Synthesis of unexpected pyrrolo[2,3- b ]quinoxaline-2-carbaldehydes via sonogashira coupling reaction

Article abstract of DOI:10.1055/s-0029-1218715

The reaction of a number of N-alkyl-3-chloroquinoxaline-2-amines with propargyl bromide in the presence of PdClPPhand copper(I) iodide in wet morpholine leads to the formation of the unexpected N-substituted pyrrolo[2,3-b]quinoxaline-2-carbaldehydes in good yields. A possible mechanism for the conversion is suggested. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218715

PAPER
1599
Synthesis of Unexpected Pyrrolo[2,3-b]quinoxaline-2-carbaldehydes via
Sonogashira Coupling Reaction
S
ynthesisof Pyrro
l
lo[2,3-
b
i
]quinoxaline
K
School of Chemistry, Shahrood University of Technology, Shahrood, Iran
Fax +98(273)3395441; E-mail: akeivanloo@yahoo.com; E-mail: keivanloo@shahroodut.ac.ir
Received 6 January 2010; revised 16 February 2010
N
Cl
Abstract: The reaction of a number of N-alkyl-3-chloroquinoxa-
line-2-amines with propargyl bromide in the presence of
PdCl2(PPh3)2 and copper(I) iodide in wet morpholine leads to the
formation of the unexpected N-substituted pyrrolo[2,3-b]quinoxa-
line-2-carbaldehydes in good yields. A possible mechanism for the
conversion is suggested.
+
Br
N
NHR
2a–f
3
PdCl2(PPh3)2, CuI
morpholine–H2O, 70 °C
Key words: palladium catalyst, pyrroloquinoxaline, Sonogashira
coupling, propargyl bromide
N
N
O
N
H
The quinoxaline nucleus is present in many pharmaceuti-
cal agents that display a broad spectrum of biological ac-
tivity.1 Furthermore, pyrrolo[1,2-a]quinoxalines have
been shown to be potent and selective 5-HT3 receptor
ligands.2 For this reason, a variety of conventional meth-
ods have been developed for the preparation of quinoxa-
line derivatives.3
R
4a–f
R = Bn, Pr, i-Pr, Bu, i-Bu, Me
Scheme 1 Heterocyclization during the Sonogashira coupling of N-
alkyl-3-chloroquinoxalin-2-amines 2a–f with propargyl bromide (3)
in the presence of morpholine and catalyst
The palladium-catalyzed coupling of terminal alkynes
with aryl and vinyl halides in the presence of a catalytic
amount of copper(I) iodide and a nitrogen base (the Sono-
gashira reaction) is one of the most important and widely
used C–C bond-forming reactions in organic synthesis.4
Several research groups have also described annulation in
heterocyclization-based Sonogashira coupling.5 We have
previously reported the preparation of fused heterocycles
via tandem Sonogashira coupling followed by intramolec-
ular alkyne-hydroamination.6
Although a few methods have been reported7 for the syn-
thesis of pyrrolo[2,3-b]quinoxalines, a literature survey
showed that there are no references concerning its carbal-
dehyde derivatives. Herein, we wish to report the synthe-
sis of pyrrolo[2,3-b]quinoxaline-2-carbaldehydes 4a–f by
the reaction of N-alkyl-3-chloroquinoxalin-2-amines 2a–f
with propargyl bromide (3) in the presence of palladium
and copper catalysts (Scheme 1).The reactions were car-
ried out in wet morpholine (with only a few drops of wa-
ter) as solvent, however, the solvent must be degassed and
under an argon atmosphere.
of choice and a nucleophile; the use of morpholine gave
cleaner products and better yields.
The starting materials 2a–f were prepared from 2,3-
dichloroquinoxaline (1) and primary alkylamines in re-
fluxing ethanol (Table 1).
Table 1 Nucleophilic Substitution of 2,3-Dichloroquinoxaline with
Aliphatic Amines To Give N-Alkyl-3-chloroquinoxalin-2-amines
2a–fa
N
Cl
N
Cl
RNH2
EtOH
N
NHR
N
Cl
1
2a–f
R = Bn, Pr, i-Pr, Bu, i-Bu, Me
Entry
R
Mp (°C)
69–71
43–45
54–56
37–39
40–41
89–90
Product
2a
Yield (%)
1
2
3
4
5
6
Bn
92
80
76
83
86
95
n-Pr
i-Pr
n-Bu
i-Bu
Me
2b
2c
Although bis(triphenylphosphine)palladium(II) chloride
is the catalyst of choice, it seems, however, that copper(I)
iodide is essential as a co-catalyst; reactions carried out
with PdCl2(PPh3)2 alone give a mixture of products in low
or very poor yields. Morpholine was found to be the base
2d
2e
2f
a Conditions: 2,3-dichloroquinoxaline (1, 0.01 mmol), RNH2 (0.02
mmol), EtOH, reflux.
SYNTHESIS 2010, No. 10, pp 1599–1602
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x.
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x
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0
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Advanced online publication: 24.03.2010
DOI: 10.1055/s-0029-1218715; Art ID: P00310SS
© Georg Thieme Verlag Stuttgart · New York

1600
A. Keivanloo et al.
PAPER
When 2a (R = benzyl) was treated with propargyl bro- action temperature was necessary, when the temperature
mide (3) and morpholine in the presence of bis(triphe- increased to 100 °C, nucleophilic aromatic displacement
nylphosphine)palladium chloride and copper(I) iodide took place and N-benzyl-3-(morpholin-4-yl)quinoxalin-2-
and few drops of water at 70 °C, a single compound was amine was formed as the major product (Scheme 3).
obtained by TLC. After aqueous work up, the reaction
mixture was subjected to column chromatography and a
single pure product was isolated. 1H NMR spectrum of the
product showed a singlet resonance at d = 10.21 which
was assigned to aldehydic proton (it did not disappear
with the addition of D2O). Signals at d = 7.18–8.25 and at
5.93 are due to aromatic and benzylic protons respective-
ly. Its infra red spectrum showed the carbonyl group of a
conjugated aldehyde at 1670 cm–1.
N
Cl
O
+
+
Br
N
NH
N
H
catalyst
100 °C
O
N
N
N
In order to explore the scope and generality of this proto-
col, several N-alkyl-3-chloroquinoxalin-2-amines 2a–f
were prepared, and reacted with propargyl bromide (3) to
afford pyrrolo[2,3-b]quinoxaline-2-carbaldehydes 4a–f in
good to high yields (Table 2). The structures of the prod-
ucts were assigned on the basis of IR and 1H NMR spec-
troscopy and elemental analysis.
NH
Scheme 3 Nucleophilic aromatic substitution takes place when the
temperature rises to 100 °C
Mechanistically, the formation of pyrrolo[2,3-b]quinoxa-
line-2-carbaldehydes 4 involves the following steps
(Scheme 2): (i) formation of propargylamine [A] through
nucleophilic substitution of morpholine at propargyl bro-
mide, (ii) palladium–copper-catalyzed Sonogashira cross-
coupling between N-alkyl-3-chloroquinoxalin-2-amine 2
and propargylamine [A] affording [B], (iii) isomerization
to the allenic intermediates [C], (iv) ring closure by hy-
droamination/cyclization giving compound [D], (v) imine
formation by protonation, (vi) hydrolysis to aldehyde [E],
and (vii) air oxidation to aromatic products 4.
In conclusion, the chemistry outlined here provides a sim-
ple and straightforward method for the synthesis of func-
tionalized pyrrolo[2,3-b]quinoxalines 4a–f via the
palladium-catalyzed reaction of readily available N-alkyl-
3-chloroquinoxalin-2-amines 2a–f with propargyl bro-
mide (3) in morpholine.
All the reagents used were of commercial reagent grade. IR spectra
were obtained as KBr pellets in the range of 400–4000 cm–1 on a
Shimadzu Model 460 spectrophotometer. 1H NMR spectra were re-
corded on a Bruker BRX 500 AVANCE spectrometer. Elemental
analyses were performed on a Thermo Finnigan Flash EA microan-
alyzer.
It seems that air oxidation takes place during aqueous
work up and column chromatography. Control of the re-
O
O
N
+
N
Br
H
[A]
N
N
N
N
N
Cl
O
Sonogashira coupling
O
N
+
NH
R
O
NH
R
O
N
H
O
[B]
+
N
O
O
N
N
N
N
N
H
H
N
N+
heteroannulation
N
N
N
R
NH
N
R
R
[D]
[C]
H2O
N
N
O
N
O
air oxidation
– 2 H
N
R
H
N
H
N
R
[E]
4a–f
Scheme 2 Proposed mechanism for the formation of pyrrolo[2,3-b]quinoxaline-2-carbaldehydes 4a–f
Synthesis 2010, No. 10, 1599–1602 © Thieme Stuttgart · New York

PAPER
Synthesis of Pyrrolo[2,3-b]quinoxaline-2-carbaldehydes
1601
Anal. Calcd for C11H12N3Cl: C, 59.60; H, 5.46; N, 18.95. Found: C,
61.22; H, 5.31; N, 18.77.
Table 2 Melting Points and Yields of Pyrrolo[2,3-b]quinoxaline-2-
carbaldehydes 4a–fa
Entry
Product
Mp (°C)
Yield (%)
3-Chloro-N-isopropylquinoxalin-2-amine (2c)
Yield: 76%; mp 54–56 °C.
IR (KBr): 3400 (NH), 1550, 1520, 1080, 750 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 1.17 (d, J = 6.54 Hz, 6 H, 2
CH3), 4.30 (m, 1 H, CH), 7.54–8.16 (m, 5 H, ArH, NH).
N
N
O
H
1
181
82
N
Bn
Anal. Calcd for C11H12N3Cl: C, 59.60; H, 5.46; N, 18.95. Found: C,
57.76; H, 5.60; N, 19.21.
4a
4b
4c
4d
4e
4f
N
N
O
H
2
3
4
5
6
137
134
146
142
176
68
62
71
77
87
N-Butyl-3-chloroquinoxalin-2-amine (2d)
Yield: 83%; mp 37–39 °C.
IR (KBr): 3420 (NH), 2800, 1560, 1520, 1080 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 0.9 (t, J = 6.86 Hz, 3 H, CH3),
1.21–1.82 (m, 4 H, 2 CH2), 3.45 (t, J = 5.82 Hz, 2 H, CH2), 7.15–
7.94 (m, 5 H, ArH, NH).
N
Pr
N
N
O
H
N
i-Pr
Anal. Calcd for C12H14N3Cl: C, 61.15; H, 5.99; N, 17.83. Found: C,
61.33; H, 5.81; N, 17.68.
N
N
O
H
3-Chloro-N-(2-methylpropyl)quinoxalin-2-amine (2e)
Yield: 86%; mp 40–41 °C.
IR (KBr): 3400 (NH), 2800, 1540, 1520, 1080 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 0.95 (d, J = 6.62 Hz, 6 H, 2
CH3), 2.14 (m, 1 H, CH), 3.23 (d, J = 5.78 Hz, 2 H, CH2), 7.13–8.12
(m, 5 H, ArH, NH).
N
Bu
N
N
O
H
N
Anal. Calcd for C12H14N3Cl: C, 61.15; H, 5.99; N, 17.83. Found: C,
60.98; H, 5.98; N, 18.01.
i-Bu
N
N
O
H
3-Chloro-N-methylquinoxalin-2-amine (2f)
Yield: 95%; mp 89–90 °C.
IR (KBr): 3410 (NH), 1580, 1080, 750 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 3.07 (d, J = 5.25 Hz, 3 H,
CH3), 7.51–8.13 (m, 5 H, ArH, NH).
N
Me
a Reaction conditions: 2 (0.56 mmol), propargyl bromide (3, 1.12
mmol), CuI (0.056 mmol), PdCl2(PPh3)2 (0.028 mmol), morpholine
(2 mL), H2O (few drops), 70 °C.
Anal. Calcd for C9H8N3Cl: C, 55.83; H, 4.16; N, 21.70. Found: C,
55.68; H, 4.31; N, 21.88.
Pyrrolo[2,3-b]quinoxaline-2-carbaldehydes 4a–f; General Pro-
cedure
N-Alkyl-3-chloroquinoxalin-2-amines 2a–f; General Procedure
A mixture of 2,3-dichloroquinoxaline (0.01 mmol) and primary
alkylamine (0.02 mmol) in EtOH (5 mL) was heated under reflux
for 4 h. The solvent was evaporated under reduced pressure, and the
resultant solid was washed with H2O and dried. The crude product
obtained was of sufficient purity that further purification was not
necessary (Table 1).
A mixture of N-alkyl-3-chloroquinoxalin-2-amine 2a–f (0.56
mmol), PdCl2(PPh3)2 (0.028 mmol, 5 mol%), and CuI (0.056 mmol,
10 mol%) was stirred in morpholine (2 mL,) and H2O (few drops)
at 0 °C under an argon atmosphere. Propargyl bromide (1.12 mmol)
was then added slowly and the mixture was stirred at 70 °C for 20
h. When the reaction was complete, the resulting soln was concen-
trated in vacuo, 10% HCl (5 mL) was added, and the mixture was
extracted with CHCl3. The CHCl3 layer was separated and concen-
trated, and the crude product was subjected to column chromatog-
raphy (CHCl3–MeOH, 98:2) to afford the product (Table 2).
N-Benzyl-3-chloroquinoxalin-2-amine (2a)
Yield: 92%; mp 69–71 °C.
IR (KBr): 3400 (NH), 1550, 1500, 1065 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 4.75 (d, J = 6.12 Hz, 2 H,
1-Benzyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde (4a)
CH2), 7.12–8.23 (m, 10 H, ArH, NH).
IR (KBr): 1670 (C=O), 1530, 1500, 1430, 1093, 756 cm–1.
Anal. Calcd for C15H12N3Cl: C, 66.79; H, 4.48; N, 15.58. Found: C,
66.61; H, 4.61; N, 15.66.
1H NMR (500 MHz, DMSO-d6): d = 5.93 (s, 2 H, CH2), 7.18 (d,
J = 7.34 Hz, 2 H, 2 CH), 7.22 (t, J = 6.92 Hz, 1 H, CH), 7.28 (t,
J = 7.25 Hz, 2 H, 2 CH), 7.82 (t, J = 7.72 Hz, 1 H, CH), 7.88 (t,
J = 7.67 Hz, 1 H, CH), 7.91 (s, 1 H, CH pyrrole), 8.15 (d, J = 8.49
Hz, 1 H, CH), 8.25 (d, J = 8.42 Hz, 1 H, CH), 10.21 (s, 1 H, CHO).
3-Chloro-N-propylquinoxalin-2-amine (2b)
Yield: 80%; mp 43–45 °C.
IR (KBr): 3390 (NH), 2900, 1555, 1525, 1080 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 1.04 (t, J = 7.25 Hz, 3 H, CH3),
1.75 (m, 2 H, CH2), 3.56 (t, J = 6.57 Hz, 2 H, CH2), 7.11–7.84 (m,
5 H, ArH, NH).
Anal. Calcd for C18H13N3O: C, 75.25; H, 4.56; N, 14.62. Found: C,
75.64; H, 4.42; N, 14.75.
1-Propyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde (4b)
IR (KBr): 1675 (C=O), 1450, 1100, 765 cm–1.
Synthesis 2010, No. 10, 1599–1602 © Thieme Stuttgart · New York

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A. Keivanloo et al.
PAPER
1H NMR (500 MHz, DMSO-d6): d = 0.87 (t, J = 7.43 Hz, 3 H, CH3),
1.82 (m, J = 7.31 Hz, 2 H, CH2), 4.65 (t, J = 7.24 Hz, 2 H, CH2),
7.79 (t, J = 7.65 Hz, 1 H, CH), 7.83 (s, 1 H, CH pyrrole), 7.86 (m, 1
H, CH), 8.15 (d, J = 8.52 Hz, 1 H, CH), 8.22 (d, J = 8.50 Hz, 1 H,
CH), 10.24(s, 1 H, CHO).
References
(1) (a) Smits, R. A.; Lim, H. D.; Hanzer, A.; Zuiderveld, O. P.;
Guaita, E.; Adami, M.; Coruzzi, G.; Leurs, R.; de Esch, I. J.
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42, 4362.
Anal. Calcd for C14H13N3O: C, 70.28; H, 5.48; N, 17.56. Found: C,
69.92; H, 5.34; N, 17.68.
1-Isopropyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde (4c)
IR (KBr): 1675 (C=O), 1445, 1090, 775 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 1.71 (d, J = 6.92 Hz, 6 H, 2
CH3), 5.75 (m, J = 7.45 Hz, 1 H, CH), 7.80 (t, J = 7.65 Hz, 1 H,
CH), 7.85 (s, 1 H, CH pyrrole), 7.87 (t, J = 7.67 Hz, 1 H, CH), 8.15
(d, J = 8.5 Hz, 1 H, CH), 8.21 (d, J = 8.5 Hz, 1 H, CH), 10.23 (s, 1
H, CHO).
Anal. Calcd for C14H13N3O: C, 70.28; H, 5.48; N, 17.56. Found: C,
70.58; H, 5.58; N, 17.38.
1-Butyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde (4d)
IR (KBr): 1680 (C=O), 1450, 1090, 770 cm–1.
(3) (a) Kreher, R.; Use, G. Tetrahedron Lett. 1978, 19, 4671.
(b) Ames, D. E.; Mitchell, J. C.; Takundwa, C. C. J. Chem.
Res. 1985, 144. (c) Sugita, M.; Mitsuhashi, K. J. Heterocycl.
Chem. 1992, 29, 771. (d) Zhang, X.-C.; Huang, W.-y.
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T.; Irisawa, S.; Matsumoto, T.; Morikawa, O.; Konishi, H.
Chem. Lett. 1998, 551. (f) Kobayashi, K.; Matsumoto, T.;
Irisawa, S.; Yoneda, K.; Morikawa, O.; Konishi, H.
Heterocycles 2001, 55, 973.
1H NMR (500 MHz, DMSO-d6): d = 0.89 (t, J = 7.46 Hz, 3 H, CH3),
1.28 (m, J = 7.61 Hz, 2 H, CH2), 1.78 (m, J = 7.49 Hz, 2 H, CH2),
4.70 (t, J = 7.45 Hz, 2 H, CH2), 7.80 (t, J = 8.31 Hz, 1 H, CH), 7.85
(s, 1 H, CH pyrrole), 7.87 (t, J = 8.92 Hz, 1 H, CH), 8.16 (d, J = 8.61
Hz, 1 H, CH), 8.23 (d, J = 8.11 Hz, 1 H, CH), 10.24 (s, 1 H, CHO).
Anal. Calcd for C15H15N3O: C, 71.12; H, 5.97; N, 16.59. Found: C,
70.71; H, 6.16; N, 16.37.
(4) (a) Sonogashira, K. Comprehensive Organic Synthesis, Vol.
3; Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford,
1991, Chap. 2.4, 521. (b) Bumagin, N. A.; Sukhomlinova,
L. I.; Luzikova, E. V.; Tolstaya, T. P.; Beletskaya, I. P.
Tetrahedron Lett. 1996, 37, 897. (c) Hundertmark, T.;
Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett. 2000, 2,
1729. (d) Erdelyi, M.; Gogoll, A. J. Org. Chem. 2001, 66,
4165.
(5) (a) Paterson, I.; Davies, R. D. M.; Marquez, R. Angew.
Chem. Int. Ed. 2001, 40, 603. (b) Toyota, M.; Komori, C.;
Ihara, M. J. Org. Chem. 2000, 65, 7110. (c) Ioshimura, F.;
Kawata, S.; Hirama, M. Tetrahedron Lett. 1999, 40, 8281.
(d) Nicolaou, K. C.; Dai, W.-M. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1387.
1-(2-Methylpropyl)-1H-pyrrolo[2,3-b]quinoxaline-2-carbalde-
hyde (4e)
IR (KBr): 1680 (C=O), 1440, 1095, 775 cm–1.
1H NMR (500 MHz, DMSO-d6): d = 0.83 (d, J = 6.63 Hz, 6 H, 2
CH3), 2.20 (m, J = 6.82 Hz, 1 H, CH), 4.50 (d, J = 7.47 Hz, 2 H,
CH2), 7.78 (t, J = 7.34 Hz, 1 H, CH), 7.82 (s, 1 H, CH pyrrole), 7.85
(t, J = 7.43 Hz, 1 H, CH), 8.12 (d, J = 8.54 Hz, 1 H, CH), 8.20 (d,
J = 8.44 Hz, 1 H, CH), 10.25 (s, 1 H, CHO).
Anal. Calcd for C15H15N3O: C, 71.12; H, 5.97; N, 16.59. Found: C,
71.46; H, 5.83; N, 16.78.
1-Methyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde (4f)
IR (KBr): 1685 (C=O), 1450, 1090, 770 cm–1.
(6) (a) Heravi, M. M.; Keivanloo, A.; Rahimizadeh, M.;
Bakavoli, M.; Ghassemzadeh, M. Tetrahedron Lett. 2004,
45, 5747. (b) Heravi, M. M.; Kivanloo, A.; Rahimzadeh, M.;
Bakavoli, M.; Ghassemzadeh, M.; Neumüller, B.
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Isfahani, H. N.; Keivanloo, A.; Doostmohammadi, N.
Tetrahedron Lett. 2008, 49, 3819. (d) Bakherad, M.;
Isfahani, H. N.; Keivanloo, A.; Sang, G. Tetrahedron Lett.
2008, 49, 6188.
1H NMR (500 MHz, DMSO-d6): d = 4.13(s, 3 H, CH3), 7.77 (s, 1 H,
CH pyrrole), 7.78 (t, J = 7.65 Hz, 1 H, CH), 7.86 (t, J = 7.68 Hz, 1
H, CH), 8.15 (d, J = 8.52 Hz, 1 H, CH), 8.21 (d, J = 8.52 Hz, 1 H,
CH), 10.23 (s, 1 H, CHO).
Anal. Calcd for C12H9N3O: C, 68.24; H, 4.29; N, 19.89. Found: C,
67.78; H, 4.47; N, 19.66.
Acknowledgment
(7) (a) Li, J. J. J. Org. Chem. 1999, 64, 8425.
(b) Andreichikov, Yu. S.; Pitirimova, S. G.; Saraeva, R. F.;
Goleneva, A. F. Pharm. Chem. J. (Engl. Transl.) 1979, 13,
1141. (c) Cacchi, S.; Fabrizi, G.; Parisi, L. M.; Bernini, R.
Synlett 2004, 287.
We are grateful to the Research Council of Shahrood University of
Technology for the financial support of this work.
Synthesis 2010, No. 10, 1599–1602 © Thieme Stuttgart · New York