Copper(II) supported on magnetic chitosan: A green nanocatalyst for the synthesis of 2,4,6-triaryl pyridines by C-N bond cleavage of benzylamines

Article abstract of DOI:10.1039/c6ra00102e

In this paper, Cu/magnetic chitosan has been synthesized and used as a new green nanocatalyst for highly efficient synthesis of 2,4,6-triaryl pyridines via C-N bond cleavage of benzylamines under aerobic oxidation at 90 °C. The chemical and structural properties of the synthesized catalyst were determined by scanning electron microscopy, energy-dispersive X-ray, X-ray powder diffraction, thermogravimetric analysis and flame atomic absorption spectroscopies. It is found that the catalyst can be easily separated from the reaction mixture by an external magnetic field and recycled several times without a significant loss in activity.

Full text of DOI:10.1039/c6ra00102e

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Copper(ΙΙ) supported on magnetic chitosan: A green nanocatalyst for the synthesis of 2,4,6ꢀ
triaryl pyridines by CꢀN bond cleavage of benzylamines
Ahmad Shaabani,* Mahmoud Borjian Boroujeni, Mohmmad Sadegh Laeini
Faculty of Chemistry, Shahid Beheshti University, G. C., P. O. Box 19396ꢀ4716, Tehran, Iran
*
Email address: aꢀshaabani@sbu.ac.ir (Ahmad Shaabani)
Abstract
In this paper, Cu/magnetic chitosan has been synthesized and used as a new green
nanocatalyst for a highly efficient synthesis of 2,4,6ꢀtriaryl pyridines via a CꢀN bond cleavage of
benzylamines under aerobic oxidations at 90 °C. The chemical and structural properties of the
synthesized catalyst were determined by scanning electron microscopy, energyꢀdispersive Xꢀray,
Xꢀray powder diffraction, thermogravimetric analysis and flame atomic absorption
spectroscopies. It is found that the catalyst can be easily separated from the reaction mixture by
an external magnetic field and be recycled for several times without a significant loss of activity.
Keywords
:
Chitosan, heterogeneous catalyst, copper, magnetic, triarylpyridines, CꢀN bond cleavage
1. Introduction
In recent years, developing routs to produce materials based on green chemistry which
minimizes pollution have been focused.1 To reach these goals, the development of heterogeneous
catalysts in the synthesis of organic compounds is highly investigated due to the ease of
handling, reusability and simple workꢀup.2 Immobilization of preciousꢀmetal and nonpreciousꢀ

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metal catalysts on various inorganic, petrochemical derived polymers and other synthetic
polymers to produce heterogeneous catalytic systems has been widely reported.3ꢀ9 Nevertheless
the enthusiasm for a cleaner and sustainable chemistry has resulted in exploiting abundant
natural polymers such as chitosan (Cs) for catalytic applications due to its high surface area, low
cost, biocompatibility, biodegradability, and nonꢀtoxicity.10ꢀ12
Chitin is the second most abundant natural polymer after cellulose which can be found in
insects, fungi, shrimps and crabs.13 Chitosan (Cs), a chemically stable, nonꢀtoxic and
biodegradable polysaccharide prepared from chitin, is an excellent candidate to be used as a
support for copper,14ꢀ17 ruthenium,18 rhodium,19 cerium20 and other transition metals due to its
insolubility in organic solvents and the presence of functionalizable amino groups in the
structure. In addition, mesoporous, nitrogenꢀcontaining carbon materials obtained directly from
pure chitin are used as an absorbent to remove toxic heavy metals and as a catalyst for
epoxidation of styrene.21 Although chitin and chitosan have been used in industrial chemistry
such as textile, cosmetics and biomedicine, they also have a significant potential for a sustainable
production of small nitrogenꢀcontaining chemicals such as ethanolamine.22 Nowadays, various
investigations have been carried out to develop new protocols for an efficient conversion of
chitin to value–added chemicals.23ꢀ26
In recent decade, the synthesis of supported magnetic nanoparticles has attracted a
considerable attention in catalyst science due to their availability, low toxicity, high catalytic
activity, excellent surface area and magnetic separability thereby eliminating the filtration
process after the end of reactions.27 Immobilization of iron oxide nanoparticles on biodegradable
polymers such as cellulose and chitosan has produced highly efficient adsorbents for the removal
of metals, drug carriers and reusable nano catalysts.28ꢀ32 To the best of our knowledge, there is no

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report in which copper(ΙΙ) magnetic chitosan nanocomposite has been used for an organic
transformation.
The CꢀN bond cleavage is an important synthetic strategy that has been widely
investigated.33 It is known that transition metals, such as palladium,34 ruthenium,35 and copper36
are catalysts which promote the CꢀN bond cleavage and the CꢀN bond formation. Specially, the
application of copperꢀbased catalysts in this reaction is remained as a considerable topic due to
the low toxicity and cost of copper compared to other more common noble metals. Recently
Wang et al. reported the C−N bond cleavage and the formation for the synthesis of
benzimidazo[1,2-a]quinazoline derivatives using CuI/Lꢀproline.37 Jiang et al. developed an
efficient copperꢀcatalyzed C−N bond cleavage of aromatic methylamines to construct pyridine
derivatives,38.
Pyridine rings are one of the most important heterocyclic moieties which have many
applications in natural products, organic and medical chemistry and functional materials.39
Pyridines synthesis is an interesting topic in modern synthetic chemistry among which, 2,4,6ꢀ
trisubstituted pyridine derivatives have been used in supramolecular chemistry due to their pꢀ
stacking ability along with Hꢀbonding capacity.40 The development of new heterogeneous
catalysts for the synthesis of multiꢀsubstituted pyridines is in high demand.41ꢀ42
In continuation of our interest in the sustainable benign pathway for organic
transformations and nanocatalysis,43ꢀ48 we wish to introduce the synthesis of 2,4,6ꢀtriaryl
pyridine derivatives by the CꢀN bond cleavage of benzyl amines in the presence of Cu (ΙΙ)
immobilized on magnetic chitosan as a nanocatalyst. The obtained catalytic system, Cu/magnetic
chitosan, exhibits a high activity and selectivity for the synthesis of 2,4,6ꢀtriaryl pyridine
derivatives under aerobic conditions. The catalyst can be separated from the reaction mixture

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using an external magnetic field and be reused for five consecutive reaction times without a
significant decrease in the catalytic activity.
2. Experimental
2.1.Materials and Methods
Chitosan (molecular weight: 100,000ꢀ300,000) was purchased from Acros Organics.
Iron(III) chloride hexahydrate, iron(II) chloride tetrahydrate, copper(II) chloride dehydrate and
sodium hydroxide were purchased from Sigma Aldrich. Dimethylsulfoxide (DMSO),
dimethylformamide
chloroacetophenone,
(DMF),
toluene,
acetophenone,
benzylamine,
4′ꢀmethylacetophenone,
4ꢀmethylbenzylamine,
4′ꢀ
4ꢀ
4′ꢀbromoacetophenone,
chlorobenzylamine, 4ꢀmethoxybenzylamine, and 2ꢀpicolylamine were purchased from Sigma
Aldrich. All chemicals were used without further purifications.
The FTꢀIR spectra were recorded on a Bomem MBꢀSeries FTꢀIR spectrometer. The
thermal analysis (TGAꢀDTA) was carried out using a Bahr STAꢀ503 instrument at a heating rate
of 10 °C minꢀ1 in air. The Xꢀray powder diffraction (XRD) pattern was recorded on a STOE
diffractometer with CuꢀKα radiation (λ= 015418 nm). The scanning electron microscopy (SEM)
and the energy dispersive Xꢀray spectroscopy (EDS) characterization of catalyst were performed
using an electron microscopy Philips XLꢀ30 ESEM. All the samples were sputtered with gold
1
before observation. The H NMR spectra were recorded on a BRUKER DRXꢀ300 AVANCE
spectrometer. The NMR spectra were obtained in CDCl3 and DMSOꢀd6 using the tetramethyl
silane (TMS) as internal standard. The melting points of the products were measured by an
electrothermal 9100 apparatus. The concentrations of copper and iron were estimated using
Shimadzu AAꢀ680 flame atomic absorption spectrophotometer and an inductively coupled
plasma optical emission spectrometer (ICPꢀOES) Varian Vista PRO Radial. The XPS analysis

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was performed using a Gammadataꢀscientifica ESCA 200 hemispherical analyzer equipped with
an Al Kα (1486.6 eV) Xꢀray source.
2.2.General procedure for synthesis of magnetic chitosan nanoparticles (MCs)
The synthesis of Fe3O4 nanoparticles with Cs was carried out according to the literature
which has described procedures.32 An aqua solution of FeCl3·6H2O (27.0 mg) and FeCl2·4H2O
(9.95 mg) with a molar ratio of 2:1 in 5 mL distilled water was prepared and added to a
suspension of the Cs (30.0 mg) in 20 mL distilled water under a vigorous stirring under nitrogen
atmosphere. After stirring for 10 min, the sodium hydroxide solution was added to the mixture
and the stirring was strongly continued for 4 h at 80 ºC. After cooling the suspension to the room
temperature, the MCs was separated by the magnet decantation, was washed with distilled water,
ethanol and, finally was dried under the vacuum at the room temperature.
2.3.General procedure for synthesis of MCsꢀCu(ΙΙ) nanocatalyst
The MCs (0.1 g) was suspended in 20 mL of distilled water and 0.03 g of CuCl2.2H2O
was added to the mixture and stirring was continued for 3h. The catalyst was separated using a
magnet and was dried under the vacuum at 50 °C.
2.4.General procedure for synthesis of copper nanoparticles@magnetic chitosan
The copper nanoparticles@magnetic chitosan was synthesized according to the pervious
report.49 Briefly, 0.5 g of the magnetic chitosan was dispersed in 25 ml of ethanol in a 100 ml
roundꢀbottomed flask, after which 25 ml (0.25 mmol) of the ethanolic solution of the copper(II)
acetate was added dropwise under vigorous stirring. After 4h of stirring, the copper ions were
completely adsorbed by chitosan and the MCsꢀCu(CH3COO)2 was separated by an external
magnet and was dried in a desiccator. The dried MCsꢀCu(CH3COO)2 was again dispersed in 50

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ml of methanol and 15 ml of NaBH4 (5 mmol) solution was added slowly under vigorous string
under nitrogen atmosphere. The copper nanoparticles@magnetic chitosan were separated by an
external magnet and washed several times with methanol and then dried in the desiccator.
2.5.General procedure for preparation of 2,4,6ꢀtriaryl pyridines
A ketone (2 mmol), benzylamine (1.2 mmol), MCsꢀCu (ΙΙ) nanocatalyst (0.064 g, 5 mol
%) were added to a 25 mL tube. The mixture was stirred for 8 h at 90 °C. Upon completion;
acetone was added to the mixture and the catalyst was separated by an external magnet. The
solid product was recrystallized from acetoneꢀwater 9:1 (v/v) to produce the pure product.
2.6.Product characterization data of 2,4,6ꢀtriaryl pyridines
2,4,6ꢀTriphenylpyridine (3a), yield: 90, C23H17N, white solid, FTꢀIR (KBr) cmꢀ1: 1450ꢀ1600,
1
758, 692 H NMR (300.13 MHz, CDCl3), δ 8.23 (d, J= 7.5 Hz, 4H), 7.9 (s, 2H), 7.7 (d, J= 6.9,
2H), 7.2ꢀ7.9 (9H, m, HꢀAr), 13C NMR (75.47 MHz, CDCl3): 117.02, 127.40, 128.72, 129.68,
129.73, 139.19, 139.26, 150.06, 157.02.
2,6ꢀDiphenylꢀ4ꢀ(pꢀtolyl)pyridine (3b), yield:91, C24H19N, white solid, FTꢀIR (KBr) cmꢀ1: 3020,
1
2950, 1450ꢀ1600, 759, 692. H NMR (300.13 MHz, CDCl3), δ 8.2 (d, 4H), 7.9 (s, 2H), 7.7 (d,
2H), 7.2ꢀ7.9 (9H, m, HꢀAr), 13C NMR (75.47 MHz, CDCl3): 21.25, 116.66, 127.54, 129.17,
129.63, 130.14, 135.18, 139.41, 156.96.
4ꢀPhenylꢀ2,6ꢀdiꢀpꢀtolylpyridine (3c), yield: 90, C25H21N, white solid, FTꢀIR (KBr) cmꢀ1: 3022,
1
2950, 1450, 1600, 757, 695. H NMR (300.13 MHz, CDCl3), δ 8.13 (d, J= 7.8 Hz, 4H), 7.87 (s,
2H), 7.76 (d, J=7.2 Hz, 2H), 7.47ꢀ7.58 (m, 3H), 7.34 (d, J=7.8 Hz, 4H), 2.46 (s,6 Hz), 13C NMR
(75.47 MHz, CDCl3): 21.30, 116.42, 126.90, 127.12, 128.82, 129.10, 129.35, 136.80, 138.92,
139.20, 149.93, 157.32

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2,4,6ꢀTriꢀpꢀtolylpyridine (3d), yield: 92, C26H23N, white solid, FTꢀIR (KBr) cmꢀ1: 3022, 2950,
1
1450, 1600, 757, 695. H NMR (300.13 MHz, CDCl3), δ 8.12 ( d, J= 7.8 Hz, 4H), 7.86 (s, 2H),
7.78 (d, J=7.8 Hz, 2H), 7.33ꢀ7.36 (m, 3H), 2.46 (s,9 Hz), 13C NMR (75.47 MHz, CDCl3): δ
21.35, 21.43, 116.44, 127.07, 127.12, 129.48, 129.87, 136.20, 136.83, 139.04, 139.07, 150.03,
150.326
2',6'ꢀDiphenylꢀ2,4'ꢀbipyridine (3e), yield: 95, C22H16N2, yellow solid, FTꢀIR (KBr) cmꢀ1: 3059,
1
1594, 1488, 1321, 830, 770, 690. H NMR (300.13 MHz, CDCl3), δ 8.18ꢀ8.28 (m, 6H), 7.85ꢀ
7.93 (m, 2H), 7.76 (t, J=7.5 Hz, 2H), 7.47ꢀ7.59 (m,6 Hz), 13C NMR (75.47 MHz, CDCl3): δ
119.13, 126.43, 127.20, 127.52, 127.70, 128.92, 129.49, 129.54, 129.87, 137.09, 139.40, 148.01,
157.33.
2,6ꢀBis(4ꢀchlorophenyl)ꢀ4ꢀphenylpyridine (3f), yield: 90, C23H15Cl2N, white solid, FTꢀIR (KBr)
1
cmꢀ1: 3052, 1489, 1486, 1597, 1524, 760, 694, H NMR (300.13 MHz, DMSOꢀd6), δ 8.36 (d, J=
13
8.4 Hz, 4H), 8.23 (s, 2H), 8.04 (d, J=7.6 Hz, 2H), 7.50ꢀ 7.61 (m, 7H), C NMR (75.47 MHz,
DMSOꢀd6): δ, 117.31, 127.87, 129.20, 129.55, 129.71, 129.91, 134.65, 137.86, 150.31, 155.75.
4ꢀ(4ꢀChlorophenyl)ꢀ2,6ꢀdiphenylpyridine (3g), yield: 95, C23H16ClN, white solid, FTꢀIR (KBr)
cmꢀ1: 3061, 1599, 1545, 1489, 775, 692, 1H NMR (300.13 MHz, DMSOꢀd6), δ 8.12 (d, J=7.2 Hz,
13
4H), 8.14 (s, 2H), 7.91 (d, J=8 Hz, 2H), 7.41 (d, J=6.8 Hz, 2H), 7.28ꢀ7.36 (m, 6H), C NMR
(75.47 MHz, DMSOꢀd6): δ, 116.88, 127.44, 129.17, 129.47, 129.64, 134.70, 136.94, 139.18,
148.64, 157.05.
4ꢀ(4ꢀMethoxyphenyl)ꢀ2,6ꢀdiphenylpyridine (3h), yield: 85, C24H19NO, white solid, FTꢀIR (KBr)
1
cmꢀ1: 3035, 1596, 1547, 1486, 750, 690, H NMR (300.13 MHz, CDCl3), δ 8.30 (d, J=7.2 Hz,

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4H), 8.15 (s, 2H), 7.93 (d, J=8 Hz, 2H), 7.45ꢀ7.55 (m, 6H), 7.34ꢀ7.36 (d, J=8 Hz, 2H), 3.36
(s,3H), (13C NMR 75.47 MHz, CDCl3): δ, 55.80, 114.80, 118, 127.60, 128.40, 129.30, 130.20,
152, 155.20, 161.10.
3. Results and discussion
The magnetic chitosan nanoparticles were prepared by the chemical coꢀprecipitation of
Fe 3+ and Fe 2+ ions with a molar ratio 2:1.32 Then, the immobilization of copper on the magnetic
chitosan was carried out in an aqueous solution of copper (ΙΙ) chloride for 3 h under neutral
conditions.17
Figure 1 shows the XRD patterns of the chitosan and the MCsꢀCu (ΙΙ) nanocatalyst.
There are six characteristic peaks at 2θ= 30.04 °, 35.5°, 43.12°, 53.44°, 57.04° and 62.8° in the
XRD pattern of the MCsꢀCu (ΙΙ) nanocatalyst that confirm the standard pattern with a spinel
structure for the crystalline magnetite (JCPDS card no. 01ꢀ1111). Because of the diffraction
peaks of Fe3O4 are stronger than the diffraction peaks of chitosan, the peaks related to the
chitosan were not observed.32 According to the Scherrer’s equation the mean crystallite size
calculated was 23 nm.

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10
20
30
40
50
60
70
80
2 Theta/ Degree
Figure 1. The XRD patterns of chitosan (red) and the MCsꢀCu (ΙΙ) nanocatalyst (black)
The Cu (II) loading on the MCsꢀCu(ΙΙ) nanocatalyst was determined as 5 wt % based on
the FAAS analysis. The Fe loading level of catalyst was measured by ICPꢀAES and it was
obtained 16.67 wt %.
Thermogravimetric analysis (TGA) was used in order to obtain information on the
thermal stability of the chitosan and the MCsꢀCu (ΙΙ) nanocatalyst (Figure 2). The TGA results
confirm that the Cs was stable over 200 °C and there were three steps of mass losses for the
catalyst in the temperature range of 50ꢀ260 °C, 260ꢀ315 °C and 315ꢀ520 °C. The weight loss of
the catalyst is about 74 % that corresponding to the thermal decomposition of the chitosan.

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0
ꢀ10
ꢀ20
ꢀ30
ꢀ40
ꢀ50
ꢀ60
ꢀ70
ꢀ80
ꢀ90
ꢀ100
0
100
200
300
400
500
600
Temperature/ °C
Figure 2. The TGA thermogram of the chitosan (black) and the MCsꢀCu(ΙΙ) nanocatalyst (red)
The SEM analysis was used to study the morphology and the structure of the MCsꢀCu
(ΙΙ) nanocatalyst. The SEM images show an excellent dispersity of Fe3O4 nanoparticles on the Cs
(Figure 3). Also, the energy dispersive spectroscopy (EDS) analysis clearly illustrates the
presence of iron and copper in the nanocatalyst (Figure 4).
Figure 3. The SEM image of the MCsꢀCu(ΙΙ) nanocatalyst (A,B)

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Figure 4. The (EDS) analysis of The MCsꢀCu(ΙΙ) nanocatalyst
To show the efficiency of the MCsꢀCu(ΙΙ) as a nanocatalyst, we investigated the reaction
of acetophenone (2 mmol) and benzylamine (1.2 mmol) for the synthesis of 2,4,6ꢀtriaryl pyridine
derivates in toluene for 12 h under aerobic oxidation in the presence of various amounts of the
catalyst (Table 1). The reaction in the absence of the catalyst did not result in the foremetioned
product (Table1, Entry 1). In addition, in the presence of the magnetic chitosan, no reaction
occurred even after 8 h (Table1, Entry 2). As the amount of the catalyst was increased (5 mol%),
the reaction went to completion at 90 °C (Table1, Entries 3ꢀ6) . This reaction in the presence of
CuCl2 as a homogeneous catalyst in the air gave the 2,4,6ꢀtriaryl pyridines in 40% yield (Table
1, Entry 7). However, the separation of the catalyst from the reaction mixture was difficult. The
yield of reaction in oxygen atmosphere was not increased (Table1, Entry 8).
49
For more investigation, copper nanoparticles@magnetic chitosan were prepared and
used for this reaction. As shown in Table 1, the yield of the reaction in the presence of the copper
nanoparticles@magneticꢀchitosan was not significantly changed compared to that in the presence
of the MCsꢀCu(ΙΙ) nanocatalyst. Therefore, the MCsꢀCu(ΙΙ) nanocatalyst was found to be a better
catalyst than the copper nanoparticles@magnetic chitosan due to its simple preparation. (Table 1,

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Entry 9). Next, various solvents were used to study their effect on the yield of product.
Surprisingly, the reaction did not show a strong solvent dependence. Also, using toluene had no
efficient influence on the transformation (Table1, Entry 10ꢀ14). Thus, the reaction was carried
out under solvent free conditions. After screening different temperatures, 90 °C was obtained as
the best temperature for the reaction. The optimized conditions for the synthesis of the 2,4,6ꢀ
triaryl pyridines were 5 mol% of the catalyst in the absence of any additive or solvent under the
air at 90 °C.
Table 1. Screening the reaction conditions a
Entry
Catalyst (mol%)
None
Solvent Oxidant
Temperature (°C)
Yield b ( %)
1
2
3
4
5
6
7
8
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
air
air
air
air
air
air
air
O2
90
90
90
90
90
90
90
90
0
MCs
0
MCsꢀCu(ΙΙ) nanocatalyst (1)
MCsꢀCu(ΙΙ) nanocatalyst (3)
MCsꢀCu(ΙΙ) nanocatalyst (5)
MCsꢀCu(ΙΙ) nanocatalyst (8)
CuCl2 (5)
30
60
90
90
40
90
MCsꢀCu(ΙΙ) nanocatalyst (5)
9
Copper nanoparticles@magneticꢀ
Chitosan (5)
toluene
air
90
90
10
MCsꢀCu(ΙΙ) nanocatalyst (5)
neat
air
90
90

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11
12
13
14
15
16
MCsꢀCu(ΙΙ) nanocatalyst (5)
MCsꢀCu(ΙΙ) nanocatalyst (5)
MCsꢀCu(ΙΙ) nanocatalyst (5)
MCsꢀCu(ΙΙ) nanocatalyst (5)
MCsꢀCu(ΙΙ) nanocatalyst (5)
MCsꢀCu(ΙΙ) nanocatalyst (5)
toluene
water
DMSO
DMF
neat
air
air
air
air
air
air
90
90
88
60
70
40
60
90
90
90
70
neat
110
a Reaction conditions: acetophenone (2.00 mmol), benzylamine (1.2 mmol), catalyst, air (1 atm), 8 h.
b isolated yield
With the optimized reaction conditions established, the scope of benzylamines and
methyl ketones was examined to explore the generality of the 2,4,6ꢀtriaryl pyridines synthesis
under the aerobic conditions.
Acetophenones derivatives with a wide array of functional groups such as chloro, bromo
and methyl on the aryl group were employed to synthesis of the 2,4,6ꢀtriaryl pyridines. Phenyl
methyl ketones with electronꢀwithdrawing or electronꢀdonating substituents on the aromatic ring
result in the forementioned product. Using nonmethyl ketones such as cyclohexanone did not led
to the desired product. Substituted benzyl amines were employed to give the desired pyridines in
good yields. According to theprevious report,38 under the homogeneous copperꢀcatalysed system,
2ꢀpyridylmethylamine with methyl ketone could not produce the 2,4,6ꢀtriaryl pyridines.38
However, in the case of our reaction conditions, using the MCsꢀCu(ΙΙ) nanocatalyst, the 2,4,6ꢀ
triaryl pyridines were produced from the reaction of 2ꢀpyridyl methylamines with methyl ketones
(Table 2, Entry 5).

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Table 2. Synthesis of 2,4,6ꢀtriaryl pyridines using MCsꢀCu(ΙΙ) nanocatalyst a
Entry R1
R2
Product
3a
Yield % b
90
Melting point
1
2
H
H
135ꢀ138
H
CH3
H
3b
85
118ꢀ120
3
CH3
CH3
H
3c
85
154 ꢀ 156 °C
174 ꢀ 176 °C
98ꢀ100 °C
175 ꢀ 178
127ꢀ128
4
CH3
H, X=N
H
3d
80
5
3e
90
6
Cl
H
3f
80
7
Cl
3g
85
8
H
OMe
OMe
H
3h
80
99ꢀ101
9
Cl
Br
3i
75
190ꢀ192
10
3j
70
98ꢀ100
a Reaction conditions: 1(2 mmol), 2 (1.2 mmol), MCsꢀCu(ΙΙ) nanocatalyst (0.064 g, 5 mol %), air, at 90 °C for 8 h. b isolated yield
It is important to note that the cross reactions with two different methyl ketones provided
asymmetric 2,4,6ꢀtriaryl pyridines as a major product. For example, when benzylamine (1.2
mmol) and pꢀmethylacetophenone (1 mmol) were treated with acetophenone (1 mmol), the 2,4,6ꢀ
triaryl pyridines were synthesized in a moderate yield. The results are summarized in Table 3.

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Table 3. The Synthesis of the asymmetric 2,4,6ꢀtriaryl pyridines using the MCsꢀCu(ΙΙ) as a nanocatalyst a
Entry R1
R2
R3
product
4a
Melting point
123 ꢀ 124
Yield % b
1
2
H
H
H
CH3
CH3
60
50
CH3
4b
133ꢀ135
a Reaction conditions: 1(1 mmol), 2 (1.2 mmol), 3 (1 mmol), MCsꢀCu(ΙΙ) nanocatalyst (0.064 g, 5 mol %), air, at 90 °C for 8 h. b isolated yield
38,50ꢀ58
Based on the previous reports,
a plausible mechanism was proposed in Scheme 1.
At first, singleꢀelectron transfer oxidation of benzylamine occurred in the presence of the MCsꢀ
Cu(ΙΙ) nanocatalyst and then imine was produced from the aminolysis reaction. Next the
hydrolysis of imine as a reversible process could provide benzylamine and benzaldehyde. The
condensation of benzaldehyde, ketone, and benzylamine in the presence Cu(ΙΙ) as a Lewis acid
produced 1,4ꢀdihydropyridine intermediate. Finally, the 2,4,6ꢀtriaryl pyridine was produced from
the oxidation of 1,4 dihydropyridine intermediate under the catalytic aerobic conditions.

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Scheme 1. Plausible reaction mechanism
The stability of the MCsꢀCu(ΙΙ) nanocatalyst toward the leaching of copper ions was
examined, as well. Briefly, a mixture of ketone and benzylamine in the presence of catalyst was
stirred in toluene at 90 °C for 15 min, next the catalyst was separated by an external magnet from
the reaction mixture and the reaction was continued by catalystꢀfree mixture under the same
conditions for an additional 8 h. the yield of the reaction was 10 % before the filtration of the
catalyst and the final yield was 11%. Also, according to the FAAS analysis results, the leaching
of the copper ions in to a solution phase was less than 0.5% . This means that the leaching of
copper should be negligible.
The recyclability of the MCsꢀCu(ΙΙ) nanocatalyst was surveyed for the synthesis of the
2,4,6ꢀtriaryl pyridines under the optimized conditions at 90 °C. After the reaction time, the solid
catalyst was separated by an external magnet, was washed and dried at 80 °C and was used in the
next run without further treatment. It was observed that in the next five consecutive uses of the

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MCsꢀCu(ΙΙ) nanocatalyst, the catalytic activity and selectivity did not significantly decrease
(Figure 5).
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Run
Figure 5. Recycle of the MCsꢀCu(ΙΙ) nanocatalyst for the synthesis of 2,4,6ꢀtriaryl pyridines
The XPS analysis of the catalyst after five runs of the reaction was shown in Figure 6.
The Cu 2p
and Cu 2p
were clearly observed at 934.05 and 954.12 eV, respectively, that
1/2
3/2
confirm the Cu(ΙΙ) ions in the catalyst were stable under the reaction conditions.59

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Figure 6. The cu 2p coreꢀlevel XPS spectrum of the MCsꢀCu(ΙΙ) nanocatalyst
4. Conclusion
The MCsꢀCu(ΙΙ) nanocatalyst was synthesized by a uniform distribution of Fe3O4
nanoparticles on the chitosan matrix. The new catalyst successfully promoted the synthesis of the
2,4,6ꢀtriaryl pyridines via a CꢀN bond cleavage of benzylamine under aerobic oxidations with
fairly good yields. The reaction was carried out under environmentally benign, solventꢀfree and
aerobic conditions. The structure of the catalyst was confirmed by the XRD, TGA, EDS and the
SEM. The MCsꢀCu(ΙΙ) nanocatalyst showed advantages such as magnetic separablitiy,
recyclability, good chemical stability, low solubility in organic solvents, and a uniform
distribution of the nanoparticles on the chitosan. The leaching tests indicated that the reaction
was mainly heterogeneous in nature.
Acknowledgments

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We gratefully acknowledge financial support from the Catalyst Center of Excellence (CCE)
of Shahid Beheshti University and the Iran National Elites Foundation (INEF).
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Copper(ΙΙ) supported on magnetic chitosan: A green nanocatalyst for the synthesis
of 2,4,6ꢀtriaryl pyridines by CꢀN bond cleavage of benzylamines
Ahmad Shaabani,* Mahmoud Borjian Boroujeni, Mohmmad Sadegh Laeini