Transition-Metal-Free Synthesis of Aryl Trifluoromethyl Thioethers through Indirect Trifluoromethylthiolation of Sodium Arylsulfinate with TMSCF3

Article abstract of DOI:10.1021/acs.orglett.1c02656

Herein, we report an indirect trifluoromethylthiolation of sodium arylsulfinates. This transition-metal-free reaction significantly provides an environmentally friendly and practical synthetic method for aryl trifluoromethyl thioethers using commercial Ruppert-Prakash reagent TMSCF3. This approach is also a potential alternative to the current industrial production method owing to facile substrates, excellent functional group compatibility, and operational simplicity.

Full text of DOI:10.1021/acs.orglett.1c02656

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Letter
Transition-Metal-Free Synthesis of Aryl Trifluoromethyl Thioethers
through Indirect Trifluoromethylthiolation of Sodium Arylsulfinate
with TMSCF3
Changge Zheng,* Chao Jiang, Shuai Huang, Kui Zhao, Yingying Fu, Mingyu Ma, and Jianquan Hong*
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ABSTRACT: Herein, we report an indirect trifluoromethylthiolation
of sodium arylsulfinates. This transition-metal-free reaction signifi-
cantly provides an environmentally friendly and practical synthetic
method for aryl trifluoromethyl thioethers using commercial
Ruppert−Prakash reagent TMSCF3. This approach is also a potential
alternative to the current industrial production method owing to
facile substrates, excellent functional group compatibility, and operational simplicity.
luorinated organic compounds have attracted significant
interest in the fields of pharmaceutical,1 agrochemical,2
drastically improve the reaction efficiency and operational
safety through transition-metal-catalyzed/mediated reaction
utilizing palladium,12,13 nickel,14 copper,15 and silver16 with
MSCF3 reagents. Despite SCF3 sources being expensive,
environmentally unfriendly, and instable, the synthetic strategy
has aroused interest through direct introduction of the -SCF3
group into the aromatic ring.17
Trifluoromethyltrimethylsilane (TMSCF3), Ruppert−Pra-
kash reagent, is most widely used as a trifluoromethyl
nucleophile due to its commercial availability, bench stability,
and operational convenience. The combination of TMSCF3
with sulfur compounds, such as S8,18 CuSCN/NaSCN,19
Na2S2O3,20 and DTSA,21 has been also employed to introduce
the -SCF3 group into aromatic substrates. The reactions
proceeded smoothly to generate the desired products with
broad functional group tolerance, but these methods were still
restricted by the starting substrates, transition-metal-catalyzed
reagent, and/or toxic byproducts.
F
and material sciences.3 As a consequence, the synthesis of the
fluorinated compounds by different strategies has become a
hot spot in modern organic chemistry.4 Due to the improved
lipophilicity and suppressed metabolic detoxification,5 the
trifluoromethylthio group (SCF3) is highly valuable for its
advantageous effects on the in vivo lifetime of a drug upon
incorporation of SCF3 into organic molecules. As early as 1960,
the aryl trifluoromethyl thioethers (ArSCF3) had been
obtained through the hazardous chlorination of ArSCH3 and
the following Cl−F exchange of ArSCCl3 with SbF3.6
Generally, there are two synthetic strategies for aryl
trifluoromethyl thioethers,7 direct and indirect insertions of
the -SCF3 group into organic molecules (Scheme 1). Although
Scheme 1. Synthetic Strategies of Aryl Trifluoromethyl
Thioethers
Inspired by the recent advance in the synthesis of thioether
from sodium arylsulfinate,22 aryl disulfides,23 and ethyl
arylsulfinates,24 we have been trying to develop a green,
economic, and efficient approach for the synthesis of aryl
trifluoromethyl thioethers. Herein, the transition-metal-free
reaction has been carried out using Ruppert−Prakash reagent
and stable, environmentally friendly sodium arylsulfinate as
starting substrates,11 with high efficiency, excellent functional
group compatibility, and operational simplicity.
ArSCF3 can be obtained through the reaction between CF3
sources and aryl substrates such as thiols,8 thiocyanates,9
disulfides,10 and thiosulfinates,11 the synthetic strategies have
several disadvantages such as the foul-smelling odors and air
sensitivity of thiols, the stoichiometric toxic byproducts from
thiocyanates, and the atom economy of disulfides and
thiosulfonates.
Received: August 10, 2021
In 2011, direct trifluoromethylthiolation of the sulfur-free
substrates was reported by Buchwald and co-workers12 to
https://doi.org/10.1021/acs.orglett.1c02656
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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We initiated our investigation using sodium 4-biphenylsulfi-
nate 1a as a model substrate, TMSCF3 as the fluorinated
reagent, and triphenylphosphine as the additive under a
nitrogen atmosphere (Table 1). No desired product was found
corresponding desired products in moderate to good yields
(Scheme 2). With the electron-donating substituents on the
Scheme 2. Transition-Metal-Free Indirect
Trifluoromethylthiolation of Monosubstituted Sodium
Arylsulfinate with TMSCF3
a
a
Table 1. Optimization of the Reaction Conditions
b
I2
PPh3
temp time yield
entry (equiv) (equiv)
additive
solvent
(°C)
(h)
(%)
1
2
3
4
5
6
7
8
−
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
−
−
−
KF
KF
DMSO
DMSO
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
25
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
18
12
0
8
11
31
39
30
36
27
11
68
0
83
30
35
20
49
19
0
K3PO4
CS2CO3
Na2CO3
K2CO3
NaF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
9
10
11
12
13
14
15
16
17
18
19
20
21
DMF
DMF
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
DMSO
DMAc
NMP
CH3CN
toluene
H2O
DMF
DMF
DMF
21
81
59
100
100
c
a
Reaction yields were determined by 19F NMR spectroscopy using
a
Reaction conditions: 1a (0.10 mmol), TMSCF3 (4.0 equiv), I2,
4,4′-difluorobiphenyl as the internal standard. Values in parentheses
PPh3, additive (4.0 equiv), 4,4′-difluorobiphenyl (0.1 mmol, internal
are isolated yields using column chromatography.
b
standard), solvent (1.5 mL). Yields determined by 19F NMR
c
spectroscopy based on 1a. Under air conditions.
aromatic ring such as aryl, alkyl, alkoxy, and methylthio, the
sodium arylsulfinate salts were well tolerated under the
reaction conditions to give the corresponding target products
with moderate to good yields of 43−83% (2a−2j). With a
halogen substituent on the aromatic ring such as Cl and Br, the
sodium arylsulfinate substrates can tolerate the reaction
system, providing the corresponding products in moderate
yields (2k and 2l in 34% and 44% yields, respectively).
Remarkably, the iodo group in substrate 1m can significantly
survive the standard reaction conditions, affording the desired
product 2m in a moderate yield of 52%. Compared with the
substrates with electron-donating substituents on the aromatic
ring, the sodium arylsulfinates with electron-withdrawing
substituents such as alkoxalyl, acetyl, cyano, and nitro (2n−
2v) afforded the desired products in slightly lower yields of
47−67%. In comparison with the nitro substituent in the para
(2t, 58%) and meta (2u, 52%) position, the slightly lower yield
from sodium arylsulfinate with the nitro substituent in the
ortho position (2v, 47%) indicated that steric effects have an
important influence on the reaction system.
in DMSO at 100 °C in the absence of the oxidant iodine or an
additive (Table 1, entry 1). To our delight, the desired product
(2a) was favorably detected in 8% yield in the presence of I2 at
100 °C, determined by 19F NMR spectroscopy with 4,4′-
difluorobiphenyl as the internal standard (entry 2). The
addition of KF as an additive was beneficial for improving the
reactivity, showing a slightly higher yield (entry 3). Compared
with additives KF, K3PO4, CS2CO3, Na2CO3, K2CO3, and NaF
(11−39%, entries 4−9, respectively), CsF is the best choice for
the reaction, providing the desired product 2a in the highest
yield (68%, entry 10). The investigation of oxidant I2 and PPh3
revealed that the optimum molar ratio of I2 to PPh3 is 1 to 1.2
for the reaction in 83% yield (entry 12). Among the polar or
less polar solvents (DMSO, DMAc, NMP, CH3CN, toluene,
and DMF, entries 12−17, respectively), the DMF solvent is the
best reaction conditions to form the product in 12 h.
Unfortunately, the desired product could not be obtained
using H2O as the solvent (entry 18). Although the reaction can
also be performed at room temperature, a higher temperature
of ≤100 °C is necessary for a high yield (Table 1, entry 19).
The reaction yield decreases dramatically in the presence of air
(entry 21).
Similarly, the polysubstituted sodium arylsulfinates with
more electron-donating substituents were well tolerated under
the standard reaction conditions to give the desired products in
higher yields than the corresponding monosubstituted sodium
arylsulfinates (Scheme 3a). With two more methyl groups in
the meta position on the aromatic ring, sodium 2,4,6-
With the optimal reaction conditions determined, the
substrate scope of sodium arylsulfinate was explored. All
monosubstituted sodium arylsulfinate substrates afforded the
B
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Inspired by its applicability to various substrates, we further
applied the synthetic method to more complex compounds,
considering the potential pharmacological activity of these
molecules after the insertion of the SCF3 group. It was found
that the synthetic method could be applied conveniently to the
natural product and the drug to provide the corresponding
trifluoromethylthiolated compounds (Scheme 3b). Estrone, a
natural estrogenic hormone, could be easily transformed into
the corresponding sulfinate to afford trifluoromethylthiolated
estrone 4h in 62% isolated yield, followed by the indirect
trifluoromethylthiolation method. Trifluoromethylthio-modi-
fied sildenafil 4i could be derived efficiently from benzene-
sulfonyl chloride, a pharmaceutical intermediate, through a
similar transformation in 75% yield. The efficiency of this
indirect trifluoromethylthiolation reaction was further demon-
strated by running the transformation on a laboratory scale.
For example, the reaction of 1a with TMSCF3 was carried out
on a gram scale (7.2 mmol), and the transformation proceeded
successfully to give the corresponding product 2a (1.196 g,
65%) in good isolated yield (see the Supporting Information).
To gain insights into the reaction mechanism and to
understand the role of each component in the transformation,
the control experiments were performed using sodium 4-
biphenylsulfinate 1a as a model substrate (see the Supporting
Information). In the absence of the oxidant iodine, the reaction
of 1a with TMSCF3 cannot provide the target compound in
the reaction system of PPh3, CsF, and DMSO at 100 °C. In
fact, sodium 4-biphenylsulfinate 1a can transform into
diphenyl disulfide intermediate I in DMSO at 100 °C in
81% isolated yield in the presence of reductant PPh3. Indeed,
4,4′-dichlorodiphenyl disulfide has been isolated and identified
as the main byproduct in the synthesis of 4-chlorophenyl
trifluoromethyl thioether (see the Supporting Information).
With oxidant I2 and additive CsF, the reaction of diphenyl
disulfide I with TMSCF3 can give aryl trifluoromethyl
thioether 2a in DMSO at 100 °C in 75% yield, determined
by 19F NMR spectroscopy.
Scheme 3. Transition-Metal-Free Indirect
Trifluoromethylthiolation of Di- or Trisubstituted Sodium
Arylsulfinates and Other Sodium Sulfinates with TMSCF3
a
a
Reaction yields were determined by 19F NMR spectroscopy using
4,4′-difluorobiphenyl as the internal standard. Values in parentheses
are isolated yields using column chromatography.
On the basis of the results presented above and previous
relevant mechanistic studies,25 a plausible reaction mechanism
is proposed as shown in Scheme 4. Initially, the reduction of
trimethylbenzenesulfinate 1′a afforded 3a in a yield higher
than that of sodium 4-methylbenzenesulfinate 1c (3a to 2c,
74% to 54%). Nevertheless, the polysubstituted sodium
arylsulfinates with both electron-donating and electron-with-
drawing substituents can tolerate the standard conditions,
providing the target product in slightly higher or lower yield
(3f, 57%) than the corresponding electron-withdrawing or
electron-donating monosubstituted sodium arylsulfinate sub-
strate (2f or 2v in 74% or 47% yield, respectively). Overall, the
electron-rich substrates always show relatively better reactivity
than the electron deficient substrates.
Scheme 4. A Plausible Mechanism for Transition-Metal-
Free Indirect Trifluoromethylthiolation
On the basis of the results presented above, the good
substrate scope and functional group compatibility of this
reaction, we next turned our attention to the polycyclic and
heterocyclic sodium arylsulfinates bearing naphthaline, pyr-
idine, and thiophene moieties. As shown in Scheme 3b, the
transformation proceeded smoothly under the optimal
conditions to give the corresponding target products in
moderate to good yields in a range of 31−71% (4a−4f).
Unfortunately, sodium acylsulfonate substrate 1″g was not
suitable under this reaction condition. Obviously, the
conjugated structure of the substrates is the most important
requirement for the reaction process.
sodium arylsulfinate 1 by triphenylphosphine affords diphenyl
disulfide A. Then, oxidation of A by iodine generates the active
species ArSI B. Finally, attacked by CF3 generated from the
reaction between TMSCF3 and CsF, ArSI B was transformed
into the target product aryl trifluoromethyl thioethers 2.
In summary, we have developed a facile synthetic method
for the aryl trifluoromethyl thioethers employing sodium
arylsulfinate substrate and TMSCF3. Through environmentally
−
C
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friendly transition-metal-free reaction, the stable sodium
arylsulfinates have been transformed efficiently into aryl
trifluoromethyl thioethers via consecutive processes of the
reduction of arylsulfinate by PPh3, the subsequent oxidation by
I2, and then the construction of S−CF3 bonds with TMSCF3.
This approach represents a new and efficient strategy for the
synthesis of aryl trifluoromethyl thioethers with potential
application in the fields of medicine, pesticides, and materials.
(21971089, 21562041, and 21502070) and the Fundamental
Research Funds for the Central Universities.
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02656.
Experimental procedures and spectroscopic character-
1
ization data and H and 13C NMR spectra of the new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Changge Zheng − Key Laboratory of Synthetic and Biological
Colloids, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu
214122, P. R. China; School of Chemical Engineering,
Xinjiang Agricultural University, Urumqi 830052 Xinjiang
Uygur Autonomous Region, P. R. China; orcid.org/0000-
0001-7047-1594; Email: cgzheng@jiangnan.edu.cn
Jianquan Hong − Key Laboratory of Synthetic and Biological
Colloids, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu
214122, P. R. China; Email: hongjq@jiangnan.edu.cn
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Authors
Chao Jiang − Key Laboratory of Synthetic and Biological
Colloids, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu
214122, P. R. China
Shuai Huang − Key Laboratory of Synthetic and Biological
Colloids, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu
214122, P. R. China
Kui Zhao − Key Laboratory of Synthetic and Biological
Colloids, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu
214122, P. R. China
Yingying Fu − Key Laboratory of Synthetic and Biological
Colloids, Ministry of Education, School of Chemical and
Material Engineering, Jiangnan University, Wuxi, Jiangsu
214122, P. R. China
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ACKNOWLEDGMENTS
The authors acknowledge the financial support for this work
from the National Natural Science Foundation of China
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