Gold Nanoparticle-Polydopamine-Reduced Graphene Oxide Ternary Nanocomposite as an Efficient Catalyst for Selective Oxidation of Benzylic C(sp3)-H Bonds under Mild Conditions

Article abstract of DOI:10.1002/cctc.201600136

A ternary nanocomposite comprising of Au nanoparticles (NPs), polydopamine, and reduced graphene oxide exhibits excellent activity and selectivity towards the oxidation of C-H bonds in benzylic hydrocarbons under mild conditions in the presence of N-hydroxyphthalimide. All the components in the nanocomposite play an important role in the effectiveness of the catalyst. Sufficient electron transfer from polydopamine to the Au NPs afforded a more negatively charged nanoparticle surface and was favorable for molecular oxygen activation, leading to C-H bond oxidation. The reaction followed a free radical pathway as shown by detailed mechanistic studies. Further, easy separation and excellent reusability without significant loss in activity over several iterations make the ternary nanocomposite an excellent heterogeneous catalyst for C-H oxidation reactions. Cooperativity for C-H oxidation: Au nanoparticles embedded on reduced graphene oxide and polydopamine 2D support acts as efficient and selective heterogeneous catalyst for benzylic C-H oxidation under mild reaction conditions in the presence of N-hydroxyphthalimide.

Full text of DOI:10.1002/cctc.201600136

DOI: 10.1002/cctc.201600136
Full Papers
Gold Nanoparticle–Polydopamine–Reduced Graphene
Oxide Ternary Nanocomposite as an Efficient Catalyst for
3
Selective Oxidation of Benzylic C(sp )ÀH Bonds Under Mild
Conditions
Biju Majumdar, Tamalika Bhattacharya, and Tridib K. Sarma*
[
a]
A ternary nanocomposite comprising of Au nanoparticles
NPs), polydopamine, and reduced graphene oxide exhibits ex-
surface and was favorable for molecular oxygen activation,
leading to CÀH bond oxidation. The reaction followed a free
radical pathway as shown by detailed mechanistic studies. Fur-
ther, easy separation and excellent reusability without signifi-
cant loss in activity over several iterations make the ternary
nanocomposite an excellent heterogeneous catalyst for CÀH
oxidation reactions.
(
cellent activity and selectivity towards the oxidation of CÀH
bonds in benzylic hydrocarbons under mild conditions in the
presence of N-hydroxyphthalimide. All the components in the
nanocomposite play an important role in the effectiveness of
the catalyst. Sufficient electron transfer from polydopamine to
the Au NPs afforded a more negatively charged nanoparticle
Introduction
Heterogeneous catalysis is at the heart of green chemical path-
ways as it addresses most of the requirements of green proto-
cols necessary in the manufacturing of diverse ranges of chem-
ous catalysts for selective oxidation of CÀH bonds under
[
5]
milder reaction conditions is crucial for effective applications.
It has been demonstrated that in the presence of an oxygen
donor such as tert-butyl hydroperoxide (TBHP), AuPd nanopar-
ticles facilitate the oxidation of hydrocarbons at relatively mild
3
icals. Selective oxidation of primary sp CÀH bonds by using
molecular oxygen to create useful functionalized chemicals is
[1]
[6]
an important class of chemical transformations. This challeng-
ing transformation is significant not only in the efficient and
sustainable exploitation of organic feed-stocks, but also in un-
derstanding the intrinsic features of the common CÀH bonds
in organic molecules in terms of their accessibility, activity, and
selectivity. Several protocols for highly efficient CÀH transfor-
mation catalyzed by transition-metal complexes such as those
conditions (808C). The capability of the nanoparticle surfaces
in stabilizing free radicals that have longer half-lives than free
radicals in solution induces oxygen activation, leading to effi-
cient oxidation of hydrocarbons at milder reaction conditions.
Whereas focus has been on the structural morphology of the
bimetallic nanoparticles and their interaction with support ma-
terials, we envisioned that even monometallic Au nanoparticles
(NPs) embedded on conducive supports could function effec-
tively as catalysts for challenging organic transformation such
as oxyfunctionalization of inert CÀH bonds under mild condi-
tions in the presence of a free radical promoter. Qualitative ex-
plorations for the role of supports in catalysis have been car-
ried out extensively; however, applying these modifications in
the selective oxidation of hydrocarbons is still a challenge. Gra-
phene oxide (GO), the two-dimensional aromatic scaffold with
both hydrophilic oxygenated functionalities and hydrophobic
nanographene domains on the basal plane, has been demon-
[
2]
of Pd, Pt, Ru, Rh, Re, Fe, Au, etc., have been developed. On
the other hand, the development of noble-metal nanoparticle-
based heterogeneous catalytic systems for important organic
transformations has taken center stage owing to several ad-
vantages such as large surface area availability, the possibility
of easy separation from the reaction mixture, and further reus-
[
3]
ability. Following the seminal work of Hutching’s group on
the oxidation of CÀH bonds in toluene, leading to several oxy-
genates by using alloyed AuPd nanoparticles supported on
[
4]
carbon or TiO2, there has been significant focus on the devel-
opment of nanoparticle-based heterogeneous thermocatalytic
systems. However, the development of Au-based heterogene-
[
7]
[8]
strated as an excellent support for metals, metal oxides,
[
9]
and biomolecules like hemin, etc., for effective catalysis. The
inherent catalytic properties of GO have been demonstrated
for several organic transformations such as alcohol oxidation,
hydration reactions, aza-Michael additions, ring-opening poly-
[
a] B. Majumdar, T. Bhattacharya, Dr. T. K. Sarma
Discipline of Chemistry, School of Basic Sciences
Indian Institute of Technology Indore
Simrol, Khandwa Road, Indore 452020 (India)
E-mail: tridib@iiti.ac.in
[
10]
merization, and Friedel–Crafts reactions. However, for the ox-
idation of inert CÀH bonds, the activity of pure GO was found
to be sluggish. GO can function as an electron shuttle across
its surface as well as transfer electrons to NPs embedded on
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201600136.
[
11]
it. This capability of GO has propelled GO and the reduced
ChemCatChem 2016, 8, 1825 – 1835
1825
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
version, rGO, to be used as an attractive platform for nanopar-
conditions through a pathway that involved free radicals.
Moreover, the recycling effectiveness of the nanocomposite
was demonstrated without significant loss in catalytic activity
over several cycles, making the nanocatalytic system highly
sustainable.
[12]
ticle immobilization and catalytic studies. Among the free
radical promoters, N-hydroxyphthalimide (NHPI) has shown ex-
cellent activity towards CÀH oxygenation of hydrocarbons
when it is combined with a co-catalyst in the presence of
[
13]
oxygen. It is believed that the highly electrophilic phthali-
mide N-oxyl (PINO) radical, which is an in situ formed one-elec-
tron oxidized form of NHPI, initiates the radical propagation of Results and Discussion
autoxidation, thus efficiently promoting hydrocarbon oxyfunc-
Synthesis and characterization of the nanocomposite
tionalization with reactive O2 present. Ishii and co-workers
showed that a combination of NHPI with transition metals, no-
tably cobalt (known as the Ishii system), or with polyoxometa-
lates gives an excellent catalytic system for the autoxidation of
[
20]
The mussel-inspired synthetic method for the growth of sup-
ported Au nanoparticle composites has been described in
detail in the Supporting Information. Briefly, GO nanosheets
synthesized by the modified Hummer’s method were first func-
tionalized with polydopamine (pDA) by polymerization of
dopamine at elevated temperature (Figure S2, in the Support-
[14]
a broad range of organic substrates.
Even non-metallic compounds such as alkyl hydroperox-
[15]
[16]
[17]
[18]
ides,
a,a-azobisisobutyronitrile,
aldehydes,
and NO2
[
21]
have been used as mediators for NHPI-based oxidation. The
biomimetic catalytic system involving anthraquinone deriva-
tives, zeolites, and NHPI has also shown superior activity to-
ing Information). During the process, GO was reduced by
DA. The pDA layer (formed by polymerization of dopamine) on
the surface of rGO was used as a nucleating site and reducing
agent for the synthesis of uniform Au nanoparticles well dis-
persed on the rGO surface. Transmission electron microscopy
(TEM) studies revealed the successful formation of the Au NPs,
which were decorated on the sheets of graphene uniformly
(Figure 1A). The high-resolution TEM image (HR-TEM) showed
a lattice separation of 0.23 nm, corresponding to the (111)
plane of Au (inset, Figure 1A). The selected-area electron dif-
fraction pattern of the Au–pDA–rGO showed crystallinity (Fig-
ure S3, in the Supporting Information). The formation of the
Au NPs on the reduced graphene surface was further con-
firmed by atomic force microscopy (AFM; Figure S5, in the
Supporting Information). The UV/Vis spectrum of the Au–pDA–
rGO composite showed a broad plasmon resonance band at
around 530 nm, confirming the formation of Au NPs. Moreover,
the peak at 230 nm for GO, corresponding to the p!p* transi-
tions of aromatic CÀC bonds, was significantly redshifted
(~15 nm) in the case of the Au–pDA–rGO composite, indicat-
ing the restoration of electronic conjugation within the gra-
[19]
wards CÀH oxygenation of hydrocarbons. However, use of
expensive radical mediators and harsh reaction conditions
such as higher temperature, halogenated solvents, etc., limits
the industrial application of such systems. Moreover, one of
the major issues associated with these catalytic systems is that
the radical mediators are either consumed during the oxida-
tion reaction or present difficulties during the recovery pro-
cess. Thus, development of a catalytic system with an exten-
sive heterogeneous hallmark and environmentally benign reac-
tion conditions is highly appealing for this NHPI-based oxida-
tion reaction.
Herein, we report the catalytic activity of Au NPs anchored
on polydopamine (pDA) and rGO for benzylic CÀH oxidation
reactions. The catalytic system involves a simple and facile
strategy for the generation of Au nanoparticles (NPs) on the
surface of GO by using dopamine hydrochloride (DA) as a re-
ducing agent and nucleation site for the growth of nanocrys-
tals (Scheme 1). In combination with N-hydroxyphthalimide
[
22]
(
NHPI) as a radical initiator, the resulting Au–pDA–rGO nano-
phene sheets (Figure 1B). The powder XRD spectrum of the
Au–pDA–rGO composite consisted of peaks at 2q values of
38.2, 44.4, 64.6, and 77.78, corresponding, respectively, to the
(111), (200), (220), and (311) facets of the face-centered cubic
(fcc) structure of Au. In addition, a broad peak at 2q=22.68, at-
composite exhibited excellent catalytic activity and selectivity
3
towards the oxidation of benzylic sp CÀH bonds in hydrocar-
bons. The high efficiency of the nanocatalytic system resulted
in 80–90% conversion of the substrates under mild reaction
[
23]
tributed to reduced graphene oxide (rGO),
was also ob-
served (Figure 1C). The effects of the growth of Au NPs on the
electronic structure of rGOs were also investigated by Raman
spectroscopy (Figure 1D). As shown, the characteristic D and G
À1
bands at approximately 1328 and 1594 cm , respectively, for
À1
GO were shifted to 1325 and 1572 cm , respectively, for Au–
[
24]
pDA–rGO. Further, the ratio of intensities of the D band to
that of the G band (I /I ) decreased from approximately 1.14
D
G
for GO to about 0.92 for the Au–pDA–rGO composite. This sig-
nifies the formation of more extended networks of conjugated
2
sp carbon atoms towards a more locally ordered graphene lat-
tice.
The reduction of GO to rGO and modification with the pDA
layer was further verified by X-ray photoelectron spectroscopy
(XPS). Figure 2 shows the XPS analysis of GO, pDA–rGO, and
Scheme 1. Schematic of the ternary nanocomposite consisting of Au nano-
particles supported on polydopamine and reduced graphene oxide as cata-
lysts for benzylic CÀH bond oxidation.
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1826
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Figure 1. A) TEM and HR-TEM (inset) image of the Au–pDA–rGO nanocomposite; B) UV/Vis spectra (inset: enlarged view of Au surface Plasmon resonance
band); C) powder X-ray diffraction pattern; and D) Raman spectra of graphene oxide (black line) and Au–pDA–rGO nanocomposite (red line).
Au–pDA–rGO. The XPS survey spectra (Figure 2A) shows the
presence of the N1s peak in the spectra of pDA–rGO and
Au4f peak in Au–pDA–rGO. The C1s core level spectrum of
GO (Figure 2B) was fitted into four components with binding
energies (BEs) at about 284.5, 286.4, 287.6, and 288.9 eV, which
correspond to CÀC, CÀO, C=O, and OÀC=O species, respective-
ly. The pDA–rGO C1s core level spectrum (Figure 2C) was
fitted into five peak components with BEs at about 284.4,
Catalytic performance of the Au–pDA–rGO composite at
variable Au loadings
We evaluated the catalytic efficiency of the heterogeneous
Au–pDA–rGO nanocomposite for the oxidation of benzylic
CÀH bonds. Until now, development of Au-based heterogene-
ous nanocatalysts for CÀH oxidations have been focused on bi-
metallic nanostructures, either in alloy or core–shell form,
where the structure and synergy between metallic constituents
2
85.5, 286.4, 287.8, and 288.9 eV attributed to CÀC, CÀN, CÀO,
[
4,6,26]
C=O, and OÀC=O species, respectively. Compared with GO,
a significant decrease of the CÀO peak component in pDA–
rGO was observed, which clearly indicated the reduction of GO
by dopamine. The appearance of the CÀN peak component at
the BE of 285.5 eV in the C1s core level spectrum of pDA–rGO
and an N1s core level spectrum at the BE of ~400 eV (inset of
Figure 2C) are consistent with the presence of a surface-modi-
play a critical role.
To our surprise, even the monometallic
Au NPs embedded on the pDA–rGO layer could function as an
effective catalyst in combination with NHPI and molecular
oxygen under mild reaction conditions. For evaluating the op-
timization conditions, we selected diphenylmethane as
a model substrate and evaluated its oxidation to diphenylke-
tone under various optimized conditions such as catalytic load-
ing, temperature, solvents, supports, oxidants, catalyst amount,
etc. First, we evaluated the variation in catalytic activity with
respect to Au loading on the pDA–rGO surface. A series of
nanocatalysts were prepared by varying the concentration of
[
25]
fied pDA layer. Figure 2D shows the high-resolution Au4f
core level spectrum. The binding energies at 83.6 and 87.3 eV
are ascribed to Au4f7/2 and Au4f5/2 of metallic Au, respectively.
Further investigation of the molecular structural changes as-
sociated with the formation of Au–pDA–rGO was carried out
by using FTIR spectroscopy (Figure S6, in the Supporting Infor-
mation). The characteristic vibration peak owing to C=O in GO
the metal precursor (HAuCl ), keeping the concentration of
4
pDA–rGO constant. The amount of Au in the composites was
evaluated by using inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES; Table S2, in the Supporting Infor-
mation). While increasing the Au loading from 0.5 to 6 wt%,
we observed that average diameter of the Au NPs increased
congruently. From the TEM image and corresponding particle
size distribution histogram (Figure S7, in the Supporting Infor-
À1
at 1720 cm decreased dramatically in the Au–pDA–rGO com-
posite, signifying the conversion of GO to rGO. Further, the ap-
À1
pearance of a new peak at 1570 cm corresponding to an
NÀH bending vibration confirmed the presence of polydopa-
mine in the composite matrix.
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1827
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Figure 2. A) XPS survey spectra of (a) GO, (b) pDA–rGO, and (c) Au–pDA–rGO. B) and C) XPS C1s core level spectrum of GO and pDA–rGO, respectively.
D) Au4f core level spectrum.
mation), the average diameter of Au NPs in 0.5, 2, 4, and
loading showed the highest turnover number (TON; Table S3,
in the Supporting Information) when the catalytic reactions
were performed by keeping the substrate-to-metal ratio con-
stant. This was expected considering that the smallest particle
size of Au NPs was formed in the case of 0.5 wt% Au loading
6
0
wt% Au-loaded nanocatalysts were calculated to be 4.5Æ
.5, 6.8Æ0.7, 7.9Æ0.7, and 9.6Æ1.0 nm, respectively. With the
different Au-loaded Au–pDA–rGO composites, we performed
the aerobic oxidation of diphenylmethane by using acetonitrile
as the solvent and in the presence of NHPI (Table 1), under the
best optimized conditions (as discussed in Table 2). For all the
Au–pDA–rGO composites with variable Au loading, diphenylke-
tone was formed as the major product along with a small
amount of diphenylmethanol as a side product. It was ob-
served that Au–pDA–rGO nanocomposites with 0.5 wt% Au
3
+
of all the NPs synthesized at various rGO–pDA/Au ratios. In-
creasing the Au loading resulted in a decrease in TON, sug-
gesting that the catalytic performance of the NPs was depen-
dent on particle size of the Au NPs. However, when we evalu-
ated the catalytic conversion of our model reaction with re-
spect to the weight of the catalytic mass of the composite, the
Au–pDA–rGO with 2 wt% Au loading offered the best catalytic
conversion with high selectivity under the standard reaction
conditions (Table 1). The nanocomposite with 0.5 wt% Au load-
ing afforded moderate catalytic conversion. On the other hand,
with decreasing rGO/Au ratio (up to 2 wt% Au loading), the
conversion improved significantly. This result suggested that
a higher rGO/Au ratio might have a detrimental effect on the
oxidation reaction. Further decreasing the rGO/Au ratio (for 4
and 6 wt% Au loading) did not result in any improvement. Al-
though increasing temperature resulted in higher conversion
in a shorter time, the selective formation of diphenylketone
suffered (Table 1, entry 6). Based on these studies, we found
that the nanocomposite with 2 wt% Au loading offered the
best catalytic conversion and high selectivity with respect to
catalyst weight of the composite under the present reaction
Table 1. Effect of Au loading on the aerobic oxidation of diphenyl-
[a]
methane.
Entry
Au loading
%]
t
[h]
Conversion
[%]
Selectivity
[%]
[
1
2
3
4
5
0.5
1
2
4
6
14
13
12
12
11
8
44.2
62.2
92.4
93.3
90.1
91.2
98.9
96.5
97.9
95.8
92.6
87.4
[b]
6
2
[
a] Reaction conditions: 1.0 mmol diphenylmethane, catalyst mass 0.02 g,
reaction temperature 608C, stirring rate 1000 rpm, O pressure 10 bar,
CH CN 5 mL, NHPI 10 mol%. [b] Reaction performed at 1008C.
2
3
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1828
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
conditions. From these results, it can be concluded that the
rGO/Au ratio had a significant effect on the catalytic activity of
the nanocomposite (Table S4, in the Supporting Information).
for the reaction and gave unfruitful results. Au NPs synthesized
by using only pDA as a reducing and stabilizing agent in the
absence of GO (Au–pDA) showed much lower catalytic activity
compared with Au–pDA–rGO (Table 2, entry 20). TEM images
of the Au–pDA composite showed the formation of agglomer-
ated nanoparticles with an average particle size of approxi-
mately 20 nm (Figure S8, in the Supporting Information).
Hence, the deposition of pDA on GO was crucial for controlling
the size of the Au NPs in the composite. On the other hand,
the Au–rGO composite without pDA (synthesized by using
Optimization and scope of reaction
To optimize the reaction conditions to obtain high conversion
and selectivity for the CÀH oxidations, we performed several
controlled experiments by using diphenylmethane as a model
substrate (Table 2). The Au–pDA–rGO nanocomposite did not
afford high conversion in the presence of molecular oxygen as
the single oxidant, even at elevated temperature. Therefore, it
was necessary to look for a secondary oxidant to facilitate
higher conversion. Addition of commonly available oxidants
such as H O or tert-butylhydroperoxide (TBHP) to the reaction
NaBH as the reducing agent) afforded only 38% yield of the
4
desired product under the optimized reaction conditions. It is
well known that several other supports such as polyvinyl pyr-
rolidone (PVP), activated carbon, and metal oxides such as TiO2
and CeO are often used for anchoring nanoparticles. To gain
2
2
2
mixture afforded moderate yields of the desired product di-
phenylketone along with the formation of several byproducts.
However, in combination with NHPI, Au–pDA–rGO afforded
high conversion and selectivity under mild reaction conditions
a better insight into the catalytic efficacy of the Au–pDA–rGO
nanocomposites, we performed the catalytic studies with Au–
PVP, Au–C, Au–TiO , and Au–CeO nanocomposites in the pres-
2
2
ence of NHPI. When using Au–PVP as a catalyst under the stan-
dard reaction conditions, the conversion was moderate and
the selectivity was poor as several other byproducts, including
diphenylmethanol and benzoic acid, were observed. Further,
the nanoparticles were agglomerated after just one cycle of
the reaction. Au NPs stabilized with activated carbon (Au–C)
did not afford higher yields of the oxidized product. In the
case of Au–TiO or Au–CeO as the catalyst in the presence of
(
608C). Use of 10 mol% of NHPI was found to be optimum for
our catalytic reaction (Table 2, entries 9–11). Among the sol-
vents, CH CN was found to be most suitable for the reaction,
3
as other solvents such as water, acetone, and DMSO only af-
forded low yields. Further, increasing the reaction temperature
from 258C to 608C resulted in tremendous enhancement in
conversion (Table 2, entries 8 and 11). Further increases in tem-
perature, however, led to a decrease in conversion and selec-
tivity (Table 2, entry 12). To evaluate the role of the individual
constituents in the ternary composite, several controlled ex-
periments were performed under the standard reaction condi-
tions. NHPI, GO, pDA or combinations of them were inefficient
2
2
NHPI, the oxidized products were obtained with moderate
yields at 608C (Table 2, entries 18 and 19), however, better con-
version was observed at elevated temperature (Table S5, in the
Supporting Information). From these studies, it was apparent
that Au–pDA–rGO exhibited the best catalytic activity when
coupled with NHPI, resulting in successful oxidation
[
a]
of diphenylmethane with 92% conversion and 97%
Table 2. Optimization of reaction conditions.
selectivity to diphenylketone in 12 h under mild con-
Entry Catalyst
Additive
mol%]
Reaction
conditions
Conversion
[%]
Selectivity
[%]
ditions.
The scope of the oxidation reactions was surveyed
[
b[b]
1
2
3
4
5
6
7
8
9
1
Au–rGO
NHPI (10) CH
3
CN, 608C, 12 h
CN, 608C, 24 h
38.1
22.8
24.4
38.2
41.2
61.3
52.5
49.4
74.2
92.5
96.6
98.5
100
82.8
81
79.3
80.9
98
98.9
97.8
97.4
89.6
87.8
96.7
99
by using Au–pDA–rGO as the catalyst in the pres-
ence of NHPI to catalyze oxygenations of various hy-
drocarbons under the same conditions except for
the reaction time (Table 3). In general, the CÀH bond
oxidation of benzylic compounds occurred smoothly
under mild conditions to give ketone products. For
example, tetralin was oxygenated with high conver-
sion and selectivity to tetralone with 85% yield.
Indan, fluorene, and xanthene were oxidized with
good conversions (80–90%, Table 3, entries 2, 4, and
Au–pDA–rGO
Au–pDA–rGO
Au–pDA–rGO
Au–pDA–rGO
Au–pDA–rGO TBHP (10) CH
Au–pDA–rGO TBHP (5) CH
Au–pDA–rGO NHPI (10) CH
–
K
CH
3
2
3
CO (10) water, 708C, 24 h
H
H
2
O
O
2
(10) CH
(15) CH
3
CN, 608C, 12 h
3
CN, 608C, 12 h
3
CN, 608C, 12 h
3
CN, 608C, 12 h
3
CN, 258C, 12 h
3
CN, 608C, 12 h
3
CN, 608C, 12 h
3
CN, 608C, 12 h
3
CN, 808C, 12 h
2
2
Au–pDA–rGO NHPI (5)
CH
0
Au–pDA–rGO NHPI (15) CH
Au–pDA–rGO NHPI (10) CH
Au–pDA–rGO NHPI (10) CH
Au–pDA–rGO NHPI (10) water, 808C, 12 h
Au–pDA–rGO NHPI (10) acetone, 608C, 12 h 45.6
Au–pDA–rGO NHPI (10) DMSO, 608C, 24 h
Au–PVP
Au–C
[
c]
[d]
[e]
1
1
92, 52 , 65 , 93
1
1
1
1
1
1
1
1
2
2
3
4
5
6
7
8
9
0
90.7
25.1
9
). An exception was the case of 9,10-dihydroanthra-
<1.0
34.0
22.1
48.8
43.6
12
cene, where the dehydrogenation product was fa-
vored over the oxidation product (Table 3, entry 8).
In the case of toluene, the major oxidation product
was benzoic acid, probably because aldehydes are
known to be a radical initiator for NHPI-based oxy-
genations and are not stable in such radical reactions
NHPI (10) CH
NHPI (10) CH
NHPI (10) CH
NHPI (10) CH
NHPI (10) CH
3
CN, 608C, 12 h
3
CN, 608C, 12 h
3
CN, 608C, 15 h
3
CN, 608C, 14 h
3
CN, 608C, 12 h
87.5
100
97
95
Au–TiO
2
Au–CeO
Au–pDA
2
96.8
[
a] Reaction conditions: Unless otherwise specified, all the reactions were carried out
[
19]
(
Table 3, entry 10).
The oxidation of cyclohexane
with diphenylmethane (1.0 mmol); Au nanoparticle composite 0.02 g (Au loading
2
1
wt%); additive (5–15 mol%); under magnetic stirring; solvent 5 mL; O
2
pressure
was found to be sluggish, resulting in very low con-
version under similar reaction conditions (Table 3,
entry 11).
b
3+
0 bar; stirring rate 1000 rpm. [b] Au–rGO =GO and Au
reduced by NaBH .
4
[
c,d,e] Au–pDA–rGO catalyst amount was 0.01, 0.015, and 0.03 g, respectively.
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1829
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[a]
Table 3. The oxidation of benzylic hydrocarbons catalyzed by Au–pDA–rGO nanocomposite.
Entry
Substrate
Product
t
Conversion
[%]
Selectivity
[%]
TOF
À1
[h]
[h
]
1
2
3
12
92.3
80.1
85.1
97.4
94.3
98.7
117
10
12
80
100
4
5
15
16
89.4
95.4
97.6
98.4
87
105
6
7
14
14
87.5
94.2
93.4
96.8
112
122
8
2
0
0
928
232
8
9
4
99.5
10
24
24
90.5
60.1
4.8
98.8
99.6
99.3
137
30
1
0
[
b]
11
17.8
[
6
a] Reaction conditions: Substrate (1.0 mmol); Au–pDA–rGO catalyst 0.02 g (2 wt% Au loading); NHPI (10 mol%); CH
3
CN (5 mL); reaction temperature
À1
À1
08C; O
2
pressure 10 bar; stirring rate 1000 rpm. Turnover frequency (TOF, h ) was calculated after 4 h of reaction. [b] TOF (h ) was calculated after 0.5 h
of reaction.
CO2 formation is a frequently observed phenomenon in
aerobic oxidation reactions either owing to over-oxidation of
the reaction mass caused by higher temperature or because of
porting Information). With the rest of the compounds also
(Table 3), we found 100% mass balances and no CO formation
2
was detected. Moreover, the stability of the pDA layer on the
GO surface was evaluated by performing a blank reaction with
[4,27]
the volatile nature of the oxidizible compounds.
Some of
the compounds, such as toluene and cyclohexane, owing to
their volatile nature, give rise to difficulties associated with
quantification. In such cases, determination of the conservation
the catalyst only. No evolution of CO was detected during the
2
reaction as 100% mass balance was found from the weight of
the reaction before and after the reaction, indicating high sta-
bility of the catalyst under the performed reaction conditions.
The Au–pDA–rGO nanocomposite can be easily recovered
by simple centrifugation followed by repeated washing and
drying and then can be reused for several cycles without sig-
nificant loss in activity or selectivity (Figure S10, in the Sup-
porting Information). Without supplementing any fresh cata-
lyst, the reused nanocatalyst afforded high conversion for the
oxidation reaction with insignificant loss in activity. TEM stud-
ies of the nanocatalysts recovered after the third cycle of the
catalytic reaction showed minor changes in the average parti-
[
27c,d,28]
of mass before and after the reaction is very important.
Therefore, we evaluated the mass balances for the oxidation
reaction of toluene and cyclohexane. The mass balances were
calculated from the weights of the reaction mixture before and
after the reaction based on their conversions and selectivity.
The mass balances were found to be 100% and no CO forma-
2
tion was observed in any of the reactions. To eliminate any
doubt regarding any desiccation or mineralization of cyclohex-
ane to CO during the aerobic reaction, we performed a gram-
2
scale oxidation reaction of cyclohexane (Figure S9, in the Sup-
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1830
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
cle diameter as well as slight agglomeration of the Au NPs.
Figure S11, in the Supporting Information). Wrinkling of the
(
rGO was also observed on which Au NPs were embedded. For
any heterogeneous catalysts, the primary emphasis is on to its
stability under the set of conditions for the reaction. Even
though we found no significant decrease in activity in the cata-
lytic reaction in terms of yield at the final stage, we performed
[29]
studies regarding the initial activity of the catalyst. A tempo-
ral profile comparing the extent of deactivation of Au–pDA–
rGO and Au–pDA over four consecutive catalytic cycles is
shown in Figure 3. The Au–pDA–rGO catalyst was shown to be
Figure 4. Formation of diphenylketone as a function of time under the stan-
dard conditions (red line) and after removal of the Au–pDA–rGO catalyst
after 4 h (blue line). Standard reaction conditions of diphenylmethane oxida-
tion (substrate/metal molar ratio 1000; stirring rate 1000 rpm, O
0 bar; 608C, NHPI 10 mol%) were employed in both cases.
2
pressure
1
product on the surface of the Au NPs, blocking the active sites,
cannot be discounted.
Mechanistic studies
The mechanistic insights into the activation of O by Au NPs
2
Figure 3. Percentage deactivation showing the initial activity of catalyst
upon reuse; Au–pDA–rGO (black) and Au–pDA (dashed) over three diphe-
nylmethane oxidation recycle tests. Each reaction was performed under the
standard conditions of diphenylmethane oxidation; substrate/metal molar
for oxidation reactions have been demonstrated by experimen-
[
30–32]
tal as well as density functional calculations.
O activation
2
takes place on the negatively charged Au NP surface through
electron transfer from the anionic Au center to the LUMO (p*)
2
ratio 1000; O pressure 10 bar; stirring rate 1000 rpm; reaction temperature
À
[33]
of O , generating a superoxide species (O C ).
We believe
6
08C; NHPI 10 mol%; reaction time 4 h.
2
2
that a similar mechanism takes place in the present catalytic
reaction, where electron transfer from polydopamine anchored
on rGO makes the Au NP surfaces more negatively charged,
promoting the formation of superoxide species (Figure S12, in
the Supporting Information).
catalytically stable with minimal deactivation over the four cat-
alytic cycles, indicating that the catalyst was highly stable
under the reaction conditions. Conversely, Au–pDA underwent
severe deactivation upon consecutive reaction cycles. Further,
the possibility of recovering NHPI by column chromatography
after several cycles of reaction make the catalytic system
highly effective.
Although the superoxide is capable of reacting directly with
the benzylic positions of the substrate, the conversion was
very low in the presence of O as the single oxidant, which is
2
possibly due to inertness of the CÀH bond. It is well known
that highly electrophilic radicals such as the phthalimide N-
To gain insight into the nature of the active species involved
in the catalytic reaction, we performed leaching experiments
to verify the heterogeneity of our catalytic system. No signifi-
cant metal leaching was observed when using the Au–pDA–
rGO nanocatalyst during the oxidation reaction of diphenylme-
thane under the optimized reaction conditions. The reaction
was stopped after 4 h and the catalyst was removed by centri-
fugation and then the reaction was continued with the super-
natant. We observed no further reaction in the absence of the
catalyst, which implies that no active metal species were leach-
[
19a]
oxyl radical (PINO) can easily abstract hydrogen atoms.
When NHPI was added along with O , the superoxide species
2
abstracts the hydrogen atom from NHPI to form the PINO radi-
cal. PINO then abstracts a hydrogen atom from the hydrocar-
bon, initiating the oxygenation process. NHPI alone cannot
afford CÀH oxidation as the bond dissociation energy of the
À1 [34]
OÀH bond in NHPI is quite high (~88 kcalmol ). Thus, the
initiation involves a two-step process in which oxygen activa-
tion first takes place on the surface of the Au NPs, resulting in
a superoxo–Au species, and then subsequent hydrogen ab-
straction from NHPI, resulting in the radical chain promoter
PINO. Once PINO is formed, the autoxidation can proceed fur-
ther by the chain propagating steps shown in Figure 5. To as-
certain the role of polydopamine in the catalytic process, we
performed the oxidation reaction under similar reaction condi-
tions but using another Au–rGO composite devoid of
[
4]
ed during the course of the catalytic reaction (Figure 4). To
further determine if any leaching of metal particles occurred
during the oxidation reaction, a fraction of the supernatant
liquid was analyzed by ICP-AES and the leached metal was
found to be 76 ppb (parts per billion), indicating that the
leaching was negligible. In terms of deactivation of the nano-
composite upon reuse, possible adsorption of the oxidized
b
polydopamine (Au–rGO ), which was formed by simultaneous
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1831
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Au–pDA could be due to the improved surface exposure of Au
NPs embedded on the two-dimensional exfoliated rGO sheets.
This ensures more accessibility of the reactant molecules to
the catalytically active site compared with the three-dimen-
sional surface coating of Au NPs by polymer layers. Further,
high p-electron density on the rGO surfaces might influence
the approach of the reactant molecules to the active Au NP
catalysts through hydrophobic interactions. Hence, a complex
scenario involving different effects might contribute to the en-
hanced activity and selectivity of Au–pDA–rGO nanocompo-
sites for CÀH oxidation reactions.
Performing the oxidation reaction in dimethylsulphoxide
(
DMSO), a well-known free radical scavenger, we found no oxi-
dation, proving the radical nature of the reaction pathway
Table 2, entry 15). Further evidence for free radicals was ob-
tained by performing the reaction in the presence of TEMPO
2,2,6,6-tetramethyl-1-piperidinyloxy) in which no product for-
Figure 5. Plausible mechanistic cycle for the oxidation of benzylic hydrocar-
bons by the Au–pDA–rGO/NHPI system.
(
(
[
39]
reduction of HAuCl and GO by using NaBH as the reducing
mation was observed; instead, the complex [TEMPO–hydro-
carbon] was detected by mass spectrometry (Figure S13, in the
Supporting Information). To further validate that the reaction
mechanism involves free radicals, an aliquot of the reaction in
the presence of Au–pDA–rGO was taken and the decrease in
the absorbance of 2,2-diphenyl-1-picrylhydrazyl (DPPH) was
measured. It was observed that the violet color of DPPH com-
pletely disappeared, suggesting the generation of free radicals
during the reaction, which formed an adduct with DPPH (Fig-
4
4
agent. We obtained substantially lower conversion when using
b
the Au–rGO composite (Table 2, entry 1). Again, the Au–pDA
nanocomposite without rGO afforded much lower conversion
(
Table 2, entry 20), demonstrating that all the constituents in
the nanocomposite, that is, Au NPs, pDA, and rGO, induce co-
operativity that enhances the catalytic activity.
To enable a proper understanding of the electronic structure
of the Au NPs in the composite, we performed X-ray photo-
b
[40]
electron spectroscopy (XPS) measurements of Au–rGO , Au–
ure S14, in the Supporting Information). In a negative control
pDA, and Au–pDA–rGO nanocomposites. Figure 6 shows the
experiment, no change in DPPH absorbance was observed
when a small amount of reaction mixture without Au–pDA–
rGO was added.
To confirm the involvement of free radicals in the mecha-
nism, we employed a spin trapping electron paramagnetic res-
onance technique using 5,5-dimethyl-1-pyrroline N-oxide
(
(
DMPO) as the spin trapping probe to detect the superoxide
À
O C ) radical formed over the Au surface during the reaction.
2
We found that in the presence of catalyst, the EPR signal
shows the characteristic fingerprint of spin adducts resulting
À
from the superoxide radical (O C ). In a controlled study with-
2
out any catalyst, the spin signal from the oxygen radical
[
41]
showed very low intensity (Figure 7).
b
Figure 6. Comparative Au4f core level XPS study of (a) Au–rGO , (b) Au–
pDA, and (c) Au–pDA–rGO.
XPS signature of the Au4f doublet (Au4f7/2 and Au4f5/2) for
b
the supported Au NP systems. For Au–rGO , Au peaks ap-
peared at 83.9 eV and 87.5 eV, which are shifted to lower BEs
0
compared with the corresponding peaks for metallic Au at
[
35–38]
8
4.0 eV and 87.7 eV.
In the case of the Au–pDA composite,
the Au4f7/2 and Au4f5/2 peaks were further shifted to lower
BEs (83.6 and 87.3 eV, respectively). Approximately similar bind-
ing energies for the Au4f doublets were obtained for Au–
pDA–rGO composite. The results suggest definite electron
transfer from the support to Au NP, making the NP surface
more negatively charged and favorable for oxygen activation.
The higher catalytic activity of the Au–pDA–rGO composite
compared with polymer-coated Au NPs such as Au–PVP or
Figure 7. DMPO trapped EPR spectra with or without Au–pDA–rGO. DMPO
À
binds with superoxide radicals (O
2
C ) formed on the Au NP surface.
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1832
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Synthesis of graphene oxide
Conclusions
Graphite powder was converted into graphite oxide by the modi-
fied Hummer’s procedure. In a typical synthesis, graphite powder
A ternary nanocomposite comprised of Au nanoparticles anch-
ored on a polydopamine-coated graphene oxide surface func-
tions as a catalyst for benzylic CÀH oxidation with high effi-
ciency and selectivity in the presence of N-hydroxyphthalimide
under mild reaction conditions. The monometallic Au nano-
composite along with NHPI leads to the desired oxidation
products under mild and neutral reaction conditions through
a free radical mechanism involving diacylnitroxyl radicals such
as PINO. Polydopamine and the graphene surface not only
played a crucial role in the generation and stabilization of the
active nanoparticle catalysts, but were also influential in acti-
vating the reaction by promoting the generation of superoxo
species on the nanoparticle surface. The high activity of the
nanocomposites can be attributed to the high access of reac-
tants to the catalytically active nanoparticle surface decorated
on the two-dimensional nanosheets. Furthermore, easy separa-
tion and excellent reusability without significant loss of activity
over several iterations could propel the Au–pDA–rGO compo-
site as an excellent heterogeneous catalyst for important or-
ganic transformations involving direct CÀH functionalization.
(
3.0 g) was added to 70 mL concentrated H SO in the presence of
2 4
NaNO (1.5 g). The mixture was stirred for 1 h at ambient tempera-
3
ture. The container was cooled in an ice-bath and KMnO (9.0 g)
4
was added slowly while stirring vigorously with a magnetic stirrer.
The mixture was allowed to warm up naturally to ambient temper-
ature. Double-distilled water (140 mL) was added slowly and care-
fully. This is a highly exothermic reaction and suitable precautions
should be taken. After the reaction temperature subsided to ambi-
ent conditions, another aliquot of distilled water (400 mL) was
added to the mixture. Subsequently, 30% H O (5.0 mL) was added
2
2
and the color of the suspension changed from light yellow to
brown, indicating oxidation of graphite to graphite oxide. The
product was separated by centrifugation, washed with warm water
and ethanol several times, and vacuum dried. The graphite oxide
was transferred into double-distilled water and sonicated for 3 h,
during which graphite oxide was exfoliated to graphene oxide
(
GO).
Synthesis of the Au–pDA–rGO nanocomposite
Graphene oxide (100 mg) was added to water (320 mL) and the
suspension was dispersed by sonication for 1 h. The color of the
mixture turns brown on sonication. Dopamine hydrochloride
(
100 mg) was added to the mixture, which was stirred for another
2
1
h. Then, the mixture was heated at vigorous reflux at 1008C for
2 h. The brown-colored solution turns black, signifying the forma-
Experimental Section
Materials
tion of reduced graphene oxide. The mixture was centrifuged and
washed with water several times. Finally, the precipitate was dis-
persed in water and dialyzed against water for 12 h to remove un-
reacted dopamine. Then, the purified solution was transferred to
a 500 mL round-bottom flask and mixed with HAuCl4 solution
Graphite powder, dopamine hydrochloride, N-hydroxyphthalimide,
and hydrogen tetrachloroaurate(III) hydrate were purchased from
Sigma–Aldrich. NaNO , H O , KMnO , and concentrated H SO were
from Merck India. All other chemicals were purchased from Sigma–
Aldrich or Alfa Aesar and used without further purification. We
used Millipore water (ultrapure level) throughout the experiments.
3
2
2
4
2
4
(
2
1 mL, 0.023m). The mixture was stirred at room temperature for
h. After the reaction, the resulting Au NPs-deposited on the poly-
dopamine-coated reduced graphene oxide surfaces were separated
from the suspension by centrifugation and washed with water
three times.
Instrumentation and characterization
b
Synthesis of Au–rGO composite (Au–rGO )
The powder XRD measurements were carried out by using
a Bruker D8 Advance X-ray diffractometer with CuKa source (wave-
length=0.154 nm). TEM images were obtained by using a JEOL-
JEM-2100 microscope operated at 200 kV. Atomic force microscopy
was carried out by using an AIST-NT instrument (model SMART
SPM 10000, Tapping mode), the samples were prepared by drop
casting a DMF suspension on mica. UV/Vis measurements were
performed by using a Varian Cary 100 Bio Spectrophotometer. FTIR
spectra were recorded with KBr pellets by using a Bruker Tensor 27
instrument. Raman spectra were recorded on an integrated Raman
system from Jobin Yvon Horiba LABRAM-HR with a 632.8 nm He/
Ne laser beam. XPS spectra were recorded by using an ESCA instru-
ment, VSW of UK make. EPR measurements were done by using
Au–rGO composites were prepared by simultaneous reduction of
HAuCl and GO by using NaBH . AuCl (0.077 g) was dissolved in
4
4
3
double-distilled water (100 mL) and GO (50 mg) was added. The
contents were sonicated for 15 min, and then Na CO solution
2
3
(
5 wt%, 20 mL) was added dropwise with stirring. NaBH (0.6 g)
4
3
+
was added to reduce both GO and Au simultaneously while stir-
ring the contents with a magnetic stirrer. The mixture was kept at
8
08C and stirred for 1 h. The product of Au-decorated rGO was
separated by centrifugation, washed with double-distilled water
and ethanol, and dried at 608C for 12 h. The Au loading in the
composite was found to be 2.2 wt% as measured by ICP-AES spec-
troscopy.
1
13
a JEOL spectrometer (Model: JES-FA200). H and C NMR spectra
were recorded with Bruker Advance (III) 400 MHz or 100 MHz spec-
Acknowledgements
1
trometers, respectively. Data for H NMR spectra are reported as
chemical shift (d ppm), multiplicity (s=singlet, d=doublet, t=trip-
let, m=multiplet), coupling constant (J Hz), integration, and as-
signment; data for C NMR spectra are reported as a chemical
shift. ICP-AES measurements were performed by using a Spectro
analytical simultaneous ICP spectrometer (model ARCOS)
We acknowledge the esteemed reviewers for their valuable in-
sights on improving this manuscript. This work was supported by
the Science and Engineering Research Board (SERB), DST, India
(SR/S1/PC-32/2010). B.M. thanks UGC for a fellowship. Instrumen-
1
3
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1833
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
tation support from SIC, IIT Indore, SAIF, NEHU, Shillong, SAIF, IIT
Bombay, UGC-DAE Consortium for Scientific Research, Indore,
and IISER Bhopal is acknowledged. The authors thank Bhagwati
Sharma, Sonam Mandani, Dr. Deepa Dey, and Dr. Rajendar
Nasani for their help and suggestions.
ira, C. W. Bielawski, Polym. Chem. 2012, 3, 757–766; d) A. Vijay Kumar,
K. R. Rao, Tetrahedron Lett. 2011, 52, 5188–5199.
11] P. V. Kamat, J. Phys. Chem. Lett. 2011, 2, 242–251.
12] J. Wang, F. J. Trindade, C. B. De Aquino, J. C. Pieretti, S. H. Domingues,
R. A. Ando, P. H. C. Camargo, Chem. Eur. J. 2015, 21, 9889–9894.
[
[
[13] a) B. Saha, N. Koshino, J. H. Espenson, J. Phys. Chem. A 2004, 108, 425–
31; b) X. Baucherel, L. Gonsalvi, I. W. C. E. Arends, S. Ellwood, R. A. Shel-
4
don, Adv. Synth. Catal. 2004, 346, 286–296; c) F. Minisci, C. Punta, F. Re-
cupero, F. Fontana, G. F. Pedulli, J. Org. Chem. 2002, 67, 2671–2676;
d) N. Koshino, Y. Cai, J. H. Espenson, J. Phys. Chem. A 2003, 107, 4262–
Keywords: graphene
nanoparticles · oxidation · supported catalysts
·
heterogeneous
catalysis
·
4267; e) R. Arnaud, A. Milet, C. Adamo, C. Einhorn, J. Einhorn, J. Chem.
Soc. Perkin Trans. 2 2002, 1967–1972.
[
1] a) Activation and Functionalization of CÀH bonds, ACS Symposium Series
85, (Eds.: K. I. Goldberg, A. S. Goldman), American Chemical Society,
Washington, DC, 2004; b) Modern Oxidation Methods, (Ed.: J.-E. Bäck-
vall), Wiley-VCH, Weinheim, 2004; c) Z. Shi, C. Zhang, C. Tang, N. Jiao,
Chem. Soc. Rev. 2012, 41, 3381–3430; d) T. Newhouse, P. S. Baran,
Angew. Chem. Int. Ed. 2011, 50, 3362–3374; Angew. Chem. 2011, 123,
[14] a) L. Schmieder-van de Vondervoort, S. Bouttemy, F. Heu, K. Weissen-
bçck, P. L. Alsters, Eur. J. Org. Chem. 2003, 578–586; b) Y. Ishii, T. Iwaha-
ma, S. Sakaguchi, K. Nakayama, Y. Nishiyama, J. Org. Chem. 1996, 61,
4520–4526; c) T. Iwahama, K. Syojyo, S. Sakaguchi, Y. Ishii, Org. Process
Res. Dev. 1998, 2, 255–260; d) N. Sawatari, T. Yokota, S. Sakaguchi, Y.
Ishii, J. Org. Chem. 2001, 66, 7889–7891; e) Y. Ishii, S. Sakaguchi, T. Iwa-
hama, Adv. Synth. Catal. 2001, 343, 393–427; f) Y. Yoshino, Y. Hayashi, T.
Iwahama, S. Sakaguchi, Y. Ishii, J. Org. Chem. 1997, 62, 6810–6813.
[15] a) R. Amorati, M. Lucarini, V. Mugnaini, G. F. Pedulli, J. Org. Chem. 2003,
68, 1747–1754; b) I. W. C. E. Arends, M. Sasidharan, A. Kühnle, M. Duda,
C. Jost, R. A. Sheldon, Tetrahedron 2002, 58, 9055–9061.
8
3
422–3435; e) K. Godula, D. Sames, Science 2006, 312, 67–72;
f) H. M. L. Davies, J. R. Manning, Nature 2008, 451, 417–424; g) T. Pun-
niyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 2005, 105, 2329–2364;
h) J. Genovino, S. Lutz, D. Sames, B. B. Toure, J. Am. Chem. Soc. 2013,
1
35, 12346–12352; i) J. A. Labinger, J. E. Bercaw, Nature 2002, 417, 507–
5
14.
[16] Y. Aoki, S. Sakaguchi, Y. Ishii, Adv. Synth. Catal. 2004, 346, 199–202.
[17] a) C. Einhorn, J. Einhorn, C. Marcadal, J. L. Pierre, Chem. Commun. 1997,
447–448; b) S. Tsujimoto, S. Sakaguchi, Y. Ishii, Tetrahedron Lett. 2003,
44, 5601–5604.
[18] a) M. Eikawa, S. Sakaguchi, Y. Ishii, J. Org. Chem. 1999, 64, 4676–4679;
b) Y. Nishiwaki, S. Sakaguchi, Y. Ishii, J. Org. Chem. 2002, 67, 5663–5668.
[19] a) G. Yang, Y. Ma, J. Xu, J. Am. Chem. Soc. 2004, 126, 10542–10543; b) G.
Yang, Q. Zhang, H. Miao, X. Tong, J. Xu, Org. Lett. 2005, 7, 263–266.
[20] a) E. K. Jeon, E. Seo, E. Lee, W. Lee, M. K. Um, B. S. Kim, Chem. Commun.
2013, 49, 3392–3394; b) L. Guo, Q. Liu, G. Li, J. Shi, J. Liu, T. Wang, G.
Jiang, Nanoscale 2012, 4, 5864–5867.
[21] W. Zheng, H. Fan, L. Wang, Z. Jin, Langmuir 2015, 31, 11671–11677.
[22] a) D. Li, M. B. Müller, S. Gilje, R. B. Kaner, G. G. Wallace, Nat. Nanotechnol.
2008, 3, 101–105; b) J. Li, C. Y. Liu, Y. Liu, J. Mater. Chem. 2012, 22,
8426–8430.
[23] H. Yu, P. Xu, D. W. Lee, X. Li, J. Mater. Chem. A 2013, 1, 4444–4450.
[24] a) N. Mohanty, A. Nagaraja, J. Armesto, V. Berry, Small 2010, 6, 226–231;
b) H. Zhang, D. Hines, D. L. Akins, Dalton Trans. 2014, 43, 2670–2675.
[25] L. Q. Xu, W. J. Yang, K. Neoh, E. Kang, G. D. Fu, Macromolecules 2010, 43,
8336–8339.
[
2] a) T. W. Lyons, M. S. Sanford, Chem. Rev. 2010, 110, 1147–1169; b) S.
Conejero, M. Paneque, M. L. Poveda, L. L. Santos, E. Carmona, Acc.
Chem. Res. 2010, 43, 572–580; c) D. A. Colby, R. G. Bergman, J. A.
Ellman, Chem. Rev. 2010, 110, 624–655; d) S. Yang, Z. Li, X. Jian, C. He,
Angew. Chem. Int. Ed. 2009, 48, 3999–4001; Angew. Chem. 2009, 121,
4
1
059–4061; e) C.-L. Sun, B. J. Li, Z.-J. Shi, Chem. Rev. 2011, 111, 1293–
314; f) H. Peng, A. Lin, Y. Zhang, H. Jiang, J. Zhou, Y. Cheng, C. Zhu, H.
Hu, ACS Catal. 2012, 2, 163–167.
3] a) A. S. K. Hashmi, G. J. Hutchings, Angew. Chem. Int. Ed. 2006, 45, 7896–
[
7
936; Angew. Chem. 2006, 118, 8064–8105; b) A. Corma, H. Garcia,
Chem. Soc. Rev. 2008, 37, 2096–2126; c) Nanoparticles and Catalysis
Ed.: D. Astruc), Wiley-VCH, Weinheim, 2008; d) D. Astruc, F. Lu, J. R. Ara-
nzaes, Angew. Chem. Int. Ed. 2005, 44, 7852–7872; Angew. Chem. 2005,
17, 8062–8083; e) T. Ishida, M. Haruta, Angew. Chem. Int. Ed. 2007, 46,
154–7156; Angew. Chem. 2007, 119, 7288–7290; f) L. Prati, A. Villa,
Acc. Chem. Res. 2014, 47, 855–863; g) F. Porta, L. Prati, J. Catal. 2004,
(
1
7
224, 397–403; h) S. Biella, L. Prati, M. Rossi, J. Catal. 2002, 206, 242–
247; i) K. Layek, H. Maheswaran, M. L. Kantam, Catal. Sci. Technol. 2013,
3, 1147–1150.
[
[
[
4] L. Kesavan, R. Tiruvalam, M. H. Ab Rahim, M. I. bin Saiman, D. I. Enache,
R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, D. W.
Knight, C. J. Kiely, G. J. Hutchings, Science 2011, 331, 195–199.
[26] a) M. D. Hughes, Y. J. Xu, P. Jenkins, P. McMorn, P. Landon, D. I. Enache,
A. F. Carley, G. A. Attard, G. J. Hutchings, F. King, E. H. Stitt, P. Johnston,
K. Griffin, C. J. Kiely, Nature 2005, 437, 1132–1135; b) J. Long, H. Liu, S.
Wu, S. Liao, Y. Li, ACS Catal. 2013, 3, 647–654.
[27] a) S. Goergen, C. Yin, M. Yang, B. Lee, S. Lee, C. Wang, P. Wu, M. B.
Boucher, G. Kwon, S. Seifert, R. E. Winans, S. Vajda, M. F. Stephanopou-
los, ACS Catal. 2013, 3, 529–539; b) H. Hattori, Y. Ide, S. Ogo, K. Inu-
maru, M. Sadakane, T. Sano, ACS Catal. 2012, 2, 1910–1915; c) J. Sµ, M.
Ace, J. J. Delgado, A. Goguet, C. Hardacre, K. Morgan, ChemCatChem
2011, 3, 394–398; d) N. Dimitratos, J. A. Lopez-Sanchez, S. Meenakshi-
sundaram, J. M. Anthonykutty, G. Brett, A. F. Carley, S. H. Taylor, D. W.
Knight, G. J. Hutchings, Green Chem. 2009, 11, 1209–1216; e) C. D. Pina,
E. Falletta, M. Rossi, Chem. Soc. Rev. 2012, 41, 350–369.
5] a) B. G. Donoeva, D. S. Ovoshchnikov, V. B. Golovko, ACS Catal. 2013, 3,
2
986–2991; b) Y. Liu, H. Tsunoyama, T. Akita, S. Xie, T. Tsukuda, ACS
Catal. 2011, 1, 2–6.
6] M. I. Bin Saiman, G. L. Brett, R. Tiruvalam, M. M. Forde, K. Sharples, A.
Thetford, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, D. M. Murphy,
D. Bethell, D. J. Willock, S. H. Taylor, D. W. Knight, C. J. Kiely, G. J. Hutch-
ings, Angew. Chem. Int. Ed. 2012, 51, 5981–5985; Angew. Chem. 2012,
1
24, 6083–6087.
7] a) X. Chen, G. Wu, J. Chen, X. Chen, Z. Xie, X. Wang, J. Am. Chem. Soc.
011, 133, 3693–3695; b) R. Muszynski, B. Seger, P. V. Kamat, J. Phys.
[
2
Chem. C 2008, 112, 5263–5266; c) P. Kundu, C. Nethravathi, P. A. Desh-
pande, M. Rajamathi, G. Madras, N. Ravishankar, Chem. Mater. 2011, 23,
[28] B. H. Monjezi, M. E. Yazdani, M. Mokfi, M. Ghiaci, J. Mol. Catal. A 2014,
383- 384, 58–63.
2
772–2780; d) S. Liu, J. Tian, L. Wang, H. Li, Y. Zhang, X. Sun, Macromo-
[29] J. Wang, S. A. Kondrat, Y. Wang, G. L. Brett, C. Giles, J. K. Bartley, L. Lu, Q.
Liu, C. J. Kiely, G. J. Hutchings, ACS Catal. 2015, 5, 3575–3587.
[30] a) H. Tsunoyama, N. Ichikuni, H. Sakurai, T. Tsukuda, J. Am. Chem. Soc.
2009, 131, 7086–7093; b) T. Tsukuda, H. Tsunoyama, H. Sakurai, Chem.
Asian J. 2011, 6, 736–748.
[31] a) M. Boronat, A. Corma, Dalton Trans. 2010, 39, 8538–8546; b) M. Oku-
mura, Y. Kitagawa, T. Kawakami, M. Haruta, Chem. Phys. Lett. 2008, 459,
133–136; c) N. C. Hernµndez, J. F. Sanz, J. A. Rodriguez, J. Am. Chem.
Soc. 2006, 128, 15600–15601.
lecules 2010, 43, 10078–10083.
8] a) C. Chen, W. Cai, M. Long, B. Zhou, Y. Wu, D. Wu, Y. Feng, ACS Nano
2
S. Zhang, S. Wang, Carbon 2011, 49, 3538–3543; c) S. Chen, J. Zhu, X.
Wu, Q. Han, X. Wang, ACS Nano 2010, 4, 2822–2830.
9] T. Xue, S. Jiang, Y. Qu, Q. Su, R. Cheng, S. Dubin, C.-Y. Chiu, R. Kaner, Y.
Huang, X. Duan, Angew. Chem. Int. Ed. 2012, 51, 3822–3825; Angew.
Chem. 2012, 124, 3888–3891.
[
[
010, 4, 6425–6432; b) J. Zhang, X. Liu, L. Wang, T. Yang, X. Gou, S. Wu,
[
10] a) D. R. Dreyer, H. P. Jia, C. W. Bielawski, Angew. Chem. Int. Ed. 2010, 49,
813–6816; Angew. Chem. 2010, 122, 6965–6968; b) S. Verma, H. P.
[32] P. Ionita, B. C. Gilbert, V. Chechik, Angew. Chem. Int. Ed. 2005, 44, 3720–
3722; Angew. Chem. 2005, 117, 3786–3788.
6
Mungse, N. Kumar, S. Choudhary, S. L. Jain, B. Sain, O. P. Khatri, Chem.
Commun. 2011, 47, 12673–12675; c) D. R. Dreyer, K. A. Jarvis, P. J. Ferre-
[33] M. Kotobuki, R. Leppelt, D. A. Hansgen, D. Widmann, R. J. Behm, J.
Catal. 2009, 264, 67–76.
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1834
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[
[
34] F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti, C. Punta, R. Faletti,
R. Paganelli, G. F. Pedulli, Eur. J. Org. Chem. 2004, 109–119.
35] a) R. Leppelt, B. Schumacher, V. Plzak, M. Kinne, R. J. Behm, J. Catal.
[40] a) O. Friaa, D. Brault, Org. Biomol. Chem. 2006, 4, 2417–2423; b) R.
Amorati, S. Menichetti, C. Viglianisi, M. C. Foti, Chem. Commun. 2012,
48, 11904–11906; c) K. U. Ingold, D. A. Pratt, Chem. Rev. 2014, 114,
9022–9046.
2006, 244, 137–152; b) H. G. Boyen, G. Kästle, F. Weigl, B. Koslowski, C.
Dietrich, P. Ziemann, J. P. Spatz, S. Riethmüller, C. Hartmann, M. Mçller,
G. Schmid, M. G. Garnier, P. Oelhafen, Science 2002, 297, 1533–1536.
36] Z. Xiong, L. L. Zhang, J. Ma, X. S. Zhao, Chem. Commun. 2010, 46, 6099–
[41] a) M. Conte, H. Miyamura, S. Kobayashi, V. Chechik, J. Am. Chem. Soc.
2009, 131, 7189–7196; b) C. Su, M. Acik, K. Takai, J. Lu, S. J. Hao, Y.
Zheng, P. Wu, Q. Bao, T. Enoki, Y. J. Chabal, K. P. Loh, Nat. Commun.
[
2012, 3, 1298; c) A. Amorelli, J. C. Evans, C. C. Rowlands, J. Chem. Soc.
6
101.
Faraday Trans. 1 1988, 84, 1723–1728.
[
[
[
37] X. J. Wu, X. C. Zeng, Nano Lett. 2009, 9, 250–256.
38] Y. C. Yeo, T. J. King, C. M. Hu, J. Appl. Phys. 2002, 92, 7266–7271.
39] S. Ding, N. Jiao, Angew. Chem. Int. Ed. 2012, 51, 9226–9237; Angew.
Chem. 2012, 124, 9360–9371.
Received: January 29, 2016
Published online on April 25, 2016
ChemCatChem 2016, 8, 1825 – 1835
www.chemcatchem.org
1835
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim