Polystyrene-supported AlCl3 as a highly active and reusable heterogeneous lewis acid catalyst for the one-pot synthesis of quinoxalines

Article abstract of DOI:10.1002/hc.21039

A new and environmentally benign protocol for the synthesis of quinoxaline derivatives through the condensation reactions of 1,2-diketones and 1,2-phenylenediamines using cross-linked polystyrene-supported aluminum chloride (PS/AlCl₃) as a highly active and reusable heterogeneous Lewis acid catalyst is described. This polymeric catalyst is stable and can be easily recovered and reused without appreciable change in its efficiency. Copyright

Full text of DOI:10.1002/hc.21039

Heteroatom Chemistry
Volume 00, Number 0, 2012
Polystyrene-Supported AlCl as a Highly
3
Active and Reusable Heterogeneous Lewis
Acid Catalyst for the One-Pot Synthesis of
Quinoxalines
Ali Rahmatpour
Polymer Science and Technology Division, Research Institute of Petroleum Industry, 14665-1137
Tehran, Iran
Received 20 December 2011; revised 24 April 2012
of quinoxaline derivatives have been reported in the
ABSTRACT: A new and environmentally benign
literature [8]. Among them, the condensation of 1,2-
protocol for the synthesis of quinoxaline deriva-
aryldiamine with 1,2-diketone in refluxing ethanol
tives through the condensation reactions of 1,2-
or acetic acid for 2–12 h is a general approach
diketones and 1,2-phenylenediamines using cross-
[9]. In addition to common homogeneous Lewis
linked polystyrene-supported aluminum chloride
acids, many other catalysts including I2/DMSO
(PS/AlCl3) as a highly active and reusable heteroge-
and CH3CN [10], sulfamic acid/CH3OH [11],
neous Lewis acid catalyst is described. This polymeric
CuSO4·5H2O [12], polyaniline-salt/dichloroethane
catalyst is stable and can be easily recovered and
[13], MnCl2[14], Zn[(l)proline]/HOAc [15], CAN
reused without appreciable change in its efficiency.
[16], montmorillonite K-10 [17], H6P2W18O62·24H2O
ꢀ
C
2012 Wiley Periodicals, Inc. Heteroatom Chem 00:1–6,
[18], p-TSA [19], Bi(OTf)3[20], and NbCl5[21] have
2012; View this article online at wileyonlinelibrary.com.
been used to promote these condensations. Ox-
DOI 10.1002/hc.21039
idative couplings of epoxides and ene-1,2-diamines
[22] catalyzed by Bi(0), Pd(OAc)2, RuCl2-(PPh3)3-
TEMPO, and MnO2 have been reported [23]. The
INTRODUCTION
condensation has also been accomplished under
catalyst-free conditions, but needs microwave and
ultrasound irradiations [24]. In addition, the above
cyclocondensation process could proceed in ionic
liquid [25]. However, some of the reported proce-
dures suffer from one or more drawbacks, such
as the use of harmful organic solvents (pollution),
harsh reaction conditions, unsatisfactory product
yields, tedious work-up processes (especially in po-
lar solvents), long reaction times, the use of toxic
and detrimental metal precursors as catalysts, and
relatively expensive reagents, requirement for spe-
cial apparatus. The main disadvantage of almost all
existing methods is that the catalysts are destroyed
in the work-up procedure, and their recovery and
reuse is often impossible, which limit their use un-
der environmentally benign processes. Owing to the
Functionalized quinoxalines are an important class
of nitrogen-containing heterocycles, and they con-
stitute useful intermediates in organic synthesis.
Quinoxaline derivatives have broad biological activ-
ities and have been used as anticancer [1], antivi-
ral [2], anti-inflammatory [3], antibacterial agents
[4], and kinase inhibition agents [5]. In addition to
the medicinal applications, quinoxalines have been
used as dyes [6] and key intermediates in the synthe-
sis of organic semiconductors [7]. Over the years,
a number of synthetic methods for the preparation
Correspondence to: A. Rahmatpour; e-mail: rahmatpoura@
ripi.ir.
c
ꢀ
2012 Wiley Periodicals, Inc.
1

2
Rahmatpour
importance of quinoxalines from pharmaceutical,
industrial, and synthetic points of view, introduction
of a simple, efficient, versatile, and environmentally
friendly method for the preparation of these com-
pounds is still in demand.
divinyl benzene, grain size 0.25–0.65 mm) in car-
bon disulfide under reflux conditions. The loading
capacity of the polymeric catalyst obtained by using
the gravimetric method and checked by an atomic
absorption technique was 0.485 mmol AlCl3/g of
complex beads catalyst [31]. The data obtained by
these two techniques showed, within experimental
error, that the catalyzing species are in the form
of AlCl3 supported on the polymeric support. The
UV spectrum of the solution of the PS–AlCl3 com-
plex in CS2 showed a new strong band at 480 nm,
which is due to the formation of a stable π → p type
coordination complex between the benzene rings
in the PS carrier with aluminum trichloride. The
IR spectrum of PS/AlCl3 showed new absorption
peaks due to the C C-stretching vibration and the
C H-bending vibration of the benzene ring at 1550–
1650 and 400–800 cm−1, by which complex forma-
tion was demonstrated. Although AlCl3 is a water-
sensitive, corrosive, and environmentally harmful
compound, the PS–AlCl3 complex is a stable and
water-tolerant species. This polymeric catalyst is
easy to prepare, stable in air for a long time with-
out any change, easily recycled, and reused without
appreciable loss of its activity.
At the onset of the research, the model reaction
between 1,2-phenylenediamine (2a) and benzil (1a)
was investigated to find the most appropriate reac-
tion conditions and evaluate the catalytic efficiency
of the PS/AlCl3 catalyst. The results are summarized
in Table 1. It was found that polar solvents such
as CH3CN and C2H5OH were better than nonpolar
solvents. No reaction occurred when H2O was used
as a solvent (Table 1, entry 6); it may be because
of the aggregation of the catalyst caused by its hy-
drophobic nature, leading to inadequate access of
substrates to the active sites of the catalyst. We there-
fore selected ethanol as a solvent. Further studies
showed that the reaction temperature has a great
influence on the model reaction, and the best re-
sults were observed when the reaction temperature
was 78◦C (Table 1, entries 9,12,13). Consequently, we
chose 78◦C as the optimal temperature for the model
In the past few decades, there have been sig-
nificant developments in the application of effec-
tive and safe heterogeneous catalysts for the pro-
duction of organic chemicals [26]. Heterogeneous
catalysts, which are widely used in organic transfor-
mations, have good thermal stability, can be easily
separated from the reaction mixture, and can be of-
ten regenerated and reused. Therefore, heterogeniz-
ing of a homogeneous metal species by supporting it
on an insoluble support has attracted a lot of inter-
est as a suitable method for solving many practical
problems including recovery of the catalyst from a
reaction mixture and recycling. The use of polymer-
supported catalysts in organic transformations and
the design of functionalized polymers carrying metal
species have gained importance in organic synthesis
since they offer economic advantage in developing
reusable catalysts and environmental friendly pro-
cesses [27]. Polystyrene-supported aluminum chlo-
ride, PS/AlCl3, which is a tightly bound complex
between anhydrous AlCl3 and PS-divinylbenzene
copolymer, has been described for the first time by
Neckers and coworkers [28a–28d]. In recent years,
a PS–AlCl3complex has been reported to be an effi-
cient catalyst for some organic transformations [28].
As part of our continuing interest in heterogeneously
catalyzed organic reactions [29,30], we report herein
an efficient and ecofriendly procedure for the synthe-
sis of quinoxalines by the one-pot condensation re-
action of 1,2-phenylenediamines with 1,2-diketones
catalyzed by PS-supported AlCl3 as a reusable het-
erogeneous Lewis acid catalyst (Scheme 1).
RESULTS AND DISCUSSION
For our investigations, cross-linked PS-supported
aluminum chloride (PS/AlCl3) was prepared by ad-
dition of anhydrous aluminum chloride to PS (8%
H N
2
R2
R1
R1
R2
R1
R1
N
O
PS/AlCl (10 mol%)
3
+ 2H O
+
2
C H OH, Reflux
2
5
H N
2
N
O
3a–w
1
2
1
R
= Ph, 4-(Cl)-Ph,4-(Br)-Ph,4-(MeO)-Ph, 4-(Me)-Ph, Me, 2-Furil, H
2
= H, Me, NO , Cl, PhCO
2
R
SCHEME 1 Synthesis of quinoxalines catalyzed by PS/AlCl3.
Heteroatom Chemistry DOI 10.1002/hc

Polystyrene-Supported AlCl3 as a Highly Active and Reusable Heterogeneous Lewis Acid Catalyst
3
TABLE 1 PS/AlCl3-Catalyzed Condensation Reaction of
ing the heterogeneous catalysts is their deactivation
and reusability. To test this, a series of five consec-
utive runs were carried out with the same catalyst
PS/AlCl3 sample, without a noticeable decrease in
the activity, furnishing the corresponding product
with 96%, 93%, 91%, 90%, and 88% isolated yields
(Table 1, entry 9). This reusability demonstrates the
high stability and turnover of a polymeric Lewis acid
catalyst under operating conditions. After using the
catalyst PS/AlCl3, the solid was simply filtered off,
washed with ethanol and ether, and reused.
To evaluate the efficiency and generality of
this methodology, we next investigated the scope
and limitation of this condensation reaction un-
der optimized conditions (C2H5OH, 10 mol% of
PS/AlCl3, reflux), and the results are summarized
in Table 2. As can be seen from Table 2, a va-
riety of structurally diverse 1,2-phenylenediamines
and 1,2-diketones underwent the condensation reac-
tion smoothly to afford the corresponding quinoxa-
line derivatives (3) in excellent yields. The electronic
property of the substituents on the aromatic ring of
the diamine part had an obvious effect on reaction
time under the above optimal reaction conditions.
Benzil (1a, 1 mmol) with 1,2-Phenylenediamine (2a, 1 mmol)
under Different Reaction Conditions
Catalyst Time
Entry
Solventa
Toluene
Cyclohexane
CH2Cl2
CH3Cl
CH3CN
H2O
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
(mol%)
(min)
Yield (%)b
1
2
3
4
5
6
7
8
10
10
10
10
10
10
–
60
60
60
60
60
60
10
10
10
10
10
10
10
10
40
<5
15
15
71
NRc
NR
63
5
9d
10
11e
12f
13g
14h
10
20
–
10
10
10
96, 93, 91, 90,88
95
NR
54
80
NR
aAll reactions were carried out under reflux conditions with 10 mol%
of catalyst in 3 mL of solvent.
bIsolated yield.
cNR: No reaction.
dCatalyst was reused for five times.
ePS was used as a catalyst.
f ,gTwo reactions were carried out in ethanol at 45 and 60◦C, respec-
tively.
hCatalyst was removed by filtration after 10 min.
TABLE 2 Preparation of Quinoxaline Derivativesa Catalyzed
by PS/AlCl3 Using Different Aromatic 1,2-Phenylenediamines
(2) and 1,2-Diketones(1)
reaction. Next, the amount of the catalyst was exam-
ined and we found that the yields were obviously
affected by different amounts of catalyst. No reac-
tion was observed in the absence of catalyst, whereas
10 mol% of PS/AlCl3 was sufficient to complete the
reaction and excessive amount of catalyst did not in-
crease the yields significantly (Table 1, entries 7–10).
To confirm the activity of PS-supported AlCl3,
a comparison experiment was carried out by using
cross-linked PS alone under the same experimen-
tal conditions. No product was isolated when us-
ing PS as a catalyst, which indicated that PS itself
did not promote the reaction (Table 1, entry 11). To
demonstrate that the reaction catalyzed by the cata-
lyst PS/AlCl3 is really a heterogeneous process and to
find out whether the reaction occurred in the solid
matrix of the supported catalyst PS/AlCl3 or whether
aluminum chloride simply released into the ethanol
was responsible for the reaction, PS/AlCl3 was heated
under reflux in ethanol for 10 min and then isolated
by filtration. When reactants were added to the fil-
trate and heated under reflux for 10 min, no reaction
took place (Table 1, entry 14). Therefore, we may
conclude that any AlCl3 that leached into the reaction
mixture is not an active homogeneous catalyst and
that the observed catalysis is truly heterogeneous in
nature. However, one of the important points regard-
Time Yield
Entry
R1
R2
Product (min) (%)b Ref.
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
10
8
96
97
90
91
96
95
94
93
89
88
84
87
92
92
85
91
91
92
94
95
96
95
94
[16]
[16]
[16]
[16]
[15]
[15]
[21]
[21]
[15]
[15]
[15]
[15]
[11]
[11]
[11]
[11]
[21]
[32]
[32]
[16]
[16]
[33]
[33]
Me
NO2
Cl
H
Me
H
40
20
15
15
15
10
25
30
60
35
25
25
55
35
55
20
20
10
10
15
12
4-(Cl)C6H4
4-(Cl)C6H4
4-(Br)C6H4
4-(Br)C6H4
4-(OMe)C6H4
4-(OMe)C6H4 Me
4-(OMe)C6H4 NO2
4-(OMe)C6H4 Cl
4-(Me)C6H4
4-(Me)C6H4
4-(Me)C6H4
4-(Me)C6H4
Ph
Me
H
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H
Me
NO2
Cl
PhCO
H
Me
H
Me
H
Me
Me
2-Furil
2-Furil
H
3s
3t
3u
3v
3w
H
Me
aReaction conditions: diamine (1 mmol), 1,2-diketone (1 mmol),
PS/AlCl3(10 mol%), ethanol (3 mL), reflux.
bIsolated yield.
Heteroatom Chemistry DOI 10.1002/hc

4
Rahmatpour
It was observed that electron-withdrawing groups
(Table 2, entries 3,4,11,12,15–17) associated with
1,2-phenylenediamines decreased the reactivity of
the substrate, and long reaction times were required.
In contrast, electron-donating groups at the phenyl
ring of 1,2-diamine favored the formation of prod-
uct (Table 2, entry 2). On the other hand, the ef-
fect of electronic factors associated with aromatic
1,2-diketones is opposite. Since only symmetric 1,2-
diketones were used for the condensation reactions,
no regioisomers were generated as the products.
To check the versatility of this method, other 1,2-
dicarbonyls such as 2,3-butanedione (Table 2, en-
tries 18,19), furil (Table 2, entries 20,21), and glyoxal
(Table 2, entries 22,23) were subjected to a conden-
sation reaction and the corresponding products were
obtained with excellent yields.
The actual mechanism of the reaction is unclear.
However, based on the experimental results and by
referring to the literature [14,16], the mechanism
of the quinoxaline formation proceeds by the usual
pathway proposed using Lewis acids, involves the
complexation of aluminum with diketone by acting
as a Lewis acid and also plays a role in promoting
the dehydration steps.
polymer-supported Lewis acid has proved to be an
efficient catalyst for the efficient synthesis of quinox-
aline derivatives. The short reaction times, simple
experimental procedure and product isolation, wide
applicability, high yields, mild reaction conditions,
low cost, and easy preparation and safe handling
of the catalyst are important features of this new
methodology. In addition, the use of PS/AlCl3 has re-
sulted in a reduction in the unwanted and hazardous
waste that is produced during conventional homoge-
neous processes. Finally, this polymeric Lewis acid
catalyst can be recovered and reused at least five
times with negligible loss in its activity.
EXPERIMENTAL
Materials and Methods
All chemical used were obtained from commer-
cial suppliers and used without further purification.
Melting points were measured on an Electrother-
1
mal 535 apparatus and were uncorrected. H and
13C NMR spectra were recorded on a Bruker DPX-
250 Avance spectrometer (Karlsruhe, Germany) at
250.13 MHz. IR spectra were recorded with a Uni-
cam Matteson 1000 spectrophotometer. Reaction
monitoring and purity determination of the products
were accomplished by thin layer chromatography
(TLC) on silica gel polygram SILG/UV 254 plates.
The capacity of the catalyst was determined by the
Mohr titration method and atomic absorption tech-
nique using a Philips atomic absorption instrument.
All yields refer to isolated products. Elemental anal-
yses were performed using a Heraeus CHN-O-Rapid
analyzer (Hanau, Germany) at Research Institute of
Finally, the efficacy of PS/AlCl3 was compared
with that of other catalysts reported earlier. The syn-
thesis of 3a was considered as a representative ex-
ample (Table 3). As indicated in Table 3, in addi-
tion to having the general advantages attributed to
polymeric-supported catalysts, PS/AlCl3 is an equally
or more efficient catalyst for this condensation reac-
tion in terms of the yield and reaction rate.
In conclusion, PS/AlCl3 as a noncorrosive, en-
vironmentally benign, and stable heterogeneous
TABLE 3 Comparison of some previously reported procedures with the present method for the synthesis of 3a
Entry
Catalyst (Loading)
Reaction Conditions
Time (min)
Yield (%)a
Ref.
1
2
3
4
5
6
7
8
Montmorillonite K-10 (10% w/w)
Polyaniline sulfate (5% w/w)
Zn[(l)proline] (10 mol%)
Amberlyst-15 (24 mol%)
MnCl2(10 mol%)
I2(10 mol%)
p-TSA (10 mol%)
H2O, rt
2.5 h
20
10
19
17
30
5
40
40
5
5
6
7
6
100
95
94
98
94
90
90
90
90
35
24
34
32
66
57
96
[17]
[13]
[15]
[35]
[14]
[10]
[19]
[10]
[34]
[19]
[19]
[19]
[19]
[19]
[19]
Table 2
CH2ClCH2Cl, rt
HOAc, rt
H2O, 70◦C
C2H5OH, rt
CH3CN, rt
Solvent free, rt
DMSO, rt
I2(10 mol%)
9
ZrO2/Ga2O3/MCM-41
FeCl3·6H2O (10 mol%)
CoCl2·6H2O (10 mol%)
AlCl3(10 mol%)
CH3CN, rt
10
11
12
13
14
15
16
Solvent free,rt
Solvent free,rt
Solvent free, rt
Solvent free, rt
Solvent free, rt
Solvent free, rt
C2H5OH, reflux
SnCl4·5H2O (10 mol%)
HCl (10 mol%)
H2SO4(10 mol%)
PS/AlCl3(10 mol%)
5
10
aIsolated yields.
Heteroatom Chemistry DOI 10.1002/hc

Polystyrene-Supported AlCl3 as a Highly Active and Reusable Heterogeneous Lewis Acid Catalyst
5
1
1245, 1175, 832 cm−1; H NMR (250 MHz, CDCl3):
δH 8.10 (d, J = 2.30 Hz, 1H), 8.05 (d, J = 8.90 Hz,
1H), 7.64 (dd, J = 2.20 Hz, 1H), 7.50 (m, 4H), 6.88
(d, J = 8.70 Hz, 4H), 3.80 (s, 6H, OCH3) ppm; 13C
NMR (62.50 MHz, CDCl3): δC 160.45, 160.35, 153.82,
141.32, 139.54, 135.15, 131.38, 131.33, 131.25,
130.46, 130.22, 127.88, 113.85, 55.35 ppm.
Petroleum Industry (Tehran, Iran), and the results
agreed favorably with calculated values.
Typical Procedure for Preparation of PS/AlCl3
Anhydrous AlCl3 (4.5 g) was added to PS (8% divinyl-
benzene, grain size 0.25–0.68 mm, 3.5 g) in carbon
disulfide (25 mL) as the reaction medium. The mix-
ture was stirred using a magnetic stirrer under reflux
condition for 1 h, cooled, and then water (50 mL)
was cautiously added to hydrolyze the excess AlCl3.
The mixture was stirred until the deep orange color
disappeared, and the polymer became light yellow.
The polymer beads were collected by filtration and
washed with water (300 mL) and then with ether (30
mL) and acetone (30 mL). The catalyst was dried in a
vacuum oven overnight at 50◦C. Complex formation
was demonstrated by a new band at 1630 cm−1in
the IR spectrum of polymer. The chlorine content
of PS/AlCl3 was 5.16% analyzed by the Mohr titra-
tion method [31], and the loading capacity of AlCl3
on the polymeric catalyst was calculated to be 0.485
mmol/g. One gram of solid catalyst (PS/AlCl3) was
decomposed by burning with Na metal, extracted
with 10 mL of water and filtered. The chlorine con-
tent of the filtrate was determined by the Mohr titra-
tion method.
6-Methyl-2,3-di-p-tolylquinoxaline (3n, C23H20N2).
Pale yellow solid; mp 136–137◦C; IR (KBr): 3430,
2912, 1611, 1450, 1336, 820 cm−1; 1H NMR (250
MHz, CDCl3): δH 8.06 (d, J = 8.50 Hz, 1H), 7.93 (s,
1H), 7.55 (dd, J = 8.50 Hz, 1H), 7.45 (d, J = 7.95
Hz, 4H), 7.14 (d, J = 7.90 Hz, 4H), 2.59 (s, 3H),
2.36 (s, 6H) ppm; 13C NMR (62.50 MHz, CDCl3):
δC 153.34, 152.60, 141.26, 140.11, 139.68, 138.60,
138.52, 136.63, 132.02, 129.85, 129.81, 128.98,
128.70, 128.02, 21.90, 21.40 ppm.
6-Methyl-2,3-dimethylquinoxaline (3s, C11H12N2).
Pale yellow oil; IR (KBr): 2970, 1622, 1565, 1452, 820
cm−1; H NMR (250 MHz, CDCl3): δH 7.90 (d, J =
1
8.50, 1H), 7.79 (s, 1H), 7.53 (d, J = 8.50 Hz, 1H),
2.75 (s, 6H), 2.60 (s, 3H) ppm; 13C NMR (62.50 MHz,
CDCl3): δC 153.70, 152.80, 141.60, 139.90, 139.50,
131.40, 128.20, 127.70, 23.60, 23.50, 22.10 ppm.
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In a round-bottomed flask (25 mL) equipped with a
condenser and a magnetic stirrer, a mixture of 1,2-
phenylenediamine (2, 1 mmol), 1,2-diketone (1, 1
mmol), and PS/AlCl3 (0.206 g, 0.1 mmol of AlCl3) in
ethanol (3 mL) was heated under reflux for an appro-
priate time as indicated by TLC. After completion
of the reaction, hot ethanol was added to the mix-
ture and the catalyst was collected by filtration and
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concentrated on a rotary evaporator under reduced
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afford the pure products 3a–w. The products were
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151◦C; IR (KBr): 2935, 2835, 1605, 1510, 1345, 1292,
Heteroatom Chemistry DOI 10.1002/hc

6
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Heteroatom Chemistry DOI 10.1002/hc