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N. Liu et al. / Journal of Molecular Structure 1218 (2020) 128537
Table 3
Comparison between Pd/a-diimine/GO and catalysts from different literatures.
Entry
Catalyst
Pd/ -diimine/GO
GOeNHCePd
SWCNT-Met/Pdþ2
Pd NPs-GO
PdeFe3O4/rGO
SBA-15/di-urea/Pd
LDH/Tris/Pd
MNP@SPGMA@AP@Pd
XePd3L
Pd (mol%)
Reaction conditions
Yield(%)
TOF(hꢁ1
)
Ref.
1
2
3
4
5
6
7
8
9
a
0.018
1
1.8
0.1
0.1
0.25
0.26
0.05
0.1
K2CO3,H2O/EtOH(1:1), 60 ꢀC, 0.5 h
CsCO3,DMF/H2O(1:1), 50 ꢀC, 1 h
K2CO3, H2O/EtOH(1:1), 50 ꢀC, 1 h
K2CO3, H2O/EtOH(1:1), 60 ꢀC, 1.5 h
K2CO3, H2O/MeOH(1:3, 80 ꢀC, 20 h
K2CO3, H2O/EtOH(1:1), 80 ꢀC, 1 h
K2CO3, H2O/EtOH(1:2), 70 ꢀC, 2 h
K2CO3, H2O/EtOH(1:1), rt, 1.25 h
K2CO3, H2O/DMF(1:1), 70 ꢀC, 1 h
K2CO3, EtOH, 70 ꢀC, 2 h
97
81
90
96
94
89
95
87
98
99
10778
81
50
640
47
356
183
1392
980
9900
This work
10
0.005
the support, PdCl2, and Pd-
a
-diimine (without GO) with the same
quantitative yield after repeated use for 6 times (Fig. 4). By
measuring the palladium content after each run (ICP-AES), no
leaching palladium species was detected in the filtrate and palla-
dium content of the catalyst remained basically unchanged after
the catalyst was used for 6 times. The date clearly indicate that the
Pd quantities, were applied as a homogeneous catalyst (entry 4 and
5), yields of 58% and 30% were obtained under the same reaction
conditions. Furthermore, carbon nanotubes (CNTs), widely used as
carbon supports, were also applied in here for comparison. Pd/
diimine/CNTs was prepared by the method similar to Pd/ -diimine/
GO and showed rather lower catalytic performance than Pd/ -dii-
mine/GO (entry 6 and 7). During all the catalysts, Pd/ -diimine/GO
exhibited the highest activity. All the results fully highlight the
importance of the -diimine and GO support.
a-
a
Pd/
excellent stability and reusability.
To sum up, bulky -diimine ligand based Pd catalyst supported
a-diimine/GO catalyst synthesized by our group provide the
a
a
a
on GO synthesized here is an effective heterogeneous catalyst for
Suzuki coupling reaction. To assess the present protocol with
respect to other reported methods, some comparison had been
done (Table 3). p-methoxybromobenzene and phenylboronic acid
was chosen as reaction substrate to increase the differentiation
capacity. From Table 3, it can be seen that the present catalyst
exhibited higher catalytic performance compared to the other re-
ported systems. The TOF reached as high as 10778 hꢁ1 under mild
reaction conditions.
a
Having established the optimal reaction conditions, we sought
to evaluate the substrate scope and limitation of the present
transformation. As shown in Table 2, a diversity of aryl bromides
was used to react with phenylboronic acid. Gratifyingly, the elec-
tronic variation of the aryl bromides slightly influenced the reac-
tion efficiency. Electron-deficient substituents including Aldehyde,
acetyl, cyano, nitro groups, afforded the corresponding biaryls (3a,
3d, 3e, 3h) in good to excellent yields of 75e99%. In parallel, the
conditions of the cross-coupling were compatible with electron-
rich groups including methyl, tert-butyl, and methoxy, which pro-
vided the desired products of (3m-3o and 3q) in nearly quantitative
yields (97e99%). To our delight, the substituents on the ortho-,
meta- and para-position of the aryl bromides hardly influence the
activities, which demonstrated that the adequate room provided by
the palladium intermediates in the process of the oxidative addi-
tion. Moreover, some important functional groups, such as CHO,
NH2 and OH, which could be further transformed, were tolerated in
the cross coupling reactions. However, heteroaryl bromides such as
2-bromopyridine and 2-bromothiophene were not effective to
experience present coupling, giving the corresponding products (3s
and 3t) in much less yields of 36 and 3%, respectively.
Based on the above analysis and results, The Pd/a-diimine/GO
catalyst prepared in this work provided excellent catalytic activity
for Suzuki coupling reactions. The reasons maybe lies in several
notable features. (i) Large steric bulky construction of
a-diimine
with Acenaphthenequinone framework provides good electron-
donor capacity and then enhances the rate of oxidation-addition
step, which is usually thought as a rate-determining step in the
mechanism of Suzuki coupling reaction. (ii) Large sterically blocked
benzhydryl in the opposite of N-aryl facilitates reductive elimina-
tion step of Suzuki- Miyaura reaction. (iii) High stability of Pd(II)
covalently immobilized on GO impede the leak and conglomeration
of Pd nanoparticles.
Inspired by the excellent performance of the supported palla-
dium catalyst, we interested in exploring the feasibility of aryl-
boronic acids. The arylboronic acids bearing substituents of
methoxyl and methyl, were selected as nucleophilic reagents to
react with aryl bromides. Pleasingly, these reactions proceeded
smoothly to produce the desired compounds in good to excellent
yields (3u-3ad). Moreover, it is noteworthy that 2-
bromobenzonitrile and 4-methyphenylbronic acid was success-
fully coupled to synthesize 3u, the key Sartan drug intermediate in
a high yield of 97%.
4. Conclusion
In summary, we developed a new type of heterogeneous cata-
lyst for Suzuki-Miyaura reaction. a-diimine complex bearing bulky
phenyl and amine moiety anchored successfully on GO support was
applied for the palladium-catalyzed Suzuki-Miyaura reaction. The
catalysts demonstrated extremely high catalytic activity for the
reaction of aryl bromides and arylboronic acids with a variety of
substituents in mild reaction conditions (60 ꢀC). The protocol of Pd/
a-diimine/GO could be reused 6 times without obvious loss of ac-
An important advantage of heterogeneous catalysts compared
to homogeneous catalysts is its recyclability, which play an
important role on the practical application. A catalyst cycle exper-
iment by coupling reaction of 4-bromotoluene (1 mmol) and phe-
nylboronic acid (1.2 mmol) (60 ꢀC, 1 h) as a model reaction was
tivity. GO supported -diimine palladium catalyst could provide a
a
general guideline for future development and rational design of
palladium catalysts and would be a valuable alternative choice for
the application of palladium-catalyzed reactions.
carried out with Pd/a-diimine/GO as catalyst. After the reaction
completion, the catalyst was recovered by centrifugation with
ethanol and water. After dried under vacuum, the recovered cata-
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
lyst was reused for the next cycle. As expected, the Pd/
a-diimine/
GO catalyst exhibits good cycle stability and maintains
a