[Cu(acac)2]·H2O-catalyzed sonogashira-type couplings of aryl halides and terminal alkynes

Article abstract of DOI:10.1002/asia.201000923

Highly selective coupling of aryl halides with terminal alkynes can be conducted using low-cost and readily available [Cu(acac)₂] ·H₂O under palladium- and amine-free conditions. This protocol makes the typical Sonogashira reaction more practical and useful (TBAB=tetrabutyl ammonium bromide; DMSO=dimethyl sulfoxide; see scheme). Copyright

Full text of DOI:10.1002/asia.201000923

DOI: 10.1002/asia.201000923
[CuACHTUNGTRENNUNG
Tingyi Li, Xiaoming Qu, Guanlei Xie, and Jincheng Mao*[a]
Transition-metal-catalyzed cross-coupling reactions have
diagnostics, and localization as well as other applications,
which include drug delivery and material science based on
high affinity between biotin-(strept)avidin.[5] Compounds 5–
6 were investigated as conjugated oligomers for research on
electrical conductance at the single-molecule level.[6] Com-
pound 7 was synthesized as a norbornene block copolymer
containing oligonucleotide and ferrocenyl side chains and
used in the electrochemical detection of DNA.[7]
become one of the most powerful tools for the construction
[1]
À
of carbon carbon bonds. Among these, the Sonogashira-
type coupling between terminal alkynes with aryl or vinyl
halides is commonly employed for the synthesis of mole-
cules containing an acetylenic moiety, which is found in
many fine chemicals and biologically active substances.[2]
Successful examples include compound 1, a potent and se-
lective agonist for the free fatty acid receptor 1 (FFA1/
GPR40), and a potential target for the treatment of type II
diabetes.[3] Compounds 2–3 were used as optical brighteners
for synthetic and natural fibers in textile industry.[4] Com-
pound 4 was employed as a carbon-linked biotin derivative
for use in affinity isolation and purification, immunoassay,
The typical Sonogashira coupling reaction is performed in
the presence of [PdCl2ACHTUNGTRENNUG(PPh3)2], PdCl2/PPh3, or PdACHTUNTGREN(NGUN PPh3)4 to-
gether with copper iodide as the co-catalyst and a large
amount of amine as the solvent or co-solvent.[8] However,
these protocols were limited to academic use owing to the
high cost of palladium complexes. There are two routes to
solve this problem: one method
is to reduce the catalytic load-
ing of the palladium catalyst,[9]
or
cheaper
alternatively
and
introduce
lower-toxicity
metals as catalysts. Thus,
copper salts in combination
with appropriate ligands have
been developed,[10] and numer-
ous metal complexes or nano-
particles, such as iron,[11]
gold,[12] indium,[13] nickel,[14]
cobalt,[15] ruthenium[16] together
with ligands. Recently, we re-
ported an efficient example of a
samarium powder-catalyzed So-
nogashira coupling in the ab-
sence of ligands.[17] It can be
concluded that iron catalysis
could be promising due to the readily availability of resour-
ces and its environmental friendliness. However, in many
cases, for iron-catalyzed reactions, it is necessary to add
some copper salts as the co-catalyst for high efficiency. This
protocol was reported by ourselves and other research
groups.[18]
[a] T. Li, X. Qu, G. Xie, Prof. J. Mao
Key Laboratory of Organic Synthesis of Jiangsu Province College of
Chemistry
Chemical Engineering and Materials Science
Soochow University
Suzhou 215123 (P. R. China)
Fax : (+86)512-6588-0403
E-mail: jcmao@suda.edu.cn
Chem. Asian J. 2011, 6, 1325 – 1330
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1325

COMMUNICATION
Actually, copper catalysis has already been reported to be
a reaction performed in PEG-200 (polyethyleneglycol) af-
forded essentially no product (Table 1, entry 1). Under the
same conditions, but in the presence of 10 mol% of CuI, the
desired product was obtained in 26% yield (entry 2).
Screening of solvents led us to identify dimethyl sulfoxide as
a suitable solvent (entries 2–4). Different copper salts were
then evaluated (entries 5–11). Generally, cupric salts per-
[19]
À
À
À
powerful in C C, C N, and C O(S) coupling reactions.
Among these, the copper-catalyzed Sonogashira coupling
has gained a great deal of interest over the past several
years.[20] It is noteworthy that suitable ligands are usually re-
quired. Thus, various ligands have been developed corre-
spondingly for the highly effective coupling reactions be-
tween aryl halides and terminal alkynes. However, to further
reduce the cost of the systems, two approaches can be envis-
aged: low catalyst loading and/or in the absence of ligands.
Recently, Bolm and co-workers have demonstrated that the
coupling of aryl iodides and terminal alkynes could readily
be performed by using a sub-loading amount of [Cu-
formed best; [CuACHTNUTRGNE(UNG acac)2]·H2O proved to be the most effec-
tive (96% yield; entry 9). To recycle the catalytic system,
the reactions were performed in PEG-400, MPEG-350
(polyethylene glycol monomethyl ether), or TBAF (tetra-n-
butylammonium fluoride). However, inferior results were
obtained (entries 12–14). Furthermore, in order to make
sure of the validity of these results, various researchers in
our laboratory independently carried out these experiments,
in some cases using different reagents from various known
chemical corporations; the results obtained were similar in
all cases.[24] To control potential metal contamination, we
carried out experiments in new flasks with new stirring bars
ACHTUNGTRENNUNG(DMEDA)2]Cl2·H2O in the presence of 30 mol% of N,N’-di-
methylethylenediamine (DMEDA) as a ligand.[21] In con-
trast, Rothenberg et al. have described that copper nano-
clusters catalyze the ligand-free cross-coupling of alkynes
and aryl halides to give the corresponding disubstituted al-
kynes in good yields and high selectivity.[22] This raises one
practical question: how about the readily available copper
salts as catalysts?
and new caps. [CuACTHNUTRGNEUNG(acac)2] with higher purity from Aldrich
(99.999%) could catalyze the coupling in slightly higher
yield (98%, entry 15; for certificate of analysis, see the Sup-
porting Information).[25] In view of practicability, [Cu-
Based on our interest in various cross-coupling reacti-
ons,[17,18,20a,23] we describe herein a high yielding Sonogashira
coupling of various aryl halides and alkynes using [Cu-
AHCTUNGRTENGUN(N acac)2]·H2O was directly employed for researching the
A
cross-coupling reactions.
ligand. We selected the cross-coupling of 4-iodoanisole and
phenylacetylene as the model reaction to identify the cata-
lysts and to optimize the reaction conditions. The related re-
sults are listed in Table 1. In the absence of copper catalysts,
As shown in Scheme 1, performing the reaction in air
rather than argon or dinitrogen slightly decreased with con-
comitant formation of the Glaser-type reaction product
through homocoupling of phenylacetylene;[26] this was con-
firmed by GC-MS.
Table 1. Screening catalytic conditions for copper-catalyzed Sonogashira
coupling between 4-iodoanisole and phenylacetylene.[a]
Scheme 1. [CuACTHNUGRTNEUNG(acac)2]·H2O-catalyzed Sonogashira coupling performed in
argon, dinitrogen, or air.
Entry
Cu cat. [mol%]
Solvent
Yield [%][b]
With a set of reaction conditions in hand, we screened a
series of aryl iodides against terminal alkynes, and this re-
sulted in the formation of desired products in excellent
yields (Table 2). In the presence of 4-iodoanisole, both elec-
tron-rich and electron-deficient para-substituted phenylace-
tylenes yielded the corresponding coupling products
(Table 2, entries 1–3). Among the aryl iodides, the coupling
reaction proceeds very readily with aryl alkynes, thus giving
excellent yields of the corresponding products (entries 4–
12). In addition, several aryl halides with sensitive, polar
functionalities (ester, nitro group, ketone, and amine) also
afforded good results (entries 13–16), which are definitely of
paramount importance with respect to envisioned applica-
tions. The reaction with alkylalkynes was found to be inferi-
or compared to the arylalkynes (entries 17–19). Neverthe-
less, no homocoupling product was detected by GC-MS
under these conditions.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15[c]
–
PEG-200
PEG-200
DMF
trace
26
42
77
40
69
10
88
96
72
66
<10
trace
35
CuI (10)
CuI (10)
CuI (10)
CuBr (10)
CuCl (10)
Cu (10)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
PEG-400
MPEG-350
TBAF
Cu
[Cu
Cu
CuSO4·5H2O
(acac)2]·H2O (10)
(acac)2]·H2O (10)
(acac)2]·H2O (10)
(acac)2] (10)
G
G
E
[Cu
[Cu
[Cu
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[Cu
N
DMSO
98
[a] Reaction conditions: Ar, copper cat. (10 mol%), 4-iodoanisole
(0.3 mmol), phenylacetylene (0.4 mmol), K2CO3 (0.6 mmol), solvent
(2 mL), 1408C, 24 h. [b] Yield of isolated product (based on 4-iodoani-
sole). [c] [Cu
(99.999%).
ACHTUNGTRENNUNG(acac)2] was purchased from Aldrich with high purity
1326
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1325 – 1330

Sonogashira-Type Couplings
Table 2. [CuACHTUNGTRENNUNG
(acac)2]·H2O-catalyzed Sonogashira coupling of alkynes and aryl iodides.[a]
tive. Furthermore, in some in-
stances, addition of two equiva-
lents of NaI could result in
markedly higher yields, which is
possibly due to the aryl bro-
mides being converted into
their corresponding iodides in
situ by Br/I exchange during
the catalytic reaction.[27]
Entry
RX
R’CꢀCH
Product
10
t [h]
Yield [%][b]
1
24
96
2
3
11
12
13
14
15
16
17
18
14
19
20
21
24
24
24
24
48
24
24
48
24
48
48
48
97
99
95
92
90
99
90
94
90
93
70
78
To extend the scope of our
methodology, we attempted to
apply this effective catalyst to
4
other
coupling
reactions,
À
namely, the formation of C O,
5
À
À
C N
C S,
and
bonds
(Scheme 3). Under unoptimized
conditions, treatment of iodo-
benzene with thiophenol yields
the corresponding product in
>99% yield. In contrast, the
6
7
8
À
related C O bond did not pro-
9
ceed in good yield with our cat-
alytic system. Furthermore, the
coupling between iodobenzene
and imidazole afforded the de-
sired product almost quantita-
tively. It thus appears that our
catalytic system may be effi-
cient in other coupling reac-
tions and further work is cur-
rently underway in our labora-
tory.
10
11
12
13
14
15
16
17
22
23
24
25
48
24
24
48
70
86
61
78
These findings raise several
interesting mechanistic ques-
tions. We hypothesized that
acetylacetone is a non-innocent
ligand that perhaps remains
bound to the copper during the
catalytic cycle. To address this
question, we tested various
copper salts with and without
the addition of acetylacetone
(Scheme 4). For both CuSO4
and CuO, the addition of acety-
lacetone had a beneficial effect
18
19
26
27
48
48
50
77
[a] Reaction conditions: Ar, Cu
ACHTUGNTRNEN(NUG acac)2·H2O (10 mol%), aryl iodide (0.3 mmol), alkyne (0.4 mmol), K2CO3
(0.6 mmol), DMSO (2 mL), 1408C, 24–48 h. [b] Isolated yield based on aryl halide (average of two runs).
In addition, the coupling reactions of aryl diiodides were
carried out using the [Cu(acac)2]·H2O catalytic system.
on the conversion. From Scheme 4, it can be seen that in the
absence of ligand, the desired product was afforded in poor
yields (66% and 15%, respectively), while the product was
obtained in almost quantitative yield for the CuSO4 system
in the presence of acetylacetone as the ligand. As copper(I)
complexes could also promote the alkynylation in low yield,
it is proposed that under the reaction conditions the initia-
tion could be the oxidative dimerization of a minor amount
of alkyne followed by comproportionation of the resulting
copper(0) and the residual copper(II) species. This can be
implied by the formation of diphenylbutadiyne from the re-
action system using copper(I), which was detected by GC-
AHCTUNGTRENNUNG
Screening with different aryl diiodides, such as 1,3-diiodo-
benzene afforded the best yield (97% yield). Furthermore,
one unsymmetrical diarylalkyne was acquired in near quan-
titative yield by coupling between 1-iodonaphthalene and
phenylacetylene using the copper(II) catalytic system as
shown in Scheme 2.
As presented in Table 3, treatment of aryl bromides with
phenylacetylene afforded the products in moderate to good
yields (53–94% yields). After screening different additives,
TBAB (tetrabutyl ammonium bromide) was the most effec-
Chem. Asian J. 2011, 6, 1325 – 1330
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1327

J. Mao et al.
COMMUNICATION
Scheme 3. The various coupling reactions catalyzed by the [Cu-
AHCTUNGRTEGN(NNU acac)2]·H2O system.
um instead of potassium carbonate, and the results are simi-
lar. Therefore, in the actual catalytic reaction, dimsyl potas-
sium is probably the effective base.[28,29] All in all, compared
to widely used ligands in the Sonogashira coupling,[20] this
cheap alternative opens fascinating perspectives towards the
use of this versatile methodology on large scale.
In summary, we have developed an efficient protocol for
the coupling between aryl halides with various terminal al-
kynes, thereby affording desired compounds in moderate to
excellent yields. In addition, when carried out under argon
or dinitrogen (much cheaper), no homocoupling product
was detected. It is noteworthy
Scheme 2. Coupling of 1,3-diiodobenzene or 1-iodonaphthalene with phe-
nylacetylene using [CuACHTUNGTRENNUNG(acac)2]·H2O.
MS.[28] In view of the special effect of dimethyl sulfoxide, it
may work not only as the solvent itself but also as an oxi-
dant or a ligand. Furthermore, the presence of potassium
carbonate triggers the deprotonation of dimethyl sulfoxide.
Thus, dimsyl potassium could possibly be generated in situ.
A parallel experiment was carried out using dimsyl potassi-
that our catalytic system is very
stable, low cost, palladium free,
Table 3. [CuACHTUNGTRENNUNG
(acac)2]·H2O-catalyzed coupling of alkynes and aryl bromides.[a]
amine free, and easily accessi-
ble. Studies into the mechanism
and further application of the
reaction will be the focus of
future work.
Entry
RX
R’CꢀCH
Product
10
Yield [%][b]
1[c]
72
2
13
28
53
91
Experimental Section
General Experimental
3[c]
All reactions were carried out under
an argon atmosphere. Solvents were
dried and degassed by the standard
methods and all aryl halides were pur-
chased from Aldrich or Alfa. Aryl al-
kynes and various copper salts were
purchased from known chemical cor-
porations. Flash column chromatogra-
phy was performed using silica gel
(300–400 mesh). Analytical thin-layer
chromatography was performed using
glass plates pre-coated with 200–
300 mesh silica gel impregnated with a
fluorescent indicator (254 nm). NMR
spectra were recorded in CDCl3 on a
Varian Inova-400 NMR spectrometer
(400 MHz) with TMS as an internal
reference. Products were characterized
by comparison of 1H NMR, 13C NMR,
and TOF-MS data in the literature.
4[c]
29
94
5[c]
30
31
32
33
70
70
86
80
6
7[c]
8[c]
[a] Reaction conditions: Ar, Cu
(0.6 mmol), TBAB (0.3 mmol), DMSO (2 mL), 1408C, 48 h; [b] Isolated yield based on aryl halide (average of
two runs); [c] 2 equiv of NaI were employed as the additive.
(acac)2·H2O (10 mol%), aryl bromide (0.3 mmol), alkyne (0.4 mmol), K2CO3
1328
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Chem. Asian J. 2011, 6, 1325 – 1330

Sonogashira-Type Couplings
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systems.
General Procedure for [Cu
Reaction
ACHTUNGRTNEN(UNG acac)2]·H2O-Catalyzed Sonogashira Coupling
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(10 mol%), K2CO3 (2 equiv), and DMSO (2 mL) in a Schlenk tube was
stirred under an argon atmosphere at 1408C for the desired time until
complete consumption of starting material, as monitored by TLC. After
the mixture was poured into ether, then washed with water, it was ex-
tracted with ethyl acetate (3ꢁ15 mL), dried by anhydrous MgSO4, then
filtered and evaporated under vacuum. The residue was purified by flash
column chromatography (petroleum ether or petroleum ether/ethyl ace-
tate 6:1) to afford the corresponding coupling products. For example: 1-
(2-(4-methoxyphenyl)ethynyl)benzene (3): 1H NMR (400 MHz, CDCl3):
d=7.50–7.52 (m, 2H), 7.47 (d, J=8.8 Hz, 2H), 7.32–7.36 (m, 3H), 6.88
(d, J=8.8 Hz, 2H), 3.83 ppm (s, 3H, CH3); 13C NMR (100 MHz, CDCl3):
d=161.2 (C), 134.7 (CH), 133.1 (CH), 130.0 (CH), 129.6 (C), 125.2 (CH),
117.0 (C), 115.6 (CH), 91.0 (C), 89.7 (C), 56.9 ppm (CH); HRMS (ESI+):
calcd for [C15H12O]+ requires m/z 208.0888; found 208.0896. See the Sup-
porting Information for full details.
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We are grateful to the grants from the Natural Science Foundation of
China (20802046) and the Key Laboratory of Organic Synthesis of Jiang-
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sured by ICP-MS.
Received: December 23, 2010
Published online: March 17, 2011
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