A simple and efficient metal-free synthesis of tetrasubstituted pyrroles by iodine-catalyzed four-component coupling reaction of aldehydes, amines, dialkyl acetylenedicarboxylates, and nitromethane

Article abstract of DOI:10.1055/s-0030-1260228

The four-component coupling reaction of aldehydes, amines, dialkyl acetylenedicarboxylates, and nitromethane in the presence of molecular iodine as a catalyst furnished the corresponding 1,2,3,4-tetrasubstituted pyrroles under reflux. The products are formed in high yields (65-88%) within 8 hours. The method is simple, efficient, cost-effective, and metal-free. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0030-1260228

PAPER
3471
A Simple and Efficient Metal-Free Synthesis of Tetrasubstituted Pyrroles by
Iodine-Catalyzed Four-Component Coupling Reaction of Aldehydes, Amines,
Dialkyl Acetylenedicarboxylates, and Nitromethane1
Synthesis of
Tetri
s
uted Pyrr
w
oles anath Das,* Nisith Bhunia, Maram Lingaiah
Organic Chemistry Division 1, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91(40)27193198; E-mail: biswanathdas@yahoo.com
Received 14 July 2011; revised 16 August 2011
eroaromatic aldehydes with different substituents were
utilized. The aromatic aldehydes contained electron-
Abstract: The four-component coupling reaction of aldehydes,
amines, dialkyl acetylenedicarboxylates, and nitromethane in the
presence of molecular iodine as a catalyst furnished the correspond-
ing 1,2,3,4-tetrasubstituted pyrroles under reflux. The products are
formed in high yields (65–88%) within 8 hours. The method is sim-
ple, efficient, cost-effective, and metal-free.
R1
CO2R
RO2C
I2 (20 mol%)
R1CHO
R2NH2
MeNO2
RO2C
N
R2
CO2R
reflux, 8 h
Key words: 1,2,3,4-tetrasubstituted pyrroles, four-component cou-
pling reaction, iodine, metal-free synthesis
1
2
3
4
65–88%
Scheme 1 Four-component reaction leading to 1,2,3,4-tetrasubsti-
tuted pyrroles
Pyrrole derivatives exhibit various valuable biological
properties such as antitumor,2a antibacterial,2b and im-
mune suppressant activities.2c The pyrrole ring is the
structural unit of different naturally occurring important
compounds such as chlorophyll, bile pigments, heme, and
vitamin B12.3 Substituted pyrroles are used as intermedi-
ates in organic synthesis.4 They are also employed as or-
ganic conducting materials.5 As a result, several methods
have been developed for the synthesis of pyrroles.6 How-
ever, many of these methods suffer from various disad-
vantages such as multi-step operation, drastic reaction
conditions, application of toxic metal derived catalysts,
long reaction times, and low yields. Here we report a sim-
ple metal-free method for the synthesis of 1,2,3,4-tetra-
substituted pyrroles.
In continuation of our work7 on the development of useful
synthetic methodologies, we have discovered that the
four-component coupling of aldehydes 1, amines 2, di-
alkyl acetylenedicarboxylates 3, and nitromethane in the
presence of molecular iodine as a catalyst afforded the
corresponding 1,2,3,4-tetrasubstituted pyrroles 4 under
reflux (Scheme 1). The reaction of benzaldehyde, aniline,
with dimethyl acetylenedicarboxylate was initially carried
out in nitromethane using different catalysts (Table 1). Io-
dine (20 mol%) was found to be most effective to catalyze
the reaction under reflux. The yield of the product was
84% in 8 hours. At room temperature or in the absence of
any catalyst the yields were very low even after 24 hours.
Table 1 Optimization of Reaction Parameters for the Synthesis of
1,2,3,4-Tetrasubstituted Pyrrolesa,b
CO2Me
Ph
MeO2C
NH2
Lewis acid
PhCHO
MeNO2
reflux
MeO2C
N
CO2Me
Ph
Entry
1
Catalyst (mol%)
none
Time (h)
Yield (%)c
24
8
14
50
55
57
43
36
25
63
65
59
81
79
88
74
2
InCl3 (10)
InBr3 (10)
In(OTf)3 (10)
YbCl3 (10)
ZrCl4 (10)
PTSA (10)
FeCl3 (10)
FeCl3 (10)
Fe(OAc)3 (10)
I2 (10)
3
12
12
8
4
5
6
12
12
8
7
8
9
12
12
8
10
11
12
13
14
I2 (10)
12
8
A series of 1,2,3,4-tetrasubstituted pyrroles were subse-
quently prepared (Table 2) from various aldehydes,
amines, and dialkyl acetylenedicarboxylates following
I2 (20)
I2 (20)
12
the above conditions (Scheme 1). Both aromatic and het- a Reaction conditions: benzaldehyde (1 mmol), dimethyl acetylenedi-
carboxylate (1.2 mmol), aniline (1.3 mmol), nitromethane (2 mL),
and catalyst under reflux.
SYNTHESIS 2011, No. 21, pp 3471–3474
Advanced online publication: 13.09.2011
DOI: 10.1055/s-0030-1260228; Art ID: Z69711SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
1
1
b The structures of the products were established from their spectral
(IR, 1H and 13C NMR, and ESI-MS) data.
c Isolated yields after purification.

3472
B. Das et al.
PAPER
Table 2 Synthesis of 1,2,3,4-Tetrasubstituted Pyrroles by Four-
donating as well as electron-withdrawing groups. However,
with an aliphatic aldehyde only a trace amount of product
was detected. On the other hand, in the case of amines,
both aromatic and aliphatic substrates underwent the con-
version smoothly. The pyrroles were efficiently prepared
by using dimethyl and diethyl acetylenedicarboxylates.
The conversion was complete in eight hours and the prod-
ucts were formed in high yields (65–90%). Nitromethane
was used as a reagent and also as a solvent. When nitro-
ethane was used instead of nitromethane the products
were formed only in trace amount. The structure of the
prepared pyrroles were established from their spectral (IR,
1H and 13C NMR and MS) and analytical data.
Component Coupling Reactions Using Iodine as Catalysta,b
CO2R3
R1
R3O2C
I2 (20 mol%)
+
R1CHO
R2NH2
R4CH2NO2
reflux, 8 h
R4
R3O2C
N
R2
CO2R3
4a–q
Entry R1
R2
R3
R4
Prod. Yield
(%)c
1
2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4a
4b
4c
4d
4e
4f
88
76
85
78
75
72
79
81
68
65
77
74
4-ClC6H4
Iodine, used as a catalyst here, is less expensive and easily
available. It acts as a Lewis acid. The plausible mecha-
nism of the present conversion involves the initial forma-
tion of the intermediates, alkenyl amine A and
nitrostyrene B catalyzed by iodine (Scheme 2). Iodine ac-
tivates the triple bond and carbonyl group of the alde-
hydes to accelerate the formation of A and B,
respectively. These two intermediates A and B undergo
Michael reaction and cyclization to afford the substituted
pyrroles. When the intermediates A and B were prepared
separately and treated in the presence of catalytic amount
of iodine, the desired 1,2,3,4-tetrasubstituted pyrroles
were formed under reflux within one hour.
3
4-BrC6H4
4
4-FC6H4
5
4-MeC6H4
4-MeOC6H4
3,4-MeOC6H3
6
7
4g
4h
4i
8
2,4,6-(MeO)3C6H2 Ph
9
4-O2NC6H4
2-O2NC6H4
thiophene
Ph
Ph
10
11
12
13
14
15
16
17
Ph
4j
Ph
4k
4l
In conclusion, we have developed a simple, efficient,
high-yielding, and cost-effective synthesis of 1,2,3,4-tet-
rasubstituted pyrroles by direct four-component coupling
reaction of aldehydes, amines, dialkyl acetylenedicarbox-
ylates, and nitromethane using iodine as a catalyst.
4-BrC6H4
Ph
4-MeOC6H4 Me
4m 83
Ph
Ph
Et
4n
4o
4p
86
3,4-(MeO)2C6H3 PhCH2
Me
Me
81
Silica gel F254 plates were used for TLC; the spots were examined
under UV light and then developed with I2 vapor. Column chroma-
tography was performed with silica gel (BDH 100–200 mesh). Sol-
vents were purified according to standard procedures. The spectra
were recorded with the following instruments; IR: Perkin-Elmer
RX FT-IR spectrophotometer; NMR: Varian Gemini 200 MHz (1H)
and 50 MHz (13C) spectrometer; ESIMS: VG-Autospec micromass.
Me(CH2)4
Ph
Ph
Ph
traced
traced,e
Me Me 4q
a Reaction conditions: aldehyde (1 mmol), dialkyl acetylenedicarbox-
ylate (1.2 mmol), aniline (1.3 mmol), nitromethane (2 mL), and I2 (20
mol%) under reflux for 8 h.
b The structures of the products were established from their spectral
(IR, 1H and 13C NMR, and ESI-MS), and analytical data.
c Isolated yields after purification.
Four-Component Reaction of Aldehydes, Amines, Dialkyl Acet-
ylenedicarboxylates, and Nitromethane; General Procedure
To a stirred solution of amine (1.3 mmol), dialkyl acetylenedicar-
boxylate (1.2 mmol), and aldehyde (1.0 mmol) in nitromethane (2
mL) was added I2 (20 mol%). The reaction mixture was heated
slowly to reflux for 8 h. After completion of the reaction as moni-
d Reaction carried out for 24 h.
e Nitroethane instead of nitromethane was used.
CO2R3
R1
R3O2C
I2
I2
R2NH2
+
R1CHO
+
MeNO2
+
R3O2C
CO2R3
NH
R2
NO2
A
B
R1
R3O2C
R3O2C
R3O2C
1
R1
R3O2C
H
R
R3O2C
N
OH
OH
NH
R2
R3O2C
O
N
N
R2
N
R2
OH
Scheme 2 Proposed reaction mechanism
Synthesis 2011, No. 21, 3471–3474 © Thieme Stuttgart · New York

PAPER
Synthesis of Tetrasubstituted Pyrroles
3473
tored by TLC (hexane–EtOAc, 5:1), the mixture was cooled to r.t.,
and extracted with EtOAc (3 × 10 mL). The combined extracts were
washed with sat. aq Na2S2O3 (2 mL), dried (Na2SO4), and concen-
trated under reduced pressure. The residue was subjected to column
chromatography (hexane–EtOAc, 10:1) to afford the corresponding
tetrasubstituted pyrrole (Table 2).
13C NMR (50 MHz, CDCl3): d = 166.5, 160.8, 159.7, 139.1, 129.2,
129.1, 128.6, 126.2, 125.5, 120.8, 114.0, 55.4, 52.2, 51.8.
ESIMS: m/z = 366 [M + H]+.
Anal. Calcd for C21H19NO5: C, 69.03; H, 5.24; N, 3.83. Found: C,
69.00; H, 5.17; N, 3.88.
The spectral data of unknown products are given below.
Dimethyl 4-(3,4-Dimethoxyphenyl)-1-phenyl-1H-pyrrole-2,3-
dicarboxylate (4g)
Dimethyl 4-(4-Chlorophenyl)-1-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4b)
IR (neat): 1719, 1586, 1491, 1436, 1294 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.48–7.31 (m, 5 H), 7.00 (d,
J = 2.0 Hz, 1 H), 6.95 (dd, J = 8.0, 2.0 Hz, 1 H), 6.92 (s, 1 H), 6.81
(d, J = 8.0 Hz, 1 H), 3.89 (s, 6 H), 3.81 (s, 3 H), 3.69 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 167.2, 160.24, 149.3, 148.5, 139.8,
129.4, 129.0, 126.1, 126.0, 125.8, 124.9, 123.1, 122.1, 120.3, 111.2,
56.0, 52.3, 51.9.
IR (neat): 1730, 1696, 1593, 1450 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.48–7.29 (m, 9 H), 6.91 (s, 1 H),
3.81 (s, 3 H), 3.70 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 166.2, 160.1, 139.0, 132.9, 131.2,
129.4, 129.3, 129.0, 126.0, 125.7, 124.2, 52.3, 51.9.
ESIMS: m/z = 370, 372 [M + H]+.
ESIMS: m/z = 396 [M + H]+.
Anal. Calcd for C20H16ClNO4: C, 64.96; H, 4.36; N, 3.79. Found: C,
64.89; H, 4.39; N, 3.82.
Anal. Calcd for C22H21NO6: C, 66.83; H, 5.35; N, 3.54. Found: C,
66.79; H, 5.29; N, 3.51.
Dimethyl 4-(4-Bromophenyl)-1-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4c)
Dimethyl 1-Phenyl-4-(3,4,5-trimethoxyphenyl)-1H-pyrrole-2,3-
dicarboxylate (4h)
IR (neat): 1728, 1698, 1594, 1448 cm–1.
IR (neat): 1724, 1592, 1496, 1447, 1293 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.50–7.31 (m, 7 H), 7.06–6.98 (m,
2 H), 6.90 (s, 1 H), 3.80 (s, 3 H), 3.69 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 166.0, 160.3, 138.9, 133.0, 131.4,
129.2, 129.1, 128.9, 125.9, 125.5, 124.0, 52.1, 51.6.
1H NMR (200 MHz, CDCl3): d = 7.46–7.31 (m, 5 H), 6.95 (s, 1 H),
6.65 (s, 2 H), 3.88 (s, 6 H), 3.84 (s, 3 H), 3.83 (s, 3 H), 3.71 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 167.0, 160.1, 153.8, 139.8,
137.129.1, 129.0, 128.8, 126.1, 125.8, 125.0, 123.1, 122.0, 104.8,
60.8, 56.1, 52.8, 51.9.
ESIMS: m/z = 416 [M + H]+.
Anal. Calcd for C20H16BrNO4: C, 57.99; H, 3.89; N, 3.38. Found: C,
57.93; H, 3.87; N, 3.35.
ESIMS: m/z = 426 [M + H]+.
Anal. Calcd for C23H23NO7: C, 64.93; H, 5.45; N, 3.29. Found: C,
64.96; H, 5.41; N, 3.24.
Dimethyl 4-(4-Fluorophenyl)-1-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4d)
Dimethyl 4-(4-Nitrophenyl)-1-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4i)
IR (neat): 1731, 1701, 1603,1447 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.51–7.32 (m, 7 H), 7.09–7.00 (m,
2 H), 6.90 (s, 1 H), 3.80 (s, 3 H), 3.69 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 167.2, 162.5 (d, J = 180.0 Hz),
162.2, 128.6, 128.1, 127.2, 125.9, 125.2, 115.9 (d, J = 10.0 Hz),
52.2, 52.0.
IR (neat): 1730, 1616, 1521, 1447, 1251 cm–1.
1H NMR (200 MHz, CDCl3): d = 8.22 (d, J = 8.0 Hz, 2 H), 7.59 (d,
J = 8.0 Hz, 2 H), 7.51–7.42 (m, 1 H), 7.39–7.32 (m, 2 H), 7.04 (s, 1
H), 3.81 (s, 3 H), 3.72 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 166.4, 160.2, 146.9, 140.2, 139.0,
130.8, 129.5, 129.4, 128.8, 126.1, 125.9, 123.5, 122.2, 116.3, 52.8,
52.1.
ESIMS: m/z = 376 [M + H]+.
Anal. Calcd for C20H16FNO4: C, 67.98; H, 4.56; N, 3.96. Found: C,
67.94; H, 4.59; N, 4.01.
ESIMS: m/z = 381 [M + H]+.
Anal. Calcd for C20H16N2O6: C, 63.16; H, 4.24; N, 7.37. Found: C,
63.17; H, 4.28; N, 7.36.
Dimethyl 1-Phenyl-4-p-tolyl-1H-pyrrole-2,3-dicarboxylate (4e)
IR (neat): 1721, 1689, 1596, 1441 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.44–7.21 (m, 7 H), 7.12 (d,
J = 8.0 Hz, 2 H), 6.92 (s, 1 H), 3.82 (s, 3 H), 3.70 (s, 3 H), 2.38 (s,
3 H).
Dimethyl 4-(2-Nitrophenyl)-1-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4j)
IR (neat): 1728, 1612, 1522, 1443, 1254 cm–1.
13C NMR (50 MHz, CDCl3): d = 166.9, 160.5, 139.9, 137.0, 130.0,
129.1, 129.0, 128.9, 128.5, 127.5, 125.1, 125.0, 124.2, 120.4, 52.2,
52.0, 21.2.
1H NMR (200 MHz, CDCl3): d = 7.99 (d, J = 8.0 Hz, 1 H), 7.54 (d,
J = 8.0 Hz, 1 H), 7.50–7.35 (m, 7 H), 6.88 (m, 1 H), 3.74 (s, 3 H),
3.65 (s, 3 H).
ESIMS: m/z = 350 [M + H]+.
13C NMR (50 MHz, CDCl3): d = 166.5, 160.3, 146.5, 139.1, 135.3,
132.7, 129.5, 129.3, 128.7, 125.6, 124.4, 121.8, 115.7, 52.5, 52.1.
Anal. Calcd for C21H19NO4: C, 72.19; H, 5.48; N, 4.01. Found: C,
67.22; H, 5.42; N, 3.98.
ESIMS: m/z = 381 [M + H]+.
Anal. Calcd for C20H16N2O6: C, 63.16; H, 4.24; N, 7.37. Found: C,
63.12; H, 4.21; N, 7.31.
Dimethyl 4-(4-Methoxyphenyl)-1-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4f)
IR (neat): 1730, 1699, 1598, 1452 cm–1.
Dimethyl 1-Phenyl-4-(thiophen-2-yl)-1H-pyrrole-2,3-dicarbox-
ylate (4k)
1H NMR (200 MHz, CDCl3): d = 7.48–7.30 (m, 7 H), 6.89 (d,
J = 8.0 Hz, 2 H), 6.84 (s, 1 H), 3.81 (s, 3 H), 3.70 (s, 3 H).
IR (neat): 1728, 1612, 1522, 1443, 1254 cm–1.
Synthesis 2011, No. 21, 3471–3474 © Thieme Stuttgart · New York

3474
B. Das et al.
PAPER
1H NMR (200 MHz, CDCl3): d = 7.42–7.09 (m, 8 H), 7.01 (s, 1 H),
Acknowledgment
3.89 (s, 3 H), 3.69 (s, 3 H).
The authors thank CSIR, New Delhi for financial assistance.
13C NMR (50 MHz, CDCl3): d = 166.9, 160.0, 139.2, 135.0, 129.2,
129.1, 128.9, 127.8, 126.0, 125.9, 124.8, 124.1, 52.5, 52.1.
ESIMS: m/z = 342 [M + H]+, 364 [M + Na]+.
References
Anal. Calcd for C18H15NO4S: C, 63.33; H, 4.43; N, 4.10. Found: C,
63.39; H, 4.39; N, 4.17.
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Dimethyl 1-(4-Bromophenyl)-4-phenyl-1H-pyrrole-2,3-dicar-
boxylate (4l)
IR (neat): 1723, 1700, 1598, 1436 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.57 (d, J = 8.0 Hz, 2 H), 7.40–
7.22 (m, 7 H), 6.91 (s, 1 H), 3.81 (s, 3 H), 3.72 (s, 3 H).
13C NMR (50 MHz, CDCl3): d = 166.2, 160.2, 138.4, 133.0, 132.1,
128.9, 128.0, 127.9, 127.6, 125.5, 125.0, 122.6, 122.0, 52.1, 51.8.
ESIMS: m/z = 414, 416 [M + H]+.
Anal. Calcd for C20H16BrNO4: C, 57.99; H, 3.89; N, 3.38. Found: C,
57.91; H, 3.83; N, 3.33.
(5) (a) Ramanavicius, A.; Ramanaviciene, A.; Malinauskas, A.
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L.; Xu, J. Org. Lett. 2007, 9, 2139.
Diethyl 1,4-Diphenyl-1H-pyrrole-2,3-dicarboxylate (4n)
IR (neat): 1728, 1612, 1522, 1443 cm–1.
(6) Some recent examples: (a) Shiner, C. M.; Taner, T. D.
Tetrahedron 2005, 61, 11628. (b) Minetto, G.; Raveglia, L.
F.; Sega, A.; Taddei, M. Eur. J. Org. Chem. 2005, 34, 5277.
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2005, 44, 5664. (d) Istrate, F. M.; Gagosz, F. Org. Lett.
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Ryabukhin, S. V.; Volochnyuk, D. M.; Shivanyuk, A. N.;
Tolmachev, A. A. Tetrahedron 2008, 64, 5933.
1H NMR (200 MHz, CDCl3): d = 7.44–7.21 (m, 10 H), 6.92 (s, 1 H),
4.26 (q, J = 7.0 Hz, 4 H), 1.23 (t, J = 7.0 Hz, 3 H), 1.14 (t, J = 7.0
Hz, 3 H).
13C NMR (50 MHz, CDCl3): d = 166.1, 160.0, 139.5, 133.1, 129.1,
128.8, 128.4, 127.4, 126.2, 125.7, 124.8, 61.2, 60.4, 14.1, 14.0.
ESIMS: m/z = 364 [M + H]+.
Anal. Calcd for C22H21NO4: C, 72.71; H, 5.82; N, 3.85. Found: C,
72.75; H, 5.86; N, 3.81.
(h) Alizadeh, A.; Rezvanian, A.; Bijanzadeh, H. R. Synthesis
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Catal. 2009, 351, 2063.
Dimethyl 1-Benzyl-4-(3,4-dimethoxyphenyl)-1H-pyrrole-2,3-
dicarboxylate (4o)
IR (neat): 1717, 1565, 1491, 1449, 1296 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.32–7.21 (m, 3 H), 7.13 (dd,
J = 8.0, 2.0 Hz, 2 H), 6.93 (d, J = 8.0 Hz, 1 H), 6.87 (dd, J = 8.0,
2.0 Hz, 1 H), 6.83 (s, 1 H), 6.78 (d, J = 8.0 Hz, 1 H), 5.50 (s, 2 H),
3.84 (s, 6 H), 3.79 (s, 6 H).
13C NMR (50 MHz, CDCl3): d = 167.7, 160.2, 148.8, 148.1, 137.0,
128.9, 128.1, 127.2, 126.0, 125.1, 123.7, 121.0, 119.8, 111.2, 111.0,
55.9, 52.2, 51.8.
(7) (a) Das, B.; Ramu, R.; Reddy, M. R.; Mahender, G.
Synthesis 2005, 250. (b) Das, B.; Krishnaiah, M.;
Venkateswarlu, K. Tetrahedron Lett. 2006, 47, 4457.
(c) Das, B.; Damodar, K.; Bhunia, N. J. Org. Chem. 2009,
74, 5607. (d) Das, B.; Reddy, G. C.; Balasubramanyam, P.;
Veeranjaneyulu, B. Synthesis 2010, 1625.
ESIMS: m/z = 432 [M + Na]+.
Anal. Calcd for C23H23NO6: C, 67.47; H, 5.66; N, 3.42. Found: C,
67.42; H, 5.63; N, 3.48.
Synthesis 2011, No. 21, 3471–3474 © Thieme Stuttgart · New York