CuO hollow nanostructures catalyze [3 + 2] cycloaddition of azides with terminal alkynes

Article abstract of DOI:10.1039/b917781g

CuO hollow nanostructures have been used for the catalytic [3 + 2] cycloaddition of azides with terminal alkynes to provide the products in good yields with high regioselectivity.

Full text of DOI:10.1039/b917781g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
CuO hollow nanostructures catalyze [3 + 2] cycloaddition
of azides with terminal alkynesw
a
b
a
b
a
Jee Young Kim, Ji Chan Park, Hyuntae Kang, Hyunjoon Song* and Kang Hyun Park*
Received (in Cambridge, UK) 28th August 2009, Accepted 23rd October 2009
First published as an Advance Article on the web 13th November 2009
DOI: 10.1039/b917781g
CuO hollow nanostructures have been used for the catalytic
3 + 2] cycloaddition of azides with terminal alkynes to provide
mixed Cu/Cu-oxide, reported by the Ponti group, revealed
that it produced very good results—87 to 95% yield—for three
[
8
c
the products in good yields with high regioselectivity.
kinds of azide and ten kinds of internal and terminal alkynes.
In addition, heterogeneous systems such as copper-in-charcoal,
alumina-supported copper nanoparticles, and Cu(I)-modified
As metal oxide nanoparticles are very stable both physically
and chemically, they have recently been used frequently as
9
zeolites have been reported. Accordingly, this communication
1
metal catalysts. In addition, their distinct qualities as nano-
shows the best results of click reactions using well-designed,
uniform, hollow-structured CuO nanoparticles. Various
types of Cu nanoparticles are tested and help in reviewing
the efficiency of the reaction in terms of the shape of the
CuO nanoparticles and understanding the behavior of the
catalysts.
particles, such as large surface area, makes them applicable to
a wide range of fields. Among the metal oxide nanoparticles,
copper oxides (Cu O, CuO) are p-type semiconductor materials
2
with a low band gap energy. Recently, Tarascon’s group used
copper oxide (Cu O, CuO) nanoparticles as an anode for
2
2
lithium ion cells, while Izaki’s group employed them with
In the present study, an approach for gram-scale synthesis
ZnO, a n-type semiconductor material, for solar cell plates,
3
demonstrating their highly useful electrochemical characteristics.
of uniform Cu
was used (see ESIw) The CuO hollow nanostructures were
prepared by adding aqueous ammonia solutions to Cu
2
O nanocubes by a one-pot polyol process
10
In addition, copper oxide (Cu
2
O, CuO) nanoparticles have
O
2
sufficient space to adsorb harmful gases, as proven by the
application of copper oxide as a gas sensor by Yadong’s
nanocube colloidal solutions. Such hollow morphologies were
achieved mainly through a sequential dissolution–precipitation
process under precise control by changing the pH of the
reaction mixture in air. Increasing the pH of the solution led
to formation of hollow cubes, hollow spheres and urchin-like
particles (ESIw).
4
group. In addition, Cu(II) nanoparticles are non-toxic,
environmentally friendly, and highly stable, and as such, are
recyclable. The research presented within intends to use Cu(II)
nanoparticles for click chemistry.
Click chemistry is a chemical philosophy introduced in 2001
by Sharpless, and is important in understanding the behavior
Research studies testing the effectiveness of the catalysts
have used benzyl azide and phenylacetylene as the benchmark
substrates. The cycloaddition reaction of benzyl azide (1 mmol)
and phenylacetylene (1.5 mmol) with the prepared CuO
5
of low-weight molecules. Click chemistry has had a very big
impact on organic synthesis, drug discovery, and biological
6
applications. The desired triazole-forming cycloaddition may
nanostructures (4 mg, 5 mol%) in H O/t-BuOH (1.6 mL/
2
need high temperatures and, usually results in a mixture of the
0.8 mL) afforded 1,4-disubstituted 1,2,3-triazoles as a single
regioisomer. The CuO nanostructures catalyzed the reaction
sequence which regiospecifically combines azides and terminal
acetylenes to give only 1,4-disubstituted 1,2,3-triazoles.
As shown in Table 1, in order to determine the solvent
system most suitable for the catalyst, several experiments were
conducted. Considering the hygroscopic properties of THF,
dioxane and toluene, it is expected that the reaction conditions
will be affected by the amount of water present (entries 1–4,
1
,4 and 1,5 regioisomers (eqn (1)).
In general, as a catalyst, a Cu(I) salt is directly used, or Cu(II)
7
after reduction. Recently, reactions capitalizing upon the
11
Table 1). Both t-BuOH and water as solvents gave high
advantages of Cu(I) or Cu(II) nanoparticles have been
8
reported. There is a case where, from Cu nanoclusters of
yields under mild conditions (entries 5 and 6) though best
results were obtained with a solvent mixture of t-BuOH and
Cu(0), Cu(I) is generated in situ and Cu/Cu-oxide nanoparticles
H O (2 : 1) (entry 4), which indicates that solubility and
2
are used. In particular, the example of using air protected
hygroscopic properties are all important factors.
In the case of using commercially available CuO, less than
a
Department of Chemistry and Chemistry Institute for Functional
1
% yield was found under the same conditions and upon
Materials, Pusan National University, Busan 609-735, Korea.
E-mail: chemistry@pusan.ac.kr; Fax: 81 51 980 5200;
Tel: 82 51 510 2238
Department of Chemistry, Korea Advanced Institute of Science and
Technology, Daejeon, Korea
extending the reaction for 24 h, only a yield of 35% was
achieved. In addition, under optimum reaction conditions,
no reaction occurred without catalyst (entries 7–9, Table 1).
Next, in order to examine the characteristics of the catalyst
according to the various nanostructure morphologies, three
different types of CuO(II) nanostructures were tested.
b
w Electronic supplementary information (ESI) available: I: Experimental.
II: Characterization: H and C NMR spectra. III: Figures. IV: H and
C NMR spectra of products. See DOI: 10.1039/b917781g
1
13
1
13
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 439–441 | 439

View Article Online
Table 1 Optimization of click reaction catalyzed by various CuO(II) nanostructures
a
Conv. (%)
Entry
Catalyst (5 mol%)
T/1C
t/h
Solvent
THF–H
Dioxane/H
Toluene/H
1
2
3
4
5
6
7
8
9
CuO urchins
CuO urchins
CuO urchins
CuO urchins
CuO urchins
CuO urchins
60
100
110
25
25
25
25
25
25
25
12
12
12
3
3
3
24
3
3
3
3
2
O (24 : 1)
O (24 : 1)
O (24 : 1)
4
61
93
96
90
71
35
o1
0
2
2
H
H
2
O/t-BuOH (2 : 1)
O
2
t-BuOH
b
Commercial CuO
Commercial CuO
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O/t-BuOH (2 : 1)
O/t-BuOH (2 : 1)
O/t-BuOH (2 : 1)
O/t-BuOH (2 : 1)
O/t-BuOH (2 : 1)
O/t-BuOH (2 : 1)
O/t-BuOH (2 : 1)
b
—
CuO urchins
CuO hollow spheres
CuO hollow cubes
CuO hollow spheres
c
10
11
12
13
93
100c
c
25
25
25
3
0.5
94
98
d
a
1
Determined by H NMR spectra. Purchased from Aldrich (nanopowder, cat No. 544868). Conversion based upon an average of two runs.
b
c
d
3
In the presence of 1.0 eq. Et N.
In general, the effect of catalytic performance can be viewed
benzyl azide. The corresponding triazoles 1-benzyl-4-(phenoxy-
methyl)-1H-1,2,3-triazole were obtained in high yields
(entry 3). Hydroxy-substituted alkynes such as propargyl
alcohol and 1-phenyl-2-propyn-1-ol also gave the expected
adducts (1-benzyltriazol-4-yl)methanol and (1-benzyl-1H-
1,2,3-triazol-4-yl)(phenyl)methanol as single regioisomers in
good to high yields (entries 4 and 5). The reaction with alkynes
containing electron-withdrawing substituents such as ethyl
propiolate and methyl propiolate gave distinctly high yields
(entries 6 and 7) while reactions with aliphatic alkynes such as
ethynyltrimethylsilane was relatively sluggish (entry 8). In a
second series of experiments, various azides bearing different
groups were reacted with phenylacetylene. When the phenyl
group is directly linked to the reactive azide, Phenyl azide or
its analogue with a para-methoxy group gave the expected
triazoles 3i and 3j as single regioisomers. A single regioisomer
was always produced whatever the substitution and yields of
isolated products remained excellent.
according to the shape of nanoparticles in terms of: (1) surface
area; (2) the atoms on corners and edges; (3) exposed crystal
1
2
faces. The activity trend is: CuO hollow spheres > CuO
hollow cubes > CuO urchins. In the case of the CuO(II)
nanostructures used herein, it is suggested that this is simply
a simple surface area effect, rather than an increase in reactivity
at certain defect sites (entries 10–12, Table 1). The active
surface areas and the crystallite sizes in the different CuO
nanostructures would be main factors to decide their catalytic
activities. The CuO nanostructures with different shapes were
obtained by controlling the dissolution and precipitation rates
of Cu O nanocubes under basic condition. The single crystal-
2
line domain sizes of the CuO nanostructures were measured to
be 2 nm for hollow cubes, 4 nm for hollow spheres, and >20 nm
for urchin-like particles, respectively. The fast precipitation of
CuO hollow cubes compared to urchin-like particles leads to
smaller crystallite sizes (B2 nm), maintaining the original cube
frame in the hollow particles. The slow CuO growth and
precipitation rates of urchin-like particles lead to the largest
single crystalline domains (>20 nm). Generally, the smaller
the size of the particles the higher is the active surface area
leading to enhanced activity in catalysis. However, CuO
hollow spheres showed slightly better activity than that of
hollow cubes in Table 1. The lower activity of hollow cubes
with smallest crystalline domains than that of hollow spheres
can explained by their effective surface areas. The BET
Additionally, any changes in the shape of the catalyst after
the reaction was examined. As shown in Fig. 1 (and ESIw), the
structure of CuO hollow spheres remained unchanged after
the reaction, showing recyclability of the catalyst. After the
reaction, the CuO hollow spheres was separated through
centrifugation and used in click reactions with phenylacetylene
at least three times without loss of catalytic activity. However,
for more recycling, the collected quantity of the catalyst
decreased (the quantity of the first use was 4.0 mg), and thus
the yields were lowered. In order to compensate for this,
current work in this lab is focused on immobilizing CuO
hollow nanostructures on a support material such as charcoal
for use as an improved heterogeneous catalyst.
(
Brunauer–Emmett–Teller) surface areas of CuO nanostructures
2
measured by nitrogen sorption experiments are 79 m g for
À1
2
hollow cubes, 113 m g for hollow spheres, and 81 m g
À1
2
À1
for urchin-like particles, respectively, and it is quite reasonable
that the catalytic activities are also dependent on the active
In summary, oxidation of Cu O nanocubes has been
2
surface areas of the catalysts. When Et N (1.0 eq.) was
3
controlled to yield CuO hollow cubes, hollow spheres or urchin-
like particles, through a sequential dissolution–precipitation
process. Various types of CuO nanoparticles have been used
for the catalytic [3 + 2] cycloaddition of azides with terminal
alkynes to provide products in good yields with high regio-
selectivity. Reaction conditions were substantially improved in
added, the reaction was faster and was complete within
3
0 min (entry 13, Table 1).
Furthermore, good results were achieved using various
terminal alkynes (Table 2). Acetylenes conjugated with an
ester group such as phenyl propargyl ether reacted readily with
4
40 | Chem. Commun., 2010, 46, 439–441
This journal is ꢀc The Royal Society of Chemistry 2010

View Article Online
Table 2 [3 + 2] Cycloaddition of aides with terminal alkynes in the
presence of CuO hollow spheres
Conv
(%)
a
Entry Azide
1
Alkyne
Product
100
2
3
4
5
6
7
42
100
100
62
Fig. 1 TEM images of CuO hollow spheres before (a) and after use
(
b). The scale bars represent 200 nm. Recycling of click reactions
catalyzed by CuO hollow spheres (c).
Notes and references
100
100
1
C. Burda, X. B. Chen, R. Narayanan and M. A. El-Sayed, Chem.
Rev., 2005, 105, 1025.
2
P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.-M. Tarascon,
Nature, 2000, 407, 496.
3 M. Izaki, T. Shinagawa, K. Mizuno, Y. Ida, M. Inaba and
A. Tasaka, J. Phys. D: Appl. Phys., 2007, 40, 3326.
J. Zhang, J. Liu, Q. Peng, X. Wang and Y. Li, Chem. Mater., 2006,
8, 867.
H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int.
Ed., 2001, 40, 2004.
(a) K. H. Bleicher, H. J. Bohm, K. Muller and A. I. Alanine,
Nat. Rev. Drug Discovery, 2003, 2, 369; (b) J. F. Pritchard,
M. J. Komet, M. L. J. Reimer, E. Mortimer, B. Rolfe and
M. N. Cayen, Nat. Rev. Drug Discovery, 2003, 2, 542.
(a) L. V. Lee, L. Michaell, S. J. Huang, V. V. Fokin,
K. B. Sharpless and C. H. Wong, J. Am. Chem. Soc., 2003, 125,
4
5
6
1
8
9
45
100
7
8
9
588; (b) P. Appukkuttan, W. Dehaen, V. V. Fokin and V. Eycken,
10
100
Org. Lett., 2004, 6, 4223; (c) A. K. Feldman, B. Colasson and
V. V. Fokin, Org. Lett., 2004, 6, 3897.
(a) L. D. Pachon, J. Maarseveen and G. Rothenberg, Adv. Synth.
´
a
1
Determined by H-NMR. Yields are based on the amount of azide used.
Catal., 2005, 347, 811; (b) M. L. Kantama, V. S. Jayaa,
B. Sreedhara, M. M. Raoa and B. M. Choudaryb, J. Mol.
Catal. A: Chem., 2006, 256, 273; (c) G. Molteni, C. L.
Bianchi, G. Marinoni, N. Santo and A. Ponti, New J. Chem.,
2006, 30, 1137.
comparison with previous catalytic systems and commercially
available micro-size CuO. In addition, reactions were
9
B. H. Lipshutz and B. R. Taft, Angew. Chem., Int. Ed., 2006, 45,
3
accelerated with stoichiometric Et N. Efforts are ongoing to
8
235.
0 J. C. Park, J. Kim, H. Kwon and H. Song, Adv. Mater., 2009, 21,
explore the scope and limitations of the heterogeneous version
of CuO hollow catalysis of the click reaction.
1
803.
11 H. A. Orgueira, D. Fokas, Y. Isome, P. C. Chan and
C. M. Baldino, Tetrahedron Lett., 2005, 46, 2911.
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
1
2 (a) R. Narayanan and M. A. Sayed, Nano Lett., 2004, 4, 1343;
b) R. Narayanan and M. A. Sayed, J. Am. Chem. Soc., 2004, 126,
194; (c) K. H. Park, K. H. Jang, H. J. Kim and S. U. Son, Angew.
Chem., Int. Ed., 2007, 46, 1152.
(
NRF) funded by the Ministry of Education, Science and
(
7
Technology (2009–0070926) and by the Korean Research
Foundation Grant (KRF-2006-312-C00565).
This journal is ꢀc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 439–441 | 441