Copper ferrite nanoparticle-mediated N-arylation of heterocycles: A ligand-free reaction

Article abstract of DOI:10.1016/j.tetlet.2011.02.050

The synergistic effects of iron and copper in copper ferrite nanoparticles for the N-arylation of heterocycles with aryl halides were demonstrated. The magnetic nature of the catalyst facilitates its removal from the reaction medium for further use. Negligible leaching of Cu and Fe in consecutive cycles makes the catalyst economical and environmentally benign for C-N cross-coupling reactions.

Full text of DOI:10.1016/j.tetlet.2011.02.050

Tetrahedron Letters 52 (2011) 1924–1927
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Copper ferrite nanoparticle-mediated N-arylation of heterocycles:
a ligand-free reaction
⇑
Niranjan Panda , Ashis Kumar Jena, Sasmita Mohapatra, Smruti Ranjan Rout
Department of Chemistry, National Institute of Technology, Rourkela 769 008, Odisha, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synergistic effects of iron and copper in copper ferrite nanoparticles for the N-arylation of heterocy-
cles with aryl halides were demonstrated. The magnetic nature of the catalyst facilitates its removal from
the reaction medium for further use. Negligible leaching of Cu and Fe in consecutive cycles makes the cat-
alyst economical and environmentally benign for C–N cross-coupling reactions.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 10 December 2010
Revised 8 February 2011
Accepted 10 February 2011
Available online 15 February 2011
Keywords:
Ullmann coupling
C–N cross-coupling
N-Arylation
N-Heterocycles
2 4
CuFe O nanoparticles
Since 1903, the copper-catalyzed Ullmann reaction1 has been
widely applied for the N-arylation of nitrogen-containing hetero-
cycles. The classic Ullmann reaction normally requires harsh con-
temperature (i.e., 90 °C), which was found to be economical and
1
0c
encouraging.
2
Quite recently, several CuI/Cu O-catalyzed ‘li-
2
12
gand-free’ catalytic systems have emerged. More importantly,
1
3
ditions, such as high temperature (ꢀ200 °C), stoichiometric
amounts of copper and selective halide substrates, which is prob-
lematic for industrial use due to high cost and waste disposal. To
overcome these drawbacks, a number of efforts have been made
recently to develop a new catalytic system. Successful methods in-
clude the use of catalytic amounts of copper and arylboronic acids
Punniyamurthy and co-workers exploited the high surface area
and reactive morphology of the CuO nanoparticles for successful
C–N, C–O, and C–S cross-coupling reactions. Park and co-workers14
also used catalytic amounts of acetylene-carbon-immobilized CuO-
hollow nanospheres for N-arylation reactions at high temperature
(i.e., 180 °C) in a stainless steel reactor. Although these results are
promising, there is still room for further development of a ligand-
free, environmentally friendly, less expensive and easily separable
catalytic system for the N-arylation of heterocycles with even less
reactive aryl chlorides and sulfonates.
3
–5
in place of aryl halides.
However, the relative instability of boro-
nic acids and the tedious purification procedure limit their further
application. Buchwald and Hartwig7 independently established
the broad applicability of palladium catalysts for N-arylation of
amines with aryl halides. The toxicity and high cost of Pd catalysts
restrict their use on the industrial scale. Thus, researchers have
turned their attention toward the use of less expensive, less toxic,
6
In recent years, magnetic nanoparticles have been extensively
studied for various biological applications, such as magnetic reso-
1
5
16
17
nance imaging, drug delivery, biomolecular sensors, biosepa-
8
9
18
19
and more efficient metals to replace Pd. Indeed, Buchwald and
Taillefer1 independently made a significant breakthrough in the
copper-catalyzed cross-coupling of N–H heterocycles with aryl
bromides and iodides in the presence of chelating ligands. Other
groups have also utilized various ligands, such as diamines, amino
acids, b-ketoesters, 1,10-phenanthroline derivatives, poly(ethylene
glycol), ninhydrin, and other nitrogen- and/or oxygen-containing
ligands as chelating agents11 under homogeneous conditions.
Taillefer et al. used a Fe–Cu co-catalytic route for the N-arylation
of numerous heterocycles with aryl bromides at relatively low
ration, and magneto-thermal therapy. Despite their wide use in
biological systems, much less attention has been focused on the
catalytic behavior of magnetic nanoparticles in organic transfor-
mations. Here, for the first time, we report the efficient catalytic
0
2 4
activity of magnetic copper ferrite (CuFe O ) nanoparticles toward
the N-arylation of various N-heterocycles with aryl halides under
‘ligand-free’ conditions.
To develop the best magnetically separable catalyst for the N-
3 4
arylation reaction, Fe O and different substituted ferrite nanopar-
2
+
2+
2+
2 4
ticles, MFe O (M = Cu , Co and Ni ), have been synthesized by
thermal decomposition and were subsequently screened. Initially,
we employed pyrrole and bromobenzene as model substrates for
the development of optimized conditions (Scheme 1). As summa-
⇑
Corresponding author. Tel.: +91 661 2462653; fax: +91 661 2462651.
E-mail addresses: npanda@nitrkl.ac.in, niranjanpanda@gmail.com (N. Panda).
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.050

N. Panda et al. / Tetrahedron Letters 52 (2011) 1924–1927
1925
Table 3
catalyst
N H + Br
CuFe O4 catalyzed N-arylation of N-heterocycles with bromobenzene
N
2
base,solvent
1
CuFe O
2 4
tBuOK, DMF
reflux
N H + Br
N
Scheme 1.
Entry
1
Azole
Product
Yielda (%)
Table 1
N-Arylation of pyrrole with bromobenzene in various solvents and bases in the
presence of different ferrites
NH
N
98
97
Entry
Catalyst
Solvent
Base
Yielda (%)
1
8
t
1
2
3
4
5
6
7
8
9
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
CuFe
2
2
2
2
2
2
2
2
2
2
2
2
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
O
4
DMF
DMF
DMF
DMF
DMF
THF
1,4-Dioxane
DMSO
BuOK
98
35
65
35
10
00
25
38
25
00
65
10
00
00
18
68
00
00
65
N
N
N
K
2
CO
NaOAc
Cs CO
3
2
3
HN
2
3
NaHCO
3
NH
N
N
N
t
BuOK
83
90
t
BuOK
t
BuOK
9
t
3
CH CN
BuOK
t
10
11
12
13
14
15
16
17
18
19
MeOH
Ethanolamine
DMF
DMF
DMF
DMF
DMF
—
DMF
BuOK
t
BuOK
4
N
H
pyridine
t
N
N
Fe
3
O
4
BuOK
1
0
t
CoFe
2
O
4
BuOK
t
N
N
NiFe
CuO
CuFe
—
2
O
4
BuOK
t
BuOK
t
2
O
4
4
BuOK
5
6
7
N
75
59
50
t
BuOK
H
t
N
N
CuFe
2
O
Toluene
BuOK
11
a
Reaction conditions: 1.49 mmol of pyrrole, 1.52 mmol of bromobenzene,
0 mol % of catalyst, 2.0 equiv of base, 5 mL of solvent, 24 h reflux under N
1
2
NH
N
N
N
atmosphere.
CH3
CH3
12
CH3
NH
CH3
Table 2
CuFe
2
O
4
catalyzed N-arylation of pyrrole with aryl halides
N
CuFe O4
2
N
H
+
X
H C
3
N
N
N
tBuOK, DMF
H3C
1
3
R
R
reflux
NH
N
Yielda (%)
8
9
85
80
Entry
X
R
Product
1
4
1
2
3
4
5
6
7
8
9
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
I
H
1
2
3
4
5
6
1
4
6
1
7
98
58
68
68
75
71
48
40
20
99
72
O
O
4-NO
4-COMe
4-CO Et
4-NH
2
O
N
2
OH
N
N
2
4-OMe
H
1
5
2
4-CO Et
4-OMe
H
N
N
1
1
0
1
N
H
10
85
82
I
4-COOH
a
Reaction conditions:1.49 mmol of pyrrole, 1.52 mmol of aryl halide, 10 mol % of
16
t
CuFe
2
O
4
nanoparticles, 2.0 equiv of BuOK, 5 mL of DMF, 24 h reflux under N
2
atmosphere.
rized in Table 1, it was found that Fe
nanoparticles were inactive, whereas CuFe O nanoparticles cat-
2 4
3
4
O , CoFe
2
O
4
, and NiFe
2
O
4
11
N
N
2
0
H
alyzed the N-arylation reaction (98% yield) in DMF at 155 °C in the
presence of 2 equiv of BuOK. The magnetic nature of the copper
ferrite nanoparticles facilitates their easy and quantitative removal
from the reaction medium in the presence of an external magnetic
field for further use.
17
t
a
Reaction conditions: 100 mg of azole, 1.02 equiv of bromobenzene, 10 mol % of
t
CuFe
atmosphere.
2 4 2
O nanoparticles, 2.0 equiv of BuOK, 5 mL of DMF, 24 h reflux under N
When the reaction was carried out in different polar and non-
polar solvents, such as THF, toluene, DMSO, CH CN, 1,4-dioxane,
3
ethanolamine, and MeOH, under reflux conditions only 0–40% of
the N-arylated product and the remaining starting material was
sence of solvent did not yield any coupling product (Table 1, entry
17). Thus, DMF may chelate the Cu centers in CuFe and catalyze
The reactivity of the catalyst in DMF
in the presence of different bases was also investigated. Among the
2 4
O
1
2b,11c
isolated. Moreover, DMF provided the best results. Direct heating
the N-arylation reaction.
t
of pyrrole, bromobenzene, CuFe
2
O
4
, and BuOK at 155 °C in the ab-

1
926
N. Panda et al. / Tetrahedron Letters 52 (2011) 1924–1927
Table 4
2 4
Reusability of CuFe O nanoparticles and leaching of Cu and Fe in multi-cycle arylation reactions
Cycle
Recovered CuFe
2
O
4
(%)
Product yielda (%)
Cu leakage (in ppm)
Fe leakage (in ppm)
1
2
3
—
97
95
98
96
96
0.45
0.4
0.2
0.08
0.02
0.02
a
Reaction conditions: 1.49 mmol of pyrrole, 1.52 mmol of bromobenzene, 10 mol % of CuFe
2
O
4
nanoparticles (for cycle 1 and the remaining recovered amount of the
t
catalyst was used for subsequent cycles), 2.0 equiv of BuOK, 5 mL of DMF, 24 h reflux under N
2
atmosphere.
t
tested bases ( BuOK, Cs
2
CO
3 2
, K CO
3
, Et
3
N, pyridine, NaHCO
3
, NaO-
centration of N-aryl pyrrole was not increased further even after
heating for an additional 24 h.
In conclusion, for the first time, we have demonstrated the
application of copper ferrite nanoparticles for ‘ligand free’ N-aryla-
tion of various N-heterocycles from differently substituted aryl ha-
t
Ac), BuOK was found to be superior for the highest yield of N-phe-
2
1
nyl pyrrole. A decrease in the catalyst loading from 10 to 5 to
mol % afforded the product in lower yield, and 10 mol % of the
catalyst was found to be optimum. It is worth mentioning that
0 mol % of the CuO nanoparticles catalyzed the N-arylation reac-
1
1
2 4
lides (X = I, Br, Cl). The magnetic nature of CuFe O nanoparticles is
tion with an appreciable yield (60–68%, Table 1, entry 16) over
multiple runs, whereas our optimum conditions resulted in signif-
icantly higher yields of the product (98% over multiple runs even
on a 15 mmol scale). Thus, it may be concluded that the synergistic
particularly advantageous for easy, quick, and quantitative separa-
tion of the catalyst for reuse. This catalytic process is simple, effi-
cient,
economical,
and
environmentally
safe.
Further
investigation of this catalytic system for C–C, C–O and C–S bond
formation is underway in our laboratory.
effects of Fe and Cu in CuFe
2 4
O co-catalyze the N-arylation reaction
10c
and are in line with Taillefer’s report.
After determining the optimized conditions, we then investi-
gated the scope of the magnetic catalyst for the C–N cross-coupling
reaction of a diverse range of halides with pyrrole (Table 2). As ex-
pected, aryl iodides and bromides gave excellent yields of the cou-
Acknowledgment
DST (Ref. SR/FTP/CS-101/2006), Govt. of India is gratefully
acknowledged for financial support.
2 4
pled product. CuFe O was found to be quite efficient in yielding
the cross-coupled product with less reactive aryl chlorides (Table 2,
entries 7–9), and moderate yields were obtained. It may be note-
worthy that C–N cross-coupling reactions with aryl chlorides are
rarely reported and, as mentioned by Taillefer, are significant chal-
lenges in Ullmann coupling reactions.1 Coupling of pyrrole with
different aryl halides having both electron-donating and -with-
drawing groups resulted in products with moderate to good yields
Supplementary data
Supplementary data (synthesis and characterization of the cat-
alyst and all other compounds with detailed experimental proce-
dures) associated with this Letter can be found, in the online
version, at doi:10.1016/j.tetlet.2011.02.050.
1c
(Table 2). More interestingly, electron-rich bromides led to N-aryl
References and notes
pyrroles (Table 2, entries 5 and 6) in good yield; however, the tran-
sition metal-catalyzed reactions with these electron-rich arylating
agents are traditionally less straightforward.
1. (a) Ullmann, F. Ber. Dtsch. Chem. Ges. 1903, 36, 2389; (b) Ullmann, F. Ber. Dtsch.
Chem. Ges. 1904, 37, 853; (c) Goldberg, I. Ber. Dtsch. Chem. Ges. 1906, 39, 1691.
2.
For reviews, see: (a) Hassan, J.; Sévignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. Rev. 2002, 102, 1359; (b) Schnurch, M.; Flasik, R.; Khan, A. F.; Spina, M.;
Mihovilovic, M. D.; Stanetty, P. Eur. J. Org. Chem. 2006, 3283.
2 4
The catalytic activity of CuFe O nanoparticles in N-arylation
reactions of other nitrogen-containing heterocycles was investi-
gated. Pyrazole, imidazole, indole, benzotriazole, carbazole, and
others underwent coupling with bromobenzene under standard
experimental conditions and provided products in 75–98% yield
3
.
.
Chan, D. M. T.; Monaco, K. L.; Wang, R.-P.; Winters, M. P. Tetrahedron Lett. 1998,
3
9, 2933.
4
Evans, D. A.; Katz, J. L.; West, T. R. Tetrahedron Lett. 1998, 39, 2937.
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6.
(a) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem. 2002, 219, 131; (b) Wolfe, J. P.;
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pyrazole and imidazole (Table 3, entries 6 and 7) gave the required
products in lower yields (50–60%).
7. (a) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046; (b) Hartwig J. F. In
Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-i., de
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Chem. Res. 2008, 41, 1450; (c) Evano, G.; Blanchard, N.; Toumi, M. Chem. Rev.
2 4
Next, we studied the reusability of a heterogeneous CuFe O
catalyst in C–N coupling reactions (Table 4). After completion of
the reaction, the catalyst was recovered by the application of an
external magnet, was washed with ethyl acetate and then acetone,
and was dried in a hot air oven at 120 °C for 2 h. The recovered cat-
alyst was reused under similar conditions for the next run, and the
2008, 108, 3054; (d) Monier, F.; Taillefer, M. Angew. Chem., Int. Ed. 2009, 48,
6954; (e) Zhu, L.; Cheng, L.; Zhang, Y.; Xie, R.; You, J. J. Org. Chem. 2007, 72,
2737; (f) Sreedhar, B.; Shiva Kumar, K. B.; Srinivas, P.; Balasubrahmanyam, V.;
Venkanna, G. T. J. Mol. Catal. A: Chem. 2007, 265, 183.
catalytic behavior of the CuFe
unaltered (yield >95%), even up to three consecutive cycles.
2 4
Then, the possibility of Fe and Cu leakage from CuFe O to the
2 4
O nanoparticles was found to be
9.
(a) Klapars, A.; Antilla, J. C.; Huang, X.; Buchwald, S. L. J. Am. Chem. Soc. 2001,
1
23, 7727; (b) Antilla, J. C.; Baskin, J. M.; Barder, T. E.; Buchwald, S. L. J. Org.
Chem. 2004, 69, 5578; (c) Altman, R. A.; Buchwald, S. L. Org. Lett. 2006, 8, 2779.
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607; (b) Cristau, H.-J.; Cellier, P. P.; Spindler, J.-F.; Taillefer, M. Eur. J. Org.
Chem. 2004, 695; (c) Taillefer, M.; Xia, N.; Ouali, A. Angew. Chem., Int. Ed. 2007,
6, 934; (d) Taillefer, M.; Cristau, H.-J; Cellier, P. P.; Spindler, J.-F; Ouali, A. Fr
medium during the reaction was investigated. After completion
of the reaction, the supernatant was collected and tested for Fe
and Cu by atomic absorption spectroscopy (AAS). The leaching of
Cu and Fe in three consecutive cycles was found to be 60.5 ppm
5
4
2002 06717 (Fr2840303-WO03101966).; (e) Taillefer, M.; Cristau, H.-J; Cellier,
P. P.; Spindler, J.-F. patents Fr 2001 16547 (Fr 2833947-WO0353225).
(
Table 4), which is well below the permissible level concerning
11. For reviews see: (a) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003, 42,
2
2
the toxicity in humans.
5400; (b) Sorokin; V. I. Mini-Rev. Org. Chem. 2008, 5, 323.; (c) Monnier, F.;
In order to investigate the catalytic effect of leached ions in N-
arylation reactions, a controlled experiment was carried out under
the optimum conditions. After 12 h of heating, the catalyst was re-
moved by application of an external magnet and the remaining
reaction mixture was again heated. It was observed that the con-
Taillefer, M. Angew. Chem., Int. Ed. 2009, 48, 6954.
1
2. (a) Taillefer, M.; Cristau, H. J.; Cellier, P. P.; Spindler, J.-F. (Rhodia Chimie,
France), French Patent Application 2859205A1 20050304, 2005.; (b)
Yasutsugu, U.; Mayumi N. (Koei Chem. Co. Ltd), Japanese Patent Application
JP 2006–342127, 2006.; (c) Zhu, R.; Xing, L.; Wang, X.; Cheng, C.; Su, D.; Hu, Y.
Adv. Synth. Catal. 2008, 350, 1253; (d) Correa, A.; Bolm, C. Adv. Synth. Catal.

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2
2
007, 349, 2673; (e) Tao, C. Z.; Li, J.; Cui, X.; Fu, Y.; Guo, Q. X. Chin. Chem. Lett.
007, 18, 1199; (f) Choudary, B. M.; Sridhar, C.; Kantam, M. L.; Venkanna, G. T.;
sodium hydroxide. Briefly, to a solution of Fe(NO
and Cu(NO
Á3H
3
)
3
Á9H
2
O (3.34 g, 8.2 mmol)
)
3 2
2
O (1 g, 4.1 mmol) in 75 ml of distilled water, 3 g (75 mmol) of
Sreedhar, B. J. Am. Chem. Soc. 2005, 127, 9948; (g) Kantam, M. L.; Venkanna, G.
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NaOH dissolved in 15 ml of water was added at room temperature over a
period of 10 min. during which reddish-black precipitate was formed. Then 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 (3 Â 30 ml) and catalyst
was kept in air oven for overnight at 80 °C. Then the catalyst was ground in a
mortar-pestle and kept in a furnace at 700 °C for 5 h (step up temperature
20 °C/min) and then cooled to room temperature slowly. 820 mg of magnetic
7
366.
1
1
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1
1
3
CuFe
CuFe
2
O
O
4
particles of size 10–25 nm were obtained. Copper and iron content in
nanoparticles were estimated to be 27.5% and 47.2%, respectively,
6. (a) Gupta, A. K.; Curtis, A. S. G. J. Mater. Sci. Mater. Med. 2004, 15, 493; (b)
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2
4
from AAS (see Supplementary data for further characterization).
21. General procedure for N-arylation reaction. To solution of N-heterocycle
(1 equiv), bromobenzene (1.02 equiv) and BuOK (2 equiv) in dry DMF, CuFe
(10 mol %) was added and heated at reflux for 24 h under N atmosphere. After
a
t
1
1
1
2
2 4
O
2
cooling to room temperature, the mixture was diluted with ethyl acetate and
the catalyst was separated by a magnetic separator. The catalyst was washed
with ethyl acetate. The combined ethyl acetate layer was washed with water
2 4
(twice), dried over anhydrous Na SO , and concentrated to yield the crude
9. (a) Hiergeist, R.; Andra, W.; Buske, N.; Hergt, R.; Hilger, I.; Richter, U.; Kaiser, W.
J. Magn. Magn. Mater. 1999, 201, 420; (b) Jordan, A.; Scholz, R.; Wust, P.;
Fahling, H.; Felix, R. J. Magn. Magn. Mater. 1999, 201, 413.
product, which was further purified by silica gel column chromatography using
petroleum ether/ethyl acetate to yield N-arylated product.
22. Permissible limit of Cu and Fe leaching is 2 ppm and 2 ppm respectively, see:
(a) Poetra, C. Environ. Health Prospect. 2004, 112, A568; (b) Sabhi, S.; Kiwi, J.
Water Res. 2001, 35, 1994.
0. Catalyst preparation. CuFe
2
O
4
nanoparticles of size 15–20 nm were prepared by
and Fe(NO in water in the presence of
thermal decomposition of Cu(NO
3
)
2
3 3
)