Bromotrifluoromethane: A Useful Reagent for Hydrotrifluoromethylation of Alkenes and Alkynes

Article abstract of DOI:10.1055/s-0036-1591944

Bromotrifluoromethane (CF ₃ Br) is a simple, inexpensive and abundant industrial material employed as a trifluoromethylating reagent. However, only limited strategies using CF ₃ Br as a fluorine source are reported. Herein, we describe a visible-light-induced hydrotrifluoromethylation of alkenes and alkynes with CF ₃ Br. The reaction proceeds under mild conditions with good functional group tolerance, providing a new route for the application of BrCF ₃ in organic synthesis.

Full text of DOI:10.1055/s-0036-1591944

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© Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
letter
A
de
Y.-Y. Ren et al.
Letter
Synlett
Bromotrifluoromethane: A Useful Reagent for Hydrotrifluoro-
methylation of Alkenes and Alkynes
Yu-Yan Rena
Blue LED
Xing Zheng*a
Xingang Zhang*b
H
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
0
0
0
0
0-
0
0
2
4-
4
0
6
6-
5
3
3
CF3
Alkyl
CF3Br
Alkyl
+
THF, r.t., 30 h
a Institute of Pharmacy and Pharmacology, Hunan Province Cooperative
Innovation Center for Molecular Target New Drug Study, University of
South China, 28 Western Changsheng Road, Hengyang, Hunan 421001,
P. R. of China
15 examples
yield 40–80%
b Key Laboratory of Organofluorine Chemistry, Center for Excellence in
Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 Lingling Road, Shanghai, 200032,
P. R. of China
Zhengxing5018@yahoo.com
xgzhang@mail.sioc.ac.cn
Received: 21.12.2017
Accepted after revision: 29.01.2018
Published online: 07.03.2018
arenes. Alternatively, the direct introduction of CF3 into
(hetero)arenes with CF3Br based on a sulfinatodehalogena-
tion reaction8 or through transition-metal-induced single-
electron transfer (SET)9 have emerged as more straightfor-
ward strategies. However, to the best of our knowledge, the
reaction of CF3Br with alkenes and alkynes has not been re-
ported.
DOI: 10.1055/s-0036-1591944; Art ID: st-2017-w0915-l
Abstract Bromotrifluoromethane (CF3Br) is a simple, inexpensive and
abundant industrial material employed as a trifluoromethylating re-
agent. However, only limited strategies using CF3Br as a fluorine source
are reported. Herein, we describe a visible-light-induced hydrotrifluoro-
methylation of alkenes and alkynes with CF3Br. The reaction proceeds
under mild conditions with good functional group tolerance, providing
a new route for the application of BrCF3 in organic synthesis.
Given the importance of trifluoromethylated alkanes
and alkenes in life and materials science, we envisioned the
feasibility of hydrotrifluoromethylation of alkenes and
alkynes with CF3Br. Although such a transformation with a
variety of trifluoromethylating reagents including TMSCF3,
CF3SO2Na, Umemoto’s reagent, Togni’s reagent, etc., has
been reported,10 most of the reagents used are expensive
and require additional step(s) to prepare from CF3Br.3,6a,11
Therefore, from the point of view of step-economy and
cost-efficiency, the use of CF3Br as a trifluoromethylating
reagent to access trifluoromethylated alkanes and alkenes
would be a promising alternative. To continue our interest
in catalytic fluoroalkylations with inexpensive and readily
available fluoroalkyl halides,12 herein, we report the first
example of visible-light-induced hydrotrifluoromethyla-
tion of alkenes and alkynes with CF3Br. The reaction pro-
ceeds under mild conditions with good functional group
compatibility.
We began our initial studies with the reaction of but-3-
en-1-yl benzoate (1a) (0.3 mmol, 1.0 equiv) and CF3Br (2)
(1.0 atm) under irradiation with a blue light-emitting diode
(LED) (12 W, λ = 425 nm) in the presence of the photoredox
catalyst fac-Ir(ppy)3 (0.5 mol%) and a base in DMF at room
temperature (Table 1). Initially, a 46% yield of hydrotrifluo-
romethylated product 3a accompanied with bromotrifluo-
romethylated by-product 4a (10%) and hydrodebrominated
CF3H (5) (15%) were observed when 1 equivalent of K2CO3
was used as the base (Table 1, entry 1). After a survey of dif-
ferent bases, we found that K2HPO4 provided a comparable
Key words bromotrifluoromethane, hydrotrifluoromethylation, alkenes,
alkynes, visible-light-induced
The increasing importance of trifluoromethylated com-
pounds in pharmaceuticals, agrochemicals and advanced
functional materials has triggered significant endeavors in
developing general and efficient methods for incorporation
of a trifluoromethyl group (CF3) into organic molecules.
Over the past decades, impressive achievements have been
made in this area.1 However, developing efficient and
straightforward trifluoromethylation methods with an in-
expensive and abundant trifluoromethylating reagent re-
mains highly desirable. Bromotrifluoromethane (CF3Br) is a
simple, abundant and inexpensive industrial material pre-
viously used in fire extinguishers,2 and represents a
straightforward fluorine source for trifluoromethylations.3
However, only limited strategies for the transformation of
CF3Br into trifluoromethylated compounds have been re-
ported thus far. One common strategy for the transforma-
tion of CF3Br is via its conversion into a nucleophile with
Zn,4 Al5 or P(NEt2)3,6 followed by reaction with electro-
philes, such as aldehydes, ketones, TMSCl, etc. The electro-
chemical trifluoromethylation of (hetero)aryl halides with
CF3Br is another strategy to transform CF3Br,7 providing an
efficient approach to access trifluoromethylated (hetero)-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

B
Y.-Y. Ren et al.
Letter
Synlett
Table 1 Representative Results for the Optimization of the Visible-Light-Induced Hydrotrifluoromethylation of Alkene 1a with CF3Bra
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3
O
O
CF3H
+
CF3Br
+
CF3
solvent, r.t., 30 h
O
O
X
1a
2
3a, X = H
4a, X = Br
5
Entry
Base (equiv)
Solvent (mL)
Yield (%)b 3a/4a/5
1
2
K2CO3 (1)
Na2CO3 (1)
Cs2CO3 (1)
K3PO4 (1)
K2HPO4 (1)
–
DMF (2 mL)
DMF (2 mL)
DMF (2 mL)
DMF (2 mL)
DMF (2 mL)
DMF (2 mL)
DMF (2 mL)
DMF (2 mL)
THF (2 mL)
46/10/15
35/31/17
24/33/17
27/8/8
3
4
5
46/20/18
trace/29/4
46/trace/11
52/5/27
6
7
K2CO3 (2)
K2CO3 (3)
K2CO3 (3)
K2CO3 (3)
8
9
65/6/51
10
THF (1.5 mL)
70 (63)/4/82
a Reactions were carried out using 1a (0.3 mmol, 1.0 equiv), CF3Br (1 atm), fac-Ir(ppy)3 (0.5 mol%), blue LED (12 W), r.t., 30 h.
b Determined by 19F NMR using fluorobenzene as an internal standard; the number in parentheses is the isolated yield. The yield of 5 was determined based on
1a.
yield of 3a, but led to 4a in a 20% yield (entry 5). The other
bases tested, Na2CO3, Cs2CO3 and K3PO4, were all less effec-
tive (entries 2–4). Furthermore, the absence of base led to
only a trace amount of 3a. However, the role of K2CO3 in the
current reaction remains elusive at this stage and will be
addressed in the future. Further investigations of a series of
reaction parameters, such as the amount of base, the type
of photoredox catalyst and the solvent, to improve the yield
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
CF3
+
CF3Br
R
R
THF, r.t., 30 h
1
2
3
O
O
O
CF3
CF3
CF3
O
O
O
MeO
F
Br
3a, 63%
3b, 60%
3c, 60%
O
O
F
O
CF3
CF3
CF3
O
3d, 59%
O
O
O
F
F
NC
3e, 52%
3f, 53%
O
H
N
CF3
CF3
CF3
O
O
AcHN
NHBoc
3g, 41%
3h, 40%
3i, 65%
O
O
CF3
CF3
S
CF3
O
BocN
N
H
N
3j, 60%
3k, 40%
3l, 76%
Scheme 1 Substrate scope of the visible-light-induced hydrotrifluoromethylation of alkenes with CF3Br. Reaction conditions: 1 (0.3 mmol, 1.0 equiv),
CF3Br (1 atm), fac-Ir(ppy)3 (0.5 mol%), K2CO3 (3.0 equiv), THF (1.5 mL), blue LED (12 W), r.t., 30 h.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

C
Y.-Y. Ren et al.
Letter
Synlett
of 3a showed that increasing the loading amount of K2CO3
to 3 equivalents increased the yield of 3a to 52% (entry 8).
Importantly, the formation of 4a could be significantly sup-
pressed under these conditions. Among all the tested iridi-
um catalysts, fac-Ir(ppy)3 was the optimum choice (for de-
tails, see the Supporting Information). The reaction was
also sensitive to the solvent and the use of THF resulted in
the formation of 3a in 65% yield (entry 9) (for details, see
the Supporting Information). We believe that THF probably
serves as a good hydrogen atom source in the current reac-
tion. Finally, the optimized reaction conditions were identi-
fied by increasing the reaction concentration, providing 3a
in an isolated yield of 63% (entry 11).
With optimized reaction conditions in hand, a range of
alkenes was examined (Scheme 1). Generally, moderate to
good yields of products 3 were obtained. The reaction ex-
hibited good functional group tolerance, including ester,
amide, cyano and bromide (3a–h), offering good opportu-
nities for further transformations. Importantly, the amino
acid based substrate 1i was also a suitable substrate, thus
providing a potential application in chemical biology.
Alkenes bearing heteroaryl, such as pyridyl and thienyl,
were also applicable to the reaction and produced the cor-
responding products 3j and 3k in moderate to good yields.
Notably, higher reactivity was observed when the external
cyclic alkene 1l was utilized.
alkyne bearing a sterically bulky group favored formation
of the E-trifluoromethylated alkene (Scheme 2, c), probably
due to steric effects.
To gain some mechanistic insights into the reaction,
several experiments were conducted. As shown in Table 2,
the results of radical inhibition experiments suggest that a
trifluoromethyl radical may be involved in the reaction.
When an electron transfer scavenger (1,4-dinitrobenzene)
or a radical inhibitor (hydroquinone, 1 equiv) was added to
the reaction of 1a with CF3Br under standard reaction con-
ditions,13 the formation of 3a was totally suppressed.
The radical clock experiment further confirmed the ex-
istence of a trifluoromethyl radical in the reaction (Scheme
3, a). When the diene compound 8 was subjected to CF3Br
under the standard reaction conditions, cyclized compound
9 (45% yield) was obtained, demonstrating that a radical
pathway was involved in the reaction. To probe whether the
hydrogen was derived from the solvent, deuterated tetrahy-
drofuran-d8 (THF-d8) was used as the reaction medium
(Scheme 3, b). We found that the deuterated product 10
was formed when the reaction of 1a with CF3Br was per-
formed in THF-d8 under standard reaction conditions, thus
demonstrating that the solvent can serve as a hydrogen
atom source.
On the basis of above results and previous reports,14
a
plausible reaction mechanism can be proposed (Scheme 4).
Firstly, the photocatalyst fac-IrIII(ppy)3 A was excited to fac-
IrIII(ppy)3* B under irradiation by the blue LED, and subse-
quent single-electron transfer from B to CF3Br generated a
trifluoromethyl radical and fac-IrIV(ppy)3 C. The resulting
trifluoromethyl radical reacted with the alkene or alkyne to
deliver a new alkyl or vinyl radical, which then abstracted a
hydrogen atom from the solvent, providing the hydrotriflu-
The reaction can also be extended to alkynes. However,
only a 32% yield of 7a in a mixture of E/Z isomers (E/Z =
4.3:1) was obtained under the standard reaction condi-
tions. Switching the solvent from THF to DMF significantly
improved the yield to 65% with an E/Z ratio of 3.1:1
(Scheme 2, a). Good functional group tolerance was also ob-
served as demonstrated by the syntheses of 7b and 7c
(Schemes 2, b and c). Furthermore, we found that the
blue LED
fac-Ir(ppy)3 (0.5 mol%)
O
O
K2CO3 (3.0 equiv)
+
N
(a)
CF3Br
N
DMF, r.t., 30 h
CF3
O
O
6a
2
7a
65% (E/Z = 3.1:1)
(0.3 mmol, 1.0 equiv)
(1 atm)
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
O
O
O
O
(b)
+
CF3Br
DMF, r.t., 30 h
CF3
NC
NC
6b
2
7b
(0.3 mmol, 1.0 equiv)
(1 atm)
55% (E/Z = 5:1)
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
O
O
Cl
Cl
N
H
N
H
CF3
+
(c)
CF3Br
DMF, r.t., 30 h
Cl
Cl
6c
2
7c
(0.3 mmol, 1.0 equiv)
(1 atm)
80% (E/Z = 8:1)
Scheme 2 Visible-light-induced hydrotrifluoromethylation of alkyne 6 with CF3Br
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

D
Y.-Y. Ren et al.
Letter
Synlett
Table 2 Radical Inhibition Experiments
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
O
O
+
CF3Br
CF3
additive
THF, r.t., 30 h
O
O
1a
2
3a
Entry
Additive (equiv)
none
Yield (%)b of 3a
1
2
3
4
70 (63)
1,4-dinitrobenzene (0.2)
hydroquinone (0.2)
hydroquinone (1)
0
11
0
a Reactions were carried out using 1a (0.3 mmol, 1.0 equiv), CF3Br (1 atm).
b Determined by 19F NMR using fluorobenzene as an internal standard; the number in parentheses is the isolated yield.
(a) Radical clock experiment
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
CF3
+
CF3Br
THF, r.t., 30 h
N
N
Ts
Ts
8
2
9, 45%
(b) Deuteration experiment
blue LED
fac-Ir(ppy)3 (0.5 mol%)
K2CO3 (3.0 equiv)
O
O
+
CF3Br
CF3
THF-d8
r.t., 30 h
O
O
D
1a
2
10, 45%
Scheme 3 Mechanistic studies
oromethylated product and the solvent-derived radical. Fi-
nally, the solvent-derived radical was oxidized by C to re-
generate A and other solvent-derived by-products.
In conclusion, we have developed the first example of
the visible-light-induced hydrotrifluoromethylation of
alkenes and alkynes with CF3Br.15,16 The reaction proceeds
under mild conditions with good functional group compat-
ibility, providing a new route for the application of BrCF3 in
organic synthesis. Preliminary mechanistic studies revealed
that a trifluoromethyl radical is involved and that the sol-
vent can serve as a hydrogen source for the reaction.
fac-IrIII(ppy)3*
CF3Br
B
Blue LED
SET
photoredox
catalysis
CF3
Funding Information
fac-IrIV(ppy)3
fac-IrIII(ppy)3
This work was financially supported by the National Natural Science
Foundation of China (21425208, 21672238, 21332010, and 2142002),
the Strategic Priority Research Program of the Chinese Academy of
Sciences (No. XDB20000000), the Key Project of Hunan Province Sci-
ence and Technology Department (2016DK2001), the Key Project of
Hengyang Science and Technology Department (2017KJ166) and
the Shanghai Institute of Organic Chemistry, Chinese Academy of Sci-
R
R
C
A
or
solvent
base
SET
solvent derivatives
CF3
CF3
or
R
R
or
R
R
CF3
CF3
ences.
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oaitn
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l
N
a
utra
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Secince
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d
oaitn
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n
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oaitn
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i
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n
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oaitn
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h
n
i
a
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0
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utarl
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ecin
c
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o
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n
d
oaitn
of
C
h
n
i
a
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1
4
2
0
0
2)
Scheme 4 Proposed mechanism
Acknowledgment
We thank Mr. Zhi-Ji Luo for helpful discussions.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

E
Y.-Y. Ren et al.
Letter
Synlett
Supporting Information
2505. (d) Yu, P.; Zheng, S.-C.; Yang, N.-Y.; Tan, B.; Liu, X.-Y.
Angew. Chem. Int. Ed. 2015, 54, 4041. (e) Cheng, Y.; Yu, S. Org.
Lett. 2016, 18, 2962.
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1591944.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
(11) (a) Tordeux, M.; Langlois, B.; Wakselman, C. J. Org. Chem. 1989,
54, 2452. (b) Naumann, D.; Tyrra, W.; Kock, B.; Rudolph, W.;
Wilkes, B. J. Fluorine Chem. 1994, 67, 91. (c) Paratian, J. M.;
Labbé, E.; Sibille, S.; Nédélec, J. Y.; Périchon, J. J. Organomet.
Chem. 1995, 487, 61. (d) Nagasaki, N.; Morikuni, Y.; Kawada, K.;
Arai, S. Catal. Today 2004, 88, 121.
(12) (a) Feng, Z.; Chen, F.; Zhang, X. Org. Lett. 2012, 14, 1938.
(b) Min, Q.-Q.; Yin, Z.; Feng, Z.; Guo, W.-H.; Zhang, X. J. Am.
Chem. Soc. 2014, 136, 1230. (c) Feng, Z.; Min, Q.-Q.; Xiao, Y.-L.;
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(15) Hydrotrifluoromethylation with CF3Br;Typical Procedure
To a 25 mL Schlenk tube were added fac-Ir(ppy)3 (0.5 mol%) and
K2CO3 (126 mg, 0.9 mmol, 3.0 equiv). The mixture was then
evacuated and backfilled with argon (3 times). The mixture was
evacuated again and backfilled with CF3Br (1 atm), followed by
the addition of alkene 1b (0.3 mmol, 1.0 equiv) and anhydrous
THF (1.5 mL). The tube was screw-capped and irradiated with a
blue LED (12 W) at room temperature. After stirring for 30 h,
the reaction mixture was diluted with EtOAc (2 mL) and filtered
through a pad of Celite. The filtrate was concentrated, and the
residue was purified by silica gel chromatography (hex-
ane/EtOAc, 20:1) to give product 3b (50 mg, 60% yield) as a col-
orless oil.
References and Notes
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1H NMR (400 MHz, CDCl3): δ = 7.98 (d, J = 8.3 Hz, 2 H), 6.91 (d,
J = 8.4 Hz, 2 H), 4.29 (t, J = 5.9 Hz, 2 H), 3.83 (s, 3 H), 2.23–2.06
(m, 2 H), 1.86–1.76 (m, 2 H), 1.76–1.65 (m, 2 H); 13C NMR (101
MHz, CDCl3): δ = 166.2, 163.4, 131.5, 127.0 (q, J = 276.4 Hz),
122.5, 113.6, 63.8, 55.4, 33.4 (q, J = 28.7 Hz), 27.9, 18.8 (q, J = 3.0
Hz); 19F NMR (376 MHz, CDCl3): δ = –66.5 (t, J = 10.8 Hz, 3 F);
MS (EI): m/z (%) = 276 [M]+, 135 (100); HRMS (EI): m/z [M]+
calcd for C13H15O3F3: 276.0973; found: 276.0978.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E