Environmentally friendly cyclotrimerization of substituted acetophenones catalyzed by a new nanocomposite of γ-Al2O3 nanoparticles decorated with H5PW10V2O40

Article abstract of DOI:10.1039/c6ra07141d

A highly efficient and simple procedure for the synthesis of substituted benzenes is described. A range of aryl and alkyl ketones were used to conduct triple condensation reactions in the presence of a catalytic amount of nano-Al₂O₃/

Full text of DOI:10.1039/c6ra07141d

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Environmentally friendly cyclotrimerization of
substituted acetophenones catalyzed by a new
nanocomposite of g-Al2O3 nanoparticles
decorated with H5PW10V2O40†
a
Cite this: RSC Adv., 2016, 6, 55319
Reza Tayebee, Kokab Savoji,b Maryam Kargar Razib and Behrooz Malekia
*
A highly efficient and simple procedure for the synthesis of substituted benzenes is described. A range of
aryl and alkyl ketones were used to conduct triple condensation reactions in the presence of a catalytic
amount of nano-Al2O3/H5PW10V2O40 as an efficient and recyclable catalyst under solvent-free
conditions. The supported heterogeneous catalyst was characterized using SEM, XRD, DLS, UV-Vis and
FT-IR spectroscopy. All condensation reactions were completed in a suitable time compared to existing
methods without formation of any undesirable 1,2,4-trisubstituted by-products. The entire synthetic
sequence is cost-effective, environmentally benign, and possesses convenient generality, which makes
the method meritorious as a valid contribution to the existing methods in the field of highly substituted
benzene synthesis.
Received 18th March 2016
Accepted 23rd May 2016
DOI: 10.1039/c6ra07141d
www.rsc.org/advances
in the presence of an acidic catalyst such as HCl,19 SiCl4,20
TiCl4,21 para-toluenesulfonic acid,22 Bi(OTf)3$4H2O,23 zircono-
cenebis(peruoroctanesulfonate)s,24 ethylenediamine or tri-
1. Introduction
Screening new efficient, high yielding, and environmentally
benign synthetic transformations based on the principles of
green chemistry and sustainability has been one of the major
challenges of the past decades.1–4 Moreover, in the context of
extending green synthetic strategies, solvent-free conditions
are also a promising and essential facet of green chemistry
through diminishing the consumption of hazardous organic
solvents in a scaled-down reaction and achieving energy
savings.
Polyarylated benzene propellers have attracted a great deal of
enthusiasm in cutting edge carbon nanotechnology, developing
new efficient molecular rotors, and production of new electro-
luminescent materials for at panel displays.5 1,3,5-Triar-
ylbenzenes are important polycyclic aromatic compounds
displaying potential applications in chemical industries such as
nanomaterials,6 photovoltaic resisting materials,7 conducting
polymers,8 electroluminescent devices,9 and various conjugated
polyaromatics.10 A number of synthetic strategies have been
developed previously to construct 1,3,5-triarylbenzenes. The
most common methods include the metal catalyzed cyclo-
trimerization of alkynes,11–14 cross coupling reactions of aryl
halides,15–18 and triple self-condensation of aryl methyl ketones
uoroacetic acid.25 Condensation of aryl acetones mediated by
Lewis acids provides an alternative pathway which only supplies
C3-symmetrical 1,3,5-triarylbenzenes. However, in spite of their
potential utility, these methods typically suffer from one or
more disadvantages such as problematic substrates, use of
a stoichiometric amount of catalyst, production of excessive
waste, moisture sensitivity, specialized handling requirements,
tedious workup, lack of generality, requirement of a transition
metal as well as complicated ligands, and non-recyclability of
the catalyst. Therefore, investigation of green routes by using
renewable catalytic systems to synthesize 1,3,5-triarylbenzenes
is highly desirable.
Heteropolyacids are strong and environmentally benign acid
catalysts which contain sufficient acidic sites and have been
explored as prominent catalysts in organic transformations.26–28
These inorganic metal–oxygen clusters bear high dispersion of
negative charge over many atoms of the polyanion by exploiting
strong Brønsted acid sites over the outer surface of the poly-
anion. However, some of their inherent drawbacks such as high
solubility in aqueous solution and continuous leakage during
operation limit the scope of their practical applications. To
overcome these restrictions, heterogenization of the hetero-
polyacid through immobilization onto different weakly acidic or
non-basic carriers is recommended. Up to now, several solid
acids such as zeolites,27,28 MCM-41,29 MCM-48,30 and SBA-15
(ref. 31) have been reported to support heteropolyacids via
immobilization.
aDepartment of Chemistry, Hakim Sabzevari University, Sabzevar, Iran. E-mail:
Rtayebee@hsu.ac.ir; Fax: +98-51-44410300; Tel: +98-51-44410310
bDepartment of Chemistry, Islamic Azad University of Tehran North Branch, Tehran,
Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07141d
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determined using Inductively Coupled Plasma-Mass Spec-
trometry (ICP-MS) conducted on a Varian Vista-Pro model ICP-
MS spectrometer. A freeze dryer, Model FD-10, Pishtaz Equip-
ment Engineering Co, Iran, was used for drying the prepared
nanocomposites. 1H and 13C NMR spectra were recorded on
a Bruker AVANCE instrument operated at 300 MHz using TMS
as an internal reference. All products were identied by
comparison of their spectral and physical data with those
previously reported.36 H5PW10V2O40$30H2O was prepared
according to reported procedures.37
Scheme
acetophenones.
1 General formula for the cyclotrimerization of
g-Al2O3 is perhaps one of the most important nanomaterials
employed as support for metal catalysts and is considered as
a promising material for a variety of applications due to its
distinctive chemical, mechanical, and thermal properties.32
Therefore, g-Al2O3 could be a good candidate to accommodate
and deposit heteropolyacids and produce effective catalysts in
organic synthesis by consideration of either Lewis and/or
Brønsted acidic characteristics.32
To continue our efforts in the development of one-pot
syntheses33–35 for various biologically important compounds
and owing to the importance of 1,3,5-triarylbenzenes, herein an
effective route for the preparation of these compounds is dis-
closed via the triple self-condensation of aryl methyl ketones in
the presence of a new heterogeneous catalyst composed of
nano-g-Al2O3 and H5PW10V2O40 (HPA) under solvent-free
conditions (Scheme 1). The present solvent-free methodology is
effective and provides only the desired C3-symmetrical 1,3,5-
triarylbenzenes in high yields by employing a simple, economic,
and reusable heterogeneous catalyst without producing waste.
2.2. Surface modication of g-alumina nanoparticles with
different modiers
Surface modication was performed by adding 0.9 g of nano-
alumina to 0.1 g of the modier dissolved in 5 ml of methanol.
Then, the resulting solution was stirred for 6 h at room
temperature. Finally, the mixture was heated gently to 40 ꢀC
until a white solid material was achieved. The resulting
ꢀ
composites were dried at 80 C for 8 h.
2.3. Preparation of g-Al2O3/H5PW10V2O40 nanocomposite
H5PW10V2O40 was supported on g-Al2O3 nanoparticles via wet
impregnation method. In this procedure, 0.9 g of nano-alumina
was dispersed in 5 ml of a methanolic solution of HPA (0.1 g/5
ml). This mixture was stirred at room temperature for 6 h, which
was supposed to be long enough to attain equilibrium of the
adsorption–desorption processes. The resultant mixture was
ꢀ
dried slowly at 40 C until a white solid material was attained.
Then, the sample was kept at 80 ꢀC for 8 h for complete drying.
To study effect of drying conditions on the catalytic efficacy of
the nanocomposite, a freeze drying technique was compared
with the usual classic drying in an oven at 80 ꢀC under air. The
results obtained demonstrate no obvious improvement in
2. Experimental
2.1. Materials and methods
All reagents and starting materials were commercially available catalyst efficiency under the freeze drying conditions.
and used as received. Hydrophilic g-aluminum oxide nano-
powder (>99%) was purchased from NanoSany Corporation,
Iran. The progress of reactions was monitored by TLC on silica
gel polygram SIL G/UV 254 plates. Melting points were recorded
on a Bamstead Electrothermal type 9200 melting point appa-
ratus. Scanning electron microscope (SEM) micrographs and
energy-dispersive X-ray (EDX) analysis were obtained using
a LEO 440i microscope (acceleration voltage 26 kV). Trans-
mission electron microscopy (TEM) images were obtained using
a Philips Zeiss-EM10C transmission electron microscope with
an accelerating voltage of 80 kV. X-ray diffraction patterns were
obtained on a PW1840 (PHILIPS) diffractometer with Cu Ka
radiation at 40 keV and 30 mA with a scanning rate of 3ꢀ minꢁ1
in a 2q range from 1ꢀ to 80ꢀ. Hydrodynamic particle size analysis
of nano-g-Al2O3/H5PW10V2O40 in an ethanol suspension was
determined by measuring dynamic light scattering using a Zeta
Sizer-HT (Brookhaven). Fourier transform infrared (FT-IR)
spectra were recorded on an 8700 Shimadzu Fourier Transform
spectrophotometer in the region of 400 to 4000 cmꢁ1 using KBr
2.4. General procedure for the cyclotrimerization of
acetophenones into 1,3,5-triphenylbenzenes
In a typical reaction, acetophenone (1 mmol) and g-Al2O3/HPA
(0.02 g) were added to a small test tube and the reaction mixture
ꢀ
was stirred at 90 C for the required time. Aer completion of
the reaction, as indicated by TLC using n-hexane/ethyl acetate
3 : 1 as an eluent, the reaction mixture was cooled to 25 ꢀC, then
hot ethanol was added and the mixture was stirred for 5 min.
The insoluble catalyst was isolated via simple ltration. The
ltrate containing water, as the only by-product of the cyclo-
trimerization, was concentrated under reduced pressure and
nally the obtained crude product was puried through re-
crystallization in EtOH : H2O (3 : 1) as conrmed by an intense
single spot in TLC. The pure products were specied based on
the spectral data and determination of their melting points.36
2.5. Calculation of pHpzc (point of zero charge pH)
pellets. Ultraviolet-visible spectra were recorded on a Photonix The pHpzc is a point at which the surface acidic or basic func-
UV-Vis Array spectrophotometer, Model Ar 2015, Iran. The tional groups no longer affect the pH value of the solution. The
tungsten and vanadium content of the nanocatalyst was effect of surface modication with HPA on the acidity of g-Al2O3
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was studied by calculating pHpzc for both g-Al2O3/HPA and g-
Al2O3 nanoparticles. The pH of a series of NaCl solutions (50 ml,
0.01 M) was adjusted to a value between 2 and 12 by the addition
of HCl (0.1 M) or NaOH (0.1 M) solutions in closed Erlenmeyer
asks. Then, pH of the solutions was dened as the initial pH
(pHI). Thereaer, 0.2 g of the modied g-Al2O3/HPA (or
unmodied g-Al2O3) was added and the nal pH (pHF) was
measured aer 48 h. Finally, the values of pHF vs. pHI and also
pHI vs. pHI were plotted and the intersect of the lines provided
38
the pHpzc
.
2.6. Representative spectral data of 1,3,5-triarylbenzenes
2.6.1. 1,3,5-Triphenylbenzene. Light yellow solid; mp 172–
ꢀ
174 C. IR (KBr): vmax ¼ 3051, 1650, 1496, 1432, 1126, 759, 699
cmꢁ1. 1H NMR (CDCl3): d ¼ 7.18 (3H, s); 7.72 (6H, m); 7.50 (6H,
m); 7.42 (3H, m).
2.6.2. 1,3,5-Tris(4-methylphenyl)benzene. White solid; mp
177–179 ꢀC. IR (KBr): vmax ¼ 3018, 2921, 1611, 1512, 1489, 1110,
813 cmꢁ1. 1H NMR (CDCl3): d ¼ 2.43 (s, 9H, CH3), 7.29 (d, J ¼ 8.0
Hz, 6H, ArH), 7.60 (d, J ¼ 8.0 Hz, 6H, ArH), 7.74 (s, 3H, ArH).
2.6.3. 1,3,5-Tris(4-methoxyphenyl)benzene. White solid;
Fig. 1 SEM micrograph of g-Al2O3 (a), and SEM (b) and TEM (c) images
of g-Al2O3/H5PW10V2O40. (a) is reproduced with permission.48
ꢀ
mp 141–143 C. IR (KBr): vmax ¼ 2954, 2917, 2849, 1603, 1463,
1
1377, 1261, 1099, 803 cmꢁ1. H NMR (CDCl3): d ¼ 3.81 (s, 9H,
OCH3), 6.92–7.60 (m, 12H, ArH), 7.75 (s, 3H, ArH).
1070–1090 cmꢁ1 are associated with M]O and P–O bonds,
respectively, which distinctively indicate the neat structure of
the Keggin framework for the heteropolyacid. In the FT-IR
spectrum of g-Al2O3/H5PW10V2O40, these four peaks slightly
overlap with those of g-Al2O3. However, spectrum (B) conrms
that loading of HPA occurred and H5PW10V2O40 was success-
fully immobilized on the pores of the nano-g-Al2O3 support.
The morphology of the g-Al2O3/H5PW10V2O40 nanocatalyst
was investigated using SEM and TEM microscopy (Fig. 1). The
TEM and SEM images show that the mean particle size of the
catalyst was clearly <40 nm. According to the TEM micrograph,
the two observed morphologies are due to g-Al2O3 (rod-like) and
H5PW10V2O40 (sheet-like) nanoparticles (Fig. 1c). Actually, by
introducing HPA into g-Al2O3, no signicant effect on the
homogeneity of the material occurred and the structure of the
nano-alumina was maintained. Concerning the chemical
composition of g-Al2O3/H5PW10V2O40, energy dispersive X-ray
(EDX) analysis was obtained (Fig. S2†). The analysis of EDX
spectrum proves the existence of tungsten, vanadium, oxygen,
and aluminium in the nanocatalyst; therefore, immobilization
of the heteropolyacid onto the nano g-Al2O3 is justied.
Fig. 2 displays wide-angle XRD patterns of the pure hetero-
polyacid (a), bare g-Al2O3 (b) and heteropolyacid loaded g-Al2O3/
HPA (c). The unmodied g-Al2O3 exhibits peaks at 37.5, 39.6,
45.7 and 67.5ꢀ which are attributed to the typical pattern of g-
alumina (Fig. 2b). These peaks are also evident for the modied
g-Al2O3/HPA (Fig. 2c). However, most of the peaks correspond-
ing to the heteropolyacid are absent or are strongly weakened in
the XRD pattern of g-Al2O3/HPA. These ndings indicate that
HPA clusters were too small or were successfully nely
dispersed onto the g-alumina surface rather than existing as the
free crystalline solid acid.40
2.6.4. 1,3,5-Tris(4-chlorophenyl)benzene. Light yellow
ꢀ
solid; mp 247–249 C. IR (KBr): vmax ¼ 3046, 1599, 1492, 1382,
1092, 1011, 814 cmꢁ1. 1H NMR (CDCl3): d ¼ 7.46 (d, J ¼ 8.4 Hz,
6H, ArH), 7.61 (d, J ¼ 8.4 Hz, 6H, ArH), 7.70 (s, 3H, ArH).
2.6.5. 1,3,5-Tris(4-bromophenyl)benzene. White solid; mp
263–264 ꢀC. IR (KBr): vmax ¼ 3043, 1595, 1489, 1379, 1075, 1007,
809 cmꢁ1. 1H NMR (CDCl3): d ¼ 7.54 (d, J ¼ 8.1 Hz, 6H), 7.61 (d, J
¼ 8.2 Hz, 6H), 7.69 (s, 3H).
2.6.6. 1,3,5-Tris(4-hydroxyphenyl)benzene. White solid; mp
237–239 ꢀC. IR (KBr): vmax ¼ 2953, 2918, 2848, 1606, 1460, 1378,
1
1263, 1097, 802 cmꢁ1. H NMR (CDCl3): d ¼ 3.81 (s, 9H), 6.94–
7.41 (m, 12H), 7.79 (s, 3H).
2.6.7. 1,3,5-Tris(4-nitrophenyl)benzene. White solid; mp
151–152 ꢀC. IR (KBr): vmax ¼ 3043, 1595, 1489, 1379, 1085, 1007,
849 cmꢁ1. 1H NMR (CDCl3): d ¼ 7.56 (d, J ¼ 8.1 Hz, 6H), 7.62 (d, J
¼ 8.2 Hz, 6H), 7.70 (s, 3H).
3. Results and discussion
3.1. Characterization and physicochemical properties of g-
Al2O3/HPA nanoparticles
The physicochemical and morphological properties of the
supported g-Al2O3/H5PW10V2O40 catalyst were investigated by
means of FT-IR, UV-Vis, SEM, XRD, and DLS. The most infor-
mative technique for the investigation of the surface interac-
tions between H5PW10V2O40 and nano-g-Al2O3 is FT-IR. The
spectra of free H5PW10V2O40 (A), HPA impregnated nano-g-
Al2O3 (B), and nano-g-Al2O3 (C) were recorded for structural
characterization purposes (Fig. S1†). The Keggin heteropolyacid
shows four characteristic bands in the range of 750–1100
cmꢁ1
.
The observed bands at around 778–785 and 850–890
39
cmꢁ1 are assigned to W–O–W. The peaks about 950–998 and
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Fig. 3 Stacked plot of UV-Vis spectral changes with time after the
addition of H5PW10V2O40 to g-Al2O3.
size histogram of the nanocatalyst shows that most of the
particles range in size from 25–60 nm and possess an average
size of 30 nm.
3.2. Equilibrium studies and adsorption isotherms
The equilibrium relationships between g-Al2O3, as the adsor-
bent, and H5PW10V2O40, as the adsorbate, and the adsorption
behavior of the heteropolyacid could be described by an
adsorption isotherm. The shape of the isotherm not only
provides information on the adsorption affinity of HPA, but also
provides relevant information about adsorption spontaneity
and the stability of adsorbent–adsorbate interactions.
Fig. 2 X-ray diffraction patterns of pure H5PW10V2O40 (a), g-Al2O3 (b),
and g-Al2O3/H5PW10V2O40 (c) nanoparticles.
The amount of the Keggin heteropolyacid adsorbed onto g-
Al2O3 nanoparticles was calculated by studying the adsorption
isotherm based on Freundlich and Langmuir patterns and the
experimental data were tted linearly.41,42 The Langmuir
isotherm theory assumes monolayer coverage of the adsorbate
over a homogenous adsorbent surface leading to surface
homogeneity of the adsorbent; whereas the Freundlich
adsorption isotherm is an indication of the surface heteroge-
neity of the adsorbent. According to the Langmuir adsorption
model, the maximum adsorption would be due to the saturated
monolayer of solute molecules on the adsorbent surface. The
non-linear expression of the Langmuir model is given by eqn
(1):
The UV-Vis spectrum of HPA in methanol displays an
absorption peak at 236 nm, which is assigned to the charge-
transfer absorption in the heteropoly cage (Fig. 3). To nd the
extent of H5PW10V2O40 that can be loaded on the support, UV-
Vis spectra of solutions containing HPA before and aer addi-
tion of g-alumina were obtained. For this purpose, 0.1 g of g-
Al2O3 was suspended in a 40 ml methanol solution of HPA with
an initial concentration of 2500 ppm. The suspension was
stirred for 120 min and absorption spectra were recorded for the
solution aer removing the solid catalyst by ltration (Fig. 3).
Evidently, most of the heteropolyacid content was adsorbed on
the alumina surface aer 90 min. Aer stirring the solution and
comparing with calibration curves for standard solutions, 0.026
mmol of H5PW10V2O40 was loaded per gram of the g-Al2O3
support.
q ¼ abCe/(1 + bCe)
(1)
For conrmation of HPA loading on the support, 0.01 g of
the heterogeneous nanocatalyst Al2O3/H5PW10V2O40 was
analyzed by ICP-MS. The results indicate the presence of 0.22
mmol H5PW10V2O40. This nding is in good agreement with the
results from UV-Vis analysis.
Finally, hydrodynamic particle size analysis of g-Al2O3/
H5PW10V2O40 was performed by measuring the dynamic light
scattering of the nanoparticles in ethanol (Fig. S3†). The particle
where constants “a” and “b” are the Langmuir constants
showing the capacity and energy of adsorption, respectively,
and can be calculated from non-linear tting. Therefore, “a” is
the maximum amount of adsorbate per unit weight of sorbent
to form a complete monolayer on the surface (mg gꢁ1) and
corresponds to the free energy or net enthalpy of adsorption.
Moreover, “b” is a constant related to the energy of adsorption
and indicates the stability of the adsorbate–adsorbent
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composite. The applicability of Langmuir and Freundlich (entry 4); but, less than nano-alumina modied with the two
adsorption isotherms in the interaction of HPA with g-alumina above heteropolyacids.
is determined based on the correlation coefficient (R2). The
3.4.2. Studying catalyst amount on the cyclotrimerization
calculations indicate that the reported experimental data reaction. To nd the optimized reaction conditions and
correlates better with the Langmuir isotherm (Fig. S4 and Table studying the catalytic activity of g-Al2O3/H5PW10V2O40
S1†).
compared with bulk g-Al2O3, a model reaction with acetophe-
none was explored with different amounts of the above catalysts
(Table 2). First of all, the condensation reaction was inefficient
in the absence of catalyst (entry 1) and <1% of product was
attained aer 2.5 h. Moreover, 5 mg of bulk alumina was inef-
cient and did not attain a detectable amount of product aer
2.5 h, whereas the same amount of g-Al2O3/H5PW10V2O40
provided a 50% yield under similar reaction conditions aer 2.5
h. However, the yield was increased by enhancing the catalyst
amount for both the bulk and surface modied nano-alumina.
Results shown in Table 2 clearly conrm that the modied
nano-alumina behaved better than the bare g-Al2O3. The best
yield was achieved in the presence of 20 mg of the heteroge-
neous catalyst, due to the proportional increase in the number
of active sites. However, beyond a catalyst loading of 20 mg, no
signicant improvement in percentage yield was observed.
3.4.3. Effect of temperature on the cyclotrimerization
reaction. The effect of temperature on the condensation of
acetophenone was investigated under the standard reaction
conditions. Expectedly, the yield of product was negligible at 25
ꢀC and <25% yield was attained aer 2.5 h; whereas, percentage
yield was increased with increasing the reaction temperature
and the best yield of 85% was attained at 90 ꢀC (Fig. S6†). A
further increase in temperature to 120 ꢀC did not afford
a signicant enhancement in product yield, theꢀrefore because
of the technical and experimental facilities, 90 C was chosen
for all condensation reactions.
3.3. Calculating the pHpzc of g-Al2O3
Point of zero charge (pHpzc) is the value at which the surface of
a solid inorganic material bears a net neutral charge. At pHpzc
,
the surface acidic (or basic) functional groups no longer
contribute to the pH value of the solution.43 It encompasses
positive charge in solution with pH < pHpzc; whereas, in solu-
tion with pH > pHpzc negative charge would be developed at the
surface of the mineral compound. Therefore, changes to the
protonation–deprotonation characteristics of Al–OH groups in
the framework of g-Al2O3 as a result of HPA loading caused
some structural transformation which altered the pHpzc and
therefore the equilibrium pH of the suspension.44 The pHpzc for
g-Al2O3 was analyzed to evaluate the effect of HPA loading on
the acidity of the nano g-alumina surface. The lower pHpzc for
the modied g-Al2O3/HPA (pHpzc ¼ 5.7) compared to unmodi-
ed g-Al2O3 conrms an increment in the acidity of nano-
alumina aer surface modication with the heteropolyacid
(Fig. S5†).
3.4. Catalytic tests
3.4.1. Effects of some surface modiers on the catalytic
activity of g-Al2O3. Effects of the nature and chemistry of
different surface modiers on the catalytic activity of g-Al2O3 in
the preparation of 1,3,5-triphenylbenzene were subjected to
study. The two Keggin heteropolyacids H3PW12O40 and
H5PW10V2O40 with comparable acidity were loaded on g-Al2O3
through a simple wet impregnation method, then the prepared
heterogeneous catalysts were applied in the above-mentioned
condensation reaction. The results show their pronounced
catalytic activity compared to bare g-Al2O3 (Table 1, entry 1) and
free heteropolyacids (entries 5–6). Moreover, the surface of g-
Al2O3 was also modied with CeO2. The nanocomposite (CeO2/
Al2O3) has an improved catalytic activity over that of g-Al2O3
3.4.4. Effect of reaction time on the cyclotrimerization
reaction. The effect of reaction time was also studied in the
condensation of acetophenone under the optimized reaction
conditions. As expected, the yield was increased with time and
most of the reaction progress was attained aer 90 min,
however, the time of 150 min was selected in all cases to reach
the best yield. A further increase in reaction time did not
enhance percentage yield signicantly (Fig. S7†).
Table
2 Cyclotrimerization of acetophenone with modified and
unmodified g-Al2O3 nanoparticlesa
Table 1 Effects of some surface modifiers on the catalytic activity of
g-Al2O3 nanoparticles in the preparation of 1,3,5-triphenylbenzenea
Yield (%)
Nanocatalyst
(mg)
HPA (mmol)
content
g-Al2O3
(nano)
g-Al2O3/HPA
(nano)
Entry
Modier
Nanocomposite
Yield%
Entry
1
2
3
4
5
6
—
g-Al2O3
H3PW12O40/Al2O3
H5PW10V2O40/Al2O3
CeO2/Al2O3
—
—
35
75
85
70
<3
<3
1
2
3
4
5
6
—
5
10
15
20
30
—
n. d.
—
15
22
35
n. d.
50
63
77
85
H3PW12O40
H5PW10V2O40
CeO2
H3PW12O40
H5PW10V2O40
0.11
0.22
0.33
0.44
0.66
b
b
38
85
a
a
Reaction conditions: 1 mmol of acetophenone was mixed with 0.02 g
Reaction conditions: 1 mmol of acetophenone was mixed with the
desired amount of catalyst under solvent-free conditions at 90 ꢀC for
2.5 h.
of the nanocatalyst under solvent free conditions at 90 ꢀC for 2.5 h.
b
0.45 mmol of the heteropolyacids were used. Note that the mean
value of 0.024 mmol HPA is considered per 1.07 g of the nanocatalyst.
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reported catalysts in terms of mol% of the catalyst, temperature,
reaction time, and percentage yield (Table 4). Accordingly, the
present methodology is clearly superior over the mentioned
protocols considering the above variables.
3.4.5. Studying the generality of the cyclotrimerization
reaction. In the next step, to assess the scope and generality of
the present methodology, different acetophenones were sub-
jected to the cyclotrimerization reaction under the optimized
conditions. Thus, a range of structurally different substituted
acetophenones were condensed to supply the corresponding
products. As shown in Table 3, both electron withdrawing and
electron releasing substituents on the aromatic ring show good
activity towards the condensation reaction. Noteworthy, the
present method was effective in the cyclotrimerization of
3.4.7. Studying the reusability of g-Al2O3/H5PW10V2O40. To
explore the reusability of g-Al2O3/H5PW10V2O40 in the conden-
sation reaction, cyclotrimerization of acetophenone was studied
under the optimized reaction conditions. The catalyst was easily
separated from the reaction mixture. Then, aer washing with
chloroform, the catalyst was dried in air and was further dried
in a vacuum oven at 100 ꢀC for 8 h. Finally, the recycled catalyst
was reused in another condensation reaction. The ndings
prove that catalytic activity similar to that of the fresh catalyst
was observed and no signicant loss of activity was detected
(Fig. 4). Moreover, to ensure reproducibility of the trans-
formation, repeated typical experiments were carried out under
identical reaction conditions. The obtained yields were found to
be reproducible within ꢂ3% variation.
p-nitro-acetophenone, as
a strongly electron withdrawing
substituted acetophenone, which has been difficult to cyclo-
trimerize by other methods (entry 4).15
3.4.6. Comparison of the efficacy of the present method-
ology with some reported methods. To explore the capability of
the present methodology among that of other reported methods
for the preparation of the title compounds, the synthesis of
1,3,5-triphenylbenzene was considered in the presence of some
3.4.8. Hot ltration test. To conrm that the catalytic
activity was generated from the heterogeneous nanocatalyst,
and not from leached heteropolyacid in the reaction mixture,
a hot ltration test was undertaken. In this technique, the
condensation reaction of acetophenone (1 mmol) in the pres-
ence of catalyst (g-Al2O3/HPA, 0.02 g) was performed under
solvent-free conditions at 90 ꢀC for 0.5 h. In this step, the yield
of the product was 31%. Then, the solid catalyst was ltered off
Table
3 Synthesis of different 1,3,5-triphenylbenzene derivatives
a
catalyzed by surface modified g-Al2O3/H5PW10V2O40
Entry
R
Time (h) Yield (%) Mp (ꢀC) Product
Ref.
1
2
3
4
5
6
7
H
2.5
85
60
85
68
82
77
93
172–174 (Ph)3C6H3
36d
4-OMe 2.5
4-Br
4-NO2 2.5
4-Me
4-OH
4-Cl
141–143 (4-OMePh)3C6H3 36d
263–264 (4-BrPh)3C6H3 23
151–152 (4-NO2Ph)3C6H3 36a
177–179 (4-MePh)3C6H3
237–239 (4-OHPh)3C6H3
247–249 (4-ClPh)3C6H3
2
3
3.5
2.5
23
36a
36d
a
Reaction conditions: substrate (1 mmol), 90 ꢀC, 0.02 g of g-Al2O3/
H5PW10V2O40, solvent-free conditions. All products were known and
were identied by comparison of their spectral and physical data with
those previously reported.
Fig. 4 Reusability of g-Al2O3/H5PW10V2O40 in the cyclotrimerization
of acetophenone.
Table 4 Comparison of the catalytic activity of g-Al2O3/H5PW10V2O40 with some reported catalysts towards the cyclotrimerization of
acetophenone
Entry
Catalyst
Catalyst (mol%)
Solvent
Time (h)
Temp. (ꢀC)
Yield (%)
Ref.
1
2
3
4
5
6
7
H3PMo12O40
SnCl4
5
10
8
20
10
30
0.02 g
Ethanol
Ethanol
Free
5
24
4
Reux
Reux
155
87
55
76
60
45
60
85
21
45
46
47
CuCl2
DBSAa
Free
3
130
HCl
Amberlyst 15
g-Al2O3/HPA
Ethanol
Toluene
Free
11
10
2.5
Reux
Reux
90
46
36b
This work
a
DBSA: 4-dodecylbenzenesulfonic acid.
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under hot conditions and with the ltrate, which was obtained
aer separation of the catalyst, the reaction was continued for
another 2 h at the same reaction temperature. However, an
inappreciable increase in the yield (35%) was observed. This
result conrmed the heterogeneous nature of the nanocatalyst
in this condensation reaction and that no signicant leaching
of the heteropolyacid occurred during the course of the
reaction.
3.4.9. Suggesting a plausible reaction pathway. A plausible
reaction pathway for the synthesis of 1,3,5-triarylbenzenes is
proposed in Scheme S1.† The reaction possibly proceeded
through protonation of the aryl methyl ketone to form the
intermediates (A) and (B) in the presence of the nanocatalyst.
The reaction between these intermediates followed by dehy-
dration, produced an a,b-unsaturated carbonyl compound (C).
Activation of (C) by the catalyst and subsequent reaction with (A)
led to the generation of (D) that on subsequent dehydration
followed by prototropic shi in the presence of catalyst afforded
(E). 6-p electrocyclization of (E) led to (F) which upon dehy-
dration afforded the desired product (G) and released the
catalyst for the next catalytic cycle.27
References
1 P. T. Anastas and T. C. Williamson, Green Chemistry:
Frontiers in Benign Chemical Syntheses and Processes, Oxford
Science Publications, New York, 1998.
2 A. S. Matlack, Introduction to Green Chemistry, Marcel
Dekker, Inc., New York, 2001.
3 P. Tundo, A. Perosa and F. Zecchini, Methods and Reagents for
Green Chemistry, John Wiley and Sons, Oxford, UK, 2007.
4 K. Tanaka and F. Toda, Chem. Rev., 2000, 100, 1025.
5 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,
R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos,
I. L. Bredas, M. Logdlung and W. R. Salaneck, Nature,
1999, 397, 121.
6 T. Kadota, H. Kayeyama, F. Wakaya, K. Gamo and Y. Shirota,
Chem. Lett., 2004, 33, 06.
7 C. Li, M. Liu, N. G. Pschirer, M. Baumgarten and K. Mullen,
¨
Chem. Rev., 2010, 110, 6817.
8 R.
Ghanbaripour,
I.
Mohammad
poor-Baltork,
M. Moghadam, A. R. Khosropour, S. Tangestaninejad and
V. Mirkhani, J. Iran. Chem. Soc., 2012, 9, 791.
9 M. J. Plater, M. Mckay and T. Jackson, J. Chem. Soc., Perkin
Trans. 1, 2000, 2695.
g-Al2O3 nanoparticles with a relatively high specic surface
of >138 m2 gꢁ1 have two types of Lewis and Bronsted acidic
¨
sites.48 The catalytic activity of the nanoparticles would be
associated with the improved acidic properties of the surface
aer modication with the heteropolyacid. Lewis-type acid sites
of alumina are formed during the dehydration process by
combination of the two surface hydroxyl groups.49 Considering
previous work,50 it is assumed that the heteropolyacid is phys-
10 (a) B. P. Dash, R. Satapathy, E. R. Gaillard, J. A. Maguire and
N. S. Hosmane, J. Am. Chem. Soc., 2010, 132, 6578; (b)
Q. Wang, C. Zhang, B. C. Noll, H. Long, Y. Jin and
W. Zhang, Angew. Chem., Int. Ed., 2014, 53, 10663.
11 K. Yoshida, I. Morimoto, K. Mitsudo and H. Tanaka,
Tetrahedron, 2008, 64, 5800.
¨
isorbed onto the surface of g-Al2O3. However, strong Bronsted
12 G. Hilt, C. Hengst and W. Hess, Eur. J. Org. Chem., 2008,
2293.
13 Y. Xu, Y. Pan, Q. Wu, H. Wang and P. Liu, J. Org. Chem., 2011,
76, 8472.
14 A. Ciammaichella, A. Leoni and P. Tagliatesta, New J. Chem.,
2010, 34, 2122.
acid sites may be formed by the reaction between H5PW10V2O40
and the hydroxyl groups present on the surface of g-Al2O3.
Therefore, the approach of modifying a g-Al2O3 surface with
HPA can increase the concentration of the surface acidic
sites.51,52
15 S. Shi and Y. Zhang, J. Org. Chem., 2007, 72, 5927.
16 L. Bai and J. X. Wang, Adv. Synth. Catal., 2008, 350, 315.
17 M. Mora, C. Jimenez-Sanchidrian and J. R. Ruiz, Appl.
Organomet. Chem., 2008, 22, 122.
18 S. Paul and J. H. Clark, Green Chem., 2003, 5, 635.
19 R. E. Lyle, E. J. DeWitt, N. M. Nichols and W. Cleland, J. Am.
Chem. Soc., 1953, 75, 5959.
20 M. J. Plater and M. Praveen, Tetrahedron Lett., 1997, 38, 1081.
21 Z. Li, W. H. Sun, X. Jin and C. Shao, Synlett, 2001, 1947.
22 Y. Zhao, J. Li, C. Li, K. Yin, D. Ye and X. Jia, Green Chem.,
2010, 12, 1370.
23 F. Ono, Y. Ishikura, Y. Tada, M. Endo and T. Sato, Synlett,
2008, 15, 2365.
4. Conclusions
In conclusion, g-Al2O3/H5PW10V2O40 is recommended as
a useful, efficient, and recyclable heterogeneous catalyst for the
cyclotrimerization of substituted aacetophenones to afford only
C3-symmetrical 1,3,5-triarylbenzenes in high yields. The pre-
sented simple protocol shows several advantages over some
reported literature methods from economic and environmental
points of view, such as operational simplicity, no need to be
protected from the atmosphere, short reaction time, mild
reaction conditions, and good yields. Moreover, water was the
only by-product of this reaction and the used catalyst was
recyclable. This safe and clean procedure would be an accept-
able candidate for automated applications.
24 G. P. Zhang, R. H. Qiu, X. H. Xu, H. H. Zhou, Y. F. Kuang and
S. H. Chen, Synth. Commun., 2012, 42, 858.
25 S. Zhang, Z. Xue, Y. Gao, S. Mao and Y. Wang, Tetrahedron
Lett., 2012, 53, 2436.
26 R. Tayebee, N. Abdollahi and M. Ghadamgahi, J. Chin. Chem.
Soc., 2013, 60, 1014.
Acknowledgements
Partial nancial support from the Research Councils of Hakim
Sabzevari University and Islamic Azad University of Tehran,
North Branch is greatly appreciated.
27 R. Tayebee, M. M. Amini, F. Nehzat, O. Sadeghi and
M. Armaghan, J. Mol. Catal. A: Chem., 2013, 366, 140.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 55319–55326 | 55325

View Article Online
RSC Advances
Paper
28 R. Tayebee, M. M. Amini, N. Abdollahi, A. Aliakbari, S. Rabiei 37 (a) E. R. Cadot, A. T. M. Thouvenot and G. Herve, Inorg.
and H. Ramshini, Appl. Catal., A, 2013, 468, 75.
29 R. Tayebee, M. M. Amini, M. Akbari and A. Aliakbari, Dalton
Trans., 2015, 44, 9596.
Chem., 1992, 31, 4128; (b) G. A. Tsigdinos and
C. J. Hallada, Inorg. Chem., 1968, 7, 437.
38 P. C. C. Faria, J. J. M. Orfao and M. F. R. Pereira, Water Res.,
2004, 38, 2043.
30 R. Tayebee and B. Maleki, J. Chem. Sci., 2013, 125(2), 335.
31 R. Tayebee, M. M. Amini, M. Ghadamgahi and 39 (a) P. Karandikar, K. C. Dhanya and S. Deshpande, Catal.
M. Armaghan, J. Mol. Catal. A: Chem., 2013, 366, 266.
32 (a) R. Rohman, H. Mecadon, A. T. Khan and B. Myrboh,
Tetrahedron Lett., 2012, 53, 5261; (b) L. Cheng and X. P. Ye,
Commun., 2004, 69; (b) R. Tayebee, F. Nehzat, E. Rezaei-
Seresht, F. Z. Mohammadi and E. Raee, J. Mol. Catal. A:
Chem., 2011, 351, 1837.
Catal. Lett., 2009, 130, 100; (c) A. Eliassi and M. Ranjbar, 40 (a) H. Cai, J. Mao, Y. Tao and X. Zheng, React. Kinet. Catal.
International Journal of NanoScience and Nanotechnology,
2014, 10, 13; (d) B. F. Mirjalili, A. Bamoniri and
Lett., 2009, 97, 43; (b) M. G. Kulkarni, R. Gopinath,
L. C. Meher and A. K. Dalai, Green Chem., 2006, 8, 1056.
S. M. Nezamalhosseini, J. Nanostruct. Chem., 2015, 5, 367; 41 E. Korngold, N. Belayev and L. Aronov, Sep. Purif. Technol.,
(e) D. Salari, A. Niaei, S. A. Hosseini, R. Aleshzadeh and 2003, 33, 179.
H. Afshary, J. Nanosci. Nanotechnol., 2010, 6, 23; (f) 42 J. Febrianto, A. Kosasih, J. Sunarso, Y. Ju, N. Indraswati and
D. R. Abd El-Haz, M. Riad and S. Mikhail, J. Nanostruct. S. Ismadji, J. Hazard. Mater., 2009, 162, 616.
Chem., 2015, 5, 393; (g) M. Salehi and E. Arabsarhangi, Int. 43 O. K. Mahadwad, P. A. Parikh, R. V. Jasra and C. Patil, Bull.
Nano Lett., 2015, 5, 141; (h) A. Badiei, M. Azmard, Mater. Sci., 2011, 34, 551.
M. Karimi and P. Zarabadi-poor, J. Nanostruct. Chem., 44 H. Guan, E. Bestland, C. Zhu, H. Zhu, D. Albertsdottir,
2014, 4, 259; (i) S. Sadjadi and S. Rasouli, Int. J. Nano
Dimens., 2011, 1, 177; (j) S. V. Atghia and S. S. Beigbaghlou,
J. Nanostruct. Chem., 2013, 3, 38.
J. Hutson, C. T. Simmons, M. Ginic-Markovic, X. Tao and
A. V. Ellis, J. Hazard. Mater., 2010, 183, 616.
45 K. Phatangare, V. Padalkar, D. Mhatre, K. Patil and
A. Chaskar, Synth. Commun., 2009, 39, 4117.
46 N. O. Mahmoodi and N. Hajati, J. Chin. Chem. Soc., 2002, 49,
91.
47 D. Prasad, A. Preetam and M. Nath, C. R. Chim., 2013, 16,
252.
48 Iranian Nanomaterials Pioneers Nanosany Group, https://
www.sites.google.com/site/nanosany.
33 R. Tayebee, M. M. Amini, S. Pouyamanesh and A. Aliakbari,
Dalton Trans., 2015, 44, 5888.
34 R. Tayebee, B. Maleki, F. M. Zonoz, R. M. Kakhki and
T. Kunani, RSC Adv., 2016, 6, 20687.
35 R. Tayebee, M. M. Amini, H. Rostamian and A. Aliakbari,
Dalton Trans., 2014, 43, 1550.
36 (a) H. R. Safaei, M. Davoodi and M. Shekouhy, Synth.
Commun., 2013, 43, 2178; (b) A. Kumar, M. Dixit, 49 E. J. M. Hensen, D. G. Poduval, V. Degirmenci,
S. P. Singh, A. Goel, R. Raghunandan and P. R. Maulik,
D. A. J. M. Ligthart, W. Chen, F. Mauge, M. S. Rigutto and
Tetrahedron Lett., 2009, 50, 4335; (c) X. Jing, F. Xu, Q. Zhu,
J. A. R. V. Veen, J. Phys. Chem. C, 2012, 116, 21416.
X. Ren, C. Yan, L. Wang and J. Wang, Synth. Commun., 50 R. Feng, S. Liu, P. Bai, K. Qiao, Y. Wang, H. A. Al-Megren,
2005, 35, 3167; (d) R. Ghanbaripour, I. Mohammadpoor- M. J. Rood and Z. Yan, J. Phys. Chem. C, 2014, 118, 6226.
Baltork, M. Moghadam, A. R. Khosropour, 51 F. Dumeignil, M. Rigole, M. Guelton and J. Grimblot, Chem.
S. Tangestaninejad and V. Mirkhani, Polyhedron, 2012, 31, Mater., 2005, 17, 2369.
721; (e) Q. Gao, F. Bao, X. Feng, Y. Pan, H. Wang and D. Li, 52 S. A. El-Hakam and E. A. El-Sharkawy, Mater. Lett., 1998, 36,
ARKIVOC, 2013, 3, 49.
167.
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