F. Wang et al.
Catalysis Communications 153 (2021) 106296
3.2. Optimization of the reaction conditions
Table 1
–
C H
Optimization of reaction conditions for the CuPd@rGO-catalyzed
acyloxylation.a
Entry
To investigate the catalytic activity of CuPd alloy NPs on C ꢀ H to C
ꢀ O transformation, we first chose 8-methylquinoline (1a) and benzoic
acid (2a) as model substrates and utilize PhI(OAc)2 as oxidant to
establish the optimal reaction conditions. Initially, the reaction of 1a
and 2a was carried out in chlorobenzene (PhCl) in the presence of 10
mol% Pd(OAc)2 and 1 equiv. of PhI(OAc)2 under an air atmosphere at
130 ◦C for 3 h, which afforded 3aa in 63% yield (Table 1, entry 1).
Comparing to homogeneous catalyst and commercial Pd/C, rGO-
supported CuxPdy alloy NPs gave better result when 5 mg (1.95 mol%
Pd) of Cu1Pd1/rGO was employed as the catalyst (Table 1, Entries 1–4).
The fact that Cu/rGO was catalytically inert in the acyloxylation showed
the presence of a synergistic effect between Cu and Pd, introducing
copper could disperse palladium nanoparticles and reduce the amount
of palladium.
Catalyst
Oxidant
Solvent
Yieldb (%)
c
1
Pd(OAc)2
5% Pd/C
Pd/rGO
PhI(OAc)2 (1)
PhI(OAc)2 (1)
PhI(OAc)2 (1)
PhI(OAc)2 (1)
PhI(OAc)2 (1)
PhI(OAc)2 (1)
K2S2O8 (1)
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
Toluene
DMF
DMSO
PhCl
PhCl
63
2
58
3
62
4
Cu1Pd1/rGO
Cu/rGO
76
5
Trace
67
6
Cu3Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
Cu1Pd1/rGO
7
ꢀ
8
Ag2CO3 (1)
ꢀ
9
O2 (1.0 atm)
ꢀ
10
11
12
13
14
15e
16f
PhI(OAc)2 (1.1)
PhI(OAc)2 (1.5)
PhI(OAc)2 (1.1)
PhI(OAc)2 (1.1)
PhI(OAc)2 (1.1)
PhI(OAc)2 (1.1)
PhI(OAc)2 (1.1)
81(78)d
65
67
ꢀ
ꢀ
70
Moreover, the atomic ratio of Cu to Pd played an important role in
CuxPdy alloy, and the Cu1Pd1 provided the best catalytic efficiency
(Entry 6). Based on this result, various oxidants were examined, and it
was found that PhI(OAc)2 was the most effective one and played an
indispensable role in the reaction (Entries 7–8). No desired acylox-
ylation product was observed when the reaction was performed by using
O2 as solo oxidant (Entry 9). With a slight increase in the amount of
oxidant from 1.0 equiv. to 1.1 equiv., the yield increased to 81%,
whereas the yield decreased to 65% when the amount of oxidant further
increased to 1.5 equiv. (Entries 10 and 11). Screening of solvents
revealed that chlorobenzene is the optimal solvent for the formation of
the desired product (Entries 12–14). Decreasing the amount of benzoic
acid resulted in the decreased yield of 3aa (Entry 15). Finally, the yield
of 3aa was decreased to 33% when the reaction was performed under Ar
atmosphere, indicating that O2 played an important role in this reaction
(Entry 16).
33
a
Reaction conditions: 1a (0.2 mmol), 2a (0.7 mmol), catalyst (5 mg) and
solvent (2 mL) were stirred at 130 ◦C for 3 h under an air atmosphere.
b
Yields were determined by using GC–MS with dodecane as an internal
standard.
c
Pd(OAc)2 (10 mol%).
d
Isolated yield.
e
2a (0.6 mmol).
f
Under Ar atmosphere.
carbonate and extracted with ethyl acetate (3 × 10 mL). The combined
organic phases were dried over anhydrous Na2SO4 and filtered. After-
ward, the solution was evaporated under vacuum. The resulting residue
was purified by flash chromatography on silica gel with petroleum ether:
ethyl acetate (10:1) as as an eluent to afford the corresponding products.
3. Results and discussion
3.3. Synthesis of esters
3.1. Characterization of the catalysts
By following the optimized conditions, the scope of acids and 8-
methylquinolines were explored (Scheme 1). To our delight, the reac-
tion displayed broad applicability and a series of substituted benzoic
acid derivatives were successfully employed. Excellent yields were ob-
tained when aliphatic acids were used as acyloxyl sources (3ab, 3ac).
Both electron-rich (3adꢀ 3ag) and electron-deficient (3ahꢀ 3al) benzoic
products in good yields, and this reaction were compatible with fluoro
and trifluoromethyl groups (3ak and 3al). Notably, 2-naphthoic acid
and thiophene-2-carboxylic acid were also well tolerated and gave the
desired product in high yields (3am and 3an). 8-Methylquinolines
bearing bromo group at the 4-, 5- and 7-positions of the quinoline ring
gave the desired products in good to excellent yields (3ba, 3ca and 3da).
However, 8-ethylquinoline exhibited lower reactivity, which is most
likely due to the effect of the methyl substituent steric hindrance (3ea).
Importantly, this catalytic system was successful for gram scale
synthesis of ester. This capability was demonstrated by reaction of 10
mmol of 1a in the model reaction, which produces 3ab in 83% yield
(Scheme S1, Eq. (1)). To demonstrate the potential utility of esters,
further transformations of the products were carried out. The ester
group and the pyridine ring were reduced when using NaBH4 as
reductant and Cu1Pd1/rGO as catalyst. The hydroxylation product was
obtained by the hydrolyzation of 3ab. Quinolin-8-ylmethanol was
oxidized to quinoline-8-carboxaldehyde in high yield by using TBHP as
oxidant in the presence of Cu1Pd1/rGO (Scheme S1, Eq. (2)).
As shown in transmission electron microscopy (TEM) image
(Fig. 1a), the obtained CuPd NPs were well dispersed on the surface of
rGO with an average diameter of approximately 4 nm. Also, the lattice
fringes in the high resolution transmission electron microscopy
(HRTEM) image is 0.228 nm, which is consistent with the (111) crystal
plane of CuPd alloy (Fig. 1b) [29]. X-ray diffraction (XRD) patterns of as-
prepared metallic NPs were provided in Fig. 1c. The diffraction peaks of
the CuPd alloy were positioned between the corresponding peaks of
monocomponent Cu and Pd, which confirmed the formation of CuPd
alloy. The composition and surface chemical state of the alloy nano-
particles was further evidenced by X-ray photoelectron spectroscopy
(XPS). As presented in Fig. 1d, the corresponding peak intensities
demonstrate the predominant metallic Pd0 and Cu0 species in the sam-
ple. The binding energy of Pd 3d5/2 (335.5 eV) and Cu 2p3/2 (931.5 eV)
were slightly different from the bulk Pd (334.9 eV) and Cu (932.4 eV)
[26,30]. The shift of binding energy of Cu and Pd in the Cu1Pd1/rGO
composite can be ascribed to the charge transfer between Cu and Pd
atoms, which may enhance the catalytic efficiency of the Cu1Pd1/rGO
composite. The metal loading of Cu and Pd in CuPd/rGO was deter-
mined to be 5.71% and 8.3% by using inductively coupled plasma
atomic emission spectroscopy (ICP-AES). The molar ratio of Cu to Pd is
1/1.15. The line scan analysis further confirms that Cu1Pd1/rGO is alloy
structure (Fig. S1).
Cu/rGO and Pd/rGO were also prepared by using the same reducing
method, and their representative TEM images and particle size distri-
bution were shown in the Supporting Information (Fig. S2). The
composition of the CuPd/rGO was controlled by changing the molar
ratios of the Cu(NO3)2⋅3H2O and Na2PdCl4, and the obtained samples
were carefully characterized (Fig. S3).
3.4. Recyclability of the catalyst
We turn to investigate the reusability of the Cu1Pd1/rGO catalyst
under optimal reaction conditions. The results showed that no obvious
decrease in catalytic activity was observed after five cycles (Fig. S4a). In
addition, the recycled Cu1Pd1/rGO was characterized by TEM and XRD
3