BF3/MCM-41 as nano structured solid acid catalyst for the synthesis of 3-iminoaryl-imidazo[1,2-a]pyridines

Article abstract of DOI:10.1039/c3cy00947e

Multi-component reaction of various types of aldehydes, 2-aminopyridines and trimethylsilyl cyanide was carried out in the presence of MCM-41 supported boron trifluoride (BF₃/MCM-41) as a nanostructured solid acid catalyst for the synthesis of 3-iminoaryl-imidazo[1,2-a]pyridine derivatives. MCM-41 nanoparticles were synthesized by a sol-gel method and BF ₃/MCM-41 samples with various loading amounts of BF₃ and different calcination temperatures were prepared and characterized by XRD, SEM and FT-IR techniques. The catalytic performance experiments show that BF ₃/MCM-41 calcined at 400 °C has the best catalytic activity. The recyclability and reusability experiments show that the catalyst is reusable many times with a moderate decrease in activity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy00947e

Catalysis
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BF3/MCM-41 as nano structured solid
acid catalyst for the synthesis of
3-iminoaryl-imidazo[1,2-a]pyridines
Cite this: Catal. Sci. Technol., 2014,
4, 1151
*
Mohammad Abdollahi-Alibeik and Ali Rezaeipoor-Anari
Multi-component reaction of various types of aldehydes, 2-aminopyridines and trimethylsilyl cyanide was
carried out in the presence of MCM-41 supported boron trifluoride (BF3/MCM-41) as a nanostructured
solid acid catalyst for the synthesis of 3-iminoaryl-imidazo[1,2-a]pyridine derivatives. MCM-41 nanoparticles
were synthesized by a sol–gel method and BF3/MCM-41 samples with various loading amounts of BF3 and
different calcination temperatures were prepared and characterized by XRD, SEM and FT-IR techniques.
The catalytic performance experiments show that BF3/MCM-41 calcined at 400 °C has the best catalytic
activity. The recyclability and reusability experiments show that the catalyst is reusable many times with a
moderate decrease in activity.
Received 20th November 2013,
Accepted 19th January 2014
DOI: 10.1039/c3cy00947e
www.rsc.org/catalysis
the catalyst. A possibility for developing solid acids is surface
modification of solid supports such as mineral oxides.
1. Introduction
Compounds with an imidazo[1,2-a]pyridine scaffold have
been shown to possess a broad spectrum of biological activi-
ties, such as anti-inflammatory,1,2 antifungal,3 antiviral,4 anti-
convulsant5 and hypnotic6 activity. The general method for
the synthesis of imidazo[1,2-a]pyridines involves isocyanide-
based multi-component condensation reactions of various
isocyanides, aldehydes and 2-aminopyridines in the presence
of various catalysts and conditions, such as synthesis of
3-aminoimidazo[1,2-a]azines in the presence of supported
scandium triflate on solid,7 combinatorial synthesis of fused
3-aminoimidazoles8 and Ugi reaction for synthesis of
3-aminoimidazo[1,2-a]pyridines and imidazo[1,2-a]pyrazines.9
Among them, MCM-41 with highly ordered hexagonal
mesopores (1–10 nm), high surface area (~1000 m2 g−1) and
high thermal stability has been the focus of recent applica-
tions of catalysts and catalyst supports.14–20
In this paper we aim to report the preparation, charac-
terization and catalytic application of MCM-41 supported
boron trifluoride (BF3/MCM-41) as a solid acid catalyst for
the synthesis of imidazo[1,2-a]pyridines by a one pot, multi-
component reaction of TMSCN, aldehydes and 2-aminopyridines
(Scheme 1).
Recently, a new protocol has been developed for the synthesis 2. Experimental
of imidazo[1,2-a]pyridines using trimethylsilyl cyanide (TMSCN)
2.1. Material and methods
as a traditional cyanide source instead of isocyanide.10,11 This
All chemicals were commercial products. All reactions were
new protocol is analogous to the classical Strecker and Ugi
reactions; however, microwave irradiation is required.
Since the discovery of the latter method, many catalytic
systems have been developed for the multi-component reac-
tion of TMSCN, aldehydes and 2-aminopyridines, such as
1-butyl-3-methylimidazolium bromide [Bmim][Br]12 and silica–
sulfuric acid.13
The use of solid acid catalysts has attracted considerable
attention in organic transformations due to their advantages
such as ease of handling, simple work-up and reusability of
monitored by TLC and all yields refer to isolated products.
Melting points were obtained on Buchi B-540 apparatus and
are uncorrected. 1H and 13C NMR spectra were recorded in
DMSO-d6 on a Bruker Avance III 400 (400 MHz for 1H and
100 MHz for 13C) spectrometer. Infrared spectra of the cata-
lysts and reaction products were recorded on a Bruker FT-IR
Equinax-55 spectrophotometer in KBr pellets. XRD patterns
were recorded on a Bruker D8 ADVANCE X-ray diffracto-
meter using nickel filtered Cu Kα radiation (λ = 1.5406 Å).
The morphology was studied using a Philips XL30 scanning
electron microscopy. A Metrohm 691 pH meter with a Metrohm
fluoride ion-selective electrode (6.0502.120) coupled with
Metrohm reference electrode Ag/AgCl (6.0729.100) was used
for fluoride determination.
Department of Chemistry, Yazd University, Yazd 89195-741, Iran.
E-mail: abdollahi@yazduni.ac.ir; Fax: +98 351 8210644; Tel: +98 351 8122659
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Scheme 1
2.2. Preparation of MCM-41 nanoparticles
4b: m.p.: 135–138 °C, FT-IR (KBr) νmax: 3050, 2950, 1601,
1520, 1445, 1390, 1233,1021, 690, cm−1; 1H NMR (400 MHz,
CDCl3): δ = 3.02 (s, 3H), 6.57 (d, 1H, J = 4.8 Hz), 7.09 (t, 1H,
J = 6.6 Hz), 7.32 (d, 1H, J = 5.4 Hz), 7.40 (t, 2H, J = 5.5 Hz),
7.46 (m, 4H), 7.71 (d, 2H J = 5.2 Hz ), 7.75 (d, 2H J = 5.4 Hz),
8.51 (s, 1H).
The synthesis of nanosized MCM-41 was carried out using
tetraethylorthosilicate (TEOS) as the Si source, cetyltrimethyl-
ammonium bromide (CTAB) as the template and ammonia
as the pH control agent with the gel composition of
SiO2: CTAB : NH4OH : H2O = 1 : 0.127 : 1.261 : 476.5.
4c: m.p.: 200–204 °C, IR (KBr) νmax: 3090, 3035, 1606, 1572,
1500, 1487, 1447, 1408, 1399, 1277, 1233, 1193, 1078, 1023,
821, 765, 700 cm−1; 1H NMR (400 MHz, CDCl3): δ = 6.97 (t, 1H,
J = 6.4 Hz), 7.33 (t, 1 H, J = 8.8 Hz), 7.42 (t, 2H, J = 6.0 Hz),
7.64 (d, 1H, J = 8.4 Hz), 8.17 (d, 1H, J = 8.0 Hz), 8.22 (d, 1 H,
J = 7.2 Hz), 8.47 (d, 1H, J = 7.2 Hz), 8.65 (m, 1H ), 8.71 (m, 1H ),
8.8 (s, 1H), 8.96 (s, 1H), 9.1 (s, 1H); 13C NMR (100 MHz, CDCl3):
δ = 113.13, 117.71, 123.48, 123.76, 123.84, 126.04, 128.98,
130.86, 131.85, 134.51, 135.81, 143.78, 149.20, 149.43, 150.34,
152.01, 153.43.
In a typical procedure, 1.04 g of CTAB was added to deion-
ized water (200 mL) at 70 °C and then 5 mL TEOS was added
dropwise for 1 h and the mixture was allowed to cool to room
temperature. Then aqueous ammonia (25 wt.%) was added
until the pH of the solution was adjusted to 10.5 and the mix-
ture was stirred for 12 h. The gel was separated by centrifuge
and washed with distilled water (20 mL) and EtOH (2 × 10 mL),
respectively. The gel was dried in an oven at 120 °C for 1 h
and then calcined at 550 °C for 4 h.
4d: m.p.: 155–157 °C, IR (KBr) νmax: 3080, 1600, 1507, 1478,
1247, 1214, 827, 749 cm−1.
2.3. Preparation of BF3/MCM-41
A mixture of MCM-41 (1 g), toluene (10 mL) and BF3·Et2O
(3 mmol) was stirred for 12 h at room temperature. The sus-
pension was separated by centrifuge and washed with toluene
(10 mL). The solid was dried in an oven at 120 °C for 1 h and
then calcined at 120, 200, 300, 400 or 500 °C for 2 h. The
obtained samples were denoted as BM-120, BM-200, BM-300,
BM-400 and BM-500, respectively.
4e: m.p.: 172–176 °C, IR (KBr) νmax: 3080, 1600, 1570, 1535,
1512, 1247. 1160. 1021, 783 cm−1.
4f: m.p.: 158–160 °C, IR (KBr) νmax: 3035, 1740, 1363, 1217,
1
1033, 1214 cm−1; H NMR (400 MHz, CDCl3): δ = 6.90 (t, 1H,
J = 4.8 Hz), 7.15 (m, 4H, J = 6.0 Hz), 7.25 (d, 1H, J = 6.9 Hz),
7.81 (m, 4H), 8.42 (d, 1H, J = 5.0 Hz), 8.71 (s, 1H).
4g: m.p.: 135–138 °C, IR (KBr) νmax: 3031, 1641, 1606, 1493,
1271, 730, 690 cm−1; 1H NMR (400 MHz, CDCl3): δ = 2.44 (s, 3H),
6.72 (d, 1H, J = 4.2 Hz), 7.34 (s, 2H), 7.46 (m, 5H), 7.82 (m, 4H),
8.34 (d, 1H, J = 4.8 Hz), 8.77 (s, 1H).
2.4. General procedure for the synthesis of imidazo[1,2-a]pyridines
in the presence of BF3/MCM-41
4h: m.p.: 211–213 °C, IR (KBr) νmax: 3070, 1656, 1604, 1484,
1450, 1240, 1038, 749 cm−1.
A mixture of 2-aminopyridine (1.1 mmol), aromatic aldehyde
(2 mmol), TMSCN (1 mmol) and BM-400 (50 mg) was refluxed
in EtOH (2 mL). After completion of the reaction (monitored
by TLC, eluent: n-hexane : EtOAc, 8 : 2), the catalyst was sepa-
rated and washed with EtOH (3 × 2 mL). After addition of
water, the product was precipitated with high purity. Further
purification was achieved by recrystallization in EtOH. The
pure imidazo[1,2-a]pyridines were obtained in 75–95% yields.
4i: m.p.: 150–152 °C, IR (KBr) νmax: 3041, 1640, 1590, 1520,
1491, 1231, 1089, 832 cm−1; 1H NMR (400 MHz, CDCl3):
δ = 2.44 (s, 3H), 6.74 (d, 1H, J = 5.1 Hz), 7.33 (s, 1H), 7.43 (t, 4H,
J = 6.3 Hz), 7.77 (d, 4H, J = 5.0 Hz), 8.30 (d, 1H, J = 5.4 Hz),
8.69 (s, 1H).
3. Results and discussions
2.5. Physical and spectroscopic data for selected compounds
3.1. Catalyst characterization
4a: m.p.: 160–163 °C, FT-IR (KBr) νmax: 3042, 2927, 1606, 1598,
Particle morphology of MCM-41 and BM-400 was studied by
scanning electron microscopy (SEM). The SEM images show
1499, 1447, 1351, 1241, 1241, 1028, 690 cm−1.
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spherical MCM-41 and BM-400 nanoparticles with a size of
<100 nm (Fig. 1).
The FT-IR spectra of MCM-41, BM-120 and BM-400 are
shown in Fig. 2. As shown in Fig. 2a, MCM-41 shows charac-
teristic peaks at 1200, 1082 and 810 cm−1, which are assigned
to asymmetric and symmetric stretching vibrations of Si–O–Si,
and a weaker peak at 966 cm−1 due to Si–OH groups.21,22 The
peak at 460 cm−1 is assigned to the bending vibration of Si–O–Si.
In the spectra of BM-120 and BM-400 (Fig. 2b, c), apart from the
main peaks of MCM-41, there are peaks at 1392 and 1408 cm−1,
which are assigned to B–O stretching of BM-120 and BM-400,
respectively.23 In the spectrum of BM-400, an increase in the
intensity of the peak at 966 cm−1 relative to the same peak in
the spectrum of BM-120 may be due to an increase in the
number of Brønsted sites (Si–OH) provided during B–O covalent
bond formation after calcination.
The low angle XRD patterns of MCM-41, BM-120 and BM-400
are shown in Fig. 3. The characteristic peaks of MCM-41 are
at 2θ = 2.3°, 4° and 4.6° (Fig. 3a).21,24 In Fig. 3b and c, after
BF3 loading, the intensity of main peak was decreased and
width of the peak was increased in both BM-120 and BM-400
samples. This is due to a decrease in the long-range order of
the hexagonal array of mesostructure of MCM-41 after loading
of BF3 into the mesoporous channels of MCM-41. However,
an increase in sharpness and intensity of these peaks was
observed in the pattern of BM-400, which is a result of the
Fig. 2 FT-IR spectra of (a) MCM-41, (b) BM-120 and (c) BM-400.
regulation of mesostructure of BM-400 relative to BM-120
(Fig. 2c), maybe due to covalent bonding between boron and
SiOH of MCM-41 during calcination.
The catalytic acidity was determined by potentiometric
titration of samples with 0.02 N solution of n-butyl amine in
acetonitrile. According to this method, the initial electrode
potential (Ei) indicates the maximum acid strength and the
range of where a plateau is reached (meq of the used n-butyl
amine per gram of the catalyst) indicates the total number of
acid sites.25 As shown in Fig. 4a, the very low initial potential
shows that the acid strength of BM samples is higher than
MCM-41. As shown in Fig. 4, the strength of BM-200 and BM-300
is close to BM-400 but with a higher number of acid sites
than BM-400. These results may be due to leaching of non-
bonded BF3 into solution at lower calcination temperatures.
However, a lower initial potential for BM-500 shows weaker
acid strength for this sample, which is in agreement with its
catalytic activity.
The distribution of both Lewis and Brønsted acid sites of
BM-400 was investigated using FT-IR spectroscopy by means of
pyridine absorption. The results are shown in Fig. 5. The FT-IR
spectrum of pyridine-adsorbed BM-400 before heat treatment
Fig. 1 The SEM image of (a) MCM-41 and (b) BM-400.
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Fig. 4 Potentiometric titration of (○) MCM-41, (Δ) BM-200, (◇) BM-300,
(●) BM-400 and (□) BM-500.
Lewis acid sites and a weaker peak of Brønsted acid sites
(Fig. 6a–d). However, after heating at 200 °C, only BM-400
shows the strong peaks of Lewis acid sites (Fig. 6h) and the
other spectra show desorption of pyridine at this temperature
(Fig. 6e–g). These results confirm strong acid sites of BM-400
compared with other samples.
Fig. 3 XRD patterns of (a) MCM-41, (b) BM-120 and (c) BM-400.
(Fig. 5b) shows the contribution of pyridine adducts in the
region of 1400–1650 cm−1. In this spectrum, the peaks at 1448
and 1598 cm−1 are attributed to pyridine bonded Lewis acid
sites of the BM-400. These peaks are overlapped with the
peaks of O–B at 1408 cm−1 and the peak of water at 1632 cm−1,
respectively. The weak peak at 1478 cm−1 is the combination
band of pyridine bonded to Lewis and Brønsted acid sites. The
characteristic peak of Brønsted acid site (1543 cm−1) is not
detected in the spectrum. This is due to the presence of much
higher numbers of boron-containing Lewis acid sites relative
to Brønsted acid sites. The peak at 1635 cm−1 in the spectrum
of the catalyst before pyridine adsorption (Fig. 5a) and also
after pyridine adsorption is due to the presence of water on
the preparation of the pellet sample. This sharp peak may
overlap with the weak peak of the Brønsted acid site at 1635 cm−1.
These results suggest that the Lewis acid sites have a domi-
nant role in the catalytic activity.
The loading amount of BF3 on the BM-400 was determined
by a spectrophotometric method26 and results are shown in
Table 1. To investigate the bonding state of BF3 and B/F mole
ratios on the MCM-41 before and after heat treatment, the
fluoride contents of BM-120 and BM-400 were measured by a
potentiometric method using a fluoride ion-selective electrode.27
By this method, the B/F mole ratios for BM-120 and BM-400
were obtained as 1/1.98 ≈ 1/2 and 1/2.91 ≈ 1/3, respectively.
These results confirm the presence of BF3 on the surface of
BM-120 with bonding interaction between the oxygen of
silanol or Si–O–Si and the boron of BF3. The B/F mole ratio
of 1/2 in BM-400 also confirms the presence of a covalent
bond between the oxygen of Si–O and boron and formation
of –O–BF2 due to evolution of HF during calcination. Thus,
Si–O–BF2 is the final structural form of the catalyst in BM-400.
This results are confirmed by the shift of the B–O stretching
vibration from 1392 cm−1 in BM-120 (Fig. 2b) to 1408 cm−1 in
BM-400 (Fig. 2c), which is due to transformation of the weaker
coordinated bond of O–B in BM-120 to a stronger covalent
bond in BM-400.
In order to compare the strength of the acidic sites of BM
samples, FT-IR spectra of pyridine-adsorbed BM samples
before heat treatment and after heating at 200 °C are shown
in Fig. 6. All samples before heat treatment show peaks of
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Fig. 5 FT-IR spectra of (a) BM-400 and pyridine-adsorbed BM-400 at:
(b) 25 °C, (c) 100 °C, (d) 200 °C, (e) 300 °C, (f) 400 °C and (g) 500 °C.
Fig. 6 FT-IR spectra of pyridine-adsorbed (a) BM-120, (b) BM-200, (c)
BM-300 and (d) BM-400 at ambient temperature and (e) BM-120, (f)
BM-200, (g) BM-300 and (h) BM-400 at 200 °C.
3.2. Catalytic activity of BF3/MCM-41
The catalytic activity of BF3/MCM-41 nanoparticles was
investigated in the synthesis of 3-imidazo[1,2-a]pyridines.
Initially, the reaction of benzaldehyde, trimethylsilyl cyanide
and 2-aminopyridine was selected as the model reaction. The
maximum loading amount of BF3 on the MCM-41 (20 wt.%)
was achieved by impregnation of MCM-41 using a suspen-
sion containing BF3: MCM-41 mole ratio of 5 : 1 (Table 1). As
shown in Table 1, a higher concentration of BF3·Et2O in the
impregnation media cannot enhance the loading amount of
BF3 on the MCM-41. The effect of BF3 loading amount in
the catalytic activity of prepared samples was investigated in
the model reaction. The results show that the catalytic activity
in terms of reaction time and yields of the products were
increased by an increase in the loading of BF3 up to 20 wt.%
(Table 1, entry 3).
To investigate the effect of the calcination temperature on
the catalytic activity, the model reaction was carried out in
the presence of 100 mg of BM samples (Table 2, entries 1–5).
As shown in Table 2 (entry 5), with increase in calcination
temperature up to 400 °C, the reaction times were decreased
and the yields were increased. The reaction time was increased
and the yield was decreased with further increase in calcination
temperature (Table 2, entry 6). These results indicate that
BM-400 has the best catalytic activity.
In order to optimize the amount of BM-400 on the cata-
lytic performance, the model reaction was also carried out in
the presence of various amounts of BM-400 and the results
show that in terms of reaction time and the yield, the use of
50 mg of BM-400 has the best performance (Table 2, entry 8).
To investigate the effect of the solvent on the catalytic
reaction, the model reaction in the presence of 50 mg of
Thus, further optimization experiments were performed
using BF3/MCM-41 prepared with 1 : 5 mole ratio (BM samples)
and the results are shown in Table 2.
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Table 1 Catalytic activity of 100 mg BM-400 with various loading
amounts of BF3 in the synthesis of imidazo[1,2-a]pyridine
showed a turnover number (TON) of 7.14 moles of product
per moles of catalyst.
BF3 loadingb
The scope and generality of this type of reaction is illus-
trated with respect to the reaction of different aldehydes and
2-aminopyridines with TMSCN and the results are summa-
rized in the Table 3.
BF3: MCM-41
(mole ratio)a
Time
(min)
Yield
(%)
Entry
(Mole ratio)
(wt.%)
1
1 : 15
1 : 10
1 : 5
1 : 4
1 : 3
—
0.053
0.11
0.181
0.181
0.181
0.181
6
13
20
20
20
20
70
60
45
45
45
60
75
85
95
95
95
90
2
3c
4
As shown in Table 3, various type of aldehydes with both
electron-donating and electron-withdrawing substituents were
reacted with TMSCN and 2-aminopyridine under the same
reaction conditions and the corresponding imidazopyridines
were obtained in high yields (Table 3, entries 4a–i). The work-up
and recovery of the catalyst are simple. After completion of the
reaction (monitored by TLC; eluent: EtOAc : n-hexane, 25 : 75),
the catalyst was separated by centrifuge and was washed by
EtOH (3 × 5 mL). The crude product was recrystallized by
EtOH to obtain pure imidazo[1,2-a]pyridine.
Reusability is one of the most important features of a
solid acid catalyst. To investigate this trait, the recovered cat-
alyst from the model reaction was dried in an oven at 120 °C
for 2 h. The recovered catalyst was reused in the model reac-
tion. The obtained result shows a moderate increase in the
reaction time and yield of the product (Table 4, entry 2).
This moderate deactivation may be due to partial blockage
of the active sites of the catalyst or leaching of boron from
the surface. To clarify the reason for the decline in activity,
the BF3 loading of the recovered catalyst was determined
and the results show 20 wt% BF3 on the MCM-41 (Table 1,
entry 6). This result confirms that leaching is not the reason
for deactivation. However, the recovered catalyst in run 4 was
calcined at 400 °C for 2 h and then reused in the same reac-
tion. The result shows that the catalytic activity was increased
almost to that of the fresh catalyst. This confirms that deac-
tivation is due to blockage of the active sites of the catalyst. It
should be noted that at 400 °C the calcination may be incom-
plete and partial blockage may remain.
5
6d
a
b
Mole ratio in the sample preparation media. Loading of BF3 on
the MCM-41 was determined by spectrophotometric method.26
Selected catalyst. d Reused catalyst.
c
BM-400 was carried out in various solvents under reflux condi-
tions (Table 2, entries 8, 10–13). The result show that the
EtOH is the best solvent in terms of the time and yield
(Table 2, entry 8). The model reaction was also performed
under solvent-free conditions and also at r.t. in EtOH. Results
show low efficiency of these reactions (Table 2, entries 14–15 ).
The efficiency of the BF3 on the support was investigated by
performing the reaction in the presence of pure MCM-41 and
a low yield of the product was obtained (Table 2, entry 16). To
show the effect of the support on the catalytic activity of BF3,
the model reaction was carried out in the presence of 11 mg
BF3·Et2O as homogeneous analog and results show that a
lower yield of product was achieved after a long time (Table 2,
entry 17). The efficiency of the catalyst was also examined in a
blank reaction, without any catalyst. The reaction was com-
pleted after 300 min with 50% yield of the product.
According to the optimization results, the reaction of
benzaldehyde (2 mmol), TMSCN (1 mmol) and 2-aminopyridine
(1 mmol) in the presence of 50 mg of BM-400 in EtOH under
reflux conditions is best for the synthesis of imidazopyridines
(Table 2, entry 8). The BM-400 catalyst under these conditions
Table 2 Optimization of the reaction conditions for the synthesis of imidazo[1,2-a]pyridine in the presence of BF3/MCM-41
Entry
Catalsyst
Solvent
Catalyst amount (mg)
Time (min)
Yield (%)
1
BM-120
BM-120
BM-200
BM-300
BM-400
BM-500
BM-400
BM-400
BM-400
BM-400
BM-400
BM-400
BM-400
BM-400
BM-400
MCM-41
BF3·Et2O
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
CH3CN
CHCl3
CH2Cl2
—
100
100
100
100
100
100
25
50
75
50
50
50
50
50
50
50
21
40
100
135
120
45
55
120
50
50
50
240
90
90
95
85
80
85
95
85
95
95
95
95
40
55
58
85
85
50
74
2a
3
4
5
6
7
8
9
10
11
12
13
14b
15c
16
17
60
EtOH
EtOH
EtOH
240
240
180
a
c
Results of reused BM-120. b The reaction was performed at 80 °C. The reaction was performed at room temperature.
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Table 3 Reaction of aldehydes, 2-aminopyridines and TMSCN for the synthesis of 3-imidazo[1,2-a]pyridines in the presence of BM-400
Paper
Entry
Aldehyde (1)
2-Aminopyridine (2)
Imidazo[1,2-a]pyridine (4)
Time (min)
Yield (%)
Mp (°C) Ref.
4a
50
95
160–16328
4b
180
75
135–13813
4c
50
85
200–20411
4d
90
80
155–15728
4e
75
80
172–17628
4f
90
80
158–16029
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Table 3 (continued)
Entry
Aldehyde (1)
2-Aminopyridine (2)
Imidazo[1,2-a]pyridine (4)
Time (min)
Yield (%)
Mp (°C) Ref.
4g
70
85
135–13828
4h
50
80
211–21313
4i
90
75
150–15228
Table
4 Reusability of BM-400 in reaction of 2-aminopyridine,
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