Hydrosilylation of Allenes Over Palladium–Gold Alloy Catalysts: Enhancing Activity and Switching Selectivity by the Incorporation of Palladium into Gold Nanoparticles

Article abstract of DOI:10.1002/ejoc.201800224

An efficient synthetic route to alkenylsilanes involving the hydrosilylation of allenes with supported Pd–Au alloy catalysts has been developed. The incorporation of Pd atoms into Au nanoparticles remarkably enhanced the catalytic activity and product selectivity. Pd–Au alloy catalysts with a low Pd/Au ratio were highly effective for the hydrosilylation at an ambient temperature, and the corresponding β-alkenylsilanes were obtained as a main product in good to high yields.

Full text of DOI:10.1002/ejoc.201800224

DOI: 10.1002/ejoc.201800224
Communication
Selective Hydrosilylation
Hydrosilylation of Allenes Over Palladium–Gold Alloy Catalysts:
Enhancing Activity and Switching Selectivity by the
Incorporation of Palladium into Gold Nanoparticles
Hiroki Miura,*[a,b,c] Suguru Sasaki,[a] Ryoichi Ogawa,[a] and Tetsuya Shishido*[a,b,c]
Abstract: An efficient synthetic route to alkenylsilanes involv- ity and product selectivity. Pd–Au alloy catalysts with a low Pd/
ing the hydrosilylation of allenes with supported Pd–Au alloy Au ratio were highly effective for the hydrosilylation at an ambi-
catalysts has been developed. The incorporation of Pd atoms ent temperature, and the corresponding ꢀ-alkenylsilanes were
into Au nanoparticles remarkably enhanced the catalytic activ- obtained as a main product in good to high yields.
Introduction
The hydrosilylation of unsaturated organic molecules is the
most straightforward and convenient method for the synthesis
of organosilicon compounds.[1] Particularly, alkyne hydrosilyl-
ation by transition-metal catalysts is a well-established tool for
the preparation of alkenylsilanes,[2] which are important scaf-
folds due to their utility in organic synthesis, such as in Hiyama
cross-coupling[3] and Tamao-Fleming oxidation.[4] The hydrosil-
ylation of allenes is an alternative route for the synthesis of
alkenylsilanes. In this case, considerable attention must be paid
to controlling the regiochemistry of the silylated adducts due
to the presence of two contiguous π-systems in allenes
(Scheme 1). In fact, previous studies have demonstrated the
selective formation of allylsilanes by Pd-[5] Co-[6] or Mo-cata-
lyzed[7] hydrosilylation involving protonation of the allene cen-
ter carbon (Scheme 1a and 1b). Despite the fact that allene
hydrosilylation under Ni,[5] Pd,[5] Al[8] and Au[9] catalysis has
emerged as an effective route to terminal alkenylsilanes
(Scheme 1c), an synthetic route to internal alkenylsilanes via
allene hydrosilylation has scarcely been explored (Scheme 1d).
Scheme 1. Formation of allylsilanes and alkenylsilanes via the hydrosilylation
of allene.
On the other hand, increasing attention has recently been
paid to the development of a novel environmentally friendly
catalytic system.[10] In this respect, the use of supported metal
nanoparticles (NPs) as a heterogeneous catalyst is intriguing
due to their high stability and high reusability, and the ease
with which the catalysts can be separated from the
products.[11] We recently reported that supported Pd–Au alloy
NPs functioned as an efficient catalyst for the hydrosilylation of
α,ꢀ-unsaturated ketones and alkynes under mild reaction con-
ditions, whereas monometallic Pd and Au NP catalysts were to-
tally ineffective.[12] A detailed characterization of supported Pd–
Au alloy catalysts revealed that Pd species that were atomically
incorporated into Au NPs served as the main active site for
efficient hydrosilylation.
[a] Department of Applied Chemistry, Graduate School of Urban
Environmental Sciences, Tokyo Metropolitan University
Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
E-mail: shishido-tetsuya@tmu.ac.jp
http://www.comp.tmu.ac.jp/shishidolab/
[b] Research Center for Hydrogen Energy-based Society
Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
[c] Elements Strategy Initiative for Catalysts & Batteries, Kyoto University
1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201800224.
Herein, we describe the hydrosilylation of allenes in the pres-
ence of supported Pd–Au alloy catalysts. The reactions took
place under ambient temperature through the use of Pd–Au
alloy catalysts with a low Pd/Au atomic ratio to afford internal
alkenylsilanes as the main products.
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Results and Discussion
tion of supported Pd–Au alloy catalysts revealed the formation
of isolated single Pd atoms on the surface of Pd–Au alloy NPs,
which are believed to act as the main active site for the present
allene hydrosilylation.[14] A survey to identify the optimal sup-
port for the 1Pd5Au alloy revealed that Al2O3-supported Pd–Au
catalysts gave the highest total yield of the products (entry 6).
The reaction with 1Pt5Au/Al2O3 resulted in a low total yield of
the products. Notably, no formation of allylsilanes was observed
in the reactions with PdAu alloy catalysts.
The reactions of phenylallene (1a) with triethylsilane (2a) in
CH3CN in the presence of the catalyst with different Pd/Au ra-
tios were investigated (Scheme 2).[13] As previously demon-
strated by Stratakis et al., the reaction at 65 °C with Au/TiO2
catalyst proceeded smoothly to give the corresponding termi-
nal alkenylsilane (4a) as a main product with high selectivity.
However, only a trace amount of products was obtained by the
reaction with Au/TiO2 at room temperature.
Under the optimized reaction conditions, the reactions of a
variety of allenes with triethylsilane were investigated (Table 2).
Table 2. Scope of allenes.[a]
Scheme 2. Hydrosilylation of phenylallene (1a) by TiO2-supported Au, Pd or
Pd–Au catalysts.
In contrast, 1Pd5Au/TiO2 showed high activity even under
ambient conditions to afford alkenylsilanes in total yields of
58 %. Remarkably, the incorporation of Pd atoms into Au NPs
drastically changed the regiochemistry of the products, and the
main product in the reaction with PdAu alloy catalysts turned
out to be the internal alkenylsilane with an E configuration
[3a(E)]. No reaction took place in the presence of Pd/TiO2 cata-
lyst under the present conditions.
Table 1 summarizes the effects of the Pd/Au molar ratio and
the support for Pd–Au alloy on the reaction of 1a with 2a. As
in the hydrosilylation of unsaturated ketones and alkynes we
previously reported,[12] Pd–Au catalysts with a low Pd/Au ratio
showed high activity for the present catalytic reactions, and the
reaction catalyzed by 1Pd5Au alloy gave the highest total yield
of the products (entry 2), whereas the reaction with Pd–Au cata-
lysts with a high Pd/Au ratio resulted in very low yields of the
products (entries 4 and 5). Our detailed structural characteriza-
Table 1. Optimization of catalysts.[a]
Entry
Catalyst
Yield [%][b]
3a(E)/3a(Z)/4[b]
1
2
3
4
5
6
7
8
1Pd10Au/TiO2
1Pd5Au/TiO2
1Pd3Au/TiO2
1Pd1Au/TiO2
3Pd1Au/TiO2
1Pd5Au/Al2O3
1Pd5Au/ZrO2
1Pd5Au/Nb2O5
1Pd5Au/CeO2
1Pd5Au/SiO2
1Pt5Au/Al2O3
7
79:6:15
62:9:29
61:8:31
68:8:24
–
57:8:35
60:7:33
64:6:30
64:7:29
77:4:19
67:0:33
58
53
3
0
73
64
62
57
11
23
9
10
11
[a] Reaction conditions: 1a (0.50 mmol), 2a (0.60 mmol), catalyst (1.0 mol-%
as a total amount of Pd and Au), CH3CN (3 mL), at room temp. [b] Yields and
selectivities of the products were determined by GC analysis by using bi-
phenyl as internal standard.
[a] Reaction conditions: 1 (0.50 mmol), 2a (0.60 mmol), 1Pd5Au/Al2O3
(1.0 mol-% as a total amount of Pd and Au), CH3CN (3 mL), at room temp.
[b] Isolated yield. [c] Selectivity of the products were determined by 1H NMR
analysis. Ts = 4-toluenesulfonyl.
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The reactions of allenes with an aromatic substituent gave the
corresponding alkenylsilanes in good to high yields, and inter-
nal alkenylsilanes with an E-configuration were obtained as the
main products (entries 1–3). Alkyl-substituted allenes were also
good substrates in the present catalytic system to afford the
silylated adducts in satisfactory yields, whereas the selectivities
of internal alkenylsilane were decreased (entries 4 and 5). Tri-
substituted internal alkenes bearing two different heteroatoms
could be synthesized in high yields by the reactions of N- or B-
atom-substituted allenes (entries 6 and 7). In the reaction of
ester-substituted allenes, increased selectivities for internal silyl-
alkenes were observed without significant decreases in the total
yields of the product (entries 8 and 9). Although tri-substituted
alkenylsilanes can be prepared by the hydrosilylation of internal
alkynes, this protocol could not be used to synthesize tetra-
substituted alkenes bearing silyl groups. Despite the unsatisfac-
tory yield and selectivity of the product, the hydrosilylation of
1,1-substituted allene with the present PdAu alloy catalytic sys-
tem provides a novel route to tetra-substituted silylalkenes (en-
try 10). Internal allene 1l could participate to give the corre-
sponding alkenylsilane 3l in a moderate yield (entry 11).
Scheme 3. Reuse of 1Pd5Au/Al2O3 for the hydrosilylation of 1a with 2a.
taneously activated by the metal center 5 to give π-allyl metal
hydride intermediate 6, which is a key intermediate for the
formation of alkenylsilanes. Subsequent reductive elimination
provides terminal alkenylsilanes. In this Pd or Ni catalysis, bulky
N-heterocyclic carbene ligands play a key role for the formation
of Si-bearing π-allyl species to avoid steric repulsion between
silyl group and the bulky metal center. Xie and co-workers theo-
retically predicted that the rate-limiting step in these catalysis
is the formation of π-allyl metal hydride species.[15] Despite
such deep consideration regarding the formation of terminal
alkenylsilanes via allene hydrosilylation, the formation of inter-
nal alkenylsilanes was not observed in these catalysis. Thus, ki-
netic studies on the allene hydrosilylation with the use of sup-
ported Pd–Au catalysts were performed to gain insight into the
reaction mechanism.
The scope of hydrosilanes was also investigated (Table 3).
Various tertiary silanes (2a) participated in the present catalytic
system to afford the corresponding alkenylsilanes in moderate
yields (entries 1–4). The reaction of diphenylsilane did not occur
with the present supported PdAu catalyst (entry 5).
Table 3. Scope of hydrosilane.[a]
Entry Hydrosilane
t [h]
Product
Yield [%][b]
3E/3Z/4[c]
1[d]
2
HSi(nBu)3 (2b)
HSi(OEt)3 (2c)
HSiMe2Ph (2d)
HSiMe2tBu (2e)
H2SiPh2 (2f)
5
4
5
8
5
3m(E)/3m(Z)/4m 37
65:7:28
40:29:31
66:0:34
80:0:20
–
Scheme 4. Possible reaction pathway to a terminal alkenylsilane via allene
hydrosilylation by Pd or Ni complex catalysis.
3n(E)/3n(Z)/4n
3o(E)/3o(Z)/4o
3p(E)/3p(Z)/4p
3q(E)/3q(Z)/4q
43
45
21
0
3
4[d]
5
The reaction orders of substrates and catalysts in the reac-
tion of 1a with 2a over 1Pd5Au/Al2O3 catalyst were found to
be almost zero-order for both substrates and first-order for the
catalyst. This suggests that the rate-limiting step in the reaction
over supported PdAu catalysts should not be the formation of
allyl metal species, but rather C–H bond formation (reductive
[a] Reaction conditions: 1a (0.50 mmol), 2 (0.60 mmol), 1Pd5Au/Al2O3
(1.0 mol-% as a total amount of Pd and Au), CH3CN (3 mL), at room temp.
[b] Isolated yield. [c] Selectivity of the products was determined by 1H NMR
analysis. [d] 2 mol-% metal was used.
The high environmental compatibility of the present cata- elimination). A similar tendency regarding for the concentra-
lysts is reflected in their reusability. Significant decreases in the tions of substrates and catalysts was observed in the reaction
yield of the product were not observed during three consecu- over Au/Al2O3 catalysts, which indicates that the incorporation
tive catalytic tests (Scheme 3). Filtration of the supported PdAu of Pd atom into Au NPs does not change the entire reaction
catalyst during the reaction of 1a with 2a completely retarded mechanism, but rather promotes the rate-limiting step of C–H
the further progress of the reaction. Furthermore, no leaching bond formation.
of either of the precious metals into the reaction mixture was
observed by atomic emission spectroscopic analysis of the fil-
trate. These results clearly indicate that supported PdAu alloy
catalyst worked heterogeneously and the reactions took place
on the surface of alloy nanoparticles.
Based on these results, a possible reaction mechanism for
the hydrosilylation of terminal allenes over supported PdAu al-
loy catalysts was depicted in Scheme 5. At first, the dissociative
adsorption of hydrosilanes on adjacent Pd and Au atoms takes
place to form Pd-H and Au-Si species. Subsequent silyl-metalla-
Scheme 4 shows a possible reaction mechanism for the Pd tion of the terminal C=C bond of allene gives primary Si-bear-
or Ni complex-catalyzed hydrosilylation of terminal allenes to ing σ-allyl Au species (A). Finally, surface C–H coupling provides
the corresponding terminal alkenylsilanes, as proposed in ear- internal alkenylsilanes 3. In contrast, the isomerization of (A)
lier reports. Hydrosilanes and allenes are subsequently or simul- generates secondary σ-allyl Au intermediate (C) via the forma-
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tion of a π-allyl Au intermediate (B). Subsequent reductive elim- and the reaction over PdAu alloy at room temperature effi-
ination from Pd-H and σ-allyl Au species (C) furnishes terminal ciently proceeded to give the corresponding alkenylsilanes as a
alkenylsilanes as a product. In this supported catalysis, the de- main product. A kinetic study revealed that the incorporation
termining factor for both the reaction efficiency and the prod- of Pd atoms into Au NPs promoted rate-limiting surface C–H
uct selectivity is the reactivity of the surface hydride and allyl coupling. Further applications of supported Pd–Au alloy cata-
species. Stratakis et al. experimentally proved that allyl species lysts in other synthetic reactions as well as a theoretical study
on Au NPs exhibited a carbocationic character.[9] From these on the detailed reaction mechanism are currently underway in
facts, striking effects of the incorporation of Pd atom into Au our laboratory.
NPs on drastic enhancement of catalytic activity and change of
product selectivity are based on the formation of highly nucleo-
philic hydride species on Pd atoms, which in turn facilitates
C–H coupling with cationic σ-allyl species on Au atoms (A) to
provide internal alkenylsilanes. Conversely, hydride species
Experimental Section
Typical Reaction Procedure: Allenes 1b (0.50 mmol) and CH3CN
(3.0 mL) were added to a Schlenk tube containing the supported
Pd–Au catalyst (1.0 mol-% as a total amount of Pd and Au) under
an argon atmosphere. The reaction was initiated by the injection of
formed on pure Au NPs exhibit low nucleophilicity due to the
strong electron-withdrawing nature of Au atoms.[16] As a result,
isomerization of primary σ-allyl Au species (A) predominantly
hydrosilane 2a (0.60 mmol) at room temperature. After the allenes
occurs to give stable secondary σ-allyl Au species (C), and sub-
sequent C–H coupling provides terminal alkenylsilanes as a sole
product. These suppositions can be supported by the fact that
the reactions of allene bearing electron-withdrawing substitu-
ents showed increased selectivities for internal alkenylsilanes
(Table 2, entries 8 and 9). On the other hand, the remarkable
decreases in the activity of Pd–Au catalysts with high Pd/Au
ratios are probably due to the generation of electron-rich Au
species by excess charge-transfer from Pd to Au, which de-
creases the electrophilicity of cationic allyl species on Au atoms.
In fact, a change in the electronic state of Au species caused
by a change in the Pd/Au ratio was confirmed in XAS and XPS
studies (Figures S8 and S11 in the Supporting Information).
were completely consumed, the solid catalyst was removed by cen-
trifugation. The remaining solution was concentrated under re-
duced pressure and purified through silica gel column chromatog-
raphy (hexane) to give the product in a total yield of 79 %.
Acknowledgments
This study was partially supported by the Program for Elements
Strategy Initiative for Catalysts & Batteries (ESICB). This work
was supported in part by Grants-in-Aid for Scientific Research
(B) (Grant 17H03459) and Scientific Research on Innovative
Areas (Grant 17H06443) commissioned by the Ministry of Edu-
cation, Culture, Sports, Science and Technology (MEXT) of Ja-
pan. The XAFS experiments at SPring-8 were conducted with
the approval (No. 2016B1224) of the Japan Synchrotron Radia-
tion Research Institute (JASRI).
Keywords: Fluorinated compounds · Alloy catalysts ·
Hydrosilylation · Allenes · Alkenylsilane · Regioselective
addition
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