An efficient procedure for the synthesis of polysubstituted pyrroles in an ionic liquid

Article abstract of DOI:10.1515/znb-2011-0212

The ionic liquid 1-butyl-3-methyl-imidazolium hydrogen sulfate, [bmim]HSO₄, was used as a catalyst and reaction medium for the pyrrole synthesis, and a wide range of aliphatic, aromatic, heteroaromatic and carboxylic 1,4-diketones easily underwent condensations with aniline and ethylenediamine to form polysubstituted pyrroles. Sequential decarboxylation/Paal-Knorr pyrrole condensation was observed, which provides a new and facile approach to monoester pyrroles from 1,4-diketo-2,3-dicarboxylic acid esters.

Full text of DOI:10.1515/znb-2011-0212

An Efficient Procedure for the Synthesis of Polysubstituted Pyrroles
in an Ionic Liquid
Yan-Hong He, Gang-Qiang Wang, Ke-Ling Xu, and Zhi Guan
School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715,
PR China
Reprint requests to Prof. Zhi Guan. Fax: +86 23 68254091. E-mail: guanzhi@swu.edu.cn
Z. Naturforsch. 2011, 66b, 191 – 196; received October 9, 2010
The ionic liquid 1-butyl-3-methyl-imidazolium hydrogen sulfate, [bmim]HSO4, was used as a cata-
lyst and reaction medium for the pyrrole synthesis, and a wide range of aliphatic, aromatic, heteroaro-
matic and carboxylic 1,4-diketones easily underwent condensations with aniline and ethylenediamine
to form polysubstituted pyrroles. Sequential decarboxylation/Paal-Knorr pyrrole condensation was
observed, which provides a new and facile approach to monoester pyrroles from 1,4-diketo-2,3-di-
carboxylic acid esters.
Key words: Ionic Liquid, Paal-Knorr Reaction, Polysubstituted Pyrrole, 1,4-Diketone,
Decarboxylation
Introduction
heterogeneous catalyst [16]. However, further develop-
ment of new methods for the preparation of pyrroles
bearing various substituents is still needed in organic
synthesis [17,18].
Pyrrole is one of the most important heterocyclic
compounds in pharmaceutical [1, 2] and material sci-
ences [3, 4]. As a consequence, many synthetic meth-
Ionic liquids have recently emerged as environ-
ods are known for the construction of the pyrrole mentally benign alternatives for organic solvents [19].
ring [5]. The most frequently used methods include However, their use in Paal-Knorr pyrrole synthesis re-
conjugate addition reactions [6], transition metal- mains largely unexplored. To the best of our knowl-
mediated reactions [7], reductive couplings [8], aza- edge, only two articles in this field have been pub-
Wittig reactions [9], multicomponent coupling reac- lished. Yadav and coworkers used Bi(OTf)3 immobi-
tions [10], and other multistep operations [11,12]. De- lized in [bmim]BF4 as a catalytic system for pyrrole
spite these extensive developments, the Paal-Knorr re- synthesis [20], and Wang and coworkers employed
action is considered to be the most attractive method [bmim]I in the reactions of 2,5-hexanedione with dif-
for the synthesis of pyrroles [13–15], which involves ferent amines [21]. Hence, the development of new
the reaction of 1,4-dicarbonyl compounds and their and convenient strategies in this field especially for the
masked equivalents with primary amines. Although synthesis of polysubstituted pyrroles is desirable. We
the Paal–Knorr reaction has been studied with vari- recently found that the ionic liquid 1-butyl-3-methyl-
ous amines, the most often used diketone was 2,5-hex- imidazolium hydrogen sulfate, [bmim]HSO4, can be
anedione, while differently substituted 1,4-diketones used as an efficient catalyst and reaction medium for
have been used less frequently. Thus, it is still chal- furan synthesis [22]. Based on the previous study, we
lenging to prepare polysubstituted pyrroles with vari- further explored the use of this ionic liquid in pyrrole
ous substituents directly from readily available build- synthesis. Herein, we wish to report a high-yield syn-
ing blocks. Furthermore, most current methods are thesis of polysubstituted pyrroles using the ionic liquid
not applicable to sterically hindered aromatic and het- [bmim]HSO4 as an efficient catalyst and green reaction
eroaromatic diketones. In addition, the effect of sub- medium for the condensation of different aliphatic,
stituents on the starting diketone has not received much aromatic, heteroaromatic, and carboxylic diketones
attention. We recently reported the synthesis of N-sub- with aniline and ethylenediamine. Notably, a new and
stituted pyrroles from different 1,4-diketones and ani- facile approach to monoester pyrroles from easily ac-
line by using a cation exchange resin as a recyclable cessible 1,4-diketo-2,3-dicarboxylic acid esters was
c
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192
Y.-H. He et al. · Polysubstituted Pyrroles in an Ionic Liquid
Table 1. Comparison of the effect of different reaction conditions for the condensation of 1a and aniline.
Entry
Temp. (◦C)
50
Equiv. of aniline
Time (h)
Yield 2a (%)a
Yield 3a (%)a
1
2
3
4
5
1
2
1
2
1
2
36
36
12
12
6
45
38
43
76
78
96
47
56
54
17
10
0
50
100
100
150
6
150
3
a
Refers to yield of isolated products after flash chromatography.
developed via sequential decarboxylation/Paal–Knorr
pyrrole condensation.
Interestingly, the 2-carboxylate-substituted 1,4-
diones 1i – 1k provided the corresponding carboxylate-
substituted pyrroles 2i, 2j, and 2l as major products
(75 – 79 % yield), but decarboxylated pyrroles 2g, 2k,
and 2f were also obtained (8 – 12% yields). However,
the 2,3-dicarboxylate-substituted 1,4-diones 1l and 1m
did not give the corresponding pyrrole dicarboxylates,
and instead gave the pyrrole monoesters 2m and 2l as
the major products in yields of 75 % and 86%, respec-
tively. The carboxylate-free products 2e and 2f were
also obtained in yields of 8 % and 5 %, respectively.
This provides a new and facile approach to pyrrole mo-
noesters from easily accessible 1,4-diketo-2,3-dicarb-
oxylates. We reasoned that the decarboxylated pyrroles
were formed probably through initial decarboxylation
and subsequent Paal–Knorr condensation with aniline.
Otherwise, if the condensation step to form a pyrrole
ring takes place before decarboxylation, the resulting
conjugated system of a pyrrole carboxylate would be
robust enough to prevent it from decarboxylation un-
der the applied reaction conditions. This assumption
was supported by the fact that no such transformation
was observed for the carboxylate-substituted pyrrole 2j
under the given reaction conditions. In addition, it was
observed that 1,4-diketo-2,3-dicarboxylates are more
likely to eliminate one carboxylate group compared to
monocarboxylate-substituted 1,4-diones.
Results and Discussion
In order to study the effect of temperature on
the reaction, the condensation of the aromatic 1,4-
dione 1a with aniline was carried out in◦the ionic liq-
uid [bmim]HSO4 at 50, 100, and 150 C (Table 1).
The results have revealed that the formation of pyrrole
was temperature-dependent. At lower temperature the
pyrrole was only obtained in low to moderate yields,
and furan was formed in a competitive reaction (Ta-
◦
ble 1, entries 1 – 4). However, at 150 C the pyrrole
was obtained as the major product after a shorter reac-
tion time (Table 1, entry 5). Besides, use of 2 equiva-
◦
lents of aniline at 150 C provided N-substituted pyr-
role 2a exclusively in excellent yield of 96 % (Table 1,
entry 6).
To test the generality of the reaction, we fur-
ther investigated the condensations of different dike-
tone compounds and aniline in the ionic liquid
[bmim]HSO4 at 150 ◦C. A wide range of di-, tri- and
tetra-substituted aliphatic, aromatic, heteroaromatic,
and carboxylic 1,4-diketones underwent smoothly the
condensation with aniline to provide N-substituted
pyrroles in good yields (Table 2). The method was ef-
ficient and easy to carry out. The reactions were com-
To improve the scope of this ionic liquid-catalyzed
plete within 3 h. When 1,4-diphenyl-1,4-diones (1a, method of pyrrole formation, the aliphatic amine
1c, 1d, and 1f, Table 2) were used, excellent yields ethylenediamine was also used (Scheme 1). The re-
◦
were obtained. Triphenyl-1,4-dione 1h and di(2-thio- action was carried out in the ionic liquid at 100◦ C
phenyl)-1,4-dione 1b gave excellent yields of 97 % because of the relatively low boiling point (117 C)
and 95 %, respectively (Table 2). Methyl-substituted of ethylenediamine, and two equivalents of amine
1,4-diones 1e and 1g provided the products in slightly were used. A clean formation of the pyrrole 2o was
lower yields of 88 % and 85 %, respectively. It seems achieved in good yield of 89 % with 1,2,4-triphenyl-
that aryl-substituted 1,4-dicarbonyl compounds per- 1,4-dione (1h). The product 2n was obtained in lower
form better than alkyl-substituted ones in this ionic yield of 74% when 1,4-diphenyl-1,4-dione (1f) was
liquid-catalyzed condensation.
used as the ketone component (Scheme 1). It is some-
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Y.-H. He et al. · Polysubstituted Pyrroles in an Ionic Liquid
193
Table 2. Pyrroles from the condensation of 1,4-diketones and aniline in the ionic liquid [bmim]HSO4.
Entry
1,4-Diketone
Pyrrole
Time (h)
Yield (%)a
1
3
96
95
97
2
3
3
3
4
5
6
3
2
3
94
88
96
7
8
2
3
85
97
2i: 76
2g: 8
9
3
3
2j: 79
2k: 11
10
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194
Y.-H. He et al. · Polysubstituted Pyrroles in an Ionic Liquid
Table 2 (continued).
Entry
1,4-Diketone
Pyrrole
Time (h)
3
Yield (%)a
2l: 75
2f: 12
11
2m: 75
2e: 8
12
3
3
2l: 86
2f: 5
13
a
Yield refers to isolated product after flash
ter was required to dissolve this curd before workup.
Thus, we did not recover the ionic liquid.
Conclusion
We have developed an efficient method for the
preparation of polysubstituted pyrroles with various
substituents. The present method avoids the use of
mineral acids and organic solvents. Instead, the ionic
liquid [bmim]HSO4 was used as a catalyst and reac-
tion medium. The reactions were high-yielding, and
the procedure is applicable to a wide range of steri-
cally hindered aliphatic, aromatic, heteroaromatic and
carboxylic diketones. Notably, the sequential decarb-
oxylation/Paal–Knorr condensation was also investi-
gated, which provides a new and facile approach to
pyrrole monocarboxylic esters from easily accessible
1,4-diketo-2,3-dicarboxylic esters. The present proce-
dure might be a potentially useful synthetic method for
organic chemistry.
Scheme 1. Condensation of diketones with ethylenediamine.
what surprising that the more hindered diketone 1h
gave a higher yield than 1f. According to Rao and
coworker’s conformational analysis of 1h [23], the two
1,4-benzoyl groups adopt a gauche rather than the anti
conformation. The dihedral angle between the CO–C2
bond and the CO–C3 bond in the lowest energy con-
former was 65◦ (the numbering of carbons is shown in
Scheme 1). We think that the demand for this dihedral
angle could facilitate the formation of the pyrrole ring.
Experimental Section
Synthesis of 2a: Typical experimental procedure for synthesis
of N-substituted pyrroles
During the course of the reaction, we observed that
the ionic liquid turned sticky when the amine was
added. This is probably due to the formation of an
amine salt between the ionic liquid [bmim]HSO4 and
A solution of 1,4-bis(4-bromophenyl)butane-1,4-dione
(1a, 75 mg, 0.2 mmol) and aniline (0.06 mL, 0.54 mmol)
in 2 g (8.4 mmol) of the ionic liquid [bmim]HSO4 was stirred
◦
◦
the amine. When the temperature was above 100 C,
at 150 C for 3 h. After cooling to r. t., 10 mL of H2O was
the reaction mixture turned into a less viscous solu-
tion, which could be stirred smoothly. However, after
the reaction was complete and the reaction mixture was
added to the reaction mixture, and then thoroughly extracted
with diethyl ether. The combined extracts were washed with
water and brine, dried (Na2SO4), and filtered. The solvents
cooled to r. t., it changed into curd. The addition of wa- were removed and the residue purified by flash chromatogra-
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Y.-H. He et al. · Polysubstituted Pyrroles in an Ionic Liquid
195
phy (petroleum ether / CH2Cl2 2 : 1) to give 2,5-bis(4-bromo-
Ethyl 2-methyl-5-(3-nitrophenyl)-1-phenyl-1H-pyrrole-3-
phenyl)-1-phenyl-1H-pyrrole 2a as a colorless solid (82 mg, carboxylate (2j). Oil. – 1H NMR (300 MHz, CDCl3): δ =
96 % yield). M. p.: 300 – 303 ◦C. – 1H NMR (300 MHz, 7.96 – 7.89 (m, 2H), 7.44 – 7.17 (m, 7H), 6.95 (s, 1H), 4.37 –
CDCl3): δ = 7.30 – 7.26 (m, 7H), 7.02 – 6.99 (m, 2H), 6.91 – 4.33 (m, 2H), 2.42 (s, 3H), 1.42 – 1.37 (m, 3H), identical to
6.88 (m, 4H), 6.47 (s, 2H). – 13C NMR: δ = 134.88, 131.87, that previously reported [30].
131.04, 130.00, 129.05, 128.70, 127.66, 110.27. – HRMS
2-Methyl-5-(3-nitrophenyl)-1-phenyl-1H-pyrrole
(2k).
(ESI): m/z = 450.9579 (calcd. 450.9571 for C22H15Br2N, M. p.: 112 – 114 ◦C, reported [30]: 114 – 115 ◦C. – 1H
[M]+).
1-Phenyl-2,5-di(thiophen-2-yl)-1H-pyrrole (2b). M. p.:
NMR (300 MHz, CDCl3): δ = 7.90 – 7.19 (m, 9H), 6.50 (s,
1H), 6.14 (s, 1H), 2.15 (s, 3H), identical to that previously
reported [30].
Ethyl 1,2,5-triphenyl-1H-pyrrole-3-carboxylate (2l).
M. p.: 132 – 134 ◦C, reported [31]: 130 – 131 ◦C. – 1H NMR
(300 MHz, CDCl3): δ = 7.26 – 7.09 (m, 13H), 6.94 – 6.93
(m, 3H), 4.18 – 4.16 (m, 2H), 1.26 (m, 3H), identical to that
previously reported [31].
◦
◦
1
181 – 182 C, reported [24]: 180 C. – H NMR (300 MHz,
CDCl3): δ = 7.47 – 7.39 (m, 3H), 7.32 – 7.25 (m, 2H), 7.05 –
7.04 (m, 2H), 6.82 – 6.79 (m, 2H), 6.54 (s, 2H), 6.51 – 6.50
(m, 2H), identical to that previously reported [24].
1-Phenyl-2,5-di(p-tolyl)-1H-pyrrole (2c). M. p.: 208 –
◦
◦
1
210 C, reported [25]: 208 – 210 C. – H NMR (300 MHz,
CDCl3): δ = 7.24 – 6.95 (m, 13H), 6.43 (s, 2H), 2.27 (s, 6H),
identical to that previously reported [25].
Ethyl 2,5-dimethyl-1-phenyl-1H-pyrrole-3-carboxylate
(2m). M. p.: 45 – 47 ◦C, reported [32]: 45 – 46 ◦C. – 1H NMR
(300 MHz, CDCl3): δ = 7.47 – 7.18 (m, 5H), 6.37 (s, 1H),
4.29 – 4.27 (m, 2H), 2.29 (s, 3H), 1.97 (s, 3H) 1.35 (m, 3H),
identical to that previously reported [32].
2,5-Di(4-chlorophenyl)-1-phenyl-1H-pyrrole (2d). M. p.:
264 – 266 ◦C, reported [25]: 264 – 265 ◦C. – 1H NMR
(300 MHz, CDCl3): δ = 7.29 – 7.26 (m, 2H), 7.15 – 7.12 (m,
4H), 7.01 – 6.95 (m, 7H), 6.46 (s, 2H), identical to that pre-
viously reported [25].
2,5-Di(4-bromophen◦yl)furan (3a). M. p.: 205 – 207 ◦C, re-
ported [33]: 206 – 208 C. – 1H NMR (300 MHz, CDCl3):
δ = 7.61 – 7.51 (m, 8H), 6.74 (s, 2H), identical to that previ-
ously reported [33].
2,5-Dimethyl-1-phenyl-1H-pyrrole (2e). M. p.: 49 – 51 ◦C,
◦
reported [26]: 50 C. – 1H NMR (300 MHz, CDCl3): δ =
2-(2,5-Diphenyl-1H-pyrrol-1-yl)ethanamine (2n). M. p.:
80 – 82 ◦C, reported [34]: 78 ◦C. – 1H NMR (300 MHz,
CDCl3): δ = 7.46 – 7.28 (m, 10H), 6.28 (s, 2H), 4.15 –
4.11(m, 2H), 2.43 – 2.38 (m, 2H), 0.96 (s, 2H). – 13C NMR:
δ = 136.94, 133.88, 128.89, 128.64, 127.08, 109.89, 48.16,
42.44. – HRMS (ESI): m/z = 263.1542 (calcd. 263.1548 for
C18H19N2, [M+H]+).
7.45 – 7.44 (m, 3H), 7.26 – 7.22 (m, 2H), 5.90 (s, 2H), 2.21
(s, 6H), identical to that previously reported [26].
◦
1,2,5-Triphenyl-1H-pyrrole (2f). M. p.: 232 – 234 C, re-
◦
ported [25]: 233 – 234 C. – 1H NMR (300 MHz, CDCl3):
δ = 7.25 – 7.19 (m, 3H), 7.17 – 7.15 (m, 7H), 7.08 – 7.02 (m,
5H), 6.49 (s, 2H), identical to that previously reported [25].
2-Methyl-1,5-diphenyl-1H-pyrrole (2g). M. p.: 83 – 84 ◦C,
2-(2,3,5-Triphenyl-1H-pyrrol-1-yl)ethanamine
(2o).
◦
reported [27]: 81 – 82 C. – 1H NMR (300 MHz, CDCl3):
M. p.: 85 – 87 ◦C. – 1H NMR (300 MHz, CDCl3): δ =
7.53 – 7.39 (m, 10H), 7.21 – 7.11 (m, 5H), 6.50 (s, 1H),
4.06 – 4.02 (m, 2H), 2.46 – 2.42 (m, 2H), 1.78 (s, 2H). –
13C NMR: δ = 135.99, 135.44, 133.50, 132.10, 131.09,
128.95, 128.76, 128.64, 128.01, 127.72, 127.70, 127.22,
125.17, 123.31, 109.95, 47.20, 42.09. – HRMS (ESI): m/z =
339.1868 (calcd. 339.1861 for C24H23N2, [M+H]+).
δ = 7.31 – 7.06 (m, 10H), 6.36 (s, 1H), 6.09 (s, 1H), 2.13 (s,
3H), identical to that previously reported [27].
1,2,3,5-Tetraphenyl-1H-pyrrole (2h). M. p.: 200 – 203 ◦C,
reported [28]: 200 – 201 ◦C. – 1H NMR (300 MHz, CDCl3):
δ = 7.26 – 7.18 (m, 16H), 7.07 – 7.00 (m, 4H), 6.73 (s, 1H),
identical to that previously reported [28].
Ethyl 2-methyl-1,5-diphenyl-1H-pyrrole-3-carboxylate
(2i). M. p.: 99 – 101 ◦C, reported [29]: 99 – 100 ◦C. – 1H
NMR (300 MHz, CDCl3): δ = 7.36 – 7.04 (m, 10H), 6.80 (s,
1H), 4.35 – 4.31 (m, 2H), 2.40 (s, 3H), 1.39 – 1.36 (m, 3H),
identical to that previously reported [29].
Acknowledgement
Financial support from the Natural Science Foundation
Project of CQ CSTC of 2009BA5051 is gratefully acknowl-
edged.
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