Fast and high-efficiency synthesis of 2-substituted benzothiazoles via combining enzyme catalysis and photoredox catalysis in one-pot

Article abstract of DOI:10.1016/j.bioorg.2020.104607

An efficient and green method, combining enzymatic and visible-light catalysis for synthesis of the widely applicable 2-substituted benzothiazoles, has been developed. This method features a relay catalysis protocol consisting of biocatalytic promiscuity

Full text of DOI:10.1016/j.bioorg.2020.104607

Bioorganic Chemistry 107 (2021) 104607
Contents lists available at ScienceDirect
Bioorganic Chemistry
journal homepage: www.elsevier.com/locate/bioorg
Fast and high-efficiency synthesis of 2-substituted benzothiazoles via
combining enzyme catalysis and photoredox catalysis in one-pot
Zeng-Jie Yang, Qing-Tian Gong, Yuan Yu, Wei-Fan Lu, Zhe-Ning Wu, Na Wang, Xiao-Qi Yu*
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
An efficient and green method, combining enzymatic and visible-light catalysis for synthesis of the widely
applicable 2-substituted benzothiazoles, has been developed. This method features a relay catalysis protocol
consisting of biocatalytic promiscuity and visible-light-induced subsequent oxidization of 2-phenyl benzothia-
zolines. The whole reaction process is very high-efficiency, achieving 99% yield in just 10 min, under an air
atmosphere, nearly 100% atomic utilization, and the 2-substituted benzothiazole products were obtained in good
to excellent yields with a wide range of substrates. This reaction is the other example of combining the non-
natural catalytic activity of hydrolases with visible-light catalysis for organic synthesis and the catalytic sys-
tem does not require additional oxidants or metals, which is good for the environment.
Eco-friendly
Photocatalytic reaction
Visible-light
Benzothiazoles
One-pot synthesis
1. Introduction
compounds are usually used as photoredox catalysts. As a powerful and
sustainable chemical reaction method, photoredox catalysis has been
Enzyme is as an efficient, environmentally friendly and biodegrad-
able catalyst in organic synthesis [1–3]. Compared with conventional
chemical catalytic methods, enzymatic catalysis has many advantages,
such as good selectivity, high catalytic efficiency, mild reaction condi-
tions, fewer by-products production and fewer synthetic reaction steps
[4–7]. In addition, some enzymes (especially hydrolases) have the
ability to catalyze reactions that are completely different from those
catalyzed in nature. This behavior is coined enzyme promiscuity, and it
greatly enriches all types of enzymatic reactions [8,36–39]. In recent
years, there have been more and more reports on enzymatic catalysis
combining with other catalysis methods, such as photocatalysis,
organometallic catalysis, and electrosynthesis [9–12]. Among them, the
combination of photocatalysis and enzymes has become a very impor-
tant point. Hyster and co-workers have reported the light-induced non-
natural catalytic activity of NAD(P)-depended KRED with high stereo-
selectivity [13]. It illustrates how powerful the methods would be when
combining light and enzymes.
revived in the past decade [16,17]. In fact, the earliest application of
photoredox catalysis should be in nature, where saccharides are first
constructed by CO2 and H2O through photosynthesis, and then are used
as raw materials to generate other structures in the process of meta-
bolism through enzymes [18]. This shows that the combination of
photoredox catalysis activity and enzyme promiscuity can promote the
synthesis of value-added chemicals and may reveal new catalytic func-
tions of enzymes. In 2020, Zhao and co-workers reported a new-to-
nature, visible-light-induced ene-reductase catalysed intermolecular
radical hydroalkylation of terminal alkenes with readily available α-halo
carbonyl compounds [19]. This method provides an efficient approach
to various carbonyl compounds bearing a γ-stereocentre with excellent
yields and enantioselectivities (up to 99% yield, 99% ee), which
otherwise are difficult to access by chemocatalysis. This work further
expands the reactivity repertoire of biocatalytic, synthetically-useful
asymmetric transformations by the merger of photocatalysis and
enzyme catalysis. In 2019, the Guan group reported a combination of
photoredox catalysis and enzymatic catalysis for the direct asymmetric
one-pot synthesis of 2, 2-disubstituted indol-3-ones [20]. In the same
year, Wu studied demonstrated a new strategy of direct photoinduced
KR of a-functionalized carboxylic acids by an engineered CvFAP without
dependence on electron transfer by NADPH or prior preparation of es-
ters, which are required in previous biocatalytic approaches [21,22].
Light energy, as a clean, inexpensive, sustainable, and environmen-
tally friendly energy, has attracted wide attention [14,15]. Visible-light
photoredox catalysis, refers to a type of catalysis that uses visible light
energy to accelerate chemical reactions through a single-electron
transfer (SET) process. In this catalytic method, the transition metal
complex, organic dyes, semiconductor, and other photosensitive
* Corresponding author.
E-mail addresses: wnchem@scu.edu.cn (N. Wang), xqyu@scu.edu.cn (X.-Q. Yu).
https://doi.org/10.1016/j.bioorg.2020.104607
Received 11 November 2020; Received in revised form 22 December 2020; Accepted 24 December 2020
Available online 31 December 2020
0045-2068/© 2020 Elsevier Inc. All rights reserved.

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
Fig. 1. Selected examples of benzothiazole derivatives embodied with biological activity and drugs.
This simultaneous photocatalytic and biocatalytic mode provides a
gentle and effective strategy for the one-pot enantioselective synthesis of
complex compounds. Therefore, combining photoredox catalysis and
enzyme is worth developing with great potential. Initiating organic re-
actions using visible photoredox catalysis and enzyme promiscuity could
be a green and powerful tool in synthetic organic chemistry.
Siva S. Panda and his colleagues utilized microwave irradiation to form
benzothiazoles [33]. In 2018, Maphupha and co-authors used laccase as
a catalyst at room temperature for the synthesis of 2-arybenzothiazole
erivatives [34]. Although these methods are very effective and satis-
factory, they are also subject to some limitations in aspects of high
temperature, long reaction time, low atomic economy, complex syn-
thesis process and unfriendly environmental impact. Considering the
medicinal value of benzothiazoles, the presence of metals should also be
avoided in the catalytic system. Therefore, there is still a great need to
find a gentle and applicable green synthesis method.
Analogies and derivatives of benzothiazole compound have received
extensive attention due to their good biological and pharmacological
properties [23,24]. When biologists discovered the pharmacological
characteristics of riluzole (6-trifluoromethoxy-2-benzothiazolamide) for
the treatment of amyotrophic lateral sclerosis, they began to focus on
this series of drugs (Fig. 1) [25], for example, IDD552 is a well-known
aldolreductase inhibitor [26], RWJ-51084 is an effective cationic
trypsin precursor and Fanetizole is an anti-inflammatory agent [27].
Therefore, synthesis of benzothiazole has attracted much attention due
to its effective and significant biological activity and great pharmaceu-
tical value. Although many strategies have been used to synthesize
benzothiazole to date, the condensation reaction of 2-amino-
benzenethiol with carbonyl or cyano-containing substances is the most
commonly used method. However, this method should be carried out
under high temperature or metal catalyst conditions to ensure the
smooth progress of the reaction [28,29]. Therefore, in order to over-
come the above shortcomings, many new synthetic methods have been
reported in recent years. For example, in 2010, Fan and co-workers re-
ported that Ru(III) chloride is an effective catalyst for the formation of
benzothiazole in (bmim)PF6(ILs) medium [30]. In 2012, Wu’s group
reported I2 promoted domino protocol and multipathway coupled
domino strategy for synthesis of 2-acylbenzothiazoles [31,32]. In 2013,
In recent years, our group has been committed to green enzyme
catalysis methodology and some achievements have been made
[35–39]. We were also attracted by the medicinal value of benzothia-
zoles, and considering the lack of green synthesis methods, we tried to
synthesize benzothiazoles using enzyme-catalyzed methods. Unfortu-
nately, the enzyme-catalyzed method can only obtain intermediate
products. At this time, we thought of the same green photocatalytic
oxidation method. When the two methods were combined, “one plus one
is greater than two” was successfully realized.
Compared with the previously reported synthetic methods, this
method is more efficient, green, convenient and does not require an
additional oxidant or metal. The compatibility of trypsin and photo-
catalyst Solvent Red 43 in organic solvents under λ = 450 nm light
irradiation, water content, amount of catalyst, and a series of experi-
mental conditions were studied. The performance of the reaction system
is promising, and under a variety of substrate conditions, a series of 2-
substituted benzothiazoles are obtained with good to excellent yields
(up to 99%), under an air atmosphere, in just 10 min. After a simple
Scheme 1. Enzyme and visible light co-catalyzed a two-step reaction, in one pot.
2

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
Table 1
The catalytic effect of different commercial enzymes on the model reaction.a
Entry
Enzyme
3a Yield(%)b
4a Yield(%)b
1
Blank
10
28
33
32
50
30
72
57
65
58
8
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
2
Lipase from Candida antarctia B (CAL-B)
Lipase from Porcine pancreas (PPL)
Bovine serum albumin (BSA)
Lipase from Mucor miehei (MML)
Amano Lipase M from Mucor javanicus (MJL)
Trypsin (bovine pancreas)
Pepsin
3
4
5
6
7
8
9
Lipase from Candida rugosa (CRL)
Bovine pancreatic lipase (BPL)
Urea (200 mg)
10
11
12
13
14
15
Trypsin (pretreated with urea)
CuSO4 (39.9 mg)
7c
5
Trypsin (pretreated with 250 mM Cu2+
Denatured Trypsin
)
6d
9e
a
Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), enzymes (5 mg), acetonitrile (1 mL) were added to 10 mL quartz tube, and at 200 rpm at λ = 450 nm 20 W for
10 min. bAll yields were determined by HPLC. cTrypsin (10 mg) in urea solution (6.7 M) [urea (400 mg) in deionized water (1 mL)] was stirred at 25 ◦C for 24 h, and
then water was removed by lyophilization before use. dTrypsin (10 mg) in Cu2+ solution (250 mM) [CuSO4 (39.9 mg) in deionized water (1 mL)] was stirred at 25 ◦C
for 24 h, and then water was removed by lyophilization before using. eTrypsin 120 ◦C for 24 h.
reaction conditions, only the target product and water are produced,
which has high atom economy and environment-friendly. The combi-
nation of enzyme-catalysis and photoredox catalysis through cascade
reaction for the synthesis of 2-substituted benzothiazoles can be used as
a supplement to current methods (Scheme 1).
3. Results and discussion
3.1. Catalytic activity of different enzymes.
Initially, we chose the reaction of the substrates 2-aminothiophenol
(1a) and p-nitrobenzaldehyde (2a) in acetonitrile as the model reaction.
In order to select a suitable enzyme to catalyze the synthesis of 2-
substituted benzothiazole compounds, the catalytic effect of different
commercial enzymes on the model reaction was first studied, including
routine, promiscuous activities, and some control experiments. The re-
sults are shown in Table 1. As shown in Table 1, all commercial enzymes
cannot catalyze this reaction to form the final product, only the inter-
mediate 2, 3-dihydro-2-(4-nitrophenyl) benzothiazole is produced
(Figure S1A). Here, we mainly discuss the effect of different enzymes on
the yield of intermediate. The catalytic activity of enzymes from
different sources is quite different. The highest yield of 72% was ach-
ieved using trypsin (bovine pancreas) (Table 1, entry 7). Three kinds of
enzymes like lipase from Mucor miehei (MML), showed moderate cata-
lyst activity (Table 1, entries 6, 9–10). As for the four types of enzymes
including lipase from Candida antarctia B (CAL-B), the catalytic yields
are all around 30%, which is a bit lower than the highest yield, but it is
still three times higher than the blank control reaction (Table 1, entries
2–4, 6). In order to prove the specific catalytic effect of trypsin on this
reaction, a series of controlled experiments were carried out. We con-
ducted a model reaction in the absence of trypsin and using thermally
denatured trypsin. At the end of the reaction, only negligible products
were observed (Table 1, entries 1 and 15), indicating that trypsin can
initiate the reaction. In addition, trypsin was pretreated with urea as a
denaturant and then used to catalyze the model reaction, producing only
a very low yield (Table 1, entry 12). To determine the effect of urea on
the model reaction, urea was used to catalyze the model reaction, and no
product was observed in urea (Table 1, entry 11). These results indicate
that after heat or urea denaturation, trypsin loses its natural structure
and fails to complete the catalysis. Heavy metal ions can also inactivate
enzymes by reacting with structural groups such as thiol groups, or with
certain amino acid residues, causing tertiary structural changes.
Therefore, Cu2+ was used for trypsin pretreatment, respectively. Similar
to the denatured enzyme diagram below, after incubation with Cu2+, the
enzyme loses catalytic activity for the reaction (Table 1, entry 14). A
blank reaction using only Cu2+ as the catalyst failed to obtain the
product (Table 1, entry 13). The above control experiments showed that
2. Experimental section
2.1. Materials and analytical methods
All reagents were used without further purification unless otherwise
noted. CRL (lipase from Candida rugosa), PPL (lipase from Porcine
pancreas, 6.8 U/mg), BPL (lipase from Bovine pancreas, 15–35 u/g),
MJL (Amano lipase from Mucor javanicus, 10,000 U/mg), CAL-B (lipase
from Candida antarctia B, 2 U/mg), MML (lipase from Mucor miehei 1 U/
mg), were purchased from Sigma-Aldrich and. Pepsin were purchased
from Fluka. BSA (bovine serum albumin), trypsin (bovine pancreas), 2-
aminothiophenol, p-nitrobenzaldehyde, Solvent Red 43, Tris (2, 2-
bipyridyl) ruthenium (II) chloride hexahydrate, Rhodamine B, Fluores-
cein were purchased from Aladdin. The NMR spectra were obtained on
an Agilent 400-MR DD2 spectrometer. The 1H NMR (400 MHz) chemical
shifts were measured relative to CDCl3 or DMSO‑d6 as the internal
reference. The 13C NMR (100 MHz) chemical shifts were given using
CDCl3 or DMSO‑d6 as the internal standard. HPLC experiments were
performed on Waters instrument (Waters e2695, 2998) using a C18
column with MeOH/water = 70:30 (v/v), 1 mL/min, λ max = 254 nm,
and 30 ◦C. High resolution mass spectra (HR-MS) were obtained with a
Waters-Q-TOF-Premier (ESI).
2.2. The typical procedure for synthesis of 2-substituted benzothiazoles.
2-aminothiophenol (0.2 mmol), p-nitrobenzaldehyde (0.2 mmol),
Solvent Red 43 1% mol trypsin (10 mg), and toluene (2 mL) were added
to 10 mL quartz tube, at 200 rpm and at λ = 450 nm, 20 W for 10 min.
The reaction was completed by filtering the enzyme. The crude products
were purified by silica gel column chromatography (200–300 mesh)
with an eluent consisting of ethyl acetate-petroleum. Product-contained
fractions were combined, concentrated, and dried to give respective
product.
3

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
trypsin can catalyze a model reaction to produce intermediate.
3.2. Wavelength and photosensitizers screen
After determining the optimal enzyme, wavelength, and photosen-
sitizers were screened in order to achieve the oxidation process. The
results are shown in Table S1 and Fig. 2. Through Table S1, we deter-
mined that λ = 450 nm is the best catalytic wavelength. As illustrated in
Fig. 2, it is worth noting that in the entries without trypsin, the yield was
reduced by 70%. This shows that trypsin plays a vital role in this reac-
tion among the four photosensitizers with enzymes, the reaction result
of Solvent Red 43 is the best, the yield is greater than 95%. Compared
with Solvent Red 43, the yield of Fluorescein also reached 89%. This
may be related to the similar structure of the two photosensitizers. After
all, they differ by only 4 bromine atoms. The worst reaction is Tris (2, 2-
bipyridyl) ruthenium (II) chloride hexahydrate, with only 69% yield.
For items that do not contain trypsin, the situation is basically the same
as that containing trypsin, with Solvent Red 43 giving the best response,
followed by Fluorescein, and Tris (2, 2-bipyridyl) ruthenium (II) chlo-
ride hexahydrate being the lowest yield. Therefore, we chose the Solvent
Red 43 as the best photosensitizer to screen the subsequent reaction
conditions.
Fig. 2. Photosensitizers were screened in order to achieve the oxidation pro-
cess. Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), photosensitizer (1%
mol), trypsin (5 mg) andacetonitrile (1 mL) were added to 10 mL quartz tube,
and at 200 rpm at λ = 450 nm. All yields were determined by HPLC.
3.3. The influence of reaction medium
Since the reaction medium can directly or indirectly affect the pho-
toenzymatic activity, the choice of the solvent for the photoenzymatic
reaction is also one of the key factors for the reaction to be effective. We
selected a series of solvents to examine their effects on the template
reaction. The results are shown in Table 2. The results showed that the
solvent played an important role in the model reaction. When water was
used as the reaction solvent, only 24% target product was formed
(Table 2, entry 8). This may be attributed to the poor solubility of
organic compounds in water. In methanol, ethylene glycol and DMSO,
the yield of the model reaction is about 50% (Table 2, entries 1, 2, 5). In
DMF, THF and trichloromethane, the yield of model reaction achieved a
moderate yield (Table 2, entries 3, 4, 7). Although a 95% yield has been
achieved in acetonitrile (Table 3, entry 9), but in toluene, the yield is
99% (Table 2, entry 6). After determining the solvent, we explored the
effect of different water content on the reaction, the results are shown in
Figure S2. It can be seen from the figure that as the water content in-
creases, the yield decreases in turn. This may be related to the solubility
of the reactants in water. Therefore, we used toluene as the optimal
reaction solvent for the photoenzymatic reaction.
Table 2
Solvent screening.a
Entry
Solvents
Yield b (%)
1
2
3
4
5
6
7
8
9
Methanol
Ethylene glycol
DMF
51
55
78
67
51
99
60
24
95
THF
DMSO
Toluene
Trichloromethane
H2O
CH3CN
a
Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), Solvent Red 43 1% mol
trypsin (5 mg) and solvent (1 mL) were added to 10 mL quartz tube, and at 200
rpm at λ = 450 nm, 20 W for 10 min. bAll yields were determined by HPLC.
3.4. The molar ratio, amount of enzyme and photocatalysis screen
Next, we further explored the effect of the molar ratio between p-
Table 3
Control experiments.a
Entry
Trypsin
λ = 450 nm
Solvent Red 43
4a Yield (%)b
1
2
3
4
5
6
7
8
+
–
+
+
–
+
+
+
–
95
23
+
+
–
Trace
Trace
0
+
–
+
+
–
–
Trace
0
–
+
–
–
–
–
0
a
Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), photosensitizer (1% mol), trypsin (5 mg) and acetonitrile (1 mL) were added to 10 mL quartz tube, and at 200
rpm at λ = 450 nm, 20 W for 10 min. bAll yields were determined by HPLC.
4

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
Fig. 3. The effect of the amount of Solvent Red 43 on the reaction. a)Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), Solvent Red 43 0.25–2% mol, trypsin (5
mg), toluene (1 mL) were added to 10 mL quartz tube, and at 200 rpm at λ = 450 nm, 20 W for 10 min. All yields were determined by HPLC. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)
nitrobenzaldehyde (1a) and 2-aminothiophenol (2a) on the reaction.
The results are shown in Figure S3. It can be observed that when p-
nitrobenzaldehyde is excessive, the yield begins to decrease, and as the
increment increases, the yield decreases. When 2-aminothiophenol is
excessive, it illustrated the same trend. When the molar ratio between
the p-nitrobenzaldehyde and 2-aminothiophenol is 1:1, the highest yield
of 99% can be obtained. Therefore, we chose molar ratio 1a:2a = 1:1 as
the best molar ratio for subsequent studies.
can be seen in 4g, 4h, and 4i. As the electron-richness of aldehyde
compounds increases, the yield decreases in sequence, which are 98%,
95%, and 89%, respectively. For methyl-substituted benzaldehyde, the
effect of steric hindrance on yield is negligible, which can be seen from
4c, 4d, and 4e. When the benzaldehyde was replaced with a larger
volume of 1-naphthaldehyde, the yield decreased significantly, only
78% 4s, indicating that the steric effect is present in this reaction. On the
contrary, when benzaldehyde is replaced by 2-thenaldehyde, although
the volume of 2-thenaldehyde is small, 4r, the electron-rich effect in-
creases due to the presence of S atoms, and the yield decreases. Finally,
when the reaction is carried out with phenylacetaldehyde 4t, the yield is
only 80%. The possible reason is that due to the increase of carbon atoms
in the middle, it is difficult to conjugate the aldehyde group with the
benzene ring, and the yield decreases.
Subsequently, we studied the effects of the amount of enzyme on the
reaction, and the results are shown in Figure S4. It can be observed from
Figure S4 that when the amount of the enzyme is increased from 0 mg to
30 mg, the yield of the target product is increased from 0% to 99%.
When the enzyme amount is 5 mg, the yield reaches a maximum of 99%.
However, as the amount of enzyme loading is further increased, the
yield remains substantially unchanged. Therefore, we chose the most
optimal reaction conditions with an enzyme content of 5 mg to carry out
the subsequent reaction.
3.6. Control experiments
Similarly, the amount of photocatalysis also has a greater impact on
the reaction. We investigated the effects of different amounts on the
reaction, and the results are shown in Fig. 3. When the photocatalyst is
0.25%mol, the yield is only 60%. As the photocatalyst content increases,
the yield increases. When the content increases to 2%mol, the yield no
longer increases. Therefore, we chose 1%mol as the optimal reaction
condition for the subsequent experiment. In the time curve (Figure S5),
we found that the highest yield can be achieved within 10 min of this
reaction.
To verify this photobiocatalytic concurrent reaction, some control
experiments were conducted (Table 3, Fig. 4 and Scheme S1). Oxygen is
crucial in the reaction process (Scheme S1). Even without photosensi-
tizer and light, there is a small increase in yield in an oxygen environ-
ment. The reaction cannot continue under argon environment, which is
consistent with the single enzyme yield. Under the reaction conditions
that all three have, the model reaction gets 4a in 95% yield (Table 3,
entry 1). In the absence of Trypsin, with Solvent Red 43 under light only
a 23% yield was obtained (Table 3, entry 2). In the absence of light
irradiation or Solvent Red 43, no reaction was observed (Table 3, entries
3 and 4). As expected, no reaction occurred in the absence of both
photocatalyst and enzyme (Table 3, entry 5). Similarly, in the absence of
the other two reaction conditions, the target product was not obtained
(Table 3, entries 6 and 7). Not to mention reactions that do not have the
three reaction conditions, the yield is 0% (Table 3, entry 8). These re-
sults indicated that light, trypsin, and Solvent Red 43 are indispensable
for this photoenzymatic concurrent reaction.
3.5. Substrate scope of the 2-substituted benzothiazoles
Based on the optimal reaction conditions for the above stencil re-
action, we used a series of aromatic aldehydes for the conversion of this
reaction. The results are shown in scheme 2. It can be seen from the table
that the electronic effect on the substituents on the benzene ring imposes
a great influence on the reaction. Although most electron-deficient or
electron-rich aldehyde compounds can be smoothly converted to the
corresponding products, the yields obtained are quite different. It can be
observed from the table that aldehyde compounds containing electron-
deficient like 4a, 4g, 4j can obtain a good yield of 98–99%, while the
electron-rich pair is like 4m. The aldehyde compounds gave only a yield
of 75%. When the electron-richness is further increased, the yield
reduction is more obvious, only 64%, 4p (single crystal X-ray diffraction
analysis confirmed the structure, CCDC number: 2004038). The same
In order to verify the effect of light irradiation, we conducted an on/
off light experiment Fig. 4. In the absence of light, the reactions of 1a
and 2a are very slow. Obvious acceleration was observed under light
irradiation. In the first minute, the reaction rate is the fastest. With the
consumption of A and B, the reaction rate gradually decreases, which
conforms to the characteristics of reaction kinetics. The yield reached
the maximum when the total reaction time was 18 min, was in good
agreement with the reaction time curve (Figure S5).
5

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
Scheme 2. Synthesis of 2-substituted benzothiazoles under the optimal conditions. Reaction conditions: 2-aminothiophenol (0.2 mmol), p-nitrobenzaldehyde (0.2
mmol), Solvent Red 43 1% mol trypsin (10 mg) and toluene (2 mL) were added to 10 mL quartz tube, at 200 rpm and at λ = 450 nm, 20 W for 10 min. The crude
products were purified by silica gel column chromatography (200–300 mesh) with an eluent consisting of ethyl acetate-petroleum. Product-contained fractions were
combined, concentrated, and dried to give respective product.
3.7. Proposed reaction mechanism
nitrogen accepts electrons to generate nitrogen anions. The hydrogen
ion of histidine residue is obtained by the nitrogen anion, the activity of
the enzyme catalyst is restored, and the intermediate product 3a’ is
generated. Subsequently, SR 43 (Solvent Red 43) is excited to the
excited state SR 43* under λ = 450 nm irradiation. A single electron
oxidation of 3a by the excited state SR 43* generated radical cation3a’
and the reduced SR 43•- species. This charge-separated state species is
oxidatively quenched by O2 to produce SR43 and superoxide. At last,
deprotonation of 3a’ by superoxide followed by hydrogen atom
abstraction by hydroperoxyl radical (HOO•) yields the desired benzo-
thiazole 4a.
Based on the results of the reaction and similar transformations re-
ported in the literature, we have reasonably speculated about the
possible mechanism of action of this reaction, as shown in scheme 3.
First, p-nitrobenzaldehyde (2a) enters the binding pocket of the enzyme
and binds with the enzyme through hydrogen bonding. At this time, 2-
aminothiophenol (1a) also enters the binding pocket and undergoes a
nucleophilic addition reaction to the carbonyl carbon. Then one mole-
cule of water is removed to form a Schiff base compound. The histidine
residue in the enzyme takes the hydrogen from the thiol to generate
sulfur anion. Sulfur anions attack carbon–nitrogen double bonds,
6

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
Fig. 4. ON/OFF light experiment. a)Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), Solvent Red 43 1% mol, trypsin (5 mg), toluene (1 mL) were added to 10 mL
quartz tube, and at 200 rpm at λ = 450 nm, 20 W. All yields were determined by HPLC. (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)
4. Conclusion
compounds using enzymes and light energy and explores an environ-
mentally friendly, simple, and convenient synthetic route in organic
chemical synthesis.
In summary, this work combinated photoredox enzymatic and
visible-light catalysis for synthesis of the widely applicable 2-substituted
benzothiazoles between a 2-aminothiophenol(1a) and a series of alde-
hyde compounds in the first time. The optimal reaction conditions were
obtained by investigating factors, such as types of enzymes, reaction
solvent, molar ratio, enzyme amount, and reaction time. A series of
substrates are expanded according to the optimal reaction conditions.
Although the yield of a single target compound is low, the overall per-
formance is medium to good, with a maximum yield of 99% in just 10
min. This novel method provides an example for the synthesis of target
CRediT authorship contribution statement
Zeng-Jie Yang: Conceptualization, Methodology, Writing - original
draft, Writing - review & editing, Validation. Qing-Tian Gong: Writing -
review & editing, Formal analysis. Yuan Yu: Formal analysis, Writing -
review & editing. Wei-Fan Lu: Writing - review & editing, Formal
analysis. Zhe-Ning Wu: Writing - review & editing, Formal analysis. Na
Wang: Writing
-
review
&
editing, Supervision, Visualization,
Scheme 3. Proposed mechanism of the reaction.
7

Z.-J. Yang et al.
Bioorganic Chemistry 107 (2021) 104607
[16] N.A. Romero, D.A. Nicewicz, Organic photoredox catalysis, Chem. Rev. 116 (2016)
10075–10166.
Resources. Xiao-Qi Yu: Funding acquisition, Project administration,
Supervision,
Visualization,
Writing
-
review
&
editing,
[17] C.K. Prier, D.A. Rankic, D.W.C. MacMillan, Visible light photoredox catalysis with
transition metal complexes: applications in organic synthesis, Chem. Rev. 113
(2013) 5322–5363.
Conceptualization.
[18] D.A. Nicewicz, D.W.C. MacMillan, Merging photoredox catalysis with
organocatalysis: the direct asymmetric alkylation of aldehydes, Science 322 (2008)
77–80.
Declaration of Competing Interest
[19] X. Huang, B. Wang, Y. Wang, G. Jiang, J. Feng, H. Zhao, Photoenzymatic
enantioselective intermolecular radical hydroalkylation, Nature 69–74 (2020).
[20] X. Ding, C.L. Dong, Z. Guan, Y.H. He, Concurrent asymmetric reactions combining
photocatalysis and enzyme catalysis: direct enantioselective synthesis of 2,2-
disubstituted indol-3-ones from 2-arylindoles, Angew. Chem. Int. Edit. 58 (2019)
118–124.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
[21] J. Xu, Y.J. Hu, J.J. Fan, M. Arkin, D.Y. Li, Y.Z. Peng, W.H. Xu, X.F. Lin, Q. Wu,
Light-driven kinetic resolution of α-functionalized carboxylic acids enabled by an,
Engineered Fatty Acid Photodecarboxylase 58 (2019) 8474–8478.
[22] Y.Z. Peng, D.Y. Li, J.J. Fan, W.H. Xu, J. Xu, H.L. Yu, X.F. Lin, Q. Wu,
Enantiocomplementary C-H bond hydroxylation combining photo-catalysis and
whole-cell biocatalysis in a one-Pot, Cascade Process (2020,) 821–825.
[23] D.A. Horton, G.T. Bourne, M.L. Smythe, The combinatorial synthesis of bicyclic
privileged structures or privileged substructures, Chem. Rev. 103 (2003) 893–930.
[24] A.A. Weekes, A.D. Westwell, 2-Arylbenzothiazole as a privileged scaffold in drug
discovery, Curr. Med. Chem. 16 (2009) 2430–2440.
This work was financially supported by the National Key R&D Pro-
gram of China (Grant No. 2018YFA0901600). We also thank the
Comprehensive Training Platform of Specialized Laboratory, College of
Chemistry, Sichuan University.
Appendix A. Supplementary material
[25] H.M. Bryson, B. Fulton, P. Benfield, Riluzole – A review of its pharmacodynamic
and pharmacokinetic properties and therapeutic potential in amyotrophic lateral
sclerosis, Drugs 52 (1996) 549–563.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.bioorg.2020.104607.
[26] F. Ruiz, I. Hazemann, A. Mitschler, A. Joachimiak, T. Schneider, M. Karplus,
A. Podjarny, The crystallographic structure of the aldose reductase-IDD552
complex shows direct proton donation from tyrosine 48, Acta. Crystallogr. D. 60
(2004) 1347–1354.
References
[27] B. Gong, F. Hong, C. Kohm, L. Bonham, P. Klein, Synthesis and SAR of 2-aryl-
benzoxazoles, benzothiazoles and benzimidazoles as inhibitors of lysophosphatidic
acid acyltransferase-β, Bioorg. Med. Chem. Lett. 14 (2004) 1455–1459.
[28] A.J. Blacker, M.M. Farah, M.I. Hall, S.P. Marsden, O. Saidi, J.M.J. Williams,
Synthesis of benzazoles by hydrogen-transfer catalysis, Org. Lett. 11 (2009)
2039–2042.
[1] A.M. Knight, S.B.J. Kan, R.D. Lewis, O.F. Brandenberg, K. Chen, F.H. Arnold,
Diverse engineered heme proteins enable stereodivergent cyclopropanation of
unactivated alkenes, Acs. Central. Sci. 4 (2018) 372–377.
[2] U.T. Bornscheuer, G.W. Huisman, R.J. Kazlauskas, S. Lutz, J.C. Moore, K. Robins,
Engineering the third wave of biocatalysis, Nature 485 (2012) 185–194.
[3] M.T. Reetz, Biocatalysis in organic chemistry and biotechnology: past, present, and
future, J. Am. Chem. Soc. 135 (2013) 12480–12496.
[29] G.-F. Chen, H.-M. Jia, L.-Y. Zhang, B.-H. Chen, J.-T. Li, An efficient synthesis of 2-
substituted benzothiazoles in the presence of FeCl3/Montmorillonite K-10 under
ultrasound irradiation, Ultrason. Sonochem. 20 (2013) 627–632.
[4] Y.H. He, W. Hu, Z. Guan, Enzyme-catalyzed direct three-component Aza-diels-alder
reaction using hen egg white lysozyme, J. Org. Chem. 77 (2012) 200–207.
[5] S. Akai, K. Tanimoto, Y. Kita, Lipase-catalyzed domino dynamic kinetic resolution
of racemic 3-vinylcyclohex-2-en-1-ols/intramolecular diels-alder reaction: one-pot
synthesis of optically active polysubstituted decalins, Angew. Chem. Int. Edit. 43
(2004) 1407–1410.
[30] X. Fan, Y. Wang, Y. He, X. Zhang, J. Wang, Ru(III)-catalyzed oxidative reaction in
ionic liquid: an efficient and practical route to 2-substituted benzothiazoles and
their hybrids with pyrimidine nucleoside, Tetrahedron. Lett. 51 (2010)
3493–3496.
[31] Y.P. Zhu, F.C. Jia, M.C. Liu, A.X. Wu, A multipathway coupled domino strategy:
metal-free oxidative cyclization for one-pot synthesis of 2-acylbenzothiazoles from
multiform substrates, Org. Lett. 14 (2012) 4414–4417.
[6] S.J. Chai, Y.F. Lai, J.C. Xu, H. Zheng, Q. Zhu, P.F. Zhang, One-pot synthesis of
spirooxindole derivatives catalyzed by lipase in the presence of water, Adv. Synth.
Catal. 353 (2011) 371–375.
[32] Y.P. Zhu, M. Lian, F.C. Jia, M.C. Liu, J.J. Yuan, Q.H. Gao, A.X. Wu, I2 promoted
domino oxidative cyclization for one-pot synthesis of 2-acylbenzothiazoles via
metal-free sp3 C-H functionalization, Chem. Commun. 48 (2012) 9086–9088.
[33] S.S. Panda, M.A. Ibrahim, A.A. Oliferenko, A.M. Asiri, A.R. Katritzky, Catalyst-free
facile synthesis of 2-substituted benzothiazoles, Green. Chem. 15 (2013)
2709–2712.
[7] Z.B. Xie, N. Wang, L.H. Zhou, F. Wan, T. He, Z.G. Le, X.Q. Yu, Lipase-catalyzed
stereoselective cross-aldol reaction promoted by water, ChemCatChem 5 (2013)
1935–1940.
[8] A. Domling, W. Wang, K. Wang, Chemistry and biology of multicomponent
reactions, Chem. Rev. 112 (2012) 3083–3135.
[34] M. Maphupha, W.P. Juma, C.B. de Koning, D. Brady, A modern and practical
laccase-catalysed route suitable for the synthesis of 2-arylbenzimidazoles and 2-
arylbenzothiazoles, RSC. Adv. 8 (2018) 39496–39510.
[9] K.F. Biegasiewicz, S.J. Cooper, M.A. Emmanuel, D.C. Miller, T.K. Hyster, Catalytic
promiscuity enabled by photoredox catalysis in nicotinamide-dependent
oxidoreductases, Nat. Chem. 10 (2018) 770–775.
[35] K. Li, T. He, C. Li, X.W. Feng, N. Wang, X.Q. Yu, Lipase-catalysed direct Mannich
reaction in water: utilization of biocatalytic promiscuity for C-C bond formation in
a “one-pot” synthesis, Green. Chem. 11 (2009) 777–779.
[10] C. Kohlmann, W. Markle, S. Lutz, Electroenzymatic synthesis, J. Mol. Catal. B-
Enzym. 51 (2008) 57–72.
[11] H.M. Key, P. Dydio, D.S. Clark, J.F. Hartwig, Abiological catalysis by artificial
haem proteins containing noble metals in place of iron, Nature 534 (2016)
534–537.
[36] W. Zhang, N. Wang, Y.H. Liu, S.Y. Jiao, W.W. Zhang, X.M. Pu, X.Q. Yu, 1,4-
Dihydropyridines: discovery of minimal AIEEgens and their mitochondrial imaging
applications, J. Mater. Chem. B. 5 (2017) 464–469.
[12] P. Dydio, H.M. Key, A. Nazarenko, J.Y.E. Rha, V. Seyedkazemi, D.S. Clark, J.
F. Hartwig, An artificial metalloenzyme with the kinetics of native enzymes,
Science 354 (2016) 102–106.
[37] W.X. He, X. Xing, Z.J. Yang, Y. Yu, N. Wang, X.Q. Yu, Biocatalytic one-pot three-
component synthesis of indoloquinolizines with high diastereoselectivity, Catal.
Lett. 149 (2019) 638–643.
[13] M.A. Emmanuel, N.R. Greenberg, D.G. Oblinsky, T.K. Hyster, Accessing non-
natural reactivity by irradiating nicotinamide-dependent enzymes with light,
Nature 540 (2016) 414–417.
[38] W. Zhang, N. Wang, Z.J. Yang, Y.R. Li, Y. Yu, X.M. Pu, X.Q. Yu, Lipase-initiated
tandem biginelli reactions via in situ-formed acetaldehydes in one pot: discovery of
single-ring deep blue luminogens, Adv. Synth. Catal. 359 (2017) 3397–3406.
[39] Z.J. Yang, Q.T. Gong, Y. Wang, Y. Yu, Y.H. Liu, N. Wang, X.Q. Yu, Biocatalytic
tandem multicomponent reactions for one-pot synthesis of 2-Amino-4H-Pyran
library and in vitro biological evaluation, Mol. Catal. 491 (2020), 110983.
[14] J.M.R. Narayanam, C.R.J. Stephenson, Visible light photoredox catalysis:
applications in organic synthesis, Chem. Soc. Rev. 40 (2011) 102–113.
[15] J. Xuan, W.J. Xiao, Visible-light photoredox catalysis, Angew. Chem. Int. Edit. 51
(2012) 6828–6838.
8