Selenium-catalyzed carbonylation of 2-aminothiazole with nitro aromatics to N-aryl-N′-2-thiazolylureas

Article abstract of DOI:10.1016/j.tetlet.2016.10.046

An efficient, economical, and phosgene-free procedure for the synthesis of N-aryl-N′-2-thiazolylureas is reported. With cheap and recyclable nonmetal selenium instead of noble metals as the catalyst, carbon monoxide instead of virulent phosgene as the carbonylation agent, the selenium-catalyzed carbonylation reaction of 2-aminothiazole can proceed smoothly in one-pot manner with a variety of nitro aromatics in the presence of triethylamine to afford the desired N-aryl-N′-2-thiazolylureas mostly in moderate to good yields. Selenium catalyst can be easily recovered due to its phase-transfer catalytic ability and reused without any significant degradation of its catalytic performance.

Full text of DOI:10.1016/j.tetlet.2016.10.046

Accepted Manuscript
Selenium-catalyzed carbonylation of 2-aminothiazole with nitro aromatics to
N-aryl-N’ -2-thiazolylureas
Xiaopeng Zhang, Zhaojing Tang, Xueli Niu, Zhengwei Li, Xuesen Fan,
Guisheng Zhang
PII:
S0040-4039(16)31360-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.10.046
TETL 48216
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 September 2016
10 October 2016
14 October 2016
Please cite this article as: Zhang, X., Tang, Z., Niu, X., Li, Z., Fan, X., Zhang, G., Selenium-catalyzed carbonylation
of 2-aminothiazole with nitro aromatics to N-aryl-N’ -2-thiazolylureas, Tetrahedron Letters (2016), doi: http://
dx.doi.org/10.1016/j.tetlet.2016.10.046
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Graphical Abstract
Selenium-catalyzed carbonylation of 2-
aminothiazole with nitro aromatics to N-
aryl-N’-2-thiazolylureas
Leave this area blank for abstract info.
Xiaopeng Zhang*, Zhaojing Tang, Xueli Niu, Zhengwei Li, Xuesen Fan, Guisheng Zhang*
H
H
N
S
Et3N
N
N
N
+
+ ArNO2
NH2
CO
Ar
Cat. Se
S
O
Cheap catalytic system & reactants!
14 examples
up to 87% yield
One-pot phosgene-free manner!
Phase-transfer catalysis!
Nice
recyclability

1
Tetrahedron Letters
Selenium-catalyzed carbonylation of 2-aminothiazole with nitro aromatics to N-aryl-
N’-2-thiazolylureas
Xiaopeng Zhang, Zhaojing Tang, Xueli Niu, Zhengwei Li, Xuesen Fan, Guisheng Zhang*
Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions,
Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, PR China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient, economical and phosgene-free procedure for the synthesis of N-aryl-N’-2-
thiazolylureas is reported. With cheap and recyclable nonmetal selenium instead of noble metals
as the catalyst, carbon monoxide instead of virulent phosgene as the carbonylation agent, the
selenium-catalyzed carbonylation reaction of 2-aminothiazole can proceed smoothly in one-pot
manner with a variety of nitro aromatics in the presence of triethylamine to afford the desired N-
aryl-N’-2-thiazolylureas mostly in moderate to good yields. Selenium catalyst can be easily
recovered due to its phase-transfer catalytic ability and reused without any significant
degradation of its catalytic performance.
Available online
Keywords:
Selenium
Carbonylation
N-Aryl-N’-2-thiazolylureas
2-Aminothiazole
Phase-transfer catalysis
2009 Elsevier Ltd. All rights reserved.
N-Aryl-N’-2-thiazolylureas, as
a
significant class of
also emerged as a practical approach to unsymmetrical N-
arylureas (such as N-aryl-N’-2-thiazolylureas),13 but both the
catalyst and the ligand required for this approach are expensive
(Scheme 1). Despite the above developments, there is still no
compounds possessing
a urea linkage and a 5-member
heterocycle, have attracted wide attention due to their broad
spectrums of pharmacological and biological activities, such as
antioxidant,1 anticholinesterase,1 anticancer,2 antibacterial,3
antiviral,4 myorelaxant,5 antiplasmodial,6 and cytokinin activity,7
etc (Figure 1). Hence, many efforts have been made towards their
synthesis. The classical method relies on the phosgenation of 2-
aminothiazole with aromatic amines.8 However, the virulent
nature of phosgene and the generation of corrosive HCl are the
main drawbacks associated with it. Carbonyldiimidazole,4,9
chloroformate,10 and triphosgene11 can also be applied as the
substitutes for phosgene for this purpose, but they are limited by
the low atom economy, high cost of these carbonylation reagents
and the generation of corrosive HCl in some cases. Addition of 2-
aminothiazole to the corresponding isocyanates provides an
H
H
N
N
N
H
H
N
N
N
OCH3
S
O
O
O
S
O
O
Antioxidant activity
Anticancer activity
O
O
H
H
H
H
N
N
N
F
F
N
N
N
O2N
S
O
S
O
NC
F
Anticholinesterase activity
Antibacterial activity
H
H
N-aryl-N’-2-thiazolylureas.1a,5,6
H
H
N
N
N
alternative
approach
to
N
N
N
H3CO
N
Unfortunately, the significant defects accompanied with the
corresponding isocyanates such as high cost, low availability,
strong excitant nature, and harsh reaction conditions limit its
application. Aminolysis of the corresponding carbamates with 2-
aminothiazole can also realize this transformation,10a,12 howerer,
S
O
S
O
Antiviral activity
Antiplasmodial activity
Figure 1. Select examples of bioactive N-aryl-N’-2-thiazolylureas.
this method suffers from the drawbacks such as low atom
economy, high cost and low availability of the carbamates.
Recently, palladium-catalyzed cross-coupling of aryl chlorides
with sodium cyanate in the presence of a biarylphosphine ligand
economical and eco-friendly protocol for the synthesis of N-aryl-
N’-2-thiazolylureas.
———
 Corresponding author. Tel./fax: +86-373-3325250; e-mail: zhangxiaopengv@sina.com (X. Zhang), zgs@htu.cn (G. Zhang)

2
Tetrahedron Letters
O
N
S
N
N
S
O
+
+
+
+
+
ArNCO
+
+
+
+
NH2
NH2
NH2
NH2
NH2
ArNH2
ArNH2
ArNH2
ArNH2
N
N
Cl
Cl
S
N
N
r
9
e
,
4
f
.
ref. 1a, 5, 6
.
8
f
e
-
r
H
NH
C
l
N
-
H
H
N
S
N
S
O
O
N
ref. 10a, 12
- ROH
N
N
ref. 10
- HCl, - ROH
+
Ar
NH2
RO
Cl
ArNH
OR
S
O
r
e
3
f
d
1
.
n
.
f
1
a
1
e
g
-
H
r
i
L
,
C
This work Cat. Se
l
a 3
b
l
d
C
d 2
H
P
-
N
S
N
S
N
S
O
NaOCN +
+
NH2
Ar Cl
+ CO +
ArNO2
NH2
Cl3CO
OCCl3
Scheme 1. Previous and present synthetic approaches to N-aryl-N’-2-thiazolylureas.
Over the past decades, catalyzed by transition metals, carbon
with Se/CO catalytic system. Herein, we wish to report an
efficient and economical approach to these target products via
one-pot carbonylation of 2-aminothiazole with nitro aromatics
using cheap and easily available selenium as the catalyst and
carbon monoxide as the carbonylation agent in the presence of
triethylamine (Scheme 1).
monoxide has been used as a readily available and cheap C1
source to prepare various carbonyl containing products.14
However, high cost and poor recyclability of the transition metal
catalysts are the unavoidable weakness presented in this catalytic
system. Thus, further pursuit of a cheap and recyclable catalyst
for this carbonylation approach is highly desirable. Fortunately,
cheap and readily available nonmetal selenium was found by
Sonoda15 to be an effective catalyst for carbon monoxide at this
stage. Henceforth, Se/CO catalytic system has been widely used
for the synthesis of a variety of carbonyl containing products
such ureas,16 carbamates,16a,17 thiocarbamates,16a,18 and
carbonates,16a,19 etc. To the best of our knowledge, there are no
literature examples of the synthesis of N-aryl-N’-2-thiazolylureas
Table 1. Optimization of the reaction conditionsa
Initially, the carbonylation of 2-aminothiazole with
nitrobenzene (1a) was chosen as a model reaction for optimizing
the reaction conditions (Table 1). The reaction failed to proceed
in the absence of selenium (Table 1, entry 1), which indicated
that selenium catalyst was essential for this carbonylation
reaction. Next, the selenium load was screened and the results
revealed that 0.25 mmol was the best choice (Table 1, entries
H
H
N
N
N
N
Cat. Se
NH2
+
+
NO2
CO
S
S
O
1a
2a
Entry
Se (mmol)
Base
Temperature (°C)
Material ratiob
CO pressure (MPa)
Solvent
Yieldc (%)
1
-
Et3N
Et3N
Et3N
Et3N
130
130
130
130
130
130
130
130
90
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:2
2:1
1:1
1:1
1:1
1:1
1:1
3
3
3
3
3
3
3
3
3
3
3
3
3
2
4
3
3
3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
EtOAc
0
2
0.15
0.25
0.35
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
62
77
75
0
3
4
5
-
6
NaOH
K2CO3
49
53
72
39
68
78
23
78
55
74
35
52
50
7
8
Pyridine
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
9
10
11
12
13
14
15
16
17
18
110
150
130
130
130
130
130
130
130
THF
Acetone
a Reaction conditions: 2-aminothiazole (5 mmol), base (10 mmol), solvent (10 mL), 8 h.
b n(2-aminothiazole) : n(nitrobenzene).
c Isolated yield of N-phenyl-N’-2-thiazolylurea.
2-4). Generally, selenium-catalyzed carbonylation reaction
should proceed in a proper alkaline condition, which is believed
to promote the formation of the active carbonyl selenide (COSe).
As expected, no desired product was obtained when the reaction
conducted in the absence of a base (Table 1, entry 5). Both the
common inorganic bases such as NaOH and K2CO3 and organic
bases such as Et3N and pyridine were test for the present catalytic
system, and among them, Et3N was found to be most effective

3
(Table 1, entries 3, 6-8). The reaction seemed to be very sensitive
to temperature. When it proceeded at 90 °C, only 39% of the
desired product was obtained (Table 1, entry 9). The product
yield increased with the rise of temperature and could reach a
satisfactory value at 130 °C. Further increase of the reaction
temperature failed to improve the product yield significantly
(Table 1, entries 3, 10, 11). Next, the raw material ratio of 2-
aminothiazole to nitro benzene was investigated and 1:1 was
found a proper proportion (Table 1, entries 3, 12, 13). The
influence of CO pressure was also checked and the results
indicating that 3 MPa could satisfy the reaction (Table 1,
entries3, 14, 15). Finally, the solvents such as toluene, EtOAc,
THF and acetone were screened for this reaction and toluene
provided the best results (Table 1, entries 3, 16-18).
OCH3
NO2
10
1j
2j
47
11
12
13
14
1k
1l
2k
2l
82
54
65
72
H3CO
Br
NO2
NO2
NO2
1m
1n
2m
2n
Br
NO2
a Reaction conditions: 2-aminothiazole (5 mmol), nitro aromatic (5 mmol), Se
(0.25 mmol), Et3N (10 mmol), CO (3 MPa), toluene (10 mL), 130 °C, 8 h.
b Isolated yield of N-aryl-N’-2-thiazolylurea.
Compared to the difficulty in recovering the catalyst from the
common homogeneous system, one prominent advantage of the
present catalytic system is that selenium catalyst can be easily
recovered for further use due to its role as a phase-transfer
catalyst. Specifically, selenium power is insoluble in the catalytic
system prior to the carbonylation reaction; then selenium reacts
with CO to from carbonyl selenide in situ to initiate the
carbonylation reaction, which can dissolve in the reaction
mixture to form a homogeneous catalytic system during the
reaction process; after the reaction, selenium power can be
precipitate out of the reaction medium conveniently by
oxidization. Thus, catalyst selenium can easily be recovered by
simple filtration and drying, and more importantly, which can be
recycled in subsequent reactions. The reusability of the recovered
selenium was tested by taking the carbonylation reaction of 2-
aminothiazole with nitrobenzene as an example. As shown in
Table 3, the yield of the desired N-phenyl-N’-2-thiazolylurea
only dropped from 77% to 73% after four cycles, indicating that
the catalytic performance of the recovered selenium was almost
the same as that of the fresh selenium considering its small loss
in each recovery step.
With the optimized reaction conditions in hand, we then
turned to explore the scope and efficiency of the present catalytic
system (Table 2). In general, the selenium-catalyzed
carbonylation reaction of 2-aminothiazole could proceed
smoothly with a variety of nitro aromatics, affording the
corresponding N-aryl-N’-2-thiazolylureas mostly in moderate to
good yields. And accompanied by the target products, a little
symmetrical diaryl urea and di(2-thiazo1yl)urea could be
detected existing in the reaction mixture, which indicated that the
competitive self-carbonylation of nitro aromatics and 2-
aminothiazole also occurred. The present carbonylation reaction
seemed very sensitive to steric factors. Typically, ortho-
substituted nitro benzenes led to lower product yields (Table 2,
entries 2, 6, 10) compared to their meta- and para-substituted
analogues (Table 2, entries 3, 4, 7, 8, 11). The higher the steric
hindrance, the lower the product yields. As a result, it was
understandable that no desired products were obtained when the
carbonylation reaction proceed with 1e and 1i (Table 2, entries 5,
9). In addition, electronic effects of the nitro substrates also
exerted an important influence on the present reaction. According
to the results, the nitro benzenes with electron-donating groups
(Table 2, entries 6-8, 10, 11) could proceed more efficiently than
those with electron-withdrawing groups (Table 2, entries 2-4, 12,
13) in higher yields. In addition to the benzenoid nitro aromatics,
1-nitronaphthalene was also tested as an typical example of
polycyclic nitro aromatics and was found to be amenable to the
carbonylation reaction well (Table 2, entry 14).
Table 3. Recyclability test of selenium catalysta
Entry
Cycle
Yieldb (%)
1
2
3
4
5
0
1
2
3
4
77
75
74
74
73
Table 2. Selenium-catalyzed carbonylation of 2-aminothiazole
with nitro aromaticsa
H
H
N
N
N
N
Cat. Se/Et3N
Ar
a Reaction conditions: 2-aminothiazole (5 mmol), nitrobenzene (5 mmol), Se
(0.25 mmol), Et3N (10 mmol), CO (3 MPa), toluene (10 mL), 130 °C, 8 h.
+
+
NH2 CO ArNO2
S
S
O
1
2
b Isolated yield of N-phenyl-N’-2-thiazolylurea.
Entry
1
Substrate
Product
Yieldb (%)
A
possible mechanism for this selenium-catalyzed
NO2
1a
1b
2a
77
35
carbonylation reaction has been outlined in Scheme 2. First, the
carbonylation reaction is initiated by the active species carbonyl
selenide (A), which is generated in situ by the reaction of carbon
monoxide with selenium in the presence of triethylamine.16a,20
Then, nitro aromatic reacts with A to afford the corresponding
isocyanate (B), accompanied by the releasing of carbon dioxide,
while A is transformed back to selenium for the coming catalytic
cycle.21 Finally, the addition of 2-aminothiazole to B generates
the desired product N-aryl-N’-2-thiazolylurea (C).
Cl
2
2b
NO2
Cl
3
4
1c
2c
51
64
NO2
NO2
1d
2d
Cl
Cl
Cl
5
6
1e
1f
2e
2f
0
NO2
Cl
CH3
Se
ArNCO
CO2
43
B
NO2
N
S
H3C
H3C
7
8
1g
1h
2g
2h
69
87
CO
NH2
NO2
NO2
OCH3
NO2
OCH3
H
N
H
N
N
9
1i
2i
0
Ar
COSe Et3N
ArNO2
S
O
C
A
Scheme 2. Proposed reaction pathway to N-aryl-N’-2-thiazolylureas.

4
Tetrahedron
Raynaud, F. I.; Kirk, R.; Whittaker, S.; Marais, R.; Springer, C. J.
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Lett. 2004, 14, 5521–5525.
In conclusion, an efficient, economical and phosgene-free
procedure for the synthesis of N-aryl-N’-2-thiazolylureas has
been established. With cheap and recyclable nonmetal selenium
instead of noble metals as the catalyst, carbon monoxide instead
of virulent phosgene as the carbonylation agent, the selenium-
catalyzed carbonylation reaction of 2-aminothiazole can proceed
smoothly in one-pot manner with a variety of nitro aromatics in
the presence of triethylamine to afford the desired N-aryl-N’-2-
thiazolylureas mostly in moderate to good yields. Low cost, high
atom economy, phosgene-free conditions, no generation of
corrosive waste, and simple procedure should make this approach
very promising.
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The financial support of this work from Program for
Changjiang Scholars and Innovative Research Team in
University (IRT1061), Program for Innovative Research Team in
Science and Technology in University of Henan Province
(15IRTSTHN003), The Education Department of Henan
Province, China (2013GGJS-059, Young Backbone Teachers
Training Fund) and Henan Normal University (2011-8, Young
Backbone Teachers Training Fund) are gratefully acknowledged.
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Supplementary data
Supplementary data (experimental procedures, compound
characterization data and copies of 1H NMR spectra for all
products) associated with this article can be found, in the online
version, at http://dx.doi.org/
9.
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10. (a) Suijkerbuijk, B. M. J. M.; Niculescu-Duvaz, I.; Gaulon, C.;
Dijkstra, H. P.; Niculescu-Duvaz, D.; Ménard, D.; Zambon, A.;
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Highlights
1. Cheap catalytic system and reactants!
2. One-pot phosgene-free manner!
3. Phase-transfer catalysis!
4. Nice recyclability of catalyst selenium!
5. Broad substrate scope!