International Journal of Organic Chemistry, 2013, 3, 185-189
http://dx.doi.org/10.4236/ijoc.2013.33023 Published Online September 2013 (http://www.scirp.org/journal/ijoc)
Salen-Cu(II) Complex Catalyzed N-Arylation of
Imidazoles under Mild Conditions
Yan Liu, Qin Zhang, Xiaowei Ma, Ping Liu*, Jianwei Xie, Bin Dai, Zhiyong Liu
School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of Chemical Engineering of
Xinjiang Bingtuan, Shihezi University, Shihezi, China
Received June 26, 2013; revised July 28, 2013; accepted August 8, 2013
Copyright © 2013 Yan Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Three inexpensive and air-/moisture-stable Salen-Cu complexes 1-3 were evaluated to be a novel class of catalysts for
the N-arylation of imidazoles with aryl halides. A v ariety of aryl iodides, bromides underwent the coupling with imida-
zoles, prom ot ed by the complex 3, in moderate to excellent yields without the protection by an inert gas.
Keywords: Salen-Cu Complex; N-Arylation; Imidazole; Catalyze
1. Introduction
N-Aryl imidazoles and its derivatives are prevalent buil-
ding blocks of numerous drugs, natural products and en-
ergetic materials, [1-5] and have been exploited as im-
portant precursors in N-heterocyclic carbene chemistry.
[6,7] Therefore, their preparation has been attracted
much attention. In recent years, the transition-metal pal-
ladium catalyzed N-arylation of imidazoles has made
remarkable achievements, and shows relative mild reac-
tion conditions, broad substrate scope and excellent func-
tional-group tolerance [8-10]. However, in comparison
with the use of costly palladium, it is desirable to develop
more effective copper catalytic systems for N-arylation
of imidazoles. The breakthroughs in this area, which
were achieved by two research groups of Buchwald
[11,12] and Taillefer, [13] respectively, have typically
been driven by the implementation of new class of
ligands and only catalytic amounts of copper metal under
mild conditions. Following these pioneering works, sev-
eral classes of mono-, bidentate, and polydentate chela-
tors have thereby been developed to expedite the reaction
rates and substantially lower the reaction temperature of
Cu-based C-N coupling reaction [14-27]. In spite of the
significant progress made in the aforementioned trans-
formation, more efficient, air stable and cheaper ligands
or metal-complexes for facilitating these coupling reac-
tions under relatively milder conditions are still in de-
mand.
Recently, our group had developed a series of effective
catalysts, pyrrolecarbaldiminato-Cu complexes for C-N
coupling reaction [28], and we previously reported Su-
zuki-Miyaura reaction catalyzed by Salen and half-salen
palladium(II) complexes [29]. Although Salen-Pd com-
plexes show low catalytic activity in the C-C coupling
reaction, we reasonable assumed that Salen-Cu com-
plexes might be a class of effective catalysts for the C-N
coupling reaction. Herein, we wish to report Salen-Cu
complexes as catalysts for the N-arylation of imidazoles
and its derivatives. This system contains several advan-
tages as follows: 1) the complexes were easily synthe-
sized from cheap starting materials, and stable in air and
moisture; 2) the reaction condition was relatively milder
and did not require the protection by an inert atmos-
phere; 3) the complexes worked well for aryl iodides,
and bromides with moderate to excellent yields.
2. Results and Discussion
Initially, the catalytic activity of the complexes 1-3
evaluated by using the C-N coupling of 4-iodotoluene
with imidazole as a model reaction in the presence of
NaOH at 120˚C for 12 h in DMSO (Scheme 1). As ex-
pected, the three Salen-Cu complexes all exhibited high
catalytic activity for this process, and which gave the
desired product in 92% - 94% isolated yields (Table 1,
entries 1-3). The coupling reaction did not occur in the
absence of any catalyst (Table 1, entry 4). Subsequently,
we select the complex 3 as catalyst to further investigate
the effects of the other reaction conditions on the
*Corresponding a uthor.
C
opyright © 2013 SciRes. IJOC

Y. LIU ET AL. 186
NN
O O
Cu
21
H H
NN
OO
Cu NN
O O
Cu
3
Scheme 1. Structure of complexes 1 - 3.
Table 1. Optimization of the reaction conditionsa.
Entry Complex Base Solvent Temp (˚C) Yield (%)b
1 - NaOH DMSO 120 0
2 1 NaOH DMSO 120 92
3 2 NaOH DMSO 120 94
4 3 NaOH DMSO 120 94
5 3 NaOH DMSO 100 94
6 3 NaOH DMSO 80 88
7 3 Na2CO3 DMSO 100 0
8 3 K3PO4 DMSO 100 70
9 3 Cs2CO3 DMSO 100 95
10 3 NEt3 DMSO 100 0
11 3 NaOH DMF 100 43
12 3 NaOH DMA 100 76
13 3 NaOH H2O 100 0
14 3 NaOH DMSO 100 82c
a Reaction conditions: 4-ioidotoluene (0.5 mmol), imidazole (1.0 mmol),
complex 1-3 (10 mol%), base (1.0 mmol), and solvent (1 ml), reaction time
12 h. b Isolated yields. c Complex 3 (5.0 mol%).
N-arylation reaction, including reaction temperature, base,
solvent and catalyst loading. The results showed that
100˚C was enough for the coupling reaction to give 94%
yield of the product (Table 1, entry 5), and the lower
temperature decelerated the reaction rate. For example,
88% yield was obtained when the reaction was carried
out at 80˚C (Tabl e 1, entry 6). Base also plays an impor-
tant role in the catalyst systems. Among various bases
examined, K3PO4, and Cs2CO3 were all effective for the
catalysis, and Cs2CO3 demonstrated the best improve-
ment to give the corresponding product in 95% yield
(Table 1, entries 8 and 9), but the use of Na2CO3 and
organic base NEt3 led to low er yields (Tab le 1, entries 7
and 10). However, NaOH was used in the following
studies because it was less expensive than Cs2CO3. Sol-
vent is another important factor affecting catalysis. It was
found that DMSO performed as the prime solvent. Both
DMF and DMA were not as good as DMSO. Meanwhile,
H2O was not suitable as a solvent (Table 1, entries 11-
13). Furthermore, decreasing the loading of complex 3
from 10 mol% to 5 mol% led to a decrease of the yield
(Table 1, entry 14). Finally, the combination of Salen-Cu
complex 3 (10 mol%), NaOH (2 equiv.) at 100˚C for 12
h in DMSO was chosen as the optimal conditions for
N-arylation of imidazole with 4-iodotoluene.
The scope of substrates was then investigated by using
this catalytic system under the optimized reaction condi-
tions. As shown in Table 2, In general, most of aryl io-
dides reacted with imidazole smoothly afforded the de-
sired products in moderate to excellent yields. For exam-
ple, 1-chlor o-4-iodoben zene, 1-fluoro -4-iodoben zene and
4-iodo-1,1’-biphenyl led to the N-arylated products in
90% - 95% yields (Table 2, entries 6-8). When 1-(4-io-
dophenyl)ethanone and 1-iodo-4-nitrobenzene as cou-
pling partners, the yields dropped to 72% and 60% re-
spectively (Table 2, entries 4 and 5). Furthermore, the
catalytic system could tolerate a variety of functional
groups including the nitro, acetyl, and ether groups. No-
tably, sterically demanding ortho substituents such as
1-iodo-2-methylbenzene did not hamper the arylation
reaction (Table 2, entry 3). Next, we were intrigued by
the possibility of using aryl bromides as coupling part-
ners. However, low yields were found under the previ-
ously optimized reaction conditions (Table 2, entries 9
and 10). In an endeavor to expand the scope of the
methodology, this new catalytic system was applied to a
variety of imidazole derivatives. To our delight, most of
the aryl iodides reacted with the 1H-benzo[ d]imidazole
to provide the corresponding products in good to excel-
lent yields (80% - 92%) under the optimized reaction
conditions. Electron-withdrawing groups seemed to be a
little beneficial for the catalysis compared to electron-
donating ones. For example, 1-chloro-4-iodoben-zene,
1-fluoro-4-iodobenzene and 1-iodo-4-nitrobenzene af-
forded the co rrespond ing arylated produ cts in 80% - 92%
yields (Table 2 , entries 14, 16 and 17). Furthermore, aryl
iodides with electron-donating could also be coupled
with imidazole to give the products in good yields (Table
2, entries 11 and 15). Sterically hindered 1-iodo-2-me-
thylbenzene afforded the product in low yield (Table 2,
entry 13).

Y. LIU ET AL. 187
Table 2. N-Arylation of imidazole with aryl halides cataly-
zed by complex 3a.
Entry ArX R, X HeT-NH ProductYield
(%)b
1 H, I (4b) imidazole (5a) 6b 98
2 4-OEt, I (4c) 5a 6c 83
3 2-Me, I (4d) 5a 6d 59
4 4-COMe, I (4e) 5a 6e 72
5 4-NO2, I (4f) 5a 6f
60
6 4-Ph, I (4g) 5a 6g 95
7 4-Cl, I (4h) 5a 6h 90
8 4-F, I (4i) 5a 6i 98
9 4-Me, Br (4j) 5a 6a 19
10 4-NO2, Br (4k) 5a 6f 49
11 Me, I (4a) 1H-benzo[d]imidazole
(5b) 6j 87
12 H, I (4b) 5b 6k 83
13 2-Me, I (4d) 5b 6l 20c
14 4-NO2, I (4f) 5b 6m
89
15 4-Ph, I (4g) 5b 6n 82
16 4-Cl, I (4h) 5b 6o 80
17 4-F, I (4i) 5b 6p 92
18 4-F, Br (4l) 5b 6p 21
a Reaction conditions: aryl halides (0.5 mmol), imidazoles (1.0 mmol),
complex 3 (10 mol%), NaOH (1.0 mmol), and DMSO (1 ml), 100˚C, 12 h. b
Isolated yields.
3. Conclusion
In summary, we have developed a novel and general
catalytic method for N-arylation of imidazoles promoted
by Salen-Cu(II) complex 3. The system is efficient for
the coupling of imidazoles and its derivatives with ArX
(X = I, Br) to give moderation to excellent yields. The
easy availability of the catalyst, mild reaction conditions,
experimental simplicity, and broad substrate scope are
the features of the catalytic method presented in the cur-
rent paper. Further application of these Salen-Cu(II)
complexes catalyzed organic reaction is currently ongo-
ing in our laboratory.
4. Experimental
4.1. Materials and Instruments
All reactions were carried out under air using magnetic
stirring unless otherwise noted. 1H NMR spectral data
were recorded on a Bruker DPX-400 spectrometer using
TMS as internal standard and CDCl3 as solvent. Mass
spectra were recorded on GC-MS (Agilent 7890A/5975C)
instrument under EI model. All other reagents were of
analytical grade quality purchased commercially and
used.
4.2. Synthesis of Complexes 1-3
Cu(OAc)2·H2O (0.012 mol, 2.40 g) was added to a solu-
tion of substituted ethane-1,2-diamine (0.01 mol) and
2-hydroxybenzaldehyde (0.02 mol) in 35 ml methanol.
The mixture was stirred at 60˚C for 5 h and then filtered.
The precipitate was washed with dichloromethane. The
solid product was collected and dried under vacuum to
afford the de sired complex 1-3.
Complex 1 [30,32]: yield 65%. Anal. Calcd. for
C16H14CuN2O2, %: C, 58.26; H, 4.28; N, 8.49; Found, %:
C, 57.59; H, 4.38; N, 8.37.
Complex 2 [31,33,34]: yield 70%. Anal. Calcd. for
C20H20CuN2O2, %: C, 62.57; H, 5.25; N, 7.30; Found, %:
C, 62.72; H, 5.26; N, 7.49.
Complex 3 [34-37]: yield 72%. Anal. Calcd. for
C20H14CuN2O2, %: C, 63.57; H, 3.73; N, 7.41; Found, %:
C, 63.45; H, 3.81; N, 7.45.
4.3. General Procedure for N-Arylation of
Imidazole with 4-Iodotoluene
To a 10 ml of sealed tube was added complex 3 (37.8 mg,
0.05 mmol), 4-iodotoluene (109 mg, 0.5 mmol), imida-
zole (68 mg, 1.0 mmol), NaOH (40 mg, 1.0 mmol), and
DMSO (1 ml). The reaction mixture was reacted at
100˚C in a preheated oil bath for 12 h. The reaction mix-
ture was cooled to r.t., diluted with 10 mL H2O, and then
the mixture was extracted with ethyl acetate (3 × 20 mL).
The combined organic phases was washed with water
and brine, dried over anhydrous Na2SO4, and concen-
trated in vacuo. The residue was purified by flash column
chromatograph on silica gel (ethyl acetate/petroleum
ether, 2:1 to pure ethyl acetate) to afford the target prod-
uct (75 mg, 95% yield). 1-p-Tolyl-1H-imidazole (6a)
[38-40], 1H NMR (400 MHz, CDCl3): δ 7.81 (s, 1H),
7.27 (s, 4H), 7.24 (t, J = 1.2 Hz, 1H), 7.19 (s, 1H), 2.40
(s, 3H). GC-MS (EI): m/z = 158 [M]+.
5. Acknowledgements
We gratefully acknowledge financial support of this
work by the National Basic Research Program of China
(973 Program: 2012CB722603), the National Natural
Science Foundation of Ch ina (No. 21103114), the Min is-
try of Education Innovation Team (No. IRT1161), and
Start-Up Foundation for Young Scientists of Shihezi

Y. LIU ET AL. 188
University (RCZX201012, RC ZX201 014, RCZ X201015).
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