Ionic liquid promoted one-pot three-component reaction: Synthesis of annulated imidazo[1,2-a]azines using trimethylsilylcyanide

Article abstract of DOI:10.1007/s00706-006-0567-0

Imidazo[1,2-a]azine derivatives are synthesized by a one-pot three-component reaction of an 2-aminoazine, an aldehyde, and trimethylsilylcyanide in the presence of 1-n-butyl-3-methylimidazolium bromide as a recoverable ionic liquid, in moderate to excelle

Full text of DOI:10.1007/s00706-006-0567-0

Monatshefte f €u r Chemie 138, 51–56 (2007)
DOI 10.1007/s00706-006-0567-0
Printed in The Netherlands
Short Communication
Ionic Liquid Promoted One-Pot
Three-Component Reaction: Synthesis
of Annulated Imidazo[1,2-a]azines Using
Trimethylsilylcyanide
ꢀ
Ahmad Shaabani and Ali Maleki
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received October 16, 2006; accepted (revised) October 23, 2006
Published online December 8, 2006 # Springer-Verlag 2006
Summary. Imidazo[1,2-a]azine derivatives are synthesized by a one-pot three-component reaction of
an 2-aminoazine, an aldehyde, and trimethylsilylcyanide in the presence of 1-n-butyl-3-methylimida-
zolium bromide as a recoverable ionic liquid, in moderate to excellent yields with relatively short
reaction times.
Keywords. Ionic liquid; 1-n-Butyl-3-methylimidazolium bromide; Multi-component reaction;
Trimethylsilylcyanide; 3-Aminoimidazo[1,2-a]azine.
Introduction
Imidazo[1,2-a]pyridines have emerged as versatile biologically active compounds
spanning applications in anti-inflammatory [1a, 1b] and antibacterial agents [1c],
as inhibitors of gastric acids secretion [1d], as calcium channel blockers [1e], and in
antiulcer based therapies [1f].
The classical synthesis of imidazo[1,2-a]azines involves the condensation of ꢀ-
haloketones with 2-aminoazines [2]. Being only a two-component condensation,
this reaction is less suitable for the generation of a large ensemble of compounds.
Several isocyanide-based multi-component reactions (MCRs) have been reported
for the synthesis of these compounds by the condensation of an aldehyde, an iso-
cyanide, and a 2-aminoazine in the presence of strong protic (AcOH, HClO ) [3–6]
4
or Lewis acids (Sc(OTf ) ) [5, 6]. However, these reactions require long times for
3
completion and isolation and recovery procedures are complicated.
ꢀ Corresponding author. E-mail: a-shaabani@cc.sbu.ac.ir

5
2
A. Shaabani and A. Maleki
Scheme 1
The synthesis of imidazo[1,2-a]pyridines has been reported to proceed under
microwave irradiation in the presence of montmorillonite K [7] or Sc(OTf ) [8],
however, special instrumentation is required.
1
0
3
Very recently, Hulme has discovered a convenient route for the synthesis of
-aminoimidazo[1,2-a]azines via the application of trimethylsilylcyanide (3), as
a non-classical isocyanide equivalent [9]. Reactions were performed in metha-
3
nol by microwave irradiation and catalysis by Sc(OTf ) with isolated yields in
3
the 30–70% range. In spite of their potential utility and novelity of this approach
the yields are relatively low and it requires an expensive catalyst and special
instrumentation.
During recent years, ionic liquids have attracted interest as environmentally
benign reagents due to their favorable properties, and a variety of catalytic reac-
tions have been successfully described using ionic liquids [10]. The solvophobic
properties of ionic liquids are able to generate an internal pressure and promote the
association of the reactants in a solvent cavity during the activation process and
thus accelerate the reaction. This property of ionic liquids is very efficient for mul-
ticomponent reactions in which the entropy of reaction is decreased for the transi-
tion state.
In connection with our previous work using ionic liquids as reaction media and
our interest in multi-component reactions [11], we now report herein the facile
synthesis of 3-aminoimidazo[1,2-a]pyridines 4, 3-aminoimidazo[1,2-a]pyrazines
4, and 3-aminoimidazo[1,2-a]pyrimidines 4 by a one-pot three-component conden-
sation of an aldehyde 1, a 2-aminoazine 2, and trimethylsilylcyanide 3, as an
isocyanide equivalent, in the presence of 1-n-butyl-3-methylimidazolium bromide
([bmim]Br) as a promoter under classical heating conditions in high yields with
rather short reaction times (1–2 h) (Scheme 1).
Results and Discussion
In order to optimize the reaction conditions, we conducted the condensation of
4
-chlorobenzaldehyde (1 mmol), 2-amino-5-methylpyridine (1 mmol), and 3
ꢁ
(1.2 mmol) with stirring under heating conditions at 80 C in various solvents
and ionic liquids. The results showed that the efficiency and the yield of the
reaction in [bmim]Br was higher than those obtained in other solvents, such as

Ionic Liquid Promoted One-Pot Three-Component Reaction
Table 1. Synthesis of 3-aminoimidazo[1,2-a]azines in the presence of [bmim]Br at 80 C
53
ꢁ
1
2
a
Product
R
R
X
Y
Time=h
Yield=%
b
4
4
a
4-ClC H
6
Me
Me
Me
Me
Me
Me
Br
H
CH
CH
CH
CH
CH
CH
CH
N
CH
CH
CH
CH
CH
CH
CH
CH
CH
N
1
92 (90, 86, 85, 80)
4
b
4-CH C H
6 4
1.2
1.2
1.5
2
87
85
77
80
62
83
60
75
72
84
3
4
c
3-O NC H
2 6 4
4
d
Ph
4
e
4-pyridyl
2-pyridyl
Ph
4
f
2
4
4
g
1.5
2
h
4-CH OC H
3 6
4
4
4
i
4-O NC H
6 4
H
N
2
2
4
j
4-CH OC H
3
H
CH
CH
1.5
1.5
6
4
k
4-O NC H
6 4
H
N
2
a
b
Isolated yield; the same ionic liquid was used for all of the five runs
MeOH, EtOH, CH Cl , and toluene and other ionic liquids like [bmim]PF and
2
2
6
[
bmim]BF4.
To illustrate the need of the ionic liquid for these reactions an experiment was
conducted in the absence of an ionic liquid. The yield in this case was only 5%
ꢁ
after heating at 80 C for 3 h. Obviously, the ionic liquid is an important component
of the reaction.
One of the advantages of ionic liquids is their ability to function as a recyclable
reaction medium. We were able to separate [bmim]Br from the reaction medium
easily by washing with water and evaporating the solvent under vacuum, and we
could reuse it for subsequent reactions (Table 1, Entry 1).
Further, we examined the effect of a variety of aldehydes and aminoazines in
this reaction. The results reported in Table 1 show that in all cases the products
were obtained efficiently. It is worth mentioning that the reaction conditions are
ꢁ
attractive: they are mild (approximately neutral conditions at 80 C in common vessels
for 1–2 h) and only a slight excess of reagent 3 is required (1.2 mmol). Thus, this
process could be also interesting for large-scale synthesis.
In conclusions, we report a useful method for the synthesis of 3-aminoimi-
dazo[1,2-a]azine derivatives via a one-pot three-component condensation reaction
of 2-aminoazines, aldehydes, and 3 (as an isocyanide equivalent) using the ionic
liquid [bmim]Br as a promoter in moderate to excellent yields with relatively short
reaction times.
Experimental
Melting points were measured on an Electrothermal 9200 apparatus. IR spectra were recorded on a FT-
IR 102MB BOMEM apparatus. Mass spectra were recorded on a FINNIGAN-MAT 8430 mass
1
spectrometer operating at an ionization potential of 70eV. H and C NMR spectra were recorded
13
1 13
on a BRUKER DRX-300 AVANCE spectrometer at 300.13 and 75.47 MHz. H and C NMR spectra
were obtained on solutions in CDCl and DMSO-d . All chemicals were obtained from Fluka, Merck,
3
6
and Aldrich and were used without purification. All products (except 4d) are new compounds, which
1
13
were identified by IR, H and C NMR spectral data, and mass spectroscopy.

5
4
A. Shaabani and A. Maleki
General Procedure
A mixture of 1 mmol 2-aminoazine and 1 mmol aldehyde in the presence of 0.3 g [bmim]Br in a screw-
3
capped vial was stirred for 15 min. Then 0.15cm 3 (1.2mmol) were added and the reaction was
ꢁ
heated at 80 C for 45min. After completion of the reaction (monitoring by TLC, ethyl acetate=
n-hexane, 3=1), the reaction mixture was cooled to room temperature and washed with H O. The
2
solid residue was crystallized from ethyl acetate to obtain the pure product 4. The ionic liquid was
recovered by evaporation of the filtrate in vacuum.
2
-(4-Chlorophenyl)-6-methylimidazo[1,2-a]pyridin-3-amine (4a, C H ClN )
14 12 3
ꢁ
White solid (0.24 g, 92%), mp 248–250 C (dec); IR (KBr): ꢁꢀ ¼ 3350, 3285, 2923, 1661, 1611, 1480,
ꢂ1
1
1
320 cm ; H NMR (DMSO-d ): ꢂ ¼ 2.32 (s, CH ), 5.48 (br, NH ), 7.19 (d, J ¼ 8.4 Hz, H–Ar), 7.42
6
3
2
(
d, J ¼ 8.4 Hz, H–Ar), 7.50 (d, J ¼ 6.4 Hz, 2H–Ar), 7.97 (d, J ¼ 6.4Hz, 2H–Ar), 8.23 (s, H–Ar) ppm;
1
3
C NMR (DMSO-d ): ꢂ ¼ 18.30, 114.54, 121.17, 122.07, 122.78, 127.55, 128.16, 128.99, 129.22,
6
þ
þ
1
31.59, 136.98, 148.73, 155.85ppm; MS (EI, 70 eV): m=z (%) ¼ 258 (M þ 1, 75), 257 (M , 100), 93
(64), 92 (35), 65 (35), 39 (20).
6
-Methyl-2-p-tolylimidazo[1,2-a]pyridin-3-amine (4b, C H N )
1
5 15 3
ꢁ
Yellow solid (0.21 g, 87%), mp 230 C (dec); IR (KBr): ꢁꢀ ¼ 3352, 3290, 2925, 1661, 1612, 1481,
ꢂ1
1
1
322 cm ; H NMR (DMSO-d ): ꢂ ¼ 1.88 (s, CH ), 2.35 (s, CH ), 3.67 (br, NH ), 7.01–7.14 (m, 7H–
6
3
3
2
13
Ar) ppm; C NMR (DMSO-d ): ꢂ ¼ 16.58, 18.23, 112.35, 123.62, 125.36, 126.30, 127.25, 129.63,
6
þ
þ
1
1
33.17, 134.25, 136.12, 147.29, 152.06 ppm; MS (EI, 70eV): m=z (%) ¼ 238 (M þ 1, 68), 237 (M ,
00), 93 (45), 65 (44), 39 (27).
6
-Methyl-2-(3-nitrophenyl)imidazo[1,2-a]pyridin-3-amine (4c, C H N O )
1
4 12 4 2
ꢁ
Yellow solid (0.23 g, 85%), mp 220–223 C (dec); IR (KBr): ꢁꢀ ¼ 3350, 3280, 2923, 1665, 1535, 1485,
ꢂ1
1
1
7
8
1
356, 1320cm ; H NMR (CDCl ): ꢂ ¼ 2.35 (s, CH ), 3.25 (br, NH ), 7.04 (d, J ¼ 9.2 Hz, H–Ar),
3
3
2
.47 (d, J ¼ 9.2 Hz, H–Ar), 7.63 (dd, J ¼ 8.0, 8.0 Hz, H–Ar), 7.98 (s, H–Ar), 8.22 (d, J ¼ 8.0 Hz, H–Ar),
1
3
.63 (d, J ¼ 8.0 Hz, H–Ar), 8.75 (s, H–Ar) ppm; C NMR (CDCl ): ꢂ ¼ 16.45, 116.20, 121.23, 121.38,
3
21.70, 122.20, 125.70, 128.35, 129.43, 132.76, 133.12, 135.60, 140.36, 148.32ppm; MS (EI, 70 eV):
þ
þ
m=z (%) ¼ 269 (M þ 1, 35), 268 (M , 100), 92 (65), 65 (38), 39 (23).
6
-Methyl-2-phenylimidazo[1,2-a]pyridin-3-amine (4d, C H N )
1
4 13 3
ꢁ
White solid (0.18 g, 77%), mp 250 C (dec); IR (KBr): ꢁꢀ ¼ 3352, 3275, 2927, 1667, 1625, 1472,
ꢂ1
1
1
333 cm ; H NMR (DMSO-d ): ꢂ ¼ 2.35 (s, CH ), 5.44 (br, NH ), 7.21–7.72 (m, 5H–Ar), 7.45
6
3
2
13
(
d, J ¼ 7.6 Hz, H–Ar), 7.96 (d, J ¼ 7.6 Hz, H–Ar), 8.26 (s, H–Ar) ppm; C NMR (DMSO-d ):
6
ꢂ ¼ 17.80, 112.28, 114.34, 121.20, 122.87, 126.64, 127.74, 127.37, 129.54, 132.48, 136.77,
þ
þ
1
44.48 ppm; MS (EI, 70 eV): m=z (%) ¼ 224 (M þ 1, 65), 223 (M , 100), 92 (72), 65 (55), 39 (23).
6
-Methyl-2-(pyridin-4-yl)imidazo[1,2-a]pyridin-3-amine (4e, C H N )
1
3 12 4
ꢁ
Yellow solid (0.18 g, 80%), mp 182–185 C; IR (KBr): ꢁꢀ ¼ 3338, 3254, 2927, 1659, 1586, 1482,
ꢂ1
1
1
340 cm ; H NMR (CDCl ): ꢂ ¼ 2.39 (s, CH ), 3.42 (br, NH ), 7.03 (d, J ¼ 9.2 Hz, H–Ar), 7.44
3
3
2
(
d, J ¼ 9.2 Hz, H–Ar), 7.85 (s, H–Ar), 8.05 (d, J ¼ 5.8 Hz, 2H–Ar), 8.64 (d, J ¼ 5.8 Hz, 2H–Ar) ppm;
1
3
C NMR (CDCl ): ꢂ ¼ 18.42, 116.75, 120.32, 120.83, 122.16, 126.55, 128.26, 133.12, 140.79, 142.30,
3
þ
þ
1
48.24 ppm; MS (EI, 70 eV): m=z (%) ¼ 225 (M þ 1, 57), 224 (M , 100), 92 (78), 65 (46), 39 (34).
6
-Methyl-2-(pyridin-2-yl)imidazo[1,2-a]pyridin-3-amine (4f, C H N )
1
3 12 4
ꢁ
Yellow solid (0.14 g, 62%), mp 201–204 C; IR (KBr): ꢁꢀ ¼ 3336, 3252, 2927, 1657, 1585, 1487,
ꢂ1
1
1
342 cm ; H NMR (CDCl ): ꢂ ¼ 2.35 (s, CH ), 3.43 (br, NH ), 7.10 (d, J ¼ 9.2 Hz, H–Ar), 7.46
3
3
2
13
(
d, J ¼ 9.2 Hz, H–Ar), 7.82 (s, H–Ar), 7.52–8.57 (m, 4H–Ar) ppm; C NMR (CDCl ): ꢂ ¼ 18.43,
3
1
16.85, 118.92, 119.85, 120.16, 121.32, 122.16, 125.64, 131.26, 137.12, 137.85, 148.65, 157.11 ppm;
þ
þ
MS (EI, 70 eV): m=z (%) ¼ 225 (M þ 1, 46), 224 (M , 100), 92 (66), 65 (44), 39 (28).

Ionic Liquid Promoted One-Pot Three-Component Reaction
-Bromo-2-phenylimidazo[1,2-a]pyridin-3-amine (4g, C H BrN )
55
6
1
3
10
3
ꢁ
ꢂ1
1
Yellow solid (0.14 g, 60%), mp 230 C (dec); IR (KBr): ꢁꢀ ¼ 3340, 3260, 2930, 1660 cm ; H NMR
1
3
(
CDCl ): ꢂ ¼ 4.72 (br, NH ), 7.23–8.45 (m, 8H–Ar) ppm; C NMR (CDCl ): ꢂ ¼ 106.65, 117.59,
3
2
3
1
23.13, 125.32, 127.03, 127.77, 128.66, 128.64, 133.36, 136.86, 139.52 ppm; MS (EI, 70eV): m=z
81
þ
þ
79
(
%) ¼ 288 (M þ 1, Br, 28), 286 (M , Br, 30), 158 (66), 156 (75), 76 (25).
2
-(4-Methoxyphenyl)imidazo[1,2-a]pyrimidin-3-amine (4h, C H N O)
3 12 4
1
ꢁ
Yellow solid (0.14 g, 60%), mp 212–214 C (dec); IR (KBr): ꢁꢀ ¼ 3337, 3256, 2935, 1658, 1467,
ꢂ1
1
1
7
1
352 cm ; H NMR (DMSO-d ): ꢂ ¼ 3.85 (s, OCH ), 4.25 (br, NH ), 7.14 (d, J ¼ 8.9 Hz, 2H–Ar),
6
3
2
1
3
.66 (d, J ¼ 8.9Hz, 2H–Ar), 7.88–8.46 (m, 3H–Ar) ppm; C NMR (DMSO-d ): ꢂ ¼ 55.30, 114.54,
6
20.17, 122.17, 124.78, 128.84, 135.28, 136.24, 148.73, 157.85, 161.14 ppm; MS (EI, 70eV): m=z
þ
þ
(%) ¼ 241 (M þ 1, 46), 240 (M , 100), 112 (35), 93 (35), 39 (20).
2
-(4-Nitrophenyl)imidazo[1,2-a]pyrimidin-3-amine (4i, C H N O )
1
2 9 5 2
ꢁ
ꢂ1
;
Yellow solid (0.19 g, 75%), mp 215–218 C (dec); IR (KBr): ꢁꢀ ¼ 3330, 3252, 1672, 1547, 1358cm
1
H NMR (DMSO-d ): ꢂ ¼ 4.35 (br, NH ), 7.67 (d, J ¼ 9.02Hz, 2H–Ar), 8.14 (d, J ¼ 9.02 Hz, 2H–Ar),
6
2
1
3
7
.38 (dd, J ¼ 7.15 Hz, J ¼ 7.15Hz, H–Ar), 8.76 (d, J ¼ 7.15Hz, 2H–Ar) ppm; C NMR (DMSO-d ):
6
ꢂ ¼ 120.65, 121.57, 122.37, 128.48, 135.68, 136.28, 138.24, 148.33, 148.62, 158.85 ppm; MS
þ
þ
(EI, 70 eV): m=z (%) ¼ 256 (M þ 1, 42), 255 (M , 100), 134 (44), 122 (25), 93 (35), 46 (30).
2
-(4-Methoxyphenyl)imidazo[1,2-a]pyrazin-3-amine (4j, C H N O)
1
3 12 4
ꢁ
Yellow solid (0.17 g, 72%), mp 205–207 C (dec); IR (KBr): ꢁꢀ ¼ 3330, 3253, 2931, 1653, 1464,
ꢂ1
1
1
7
1
340 cm ; H NMR (DMSO-d ): ꢂ ¼ 3.86 (s, OCH ), 4.36 (br, NH ), 7.01 (d, J ¼ 8.5 Hz, 2H–Ar),
6
3
2
1
3
.33 (d, J ¼ 8.5 Hz, 2H–Ar), 8.42–8.46 (m, 3H–Ar) ppm; C NMR (DMSO-d ): ꢂ ¼ 55.63, 114.54,
6
20.47, 122.15, 124.68, 128.35, 135.32, 136.24, 143.24, 144.73, 161.24 ppm; MS (EI, 70eV): m=z
þ
þ
(%) ¼ 241 (M þ 1, 35), 240 (M , 100), 112 (31), 93 (42), 39 (36).
2
-(4-Nitrophenyl)imidazo[1,2-a]pyrazin-3-amine (4k, C H N O )
1
2 9 5 2
ꢁ
ꢂ1
;
Yellow solid (0.21 g, 84%), mp 230–232 C (dec); IR (KBr): ꢁꢀ ¼ 3335, 3250, 1674, 1537, 1353cm
1
H NMR (DMSO-d ): ꢂ ¼ 4.25 (br, NH ), 7.73 (d, J ¼ 9.14Hz, 2H–Ar), 8.22 (d, J ¼ 9.14 Hz, 2H–Ar),
6
2
1
3
8
1
1
.58–8.66 (m, 3H–Ar) ppm; C NMR (DMSO-d ): ꢂ ¼ 120.15, 121.67, 122.23, 128.42, 135.58, 136.48,
6
þ
þ
39.24, 143.21, 144.33, 148.62 ppm; MS (EI, 70eV): m=z (%) ¼ 256 (M þ 1, 46), 255 (M , 100),
34 (34), 122 (35), 93 (32), 46 (20).
Acknowledgements
We gratefully acknowledge the financial support from the Research Council of Shahid Beheshti
University.
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