Copper(ii) ions supported on functionalized graphene oxide: an organometallic nanocatalyst for oxidative amination of azolesviaC-H/C-N bond activation

Article abstract of DOI:10.1039/d0nj02385j

Graphene oxide (GO) was chemically modified withpara-aminobenzoic acid (PABA) to immobilize copper(ii) ions on its surface and used as a nanocatalyst for the oxidative C(sp²)-H bond amination reaction. A practical method to prepare Cu²⁺supported onpara-aminobenzoic acid grafted on GO was reported. The prepared Cu²⁺@GO/PABA was characterized by FT-IR, XRD, SEM, AFM, TEM, UV-Vis, and ICP techniques. The results showed that the morphology, distribution, and loading of copper ions could be well-adjusted by grafting of PABA on GO. Moreover, just 2 mol% of Cu²⁺@GO-PABA could catalyze the C-H activation reaction of benzoxazole and benzothiazole with secondary amines in >94% yields. Also, the catalyst showed very good recyclability and much less leaching of the Cu into the reaction solution. The high activity of Cu²⁺@GO-PABA can be ascribed to the good synergistic effects of Cu²⁺andpara-aminobenzoic acid grafted on graphene oxide.

Full text of DOI:10.1039/d0nj02385j

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ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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DOI: 10.1039/D0NJ02385J
Copper (II) ions Supported on Functionalized Graphene Oxide: An
Organometallic Nanocatalyst for Oxidative Amination of Azoles
via C‒H/ C-N Bond Activation
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Masoumeh Behzadi,*a Mohammad Mahmoodi Hashemi,*a Mostafa Roknizadeh,a Shahrokh
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Nasiri,a Ahmad Ramazani Saadatabadib
Graphene oxide (GO) was chemically modified with para-aminobenzoic acid (PABA) to immobilize copper (II) ions to its
surface and used as a nanocatalyst for the oxidative C (sp2)‒H bond amination reaction. A practical method to prepare Cu+2
supported on para-aminobenzoic acid grafted on GO was reported. Prepared Cu+2@GO/PABA was characterized by FT-IR,
XRD, SEM, AFM, TEM, UV-Vis, and ICP techniques. The results showed that the morphology, distribution, and loading of
copper ions could be well-adjusted by grafting of PABA on GO. Moreover, just 2 mol % of Cu+2@GO-PABA could catalyze
the C‒H activation reaction of benzoxazole and benzothiazole with secondary amines in >94% yields. Also, the catalyst
showed very good recyclability and very less leaching of the Cu to the reaction solution. The high activity of Cu+2@GO-
PABA can be ascribed to the good synergistic effects of Cu+2 and para-aminobenzoic acid grafted on graphene oxide.
oxidation of the graphite is the most compatible and easily large-
scale procedure to prepare GO.27 Due to the abundant oxygen
containing functional groups on its basal sheet (phenol, hydroxyl,
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1. Introduction
The transition-metal-catalyzed selective C-N bond formation
and epoxide groups) and at its edges (carboxylic groups),
Conspicuously, GO displays a remarkable hydrophilic inherent and
significantly distributable in water and many organic solvents.28 As a
result, the defects and multiple functional groups, authorize GO to
be a preferential candidate for further chemical functionalization
with different materials such as, surfactants, polymer, alkaloid,
organometallic compounds, and metal nanoparticles. 29-32 which are
greatly applicable as catalysts or supports. In addition, setting down
the organometallic compounds, nanoparticles or metal ions on
functionalized graphene oxide makes also a significant state for
recovery and recyclability of the catalysts, forming cross-linked
structures, with substantially improved mechanical and
electronically properties.
reaction of azoles is an efficient approach in synthetic organic
chemistry, whereas five-membered heterocyclic componds with
amine groups are widely applicable in biological, pharmaceutical,
and material sciences.1 Many synthetic methods, including cyclo
condensation reactions from two functionalized precursors (e.g.
Hantszch aminothiazole synthesis),2 palladium-catalyzed Buchwald-
Hartwig coupling3 and copper-catalyzed Ullmann and Goldberg
coupling, have been developed to access this vital motif.
With great advancement in the C-H activation reaction, it seems
some difficulties, such as high-temperature, long reaction-time,
toxic functionalization, and use of a large amount of expensive
metal oxidant and strong acid or base, have not been provided yet.
A
pioneer and engaged two-dimensional (2D) carbon
nanostructure, graphene oxide (GO), has fascinated exclusive
In this study, we investigated a C-H activation, C-N bond formation
of benzoxazole and benzothiazoles with secondary amines and O2
as a green oxidant. However, the success of this synthetic
attention in catalysis, photochemistry, electrochemistry, and
4-8
organic catalysis, during the past decays.
The substances based
procedure relies on employing
a recoverable organometallic
on graphene material keep incomparable structural features and
generating extraordinary physical properties, such as high surface
area, high water dispersal status, innate low mass, easy surface
nanocatalyst which is used in a few amounts. To achieve this goal,
we surveyed if the exploiting of chemical optimized-graphene oxide
with primary amine to immobilize copper (II) ions to accomplishing
a better function of catalytic for C (sp2) ‒H bond amination reaction.
So, a facile and novel method to prepare copper salt of para-
aminobenzoic acid (PABA) grafted on graphene oxide materials
(Cu+2@GO-PABA) was firstly reported which could catalyze C-H/ C-N
reaction of benzoxazole and benzothiazoles with various secondary
amines with good yields.
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adjustments, and plenty of oxygen-carrying functionalities,
which are promising application as catalysts or supports.
Graphene, a single layer of carbon atoms arranged in a honeycomb
lattice, regarded as an efficient applicant for abundant state-of-the-
art technologies, such as nanoelectronics,12 transparent conducting
electrodes,13 composites,14 supercapacitors,15 gas sensors,16 and
17
hydrogen storage
because of its fascinating electronic,
mechanical, thermal properties, and large specific surface areas.18-26
Between the chemical approaches of GO generation, rough
2. Experimental section
2.1 General Information
The extra pure graphite powder, CuCl2 .2H2O, KMnO4, H3PO4, H2SO4
(96-98%) and HCl (37%), H2O2 (30%), benzoxazole, para
aminobenzoic acid, sodium carbonate, various amiones, and all of
solvents and other marerials were purchased from Sigma-Aldrich
and Merck companies. Ultrapure deionized water (resistivity >18
a. Department chemistry, Sharif university of Technology, Tehran, Iran. Email:
masome.behzadi@gmail.com
b. Department of Chemical and Petroleum Engineering, Sharif university of
Technology, Tehran, Iran
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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MΩ) was obtained from a Mili-Q Biocel system. The morphology give the black sodium salt. This result dried and soxhelet with DI
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and size of Cu@GO-PABA were studied by Scanning Electron water to remove of sodium adsorbed onDtOheI: 1G0.O10/3P9A/BDA0NsJu0r2f3a8c5eJ.
Microscopy (SEM; TESCAN–MIRA 3) and Transmission electron Finally, 2.5 gr black powder product was obtained. Furthermore, for
microscopy (TEM, CM30 300kV). The XRD patterns of Cu-GO/PABA preparation of Cu-GO-PABA nanocatalyst, sodium salt of GO/PABA
nanohybride were determined via INEL EQUINOX 3000 X-ray (0.3 gr) was dispersed in 10 ml DI water in 10 minutes in a round-
diffractometer, using Cu Kα radiation, to determine structural bottomed flask to obtain the homogenous suspension and then
changes and d-spacing. EDAX analysis was performed using treated with 10 ml solution of CuCl2 ( 10 wt %) and refluxed at
MIRAPLMU for elemental analysis. A Raman spectrometer (Takram 100˚C for 2h. Subsequently, the solution stirred at ambient
p50C0R10) equipped with a laser with a working frequency of 532 temperature for 24 hr. after completion of the reaction, the crude
nm as its excitation source was used to approve the Cu@GO/PABA product filtered and dried in room temprature. The obtained gray-
compared to GO and GO-PABA structures. UV visible spectra were black solid was washed with deionized water for 10 hrs (soxhelet)
obtained using a Unico 4802 spectrophotometer. IR spectra were and dried in an oven at 40˚C for 5 hours to give 0.63 gr copper
recorded on Bruker Tensor-27 FTIR spectrometer using KBr pellets. nanocatalyst.
The amount of copper in catalysts was identified by inductively 2.5 General procedure for copper-catalyzed oxidative C-H
coupled plasma atomic emission spectroscopy (ICPAES WFX-120). activation-direct amination of benzoxazoles and benzothiazole
All of the known C-H amination of benzoxazole compounds were with secondary amines
identified by comparison of their spectral data and physical In a round bottom flask equipped with a stirring magnetic bar and a
properties with those of the authentic samples. Other new products condenser, azoles (0.5 mmol) 1, amine (0.6 mmol) 2a-i,
were characterized with 1HNMR and 13CNMR spectra (Bruker DRX- nanocatalyst Cu+2-GO-PABA (2 mol %), CH3COOH (120 µl), and
300 AVANCE NMR spectrometer) using CDCl3 as a solvent and using CH3CN (2 mL) were mixed. Then the pure O2 injected to the
tetramethylsilane (TMS) as an internal standard. Chemical shifts reaction mixture. The suspension was stirred and refluxed at 70 °C
were reported in parts per million (δ) downfield from TMS. All for 12 h. After completion of the reaction, the mixture was cooled
coupling constants (J) are in hertz. Also, Mass spectrometer to room temperature, filtered from nanocatalyst and washed with
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operating at an ionization potential of 70 ev.
10 ml chloroform to extract the pure products with (32-94%) yield.
2.2 Synthesis of GO
Graphene oxide (GO) was synthesized from the oxidation of extra
pure graphite powder by the modified Hummer’s method.33 Briefly,
1.0 g graphite powder was mixed to 120 mL concentrated H2SO4
and 13 mL H3PO4 85% was stirred in a room temperature for 1 h.
Then 6.0 g KMnO4 were added stepwise to this solution during the
30 minutes with stirring at 50˚C. Then the reaction maintained
about 50˚C and stirred for 12h. After this time, 200-250 mL distilled
water slowly was added to the green suspension with stirring while
the reaction temperature kept in 0˚C. Next, 2-3 mL H2O2 30% was
added slowly to reduce the excess KMnO4 whereas color of solution
rapidly changed from green to yellow. The prepared graphene oxide
could be easily centrifuged at 5,000 rpm and washed with HCl
solution (30 mL, 1 M) and DI water 3 times successively. Finally, the
crude brown product was dried in vacuum oven 24h at 40˚C. The
weight of dried GO was 2.30 g.
3. Results and discussion
The designed synthetic process for functionalized-GO/metal
nanocomposite is shown in Scheme 1. This strategy for the
synthesis of the activated nanocatalyst relies on three simple steps.
First, the exfoliated graphene oxide (GO) nanosheets were
prepared from natural graphite by the modified Hummer’s method
without using sodium nitrate as an additive reagent.33 Furthermore,
to eliminate the generated toxic gases, a mixture of H2SO4/H3PO4
(9:1 volume ratio) was used instead of H2SO4 as
a novel
approach.41 In this method graphene oxide (GO) nanosheets have
usually a size between tens to several hundreds of square
nanometers.
Second, In the process of fabricating the new activated GO-Cu
nanocatalyst, we used para-aminobenzoic acid (PABA) to adjust
functionalization of graphene oxide and immobilization of Cu+2 on
the surface of the functionalized GO-PABA is chemically grafted on
surface GO and increase the carboxyl functional group for
covalently chelating Cu on the GO surfaces.
The third key process for preparing GO nanocomposite, the para-
aminobenzoic acid supported on GO is reacted with CuCl2 to give its
copper salts as a GO-PABA/Cu+2 which is new and efficient
nanocatalyst.
As shown in Scheme 1, the oxygen-containing groups on GO, such
as carboxyl and hydroxyl groups, were removed under thermal
treatment as a reducing agent. The color of the solution changed
from brown-yellow to dark brown, indicating the removal of some
oxygen functional group. To increase the loading Cu on the GO, and
preventing of releasing various oxygen functional groups, we
improved the activity of GO substrate with PABA that keep the
especially carboxylic acid group on the GO surface, as a result of
chemical bonds formed between functional groups of GOs and Cu
2.3 Synthesis of Graphene Oxide para-Amino Benzoic Acid (GO-
PABA)
Prepared GO (2.30 g) dispersed in ethyl acetate (30 ml) by
sonication bath for 5 minutes. Then para-aminobenzoic acid (1.75 g,
7.5 mmol) was added to exfoliated GO. The reaction mixture was
then refluxed while being stirred magnetically at 80 °C for 16 h. The
mixture was cooled to room temperature and filtered. The solid
mass was washed with ethyl acetate by a soxhlet for 10 h in order
to remove all unreacted para-aminobenzoic acid. The solid was
dried in an oven at 100 °C to gain 3.56 g black solid product.
2.4 Grafting of Cupper ion nanoparticle on Amine functionalized
Graphene Oxide (Cu+2@GO-PABA nanocatalyst)
This process has 2 steps. All of procedure summarized in scheme 1.
Firstly, sodium salt of the para-aminobenzoic acid (GO-PABA-Na+)
was prepared via dispersing of 1.15 gr GO/PABA in 30 ml of DI
water for 30 minute in ultra-sonication bath then a saturated
solution of sodium carbonate (4.11 gr, 30 ml DI water) was added to
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ions. Also, metal leaching is decreased due to this chemical carboxylic functional group in 1703 cm-1 approved the coordination
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reaction.
between carboxylic acid and a Cu2+ ion. InDaOdId: i1t0io.1n0,39b/yD0trNeJa0t2m38e5nJt
with Cu2+, the epoxide group participates in the ring-opening
reaction, then the its relative intensity of stretching vibration
decreased, as a result, the intensity of alkoxy stretch enhanced.35
On the other hand, vanishing OH broad peak at about 3371 cm-1 is
related to interaction between copper (II) ions with OH functional
groups, and peaks in 3447 and 3337 cm-1 are contributed to the N-H
stretching vibration. These results support that chemical
interactions between GO and Cu+2 occurred and due to bearing the
plenty of active oxygenating functional groups (epoxide, hydroxyl,
alkoxy, and carboxylic acid), both the basal plane and edges of GO,
completely attached to Cu salts. Consequently, Cu nanoparticles are
nucleated in whole of GO surface and edges.
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para-Aminobenzoic acid
1. Na2CO3
2. CuCl2.H2O
O
OH
O
HO
HN
OH
Scheme 1. Synthesis steps of Cu@GO-PABA nanocatalyst.
Morphological, compositional, and structural characterization of
the nanocomposite were analyzed with a number of methods
including scanning electron microscopic (SEM), transmission
electron microscopic (TEM), XRD, EDX, Raman spectroscopy, Uv-
visible and FT-IR analyse. To investigate the chemical structure
changes of GO after functionalization, the FTIR spectrum of the
nanocomposite powders in each processing step for GO, GO-PABA,
and GO-PABA-Cu+2 were taken which were shown in Figure 1.
Considerable changes can be observed among the FT-IR spectra of
GO and GO-PABA. GO has some characteristic peaks, O-H stretching
vibration peak was observed at 3371, while carboxyl/carbonyl
groups had stretching peak at 1732, C=C aromatic bands displayed
at 1627, C-O related epoxide/ether group was at 1285, and C-O at
alkoxy functional group was conceived at 1174 cm−1. After grafting
PABA on the GO, the carboxyl (C=O, 1732) and O–H stretching
vibration peaks (3371) of GO were disappeared while epoxy and
alkoxy bands at 1285 and 1122 cm-1 appeared and intensified.
Upon treatment with PABA, two new bands at 1561, and 1516 cm-1
appeared in the spectrum, which can be assigned to the coupling of
the C-N stretching vibration with N-H deformation vibration while
broadband peak at 3012 cm-1 is contributed to the N-H stretching
vibration that is overlapped with O–H stretching vibration in
carboxylic acid functional group on PABA and other hydroxyl groups
in GO support, also, strong peak at 1698 cm−1 is attributed to C=O
stretching in carboxylic group on PABA. So, the broad peaks at 3012
and strong peak 1698 cm−1 are attributed to the characteristic
carboxylic group on the para-aminobenzoic acid. Furthermore, the
presence of peaks at 2890 cm-1 was associated with the stretching
vibrations of C-H aliphatic on GO/PABA. These results indicate the
successful attachment of PABA units on the GO nanosheet surface.
When GO chemically reacted and bonded to Cu salts, its
characteristic peaks were vanished. For example, the C=O, OH, and
epoxide vibration stretching peaks disappeared whereas the
intensity of alkoxy (C-O) band increased. Reducing carbonyl bond in
Figure 1. FT-IR spectra of (a) GO, (b) GO-PABA and (c) Cu@GO-PABA
The pristine GO distinguished through two fingerprint peaks in Uv-
vis, 251 nm and 300 nm (shoulder peak), related to the C=C and
C=O transmission. As demonstrated conspicuously in Figure 2., one
of GO discriminative peaks that attributed to C=O and other oxygen
functional groups, has been improved in GO-PABA and also the
absorption is red-shifted with a maximum at 269 nm. The visible
wavelength absorption and dark color of GO solution imply the
regeneration of the conjugated C=C lattice clearly.36
Figure 2. Photographs and UV-vis spectra of 0.mg/mL solution of
GO, GO-PABA, and Cu@ GO-PABA in water.
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The functional groups bearing heteroatoms (oxygen, nitrogen) on The morphology and structural features of the GO and Cu@PABA-
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DOI: 10.1039/D0NJ02385J
GO and GO-PABA surfaces have
a significant role in the GO composites were examined highly by some spectroscopic
immobilization of metal ions. It is obvious that the negative charges analysis such as, XRD, scanning electron microscopy (SEM), and
on the GO substrate develops a nucleus for the growth of positive transmission electron microscopy (TEM). Figure 4 shows the XRD
metal ions on the GO and GO-PABA. UV-vis demonstrates that Cu patterns of the as-prepared Cu@GO/PABA nanocomposites. The
ions can be easily grafted on the GO-PABA surface. The diffraction peaks at around 2θ= 16.1° and 17.5˚ correspond to the
characteristic dark color of Cu materials in solution with no distinct (001) GO XRD pattern. The interlayer of GO (0.54 nm) was
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absorption band at 300−1000 nm is also evident in Cu+2@GO-PABA.
Raman spectroscopy of the nanocomposite powders indicates
changes of oxidation states in the GO during processing
functionalization and coating with the metal process (Figure 3). In
the Raman spectrum, GO has two characteristic peaks, G-band and
D-band. The G-band comes from the E2g symmetry related to sp2
carbons (graphite network), and the disorder ones, D-band
generated through the chemical defects that the oxygenated
functional groups on basal and edges surface create on graphene
lattice.37
As shown in Figure 3, the peaks in GO (1350 cm−1), GO-PABA (1355),
and Cu@GO-PABA (1350 cm−1) are characterized as a D-band. In
addition, the peaks at 1602 cm-1 in all panels, related to G-band that
makes by the the E2g symmetry of the sp2 C atoms.
Furthermore, the ratio of the D band to the G band is a measure of
defects present in the GO, GO-PABA, and Cu@GO-PABA. This
amount is 0.877 for GO, 0.875 in GO-PABA and 0.89 after grafting
Cu on GO/PABA surface. A higher value may indicate higher defect
density. Based on figure 3, in GO and GO/PABA peaks this value is
relatively fixed that indicates after grafting para-aminobenzoic acid
to GO substrate, any more defection has not been observed at
graphene domains.
The ID /IG ratio increased from 0.87 ( GO and GO/PABA) to 0.89 for
Cu@GO/PABA because of the covalent chemical banding between
GO-PABA and Cu ions that are resulted from the further defect and
damage in the graphene sp2 bonding lattice. As shown in figure 3,
the oxidation process makes a continuous and adaptable coating of
Cu+2 on the GO-PABA flakes that heighten the ID /IG ratios of the GO-
PABA/Cu nanocomposite powders markedly due to the
functionalization GO with PABA and grafting with Cu+2 process
enhanced the functional groups and partially recovered the
graphene oxide structure. Remaining defects in the GO serve as
chemical bonding sites between the GO-PABA and Cu (II) ions. Also,
in panel c, Raman bands at 97, 211 cm−1 proves the stable
formation of continuous Cu ions on GO.38
calculated by the Scherer equation, which was much more larger
than fresh graphite that is about 0.34 nm. Indeed, increasing the
interlayer space in GO material resulted from to the presenting of
oxygen-containing functional groups on the graphene sheet .39
The peaks at 32.2° (220), 32.5 (220), 39.6˚ (200) and 36.5˚ (111)
were fixed with CuO and Cu2O forms, respectively, and the
coalescence of two crystal structures was identified at 48.2° (200) in
Figure S6 of the Supporting Information.40 All the peaks in the
spectra can be assigned to orthorhombic CuO and Cu2O (a = 6.8920
Å, b = 9.0800 Å, c= 6.0550 Å, α= 90.0000˚, β = 90.000°, γ=
90.000˚). None of obvious typical peaks belonging to rGO are
observed in the XRD spectra of Cu@GO/PABA nanocomposites. It
can attributed in slight amount and little diffraction intensity of
rGO. In addition, the spectrum displays more negligible peak
intensity than the CuO for the Cu2O species, which suggest the CuO
in the Cu@GO/PABA nanocomposites, comparatively has high
crystalline structure, bearing high amount of oxygen functional
groups (carboxylic, hydroxyl, and epoxy), intensifying the diffusion
and crystallization of CuO grains on the GO surface.40
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Figure 4. XRD image of Cu@GO-PABA
Figure 5 shows SEM images and EDX surface elemental analysis of
copper catalyst nanostructures.
As can be shown, the 2D composite nanosheets with wrinkled and
folded features could be observed in the PABA−GO-Cu material.
However, the chemical interaction between amino groups at the
edge and basal plane of GO and copper ions, enhances the stacking
of PABA−GO-Cu hybrids more than that of GO.
Most of the Cu+2@GO-PABA nanocatalyst are uniformly distributed
on the GO sheet and keep away from the aggregation, which will
greatly improve their catalytic properties. The GO-Cu (II)-PABA
(Figures 5a) exhibits a rough bulky surface, probably due to the
complexation with Cu (II) and coordination with the ligands. It can
Figure 3. Raman spectra of GO (a), GO-PABA (b), and Cu@GO/PABA
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be observed from the SEM images (Figure 5a) that the Moreover, to optimize the amount of the catalyst, we performed a
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Cu+2@GO/PABA are nearly spherical with an average size of 6-15 series of reactions with varying amounts of Cu @GO/PABA (0.5- 10
+2
DOI: 10.1039/D0NJ02385J
nm, respectively. mol %). The data is given in Table 1. Astonishingly, the catalyst was
The EDX surface elemental analyses (Figures 5b and S5) further found to be efficient (18-84 % yield) even in its minimum quantity
proved the presences of Cu and N atoms, indicating that the GO is (0.5 mol %). Hence, this amount of the catalyst was considered as
bounded to copper, which in turn is coordinated to the energetic its minimum quantity for a successful copper catalyzed C-H
ligands.
activation reaction with excellent yield and high purity. As shown in
Furthermore, according to the results of XRD characterization, the Table 1, the highest yield was obtained when 2 mol% of copper
morphology of Cu materials on the GO-PABA support, were also nanocomposite was used which was the optimize amount for our
recognized by TEM (as described in Figure 6). Thus, the catalyst (entry 3). An increase in the amount of catalyst did not
immobilization of Cu NPs on the PABA-GO-Cu+2 surface improve the yield of the reactions (entry 4-6).
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principally were spherical (all patterns in Figure 6).
Notably, in the controlled experiments of the reaction conditions,
The Cu NPs on the surface of the PABA-GO-Cu+2 catalyst were the optimal system for the coupling reactions was obtained using
mainly spherical (all panels Figure 6). However, a homogeneous CH3COOH (2 equiv) as an additive, CH3CN as solvent at 70 °C and 1
dispersion of Cu ions was observed for our nanocatalyts at edge and atmosphere O2.34 The copper and the oxidizing reagent, O2 have
base of GO surface (panel a, b). TEM showed the presence of 8.8- substantial role in the reaction procedure, so that in the lack of
10.5 nm size Cu ions in large numbers on the aminated graphene copper source and oxidant, the expected product yields reduced
oxide surface. Based on the panel c and d, a few Cu nanostructure from 84% to 18 or 0%, respectively (Table 1, entry 7, 8).
like worm pattern can be seen, which implies that para-
aminobenzoic acid reacted to graphene oxide has plentiful active Table 1. Optimization of Reaction Conditions of Benzoxazole and
sites including N and a π-conjugated system, and the multiple Pipyridine (Model reaction)
connection sites that result in massive aggregation of Cu
nanomaterials. Also, GO has more oxygenated functional groups
and the aggregation of Cu nanocatalyst was inevitable on these
materials with rich O, N sites or a π-conjugated system. Also, to
elucidate the quantitative correlation between O, N active groups
and copper species, copper loading in the graphene oxide-based
composites, was determined from the inductively coupled plasma
analysis (ICP). However, loading of copper in these materials was
about 14.5 wt %. The high amount of copper for PABA-GO-Cu nano
catalyst may be attributed to rich amine group, oxygen active sites
or aromatic rings, which could be created the main driving force for
the affinity for Cu2+ ions.
In order to preparation of amino-benzoxazole and amino
benzothiazole compounds and evaluate the catalytic activity of the
PABA−GO−Cu nano composite in organic synthesis, direct oxidative
C-H amination of benzoxazoles and benzothiazoles with secondary
amines as a nitrogen source was investigated in Scheme 2.
Entry
Cat.
Oxidant
Conversion Yield
(%)
(mol %) (1atm)
(%)
1
2
3
4
5
6
7
0.5
1
O2
O2
O2
O2
O2
O2
-
60
31
52
84
73
70
70
18
90
2
100
95
3
5
95
10
80
Opt
amount
trace
8
-
O2
N.R
N.R
R1
N
X
N
X
R1
R2
Cu+2@GO-PABA
+
HN
N
Reaction conditions: Benzoxazole (0.5 mmol), pipiridine (0.6 mmol),
CH3CO2H (2 eq), 70˚C, 12 h.
R2
CH3COOH (2 equiv), O2
CH3CN, 70°C, 12h
X= O, S
3, 4
1
2
Scheme 2. Oxidative amination of azoles with secondary amine
To obtain the optimize reaction conditions, a set of experiments, as
a benchmark reaction, were performed using benzoxazole (0.5
mmol)
1 and pypiridine (0.6 mmol) 2a for synthesis of
corresponding N, N-2-amine benzoxazole 3a (Scheme 3).
Cu+2@GO-PABA
N
N
O
N
HN
+
O
CH3COOH (2 equiv)
CH3CN, O2, 70°C, 12h
1
2a
3a
Scheme 3. Model reaction for optimization of reaction condition
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/D0NJ02385J
5
6
1200
C
7
8
9
1100
1000
900
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800
Cu
700
600
O
500
400
300
200
100
0
Cl
Cl
N
Cu
Cu
keV
1 µm
0
5
10
Figure 5. (a) SEM images of the Cu@GO-PABA catalysts, at 200 nm-1 μm size magnification (b) EDX images of the Cu@GO-PABA
6 a
6 b
6 c
6 d
Figure 6. TEM images of PABA-GO-Cu
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With best conditions in hand, direct amination of benzoxazole and 4. CONCLUSION
benzothiazole was carried out with various amines (Table 2). The In Summary, a green, effective and environmentally friendly
runs of this reaction were performed with aliphatic and procedure was reported for the development of heterogeneous Cu
cycloaliphatic amines including diethylamine, dipropylamine, (II)-graphene oxide system to carrying out the oxidative sp2 C-H
diisopropylamine, piperidine, morpholine, pipirazine, N-methyl coupling of benzoxazole and benzothiazole with secondary dialkyle
pipirazine and pyrrolidine. This C-H activation reaction proceeded amines under mild condition in O2 as oxident. Interestingly, the
smoothly to produce the desired products 3a-i in good yields (30- morphology, distribution, and loading of Cu ions could be well-
94%). Some of this derivatives with steric hinderance give controlled through grafting the graphene oxide with para-amino
corresponding secondary amine in low yields (pyrrolidine and benzoic acid, and the enhanced catalytic performance was achieved
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diisopropylamine).
for C-H/C-N bond activation reaction. N-Heteroatoms in the para-
The stability and reactivity of 2 mol% Cu+2@GO-PABA nanocatalyst aminobenzoic acid can accelerate the immobilization of Cu (II) on
were investigated during the four consecutive runs in model the graphene oxide surface. Accordingly, the high activity of
reaction. The stability of copper nanocatalyst displayed any Cu+2@GO-PABA can be ascribed to the good synergistic effects of
remarkable leaching of the metal in the reactions medium, also a Cu+2 and para-amino benzoic acid grafted on graphene oxide. Also,
slight activity decrease was detected (Figure 7).
Easy work-up, high yield, short reaction time and single-step nature
of the methodology are among the main advantages of this
procedure.
Conflicts of interest
There are no conflicts to declare.
Figure 7. Catalyst reactivity performance
The plausible mechanism has been suggested in Scheme 4, which is
accordance with the reaction of tertiary amine reported by
Huang.41 this pathway has three steps. First, the copper species
[GO/PABA@Cun+1] that is oxidized by oxygen, coordinates with the
secondary amines to form the intermediate A that is hydrolyzed via
CH3COOH. Then intermediate A coordinates to azole to product B,
followed by the subsequent deprotonation/ rearrangment to afford
C and recovered CH3COOH. Finally, the reductive elimination
process of C forms the desired product and regenerates the copper
catalyst to complete the catalytic cycle of Cu@GO-PABA.
R1
N
X
N
R2
N
[Cun@GO/PABA]
CuLn
N R1
R2
O2
X
C
[Cun+1@GO/PABA]
NHR1R2
CH3COOH
R2
N
CuLn
R1
CH3COOH
R1
-H2O
LnCu
N
CH3COO
N
H
X
R2
CH3COO
A
B
N
X
H
Scheme 4. Proposed mechanism for copper-catalyzed direct
oxidative C-H/C-N reaction of azoles.
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Table 2. Copper-Catalyzed Direct C-H Amination of azole
View Article Online
DOI: 10.1039/D0NJ02385J
Entry
Product (yield)
6
7
8
9
1
N
O
N
S
N
N
N
N
N
N
3a (80%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
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57
58
59
60
4a (94%)
2
3
N
O
N
S
3b (40%)
3c (30%)
3d (84%)
4b (45%)
N
O
N
S
4c (30%)
4
5
N
O
N
S
N
N
4d (94%)
N
O
N
S
N
O
N
O
3e (53%)
4e (90%)
4f (80%)
4g (70%)
4h (50%)
N
N
6
7
8
9
N
NH
N
NH
O
S
3f (75%)
O
N
N
S
N
N
N
N
3g (70%)
3h (50%)
N
O
N
N
N
N
N Ph
S
N
N
S
N
N
O
4i (40%)
3i (32%)
Reaction conditions: azoles (0.5 mmol), amides (2 mL), Cu@PABA-GO (2 mol %), CH3CO2H (120 µL), O2 atmosphere, 70˚C, 12 h.34
Purification was carried out in chloroform as a solvent.
3 A. R. Muci, S. L. Buchwald, Top. Curr. Chem. 2002, 219, 131; D. S.
Surry, S. L. Buchwald, Angew. Chem., Int. Ed., 2008, 47, 6338; J.
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Graphical Abstract
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Copper (II) ions Supported on Functionalized Graphene Oxide: An Organometallic
Nanocatalyst for Oxidative Amination of Azoles via C‒H/ C-N Bond Activation azoles
8
9
Masoumeh Behzadi,*a Mohammad Mahmoodi Hashemi,*a Mostafa Roknizadeh,a Shahrokh Nasiri,a
Ahmad Ramazani Saadatabadib
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aDepartment of Chemistry, Sharif University of Technology,Tehran, Iran.
bDepartment of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran.
Oxidation
Graghite
GO
para-Aminobenzoic acid
CuCl2.H2O
Cu+2@GO-PABA
GO-PABA
N
Cu
O
OH
O
N
R1
N
R2
N
X
R1
R2
+
H
HN
X
O
X= O, S
HO
NH
OH

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