Reusable proline-based ionic liquid catalyst for the simple synthesis of 2-arylbenzothiazoles in a biomass medium

Article abstract of DOI:10.1007/s11164-015-2133-z

A simple, efficient, and eco-friendly aerobic procedure for synthesis of 2-substituted benzothiazoles catalyzed by facile prepared [N₂₂₂₂][Pro] in a biomass medium (ethyl lactate) has been developed. The reactions afford the target 2-arylbenzot

Full text of DOI:10.1007/s11164-015-2133-z

Res Chem Intermed
DOI 10.1007/s11164-015-2133-z
Reusable proline-based ionic liquid catalyst
for the simple synthesis of 2-arylbenzothiazoles
in a biomass medium
1
1
1
•
Zhi-Yu Yu
Zhi-Bin Song
•
Qiu-Sheng Fang
•
Jia Zhou
1
Received: 4 April 2015 / Accepted: 6 June 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A simple, efficient, and eco-friendly aerobic procedure for synthesis of
-substituted benzothiazoles catalyzed by facile prepared [N2222][Pro] in a biomass
2
medium (ethyl lactate) has been developed. The reactions afford the target 2-aryl-
benzothiazoles in good to excellent yields ([80 %) under air condition and was
observed without use of chromatography. In addition, this proline-based ionic liquid
catalyst can be easily recycled six times without loss of catalytic activity in gram
scale synthesis.
Keywords Ionic liquids Á Amino acid Á 2-Arylbenzothiazoles Á
Aerobic condensation
Introduction
Benzothiazoles are the most important class of heterocyclic compounds because of
their various applications in biology and functional materials [1, 2]. In industry,
benzothiazoles are also used as antioxidants and vulcanization accelerators. These
special utilities of benzothiazoles highlight the importance of their synthesis [3].
Numerous methods have been reported in the literature for the synthesis of
benzothiazoles [4–9]. Generally, the most popular approaches involve condensation
with aldehydes under oxidative conditions [10, 11] (Scheme 1). Many oxidants have
been employed in the synthesis of benzothiazoles such as DDQ [12], PhI (OAc)2
[
13], CAN [14], BaMnO [15]. Although these oxidants are effective, they usually
4
&
Zhi-Bin Song
zbsong@jxnu.edu.cn
1
Key Laboratory of Functional Small Organic Molecules, Ministry of Education, College of
Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, Jiangxi,
People’s Republic of China
123

Z.-Y. Yu et al.
NH2
SH
Proline-based ILs
S
CHO
R
R
biomass medium
N
o
8
0 C air
R= aryl
without the use of chromatography
Scheme 1 Synthesis of 2-substituted benzothiazoles from various substituted aldehydes and
o-aminothiophenol using proline-based ILs at 80 °C
require excess or stoichiometric amounts of strong oxidants resulting in a large
amount of waste.
Recently, renewed interests have focused on aerobic oxidations by employing
oxygen as the ultimate oxidant. Several examples have been reported in the
synthesis of benzo-fused azole compounds under aerobic oxidation conditions [16–
2
0]. With the requirement of sustainable chemistry, some green approaches for the
preparation of 2-arylbenzothiazoles via aerobic condensation reaction have also
been reported, such as ‘‘on water’’ synthesis [21], catalyst-free synthesis [22],
photocatalytic synthesis [23]. Room temperature ionic liquids (ILs) as reusable,
homogeneous catalysts, reaction media, and reagents with ‘‘green’’ aspects have
been applied to the synthesis of heterocyclic compounds due to their remarkable
properties such as designable structure, low volatility, and chemical and high
thermal stability [24, 25].
Table 1 Effect of catalyst on
synthesis of
a
Entry
Catalyst (mol%)
Yield of product (%)
2
-phenylbenzothiazole
1
2
3
4
5
6
7
8
9
Blank
trace
trace
trace
trace
trace
trace
50
NaOH(30)
NaHCO
NaOAc(30)
Et N(30)
3
(30)
3
Pyridine(30)
[N2222][Pro](5)
[N2222][Pro](10)
[N2222][Pro](15)
[N2222][Pro](20)
[N2222][Pro](25)
[N2222][Pro](30)
[N2222][Val](30)
[N2222][Ala](30)
[N2222][Gly](30)
[N2222][Ser](30)
L-proline(30)
60
80
Reaction conditions:
benzaldehyde (1.98 mmol),
o-aminothiophenol (2.28 mmol)
1
1
0
1
90
91
in ethyl lactate (5 mL), reaction 12
92
temperature 80°C, reaction time 13
4
40
h
1
1
1
1
4
5
6
7
45
a
Yield of isolated product
60
b
Catalyst was replaced with
N2222][OH] (25 %aq,
50
[
63
0
.4 mmol) and L-proline
(
0.4 mmol)
18
19
Glycine
32
c
b
Catalyst was replaced with
NaOH (0.4 mmol) and L-proline
0.4 mmol)
[N2222][Pro](20)
c
NaPro (20)
91
2
0
49
(
1
23

Reusable proline-based ionic liquids catalyst for the…
As our interests in developing organic synthesis with better sustainability by
ionic liquids catalysts, we report herein a green and facile aerobic procedure for
preparation of 2-arylbenzothiazoles in biomass medium using proline-based ionic
liquids as a catalyst (Scheme 1).
Results and discussion
A series of tetraethylammonium amino acid ionic liquids ([N2222][AA]) were
synthesized via simple neutralization reactions. In order to investigate the
performances of [N2222][AA] catalysts and establish the optimum conditions in
this aerobic condensation reaction, we took the reaction of benzaldehyde 1a with
o-aminothiophenol as model reaction.
Firstly, the effect of catalysts on synthesis of 2-phenylbenzothiazole was tested
and is summarized in Table 1. The target products were almost not formed in the
present of common inorganic and organic bases (Table 1, entries 1–6). In addition,
the 2-arylbenzothiazoles could be obtained in the present of [N2222][AA], L-proline,
glycine, and NaPro as catalysts (Table 1, entries 7–20). This can be ascribed to the
formation of imino intermediates from the amino group of catalysts and aldehyde.
Compared with other amino acid ILs, [N2222][Pro] showed superior catalytic
activity in this reaction. In other words, the secondary amine group showed a
relatively high catalytic activity. This is probably because the imium intermediates
that are formed from the aldehyde group and secondary amine gives more reactivity
[
26]. In order to verify the better catalytic performance of secondary amine, we also
tested the catalytic activities of L-proline and glycine (Table 1, entries 17–18). This
result further confirmed that the secondary amine gave a better yield than the
primary amino acid. But in this condensation reaction, it seemed that L-proline was
not a very ideal catalyst because it is weakly alkaline. For comparison, NaOH and
[
N2222][OH] as base were added to the reaction mixture (Table 1 entries 19–20).
Table 2 Optimization results
for the reaction of benzaldehyde
and o-aminothiophenol in
presence of [N2222][Pro]
a
Entry Medium
Temperature (°C) Time (h) Yield (%)
1
Benzene
80
80
80
80
80
80
80
80
80
20
40
60
4
4
4
4
4
4
2
6
8
4
4
4
4
50
40
45
65
75
90
52
91
92
(20 mol%)
2
Xylene
3
1.4-dioxane
MeOH
4
5
EtOH
6
Ethyl lactate
Ethyl lactate
Ethyl lactate
Ethyl lactate
Ethyl lactate
Ethyl lactate
Ethyl lactate
7
8
9
Reactions performed at 2 mmol
scale of all reactants
b
10
11
7
a
b
Yield of isolated product
26
58
91
b
The products were purified by 12
silica gel column
chromatography
1
3
Ethyl lactate 100
123

Z.-Y. Yu et al.
The results indicated that the catalytic performance of L-proline by using NaOH as
base was not improved, probably due to the poor solubility of NaPro in organic
solvent. Therefore, [N2222][Pro] was finally chosen as the catalyst for further
studies.
Subsequently, the effect of catalyst loading on this reaction was evaluated. It was
found that catalyst loading had an obvious influence on the formation of product
(
Table 1, entries 7–12). The yield of 2-phenylbenzothiazole increased significantly
50–90 %) with the rise of catalyst loadings from 5 to 20 mol%. However, while the
(
catalyst loading further increased to 30 mol%, the yield was almost no change (from
to 91 %). Thus, considering the cost of ILs, 20 mol% was taken as the optimum
catalyst loading for further investigation. In addition, excellent yield (90 %) of
-phenylbenzothiazole has been obtained at a 1.1 molar ratio of o-aminothiophenol
9
2
to benzaldehyde. Further increase on the molar ratio is not necessary.
As is well known, many organic solvents are harmful to the environment. Thus, it
is necessary to seek an eco-friendly solvent for this reaction. It can be seen that
alcohols showed better results than other organic solvents in the presence of
[
N2222][Pro] (Table 2, entries 1–6). Ethyl lactate (EL) gave the best yield (90 %) in
the synthesis of 2-phenylbenzothiazole. This may be the reason for the good
solubility of the raw materials and catalyst in alcohols. Except for high product
yield, ethyl lactate can be completely biodegraded and is easily obtained from the
biomass at low cost [27]. Thus, ethyl lactate as a biodegradable and facile solvent
was finally chosen in this aerobic condensation reaction.
The following examination on reaction time showed that the yield of
-phenylbenzothiazole was almost no change, even when the reaction time was
2
extended from 4 to 8 h (Table 2, entries 6–9), but 2 h of reaction time was not
Table 3 [N2222][Pro]-catalyzed synthesis of 2-substituted benzothiazoles from various substituted
aldehydes
a
Entry
R
Product
Time (h)
Temperature (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
Ph
2a
2b
2c
2d
2e
2f
4
80
90
70
70
90
85
80
80
80
80
90
82
92
91
85
92
86
85
83
4-MeC
6
H
4
6
4-FC
4-CNC
4-MeOC
4-(Me)
2-ClC
6
H
4
3
6
H
4
2.5
6
6
H
4
2
NC
6
H
4
5
6
H
4
2g
2h
2i
4
4-BrC
6
H
4
4
thieny
4
b
55
10
n-Butyl
2j
6
Reaction conditions: reactants 2 mmol, catalyst 0.4 mmol
a
Yield of isolated product
b
The products were purified by silica gel column chromatography
1
23

Reusable proline-based ionic liquids catalyst for the…
enough for complete conversion by TLC analysis. Therefore, the reaction time was
preferably 4 h.
Reaction temperature is very important for this aerobic condensation reaction.
Thus, the investigation on the effect of temperature is essential for the synthesis of
2
-arylbenzothiazoles. The model reaction was performed in the presence of
N2222][Pro] catalyst at temperatures ranging from 20 to 100 °C (Table 2, entries
0–13). The yield increased obviously from 7 to 90 % with the rise of temperature
[
1
from 20 to 80 °C. While the reaction temperature was further increased to 100 °C,
the yield was only improved with a limited extent. Therefore, an optimized reaction
temperature was chosen to be 80 °C.
With the optimized reaction condition in hand, the scope of application by using
[
N2222][Pro] as catalyst was also explored. The reaction of o-aminothiophenol with
various aromatic aldehydes was further examined. The results are summarized in
Table 3. It was observed that all aromatic aldehydes can be successfully converted
to the corresponding 2-arylbenzothiazoles. Most of these products could be obtained
with excellent yield simply by crystallization. It was also observed that the
electronic nature of the substituents on the phenyl ring had some impact on the yield
of products. Excellent yields of the 2-arylbenzothiazoles with electron-withdrawing
substituents were obtained, possibly because benzaldehydes with electron-with-
drawing substituents were more activated than other substituents. The n-butylalde-
hyde was also tested in this catalytic system. It seemed that this procedure was not
very conducive to aliphatic aldehydes. The structures of all products were confirmed
1
by H NMR, mass spectrometry and comparing the melt points with literature
results.
The recovery and reuse of catalysts is highly preferable for sustainable chemical
process. Thus, the [N2222][Pro] catalyst was recycled and reused for gram scale
synthesis. It was shown that this catalytic system is typically recovered and the
catalyst can be easily reused for subsequent reactions with little decrease in yields
because of catalyst loss in the recycling process (Table 4). After completion of the
reaction, water was added to the reaction mixture. The reaction mixture was then
filtered off and the IL catalyst was recovered by evaporating the solvent under
vacuum. The catalytic activity of recovered catalyst was retained up to the sixth
cycle. In addition, compared with the fresh [N2222][Pro] catalyst, the characteristic
bands of reused [N2222][Pro] catalyst was the same in the IR spectrum (Fig. 1). This
result proved that [N2222][Pro] catalyst is very stable in this condensation reaction
for several runs.
Based on experimental observations and literature reports of similar reaction [28–
0], a plausible reaction mechanism was proposed for proline-based ILs catalyzed
3
Table 4 Reuse of the catalyst for scale-up synthesis of 2a
Run
1
2
3
4
5
6
a
Yield (%)
90
88
85
83
83
81
Reactions performed at 20 mmol scale of all reactants
a
Isolated yield
123

Z.-Y. Yu et al.
Fig. 1 FT-IR spectra of fresh [N2222][Pro] catalyst (a) and used [N2222][Pro] catalyst after six runs (b)
the aerobic condensation of aromatic aldehydes with o-aminothiophenol
(
Scheme 2). Firstly, proline-based ILs reacted with the aldehydes group to form
intermediate A. The next step involved nucleophilic addition with an amine to
generate tetrahedral intermediate B. The thiol of intermediate B is activated by the
formation of intramolecular hydrogen bond between the thiol and carboxylate anion
in the alkaline condition leading to the formation of intermediate C. This activated
thiol group makes nucleophilic attack more easily. Subsequently, cyclization
O
H
N
N
S
O2
H
O
-H2O
S
N
H
N2222
R
D
O
O
O
N2222
H
¦
Ä
O
N
N2222
O
H
¦
Ä
S
N
A
R
N
H
O
H N
2
C
N2222
N
O
HS
NH
HS
B
Scheme 2 Proposed mechanism for proline-based ILs catalyzed synthesis of 2-arylbenzothiazoles
1
23

Reusable proline-based ionic liquids catalyst for the…
resulted in another intermediate D, then the catalyst [N2222][Pro] was regenerated.
Finally, its oxidative dehydrogenation provided the product. Therefore, it is
obviously demonstrated that the [N2222][Pro] catalyst are highly favorable for this
aerobic condensation and have good yields of 2-arylbenzothiazoles.
Conclusions
We have developed an efficient, green, and gram-scale methodology for the
synthesis of 2-arylbenzothiazoles by using proline-based ILs as catalyst. The
advantages of this procedure are the simplicity of reaction conditions, nontoxic
solvents, and easy workup. This [N2222][Pro] IL catalyst can be also simply
recovered and reused for six runs without loss of catalytic activities on the gram-
scale synthesis.
Experimental
Apparatus and analysis
Thin-layer chromatography (TLC) was used to monitor the reaction progress.
Melting points were recorded in open capillaries and are uncorrected. IR spectra
1
were recorded on an Nicolet 6700 FT-IR spectrophotometer (Thermo Nicolet). H
Fig. 2 FT-IR spectra of [N2222][AA] ILs
123

Z.-Y. Yu et al.
NMR spectra were recorded at 400 MHz on a BRUKER AVANCE 400 MHz
spectrophotometer using D O, CDCl and DMSO-d6 as solvent and tetramethyl-
2
3
silane (TMS) as internal standard. Elemental analyses were recorded on a
EuROVECTOR EA3000 CHNS/O elemental analyzer. High resolution mass
spectrometry (HRMS) spectra analysis was performed by electrospray ionization
(
(
ESI-micrOTOF). Tetraethylammonium hydroxide (25 % aq) and ethyl lactate (EL)
purity C 99 %) were purchased from Aladdin (Shanghai, China). Other reagents
such as aromatic aldehydes, o-aminothiophenol, amino acid, and solvents were of
analytical grade and used without any further purification.
Preparation and characterization of [N2222][AA]
Five [N2222][AA] ILs, [N2222][Ser], [N2222][Pro], [N2222][Val], [N2222][Gly], and
[
N2222][Ala] were synthesized via simple acid–base neutralization reactions with a
slight excess of amino acid by stirring at room temperature for 10 h (Fig. 1). The
mixture was dried at 60 °C under vacuum, and ethanol was added to the residue.
The solution was agitated completely so that the excess amino acids were deposited.
After filtration, the solvent was removed by evaporation. The pure [N2222][AA]
(
The structures of [N
C80 % yields) was obtained in a vacuum oven containing P O at 90 °C for 48 h.
2
5
1
][AA] ILs were confirmed by H NMR, elemental analysis,
222
2
FT-TR (Fig. 2), and HRMS.
1
[
N2222][Pro]: H NMR (D O) 1.23 (12H, CH ), 1.68 (3H, NCH CH CH CH),
2 3 2 2 2
2
CH CH), 3.22 (8H, CH CH N), 3.43 (1H, NCH CH CH CH) ppm. Elemental
.06 (1H, NCH CH CH CH), 2.71 (1H, NCH CH CH CH), 3.00 (1H, NCH CH
2 2 2 2 2 2 2 2-
2
3
2
2
2
2
analyses: calculated for C H N O : C, 63.93; H, 11.48; N, 11.48 %. Found: C,
1
3 28 2 2
?
64.10; H, 11.32; N, 11.61 %. HRMS for C H N O [M], [M ? H-Proline]
13 28 2 2
calculated: 130.1590; found: 130.1596. [M–N2222
-
]
calculated: 114.0561; found:
1
14.0565. Yield: 87 %.
1
[
COO), 3.24 (8H, CH CH N) ppm. Elemental analyses: calculated for C H N O :
N2222][Gly]: H NMR (D O) d = 1.24 (12H, CH CH N), 3.15 (2H, NCH
3 2-
2
2
3
2
10 24 2 2
C, 58.82; H; 11.76; N; 13.72 %. Found: C, 59.87; H; 11.69; N; 13.83 %. HRMS for
?
C H N O [M], [M ? H-Glycine] calculated: 130.1590; found: 130.1594. [M-
1
0 24 2 2
-
N2222] calculated: 74.0248; found: 74.0251. Yield: 86 %.
1
[
N2222][Ala]: H NMR (D O) d = 1.20 (3H, CH CH), 1.25 (12H, CH CH N),
2
3 3
2
3
C H N O ; C, 57.78; H, 11.40; N, 12.28 %. Found: C, 58.01; H, 11.32; N,
.24 (8H, CH CH N), 3.29 (1H, CH CH) ppm. Elemental analyses: calculated for
3 2 3
1
1 26 2 2
?
12.33 %. HRMS for C H N O [M], [M ? H-Alanine] calculated: 130.1590;
11 26 2 2
found: 130.1593. [M-N
-
] calculated: 88.0404; found: 88.0406. Yield: 85 %.
222
2
1
[N2222][Val]: H NMR (D O) d = 0.85 (3H, CH3CH), 0.93 (3H. CH3CH), 1.24
2
(
CH CH2 N) ppm. Elemental analyses: calculated for C H N O : C, 63.41; H,
s, 12H, CH CH N), 1.93 (1H, CH(CH3)2), 3.06 (1H, CHNH2), 3.25 (8H,
3 2
3
13 30 2 2
1
2.19; N, 11.38 %. Found: C, 63.56; H, 12.12; N, 11.43 %. HRMS for C H N O
13 30 2 2
?
-
M], [M ? H-Valine] calculated: 130.1590; found: 130.1593. [M–N2222] calcu-
[
lated: 116.0717; found: 116.0722. Yield: 84 %.
1
[
N2222][Ser]: H NMR (D O) d = 1.25 (12H, CH CH N), 3.26 (8H, CH CH N),
3 2
2
2
3
3
.37 (1H, HOCH CHCOO), 3.74 (2H, HOCH CHCOO) ppm. Elemental analyses:
2 2
1
23

Reusable proline-based ionic liquids catalyst for the…
calculated for C H N O : C, 56.41; H, 11.11; N, 11.97 %. Found: C, 56.55; H,
1
1 26 2 3
?
1
0.95; N, 12.07 %, HRMS for C H N O [M], [M ? H-Serine] calculated:
11 26 2 3
-
130.1590; found: 130.1594. [M–N2222
Yield: 88 %.
]
calculated: 104.0353; found: 104.0358.
General procedures for the synthesis of 2-arylbenzothiazoles
[
N2222][AA] catalysts (0.4 mmol), benzaldehydes (1.98 mmol), o-aminothiophenol
2.28 mmol), and solvent (5 ml) were added into a round-bottom flask (25 ml) with
(
a magnetic stirrer and condenser. The temperature of the reaction mixture was
controlled by using a temperature controller with an accuracy of ± 0.01 °C. Then,
the reactor was heated to the designated temperature in an oil bath with stirring.
After the reaction was completed, deionized water (10 ml) was added to the reaction
mixture. The products were precipitated out, and the aqueous phase containing
[
N2222][AA] ILs was isolated simply by filtration, thus were recovered and reused in
the next run after heat treatment to remove water under vacuum at 90 °C for 12 h.
The pure products were obtained by recrystallization with EL/water or silica gel
column chromatography.
1
2
-Phenylbenzothiazole(2a) H NMR (400 MHz, CDCl ) d = 7.34–7.38 (m, 1H),
3
?
7
.47–7.51 (m, 4H), 7.85–7.88 (m, 1H), 8.07–8.09 (m, 3H) ppm; MS m/z (MH)
calcd.: 212.3, found: 212.5; m.p. (°C): 114–116, (lit. 110–112) [1].
0
1
2
3
1
-(4 -Methylphenyl)benzothiazole(2b) H NMR (400 MHz, CDCl ) d = 2.43 (s,
3
H) 7.31 (d, J = 8 Hz, 2H) 7.36–7.40 (m, 1H) 7.47–7.51 (m, 1H) 7.89–7.91 (m,
?
H) 7.99 (d, J = 8 Hz, 2H) 8.06–8.08 (m, 1H) ppm. MS m/z (MH) calcd.: 226.3,
found: 226.6; m.p. (°C): 99–101, (lit. 100–102) [1].
0
1
2
-(4 -Fluorophenyl)benzothiazole(2c)
H
NMR
(400 MHz,
CDCl3)
d = 6.57–6.61 (m, 2H) 6.70–6.72 (m, 2H) 7.14–7.17 (m, 4H) ppm. MS m/z
?
(
MH) calcd.: 230.3, found: 230.1; m.p. (°C): 78–79, (lit. 85–86) [1].
0
1
2
-(4 -Cyanophenyl)
benzothiazole(2d)
H
NMR
(400 MHz,
CDCl3)
d = 7.25–7.27 (m, 2H) 7.57–7.60 (m, 2H) 7.69 (d, J = 8 Hz, 2H) 8.10 (d,
?
J = 8 Hz, 2H) ppm. MS m/z (MH) calcd.: 237.3, found: 237.6; m.p. (°C): 91–92,
(
lit. 94–96) [6].
0
1
2
-(4 -Methoxylphenyl)benzothiazole(2e) H NMR (400 MHz, CDCl ) d = 3.89
3
(
s, 3H) 7.01 (d, J = 8 Hz, 2H) 7.34–7.38 (m, 1H) 7.46–7.50 (m, 1H) 7.87–7.89 (m,
?
H) 8.05 (d, J = 8 Hz, 2H) 8.06–8.08 (m, 1H) ppm. MS m/z (MH) calcd.: 242.3,
1
found: 242.2; m.p. (°C): 121–123, (lit. 120–124) [2].
0
-(4 -dimethylaminophenyl)benzothiazole(2f) H NMR (400 MHz, d -DMSO)
6
1
2
d = 3.00 (s, 6H) 6.84 (d, J = 8 Hz, 2H) 7.12–7.15 (m, 2H) 7.50–7.52 (m, 2H)
?
7
.99 (d, J = 8 Hz, 2H) ppm. MS m/z (MH) calcd.: 255.4, found: 255.8; m.p. (°C):
1–62, (lit. 63–65) [3].
6
123

Z.-Y. Yu et al.
0
-(2 -Chlorophenyl)benzothiazole(2g) H NMR (400 MHz, CDCl ) d = 7.38–7.43
?
1
2
3
(
m, 4H) 7.51–7.56 (m, 2H) 8.12–8.14 (m, 1H) 8.21–8.23 (m, 1H) ppm. MS m/z (MH)
calcd.: 246.7, found: 246.5; m.p. (°C): 82–83, (lit. 83–84) [4].
0
1
2
-(4 -Bromophenyl)benzothiazole(2h) H NMR (400 MHz, CDCl ) d = 7.39–7.42
3
(m, 1H) 7.49–7.53 (m, 1H) 7.63 (d, J = 8 Hz, 2H) 7.91 (d, J = 8 Hz, 1H) 7.96 (d,
?
J = 8 Hz, 2H) 8.077.91 (d, J = 8 Hz, 1H) ppm. MS m/z (MH) calcd.: 291.2, found:
2
91.0; m.p. (°C): 133–135, (lit. 129–131) [1].
0
1
2
-(Thiophen-2 -yl)benzothiazole(2i) H NMR (400 MHz, CDCl ) d = 7.05–7.08
3
(
m, 1H) 7.27–7.31 (m, 1H) 7.38–7.44 (m, 2H) 7.58–7.60 (m, 1H) 7.78 (d, J = 8 Hz,
?
H) 7.96 (d, J = 8 Hz, 1H) ppm. MS m/z (MH) calcd.: 218.3 found: 218.2; m.p.
1
(
°C): 99–100, (lit. 99) [5].
1
2
J = 8.0 Hz 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.40 (t, J = 7.5 Hz, 1H), 7.30 (t,
-Butylbenzothiazole(2j) H NMR (400 MHz, CDCl , ppm): d 7.92 (d,
3
J = 7.3 Hz, 1H), 3.08 (t, J = 7.6 Hz, 2H), 1.86–1.80 (m, 2H), 1.45 (t, J = 7.6 Hz,
?
H), 0.92 (t, J = 7.6 Hz, 3H); MS m/z (MH) calcd.: 192.3 found: 192.6. Yellow
2
oil.
Acknowledgments This work is supported by the Department of Education of Jiangxi Province and
Key Laboratory of Functional Small Organic Molecules, Ministry of Education (No. GJJ13214, KLFS-
KF-201229).
References
1
. S. No e¨ l, S. Cadet, E. Gras, C. Hureau, Chem. Soc. Rev. 42, 7747 (2013)
. S. Massari, D. Daelemans, M.L. Barreca, A. Knezevich, S. Sabatini, V. Cecchetti, A. Marcello, C.
Pannecouque, O. Tabarrini, J. Med. Chem. 53, 641 (2010)
2
3
4
5
6
7
8
9
. I. Navizet, Y.J. Liu, N. Ferreo, H.Y. Xiao, W.H. Fang, R. Lindh, J. Am. Chem. Soc. 132, 706 (2010)
. D. Ma, S. Xie, P. Xue, X. Zhang, J. Dong, Y. Jiang, Angew. Chem. Int. Ed. 48, 4222 (2009)
. X.J. Mu, J.P. Zou, R.S. Zeng, J.C. Wu, Tetrahedron Lett. 46, 4345 (2005)
. J. Hyvl, J. Srogl, Eur. J. Org. Chem. 75, 2849 (2010)
. A. Yokooji, T. Okazawa, T. Satoh, M. Miura, M. Nomura, Tetrahedron 59, 5685 (2003)
. X.F. Wu, P. Anbarasan, H. Neumann, M. Beller, Angew. Chem. Int. Ed. 49, 7316 (2010)
. J. Wang, X.-Z. Zhang, S.-Y. Chen, X.-Q. Yu, Tetrahedron 70, 70245 (2014)
1
1
1
1
1
1
1
1
1
1
2
2
0. K. Bahrami, M.M. Khodaei, F. Naali, J. Org. Chem. 17, 6835 (2008)
1. B.S. Londhe, U.R. Pratap, J.R. Mali, R.A. Mane, Bull. Korean Chem. Soc. 31, 2329 (2010)
2. J. Chang, K. Zhao, S. Pan, Tetrahedron Lett. 43, 951 (2002)
3. R.S. Varma, R.K. Saini, O. Prakash, Tetrahedron Lett. 38, 2621 (1997)
4. H.-L. Jin, T.-X. Cheng, J.-X. Chen, Appl. Organomet. Chem. 25, 238 (2011)
5. R.G. Srivastava, P.S. Venkataramani, Synth. Commun. 18, 1537 (1988)
6. N. Parikh, D. Kumar, S.R. Roy, A.K. Chakraborti, Chem. Commun. 47, 1797 (2011)
7. M. Lei, L. Ma, L. Hu, Synth. Commun. 42, 2981 (2012)
8. W.-H. Chen, Y. Pang, Tetrahedron Lett. 50, 6680 (2009)
9. W.-J. Yoo, H. Yuan, H. Miyamura, S. Kobayashi, Adv. Synth. Catal. 353, 3085 (2011)
0. M. Behrooz, S. Hafezeh, Eur. J. Chem. 1, 377 (2010)
1. A.K. Chakraborti, S. Rudrawar, K.B. Jadhav, G. Kaur, S.V. Chankeshwara, Green Chem. 9, 1335
(
2007)
2. K.U. Sadek, R.A. Mekheimer, A.M.A. Hameed, F. Elnahas, M.H. Elnagdi, Molecules 17, 6011
2012)
3. S. Samanta, S. Das, P. Biswas, J. Org. Chem. 78, 11184 (2013)
2
2
(
1
23

Reusable proline-based ionic liquids catalyst for the…
2
2
2
2
2
2
3
4. C. Maton, N. De Vos, C.V. Stevens, Chem. Soc. Rev. 42, 5963 (2013)
5. P.J. Hallett, T. Welton, Chem. Rev. 111, 3508 (2011)
6. F. Ouyang, Y. Zhou, Z.-M. Li, N. Hu, D.J. Tao, Korean J. Chem. Eng. 31, 1377 (2014)
7. Y.Y. Liu, H. Wang, C.P. Wang, J.-P. Wan, RSC Adv. 3, 21369 (2013)
8. S.N. Rao, D.C. Mohan, S. Adimurthy, Org. Lett. 15, 1496 (2013)
9. M. Venkatanarayana, P.K. Dubey, Synth. Commun. 42, 1746 (2012)
0. M.K. Ghorai, S. Samanta, S. Das, Asian J. Org. Chem. 2, 1026 (2013)
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