Synthesis of CuO on mesoporous silica and its applications for coupling reactions of thiols with aryl iodides

Article abstract of DOI:10.1039/b918117b

Novel CuO on mesoporous silica is prepared under a convenient approach, and has been shown to be an efficient catalyst for cross-coupling reactions of thiols with aryl iodides with only 1.0-5.0 mol% catalyst loading.

Full text of DOI:10.1039/b918117b

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Synthesis of CuO on mesoporous silica and its applications for coupling
reactions of thiols with aryl iodidesw
Chin-Keng Chen,a Yan-Wun Chen,b Che-Hung Lin,a Hong-Ping Lin*b and Chin-Fa Lee*a
Received (in Cambridge, UK) 3rd September 2009, Accepted 12th November 2009
First published as an Advance Article on the web 25th November 2009
DOI: 10.1039/b918117b
Novel CuO on mesoporous silica is prepared under a convenient
approach, and has been shown to be an efficient catalyst for
cross-coupling reactions of thiols with aryl iodides with only
1.0–5.0 mol% catalyst loading.
prepare the silicate stock solution, a mixture of 4.0 g of sodium
silicate (SiO2: 27 wt%, Aldrich) and a 50.0 g of water was
added into a 25 mL of 1.50 M H2SO4 aqueous solution, and
the pH of the acidified silicate solution was adjusted to 5.0 at
40 1C by using 0.1 M NaOH aqueous solution. Then the pH
value of the resultant gel solution was raised to approximately
7.0 and further stirred for 2.0 h. Finally, that gel solution was
poured into a polypropylene bottle and hydrothermally
treated at 100 1C for 24 h without stirring. Filtration, drying
and calcination at 550 1C in air for 6 h gave the CuO on
mesoporous silica.
Aryl thioethers are important compounds in biological
chemistry,1,2 and the transition-metal-catalyzed cross-coupling
reactions of thiols with aryl halides represents the most common
strategy for constructing C–S bonds.3–8 The combination of
palladium,3 nickel,4 cobalt,5 iron6,7 with phosphines; indium8
with N,N,N0,N0-tetramethylethylenediamine (TMEDA) or
copper9 with a variety of ligands are well-known for C–S
bond forming reactions. Among them, the copper-catalyzed
(Ullmann-type reaction) C–S coupling is the most popular
approach in this field due to its low cost when compared with
most other metal catalysts.
Owing to the chelating capability of the silanol (–Si–OH)
and silanolate (–Si–Oꢀ) groups, hydrated copper ions could be
incorporated on the surface of the mesoporous silica at
pH = 7.0 (Scheme 1).
After calcination almost all of the CuO active sites were well
dispersed and strongly bound on the high-surface-area
mesoporous silica. TEM studies (Fig. 1A and B), depict a
wormhole-like mesostructure where CuO nanoparticles were
seldom observed. At higher magnification, the amorphous
structure was observed without any CuO lattice fringes. The
existence of two broad and low-intensity XRD diffraction
peaks at high-angle indicates that the CuO is well dispersed
onto the mesoporous silica matrix instead of forming CuO
nanoparticles from self-condensation (Fig. 1C). To further
investigate the dispersion and structure of the calcined CuO
on mesoporous silica, TEM and SEM EDX-mapping and
selected area electron diffraction were used. The similar
distribution profiles of Si, O and Cu elements and diffuse
ring-diffraction pattern indicate a good distribution of the
CuO active sites on the mesoporous silica in accordance with
the XRD result (Fig. S1 and S2, see ESIw). In appearance,
the CuO on the mesoporous silica sample was uniformly
aquamarine, rather than gray or black that was observed for
CuO nanoparticle-containing mesoporous silica prepared by
employing the impregnation method (Fig. S3, ESIw).
Recent studies have revealed that copper salts could be
reduced to 0.1 mol% level and still catalyze C–S coupling
reactions of thiols with aryl iodides,9n,o however, 20 mol% of
ligand is critical for this reaction. The authors claimed that the
excess ligand is needed to prevent the aggregation of the metal
during the catalysis. A similar phenomenon has been reported
in palladium catalysis.10 However, indium oxide11 and copper
oxide12 nanoparticles have been reported to promote the C–S
arylations without the assistance of additional ligand, but there
remains some drawbacks in these systems. First, KOH has been
shown to be the best base for these reactions and this might
seriously limit the tolerance towards base-sensitive functional
groups. Furthermore, poor yields were observed when electron-
rich iodides (for example, 4-iodoanisole) were used.
Herein we report the synthesis of a novel CuO on mesoporous
silica prepared via a convenient sol–gel method. These
well-dispersed CuO active sites appear to be highly active in
this transition-metal-catalyzed cross-coupling reaction.13 To
synthesize the CuO on mesoporous silica sample, 1.00 g of
gelatin (Acros) and 0.19 g of copper nitrate pentahemihydrate
(Sigma-Aldrich) was dissolved in 40.0 g of deionized water to
form a clear solution at 40 1C. Then, the Cu2+-gelatin solution
was added directly into a silicate stock solution under stirring,
and a light-blue precipitate was generated in a few minutes. To
a Department of Chemistry, National Chung Hsing University,
Taichung, Taiwan 402, ROC. E-mail: cfalee@dragon.nchu.edu.tw;
Fax: +886 4 2286-2547; Tel: +886 4 2284-0411 ext. 810
b Department of Chemistry, National Cheng Kung University, Tainan,
Taiwan 701, ROC. E-mail: hplin@mail.ncku.edu.tw;
Fax: +886 6 2740-552; Tel: +886 6 2757-575 ext. 65342
w Electronic supplementary information (ESI) available: Experimental
details, spectral data for products and TEM images of the CuO on
mesoporous silica after fourth run reaction. See DOI: 10.1039/b918117b
Scheme 1 Formation mechanism of the CuO on mesoporous silica
via a sol–gel reaction.
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Table 2 CuO on mesoporous silica-catalyzed S-arylation of aryl
iodides with thiolsa
Product, yield (%)
Product, yield (%)
13
14
15
1
2
3b, 93
3c, 72
3n, 96
3o, 95
3
4
3d, 69
3p, 66
3q, 80
16
17
3e, 86b,c
Fig. 1 (A) TEM image, (B) high-resolution TEM photograph, (C)
high-angle XRD pattern and (D) N2 adsorption–desorption isotherm
of the calcined CuO on mesoporous silica templated with gelatin.
5
6
7
3f, 79
3r, 87
3s, 74
3t, 87
18
19
20
Analyzing the N2 adsorption–desorption isotherm (Fig. 1D),
the BET surface area was about 285 m2 gꢀ1; and the pore size
calculated by the BJH method was centered at 7.7 nm.
Accordingly, the CuO that is incorporated into the mesoporous
silica became highly accessible to the environment, and well
dispersed active sites had been conveniently prepared and
should exhibit high activity for coupling reactions with the
added feature of recyclability.14,15
3g, 97b,c
3h, 68
8
9
3i, 95
3j, 73
3u, 72
3v, 77
Initially, 4-iodotoluene and 1-dodecanethiol were screend as
substrates in order to determine optimal reaction conditions.
The results are summarized in Table 1, the screening of bases
(entries 1–7) and solvents (entries 8–11; DME = dimethoxy-
ethane; dioxane = 1,4-dioxacyclohexane; DMF = N,N-di-
methylformamide) revealed that satisfactory results were
achieved when the reaction was performed with Cs2CO3 in
DMSO or dioxane at 110 1C.
21
22
23
10
11
3k, 95
3w, 76b,c
3x, 93
3l, 82b,c
3m, 82b,c
Table
1 Optimization of CuO on mesoporous silica-catalyzed
coupling of 4-iodotoluene with 1-dodecanethiola
24
12
3y, 72c
a
Reaction conditions unless otherwise stated: CuO on mesoporous
silica (0.01 mmol, 1.0 mol%), Cs2CO3 (1.5 mmol) in DMSO (1.0 mL).
Entries 1–12: aryl iodide (1.0 mmol), alkyl thiol (1.2 mmol); entries
b
13–24: aryl iodide (1.1 mmol), aryl thiol (1.0 mmol). 5 mol% of CuO
c
on mesoporous silica. Dioxane.
Base
Solvent
Yield (%)
1
2
3
4
5
6
7
8
KOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Dioxane
Toluene
DMF
81
35
Trace
85
81
Na2CO3
K2CO3
Cs2CO3
KOtBu
K3PO4
Et3N
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
In order to extend the scope of this catalytic system, we
examined a wide array of aryl iodides and thiols. The results
are summarized in Table 2, alkyl and aryl thiols can be
coupled with a series of electron-donating and -withdrawing
aryl iodides. The functional groups possessing free amine
(entry 3), free alcohols (entries 2, 8 and 16), nitro (entries 10
and 23) and nitrogen-containing heterocycles (entries 17–21)
were coupled smoothly with a wide selection of thiols. More
importantly, our system employed weak bases, tolerant of
functional groups such as hydrolyzable esters (entries 4, 5, 9,
12 and 22) and enolizable ketones (6, 11, 21 and 24).
55
Trace
85
80
62
27
9
10
11
H2O
a
Reaction conditions: CuO on mesoporous silica (0.01 mmol,
1.0 mol%), 4-iodotoluene (1.0 mmol), 1-dodecanethiol (1.2 mmol),
base (1.5 mmol) in 1.0 mL solvent.
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Table 3 Reuse of CuO on mesoporous silicaa
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an oven.
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´
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model to test the recyclability of CuO on mesoporous silica.
The results in Table 3 show the catalyst could be reused several
runs without loss of reactivity. According to the EDS analysis,
the Cu/Si atomic ratio of the sample before and after the
fourth run were almost the same (E1/20). From the TEM
images of the CuO on mesoporous silica after the fourth
reaction (Fig. S4, see ESIw), the mesostructures were clearly
visible and the presence of CuO nanoparticles (from aggregation)
were rarely found. These results indicated that the CuO active
sites remained highly stabilized on the mesoporous silica
during multiple catalytic trials.
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In conclusion, for the first time well dispersed CuO on
mesoporous silica with large surface area and robust active
sites has been prepared, by using natural polymer gelatin at
neutral pH. In addition, the CuO active sites were readily
accessible and are shown to be highly active catalysts for
coupling reactions between thiols and aryl iodides under
ligand-free conditions. Based on the high chelating capability
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The National Science Council, Taiwan (NSC 97-2113-M-
005-006-MY2 and NSC95-2113-M-006-011-MY3); the
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284 | Chem. Commun., 2010, 46, 282–284