Polystyrene-supported GaCl3 as a highly efficient and recyclable heterogeneous Lewis acid catalyst for one-pot synthesis of N-substituted pyrroles

Article abstract of DOI:10.1016/j.jorganchem.2012.03.025

A new and environmentally benign method for the preparation of N-substituted pyrroles from one-pot condensation reaction of 2,5-hexanedione with amines and diamines in the presence of polystyrene-supported gallium trichloride (PS/GaCl₃) as a highly active and reusable heterogeneous Lewis acid catalyst is presented. This new protocol has the advantages of easy availability, stability, reusability and eco-friendly of the catalyst, high to excellent yields, simple experimental and work-up procedure.

Full text of DOI:10.1016/j.jorganchem.2012.03.025

Journal of Organometallic Chemistry 712 (2012) 15e19
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Journal of Organometallic Chemistry
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Polystyrene-supported GaCl3 as a highly efficient and recyclable heterogeneous
Lewis acid catalyst for one-pot synthesis of N-substituted pyrroles
*
Ali Rahmatpour
Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI), 14665-1137 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new and environmentally benign method for the preparation of N-substituted pyrroles from
one-pot condensation reaction of 2,5-hexanedione with amines and diamines in the presence of
polystyrene-supported gallium trichloride (PS/GaCl3) as a highly active and reusable heterogeneous
Lewis acid catalyst is presented. This new protocol has the advantages of easy availability, stability,
reusability and eco-friendly of the catalyst, high to excellent yields, simple experimental and work-up
procedure.
Received 27 December 2011
Received in revised form
10 March 2012
Accepted 21 March 2012
Keywords:
Polymer-supported catalyst
Pyrrole
Ó 2012 Elsevier B.V. All rights reserved.
PaaleKnorr condensation reaction
Heterogeneous Lewis acid catalyst
1. Introduction
these methodologies for the synthesis of pyrroles are associated
with several shortcomings such as low yields, prolonged reaction
Functionalized pyrroles are an important class of nitrogen-
containing heterocyclic compounds. They constitute the core unit
of many natural products, synthetic materials, and serve as building
blocks for porphyrin synthesis [1,2]. Members of this family have
wide applications in medicinal chemistry, being used as antima-
larial, antiflammatory agents, antibacterial, and antiviral [3e5].
These compounds can be prepared from the classical Hantzsch
procedure [6], 1,3-dipolar cycloaddition reactions [7], aza-Wittig
reactions [8], annulations reactions [9], and other multistep oper-
ations [10]. Despite these new developments, the PaaleKnorr
reaction remains one of the most significant and simple methods
which consists the cyclocondensation of primary amines with
1,4-dicarbonyl compounds to produce N-substituted pyrroles.
Several catalysts have been used to promote this reaction including
HCl [11], p-TSA [12], H2SO4 [13], Sc(OTf)3 [14], Bi(NO3)3$5H2O [15],
SnCl2$2H2O [16], Ti(OPri)4 [17], RuCl3 [18], InCl3, InBr3, In(OTf)3 [19],
zeolite [20], Al2O3 [21], montmorillonite K10 [22], silica sulfuric
acid [23], layered zirconium phosphate and phosphonate [24],
Fe3þ-montmorillonite [25], montmorillonite KSF-clay and I2 [26].
Additionally, the above cyclocondensation process could proceed in
ionic liquid [27] or ultrasonic and microwave irradiation [28].
However, despite the potential utility of these catalysts, many of
time, harsh reaction conditions, the requirement of excess of
catalysts, the use of toxic and detrimental metal precursors as
catalysts, and relatively expensive reagents and high temperature,
and tedious work-up leading to the generation of large amounts of
toxic metal-containing waste. 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 under the aspect of environmentally benign
processes.
Heterogeneous supported catalysts have been gained much
attention in recent years, as they possess a number of advantages in
preparative procedures [29,30]. Immobilization of catalysts on solid
support improves the available active site, stability, hygroscopic
properties, handling, and reusability of catalysts which all factors
are important in industry [31]. Therefore, use of supported and
reusable catalysts in organic transformations has economical and
environmental benefits. A large number of polymer supported
Lewis acid catalysts have been prepared by immobilization of the
catalysts on polymer via coordination or covalent bonds [32]. Such
polymeric catalysts are usually as active and selective as their
homogeneous counterparts while having the distinguishing char-
acteristics of being easily separable from the reaction mixture,
recyclability, easier handling, non-toxicity, enhanced stability, and
improved selectivity in various organic reactions. Polystyrene is
one of the most widely studied heterogeneous and polymeric
supports due to its environmental stability and hydrophobic nature
* Tel.: þ98 21 44739518; fax: þ98 21 44739517.
E-mail address: rahmatpoura@ripi.ir.
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.03.025

16
A. Rahmatpour / Journal of Organometallic Chemistry 712 (2012) 15e19
which protects water-sensitive Lewis acids from hydrolysis by
atmospheric moisture until it is suspended in an appropriate
oven overnight at 50 ꢁC before use. The chlorine content of PS/
GaCl3 was 4.17% analyzed by the Mohr titration method [37] and
the loading capacity of GaCl3 on the polymeric catalyst or the
amount of GaCl3 complexed with polystyrene was calculated to be
0.391 mmol/g [38].
solvent where it can be used in
Polystyrene-supported gallium chloride, PS/GaCl3, which is
tightly bound complex between anhydrous GaCl3 and
a chemical reaction [33].
a
polystyrene-divinylbenzene copolymer beads, has been described
for the first time by Ruicheng and co-workers [34]. The use of PS/
GaCl3 has several advantages over conventional Lewis acid catalyst
like its cost-effectiveness, ease of handling, recyclability, and
tunable Lewis acidity.
As part of our continuing interest in heterogeneously catalyzed
organic reactions [35,36], we report herein an efficient and
eco-friendly procedure for the synthesis of pyrroles from
2,5-hexanedione and primary amines catalyzed by polystyrene-
supported GaCl3 as a reusable heterogeneous Lewis acid catalyst
(Scheme 1). Tothe best of our knowledge, this is only second example
of PS/GaCl3 as a catalyst for any organic transformation.
2.3. General experimental procedure for the synthesis of pyrroles (3)
In a round-bottom flask (25 mL) equipped with a condenser
and a magnetic stirrer, a stirring mixture of amine (1, 1 mmol),
2,5-hexanedione (2a, 1 mmol) and PS/GaCl3 (2.55 g, 1 mmol of
GaCl3) in CH3CN (10 mL) was heated under reflux for an appro-
priate time as indicated by TLC (hexane/ethyl acetate, 4/1). After
completion of the reaction, the catalyst was collected by filtration
and then washed with chloroform and ether (2 ꢂ 5 mL). The filtrate
was concentrated on a rotary evaporator under reduced pressure
and the solid product obtained was purified by short silica gel
column chromatography and by recrystallization to afford the pure
product. The spent catalyst from different experiments was
washed with chloroform and ether, dried and used again. Except
for compounds 3d, 3f, which no spectroscopic data were found in
the literature, all the pyrrole products are known compounds and
were characterized by IR and NMR spectroscopies data and their
melting points, which agreed with reported values
[15,19,23,25e27,42,43].
2. Experimental section
2.1. Materials and instruments
All chemical reagents were obtained from Fluka and Merck
chemical companies and were used without further purification.
Cross-linked polystyrene (8% divinylbenzene, grain size range:
0.25e0.68 mm) was prepared via suspension polymerization as
reported in the literature [35]. PS/AlCl3 and PS/Al(OTf)3 were also
prepared as previously reported [35,40]. 1H and 13C NMR spectra
were recorded on a Bruker DPX-250 Avance spectrometer at
250.13 MHz. FT-IR spectra of the samples were recorded from 400
to 4000 cmꢀ1 on a Unicam Matteson 1000 spectrophotometer. UV
spectra were taken using a Pharmacia Biotech Ultraspec 3000
model 80-2106-20 spectrometer. Mass spectra were recorded on
a Fisons instrument. Elemental analyses were performed using
a Heraeus CHN-O-Rapid analyzer by RIPI and the results agreed
favorably with calculated values. The capacity of the catalyst was
determined by the Mohr titration method and atomic absorption
technique using a Philips atomic absorption instrument. Reaction
monitoring and purity determination of the products were
accomplished by TLC on silica gel polygram SILG/UV254 plates. All
yields refer to isolated products.
2.4. The spectral data for selected compounds
2.4.1. N-(20,50-Dimethylphenyl)-2,5-dimethylpyrrole (3d)
dH (250 MHz, CDCl3) 1.91 (s, 3H), 1.95 (s, 6H), 2.38 (s, 3H), 5.93 (s,
2H, Pyrrole ring), 7.0 (s, 1H), 7.16 (1H, d, J ¼ 15.5 Hz), 7.21 (1H, d,
J ¼ 15.5 Hz); 13C NMR (62.9 MHz, CDCl3) dC 12.6 (2C), 16.6, 20.8,
105.2 (2C), 128.1, 129.1, 129.3 (2C), 129.6, 133.7, 136.3, 137.9; MS (EI),
m/z 200 (10), 199 (Mþ, 90), 198 (50), 184 (100), 169 (15), 77 (20) %.
2.4.2. N-(40-Cyanophenyl)-2,5-dimethylpyrrole (3g)
nmax/cmꢀ1 2224 (CN); dH (250 MHz, CDCl3) 2.04 (s, 6H), 5.93 (s,
2H, Pyrrole ring), 7.32 (2H, d, J ¼ 16.8 Hz), 7.77 (2H, d, J ¼ 16.8 Hz);
13C NMR (62.9 MHz, CDCl3) dC 13.0, 99.8, 107.1, 114.4, 120.2, 128.9,
133.7, 150.5; MS (EI), m/z 196 (Mþ, 70), 195 (100), 181 (10), 154 (5),
102 (15), 92 (10), 75 (15) %.
3. Results and discussion
2.2. Preparation of PS/GaCl3
PS/GaCl3 was prepared by addition of anhydrous gallium chlo-
ride to polystyrene (8% divinylbenzene) in carbon disulfide under
reflux conditions. The loading capacity of the polymeric catalyst
obtained by gravimetric method and checked by atomic absorption
technique was 0.391 mmol GaCl3/g of complex beads catalyst [38].
The data obtained by these two techniques showed, within
experimental error, that the catalyzing species are in the form of
GaCl3 supported on the polymeric support. The UV spectrum of the
solution of PSeGaCl3 complex in CS2 showed a new strong band at
Anhydrous GaCl3 (4 g) was added to polystyrene (8% divinyl-
benzene, grain size range: 0.25e0.68 mm, 8 g) in carbon disulfide
(30 mL) as the reaction medium. The mixture 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 GaCl3.
The mixture was stirred until the bright red color disappeared, and
the polymer became yellow. The polymer beads were collected by
filtration and washed with water (300 mL) and then with ether
(30 mL) and chloroform (30 mL). The catalyst was dried in a vacuum
470 nm, which is due to the formation of a stable
p / p type
coordination complex between the benzene rings in the poly-
styrene carrier with gallium trichloride. The IR spectrum of PS/
GaCl3 showed new absorption peaks due to the CeC stretching
vibration and the CeH bending vibration of the benzene ring at
1500e1560 and 400e800 cmꢀ1, by which complex formation was
demonstrated. The structure of the PSeGaCl3 complex is similar to
that of the PSeAlCl3 complex as suggested by Neckers and co-
workers [39], because the Lewis acid GaCl3 is complexed with the
benzene rings of the polystyrene and the GaCl3 is stabilized due to
the decreased mobility of the benzene rings hindered by the long
polystyrene chain. The PSeGaCl3 complex is a non-hygroscopic,
O
Me
PS/GaCl (10 mol%)
3
Me
Me
2H O
R +
H NR
N
+
2
2
CH CN, Reflux
3
R= alkyl, Ar, ArCO, ArSO
Me
2
O
3
1
Scheme 1. Synthesis of pyrroles catalyzed by PS/GaCl3.

A. Rahmatpour / Journal of Organometallic Chemistry 712 (2012) 15e19
17
water tolerant, and especially stable species. In addition, this
polymeric catalyst is easy to prepare, stable in air for a long time
(over 2 years) without any change, easily recycled and reused
without appreciable loss of its activity.
when heterogeneous catalysts are used, particularly for practical
applications of the reaction. Our preliminary investigations
demonstrated that PS/GaCl3 catalyst is very stable to air and
moisture. Moreover, in a separate experiment the catalyst was
filtered off after heating for 15 min at the reaction temperature.
Further treatment of the filtrate (the catalyst-free liquid) under
similar reaction condition did not proceed significantly (Table 1,
entry 16). On the other hand, after each run the filtrates were used
for determination of catalyst leaching (gallium content), which
showed a negligible release of GaCl3 by atomic absorption spec-
troscopy or ICP measurement. The capacity of the catalyst after six
uses was 0.381 mmol of GaCl3 per gram. Also, the catalytic behavior
of the separated liquid was tested by addition of fresh aniline (1a)
and 2,5-hexanedione (2a) to the filtrates after each run. Execution
of the condensation reaction under the same reaction conditions, as
with catalyst, showed that the obtained results were as same as
blank experiments. Therefore, we may conclude that PS/GaCl3 is
stable and no significant leaching of Lewis acid moieties is oper-
ating under our reaction conditions and any gallium species 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 regarding the heteroge-
neous catalysts is their deactivation and reusability. To test this,
a series of five consecutive runs were carried out with the same
catalyst PS/GaCl3 sample, without noticeable decrease in the
activity, furnishing the corresponding product with 92, 90, 90, 89,
88% isolated yields (Table 1, entry 10). This reusability demonstrates
the high stability and turnover of polymeric Lewis acid catalyst
under operating condition. After using the catalyst PS/GaCl3, the
solid was simply filtered off, washed with chloroform, and ether
and reused.
Under the optimized conditions, the substrate scope of the
PaaleKnorr cyclocondensation reaction was investigated, and
a range of pyrroles were prepared (Table 2). It is evident that our
methodology is reasonably general and can be applied to several
amines. As indicated in Table 2, in all cases, the PS/GaCl3-catalyzed
PaaleKnorr condensation reaction proceeded smoothly, completed
within 8e80 min, gave the corresponding pyrrole products in good
to excellent yields and also avoided the problems associated with
homogeneous metal catalyst use, such as cost, handling, safety
and pollution. As shown in Table 2, a series of aromatic amines
bearing either electron-donating (Table 2, entries 2e5, 8) or
electron-withdrawing (Table 2, entries 6, 7, 9) groups on aromatic
ring was investigated and the results show they are both effective in
the PaaleKnorr condensation reaction. Moreover, we also examined
the reaction of acyclic and cyclic aliphatic amines with 2a (Table 2,
entries 10e13, 14). Similarly, the desired pyrrole products were
obtained with excellent yields. It is well known that the reactivity of
aliphatic amines is more potent compared with that of aromatic
amines, thus aliphatic amines give higher yields or shorter reaction
times than are generally observed in other systems [41], and both
lead to high yields of the pyrrole products. The present protocol was
also applied to less nucleophilic aromatic amine such as 1-
aminonaphthalene and the result shows that this catalyst provides
higher yields in shorter reaction times in comparison to the previ-
ously reported methods using I2 [26], montmorillonite KSF [26] and
Bi(NO3)3$5H2O [15] (Table 2, entry 15). That means that a better
yield was obtained in a shorter time. Next, to extend the scope of the
PS/GaCl3-catalyzed method of pyrrole synthesis, we investigated
the reaction of diamines with 2a in the same conditions (Table 2,
entries 16, 17). In this reaction, 2 equivalents of 2a were required in
order to obtain a complete conversion of diamines. When aliphatic
diamines such as ethylene diamine and butylene diamine were
examined, the corresponding bis-pyrrole products (3p,3q) were
In order to find the most appropriate reaction conditions and
evaluate the catalytic efficiency of PS/GaCl3 catalyst, initially
a model study to screen the best conditions was carried out on the
synthesis of 1-phenyl-2,5-dimethylpyrrole 3a. The results are
summarized in Table 1. It was found that among the tested
solvents, polar solvents such as CH3CN and C2H5OH were better
than non-polar solvents. No reaction occurred when H2O was used
as solvent (Table 1, entry 7), may be because of the aggregation of
the catalyst caused by its hydrophobic nature, leading to inade-
quate access of substrates to the active sites of the catalyst [40]. We
therefore selected CH3CN as solvent. Further studies showed that
the reaction temperature has a great influence on the model
reaction and the best results were observed when the reaction
temperature was 82 ꢁC (Table 1, entries 10, 13, 14). Consequently,
we chose 82 ꢁC as the optimal temperature for the model reaction.
Next, the amount of the catalyst was examined and we found that
the yields were obviously affected with different amount of cata-
lyst. No reaction was observed in the absence of catalyst, while
10 mol% of PS/GaCl3 was sufficient to drive the reaction completely
and excessive amount of catalyst did not increase the yields
significantly (Table 1, entries 8e11). The key role played by the
Lewis acidity of the heterogeneous catalyst PS/GaCl3 was proved by
employing the cross-linked polystyrene beads (PS) (Table 1, entry
12) and the GaCl3ebenzene complex as catalysts under the same
experimental conditions. In fact, while in the former case no
reaction occurred, which indicated that polystyrene itself did not
promote the reaction, in the later, the desired product was isolated
in low yield 49% (Table 1, entry 13). Also, PS/GaCl3 was found to be
a more effective catalyst than PS/AlCl3 and PS/Al(OTf)3 in terms of
reaction time and the yield of the product 3a under identical
conditions (Table 1, entries 17, 18). The lifetime and leaching of
active metal species into solution are important issues to consider
Table 1
PS/GaCl3-catalyzed condensation reaction of 2,5-hexanedione (2a, 1 mmol) with
aniline (1a, 1 mmol) under different reaction conditions.
Entry
Solventa
Catalyst (mol%)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
Toluene
Cyclohexane
CH2Cl2
CH3Cl
EtOAc
C2H5OH
H2O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
10
10
10
10
10
10
e
60
60
60
60
60
20
60
20
20
20
20
20
20
20
20
15
60
60
49
51
25
20
69
83
NRf
NR
63
9
5
10c
11
12d
13d
14e
15e
16g
17h
18h
10
20
e
92,90,90,89,88
92
NR
49
54
80
NR
78
85
e
10
10
10
10
10
a
All reactions were carried out under reflux conditions with 10 mol% of catalyst
in10 mL of solvent.
b
Isolated yield.
Catalyst was reused for five times.
PS and the tolueneeGaCl3 complex were used as catalysts, respectively.
Two reactions were carried out in CH3CN at 45, 65 ꢁC, respectively.
NR: No reaction.
Catalyst was removed by filtration after 15 min.
PS/AlCl3 and PS/Al(OTf)3 were used as catalysts, respectively.
c
d
e
f
g
h

18
A. Rahmatpour / Journal of Organometallic Chemistry 712 (2012) 15e19
Table 2
Table 3
PS/GaCl3-catalyzed synthesis of pyrroles.a
Comparison of some other procedures with the present method for the synthesis
of 3a.
Entry Amine (1)
Product (3)
Time Yieldb Ref.
(min) (%)
Entry Catalyst (loading)
Media
Time
Yielda Ref.
(%)
1
C6H5NH2
3a
3b
3c
3d
3e
3f
3g
3h
3i
20 92
20 92
22 91
30 90
10 94
40 83
40 85
10 94
25 92
[15,19]
[15,19]
[15,27]
2
3
4
5
6
7
8
9
10
11
12
13
p-(CH3)C6H4NH2
o-(CH3)C6H4NH2
2,5-(CH3)2C6H3NH2
p-(OCH3)C6H4NH2
p-(NO2)C6H4NH2
p-(CN)C6H4NH2
p-(OH)C6H4NH2
p-(Cl)C6H4NH2
C6H5CH2NH2
1
2
3
4
5
6
7
8
p-TSA
I2 (0.1 mmol, 0.025 g)
Bi(NO3)3$5H2O (50 mol%) CH2Cl2, r.t.
InBr3 (5 mol%)
Sc(OTf)3 (5 mol%)
Sc(OTf)3 (5 mol%)
In(OTf)3 (5 mol%)
Cu(OTf)2 (5 mol%)
CuCl2 (40 mol%)
Mg(OTf)2 (5 mol%)
ZnCl2 (5 mol%)
C6H6/80 ꢁC
THF, r.t.b
1 h
9 h
10 h
1 h
88
89
95
84
94
[12]
[26]
[15]
[19]
[14]
[14]
[19]
[14]
[14]
[14]
[19]
[19]
[19]
[27]
[24]
[26]
[15,19]
[15,19]
CH2Cl2
Solvent-free/30 ꢁC 0.5 h
CH2Cl2/30 ꢁ
CH2Cl2
C
25 min 77
0.5 h
0.5 h
25 min 34
25 min 47
1.5 h
2 h
3 h
3 h
2 h
10 h
[15,27]
[15,19]
[15,23]
[15,19]
[27]
90
78
Solvent-free,r.t.
Solvent-free,r.t.
Solvent-free,r.t.
Solvent-free, r.t.
Solvent-freer.t.
Solvent-freer.t.
Solvent-free,r.t.
Solvent-free,r.t.
CH2Cl2, r.t.
3j
8
97
9
p-(OCH3)C6H4CH2NH2 3k
CH3CH2CH2NH2
CH3(CH2)3NH2
10 96
10
11
12
13
14
15
16
3l
3m
8
8
97
96
51
47
39
96
78
95
[27]
CoCl2 (5 mol%)
NiCl2 (5 mol%)
[Bmim]I ionic liquid
14
15
3n
3o
10 96
[27]
NH2
NH2
a
-Zr(KPO4)2 (6 mol%)
Montmorillonite,
KSF (1 g)
45 89
60 85
660 83
660 83
[15]
[26]
[25]
[15]
17
18
Fe3þ-montmorillonite
Cationic exchange
resin (Dowex 50w, 1.63 g)
PS/AlCl3 (15 mol%)
Microwave
CH2Cl2, r.t.
H2O/130 ꢁC
3 h
5 h
96
85
[25]
[44]
19
20
21
CH3CN, Reflux
Ether
CH3CN, Reflux
1.25 h 90
0.5 h 90
20 min 92
[35]
[30]
Table 2
CH3
N
H3C
PS/GaCl3 (10 mol%)
(CH )
2 2 N
16
NH2(CH2)2NH2
3p
8
96
[19,23]
a
Isolated yields.
r.t., room temperature.
b
H3C
CH3
CH3
H3C
1-phenylpentane-1,4-dione (2b) and 1,4-diphenylbutane-1,4-dione
(2c) were used. Under identical conditions, formation of pyrroles
(3v,3w) was observed with 2b, 2c as the diketone components
(Scheme 2). However, the formation of 3w from 2c required longer
time.
Finally, to show the advantage of this work in comparison with
previously reported procedures, synthesis of 3a was considered as
a representative example (Table 3). As indicated in Table 3, in
addition to having the general advantages attributed to polymeric
supported catalysts, our present method is better or comparable
with others in terms of yield and reaction rate.
17
NH2(CH2)4NH2
C6H5CONH2
p-(OCH3)C6H4CONH2 3s
C6H5SO2NH2 3t
p-(OCH3)C6H4SO2NH2 3u
3q
3r
8
95
[19,23]
(CH2)
N
N
4
CH3
H3C
18
19
20
21
45 85
30 90
80 83
80 84
[42]
[42]
[43]
[43]
a
Reaction conditions: amine (1, 1 mmol); 2,5-hexanedione (2a, 1 mmol); PS/
GaCl3 (10 mol%); CH3CN (10 mL), reflux.
b
Isolated yield.
obtained in 96 and 95% yields, respectively. This method was also
applicable to primary aromatic amides and primary aromatic
sulfonamide such as benzamide, 4-methoxybenzamide and ben-
zenesulfonamide, 4-methoxybenzenesulfonamide (Table 2, entries
18e21) under identical reaction conditions to afford the corre-
sponding N-acyl pyrroles and N-sulfonyl pyrroles (3r,3s and 3t,3u)
in good yields, respectively.
4. Conclusion
In summary, PS/GaCl3 was found to be highly efficient and
recyclable heterogeneous Lewis acid catalyst for the preparation of
N-substituted pyrroles. The short reaction times, simple work-up
procedure, general applicability, high yields, mild operating
conditions, low cost, easy preparation, and safe handling of the
To further extend the scope and utility of this PS/GaCl3-catalyzed
procedure of pyrrole formation, other substituted diketones such as
Ph
N
O
Ph
Me
PS/GaCl (10 mol%)
3
2b
2c
3v , 84 %, 70 min
O
O
1a
+
CH CN, Reflux
3
O
Ph
Ph
Ph
N
Ph
3 w , 80 %, 90 min
Scheme 2.

A. Rahmatpour / Journal of Organometallic Chemistry 712 (2012) 15e19
19
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addition, the use of water-tolerant PS/GaCl3 has resulted in
a reduction in the unwanted and hazardous waste and minimum
amount of product contamination with metal that is produced
during conventional homogeneous processes. Finally, recovery and
reuse of this polymeric catalyst is also satisfactory, which demon-
strates the cost efficiency and green aspect of our methodology.
(b) M. Abid, A. Spaeth, B. Török, Adv. Synth. Catal. 348 (2006) 2191e2196.
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