Ag1Pd1 Nanoparticles–Reduced Graphene Oxide as a Highly Efficient and Recyclable Catalyst for Direct Aryl C?H Olefination

Article abstract of DOI:10.1002/chem.201704056

The efficient and selective palladium-catalyzed activation of C?H bonds is of great importance for the construction of diverse bioactive molecules. Despite significant progress, the inability to recycle palladium catalysts and the need for additives imped

Full text of DOI:10.1002/chem.201704056

A Journal of
Accepted Article
Title: Ag1Pd1 nanoparticles-reduced graphene oxide as highly efficient
and recyclable catalyst for direct aryl C-H olefination
Authors: Qiyan Hu, Xiaowang Liu, Guoliang Wang, Feifan Wang, Qian
Li, and Wu Zhang
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To be cited as: Chem. Eur. J. 10.1002/chem.201704056
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Ag1Pd1 nanoparticles-reduced graphene oxide as highly efficient
and recyclable catalyst for direct aryl C–H olefination
Qiyan Hu, Xiaowang Liu,* Guoliang Wang, Feifan Wang, Qian Li and Wu Zhang*[a]
Abstract: The efficient and selective palladium-catalyzed activation
of C–H bonds is of great importance for the construction of diverse
bioactive molecules. Despite significant progress, the inability to
recycle palladium catalysts and the need for additives impedes the
practical applications of these reactions. Here, we report the use of
Ag1Pd1 nanoparticles-reduced graphene oxide (Ag1Pd1-rGO) as
highly efficient and recyclable catalyst for the chelation-assisted
ortho C–H bond olefination of amides with acrylates in good yields
with a broad substrate scope. The catalyst can be recovered and
reused at least 5 times without losing their activity. A synergistic
effect between the Ag and Pd atoms on the catalytic activity was
found, and a plausible mechanism for the AgPd-rGO catalyzed C–H
olefination is proposed. These findings suggest that search of Pd-
based bimetallic alloy nanoparticles represent a new method to the
development of superior recyclable catalysts for direct aryl C−H
functionalization under mild conditions.
including carbon nanotube and reduced graphene oxide
(rGO).[5] In addition, the introduction of other transition
metal atoms into Pd nanoparticles not only lowers the cost of
the catalyst but also enhances the catalytic activity owing to
the synergistic effects between the two distinct metals.[6]
Recently, activated bimetallic alloy nanoparticle catalysts
have been developed on catalytic coupling reactions such as
Suzuki–Miyaura, Heck and Sonogashira reactions.[7] However,
the use of Pd based bimetallic nanoparticles in direct C–H
functionalization by using only C–H bonds under oxidative
conditions has been rarely explored.[8]
Herein, we report the benefits of using Ag1Pd1-rGO as a
recyclable catalyst for the directing-group-assisted ortho C–H
olefination of amides with acrylates (Scheme 1). We found a
synergistic effect between the constituent Ag and Pd atoms
in promoting the catalytic activity toward ortho C–H
activation, and the synergistic effect is dependent on the
molar ratio of Ag to Pd in the nanoparticles. The highest
Over the last two decades, tremendous effort has been
devoted to transition-metal-catalyzed C–H activation due to
its ability to promote the formation of carbon–heteroatom
and carbon–carbon (C–C) bonds, allowing the incorporation
of complexity into small molecules.[1] Among the metallic
catalysts that have been developed, palladium-based
performing recyclable Ag1Pd1-rGO catalyst exhibited
a
negligible decrease in the catalytic activity after each cycle of
use. It is worth noting that this observation was mainly a
result of mild reaction conditions, especially in the absence of
base and additives.
catalysts have become
a hot topic because of their
outstanding performance for high regio- and stereo-
selectivity C–H activation.[2] Despite their benefits,
homogeneous palladium catalysts are relatively expensive
and sensitive to air and moisture. Moreover, they require the
addition of additives, such as phosphine or amine ligands,
that not only pollutes the environment but also causes issues
for product isolation. The inability to recycle the
homogeneous catalysts obviously raises economic concerns,
thereby hindering transition-metal-catalyzed C–H activation
reactions in practical applications.[3]
Scheme 1 Ortho C–H Olefination.
The development of palladium nanoparticles (Pd NPs) as
catalysts may provide a much-needed solution for the
abovementioned problems due to high surface area,
abundance of active catalytic sites and easy recovery.[4] Of
particular note is the further improvement in the catalytic
activity and prevention of aggregation of the palladium by
dispersion of Pd NPs on variety of supporting materials,
We commenced our study with the preparation of AgxPdy
nanoparticles-rGO (x/y = 1/1, 1/3 and 3/1), Ag nanoparticles-
rGO (Ag-rGO) and Pd nanoparticles-rGO (Pd-rGO) via an in
situ reduction method.[9] The composition of the AgPd NPs-
rGO was controlled by varying the molar ratios of the AgNO3
and Na2PdCl4, and the obtained samples were carefully
characterized. For example, Ag1Pd1-rGO was characterized by
the X-ray powder diffraction (XRD) (Figure S1), transmission
electron microscopy (TEM) (Figure 1a), high-resolution TEM
(insert, Figure 1a), energy dispersive X-ray spectroscopy
(Figure 1b) and X-ray photoelectron spectroscopy (XPS,
Figure 1c). The results show that the formed alloy AgPd
nanoparticles are uniformly distributed on the surface of the
rGO, with an average diameter of approximately 6 nm, and
[a]
Q. Hu, Dr. X. Liu, G. Wang, F. Wang, Q. Li and Prof. Dr.Wu Zhang
Key Laboratory of Functional Molecular Solids, Ministry of
Education; Anhui Laboratory of Molecular-Based Materials; College
of Chemistry and Materials Science, Anhui Normal University
No.1 Beijing East Road, Wuhu, Anhui Province 241000 (China)
E-mail: zhangwu@mail.ahnu.edu.cn; xwliu601@mail.ahnu.edu.cn
Supporting information for this article can be found under:
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Table 1. Optimization of reaction conditions for the AgPd-rGO-catalyzed
C–H olefination.[a]
Temp
(°C)
Yield[b]
(%)
Entry
Catalyst
Solvent
Oxidant
[c]
1
2
Pd(OAc)2
Pd-rGO
DCE
DCE
BQ
BQ
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
120
90
48
39
85
NR
60
28
35
86
64
NR
35
20
66
83
31
55
3
Ag1Pd1-rGO
Ag-rGO
DCE
BQ
4
DCE
BQ
5
Ag1Pd3-rGO
Ag3Pd1-rGO
Ag1Pd1-rGO[d]
Ag1Pd1-rGO[e]
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
DCE
BQ
6
DCE
BQ
7
DCE
BQ
8
DCE
BQ
9
DMSO
toluene
DCE
BQ
Figure 1. (a) TEM image. (b) EDS and (c) XPS profiles of the as-prepared
Ag1Pd1-rGO. The insets in (a) and (c) show an HRTEM image of an Ag1Pd1
nanoparticle and XPS spectra (Ag 3d, left panel and Pd 3d, right panel) of the
as-prepared Ag1Pd1-rGO.
10
11
12
13[f]
14[g]
15
16
17
18
BQ
K2S2O8
Ag2CO3
BQ
DCE
DCE
Ag, Pd should be in oxidation states of 0. [10] The inductively
coupled plasma optical emission spectrometry (ICP-OES)
results indicate that the molar ratio of Ag to Pd was 1.27/1.
We chose the olefination of the aryl C–H of 2-phenyl-N-
(quinolin-8-yl)acetamide (1a) with methyl acrylate (2a) to
evaluate the catalytic activity of the as-prepared AgPd-rGO
with conventional homogenous catalysts. Initially, the
reaction of 1a and 2a was carried out in dichloroethane (DCE)
in the presence of 10 mol % Pd(OAc)2 and 2 equiv of
benzoquinone (BQ) under an air atmosphere at 100 °C, which
afforded 3aa in 48% yield (Table 1, entry 1). A similar yield
was obtained by changing the catalyst to the Pd-rGO
nanocomposite (Table 1, entry 2). To our delight, the yield
increased substantially to 85% when 5 mg (3.3 mol % Pd) of
Ag1Pd1-rGO was employed as the catalyst (Table 1, entry 3).
The fact that Ag-rGO was catalytically inert (Table 1, entry 4)
DCE
BQ
DCE
BQ[h]
BQ[i]
BQ
DCE
DCE
77
63
DCE
BQ
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (5 mg), and
oxidant (0.4 mmol) were stirred at 100 °C for 12 h under an air atmosphere. [b]
Yields were determined using GC with dodecane as an internal standard. [c]
Catalyst 10 mol %. [d] 2 mg. [e] 10 mg. [f] Under argon. [g] O2 balloon. [h] 1.0
equiv BQ and [i] 1.5 equiv BQ.
and 16).
With the optimized conditions in hand, we then turned our
attention toward the scope of the Ag1Pd1-rGO catalyzed C–H
olefination (Scheme 2). The results suggest that both
electron-donating substituents, such as methyl and methoxy,
and electron-withdrawing substituents, such as Cl and Br on
the amide phenyl ring, were well tolerated during the C–H
olefination. Moreover, a noticeable steric effect from the
directing group on the C–H activation was found based on
the obvious decrease in the yield from 77% to 62% when R2
was changed from a methyl to an ethyl group (3ia and 3ja).
The scope of the acrylates was also evaluated in the
Ag1Pd1-rGO-catalyzed aryl C–H olefination under the
optimized reaction conditions (Scheme 2). The replacement
of the methyl group in the acrylate with ethyl, butyl or
isobutyl groups led to the desired olefination products in
good yields. However, the substitution of the methyl group
with a tert-butyl group resulted in a lower yield, likely due to
steric hindrance. Moreover, considerable steric hindrance
was observed when a methyl side group was introduced to
the acrylate, providing moderate yields in the range of 40 to
during this olefination evidenced the presence of
a
synergistic effect between the constituent Ag and Pd atoms
in the alloy nanoparticles, which accounted for the excellent
catalytic activity. The other control experiments showed a
composition-dependent synergistic effect in the as-prepared
AgPd-rGO catalysts (Table 1, entries 5 and 6). The optimal
catalytic activity appeared when the molar ratio of Ag to Pd
in the alloy nanoparticles approached 1:1. The product yield
decreased to 35% in the presence of 2 mg of catalyst, while
significant changes in the yield were not observed when 10
mg of the catalyst was used. In addition to the attributes of
the catalyst, oxidant, solvent and reaction temperature were
optimized: Ag1Pd1-rGO (3.3 mol % Pd) as the catalyst, DCE as
the solvent and BQ as the oxidizing agent under an air
atmosphere at 100 °C. Notably, the use of 2 equiv of BQ was
essential for increasing the yield to 85% (Table 1, entries 15
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Scheme 3 Recyclability of the Ag1Pd1-rGO during the olefination of 1a and 2a.
cubic phase and maintained their highly crystalline nature.[11]
The XPS measurements (Figure S4c-d) suggested slight
oxidation of the surface Ag and Pd atoms into Ag(I) and Pd(II),
respectively.[12] The ICP-OES analysis after removing the solid
catalyst did not indicate obvious Pd or Ag leaching in the
reaction system (<1 ppm), suggesting that the catalytic
activity originated from Ag1Pd1 nanoparticles rather than
from the leached Pd. These findings indicate that the Ag1Pd1-
rGO catalyst was stable under the C–H olefination conditions
and could be reused for multiple cycles while maintaining its
high catalytic activity.
Based on the abovementioned findings and earlier
precedent,[13] a plausible mechanism for the Ag1Pd1-rGO
catalyzed heterogeneous ortho C–H bond activation of
amides with acrylates was proposed in Scheme 4. First, the
surface Pd atoms of the catalyst coordinated with the two
nitrogen atoms of the amide and served as a directing group
for the formation of intermediate
followed by the oxidation of with BQ and air to afford
intermediate . The strong coordination of the in situ-formed
Pd(II) species with the intramolecular aminoquinoline, with
the concurrent loss of the hydroquinone, enabled the
A. This process was
Scheme 2 Scope of the Ag1Pd1-rGO-catalyzed C–H olefination. Reaction
conditions: 1 (0.2 mmol), 2 (0.3 mmol), 5 mg Ag1Pd1-rGO (3.3 mol %) and BQ
(0.4 mmol) were stirred at 100 °C under an air atmosphere. Isolated yields are
listed. [a] Acrylate 2 (0.6 mmol) for 24 h.
A
B
generation of cyclopalladated intermediate
of hydroquinone was confirmed using GC-MS. Intermediate
was then proposed to coordinate to the acrylate, forming
intermediate . The insertion of the C=C bond into the C–Pd
bond resulted in the formation of intermediate . The
C. The generation
50% (3ah and 3ai). Besides acrylate, phenyl vinyl sulfone was
tolerated and gave the desired product in 80% yield.
C
The success of the mono-olefination inspired us to explore
the possibility of achieving diolefination of the amides
(Scheme 2). The results show that the use of twice the
amount of acrylate (0.6 mmol) for an extended time (24 h)
led to diolefinated products in good yields. These findings
suggest the marginal effect of the electron-withdrawing or
donating groups during the diolefination.
D
E
To probe the recyclability of the as-prepared Ag1Pd1-rGO
catalyst in the C–H olefination, we carried out consecutive
reactions of 1a with 2a 5 times using the recovered catalyst.
We discovered that the catalytic activity of the supported
Ag1Pd1-nanoparticles decreased negligibly after each cycle of
use, and the average yield after 5 consecutive rounds was 83%
(Scheme 3). The characterization of the recycled catalyst
revealed the origin of the retention of the high catalytic
activity. The TEM image (Figure S4a) shows a slight increase
in the diameter of the supported Ag1Pd1 nanoparticles after
being used for 5 runs, but the particles were still well-
dispersed on the surface of the rGO. The XRD pattern (Figure
S4b) shows that the Ag1Pd1 nanoparticles were still in the
Scheme 4 Plausible mechanism for the Ag1Pd1-rGO catalyzed C–H olefination.
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This study was supported by the National Natural Science
Foundation of China (Nos. 21272006, 21471007).
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The authors declare no conflict of interest.
Keywords: Ag1Pd1 nanoparticles • reduced graphene oxide •
recyclable catalyst • C–H olefination • heterogeneous catalysis
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Entry for the Table of Contents
COMMUNICATION
The first example of supported
bimetallic alloy nanoparticles
catalyzed direct aryl C–H activation
is described. Ag1Pd1-rGO can be
used as a high efficient and
Qiyan Hu, Xiaowang Liu,* Guoliang
Wang, Feifan Wang, Qian Li, and
Wu Zhang*
Page No. – Page No.
recyclable catalyst for ortho C–H
olefination of amides with acrylates
in good yields under base-free and
additives-free conditions in air.
Ag1Pd1 nanoparticles-reduced
graphene oxide as highly efficient
and recyclable catalyst for direct
aryl C–H olefination
((Insert TOC Graphic here))
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