Ag1Pd1-rGO nanocomposite as recyclable catalyst for CDC reactions of 2-arylpyridines with aldehydes

Article abstract of DOI:10.1016/j.catcom.2018.05.007

Ag₁Pd₁ nanoparticle-reduced graphene oxide (Ag₁Pd₁-rGO) nanocomposite was used as an efficient catalyst for the synthesis of aromatic ketones via cross dehydrogenative coupling (CDC) reactions of 2-arylpyridines

Full text of DOI:10.1016/j.catcom.2018.05.007

CatalysisCommunications113(2018)27–31
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Catalysis Communications
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Short communication
Ag1Pd1-rGO nanocomposite as recyclable catalyst for CDC reactions of 2-
arylpyridines with aldehydes
Qiyan Hua,b, Xiaowang Liua,⁎, Fei Huanga, Feifan Wanga, Qian Lia, Wu Zhanga,⁎
a
College of Chemistry and Materials Science, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials,
Centre for Nano Science and Technology, Anhui Normal University, Wuhu 241000, PR China
b
School of Pharmacy, Wannan Medical College, Wuhu 241002, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ag1Pd1 nanoparticle-reduced graphene oxide (Ag1Pd1-rGO) nanocomposite was used as an efficient catalyst for
the synthesis of aromatic ketones via cross dehydrogenative coupling (CDC) reactions of 2-arylpyridines with
aldehydes. The catalyst can be reused for 5 cycles without significantly losing its catalytic activity. On the basis
of the obtained experimental evidence, we proposed a possible mechanism for the heterogeneous catalysis. We
find that the supported catalysts exhibit a set of benefits, including extraordinarily catalytic activity, high
reusability and remarkable tolerance of a variety of substrates.
Heterogeneous catalysis
Ag1Pd1 alloy nanoparticle
Reduced graphene oxide
Recyclable catalyst
C − H acylation
1. Introduction
alcohols [16], toluenes [17], α-oxocarboxylic acids [18], α-diketones
[19], benzylamines [20] and alkenes/alkynes [21,22]. The reactions
A major theme in organic synthetic community is to develop eco-
logically and economically advantageous reactions which have the
ability to transform simple precursors to multifunctional compounds or
natural products [1]. Due to the abundance of CeH bonds in the
starting substrates, direct and efficient conversion of these CeH bonds
to CeC bonds is of significant importance [2–5]. Indeed, the past
decade has witnessed the rapid development in CeH activation capable
of leading to arene-arene or arene-alkane linkages [6]. The reported
CeH activation strategies oftentimes relies on directing-group assisted
CeH bond functionalization [7] or dehydrogenative cross-coupling
[8,9]. In both strategies, catalyst, functionalized partner, and oxidant
should be screened carefully to proceed the CeH bond cleavage and
subsequent CeC bond formation effectively [10,11].
Aromatic ketones are an important class of chemicals which have
widespread applications in the synthesis of pharmaceuticals, fragrance,
dye and agrochemical industries [12]. The classical synthesis of aryl
ketones is based on the use of Friedel-Crafts acylation of aromatics in
the presence of solid acids, such as acid-treated metal oxides and het-
eropoly acids [13]. In comparison, the direct introduction of carbonyl
functionality into aromatic motifs via CeH bond cleavage is more eco-
friendly alternative. In fact, the synthesis of such compounds has
achieved considerable advances via directed CeH bond activation [14].
A typical strategy is by use of direct acylation of 2-phenyl pyridines
with a variety of potential acyl sources, such as aldehydes [15],
are commonly carried out in the presence of transition metal salts as
homogenous catalysts, including Pd(OAc)2 [15,17,19,22], PdCl2
[16,20,21], Pd(PhCN)2Cl2 [18]. While the use of such homogeneous
catalysts usually offers high selectivity and efficiency, the unrecyclable
nature of the metal salts tremendously increases the economic cost in
the practical use [23]. In addition, utilization of homogeneous catalysts
definitely complicates the step of product purification and sometimes
can lead to environmental concerns, especially for the case of needing
for toxic phosphine compounds as ligands [24].
The use of Pd nanoparticles for CeH bond activation is expected to
provide an alternative pathway to address the aforementioned issues
[25]. In addition to catalyzing CeC coupling [26,27] and alkene hy-
drogenation [28], Pd nanoparticles have proven to possess comparable
catalytic activity to palladium salts in CeH bond functionization. For
example, the use of Pd/γ-Al2O3 (3 wt%) as catalyst led to the reaction of
2-phenylpyridine with benzaldehyde with a yield up to 88% in the
presence of tert-butyl peroxybenzoate as oxidant [29]. However, this
reaction applied unsuccessfully to aliphatic aldehydes, and the recycl-
ability of the catalyst was not good. We reason that the catalytic per-
formance of Pd nanoparticles in the CeH bond activation allows to be
further improved by doping another component in the Pd lattice to
form alloy nanoparticles in which a synergistic effect between the
constituting elements appears [30]. By making use of Ag1Pd1 alloy
nanoparticles on reduced graphene oxide (rGO) as catalysts, here we
⁎
Corresponding authors at: 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, PR China.
E-mail addresses: xwliu601@mail.ahnu.edu.cn (X. Liu), zhangwu@mail.ahnu.edu.cn (W. Zhang).
https://doi.org/10.1016/j.catcom.2018.05.007
Received 22 January 2018; Received in revised form 10 May 2018; Accepted 14 May 2018
1566-7367/©2018ElsevierB.V.Allrightsreserved.

Q. Hu et al.
CatalysisCommunications113(2018)27–31
metals, respectively [34]. Other alloy nanoparticles and pure metal
nanoparticles were characterized by a similar way before catalytic
studies.
2.2. Optimization of the reaction conditions
We carried out catalyst screening for sp2 CeH bond acylation of 2-
phenylpyridine (1a) and benzaldehyde (2a) in the presence of tert-butyl
hydroperoxide (TBHP) as an oxidant. We found the use of Pd(OAc)2
(10 mol%) and Pd(PPh3)4 (3 mol%) offered a yield of 39% and 47%,
respectively, after keeping the reaction at 110 °C for 18 h (Table 1,
entries 1 and 2). A control experiment showed that indeed no reaction
took place in the absence of catalyst (Table 1, entry 3). When replacing
Pd(OAc)2 and Pd(PPh3)4 with heteroneous catalysts of Pd-rGO, a
comparable yield was obtained (Table 1, entry 4). To our delight, the
yield was substantially elevated to 87% when Ag1Pd1-rGO (10 mg,
2.6 mol% Pd) was employed as a heterogeneous catalyst (Table 1, entry
5). The inability of Ag-rGO toward the CeH bond activation (Table 1,
entry 6), together with the low catalytic activity of the Pd-rGO, implied
a synergistic effect between the Ag and Pd atoms in the acylation of 2-
phenylpyridine. The inferior catalytic performance of Ag1Pd3-rGO and
Ag3Pd1-rGO in the model reaction suggested that the synergistic effect
is highly dependent on the composition of the alloy nanoparticles
(Table 1, entries 7 and 8). On a separate note, the application of
commercialized Pd/C as catalyst led to a yield of 5% (Table 1, entry
22), indicating that ordinary Pd0 heterocatalysts are likely to be in-
effective for the cross-dehydrogenative coupling reactions.
Reaction conditions: 1a (0.5 mmol), 2a (0.75 mmol), catalyst
Ag1Pd1-rGO (10 mg), and TBHP (0.75 mmol), 110 °C, 18 h under an air
atmosphere. Yields of isolated product are listed.
Besides the nature of the catalyst, other reaction parameters, in-
cluding the amount of catalyst, solvent, oxidant, temperature and at-
mosphere, also exert a profound impact on the yield. The reaction
conditions were optimized as follows: 10 mg of catalyst Ag1Pd1-rGO
(2.6 mol% Pd), toluene as solvent, TBHP as oxidant, 110 °C under air
atmosphere (Table 1, entries 9–21).
Fig. 1. (a and b) TEM and EDS profile of as-prepared Ag1Pd1-rGO nano-
composites, inset in a showing HRTEM image of an Ag1Pd1 nanoparticle on
rGO. (c) XRD comparison of as-prepared Ag1Pd1-rGO, Pd-rGO and Ag-rGO
nanocomposites.
report a cooperative effect between the elements of Ag and Pd in cross
dehydrogenative coupling reactions of 2-arylpyridines with aldehydes.
We find a negligible loss in the catalytic activity of the supported cat-
alyst after 5 successive catalytic cycles. We demonstrate that the het-
erogeneous catalysts not only have extraordinarily catalytic activity,
high reusability, but also remarkable tolerance of a variety of sub-
strates. By combining our experimental evidence with previous find-
ings, we propose a possible mechanism accounting for the hetero-
geneous catalysis process.
2.3. Synthesis of aromatic ketones
Under the optimized reaction conditions, the scope of the acylation
reaction of substituted 2-arylpyridines and various benzaldehyde de-
rives was examined (Scheme 1). A variety of benzaldehydes with
electron-withdrawing and electron-donating groups, including methyl,
methoxy, chlorine and trifluoromethyl, are suitable for this reaction,
and the corresponding products (3a(a–h)) were obtained in good to
moderate yields. In general, the reactivity of the electron-withdrawing
group-substituted benzaldehydes is higher than that of the electron-
withdrawing precursors. This may attribute to the difference in the
activity of the substituted benzoyl radicals in situ formed in the reac-
tions. Notably, the yields of 4-, 3-, 2-methoxy-substituted benzalde-
hydes were decreased in turn, which should be a result of a gradual
increase in the steric effect. In addition, various arylpyridines bearing
substituents on the benzene rings were also examined, and the results
suggested that the functional groups, including electron-donating and
-withdrawing ones, were tolerated (3(b–h)a). The CDC reaction was
also applicable to other substrates, such as 7,8-benzoquinoline and 1-
phenyl-pyrazole (scheme 1, 3ia, 3ic, 3if, 3ja and 3jc), to afford cor-
responding compounds in good yields. To our delight, good yields were
obtained as aliphatic aldehydes were used in the Ag1Pd1-rGO catalyzed
CeH bond acylation reaction (scheme 1, 3ak, 3al, 3bl and 3il).
It is worth noting that scale-up reaction (15 times) of 2-phe-
nylpyridine (1a) with benzaldehyde (2a) afforded a yield of 79%,
showing the potential utility of the as-prepared catalyst in practical
chemical industry.
2. Results and discussion
2.1. Characterization of the catalysts
We first synthesized AgxPdy-rGO (x/y = 1/1, 1/3 and 3/1) as well
as Ag-rGO (or Pd-rGO) via our reported method (see supporting ma-
terials, Fig. S1) [31]. Transmission electron microscopy (TEM) showed
that the surface of rGO was homogeneously decorated with Ag1Pd1
nanoparticles with an average diameter of 6 nm (Fig. 1a). The high-
resolution TEM (HRTEM) image (inset in Fig. 1a) reveals high crystal-
linity of the loaded nanoparticles, and the d-spacing of the lattice
fringes (0.234 nm) is in accordance with the lattice parameter of the
AgPd alloy [32]. Energy dispersive X-ray spectroscopy (EDS, Fig. 1b)
suggested that the atomic ratio of Ag-to-Pd in the alloy nanoparticles is
about 1.24/1 that closely matches to the value obtained by inductively
coupled plasma optical emission spectrometry (ICP-OES, mAg
/
m
Pd = 17.7%/13.9%, nAg/nPd = 1.26/1). X-ray power diffraction
measurement (Fig. 1c) confirmed that the nanoparticles are alloy in-
stead of a mixture of isolated Ag and Pd segments due to the presence of
obvious peak shift in the diffraction patterns [33]. Meanwhile, the
strong diffraction profiles implied the high crystalline nature of the as-
prepared alloy nanoparticles. The character of Ag (0) and Pd (0) in the
nanoparticles was further evidenced by X-ray photoelectron spectro-
scopy (XPS, Fig. S2c). The binding energies for Ag 3d3/2 (373.6 eV),
3d5/2 (367.6 eV) and Pd 3d3/2 (340.2 eV), 3d5/2 (334.9 eV) are in good
agreement with those 3d binding energies for bulk pure Ag and Pd
We also investigated the possibility of using benzyl alcohol and
toluene as potential acylation agent under the same conditions (TBHP:
28

Q. Hu et al.
CatalysisCommunications113(2018)27–31
Table 1
Optimization of reaction conditions for the AgPd-rGO-catalyzed CeH acylation.a
Entry
Catalyst
Solvent
Oxidant
Temp (°C)
Yieldb (%)
c
1
2
3
4
5
6
7
8
Pd(OAc)2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DMSO
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHPh
DTBP
IBXi
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
120
100
110
39
47
NR
44
87
NR
48
31
59
30
86
Trace
80
41
75
NR
NR
63
74
79
70
5
d
Pd(PPh3)4
–
Pd-rGO
Ag1Pd1-rGO
Ag-rGO
Ag1Pd3-rGO
Ag3Pd1-rGO
e
9
Ag1Pd1
10
11
12
13
14
15
16
17j
18k
19j
20
21
22l
Ag1Pd1-rGOf
Ag1Pd1-rGOg
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
Ag1Pd1-rGO
5% Pd/C
PhCl
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
O2
TBHP
TBHP
TBHP
TBHP
TBHP
a
Reaction conditions: 1a (0.5 mmol), 2a (0.75 mmol), catalyst (10 mg), and oxidant (0.75 mmol), 110 °C, 18 h under an air atmosphere.
b
c
d
e
f
Yields were determined by GC through use of dodecane as an internal standard.
10 mol%.
3 mol%.
Pure Ag1Pd1 (3 mg).
Catalyst: 5 mg.
Catalyst: 20 mg.
0.5 mmol TBHP.
2-iodoxybenzoic acid.
O2 balloon.
g
h
i
j
k
l
Under argon.
30 mg.
5equiv). However, the obtained yields of 3aa were estimated to be only
2.5. Plausible reaction mechanism
37% and less than 10%, respectively. The lower yields were likely due
to the poor ability of experimental conditions to complete both oxida-
tion and coupling in a short time.
To gain more insight into the mechanism of Ag1Pd1-rGO catalyzed
CDC reaction of 2-phenylpyridine with aldehyde, several control ex-
periments were carried out, as shown in supporting materials. First, the
effects of the directing group on the efficiency of the CDC reaction was
investigated, the results suggest that the presence of a 2-pyridyl moiety
in the starting compound is essential for the CeH bond activation
process. Second, a deuterium-labeling experiment was carried out. The
kinetic isotope effect (KIE) value of 1.2 was observed from the parallel
reactions of 1a and 1a-d5 respectively. The results indicated that the
CeH cleavage of 2-phenylpyridine is not involved in the rate de-
termining step. Furthermore, the desired acylation product (3aa) could
not be obtained when radical inhibitor 2,2,6,6- tetramethyl piperidine-
N-oxide (TEMPO, 2 equiv) was added to the standard reaction. This
observation suggests a possible radical acylation process. Combing
previous findings and our experimental evidence, we proposed a
plausible mechanism as illustrated in Scheme 3. Initially, the surface Pd
atoms of the alloy nanoparticles chelate with the nitrogen atom of 2-
phenylpyridine. The surface Pd atoms are then oxidized into PdII upon
heating at 110 °C in the presence of TBHP and air. The in-situ formed
PdII species play an important role in the activation of the ortho CeH
2.4. Recyclability of the catalyst
By use of the reaction of 2-phenylpyridine 1a and benzaldehyde 2a
(Scheme 2) as a model system, we examined the recyclability of the
catalyst. We discovered that the catalyst can be reused at least for five
cycles with only 9% decline in the activity. The retention of the high
catalytic activity of the catalyst is likely due to a marginable increase in
the size of Ag1Pd1, as evidenced by TEM of the used Ag1Pd1-rGO cat-
alyst (Fig. S2a). Besides, no obvious loss and aggregation of the sup-
ported Ag1Pd1 nanoparticles should make contribution of the main-
tained high catalytic activity as well. Comparative XRD analysis
suggested no change in the crystallinity of these nanoparticles after
catalytic use (Fig. S2b). However, XPS measurement (Fig. S2c and d)
implied the oxidation of the surface Ag and Pd atoms to Ag(I) and Pd(II)
after the catalytic use, respectively.
29

Q. Hu et al.
CatalysisCommunications113(2018)27–31
Scheme 3. Plausible mechanism for the Ag1Pd1-rGO catalyzed CeH acylation.
anchored on rGO can be used as recyclable catalyst for the direct aryl
CeH acylation to efficient synthesis of aromatic ketones. We find a
synergetic effect from the constituent atoms toward the CeH bond
acylation reactions, and a possible mechanism was proposed. This work
suggests that alloying of Pd and other transition metals represents a
new strategy to develop high-performance and recyclable nanoparticles
toward direct aryl CeH functionalization with great potential practical
applications.
Acknowledgements
Scheme 1. Scope of the Ag1Pd1-rGO-catalyzed CeH acylation. Reaction con-
ditions: 1a (0.5 mmol), 2a (0.75 mmol), catalyst Ag1Pd1-rGO (10 mg), and
TBHP (0.75 mmol), 110 °C, 18 h under an air atmosphere. Yields of isolated
product are listed.
This study was supported by the National Natural Science
Foundation of China (Nos. 21471007, 21272006).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2018.05.007.
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