Directed Aromatic C-H Activation/Acetoxylation Catalyzed by Pd Nanoparticles Supported on Graphene Oxide

Article abstract of DOI:10.1021/acs.orglett.7b02967

The first solid-supported directed aromatic C-H activation/acetoxylation has been successfully developed by using palladium nanoparticles supported on graphene oxide (PdNPs/GO) as a catalyst. The practicability of this method is demonstrated by simple preparation of catalyst, high catalytic efficiency, wide functional group tolerance, and easy scale up of the reaction. A hot filtration test and Hg(0) poisoning test indicate the heterogeneous nature of the catalytic active species.

Full text of DOI:10.1021/acs.orglett.7b02967

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Directed Aromatic C−H Activation/Acetoxylation Catalyzed by Pd
Nanoparticles Supported on Graphene Oxide
Yi Zhang,‡ Yu Zhao,‡ Yu Luo, Liuqing Xiao, Yuxing Huang, Xingrong Li, Qitao Peng, Yizhen Liu,
Bo Yang, Caizhen Zhu,* Xuechang Zhou, and Junmin Zhang*
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, P. R. China
S
* Supporting Information
ABSTRACT: The first solid-supported directed aromatic C−
H activation/acetoxylation has been successfully developed by
using palladium nanoparticles supported on graphene oxide
(PdNPs/GO) as a catalyst. The practicability of this method is
demonstrated by simple preparation of catalyst, high catalytic
efficiency, wide functional group tolerance, and easy scale up of
the reaction. A hot filtration test and Hg(0) poisoning test
indicate the heterogeneous nature of the catalytic active species.
Scheme 1. Solid-Supported Directed C−H Activation for
Construction of C−O Bonds
atalytic C−H activation has become a powerful tool for
C
modern organic synthesis.1 The directed functionalization
of a specific C−H bond is the most atom-economical strategy for
the construction of C−C and C−heteroatom bonds.1 One of the
most important methods to achieve selectivity in the C−H
activation is to use palladium catalysis combined with an
intramolecular directing group, such as a pyridinyl or other basic
functional group.2 Considering the expensive price and high
toxicity of palladium catalyst, solid-supported palladium-
catalyzed directed C−H activation has appeared as a valuable
alternative to the homogeneous one. There are fewer studies on
solid-supported palladium-catalyzed C−H activations than there
are of homogeneous reactions.3 Most of the protocols focus on
the activation of heterocycles with an “acidic” C−H bond or
intramolecular functionalization.4 Solid-supported directed
aromatic C−H activation is more challenging. Until now, only
limited arylations,5 cyanation,6 alkylation/alkenylation,7 alkyny-
lation,8 halogenation,9 borylation,10 and cyclization11 reactions
have been reported by several groups. Good to excellent
efficiency yield frequently required a fairly tedious preparation
procedure that included catalyst, large loading of catalyst, and
low functional group tolerance.
The directed C−H activation/oxidation reaction to construct
the C−O bond using solid-supported catalysts is rare. Recently,
the Cohen group developed solid-supported C−H alkoxylation
using a metal−organic framework supported palladium catalyst
(Pd/MOF) (Scheme 1, a).9a The Ellis group developed C−H
alkoxylation and halogenation with a multiwalled carbon
nanotube supported palladium nanoparticle catalyst (Pd/
MWCNT) (Scheme 1, b).9b From the Ellis group’s work, only
one single example of Csp3−H acetoxylation12 was achieved,
which came from benzylic C−H activation with 8-methylquino-
line as substrate (Scheme 1, b-2).9b However, the effort to
achieve aromatic C−H acetoxylation by the same Pd/MWCNT
catalyst failed with no formation of the desired product (Scheme
1, b-3).9b To the best of our knowledge, there is no report on
successful solid-supported directed aromatic C−H acetoxyla-
tion.13
Graphene oxide (GO) coupled with high chemical, mechan-
ical stability, as well as excellent dispersibility is a highly desirable
two-dimensional support for metal nanoparticles (MNPs).
Received: September 21, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02967
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Various MNPs supported on GO were used as the catalysts in
diverse organic transformations.14 However, no C−H activation
was realized by GO-based solid-supported catalyst. Herein, we
report the first solid-supported directed aromatic C−H
acetoxylation with moderate to excellent yields by using
palladium nanoparticles supported on graphene oxide
(PdNPs/GO) as the catalyst (Scheme 1, c).
The preparation of PdNPs/GO is a process of cation exchange
and absorption with GO support.15 PdNPs/GO maintained the
original layer structure as shown in Figure 1a, which is favorable
Table 1. C−H Acetoxylation of 2-(o-Tolyl)pyridine
b
entry
deviation from standard conditions
GC yield (%)
c
d
1
none
99 (98 , TOF : 24.8 h−1
)
2
toluene instead of DMF
AcOH instead of DMF
DMSO instead of DMF
DCM instead of DMF
DCE instead of DMF
dioxane instead of DMF
1.5 equiv of PhI(OAc)2
80 °C
21
73
25
4
3
4
5
6
84
71
65
72
trace
7
8
9
10
11
60 °C
0.8 mol % of Pd(OAc)2
76 (TOF: 19.0 h−1
)
a
Standard conditions: 1a (0.25 mmol), PdNPs/GO (0.8 mol %),
PhI(OAc)2 (2 equiv), DMF (1.1 mL), argon atmosphere, 100 °C, 5 h.
b
c
Determined by GC using biphenyl as an internal standard. Isolated
d
yield. TOF: turnover frequency.
The TOF value of PdNPs/GO (24.8 h−1) was higher than that of
Pd(OAc)2, as 0.8 mol % Pd(OAc)2 instead of PdNPs/GO gave
only 76% yield (TOF: 19.0 h−1) (Table 1, entry 11), which
indicated that GO is an ideal support for Pd nanoparticle catalyst.
With the optimized conditions in hand, we next examined the
substrate scope. A wide range of different functional groups were
tolerated under the oxidative conditions (Scheme 2). Organic
halides such as aryl bromides (3e, 3i) and aryl chloride (3h) were
tolerated in this catalytic system, so that they can be used as a
Figure 1. Field emission scanning electron microscopy image of
PdNPs/GO (a). TEM images (b, c) and HRTEM image (d) of the
catalysts in various scales.
for the dispersion in the reaction system. To further investigate
the morphology of Pd nanoparticles, the dispersion of Pd
nanoparticle was observed via transmission scanning electron
microscopy (TEM) (Figure 1b,c). Palladium nanoparticles are
well embedded in GO support without aggregation. The
palladium nanoparticles had a spherical shape with an average
diameter of 4 nm, which was confirmed by microtome cuts of the
embedded samples. Figure 1d showed the high-resolution
transmission electron microscope (HRTEM) image of palladium
particles, which exhibited an atomic lattice fringe spacing of 0.226
nm corresponding well to the (111) plane of the palladium. The
Pd content was found to be 3.9%, which was determined by an
inductively coupled plasma optical emission spectrometer (ICP-
OES). X-ray photoelectron spectroscopy (XPS) showed that
palladium nanoparticles existed as Pd(II) and Pd(0) on the GO
support.
a
Scheme 2. Substrate Scope
Our study began with 2-(o-tolyl)pyridine as a model substrate
by using PdNPs/GO (0.8 mol %) as catalyst, and a variety of
solvents, amounts of oxidant, and reaction temperatures were
investigated. Among several solvents screened, DMF was found
to be superior to other solvents with 98% isolated yield (Table 1,
entries 1−7). It might be that polar DMF was a more suitable
solvent for dispersion of the catalyst.16 PdNPs/GO could be
distributed uniformly in DMF with stirring for only 1 min at
room temperature, but the catalyst could not be scattered in
toluene even for 1 h at 100 °C. When the quantity of
iodosobenzene diacetate was increased from 1.1 to 2 equiv, a
nearly quantitative yield of acetoxylated product was achieved.
Further decreasing the reaction temperature to 80 or 60 °C
resulted in lower conversion or even trace product (Table 1,
entries 9 and 10). In the absence of solid-supported Pd catalyst or
with only addition of graphene oxide support, no desired product
was observed. Interestingly, we discovered that the catalytic
efficiency of PdNPs/GO was superior to that of free Pd(OAc)2.
a
General reaction conditions: 1 (0.25 mmol), PdNPs/GO (0.8 mol
%), PhI(OAc)2 (2 equiv), DMF (1.1 mL), argon atmosphere, 100 °C,
5 h; isolated yields are given; the yields in parentheses were obtained
using free Pd(OAc)2 (0.8 mol %) instead of PdNPs/GO as the
b
catalyst. Iodobenzene dipivalate as oxidant.
B
DOI: 10.1021/acs.orglett.7b02967
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Organic Letters
Letter
handle for further transformation. The trifluoromethoxy group
(3f, 3g), alkyl group (3a, 3c, 3j), methoxyl group (3b, 3d), and
mesyl group (3l) were compatible. Substrates with an electron-
donating group were particularly effective; however, decreased
yields were obtained when electron-withdrawing groups were
present on the aromatic ring. Interestingly for meta-substituted
arenes, the transformation proceeded with excellent regioselec-
tivity for functionalization of the less sterically hindered ortho-
C−H bond. The electron deficient para-substituted substrates
afforded monoacetoxylated products (3l), while the electron rich
one gave mixtures of mono- and diacetoxylated products even
when PhI(OAc)2 was increased to 5 equiv (3j). This could be
explained by the sterically hindrance from the support graphene
oxide. For bulky iodobenzene dipivalate, excellent yield was
achieved (3o). The scope of the directing groups was also
explored. Substituted pyridines (3m, 3n) showed good reactivity
for this functionalization. However, low yield (3p) was obtained
for benzo[h]quinolone because of the steric hindrance.
was supported by the XPS study result, which showed that after
completion of the first reaction, Pd(0) was converted to Pd(II)
under oxidative conditions (see the SI).
Besides the acetoxylation, we also tested halogenations using
PdNPs/GO as the catalyst with high yield for bromination
(81%), low yield for chlorination (15%), and no desired product
for iodation (Scheme 3, a). To test the feasibility of scaling up the
Scheme 3. Halogenation and Gram-Scale Acetoxylation
We also compared the catalytic activity of PdNPs/GO with
free Pd(OAc)2 under the same reaction conditions. The
experimental results showed that Pd(OAc)2 was not very
effective for electron-poor substrates, as very low yields (5−
18%) were obtained in most cases (Scheme 2, entries 3e−i, 3l,
and 3n). Gratifyingly, we found out that PdNPs/GO is superior
to Pd(OAc)2 for electron-deficient substrates (with up to 9-fold
yield compared to Pd(OAc)2) (Scheme 2, entry 3n). The yields
of electron-rich substrates are close for PdNPs/GO and
Pd(OAc)2, with PdNPs/GO still affording slightly higher yields.
The above comparison results have demonstrated GO not only is
an ideal support for Pd nanoparticle catalyst, but also can
efficiently improve the activity of Pd catalyst. The higher catalytic
activity could be attributed to the high dispersibility of PdNPs
and oxygen functional groups on GO nanosheets, as proposed by
Nishina et al.17
To obtain a better understanding of this transformation, we
used a hot filtration test.18 After removing the catalyst by hot
filtration after the first hour of the reaction, no further conversion
to product was observed, which indicated the removal of active
catalytic species during filtration and probably that the catalytic
species were heterogeneous. Hg(0) poisoning test results
corroborated this proposal (see the Supporting Information for
details). However, the three-phase test failed, so the PdNPs/GO
was named a solid-supported catalyst at the current stage. Then
the recyclability of PdNPs/GO was investigated. With use of 0.8
mol % catalyst, the reaction could be recycled two times without
a significant decrease in yields; however, a drastic decrease of
yield was observed in the third run (only 8% yield) (see the SI for
details). The unsatisfactory recyclability of the catalyst is similar
to other reports on solid-supported C−H activation reac-
tions.4f,h,7,11,19 We removed the catalyst by filtration over Celite
from the reaction mixture and measured the palladium content in
reaction solution by ICP-MS after the first circle. The palladium
content of the reaction solution was found to be 85 ppm; about
40% of palladium on catalyst leached into reaction solution, so it
should be the major reason for the loss of catalytic activity.
Moreover, the aggregation of palladium nanoparticles after the
reaction which was confirmed by TEM results (see SI) was
another reason for the loss. It should also be pointed out that the
yield for the second run is still high; this could be attributed to the
oxidation of Pd(0) in initial PdNPs/GO catalyst to Pd(II), so as
to guarantee the enough concentration of effective Pd(II)
species, which could start the efficient Pd(II)/Pd(IV) or Pd(II)/
Pd(III) catalytic cycle of C−H acetoxylation.20 The assumption
current reaction, a gram-scale C−H acetoxylation was conducted
(Scheme 3, b). 2-(o-Tolyl)pyridine was directly scaled up 23.6
times to give the corresponding product (1.2 g) with 89%
isolated yield.
In conclusion, we have prepared a solid-supported nano-
palladium catalyst with commercial graphene oxide as the
support in a convenient procedure. The physicochemical
properties of PdNPs/GO catalyst were characterized by SEM,
TEM, ICP-OES, and XPS. The first solid-supported directed
aromatic C−H acetoxylation has been developed successfully
with PdNPs/GO as the catalyst. PdNPs/GO showed excellent
catalytic efficiency (with much better catalytic activity than free
Pd(OAc)2) for acetoxylation and good catalytic activity for
bromination. The low catalyst loading, wide substrate scope, and
easy scale up of the reaction make this method more practical in
organic synthesis. Mechanistic studies (including hot filtration
test and Hg(0) poisoning test) suggest a heterogeneous
mechanism.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02967.
Experimental procedures, characterization of products,
and 1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: czzhu@szu.edu.cn.
*E-mail: zhangjm@szu.edu.cn.
ORCID
Yuxing Huang: 0000-0001-6012-8100
Yizhen Liu: 0000-0002-8334-6124
Xuechang Zhou: 0000-0002-1558-9423
Junmin Zhang: 0000-0003-4061-8350
Author Contributions
‡Y.Z. and Y.Z. contributed equally.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.7b02967
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Burkholder, M.; Gilliland, S. E.; Brinkley, K.; Gupton, B. F.; Ellis, K. C.
Chem. Commun. 2017, 53, 7022.
(6) Kishore, R.; Yadav, J.; Venu, B.; Venugopal, A.; Lakshmi Kantam,
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21402124, and 51673117), the Training Foundation for
Outstanding Young Teachers in Higher Education Institutions of
Guangdong (YQ2015145), the Shenzhen Science and Technol-
o g y F o u n d a t i o n ( J C Y J 2 0 1 5 0 3 2 4 1 4 1 7 1 1 7 0 2 ,
JCYJ20170302143658923, JCYJ20150529164656097, and
JCYJ20150625102750478), and the Natural Science Foundation
of SZU (827000038). We also appreciate the thoughtful
suggestions from the reviewers.
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