Ligand-free palladium-catalyzed decarboxylative coupling reactions of aryl iodides and alkynyl carboxylic acids

Article abstract of DOI:10.1016/j.jorganchem.2012.11.029

Pd-CNT nanocomposite was prepared from the composite of thiolated MWNT and Na₂PdCl₄. The palladium nanoparticles possess an average diameter of 2.9 nm with a narrow size distribution for the Pd-CNT. This Pd-CNT showed good activities in the coupling reactions of aryl iodides and aryl alkynyl carboxylic acids to produce the diaryl alkynes in good yields without any additional ligand.

Full text of DOI:10.1016/j.jorganchem.2012.11.029

Journal of Organometallic Chemistry 724 (2013) 271e274
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Journal of Organometallic Chemistry
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Note
Ligand-free palladium-catalyzed decarboxylative coupling reactions of aryl
iodides and alkynyl carboxylic acids
**
*
Ayoung Pyo, Ji Dang Kim, Hyun Chul Choi , Sunwoo Lee
Department of Chemistry, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd-CNT nanocomposite was prepared from the composite of thiolated MWNT and Na2PdCl4. The
palladium nanoparticles possess an average diameter of 2.9 nm with a narrow size distribution for the
Pd-CNT. This Pd-CNT showed good activities in the coupling reactions of aryl iodides and aryl alkynyl
carboxylic acids to produce the diaryl alkynes in good yields without any additional ligand.
Ó 2012 Elsevier B.V. All rights reserved.
Received 16 September 2012
Received in revised form
12 November 2012
Accepted 20 November 2012
Keywords:
Palladium
CNT
Decarboxylative coupling
Alkynyl carboxylic acid
Aryl iodides
1. Introduction
respectively [33,42]. P. H. Lee and Tunge used PPh3 as a ligand in the
intramolecular and intermolecular decarboxylative coupling reac-
The structure of aryl alkyne is one of the most important
building blocks in the pharmaceutical and electro material fields
[1e9]. For the synthesis of aryl alkyne moiety, the Sonogashira
coupling reactions have been widely used in the synthetic chem-
istry field. The typical reaction method of Sonogashira reaction is
that aryl halides and terminal alkynes are reacted with amine base
in the presence of palladium catalyst, phosphine ligand and copper
co-catalyst [10e14]. A number of improved reaction methods such
as ligand-free [15], copper and/or amine-free [16e19] methods and
the expansion of the scope of substrates [20,21] have been re-
ported. In addition, a variety of alkynyl compounds have been
employed as an alkyne source instead of terminal alkyne [22e28].
Among them, alkynyl carboxylic acids have been widely used since
we first reported the decarboxylative coupling reaction [29],
because they have several advantages such as easy storage,
handling and preparation [30e44]. In most cases, the reactivity of
catalyst depends on the properties of the ligand. We used chelating
ligands such as dppb (1,4-bis(diphenylphosphino)butane) and dppf
tions, respectively [30,45]. Although all ligands are commercially
available, more general condition not requiring any ligands are still
needed. To the best of our knowledge, investigation on the devel-
opment and application of ligand-free catalytic system has never
been accomplished in the decarboxylative coupling reaction.
To use palladium catalyst without any ligand, it is important to
stabilize the active palladium species which is generally palla-
dium(0). It is known that palladium(0) is easily aggregated to form
palladium black and increasing its size in the absence of ligand. The
increasing size of the catalyst causes the surface area of catalyst to
decrease, which means that the activity of heterogeneous catalyst
will decrease. To prevent aggregation and stabilize palladium(0),
carbon nanotubes (CNTs) have been employed as palladium
supports [46e48]. Carbon nanotubes (CNTs) are considered to be
the ideal catalyst supports in both heterogeneous catalysis and
electrocatalysis, due to their high mechanical strength, large
surface area, good electrical conductivity, and durability under
harsh conditions. However, there are difficulties in dispersing metal
nanoparticles with a uniform dispersion and regular size onto CNTs,
due to their hydrophobic nature and their tendency to agglomerate.
To solve these problems, the surface modification of CNTs by
various non-covalent and covalent methods has been developed. In
this work, we prepared Pd-decorated CNTs by depositing Pd
nanoparticles on thiolated CNTs. Sodium hydrosulfide (NaSH) was
used as a linker to secure the Pd nanoparticles on the CNTs.
(1,10-bis(diphenylphosphino)ferrocene). Li and Goo
ben employed
sterically bulky phosphine ligands such as XPhos and SPhos,
* Corresponding author. Tel.: þ82 62 530 3385, fax: þ82 62 530 3389.
** Corresponding author. Tel.: þ82 62 530 3491, fax: þ82 62 530 3389.
E-mail addresses: chc12@chonnam.ac.kr (H.C. Choi), sunwoo@chonnam.ac.kr
(S. Lee).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.11.029

272
A. Pyo et al. / Journal of Organometallic Chemistry 724 (2013) 271e274
Fig. 1. (a) TEM image of pristine CNT. (b) TEM image of the Pd-CNT catalyst. (c) EDS spectrum of the Pd-CNT catalyst. (d) Magnified TEM image of the Pd-CNT catalyst. (e) Size
distribution histogram of nanoparticles in Pd-CNT. The solid line in the histogram graph represents a Gaussian fitting curve.
Recently, we synthesized the Pd-CNT nanocomposites and
applied them as catalysts in the Pd-catalyzed coupling reactions
[49e51]. They showed good activities in the hydrodehalogenation,
Suzuki coupling and Heck reactions. To expand the application of
Pd-CNT nanocomposite and develop the ligand-free system for the
decarboxylative coupling reaction, we report on a Pd-CNT nano-
composite catalytic system for the coupling of aryl iodides and aryl
alkynyl carboxylic acids.
2. Results and discussion
First, we prepared Pd-CNT nanocomposite by the modified
method. Na2PdCl4 (ca. 4.0 mg/mL) was dissolved in distilled water
and stirred vigorously. Then, 0.1 M NaBH4 solution was added all at
once with continuous stirring. After the reduction reaction, thiolated
MWNTs were subsequently added to this solution. The resultant
suspension was filtered, washed, and dried under vacuum at 50 ꢀC
Table 1
Optimized conditions for the decarboxylative coupling reaction in the presence of Pd-CNT catalyst.a
Entry
Pd (mol%)
Temp. (ꢀC)
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
5
2.5
1
5
5
5
5
5
110
110
110
90
70
90
DMSO
DMSO
DMSO
DMSO
DMSO
NMP
97
91
64
95
trace
76
12
90
90
Toluene
Diglyme
24
a
Reaction conditions: Phenyl propiolic acid (0.33 mmol), iodobenzene (0.30 mmol), DBU (0.60 mmol) were reacted with Pd-CNT.
Yield was determined by gas chromatography with internal standard.
b

A. Pyo et al. / Journal of Organometallic Chemistry 724 (2013) 271e274
273
for 24 h Fig. 1(a) shows a typical TEM image of pristine CNT with
diameter in the range of 10e30 nm. All of the tubes had a clean
surface. For the prepared sample, the TEM image clearly shows that
nano-sized particles were highly dispersed on the CNTs (Fig. 1(b)).
Smaller and highly dispersed nanoparticles were much more
abundant than larger aggregated ones. According to EDS analysis
(Fig. 1(c)), the species supported on the CNT was Pd. The magnified
TEM images show that similarly sized and quasiespherical nano-
particles were anchored on the surface of the CNTs (Fig. 1(d)).
Fig. 1(e) shows a particle size histogram for Pd-CNT, in which the
histogram was fitted with a curve of a Gaussian shape to obtain the
particle size distribution. The nanoparticles possess an average
diameter of 2.9 nm with a narrow size distribution. According to ICP
experiments, the amount of Pd contained in the sample was 6.7 wt.%.
With this prepared nanocomposite, a variety of reactions
conditions were tested. The results are summarized in Table 1.
When 5 mol% and 2.5 mol% palladium were used, the desired
product was obtained in 97% and 91% yields, respectively (entries 1
Table 2
The decarboxylative coupling reactions of aryl iodides and aryl alkynyl carboxylic acids.a
,c
Entry
1
Ar1
Ar2
Product
Yield (%)b
C6H5
C6H5
95
2
3
4-MeC6H4
C6H5
C6H5
70
72
4-MeOC6H4
4
2,4-Me2C6H3
C6H5
65
5
6
7
4-BrC6H4
C6H5
C6H5
C6H5
87
92
58
4-MeCOC6H4
4-CNC6H4
8
9
1-Naphthyl
2-thiophene
C6H5
C6H5
77
64
10
11
C6H5
C6H5
4-MeC6H4
94
84
4-MeOC6H4
12
13
C6H5
1-Naphthyl
85
C6H5
C6H5
4-MeCOC6H4
2-Thiophene
64
45
14
a
Reaction condition: Aryl iodide (3.0 mmol), aryl alkynyl carboxylic acid (3.3 mmol), Pd-CNT (237 mg, 5 mol% based on 6.7 wt.%) and DUB (6.0 mmol) were reacted in DMSO
at 90 ꢀC for 12 h.
b
Isolated yield.
c
All compounds are characterized by comparison of 1H and 13C NMR spectra with authentic samples or literature data.

274
A. Pyo et al. / Journal of Organometallic Chemistry 724 (2013) 271e274
and 2). The yield was decreased to 64% when the amount of
palladium was reduced to 1 mol% (entry 3). Keeping 2.5 mol% of
palladium, we tried decreasing the reaction temperature to 90 ꢀC
and 70 ꢀC, and the coupled product was obtained in good yield at
90 ꢀC, but only a trace amount of product was formed at 70 ꢀC
(entries 4 and 5). This result is consistent with the need for
temperature over 80 ꢀC for the decarboxylation of alkynyl carbox-
ylic acid. Among the solvents tested, DMSO showed the best result,
and NMP, toluene, and diglyme afforded 76%, 12% and 24% yields,
respectively (entries 6e8).
Next, we expand the scope of aryl iodides and alkynyl carboxylic
acids in the decarboxylative coupling reactions. A variety of aryl
alkynyl carboxylic acids were easily prepared by our previously
reported method of palladium-catalyzed site-selective coupling
reaction of propiolic acid and aryl iodides. As shown in Table 2, the
reactions using aryl iodides gave the desired product in moderate
to good yields. As expected, iodobenzene afforded diphenylacety-
lene in 95% yield (entry 1). 4-Iodotoluene and 4-iodoanisole
produced the corresponding diaryl alkyne in 70% and 72% yields,
respectively (entries 2 and 3). Ortho-substituted iodobenzene such
as 1-iodo-2,4-dimethylbenzene showed lower yield than para-
substituted (entry 4). Iodobenzenes bearing electron-withdrawing
groups afforded the desired product in higher yields than others
(entries 5 and 6). However, 4-iodobenzonitrile showed a low yield
of product (entry 7). 1-Iodonaphthalene and 2-iodothiophene gave
the corresponding diaryl alkynes in 77% and 64% yields, respec-
tively (entries 8 and 9). Keeping iodobenzene as the coupling
partner, we employed a variety of aryl alkynyl carboxylic acids in
the decaroboxylative coupling reactions. P-Tolylpropiolic acid
and 3-(4-methoxyphenyl)propiolic acid showed 94% and 84%
yields, respectively (entries 10 and 11). 1-Naphthylpropiolic acid
produced the desired product in 85% yield (entry 12). However,
3-(4-acetylphenyl)propiolic acid and 3-(thiophen-2-yl)propiolic
acid showed somewhat lower yields (entries 13 and 14).
Acknowledgments
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2012R1A1B3000871)
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