Angewandte
Chemie
DOI: 10.1002/anie.200705126
Homogeneous Catalysis
Efficient Oxidative Alkyne Homocoupling Catalyzed by a Monomeric
Dicopper-Substituted Silicotungstate**
Keigo Kamata, Syuhei Yamaguchi, Miyuki Kotani, Kazuya Yamaguchi, and Noritaka Mizuno*
The versatility and accessibility of polyoxometalates have led
to various applications in the fields of analytical chemistry,
medicine, electrochemistry, photochemistry, and catalysis,[1]
particularly in the field of oxidation catalysis. Interest in
catalysis by metal-substituted polyoxometalates has grown
significantly because of the unique reactivity that results from
the composition and structure of their active sites. To date,
various kinds of metal-substituted polyoxometalates have
been synthesized and applied in selective oxidation reac-
tions.[1,2]
This reaction mechanism has generally been accepted,
although some detailed mechanistic workis still necessary.
[7]
Thus, although it is expected that the homocoupling reaction
should proceed efficiently in the presence of catalysts with a
dicopper(II) core on the basis of this mechanism, an alkyne
homocoupling reaction catalyzed by complexes with a
dicopper(II) core is as yet unknown.[4–8]
Herein we report that the dicopper-substituted g-Keggin
silicotungstate
TBA4[g-H2SiW10O36Cu2(m-1,1-N3)2]
(I,
Figure 1; TBA = tetra-n-butylammonium)[9] is an effective
1,3-Diyne derivatives are very important materials in
biological, polymer, and materials science because they can
be converted into various structural entities, especially
substituted heterocyclic compounds.[3] Oxidative alkyne–
alkyne coupling is a good candidate for the synthesis of 1,3-
diyne derivatives. Copper salts (stoichiometric amounts,
Glaser conditions),[4] copper salts with appropriate nitrogen
bases and molecular oxygen (catalytic, Hay conditions),[5] and
a combination of copper and palladium salts (catalytic)[6] have
commonly been used to promote oxidative alkyne–alkyne
coupling.[7] However, most copper-catalyzed systems have
shortcomings, especially their low turnover numbers, the
formation of significant amounts of by-products, severe
catalyst deactivation, narrow applicability to a limited
number of alkynes, and/or the need for additives such as
bases and co-catalysts.
Figure 1. Polyhedral and ball-and-stick representation of the anion in
TBA4[g-H2SiW10O36Cu2(m-1,1-N3)2] (I). The {WO6} and {SiO4} units are
shown as gray octahedra and a blue tetrahedron, respectively. Blue and
green spheres show the copper and nitrogen atoms, respectively.
Dimeric dicopper-substituted silicotungstates with azide ligands have
been reported by Mialane and co-workers.[9]
In 1964 Bohlmann and co-workers proposed that the
copper(II)-catalyzed alkyne homocoupling reaction proceeds
via the formation of the alkynyldicopper(II) intermediate
ꢀ
{Cu2(m-C CR)2}, which would react further to give the 1,3-
diyne products directly (see the Supporting Information).[8]
[*] Dr. K. Kamata, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
homogeneous catalyst for the oxidative homocoupling of
various kinds of structurally diverse alkynes [Eq. (1)]. Cata-
lyst I can easily be recovered after the reaction and reused
with retention of its high catalytic performance. The mech-
anism of the present homocoupling reaction is also inves-
tigated.
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)358-417-220
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Dr. K. Kamata, Dr. S. Yamaguchi, M. Kotani, Dr. K. Yamaguchi,
Prof. Dr. N. Mizuno
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
I
2 RCꢀCH þ 1=2 O2
RCꢀCÀCꢀCR þ H O
ð1Þ
!
2
The oxidative homocoupling of phenylacetylene (1a) to
give 1,4-diphenyl-1,3-butadiyne (2a) was carried out first
under various conditions (Table 1). Among the solvents
tested, benzonitrile gave 2a in the highest yield (91%;
Table 1, entry 1).[10] The reaction proceeded efficiently even
under 1 atm of air, although a longer reaction time was
required (Table 1, entry 2). Polar solvents such as DMSO,
DMF, and acetonitrile gave 2a in 39, 39, and 15% yields,
[**] This work was supported by the Core Research for Evolutional
Science and Technology (CREST) program of the Japan Science and
Technology Agency (JST) and a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Science, Sports and
Technology of Japan. We are grateful to Dr. S. Shinachi, Y. Fujita, and
T. Katayama (University of Tokyo) for their experimental help.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2008, 47, 2407 –2410
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2407