Catalyst-Free Electrosynthesis of Benzimidazolones through Intramolecular Oxidative C?N Coupling

Article abstract of DOI:10.1002/adsc.202000198

The electrochemical synthesis of N, N’-disubstituted benzimidazolones from ureas through an intramolecular anodic dehydrogenative N?H/C?H coupling has been developed. The reaction undergoes under the undivided electrolysis conditions and obviates the need for any catalysts and chemical oxidants. (Figure presented.).

Full text of DOI:10.1002/adsc.202000198

Title: Catalyst-Free Electrosynthesis of Benzimidazolones through
Intramolecular Oxidative C-N Coupling
Authors: Jiang-Sheng Li, Pan-Pan Yang, Xin-Yun Xie, Si Jiang, Li Tao,
Zhi-Wei Li, Cui-Hong Lu, and Wei-Dong Liu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000198
Link to VoR: http://dx.doi.org/10.1002/adsc.202000198

10.1002/adsc.202000198
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Catalyst-Free Electrosynthesis of Benzimidazolones through
Intramolecular Oxidative C-N Coupling
Jiang-Sheng Li,a* Pan-Pan Yang,a Xin-Yun Xie,a Si Jiang,a Li Tao,b Zhi-Wei Li,a Cui-
Hong Lua and Wei-Dong Liuc
a
Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, School of Chemistry
and Food Engineering, Changsha University of Science & Technology, Changsha, 410114, China
Fax: (+86)-73185258733; Phone: (+86)-73185258733; e-mail: jsli@csust.edu.cn
State Grid Hunan Electric Power Company Limited Research Institute, Changsha, 410004, China
National Engineering Research Center for Agrochemicals, Hunan Research Institute of Chemical Industry, Changsha
b
c
410007, China.
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: The electrochemical synthesis of N, N’-
disubstituted benzimidazolones from ureas through an
intramolecular anodic dehydrogenative N-H/C-H coupling
has been developed. The reaction undergoes under the
undivided electrolysis conditions and obviates the need for
any catalysts and chemical oxidants.
Electrochemical synthesis constitutes a convenient,
efficient, and sustainable avenue for the creation of
new chemical bonds.[6] In 2018, Zeng and his co-
authors reviewed the use of electrochemical
techniques in the establishment of heterocyclic
structures through intra- and intermolecular
annulation processes reported from 2000 to then.[6f]
This compilation substantiates that the organic
electro-oxidative synthesis in the anode of an
undivided cell has been developed into an appealing
tool for the synthesis of potentially bioactive
heterocyclic compounds.[7] It is noted that anodic
oxidation enables a direct functionalization of N-H
bonds through the formation of N-centered radicals
under constant current electrolysis,[8] and obviates the
requirement of difficult-to-prepare N-X bonds (X =
halo, aryloxyl) and the production of undesired
byproducts. To our knowledge, the utilization of
amides, amidines, carbamates, and ureas as the N-
centered radical precursors to realize the construction
of nitrogen-containing heterocycles, for example,
pyrazolones,[8b] pyrrolidinones,[8c] oxazolones,[8d]
imidazolones,[8d] fused azoles,[8e-h] indoles,[8i] and
lactams,[8j] has been demonstrated. Very recently,
Keywords: Electrosynthesis; Dehydrogenative coupling;
C-N formation; Benzimidazolones; Ureas
Benzimidazolones are widely found as privileged
structures in numerous natural alkaloids,[1] bioactive
molecules,[2] and dye pigments.[3] To date, a diverse
array of efficient approaches have been documented
for ready access to such scaffolds.[4] Among them, the
intramolecular oxidative aryl C-H amidation of ureas,
prefabricated or generated in situ by the addition of
aniline to isocyanates, is one of the most fascinating
protocols in terms of high atom-economy.[5] In 2015,
Fu’s group reported a metal-free oxidative C-H
amidation annulation of ureas mediated by PhI(OAc)2
along with Cs2CO3 to access benzimidazolone
derivatives.[5b] Later on, Youn and Kim disclosed a
one-pot sequential addition and oxidative cyclization
from anilines and isocyanates promoted by Pd(OAc)2
as a catalyst and Ag2CO3 as a terminal oxidant.[5c] In
2018, Chang and his collaborators utilized the
combination of iodine and Cs2CO3 to enable this
Xu’s
group
presented
an
electrochemical
dehydrogenative cascade cyclization for the efficient
synthesis of highly substituted benzimidazolones
from an acyclic precursor, urea-tethered 1,5-enyne.[9]
However, benzimidazolones have not yet been
achieved from simple aryl ureas by means of this
sustainable strategy so far.
In our continuous efforts towards the construction
of heterocycle skeletons,[10] we herein report an
electrochemical synthesis of benzimidazolones from
trisubstituted ureas through an anodic oxidative C-N
transformation
under
the
reflux
of
1,2-
dichloroethane.[5d] Nevertheless, these methods still
suffer from some drawbacks such as the requirement
of noble-metal catalysts, and the need for a large
excess of chemical oxidants (e.g. PhI(OAc)2, Ag2CO3,
I2) and bases (e.g. Cs2CO3). Thus, it remains desirable
to develop an eco-friendly and efficient strategy to
achieve the assembly of benzimidazolone scaffolds.
1
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10.1002/adsc.202000198
Advanced Synthesis & Catalysis
o
of NaOMe (1 equiv) as a base in dry MeOH at 60 C
for 8h, wherein the current was kept constant at 2 mA,
and Me4NBF4 (1 equiv) was used as the electrolyte.
To our delight, product 2a was achieved in 66% yield
under these conditions (Table 1, entry 2). Other
electrolytes were tested, which are composed of
different cations (Me4N+, Et4N+, and n-Bu4N+) and
-
-
counterions (BF4 , PF6 ). It was found that 2a was
obtained in slightly lower yields (Table 1, entry 2 &
3). Thus, the use of Me4NBF4 was of the best choice
for this electrochemical reaction. Decreasing the
amount of Me4NBF4 to 0.2 equivalent, or even to zero,
delivered the yield of 60%, 40%, respectively (Table
1, entries 4 & 5). Then, other solvent systems were
considered, such as MeOH and its mixtures, MeCN,
HFIP, and i-PrOH without further treatment.
Surprisingly, the use of undried MeOH afforded the
highest reaction efficiency (Table 1, entry 1 vs 6, 7).
Reducing the dose of base or changing the base kinds
led to a remarkable decline in the yield (Table 1,
entries 8 & 9). Besides, changing the current density,
the electrolysis temperature, or the reaction
atmosphere exerted awful impacts on the electrolysis
(Table 1, entries 10-12). The use of other electrode
materials instead gave negative results (Table 1, entry
13).
Scheme 1 Access to benzimidazolones via intramolecular
N-H/C-H coupling.
coupling, which is devoid of transition-metal reagents
and chemical oxidants (Scheme 1).
Initially, we chose the urea 1a as the precursor for
this investigation on electrochemical intramolecular
cyclization conditions, so as to obtain our desired
potentially bioactive benzimidazolone nucleus.
Electrolysis was conducted in a three-necked flask as
an undivided cell equipped with a Reticulated
Vitreous Carbon (RVC) plate as the anode and a Pt
plate as the cathode. Precursor 1a underwent an
electrochemical oxidative annulation in the presence
Having established the standard electrolysis
conditions, the cyclization precursor scope was
explored. Firstly, keeping the –Ar part of urea 1 as 4-
chlorophenyl, secondary amine fragments were taken
into account for the reaction compatibility, and the
results are depicted in Table 2. Overall, all the ureas
1a-o tested proceeded smoothly to provide their
corresponding products 2a-o in moderate to good
yields. The substituents R1 of the secondary amines
could be an alkyl (–Me, –Et, and –n-Pr) or aryl group
(–Ph), giving rise to the corresponding annulated
products in the range of 78%–87% yield (2a-2d). A
diverse array of R2 substituents on the aromatic ring
of precursors 1 were also compatible with this
electrolysis (2e-2m), which include methyl, methoxy,
phenyl, formyl, ester, halo, and trifluoromethyl.
Noteworthy, a formyl group was found to be inert
Table 1 Optimization of the electrolysis conditions a
without
undergoing
electro-oxidation
(2h).
Furthermore, the electron properties of the
substituents R2, either electron-rich or electron-poor,
imposed extremely limited impacts on the cyclization
efficacy (2f vs 2i). When urea 1n derived from N-
methyl 2-naphthylamine was used instead in the
electrolysis, product 2n was obtained in good yield
and high regioselectivity. In addition, the urea
containing an N-methyl-pyridyl-2-amine moiety was
identified to be an effective substrate, thus generating
its pyrido-fused imidazolone 2o in 77% yield.
Next, we turned our attention to investigating the
effect of the primary amine moiety in precursors 1 on
the electrolysis reaction (Table 3). Under our standard
electrolysis conditions, the primary aniline fragments
were first taken into account. The ureas 1 bearing an
electron-donating (–Me) or electron-withdrawing
group (–Cl, –CF3) underwent a smooth electrolysis,
thereby generating their corresponding products 2 in
aReaction conditions: undivided cell, RVC anode (100 PPI,
1.2 cm × 1 cm × 1 cm), Pt cathode, constant current = 2
mA, 1a (0.1 mmol), base (1.0 equiv), electrolyte (1.0
equiv), solvent (6 mL), 60 oC, under Ar for 8 h. b Yields are
for isolated products. cDry solvent. HFIP
=
hexafluoroisopropanol.
2
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10.1002/adsc.202000198
Advanced Synthesis & Catalysis
Table 2. Scope with respect to secondary amine fragments Table 3 Scope with respect to primary amine fragments of
of ureas.a
ureas.
Scheme 2 Radical-trapping experiment.
To shed light on this electrolysis mechanism, a
radical-trapping experiment was conducted under the
standard electrolysis conditions (Scheme 2). It turned
out that no product 2a was detected in the presence of
TEMPO (2 equiv), which probably indicates that a
radical process was involved in this electrochemical
oxidation.[11] Moreover, the fact that the addition of
NaOMe as a base could facilitate the electrolysis,
implied that a deprotonation step occurs to generate
an anion during the cyclization process, which can
then be readily oxidized through the single-electron
transfer on the anodic surface.
aStandard conditions: undivided cell, RVC anode, Pt
cathode, constant current = 2 mA, 1 (0.1 mmol), NaOMe
(1.0 equiv), Me4NBF4 (1.0 equiv), undried MeOH (6 mL),
60 oC, under Ar for 8 h. Isolated yields are given.
moderate to good yields. However, the introduction of
the -Cl group to the ortho-position caused the failure
of the electrolysis (2q), suggesting that the substituent
steric hindrance might govern this oxidation process.
The replacement of the Cl group in 1a with a strong
electron-pulling CF3 resulted in a slight decay of the
yield (2t, 78%).
Furthermore, we carried out the cyclic voltammetry
(CV) experiments to figure out the role of the base
added in the electrolysis. As depicted in Figure 1, no
obvious oxidation peaks were observed for substrate
1a (green) and NaOMe (red), respectively, in the
range of 0-0.9 V vs Ag/AgCl, whereas a mixture of
1a with NaOMe displayed a strong oxidation signal at
Ep = 0.72 V, which is ascribed to the anodic oxidation
of a nitrogen anion to its nitrogen radical. Evidently,
as is in accordance with the results in Table 1 (entry 1
vs 8), the addition of NaOMe promotes the
electrolysis by deprotonation of ureas.
This electrosynthetic method was then extended to
ureas with a primary aliphatic amine, a challenging
substrate for the previously reported oxidative
cyclizations.[5b-d] Unfortunately, negative results for
the substrate with a c-hexyl (1u) were observed under
our
standard
conditions,
and
re-screening
electrochemical conditions, including various bases
and Lewis acids, failed to improve the reaction
(please see ESI, Section 3). Nontheles, N-methoxy
urea 1v was found to be an effective precursor in the
transformation, thereby delivering N-methoxy-N-
methyl benzimidazolone (2v) in 68% yield.
Based on previous work[8] and the aforementioned
observations, a plausible electrolysis mechanism for
this dehydrogenative annulation of ureas is proposed
3
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10.1002/adsc.202000198
Advanced Synthesis & Catalysis
condenser, a RVC plate (100 PPI) anode (1.2 cm × 1 cm ×
1 cm) and a platinum plate (1 cm × 1 cm) cathode and then
flushed with argon. MeOH (6 mL) was added with a
syringe. The constant current (2 mA) electrolysis was
o
carried out at 60 C (oil bath temperature) until complete
consumption of 1. The reaction mixture was cooled to
room temperature, followed by addition of water (10 mL)
and ethyl acetate (10 mL). The phases were separated and
the aqueous phase was extracted with ethyl acetate (3 × 20
mL). The combined organic solution was dried over
anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. The residue was chromatographed
through silica gel eluting with ethyl acetate/petroleum ether
to give the desired product 2.
Acknowledgements
Figure 1. Cyclic voltammograms recorded in n-
Me4NBF4/MeOH (0.1 M): (a) blank, (b) 1a (0.006 M), (c)
NaOMe (0.015M), (d) 1a (0.006 M) + NaOMe (0.015 M).
We would thank the Financial support from National Key R&D
Program of China (No. 2017YFB0307200), Changsha Municipal
Science and Technology Project (kq1801053), Young Teachers
Development Project of CSUST (2019QJCZ039), International
Cooperative Extented Project for “Double First-Class”, CSUST
(2018IC21), Hunan Provincial Graduate Student Teaching
Reform and Research Project
(JG2018B081) and Hunan
Provincial Graduate Student Scientific Research Innovation
Project (CX20190698). We also appreciate Dr. Alejandra
Dominguez-Huerta at the group of Prof. Chao-Jun Li in McGill
University for polishing language.
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Typical Procedure for the Electrosynthesis of the
Cyclization Products.
A 10 mL three-necked round-bottom flask was charged
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4
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10.1002/adsc.202000198
Advanced Synthesis & Catalysis
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5
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Advanced Synthesis & Catalysis
COMMUNICATION
Catalyst-free Electrosynthesis of Benzimidazolones
through Intramolecular Oxidative C-N Coupling
Adv. Synth. Catal. 2020, Volume, Page – Page
Jiang-Sheng Li,a* Pan-Pan Yang,a Xin-Yun Xie,a Si
Jiang,a Li Tao,b Zhi-Wei Li,a Cui-Hong Lua and
Wei-Dong Liuc
6
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