Bromodimethylsulfonium bromide (BDMS) catalyzed synthesis of imidazo[1,2-a]pyridine derivatives and their fluorescence properties

Article abstract of DOI:10.1016/j.tetlet.2012.02.078

A convenient synthetic protocol for the synthesis of imidazo[1,2-a]pyridine has been developed by employing one-pot three-component Ugi reaction by employing aromatic amidine, aromatic aldehyde, and isocyanide using 5 mol % of bromodimethylsulfonium bromide (BDMS) at room temperature. In addition, they also exhibit interesting fluorescence properties, which may be useful for fluorescent probe. Mild reaction conditions, non-aqueous work-up procedure, good yields, short reaction time, and no need of chromatographic separation are some of the salient features of the present protocol.

Full text of DOI:10.1016/j.tetlet.2012.02.078

Tetrahedron Letters 53 (2012) 2211–2217
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bromodimethylsulfonium bromide (BDMS) catalyzed synthesis
of imidazo[1,2-a]pyridine derivatives and their fluorescence properties
⇑
Abu T. Khan , R. Sidick Basha, Mohan Lal
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient synthetic protocol for the synthesis of imidazo[1,2-a]pyridine has been developed by
employing one-pot three-component Ugi reaction by employing aromatic amidine, aromatic aldehyde,
and isocyanide using 5 mol % of bromodimethylsulfonium bromide (BDMS) at room temperature. In addi-
tion, they also exhibit interesting fluorescence properties, which may be useful for fluorescent probe.
Mild reaction conditions, non-aqueous work-up procedure, good yields, short reaction time, and no need
of chromatographic separation are some of the salient features of the present protocol.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 16 December 2011
Revised 13 February 2012
Accepted 16 February 2012
Available online 22 February 2012
Keywords:
Multi-component reactions (MCRs)
Bromodimethylsulfonium bromide (BDMS)
Aromatic amidine
Isocyanide
Aromatic aldehyde
Imidazo[1,2-a]pyridine derivatives
Fluorescent probe
lonite clay K107a or Sc(OTf)3 solid supported p-toluenesulfonic
7b
Multicomponent reactions (MCRs) are found to be a useful
strategy for the construction of architecturally complex molecules
having a wide range of biological activities from the readily avail-
able starting materials in a single step.1 Due to their simplicity,
high selectivity, good yield, superior atom-economy, high variabil-
ity, less time consuming, and avoidance of costly purification pro-
cesses,2,3 these reactions have gained considerable attention in
recent times. The catalysts also play a crucial role in various mul-
ti-component reactions in determining the nature of the product,
yield, reaction time, and selectivity.
Heterocyclic compounds always attribute remarkable attention
in pharmaceutical industry due to their wide therapeutic values.
Among them, imidazo[1,2-a]pyridine derivatives display a diverse
range of biological activities such as antiviral,4 antiulcer,5a hyp-
notic, and diabetic agent5b as depicted in Figure 1.
9
acid8a and glyoxalic acid8b or Sc(OTf)3,8c ZnCl2, and ionic liquid.10
Recently, Adib et al. reported the synthesis of imidazo[1,2-a]
pyridine scaffolds using Ugi reaction (3-CR) without any catalyst11
and by employing modified three-component Ugi reaction (3-CR)
of aromatic amidines, benzyl halide, and isocyanide in the presence
of DMSO and K2CO3.11b However, some of the above methods have
some demerits such as the requirement of expensive and excess
amount of catalyst,6 longer time,6,7a,11 difficulties in work-up
procedure, and harsh reaction conditions.7 Though these protocols
are quite useful, still there is a further scope to develop a new
methodology which might work even under milder reaction
conditions. Recently we have reported12 the usefulness of bro-
modimethylsulfonium bromide (BDMS) as a catalyst as well as bro-
minating reagent in the organic synthesis. As a part of our ongoing
efforts on the development of new synthetic protocols for the syn-
thesis of biologically active heterocyclic compounds through
MCRs,13 we conceived that BDMS can be used as an efficient catalyst
for the synthesis of imidazo[1,2-a]pyridine derivatives. In this Let-
ter, three-component condensation (3CC) of aromatic aldehydes,
aromatic amidines, and isocyanides is reported for the synthesis
of imidazo[1,2-a]pyridine derivatives using BDMS as a catalyst as
shown in Scheme 1.
These compounds can be prepared traditionally in two ways: (i)
by two component condensation of an amidine and
a-haloke-
tones4,5a (ii) based on three-component Ugi reaction (3-CR) of aro-
matic amidines with aromatic aldehyde and isocyanide.6 For the
second approach, numerous catalysts have been used over the years
such as using microwave irradiation in the presence of Montmoril-
For the present study, the catalyst bromodimethylsulfonium
bromide (BDMS) was prepared by following the literature proce-
dure.12a In the beginning, the model reaction of p-chlorobenzalde-
⇑
Corresponding author. Tel.: +91 361 2582305; fax: +91 361 2582349.
E-mail address: atk@iitg.ernet.in (A.T. Khan).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.078

2212
A. T. Khan et al. / Tetrahedron Letters 53 (2012) 2211–2217
NH2
N
O
N
N
N
F
S
N
A
NH2
F
N
Antiviral Agent
D
O
F
O
N
Type 2 -Diabetiec Agent
NH
HO
N
N
NCN
NH
N
N
O
NH
S
C
Soraprazan Phase II
N
B
E
Antiulcer Agent
zolpidem - Hypnotic Agent
Figure 1. Some of the biologically active compounds containing imidazo[1,2-a]pyridine framework.
R4
of 5 mol % BDMS in acetonitrile at room temperature (Table 1, entry
NH2
N
R3
R2
R4
R3
N
10). Other Lewis acids such as ceric ammonium nitrate (CAN) and
Cu(OTf)2 in MeCN (Table 1, entries 2 and 3) were also examined,
which afforded relatively lower yield of compound 4a. To examine
the suitability of various solvent(s) system, it was observed that ace-
tonitrile gave the best result in terms of reaction time and yield.
After optimizing the reaction conditions, a mixture of 4-
bromobenzaldehyde, 2-aminopyridine, and tert-butyl isocyanide
was kept for stirring with 5 mol % BDMS at room temperature
and the desired product 4b was obtained in 85% yield. Likewise,
a mixture of 3-hydroxybenzaldehyde, 2-aminopyridine, and tert-
butyl isocyanide also provided the required product 4c in 90% yield
under identical reaction conditions. For generality of the present
protocol, we have scrutinized several reactions with various aro-
matic aldehydes having substituent in the aromatic ring with
2-amino-5-methylpyridine and tert-butyl isocyanide smoothly un-
der similar reaction conditions and all these reactions afforded the
products 4d–k in good yields. It was noted that the aldehyde con-
taining electron-withdrawing group takes relatively shorter reac-
tion time than the aromatic aldehyde having electron-donating
group (Table 1, entry 6).
5mol% BDMS
MeCN, rt
Ar
+
R5NC
+
ArCHO
N
R1
NH
R5
3
1
R1
R2
2
4
Amidines
2a: R1= R2=R3=R4=H
2b: R1=R3=R4=H, R2=Me
2c: R1= R2=R3= H, R4=Me
2d: R1=R3=Me,R2=Br,R4=H
Isocyanides
3a: tert-Butyl
3b: Cyclohexyl
3c: 2-Morpholinoethyl
Scheme 1. Synthesis of imidazo[1,2-a]pyridine derivatives.
hyde (1a), amidine (2a) and tert-butyl isocyanide (3a) was con-
ducted to find out the optimal reaction conditions and the results
were mentioned in Table 1. We executed various trial reactions
and noticed that the reaction provided best result in the presence
Table 1
Optimization of the reaction conditionsa
CHO
NH2
N
N
Catalyst
NC
Cl
+
+
N
Solvent, r.t
NH
Cl
1a
2a
3a
4a
Entry
Catalyst
—
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
CH3CN
CH3CN
CH3CN
MeOH/DCM
CH3CN/MeOH
CH3CN/DCM
THF
1.4 Dioxane
DMF
14
12
9
3
2
2.5
3.5
3
3.5
0.5
26
64
55
75
85
80
77
72
68
95
10 mol % CAN
10 mol % Cu(OTf)2
5 mol % BDMS
5 mol % BDMS
5 mol % BDMS
5 mol % BDMS
5 mol % BDMS
5 mol % BDMS
5 mol % BDMS
CH3CN
a
The reaction was performed using 1 mmol scale of p-chlorobenzaldehyde, 2-amino pyridine, and tert-butyl isocyanide at
room temperature.
b
Isolated yield.

A. T. Khan et al. / Tetrahedron Letters 53 (2012) 2211–2217
2213
Table 2
Synthesis of imidazo[1,2-a]pyridines15
R4
NH2
R3
R2
R4
N
N
5mol% BDMS
MeCN, rt
Ar
+
R5NC
+
ArCHO
N
R3
R1
NH
R5
3
R2
1
R1
2
4
Entry
Aldehyde
Amidine
Isocyanide
Producta
Time (h)
Yieldb (%)
CHO
N
Cl
N
N
NH
1
2a
3a
2
95
85
Cl
1a
4a
CHO
N
Br
NH
2
2a
3a
2
Br
1b
4b
CHO
OH
N
N
OH
3
4
5
2a
2b
2b
3a
3a
3a
2
90
96
93
NH
1c
4c
CHO
N
N
Me
NH
1.5
1d
4d
CHO
N
N
Me
Me
1e
1
NH
4e
N
CHO
NO2
N
NH
6
2b
3a
0.5
94
95
NO2
1f
4f
CHO
N
Me
Br
N
N
Me
Me
NH
7
8
2b
2b
3a
3a
1
Me
1g
4g
N
CHO
NH
2.5
89
Br
1h
4h
(continued on next page)

2214
A. T. Khan et al. / Tetrahedron Letters 53 (2012) 2211–2217
Table 2 (continued)
Entry
Aldehyde
Amidine
Isocyanide
Producta
Time (h)
Yieldb (%)
CHO
OH
N
N
Me
OH
9
2b
3a
1
92
NH
1i
4i
CHO
O
O
N
O
N
Me
10
2b
3a
2
85
O
NH
1j
4j
CHO
O
O
N
O
N
O
O
Me
11
2b
3a
2.5
88
O
NH
1k
4k
CHO
Cl
Me
N
Cl
N
NH
12
2c
3b
1.5
90
1l
4l
CHO
Br
Me
Br
N
Br
N
Me
NH
13
2d
3b
1.5
87
1m
4m
CHO
Me
Br
N
NO2
N
NH
14
2d
3a
1
96
Me
NO2
1n
4n
H3C
N
N
H
CHO
N
15
2b
3c
2.5
70
N
1o
O
CH3
4o
a
All the reaction was performed using (1 mmol) aldehyde, (1 mmol) amidine, and (1 mmol) isocyanide.
Isolated yield.
b
To verify further the scope of the present protocol, the reactions
were also conducted with 2-amino-3-methylpyridine, chlorobenz-
aldehyde, and cyclohexylisocyanide in the presence of 5 mol %
BDMS at room temperature, which gave the desired product in
good yield. Lastly, the reactions were carried out with sterically
hindered 2-amino-5-bromo-4,6-dimethylpyridine with aromatic

A. T. Khan et al. / Tetrahedron Letters 53 (2012) 2211–2217
2215
The formation of imidazo[1,2-a]pyridine can be explained as fol-
lows: Initially aromatic aldehyde reacts with amidine to form imine
F and water. The released water molecule reacts with BDMS which
leads to the formation of dimethyl sulfide, HOBr, and HBr. Then, the
imine undergoes protonation with the liberated HBr to form G,
which reacts instantly with isocyanide in a [4+1] cycloaddition
manner to give intermediate I. The final product imidazo[1,2-a]pyr-
idines 4 was obtained from the intermediate I by 1,3-hydrogen
shift, as shown in Scheme 2. To compare the efficiency of the pres-
ent protocol, we have also examined the reaction with a mixture of
4-chlorobenzaldehyde, 2-aminopyridine, and tert-butyl isocyanide
in the presence of 48% aqueous HBr (0.1 mL) at room temperature.
The desired product 4a was obtained in 65% yield after 4 h of stir-
ring, which is lower as compared to BDMS-catalyzed reaction.
Recently, Balakirev et al. reported that some of the imidazo[1,2-
a]pyridine derivatives are potential candidate for fluorescent
probes, which can be used for fluorescence imaging in clinical diag-
nostics and biomedical research.14 Therefore, we felt that the com-
pounds prepared by us may also exhibit similar properties and
their results of the photophysical properties of the imidazo[1,2-
a]pyridine are summarized in Table 3. Moreover, the UV–visible
spectra of the imidazo[1,2-a]pyridine derivatives contain intense
absorption maxima at 250 5 and at 335 2 nm. At C2 position
of the imidazo[1,2-a]pyridine which bears a chlorine substituent
in the aromatic ring 4a results in a shift of the absorption maxima
to longer wavelength. These compounds exhibit Stokes shift in the
range of 122–137 nm.
Figure 2. X-ray crystal structure of 4m (CCDC No. 848594).
aldehydes and isocyanides under identical conditions and they also
gave products 4m and 4n in 87% and 96% yields, respectively. To
verify the present protocol with aliphatic aldehyde, we have per-
formed reaction with valeraldehyde, 2-amino-5-methylpyridine
and 2-morpholinoethyl isocyanide in the presence of 5 mol %
BDMS under identical reaction conditions and it provided the
desired product 4o in 70% yield (Table 2, entry 15). From these
results, it is quite obvious that the present protocol is a general
one, which works with various amidines as well as with different
aromatic/aliphatic aldehydes.
The output of the fluorescence results are displayed in Table 3,
which show that among the electron donating group on the aro-
matic ring at C2 position of the imidazo[1,2-a]pyridine, 2,4-OMe
group on the aromatic ring 4j enhances the fluorescence wave-
length maxima.
All the isolated products were fully characterized by IR, and
NMR spectroscopy. In addition, the structure of compound 4m
was also determined using single-crystal X-ray data (as shown in
Fig. 2).16
Furthermore, the fluorescence of the imidazo[1,2-a]pyridine
derivatives 4 is shown in Figures 3 and 4, respectively.
Scheme 2. Plausible mechanism for the formation of imidazo[1,2-a]pyridines.

2216
A. T. Khan et al. / Tetrahedron Letters 53 (2012) 2211–2217
Table 3
Photo physical data of imidazo[1,2-a]pyridine in CH2Cl2
b
Entry
Imidazo[1,2-a]pyridine
Absorptiona
Fluorescenceb kem
kabs (nm)
e
(MÀ1 cm1)
N
N
Cl
Br
253
337
133,400
28,110
NH
1
460
4a
N
N
254
336
166,120
35,200
NH
2
3
458
461
4b
N
Me
N
Me
245
335
230,100
85,500
NH
4g
OH
N
N
Me
249
332
58,630
13,050
4
467
NH
4i
O
N
O
N
252
333
47,430
13,560
Me
5
470
NH
4j
a
Measured at a concentration of 1.0 Â 10À5 mol dmÀ3 at 25 °C.
b
Emission maxima upon excitation at 320 nm.
Figure 4. Fluorescence of 4 in CH2Cl2.
In summary, we have devised a simple and an efficient
protocol for the one-pot synthesis of imidazo[1,2-a]pyridine
derivatives catalyzed by BDMS under mild reaction conditions.
Moreover, the present protocol possesses several unique merits
such as simplicity, non aqueous work-up, and most importantly
Figure 3. Fluorescence spectra of imidazo[1,2-a]pyridine derivatives.

A. T. Khan et al. / Tetrahedron Letters 53 (2012) 2211–2217
2217
9. Rousseau, A. L.; Matlaba, P.; Parkinson, C. J. Tetrahedron Lett. 2007, 48,
4079.
10. Shaabani, A.; Soleimani, E.; Maleki, A. Tetrahedron Lett. 2006, 47, 3031.
11. (a) Adib, M.; Mahdavi, M.; Noghani, M. A.; Mirzaei, P. Tetrahedron Lett. 2007, 48,
7263; (b) Adib, M.; Sheikhi, E.; Rezaei, N. Tetrahedron Lett. 2011, 52,
3191.
12. (a) Choudhury, L. H.; Parvin, T.; Khan, A. T. Tetrahedron 2009, 65, 9513; (b)
Khan, A. T.; Parvin, T.; Choudhury, L. H. J. Org. Chem. 2008, 73, 8398; (c) Khan, A.
T.; Ali, M. A.; Goswami, P.; Choudhury, L. H. J. Org. Chem. 2006, 71, 8961.
13. (a) Khan, A. T.; Lal, M.; Ali, S.; Khan, M. M. Tetrahedron Lett. 2011, 52, 5327; (b)
Khan, A. T.; Das, D. K.; Khan, M. M. Tetrahedron Lett. 2011, 52, 4539; (c) Khan, A.
T.; Khan, M. M. Tetrahedron Lett. 2011, 52, 3455; (d) Khan, A. T.; Khan, M. M.;
Bannuru, K. K. R. Tetrahedron 2010, 66, 7762; (e) Khan, A. T.; Lal, M.; Khan, M.
M. Tetrahedron Lett. 2010, 51, 4419.
high yield of the products. These new classes of fluorescence imi-
dazo[1,2-a]pyridine derivatives may be of great interest in clinical
diagnostics and biomedical application in future. As a result, this
protocol may provide an access to a diverse array of medicinal
scaffold of imidazoheterocycles. Furthermore, the studies on the
fluorescence properties of the imidazo[1,2-a]pyridine in biomedi-
cal application is under process, which will be reported in due
course of time.
Acknowledgments
14. Burchak, O. N.; Mugherli, L.; Ostuni, M.; Lacapère, J. J.; Balakirev J. Am. Chem.
Soc. 2011, 133, 10058.
R.S.B. and M.L. are thankful to the CSIR, New Delhi, India for
their research fellowships. The authors are grateful to the Director,
IIT Guwahati for providing the general facilities to carry out this
work. We are also thankful for the referees’ valuable comments
and suggestions.
15. General procedure for the synthesis of imidazo[1,2-a]pyridine:
A mixture of
aromatic aldehyde (1 mmol), amidine (1 mmol), and isocyanide (1 mmol) was
taken in 2 mL acetonitrile. Then, the catalyst bromodimethylsulfonium
bromide (0.011 g, 0.05 mmol) was added into it and the reaction mixture
was kept for stirring at room temperature till the completion of the reaction as
indicated by TLC. The solid product came out slowly which was filtered
through
a Buchner funnel and the solid precipitate was washed with
Supplementary data
acetonitrile. Finally it was dried under reduced pressure.
Spectral data of some of the compounds: N-tert-butyl-2-(4-chlorophenyl)H-
imidazo[1,2-a]pyridin-3-amine (4a): Brown solid: mp 146 °C. IR (KBr): 2961,
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.02.078.
2925, 2854 cmÀ1 1H NMR (400 MHz, CDCl3): d 0.99 (s, 9H), 3.0 (br s, NH, 1H),
.
6.70 (t, J = 6.8 Hz, 1H), 7.07 (t, J = 7.2 Hz, 1H), 7.32 (d, J = 8.4 Hz, 2H), 7.47 (d,
J = 8.8 Hz, 1H), 7.87 (d, J = 8.4 Hz, 2H), 8.11 (d, J = 6.8 Hz, 1H). 13C NMR
(100 MHz, CDCl3): d 30.5, 56.5, 111.5, 117.4, 123.5, 123.6, 124.3, 128.5, 129.4,
133.1, 133.9, 138.4, 142.1. Anal. Calcd for C17H18ClN3: C, 68.11; H, 6.05; N,
14.02. Found: C, 67.95; H, 5.99; N, 13.91. N-tert-Butyl-6-methyl-2-phenylH-
imidazo[1,2-a]pyridin-3-amine (4d): White solid: mp 215–217 °C. IR (KBr):
References and notes
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.
3H), 3.0 (br s, NH, 1H), 6.90 (d, J = 9.2 Hz, 1H), 7.22 (t, J = 7.2 Hz, 1H), 7.31–7.39
(m, 3H), 7.82 (d, J = 7.6 Hz, 2H), 7.91 (s, 1H). 13C NMR (100 MHz, CDCl3): d 18.5,
30.4, 56.5, 116.8, 121.0 (2C), 121.3, 123.4, 127.3, 128.2, 128.3, 135.6, 139.5,
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.
(m, 1H), 2.68 (br s, NH, 1H), 3.14 (s, 3H), 3.45 (s, 3H), 7.52 (d, J = 8.4 Hz, 2H),
7.77 (d, J = 8.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): d 18.6, 24.1, 24.9, 25.8, 33.2,
58.6, 114.7, 114.8, 121.5, 127.0, 129.3, 131.5, 133.7, 134.8, 135.5, 138.4, 142.3.
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Cambridge
CB2
1EZ,
UK
(fax:
+44
1223
336033,
e-mail:
deposit@ccdc.cam.ac.uk or via: www.ccdc.cam.ac.uk.