C2-arylacylation of 2H-benzothiazoles with methyl arenes via Selectfluor oxidation

Article abstract of DOI:10.1016/j.tetlet.2021.153184

An efficient Selectfluor-oxidative protocol was developed for the direct C2-arylacylation of 2H-benzothiazoles via the radical reaction of 2H-benzothiazoles with methyl arenes. Selectfluor can effectively promote the oxidative cross-coupling without an ex

Full text of DOI:10.1016/j.tetlet.2021.153184

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
C2-arylacylation of 2H-benzothiazoles with methyl arenes via
Selectfluor oxidation
⇑
Qi Lu, Xiao-Tong Sun, Yao-Lei Kong, Jin-Chuan Liu, Bo Chen, Jian-Quan Weng
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient Selectfluor-oxidative protocol was developed for the direct C2-arylacylation of 2H-benzoth-
iazoles via the radical reaction of 2H-benzothiazoles with methyl arenes. Selectfluor can effectively pro-
mote the oxidative cross-coupling without an external catalyst. This arylacylation reaction tolerates a
wide range of functional groups affording 28 examples of the arylacylated products in 39–81% yield.
Ó 2021 Elsevier Ltd. All rights reserved.
Received 10 March 2021
Revised 8 May 2021
Accepted 14 May 2021
Available online xxxx
Keywords:
Arylacylation
2H-Benzothiazole
Methyl arenes
Selectfluor
Oxidation
Introduction
O
OMe
OMe
O
OH
Cl
OMe
Aryl ketones, including arylacylated heterocycles, are important
structural motifs found in various bioactive compounds, pharma-
ceuticals and natural products [1–4] (Fig. 1). Amongst them, C2-
arylacylated 2H-benzothiazoles exhibit various pharmacological
effects, such as antiviral [5], antitumor [6–8] and antidiabetic
activities [9,10].
Therefore, the development of efficient strategies for the syn-
thesis of C2-arylacylated 2H-benzothiazoles has received much
attention. The reported methods mostly depend on the tandem
reactions of 2-aminothiophenols, 2-halonitroarenes, 2-haloanilines
or anilines with diverse substrates including phenyl-acetaldehydes
[11], 1,1-dibromoethenes [12], acetophenones [13–16], ary-
OH
MeO
MeO
HO
Me
OH
Griseophenone B
Tatarinoid A
HO
HO
OH
O
S
N
N
OH
OH
O
OH
17-HSD1 inhibitor
IGFBPs inhibitor
lacetylenes [17–19], -hydroxyacetophenones [19,20], enami-
a
nones [21] and a,a-dihaloketones [22]. However, these
Fig. 1. Selected pharmaceuticals and natural products containing the aryl ketone
methodologies generally require prefunctionalized reactants and
complicated handling procedures, which limits their applications.
In 2013, Wu and co-workers [23] reported the I2/KOH-catalyzed
direct ring-opening arylacylation of 2H-benzothiazoles with aryl
ketones at 100 °C (Scheme 1a). Chen and co-workers [24] also
reported the C2-arylacylation of 2H-benzothiazoles using K2S2O8
as the oxidant and FeCl3Á6H2O as the catalyst at 100 °C
(Scheme 1b). In 2014, a CuI-catalyzed reaction under a nitrogen
atmosphere was developed by Song and co-workers [25]
moiety.
(Scheme 1c). In 2019, Yin and co-workers [26] reported the C2-ary-
lacylation of 2H-benzothiazoles under the synergistic catalysis of i-
PrMgClÁLiCl and CDI (Scheme 1d). In 2020, Ablajan and co-workers
[27] reported a I2/TBHP synergistically catalyzed method for the
synthesis of 2-arylacylated 2H-benzothiazoles (Scheme 1e).
It should be noted that these methods typically use aryl ketones
as the carbonyl sources, which are sensitive to oxidants and reduc-
tants. Selectfluor is known as a powerful oxidant (SCE = 0.33 V),
possessing notable properties such as high thermal stability, low
⇑
Corresponding author.
E-mail address: jqweng@zjut.edu.cn (J.-Q. Weng).
https://doi.org/10.1016/j.tetlet.2021.153184
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
Please cite this article as: Q. Lu, Xiao-Tong Sun, Yao-Lei Kong et al., C2-arylacylation of 2H-benzothiazoles with methyl arenes via Selectfluor oxidation,
Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2021.153184

Q. Lu, Xiao-Tong Sun, Yao-Lei Kong et al.
Tetrahedron Letters xxx (xxxx) xxx
Previous Work
toxicity, good solubility and stability in polar solvents, and signifi-
cant attention has been paid to its applications in organic chem-
istry [28–30]. Based on our previous work [31], herein we report
a convenient and efficient approach for the direct C2-arylacylation
of 2H-benzothiazoles with methyl arenes via Selectfluor oxidation.
Compared with aryl ketones, methyl arenes are more readily avail-
able, more chemically stable and less expensive. Our process also
has the advantages of mild reaction conditions and being transition
metal-free.
a) Wu and co-workers
[23]
O
O
N
S
N
I2 KOH
,
R1
R1
+
R2
DMSO:H2O=3:1
100 °C
R2
S
R2=(hetero)aryl
b) Chen and co-workers
[24]
[25]
O
N
O
R1
N
FeCl3•6H2O
R1
+
S
R2
DMSO:H2O=2:1
S
R2
K2S2O
8 , 100 °C
Results and discussion
c) Song and co-workers
O
O
N
N
R1
CuI, HBF
R2 DMSO, 130 °C
4, N2
Initially, we chose 2H-benzothiazole (1a) and toluene (2a) as
model substrates to optimize the reaction conditions (Table 1).
When Selectfluor (2 eq.) was used as the oxidant, trifluoroacetic
acid (TFA) (1.5 eq.) as the acid and acetonitrile (MeCN) as the sol-
vent at 80 °C for 8 h, the desired product (3aa) was obtained in 47%
yield (Entry 1). Next, various solvents were screened. However, the
reactions without solvent or with solvents such as N,N-dimethyl-
formamide (DMF), dimethyl sulfoxide (DMSO) and H2O did not
give any product (Entries 2–5). The mixed solvents CH3CN/H2O
with different ratios gave lower yields compared to that of CH3CN
(Entries 6–7). Other acid such as p-toluenesulfonic acid monohy-
drate (TsOHÁH2O), hydrochloric acid (HCl) and acetic acid (AcOH)
were less effective (Entries 8–10). In addition, the amounts of the
oxidant, acid and solvent were examined; however, lower yields
were found in all cases (Entries 11–18). Furthermore, lower yields
were obtained when the reaction temperature was decreased to
70 °C or increased to 90 °C (Entries 19–20). Gratifyingly, the yield
increased to 58% when the reaction time was extended from 8 h to
10 h, but no significant improvement was observed when the reac-
tion time was increased to 12 h (Entries 21–22). The reaction con-
ducted under a N2 atmosphere resulted in a lower yield of the
desired product, which showed that air was necessary for the pro-
R1
+
S
S
R2
d) Yin and co-workers
[26]
O
O
O
N
S
N
i-PrMgCl•LiCl
R1
R1
+ HO
R2
, THF, r.t.
CDI
S
R2
e) Ablajan and co-workers
[27]
O
N
S
N
I2 TBHP
,
R1
, N2
R1
+
R2 DMSO, 60 °C
S
R2
This Work
O
N
S
R1
Selectfluor, TFA
MeCN, 80 °C, air
N
S
R1
+
R2
R2
Scheme 1. Selected arylacylation reactions of 2H-benzothiazoles.
Table 1
Optimization of the reaction conditions.a
O
N
S
N
S
Selectfluor, Acid
Solvent
H
H
+
1a
2a
3aa
Entry
Oxidant (eq.)
Acid (eq.)
Solvent (mL)
Yield 3aab (%)
1
2
3
4
5
6
7
8
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (1.5)
Selectfluor (2.5)
Selectfluor (3)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
Selectfluor (2)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TsOHÁH2O (1.5)
HCl (1.5)
AcOH (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (0.5)
TFA (1.0)
TFA (2.0)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
TFA (1.5)
MeCN (2)
None
DMF (2)
DMSO (2)
H2O (2)
MeCN:H2O = 1:1 (2)
47%
n.r.
n.r.
n.r.
n.r.
17%
45%
5%
n.r.
n.r.
17%
25%
19%
22%
35%
33%
25%
32%
43%
45%
58%
55%
28%
n.r.
MeCN:H2O = 100:1 (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (1)
MeCN (3)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
MeCN (2)
9
10
11
12
13
14
15
16
17
18
19c
20d
21e
22f
23e,g
24e,h
2

Q. Lu, Xiao-Tong Sun, Yao-Lei Kong et al.
Tetrahedron Letters xxx (xxxx) xxx
a
Reagents and conditions: 2H-benzothiazole (1a, 0.3 mmol), toluene (2a, 1.5 mL), Selectfluor, acid, solvent, 80 °C, 8 h.
Isolated yield based on 2H-benzothiazole.
70 °C.
90 °C.
10 h.
12 h.
N2 atmosphere.
b
c
d
e
f
g
h
Irradiated with 30 W Blue LEDs (k = 405 nm) at room temperature.
tocol (Entry 23). Further investigation demonstrated that the
oxidative coupling irradiated with visible light at room tempera-
ture was ineffective (Entry 24).
Moreover, a gram-scale synthesis was carried out. Gratifyingly,
the reaction efficiency was not significantly affected, and the target
product 3aa was obtained in 50% yield when the model reaction
was performed on a 10 mmol scale (Scheme 2).
O
N
S
N
Selectfluor (2.0 equiv.)
H
H
+
S
TFA (1.5 equiv.)
MeCN, 80 °C, air
1a,
1.35 g
2a,
15 mL
3aa,
50%
Scheme 2. Gram-scale synthesis.
Table 2
Substrate scope of the methyl arenes.
O
N
S
N
Selectfluor (2.0 equiv.)
H
H
+
R
TFA (1.5 equiv.)
MeCN, 80 °C, air
S
1a
3
R
2
O
O
N
N
S
O
N
S
S
3aa
, 58%
3ac
3ab
, 74%
, 81%
O
O
O
N
S
N
S
O
O
O
N
S
Cl
3ad
, 63%
3ae
, 58%
3af
, 60%
Cl
Br
F
N
N
S
O
O
N
S
Cl
Br
S
Br
F
3ag
, 47%
3ah
, 56%
3ai
, 55%
O
N
N
S
N
S
S
3al
, 61%
3ak
3aj
, 57%
, 45%
O
O
N
S
N
S
O
N
S
F
3am
, 50%
3an
, trace
3ao
, n. d.
OMe
O
N
S
O
N
S
O
N
S
OH
OH
3ap
3aq
, n. r.
, n. r.
3ar
, n. r.
OH
aReagents and conditions: 2H-benzothiazole (1a, 0.3 mmol), methyl arenes (2, 1.5 mL), Selectfluor (2.0 eq.,
0.6 mmol), TFA (1.5 eq., 0.45 mmol), MeCN (2 mL), 80 °C, 10 h.
3

Q. Lu, Xiao-Tong Sun, Yao-Lei Kong et al.
Tetrahedron Letters xxx (xxxx) xxx
With the optimized reaction conditions in hand, the scope of
substituted methyl arenes (2) was first explored for the arylacyla-
tion of 2H-benzothiazole (1a). As shown in Table 2, monosubsti-
tuted toluene derivatives bearing a methyl group or halogens
were compatible with this oxidative coupling system (3ab-am).
The arylacylation of 2H-benzothiazole with toluene derivatives
containing the electron-donating methyl group gave higher yields
than those containing electron-withdrawing halogens (3ab-ad vs
3ae-am). Notably, substituents at the meta- and para-positions
have no significant effect on the yields (3ab, 3ac, 3ae, 3af, 3ah,
3ai, 3ak and 3al). However, substitution at the ortho-position gen-
erally gave the corresponding arylacylated products in lower yields
(3ad, 3ag, 3aj and 3am). Unfortunately, the reaction of 2H-ben-
zothiazole with mesitylene only gave trace amounts of the target
product (3an) and none of the desired product was detected using
1-methoxy-4-methylbenzene (3ao). Also, p-cresol, m-cresol and
3,5-dimethylphenol were unreactive under the current reaction
conditions (3ap, 3aq, 3ar).
We further investigated the substrate scope of various substi-
tuted 2H-benzothiazoles (1) and substituted methyl arenes (2)
(Table 3). Gratifyingly, toluene reacted with substituted 2H-ben-
zothiazoles containing electron-withdrawing substituents (nitro,
chloro, cyano and acetyl), affording the desired products in 39–
68% yield (3ba-bd). Meanwhile, 2H-benzothiazoles bearing elec-
tron-donating groups (6-methoxy, 7-methoxy) gave the desired
products in 70–75% yield (3be, 3bf). The reactions of substituted
2H-benzothiazoles with various methyl arenes were also investi-
gated and the desired products were obtained in moderate to good
yields (3bg-bm). These results showed that the yields of 2H-ben-
zothiazoles with electron-donating groups were generally higher
than those of 2H-benzothiazoles with electron-withdrawing
groups. Unfortunately, the desired reaction did not occur between
6-aminobenzothiazole and toluene (3bo), presumably because the
amino group is readily oxidizable. In addition, the desired reaction
also did not occur between 6-hydroxybenzothiazole and toluene
(3bp).
To gain further insights into the reaction mechanism, a series of
control experiments were carried out. A radical trapping experi-
ment was performed by the addition of TEMPO (2,2,6,6-tetram-
ethyl-1-piperidinyloxy) under the optimized reaction conditions
(Scheme 3A). This result showed that when 2 equivalents of
TEMPO were added to the arylacylation reaction, the coupling pro-
cess was completely inhibited. At the same time, a TEMPO-trapped
complex from benzyl radical (4c) was detected by LC-MS, which
indicated that a radical process may be involved in the coupling.
Notably, the TEMPO-trapped complexes of 2H-benzothiazole radi-
cal (4a) and benzaldehyde radical (4b) were not observed. Next,
TEMPO was added to the catalytic system, and the fluorine radical
adduct TEMPO-F was detected by LC-MS. This result demonstrated
that the N–F activation of Selectfluor could be achieved under
heating (Scheme 3B) [32]. The reaction of 2H-benzothiazole with
toluene gave 2-benzylbenzo[d]thiazole (1ac) in 70% yield under a
N2 atmosphere (Scheme 3C). 2-Benzylbenzo[d]thiazole (1ac)
underwent oxidation in air to give the target product 3aa in 72%
yield (Scheme 3D). This result indicated that air is required in this
reaction (see the ESI for further details).
Based on the above experimental results, a plausible mecha-
nism for this arylacylation reaction was proposed (Scheme 4). It
was previously reported that Selectfluor could serve as an efficient
Lewis acid catalyst and powerful oxidant [33–35]. Thus, the N-F
bonds of Selectfluor cleaved homolytically upon heating producing
an N radical cation and an F radical [36,37]. The electrophilic N-
radical cation then abstracts a hydrogen atom from toluene 2a to
provide methyl radical 2aa [38,39]. Meanwhile, 2H-benzothiazole
1a is protonated by acid to form 1aa, which can capture the rela-
tively nucleophilic methyl radical 2aa and provides the corre-
sponding radical adduct 1ab [40]. Oxidation and deprotonation
of 1ab by another molecule of Selectfluor then affords the coupling
Table 3
Substrate scope of the 2H-benzothiazoles.
O
N
S
R1
N
Selectfluor (2.0 equiv)
H
R1
H
+
R2
TFA (1.5 equiv)
MeCN, 80 °C, air
S
R2
3
1
2
O
O
N
O
Cl
N
O
N
S
O
O
N
S
S
O2N
NC
Cl
S
3bd
, 66%
3bc
, 68%
3ba
, 39%
3bb
, 63%
O
O
N
N
N
S
O
N
S
S
S
MeO
MeO
MeO
Cl
OMe
3bf
3bg
, 67%
3bh
, 70%
3be
, 75%
, 70%
O
N
S
O
N
S
Cl
O
N
S
O
N
S
MeO
3bi
, 63%
3bk
3bl
, 53%
, 61%
3bj
, 65%
Cl
Cl
Br
Br
Cl
O
O
N
S
N
S
O
O
N
S
N
S
MeO
H2N
HO
3bm
, 45%
3bn
, 50%
3bo
, n. d.
3bp
, n. d.
F
F
aReagents and conditions: 2H-benzothiazole (1a, 0.3 mmol), methyl arenes (2, 1.5 mL), Selectfluor (2.0 eq.,
0.6 mmol), TFA (1.5 eq., 0.45 mmol), MeCN (2 mL), 80 °C, 10 h.
4

Q. Lu, Xiao-Tong Sun, Yao-Lei Kong et al.
Tetrahedron Letters xxx (xxxx) xxx
O
N
S
N
S
3aa,
n. d.
TEMPO (2.0 equiv.)
Selectfluor (2.0 equiv.)
TFA (1.5 equiv.)
N
S
1a
+
O
(A)
+
O
N
N
MeCN, 80 °C, air
4c
4a,
n. d.
O
Detected by LC-MS
2a
O
N
4b,
n.d.
TEMPO (2.0 equiv.)
TFA (1.5 equiv.)
N
O
N
N F
(B)
F
Cl
MeCN, 80 °C, air
Selectfluor
TEMPO-F
Detected by LC-MS
Selectfluor (2.0 equiv.)
TFA (1.5 equiv.)
MeCN, 80 °C, N2
N
S
N
+
(C)
(D)
S
1a
2a
1ac,
70%
Selectfluor (2.0 equiv.)
TFA (1.5 equiv.)
N
S
O
N
S
MeCN, 80 °C, air
1ac
3aa,
72%
Scheme 3. Mechanistic experiments.
2a
N
N F
N
N
Cl
Cl
2aa
H
N
F
N
N H
Cl
S
H
N
N
S
1ab
TFA
S
1a
1aa
- e- and - 2H+
O
N
N
S
air
Selectfluor
S
3aa
1ac
Scheme 4. Plausible reaction mechanism.
product 1ac. Finally, oxidation of 1ac in air affords the desired ary-
lacylated product 3aa [41].
protocol practical for the synthesis of C2-arylacylated 2H-
benzothiazoles.
In conclusion, we have developed a convenient and efficient
approach for the direct C2-arylacylation of 2H-benzothiazoles with
various methyl arenes via Selectfluor oxidation. This arylacylation
reaction tolerates a wide range of functional groups affording 28
examples of the arylacylated products in 39–81% yield. Moreover,
the direct coupling of the 2H-benzothiazoles without prefunction-
alized substrates and without transition-metal catalysts make this
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
5

Q. Lu, Xiao-Tong Sun, Yao-Lei Kong et al.
Tetrahedron Letters xxx (xxxx) xxx
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153184.
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