Microwave efficient S-arylation of thiols with aryl iodides using supported metal nanoparticles

Article abstract of DOI:10.1039/b818767c

An efficient and green microwave-assisted heterogeneous S-arylation protocol, reported for the first time, provided excellent coupling conversions and selectivities in minutes of reaction using Fe (homocoupling) and Cu (cross-coupling) supported metal nanoparticles.

Full text of DOI:10.1039/b818767c

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Microwave efficient S-arylation of thiols with aryl iodides using
supported metal nanoparticlesw
Camino Gonzalez-Arellano, Rafael Luque*z and Duncan J. Macquarrie
Received (in Cambridge, UK) 23rd October 2008, Accepted 10th December 2008
First published as an Advance Article on the web 22nd January 2009
DOI: 10.1039/b818767c
An efficient and green microwave-assisted heterogeneous
S-arylation protocol, reported for the first time, provided
excellent coupling conversions and selectivities in minutes of
reaction using Fe (homocoupling) and Cu (cross-coupling)
supported metal nanoparticles.
and supported metal nanoparticles is the way forward in
future green catalytic protocols.
Inspired by the results of Correa et al. with the use of Fe as
catalyst for C–S couplings, we have prepared a series of supported
Fe and Cu supported metal nanoparticles (SMNP) and investi-
gated their activity and selectivity in the heterogeneously-
catalysed S-arylation model (Scheme 1, thiophenol with phenyl
iodide) under microwave irradiation.
C–S couplings are of great importance in pharmaceutically active
1
2
compounds and polymeric materials. A range of transition
metal catalysts have been investigated in this cross-coupling
reaction that involves the coupling of an aryl halide with a thiol
to give a thioether (Scheme 1). These include homogeneous
To the best of our knowledge, this is the first example of a
heterogeneously catalysed microwave C–S coupling protocol
using supported metal nanoparticles.
3
3a,4
1c,5
6
7
palladium, , nickel, copper, cobalt and more recently iron
catalysts. However, S-arylation has been comparatively less
studied than other coupling reactions due to the tendency of
In a typical synthesis of catalytically active materials,
0.6 mmol n-dodecylamine (0.14 mL), 2 mmol tetraethoxyortho-
silicate (TEOS, 0.45 mL), 0.1–0.2 g metal salt (i.e. FeCl
CuCl ), 2 mL water and 2 mL acetonitrile were added to a
or
2
7
thiols to undergo oxidative homocoupling S–S reactions.
Furthermore, organic sulfur compounds have also a significant
metal binding efficiency that leads to catalyst modification and/or
2
microwave tube and microwaved for 1 to 15 min at 200 W
(maximum temperature reached 40–110 1C depending on the
material). Materials were denoted as M-MW-HMS where M
stands for the metal (Fe or Cu employed as precursor), MW for
microwave protocol and HMS for hexagonal microporous
silica as indicated in powder X-ray diffraction patterns
(Fig. 1S, ESIw). Alternatively, the preparation of the materials
was carried out under conventional heating (70 1C), under
identical conditions to those reported in the microwave
protocol, to compare results. These solids were similarly
denoted as M-HMS.
2a
deactivation. Correa et al. recently reported the iron catalysed
S-arylation using homogeneous Fe systems as catalyst, achieving
7
high selectivities to the cross-coupling product. However, the
catalysts are not reusable and the use of NaOtBu as base requires
the use of dried organic solvents (e.g. toluene) under inert atmo-
sphere. Furthermore, the energy efficiency of the reaction is
relatively low as high temperatures (4130 1C) and long times
of reaction (24 h) are needed to achieve satisfactory yields.
Consequently, there is a need to develop alternative efficient
and facile protocols aiming to provide more efficient and
environmentally friendly reactions. We have recently reported
the preparation of a range of supported metal nanoparticles
TEM micrographs of some of the silica materials are shown
in Fig. 1 and Fig. 4S (ESIw). Nanoparticles (2–3 nm average
size) could be clearly seen in the prepared materials, in good
agreement with our previously reported results using similar
(e.g. Pd, Au, Ag, Fe) on porous materials with a variety of uses
in catalysis. These promising supported nanoparticles have
8
8
reported protocols. Textural properties of the supported
8a
been shown to be highly active and selective in C–C couplings
metal nanoparticles have been summarised in Table 1S (ESIw).
Nitrogen adsorption measurements showed that materials had
a noticeably marked microporosity with a 0.8–1.2 nm average
8b,c
and oxidations due to their outstanding properties including
a large surface to volume ratio, tiny particle sizes (o10 nm)
and narrow particle size distributions as well as high metal
dispersion on the porous surfaces.
2
ꢀ1
pore diameter and high surface areas (200–600 m g ) with a
significant contribution of macroporosity (over 30 nm) found
Microwaves have also been reported to speed up the rates of
reactions (from hours to minutes) as well as improving yields
9
and influencing selectivities. The combination of microwaves
Green Chemistry Centre of Excellence, The University of York,
Heslington, York, UK. E-mail: q62alsor@uco.es;
Fax: +34 957 212066; Tel: +34 957 218623
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b818767c
z Current address: Dpto. Quı
Campus de Rabanales, Edificio Marie Curie, Ctra Nnal IV, Km 396,
E14014, Cordoba, Spain.
mica Organica, Universidad de Cordoba,
´ ´ ´
´
Scheme 1 S-arylation reaction of thiophenol with phenyl iodide.
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Fig.
1
Transmission electron microscopy (TEM) image of
Cu-MW-HMS.
Fig. 2 XPS spectra of Fe-HMS; survey (left) and high resolution
for the Fe materials (Fig. 2S, ESIw). Typical metal content was
found to be around 0.5% as determined by ICP (Table 1). XPS
were recorded to study the Fe and Cu species on the catalyst
surface. Results have been included in Fig. 2 and 3.
spectrum of the Fe 2p region (right).
3+
Fe materials were found to be mostly composed of Fe
Fe O , 75–90%) and metallic Fe (10–25%) in different propor-
(
2
3
tions depending on the material, pointing to the existence of
nanoparticles with a metallic iron core covered with iron oxide.
These evidences are supported (Fig. 2) by the Fe2p bands at
7
10.6 (Fe 2p3/2) and 723.6 eV (Fe 2p1/2), with the characteristic
broad satellite at 719.8 eV, typical of the reported Fe 2p binding
1
0
2 3
energies of Fe O .
The small contribution at lower binding
energies (ca. 706.9 eV) was found to be very close to that of
1
0,11
metallic iron, in good agreement with literature data.
Cu
XPS spectrum confirmed the presence of a majority of metallic
copper (in general, over 60% of the total copper content, peak
1
2
centered at 931.8 eV ) together with minor quantities of copper
Fig. 3 XPS spectra of Cu-MW-HMS; survey (left) and high resolu-
+
oxides (Cu and Cu at 934.1 and 942.2 eV, respectively
2+
12b,13
)
tion spectrum of the Cu 2p region (right).
as shown in Fig. 3. These oxide species were also in good
agreement with the complex O 1s spectrum that shows at least
three types of oxygen in the materials (Fig. 6S, ESIw).
Poor conversions of starting material were found for all
catalysts except for the Cu-MW-HMS that exhibited a
quantitative conversion of starting material but a complete
selectivity to the disulfide (Scheme 1, product B) after 15 min
at 60 1C. We were then prompted to investigate the potential
effect of microwaves in the activity and selectivity in the
systems. The experiments carried out under microwave
irradiation are summarised in Table 1.
The S-arylation reaction of phenyl iodide and thiophenol
was initially run under conventional heating to check the
activity of the materials. In this reaction, the challenge lies in
the suppression of the known ability of Fe and Cu in promot-
7
,14
ing the homocoupling reaction to form disulfides.
Table 1 Catalytic activity of materials in the S-arylation of thio-
phenol with phenyl iodide (Scheme 1) under microwave irradiation
a
Fe materials exhibited a poor activity in the cross-coupling,
with the exception of the iron salt (Table 1, entry 4), and almost
complete selectivities to diphenyl disulfide (Scheme 1, product B).
Comparatively, very interesting results were found for the Cu
catalysts. While the copper salt gave quantitative conversion of
thiophenol to the disulfide, the heterogenisation of copper in both
Cu-MW-HMS materials provided quantitative conversions of
starting material and good to excellent selectivities to the cross-
coupled product A (Scheme 1). We believe the presence of active
Metal
content
(wt%)
Selectivity
(A : B ratio)
(mol%)
b
Conversion
(mol%)
Entry Material
1
2
3
4
5
Blank —
Fe-MW-HMS 0.48
o5
37
1 : 2
1 : 2
1 : 9
1 : 99
22 : 1
Fe-HMS
FeCl
0.59
—
o10
c
3
499
Cu-MW-
HMS
Cu-HMS
0.51
499
0
Cu SMNP (Fig. 3, Cu 2p) coupled with the use of microwave
6
7
0.57
—
96
499
6 : 1
1 : 99
c
CuCl
2
irradiation made possible the high activity and switch in selectivity.
Most importantly, the catalysts were highly reusable in the C–S
coupling under the reaction conditions preserving most of their
initial catalytic activity after three uses (Table 2). No significant
quantities of Cu (o3 ppm) were found in solution as determined
by ICP, in good agreement with the reusability results obtained for
the materials.
a
1
2 3
mmol thiophenol, 0.5 mmol phenyl iodide, 1 mmol K CO , 2 mL
acetonitrile, 0.05 g catalyst, microwaves, 80 W (60–100 1C maximum
temperature reached), 10 min. Conversion values were based on the
b
c
disappearance of phenyl iodide in the coupling reaction. Quantita-
tive conversion of thiophenol and complete selectivity to the diphenyl
disulfide (homocoupling product).
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Table 2 Catalytic reuses of Cu-MW-HMS in the S-arylation of
thiophenol with phenyl iodide under microwave irradiation
Having proved the activity and stability of the Cu-MW-HMS
materials, the scope of the cross-coupling reaction was sub-
sequently extended to a range of aryl iodides and thiols
in the C–S coupling. Results are included in Table 3 for
Cu-MW-HMS, chosen due to their higher A:B ratios (Table 3).
In general, very good conversions and selectivities to the
thioethers were obtained with the exception of aliphatic thiols
(Table 3, entry 4 and others not included) for which traces of
cross-coupled products were only found. These findings were
a
b
Reaction conditions
Conversion (mol%)
1st run (103 1C, 10 min)
1st reuse (80 1C, 20 min)
2nd reuse (87 1C, 20 min)
3rd reuse (90 1C, 20 min)
499
96
95
93
a
Reaction conditions: 1 mmol thiophenol, 0.5 mmol phenyl iodide,
mmol K CO , 0.05 g catalyst, 2 mL acetonitrile, microwaves 80 W,
0–100 1C, 10–20 min. Conversion values were based on the
1
8
2
3
b
7
in good agreement with results reported by Correa et al.
However, the tolerance of substituents in the aryl iodides and
the range of thiols employed are remarkable. Electron with-
consumption of pheny iodide in the coupling reaction.
2
drawing groups (e.g. NO , Cl, F) in the aryl iodides favoured
the coupling reaction while systems with electron donating
groups (e.g. CH , CH O) gave lower coupling yields.
Table 3 Catalytic activity of Cu-MW-HMS in the S-arylation of aryl
iodides with thiols under microwave irradiation
a
3
3
In conclusion, we have developed an efficient protocol for
C–S coupling reactions using SMNP and microwave irradiation.
Cu-MW-HMS were highly active and reusable in the S-arylation
of thiols with aryl iodides. We believe the supported systems can
greatly contribute to advance the development of active and
selective supported materials in continuous processes owing to
their reusability, availability, ease of preparation and low cost as
well as promoting the green credentials of future protocols in
terms of sustainability and environmentally friendliness.
˜
C. G. A. would like to thank Fundacion Espanola para la
Ciencia y Tecnologıa (FECYT) for a funded fellowship.
´
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