Tf2O-Promoted Trifluoromethythiolation of Various Arenes Using NaSO2CF3

Article abstract of DOI:10.1002/adsc.201800702

A sulfonic anhydride-promoted direct trifluoromethylthiolation using NaSO₂CF₃ has been developed. The simple and practical strategy enabled the direct introduction of SCF₃ moiety into unexplored arene scaffolds under reductant- and metal-free condition at room temperature. (Figure presented.).

Full text of DOI:10.1002/adsc.201800702

Title: Tf2O-Promoted Trifluoromethythiolation of Various Arenes Using
NaSO2CF3
Authors: Jie Liu, Xiaochun Zhao, Lvqi Jiang, and Wen-bin Yi
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800702
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10.1002/adsc.201800702
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Tf2O-Promoted Trifluoromethythiolation of Various
Arenes Using NaSO2CF3
Jie Liu, Xiaochun Zhao, Lvqi Jiang, Wenbin Yi*
a
Chemical Enginering college, Nanjing University of Science and Technology, 200 Xiao Ling Wei Street, Nanjing,
Jiangsu, People’s Republic of China.
E-mail :yiwenbin@njust.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract:
A
sulfonic anhydride-promoted direct
trifluoromethylthiolation using NaSO2CF3 has been
developed. The simple and practical strategy enabled the
direct introduction of SCF3 moiety into unexplored arene
scaffolds under reductant- and metal-free condition at
room temperature.
Keywords:
trifluoromethylthiolation,
sodium
trifluoromethanesulfinate,
trifluoromethanesulfonic
anhydride, arenes
Figure 1. Biologically active compounds
containing an arene-SCF3 group
The growing importance of fluorinated organic
compounds in pharmaceuticals, agrochemicals and
materials has driven the development of new methods
for the introduction of fluorine into small molecules.[1]
In the organofluorine family, the trifluoromethylthio
group (SCF3) has attracted special interests in both
academia and the pharmaceutical industry due to its
high lipophilicity and high electron-withdrawing
character.[2] Consequently, compounds bearing this
functional group, especially aromatic derivatives, are
potentially in the pharmaceutical and agrochemical
fields (Figure 1).[3]
negative aspects, it is still necessary to develop an
efficient method to introduce the SCF3 moiety
directly.
The development of general and practical
methods for the trifluoromethylthiolation of
unactivated arenes is highly desirable. In moving
towards this goal, Our group,[10] Yang/Vicic,[11]
Shibata/Cahard,[12] Lin/Liu[13] and Zhao/Lu[14]
disclosed CF3SO2Na and CF3SO2Cl which could
conceivably generate CF3S+ for the C(sp2)-H
direct trifluoromethylthiolation in the presence of
phosphorus reductant. However, the substrate
scope is extremely limited to electron-rich
heterocycles such as indoles, pyrroles and some
activated arenes. Morever, requirement of
sensitive phosphorus reductant decreases the
overall synthetic efficiency to some extent.
In the last few decades, numerous methods for the
introduction of SCF3 group into organic compounds
have been developed. The most efficient and elegant
strategies are direct methods. Nucleophilic and
radical sources of the SCF3 group are often copper- or
silver-based metallic reagents,[4] or [Me4N][SCF3].[5]
In particular, the popularity of electrophilic
trifluoromethylthiolation has grown remarkably with
As shown in Figure 1, Ar-SCF3 components
occurred widely in a number of pesticides and drugs.
shelf-stable
CF3SN-[6]
or
CF3SO-based[7]
In
the
quest
of
direct
aromatic
trifluoromethylthiolating reagents in recent years. In
addition, trifluoromethylthiolation via C-H activation
was explored under the help of directing-groups such
as 8-aminoquinoline[8] and pyridine.[9] However,
synthesis of these trifluoromethylthiolating reagents
also presents a great challenge: the multiple steps
needed prior to their use and the possible necessity of
expensive reagents. Because of these limitations and
trifluoromethylthiolation, we envisioned that
CF3SO2Na could be used as trifluoromethylating
sources which applied to more general arenes. In
2006, Magnier and co-workers reported a novel
preparation of various aryl trifluoromethyl sulfoxides
through aromatic compound mixed with potassium
trifluoromethanesulfinate
CF3SO2K
and
1
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10.1002/adsc.201800702
Advanced Synthesis & Catalysis
trifluomethanesulfonic acid TfOH.[15a] Then in 2009,
additives including Ac2O, TFAA, (CH3SO2)O did not
give any desired products
the same group found
a
small amount of
trifluoromethylthiolation traces in the process of
preparing Yagupol’skii-Umemoto reagent with
CF3SO2K and trifluoromethanesulfonic anhydride
Tf2O.[15b] Moreover, CF3SO2Na was also reported to
Table 2. Scope of Trifluoromethylthiolation of arene
derivatives[a]
prepare
trifluoromethyl
sulfoxide[16]
and
Yagupol’skii-Umemoto reagent[17] in the presence of
TfOH or Tf2O. However, the method for preparing
sulfonium salts required benzene or biphenyl
derivatives with special directing substituents as
substrates. Therefore, we considered that there should
be an efficient and direct approach for the preparation
of aromatic trifluoromethyl sulfides under the
conditions of CF3SO2Na/Tf2O.
Table 1. Optimization of trifluoromethylthiolation with
a
NaSO2CF3
Entry
1
x
1
1
1
2
3
2
2
2
2
2
2
2
2
y
1
2
3
2
2
2
2
2
2
2
2
2
2
Solvent
DCM
additive
Tf2O
Yield(%)b
30
2
DCM
Tf2O
48
3
DCM
Tf2O
48
4
DCM
Tf2O
80
5
DCM
Tf2O
78
6
MeCN
1,4-dioxane
DMF
Tf2O
＜5
19
7
Tf2O
8
Tf2O
n.r
9
DMSO
DCM
Tf2O
n.r
10
11
12
13
Ac2O
TFAA
(CH3SO2)O
Tf2O
n.r
DCM
n.r
DCM
n.r
71c
DCM
[a] Reaction conditions: 1a (0.2 mmol, 1.0 equiv), solvent
(1.0 mL) at room temperature under an atmosphere of
argon for 24h.
[a]
Reaction conditions: 1 (0.5 mmol), NaSO2CF3 (1.0
mmol), and Tf2O (1.0 mmol) in DCM at room temperature
under an atmosphere of argon for 24h.
[b]
Yield was determined by 19F NMR, hexafluorobenzene
[b] Yield was determined by 19F NMR, hexafluorobenzene
used as internal standard.
used as internal standard.
[c] Yield was obtained at gram scale (2g).
We chose cyclohexylbenzene (1a) as a model
substrate to search for the optimal reaction conditions.
A reaction of 1.0 equiv of NaSO2CF3 in the presence
of 1.0 equiv of Tf2O in dichloromethane without any
catalyst delivered trifluoromethylthiolation product
2a in 30% yield at room temperature for 24 h (Table
1, entry 1), which was similar to the result of
Magnier’s work.[15b] It was found that when
increasing the amount of trifluoromethanesulfonic
anhydride and sodium trifluoromethanesulfinate to
2.0 equiv respectively, the yield significantly
increased to 80% (entries 2-5). Further studies
indicated that DCM was the best choice of solvents,
consequently providing a homogeneous reaction
mixture (entries 6-9). Finally, switching Tf2O to other
(entries 10-12). The method is effective on the gram
scale, which highlights the inherent practicality of
this synthetic transformation (entry 13).
Having established the optimized reaction
conditions, we examined the scope of substrates with
structurally diverse arenes (Table 2). A broad range
of functional groups, such as alkyl, alkoxy, halides,
ester, hydroxyl and amino were tolerated under the
standard reaction conditions. Alkoxy (2b), phenoxy
(2c), methyl (2h), bromo (2i) and polysubstituted
benzenes (2d-g) provided the expected products in
moderate to good yields. Trifluoromethylthiolation
occurred at the position para to the substituent due to
the electronic effects. Additionally, potentially
bioactive compounds[18] with acetoxy (2j) and
acetamido (2k) were obtained in moderate yields.
2
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10.1002/adsc.201800702
Advanced Synthesis & Catalysis
The acetanilide led to the selective formation of the
product 2l (46%), thus confirming that under
superacid conditions the amide function is protonated
and that the methyl group directs the electrophilic
substitution.[18] Notably, reactions of electron-rich
benzenes afforded the products with two SCF3
moieties in good yields at room temperature for a
short time (2m-p) as main products. This
demonstrated the potentiality of this strategy for
multiple trifluoromethylthiolation of aromatic
derivatives. Unfortunately, arenes bearing electron-
withdrawing groups such as nitro and cyano groups
only gave trace of SCF3 products.
In
a
context
where
direct
late-stage
functionalization of bioactive molecules is being fully
embraced as an innovative approach in medicinal
chemistry, we decided to investigate the possibility of
late-stage trifluoromethylthiolation using our method
(Scheme 1).[18] The direct trifluoromethythiolation of
xanthotoxin, which was the intermediate of
“Methoxsalen”[19], was selectively achieved (2aj). To
evaluate the ability to extend our strategy to aromatic
steroids, our method was therefore extended to the
protected 17β-estradiol to generate its SCF3 analogue
(2ak) in 40% yield. The selectivity of the substrate
was unambiguously confirmed by single crystal X-
ray analysis.
Other than single benzene structure, biphenyls (2q-
u)
proceeded
smoothly
to
give
trifluoromethylthiolation products in moderate to
good yields. Since the steric hindrance and the
electronic effects, 4-SCF3-substituted products were
formed. It was worth noting that sulfonium salts were
also observed in <10% yield determined by 19F NMR
as side products. Different from Magnier’s
observasion,[15b] the biphenyls selected in this work
mainly led to SCF3 products, the formation of
sulfonium salts depend on the directing influence of
the substituents located on the phenyl groups. The
standard conditions were compatible with the active
α-position of the naphthalenes bearing methyl (2aa),
bromo (2ab), methoxy (2ac), hydroxyl (2ad), and
delivered the products in good yields. Importantly,
multi-ring aromatic compounds including 9-
methylanthracene and 9-bromophenanthrene were
good substrates to give corresponding products 2ae
and 2af in 69% and 62% yields respectively.
Fluorene, N-methylcarbazole and 9,9’-spirobi[9H-
fluorene] were also successfully subjected to this
simple protocol to access their trifluoromethylated
derivatives with 48%-75% yields (2ag-2aj).
Scheme 2. Reaction of cyclohexylbenzene CF3-
sulfoxide with triflic anhydride.
To gain some insights into the formation of the
SCF3 products, we turned our attention to the
transformation of Ar-SOCF3 to Ar-SCF3. Aryl
trifluoromethyl sulfoxide (3a)[15a] was prepared for
the reaction with TfOH or NaSO2CF3. It was found
that trifluoromethyl sulfoxide 3a was converted to the
target product and side-product sulfinate at a ratio of
1 : 1 (Scheme 2). This indicated that Tf2O could
promote disproportionation of trifluoromethyl
sulfoxide to the SCF3 product.
Scheme 3. Proposed reaction mechanism
Although the details of the reaction process are not
clear, the mechanism was hypothesized as shown in
Scheme 3 on the basis of the control experimental
results and literature information.
A
close
examination of the literature shows that the
cornerstone of the previously described synthesis is
the formation of an intermediate sulfonate sulfinate
anhydride 5.[15a] Two potential paths may achieve
final result. As shown in path I, trifluoromethyl
sulfoxide 3, which was detected by GC-MS, partly
contributed to the formation of SCF3 product via
disproportionation. From path I, the sulfoxide 3 can
disproportionate itself under the anhydride system to
affort the product 2. Therefore, we suspect that the
Scheme 1. TfO2-promoted trifluoromethylthiolation of
drug-like molecules and X-ray structure of 2ak
3
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10.1002/adsc.201800702
Advanced Synthesis & Catalysis
intermediate 5 also undergoes self-disproportionation.
Half of intermediate 5 was oxidized to form TfO-
SO2CF3, and another part was reduced to form the
true reactive species, the intermediate 6. To gain
more insights into the intermediates 5 and 6, we
performed a control experiment only in the presence
of NaSO2CF3 and Tf2O. 19F analysis of the reaction
mixture after 1 h gave four new signals (δ= -37.00, -
77.67, -79.51 and -80.23ppm). Based on their peak
integrations and comparison of the 19F NMR of
reported similar compounds,[17c, 20] the four signals
were suggested to be the intermediates 5 (δ= -77.67
and -80.23ppm) and 6 (δ= -37.00 and -79.51 ppm).
The two intermediates were also confirmed by MS
Synthesis of 1ak
To a ice cold solution of 17β-estradiol (3.0g,
11.1mmol) in pyridine (15 mL) added acetic anhydride
(7.36 g, 72.1mmol) and stirred at room temperature 16
h. Reaction mixture poured on ice water mixture (300
mL) and resulting solid filtered, washed with water
and dried to give pure product (1ak) (3.8g, 98%).
CCDC 1856060 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge
Crystallographic
www.ccdc.cam.ac.uk/data_request/cif
Data
Centre
via
study.[21]
Finally,
the
transfer
trifluoromethylthiolation from 6 to the nucleophile
via an electrophilic path which mainly contributed to
the formation of product 2.
Acknowledgements
In summary, we have developed a direct Tf2O-
We gratefully acknowledge the National Natural
Science Foundation of China (21776138, 21476116),
Fundamental Research Funds for the Central
Universities (30916011102, 30918011314), Qing Lan
and Six Talent Peaks in Jiangsu Province, and
Priority Academic Program Development of Jiangsu
Higher Education Institutions.
promoted
trifluoromethylthiolation
using
CF3SO2Na. This system has been successfully
applied to a wide range of benzenes by a one-pot
strategy for conventional laboratory applications.
This use-friendly reagent NaSO2CF3 has a great
potential for the preparation of more complicated,
densely functionalized drug molecules with SCF3
moiety.
Notes and references
Experimental Section
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General Procedure for the Formation of
Product 2a-2aj
A 25mL oven-dried reaction vessel was charged with
aromatic
compounds
(0.5mmol),
sodium
trifluoromethanesulfinate (156.0 mg, 1.0 mmol),
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mmol)
under
an
atmosphere
of
argon.
Dichloromethane (1.0 mL) was added to the sealed
reaction vessel by syringe. The mixture was stirred
vigorously at room temperature for 24 h. After
consumption of the starting material, it was quenched
with brine. The aqueous layer was extracted with
dichloromethane (3×50 mL), then organic layers was
neutralized with NaHCO3 and dried with Na2SO4. The
residue was purified by column chromatography to
give the corresponding SCF3 substituted products.
Synthesis of 3a
Under argon, a 25mL oven-dried reaction vessel was
charged with sodium trifluoromethanesulfinate (1.2 g, 7.6
mmol) and trifluoromethanesulfonic acid (4.10 mL, 0.046
mol, 6 equiv). Then, cyclohexylbenzene (1.94 mL, 11.5
mmol, 1.5 equiv) was added. Dichloromethane (8 mL)
was added to the sealed reaction vessel by syringe.
After 24 h stirring at room temperature, the resulting
mixture was hydrolyzed with ice-water, neutralized with
NaHCO3, extracted with dichloromethane (3 x 50 mL).
The organic phases were mixed, dried over Na2SO4 and
concentrated under reduced pressure. Distillation of the
crude mixture afforded 1.59 g (50%) of the desired
compound.
4
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10.1002/adsc.201800702
Advanced Synthesis & Catalysis
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intermediates 5 and 6, see supporting information.
5
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10.1002/adsc.201800702
Advanced Synthesis & Catalysis
UPDATE
Title Text
Adv. Synth. Catal. Year, Volume, Page – Page
Jie Liu, Xiaochun Zhao, Lvqi Jiang, Wenbin Yi*
6
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