Magnetically separable nano CeO2: A highly efficient catalyst for ligand free direct C-H arylation of heterocycles

Article abstract of DOI:10.1016/j.tetlet.2014.12.069

An efficient, ligand free, nano CeO₂-Fe₃O₄ catalyzed direct C-H arylation of heteroarenes such as benzoxazole and benzothiazole has been carried out with aryl halides in DMSO or arenediazonium salts in water. The catalyst exhibited high activity with moderate to excellent product yields under mild reaction conditions. Nano CeO₂-Fe₃O₄ was synthesized and characterized by SEM, TEM, EDAX, XRD, FTIR, DSC-TGA, and ICP-MS analyses. The catalytic activity and stability of the catalyst were excellent, even after the ten cycle of recyclability

Full text of DOI:10.1016/j.tetlet.2014.12.069

Accepted Manuscript
Magnetically separable nano CeO : A highly efficient catalyst for ligand free
2
direct C–H arylation of heterocycles
Radheshyam S. Shelkar, Kishor E. Balsane, Jayashree M. Nagarkar
PII:
S0040-4039(14)02135-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.12.069
TETL 45593
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
18 August 2014
10 December 2014
12 December 2014
Please cite this article as: Shelkar, R.S., Balsane, K.E., Nagarkar, J.M., Magnetically separable nano CeO : A highly
2
efficient catalyst for ligand free direct C–H arylation of heterocycles, Tetrahedron Letters (2014), doi: http://
dx.doi.org/10.1016/j.tetlet.2014.12.069
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Magnetically separable nano CeO : A highly
efficient catalyst for ligand free direct C-H
arylation of heterocycles
2
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a
Radheshyam S Shelkar , Kishor E Balsane , Jayashree M Nagarkar ∗
a
a,

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Magnetically separable nano CeO : A highly efficient catalyst for ligand free direct
2
C-H arylation of heterocycles
a
Radheshyam S Shelkar , Kishor E Balsane , Jayashree M Nagarkar ∗
a
a,
a
Department of Chemistry, Institute of Chemical Technology,Matunga, Mumbai-400 019.
ARTICLE INFO
ABSTRACT
Article history:
Received
2 3 4
An efficient, ligand free, nano CeO -Fe O catalyzed direct C-H arylation of heteroarenes such
as benzoxazole and benzothiazole has been carried out with aryl halides in DMSO or
arenediazonium salts in Water. The catalyst exhibited high activity with moderate to excellent
Received in revised form
Accepted
2 3 4
product yields under mild reaction conditions. Nano CeO -Fe O was synthesized and
Available online
characterized by SEM, TEM, EDAX, XRD, FTIR, DSC-TGA and ICP-MS analysis. The
catalytic activity and stability of the catalyst was excellent, even after fifth cycles of
recyclability.
Keywords:
C-H arylation
Heterocycles
2
009 Elsevier Ltd. All rights reserved.
Arenediazonium Salts
2 3 4
Nano CeO -Fe O
Heterogeneous Catalyst
1
. Introduction
catalytic system is highly desired to overcome the drawbacks of
earlier reported methods.
Heterocycles are the key intermediates in synthesis of various
1
pharmaceutical and biologically active products. Therefore, due
to the importance and their great applications in medicinal
chemistry, the synthesis of these structural motifs is more
The first report has been demonstrated by Zhang and co-
workers using CuO nanospindles as a catalyst for direct C-H
arylation reaction between benzoxazole and iodobenzene under
2
7
valuable. The various methods have been developed to
3
synthesize these heterocycles. Among these developed methods,
the heterogeneous conditions. Herein we report for the first time,
a very simple, economical and environmentally compatible
method for the direct C-H arylation of benzoxazole and
benzothiazole using nano CeO -Fe O as a catalyst. Nano CeO -
the transition metal catalyzed C-H bond functionalization
4
reaction of arenes and heteroarenes have gained more attention.
2
3
4
2
One of the conventional method for the synthesis of these
heterocycles involves the palladium catalyzed cross coupling
reactions of either heteroaryl halides with aryl metals or
3 4
Fe O catalyst is superior to the reported CuO nanospindle
catalyst regarding reaction temperature, reaction time, catalyst
concentration, recyclability, requirement of inert atmosphere and
prominently substrate study. The magnetic nanoparticles as a
catalyst support have been attracting more attention because of
high surface area and easy recovery of the catalyst from reaction
mass simply by external magnet. This magnetic support proved
5
heteroaryl metals with aryl halides. As compared to above
methods, the direct C-H functionalization reactions of arenes and
heteroarenes provide both ecological and economical benefits.
This direct functionalization method is more efficient, as it
avoids the pre-functionalization step and the use of
stoichiometric amounts of organometallic reagents which pose
problems with respective to their synthesis and stability. The
reported methods for the synthesis of these heterocycles via
direct C-H functionalization of arenes and heteroarenes suffer
from certain limitations, which include use of expensive catalyst,
high amount of catalyst loading, use of strong base, use of
7
to be a better choice over the non magnetic materials as it
eliminates the tedious workup, centrifugation, filtration and
thereby loss in the amount of catalyst. Fe O is an ideal material
to synthesize the various magnetic nanoparticles supported
3
4
8
catalysts. In catalysis, ceria (CeO ) has gained wide scope due to
2
its efficiency and stability. The magnetic nanoparticles supported
ceria have been used for various organic transformations such as
6
9
oxidation, degradation
10
organic ligands and drastic reaction conditions. These issues
and in synthesis of carbonyl
compounds. In addition to this, ceria has been effectively used
1
1
need to be tackled from ‘green chemistry’ point of view.
Importantly, the recyclability of catalyst is also of great concern
for industrial applications which limits the use of homogeneous
catalyst. Thus, the development of more efficient,
environmentally and economically feasible heterogeneous
1
as catalyst in various coupling reactions. In continuation, we
2
report the direct C-H arylation reactions using magnetic
supported CeO
of magnetically separable nano CeO -Fe O catalyst for direct C-
2
2
nanoparticles. To the best of our knowledge, use
3
4
—
——
H arylation reaction is not yet reported. The nano CeO
2 3 4
-Fe O
∗
Corresponding author. Tel.: +0-000-000-0000; fax: +0-000-000-0000; e-mail: author@university.edu

2
13Tetrahedron
catalyst was synthesized by simple impregnation method. The
obtained catalyst was characterized by SEM, TEM, EDAX,
XRD, FT-IR, DSC-TGA and ICP-MS analysis.
presence of Fe
3
O
4
. The slight shift in absorption peaks clearly
on surface of ferrite.
The ICP-MS analysis showed the net Ce content equal to 7.44%.
indicates successful immobilization of CeO
2
2 3 4
The activity and stability of synthesized nano CeO -Fe O
catalyst was examined by applying it for direct C-H arylation of
benzoxazole or benzothiazole with aryl halides to obtain 2-aryl
substituted benzoxazoles or 2-aryl substituted benzothiazoles
(
Fig. 1). The direct arylation reaction can also be achieved by
replacing aryl halides with easily available low cost
arenediazonium salts in aqueous medium, using nano CeO
Fe as a catalyst. The catalyst showed excellent catalytic
2
-
3
O
4
activity for C-H arylation reaction due to the high surface area
and greater stability.
Figure 3. EDAX image of CeO -Fe O .
2
3
4
3. Nano CeO
benzoxazole
2 3 4
-Fe O catalyzed direct C-H arylation of
2 3 4
In order to evaluate the catalytic activity of nano CeO -Fe O
catalyst, our initial studies have been focused on the optimization
of reaction parameters. The direct C-H arylation of benzoxazole
with iodobenzene was chosen as the model reaction (Scheme 1).
Figure 1. Nano CeO -Fe O catalyzed direct C-H arylation of
2
3
4
heteroarenes with arenes.
2
. Characterization of nano CeO -Fe O catalyst
2
3
4
2 3 4
The obtained nano CeO -Fe O catalyst was subjected to SEM
and TEM analysis to examine the particle size and morphology
of nanoparticles Fig.2. Fig. 2b shows that the particles are
spherical in shape with size ranging from 3-25nm. The particle
Scheme 1. Direct C-H arylation of benzoxazole with iodobenzene.
size of CeO
2
-Fe
3
O
4
obtained from TEM analysis matches with
The influence of solvents has been explored for the model
reaction and it was found that amongst the DMSO, DMF, NMP,
1, 4-dioxane and toluene, DMSO was the best suitable solvent
which gave excellent yield of coupling product (Table1, entries
that calculated from XRD results using Scherrer equation. The
EDAX image exhibits elemental peaks of Ce, Fe and O which
confirms presence of nanoparticles of Fe O and CeO (Fig. 3).
3
4
2
1
-5). Screening of the various bases such as K
and NaOH was also carried out and higher yield was observed
with K CO base (Table 1, entries 1, 6-8). Furthermore, we
2 3 3 4 3
CO , K PO , NEt
2
3
examined the effect of temperature on model reaction and 100 °C
was found to be the optimum temperature for maximum
conversion (Table 1, entries 1, 9-11). The progress of the reaction
was monitored by GC analysis. Fig. 4 depicts the progress of
reaction with respect to time. It was observed that, the maximum
yield of product for model reaction was obtained in 4 hours.
Considering the importance of catalyst loading, we employed
various amounts of catalyst, ranging from 1 mol% to 7 mol% for
model arylation reaction (Table 1, entries 1, 12-14). The yield of
arylated product was increased on increasing the catalyst loading
from 1 mol% to 5 mol%. Further increase in catalyst loading to 7
mol% showed only 2% increase in the yield. Hence 5 mol% was
found as the optimum concentration of the catalyst.
2 3 4
Figure 2. (a) SEM image of CeO -Fe O . (b) TEM image of fresh
catalyst.
The XRD pattern of Fe O and CeO -Fe O nanoparticles were
3
4
2
3
4
shown in Fig. S1. It could be seen that the strong characteristic
defraction peaks at 2θ of 33.1° (220), 35.6° (311), 40.8° (400),
Table 1. Optimization of reaction conditions for direct C-H
a
arylation of benzoxazole with iodobenzene .
54.0° (422), 57.5° (511), 62.4° (440), 71.9° (620) correspond to
Fe O . These peaks resemble that of Fe O in cubic phase. The
Entry Solvent Base
Catalyst Temperature Time Yield
3
4
3
4
b
loading
(h)
4
(%)
96
92
83
62
33
78
45
31
00
21
67
peaks at 2θ of 28.5° (111), 30.0° (200), 43.2° (220), 55.8° (311),
and 56.8° (222) correspond to the reflection of Ce. This XRD
°
1
2
3
.
.
.
DMSO
DMF
K
K
K
K
K
K
2
2
2
2
2
3
CO
CO
CO
CO
CO
3
3
3
3
3
5 mol% 100 C
5 mol% 100°C
5 mol% 100°C
5 mol% 100°C
5 mol% 100°C
5 mol% 100°C
5 mol% 100°C
4
pattern revealed that the nanocatalyst is composed of Fe
CeO . In order to find out surface modifications, FT-IR analysis
2
3 4
O and
NMP
4
-
was carried out in the range of 400 to 4000 cm . Fig. S2 shows
1
4.
Dioxane
Toluene
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
4
the FT-IR spectrum of synthesized Fe O and CeO -Fe O . The
3
5.
4
4
2
3
4
spectra show presence of characteristic peaks for Fe
3
O
4
at 3380-
151, 1621, 1402, 632 cm and for CeO -Fe O at 3367, 1647,
6.
PO
4
4
-
1
3
1
7.
NEt
3
4
2
3
4
-
498, 644 cm . The peaks in the range of 3380-3151 cm are
1
-1
8.
NaOH 5 mol% 100°C
K CO 5 mol% RT
4
due to the stretching vibration of –OH group. The absorption
9.
24
4
2
3
-
peaks at 1621 cm and 1647 cm correspond to the bending
1
-1
10.
11.
K CO 5 mol% 60°C
3
2
1
vibration of H O. The band appeared between 500-700 cm
4
-1
2
K
2
CO
3
5 mol% 80°C
4
corresponds to metal-oxygen (Fe-O) bond which confirmed the

3
1
1
1
2.
3.
4.
DMSO
DMSO
DMSO
K
K
K
2
2
2
CO
CO
CO
3
3
3
1 mol% 100°C
3 mol% 100°C
7 mol% 100°C
4
4
4
54
72
98
c
c
c
c
c
c
7.
8.
4
4
4
4
4
4
4
5
5
5
8
93(89)
91(85)
92(87)
90(84)
85(80)
82(77)
76
a
Reaction conditions: Benzoxazole (1mmol), Iodobenzene (1mmol), Base
b
2mmol), Solvent (1mL). GC yield.
(
9
1
1
1
.
0.
1.
2.
Figure 4. Plot of GC yield versus time for Scheme 1.
The electronic effects of various substituted arenes on arylated
product yields have also been studied for this reaction under the
established optimal reaction conditions. The arylation reaction
has been carried out with substituted iodobenzenes as well as
substituted bromobenzenes. This reaction was significantly
influenced by electronic nature and position of substituted
electron-withdrawing or electron-donating groups on aromatic
ring of aryl halides. It was found that iodobenzene derivatives
undergo arylation reaction more efficiently with high product
yield than the bromobenzene derivatives. The aryl halides
containing electron-withdrawing substituents such as 4-
nitroiodobenzene, 4-cynoiodobenzene, 3-nitroiodobenzene, 2-
nitroiodobenzene, 4-nitrobromobenzene, 4-cynobromobenzene
gave excellent yield of desired products (Table 2, entries 1-8).
Electron-donating substituents containing aryl halides such as 4-
13.
14.
84
c
c
1
1
1
5.
6.
7.
85(76)
80(72)
56
O
N
F
a
Reaction conditions: Benzoxazole (1 mmol), Aryl halides (1 mmol), K
2
CO
(5 mol%), DMSO (1 mL) at 100 °C. GC yield.
Isolated yield in parentheses.
3
hydroxyiodobenzene,
aminoiodobenzene,
methoxybromobenzene,
4-methoxyiodobenzene,
4-hydroxybromobenzene,
4-fluorobromobenzene,
4-
4-
3-
b
(
c
2 3 4
2 mmol), Nano CeO -Fe O
hydroxyiodobenzene and 2-hydroxyiodobenzene also afforded
good to moderate yields (Table 2, entries 9-16). Heterocyclic aryl
halide 3-bromofuran was less reactive and yielded 56% product
and that too with longer reaction time (Table 2, entry 17).
4. Nano CeO -Fe O catalyzed direct C-H arylation of
benzothiazole
2 3 4
The study was further extended to explore the catalytic
activity of nano CeO -Fe for arylation reactions of
2
3 4
O ,
Table 2 Direct C-H arylation reaction of benzoxazole with
a
various aryl halides .
benzothiazole with various aryl halides (Scheme 2).
Entry Aryl halides
Products
Time Yield
b
(
h) (%)
c
1
2
3
4
5
6
.
.
.
.
.
.
4
4
4
4
4
4
96(91)
Scheme 2. Direct C-H arylation of benzothiazole with iodobenzene.
c
98(94)
Initially, reaction between benzothiazole and iodobenzene was
chosen as model reaction to get the best reaction parameters. The
results are summarized in Table S1.The model reaction was
monitored by using GC analysis and the progress of the reaction
was studied by plotting the graph of GC yield verses time (Fig.
c
97(92)
2 3
5). The DMSO solvent (1 mL), K CO base (2 mmol), 5 mol%
c
catalyst, at 120 °C temperature and 5h reaction time are the
optimized reaction parameters which gave maximum product
yield for the model reaction. We next screened various
substituted aryl halides with benzothiazole under the optimized
reaction conditions. The catalyst was more reactive towards aryl
halides containing electron-withdrawing substituents, giving
excellent product yields as compared to electron-donating ones
93(88)
87
c
89(83)
(Table 3, entries 1-16). Similarly, heterocyclic aryl halide also
coupled with benzothiazole giving corresponding product with

4
Tetrahedron
moderate yields (Table 3, entry 17).
Table 3 Direct C-H arylation reaction of benzothiazole with
a
various aryl halides .
Entry Aryl halides
Products
Time Yield
b
(
h) (%)
c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
2
.
5
5
5
5
6
6
5
5
6
6
6
6
6
7
7
6
8
83(79)
c
.
89(83)
S
N
Figure 5. Plot of GC yield versus time for Scheme 2.
5. Nano CeO -Fe catalyzed direct C-H arylation of
benzoxazole and benzothiazole with arenediazonium salts
c
.
CN
86(82)
2
3 4
O
c
.
81(76)
.
75
Scheme 3. Direct C-H arylation of benzoxazole and benzothiazole
c
with arenediazonium salts.
.
80(74)
Table 4. Direct C-H arylation reaction of benzoxazole and
benzothiazole with various arenediazonium salts .
a
c
.
84(77)
Entry Arenediazonium
salts
Products
Time Yield
b
(h) (%)
S
N
c
.
CN
78(73)
c
c
c
1
2
3
4
5
6
7
8
9
.
.
.
.
.
.
.
.
.
3
3
3
3
3
3
4
5
3
87(83)
73(66)
92(87)
74
c
.
72(68)
c
0.
1.
2.
3.
4.
5.
6.
0.
70(64)
O
N
F4BN2
F4BN2
NO2
NO2
NO2
c
67(61)
64
c
78(70)
65
59
c
63(57)
57
c
59(52)
c
53(47)
c
61(58)
F4BN2
CH3
64
41
a
Reaction conditions: Benzoxazole/Benzothiazole (1 mmol), Arenediazonium
salts (1 mmol), K CO (2 mmol), Nano CeO -Fe (5 mol%), Water (1 mL)
at 80 °C. GC yield. Isolated yield in parentheses.
2
3
2
3 4
O
a
Reaction conditions: Benzothiazole (1 mmol), aryl halides (1 mmol), K
2
CO
(5 mol%), DMSO (1 mL) at 120 °C. GC yield.
Isolated yield in parentheses.
3
b
c
b
(
c
2 3 4
2 mmol), Nano CeO -Fe O
Iodobenzene and bromobenzene derivatives are more expensive.
Keeping this in mind, we have employed arenediazonium salts
which are easily available and cheaper for arylation of
benzoxazole or benzothiazole (Scheme 3). The arylation reaction

5
of benzoxazoles or benzothiazoles with arenediazonium salts in
aqueous medium is the greener route which is the merit of nano
catalyst showed recyclability up to ten cycles, with marginal
loss in the catalytic activity.
2 3 4
CeO -Fe O catalyst. It was observed that coupling reactions
Acknowledgments
between benzoxazoles or benzothiazole with various
arenediazonium salts are effectively carried out by using nano
The authors are very thankful to the UGC-UPE-2756, Green
Technology centre, New Delhi, India for awarding the
fellowship.
CeO
2 3 4
-Fe O (Table 4, entries 1-9).
6. Recyclability of nano CeO -Fe O in direct C-H arylation
reaction of benzoxazole with iodobenzene.
2
3
4
References and notes
2
3 4
Reusability of nano CeO -Fe O catalyst was tested as it is one
of the important parameters regarding industrial application of
heterogeneous catalytic systems. Initially the fresh catalyst was
employed to arylation reaction of benzoxazole with iodobenzene
under the optimized reaction conditions. After completion of
reaction, the catalyst was separated from reaction mass by using
external magnet and washed with ethanol followed by drying.
The recovered catalyst was reused for next batch of same
reaction under similar reaction conditions using fresh starting
substrates. This procedure was repeated up to ten cycles. The
results obtained from GC analysis clearly indicate that the
catalyst has fairly good activity over the ten cycles of the reaction
1
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Figure 6. Recyclability of nano CeO -Fe O in arylation reaction.
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1
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2
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Figure 7. Hot filtration curve.
In conclusion, we are presenting the first report on the
9
1
.
magnetically separable, nanosized CeO -Fe catalyzed direct
2
3 4
O
2
002, 38, 321-329; Akhtara, J.; Amina, N. S.; Arisb, A. Chem.
C-H arylation reaction of benzoxazole or benzothiazole with
various arenes under the ligand free conditions. The catalyst
preparation and arylation reaction methodology were very
simple, economical and environmentally favourable, giving
excellent yields of arylated products. In addition to this, the
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Huang, W. Catal. Lett. 2008, 125, 160-167.
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3-65.
5

6
Tetrahedron
1. Kamimura, Y.; Sato, S.; Takahashi, R.; Sodesawa, T.; Akashi, T.
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1
1
2. Diez, A. S.; Mayer, M. G.; Radivoy, G.; Volpe, M. A. Appl. Catal.
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B.; Kantam, M. L.; Bhargava, S. Catalysis Today, 2012, 198, 35-
4
5
4; Agawane, S. M.; Nagarkar, J. M. Tetrahedron Lett., 2011, 52,
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1
1
3. Gawande, M. B.; Bonifacio, V. D. B.; Verma, R. S.; Nogueira, I.
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5879-5885.

8
Tetrahedron
Highlights
•
•
•
•
•
Nano CeO
2
-Fe
3
O
4
was used for direct C-H arylation of heterocycles.
The system was optimized with respect to various reaction parameters.
Arylation reaction was also carried out with arenediazonium salts in water.
The simple methodology gives excellent yield of arylated products.
The catalyst should be reused up to five cycles.