Lipase and deep eutectic mixture catalyzed efficient synthesis of thiazoles in water at room temperature

Article abstract of DOI:10.1007/s10562-012-0902-5

Lipase bio-catalyst or ammonium based deep eutectic mixture efficiently catalyzed aqueous phase synthesis of methyl-thiazole and amino-thiazole derivatives. The simple ammonium deep eutectic catalyst, easily synthesized from choline chloride and urea, is inexpensive, recyclable and bio-degradable, making it suitable for industrial applications. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10562-012-0902-5

Catal Lett (2012) 142:1369–1375
DOI 10.1007/s10562-012-0902-5
Lipase and Deep Eutectic Mixture Catalyzed Efficient Synthesis
of Thiazoles in Water at Room Temperature
•
Hyacintha Rennet Lobo Balvant Shyam Singh
•
Ganapati Subray Shankarling
Received: 3 July 2012 / Accepted: 26 August 2012 / Published online: 18 September 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Lipase bio-catalyst or ammonium based deep
eutectic mixture efficiently catalyzed aqueous phase syn-
thesis of methyl-thiazole and amino-thiazole derivatives.
The simple ammonium deep eutectic catalyst, easily syn-
thesized from choline chloride and urea, is inexpensive,
recyclable and bio-degradable, making it suitable for
industrial applications.
Since then, many improved methods have been reported
for the synthesis of thiazoles using catalyst such as:
(a) ammonium-12-molybdophosphate in methanol [11],
(b) iodine [12], (c) a-tosyloxyketones with thioureas in
PEG-400 [13], (d) Cu(OTf)2-catalyzed coupling of a-dia-
zoketones with thiourea [14], (e) b-cyclodextrin [15].
Although these methods are efficient in terms of yields and
some even use greener methods, however, most of these
methods hold some demerits which we intend to over-
come. The method (a) uses heteropoly acid as an efficient
catalyst but uses methanol as solvent whereas method
(b) requires overnight heating at 100 °C and method
(c) uses larger quantity of PEG-400 (as reaction medium).
The method (d) involving diazoketones require toxic metal
catalysts in addition to consuming comparatively longer
reaction time (2–3 h) and higher temperature (80 °C). The
method (e) was greener but no study of recyclability was
demonstrated. We have tried to overcome the limitations
and retain the benefits of the reported methods by using
bio-catalysts or simple deep eutectic mixtures as catalysts
in water. Srinivasan and co-workers [16] reported synthe-
sis of amino-thiazoles in aqueous media, but we employ
greener catalysts to make this aqueous process even more
efficient.
Keywords Thiazole Á Deep eutectic mixtures Á Lipase Á
Phenacyl bromide Á Urea
1 Introduction
Thiazole derivatives are found to be associated with several
biological activities which has made them extremely useful
in the treatment of hypertension [1], schizophrenia [2],
inflammation [3], and HIV [4] infections. Its derivatives
like aminothiazoles are known to be ligands of estrogen
receptors [5] and also adenosine receptor antagonists [6].
Drug formulations containing 2-aminothiazoles are cur-
rently available for the treatment of inflammation, breast
cancer, HIV-1 and rheumatoid arthritis [7–9]. Among the
synthetic methods, Hantzsch synthesis [10] is a direct
process involving one-pot condensation of a-haloketones,
and thiourea or thioamides in refluxing alcohol.
Bio-catalysis is known to be an efficient and green tool
for organic synthesis due to its high selectivity and
requirement of mild reactions conditions [17]. Lipases are
ubiquitous enzymes of considerable physiological impor-
tance and industrial potential. Lipases catalyze a number of
useful reactions including esterification [18], transesterifi-
cation [19], regioselective acylation of glycols and men-
thols [20], and synthesis of peptides [21]. Although the
utility of lipases has been observed for various reactions as
discussed above, its effectiveness for thiazole synthesis has
not yet been explored to the best of our knowledge.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-012-0902-5) contains supplementary
material, which is available to authorized users.
H. R. Lobo Á B. S. Singh Á G. S. Shankarling (&)
Department of Dyestuff Technology, Institute of Chemical
Technology, N. P. Marg, Matunga, Mumbai 400019, India
e-mail: gsshankarling@gmail.com;
gs.shankarling@ictmumbai.edu.in
123

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H. R. Lobo et al.
Thiazole synthesis has also been performed in ionic
reported in the literature [29]. The preparation involved
reaction of choline chloride 1 (1 mol) with urea 2 (2 mol)
at 74 °C (Scheme 1) till a clear solution was obtained
which was used for reactions without any purification. This
method gives deep eutectic solvent 3 with 100 % atom
economy without any other by-product formation.
liquids [22]. However, the ionic liquids based on imidazole
and fluorinated anions suffer from the demerits of being
toxic and commercially expensive [23]. In this context,
deep eutectic mixtures could be a good alternative that
include simple eutectic combinations of ammonium salts
such as choline chloride with different hydrogen bond
donors like urea or lewis acids such as zinc chloride.
Choline is a naturally occurring bio-compatible compound
and choline chloride is also commercially produced on a
large scale as a chicken feed additive whereas urea is found
to be present in animal waste product [24]. The deep
eutectic solvents would thus be an environmental-friendly
and economically viable alternative for conventional cat-
alysts not only on lab scale but also with regard to indus-
trial applications. In the past few years, our research group
has focused towards application of deep eutectic mixtures
as efficient solvents or catalysts [25–28].
2.2 Typical Procedure for Lipase-Catalyzed Reaction
Phenacyl bromide 4 (R’=H) (1 g, 5.0 mmol) was added to
water (10 mL, 10 vol) containing lipase catalyst (100 mg,
10 % by weight of phenacyl bromide derivative) to which
thiourea 5 (R=NH2) (0.40 g, 5.0 mmol) was added. The
reaction mixture was stirred in at room temperature under
vigorous stirring for 20 min. The progress of the reaction was
monitored by TLC. After completion of the reaction, the
reaction mass was filtered through a Whatmann No. 42 under
suction to remove water. The solid compound on filter paper
containing both product and lipase catalyst was added to
methanol (7 mL). This solution was then filtered off through
a Whatmann No. 42 filter paper to recover the bio-catalyst.
The filtrate containing methanol was evaporated under
reduced pressure to obtain the crude solid product. The crude
product was further purified by column chromatography
using toluene as eluent to afford the pure product 6.
In viewoftheemergingimportance ofgreenercatalysts, we
have explored for the first time, the role of lipase catalyst and a
deep eutectic mixture, prepared from choline chloride and
urea, as efficient and recyclable catalyst in water for synthesis
of different thiazole derivatives at room temperature.
2 Experimental
2.3 Typical Procedure for Deep Eutectic
Mixture-Catalyzed Reaction
The enzyme lipase was acquired from Sigma Aldrich. It was
obtained from Pseudomonas sp. strain with particle size of
less than 60–100 mesh and activity of 30,000 u g-1. FT–IR
spectrums were recorded on a Bomem Hartmann and Braun
A mixture of phenacyl bromide 4 (R’=H) (1 g, 5.0 mmol) and
thiourea 5 (R=NH2) (0.40 g, 5.0 mmol) was stirred in water
(10 mL) containing 20 % deep eutectic mixture catalyst
(2 mL) at room temperature under vigorous stirring for
20 min. The progress of the reaction was monitored by TLC.
After completion of the reaction, the solid product was fil-
tered off and the aqueous layer was evaporated under
reduced pressure to recover the deep eutectic mixture. The
crude product was further purified by column chromatogra-
phy using toluene as eluent to afford the pure product 6.
Both the methods were scaled-up to 50 g to obtain 94 %
yield in lipase-catalyzed reactions and 95 % yield in deep
eutectic mixture catalyzed reactions.
1
MB-Series FT–IR spectrometer. H NMR spectrums were
recorded on Varian 300 MHz mercury plus spectrometer and
mass spectral data were obtained with a micromass-Q-TOF
(YA105) spectrometer. Common reagent grade chemicals
were procured from M/s S.D. Fine Chemical Ltd. India and
were used without further purification.
2.1 Preparation of Deep Eutectic Solvent
In this study, two deep eutectic solvents (deep eutectic
mixture) were synthesized according to the procedures
Scheme 1 Preparation of deep
eutectic mixture from choline
chloride and urea
CH3
N
CH3
O
H3C
CH3
H3C
CH3
Heated at 74 oC
N
O
Cl-
Cl-
+
H2N
NH2
H2N
NH2
OH
OH
Choline Chloride (1 mole)
Urea (2 moles)
Deep eutectic solvent
1
2
3
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Lipase and Deep Eutectic Mixture Catalyzed Efficient Synthesis of Thiazoles
1371
Scheme 2 Aqueous-phase
synthesis of thiazole in presence
R'
O
S
of deep eutectic mixture/lipase
catalyst
Br
catalyst
N
S
R
NH2
water
room temperature
R
R'
5
4
6
where R=NH2, CH3, NH-Ph, NH-Ph-OCH3
R'=H, NO2, Br, OCH3
Table 1 Optimization of reaction under different quantities of catalyst in thiazole synthesis from phenacyl bromide with thioamide derivatives
Entry
Solvent
Catalyst
Thiourea yielda (%)
Thioacetamide yielda (%)
1
2
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
–
63
82
85
90
95
96
67
89
94
95
55
60
68
80
85
87
60
78
82
82
5 % DES (ChCl:urea)b
10 % DES (ChCl:urea)b
15 % DES (ChCl:urea)b
20 % DES (ChCl:urea)b
30 % DES (ChCl:urea)b
30 % DES (ChCl:glycerol)b
Lipase (5 % by weight)c
Lipase (10 % by weight)c
Lipase (15 % by weight)c
3
4
5
6
7
8
9
10
All reactions were carried out with phenacyl bromide (5.0 mmol), thiourea/thioacetamide (5.0 mmol), water (10.0 vol); reaction time: 25 min
a
Isolated yields of thiazole derivatives
b
v/v% of DES in water
c
Percentage by weight of phenacyl bromide
Spectral data for representative compounds.
(aromatic C–C ring stretch) 1324, 1199 (C–N stretch band),
1296 (C–O stretch); Anal. Calcd for C16H14N2OS: C,
68.06; H, 5.00; N, 9.92; O, 5.67; S, 11.36. Found: C, 67.97;
H, 4.21; N, 10.06; S, 11.07.
4-Phenylthiazol-2-amine (6a): dH(300 MHz; CDCl3;
Me4Si) 7.76 (s, 1H), 7.32 (s, 2H), 7.19 (m, 5H); mmax cm-1
3437 (N–H Stretch), 3114 (C–H Stretch), 1598 (N–H
bend), 1531, 1515 and 1482 (aromatic C–C ring stretch),
1330 (C–N stretch band); Anal. Calcd for C9H8N2S: C,
61.34; H, 4.58; N, 15.90; S, 18.19. Found: C, 62.10; H,
4.73; N, 15.41; S, 18.86.
4-(4-Nitrophenyl)thiazol-2-amine (6e): dH(300 MHz;
CDCl3; Me4Si) 8.21–8.03 (m, 4H), 7.39 (s, 1H), 7.21 (s, 2H);
mmax cm-1 3396 (N–H Stretch), 3114 (C–H Stretch), 1591
(N–H bend), 1536, 1319 (N–O stretch), 1640, 1482 (aro-
matic C–C ring stretch), 1410, 1108 (C–N stretch band);
Anal. Calcd for C9H7N3O2S: C, 48.86; H, 3.19; N, 18.99; O,
14.46; S, 14.49. Found: C, 47.77; H, 2.25; N, 17.24; S, 14.45.
2-Methyl-4-phenylthiazole (6b): dH(300 MHz; CDCl3;
Me4Si) 7.30–7.87 (m, 6H), 2.78 (s, 3H); mmax cm-1 3106
(C–H Stretch), 1498 (aromatic C–C ring stretch), 1439 (C=N
stretch); Anal. Calcd for C10H9NS: C, 68.53; H, 5.18; N,
7.99; S, 18.30. Found: C, 68.21; H, 4.49; N, 8.22; S, 18.44.
N,4-Diphenylthiazol-2-amine (6c): dH(300 MHz; CDCl3;
Me4Si) 7.76–7.19 (m, 10H), 7.06 (s, 1H), 6.72 (s, 1H); mmax
cm-1 3300 (N–H Stretch), 2957 (C–H Stretch), 1598 (N–H
bend), 1499 (C–C ring stretch), 1314 (C–N stretch band);
Anal. Calcd for C15H12N2S: C, 71.40; H, 4.79; N, 11.10; S,
12.71. Found: C, 70.45; H, 4.15; N, 10.97; S, 12.30.
3 Results and Discussion
The reaction for synthesis of thiazole from phenacyl bro-
mide and thioamide derivatives (Scheme 2) was initially
optimized for deciding the amount of lipase or deep
eutectic mixture catalyst. In case of lipase-catalyzed reac-
tions, 10 % lipase by weight of phenacyl bromide was
found to give the best results. It could also be observed
from Table 1 that 20 and 30 % deep eutectic mixture cat-
alyst (v/v% of deep eutectic mixture in water) gave almost
N-(4-Methoxyphenyl)-4-phenylthiazol-2-amine (6d): dH
(300 MHz; CDCl3; Me4Si) 7.82–6.9 (m, 10H), 6.72 (s,
1H), 3.83 (s, 3H); mmax cm-1 3379 (N–H Stretch), 2837
(C–H Stretch), 1597 (N–H bend), 1556, 1508 and 1480
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H. R. Lobo et al.
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Lipase and Deep Eutectic Mixture Catalyzed Efficient Synthesis of Thiazoles
1373
The recycling of lipase and deep eutectic mixture cata-
lysts were studied up to four runs considering the amino-
thiazole synthesis from phenacyl bromide and thiourea as
the standard reaction. The results of recycling studies are
summarized in Fig. 1. It could be observed that both the
catalysts can be recycled efficiently at least up to four times
with lipase showing reduction in activity after second run
whereas deep eutectic mixture gives quite high yields even
after four runs. However, the color of DES gets slightly
darkened after each run.
In lipase-catalyzed reaction, methanol was added for
solubilizing the product that would enable the separation of
lipase catalyst. The methanol was recovered by distillation
of the filtrate under vacuum. This process also uses only
methanol which is greener than many organic volatile
solvents. In deep eutectic mixture-catalyzed reaction, the
reaction mass was simply filtered through a filter paper to
obtain crude solid product where the deep eutectic catalyst
could be recovered in case of scale-up batches by removing
water under vacuum from the filtrate. The work-up, thus,
did not involve any volatile organic solvent and is com-
pletely eco-friendly in nature.
Fig. 1 Recycling studies of DES and lipase catalyst in thiazole
synthesis from phenacyl bromide and thiourea
similar results. Hence, the quantity of deep eutectic mixture
was optimized to 20 %. Thus, the addition of catalysts
(lipase or deep eutectic mixture) improved the yields in
comparison to reaction in water.
A variety of amino and methylthiazole derivatives were
synthesized in water in presence of two bio-degradable
catalysts including lipase or deep eutectic mixtures as
summarized in Table 2. The reaction conditions were mild
(room temperature) and the products were obtained within
much shorter times and in excellent yields thus giving
improved results as compared to previous procedures
reported in water.
Although the mechanism illustrating the role of such
deep eutectic solvents or lipase catalyst in thiazole syn-
thesis is yet to be confirmed, we suggest a mechanism
predicting their probable role in synthesis. In case of lipase
catalyst, the imidazole ring of the Asp-His dyad present in
lipase catalyst could abstract a proton from the thioamide
moiety thus enabling its attack on the phenacyl bromide
His
His
His
Asp
Asp
O
N
N
Asp
H
O
N
N
H
O
O
N
N
H
O
O
H
O
H
H
N
- HBr
NH
Ar
S
R
NH
O
Ar
-S
R
S
R
Proton abstraction of thioamide by Asp-His dyad
Br
His
His
His
Ar
Asp
Asp
Asp
N
O
N
N
O
N
N
O
N
N
H
H
H
H
- H2O
O
O
O
R
S
Ar
H
R
N
Ar
Ar
O
HO
N+
N
H
N
H
N
N
S
R
O
H
H
N
S
N+
S
R
oxyanion
hole
oxyanion
hole
H
H
N+
Negative charge is stabilized by oxyanion hole
Scheme 3 Proposed mechanism for thiazole synthesis in lipase-catalyzed medium
123

1374
H. R. Lobo et al.
CH3
N
H3C
CH3
O
R
H
NH2
N
H
H
H
Cl-
OH
N
N
H
H
O
CH3
N
O
O
H
H3C
CH3
H
H
N
N
Br
H
H
R
R
R
Cl-
O
OH
O
O
H
OH
N
H
N
NH2
N
N
H
CH2
R1
S
R1
(-HBr)
R1
S
S
(-H2O)
S
C
H
R
R1
N
H
N
R1
S
Scheme 4 Proposed mechanism for thiazole synthesis in deep eutectic mixture-catalyzed medium
derivative as shown in Scheme 3. The negative charge
developed on oxygen atom during cyclization step is fur-
ther stabilized by the oxyanion hole.
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