Silver-catalyzed [3 + 2] domino reaction: An efficient strategy to synthesize imidazole-5-carbaldehydes

Article abstract of DOI:10.1039/c7ob01242j

An unprecedented regioselective silver-catalyzed [3 + 2] domino reaction of amidines and ynals for the formation of C-N bonds has been developed. The reaction provided a new route to prepare imidazole-5-carbaldehydes which are important intermediates for the construction of fine chemicals. The reaction proceeds smoothly with a broad range of substrates to give imidazoles in good yields.

Full text of DOI:10.1039/c7ob01242j

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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Silver-Catalyzed [3+2] Domino Reaction: An Efficient Strategy to
Synthesize Imidazole-5-carbaldehydes
Changcheng Wang,a Hangqi Jiang,a Weifeng Chen,a Jun Dong,b* Zhengwang Chen,c and Hua Caoa *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unprecedented regioselective silver-catalyzed [3+2] domino
reaction of amidines and ynals for the formation of C-N bonds has
been developed. The reaction provided a new route to prepare
imidazole-5-carbaldehydes which are important intermediate for
the construction of fine chemicals. The reaction proceeds
smoothly with a broad range of substrates to give imidazoles in
good yields.
pollution problem in most of organic transformation is
caused by organic solvents, the development of efficient
synthetic methods for organic transformation with nonꢀ
toxic solvents is an important goal. It is well known that
water is a nonꢀtoxic, nonꢀhazardous, nonꢀflammable,
uniquely redoxstable, inexpensive green solvent,
therefore, using water as the reaction solvents8 has
attracted considerable attention.
Transition
metalꢀcatalyzed
organic
synthetic
reactions1 are powerful tools for the formation of new
carbon–carbon and carbonꢀhetero bonds in both
industrial applications and academic research. A lot of
great and successful methods, such as Heck reaction,2
Suzuki reaction,3 Negishi coupling,4 Sonogashira
coupling5 and BuchwaldꢀHartwig reaction,6 have been
developed for the synthesis of complex molecules from
readily accessible starting materials under mild
conditions. On the basis of these classical
transformations, new and facile routes have been
developed to prepare fine chemicals. These
advancements have encouraged chemists to design an
inexpensive, environmentally friendly and nontoxic
catalytic system. With the development of green
chemistry and increased concern over environmental
issues,7 one of the current challenges in organic
synthesis is to develop green methods. As the biggest
Imidazole derivatives recognized as a significant class
of skeleton in many natural products, pharmaceuticals,
and biologically active molecules. The commercially
9
available
drugs,
for
instance,
Ketoconazole,
Clotrimazole, Miconazole and Tinidazole, are derived
from imidazole scaffolds. One of the most important
reaction is the DebusꢀRadziszewski method10 for the
construction of imidazole derivatives. As far as
transition metalꢀcatalyzed strategies to access
imidazoles are concerned, significant advances based on
Pdꢀ, Cuꢀ, and Feꢀcatalyzed reactions have been achieved
11
in the field.
Although many efforts have been
devoted to prepare imidazole derivatives,12 the search
for the development of more environmentally friendly
reaction is still highly desirable.
Combined with our previous work on the synthesis of
furans13. and imidazo[1,2ꢀa]pyridines14, herein, we
report an efficient Agꢀcatalyzed [3+2] domino process
to synthesize imidazoleꢀ5ꢀcarbaldehydes from amidins
and ynals.
aSchool of Chemistry and Chemical Engineering and Guangdong Cosmetics
Engineering & Technology Research Center, Guangdong Pharmaceutical University,
Zhongshan 528458, China
b School of Traditional Chinese Medicine, Guangdong Pharmaceutical University,
Guangzhou 510006, China
c Key Laboratory of Organo-pharmaceutical Chemistry of Jiangxi province, Gannan
Normal University, Ganzhou, 341000, China
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Table 1. Optimization of the Reaction Conditionsa
including AgNO3, Ag2O, AgOAc, AgSbF6, and AgBF4
(Table 1, entries 2ꢀ6). It was found that a decreased
yield was obtained in the reaction. The poor yields were
observed when copperꢀ (Cu(OAc)2, CuI) and gold
(AuCl, AuCl3) salts were employed (Table 1, entries 7ꢀ
10). Furthermore, Unsatisfactory yields were formed
when TsOH or TFA was used as additive (Table 1,
entries 11ꢀ12). The effect of solvents were next tested.
Other solvents, such as DMA, DMF, or toluene, did not
lead to any improved result (Table 1, entries 13ꢀ15). We
then attempted to improve the yield of product 3a by
adding water that restrained dehydration of 1a and 2a to
produce intermediate I and later form 4a . As we
expected, the desired product 3a was generated in 77ꢀ
80% yields when the reaction carried out by using
DMSO and H2O as the coꢀsolvent (Table 1, entries 16ꢀ
18). However, no imdazoles formation was observed by
using H2O as the solvent (Table 1, entry 19). Then,
different oxidants (DDQ, K2S2O8) were also employed
and only trace amount of 3a was formed (Table 1,
entries 20ꢀ22).
Ph
CHO
CHO
H
N
catalyst, oxidant
N
Ph
Ph
+
Ph
solvent, additive, 10h
N
NH
1a
Ph
3a
Ph
2a
entry
catalyst
AgOTf
AgNO3
AgOAc
Ag2O
additive
AcOH
AcOH
AcOH
AcOH
solvent
DMSO
DMSO
DMSO
DMSO
oxidant
O2
yieldb (%)
1
2
3
4
63
39
22
25
O2
O2
O2
5
6
AgSbF6
AgBF4
Cu(OAc)2
CuI
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
TFA
DMSO
DMSO
O2
O2
trace
trace
n.d.
12
7
DMSO
O2
8
DMSO
O2
9
AuCl
DMSO
O2
trace
trace
13
10
11
12
13
14
15
16
17
18
19
20
21
22
23c
24d
AuCl3
DMSO
O2
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
DMSO
O2
TsOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
DMSO
O2
trace
41
DMA
O2
DMF
O2
33
toluene
O2
<5
DMSO/H2O=5:1
DMSO/H2O=2:1
DMSO/H2O=1:1
H2O
O2
78
O2
80
O2
77
O2
trace
trace
trace
trace
n.d.
n.d.
Ph
CHO
H
DMSO/H2O=2:1
DMSO/H2O=2:1
DMSO/H2O=2:1
DMSO/H2O=2:1
DMSO/H2O=2:1
DDQ
K2S2O8
TBHP
-
AcOH (5mol%)
AgOTf (5mol%)
N
N
Ar2
Ar1
+
Ar2
CHO
N
Ar1
NH
DMSO:H2O=2:1
Ph
O ,120
℃, 10 h
2
1
2a
3
-
Ph
Ph
Ph
Ph
N
N
N
N
N
N
N
aReaction conditions: 1a (0.25 mmol), 2a (0.3 mmol), catalyst (5 mol %), additive
(5 mol %), and oxidant (2.0 equiv) in solvent (2 mL) at 120 °C for 8 h. b Determined
by GC analysis.cReaction conducted at N2 atmosphere. dReaction conducted at Ar
atmosphere.
CHO
N
CHO
CHO
CHO
H3C
3a
3c
69%
78%
3b 71%
Ph
H3C
3d
70%
F
Ph
Ph
Ph
N
N
N
N
N
N
N
CHO
N
CHO
CHO
CHO
N
Ph
Ph
CHO
H
N
AcOH (5mol%)
AgOTf (5mol%)
N
Cl
[Ag]
H3CO
Ph
+
Ph
Ph
N
Ph
Cl
3e 72%
3g 70%
3f
N
N
71%
H3CO
3h
N
72%
DMSO, O ,120
℃, 10 h
NH
2
O
Ph
Ph
H3C
Ph
4a
Ph
Ph
N
N
Ph
2a
N
N
N
Ph
1a
I
CHO
CHO
N
CHO
CHO
H3C
H3C
Scheme 1.The formation of byproduct 4a
.
H3C
H3C
Cl
3j
83%
3l 77%
H3C
3k
3i
80%
Ph
73%
F
Initially, Nꢀphenylꢀbenzamidine (1a) and phenylꢀ
propynal (2a) were chosen as model substrates, and the
results are described in Table 1. The transformation was
carried out in the presence of AgOTf (5 mol %) and
AcOH (5 mol %) in DMSO (2 mL) under O2 as the
oxidant at 120 °C (Table 1, entry 1). After 12 h, we
were pleased to find that the product 3a was generated
in 63% yield with a little byproduct 4a in 12% yields
(Scheme 1). According to the preliminary result in entry
1, we next evaluated the influence of other silver,
Ph
N
N
Ph
Ph
N
N
MeO
CHO
N
CHO
N
N
N
CHO
CHO
Br
Br
H3
C
3p
80%
3m
F
3o 78%
75%
3n
83%
a
Isolated yields
Scheme 2. Scope of the AgOTf-Catalyzed Reaction.
With the optimized catalytic system in hand, we
then explored the substrate scope of amidines with
diverse substituents. The results are shown in Scheme 2.
2 | J. Name., 2012, 00, 1-3
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The reaction was successfully extended to various Nꢀ
We were also pleased to find that this Agꢀ
substituted amidines (Ar1). All of the reactions catalyzed process could be successfully applied to
proceeded smoothly under the optimized conditions and different ynals and the results are described in Scheme
gave the substituted imidazole aldehydes (3a-3i). This 4. A variety of ynals, bearing either electronꢀdonating
transformation tolerated different substituents, such as groups
(4ꢀCH3,
4ꢀOMe,
3,4ꢀdimethyl)
or
electronꢀrich (CH3, OCH3) and electronꢀpoor (F, Cl), on electronwithdrawing groups (Br) on the phenyl ring,
the benzene ring of Nꢀsubstituted amidines (Ar1). The reacted smoothly with various amidines, affording the
effect of Ar2 was then investigated for this Agꢀcatalyzed corresponding products (3q-3y) in 60−82% yields.
strategy. The substituted groups on the arene (Ar2) such Notably, the bromo substituent products could be
as CH3 and OCH3 remained unaffected and were also subjected to further transformations to prepare diverse
wellꢀtolerated under optimized conditions (3j-3p). All molecules. The molecular structure of product 3t was
those reactions afforded the corresponding products in determined by Xꢀray crystallography (Figure 1).
moderate to good yields. To further investigate useful
substrate, we use acetimidamide. But unfortunately, the
desired product was not detected in this reaction
(Scheme 3).
AcOH (5mol%)
AgOTf (5mol%)
CHO
Ph
CHO
NH2
N
+
NH
1q
DMSO, O ,120
℃
2
, 10 h
N
H
Ph
2a
Figure 1. X-ray structure of 3t
.
3q
Ph
CHO
Scheme 3. Synthesis of imidazole by using acetimidamide
CHO
N
Ph
N
Ph
Ph 3a
[Ag]
2a
O2
Ar
CHO
H
AcOH (5mol%)
AgOTf (5mol%)
N
N
Ar2
NH
Ar1
+
Ar2
CHO
Br
N
Ar1
[Ag]
DMSO:H2O=2:1
CHO
[Ag]
Ar
2
CHO
Ph
O ,120
2
℃, 10 h
Ph
1
NH
3
Ph
Ph
NH
NH
Ph
N
R
A
1a
B
N
N
N
N
N
CHO
N
CHO
CHO
Scheme 5. Proposal Mechanism
3q
3r
68%
68%
3s
75%
OCH3
On the basis of above results, a plausible mechanism
for this reaction is illustrated in Scheme 5. For the
synthesis of 3a, the carbonꢀcarbon triple bond of 2a is
activated by coordination with Agꢀsalt to give the
N
N
N
N
CHO
N
N
CHO
CHO
Br
intermediate
triple bond to give the intermediate
then undergoes dehydrogenation oxidation16 in the
A,
15 followed 1a attack the Agꢀcoordinated
. The intermediate
3v
66%
3u
73%
3t
60%
B
N
B
N
N
N
N
presence of oxygen to give the final product 3a and
N
CHO
CHO
82%
H3CO
CHO
releases the Ag catalyst.
3y
73%
3w
3x
63%
Conclusions
a Isolated yields
Scheme 4. AgOAc-Catalyzed Synthesis of Imidazoles
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In summary, we have successfully developed a facile
regioselective silverꢀcatalyzed [3+2] domino reaction of
amidines and ynals for the formation of CꢀN bonds.
This strategy has provided a new route for the synthesis
of imidazoleꢀ5ꢀcarbaldehydes that are important
materials and intermediates for the construction of
complex organic compounds. This reaction has several
advantages: 1) the reaction has employed O2 as a clean
oxidant instead of other chemical oxidants; 2) the
employment of H2O and DMSO as the coꢀsolvents is
cheaper; 3) this reaction has showed good selectivity
and susceptible aldehyde products have been formed
without further oxidation.
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This research was financially Supported by National Natural
Science Foundation of China (41271263), the Innovation and
Strong School Project of Guangdong Pharmaceutical University
(2015cxqx212), the NSF of Jiangxi Provincial Education
Department (GJJ160924) and Project of Innovation for
Enhancing Guangdong Pharmaceutical University, Provincial
Experimental Teaching Eemonstration Center of Chemistry &
Chemical engineering.
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