Visible-light induced decarboxylative C2-alkylation of benzothiazoles with carboxylic acids under metal-free conditions

Article abstract of DOI:10.1039/c8ob02476f

An effective protocol of photoredox catalyzed C2-alkylation of benzothiazoles with aliphatic carboxylic acids was disclosed. In the presence of an acridinium salt as a photocatalyst and air as an oxidant, a wide range of secondary or tertiary aliphatic ca

Full text of DOI:10.1039/c8ob02476f

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Visible-light induced decarboxylative
C2-alkylation of benzothiazoles with carboxylic
Cite this: DOI: 10.1039/c8ob02476f
acids under metal-free conditions†
a
a
a
a
a,b
Bin Wang, Pinhua Li, * Tao Miao, Long Zou and Lei Wang
*
Received 5th October 2018,
Accepted 30th November 2018
An effective protocol of photoredox catalyzed C2-alkylation of benzothiazoles with aliphatic carboxylic
acids was disclosed. In the presence of an acridinium salt as a photocatalyst and air as an oxidant, a wide
range of secondary or tertiary aliphatic carboxylic acids were employed as alkylation reagents, providing the
desired products in good to excellent yields under mild reaction conditions with a broad substrate scope.
DOI: 10.1039/c8ob02476f
rsc.li/obc
alkylated benzothiazoles with various functional groups under
metal-free conditions remains highly desirable.
Introduction
Benzothiazoles are widespread in various natural products
Carboxylic acids are abundant, non-toxic and renewable
from agriculture and pharmaceutical agents to materials feedstocks in organic synthesis, and are regarded as con-
1
4
sciences. Synthetic methods which enable the selective venient radical precursors. Moreover, radicals are produced
functionalization of the C–H bonds of benzothiazoles could upon decarboxylation, and the elimination of CO , as a trace-
2
greatly facilitate their applications in these areas. Recently, less by-product, does not have an impact on their reactivity.
transition-metal-catalyzed direct introduction of secondary Decarboxylative radical generation dated back to the classical
and tertiary alkyl groups into benzothiazoles offered a power- studies of Hunsdiecker, who found that aliphatic, unsaturated,
2
ful tool for the synthesis of alkyl-substituted benzothiazoles.
and aromatic carboxylic acids could efficiently undergo radical
5
For example, Miura and co-workers reported an efficient alkyl- halogenative decarboxylation in the presence of silver salts.
ation protocol of benzothiazoles with N-tosylhydrazones Subsequently, radical decarboxylative functionalization was
bearing unactivated alkyl groups using nickel and cobalt as widely used in organic synthesis, but an excess amount of
catalysts. In 2014, Zhao’s group developed an Ag-catalyzed strong oxidant and UV-light irradiation were usually applied as
alkylation of benzothiazoles with carboxylic acids through a radical initiation conditions. In the past few years, visible-
direct decarboxylative cross-coupling reaction. Nevertheless, light photoredox catalysis has emerged as a valuable platform
most of the strategies were carried out by utilizing precious for the design of a variety of radical reactions under mild reac-
and/or toxic metal complexes as catalysts or oxidants, leading tion conditions and low-energy irradiation. Recently, the
2
a
6
2
b
7
to the difficult purification of the desired products from application of this strategy to radical decarboxylation reactions
residual metal catalysts. Furthermore, some groups also has gathered much attention, and remarkable achievements
8
described examples of direct C–H bond functionalization via have already been demonstrated. Among these, several
cross dehydrogenative coupling processes without the use of a examples of the visible-light-induced decarboxylation of ali-
transition metal catalyst, yet these reactions often rely on the phatic acids have been reported for cross-coupling reactions to
9
additives in the system to promote the reactions, and are date. However, the reactions often involve the use of Ir- and
limited in the substrate scope to cyclic and benzyl frameworks Ru-complexes as photosensitizers and hypervalent iodine
3
(
cycloalkanes and toluene derivatives). Therefore, the develop- reagents as stoichiometric sacrificial oxidants in most of the
2
ment of efficient methods for the straightforward synthesis of cases. Furthermore, molecular oxygen (O ), as an inexpensive,
available and environmentally friendly oxidant, has scarcely
been investigated in the visible-light-induced decarboxylative
cross-coupling reactions under metal-free conditions.
Herein, we wish to report an effective photoredox decarbox-
ylative method for the direct C2-alkylation of benzothiazoles
a
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000,
P. R. China. E-mail: pphuali@126.com, leiwang@chnu.edu.cn
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
with readily available aliphatic carboxylic acids using
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
+
9
-mesityl-10-methylacridinium perchlorate ([Acr -Mes]ClO ) as
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
4
c8ob02476f
the photocatalyst and air as the oxidant under blue LED
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a
Table 1 Optimization of the reaction conditions
b
Entry Photocatalyst
Base
Solvent
Yield (%)
+
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
[Acr -Mes]ClO
4
4
4
Na
Na
Na
Na
Na
Na
Na
Na
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
HPO
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
H
2
O
35
58
83
0
0
0
0
0
0
25
24
55
51
31
40
0
+
[Acr -Mes]ClO
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
CN
+
[Acr -Mes]ClO
CN/H
CN/H
CN/H
CN/H
CN/H
CN/H
CN/H
2
2
2
2
2
2
2
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
O (1 : 1)
3 2
Ru(bpy) Cl
fac-Ir(ppy)
Eosin Y
Rose bengal
Acridine red
Scheme 1 Decarboxylative C2-alkylation of benzothiazoles with car-
boxylic acids.
3
irradiation at room temperature, providing the desired pro-
ducts in good to excellent yields with a broad substrate scope
Methylene blue Na
+
0
1
2
3
4
5
6
7
[Acr -Mes]ClO
4
4
4
4
4
4
4
4
4
4
4
4
4
4
Na
Na
Na
Na
Na
Na
Na
DMSO/H
DMF/H O (1 : 1)
Acetone/H O (1 : 1)
OH/H O (1 : 1)
THF/H O (1 : 1)
Dioxane/H O (1 : 1)
2
O (1 : 1)
+
[Acr -Mes]ClO
2
(Scheme 1).
+
[Acr -Mes]ClO
2
+
[Acr -Mes]ClO
C
2
H
5
2
+
[Acr -Mes]ClO
2
+
[Acr -Mes]ClO
2
Results and discussion
+
[Acr -Mes]ClO
CO
3
CH
CH
3
CN/H
CN/H
2
O (1 : 1)
O (1 : 1)
+
[Acr -Mes]ClO
K
2
CO
3
3
2
0
+
Initially, a model reaction of benzothiazole (1a) and pivalic 18
acid (2a) was chosen to optimize the reaction conditions, and
[Acr -Mes]ClO
Et N
CH CN/H O (1 : 1)
56
39
3
3
3
3
3
2
2
2
2
+
19
20
21
[Acr -Mes]ClO
DIPEA
Na
Na
CH
CH
CH
CN/H
CN/H
CN/H
O (1 : 1)
O (1 : 1)
O (1 : 1)
+
c
[Acr -Mes]ClO
2
HPO
HPO
4
4
4
4
4
0
84
68
0
the results are shown in Table 1. In the presence of 9-mesityl-
+
d
[Acr -Mes]ClO
2
+
+
e
1
0-methyl acridinium perchlorate ([Acr -Mes]ClO
4
, 3 mol%) as 22
[Acr -Mes]ClO
Na HPO
CH CN/H O (1 : 1)
2
3
3
3
3
2
2
2
2
+
f
23
24
25
[Acr -Mes]ClO
Na
Na
—
2
HPO
HPO
CH
CH
CH
CN/H
CN/H
CN/H
O (1 : 1)
O (1 : 1)
O (1 : 1)
2 4 2
a photocatalyst and Na HPO ·12H O as a base, this alkylation
—
2
0
0
reaction could occur in water at room temperature by
+
[Acr -Mes]ClO
4
+
g
irradiation with a blue light emitting diode (LED, 410–415 nm, 26
.5 W), generating the desired product 3a in 35% yield
Table 1, entry 1). By using CH CN instead of H O as a solvent,
[Acr -Mes]ClO
Na HPO
CH CN/H O (1 : 1)
0
4
2
4
3
2
1
a
Reaction conditions: Benzothiazole (1a, 0.5 mmol), pivalic acid (2a,
1.5 mmol), photocatalyst (3 mol%), base (3.0 equiv.) and solvent
8% yield of 3a was obtained (Table 1, entry 2). Much to our (3 mL) at room temperature under blue LED (410–415 nm, 1.5 W)
(
3
2
5
b
c
irradiation for 36 h in air. Isolated yield based on 1a.
N
2
atmo-
pleasure, an isolated yield of 83% was achieved when a
d
e
f
sphere.
LED (530 ± 5 nm). Without light irradiation. Commercially available
HPO ·12H O was used in all reactions.
O atmosphere. 1.5 W blue LED (450 ± 5 nm). 1.5 W green
2
mixture of acetonitrile and H O (1 : 1 volume ratio) was used
g
2
as the solvent (Table 1, entry 3). Other generally used photoca- Na
2
4
2
talysts, such as Ru(bpy)
3 2 3
Cl , fac-Ir(ppy) , eosin Y, acridine red
or methylene blue was employed as the photocatalyst instead
+
of [Acr -Mes]ClO
in the model reaction, but no desired (530 ± 5 nm) (Table 1, entry 23). It should be noted that no
4
product 3a was observed (Table 1, entries 4–9). The effect of product was detected in the absence of a photocatalyst, visible
solvent was also investigated in detail. The mixed solvent of light or a base (Table 1, entries 24–26). All these features
H
2
O with DMSO (dimethyl sulfoxide), DMF (N,N-dimethyl- demonstrate that this radical reaction is a photochemical
formamide), acetone, ethanol, THF (tetrahydrofuran) or 1,4- process. In addition, the reactions were carried out in an open
dioxane in the 1 : 1 volume ratio showed poor performance to air tubed vessel in our experiments. The internal temperature of
the model reaction, providing product 3a in 24–55% yields the reaction mixture was measured and found to be 30 ± 1 °C.
(Table 1, entries 10–15). Further exploration of a variety of
With the optimized reaction conditions in hand, a series of
commercially available bases indicated that Na HPO ·12H O substituted benzothiazoles were examined to establish the sub-
2
4
2
was superior to the others, including Na
2
CO
3
, K
2
CO
3
, Et
3
N and strate scope. As can be seen from Scheme 2, benzothiazoles
DIPEA (Table 1, entries 16–19). Inorganic bases Na
2 3
CO
and substituted at the C6-position with electron-donating groups
K CO were not suitable ones and the model reaction was pro- (Me, MeO and AcNH) provided the desired products 3b–3d in
2
3
hibited completely. Organic bases Et
3
N and DIPEA gave moderate to good yields. It is important to note that this reac-
inferior results and led to the formation of 3a in 56% and 39% tion was compatible with electron-withdrawing substituents
yields, respectively. Furthermore, the role of atmospheric including Br and Cl on the aromatic rings, furnishing 76–90%
oxygen was also observed as an essential parameter in the for- yields of products 3e–3i, which can undergo further transform-
mation of 3a because there was no reaction under the N
sphere under the given conditions (Table 1, entries 20 and 21). benzothiazoles bearing a trifluoromethyl (CF
Under the irradiation of a 450–455 nm LED, 68% yield of 3a a useful structural motif in biologically active molecules,
2
atmo- ation for the synthesis of more complex molecules. Notably,
) group, which is
3
1
0
was obtained (Table 1, entry 22). Similarly, the reaction also reacted with 2a to generate the corresponding product 3j in
did not proceed under irradiation with a 1.5 W green LED 75% yield. Furthermore, 5-phenyl benzothiazole was also suc-
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Scheme 2 The scope of benzothiazoles [reaction conditions: ben-
zothiazole (1, 0.5 mmol), pivalic acid (2a, 1.5 mmol), 9-mesityl-10-
+
methyl
Na HPO
acridinium
perchlorate
([Acr -Mes]ClO
4
,
3
mol%),
2
4
·12H O (3.0 equiv.) and CH
2
3
CN/H
2
O (1 : 1, 3 mL) at room temp-
erature under 1.5 W blue LED (410–415 nm) irradiation for 36 h in air;
isolated yield of the product].
Scheme 3 The scope of carboxylic acids [reaction conditions: ben-
zothiazole (1, 0.5 mmol), carboxylic acid (2, 1.5 mmol), 9-mesityl-10-
+
methyl
Na HPO
acridinium
perchlorate
([Acr -Mes]ClO
4
,
3
mol%),
cessful in this transformation and 70% yield of the corres-
ponding product 3k was isolated.
2
4
·12H O (3.0 equiv.) and CH
2
3
CN/H
2
O (1 : 1, 3 mL) at room temp-
erature under 1.5 W blue LED (410–415 nm) irradiation for 36 h in air;
Next, the scope of the readily available aliphatic carboxylic
acids was investigated. The reaction of tertiary alkyl carboxylic
acids proceeded smoothly with the construction of a quatern-
ary carbon center to generate the corresponding products 3l–
isolated yield of the product].
under the optimized conditions (Scheme 3). No desired
product 3a was found upon the addition of TEMPO, whereas
the tert-butyl radical of pivalic acid formed in situ was trapped
by TEMPO to form a radical adduct (4), which was determined
by HPLC-HRMS analysis (Scheme 4). The result implied that a
radical process was involved in this transformation.
3
v in good to excellent yields. It should be noted that adaman-
tane-1-carboxylic acid is an excellent reaction partner, and
2
7
-adamantan-1-yl-benzothiazoles (3o–3v) were obtained in
8–92% yields under the present reaction conditions. Due to
the steric bulkiness of adamantane, radical chemistry is very
convenient for its transformations and this moiety has been
quintessential for the development of new drugs and cata-
On the basis of the above results, fluorescence quenching
experiments and previous reports, a plausible mechanism
13
12
lysts. Furthermore, tolerance to a variety of functionalities
such as Br, Cl and ether is particularly significant. In addition,
secondary radicals also reacted well to form 3w–3ab in moder-
ate to good yields. For example, secondary alkyl groups includ-
ing 2-butyl, 2-pentyl, 3-pentyl, 2-hexyl and cyclohexyl were suc-
cessfully incorporated into benzothiazoles, forming the
desired products 3w–3ab. When a primary carboxylic acid,
such as n-butyric acid was used in the reaction, no desired
product (3ac) was isolated. These results demonstrate that the
present photocatalytic system enables the selective generation
of various alkyl radicals from the corresponding aliphatic car-
boxylic acids except for the primary carboxylic acid.
for this reaction is proposed in Scheme 5. Initially, a single
electron oxidation of aliphatic carboxylic acid (2) by an excited-
+
state [Acr -Mes]* formed under blue LED irradiation and
sequential decarboxylation occur to give the alkyl radical (A)
+
•
and [Acr -Mes] , which was then oxidized by O to complete
2
To clarify the reaction mechanism, 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO), as a radical-trapping reagent, was
added into the reaction system to capture the intermediate Scheme 4 The radical-trapping experiment.
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All reactions were set up in air. Methyl, bromo- and chloro-
substituted benzothiazoles were prepared from the corres-
1
1
ponding substituted 2-aminobenzothiazoles.
All other
chemicals were purchased commercially and used without
further purification. Products were purified by flash chromato-
graphy on 200–300 mesh silica gels, SiO . Melting points were
2
determined on a digital melting point apparatus and tempera-
tures were uncorrected.
Typical procedure for the photoredox induced decarboxylative
C2-alkylation of benzothiazoles
A tubed vessel containing a stir bar was charged with ben-
+
zothiazole (0.50 mmol), carboxylic acid (1.5 mmol), [Acr -Mes]
Scheme 5 Proposed mechanism.
ClO (3 mol%), Na HPO ·12H O (1.5 mmol) and CH CN/H O
4
2
4
2
3
2
(
1 : 1, 3.0 mL). The reaction vessel was exposed to a blue LED
•−.
(410–415 nm, 1.5 W) at room temperature for 36 h. Upon com-
pletion of the reaction, the mixture was diluted with water
the photocatalytic cycle along with the generation of O
2
Subsequently, the obtained A was added to the 2-position of
the excited state 1a* which was from the reaction of 1a and
Acr -Mes]* via energy transfer, providing intermediate B. B
underwent an aromatization through an oxidation in the pres-
ence of O2 to generate the desired product 3 via intermediate
C. Notably, alkyl radical (A) was trapped by TEMPO to form a
radical adduct (4), determined by HPLC-HRMS analysis. After
the reaction was complete, the reaction solution was detected
with a starch potassium iodide test paper and the test paper
(
3 mL) and extracted with ethyl acetate (10 mL × 3). The
+
organic layer was combined, dried over anhydrous Na SO ,
2
4
[
and filtered. The solvent was removed via a rotary evaporator
under reduced pressure and the residue was purified by flash
chromatography (silica gel, a gradient eluent of petroleum
ether/ethyl acetate) to yield the desired product.
•
−
Characterization data for the products
2
-(tert-Butyl)benzo[d]thiazole (3a). Yellow liquid (79.4 mg,
changed into blue colour, indicating that H
the reaction.
2 2
O was formed in
1
8
3%). H NMR (CDCl
3
, 400 MHz) δ: 7.99 (d, J = 8.2 Hz, 1H),
7
.84 (d, J = 7.8 Hz, 1H), 7.43 (td, J = 7.7, 1.1 Hz, 1H), 7.32 (td,
1
3
J = 7.6, 1.1 Hz, 1H), 1.52 (s, 9H); C NMR (CDCl , 100 MHz)
3
δ: 181.8, 153.3, 135.0, 125.7, 124.5, 122.7, 121.4, 38.3, 30.7.
Conclusions
+
HRMS (ESI) [M + H] calcd for C11
H
14NS: 192.0841, found:
1
92.0841.
In summary, a novel and efficient photoredox induced decar-
boxylative C2-alkylation of benzothiazoles was developed by
using a catalytic amount of 9-mesityl-10-methyl acridinium
perchlorate (3 mol%) as a photocatalyst and air as an oxidant
with the irradiation of a blue LED under transition-metal free
conditions at room temperature. A variety of secondary and ter-
tiary alkyl groups could be efficiently incorporated into ben-
zothiazoles at the C2-position in good yields. The approach
has advantages in terms of experimental simplicity, mild reac-
tion conditions and easy work-up. Further expansion of this
method to a broader spectrum of substrates is underway.
2-(tert-Butyl)-6-methylbenzo[d]thiazole (3b). Yellow liquid
1
(
77.1 mg, 75%). H NMR (CDCl
3
, 400 MHz) δ: 7.86 (d, J =
8
3
1
.3 Hz, 1H), 7.61 (s, 1H), 7.23 (dd, J = 8.3, 1.1 Hz, 1H), 2.45 (s,
H), 1.50 (s, 9H); C NMR (CDCl
35.1, 134.4, 127.2, 122.1, 121.1, 38.2, 30.7, 21.4. HRMS (ESI)
1
3
3
, 100 MHz) δ: 180.6, 151.3,
+
[M + H] calcd for C H NS: 206.0998, found: 206.0999.
12 16
2-(tert-Butyl)-6-methoxybenzo[d]thiazole (3c). Yellow liquid
1
(
66.4 mg, 60%). H NMR (CDCl
3
, 400 MHz) δ: 7.86 (d, J =
8
1
.9 Hz, 1H), 7.30 (d, J = 2.5 Hz, 1H), 7.03 (dd, J = 8.9, 2.5 Hz,
H), 3.85 (s, 3H), 1.50 (s, 9H); C NMR (CDCl , 100 MHz)
3
1
3
δ: 179.2, 157.2, 147.7, 136.2, 123.1, 114.8, 104.2, 55.7, 38.1,
+
30.7. HRMS (ESI) [M + H] calcd for C H NOS: 222.0947,
12 16
found: 222.0942.
N-(2-(tert-Butyl)benzo[d]thiazol-6-yl)acetamide (3d). Yellow
solid (74.4 mg, 60%). M.p. 114–115 °C. H NMR (CDCl3,
Experimental section
General remarks
1
1
13
All H NMR and C NMR spectra were recorded on a 400 MHz 400 MHz) δ: 8.39 (d, J = 1.9 Hz, 1H), 8.37 (s, 1H), 7.83 (d, J =
Bruker FT-NMR spectrometer (400 MHz or 100 MHz, respect- 8.7 Hz, 1H), 7.28 (dd, J = 8.7, 2.1 Hz, 1H), 2.18 (s, 3H), 1.49
1
3
ively). All chemical shifts are given as δ values (ppm) with (s, 9H); C NMR (CDCl , 100 MHz) δ: 181.5, 168.8, 149.7,
3
reference to tetramethylsilane (TMS) as an internal standard. 135.7, 134.9, 122.3, 118.6, 112.6, 38.2, 30.6, 24.4. HRMS (ESI)
+
The peak patterns are indicated as follows: s, singlet; d, [M + H] calcd for C13
H
17
N
2
OS: 249.1056, found: 249.1056.
doublet; t, triplet; m, multiplet; q, quartet. The coupling con-
stants, J, are reported in hertz (Hz). High resolution mass spec- (85.5 mg, 76%). H NMR (CDCl
troscopy data of the products were collected on an Agilent 8.0 Hz, 1H), 7.44 (dd, J = 7.8, 0.6 Hz, 1H), 7.23 (t, J = 7.9 Hz,
2-(tert-Butyl)-4-chlorobenzo[d]thiazole (3e). Yellow liquid
1
3
, 400 MHz) δ: 7.72 (d, J =
1
3
Technologies 6540 UHD Accurate-Mass Q-TOF LC/MS (ESI).
1H), 1.53 (s, 9H); C NMR (CDCl , 100 MHz) δ: 182.7, 150.2,
3
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1
36.5, 127.5, 126.0, 125.0, 120.0, 38.6, 30.7. HRMS (ESI)
2-(2-(4-Chlorophenoxy)propan-2-yl)benzo[d]thiazole
(3n).
+
1
[M + H] calcd for C H ClNS: 226.0452, found: 226.0451.
11 13
Yellow liquid (113.5 mg, 75%). H NMR (CDCl , 400 MHz) δ:
3
2
-(tert-Butyl)-5-chlorobenzo[d]thiazole (3f). Colorless oil 8.03 (d, J = 8.2 Hz, 1H), 7.88 (d, J = 7.9 Hz, 1H), 7.49 (td, J = 7.7,
1
(
3
91.5 mg, 81%). H NMR (CDCl , 400 MHz) δ: 7.97 (d, J = 1.0 Hz, 1H), 7.39 (td, J = 7.6, 0.8 Hz, 1H), 7.14 (d, J = 8.9 Hz,
.0 Hz, 1H), 7.74 (d, J = 8.5 Hz, 1H), 7.30 (dd, J = 8.5, 2.0 Hz, 2H), 6.79 (d, J = 8.9 Hz, 2H), 1.85 (s, 6H); C NMR (CDCl3,
H), 1.51 (s, 9H); C NMR (CDCl
54.1, 133.2, 131.7, 125.0, 122.6, 122.1, 38.5, 30.6. HRMS (ESI) 125.2, 123.3, 122.4, 121.8, 81.5, 28.3. HRMS (ESI) [M + H]
M + H] calcd for C H ClNS: 226.0452, found: 226.0446.
1
3
2
1
1
1
3
3
, 100 MHz) δ: 183.9, 100 MHz) δ: 177.5, 153.5, 152.9, 135.7, 129.1, 128.1, 126.0,
+
+
[
calcd for C H ClNOS: 304.0559, found: 304.0557.
1
1
13
16 15
4
-Bromo-2-(tert-butyl)benzo[d]thiazole (3g). Colorless oil
2-(Adamantan-1-yl)benzo[d]thiazole (3o). Yellow solid
1
1
(
107.9 mg, 80%). H NMR (CDCl
3
, 400 MHz) δ: 7.76 (dd, J = (123.5 mg, 92%). M.p. 77.1–77.8 °C. H NMR (CDCl
3
,
7
7
1
.9, 0.9 Hz, 1H), 7.62 (dd, J = 7.8, 0.9 Hz, 1H), 7.16 (t, J = 400 MHz) δ: 7.99 (d, J = 8.1 Hz, 1H), 7.85 (d, J = 7.8 Hz, 1H),
1
3
.9 Hz, 1H), 1.53 (s, 9H); C NMR (CDCl
3
, 100 MHz) δ: 182.3, 7.43 (td, J = 7.7, 1.2 Hz, 1H), 7.32 (td, J = 7.6, 1.0 Hz, 1H), 2.15
1
3
51.4, 136.0, 129.2, 125.3, 120.6, 116.3, 38.7, 30.7. HRMS (ESI) (s, 9H), 1.82 (s, 6H); C NMR (CDCl
3
, 100 MHz) δ: 182.1,
153.2, 134.4, 125.6, 124.4, 122.6, 121.5, 43.0, 40.2, 36.5, 28.6;
+
[M + H] calcd for C H BrNS: 269.9947, found: 269.9942.
11 13
+
5
-Bromo-2-(tert-butyl)benzo[d]thiazole (3h). White solid HRMS (ESI) [M + H] calcd for C17H20NS: 270.1311, found:
1
(121.2 mg, 90%). M.p. 88.5–90.0 °C. H NMR (CDCl
3
,
270.1314.
2-(Adamantan-1-yl)-6-methylbenzo[d]thiazole (3p). Yellow
solid (114.6 mg, 81%). M.p. 134.4–134.8 °C. H NMR (CDCl ,
3
4
7
1
3
2
00 MHz) δ: 8.14 (d, J = 1.9 Hz, 1H), 7.69 (d, J = 8.5 Hz, 1H),
.43 (dd, J = 8.5, 1.9 Hz, 1H), 1.51 (s, 9H); C NMR (CDCl
00 MHz) δ: 183.7, 154.5, 133.8, 127.6, 125.6, 122.5, 119.2, 400 MHz) δ: 7.86 (d, J = 8.3 Hz, 1H), 7.61 (s, 1H), 7.22 (dd, J =
8.5, 30.6. HRMS (ESI) [M + H] calcd for C H BrNS: 8.3, 1.2 Hz, 1H), 2.44 (s, 3H), 2.13 (s, 9H), 1.79 (s, 6H);
1
3
1
3
,
+
13
C
1
1
13
69.9947, found 269.9942.
3
NMR (CDCl , 100 MHz) δ: 180.9, 151.2, 134.4, 134.2, 127.1,
+
6
-Bromo-2-(tert-butyl)benzo[d]thiazole (3i). Colorless oil 122.1, 121.2, 42.9, 40.0, 36.5, 28.5, 21.3. HRMS (ESI) [M + H]
1
(114.4 mg, 85%). H NMR (CDCl , 400 MHz) δ: 7.95 (d, J = 1.7 calcd for C H NS: 284.1467, found: 284.1460.
3 18 22
Hz, 1H), 7.82 (d, J = 8.7 Hz, 1H), 7.52 (dd, J = 8.7, 1.9 Hz, 1H),
2-(Adamantan-1-yl)-6-methoxybenzo[d]thiazole (3q). Yellow
1
3
1
1
1
.50 (s, 9H); C NMR (CDCl
29.1, 123.9, 123.7, 117.9, 38.3, 30.6. HRMS (ESI) [M + H]
13BrNS: 269.9947, found: 269.9947.
-(tert-Butyl)-5-(trifluoromethyl)benzo[d]thiazole
3
, 100 MHz) δ: 182.3, 152.1, 136.6, solid (116.8 mg, 78%). M.p. 119.2–119.9 °C. H NMR (CDCl
3
,
+
400 MHz) δ: 7.87 (d, J = 8.8 Hz, 1H), 7.30 (s, 1H), 7.03 (d, J =
1
3
calcd for C11
H
8.8 Hz, 1H), 3.83 (s, 3H), 2.12 (s, 9H), 1.80 (s, 6H); C NMR
(3j). (CDCl , 100 MHz) δ: 179.5, 157.1, 147.6, 135.5, 123.0, 114.7,
Colorless oil (97.1 mg, 75%). H NMR (CDCl , 400 MHz) 104.2, 55.7, 42.9, 39.9, 36.4, 28.5. HRMS (ESI) [M + H] calcd
2
3
1
+
3
δ: 8.27 (s, 1H), 7.94 (d, J = 8.4 Hz, 1H), 7.57 (dd, J = 8.4, 1.0 Hz, for C18
H
22NOS: 300.1417, found: 300.1412.
1
3
1
1
1
3
H), 1.53 (s, 9H); C NMR (CDCl , 100 MHz) δ: 184.1, 152.9,
2-(Adamantan-1-yl)-4-chlorobenzo[d]thiazole (3r). White
38.5, 128.4 (q, J = 32.3 Hz), 124.3 (q, J = 270.5 Hz), 122.0, solid (124.2 mg, 82%). M.p. 123.9–124.1 °C. H NMR (CDCl ,
20.9 (q, J = 3.5 Hz), 119.9 (q, J = 4.1 Hz), 38.6, 30.6. HRMS (ESI) 400 MHz) δ: 7.71 (dd, J = 8.0, 1.0 Hz, 1H), 7.42 (dd, J = 7.8, 1.0
1
3
+
13
13 3
[M + H] calcd for C12H F NS: 260.0715, found: 260.0710.
Hz, 1H), 7.21 (t, J = 7.9 Hz, 1H), 2.14 (s, 9H), 1.80 (s, 6H);
C
2
-(tert-Butyl)-5-phenylbenzo[d]thiazole (3k). Yellow liquid NMR (CDCl , 100 MHz) δ: 182.8, 150.1, 136.0, 127.5, 125.8,
3
1
+
(
0
7
7
93.8 mg, 70%). H NMR (CDCl
3
, 400 MHz) δ: 8.23 (d, J = 124.8, 120.0, 42.9, 40.5, 36.4, 28.5. HRMS (ESI) [M + H] calcd
.8 Hz, 1H), 7.85 (d, J = 8.3 Hz, 1H), 7.65 (d, J = 7.4 Hz, 2H), for C17 19ClNS: 304.0921, found: 304.0916.
.56 (dd, J = 8.3, 1.6 Hz, 1H), 7.44 (t, J = 7.6 Hz, 2H), 7.34 (t, J = 2-(Adamantan-1-yl)-5-chlorobenzo[d]thiazole (3s). White
H
1
3
1
.3 Hz, 1H), 1.53 (s, 9H); C NMR (CDCl
3 3
, 100 MHz) δ: 182.6, solid (121.3 mg, 80%). M.p. 117.2–117.4 °C. H NMR (CDCl ,
1
3
54.0, 140.9, 139.3, 133.9, 128.8, 127.3, 123.9, 121.5, 121.0, 400 MHz) δ: 7.98 (d, J = 1.9 Hz, 1H), 7.75 (d, J = 8.5 Hz, 1H),
+
13
8.4, 30.7. HRMS (ESI) [M + H] calcd for C H NS: 268.1154, 7.30 (dd, J = 8.5, 2.0 Hz, 1H), 2.13 (s, 9H), 1.81 (s, 6H);
C
1
7
18
found: 268.1139.
3
NMR (CDCl , 100 MHz) δ: 184.2, 154.1, 132.7, 131.6, 124.8,
+
2
-(tert-Pentyl)benzo[d]thiazole (3l). Yellow liquid (82.1 mg, 122.5, 122.2, 42.9, 40.3, 36.4, 28.5. HRMS (ESI) [M + H] calcd
1
8
7
0%). H NMR (CDCl , 400 MHz) δ: 8.00 (d, J = 8.1 Hz, 1H), for C H ClNS: 304.0921, found: 304.0917.
3
17 19
.85 (d, J = 7.9 Hz, 1H), 7.44 (td, J = 7.7, 1.1 Hz, 1H), 7.33 (td,
2-(Adamantan-1-yl)-4-bromobenzo[d]thiazole (3t). White
1
J = 7.6, 1.0 Hz, 1H), 1.89–1.83 (m, 2H), 1.48 (s, 6H), 0.86 (t, J = solid (152.9 mg, 88%). M.p. 118.2–119 °C. H NMR (CDCl
3
,
1
3
7
.5 Hz, 3H); C NMR (CDCl , 100 MHz) δ: 181.2, 153.2, 135.0, 400 MHz) δ: 7.76 (d, J = 8.0 Hz, 1H), 7.60 (dd, J = 7.8, 0.6 Hz,
3
1
3
1
25.6, 124.4, 122.7, 121.4, 41.7, 36.7, 28.0, 9.0. HRMS (ESI) 1H), 7.14 (t, J = 7.8 Hz, 1H), 2.14 (s, 9H), 1.80 (s, 6H); C NMR
+
[M + H] calcd for C12
H
16NS: 206.0998, found: 206.0999.
3
(CDCl , 100 MHz) δ: 182.5, 151.3, 135.4, 129.0, 125.1, 120.6,
+
6
-Methyl-2-(tert-pentyl)benzo[d]thiazole (3m). Yellow liquid 116.3, 42.9, 40.5, 36.4, 28.5. HRMS (ESI) [M + H] calcd for
1
(
80.2 mg, 73%). H NMR (CDCl
3
, 400 MHz) δ: 7.87 (d, J =
.3 Hz, 1H), 7.62 (s, 1H), 7.23 (d, J = 8.5 Hz, 1H), 2.45 (s, 3H),
.86–1.81 (m, 2H), 1.46 (s, 6H), 0.85 (t, J = 7.4 Hz, 3H); solid (137.6 mg, 79%). M.p. 104.5–105 °C. H NMR (CDCl3,
C
17
H
19BrNS: 348.0416, found: 348.0411.
8
1
2-(Adamantan-1-yl)-5-bromobenzo[d]thiazole (3u). White
1
1
3
C NMR (CDCl
3
, 100 MHz) δ: 180.0, 151.3, 135.1, 134.4, 127.1, 400 MHz) δ: 8.14 (d, J = 1.7 Hz, 1H), 7.68 (d, J = 8.4 Hz, 1H),
+
13
1
22.1, 121.1, 41.6, 36.7, 28.0, 21.4, 9.0. HRMS (ESI) [M + H]
7.42 (dd, J = 8.4, 1.7 Hz, 1H), 2.12 (s, 9H), 1.80 (s, 6H);
C
calcd for C H NS: 220.1154, found: 220.1147.
NMR (CDCl , 100 MHz) δ: 184.0, 154.4, 133.2, 127.4, 125.5,
1
3
18
3
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+
13
1
22.5, 119.2, 42.8, 40.3, 36.4, 28.5. HRMS (ESI) [M + H] calcd (m, 2H), 1.50–1.39 (m, 2H), 1.37–1.26 (m, 1H); C NMR
for C H BrNS: 348.0416, found: 348.0414.
(CDCl , 100 MHz) δ: 177.5, 153.1, 134.5, 125.7, 124.5, 122.5,
3
1
7
19
+
2
-(3-Bromoadamantan-1-yl)benzo[d]thiazole (3v). White 121.5, 43.4, 33.4, 26.0, 25.8. HRMS (ESI) [M + H] calcd for
1
solid (135.3 mg, 78%). M.p. 125.8–126.2 °C. H NMR (CDCl
3
,
13
C H15NS: 218.0998, found: 218.1000.
4
7
00 MHz) δ: 8.00 (d, J = 8.2 Hz, 1H), 7.87 (d, J = 7.9 Hz, 1H),
.46 (td, J = 7.7, 1.1 Hz, 1H), 7.35 (td, J = 7.6, 1.0 Hz, 1H), 2.74
(
s, 2H), 2.42 (d, J = 2.8 Hz, 4H), 2.33 (s, 2H), 2.20–2.12 (m, 4H), Conflicts of interest
1
3
1
1
3
.80 (s, 2H); C NMR (CDCl , 100 MHz) δ: 178.9, 153.0, 134.2,
3
There are no conflicts to declare.
26.0, 124.7, 122.8, 121.6, 63.5, 53.3, 48.1, 44.2, 41.1, 34.5,
+
2.2. HRMS (ESI) [M + H] calcd for C17
H
19BrNS: 348.0416,
found: 348.0411.
-(sec-Butyl)benzo[d]thiazole (3w). Yellow liquid (45.9 mg,
Acknowledgements
2
1
4
8%). H NMR (CDCl
3
, 400 MHz) δ: 7.98 (d, J = 8.2 Hz, 1H),
We gratefully acknowledge the National Natural Science
Foundation of China (21772062, 21602072, and 21572078).
7
.85 (dd, J = 7.0, 0.4 Hz, 1H), 7.45 (td, J = 7.7, 1.2 Hz, 1H), 7.34
(
1
td, J = 7.6, 1.1 Hz, 1H), 3.26–3.17 (m, 1H), 1.97–1.73 (m, 2H),
.45 (d, J = 6.9 Hz, 3H), 0.98 (t, J = 7.4 Hz, 3H); C NMR
13
(
1
C
CDCl , 100 MHz) δ: 177.9, 153.1, 134.7, 125.8, 124.5, 122.6,
3
Notes and references
+
21.5, 41.1, 30.6, 20.7, 11.8. HRMS (ESI) [M + H] calcd for
14NS: 192.0841, found: 192.0842.
-(Pentan-2-yl)benzo[d]thiazole (3x). Yellow liquid (51.2 mg,
11
H
1 (a) É. Bokor, S. Kun, T. Docsa, P. Gergely and L. Somsák,
ACS Med. Chem. Lett., 2015, 6, 1215; (b) A. L. Kantsadi,
2
1
5
0%). H NMR (CDCl
3
, 400 MHz) δ: 7.98 (d, J = 8.1 Hz, 1H),
É.
Bokor,
S.
Kun,
G.
A.
Stravodimos,
7
.85 (dd, J = 8.0, 0.4 Hz, 1H), 7.44 (td, J = 7.7, 1.2 Hz, 1H), 7.34
D. S. M. Chatzileontiadou, D. D. Leonidas, A. Szakács,
G. Batta, T. Docsa, P. Gergely and L. Somsák, Eur. J. Med.
Chem., 2016, 123, 737; (c) R. A. Hughes and C. J. Moody,
Angew. Chem., Int. Ed., 2007, 46, 7930.
(td, J = 7.6, 1.1 Hz, 1H), 3.35–3.26 (m, 1H), 1.91–1.68 (m, 2H),
1
3
1
1
3
3
.42–1.35 (m, 2H), 0.93 (t, J = 7.3 Hz, 3H); C NMR (CDCl ,
00 MHz) δ: 178.1, 153.1, 134.7, 125.8, 124.5, 122.6, 121.5,
+
9.8, 39.3, 21.1, 20.5, 13.9. HRMS (ESI) [M + H] calcd for
2 (a) T. Yao, K. Hirano, T. Satoh and M. Miura, Angew. Chem.,
Int. Ed., 2012, 51, 775; (b) W.-M. Zhao, X.-L. Chen,
J.-W. Yuan, L.-B. Qu, L.-K. Duan and Y.-F. Zhao, Chem.
Commun., 2014, 50, 2018; (c) P. Y. Xin, H. Y. Niu, G. R. Qu,
R. F. Ding and H. M. Guo, Chem. Commun., 2012, 48, 6717;
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2010, 12, 1360; (e) G. A. Molander, V. Colombel and
V. A. Braz, Org. Lett., 2011, 13, 1852; (f) P. Ren, I. Salihu,
R. Scopelliti and X. L. Hu, Org. Lett., 2012, 14, 1748;
(g) Z. Xie, Y. Cai, H. Hu, C. Lin, J. Jiang, Z. Chen, L. Wang
and Y. Pan, Org. Lett., 2013, 15, 4600.
3 (a) R. Xia, H.-Y. Niu, G.-R. Qu and H.-M. Guo, Org. Lett.,
2012, 14, 5546; (b) Z. Li, L. Jin and C. Cai, Org. Chem.
Front., 2017, 4, 2039.
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40, 5030; (b) P. Gallezot, Chem. Soc. Rev., 2012, 41,
1538; (c) A. J. J. Straathof, Chem. Rev., 2014, 114,
1871.
C
12
H
16NS: 206.0998, found: 206.0999.
5
-Bromo-2-(pentan-3-yl)benzo[d]thiazole (3y). Yellow liquid
1
(86.4 mg, 61%). H NMR (CDCl , 400 MHz) δ: 8.13 (d, J =
3
1
1
6
1
.8 Hz, 1H), 7.69 (d, J = 8.4 Hz, 1H), 7.44 (dd, J = 8.5, 1.9 Hz,
H), 3.02–2.95 (m, 1H), 1.87–1.78 (m, 4H), 0.91 (t, J = 7.4 Hz,
H); C NMR (CDCl , 100 MHz) δ: 178.6, 154.3, 133.5, 127.6,
25.5, 122.5, 119.3, 48.7, 48.6, 28.8, 11.8. HRMS (ESI) [M + H]
13
3
+
calcd for C12
H
15BrNS: 284.0103, found: 284.0106.
(3z).
, 400 MHz) δ: 7.86 (d,
J = 8.9 Hz, 1H), 7.31 (d, J = 2.5 Hz, 1H), 7.04 (dd, J = 8.9,
5
-Methoxy-2-(pentan-3-yl)benzo[d]thiazole
Yellow
1
liquid (76.3 mg, 65%). H NMR (CDCl
3
2
0
1
.5 Hz, 1H), 3.86 (s, 3H), 2.98–2.91 (m, 1H), 1.86–1.76 (m, 4H),
.92 (t, J = 7.4 Hz, 6H); C NMR (CDCl
1
3
3
, 100 MHz) δ: 174.2,
57.3, 147.5, 135.9, 123.0, 114.8, 104.4, 55.8, 48.6, 28.9, 11.9.
+
HRMS (ESI) [M + H] calcd for C H NOS: 236.1104, found:
2
1
3
18
36.1099.
-(Hexan-2-yl)benzo[d]thiazole
2
(3aa).
Orange
liquid
1
(56.0 mg, 51%). H NMR (CDCl , 400 MHz) δ: 7.98 (d, J =
5 (a) H. Hunsdiecker and C. Hunsdiecker, Chem. Ber., 1942,
75, 291; (b) H. Kolbe, Liebigs Ann. Chem., 1849, 69, 257;
(c) H. Kolbe, Liebigs Ann. Chem., 1848, 64, 339.
3
8
1
.1 Hz, 1H), 7.84 (d, J = 7.8 Hz, 1H), 7.44 (td, J = 7.7, 1.2 Hz,
H), 7.33 (td, J = 7.6, 1.0 Hz, 1H), 3.32–3.24 (m, 1H), 1.92–1.68
(
7
1
m, 2H), 1.45 (d, J = 6.9 Hz, 3H), 1.38–1.29 (m, 4H), 0.88 (t, J =
6 (a) R. G. Johnson and R. K. Ingham, Chem. Rev., 1956, 56,
219; (b) D. Budac and P. J. Wan, Photochem. Photobiol., A,
1992, 67, 135; (c) A. G. Griesbeck, W. Kramer and
M. Oelgemoller, Synlett, 1999, 1169.
1
3
.0 Hz, 3H); C NMR (CDCl
3
, 100 MHz) δ: 178.1, 153.1, 134.6,
25.7, 124.5, 122.6, 121.5, 39.5, 37.4, 29.5, 22.6, 21.6, 13.9.
+
HRMS (ESI) [M + H] calcd for C H NS: 220.1154, found:
1
3
18
2
20.1157.
-Cyclohexylbenzo[d]thiazole (3ab). Yellow liquid (56.7 mg,
7 For selected reviews, see: (a) T. P. Yoon, M. A. Ischay and
J. Du, Nat. Chem., 2010, 2, 527; (b) J. M. R. Narayanam and
C. R. J. Stephenson, Chem. Soc. Rev., 2011, 40, 102;
(c) J. Xuan and W.-J. Xiao, Angew. Chem., Int. Ed., 2012, 51,
6828; (d) C. K. Prier, D. A. Rankic and D. W. C. MacMillan,
Chem. Rev., 2013, 113, 5322; (e) M. L. Marin, L. Santos-
2
1
5
7
2%). H NMR (CDCl , 400 MHz) δ: 7.97 (d, J = 8.1 Hz, 1H),
3
.84 (d, J = 8.0 Hz, 1H), 7.43 (td, J = 7.7, 1.1 Hz, 1H), 7.33 (td,
J = 7.6, 1.0 Hz, 1H), 3.14–3.10 (m, 1H), 2.20 (dd, J = 13.5,
.0 Hz, 2H), 1.91–1.86 (m, 2H), 1.79–1.74 (m, 1H), 1.69–1.59
2
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This journal is © The Royal Society of Chemistry 2018
Org. Biomol. Chem.