Synthesis of a polymer-capped palladium nanoparticles and its application as a reusable catalyst in oxidative coupling reaction of α-hydroxyketones and 1,2-diamines for preparation of pyrazines and quinoxalines

Article abstract of DOI:10.1007/s13738-018-1290-6

A novel method for the synthesis of pyrazines and quinoxalines has been developed using α-hydroxyketones and 1,2-diamines in the presence of cross-linked poly(4-vinylpyridine)-stabilized Pd(0) nanoparticles, [P4-VP]-PdNPs. The catalyst was easily prepared and characterized using various techniques such as FT-IR and UV–Vis spectroscopy, AAS, TEM, FESEM, EDX analysis and XRD. The results confirm a good dispersion of palladium nanoparticles on the polymer support. The catalyst displayed good catalytic activity when applied to the synthesis of quinoxalines via condensation of α-hydroxyketones with 1,2-diamines. A few pyrazine derivatives and various quinoxalines are prepared via coupling reaction of α-hydroxyketones and 1,2-diamines in high–excellent yields (81–99%) with short reaction times. The quinoxalines products were characterized by FT-IR, ¹H and ¹³C NMR spectroscopy, and the physical properties were compared to the literature values of known compounds. The advantages of the present method over conventional classical methods are rapid and very simple work-up, and the catalyst is reusable many times without a significant loss in its activity.

Full text of DOI:10.1007/s13738-018-1290-6

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1290-6
ORIGINAL PAPER
Synthesis of a polymer‑capped palladium nanoparticles and its
application as a reusable catalyst in oxidative coupling reaction
of α‑hydroxyketones and 1,2‑diamines for preparation of pyrazines
and quinoxalines
Mohammad Ali Karimi Zarchi1 · Seyed Shahab Addin Darbandizadeh Mohammad Abadi1
Received: 19 November 2017 / Accepted: 2 January 2018
© Iranian Chemical Society 2018
Abstract
A novel method for the synthesis of pyrazines and quinoxalines has been developed using α-hydroxyketones and 1,2-diamines
in the presence of cross-linked poly(4-vinylpyridine)-stabilized Pd(0) nanoparticles, [P4-VP]-PdNPs. The catalyst was easily
prepared and characterized using various techniques such as FT-IR and UV–Vis spectroscopy, AAS, TEM, FESEM, EDX
analysis and XRD. The results confrm a good dispersion of palladium nanoparticles on the polymer support. The catalyst
displayed good catalytic activity when applied to the synthesis of quinoxalines via condensation of α-hydroxyketones with
1,2-diamines. A few pyrazine derivatives and various quinoxalines are prepared via coupling reaction of α-hydroxyketones
and 1,2-diamines in high–excellent yields (81–99%) with short reaction times. The quinoxalines products were character-
ized by FT-IR, 1H and 13C NMR spectroscopy, and the physical properties were compared to the literature values of known
compounds. The advantages of the present method over conventional classical methods are rapid and very simple work-up,
and the catalyst is reusable many times without a signifcant loss in its activity.
Keywords Pyrazine · Quinoxaline · Characterization · Benzoin · Palladium nanoparticle
Introduction
in macrocyclic receptors in molecular recognition [9]. In
addition, quinoxaline derivatives have also found applica-
tions in dyes, organic semiconductors, chemically control-
lable switches and efcient electron luminescent materials
[10–13]. Considering their chemical and biological signif-
cance, numerous synthetic strategies have been developed
for the preparation of quinoxaline derivatives and the vast
scope of synthesized quinoxaline and derivatives potential
is well referenced and published in a wide range of scientifc
journals. Scientifc data concerning the potential relevance
of quinoxaline and derivatives in the literature were ana-
lyzed. The most common method for the synthesis of qui-
noxaline derivatives is the coupling reaction of 1,2-dicarbo-
nyles and o-phenylenediamines in the presence of an acidic
catalysts [14–18]. Also, numerous methods are available for
the synthesis of quinoxaline derivatives which involve con-
densation of 1,2-diamines with α-diketones [19–21], oxida-
tive coupling of epoxides with ene-1,2-diamines [22, 23]
and cyclization–oxidation of phenacyl bromides [24, 25].
Also, α-hydroxyketones react with o-phenylenediamines in
the presence of transition metals such as Ru and Pd to give
Quinoxaline derivatives are an important class of heterocy-
clic compounds, in which two nitrogen atoms are replaced
by carbon atoms at 1 and 4 positions of the naphthalene ring.
The study of quinoxaline and its derivatives has become a
subject of interest in recent years due to their wide variety
of biological activities as well as therapeutic applications
[1, 2]. For example, they can be used against bacteria, fungi,
virus, leishmania, tuberculosis, malaria, cancer, depression,
neurological activities, among others [3–6].
Also, quinoxaline moieties are present in the structure of
various antibiotics such as echinomycin, levomycin and acti-
noleutin, which are known to inhibit the growth of gram-pos-
itive bacteria, and they are active against various transplant-
able tumors [7, 8]. They also serve as useful rigid subunits
* Mohammad Ali Karimi Zarchi
makarimi@yazd.ac.ir
1
Department of Chemistry, College of Science, Yazd
University, P. O. Box 89195-741, 8915813149 Yazd, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
quinoxalines [26]. However, many of these processes sufer
from one or more limitations such as low product yields,
tedious work-up procedures, drastic reaction conditions, the
use of toxic metal salts as catalysts, and relatively expensive
reagents.
coupling reaction of α-hydroxyketones and 1,2-diamines.
Hence, in continuation of our studies on organic transfor-
mations using polymer-supported catalysts, reagents and
scavengers based on commercially available cross-linked
poly(4-vinylpyridine) [32–50], in the present study, we wish
to report preparation, characterization and catalytic applica-
tion of cross-linked poly(4-vinylpyridine)-supported palla-
dium nanoparticles abbreviated as [P4-VP]-PdNPs] as a new
recyclable catalyst in organic transformations. Also, herein
we wish to report a one-pot oxidation–condensation method
for the synthesis of pyrazines and quinoxalines from read-
ily available benzoins and 1,2-diamines by heating in DMF
(Scheme 1).
Palladium nanoparticles used in C–C coupling reac-
tions are usually formed by metal salt reduction and are
generally stabilized by polymeric molecules, tetraalkylam-
monium salts and ionic liquids. Very recently, palladium
dispersions containing small metal particles (20–80 Å)
have been of increasing scientifc interest as a colloidal
catalyst for organic C–C bond formation reactions (Heck
reactions) [27, 28]. In recent years, the polymeric catalysts,
reagents and scavengers have been widely applied in organic
transformations [29–69]. Also there are many reports on
polymer-supported palladium in the literature [51–69].
Poly(vinylpyrrolidone) (PVP) has been shown to be an efec-
tive matrix in stabilizing palladium nanoparticles as catalyst
for cross-coupling reactions such as coupling of aryl iodides
with arylboronic acid [52, 53], Suzuki reactions [54–57],
Sonogashira reaction [58], Heck and carbonylation reactions
[55, 59, 60], hydrogenation of 2-ethyl-9,10-anthraquinone
[61], electroless copper deposition [62], nitrate and nitrite
reduction [63] and H–D exchange between N-heterocyclic
compounds and D2O [64]. Palladium nanoparticles sup-
ported on poly(N-vinylpyrrolidone)-grafted silica as new
recyclable catalyst for Heck cross-coupling reactions also
was reported by Tamami et al. [66]. Also in 2011, Iyer and
co-workers reported synthesis of palladium nanoparticles
assembled in a polymeric nanosphere as catalyst for the Miz-
oroki–Heck cross-coupling reaction [67]. In 2014, Kaur and
Shah reported the microwave-assisted fuoride-free Hiyama
cross-coupling reaction catalyzed by Pd (0)–PVP nanopar-
ticles [68]. In 2014, Leferts and co-workers reported PVP-
stabilized Pd nanoparticles as catalyst for nitrite hydrogena-
tion in aqueous phase [69].
Experimental
Material
Chemicals were either prepared in our laboratory or were
purchased from Fluka (Buchs, Switzerland), Aldrich (Mil-
waukee, WI) and Merck chemical companies and were
used without further purifcation. Poly(4-vinylpyridine)
cross-linked with 2% divinyl benzene (DVB), [P4-VP] 2%
DVB, was purchased from Fluka company (Busch, Switzer-
land), and [P4-VP]-PdNPs was prepared in our laboratory
as described below and characterized by FT-IR, ultravio-
let–visible spectrophotometry (UV–Vis), atomic absorp-
tion spectrometer (AAS) transmission electron microscopy
images (TEM), feld emission scanning electron microscopy
(FESEM) images, energy-dispersive X-ray analysis spectros-
copy (EDX) and X-ray powder difraction (XRD). The reac-
tions were monitored by thin layer chromatography (TLC)
using silica gel Poly Gram SIL G/UV 254 plates. All prod-
ucts were characterized by comparison of their FT-IR, pro-
ton and carbon thirteen nuclear magnetic resonance (1H– and
13C NMR) spectra and physical data with pure compounds.
All yields refer to the isolated products. The FT-IR spectra
were obtained with a Bruker Equinox (model 55, Germany),
and the NMR spectra were recorded on a Bruker AC 400
Avance DPX spectrophotometer (Germany) at 400 MHz for
1H and at 100 MHz for 13C NMR in CDCl3 solutions and
on a Bruker (DRX-500 AVANCE) 500 MHz spectrometer
at 500 MHz for 1H and at 125 MHz for 13C NMR in CDCl3
(using tetramethylsilane as internal reference). Capacity
The recent developments in polymer-supported reactions
have led to the propagation of combinatorial chemistry as
a method for the rapid and efcient preparation of novel
functionalized molecules [29]. A fast growing branch of this
area is polymer-supported catalysts, reagents and scavengers
[30]. Although there are numerous applications of polymeric
catalysts in literature but, to the best of our knowledge, there
is no report for the synthesis of quinoxaline derivatives by
using polymer-supported palladium particles via oxidative
Scheme 1 Synthesis of pyra-
zines and quinoxalines
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Journal of the Iranian Chemical Society
measurements were carried out with an Analytik Jena nova
300 (model 330, Germany) atomic absorption spectrom-
eter utilizing an air-acetylene fame atomizer. The UV–Vis
absorption spectra were recorded on an Avantes photodiode
array spectrophotometer model AvaSpec-2048 equipped
with a source model of AvaLight-DH-S-BAL. TEM images
were recorded using ZEISS 10A conventional TEM model
Carl Zeiss-EM10C-100 kV (Germany). FESEM images
were recorded using FESEM MIRA 3-XMU with a high-
energy beam of electrons from Razi Metallurgical Research
Center. EDX analysis was performed using ZEISS (model
SIGMA VP-500, Germany) with Oxford Instrument detector
(England). XRD patterns were recorded on a PANalytical
(model X’Pert PRO) X-ray difractometer. The uncorrected
melting points of compounds were taken by a Buchi melting
point B-540 B.V. CHI apparatus.
by TLC, eluent: n-hexane/EtOAc = 9/1), the catalyst was
separated and washed with EtOH (2 × 2 mL). After the addi-
tion of water, the product was precipitated with high purity.
The pure quinoxalines were obtained in 95% isolated yield.
The characteristic spectral data of some quinoxalines
products are given below, and the FT-IR, 1H and 13C NMR
spectra of 6-benzoyl-2,3-difuran-2-yl-quinoxaline (3i), for
example, are given in Fig. 1a–c.
2,3‑Diphenylquinoxaline (3a) White solid, m.p. = 127–
128 °C (lit; 127–129 [70], 126–127 °C [71] and 128–129 °C
[72]); FT-IR (neat), νmax (cm−1) = 3053, 3026, 1620, 1556,
1533, 1483, 1445, 1344, 1308, 1202, 1138, 1061, 1022, 980,
833, 817, 775, 752, 704, 696, 596, 546; 1H NMR (400 MHz,
CDCl3), δ (ppm) = 8.19–8.23 (m, 2H), 7.78–7.83 (m, 2H),
7.52–7.57 (m, 4H), 7.34–7.41 (m, 6H); 13C NMR (100 MHz,
CDCl3), δ (ppm) = 153.49, 141.25, 139.09, 129.98, 129.85,
129.22, 128.82, 128.29.
Preparation of [P4‑VP]‑PdNPs
The [P4-VP]-PdNPs were prepared by the following pro-
cedure: In the frst step, aqueous solution of H2PdCl4 was
prepared by mixing 106.4 mg of PdCl2 (0.6 mmol), 6.0 mL
of 0.2 M HCl and 294 mL of distilled water. In the second
step, a mixture of 15 mL solution of H2PdCl4 (prepared in
the frst step), 21 mL of methanol and 0.2668 g of [P4-VP]
2% DVB was refuxed in a 100-mL fask for 1 h. In the
fnal step, 15 ml of 0.015 M solution of sodium borohydride
in methanol was added into the above mixture dropwise
immediately. The abrupt color change from pale green to
dark brown indicates that Pd2+ reduced to Pd(0) nanoclus-
ters stabilized by [P4-VP] 2% DVB, and consequently, the
[P4-VP]-PdNPs were formed. Methanol was removed from
the solution by evaporation in a rotary evaporator (Heidolph
Laborata-4000), and [P4-VP]-PdNPs were washed with
excess acetone to remove the excess residuals.
6‑Methyl‑2,3‑diphenylquinoxaline (3b) White solid,
m.p. = 115–117 °C (lit. 116–117 °C [71] and 117–118 °C)
[72]; FT-IR (neat), νmax (cm−1) = 3055, 2924, 2853, 1556,
1541, 1440, 1346, 1219, 1059, 978, 770, 760, 731, 700,
1
600, 542; H NMR (400 MHz, CDCl3), δ (ppm) = 2.6 (s,
3H), 7.32 (m, 6H), 7.52 (m, 4H), 7.57 (d, J = 8.75 Hz, 1H),
7.96 (s, 1H), 8.05 (d, J = 8.5 Hz, 1H); 13C NMR (100 MHz,
CDCl3), δ = 21.9, 127.1, 128.2, 128.6, 128.7, 129.8, 132.3,
139.2, 139.7, 140.4, 141.3, 152.5, 153.3.
6‑Nitro‑2,3‑diphenylquinoxaline (3c) White solid,
m.p. = 193–194 °C (lit. 192–193 °C [71], 193–194 °C [72]
and 183 °C [73]); FT-IR (neat), νmax (cm−1) = 3090, 3055,
2953, 2924, 2853, 1616, 1570, 1520, 1466, 1435, 1398,
1336, 1310, 1285, 1244, 1207, 1186, 1163, 1128, 1072,
1055, 1026, 980, 907, 812, 798, 745, 700, 635, 598, 540; 1H
NMR (400 MHz, CDCl3), δ (ppm) = 7.43 (m, 6H), 7.60 (t,
J = 6.5 Hz, 4H), 8.33 (d, J = 9.1 Hz, 1H), 8.56 (dd, J = 9.1,
2.2 Hz, 1H), 9.19 (d, J = 2.3 Hz, 1H); 13C NMR (100 MHz,
CDCl3), δ (ppm) = 123.73, 126.06, 128.89, 130.07, 130.20,
130.26, 130.33, 131,192, 138.47, 138.54, 140.42, 144.02,
148.32, 156.13, 156.75.
General procedure for synthesis of pyrazine
or quinoxaline derivatives
A mixture of benzoin (1 mmol), o-phenylenediamine
(1.1 mmol) and [P4-VP]-PdNPs (120 mg) was refuxed in
DMF (3 mL). After completion of reaction (monitored by
TLC, eluent: n-hexane/EtOAc = 9/1), the catalyst was sepa-
rated and washed with EtOH (2 × 2 mL). After addition of
water the product was precipitated with high purity. The
pure quinoxalines were obtained in 81–99% isolated yields.
6‑Benzoyl‑2,3‑diphenylquinoxaline (3d) White solid,
m.p. = 149–151 °C (lit. 139–140 °C [73]); FT-IR (neat),
νmax (cm−1) = 3055, 1661, 1597, 1445, 1346, 1321, 1310,
1288, 1265, 1246, 1225, 1198, 1180, 1124, 1074, 1057,
1022, 980, 907, 891, 793, 770, 733, 716, 696, 675, 600,
452; 1H-NMR (400 MHz, CDCl3), δ (ppm) = 7.30 (m, 6H),
7.47 (m, 7H), 7.83 (m, 2H), 7.83 (dd, J = 8 and 1.4 Hz, 2H),
8.21 (m, 2H), 8.47 (s, 1H); 13C-NMR (100 MHz, CDCl3),
δ (ppm) = 128.41, 128.55, 129.21, 129.36, 129.70, 129.79,
129.92, 130.16, 132.46, 132.85, 137.18, 138.36, 138.52,
138.58, 140.19, 142.92, 154.63, 155.14, 195.82.
Synthesis of 2,3‑diphenylquinoxaline as a typical
procedure (3a)
A mixture of benzoin (1 mmol), o-phenylenediamine
(1.1 mmol) and [P4-VP]-PdNPs (120 mg) was refuxed in
DMF (3 mL). After the completion of reaction (monitored
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Journal of the Iranian Chemical Society
Fig. 1 FT-IR, 1Hand 13C NMR spectra of 6-benzoyl-2,3-difuran-2-yl-quinoxaline (a–c)
2,3‑Dihydro‑5,6‑diphenylpyrazine (3e) White solid,
m.p. = 160–162 °C (lit. 158 °C [17] and 160–163 [70]);
FT-IR (neat), νmax (cm−1) = 3090, 3055, 2953, 2924, 1616,
1570, 1520, 1466, 1435, 1398, 1338, 1309, 1284, 1244,
1207, 1186, 1163, 1128, 1072, 1055, 1026, 980, 906,
6‑Methyl‑2,3‑difuran‑2‑yl‑quinoxaline (3g) Pale brown solid,
m.p. = 119–120 °C (lit. 119–120 °C [70]); FT-IR (neat),
νmax (cm−1) = 3111, 2922, 1618, 1570, 1533, 1491, 1458,
1404, 1337, 1217, 1167, 1150, 1134, 1076, 1062, 1018,
991, 912, 887, 829, 820, 800, 758, 748, 598, 582, 490; 1H-
NMR (400 MHz, CDCl3), δ (ppm) = 2.52 (s, 3H), 6.48–6.49
(m, 2H), 6.5 (m, 2H), 7.51 (dd, J = 8.6, 1.8 Hz, 2H), 7.54
(m, 1H), 7.84 (s, 1H), 7.95 (d, J = 8.4 Hz, 1H); 13C-NMR
(100 MHz, CDCl3), δ (ppm) = 21.95, 111.87, 111.89,
112.58, 112.83, 127.99, 128.64, 132.80, 139.12, 140.74,
141.14, 141.87, 142.62, 144.01, 144.14, 150.92.
1
812, 768, 745, 700, 635, 598, 540; H NMR (400 MHz,
CDCl3), δ (ppm) = 3.73 (s, 4H), 7.28 (t, J = 7.5 Hz, 4H),
7.34 (t, J = 7 Hz, 2H), 7.43 (d, J = 7.5 Hz, 4H); 13C NMR
(100 MHz, CDCl3), δ (ppm) = 46.25, 128.33, 128.54,
130.05, 138.21, 160.73.
2,3‑Difuran‑2‑yl‑quinoxaline (3f) Pale brown solid,
m.p. = 132–134 °C (lit. 132–134 °C [70]); FT-IR (neat),
νmax (cm−1) = 3107, 1570, 1537, 1479, 1400, 1337, 1327,
1163, 1128, 1076, 1059, 1034, 1009, 993, 912, 762, 752,
596, 494; 1H-NMR (400 MHz, CDCl3), δ (ppm) = 6.59 (m,
2H); 6.7 (d, J = 3.31 Hz, 2H), 7.6 (d, J = 1.02 Hz, 2H),
7.78 (m, 2H); 8.17 (m, 2H); 13C NMR (125 MHz, CDCl3),
δ (ppm) = 112.35, 113.43, 129.56, 130.83, 141.074, 143.09,
144.66, 151.25.
6‑Nitro‑2,3‑difuran‑2‑yl‑quinoxaline (3h) Orange solid,
m.p. = 167–169 °C (lit. 171–173 °C [70] and 164–166 °C
[73]); FT-IR (neat), νmax (cm−1) = 3119, 2924, 1616, 1566,
1519, 1485, 1402, 1387, 1346, 1296, 1254, 1215, 1157,
1080, 1063, 1022, 995, 951, 889, 841, 826, 754, 733, 592,
484; 1H-NMR (400 MHz, CDCl3), δ (ppm) = 6.65 (m, 2H),
6.85 (dd, J = 3.4, 0.6 Hz, 1H), 6.91 (dd, J = 3.4, 0.6 Hz, 1H),
7.70 (m, 2H), 8.25 (d, J = 9.2 Hz, 2H), 8.51 (dd, J = 9.2,
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Journal of the Iranian Chemical Society
3.2 Hz, 2H), 9.04 (d, J = 3.2 Hz, 1H); 13C NMR (100 MHz,
CDCl3), δ (ppm) = 112.31, 112.44, 114.51, 115.37, 123.66,
125.37, 130.47, 139.26, 143.02, 144.23, 144.73, 145.01,
145.49, 147.95, 149.40, 150.17.
1306, 1296, 1260, 1248, 1227, 1173, 1161, 1026, 835, 725,
712.
5,6‑Bis‑(4‑methoxyphenyl)‑2,3‑dihydro‑pyrazin (3n) Very
pale brown solid, m.p. = 157–160 °C; FT-IR (neat), νmax
(cm−1) = 3055, 1556, 1541, 1479, 1346, 1219, 1059, 978,
770, 760, 731, 700, 600, 542.
6‑Benzoyl‑2,3‑difuran‑2‑yl‑quinoxaline (3i) Pale brown solid,
m.p. = 119 °C (lit. 118–120 °C [70]); FT-IR (neat), νmax
(cm−1): 3130, 3107, 1657, 1614, 1597, 1570, 1533, 1485,
1406, 1341, 1310, 1294, 1267, 1256, 1227, 1204, 1155,
1134, 1117, 1087, 1067, 1032, 1018, 912, 874, 766, 750,
Results and discussion
1
731, 710, 698, 648, 594; H-NMR (400 MHz, CDCl3), δ
(ppm) = 6.60–6.63 (m, 2H), 6.76–6.79 (m, 2H), 7.53–7.57
(m, 3H), 7.64–7.69 (m, 2H), 7.90–7.92 (m, 2H), 8.27 (d,
J = 1.6, 2H), 8.5 (t, J = 1.2, 1H); 13C NMR (100 MHz,
CDCl3), δ (ppm) = 112.09, 112.22, 113.62, 114.26, 128.55,
129.59, 130.11, 132.29, 132.84, 137.11, 138.55, 139.54,
142.42, 143.47, 143.97, 144.56, 144.89, 150.52, 195.67.
The palladium nanoparticles immobilized on cross-linked
poly(4-vinylpyridine) was easily prepared in the two-step
procedure. In the frst step, aqueous solution of H2PdCl4
was prepared by mixing PdCl2, HCl and distilled water (step
1, Scheme 2). Then, methanol and [P4-VP] 2% DVB were
added to the above mixture and refuxed for 1 h. The UV–Vis
electronic absorption spectrum of this solution shows that no
reduction of Pd2+ species to Pd(0) occurs during the refux
(Fig. 2).
2,3‑Bis(4‑methoxyphenyl)quinoxaline (3j) White solid,
m.p. = 149–151 °C (lit. 151–152 °C [70], 148–150 °C
[73] and 151–152.5 °C [72, 74]); FT-IR (neat), νmax
(cm−1) = 3063, 3006, 2967, 2887, 2837, 1665, 1601,
1574, 1536, 1512, 1477, 1458, 1440, 1423, 1392, 1346,
1296, 1284, 1244, 1224, 1170, 1161, 1138, 1109, 1086,
1068, 1028, 977,879, 829, 804, 786, 733, 670, 610, 588,
633; 1H NMR (500 MHz, CDCl3), δ (ppm) = 3.8 (s, 6H),
6.91 (d, J = 6.73 Hz, 4H), 7.53 (d, J = 6.63 Hz, 4H), 7.76
(dd, 2H), 8.16 (dd, 2H); 13C NMR (125 MHz, CDCl3), δ
(ppm) = 55.74, 114.21, 129.44, 129.95, 131.68, 132.17,
141.51, 153.46, 160.60.
We also performed experiments to understand the efect
of refux in methanol on reduction of Pd2+ precursor vary-
ing the refux time up to 3 h, but there was no reduction
observed. It is clearly seen that refux in methanol alone
is not enough to reduce Pd2+ precursor to zero valent pal-
ladium. Therefore, sodium borohydride solution in metha-
nol was added dropwise to the mixture to reduction of Pd2+
precursors to Pd(0) occurs. An abrupt color change was
observed from pale yellow to dark brown immediately upon
the addition of sodium borohydride, indicating decreasing
cluster size and the formation of Pd(0) nanoclusters on the
surface or in the pores of the polymer. But, when refux
in ethanol was used instead of methanol, we observed that
reduction of Pd2+ species to Pd(0) occurs during the refux
and stabilized by polymer to produce the [P4-VP]-PdNPs
catalyst and we also observed (as expected) the C=O stretch-
ing frequency of acetaldehyde at 1655 cm−1 in the FT-IR
spectrum of the fltrates after the catalyst was separated by
fltration. Our initial investigation on the evaluation of the
catalytic activity of [P4-VP]-PdNPs started with the coupling
reaction between α-hydroxyketones and 1,2-diamines for
synthesis of quinoxalines, and we found that [P4-VP]-PdNPs
are a suitable and reusable catalyst for synthesis of qui-
noxalines (step 3, Scheme 2) and also we found that the
[P4-VP]-PdNPs are stable for months at room temperature in
air and can be recovered and reused several cycles.
6‑Methyl‑2,3‑bis(4‑methoxyphenyl)quinoxaline (3k) White
solid, m.p. = 128–130 °C (lit, 123–126 °C [70] and 125–
127 °C [72, 74]); FT-IR (neat), νmax (cm−1) = 2953, 2926,
2837, 1655, 1607, 1573, 1512, 1187, 1458, 1344, 1288,
1248, 1178, 1163, 1028, 833; 1H NMR (400 MHz, CDCl3),
δ (ppm) = 2.52 (s, 3H), 3.75 (s, 6H), 6.79 (d, J = 7.2, 4H),
7.40 (d, J = 7.2, 4H), 7.47 (d, J = 7.6 Hz, 1H), 7.84 (s, 1H),
7.93 (d, J = 7.6 Hz, 1H); 13C NMR (100 MHz, CDCl3),
δ (ppm) = 21.89, 55.31, 113.74, 127.88, 128.53, 131.20,
131.24, 132.87, 139.54, 140.00, 141.14, 152.17, 152.90,
160.01.
6‑Nitro‑2,3‑bis(4‑methoxyphenyl)quinoxaline (3l) Orange
solid, m.p. = 192–193 °C (lit; 192–194 °C [72] and 192–
194 °C [74]); FT-IR (neat), νmax (cm−1) = 3005, 2926, 2853,
1607, 1574, 1516, 1466, 1435, 1342, 1305, 1294, 1251,
1177, 1020, 978, 847, 826, 679, 538.
The catalyst was characterized using several techniques.
The morphology and structure of the samples were exam-
ined by UV–Vis absorption spectra (Fig. 2), FT-IR spectros-
copy (Fig. 3), TEM images (Fig. 4a, b), FESEM (Fig. 5a–c),
EXD pattern (Fig. 5d) and XRD spectroscopy (Fig. 6) and
AAS experimental techniques.
6‑Benzoyl‑2,3‑bis(4‑methoxyphenyl)quinoxaline (3m) White
solid, m.p. = 147–148 °C; FT-IR (neat), νmax (cm−1) = 2955,
2924, 2853, 1653, 1605, 1574, 1512, 1458, 1348, 1334,
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Journal of the Iranian Chemical Society
Scheme 2 Synthesis of [P4-VP]-PdNPs
Figure 2 shows the UV–Vis absorption spectra of (a)
H2PdCl4, (b) [P4-VP] 2% DVB after refuxed in methanol
(1 h) in the presence of H2PdCl4 before the addition of
sodium borohydride [(I), in Scheme 2] and (c) polymer after
addition of sodium borohydride, [P4-VP]-PdNPs (Scheme 2).
As Fig. 2 reveals, two absorption bands observed at 250 and
325 nm attributed to the [PdCl4]2− anions (a and b) that
disappeared when sodium borohydride was added to the
mixture (c). This disappearance indicates that the reduc-
tion of Pd2+ precursor to Pd(0) occurs and stabilized by
[P4-VP] 2% DVB, and consequently, the [P4-VP]-PdNPs
catalyst forms. The free pyridine pendant group of [P4-VP]
2% DVB can also be protonated by adsorbing the liber-
ate HCl as a toxic waste. The structures of [P4-VP] 2%
DVB and [P4-VP]-PdNPs and the simplifed structure of
[P4-VP]-PdNPs are shown in Scheme 2.
Vibrational spectroscopy is a suitable tool for charac-
terization of polymer-transition metal ions systems. It is
known that the C=N and C=C stretching frequencies in
vinylpyridine polymers shift to higher frequencies upon
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Journal of the Iranian Chemical Society
corresponding salt (I) and [P4-VP]-PdNPs in Scheme 2. The
formation of the polymer salt is approved with observation
of the N–H stretching frequency at 3415 cm−1 in the FT-IR
of [P4-VP]-PdNPs (b, in Fig. 3).
During our study, we found that, 1-h refux is necessary
for good dispersion of the polymer in the mixture to pro-
vide better interaction between the metal and the stabilizer
to prevent agglomeration after reduction. When H2PdCl4 in
methanol is refuxed in the presence of [P4-VP] 2% DVB,
probably the complexation takes place between Pd2+ ions
and the nitrogen atoms of the pyridine pendant group of
polymer [(I) in Scheme 2], and this can be to prevent the
agglomeration after reduction of Pd2+ to Pd(0) occurs by
NaBH4. [P4-VP]-PdNPs could be isolated as a dark brown
solid from the reaction mixture by removing the volatiles in
vacuum. Also as Fig. 3 reveals, the C=C (1414 cm−1) and
C=N (1595 cm−1) stretching frequencies of pyridine pendant
groups in [P4-VP] 2% DVB (a, in Fig. 3) and [P4-VP]-PdNPs
(b, in Fig. 3) are the same, and it can be concluded that the
Pd2+ ions reduced to Pd(0) and absorbed on the surface or in
the pores of the polymer, while upon polymer complexation
with Pd2+ ions, the C=N and C=C stretching frequencies in
vinylpyridine of catalyst must be shifted to higher frequen-
cies as reported by Hasik et al. [71].
Fig. 2 UV–Vis absorption spectra of a H2PdCl4, b [P4-VP] 2% DVB
after refuxed (1 h) in H2PdCl4 solution before the addition of sodium
borohydride [(I), in Scheme 2] and c [P4-VP]-PdNPs
complexation with metal ions such as Cu2+ [37, 75] and
Pd2+ ions [76]. Hasik et al. have studied the palladium–nitro-
gen-containing polymers interactions, and they have been
found that, in the PdCl2 solutions of high HCl concentra-
tion containing [PdCl4]2− and [PdCl3(H2O)]− as the major
species acid–base type reaction (protonation) as well as
the coordination of Pd2+ ions by the nitrogen atoms of the
polyvinylpyrrolidone take place. In the present study also,
the nitrogen atoms of the pyridine rings in the structure of
[P4-VP] 2% DVB constitute potential sites for polymer–pal-
ladium complexes interactions and also can be protonated
by adsorbing the liberates HCl as a toxic waste to give the
In order to obtain the microstructure of the
[P4-VP]-PdNPs nanocomposites, the typical TEM images
(high resolution) are clearly shown in (Fig. 4a, b). It could be
seen that the Pd nanoparticles in the range of 4–6 nm were
uniformly attached on the surface of polymer.
The XRD diffraction patterns of [P4-VP]-PdNPs are
shown in Fig. 4c. Three observed peaks in XRD pattern at
2θ = 39, 46 and 69 are attributed to elemental palladium.
The difraction peaks were broadened owing to the very
small crystallite size. The random orientation of particles
Fig. 3 FT-IR spectra of a
[P4-VP] 2% DVB and b
[P4-VP]-PdNPs
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Journal of the Iranian Chemical Society
Fig. 4 a, b TEM images, c XRD difractogram of Pd nanoparticles in [P4-VP]-PdNPs and d nanoparticles size distributions
allows for a statistical measure of the size distribution to be
generated (Fig. 4d).
form. But, when sodium borohydride is added, reduction
of Pd2+ to Pd(0) by sodium borohydride and stabilization
of Pd(0) by polymer occurs faster, and consequently, Pd
nanoparticles in the [P4-VP]-PdNPs have very small size.
In this case, the FESEM micrographs [(c) Fig. 5] show a
large number of uniformly distributed Pd nanoparticles
and have very small size distribution with a rough sur-
face. After immersing these catalysts in organic solvents
for a few days and then repeatedly washing with water
and organic solvents (ethanol, toluene and acetone), the
FESEM of these nanoparticles before and after the reac-
tion are basically identical.
The morphology of the [P4-VP]-PdNPs was studied by
FESEM (at magnifcation of 100 KX) image (Fig. 5). One
of the applications of FESEM includes small contamina-
tion feature geometry and elemental composition meas-
urement. The FESEM micrographs of [P4-VP] 2% DVB
after refuxed (1 h) in methanol in the presence of H2PdCl4
before the addition of sodium borohydride do not show
any Pd nanoparticles on the surface of the polymer [(a)
in Fig. 5], while when [P4-VP] 2% DVB is refuxed (1 h)
in ethanol in the presence of H2PdCl4 before addition of
sodium borohydride, the FESEM micrographs show the Pd
nanoparticles on the surface of the polymer [(b) in Fig. 5].
This means that Pd2+ reduces to Pd(0) by ethanol and sta-
bilizes by polymer, and consequently, [P4-VP]-PdNPs
EDX analysis of [P4-VP]-PdNPs was used to analyze the
composition of samples and is shown in Fig. 5e. This analy-
sis shows considerable content of Pd (2.3%).
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Journal of the Iranian Chemical Society
Fig. 5 FESEM images of a [P4-VP] 2% DVB after refuxed (1 h) in
methanol in the presence of H2PdCl4 before the addition of sodium
borohydride, b [P4-VP] 2% DVB after refuxed (1 h) in ethanol in
the presence of H2PdCl4 before the addition of sodium borohydride,
[P4-VP]-PdNPs, c [P4-VP] 2% DVB after refuxed (1 h) in methanol
in the presence of H2PdCl4 after the addition of sodium borohydride,
[P4-VP]-PdNPs and d EDX analysis of [P4-VP]-PdNPs
Fig. 6 Recovery studies: suc-
cessively catalyzed by the same
recovered [P4-VP]-PdNPs under
the same reaction conditions
(model reaction)
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Journal of the Iranian Chemical Society
However, the capacity of [P4-VP]-PdNPs was determined
by AAS using Analytik Jena nova 300 (model 330, Ger-
many). After the fltration of the mixture in step 2 for the
synthesis of [P4-VP]-PdNPs, the excessive palladium ions
in the fltrates determination were carried out on a Buck
Scientifc atomic absorption spectrometer (Model 210 VGP,
USA) with a hollow cathode lamp using air-acetylene fame.
The single-line fow-injection system consisting of peristaltic
pump (Ismatic, MS-REGLO/8-100, Switzerland) and rotary
injection valve (Rheodyne, CA, USA) with a loop of 100-μL
capacity was used for the efective control of the amount of
sample and reproducibility of the measurements. The absorb-
ance time response was monitored on an x-t-chart recorder
(L-250), and consequently, the capacity of the polymer was
determined to be 0.10 mmol of Pd g−1 of the polymer.
After characterization of the catalyst structure, its cata-
lytic activity was studied in coupling reaction between
1,2-diketones and 1,2-diamines for synthesis of pyrazines
and quinoxalines and we found that [P4-VP]-PdNPs is as a
suitable and reusable catalyst for synthesis of pyrazines and
quinoxalines (Scheme 1).
model reaction behavior was studied under a variety of con-
ditions, and the results are summarized in Table 1. First, we
began to test in diferent solvents such as DMF (3 mL) in
diferent temperatures and refux in 3 mL of other solvents
such as ethanol, methanol, acetonitrile, dichloromethane and
chloroform, and the results are summarized in Table 1. Dur-
ing our optimization studies, it was found that 120 mg of
[P4-VP]-PdNPs in 3 mL of DMF at 120 °C was the best con-
ditions for complete conversion in a short reaction time, to
achieve the highest yield of 2,3-diphenylquinoxaline product
(Entry 3). The efciency of the [P4-VP]-PdNPs compared to
various catalysts was also examined (Table 1, entries 14–19),
and it was found that [P4-VP]-PdNPs were an efcient and
superior catalyst compared to other heterogeneous catalysts
with respect to time and yield of product. During our optimi-
zation studies, we also found that the [P4-VP]-PdNPs (a dark
brown color catalyst) are stable for months at room tempera-
ture. As Table 1 reveals, Pd(OAc)2 and PdCl2 reacted faster
than [P4-VP]-PdNPs (entries 14 and 15) as expected, but
they were not reusable catalysts.
After the optimized reaction conditions, the condensation
reaction of 1,2-diamines such as ethylenediamine and dif-
ferent o-phenylenediamines with α-hydroxyketones such as
benzoin, furoin and anisoin was explored for the synthesis
of a few pyrazine derivatives and various quinoxalines, and
the results are summarized in Table 2. We have also suc-
cessfully applied this new method on a nearly large scale;
In order to increase the yields of products, optimiza-
tion of the reaction conditions was accomplished. Benzoin
(1 mmol) and o-phenylenediamine (1.1 mmol), in the pres-
ence of [P4-VP]-PdNPs under heating in dimethyl forma-
mide (DMF), were chosen as model reaction conditions and
were converted to 2,3-diphenylquinoxaline product. The
Table 1 Synthesis of
2,3-diphenylquinoxaline
using benzoin and
Entry
Catalyst (mol%)
Solvent
Temperature (°C)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.013 mol%)
[P4-VP]-PdNPs (0.011 mol%)
[P4-VP]-PdNPs (0.010 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
[P4-VP]-PdNPs (0.012 mol%)
Pd(OAc)2 (3)
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
EtOH
MeOH
CH3CN
CHCl3
CH2Cl2
DMF
DMF
DMF
DMF
DMF
DMF
90
100
110
120
130
110
110
110
Refux
Refux
Refux
Refux
Refux
110
110
110
110
110
5
89
92
95
90
85
95
90
91
91
80
82
85
60
97
97
70
80
92
90
1,2-phenylenediamine as a
model reaction under various
conditions
2.5
1.5
1.5
1.6
1.5
1.8
2
9
5
7
10
11
12
13
14
15
16
17
18
19
10
10
12
0.5
0.5
2.5
7
PdCl2 (2.3)
H2PdCl4 (3.4)
[P4-VP] 2% DVB (0.012 mol%)
[P4-VP]-ZnCl2 (0.012 mol%)
[P4-VP]-CuI (0.012 mol%)
4
4
110
All the reactions were performed using benzoin (1 mmol) and 1,2-phenylenediamine (1.1 mmol) under
heating in a solvent
aYields refer to isolated pure product
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Journal of the Iranian Chemical Society
Table 2 Synthesis of pyrazine and quinoxaline derivatives from α-hydroxyketones (1 mmol) and 1,2-diamines (1.1 mmol) using [P4-VP]-PdNPs
(0.0120 mol%) in DMF at 110 °C
Entry α-Hydroxyketone Diamine
Product
Time Yielda
(min) (%)
1
Benzoin
1.5
95
2
Benzoin
1.3
97
7
95
3
Benzoin
4
Benzoin
6
95
5
Benzoin
0.5
96
1.2
0.8
89
99
6
7
Furoin
Furoin
8
Furoin
5
97
4.5
7
95
94
9
Furoin
10
Anisoin
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Journal of the Iranian Chemical Society
Table 2 (continued)
11
12
13
Anisoin
Anisoin
Anisoin
5
94
87
81
11
12
14
Anisoin
0.5
98
aIsolated yields
for example, up to 15 mmoles of benzoin and o-phenylene-
diamine (Table 2, entry 1) could be converted into the corre-
sponding quinoxaline product (3a) in 95% isolated yield. In
general, all reactions were very clean and the products were
obtained in high–excellent yields (81–99%). All the products
were cleanly isolated with simple fltration and precipita-
tion in water. All products were characterized by FT-IR, 1H
and 13C NMR spectra, melting point and compared with
literature data.
As Table 2 reveals, both electron-rich and electron-def-
cient o-phenylenediamines were efective in this process.
When o-phenylenediamines are substituted at the 4-posi-
tion with electron-donating groups, higher rates and yields
are observed than the ones bearing electron-withdrawing
groups at that position (entries 2–4, 7–9 and 11–13). For
example, 4-nitro-1,2-phenylenediamine aforded the product
in 6 h with 95% yield, when reacted with benzoin (entry
4), with furoin in 4.5 h with 95% yield (entry 9) and with
Scheme 3 The plausible
mechanism of the reaction
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Journal of the Iranian Chemical Society
anisoin in 11 h with 87% yield (entry 12), while 4-methyl-
1,2-phenylenediamine aforded the product in 1.3 h with
97% yield (entry 2), with furoin in 0.8 h with 99% yield
(entry 7) and with anisoin in 5 h with 94% yield (entry 11).
All the reactions proceeded very efciently and no undesir-
able side-reactions were observed, although the yields were
highly dependent on the substrate used.
completion of the model reaction, the recovered catalyst was
washed with DMF and after dryness was reused in the next
similar run. This procedure was repeated for 6 consecutive
runs, and the results of this study are shown in Fig. 5. As
Fig. 5 reveals, when the polymer was reused after six runs in
the same reaction and under identical conditions, the isolated
yield of the product was decreased only a few percent (4.2%)
which indicates the low leaching amount of Pd catalyst into
the reaction mixture. No decomposition of the stabilizer was
observed in the reaction, and [P4-VP]-PdNPs are found to be
stable to metal leaching throughout the reaction. Moreover,
the recyclability of the supported catalyst is also important.
Based on the previous reports, our observations and
obtained results, the plausible mechanism of the reac-
tion catalyzed by [P4-VP]-PdNPs is shown in Scheme 3.
In path (A), [P4-VP]-PdNPs promoted the condensation
reaction of 1,2-diamine and α-hydroxyketone via cycliza-
tion and dehydration, and consequently, the corresponding
The main advantage of polymer-supported catalyst is its
insolubility in the reaction medium and consequently its
easier work-up by a simple fltration. A few pyrazine deriva-
tives such as 2,3-dihydro-5,6-diphenylpyrazine and 5,6-bis-
(4-methoxyphenyl)-2,3-dihydro-pyrazin and various quinox-
alines were prepared by coupling reaction of 1,2-diamines
and α-hydroxyketones. The process is experimentally simple
and appears to have enormous scope.
The reusability of the catalyst was investigated.
[P4-VP]-PdNPs can be easily recovered and reused many
times without signifcant loss of catalytic activity. After
Table 3 Comparison of the
model reaction with other
reported methods for synthesis
of 2,3-diphenylquinoxaline
Entry
Reaction conditions
Time (h)
Yield (%)a
References
1
2
3
4
5
6
7
8
[(NH4)6Mo7O24·4H2O-PEG 300], MWIb (420 W)/90 °C
AcOH, refux
Ga(ClO4)3, EtOH, refux
0.25
2
4
4
4
4
4
26
6
1
3
3
3
3
98
96
70
85
93
83
91
96
93
47
20
30
50
95
25
87
95
95
95
34
74
92
72
95
[14]
[19]
[77]
[78]
[78]
[78]
[79]
[80]
[81]
[82]
[83]
[83]
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[85]
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[86]
[87]
[87]
[88]
[89]
g
Yb/NaY, HOAc, 80 °C
Yb/NaY, morpholine (35 mol%), HOAc, 80 °C
Yb/NaY, PhI(OAc)2 (35 mol%), 80 °C
Au-NPs, K2CO3, H2O, air, 80 °C
Au-CNTc, toluene/H2O (1/1)/NaOH, r.t.
CaO (5 mol%), ethylene glycol, O2 (1 atm.), 130 °C
K-OMS-2 catalysts, toluene, refux
CH3CN, O2, refux
9
10
11
CH3CN, O2, FeCl3, refux
12
13
14
15
16
17
18
19
20
21
22
23
CH3CN, O2, MPAd, refux
CH3CN, O2, FeMPA, refux
CH3CN, O2, Fe(NO3)3, refux
l-proline, air, H2O, refux
FeCl3 (10 mol%), morpholine (20 mol%), EtOH, 80 °C
Fe-molybdophosphoric acid (50 mg)/O2/CH2Cl2/refux
Amberlite IR-120H (200 mg), 110 °C
MW (15 min), open vessel
3
12
1.5
3
5
0.25
0.25
22
24
1.5
MW (15 min), closed vessel
I2 (25 mol%), DMSO (2 mL), 100 °C
Pd(OAc)2 (2 mol%)/MSf/toluene/Et3N/THF/refux
[P4-VP]-PdNPs (1.2 mol%), DMF/120 °C
aYields refer to isolated products
bMWI microwave irradiation
cCNT carbon nanotube
dMPA 12-molybdophosphoric acid
ePTSA para-toluenesulfonic acid
fMS molecular sieves
gPresent method (Table 2, entry 1)
1 3

Journal of the Iranian Chemical Society
1,2-dihydroquinoxaline is produced. Then, aerobic oxidation
of 1,2-dihydroquinoxaline in the presence of [P4-VP]-PdNPs
occurs and the corresponding quinoxaline product is pro-
duced. In path (B), benzoin is oxidized to benzil by aerobic
oxidation in the presence of [P4-VP]-PdNPs and then by con-
densation reaction of 1,2-diamine and benzil, via cyclization
and dehydration, and consequently the corresponding qui-
noxaline formation occurs (Scheme 3). In fact, the reaction
occurs on the surface or in the pores of the polymer.
In order to show the advantages and limitations of
this protocol, we have compared the results of our model
reaction with those reported in the literature (Table 3).
[P4-VP]-PdNPs are an efective catalyst for this reaction in
terms of the amount of catalyst (1.2 mol%) and the reac-
tion time (1.5 h) in comparison with other catalysts [14, 19,
77–88].
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The superiority of this work is that catalyst is recoverable
and reusable for many times without signifcant loss of their
activity. However, the yield of the product and the reaction
time of our procedure are comparable with reported meth-
ods, while the reaction time for our method is shorter than
many of the other previously reported methods. This can
probably be attributed to the local concentration of palla-
dium nanoparticle species on the surface or inside the pores
of the polymer that can be interacted with 1,2-diamines and
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The main advantage of polymeric catalysts over no poly-
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and consequently their easier work-up by a simple fltration.
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amounts of supported catalyst into the reaction mixture.
Moreover, high yields of products, short reaction times
and ease of work-up will make the present method a use-
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for the synthesis of quinoxalines. In addition, there is cur-
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