Paal-Knorr pyrrole synthesis using recyclable amberlite IR 120 acidic resin: A green approach

Article abstract of DOI:10.1080/00397911.2010.540766

Amberlite IR 120 acidic resin, a polymer matrix, has been demonstrated as a catalyst for Paal-Knorr condensation of 2,5-hexadione with primary amines under solvent-free conditions. This is an efficient, mild, and green methodology for N-substituted pyrrole derivatives. Copyright Taylor & Francis Group, LLC.

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Paal–Knorr Pyrrole Synthesis Using
Recyclable Amberlite IR 120 Acidic
Resin: A Green Approach
Aarti Devi a , Shallu a , M. L. Sharma a & Jasvinder Singh a
a Department of Chemistry and Centre for Advanced Studies in
Chemistry, Panjab University, Chandigarh, India
Accepted author version posted online: 17 Nov 2011.Version of
record first published: 25 Jan 2012.
To cite this article: Aarti Devi , Shallu , M. L. Sharma & Jasvinder Singh (2012): Paal–Knorr
Pyrrole Synthesis Using Recyclable Amberlite IR 120 Acidic Resin: A Green Approach, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
42:10, 1480-1488
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Synthetic Communications1, 42: 1480–1488, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.540766
PAAL–KNORR PYRROLE SYNTHESIS USING
RECYCLABLE AMBERLITE IR 120 ACIDIC RESIN:
A GREEN APPROACH
Aarti Devi, Shallu, M. L. Sharma, and Jasvinder Singh
Department of Chemistry and Centre for Advanced Studies in Chemistry,
Panjab University, Chandigarh, India
GRAPHICAL ABSTRACT
Abstract Amberlite IR 120 acidic resin, a polymer matrix, has been demonstrated as a cata-
lyst for Paal-Knorr condensation of 2,5-hexadione with primary amines under solvent-free
conditions. This is an efficient, mild, and green methodology for N-substituted pyrrole deri-
vatives.
Keywords Amberlite IR 120; Paal–Knorr condensation; pyrrole; solvent free
INTRODUCTION
Pyrroles are an important class of heterocyclic compounds with different
biological activities.[1] Many members of this family are used as antimalarial,
anti-inflammatory, anti-asthmatic, antibacterial, antihypertensive, and tyrosine
kinase inhibiting agents,[2] for example, lipitor[3] (cholesterol-lowering drug) and
amtolmetin[4] (anti-inflammatory agent). Functionalized pyrroles are abundant in
nature, forming characteristic subunits of haeme, chlorophyll, bile pigments, and vit-
amin B12.[5] Despite their importance from pharmacological, industrial, and syn-
thetic points of view, relatively few methods for their preparation have been
reported.[6] Of the current methods, such as Hantzsch,[7] Knorr,[8] and aza-Wittig[9]
reactions, Paal–Knorr[10] has experienced a rekindled interest from synthetic per-
spectives because it is simple and straightforward. Various catalysts reported for
Paal–Knorr reaction, such as Ti(OiPr)4.[11] Al2O3,[12] Bi(NO3)3,[13] Bi(OTf)3,[14]
Sc(OTf)3,[15] montmorillonite-KSF,[16] sulfamic acid,[17a] CoCl2,[17b] Indium (III)[17c]
Received August 23, 2010.
Address correspondence to Jasvinder Singh, Department of Chemistry and Centre of Advanced
Studies in Chemistry, Panjab University, Chandigarh 160014, India. E-mail: jsbrar_pu@yahoo.com
1480

PAAL–KNORR PYRROLE SYNTHESIS
1481
Scheme 1. Paal–Knorr pyrrole condensation. R ¼ alkyl, aryl, heteroaromatic or polyaryl group.
N,N,N0,N0-tetrachlorobenzene-1,3-disulfonamide,
and
N,N0-diiodo-N,N0-1,2-
ethanediylbis(p-toluenesulfonamide)[17d] are associated with one or other of these
drawbacks: poor yield, harsh reaction conditions, tedious workup leading to toxic-
metal-containing waste, requirement for inert atmosphere or high temperature, use
of stoichiometric or expensive reagents, destruction of catalyst, and use of toxic sol-
vents such as dimethylsulfoxide (DMSO) and dimethylformamide (DMF). Therefore,
the search continues for a better catalyst for the synthesis of pyrroles in terms of oper-
ational simplicity, reusability, economic viability, and greater selectivity.
The development of new catalytic methods without use of any metal catalyst or
toxic solvents, with efficient recycling of catalyst, and avoiding stringent reaction
conditions is a conditio sine quo non in chemical research for economically and envir-
onmentally attractive processes worldwide. One such method in organic synthesis is
the use of reagents and catalysts that are bound to a polymer matrix insoluble in the
reaction media, so that they can be recycled.[18a] Use of styryl resin (D001) has been
reported in literature.[18b] Amberlite IR 120 acidic resin is polystyrene divinylbenzene
polymer with supported –SO3H functional group. Here, this resin has been used as an
efficient, cost-effective, and recyclable heterogeneous catalyst for Paal-Knorr pyrrole
synthesis under solvent-free conditions.
RESULTS AND DISCUSSION
Recently, the use of solid acids as heterogeneous catalysts has received tremen-
dous interest in organic synthesis[19] because these can be easily separated from the
reaction mixture by simple filtration and can be reused with or without activation,
making the process economically more viable. During the past few years, amberlite
IR 120 acidic resin has emerged as a substitute for conventional acidic catalysts in
various organic reactions such as alcohol oxidation,[20] a-amino phosphonates syn-
thesis,[21] glycosyl dihydropyridones synthesis,[22] and synthesis of 2-aryl-1-aryl-
methyl-1H-benzimidazole.[23] In continuation of our work on environmentally
friendly synthetic strategies,[24] we herein report a facile method of pyrrole synthesis
by condensation of 1,4-diketones with primary amines in the presence of amberlite
IR 120 acidic resin.
In conventional systems, an excess of amine usually has to be used to promote
condensation. From a recent atom-economical standpoint, the use of equimolar sub-
strates is strongly required. Thus, initially we treated aniline with nearly equimolar
2,5-hexadione to obtain exclusive product N-phenyl-2,5-dimethylpyrrole in 99%
yield after 18 min. Under microwave irradiation, we get 98% yield for the reaction
in 12 min, but all the reported reactions were carried out at room temperature to
minimize energy consumption, which is one of the principles of green chemistry.
The results obtained with various amines are shown in Table 1. It is clear from

1482
A. DEVI ET AL.
Table 1. Paal–Knorr pyrrole synthesis using amberlite IR 120
No.
Amine
Product
Time (min)
Mp (ꢀC)
Yield (%)
1
18
—
96
2
20
20
30
60–61
45–46
75–76
92
3
99
93
91
4
5
28
56–57
6
32
—
70
(Continued )

PAAL–KNORR PYRROLE SYNTHESIS
1483
Table 1. Continued
No.
Amine
Product
Time (min)
Mp (ꢀC)
Yield (%)
7
30
36
—
99
98
8
84–85
9
30
36
48
16
145–146
97
98
93
10
11
12
—
—
47–48
99
99
13
14
14
40
—
120–121
99
(Continued )

1484
A. DEVI ET AL.
Table 1. Continued
No.
Amine
Product
Time (min)
Mp (ꢀC)
Yield (%)
15
16
6
—
99
5
—
99
Note. All the compounds were characterized by 1H NMR, 13C NMR, and mass spectra and verified by
comparison with literature=authentic samples.
the Table 1 that the reactions proceed very well in both aliphatic and aromatic
amines. Less nucleophilic aromatic amines require longer times for completion of
reaction compared to more nucleophilic aliphatic amines. A structure–activity
relationship study in aromatic amines shows that aromatic amines with electron-
donating groups react faster to aromatic amines with electron-withdrawing groups.
The substitution pattern on the aromatic ring makes a large impact on reactivity as
shown by 2, 3, and 4-nitro anilines, where 2-nitro aniline did not react at all, 4-nitro
aniline gives 30% yield, and 3-nitro aniline gives 98% yield of product. Similarly,
4-amino benzoic acid did not react at all, but 3-aminobenzoic acid gives corres-
ponding pyrrole in 97% yield. Sterically very hindered 2-amminoacetophenone
and 2-aminothiophenone did not yield any product. Heteroaromatic 3-amino pyri-
dine reacted excellently to give corresponding pyrrole in 92% yield. With 1,3-diami-
nopropane, bispyrrole was obtained in 99% yield. It is gratifying that the present
methodology is compatible with various functionalities such as methoxy, nitro,
-SH, and halo groups.
For optimizing the recyclability of the catalyst, experiments were performed
over aniline and 2,5-hexadione. Fresh catalyst gave a yield of 96%, and when the
same catalyst was repeatedly used up to six times under similar conditions, the yields
were 96, 96, 96, 95, 96, and 95% respectively. Thus, the catalyst was recycled and
reused for six times with slight degradation on each step but without any loss of
activity of undegraded resin.
CONCLUSION
In conclusion, we have demonstrated a mild, efficient, and environmentally
friendly approach to the synthesis of substituted pyrroles using amberlite IR 120
acidic resin as heterogeneous catalyst. Significant features of this strategy include
operational simplicity, solvent-free conditions, good yields, no side products, no
requirement for extra source of energy such as microwave or ultrasound, and toler-
ance of various functionalities.

PAAL–KNORR PYRROLE SYNTHESIS
1485
EXPERIMENTAL
NMR spectra were recorded on a Jeol AL 300-MHz Fourier transform (FT)
NMR instrument. Mass spectra were recorded on a Q-TOF Micro (Waters) mass
spectrometer. Melting points were recorded on a Buchi R-535 apparatus and are
uncorrected. Infrared (IR) spectra were recorded on a Perkin-Elmer IR spectrophot-
ometer. All solvents and reagents were purchased from Aldrich with high quality and
were used without any further purification. All yields refer to isolated products.
Typical Procedure
A mixture of 2,5-hexadione (570 mg, 5 mmol), aniline (558 mg, 6 mmol), and
amberlite IR 120 (200 mg) was stirred at room temperature under solvent-free con-
ditions for 18 min. After completion of reaction, as monitered by thin-layer chroma-
tography (TLC), the reaction mixture was diluted with diethyl ether and filtered to
recover the catalyst. The ether layer was washed with saturated sodium bicarbonate
solution and brine solution, dried (anhydrous Na2SO4), and concentrated in vacuo.
The residue was purified by silica-gel column chromatography (eluting with 20%
ethyl acetate in hexane) to afford pure product N-phenyl-2,5-dimethylpyrrole
(846 mg) in 99% yield. The recovered catalyst was washed with 5% diluted HCl
and dried prior to reuse. The IR spectrum of the recovered catalyst was identical
to that of the commercially available catalyst, which was reused for further reactions
(six times) without loss of activity. All the compounds are known and were identified
by comparison with those of the authentic samples.
Product Characterization Data[17]
2,5-Dimethyl-1-phenyl-1H-pyrrole (Entry 1). 1H NMR (300 MHz, CDCl3)
d 1.99 (s, 6H), 5.80 (s, 2H), 7.17 (m, 2H), 7.42 (m, 3H). 13C NMR (300 MHz, CDCl3)
d 10.86, 103.81, 125.66, 126.29, 126.56, 126.86, 136.98.
1-(4-Methoxyphenyl)-2,5-dimethyl-1H-pyrrole (Entry 2). Mp 60–61 ꢀC;
1H NMR (300 MHz, CDCl3) d 2.01 (s, 6H), 3.84 (s, 3H), 5.77 (s, 2H), 6.94 (d,
2H), 7.23 (d, 2H). 13C NMR (300 MHz, CDCl3) d 10.40, 52.58, 103.00, 111.53,
125.91, 126.61, 129.32, 156.21.
2,5-Dimethyl-1-p-tolyl-1H-pyrrole (Entry 3). 1H NMR (300 MHz, CDCl3)
d 2.01 (s, 6H), 2.40 (s, 3H), 5.81 (s, 2H), 7.07 (d, 2H), 7.22 (d, 2H). 13C NMR
(300 MHz, CDCl3) d 10.56, 18.67, 103.21, 125.55, 125.98, 127.21, 134.05, 134.69.
1-(4-Bromophenyl)-2,5-dimethyl-1H-pyrrole
(Entry
4). 1H
NMR
(300 MHz, CDCl3) d 1.99 (s, 6H), 5.78 (s, 2H), 7.06 (d, 2H), 7.55 (d, 2H). 13C
NMR (300 MHz, CDCl3) d 10.90, 104.46, 119.32, 125.90, 127.71, 130.07, 136.06.
1-(4-Chlorophenyl)-2,5-dimethyl-1H-pyrrole
(Entry
5). 1H
NMR
(300 MHz, CDCl3) d 1.99 (s, 6H), 5.83 (s, 2H), 7.09 (d, 2H), 7.37 (d, 2H). 13C
NMR (300 MHz, CDCl3) d 10.85, 104.32, 126.14, 127.15, 127.41, 131.34.
1-(2-Chlorophenyl)-2,5-dimethyl-1H-pyrrole
(Entry
6). 1H
NMR
(300 MHz, CDCl3) d 1.96 (s, 6H), 5.83 (d, 2H), 7.25 (m, 1H), 7.34 (m, 2H), 7.50

1486
A. DEVI ET AL.
(m, 1H). 13C NMR (300 MHz, CDCl3) d 10.35, 103.87, 116.78, 125.35, 126.02,
127.32, 128.53, 131.84, 140.95.
2,5-Dimethyl-1-(2-fluorophenyl)-1H-pyrrole
(Entry
7). 1H
NMR
(300 MHz, CDCl3) d 2.09 (s, 6H), 5.98 (d, 2H), 7.32 (m, 3H), 7.47 (m, 1H). 13C
NMR (300 MHz, CDCl3) d 10.25, 104.05, 116.23, 122.23, 124.63, 126.66, 127.52,
128.45, 157.91.
2,5-Dimethyl-1-(3-nitrophenyl)-1H-pyrrole (Entry 8). 1H NMR (300 MHz,
CDCl3) d 2.04 (s, 6H), 5.84 (d, 2H), 7.56 (d, 1H), 7.68 (t, 1H), 8.07 (s, 1H), 8.24 (d,
1H). 13C NMR (300 MHz, CDCl3) d 10.49, 104.84, 120.65, 125.66, 126.13, 127.28,
131.60, 137.81, 146.18.
3-(2,5-Dimethyl-pyrrol-l-yl)benzoic Acid (Entry 9). 1H NMR (300 MHz,
CDCl3) d 2.10 (s, 6H), 5.71 (d, 2H), 7.76 (m, 2H), 8.25 (m, 2H), 12.21 (bs, 1H).
13C NMR (300 MHz, CDCl3) d 10.51, 105.83, 120.32, 125.17, 126.23, 128.19,
130.26, 132.61, 139.52, 166.34.
2-(2,5-Dimethyl-pyrrol-l-yl)benzenethiol (Entry 10). 1H NMR (300 MHz,
CDCl3) d 1.80 (s, 3H), 1.95 (s, 3H), 2.76 (bs, 1H), 5.87 (d, 2H), 7.09 (m, 2H), 7.28
(m, 2H). 13C NMR (300 MHz, CDCl3) d 10.25, 10.44, 104.43, 116.34, 123.83,
125.69, 125.96, 129.19, 129.32, 145.63.
3-(2,5-Dimethyl-pyrrol-l-yl)pyridine (Entry 11). 1H NMR (300 MHz,
CDCl3) d 2.03 (s, 6H), 5.83 (d, 2H), 7.40 (m, 1H), 7.55 (m, 1H), 8.50 (d, 1H), 8.62
(m, 1H). 13C NMR (300 MHz, CDCl3) d 13.13, 107.13, 123.40, 128.40, 135.28,
135.73, 148.66, 149.53. EIMS (70 eV) m=z 172.1 [Mþ], 171.1, 156.1.
1-Benzyl-2,5-dimethyl-1H-pyrrole (Entry 12). Mp 43–44 ꢀC; 1H NMR
(300 MHz, CDCl3) d 2.10 (s, 6H), 5.11 (s, 2H), 5.77 (s, 2H), 6.81 (m, 2H), 7.23
(m, 3H). 13C NMR (300 MHz, CDCl3) d 10.48, 44.65, 103.82, 123.59, 125.95,
126.92, 128.64, 136.66.
2,5-Dimethyl-1-phenethyl-1H-pyrrole (Entry 13). 1H NMR (300 MHz,
CDCl3) d 2.15 (s, 6H), 2.85 (t, 2H, J ¼ 7.50 Hz), 3.91 (t, 2H, J ¼ 7.50 Hz), 5.74 (s,
2H), 7.03 (m, 2H), 7.25 (m, 3H). 13C NMR (300 MHz, CDCl3) d 10.21, 35.46,
43.02, 103.25, 124.43, 124.80, 126.40, 126.66, 136.31.
2,5-Dimethyl-1-(naphthalene-1-yl)-1H-pyrrole (Entry 14). Mp 120–121 ꢀC;
1H NMR (300 MHz, CDCl3) d 1.83 (s, 6H), 5.90 (s, 2H), 7.20 (m, 1H), 7.51 (m, 4H),
7.58 (d, 2H). 13C NMR (300 MHz, CDCl3) d 10.58, 103.82, 107.39, 116.80, 121.40,
122.52, 125.93, 126.45, 129.97, 132.14, 133.96.
2,5-Dimethyl-1-(2-methylpropyl)-1H-pyrrole (Entry 15). 1H NMR
(300 MHz, CDCl3) d 1.07 (d, 6H, J ¼ 6.60 Hz), 2.17 (m, 1H), 2.36 (s, 6H), 3.67 (d,
2H, J ¼ 6.60 Hz), 5.85 (s, 2H). 13C NMR (300 MHz, CDCl3) d 11.33, 18.47, 28.39,
49.02, 104.09, 125.27.
1,3-Bis-(2,5-dimethyl-pyrrol-1-yl)propane
(Entry
16). 1H
NMR
(300 MHz, CDCl3) d 1.88 (m, 2H), 2.13 (s, 12H), 3.71 (t, 4H), 5.67 (s, 4H). 13C
NMR (300 MHz, CDCl3) d 10.09, 10.37, 30.38, 40.06, 103.27, 103.96, 123.88, 124.33.

PAAL–KNORR PYRROLE SYNTHESIS
1487
ACKNOWLEDGMENT
The authors are very grateful to the University Grants Commission India
(UGC) for financial assistance.
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