Aluminum chloride-catalyzed C-alkylation of pyrrole and indole with chalcone and bis-chalcone derivatives

Article abstract of DOI:10.1080/00397911.2015.1136644

The AlCl₃-catalyzed alkylation of pyrrole (2) with chalcone (1a-i) at a ratio of 8:1 in the presence of 10 mol% AlCl₃ gave the solely 2-alkyl pyrroles (3a-i) at room temperature for 12 in good yields. The same reaction was performed with pyrrole (2) and chalcone at a ratio of 1:3 in CH₃CN at rt for 3 h to achieve 2,5-dialkyl pyrroles (4a-f). In addition, the reaction of the pyrrole (2) and indole (7) on 1,4-phenylene bis-chalcones (5a-g) at the ratio of 8:1 at rt for 24 h gave the double-addition products 6a-g and 8a-g in good yields, respectively. The structure of the products was confirmed by ¹H and ¹³C NMR spectroscopy and elemental analysis.

Full text of DOI:10.1080/00397911.2015.1136644

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Aluminum Chloride–Catalyzed C-Alkylation of
Pyrrole and Indole with Chalcone and Bis-Chalcone
Derivatives
Meliha Burcu Gürdere, Oguz Özbek & Mustafa Ceylan
To cite this article: Meliha Burcu Gürdere, Oguz Özbek & Mustafa Ceylan (2016): Aluminum
Chloride–Catalyzed C-Alkylation of Pyrrole and Indole with Chalcone and Bis-Chalcone
Derivatives, Synthetic Communications, DOI: 10.1080/00397911.2015.1136644
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Date: 29 January 2016, At: 06:44

Aluminum Chloride–Catalyzed C-Alkylation of Pyrrole and Indole with Chalcone
and Bis-Chalcone Derivatives
Meliha Burcu Gürdere1, Oguz Özbek1, Mustafa Ceylan1
1Department of Chemistry, Faculty of Arts and Sciences, Gaziosmanpasa University,
Tokat, Turkey
Corresponding author Meliha Burcu Gürdere E-mail: burcu.gurdere@gop.edu.tr
Abstract
The AlCl3-catalyzed alkylation of pyrrole (2) with chalcone (1a-i) at a ratio of 8:1 in the
presence of 10 mol % AlCl3 gave the solely 2-alkyl pyrroles (3a-i) at room temperature
for 12 hours in good yields. The same reaction was performed with pyrrole (2) and
chalcone at a ratio of 1:3 in CH3CN at r.t. for 3 h. to achieve the only 2,5-dialkyl pyrroles
(4a-f). In addition, the reaction of the pyrrole (2) and indole (7) on 1.4-phenylene bis-
chalcones (5a-g) at the ratio of 8:1 at r.t. for 24 h. gave the double-addition products 6a-g
and 8a-g in high yields, respectively. The structure of the products was confirmed by 1H
and 13C NMR spectroscopy and the elemental analysis.
KEYWORDS: Aluminum (III) chloride, Michael addition, Pyrrole, Indole, Chalcone,
Bis-chalcone
1

1. INTRODUCTION
Pyrrole, indole, and their C-alkylated derivatives are important building blocks in respect
to many biologically active compounds such as chlorophyll, porphyrin, hemoglobin,
Vitamin B12, indigo, and bile pigment.[1,2] The alkylation of pyrrole and indole have thus
been attracting much attention in recent years.[3,4] The Michael addition is one of the
most useful reactions in the alkylation of pyrrole and indole.[5,6] This reaction is
traditionally catalyzed by either strong bases or acids. Recently, a variety of Lewis acids
have been found to catalyze this reaction and have been used for the alkylation of pyrrole
with
-unsaturated ketones. These acids are InCl3,[7,8] InBr3,[9] BiCl3,[10] Bi(NO3)3,[11,12]
Bi(OTf)[3,13] Yb(OTf)3,[14] CuBr2,[15,16] GaI3,[17] SmI3,[18] and HfCl4.[19] In the alkylation of
pyrrole with these catalyses, the 2- and 2,5-dialkylated products have generally been
obtained as a mixture. In addition, a number of these procedures have had some
disadvantages such as a longer reaction time, strong acidic conditions, the use of an
excessive expensive catalyst, and harsh reaction conditions.
Although several methods have been developed for the indirect synthesis of 2-alkyl
pyrroles, in the literature there are few methods for the direct synthesis of 2-alkyl
pyrrole.[4,20] Thus, developing a useful method for the direct synthesis of 2-alkyl pyrroles
still remains a challenge for the synthetic chemist.
Aluminum chloride was widely used as a powerful and effective catalyst for various
organic transformations.[21] For example, AlCl3 has been used as a catalyst for Michael
addition of indoles and pyrroles to maleimides.[22] However, there has been no report
2

related to AlCl3 when used as a catalyst in addition to pyrrole and indole with chalcones.
In this paper, an efficient method was presented for the alkylation of pyrrole and indole
by chalcone and 1.4-phenylene bis-chalcones using aluminum chloride.
2. RESULTS AND DISCUSSION
The general synthetic strategy employed to prepare the chalcone and bis-chalcone
derivatives were based on the Claisen-Schmidt condensation, which was previously
reported.[23,24] The chalcone derivatives were synthesized by adding acetophenone
derivatives to the benzaldehyde derivatives at the basic medium (ethanol-NaOH) at room
temperature in high yields. Additionally, 2 mol acetophenone derivatives were used in the
case of the bis-chalcone. The chalcone derivatives 1a,[25] 1b,[26] 1c,[23] 1d,[23] 1e,[27] 1f,[23]
1g,[28] 1h[29] and 1i[29] are well-known in the literature. Bis-chalcones 5f except for; 5a-
d,[30] 5e[31] and 5g.[32]
In previous reported studies, the 2-alkyl and 2,5-dialkyl pyrroles have been generally
obtained as a mixture in the alkylation of pyrrole by chalcone. In this study, the synthesis
of the 2-alkyl and 2,5-dialkyl pyrroles as a sole product was focused upon separately.
Hence, a systematic study was carried out for determination of optimized conditions.
Initially, the pyrrole was reacted with chalcone (in a ratio of pyrrole:chalcone 1:1, 2:1 and
4:1) in the presence of AlCl3 in various solvents such as CH2Cl2, EtOH, THF and
CH3CN, and also without solvent. All the reactions performed in the solvent gave the
mixture of 2- and 2,5-dialkyl pyrroles in low yields, and the chalcone remained
unconsumed; however, in contrast, the products’ yields increased in the solvent-free
3

reaction. Thus, the reactions were examined increasing the pyrrole ratio to consume the
chalcone under solvent-free conditions. The reactions were performed with the
pyrrole:chalcone 6:1, 8:1 and 10:1 ratios and 5%, 10% and 15% mol of the AlCl3 under
solvent-free conditions. The best condition to approach 2-alkyl pyrroles was obtained:
solvent-free, the ratio of pyrrole:chalcone 8:1, 10% mol of the AlCl3, reaction
temperature of r.t. and reaction time of 12 hours.
In these optimized conditions, the 2-alkyl pyrroles (3a-i) were synthesized in high yields
of 84%-95% (Sch. 1, Tab. 1).
The structures of compounds (3a-i) were explained on the basis of the spectral (NMR and
elemental analysis) data and a comparison with the literature data.[10,15,33]
After, in order to obtain the 2,5-dialkyl pyrroles (4a-f) as a sole product, some reactions
were examined by increasing the chalcone ratio in the different solvents. The reactions of
the pyrrole with chalcone at the ratio of 1:3 were performed in the presence of 10% mol
of AlCl3 in various solvents such as CH2Cl2, EtOH, THF, and CH3CN at room
temperature. Among these reactions, the 2,5-dialkyl pyrroles was obtained as a sole
product with the best yield in CH3CN at room temperature for 3 hours. In these
conditions, the 2,5-dialkyl pyrroles (4a-f) were obtained in yields of 76%-93% (Sch. 2,
Tab. 2). The structures of compounds (4a-i) were explained on the basis of the spectral
(NMR and elemental analysis) data and comparison with the literature data. [10,15,33]
4

This study was extended to the alkylation of the pyrrole and indole with 1,4-phenylene
bis-chalcones. The reactions of the pyrrole and/or indole with 1,4-bis-chalcones at a ratio
of 8:1 and in the presence of 10% mol of AlCl3 gave the double-addition products 6a-g
(67-89%) (Sch. 3, Tab. 3) and 8a-g (69-83 %) (Sch. 4, Tab. 4), respectively, in CH3CN at
room temperature for 24 hours.
The structure of all the synthesized compounds was confirmed by 1H-, 13C-NMR
spectroscopy and the elemental analysis.
3. CONCLUSION
As a consequence, in this paper, a useful method was reported for the direct synthesis of
2- and 2,5-dialkyl pyrroles from the AlCl3-catalyzed addition of pyrrol to chalcone
derivatives. In addition, the AlCl3 was used as the catalyst for the first time in this
reaction. The 2-alkyl pyrrolles were obtained in pyrrole, chalcone and AlCl3 8:1:10% mol
ratios, respectively, at room temperature for 12 hours in high yields (84%-95%). When
the same reaction was performed using the ratio of pyrrole, chalcone and AlCl3 at
1:3:10% mol, respectively, in CH3CN at room temperature for 3 hours, the 2,5-dialkyl
pyrroles were obtained in good yields (76-93%). Moreover, the addition of pyrrole and
indole to 1,4-phenylene bis-chalcone was performed for the first time. The reaction of
pyrrole and/or indole with 1,4-phenylene bis-chalcone at a ratio of 8:1 in the presence of
10% mol of AlCl3 at room temperature for 24 hours gave the double additional products
in good yields (67-89% and 69-83%, respectively).
5

4. EXPERIMENTAL
The melting points were determined on a Electrothermal 9100 apparatus and were
uncorrected. The 1H and 13C NMR spectra were recorded on a Bruker Avance DPX-400
instrument. As internal standards served TMS ( 0.00) for 1H NMR and CDCl3 ( 77.0)
for 13C NMR spectroscopy, the J values are given in Hz. The multiplicities of the signals
in the 1H NMR spectra were abbreviated by s (singlet), d (doublet), t (triplet), q (quartet),
m (multiplet), br (broad) and combinations thereof. The elemental analyses were obtained
from a LECO CHNS 932 Elemental Analyzer.
4.1. General Procedure For The Synthesis Of 3a-I:
A mixture of pyrrole (2) (10 mmol) and chalcone (1a-i) (1 mmol) and AlCl3 (10%, mol)
was stirred at room temperature for 24 h until the starting material disappeared on the
TLC. After the completion of the reaction, the catalyst was separated by filtration. The
reaction mixture was then diluted with water and extracted with CHCl3. Subsequently,
the organic layer was dried over anhydrous sodium sulfate. The evaporating of the
solvent under reduced pressure gave the corresponding viscous oil products.
4.1.1. 1,3-Di(Furan-2-Yl)-3-(1H-Pyrrol-2-Yl)Propan-1-One (3a)
Viscous oil; Yield (84%). 1H-NMR (400 MHz, CDCl3) (ppm): 8.63 (s, 1H), 7.59 (br s,
1H), 7.40 (br s, 1H), 7.23 (d, J = 3.2 Hz, 1H), 6.71 (t, J = 1.2 Hz, 1H), 6.56 (t, J = 1.6
Hz, 1H), 6.33 (t, J = 1.4 Hz, 1H), 6.18-6.14 (m, 2H), 5.98 (br s, 1H), 4.89 (t, J = 7.0, Hz,
1H), 3.58 (dd, J = 6.4; 4.0; 2.4 Hz, 2H). 13C-NMR (100 MHz, CDCl3) (ppm): 187.4,
157.4, 152.5, 151.5, 146.7, 141.6, 131.5, 117.2, 112.5, 110.3 (2C), 105.5, 105.3, 44.1,
6

33.4. Anal. cald. for C15H13NO3: C, 70.58; H, 5.13; N, 5.49. Found: C, 70.46; H, 5.09; N,
5.37.
4.2. General Procedure For The Synthesis Of 4a-F
To a mixture of pyrrole (2) (1 mmol) and chalcone (1) (3 mmol) in CH3CN (5 mL) and
AlCl3 (10%, w/w) was added, and stirred at room temperature for 3 h. The reaction was
followed by TLC. After the completion of the reaction, the catalyst was separated by
filtration. The reaction mixture was then diluted with water and extracted with CHCl3.
Subsequently, the organic layer was dried over anhydrous sodium sulfate. The
evaporating of the solvent under reduced pressure gave the corresponding viscous oil
products.
4.2.1. 3,3'-(1H-Pyrrole-2,5-Diyl)Bis(1,3-Bis(4-Methoxyphenyl)Propan-1-One) (4a)
Viscous oil; Yield (85%). 1H-NMR (400 MHz, CDCl3) (ppm): 8.53 (br s, 1H), 8.05 (d,
J = 8.8 Hz, 4H), 7.87 (d, J = 8.4 Hz, 4H), 6.95 (d, J = 8.8 Hz, 4H), 6.84 (d, J = 8.4 Hz,
4H), 5.79 (d, J = 2.4 Hz, 1H), 5.75 (d, J = 4.0 Hz, 1H), 4.76 (t, J = 6.8 Hz, 2H), 3.79 (s,
12H), 3.65 (dd, J = 17.2, 7.6 Hz, 2H), 3.45 (dd, J = 16.8, 6.4 Hz, 2H). 13C-NMR (100
MHz, CDCl3) (ppm): 197.4 (2C), 163.5 (2C), 158.1 (2C), 138.1 (2C), 135.5 (2C), 130.7
(2C), 128.9 (4C), 127.7 (4C), 114.4 (4C), 114.0 (4C), 107.7 (2C), 55.3 (2C), 55.0 (2C),
44.8 (2C), 38.9 (2C). Anal. cald. for C38H37NO6: C, 75.60; H, 6.18; N, 2.32. Found: C,
74.98; H, 6.07; N, 2.28.
4.3. General Procedure For The Synthesis Of Bis-Chalcone Derivatives (5a-G)
7

To a solution of acetophenone derivatives (2 mmol) in EtOH (20 ml) was added NaOH
(2.5 M, 6 ml). The mixture was stirred for 3 min and teraftaldehit was added (1 mmol)
and stirred at r.t. for 4 h. After the reaction, the precipitate was filtered and washed with
ethanol several times. The solid was crystallized by methylene chloride / hexane (3/7).
4.3.1. 3,3'-(1,4-Phenylene)Bis(1-(Thiophene-3-Yl)Prop-2-En-1-One (5f)
Yield (90%); mp: 219-221 oC. 1H-NMR (400 MHz, CDCl3) (ppm): 8.85 (s, 2H), 7.97
(s, 4H), 7.95-7.91 (d, J= 15.6 Hz, 2H), 7.76-7.72 (d, J= 15.6 Hz, 2H), 7.70-7.69 (m, 4H).
13C-NMR (100 MHz, CDCl3) (ppm): 183.3, 143.3, 142.5, 137.0, 135.0, 129.7, 128.2,
127.5, 124.5. IR Spektrumu (KCl, cm-1): 3077, 1656, 1600, 1511, 1423, 1396, 1230,
1203, 1072, 1035, 981, 804, 713, 690. Anal. cald. for C20H14O2S2: C, 68.54; H, 4.03.
Found: C, 68.28; H, 4.02.
4.4. General Procedure For The Synthesis Of 6a-G And 8a-G
To a solution of pyrrole (2) (8 mmol) or indole (7) (8 mmol) and bis-chalcone (5a-g) (1
mmol) in CH3CN (5 mL) was added AlCl3 (10%, w/w) and the mixture was stirred at
room temperature for 24 h. The reaction was followed by TLC. After the completion of
the reaction, the catalyst was separated by filtration. The reaction mixture was then
diluted with water and extracted with CHCl3. Subsequently, the organic layer was dried
over anhydrous sodium sulfate. The evaporating of the solvent under reduced pressure
gave the corresponding solid products. The crude products were purified by
recrystallization from n-hexane-CHCl3.
8

4.4.1. 3,3'-(1,4-Phenylene)Bis(1-Phenyl-3-(1H-Pyrrol-2-Yl)Propan-1-One) (6a)
Yield (85%); mp: 212-214 oC. 1H-NMR (400 MHz, CDCl3) (ppm): 8.43 (br s, 2H),
7.97 (d, J = 7.2 Hz, 4H), 7.60-7.57 (m, 2H), 7.48 (t, J = 7.2 Hz, 4H), 7.27 (s, 4H), 6.68
(s, 2H), 6.11 (s, 2H), 5.87 (s, 2H), 4.77 (d, J = 7.2 Hz, 2H), 3.83-3.79(dd, J = 8.0, 17.6
Hz, 2H), 3.64-3.58 (dd, J = 5.6, 17.6 Hz, 2H). 13C-NMR (100 MHz, CDCl3) (ppm):
197.7 (2C), 141.4 (2C), 139.8 (2C), 135.1 (2C), 134.1 (2C), 129.5 (4C), 128.9 (4C),
128.3 (4C), 117.2 (2C), 107.9 (2C), 105.4 (2C), 45.1 (2C), 39.0 (2C). Anal. cald. for
C32H28N2O2: C, 81.33; H, 5.97; N, 5.93. Found: C, 81.27; H, 5.91; N, 5.86.
4.4.8. 3,3'-(1,4-Phenylene)Bis(3-(1H-Indol-3-Yl)-1-Phenylpropan-1-One) (8a)
Yield (75%); mp: 230-232 oC. 1H-NMR (400 MHz, CDCl3) (ppm): 10.81 (s, 2H),
-7.58 (m, 2H), 7.46 (t, J = 7.6 Hz, 4H), 7.39-7.36 (m, 2H), 7.26 (s,
8H), 7.01-6.97 (m, 2H), 6.86-6.82 (m, 2H), 4.79 (t, J= 7.2 Hz, 2H), 3.93-3.87 (dd, J=
7.6, 17.2 Hz, 2H), 3.69-3.63 (dd, J= 6.8, 17.2 Hz, 2H). 13C-NMR (100 MHz, CDCl3)
(ppm): 199.3 (2C), 143.6 (2C), 137.8 (2C), 137.2 (2C), 134.0 (2C), 129.5 (4C), 128.9
(4C), 128.4 (4C), 127.3 (2C), 122.7 (2C), 121.8 (2C), 119.6 (2C), 119.1 (2C), 118.9
(2C),112.2 (2C), 45.3 (2C), 38.1 (2C). Anal. cald. for C40H32N2O2: C, 83.89, H, 5.63; N,
4.89. Found: C, 83.77; H, 5.58; N, 4.81.
ACKNOWLEDGMENTS
We are indepted to department of Chemistry of Gaziosmanpaşa University.
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Supplemental data for this article can be accessed on the publisher’s website.
12

Table 1. AlCl3-Catalyzed addition of pyrrol to chalcones in ratio of 8:1
Entry Product
Yield (%)
1
84
2
3
89
95
4
93
5
6
91
89
7
8
93
84
13

9
86
14

Table 2. AlCl3-Catalyzed addition of pyrrol to chalcones in ratio of 1:3
Entry
Product
Yield (%)
1
85
2
93
3
4
5
76
82
85
6
84
15

Table 3. AlCl3-Catalyzed addition of pyrrol to bis-chalcones
Entry
Product
Yield (%)
1
85
2
3
89
74
4
71
5
6
7
89
76
81
16

Table 4. AlCl3-Catalyzed addition of indole to bis-chalcones
Entry
Product
Yield (%)
1
75
2
3
69
73
4
5
6
7
81
77
83
78
17

Scheme 1.
18

Scheme 2.
19

Scheme 3.
20

Scheme 4.
21