Copper and Zinc Copromoted Bromo(chloro)trifluoromethylation of Alkenes and Alkynes with Trifluoromethanesulfonic Anhydride

Article abstract of DOI:10.1021/acs.orglett.0c03855

The first copper and zinc copromoted trifluoromethylation using trifluoromethanesulfonic anhydride (Tf2O) as a trifluoromethylating reagent has been developed. The reaction of alkenes or alkynes with Tf2O in the presence of CuX2 (X = Br, Cl), Zn powder, and 2,2′-bipyridine affords bromo(chloro)trifluoromethylated products in good yields. CuX2 plays a dual role as the catalyst and halide source, whereas 2,2′-bipyridine acts as both the activation reagent and ligand.

Full text of DOI:10.1021/acs.orglett.0c03855

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Letter
Copper and Zinc Copromoted Bromo(chloro)trifluoromethylation of
Alkenes and Alkynes with Trifluoromethanesulfonic Anhydride
Yao Ouyang, Chao-Lai Tong, Xiu-Hua Xu, and Feng-Ling Qing*
Cite This: Org. Lett. 2021, 23, 346−350
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ABSTRACT: The first copper and zinc copromoted trifluoromethylation using
trifluoromethanesulfonic anhydride (Tf2O) as a trifluoromethylating reagent has been
developed. The reaction of alkenes or alkynes with Tf2O in the presence of CuX2 (X =
Br, Cl), Zn powder, and 2,2′-bipyridine affords bromo(chloro)trifluoromethylated
products in good yields. CuX2 plays a dual role as the catalyst and halide source,
whereas 2,2′-bipyridine acts as both the activation reagent and ligand.
rifluoromethanesulfonic anhydride ((CF3SO2)2O, Tf2O)
is an inexpensive and abundant chemical. Normally, Tf2O
is used as an electrophilic triflating reagent to transfer the
−SO2CF3 group.1 We recently reported an unprecedented use
of Tf2O as a radical CF3 source by merging pyridine activation
and photoredox catalysis for C−H trifluoromethylation of
(hetero)arenes (Scheme 1a) and trifluoromethyltriflation of
Over the past decade, the copper-catalyzed radical
trifluoromethylation of unsaturated compounds with Togni
reagents6,7 or the Umemoto reagent8 has been extensively
investigated. The SET reduction of Togni reagents or the
Umemoto reagent with Cu(I) catalyst gave CF3 radical. As the
reduction potentials of N-triflylpyridinium salts2 are close to
those of Togni reagent II9 (Figure 1), we envisioned that the
T
Scheme 1. Trifluoromethylation with Tf2O under Pyridine
Activation
Figure 1. Reduction potentials of trifluoromethylating reagents.
copper-catalyzed radical trifluoromethylation with N-triflylpyr-
idinium salts would be feasible. Herein we disclose the first
copper and zinc copromoted trifluoromethylation of alkenes
and alkynes with Tf2O for the preparation of bromo(chloro)-
trifluoromethylated products (Scheme 1d).
We began our investigation of bromotrifluoromethylation
with the model substrate but-3-enylbenzene (1a).8b,10 The
reaction of 1a with Tf2O in the presence of different copper
catalysts, pyridine derivatives, and bromide sources was
examined (Table 1). To our disappointment, none of the
alkynes (Scheme 1b).2 In those reactions, the trifluoromethyl
radical was formed by single electron transfer (SET) reduction
of the in situ-generated N-triflylpyridinium salt3 with the
excited-state photocatalyst.4 Very recently, Hong and co-
workers described the introduction of both CF3 and pyridyl
groups into olefins using N-triflylpyridinium salts as modular
bifunctional reagents under photocatalysis (Scheme 1c).5
These works demonstrated that Tf2O is an attractive
alternative trifluoromethyl radical reagent through a combina-
tion of pyridine activation and photoredox catalysis.
Received: November 22, 2020
Published: December 28, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03855
© 2020 American Chemical Society
Org. Lett. 2021, 23, 346−350
346

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Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope of Bromotrifluoromethylation of
Unactivated Alkenes
a
Cu
catalyst
yield
b
entry
pyridine derivative Br source additive
(%)
1
2
3
4
5
6
7
8
9
CuI
CuBr
pyridine
pyridine
pyridine
pyridine
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
n-Bu4NBr
NaBr
NaBr
NaBr
NaBr
―
―
−
−
−
−
−
−
−
−
0
0
0
5
trace
trace
16
0
CuOTf
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
2-fluoropyridine
4-phenylpyridine
2,2′-bipyridine
f
phen
2,2′-bipyridine
2,2′-bipyridine
2,2′-bipyridine
2,2′-bipyridine
2,2′-bipyridine
2,2′-bipyridine
2,2′-bipyridine
−
6
10
11
Zn
Fe
Zn
Zn
Zn
Zn
34
18
60
62
55
78
c
12
d
13
14
c
c e
,
15
a
Reaction conditions: 1a (0.2 mmol), Tf2O (0.6 mmol), Br source
(0.4 mmol), Cu catalyst (0.02 mmol), pyridine derivative (0.6 mmol),
b
additive (0.2 mmol), DCE (2.0 mL), N2, rt, overnight. Yields were
determined by 19F NMR spectroscopy using trifluoromethylbenzene
c
d
as an internal standard. CuBr2 (0.1 mmol). CuBr2 (0.16 mmol).
e
f
Tf2O (0.8 mmol), 2,2′-bipyridine (0.8 mmol). phen = 1,10-
phenanthroline.
desired product 2a was detected in the presence of Cu(I)
catalysts, including CuI, CuBr, and CuOTf (entries 1−3). 2a
was formed in 5% yield when CuBr2 was used as the catalyst
(entry 4). The screening of a variety of pyridine derivatives
indicated that 2,2′-bipyridine was optimal, giving 2a in 16%
yield (entries 5−8). We reasoned that 2,2′-bipyridine probably
was used as both the activating reagent of Tf2O and the ligand
to coordinate to CuBr2. The reaction efficiency was not
improved when n-Bu4NBr instead of NaBr was used as
bromide source (entry 9). Subsequently, the yield of 2a was
increased in the presence of Zn or Fe powder as a reductant
(entries 10 and 11), with Zn leading to a higher yield. The
yield was improved to 60% by increasing the amount of CuBr2
to 0.5 equiv (entry 12). However, a further increase in
equivalents of CuBr2 had little effect on the yield (entry 13).
Notably, even without NaBr, 2a was formed in 55% yield
(entry 14). This result showed that CuBr2 played a dual role as
the catalyst and bromide source. Lastly, the yield of 2a was
improved to 78% when larger amounts of Tf2O and 2,2′-
bipyridine were employed (entry 15).
Having optimized the reaction conditions, we then explored
the substrate scope of this copper and zinc copromoted
bromotrifluoromethylation reaction (Scheme 2). A variety of
unactivated alkenes (1a−o) underwent this reaction smoothly
to give the bromotrifluoromethylated products (2a−o) in
moderate to excellent yields. The reaction of 1a was easily
scaled up to 1.0 mmol. The mild conditions tolerated a range
of functional groups, including ether, ester, amide, sulfonate,
sulfamide, chloro, bromo, and nitro. 1,1-Disubstituted alkene
1m was efficiently converted to the desired product 2m in
good yield. However, internal alkenes were not suitable
substrates. Alkenes derived from biologically relevant com-
a
Reaction conditions: 1 (0.4 mmol), Tf2O (1.6 mmol), 2,2′-
bipyridine (1.6 mmol), CuBr2 (0.2 mmol), Zn (0.4 mmol), DCE
(4.0 mL), N2, rt, overnight. Isolated yields are shown. The reaction
was performed on a 1.0 mmol scale.
b
pounds including, dicamba (1n) and estrone (1o), were also
compatible with the reaction conditions. In the case of
substrate 1o containing a cyclic ketone, electrophilic triflation
also took place to form enol triflate 2o.
To expand the scope of the method, styrenes and alkynes
were tested under the standard reaction conditions (Table 1,
entry 15). Styrenes 1p−t bearing different substituents on the
benzene ring furnished the corresponding products 2p−t in
moderate yields (Scheme 3). Bromotrifluoromethylation of
heterocyclic substrates such as 2-vinylthiophene and 2-
vinylpyridine failed to provide any of the desired products.
Furthermore, terminal and internal alkynes 3a−d were
smoothly converted to substituted CF3-containing alkenes
4a−d with excellent regio- and stereoselectivities (Scheme 4).
Subsequently, this copper and zinc copromoted protocol
was successfully extended to chlorotrifluoromethylation
reactions11 by replacing CuBr2 with CuCl2 and n-Bu4NCl
(for details, see the Supporting Information). As shown in
Scheme 5, alkenes 1a−o underwent this reaction smoothly to
give the chlorotrifluoromethylated products 5a−o in good
yields. Unfortunately, the analogous fluoro- and iodotrifluor-
omethylation reactions were unsuccessful.
Mechanistic experiments were then performed to gain
insight into the reaction mechanism. The reaction of 1a was
completely blocked when 2,2,6,6-tetramethylpiperidin-1-oxyl
(TEMPO) was added to the standard conditions, whereas the
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Scheme 3. Substrate Scope of Bromotrifluoromethylation of
Styrenes
Scheme 6. Mechanistic Experiments
a
a
Reaction conditions: 1 (0.4 mmol), Tf2O (1.6 mmol), 2,2′-
bipyridine (1.6 mmol), CuBr2 (0.2 mmol), Zn (0.4 mmol), DCE
(4.0 mL), N2, rt, overnight. Isolated yields are shown.
(Scheme 6b,c). The reaction proceeded when n-Bu4NBr was
used instead of CuBr2 in the absence of Cu salt, affording 2a in
58% yield along with the formation of dibrominated byproduct
6 (Scheme 6d). This result revealed that the Zn promoted
bromotrifluoromethylation reaction was feasible but less
efficient than the copper and zinc copromoted bromotri-
fluoromethylation reaction (Table 1, entry 15).
Scheme 4. Substrate Scope of Bromotrifluoromethylation of
Alkynes
a
On the basis of the above results and previous
reports,2,5−8,10,11 a plausible mechanism for this reaction is
proposed in Scheme 7. Initially, the reaction of Tf2O and 2,2′-
Scheme 7. Proposed Reaction Mechanism
a
Reaction conditions: 3 (0.4 mmol), Tf2O (1.6 mmol), 2,2′-
bipyridine (1.6 mmol), CuBr2 (0.2 mmol), Zn (0.4 mmol), DCE
b
(4.0 mL), N2, rt, overnight. Isolated yields are shown. The Z/E ratio
was determined by 19F NMR spectroscopy of the crude reaction
mixture.
Scheme 5. Substrate Scope of Chlorotrifluoromethylation of
a
Alkenes
bipyridine generates the corresponding N-triflylpyridinium salt
A. Then SET reduction of A with Zn powder affords radical
species B, which rapidly collapses to release 2,2′-bipyridine,
SO2, and CF3 radical. On the other hand, LCuX2 (L = 2,2′-
bipyridine, X = Br or Cl) is reduced by Zn powder or other
factors12 to deliver LCuX, which might also act as a reductant
and then enable the conversion of N-triflylpyridinium salt A to
radical species B. Subsequently, the addition of CF3 radical to
the unsaturated substrate (1 or 3) gives β-trifluoromethyl
carbon radical C. There might be two possible pathways for
the conversion of C to the final bromo(chloro)-
trifluoromethylated products (2 or 4 or 5). First, reaction of
LCuX2 and intermediate C could furnish Cu(III) intermediate
D, which would undergo reductive elimination to afford the
desired products and regenerate LCuX. The alternative way
would involve oxidation of intermediate C by N-triflylpyr-
a
Reaction conditions: 1 (0.4 mmol), Tf2O (1.6 mmol), 2,2′-
bipyridine (1.6 mmol), CuCl2 (0.2 mmol), Zn (0.4 mmol), n-
Bu4NCl (0.8 mmol), DCE (4.0 mL), N2, rt, overnight. Isolated yields
are shown.
TEMPO−CF3 adduct was formed in low yield (Scheme 6a).
These results indicated that CF3 radical is involved in this
reaction. Furthermore, the control experiments indicated that
both of activator (2,2′-bipyridine) and reductant (Zn powder)
are crucial for the bromotrifluoromethylation reaction
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Letter
idinium salt A13 or LCuX2 to give carbocation intermediate E
followed by nucleophilic attack by bromide or chloride anion.
In conclusion, we have developed a unique reaction system
to realize the copper and zinc copromoted trifluoromethylation
using low-cost and abundant Tf2O as the trifluoromethylating
reagent. The combination of CuX2 (X = Br, Cl), Zn powder,
and 2,2′-bipyridine enables various unactivated alkenes,
styrenes, and alkynes to be transformed to the halotrifluor-
omethylated products.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03855.
Experimental procedures, characterization data, and
1
copies of H, 19F, and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Feng-Ling Qing − Key Laboratory of Organofluorine
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy of Sciences, Chinese Academy of Sciences,
Shanghai 200032, China; orcid.org/0000-0002-7082-
756X; Email: flq@mail.sioc.ac.cn
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as Radical Reservoirs in Organic Synthesis. ACS Catal. 2019, 9,
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G.; Carreira, E. M.; Togni, A. Pyridinium Salts as Redox-Active
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Authors
Yao Ouyang − Key Laboratory of Organofluorine Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai
Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, Shanghai
200032, China
Chao-Lai Tong − Key Laboratory of Organofluorine
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy of Sciences, Chinese Academy of Sciences,
Shanghai 200032, China
Xiu-Hua Xu − Key Laboratory of Organofluorine Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai
Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, Shanghai
200032, China; orcid.org/0000-0002-0759-2286
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03855
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The National Natural Science Foundation of China (21421002
and 21991211) and the Strategic Priority Research Program of
the Chinese Academy of Sciences (XDB20000000) are
gratefully acknowledged for funding this work.
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