External-Oxidant-Free Electrochemical Oxidative Trifluoromethylation of Arenes Using CF3SO2Na as the CF3 Source

Article abstract of DOI:10.1002/cjoc.201900168

An efficient and environmentally benign electrochemical oxidative radical C—H trifluoromethylation of arenes by employing Langlois reagent as the CF₃ source was developed in this work. Neither transition metal catalysts nor external chemical oxidants were required in this trifluoromethylation process. The reaction could be conducted in gram scale with high reaction efficiency.

Full text of DOI:10.1002/cjoc.201900168

Chinese Journal
of Chemistry
CJC
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Accepted Article
Title: External-Oxidant-Free Electrochemical Oxidative Trifluoromethylation of
Arenes Using CF3SO2Na as the CF3 Source
Authors: Yong Deng, Fangling Lu, Siqi You, Tianrui Xia, Yifan Zheng, Cuifen Lu,
Guichun Yang, Zuxing Chen, Meng Gao,* and Aiwen Lei *
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This work may now be cited as: Chin. J. Chem. 2019, 37, 10.1002/cjoc.201900168.
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External-Oxidant-Free Electrochemical Oxidative Trifluoromethylation of
Arenes Using CF3SO2Na as the CF3 Source
Yong Deng, †,a Fangling Lu,†,b Siqi You,a Tianrui Xia, a Yifan Zheng,a Cuifen Lu,a Guichun Yang,a Zuxing Chen,a Meng
Gao,*,a, b and Aiwen Lei *,c
a Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecule & School of Chemistry and Chemical Engineering,
Hubei University, Wuhan 430072, P. R. China.
b National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China.
c College of Chemistry and Molecular Sciences, the Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072, P. R. China
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion An efficient and environmentally benign electrochemical oxidative radical C–H trifluoromethylation of
arenes by employing Langlois reagent as the CF3 source was developed in this work. Neither transition metal catalysts nor external chemical oxidants
were required in this trifluoromethylation process. The reaction could be conducted in gram scale with high reaction efficiency.
radical could be generated in a low concentration from continued
electrolysis of Zn(SO2CF3)2. However, divided cells, along with an
Background and Originality Content
Introduction of the trifluoromethyl group onto organic
expensive Et4NClO4/DMSO supporting electrolyte, were required
to give acceptable yields.[6] Thus, there is still great demand for
compounds can often impart several desirable features
associated with the lipophilicity, bioavailability and metabolic
the development of electrochemical trifluoromethylation
stability of bioactive molecules. As
a
result, synthetic
methods that function under mild and simple conditions. Very
recently, the groups of Zeng, Ackermann, Ruan and Mo
respectively, realized the direct electrolysis of CF3SO2Na to
trifluoromethylated oxindoles. [7, 8] As part of our recent studies in
the area of the electrochemical oxidative functionalization of
alkenes and arenes,[9] we report here an efficient and
trifluorinated compounds have been widely applied in various
fields, especially in biochemical and medicinal science.[1] Among
all the trifluoromethylated compounds, trifluoromethylated
aromatics are core building blocks in both pharmaceuticals and
agrochemicals, due to the stereoelectronic property of the CF3
moiety and the increased bioavailability supported by this group.
Hence, numerous processes have been developed for the
trifluoromethylation of arenes and heteroarenes,[2] although most
of these methods are limited to the use of stoichiometric
amounts of metal complexes, preactivated substrates, or
directing groups. Recently, radical trifluoromethylation has been
proven as a powerful tool for the direct C-H trifluoromethylation
of arenes.[3] The significant breakthrough made by MacMillan and
Baran showed that either visible light or peroxide is amenable to
environmentally
benign
electrochemical
oxidative
trifluoromethylation of arenes by employing CF3SO2Na (Langlois
reagent) as the CF3 source (Scheme 1). The reaction was
performed with a constant current of 10 mA in an undivided
n
electrolytic cell using Bu4NBF4, MeCN and H2O as the supporting
electrolyte.
initiate CF3 radical formation from eithr CF3SO2Cl or CF3SO2Na
3c]
(Langlois reagent).[3b,
However, despite these indisputable
advances, expensive photoredox catalysts or strong chemical
oxidants, such as hypervalent iodine and peroxide, are often
required in most of these cases. Since CF3SO2Na is easily
converted to CF3SO3Na in the presence of O2 or other chemical
oxidants. and CF3SO3Na could not be further converted to CF3
radical by a desulfurization event (Scheme 1), a great number of
2f, 3j]
CF3SO2Na is usually required in these reactions.[2a,
Electrochemical synthesis has been recognized as an efficient and
environmentally benign synthetic strategy and minimized
byproduct formation.[4, 5] Recently, Baran and co- workers have
developed
a metal-free radical C−H trifluoromethylation of
heteroarenes under electrochemical conditions, in which the CF3
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First author et al.
Report
Scheme 1 External-Oxidant-Free Electrochemical Trifluoromethylation.
Table 1 Optimization of reaction conditionsa
CF3
:
=
mA
Fe
I
10
C
(+) |
(-)
(50
25 °C, 5 h
MeO
OMe
MeO
OMe
nBu4NBF4
mol%)
O
H
CF3SO2Na
O
N
NO2
MeCN/H
2O,
undivided
O
O
N
cell
HCl
F3C
OMe
O
CF3
OMe
F3C
NO2
1a
2a
3a
Fluoxetine
Trifluralin
(Prozac)
Picoxystrobin
yield (%)b
Entry
Variation from the standard conditions
HO
O
O
O
H
F3C
NC
N
S
3a
F
1
2
None
Platinum cathode
Carbon cathode
90
89
64
84
91
65
82
85
80
39
40
n.d.
Bicalutamide
(Casodex)
3
or
O2 other oxidants
CF3
CF3SO2Na
CF3SO3Na
4
Nickel cathode
5
3.0 equiv. of CF3SO2Na
6 mA, 8 h
6
work
This
7
18 mA, 3h
CF3
nBu4NPF6 instead of nBu4NBF4
LiClO4 instead of nBu4NBF4
DMSO instead of MeCN
DMF instead of MeCN
No electric current
C
Fe
+
8
CF3SO2Na
R
R
9
catalyst-free
external-oxidant-free
mild conditions
10
11
12
.
a
Reaction conditions: carbon rod anode, iron plate cathode, constant
Results and Discussion
current = 10 mA, 1a (0.50 mmol), 2a (purity: 95%, 1.00 mmol), nBu4NBF4
(0.25 mmol), MeCN (10.0 mL), H2O (1.0 mL), 25 oC, N2, 5 h. b Isolated yields.
n.d. = not detected.
We initially tested our design with 1, 3, 5-trimethoxybenzene
(1a) and sodium trifluoromethanesulfinate (2a) as the model
substrates (Table 1). Encouragingly, when reaction was performed
with a constant current of 10 mA in an undivided electrolytic cell
(see Table 1, entry 1), 90% yield of the desired
trifluoromethylated compound 3a could be isolated. We next
tested the reaction under various conditions, including different
current, electrode material, electrolyte, solvent, and the amount
of trifluoromethanesulfinate (3a). The electrode materials were
important for achieving good reaction efficiency. Platinum plate
cathode showed similar reactivity with iron plate cathode (Table 1,
entry 2). However, nickel plate and carbon rod cathodes were less
efficient for promoting this trifluoromethylation (Table 1, entries
3 and 4). Further explorations showed that 2.0 equivalent of 2a is
the best choice, increasing the amount of 2a to 3.0 equivalent
could only slightly increase the yield. 65% yield of
trifluoromethylated compound 3a was isolated when decreasing
the operating current to 6 mA, while increasing the operating
current to 18 mA led to an obvious loss in yield (see Table 1,
entries 2 and 3). As for the choice of supporting electrolyte,
nBu4NPF6 demonstrated similar reactivity to nBu4NBF4 (Table 1,
entry 8) while LiClO4 was less effective than nBu4NBF4 (Table 1,
entry 9). The effect of solvent was also explored. Using methanol
or DMSO instead of acetonitrile also showed decreased reaction
efficiency (entry 14). As was expected, no desired product could
be obtained without electric current (Table 1, entry 12).
Since the method had been established, efforts were then
paid to explore the scope and limitations of the substrates.
Electron-rich arenes bearing methyl, carboxyl, Br and aldehyde
groups gave the corresponding products in moderate to good
yields (Table 2, 3a-3l). For unsymmetrical aromatics with more
than one reactive site, products may be formed as isomeric
mixtures (Table 2, 3i). We got poor or only trace amounts of
product when benzene and anisole were used as substrates,
nitrobenzene and analine did not react at all (Table 2, 3p-3s).
Moreover, some substituted heteroarenes, such as bioactive
theophylline,
3,4-ethylenedioxythiophene
and
N-(quinolin-8-yl)benzamide were selectively functionalized and
3m, 3n, 3l were obtained in 78%, 62% and 52% yield,
respectively. The synthetic potential of this transformation was
then evaluated by performing a gram scale reaction. By utilizing
20 mA constant current, 0.99 g of 3a could be synthesized by
performing a 5 mmol scale reaction after 36 h electrolysis
(Scheme 2).
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Running title
Chin. J. Chem.
Table 2 Electrochemical Oxidative Radical C–H Trifluoromethylation of
performed, respectively. An oxidation peak of 2a was observed at
1.6 V while the oxidation peak of 1a was observed at 2.6 V (Figure
1). This result indicated that 2a was oxidized preferentially at the
anode. Moreover, the BHT was selected as the radical scavengers
because its oxidation occurred at lower potential than for the
substrate 1a but a higher potential than for the 2a (Figure 1). The
trifluoromethylation of substrate 1a was fully suppressed upon
the addition of the radical scavenger BHT with a trace of desired
product formed，which reveals a single electron transfer (SET)
process may be involved in the reaction. (Scheme 3).
Arenes/Heteroarenes.a
C
Fe
cell
Ar-CF3
+
Ar-H
Na
CF3SO2
undivided
1
2a
3
OMe
OMe
MeO
OMe
CF3
MeO
MeO
MeO
MeO
CF3
O
CF3
CF3
OMe
3a
On the basis of the above results and previous report,[8]
a
3b
3c
,
90%
79%
OMe
60%
OMe
,
,
possible mechanism for the electrochemical oxidative
trifluoromethylation of arenes was illustrated as follows (see
Scheme 4). Firstly, the anodic oxidation of the sulfinate anion
produces the corresponding radical that decomposes to the CF3
radical in a desulfurative manner. Addition of the CF3 radical to
1,3,5-trimethoxybenzene (1a) formed the key carbon-centered
radical B, Subsequently, intermediate B underwent further single
electron transfer (SET) oxidation and finally resulted in the
desired product 3a after a deprotonation process. Concomitant
cathodic reduction of protons led to hydrogen liberation.
OMe
MeO
MeO
3d
CF3
Br
MeO
MeO
MeO
MeO
3e
CF3
CO2Me
3f
80%
,
58%
OMe
67%
,
,
OMe
OMe
∗
MeO
Br
CF3
OMe
MeO
CF3
OMe
CF3
Scheme 2 Reaction scale-up.
3i
3g
3h
, 50%
54%
63%
,
,
CF3
:
=
mA
I
20
C
Fe
(3:2)
(+) |
(-)
(25
MeO
OMe
MeO
OMe
nBu4NBF4
mol%)
CF3SO2Na
OMe
OMe
OMe
MeCN/H
25 °C, 36 h
2O,
MeO
cell
MeO
MeO
undivided
MeO
MeO
OMe
mmol
OMe
mmol
1a 5.0
2a 10.0
3a 0.99 84%
g,
CF3
COOH
yield
CHO
CF3
CF3
3j
3k
3l 55%
,
50%
70%
Figure 1. Cyclic voltammograms of 1a, BHT and 2a.
,
,
O
O
N
0.2
0.0
CF3
S
HN
O
N
N
CF3
N
N
H
O
O
-0.2
-0.4
-0.6
CF3
F3C
3mb
3nc
3o
52%
,
78%
62%
,
,
-0.8
examples
Some inert
blank
-1.0
-1.2
-1.4
-1.6
1a
2a
BHT
H2N
CF3
N
O2
CF3
CF3
0%
CF3
3p trace
OMe
10%
3q
3r
3s
,
0%
,
,
,
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Potential / V(vs Ag/AgCl)
a
Reaction conditions: carbon rod anode, iron plate cathode, constant
current = 10 mA, 1 (0.50 mmol), 2a (purity: 95%, 1.00 mmol), nBu4NBF4
(0.25 mmol), MeCN (10.0 mL), H2O (1.0 mL), 25 oC, 8 h, isolated yield.
b
Scheme 3 Control experiment.
c
LiClO4 (0.25 mmol), 9 h.
equivalents of CF3SO2Na. * Minor isomeric product.
The reaction was carried out with 3.0
CF3
MeO
OMe
MeO
OMe
Standard condition
2.0 BHT
CF3SO2Na
equiv.
In the next step, efforts were taken to understand the
reaction mechanism using 1a and 2a as reaction partners. First,
cyclic voltammetry (CV) experiments on 1a, 2a and BHT were
OMe
OMe
1a
2a
3a trace
Chin. J. Chem. 2019, 37, XXX－XXX
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First author et al.
Report
Scheme 4 Proposed mechanism.
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In summary, we have disclosed a mild and reagent-free
electrochemical oxidative radical C–H trifluoromethylation of
arenes/heteroarenes by employing Langlois reagent as the CF3
source. A series of C–H trifluoromethylated products were
produced in moderate to high yields. Neither transition metal
catalysts nor external chemical oxidants were required to
facilitate the trifluoromethylation process. Notably, the reaction
could be conducted in gram scale with high reaction efficiency.
Experimental
In an oven-dried undivided three-necked bottle (25 mL)
equipped with a stir bar, 1,3,5-Trimethoxybenzene (84 mg，0.5
mmol), CF3SO2Na (156 mg，1 mmol), nBu4NBF4 (165 mg, 0.5 mmol)
were combined and added. Under the protection of N2, CH3CN
(10 mL) and H2O (1mL) were injected respectively into the tubes
via syringes. The bottle was equipped with graphite rod (ϕ 6 mm,
about 15 mm immersion depth in solution) as the anode and iron
plate (15 mm×15 mm×1.0 mm) as the cathode. The reaction
mixture was stirred and electrolyzed at a constant current of 10
mA under room temperature for 8 h. After completion of the
reaction, as indicated by TLC and GC-MS, The pure product (yield
90%, 106.2 mg) was obtained by flash column chromatography on
silica gel.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2019xxxxx.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (21702081) and Jiangxi Provincial Education
Department Foundation (GJJ160325).
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This article is protected by copyright. All rights reserved.

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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2019
Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
Chin. J. Chem. 2019, 37, XXX－XXX
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
This article is protected by copyright. All rights reserved.

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External-Oxidant-Free
Electrochemical
Oxidative Trifluoromethylation of Arenes Using
CF3SO2Na as the CF3 Source
CF3
C
Fe
+
Na
CF3SO2
R
R
catalyst-free
external-oxidant-free
mild conditions
Yong Deng Fangling Lu Siqi You Tianrui Xia Yifan
Zheng Cuifen Lu Guichun Yang Zuxing Chen
Meng Gao* and Aiwen Lei *
An efficient and environmentally benign electrochemical oxidative radical C–H
trifluoromethylation of arenes by employing Langlois reagent as the CF3 source was developed in
this work.
www.cjc.wiley-vch.de
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.