Microwave Energized Synthesis of 2-Aroylindole Derivatives: Piperidine/DMF as an Effective Medium

Article abstract of DOI:10.1002/jhet.2334

A convenient and efficient protocol has been developed for the synthesis of 2-aroylindoles (3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h, 3i, 3j) in good yields by the reaction of 2-aminoketones (1a, 1b, 1c) with phenacyl bromides (2a, 2b, 2c, 2d). The reaction success varied with different bases and solvents in both conventional and microwave methods. But finally, it was established that piperidine in DMF was an effective medium to carry out the reaction under microwave irradiation conditions.

Full text of DOI:10.1002/jhet.2334

Month 2015
Microwave Energized Synthesis of 2-Aroylindole Derivatives:
Piperidine/DMF as an Effective Medium
Jayaprakash Soora Haranath, Satheesh Krishna Balam, Syama Sundar Chereddy, Siva Prasad Sthanikam,
*
Santhisudha Sarva, Mohan Gundluru, and Suresh Reddy Cirandur
Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, India
*
E-mail: csrsvu@gmail.com
Received June 9, 2014
DOI 10.1002/jhet.2334
Published online 00 Month 2015 in Wiley Online Library (wileyonlinelibrary.com).
A convenient and efficient protocol has been developed for the synthesis of 2-aroylindoles (3a–j) in good
yields by the reaction of 2-aminoketones (1a–c) with phenacyl bromides (2a–d). The reaction success varied
with different bases and solvents in both conventional and microwave methods. But finally, it was
established that piperidine in DMF was an effective medium to carry out the reaction under microwave
irradiation conditions.
J. Heterocyclic Chem., 00, 00 (2015).
INTRODUCTION
synthetic methods have been developed for the synthesis of
indole frameworks over the past hundred years right from
fisher indole synthesis [13–17]. Despite these advances,
development of new and convenient methods for 2,
3-disubstituted indoles [18–20], from simple and readily
available starting materials, still remains to be accomplished.
In recent years, microwave accelerated chemical reactions
are growing at a rapid rate. This technique provides an effec-
tive alternative energy source for effecting chemical reactions
and processes [21]. Thus, application of microwave energy to
accelerate organic reactions is of increasing interest and offers
several advantages over the conventional techniques. Micro-
wave reactions are environmentally benign and finds valuable
applications in organic [22], peptide synthesis [23], nanotech-
nology [24], polymer chemistry [25], and biochemical pro-
cesses [26,27]. Herein, we report piperidine-promoted
synthesis of 2, 3-substituted indoles in good to excellent yields
by the reaction of phenacyl bromides with 2-aminoketone s in
DMF under microwave irradiation conditions.
Indole framework is widely found in natural products, phar-
maceuticals, and other synthetic compounds [1]. Owing to the
great diversity of bioactivity, the indole ring system has
become an important structural component in most of the
biologically active alkaloids. Considering the significance of
the indole pharmacophore, medicinal chemists repeatedly turn
to indole-based compounds as they act as targets for the
development of potential therapeutic agents [2].
Indole and its synthetic analogs are known to possess prom-
ising anticancer, antipyretic, antiHIV, analgesic, antihyperten-
sive, and anticonvulsant [3–6] activity. Some examples in this
class, which shows potential biological activity against anti-
cancer, antiHIV, and antihypertensive, are 2-aroylindole (I),
Delavirdine (II), and Pindolol (III), shown in Figure 1.
The development of new efficient synthetic methods lead-
ing to indole analogs continues to receive much attention in
organic synthesis. Substituted indoles have been referred as
“privileged structures” because they are capable of binding
many receptors with high affinity [7,8]. Several multi com-
ponent reactions have been reported for the synthesis of
substituted indoles [9–11]. Among them, 2, 3-disubstituted
indoles are important subclass of indoles, which are the core
nucleus of many promising therapeutic agents [12]. Con-
sidering the significance of these compounds, a variety of
RESULTS AND DISCUSSION
2-Aroylindoles (3a–j) are obtained by the reaction of
2-aminoketone s with phenacyl bromides using piperi-
dine base in DMF solvent under microwave irradiation
conditions (Scheme 1).
© 2015 HeteroCorporation

J. Soora Haranath, S. K. Balam, S. S. Chereddy, S. P. Sthanikam, S. Sarva, M. Gundluru,
and S. R. Cirandur
Vol 000
Figure 1. Important indole drugs. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Scheme 1. 2-Aroylindoles synthesis from 2-aminoketone s and phenacyl bromides (3a–j).
To establish the appropriate reaction conditions for the
synthesis of 2-aroylindoles, we have chosen the reaction be-
tween phenacyl bromide (2a) and 2-aminoacetophenone
(1a) as a model reaction (Table 2). We carried out the model
reaction and studied the effect of solvent, base, and reaction
time under conventional and microwave conditions. We
found significant improvement in the yield of the product
with short reaction time by using microwave technique as
compared to conventional technique. Overall, the best results
were obtained by performing the model reaction in the pres-
ence of 10mol% of piperidine in DMF under microwave
conditions to yield the product 3a in good yield (Table 1, en-
try 8). Note that increasing the amount of piperidine failed to
improve the yield (Table 1, entries 16 and 17) while decreas-
ing these parameters led to reduced yield (Table 1, entry 15).
We did not found any improvement of the yield of the
product by increasing the reaction time (Table 1, entries
18, 19, and 20). The results are summarized in Table 1.
Further, we repeated the model reaction at various micro-
wave power ranges starting from 140 to 700 W to test its ef-
fect on model reaction, and the results are shown in Figure 2.
It represents that performance of this reaction with irradiation
Table 1
Scrutinization of solvent and base for the synthesis of 3a.
Conventional heating
Microwave conditionc
Time (min)
Yield (%)a
Entry
Solvent
Base (mol%)
Time (min)
Yield (%)a
Trace
1
2
3
Acetonitrile
Ethanol
THF
Et3N (10)
Et3N (10)
Et3N (10)
180
180
180
180
180
180
180
180
180
180
180
180
180
180
180
180
180
210
240
300
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
30
45
60
14
—
Trace
34
53
14
44
76
39
b
b
—
—
28
44
Trace
32
55
33
—
b
4
DMF
Et3N (10)
5
6
7
8
Acetonitrile
Ethanol
THF
Piperidine (10)
Piperidine (10)
Piperidine (10)
Piperidine (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
K2CO3 (10)
No base
DMF
9
Acetonitrile
Ethanol
THF
DMF
DMF
No solvent
DMF
DMF
DMF
DMF
b
b
10
11
12
13
14
15
16
17
18
19
20
—
Trace
43
12
18
52
26
b
b
No base
—
—
Piperidine (5)
Piperidine (15)
Piperidine (20)
Piperidine (10)
Piperidine (10)
Piperidine (10)
46
55
55
55
55
55
63
76
76
76
76
76
DMF
DMF
aIsolated yield.
bNo reaction occurred.
cReactions are carried out at 280 watts of microwave power.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2015
Synthesis of 2-Aroylindoles under Microwave Conditions
regenerates piperidine. The cyclized intermediate (III) un-
dergoes dehydration and ultimately forms 2-aroylindole.
The regenerated piperidine is again involved in reaction
cycle (Scheme 3).
CONCLUSION
We described a simple and an efficient pathway for the
synthesis of 2-aroylindoles (3a–k) using piperidine in
DMF medium under MWI conditions within 25–45 min
from readily available substrates. From the synthetic point
of view, the present protocol is very simple, the conversion
proceeds smoothly, and has no effect of air and moisture
resulting 2-aroylindoles in moderate to good yields. We
believe that the present approach will be better and more
practical alternative to the existing methodologies for the
synthesis of 2-aroylindoles.
Figure 2. Effect of microwave power on model reaction.
power of 140, 210, 280, 350, and 420 W afforded the corre-
sponding products with yields of 33%, 59%, 77%, 84%, and
84%, respectively. Further increase of the microwave oven
power (560 and 700 W) showed decrease in the product yield
with 72% and 66%, respectively. Perhaps this may be due to
the formation of complex reaction mixtures at high MW
power. Hence, 350 W of MW power is suitable to carry out
this reaction for better results (Fig. 2).
EXPERIMENTAL
General. Solvents and reagents were procured from Sigma-
Aldrich (Hyderabad, India) and Merck (Mumbai, India) and were
used as such without further purification. The reactions were
carried out on Microwave Oven, CATALYST-4R, Research
Model, Made in India. Melting points were determined using a
calibrated thermometer by Guna Digital Melting Point apparatus
(Chennai, India). IR spectra of samples were recorded as
potassium bromide pellet on a Bruker Vector 21 FT-IR
spectrophotometer (France). 1H and 13C NMR spectra were
recorded as solutions in CDCl3 on a Bruker AMX 500 MHz
spectrometer operating at 300 MHz for 1H and 75.4 MHz for
After optimizing the experimental conditions for this
reaction, we investigated its scope by reacting different
phenacyl bromides and 2-aminoketones. The results are
summarized in Table 2. Various phenacyl bromides worked
well under the standardized reaction conditions. Particularly,
trimethoxy phenacyl bromide (2c) reacted well and gave
good product yields (Table 2, entries 3c, 3g, and 3j). The ef-
fect of the electronic properties of the substituents on the
phenyl ring of the 2-aminoketone s was also investigated.
4, 5-dimethoxy 2-aminoacetophenone (1b) shows good
reactivity and provides comparatively good yields than
2-aminoacetophenone (1a); this might be due to the presence
of electron donating groups. 2-Aminobenzophenone (1c)
exhibited moderate reactivity than 2-aminoacetophenones
(1a). This might be due to steric interference of the phenyl
group during cyclization. Consequently, this produced low
product yields. All the results are summarized in Table 2.
Further, we extended the scope of the reaction with
aliphatic acyl bromide (2e) for the synthesis of 2-methyl-
1-(3-methyl-1H-indol-2-yl) propan-1-one (3k). Accordingly,
we treated 2-amino acetophenone (1a) with 1-bromo-3-
methylbutan-2-one (2e) in the presence of 10 mol% piperidine
in DMF under microwave conditions (350 MW). In contrast,
aliphatic acyl bromide gave lower product yield compared to
aromatic phenacyl bromides (Scheme 2).
1
13C. For H and 13C chemical shifts, tetramethylsilane was used
as an internal standard. ESI mass spectra were recorded on a
Micromass Quattro LC instrument at Indian Institute of
Technology (IICT), Hyderabad, India (Sparta, NJ). Elemental
analysis was performed on Thermo Finnigan Instrument at the
University of Hyderabad, India (San Jose, CA).
General procedure for the synthesis of 2-aroylindoles
(3a–k).
In a Pyrex tube 2-Aminoacetophenone (1a,
1 mmol), Phenacyl bromide (2a, 1 mmol) and piperidine
(10 mol%) were taken and were exposed to microwaves until
completion of the reaction (Table 2). After completion of the
reaction as indicated by TLC analysis, the reaction mixture
was cooled to room temperature and added 20 ml of water
and extracted with ethyl acetate and concentrated by rotary
evaporator. The residue was purified by silica gel column
chromatography on silica gel using petroleum ether/ethyl
acetate as eluent to afford the product 3a. The same
procedure was used for the synthesis of (3b–k).
A plausible mechanism for the synthesis of 2-aroylindoles
is described in Scheme 3. Probably, the mechanism proceeds
via the initial attack of piperidine to the acidic proton of
phenacyl bromide to produce carbanion (I), which is in equi-
librium with oxonium ion (II). The oxonium ion attacks at
carbonyl carbon of 2-amino acetophenone, and subsequent
protonation, dehydrobromination reproduces the cyclic
intermediate 2-acyl-3-hydroxy-dihydro indole (III) and
Spectral data of the titled compounds (3a–k). (3-Methyl-
1H-indol-2-yl)(phenyl)methanone (3a).
mp 112–114°C; 1H
NMR (300 MHz, CDCl3) δ [ppm]: 2.25 (s, 3H), 7.05–7.15 (m,
1H), 7.27–7.38 (m, 2H), 7.45–7.66 (m, 4H), 7.63–7.81 (m, 2H),
8.90 (brs, 1H); 13C NMR (75 MHz, CDCl3) δ [ppm]: 11.2, 111.8,
120.1, 121.2 126.4, 127.9 128.4, 128.7, 129.6, 131.5, 131.9,
136.5, 139.3, 189.3; IR (KBr, cmÀ1) 3315, 2920, 1622, 1447,
1336; MS (ESI): m/z 236 [M+ H]+. Anal. Calcd for C16H13NO:
C, 81.68; H, 5.57; N, 5.95. Found: 81.57; H, 5.49; N, 5.88.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

J. Soora Haranath, S. K. Balam, S. S. Chereddy, S. P. Sthanikam, S. Sarva, M. Gundluru,
and S. R. Cirandur
Vol 000
Table 2
Synthesis of 2-aroylindoles (3a–j).
Entry
1
2-Aminoketone (4a–c)
Bromo ketone (5a–d)
Product (6a–j)
Time (min)
30
Yield (%)a
84
86
2
30
3
25
89
4
30
76
5
6
30
30
78
81
7
25
87
(Continued)
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2015
Synthesis of 2-Aroylindoles under Microwave Conditions
Table 2
(Continued)
Entry
8
2-Aminoketone (4a–c)
Bromo ketone (5a–d)
Product (6a–j)
Time (min)
45
Yield (%)a
68
9
45
30
72
76
10
aIsolated yield.
(brs, 1H); 13C NMR (75MHz, CDCl3) δ [ppm]: 11.1, 21.2, 111.8,
120.0, 121.1, 126.0, 126.3 128.2, 128.4, 128.9, 129.3, 130.0, 132.7,
138.2, 139.3, 194.0; IR (KBr, cmÀ1) 3313, 2924, 2863, 1618, 1448,
1324; MS (ESI): m/z 250 [M + H]+. Anal. Calcd for C17H15NO: C,
81.90; H, 6.06; N, 5.62. Found: C, 81.84; H, 5.94; N, 5.54.
Scheme 2. Synthesis of 2-methyl-1-(3-methyl-1H-indol-2-yl) propan-1-
one (3k).
(3-Methyl-1H-indol-2-yl)(3,4,5-trimethoxyphenyl)methanone
(3c). mp 129–131°C; 1H NMR (300MHz, CDCl3) δ [ppm]: 2.38
(s, 3H), 3.85 (s, 6H), 3.90 (s, 3H), 7.02 (s, 2H), 7.10–7.17 (m, 1H),
7.27–7.40 (m, 2H), 7.60–7.63 (m, 1H), 8.84 (brs, 1H); 13C NMR
(75 MHz, CDCl3) δ [ppm]: 11.4, 56.2, 60.9, 106.5, 111.8, 119.9,
120.2, 121.1, 126.3, 128.8, 134.2, 136.5, 141.5, 153.0, 188.3; IR
(KBr, cmÀ1) 3324, 2926, 2853, 1618, 1433, 1337, 1132; MS
(3-Methyl-1H-indol-2-yl)(p-tolyl)methanone (3b).
mp 123–
125°C; 1H NMR (300 MHz, CDCl3) δ [ppm]: 2.28 (s, 3H), 2.47 (s,
3H), 7.07–7.14 (m, 1H), 7.30–7.39 (m, 4H), 7.51–7.65 (m, 3H), 8.85
Scheme 3. Plausible mechanistic pathway for the reaction.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

J. Soora Haranath, S. K. Balam, S. S. Chereddy, S. P. Sthanikam, S. Sarva, M. Gundluru,
and S. R. Cirandur
Vol 000
(ESI): m/z 326 [M + H]+. Anal. Calcd for C19H19NO4: C, 70.14; H,
5.89; N, 4.31. Found: C, 70.11; H, 5.78; N, 4.24.
(ESI): m/z 388 [M + H]+. Anal. Calcd for C24H21NO4: C, 74.40;
H, 5.46; N, 3.62. Found: C, 74.31; H, 5.36; N, 3.48.
(4-Bromophenyl)(3-methyl-1H-indol-2-yl)methanone (3d). mp
174–176°C; 1H NMR (300 MHz, CDCl3) δ [ppm]: 2.32 (s, 3H),
7.12–7.14 (m, 1H), 7.34–7.37 (m, 1H), 7.39–7.41 (m, 1H), 7.62–
7.68 (m, 5H), 8.91 (brs, 1H); 13C NMR (75 MHz, CDCl3) δ [ppm]:
11.4, 112.7, 119.9, 120.7, 121.6, 127.3, 127.5, 130.0, 131.1, 132.2,
137.4, 138.9, 187.6; IR (KBr, cmÀ1) 3338, 2932, 1623, 1435, 1342,
588; MS (ESI): m/z 314 [M+ H]+. Anal. Calcd for C16H12BrNO: C,
61.17; H, 3.85; N, 4.46. Found: C, 61.09; H, 3.78; N, 4.38.
2-Methyl-1-(3-methyl-1H-indol-2-yl) propan-1-one (3k).
mp
119–121°C; 1H NMR (300 MHz, CDCl3) δ [ppm]: 1.28 (d,
J= 6.7 Hz, 6H), 3.39–3.51 (m, 1H), 7.06–7.11 (m, 1H), 7.27–7.36
(m, 2H), 7.63 (d, J= 8.3 Hz, 1H), 9.13 (brs, 1H), 13C NMR (75MHz,
CDCl3) δ [ppm]: 10.9, 18.8, 37.1, 111.7, 120.0, 121.1, 126.3, 129.1,
131.4, 136.0, 197.6; IR (KBr, cmÀ1) 3323, 2958, 1632, 1423, 1329;
MS (ESI): m/z 202 [M + H]+. Anal. Calcd for C13H15NO: C, 77.58;
H, 7.51; N, 6.96. Found: C, 76.91; H, 7.36; N, 6.45.
(5,6-Dimethoxy-3-methyl-1H-indol-2-yl)(phenyl)methanone
1
(3e).
mp 163–165°C; H NMR (300 MHz, CDCl3) δ [ppm]:
2.19 (s, 3H), 3.91 (s, 6H), 6.78 (s, 1H), 6.90 (s, 1H), 7.44–7.58
(m, 3H), 7.68–7.70 (m, 2H), 8.79 (brs, 1H); 13C NMR (75 MHz,
CDCl3) δ [ppm]: 11.4, 56.0, 56.2, 93.6, 101.0, 121.1, 122.0,
128.3, 128.6, 128.8, 129.4, 131.5, 132.3, 139.8, 145.9, 146.4,
188.1; IR (KBr, cmÀ1) 3319, 2936, 2863, 1621, 1438, 1321,
1160; MS (ESI): m/z 296 [M + H]+. Anal. Calcd for C18H17NO3:
C, 73.20; H, 5.80; N, 4.74. Found: C, 73.12; H, 5.74; N, 4.63.
(5,6-Dimethoxy-3-methyl-1H-indol-2-yl)(p-tolyl)methanone
(3f). mp 141–143°C; 1H NMR (300 MHz, CDCl3) δ [ppm]: 2.12
(s, 3H), 2.43 (s, 3H), 3.92 (s, 6H), 6.75 (s, 1H), 6.91 (s,1H), 7.30–
7.39 (m, 2H), 7.49–7.60 (m, 2H), 8.68 (brs, 1H); 13C NMR
(75 MHz, CDCl3) δ [ppm]: 11.3, 21.3, 56.1, 56.2, 93.6, 101.2,
125.8, 128.2, 129.1, 132.2, 138.2, 139.8, 144.6, 146.0, 166.5,
176.4; IR (KBr, cmÀ1) 3316, 2924, 2862, 1621, 1442, 1321,
1146; MS (ESI): m/z 310 [M+ H]+. Anal. Calcd for C19H19NO3:
C, 73.77; H, 6.19; N, 4.53. Found: C, 73.67; H, 6.08; N, 4.43.
(5,6-dimethoxy-3-methyl-1H-indol-2-yl)(3,4,5-trimethoxyphenyl)
methanone (3g). mp 190–192°C; 1H NMR (300 MHz, CDCl3) δ
[ppm]: 2.29 (s, 3H), 3.87 (s, 6H), 3.89 (s, 3H), 3.91 (s, 6H), 6.79 (s,
1H), 6.91 (s, 1H), 6.99 (s, 2H), 8.83 (brs, 1H); 13C NMR (75MHz,
CDCl3) δ [ppm]: 11.6, 56.0, 56.1, 56.2, 60.9, 93.5, 100.9, 106.3,
120.5, 121.9, 130.6, 132.2, 134.7, 141.1, 145.9, 151.1, 153.0,
187.2; IR (KBr, cmÀ1) 3352, 2931, 1619, 1452, 1318, 1128; MS
(ESI): m/z 386 [M + H]+. Anal. Calcd for C21H23NO6: C, 65.44; H,
6.02; N, 3.63. Found: C, 65.38; H, 5.88; N, 3.52.
Acknowledgments. We thank Professor C. Devendranath Reddy,
Department of Chemistry, Sri Venkateswara University, Tirupati,
for his helpful discussions and University Grants Commission
(UGC), New Delhi, for providing Senior Research Fellowship
to one of the authors Mr. S. H. Jayaprakash and for funding a
research project (F. No.: 42-281/2013 (SR), Dated:12-03-2013).
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1
194°C; H NMR (300 MHz, CDCl3) δ [ppm]: 7.01–7.07 (m,2H),
7.08–7.21 (m, 4H), 7.21–7.25 (m, 1H), 7.32–7.38 (m,1H), 7.41–7.53
(m, 4H), 7.62–7.70 (m, 2H), 9.29 (brs, 1H); 13C NMR (75MHz,
CDCl3) δ [ppm]: 111.9, 121.1, 122.0, 126.5, 126.7, 127.4, 127.9,
127.9, 128.8, 129.4, 130.1, 130.6, 130.8, 131.6, 132.1, 133.6, 134.2,
134.5, 143.1, 189.5; IR (KBr, cmÀ1) 3321, 2926, 1619, 1338; MS
(ESI): m/z 298 [M + H]+. Anal. Calcd for C21H15NO: C, 84.82; H,
5.08; N, 4.71. Found: C, 84.69; H, 4.87; N, 4.63.
(3-Phenyl-1H-indol-2-yl)(p-tolyl)methanone (3i). mp 188–
190°C; 1H NMR (300 MHz, CDCl3) δ [ppm]: 2.25 (s, 3H), 7.05–
7.15 (m, 1H), 7.27–7.38 (m, 2H), 7.45–7.66 (m, 4H), 7.63–7.81
(m, 2H), 8.90 (brs, 1H); 13C NMR (75 MHz, CDCl3) δ [ppm]:
21.7, 111.8, 120.9, 122.3, 127.1, 127.6, 128.2, 129.1, 129.6,
130.2, 130.8, 131.7, 132.3, 133.9, 134.7, 143.3, 187.1; IR (KBr,
cmÀ1) 3311, 2923, 2892, 1698, 1426, 1332; MS (ESI): m/z 312
[M + H]+. Anal. Calcd for C22H17NO: C, 84.86; H, 5.50; N,
4.50. Found: C, 84.73; H, 5.42; N, 4.37.
(3-Phenyl-1H-indol-2-yl)(3,4,5-trimethoxyphenyl)methanone
(3j). mp 139–141°C; 1H NMR (300MHz, CDCl3) δ [ppm]: 3.58
(s, 6H), 3.75 (s, 3H), 6.73 (s, 2H), 7.10–7.17 (m, 4H), 7.19 (s, 1H),
7.23–7.65 (m, 4H), 9.22 (brs, 1H); 13C NMR (75 MHz, CDCl3) δ
[ppm]: 155.9, 60.7, 107.2, 113.1, 121.3, 123.8, 126.9, 127.3,
128.2, 128.4, 129.8, 130.5, 131.6, 132.2, 133.4, 134.6, 141.7,
152.5, 188.2; IR (KBr, cmÀ1) 3315, 2920, 1622, 1447, 1336; MS
[27] Rahman, K. M.; Thurston, D. E. Chem Commun 2009, 20,
2875.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet