COMMUNICATION
An efficient approach to homocoupling of terminal alkynes: Solvent-free
synthesis of 1,3-diynes using catalytic Cu(II) and base†
Dong Wang, Jihui Li, Na Li, Tingting Gao, Sihua Hou and Baohua Chen*
Received 24th August 2009, Accepted 19th October 2009
First published as an Advance Article on the web 30th October 2009
DOI: 10.1039/b917448f
We report an environmentally friendly, efficient method for
transforming terminal acetylenes into 1,3-diynes based on
catalytic amounts of a Cu(II) salt and base under solvent-
free conditions. The developed process conforms to the
principles of ‘green’ chemistry and addresses the shortage of
such methods for the synthesis of 1,3-diynes. The reaction
is quite general and results in good yields. Interestingly, the
system also allows the synthesis of unsymmetric 1,3-diynes
by cross-coupling of two different terminal alkynes. Finally,
the catalyst can also be recycled.
in supercritical carbon dioxide.9 These Pd-free catalytic systems
are efficient and economic, but their imperfections include the
requirements of stoichiometric amounts of amine reagents, high
pressure, high temperature, utilization of a co-catalyst and an
oxygen atmosphere.5h,7b,7c,10 Moreover, it should be pointed out
that the classical syntheses of conjugate 1,3-diynes (including the
Pd-catalyzed and the Pd-free systems) generally involve organic
solvents such as methanol, acetone, pyridine, methyl cellosolve
(2-methoxyethanol) and toluene. Use of organic solvents is en-
vironmentally unfriendly, and so it is highly desirable to develop
more environmentally friendly and economic methodologies for
synthesizing conjugate 1,3-diynes. To achieve this, we report an
environmentally friendly, economic, efficient and simple solvent-
free system that allows the homocoupling reactions of terminal
alkynes based on catalytic amounts of CuCl2 and triethylamine
at 60 ◦C in air. The method is also useful for the synthesis
of unsymmetric 1,3-diynes by cross-coupling of two different
terminal alkynes. The results are summarized below.
Introduction
One of the challenges facing chemists this century is to de-
velop new transformations that are not only efficient, selective,
and high-yielding but that are also environmentally benign.1
During the last decade, the topic of ‘green’ chemistry has
received increasing attention.1,2 ‘Green’ chemistry aims at the
total elimination (or at least the minimization) of waste, and
the implementation of sustainable processes.1a The utilization
of nontoxic chemicals, renewable materials and solvent-free
conditions are the key issues in a green synthetic strategy. In
the present work, we describe a green approach towards the
synthesis of conjugate 1,3-diyne.
Conjugate 1,3-diyne derivatives are very important materials
in the fields of biology and materials science, because they can be
converted into various structural entities, especially substituted
heterocyclic compounds.3 Traditional methods for the synthesis
of 1,3-diynes include Glaser oxidative dimerization of terminal
alkynes,4a various improved Glaser oxidative homocoupling re-
actions of terminal alkynes,4b–e and Sonogashira coupling.4f The
catalyst system used commonly is the Pd,5 which involves Cu(I)
salts as co-catalyst. Although Pd catalysts play a crucial role
because of their mild, efficient and selective properties, they are
expensive and often require phosphine or amine reagents.5e,5g,6
To address this, several groups have reported homocoupling
reactions of terminal alkynes using a Pd-free catalytic system.7
For example, D. F. Li et al. described the reaction in the
presence of CuI/I2,8 and H. F. Jiang et al. reported the Cu(II)-
promoted oxidative homocoupling reaction of terminal alkynes
Results and discussion
In order to identify the optimal reaction conditions, pheny-
lacetylene was chosen as a test substrate. In this preliminary
experiment, the homocoupling of phenylacetylene was carried
out in various solvents, with CuCl2 (3 mol%) as a catalyst and
Et3N (3 mol%) as a base, in air at 60 ◦C for 6 h (Table 1). It was
found that the reaction proceeded perfectly in toluene or benzene
(Table 1, entries 1 and 2), but that the yields decreased when the
reaction was carried out in other solvents (Table 1, entries 3–11).
However, the reaction surprisingly proceeded in excellent yields
(>96%) under solvent-free conditions (Table 1, entry 12).
The pure product was purified by column chromatography
or reduced-pressure distillation. Considering our goal of an
economic and environmentally friendly reaction, these solvent-
free conditions are clearly the most favorable.
Then we examined the influence of the catalysts on the yields.
The reaction did not occur without a catalyst (Table 2, entry 1),
and CuCl2 was more efficient than other Cu(0), Cu(I) and Cu(II)
catalysts (Table 2, entries 2–12). It was noted that Cu(OH)x/TiO2
produced the desired product in 65% yield in the absence of base
at 100 ◦C under an oxygen atmosphere (entry 2).
In the third set of experiments, we performed the reaction
with various bases under solvent-free conditions in air (Table 3).
As can be seen, the reaction could not be performed without a
base (Table 3, entry 1). Organic bases including primary amines,
secondary amines and tertiary amines were more effective than
inorganic bases, with triethylamine being the best. There was
Key Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province, Lanzhou, 730000, P. R. China; State Key
Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, 730000, P. R. China. E-mail: chbh@lzu.edu.cn
† Electronic supplementary information (ESI) available: Additional
experimental information. See DOI: 10.1039/b917448f
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 45–48 | 45
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