Graphene-supported RuO2 nanoparticles for efficient aerobic cross-dehydrogenative coupling reaction in water

Article abstract of DOI:10.1021/ol3028785

A cross-dehydrogenative coupling (CDC) reaction between tertiary amines and nitroalkanes has been realized under an oxygen atmosphere in water simply by using graphene-supported RuO₂ as the catalyst, which was made from water-soluble graphene w

Full text of DOI:10.1021/ol3028785

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Graphene-Supported RuO2
Nanoparticles for Efficient Aerobic
Cross-Dehydrogenative Coupling
Reaction in Water
Qing-Yuan Meng,† Qiang Liu,†,‡ Jian-Ji Zhong,† Hui-Hui Zhang,† Zhi-Jun Li,†
Bin Chen,† Chen-Ho Tung,† and Li-Zhu Wu*,†
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,
People’s Republic of China, and State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
People’s Republic of China, and
lzwu@mail.ipc.ac.cn
Received October 21, 2012
ABSTRACT
A cross-dehydrogenative coupling (CDC) reaction between tertiary amines and nitroalkanes has been realized under an oxygen atmosphere in
water simply by using graphene-supported RuO2 as the catalyst, which was made from water-soluble graphene with sulfonic groups and
RuCl3 nH2O to form RuO2 nH2O nanocomposites in situ. In contrast to RuCl3 nH2O and RuO2 nH2O, the graphene-supported RuO2
3
3
3
3
nanoparticles exhibited higher activity and stability for the aerobic CDC reaction in water.
Transition-metal-catalyzed activation of CÀH bonds
for CÀC bond formation has always been an important
concern of organic chemistry.1 As exemplified by one of the
most successful reaction protocols, cross-dehydrogenative
coupling (CDC) reaction that avoids prefunctionalization
and defunctionalization has aroused much interest in recent
years.2 With the aid of a sacrificial oxidant, inorganic metal
salts3 and organic oxidants4 have been successfully em-
ployed for such organic transformation.2À5 Among various
oxidants, molecular oxygen5 is one of the best choices due to
† Chinese Academy of Sciences.
‡ Lanzhou University.
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10.1021/ol3028785
XXXX American Chemical Society

it being abundant, clean, and atom-efficient. Although
considerable progress has been made, most of the aerobic
CDC reactions are carried out in organic solvents. It seems
difficult to realize a CDC reaction in water that is safe and
environmentally benign. In 2006, Li and Wang reported an
efficient “on water”-promoted direct coupling of 1,4-ben-
zoquinones with indole compounds.6 Later, Li et al. found
that copper salts could catalyze the aerobic CDC reaction
of tertiary amines with nitroalkanes or dialkyl malon-
ates to proceed efficiently in water.7 Lipshutz et al. recently
employed cationic [Pd(MeCN)4](BF4)2 salts as a catalyst
to activate aromatic CÀH bonds for the FujiwaraÀ
Moritani reaction of anilides in water.8 Nonetheless, exam-
ples on aerobic CDC reactions in water6À9 that are as
stable and efficient as those in organic solvents are
quite rare.
In this contribution, we wish to report an aerobic CDC
reaction in water, in which graphene, a two-dimensional
sp2-hybridized carbon network, was incorporated into the
transition-metal-catalyzed CDC reaction. It is anticipated
that the excellent electronic, mechanical, and thermal
properties10 of graphene would be useful not only to
provide a support for the transition metal catalyst but also
to improve the performance of the CDC reaction. In order
to make the whole system water-soluble, sulfonic groups
were used to functionalize graphene in this work. Inspired
by the work of Murahashi,2a who discovered RuCl3-
catalyzed oxidation of tetrahydroisoquinolines by oxygen,
we expected that RuCl3 could be successfully anchored on
the surface of the water-soluble graphene with sulfonic
groups. However, we found that RuO2 nH2O rather than
3
RuCl3 was formed on the surface of the graphene in situ.
More importantly, graphene-supported RuO2 (G-RuO2)
isabletocatalyzethe CDC reactionbetween tertiary amine
and nitroalkane under an oxygen atmosphere in water.
A comparison of the performance revealed that G-RuO2
Scheme 1. Preparation of the Water-Soluble Nanocomposites
nanocomposites are more efficient than RuCl3 nH2O and
3
RuO2 nH2O under the same condition. Moreover, G-RuO2
catalyst could be recycled simply by filtration.
3
The synthetic route to the nanocomposites is shown in
Scheme 1. Graphite oxide was prepared by a modified
Hummers’ method from graphite powder11 and used as the
starting material. Under the reduction by NaBH4, the ma-
jority of the oxygen functional groups on the graphite oxide
was removed, and sulfonation of the preliminarily reduced
graphene by aryl diazonium salt of sulfanilic acid12 afforded
water-soluble graphene. After addition of RuCl3 nH2O into
3
the aqueous solution of the sulfonated graphene, sodium
citrate aqueous solution was further added dropwise to
reduce the graphene under heat for 10 h. Then centrifugation
was carried out to obtain the nanocomposite.
Transmission electron microscopy (TEM) equipped
with energy-dispersive spectroscopy (EDS) and X-ray
photoelectron spectroscopy (XPS) were used to study the
chemical nature of the catalyst. As shown in Figure 1a,b,
the reduced graphene oxide sheets are slightly wrinkled
and folded on the ultrathin carbon membrane. The nano-
particles were about 2 nm in size and well-dispersed on the
surface of the transparent carbon sheet. Almost no particle
was found to scatter out of the surface of graphene,
indicating the strong interaction between the graphene
and the particles. The EDS images confirmed the existence
of S and Ru elements in the nanocomposite (Figure 1c).
The nature of nanoparticles on the surface of graphene was
further investigated by XPS measurements. Note that the
binding energies of Ru 3p3/2 and 3p1/2 at 463.4 and 486.3 eV
(Figure 1d), respectively, are consistent with the character of
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B
Org. Lett., Vol. XX, No. XX, XXXX

hydrous RuO2 (also written as RuOxHy).13 The absence of
RuCl3 nH2O might be a result of hydrolysis during reaction
3
under an alkaline environment.14 The alkaline environment
is related to sodium citrate, which could act as a capping
reagent to interact with the surface of sulfonic-group-func-
tionalized graphene.15 As a result, the real catalytic center on
the graphene is RuO2 rather than RuCl3. The prepared
nanocomposition is stable and soluble in water without the
need for any polymeric or surfactant stabilizers.
Table 1. Screening of Reaction Conditions for Aerobic CDC
Reaction of 1a and 2a Catalyzed by G-RuO2 in Watera
entry
catalyst
solvent
gas (1 atm)
yield (%)b
c
1
2
3
4
5
6
G-RuO2
H2O
H2O
H2O
H2O
H2O
H2O
O2
O2
O2
air
O2
N2
78
43
54
36
0
RuCl3 nH2O
3
RuO2 nH2O
3
c
G-RuO2
no
c
G-RuO2
0
a N-phenyl-1,2,3,4-tetrahydroisoquinoline (0.1 mmol), nitromethane
(0.3 mmol), and 0.002 mmol of catalyst (the amount of ruthenium) were
stirred in 0.6 mL of water under the corresponding gas for 16 h. b Isolated
yield based on N-phenyl-1,2,3,4-tetrahydroisoquinoline. c Sonicated in
water for 20 min before utilization.
in 24 h and the CDC reaction product was obtained in a
yield of 89%. Clearly, the catalytic activity of G-RuO2
nanocomposite is much better than that of RuCl3 nH2O
3
and RuO2 nH2O for the reaction.
3
Figure 1. (a,b) TEM images of the G-RuO2 at different magni-
fications; (c) typical EDS spectrum of G-RuO2; (d) Ru 3p3/2 and
3p1/2 XPS spectrum of G-RuO2 composites.
Our preliminary studies focused on the CDC reation of
N-phenyl-1,2,3,4-tetrahydroisoquinoline (1a) and nitro-
methane (2a) in water. Typically, an aqueous solution of
G-RuO2 (2 mol %) containing 1a and 2a (3.0 equiv) was
heated at 60 °C in an oxygen atmosphere for 16 h. It was a
pleasure to see that 78% yield of cross-coupling product
Figure 2. Comparison of RuCl3 nH2O, RuO2 nH2O, and
G-RuO2 with reaction time.
3
3
was obtained (Table 1, entry 1). In contrast, either RuCl3
nH2O or RuO2 nH2O gave lower yields under the same
3
3
condition (Table 1, entries 2 and 3). The water-soluble
graphene G-RuO2 is indeed important for the reaction
performance. Control experiments showed that G-RuO2
catalyst and oxygen are all essential for the CDC reaction
in water. Replacement ofoxygen by air caused a significant
drop in the reaction yield (Table 1, entry 4). Furthermore,
the absence of any of G-RuO2 and oxygen led to no
product formation (Table 1, entries 5 and 6).
As tabulated in Table 2, the scope of the CDC reaction
between tetrahydroisoquinoline derivatives and nitroal-
kanes was studied. Using G-RuO2 ascatalyst, the reactions
proceeded and the desired coupling products were ob-
tained in good to excellent yields. In the case of tetrahy-
droisoquinoline derivatives with a methoxy group either at
the 4-position of N-phenyl (Table 2, entry 5) or at the 5,6-
position of isoquinoline (Table 2, entry 6), the yields were
obviously decreased. This is possibly due to the steric and
electronic effect of the methoxy group. Additionally, the
reaction system works well when less reactive nitroethane
(Table 2, entries 7À9) or nitropropane (Table 2, entry 10)
was used as an electrophilic reagent.
Time course of the yield for coupling production in the
reaction was also investigated (Figure 2). Both RuCl3
3
nH2O and RuO2 nH2O were found to lose their catalytic
activity in water within 5 h. However, when G-RuO2 was
used to catalyze the reaction, the yield of 3a was increased
3
To evaluate the catalytic stability of G-RuO2, we re-
cycled the catalyst, and the result is shown in Figure 3.
Although the yield was decreased from 89 to 64% after five
(13) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W.
Langmuir 1999, 15, 774.
(14) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995,
142, 2699.
(15) Zhang, Z.; Chen, H.; Xing, C.; Guo, M.; Xu, F.; Wang, X.;
Gruber, H.; Zhang, B.; Tang, J. Nano Res. 2011, 4, 599.
runs, the catalytic activity was still superior to RuCl3
nH2O (Table 1, entry 2). From TEM and fluorescence
3
Org. Lett., Vol. XX, No. XX, XXXX
C

Table 2. G-RuO2-Catalyzed Aerobic CDC Reactions between
N-Phenyltetrahydroisoquinoline Derivatives and Nitroalkane
in Watera
Scheme 2. Proposed Mechanism
entry
R
R1
R2
R3
product
yield (%)b
1
2
H
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3f
89
H
H
Me
F
88
3
H
H
88
4
H
H
OMe
H
90
5
H
OMe
H
72
6
OMe
H
H
67
7
Me
H
H
3g
3h
3i
87c
91d
92d
91d
8
Me
H
H
Me
Br
H
afford coupling product 3, reduced Ru species 6, and
water. In the presence of O2, the low-valent Ru species 6
is able to further react with the other amine, leading to the
formation of iminium ion RuIIOOH 7. Subsequently,
nucleophilic attack of iminium 7 with nitroalkane gave
the desired product 3, RuO2 4, and water to complete
the catalytic cycle. The better performance of G-RuO2
9
Me
H
H
10
C2H5
H
H
3j
a N-phenyltetrahydroisoquinoline derivatives (0.1 mmol), nitroalk-
ane (0.3 mmol), G-RuO2 (2.5 mg, sonicated in 0.6 mL of water for 20 min
before use), under O2 (1 atm) at 60 °C for 24 h. b Isolated yield. c Ratios of
the two diastereoisomers were 3:2. d Ratios of the two diastereoisomers
were 2:1.
over RuO2 nH2O implies that the electronic conductivity
3
of graphene10 may accelerate the electron transfer to the
catalytic species, like RuIV=O and RuII, which might
further suppress the aggregation of the nanoparticles on
the surface of graphene during the reaction.
In summary, the nanocomposite G-RuO2 has been
made in situ from sulfonic-group-functionalized graphene
and RuCl3 hydrate. The well-dispersed G-RuO2 has been
demonstrated to be a robust catalyst for the CDC reaction
between N-phenyl-1,2,3,4-tetrahydroisoquinoline deriva-
tives and nitroalkanes in water under an oxygen atmo-
sphere. The higher efficiency of G-RuO2 than either
RuCl3 nH2O or RuO2 nH2O highlights the potential
3
3
application of transition-metal-functionalized graphene
in green and sustainable chemistry. Research is currently
underway to functionalize graphene with other metal
nanoparticles, especially cheap metals, and to apply these
composites in organic transformation.
Figure 3. Reaction yield of CDC reaction using the recycled
G-RuO2 catalyst.
microscopic images, we inferred that the decreased cataly-
tic activity of G-RuO2 is a result of leaching of RuO2
3
nH2O nanoparticles from the surface of graphene, while
that of RuO2 nH2O is the aggregation after reaction
(Figures S1 and S2, Supporting Information).
Acknowledgment. Financial support from the Ministry
of Science and Technology of China (2009CB220008,
2013CB834505, and 2013CB834800), the National Natur-
al Science Foundation of China (21090343, 21172102, and
91027041), and the Chinese Academy of Sciences is grate-
fully acknowledged.
3
On the basis of the above observation, we tentatively
proposed the reaction mechanism shown in Scheme 2. The
high-valent RuO2 nH2O 4 on the surface of water-soluble
3
graphene could coordinate to tertiary amine to generate
the iminium ion RuIIOH 5 by abstracting an electron
and one hydrogen atom directly from the amine.2a,g,16
The intermediate 5 would be trapped by nitroalkane to
Supporting Information Available. Experimental details
and characterization of all compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) Murahashi, S.; Naota, T.; Kuwabara, T.; Saito, T.; Kumobayashi,
H.; Akutagawa, S. J. Am. Chem. Soc. 1990, 112, 7820.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX