A Diheteroatom Fluoroalkylation Reagent for Preparation of S- and N-Containing Fluoroalkyl Compounds and Sulfonic Acid Polymer

Article abstract of DOI:10.1021/acs.orglett.7b00347

The first stable diheteroatom fluoroalkylation reagent, 2-((2-azido-1-chloro-1,2,2-trifluoroethyl)thio)pyrimidine (ACTP), has been prepared by a novel method. By using this reagent, various fluorinated thioethers and sulfones have been successfully prepared. The dearylation and dearylation-oxidation of fluoroalkyl 2-pyrimidyl sulfone in one-pot reaction were investigated systematically, and the results demonstrated that both fluoroalkyl sulfinates and sulfonates could be obtained in high yields. In addition, ACTP proved to be useful for the preparation of a fluorinated sulfonic acid proton-exchange membrane.

Full text of DOI:10.1021/acs.orglett.7b00347

Letter
pubs.acs.org/OrgLett
A Diheteroatom Fluoroalkylation Reagent for Preparation of S- and
N‑Containing Fluoroalkyl Compounds and Sulfonic Acid Polymer
Jingwen Dai,† Zili Li,† Taisheng Wang,† Wei Bai,*,‡ and Ruke Bai*,†
†CAS Key Laboratory of Soft Matter Chemistry and Department of Polymer Science and Engineering, University of Science and
Technology of China, Jinzhai Road 96, Hefei, Anhui Province, People’s Republic of China
‡Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
S
* Supporting Information
ABSTRACT: The first stable diheteroatom fluoroalkylation
reagent, 2-((2-azido-1-chloro-1,2,2-trifluoroethyl)thio)-
pyrimidine (ACTP), has been prepared by a novel method. By
using this reagent, various fluorinated thioethers and sulfones
have been successfully prepared. The dearylation and dearyla-
tion−oxidation of fluoroalkyl 2-pyrimidyl sulfone in one-pot
reaction were investigated systematically, and the results
demonstrated that both fluoroalkyl sulfinates and sulfonates
could be obtained in high yields. In addition, ACTP proved to be
useful for the preparation of a fluorinated sulfonic acid proton-
exchange membrane.
n recent years, methods for preparation of fluoroalkyl thio-
and efficient pathway for large-scale preparation of difluoro-
Icompounds have been of growing interest because the methanesulfinate salts via NaBH4-mediated reduction of benzo-
[d]thiazol-2-yl sulfones.13
simultaneous incorporation of both sulfur and fluorine into the
To date, investigation of fluoroalkyl thio-compounds including
thioethers, sulfones, andsulfonatesisstilllimitedduetothelackof
practical synthetic methods. Therefore, it is of high demand to
develop facile, efficient methods for the synthesis of fluoroalkyl
thio-compounds as the possibility to modulate the intrinsic
properties of molecules, which can provide powerful tools for the
discovery of new bioactive molecules. Recently, we have prepared
a stable and versatile heterobifunctional fluoroalkylation reagent
that was successfully used in the synthesis of the different
fluorinated compounds with an asymmetric molecular struc-
ture.14 Inspired by the achievement of the previous work, we tried
to develop a versatile method for preparation of new fluoroalkyl
thio-compounds. Meanwhile, we noticed that, to date, there is no
report published on a fluoroalkylation reagent containing both S
and N atoms, which can be used to simultaneously incorporate
both sulfur and nitrogen atoms into one fluoroalkyl molecule.15
Herein, we report a fluoroalkylation reagent bearing both azide
and thiother groups, and its application in the synthesis of various
fluorinated thioethers, sulfones, and sulfonates.
target molecules can give rise to attractive new properties, leading
to a wide range of applications in many areas.1 An interesting
exampleisfromtheareaofmedicalchemistry:theCF3SorHCF2S
moiety as an intriguing structural motif has been used to improve
the membrane permeability of drug candidates owing to the high
stability and lipophilicity.2 Consequently, a number of methods
have been developed for the preparation of the fluoroalkyl thio-
compounds. Among these methods, thiofluoroalkylation re-
agents play a very important role. By employing these reagents,
both S and F can be incorporated into molecules directly and
efficiently.3 Recently, several thiofluoroalkylation reagents have
been reported, such as the trifluoromethanesulfenamide,4 N-
(trifluoromethylthio)phthalimide,5 trifluoromethanesulfonyl hy-
pervalent iodoniumylide,6 N-difluoromethylthiophthalimide,7
and phenylthiolfluoroalkyl bromides.8
On the other hand, fluorinated sulfonic acids as a class of strong
Bronstedacidsareveryusefulforpreparationofhighperformance
polymer materials, such as a proton-exchange membrane in fuel
cells.9 However, very few methods are available for the synthesis
of fluorinated sulfonic acids and their derivatives. In 1981, Huang
reported an approach for the preparation of fluorinated sulfinates
and sulfonates by sulfinatodehalogenation via the reaction
between fluoroalkyl iodides and sodium dithionite.10 Then,
Prakash et al. prepared fluorinated sodium sulfinates by the
reaction of difluoroalkyl 2-pydidyl sulfone with EtSNa, which was
further converted into difluoroalkyl sulfonates in excellent yield
by an oxidation reaction.11 Sanchez et al. synthesized perfluoro-
alkanesulfonyl fluorides from perfluoro-alkyl trimethyl silanes in
several successive steps.12 More recently, Hu reported a simple
To obtain the new diheteroatom bifunctional fluoroalkylation
reagents, we carefully investigated the three-component reaction
of NaN3, chlorotrifluoroethylene (CTFE), and 2-thiocyanato-
pyrimidine in DMF at rt (Table 1, entries 1−2). Initially, the
desired product, 2-((2-azido-1-chloro-1,2,2-trifluoroethyl)thio)-
pyrimidine (ACTP), was obtained in a yield of 21% when 2 equiv
of NaN3 were used without any catalyst or promoter (entry 2).
Received: February 2, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.7b00347
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Organic Letters
Letter
a
molar ratio of NaN3 to 1a was 2.5 using 0.5 equiv of CuI/Phen as
thepromotor(entries15−19). Notably,comparedtothereaction
at rt, the yield of the product was decreased significantly when the
reaction was conducted at either −30 or 50 °C (entries 20−21).
Other ligands such as PMDETA and Bpy gave a slightly lower
yield (entries 22−23). With the optimized reaction conditions in
hand, we further investigated the behaviors of other thiocynates
1b−1d and found that all the corresponding products could be
obtained in modest yields (entries 24−26). Consequently, a new
diheteroatom bifunctional S- and N-containing fluoroalkylation
reagent ACTP was synthesized successfully. Moreover, the
stability of ACTP was examined by 19F NMR spectroscopy, and
the result indicates thatACTP isa fairlystable colorless liquid at rt
in the dark. No detectable decomposition was observed after
storage in a glass vial for 6 months in the dark at rt. To the best of
our knowledge, it is the first diheteroatom bifunctional
fluoralkylation reagent containing both S and N atoms.
It is well-known that the Cu(I)-catalyzed azide−alkyne
cycloaddition (CuAAC) reaction is commonly described as a
“click reaction” and widely used in the synthesis of organic
compounds, polymers and biomaterials.17 In our previous work,
we have demonstrated that CuI and TBAA as a combined catalyst
was highly effective for the CuAAC reaction of fluoroalkyl
azides.14 To explore the reactivity of azide group in the new
diheteroatomfluoroalkylationreagent, weconductedtheCuAAC
reactions of azide with different alkynes using CuI/TBAA as the
catalyst in toluene (Scheme 1). The target products can be
Table 1. Optimization of Reaction Conditions
b
NaN3
(equiv)
ligands
(equiv)
yield
entry
[Cu] (equiv)
solvent
(%)
1
2
3
4
1.0
2.0
2.0
2.0
−
−
−
−
DMF
trace
21
DMF
DMF
DMF
CuSCN (1)
Phen (1)
54
Cu(BF4)·
Phen (1)
46
4MeCN (1)
5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
CuBr (1)
CuI (1)
Phen (1)
Phen (1)
−
Phen (1)
Phen (1)
Phen (1)
Phen (1)
Phen (1)
Phen (1)
Phen (1)
DMF
52
58
18
22
17
0
6
DMF
7
Br(PPh3)3Cu (1)
CuBr2 (1)
Cu (1)
DMF
8
DMF
9
DMF
10
11
12
13
14
CuI (1)
CH3CN
dioxane
THF
CuI (1)
0
CuI (1)
0
CuI (1)
DMSO
47
67
CuI (1)
DMSO/
DMF
15
16
17
18
19
2.0
2.0
2.0
2.5
3.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
CuI (0.1)
CuI (0.3)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
CuI (0.5)
Phen (0.1)
Phen (0.3)
Phen (0.5)
Phen (0.5)
Phen (0.5)
Phen (0.5)
Phen (0.5)
DMSO/
DMF
19
36
74
78
75
32
38
67
74
60
63
51
DMSO/
DMF
ab
,
DMSO/
DMF
Scheme 1. Substrate Scope of Alkynes
DMSO/
DMF
DMSO/
DMF
c
20
DMSO/
DMF
d
21
DMSO/
DMF
22
23
PMDETA
(0.5)
DMSO/
DMF
Bpy (0.5)
Phen (0.5)
Phen (0.5)
Phen (0.5)
DMSO/
DMF
e
24
DMSO/
DMF
f
25
DMSO/
DMF
g
26
DMSO/
DMF
a
a
Reaction used ACTP (1 mmol) and alkynes (1.1 mmol) with CuI
(0.05 mmol) and TBAA (0.1 mmol) in 1 mL of toluene at rt under N2
Reaction conditions: 2-thiocyanatopyrimidine 1a (2 mmol), NaN3 (4
mmol), CTFE (1.5 g, 0.2 MPa), CuI (1 mmol) and phen (1 mmol) in
2 mL of solvents (DMSO/DMF = 1/1) at rt for 12 h. Yields refer to
isolation after chromatography. This reaction was performed at 50 °C.
This reaction was performed at −30 °C. 1b was used instead of 1a.
1c was used instead of 1a. 1d was used instead of 1a.
b
b
for 4 h. Yields refer to isolation after chromatography.
c
d
e
obtained in excellent yields for both the aromatic alkynes with
electron-withdrawing or -donating groups and aliphatic alkynes,
tolerating various functional groups. Obviously, this approach is
more feasible and efficient to synthesize fluoroalkyl thioethers
than a previously published method,14 the latter requiring two-
step chemical reactions.
In the past decade, increasing attention has been focused
toward the synthesis of fluorinated sulfones because they have
shown wide applications for the incorporation of diverse
fluoroalkyl groups into organic molecules.18 We tried to convert
the substituted fluoroalkyl 2-pyrimidyl thioethers to correspond-
ing sulfones by an oxidation reaction. The reactions were
performed by using RuCl3/NaIO4 as the oxidation reagents in the
combined solvents of CH3CN, CCl4, and H2O at rt, and the
f
g
Copper salts as promoters are usually used in nucleophilic
fluoroalkylation reactions, due to their ability to stabilize the
fluorocarbon anion.16 When the reaction was performed with a
mixture of CuSCN (1 equiv) and ligand Phen, the yield of the
product was increased to 54% (entry 3). The results of the
screening of promoters revealed that CuI/Phen was the most
efficientpromoter forthereaction(entries4−9). Moreover, polar
solvents were favorable for this reaction (entries 10−13). ACTP
was obtained in 67% yield in a mixed solvent of DMSO/DMF (1/
1, v/v)(entry14). Theproduct yieldwasrelated totheratio ofthe
reactants, and the highest yield (78%) was obtained when the
B
DOI: 10.1021/acs.orglett.7b00347
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Letter
results of the oxidation reactions are summarized in Scheme 2. All
the expected products were obtained in moderate yields (4a−4j),
fluorinated sulfones into a fluorinated sulfinate via reductive
dearylation, and the desired sulfinate could be obtained in 88%
yield(entry5).Wealsonoticedthatsolventhadasignificanteffect
on the reaction. For example, the fluorinated sulfinate could be
obtained in 99% yield in a mixture of CH3OH/THF(1/1, v/v)
with NaBH4 as the reductant (entry 6). In contrast, when other
reductants, such as NaBH3CN and (CH3COO)3BHNa were
used, no products could be detected in the reactions (entries 7−
8). Moreover, the behavior of the reductive dearylation reaction is
also highly related to the structure of the fluorinated sulfones.
When the sulfonyl group was linked to 2-pyridyl, phenyl, and
benzothiazol-2-yl, respectively, instead of pyrimidyl, the reactions
of dearylation were failed (entries 9−11). This result indicates
thattherationaldesignofthemolecularstructuresisacrucialissue
for preparation of fluorinated sulfinate from fluorinated sulfones.
Encouraged by the initial success, we further investigated the
dearylation reactions of common perfluoroalkyl sulfones with
different Ar groups (Table S1, Figure S1 in Supporting
Information (SI)). Perfluorohexylsulfones were chosen as the
model substrates to test the reactivity under the dearylation
reaction conditions optimized for 4a. The comparisons of the
different perfluorohexylsulfones proved that only 2-pyrimidyl
sulfones could be transformed into the corresponding sulfinate
effectively. These results demonstrate that the NaBH4-mediated
dearylation reactionof fluoroalkyl 2-pyrimidyl sulfonesprovides a
general and efficient synthetic method for the preparation of
fluoroalkyl sulfinates.
Scheme 2. Oxidation Reaction of Fluoroethyl 2-Pyrimidyl
ab
,
Thioether Derivatives
a
Reaction used 3 (1 mmol), NaIO4 (4.5 mmol), and 3 mg of RuCl3·
3H2O in a solution of 1.5 mL of CH3CN, 1.5 mL of CCl4, and 3.2 mL
b
of H2O at rt for 8 h. Yields refer to isolation after chromatography.
except 4k and 4j, in which the hydroxyl and aldehyde groups on
the substrates were oxidized into carboxyl groups.
Based on the achievement of the fluorinated sulfinate, we
further tried to prepare the fluorinated sulfonates in a one-pot
reaction by a combined dearylation−oxidation procedure. The
results showed the fluoroalkyl sulfones with 2-pyridyl groups
could be transformed into sulfonates in excellent yield by the one-
pot approach (Scheme 3, 5a−5d). Thus, this is a new facile and
efficient method for the preparation of fluoroalkyl sulfonic acids.
Subsenquently, the transformation of fluorinated sulfones to
corresponding fluorinated sulfinates was investigated. The
compound 4a was selected as the model substrate. A series of
nucleophiles were examined, and the results showed that
CH3ONa was more effective than others. However, the yield of
sulfinate was not satisfactory (Table 2, entries 1−4). Fortunately,
we found that NaBH4 was an excellent reagent to transform the
a b
,
Scheme 3. One-Pot Synthesis of Fluorinated Sulfonates
a
Table 2. Screening of Conditions for Dearylation
b
yield
entry sulfone
reagent (equiv)
C2H5SNa (2.0)
solvent
t (h)
(%)
1
4a
C2H5SH/THF
(1/2)
12
0
2
3
4
5
6
4a
4a
4a
4a
4a
C2H5SNa (2.0)
CH3ONa (1.5)
CH3ONa (1.5)
NaBH4 (1.5)
THF
12
0.5
0.5
1
0
a
b
Determined by 19F NMR. Yields refer to isolation after
THF
79
85
88
99
chromatography.
CH3OH
CH3OH
To understand the relationship between the structure and
reactivity of the fluoroalkyl sulfones, theoretical calculations
based on the density functional theory (DFT) were performed at
the B3LYP/6-31G+(d) levels (Tables 3 and S2 in SI). The results
reveal that, compared to other fluoroalkyl analogues, fluoroalkyl
2-pyrimidyl sulfones possess a longer bond length and a lower
bond order, implying the weaker C−S bond in the S-CFCl and S-
CF2 moieties; thus, 2-pyrimidyl is easily removed by the
dearylation reaction. Moreover, the Mulliken charges of the aryl
carbon associated with the sulfur were higher than that of others,
thus showing a higher electrophilicity.19
NaBH4 (1.5)
CH3OH/THF
(1/1)
1
7
8
4a
4a
NaBH3CN (1.5)
THF
THF
12
12
0
0
(CH3COO)3BHNa
(1.5)
9
4ab
4ac
4ad
NaBH4 (1.5)
NaBH4 (1.5)
NaBH4 (1.5)
CH3OH/THF
(1/1)
12
12
12
0
0
0
10
11
CH3OH/THF
(1/1)
CH3OH/THF
(1/1)
a
Synthesis of new fluorinated sulfonic acid polymers has
received considerable attention since the use of a Nafion
Reaction used 4a (0.2 mmol) and NaBH4 (0.3 mmol) in a 0.5 mL
b
solution of CH3OH and THF at rt. Determined by 19F NMR.
C
DOI: 10.1021/acs.orglett.7b00347
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Table 3. Structural Parameters Associated with Fluorinated
Sulfone
AUTHOR INFORMATION
Corresponding Authors
■
a
*E-mail: baiwei81@gmail.com.
*E-mail: bairk@ustc.edu.cn.
ORCID
Ruke Bai: 0000-0002-1310-316X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
WethanktheNationalNaturalScienceFoundationofChina(No.
21174136) for financial support.
■
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tionreagentACTPbyafacileandhighlyefficientapproach. Asthe
first diheteroatom bifunctional fluoroalkylation agent, ACTP can
be used to simultaneously incorporate both S and N atoms into
fluorinated compounds. Therefore, a variety of organic
fluorinated thio-compounds with N-containing functional
groups, such as fluorinated ethers and sulfones with a trizole
group, were successfully prepared. These fluorinated compounds
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ASSOCIATED CONTENT
* Supporting Information
■
S
(19) (a) Lu, G.; Dang, Z.; Fennell, D. E.; Huang, W.; Li, Z.; Liu, C. J.
Hazard. Mater. 2010, 177, 1145. (b) Du, X.; Gao, X.; Qiu, K.; Luo, Z.;
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TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00347.
Experimental procedures and spectral data (PDF)
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DOI: 10.1021/acs.orglett.7b00347
Org. Lett. XXXX, XXX, XXX−XXX