Aqueous-phase synthesis of 2-aminothiazole and 2-iminothiazolidine derivatives catalyzed by diammonium hydrogen phosphate and DABCO

Article abstract of DOI:10.1080/00397910802219155

2-Aminothiazole and 2-iminothiazolidine derivatives were synthesized from the reaction of phenacyl bromide, thiourea, and its derivatives in aqueous media catalyzed by diammonium hydrogen phosphate (10%) and DABCO (10%) at room temperature in an efficient and simple procedure. Copyright Taylor & Francis Group, LLC.

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Aqueous-Phase Synthesis
of 2-Aminothiazole and 2-
Iminothiazolidine Derivatives
catalyzed by Diammonium
Hydrogen Phosphate and
DABCO
Saeed Balalaie a , Sahar Nikoo b & Shokat Haddadi b
a Department of Chemistry, K. N. Toosi University of
Technology, Tehran, Iran
b Department of Chemistry, Islamic Azad University,
North Tehran Branch, Tehran, Iran
Version of record first published: 09 Sep 2008
To cite this article: Saeed Balalaie, Sahar Nikoo & Shokat Haddadi (2008): Aqueous-
Phase Synthesis of 2-Aminothiazole and 2-Iminothiazolidine Derivatives catalyzed
by Diammonium Hydrogen Phosphate and DABCO, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:15,
2521-2528
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Synthetic Communications1, 38: 2521–2528, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802219155
Aqueous-Phase Synthesis of 2-Aminothiazole and
2-Iminothiazolidine Derivatives catalyzed by
Diammonium Hydrogen Phosphate and DABCO
Saeed Balalaie,1 Sahar Nikoo,2 and Shokat Haddadi2
1Department of Chemistry, K. N. Toosi University of Technology, Tehran, Iran
2Department of Chemistry, Islamic Azad University, North Tehran Branch,
Tehran, Iran
Abstract: 2-Aminothiazole and 2-iminothiazolidine derivatives were synthesized
from the reaction of phenacyl bromide, thiourea, and its derivatives in aqueous
media catalyzed by diammonium hydrogen phosphate (10%) and DABCO
(10%) at room temperature in an efficient and simple procedure.
Keywords: 2-Aminothiazole, aqueous media, DABCO, diammonium hydrogen
phosphate, 2-iminothiazolidine, phenacyl bromide, thiourea
INTRODUCTION
One of the main aims of green chemistry is the reduction of the use of
organic solvents because of the economical and environmental concerns
associated with them. Recently, organic reactions in aqueous media have
been of interest in organic synthesis.[1] In this article, we describe a user-
friendly method for the synthesis of the thiazole ring system, which is a
useful structure motif found in numerous biologically active molecules.[2]
This structure has found applications in drug development for treatment
of allergies,[3] hypertension,[4] schizophrenia,[5] inflammations,[6] and
bacterial[7] and HIV[8] infections. The 2-aminothiazole ring is a useful
structural element in medicinal chemistry. Aminothiazoles are known
to be ligands of esterogen receptors[9] as well as a novel class of adenosine
Received in the U.K. August 7, 2007
Address correspondence to Saeed Balalaie, Department of Chemistry, K. N.
Toosi University of Technology, P. O. Box 15875-4416, Tehran, Iran. E-mail:
balalaie@kntu.ac.ir
2521

2522
S. Balalaie, S. Nikoo, and S. Haddadi
receptor antagonists,[10] whereas other analogs are used as fungicides,
for in vivo growth of xanthomonas, and as an ingredient of herbicidesor
as schistosomicidal and anthelmintic drugs.[11] There are some
methods for the synthesis of thiazole derivatives based on reaction of
phenacyl bromide and thiourea in organic solvents such as DMF[12]
and 1,4-dioxane or using solid supports.[13]
Recently, synthesis of thiazole ring was reported in aqueous media in
the presence of b-cyclodextrin at 50 ꢀC.[14] All of the reported methods
have some disadvantages such as high temperatures, long reaction times,
use of hazardous solvents, and requirements for cleavage of the solid sup-
port using acids.
Following our research work to find novel natural catalysts[15] and
also to overcome some of the drawbacks in the existing methodologies,
we report here the synthesis of 2-aminothiazole and 2-iminothiazolidine
derivatives in aqueous media catalyzed by 10% diammonium hydrogen
phosphate or DABCO (10%) (Scheme 1).
RESULTS AND DISCUSSION
Reaction of phenacyl bromide and thiourea in aqueous media catalyzed
by diammonium hydrogen phosphate (10%) (method A) or DABCO
(10%) (method B) leads to 2-amino thiazole (Table 1, entries 1, 2). Mean-
while, unsymmetric N,N0-disubstituted thiourea derivatives were synthe-
sized by reaction of phenylisothiocyanate and primary amines. Reaction
of N,N0-disubstituted thiourea with phenacyl bromide leads to iminothia-
zolidine derivatives.
Diammonium hydrogen phosphate is a water-soluble catalyst and
has been used in the synthesis of heterocyclic compounds[16,17] and
alkenes.[17a] Diammonium hydrogen phosphate (NH4)2HPO4 is a very
cheap, nontoxic, and water-soluble salt whose pH in aqueous solution
(1 g=100 ml) is in the range of 7.6–8.4. This reagent has been used in
Scheme 1. (a) (NH4)2HPO4 10% mol, EtOH–H2O, rt, 30 mins; (b) DABCO 10%
mol, ETOH–H2O, rt, 30 min.

2-Aminothiazole and 2-Iminothiazolidine Derivatives
2523
Table 1. Synthesis of 2-aminothiazole and 2-iminiothiazoline derivatives
catalyzed by diammonium hydrogen phosphate and DABCO in aqueous media
Entry
Ar
R
R0
Product Yield (%) (A)a Yield(%) (B)a
1
2
3
4
5
6
C6H5
4-br-C6H4
C6H5
C6H5
C6H5
C6H5
H
H
H
H
3a
3b
3c
3d
3e
3f
87
98
70
85
76
86
96
97
55
87
76
85
CH3 CH3
Ph
Ph
Ph
CH3
C2H5
Ph
aIn all cases, the yields are related to pure isolated compounds.
the manufacturing of many important materials such as fireproof textiles,
paper, wood, and vegetable fibers.[18] It has also been used as a corrosion
inhibitor and fertilizer.[19] Diammonium hydrogen phosphate is a com-
mercially available compound that can be used in the laboratory without
any special precaution. DABCO is the most commonly used catalyst in
the Baylis–Hillman reaction.[20]
Table 1 summarizes the results of our study. The structures of
compounds were deduced from their elemental analysis and their IR
and 1H NMR spectra. For example, 1H NMR of 3c exhibited two singlets
=
identified as methyl (d 3.06 and 3.42 ppm) and an CH proton along with
singlet (d 7.10 ppm) and multiplets at d 7.52 ppm for the aromatic
protons. Diammonium hydrogen phosphate and DABCO in catalytic
amounts can provide a basic solution that activates thiourea and its
derivatives to react with phenacyl bromide in aqueous media at room
temperature.
A proposed mechanism for the reaction is outlined in Scheme 2.
Based on this mechanism, the basic media provided by DABCO and
diammonium hydrogen phosphate in aqueous media makes the
assumption reasonable that thiourea is activated in basic media and
could be added to phenacyl bromide. The intermediate 4 could be acti-
vated and cyclized to compound 5, which could be converted to our
desired product 3a, b after dehydration and proton transfer. A similar
mechanistic trend is suggested for the reaction of unsymmetric N,N0-
disubstituted thiourea and phenacyl bromide, which is shown in
Scheme 2.
In conclusion, we have introduced here a new method for the
synthesis of 2-aminothiazole and 2-iminothiazolidine derivatives that
can be promoted by DABCO and diammonium hydrogen phosphate in
aqueous media at room temperature. Easy workup, short reaction times,

2524
S. Balalaie, S. Nikoo, and S. Haddadi
Scheme 2. Proposed mechanism for the synthesis of 2-aminothiazole and 2-
iminodihydrothiazole derivatives catalyzed by DABCO or diammonium hydrogen
phosphate in aqueous media.
high yields, and environmentally friendliness are advantages of our
method over previously reported methods.
EXPERIMENTAL
Melting points were recorded on an Electrothermal 9100 melting-point
apparatus and are uncorrected. IR spectra were recorded on a FT-IR
1
Perkin-Elmer GX spectrophotometer using KBr disks. H NMR spectra
were recorded on Bruker DRX-300 and DRX-500 Avance spectrometer
at 300 and 500 MHz in DMSO-d6 using TMS as the internal standard.
Elemental analyses (C, H, N) were performed using a Heraeus CHN
Rapid analyzer.

2-Aminothiazole and 2-Iminothiazolidine Derivatives
2525
General Procedure for the Synthesis of Compounds 3a–f
Method A
A solution of phenacyl bromide (1 mmol, 199 mg), thiourea or disubsti-
tuted thiourea (1.2 mmol), and diammonium hydrogen phosphate (14 mg,
10%) in a 20-ml solution of water–ethanol (1:1) was stirred at room
temperature for 30 min. The progress of reaction was monitored by thin-
layer chromatography (TLC, eluent: ethyl acetate). The precipitate was
collected and washed with water. The crude products were purified by
crystallization from EtOH.
Method B
A solution of phenacyl bromide (1 mmol, 199 mg), thiourea or disubsti-
tuted thiourea (1.2 mmol), and DABCO (11 mg, 10%) in a 20-ml solution
of water-ethanol (1:1) was stirred at room temperature for 30 min. The
progress of reaction was monitored by TLC (eluent: ethyl acetate). The
precipitate was collected and washed with water. The crude products
were purified by crystallization from EtOH.
Selected Data for Compounds 3a–f
Compound 3a: 2-amino-4-phenylthiazole; mp 150–151 ꢀC (151–152 ꢀC)[21]
;
IR (KBr, cmꢁ1): 3434, 1618, 1519; 1H NMR (300 MHz, DMSO-d6) d: 6.90
=
(d, 1H, J ¼ 2.8 Hz, CH), 7.05 (s, 2H, NH2), 7.23 (t, 1H, J ¼ 6.3 Hz, H-
Ar), 7.34 (t, 2H, J ¼ 6.9 Hz, H-Ar), 7.78 (d, 2H, J ¼ 7.5 Hz, H- Ar).
Compound 3b: 2-amino-4-(4-bromophenyl)-thiazol; mp 179–181 ꢀC (180–
ꢀ
1
181 C)22; IR ( KBr, cmꢁ1): 3425, 1636, 1617, 1533; H NMR (500 MHz,
=
DMSO-d6) d: 7.09 (s, 1H, CH), 7.11 ( s, 2H, NH2), 7.55 (d, 2H,
J ¼ 7.5 Hz , H- Ar), 7.74 (d, 2H, J ¼ 7.5 Hz, H-Ar).
Compound 3c: N-(3-methyl-4-phenylthiazol-2(3H)-ylidene) methan
amine; mp 156–157 ꢀC, IR (KBr, cmꢁ1): 1638, 1618; 1H NMR
(300 MHz, DMSO-d6) d: 3.06 (s, 3H, CH3), 3.42 (s, 3H, CH3), 7.10 (s,
=
1H, CH), 7.52 (m, 5H, H-Ar), 9.87 (s, NH), C11H12N2S calcd. C,
64.67; H, 5.92; N, 13.71; S, 15.69, Found: C, 64.55; H, 6.61; N, 13.64;
S, 15.52.
Compound 3dꢀ: N-(3-methyl-4-phenylthiazol-2(3H)-ylidene) phenyl amine;
mp 169–170 C; IR (KBr, cmꢁ1): 1610, 1579; 1H NMR (500 MHz,
=
DMSO-d6) d: 3.28 (s, 3H, CH3), 6.22 (d, 2H, J ¼ 0.93 Hz, CH), 7.01 (t,
3H, J ¼ 7.35 Hz, Ar), 7.33 (t, 2H, J ¼ 7.26 Hz, Ar), 7.50 (s, 5H, Ar).

2526
S. Balalaie, S. Nikoo, and S. Haddadi
C16H14N2S calcd. C, 72.15; H,5.30; N, 10.52; S, 12.04 Found: C, 71.90,
H, 5.41; N, 10.46; S, 12.10.
Compoꢀund 3e: N-(3-ethyl-4-phenylthiazol-2(3H)-ylidene) phenyl amine;
1
mp 72 C; IR (KBr, cmꢁ1): 1616, 1555; H NMR (500 MHz, DMSO-d6)
d: 1.1 (t, 3H, J ¼ 6.92 Hz, CH3), 3.8 (q, 2H, J ¼ 6.92 Hz, CH2), 6.17
=
(s, 1H, CH), 7.01 (t, 3H, J ¼ 7.74 Hz, Ar), 7.33 (t, 2H, J ¼ 7.56 Hz,
Ar), 7.5 (m, 5H, Ar). C17H16N2S calcd. C, 72.82; H, 5.75; N, 9.99; S,
11.44, Found: C, 72.73; H, 5.71; N, 9.99; S, 11.30.
Compound 3f: N-(3,4-diphenylthiazol-2(3H)-ylidene) phenyl amine; mp
ꢀ
1
192–193 C; IR (KBr,cmꢁ): 1617, 1578; H NMR (500 MHz, DMSO-d6)
=
d: 6.49 (s, 1H, CH), 6.92 (d, 2H, J ¼ 7.45 Hz, H-Ar), 7.02 (t, 1H,
J ¼ 7.36 Hz, H-Ar), 7.16 (m, 2H,H-Ar), 7.24 (m, 3H, H-Ar), 7.29
(m, 5H, H-Ar), 7.36 (t, 2H, J ¼ 7.46 Hz, H-Ar). C 21H16N2S calcd. C,
76.78; H, 4.90; N, 8.53; S, 9.76, Found: C, 76.38; H, 5.10; N, 8.41; S, 9.60.
ACKNOWLEDGMENTS
Saeed Balalaie is grateful to Iran National Science Foundation (INSF)
(Grant No.85043=11) for the financial support.
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