Microwave-induced calcium(II) chloride-catalyzed Paal–Knorr pyrrole synthesis: a safe, expeditious, and sustainable protocol

Article abstract of DOI:10.1007/s11164-018-3355-7

Abstract: Among various alkali (Na, K) and alkaline-earth (Ca, Mg, Sr, Ba) chlorides, calcium(II) chloride was found to be a cost-effective Lewis acid catalyst for solvent-free synthesis of pyrroles from primary aromatic and aliphatic amines under open-vessel focused microwave irradiation. The salient features of this environmentally benign method are high to quantitative conversion, short reaction time, safe and clean reaction profile, possibility of scale-up to multigram quantities, and use of a low-cost, widely available, nontoxic catalyst.

Full text of DOI:10.1007/s11164-018-3355-7

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3355-7
Microwave-induced calcium(II) chloride-catalyzed
Paal–Knorr pyrrole synthesis: a safe, expeditious,
and sustainable protocol
Kioumars Aghapoor1 Farshid Mohsenzadeh Hossein Reza Darabi
Saeed Rastgar1
1
1
•
•
•
Received: 12 November 2017 / Accepted: 24 February 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Among various alkali (Na, K) and alkaline-earth (Ca, Mg, Sr, Ba) chlorides,
calcium(II) chloride was found to be a cost-effective Lewis acid catalyst for solvent-free
synthesis of pyrroles from primary aromatic and aliphatic amines under open-vessel
focused microwave irradiation. The salient features of this environmentally benign
method are high to quantitative conversion, short reaction time, safe and clean reaction
profile, possibility of scale-up to multigram quantities, and use of a low-cost, widely
available, nontoxic catalyst.
Graphical abstract
Keywords Calcium(II) chloride Á Ecofriendly method Á Microwave
heating Á Paal–Knorr reaction Á 2,5-Dimethylpyrrole
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3355-7) contains supplementary material, which is available to authorized users.
&
Kioumars Aghapoor
k.aghapoor@yahoo.com; aghapoor@ccerci.ac.ir
1
Applied Chemicals Synthesis Lab, Chemistry and Chemical Engineering Research Center of
Iran, Pajoohesh Blvd., 17th Km of Tehran-Karaj Highway, Tehran 14968-13151, Iran
123

K. Aghapoor et al.
Introduction
Microwave-induced organic reaction enhancement (MORE) is now a well-
established tool in development of clean organic transformations. This scientific
tool has the merits of user-friendliness, operational simplicity, uniform heating, and
greater reproducibility of chemical reactions. The ability to adjust various
parameters (microwave power, reaction time, and temperature), accelerate the
reaction rate, minimize byproduct generation, and significantly reduce energy
consumption and reaction time (compared with conventional thermal heating)
makes it a valuable asset for organic chemists aiming to implement green chemical
processes [1].
Pyrrole moieties are one of the most important classes of heterocyclic
compounds, being present in a large number of naturally occurring, biological,
and drug molecules [2–9]. The growing importance of these heterocycles as
intermediates in pharmaceutical and chemical processes has led to considerable
effort to develop a variety of methodologies for their synthesis. Among the various
methods developed for straightforward synthesis of pyrrole ring, Paal–Knorr
cyclization and the Clauson-Kaas reaction have received considerable attention due
to their use of readily available starting materials [10, 11]. Among catalysts reported
for the Paal–Knorr reaction, Lewis acid catalysts are the most widely adopted, e.g.,
Bi(NO3)3Á5H2O [12], Sc(OTf)3 [13], SnCl2Á2H2O [14], RuCl3 [15], CoCl2 [16],
InCl3, InBr3 or In(OTf)3 [17], ZrCl4 [18], Ga(OTf)3 [19], AlCl3/PS [20],
ZrOCl2Á8H2O [21], UO2(NO3)2Á6H2O [22], CuI/C [23], SbCl3/SiO2 [24], BiCl3/
SiO2 [25], GaCl3/PS [26], Al(DS)3 [27], MgI2Á(OEt2)n [28], FePO4 [29], and MgI2
[30]. Besides, some ecofriendly protocols have recently been reported for this
purpose, e.g., mechanical activation [31], use of nanomagnetically modified sulfuric
acid [32], and a deep eutectic solvent under ultrasonic activation [33]. However, the
search for inexpensive, abundant, and greener Lewis acid catalysts is still being
actively pursued.
Calcium(II) chloride is an abundant moisture-tolerant salt having several
industrial applications, e.g., deicing of roads, dust control, and imparting stability
to roads and buildings. It is also used as a desiccant for dehydrating gases and
liquids, and as a calcium source in liquid feed supplements for dairy cattle [34]. It is
used as catalyst for preparation of tetrahydropyranyl ethers [35], 3,4-dihydropy-
rimidin-2(1H)-ones [36, 37], a-aminophosphonic esters [38], carbamoyl and
thiocarbamoyl phosphonates [39], b-aminoketones [40], 4H-pyrans [41], 9-aryl-
1,8-dioxooctahydroxanthene [42], 3-(aryliminomethyl) chromones [43], allylic
oxidation of alkenes [44], esterification of aromatic aldehydes [45], and C–F
functionalization of fluoroarenes [46]. Its conjunction with chiral ligands has also
proved to be effective for asymmetric reactions [47–49].
Microwave-assisted organic synthesis (MAOS) has had a tremendous impact in
generation of N-heterocycle libraries [50–55]. Its contribution to sustainable
development of the pharmaceutical industry and drug discovery is now undeniable
[56–58]. As part of our continuing efforts to develop sustainable processes for
pyrrole synthesis [24, 25, 59–61], we introduce herein calcium(II) chloride as an
123

Microwave-induced calcium(II) chloride-catalyzed Paal…
inexpensive, environmentally benign, commercially available catalyst for rapid
Paal–Knorr pyrrole cyclocondensation under microwave irradiation.
Results and discussion
Initially, condensation of hexane-2,5-dione (1.2 mmol) with 4-bromoaniline
(1 mmol) was examined in presence of catalytic amount (20 mol%) of various
alkali and alkaline-earth chlorides (Scheme 1). The reaction was screened under
microwave-induced solvent-free condition within 10 min. Among these Lewis acid
salts, NaCl, KCl, and BaCl2 showed poor catalytic activity (Table 1, entries 2, 3, 7)
whereas MgCl2Á6H2O, CaCl2Á2H2O, and SrCl2Á6H2O (Table 1, entries 4–6)
catalyzed the reaction to afford the desired product in excellent yield.
Synthesis of 1j was also performed in presence of various calcium salts as readily
available catalysts (Table 2). CaSO4, CaCO3, Ca(OH)2, and CaO exhibited poor
catalytic activity (Table 2, entries 4–7). While CaHPO4 gave good yield of product
(Table 2, entry 3), calcium chloride catalyzed the reaction to afford the desired
product in excellent yield (Table 2, entries 1, 2). Consequently, CaCl2Á2H2O was
chosen as catalyst of choice due to its easy availability, nontoxicity, ecofriendliness,
and quantitative yield of 1j (Table 2, entry 1). The classical reaction was also
studied in presence of CaCl2Á2H2O at 25 and 70 °C for 1 h (Table 2, entries 12, 13).
The same results were obtained under microwave irradiation and classical heating at
70 °C (Table 2, entries 1, 12) to highlight the role of the catalyst.
The effect of the microwave power in presence of CaCl2Á2H2O was also
investigated. The results revealed that decrease of the microwave power led to lower
conversion (Fig. 1).
To evaluate the performance and generality of CaCl2Á2H2O applied as catalyst,
various primary amines were tested with hexane-2,5-dione under the optimized
conditions; the results are presented in Table 3. In most cases, the reaction
proceeded expeditiously, although the yield was highly dependent on the substrate
used (Table 3).
It was found that aniline derivatives bearing either electron-donating (such as
methoxy, methyl, and dimethyl) or electron-withdrawing (such as nitro, bromo,
chloro, dichloro, and cyano) substituents reacted smoothly to give corresponding N-
substituted pyrroles (Table 3, entries 1a–1o) in high to excellent yield. An
exception to this rule was observed for 2-nitroaniline as substrate, for which strong
Scheme 1 Catalyst screening of alkali and alkaline-earth salts as potential Lewis acids for synthesis of 1j
123

K. Aghapoor et al.
Conversion (%)a
Table 1 Catalyst effect on
synthesis of 1j under microwave
heating
Entry
Catalyst (mol%)
1
2
3
4
5
6
7
–
24
28
NaCl (20)
KCl (20)
15
MgCl2Á6H2O (20)
CaCl2Á2H2O (20)
SrCl2Á6H2O (20)
BaCl2 (20)
100
100
96
Microwave irradiation: 420 W
aGas chromatography assay (%)
31
Table 2 Screening of various Ca salts for synthesis of 1j
Entry
Catalyst (mol%)
Conditions
Time (min)
Conversion (%)
1
CaCl2Á2H2O (20)
CaCl2 anhydrous (20)
CaHPO4 (20)
MWI
MWI
MWI
MWI
MWI
MWI
MWI
MWI
MWI
MWI
MWI
70 °C
25 °C
10
10
10
10
10
10
10
10
10
8
100
100
82
2
3
4
CaSO4 (20)
39
5
CaCO3 (20)
18
6
Ca(OH)2 (20)
29
7
CaO (20)
13
8
CaCl2Á2H2O (15)
CaCl2Á2H2O (10)
CaCl2Á2H2O (20)
CaCl2Á2H2O (20)
CaCl2Á2H2O (20)
CaCl2Á2H2O (20)
99
9
98.5
96
10
11
12
13
5
55
60
60
100
75
Fig. 1 Effect of microwave
power in presence of
CaCl2Á2H2O (20 mol%) as
catalyst in synthesis of 1j within
10 min
123

Microwave-induced calcium(II) chloride-catalyzed Paal…
Table 3 Microwave-induced synthesis of N-substituted pyrroles catalyzed by CaCl2Á2H2O (20 mol%)
Time
(min)
Conversion
(%)a
Yield
(%)b
m.p.
(°C)
Entry
1a
Amine
Product
10
10
10
10
100
100
100
100
96
95
94
94
58–59
50–51
46–47
51–52
1b
1c
1d
10
10
100
95
94
90
Oil
Oil
1e
1f
10
10
91
86
93
145–146
85–86
1g
1h
100
10 / 20
10 / 14
5
87–88
1i
10
10
100
100
95
95
74–75
56–57
1j
1k
10
100
94
48–49
1l
123

K. Aghapoor et al.
Table 3 continued
10
10
100
100
94
102–103
138–139
1m
95
1n
10
10
89
92
80
84
84–85
Oil
1o
1p
10
10
100
100
97
94
44–45
Oil
1q
1r
10
89
74
Oil
1s
aGas chromatography assay (%)
bYields refer to pure isolated product
electron-withdrawing nature and steric hindrance led to poor conversion
[24, 25, 59–61], even after extended irradiation time (Table 3, entry 1i).
The reaction was also carried out with 2-aminopyridine and aliphatic amines
such as benzylamine, n-hexylamine, and cyclohexylamine, giving high to excellent
conversion (Table 3, entries 1p–1s).
To investigate the viability of this protocol in pharmaceutical industry, gram-
scale synthesis of 1j (using 20 mmol 4-bromoaniline) was also carried out under the
same optimized conditions. The synthesis was successfully scaled up to produce N-
(40-bromophenyl)-2,5-dimethylpyrrole on gram scale with almost the same isolated
yield (96 %) of 1j as in the small-scale reaction (Table 3, entry 1j, yield 95 %).
Moreover, the catalyst could be readily recovered. After reaction completion, ethyl
acetate was added and the catalyst together with water produced from the reaction
was separated. The aqueous solution was evaporated under reduced pressure or put
in the microwave oven (5 min, 420 W), and the catalyst was reused in the
subsequent run with little loss of activity.
123

Microwave-induced calcium(II) chloride-catalyzed Paal…
A plausible mechanism for calcium(II) chloride-catalyzed synthesis of pyrroles is
depicted in Scheme 2. Ca2? ion interacts with hexane-2,5-dione and activates the
oxygen of carbonyl groups to promote the reaction via nucleophilic attack of
primary amines. Eventually, after completion of cyclocondensation, detachment of
CaCl2 occurs with subsequent loss of water, leading to formation of the pyrrole ring.
Conclusions
Catalyst screening of various alkali (Na, K) and alkaline-earth (Ca, Mg, Sr, Ba) salts
was performed for microwave-induced Paal–Knorr pyrrole cyclocondensation of
4-bromoaniline and hexane-2,5-dione under solvent-free condition. Among these
potential catalysts, CaCl2Á2H2O led to quantitative conversion and was chosen as
catalyst of choice due to its inexpensiveness and innocuous nature. The scope and
generality of this protocol were explored with respect to various primary aromatic
and aliphatic amines, revealing high to excellent yields. The combination of
microwave irradiation with a nontoxic and abundant catalyst under solvent-free
condition, together with the possibility of scale-up to multigram quantities of
Scheme 2 Plausible mechanism for calcium(II) chloride-catalyzed Paal–Knorr pyrrole
cyclocondensation
123

K. Aghapoor et al.
desired pyrroles, makes this protocol a useful asset for generation of pyrrole
libraries.
Experimental
Materials and methods
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
300 and 500 MHz spectrometer. All NMR samples were run in CDCl3, and
chemical shifts are expressed as ppm relative to internal Me4Si. Mass spectra were
obtained on a Fisons instrument. Substrates are commercially available and used
without further purification. A laboratory microwave oven (MW 3100, Landgraf
Laborsysteme HLL GmbH, Langenhagen, Germany) equipped with magnetic stirrer
operating at 2450 MHz was used for synthesis of 2,5-dimethylpyrroles.
General procedure for preparation of 2,5-dimethylpyrroles
In a typical reaction, primary amine (1.0 mmol), hexane-2,5-dione (1.2 mmol), and
CaCl2Á2H2O (0.03 g, 0.2 mmol) were mixed thoroughly for 5 min. The mixture was
taken in an open vessel and irradiated at 420 W (70 % of maximum power) for
10 min. Ethyl acetate (5 mL) and water (2 9 10 mL) were added to the reaction
mixture. The organic layer was separated and dried over Na2SO4. After filtration,
the recovered organic medium was subjected to gas chromatography (GC) analysis
to determine the conversion. The solvent was then evaporated under reduced
pressure to obtain the corresponding product. In cases where the reaction did not
proceed to completeness, crude product was passed through a short column of
neutral alumina [eluted with ethyl acetate/petroleum ether (3:7)] to give pure
pyrrole 1.
Selected spectroscopic data
N-(40-Methoxyphenyl)-2,5-dimethylpyrrole (1a) [62] [CAS registry no. 5044-27-
9] 1H NMR (300 MHz, CDCl3): d = 7.13 (d, J = 8.7 Hz, 2H), 6.97 (d,
J = 8.7 Hz, 2H), 5.89 (s, 2H), 3.87 (s, 3H), 2.02 (s, 6H). 13C NMR (75 MHz,
CDCl3): d = 158.9, 131.7, 129.2, 129.0, 114.2, 105.3, 60.4, 14.2. MS (EI): m/z (rel.
intensity %) = 201 (M?, 100), 200 (48), 186 (23), 154 (7), 145 (10).
N-Phenyl-2,5-dimethylpyrrole (1b) [63] [CAS registry no. 83-24-9] 1H NMR
(500 MHz, CDCl3): d = 7.52–7.48 (m, 2H), 7.45–7.42 (m, 1H), 7.30–7.25 (d, 2H),
5.95 (s, 2H), 2.08 (s, 6H). 13C NMR (125 MHz, CDCl3): d = 138.9, 128.9, 128.8,
128.2, 127.6, 105.6, 12.9. MS (EI): m/z (rel. intensity %) = 171 (M?, 100), 154
(13).
N-(20,50-Dichlorophenyl)-2,5-dimethylpyrrole (1n) [64] 1H NMR (300 MHz,
CDCl3): d = 7.49 (d, J = 8.4 Hz, 1H), 7.41–7.34 (m, 2H), 5.95 (s, 2H), 1.99 (s,
123

Microwave-induced calcium(II) chloride-catalyzed Paal…
6H). 13C NMR (75 MHz, CDCl3): d = 138.0, 132.9, 132.3, 131.0, 130.7, 130.2,
128.8 (2C), 106.2 (2C), 12.5 (2C). MS (EI): m/z (rel. intensity %) 241 (45), 240
(73), 239 (M?, 68), 238 (100), 224 (7), 203 (7), 168 (18), 109 (7), 83 (20).
N-(20-Cyanophenyl)-2,5-dimethylpyrrole (1o) [65] [CAS registry no. 124678-40-
6] 1H NMR (300 MHz, CDCl3): d = 7.81 (d, J = 7.8 Hz, 1H), 7.74 (t,
J = 7.8 Hz, 1H), 7.56 (t, J = 7.5 Hz, 1H), 7.38 (d, J = 8.1 Hz, 1H), 5.98 (s,
2H), 2.04 (s, 6H). 13C NMR (75 MHz, CDCl3): d = 142.0, 133.6, 130.1, 128.8,
128.6, 115.9, 113.6, 107.1, 106.8, 12.5. MS (EI): m/z (rel. intensity %) = 196 (M?,
80), 195(100), 181 (35), 154 (10), 102 (15), 92 (10), 75 (5).
N-Benzyl-2,5-dimethylpyrrole (1q) [66] [CAS registry no. 5044-20-2] 1H NMR
(500 MHz, CDCl3): d = 7.44–7.34 (m, 3H), 7.02 (d, J = 9.0 Hz, 2H), 6.00 (s, 2H),
5.14 (s, 2H), 2.27 (s, 6H). 13C NMR (125 MHz, CDCl3): d = 139.1, 129.2, 128.5,
127.4, 126.1, 105.9, 47.2, 12.7; MS (EI): m/z (rel. intensity %) = 185 (M?, 70), 91
(100).
N-Cyclohexyl-2,5-dimethylpyrrole (1s) [67] [CAS registry no. 24836-02-0] 1H
NMR (500 MHz, CDCl3): d = 5.76 (s, 2H), 3.94 (m, 1H), 2.33 (s, 6H), 1.98–1.23
(m, 10H). 13C NMR (125 MHz, CDCl3): d = 128.3 (2C), 106.5 (2C), 56.9, 32.9
(2C), 27.1 (2C), 26.1 (2C), 14.9. MS (EI): m/z (rel. intensity %) = 178 (40), 177
(M?, 100), 94 (10).
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