Efficient iron/copper cocatalyzed N-arylation of arylamines with bromoarenes

Article abstract of DOI:10.1055/s-0030-1260534

Fe(acac)₃ and Cu(OAc)₂H₂O were found to effectively promote the C-N cross-coupling reaction in the presence of K ₂CO₃ as the base. A series of diaryl amine with different substituents can be synthesized in moderate to good yields. This efficient and economic method is attractive for applications on an industrial scale. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1260534

LETTER
1137
Efficient Iron/Copper Cocatalyzed N-Arylation of Arylamines with
Bromoarenes
Iron/Copper
C
oca
t
i
alyzed
a
N-Arylatio
o
n
of Arylami
y
nes
w
ithBro
a
moarenes n Liu, Songlin Zhang*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Dushu Lake Campus, Soochow University, Suzhou 215123, P. R. of China
Fax +86(512)65880352; E-mail: zhangsl@suda.edu.cn
Received 18 January 2011
products were obtained. The catalyst system was also suit-
able for the cross-coupling of heteroarylamines with aryl
bromides.
Abstract: Fe(acac)3 and Cu(OAc)2⋅H2O were found to effectively
promote the C–N cross-coupling reaction in the presence of K2CO3
as the base. A series of diaryl amine with different substituents can
be synthesized in moderate to good yields. This efficient and eco-
nomic method is attractive for applications on an industrial scale.
Table 1 Optimization of Reaction Conditionsa
Key words: catalysis, iron, copper, diaryl amine
Entry Fe source
Cu source
Base
Yield
(%)b
1
2
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(NO3)3·9H2O
Fe2O3
CuI
Cs2CO3
K2CO3
Et3N
55
60
0
Diaryl amines are prevalent functional units in com-
pounds that are of numerous drugs, materials, and natural
products.1 Therefore, considerable effort has been made
in the development of efficient strategies for their con-
structions. The transition-metal-catalyzed cross-coupling
of anilines and aryl halides is one of the most powerful
and straightforward methods for the preparation of diaryl
amines. During the past years, significant improvements
have been achieved in the copper-catalyzed Ullmann
reactions2,3 and the palladium-catalyzed Buchwald–
Hartwig reactions.4,5 But these reactions often require sto-
ichiometric amounts of copper reagents or ligands to pro-
mote the reactions. So it is a major goal for organic
synthesis to develop less expensive and environmentally
more benign catalysts. Iron salts are inexpensive, relative-
ly less toxic, and more environmentally benign in compar-
ison to other transition metals. Following the pioneering
work by Tamura and Kochi,6 the iron salt catalyzed C–N
bond-forming reaction is an alternative and promising
strategy.7 However, to our knowledge, the use of iron
complexes associated with other metals to form diaryl
amine product is rare.8 Recently, Fu and co-workers8a de-
veloped an iron/copper cooperative catalysis for N-aryla-
tion process of aromatic amines by using the FeCl3, CuO,
and rac-BINOL as the catalyst system. Liu and co-
workers8b disclosed a novel and versatile arylation proto-
col involving the use of Fe2O3, Cu(acac)2, and Cs2CO3 un-
der microwave irradiation. However, the yield of N-
arylation of aryl iodide with aryl amines was low. Appar-
ently, an effective method of N-arylation of arylamines is
still to be found. Herein, we reported C–N cross-coupling
reactions of arylamines with bromoarenes based on sim-
ple and inexpensive Fe(acac)3, Cu(OAc)2⋅H2O, and K2CO3
(Scheme 1). A variety of useful functional groups were
tolerated, and moderate to good yields of the diaryl amine
CuI
3
CuI
4
CuI
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
58
55
66
45
0
5
CuSO4·5H2O
Cu(OAc)2·H2O
Cu
6
7
8
–
9
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(acac)2
Cu(OAc)2·H2O
0
10
11
12
13
14
15
10
6
–
Fe(acac)3
Fe(acac)3
–
75c
89d
37e
acac
15f
52g
58h
16
acac
Cu(acac)2
K2CO3
53i
58j
a The reaction was performed with bromobenzene (0.5 mmol), p-tolu-
idine (0.75 mmol), Fe source (10 mol%), Cu source (10 mol%), and
base (1.0 mmol) in DMF (0.8 mL) at 135 °C for 20 h.
b Isolated yield based on 1a.
c Conditions: Fe source (10 mol%), Cu source (20 mol%).
d Conditions: Fe source (20 mol%), Cu source (20 mol%).
e Conditions: Cu source (20 mol%).
f Conditions: acac = acetylacetonate (20 mol%), Cu source (20
mol%).
g Conditions: acac (40 mol%), Cu source (20 mol%).
h Conditions: acac (60 mol%), Cu source (20 mol%).
i Conditions: acac (10 mol%), Cu(acac)2 (20 mol%), K2CO3 (2 equiv).
j Conditions: acac (10 mol%), Cu(acac)2 (20 mol%), K2CO3 (3 equiv).
SYNLETT 2011, No. 8, pp 1137–1142
x
x.
x
x.
2
0
1
1
Advanced online publication: 18.04.2011
DOI: 10.1055/s-0030-1260534; Art ID: W00811ST
© Georg Thieme Verlag Stuttgart · New York

1138
X. Liu, S. Zhang
LETTER
sources Fe(NO3)3·9H2O or Fe2O3 together with
Cu(OAc)2·H2O gave no product or the product in poor
yield, respectively (Table 1, entries 9 and 10), while cop-
per sources CuSO4·5H2O and Cu power gave the product
in moderate yields (Table 1, entries 5 and 7). Subsequent-
ly, a blank test was performed with only an iron source (no
copper source) and showed that the arylation did not pro-
ceed at all (Table 1, entry 8). Low yield of diaryl amine
was obtained when using the copper source in the absence
of the iron salt (Table 1, entries 11 and 14). When the
ligand acac (acetylacetonate) was added instead of
Fe(acac)3, we only got the product at low yields (Table 1,
entry 15). And when Cu(acac)2 was used in combination
with acac, the product was obtained in moderate yield. We
then increased the amount of the base from 2 to 3 equiva-
lents but with little improvement in yield (Table 1, entry
16). Finally, it was concluded that the optimal conditions
for the diaryl amines involved the use of Fe(acac)3,
Cu(OAc)2·H2O, and K2CO3 in DMF at 135 °C.
H
N
NH2
Br
[Fe], [Cu]
base
+
1a
2a
3a
Scheme 1
In our initial study, the reaction between aryl bromoarene
(1a) and p-toluidine (2a) was chosen as a model for the
coupling reaction. As shown in Table 1, a series of exper-
iments were carried out to evaluate the ability of various
iron sources, Cu sources, and bases for the N-arylation
process. By investigating into the base system, no product
was obtained when Et3N was used as the base (Table 1,
entry 3). Moderate yields were obtained when K3PO4,
K2CO3, or Cs2CO3 was used as the base (Table 1, entries
1, 2, and 4). So the inexpensive K2CO3 was used in exper-
iments to investigate the effect of iron sources and copper
sources on the coupling reaction. Cu(OAc)2·H2O and
Fe(acac)3 was found to be the best combination to catalyze
this reaction (Table 1, entries 2 and 5–15). Other iron
Table 2 Fe(acac)3/Cu(OAc)2·H2O-Catalyzed C–N Cross-Coupling of Various Anilines and Aryl Bromoarenesa
H
Fe(acac)3
Cu(OAc)2⋅H2O
Br
NH2
N
+
K2CO3, DMF
R1
R2
R2
R1
1
2
3
Entry
1
Substrate 1
Substrate 2
Product 3
Yield (%)b
89
H
N
NH2
Br
H
N
NH2
Br
Br
2
3
91
65
MeO
OMe
H
N
NH2
OMe
OMe
Br
Br
H
N
NH2
4
5
87
44
H
N
NH2
Cl
Cl
H
N
Br
NH2
6
7
85
84
O
O
H
Br
NH2
N
Cl
Cl
Synlett 2011, No. 8, 1137–1142 © Thieme Stuttgart · New York

LETTER
Iron/Copper Cocatalyzed N-Arylation of Arylamines with Bromoarenes
1139
Table 2 Fe(acac)3/Cu(OAc)2·H2O-Catalyzed C–N Cross-Coupling of Various Anilines and Aryl Bromoarenesa (continued)
H
Fe(acac)3
Cu(OAc)2⋅H2O
Br
NH2
N
+
K2CO3, DMF
R1
R2
R2
R1
1
2
3
Entry
8
Substrate 1
Substrate 2
Product 3
Yield (%)b
74
H
N
Br
Br
NH2
H
N
NH2
NH2
9
85
73
H
Br
Br
N
10
H
NH2
N
O
O
O
O
11
12
67
92
MeO
MeO
OMe
H
N
NH2
NH2
NH2
Br
OMe
H
Br
N
13
14
82
90
OMe
MeO
MeO
O
O
H
Br
N
Cl
Cl
OMe
NO2
H
N
NH2
Br
56
21
15
O2N
N
NO2
H
N
NO2
H
N
Br
52
21
16
17
O
O2N
N
NO2
H
NH2
Br
N
N
64
N
MeO
OMe
Synlett 2011, No. 8, 1137–1142 © Thieme Stuttgart · New York

1140
X. Liu, S. Zhang
LETTER
Table 2 Fe(acac)3/Cu(OAc)2·H2O-Catalyzed C–N Cross-Coupling of Various Anilines and Aryl Bromoarenesa (continued)
H
Fe(acac)3
Cu(OAc)2⋅H2O
Br
NH2
N
+
K2CO3, DMF
R1
R2
R2
R1
1
2
3
Entry
18
Substrate 1
Substrate 2
Product 3
Yield (%)b
H
N
NH2
Br
78
78
N
N
Br
H
N
NH2
19
H
N
NH2
NH2
I
20
21
92
0
H
N
Cl
a The reaction was performed with aryl bromoarenes (0.5 mmol), anilines (0.75 mmol), K2CO3 (1.0 mmol), Fe(acac)3 (20 mol%),
Cu(OAc)2·H2O (20 mol%) in DMF (0.8 mL) at 135 °C for 20 h.
b Isolated yield based on aryl bromoarenes.
Under the optimized conditions, we set out to study the ef- 17). Additionally, phenyl iodide was found to be more ef-
fect of substituent for the coupling reactions which in- ficient than phenyl bromide while phenyl chloride failed
volved various aryl bromides and aryl amines (Table 2). to offer the amination product (Table 2, entries 20 and
Firstly, the reaction of substituted arylamines with aryl 21).
bromoarene were studied. 4-Methyl-, 4-methoxy-, 2-
These reaction conditions were also suitable for the cross-
methoxy-, and 4-chloroaniline underwent reaction with
coupling of heteroarylamines with aryl bromides. A vari-
bromobenzene to give products in 44–91% yields
ety of substituted aryl bromides were tested under the op-
(Table 2, entries 1–5). These reactions indicated that aryl-
timized reaction conditions with imidazole, which
amines having electron-donating groups showed greater
afforded the corresponding coupling products in excellent
reactivity in comparison to those with electron-withdraw-
yields (Table 3, entries 1–3). Hence, imidazole deriva-
ing groups (Table 2, entries 1, 2, 4 and 5). The steric hin-
tives 1H-imidazole-4-carboxylic acid and ethyl 1H-
imidazole-4-carboxylate reacted with 1-bromo-4-methyl-
drance of anilines also exerted some effect on this reaction
as indicated from the reaction of 4-methoxyaniline, and 2-
benzene to give the products in good to moderate yields
methoxyaniline gave high yields with bromoarenes
(Table 3, entries 4 and 5). Furthermore, triazole, pyrazole,
(Table 2, entries 2 and 4). As for bromoarenes, electron-
and indole underwent reaction to provide coupling prod-
neutral (Table 2, entries 6, 7, 13, and 14) and electron-rich
ucts in 61–93% yields (Table 3, entries 6–8).
ones (Table 2, entries 8–12) offered the desired products
In summary, we have demonstrated an efficient iron/
in good yields. The reaction of 4-nitroaniline with 1-bro-
copper-catalyzed N-arylation of arylamines with aryl bro-
mo-4-methylbenzene provided the target product in mod-
moarenes. From a synthesis point of view, a series of dia-
erate yield together with a significant quantity of
ryl amines was obtained in moderate to good yields. Due
triarylamine (Table 2, entry 15). Because of the basic con-
to the relatively low cost and commercial availability of
ditions and the reaction temperature of 135 °C, the acetyl
the metal salts and inorganic base, the reaction was very
convenient and could be easily applied on an industrial
scale.
group of N-(4-nitrophenyl)acetamide was cleaved to pro-
vide the above products (Table 2, entry 16). We were
pleased to note the reaction conditions were also suitable
for the cross-coupling of pyridin-3-amine and phenyl-
methanamine with bromobenzene (Table 2, entries 18 and
19), and 2-bromopyridine with 4-methoxyaniline could
also give the product in moderate yield (Table 2, entry
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Synlett 2011, No. 8, 1137–1142 © Thieme Stuttgart · New York

LETTER
Iron/Copper Cocatalyzed N-Arylation of Arylamines with Bromoarenes
1141
Table 3 Fe(acac)3/Cu(OAc)2·H2O-Catalyzed C–N Cross-Coupling of Heteroarylamines and Aryl Bromoarenesa
Fe(acac)3
Cu(OAc)2⋅H2O
Br
hetero-
cycle
hetero-
cycle
+
N
NH
K2CO3, DMF
R
R
1
4
5
Entry
1
Substrate 1
Substrate 4
Product 5
Yield (%)b
98
N
Br
N
N
N
H
N
Br
N
N
2
92
95
82
52
Cl
N
Cl
H
N
Br
N
N
3
N
H
N
N
HOOC
N
Br
Br
HOOC
4
N
N
N
N
N
EtOOC
N
EtOOC
59
N
N
N
Br
Br
N
N
6
7
81
93
N
N
H
N
N
N
N
H
Br
N
8
61
N
H
a The reaction was performed with aryl bromoarenes (0.5 mmol), heteroarylamines (0.75 mmol), K2CO3 (1.0 mmol), Fe(acac)3 (20 mol%),
Cu(OAc)2·H2O (20 mol%) in DMF (0.8 mL) at 135 °C for 20 h.
b Isolated yield based on aryl bromoarenes.
Punniyamurthy, T. Org. Lett. 2007, 9, 3397. (j) He, C.;
Chen, H.; Cheng, J.; Liu, C.; Liu, W.; Li, Q.; Lei, A. Angew.
Chem. Int. Ed. 2008, 47, 6414. (k) Altman, R. A.;
Anderson, K. W.; Buchwald, S. L. J. Org. Chem. 2008, 73,
Acknowledgment
The work was partially supported by the National Natural Science
Foundation of China (No. 21072143).
5167. (l) Yang, C.-T.; Fu, Y.; Huang, Y.-B.; Yi, J.; Guo,
Q.-X.; Liu, L. Angew. Chem. Int. Ed. 2009, 48, 7398.
(m) Lin, B.; Liu, M.; Ye, Z.; Ding, J.; Wu, H.; Cheng, J. Org.
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LETTER
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(9) General Procedure for the Synthesis of Diaryl Ethers
A Schlenk tube equipped with a magnetic stir bar was
charged with p-toluidine (2a, 0.75 mmol, 80 mg), K2CO3
(138 mg, 1 mmol), Fe(acac)3 (34.4 mg, 20 mo%), and
Cu(OAc)2·H2O (20 mg, 20 mol%). Under a nitrogen
atmosphere, bromobenzene (1a, 0.5 mmol, 78.0 mg) was
added followed by dry DMF (0.8 mL). The reaction mixture
was then heated in an oil bath of 135 °C. After being stirred
at this temperature for 20 h, the mixture was cooled to r.t.
and diluted with Et2O. The resulting suspension was directly
filtered through a pad of Celite, the filtrate was concentrated,
and the crude mixtures were purified by column chroma-
tography on silica gel using PE–EtOAc solvent mixture as
the eluent.
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Scholz, U. Adv. Synth. Catal. 2006, 348, 23. (c) Surry, D.
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2007, 46, 934. (c) Song, R.; Deng, C.; Xie, Y.; Li, J.
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Xu, J.; Jiang, H.; Liu, H. Org. Lett. 2008, 10, 4513. (e) Du,
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2007. (f) Wang, Z.; Zhang, Y.; Fu, H.; Jiang, Y.; Zhao, Y.
4-Methyl-N-phenylaniline (3a)
1H NMR (400 MHz, CDCl3): d = 7.22 (t, J = 7.7 Hz, 2 H),
7.07 (d, J = 8.3 Hz, 2 H), 6.94–7.00 (m, 4 H), 6.87 (t, J = 7.3
Hz, 1 H), 5.58 (br s, 1 H), 2.29 (s, 3 H). 13C NMR (100 MHz,
CDCl3): d = 144.35, 140.68, 131.34, 130.30, 129.75, 120.71,
119.31, 117.26, 21.15. HRMS (EI+): m/z calcd for C13H13N
[M+]: 183.1048; found: 183.1051.
Ethyl 1-p-Tolyl-1H-imidazole-4-carboxylate (Table 3,
Entry 5)
1H NMR (400 MHz, CDCl3): d = 7.93 (s, 1 H), 7.82 (s, 1 H),
7.30 (s, 4 H), 4.41 (q, J = 7.1 Hz, 2 H), 2.42 (s, 3 H), 1.41 (t,
J = 7.1 Hz, 3 H). 13C NMR (75 MHz, CDCl3): d = 163.25,
138.99, 136.77, 135.30, 134.50, 131.02, 124.56, 122.02,
61.17, 21.481, 14.87. HRMS (EI+): m/z calcd for
C13H14N2O2 [M+]: 230.1055; found: 230.1058.
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