Silylation of Aryl Chlorides by Bimetallic Catalysis of Palladium and Gold on Alloy Nanoparticles

Article abstract of DOI:10.1002/adsc.202000045

Supported palladium-gold alloy-catalyzed cross-coupling of aryl chlorides and hydrosilanes enabled the selective formation of aryl-silicon bonds. Whereas a monometallic palladium catalyst predominantly promoted the hydrodechlorination of aryl chlorides and gold nanoparticles showed no catalytic activity, gold-rich palladium-gold alloy nanoparticles efficiently catalyzed the title reaction to give arylsilanes with high selectivity. A wide array of aryl chlorides and hydrosilanes participated in the heterogeneously-catalyzed reaction to furnish the corresponding arylsilanes in 34–80% yields. A detailed mechanistic investigation revealed that palladium and gold atoms on the surface of alloy nanoparticles independently functioned as active sites for the formation of aryl nucleophiles and silyl electrophiles, respectively, which indicates that palladium and gold atoms on alloy nanoparticles work together to enable the selective formation of aryl-silicon bonds. (Figure presented.).

Full text of DOI:10.1002/adsc.202000045

Title: Silylation of Aryl Chlorides by Bimetallic Catalysis of Palladium
and Gold on Alloy Nanoparticles
Authors: Hiroki Miura, Yosuke Masaki, Yohei Fukuta, and Tetsuya
Shishido
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
VERY IMPORTANT PUBLICATION
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Silylation of Aryl Chlorides by Bimetallic Catalysis of
Palladium and Gold on Alloy Nanoparticles
Hiroki Miura,a,b,d,* Yosuke Masaki,a Yohei Fukutaa and Tetsuya Shishidoa,b,c,d,
*
a
Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo
Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
E-mail: miura-hiroki@tmu.ac.jp, shishido-tetsuya@tmu.ac.jp
Research Center for Hydrogen Energy-based Society, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji,
Tokyo 192-0397, Japan
Research Center for Gold Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397,
Japan
Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan
b
c
d
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. Supported palladium–gold alloy-catalyzed cross-coupling of aryl chlorides and hydrosilanes enabled the
selective formation of aryl–silicon bonds. Whereas a monometallic palladium catalyst predominantly promoted the
hydrodechlorination of aryl chlorides and gold nanoparticles showed no catalytic activity, gold-rich palladium–gold
alloy nanoparticles efficiently catalyzed the title reaction to give arylsilanes with high selectivity. A wide array of aryl
chlorides and hydrosilanes participated in the heterogeneously-catalyzed reaction to furnish the corresponding
arylsilanes in 34-80% yields. A detailed mechanistic investigation revealed that palladium and gold atoms on the surface
of alloy nanoparticles independently functioned as active sites for the formation of aryl nucleophiles and silyl
electrophiles, respectively, which indicates that palladium and gold atoms on alloy nanoparticles work together to
enable the selective formation of aryl–silicon bonds.
Keywords: silylation; cross-coupling; aryl chloride; alloy nanoparticles; heterogeneous catalyst
Introduction
(a) under single metal or acid-base catalysis.
Arylsilanes are an important class of molecules due to
their property for components of organic light
emitting diodes.[1–3] Furthermore,
established
synthetic methods, such as Hiyama cross-coupling
reactions, enable facile transformation of these
compounds to high value-added compounds, such as
natural products, drugs, and functional materials.[4]
Despite the high efficiency of the nucleophilic attack
of Grignard and organolithium reagents to Si
electrophiles (Scheme 1a, route 1), severe obstacles
regarding stoichiometric amount of metallic reagents,
the limitations of functional groups and the
prefunctionalization of substrates reduce their
reliability for introducing a silyl moiety into
aromatics. Consequently, cross-coupling reactions of
aryl and silyl moieties under the influence of
transition-metal catalysts have appeared to be a
sustainable tool for the formation of aryl–Si bonds.[5–
9] While the direct silylation of an aromatic C–H bond
with hydrosilanes is the most straightforward and
atom-economical route to arylsilanes (Scheme 1a,
route 2),[10–13] the inability to control the
regiochemistry and the narrow substrate scope
decrease their versatility. In contrast, the
(b) under bimetallic catalysis.
Scheme 1. Synthetic routes for arylsilanes
substitution of an aryl-halide bond to an aryl–Si bond,
namely the cross-coupling of aryl halides with
hydrosilanes, is the most reliable method for the
regio-controlled silylation of arenes. Particularly,
1
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
appropriate combinations of transition metals, such as selective formation of aryl–Si bonds while avoiding
Pd,[14–21] Rh[22–28] and Pt,[29,30] and bulky phosphine the reduction of aryl–Cl bonds.
ligands enable the catalytic silylation of aryl halides
bearing various functional groups while avoiding
undesired hydrodehalogenation (Scheme 1a, route 3).
Results and Discussion
However, nearly all investigations of the title reaction
have been limited to the use of reactive bromides,
iodides and triflates as an aryl source. Although aryl
chlorides are undoubtedly expedient substrates in
view of their low cost and wide availability, their
application is still difficult due to the higher
dissociation energy of the C–Cl bond compared to
those of C–Br and C–I.[31,32] Although the cross-
coupling reaction of aryl chlorides and disilanes has
been reported as an alternative method for the
silylation of aryl chlorides (Scheme 1a, route 4),[33,34]
the limited variety of disilanes still requires an
efficient silylation via the cross-coupling of aryl
chlorides and hydrosilanes. However, the fact that
metal complex catalysts with activities that can be
tuned by the choice of suitable ligand have not
provided definite solutions to date implies that some
as-yet unexplored approach may be needed to devise
a novel protocol for C–Si bond formation. On the
other hand, bimetallic catalysis has recently attracted
considerable attention as a powerful tool for realizing
unprecedented molecular transformations.[35–39] The
groups of Martin and Rueping independently reported
efficient aryl–Si bond formation via the coupling of
aryl esters and silyl boranes under Ni/Cu bimetallic
catalysis,[40,41] which includes C–O bond and Si–B
bond activation by Ni and Cu, respectively (Scheme
1b, routes 5 and 6). On the other hand, much attention
has been focused on the reaction under the influence
of heterogeneous catalysts from the perspective of
green and sustainable chemistry thanks to high
reusability of the catalysts and low contamination of
toxic metals into valuable chemicals. We recently
demonstrated the unique catalytic functions of
bimetallic nanoparticles (NPs) for efficient organic
transformations.[42–45] In the course of our study on the
PdAu alloy-catalyzed hydrosilylation of unsaturated
organic molecules, we found that highly electrophilic
silyl species could form on Au atoms on the surface
of NPs, which realized an efficient silylation of
organic molecules.[42,43] Furthermore, Sakurai and co-
workers reported the low-temperature activation of an
aryl–Cl bond under the catalysis of well-defined
PdAu bimetallic nanoclusters.[46–49] These findings
enabled us to envision an efficient formation of aryl–
Si bonds from aryl chlorides under the bimetallic
catalysis of adjacent Pd and Au atoms on the surface
of PdAu alloy NPs(Scheme 1b, route 7). Herein, we
describe an efficient method for synthesizing
arylsilanes via the cross-coupling of aryl chlorides
and hydrosilanes in the presence of supported PdAu
alloy NPs catalysts. A variety of arylsilanes could be
synthesized by using PdAu bimetallic catalysis
without the aid of phosphine ligands. The surface
composition of Pd and Au atoms on the alloy NPs
affected the selectivity for arylsilanes, and alloy NPs
with high Au concentration were crucial for the
Supported PdAu alloy catalysts (xPdyAu/support: x
and y denote the molar ratio of Pd to Au.) were
prepared through a sol-immobilization method. The
PdAu NPs with mean diameters of about 3 nm were
highly dispersed on the surface of each support, and
no significant differences in the mean diameter of
alloy NPs were confirmed when the Pd/Au ratio of
alloy NPs was varied.[42,50] Table 1 summarizes the
effect of metallic components of NPs and their
support on the cross-coupling of chlorobenzene (1a)
with triethylsilane (2a) in the presence of Na2CO3 (2.0
eqiuv) as a base. Monometallic Pd NP catalyst
predominantly underwent hydrodechlorination to
form benzene (4a), resulting in a quite low yield of
the target product (3aa) (entry 1). On the other hand,
alloying Pd with Au improved the selectivity for 3aa,
and the values increased with an increase in the Au
concentration in PdAu alloy NPs (entries 2–5).
Although the conversion rate was decreased, the
highest selectivity was obtained in the reaction with
the 1Pd6Au catalyst. Monometallic Au NPs showed
no
Table 1. Silylation of chlorobenzene over supported
catalysts.
[a]
Reaction conditions: 1a (0.5 mmol), 2a (2.5 mmol),
catalyst (2.0 mol% as metal), Na2CO3 (1.0 mmol), DMA (1
mL), at 100 ℃. [b] Yields and selectivities of the products
were determined by GC analysis by using n-decane as
internal standard. [c] Reaction for 6 h. [d] Reaction for 3 h.
activity for the cross-coupling reaction (entry 6). Our
previous works on the detailed characterization of
2
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
[c]
PdAu alloy NPs revealed that Pd and Au were internal standard. Biphenyl was formed as a by-product
uniformly mixed in the nanoparticle to random alloy in 31% yield.
structure.[42–45, 50] PdPt NPs have also been reported to
form random alloy, while PdPt NPs were totally
benzene and biphenyl, respectively. These results
ineffective toward the reaction (entry 7). Furthermore, suggest that specific interaction between PdAu NPs
supported NiAu and CuAu catalysts showed quite low and chloride anion should be involved in the catalytic
activity. These results clearly indicate that the cycles for the selective formation of aryl–Si bonds
significance of the coexistence of Pd and Au for (vide infra).With the optimized catalyst in hand, the
realizing the selective formation of an aryl–Si bond. scope of substrates in the cross-coupling of aryl
Interestingly, although the reaction under the catalysis chlorides with hydrosilanes was examined (Table 3).
of soluble Pd(0) complex resulted in a low selectivity A series of triethylsilyl arenes could be synthesized.
for 3aa (entry 8), the addition of Au/C to the Furthermore, the present PdAu catalytic system could
homogeneous Pd-catalyzed reaction improved the be employed on a gram scale to furnish 3ba in a yield
arylsilane selectivity (entry 9). If we consider the of 55% (1.13 g of 3ba was isolated). The electronic
quite low activity for activating the C-Cl bond and nature of the substituents on arenes slightly
significant activity for cleaving the Si-H bond of Au influenced both the conversion rate of aryl chlorides
NPs,[51–55] Pd and Au are anticipated to activate aryl and the yields of the silylated products. As generally
chloride and hydrosilane, respectively. After careful observed in the present catalytic reactions, aryl
screening of the supporting materials for 1Pd6Au NPs chlorides bearing electron-donating substituents, such
(entry 10–13), ZrO2 was found to be the optimal as methyl and methoxy groups, gave the
support to give 3aa in 77% yield after the reaction for corresponding products (3ba–3ea) in high yields,
3 h at 100 °C. Evaluation of the adsorption rate of although the reactions required a long time to run to
chlorobenzene (1a) onto supports helped to elucidate completion. A fluoro substituent positively affected
the effect of the supports (Figure S1). The adsorption the yield of the product
rate of 1a onto supports positively correlated with the
formation rate of both 3aa and 4a. Moreover, we Table 3. Scope of substratesa)
revealed that the formation-rate limiting step for
arylsilanes in the title reaction was oxidative addition
of aryl chlorides (vide infra). These facts enabled us
to deduce that acidic or basic property of ZrO2
increased the concentration of aryl chlorides at around
catalytically active PdAu NPs, thereby leading high
catalytic performance of PdAu/ZrO2. Notably, DMA,
DMF, NMP and CH3CN, were suitable solvents for
the present reaction, while toluene, 1,4-dioxane and
DMSO were totally ineffective.[56]
The unique catalysis by PdAu NPs for synthesizing
arylsilanes is reflected only in the reaction of aryl
chlorides as starting substrates (Table 2). The reaction
of bromobenzene (1a’) with 2a resulted in a moderate
selectivity for 3aa. Besides, the reaction of
iodobenzene (1a’’) did not afford 3aa since reduction
or homo-coupling proceeded predominantly to give
Table 2. Silylation of aryl halides over supported
PdAu catalysts.a)
[a]
Reaction conditions: 1 (0.5 mmol), 2a (2.5 mmol),
1Pd6Au/C (2.0 mol% as metal), Na2CO3 (1.0 mmol), DMA
(1 mL), at 100 ℃. [b]Yields and selectivities of the products
were determined by GC analysis by using n-decane as
3
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
[a]Reaction conditions: 1 (0.50 mmol), 2 (2.5 mmol), Scheme 2. The model structure of 1Pd6Au alloy
1Pd6Au/ZrO2 (2.0 mol% as metal), Na2CO3 (1.0 mmol), nanoparticles and possible reaction pathway for the
DMA (1 mL), at 100 oC. Isolated yields were given. coupling reaction of aryl chlorides with hydrosilanes over
[b]Reaction run at 20 times the scale as that used in the PdAu catalysts.
standard reaction conditions. [c]Hydrosilane was added
dropwise.
To gain insight into the reaction mechanism and the
3fa. Silyl naphthalene (3ga) was also obtained in 64% bimetallic effects on product selectivity, we
yield. In contrast, the reactions of aryl chlorides with postulated the reaction pathways for both the
electron-withdrawing substituents afforded the silylation and hydrodehalogenation of aryl halides, as
corresponding arylsilanes (3ha–3ma) in slightly shown inScheme 2. Our previous detailed study on
reduced yields, and the reactions went to completion the structural characterization of PdAu NPs revealed
quickly. Importantly, reactive functionalities, such as that Pd and Au atoms were homogeneously mixed to
ester, nitrile and ketone, remained intact during the form random alloy NPs.[42–45,50] Furthermore, hot-
PdAu-catalyzed silylation. The position of a methyl filtration of supported PdAu catalysts completely
or ester substituent on arenes did not affect the yield suppressed further progress of the reaction (see Fig.
of the final product (3ba–3da and 3ha–3ja), S4 in Supplementary Information), clearly indicating
suggesting that steric hindrance around the C–Cl bond that the reaction took place on the surface of PdAu
is not
a
crucial factor for determining the alloy NPs. The routes to arylsilanes and reduced
chemoselectivity. The significance of the present arenes must include the dissociative adsorption of
protocol was well-reflected in the formation of hydrosilanes and aryl halides on the surface of PdAu
heteroaryl–Si bonds. Silyl pyridine, pyrazine and NPs, and H–Si bond activation should take place prior
thiophene were obtained in around 80% yields. Facile to cleavage of a C–X bond in view of the high
adsorption of heteroaryl substrates onto PdAu NPs dissociation energy of the C–X bond. The subsequent
due to the positive interaction of N and S atoms with bond-forming steps (Paths 1–4 in Scheme 2) could be
PdAu NPs would promote the rate-limiting step in the classified into two categories, for hydrodechlorination
present catalytic reaction. Several silyl coupling and silylation, each of which should be divided into
partners other than triethylsilane could also be two bond-forming paths, Path 1 (aryl–H → Si–X),
accommodated to give the corresponding arylsilanes, Path 2 (Si–X → aryl–H), Path 3 (H–X → aryl–Si) and
while aryl-substituted hydrosilanes, such as Path 4 (aryl–Si → H–X), and the rate for the first
dimethylphenylsilane,
diphenylsilane
and bond-forming step should dominate the selectivity for
phenylsilane, and silanes with bulky substituents, the products. We performed the following studies to
such as dimethyl tert-butylsilane were not good uncover the factors for dominating the product
substrates in the PdAu NPs-catalyzed reaction. selectivity.
Notably, excess hydrosilanes were required to obtain Figure 1A shows the effect of the type of carbonate
arylsilanes in satisfactory yields, since water that is base on the formation rate and selectivity of arylsilane
adsorbed on the support promotes the rapid hydrolytic 3aa in the cross-coupling of chlorobenzene (1a) and
conversion of hydrosilanes to silanols or siloxanes as 2a. Basicity affected the formation rate of 3aa, and
the main side reaction to reduce the product yields. alkaline-metal carbonates with weak basicity, such as
High temperatures that can remove the adsorbed lithium and sodium salt, exhibited high formation
water on the surface of solid catalysts cause the rates for 3aa. In contrast, there were no significant
aggregation and segregation of PdAu NPs. Thus, we differences in the selectivity for 3aa. Furthermore, as
used the catalysts dried in oven below 100 oC.
shown in Figure 1B, the amount of base also did not
affect either the formation rate or the selectivity for
3aa. These results suggest that the base does not
participate
■ : 3aa ■ : 4a
■ : 3aa ■ : 4a
▲ : Selectivity for 3aa
▲ : Selectivity for 3aa
50
40
30
20
10
0
100
80
60
40
20
0
50
40
30
20
10
0
100
80
60
40
20
0
(A)
(B)
Pd
Au
Li2CO3 Na2CO3 K2CO3 Cs2CO3
0.0
0.5
1.0
1.5
Na2CO3 / mmol
4
1Pd6Au alloy NPs
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
Figure 1. Effect of (A) a sort of carbonate base and (B) the Another key factor for product selectivity is the PdAu
amount of Na2CO3 on the formation-rate and the selectivity atomic ratio of PdAu alloy NPs, as shown in Table 1.
for the products (3aa and 4a) in the coupling reaction of 1a As we previously reported regarding the
and 2a. Reaction condition (A); 1a (0.50 mmol), 2a (2.5 hydrosilylation of unsaturated organic molecules over
mmol), 1Pd6Au/C (2.0 mol% as metal), base (1.0 mmol), supported PdAu NPs,[42,43] the electronic state of Au
o
DMA (1 mL), at 100 C, for 1 h. (B) 1a (0.50 mmol), 2a in NPs can be varied by changing the Pd/Au atomic
(2.5 mmol), 1Pd6Au/C (2.0 mol% as metal), DMA (1 mL), ratio.[50] Furthermore, electron-deficient Au atoms can
at 100 oC, for 1 h. GC yields were given.
be present in alloy PdAu NPs with a high Au
concentration, and thus can form highly electrophilic
silyl species on Au atoms in alloy PdAu NPs. As in
in the selectivity-determining step of the reaction of the previous hydrosilylation over PdAu catalysts,[42,43]
aryl chlorides and hydrosilanes. On the other hand, the formation of highly electrophilic silyl species on
the substituent on the aromatic ring influenced the Au atoms facilitated aryl-Si bond formation to realize
selectivity of the products (Figure 2). The formation the highly selective silylation of aryl chlorides.
rates of both arylsilane and reduced arene in the Notably, the reaction-orders of 1a, 2a and Na2CO3
reaction of aryl chloride with an electron-withdrawing estimated from the formation rate of 3aa were 0.6,
ester group (1h) were increased in comparison to that ˗0.4 and ˗0.1, respectively. This indicates that the
with an electron-donating methyl group (1b). In formation rate-determining step for arylsilanes in the
contrast, increased selectivity for arylsilane was reaction of aryl chlorides is undoubtedly oxidative
achieved in the reaction of 1b. This is consistent with addition of aryl chlorides, whereas the selectivity for
the results shown in Table 3 and indicates that the arylsilanes should be determined in the step for
electronic state of the aryl species formed on Pd atom desorbing each fragment.
of PdAu NPs through the cleavage of aryl–Cl bonds We next focused on understanding the specific effect
should be the dominant factor for determining the of halides in the reaction over PdAu alloy NPs. As
product selectivity in the coupling reaction of aryl shown in Table 2, highly selective silylation of aryl
chlorides and hydrosilanes. Based on these results, the chlorides was seen only in the reaction of aryl
silylation and hydrodechlorination of aryl chlorides chlorides, whereas the reactions of aryl bromides and
are expected to proceed preferably through Paths 4 iodides resulted in quite low arylsilane selectivity.
and 1, respectively. Hence, electron-rich aryl species The reaction-orders of bromobenzene (1a’), 2a and
formed on Pd atoms of PdAu NPs are considered to Na2CO3 estimated from the formation rate of 3aa
rapidly react with electrophilic silyl species formed were completely different from those for the reaction
on adjacent Au atoms, which leads to highly selective of aryl chlorides, with values of ˗0.4, 0.0 and 0.4,
aryl–Si bond formation.
respectively. This indicates that both the formation-
limiting and selectivity-determining steps in the
reaction of aryl bromides should be different from
those of aryl chlorides, and particularly, the rate-
limiting step of the reaction would not be cleavage of
the aryl–Br bond, but rather the bond-forming step.
As shown in Figure 3A, the amount of base affected
the selectivity for the product, and an increase in the
amount of base increased the selectivity for arylsilane.
In contrast, the substituents on the aromatic ring did
not impact either the formation rate or the selectivity
for arylsilane. These results suggest that the product
selectivity in the reaction of aryl bromides is
dominated, not by the electronic state of aryl species,
but rather by the base, and that arylsilanes and
reduced arenes are likely to be formed preferentially
through Paths 3 and 2 in Scheme 2, respectively.
■ : 3 ■ : 4 ▲ : Selectivity for 3
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
■ : 3aa ■ : 4a
■ : 3 ■ : 4
▲ : Selectivity for 3aa
▲ : Selectivity for 3
100
80
60
40
20
0
100
80
60
40
20
0
80
60
40
20
0
60
40
20
0
(A)
(B)
Figure 2. Effect of substituents on aryl chloride on the
formation rate and selectivity for the products (3 and 4) in
the coupling reaction of 1 and 2. Reaction conditions; 1
(0.50 mmol), 2 (2.5 mmol), 1Pd6Au/ZrO2 (2.0 mol% as
metal), Na2CO3 (1.0 mmol), DMA (1 mL), at r.t., for 1 h.
GC yields are given.
0
0.5
1
1.5
Na2CO3 / mmol
5
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
Figure 3. Effects of (A) the amount of Na2CO3 and (B) the interaction between PdAu NPs and chloride anion
substituents on aryl bromide on the formation rate and enabled the highly selective silylation of aryl
selectivity for the products (3 and 4) in the coupling chlorides. Further applications of supported PdAu
reaction of bromobenzene and 2a. Reaction conditions; 1 alloy catalysts in other synthetic reactions involving
(0.50 mmol), 2a (2.5 mmol), 1Pd6Au/ZrO2 (2.0 mol% as silylation as well as a theoretical study to reveal the
o
metal), base (1.0 mmol), DMA (1 mL), at 100 C, for 40 detailed reaction pathway are currently under
min. GC
investigation in our laboratory.
Although the exact reason for the specific effect of
halogen in the reaction over PdAu alloy NPs is still
not fully understood, we surmise that the difference in
the ion radius between bromide and chloride ions
remarkably affects their reactivities. The selectivity-
determining step in the reaction of aryl bromides
includes the desorption of Br from PdAu alloy NPs
(Paths 2 and 3 in Scheme 2). In contrast, desorption
of Cl is not involved in either Path 1 or Path 4 of the
selectivity-determining step of silylation of aryl
chlorides. This is probably because large bromide and
silyl ions preferentially desorb from the surface to
avoid steric repulsion between bulky PdAu NPs. In
fact, the reaction of 1-chloro-4-iodobenzene and
triethylsilane (2a) by 1Pd6Au/ZrO2 catalyst
predominantly gave chlorobenzene, which suggests
that hydrodeiodation proceeded much faster than the
silylation of C-Cl bond. In contrast, detailed
theoretical studies on the homocoupling of aryl
chloride over PdAu clusters by Ehara and Sakurai et
al. proposed that the chloride ion was adsorbed at the
interface of Pd and Au atoms. This suggests that such
Experimental Section
Materials.
PdCl2 and HAuCl4·3H2O were purchased from KOJIMA
CHEMICALS Co., Ltd. Al2O3 (Sumitomo Chemical Co.,
Ltd, AKP-G015; JRC-ALO-8 equivalent), TiO2 (JRC-TIO-
4), and ZrO2 (JRC-ZRO-3) were obtained from the
Catalysis Society of Japan. Nb2O5·nH2O was kindly
provided by CBMM. Nb2O5 was obtained by the
calcination of Nb2O5·nH2O at 550 °C for 3 h under air flow.
Polyvinyl alcohol (PVA) was purchased from Wako
Chemicals. Solvents with super dehydrated grade were
used. Aryl chlorides and hydrosilanes were of analytical
grade and used as received without further purification.
Physical and Analytical Measurements.
The products of the catalytic runs were analyzed by GC-
MS (Shimadzu GCMS-QP2010, CBP-10 capillary column,
i.d. 0.25 mm, length 30 m, 50250 °C) and gas
chromatography (Shimadzu GC-2014, CBP-10 capillary
column, i.d. 0.25 mm, length 30 m, 50250 °C). NMR
specific interaction between chloride ion and PdAu spectra were recorded on a JMN-ECS400 (FT, 400 MHz
(1H), 100 MHz (13C)) instrument (JEOL Ltd., Tokyo,
NPs helps to stabilize chloride anion at the surface of
PdAu, thus resulting in highly selective silylation of
Japan). Chemical shifts (δ) of 1H and 13C{1H} NMR
spectra are referenced to SiMe4. The supported catalysts
were analyzed by TEM, XRD, XPS and XAFS. High angle
annular dark field-scanning transmission electron
microscope (HADDF-STEM) images were recorded using
a JEOL JEM-3200FS transmission electron microscope.
The samples were prepared by depositing drops of ethanol
suspensions containing small amounts of the powders onto
carbon-coated copper grids (JEOL Ltd.) followed by
evaporation of the ethanol in air. X-ray powder diffraction
analyses were performed using Cu Kα radiation and a one-
dimensional X-ray detector (XRD: SmartLab, RIGAKU).
The samples were scanned from 2 Ө =36° to 42° at a
scanning rate of 0.067 s-1 and a resolution of 0.01°. X-ray
photoelectron spectroscopy (XPS) of the catalysts was
performed using a JPS-9010 MX instrument. The spectra
were measured using MgKα radiation (15 kV, 400 W) in a
chamber at a base pressure of ~10-7 Pa. All spectra were
calibrated using C1s (284.6 eV) as a reference. Pd K-edge
and Au L3-edge XAFS measurements were performed at
aryl chlorides. Detailed theoretical investigations on
the interaction of halogen and PdAu alloy NPs in the
present reactions are currently underway in our
laboratory.
Conclusion
We have described a highly selective silylation of aryl
chlorides over supported PdAu NPs catalysts.
Whereas monometallic Pd NPs predominantly
promoted the hydrodechlorination of aryl chlorides to
give reduced arenes and monometallic Au NPs
showed no catalytic activity, PdAu alloy catalysts
with high Au concentrations promoted the highly
selective silylation of aryl–Cl bonds via the cross-
coupling of aryl chlorides and hydrosilanes without
the aid of phosphine ligands. A detailed mechanistic
investigation unveiled that nucleophilic aryl species the BL01B1 beam line at SPring-8 operated at 8 GeV using
a Si(311) two-crystal monochromator. XAFS spectra were
obtained at room temperature. XANES were analyzed
using REX2000 version 2.5 (Rigaku). The actual contents
of palladium and gold species immobilized on the supports
were determined by atomic emission spectroscopic analysis
with a SHIMADZU AA-6200.
and electrophilic silyl species formed on Pd and Au
atoms on the surface of PdAu NPs, respectively,
which enabled the efficient formation of aryl–Cl
bonds by working in concert. Interestingly, the unique
catalysis of PdAu alloy NPs was positively reflected
in the silylation of aryl chlorides, where the reaction
of aryl bromides and iodides resulted in low
selectivity for the formation of aryl-Si bonds. The
results of a kinetic study suggested that specific
Experimental procedure.
6
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
Preparation of supported PdAu alloy NPs catalysts by a References
sol-immobilization method.
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8
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10.1002/adsc.202000045
Advanced Synthesis & Catalysis
FULL PAPER
Silylation of Aryl Chlorides by Bimetallic
Catalysis of Palladium and Gold on Alloy
Nanoparticles
19 examples
34–85%
R’ = Et, nBu, iPr, OEt
Adv. Synth. Catal. Year, Volume, Page – Page
Pd
Au
H. Miura,* Y. Masaki, Y. Fukuta, T. Shishido*
R = H, Me, OMe, F,
CO2Me, CF3, CN
PdAu alloy
nanoparticle
9
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