A solvent and transition metal-free, highly efficient Br?nsted acidic ionic liquid promoted one-potthree-component reactions for the synthesis of tetrasubstituted pyrroles

Article abstract of DOI:10.1080/00397911.2019.1593460

The present protocol describes a highly efficient, transition metal and solvent-free one-pot annulation of amines, dialkyl acetylenedicarboxylates and β-nitrostyrene using imidazolium Br?nsted acidic ionic liquid for the synthesis of tetrasubstituted pyrroles. A diverse functionalized tetrasubstituted pyrroles derivatives were obtained with excellent yields under transition metal and solvent-free conditions. The Br?nsted acidic ionic liquid can be easily recovered and reused for the next reaction without any effects on the yields of the products. This green protocol provides structurally complex, medicinally important tetrasubstituted pyrroles with various substitution patterns in a one-pot operation.

Full text of DOI:10.1080/00397911.2019.1593460

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
A solvent and transition metal-free, highly efficient
Brønsted acidic ionic liquid promoted one-
potthree-component reactions for the synthesis of
tetrasubstituted pyrroles
Amol Balu Atar, Eunbi Han, Dae Hyup Sohn & Jongmin Kang
To cite this article: Amol Balu Atar, Eunbi Han, Dae Hyup Sohn & Jongmin Kang (2019): A solvent
and transition metal-free, highly efficient Brønsted acidic ionic liquid promoted one-potthree-
component reactions for the synthesis of tetrasubstituted pyrroles, Synthetic Communications, DOI:
10.1080/00397911.2019.1593460
To link to this article: https://doi.org/10.1080/00397911.2019.1593460
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1593460
A solvent and transition metal-free, highly efficient
Brønsted acidic ionic liquid promoted one-pot
three-component reactions for the synthesis of
tetrasubstituted pyrroles
Amol Balu Atar, Eunbi Han, Dae Hyup Sohn, and Jongmin Kang
Department of Chemistry, Sejong University, Seoul, Republic of Korea
ABSTRACT
ARTICLE HISTORY
Received 28 January 2019
The present protocol describes a highly efficient, transition metal
and solvent-free one-pot annulation of amines, dialkyl acetylenedi-
carboxylates and b-nitrostyrene using imidazolium Brønsted acidic
ionic liquid for the synthesis of tetrasubstituted pyrroles. A diverse
functionalized tetrasubstituted pyrroles derivatives were obtained
with excellent yields under transition metal and solvent-free condi-
tions. The Brønsted acidic ionic liquid can be easily recovered and
reused for the next reaction without any effects on the yields of the
products. This green protocol provides structurally complex, medicin-
ally important tetrasubstituted pyrroles with various substitution pat-
terns in a one-pot operation.
KEYWORDS
Transition metal
free;ꢀBrønsted acid ionic
liquid;ꢀtetrasubstituted
pyrroles;ꢀGreen
Media ꢀMCRs
GRAPHICAL ABSTRACT
Introduction
Ionic liquids (ILs) as a green media has emerged powerful tool for the organic chemist
in the field of modern organic chemistry and are one of the mainstream research areas
of the scientific community that drives increasing efforts.[1] ILs catalyzed reactions show
significant efforts in many useful organic transformations due to their unique properties
such as dual role of solvent and catalyst, low vapor pressure, excellent chemical stability,
CONTACT Amol Balu Atar
amol.atar@sejong.ac.kr; Jongmin Kang
kangjm@sejong.ac.kr
Department of
Chemistry, Sejong University, Seoul 143-747, Republic of Korea.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
A. B. ATAR ET AL.
inexpensive, safe, and environmentally non-hazardous.[2] Most importantly they are
immiscible with a number of organic solvents which provide nonaqueous, polar alterna-
tive for two-phase systems instead of the use of water. Recently ionic liquids have been
considered as a new class of green solvents.[3] Moreover, ionic liquids have great ability
to control the selectivity of chemical transformations as well as reduce the time of com-
pletion reaction. Therefore, the development of synthetically useful reactions in ILs is of
considerable interest in the scientific community.[4]
Currently, there has been increasing concern with regard to the tight legislation on
the maintenance of “greenness” in synthetic pathways and processes. Green chemistry
strongly influences chemical research, and there is an insistence on the use of “greener”
reaction conditions.[5] Indeed, the use of ILs, as a green solvents, in organic reactions
has improved not only the aspect of the reactions from the viewpoint of green and sus-
tainable properties but also the synthetic efficiency by stabilizing the catalyst, changing
the reaction selectivity or facilitating product isolation.[6–14] Particularly, ILs, as a green
solvents, showed a great ability for multicomponent reaction (MCRs).[15] In contrast of
conventional multistep synthetic protocols, the MCRs using ILs deliver outstanding ben-
efits, such as simple experimental procedures, creation of several carbon–carbon and
carbon–heteroatom bonds in a single operation, high atom economy, high yields, less
formation of by products, short reaction times, and restraint of complex isolation and
purification of intermediates.[16]
Pyrroles are one of the major classes of heterocyclic compounds, having increasing
importance in medicinal chemistry and organic synthesis. Several pyrroles as core
structures are found in natural products, bioactive compounds, drug molecules, and
optoelectronic materials. The pyrroles structural motifs have shown a wide range of
biological properties such as antibacterial,[17] antioxidant,[18] anti-inflammatory,[19]
antifungal agents,[20] and antitumor.[21] In addition, they are used as a chromophores
in an electro-optic (EO) devices. Recently Nishihara et al. showed pyrroles based
p-conjugated system as a candidates for molecular flat-band ferromagnets.[22] The
pyrroles derivatives have significant importance in the pharmaceutical industry and
the synthesis of these novel type of heterocycles desirable. Numerous elegant
approaches have been developed to access for the pyrroles derivatives. These include
traditional methods such as classical Knorr reaction, the Hantzsch reaction, and the
Paal ꢁ Knorr condensation reaction, multicomponent coupling, tandem reactions,
transition-metal-catalyzed cyclization, and catalytic C–H bond functionalization
strategies.[23]
Until now, a three-component reaction of amines, aldehydes, and b-nitrostyrene is
reported to give tetrasubstituted pyrroles derivatives using FeCl3 as a transition metal
catalyst.[24] But unfortunately, the tetrasubstituted pyrroles derivatives has basically been
catalyzed by transition metals or carried out in toxic solvents whose uses have been lim-
ited from its noncompliance with the requirements of green chemistry. Thus, the search
for green solvent as well as convenient catalyst is desirable. In continuation of our
ongoing research to development of greener methods for the synthesis interesting bio-
logically active heterocycles,[25–28] we disclose herein for the first time a greener method
one-pot multicomponent synthesis of tetrasubstituted pyrroles derivatives using imida-
zolium Brønsted acidic ionic liquid (Scheme 1).

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SYNTHETIC COMMUNICATIONSV
3
Ar
CO2R2
CO2R2
R2O2C
BAIL
Ar
NH2
H
NO2
R1
N
RT
30-60 Min
R2O2C
R1
N
SO3H
N
Me
BAIL:
OTf
Scheme 1. BAIL-catalyzed one pot annulation for the synthesis of tetrasubstituted pyrroles
derivatives.
Result and discussion
To optimize the best reaction conditions, we started our study by reacting trans b-nitro-
styrene 1a, 4-methoxyaniline 2a and diethyl acetylenedicarboxylate 3a as the model sub-
strates by using various parameters like different catalysts, temperature, and solvents
were carried out (Table 1). Initially, using model substrate, the reaction was carried out
in water as a green solvent at room temperature. The result shows that there was no
any product formation and TLC indicating only starting materials (Table 1, entry 1).
Then, further using water as solvent, reaction was carried out at refluxed condition.
These results show that there is no any progress in reaction and indicated that need
catalyst to proceed the reaction (Table 1, entry 2). Then, we screened different acid pro-
moter catalysts for this one-pot annulation using water as a solvent. Unfortunately,
when various acid promoters catalysts, such as TsOH, HCl, TFA, HOAc, and
CF3COOH (Table 1, entries 3–7), were evaluated as highly efficient promoters for the
reaction, the desired product was obtained only in 12–34% yields.
Next, we decided to investigate to use different Brønsted acidic ionic liquid as a cata-
lyst as well as solvent. To our surprise, when BAIL-1 used as catalyst and solvent for
this one-pot annulation using model substrates, the desired product 4a was obtained in
54% yield at room temperature (Table 1, entry 8). Inspired by this result, we screened
different types of Brønsted acidic ionic liquid for this one-pot reaction (Table 1).
Among all screened Brønsted acidic ionic liquid imidazolium-based BAIL-2 as a catalyst
as well as solvent for the model reaction, and gratifyingly, 98% of the desired product
4a was obtained within 30 min (Table 1, entry 9). In contrast, another Brønsted acidic
ionic liquid was not highly effective for this transformation (Table 1, entries 10–13).
Also, we screened different temperature system, and room temperature was best system
in term of yield of desired product and reaction time (Table 1, entries 8, 14, and 15).
The acid strength values (H0) of the ionic liquids were determined using the UV-visible
spectroscopy combined with Hammett indicator (p-nitroaniline) method at room tem-
perature. The acidity of BAIL decreases in the order: BAIL-2 > BAIL-6ꢂ BAIL-3>
BAIL-1> BAIL-4> BAIL-5. It is concluded that the sequence of the catalytic perform-
ance observed for the synthesis of tetrasubstituted pyrroles derivatives is in good agree-
ment with the Brønsted acidity order determined by the Hammett method, and BAIL-2
was determined to be the most suitable catalyst for this one-pot reaction. We also
checked the effect of amount of BAIL-2 loaded for this reaction. When 5, 10, and

4
A. B. ATAR ET AL.
Table 1. Screening of various types of catalysts and solvents for the synthesis of compound 4a.a
NH2
CO2Et
EtO2C
Catalyst
H
N
EtO2C
4a
NO2
CO2Et
3a
O
1a
2a
O
SO3H
N
SO3H
N
N
Me
N
N
Me
HSO4
BAIL-1
OTf
Br
BAIL-2
BAIL-3
H
N
N
Br
BAIL-6
Temperature
H
Me
Cl
Br
H
N
N
BAIL-4
BAIL-5
Solvent
Entry
Catalyst (10 mol%)
Time
24 h
Yieldb (%)
1
2
—
—
TsOH
HCl
TFA
HOAc
CF3COOH
BAIL-1
BAIL-2
BAIL-3
BAIL-4
BAIL-5
BAIL-6
BAIL-2
BAIL-2
BAIL-2 (5 mol%)
BAIL-2 (20 mol%)
Water
Water
Water
Water
Water
Water
Water
—
—
—
—
—
RT
0c
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
RT
RT
RT
RT
RT
24 h
12 h
16 h
24 h
20 h
18 h
2 h
30 min
40 min
70 min
100 min
80 min
30 min
30 min
30 min
30 min
0c
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
17
15
12
34
22
54
98
62
50
45
66
85
78
96
95
—
—
—
—
RT
50 C
80 C
0
0
RT
RT
—
aReactions and conditions: The reactions were carried out in a pressure vial in the presence of trans b-nitrostyrene 1a
(1 mmol), 4-methoxyaniline 2a (1 mmol) and diethyl acetylenedicarboxylate 3a (1 mmol).
bIsolated yields.
cStarting material recovered.
20 mol% of BAIL-2 were used, the yields were obtained 96, 98, and 95%, respectively
(Table 1, entries 9, 16, and 17). This results show that 10 mol% of BAIL-2 was sufficient
and optimal quantity for the completion of the reaction (Table 1, entry 9).
Under the above-optimized conditions, the substrate scope of this reaction was exam-
ined by using various aryl substituted b-nitrostyrene, amines, and dialkyl acetylenedicar-
boxylate (Table 2). The results show that aryl substituted b-nitrostyrene substituted with

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5
Table 2. One pot annulation of amines, dialkyl acetylenedicarboxylate and b-nitrostyrenes for the
synthesis of tetrasubstituted pyrroles derivatives catalyzed by BAIL.a
Ar
CO2R2
R2O2C
R2O2C
BAIL
NH2
H
R1
Ar
RT
30-60 Min
N
NO2
CO2R2
3
R1
1
4
2
N
SO3H
N
Me
BAIL-
OTf
No.
b-nitrostyrene
R1
R2
Product
Time(Min) Yieldb
EtO2C
EtO2C
NH2
CO2Et
CO2Et
H
N
4a
NO2
OMe
30
30
25
35
98%
95%
96%
OMe
EtO2C
EtO2C
NH2
CO2Et
CO2Et
H
OMe
N
OMe
4b
4c
4d
NO2
NO2
NO2
EtO2C
EtO2C
NH2
Cl
CO2Et
CO2Et
H
N
Cl
EtO2C
EtO2C
NH2
Br
CO2Et
CO2Et
H
N
Br
92%
(continued)

6
A. B. ATAR ET AL.
EtO2C
EtO2C
Me
NH2
CO2Et
CO2Et
H
N
Me
4e
NO2
OMe
50
96%
OH
H
HO
NH2
Me
CO2Me
CO2Me
MeO2C
MeO2C
4f
NO2
N
45
90%
Me
F
NH2
CO2Me
CO2Me
MeO2C
MeO2C
H
4g
F
N
NO2
OMe
50
93%
OMe
EtO2C
EtO2C
NH2
CO2Et
CO2Et
H
Me
N
Me
4h
NO2
60
89%
EtO2C
EtO2C
NH2
CO2Et
CO2Et
H
N
4i
NO2
Me
Me
55
85%
(continued)

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7
EtO2C
EtO2C
NH2
Me
CO2Et
CO2Et
H
N
4j
NO2
40
88%
Me
EtO2C
NH2
CO2Et
CO2Et
H
N
EtO2C
4k
NO2
OMe
OMe
35
91%
F
F
F
NH2
CO2Me
CO2Me
MeO2C
MeO2C
F
H
4l
N
NO2
OMe
35
91%
OMe
NH2
CO2Me
CO2Me
MeO2C
MeO2C
4m
NO2
H
N
60
84%
OEt
H
EtO
H2N
4n
CO2Me
CO2Me
MeO2C
MeO2C
NO2
N
60
80%
(continued)
either electron-donating or electron-withdrawing groups proceeded the reaction
smoothly and gave the products in good yields (Table 2, entries 4a, 4e, 4f, 4g, 4l, 4n,
4p, 4s, and 4v). The present methods also work well both aromatic and aliphatic amines
as well as dialkyl acetylenedicarboxylate (Table 2, entries 4a, 4x, and 4y). We
did not observe much difference in the yield of 4, obtained by electron-donating

8
A. B. ATAR ET AL.
EtO2C
EtO2C
NH2
Me
CO2Et
CO2Et
N
40
NO2
Me
45
83%
Me
Me
Br
H2N
Br
CO2Me
CO2Me
MeO2C
MeO2C
H
4p
NO2
N
40
90%
F
F
H2N
CO2Me
CO2Me
MeO2C
MeO2C
4q
H
N
NO2
60
81%
EtO2C
EtO2C
4r
NH2
Me
CO2Et
CO2Et
H
N
NO2
Me
60
79%
Me
Me
Me
H2N
4s
CO2Me
CO2Me
Me
MeO2C
MeO2C
H
NO2
N
30
95%
EtO2C
EtO2C
NH2
CO2Et
CO2Et
H
N
4t
NO2
Me
Me
30
86%
Me
Me
(continued)

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9
CO2Et
CO2Et
EtO2C
EtO2C
4u
NH2
N
H
N
NO2
N
60
nr
F
F
MeO2C
MeO2C
F
NH2
CO2Me
CO2Me
H
F
N
4v
NO2
60
81%
OMe
OMe
EtO2C
4w
NH2
Me
CO2Et
CO2Et
H
Me
Me
N
EtO2C
Me
Me
NO2
60
78%
Me
Br
H2N
4x
CO2Et
CO2Et
Br
EtO2C
EtO2C
NO2
N
35
91%
CO2Et
CO2Et
EtO2C
EtO2C
NH2
4y
NO2
H
N
45
84%
aReactions and conditions: The reactions were carried out in a pressure vial in the presence of trans b-nitrostyrene 1
(1 mmol), amines 2 (1 mmol) and dialkyl acetylenedicarboxylate 3 (1 mmol)
bIsolated yields. Figure 1 illustrated the plausible mechanism of this one-pot reaction is expected on the basis of the
reported literature.[16] Initially, enaminone A was formed by reaction of amines 2 with dialkyl acetylenedicarboxylate
3. Then b-nitrostyrene 1 undergo the nucleophilic addition with enaminone to form intermediate B. The subsequent
cyclization of intermediate B to form C adduct. Finally, the title compounds 4 were formed by removal of nitro group
by oxidative aromatization.
and-withdrawing amines. When we used -o, -m, and -p substituted amines for the reac-
tion, an excellent yield of 4 was obtained in each case (Table 2, entries 4a, 4b, 4c, 4d,
4h, 4i, 4k, 4o, and 4w). Furthermore, these methods extended to heterocyclic amines,
but unfortunately, the reaction was not worked (Table 2, entry 4u).

10
A. B. ATAR ET AL.
Table 3. Recycling and reuse of imidazolium Brønsted acidic
ionic liquid.
Entry
Reaction cycle
Yielda (%)
1
2
3
4
First (fresh run)
Second cycle
Third cycle
98
96
95
95
Fourth cycle
aIsolated yield.
The reusability of the imidazolium Brønsted acidic ionic liquid is also important
from the perspective of commercial applications. Therefore, the recovery and reusability
of the catalyst were explored for the synthesis of 4a by using model reaction (Table 3).
After completion of the reaction as monitored using TLC, the crude product was
extracted with ether. The recycled catalyst was washed with ether and dried under vac-
uum. The recovered catalyst could be recycled efficiently without any remarkable loss in
catalytic activity. Recycling and reuse of the catalyst show had high activity and gave
the desired product in excellent yield (Table 3).
The synthesized title compounds were further characterized by their spectral proper-
ties (1H, 13C NMR, and HRMS).
Experimental
Material and methods
Chemicals were purchased from Aldrich and Alfa Aesar chemical companies and used
as it is. The NMR spectra were recorded in CDCl3 on a Jeol JNMECP 400 NMR instru-
ment using TMS as an internal standard. The HRMS was recorded on a Jeol JMS-700
mass spectrometer.
General procedure for the synthesis of fully substituted pyrrole functionalities
To a mixture of amines (1 mmol), dialkyl acetylenedicarboxylates (1 mmol) and b-nitro-
styrene (1 mmol) were added 10 mol % BAIL, and the reaction mixture was kept room
temperature with constant stirring for stipulated time (see Table 2). After completion of
the reaction as monitored using TLC, the crude product was extracted with ether. The
combined organic layers were washed with brine solution and dried over Na2SO4. The
Na2SO4 was filtered off and the solvent was removed under vacuum. The crude product
obtained was then purified using silica gel column chromatography (100–200 mesh)
using ethyl acetate: hexane as eluent to get corresponding products. The spectral and
analytical data of the model representative compound is given here:
4a. Diethyl 1-(4-methoxyphenyl)-4-phenyl-1H-pyrrole-2, 3-dicarboxylate
1
Yellow sticky liquid; H NMR (400 MHz, CDCl3) d (ppm) 7.46–7.43 (m, 2H), 7.35 (t,
J ¼ 12Hz, 2H), 7.29–7.24 (m, 3H), 6.96–6.92 (m, 3H), 4.34–4.28 (m, 2H), 4.19–4.14 (m,
2H), 3.83 (s, 3H), 1.28 (t, J ¼ 12Hz, 3H), 1.17 (t, J ¼ 12Hz, 3H); 13C NMR (CDCl3,
100 MHz) d (ppm) 166.31, 159.92, 159.49, 133.33, 132.49, 128.47, 127.74, 127.42, 127.00,

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11
Me
N
N
O
S
O
O
H
OTf
R2O2C
Ar
CO2R2
CO2R2
BAIL
BAIL
H
NH2
R1
NH
R1
R2O2C
NH
R1
NO2
CO2R2
3
CO2R2
3
A
1
2
2
H
R2O2C
R2O2C
Ar
R2O2C
R2O2C
Ar
R2O2C
R2O2C
Ar
Oxidation
Aromatization
BAIL
OH
OH
N
N
N
H
NH
N
O
OH
R1
R1
R1
4
C
B
Figure 1. Plausible mechanism for the formation of substituted pyrrole derivatives 4.
126.04, 124.46, 123.41, 121.77, 61.26, 60.74, 55.50, 14.05, 13.95; HRMS m/z calcd for
C23H23NO5 [Mþ] 393.4324, found 393.4321.
Conclusion
In summary, we have disclosed greener method of a three component one pot coupling
reaction of amines, dialkyl acetylenedicarboxylate, and b-nitrostyrene for the synthesis
of tetrasubstituted pyrroles derivatives using imidazolium Brønsted acidic ionic liquid as
a catalyst as well as solvent. The present protocol demonstrated a clean reaction, a tran-
sition metal-free and solvent-free process, easily accessible reactants, and environmen-
tally friendly reaction conditions. Further application of this green method for the
synthesis of various biologically important and optoelectronic relevant heterocyclic com-
pounds are ongoing in our laboratory.
Funding
This research was supported by Basic Science Research Program through the National Research
Foundation of Republic of Korea (NRF) funded by the Ministry of Education, Science and
Technology (2016R1D1A1B01007177).
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