Highly efficient heterogeneous copper-catalysed coupling of oxime acetates with isothiocyanates leading to 2-aminothiazoles

Article abstract of DOI:10.3184/174751917X14894997017739

The heterogeneous coupling reaction of oxime acetates with isothiocyanates was achieved at 110°C in toluene in air in the presence of a bidentate nitrogen-functionalised MCM-41-immobilised copper(I) complex (MCM-41-2N-CuI) with Cs₂CO₃ as base to afford a variety of 2-aminothiazoles in good yields. The MCM-41-2N-CuI catalyst can be easily recovered by a simple filtration and reused at least eight times without significant loss of activity.

Full text of DOI:10.3184/174751917X14894997017739

JOURNAL OF CHEMICAL RESEARCH 2017
VOL. 41 APRIL, 225–229
RESEARCH PAPER 225
Highly efficient heterogeneous copper-catalysed coupling of oxime acetates
with isothiocyanates leading to 2-aminothiazoles
a
b
a
a
Yuxin Tuo , Fang Yao , Yang Liao and Mingzhong Cai *
a
Key Laboratory of Functional Small Organic Molecule, Ministry of Education and Department of Chemistry, Jiangxi Normal University, Nanchang 330022,
P.R. China
b
College of Chemical and Material Engineering, Quzhou University, Quzhou 324000, P.R. China
The heterogeneous coupling reaction of oxime acetates with isothiocyanates was achieved at 110 °C in toluene in air in the presence of
a bidentate nitrogen-functionalised MCM-41-immobilised copper(I) complex (MCM-41-2N-CuI) with Cs CO as base to afford a variety of
2
3
2
-aminothiazoles in good yields. The MCM-41-2N-CuI catalyst can be easily recovered by a simple filtration and reused at least eight times
without significant loss of activity.
Keywords: copper, oxidative coupling, functionalised MCM-41, oxime acetate, heterogeneous catalysis
The 2-aminothiazole motif is a useful structural element
in medicinal chemistry and has been widely used in drug
development for the treatment of allergies, hypertension,
inflammation, schizophrenia and bacterial and HIV
environmental and economic concern in large-scale syntheses
and in industry. To overcome these problems, from the
standpoint of green and sustainable chemistry, the development
of highly efficient and recyclable heterogeneous catalysts, for
example by immobilisation of catalytically active species onto
1–3
infections. Many methods have been developed for the
synthesis of 2-aminothiazole and their derivatives, such as those
an ideal solid support to produce a molecular heterogeneous
4
18–20
of such as Hantzsch, Cook–Heilbron and Tchernic. Among
catalyst, is essential.
The use of immobilised catalysts could
these, the Hantzsch reaction, which uses α-halocarbonyl
result in easy recovery and reusability of the copper catalyst,
thereby minimising copper contamination of the desired
isolated products and of wastes derived from reaction workup.
The mesoporous MCM-41 material has emerged as an ideal
heterogeneous support for the immobilisation of homogeneous
catalysts due to its high surface area, large and defined pore
size, big pore volume and rich silanol groups in the inner
compounds to couple with thiourea or thioamides, is one of
5
–8
the most widely used protocols. Recently, 2-aminothiazole
derivatives were also prepared by using various catalysts
9
10
such as iodine, silica chloride, 1,3-di-n-butylimidazolium
11
tetrafluoroborate, ammonium 12-molybdophosphate and
12,13
cyclodextrin.
Reactions starting directly from ketones
21–23
24–27
and thioureas have also been reported for the synthesis
channel walls.
To date, MCM-41-immobilised palladium,
14–16
28
29
30–32
33–35
of 2-aminothiazoles.
However, they suffer from some
rhodium, molybdenum, gold
and copper
complexes
drawbacks such as harsh reaction conditions, poor functional
group tolerance or limited starting materials. Therefore, the
development of more green reagents and new efficient methods
for the synthesis of 2-aminothiazoles is highly desirable.
Very recently, Jiang and co-workers reported a novel route to
have been successfully utilised as potentially green and
sustainable catalysts in organic synthesis. Recently, we reported
the first synthesis of an MCM-41-immobilised bidentate nitrogen
copper(I) complex (MCM-41-2N-CuI) and its successful
33
3
application to the N-arylation of indoles and the A reaction of
35
2
-aminothiazoles by the copper-catalysed coupling of oxime
tetrahydroisoquinoline, aldehydes and alkynes. In continuing
our efforts to develop greener synthetic pathways for organic
transformations, we wish to report here a highly efficient
heterogeneous copper-catalysed coupling reaction of oxime
acetates with isothiocyanates using 20 mol% of the MCM-41-2N-
CuI complex as catalyst and yielding a variety of 2-aminothiazoles
in good isolated yields (Scheme 1). This heterogeneous copper
catalyst can easily be recovered by a simple filtration of the
reaction solution and its catalytic efficiency remains essentially
unaltered even after recycling eight times.
17
acetates with isothiocyanates under mild conditions. The
proposed mechanism involved an oxidative reaction of an
intermediate due a to Cu(II) species. Although the Cu-catalysed
oxidative coupling of oxime acetates with isothiocyanates
was highly efficient for the construction of 2-aminothiazoles,
the use of 0.2 equiv. of CuI was required to obtain good
yields. Moreover, a homogeneous copper salt is difficult to
separate from the reaction mixture and it cannot be reused
in consecutive reactions. It is well-known that homogeneous
catalysis may result in heavy metal contamination of the
desired isolated product due to the coordination of the product
to the metal, which restricts the application of such systems in
electronics and biomedicine. These problems are of particular
Results and discussion
A series of MCM-41-immobilised bidentate nitrogen copper(I)
or copper(II) complexes (MCM-41-2N-CuX ) were easily
n
c
OA
N
MCM-41-2N-CuI
20 mol%)
NHR2
N
R
(
+
R2 N=C=S
o
Cs CO , toluene, 110 C
S
2
3
R
R1
R1
Scheme 1
*
Correspondent. E-mail: caimzhong@163.com

226 JOURNAL OF CHEMICAL RESEARCH 2017
OH
OH
OSiMe3
1
. (MeO) Si(CH ) NH(CH ) NH
3
2 3
o
2 2
2
O
Toluene, 100 C, 24 h
Si
N
NH2
OH
O
H
2
. Me SiCl, rt, 24 h
3
OMe
MCM-41
MCM-41-2N
MCM-41-2N-CuCl [A], CuX = CuCl
n
OSiMe3
MCM-41-2N-CuBr
[B]
, CuX = CuBr
n
O
H
CuX
n
[
C], CuX = CuI
Si
N
MCM-41-2N-CuI
n
NH2
O
MCM-41-2N-CuCl [D], CuX
n
= CuCl2
= CuBr2
2
DMF, r.t., 7 h
OMe
CuX
n
MCM-41-2N-CuBr [E], CuX
2
n
MCM-41-2N-Cu(OAc) [F], CuX
= Cu(OAc)2
n
2
MCM-41-2N-Cu(OTf)
2
[G], CuX
n
= Cu(OTf)
2
Scheme 2
a
Table 1 Optimisation of reaction conditions
OAc
N
N
Ph
NHPh
Copper catalyst (20 mol%)
+
Ph N=C=S
o
Base, solvent, 110 C
Ph
S
1a
2a
3a
b
Entry
Copper catalyst
Base
Solvent
Yield /%
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
G
C
C
C
C
C
C
C
C
C
−
C
Cs CO
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
1,4-Dioxane
DMF
38
73
79
11
15
66
62
16
13
40
12
25
32
0
2
3
3
3
3
3
3
3
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
Cs CO
2
K CO
2
2
3
Na CO
3
3
t
1
0
KOBu
DBU
1
1
1
2
Na SO
2
1
3
Cs CO
2
3
1
4
Cs CO
2
3
1
5
Cs CO
DMSO
0
2
3
1
6
−
Toluene
Toluene
Toluene
0
1
7
Cs CO
0
2
3
3
c
1
8
Cs CO
80
2
a
b
c
All reactions were performed using 1a (0.5 mmol), 2a (0.5 mmol), base (0.25 mmol) and copper catalyst (20 mol%) in solvent (3.0 mL) at 110 °C in air for 12 h.
Isolated yield.
Under Ar.
prepared starting from commercially available and inexpensive
MCM-41-2N-CuBr [B], MCM-41-2N-CuI [C], MCM-41-2N-
Cu(OAc) [F] or MCM-41-2N-Cu(OTf) [G] were used as the
3
-(2-aminoethylamino)propyltrimethoxysilane and simple
2
2
copper salts such as CuX (X = Cl, Br, I, OAc, OTf) according
catalyst and MCM-41-2N-CuI [C] gave the best result (Table 1,
n
33
to our previous procedure (Scheme 2). Firstly, the mesoporous
material MCM-41 was condensed with 3-(2-aminoethylamino)
propyltrimethoxysilane in toluene at 100 °C for 24 h, followed by
entry 3), whilst MCM-41-2N-CuCl [A], MCM-41-2N-CuCl [D]
2
and MCM-41-2N-CuBr [E] were substantially less efficient.
2
MCM-41-2N-CuI [C] was therefore selected as the catalyst for
the reaction. Our next studies focused on the effects of bases
and solvents on the model reaction. For the bases tested (K CO ,
silylation with Me SiCl in dry toluene at room temperature for 24
3
h to afford 3-(2-aminoethylamino)propyl-functionalised MCM-
2
3
t
41 (MCM-41-2N). The latter was subsequently treated with
Cs CO , Na CO , KOBu , DBU and Na SO ), Cs CO gave the
2 3 2 3 2 3 2 3
various copper salts in DMF at room temperature for 7 h to give
a series of MCM-41-immobilised bidentate nitrogen copper(I) or
best result and other bases afforded low yields (Table 1, entries
3, 8–12). When DMSO or DMF was used as solvent, the reaction
did not occur and 1,4-dioxane gave a low yield (Table 1, entries
13–15). The control experiments showed that no desired product
3a was detected in the absence of a base or a copper catalyst
(Table 1, entries 16 and 17). When the reaction was carried
out under an argon atmosphere, the result was not improved
significantly (Table 1, entry 18). Therefore, the optimal catalytic
system involved the use of MCM-41-2N-CuI [C] (20 mol%) and
Cs CO (0.5 equiv.) in toluene at 110 °C in air for 12 h (Table 1,
(
II) complexes (MCM-41-2N-CuX ) as pale blue powders.
n
In our initial screening experiments, the reaction of
acetophenone oxime acetate (1a) with phenyl isothiocyanate
2a) was investigated to optimise the reaction conditions and
(
the results are summarised in Table 1. First, the effect of various
immobilised copper complexes on the model reaction was
examined by using Cs CO as base and toluene as solvent at
2
3
110 °C and a significant catalytic effect was observed (Table 1,
2
3
entries 1–7). It is evident that good yields were obtained when
entry 3).

JOURNAL OF CHEMICAL RESEARCH 2017 227
a
Table 2 Heterogeneous copper-catalysed synthesis of 2-aminothiazoles from various oxime acetates and isothiocyanates
N
OAc
NHR2
R
N
MCM-41-2N-CuI (20 mol%)
+
R2 N=C=S
R1
o
Cs CO , toluene, 110 C, 12 h
S
2
3
R
R1
2
3
1
1
2
b,c
Product
R
R
R
Yield /%
M.p./°C
3
8
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
a
b
c
d
e
f
Ph
H
Ph
79 (80)
84 (86)
81 (84)
75 (78)
72 (74)
76 (75)
71 (70)
72 (75)
69 (72)
78 (81)
73 (75)
81 (83)
84 (85)
80 (82)
72 (74)
83 (82)
78 (80)
75 (76)
77 (75)
86 (88)
76 (77)
86 (85)
88 (90)
75 (73)
135–136 (lit. 136–137)
38
4-MeOPh
4-MePh
4-FPh
H
Ph
141–142 (lit. 142–143)
39
H
Ph
95–96 (lit. 94–95)
38
H
Ph
125–126 (lit. 126–127)
40
4-ClPh
3-ClPh
3-BrPh
2-FPh
H
Ph
142–143 (lit. 141–142)
17
H
Ph
125–126 (lit. 127–128)
Oil
g
h
i
H
Ph
17
H
Ph
105–107 (lit. 104–105)
17
2-ClPh
H
Ph
128–129 (lit. 130–131)
Oil
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
3,4-Me Ph
H
Ph
2
17
2-Thienyl
Ph
H
Ph
90–91 (lit. 89–90)
17
Me
Me
Et
Ph
160–162 (lit. 161–162)
17
4-MePh
Ph
Ph
162–163 (lit. 164–165)
17
Ph
156–158 (lit. 156–157)
17
–(CH ) –
Ph
102–103 (lit. 103–104)
17
2
5
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
H
H
H
H
4-MePh
4-FPh
4-ClPh
4-BrPh
4-MeOPh
3-MePh
103–104 (lit. 102–103)
17
141–142 (lit. 142–143)
17
138–139 (lit. 140–141)
3
9
144–145 (lit. 146–147)
39
143–144 (lit. 142–143)
Oil
Oil
Oil
Oil
PhCH
2
Et
Me CH
2
a
b
c
The reactions were performed at 110 °C using 1 (0.5 mmol), 2 (0.5 mmol), MCM-41-2N-CuI (20 mol%) and Cs CO (0.25 mmol) in toluene (3 mL) in air for 12 h.
2
3
Isolated yield.
Isolated yield in parentheses was obtained with CuI (20 mol%) and Cs CO (0.25 mmol) in toluene at 105 °C for 8 h.
17
2
3
With the optimal reaction conditions established, we tried
isothiocyanate was used as the substrate, the desired product 3u
was obtained in 76% yield. Additionally, the reactions of alkyl
or benzyl isothiocyanates with acetophenone oxime acetate
also proceeded effectively to afford the corresponding products
3v–x in 75–88% yields.
to investigate the scope and limitations of this heterogeneous
copper-catalysed coupling reaction of oxime acetates with
isothiocyanates and the results are summarised in Table 2 and
17
yields compared with the homogeneous method. Firstly, the
reactions of various oxime acetates with phenyl isothiocyanate
were examined. The reactions of para-substituted acetophenone
oxime acetates bearing both electron-donating groups and
electron-withdrawing groups with phenyl isothiocyanate
proceeded smoothly, leading to the desired 2-aminothiazoles
To determine whether the observed catalysis was due to
the heterogeneous catalyst MCM-41-2N-CuI or to a leached
3
6
copper species in solution, a hot filtration test was performed.
We focused on the coupling reaction of acetophenone oxime
acetate (1a) with phenyl isothiocyanate (2a). The MCM-41-2N-
CuI complex was filtered off after 5 h of reaction time and the
filtrate was allowed to react further by addition of 0.5 equiv. of
Cs CO . The catalyst filtration was performed at the reaction
3
b–e in good yields. The results indicated that the electronic
natures of the substituents on acetophenone oxime acetates
have limited influence on this heterogeneous copper-catalysed
coupling reaction. Besides, acetophenone oxime acetates
with a fluoro, chloro or bromo group at the meta- or ortho-
position could also participate in this reaction well to give the
corresponding coupled products 3f–i in good yields. In general,
electron-rich acetophenone oxime acetates showed higher
reactivity than electron-deficient ones. However, a strong
electron-withdrawing group such as a nitro group was not
tolerated in this transformation. Ester and cyano groups were
also unsuitable substituents. The disubstituted acetophenone
oxime acetate proved to be a suitable substrate and the
desired product 3j was formed in 78% yield. It is noteworthy
that a heteroaryl-containing oxime acetate was compatible
for this reaction and was transformed into the corresponding
product 3k in 73% yield. To our delight, 4,5-disubstituited
2
3
temperature (110 °C) in order to avoid possible recoordination
or precipitation of soluble copper upon cooling. We found
that, after this hot filtration, no further reaction was observed,
indicating that leached copper species from the catalyst (if any)
are not responsible for the observed activity.
For a heterogeneous transition metal catalyst system, it is
important to examine its ease of separation, recoverability and
reusability. This heterogeneous copper catalyst can be easily
recovered by simple filtration of the reaction solution. We next
investigated the recyclability of the catalyst by using the coupling
reaction of 4'-methoxyacetophenone oxime acetate with phenyl
isothiocyanate in air. After carrying out the reaction, the catalyst
was separated by simple filtration and washed with distilled water,
DMF and ethanol. After being air-dried, it can be reused directly
without further purification. The recovered catalyst was used
in the next run and almost consistent activity was observed for
eight consecutive cycles (Fig. 1). The high stability and excellent
reusability of the catalyst should result from the chelating action
of the bidentate nitrogen ligand on copper and the mesoporous
structure of the MCM-41 support. The result is important from
industrial and environmental points of view.
2
-aminothiazoles 3l–o could be easily obtained in good yields
from other oxime acetates. Encouraged by the above results, we
next examined the scope of isothiocyanates for this reaction. For
various para-substituted phenyl isothiocyanates bearing either
electron-donating or electron-withdrawing substituents on the
aromatic ring, the reactions proceeded smoothly to give the
expected products 3p–t in high yields. When 3-methylphenyl

228 JOURNAL OF CHEMICAL RESEARCH 2017
(10 mL). The mixture was stirred at room temperature for 7 h under
an argon atmosphere. The solid product was filtered off by suction,
washed with DMF and acetone and dried at 40 °C/26.7 Pa under Ar
for 5 h to give a pale blue copper complex (MCM-41-2N-CuI) (1.07 g).
8
0
–1
The nitrogen and copper contents were found to be 1.67 mmol g and
60
–1
0
.45 mmol g respectively.
Coupling reaction of oxime acetates with isothiocyanates; general
procedure
40
A mixture of the oxime acetate (0.5 mmol), the isothiocyanate (0.5
mmol), Cs CO (0.25 mmol), toluene (3 mL) and the MCM-41-2N-
2
3
CuI complex (222 mg, 20 mol%) was stirred at 110 °C in air for 12 h.
The reaction mixture was cooled to room temperature and quenched
20
with H O (10 mL) and extracted with EtOAc (3 × 10 mL). The MCM-
2
4
1-2N-CuI catalyst was recovered by filtration, washed with distilled
0
water (2 × 5 mL), DMF (2 × 5 mL) and EtOH (2 × 5 mL) and could be
reused in the next run. The combined organic layers were dried over
MgSO and the solvent was removed under reduced pressure. The
1
2
3
4
5
6
7
8
Recycling times
4
crude product was purified by column chromatography (hexane/ethyl
acetate) on silica gel.
All the products 3a–x are known compounds and the melting points
of the solids are given in Table 2 and compared with literature values.
Fig. 1 Recycling of the MCM-41-2N-CuI complex.
Where 1 indicates the original catalyst before the first recycling.
In conclusion, we have developed a novel, environmentally
Acknowledgement
friendly and economic catalyst system for the oxidative coupling
reaction of oxime acetates with isothiocyanates by using an
MCM-41-immobilised bidentate nitrogen copper(I) complex
We thank the National Natural Science Foundation of China
(21462021) for financial support.
(
MCM-41-2N-CuI) as catalyst. The reactions generated a
Electronic Supplementary Information
variety of 2-aminothiazoles in good yields and were applicable
to various oxime acetates and different isothiocyanates. This
heterogeneous copper catalyst could be easily recovered by a
simple filtration of the reaction solution and recycled at least eight
times without significant loss of catalytic activity. The present
methodology provides a green and practical route to 4-substituted
or 4,5-disubstituted 2-aminothiazoles.
The ESI (FTIR and NMR data of the products) is available
through:
stl.publisher.ingentaconnect.com/content/stl/jcr/supp-data
Received 25 December 2016; accepted 4 March 2017
Paper 1604502
https://doi.org/10.3184/174751917X14894997017739
Published online: 3 April 2017
Experimental
All reagents were used as received without further purification. The MCM-
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2
2
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33
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–
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–1
–1
–1
–1
0
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–1
4
and 0.50 mmol g respectively. Oxime acetates were prepared according to
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37
5
6
7
8
9
1
MAGNA-IR 750 spectrometer. H NMR spectra were recorded on a Bruker
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13
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3
1
0
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3
11
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7
(
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18
1
00 °C. The solid was filtered off, washed with CHCl (2 × 20 mL)
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19
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(
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(
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2
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