Ligand-free copper-catalyzed O-arylation of arenesulfonamides with phenols: An unusual approach to biaryl ether synthesis

Article abstract of DOI:10.1080/00397911.2019.1574833

An unprecedented ligand-free copper-catalyzed O-arylation of arenesulfonamides with phenols has been developed. Thus, a range of unsysmmetric biaryl ethers were synthesized in excellent yields. The reaction occurs efficiently with excellent regioselectivity through the cleavage of C_ary–S bond and with a good tolerance of functional groups on the phenyl ring of phenols. The reaction is appreciated for its readily accessible substrates, mild conditions, and simple operation under air.

Full text of DOI:10.1080/00397911.2019.1574833

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Ligand-free copper-catalyzed O-arylation of
arenesulfonamides with phenols: An unusual
approach to biaryl ether synthesis
Junmin Chen, Junmin Wang, Xida Chen, Yuming Huang & Pu Shouzhi
To cite this article: Junmin Chen, Junmin Wang, Xida Chen, Yuming Huang & Pu Shouzhi (2019):
Ligand-free copper-catalyzed O-arylation of arenesulfonamides with phenols: An unusual approach
to biaryl ether synthesis, Synthetic Communications, DOI: 10.1080/00397911.2019.1574833
To link to this article: https://doi.org/10.1080/00397911.2019.1574833
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https://doi.org/10.1080/00397911.2019.1574833
Ligand-free copper-catalyzed O-arylation of
arenesulfonamides with phenols: An unusual approach to
biaryl ether synthesis
a,b
b
b
b
a
Junmin Chen , Junmin Wang , Xida Chen , Yuming Huang , and Pu Shouzhi
a
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University,
b
Nanchang, Jiangxi, P. R. China; Key Laboratory of Functional Small Organic Molecules Ministry of
Education and College of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang,
Jiangxi, P. R. China
ABSTRACT
ARTICLE HISTORY
An unprecedented ligand-free copper-catalyzed O-arylation of arene-
sulfonamides with phenols has been developed. Thus, a range of
unsysmmetric biaryl ethers were synthesized in excellent yields. The
reaction occurs efficiently with excellent regioselectivity through the
cleavage of Cary–S bond and with a good tolerance of functional
groups on the phenyl ring of phenols. The reaction is appreciated
for its readily accessible substrates, mild conditions, and simple oper-
ation under air.
Received 16 August 2018
KEYWORDS
Arenesulfonamides; biaryl
ether synthesis; copper-
catalyzed; C–O
bond-forming
GRAPHICAL ABSTRACT
OH
O O
S
O
R
2
2 2
Cu(OAc) .H O (10 mol%)
N
R3
R3
R
1
Cs CO3 (2 equiv)
2
NO2
NO
2
DMF
1
2
80
°
C, 8h
3
up to 96% yields
Introduction
Diaryl ethers represent an important and versatile structure motif that is frequently pre-
sent in natural products; materials, pharmaceuticals, and other biologically active mole-
[
1–6]
cules.
Because of their great importance, a wide variety of synthetic methodologies
have been developed for the synthesis of diaryl ethers. Conventionally, the classic
Ullmann ether synthesis is carried out under relatively harsh conditions, such as high
ꢀ
[7]
temperatures (125–220 C) and stoichiometric amounts of copper reagents. In the last
[
8–10]
few decades, much progress has been made using Pd/ligand systems.
Although Pd-
catalyzed systems show high efficacy in terms of turnover in cross-coupling reactions,
[
11]
costly and toxic Pd limits its applications to large scale production.
Significantly, Cu-
catalyzed Ullmann coupling between an aryl halide and phenol has been successfully
[
12–18]
developed under mild conditions by using some special ligands.
Although
CONTACT Pu Shouzhi
Technology Normal University, Nanchang, Jiangxi, P. R. China.
Supplemental data for this article can be accessed on the publisher's website.
pushouzhi@tsinghua.org.cn
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science
&
ß 2019 Taylor & Francis Group, LLC

2
J. CHEN ET AL.
significant progress has been achieved in the ether formation reaction, there remain
some limitations. For example, the electrophilic coupling partners only were limited to
[
19]
aryl halides and some auxiliary ligands were needed to promote the transformation.
To increase the applicability of the method, there is interest in extending cross-coupling
reactions to a broader range of electrophiles. Recently, Wu et al. reported for first time
nitroarenes as a powerful electrophilic coupling partner and unsymmetrical diaryl ethers
[
20]
were obtained via Rh-catalyzed coupling of nitroarenes with arylboronic acids.
Latterly, some Pd and Cu catalyst were also found to activate the Ar-NO bonds of
2
[
21,22]
[23–26]
nitroarenes towards cross-coupling with arylboronic acids
and phenols.
Similarly, arenesulfonamides are valuable intermediates in synthetic organic chemistry
and were extensively used as pharmaceutical agents because of their diverse biological
properties. To our knowledge, arenesulfonamides as coupling partners were less investi-
gated, the reactions mainly focused on the development of C–C bond-forming transfor-
mations through the cross-coupling of vinyl sulfones or aryl sulfonates with Grignard
[
27–31]
reagents.
. Recently, we have successfully developed the copper(I)-catalyzed cross-
coupling of 2-nitro benzenesulfonamides with thiol to form unsymmetrical sulfides
[
32]
under mild conditions with excellent yields.
Inspired by our previous work, we envi-
sion the C–O bond-forming transformations can be carried out via copper-catalyzed O-
arylation of arenesulfonamides with phenols through the cleavage of C –S bonds. In
ary
our continuation of efforts to develop arenesulfonamides as a new coupling partner in
cross-coupling reactions, we wish to report the new protocol of copper-catalyzed syn-
thesis of unsymmetrical diaryl ethers via direct coupling reaction of arenesulfonamides
with phenols, which could be a synthetically alternative route that is environmentally
friendly, avoiding the use of halides.
Results and discussion
To test the validity of the proposed Ulmman coupling reactions, we chose 2-nitro ben-
zylsulfonamide (1a) and 4-methylphenol (2a) as the model substrates to optimize the
reaction conditions (Table 1). In our initial investigation, the reaction was performed
ꢀ
with Cu(OAc) ÁH O (10% mol), Na CO (2 equiv) in DMF, at 80 C for 8 h under air,
2
2
2
3
[
32]
which is the similar conditions used by Chen et al.
To our delight, the target product
(3a) was obtained with 67% yield (Table 1, entry 1). It should be noted that this reac-
tion is highly regioselective and the NO group was not cleaved. Subsequently, we opti-
2
mized the conditions by varying the Cu salt, solvent, base, and temperature. Firstly, we
explored the effects of the base in the arylation coupling reaction. The results revealed
t
that the use of K CO or LiO Bu as a base in the presence of Cu(OAc) .H O can also
2
3
2
2
perform the desired arylated product 3a in 62% or 71% yield, respectively (Table 1,
entries 2 and 3). However, the trace amount of product 3a was obtained by using
K PO or KOAc as a base (Table 1, entries 4 and 5). Significantly, the excellent yield
3
4
was obtained when Cs CO was used under similar conditions (Table 1, entry 6). To
2
3
further optimize the reaction conditions, the various copper courses were screened in
ꢀ
the presence of Cs CO (2 equiv.) with DMF as solvent at 80 C for 8 h under air.
2
3
However, CuO, CuI, CuI, CuSO , CuCl and Cu O produced the desired product 3a in
4
2
2
moderate to good yields (Table 1, entries 7–11) and none of them were superior to

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3
Table 1. Optimization of conditions.
OH
O
O
S
CH3
O
N
Cu salt (10% mol )
CH
3
NO
base (2 equiv)
solvent
2
NO2
3a
CH3
CH3
°
80 C, 8h
1a
2a
a
b
Entry
Copper salt
Base
Na CO
CO
Solvent
Yield (%)
1
2
3
4
5
6
7
8
9
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
CuO
2
2
2
2
2
2
.H
2
.H
2
.H
2
.H
2
.H
2
.H
2
O
O
O
O
O
O
2
3
DMF
67
62
71
Trace
Trace
93
78
47
78
82
51
66
47
34
35
53
14
73
K
2
3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
t
LiO Bu
PO
KOAc
K
3
4
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
/
3
3
3
3
3
3
3
3
3
3
3
3
3
3
CuI
CuSO
CuCl
Cu
4
10
11
12
13
14
15
16
17
18
19
20
a
2
2
O
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
/
2
2
2
2
2
2
2
.H
2
.H
2
.H
2
.H
2
.H
2
.H
2
.H
2
O
O
O
O
O
O
O
3
CH CN
1,4-Dioxane
H
2
O
c
DMF
DMF
DMF
DMF
DMF
d
e
Trace
Trace
Cu(OAc)
2
.H
2
O
Reaction conditions: 2-nitro benzylsulfonamide (1a) (0.25 mmol), 4-methylphenol (2a) (0.30 mmol), copper salt
10 mol%), base (0.5 mmol), and solvent (0.5 mL), at 80 C for 8 h.
Isolated yield.
Reaction at 50 C.
Reaction at 25 C.
2 2
In the presence of 5 mol% Cu(OAc) .H O.
ꢀ
(
b
c
ꢀ
ꢀ
d
e
Cu(OAc) ÁH O (Table 1, entry 6). Secondly, the solvents were screened. It was found
2
2
that reaction works in various solvents including DMSO, CH CN, H O or 1,4-dioxane,
3
2
however, the yields are less efficient compared to DMF under the similar conditions
(
Table 1, entries 12–15). The temperature had an obvious effect on the reaction effi-
ꢀ
ciency. When the reaction was proceeded at 50 and 25 C respectively, the yields
reduced to 53%, and 14%, respectively (Table 1, entries 16 and 17). Additionally, lower-
ing the amount of the Cu source (5 mol %) led to the formation of 3a in a drastically
decreased yield (Table 1, entry 18). Finally, control experiment confirmed that a trace
amount of desired product 3a were observed without the Cu source or base (Table 1,
entries 19 and 20). which also further proved that the present reaction was not classic
aromatic nucleophilic substitution (SN ) reactions. Thus, the optimized reaction condi-
Ar
tions were as follows: 10 mol% of Cu(OAc) .H O, and 2 equiv. of Cs CO in DMF (as
2
2
2
3
ꢀ
a solvent) reacting at a temperature of 80 C for 8 h under air.
With the optimized reaction conditions in hand, we set out to explore the scope of
the ligand-free copper-catalyzed transformation of arenesulfonamides 1 with phenol 2
to unsymmetrical diaryl ethers 3 (Table 2). Primarily, we set on to evaluate 2-nitro
benzenesulfonamide 1a and various phenols 2. Phenols with electron-neutral, electron-
donating or electron-withdrawing groups reacted smoothly with 2-nitro benzenesulfona-
mide 1a, and high to excellent yields were achieved (Table 2, entries 1–11). For

4
J. CHEN ET AL.
Table 2. Copper-catalyzed cross coupling of 2-nitro benzenesulfona-
mide 1 with various phenols 2.
OH
O
O
O
S
R1
Cu(OAc)
2
.H
O (10mol%)
2
N
R3
R
3
R
2
Cs CO
3
(2 equiv)
NO2
1
NO
2
DMF
80 oC, 8h
1
2
3
a
b
Entry
Substrate 1
Substrate 2
Product 3
Yield (%)
O
O
O
1
OH
93
S
Me
N
3
Me
a
2
b
Me 1a
Me
NO
2
NO
2
O
O
S
OH
2a
O
2
3
4
91
89
87
Me
N
3
b
Me 1a
NO2
O
NO
2
O
O
Me
OH
2c
Me
S
Me
N
3c
Me 1a
NO
2
NO
2
O
O
S
Me
Me
Me
OH
N
O
Me 1a
2
d
3d
Me
NO
2
NO
O
2
O
O
O
O
O
Me
OH
2e
5
6
7
8
83
86
84
81
S
Me
3
e
N
NO
2
Me 1a
NO
2
Me
Me
O
S
OH
O
Me
N
3f
OMe
2
f
Me 1a
MeO
F
NO
2
NO
2
O
OH
O
S
Me
N
3
g
Me 1a
2g
NO
F
2
NO
2
O
S
F
F
Me
OH
N
O
Me 1a
2
h
3h
NO
2
NO2
O
O
O
OH
9
83
91
S
Me
N
3i
Br
Me 1a
2i
Br
NO
2
NO
2
O
10
O
O
OH
2b
S
CH
3
3a
Me
N
H
Me
NO
2
1
b
NO
2
O
O
S
OH
2b
O
96
23
1
1
2
Et
3a
Me
N
Me
Me
NO
2
Et 1c
NO2
O
O
OH
2b
O
1
S
3a
Me
NH
2
NO
2
1
d
NO2
O
O
OH
O
13
trace
S
Me
N
3
j
2
j
Me 1a
2
O N
NO
2
NO
2
NO
2
a
Reaction conditions: 2-nitro benzenesulfonamides (1) (0.25 mmol), phenol (2)
(
8
0.30 mmol), Cu(OAc)
0 C for 8 h under air.
2
.H
2
O (10 mol%), Cs
2
CO
3
(0.5 mmol), and DMF (0.5 mL), at
ꢀ
b
Isolated yield.

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5
Cu(OAc) .H O.
2
2
reduction
O O
S
Ar
O
NR2
Cu(I)
NO2
NO2
1
3
reductive elimination
oxidation reaction
OAr
SO NR2
2
Cu(II)
Cu(II)
O
O
N
N
B
A
O
O
+
-
H
base
Ar
OH
2
Scheme 1. Plausible reaction mechanism.
example, the reacted with unsubstituted phenol 2b to give the desired product 3b in
1% yield (Table 2, entry 2). The steric effects of substituents on the phenyl ring of phe-
9
nols had no some effect on the reaction, for example, meta and ortho-methyl substituted
phenols 2b and 2c were proved to be good substrates for this transformation, affording
the corresponding products 3c, and 3d in excellent yields (Table 2, entries 3 and 4).
Para-methoxyl substituted phenol 3e can afford the desired coupling product 3e in 86%
yield (Table 2, entry 5). Moreover, the two methyl-substituted phenol 3f was proved to
be good substrate and afforded the desired 3f in a very high yield (Table 2, entry 6). It
should be mentioned that halo-substituted phenols have tolerated well to give the corre-
sponding products, for example, para, and ortho-halgon substituted phenols 2g, 2h, and
2
i can offer the corresponding product 3g, 3h, and 3i in 84, 81, and 83% yields, respect-
ively (Table 2, entries 7–9), which could offer platforms for further functionalization as
they contain halogens. Furthermore, the secondary 2-nitro benzenesulfonamide deriva-
tive 1b served as a suitable substrate and provided the corresponding product 3b in
excellent yield (Table 2, entry 10). The 2-nitro benzenesulfonamide derivative 1c bearing
a diethyl amino group can perform the desired product in 91% yield (Table 2, entry
1
1). Curiously, the primary 2-nitro benzenesulfonamide 1d, which is a good substrate
[
32]
in C-S bond-forming transformations,
only gave the desired product 3b in a very
low yield (Table 2, entry 12). Furthermore, a very low yield of product 3j was observed
by TLC when 4-nitro phenol was used as a substrate with 2-nitro benzenesulfonamide
derivative 1a, and no attempt was made to isolate the product from this reaction
(Table 2, entry 13). However, we attempted to use various benzenesulfonamide 1 bear-
ing nitro group at meta- or para position as the substrate with 4-methylphenol (2a),
and only trace amount of the desired product was obtained. Based on these results, we
concluded that the NO groups at ortho position play a coordination directing and
2

6
J. CHEN ET AL.
electron-withdrawing double roles for controlling the regioselectivity and reactivity in
this C–O bond coupling reaction.
We have formulated a possible mechanism based on previously reported mechanism
[
33]
of copper-catalyzed O-arylation reactions (Scheme 1).
Firstly, the Cu(II) salt was
reduced to form Cu(I) species, and the subsequent oxidative addition of 2-nitro benze-
nesulfonamides 1 formed a chelated five-membered reactive species A, in which the
oxygen of the NO group may coordinate to Cu to provide additional stabilization.
2
Secondly, in the presence of a base, the non-nucleophilic phenol 2 could be converted
to intermediate B by nucleophile displacement of a sulfonamide group. Finally, the
reductive elimination of B gave the diaryl ether 3 and regenerated the copper catalyst.
Conclusion
In summary, an efficient, ligand-free copper-catalyzed method for the O-arylation of
arenesulfonamides with phenols was developed. This protocol provided a new avenue
for developing C-O bond-forming reactions and led to access unsymmetrical diaryl
ethers. Notably, this method furnished the direct synthesis of unsymmetrical diaryl
ethers through the regioselective cleavage of C –S bond without the cleavage of C -N
ary
ary
bond. Efforts to extend the applications of arenesulfonamides as a new coupling partner
to other types of trans-metal-catalyzed coupling reactions are currently underway in our
laboratory and will be reported in due course.
Experimental
All reactions were performed under air. Chemicals were purchased from Aldrich, Acros,
or Alfa Asar, and, unless otherwise noted, were used without further purification. Flash
chromatography was performed on silica gel (silica gel, 200–300 mesh). 1H NMR spec-
tra were recorded on Bruker 400 MHz spectrometers with CDCl as the solvent.
3
[
31]
Compounds 1a-c were prepared according to the literature.
General procedure for the synthesis of unsymmetrical diaryl ethers 3
A mixture of 2-nitro benzylsulfonamide 1 (0.25 mmol), phenol 2 (0.30 mmol),
Cu(OAc) .H O (0.025 mmol, 5 mg), Cs CO (0.5 mmol,163 mg), and DMF (0.5 mL)
2
2
2
3
ꢀ
was placed in a 25-mL roundbottom flask. The mixture was stirred at 80 C for 8 h
under air. Then, the reaction mixture was cooled, diluted with ethyl acetate, filtered
through celite, and concentrated in vacuo. The residue was purified by silica gel column
chromatography with ethyl acetate/petroleum ether to afford the desired product 3.
Funding
We are grateful for the financial support from the National Natural Science Foundation of China
[
Grant No. 21762022] and Foundation of Jiangxi Educational Committee [Grant No. GJJ160287].

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