Visible-Light Carbon Nitride-Catalyzed Aerobic Cyclization of Thiobenzanilides under Ambient Air Conditions

Article abstract of DOI:10.1021/acs.orglett.1c01571

A metal-free heterogeneous photocatalysis has been developed for the synthesis of benzothiazoles via intramolecular C-H functionalization/C-S bond formation of thiobenzanilides by inexpensive graphitic carbon nitride (g-C3N4) under visible-light irradiation. This reaction provides access to a broad range of 2-substituted benzothiazoles in high yields under an air atmosphere at room temperature without addition of a strong base or organic oxidizing reagents. In addition, the catalyst was found to be stable and reusable after five reaction cycles.

Full text of DOI:10.1021/acs.orglett.1c01571

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Letter
Visible-Light Carbon Nitride-Catalyzed Aerobic Cyclization of
Thiobenzanilides under Ambient Air Conditions
Jin Bai,§ Sijia Yan,§ Zhuxia Zhang, Zhen Guo,* and Cong-Ying Zhou*
Cite This: Org. Lett. 2021, 23, 4843−4848
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ABSTRACT: A metal-free heterogeneous photocatalysis has been developed for the synthesis of benzothiazoles via intramolecular
C−H functionalization/C−S bond formation of thiobenzanilides by inexpensive graphitic carbon nitride (g-C3N4) under visible-
light irradiation. This reaction provides access to a broad range of 2-substituted benzothiazoles in high yields under an air
atmosphere at room temperature without addition of a strong base or organic oxidizing reagents. In addition, the catalyst was found
to be stable and reusable after five reaction cycles.
Scheme 1. Properties of Carbon Nitride Materials
he visible-light-driven photocatalysis not only offers an
Tappealing approach to a green chemical process but also
provides a vast opportunity to develop new synthetic
methodologies for organic synthesis.1 The past decades have
witnessed the rapid development of photoredox catalysis,
especially in homogeneous catalysis with transition metal
complexes based on Ru and Ir,2 as well as with organic dyes
like acridinium salts,3 riboflavins,4 and chromones.5 Heteroge-
neous photocatalysts, which have been often used in water
splitting,6 organic pollutant degradation,7 and CO2 reduction,8
have found increasing applications in organic transformations
in recent years due to their low cost, recyclability, and excellent
thermal and chemical stability.9 Among heterogeneous photo-
catalysts, graphite carbon nitride materials (g-CNs) are
particularly prominent because of their unique properties as
described here10 (Scheme 1). (1) g-CNs are metal-free
organocatalysts, which can avoid the introduction of transition
metals into the reaction.10a (2) g-CNs can be easily
synthesized from commodity chemicals, and their properties
can be readily tuned by synthetic modifications.10b (3) g-CNs
have a suitable band gap (2.7 eV, corresponding to 460 nm)
and favorable positions of the valence and conduction band,
allowing for controlled oxidation and reduction of many
substrates.10a (4) g-CNs display exceptionally high chemical
stability in the presence of reactive nucleophiles, electrophiles,
and radicals.10b (5) g-CNs as heterogeneous photocatalysts are
two-dimensional materials with rich surface characteristics and
have catalytic properties comparable to those of homogeneous
photocatalysts.10a Because of these advantages, g-CNs have
received a great deal of attention recently as a synthetically
useful heterogeneous photocatalyst.11
Benzothiazole is a privileged structure frequently found in
pharmaceuticals and biologically active natural products.12
Received: May 8, 2021
Published: June 2, 2021
https://doi.org/10.1021/acs.orglett.1c01571
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4843−4848
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Letter
Generally, benzothiazoles are prepared via oxidative cyclization
of thiobenzanilides using various oxidants such as hypervalent
iodine, bromine, or metal salt.13 Though convenient, the
synthetic method often displays low functional group tolerance
or requires stoichiometric or excess amounts of toxic oxidants.
In recent years, visible-light-driven synthesis as a green
approach to this kind of compound has attracted an increasing
amount of attention (Scheme 2a).14 Li et al. reported a visible-
Table 1. Optimization of Reaction Conditions
a
entry
catalyst
solvent
atmosphere
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
g-C3N4
−
DMF
air
air
air
air
air
air
air
air
air
N2
air
air
air
air
39
18
trace
20
95
57
80
69
78
trace
no reaction
6
12
17
CH3CN
CHCl3
DCE
DMSO
THF
CH3OH
EtOH
H2O
DMSO
DMSO
DMSO
DMSO
DMSO
Scheme 2. Synthesis of 2-Substituted Benzothiazoles via
Visible-Light Photocatalysis
b
Ru(bpy)3Cl2 (5 mol %)
Ir(ppy)3 (5 mol %)
a
Reaction conditions: 1aa (0.1 mmol) and g-C3N4 (10 mg) in solvent
(3 mL) irradiated by a 10 W blue LED for 8 h at rt. Yields were
determined by HPLC. Without irradiation.
b
light-driven synthesis of benzothiazoles using Ru(bpy)3(PF6)2
as a photocatalyst, dioxygen as the terminal oxidant, and DBU
as a base.14a Lei et al. developed an external oxidant-free
oxidative coupling for the synthesis of benzothiazoles by the
Ru(bpy)3(PF6)2/Co dual catalytic system.14b Gustafson et al.
reported a similar synthesis of benzothiazoles using Ru-
(bpy)3Cl2·6H2O as a photocatalyst and Na2S2O8 as an
oxidant.14c By using TEMPO as an oxidant, Li and Lang et
al. achieved a visible-light-driven cyclization of thiobenzanilides
without addition of a photosensitizer.14d Schmidt et al.
employed riboflavin as a photocatalyst and potassium
peroxydisulfate as a sacrificial oxidizing agent to synthesize 2-
substituted benzothiazoles via cyclization of thiobenzanilid-
es.14e Despite these advances, there is still a great demand for
photoredox synthesis of benzothiazoles using a green oxidant
and a recyclable catalyst under mild conditions. Herein, we
report the first heterogeneous visible-light-driven aerobic
synthesis of benzothiazoles via intramolecular C−H function-
alization/C−S bond formation of thiobenzanilides under
ambient air conditions using carbon nitride as a photocatalyst.
At the outset, we chose N-phenylbenzothioamide (1aa) as
the substrate and g-C3N4 as the photocatalyst to optimize the
conditions. g-C3N4 can be readily prepared by thermal
polycondensation of urea (for details, see the Supporting
Information). Initially, the photoredox cyclization of 1aa was
performed under an air atmosphere and irradiation of a 10W
blue LED (410 nm) at room temperature with dimethylfor-
mamide (DMF) as the solvent. To our delight, cyclization
product 2-phenyl benzothiazole 2aa was obtained in 39% yield
(Table 1, entry 1). Solvent screening showed that dimethyl
sulfoxide (DMSO) was the best, giving 2aa in 95% yield
(entries 2−9). It is worth noting that when H2O was used as
the solvent, 2aa was obtained in 78% yield (entry 9). Under a
N2 atmosphere, no reaction was observed, indicating O2 in air
is essential for the cyclization reaction (entry 10). No reaction
occurred in the absence of blue-light irradiation, and a 6%
product yield was obtained in the absence of g-C3N4 (entries
11 and 12). The two experiments revealed that g-C3N4 and
light are indispensable in the photochemical C−H thiolation
reaction. Interestingly, Ru(bpy)3Cl2 and Ir(ppy)3, which are
commonly used in homogeneous photocatalysis, were found to
be less efficient under the same condition (entries 13 and 14,
respectively).
With the optimized condition in hand, we investigated the
scope of this reaction (Table 2). We first examined the
a
Table 2. Synthesis of Substituted 2-Arylbenzothiazoles
a
Reaction conditions: 1 (0.1 mmol) and g-C3N4 (10 mg) in DMSO
(3 mL) irradiated by a 10 W blue LED under an air atmosphere at rt.
Isolated yields are shown.
substituent (R1) effect of the N-aryl group of 1aa on the
reaction. Both electron-donating and electron-withdrawing
groups are well tolerated to afford the desired products (2aa−
2ah) in 66−88% yields. The reactivity of the electron-donating
groups is higher than that of electron-withdrawing groups,
completing the reaction in shorter period of time. The reaction
showed excellent combability with halogen groups (F, Cl, and
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Br) without dehalogenation being observed. It is worth noting
that p-NO2-containing thiobenzanilide, which is a challenging
substrate for the synthesis of the corresponding benzothiazo-
le,14e reacted efficiently under our condition to provide the
desired product (2ae) in 66% yield. When an N-aryl ring was
replaced with a naphthyl ring, the reaction gave the
corresponding product in 84% yield.
We further investigated the substitution (R2) effect of the 2-
aryl group on the reaction (Table 2, 2ba−2bp). Excellent
yields were obtained with both electron-rich and electron-poor
substituents on the 2-phenyl ring (82−90%). All of the
substituent positions (ortho, meta, and para) are compatible
with the reaction (82−90%).
Heterogeneous photocatalysis can also be applied to N-aryl
alkylthioamides for the synthesis of 2-alkylbenzothiazoles. As
shown in Table 3, both bulky and small alkyl substituents are
a
Table 3. Synthesis of Substituted 2-Alkylbenzothiazolesa
Figure 1. Light on/off experiments.
Furthermore, the steady-state emission of g-C3N4 is
conducted in the presence of N-phenylbenzothioamide (see
Figure S8). The red shift was observed with the increase in the
concentrations of N-phenylbenzothioamide in the experiment,
indicating N-phenylbenzothioamide adsorbs on the surface of
g-C3N4 to form a complex that has a new electron donor level
above the original valence band of g-C3N4.9a,b,15
The effect of different scavengers was investigated to gain
further information about the reaction mechanism (Table 4).
a
Reaction conditions: 1d (0.1 mmol) and g-C3N4 (10 mg) in DMSO
Table 4. Active Species Trapping Reaction for the
Cyclization of Thiobenzanilides
(3 mL) irradiated by a 10 W blue LED under an air atmosphere at rt.
Isolated yields are shown.
a
compatible with our protocol, leading to the desired products
in excellent yields. It is worth noting that thioamides with a
small alkyl group are poor substrates for the synthesis of the
corresponding 2-alkylbenzothiazoles in previous reports.14b,d,e
To demonstrate the practicality of this method, a gram-scale
reaction with 1aa was performed. Treatment of 1aa (1.00 g)
with g-C3N4 by irradiation of blue LED under an air
atmosphere gave desired product 2aa in 62% yield (Scheme 3).
entry
quencher
equiv atmosphere quenching group yield (%)
̅
•−
1
2
benzoquinone
AgNO3
1.0
3.0
air
N2
O2
e
93
90
a
Reaction conditions: 1aa (0.1 mmol) and g-C3N4 (10 mg) in DMSO
(3 mL) irradiated by a 10 W blue LED for 10 h at rt. Yields were
determined by HPLC.
Scheme 3. Gram-Scale Reaction
When superoxide radical (O2•−) scavenger benzoquinone was
added,11 the product yield was not obviously changed. Our
EPR experiment has demonstrated the existence of superoxide
radical in the reaction (see Figure S10). Furthermore, when
the reaction was performed under a N2 atmosphere with 3
equiv of AgNO3, an electron scavenger,16 product 2aa was
obtained in 90% yield. These two experiments indicated that
molecular oxygen in air acts as an electron scavenger to
promote the photoredox reaction.
The intramolecular kinetic isotope effect (KH/KD = 0.939)
was obtained by using deuterated substrate 1e-D (Scheme 4),
which suggests that the C−H bond cleavage might not be the
rate-determining step. In addition, no reaction occurred when
N-methyl-N-phenylbenzothioamide 1f was used as a substrate
under the same reaction condition (Scheme 5), indicating that
the deprotonation of the N−H bond is involved in the
photoredox cyclization.
To get insight into the reaction mechanism, several control
experiments were carried out. Light on/off experiments were
performed to investigate the effect of light irradiation on the
reaction (Figure 1). It was found that the cyclization
proceeded smoothly under blue-light irradiation, but no
reaction occurred when the light was turned off. This
experiment revealed that light is indispensable for the reaction
and the reaction should not be a radical chain reaction initiated
by light.
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The recyclability of g-C3N4 was investigated (Table 5).
Treatment of 1aa with g-C3N4 gave 2aa in 95% yield in the
Scheme 4. Intramolecular Kinetic Study
Table 5. Recyclability of Catalyst g-C3N4
Scheme 5. Control Experiment
no. of cycles
yield (%)
1
95
2
96
3
95
4
93
5
94
first run. The heterogeneous photocatalyst can be separated
easily from the reaction mixture by simple centrifugation. The
recovered catalyst was reused five times without a loss of
catalytic activity. Furthermore, the stability of g-C3N4 was
investigated by comparison of the recycled catalyst and the
fresh catalyst. Examination of the two catalysts by X-ray
powder diffraction (XRD), Fourier-transform infrared spec-
troscopy (FTIR), and scanning electron microscopy (SEM)
shows that a recycled catalyst did not change obviously in
morphology or structure after five recycling steps (see Figure
S5).
According to the experiments described above and the
literature,14a,b a plausible reaction mechanism is proposed
(Scheme 6). Initially, N-substituted benzothioamide 1 adsorbs
Scheme 6. Proposed Reaction Mechanism
In conclusion, we have developed a metal-free, heteroge-
neous photocatalysis for the synthesis of 2-substituted
benzothiazoles via C−H functionalization/C−S bond for-
mation of thiobenzanilides using inexpensive graphitic carbon
nitride (g-C3N4) as the catalyst and visible light as the energy
source. This reaction is tolerant to a variety of functional
groups and can be carried out under an air atmosphere at room
temperature without the need for a strong base or an organic
oxidizing agent. In addition, the catalyst was found to be stable
and reusable. A mechanistic study revealed that absorption of
the substrate to g-C3N4 took place and facilitated the
photoredox transformation; cleavage of the C−H bond of
thiobenzanilides may not be the rate-determining step, and O2
in air acts as an electron scavenger to promote the photoredox
reaction. We believe that this green protocol would find
applications in the pharmaceutical industry.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01571.
to the surface of g-C3N4 to form a complex A with the
formation of a new donor level above the valence band of g-
C3N4. Visible-light irradiation of A drives the promotion of an
electron to the conduction band of g-C3N4 with the generation
of radical cation B. The photogenerated electron quenching by
O2 and loss of a proton from B produce radical C. The sulfur-
centered radical of C intramolecularly attacks the benzene ring
to form aryl radical D, which releases an electron to the
conduction band of g-C3N4 to generate a cation E under
visible-light irradiation. E undergoes deprotonation and
rearomatization to provide final product 2, which is consistent
with the KIE study and previous findings.14b It has been
reported that the direct conversion of aryl radical D to 2 via
hydrogen atom transfer (HAT) with HOO• has a KIE of 5,
where cleavage of the C−H bond is considered to be the rate-
determining step.14a In our reaction, O2 acts as an electron
scavenger to be reduced to O2•− that may react with protons to
form HOO•; the latter is eventually converted to H2O2 and
O2.17
1
Experimental procedures, characterization data, and H
and 13C NMR spectral data of products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Zhen Guo − College of Materials Science & Engineering, Key
Laboratory of Interface Science and Engineering in Advanced
Materials, Ministry of Education, Taiyuan University of
Technology, Shanxi 030024, People’s Republic of China;
orcid.org/0000-0001-9528-1383; Email: guozhen@
tyut.edu.cn
Cong-Ying Zhou − College of Chemistry and Materials
Science, Guangdong Provincial Key Laboratory of Functional
Supramolecular Coordination Materials and Applications,
Jinan University, Guangzhou 510632, People’s Republic of
China; orcid.org/0000-0002-2235-6070;
Email: zhoucy2018@jnu.edu.cn
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Authors
pubs.acs.org/OrgLett
Letter
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Jin Bai − College of Materials Science & Engineering, Key
Laboratory of Interface Science and Engineering in Advanced
Materials, Ministry of Education, Taiyuan University of
Technology, Shanxi 030024, People’s Republic of China
Sijia Yan − College of Chemistry and Materials Science,
Guangdong Provincial Key Laboratory of Functional
Supramolecular Coordination Materials and Applications,
Jinan University, Guangzhou 510632, People’s Republic of
China; orcid.org/0000-0002-3957-0495
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Key Laboratory of Interface Science and Engineering in
Advanced Materials, Ministry of Education, Taiyuan
University of Technology, Shanxi 030024, People’s Republic
of China; orcid.org/0000-0003-3536-6363
Complete contact information is available at:
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Author Contributions
§J.B. and S.Y. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are sincerely thankful for the financial support
from the 100-Talent Program in Shanxi province, the Start-up
fund of Jinan University, the National Natural Science
Foundation of China (21903059 and 21472159), the Natural
Science Foundation of Shanxi Province (201901D111113,
201901D111109, and 201901D211093), and the Guangdong
Science and Technology Department (2021A1515012023).
This study was also partially supported by the National Natural
Science Foundation of China (41877276).
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Org. Lett. 2021, 23, 4843−4848