Copper-Catalyzed Three-Component Azidotrifluoromethylation/Difunctionalization of Alkenes

Article abstract of DOI:10.1002/cjoc.201400167

A novel three-component strategy for the azidotrifluoromethylation of alkenes has been presented here. The reaction proceeded smoothly under gentle temperature and gave the bifunctional olefins in high yields. Furthermore, 1,3-dipolar reactions between az

Full text of DOI:10.1002/cjoc.201400167

COMMUNICATION
DOI: 10.1002/cjoc.201400167
Copper-Catalyzed Three-Component Azidotrifluoromethyl-
ation/Difunctionalization of Alkenes
Mingbo Yang,*,a Wenlu Wang,b Yin Liu,a Lijie Feng,a and Xuexia Jua
a Gansu Research Institute of Chemical Industry, Lanzhou, Gansu 730020, China
b Gansu Province Environmental Monitoring Center, Lanzhou, Gansu 730020, China
A novel three-component strategy for the azidotrifluoromethylation of alkenes has been presented here. The re-
action proceeded smoothly under gentle temperature and gave the bifunctional olefins in high yields. Furthermore,
1,3-dipolar reactions between azide-containing products and phenylacetylene revealed great potential in molecular
modification by using this method.
Keywords azidotrifluoromethylation, alkenes, azide-containing
Introduction
of alkenes to construct two vicinal chemical bonds un-
der copper catalysis (Scheme 1).
Transition-metal-catalyzed alkene difunctionaliza-
tion has been proved to be a powerful strategy for con-
structing carbon-carbon/carbon-heteroatom bonds in a
single step and has been widely used for the preparation
of functionalized organic compounds.[1] Compounds
containing a trifluoromethyl group, especially heterocy-
cles played an important role as pharmaceuticals, agro-
chemicals and functional materials due to their signifi-
cantly increased lipophilicity, metabolic stability, mem-
brane permeability and bioavailability.[2,3] Thus, much
attention has been given to the introduction of trifluoro-
methyl group into organic molecules to improve their
properties.[4,5] Indeed, a series of transition-metal-cata-
lyzed difunctionalization of alkenes reactions have been
reported, in particular reactions involving trifluoro-
methyl group, such as halotrifluoromethylation,[6] carbo-
trifluoromethylation,[7] oxytrifluoromethylation,[8] hy-
drotrifluoromethylation,[9] and aminotrifluoromethyl-
ation.[10] However, many traditional approaches to the
generation of these CF3-containing compounds based on
the difunctionalization strategy were still limited to in-
tramolecular process[11] or required photoredox catalytic
system.[12] Seeking transition-metal-catalyzed multi-
component reactions is a highly attractive and sustain-
able approach for introducing the trifluoromethyl func-
tionality in synthetic chemistry. To meet this demand,
Loh and Qing groups,[13] independently, reported inter-
molecular Cu-catalyzed oxytrifluoromethylation of al-
kenes by using Togni’s reagent and sodium trifluoro-
methane sulfinate. In 2013, Szabó and Liang group[14]
reported the three-component cyanotrifluoromethylation
Scheme 1 Intermolecular difunctionalization of alkenes
H
R2
CF3
R1
R3
NHR
R3
CN
R2
R1
R2
R1
CF3
CF3
C
u
d
c
R3
p
h
o
t
-
e
z
a
y
t
l
a
t
a
o
a
l
y
r
e
d
e
t
z
d
c
e
o
-
d
a
i
x
u
C
R2
e
-
d
c
a
t
a
m
-
l
r
y
z
o
e
d
R1
d
R3
C
u
p
-
t
c
h
h
a
e
o
g
i
T
z
t
t
a
l
o
y
E
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e
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t
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b
z
e
i
a
P
s
i
c
o
-
d
OR
-
O
v
x
x
-
N
o
c
a
R2
R1
d
a
e
t
r
M
a
CF3
o
t
l
y
z
e
X
o
d
h
e
d
R2
R1
i
a
p
t
e
R3
CF3
d
R3
Nitrogen-containing compounds, which were found
in many bioactive chemicals and drug molecules, have
been also studied extensively, and numerous synthetic
methods have been developed to construct carbon-ni-
trogen bonds.[15] As one of the most important nitrogen
sources, organic azides were widely applied in synthetic
chemistry.[16] For instance, they could be reduced to
amines under various reducing agents. The click chem-
istry, in particular the copper-catalyzed azide-alkyne [3
＋2] cycloaddition (CuAAC) reaction has attracted in-
terests of many chemists. On account of our research in
alkene functionalization, we envisioned that the addition
*
E-mail: yangmb05@163.com
Received March 20, 2014; accepted June 13, 2014; published online XXXX, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400167 or from the author.
Chin. J. Chem. 2014, XX, 1—5
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1

Yang et al.
COMMUNICATION
of azide group and trifluoromethyl group across a dou-
ble bond could be achieved in one step. Herein, we re-
ported a novel three-component azidotrifluoromethyla-
tion of alkenes with Togni’s reagent (2)[17] and trime-
thylsilyl azide (TMSN3) by copper catalysis under mild
conditions (Eq. 1). This methodology allows a wide
range of substrate scope to access azidotrifluoromethy-
lation products. It should be indicated that, when we
finished our work, Liu and co-workers[18] reported the
trifluoromethylazidation of alkenes.
come (Table 1, Entry 13). We also tested NaN3 as the
azide source and the reaction proceeded smoothly as
well, leading to the desired azidotrifluoromethylation
product in 74% yield. The reaction did not occur with-
out copper catalyst (Table 1, Entry 15). Therefore,
heating the reactants at 40 ℃ for 1.5 h and using 10
mol% CuBr as the catalyst under argon atmosphere
were chosen as the optimized reaction conditions.[19]
Table 1 Screening of reaction conditionsa
CF3
CF3
N3
R2
N3
F3C
O
CuBr (10 mol%) R2
I
O
I
CF3
(1)
R1
O
catalyst
TMSN3
+
R1
+
+
+
TMSN3
CH3CN, Argon
40 oC, 1.5 h
R3
R3
3
solvent, argon
O
1
2
2
1a
3a
Experimental
Entry
1
Catalyst
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OAc)2
CuF2
Solvent
T/h Yieldb/%
DMF
DMSO
CH3CN
DCE
3.0
3.0
3.0
3.0
Trace
Trace
25
An oven-dried tube was charged with Togni’s re-
agent 2a (75.8 mg, 0.24 mmol) and CuBr (2.86 mg, 0.02
mmol). The tube was evacuated and backfilled with ar-
gon (repeated three times). Then, TMSN3 (0.5 mmol)
and alkenes (0.2 mmol) dissolved in CH3CN (2 mL)
were added into the tube. The reaction mixture was
stirred at 40 ℃ for 1.5 h and extracted with DCM. The
combined corganic layers were washed with saturated
brine, dried over Na2SO4, concentrated in vacuum (Note:
the control of temperature and pressure is very impor-
tant) and purified by flash column chromatography (sil-
ica gel) to afford the product.
Azide 3 (0.1 mmol) and phenylacetylene (0.25 mmol)
was suspended in a mixture of t-BuOH (0.5 mL) and
water (0.5 mL) in a Schlenk tube with a magnetic bar.
Then CuSO4 (0.01 mmol) and sodium ascorbate (0.01
mmol) were added to the mixture. The resulting mate-
rial was stirred for 4 h at 70 ℃ before it was diluted
with EtOAc. Water (10 mL) was added and the mixture
was extracted with EtOAc. The organic layer was com-
bined and washed with brine (10 mL) and dried over
anhydrous MgSO4. The solvent was removed and the
residue was purified by column chromatography to give
4.
2
3
4
Trace
Trace
75
5
1,4-Dioxane 3.0
6
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
3.0
3.0
3.0
3.0
3.0
3.0
2.0
1.5
1.5
1.5
7
76
8
Cu(MeCN)4PF6
CuCl
71
9
84
10
11
12
13
14c
15d
CuBr
86
CuI
78
CuBr
85
CuBr
CuBr
86
74
－
NR
a
Reaction conditions: 1a (0.2 mmol), Togni’s reagent (0.24
mmol), TMSN3 (0.5 mmol), copper catalyst, solvent (2.0 mL), 40
℃, under argon. b Isolated yield. NaN3 was used. Without cop-
c
d
per catalyst.
Under the optimal reaction conditions, the scope of
alkenes was investigated. As demonstrated in Table 2, a
series of alkenes bearing both electron-donating and
electron-withdrawing groups at the aromatic rings were
converted into the corresponding difunctionalization
products in good to excellent yields. Naphthalenes
bearing vinyl groups at 2-position furnished the desired
product 3p in 87%. Notably, α-methylstyrene was still
very effective in our catalytic system, which gave the
difunctionalization product in high yield. The hindered
1,2-disubstituted alkene 1r reacted smoothly, gaving a
mixture of diastereoisomers (d.r.＝3.3∶1) in 67%. The
success of azidotrifluoromethylation of alkenes encour-
aged us to further investigate the scope of this reaction.
We found that the linear olefins were also well tolerated,
with the corresponding product obtained in good yields.
Results and Discussion
To probe the feasibility of our envisioned strategy,
we started the investigation by exploring the reaction of
styrene 1a with Togni’s reagent and TMSN3 in the
presence of 10 mol% Cu(OTf)2 at 40 ℃ under argon
conditions. Unfortunately, only trace amount of the de-
sired product 3a was obtained in DMF (Table 1, Entry
1). However, screening of the solvent showed that
CH3CN was effective and the product 3a was obtained
in 25% yield (Table 1, Entry 3). Inspired by this finding,
further screening of different copper catalysts exhibited
that CuBr was the best and increased the yield to 86%
(Table 1, Entry 10). The reaction time could be de-
creased to 1.5 h with no influence on the reaction out-
2
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Copper-Catalyzed Three-Component Azidotrifluoromethylation/Difunctionalization of Alkenes
For simple alkenes 1s and 1t, the desired products 3s
and 3t were afforded in 76% and 77% yields, respec-
Continued
Yield/%
Compound 3
tively. Terminal olefins bearing ester (1v) and benzoate
(1w) groups reacted equally well to give 3v and 3w in
high yields. To verify the synthetic usefulness of this
process, we derivatized some of our azidotrifluorometh-
ylation products in click reaction (Table 3). The cyclo-
addition occurred when the azidotrifluoromethylation
compounds reacted with phenylacetylene in the pres-
ence of CuSO4 (10 mol%) and sodium ascorbate (10
mol%) at 70 ℃ for 4 h, the yields of CF3-containing
1,2,3-triazoles were excellent. In addition, the azido-
trifluoromethylation product 3a was efficiently reduced
to the corresponding amine 5 in the presence of PPh3 or
NaBH4 as the reductant (Scheme 1). All of these illus-
trated the potential applications in discoveries of lead
compounds, synthesis of biologically active compounds,
and other useful CF3-containing heterocycles.
N3
H
CF3
67
H
b
3
r (d.r . = 3.3 : 1)
N3
CF3
N3
76
77
61
86
3s
CF3
3t
O
N3
S
O
CF3
O
3u
Table 2 Substrate scope of the azidotrifluoromethylation of
alkenesa
O
EtO
CF3
Compound 3
3a R＝H
Yield/%
86
N3
3v
3b R＝Me
3c R＝OMe
3d R＝t-Bu
3e R＝OCOMe
3f R＝Ph
O
88
89
O
CF3
N3
81
N3
CF3
91
3w
85
R
a Reaction conditions: 1 (0.2 mmol), Togni’s reagent (0.24 mmol),
TMSN3 (0.5 mmol), CuBr (0.02 mmol), CH3CN (2 mL), 40 ℃,
1.5 h, under argon, isolated yield. b The ratio of diastereomers
was determined by crude 19F NMR. Structure of the major di-
astereomer is shown.
3a 3h
－
83
3g R＝F
82
3h R＝Cl
79
N3
MeO
CF3
Scheme 1 Reduction of the azidotrifluoromethylation products
81
OMe
3i
N3
3j R＝Me
3k R＝OMe
3l R＝Cl
88
90
79
73
cat. (10 mol%)
TMSN3
Togni's reagent
86%
CF3
R
NH2
3j 3m
－
3m R＝Br
N3
CF3
CuSO4 (10 mol%)
CF3
N3
3n R＝Me
3o R＝Cl
84
74
NaBH4, MeOH
0 oC - r.t.
CF3
5
PPh3 (2.5 equiv.)
THF/H2O (V : V = 4 : 1)
60 °C, 4 h
77%
R
3n, 3o
93%
N3
NH2
CF3
CF3
87
80
3p
5
N3
CF3
To elucidate the mechanism of the above reactions, a
stoichiometric amount of 2,2,6,6-tetramethyl-1-piperi-
dinyloxy (TEMPO) was used in the reaction. A signifi-
3q
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Yang et al.
COMMUNICATION
Table 3 Cycloaddition of phenylacetylene to azidotrifluoro-
cant drop in yield (39%) was obtained and the TEMPO-
CF3 adduct was detected in 49% using GC-MS, indicat-
ing that the present reaction included a radical pathway.
On the basis of the above result and previous studies,[20]
the mechanism for the azidotrifluoromethylation proc-
ess is illustrated in Scheme 2. Togni’s reagent 2 was
activated by copper catalysis, leading to the CF3-con-
taining radical species. Then, a single-eletron oxidation
B of a radical intermediate was formed from the radical
intermediate A, which was further trapped by TMSN3 to
led to the difunctionalization product 3a (Path a). The
radical intermediate A reacted with the copper species
and TMSN3 to afford the copper(III) azide complex C.
Subsequent elimination of the copper(III) azide complex
C would afford the desired azidotrifluoromethylation
product 3a (Path b).
methylation productsa
CuSO4 (10 mol%)
sodium ascorbate (10 mol%)
N3
N
R2
R1
N
CF3
N
H2O/t-BuOH (V : V = 1 : 1)
70 oC, 4 h
R2
R1
R3
CF3
3
R3
4
Compound 4
Yield/%
N
N
MeO
N
92
85
85
93
86
CF3
4c
N
N
MeOCO
N
Scheme 2 Proposed reaction mechanism
CF3
CF3
CF3
4e
N
N
A
1a
N
MeO
CF3
path a
- e
4k
TMSN3/CuBr
path b
CF3
N
N
N3 Cu(III)Br
CF3
N
B
Cl
CF3
TMSN3
N3
4l
C
N
N
CF3
N
3a
CuBr
CF3
4n
Conclusions
N
N
In conclusion, we have developed copper-catalyzed
three-component azidotrifluoromethylation of alkenes.
The mild reaction conditions allow a wide range of sub-
strate scope to achieve the difunctionalization. Accom-
panied with the subsequent transformation, such as
CuAAC reaction and reduction of azide, it would be an
excellent strategy in olefin modification. Further inves-
tigations on the scope and synthetic applications of this
reaction are currently underway in our group.
N
88
86
CF3
4p
N
N
N
CF3
Acknowledgement
4t
We thank the Gansu Province Key Laboratory of
Fine Chemical (No. 1206RTSA022) and the Gansu
Province Science and Technology Innovation Service
Platform Construction Plan (No. 1306TTPA024) for
financial support. We also acknowledge support from
the Gansu Province Containing Fluorine Meticulous
Chemicals R&D Platform Project and Program for
Pharmaceutical Intermediates Innovation Research
Team in Gansu Province (No. 1207TTCA009).
O
O
CF3
N
N
81
N
4w
4
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Copper-Catalyzed Three-Component Azidotrifluoromethylation/Difunctionalization of Alkenes
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