Silver-catalyzed decarboxylative homocoupling reaction for the construction of tetrafluoroethylene-bridging aromatic compounds

Article abstract of DOI:10.1080/00397911.2019.1651865

The Ag(I)-catalyzed decarboxylative homocoupling from the difluoroacetate has been developed to the synthesis of symmetric CF₂–CF₂ containing dimers. This radical dimerization overpasses the prefunctionalization of the substrate and provides a direct and efficient method for construction of tetrafluoroethylene bridge-linked homodimers.

Full text of DOI:10.1080/00397911.2019.1651865

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
Silver-catalyzed decarboxylative
homocoupling reaction for the construction of
tetrafluoroethylene-bridging aromatic compounds
Yong Wang, Huaxin Zhao, Xiaojuan Xie, Haizhen Jiang, Hongmei Deng, Jian
Hao & Wen Wan
To cite this article: Yong Wang, Huaxin Zhao, Xiaojuan Xie, Haizhen Jiang, Hongmei Deng,
Jian Hao & Wen Wan (2019): Silver-catalyzed decarboxylative homocoupling reaction for the
construction of tetrafluoroethylene-bridging aromatic compounds, Synthetic Communications, DOI:
10.1080/00397911.2019.1651865
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https://doi.org/10.1080/00397911.2019.1651865
Silver-catalyzed decarboxylative homocoupling reaction for
the construction of tetrafluoroethylene-bridging
aromatic compounds
Yong Wanga, Huaxin Zhaoa, Xiaojuan Xiea, Haizhen Jianga, Hongmei Dengb,
Jian Haoa,c, and Wen Wana
b
aDepartment of Chemistry, Shanghai University, Shanghai, China; Laboratory of Microstructures,
c
Shanghai University, Shanghai, China; Key Laboratory of Organofluorine Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China
ABSTRACT
ARTICLE HISTORY
Received 3 April 2019
The Ag(I)-catalyzed decarboxylative homocoupling from the difluor-
oacetate has been developed to the synthesis of symmetric CF2–CF2
containing dimers. This radical dimerization overpasses the prefunc-
tionalization of the substrate and provides a direct and efficient
method for construction of tetrafluoroethylene bridge-linked homo-
dimers.
KEYWORDS
Decarboxylative homocou-
pling; difluoroacetate;
radical dimerization;
tetrafluoroethylene bridge
GRAPHICAL ABSTRACT
Introduction
Organofluorine compounds have attracted much attention because of their remarkable
applications in medicinal, agrochemical and new-material chemistries.[1] Among various
fluorinated moieties, gem-difluoromethylene unit (CF2), which has a steric profile simi-
lar to the methylene unit, is of great interest due to its unique stability and isosteric
property as an ethereal oxygen atom or a carbonyl group.[2] Moreover, the dimeric moi-
ety of tetrafluoroethylene bridge (–CF2CF2–) has been regarded as a key structure in
insecticides,[3] chemical vapor deposition (CVD) precursors of thin-film polymers,[4] as
well as in the liquid-crystalline compounds because of its potential to influence the
physical properties of these materials, such as clearing point, mesophase sequence, and
rotational viscosity.[5] Thus, the development of an efficient and straightforward method
to the formation of tetrafluoroethylene bridge has been in high demand.
CONTACT Wen Wan
wanwen@shu.edu.cn, jhao@shu.edu.cn
Department of Chemistry, Shanghai University,
Shangda Road 99, Shanghai, 200444, China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
Y. WANG ET AL.
Scheme 1. Strategies for transition-metal-catalyzed decarboxylative homo-coupling to form symmet-
rical difluoromethylated derivatives.
In general, the construction of a tetrafluoroethylene-bridging structure ArCF2CF2Ar’
is achieved by fluorination of a carbon-carbon triple bond with F2 gas or via the deoxo-
fluorination of 1,2-diketones with a fluorination reagent such as DAST (Scheme 1a).[6,7]
Recently, Hu disclosed a fluoroalkylation process through a copper(0) mediated reduc-
tive cross-coupling between aryl idoides with structurally diverse 1-bromo-1,1,2,2-teta-
fluoroethyl compounds RCF2CF2Br.[8] Ogoshi described an efficient construction of a
tetrafluoroethylene-bridging structure through a copper-mediated transformation of
tetrafluoroethylene (TFE) into aromatic compounds by the carbocupration of TFE with
arylboronates in the presence of [CuOt-Bu] (Scheme 1b).[9] However, these methods
have drawbacks including low functional group tolerance, the use of toxic, costly fluor-
ination reagents, or the requirement of prefunctionalization of the substrates, which
limit the widespread application of the protocols in synthesis and industrial production.
Since the pioneering work of Myers and Goossen, catalyzed decarboxylative coupling
reactions have emerged as a powerful tool for the formation of C–C bonds in the past
decade.[10] Decarboxylative homocoupling has been applied as an efficient protocol to
construct the symmetrical molecules, which constitute an important motif in chiral
ligands, monomers of conductive polymers, liquid crystal precursors, natural products,
pharmaceuticals, and pesticides.[11] Transition metal-mediated decarboxylative homo-
couplings have been highly successful in the formation of symmetrical molecules, which
are mainly related to the tethering of sp2 or sp hybridized carbon centers.[12]
Due to its notable advantages, transition-metal-catalyzed oxidative decarboxylative
radical gem-difluoromethylenation has recently emerged as an attractive and alternative
approach for the direct construction of the C–CF2 bond, because of the ready availabil-
ity and low cost of difluorocarboxylic acids as well as the extrusion of innocuous CO2
as a by-product in these reactions.[13] Very recently, our group developed a mild and

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3
efficient method for the Ag(I) or Pd(II)-catalyzed oxidative decarboxylative gem-
difluoromethylenation, affording Csp2–CF2 bond and Csp3–CF2 bonds in the construc-
tion of meta–C–H/difluoromethylenated aromatic derivatives, as well as phenanthridine
and 2-oxindoles.[14] Hashmi and Wu described an efficient and direct decarboxylative
cross-coupling of a,a-difluoroarylacetic acids and ethynylbenziodoxolone reagents to
give difluoromethylated alkynes, providing the Csp–CF2 bond.[15] Qing developed a vis-
ible-light-induced hydroaryl difluoromethylenation of alkenes with a,a-difluoroarylacetic
acids through decarboxylation and subsequent radical hydroaryldifluoromethylenation
to form Csp3–CF2 bond.[16]
During our study on the reactivity of difluoromethyl radical generated from the read-
ily available difluoroacetic acids through the oxidative decarboxylative process, we envis-
aged that the difluoromethyl radical intermediates could be applied to construct the
C
(sp3)F2–C(sp3)F2 bond through the valuable and effective tool of homocoupling reaction
by radical dimerization (Scheme 1c). To the best of our knowledge, the construction of
sp3F2–Csp3F2 bonds through decarboxylative homocoupling strategy of difluoroacetate
C
has not been reported. Such an environmental-friendly methodology would provide
easy access to a variety of symmetric tetrafluoroethylene bridge-linked homodimers
starting from inexpensive and stable aryl difluorocarboxylic acids with only innocuous
carbon dioxide as a by-product.
Results and discussion
Our investigation commenced by taking the decarboxylative homocoupling of difluoroa-
cetate 1a as the model reaction in the presence of catalyst and oxidant for the optimiza-
tion studies. As we expected, decarboxylative homocoupling of 1a in dry DMSO under
a nitrogen atmosphere at 80 ꢀC for 2 h afforded the desired homodimer in modest yield
of 31% (Table 1, entry 1). Among the silver salts and oxidants examined, the combin-
ation of AgNO3 and (NH4)2S2O8 turned out to be the most effective, which afforded up
to 48% yield (entries 2–8). Using Ag2CO4, AgBF4, AgOAc or AgSO3CF3 as the catalysts
and K2S2O8, Na2S2O8 or PhI(OAC)2 as oxidant instead of AgNO3 and (NH4)2S2O8 led
to the decreased yields. Replacement of silver salts with copper (I) or (II) salts com-
pletely suppressed reaction (entry 9). Sequential screening of solvents revealed that
DMSO was crucial for this reaction, while other solvents, such as DMF, NMP, 1,4-
dioxane or toluene, were of no use for this transformation. The optimal temperature for
the current reaction was found to be 120 ꢀC, giving good conversion to the expected
product in 68% yield (entries 10–12). Lowering the reaction temperature reduced con-
version and gave low yield (entry 13). The influence of bases, such as KHCO3, KOH,
K2CO3, and Cs2CO3 were further investigated. Other bases except for KHCO3 or the
absence of base reduced the reaction efficiency (entries 14–17). Longer or shorter reac-
tion times also gave negative results (entries 18–21). Increasing AgNO3 loading to two
equivalents or lowering AgNO3 loading to 0.3 equivalents was also found to be counter-
productive. Control experiments revealed that the reaction still gave 34% yield even
without Ag(I) catalyst (entry 22). However, no reaction occurred in the absence of the
oxidant (entry 23).

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Y. WANG ET AL.
Table 1. Optimization of reaction conditions.a
Entry
Catalyst
AgNO3
AgNO3
AgNO3
Ag2CO3
AgBF4
Oxidant
Base
Temp. (oC)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
PhI(OAC)2
Na2S2O8
K2S2O8
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
K2CO3
KOH
80
80
80
80
80
80
80
80
80
100
120
140
60
120
120
120
120
120
120
120
120
120
120
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
31
42
34
30
43
34
36
48
0
49
68
46
12
40
38
36
28
51
40
33
31
34
0
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
—
AgOAc
AgSO3
AgNO3
Cu(I) or Cu(II)
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
—
9
10
11
12
13
14
15
16
17c
18
19
20
21
22d
23e
Cs2CO3
—
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
4
6
1
1/3
2
2
AgNO3
aReaction conditions: difluoroacetate (0.6 mmol, 1.0 equiv.), AgNO3 (0.3 mmol, 0.5 equiv.), (NH4)2S2O8 (1.2 mmol, 2.0
equiv.), KHCO3 (0.3 mmol, 0.5 equiv.) under N2 atmosphere.
bYields determined by 19F NMR analysis with PhCF3 as the internal standard;
cwithout base;
dwithout catalyst;
ewithout oxidant.
With the optimized reaction conditions in hand (Table 1, entry 11), the decarboxyla-
tive homocoupling reactions of difluoroacetates were explored in order to study the sub-
strate scope and limitation of this transformation. As depicted in Table 2, a variety of
substrates with para-substituted groups, such as electron-donating groups (CH3, OCH3,
t-Bu) 2a, 2b, 2c or electron-withdrawing halogen groups (F, Cl, Br) 2d, 2e, 2f afforded
the homodimers in moderate to satisfactory yields. The halogen moiety provided the
possibility for further modification. A mere 9% yield of the product dimer 2g was
obtained when the extremely electron-poor cyano group CN was incorporated in the
para-position of the aromatic ring. Phenyl difluoromethyl radicals released from phenyl-
difluoroacetate 1h combined more effectively to furnish diphenyl perfluoroethane 2h in
yield of 48%. The substrates with ortho-substituted methyl or methoxyl group 2i and 2j
also afforded moderate yield. It was interesting to find that though dimethylated sub-
strate 1k gave a moderate yield of 46%, the trimethylated substrate 1l produced the
desired dimer 2l in excellent yield (85%), indicative of the insensitivity of this trans-
formation to steric hindrance. Increasing the size of the aromatic units with naphtha-
lenyl difluoroacetate 1m gave a reasonable yield of 32%. In this work, the possibility of

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Table 2. Synthesis of symmetric tetrafluoroethylene bridge-linked homodimers.a,b
aReaction condition: difluoroacetate (0.6 mmol, 1.0 equiv.), AgNO3 (0.3 mmol, 0.5 equiv.), (NH4)2S2O8 (1.2 mmol, 2.0
equiv.), KHCO3 (0.3 mmol, 0.5 equiv.) under N2 atmosphere at 120 ꢀC.
bYields of isolated products.
the homocoupling of aryldifluoromethyl radicals and/or the reaction of aryldifluoro-
methyl radicals with aryldifluoroacetate derivatives could occur on their benzene rings,
resulting in these regioisomers undergoing aromatization to afford bis(difluoromethyl)-
biaryls and/or (difluoromethylaryl)aryldifluoromethanes. However, only homocoupling
products have been observed according to the careful 19F NMR analysis. Unfortunately,
an attempt to employ furan derivative 1n failed to return any dimer. The starting
material 1n was found to be completely consumed according to the 19F NMR analysis,
but only thiophene-2-carboxylic acid was obtained. The acid was probably formed by
hydrolysis of the free difluoromethyl radical. To our surprise, while the
mono-methyoxylated phenyldifluoroacetate 1a and 1j gave the corresponding tetra-
fluoroethylene-linked dimers in moderate yields, the substrate of 2,4-dimethoxyphenyl-
2,2-difluoroacetate 2o afforded instead the 2,4-dimethoxybenzoic acid as the main
product, along with, for the first time, a minor product of defluoro homocoupling prod-
uct after decarboxylative hydrolysis under the standard reaction conditions (See
Supporting Information).
To figure out whether the in situ generated aryl difluoromethyl radical (ArCF2.) is
involved in the reaction, some inhibition experiments using 2,2,6,6-tetramethyl-1-

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Y. WANG ET AL.
Table 3. Effect of additives on Ag(I)-catalyzed decarboxylative homocoupling reaction.a
Entry
Additive
Yield (%)b
1
2
3
4
1.0 equiv. 1,4-dinitrobenzene
1.0 equiv. hydroquinone
1.0 equiv. TEMPO
57
63
4
2.0 equiv. TEMPO
0
aReaction condition: 1a (0.6 mmol), AgNO3 (0.3 mmol), and (NH4)S2O8 (1.2 mmol), KHCO3 (0.3 mmol) under N2 at 120 ꢀC
for 2 h.
bYields determined by 19F NMR analysis with PhCF3 as the internal standard.
Scheme 2. Proposed mechanism for transition-metal-catalyzed decarboxylative homo-cou-
pling reaction.
piperidinyloxy (TEMPO), 1,4-dinitro-benzene and hydroquinone as the radical scaven-
ger were applied under the standard reaction conditions (Table 3). Significant suppres-
sion of the formation of the desired product 2a suggested that the ArCF2. radical
species acts as an important role in the current reaction. Additionally, control experi-
ments (Table 1, entries 22, 23) provided evidence of the importance of the combination
of AgNO3 and (NH4)2S2O8 for the efficient generation of ArCF2. radical intermediate.
A plausible mechanism for this decarboxylative homocoupling reaction can be pro-
posed (Scheme 2).[17] Initially, the active Ag2þ species is generated by oxidation of the
Agþ species with persulfate. Subsequently, the difluoroacetate undergoes single electron
oxidative decarboxylation with the active Ag2þ species to afford the difluoromethyl rad-
ical intermediate and regenerate the Agþ species. Finally, termination occurs by homo-
coupling of two difluoromethyl radicals producing the desired homodimer containing
CF2–CF2 bridge.
Conclusion
In summary, the decarboxylative homocoupling reaction in this work provides a valu-
able and effective tool for the synthesis of symmetric dimers linked by tetrafluoroethyl-
ene-bridge. This decarboxylative homocoupling from the difluoroacetate bypasses the
prefunctionalization of the substrate to efficiently construct the CF2–CF2 containing

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7
homodimers. The primary mechanistic investigation suggests that the aryl difluoro-
methyl radical is involved in the homocoupling process.
Experimental section
General
The reagents used for experiments were commercially available and were used as
received unless otherwise noted. Column chromatography was performed on silica gel
1
300–400 mesh. H, 13C and 19F NMR spectra were recorded in CDCl3 on a Bruker AV-
l
500 spectrometer (Bruker, Billerica, MA). Chemical shifts for H NMR spectra are
1
reported in ppm relative to residual CHCl3 as internal reference (d 7.26 ppm for H)
downfield from TMS, chemical shifts for 13C NMR spectra are reported in ppm relative
to internal CDCl3 (d 77.16 ppm for 13C). 19F NMR chemical shifts are determined rela-
tive to CFCl3 at d 0.0 as the outside standard and low field is positive. Coupling con-
stants (J) are given in Hertz (Hz). The terms m, s, d, t, q refer to multiplet, singlet,
doublet, triplet, quartet, respectively; br refers to a broad signal. High-resolution mass
spectra (HRMS) and Mass spectra (MS) were recorded using an Electron impact (EI) or
Electrospray ionization (ESI) techniques. High-resolution mass spectra (HRMS) was
recorded on Agilent G6500 iFunnel Q-TOF LC/MS (Agilent, Santa Clara, CA).
Typical procedure for the decarboxylative homocoupling of difluoroacetates
to homodimers
A mixture of aryl difluoroacetates (0.6 mmol), AgNO3 (0.3 mmol), and (NH4)S2O8
(1.2 mmol), and KHCO3 (0.3 mmol) was added to in 3.0 mL of DMSO under N2 atmos-
phere. The solution was stirred at 120 ꢀC for 2 h. Then the reaction mixture was
quenched by water and extracted with ethyl acetate. The combined organic layer was
dried over anhydrous Na2SO4, filtered and concentrated in vacuo. The residue was puri-
fied by flash column chromatography on silica gel to give the desired homodimers
(2a–2m) with ethyl acetate/petroleum ether as eluent.
1,2-bis(4-methylphenyl)-1,1,2,2-tetrafluoroethane (2b)
1
White solid, mp 137–140 ꢀC, 53% yield. H NMR (CDCl3) d: 7.33 (d, J ¼ 8.3 Hz, 4H),
7.21 (d, J ¼ 8.3 Hz, 4H), 2.39 (s, 6H). 19F NMR (CDCl3) d: ꢁ111.36 (s, 4F, CF2). 13C
NMR (CDCl3) d: 141.0, 128.8, 128.1–128.3 (m), 126.9 (m), 116.8 (tt, 1JCF ¼ 250.7 Hz,
2JCF ¼ 36.4 Hz), 21.4. HRMS (EI): calcd. for C16H14F4 (Mþ) 282.1026; found 282.1029.
1
Full experimental details of H, 13C, and 19F NMR spectra can be found through the
“Supplementary Content” section of this article’s webpage.
Acknowledgments
The authors thank the Laboratory of Micro-structures of Shanghai University for struc-
ture analysis.

8
Y. WANG ET AL.
Disclosure statement
We state that none of the authors have any conflict of interest in the context of this
communication.
Funding
This work was financially supported by the National Natural Science Foundation of China
[21572128, 21672139].
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