Infrared heat aided solid state synthesis of pyrroles from 1,4-diketones and ammonium acetate

Article abstract of DOI:10.1002/jhet.744

A facile amination of 1,4-diketones with ammonium acetate has been achieved by mechanochemical grinding assisted by infrared irradiation in the presence of silica gel. The wastes are reduced and the energy resources saved..

Full text of DOI:10.1002/jhet.744

204
Infrared Heat Aided Solid State Synthesis of Pyrroles from
1,4-Diketones and Ammonium Acetate
Vol 49
Chun Zhang, Jian Wang, and Jing-Hua Li*
College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310032, China
*E-mail: lijh@zjut.edu.cn
Received July 29, 2010
DOI 10.1002/jhet.744
Published online 13 September 2011 in Wiley Online Library (wileyonlinelibrary.com).
A facile amination of 1,4-diketones with ammonium acetate has been achieved by mechanochemical
grinding assisted by infrared irradiation in the presence of silica gel. The wastes are reduced and the
energy resources saved.
J. Heterocyclic Chem., 49, 204 (2012).
time. Various catalysts were investigated and the results
were summarized in Table 1.
INTRODUCTION
The pyrrole unit is one of the most abundant hetero-
cycles in natural products [1] and medicinal agents [2]. As
versatile building blocks, pyrroles have been widely used
in heterocyclic chemistry [3] and materials science [4].
Hitherto, a great many of methods have been developed
for the synthesis of these five-membered heterocycles [5],
including classical Knorr reaction [6], Hantzsch reaction
[7], Paal-Knorr condensation reaction [8] and other new
strategies [9]. However, upon these well-established proce-
dure, some drawbacks still exists, for instance, high tem-
peratures [10], long reaction times [11,8a], low yields [12]
and complicated workup procedures [13].
Generally used catalysts such as triflates, KHSO4, sulfu-
ric acid, p-toluenesulfonic acid and formic acid were firstly
employed in our model reaction. However, poor to moder-
ate yields were given (Table 1, entries 1ꢀ7). According to
our previous work [15], we chose silica gel as a catalyst to
explore the solid state organic reactions. To our surprise,
as a weak Lewis acid, silica gel showed high activity in
the reactions. The best result was obtained in the case of
20 mol % silica gel employed as a catalyst, and the con-
version of diketone was proceed smoothly in 96% yield
(Table 1, entry 9). the dosage of silica gel, whether
increasing or decreasing, did not improve the reaction
noticeably (Table 1, entries 10, 11). As expected, the blank
reaction without addition of any catalyst gives low yield
(Table 1, entry 8). Furthermore, silica gel was easy to be
recovered and reused with only a little loss of its activity
(Table 1, entry 12). Though good yield was given by con-
ventional way of refluxing in acetic acid, it takes long
reaction time. Above all, silica gel catalyzed grinding syn-
thesis of pyrroles from 1,4-diketones and ammonium salts,
assisted by infrared irradiation, were superior to other reac-
tion systems known, because of the advantage of excellent
yield, short reaction time and no metal pollution.
With growing interest in better methodologies for the
preparation of pyrroles, it is quite necessary to explore fac-
ile, green and practical methods. In recent years, organic
chemists have paid much attention to solvent-free reactions
[14] owing to the easy isolation and purification of the
products as well as the low environment pollution. Based
on our previous work, a well-established method for syn-
thesis of b-enamino ketones from 1,3-diketones at solid
state [15], we turned our interest to preparation of pyrroles.
In this article, we report that an infrared aided synthesis
of pyrroles was accomplished using inorganic ammonium
salts as a nitrogen source and silica gel as a catalyst.
Under the similar conditions, a variety of ammonium
salts were screened, and the corresponding results were
listed in Table 2. As illustrated, ammonium salt of weak
acids such as NH4HCO3, NH4OAc, (NH4)2CO3, and
NH4OOCH exhibited good activity because of their easy
release of NH3 during the reactions. In contrast, when
ammonium salt of strong acids, such as NH4Cl,
(NH4)2SO4, were used, the low yields were given.
RESULTS AND DISCUSSION
Initially, a model reaction was established just by
grinding a mixture of 1,4-diketone and ammonium ace-
tate under radiation of infrared lamp for appropriate
C
V
2011 HeteroCorporation

January 2012
Infrared Heat Aided Solid State Synthesis of Pyrroles from
1,4-Diketones and Ammonium Acetate
205
Table 1
CONCLUSION
Condensation of 1,4-diketone 1f with ammonium acetate in the
presence of various acids under different reaction conditions.a
In summary, we have developed an efficient and sol-
vent-free Paal-Knorr synthesis of pyrroles under mild
conditions. This simple catalytic method represents an
attractive pathway to synthesis of pyrroles. Compared
with the traditional method, there are at least four advan-
tages: (1) the way of energy supply (infrared heat) is
harmless; (2) the reaction was carried out in solid state
which means that no volatile organic compounds
(VOCs) was used in this method; (3) the reaction time is
short (in most cases less than 30 minutes); (4) It avoids
the usage of any metal compound and corrosive acid
which may cause the pollution.The only catalyst used in
this reaction is silica gel, which can be easily recycled.
Entry
Catalyst
Time (min)
Yieldb (%)
1
2
3
Sm(OTf)3
Mn(OTf)3
Pr(OTf)3
KHSO4
40
35
38
30
35
30
30
40
20
35
20
25
80
180
82
68
84
76
43
67
89
48
96
84
95
91
32
92
4
5
HCOOHc
d
6
7
H2SO4
p-TsOH
8
Neat
9
Silica gele
Silica gelf
Silica gelg
Silica gelh
Silica geli
CH3COOHj
10
11
12
13
14
EXPERIMENTAL
Melting points were determined on a Bu¨chi B-540 melting
1
apparatus and are uncorrected. H NMR and 13C NMR spectra
were recorded on a Bruker Model Avance III 500 MHz spec-
trometer in CDCl3 or DMSO-d6, using TMS as the internal
standard and chemical shifts are given in d relative to the sol-
vent peak. Mass spectra were measured with Agilent 6210
TOF LC/MS (APCI, ESI) mass spectrometer. TLC was per-
formed on silica gel GF 254. Infrared lamp was used with 250
W and the silica gel was 200–300 mesh heated under 120ꢁC
for half an hour.
General procedure for the preparation of pyrroles from
1,4-diketones and ammonium salts. A mixture of 1,4-dike-
tones (1.0 mmol), ammonium acetate (116 mg, 1.5 mmol) and
20 mol % silica gel (13 mg) was grinded in mortar at room
temperature for 1 minute. Then let it under infrared lamp for
several minutes. After completion of the reaction as indicated
by TLC, the reaction mixture was extracted with CH2Cl2 twice
aGeneral process: grinding reactants with catalyst together for 1 minute
then allowed to stand under infrared lamp for appropriate times.
bIsolated products.
c10% mol HCOOH as catalyst.
d5% mol H2SO4 as catalyst.
e20 mol % catalyst was used.
f10 mol % catalyst was used.
g30 mol % catalyst was used.
hThe catalyst was reused after heated at 120ꢁC for half an hour.
iGrinding reactants with catalyst together at room temperature.
jCH3COOH and CH3OH as solvent at reflux temperature.
To evaluate the scope and limitations of this solid
state reaction system, different substituted 1,4-diketones
were tested, from which functionalized pyrroles were
synthesized under the optimized reaction conditions (Ta-
ble 3). As shown in Table 3, this system is suitable for
both symmetrical 1,4-diketones (Table 3, entries 1,2)
and unsymmetrical ones (Table 3, entries 3–11).
Table 2
The result of solid state reactions of 1,4-diketone 1a with different
ammonium salts.a
It is noticed that pyrroles can be synthesized smoothly
from ammonium acetate and 1,4-diketones (Table 3,
entries 1–11) except diphenyl diketone 1m (Table 3,
entry 12). And it seems like that the electron withdraw-
ing substituents in R2 and R3 position of 1 promote the
reaction (Table 3, compare entries 3, 4 with entries 10,
11), and the aryl groups in R1 and R4 positions deacti-
vate the reactions.
As a contrast, the reactions were carried out in 70ꢁC
heat oven (Table 3, yields in the parentheses) instead of
under infrared heat. It is observed that infrared heat
facilitates promoting amination process. The experimen-
tal results show that infrared heat can reach or be supe-
rior to the yield of conventional heating procedure, but
the reaction time is greatly reduced which also improves
the efficiency of the experiment.
b
Entry
Ammonium salt
Time (min)
2
Yield (%)c
1
2
3
4
5
6
NH4HCO3
NH4OAc
(NH4)2CO3
NH4OOCH
NH4Cl
7
3
13
5
40
40
2a
2a
2a
2a
2a
2a
98
100
97
99
41
(NH4)2SO4
26
a General process: grinding reactants with amount of silica gel together
for 1 minute then allowed to stand under infrared lamp for appropriate
times.
b The time is the best reaction time.
c Isolated yields.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

206
C. Zhang, J. Wang, and J.-H. Li
Vol 49
Table 3
The result of different 1,4-diketones with ammonium acetate.a
Entry
1, 1,4-Diketone R1, R2,R3, and R4
Time/minb
2
Yieldc (%)
1
2
3
1a, R1¼R4¼CH3, R2¼R3¼CO2Et
3 (25)
13 (60)
24 (75)
30 (105)
16 (80)
20 (120)
13 (70)
12 (65)
20 (70)
14 (50)
40 (130)
60
2a
2b
2c
2d
2e
2f
2g
2h
2i
100 (98)
97 (92)
96 (94)
91 (87)
97 (98)d
96 (97)d
96 (98)
94 (91)
89 (87)d
81 (79)
83 (76)
Trace
1b, R1¼R4¼CH3, R2¼R3¼CO2Me
1c, R1¼CH3, R2¼CO2Me, R3¼H, R4¼Ph
1d, R1¼CH3, R2¼CO2Et, R3¼H, R4¼Ph
1e, R1¼CH3, R2¼CO2Me, R3¼H, R4¼4-FC6H4
1f, R1¼CH3, R2¼CO2Et, R3¼H, R4¼4-FC6H4
1g, R1¼CH3, R2¼CO2Me, R3¼H, R4¼4AOHC6H4
1h, R1¼CH3, R2¼Ac,R3¼H, R4¼Ph
1i, R1¼CH3, R2¼Ac, R3¼H, R4¼4AFC6H4
1j, R1¼Ph, R2¼ R3¼H, R4¼ CH3
4
5
6
7
8
9
10
11
12
2j
2k
/
1k, R1¼CH3, R2¼COPh, R3¼H, R4¼Ph
1m R1¼R4¼Ph, R2¼ R3¼H
a General process: 1,4-diketones (1mmol), ammonium acetate (1.5 mmol) and silica gel (20 mol %) under infrared heat (the temperature detected
was below 70ꢁC), yields are given for isolated products.
b The data showed in the parentheses were performed at 70ꢁC in common oven.
c Yields are given for isolated products.
d 1,4-Diketones (1 mmol), ammonium salts (2.5 mmol).
(2 ꢂ 8 mL). The combined organic layers were dried over
MgSO4, concentrated under reduced pressure to give a product
with high purity. The solid product could be further purified
by recrystallization in CH2Cl2.
Hz, 1H), 7.04–7.08 (m, 2H), 7.39–7.42 (m, 2H), 8.48 (s, 1H).
13C NMR (500 MHz, CDCl3): d ¼ 13.32, 50.94, 107.17,
113.10, 115.84, 116.01, 125.38, 125.44, 128.15, 129.23,
136.28, 160.70, 162.66, 165.90. hrms (ESI) [M-H]ꢀ m/z Calcd
for C13H11FN O2: 232.0774; Found: 232.0783.
5-(4-fluorophenyl)-2-methyl-1H-pyrrole-3-carboxylic acid
ethyl ester (2f). Yield: 96%; mp 138–140ꢁC; 1H NMR (500
MHz, CDCl3): d ¼ 1.36 (t, J ¼ 7.0 Hz, 3H), 2.58 (s, 3H),
4.27–4.31 (q, 2H, J ¼ 7.0 Hz), 6.76 (d, J ¼ 3.0 Hz, 1H),
7.04–7.07 (m, 2H), 7.40–7.42 (m, 2H), 8.52 (s, 1H). 13C NMR
(500 MHz, CDCl3): d ¼ 13.29, 14.46, 59.63, 107.16, 113.23,
115.70, 115.87, 125.39, 125.46, 128.24, 128.25, 129.30,
136.34, 160.63, 162.59, 165.82.
2,5-Dimethyl-1H-pyrrole-3,4-dicarboxylic acid diethyl ester
1
(2a). Yield: 100%; mp 85–86ꢁC; ref. [16a] 96–98ꢁC; H NMR
(500 MHz, CDCl3): d ¼ 1.32 (t, J ¼ 7.0 Hz, 6H), 2.35 (s,
6H), 4.24–4.29 (q, J ¼ 7.0, 14.5 Hz, 4H), 8.30 (s, 1H). 13C
NMR (500 MHz, CDCl3): d ¼ 12.17, 14.25, 60.02, 112.24,
132.47, 165.74.
2,5-Dimethyl-1H-pyrrole-3,4-dicarboxylic acid dimethyl
ester (2b). Yield: 97%; mp 114–115ꢁC; ref. [16b] 115–117ꢁC;
1H NMR (500 MHz, CDCl3): d ¼ 2.36 (s, 6H), 3.81 (s, 6H),
8.37 (s, 1H). 13C NMR (500 MHz, CDCl3): d ¼ 12.37, 51.27,
112.27, 132.54, 165.89.
5-(4-hydroxyphenyl)-2-methyl-1H-pyrrole-3-carboxylic acid
1
methyl ester (2g). Yield: 96%; mp 167–1688C; H NMR (500
2-Methyl-5-phenyl-1H-pyrrole-3-carboxylic acid methyl
ester (2c). Yield: 96%; mp 138–139ꢁC; 1H NMR (500 MHz,
CDCl3): d ¼ 2.59 (s, 3H), 3.83 (s, 3H), 6.82 (d, J ¼ 3.0 Hz,
1H), 7.21–7.24 (m, 1H), 7.34–7.37 (m, 2H), 7.44–7.46 (m,
2H), 8.53 (s, 1H). 13C NMR: (500 MHz, CDCl3) d ¼ 13.28,
50.85, 107.34, 113.07, 123.71, 126.57, 128.91, 130.12, 131.85,
136.34, 166.04.
2-Methyl-5-phenyl-1H-pyrrole-3-carboxylic acid ethyl ester
(2d). Yield: 91%; mp 108–110ꢁC; ref. [16c] 112–113ꢁC; 1H
NMR (500 MHz, CDCl3): d ¼ 1.36 (t, J ¼ 7.5 Hz, 3H), 2.59
(s, 3H), 4.28–4.32 (q, J ¼ 7.0 Hz, 2H), 6.84 (d, J ¼ 3.0 Hz,
1H), 7.21–7.24 (m, 1H), 7.35–7.38 (m, 2H), 7.44–7.46 (m,
2H), 8.47 (s, 1H). 13C NMR (500 MHz, CDCl3): d ¼ 13.34,
14.51, 59.52, 107.39, 113.41, 123.71, 126.52, 128.89, 130.03,
131.90, 136.20, 165.66.
MHz, DMSO-d6): d ¼ 2.45 (s, 3H), 3.69 (s, 3H), 6.54 (d, J ¼
3.0 Hz,1H), 6.74–6.77 (m, 2H), 6.41–6.44 (m, 2H), 9.41 (s,
1H), 11.41 (s, 1H). 13C NMR (500 MHz, DMSO-d6): d ¼
12.77, 50.39, 104.42, 111.33, 115.55, 123.24, 125.06, 130.16,
135.67, 156.03, 165.15. hrms (ESI) [M-H]ꢀ m/z Calcd for C13
H12 N O3: 230.0817; Found: 230.0824.
3-Acetyl-2-methyl-5-phenyl-1H-pyrrole (2h). Yield: 94%;
mp 185–1868C; ref. [16d] 179–181ꢁC; 1H NMR (500 MHz,
CDCl3): d 2.46 (s, 3H), 2.62 (s, 3H), 6.78 (d, J ¼ 3.0 Hz, 1H),
7.23–7.26 (m, 1H), 7.36–7.39 (m, 2H), 7.47–7.49 (m, 2H),
8.82 (s, 1H); 13C NMR (500 MHz, CDCl3) d 14.09, 28.45,
107.45, 122.26, 123.78, 126.71, 128.97, 129.89, 131.73,
136.02, 195.23.
3-Acetyl-2-methyl-5-(4-fluorophenyl)-1H-pyrrole (2i). Yield:
89%; mp 232ꢁC; ref. [16e] 220ꢁC; 1H NMR (500 MHz,
CDCl3): d ¼ 2.45 (s, 3H), 2.61 (s, 3H), 6.70 (d, J ¼ 3.0
Hz,1H), 7.07–7.10 (m, 2H), 7.41–7.44 (m, 2H), 8.43 (s, 1H).
13C NMR (500 MHz, DMSO-d6): d ¼ 13.31, 14.04, 20.70,
5-(4-fluorophenyl)-2-methyl-1H-pyrrole-3-carboxylic acid
1
methyl ester (2e). Yield: 97%; mp 178–180ꢁC; H NMR (500
MHz, CDCl3): d ¼ 2.58 (s, 3H), 3.83 (s, 3H), 6.75 (d, J ¼ 3.0
Journal of Heterocyclic Chemistry
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Infrared Heat Aided Solid State Synthesis of Pyrroles from
1,4-Diketones and Ammonium Acetate
207
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