Ionic liquid catalyzed one pot four-component coupling reaction for the synthesis of functionalized pyrroles

Article abstract of DOI:10.1016/j.molliq.2014.07.038

A facile and convenient one-pot tandemsynthesis of functionalized pyrroles using acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate as a catalyst has been achieved. It involves four-component reaction of amines, aldehydes, 1,3-dicarbonyl compounds and nitroalkane. This method has the advantage of simple and readily available starting materials, low cost, good yields and recyclability of catalyst.

Full text of DOI:10.1016/j.molliq.2014.07.038

MOLLIQ-04373; No of Pages 4
Journal of Molecular Liquids xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Ionic liquid catalyzed one pot four-component coupling reaction for the
synthesis of functionalized pyrroles
⁎
Neeru Gupta, Kamal N. Singh, Jasvinder Singh
Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and convenient one-pot tandem synthesis of functionalized pyrroles using acidic ionic liquid 1-butyl-
3-methylimidazolium hydrogen sulfate as a catalyst has been achieved. It involves four-component
reaction of amines, aldehydes, 1,3-dicarbonyl compounds and nitroalkane. This method has the advantage
of simple and readily available starting materials, low cost, good yields and recyclability of catalyst.
© 2014 Elsevier B.V. All rights reserved.
Received 20 May 2014
Received in revised form 15 July 2014
Accepted 23 July 2014
Available online xxxx
Keywords:
Pyrrole
Ionic liquid
One-pot synthesis
Four-component reaction
Reusable media
1. Introduction
et al. synthesized substituted pyrroles via four-component coupling
reaction of amines, aldehydes, 1,3-dicarbonyl compounds and
Pyrroles are an important class of heterocycles found in many
natural products [1] and have pharmacological activities [2,3]. It has
wide application as antitumor [4], anti-inflammatory [5], antibacterial
[6] and antioxidant [7] activities. Moreover, they are widely used in
the field of material chemistry [8,9]. Thus, several methods for synthesis
of pyrroles have been developed. The commonly used methods are
Hantzsch [10,11], Knorr [12–14], Paal–Knorr [15,16] and Clausen Kaas
condensation [17] reactions. These classical reactions are very useful
but have several drawbacks like multistep synthetic operations,
harsh reaction conditions and availability of starting materials. To
overcome these problems transition metal [18,19] catalysis and multi-
component [20–23] reactions (MCRs) have been developed. MCRs
emerged as fast and experimentally simple way of synthesis of complex
molecules without the need of isolation and purification of any interme-
diate which results in minimization of waste, time and cost. Recently
functionalized pyrroles have been synthesized via multi component
coupling reaction using various catalysts and reagents like Fe(III)-
catalyst [24], Pd catalyst [25], iodine [26], heterogenized tungsten
complex [27], NiCl2 [27], gluconic acid [29], amberlyst-15 (under ultra-
sound) [30], nano-CoFe2O4 supported Mo catalyst [31], CeCl3·7H2O
(under microwave) [32] and nano-CoFe2O4 supported Sb(III) catalyst
[33]. These methods have drawbacks such as use of metal catalyst,
tedious workup and long reaction time. In a recent article, Meshram
nitroalkane using a huge quantity (5 ml) of ionic liquid [Hbim]BF4
[34] as solvent. Also the authors have not given any clue about the
behavior of aromatic amines under the set of reaction conditions
described. So, we felt that still there could be a scope of improvement
in this reaction so as to overcome the drawbacks of many reported
methods using ionic liquid in catalytic amount. More recently,
an efficient, mild and green methodology for N-substituted pyrrole
derivatives using amberlite IR 120 acidic resin, as a catalyst for Paal–
Knorr condensation of 2,5-hexadiene with primary amines under
solvent-free condition has also been reported by us [35].
Room temperature ionic liquids are a subject of continuous attention
as either solvents or catalysts [36]. Room temperature ionic liquid 1-
butyl-3-methylimidazolium hydrogen sulfate ([bmim]HSO4) has
attracted much attention due to its bronsted acidity, recyclability and
efficiency in various organic transformations [37–40]. In continuation
with our studies towards exploring the use of [bmim]HSO4 [41–45]
we synthesized functionalized pyrroles via one-pot tandem reaction.
2. Experimental
The acidic ionic liquid [bmim]HSO4 was prepared according to our
previously reported method in literature [41]. IR spectra were recorded
on Perkin Elmer model 1430 spectrometer. 1H NMR (300 MHz) and 13
C
NMR (75 MHz) spectra were recorded using Joel 300 MHz spectrome-
ter. Chemical shifts (δ) are reported in ppm with tetramethylsilane as
internal standard. Melting points were determined with Sunbeam
melting point apparatus and are uncorrected. High resolution mass
⁎
Corresponding author.
E-mail address: jsbrar_pu@yahoo.com (J. Singh).
http://dx.doi.org/10.1016/j.molliq.2014.07.038
0167-7322/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: N. Gupta, et al., Ionic liquid catalyzed one pot four-component coupling reaction for the synthesis of functionalized
pyrroles, Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.038

2
N. Gupta et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Scheme 1. Synthesis of functionalized pyrroles.
spectra (HRMS) were recorded using Waters Micromass Q-Tf Micro
instrument.
126.3, 127.9, 128.2, 129.4, 130.6, 131.4, 135.6, 138.8, 155.4,
198.3 ppm. HRMS: m/z calcd for C19H18NO2 (M + 1) 292.1332; found
292.1366.
1-[4-(3-Bromophenyl)-2-methyl-1-phenyl-1H-pyrrol-3-yl]
ethanone (5f): Orange sticky liquid. IR (neat)/υmaxcm−1: 3055, 2959,
2926, 1723, 1651, 1595, 1504, 1402, 1222. 1H NMR (CDCl3, 300 MHz)
δ: 2.01 (s, 3H, \C_C(CH3)-N), 2.32 (s, 3H, CH3-C(_O)-), 6.58 (s,1H,
\C_C(H)-N), 7.16–7.26 (m, 4H, ArH), 7.34–7.46 (m, 5H, ArH) ppm.
13C NMR (CDCl3, 75 MHz) δ: 13.0, 31.2, 120.8, 122.5, 125.0, 126.4,
128.1, 128.3, 129.5, 129.7, 129.9, 132.2, 138.4, 196.3 ppm. HRMS: m/z
calcd for C19H17BrNO (M + 1) 354.0488; found 354.0512.
2.1. General procedure for the synthesis of functionalized pyrrole
derivatives (5)
To a stirred solution of amine 1 (1.0 mmol), aldehyde 2 (1.0 mmol),
1,3-dicarbonyl compound 3 (1.0 mmol) and nitromethane 4 (1.0 mL)
were added [bmim]HSO4 (0.20 mmol) (Scheme 1). The mixture was
then heated to 90–95 °C for a set period of time (monitored by TLC)
without an inert atmosphere. After completion of the reaction, reaction
mixture was cooled to room temperature and extracted with ethyl
acetate (3 × 5 mL) leaving ionic liquid as residue. The combined organic
layers were washed with water (2 × 10 mL), brine (5 mL), dried over
Na2SO4, filtered and concentrated under vacuum. The crude product
was purified by silica gel column chromatography to afford the desired
compound. To recover the catalyst the residual ionic liquid was washed
with diethyl ether (5 mL) for 30 min and the ether layer was decanted.
The ionic liquid was then dried under vacuum and reused. The
formation of known products was related by comparison of melting
point, IR and NMR data with literature data. HRMS data for unknown
compounds was taken.
1-[1-(4-Hydroxyphenyl)-2-methyl-4-phenyl-1H-pyrrol-3-yl]
ethanone (5i): Orange solid, mp 155–158 °C. IR (nujol)/υmaxcm−1
:
3234, 3018, 2922, 1620, 1517, 1410, 1273, 1219. 1H NMR (CDCl3,
300 MHz) δ: 2.00 (s, 3H, \C_C(CH3)-N), 2.30 (s, 3H, CH3-C(_O)-),
4.59 (bs, 1H, Ar-OH, D2O exchangeable), 6.47 (s, 1H, \C_C(H)-N),
6.85 (d, J = 8.4 Hz, 2H, ArH), 7.00 (d, J = 8.4 Hz, 2H, ArH), 7.16–7.21
(m, 3H, ArH) ppm. 13C NMR (CDCl3, 75 MHz) δ: 13.3, 30.8, 116.3,
121.4, 121.8, 126.5, 127.0, 127.4, 128.3, 129.5, 130.3, 136.1, 136.9,
157.4, 198.7 ppm. HRMS: m/z calcd for C19H18NO2 (M + 1) 292.1332;
found 292.1239.
3. Results and discussion
2.2. Spectral data for the synthesis of functionalized pyrrole derivatives
(5) of three new compounds
It is known that pyrroles can be obtained via Michael reaction of
β-enamino ketones or esters and nitroolefins followed via cyclization
[24–31,34]. β-Enamino ketones or esters can be obtained from reaction
of β-dicarbonyl compounds with amines and nitroolefins can be obtain-
ed from reaction of aldehyde and nitroalkane which can be catalyzed by
ionic liquids [34,46,47]. So first we examined the reaction of amine 1a,
aldehyde 2b, active methylene compound 3a and nitromethane 4
using various catalysts (Table 1). The reactions were carried out at
90–95 °C except for one reaction with [bmim]HSO4 (entry 6, Table 1)
which was carried out at room temperature. The use of SiO2 (entry 1,
Table 1), acidic Al2O3 (entry 2, Table 1), p-toluenesulfonic acid (p-
TSA) (entry 3, Table 1), hydrochloric acid (HCl) (entry 4, Table 1) and
acidic resin amberlist-15 (entry 5, Table 1) was examined but the
product 5b was isolated only in 11–25% yield. In the absence of a
catalyst (entry 6, Table 1) only 10% yield was obtained. The use of
acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate
([bmim]HSO4) (entry 10, Table 1) increased the product yield to
88%. Good yield was obtained using 20 mol% of [bmim]HSO4
(entry 9, Table 1). On increasing [bmim]HSO4 to 25 mol% (entry
10, Table 1) no significant improvement in yield was observed
while the yield decreased when 15 mol% of [bmim]HSO4 (entry 8,
Table 1) was used or the reaction was carried out at room
temperature (entry 7, Table 1). This reaction was also studied
in various solvents like dichloromethane, acetonitrile, N,N-
dimethylformamide and tetrahydrofuran but the yield of desired
product decreased.
1-[4-(4-Hydroxyphenyl)-2-methyl-1-phenyl-1H-pyrrol-3-yl]
ethanone (5c): Orange solid, mp 170–171 °C. IR (nujol)/υmaxcm−1
:
3371, 3015, 2926, 1647, 1507, 1406, 1219. 1H NMR (CDCl3, 300 MHz)
δ: 1.98 (s, 3H, \C_C(CH3)-N), 2.34 (s, 3H, CH3-C(_O)-), 5.19 (bs, 1H,
Ar-OH, D2O exchangeable), 6.75 (d, J = 8.7 Hz, 2H, ArH), 7.13 (d, J =
8.4 Hz, 2H, ArH), 7.25 (d, J = 6.9 Hz, 2H, ArH), 7.30–7.43 (m, 3H, ArH)
ppm. 13C NMR (CDCl3, 75 MHz) δ: 13.1, 31.0, 115.4, 120.6, 122.6,
Table 1
Screening of different catalysts for the preparation of a pyrrole derivativea.
S.No.
Catalyst(mol%)
Temp(°C)
Time (h)
% Yieldb
1.
2.
3.
4.
5.
6.
7.
8.
SiO2 (20)
Acidic Al2O3 (20)
p-TSA (20)
HCl (20)
Amberlist-15 (20)
No catalyst
[bmim]HSO4 (15)
[bmim]HSO4 (15)
[bmim]HSO4 (20)
[bmim]HSO4 (25)
[bmim]BF4 (20)
[bmim]I (20)
90–95 °C
90–95 °C
90–95 °C
90–95 °C
90–95 °C
90–95 °C
Room temp
90–95 °C
90–95 °C
90–95 °C
90–95 °C
90–95 °C
14
14
14
14
24
24
14
3
3
3
3
3
15
16
25
12
20
10
52
79
87
88
72
65
9.
10.
11.
12.
a
All reactions were carried out using amine (1 mmol), aldehyde (1 mmol) and active
methylene compound (1 mmol) with nitromethane (1 mL).
Following the optimized reaction conditions, a variety of functional-
ized pyrroles (5) was synthesized (Table 2) in good yields. Both the
b
Isolated yield.
Please cite this article as: N. Gupta, et al., Ionic liquid catalyzed one pot four-component coupling reaction for the synthesis of functionalized
pyrroles, Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.038

N. Gupta et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
3
Table 3
Reusability of catalyst.
electron rich aromatic aldehydes (Table 2, entries 1–3, 5 and 7) and also
weakly electron deficient aromatic aldehydes (Table 2, entries 4 and 6)
gave the corresponding pyrroles in good yield. However, in case of a
strong electron withdrawing group such as −NO2 on aromatic alde-
hydes and using aliphatic aldehydes the yield was not satisfactory and
the reaction was not studied further. Similarly electron rich (Table 2,
Sr. No.
Run
Yield (%)
1.
2.
3.
4.
5.
1(fresh)
87
85
81
78
65
2
3
4
5
Table 2
Four component coupling synthesis of functionalized pyrrolea.
S.No. R1
1.
Substrate R2 R3, R4
Time (h) Products %Yieldb [Ref]
entries 1, 8 and 9) and weakly electron deficient (Table 2, entry 10) ar-
omatic amines gave the corresponding pyrroles in good yield, while in
case of nitro anilines no satisfactory yield was obtained. The reaction
went well with different dicarbonyl compounds such as acetyl acetone
(3a), ethyl acetoacetate (3b) and methyl acetoacetate (3c).
After completely establishing this method for successful conversion
of aromatic amines to corresponding pyrroles, we also tried to screen
the aliphatic amines and benzyl amines under similar set of conditions
as these are more reactive for enaminone [46,47] and pyrrole [34]
formations. The expected behavior could not be showed by these
substrates and no product was formed in this case. So, we can deduce
that this particular ionic liquid is specific towards the reaction of
aromatic amines.
CH3, CH3
3
5a
90 [24]
3a
3a
1a
2a
2.
1a
3
5b
87 [29]
2b
2c
3.
4.
1a
1a
3a
3a
3
4
5c
83
A plausible mechanism for the ionic liquid catalyzed tandem
reaction was proposed (Scheme 2) in accordance with recent reports
from literature [24–31,34].
5d
77 [29]
To investigate the reusability of a catalyst we have taken reaction
of amine 1a, aldehyde 2b, active methylene compound 3a and nitro-
methane 4 as model (Table 3). The residual ionic liquid was washed
with diethyl ether and the ether layer was decanted. The ionic liquid
was then dried under vacuum and reused. We were able to recycle
the ionic liquid thrice, after that there was a sharp decrease in
product yield.
2d
2e
5.
6.
1a
1a
3a
3a
6
5
5e
5f
70 [29]
70
4. Conclusions
In conclusion, we have developed an operationally simple, eco-
nomical and eco-friendly one-pot four-component coupling reac-
tion for synthesis of functionalized pyrroles using an acidic ionic
liquid as a powerful and recyclable catalyst. This method has sever-
al advantages of high catalytic activity, mild reaction conditions,
easily available starting materials, less amount of ionic liquid
used and metal free synthesis. Thus, this procedure clearly repre-
sents an appealing methodology for the synthesis of functionalized
pyrroles.
2f
7.
8.
1a
3a
3a
2
3
5g
5h
93 [29]
89 [29]
2g
2a
1b
1c
9.
2a
2a
3a
3a
3
5
5i
5j
88
Acknowledgment
We are grateful to the University Grants Commission (UGC) for
financial support (8-26/SC/SA). Neeru Gupta thanks to the Council
of Scientific and Industrial Research (CSIR), New Delhi, India, for
the award of senior research fellowship (09/135(0614)/2010-
EMR-I).
10.
68 [29]
1d
1a
11.
12.
2a
2a
CH3, OEt
3b
CH3, OMe
3c
3
3
5k
5l
95 [29]
91 [29]
Appendix A. Supplementary data
1a
Supporting Information: Experimental details and analytical
data of all compounds as well as copy of 1H NMR and 13C NMR spec-
tra. Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molliq.2014.
07.038.
a
All reactions were carried out using amine 1 (1.0 mmol), aldehyde 2 (1.0 mmol),
1,3-dicarbonyl compound 3 (1.0 mmol) and nitromethane 4 (1.0 mL) in the presence of
[bmim]HSO4 (0.20 mmol) at 90–95 °C.
b
Isolated yield.
Please cite this article as: N. Gupta, et al., Ionic liquid catalyzed one pot four-component coupling reaction for the synthesis of functionalized
pyrroles, Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.038

4
N. Gupta et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Scheme 2. Plausible mechanism for synthesis of pyrrole derivatives.
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Please cite this article as: N. Gupta, et al., Ionic liquid catalyzed one pot four-component coupling reaction for the synthesis of functionalized
pyrroles, Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.038