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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 This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eur. J. 10.1002/chem.201704056 Link to VoR: http://dx.doi.org/10.1002/chem.201704056
Supported by
10.1002/chem.201704056
Chemistry - A European Journal
COMMUNICATION 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 Pdbased bimetallic alloy nanoparticles represent a new method to the development of superior recyclable catalysts for direct aryl C−H functionalization under mild conditions.
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 [1] of complexity into small molecules. Among the metallic catalysts that have been developed, palladium-based catalysts have become a hot topic because of their outstanding performance for high regio- and stereo[2] selectivity C–H activation. 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 [3] reactions in practical applications. The development of palladium nanoparticles (Pd NPs) as catalysts may provide a much-needed solution for the abovementioned problems due to high surface area, [4] abundance of active catalytic sites and easy recovery. 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,
[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: [email protected]; [email protected] Supporting information for this article can be found under:
including carbon nanotube and reduced graphene oxide [5] (rGO). 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 [6] the synergistic effects between the two distinct metals. Recently, activated bimetallic alloy nanoparticle catalysts have been developed on catalytic coupling reactions such as [7] Suzuki–Miyaura, Heck and Sonogashira reactions. However, the use of Pd based bimetallic nanoparticles in direct C–H functionalization by using only C–H bonds under oxidative [8] conditions has been rarely explored. Herein, we report the benefits of using Ag 1Pd1-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 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.
Scheme 1 Ortho C–H Olefination.
We commenced our study with the preparation of Ag xPdy nanoparticles-rGO (x/y = 1/1, 1/3 and 3/1), Ag nanoparticlesrGO (Ag-rGO) and Pd nanoparticles-rGO (Pd-rGO) via an in [9] situ reduction method. The composition of the AgPd NPsrGO 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
This article is protected by copyright. All rights reserved.
10.1002/chem.201704056
Chemistry - A European Journal
COMMUNICATION Table 1. Optimization of reaction conditions for the AgPd-rGO-catalyzed [a] C–H olefination.
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.
Entry
Catalyst
Solvent
Oxidant
Temp (°C)
Yield[b] (%)
1
Pd(OAc)2[c]
DCE
BQ
100
48
2
Pd-rGO
DCE
BQ
100
39
3
Ag1Pd1-rGO
DCE
BQ
100
85
4
Ag-rGO
DCE
BQ
100
NR
5
Ag1Pd3-rGO
DCE
BQ
100
60
6
Ag3Pd1-rGO
DCE
BQ
100
28
7
Ag1Pd1-rGO[d]
DCE
BQ
100
35
8
[e]
DCE
BQ
100
86
9
Ag1Pd1-rGO
DMSO
BQ
100
64
10
Ag1Pd1-rGO
toluene
BQ
100
NR
11
Ag1Pd1-rGO
DCE
K2S2O8
100
35
12
Ag1Pd1-rGO
DCE
Ag2CO3
100
20
[f]
Ag1Pd1-rGO
DCE
BQ
100
66
14[g]
Ag1Pd1-rGO
DCE
BQ
100
83
[h]
13 [10]
Ag, Pd should be in oxidation states of 0. 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) 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
Ag1Pd1-rGO
15
Ag1Pd1-rGO
DCE
BQ
100
31
16
Ag1Pd1-rGO
DCE
BQ[i]
100
55
17
Ag1Pd1-rGO
DCE
BQ
120
77
18
Ag1Pd1-rGO
DCE
BQ
90
63
[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 2 the obvious decrease in the yield from 77% to 62% when R 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
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10.1002/chem.201704056
Chemistry - A European Journal
COMMUNICATION
Scheme 3 Recyclability of the Ag1Pd1-rGO during the olefination of 1a and 2a.
[11]
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.
50% (3ah and 3ai). Besides acrylate, phenyl vinyl sulfone was tolerated and gave the desired product in 80% yield. 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. 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 welldispersed on the surface of the rGO. The XRD pattern (Figure S4b) shows that the Ag1Pd1 nanoparticles were still in the
cubic phase and maintained their highly crystalline nature. The XPS measurements (Figure S4c-d) suggested slight oxidation of the surface Ag and Pd atoms into Ag(I) and Pd(II), [12] respectively. The ICP-OES analysis after removing the solid catalyst did not indicate obvious Pd or Ag leaching in the reaction system (
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 This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eur. J. 10.1002/chem.201704056 Link to VoR: http://dx.doi.org/10.1002/chem.201704056
Supported by
10.1002/chem.201704056
Chemistry - A European Journal
COMMUNICATION 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 Pdbased bimetallic alloy nanoparticles represent a new method to the development of superior recyclable catalysts for direct aryl C−H functionalization under mild conditions.
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 [1] of complexity into small molecules. Among the metallic catalysts that have been developed, palladium-based catalysts have become a hot topic because of their outstanding performance for high regio- and stereo[2] selectivity C–H activation. 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 [3] reactions in practical applications. The development of palladium nanoparticles (Pd NPs) as catalysts may provide a much-needed solution for the abovementioned problems due to high surface area, [4] abundance of active catalytic sites and easy recovery. 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,
[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: [email protected]; [email protected] Supporting information for this article can be found under:
including carbon nanotube and reduced graphene oxide [5] (rGO). 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 [6] the synergistic effects between the two distinct metals. Recently, activated bimetallic alloy nanoparticle catalysts have been developed on catalytic coupling reactions such as [7] Suzuki–Miyaura, Heck and Sonogashira reactions. However, the use of Pd based bimetallic nanoparticles in direct C–H functionalization by using only C–H bonds under oxidative [8] conditions has been rarely explored. Herein, we report the benefits of using Ag 1Pd1-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 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.
Scheme 1 Ortho C–H Olefination.
We commenced our study with the preparation of Ag xPdy nanoparticles-rGO (x/y = 1/1, 1/3 and 3/1), Ag nanoparticlesrGO (Ag-rGO) and Pd nanoparticles-rGO (Pd-rGO) via an in [9] situ reduction method. The composition of the AgPd NPsrGO 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
This article is protected by copyright. All rights reserved.
10.1002/chem.201704056
Chemistry - A European Journal
COMMUNICATION Table 1. Optimization of reaction conditions for the AgPd-rGO-catalyzed [a] C–H olefination.
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.
Entry
Catalyst
Solvent
Oxidant
Temp (°C)
Yield[b] (%)
1
Pd(OAc)2[c]
DCE
BQ
100
48
2
Pd-rGO
DCE
BQ
100
39
3
Ag1Pd1-rGO
DCE
BQ
100
85
4
Ag-rGO
DCE
BQ
100
NR
5
Ag1Pd3-rGO
DCE
BQ
100
60
6
Ag3Pd1-rGO
DCE
BQ
100
28
7
Ag1Pd1-rGO[d]
DCE
BQ
100
35
8
[e]
DCE
BQ
100
86
9
Ag1Pd1-rGO
DMSO
BQ
100
64
10
Ag1Pd1-rGO
toluene
BQ
100
NR
11
Ag1Pd1-rGO
DCE
K2S2O8
100
35
12
Ag1Pd1-rGO
DCE
Ag2CO3
100
20
[f]
Ag1Pd1-rGO
DCE
BQ
100
66
14[g]
Ag1Pd1-rGO
DCE
BQ
100
83
[h]
13 [10]
Ag, Pd should be in oxidation states of 0. 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) 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
Ag1Pd1-rGO
15
Ag1Pd1-rGO
DCE
BQ
100
31
16
Ag1Pd1-rGO
DCE
BQ[i]
100
55
17
Ag1Pd1-rGO
DCE
BQ
120
77
18
Ag1Pd1-rGO
DCE
BQ
90
63
[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 2 the obvious decrease in the yield from 77% to 62% when R 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
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10.1002/chem.201704056
Chemistry - A European Journal
COMMUNICATION
Scheme 3 Recyclability of the Ag1Pd1-rGO during the olefination of 1a and 2a.
[11]
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.
50% (3ah and 3ai). Besides acrylate, phenyl vinyl sulfone was tolerated and gave the desired product in 80% yield. 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. 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 welldispersed on the surface of the rGO. The XRD pattern (Figure S4b) shows that the Ag1Pd1 nanoparticles were still in the
cubic phase and maintained their highly crystalline nature. The XPS measurements (Figure S4c-d) suggested slight oxidation of the surface Ag and Pd atoms into Ag(I) and Pd(II), [12] respectively. The ICP-OES analysis after removing the solid catalyst did not indicate obvious Pd or Ag leaching in the reaction system (
