An efficient magnetic copper ferrite nanoparticle: For one pot synthesis of 2-substituted benzoxazole via redox reactions

Article abstract of DOI:10.1016/j.tetlet.2014.11.065

A new, green and sustainable approach for the synthesis of 2-substituted benzoxazole by using a one pot redox cascade condensation reaction of benzyl amine and 2-nitro phenol, catalysed by Cu Ferrite NPs is reported. Cu Ferrite NPs are magnetically separable, air stable and can be recycled up to fifth cycle without a significant loss in catalytic activity. The catalyst is characterised by FEG-SEM, TEM, EDAX and XRD.

Full text of DOI:10.1016/j.tetlet.2014.11.065

Tetrahedron Letters 56 (2015) 206–210
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An efficient magnetic copper ferrite nanoparticle: for one pot
synthesis of 2-substituted benzoxazole via redox reactions
⇑
Sachin A. Sarode, Jeevan M. Bhojane, Jayashree M. Nagarkar
Department of Chemistry, Institute of Chemical Technology (Deemed University), Nathalal Parekh Marg, Matunga, Mumbai 400016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new, green and sustainable approach for the synthesis of 2-substituted benzoxazole by using a one pot
redox cascade condensation reaction of benzyl amine and 2-nitro phenol, catalysed by Cu Ferrite NPs is
reported. Cu Ferrite NPs are magnetically separable, air stable and can be recycled up to fifth cycle with-
out a significant loss in catalytic activity. The catalyst is characterised by FEG-SEM, TEM, EDAX and XRD.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 29 August 2014
Revised 13 November 2014
Accepted 14 November 2014
Available online 27 November 2014
Keywords:
2-Substituted benzoxazole
Redox condensation
Copper ferrite NPs
Heterogeneous catalyst
Magnetically retrievable
Benzoxazoles have a wide range of applications in medicinal
chemistry and are found in many biologically active natural prod-
ucts and pharmaceutical targets.1 These compounds have been
extensively studied in HIV-reverse transcriptase inhibitors2 and
anticancer drugs.3 Derivatives of benzoxazole have applications
in various dyes due to their fluorescent nature.4 Benzoxazole moi-
eties also have a number of applications in medicinal chemistry,
particularly in drug discovery. Hence development of an efficient
methodology for the preparation of benzoxazole and its derivatives
has gained a lot of importance in current research. Various conven-
tional methods have been developed for the synthesis of 2-substi-
tuted benzoxazole such as the condensation of 2-aminophenol
with aldehydes or carboxylic acid,5 direct C–H activation of benz-
oxazole6 and metal catalysed intermolecular cyclisation of o-halo-
anilides.7 There are few reports on the synthesis of 2-substituted
benzoxazole using the oxidative coupling of 2-amino phenol with
benzyl amines or benzyl alcohol.8 However, some of the reported
methods suffer from drawbacks such as either the use of highly
strong acids or ligands. Harsh reaction conditions and the use of
strong oxidising reagents also employed in some of the methods.
Recently the redox condensation reactions, in synthesising
nitrogen containing heterocyclic compounds have attracted much
importance because of the efficient, direct and atom economical
approach.9 Some of the direct redox condensation reactions are
carried out by using the external reducing and oxidising reagents,
noble metals and ligands.10 2-substituted benzoxazole, are syn-
thesised by redox condensation reactions.
2-Nitrophenol was coupled with aromatic aldehydes using a
large amount of external reducing agent,11a,b with benzyl amine
at very high temperature, (more than 200 °C) giving low yields,11b
and also with benzyl alcohol using dppf as a homogenous catalyst
at 150 °C.11d Very recently, Tang et al. have reported the synthesis
of 2-substituted benzoxazole by using redox condensation of ben-
zyl alcohol and 2-nitrophenol on Au/TiO2.11e Transition metals and
particularly bimetallic salts and oxides have been used as catalyst
due to their activity as reducing and oxidising agents.12
One of the best strategies for green and sustainable chemistry is
the recovery and reusability of catalyst, without loss in catalytic
activity.13 Many organic reactions have been carried out by using
supported metal catalysts. Magnetic nanocatalysts have great
advantages in activity, separation and recyclability.13 In particular,
copper ferrite nanoparticles have been used in many organic
transformations such as Ullmann coupling, Sonogashira reaction
and C–N, C–S bond formations. Moreover, they also showed good
air stability in various organic transformations.14
Herein, we report the synthesis of 2-substituted benzoxazole by
using 2-nitrophenol and benzyl amine as starting materials in the
presence of copper ferrite nanoparticles (copper ferrite NPs).
Copper Ferrite NPs were prepared by using co-precipitation and
the thermal decomposition method.14,16a Cu Ferrite NPs were char-
acterised by using the XRD (X-ray diffraction), TEM (Transmission
electron spectroscopy), FEG-SEM (Field emission Gun-Scanning
⇑
Corresponding author. Tel.: +91 22 2361 1111; fax: +91 22 2414 5614.
E-mail addresses: jm.nagarkar@ictmumbai.edu.in, jayashreenagarkar@yahoo.
com (J.M. Nagarkar).
http://dx.doi.org/10.1016/j.tetlet.2014.11.065
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

S. A. Sarode et al. / Tetrahedron Letters 56 (2015) 206–210
207
Figure 1. (A) XRD of copper ferrite NPs. (B) TEM of copper ferrite NPs catalyst.
Figure 2. FEG-SEM image of (A) Fresh copper ferrite NPs catalyst. (B) Copper ferrite NPs after the 5th recycled.
O2
N
Table 1
N
NH2
Reaction condition
Screening of reaction conditions for redox condensation of benzyl amine with
2-nitrophenola
+
O
HO
Entry
Solvent
Temperature (°C)
Mol % of nano
copper ferrite
Yield(%)b
3a
1a
2a
Scheme 1. Copper ferrite catalysed redox condensation.
1
2
3
4
5
6
7
8
Neat reaction
—
NMP
Toluene
THF
DMSO
DMF
NMP
NMP
NMP
NMP
130
130
130
110
80
130
130
100
110
120
130
130
130
130
No Catalyst
No reaction
64
92
No reaction
No reaction
76
82
52
65
85
72
80
86
10
10
10
10
10
10
10
10
10
2.5
5
Electron Microscopy) and EDS (Energy-dispersive X-ray spectros-
copy) techniques. The crystallinity and phase purity was deter-
mined by using the XRD (Fig. 1A) characterisations. XRD pattern
of prepared copper ferrite nanoparticles shows the tetragonal
structure with good crystallinity (JCPDS card No. 034-0425).14
The average particle size calculated by using the Scherrer formula
was 12.99 nm. The particle size was also measured by using TEM
(Fig. 1B), and was found to be in the range of 11–14 nm. FEG-
SEM showed that the copper ferrite NPs remain in the same state
after the fifth cycle with some agglomeration of particles (Fig. 2).
The optimisation of reaction conditions is an integral part in the
synthesis of any molecule. In this redox reaction, we have chosen
benzyl amine and o-nitrophenol as starting substrates to optimise
the reaction condition for the synthesis of 2-phenylbenzo[d]oxa-
zole. (Scheme 1)16b Solvent, temperature and catalyst loading are
the important parameters which affect the yield of desired product
to a great extent. Initially the reaction was carried out without
solvent and without catalyst which did not result in any desired
9
10
11
12
13
14
NMP
NMP
NMP
7.5
10
c
No reaction
a
Reaction condition: 2-nitrophenol (1 mmol), benzyl amine (1.5 mmol), solvent
(1 mL) catalyst copper ferrite NPs.
b
Isolated yield.
Under inert condition.
c
product (Table 1 entry 1). Reaction with non polar solvents gave
negligible yields of the product (Table 1 entry 4). DMSO and DMF
solvents gave moderate yields, whereas reaction in THF did not

208
S. A. Sarode et al. / Tetrahedron Letters 56 (2015) 206–210
Table 2
Copper ferrite NPs catalysed redox condensation of benzyl amine (reactant I) with 2-nitrophenol (reactant II)a
O2
N
Nano Copper ferrite
130oC
N
O
NH2
+
NMP,16h
HO
Entry
1
Reactant I
Reactant II
Products
Yieldb(%)
92
HO
O
N
NH2
O2N
HO
OH
HO
NH2
O
2
84
O2N
N
O
HO
NH2
OMe
Cl
3
4
5
89
85
78
N
O
O2
N
MeO
F
HO
NH2
N
O2N
HO
O
NH2
Br
Br
O2N
N
HO
NH2
O
6
7
8
90
87
81
O2
N
N
O
HO
NH2
N
O2N
N
O
N
N
MeO
HO
OMe
OMe
NH2
O2N
OMe
NO2
NH2
OH
O
N
9
84
80
NO2
O
N
NH2
OH
10
Cl
Cl
NO2
O
N
NH2
OH
11
90
NO2
O
N
NH2
OH
OH
12
13
86
83
Br
Br
NO2
NH2
O
N
Cl
Cl

S. A. Sarode et al. / Tetrahedron Letters 56 (2015) 206–210
209
Table 2 (continued)
Entry
Reactant I
Reactant II
Products
Yieldb(%)
OH
O
N
NH2
14
82
NO2
a
Reaction condition: Benzyl amine (1.5 mmol), 2-nitrophenol (1 mmol), copper ferrite NPs (10 mol %), NMP (2 mL),130 °C,16 h.
Isolated yield.
b
Recyclability study
Table 3
Recyclability study of copper ferrite NPs
We have examined the recyclability of copper ferrite NPs
catalyst for the model reaction. The study indicated that the cata-
lyst can be reused up to the 5th cycle (Table 3) under optimised
reaction conditions without leaching of the Cu and Fe metals.
The catalyst was separated by using a magnet after completion
of the reaction washed with deionised water followed by ethyl
acetate, dried at 100 °C and reused for the next cycle. The leaching
of the metal was checked by using the standard hot filtration
method which confirms the heterogeneous nature of the catalyst.
Atomic absorption spectroscopy (AAS) of the filtrate clearly
indicated the absence of the Cu and Fe metals.
We proposed the plausible reaction mechanism (Fig. 3) on the
basis of the literature reports11,15 and the above mentioned results.
Cu Ferrite NPs provide the surface for the simultaneous oxidation
and reduction of the reactants. Benzyl amine on autoxidation
forms the corresponding benzaldehyde and releases ammonia
which was confirmed by the litmus paper test after the workup
of the reaction. Here ammonia acts as a hydrogen source for the
reduction of the nitro group of the o-nitro phenol. It reduces to
2-amino phenol which was then condensed with benzaldehyde
and gives 2-phenylbenzo[d]oxazole.
In conclusion, we have developed a simple, green and efficient
magnetic copper ferrite nanoparticle catalysed method for the syn-
thesis of substituted benzoxazoles by redox condensation. This
method provides the application of the air stable, cheap, magnetic
Copper ferrite nanoparticle in the redox type reaction. In this
protocol substituted benzoxazoles were synthesised in a one pot
method avoiding multi steps. Also the strategy gives further scope
to the mechanistic study of the redox reaction on copper ferrite
nanoparticles.
Run
1
2
3
4
5
Yield
92
92
90
88
84
give the desired product (Table 1 entries 5–7). It was observed that
NMP gave maximum yields of the desired product (Table 1 entry
3). This may be due to the appreciable solubility of oxygen in polar
solvent which may promote the redox reaction. The reaction did
not give the desired product under inert conditions which also sup-
ports the role of oxygen for carrying out the reaction (Table 1 entry
14). The effect of the temperature on the reaction was examined by
increasing the temperature from 100 °C to 130 °C (Table 1 entries
3, 8–10). The product yield increases with increase in temperature
up to 130 °C. Hence the next parameter catalyst loading was opti-
mised at 130 °C. It was observed that 10 mol % catalyst loading is
sufficient to give 92% yield of the products (Table 1 entries 3,
11–13). NMP as a solvent, 10 mol % of copper ferrite NPs and
130 °C temperature were the optimised reaction conditions for
the model reaction (Table 1 entry 3).
We applied these optimised reaction conditions to the various
substrates of 2-nitro phenol and benzylamine to explore the cata-
lytic activity of Cu-ferrite NPs. The various functional groups on the
primary benzyl amines and o-nitrophenols were well tolerated
affording good to excellent yields of the corresponding products.
The results are summarised in Table 2. Moderate to good yields
of the desired product was obtained using electron withdrawing
and electron donating substrates of the benzyl amines and 2-nitro
phenols, respectively, (Table 2 entry no. 1–8).Also o-nitro phenol
bearing chloro, methyl, bromo and substituents at different posi-
tions afforded good yields of the products (entries 10–14).
NH2
O2
O
H2O
O
N
CuFe2O4
NH3
CuFe2O4
OH
NH2
OH
NO2
Figure 3. Plausible reaction mechanism.

210
S. A. Sarode et al. / Tetrahedron Letters 56 (2015) 206–210
Cao, X.; Xiao, F.; Liu, S.; Deng, G. Org. Lett. 2013, 15, 4900; (e) Wu, M.; Hu, X.;
Acknowledgment
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The authors are thankful to the UGC-SAP, New Delhi, India for
the award of fellowship.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.
11.065.
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16. (a) Procedure for synthesis of CuFe2O4 nanoparticles
Copper ferrite NPs were prepared by thermal decomposition of Cu(NO3)2 and
Fe(NO3)3 in water in the presence of 1 M sodium hydroxide. To a solution of
Fe(NO3)3Á9H2O (8.2 mmol) and Cu(NO3)2Á3H2O (4.1 mmol) in 100 mL of
distilled water, 1 M of NaOH added dropwise under stirring to the above
solution till pH 10 .Then reddish-black precipitate was formed at room
temperature (30 °C). The reaction mixture was warmed to 90 °C and stirred.
After 2 h, it was cooled to room temperature and the magnetic particles so
formed were separated by a magnetic separator. It was then washed with
water and ethanol and catalyst was kept in air oven for overnight at 80 °C. Then
the catalyst was grinded in a mortar-pestle and kept in a furnace at 700 °C for
5 h. Then it was cooled to room temperature slowly which gave the magnetic
Copper ferrite NPs . The catalyst is characterised by FEG-SEM, EDAX and XRD.
(b) Typical reaction procedure for the synthesis of 2-phenylbenzoxazole:
An oven dried Schlenk tube equipped with a magnetic stirring bar charged
under air with 2-nitrophenol (1 mmol), benzyl amine (1.5 mmol) and Copper
ferrite NPs (10%),followed by 2 mL of NMP (N-methyl pyrrolidone) as a solvent.
Reaction mixture was heated in an oil bathe at 130 °C and was stirred for 16 h.
The reaction was monitored by GC and TLC. After completion of reaction the
reaction mixture was cooled to room temperature and the reaction mass was
diluted with ethyl acetate .Copper ferrite NPs was separated by using external
magnet. Then the solution was filtered through the plug of cellite. The filtrate
was washed with water (3 Â 10 mL). The organic layer was separated and
dried over anhydrous sodium sulphate. The solvent was removed under
vacuum to get the crude product which was purified by column
chromatography on silica gel eluting with the mixture of pet ether/ethyl
acetate (20:1) mixture to afford pure product.
The purity and identity of known product are confirmed by 1H NMR and GC–
MS spectroscopic Techniques.
2-(3,5-Dimethoxyphenyl)benzo[d]oxazole: White solid compound. Melting
Point: 105 °C
1H NMR (300 MHz, CDCl3): d 7.76–7.78 (m, 2H), 7.59–7.56 (m, 1H), 7.41(m 1H)
7.26–7.35(m, 2H), 6.24–6.63 (m, 1H), 3.89 (s, 6H)
13C NMR (75 MHz DMSO) d 162.42, 161.3, 150.6, 141.8, 128, 125.9, 125.5,
120.2, 105, 104, 55.75.
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