New, simple, and effective thiosemicarbazide ligand for copper(II)-catalyzed N-arylation of imidazoles

Article abstract of DOI:10.1080/00397911.2010.537425

A new methodology of copper-catalyzed coupling reaction has been developed, involving imidazoles react with aryl halides using CuCl₂ ·H₂O as catalyst and 1-(4- hydroxy-3-me thoxybenzylidene) thiosemicarbazide (L1) as ligand. It is th

Full text of DOI:10.1080/00397911.2010.537425

1
Synthetic Communications , 42: 1192–1199, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.537425
NEW, SIMPLE, AND EFFECTIVE THIOSEMICARBAZIDE
LIGAND FOR COPPER(II)-CATALYZED N-ARYLATION
OF IMIDAZOLES
1
Yuefeng Shan, Ying Wang, Xiaolei Jia,
2
1
1
Wenwen Cai, and Jiannan Xiang
1
1
College of Chemistry and Chemical Engineering, Biomedical Engineering
Center and State Key Laboratory of Chemo=biosensing and Chemometrics,
Hunan University, Changsha, Hunan, China
2
Institute of Cancer Research, Southern Medical University, Guangzhou,
China
GRAPHICAL ABSTRACT
Abstract A new methodology of copper-catalyzed coupling reaction has been developed,
involving imidazoles react with aryl halides using CuCl
2
2
ꢀ H O as catalyst and 1-(4-
hydroxy-3-me thoxybenzylidene)thiosemicarbazide (L1) as ligand. It is the first time that
the complex of new ligand L1 and copper(II) presents better catalytic effects than with cop-
per(I). Because of the stability and economy of copper(II), this methodology will be a
promising approach in preparing N-arylimidazoles.
Keywords Aryl halide; copper(II); coupling reaction; L1; N-arylation of imidazole
INTRODUCTION
N-Aryl imidazoles and benzimidazoles structures are found in many biologically
active compounds. Although the traditional preparation of these moieties, including
nucleophilic aromatic substitution of an activated aryl halide and copper-mediated
coupling of the heterocycle with an aryl iodide, can give access to a wide variety of
N-arylated products, these methods suffer from significant limitations: exposure of
Received April 22, 2010.
Address correspondence to Jiannan Xiang, College of Chemistry and Chemical Engineering,
Hunan University, Changsha, China. E-mail: Jnxiang@hnu.cn
1
192

COPPER(II)-CATALYZED N-ARYLATION OF IMIDAZOLES
1193
ꢁ
substrates to high temperatures, typically 150–200 C, extended periods of time, and
[
1,2]
use of stoichiometric quantities of copper compound.
a mild transition metal–catalyzed coupling reaction has been developed to comp-
Therefore, in recent years,
[3]
lement the traditional preparations.
Despite the continued development
of hindered biaryl monophosphines and other ligands for improved Pd-catalyzed C-
N bond-forming reactions, no Pd-based catalysts display good generality for the
N-arylation of imidazoles. Thus, copper-based catalysts have been continuing to pro-
[
4–6]
vide the most effective systems for the N-arylation of imidazoles.
[
7]
Although copper(I) compounds showed more efficiency in most cases, their
stability and economy are worse than copper(II) compounds. Meanwhile, it has
never before been reported that copper(II) compounds can provide better yield than
copper(I) compounds.
In recent years, thiosemicarbazide compounds (L-type compounds), like 1-(4-
hydroxy-3-methoxybenzylidene)thiosemicarbazide (L1), have generated considerable
interest for various reasons, such as variable bonding properties, several donor sites,
[
8–11]
structural diversity, and pharmacological aspects.
L-Type compounds have
never been applied to the copper-based system as ligands. According to these consid-
erations, in this study we synthesized a series of N-arylation of imidazole compounds
using copper-based systems containing L-type ligands and studied the catalytic effect
of copper-based system on the preparation of N-arylation of imidazoles. We also
tried to examine the influence of copper-based catalysts with different valence on
the yields of products and made a comparison of activities of L-type ligands.
First, 1-iodo-3-nitrobenzene reacted with imidazole using copper(I) or cop-
per(II) as catalyst, combining with L-type ligands under basic condition (Table 1).
As we can see from entries 1 and 5, it was obvious that L1 showed greater catalytic
activity than any other L-type ligands with copper(I). Compared to the structure of
other L-type ligands, L1 with the strong electron-withdrawing substitution groups
on the benzene ring increased the ligand activity. The coupling reaction was then
allowed to proceed under copper(II) and L1 ligand catalyst system (Table 1, entries
6
ꢂ11). Different copper(II) compound catalysts, such Cu(OAc) , CuSO ꢀ 5H O, and
2 2
2
4
2
CuCl ꢀ 2H O, were investigated. The greater product yields were found under these
conditions, especially when using CuCl ꢀ 2H O as catalyst. In addition, the solvent
2
2
effect was screened. Although recent reports indicated the existence of trace H O
2
[12,13]
could improve the reaction efficiency,
mal solvent in our catalytic system. Therefore, the optimized catalyst system was the
dimethylformamide (DMF) was the opti-
combination of CuCl ꢀ 2H O and L1 ligand in DMF using K CO as base.
2
2
2
3
Having optimized the reaction conditions for 1-iodo-3-nitrobenzene and imi-
dazole, we further extended the reaction scope and examined the influence of sub-
strates in the reaction (Table 2). Meanwhile, we tried to investigate the possible
mechanism of copper-mediated coupling reaction. In Table 2 (entries 1ꢂ7), aryl
iodides showed better activity than aryl bromides. Entries 2 and 3 gave better yield,
demonstrating the nucleophilic activity of halides in C-X played an important role in
this coupling reaction and aryls with electron-deficient substitutions could increase
the coupling reaction activity. Benziminazole as N-heterocycle substrate reacted with
aryl halides (Table 2, entries 9ꢂ12). Compared with aryl bromides, aryl iodides
showed better activity in reaction with N-heterocycle, in accordance with results of
the former experiments. However, compared with the reaction using imidazole as

1194
Y. SHAN ET AL.
a
Table 1. Screening of conditions for the coupling reaction of 1-iodo-3-nitrobenzene with imidazole
b
Entry
Copper salts
Ligands
Solvents
DMF
NMP
Yields (%)
1
2
3
4
5
6
7
8
9
CuI
CuI
CuI
CuI
CuBr
L1
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
67
62
57
43
63
76
79
83
51
80
82
DMF
DMF
DMF
DMF
CuSO
4
ꢀ 5H
2
O
Cu(OAc)
2
DMF
DMF
CuCl
CuCl
CuCl
CuCl
2
2
2
2
ꢀ 2H
ꢀ 2H
ꢀ 2H
ꢀ 2H
2
2
2
2
O
O
O
O
1,4-Dioxane
DMF
1
1
0
1
2
DMF:H O(10:1)
a
Unless otherwise indicated, the reaction conditions were as follows: 3-iodo-1-nitrobenzen (2.4 mmol),
ꢁ
imidazole (2 mmol), copper salt (0.2 mmol), ligand (0.4 mol), base (4 mmol), DMF (3 mL), 110 C, 24 h.
CuI and CuBr were proceeded under N protection.
2
b
Isolated yield based on imidazole.
substrate, lower yield was received. It may be the conjugation effect of the benzene
ring leading to weaken the electron density of imidazole. Meanwhile, two white
powder products, N-naphthalene of imidazoles, were obtained with moderate yields
(Table 2, entries 7 and 12). This suggested that thiosemicarbazide ligands could be a
readily available ligand in the preparation of N-naphthalene of imidazoles.
The probable mechanism of the reaction is shown in Scheme 1. Lobana and
coworkers reported that copper(II) can theoretically form a series of complexes with
[14]
thiosemicarbazides, but the exact mechanism for the reaction was unknown. The
screening of catalysis condition (Table 1) demonstrates that 1-(4-hydroxy-3-methoxy
benzylidene)thiosemicarbazide (L1) in combination with CuCl ꢀ 2H O provided a
2
2
optimized catalyst system (Scheme 1, I), indicating the reaction follows the oxidative
addition–reductive elimination principle. Because the electron-rich ligands could sta-
bilize the copper intermediate (Scheme 1, IV) and decrease the oxidation potential
for the copper(II)–copper(IV) redox pair, the accelerating rate limited aryl halide
activation. Therefore, it may explain why the complex copper(II) with L1 had the
best catalytic efficiency in the preparation of N-arylation of imidazoles.
The results implied that the methodology was feasible for a range of aryl iodide
and imidazole substrates with good product yields. L1 was considered as a good

COPPER(II)-CATALYZED N-ARYLATION OF IMIDAZOLES
1195
Table 2. The coupling reaction of imidazole or benzimidazole with aryl halides H ꢃ NHetþ
CuCl2 L1 K2CO3
Ar ꢃ X
ꢃ!
Ar ꢃ NHet
ꢁ
DMF 110 C 24 h
a
Entry
H-NHet
Ar-X
Products
Yields (%)
77
1
2
3
4
5
6
81
83
73
56
42
7
8
9
51
trace
63
1
1
1
0
1
2
74
52
48
a
Isolated yield based on imidazole or benzimidazole.
bidentate ligand applied to a copper-based catalytic system. It can facilitate CuCl to
2
perform better catalytically than any other copper catalysts.
In conclusion, we have developed a new and simple methodology for
N-arylation of imidazoles in good yield using the complex 1-(4-hydroxy-3-methoxy-
benzylidene)thiosemicarbazide (L1) with copper(II) as catalyst. The most important

1196
Y. SHAN ET AL.
Scheme 1. Outline of the reaction scheme for coupling reaction of imidazole with aryl halides under
copper-based catalyst system.
point is that it is the first time that the new ligand L1, combining with copper(II) as
catalyst system, performed better than combining with copper(I) catalysts in an
N-arylation reaction. Nevertheless, the better catalytic yields were obtained and
further investigation is now under study in our laboratory.
EXPERIMENTAL
All reactions were manipulated in an opening system without nitrogen protect.
13
H NMR and C NMR spectra were recorded on 400=125-MHz spectrometer.
1
Chemical shifts were reported in parts per million (ppm) downfield from tetramethyl-
silane (TMS) with the solvent resonance as the internal standard. The type of elec-
trospray ionization–mass spectrometry (EI-MS) instrument is MAT-95 XP.
Column chromatography was performed using EM silica gel 60 (300ꢃ400 mesh),
[
15]
and ligands were prepared according to the literature.
The infrared (IR) spectra
were recorded on a Perkin-Elmer Spectrum One using KBr discs.
General Procedure for the Ligand L1
[
15]
L1 ligands were prepared according to the literature. Briefly, 4-hydroxy-3-
methoxybenzaldehyde (70 mmol) was dissolved in 60 mL ethanol. Aminothiourea

COPPER(II)-CATALYZED N-ARYLATION OF IMIDAZOLES
1197
(20 mmol) and acetate (4 mL) were dissolved in 60 mL water, and then the mixture
was added to the ethanol solution followed by reflux for 60 min. The crude product
was recrystallized in ethanol.
General Procedure for the Copper(II)-Catalyzed N-Arylation
Imidazoles
CuCl (0.2 mmmol) and L1 (0.4 mmol) were dissolved in 3 mL dimethylforma-
2
ꢁ
mide (DMF), and the solution was stirred at 70 C until it turned clear.
3
-Iodo-nitrobenzene (2.4 mmol), imidazole (2 mmol), and K CO (4 mmol) were then
2 3
ꢁ
added. The reaction was allowed to proceed for 24 h under 110 C. The reaction was
monitored by thin-layer chromatography (TLC) using 0.25-mm E. Merck silica-gel
60 F254 precoated plates, which were visualized with ultraviolet light. The chromato-
graphic purification (ethyl acetate=hexane 1:3) provided compound.
Data
1
-(3-Nitrophenyl)-1H-imidazole (3a). Slight yellow oil; IR (KBr) 3238,
ꢃ1
1
3
7
1
116, 1616, 1520, 1401, 1119 cm ; H NMR (400 MHz, CDCl ) d 7.93 (s, 1H),
.48–7.51 (m, 3H), 7.37–7.42 (m, 4H); C NMR (125 MHz, CDCl ) d 114.7,
3
1
3
3
21.3, 127.0, 129.1, 130.3, 134.6, 136.2; HRMS (ESI): m=z calcd. for C H N
þ
9
8
2
[
M þ 1] : 145.0687; found: 145.0629.
-(4-Nitrophenyl)-1H-imidazole (3b). Yellow solid; mp 202–204 C (lit.
ꢁ
[16]
1
mp 203–205 C); IR (KBr) 3134, 3026, 1631, 1609, 1527, 1398, 1057 cm
ꢁ
ꢃ1
;
1
H NMR (400 MHz, CDCl ) d 8.4 (d, J ¼ 8.8 Hz, 2H), 7.99 (s, 1H), 7.59 (d,
3
1
3
J ¼ 9.2 Hz, 2H), 7.39 (s, 1H), 7.29 (s, 1H); C NMR (125 MHz, CDCl ) d 109.8,
3
1
17.6, 121.0, 125.8, 131.7, 135.4, 142.0; HRMS (ESI): m=z calcd. for C H N O
9 7 3
2
þ
[
M þ 1] : 190.0538; found: 190.0517.
-(3-Nitrophenyl)-1H-imidazole (3c). Yellow solid; mp 110–111 C (lit.
ꢁ
[17]
1
mp 109–110 C); IR (KBr) 3127, 3023, 1638, 1604, 1520, 1404, 1110 cm
ꢁ
ꢃ1
;
1
H NMR (400 MHz, CDCl ) d 8.30 (t, J ¼ 1.6 Hz, 1H), 8.25 (ddd, J ¼ 1.2 Hz,
3
2
(
1
.0 Hz, 7.6 Hz, 1H), 7.97 (s, 1H), 7.78 (ddd, J ¼ 0.8 Hz, 1.6 Hz, 8.4 Hz, 1H), 7.72
3
1
3
t, J ¼ 8 Hz, 1H), 7.38 (s, 1H), 7.29 (s, 1H); C NMR (125 MHz, CDCl ) d 116.2,
17.9, 122.0, 126.8, 131.0, 131.5, 135.4; HRMS (ESI): m=z calcd. for C H N O
3
9
7
2
þ
[
M þ 1] : 190.0538; found: 190.0584.
-p-Tolyl-1H-imidazole (3d). Slight yellow solid; mp 63–64 C (lit.
ꢁ
ꢃ1
[18]
mp
H NMR
1
ꢁ
1
6
3–65 C); IR (KBr) 3114, 3016, 1601, 1507, 1397, 1053 cm
;
(
3
1
400 MHz, CDCl ) d 7.83 (s, 1H), 7.28 (s, 4H), 7.26 (s, 1H), 7.2 (s, 1H), 2.41 (s,
3
1
3
H); C NMR (125 MHz, CDCl ) d 20.9, 118.4, 121.4, 130.1, 130.3, 134.9, 135.6,
3
þ
37.5; HRMS (ESI): m=z calcd. for C H N [M þ 1] : 159.0844; found: 159.0877.
1
0
10
2
Methyl 6-(1H-imidazol-1-yl)-2-naphthoate (3e). White solid; mp
ꢃ1
ꢁ
1
21–223 C; IR (KBr) 3234, 3126, 1750, 1638, 1601, 1521, 1408, 1115 cm ; H
2
NMR (400 MHz, CDCl ) d 8.66 (s, 1H), 8.15 (dd, J ¼ 1.6 Hz, 8.8 Hz, 1H), 8.09 (d,
3
J ¼ 8.4 Hz, 1H), 8.02 (s, 1H), 7.93 (d, J ¼ 8.8 Hz, 1H), 7.87 (d, J ¼ 1.6 Hz, 1H),
1
3
7
.61 (dd, J ¼ 2.4 Hz, 9.2 Hz, 1H), 7.34 (s, 1H), 7.28 (s, 1H), 4.00 (s, 3H); C NMR

1198
Y. SHAN ET AL.
(
125 MHz, CDCl ) d 51.5, 115.7, 118.3, 120.6, 126.1, 128.0, 128.5, 129.8, 130.2,
3
1
30.4, 132.6, 135.2, 136.5, 136.6, 166.8; HRMS (ESI): m=z calcd. for C H N O
15 12 2 2
þ
[
M þ 1] : 253.0899; found: 253.0865.
-p-Tolyl-1H-benzoimidazole (3f). Slight yellow solid; mp 108–109 C
ꢁ
1
[
18]
ꢁ
ꢃ1
1
mp 109 C); IR (KBr) 3115, 3074, 1611, 1547, 1420, 1098 cm ; H NMR
(lit.
(
400 MHz, CDCl ) d 8.10 (s, 1H), 7.87–7.89 (m, 1H), 7.51–7.53 (m, 1H), 7.39 (dd,
3
1
3
J ¼ 15 Hz, 8.5 Hz, 4H), 7.32–7.34 (m, 2H), 2.47 (s, 3H); C NMR (125 MHz, CDCl )
3
d 23.9, 115.6, 122.3, 123.7, 130.0, 134.2, 136.2, 138.6, 139.1, 146.8; HRMS (ESI): m=z
þ
calcd. for C H N [M þ 1] : 209.1000; found: 209.0885.
1
4
12
2
1
-(4-Nitrophenyl)-1H-benzoimidazole (3g). Slight yellow solid; mp
[19]
ꢁ
ꢁ
1
1
(
7
1
[
75–176 C (lit.
1
mp 175–178 C); IR (KBr) 3105, 1595, 1510, 1418, 1300,
143 cm ; H NMR (400 MHz, CDCl ) d 8.50 (dd, J ¼ 2.4 Hz, 7.2 Hz, 2H), 8.21
ꢃ1
3
s, 1H), 7.91–7.92 (m, 1H), 7.76 (dd, J ¼ 2.4 Hz, 6.8 Hz, 2H), 7.61–7.63 (m, 1H),
1
3
.41–7.43 (m, 2H); C NMR (125 MHz, CDCl ) d 115.3, 122.6, 123.0, 123.9,
3
36.5, 138.2, 143.2, 143.5, 147.6; HRMS (ESI): m=z calcd. for C H N O
þ
1
3
9
3
2
M þ 1] : 240.0695; found: 240.0711.
-Phenyl-1H-benzoimidazole (3h). Colorless oil; mp 95 C (lit.
ꢁ
[18]
1
6 C); IR (KBr) 3134, 3012, 1632, 1530, 1418, 1104 cm ; H NMR (400 MHz,
mp
ꢁ
ꢃ1
1
9
CDCl ) d 7.81 (s, 1H), 7.48–7.51 (m, 3H), 7.36–7.41 (m, 4H), 7.28 (d, J ¼ 11.5 Hz,
3
1
3
1
1
1
H), 7.21 (s, 1H); C NMR (125 MHz, CDCl ) d 116.4, 122.4, 123.1, 128.4,
3
þ
29.8, 135.2, 137.4, 139.0, 144.3; HRMS (ESI): m=z calcd. for C H N [M þ 1] :
1
3
10
2
95.0844; found: 195.0827.
Methyl 6-(1H-benzoimidazol-1-yl)-2-naphthoate (3i). White solid; mp
ꢁ
ꢃ1
1
45–247 C; IR (KBr) 3234, 3126, 1754, 1614, 1507, 1413, 1127 cm ; H NMR
2
(
400 MHz, CDCl ) d 8.64 (s, 1H), 8.18 (s, 1H), 8.1–8.13 (m, 2H), 7.95 (s, 1H),
3
7
7
1
1
3
.90 (d, J ¼ 8.8 Hz, 1H), 7.84–7.87 (m, 1H), 7.65–7.68 (m, 1H), 7.57–7.59 (m, 1H),
1
3
.30–7.33 (m, 2H), 3.95 (s, 3H); C NMR (125 MHz, CDCl ) d 51.5, 116.0, 119.3,
3
20.5, 123.6, 125.6, 128.0, 128.3, 128.9, 130.4, 132.5, 135.2, 136.5, 136.7, 138.9,
þ
44.3, 168.1; HRMS (ESI): m=z calcd. for C H N O [M þ 1] : 303.1055; found:
1
9
14
2
2
03.1068.
ACKNOWLEDGMENTS
We express our sincere thanks to Zhengying Yu of Hunan University for
assistance in NMR measurements and Hengbin Wang of Emory University.
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