Rhodium-Catalyzed Twofold Unsymmetrical C-H Alkenylation-Annulation/Thiolation Reaction to Access Thiobenzofurans

Article abstract of DOI:10.1021/acs.orglett.0c04134

A Rh(III)-catalyzed twofold unsymmetrical C-H alkenylation-annulation/thiolation reaction has been developed, enabling the straightforward and efficient synthesis of various thiobenzofurans in one step. This robust protocol proceeds with a broad substrate scope and good functional group tolerance under relatively mild reaction conditions.

Full text of DOI:10.1021/acs.orglett.0c04134

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Letter
Rhodium-Catalyzed Twofold Unsymmetrical C−H Alkenylation−
Annulation/Thiolation Reaction To Access Thiobenzofurans
Jian Lin, Liuyu Hu, Chao Chen, Huijin Feng, Yang Yu,* Yaxi Yang,* and Bing Zhou*
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ABSTRACT: A Rh(III)-catalyzed twofold unsymmetrical C−H
alkenylation−annulation/thiolation reaction has been developed,
enabling the straightforward and efficient synthesis of various
thiobenzofurans in one step. This robust protocol proceeds with a
broad substrate scope and good functional group tolerance under
relatively mild reaction conditions.
he transition-metal-catalyzed C−H functionalization has
Scheme 1. Benzofuran Synthesis Based on C−H Activation
T
recently emerged as an important strategy of organic
synthesis.1,2 Despite the growth and success of C−H
functionalization, these methods are largely limited to ortho-
mono-C−H functionalization or symmetrical ortho-di-C−H
functionalization.1,2 The more challenging unsymmetrical
twofold C−H functionalization is poorly investigated,3
presumably due to an incomplete reaction profile, a selectivity
issue in each step of the transformations, a sensitivity of
catalyst for special functionalization, and the potential
formation of numerous byproducts. Recently, several strategies
have been developed to achieve unsymmetrical functionaliza-
tion of twofold C−H bonds.4 However, most of these methods
require additional steps, two different catalytic systems, or
different reaction conditions. Therefore, the development of
catalytic twofold unsymmetrical C−H functionalization,
especially under a single reaction condition, is highly desirable.
Benzofuran is a privileged molecular scaffold which exists in
numerous natural products and bioactive compounds (Figure
1).5 The transition-metal-catalyzed C−H functionalization of
of catalytic and efficient methods to directly access
benzofurans with good regioselectivity is highly desirable.
On the other hand, aryl sulfides are widely found in
numerous natural products, bioactive compounds, and
materials.8 The transition-metal-catalyzed C−H thiolation
has recently emerged as an efficient and environmentally
friendly approach to construct the C−S bond.9 In this context,
we wondered whether the combination of an intramolecular
C−H activation/annulation of (2-propen-1-yloxy)benzenes
and an intermolecular C−H thiolation is possible, if a suitable
catalyst system and directing group were chosen, thus leading
to a twofold unsymmetrical C−H alkenylation−annulation/
thiolation reaction to construct the valuable thiobenzofurans in
only one step.
Inherently, the twofold unsymmetrical C−H functionaliza-
tion has the following challenges.
Figure 1. Representative benzofurans.
(1) The (2-propen-1-yloxy)benzenes are very liable to
undergo an intramolecular olefin hydroarylation via C−H
activation to produce dihydrobenzofurans.10 (2) The sulfur
phenols with alkynes provides an efficient method for the
preparation of benzofurans (Scheme 1a).6 Despite remarkable
progress, poor regioselectivity was observed when unsym-
metrical dialkyl or diaryl acetylenes were employed. A
transition-metal-catalyzed intramolecular annulation of ortho-
alkynyl phenols provided an alternative strategy for the
preparation of benzofurans with excellent regioselectivity
(Scheme 1b).7 However, these methods require time-
consuming and tedious multistep preparation of substrates or
prefunctionalization of substrates. Therefore, the development
Received: December 15, 2020
Published: February 1, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04134
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1194−1198
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a
Table 1. Optimization of the Reaction Conditions
b
b
entry
substrate
catalyst
additive 1
additive 2
Solvent
3aa yield (%)
3a yield (%)
1
2
3
4
5
6
7
8
9
10
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a-1
1a-2
[Cp*RhCl2]2
[Ru(p-cymene)Cl2]2
[Cp*IrCl2]2
Cp*Co(CO)I2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
−
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
−
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
−
−
−
−
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DCE
Toluene
t-AmOH
DMSO
DMF
DMF
DMF
72
0
0
0
23
0
<5
0
0
0
c
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
56
86
87
67
<5
15
10
44
0
d
0
26
<5
74
62
28
0
d
d
d
11
12
13
14
d
d
d
[Cp*RhCl2]2
[Cp*RhCl2]2
0
<5
0
0
d
15
a
Reaction conditions: substrates (0.1 mmol), 2a (0.1 mmol), catalyst (5 mol %), AgSbF6 (20 mol %), additive 1 (0.2 mmol), and additive 2 (0.2
b
c
d
mmol) in solvent (1 mL) at 80 °C for 12 h. Isolated yield. Ag2CO3 (0.1 mmol) was used. Cu(OAc)2 (10 mol %) was used.
a
atom is able to bind to metals, resulting in the deactivation of
the metal catalyst. (3) The monofunctionalized intermediates
usually hamper the installation of the second functional group
due to the steric, electronic, or coordinating effect. (4) Each
C−H functionalization will deliver several byproducts
separately, and each step lacks selectivity.
Scheme 2. Scope of Substrates 1
To commence our studies, we reacted 1a and diphenyl
disulfide (2a) in the presence of various catalysts, AgSbF6, and
Cu(OAc)2 at 80 °C in DMF (Table 1, entries 1−4). The
catalyst [Cp*RhCl2]2 was found to be crucial to provide the
benzofuran 3aa (entry 1), while only a small amount of
twofold C−H functionalization product 3a was observed. To
our delight, addition of another oxidant Ag2CO3 (100 mol %)
significantly improved the second C−H thiolation reaction,
affording the desired product 3a in 56% yield (entry 5). The
use of 2 equiv of Ag2CO3 led to a further improved yield (entry
6). Notably, lowering the loading of Cu(OAc)2 to 10 mol %
provided an equal yield (entry 7), although the removal of the
Cu(OAc)2 led to a decreased yield (entry 8). A screen of
solvents (entries 9−12) revealed that other solvents afforded
inferior results. The Rh(III) catalyst and the directing group
were crucial for this twofold unsymmetrical C−H alkenyla-
tion−annulation/thiolation reaction, with no reactivity in its
absence (entries 13−15).
a
Reaction conditions: 1 (0.1 mmol), 2a (0.1 mmol), [Cp*RhCl2]2 (5
mol %), Ag2CO3 (2.0 equiv), Cu(OAc)2 (10 mol %), AgSbF6 (20 mol
%), DMF (1 mL), 80 °C, 12 h. Isolated yield.
With the optimal conditions in hand, we started to
investigate the scope of substrates (Scheme 2). The substrates
1 with either electron-withdrawing or electron-donating groups
at different positions of the phenyl ring effectively underwent
this twofold unsymmetrical C−H alkenylation−annulation/
thiolation reaction, providing compounds 3b−3j in good
yields. To our delight, 2-naphthalenecarboxamide was also
compatible to the current reaction conditions to afford the
desired product 3k. Notably, the various substituted allyl
groups were well tolerated and provided the 2-substituted or
1,2-disubstituted benzofurans 3l−3o in good yield.
Next, the scope of disulfides was also investigated (Scheme
3). Both electron-donating (4b and 4c) and electron-
withdrawing (4d−4g) substituents were well tolerated, giving
the corresponding products 4b−4g in good yields. The
structure of 4f was established through X-ray crystallographic
determination (CCDC 2050064). Particularly remarkable is
the participation of heterocyclic (4i and 4j) or alkyl disulfides
(4h, 4k−n) in this reaction, both of which showed good
reactivity. In addition, diphenyl diselenide failed to produce
the desired product 4o.
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a
Scheme 3. Scope of Substrates 2
Scheme 5. Control Experiments
a
Reaction conditions: 1a (0.1 mmol), 2 (0.1 mmol), [Cp*RhCl2]2 (5
mol %), Ag2CO3 (2.0 equiv), Cu(OAc)2 (10 mol %), AgSbF6 (20 mol
%), DMF (1 mL), 80 °C, 12 h. Isolated yield.
In order to demonstrate the synthetic utility of the reaction
we developed, this twofold unsymmetrical C−H alkenylation−
annulation/thiolation reaction was performed on a 2.0 mmol
scale in the presence of 2.5 mol % of Rh(III) catalyst,
providing 3a in 73% yield (Scheme 4). Notably, 3a could
undergo an intramolecular annulation in the presence of TFA
to provide the tetracyclic product 5a in 56% yield.
Scheme 6. Plausible Mechanism
Scheme 4. Scaled-Up Study and Application of 3a
To investigate the plausible mechanism of this reaction,
several experiments were performed (Scheme 5). A H/D
exchange in 1d was obtained in the presence or absence of 2a,
suggesting that the ortho C−H activation step is reversible
(Scheme 5a). Compound 3aa could be readily converted into
3a under the standard reaction conditions, suggesting that the
C−H alkenylation/annulation occurs first, followed by a C−H
thiolation (Scheme 5b). Moreover, the yield of 3a decreased in
the absence of Cu(OAc)2 and the addition of NaOAc (20 mol
%) increased the yield of 3a, indicating the Cu(OAc)2 may
provide the acetate to form the active cationic Cp*Rh(OAc)2
catalyst (Scheme 5c). In addition, the unsymmetrical C−H
difunctionalization reaction was not affected in the presence of
radical scavengers TEMPO, suggesting that a radical pathway
may not be involved (Scheme 5d).
Based on these preliminary studies, we tentatively proposed
the following plausible reaction mechanism (Scheme 6).
Initially, [RhCp*Cl2]2 reacts with the additives AgSbF6 and
Cu(OAc)2 to form an active cationic Rh(III) species A,
facilitating the ortho C−H activation of 1a to give the
rhodacycle B. An intramolecular olefin migratory insertion of B
forms C which undergoes a β-H elimination and olefin
isomerization to afford intermediate 3aa and Rh(I) that can be
oxidized with Ag2CO3 to regenerate the Cp*Rh(III) catalyst.
Intermediate 3aa undergoes an ortho C−H activation to form
D. The oxidative addition of the diphenyldisulfide to the
Rh(III) and subsequent reductive elimination gives 3a with
Rh(III) species F (path a). Alternatively, intermediate D may
undergo a nucleophilic addition with diphenyldisulfide to give
3a and species F (path b). Afterward, species F can facilitate
another C−H activation of intermediate 3aa to give
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intermediate G, which undergoes a reductive elimination to
release 3a and the Rh(I) catalyst that can be reoxidized by
Ag2CO3 to regenerate the Cp*Rh(III) catalyst.
University of Chinese Academy of Sciences, Beijing 100049,
China
Liuyu Hu − School of Chinese Materia Medica, Nanjing
University of Chinese Medicine, Nanjing, Jiangsu 210023,
China; State Key Laboratory of Drug Research, Shanghai
Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai 201203, China; University of Chinese Academy of
Sciences, Beijing 100049, China
Chao Chen − State Key Laboratory of Drug Research,
Shanghai Institute of Materia Medica, Chinese Academy of
Sciences, Shanghai 201203, China; University of Chinese
Academy of Sciences, Beijing 100049, China; Department of
Medicinal Chemistry, School of Pharmacy, Fudan University,
Shanghai 201203, China
In conclusion, a rhodium-catalyzed intramolecular C−H
activation/annulation of (2-propen-1-yloxy)benzenes followed
by a second C−H thiolation reaction was developed, enabling
the efficient synthesis of valuable thiobenzofurans in one step.
This robust protocol proceeds with a broad substrate scope
and good functional group tolerance. Given the importance of
thiobenzofurans, we expect this twofold unsymmetrical C−H
alkenylation−annulation/thiolation reaction to gain broad
synthetic utility.
ASSOCIATED CONTENT
* Supporting Information
■
Huijin Feng − State Key Laboratory of Drug Research,
Shanghai Institute of Materia Medica, Chinese Academy of
Sciences, Shanghai 201203, China
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04134.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04134
General information, preparation of substrates 1, general
procedure for synthesis of compounds 3 and 4,
mechanism studiessynthesis of compound 5a, character-
ization of compounds 3 and 4, 1H and 13C NMR spectra
of compounds (PDF)
Notes
The authors declare no competing financial interest.
Accession Codes
ACKNOWLEDGMENTS
■
CCDC 2050064 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
This work is financially supported by National Natural Science
Foundation of China (No. 81973166, 91753207, 81773568,
21702218), the National Major Science and Technology
Project “Major New Drug Creation” (2018ZX09711002-006-
001, 2018ZX09711002-008-005, 2018ZX09711002-004-012),
Youth Innovation Promotion Association (2017333), Science
and Technology Commission of Shanghai Municipality
(18431907100), and K. C. Wong Education Foundation.
AUTHOR INFORMATION
Corresponding Authors
■
Yang Yu − Center for Supramolecular Chemistry & Catalysis
and Department of Chemistry, College of Science, Shanghai
University, Shanghai 200444, China; orcid.org/0000-
0001-5698-3534; Email: yangyu2017@shu.edu.cn
Yaxi Yang − State Key Laboratory of Drug Research, Shanghai
Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai 201203, China; University of Chinese Academy of
Sciences, Beijing 100049, China; School of Pharmaceutical
Science and Technology, Hangzhou Institute for Advanced
Study, University of Chinese Academy of Sciences, Hangzhou
310024, China; Email: yangyaxi@simm.ac.cn
Bing Zhou − State Key Laboratory of Drug Research,
Shanghai Institute of Materia Medica, Chinese Academy of
Sciences, Shanghai 201203, China; School of Chinese
Materia Medica, Nanjing University of Chinese Medicine,
Nanjing, Jiangsu 210023, China; School of Pharmaceutical
Science and Technology, Hangzhou Institute for Advanced
Study, University of Chinese Academy of Sciences, Hangzhou
310024, China; University of Chinese Academy of Sciences,
Beijing 100049, China; orcid.org/0000-0003-1813-
8035; Email: zhoubing@simm.ac.cn, zhoubing2012@
hotmail.com
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