Microwave-assisted synthesis and luminescent activity of imidazo[1,2-a]pyridine derivatives

Article abstract of DOI:10.1002/jhet.3950

In this work, a series of phenacyl bromide derivatives was synthesized and employed as key intermediate for the synthesis of substituted imidazo[1,2-a]pyridines. First, phenacyl bromide molecules were obtained from the bromination reaction of acetophenones assisted by microwave irradiation, obtaining the products 4a-v in a 15 minutes reaction with yields in the range of 50% to 99%. Subsequently, the conjugation of these molecules with 2-aminopyridine conduced to the production of imidazo[1,2-a]pyridine derivatives (7a-v) in a 60-second reaction with yields of 24% to 99%. Improved yields were determined with respect to those obtained with more tedious methodologies like thermally and mechanically assisted routes. Intense luminescence emissions in the purple and blue regions of the electromagnetic spectra were observed under UV excitation according to the nature of the substituents. This environmentally friendly methodology is expected to constitute an important class of organic compounds for the development of biomarkers, photochemical sensors, and medicinal applications.

Full text of DOI:10.1002/jhet.3950

Received: 30 January 2020
DOI: 10.1002/jhet.3950
Revised: 18 February 2020
Accepted: 20 February 2020
A R T I C L E
Microwave-assisted synthesis and luminescent activity
of imidazo[1,2-a]pyridine derivatives
Juan C. Rodríguez1 | Rony A. Maldonado1 | Gonzalo Ramírez-García2
Erik Díaz Cervantes3 | Fabiola N. de la Cruz1
|
1Departamento de Química Orgánica,
Abstract
Facultad de Ciencias Químicas,
Universidad Autónoma de Coahuila, Ing.
J. Cárdenas Valdez S/N, Saltillo, Mexico
2Cátedras CONACYT, Departamento de
Biociencias y Agrotecnología, Centro de
Investigación en Química Aplicada,
Saltillo, Mexico
3Departamento de Alimentos, Centro
Interdisciplinario del Noreste (CINUG),
Universidad de Guanajuato, Guanajuato,
Mexico
In this work, a series of phenacyl bromide derivatives was synthesized and
employed as key intermediate for the synthesis of substituted imidazo
[1,2-a]pyridines. First, phenacyl bromide molecules were obtained from
the bromination reaction of acetophenones assisted by microwave irradia-
tion, obtaining the products 4a-v in a 15 minutes reaction with yields in
the range of 50% to 99%. Subsequently, the conjugation of these molecules
with 2-aminopyridine conduced to the production of imidazo[1,2-a]pyri-
dine derivatives (7a-v) in a 60-second reaction with yields of 24% to 99%.
Improved yields were determined with respect to those obtained with more
tedious methodologies like thermally and mechanically assisted routes.
Intense luminescence emissions in the purple and blue regions of the elec-
tromagnetic spectra were observed under UV excitation according to the
nature of the substituents. This environmentally friendly methodology is
expected to constitute an important class of organic compounds for the
development of biomarkers, photochemical sensors, and medicinal
applications.
Correspondence
Fabiola N. de la Cruz, Departamento de
Química Orgánica, Facultad de Ciencias
Químicas, Universidad Autónoma de
Coahuila, Ing. J. Cárdenas Valdez S/N,
Saltillo, Coahuila, MexicoMéxico.
Email: fcruz@uadec.edu.mx
Funding information
PRODEP, Grant/Award Number:
UACOAH-PTC-456
1 | INTRODUCTION
product by an oxidative cross-coupling/cyclization cat-
alyzed by silver in nitrogen atmosphere (Scheme 1D),
The chemistry of imidazopyridines has been studied
since the beginning of this century. Accordingly, the
most widely used and practical synthesis routes are
the condensation between α-halocarbonyls and
2-aminopyridine where Vieira et al.[1] employed ultra-
sound as an alternative energy in a basic medium
(Scheme 1A). On the other hand, Jadhav et al[2] used
water and polyethylene glycol as solvent in a one-pot
reaction avoiding the synthesis of the α-halocarbonyl
(Scheme 1B). In the same way, Tufail et al[3] developed
a green media using glycerol and water as solvent with
the regioselective product in position 2 (Scheme 1C).
On the other hand, He et al[4] accomplished the
using alkynes as well. Mishra et al[5] generated in a
one-pot reaction with aldehydes, 2-aminopyridine and
terminal alkynes with copper iodide and a silica-
supported Brønsted acid obtained the product with
substitution in both 2 and 3 positions (Scheme 1E).
Chunavala et al[6] reported the unexpected synthesis
between α-bromo-β-keto esters and 2-aminopyridine
under solvent-free conditions (Scheme 1F). Santra
et al[7] used nitro olefins instead of ketones to obtain
the nuclei, this reaction is catalyzed by iron (III)
(Scheme 1G). Finally, Ponnala et al[8] reported the use
of neutral alumina in room temperature as an efficient
medium for the product to be formed (Scheme 1H).
J Heterocyclic Chem. 2020;1–9.
wileyonlinelibrary.com/journal/jhet
© 2020 Wiley Periodicals, Inc.
1

2
RODRÍGUEZ ET AL.
(B)
(C)
(D)
(A)
(H)
(E)
(F)
(G)
SCHEME 1 Representative pathways for the synthesis of imidazo[1,2-a]pyridine with different substitution patterns
Although new synthetic routes have been developed,
the condensation between α-halocarbonyls and
2-aminopyridine is nowadays the most popular
strategy.[9] These molecules exhibit a wide range of
medical activities such as anti-inflammatory,[10–12]
antioxidant,[13,14] antimicrobial,[15,16] fungicide,[17,18]
and anticancer agent.[19] In addition, other applica-
tions such as fluorescent sensor[20] and biomarkers[21]
have been described.
Beside the above-described applications, imidazo
[1,2-a]pyridines and pyrimidines are also attractive due
to their physicochemical properties such as the fluores-
cent activity.[22,23] Several studies concerning the effects
of substituents on their fluorescent properties have
been reported. Generally, its solid-state emission is due
to the π-conjugated bicyclic structure, providing intense
fluorescence emissions with high quantum yields.[24]
The experimental evidence demonstrated that the sub-
stitution with phenyl or naphthyl groups in position
C2 increased their fluorescence yield.[25] Electron-
donating substituents also improved the luminescence
performance whereas electron-withdrawing substitu-
ents led to a less intense emissions.[26] While the amino
or dimethylamine substitution at the 40-position cause a
red shift of the fluorescence in polar solvents.[27] The
replacement of methyl by trifluoromethyl triggers a
blue shift.[28] Despite these observations, the develop-
ment of more efficient fluorophores and bioactive com-
pounds is a field of growing interest, making necessary
the evaluation of new methodologies for the synthesis
of imidazo[1,2-a]pyridine derivatives with varied sub-
stituents allowing the modulation of the physicochemi-
cal properties and therefore, their activity and
applications. This work reported a novel microwave
irradiation-assisted methodology for the synthesis of
imidazo[1,2-a]pyridine derivatives in eco-friendly con-
ditions, obtaining the phenacyl bromide precursors in
15 minutes process, and the substituted imidazo[1,2-a]
pyridines in a 1-minute reaction. Furthermore, the
photophysical evaluation described in this work provide
additional insight on the relationship between structure
and optical activity.

RODRÍGUEZ ET AL.
3
2 | RESULTS AND DISCUSSIONS
2.1 | Synthesis of phenacyl bromides
in a hot plate. The results of these reactions are grouped
in Table 1. When comparing the three methodologies
proposed in entries 1, 8, and 15 (compound 4a), it is
observed that the best route of synthesis is the
mechanosynthesis, obtaining an excellent yield of 98%,
compared with 78% (entry 1, MW) and 80% (entry
15, ThT). However, in the case of double substitution in
the aromatic ring (4g), the yield of the mechanosynthesis
decreases considerably (entry 14) until 65%, while the
same compound was obtained by means of the MW and
thermal paths (entries 7 and 21) with yields of 99 and
94%, respectively. In addition, the products 4b and
In order to obtain the desired products, two reaction steps
are required. The first one is the synthesis of phenacyl
bromides (4a-v). These compounds are considered useful
precursors for organic synthesis of many molecules.[29]
The synthesis was conducted by an halogenation in the
alpha position of the respective acetophenone (1a-v),
using NBS (2) as a brominating agent, and PTSA as a cat-
alyst (3). That reaction was promoted by three different
strategies: microwave irradiation (MW), mechanic inter-
action (mechanosynthesis), and thermal treatment (ThT)
4c (entries
9
and 10) were not obtained by
mechanosynthesis, while the microwave irradiation
TABLE 1 Synthesis of 4a-g
derivatives under different
methodologies
Methodology
Entry
1
4
a
b
c
d
e
f
R
Time (min)
Yield (%)a
Microwaveb
H
15
15
15
15
15
15
15
15
45
45
15
15
15
15
40
110
120
40
45
55
40
78
99
93
95
95
95
99
98
np
np
96
95
96
65
80
98
95
96
99
90
94
2
4-Br
3
4-NO2
4-Me
4-Cl
4
5
6
4-OMe
3,4-Cl
H
7
g
a
b
c
d
e
f
Mechanosynthesisc
8
9
4-Br
10
11
12
13
14
15
16
17
18
19
20
21
4-NO2
4-Me
4-Cl
4-OMe
3,4-Cl
H
g
a
b
c
d
e
f
Thermald
4-Br
4-NO2
4-Me
4-Cl
4-OMe
3,4-Cl
g
aIsolated yield.
bReaction conditions: 1a-g (1 eq), 2 (1.1 eq), 3 (1.5 eq), DCM (1 mL), 40ꢀC, 15 minute.
cReaction conditions: 1a-g (1 eq), 2 (1.1 eq), 3 (0.5 eq), neat, r.t, 15 to 45 minute.
dReaction conditions: 1a-g (1 eq), 2 (1.1 eq), 3 (0.5 eq), ACN (5 mL), 85ꢀC, 40 to 120 minute.
Note: np = the product is not formed.

4
RODRÍGUEZ ET AL.
TABLE 2 Derivatives of 4h-v
obtained by the MW assisted
methodology
Entry
1
4
h
i
R
Yield (%)a
NHPh
75
90
85
90
50
87
99
92
60
97
40
70
95
90
90
97
2
2-OMePh
2-ClPh
3
j
4
k
l
3-MePh
3-OMePh
3-ClPh
5
6
m
n
ñ
o
p
q
r
7
3-BrPh
8
2,5-OMePh
2,5-FPh
2,5-ClPh
2,6-OMePh
2,6-ClPh
3,4-OMePh
3,4-FPh
2,4-OMePh
2,4-FPh
9
10
11
12
13
14
15
16
s
t
u
v
aIsolated yield.
TABLE 3 Synthesis of 7a-c under different methodologies
Methodologies
Entry
7
a
b
c
a
b
c
a
b
c
(R)
H
Time (min)
Yield (%)a
Thermalb
1
2
3
4
5
6
7
8
9
40
80
120
1
80
4-Br
78
4-NO2
H
30
Microwavec
88
4-Br
4-NO2
H
1
85
1
41
Mechanosynthesisd
45
45
45
mix
mix
mix
4-Br
4-NO2
Note: mix = product and byproduct.
aIsolated yield.
bReaction condition: 4a-c (1 eq), 5 (1.1 eq), 6 (1.5 eq), ACN (3–5 mL), 85ꢀC.
cReaction condition: 4a-c (1 eq), 5 (1.2 eq), 6 (2.0 eq), MeOH (1 mL), 80ꢀC.
dReaction condition: 4a-c (1 eq), 5 (1.2 eq), 6 (2.0 eq), neat, r.t.

RODRÍGUEZ ET AL.
5
TABLE 4 Reaction conditions for the synthesis of 7a
Entry
NaHCO3 (eq)
2-aminopyridine (5) (eq)
Solvent
%Ra
80
76
82
82
82
88
88
90
88
81
78
1
1
1
EtOH
2
1.5
2
3
4
2
1
EtOH
5
1.1
1.2
1.2
6
7
2
EtOHb
MeOHb
ACNc
DCMd
Neatb
8
9
10
11
aIsolated yield.
bReaction temperatures with solvents: 80ꢀC.
cReaction temperatures with solvents: 90ꢀC.
dReaction temperatures with solvents: 50ꢀC.
strategy gave yields of 99% and 93% (entries 2 and 3), and
the ThT conduced to yields of 98% and 95% (entries
16 and 17). Accordingly, MW and ThT methodologies are
the best option for the synthesis.
However, the reaction time is much shorter
(15 minutes) with respect to those required in ThT (40 to
120 minutes). Thereby, the MW methodology was
selected to synthesize the α-halogenation derivatives
reported in Table 2, which were obtained with moderate
to excellent yields.
(30%-80%). Whereas, a mixture of the main product plus
different byproducts were obtained in all the cases by
means of mechanosynthesis. As for the synthesis of the
precursors reported in Tables 1 and 2, the MW strategy
allowed the synthesis of the imidazo[1,2-a]pyridine
nuclei in only 1 minute. This is remarkably faster with
respect to the ThT methodology, which required periods
between 40 and 120 minute. This represents a clear
advantage of the MW methodology in terms of yield of
the reaction and time consumption.
Once the MW irradiation was selected as the most
efficient energy source for the synthesis of imi-
dazopyridine derivatives, modifications were made on
the base equivalents of 5, solvent, and reaction time in
order to improve the yields of the reaction, and the
results are shown in Table 4. Upon modification of the
base equivalents (NaHCO3) in the range of 1 to2 eq
(entries 1-3), the best yield was obtained with 2 eq of
the base (entry 3). Similarly, better yield was obtained
when a high quantity of 2-aminopyridine equivalents
were used (entry 6). Different solvents were used
(entries 7-10) as well as neat reaction was tested (entry
11), resulting the lowest yield in absence of solvent
(78%), and the best one in presence of methanol at
80ꢀC (90%).
2.2 | Synthesis of imidazo[1,2-a]pyridine
Once the brominated compounds were obtained, the
respective imidazo[1,2-a]pyridine nuclei were synthe-
sized. Similarly, to the first step of the reaction, ThT, MW
and mechanosynthesis strategies were applied to promote
the condensation between 2-aminopyridine (5) with the
phenacyl bromide molecules 4a-c in a basic medium
(Table 3). When comparing entries 1, 4, and 7, MW
resulted as the best methodology, presenting a yield of
88% (Table 3, entry 4), while 80% and a mixture of
byproducts were obtained by ThT and mechanosynthesis
(entries 1 and 7, respectively). In general, higher yields of
the reaction were obtained by means of the MW strategy
(41%-88%) respect to those observed with the ThT path
Therefore, considering the optimized conditions, a
wide set of derivatives with structural diversity were

6
RODRÍGUEZ ET AL.
TABLE 5 Synthesis of 7a-v according to the optimized conditions under MW irradiation
Entry
1
7
R
Yield (%)a
88
7a
7b
7c
7d
7e
7f
H
2
4-Br
85
3
4-NO2
4-Me
41
4
86
5
4-Cl
95
6
4-OMe
3,4-Cl
NHPh
2-OMe
2-Cl
94
7
7g
7h
7i
95
8
np
24
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
7j
80
7k
7l
3-Me
70
3-OMe
3-Cl
50
7m
7n
7ñ
7o
7p
7q
7r
7s
7t
99
3-Br
55
2,5-OMe
2,5-F
90
84
2,5-Cl
2,6-OMe
2,6-Cl
3,4-OMe
3,4-F
99
np
70
50
60
7u
7v
2,4-OMe
2,4-F
np
98
Note: Reaction conditions: 4a-v (1 eq), 5 (1.2 eq).
Note: np = the product is not formed.
aIsolate yield.
obtained and described in Table 5. This family of imidazo
[1,2-a]pyridine nuclei is expected to constitute an impor-
tant class of organic compounds for tracking and medici-
nal applications, by which their photophysical properties
were evaluated.
the spectra of the product 7e are presented in Figure 1,
where narrow absorption bands were detected with
maximal absorbance at 250, 253, 253, and 254 nm for
MeOH, ACN, THF, and DCM, respectively. In all cases,
a secondary band was observed in the range from
280 to 360 nm. Similar patterns were observed with the
other synthesized molecules (Figures 47 and 48 in
Data S1).
2.3 | Photophysical characterization
The photoluminescent behavior of the imidazo
[1,2-a]pyridines were analyzed under 365 nm UV-
excitation (Figure 2). As observed in the inset
The UV-Vis absorbance spectra of the different mole-
cules were obtained in different solvents. For instance,

RODRÍGUEZ ET AL.
7
photographs, all the synthesized derivatives pres-
ented strong emission at the blue and violet regions
of the electromagnetic spectrum upon dispersion in
varied solvents. The wavelengths of maximal emis-
sion were registered at 395, 401, 428, 394, 398, and
400 nm for the compounds 7a, 7d, 7e, 7v, 7f, and 7g,
respectively, while the most intense emission was
presented by the methoxy-substituted derivative (7f).
The different analyzed substitutions on the central
imidazo[1,2-a]pyridine nuclei caused no deterioration
of the fluorescent property, while important shifts
were observed in the wavelength of maximal emis-
sions, including regions from 394 until 428 nm. This
strong emission was promoted by the extension of
the electronic delocalization due to the presence of
functionalized phenacyl substituents enclosed to the
imidazo[1,2-a]pyridine nuclei.
FIGURE 1 UV-Vis absorbance spectra of 7e in different
solvents
FIGURE 2 Fluorescence emission spectra of different imidazo[1,2-a]pyridine derivatives. The inset shows photographs of these molecules
in different solvents under visible and 365 nm UV irradiation. (1) Dry compound, (2) MeOH, (3) THF, (4) Hexane, (5) ACN, and (6) DCM

8
RODRÍGUEZ ET AL.
3 | CONCLUSIONS
4.3 | General procedure for the thermal
treatment method synthesis of 4a-g
The reported method for the synthesis of imidazo
[1,2-a]pyridine compounds assisted by microwave
irradiation offers different advantages respect to the
mechanosynthesis and the thermal treatment meth-
odologies such as higher yields, and very short reac-
tion times. In addition, both α-halogenation of
ketones and condensation for the imidazopyridine
derivatives production reactions assisted by micro-
wave irradiation were obtained under more environ-
mentally friendly conditions respect to the other
evaluated methodologies. The importance of these
molecules lies in their potential application as anti-
inflammatory, antioxidant, antimicrobial, anti-prolif-
erative, and fungicide agents, as well as contrast
agents in the monitoring of the progression of the
related diseases or the development of optical sensors
by considering the photoluminescence response in
the blue and purple regions.
In a ball flask were mixed NBS (1.1 eq) and PTSA (0.5 eq) in
acetonitrile at 55ꢀC, then the respective acetophenone (1 eq)
was added and reacted for 40 to 120 minute. The progress of
the reaction was monitored by TLC (hexane/EtOAc, 7:3).
The resulting reaction mixture was extracted with CH2Cl2:
H2O, thus concentrated and finally dried under vacuum.
4.4 | General procedure for the
microwave method synthesis of 4a-v
In a microwave tube were mixed acetophenone 1a-v (1 eq),
NBS 2 (1.1 eq) and p-TsOH 3 (1.5 eq) in CH2Cl2. The reac-
tion mixture was exposed to microwave irradiation in a
closed vessel (100 W) at 40ꢀC for 15 minute. The progress of
the reaction was monitored by TLC (hexane/EtOAc, 7:3).
The resulting reaction mixture was extracted with CH2Cl2:
H2O, thus concentrated and finally dried under vacuum.
4 | EXPERIMENTAL SECTION
4.1 | General considerations
4.5 | General procedure for the
mechanosynthesis method synthesis of 7a-c
Microwave-assisted reactions were performed in a closed
vessel in a CEM Discover Microwave. The progress of the
reaction was monitored by TLC (aluminum sheets, silica
gel 60 F/UV254), using a hexane/ethyl acetate system as
eluent. Visualization was carried out under UV light
(254 and 365 nm). The products were purified by recrys-
tallization with EtOH:H2O. NMR spectra were obtained
on a Bruker Ultrashield 500 MHz spectrometer in CDCl3.
Chemical shifts are reported in ppm, relative to tetra-
methylsilane (TMS) and CHCl3 used as the internal stan-
dard. Melting points were determined on a digital
Electrothermal 9200 melting point apparatus. Infrared
spectra were recorded on a Perkin-Elmer Spectrum GX
spectrophotometer by the ATR method using neat com-
pounds. UV-vis spectra were obtained on a Jenway 7315
spectrophotometer.
In
a
mortar were triturated 4a-c (1.0 eq),
2-aminopyridine 5 (1.1 eq) and sodium bicarbonate
(2.0 eq). The reaction was triturated for 45 minute. The
reaction progress was monitored by TLC in Hx:AcOEt.
4.6 | General procedure for the thermal
treatment synthesis 7a-c
In a ball flask were mixed 4a-c (1.0 eq), 2-aminopyridine
5 (1.1 eq) and sodium bicarbonate (1.5 eq) in acetonitrile
as a solvent. The reaction was heated at 85ꢀC for 40 to
120 minute. The reaction progress was monitored by TLC
in Hx:AcOEt. Upon completion, the reaction mixture was
extracted with CH2Cl2:H2O and thus concentrated. The
organic phase was recrystallized in EtOH:H2O to obtain
the respective compound.
4.2 | General procedure for the
mechanosynthesis method synthesis of 4a-g
4.7 | General procedure for the
microwave method synthesis
In a mortar were triturated acetophenone 1a-g (1 eq),
NBS (1.1 eq) and PTSA (0.1 eq) for 15 to 45 minute at
room temperature. The progress of the reaction was mon-
itored by TLC (hexane/EtOAc, 7:3). The resulting reac-
tion mixture was extracted with CH2Cl2:H2O, thus
concentrated and finally dried under vacuum.
2-phenylimidazo[1,2-a]pyridine (7a-v)
In a microwave tube were mixed 4a-v (2.57 mmol),
2-aminopyridine 5 (3.08 mmol) and sodium bicarbonate
(2.0 eq) in MeOH as a solvent. The reaction mixture was
irradiated by microwave in a closed vessel (100 W) at

RODRÍGUEZ ET AL.
9
80ꢀC for 1 minute. The reaction progress was monitored
by TLC in Hx:AcOEt. Upon completion, the reaction
mixture was extracted with CH2Cl2:H2O and thus con-
centrated. The organic phase was recrystallized in EtOH:
H2O to obtain the compounds 7a-v.
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The products obtained were dissolved in methanol,
THF, hexane, acetonitrile, and methylene chloride and
were observed under an UV-lamp at 254 and 365 nm.
2 × 10−5 M solutions of the 7e molecules were prepared
in MeOH, THF and CH2Cl2, and 4 × 10−7 M in CH3CN,
and subsequently the UV-absorbance spectra obtained
in a Jenway 7315 spectrophotometer. For the other
products, only MeOH and CH3CN were used at a
2 × 10−5 M concentration. The luminescence emission
was analyzed with a Spectrograph Spectra Acton Pro
3500i and a R955 photomultiplier tube from Hamama-
tsu. The system was PC controlled with Spectra Sense
software.
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ACKNOWLEDGMENTS
We acknowledge the financial support from the program
of Proyecto de Nuevo PTC 2018, Grant UACOAH-PTC-
456, and to Yolanda Ortega for her technical assistance.
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Gonzalo Ramírez-García https://orcid.org/0000-0003-
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How to cite this article: Rodríguez JC,
Maldonado RA, Ramírez-García G, Díaz
Cervantes E, de la Cruz FN. Microwave-assisted
synthesis and luminescent activity of imidazo[1,2-
a]pyridine derivatives. J Heterocyclic Chem. 2020;
1–9. https://doi.org/10.1002/jhet.3950