Recyclable, graphite-catalyzed, four-component synthesis of functionalized pyrroles

Article abstract of DOI:10.1080/00397911.2013.848368

A facile, convenient, and multicomponent coupling strategy for the synthesis of highly functionalized pyrroles is developed using graphite as a catalyst, but devoid of moisture-sensitive metal catalysts and corrosive acidic reagents. This method involves four-component coupling reactions of ethyl acetoacetate compound or diethyl acetylene dicarboxylate, amines, aromatic aldehydes, and nitromethane without an inert atmosphere. This approach provides easy access to highly substituted pyrroles in good yields via one-pot tandem reaction. The method is very simple, straightforward, and environmentally friendly compared to the existing methods.

Full text of DOI:10.1080/00397911.2013.848368

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Recyclable, Graphite-Catalyzed, Four-
Component Synthesis of Functionalized
Pyrroles
K. N. Nandeesh a , G. M. Raghavendra a , C. N. Revanna a , T. A.
Jenifer Vijay a , K. S. Rangappa a & K. Mantelingu a
a Department of Studies in Chemistry , University of Mysore ,
Mysore , India
Accepted author version posted online: 08 Nov 2013.Published
online: 19 Mar 2014.
To cite this article: K. N. Nandeesh , G. M. Raghavendra , C. N. Revanna , T. A. Jenifer Vijay , K.
S. Rangappa & K. Mantelingu (2014) Recyclable, Graphite-Catalyzed, Four-Component Synthesis
of Functionalized Pyrroles, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 44:8, 1103-1110, DOI: 10.1080/00397911.2013.848368
To link to this article: http://dx.doi.org/10.1080/00397911.2013.848368
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Synthetic Communications®, 44: 1103–1110, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2013.848368
RECYCLABLE, GRAPHITE-CATALYZED, FOUR-
COMPONENT SYNTHESIS OF FUNCTIONALIZED
PYRROLES
K. N. Nandeesh, G. M. Raghavendra, C. N. Revanna,
T. A. Jenifer Vijay, K. S. Rangappa, and K. Mantelingu
Department of Studies in Chemistry, University of Mysore, Mysore, India
GRAPHICAL ABSTRACT
Abstract A facile, convenient, and multicomponent coupling strategy for the synthe-
sis of highly functionalized pyrroles is developed using graphite as a catalyst, but devoid
of moisture-sensitive metal catalysts and corrosive acidic reagents. This method involves
four-component coupling reactions of ethyl acetoacetate compound or diethyl acetylene
dicarboxylate, amines, aromatic aldehydes, and nitromethane without an inert atmosphere.
This approach provides easy access to highly substituted pyrroles in good yields via one-pot
tandem reaction. The method is very simple, straightforward, and environmentally friendly
compared to the existing methods.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications® for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Carbon catalyst; graphite; multicomponent reaction; Paal–Knorr synthesis;
pyrroles
INTRODUCTION
Pyrrole is one of the most important heterocyclic compounds, with increasing
importance in medicinal chemistry and organic synthesis.[1] The pyrrole ring is an
important class of structural unit frequently found in many natural products that
have biologically and pharmaceutically active compounds.[2] It has been widely
used as antitumor,[3a] anti-inflammatory,[3b,c] antibacterial,[3c,d] antioxidant,[3e] and
Received June 9, 2013.
Address correspondence to, K. S. Rangappa or K. Mantelingu, Department of Studies in Chemistry,
University of Mysore, Manasagangothri, Mysore # 570006, India. E-mail: rangappaks@gmail.com;
kmantelingu@yahoo.com
1103

1104
K. N. NANDEESH ET AL.
antifungal,[3f] agent. Consequently, an enormous number of synthetic methodologies
have been developed for the construction of pyrroles. The classical methods for pyr-
roles synthesis are as follows:(i) the Hantzsch reaction,[4] which provides pyrroles
from the reaction of α-chloromethyl ketones with β-ketoesters and ammonia; (ii)
the Knorr reaction,[5] which assembles pyrroles by the reaction between α-amino
ketones derived from α-haloketones, ammonia, and β-ketoesters; and (iii) the
Paal–Knorr reaction,[6] one of the most common approaches in which γ-diketone
is converted into pyrroles by the reaction with primary amines in the presence of
various promoting agents. Although these methods are very useful for the synthe-
sis of pyrrole derivatives, the classical reactions involved have major drawbacks
such as availability of the starting materials, multistep synthetic operations, func-
tional group compatibitly, regiospecficity, and harsh reaction conditions, which
limit their easy operation.To overcome these limitations, various new efficient strate-
gies such as multicomponent coupling reactions have recently been introduced.[7–13]
Reddy and coworkers reported the palladium-catalyzed synthesis of functionalized
pyrroles as potential phosphodiesterase (PDE) inhibitors.[14] According to Ghabraie
et al., pyrroles can be obtained from Michael reaction of β-enamino ketones or esters
and nitromethane followed by cyclization.[15] Very recently, Guan et al. reported the
synthesis of pyrrole from enamino esters and nitroolefins (Scheme 1).[16] Moreover,
in this method it is a prerequisite to prepare nitroalkenes from aldehydes, nitrometh-
ane, and β-enamino carbonyl derivatives. Maiti et al. reported a four-component cou-
pling reaction to synthesize functionalized pyrroles catalyzed by FeCl3 employing ethyl
acetoacetate, aromatic aldehyde, amines, and nitromethane.[17] Very recently Khan et al.
have reported a four-component coupling reaction for the syntheses of functional-
ized pyrroles by NiCl2 ·6H2O.[18] Over the past few years graphite has been considered
to be an effective, alternative, and promising Lewis acid catalyst,[19] and has received
much attention because of its low price sustainability, ready availability, lack of toxic-
ity, easy handling, and environmentally friendly properties. However, to the best our
knowledge there are no methods published for the synthesis of graphite-catalyzed
pyrroles. In continuation of our work on the development of synthetic methodolo-
gies,[20–27] we perceived that graphite might be useful as a Lewis acid catalyst for the
synthesis of highly substituted pyrroles. Herein we propose asimple and useful synthetic
Scheme 1. Synthesis of pyrrole from nitroolefins and enamines.
Scheme 2. Graphite-mediated synthesis of functionalized pyrroles.

FOUR-COMPONENT SYNTHESIS OF FUNCTIONALIZED PYRROLES
1105
protocol for the synthesis of tetra-substituted pyrroles employing graphite as a cata-
lyst through one-pot, four-component condensation reaction of aromatic aldehyde,
aromatic amine, ethyl acetoacetate, and nitromethane as shown in Scheme 2.
RESULTS AND DISCUSSION
To explore the scope and optimal conditions (Table 1), graphite-catalyzed
synthesis of pyrrole 1c was selected as a model. The reaction of aniline 1a, aldehyde
1b, ethyl acetoacetate andnitromethane at 90–100°C in the presence of different sol-
vents and different concentrations of catalyst are summarized in Table 1. Increasing
the amount of graphite from 5mol% to 20mol% had a positive effect on the catalytic
activity (Table 1, entries 7–10). However, the enhancement of graphite beyond 20mol%
did not give a significant increase in the yield of pyrrole. Similarly, increase the amount
of catalyst gave a decrease in the reaction time, but with no significant increase in the
yield of pyrrole (Table 1, entries 11 and 12). Next, we examined the effect of the sol-
vents. The reaction did not proceed to completion in dichloromethane, ethanol, or
water, but in tetrahydrofuran (THF), dimethylformamide (DMF), and toluene it gave
poor yield. Interestingly, when nitro methane was used as reactant as we observed
better yield. Here nitromethane’s dual nature acts as reactant as well as solvent.It was
observed that diverse functional groups played significant roles in achieving the prod-
uct yields. Aldehydes with electron-withdrawing groups reacted well with aromatic
amines and gave the better yield (Table 2, entries 13–16). In this protocol, even ali-
phatic cyclic amines generated products in moderate amounts. Graphite accelerated
this reaction and gave good yield of product (Table 2, entries1–13). When studied in
diethyl acetylene dicarboxylate, the reaction also gave good yield of product (Table
3,entries14–16). The experimental procedure involved in this reaction was very simple
and straightforward. A mixture of ethyl acetoacetate aromatic amine (1.5mmol), aro-
matic aldehyde (1mmol), nitromethane, and graphite was heated to reflux for 4–6h
Table 1. Graphite-mediated synthesis of functionalized pyrroles with various solventsa
Entry
Solvent
Catalyst (mol %)
Reaction time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
CH2Cl2
Ethanol
H2O
THF
Toluene
DMF
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
5
5
5
5
5
24
24
24
20
16
16
12
10
10
6
No product
No product
No product
20
30
30
20
30
50
88
80
80
5
5
10
15
20
30
40
10
11
12
4
4
aAll reaction were carried out using aldehyde (1mmol), amine (1.5mmol), ethyl acetoacetate (1mmol),
and (Table 1, entries 1–6) nitromethane (3mmol) and (Table 1, entries 7–12) nitromethane (1.5ml).
bYield obtained after column chromatography.

1106
K. N. NANDEESH ET AL.
Table 2. Graphite-mediated synthesis of functionalized pyrroles with various substrates
Entry
R1
R2
Producta,b
Reaction time (h)
Yield %
5
4
4
5
4
5
4
4
4
6
6
6
6
6
6
6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Ph
Benzo[d][1,3]dioxole
4-Acetyl Ph
4-Acetyl Ph
Benzo[d][1,3]dioxole
Ph
Benzo[d][1,3]dioxole
4-Acetyl Ph
Ph
1c
2c
3c
4c
5c
6c
7c
8c
9c
10c
11c
12c
13c
14c
15c
16c
65
73
68
70
67
70
70
67
66
60
65
67
88
80
83
82
4-OMeBn
4-Me Bn
4-Et Ph
4-t-Bu Bn
4-F Ph
4-Cl Bn
Ph-O-CHF2
4-CF3 Bn
C4H7
C6H11CH2
C7H13CH2
4-Cl Ph
Ph
3-OMe Ph
3-OMe Ph
3-OMe Ph
4-Cl Ph
4-Cl Ph
4-Cl Ph
Bn
3-OMe Bn
2-F Bn
4-Cl Ph
aReaction time 4–6h.
bPurified by column chromatography.
without an inert atmosphere (Scheme 3). When the reaction was completed the prod-
uct was directly purified by column chromatography. The scope of this reaction was
examined with various aromatic amines and the results are summarized in Table 2.
All the compounds were characterized by physical and spectral data. The recy-
clability of the catalyst was also demonstrated. The reaction mixture was allowed to
cool, and the catalyst was recovered by filtration, washed with water, and dried under
reduced pressure. The catalyst was reused (Table 3, entries 13 to 16). The reaction
proceeded smoothly even after three cycles without any extension of reaction time or
any substantial loss of yield (Table 3).
A possible mechanism for the graphite-catalyzed reaction of amine, aldehyde, dieth-
ylacetalenecarboxylate, and nitromethane is proposed in Scheme 4. The reaction of amine
1 with diethylacetalenecarboxylate 3 generates β-enaminocarboxylate A and simultane-
ously nitrostyrene B is obtained by the reaction of aldehydes 2 with nitro methane 4 in one
pot in the presence of graphite as Lewis acid catalyst. If β-enaminocarboxylate A and
nitrostyrene B undergo Michel reaction to form C followed by intramolecular cyclization
to generate intermediate D, which finally undergoes aromatization to form the product E,
this could represent a possible tandem synthesis of functionalized pyrroles (Scheme 4).
In conclusion, we have developed a tandem, economical, and one-pot multicom-
ponent synthesis of functionalized pyrroles. This method has several advantages: It is
Scheme 3. Graphite-mediated synthesis of functionalized pyrroles.

FOUR-COMPONENT SYNTHESIS OF FUNCTIONALIZED PYRROLES
1107
Table 3. Recyclability of graphite
Entry
Product
Product yield (%)
Catalyst recovered (%)
13
14
15
16
13c
14c
15c
16c
88
80
83
82
95
92
90
86
inexpensive and environmentally friendly, a wide variety of functional group can
survive, and all the components are readily available. A wide variety of substituted
pyrroles were synthesized in good yield. This is the first report on functionalized pyr-
role synthesis from ethyl acetoacetate utilizing graphite as catalyst. More important is
that this method is suitable to library generation, diversity-oriented synthesis, and
drug discovery, which make the methodology more attractive for organic synthesis.
EXPERIMENTAL
All work relating to analytical thin-layer chromatography (TLC) was performed
with E. Merck silica-gel 60F254 aluminum plates and was visualized with ultraviolet
(UV) light. The following mobile phases were employed for TLC: chloroform,
Scheme 4. Strategy for the Lewis acid–mediated one-pot, four-component coupling.

1108
K. N. NANDEESH ET AL.
methanol and hexane, and ethyl acetate in different ratios. The instrumental tech-
niques employed for the characterization of the newly synthesized compounds include
1H and 13 C NMR and mass spectroscopy. 1H and 13 C NMR spectra were recorded on
a (400- and 300-MHz) Fourier transform spectrophotometer in CDCl3 or DMSO-d6
solution using tetramethylsilane (TMS) as internal standard. Chemical shifts were
recorded in parts per million (ppm) relative to TMS. Mass and purity were recorded
on a LC–MSD-Trap-XCT.
Typical Procedure
Graphite (20mol%) was added to a solution of benzo[d][1,3]dioxole-5-carbal-
dehyde (100mg, 0.666mmol), aniline (93.03mg, 1.10mmol), and ethyl acetoacetate
(86mg, 0.666mmol) in nitromethane (1.5ml). The mixture was refluxed at 90–100°C
for 5h (TLC monitoring). The reaction mixture was cooled to room temperature and
the graphite was filtered. The filtrate was concentrated under vacuum and the residue
was directly purified by column chromatography with ethyl acetate in hexanes as elu-
ent to afford the desired product. The products were further identified by 1H NMR,
13CNMR, and liquid chromatography–mass spectrometry (LCMS), which were all in
good agreement with the assigned structures.
Ethyl-4-(benzo[d][1, 3]dioxol-5-yl)-2-methyl-1-phenyl-1H-pyrrole-
3-carboxylate (1c)
1
Brown semiliquid: 0.152g (65%); IR (KBr): 1739cm−1; H NMR (500MHz,
CDCl3): δ=7.15–7.29 (m, 5H, ArH), 6.91 (s, 1H, ArH), 6.78–6.87 (d, 2H, ArH,
J=8.0Hz), 6.62 (s, 1H, ArH), 5.96 (s, 2H, CH2), 4.18 (q, 2H, OCH3, J=7.5Hz), 2.40
(s, 3H, CH3), 1.18 (t, 3H, CH3, J=7.5Hz); 13C NMR (100MHz, CDCl3): δ=165.6,
163.0, 161.0, 136.5, 134.9, 129.3, 128.0, 122.3, 120.7, 111.7, 110.1, 107.6, 100.8, 59.5,
14.1, 12.6; MS: m/z=350.20 [M++ H], Anal. calcd. for C21H19NO4:C, 72.19; H, 5.48;
N, 4.01. Found: C, 72.02; H, 5.65; N, 3.90.
Diethyl-1-benzyl-4-(4-chlorophenyl)-1H-pyrrole-2,3-dicarboxylate (14c)
Brown semiliquid: 0.236g (80%); IR (KBr): 1756cm−1; 1H NMR
(400MHz, CDCl3): δ=7.37 (d, 2H, ArH, J=7.6Hz), 7.25–7.33 (m, 5H, ArH),
7.17 (d, 2H, ArH, J=7.2Hz), 6.91 (s, 1H, ArH), 5.51 (s, 2H, CH2), 4.23 (q,
2H, OCH2, J=8.0Hz), 1.25 (t, 3H, CH3, J=6.8Hz); 13C NMR (100 MHz,
CDCl3): δ=166.4, 160.2, 136.8, 132.8, 131.9, 128.6, 127.5, 126.8, 125.2,
122.2, 121.6, 60.7, 52.4, 14.0; MS: m/z=412.10 [M++ H], Anal. calcd. for
C23H22ClNO4: C, 67.07; H, 5.38; N, 3.40. Found: C, 69.97; H, 5.62; N, 3.32.
FUNDING
K. M. acknowledges University Grants Commission No. 39-710-2011 (SR,
dated December 1, 2010).

FOUR-COMPONENT SYNTHESIS OF FUNCTIONALIZED PYRROLES
1109
SUPPORTING INFORMATION
Complete experimental procedure and relevant LCMS, 1H NMR, and 13 C NMR
spectra are available online for all compounds.
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