Copper-catalyzed trifluoromethylation of terminal alkynes using Umemoto's reagent

Article abstract of DOI:10.1016/j.tetlet.2012.03.107

A copper-catalyzed method for trifluoromethylation of terminal alkynes with Umemoto's reagent has been developed. The reaction is conducted at room temperature and shows good tolerance to a variety of functional groups.

Full text of DOI:10.1016/j.tetlet.2012.03.107

Tetrahedron Letters 53 (2012) 2769–2772
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Tetrahedron Letters
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Copper-catalyzed trifluoromethylation of terminal alkynes using Umemoto’s
reagent
⇑
⇑
Dong-Fen Luo, Jun Xu, Yao Fu , Qing-Xiang Guo
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A copper-catalyzed method for trifluoromethylation of terminal alkynes with Umemoto’s reagent has
been developed. The reaction is conducted at room temperature and shows good tolerance to a variety
of functional groups.
Received 16 January 2012
Revised 21 March 2012
Accepted 27 March 2012
Available online 30 March 2012
Ó 2012 Published by Elsevier Ltd.
Keywords:
Trifluoromethylation
Umemoto’s reagent
Terminal alkynes
Copper-catalyzed
Trifluoromethyl-containing organic compounds constitute an
important class of biologically active molecules.1,2 The introduction
of a CF3 group into organic compounds often enhances the chemical
and metabolic stability, lipophilicity, and binding selectivity.2 Due
to this reason, the development of efficient methods for the incor-
poration of trifluoromethyl group into organic structures has
received much attention recently.3 Traditional methods in this area
usually focused on non-catalytic trifluomethylation reactions,
mostly forming C(sp3)–CF3 bond.4 However, these methods suffer
from harsh reaction conditions and limited substrates scope. Re-
cently, more attention has been paid to transition metal-catalyzed
methods5 to make CF3-containing building blocks. Examples in-
clude: (1) Pd-catalyzed trifluoromethylation of C(sp2)–H and
C(sp2)–halide bonds;6–9 (2) copper-mediated10 or copper-cata-
lyzed11 trifluoromethylation of C(sp2)–X (X = halide, boronic acid)
moieties; and (3) copper-catalyzed allylic C–H bond activation/tri-
fluoromethylation reactions.12 In comparison to the previous triflu-
oromethylation methods, these newly developed reactions employ
mild reaction conditions and enjoy a broader substrate scope.
Trifluoromethylated acetylenes are a class of building blocks for
the synthesis of agrochemicals, pharmaceuticals, and functional
materials.13 Traditional methods for the synthesis of trifluorome-
thylated acetylenes include: Pd-catalyzed coupling of trifluoropro-
pynyl metal reagents with aryl iodides; dehalogenation of
trifluoromethylethenes; and electrophilic trifluoromethylation of
alkynyl metal reagents.14,15 However, these methods usually
involve tedious procedures or use toxic substrates. Recently, Chu
and Qing, developed the first example of Cu-mediated protocol
for oxidative trifluoromethylation of terminal alkynes with Me3-
SiCF3.16 This method is very useful for the synthesis of a broad range
of trifluoromethylated acetylenes. However, this method also has
some limitations, such as the requirement of a high reaction tem-
perature, use of an excess amount (5.0 equiv) of Me3SiCF3, and
use of a stoichiometric amount of copper reagents. Herein, we want
to report a mild copper-catalyzed trifluoromethylation of terminal
alkynes using Umemoto’s reagent (Eq. (1)).
[Cu]
R
R
CF3
+
ð1Þ
S
CF3
OTf
We began our study by examining the trifluoromethylation
reaction of but-3-ynyl 4-methylbenzene-sulfonate (1a) with (tri-
fluoromethyl)dibenzothiophenium triflate (2a). At first, a catalytic
amount of CuTc (20% mmol) and a cheap ligand (2,4,6-trimethyl-
pridine, L1) were tested. To our delight, the desired trifluoromethy-
lation alkyne 3a was detected, although the yield was still low
(Table 1, entry1). To optimize the reaction, we changed L1 to other
related ligands but observed only poorer performance (entries 2
and 3). In particular, the often used ligand (1,10-phenanthroline,
L4) generated no desired product (entry 4). Furthermore, the best
reaction temperature was 30 °C (entry 5), because a higher reac-
tion temperature decreases the yield of 3a (entry 1). To improve
the yield, we then tried different Cu(I) salts (entries 6–10) until
we found that CuCl increased the yield to 75%. Under the same
optimized conditions, the use of (trifluoromethyl)dibenzothiophe-
nium tetrafluoroborate (2b, entry 11) produces 3a in a similar
⇑
Corresponding authors.
E-mail address: fuyao@ustc.edu.cn (Y. Fu).
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2012.03.107

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D.-F. Luo et al. / Tetrahedron Letters 53 (2012) 2769–2772
Table 1
Optimization of the Cu-catalyzed trifluoromethylation reactiona
CuX / Ligand
TsO
TsO
+
H
CF3
S
CF3
2a X=OTf
2b X=BF4
base, solvent,
30°C, 24h
X
1a
3a
N
H
MeO
N
OMe
N
N
N
L1
L2
L3
L4
Entry
CuX (equiv)
Ligand (equiv)
Base (equiv)
Solvent
Yield of 3a
1b
2b
3b
4b
5
6
7
8
9
10
11d
12
13
14
CuTcc (0.2)
CuTc (0.2)
CuTc (0.2)
CuTc (0.2)
CuTc (0.2)
CuBr (0.2)
CuI (0.2)
CuOAc (0.2)
[Cu(OTf)]ꢀC6H6 (0.1)
CuCl (0.2)
CuCl (0.2)
CuCl (0.2)
CuCl (0.2)
—
L1 (2.0)
L2 (2.0)
L3 (2.0)
L4 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
L1 (2.0)
—
—
—
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DCE
35
2
0
K3PO4(2.0)
0
—
—
—
—
—
—
—
—
—
—
55
61
46
53
12
75
73
30
26
0
THF
DMAc
a
b
c
Reaction conditions: 1a (0.10 mmol), 2a (0.12 mmol), [Cu] (20 mol %), solvent (0.5 mL), 30 °C, 24 h under Ar atmosphere, GC yields with biphenyl as an internal standard.
Reaction was conducted at 60 °C.
TcCu = (thiophene-2-carbonyloxy) Cu(I).
2a was instead of 2b.
d
yield. Moreover, we optimized the solvent and found N,N-dimeth-
ylacetamide (DMAc) to be optimal (entries 10, 12 and 13). Note
that in the absence of copper salt we did not observe any trifluo-
romethylation product (entry 14).
presence of 2,4,6-trimethylpyridine as a base. Oxidative addition
of CFþ3 to species B then took place to form a Cu (alkynyl)(trifluoro-
methyl)complex C, which affords the product alkynyl-CF3 through
reductive elimination.17 Alternatively, C may cause an SN2-type
With the optimized set of reaction conditions in hand, we
examined the scope of the new transformation (Table 2). We found
that various terminal alkynes can be transformed to the corre-
sponding products in modest to good yields. Many synthetically
important functional groups, such as sulfonate, nitro, ester, amide,
ether, and even hydroxyl are well-tolerated under the conditions.
Moreover, arene rings carrying chloro and iodo groups do not
interfere with the transformation (entries 2 and 3), making addi-
tional modifications possible at the halogenated positions. A high
reactivity was also observed with substrates bearing sulfonate
group (entries 1–5). The 4-nitro-substituted phenylacetylene and
3-ester-substituted phenylacetylene also smoothly underwent
the reaction in good yield (entries 6 and 7). It should be noted that
2-(prop-2-ynyl) isoindoline-1,3-dione (entry 8) could be employed
as substrate to afford the target product in a moderate yield. Ether-
containing alkynes can also be trifluomethylated (entry 9). Same as
the sulfonate, benzoate can also be tolerated in this reaction and
successfully converted to the product in a moderated yield (entry
10). Finally, undec-10-yn-1-ol was suitable in this reaction to af-
ford corresponding product in a good yield (entry 11). Thus, both
of the aliphatic (entries 1–5 and 8–11) and aromatic alkynes (en-
tries 6 and 7) can afford the desired trifluoromethylated products
in moderate to good yields under the reaction conditions.
substitution (or r-bond metathesis) reaction at the CF3 center to
form the product alkynyl-CF3. Finally, Cu(I) was released regener-
ating the starting copper species A.
In conclusion, we have developed an efficient copper-catalyzed
trifluoromethylation reaction of terminal alkynes with Umemoto’s
reagent. The reaction is carried out under mild conditions and
shows good functional group compatibility even with unprotected
OH groups. The reaction provides a straightforward way to prepare
various trifluoromethylated alkynes.
Experimental
1H NMR spectra were recorded on a Bruker 400 MHz spectrom-
eter. Chemical shifts of 1H NMR spectra were reported in parts per
million relative to tetramethylsilane (d = 0). 19F NMR spectra (CFCl3
as outside standard and low field is positive) were recorded with
1H-coupling on a Bruker 376 MHz spectrometer. 13C NMR spectra
were recorded with 1H-decoupling on a Brucker 100 MHz spec-
trometer. Chemical shifts of 1C NMR spectra were reported in parts
per million relative to the solvent resonance as the internal stan-
dard (CDCl3, d 77.00 ppm). Chemical shifts (d) are reported in
ppm, and coupling constants (J) are in Hertz (Hz). The following
abbreviations were used to explain the multiplicities: s = singlet,
d = doublet, t = triplet, q = quartet, m = multiplet, br = broad. Or-
ganic solutions were concentrated under reduced pressure on a
Buchi rotary evaporator. Flash column chromatographic purifica-
tion of products was accomplished using forced-flow chromatogra-
phy on silica gel (200–300 mesh).
A plausible reaction pathway for trifluoromethylation of al-
kynes with CFþ3 is presented in Scheme 1. A ligated complex A
would be generated in situ by the reaction of CuCl with 2,4,6-
trimethylpyridine in DMAc. Subsequently, coordination/deproto-
nation of the alkyne occurs to form a copper species B in the

D.-F. Luo et al. / Tetrahedron Letters 53 (2012) 2769–2772
2771
Table 2
this tube. After the addition of all materials, the reaction mixture
was kept for 24 h at 30 °C. After the reaction, ethyl acetate was
added. The organic layer was separated, and washed with water.
The combined organic extracts were washed with brine, and dried
over MgSO4. After evaporation of the solvent, the crude product
was purified by chromatography on silica gel to give the product.
CuCl catalyzed trifluoromethylation of terminal alkynesa
CuCl (20%mol)
L1 (2equiv)
R
H
+
R
CF3
S
DMAc, 30°C, 24h
OTf
CF3
3
1
2a
5,5,5-Trifluoropent-3-ynyl 4-methylbenzenesulfonate (3a)
1H NMR (400 MHz, CDCl3) d 7.81 (d, J = 7.9 Hz, 2H), 7.37 (d,
J = 7.9 Hz, 2H), 4.14 (t, J = 6.6 Hz, 2H), 2.70 (d, J = 3.1 Hz, 2H), 2.46
(s, 3H). 13C NMR (100 MHz, CDCl3) d 145.39, 132.50, 130.04,
127.99, 111.14 (q, J = 254.8 Hz), 83.29 (q, J = 6.5 Hz), 70.24 (q,
J = 52.6 Hz), 65.72, 21.66, 19.29. 19F NMR (376 MHz, CDCl3) d
ꢁ50.28 (t, J = 3.1 Hz).
Entry Substrate
Product
Yield (%)
65
O
O
CF3
3a
S
S
1a
1
O
O
O
O
O
O
CF3
S
O
S
O
O
2
3
4
65
69
75
1b
1c
3b
O
Cl
I
Cl
O
O
CF3
5,5,5-Trifluoropent-3-ynyl 4-chlorobenzenesulfonate (3b)
1H NMR (400 MHz, CDCl3) d 7.87 (d, J = 8.6 Hz, 2H), 7.56 (d,
J = 8.6 Hz, 2H), 4.19 (q, J = 6.4 Hz, 2H), 2.74 (tq, J = 6.7, 3.4 Hz,
2H). 13C NMR (100 MHz, CDCl3) d 141.06, 134.01, 129.82, 129.37,
113.66 (q, J = 257.3 Hz), 83.07 (q, J = 6.3 Hz), 70.41 (q, J = 52.8 Hz),
66.12, 19.31. 19F NMR (376 MHz, CDCl3) d ꢁ50.30 (t, J = 3.4 Hz).
S
S
O
O
O
O
3c
O
I
O
S
O
O
S
CF3
O
1d
3d
O
F
F
O
O
CF3
S
S
O
O
5
75
5,5,5-Trifluoropent-3-ynyl 4-iodobenzenesulfonate (3c)
1H NMR (400 MHz, CDCl3) d 7.98–7.91 (m, 2H), 7.66–7.60 (m,
2H), 4.21–4.15 (m, 2H), 2.80–2.66 (m, 2H). 13C NMR (100 MHz,
CDCl3) d 138.77, 135.18, 129.14, 113.65 (q, J = 257.2 Hz), 102.10,
83.04 (q, J = 6.3 Hz), 70.40 (q, J = 52.7 Hz), 66.11, 19.29. 19F NMR
(376 MHz, CDCl3) d ꢁ50.26 (t, J = 3.3 Hz).
O
O
1e
1f
3e
CF3
3f
6
7
44
58
O2N
O2N
O
O
CF3
1g
1h
3g
O
O
5,5,5-Trifluoropent-3-ynyl 4-fluorobenzenesulfonate (3d)
1H NMR (400 MHz, CDCl3) d 8.00–7.90 (m, 2H), 7.30–7.21 (m,
2H), 4.18 (t, J = 6.5 Hz, 2H), 2.73 (tq, J = 6.8, 3.4 Hz, 2H). 13C NMR
(100 MHz, CDCl3) d 166.03 (d, J = 257.4 Hz), 131.57 (d, J = 3.3 Hz),
130.81 (d, J = 9.6 Hz), 116.82 (d, J = 22.9 Hz), 113.65 (q, J =
257.2 Hz), 83.13 (q, J = 6.3 Hz), 70.33 (q, J = 52.7 Hz), 65.97 (d,
J = 1.5 Hz), 19.28 (d, J = 1.4 Hz). 19F NMR (376 MHz, CDCl3) d
ꢁ50.30 (t, J = 3.4 Hz), ꢁ102.32 (s).
O
CF3
O
N
8
9
38
26
3h
N
O
O
CF3
O
O
1i
3i
3j
O
O
CF3
10
11
54
79
1j
O
O
5,5,5-Trifluoropent-3-ynyl 4-tert-butylbenzene-sulfonate (3e)
1H NMR (400 MHz, CDCl3) d 7.84 (d, J = 8.5 Hz, 2H), 7.58 (d,
J = 8.5 Hz, 2H), 4.19–4.13 (m, 2H), 2.78–2.64 (m, 2H), 1.35 (s, 9H).
13C NMR (100 MHz, CDCl3) d 158.31, 132.38, 127.85, 126.47,
113.71 (q, J = 257.1 Hz), 83.45 (q, J = 6.4 Hz), 70.19 (q, J = 52.7 Hz),
65.78, 35.36, 31.00, 19.29. 19F NMR (376 MHz, CDCl3) d ꢁ50.21 (t,
J = 3.4 Hz).
HO
HO
1k
CF3
3k
a
The reactions were conducted under the optimized conditions.
R
H
Cl-
base
CuI
R
L
Cu(I) OTf
L
CuICl
1-Nitro-4-(3,3,3-trifluoroprop-1-ynyl)benzene (3f)
1H NMR (400 MHz, CDCl3) d 8.28 (d, J = 7.9 Hz, 2H), 7.75 (d,
J = 8.1 Hz, 2H). 13C NMR (100 MHz, CDCl3) d 132.47, 129.07,
122.83, 122.64, 113.38 (q, J = 258.2 Hz), 82.65 (q, J = 6.0 Hz),
78.39 (q, J = 53.3 Hz). 19F NMR (376 MHz, CDCl3) d ꢁ50.60.
B
A
2a
R
CF3
R
S
TfO
L
OTf
CF3
R
CuIII
CF3
or
Cu
2-Methyl 3-(3,3,3-trifluoroprop-1-ynyl)benzoate (3g)
1H NMR (400 MHz, CDCl3) d 8.16 (s, 1H), 8.07 (d, J = 7.6 Hz, 1H),
7.66 (d, J = 7.6 Hz, 1H), 7.42 (t, J = 7.8 Hz, 1H), 3.87 (s, 3H). 13C NMR
(100 MHz, CDCl3) d 164.65, 135.32, 132.58, 130.80, 130.13, 129.93,
127.89, 112.38 (q, J = 257.1 Hz), 84.23 (q, J = 6.4 Hz), 75.11, 51.46.
19F NMR (376 MHz, CDCl3) d ꢁ50.12.
L
C
D
Scheme 1. A possible mechanism for the catalytic trlfluoromethylation reaction.
General procedure for copper-catalyzed trifluoromethylation of
terminal alkenes
2-(4,4,4-Trifluorobut-2-ynyl)isoindoline-1,3-dione (3h)
1H NMR (400 MHz, CDCl3) d 7.94–7.88 (m, 2H), 7.81–7.74 (m,
2H), 4.63–4.54 (m, 2H). 13C NMR (100 MHz, CDCl3) d 166.52,
134.55, 131.80, 123.87, 113.66 (q, J = 258.0 Hz), 80.98 (q,
J = 6.5 Hz), 70.36 (q, J = 53.2 Hz), 26.43. 19F NMR (376 MHz, CDCl3)
d ꢁ50.76 (t, J = 3.2 Hz).
CuCl (0.04 mmol), 2,4,6-trimethylpyridine (0.4 mmol), trifluo-
romethylating reagent 2a (0.24 mmol) and terminal alkynes
(0.2 mmol) were added to a Schlenk tube which was equipped with
a stirring bar. DMAc (1 mL) was added under argon atmosphere to

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D.-F. Luo et al. / Tetrahedron Letters 53 (2012) 2769–2772
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118.39, 113.78 (q, J = 258.0 Hz), 107.54, 82.15 (q, J = 6.4 Hz),
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1H), 7.50–7.42 (m, 2H), 4.47 (t, J = 6.6 Hz, 2H), 2.81 (tq, J = 7.0,
3.5 Hz, 2H). 13C NMR (100 MHz, CDCl3) d 165.13, 132.32, 128.67,
128.52, 127.47, 112.86 (q, J = 256.8 Hz), 83.78 (q, J = 6.4 Hz),
68.79 (q, J = 52.4 Hz), 60.01, 17.92. 19F NMR (376 MHz, CDCl3) d
ꢁ49.99 (t, J = 3.5 Hz).
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(m, 2H), 1.56–1.42 (m, 4H), 1.35–1.20 (m, 10H). 13C NMR
(100 MHz, CDCl3) d 113.17 (q, J = 255.9 Hz), 88.33 (q, J = 6.3 Hz),
67.35 (q, J = 51.8 Hz), 61.99, 31.75, 28.31, 28.30, 27.86, 27.65,
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Acknowledgments
The authors are grateful to the National Key Basic Research Pro-
gram of China (2012CB215305, 2012CB215306), the National Nat-
ural Science Foundation of China (21172209), Chinese Academy of
Science (KJCX2-EW-J02) and the Fundamental Research Funds for
the Central Universities for the financial support.
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