Ligand-free copper-catalyzed O-arylation of nitroarenes with phenols

Article abstract of DOI:10.1016/j.tet.2012.08.032

The first example of ligand-free copper-catalyzed O-arylation of nitroarenes with phenols was developed, achieving unsymmetrical diaryl ethers in moderate to excellent yields. This arylation proceeded smoothly without promotion of the ligands, and displayed great functional group compatibility. Thus, the method represents a new, facile, and cost-effective approach to access unsymmetrical diaryl ethers.

Full text of DOI:10.1016/j.tet.2012.08.032

Tetrahedron 68 (2012) 8905e8907
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Ligand-free copper-catalyzed O-arylation of nitroarenes with phenols
,
a
a
a
,
,
*
Jiuxi Chen a , Xingyong Wang , Xingwang Zheng , Jinchang Ding b, Miaochang Liu a, Huayue Wu a
*
a College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, PR China
b Wenzhou Vocational and Technical College, Wenzhou 325035, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2012
Received in revised form 31 July 2012
Accepted 10 August 2012
Available online 16 August 2012
The first example of ligand-free copper-catalyzed O-arylation of nitroarenes with phenols was de-
veloped, achieving unsymmetrical diaryl ethers in moderate to excellent yields. This arylation proceeded
smoothly without promotion of the ligands, and displayed great functional group compatibility. Thus, the
method represents a new, facile, and cost-effective approach to access unsymmetrical diaryl ethers.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
O-Arylation
Unsymmetrical diaryl ethers
Copper
Catalysis
1. Introduction
Previous work
Diaryl ethers are ubiquitous structural motifs that represent
a very important class of organic compounds in natural products,
polymer science, and pharmacologically active compounds.1 Tra-
ditionally, the preparation of these compounds involves Ullmann
ether formation reaction of aryl halides with phenols by using
copper reagents.2 Unfortunately, these classical methods suffer
from drawbacks (e.g., high temperature, often low yields, and the
use of stoichiometric or greater amounts copper reagents) as a re-
sult of limited synthetic application. Consequently, the de-
velopment of new methods for the preparation of diaryl ethers
under milder reaction conditions has received much attention. In
recent years, it has been reported that the use of additives or li-
gands enables the Ullmann ether formation reaction to be per-
formed under milder conditions by using a catalytic amounts of
rhodium,3 palladium,4 copper5 or iron6 catalyst (Scheme 1, Eq. 1).
Very recently, we have developed rhodium-7a or copper-cata-
lyzed7b coupling reaction of arylboronic acids using nitroarenes as
a new arylation reagent to produce diaryl ethers with yields rang-
ing from poor to good (Scheme 1, Eq. 2). In our continuation of
efforts to develop more efficient and practical metal-catalyzed or-
ganic transformations,7,8 we herein report the first example
of ligand-free copper-catalyzed synthesis of unsymmetrical
diaryl ethers via O-arylation of phenols with nitroarenes (Scheme 1,
Eq. (3)).
O
O
[M]
+
X
OH
R1
R2
(1)
(2)
M = Rh, Pd, Cu, Fe
X = I, Br, Cl
R1
R2
[M]
R1
R2
+
+
NO2
B(OH)2
M = Rh, Cu
R1
R2
our previous works: see Ref 8
This work
NO2
O
[Cu]
ligand-free
R1
R2
OH
(3)
R1
R2
Scheme 1.
2. Results and discussion
Initially, the reaction between p-nitrobenzaldehyde (1a) and
phenol (2a) was selected as a model reaction to screen the optimal
reaction conditions (Table 1). First, we explored the effects of the
solvent in the arylation. The results revealed that the use of DMF as
solvent achieved the best result with Na2CO3 as base in the pres-
ence of Cu(OAc)2$H2O, achieving the desired arylated product 3aa
in 76% yield (Table 1, entry 7). Other solvents, including THF, CH3CN,
xylene, 1,4-dioxane, toluene, and pyridine, were less efficient (Table
1, entries 1e6). With these partially optimized arylation conditions,
the effects of the base were investigated. A variety of weak
* Correspondingauthors.Tel./fax:þ865778836 8280;e-mail addresses: jiuxichen@
wzu.edu.cn (J. Chen), huayuewu@wzu.edu.cn (H. Wu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.032

8906
J. Chen et al. / Tetrahedron 68 (2012) 8905e8907
Table 2
inorganic bases (NaHCO3, K2CO3, KF, DABCO, and CsF) were all
unsatisfactory (Table 1, entries 8e12). We then investigated other
alkoxide bases, tBuONa and tBuOK, and yields of 56% and 45% were
obtained, respectively (Table 1, entries 13 and 14). However, we
were delighted to find that the yield could be improved to 92%
when the combination of Cu(OAc)2$H2O and Cs2CO3 was employed
in DMF under N2 atmosphere (Table 1, entry 15). No arylated
product was found without bases. Among the copper sources used,
Cu(I) salts, CuI, Cu2O, and CuCl, were tested in our studies and found
to be effective but a little less active than Cu(II) salts (e.g., CuCl2,
CuBr2, Cu(OAc)2$H2O, Cu(OTf)2, and Cu(acac)2) (Table 1, entries
15e19). Nevertheless, no arylated product was observed in the
absence of the Cu catalyst, which also further proved that Cu(II)
salts do play an important role in this transformation (Table 1,
entry 20).
Scope of the O-arylation of nitroarenes with phenolsa
Cu(OAc)2·H2O, Cs2CO3
Ar1 NO2
O
Ar2 OH
2
+
Ar1
Ar2
DMF, 100 oC, 4 h
1
3
Entry
Ar1 (1)
Ar2 (2)
Product
Yieldb (%)
1
2
3
4
5
6
7
8
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
p-(CHO)C6H4 (1a)
o-(CHO)C6H4 (1b)
o-(CHO)C6H4 (1b)
o-(CHO)C6H4 (1b)
o-(CHO)C6H4 (1b)
o-(CHO)C6H4 (1b)
o-(CHO)C6H4 (1b)
o-(CHO)C6H4 (1b)
p-(MeC]O)C6H4 (1c)
p-(MeC]O)C6H4 (1c)
p-(MeC]O)C6H4 (1c)
p-(MeC]O)C6H4 (1c)
p-(MeC]O)C6H4 (1c)
p-(MeC]O)C6H4 (1c)
p-(PhC]O)C6H4 (1d)
p-(PhC]O)C6H4 (1d)
p-(PhC]O)C6H4 (1d)
p-(PhC]O)C6H4 (1d)
p-(PhC]O)C6H4 (1d)
p-(CN)C6H4 (1e)
C6H5 (2a)
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
92
95
87
88
97
82
65
85
77
62
81
91
90
84
85
62
81
69
84
86
77
63
78
67
90
88
85
72
68
90
91
83
89
63
71
82
93
82
70
61
68
83c
p-(Me)C6H4 (2b)
o-(Me)C6H4 (2c)
m-(Me)C6H4 (2d)
p-(OMe)C6H4 (2e)
o-(OMe)C6H4 (2f)
p-(F)C6H4 (2g)
p-(Cl)C6H4 (2h)
p-(Br)C6H4 (2i)
p-(CF3)C6H4 (2j)
2-Naphthyl (2k)
C6H5 (2a)
p-(Me)C6H4 (2b)
o-(Me)C6H4 (2c)
p-(OMe)C6H4 (2e)
p-(F)C6H4 (2g)
p-(Cl)C6H4 (2h)
p-(Br)C6H4 (2i)
C6H5 (2a)
p-(Me)C6H4 (2b)
p-(OMe)C6H4 (2e)
p-(F)C6H4 (2g)
p-(Cl)C6H4 (2h)
p-(Br)C6H4 (2i)
C6H5 (2a)
o-(Me)C6H4 (2c)
m-(Me)C6H4 (2d)
p-(F)C6H4 (2g)
p-(Br)C6H4 (2i)
C6H5 (2a)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
3aj
3ak
3ba
3bb
3bc
3be
3bg
3bh
3bi
3ca
3cb
3ce
3cg
3ch
3ci
3da
3dc
3dd
3dg
3di
3ea
3eb
3ec
3ed
3eg
3ei
Table 1
Optimization of reaction conditionsa
[Cu]
base, solvent
+
O2N
CHO
PhOH
PhO
CHO
1a
2a
3aa
Entry
Cu sources
Base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
CuCl2
CuBr2
Cu(OTf)2
Cu(acac)2
CuI
Cu2O
CuCl
None
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaHCO3
K2CO3
THF
<5
9
8
CH3CN
Xylene
1,4-Dioxane
Toluene
Pyridine
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
11
14
21
76
45
81
35
27
41
56
45
92
85
87
78
74
49
35
59
0
9
10
11
12
13
14
15
16
17
18
19
20
18
19
20
KF
p-(CN)C6H4 (1e)
p-(CN)C6H4 (1e)
p-(CN)C6H4 (1e)
p-(CN)C6H4 (1e)
p-(CN)C6H4 (1e)
p-(CN)C6H4 (1e)
p-(NO2)C6H4 (1f)
p-(NO2)C6H4 (1f)
p-(MeO2C)C6H4 (1g)
p-(MeO2C)C6H4 (1g)
p-(CF3)C6H4 (1h)
p-(F)C6H4 (1i)
p-(Me)C6H4 (2b)
o-(Me)C6H4 (2c)
m-(Me)C6H4 (2d)
p-(F)C6H4 (2g)
p-(Br)C6H4 (2i)
2,6-(Me)2C6H3 (2l)
p-(Me)C6H4 (2b)
p-(Cl)C6H4 (2h)
p-(Me)C6H4 (2b)
p-(Cl)C6H4 (2h)
C6H5 (2a)
DABCO
CsF
tBuONa
tBuOK
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
3el
3fb
3fh
3gb
3gh
3ha
3ia
DMF
DMF
DMF
DMF
C6H5 (2a)
a
Reaction conditions:
1 (0.5 mmol), 2 (1.0 mmol), Cu(OAc)2$H2O (5 mol %),
Cs2CO3 (2 equiv), and DMF (3 mL), N2, 100 ꢀC, 4 h.
a
Reaction conditions: p-nitrobenzaldehyde (0.5 mmol), phenol (1.0 mmol), Cu
b
source (5 mol %), base (2 equiv), and solvent (3 mL), N2, 100 ꢀC, 4 h.
Isolated yield.
1-Nitro-4-phenoxybenzene was isolated.
c
b
Isolated yields.
With the optimized reaction conditions in hand, we set out to
explore the scope of the ligand-free copper-catalyzed trans-
formation of nitroarenes (1) with phenol to unsymmetrical diaryl
ethers (Table 2, entries 1e11). The electronic properties of the
groups on the phenyl ring of phenols had some effect on the re-
action (Table 2, entries 1e9). Electron-donating phenols possessing
a methoxy or methyl group at either the para or meta position
proved to be good substrates for this transformation, affording the
corresponding products 3ab, 3ad, and 3ae in higher yields than
those bearing substituents at ortho position analogues, which in-
dicated that the steric effects of substituents affected the yields of
the corresponding products 3ac and 3af to some extent (Table 2,
entries 2e6). Having an electron-withdrawing substituent (eCF3)
at the para position afforded a slightly lower yield of arylated
products under the optimized conditions (Table 2, entry 10).
Moreover, naphthalen-2-ol (2j), a less nucleophilic phenol, was also
tolerated well and afforded the arylated product 3ak in 81% yield
(Table 2, entry 11).
the effects of the O-arylation of phenols with different substituents
on the phenyl ring in the nitroarenes (Table 2, entries 12e42).
Under the optimized reaction conditions, not only a formyl group
but also other electron-withdrawing groups (e.g., acetyl, benzoyl,
cyano, nitro, methoxycarbonyl, and trifluoromethyl) gave the cor-
responding arylated products in moderate to good yields. In-
terestingly, using the present protocol, the sterically hindered
phenols, such as 2,6-dimethylphenol (2l) smoothly reacted with p-
nitrobenzonitrile (1e) in high yield (Table 2, entry 36). It is note-
worthy that the mono-arylated products 3fb and 3fh were selec-
tively produced in 93% and 82% yields, respectively, when 1,4-
dinitrobenzene (1f) was employed as a reactant (Table 2, entries
37 and 38). It would be specially mentioned that the nucleophilic
displacement of the fluoro group with phenol was observed when
1-fluoro-4-nitrobenzene (1i) was used under the standard condi-
tions, affording 1-nitro-4-phenoxybenzene (3ia) in 83% yield (Table
2, entry 42). However, nitrobenzene itself and 3-nitrobenzaldehyde
resisted the coupling etherification under the same conditions.
Notably, the fluoro, chloro, and bromo moieties (commonly used
for cross-coupling reactions) in phenols were all tolerated in our
On the basis of the broad scope this catalyst displayed for the O-
arylation of nitroarenes (1) with phenols (2), we decided to explore

J. Chen et al. / Tetrahedron 68 (2012) 8905e8907
8907
catalytic system (Table 2, entries 7e9, 16e18, 22e24, 28, 29, 34, 35,
Supplementary data
38, 40) and provided a novel route to the targeted products in
moderate to good yields, making further elaborations of the cor-
responding biaryl products possible.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.08.032.
Furthermore, the present synthetic route to unsymmetrical
diaryl ethers was successfully applied to the gram-scale operations.
For instance, the O-arylation of p-nitrobenzaldehyde (1.511 g) with
phenol (1.882 g) provided the desired product 3aa in 88% yield.
References and notes
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3. Conclusion
In summary, an efficient, ligand-free copper-catalyzed method
for the O-arylation of nitroarenes with phenols was developed. The
efficiency of this transformation was demonstrated by compati-
bility with a wide range of groups. This protocol provided a new
avenue for developing CeO bond-forming reactions leading to ac-
cess unsymmetrical diaryl ethers. Applications of this copper-
catalyzed arylation using nitroarenes as a new arylation reagent
in organic synthesis are ongoing in our laboratory.
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4.1. General
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5. For recent examples of copper-catalyzed diaryl ethers formation, see: (a)
Chemicals and solvents were either purchased or purified by
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vent with TMS as an internal standard. All reactions were con-
€
Jung, N.; Brase, S. J. Comb. Chem. 2009, 11, 47; (b) Marcoux, J.-F.; Doye, S.;
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ducted
using
standard
Schlenk
techniques.
Column
chromatography was performed using EM Silica gel 60 (300e400
mesh).
4.2. General procedure for the synthesis of unsymmetrical
diaryl ethers
Under N2 atmosphere, a Schlenk tube was charged with nitro-
arenes 1 (0.5 mmol), phenols 2 (1.0 mmol), Cu(OAc)2$H2O (5 mol
%), and Cs2CO3 (1.0 mmol) in DMF (3 mL) at room temperature.
After that, the mixture was stirred constantly at 100 ꢀC (oil bath
temperature) for 4 h. After the completion of the reaction, as
monitored by TLC and GCeMS analysis, the reaction mixture was
cooled to room temperature, diluted with ethyl acetate, and fil-
trated. The filtrate was concentrated under vacuum, and the
resulting residue was purified by silica gel column chromatography
(petroleum ether/ethyl acetate) to afford the desired arylated
product 3.
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Acknowledgements
We thank the National Natural Science Foundation of China
(Nos. 21102105 and 21172175) and Natural Science Foundation of
Zhejiang Province (No. LY12B02011) for financial support.