Copper-Mediated Decarboxylative Sulfonylation of Arylacetic Acids with Sodium Sulfinates

Article abstract of DOI:10.1021/acs.orglett.0c02516

Herein, we present a copper-mediated decarboxylative sulfonylation of arylacetic acids with sodium sulfinates that provides viable access to sulfone compounds. This protocol features readily available feedstocks, simple operations, high regioselectivities, and moderate to good yields. The newly obtained products could be converted to other useful compounds. Importantly, the products and their derivatives exhibited potent antitumor activities in vitro, which were tested by MTT assay.

Full text of DOI:10.1021/acs.orglett.0c02516

pubs.acs.org/OrgLett
Letter
Copper-Mediated Decarboxylative Sulfonylation of Arylacetic Acids
with Sodium Sulfinates
Yinrong Wu, Jiewen Chen, Lu Li, Kangmei Wen, Xingang Yao, Jianxin Pang, Ting Wu,*
and Xiaodong Tang*
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ABSTRACT: Herein, we present a copper-mediated decarboxylative
sulfonylation of arylacetic acids with sodium sulfinates that provides viable
access to sulfone compounds. This protocol features readily available
feedstocks, simple operations, high regioselectivities, and moderate to good
yields. The newly obtained products could be converted to other useful
compounds. Importantly, the products and their derivatives exhibited
potent antitumor activities in vitro, which were tested by MTT assay.
ulfones are ubiquitous in nature. They are privileged
functional groups in pharmaceuticals,1 such as tinidazole
(antiparasitic drug), dapsone (treatment of leprosy and skin
Carboxylic acids are attractive raw materials, because they
are readily available, highly stable, and inexpensive, and they
have long been utilized as a handle to construct chemical
bonds.10 In recent years, the transition-metal-catalyzed
decarboxylative couplings of carboxylic acids have achieved
great success, and gradually became alternative approaches for
carbon−carbon and carbon−heteroatom bond formations.11
For example, Gooßen and other groups utilized benzoic acids
to build C(sp2)−C, C(sp2)−N, C(sp2)−O, and C(sp2)−S
bonds via metal-catalyzed oxidative decarboxylative coupling
(ODC) reactions.12 Li, Xu, Lee, Guo and others widely
reported alkynyl and alkenyl carboxylic acids that were used as
alkynyl and alkenyl sources via decarboxylative cross-
couplings.13 Furthermore, some pioneering works of decar-
boxylative functionalization of aliphatic carboxylic acids have
been reported by Li and other groups.14
S
diseases), and apremilast (anti-inflammatory) (see Scheme 1).
Scheme 1. Structures of Representative Sulfone-Based
Drugs
We have also been working on the transformations of
sodium sulfinates.15 Previously, we developed a copper-
catalyzed oxidative coupling of oxime esters with sodium
sulfinates for β-ketosulfones, where the N−O bond was used as
an internal oxidant (Scheme 2a).15a We also reported a one-
pot synthesis of β-ketosulfones from ketones and sodium
sulfinates with DMSO as an oxidant (Scheme 2b).15b Herein,
we present a copper-mediated decarboxylative sulfonylation of
arylacetic acids with sodium sulfinates (Scheme 2c). To the
best of our knowledge, it is the first example of the
construction of the C(sp3)−SO2 bond via decarboxylative
coupling with unactivated alkyl carboxylic acids.
At the same time, they also serve as versatile synthons in
organic transformations involving alkynylation,2 alkenylation,3
Michael addition,4 and cycloaddition.5 Because of their
importance, a large of valuable protocols have been developed
for the synthesis of sulfones.6 Although some progresses have
been made toward heteroatom−SO2 and C(sp2)−SO2 bond
formations in recent years,7 C(sp3)−SO2 bond formations
remained underdeveloped.8 The traditional method for
constructing C(sp3)−SO2 bond was via nucleophilic sub-
stitution of organic halides with sodium sulfinates.9 However,
the preparation and use of the organic halides inevitably cause
environmental pollution. On the other side, some substrates
cannot be transformed into their corresponding halides,
because of the harsh conditions of halogenation. Therefore,
it is very desirable to develop new methods for constructing
the C(sp3)−SO2 bond, which widely exists in drugs and
natural products.
Received: July 28, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02516
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Scheme 2. Our Works Regarding C(sp3)−SO2 Bond
Formation
Table 1. Optimization of the Reaction Conditions
b
entry
[Cu]
solvent
additive
yield (%)
1
2
3
4
5
6
7
8
9
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
CuO
Cu(OTf)2
Cu(OTFA)2
CuBr2
CuI
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
dioxane
toluene
DCE
−
−
−
−
−
−
−
−
54
24
50
90
88
trace
trace
trace
trace
12
60
67
65
50
66
80
24
59
DMF
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
−
−
10
11
12
13
14
15
16
17
18
19
Our investigation of the copper-mediated decarboxylative
sulfonylation began with 2-(1H-indol-3-yl)acetic acid (1a) and
sodium 4-methylbenzenesulfinate (2a) as coupling partners
(see Table 1). To our delight, when we treated 1a with 2.0
equiv of 2a in the presence of 2.0 equiv Cu(OAc)2·H2O, with
dioxane as a solvent under N2 at 115 °C for 6 h, the desired
product 3a was isolated in 54% yield (Table 1, entry 1). Next,
different solvents were screened (Table 1, entries 2−5), and
we found the reaction proceeded better in polar, aprotic
solvents, such as dimethylsulfoxide (DMSO) and dimethylfor-
mamide (DMF). Switching the copper salts to CuO,
Cu(OTf)2, Cu(OTFA)2, CuBr2 or CuI, the reaction almost
could not occur (Table 1, entries 6−10). The 20 mol %
additives, including NaHCO3, K2CO3, NEt3, DBU, and
imidazole, all resulted in reduced reaction performance
(Table 1, entries 11−15). Control experiments showed that
O2 was bad for the reaction (Table 1, entries 16 and 17).
Conducting the reaction at a lower temperature or decreasing
the amount of 2a led to lower product yields (Table 1, entries
18 and 19). Furthermore, we tried to perform the reaction with
catalytic amounts of copper salts and suitable oxidants, but the
yields of the product 3a were sharply decreased (Table 1,
entries 20−24). Hence, the optimal conditions are as shown in
entry 4 by performing the reaction of 1a (0.5 mmol), 2a (1.0
mmol), Cu(OAc)2·H2O (1.0 mmol) in DMF (3 mL) at 115
°C for 6 h under N2.
NaHCO3
K2CO3
NEt3
DBU
imidazole
c
−
−
−
−
−
−
−
−
−
d
e
f
63
28
50
41
38
32
g
20
21
22
23
h
i
j
k
24
a
Unless otherwise stated, the reaction was performed with 1a (0.5
mmol), 2a (1 mmol), [Cu] (1 mmol), additive (20 mol %), in solvent
(3 mL) at 115 °C for 6 h under N2. Abbreviations used: DCE,
dichloroethane; DMF, dimethylformamide; DMSO, dimethyl sulf-
oxide; NEt3, trietylamine; and DBU, 1,8-diazabicycloundec-7-ene.
b
c
d
Isolated yield. The reaction was under air. The reaction was under
e
f
O2. Yield was with respect to the temperature at 100 °C. 0.75 mmol
g
TsNa was used. Using 20 mol % Cu(OAc)2·H2O and O2 as an
h
oxidant. Using 20 mol % Cu(OAc)2·H2O and Ag2CO3 (2 equiv) as
i
an oxidant. Using 20 mol % Cu(OAc)2·H2O and DDQ (2 equiv) as
j
an oxidant. Using 20 mol % Cu(OAc)2·H2O and K2S2O8 (2 equiv) as
k
an oxidant. Using 20 mol % Cu(OAc)2·H2O and PhI(OAc)2 (2
equiv) as an oxidant.
With the optimal reaction conditions in hand, we next
evaluated the scope of arylacetic acids with sodium 4-
methylbenzenesulfinate 2a as the sulfone reagent (Scheme
3). At first, a variety of indoleacetic acids in combination with
2a were tested. Indoleacetic acids bearing different substituents
(2-Me, 4-Cl, 5-Me, 5-OMe, 6-OMe, and 2-Me-5-OMe) gave
corresponding products 3b−3g in good to excellent yields
(63%−81%).16 Other arylacetic acids then were also tested. To
our delight, a range of 2-and 4-nitrophenylacetic acids worked
well under standard conditions. 2-(4-nitrophenyl)acetic acid
and 2-(2-nitrophenyl)acetic acid could be transformed to the
corresponding products 3h and 3i in 69% and 59% yields,
respectively. The halogens (Cl, Br) on the benzene ring of 2-
nitrophenylacetic acids were compatible in the reaction (3j,
3k), illustrating the further modifications of the products were
feasible. In addition, the reaction of 2-(4-nitrophenyl)-
propanoic acid with 2a also produced the desired product 3l
in 63% yield. Unfortunately, no NO2-substituted phenylacetic
acids such as 2-(4-cyanophenyl)acetic acid, 2-(2-
cyanophenyl)acetic acid, 2-(4-(methylsulfonyl)phenyl)acetic
acid, 2-(4-chlorophenyl)acetic acid, and 2-(p-tolyl)acetic acid
did not react under the current reaction conditions.
Subsequently, the scope of the reaction, with respect to
variation of sodium sulfinates, was examined (Scheme 4).
Various alkyl- and halogen-substituted sodium benzenesulfi-
nate smoothly gave products 3m−3p with the 2-(1H-indol-3-
yl)acetic acid. Unfortunately, sodium benzenesulfinate bearing
strong electron-withdrawing substituents such as NO2 and CF3
cannot give the desired products. Notably, sodium alkylsulfi-
nates were readily transformed to the target products in good
yields (3r, 3s, 3t). Furthermore, we also utilized 2-(4-
nitrophenyl)acetic acid as a coupling partner to react with
different sodium sulfinates. A series of substituted sodium
benzenesulfinates and sodium alkylsulfinates were proven to be
compatible in the reaction with 2-(4-nitrophenyl)acetic acid,
and the corresponding products were obtained in moderate to
good yields (3u−3z).
B
https://dx.doi.org/10.1021/acs.orglett.0c02516
Org. Lett. XXXX, XXX, XXX−XXX

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pubs.acs.org/OrgLett
Letter
Scheme 3. Exploration of the Scope of the Reaction
Employing Various Arylacetic Acids
Scheme 5. Control Experiments and Plausible Reaction
Pathway
a
was not detected (Scheme 5, eq 2). The above observations
demonstrated that the reaction possibly did not undergo a
radical pathway. Based on the above findings, a plausible
reaction mechanism was illustrated in Scheme 5. A Cu(II)
carboxylate intermediate A is formed via the reaction of 1a
with Cu(OAc)2·H2O.14b Coordination of sodium sulfinate 2a
to A produces intermediate B with the release of NaOAc.15a
Intermediate B is decarboxylated to give an active copper
specie C. Finally, reductive elimination of intermediate C
generates the desired decarboxylative sulfonylation product
3a.14b,c
a
Reaction conditions: 1 (0.5 mmol), 2a (1 mmol), Cu(OAc)2.H2O
(1 mmol), in DMF (3 mL) at 115 °C for 6 h under N2. Isolated yield.
Scheme 4. Exploration of the Scope of the Reaction
Employing Various Sodium Sulfinates
a
To illustrate the synthetic utility of the newly developed
decarboxylative sulfonylation method, some transformations of
the products were performed (Scheme 6). Benzylation of the
Scheme 6. Derivatization of Products
3a with BnBr and NaH was performed to give product 4 in
91% yield (Scheme 6, eq 1). The nitro group of 3h was easily
reduced to amino group by iron powder (Scheme 6, eq 2).
Meanwhile, 3h can be transformed to sulfonamides, using the
method developed by Wu and coworkers (Scheme 6, eq 3).17
Besides the derivatization of products, the biological
application of the products and their derivatives was also
investigated (see Table 2). The compounds were evaluated for
their in vitro cytotoxicity against the human cancer cell lines
HCT116, A549, and HepG2 by MTT assay, with 5-
fluorouracil (5-Fu) as a reference drug. To our delight, some
compounds exhibited potent inhibitory activities against the
a
Reaction conditions: 1 (0.5 mmol), 2 (1 mmol), Cu(OAc)2.H2O (1
mmol), in DMF (3 mL) at 115 °C for 6 h under N2. Isolated yield.
To probe the mechanism of the reaction, we conducted
several control experiments. Initially, we added 2.0 equiv of
radical scavenger TEMPO (2,2,6,6-tetramethylpiperidine) in
the reaction under standard conditions; product 3a was
delivered in 70% yield (Scheme 5, eq 1). Whereafter, we
researched whether sulfonyl radical produced in the reaction
by adding 2.0 equiv 1,1-diphenylethylene and vinyl sulfone 4
C
https://dx.doi.org/10.1021/acs.orglett.0c02516
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Lu Li − Guangdong Provincial Key Laboratory of New Drug
Screening, School of Pharmaceutical Sciences, Southern Medical
University, Guangzhou 510515, People’s Republic of China
Kangmei Wen − Guangdong Provincial Key Laboratory of New
Drug Screening, School of Pharmaceutical Sciences, Southern
Medical University, Guangzhou 510515, People’s Republic of
China
Xingang Yao − Guangdong Provincial Key Laboratory of New
Drug Screening, School of Pharmaceutical Sciences, Southern
Medical University, Guangzhou 510515, People’s Republic of
China
Table 2. Biological Applications
IC50(μM)
compound
HCT116
HepG2
A549
3j
28.48 2.10
22.30 1.70
33.18 2.70
>100
2.56 0.60
30.86 1.40
6.72 0.50
33.59 9.53
35.30 2.50
34.36 3.90
74.42 4.65
35.21 3.78
>100
99.54 12.56
86.77 3.80
72.94 3.14
>100
3k
3v
3w
4
5
6
>100
>100
84.69 14.57
59.75 1.20
>100
93.37 5.94
56.78 8.34
5-Fu
Jianxin Pang − Guangdong Provincial Key Laboratory of New
Drug Screening, School of Pharmaceutical Sciences, Southern
Medical University, Guangzhou 510515, People’s Republic of
China
human cancer cell lines. Among them, 4 displayed the highest
cytotoxicity against HCT116 cell (IC50 = 2.56 μM).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02516
In conclusion, we have developed a copper-mediated
decarboxylative sulfonylation of arylacetic acids and sodium
sulfinates for the sythesis of sulfone compounds with moderate
to good yields. This work provides a simple strategy for
decarboxylative couplings with unactivated alkyl carboxylic
acids. The transformation has several notable features,
including available and stable reagents, simple operations,
and good regioselectivities. Above all, we found the products
and their derivatives possessing potent antitumor activities in
vitro via MTT assay. Further efforts to expand unactivated
alkyl carboxylic acids as decarboxylative coupling partners are
now underway in our laboratory.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 21702096), Natural Science Foundation of Guangdong
Province (No. 2017A030310546), and the High-Level Talent
Introduction Foundation of Southern Medical University (No.
C1033520) for financial support.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02516.
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General information; experimental section for products
3; procedure for copper-mediated decarboxylative
sulfonylation of arylacetic acids with sodium sulfinates
in 1 mmol scale; cell culture and evaluation of the
antiproliferative activity; characterization data for all of
the products; associate references; NMR spectra for all
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Authors
Yinrong Wu − Guangdong Provincial Key Laboratory of New
Drug Screening, School of Pharmaceutical Sciences, Southern
Medical University, Guangzhou 510515, People’s Republic of
China
Jiewen Chen − Guangdong Provincial Key Laboratory of New
Drug Screening, School of Pharmaceutical Sciences, Southern
Medical University, Guangzhou 510515, People’s Republic of
China
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(16) The electron-withdrawing groups usually favor the metal-
catalyzed decarboxylation, but the electron-rich indole rings were also
compatible in our reaction. We thought the reason may be the
carbanions that were produced from indoleacetic acids via
decarboxylation were stable, because the carbanions could isomerize
to the α-site of the N atom in the indole rings.
(17) Chen, K.; Chen, W.; Han, B.; Chen, W.; Liu, M.; Wu, H. Org.
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E
https://dx.doi.org/10.1021/acs.orglett.0c02516
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