Microwave-assisted facile synthesis of trisubstituted pyrrole derivatives

Article abstract of DOI:10.1007/s11164-015-1966-9

We report efficient synthesis of pyrrole derivatives by use of microwave irradiation. Quantitative yields were obtained in short reaction times. Low yields of product were obtained from alicyclic amino unsaturated ketone derivatives; higher yields were ob

Full text of DOI:10.1007/s11164-015-1966-9

Res Chem Intermed
DOI 10.1007/s11164-015-1966-9
Microwave-assisted facile synthesis of trisubstituted
pyrrole derivatives
•
V. Hanuman Reddy G. Mallikarjuna Reddy
•
•
M. Thirupalu Reddy Y. V. Rami Reddy
Received: 26 November 2014 / Accepted: 17 February 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract We report efficient synthesis of pyrrole derivatives by use of microwave
irradiation. Quantitative yields were obtained in short reaction times. Low yields of
product were obtained from alicyclic amino unsaturated ketone derivatives; higher yields
were obtained from aliphatic amino unsaturated ketone derivatives.
Graphical Abstract
Keywords Pyrroles Á Microwave irradiation Á Time saved Á Ring effect
Introduction
In recent times, synthetic methods have been developed to save time and achieve
quantitative yields. Microwave irradiation has increased in importance because of
its several advantages, including experimental simplicity, reduced reaction time,
lower energy requirement, improved substrate reactivity, and product formation
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-1966-9)
contains supplementary material, which is available to authorized users.
V. Hanuman Reddy Á G. Mallikarjuna Reddy Á M. Thirupalu Reddy Á Y. V. Rami Reddy (&)
Department of Chemistry, Sri Venkateswara University, Tirupati, India
e-mail: dryvrsvu@gmail.com
V. Hanuman Reddy
Organic Division, Indian Institute of Chemical Science, Hyderabad, Tarnaka, Hyderabad, India
123

V. H. Reddy et al.
efficiency, because energizing the reactions leads to almost quantitative yields.
Theoretical calculations have also suggested that reactions with high activation
energies can be performed under microwave irradiation without use of harsh
reaction conditions. Nowadays, numerous important heterocycles have been
synthesized by use of microwave irradiation [1–6]. Among five-membered
heterocycles, the pyrrole core is a ubiquitous structural unit of many natural
compounds, including hemoglobin, chlorophyll, vitamin B12, L-tryptophan [7, 8],
alkaloids [9, 10], and numerous essential drugs [11, 12] and synthetic pharmaco-
logically valuable substances with antibacterial [13–15], antifungal [16, 17],
antioxidant [18], antitumor, and anti-HIV activity [19], As a result, development of
new methods for synthesis of these molecules has been an objective of organic
synthesis for several years [20–25]. N-Heterocyclic compounds are obtained by use
of several traditional routes, including the Knorr reaction [26], Paal–Knorr
cyclization [27] and Hantzsch synthesis [28]. Recently, other elegant approaches
for construction of pyrrole derivatives have been reported, for example dehydro-
genative cyclization reactions [29], oxidative coupling of enamides and alkynes
[30], multicomponent reactions [31, 32], tandem rearrangement of a-diazo oxime
ethers [33], oxidative cyclization of N-allylimines and sulfonyl migrations [34], and
domino aminocyclization/1,3-sulfonyl migrations [35]. Chiba et al. developed a
method for synthesis of polysubstituted N–H pyrroles from vinyl azides and 1,3-
dicarbonyl compounds [36]. There are, moreover, many reported methods for
synthesis of substituted pyrroles by use of phenacyl bromide, unsaturated amines,
and diketones [37–39]. Many of these methods suffer from such disadvantages as
harsh reaction conditions, tedious experimental procedures, unsatisfactory yields,
long reaction times, and use of expensive and moisture-sensitive catalysts. Hence,
there is a need for time saving and efficient methods for synthesis of pyrrole
derivatives by use of microwave irradiation. In this paper we report efficient method
for synthesis of a variety of polysubstituted pyrroles under very mild reaction
conditions.
Experimental
General considerations
1H NMR and 13C NMR spectra were recorded at 500 MHz and 75 MHz,
1
respectively. For H NMR, tetramethylsilane (TMS) served as internal standard
(d = 0). Results are reported as follows: chemical shift, integration, multiplicity
(s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad), and
coupling constant in Hz. For 13C NMR, CDCl3 (d = 77.27) was used as internal
standard and spectra were obtained with complete proton decoupling. Low-
resolution MS and HRMS data were obtained by use of electrospray ionization.
Melting point data were measured with micro melting point apparatus and were
uncorrected. Reactions were conducted in a Catalyst-4R microwave oven (a
research model made in India).
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Microwave assisted facile synthesis
General procedure for synthesis of substituted pyrrole derivatives (3a–l)
A substituted amino unsaturated ketone (2, 1 mmol) and boron trifluoride
diethyletherate (10 mol%) in dichloromethane (5 ml) were added to a solution of
substituted phenacyl bromide (1, 1 mmol) and the mixture was irradiated with
microwaves for 10–16 min at 130 °C (250 W). After completion of the reaction, as
indicated by TLC analysis, the reaction mixture was poured on to crushed ice,
neutralized with sodium bicarbonate, extracted with ethyl acetate, and the extract was
concentrated by rotary evaporation. The residue was purified by silica gel column
chromatography with hexane–EtOAc (7:3) as eluent to afford the pyrrole derivatives 3.
1-(2-Methyl-5-phenyl-1H-pyrrol-3-yl)ethanone (3a)
White solid, m.p. 118–120 °C; 1H NMR (500 MHz, CDCl3): d 8.55 (brs, 1 H), 7.42
(d, J = 6.9 Hz, 2 H), 7.36–7.32 (m, 2 H), 7.22–7.18 (m, 1 H), 6.71(s, 1 H), 2.61 (s,
3 H) 2.43 (s, 3 H) ppm; 13C NMR (75 MHz, CDCl3): d 195.2, 135.9, 131.7, 128.9,
126.7, 123.7, 107.5, 28.5, 14.1 ppm; IR (KBr): m 3260, 2921, 2855,1601, 1438,
1253, 1170, 899, 754 cm-1; MS (ESI): m/z ([M ? Na]?): 222; HRMS (ESI): m/
z calcd for (C13H13NO ? Na?): 222.089; found: 222.0875.
Methyl 2-methyl-5-phenyl-1H-pyrrol-3-carboxylate (3b)
White solid, m.p. 90–92 °C; 1H NMR (500 MHz, CDCl3): d 8.48 (brs, 1 H),
7.50–7.33 (m, 4 H), 7.25–7.20 (m, 1 H), 6.83 (d, J = 3.0 Hz, 1 H), 3.83 (s, 3 H),
2.60 (s, 3 H) ppm; 13C NMR (75 MHz, CDCl3): d 173.35, 166.0, 129.95, 129.0,
128.8, 128.6, 126.5, 123.6, 112.9, 107.2, 50.8, 22.6 ppm; IR (KBr): m 3328, 2925,
2854, 1709, 1679, 1453, 1236, 1100, 760 cm-1; MS (ESI): m/z ([M ? H]?): 216;
HRMS (ESI): m/z calcd for (C13H13O2N ? H?): 216.101; found: 216.1026.
1-(2,5-Diphenyl-1H-pyrrol-3-yl)ethanone (3c)
1
Yellow solid, m.p. 159–160 °C; H NMR (500 MHz, CDCl3): d 8.59 (brs, 1 H),
7.88–7.82 (m, 2 H), 7.57–7.42 (m, 5 H) 7.41–7.34 (m, 2 H), 6.68 (s, J = 2.2 Hz,
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V. H. Reddy et al.
1H), 2.65 (s, 3 H) ppm; 13C NMR (75 MHz, CDCl3): d 192.4, 140.9, 137.5, 133.0,
131.7, 131.2, 129.7, 129.1, 129.1, 128.9, 128.9, 128.7, 126.7, 109.1, 13.9 ppm; IR
(KBr): m 3261, 3058, 2922, 1598, 1557, 1440, 1254, 1172, 901, 756, 695 cm-1; MS
(ESI): m/z ([M ? H]?): 262; HRMS (ESI): m/z calcd for (C18H15ON ? H?):
262.122; found: 262.1238.
Ethyl 2-methyl-5-phenyl-1H-pyrrol-3-carboxylate (3d)
White solid, m.p. 94–96 °C; 1H NMR (500 MHz, CDCl3): d 8.63 (brs, 1 H),
7.51–7.43(m, 2 H), 7.39–7.32 (m, 2 H) 7.27–7.17 (m, 1 H), 6.84 (d, J = 3.0 Hz, 2
H), 4.29 (q, J = 7.2 Hz, 2 H), 2.58 (s, 3 H), 1.36 (t, J = 7.2 Hz, 3 H) ppm; 13C
NMR (75 MHz, CDCl3): d 165.7, 136.3, 134.1, 131.8, 130.0, 128.9, 128.8, 128.5,
126.3, 123.6, 107.2, 59.5, 14.5, 13.3 ppm; IR (KBr): m 3314, 2980, 2927, 1672,
1609, 1479, 1448, 1236, 1099, 780 cm-1; MS (ESI): m/z ([M ? H]?): 230; HRMS
(ESI): m/z calcd for (C14H15O2N ? H?): 230.117; found: 230.1180.
1-(2-Ethyl-5-p-tolyl-1H-pyrrol-3-yl)propan-1-one (3e)
White solid, m.p.162-164 °C; 1H NMR (500 MHz, CDCl3): d 8.44 (brs, 1 H), 7.36 (d,
J = 7.9 Hz, 2 H), 7.19 (d, J = 8.0 Hz, 2 H), 6.74 (d, J = 2.6 Hz, 1 H), 3.06 (q,
J = 7.5 Hz, 2 H), 2.83(q, J = 7.3 Hz, 2 H), 2.36 (s, 3 H), 1.29(t, J = 7.5 Hz, 3 H),
1.19 (t, J = 7.3 Hz, 3 H) ppm; 13C NMR (75 MHz, CDCl3):d 197.9, 141.4, 136.7,
129.9, 129.6, 129.0, 120.7, 123.7, 106.4, 21.2, 21.1, 13.1, 8.4 ppm; IR (KBr): m 3274,
2974, 2926, 1629, 1460, 1211, 1012, 918, 801 cm-1; MS (ESI): m/z ([M ? H]?):
242; HRMS (ESI): m/z calcd for (C16H19ON ? H?): 242.153; found: 242.1463.
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Microwave assisted facile synthesis
1-(2-Methyl-5-m-tolyl-1H-pyrrol-3-yl)ethanone (3f)
1
Yellow solid, m.p. 160–162 °C; H NMR (500 MHz, CDCl3): d 8.58 (brs, 1 H),
7.33–7.25 (m, 4 H),7.07 (s, 1 H), 6.75(s, 1 H), 2.64 (s, 3 H) 2.45 (s, 3 H) 2.40 (s, 3
H) ppm; 13C NMR (75 MHz, CDCl3): d 195.2, 138.7, 135.9, 131.8, 131.7, 129.9,
128.9, 127.5, 124.3, 107.3, 28.4, 21.5, 13.8 ppm; IR (KBr): m 3289, 2920, 2854,
1635, 1573, 1437, 1239,1170, 942, 776 cm-1; MS (ESI): m/z ([M ? H]?): 214;
HRMS (ESI): m/z calcd for (C14H15NO ? H?): 214.123; found; 214.1246.
1-(5-Isopropyl-2-methyl-1H-pyrrol-3-yl)ethanone (3g)
White solid, m.p. 116–118 °C; 1H NMR (500 MHz, CDCl3): d 7.95 (brs, 1 H), 6.09
(s, 1 H), 2.86–2.79 (m, 1 H) 2.50 (s, 3 H), 2.34 (s, 3 H), 1.25 (d, J = 7.0, 6 H) ppm;
13C NMR (75 MHz, CDCl3): d 195.1, 136.7, 136.3, 133.7, 104.9, 28.3, 26.7, 22.3,
13.9 ppm; IR (KBr): m 3228, 2963, 2925, 1618, 1463, 1262, 1024, 957, 824 cm-1
;
MS (ESI): m/z ([M ? H]?): 166; HRMS (ESI): m/z calcd for (C10H15NO ? Na?):
188.105; found: 188.1068.
1-(5-Tert-butyl-2-methyl-1H-pyrrol-3-yl)ethanone (3h)
Yellow solid, m.p. 92–94 °C; 1H NMR (500 MHz, CDCl3): d 7.94 (brs, 1 H), 6.19(s, 1
H), 2.52 (s, 3 H), 2.39 (s, 3 H), 1.55 (s, 9 H) ppm; 13C NMR (75 MHz, CDCl3): d 195.2,
139.9, 139.8, 134.1, 104.4, 31.2, 30.2, 29.7, 28.4, 26.8, 13.8 ppm; IR (KBr): m 3248,
2960, 1620, 1525, 1455, 1243, 1018, 821, 775 cm-1; MS (ESI): m/z ([M ? H]?): 180;
HRMS (ESI): m/z calcd for (C11H17ON ? H?): 180.138; found: 180.1360.
1-(2-Methyl-5-p-tolyl-1H-pyrrol-3-yl)ethanone (3i)
1
Yellow solid, m.p. 168–170 °C; H NMR (500 MHz, CDCl3): d 8.66 (brs, 1 H),
7.35–7.30 (m, 2 H), 7.16–7.13 (s, 2 H), 6.65 (s, 1 H), 2.60 (s, 3 H) 2.42 (s, 3 H) 2.36
123

V. H. Reddy et al.
(s, 3 H) ppm; 13C NMR (75 MHz, CDCl3): d 195.3, 136.4, 135.7, 130.0, 129.5,
128.9, 123.6, 106.7, 28.4, 21.0, 14.0 ppm; IR (KBr): m 3261, 2916, 1637, 1582,
1445, 1236, 1167, 942, 795 cm-1; MS (ESI): m/z ([M ? H]?): 214; HRMS (ESI):
m/z calcd for (C14H15NO ? H?): 214.125; found: 214.1236.
Methyl 2-methyl-5-(3,4,5-trimethoxyphenyl)-1H-pyrrol-3-carboxylate (3j)
White solid, m.p. 192–194 °C; 1H NMR (500 MHz, CDCl3):d 9.09(brs, 1 H), 7.16(s, 2
H), 6.66 (d, J = 1.0 Hz, 1 H), 3.95–3.82 (m, 12 H), 2.61 (s, 3H) ppm; 13C NMR
(75 MHz, CDCl3): d 174.2, 153.4, 153.1, 136.8, 136.2, 130.1, 127.8, 127.3, 112.6,
106.9, 105.2, 101.2, 60.8, 56.2, 56.0, 13.1 ppm; IR (KBr); m 3326, 2925, 2850, 1743,
1707, 1679, 1450, 1276, 1234, 1160, 1133, 760, 695 cm-1; MS (ESI): m/z ([M ? H]?):
306; HRMS (ESI): m/z calcd for (C16H19O5N ? H?): 306.136; found: 306.1330.
6,6-Dimethyl-2-phenyl-6,7-dihydro-1H-indol-4(5H)-one (3k)
1
White solid, m.p. 226–228 °C; H NMR (500 MHz, CDCl3): d 9.07 (brs, 1 H),
7.50–7.45 (m, 2 H), 7.36 (t, J = 7.5 Hz, 2 H), 7.27–7.20 (m, 1 H), 6.80 (d,
J = 3.0 Hz, 1 H), 2.73 (s, 3 H), 2.38 (s, 3 H), 1.14 (s, 6 H) ppm; 13C NMR
(75 MHz, CDCl3): d 194.1, 143.7, 133.1, 131.6, 128.8, 126.8, 124.0, 120.4, 102.1,
51.9, 36.8, 35.7, 28.5 ppm; IR (KBr): m 3259, 2954, 1626, 1489, 1224, 1162, 1060,
760, 686 cm-1; MS (ESI): m/z ([M ? H]?): 240; HRMS (ESI): m/z calcd for
(C16H17ON ? H?): 240.131; found: 240.1382.
2-Phenyl-6,7-dihydro-1H-indol-4(5H)-one (3l)
1
White solid, m.p. 226–228 °C; H NMR (500 MHz, CDCl3): d 8.87 (brs, 1 H),
7.55–7.50 (m, 2 H), 7.46 (t, J = 7.5 Hz, 2 H), 7.27–7.25 (m, 1 H), 6.85 (d,
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Microwave assisted facile synthesis
J = 3.0 Hz, 1 H), 2.78 (s, 2 H), 2.50 (s, 2 H), 2.28 (s, 2 H) ppm; 13C NMR
(75 MHz, CDCl3): d 194.7., 144.7, 132.9, 131.6, 128.9, 126.9, 124.0, 121.7, 102.3,
37.7, 23.8, 22.8 ppm; IR (KBr): m 3260, 2955, 1636, 1459, 1234, 1182, 1060, 760,
686 cm-1; MS (ESI): m/z ([M ? H]?): 240; HRMS (ESI): m/z calcd for
(C14H13ON ? H?): 212.126; found: 212.1285.
Results and discussion
In this paper we report a facile synthetic condensation reaction using microwave
irradiation as another development in organic chemical synthesis. The process is
adaptable for assembly of compound libraries, which have recently attracted much
attention in pharmaceutical research. Equimolar amounts of starting compounds 1
and 2 were placed in an open Pyrex vessel and irradiated in a microwave oven, in
the presence of boron trifluoride diethyl etherate (10 mol%) in dichloromethane
(5 ml), for 10–16 min (Scheme 1). Workup gave products 3a–l in 68–93 % yield
(Table 1). All product yields were between 80 and 93 % except for compounds 3k
(68 %) and 3l (70 %), possibly because alicyclic amino unsaturated keto
derivatives gave low product yields whereas aliphatic amino unsaturated keto
derivatives furnished higher yields. Another reason for low yields of 3k and 3l
could be that the unsaturated keto functional group is within the six membered
ring and would lead to the ring effect and low reactivity of the alicyclic amino
unsaturated ketone; the other compounds had ring-free unsaturated keto functional
groups which may have enhanced their activity. It should be noted that the desired
product 3c was obtained in high yield (93 %, entry 3), when the reaction was
conducted between phenacyl bromide (1c) and the benzene ring-substituted
unsaturated ketone (2c) under microwave irradiation in dichloromethane. The
reaction time was 10 min. Compounds 3a and 3b were obtained in excellent yield,
90 and 92 %. The other compounds were obtained in 80–90 % yield. To study the
effect of solvent and the catalyst, we treated phenacyl bromide (1) with substituted
amino unsaturated ketone (2) and a variety of acid catalysts (Table 2) in different
Scheme 1 Synthesis of trisubstituted pyrrole derivatives
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V. H. Reddy et al.
Table 1 Reaction times, and yields of products 3(a–l)
123

Microwave assisted facile synthesis
Table 1 continued
Table 2 Results from use of different catalysts and solvents
Entry
Acid catalyst
Solvent
Time in MW (min)
Yields
3a
3b
3k
45
3l
41
1
2
3
4
FeCl3
CH2Cl2
15
12
10
18
54
51
62
35
57
49
68
41
Cu(oTf)2
Cu(oTf)2
InCl3
Toluene
THF
32
54
26
49
ClCH2CH2Cl
\20
\20
solvents under microwave irradiation. As shown in Table 2, use of Cu-(OTf)2 as
catalyst and tetrahydrofuran as solvent gave the best results in terms of reaction
time and yields of compounds 3a and 3b (entry 3, Table 2). Interestingly, the
same catalyst in toluene as solvent gave only *50 % yields. Products 3a and 3b
were obtained in better yield by use of FeCl3 in dichloromethane. Dichloroethane
was ineffective in this reaction. Use of InCl3 as catalyst resulted in poor yields of
all the compounds (Table 2). Compounds 3k and 3l were obtained in low yields
with all these catalysts and solvents, as discussed above. The quantitative yields
obtained in short times were indicative of spectacular acceleration of these
reactions as a consequence of microwave-assisted three-dimensional heating of the
reaction mixture, which cannot be achieved by classical heating. Improved yields
and clean reaction pathways are additional advantages of microwave-assisted
preparation of organic compounds. Indeed, reactions that do not occur if
conventional heating is used can be effectively performed by use of microwaves.
The proposed pathway for formation of substituted pyrroles is depicted in
Scheme 2. The Lewis acid activates the carbonyl group of the phenacyl compound
which acts as an electrophile and reacts with the amino group of the enamine,
leading to the pyrrole.
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V. H. Reddy et al.
Scheme 2 Mechanism of formation of the pyrrole ring
Conclusion
Trisubstituted pyrrole derivatives have been synthesized with short reaction times
and in quantitative yields by use of microwave irradiation. The presence of alicyclic
amino unsaturated keto derivatives resulted in low yields of product whereas
aliphatic amino unsaturated keto derivatives furnished higher yields.
Acknowledgments The author V. Hanuman Reddy is grateful to UGC-BSR, New Delhi, India for
providing financial assistance in the form of a Senior Research Fellowship. The authors also thank to
Professor B.V. Subba Reddy, senior scientist, organic division, Indian Institute of Chemical Science,
Hyderabad, for providing laboratory facilities.
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