyst was separated by
20 centrifuging, washed with hot ethanol and acetone and dried in
oven. Then the recovered catalyst was used in the next run. The
results of five consecutive runs showed that the catalyst can be
reused 5 times without significant loss of its activity (Fig. 5). ICP
analysis of fresh and recycled catalyst showed 3.8% and 2.7% of
25 S content respectively.
3.2. Preparation of nano-ZrO2
Initially, a solution of ZrOCl2.8H2O (0.5 M) in EtOH/H2O
65 (40:10) was prepared. Conc. NH3·H2O was diluted to a solution
(the concentration of it was twice that of the ZrOCl2 solution)
with the same solvent and was then added dropwise at rate of 3
ml/min to the vigorously stirred ZrOCl2 solution. A white,
gelatinous hydroxide precipitate was filtrated, washed with
70 EtOH/H2O in order to remove starting materials, dried at 100 ˚C
for 2h and calcined at 600 ˚C for 2h, ZrO2 nanoparticles with
white colour were obtained.58
3.3. Preparation of nano-ZrO2-SO3H (n-ZrSA)
ZrO2 nanoparticles (1g) in dry CH2Cl2 (20 ml) was added to a
75 suction flask that was equipped with a constantꢀpressure dropping
funnel and a gas inlet tube for conducting HCl gas over an
adsorbing solution (i.e., water). Then chlorosulfonic acid (0.25
ml, 3.76 mmol) was added dropwise over a period of 20 min in
an ice bath (0 ˚C). After completion of the addition, the reaction
80 mixture is allowed to stand for 1 h at room temperature, while the
residual HCl was eliminated by suction vessel. Solid powder was
washed with water (10 mL) and dried at 80 ˚C. NanoꢀZrSA
(1.15g) was obtained as a solid acid catalyst in the organic
synthesis.
Fig. 5. Reusability of nꢀZrSA in Biginelli reaction
3. Experimental section
30 3.1. General Remarks
All chemicals reagents were purchased from Merck and Fluka
and used without any further purification. Solvents were purified
by conventional methods. Fourier transform infrared
spectroscopy (FTꢀIR) was recorded on a Shimadzu 8400s
35 spectrometer using KBr pressed powder discs. The crystal
structure of synthesized materials was obtained with a Siemens
D5000 (Siemens AG, Munich,Germany) Xꢀray diffractometer
using CuꢀKa radiation of waveꢀlength 1.54˚A. Thermal analysis
85 3.4 General procedure for the synthesis of 3,4-
dihydopyrimidin-2(1H)-ones
In a typical procedure, a mixture of aromatic aldehyde (1 mmol),
βꢀdicarbonyl compounds (1 mmol), urea (or thiourea) (1.2 mmol)
was stirred at 90 ˚C utilizing nꢀZrSA (0.034g, 10 mol%) in
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RSC Advances
Page 8 of 10
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DOI: 10.1039/C5RA19350H
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solvent free condition for the appropriate time until the reaction
was complete. Completion of the reaction was monitored by Thin
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the completion of the reaction, the reaction mixture was cooled,
eluted with hot ethanol (5 mL), centrifuged and filtrated to collect
the formed precipitate. The crude product was recrystallized from
ethanol to yield pure 3,4ꢀdihydopyrimidinꢀ2(1H)ꢀones
derivatives.
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1285ꢀ1308.
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dihydopyrimidinꢀ2(1H)ꢀones between benzaldehyde (1 mmol),
ethylacetoacetate (1 mmol) and urea (1.2 mmol) under solventꢀ
free condition at 90 ˚C. After completion of the reaction, the
heterogeneous catalyst was separated from the production
15 mixture by centrifuging, washed with hot ethanol (2*15 ml), hot
acetone (2*10 ml) to remove the organic compounds and dried in
oven to be used in the next cycles. In every run, the yield of
product was performed in constant time. This process was
repeated 5 more times, affording the desired product in good
20 yields, with undiminishing efficiency.
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4. Conclusion
In summary, for the first time nanoꢀZrO2 sulfuric acid (nꢀ
ZrSA) was introduced as a heterogeneous highly powerful solid
25 acid catalyst for the synthesis of 3,4ꢀdihydropyrimidinꢀ2(1H)ꢀ
ones under solventꢀfree conditions. All the reactions work easily
for a variety of aldehydes with both electronꢀdonating and
electronꢀwithdrawing groups to give corresponding products in
excellent yields.
30 The catalyst was reused for 5 consecutive cycles with consistent
activity. The excellent catalytic performance, easy preparation
and separation of the catalyst make it to be a good heterogeneous
solid acid nano catalyst for organic synthesis and transformations.
In addition, good yields, lowꢀcost, short reaction times, nonꢀ
35 toxicity, solventꢀfree conditions and a recyclable catalyst is
supporting methods toward the green chemistry.
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Acknowledgement
We gratefully acknowledge the department of chemistry of
34. B. Karami, Z. Haghighijou, M. Farahi and S. Khodabakhshi,
Phosphorus, Sulfur Silicon Relat. Elem., 2012, 187, 754ꢀ761.
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Shaghasemi and B. Fasahati, Adv. Synth. Catal., 2012, 354, 2001ꢀ
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Semnan University for supporting this work.
40
Notes
110
115
120
125
Department of Chemistry, Semnan University, Semnan, Iran. Fax:
(+98)ꢀ23ꢀ336ꢀ54110;
kolvari@semnan.ac.ir
Tel:
(+98)ꢀ23ꢀ336ꢀ54058;
eꢀmail:
45
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