Graphene-Oxide-Catalyzed Cross-Dehydrogenative Coupling of Oxindoles with Arenes and Thiophenols

Article abstract of DOI:10.1002/adsc.201901224

Here, we explore the GO-catalyzed cross-dehydrogenative coupling of oxindoles with arenes and thiophenols for the rapid synthesis of 3-aryloxindoles and 3-sulfenylated oxindoles. Control experiments and small-molecule mimicking studies reveal that the acidic nature and quinone-type functionalities of GO are synergistically utilized for the coupling reaction. The reaction proceeds under simple and mild reaction conditions, exhibits good functional group tolerance, and can be easily scaled up to the gram level. (Figure presented.).

Full text of DOI:10.1002/adsc.201901224

Title: Graphene-Oxide-Catalyzed Cross-Dehydrogenative Coupling of
Oxindoles with Arenes and Thiophenols
Authors: hongru wu, chuntian qiu, zhao-fei Zhang, bing zhang,
shaolong zhang, yangsen xu, Hongwei Zhou, Chenliang Su,
and Kian Ping Loh
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901224
Link to VoR: http://dx.doi.org/10.1002/adsc.201901224

10.1002/adsc.201901224
Advanced Synthesis & Catalysis
Very Important Publication - VIP
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DOI: 10.1002/adsc.201901224
Graphene-Oxide-Catalyzed Cross-Dehydrogenative Coupling of
Oxindoles with Arenes and Thiophenols
Hongru Wua, Chuntian Qiua, Zhaofei Zhanga, Bing Zhanga, Shaolong Zhanga,b,
Yangsen Xua, Hongwei Zhouc, Chenliang Sua*, Kian Ping Lohb
a
International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of
Education, Engineering Technology Research Center for 2D Materials Information Functional Devices and Systems of
Guangdong Province, Institute of Microscale Optoeletronics, Shenzhen University Shenzhen 518060, PR China
Department of Chemistry, Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
b
117543, Singapore
c
College of Biological, Chemical Science and Engineering, Jiaxing University, 118 Jiahang Road, Jiaxing 314001,
China
E-mail addresses: chmsuc@szu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Here, we explore the GO-catalyzed cross-
dehydrogenative coupling of oxindoles with arenes and
thiophenols for the rapid synthesis of 3-aryloxindoles and
3-sulfenylated oxindoles. Control experiments and small-
molecule mimicking studies reveal that the acidic nature
and quinone-type functionalities of GO are synergistically
utilized for the coupling reaction. The reaction proceeds
under simple and mild reaction conditions, exhibits good
functional group tolerance, and can be easily scaled up to
the gram level.
Keywords: Graphene Oxide; Cross-dehydrogenative
coupling; Metal-free Catalysis; 3-sulfenylated oxindoles ;
3-aryloxindoles
Graphene oxide (GO)-based materials have
emerged as promising catalyts for liquid-phase
organic reactions owing to their low-cost, production
scalability (thousand tons-scale production) and rich
chemical properties.[1] Endowed with rich oxygen
functionalities and aromatic carbon scaffolds GO
shows remarkable ability to catalyse a series of
synthetic transformations [2], such as stoichiometric
oxidations[3a-3e], solid acids-mediated reactions[3f-3g]
and oxidations[3h-3i], C-C couplings[3j-3l] etc. The latter
is currently a very active research area and the
reactions are usually catalysed by precious metals.
GO-based carbocatalyst are emerging as alternatives
to precious metal resources that are dwindling in
supply. Cross-dehydrogenative coupling (CDC)
reactions, a means of constructing carbon–carbon
bonds directly from two different C–H bonds with
Figure 1. GO-mediated CDC reactions.
extremely high atom-economy, are very attractive
targets of carbocatalysis, but rarely explored.[3i, 3l, 4] In
2016, Nishina and co-workers reported the first GO-
mediated CDC reaction between two electron-rich
arenes with GO and boron trifluoride diethyletherate
(BF3·OEt2), however GO was used as stoichiometric
reagent and the reaction scopes were mainly limited
to homocoupling (Figure 1a).[4a] Wu and co-workers
sequentially developed the GO-mediated thiolation of
indoles with thiols in water and presented a radical-
based mechanism (Figure 1b).[4b] Recently, we had
explored the carbocatalytic CH-CH type cross-
1
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10.1002/adsc.201901224
Advanced Synthesis & Catalysis
couplings of xanthenes with arenes. Both GO and
highly-reduced GO showed high reactivity for this
CDC reaction and furnished the coupling products in
good yields. Mechanistic studies suggest that
quinone-type functionalities as well as the zigzag
edges in GO materials are the catalytic active sites
(Figure 1c). [3i]
reaction with 0.2 mmol of 1a, 50 wt% of GO in 2 mL
anisole at 120 °C for 3 h.
Table 1. The dehydrogenative arylation of 1a with 2aa)
Entry Catalyst
Catalyst amount
(wt%)
Yield
(%)b)
1
2
3
4
5
6
7
8
-
-
0
91
0
0
0
0
78
62
GO
b-GO
r-GO
50
50
50
50
50
40
30
Figure 2. Representative bioactive compounds and
pharmaceutical molecules with 3-arylation 2-oxindole
skeleton: A (an anticancer agent), B (a neuroprotective
agent), C (a potent growth hormone secretagogue).
Graphite
Active carbon
GO
GO
a) The reaction of 1a (0.2 mmol) in the presence of the
3-Aryloxindoles and their derivatives are ubiquitous in
natural products as well as pharmaceutical molecules
(Figure 2).[5] The existing strategies generally rely on
utilizing pre-functionalized substrates, transition
metal catalysts or operationally tedious procedures.[6]
Herein, we devise a sustainable and metal-free
strategy for the construction of 3-aryloxindoles via
the most direct and efficient CDC coupling of
oxindoles with arenes using low-cost and commercial
available GO as the carbocatalyst. Control
experiments and small-molecule mimicking studies
reveal that that acidic groups and quinone-type
functionalities in GO are synergistically utilized for
activating the oxindoles, which allows the sequential
Friedel-Crafts-type reaction with electron-rich arenes.
To broaden the classes of CDC reactions catalyzed by
GO, we have also examined the carbocatalytic cross-
coupling of oxindoles with thiophenols to directly
construct 3-sulfenylated oxindoles for the first time,[7]
with good to excellent yields via radical-induced
coupling reactions.
First, N-methyl-3-phenyl-2-oxindole (1a) and
anisole (2a) were reacted in the presence of various
graphene-related materials, as shown in Table 1. No
product was observed when the reaction was
conducted at 120 °C in the absence of GO (entry 1).
The coupling product (3a) was formed in 91% yield
when using 50 wt% of GO (entry 2). However, b-GO
(base-treated GO, entry 3), r-GO (highly-reduced GO,
entry 4) and other carbonaceous materials, such as
graphite (entry 5) and active carbon (entry 6) that
possess much fewer oxygen functionalities, showed
no catalytic activity. These results suggest that
oxygen functional groups on the GO surface play
vital roles in this cross-coupling system. Further
studies revealed that the amount of GO had great
impact on the yield of 3a (entry 7-8). The influence
of metal impurities in GO was excluded (see Figure
S1 in the Supporting Information). Finally, the
optimal conditions were identified by performing the
catalyst in anisole (2 mL) at 120 °C for 3h under an air
1
atmosphere. b) Yield was determined by HNMR with
1,3,5-trimethoxybenzene as an internal standard substance.
Under the optimized conditions, the reaction scope
was explored with various substituted 2-oxindoles
(Scheme 1). Unsubstituted 2-oxindolegaves the
corresponding products 3b in 76% yield. Using N-
methyl-2-oxindole (R2 = H) (1c) as the substrate,
multi-steps cross-coupling of 1c with anisole gave the
di-substituted product 3c in 67% yield. When R2 was
4-CH3Ph, the reaction furnished the corresponding
product in a slightly lower yield (3d, 66%). A diverse
set of substituted N-methyl-3-benzyl-2-oxindoles
were all smoothly coupled with anisole, affording the
corresponding products 3e-3h in good to excellent
yields (73~92%). Next, the region-selective cross-
coupling reactions were examined with a variety of
electron-rich arenes 2. It is noteworthy that the
reactions showed extremely high region-selectivity to
produce one regioisomer product in good yields (3j–
3l). Hydroxy groups, which hardly survived in
traditional approaches, were well-tolerated to give the
coupling products 3m-n in moderate to good yields.
This protocol was also applicable to heterocyclic
aromatic compounds, producing the products 3o–3p
in a range of 47-67% yields. When using toluene or
chlorobenzene as arene substrates, no coupling
products were observed, which was consistent with
8]
the nature of Friedel–Crafts-type reactions.[6n,
Interestingly, the carbocatalytic CH-CH coupling
strategy could be successfully extended to α-arylation
of deoxybenzoins (3q-3s) in acceptable yields,
making this carbocatalytic CDC strategy much more
appealing.
2
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10.1002/adsc.201901224
Advanced Synthesis & Catalysis
Scheme 2. General reaction conditions: oxindoles (0.2
mmol), thiophenols (0.6 mmol) and 100 wt % of GO in
PhCl (2 mL) at 100℃ for 12h under an air atmosphere, all
yields refer to isolated products based on oxindoles.
Carbocatalysis using GO materials could be easily
scaled up as they are low-cost and abundantly
available. Herein, arylation of 3-substituted oxindoles,
a chosen model reaction, was scaled up to the gram
level by using GO (0.59 g, 50 wt%), 1e (1.185 g, 5
mmol) and anisole as the substrate, successfully
furnished the desired product 3e in 72% yield.
Scheme 1. General reaction conditions: oxindoles (0.2
mmol) and 50 wt % of GO in anisole (2 mL) at 120 °C for
3h under an air atmosphere, b) oxindoles (0.2 mmol),
aromatic substrates (0.6 mmol) and 50 wt % of GO in 1,2-
dichlorobenzene (2 mL) at 100℃ for 5h. c)deoxybenzoins
(0.2 mmol), aromatic substrates (0.6 mmol) and 100 wt %
of GO in dichloroethane (2 mL) at 85℃ for 10 h. d
isolated yields.
3-Thio-substituted oxindoles and their derivatives
are important in bioactive molecules as well as
natural products.[9-13] Traditional methods for
synthesis these include the direct sulfenylation of
oxindoles with sulfenylating agents, such as N-
Scheme 3. Gram-scale reaction of arylation of 3-
substituted oxindoles.
thioimides[12] and disulfides[13]
, or nucleophilic
To probe the origin of GO’s catalytic activity in the
arylation of 3-substituted oxindoles, the surface-
functionalities of GO was fine tuned.[1h] First, r-GO,
where most of the oxygen functionalities were
removed by high-temperature annealing (800 °C),[3i]
showed no reactivity, indicating the importance of
oxygen-functionalities. We next utilized the small
molecule analogues to mimic the active-domain in
GO to repeat the catalytic behavior of GO in such
CDC coupling. TsOH.H2O (p-Toluenesulfonic acid
monohydrate) herein was used to mimic the acidic
nature of GO; tetracene, anthraquinone, 9,10-
phenanthrenequinone and DDQ (2,3-Dichloro-5,6-
dicyano benzoquinone) were used respectively to
mimic other active sites. As shown in Table 2,
tetracene, anthraquinone, 9,10-phenanthrenequinone
and DDQ (2,3-dichloro-5,6-dicyano benzoquinone),
gave 9%, 2%, 32% and 82% yields, respectively (see
scheme 4). As expected, either DDQ or TsOH.H2O
substitution of prefunctionalized oxindoles such as 3-
hydroxy-oxindoles by acid-catalyst.[11] The direct
cross-dehydrogenative-coupling of oxindoles and
thiophenols for the construction of 3-sulfenylated
oxindoles is highly desired but still unexplored.[7 ] To
this end, we turned our attention to the CDC reactions
of oxindoles with thiophenols using the GO
carbocatalyst. Under the established conditions (see
scheme 2), thiophenols bearing electron-withdrawing
substituents such as F, Cl, Br furnished the desired
products in 57-76% yields (5a-5e). Thiophenols with
electron-donating substituents gave the products in
41-59% yields (5f-5g). A series of substituted
oxindoles were also comparable with the synthesis
conditions, producing the coupling products in good
yields (5h-5j).
3
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10.1002/adsc.201901224
Advanced Synthesis & Catalysis
alone showed no reactivity (Table 2, entries 5-6).
These experiments indicate that the acidic groups and
quinone-type functionalities in GO worked
synergistically in promoting the CDC couplings. Our
further study also confirm that hydroxyl and epoxide
oxygen-functionalities inevitably lost during the heating
process may not have contributed to the catalytic pathways,
but could be benefit to the reaction as an oxidant (See
Figure S3, Table S2).
coupling of intermediate B' with anisole (2a) provides 3-
aryloxindoles 3a via intermediate C'. Single-electron-
transfer (SET) oxidation of thiophenol promoted by GO
gives a sulfur radical 6, which undergoes a radical-induced
coupling with intermediate B' to afford the 3-sulfenylated
oxindoles 5 .[6n,8]
Scheme 4. Evaluation of small molecular analogue mimics.
Reaction conditions: oxindoles (0.2 mmol), TsOH·H2O
(0.2 mmol) and small molecular (0.5 mmol) in anisole (2
mL) at 120 °C for 3h under an air atmosphere. Yield was
1
determined by HNMR with 1,3,5-trimethoxybenzene as
an internal standard substance.
Figure 3. Plausible reaction mechanism.
In summary, we have developed GO-catalyzed
cross-dehydrogenative coupling of oxindoles with
arenes and thiophenols for the rapid construction of
3-aryloxindoles and 3-sulfenylated oxindoles via a
metal-free and sustainable process. The acidic nature
of GO as well as its quinone-type functionalities
work synergistically in promoting the CDC couplings.
This metal-free strategy has a broad substrate scope
with high atom economy and affords several
advantages, such as being metal-free, and good
scalability. This study suggests that GO materials are
expected to lead to the discovery of new organic
transformations.
Experimental Section
Scheme 5. Control experiment.
General procedure for the CDC coupling of
oxindole with anisole: A mixture of 3-substituted-2-
oxindole (0.2 mmol) and 50 wt % of GO in anisole (2 mL)
were stirred at 120 °C for 3h under an air atmosphere. The
mixture was cooled to room temperature, and the anisole
was removed under reduced pressure. The residue was
purified by flash column chromatography eluted with ethyl
acetate/petroleum ether to afford the desired product (3a-
3i).
To investigate the reaction pathway, a radical inhibition
experiment was conducted using TEMPO (2,2,6,6-
tetramethyl piperidinooxy) as blocker (Scheme 5).[14] As
expected, no desired product was detected, confirming that
the catalysis proceeds via a radical reaction pathway. The
radical intermediate could be further oxidized to the cation
species which was trapped by alcohol, which indicated the
General procedure for the CDC coupling of
oxindole with activated arenes: A mixture of oxindoles
(0.2 mmol), aromatic substrates (0.6 mmol) and 50 wt %
of GO in 1,2-dichlorobenzene (2 mL) were stirred at 100℃
for 5h. The mixture was cooled to room temperature, and
the residue was purified by flash column chromatography
eluted with ethyl acetate/petroleum ether to afford the
desired product (3j-3p).
General procedure for the CDC coupling of
deoxybenzoins with activated arenes: A mixture of
deoxybenzoins (0.2 mmol), aromatic substrates (0.6
mmol) and 100 wt % of GO in dichloroethane (2 mL) were
existence of cation species under the reaction condition.
[15]
(eq 2).
In addition, when using bis(4-chlorophenyl)
disulfide 6 instead of thiophenol, no desired product 5b
could be observed, indicating that 6 is not the intermediate
(eq 3).[4]
Based on these findings, a plausible mechanism is
proposed in Figure 3. First, the inherent surface acidity of
GO activates the oxindole substrate 1a so that it
tautomerizes to its enol form,[6n,16] which is easily oxidized
to the corresponding radical A' via a single-electron-
transfer (SET).[16] Then, radical A' is oxidized to the cation
species B' by GO. The sequential Friedel-Craft-type
4
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10.1002/adsc.201901224
Advanced Synthesis & Catalysis
stirred at 85℃ for 10h.The mixture was cooled to room
temperature, and the residue was purified by flash column
chromatography eluted with ethyl acetate/ petroleum ether
to afford the desired product (3q-3s).
General procedure for the CDC coupling of
oxindole with thiophenols: A mixture of oxindoles (0.2
mmol), thiophenols (0.6 mmol) and 50 wt % of GO in
PhCl (2 mL) at 100℃for 12h under an air atmosphere. The
mixture was cooled to room temperature, and the residue
was purified by flash column chromatography eluted with
ethyl acetate/petroleum ether to afford the desired product
(5a-5j).
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Acknowledgements
C. L. Su thanks the supports from Guangdong Special
Support Program, Pengcheng Scholar program, Shenzhen
Peacock
Plan
(KQJSCX20170727100802505
and
KQTD2016053112042971) and Educational Commission of
Guangdong Province (2016KTSCX126 and
2016KCXTD006). K. P. Loh thanks National Research
Foundation, Prime’s Minister Office for support for NRF
Investigatior award “Graphene oxide a new class of
catalytic, ionic and molecular sieving materials, award
number NRF-NRF12015-01”. H. W. Zhou thanks the
supports
from
Natural
Science Foundation of Zhe
jiang (LY19B020004).
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10.1002/adsc.201901224
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