One-pot synthesis of N-substituted pyrroles catalyzed by polystyrene-supported aluminum chloride as a reusable heterogeneous Lewis acid catalyst

Article abstract of DOI:10.1002/hc.20661

A convenient and efficient procedure is presented for the one-pot synthesis of N-substituted pyrroles by condensation of 2,5-hexandione and amines or diamines in the presence of cross-linked polystyrene-supported aluminum chloride (PS/AlCl₃) as a highly active and reusable heterogeneous Lewis acid catalyst. This polymeric solid acid catalyst is stable and can be easily recovered and reused without appreciable change in its efficiency.

Full text of DOI:10.1002/hc.20661

Heteroatom Chemistry
Volume 22, Number 1, 2011
One-Pot Synthesis of N-Substituted Pyrroles
Catalyzed by Polystyrene-Supported Aluminum
Chloride as a Reusable Heterogeneous Lewis
Acid Catalyst
Ali Rahmatpour and Jamal Aalaie
Research Institute of Petroleum Industry, Polymer Science and Technology Division,
Tehran, 14665-1137, Iran
Received 20 July 2010; revised 17 October 2010
suppressants [3,4]. In view of their high signifi-
ABSTRACT: A convenient and efficient procedure is
cance, many methodologies have been developed for
presented for the one-pot synthesis of N-substituted
the construction of the pyrrole skeleton [5]. Among
pyrroles by condensation of 2,5-hexandione and
them, the Paal–Knorr synthesis remains the most
amines or diamines in the presence of cross-linked
useful preparative method for generating pyrroles
polystyrene-supported aluminum chloride (PS/AlCl3)
[6]. In recent years, a variety of reagents such as K10
as a highly active and reusable heterogeneous Lewis
clay [7], Bi(NO3)3·5H2O [8], Dy(OTf)3 [9], PMA/SiO2
acid catalyst. This polymeric solid acid catalyst is
[10], β-CD [11], aluminum oxide [12], acidic resin
stable and can be easily recovered and reused with-
(D001) [13], zinc tetrafluoroborate [14], and I2 [15]
ꢀ
C
out appreciable change in its efficiency.
2010 Wi-
under reflux conditions and at room temperature
have been utilized for the synthesis of N-substituted
pyrroles. In addition, the above-mentioned cyclo-
condensation process could proceed in ionic liquid
[16] or ultrasonic and microwave irradiation [17].
Many of these procedures involve the use of ex-
pensive reagents, metal triflates, and extended re-
action times and produce a huge amount of corro-
sive, toxic waste products and tedious workup and
the use of extra energy source, such as high-power
microwave irradiation or ultrasound. So, a milder,
nonhazardous, recyclable, and ecofriendly organic
catalyst is still in demand.
One of the main themes of contemporary syn-
thetic organic chemistry is the development of atom-
economic and environmentally benign catalytic
systems [18]. In this context, the development of
heterogeneous catalysis is of prime importance not
only from the economic point of view but also
due to the easy workup procedures involved in the
separation of products from a catalyst. In recent
years, a large number of polymer-supported Lewis
ley Periodicals, Inc. Heteroatom Chem 22:85–90, 2011;
View this article online at wileyonlinelibrary.com. DOI
10.1002/hc.20661
INTRODUCTION
Pyrroles are important heterocyclic compounds
displaying remarkable pharmacological properties
such as antibacterial, antiviral, anti-inflammatory,
antitumoral, and antioxidant activities [1]. The pyr-
role moiety is found in many naturally occurring
compounds such as heme, chlorophyll, and vita-
min B12 [2]. Pyrroles are also present in various
bioactive drug molecules such as atorvastatin, anti-
inflammatory and antitumor agents, and immuno-
Correspondence to: Ali Rahmatpour; e-mail: rahmatpoura@
ripi.ir.
c
ꢀ
2010 Wiley Periodicals, Inc.
85

86 Rahmatpour and Aalaie
SCHEME 1 PS/AlCl3-catalyzed synthesis of pyrroles.
acids have been prepared by immobilization of the
catalysts on polymers via coordination or covalent
bonds and are used in many organic transformations
[19–22]. Such polymeric catalysts are usually as ac-
tive as their homogeneous counterparts while having
the advantages of recyclability, greater selectivity,
enhanced stability, easier handling, nontoxicity, and
noncorrosiveness. The most frequently used poly-
meric support is polystyrene; its hydrophobic nature
protects water-sensitive Lewis acids from hydroly-
sis by atmospheric moisture until it is suspended
in an appropriate solvent where it can be used in
a chemical reaction [21,23]. Recently, polystyrene-
supported aluminum chloride has emerged as a
promising heterogeneous Lewis acid catalyst for
acid-catalyzed reactions, such as Friedel–Crafts acy-
lation and sulfonylation reactions, tetrahydropy-
ranylation of alcohols and phenols, and the synthesis
of bis-indolylmethanes [19a,19b,24].
In continuation of our interest in the syn-
thesis of organic compounds from condensation
of phenols with dialdehydes [25] and our ongo-
ing research on the use of heterogeneous Lewis
acid catalysts, herein, we wish to report an effi-
cient and environmentally benign method for the
synthesis of substituted pyrrole derivatives cat-
alyzed by polystyrene-supported aluminum chloride
(PS/AlCl3) (Scheme 1). To the best of our knowledge,
polystyrene-supported aluminum chloride has not
been used in the synthesis of pyrroles.
ily recycled and reused without appreciable loss of
its activity.
In our initial research, to determine the most
appropriate reaction conditions and evaluate the
catalytic efficiency of the PS/AlCl3 catalyst, the con-
densation reaction of aniline (1 mmol) with 2,5-
hexandione (1 mmol) was carried out in different sol-
vents. Among the various solvents screened, CH3CN
was the most effective reaction media (Table 1, en-
try 6). In addition, we also studied influence of the
amount of PS/AlCl3 on the reaction yields. We found
that the use of 15 mol% of PS/AlCl3 was sufficient
to progress the reaction and gave 3a in 90% yield in
75 min under reflux conditions and also an increase
in the amount of catalyst did not improve the yield
(Table 1, entry 7). A comparative reaction was exam-
ined with aniline and 2,5-hexandione in the absence
of catalyst, and no product was observed (Table 1,
entry 11). The immobilized aluminum chloride cata-
lyst is not sensitive to air and moisture upon storage
and much easier to handle than its soluble counter-
part, AlCl3. In addition, the catalyst was not influ-
enced by water formed and not altered during the
TABLE 1 Solvent Effect on the Reaction of Aniline (1 mmol)
with 2,5-Hexandione (1 mmol) Catalyzed by PS/AlCl3 (15
mol%)a
Entry
Solvent
Conditions Time (h) Yield (%)
1
2
3
4
5
6
7
8
THF
CHCl3
CH2Cl2
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
80◦C
8
8
8
8
0
Trace
Trace
Trace
46
90
90
Trace
Trace
Trace
0
RESULTS AND DISCUSSION
For our investigations, cross-linked PS/AlCl3 was
prepared by addition of anhydrous aluminum chlo-
ride to polystyrene (8% divinylbenzene) in carbon
disulfide under reflux conditions [21b]. The capac-
ity of the polymeric catalyst was 0.48 mmol AlCl3
per gram catalyst [19]. Although AlCl3 is a water-
sensitive, corrosive, and environmentally harmful
compound, PS/AlCl3 is a stable and water-tolerant
species. This polymeric catalyst is easy to prepare,
stable in air for a long time without any change, eas-
ClCH2CH2Cl
CH3CH2OH
CH3CN(15 mol%)b
CH3CN(20 mol%)
Toluene
6
75 min
75 min
8
8
8
9
10
11
Neat
Neat
100◦C
Reflux
CH3CNc
12
aThe yields refer to the isolated pure products.
bThe catalyst loading.
cThe reaction was carried out in the absence of PS/AlCl3.
Heteroatom Chemistry DOI 10.1002/hc

One-Pot Synthesis of N-Substituted Pyrroles Catalyzed by Polystyrene-Supported Aluminum Chloride 87
TABLE 2 Preparation of Pyrroles Using PS/AlCla3 as Catalyst
Entry
Amine (1)
Product (3)
Time (min)
Yieldb (%)
c
1
2
3
4
5
6
7
8
C6H5NH
3a
3b
3c
3d
3e
3f
3g
3h
3i
75
75
70
55
135
50
70
30
35
30
40
9090,90,90,88,88,86,86
p-(CH3)C6H42NH2
o-(CH3)C6H4NH2
p-(OCH3)C6H4NH2
p-(NO2)C6H4)NH2
p-(OH)C6H4NH2
p-(Cl)C6H4NH2
90
90
92
83
93
87
94
94
95
93
C6H5CH2NH2
9
10
11
p-(OCH3)C6H4CH2NH2
CH3CH2CH2NH2
CH3(CH2)3NH2
3j
3k
NH2
12
3l
35
94
13
14
NH2(CH2)2NH2
NH2(CH2)4NH2
3n
3o
45
45
95
94
aReaction conditions: amine (1 mmol); 2,5-hexandione (1 mmol); PS/AlCl3 (15 mol%); CH3CN as solvent (10 mL); reflux condition.
bIsolated yield.
cReusability of catalyst after 8 times.
reaction, confirmed by the reusability and leach-
ing test of PS/AlCl3. To find out whether the reac-
tion takes place in the solid matrix of PS/AlCl3 or
whether released AlCl3 is responsible for the con-
densation reaction, PS/AlCl3 was added to CH3CN
and the mixture was stirred at reflux temperature
for 3 h. Then, the catalyst was filtered off and
the filtrate was analyzed for its aluminum con-
tent, which showed a negligible release of AlCl3.
The filtrate was found to be inactive for the con-
densation reaction. This observation indicates that
PS/AlCl3 is stable under the reaction conditions, and
there is no leaching of Lewis acid moieties during
reactions.
With these results in hand, other amines
(aliphatic and aromatic) have been subjected to the
above-mentioned optimized conditions, and the re-
sults are listed in Table 2. It is evident that our
methodology is reasonably general and can be ap-
plied to several amines. As indicated in Table 2, in all
cases the reaction gives the corresponding pyrroles
in good to excellent yields and completed within 30–
135 min under reflux conditions. It is well known
that the reactivity of aliphatic amines is more po-
tent compared with that of aromatic amines, thus
aliphatic amines give higher yields or shorter re-
action times that are generally observed in other
systems [26], and both lead to high yields of the
pyrrole products. As shown in Table 2, a series of
aromatic amines bearing either electron-donating
(Table 2, entries 2–4,6) or electron-withdrawing
(Table 2, entries 5,7) groups on an aromatic ring was
investigated and the results show that they are both
effective in the Paal–Knorr condensation reaction.
Moreover, we also examined the reaction of aliphatic
and cyclic aliphatic amines with 2 (Table 2, entries
8–11,12). Similarly, the corresponding pyrrole prod-
ucts were obtained with excellent yields. In contin-
uation of this study, we replaced the monoamino
instead of diamino substrates in the same condi-
tions in giving bipyrrole compounds (Table 2, entries
13,14) in excellent yields. In this reaction, 2 equiv of
2 were required to have a complete conversion of
diamines.
The stability of the catalyst and its reusability
are the main advantages of this polymeric catalyst.
Therefore, the recovery and reusability of PS/AlCl3
was examined. The catalyst can be separated and
reused after washing with CHCl3 and drying at 60◦C.
The reusability of the catalyst was investigated in
the reaction of aniline with 2,5-hexandione (Table 2,
entry 1) in the presence of 15 mol% PS/AlCl3. The
results illustrated in Table 2 showed that the cata-
lyst can be used eight times without any loss of its
activity. PS/AlCl3 could be stored and handled under
open-air conditions.
Heteroatom Chemistry DOI 10.1002/hc

88 Rahmatpour and Aalaie
TABLE 3 Synthesis of Pyrrole 3a by Reacting Aniline 1a with 2,5-Hexandione 2 under Various Lewis Acid Catalysts
Entry
Catalyst
Conditions
Time (min)
Yield (%)a
Reference
1
2
3
4
5
6
7
8
CuClb2
Cu(OTf)2
Mg(OTf)2
Bi(OTf)3
Sm(OTf)3
Eu(OTf)3
Sc(OTf)3
ZnCl2
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
CH2Cl2
Solvent-free
Solvent-free
Solvent-free
CH2Cl2
25
25
25
25
25
25
25
1.5 h
2 h
3 h
1 h
1 h
10 h
75
55
78
47
72
88
88
77
51
47
39
91
84
96
90
[28]
[28]
[28]
[28]
[28]
[28]
[28]
[27]
[27]
[27]
[27]
[27]
[8]
9
CoCl2
NiCl2
In(OTf)3
InBr3
Bi(NO3)3.5H2O
PS/AlCl3
10
11
12
13
14
Solvent-free
CH2Cl2/r.t.
CH3CN
Table 2
aIsolated yield.
b40 mol%.
The actual mechanism of the reaction is unclear.
However, based on the experimental results and by
referring to the literature [8,27,28], the mechanism
of the pyrrole formation proceeds by the usual path-
way proposed using Lewis acids.
Table 3 compares efficiency of PS/AlCl3 (time,
yield, reaction conditions) with efficiency of other
Lewis acid catalysts in synthesis of 3a obtained by
other groups.
samples. Melting points were measured on an Elec-
trothermal 535 apparatus and were uncorrected. 1H
NMR spectra were recorded on a Bruker DPX-250
Avance spectrometer at 250.13 MHz. IR spectra were
recorded on a Unicam Matteson 1000 spectropho-
tometer. Reaction monitoring and purity determi-
nation of the products were accomplished by TLC
on silicagel polygram SILG/UV254 plates. All yields
refer to isolated products.
In conclusion, PS/AlCl3 was found to be an ef-
ficient, environmentally benign, and stable hetero-
geneous polymer-supported Lewis acid catalyst for
the preparation of N-substituted pyrroles. The mild
reaction conditions, high to excellent yields, short
reaction times, simple experimental procedure and
product isolation, recyclability of the catalyst with
no loss in its activity, low cost, and easy handling of
the polymeric catalyst are important features of this
new protocol to prepare pyrroles. In addition, unlike
some other methods, no extra energy source such as
microwave irradiation or ultrasound is needed for
the success of the reaction.
Preparation of PS/AlCl3
Anhydrous AlCl3 (4.5 g) was added to polystyrene
(8% divinylbenzene, 3.5 g) in carbon disulfide
(25 mL) as the reaction medium. The mixture was
stirred using a magnetic stirrer under reflux con-
dition for 50 min, cooled and then water (40 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 then filtered and washed
with water (300 mL) and then with acetone and di-
ethyl ether. The polymer was dried in a vacuum oven
for 12 h at room temperature. The capacity of the
polymeric catalyst based on its chloride content was
0.47 mmol AlCl3/g catalyst [29].
EXPERIMENTAL
Materials and Instruments
All chemical reagents and solvents were obtained
from Fluka (Switzerland) and Merck (Germany)
chemical companies and were used without fur-
ther purification. Cross-linked polystyrene (8% di-
vinylbenzene) was prepared via suspension polymer-
ization. PS/AlCl3 was prepared using cross-linked
polystyrene as reported in the literature [21b]. The
capacity of the catalyst was determined by the Mohr
titration method. All products were known com-
pounds and were identified by comparison of their
physical and spectral data with those of the authentic
General Experimental Procedure for the
Synthesis of Pyrroles 3:
In a round-bottom flask (50 mL) equipped with a
condenser and a magnetic stirrer, a solution of the
amine (1 mmol) and 2,5-hexandione (1 mmol) in
CH3CN (10 mL) was prepared. PS/AlCl3 (15 mol%,
0.15 mmol) was added to the solution, and the reac-
tion mixture was stirred magnetically under reflux
conditions for the given time (Table 2). The progress
Heteroatom Chemistry DOI 10.1002/hc

One-Pot Synthesis of N-Substituted Pyrroles Catalyzed by Polystyrene-Supported Aluminum Chloride 89
A. R., Rees, C. W., Scriven, E. F. V. (Eds.); Pergamon
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of the reaction was monitored by TLC. After comple-
tion of the reaction, the catalyst was filtered off and
washed with CHCl3 (2 × 10 mL) to remove any ad-
hering pyrrole product and the filtrate concentrated
on a rotary evaporator under reduced pressure to
give the crude product. The residue was purified by
column chromatography and by recrystallization to
afford the pure pyrrole derivatives. The spent poly-
meric catalyst from different experiments was com-
bined, washed with CHCl3 and dried overnight in a
vacuum oven and reused. Representative examples
of spectroscopic and analytical data are given below.
Compound 3a: mp = 50◦C; νmax/cm−1 3059, 2921,
1600, 1520, 1497, 1403, 1320; δH(250 MHz, CDCl3)
2.05 (s, 6H, CH3), 5.92 (s, 2H, pyrrole ring), 7.22–7.26
(2H, m, Ar H), 7.42–7.50 (3H, m, Ar H). Found: C,
79.89; H, 7.48; N, 8.00. C12H13N requires C, 84.16; H,
7.65; N, 8.18%.
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