A carbon nitride supported copper nanoparticle composite: a heterogeneous catalyst for the N-arylation of hetero-aromatic compounds

Article abstract of DOI:10.1039/c6nj03584a

A graphitic carbon nitride supported copper nanoparticle composite (Cu-gCN) has been found to be an active catalyst for the N-arylation of hetero-aromatic systems (pyrrole, pyrazole, and substituted indole) and benzamide molecules, with high product selectivity and good to excellent yields, using substituted aryl bromide as a coupling partner. The intercalated structure and the amine functional group of the carbon nitride prevent the aggregation of the catalytically active copper species during the reaction, and the recyclability study shows the stable performance of the catalyst without a significant loss of catalytic activity.

Full text of DOI:10.1039/c6nj03584a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Carbon nitride supported copper nanoparticles: A heterogeneous catalyst for the
N-arylation of hetero-aromatic compounds
Debkumar Nandi, Samarjeet Siwal, Kaushik Mallick*
Department of Chemistry, University of Johannesburg, P.O. Box: 524, Auckland Park 2006,
South Africa.
Eꢀmail: kaushikm@uj.ac.za (K Mallick)
Abstract: Graphitic carbon nitride supported copper nanoparticle composite (CuꢀgCN) has been
found to be an active catalyst for Nꢀarylation of heteroꢀaromatic system (pyrrole, pyrazole and
substituted indole) and benzamide molecule, with high product selectivity and from good to
excellent yields, using substituted aryl bromide as a coupling partner. The intercalated structure
and the amine functional group of carbon nitride preventing the aggregation of the catalytically
active copper species during the reaction and the recyclability study shows the stable
performance of the catalyst without the significant loss of catalytic activity.
Keywords: Nꢀarylation; carbon nitride; copper nanoparticles.
1. Introduction
Substituted heteroarene molecules have enormous applicability to a variety of research areas,
ranging from biology to the materials sciences.1ꢀ5 The Nꢀarylation has been recognized as one of
the most important transformations of −NH containing heteroꢀaromatic compounds, such as,
pyrrole, pyrazole and indole etc. Copper mediated aromatic nucleophilic substitution reactions
were reported by Fritz Ullmann and Irma Goldberg.6 Stoichiometric amounts of copper salts
together with higher reaction temperature were major limitations of this methodology in spite of
its numerous industrial applications.7, 8
With time researchers have made significant progress to minimize these problems by applying
the catalytic amount of copper salt with ligands or by preformed catalysts and have achieved the
moderate optimal reaction conditions. Copper catalyzed Nꢀarylation for imidazole9 and other
nitrogen containing heterocycles10 using milder reaction condition have appeared in recent
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literatures. Some strategies have also been reported for mild reaction conditions for the title
reaction using aryl lead11 or aryl bismuth12 as the coupling partner. Utilization copper salts with
several ligands, such as, racemic Lꢀproline,13 1, 10ꢀphenanthroline14 and diꢀimine ligands15 have
also remain effective. Successful efforts have been also made for the photocatalytic Nꢀarylation
for imidazole and phenylboronic acid using copperꢀgraphene system as a catalyst under
irradiation of visible light recently.16
Again, palladium also proved as an effective catalyst for such kind of title reaction. Palladium
catalyzed intramolecular Nꢀarylation of sulfonamides has been described by Buchwald17 and
since then significant progress for palladium catalyzed Nꢀarylation of using various type of
ligands have also been reported.18 Recent literature shows that palladium catalyst, in presence of
various biaryl ligands, has the excellent performance for the Nꢀarylation of cyclopropylamines
using aryl bromide as coupling partner.19 Despite of these certain successes, in general, ligand
synthesis, moisture sensitivity, toxicity, cost effectiveness of the catalyst and the limitation of
substrate scope are still remaining the challenge and which need more modification of this
methodology.
During last few decade, with the advent of nanotechnology, nanomaterials replace the classical
strategies of several conventional research areas like catalysis, sensing, medicine and electronics.
Among them, catalysis has been emerged as the most successful branch where nanomaterials are
frequently used. Dispersed metal nanoparticles, on solid support or stabilizer, such as carbon
based materials,20 metal oxides,21 zeolites22 and clays materials,23 have high exposed surface
area, so that the reactant molecules have more exposure towards the catalyst surface, rather than
the soft stabilizers, like ligands,24 polymers,25ꢀ27 dendrimers28 and surfactants,29 where the
catalyst species are within the stabilizer and that limits the accessibility for the reactant molecule
toward the catalyst surfaces and there by the catalytic performances.30 Solid supported catalyst,
for heterogeneous liquidꢀphase organic transformation reactions, are easy to handle during the
recycling process and which is important for industrial, environmental and economical point of
view. In this current manuscript we like to report the Nꢀarylation reaction for both the heteroꢀ
aromatic and aromatic systems using various substituted aryl bromide molecules as the coupling
partner in the presence of carbon nitride supported copper nanoparticle composite catalyst.
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2. Experimental Section
2.1. Materials and instrumentation
Ultraꢀpure water was used to prepare the solution of CuSO4, 5H2O (10−2 mol dm−3). All the
chemicals as well as the solvents were purchased for this study were ‘used as received’ without
further purification.
The JEOL (JEMꢀ2100), 200keV, transmission electron microscope (TEM) with LaB₆ electron
source was used for the microscopic analysis of the sample. An ultraꢀthin windowed energy
dispersive Xꢀray spectrometer (EDS) attached to the TEM was used to determine the chemical
composition of the samples. TEM specimens were onto a carbon coated nickel TEM grids.
Infrared spectra were collected utilizing a Shimadzu IRAffinityꢀ1 with a spectral resolution of
0.5 cmꢀ1. The XRD patterns were recorded on a Shimadzu XDꢀ3A Xꢀray diffractometer
operating at 20 kV using CuꢀKα radiation (k = 0.1542 nm). The measurements were performed
over a diffraction angle range of 2θ = 20o to 80o. The XPS spectra were collected in an ultraꢀhigh
vacuum chamber attached to a Physical Electronics PHI 560 ESCA/SAM system. 1H NMR and
13C NMR spectra were recorded on Bruker Ultrashield 400 (400 MHz for 1H and 100 MHz
for 13C{1H} in CDCl3).
2.2. Synthesis:
2.2.1. General procedure for the preparation of Cu-gCN:
In a typical experiment, 15 g of urea was kept in a covered crucible at 80 ⁰C under ambient
pressure for 12h. After that the dried urea was put in a Muffle Furnace for 4h at 550 ⁰C. The
lemon yellow colored product was washed several time with distilled water to remove the
residual alkaline species (e.g. ammonia) on the sample surface and again dried at 80⁰C for 24
hours under vacuum. In the second stage of the synthesis, each 0.5 g of the resultant product was
dispersed in 10 mL of water 2 separate 25 mL of round bottom flux. A 0.1 M of CuSO4.5H2O
(3.95 mL and 7.9 mL) was added drop wise (5 wt% and 10 wt% loading of Cu) to each flux.
After that, 5 mL of 1×10ꢀ3 M NaBH4 solution was added to each flux drop wise for the reduction
of Copper salt. At the end, the material was filtered, washed with water and dried. The resultant
material, Cu(0)ꢀgCN, was characterized using different techniques and applied as catalyst for the
Nꢀarylation reaction.
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2.2.2. General procedure for the Ullmann reaction:
To a 25 mL round bottom flask fitted with magnetic stirrer were added ꢀNH containing heteroꢀ
aromatics 1 (1.0 mmol), arylhalide 2 (1.2 mmol), Cs2CO3 (650 mg, 2.0 mmol) and 5mg of Cuꢀ
gCN catalyst (5 wt% or 10 wt% loaded Cu) in dry toluene (4 mL). The reaction mixture was
stirred under reflux at 100 °C for 12h. After completion (monitored by TLC), the reaction
mixture was allowed to cool at room temperature and diluted with 20 mL of distilled water; the
solution was extracted with ethyl acetate (3 × 20 mL). The organic layer was separated and dried
over anhydrous MgSO4. Removal of the solvent resulted in a residual mass which was subjected
to column chromatography over silica gel using hexane and an increasing proportion of ethyl
acetate as eluent to provide the corresponding products.
3. Results and Discussion
3.1. Characterization of material
The TEM image (Figure 1A) of the CuꢀgCN composite, where nanoparticles are seen, with wide
range of size distribution ranging from 5 to 20 nm, on the carbon nitride support material.
The EDS spectra (Figure 1B) derived from placing the electron beam directly on a nanoparticle
in the carbon nitride support. The EDS analyses confirmed that copper was only found
associated with the nanoparticles, the nickel peak in the spectra are from the nickel grid used for
the microscopic study.
The XRD pattern (Figure 2A) of the of the Cu-gCN sample show an intense peak at 27.33°,
which indexed as the (002) stacking peak of the conjugated aromatic ring of the support
material.32 Other three peaks at 43.18º, 50.20º and 74.06º are assigned for the (111), (200) and
(220) crystal face of metallic copper. The low intensity peak at about 35.80o can be assigned for
(111) crystal face of the Cu2O, possible due to the surface oxidation of the copper nanoparticles
particles.
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Figure 1: (A) The TEM image shows the distribution of the copper nanoparticles (dark spots) on
the carbon nitride support, where the sizes of the nanoparticles are within range between 5-20
nm. (B) The EDS spectra shows the presence of copper (derived by placing the electron beam
directly on a dark spots on the carbon nitride support) and the nickel peak in the spectra are
from the nickel grid.
To identify the chemical state of the copper, Xꢀray photoelectron spectroscopy (XPS)
measurements were performed. The Cu 2p region of the XPS spectrum of the Cu-gCN composite
sample is illustrated in Figure 2B, which reveals the presence of 2p3/2 and 2p1/2 peaks at binding
energies of 932.75 and 953.4 eV, respectively. It is not clear whether the peak at 932.75 eV is
due to metallic copper or Cu2O because the Cu 2p3/2 binding energies and peak widths of these
two species are similar31 but the symmetric nature of the Cu 2p3/2 spectrum indicates the presence
of only metallic copper in the Cu-gCN sample. The evidence of formation of Cu(I) species, from
Cu2O as indicated in the XRD pattern, is not visible in the XPS signal probably due to the Xꢀrayꢀ
induced reduction of Cu(I) species.32 In the IR spectra (Figure 2C), the vibrational bands
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attributed for C=N and C–N stretching modes and falls within the range from 1240 to 1700 cm−1
for both gCN and CuꢀgCN samples. The outꢀofꢀplane bending vibration has been observed at
810 and 872 cm−1, due to the presence of typical heptazine rings, for gCN (a) and CuꢀgCN (b)
samples, respectively. The broad band, between 3000ꢀ3500 cm−1, appear for uncondensed
terminal amino groups (–NH2 or =NH groups) for both the samples. The shifting of peak
positions for all the vibrational bands have been observed towards the higher wavelength region
for the sample CuꢀgCN, as compared with gCN sample, indicates the functionalization of the
copper nanoparticle with the nitrogen from the support material.
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Figure 2: (A) The XRD pattern of the of the copper-carbon nitride composite system, where the
peak at 27.33° represent the (002) stacking peak of the conjugated aromatic ring of the carbon
nitride. The peaks at 43.18º, 50.20º and 74.06º are assigned for the (111), (200) and (220)
crystal face of metallic copper, whereas, the (111) crystal face of the Cu2O has been
represented by the low intensity peak at 35.80o. (B) The X-ray photoelectron spectroscopy
shows the presence of 2p3/2 and 2p1/2 peaks at binding energies of 932.75 and 953.4 eV,
respectively, with the symmetric nature of the Cu 2p3/2 spectrum. (C) The FTIR spectra of the
gCN (a) and Cu-gCN (b) samples.
3.2. Optimization of reaction condition
The reaction between pyrrole (1a) and 4ꢀbromobenzonitrile (2d) was taken as model reaction
system to yield the Nꢀarylated product, 4ꢀ(1Hꢀpyrrolꢀ1ꢀyl)benzonitrile (3ad) for the optimization
of reaction conditions (Table S1, supporting information). Carbonate, acetate, alkoxide and
phosphate type of bases have been screened for the reaction. Among them, Cs2CO3 appeared to
be the most suitable base for the reaction (entries 3 and 5). Various high boiling point solvents,
such as, xylene, toluene, N,Nꢀdimethylacetamide (DMAc), N,Nꢀdimethylformamide (DMF), Nꢀ
methylꢀ2ꢀpyrrolidine (NMP) and tetrahydrofuran (THF) have been tested for the solvent
optimization study and toluene was chosen for the current methodology by considering all the
entries listed in table S1. Again, by comparing the different entries in table S1, we found 5 wt%
of loading of copper nanoparticles on the support, CuꢀgCN (5 mg), was the best choice since
further increase of copper loading has not shown any improvement in terms of yield of 3ad for
o
the present methodology (entries 3 and 5). It is also important to mention that 100 C was found
o
to be optimum temperature for the current reaction since at 80 C no desired product has been
formed (entry 8), as monitored by thin layer chromatography (TLC) technique. We also have
o
observed that further increase of the temperature at 120 C does not have any positive effect on
the yield of the product. In the absence of any base molecule the homocoupling product of aryl
halide has been formed (entry 4).
3.3. Substrate scope
Based on the above optimized reaction conditions, entry 5, we have explored the versatility of
the CuꢀgCN catalyst for the Nꢀarylation of various heteroꢀaromatic system using different
substituted aryl bromide molecules as a coupling partner and the results are summarized in
Tables 1. All the substrates produced the expected Nꢀarylated product with very good to
excellent yields and selectivity.
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When pyrrole (1a) was used as a substrate, the aryl bromide molecules, such as, bromobenzene
(2a), electron withdrawing group substituted 4ꢀbromonitorbenzene (2b), 4ꢀbromobenzonitrile
(2d) and electron donating group substituted 4ꢀbromoanisole (2c) and 4ꢀbromotoluene (2e),
formed the desired Nꢀarylated products of 1ꢀphenylꢀ1Hꢀpyrrole (3aa), 1ꢀ(4ꢀnitrophenyl)ꢀ1Hꢀ
pyrrole (3ab), 1ꢀ(4ꢀmethoxyphenyl)ꢀ1Hꢀpyrrole (3ac), 4ꢀ(1Hꢀpyrrolꢀ1ꢀyl)benzonitrile (3ad) and
1ꢀpꢀtolylꢀ1Hꢀpyrrole (3ae) with the yield of 78%, 88%, 82%, 92% and 85%, respectively, table
1. The other −NH containing 5 member heteroꢀaromatic compound pyrazole (1b) was also
coupled with ꢀNO2 (2b), ꢀOMe (2C), ꢀCN (2d) and ꢀCOMe (2g) substituted bromobenzene
molecules with the formation of desired Nꢀarylated products 1ꢀ(4ꢀnitrophenyl)ꢀ1Hꢀpyrazole
(3bb), 1ꢀ(4ꢀmethoxyphenyl)ꢀ1Hꢀpyrazole (3bc), 4ꢀ(1Hꢀpyrazolꢀ1ꢀyl)benzonitrile (3bd), (4ꢀ(1Hꢀ
pyrazolꢀ1ꢀyl)phenyl)ethanone (3bg) with the yield of 85%, 93%, 95% and 90%, respectively,
table 1.
Motivated by the above observations, we have shifted our attention towards the indole (1c) and
substituted indole moieties (aldehyde, 1d, and acetyl, 1e), one of the privileged hetero aromatic
structures in the realm of organic chemistry, to imply the present methodology to get access of
their corresponding Nꢀarylated coupling products. It was observed that all the moieties have
reacted with the substituted aryl bromide molecules to produce the expected compounds from
good to excellent yield ranging from 75% to 92% of the isolated products (Table 1). In the
presence of CuꢀgCN catalyst, indole (1c) was reacted with bromobenzene (2a), 4ꢀ
bromonitorbenzene (2b), 4ꢀbromobenzonitrile (2d), 4ꢀbromoanisole (2c) and 4ꢀbromotoluene
(2e) and form the corresponding products 3ac, 3cb, 3cc, 3cd and 3ce with 80%, 85%, 80%, 92%
and 86% of isolated yield, respectively. It is important to mention that oꢀbromotoluene (2f) was
also participated in title reaction with indole and formed 1ꢀoꢀtolylꢀ1Hꢀindole (3ef), successfully,
with 78% of isolated yield.
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Table 1: Substrates scope for C-N cross coupling reaction between −NH containing hetero
aromatics and substituted aryl bromides. a, b
N
N
R
N
N
R
R
R=H, 3ca; 80%
R=NO2, 3bb; 85%
R=OMe, 3bc; 93%
R=CN, 3bd; 95%
R=COMe, 3bg; 90%
R=H, 3aa; 78%
R=4ꢀNO2, 3cb; 85%
R=4ꢀOMe, 3cc; 80%
R=4ꢀCN, 3cd; 92%
R=4ꢀMe, 3ce; 86%
R=2ꢀMe, 3cf; 78%
R=NO2, 3ab; 88%
R=OMe, 3ac; 82%
R=CN, 3ad; 92%
R=Me, 3ae; 85%
H
O
O
HN
N
Br
N
O
R
3dg; 90%
R
R=H, 3da; 82%
R=OMe, 3dc; 84%
R=CN, 3dd; 89%
R=Me, 3de; 85%
R=H, 3ea; 79%
R=NO2, 3eb; 83%
R=OMe, 3ec; 75%
R=CN, 3ed; 85%
(a) Reaction conditions: Hetero-aromatics 1(1.0 mmol), arylbromide 2 (1.2 mmol), Cs2CO3 (650
mg, 2.0 mmol), Cu-gCN catalyst (5 mg, 5 wt% of Cu) and Toluene (4mL). (b) Isolated yield.
In a similar way, indoleꢀ3ꢀcarboxaldehyde (1d) have also performed the Nꢀarylation reaction
with a set of aryl bromide molecules, namely, bromobenzene (2a), 4ꢀbromobenzonitrile (2d), 4ꢀ
bromoanisole (2c) and 4ꢀbromotoluene (2e) and resulted the arylated products 1ꢀphenylꢀ1Hꢀ
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indoleꢀ3ꢀcarbaldehyde (3da), 1ꢀ(4ꢀmethoxyphenyl)ꢀ1Hꢀindoleꢀ3ꢀcarbaldehyde (3dc), 4ꢀ(3ꢀ
formylꢀ1Hꢀindolꢀ1ꢀyl)benzonitrile (3dd) and 1ꢀpꢀtolylꢀ1Hꢀindoleꢀ3ꢀcarbaldehyde (3de) with the
isolated yield ranging from 82% to 89%. In contrary, when 4ꢀbromoacetophenone (2g) was
reacted with indoleꢀ3carboxaldehyde (1d), an aldol condensation product, (E)ꢀ1ꢀ(4ꢀ
bromophenyl)ꢀ3ꢀ(1Hꢀindolꢀ3ꢀyl)propꢀ2ꢀenꢀ1ꢀone (3dg), with 90% isolated yield has been
achieved as the main product and a trace amount of the corresponding Nꢀarylated compound was
noticed.
As expected, 3ꢀacetylindole (1e) also underwent the title reaction with bromobenzene (2a), 4ꢀ
bromonitorbenzene (2b), 4ꢀbromoanisole (2c) and 4ꢀbromobenzonitrile (2d) to produce the
corresponding Nꢀarylated products, 1ꢀ(1ꢀphenylꢀ1Hꢀindolꢀ3ꢀyl)ethanone (3ea), 1ꢀ(1ꢀ(4ꢀ
nitrophenyl)ꢀ1Hꢀindolꢀ3ꢀyl)ethanone (3eb), 1ꢀ(1ꢀ(4ꢀmethoxyphenyl)ꢀ1Hꢀindolꢀ3ꢀyl)ethanone
(3ec) and 4ꢀ(3ꢀacetylꢀ1Hꢀindolꢀ1ꢀyl)benzonitrile (3ed), with the yields of 79%, 83%, 75% and
85%, respectively (Table 1).
With the success of the Nꢀarylation of the heteroꢀaromatic system, we were interested to widen
the substrate scope, by maintaining the current optimized reaction condition, for mono arylation
at −NH2 group present in the aromatic system. We have chosen benzamide molecule (4) as the
source of −NH2 group and bromobenzene (2a) and various electron withdrawing and donating
group substituted aryl bromide molecules, such as, 4ꢀbromonitorbenzene (2b), 4ꢀ
bromobenzonitrile (2d), 4ꢀbromoanisole (2c), 4ꢀbromotoluene (2e) and oꢀbromotoluene (2f), as
the coupling partner for this reaction. The monoꢀarylated products of Nꢀphenylbenzamide (5a),
Nꢀ(4ꢀnitrophenyl)benzamide
(5b),
Nꢀ(4ꢀmethoxyphenyl)benzamide
(5c),
Nꢀ(4ꢀ
cyanophenyl)benzamide (5d), Nꢀpꢀtolylbenzamide (5e) and Nꢀoꢀtolylbenzamide has been formed
with 85%, 94%, 89%, 93%, 90% and 84% of yield, respectively (Table 2).
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Table 2: Substrates scope for C-N cross coupling reaction between benzamide and substituted
aryl bromides. a, b
O
OMe
O
O
NO2
N
H
N
N
H
H
5c; 89%
5b; 94%
5a; 85%
O
O
O
CN
N
H
N
H
N
H
5e; 90%
5f; 84
5d; 93%
(a) Reaction conditions: Benzamide 4 (121 mg, 1.0 mmol), arylbromide 2 (1.2 mmol), Cs2CO3
(650 mg, 2.0 mmol), Cu-gCN catalyst (5 mg, 5 wt% of Cu) and Toluene (4mL). (b) Isolated yield.
3.4. Scaling up, durability and the kinetic study of the catalyst
The demand of heterogeneous catalysis lies in its durability and recyclability study that could
allow the catalyst to serve the purpose of repetitive use. The reaction between pyrrole (1a) and 4ꢀ
bromobenzonitrile (2d) was chosen as the substrates for Nꢀarylation reaction to obtain the
product 4ꢀ(1Hꢀpyrrolꢀ1ꢀyl)benzonitrile (3ad) using carbon nitride supported copper catalyst by
maintaining the reaction condition as mentioned in table S1 (entry 5, supporting information).
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Figure 3: (A) The histogram from the recyclability study of the copper-carbon nitride catalytic
system. The figure shows that the used material is also active as a catalyst without a significant
loss of catalytic performance. (B) The TEM image of the used catalyst at the end of the sixth
cycle reveals the enlargement of the copper nanoparticles. (C) The kinetic study shows the
formation of the product, 4-(1H-pyrrol-1-yl) benzonitrile, as a function of time.
To perform this study for the purpose of convenience, we have increased the amount of all the
reagents, including the catalyst, by a factor of ten. After completion of the first cycle, a yield of
92% of targeted the product 3ad was achieved whereas at the end of the sixth cycle a moderate
deactivation of the reaction has been noticed with the yield of 80% of the Nꢀarylated product. At
the end of each cycle, the catalyst was recovered by filtration technique then washed with ethyl
acetate followed by drying in air and was reused for the next cycle. The graphical representation
(Figure 3A) shows a relation between the yield percentage of product (3ad) and the number of
cycles and the result demonstrates that the used material was also active as a catalyst without a
significant loss of catalytic performance. At the end of last cycle, the recovered catalyst was
characterized by TEM and a significant amount of particles enlargement has been noticed
(Figure 3B), which is possibly due to the Ostwald ripening process.33 The gradual decrease of the
product yield (%) per cycle could be accounted mainly due to two reasons, firstly, loss of
composite catalyst during the filtration and washing process and, secondly, the particle
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agglomeration, which causes the decrease of the effective surface area or the active sites of the
catalyst. To find the possibility of metal leaching from the carbon nitride support, we have
performed the inductively coupled plasma mass spectrometry (ICPꢀMS) analysis of the filtrate,
collected during the recyclability study, and we found that instrument did not able to record any
copper species within the range of its detection limit.
The stronger metalꢀsupport (CuꢀN) interaction, as evidenced by FTIR spectra (Figure 2C), could
be responsible to prevent the metal leaching phenomenon in this current study.
For the kinetic study (Figure 3C), we have followed the first cycle of the above reaction where
there was no intended product, 4ꢀ(1Hꢀpyrrolꢀ1ꢀyl)benzonitrile (3ad), has been formed for the
first two hours, which is due to the required induction time for the reaction. From the kinetic
graph it is evident that initially, for 2ꢀ4h, the rate of the reaction was slow and after that a steady
conversion has been achieved. A slower rate again observed during the later stage of the reaction
probably due to the solid product formed during the reaction block the active site or the surface
of the copper nanoparticles causes the deactivation of the reaction.
3.5. Proposed mechanism
The mechanism of the title reaction has been represented in Scheme 1. In the presence of Cuꢀ
gCN (A) the oxidative addition of aryl bromide produce the Cu(II)ꢀgCN intermediate B which
under goes base mediated (Cs2CO3) transmetallation to generate the intermediate C. Reductive
elimination occurs in the intermediate C to regenerate the active metallic copper species and the
Nꢀarylated product to complete the catalytic cycle.
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R1
Br
X
N
Cu(0)
gCN
R2
Oxidative
addition
A
R2
Reductive
elimination
R1
X
Br
(II)
Cu
(II)
Cu
R2
N
R2
B
gCN
gCN
C
Cs
N
X
CsBr
Transmetallation
R1
Scheme 1: A proposed catalytic cycle for the N-arylation reaction.
It is also important to mention some, previously reported, similar type of Nꢀarylation reactions
using copper as a catalytic.
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Table 3: Comparative literature survey of the N-arylation reaction using copper catalyst
Previous
report
Reagent and
condition
Entry
Yield (%) in
the previous
report
Yield (%) in
the present
report
Reference
34
[a]
[b]
CuO hollow
spheres,
3aa
3ac
80
78
34
82
KOtBu,
Toluene,
1800C, 18h
Copper and
Porous Zincꢀ
Based MOF,
Cs2CO3,
35
3bb
3bc
3bd
3bg
99
82
91
95
85
93
95
90
DMSO,
900C,
hydrogen
atmosphere
[c]
CuIꢀ
binaphthyl
amine
complex,
Aryl iodide,
Toluene,
Cs2CO3,
82 0C
36
80
76
76
99
80
86
3ca
3cc
3ce
4. Conclusion
In summary, a simple and straight forward procedure for synthesis of carbon nitride supported
copper nanoparticles (CuꢀgCN) has been reported where the synthesized material was
successfully implied as catalyst towards Nꢀarylation reaction. Number of hetero aromatic
compounds, such as, pyrrole, pyrazole and substituted indoles and aromatic compounds, like
benzamide molecule, show Nꢀarylation coupling with various types of aryl bromide molecules.
We also note that the reaction between 4ꢀbromoacetophenone and indoleꢀ3 carboxaldehyde
forms only the aldol condensation product. We showed the recycling competence of the
synthesized material as a catalysts and the efficiency can be retained for at least six consecutive
cycles.
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DOI: 10.1039/C6NJ03584A
Acknowledgements
The authors (DN, SS and KM) acknowledge financial support from the Research Committee and
the Faculty of Science of the University of Johannesburg. KM also acknowledge financial
support from the National Research Foundation, South Africa. DN and SS further acknowledge
financial support from the Global Excellence and Stature fellowship from the University of
Johannesburg.
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