One-pot synthesis of 2-substituted imidazo[2,1-b][1,3]benzothiazoles via coupling-cyclization under Pd-Cu catalysis in Water

Article abstract of DOI:10.2174/157017809787582717

The reaction of 2-imino-3-(2-propynyl)-1,3-benzothiazole with various aryl iodides, catalyzed by Pd-Cu, in the presence of sodium lauryl sulfate as the surfactant and potassium carbonate as the base, in water, leads to the formation of 2-substituted imida

Full text of DOI:10.2174/157017809787582717

Letters in Organic Chemistry, 2009, 6, 165-170
165
One-pot Synthesis of 2-Substituted Imidazo[2,1-b][1,3]benzothiazoles via
Coupling-Cyclization Under Pd-Cu Catalysis in Water
Mohammad Bakherad*, Ali Keivanloo and Mahdieh Hashemi
Faculty of Chemistry, Shahrood University of Technology, Shahrood, Iran
Received July 23, 2008: Revised November 20, 2008: Accepted November 25, 2008
Abstract: The reaction of 2-imino-3-(2-propynyl)-1,3-benzothiazole with various aryl iodides, catalyzed by Pd-Cu, in the
presence of sodium lauryl sulfate as the surfactant and potassium carbonate as the base, in water, leads to the formation
of 2-substituted imidazo[2,1-b][1,3]benzothiazoles.
Keywords: Sonogashira coupling, aryl iodide, imidazo[2,1-b][1,3]benzothiazole, Pd-catalyzed, potassium carbonate, sodium
lauryl sulfate.
Imidazobenzothiazole derivatives are common substruc-
tures of synthetic molecules displaying important medicinal
activities [1]. Their core ring system is present in numerous
carditonic [2] and immunosuppressive [3] drugs. Considering
the potent bioactivities of compounds possessing an imida-
zobenzothiazole core, the development of a new strategy to
synthesize 2-benzylimidazo[2,1-b][1,3]benzothiazoles effi-
ciently attracted our attention.
In continuation of our recent studies [20] on the synthesis
of fused heterocycles and the Pd-catalyzed reaction of acety-
lenes leading to heterocyclic compounds of biological sig-
nificance, we became interested in developing a synthetic
route to 2-substitured imidazo[2,1-b][1,3]benzothiazoles in
water.
In this communication, we report that treatment of 2-
imino-3-(2-propynyl)-1,3-benzothiazole [21] 2 with aryl
halides 3a-h in water and in the presence of
bis(triphenylphosphine)palladium(II) chloride, copper io-
dide, sodium lauryl sulfate, and potassium carbonate at 60
ºC. The 2-substituted imidazo[2,1-b][1,3]benzothiazoles 4a-
h were obtained in moderate to high yields (Scheme 1, Table
3). The reactions were carried out under an argon atmos-
phere, and water was degassed prior to use.
Transition metal-mediated cross-couplings have proven
to be powerful tools for mild, highly efficient carbon-carbon
bond formations [4]. Among these processes, those involv-
ing palladium catalysis especially Sonogashira coupling or
alkynylation of aryl halides [5] are particularly useful for the
synthesis of complex molecules, including natural products
[6], biologically active molecules [7], molecular electronics
[8], and polymers [9]. The original Sonogashira reaction was
generally performed in the presence of large amounts of pal-
ladium and copper(I) iodide as a co-catalyst in organic sol-
vents, which was economically and environmentally malig-
nant. Nevertheless, this palladium-catalyzed reaction has
been extensively investigated to improve the reaction condi-
tions [10], and an impressive variety of modifications have
been reported thus far. This includes the use of a variety of
additives [11], solvents [12], phase-transfer catalysts [13],
copper-free versions [14], biphasic versions [15], polymer
supported Pd-triazine complex [16], solid supported Pd-
catalyst [17], etc. The use of aqueous media in palladium-
catalyzed reactions have become popular [18] because water-
based synthetic processes are inherently safer (water is non-
toxic and non-flammable) as well as being inexpensive.
Moreover, it does not require dry solvents, and the products
could easily be isolated by extraction, which greatly facili-
tates the operation. Thus the use of water in palladium-
catalyzed reactions represents one of the most economically
and environmentally viable options for many organic trans-
formations. Several examples of Pd-catalyzed Sonogashira
reactions in aqueous media have been reported [19].
Mechanistically, formation of 2-benzylimidazo[2,1-
b][1,3]benzothiazoles involves the following steps (as shown
in Scheme 1): (i) formation of ArPdI [B] through oxidative
addition of Pd(0) [A] to ArI [5]; (ii) transmetallation of Ar-
PdI with the Cu salt of [C], generating the alkynyl palladium
species [D]; (iii) extrusion of Pd(0) to yield the alkynes [E];
(iv) isomerization to the allenic intermediates [22] [F], which
then cyclize to the 2-benzylimidazo[2,1-b][1,3]benzothia-
zoles 4a-h.
For optimization of the reaction conditions, we chose the
reaction of 2-imino-3-(2-propynyl)-1,3- benzothiazole 2 with
2-iodo-5-nitrotuluene 3a as the model reaction, and the ef-
fects of the base, surfactant and catalyst on the reaction were
examined. First several bases were screened for the reaction
in the presence of a catalytic amount of Pd(PPh3)2Cl2. As
shown in Table 1, the reaction is significantly influenced by
the base employed. The reaction works very well when inor-
ganic bases such as K2CO3 and Cs2CO3 are used (Entries 2
and 3, Table 1), with the best result obtained in the case of
potassium carbonate as the base (Entry 2, Table 1).
The influence of the amounts of copper(I) iodide, surfac-
tant, and catalyst was investigated using the reaction of com-
pound 2 with 3a. The results were tabulated in Table 2. In-
creasing the amount of the palladium catalyst shortened the
reaction time but did not increase the yield (Entry 3,
*Address correspondence to this author at the Faculty of Chemistry,
Shahrood University of Technology, Shahrood, Post Code: 3619995161,
Iran; Tel/Fax: +982733335441; E-mail: m.bakherad@yahoo.com
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

166 Letters in Organic Chemistry, 2009, Vol. 6, No. 2
Bakherad et al.
Pd (PPh3)2Cl2
a
ArI
3a-h
Pd (0)
ArPdI
CH
CCu
N
N
S
N
S
BrH2C
C
CH
b
NH2
NH
NH
EtOH
S
1
[C]
2
CuI
CHAr
Ar
C
CPdAr
N
S
N
N
S
c
NH
NH
NH
S
Pd(0)
[D]
[E]
[F]
d
CH2Ar
N
N
S
4a-h
Scheme 1. A plausible mechanism for the formation of 2-benzylimidazo[2,1-b][1,3]benzothiazoles at room temperature. Reagents and condi-
tions: (a) reduction of Pd(II) to Pd(0) with alkyne and K2CO3; (b) CuI, K2CO3; (c) isomerization to an allene with CuI, K2CO3; (d) nucleo-
philic attack on the allene (F) to generate the 2-benzylimidazo[2,1-b][1,3] benzothiazoles.
Table 1. Effect of base on the Heterocyclization During Sonogashira Coupling of Compound 2 with 2-iodo-5-nitrotoluene 3a in
Watera
Yieldb (%)
Base
Entry
35
95
87
25
40
30
25
KOH
K2CO3
1
2
3
4
5
6
7
Cs2CO3
Et3N
DIEA
Pyrrolidine
Piperidine
aReaction conditions: 2 (1mmol), 3a (1mmol), base (3mmole), Pd(PPh)3Cl2 (3 mol %), CuI (7 mol %), Sodium lauryl sulfate (7 mol %), degassed water (5 mL) at 60 ºC for 10 h.
bIsolated yields.
Table 2). Low palladium concentration prolonged the reac-
tion time and decreased the yield (Entry 1, Table 2). The use
of surfactant is also critical for the success of the reaction:
without a surfactant/phase-transfer reagent, the yield dropped
from 95% to 10% (Entries 2 and 8, Table 2). We also found
that with increase in the amount of surfactant, the reaction
yield did not increase (Entry 7, Table 2). Low surfactant
concentration decreased the yield (Entry 6, Table 2). No re-
action occurred when either Cu(II) alone or Pd(II) alone was
used as the catalyst (Entries 4 and 5, Table 2).

One-pot Synthesis of 2-Substituted Imidazo[2,1-b][1,3]benzothiazoles
Letters in Organic Chemistry, 2009, Vol. 6, No. 2 167
Table 2. Effects of the Catalyst, Co-Catalyst, and Surfactant on the Heterocyclization During Sonogashira Coupling of Compound
2 with 2-iodo-5-nitrotoluene 3a in Watera
Entry
Pd(PPh3)2Cl
CuI
Sodium lauryl sulfate
Yieldb (%)
1
2
3
4
5
6
7
8
2 mol %
3 mol %
5 mol %
3 mol %
------
5 mol %
7 mol %
10 mol %
------
7 mol %
7 mol %
7 mol %
7 mol %
7 mol %
3 mol %
12 mol %
-------
75c
95
90d
nr
7 mol %
7 mol %
7 mol %
7 mol %
nr
3 mol %
3 mol %
3 mol %
65
75
10
aReaction reagents and conditions: 2 (1mmol), 3a (1mmol), K2CO3 (3 mmol), degassed water (5 mL) at 60 ºC for 10 h.
bIsolated yields.
cReaction time: 14 h.
dReaction time: 7 h.
Table 3. Yields of 2-benzylimidazo[2,1-b][1,3]benzothiazoles 4a-ha
Entry
Ar
Product
Yield (%)
CH3
1
4a
95
NO2
3a
NO2
2
3
4b
4c
70
65
3b
NO2
3c
4
4d
80
NO2
3d
CH3
5
4e
85
Cl
3e

168 Letters in Organic Chemistry, 2009, Vol. 6, No. 2
Bakherad et al.
(Table 3). Contd…..
Entry
Ar
Product
Yield (%)
6
4f
72
68
92
NO2
Cl
3f
Cl
7
4g
NO2
3g
8
4h
COOMe
3h
aReaction reagents and conditions: 2 (1mmol), 3a-h (1mmol), K2CO3 (3 mmol), Pd(PPh)3Cl2, (3 mol %), CuI (7 mol %), Sodium lauryl sulfate (7 mol %), degassed water (5 mL) at 60
ºC for 10 h.
Subsequently, the reactions of a variety of aryl iodides
3a-h and 2- imino-3-(2-propynyl)-1,3-benzothiazole 2 were
studied under the optimal conditions. As shown in Table 3,
the presence of electron-withdrawing groups on aryl iodide
seems to be essential. When iodobenzene or p-iodoanisole
was used as the aryl iodide, Sonogashira coupling could not
be achieved.
(28). Anal. Calcd. for C17H13N3O2S: C, 63.14; H, 4.05; N,
12.99; S, 9.92%. Found: C, 62.78; H, 3.86; N, 12.63; S,
9.73%.
4b: 1H NMR ꢀ (500 MHz, DMSO-d6): 4.37 (s, 2H, CH2),
7.37-8.00 (m, 8H, ArH), 8.2 (s, 1H, CH of imidazole); 13C
NMR ꢀ (125 MHz, DMSO-d6): 34.08, 114.42, 122.12,
122.70, 123.11, 125.43, 127.61, 128.93, 129.27, 130.67,
131.13, 132.02, 134.30, 139.34, 148.20, 153.25; IR, ꢀ (KBr
disc): 1490, 1340 cm-1; MS (EI) m/z, 309 (M+,45), 308 (63),
263 (100), 186 (53), 131 (34), 108 (47). Anal. Calcd. for
C16H11N3O2S: C, 62.12; H, 3.58; N, 13.58; S, 10.37%.
Found: C, 62.54; H, 3.72; N, 13.82; S, 10.16%.
In conclusion, we have developed a successful palla-
dium-catalyzed reaction for the syntheses of 2-substituted
imidazo[2,1-b][1,3]benzothiazoles in the presence of sodium
lauryl sulfate as the surfactant and potassium carbonate as
the base in water.
4c: 1H NMR ꢀ (500 MHz, DMSO-d6): 4.18 (s, 2H, CH2),
7.39-7.81 (m, 4H, ArH), 7.97-8.09 (m, 4H, ArH), 8.20 (s,
1H, CH of imidazole); 13C NMR ꢀ (125 MHz, DMSO-d6):
34.20, 114.62, 122.40, 123.01, 126.10, 127.61, 128.05,
128.37, 129.21, 130.22, 130.76, 133.87, 138.20, 140.09,
146.80, 154.01; IR, ꢀ (KBr disc): 1510, 1340 cm-1; M (EI) S
m/z, 309 (M+,100), 261 (63), 187 (42), 130 (24), 109 (39).
Anal. Calcd. for C16H11N3O2S: C, 62.12; H, 3.58; N, 13.58;
S, 10.37%. Found: C, 61.73; H, 3.30; N, 13.25; S, 10.11%.
4d: 1H NMR ꢀ (500 MHz, DMSO-d6): 4.38 (s, 2H, CH2),
7.38-8.18 (m, 8H, ArH), 8.20 (s, 1H, CH of imidazole); 13C
NMR ꢀ (125 MHz, DMSO-d6): 34.26, 114.75, 122.70,
123.02, 125.37, 127.26, 127.95, 128.87, 130.80, 131.13,
133.15, 140.04, 147.75, 153.82; IR, ꢀ (KBr disc): 1500, 1345
cm-1; MS (EI) m/z, 309 (M+, 100), 262 (96), 187 (55), 131
(40), 108 (42).. Anal. Calcd. for C16H11N3O2S: C, 62.12; H,
3.58; N, 13.58; S, 10.37%. Found: C, 62.43; H, 3.70; N,
13.76; S, 10.56%.
General Procedure for Synthesis of 2-substituted imi-
dazo[2,1-b][1,3]benzothiazoles 4a-h
A mixture of the aryl iodide (1 mmol), (PPh3)2PdCl2 (3
mol %), CuI (7 mol %), sodium lauryl sulfate (7 mol %), and
potassium carbonate (3 mmol) was stirred in water (5 mL) at
60 ºC for 30 min under an argon atmosphere. 2-imino-3-(2-
propynyl)-1,3-benzothiazole (1 mmol) was then added and
the mixture was stirred at 60 ºC for 10 h. After completion of
the reaction, the resulting solution was concentrated in
vacuo, and the crude product was subjected to silica gel col-
umn chromatography using CHCl3-CH3OH (95:5) as eluent
to afford the pure product (Table 3).
4a: 1H NMR ꢀ (500 MHz, DMSO-d6): 2.06 (s, 3H, CH3),
4.35 (s, 2H, CH2), 7.19-7.90 (m, 7H, ArH), 8.00 (s, 1H, CH
of imidazole); 13C NMR ꢀ (125 MHz, DMSO-d6): 20.12,
33.91, 114.15, 122.21, 122.55, 122.96, 123.90, 126.02,
127.41, 127.86, 128.97, 130.37, 130.95, 133.32, 139.25,
148.03, 153.56; IR, ꢀ (KBr disc): 1520, 1340 cm-1; MS (EI)
m/z 323 (M+,100), 276 (49), 187 (24), 150 (21), 131 (37), 39
4e: 1H NMR ꢀ (500 MHz, DMSO-d6): 4.33 (s, 2H, CH2),
7.52-7.99 (m, 7H, ArH), 8.11 (s, 1H, CH of imidazole); 13C

One-pot Synthesis of 2-Substituted Imidazo[2,1-b][1,3]benzothiazoles
Letters in Organic Chemistry, 2009, Vol. 6, No. 2 169
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NMR ꢀ (125MHz, DMSO-d6): 33.77, 114.71, 123.15,
123.55, 125.87, 127.23, 128.20, 129.10, 130.17, 130.65,
131.35, 132.71, 134.32, 139.77, 149.60, 153.20; IR, ꢀ (KBr
disc): 1490 cm-1; MS (EI) m/z, 345 (M+37Cl, 14), 343
(M+35Cl, 46), 326 (100), 298 (68), 263 (93), 203 (34), 161
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C, 55.90; H, 2.93; N, 12.22; S, 9.33%. Found: C, 55.59; H,
2.66; N, 12.07; S, 9.10%.
[5]
[6]
[7]
1
4f: H NMR ꢀ (500 MHz, DMSO-d6): 4.35 (s, 2H, CH2),
7.38-8.07 (m, 7H, ArH), 8.32 (s, 1H, CH of imidazole); 13C
NMR ꢀ (125 MHz, DMSO-d6): 33.87, 114.85, 122.78,
123.67, 125.76, 126.88, 127.61, 129.17, 129.57, 130.75,
131.20, 135.02, 135.47, 139.34, 148.45, 153.27; IR, ꢀ (KBr
disc): 1510, 1335 cm-1; MS (EI) m/z, 345 (M+37Cl, 12), 343
(M+35Cl, 31), 298 (100), 262 (68), 201 (27), 132 (43), 109
(35).. Anal. Calcd. for C16H10ClN3O2S: C, 55.90; H, 2.93; N,
12.22; S, 9.33%. Found: C, 55.67; H, 2.69; N, 11.97; S,
9.41%.
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4g: 1H NMR ꢀ (500 MHz, DMSO-d6): 4.34 (s, 2H, CH2),
7.38-8.1 (m, 7H, ArH), 8.17 (s, 1H, CH of imidazole); 13C
NMR ꢀ (125 MHz, DMSO-d6): 34.10, 114.21, 122.67,
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4h: 1H NMR ꢀ (500 MHz, DMSO-d6): 3.86 (s, 3H, CH3),
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100), 306 (87), 262 (43), 186 (44), 170 (24), 147 (23), 131
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8.69; S, 9.95%. Found: C, 66.85; H, 4.22; N, 8.90; S, 9.72%.
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ACKNOWLEDGEMENT
The authors thank the Research Council of Shahrood
University of Technology for the support of this work.
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