Catalyst-free preparation of 1,2,4,5-tetrasubstituted imidazoles from a novel unexpected domino reaction of 2-azido acrylates and nitrones

Article abstract of DOI:10.1021/ol202650z

A highly efficient and convenient method for the synthesis of 1,2,4,5-tetrasubstituted imidazoles from readily accessible 2-azido acrylates and nitrones has been developed. This reaction proceeded under mild conditions without the assistance of any metal,

Full text of DOI:10.1021/ol202650z

ORGANIC
LETTERS
2011
Vol. 13, No. 24
6362–6365
Catalyst-Free Preparation of 1,2,4,5-
Tetrasubstituted Imidazoles from a Novel
Unexpected Domino Reaction of 2-Azido
Acrylates and Nitrones
Bao Hu,* Zhao Wang, Ning Ai,* Jie Zheng,† Xing-Hai Liu, Shang Shan, and
Zhongwen Wang†
College of Chemical Engineering and Materials Science, Zhejiang University of
Technology, Hangzhou 310014, PR China
aining@zjut.edu.cn; hubao001@zjut.edu.cn
Received September 30, 2011
ABSTRACT
A highly efficient and convenient method for the synthesis of 1,2,4,5-tetrasubstituted imidazoles from readily accessible 2-azido acrylates and
nitrones has been developed. This reaction proceeded under mild conditions without the assistance of any metal, acid, or base.
Imidazoles are an important class of N-heterocycles
that are finding many diverse applications,1 with exam-
ples including drug cores (e.g., angiotensin II inhibitors,2a
antiinflammatory,2b and anticancer2c agents), natural
products,3 conjugated and functional polymers,4 coordi-
nation complexes,5 important ligands in metalloenzymes,6
precursors of stable carbene ligands,7 and ionic liquids.8
This versatile applicability highlights the importance of
access to efficient synthetic routes to prepare imidazole
derivatives.9 Traditional methods1 for imidazole core
synthesis include cyclocondensation reactions between
R-diketones, R-haloketones (or their derivatives), and
formamide (Bredereck synthesis);10 the reaction of
R-diketones with aldehydes and ammonia (Debusꢀ
Radziszewski reaction);11 the reaction of R-haloketones
with amidines;12 and the base-promoted reaction of
p-tosylmethyl isocyanide and aldimines or imidoyl chlorides
(van Leusen reaction).13 However, many of these reac-
tion conditions require the use of a strong base or high
† Present address: State Key Laboratory of Elemento-Organic Chemistry,
Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071,
PR China.
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r
10.1021/ol202650z
Published on Web 11/09/2011
2011 American Chemical Society

temperature or produce acids as byproducts. As such, a
range of new synthetic routes,9 including stepwise sub-
stitution reactions on simple imidazoles,14 and catalytic
cyclizations from acyclic precursors,15 have been devel-
oped, many of which can provide easy access to these
products. However, there are still some limitations asso-
ciated with these methods, such as use of corresponding
imidazoles as a starting material; inaccessible synthetic
precursors; and hazardous, toxic, special, and often ex-
pensive reagent or transition-metal catalysts. Thus, the
discovery of new, direct, and general synthetic routes to
such heterocycles remains a formidable challenge.
Scheme 1. A Domino Process Leading to 1,2,4,5-Tetrasubsti-
tuted Imidazoles 3
As remarkably versatile intermediates in modern or-
ganic synthesis, azides participate in a wide range of reac-
tions that construct new carbonꢀnitrogen or nitrogenꢀ
heteroatom bonds.16 Recently, much attention has been
focused toward applying 2-azido acrylates as a pivotal
three-atom synthon for the formation of diverse nitrogen-
containing heterocycles including indoles, pyridines,
pyrroles, isoquinolines, 1,2,4-triazolines, pyrrolo[1,2-R]-
pyrazines, and pyrazoles, which have been synthesized
with the assistance of meal salt,17 triphenylphosphine,18
or base.19 Inspired by these results and with the interest of
developing a new type of [3 þ 3] cycloaddition of nitrones,20
we investigated the reaction of 2-azido acrylates 1 and
nitrones 2. Quite surprisingly, instead of the anticipated
[3 þ 3] cycloaddition products and/or the possible [3 þ 2]
cycloaddition21 side products, we observed an unexpected
domino process leading to 1,2,4,5-tetrasubstituted imida-
zoles 3under catalyst-free conditions (Scheme 1). To the best
of our knowledge, only a few one-step, noncatalytic reac-
tions which produce highly substituted imidazoles have been
reported.9,22 Herein, we wish to report our recent results.
Initially, we examined the reaction of 2-azido acrylate 1a
(1.0 equiv) with nitrone 2a (1.5 equiv) in 1,2-dichloroethane
(DCE) at 50 °C for 24 h and obtained the imidazole 3aa in
23% yield together with the recoveryof 51% of 1a (Table 1,
entry 1). Further screening of the solvents, reaction tem-
perature, and time (entries 2ꢀ16) established the optimal
reaction conditions: 3.0 equiv of 2a and use of anhydrous
MgSO4 (4.0 equiv) as an additive in DCE at 66 °C for 48 h
with 96% yield of 3aa (entry 11). The structure of 3aa was
established by spectroscopic analysis and further confirmed
by single-crystal X-ray analysis (Figure 1).23
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Since 2-azido acrylates24 and nitrones25 are readily
available, the domino approach to imidazoles is highly
appealing. We, therefore, extended the substrate scope
to various 2-azido acrylates 1 and nitrones 2 using the
optimized conditions. As presented in Table 2, various
substituted 2-azido acrylates 1 with nitrone 2a worked well
€
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(23) CCDC 835710 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-request/cif.
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Org. Lett., Vol. 13, No. 24, 2011
6363

Table 1. Optimization of Reaction Conditionsa
Table 2. Reactions of Various 2-Azido Acrylates 1 with
Nitrones 2aa
temp
time
(h)
conversion
(%)b
yield
(%)c
entry
solvent
(°C)
product
yield
(%)b
1
DCE
DCE
EtOAc
THF
DMF
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
50
60
60
60
60
66
80
66
66
66
66
70
83
66
66
66
24
36
36
36
36
24
24
36
48
48
48
48
17
48
48
48
49
75
23
66
65
55
58
67
22
77
83
87
96
93
44
37
71
<20
entry
1
R1
R2
3
2
1
1a
1b
1c
1d
1e
1f
C6H5
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
COMe
CHO
3aa
3ba
3ca
3da
3ea
3fa
96
98
98
95
95
91
90
91
83
96
98
41
97
98
88
0
3
73
2
3,4,5-tri-MeOC6H2
3,4-di-MeOC6H3
p-MeOC6H4
p-CH3C6H4
p-BrC6H4
p-FC6H4
4
83
3
5
67
4
6
7d
80
5
29
6
8
82
7
1g
1h
1i
3ga
3ha
3ia
9
88
8
p-CF3C6H4
o-MeOC6H4
m-MeC6H4
m-ClC6H4
C6H5CH2
C6H5
10e
11e,f
12e,f
13e,f
14e,g
15e,h
16e,i
>95
>95
>95
>95
>95
>95
>95
9
10
11
12
13
14
15
16c,d
1j
3ja
1k
1l
3ka
3la
1m
1n
1o
1p
3ma
3na
3oa
3pa
C6H5
C6H5
a Reaction conditions, unless otherwise stated: 2-azido acrylate 1a (0.30
mmol, 1.0 equiv), nitrone 2a (0.45 mmol, 1.5 equiv), 5.0 mL of solvent, Ar
atmosphere. b Determined by recovered 1a. c Isolated yields based on 1a.
d Nitrone 2a was consumed. e 3.0 equiv of 2a was added. f Anhydrous MgSO4
H
C6H5
a Reaction conditions, unless otherwise stated: 2-azido acrylates 1
(0.30 mmol, 1.0 equiv), nitrone 2a (0.90 mmol, 3.0 equiv), anhydrous
MgSO4 (150 mg), DCE (5.0 mL) at 66 °C, Ar atmosphere. b Isolated
yields based on 1. c 2-Azido acrylate 1P was consumed. d Reaction was
carried out at 40 and 66 °C respectively.
g
˚
(150 mg) was added as an additive. 4Amolecularsieves(150mg) wasadded
as an additive. h Anhydrous MgSO4 (50 mg). i Anhydrous MgSO4 (200 mg).
9 where ortho substituents on the aryl ring gave a slightly
reduced yield (83%), compared to entry 4 (95%). For alkyl
substituted 2-azido acrylate 1l, the corresponding imidazole
product 3la was obtained in 41% yield (entry 12). Instead
of 2-azido acrylates, both 3-azido but-3-en-2-one 1n and 2-
azido acrylaldehyde 1o worked well to give the correspond-
ing products in 98% and 88% yields (entries 14 and 15).
However, when R-azidostyrene 1p was subjected to reaction
conditions, no imidazole formation was observed (entry 16).
Further studies revealed that the reactions of a variety of
nitrones 2 with 2-azido acrylate 1a also proceeded smoothly
to give the corresponding products 3abꢀal in 21ꢀ98% yields
(Table 3, entries 1ꢀ11). It should be noted that substrate 2i
with methyl substituted at the 2-position on the aryl ring
gave the desired product 3ai in 84% yield (entry 8). When
applied to the cyclic nitrone 2l, the reaction also proceeded
smoothly to give the corresponding imidazole derivative 3al
in 21% yield under identical reaction conditions (entry 11).
To understand the mechanism of the domino process,
2H-azirine 426 prepared by thermolysis of 2-azido acrylate 1e
was used instead of 1e to perform the reaction (Scheme 2).
We found that it did not follow the same reaction as 1e, and
we did not obtain the desired imidazole product 3ea. These
Figure 1. ORTEP drawing of 3aa.
to provide the corresponding 1,2,4,5-tetrasubstituted imida-
zoles 3 in moderate to excellent yields. The reaction could
tolerate aromatic substituted 2-azido acrylates with various
steric and electronic properties (entries 1ꢀ11). Notably,
excellent yields of imidazoles were obtained for both 1b
bearing a strong electron-donating group (entry 2) and 1h
bearing a strong electron-withdrawing group (entry 8). The
retarding effect of sterics on the reaction is illustrated in entry
(25) For synthesis of nitrones, see: (a) Lo, M. M.-C.; Fu, G. C. J. Am.
Chem. Soc. 2002, 124, 4572. (b) Evans, D. A.; Song, H.-J.; Fandrick,
K. R. Org. Lett. 2006, 8, 3351. (c) Beckmann, E. Justus Liebigs Ann.
Chem. 1909, 365, 201. (d) Murahashi, S.; Mitsui, H.; Shiota, T.; Tsuda,
T.; Watanabe, S. J. Org. Chem. 1990, 55, 1736.
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D. Monatsh. Chem. 1972, 103, 205. (b) Knittel, D. Synthesis 1985, 186.
(c) Henn, L.; Hickey, D. M. B.; Moody, C. J.; Rees, C. W. J. Chem. Soc.,
Perkin Trans. 1 1984, 2189. (d) Alves, M. J.; Gilchrist, T. L. J. Chem.
Soc., Perkin Trans. 1 1998, 299.
6364
Org. Lett., Vol. 13, No. 24, 2011

Table 3. Reactions of 2-Azido Acrylate 1a with Several
Nitrones 2a
Scheme 3. A Plausible Mechanism for the Formation of 3aa
a Reaction conditions: 2-azido acrylate 1a (0.30 mmol, 1.0 equiv),
nitrones 2 (0.90 mmol, 3.0 equiv), anhydrousMgSO4 (150 mg), DCE (5.0
mL) at 66 °C, Ar atmosphere. b Isolated yields based on 1a. c After 57 h.
resonance form C4. It is believed that the key intermediate
C undergoes a former [3 þ 3] cycloaddition with nitrone
2a via the zwitterionic pathway or the two-electron processes,
giving intermediate D with high regioselectivity. Then, the
intermediate D undergoes a thermally induced homolytic
cleavage of the NꢀO bond,30 followed by the hydrogen shift
resulting in the formation of 5-amino ketomalonate F. D
might also directly convert to F. The intermediate G is readily
obtained via an intramolecular cyclization through nucleo-
philic addition of the amino nitrogen to the carbonyl group.
Finally, dehydration of intermediate G would afford the
desired product 3aa. Further investigation into the mechan-
ism is currently underway.
Scheme 2. Reaction of 2H-Azirine 4 and Nitrone 2a
results suggest that the present reaction does not undergo the
2H-azirine intermediate. Further investigation implied that
the reaction might go on via free radical species, which was
supported by the case that no imidazole product 3aa was
obtained with the addition of a radical-trapping compound,
2,6-di-tert-butyl-4-methylphenol (1.0 equiv), in the domino
reaction of 2-azido acrylate 1a and nitrone 2a.
In conclusion, we have developed a new and efficient
strategy to prepare highly substituted imidazoles in moderate
to excellent yields. This reaction was realized through a novel
domino process from readily available 2-azido acrylates and
nitrones. Further studies on the scope, mechanism, and
synthetic applications of this reaction are in progress.
On the basis of these results, a tentative mechanism for
the domino reaction is proposed in Scheme 3. Initially,
intramolecular cyclization of 2-azido acrylate 1a takes
place to form a triazoline A,27 which undergoes 1,5-
hydrogen shift processes to give a triazoline B. The
zwitterionic intermediate C128 (path a) or biradical inter-
mediate C229 (path b) is in situ generated from the triazo-
line intermediate B by thermal elimination of dinitrogen.
The intermediate C1 presumably exists in one resonance
form C3. The intermediate C2 might also stand in one
Acknowledgment. We thank the starter grant from
Zhejiang University of Technology (56710108007), the
Science and Technology Project of Zhejiang Province
(Y201121893), the National Natural Science Foundation
of China (No. 21006095), and the Natural Science Foun-
dation of Zhejiang Province (Y4100680) for financial
support. Dr. Xiaoliang Xu (College of Chemical Engineer-
ing and Materials, Zhejiang University of Technology) are
kindly acknowledged for helpful discussions. Manuscript
revision by Dr. Ding Du at China Pharmaceutical Uni-
versity is also gratefully acknowledged.
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via the Internet at http://pubs.acs.org.
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