Detailed Mechanism for Hiyama Coupling Reaction in Water Catalyzed by Linear Polystyrene-Stabilized PdO Nanoparticles

Article abstract of DOI:10.1021/acs.organomet.7b00170

The catalytic cycle of the Hiyama coupling reaction in water catalyzed by linear polystyrene-stabilized PdO nanoparticles (PS-PdONPs) was investigated. The formation of 4-methylbiphenyl was confirmed from the reaction of 4-bromotoluene with the catalyst recovered from the reaction of PS-PdONPs with trimethoxyphenylsilane. The catalytic activity of PS-PdONPs significantly diminished upon addition of NaI or poly(4-vinylpyridine). The Hiyama coupling reaction using PS-PdONPs as a catalyst proceeds through a different mechanism from the commonly accepted one that starts from the oxidative addition of an aryl halide to a Pd⁽⁰⁾ species.

Full text of DOI:10.1021/acs.organomet.7b00170

Article
pubs.acs.org/Organometallics
Detailed Mechanism for Hiyama Coupling Reaction in Water
Catalyzed by Linear Polystyrene-Stabilized PdO Nanoparticles
Akira Sakon,† Ryoma Ii,† Go Hamasaka,§ Yasuhiro Uozumi,§ Tsutomu Shinagawa,∥
Osamu Shimomura,† Ryoki Nomura,†,‡ and Atsushi Ohtaka*,†
̂
†Department of Applied Chemistry, Faculty of Engineering and ‡Nanomaterials and Microdevices Research Center, Osaka Institute of
Technology, 5-16-1 Ohmiya, Asahi, Osaka 535-8585, Japan
§Institute for Molecular Science (IMS), Higashiyama 5-1, Myodaiji, Okazaki 444-8787, Japan
∥Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto, Osaka, 536-8553, Japan
ABSTRACT: The catalytic cycle of the Hiyama coupling reaction in
water catalyzed by linear polystyrene-stabilized PdO nanoparticles (PS-
PdONPs) was investigated. The formation of 4-methylbiphenyl was
confirmed from the reaction of 4-bromotoluene with the catalyst
recovered from the reaction of PS-PdONPs with trimethoxyphenyl-
silane. The catalytic activity of PS-PdONPs significantly diminished
upon addition of NaI or poly(4-vinylpyridine). The Hiyama coupling
reaction using PS-PdONPs as a catalyst proceeds through a different
mechanism from the commonly accepted one that starts from the
oxidative addition of an aryl halide to a Pd(0) species.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Carbon−carbon bond formation is an important research topic
in organic synthesis. From the viewpoint of green sustainable
chemistry, it is preferable to carry out reactions in water rather
than in flammable organic solvents. An enormous number of
metal nanoparticles catalysts, which have high catalytic activity
for carbon−carbon coupling reactions such as Suzuki,1 Heck,2
Sonogashira,3 Stille,4 and Hiyama5 coupling in water, have been
developed in recent years. Research on the mechanism of the
reactions using metal nanoparticle as a catalyst has been
focused on whether the reaction proceeds in solution or on the
surface of the nanoparticles.6 Unfortunately, there are few
studies of the actual mechanism, and in most cases, those
studies concluded that the detailed mechanism is similar to that
in the case of a metal complex.
PS-PdONPs were prepared by heating the mixture of
Pd(OAc)2 and TSKgel standard polystyrene (MW = 5.06 ×
103) in 1.5 mol·L−1 K2CO3 aqueous solution at 90 °C for 5 h.
The composition (PdO) was confirmed by X-ray diffraction
(XRD) measurement (Figure 1). A transmission electron
microscopy (TEM) image of PS-PdONPs showed a fairly
uniform particle size of 2.1 0.4 nm (Figure 2). Inductively
Recently, we developed linear polystyrene-stabilized PdO
nanoparticles (PS-PdONPs) that showed high catalytic activity
for several carbon−carbon coupling reactions in water.7 In the
course of our research on the Hiyama coupling reaction
catalyzed by PS-PdONPs,8 we determined that the reaction
must occur through a different mechanism than that of Pd(0)−
Pd(II)−Pd(0) cycle by metal complex catalysts because of the
following: (1) The Hiyama coupling reaction did not take place
with polystyrene-stabilized Pd nanoparticles (PS-PdNPs, Pd(0)
species), but it proceeded smoothly with PS-PdONPs (Pd(II)
species). (2) No formation of Pd(0) species was confirmed by
XPS analysis of the recovered catalyst after the reaction. These
data prompted us to examine the detailed mechanism of the
Hiyama coupling reaction in water using PS-PdONPs as a
catalyst.
Figure 1. (a) XRD patterns of PS-PdONPs; (b) Powder Diffraction
File #41-1107 (Joint Committee on Powder Diffraction Standards) for
PdO.
Received: March 5, 2017
© XXXX American Chemical Society
A
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coupled plasma-atomic emission spectroscopy (ICP-AES)
revealed that PS-PdONPs contained an average of 2.5 mmol·
g−1 of Pd.
consistent with the fact that leaching of palladium was observed
when PS-PdONPs was heated in the presence of 4-
bromotoluene (entry 3 in Table 1). Indeed, 92% of 4-
Table 1. Leaching Test
a
entry
reagent
amount of Pd leached (%)
1
2
3
4
5
6
7
none
2.8
TBAC
2.9
p-tolyl-Br + TBAC
3.3
PhSi(OMe)3 + TBAC
<0.1
<0.1
<0.1
<0.1
p-tolyl-Br + PhSi(OMe)3 + TBAC
NaI
TBAI
a
Average weight of Pd atoms detected by ICP-AES in the supernatant
liquid after exposure of PS-PdONPs to different reagents in NaOH
aqueous solution at 80 °C for 3 h.
Figure 2. (a) TEM micrograph of PS-PdONPs (scale bar = 20 nm);
(b) size distribution of PS-PdONPs.
First, stepwise reactions were performed in order to confirm
whether the Hiyama coupling reaction catalyzed by PS-
PdONPs takes place on the surface of the catalyst or in
solution, and whether aryl halide or aryltrimethoxysilane is first
involved in the reaction (Scheme 1). After heating the mixture
of PS-PdONPs and 4-bromotoluene in 1.5 mol·L−1 NaOH
aqueous solution at 80 °C for 3 h in the presence of TBAC, the
catalyst and the aqueous phase were separated. When the
reaction of the recovered catalyst (ReCat-1) with trimethox-
yphenylsilane was performed at 80 °C for 3 h, no formation of
4-methylbiphenyl was confirmed. Additionally, no coupling
product was obtained after heating of trimethoxyphenylsilane in
the solution phase (Sol-1). Based on the hypothesis that
coupling product was not obtained because 4-bromotoluene
would be filtered out with the catalyst, Sol-1 was heated in the
presence of both 4-bromotoluene and trimethoxyphenylsilane
to give 4-methylbiphenyl in 76% yield (eq 1). This result is
methylbiphenyl was obtained from the reaction of 4-
bromotoluene with trimethoxyphenylsilane in the presence of
a water-soluble palladium species, which was prepared by
dissolving PdBr2 in 1.5 mol·L−1 NaOH aqueous solution (eq
2). Considering that the yield of 4-methylbiphenyl was low
(34%) in the case of 4-iodotoluene using water-soluble
palladium species as a catalyst (eq 3) and that no increase in
the yield of coupling product was observed in the hot filtration
test (eq 4), the reaction would not proceed only in solution if
leached palladium species was involved in the reaction cycle.
A similar stepwise experiment was conducted with the order
of 4-bromotoluene and trimethoxyphenylsilane reversed
(Scheme 2). No coupling product was obtained from the
reaction of 4-bromotoluene with the solution phase (Sol-2),
which was separated after heating the mixture of PS-PdONPs
and trimethoxyphenylsilane in 1.5 mol·L−1 NaOH aqueous
Scheme 1. Stepwise Experiment of Reacting from 4-Bromotoluene
B
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Scheme 2. Stepwise Experiment of Reacting from Trimethoxyphenylsilane
solution at 80 °C for 3 h. This result is consistent with the fact
no palladium species was detected in the solution phase after
heating of PS-PdONPs in the presence of trimethoxyphenylsi-
lane (entry 4 in Table 1). In contrast, the formation of 4-
methylbiphenyl was confirmed, although the yield was low
(3%) from the reaction of the recovered catalyst (ReCat-2)
with 4-bromotoluene. In order to confirm that the desired
coupling product was not obtained from the reaction with
physically absorbed trimethoxyphenylsilane, the following
experiment was performed. When ReCat-2 was heated in the
absence of 4-bromotoluene in 1.5 mol·L−1 NaOH aqueous
solution at 80 °C for 3 h, no biphenyl was obtained (Scheme
3). However, 4% 4-methylbiphenyl was confirmed after heating
1 in Table 1), and the peak at 360 nm corresponding to Pd(II)
was observed by UV−vis spectra (Figure 3).9 In contrast, no
Scheme 3. Formation of Coupling Product from the
Recovered Catalyst
Figure 3. UV−vis spectra of the filtrate after heating of PS-PdONPs in
1.5 mol·L−1 aqueous NaOH solution at 80 °C for 3 h.
palladium species was observed in the presence of NaI (entry
6). The activation effect of trimethoxyphenylsilane by TBAC
would also decrease because TBAI was generated by addition of
NaI into the reaction mixture,10,11 and no leaching of palladium
was confirmed in the presence of TBAI (entry 7). These results
suggest that iodide ion retards the leaching of palladium. To
further investigate the correlation between leaching of
palladium species and progress of the reaction, a catalyst-
poisoning experiment was performed. Poly(4-vinylpyridine)
(PVPy: addition of 0.5 mmol of 4-vinylpyridine unit, MW
=
16 000) was employed as the catalyst poison,12 resulting in
inhibition of the catalytic reaction. This result is consistent with
the idea that leaching of palladium species is involved in the
catalytic cycle.
of the recovered catalyst (ReCat-3) and 4-bromotoluene,
indicating that the phenyl group derived from trimethoxyphe-
nylsilane was present on the surface of the catalyst.
When the Hiyama coupling reaction of 4-bromotoluene with
trimethoxyphenylsilane was performed in the presence of NaI,
no coupling product was obtained (eq 5). To elucidate the
All of the above results strongly suggested that the Hiyama
coupling reaction using PS-PdONPs as a catalyst proceeds
through a different mechanism from the commonly accepted
one that starts from the oxidative addition of an aryl halide to a
Pd(0) species. A more plausible mechanism is a Pd(II) catalytic
cycle (Scheme 4), where aryltrimethoxysilane reacts with the
catalyst rather than aryl halide.13 Although leaching of
palladium into reaction medium occur, the leached palladium
species is immediately restabilized on the catalyst surface.
Subsequently, the reaction with aryl halide takes place on the
catalyst surface to give desired coupling product. If the reaction
proceeds via this mechanism, then the valence of palladium
remains divalent, which is consistent with the results of XPS, in
which no Pd(0) species was observed even after the reaction.8
effect of NaI, the amount of leached palladium species in the
aqueous solution was checked by ICP-AES. After heating of PS-
PdONPs in 1.5 mol·L−1 aqueous NaOH solution at 80 °C for 3
h, 2.8% palladium was detected in the aqueous solution (entry
C
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added trimethoxyphenylsilane (80 mg, 0.40 mmol), TBAC (111 mg,
0.40 mmol), and 1.5 mol·L−1 aqueous NaOH solution (2 mL). After
stirring at 80 °C for 3 h, the reaction mixture was cooled to room
temperature by immediately immersing the vial in water (∼20 °C) for
about 10 min. After separating the catalyst and the aqueous phase by
centrifugation, the aqueous phase was decanted. Recovered catalyst
was washed with H2O (3 × 3.0 mL) and diethyl ether (3 × 3.0 mL),
which were then added to the aqueous phase. The aqueous phase was
extracted eight times with diethyl ether. The combined organic
extracts were dried over MgSO4 and concentrated under reduced
pressure. The product was analyzed by 1H NMR. However, the
aqueous phase (Sol-1) which was filtered with a membrane filter was
stirred at 80 °C for 3 h after addition of trimethoxyphenylsilane (80
mg, 0.40 mmol). The mixture was cooled to room temperature by
immediately immersing the vial in water (∼20 °C) for about 10 min
and extracted eight times with diethyl ether. The organic extracts were
dried over MgSO4 and concentrated under reduced pressure. The
Scheme 4. Plausible Reaction Mechanism
1
product was analyzed by H NMR.
Typical Procedures for Leaching Tests. To a screw-capped vial
with a stirring bar were added PS-PdONPs (3.0 mg, 7.5 μmol of Pd),
4-bromotoluene (86 mg, 0.5 mmol), TBAC (142 mg, 0.5 mmol), and
1.5 mol·L−1 aqueous NaOH solution (1 mL). After stirring at 80 °C
for 3 h, the aqueous phase was filtered with a membrane filter and
adjusted to 10 mL by hydrochloric acid (1.0 mol·L−1). The amount of
Pd metal was measured by ICP-AES analysis (2.1 ppm).
Results of leaching and hot filtration tests also support this
mechanism. Furthermore, the size of nanoparticles was
maintained even after the recycle experiment, and no detectable
Pd species was observed from the filtrate in the hot filtration
test.8 These data suggest that the leaching of Pd species in this
system is “local leaching”.6a
General Procedure for Hiyama Coupling Reaction. To a
screw-capped vial with a stirring bar were added 4-bromotoluene (86.4
mg, 0.5 mmol), trimethoxyphenylsilane (152 mg, 0.75 mmol), PS-
PdONPs (2.9 mg, 1.5 mol % of Pd), TBAC (142 mg, 0.5 mmol), and
1.5 mol·L−1 aqueous NaOH solution (1 mL). After stirring at 80 °C
for 3 h, the reaction mixture was cooled to room temperature by
immediately immersing the vial in water (∼20 °C) for about 10 min.
After separating the catalyst and the aqueous phase by centrifugation,
the aqueous phase was decanted. The aqueous phase was extracted
eight times with diethyl ether. The combined organic extracts were
dried over MgSO4 and concentrated under reduced pressure. The
CONCLUSION
■
The mechanism of the Hiyama coupling reaction in water
catalyzed by PS-PdONPs was investigated. Stepwise reactions
showed that the catalyst reacted first with aryltrimethoxysilane
and that the reaction of the catalyst with aryl halide took place
on the surface of the catalyst. The existence of NaI retarded the
progress of the reaction and prevented leaching of palladium
species. The leaching of palladium species into the reaction
medium was supported by the results of a poisoning test. This
result indicates that the mechanism of the reaction using metal
nanoparticles as a catalyst may be different from that in the case
of a complex catalyst.
1
product was analyzed by H NMR.
AUTHOR INFORMATION
Corresponding Author
*E-mail: atsushi.otaka@oit.ac.jp.
■
ORCID
EXPERIMENTAL SECTION
Tsutomu Shinagawa: 0000-0001-5671-1512
Atsushi Ohtaka: 0000-0001-8518-5187
Notes
■
1H NMR spectra in CDCl3 were recorded with a 300 MHz NMR
spectrometer (UNITY 300, Varian, Palo Alto, CA) using tetrame-
thylsilane (δ = 0) as an internal standard. Inductively coupled plasma-
atomic emission spectroscopy (ICP-AES) was performed using ICPS-
8100 (Shimadzu Co., Kyoto, Japan). X-ray photoelectron spectroscopy
(XPS) analysis was carried out using a PHI 5700MC (ULVAC-PHI,
Inc., Kanagawa, Japan).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Nanomaterials and Microdevices
Research Center (NMRC) of OIT for financial and
instrumental supports. This work was supported by the Joint
Studies Program (2016) of the Institute for Molecular Science.
Preparation of Polystyrene-Stabilized PdO Nanoparticles
(PS-PdONPs). To a screw-capped vial with a stirring bar were added
polystyrene (9.0 mg , 85 μmol of styrene unit), Pd(OAc)2 (5.5 mg, 25
μmol), and 1.5 mol·L−1 aqueous K2CO3 solution (3 mL). After stirring
at 90 °C for 5 h, the reaction mixture was cooled to room temperature
by immediately immersing the vial in water for approximately 10 min.
After separating the catalyst and the aqueous phase through
centrifugation, the aqueous phase was decanted. The catalyst was
washed with water (5 × 1.0 mL) and MeOH (5 × 1.0 mL).
Typical Procedures for Stepwise Experiments. To a screw-
capped vial with a stirring bar were added PS-PdONPs (80 mg, 0.20
mmol of Pd), 4-bromotoluene (48 mg, 0.27 mmol), TBAC (81 mg,
0.29 mmol), and 1.5 mol·L−1 aqueous NaOH solution (2 mL). After
stirring at 80 °C for 3 h, the reaction mixture was cooled to room
temperature by immediately immersing the vial in water (∼20 °C) for
about 10 min. After separating the catalyst and the aqueous phase by
centrifugation, the aqueous phase was decanted. Recovered catalyst
(ReCat-1) was washed with H2O (3 × 3.0 mL) and diethyl ether (3 ×
3.0 mL). To a screw-capped vial including the recovered catalyst were
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