Graphene-Oxide-Catalyzed Direct CH?CH-Type Cross-Coupling: The Intrinsic Catalytic Activities of Zigzag Edges

Article abstract of DOI:10.1002/anie.201802548

The development of graphene oxide (GO)-based materials for C?C cross-coupling represents a significant advance in carbocatalysis. Although GO has been used widely in various catalytic reactions, the scope of reactions reported is quite narrow, and the relationships between the type of functional groups present and the specific activity of the GO are not well understood. Herein, we explore CH?CH-type cross-coupling of xanthenes with arenes using GO as real carbocatalysts, and not as stoichiometric reactants. Mechanistic studies involving molecular analogues, as well as trapped intermediates, were carried out to probe the active sites, which were traced to quinone-type functionalities as well as the zigzag edges in GO materials. GO-catalyzed cross-dehydrogenative coupling is operationally simple, shows reusability over multiple cycles, can be conducted in air, and exhibits good functional group tolerance.

Full text of DOI:10.1002/anie.201802548

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Graphene Oxide Catalyzed Direct CH−CH Type Cross-Coupling:
The Intrinsic Catalytic Activities of Zig-zag Edges
Authors: Hongru Wu, Chenliang Su, Rika Tandiana, Cuibo Liu,
Chuntian Qiu, Yang Bao, Ji’en Wu, Yangsen Xu, Jiong Lu,
Dianyuan Fan, and Kian Ping Loh
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802548
Angew. Chem. 10.1002/ange.201802548
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http://dx.doi.org/10.1002/ange.201802548

10.1002/anie.201802548
Angewandte Chemie International Edition
Angewandte Chemie
Graphene Oxide Catalyzed Direct CH−CH Type Cross-Coupling: The Intrinsic Catalytic
Activities of Zig-zag Edges
--Manuscript Draft--
Manuscript Number:
Article Type:
201802548R2
Communication
Corresponding Author:
Kian Ping Loh, Prof.
National University of Singapore
Singapore, SINGAPORE
Corresponding Author E-Mail:
chmlohkp@nus.edu.sg
Order of Authors (with Contributor Roles): Hongru Wu, Ph. D.
Chenliang Su, Ph.D.
Rika Tandiana
Cuibo Liu, Ph. D.
Chuntian Qiu, Ph. D.
Yang Bao
Ji’en Wu, Ph. D.
Yangsen Xu, Ph. D.
Jiong Lu
Dianyuan Fan
Kian Ping Loh, Ph. D.
Abstract:
The development of graphene oxide (GO)-based materials for C-C cross couplings
represents a significant advance in carbocatalysis. Although GO has been widely used
in various catalytic reactions, the scopes of reactions reported are quite narrow, and
the relationships between the type of functional groups present and the specific activity
of the GO are not well understood. Herein, we explore CH-CH type cross-couplings of
xanthenes with arenes using GO as real carbocatalysts, and not as stoichiometric
reactants. Mechanistic studies involving molecular analogs as well as trapped
intermediates were carried out to probe the active sites, which were traced to quinone-
type functionalities as well as the zigzag edges in GO materials. GO-catalyzed cross
dehydrogenative couplings are operationally simple, show multiple cycle reusability,
and can be conducted under open-air conditions, and exhibit good functional group
tolerance.
Author Comments:
To
15 April 2018
To
Editor
Angewandte Chemie International Edition
Dear Dr Kim Meyer
We received your email requesting the revision of our manuscript entitled " Graphene
Oxide Catalyzed Direct CH−CH Type Cross-Coupling: The Intrinsic Catalytic Activities
of Zig-zag Edges ".
We are pleased to note that both referees endorsed our work positively and
recommended publication. We have revised our manuscript based on the comments of
the referees and hope that the revised manuscript meets the requirement for
publication in Angewandte Chemie International Edition.
Manuscript number: 201802548
MS Type: Communication
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10.1002/anie.201802548
Angewandte Chemie International Edition
Title: "Graphene Oxide Catalyzed Direct CH−CH Type Cross-Coupling: The Intrinsic
Catalytic Activities of Zig-zag Edges"
The main revised points are as follows:
1.All the points raised by reviewers have been successfully addressed.
2.We have supplied elemental analysis data to show that the role of Mn-impurity can
be excluded.
WE HAVE ATTACHED OUR POINT BY POINT RESPONSE WORD FILE UNDER
THE "CIF" section
Corresponding author
Professor Kian Ping Loh
Department of Chemistry
National University of Singapore
3 Science Drive 3
Singapore 117543
Email: chmlohkp@nus.edu.sg
Response to Reviewers:
Thanks we have made the necessay changes and include all the data file
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Manuscript
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10.1002/anie.201802548
Angewandte Chemie International Edition
COMMUNICATION
Graphene Oxide Catalyzed Direct CH−CH Type Cross-Coupling:
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The Intrinsic Catalytic Activities of Zig-zag Edges
Hongru Wua‖, Chenliang Sua‖*, Rika Tandianaa,b, Cuibo Liua,b, Chuntian Qiua,Yang Baob, Ji’en Wub,
Yangsen Xua, Jiong Lub, Dianyuan Fana, Kian Ping Lohb*
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Abstract: The development of graphene oxide (GO)-based arenes with styrenes or alcohols[6a] and arylation of benzene with
materials for C-C cross couplings represents a significant aryl iodides[6b]
. However, the development of metal-free
advance in carbocatalysis. Although GO has been widely used carbocatalysts for CH−CH-type cross-couplings has been rarely
in various catalytic reactions, the scopes of reactions reported reported, although much desired.[1c] Herein, we report that
are quite narrow, and the relationships between the type of GO/rGO can be used as heterogeneous, cost-efficient and
functional groups present and the specific activity of the GO are metal-free catalysts for the direct functionalization of the benzylic
not well understood. Herein, we explore CH-CH type cross- C-H bond of substituted xanthenes and thioxanthene with
couplings of xanthenes with arenes using GO as real arenes in good yields and selectivity under open air. Mechanistic
carbocatalysts, and not as stoichiometric reactants. Mechanistic studies suggested that the defective domains with zigzag edges
studies involving molecular analogs as well as trapped and quinone-type functionalities in GO materials are the most
intermediates were carried out to probe the active sites, which likely active sites for these reactions.
were traced to quinone-type functionalities as well as the zigzag
Our initial study was conducted using oxygen-rich graphene
edges in GO materials. GO-catalyzed cross dehydrogenative oxide (GO-18) as the carbocatalyst for the aerobic cross-
couplings are operationally simple, show multiple cycle coupling of xanthene and 1,2-dimethoxybenzene.[7] Performing
reusability, and can be conducted under open-air conditions, the reaction at 100 °C for 24 h under solvent-free and open-air
and exhibit good functional group tolerance.
conditions afforded the coupling product in 46% yield with
xanthenone as the main byproduct. Replacing air with other
oxidants including TBHP, H2O2 and TEMPO resulted in lower
yields (Table S1). To optimize the yield, a series of organic acid
co-catalyst was screened to suppress the formation of the
byproduct (Table S1 and Scheme S1). Other classes of
carbocatalysts were also examined (Table S1). The synergistic
catalytic effect of GO-18 with TsOH·H2O provided the best
results (85% NMR yield). The recovered GO-18 could be
recycled and maintained its high catalytic activity (68%) up to the
5th run (Table S1). Performing the reaction in a glove box with
ppm levels of O2 reduced the yield to 26%, which proves that O2
is the terminal oxidant and that GO could only serve as an
oxidant to a small degree. A control experiment conducted in the
absence of catalyst produced 14% yield, while using graphite (G)
as the catalyst afforded only 6% yield, indicating that the intrinsic
properties of GO play an important role in the catalysis.
Activated carbon (AC) also showed some catalytic activity (30%),
which probably stems from its surface oxygen functionalities
or/and defects. However, base-acid treated GO (ba-GO)[3a] and
reduced GO (r-GO) that have fewer oxygen functionalities than
fully oxidized GO also showed good catalytic activity (Fig. 1 and
Fig. S1), suggesting that not all oxygen functional groups have
vital roles in this catalytic system. Finally, the role of Mn-
impurities in GO materials has also been ruled out (Fig. S2).
To determine the relationship between the catalytic
reactivity of GO and the density and types of oxygenated
functionalities on the GO surface, the oxidation of GO samples
was controlled systematically by using a reduced amount of
oxidizing agent compared to the standard Hummer’s
procedure.[8] For example, 1 g, 3 g, 6 g, and 10 g of KMnO4 were
used in place of the 18 g of KMnO4 used in the normal
Hummer’s procedure, and the resulting products were named
GO-1, GO-3, GO-6, GO-10 and GO-18 (or GO), respectively
(Characterizations: Fig. 2 and Fig. S3). Fig. 2a shows the X-ray
photoelectron spectroscopy (XPS) C1s spectra of GOs treated
with different amounts of KMnO4. The XPS C1s spectra of GO-1,
Carbocatalysis has become increasingly attractive in synthetic
chemistry due to the prospect of replacing noble metal
catalysts.[1] The use of graphene oxide (GO) and its
functionalized derivatives as carbocatalysts[2-6] is of interest since
the production of GO has entered the first phase of commercial
production on the ton scale. The first landmark study on GO-
based carbocatalysis by Bielawski reported selective oxidation
and hydration reactions.[2] GO and its derivatives also exhibit
excellent activity in the aerobic oxidation of amines[3] and other
oxidation reactions[4]. In addition, GO has been used as a
bifunctional catalytic material when hybridized with a second
metal catalyst. One major advantage of using GO compared to
heterogeneous metal catalysts is that no additional support is
needed since GO not only serves as a highly dispersible
platform for anchoring metal nanoparticles but also provides
synergetic catalytic sites.[3b]
The activation of C-H bonds by carbocatalysts to form
carbon−carbon and carbon−heteroatom bonds has recently
emerged as a hot topic in carbocatalysis.[1c] We and others have
recently reported GO-catalyzed α-position C-H bond activation
of primary amines with various nucleophiles,[3b] alkylation of
SZU-NUS Collaborative Center and International Collaborative
Laboratory of 2D Materials for Optoelectronic Science &
Technology, Engineering Technology Research Center for 2D
Material Information Function Devices and Systems of Guangdong
Province, College of Optoelectronic Engineering, Shenzhen
University, Shenzhen 518060, China
56E-mail: chmsuc@szu.edu.cn
57[b]
Department of Chemistry, National University of Singapore, 3
Science Drive 3, Singapore 117543
58E-mail: chmlohkp@nus.edu.sg
‖These authors shared co-first authorship
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Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
This article is protected by copyright. All rights reserved.

10.1002/anie.201802548
Angewandte Chemie International Edition
COMMUNICATION
GO-3 and GO-6 indicate that mild oxidation produces mainly C-
O species (i.e., hydroxyl and epoxide moieties), which increase
in density with the amount of oxidizing agent used. XPS spectra
show that GO-1 has a significantly lower content of oxygenated
functional groups compared to GO-3 and GO-6. Increasing the
amount of oxidizing agent resulted in the formation of more C=O
species (288.8 eV) than C-O species (286.6 eV), indicating the
hydroxyl and epoxide moieties were being converted to
carbonyls and carboxylic groups. To determine the effect of the
type of oxygen-functionalities present on the activity of the
catalyst, GO-1 to GO-10 were used as catalysts under the same
conditions as were used with GO-18 (Fig. 1). The results show
that the catalytic activity of GO is probably correlated to the
density of C=O (Fig. 1 and Fig. 2d) species. A more detailed
mechanistic study will be discussed in a later chapter.
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Scheme 1│The reaction was carried out with xanthene or thioxanthene (0.5
mmol), arene (1 mmol), TsOH·H2O (20 mol%) and catalyst (20 mg) at 100 °C
under solvent-free conditions. a Reaction at 60 oC. b Reaction at 75 oC.
Figure 1│Carbocatalyzed cross-coupling of xanthene and arenes: The
reaction was carried out with xanthene (0.5 mmol), 1,2-dimethoxybenzene (1
mmol), TsOH·H2O (20 mol%) and catalyst (20 mg) stirred at 100 °C for 24 h;
NMR yield with ethylbenzene as the internal standard. G = graphite; r-GO =
reduced GO; ba-GO = base-acid treated GO; AC = activated carbon.
Scheme 2│The reaction was carried out with thioxanthene (0.5 mmol), arene
(1 mmol), TsOH·H2O (20 mol%) and catalyst (20 mg) at 100 °C under solvent-
free conditions. a Reaction time 40 h. b Catalyst (50 mg).
Using the optimized conditions, the effectiveness of GO as a
catalyst for coupling various arenes with xanthene was
assessed (Scheme 1). A series of electron-rich arenes, such as
meta- and para-dimethoxybenzene and anisole was reacted with
xanthene to afford the corresponding coupling products in 45-74%
isolated yields. Acidic 2,6-xylenol and o-cresol can be smoothly
coupled with xanthene to produce the corresponding products in
80% and 52% yield, representatively. The synthetic utility of our
protocol was further extended to the direct coupling of xanthene
Figure 2│a) X-ray photoelectron spectroscopy (XPS) C1s spectra of GO
materials. b) XPS O1s spectra of GO materials. c) The concentration of
oxygen functionalities was determined from the XPS C1s spectra. d) The
relationship between the density and type of C=O functionalities and the
catalytic reactivity.
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10.1002/anie.201802548
Angewandte Chemie International Edition
COMMUNICATION
with heteroaromatic rings. High yields and excellent
regioselectivity can be achieved in the direct CH-CH coupling of
thioxanthene with a variety of electron-rich arenes including
heteroaromatic compounds (Scheme 1).
tetracene and pentacene exhibit higher reactivity. Anthraquinone,
which incorporates both the zigzag edges and the C=O species,
afforded the best performance among all the tested small-
molecule analogs. This result echoes the previous observation
that the reactivities of GO-related materials are linked to C=O
species (Table 1, entries 10-11). These findings provide strong
evidence that both the quinone-type functionalities and the
zigzag edges in GO materials promote this coupling reaction.
Quinones are often suggested as the active sites/model
catalysts for oxidations and dehydrogenations,[9] whereas the
catalytic behavior of the zigzag edges is much less studied.[1] To
investigate the exclusive role of zigzag edges in GO, the oxygen
functionalities were removed by high-temperature annealing
(>800 °C). FTIR analysis (Fig. 3a) shows greatly reduced
contents of epoxide and hydroxyl species after this treatment,
and confirms the successful removal of the C=O functionalities
(~1650-1800 cm-1); this conclusion is also supported by XPS
analysis in Fig. 3b where the ratio of C : O is now increased to
30 : 1. Due to the thermally processed desorption of oxygen
species as CO or CO2, vacancies are created. The aggregation
of these vacancies creates nanopores, which are believed to be
terminated by zigzag edges. To confirm this, we analyzed the
atomic structures of the pores using scanning tunneling
microscopy (STM). Fig. 3c reveals the honeycomb lattice of the
thermally treated GO basal plane. Most importantly, the edges of
the pores contain straight edges that are aligned with the zigzag
axis in graphene, thus these are zigzag edges (marked by white
dashed lines). An atom-resolved STM image and the structural
model of the zigzag edge are provided in Fig. 3c. The fact that
these zigzag edges play important roles in the catalysis was
confirmed by testing the catalytic reactivity of the rGO in the
cross-coupling reactions of xanthene and arenes, in which the
coupled products were obtained in 83% and 80% yields using
rGO annealed at 800 °C and 1000 °C, respectively. High-
temperature annealing at 1000 °C had removed most of the
oxygen functionalities, but the catalytic activity is only marginally
reduced relative to untreated GO, which provides further proof
that the zigzag edges play a major role in the catalysis.
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Next, the scope of xanthenes with different substituents was
examined. The catalyst shows good activity for a wide range of
electron-donating groups, such as methoxy and methyl, and the
desired coupling products were prepared in 72 to 80% yields.
Hydroxyl- and chloro-substituted xanthene produced coupling
products in good yields. Benzo-fused xanthenes gave the
corresponding products 5f-5g in moderate to excellent yields
(Scheme 2).
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Table 1 – Evaluation of small-molecular analog mimicsa
Entry
Small Molecule
Mimicking sites
Yield (%)b
Alcohol
20
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Epoxide
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Carboxylic acid
Carboxylic acid
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Conjugation domain
Conjugation domain
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Arm-chair edge
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Zig-zag edge
Zig-zag edge
Quinones
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Quinones
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a
The reaction was carried out with xanthene (0.5 mmol), arene (1 mmol),
TsOH·H2O (20 mol%) and catalyst (20 mg) at 100 °C. b NMR yield with anisole
as the internal standard.
As discussed in the above section, the reactivity of GO is
correlated to the concentration of quinone-type species (C=O)
and has no obvious relationship with the content of epoxide and
hydroxyl moieties. The use of molecular analogs allows us to
mimic their catalytic domains, and the results are summarized in
Table 1. Molecular analogs such as benzyl alcohol and 2,3-
diphenyloxirane, which mimic the hydroxyl and epoxide groups,
showed low catalytic activity (Table 1, entries 1-2). Molecules
with carboxylic acid groups, both with or without large
conjugated domains, also showed little activity (Table 1, entries
3-4). Notably, polyaromatic hydrocarbons such as pyrene,
coronene and picene, which are characterized by aromatic
structures and arm-chair edges, gave <12% yield (Table 1,
entries 5-7), whereas their zigzag edged counterparts such as
Figure 3| a) Transmission infrared spectra of GO (red) and rGO-800 (blue). b)
A typical STM image of GO on HOPG and annealed at high temperature. c)
Atom-resolved STM image of the zigzag edge marked in and the structural
model of the graphene lattice is superimposed.
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10.1002/anie.201802548
Angewandte Chemie International Edition
COMMUNICATION
A possible mechanism is presented in Fig. S4, involving the
following key steps; 1. the activation of xanthene by rGO—O2 or
tetracene—O2 intermediate, and 2. the formation of a transition
state in which the arene nucleophile is prepositioned for C–C
bond formation by π-π stacking interactions with graphene,
which is consistent with literature[6a]. rGO with zigzag edges are
expected to promote the oxidation of xanthene to a peroxide
intermediate via the following steps. First, the absorption of O2
onto the defective edges of rGO is believed to form the
intermediate rGO—O2. The conjugated electrons of xanthene
favor its absorption onto graphene and formation of a complex
with rGO—O2 to produce the xanthene peroxide intermediate.
The synergistic effect of acids and rGO helps to promote the
coupling of xanthene peroxide intermediate with arenes. The
use of tetracene as a molecular analog for the zigzag edges of
graphene allows us to probe the reaction mechanism by
isolation of the intermediates. In this case, the reduced product
5,12-dihydrotetracene confirmed the ability of the zigzag edges
to extract hydrogen (deuterium) in competition with O2, in which
the extraction may occur via the same intermediate tetracene—
O2 species. The interaction between xanthene and rGO—O2 is
further supported by the small-molecule mimicking experiments
and the detection of superoxide radical (Fig. S5 and S6). One
major difference in the catalytic activity of the tetracene analog
compared to that of graphene is that once the former is reduced,
it loses it catalytic activity, whereas the much longer conjugated
aromatic network in graphene allows the catalyst to be recycled
multiple times (Fig. S7).
(83%) was only slightly lower than that of GO (85%), which
indicated that the intrinsic catalytic activities of the zigzag edges
are quite high. Our study suggests that under acidic conditions,
porous carbon materials with a high density of zigzag edge sites
can serve as true carbocatalytic models for C-C couplings.
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Acknowledgements
C. L. Su is grateful for the financial supported from the NNSFC
(51502174),
Shenzhen
Peacock
Plan
(Grant
No.
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KQJSCX20170727100802505, KQTD2016053112042971). K.
P. Loh thanks NRF Investigator Award (Award No. NRF-
NRF12015-01) from National Research Foundation, Singapore
for support. K. P. Loh also thanks Shenzhen peacock plan
KQTD2016053112042971. Prof. Fan thanks the Educational
Commission of Guangdong Province (2016KCXTD006 and
2016KSTCX126).
Keywords: Graphene Oxide• Carbocatalysis
Catalysis •C-C coupling• Zig-zag edges
• Metal-free
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D. R. Dreyer, C. W. Bielawski. Chem. Sci. 2011, 2, 1233-1240; f) D. S.
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Theoretical calculations suggest that due to the localized 
states in zigzag edges and its closeness to the Fermi level, the
zigzag edges are radical-like[10a], thus these are potentially active
catalyst sites. Using a combination of bias-dependent STM
studies and DFT calculations, Enoki suggested that oxygenated
zigzag edges, on account of the additional  conjugation from
C=O functionalities, change the spatially localized edge state in
the zigzag edges into an extended one.[10b] The improved
“metallicity” of these oxidized edge sites[10c] should further
improve the catalytic properties of the edge sites. Therefore the
co-existence of ketonic functionalities and zigzag edges can
have synergetic effects in catalysis, although our studies show
that substantial removal of the C=O functionalities do not
degrade the catalytic activity of the zigzag edges significantly
since the latter can participate directly in dehydrogenation
reactions due to its radical-like nature. As opposed to
oxygenated functionalities in GO which sometime act as
stoichiometric reactants and become consumed in the reactions,
the zigzag sites are relatively robust. Defective edges could be
generated by heating due to the decomposition of oxygenated
functionalities and generation of pores, thus zig-zag edged
catalysts can be reused in multiple catalytic cycles. Previous
catalysis studies on GO materials have largely focused on the
role of oxygenated groups, this work suggests that more
attention should be focused on the role of zigzag edge sites
instead since these qualify as true catalytic sites in
carbocatalysts.
[2] D. R. Dreyer, H. P. Jia, C. W. Bielawski. Angew. Chem. 2010, 122,
6965-6968; Angew. Chem. Int. Ed. 2010, 49, 6813-6816.
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L. Bao, T. Enoki, Y. J. Chabal, K. P. Loh. Nat. Commun. 2012, 3, 1298-
1307; b) C. L. Su, R. Tandiana, J. Balapanuru, W. Tang, K. Pareek, C.
T. Nai, T. Hayashi, K. P. Loh. J. Am. Chem. Soc. 2015, 137, 685-690.
[4] a) Y. J. Gao, G. Hu, J. Zhong, Z. J. Shi, Y. S. Zhu, D. S. Su, J. G.
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In summary, we have developed a carbocatalyzed CH-CH-
type cross-coupling reaction of xanthenes (or thioxanthenes)
and arenes that is operationally simple and has good functional
group tolerance. Mechanistic studies revealed that the catalytic
reactivity is promoted by C=O species as well as the zigzag
edges in GO. STM studies reveal that thermally processed GO
possess a high density of zigzag edges around defective sites,
and despite its lack of C=O functionalities, its catalytic activity
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10.1002/anie.201802548
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
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Hongru Wu, Chenliang Su*, Rika
Tandiana, Cuibo Liu, Chuntian Qiu,Yang
Bao, Ji’en Wu, Yangsen Xu, Jiong Lu,
Dianyuan Fan, Kian Ping Loh*
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Graphene Oxide Catalyzed Direct
CH−CH Type Cross-Coupling: The
Intrinsic Catalytic Activities of Zigzag
Edges
Carbocatalysis: GO materials catalyzed cross dehydrogenative couplings of
xanthene (or thioxanthene) and arenes are operationally simple, show multiple
cycle reusability, and can be conducted under open-air conditions, and exhibit good
functional group tolerance. Mechanistic studies suggest that more attention should
be focused on the role of zigzag edge sites instead of oxygenated groups since
these qualify as true catalytic sites in carbocatalysts.
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