The synergistic effect of nanoporous AuPd alloy catalysts on highly chemoselective 1,4-hydrosilylation of conjugated cyclic enones

Article abstract of DOI:10.1039/c3cc49524h

The nanoporous AuPd (AuPdNPore) alloy catalyst showed superior chemoselectivity and high catalytic activity for the direct 1,4-hydrosilylation of the conjugated cyclic enones with hydrosilane in comparison with the monometallic nanoporous Au and Pd catalysts. The enhanced catalytic properties of AuPdNPore arise mainly from the nanoporous structure and the synergistic effect of the AuPd alloy. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49524h

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The synergistic effect of nanoporous AuPd
alloy catalysts on highly chemoselective
1,4-hydrosilylation of conjugated cyclic enones†
Qiang Chen,a Shinya Tanaka,a Takeshi Fujita,a Luyang Chen,a Taketoshi Minato,b
Yoshifumi Ishikawa,a Mingwei Chen,a Naoki Asao,a Yoshinori Yamamotoac and
Tienan Jin*a
Cite this: Chem. Commun., 2014,
50, 3344
Received 16th December 2013,
Accepted 10th February 2014
DOI: 10.1039/c3cc49524h
www.rsc.org/chemcomm
The nanoporous AuPd (AuPdNPore) alloy catalyst showed superior nanoporous metals and alloys with uniform nanoporous
chemoselectivity and high catalytic activity for the direct 1,4-hydro- structures.9 However, so far the nanoporous alloy catalysts have never
silylation of the conjugated cyclic enones with hydrosilane in been demonstrated to show such an alloy effect toward the selective
comparison with the monometallic nanoporous Au and Pd catalysts. organic transformations. Here, we report superior chemoselectivity and
The enhanced catalytic properties of AuPdNPore arise mainly from the high catalytic activity of the nanoporous AuPd alloy (AuPdNPore)
nanoporous structure and the synergistic effect of the AuPd alloy.
catalysts for the direct 1,4-hydrosilylation of conjugated cyclic enones
with hydrosilane, which cannot be achieved by using monometallic
Heterogeneous catalysis is widely used in chemical industrial nanoporous Au (AuNPore) and Pd (PdNPore) catalysts. The 1,4-hydro-
processes and environmental protection, which overcomes the silylation of conjugated cyclic enones is one of the direct methods for
problems of homogeneous catalysis, and therefore considerable preparing the synthetically useful intermediates of silyl enol ethers.10
efforts have been made to develop new nanostructured metals as Although a number of stoichiometric and homogeneous catalytic
powerful green catalysts.1 Particularly, among the nanoparticle methodologies have been reported,11 the catalytic protocol using a
catalysts, the bimetallic alloy catalysts exhibit enhanced stability, reusable heterogeneous catalyst has never been reported to date.12
activity, and selectivity in various chemical reactions compared
In this study, the AuPdNPore-1 alloy catalyst was fabricated by
with the monometallic counterparts,2 which are mainly attributed the selective etching of Al from the Au20Pd10Al70 (at%) ternary alloy,
to ensemble and ligand effects.2b Although much progress has been which was prepared using a high-frequency induction furnace,
made in the synthesis of such nanoparticulate catalysts, it is still using NaOH (20%) as the electrolyte at room temperature for 36 h.
difficult to control their composition, particle size, and geometry The scanning electron microscopy (SEM) and transmission electron
simultaneously to attain optimal catalytic performances.2
microscopy (TEM) images of the as-dealloyed AuPdNPore-1 (Fig. 1)
Recently, we and other groups have demonstrated that nano- catalyst showed a bicontinuous nanoporous structure and the
porous metals having a 3D nanoporous structure are promising curved metallic ligaments with an average size of B8 nm. The
green heterogeneous catalysts for various chemical/electrochemical composition of AuPdNPore-1 was Au58Pd27Al15 (at%) determined
reactions in terms of high catalytic activities, selectivities, and by energy dispersive X-ray (EDX) analysis. The AuPdNPore-1 catalyst
recyclability.3–7 Importantly, the nanoporous alloys display surface area was measured by the BET method to be 28.5 m2 gÀ1
.
enhanced electrocatalytic performances and stability in electro- For comparison, AuPdNPore-2 (Au75Pd16Al9) and AuPdNPore-3
chemical reactions compared with the monometallic nanoporous (Au35Pd47Al17) were fabricated using the same dealloying method
metals owing to the alloy effect.8 The selective etching of a less noble from the corresponding ternary alloys of Au25Pd5Al70 and
element from suitable precursor alloys possessing a homogeneous
single phase is one of the most efficient routes to fabricate
a WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University,
Sendai 980-8577, Japan. E-mail: tjin@m.tohoku.ac.jp
b Institute for International Advanced Interdisciplinary Research (IIAIR),
International Advanced Research and Education Organization, Tohoku University,
Sendai, 980-8578, Japan
c State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian 116012, China
† Electronic supplementary information (ESI) available: Experimental procedures
Fig. 1 SEM (a) and TEM (b) images of the AuPdNPore-1 catalyst.
and characterization data. See DOI: 10.1039/c3cc49524h
3344 | Chem. Commun., 2014, 50, 3344--3346
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Table 1 Screening of various nanoporous metal catalystsa
AuPdNPore-1 catalyst was further reused 2 times without a significant
decrease of yield and selectivity of 2a (entry 5). In addition, the
inductively coupled plasma (ICP-MS) analysis showed that no detect-
able Au and Pd atoms were observed from the reaction solution at the
ppb level. These results together with the leaching experiments
(Scheme S1 in ESI†) indicate that no Au and Pd catalysts were leached
into the reaction solution and the present hydrosilylation was cata-
lysed by the solid state of AuPdNPore catalysts.
The surface chemistry of the AuPdNPore-1 alloy was investigated
using STEM-energy dispersive X-ray spectroscopy (EDS) and X-ray
photoelectron spectroscopy (XPS) to understand the alloy effect on
chemoselectivity. The STEM-EDS mapping of AuPdNPore-1 did not
show detectable surface segregation of Pd, Au, and Al atoms, indicat-
ing the uniform distribution of those atoms in the ligaments of the
nanoporous alloy (Fig. S2 in ESI†). The XPS characterization showed
that the binding energy of Pd 3d5/2 for AuPdNPore-1 is 336.2 eV, which
is more positively shifted than that of PdNPore-1 (336 eV) (Fig. S3a in
ESI†). Meanwhile, the binding energy of Au 4f7/2 for AuPdNPore-1
is 83.9 eV, which is visibly lower than that of AuNPore-1 (84.0 eV)
(Fig. S3b in ESI†). With regard to the change in binding energy, the
Entry Catalyst (precursor alloy)
2ab (%)
3ab (%) 4ab (%)
1
2
3
4
5
6
7
8
PdNPore-1 (PdAl)
PdNPore-2 (PdNiP)
AuNPore-1 (AuAl)
AuNPore-2 (AuAg)
AuPdNPore-1 (Au20Pd10Al70
AuNPore-1 + PdNPore-1
Au20Pd10Al70 alloy
None
14
75
56
0
0
44
51
0
20
0
0
23
12
0
0
4
6
0
0
5
48
)
)
92 (89, 87)c
70
0
0
87
46
47
9
AuPdNPore-2 (Au25Pd5Al70
)
0
0
6
10
AuPdNPore-3 (Au10Pd20Al70
AuPdNPs on TiO2
13
4
11d
a
Reaction conditions: 1a (2.0 mmol), HSiEt3 (2.0 mmol), and nano-
porous catalyst (2.5 mol%) at 100 1C without solvents. b 1H NMR yields
determined using CH2Br2 as an internal standard. The yields of second
(10 h) and third (24 h) uses. 1 mol% of the AuPdNPs catalyst was used.
c
d
Au10Pd20Al70, respectively (Fig. S1, ESI† for TEM images). The negative shift of the Au atom and the positive shift of the Pd atom in
monometallic nanoporous Pd (PdNPore) and Au (AuNPore) AuPdNPore-1 indicate that the charge is transferred from Pd to Au
catalysts were fabricated following our reported methods.3
due to the higher electronegativity of Au. According to the catalyst
Initially, we investigated the catalytic activity and selectivity of nano- characterization and catalytic properties of AuPdNPore-1, we assumed
porous metals and alloys for the 1,4-hydrosilylation of 2-cyclohexene-1- that the more negative hydride in organosilane preferred to chemi-
one (1a) with triethylsilane at 100 1C (Table 1). The PdNPore-1 prepared sorb on the electron poor Pd atom compared with the PdNPore
from a PdAl alloy and the PdNPore-2 prepared from a PdNiP metallic catalyst and the more positive Si in organosilane chemisorbed on the
glass produced the corresponding 1,4-adduct 2a in 14% and 75% electron rich Au atom, which eliminated the possibility of the
yields, respectively, along with the by-product of phenoxysilane (4a) in adsorption of hydride on the Au atom, resulting in suppression of
comparable yields (entries 1 and 2, Scheme S2 in ESI†).3j,k The use of the formation of the 1,2-adduct 3a. Moreover, we found that both
AuNPore-1 and AuNPore-2 catalysts, which were prepared from a AuAl PdNPore-1 and AuPdNPore-1 were able to catalyse the disproportiona-
alloy and a AuAg alloy, respectively, completely suppressed the for- tion reaction of 1a to give a mixture of phenol and cyclohexanone
mation of 4a, while the 1,2-adduct 3a was obtained as a by-product products in the absence of organosilane (Scheme S4 in ESI†). In sharp
together with the desired 1,4-adduct 2a (entries 3 and 4). Interestingly, contrast, high chemoselectivity was achieved for the 1,4-hydro-
unlike those monometallic nanoporous catalysts, the AuPdNPore-1 silylation of 1a with the AuPdNPore-1 catalyst in the presence of
alloy catalyst exhibited a high catalytic activity and chemoselectivity organosilane (Table 1, entry 5). These results strongly support the
for the 1,4-hydrosilylation, affording the 1,4-adduct 2a in 92% 1H NMR preferred chemisorption of hydride on the Pd atom in AuPdNPore-1,
yield and only a small amount of 4a (4%) was obtained without which sufficiently diminished the formation of the by-product 4a.
producing the 1,2-adduct 3a (entry 5). It should be noted that the
On the basis of the catalyst characterization and properties, the
combined catalysts of AuNPore-1 and PdNPore-1 afforded a mixture of proposed mechanism is shown in Scheme 1. The dissociation of
2a, 3a, and 4a (entry 6), implying the remarkable alloy effect of the HSiEt3 on the uniformly distributed AuPdNPore-1 catalyst was pro-
AuPdNPore-1 catalyst on the selective formation of 2a. It was noted that moted by the chemisorption of hydride on the Pd atom and Si on the
the reaction did not proceed with the precursor alloy Au20Pd10Al70 as a Au atom (Schemes S3 and S4 in ESI†). The high affinity of Si for
catalyst or without catalysts (entries 7 and 8). We also found that the carbonyl oxygen of 1a, which chemisorbed on the AuPdNPore catalyst,
composition ratio of AuPdNPore alloys influences the catalytic proper- induced the selective addition of hydride to the b-position of 1a,
ties. For example, although the AuPdNPore-2 catalyst containing a affording the desired product 2a. Although the role of Al is unclear in
higher amount of Au atom than Pd atom showed a comparable the present hydrosilylation, we thought that the remaining Al atoms
catalytic performance to AuPdNPore-1, the AuPdNPore-3 catalyst having in the AuPdAl alloy catalyst should modify the electronic and
a lower amount of Au atom than Pd atom showed dramatically
decreased catalytic activity and selectivity (entries 9 and 10). The
Au–Pd nanoparticles (AuPdNPs) catalyst exhibited low activity and
selectivity in the present hydrosilylation, producing 2a as a major
product together with 3a and 4a as minor products (entry 11).13
The nanoporous skeleton catalyst can be readily recoverable by
a simple filtration of the skeleton catalyst from the reaction
mixture without cumbersome work-up procedures. The recovered Scheme 1 Proposed mechanism for the selective 1,4-hydrosilylation of 1a.
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Table 2 1,4-Hydrosilylation of various conjugated cyclic enones using the
AuPdNPore-1 catalysta,b
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a
Reaction conditions: conjugated cyclic enones 1 (2.0 mmol), HSiEt3
(2.0 mmol), and AuPdNPore-1 (2.5 mol%) at 100 1C without solvents. b Isolated
yields of products 2. c 3-Methylcyclohex-2-enone was used as a starting sub-
stance. d 5-Methylcyclohex-2-enone was used as a starting substance.
geometric structures of the nanoporous catalyst, but Al itself
may not directly incorporate in the catalysis as an active catalyst
species as indicated in entries 1–4 in Table 1.
The catalytic properties of AuPdNPore-1 were further examined
by the reaction with various conjugated cyclic enones (Table 2). The
monomethyl- and dimethyl-substituted cyclohexen-2-ones were
compatible with the present heterogeneous catalytic systems, giving
the desired silyl ethers in high yields, while both 3-methyl- and
5-methylcyclohex-2-enones produced a mixture of regioisomers 2b
and 2c. The 1,2-addition by-products 3 have never been observed
and only a less than 4% of the phenol silyl ethers 4 were observed
in the case of 2a–d. The cyclopenten-2-one and cyclohepten-2-one
also underwent selective 1,4-hydrosilylation sufficiently, affording
the corresponding silyl ethers in good to high yields.
In conclusion, we have demonstrated for the first time that the
uniformly distributed nanoporous AuPd alloy catalysts showed a
remarkable synergistic effect for the heterogeneous 1,4-hydrosilylation
of conjugated cyclic enones with organosilane in comparison with
nanoporous monometallic catalysts. The catalytic performances and
characterization disclose that the alloy effect of AuPdNPore plays a
crucial role in the reaction efficiency and product selectivity. Further
investigation of the underlying mechanism and application to
new synthetic methodologies are in progress.
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This work was supported by a Scientific Research (A) from
Japan Society for Promotion of Science (JSPS) (No. 23245020),
and World Premier International Research Center Initiative
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3346 | Chem. Commun., 2014, 50, 3344--3346
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