Synthesis of 6-substituted imidazo[2,1-b]thiazoles via Pd/Cu-mediated Sonogashira coupling in water

Article abstract of DOI:10.1016/j.tetlet.2009.07.017

The reaction of 2-amino-3-(2-propynyl)thiazolium bromide with various iodobenzenes, catalyzed by Pd/Cu, in the presence of sodium lauryl sulfate as surfactant and cesium carbonate as base, in water, leads to the formation of 6-substituted imidazo[2,1-b]th

Full text of DOI:10.1016/j.tetlet.2009.07.017

Tetrahedron Letters 50 (2009) 5459–5462
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Tetrahedron Letters
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Synthesis of 6-substituted imidazo[2,1-b]thiazoles via Pd/Cu-mediated
Sonogashira coupling in water
b
c
a
c
Taghi A. Kamali a, , Mohammad Bakherad , Mahmoud Nasrollahzadeh , Shiva Farhangi , Davood Habibi
*
a Laboratory of Organic Chemistry, Moshashimi Company, Kardikola Industrial Region, PO Box 47571-54879, Babol, Iran
b Faculty of Chemistry, Shahrood University of Technology, Shahrood, Iran
c Department of Organic Chemistry, Faculty of Chemistry, Bu Ali Sina University, Hamedan 6517838683, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 2-amino-3-(2-propynyl)thiazolium bromide with various iodobenzenes, catalyzed by Pd/
Cu, in the presence of sodium lauryl sulfate as surfactant and cesium carbonate as base, in water, leads to
the formation of 6-substituted imidazo[2,1-b]thiazoles.
Received 17 March 2009
Revised 23 June 2009
Accepted 3 July 2009
Available online 9 July 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Sonogashira coupling
Aryl iodide
Imidazothiazole
Pd-catalyzed
Sodium lauryl sulfate
Compounds containing the imidazo[2,1-b]thiazole skeleton
have been used as anthelmintic agents, anti-hypertensives, anti-
inflammatories, immunosuppressive agents, fungicides, herbicides,
antitumor agents, and cardiotonic agents.1 Considering the potent
bioactivities of compounds possessing an imidazothiazole core, the
development of a new strategy to synthesize 6-substituted imi-
dazo[2,1-b]thiazoles efficiently attracted our attention.
Although several procedures have been developed for the syn-
thesis of imidazo[2,1-b]thiazoles,2 no examples involving arylation
of a thiazolium alkyne by Pd/Cu-catalyzed (Sonogashira coupling)
reactions have been reported in the literature.
reactions.13 The use of aqueous media in palladium-catalyzed
reactions has become popular14 because water-based synthetic
processes are inherently safer (water is non-toxic and non-flam-
mable) and inexpensive. Moreover, the products can easily be iso-
lated by extraction, which greatly facilitates the operation. Several
examples of Pd-catalyzed Sonogashira reactions in aqueous media
have been reported.15
In continuation of our recent studies16 on the synthesis of het-
erocycles and the Pd-catalyzed reaction of acetylenes leading to
heterocyclic compounds of biological significance, we were inter-
ested in developing
a
synthetic route to 6-substituted
Alkynes are versatile intermediates in synthesis3,4 as well as are
important functional groups in a wide range of biologically active
compounds.5 The Sonogashira reaction typically employs a palla-
dium catalyst and copper iodide to couple a terminal alkyne with
an aryl halide.6 Several modifications of the original Sonogashira
protocol have been reported, prominent among which are phase
transfer7 and copper-free conditions,8 and the use of a more active
catalyst system, including those utilizing N-heterocyclic carbene
(NHC) ligands for reaction with less reactive bromo- and chloroa-
renes.9 The effect of different solvents has also been studied10
including aqueous–organic solvent mixtures in the presence of
water-soluble phosphine ligands11 and ionic liquids.12
imidazo[2,1-b]thiazoles using water as the solvent.
In this Letter, we report that treatment of 2-aminothiazole 1
with propargyl bromide in refluxing acetonitrile affords 2-amino-
3-(2-propynyl)thiazolium bromide 2 in good yield. The 1H NMR
spectrum of 2 showed a CH proton signal at 3.73 ppm, CH2 protons
at 5.03 ppm, and a single resonance for the NH2 group at 9.78 ppm;
this signal was removed on deuteration.
When compound 2 was reacted with aryl iodides 3a–f and ce-
sium carbonate in water in the presence of bis(triphenylphos-
phine)palladium(II) chloride, copper iodide, and sodium lauryl
sulfate at 60 °C, 6-substituted imidazo[2,1-b]thiazoles 4a–f were
obtained in moderate to high yields (Scheme 1). The reactions were
carried out under an argon atmosphere and water and cesium car-
bonate were degassed prior to use.
Driven by environmental concerns, much attention has been
paid to using water as the solvent for organic and organometallic
Mechanistically, the formation of 6-substituted imidazo[2,1-
b]thiazoles involves the following steps (as shown in Scheme 1):
(i) formation of ArPdI B through oxidative addition of Pd(0) A to
* Corresponding author. Tel.: +98 911 1276314; fax: +98 111 2524702.
E-mail address: t.azizkamali@gmail.com (T.A. Kamali).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.017

5460
T. A. Kamali et al. / Tetrahedron Letters 50 (2009) 5459–5462
(PPh3)2PdCl2
i
ArI
3a-f
Pd(0)
ArPdI
A
B
CH
CCu
NH
C
C
Br-
N
N
N
NH2
NH2
+
ii
BrH2C
C
CH
S
S
S
2
1
C
CuI
CAr
NH
CPdAr
NH
CHAr
C
C
C
S
N
..
N
N
N
NH
iii
iv
N
S
S
S
Pd(0)
F
E
D
CH Ar
-f
4a
2
d: Ar = 2-Cl-4-NO2-C6H3-
e: Ar = 4-Cl-2-NO2-C6H3-
f: Ar = 4-Cl-3-NO2-C6H3-
a: Ar = 2-NO2-C6H4-
b: Ar = 4-NO2-C6H4-
c: Ar = 2-CH3-4-NO2-C6H3-
Scheme 1. A plausible mechanism for the formation of 6-substituted imidazo[2,1-b]thiazoles at 60 °C. Reagents and conditions: (i) reduction of Pd(II) to Pd(0) with
alkyne and Cs2CO3; (ii) CuI, Cs2CO3; (iii) isomerization to an allene with CuI, Cs2CO3; (iv) intermolecular nucleophilic attack on the allene F to generate the 6-substituted
imidazo[2,1-b]thiazoles 4a–f.
ArI;17 (ii) transmetallation of ArPdI with the Cu salt of C, generating
the alkynyl palladium species D; (iii) extrusion of Pd(0) to yield the
alkyne E; and (iv) isomerization to the allenic intermediate18 F,
which then cyclizes to the 6-substituted imidazo[2,1-b]thiazoles
4a–f.
For optimization of the reaction conditions, we chose the reac-
tion of 2 with 4-chloro-1-iodo-2-nitrobenzene 3e as the model
reaction, and the effects of the base, surfactant, and catalyst 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 was influenced significantly by the base em-
ployed. The reaction worked very well when inorganic bases such
as K2CO3 and Cs2CO3 were used (Table 1, entries 2 and 3), with the
best result obtained in the case of cesium carbonate
(Table 1, entry 3).
The influence of the amount of copper(I) iodide, surfactant, and
the catalyst was investigated using the reaction of compound 2
with 3e. The results are shown in Table 2. Increasing the amount
of the palladium catalyst shortened the reaction time but did not
increase the yield (entry 3). A low palladium concentration pro-
longed the reaction time and led to a decreased yield (entry 1).
The use of surfactant was also critical for the success of the
reaction: without a surfactant/phase-transfer reagent, the yield
dropped from 90% to 10% (compare entries 2 and 8). We also found
that with an increased amount of surfactant, the reaction yield did
not increase (entry 7). A low surfactant concentration decreased
the yield (entry 6). No reaction was observed when either Cu(I)
alone or Pd(II) alone was used as the catalyst (entries 4 and 5).
Subsequently, the reactions of a variety of aryl iodides 3a–f and
2-amino-3-(2-propynyl)thiazolium bromide 2 were studied under
the optimal conditions. As shown in Table 3, the presence of elec-
tron-withdrawing groups such as NO2 and Cl on the aryl iodide was
essential for a successful reaction. When iodobenzene or p-iodoan-
isole was used as the aryl iodide, the Sonogashira coupling was not
achieved.
In conclusion, we have developed a successful palladium-cata-
lyzed reaction for the synthesis of 6-substituted imidazo[2,1-b]thi-
azoles in the presence of sodium lauryl sulfate as the surfactant
and cesium carbonate as the base in water.
2-Amino-3-(2-propynyl)thiazolium bromide 2: A mixture of 2-ami-
nothiazole 1 (2 g, 20 mmol) and propargyl bromide (2 mL, 24 mmol)
in MeCN (10 mL) was heated under reflux for 1 h. The precipitate
formed was filtered off and recrystallized from MeCN to afford the
title compound. Yield, 95%; mp 159–160 °C; 1H NMR (DMSO-d6,
500 MHz): dH = 3.73 (t, J = 2.4 Hz, 1H, CH), 5.03 (d, J = 2.3 Hz, 2H,
CH2), 7.07 (d, J = 4.5 Hz, 1H, CH of thiazole), 7.50 (d, J = 4.5 Hz, 1H,
Table 1
Effect of base on the Sonogashira coupling of compound 2 with 4-chloro-1-iodo-2-
nitrobenzene 3e in watera
Entry
Base
Yieldb (%)
1
2
3
4
5
6
7
KOH
K2CO3
Cs2CO3
Et3N
DIEA
15
78
90
25
42
28
34
Pyrrolidine
Piperidine
a
Reaction conditions:
2 (1 mmol), 3e (1 mmol), base (3 mmol), Pd(PPh)3Cl2
(3 mol %), CuI (7 mol %), sodium lauryl sulfate (7 mol %), degassed water (5 mL),
60 °C, 12 h.
b
Isolated yield.

T. A. Kamali et al. / Tetrahedron Letters 50 (2009) 5459–5462
5461
Table 2
an argon atmosphere. 2-Amino-3-(2-propynyl)thiazolium bromide
(1 mmol) was then added and the mixture was stirred at 60 °C for
12 h. After completion of the reaction, the resulting solution was
concentrated in vacuo and the crude product was subjected to sil-
ica gel column chromatography using CHCl3–CH3OH (95:5) as elu-
Effects of catalyst, co-catalyst, and surfactant on the Sonogashira coupling of
compound 2 with 4-chloro-1-iodo-2-nitrobenzene 3e in watera
Entry
Pd(PPh3)2Cl2
(mol %)
CuI (mol %)
Sodium lauryl
sulfate (mol %)
Yieldb (%)
72c
90
ent to afford the pure product (Table 3). The spectral (1H NMR, 13
C
1
2
3
4
5
6
7
8
2
3
5
3
—
3
3
3
5
7
10
—
7
7
7
7
7
7
7
7
7
3
12
—
NMR, IR and mass) data of the 6-substituted imidazo[2,1-b]thia-
zoles are given below.
85d
e
—
—
6-(2-Nitrobenzyl)imidazo[2,1-b]thiazole (4a, Table 3, entry 1). 1H
NMR (DMSO-d6, 500 MHz): dH 4.25 (s, 2H, CH2), 6.95–7.96 (m, 6H,
thiazole, ArH), 8.63 (s, 1H, CH of imidazole); 13C NMR (DMSO-d6,
125 MHz): dC 35.32, 110.46, 122.45, 124.10, 128.93, 129.82,
130.37, 131.25, 131.98, 134.24, 147.67, 154.56; IR (KBr): 1520,
1340 cmÀ1; MS (EI) m/z, 259 (M+, 45), 213 (100), 137 (20), 123
(8). Anal. Calcd for C12H9N3O2S: C, 55.59; H, 3.50; N, 16.21; S,
12.37. Found: C, 55.32; H, 3.36; N, 16.40; S, 12.21.
e
50
60
10
a
Reaction conditions:
2 (1 mmol), 3e (1 mmol), Cs2CO3 (3 mmol), degassed
water (5 mL), 60 °C, 12 h.
b
Isolated yield.
Reaction time: 14 h.
Reaction time:8 h.
No reaction.
c
d
6-(4-Nitrobenzyl)imidazo[2,1-b]thiazole (4b, Table 3, entry 2). 1H
NMR (DMSO-d6, 500 MHz): d 4.18 (s, 2H, CH2), 7.25–8.15 (m, 6H,
thiazole, ArH), 8.31 (s, 1H, CH of imidazole); 13C NMR (DMSO-d6,
e
125 MHz):
d 34.78, 109.67, 122.87, 123.05, 127.93, 128.30,
Table 3
Melting points and yields of 6-substituted imidazo[2,1-b]thiazoles 4a–fa
129.87, 130.64, 131.43, 134.08, 148.20, 154.86; IR (KBr): 1525,
1340 cmÀ1; MS (EI) m/z, 259 (M+, 100), 213 (36), 137 (18), 106
(10). Anal. Calcd for C12H9N3O2S: C, 55.59; H, 3.50; N, 16.21; S,
12.37. Found: C, 55.37; H, 3.32; N, 16.45; S, 12.18.
Entry
Product
Ar
Yieldb (%)
Mp (°C)
6-(2-Methyl-4-nitrobenzyl)imidazo[2,1-b]thiazole (4c, Table 3,
entry 3). 1H NMR (DMSO-d6, 500 MHz): d 2.50 (s, 3H, CH3), 4.10
(s, 2H, CH2), 7.22–8.06 (m, 5H, thiazole, ArH), 8.14 (s, 1H, CH of
1
4a
79
224–225
NO2
imidazole); 13C NMR (DMSO-d6, 125 MHz):
d 21.30, 35.74,
110.21, 122.13, 122.90, 123.65, 126.92, 130.12, 130.65, 131.15,
133.16, 147.43, 155.30; IR (KBr): 1530, 1345 cmÀ1; MS (EI) m/z,
273 (M+, 100), 227 (48), 137 (42), 123 (10). Anal. Calcd for
C13H11N3O2S: C, 57.13; H, 4.06; N, 15.37; S, 11.73. Found: C,
57.31; H, 3.90; N, 15.51; S, 11.61.
6-(2-Chloro-4-nitrobenzyl)imidazo[2,1-b]thiazole (4d, Table 3,
entry 4). 1H NMR (DMSO-d6, 500 MHz): d 4.28 (s, 2H, CH2), 7.24–
8.28 (m, 5H, thiazole, ArH), 8.32 (s, 1H, CH of imidazole); 13C
2
3
4b
4c
78
78
229–230
254–255
NO2
H3C
NO2
NO2
Cl
NMR (DMSO-d6, 125 MHz):
d 34.88, 109.53, 123.20, 123.78,
128.31, 130.62, 131.15, 131.85, 132.21, 133.90, 151.10, 155.06; IR
(KBr): 1525, 1340 cmÀ1; MS (EI) m/z, 295 [M+(37Cl), 10], 293
[M+(35Cl), 28], 247 (100), 212 (42), 156 (37), 137 (22). Anal. Calcd
for C12H8ClN3O2S: C, 49.07; H, 2.75; N, 14.31; S, 10.92. Found: C,
49.29; H, 2.87; N, 14.20; S, 11.02.
6-(4-Chloro-2-nitrobenzyl)imidazo[2,1-b]thiazole (4e, Table 3, en-
try 5). 1H NMR (DMSO-d6, 500 MHz): d 4.34 (s, 2H, CH2), 7.26–7.93
(m, 5H, thiazole, ArH), 8.08 (s, 1H, CH of imidazole); 13C NMR
(DMSO-d6, 125 MHz): d 35.52, 110.30, 123.60, 129.21, 130.05,
130.95, 131.46, 132.04, 132.65, 134.32, 148.90, 155.45; IR (KBr):
1520, 1340 cmÀ1; MS (EI) m/z, 295 [M+(37Cl), 7], 293 [M+(35Cl),
18], 276 (100), 248 (75), 213 (50), 153 (42), 127 (40), 111 (38).
Anal. Calcd for C12H8ClN3O2S: C, 49.07; H, 2.75; N, 14.31; S,
10.92. Found: C, 48.88; H, 2.62; N, 14.47; S, 10.77.
Cl
4
5
4d
4e
4f
81
90
82
209–210
231–232
243–244
O2N
NO2
6
Cl
6-(4-Chloro-3-nitrobenzyl)imidazo[2,1-b]thiazole (4f, Table 3, en-
try 6). 1H NMR (DMSO-d6, 500 MHz): d 4.13 (s, 2H, CH2), 7.25–7.95
(m, 5H, thiazole, ArH), 8.02 (s, 1H, CH of imidazole); 13C NMR
(DMSO-d6, 125 MHz): d 34.95, 109.85, 127.02, 129.12, 129.95,
130.64, 131.14, 134.87, 135.56, 135.98, 148.80, 154.48; IR (KBr):
1530, 1350 cmÀ1; MS (EI) m/z, 295 [M+(37Cl), 13], 293 [M+(35Cl),
40], 248 (100), 214 (35), 156 (42), 138 (15). Anal. Calcd for
C12H8ClN3O2S: C, 49.07; H, 2.75; N, 14.31; S, 10.92. Found: C,
49.23; H, 2.90; N, 14.45; S, 10.81.
a
Reaction conditions: 2 (1 mmol), 3a–f (1 mmol), Cs2CO3 (3 mmol), Pd(PPh3)2Cl2
(3 mol %), CuI (7 mol %), sodium lauryl sulfate (7 mol %), degassed water (5 mL),
60 °C, 12 h.
b
Isolated yield.
CH of thiazole), 9.78 (s, 2H, NH2); IR (KBr): 3300, 3250, 2150 cmÀ1; MS
(EI) m/z, 220 [M+(81Br), 22), 218 [M+(79Br), 22], 139 (100), 112 (18), 99
(20), 80 (22), 58 (26), 45 (27). Anal. Calcd for C6H7BrN2S: C, 32.89; H,
3.22; N, 12.79. Found: C, 32.65; H, 3.03; N, 12.55.
Acknowledgment
6-Substituted imidazo[2,1-b]thiazoles; typical procedure:
A
mixture of aryl iodide (1 mmol), (PPh3)2PdCl2 (3 mol %), CuI
(7 mol %), sodium lauryl sulfate (7 mol %), and cesium carbonate
(3 mmol) was stirred in water (5 mL) at 60 °C for 30 min under
The authors are thankful to Moshashimi Company Council for
partial support of this work.

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T. A. Kamali et al. / Tetrahedron Letters 50 (2009) 5459–5462
Felpin, F.-X.; Vo-Thanh, G.; Villeras, J.; Lebreton, J. Tetrahedron: Asymmetry
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