Microwave-assisted C-N and C-S bond-forming reactions: An efficient three-component domino sequence for the synthesis of sulfoether-decorated imidazo[1,2-A]pyridines

Article abstract of DOI:10.1039/c5ra05377c

An efficient and simple microwave-assisted three-component reaction for the formation of imidazo[1,2-a]pyridine derivatives in the presence of F₃CCO₂H has been described. The three-component reaction of 3-phenylpropiolaldehyde, pyrid

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ARTICLE TYPE
Microwave-assisted C-N and C-S Bond-Forming Reactions: An Efficient
Three-component Domino Sequence for the Synthesis of Sulfoether-
Decorated Imidazo[1,2-a]pyridines
Haiying Zhan, Hua Cao,*a Huifang Qiu, Naiying Li,a Longbin Chen,a Jingyun Liu,a Huiyin Cai,a and
5
Jingwen Tana
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
On the other hand, imidazo[1,2-a]pyridines are recognized
50 as an important class of heterocycles which exhibited
remarkable biological activities as privileged scaffolds in drug
discovery and development.12 The core structure of
imidazo[1,2-a]pyridines has been found in many drugs such as
zolpidem, alpidem, zolimidine, olprinone, saripidem,
55 necopidem13. Therefore, organic chemists have been making
An efficient and simple microwave-assisted three-component
reaction for the formation of imidazo[1,2-a]pyridine
10 derivatives in the presence of F3CCO2H has been described.
The three-component reaction of 3-phenylpropiolaldehyde,
pyridin-2-amines and thiols gives the desired products in
good yields. It represents an efficient approach for the
formation of C-N and C-S bonds under the microwave
15 irradiation.
extensive
efforts
to
prepare
imidazo[1,2-a]pyridine
derivatives14 by developing novel and convenient organic
transformations. Although many elegant processes have been
reported to form those compounds,15 the development of new
60 microwave-assisted domino reations for syntesis of
sulfoether-decorated imidazo[1,2-a]pyridines is still highly
favorable the use of thiols and alkynes as starting materials.
Our recent efforts were including the construction of
imidazo[1,2-a]pyridines16 by direct C-H functionalization or
65 multicompoent reaction. In this context, we described a novel
microwave-assisted domino reations for the formation of C-N
and C-S bonds to prepare sulfoether-decorated imidazo[1,2-
a]pyridine derivatives via three-component reactions of ynals,
pyridin-2-amines and thiols.
The formation of carbon-heteroatom bonds continues to be
an active and challenging field of chemical research.1
Transition metal-catalyzed reactions have emerged as
a
powerful methodology for the formation of carbon-heteroatom
20 bonds over the past decades.2 Many significant and favorable
processes have been developed in this field; particularly Pd,3
Ag,4 Cu,5 Au,6 Rh7 and Ru8 catalyst are favorable for the
formation of carbon-heteroatom bonds. However, those
reactions suffer from the expensive catalyst and ligand. The
25 cost, toxicity and environmental impact of these catalysts
have hindered its further application on an industrial scale.
These problems are of particular environmental and economic
concern in large-scale syntheses. Therefore, the discovery of
efficient processes that do not require a metal catalyst will be
30 of great importance because such procedures provide an
efficient route to avoid and solve these problems. It represents
one of the most fundamental and economic strategy, and has
attracted critical attention of organic chemist in recent years.9
In spite of the utility of transformation in the preparation of
35 complex molecules, environmental sustainability and
economy remains challenging.
70
Very recently, we have developed efficient three-
component reaction of ynals, pyridin-2-amines and alcohol to
construct functionalized imidazo[1,2-a]pyridine (Scheme
1).16e In our further exploration of the scope of this novel
domino reaction, we employed 3-phenylpropiolaldehyde,
75 pyridin-2-amine and 2-methyl propane-2-thiol as the
substrates and surprisingly found that the desired product was
obtained with lower yield in the presence of AcOH in CH3CN
at 80 °C for 8 h.
Microwave-assisted chemistry10 has matured into
a
Previous work:
RX
R1
CHO
promising strategy for the formation of useful moleculars. It
has shown tremendous advantages including simple easy work
40 up procedure, decreased reaction time, saving energy and cost
as well as providing clean products in good to excellent yields
in comparison to conventional methods. Multicomponent
reactions (MCRs) are very interested transformation because
of their significant advantages.11 The strategy of MCRs has
45 been developed to enable the rapid preparation of diverse
structures with an optimal number of new bonds and
functionalities from readily accessible starting materials in a
single operation under mild conditions.
AcOH, 8 h, 80oC
CH3CN or ROH
R2
+ RXH
NH2
+
N
R2
N
R1
N
X= O, S, N
80
This work:
(H3C)3CS
CHO
Ph
+ (CH3)3CSH
NH2
+
N
N
N
Ph
Scheme 1. Synthesis of imidazo[1,2-a]pyridines
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Table 1. Optimization of Reaction Conditionsa
Table 2. Microwave-assisted Synthesis of Imidazo[1,2-a]pyridines a
R1R2CS
S
CHO
Ph
Ph
CHO
F3CCO2H, DMF
R1
R1R2C SH
catalyst
+
NH2
+
N
MW, 130oC, 30min
+
R1
+
SH
N
N
solvent
N
NH2
N
Ph
1a
N
Ph
2
3
4
1a
2a
3a
4a
(H3C)3CS
(H3C)3CS
(H3C)3CS
Ph
Entry
Yield(%)b
Ph
Ph
Catalyst
Thermal/MW
Solvent
t
H3C
N
AcOH
AcOH
CH3CN
DMF
24h
10
39
22
46
83
45
17
69
84
76
80
34
79
36
72
82
80℃
1
N
N
30min
30min
30min
30min
30min
30min
30min
30min
30min
30min
30min
30min
30min
20min
50min
120℃(MW)
120℃(MW)
120℃(MW)
120℃(MW)
120℃(MW)
120℃(MW)
100℃(MW)
130℃(MW)
150℃(MW)
130℃(MW)
130℃(MW)
130℃(MW)
130℃(MW)
130℃(MW)
130℃(MW)
2
N
N
N
H3C
PhCO2H
TsOH
DMF
3
CH3
4a
89%
4b
86%
4c
DMF
82%
(H3C)3CS
4
(H3C)3CS
(H3C)3CS
F3CCO2H
HCl
DMF
Ph
5
Ph
Ph
Ph
Cl
Br
DMF
6
I
N
N
N
H2SO4
DMF
7
N
N
N
F3CCO2H
F3CCO2H
F3CCO2H
F3CCO2H
F3CCO2H
F3CCO2H
F3CCO2H
F3CCO2H
F3CCO2H
DMF
8
4f
4d
4g
4e
81%
86%
87%
DMF
9
S
S
S
DMF
10
11
12
13
14
15
Ph
Ph
Ph
N
DMSO
Toluene
DMA
dioxane
DMF
H3C
N
N
N
N
H3C
N
CH3
4i
85%
4h
83%
88%
S
(H3C)2HCS
Ph
S
Ph
DMF
16
a
N
I
Br
N
N
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), 3a (1.2 mmol),
N
b
N
catalyst (2.0 mol % ); solvent ( 3.0 mL), under microwave 500 W. ; GC-
N
CH3
4l
4j
4k
85%
89%
yield.
87%
(H3C)2HCS
(H3C)2HCS
Initially, 3-phenylpropiolaldehyde 1a, pyridin-2-amine 2a
and 2-methylpropane-2-thiol 3a were chosen as model
substrates to screen the catalysts and determine suitable
reaction conditions for the MCRs. The results are summarized
in Table 1. The desired product 4a was formed in 10% yield
in the presence of AcOH in DMF at 80 °C for 24 h(Table 1,
entry 1). Interestingly, the expected product 3-(tert-
Ph
Ph
I
Cl
N
N
5
N
N
4m
86%
4n
83%
a Isolated yields
35
Under the optimized reaction conditions, we evaluated the
scope of this novel transformation synthesis of sulfoether-
10 butylthio(phenyl)methyl) imidazo[1,2-a]pyridine 4a was
obtained in 39% yield (Table 1, entry 2) under microwave
irradiation condition at 120°C in the presence of AcOH for
30min. Encouraged by the result that microwave irradiation
could promote the reaction, other catalyst, such as PhCO2H,
15 TsOH, F3CCO2H, HCl, and H2SO4, were next to examined
(entries 3-7, Table 1). To our delight, the product 4a was
formed in 83% yield in the presence of F3CCO2H under
microwave heating condition. The results indicated that
PhCO2H, TsOH, HCl or H2SO4 could catalyze the
20 transformation for yielding the product in moderate yields. It
was interestingly found that the three-component reaction was
highly sensitive to temperature variations (Table 1, entries 8-
10). When the reaction carried out decreasing the reaction
temperature from 120 °C to 100°C, the corresponding product
25 4a was obtained in 69% yield. Furthermore, the effects of
solvents were surveyed (Table 1, entries 11-14). The results
clearly indicated that best result presented when DMF was
used as solvent. An increase of reaction time was studied and
the same efficiency was obtained after 50 min of microwaves
30 activation, while the decrease of reaction time lead to lower
yield (Table 1, entries 15-16).
decorated
imidazo[1,2-a]pyridines.
The
results
are
summarized in Table 2. First, 1a and 3a fixed as substrates to
test various substituted pyridin-2-amines. As shown in Table
40 2, a variety of substituted pyridin-2-amine derivatives were
effective substrates for this transformation and the
corresponding products (4a-4f) were obtained in good yields
in all the cases. To our delight, the sensitive functionalized
groups on the pyridine ring, such as Cl, Br and I, were also
45 tolerated to afford the corresponding in good yields.
Subsequently, cyclohexanethiol (3b) and propane-2-thiol (3c)
were also employed for this transformation. The resulsts also
indicated that all of the reactions proceeded smoothly under
the optimized conditions and provided the thioether-decorated
50 imidazo[1,2-a]pyridine derivatives (4g-4n) in good yields.
Table 3. F3CCO2H-catalyzed synthesis of imidazo[1,2-a]pyridinesa
RS
CHO
Ph
F3CCO2H, DMF
R1
+
RSH
+
N
MW, 130oC, 30min
R1
N
NH2
N
Ph
1a
2
3
5
2
| Journal Name, [year], [vol], 00–00
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H3C(H2C)11
Ph
S
H3C(H2C)11
S
H3C(H2C)11
S
H3C(H2C)11
N
S
Ph
worth pointing out that multi-halogen-substituted pyridin-2-
amines could also be achieved upon carrying out the
25 experiment under the optimized conditions. The results clearly
indicated that this strategy can be extended to a variety of
substituted thiols to form functionalized imidazo[1,2-
a]pyridine derivatives. Notably, the sole products were
detected in the reaction, which was indicated that this one pot
30 transformation was regioselective and chemoselective.
To our delight, other nucleophiles, such as CH3OH and
C2H5OH, were also performed very vell and affored the
corresponding products in 95% and 86% yields respectively
(Scheme 2).
Ph
Ph
H3C
N
N
N
N
N
N
N
H3C
CH3
5ad 88%
5aa
90 %
5ac 91%
H3C(H2C)11
5ab
89%
H3C(H2C)11
S
S
H3C(H2C)11
S
H3C(H2C)11
S
Ph
CH3
Ph
Ph
Ph
Br
F
N
N
N
N
N
N
N
N
F3C
5ag
91%
5af 89%
H3C(H2C)11
Ph
5ae 92%
5ah 90%
Ph
H3C(H2C)11
S
S
H3C(H2C)11
H3C
S
Ph
H3C(H2C)11
S
Ph
Br
N
Cl
N
I
N
N
N
I
N
N
H3C
N
R1O
Br
5ak 89%
I
I
CHO
5ai 87%
5aj
5al
89%
87%
Ph
F3CCO2H, DMF
+
R1OH
H3C(H2C)2S
H3C(H2C)2S
H3C(H2C)2S
H3C(H2C)2S
+
Ph
N
Ph
Ph
Ph
MW, 130oC, 30min
R1
N
NH2
I
Br
F
N
N
N
N
Ph
N
6a
6b
N
: CH3 92%
: C2H5 94%
N
N
N
5be 86%
5bc 90%
5ba 90%
5bd 92%
35
H3C(H2C)2S
Scheme 2. MCRs of 1a, 2a and ethanol for the synthesis of
imidazo[1,2-a]pyridines
In summary, we have reported an efficient microwave-assisted
three-component reactions synthesis of sulfoether-decorated
H3C(H2C)2S
H3C(H2C)2S
Ph
H3C(H2C)2S
Ph
Ph
Ph
Cl
H3C
N
N
N
N
N
N
N
N
H3C
I
CH3
5gh
87%
5bf 90%
5bi 92%
40 imidazo[1,2-a]pyridines. This procedure offers easy access to highly
functionalized imidazo[1,2-a]pyridines which are useful structural
motif in pharmaceuticals and natural compounds. This transformation
provides a convenient strategy for the formation of C-N and C-S
bonds to prepare sulfoether-decorated imidazo[1,2-a]pyridines. This
45 methodology has several advantages: 1) it does not require expensive
substrates or catalysts; 2) it decrease reaction time, saves energy and
cost as well as provides clean products in good to excellent yields; 3)
it has provided a wide range of substrates. Further studies and
applications of metal-free multicomponent reactions are ongoing in
50 our laboratory.
5bg 88%
H3CH2CS
H3CH2CS
H3C(H2C)2S
Ph
H3CH2CS
Ph
Ph
H3C
Ph
N
N
N
N
N
N
H3C
N
F3C
N
5cb 88%
CH3
5cc 86%
5ca
87%
5bj 83%
H3CH2CS
H3CH2CS
H3C(H2C)3S
H3CH2CS
Ph
Ph
Ph
Ph
Ph
F
Br
N
N
N
N
N
N
Cl
N
92%
N
5da
5cf87%
Ph
5ce88%
Ph
5cd 85%
Ph
S
S
S
H3C(H2C)3S
Ph
Ph
Ph
Cl
F
I
N
N
N
N
Acknowledgements
N
N
N
N
5ec 91%
5db
5eb 87%
89%
5ea 89%
This research was financially Supported by National Natural
Science Foundation of China (21302023) and the Project of
Department of Education of Guangdong Province
55 (2013KJCX0111)
a Isolated yields
Encouraged by the successful syntheses of substituted
imidazo[1,2-a]pyridines, our attention turned to the possibility
of
synthesizing
functionalized
imidazo[1,2-a]pyridine
5
derivatives by using primary thiols in this reaction under the
optimized conditions. And the results are shown in Table 3.
Dodecane-1-thiol (3d) was firstly examined. Various
substituted pyridin-2-amines reacted well with 1a, 3d and led
to the desired product 5aa−5al in good to high yields.
Notes and references
a School of Chemistry and Chemical Engineering, Guangdong
Pharmaceutical University, Zhongshan 528458, P.R. of China; Fax:
(+)86(760)88207939, caohua@gdpu.edu.cn
60
† Electronic Supplementary Information (ESI) available: Experimental
Protocols and NMR Spectra for products 4aa-4eg. See
DOI: 10.1039/b000000x/
10 Furthermore, multisubstituted pyridin-2-amine, such as 5-
chloro-3-iodopyridin-2-amine, 3,5-dibromo-4-methylpyridin-
2-amine or 3,5-diiodo-6-methylpyridin-2-amine reacted with
1a and 3d smoothly under the optimized conditions. Other
commercially available aliphatic primary thiols, such as,
15 propane-1-thiol(3e), ethanethiol (3f), butane-1-thiol (3g), and
phenylmethanethiol (3f), were also tested. We were pleased to
find that substituted thiols as substrates were also tolerated in
the reaction and provided the desired imidazo[1,2-a]pyridines
5ba−5ec in good to excellent yields. To our delight,
20 substituted thiols reacted with 1a and multisubstituted or
electron-poor groups substituted (CF3) pyridin-2-amines
smoothly and afforded the desired product in good yields. It is
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