Microwave-promoted efficient conversion of acetophenones to 1,3,5-triarylbenzenes catalyzed by H3PW12O40 and nano-silica supported H3PW12O40 as reusable catalysts

Article abstract of DOI:10.1016/j.poly.2011.10.027

H₃PW₁₂O₄₀ and nano-silica supported H ₃PW₁₂O₄₀ were found to be efficient heterogeneous catalysts for the preparation of 1,3,5-triarylbenzenes via triple self condensation of acetophenones

Full text of DOI:10.1016/j.poly.2011.10.027

Polyhedron 31 (2012) 721–728
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Microwave-promoted efficient conversion of acetophenones
to 1,3,5-triarylbenzenes catalyzed by H PW O and nano-silica
3
12 40
supported H PW O as reusable catalysts
3
12 40
⇑
⇑
Rashid Ghanbaripour, Iraj Mohammadpoor-Baltork , Majid Moghadam , Ahmad R. Khosropour,
Shahram Tangestaninejad, Valiollah Mirkhani
Catalysis Division, Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2011
Accepted 26 October 2011
Available online 4 November 2011
H PW12O40 and nano-silica supported H PW12O40 were found to be efficient heterogeneous catalysts for
3
3
the preparation of 1,3,5-triarylbenzenes via triple self condensation of acetophenones under microwave
irradiation and solvent-free conditions. High yields, short reaction times, easy work-up, easy availability
and handling, eco-friendly and reusability of the catalysts are the main aspects of the present method.
The catalytic mechanism of 1,3,5-triarylbenzenes synthesis is also proposed.
Keywords:
Acetophenones
Ó 2011 Elsevier Ltd. All rights reserved.
1
H
,3,5-Triarylbenzenes
PW12
3
O
40
Microwave irradiation
Supported catalyst
1
. Introduction
In recent years, the chemical synthesis under solvent-free condi-
catalytic activity. Due to the unique properties of HPAs, various
chemical transformations using these compounds as catalysts have
been reported in the literature during the recent years [10–16].
1,3,5-Triarylbenzenes represent an important class of organic
compounds because of their applications in the fields of electrode
and electroluminescent devices [17], resisting materials [18] and
conducting polymers [19]. These compounds can also be used as
important intermediates for the synthesis of buck minster fuller-
enes, pharmaceuticals and conjugated star polyaromatics [20,21].
On this basis, a number of synthetic methods to approach 1,3,5-tri-
arylbenzenes have been previously developed [22–34]. Although
these methods have proved to be useful for the synthesis of these
compounds, there are some limitations, including low yields, te-
dious workup, nonrecyclability of the catalyst, long reaction times
and the use of stoichiometric amounts of catalysts which are
expensive, moisture sensitive and toxic.
tions has developed into a powerful methodology as it reduces the
toxic waste produced and therefore becomes less harmful to the
environment [1]. On the other hand, one of the major challenges
facing chemists is to develop new synthetic transformations that
are not only efficient and high-yielding but that are also environ-
mentally benign [2]. Consequently, green chemistry and sustain-
ability have received great attention in organic synthesis during
the last decade [2–4].
The application of heterogeneous solid acid catalysts such as
heteropoly acids (HPAs) have attracted extensive interest in recent
times due to their easy recovery and reusability, non-corrosive-
ness, environmental benignity, non-hazardous nature, low cost
and operational simplicity [5,6]. Heteropoly acids, especially
H
3
PW12
O
40, have received great attention among the acid catalysts,
As a part of our ongoing efforts to explore environmentally
benign synthetic reactions using HPA catalysts [35–40], herein
we report a rapid, practical and efficient synthesis of 1,3,5-triaryl-
benzenes via a microwave-assisted cyclotrimerization of acetoph-
because of their high acid strength and selectivity properties [5,6].
A major disadvantage of bulk HPAs lies in their low specific surface
area, less than 10 m g that can be improved by supporting on
oxidic carriers especially SiO which is an inexpensive and non-
2
À1
2
3
enones catalyzed by H PW12O40 (HPW) and nano-silica supported
corrosive neutral solid [7–9]. The support provides an opportunity
to spread HPAs over a large surface area, which generally increases
3
H PW12O
40 under solvent-free conditions (Scheme 1).
2
. Experimental
⇑
Corresponding authors. Tel.: +98 311 7932705; fax: +98 311 6689732
M. Moghadam).
(
All starting materials were purchased from Merck or Fluka
E-mail addresses: imbaltork@sci.ui.ac.ir(I. Mohammadpoor-Baltork), moghadamm
@sci.ui.ac.ir (M. Moghadam).
chemical companies. All products were identified by comparison
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.10.027

7
22
R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
Ar
informative technique for the investigation of surface interaction
between HPW and nano-SiO is FT-IR spectroscopy (Fig. 1). The
O
2
HPW or HPW@nano-SiO2
MW, Solvent-free
3
+ 3H O
Ar
pure HPW displays bands at 1081 (P–O), 985 (W–O), 889 (W–
2
Ar
À1
Oc–W) and 801 cm (W–Ob–W). The FT-IR spectrum of HPW@na-
Ar
À1
no-SiO
2
shows bands at 982 (W–O), 899 and 801 cm (W–O–W)
Scheme 1. Synthesis of 1,3,5-triarylbenzenes.
and the band assigned to P–O vibrations is masked by nano-SiO
broad vibrations.
2
3
The UV–Vis spectra of HPW in CH CN displayed an absorption
Table 1
Optimization of the reaction conditions for triple-self condensation of acetophenone.
peak at 255 nm, which is assigned to charge-transfer absorption
in the heteropoly cage (Fig. 2A). This absorption peak was appeared
in the solid-state UV–Vis spectra of catalyst and since pure nano-
Yield (%)a
Entry
Catalyst (mmol)
T (°C)
Power (W)
2
SiO shows no UV absorption peak; therefore, these results indi-
cated that primary Keggin structure has been introduced into the
nanostructure framework (Fig. 2B).
The SEM image of the catalyst showed that the catalyst particles
have sizes in the range of nanometers (Fig. 3).
The HPW loading on the supports, which were calculated from
W content in supported HPW catalyst, was determined by neutron
activation analysis. The amount of W loading on the supported cat-
alyst was obtained about 0.068 mmol/g.
In order to optimize the reaction conditions, we carried out the
cyclotrimerization of acetophenone and studied the effect of tem-
perature, amount of catalyst and MW power (Table 1). First, the
model reaction was examined in the presence of different hetero-
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
3
4
3
3
3
3
3
3
3
3
3
3
PMo12
SiW12
O
40 (0.15)
90
90
90
90
90
100
80
90
85
90
90
90
450
450
450
450
450
450
450
500
400
450
450
450
55
70
93
93
73
93
81
93
85
95
95
91
O
40 (0.15)
PW12
PW12
PW12
PW12
PW12
PW12
PW12
PW12
PW12
PW12
O40 (0.15)
O
O
O
O
O
O
O
O
O
40 (0.17)
40 (0.12)
40 (0.15)
40 (0.15)
40 (0.15)
40 (0.15)
40@nano-SiO
40@nano-SiO
40@nano-SiO
1
1
1
0
1
2
2
2
2
(0.08)
(0.07)
(0.05)
a
Isolated yield.
3
poly acids (15 mol%) as catalysts, among which H PW12O40 proved
of their physical and spectral data with those of authentic samples.
Melting points were determined using Stuart Scientific SMP2 appa-
to be the most effective catalyst for this purpose. Investigation of
different parameters in the model reaction indicated that the best
yield of the desired product was obtained using 1:0.15 molar ratios
ratus. IR spectra were recorded on a Shimadzu IR-435 spectropho-
1
tometer. H NMR spectra were recorded in a CDCl
3
solution on a
of acetophenone and H
at 90 °C after 18 min. Under the same reaction conditions, the cat-
alytic activity of nano-silica supported H 40 was also inves-
3
PW12O40 with an applied power of 450 W
Bruker-Avance 400 MHz spectrometer. The microwave system
used for these experiments includes the following items: Micro-
SYNTH lab station, complete with glass door, dual magnetron sys-
tem with pyramid-shaped diffuser, 1000 W delivered power, ex-
haust system, magnetic stirrer, ‘‘quality pressure’’ sensor for
flammable organic solvents, ATCFO fiber optic system for auto-
matic temperature control.
3
PW12O
tigated and the best result was obtained using 0.07 mmol of the
catalyst per mmol of acetophenone. Increasing the temperature,
the amount of catalyst, reaction time and/or applying a higher
power level failed to improve the yield, while decreasing these
parameters led to reduced yield.
To explore the generality of the reaction, we extended our study
with different acetophenones. The results of synthesis of a series of
3 40
2.1. Preparation of nano-silica supported H PW12O
1
3
,3,5-triarylbenzenes in the presence of H PW12O40 (15 mol%) are
The supported H
of incipient wetness. To a solution of H
ized water (50 mL), high surface area SiO
1.6 g) was added and stirred for 24 h. The resulting pastes were
dried at 100 °C and then calcined at 250 °C for 4 h [41–44].
3
PW12
O
40 catalyst was prepared by the method
PW12 40 (0.4 g) in deion-
(Cabosil 20) support
summarized in Table 2. This protocol tolerates a variety of acetoph-
enones containing both electron-withdrawing and electron-donat-
ing substituents. The reactions are generally clean and the desired
3
O
2
(
1
,3,5-triarylbenzenes were obtained in high yields (80–93%) and in
very short reaction times (18–30 min). In order to show the influ-
ence of supporting on the reactivity of the catalyst, these reactions
were performed in the presence of nano-silica supported
H
H
2
.2. General procedure for the synthesis of 1,3,5-triarylbenzenes in the
presence of H 40 and nano-silica supported H 40 under
MW irradiation
3
PW12O
3
PW12O
PW12
PW12
O
O
40. As shown in Table 2, the comparable results as with
40 was obtained using only 7 mol% of supported catalyst.
3
3
These results clearly revealed that the supporting HPA on nano-sil-
ica increases the specific surface area and therefore, the catalytic
activity of the catalyst is increased.
In order to verify the effect of MW irradiation, some of these
reactions were also carried out under thermal conditions at the
same temperature (90 °C) used for the system under MW irradia-
tion (Table 2, entries 1, 2 and 9). It was found that significantly
lower yields (8–48%) were obtained under these conditions even
after 8 h. These clearly showed that MW irradiation improved
the results.
A mixture of acetophenone derivative (1 mmol) and H
3 40
PW12O
(
15 mol%) or nano-silica supported H 40 (7 mol%) was sub-
3
PW12O
jected to microwave irradiation (450 W) at 90 °C for the appropri-
ate time according to Table 2. The progress of the reaction was
monitored by TLC (eluent:ether/ethyl acetate, 6:1). After comple-
tion of the reaction, hot ethyl acetate (10 mL) was added and the
catalyst was separated by filtration. The solvent was evaporated
and the residue was recrystallized from ethanol or acetone to af-
ford the pure 1,3,5-triarylbenzene.
The acceleration of the reactions by microwave mainly results
from two effects: thermal effect and specific microwave effect
3
. Results and discussion
[
45,46]. Thermal effect (dielectric heating) resulting from mate-
3
.1. Synthesis of 1,3,5-triarylbenzenes in the presence of H
3
PW12
O
40
rial–microwave interaction which allows fast and uniform distri-
bution and conversion of microwave energy into heat. Specific
microwave effect can be expected for the polar mechanism, when
the polarity is increased during the reaction from the ground state
(GS) towards the transition state (TS). For the reactions under
and nano-silica supported H 40 under MW irradiation
3
PW12O
2
The supported HPW@nano-SiO catalyst was characterized by
elemental analysis, FT-IR spectroscopy and SEM. The most

R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
723
Table 2
Synthesis of 1,3,5-triarylbenzenes catalyzed by HPW and HPW@nano-SiO
2
.
Entry
1
Ar
Product
Time (min)
18 (480b)
HPW
Yield (%)
HPW@nano-SiO
Yield (%)
2
a
a
93 (45b)
95 (48b)
2
22 (480b)
83 (30b)
85 (30b)
3
20 (480b)
89 (35b)
82 (20b)
87 (32b)
91 (38b)
81 (18b)
89 (35b)
4
23 (480b)
5
20 (480b)
6
28 (480b)
82 (15b)
85 (20b)
(
continued on next page)

7
24
R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
Table 2 (continued)
Entry
Ar
Product
Time (min)
HPW
Yield (%)
HPW@nano-SiO
Yield (%)
2
a
a
7
20 (480b)
84 (30b)
86 (30b)
8
28 (480b)
25 (480b)
28 (480b)
30 (480b)
30 (480b)
80 (10b)
85 (12b)
84 (10b)
80 (8b)
84 (14b)
87 (15b)
84 (15b)
82 (10b)
85 (10b)
F
9
10
11
12
82 (8b)
a
Isolated yield.
Reaction was performed under thermal conditions.
b
investigation, it seems that stabilization of the transition state is
more effective than that of the ground state, and therefore, the
reactivity is enhanced as a result of decreased activation energy.
A plausible mechanism for the synthesis of 1,3,5-triarylbenz-
enes has been proposed in Scheme 2. The acetophenone was first
converted to intermediates A and B in the presence of the catalyst.
The reaction between these intermediates followed by dehydration
produces a,b-unsaturated carbonyl compound C. Activation of C by
the catalyst and subsequent reaction with A provides D that on
subsequent dehydration followed by prototropic shift in the pres-
ence of catalyst afforded E. 6p-electrocyclization of E leads to F
which upon dehydration produces the desired product G and re-
leases the catalyst for the next catalytic cycle. On the other hand,
the desired product was obtained by the reaction of
a,b-unsatu-

R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
725
2
Fig. 1. The FT-IR spectrum of (A) pure nano-silica, (B) pure HPW and (C) HPW@nano-SiO .

7
26
R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
3.3.3. 1,3,5-Tris(4-isopropylphenyl)benzene (Table 2, entry 3)
White solid; m.p. 166–167 °C (Lit. [24] 166 °C). IR (KBr):
À1
1
m
max = 2957, 2867, 1637, 1509, 1457, 1362, 1054, 826 cm . H
NMR (400 MHz, CDCl
3
): d = 1.32 (d, J = 6.4 Hz, 18H, CH(CH
), 7.36 (d, J = 7.6 Hz, 6H, ArH), 7.63 (d,
J = 7.6 Hz, 6H, ArH), 7.76 (s, 3H, ArH).
3 2
) ,
2
3 2
.93–3.01 (m, 3H, CH(CH )
3.3.4. 1,3,5-Tris(4-methoxyphenyl)benzene (Table 2, entry 4)
White solid; m.p. 142–143 °C (Lit. [25] 143 °C). IR (KBr):
À1
m
max = 2954, 2917, 2849, 1603, 1463, 1377, 1261, 1099, 803 cm
.
1
H NMR (400 MHz, CDCl
3
): d = 3.81 (s, 9H, OCH
3
), 6.92–7.60 (m,
1
2H, ArH), 7.75 (s, 3H, ArH).
3
.3.5. 1,3,5-Tris(4-cyclohexylphenyl)benzene (Table 2, entry 5)
Light yellow solid; m.p. 230–232 °C. IR (KBr): max = 3022, 2922,
850, 1591, 1509, 1447, 1396, 998, 906, 823, 783 cm . H NMR
): d = 1.29–1.35 (m, 6H, CH ), 1.42–1.50 (m, 12H,
), 1.91 (d, J = 11.9 Hz, 6H, CH ) 1.97 (d, J = 11.9 Hz, 6H, CH ),
m
À1
1
2
(
400 MHz, CDCl
3
2
CH
2
2
2
2
.59–2.60 (m, 3H, CH),7.35 (d, J = 8.1 Hz, 6H, ArH), 7.64 (d,
1
3
J = 8.1 Hz, 6H, ArH), 7.77 (s, 3H, ArH). C NMR (100 MHz, CDCl
3
):
d = 26.60, 27.35, 34.91, 44.71, 125.07, 127.63, 127.70, 139.24,
Fig. 2. UV–Vis spectrum of (A) pure HPW and (B) HPW@nano-SiO
2
.
+
1
42.53, 147.86. MS (EI): m/z = 552.38 (18.54, [M ], 207.16
(
17.81), 83.27 (41.67), 69.20 (52.08), 57.21 (71.67), 55.14 (100)).
3.3.6. 1,3,5-Tris(bipheny)benzene (Table 2, entry 6)
White solid; m.p. 239–240 °C (Lit. [33] 236.2–238 °C). IR (KBr):
À1
1
m
max = 3025, 1590, 1486, 1380, 1217, 1005, 829, 766, 695 cm . H
NMR (400 MHz, CDCl ): d = 7.36–7.52 (m, 10H, ArH), 7.57–7.85 (m,
7H, ArH), 7.90 (s, 3H, ArH).
3
1
3.3.7. 1,3,5-Tris(2-naphthyl)benzene (Table 2, entry 7)
Light yellow solid; m.p. 240–241 °C (Lit. [34] 241–242 °C). IR
À1
1
(
KBr):
mmax = 3050, 1598, 1507, 854, 747 cm
.
H
NMR
(
3
400 MHz, CDCl ): d = 7.45–7.55 (m, 6H, ArH), 7.91–8.02 (m, 12H,
ArH), 8.08 (s, 3H, ArH), 8.23 (s, 3H, ArH).
Fig. 3. SEM image of HPW@nano-SiO
2
.
3.3.8. 1,3,5-Tris(4-fluorophenyl)benzene (Table 2, entry 8)
White solid; m.p. 237–238 °C (Lit. [31] 238–240 °C). IR (KBr):
À1
1
mmax = 3059, 1605, 1509, 1448, 1222, 1158, 823, 772 cm
.
H
rated carbonyl compound C with acetophenone under the same
conditions. This is a good indication that the proposed mechanism
is reasonable.
3
NMR (400 MHz, CDCl ): d = 7.18 (t, J = 8.4 Hz, 6H, ArH), 7.63–766
(
m, 6H, ArH), 7.67 (s, 3H, ArH).
3
.3.9. 1,3,5-Tris(4-chlorophenyl)benzene (Table 2, entry 9)
3
.2. Catalyst reusability
White solid; m.p. 245–246 °C (Lit. [25] 246 °C). IR (KBr):
À1
1
m
(
max = 3046, 1599, 1492, 1382, 1092, 1011, 814 cm
400 MHz, CDCl ): d = 7.46 (d, J = 8.4 Hz, 6H, ArH), 7.61 (d,
J = 8.4 Hz, 6H, ArH), 7.70 (s, 3H, ArH).
. H NMR
The catalyst recovery and reuse was evaluated taking the cyclo-
3
trimerization of acetophenone as a model reaction. After comple-
tion of the reaction, hot ethyl acetate was added and the catalyst
was separated by simple filtration. The catalyst was dried at
3
.3.10. 1,3,5-Tris(4-bromophenyl)benzene (Table 2, entry 10)
1
00 °C and reused. The recovered catalyst can be used at least five
consecutive times without significant loss in product yield (Table
).
White solid; m.p. 260–261 °C (Lit. [31] 260–261 °C). IR (KBr):
À1
1
m
max = 3043, 1595, 1489, 1379, 1075, 1007, 809 cm
400 MHz, CDCl ): d = 7.55 (d, J = 8.4 Hz, 6H, ArH), 7.62 (d,
J = 8.4 Hz, 6H, ArH), 7.70 (s, 3H, ArH).
. H NMR
3
(
3
3
3
.3. Spectral data
3
.3.11. 1,3,5-Tris(4-iodophenyl)benzene (Table 2, entry 11)
.3.1. 1,3,5-Triphenylbenzene (Table 2, entry 1)
Light yellow solid; m.p. 174–175 °C (Lit. [34] 175–176 °C). IR
Dark brown solid; m.p. 264–265 °C (Lit. [24] 265 °C). IR (KBr):
À1
1
mmax = 3055, 1593, 1485, 1376, 1003, 809 cm
(400 MHz, CDCl ): d = 7.41 (d, J = 8.0 Hz, 6H, ArH), 7.69 (s, 3H,
ArH), 7.82 (d, J = 8.0 Hz, 6H, ArH).
.
H NMR
À1
1
(
KBr):
NMR (400 MHz, CDCl
J = 8.2 Hz, 6H, ArH), 7.79 (s, 3H, ArH).
m
max = 3061, 1595, 1476, 1452, 1026, 751, 699 cm
.
H
3
3
): d = 7.34–7.53 (m, 9H, ArH), 7.71 (d,
3.3.12. 1,3,5-Tris(3-chlorophenyl)benzene (Table 2, entry 12)
3
.3.2. 1,3,5-Tris(4-methylphenyl)benzene (Table 2, entry 2)
White solid; m.p. 171 °C (Lit. [29] 171–172 °C). IR (KBr):
À1
White solid; m.p. 176–177.5 °C (Lit. [24] 178 °C). IR (KBr):
m
max = 3059, 1592, 1484, 1381, 1101, 1079, 861, 771, 684 cm
.
À1
1
1
m
max = 3018, 2921, 1611, 1512, 1489, 1110, 813 cm
400 MHz, CDCl ): d = 2.43 (s, 9H, CH ), 7.29 (d, J = 8.0 Hz, 6H,
ArH), 7.60 (d, J = 8.0 Hz, 6H, ArH), 7.74 (s, 3H, ArH).
.
H NMR
H NMR (400 MHz, CDCl
3
): d = 7.39–7.42 (m, 3H, ArH), 7.45 (d,
(
3
3
J = 7.2 Hz, 3H, ArH), 7.58 (d, J = 7.2 Hz, 3H, ArH), 7.69 (s, 3H, ArH),
7.75 (s, 3H, ArH).

R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
727
H
O
O
OH
H PW12O40
3
H
Ar
Ar
Ar
CH
2
H PW12O
2
40
A
B
H PW12O
2
40
H
H
O
CH3
Ar
O
O
O
+
CH2 Ar
HO
+
H PW12O
3
40
Ar
Ar
CH3
H
H
2
PW12
O
40
OH
O
Ar
Ar
H PW12O
3
40 Ar
+
H PW12O40
3
-
H O
2
H
H
Ar
Ar
Ar
Ar
C
O
O
H
H
2
PW12O
40
O
Ar
H C
Ar
Ar
H
Ar
A
H PW12O40
3
3
-
H O
H
2
Ar
Ar
HO
D
Ar
Ar
Ar
Ar
HO
H PW12O40
2
OH
OH
2
Ar
H
H
electrocyclization
H PW12
3
O
40
Ar
Ar
Ar
Ar
Ar
Ar
E
F
Ar
-
H O
2
+
H PW12O40
3
Ar
Ar
G
Scheme 2. Plausible mechanism for synthesis of 1,3,5-triarylbenzenes.
Table 3
a
Investigation of catalyst reusability in cyclotrimerization of acetophenone.
O
HPW or HPW@nano-SiO2
CH3
+ 3 H O
2
3
o
MW (450 W), 90 C, 18 min
Run
Yield (%)b
HPW
HPW@nano-SiO
2
1
2
3
4
5
93
90
87
86
84
95
92
88
86
85
a
Reaction conditions: acetophenone (1 mmol), HPW (0.15 mmol) or HPW@nano-SiO
Isolated yield.
2
(0.07 mmol), applied power (450 W), 90 °C and 18 min.
b

7
28
R. Ghanbaripour et al. / Polyhedron 31 (2012) 721–728
[
[
[
14] A.C. Garade, V.S. Kshirsagar, C.V. Rode, Appl. Catal. A: Gen. 354 (2009) 176.
4
. Conclusion
15] J. Zhang, R. Ohnishi, T. Okuhara, Y. Kamiya, Appl. Catal. A: Gen. 353 (2009) 68.
16] V.V. Costa, K.A. da Silva Rocha, I.V. Kozhevnikov, E.V. Gusevskaya, Appl. Catal.
A: Gen. 383 (2010) 217.
In conclusion, we have demonstrated an efficient microwave-
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merization of structurally diversified acetophenones catalyzed by
H PW12O40 and nano-silica supported H PW12O40 under solvent-
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practical alternative to the existing methodologies for the synthe-
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Acknowledgement
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Financial support of this work by Center of Excellence of Chem-
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