Efficient ligand-free Hiyama cross-coupling reaction catalyzed by functionalized SBA-15-supported Pd nanoparticles

Article abstract of DOI:10.1039/c4gc00141a

A ligand-free Hiyama cross-coupling reaction catalyzed by functionalized SBA-15-supported Pd catalysts has been developed. The catalysts were prepared by depositing Pd nanoparticles preferentially in the micropores of the SBA-15 with hydrophobic trimethylsilyl or triphenylsilyl groups grafted on the mesopores. The nanocomposite catalysts showed excellent activities for the cross-coupling between various aryltriethoxysilanes and aryl halides under relatively mild (at 100 °C in air) reaction conditions. Moreover, the hydrophobic functionalization rendered the catalysts reusable without showing significant activity loss. The decreased activity after successive catalytic runs was attributed to a low level of Pd-leaching and a gradual collapse of mesopores of host silica. The cross-coupling protocol with the designed catalysts would be practical for use as an economical synthetic method for the construction of biphenyl derivatives. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00141a

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Efficient ligand-free Hiyama cross-coupling
reaction catalyzed by functionalized
SBA-15-supported Pd nanoparticles†
Cite this: Green Chem., 2014, 16,
706
2
a
a
a,b
Shao-Hsien Huang, Chun-Hsia Liu and Chia-Min Yang*
A ligand-free Hiyama cross-coupling reaction catalyzed by functionalized SBA-15-supported Pd catalysts
has been developed. The catalysts were prepared by depositing Pd nanoparticles preferentially in the
micropores of the SBA-15 with hydrophobic trimethylsilyl or triphenylsilyl groups grafted on the meso-
pores. The nanocomposite catalysts showed excellent activities for the cross-coupling between various
aryltriethoxysilanes and aryl halides under relatively mild (at 100 °C in air) reaction conditions. Moreover,
the hydrophobic functionalization rendered the catalysts reusable without showing significant activity
loss. The decreased activity after successive catalytic runs was attributed to a low level of Pd-leaching and
a gradual collapse of mesopores of host silica. The cross-coupling protocol with the designed catalysts
would be practical for use as an economical synthetic method for the construction of biphenyl
derivatives.
Received 26th January 2014,
Accepted 16th February 2014
DOI: 10.1039/c4gc00141a
www.rsc.org/greenchem
3
7–46
catalysts for the Hiyama coupling
syntheses.
and other organic
Introduction
4
7–51
The first example was reported by Rothenberg
The Hiyama cross-coupling reaction, a palladium-catalyzed and coworkers using bimetallic core–shell Ni–Pd nanoclusters
carbon–carbon bond formation between organosilanes and to catalyze the coupling of phenyltrimethoxysilane and halo-
3
7
organohalides or their equivalents, is one of the effective arenes. After that, several research groups also tried to use
methods for constructing asymmetrical biphenyls or multi- Pd nanoparticles to catalyze the same type of reactions under
2
7–29
substituted alkenes that are structural components of various ligand-free conditions.
pharmaceuticals and other functional materials.
However, the usage of the colloidal
In com- metal nanoparticles has problems of catalyst separation and
parison with Stille coupling using toxic tin reagents and recycling. These problems may be overcome by immobilizing
1
–6
4
0,41
Suzuki coupling with problems in purifying the boron or supporting Pd nanoparticles on/in dendrimers,
carbon-
3
8,42–44
39,45,46
reagents, the Hiyama coupling uses relatively stable, low- based materials,
or metal oxides.
As far as catalyst
toxicity and easily prepared organosilanes that, after reaction, support is concerned, ordered mesoporous silicas (OMSs) are
may facilely convert to relatively harmless silica waste, making highly attractive mainly because of their chemical stability,
the coupling reactions more attractive from the environmental high surface area for high metal dispersion, nanometer-sized
1
–6
point of view.
However, the Hiyama coupling is generally porosity to facilitate molecular diffusion, and versatile surface
catalyzed by Pd complexes containing phosphine ligands that functionalization. In comparison with OMSs, carbon-based
7
–20
are often toxic, air-sensitive and expensive.
It has therefore supports can also be made with high surface area but it is gen-
been of great interest to develop (phosphine) ligand-free erally difficult to disperse Pd or other metals on these supports
2
1–36
Pd catalysts for Hiyama coupling.
because of their hydrophobic nature. Nevertheless, the adsorp-
In addition to homogeneous catalysts, metallic Pd and Pd- tion of reactants on carbon supports is promoted by van der
containing nanoparticles have been explored as heterogeneous Waals interactions, leading to favorable reactant/product mass
transport. Herein we report functionalized SBA-15-supported
Pd catalysts for efficient and ligand-free Hiyama coupling reac-
a
b
tions. Mesoporous SBA-15 silica is a special type of OMSs with
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Frontier Research Center on Fundamental and Applied Sciences of Matters, National hexagonally arranged channel-type mesopores and additional
5
2–57
Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: cmyang@mx.nthu.edu.tw;
micropores in the silica walls.
lized the mesopores with hydrophobic groups,
only to deposit Pd nanoparticles preferentially in the hydro-
We selectively functiona-
Fax: +886-3-5165521; Tel: +886-3-5731282
58–62
aiming not
1
13
†
Electronic supplementary information (ESI) available: H and C NMR data
and spectra of biphenyl products of cross-coupling reactions. See DOI:
0.1039/c4gc00141a
5
8,60
1
philic micropores of SBA-15,
but also to facilitate the
2706 | Green Chem., 2014, 16, 2706–2712
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transport of hydrophobic reactants and prevent possible reac- using 4 mm MAS probes at a spinning rate of 6.5 kHz. Trans-
tions of silica walls with fluoride ions and/or other additives. mission electron microscopy (TEM) images were taken on a
The designed Pd catalysts exhibited excellent activities for FEI Tecnai 20 electron microscope operated at 200 kV and a
Hiyama coupling between various aryltriethoxysilanes and aryl field-emission JEOL JEM-3000F electron microscope operated
halides (mainly bromides) at 100 °C in air. Moreover, a low at 300 kV and equipped with a high-angle annular dark field
level of Pd-leaching was observed during the coupling reaction, detector and an energy dispersion X-ray (EDX) spectrometer.
allowing recycling of the catalysts for the construction of
biphenyl derivatives.
Catalyst activity testing
In a typical reaction, a mixture of aryl halide (0.50 mmol), aryl-
triethoxysilane (1.0 mmol), tetra-n-butylammonium fluoride
Experimental
trihydrate (TBAF·3H O, 1.0 mmol), acetic acid (AcOH,
2
Catalyst preparation and characterization
0.50 mmol), toluene (3.0 mL) and Pd@M-SBA-15 (26 mg, or
.5 mol% of Pd) was stirred at 100 °C under an air atmosphere
0
Mesoporous SBA-15 silica was synthesized at 60 °C by follow-
ing the procedure reported previously using tetraethoxysilane
as a silica source and a triblock poly(ethylene oxide)–poly(pro-
pylene oxide)–poly(ethylene oxide) copolymer Pluronic P-123
for 24 h. Alternatively, 0.5 mol% of Pd@P-SBA-15 or a commer-
cial carbon-supported Pd/C catalyst (5.4 mg, 5 wt%, Acros) was
used as a catalyst. The mixture was cooled down to room temp-
erature and then passed through a Celite pad to remove the
catalyst. The filtrate was mixed with ethyl acetate (EtOAc) and
water and was then extracted repeatedly with EtOAc. The
organic phase was dried over MgSO4 and the solvent was
removed under reduced pressure. The residue was purified
5
2,53
(EO20PO70EO20) as a structure directing agent.
The as-syn-
thesized SBA-15 was stirred in aqueous solution of sulfuric
acid (45 wt%) at 95 °C for 24 h to selectively vacate the meso-
5
6,57
pores.
The dried acid-treated SBA-15 samples were then
stirred in toluene solution of trimethylchlorosilane at room
temperature for 1 h or in pyridine solution of triphenylchloro-
silane at 120 °C for 12 h in order to graft the open mesopores
with trimethylsilyl (TMS) or triphenylsilyl (TPS) groups. Sub-
sequently, the samples were heated in air at 250 °C to remove
by column chromatography on silica gel (eluent: n-hexane–
1
EtOAc). The obtained products were pure, and their
H
1
3
6
1
and C NMR spectra data (recorded on a Bruker 300 NMR
3
spectrometer with CDCl ) were in agreement with authentic
5
6,57
compounds.
the EO fragments remaining in the micropores,
resulting
in samples referred to as M-SBA-15 (grafted with TMS groups)
and P-SBA-15 (grafted with TPS groups).
To incorporate Pd nanoparticles into the functionalized
SBA-15, 0.2 g M-SBA-15 or P-SBA-15 was impregnated with
Results and discussion
0
.2 mL acetone solution of dichlorobis(acetonitrile)palladium The SBA-15 materials had an ordered 2D-hexagonal structure
PdCl (MeCN) ), prepared by dissolving 25 mg PdCl (MeCN) and exhibited the 10, 11, and 20 reflections in its small-angle
in 1.0 mL acetone. After drying at 60 °C, the sample was PXRD pattern shown in Fig. 1a. The cell parameter a is
(
2
2
2
2
−1
heated in hydrogen flow at 300 °C (1 °C min heating ramp) 10.6 nm, which remained unchanged after subsequent treat-
for 3 h to reduce the metal. The resulting Pd-containing ments and metal deposition (cf. Fig. 1a). The sulfuric acid
5
6,57
samples are designated as Pd@M-SBA-15 and Pd@P-SBA-15. A treatment vacated the mesopores of SBA-15,
as indicated
reference catalyst was also prepared by using the pure-silica by the N2 physisorption isotherms of the treated samples
SBA-15 prepared by directly heating the acid-treated SBA-15 at (cf. Fig. 1b) showing a sharp step with an H1-type hysteresis
2
50 °C (referred to as PS-S15) as a support for Pd deposition. loop corresponding to the filling of uniform mesopores with
The reference catalyst is designated as Pd@PS-S15. open cylindrical geometry. The mesopore diameter of the acid-
Inductively coupled plasma-mass spectroscopy (ICP-MS) treated SBA-15 is 7.3 nm and is identical to that of PS-S15
data were obtained using a Perkin-Elmer SCIEX-ELAN 5000 (pure-silica sample with further 250 °C treatment). Sub-
device. Powder X-ray diffraction (PXRD) data were obtained on sequently, the mesopore surface was grafted with TMS or TPS
a Mac Science 18MPX diffractometer using Cu Kα radiation. groups and the micropores were vacated by heat treatment at
5
8–61
Nitrogen physisorption isotherms were measured at 77 K 250 °C.
The stepwise removal of P-123 from the as-syn-
using a Quantachrome Autosorb-1-MP instrument. The iso- thesized SBA-15 and surface functionalization were confirmed
1
3
29
56–60
29
therms were analyzed by the nonlocal density functional by C CP/MAS and Si MAS NMR.
Fig. 2 shows the Si
theory (NLDFT) method to evaluate pore sizes using the kernel MAS NMR spectra of M-SBA-15 and P-SBA-15 exhibiting the Q
n
of NLDFT equilibrium capillary condensation isotherms of lines (Q : Si(OSi) (OH)4−n, n = 2–4, at −90 to −112 ppm) from
n
nitrogen at 77 K on silica (adsorption branch, assuming SBA-15 silica and T lines attributed to the silicon atoms of
cylindrical pore geometry). BET surface areas were calculated TMS (14 ppm) or TPS (−18 ppm) groups. The intensity ratios
from adsorption branches in the relative pressure range of of the T and Q lines for the two samples are 9 : 100 and 8 : 100.
0
.05–0.20. Total pore volumes were evaluated at a relative The presence of TMS or TPS groups made the effective meso-
2
9
13
pressure of 0.95. Solid-state Si MAS and C CP/MAS NMR pore diameters slightly smaller and the values derived from N
spectra were measured on a Bruker DSX400WB spectrometer physisorption are 7.2 nm and 7.1 nm, respectively. Both
2
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Fig. 3 Bright-field (a) and dark-field (b) TEM images of Pd@M-SBA-15.
3
−1
samples had total pore volumes of around 1.10 cm g and
2
−1
BET surface areas of 740 cm g
.
The samples M-SBA-15 and P-SBA-15 were applied to
prepare Pd catalysts by impregnation with PdCl (MeCN) fol-
2
2
lowed by hydrogen reduction. The Pd loadings were 0.88 wt%
0.08 mmol Pd per gram of catalyst) determined by ICP-MS.
The salt could be preferentially loaded into the micropores
(
6
1,62
owing to the micropore-induced capillary forces
and poss-
ible interactions between the salt and surface silanol groups
5
8
on micropores. After hydrogen reduction, the samples gave
Fig. 1 PXRD patterns (a) and N
2
physisorption isotherms (b) of PS-S-15, featureless wide-angle PXRD patterns and show only slight
M-SBA-15 and Pd@M-SBA-15. The inset in (a): wide-angle PXRD pattern
of Pd@M-SBA-15. The isotherms in (b) are shifted by (from bottom to
top) 0, 500 and 1000 cm g STP, respectively.
changes in mesopore diameter (6.9 nm), total pore volume
3
−1
3
−1
(
1.06 cm g ) and surface area (725 cm g ) (cf. Fig. 1 for the
3
−1
data of Pd@M-SBA-15), indicating that the reduced Pd nano-
particles were very small without blocking the mesopores.
TEM investigation was consistent with PXRD results. Fig. 3
shows the bright-field (BF) and dark-field (DF) TEM images of
Pd@M-SBA-15. While fully open mesopores and no discern-
able Pd nanoparticles are observed in the BF image, rather
inhomogeneous and highly dispersed bright contrast in the
walls of mesopores observed in the DF image could be related
to the Pd nanoparticles, considering that Pd is appreciably
heavier than the other constituent elements. Semiquantitative
analysis by EDX was in agreement with the ICP-MS result.
The SBA-15-supported Pd catalysts were used as catalysts
for the Hiyama coupling reaction. We first screened the reac-
tion conditions by performing the coupling between 4-bromo-
acetophenone and phenyltriethoxysilane using Pd@M-SBA-15
as a catalyst, and the results are shown in Table 1. Under the
2
9
Fig. 2
Si MAS NMR spectra of M-SBA-15 and P-SBA-15.
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Table 1 Optimization of reaction conditions and comparison of
a
different catalysts
b
Entry
Modified conditions
Yield (%)
1
2
3
4
5
6
7
8
9
Standard conditions
THF instead of toluene
p-Xylene instead of toluene
Acetonitrile instead of toluene
Ethanol instead of toluene
KF instead of TBAF·3H
CsF instead of TBAF·3H
With 0.75 mmol of AcOH
With 0.50 mmol of AcOH
With 0.25 mmol of AcOH
Pd@P-SBA-15 was used
Pd@PS-S15 was used
55
36
14
4
4
0
Fig. 4 Reuse tests of Pd@M-SBA-15 and Pd@P-SBA-15. Reaction con-
ditions: 4-bromoacetophenone (0.50 mmol), phenyltriethoxysilane
(
(
1.0 mmol), Pd catalyst (2.5 μmol), TBAF·3H
0.5 mmol), and toluene (3.0 mL) in air at 100 °C for 24 h.
2
O (1.0 mmol), AcOH
2
O
2
O
0
88
92
74
90
56
64
94
3
1
1
1
1
1
1
0
1
2
3
4
5
micropores of SBA-15. Compared to the commercial Pd/C cata-
lyst that gave a yield of 64% (entry 13), Pd@M-SBA-15 and
Pd@P-SBA-15 were more efficient catalysts for the Hiyama
coupling. Noticeably, the usage of Pd@M-SBA-15 could be
markedly reduced from 0.5 mol% to 0.05 mol% with a similar
yield of 4-acetobiphenyl (entry 14). It might be presumably due
5% Pd/C was used
0.05 mol% Pd@M-SBA-15
0.02 mol% Pd@M-SBA-15
a
Under standard conditions, the reaction was carried out using
4
2
Pd@M-SBA-15 (0.5 mol%, 2.5 μmol), 4-bromoacetophenone to the suppression of homo-coupling of the aryl bromide.
(
(
2
0.50 mmol), phenyltriethoxysilane (1.0 mmol), TBAF·3H O
b
Further reduction to 0.02 mol% led to a dramatic decrease in
product yield (entry 15).
1.0 mmol), and toluene (3.0 mL) in air at 100 °C for 24 h. Isolated
yields.
The reuse tests of Pd@M-SBA-15 and Pd@P-SBA-15 (0.5 mol%)
using 4-bromoacetophenone and phenyltriethoxysilane as
“standard” conditions with toluene as a solvent, the corres- substrates were then examined under the optimal reaction
ponding 4-acetobiphenyl was obtained in 55% isolated yield conditions (Table 1, entry 9), and after each reaction the cata-
after 24 h in air at 100 °C (Table 1, entry 1). The reaction pro- lyst was collected by centrifugation, filtered, dried and then
gress was affected markedly by the solvent. The desired reused in the next run. As shown in Fig. 4, both catalysts could
product was obtained in moderate yield in THF (entry 2), and be reused and the catalytic activities decreased slightly in the
the reaction hardly took place in p-xylene, acetonitrile or second run. The isolated yield of 4-acetobiphenyl remained
ethanol (entries 3–5). For the coupling reaction, a fluoride acceptably high (54%) for Pd@M-SBA-15 in the third run but
source is necessary to activate organosilane and, as an activa- decreased to 23% in the fourth run. Nevertheless, the reaction
2
tor, TBAF·3H O was specifically effective probably due to its efficiency of Pd@P-SBA-15 was already reduced drastically in
high solubility in toluene. The reaction did not proceed under the third run, which might probably be related to higher ten-
2
fluoride-free conditions or in the presence of metal fluorides dency for the C(sp )–Si bonds in TPS groups to be cleaved by
3
such as KF and CsF (entries 6 and 7).
fluoride ions as compared to the C(sp )–Si bonds in TMS
6
4
On the other hand, it has been reported that the addition groups. Generally speaking, the decreased product yield after
of AcOH as a proton source may improve the yield of the successive runs might be attributed either to the leaching of
biphenyl product by suppressing the undesired coupling of Pd species or to gradual collapse of mesopores of host silica to
the ethoxide anion (generated from phenyltriethoxysilane) block the access of reactants to Pd nanoparticles. The solid
1
3,42,63
with aryl halide.
the addition of AcOH indeed enhanced the reaction (entries trate was analyzed by ICP-MS. A Pd-leaching of 5.3 ppm was
–10) and an isolated yield of 92% was obtained with an detected, which was much lower than that (60 ppm) reported
amount of 1.0 equiv. of AcOH. With AcOH (1.0 equiv.), the use for 5% Pd/C under similar reaction conditions (in the presence
Under the reaction conditions studied, catalyst was filtered after the first run of reactions and the fil-
8
4
2
of Pd@P-SBA-15 gave a similar yield (entry 11) whereas the of AcOH). The low level of Pd-leaching might be associated
reaction catalyzed by Pd@PS-S15 resulted in a significantly with the favorable hydrophilic environment in the micropores
lower yield of 56% (entry 12). The higher catalytic activities of of SBA-15 for keeping the ionic Pd species produced during
Pd@M-SBA-15 and Pd@P-SBA-15 may be associated with the the catalytic cycle from diffusing out to the relatively hydro-
presence of TMS or TPS groups on the mesopores that may phobic reaction media.
facilitate the diffusion of relatively hydrophobic reactants into
We further performed a hot filtration test, in order to
the pore system to approach Pd nanoparticles in the intra-wall examine the catalytic activity of the leached Pd species, by
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Table 2 The Hiyama cross-coupling of various aryl halides and
a
phenyltriethoxysilane
b
Entry
X
R
Yield (%)
1
2
3
4
5
6
7
8
9
Br
Br
Br
Br
Br
Br
Cl
Cl
Br
Br
Br
Br
4-Ac
92
71
78
81
73
84
0
47
75
70
63
13
4-CN
4-CHO
4-Cl
4-Me
4-OMe
4-OMe
4-NO
2-CHO
3-OMe
Naphthalene
2-OMe
Fig. 5 Profiles of product yield versus time with or without hot fil-
tration. Reaction conditions: 4-bromoacetophenone (0.50 mmol),
phenyltriethoxysilane (1.0 mmol), Pd@M-SBA-15 (5.0 µmol), TBAF·3H
1.0 mmol), AcOH (0.5 mmol), and toluene (3.0 mL) in air at 100 °C.
2
O
2
(
10
11
12
c
a
Reaction conditions: aryl halide (0.50 mmol), phenyltriethoxysilane
(
(
1.0 mmol), Pd@M-SBA-15 (0.5 mol%), AcOH (0.50 mmol), TBAF·3H O
2
b
1.0 mmol), and toluene (3.0 mL) in air at 100 °C for 24 h. Isolated
c
yields. 1-Bromonaphthalene.
reflection was observed in the PXRD pattern of Pd@M-SBA-15
after the fourth run. Obviously, even though the TMS groups
(
or TPS groups in Pd@P-SBA-15) on the mesopore surface
could protect host silica from reacting with fluoride ions from
TBAF·3H O and/or AcOH and allow the catalyst to be recycled,
2
the undesired reactions still took place slowly and inevitably
after repeated uses of the catalysts. Therefore, we could con-
clude that a low level of Pd-leaching together with gradual col-
lapse of mesopores of host silica caused the decrease in
product yield for Pd@M-SBA-15 and Pd@P-SBA-15 after suc-
cessive catalytic runs.
The substrate scope of the new cross-coupling protocol was
examined under the optimal conditions using Pd@M-SBA-15
as a catalyst. As listed in Tables 2 and 3, in most cases, the
Hiyama reactions of aryltriethoxysilanes with a variety of aryl
Fig. 6 PXRD patterns of Pd@M-SBA-15 before and after successive
catalytic runs.
using a doubled amount of Pd@M-SBA-15 to catalyze the coup- halides afforded the corresponding biphenyl derivatives in
ling reaction under optimal conditions for 1.5 h, filtering out good to excellent yields. Table 2 summarizes the results of the
the catalyst without cooling, and allowing the filtrate to react reactions of various aryl halides and phenyltriethoxysilane.
further. As shown in Fig. 5, without hot filtration, the yield of Aryl bromides possessing either an electron-withdrawing
4
-acetobiphenyl continued to increase to 70% at a reaction (acetyl, cyano, formyl or chloro group, entries 1–4) or an elec-
time of 3 h and then to 90% at 9 h. If the catalyst was filtered tron-donating (methyl or methoxy group, entries 5 and 6) func-
at 1.5 h, the yield only slightly increased from 54% at 1.5 h to tionality at the para-position were found to be good substrates
6
4% at 3 h and remained almost constant afterwards. The for the cross-coupling. We also tried the coupling reaction
results suggested that the leached Pd species in a very small with aryl chlorides that are generally much less reactive than
amount were active for the Hiyama coupling but became in- the corresponding bromides. Two chlorides, namely 4-chloro-
active after reacting for a short period of time.
anisole and 4-nitrochlorobenzene, were tested in this study.
The other possible reason for the decreased product yield Markedly, while 4-chloroanisole appeared to be a poor sub-
was the collapse of mesopores. This was indeed the case, as strate (entry 7), a fairly good yield of 47% was obtained for
indicated by the decreased intensity of X-ray reflections of 2D- 4-nitrochlorobenzene under the present reaction conditions
hexagonal structure after repeated uses. As shown in Fig. 6, no (entry 8). The yield was much higher than that (7%) obtained
2710 | Green Chem., 2014, 16, 2706–2712
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Table 3 The Hiyama cross-coupling of various aryl bromides and mesopores of host silica after successive catalytic runs. The
a
aryltriethoxysilanes
protocol using the designed catalysts provides an economical
synthetic method for the construction of biphenyl derivatives.
Acknowledgements
The authors thank the National Science Council of the Repub-
lic of China for financial support under contract no. NSC 101-
2811-M-007-089 and NSC 101-2628-M-007-001-MY2.
b
Entry
R
1
R
2
Yield (%)
1
2
3
4
5
6
4-Ac
Cl
Cl
Cl
Me
Me
Me
89
84
87
79
82
38
References
4-OMe
2-CHO
4-Ac
4-OMe
2-CHO
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a
Reaction conditions: aryl bromide (0.50 mmol), aryltriethoxysilane
(
(
1.0 mmol), Pd@M-SBA-15 (0.5 mol%), AcOH (0.50 mmol), TBAF·3H O
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2
b
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2
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7
8
9
The cross-coupling protocol could also tolerate functional
groups on the aryl ring of the aryltriethoxysilanes. As shown
in Table 3, p-chlorotriethoxysilane was effectively coupled
with 4-bromoacetophenone, 4-methoxybromobenzene and
2006, 47, 8673.
1
1
0 I. Gordillo, E. de Jesús and C. López-Mardomingo, Org.
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derivatives in excellent yields (entries 1–3). When p-tolyl-
triethoxysilane was used as a reactant, slightly lower yields
were obtained for the coupling with para-substituted bromo-
benzenes and only 38% isolated yield was obtained for the
reaction with 2-bromobenzaldehyde. The results seemed to
suggest possible electronic effects of the substituent group in
aryltriethoxysilanes on the efficiency of the cross-coupling
reaction. In addition, the steric factor seemed to become more
pronounced for the substituted aryltriethoxysilanes and
should also be considered for the present cross-coupling
system.
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