1,3-Diynes synthesis by homo-coupling of terminal alkynes using a Pd(PPh3)4/Ag2O simple catalyst system

Article abstract of DOI:10.1016/j.jorganchem.2011.01.022

Amine- and copper salt-free palladium-catalyzed homo-coupling reaction of terminal alkynes proceeded efficiently in the presence of silver(I) oxide, which served as both activator and oxidant, in tetrahydrofuran at 60 °C to achieve satisfactory yields of 1,3-diyne compounds. It was demonstrated for the first time by means of XPS analysis that Pd(0) species can be oxidated to Pd(II) by silver(I) oxide.

Full text of DOI:10.1016/j.jorganchem.2011.01.022

Journal of Organometallic Chemistry 696 (2011) 1479e1482
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
1,3-Diynes synthesis by homo-coupling of terminal alkynes
using a Pd(PPh3)4/Ag2O simple catalyst system
,
c
c
a
a,
**
Xiujuan Feng a, Ziran Zhao b , Fan Yang , Tienan Jin , Yongjie Ma , Ming Bao
*
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b Department of Plastic and Reconstructive Surgery, The First Hospital of Jilin University, Changchun 130021, China
c Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Amine- and copper salt-free palladium-catalyzed homo-coupling reaction of terminal alkynes proceeded
efficiently in the presence of silver(I) oxide, which served as both activator and oxidant, in tetrahydro-
furan at 60 ꢀC to achieve satisfactory yields of 1,3-diyne compounds. It was demonstrated for the first
time by means of XPS analysis that Pd(0) species can be oxidated to Pd(II) by silver(I) oxide.
Ó 2011 Elsevier B.V. All rights reserved.
Received 29 November 2010
Received in revised form
8 January 2011
Accepted 18 January 2011
Keywords:
1,3-Diyne synthesis
Homo-coupling
Terminal alkyne
Palladium catalyst
1. Introduction
research on the development of facile and effective carbonecarbon
bond forming reactions using palladium catalysts, we found that
Development of efficient methods for the synthesis of 1,3-diynes
has attracted considerable attention because these compounds can
be utilized as equivalents of various functional groups and valuable
intermediates in organic synthesis. Over the past four decades,
a significant number of catalyst systems have been developed for
the homo-coupling reaction of terminal alkynes, which include
palladium [1e5], nickel [6,7], copper [8e17], cobalt [18], and gold
[19] catalyst systems. Among them, the palladium catalyst systems
have been shown to be the most efficient and selective for the
synthesis of 1,3-diynes. However, the palladium-catalyzed homo-
coupling reaction of terminal alkynes often requires the use of an
excess amount of amine as a base, which has characteristic foul
smell and pungent flavor. Moreover, the presence of a stoichio-
metric quantity of an oxidant and a copper(I) salt as an activator are
required for the palladium-catalyzed homo-coupling reaction of
terminal alkynes in most cases.
homo-coupling reaction of terminal alkynes occurred under the
same reaction conditions in the absence of aryl iodide (Scheme 1). In
the current study, we report a simple palladium catalyst system for
the synthesis of 1,3-diyne compounds as well as the evidence that
Pd(0) species can be oxidated to Pd(II) by silver(I) oxide.
2. Results and discussion
2.1. Optimization of the reaction conditions
The homo-coupling reaction of phenylacetylene (1a) was chosen
as a model reaction for the optimization of the reaction conditions.
The results are shown in Table 1. When the reaction of 1a was
R2
I
R2
In 2000, A. Mori and co-workers reported that the cross-coupling
reaction of terminal alkynes with aryl iodides proceeded with high
efficiency in the presence of Pd(PPh3)4 (5 mol%) as catalyst and Ag2O
(one equivalent) as activator in tetrahydrofuran (THF) at 60 ꢀC
(Scheme 1) [20]. Interestingly, in the course of our continuous
R1
cat Pd(PPh3)4
Ag2O
R1
Reported work
THF, 60 °C
R1
Our observation
R1
* Corresponding author. Tel.: þ86 431 88782960; fax: þ86 431 85654528.
** Corresponding author. Tel.: þ86 411 39893862; fax: þ86 411 39893687.
E-mail addresses: dczhaoziran@126.com (Z. Zhao), mingbao@dlut.edu.cn (M. Bao).
Scheme 1. Palladium-catalyzed cross-coupling and homo-coupling reactions of
terminal alkynes.
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.01.022

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X. Feng et al. / Journal of Organometallic Chemistry 696 (2011) 1479e1482
Table 1
Optimization for the palladium-catalyzed homo-coupling reaction of
phenylacetylenea.
Table 2
Palladium-catalyzed homo-coupling reaction of terminal alkynesa.
Pd(PPh3)4, Ag2O
THF, 60 °C, 12 h
Pd(PPh3)4 (1 mol%)
R
R
R
Ag2O (1.0 equiv)
THF, 60 °C
1a
2a
1
2
Entry
Catalyst (mol%)
Ag2O (equiv)
Yield (%)b
Entry Substrate (1)
Time (h) Product (2) Yield (%)b
1
2
3
4
5
Pd(PPh3)4 (5)
Pd(PPh3)4 (2)
Pd(PPh3)4 (1)
Pd(PPh3)4 (0.5)
Pd(PPh3)4 (1)
1.0
1.0
1.0
1.0
0.5
95
95
91
52
66
1
1a
1b
3
3
2a
2b
92
93
a
Reaction conditions: 0.5 mmol of phenylacetylene, palladium catalyst (1 mol%,
2 mol%, and 5 mol%, respectively), Ag2O (0.5 mmol and 0.25 mmol, respectively) in
2.5 mL of THF at 60 ꢀC for 12 h.
MeO
2
b
Isolated yield.
N
carried out under the similar conditions as employed in the cross-
coupling reaction of terminal alkyne with aryl iodide [20], desired
homo-coupling product 2a was isolated in 95% yield (entry 1). The
same excellent yield was also obtained even when the catalyst
loading was decreased from 5 mol% to 2 mol% (95%, entry 2).
A slightly decreased yield was obtained when the catalyst loading
was further decreased to 1 mol% (91%, entry 3). Product 2a was
obtained in only 52% yield when the catalyst loading was decrea-
sed from 1 mol% to 0.5 mol% (entry 4). In comparison with the result
in entry 3, a moderate yield was obtained when the amount of
Ag2O was decreased from one equivalent to 0.5 equivalents (66%,
entry 5). Therefore, 1 mol% of catalyst loading was chosen and one
equivalent of Ag2O was employed in the subsequent study.
3
1c
6
3
2c
90
91
4
1d
2d
5
6
7
1e 18
1f 18
1g 18
2e
2f
78
89
90
2.2. Homo-coupling reaction of various terminal alkynes
2g
Homo-coupling reactions with a wide range of terminal alkynes
were tested, and the results are summarized in Table 2. The reac-
tions of phenylacetylene (1a) and 4-methoxy phenylacetylene (1b)
proceeded efficiently to achieve homo-coupling products 2a and 2b
in excellent yields (92% and 93%, respectively, entries 1 and 2).
Substrates 2-pyridinyl acetylene (1c) and 1-cyclohexenyl acetylene
(1d) also exhibited high reactivities toward furnishing products 2c
and 2d in 90% and 91% yields, respectively (entries 3 and 4).
Reactions of aliphatic alkynes 1eeg needed prolonged reaction
O
8
1h 18
1i 18
2h
2i
80
85
HO
9
time than those of aromatic alkynes (1aec) and
p-conjugated
alkyne (1d) to obtain homo-coupling products 2eeg in 78e90%
yields (entries 5e7). It was found that the reaction conditions
employed could tolerate ketone carbonyl group, hydroxy group,
and also an aldehyde group, which is well known to be easily
oxidated to carboxyl group in the presence of a usual oxidant. For
example, the homo-coupling reactions of hept-6-yn-2-one (1h),
prop-2-yn-1-ol (1i), and 2-ethynylbenzaldehyde (1j) proceeded
efficiently to obtain corresponding products 2hej in 80e86% yields
(entries 8e10). Subsequently, we examined the reactions of aryl
propargyl ethers 1kem, and obtained the corresponding products
2kem in 64e88% yields (entries 11e13).
10
1j
6
2j
86
64
CHO
O
11
12
1k 12
2k
O
1l 12
2l
84
88
2.3. Mechanism of the reaction
Et
O
13
1m 12
2m
X-ray photoelectron spectroscopy (XPS) experiments were
performed to reveal the reaction mechanism. After a mixture of Pd
(PPh3)4 (57.8 mg, 0.05 mmol) and Ag2O (11.6 mg, 0.05 mmol) in THF
(1.5 mL) was treated at 60 ꢀC under a nitrogen atmosphere for 12 h,
the solvent THF was evaporated completely. The residue obtained
was introduced for the determination of binding energies of pal-
ladium and silver 3d3/2, 3d5/2 [21]. Figs. 1 and 2 show the XPS
a
A mixture of alkyne (0.5 mmol), Ag2O (0.5 mmol), and Pd(PPh3)4 (1 mol%) in
tetrahydrofuran (2.5 mL) was stirred at 60 ꢀC under a nitrogen atmosphere for the
period indicated in the table.
b
Isolated yield.

X. Feng et al. / Journal of Organometallic Chemistry 696 (2011) 1479e1482
1481
R
R
Ag2O + H2O
Pd 3d
2
335.3
Pd(0)
340.6
Pd3d
5/2
Pd3d
HNO3
3/2
Ag
Ag2O
R
(a)
341.6
Pd(II)(OH)2
Pd
R
336.6
4
Pd3d
3/2
Pd3d
5/2
Ag
R
2
2 AgOH
(b)
3
H2O
H2O
Ag2O
2
R
1
330
335
340
345
350
Scheme 2. A proposed mechanism for the palladium-catalyzed homo-coupling reac-
tions of terminal alkynes.
Binding Energy (eV)
Fig. 1. Pd 3d XPS spectra. (a) Before the Pd(PPh3)4 was treated with Ag2O. (b) After the
Pd(PPh3)4 was treated with Ag2O.
which can be easily reconverted to Ag2O via an oxidation reaction
with concentrated nitric acid. The Pd(II) species would react with
alkynyl silver 3 derived from the reaction of alkyne 1 with Ag2O to
generate dialkynyl palladium intermediate 4, and regenerate Ag2O
upon transmetalation. Reductive elimination of 4 would give the
desired 1,3-diyne compound 2, and regenerate Pd(0) catalyst.
spectra of Pd(3d) and Ag(3d), respectively. The Pd 3d XPS spectra
(Fig. 1) indicated that after the treatment with Ag2O, Pd0 (corre-
sponding to the peak at 335.3 eV, spectrum (a)) was completely
oxidated to Pd2þ (corresponding to the peak at 336.6 eV, spectrum
(b)) [22]. Furthermore the Ag 3d XPS spectra in Fig. 2 reveal that in
the oxidation reaction of Pd(PPh3)4 with Ag2O, the oxidant Ag2O
(the Ag 3d5/2 binding energy for Agþ is 367.7 eV, spectrum (a)) was
reduced to metallic Ag (the Ag 3d5/2 binding energy for Ag0 is
368.1 eV, spectrum (b)) [23].
Based on the XPS experiment results described above, the
mechanism of the Pd(PPh3)4/Ag2O catalyst system- promoted
homo-coupling of terminal alkynes was proposed as shown in
Scheme 2. The oxidation reaction of Pd(0) species with Ag2O and
H2O would occur to produce a Pd(II) species along with metallic Ag,
3. Conclusion
In conclusion, we have successfully developed a facile and
efficient method for the preparation of 1,3-diynes using a Pd
(PPh3)4/Ag2O simple catalyst system. A variety of terminal alkynes
including aliphatic and aromatic acetylenes underwent the effi-
cient palladium-catalyzed homo-coupling reaction smoothly to
produce corresponding 1,3-diynes in good to excellent yields. It is
worthy to note that the reaction conditions employed can tolerate
ketone carbonyl group, hydroxy group, and an aldehyde group. The
additive Ag2O acts as both activator and oxidant. It was demon-
strated for the first time by means of XPS analysis that Pd(0) species
can be oxidated to Pd(II) by Ag2O.
Ag 3d
367.7
Ag3d
373.7
5/2
4. Experimental section
Ag3d
3/2
4.1. General methods
(a)
368.1
All organic starting materials were analytically pure and were
used without further purification. 1H and 13C NMR spectra were
recorded in CDCl3 solution on a Varian Inova-400 spectrometer
374.2
Ag3d
5/2
Ag3d
3/2
(400 MHz for 1H, 100 MHz for 13C). 1H chemical shifts (
d
) were
referenced to Me4Si, and 13C chemical shifts (
d) were referenced to
internal solvent resonance. IR spectra were recorded on a NEXUS
FT-IR spectrometer. High resolution mass spectra (HRMS) were
recorded on a Q-TOF mass spectrometry (Micromass, England)
equipped with Z-spray ionization source. X-ray photoelectron
spectra (XPS) were recorded on VG ESCALAB MK2 electron spec-
(b)
trometer. A Al-K
a
X-ray source (hv ¼ 1486.6 eV) was operated at
360
365
370
375
380
250 W and 12.5 eV. The working pressure in the analyzing chamber
was less than 5 ꢁ 10ꢂ8 Pa. The experimental resolution, charac-
terized by full width at half maximum (FWHM) of the Ag 3d5/2 line,
is 1.45 eV if the pass energy of the analyzer is equal to 50 eV.
Binding Energy (eV)
Fig. 2. Ag 3d XPS spectra. (a) Before the Ag2O was treated with Pd(PPh3)4. (b) After the
Ag2O was treated with Pd(PPh3)4.

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X. Feng et al. / Journal of Organometallic Chemistry 696 (2011) 1479e1482
4.2. Representative procedure for the palladium-catalyzed homo-
coupling reaction of terminal alkynes
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (No. 20972021) for their financial support. This work was also
supported by the Specialized Research Fund for the Doctoral
Program of Higher Education (No. 20090041110012) and the State
Key Laboratory of Fine Chemicals (KF0607).
To a mixture of Pd(PPh3)4 (5.8 mg, 0.005 mmol) and Ag2O
(115.9 mg, 0.5 mmol) in THF (2.5 mL) was added phenylacetylene
1a (50.1 mg, 0.5 mmol), and then the mixture was stirred at 60 ꢀC
under a N2 atmosphere. The reaction progress was monitored by
TLC. After the 1a consumed, the solvent was removed under
a reduced pressure. The product was filtered through a short silica
column with petroleum ether (60e90 ꢀC) to remove palladium
residue and then was purified with a silica column using petroleum
ether (60e90 ꢀC) as eluent, giving 1,3-diyne product 2a in 92% yield
Appendix. Supplementary material
Experimental procedures, characterization data for all homo-
coupling products and 1H NMR spectra for the corresponding
products. This material can be found in the online version at doi:10.
1016/j.jorganchem.2011.01.022.
(46.5 mg) as a white solid [1]. 1H NMR (400 MHz, CDCl3)
d
7.55e7.51
132.7,
(m, 4H), 7.38e7.30 (m, 6H); 13C NMR (100 MHz, CDCl3)
d
129.4, 128.6, 122.0, 81.7, 74.1.
All the homo-coupling products 2 were identified through their
NMR and/or HRMS data as well as IR spectra. Among them, the
products 2h, 2k, and 2m are unknown compounds.
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d
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d 207.9, 76.6, 65.9, 42.0, 30.0, 22.0, 18.4; IR
(neat) 2940, 2147, 1711, 1428, 1355, 1222, 1158, 950, 731 cmꢂ1
;
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