Palladium salt and functional reduced graphene oxide complex: in situ preparation of a generally applicable catalyst for C-C coupling reactions

Article abstract of DOI:10.1039/c5ra10585d

A novel Pd catalyst was designed by affording Pd²⁺ salt on the surface of functional reduced graphene oxide (FRGO), providing a new cheap and stable in situ prepared palladium catalyst for C-C coupling reactions, including the Heck reaction, Suzuki reaction, C-H bond functionalization reactions of thiophenes, and terminal alkyne C-H activation and homocoupling.

Full text of DOI:10.1039/c5ra10585d

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Palladium salt and functional reduced graphene
oxide complex: in situ preparation of a generally
applicable catalyst for C–C coupling reactions†
Cite this: RSC Adv., 2015, 5, 53935
Sheng Wang,a Donghua Hu,b Wenwen Hua,b Jiangjiang Gu,a Qiuhong Zhang,a
a
b
*
*
Xudong Jia and Kai Xi
Received 22nd April 2015
Accepted 12th June 2015
A novel Pd catalyst was designed by affording Pd2+ salt on the surface of functional reduced graphene oxide
(FRGO), providing a new cheap and stable in situ prepared palladium catalyst for C–C coupling reactions,
including the Heck reaction, Suzuki reaction, C–H bond functionalization reactions of thiophenes, and
terminal alkyne C–H activation and homocoupling.
DOI: 10.1039/c5ra10585d
www.rsc.org/advances
Many products could be synthesized by coupling reactions, extremely versatile carbon material which can be modied by
either for the rst time or in a much more efficient way than anchoring molecules with favorable properties.30,31
before.1–4 These reactions have largely been employed in
In our previous work, we had prepared an amino group
academic laboratories as well as in pharmaceutical and ne FRGO and developed a single crystal triangle Au nanoplates
chemical industries to synthesize a large variety of organic supported by this FRGO, which showed high efficiency in
molecules.4–7 In these reactions, palladium catalysts have reduction of 4-nitro phenol.32 As palladium–amine complex can
gained enormous relevance as the most commonly used cata- form on FRGO in situ by direct coordination,18,33 we use the
lysts.8,9 In the last decade, palladium nanoparticles have same FRGO dispersed in DMF to produce the catalyst, without
attracted more and more attention in the catalytic eld.10,11 extra ligand to afford palladium system. When the solution of
Compared to traditional catalysts, most nanosized catalysts palladium chloride in DMF was added into FRGO, the new
have much higher efficiency.12 Several palladium nanosized catalyst was generated. This catalyst can be widely used in
catalysts for C–C bond formation reactions have been described Suzuki–Miyaura reaction. In all the examples we have tried, the
in the literatures, but preparation of these catalysts is usually reactions are carried out mildly at room temperature with a low
complicated, in a strict process.13–16
catalyst amount, and a high yield, moreover, unaffected in the
A common and critical feature of these catalytic processes is presence of air. Meanwhile, Mizoroki–Heck reaction, C–H bond
the formation of aryl or alkyl palladium(II) intermediates, which functionalization reactions of thiophenes, as well as terminal
can be subsequently functionalized to form carbon–carbon and alkyne C–H activation leading to Glaser coupling can also be
carbon–heteroatom bonds.17,18 Palladium(II) catalysts are catalyzed. By this means, a new methodology in palladium
sometimes used to replace palladium(0) catalysts which can be catalyzed coupling reaction is developed.
prepared from cheap and stable palladium salt such as PdCl2.19
GO was rstly prepared by modied Hummers method. The
We designed to composite palladium(II) salt and supporting functionalized graphene oxide was obtained from reaction of
materials for the preparation of palladium nanoparticles to 0.1 g of GO powders and 0.2 g of N-propylethane-1,2-diamine,
absorb both of their merits. In this way, dispersion, size and then reduced by hydrazine hydrate. The as-synthesized FRGO
stabilization problems in preparation of palladium nano- (1 mL, 0.3 mg mLÀ1) and a DMF solution of PdCl2 (0.25 mL, 0.01
particles can be abstained, and high-efficiency of palladium M) were mixed at room temperature. This catalyst was prepared
nanoparticle catalysts could be reserved. Many attempts have and would be applied in reactions immediately (TEM images in
been made to immobilize Pd nanoparticles in a wide variety of Fig. 1a and b). The XPS result showed the palladium existed in
supports.20–29 Among these supports, graphene oxide (GO) is an divalent state. The palladium content of the obtained
Pd(II)–FRGO was amounted to 4.78% by weight as veried by
EDX, which could be converted into 0.59% by number of atom.
aState Key Laboratory of Coordination Chemistry, Department of Polymer Science &
Meanwhile, the amount of chloride was 0.60% by number of
Engineering, Nanjing National Laboratory of Microstructures, Nanjing University,
atom, equal to the amount of Pd practically (Fig. 1c). The
Nanjing 210093, P. R. China. E-mail: jiaxd@nju.edu.cn; Fax: +86-25-83621337
particle size distribution of the Pd2+ nanoparticles on the FRGO
bDepartment of Polymer Science & Engineering, Nanjing University, Nanjing 210093, P.
supports was determined by TEM analysis, and the average
particle diameter was 6.14 nm (Fig. 1d).
R. China. E-mail: xikai@nju.edu.cn; Fax: +86-25-83686197
† Electronic supplementary information (ESI) available: Experimental details and
supporting data. See DOI: 10.1039/c5ra10585d
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Table
1
Suzuki–Miyaura reaction of different aryl bromide and
boronic acid catalyzed by Pd(II)–FRGOa
Fig. 1 Characterization of the Pd(II)–FRGO catalyst. (a) TEM image of
the catalyst at 200 nm scale. (b) TEM image of the catalyst at 100 nm
scale. (c) EDX analysis for elements of the catalyst. (d) Particle size
distributions of the supported Pd nanoparticles.
Entry
A
B
Yield (%)
1
2
3
4
5
6
7
8
1
2
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
4
5
5
5
5
5
5
6
6
7a
7a
7b
7c
7d
7e
7f
7a
7b
7c
7e
7f
7a
7b
7c
7d
7e
7f
7a
7b
7c
7d
7e
7f
9, 96
10a, 94
10b, 85
10c, 91
10d, 92
10e, 93
10f, 84
11a, 93
11b, 90
11c, 80
11d, 97
11e, 88
12a, 96
12b, 95
12c, 90
12d, 94
12e, 90
12f, 89
13a, 89
13b, 88
13c, 93
13d, 93
13e, 93
13f, 94
14a, 84
14b, 86
Then, this in situ prepared, cheap and stable palladium
catalyst was applied to several series of C–C coupling reactions,
which showed good catalytic performance and general
applicability.
9
The palladium-catalyzed Suzuki–Miyaura reaction is one of
the most widely used synthetic protocols in modern chemistry
and in industrial application.34,35 These reactions were carried
out with Pd(II)–FRGO, boronic acid (7a–7f, 8) (0.55 mmol,
1.1 equiv.), sodium carbonate (106 mg, 1 mmol, 2 equiv.), aryl
bromide or iodide (1–6) (0.5 mmol, 1 equiv.), using ethanol
(2 mL) and water (2 mL) as the solvent.13 The 26 coupling
reactions proceeded smoothly to give the product with 84–97%
yields under nitrogen in room temperature. Further experi-
ments proved the reaction could even be carried out in atmo-
sphere. To shorten the reaction time, we chose the catalyst in
0.25 mol% of reactant to control all the reaction time in less
than 8 hours (Table 1).
Aer the reaction, the catalyst was separated and charac-
terized by EDX. Compared to the catalyst we prepared before the
reaction, the amount of Pd increased to 1.00% by number of
atom, with the chloride atom disappeared from the catalyst. In
the reaction, more palladium ion from dissociative PdCl2 could
be further xed onto the surface of FRGO, as the cavities on
FRGO also had the capacity in bonding metal ions. On the other
hand, in the basic condition, hydroxyl group from the base
would coordinate with palladium instead of chloride to lead to
the coordination state of palladium changed.18,33 The supposed
mechanism was showed in Fig. 2. In the catalytic cycle, the
addition of aryl halide onto the palladium coordination centres
formed a tetra-coordinated intermediate. Transmetallation
between palladium intermediate and the alkyl borate complex
followed by the elimination formed the C–C sigma bond. The
Pd–FRGO catalyst regenerated, with hydroxyl replaced chloride
to coordinate with palladium.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
7d
8
a
Structure of the products see in the ESI.
from the solvent with the dissociative PdCl2 removed. Aer
redispersed in DMF, the catalyst was used to the reactions of
compound 2 with 7a–f. The results had no different manifes-
tations with the previous reactions, which certied that the
Pd(II)–FRGO played a main role. Tolerance of air and water of
this catalyst was also tested. Reactions of compound 2 with 7a–f
were carried out in atmosphere successfully. The turnover
frequency of this reaction can be up to 9700.
The palladium-catalysed coupling of olens with aryl or vinyl
halides, known as the Heck reaction, is a standard method for
carbon–carbon bond formation in organic synthesis.4,36–38 Our
in situ prepared catalyst not only showed high efficiency in
Suzuki reaction, but also good performance in Heck reaction.
The reaction of iodobenzene (15) and butyl acrylate (18) was
To study the actual function of the catalyst, several reactions
were carried out. The in situ prepared catalyst was separated
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Table 3 C–H activation reaction catalyzed by Pd(II)–FRGOa,b
Entry
A
B
Yield (%)
1
2
3
4
3
4
16
17
22
22
22
22
23a, 67
23b, 83
23c, 85
23d, 77
Fig. 2 Supposed mechanism of catalyst formation and transformation
in the reaction.
Structure of the products see in the ESI. b The reaction time was 20 h,
at 140 ꢀC.
a
carried out with Pd(II)–FRGO, triethylamine and tetrabuty-
lammoniumacetate (TBAA) in DMF at 120 ꢀC.16 The coupling
proceeded smoothly to give butyl cinnamate with 84% yield.
Moreover, aryl bromides (2–5, 16, 17) were also suitable
substrates for the coupling with 62–89% yields. The coupling of
aryl halides (2–4, 15–17) and styrene (19) also proceeded
smoothly to give the corresponding stilbenes with 66–82%
yields (Table 2).
The in situ prepared catalyst was also applied to a variety of
other organic transformations. The development of C–H bond
functionalization reactions (C–H bond activation) is an impor-
tant topic.39 In the past decade, palladium-catalyzed C–H acti-
vation/C–C bond-forming reactions have emerged as promising
new catalytic transformations.17,40–42 We applied Pd(II)–FRGO for
the C–H bond functionalization reactions of thiophenes.43,44
These reactions of aryl bromide (3, 4, 16, 17) with 2-ethyl-
thiophene (22) were carried out with Pd(II)–FRGO (0.5 mol%)
and CsOAc in DMF to give products (23a–23d) with 67–85%
yields (Table 3).
Table 2 Heck reaction catalyzed by Pd(II)–FRGOa,b
Another interesting phenomenon was discovered when the
catalyst was applied to Sonogashira reaction. Phenyl acetylene
(24), aryl bromide, Pd(II)–FRGO, cuprous iodide, base and DMF
were treated under nitrogen. The product was the Glaser
coupling reaction product 1,4-diphenylbuta-1,3-diyne (25),
while aryl bromide did not take part in the reaction. When we
used 1-octyne (26) instead of phenyl acetylene, hexadeca-
7,9-diyne (27) was the only product (Scheme 1). This high effi-
ciency catalyst in Glaser reaction may be applied into new kind
of poly-alkyne materials or be used as a new click reaction
Entry
A
B
Yield (%)
1
2
3
4
5
6
7
8
2
3
4
5
15
16
17
2
3
4
18
18
18
18
18
18
18
19
19
19
19
19
19
20a, 84
20b, 64
20c, 83
20d, 62
20e, 89
20f, 86
20g, 87
21a, 71
21b, 66
21c, 82
21d, 82
21e, 72
21f, 79
9
10
11
12
13
15
16
17
a
b
Structure of the products see in the ESI. Butyl acrylate reacted with
halides for 20 h at 120 ꢀC while styrene reacted with halides for 10 h
at 100 ꢀC.
Scheme 1 Terminal alkyne C–H activation reaction catalyzed by
Pd(II)–FRGO.
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€
´
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a
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In summary, a new designed Pd(II)–FRGO complex shows
favourable catalytic property in several kinds of coupling reac-
tions. Existence form of palladium as a Pd(II) salt brings many
benets, especially, low cost, simple material, excellent storage
performance and high stability. This catalyst can be widely used
in Suzuki–Miyaura reaction and Glaser coupling reaction, in all
the examples we have tried, the reactions are carried out mildly
at room temperature with a low catalyst amount and a high
yield. In addition, the reaction is largely unaffected in the
presence of air. Meanwhile, Mizoroki–Heck reaction and C–H
bond functionalization reactions of thiophenes can also be
catalysed. By this means, a new methodology in palladium
catalysed coupling reaction is developed. Two stable and
commercial materials in the same solvent formed an effective
catalyst immediately aer mixing, which can be applied in C–C
coupling reactions without any sample pre-treatment. Low cost,
perfect stability and convenience in use makes this novel cata-
lyst to be a promising catalyst in future.
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