Copper complex of polyglycerol anchored to graphene oxide as a recyclable nanocatalyst for sonochemical green synthesis of naphthoquinones

Article abstract of DOI:10.1139/cjc-2018-0217

In the present study, Cu(II) immobilized on hyperbranched polyglycerol functionalized graphene oxide catalyst was prepared as a novel, green, highly efficient, and reusable heterogeneous catalyst. Then, the synthesis of naphthoquinone derivatives was performed using Go-HPG-IA-Cu(II) as a catalyst under ultrasound irradiation to afford the desired products in high yields and short reaction times. This catalyst was easily recovered and reused several times without significant loss of catalytic activity.

Full text of DOI:10.1139/cjc-2018-0217

Page 1 of 32
Copper complex of polyglycerol anchored onto graphene oxide as a
recyclable nanocatalyst for sonochemical green synthesis of
naphthoquinones
Hossein Naeimi* and Maryam Farahnak Zarabi
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan,
8
7317, I. R. Iran; Fax: +98-03155912397; Tel: +98-03155912388;
E-mail: naeimi@kashanu.ac.ir.

Page 2 of 32
Abstract:
In the present study, Cu (II) immobilized on hyperbranched polyglycerol functionalized
graphene oxide catalyst was prepared as a novel, green, highly efficient and reusable
heterogeneous catalyst. Then, the synthesis of naphthoquinone derivatives was performed by
the employment of Go-HPG-IA-Cu(II) as catalyst under ultrasound irradiation to afford the
desired products in high yields and short reaction times. Also, this catalyst could be easily
recovered and reused several times without any significant loss of its catalytic activity.
Keywords: Graphene oxide, Hyperbranched polyglycerol, Catalyst, Naphthoquinone,
Ultrasound,

Page 3 of 32
Introduction
Fluorescent materials are of interest in many disciplines such as emitters for
1
2
electroluminescence devices , molecular probes for biochemical research , in traditional
1
3
4
textile and polymer fields , fluorescent whitening agents and photo conducting materials .
Molecules with the napthoquinone structure were reported as fluorescence compounds 5.
Also, these molecules are interested because of widespread biological properties, their
industrial applications, and their potential as intermediates in the synthesis of heterocycles5.
Because of their common applications, several methods have been reported for synthesis of
these compounds. Hydroxyl naphthalene-1,4-dione derivatives have been synthesized in the
6
7
presence of different catalystssuch asdodecyl benzenesulfonic acid ,Ionic liquids , MCM-41
5,
8
9
4
10
p-TsOH , ZrOCl , InCl , and nano copper(II) oxide .
2
3
Graphene, a monolayer of carbon atoms packed into a dense honeycomb crystal structure,
has attached much attention due to its unique properties such as a large theoretical specific
surface area (2630 m g ), high intrinsic mobility (200 000 cm v s ) 11-12, high Young’s
2
-1
2
-1 -1
modulus (∼1.0 TPa)13 and thermal conductivity (∼5000 Wm K ) 14, and its optical
transmittance (∼97.7%) and good electrical conductivity 15-16. Graphene oxide (GO), oxidized
form of graphene, is usually produced by the chemical oxidation of graphite, with subsequent
-1
-1
1
7
dispersion and exfoliation in water or suitable organic solvents .
The structure of graphene oxide have been identified with oxygen functionalities such as
hydroxyl and epoxy groups on the basal plane, and with smaller amounts of carboxy,
carbonyl, phenol, lactone, and quinone at the sheet edges 18-20. The presence of these
functional groups can also provide sites for chemical modification or functionalization of GO
2
1
through covalent or noncovalent bonds for numerous applications . The study of GO-based
catalysts in recent years has been popular and extensive.

Page 4 of 32
Hyperbranched polymers are highly branched macromolecules with three-dimensional
dendritic architecture. Due to their unique chemical and physical properties and potential
2
2
applications in various fields is growing rapidly . A class of hyperbranched polymers are the
aliphatic polyether polyols named polyglycerol23-24. Glycidol is the basic monomer to produce
polyglycerol. It is a commercially available and highly reactive hydroxy epoxide, can be
polymerized to polyglycerol using nucleophilic initiators such as alkoxides25-27. The
distinctive characteristic of hyperbranched polyglycerol is their highly branched structure
containing many hydroxyl groups on the surface, which can increase the functionalizing with
2
8
different functional groups covalently or non-covalently to obtain novel nanostructures .
Based on the green chemistry principles, synthetic methods should be use materials that
2
9
display little or no toxicity to the environment and human health . Therefore, Cu (II) salt has
attracted attention owing to their ease of handling, low toxicity, low cost and relative
insensitivity to air and moisture.
Ultrasound irradiation has progressively been considered as a convenient and clean
3
0
synthetic approach . Ultrasonic irradiation leads to acoustic cavitation, which collapse of
bubbles generates localized hot spots with very short lifetimes and extremely high pressures
and temperatures 31. Ultrasonic activation is increased reaction rates, formation of pure
products in high yields, easier manipulation, and mild reaction conditions 32-34 which
compared with traditional methods, this technique is more suitable taking green chemistry
concepts into account 30, 35
.
Herein, we would like to describe a highly efficient method for the preparation of
naphthoquinone derivatives via the one-pot three-component condensation reactionin the
presence of Cu (II) immobilized on hyperbranched polyglycerol functionalized graphene
oxide as an efficient and reusable heterogeneous catalyst under ultrasound irradiation.

Page 5 of 32
Experimental
The chemicals were purchased from Fluka and Merck Chemical Companies and used
without purification. IR spectra were obtained as KBr pellets on a Perkin-Elmer 781
1
spectrophotometer and on an impact 400 Nicolet FT-IR spectrophotometer. H NMR was
recordedin DMSO-d solvent on a Bruker DRX-400 spectrometer with tetramethylsilane as
6
internal reference.XRD patterns were obtained by an X'PertPro (Philips) instrument with 1.54
-1
°
A wavelengths of X-ray beam and Cu anode material, at a scanning speed of 2° min from
1
0° to 80° (2ϴ). Thermogravimetric analysis (TGA) was performed on a mettler TA4000
-1
system TG-50 at a heating rate of 10 K min under N atmosphere. The SEM images of
2
catalysts were recorded on a FE-SEM Hitachi S4160 instrument. The AFM image of
catalystswas investigated using scanning probe microscopy (SPM-9600, Shimadzu). Also,
elemental analyses for catalyst with inductively coupled plasma atomic emission spectroscopy
were obtained from an ICP-OES simultaneous instrument VISTA-PRO Model. A Bandelin
ultrasonic HD 3200 with 6 mm diameter model KE 76 probe was used to generate ultrasonic
irradiation and homogenize the reaction mixture. The piezoelectric crystals in this kind of
probe normally work at approximately 700 kHz; by use of appropriate clamps. However, the
output frequency of piezoelectric crystals was controlled and reduced to 20 kHz in the
reaction mixture. Melting points were measured with a Yanagimoto micro melting point
apparatus. The purity determination of the substrates and reaction monitoring were
accomplished by TLC on silica-gel polygram SILG/UV 254 plates (from Merck Company).
Preparation of catalyst
Preparation of graphene oxide

Page 6 of 32
The graphene oxide nanosheets were synthesized by a modified Hummer's method.
Typically, in a 1000 mL round-bottom flask equipped with a magnetic bar, a mixture of 5.0 g
of natural graphite powder and 2.5 g of sodium nitrate and 115 mL of sulfuric acid (98%)
were placed in an ice bath. Then, 15.0 g of potassium permanganate was slowly added to
obtained solution and stirred for 2 h. The mixture solution was taken to a water bath (35 °C)
and stirred for 30 min. Then, deionized water (230 mL) was slowly added and the solution
temperature was about 98 °C and stirred for 15 min. Afterward, 700 mL of deionized water
and 50 mL of H O (30%) was added to the solution respectively. The resulting materials
2
2
were filtered and washed by diluted HCl (5%) and deionized water for several times. Then
graphite oxide powder was obtained after drying for 12 h at 60 °C under vacuum. The
graphite oxide powder was dispersed in distilled water to make concentration of 0.5 mg mL-1
and exfoliated by ultrasonication to obtain GO nanosheets.
Grafting of hyperbranchedpolyglycerol on GO nanosheets
The hyperbranched polyglycerol modified GO nanosheets were prepared according to the
3
6
reported protocol with minor revision . A saturated solution of potassium methoxide in
methanol was mixed with 0.2 g of graphene oxide nanosheets. The mixture was sonicated in
an ultrasonic bath for 30 min and stirred at room temperature for 2 h. Then, mixture was
refluxed at 80 °C for 4 h. After completion of the reaction, graphene oxide nanosheets were
washed with methanol and dried in a vacuum oven at 60 °C. The glycidol (4 mL) was added
slowly to deprotonated nanosheets at 100 °C and the mixture was stirred at 100 °C for 8 h.
Then it was cooled and the mixture was dissolved in methanol. After repeated wash by
methanol followed by drying overnight under vacuum, HPG grafted on graphene oxide
nanosheets (GO-HPG) was obtained. The synthesis of hyperbranched polyglycerol
functionalized graphene oxide was confirmed by IR and TGA.

Page 7 of 32
Preparation of GO-HPG-IA
In a round-bottomed flask, GO-HPG (0.2 g) and isatoic anhydride (0.2 g) was suspended
in 30 mL ethanol and subsequently, the reaction mixture was refluxed at 60 °C for 24 h.
Subsequently formed mixture was washed several times with ethanol and dried under vacuum
at 70°C for 24 h.
Preparation of GO-HPG-IA- Cu (II)
In the final of catalyst preparation, GO-HPG-IA (0.2 g) and copper (II) acetate (0.2 g)
was suspended in 20 mL acetonitrile. Then, the reaction mixture was refluxed at 60 °C for 24
h. The reaction mixture was filtered and washed with acetonitrile to ensure that the excess
copper (II) acetate was completely removed. Finally, the products were dried at 70 °C under
vacuum, the pure complex was obtained.
General procedure for the preparation of aminonaphthoquinonederivatives catalyzed by
GO-HPG-IA- Cu (II) under ultrasound irradiation
A mixture of 2-hydroxy-1,4-naphthoquinone (1 mmol), aldehyde (1 mmol), amine (1
mmol), catalyst GO-HPG-IA- Cu (II) (15 mg, 0.1 mol% Cu) in EtOH (2mL) was sonicated at
4
0 kHz at room temperature for the appropriate time according to Table 2. The progress of the
reaction was monitored by thin layer chromatography (TLC) and was used n-hexane/ethyl
acetate as an eluent. After completion of the reaction, the mixture was diluted with
dichloromethane and the catalyst was filtered and washed with chloroform and EtOH for
checking the reusability. The solution was recovered via evaporation using a rotary
evaporator. The organic residue was washed with cold ethanol for several times and dried
under vacuum to give the pure product. The products were confirmed by spectral data and
physical data.

Page 8 of 32
2
-((4-Chlorophenyl)((4-methoxyphenyl)amino)methyl)-3-hydroxynaphthalene-1,4-dione
(4a)
3
7
-1
Yellow solid, mp. 125-127 °C (lit. : 127-128 °C); IR (KBr, ν, cm ): 3428, 3070, 1685,
1
1
616, 1539, 1487, 1224, 1173 ; H NMR (400 MHz, DMSO-d ) (δ, ppm): 3.63 (s, 3H), 6.58
6
(s, 1H), 7.29-7.31 (m, 4H), 7.49-7.50 (m, NH), 7.58-7.59 (m, 4H), 7.83-7.88 (m, 2H), 7.98-
8
.0(m, 1H), 8.05-8.09 (m, 1H), 8.82 (s, OH).
2
-((4-Bromophenyl)((4-methoxyphenyl)amino)methyl)-3-hydroxynaphthalene-1,4-dione
(4b)
-1
Brown solid, mp. 135-136 °C; IR (KBr, ν, cm ): 3437, 3072, 1678, 1614, 1535, 1488, 1261,
1
3
1
175; H NMR (400 MHz, DMSO-d ) (δ, ppm): 3.64 (s, 3H), 6.58 (s, 1H), 7.25 (d, J =8.0
6
3
Hz, 2H),7.43 (s, NH), 7.55-7.64 (m, 4H), 7.74 (d, J =8.0 Hz, 2H), 7.85-7.88(m, 2H), 7.99 (d,
3
3
J =8.0 Hz, 1H), 8.09 (d, J =8.0 Hz, 1H), 8.81(s, OH).
2
-((4-Fluorophenyl)((4-methoxyphenyl)amino)methyl)-3-hydroxynaphthalene-1,4-dione
(4c)
-1
Brown solid, mp. 160-165 °C; IR (KBr, ν, cm ): 3424, 3054, 1684, 1611, 1541, 1496, 1271,
1
1
223, 1170; H NMR (400 MHz, DMSO-d ) (δ, ppm): 3.63 (s, 3H), 6.59 (s, 1H), 7.04-7.07 (s,
6
NH),7.33-7.35 (m, 4H), 7.54-7.56 (m, 2H), 7.61-7.62 (m, 2H), 7.85-7.86 (m, 2H), 7.98-8.0
m, 1H), 8.03-8.08 (m, 1H), 8.81(s, OH).
(
2
-Hydroxy-3-(((4-methoxyphenyl)amino)(p-tolyl)methyl)naphthalene-1,4-dione (4d)
-1
Brown solid, mp. 170-174°C; IR (KBr, ν, cm ): 3432, 2923, 1678, 1615, 1540, 1497, 1270,
1
1
7
8
170; H NMR (400 MHz, DMSO-d ) (δ, ppm): 2.47 (s, 3H), 3.63 (s, 3H), 6.68 (s, 1H), 7.16-
6
.17 (m, 2H), 7.32-7.34 (m, 2H), 7.55-7.64 (m, 4H), 7.85-7.88(m, 2H), 7.99-8.0 (m, 1H),
.08-8.10 (m, 1H), 8.83(s, OH).

Page 9 of 32
2
-((4-Chlorophenyl)((4-nitrophenyl)amino)methyl)-3-hydroxynaphthalene-1,4-dione (4e)
3
7
-1
Orange solid, mp. 128-130 °C (lit. : 132-133 °C); IR (KBr, ν, cm ): 3200, 3031, 1678, 1605,
1
1
4
570, 1518, 1492, 1356; H NMR (400 MHz, DMSO-d ) (δ, ppm): 6.12(s, 1H), 7.40-7.42 (m,
6
3
H),7.47-7.49 (m, 4H), 7.76-7.80 (m, 1H), 7.83-7.87 (m, 1H), 7.94 (d, J =8.0 Hz, 1H), 8.05
3
(
d, J =8.0 Hz, 1H), 9.29 (s, OH).
2
-((4-Bromophenyl)((4-nitrophenyl)amino)methyl)-3-hydroxynaphthalene-1,4-dione (4f)
-1
Orange solid, mp. 128-130 °C; IR (KBr, ν, cm ): 3191, 3062, 1667, 1593, 1574, 1504, 1347;
1
H NMR (400 MHz, DMSO-d ) (δ, ppm): 6.11(s, 1H), 7.40-7.42 (m, 4H),7.47-7.49 (m, 4H),
6
3
3
3
3
7
.78 (t, J =8.0 Hz, 1H), 7.85 (t, J =8.0 Hz, 1H), 7.94 (d, J =8.0 Hz, 1H), 8.05 (d, J =8.0 Hz,
1
H), 9.29 (s, OH).
2
-Hydroxy-3-((4-nitrophenyl)((4-nitrophenyl)amino)methyl)naphthalene-1,4-dione (4g)
3
7
-1
Yellow solid, mp. 134-137 °C (lit. : 135-136 °C); IR (KBr, ν, cm ): 3197, 3076, 1668,
1
1
4
1
609, 1591, 1516, 1346; H NMR (400 MHz, DMSO-d ) (δ, ppm): 6.11(s, 1H), 7.40-7.42 (m,
6
3
3
3
H),7.47-7.49 (m, 4H), 7.78 (t, J =8.0 Hz, 1H), 7.85 (t, J =8.0 Hz, 1H), 7.94 (d, J =8.0 Hz,
3
H), 8.05 (d, J =8.0 Hz, 1H), 9.28 (s, OH).
2
-Hydroxy-3-(((4-nitrophenyl)amino)(p-tolyl)methyl)naphthalene-1,4-dione (4h)
3
7
-1
Orange solid, mp. 146-148 °C (lit. : 145-147 °C); IR (KBr, ν, cm ): 3198, 3074, 1667,
1
1
7
1
610, 1591, 1506, 1349; H NMR (400 MHz, DMSO-d ) (δ, ppm): 2.33 (s, 3H), 6.11(s, 1H),
6
.40-7.42 (m, 4H),7.47-7.49 (m, 4H), 7.77-7.80 (m, 1H), 7.83-7.87 (m, 1H), 7.93-7.94 (m,
H), 8.04-8.06 (m, 1H), 9.28 (s, OH).
2
-((4-Chlorophenyl)(m-tolylamino)methyl)-3-hydroxynaphthalene-1,4-dione (4i)

Page 10 of 32
-1
1
Brown solid, mp. 150-153 °C; IR (KBr, ν, cm ): 3414, 3070, 1680, 1596, 1540, 1487; H
NMR (400 MHz, DMSO-d ) (δ, ppm): 2.39 (s, 3H), 6.34 (s, 1H), 6.81-6.83 (m, 1H), 7.14-
6
7
7
.16 (m, 1H), 7.30-7.32 (m, 2H), 7.47 (s, NH), 7.61-7.70 (m, 4H), 7.77-7.79 (m, 1H), 7.90-
.94 (m, 1H), 8.04-8.05 (m, 1H),8.10-8.12 (m, 1H), 8.91(s, OH).
2
-((4-Bromophenyl)(m-tolylamino)methyl)-3-hydroxynaphthalene-1,4-dione (4j)
-1
1
Yellow solid, mp. 147-150 °C; IR (KBr, ν, cm ): 3403, 3068, 1680, 1595, 1539, 1485; H
NMR (400 MHz, DMSO-d ) (δ, ppm): 2.37 (s, 3H), 6.34 (s, 1H), 6.82-6.83 (m, 1H), 7.12-
6
7
8
.13 (m, 1H), 7.23-7.27 (m, 2H), 7.67-7.69(m, 1H), 7.75-7.77 (m, 4H), 7.90-7.93 (m, 1H),
.02-8.04 (m, 1H), 8.08-8.10 (m, 1H), 8.90(s, OH).
2
-((4-Fluorophenyl)(m-tolylamino)methyl)-3-hydroxynaphthalene-1,4-dione (4k)
-1
Yellow solid, mp. 143-146 °C; IR (KBr, ν, cm ): 3382, 3061, 1685, 1600, 1541, 1509, 1220;
1
H NMR (400 MHz, DMSO-d ) (δ, ppm): 2.38 (s, 3H), 6.34 (s, 1H), 6.81-6.83 (m, 1H), 7.14
6
(m, 2H), 7.33-7.39 (m, 4H), 7.67-7.69 (m, 1H), 7.75-7.77 (m, 1H), 7.89-7.91 (m, 1H), 8.03-
8
.05 (m, 1H),8.08-8.10 (m, 1H), 8.89(s, OH).
2
-Hydroxy-3-((4-methoxyphenyl)(m-tolylamino)methyl)naphthalene-1,4-dione (4l)
-1
Brown solid, mp. 144-147°C; IR (KBr, ν, cm ): 3327, 3010, 1683, 1607, 1538, 1509, 1246,
1
1
6
7
175; H NMR (400 MHz, DMSO-d ) (δ, ppm): 2.37 (s, 3H), 3.87 (s, 3H), 6.34 (s, 1H), 6.73-
6
.77 (m, 1H), 6.80-6.83 (m, 1H), 7.10-7.14 (m, 2H), 7.20-7.23 (m, 4H), 7.67-7.68 (m, 1H),
.75-7.76 (m, 1H), 7.90-7.93 (m, 1H), 8.01-8.09 (m, 1H), 8.90(s, OH).
2
-Hydroxy-3-(m-tolyl(p-tolylamino)methyl)naphthalene-1,4-dione (4m)
-1
1
Brown solid, mp. 146-149 °C; IR (KBr, ν, cm ): 3326, 3027, 1669, 1602, 1526, 1298; H
NMR (400 MHz, DMSO-d , - CDCl ) (δ, ppm): 2.31 (s, 3H), 2.37 (s, 3H), 6.03 (s, 1H), 7.16
6
3

Page 11 of 32
3
3
3
3
(
d, J =8.0 Hz, 2H), 7.35 (d, J =8.0 Hz, 2H), 7.67 (t, J =8.0 Hz, 1H), 7.77 (t, J =8.0 Hz, 1H),
3
7
.83-7.86 (m, 1H), 7.92-7.94 (m, 2H), 8.02-8.06 (m, 2H),8.08-8.10 (d, J =8.0 Hz, 1H), 8.91
(s, OH).
Results and Discussion
The process for the preparation of GO-HPG-IA-Cu (II) catalyst is described in Scheme 1.
First, graphene oxide nanosheets were prepared using a modified Hummer’s method. There
was a great amount of active hydrogen-containing functional groups including hydroxyl and
carboxylic acid groups on graphene oxide nanosheets surface. Hyperbranched polymerization
of glycidol was initiated by these active surface groups on graphene oxide nanosheets,
producing a high hydrophilic PG layer on graphene oxide surface 38. The obtained
hyperbranched polyglycerol functionalized graphene oxide was then treated with isatoic
anhydride to generate GO-HPG-IA. Finally, the GO-HPG-IA-Cu (II) was easily prepared by
reaction of GO-HPG-IA with Cu(OAc) . After successful preparation of the GO-HPG-IA-Cu
2
(II), the catalyst was characterized by FT-IR, TGA, XRD, AFM, and FESEM.
Scheme 1. Preparation of GO-HPG-IA-Cu(II) nanocatalyst.
Figure 1a–d shows the FT-IR spectra of graphite, GO, GO-HPG, and GO-HPG-IA,
respectively. The FT-IR spectrum of graphite powder shows peaks characteristic of C=C
-1
groups at 1573 cm which the peak is not sharp (Figure 1a). Figure 1b shows the FT-IR
-1
spectrum of graphene oxide powder. The absorption band at 1578 cm is related to C=C
double bonds which this peak is sharper than graphite due to the unsymmetry of GO. The
-1
absorption peaks at 1030, 1720 and 3410 cm are assigned to C–O, carbonyl, and hydroxyl

Page 12 of 32
stretching mode of functional groups attachment to GO, respectively. Figure 1c shows the FT-
-1
-1
IR spectrum of the GO-HPG. The vibrational bands at 2922 cm and 2853 cm are attributed
to C-H stretching, which confirmed that the HPG had been successfully grafted on the surface
-1
of graphene oxide. In the FT-IR spectrum of the GO-HPG-IA, the peak at 1741 cm is
attributed to ester bonds which indicates that the terminal hydroxyl groups is replaced by
isatoic anhydride and the GO-HPG-IA had been successfully obtained (Figure 1d).
Fig. 1. FT-IR spectra of (a) graphite, (b) GO, (c)GO-HPG, and (d) GO-HPG-IA.
Figure 2 depicts XRD patterns of graphite and GO, respectively. The XRD pattern of
graphite exhibits a peak at approximately 2θ= 26.5º, corresponding to the interlayer distance
of 0.335nm (Figure 2a). In Figure 2b, the peak at 2θ = 12.7° represented graphene oxide. The
interlayer spacing (d-spacing) of GO was calculated 0.78 nm which indicated the formation of
oxygen-containing functional groups between the layers of the graphite.
Fig. 2. XRD patterns of (a) graphite, (b) GO
In Figure 3 the TGA thermograms of (a) GO, (b) GO-HPG and (c) GO-HPG-IA are
displayed. As seen from Figure 3a, the first weight loss below 110 ºC can be assigned to
trapped water between GO nanosheets. Also, there is a major weight loss between 110 °C and
3
60 °C due to the decomposition of the labile oxygen-containing functional groups on the
3
9
GO, and the third step above 400°C assigned to an unstable carbon remaining . As shown in
Figure 3b and c, the GO-HPG and GO-HPG-IA are more stable than GO, which demonstrates

Page 13 of 32
the HPG has been successfully grafted on to the GO surface. For the GO-HPG curve, the
weight loss below 100 ºC is assigned to removal of physically adsorbed water. The second
weight loss about 33% occur at the temperatures ranging from 260 ºC to 370 ºC, and it is
4
0
attributed to the chemical grafting of HPG on to the GO surface (Figure 3b) . After reacting
with isatoic anhydride, the weight loss of the curve in Figure 3c between 290–400 ºC increase
to 39%, confirmed that covalently functionalized GO-HPG.
Fig. 3. TGA graph of (a) GO, (b) GO-HPG and (c) GO-HPG-IA.
Figures 4a and 4b exhibit the FE-SEM images of GO and GO-HPG-IA-Cu(II) nanosheets
respectively. In Figure 4a, the observed well-ordered two-dimensional and flake-like sheets of
the GO nanosheets. Different morphologies were shown for GO and prepared catalyst.
Wrinkled and folded structures of GO-HPG-IA-Cu(II) nanosheets appears that polymers
covered the surfaces of graphene oxide (Figure 4b)41-42
.
Fig. 4. FE- SEM images of (a) GO and (b) GO-HPG-IA-Cu(II).
Figure 5 shows typical AFM images and the height profiles of GO and GO-HPG-IA-
Cu(II) which are occupied to measure their thickness and observe the morphology of GO
nanosheets. The AFM images of the GO and GO-HPG-IA-Cu(II) confirm corrugated nature
of the grapheme oxide nanosheets. The height profile shows that the average thickness of the
GO is about 1nm which proposed a single layer structure (Figure 5a). The thickness of GO-
HPG-IA-Cu(II) is shown in Figure 5b. In the microscope image of GO-HPG-IA-Cu(II) could

Page 14 of 32
be observed nanosheets with thickness of about 15 nm. The surface uniform coverage
illustrates that the polymer chains were nearly equally distributed over the entire surface.
The EDS analysis of GO and GO-HPG-IA-Cu(II) catalyst confirmed the presence of C,
O, N, and Cu atoms (Figure 6a, b). Also, the catalyst was evaluated by an inductively coupled
plasma (ICP) analyzer. ICP analysis indicated the presence of copper in the catalyst and the
-1
content of Cu was measured to be 1.0 mmol g (6.4%).
Fig. 5. AFM images of (a) GO nanosheets and (b) GO-HPG-IA-Cu(II).
Fig. 6. EDS patterns of (a) GO and (b) GO-HPG-IA-Cu(II).
Investigation of catalyst activity of GO-HPG-IA-Cu(II) in the synthesis of
naphthoquinone derivatives
The prepared GO-HPG-IA-Cu(II) was used as heterogeneous catalyst in the synthesis of
naphthoquinones. Initially, the reaction parameters were optimized in the one-pot three-
component reaction of 2-hydroxy-1,4-naphthoquinone, 4-chlorobenzaldehyde, 4-
methoxyanilineas a model reaction underultrasound conditions.
To observe the optimal solvents, the reaction was tested in various solvents such as
H O,EtOH, acetonitrile, n-hexane, and chloroform (Table 1, entries 1–5). As shown in Table
2
1
, entry 5, the yield of reaction in the EtOH is more than other solvents. Moreover, the
reactions were clean in EtOH compared to those in organic solvents. Therefore, EtOH were
chosen as the reaction medium. In order to obtain the optimal amount of the catalyst, the
reaction was carried out in the presence of different amounts of catalyst. The best result was

Page 15 of 32
observed with 15 mg of the catalyst (0.1 mol% Cu) and the productyield afforded 94% at 5
min in EtOH under ultrasound conditions (Table 1, entry 5). Also, the reaction was performed
in the absence of catalyst and no product was formed (Table 1, entry 6). As shown in Table 1,
we observed effecting of ultrasonic irradiation frequency (rang of 20-45 kHz) on reaction.
When the reaction was performed in the absence of ultrasonic irradiation, the product was not
obtained at room temperature for 60 min (Table 1, entry 10), while in the presence of
ultrasonic irradiation at power of 40 kHz and GO-HPG-IA-Cu(II) catalyst, the yield increased
to 94% after 5 min (Table 1, entry 5).
Therefore, the best result was achieved with 15 mg (0.1mol% Cu) of GO-HPG-IA-Cu(II)
as catalyst in EtOH under ultrasonic irradiation with power of 40 kHz afforded 2-((4-
chlorophenyl)((4-methoxyphenyl)amino)methyl)-3-hydroxynaphthalene-1,4-dione at 5 min
with 94 % yield.
Table 1. Optimizing the reaction conditions a
After optimization of the reaction conditions, synthesis of naphthoquinone derivatives
were carried out for a series of different aromatic aldehydes containing electron-releasing and
electron-withdrawing substituents to investigate the versatility of the protocol under
GO-HPG-IA-Cu(II) catalysis. As shown in Table 2, aromatic aldehydes containing electron-
withdrawing groups reacted rapidly and gave higher yields, while substitutions of electron-
rich groups on the benzene ring required longer reaction times and got lower yields. It seems
that the electron-donating groups decrease the electrophilicity of the carbonyl group, leading
to longer reaction times and little lower yields. Furthermore, the structures of these products
1
were supported by H NMR spectra.

Page 16 of 32
Table 2. Synthesis of naphthoquinone derivatives catalyzed by GO-HPG-IA-Cu(II) a.
As shown in Figure 7, we examined the reusability of GO-HPG-IA-Cu(II) as
heterogeneous catalyst for the same reactants. After completion of the reaction, the catalyst
was isolated by filtration, washed exhaustively with chloroform, and ethanol and drying at
8
0 °C in an oven for 24h for subsequent experiments to check its reusability under similar
reaction conditions. The recycled catalyst could be reused for five times without considerable
loss of its catalytic activity and gave the corresponding product in high yields. Afterwards, the
product melting point was checked to ensure that the purity remained excellent.
Fig. 7. The reusability of the catalyst GO-HPG-IA-Cu(II) in the synthesis of
naphthoquinone derivatives.
The proposed reaction mechanism
A possible mechanism for the synthesis of naphthoquinone derivatives is depicted in
Scheme 2. Firstly, the aldehyde is activated by the GO-HPG-IA-Cu(II) catalyst to give (I)
which upon nucleophilic attack of amine to form imine (II). Subsequently, 2-
hydroxynaphthalene-1,4-dione attacks imine (II) in the presence of the GO-HPG-IA-Cu(II)
catalyst to provide intermediate (III), which undergoes tautomeric proton shift to generate the
naphthoquinone derivatives and releases the catalyst for the next run.
Scheme 2. Proposed mechanism for synthesis of naphthoquinone derivatives.

Page 17 of 32
Conclusion
In conclusion, we have successfully introduced a clean, eco-friendly, efficient and cost-
effective synthesis of naphthoquinones under ultrasound irradiation. Furthermore, we have
designed a heterogeneous catalyst based on Go-HPG-IA-Cu(II). In this research, a facile
strategy for grafting of PG onto the surface of GO nanosheets is demonstrated. This PG layer
on graphene oxide surface not only stabilize Cu(II) intermediates but also advance the
catalytic process. The catalyst was easily recovered and reused five cycles without a
significant loss of catalytic activity and given products with high yields in short time.
Acknowledgements
The authors are grateful to University of Kashan for supporting this work by Grant No.
1
59148/86.
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Page 21 of 32
Fig. 1. FT-IR spectra of (a) graphite, (b) GO, (c)GO-HPG, and (d) GO-HPG-IA.
Fig. 2. XRD patterns of (a) graphite, (b) GO
Fig. 3. TGA graph of (a) GO, (b) GO-HPG and (c) GO-HPG-IA.
Fig. 4. FE- SEM images of (a) GO and (b) GO-HPG-IA-Cu(II).
Fig. 5. AFM images of (a) GO nanosheets and (b) GO-HPG-IA-Cu(II).
Fig. 6. EDS patterns of (a) GO and (b) GO-HPG-IA-Cu(II).
Table 1. Optimizing the reaction conditions a
Fig. 7. The reusability of the catalyst GO-HPG-IA-Cu(II) in the synthesis of
naphthoquinone derivatives.
Scheme 1. Preparation of GO-HPG-IA-Cu(II) nanocatalyst.
Scheme 2. Proposed mechanism for synthesis of naphthoquinone derivatives.

Page 22 of 32
O
O
O
OH
MeONa
O
O
N
H
O
O
OH
O
OH
O
HO
OH
HO
OH
O
O
HO
O
O
HO
O
O
3 2
Cu(CH COO)
H2N
O
OH HN
O
OcA
OcA
O
OH
O
OH
O
Cu
O
O
OH H N
2
HN
O
HO
O
O
HO
O
O
Scheme 1. Preparation of GO-HPG-IA-Cu(II) nanocatalyst.

Page 23 of 32
Fig. 1. FT-IR spectra of (a) graphite, (b) GO, (c)GO-HPG, and (d) GO-HPG-IA.

Page 24 of 32
Fig. 2. XRD patterns of (a) graphite, (b) GO

Page 25 of 32
Fig. 3. TGA graph of (a) GO, (b) GO-HPG and (c) GO-HPG-IA.

Page 26 of 32
a
b
Fig. 4. FE- SEM images of (a) GO and (b) GO-HPG-IA-Cu(II).

Page 27 of 32
a
b
Fig. 5. AFM images of (a) GO nanosheets and (b) GO-HPG-IA-Cu(II).

Page 28 of 32
b
a
Fig. 6. EDS patterns of (a) GO and (b) GO-HPG-IA-Cu(II).

Page 29 of 32
Table 1. Optimizing the reaction conditions a
Cl
H
O
OCH3
O
O
NH2
O
O
catalyst
N
H
OH
OH
Cl
OCH3
b
Entry
Solvent
US.F (kHz) Catalyst (mg)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
H O
40
40
40
40
40
40
40
40
40
-
15
15
15
15
15
0
10
10
20
20
5
Trace
70
35
40
94
-
2
CH CN
3
n-hexane
CHCl3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
15
10
10
5
5
78
81
94
-
10
20
15
15
15
15
1
1
1
1
0
1
2
3
60
10
10
5
20
35
45
55
80
94
a
Reaction conditions: 2-hydroxy-1,4-naphthoquinone (1 mmol), aldehyde (1 mmol), amine(1
b
mmol) ), GO-HPG-IA-Cu(II). Isolated yields.

Page 30 of 32
Table 2. Synthesis of naphthoquinone derivatives catalyzed by GO-HPG-IA-Cu(II) a.
X
H
O
O
O
NH2
O
Y
GO-HPG-IA-Cu(II)
EtOH, US
N
H
OH
OH
X
Y
O
1
2
3
4a-m
-1
b
T (min), Yield (%), TON, TOF (h )
Cl
Br
F
CH3
OCH3
OCH3
OCH3
OCH3
O
O
O
O
O
O
O
N
N
N
N
H
OH
H
H
H
OH
OH
OH
O
5
, 94, 63, 755
5, 94, 63, 755
10, 90, 60, 353
10, 84, 56, 329
Cl
Br
NO2
CH
3
NO2
NO2
NO2
NO2
O
O
O
O
O
N
N
H
OH
N
H
OH
N
H
OH
H
OH
O
O
O
1
5, 85, 57, 227
15, 85, 57, 227
10, 93, 62, 365
15, 80, 53, 213
Cl
Br
F
OCH
3
CH3
CH3
CH3
CH3
O
O
O
O
N
H
OH
N
H
N
N
H
H
OH
O
OH
OH
O
O
O
1
0, 94, 63, 369
10, 94, 63, 369
15, 84, 56, 224
15, 80, 53, 213
CH3
CH3
O
N
H
OH
O
1
0, 82, 55, 322
a
Reaction conditions: 2-hydroxy-1,4-naphthoquinone (1 mmol), aldehyde (1 mmol), amine
1 mmol), GO-HPG-IA-Cu(II) (15 mg, 0.1 mol%), EtOH, under ultrasound conditions.
(
b Isolated yields. c TON: mole of formed naphthoquinone derivatives per mole of catalyst.
-1
d TOF (h ): (mmol of product/mmol of active site of catalyst)/time of the reaction (h).