Polyethylene Glycol (PEG-400) as Methylene Spacer and Green Solvent for the Synthesis of Heterodiarylmethanes under Metal-Free Conditions

Article abstract of DOI:10.1002/ejoc.202000467

Polyethylene glycol 400 (PEG-400) has been employed as a green, biodegradable solvent as well as methylene spacer for the one-pot synthesis of heterodiarylmethanes under metal-free conditions. PEG-400 is used for the dimerization of both symmetrical and unsymmetrical fused heterocycles and arenes as methylene spacer. The reaction was facilitated by (diacetoxyiodo)benzene (PIDA) as an oxidant. This reaction proceeds without transition metal, base, ligand, and under additive-free conditions involving the coupling of sp³ and sp²-hybridized carbon atoms. The method is applicable for variety of heterocycles such as imidazopyridines, imidazothioazoles, imidazobenzothiozoles, indolizines, indole, 1-methyl-1H-indole, N,N-dimethylaniline and trimethoxybenzene. Good to excellent yields and applicability to gram-scale synthesis demonstrate the potential scope of the process for commercial/industrial applications.

Full text of DOI:10.1002/ejoc.202000467

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Polyethylene Glycol (PEG-400): As Methylene Spacer and Green
Solvent for the Synthesis of Hetero Diarylmethanes Under Metal-
Free Conditions
Authors: Rahul Kumar, Deepa Rawat, and Subbarayappa Adimurthy
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000467
Link to VoR: https://doi.org/10.1002/ejoc.202000467
01/2020

10.1002/ejoc.202000467
European Journal of Organic Chemistry
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Polyethylene Glycol (PEG-400): As Methylene Spacer and Green
Solvent for the Synthesis of Hetero Diarylmethanes Under Metal-
Free Conditions
Rahul Kumar[a], Deepa Rawat[a] and Subbarayappa Adimurthy* [a]
Abstract: Polyethylene glycol 400 (PEG-400) has been and additive-free conditions involving the coupling of
employed as a green biodegradable solvent and as well sp3 and sp2-hybridized carbons. The method is
as methylene spacer for the one-pot synthesis of hetero applicable for variety of heterocycles such as imidazo
diarylmethanes under metal-free conditions. PEG-400 is pyridines, imidazothioazoles, imidazobenzothiozoles,
used for the dimerization of both symmetrical and indolizines,
indole,
1-methyl-1H-indole,
N,N-
unsymmetrical fused heterocycles and arenes as dimethylaniline and trimethoxybenzene. Good to
methylene spacer. The reaction was facilitated by excellent yields, applicability at gram-scale synthesis
(diacetoxyiodo)benzene (PIDA) as an oxidant. This demonstrates the potential scope of the process for
reaction proceeds without transition metal, base, ligand
commercial/industrial applications.
Introduction
In recent years, polyethylene glycol 400 (PEG-400) has been
employed as a green and inexpensive solvent in various
chemical transformations.[1] Due to its low toxicity, PEG-400 is
widely used in a variety of pharmaceutical formulations.[1k]
Currently, PEG has been utilised as an alternative reaction
media and is well-recognized in various coupling reactions such
as Suzuki, Heck, Stille, Sonogashira, Hiyama and some
challenging organic reactions for carbon–carbon bond
formations.[2]
Pyridine
fused
heterocycles
such
as
imidazopyridines and their derivatives are known to exhibit
various biological activities.[3] Among the various imidazopyridine
derivatives, imidazo[1,2-a]pyridine moiety is the most important
in the area of natural products and pharmaceuticals.[4] These
derivatives display a wide range of Biological activities such as
antifungal, anti-inflammatory, antitumor, antiviral, antibacterial,
Figure 1. Structure of bioactive molecules with methylene linkage.
There are few efficient methods have been reported for the
methylene linkage at the C-3 position of fused heterocycles
antiprotozoal,
antipyretic,
analgesic,
antiapoptotic,
(Scheme-1,
upper).[9]
These
methods
employ
hypnoselective, and anxioselective
dimethylacetamide (DMA),[9a-c]
dimethyl formamide
(DMF)[9d] or dimethylsulfoxide (DMSO)[9e,f] as methylene
source in presence of metal catalysts such as vanadium,
copper, palladium. The major concerns about these
reagents are carcinogenicity and their associated
environmental hazards. Therefore, the replacement of such
harmful organic solvents/reagents with an eco-friendly
solvents/reagents is one of the major focal points of green
activities.[5] These heterocycles are also the core structure of
many marketed drugs such as alpidem, zolpidem, (the treatment
of acute heart failure), menodronic acid,[6] nicopidem, saripidem,
(sedative and anxiolytic),[7] and optically active GSK812397
candidate (HIV infection)[8] (Figure 1). Among these molecules
the key methylene linkage at the C-3 position play a key role for
further functionalization.
chemistry.[10]
Moreover,
transition-metal-catalysed
reactions also generate hazardous waste which is
environmentally problematic and hence should be avoided
wherever possible.[9a-d] Although considerably efficient
methods have been developed for the methylene linkage at
the C-3 position of imidazo[1,2-a]pyridines, little attention
has been paid to the development under transition metal-
free conditions with green solvents. In continuation of our
efforts on the green methods development[11] and selective
C-3 functionalization of imidazo[1,2-a]pyridines,[12] we
report herein a benign and sustainable method for the
[a] Mr. R. Kumar, Ms. D. Rawat and Prof. Dr. S. Adimurthy; Council of
Scientific & Innovative Research, CSIR–Central Salt & Marine
Chemicals Research Institute, G.B. Marg, Bhavnagar-364 002.
Gujarat (INDIA).
*E-mail: adimurthy@csmcri.res.in
http://adimurthy.wix.com/drsadimurthygroup
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
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selective synthesis of heterodiarylmethanes utilizing
polyethylene glycol (PEG-400) as green reaction medium
and as well as methylene source under metal-free
conditions (Scheme 1, lower). To the best of our knowledge
the direct synthesis of heterodiarylmethanes by using
polyethylene glycol (PEG-400) as eco-friendly solvent and
as a methylene source under metal-free conditions have
never been reported.
-OMe, Cl, and CN,) at the para- position of the phenyl ring,
these substrates reacted smoothly and afford the
corresponding products 2a–f in good to excellent yields
(65–90%).
Table 1. Optimization of the reaction conditions [a]
Scheme 1. Recent reports for dimerization via methylene bridge of
imidazo[1,2-a] pyridines
Results and Discussion
Initially, to optimize the reaction conditions 2-
phenylimidazo[1,2-a]pyridine 1a was chosen as a model
substrate for the conversion to bis(2-phenylimidazo[1,2-
a]pyridin-3-yl)methane 2a, in PEG-400 as solvent-come
methylene
source
with
one
equivalent
of
[a]Reaction conditions:1a (0.25 mmol) and PIDA (0.375 mmol) were
(diacetoxyiodo)benzene PIDA as oxidant at 100 °C for 24 h,
under these conditions the desired product 2a was obtained
in 60% isolated yield (Table 1, entry 1). With the feasibility
of this reaction and to improve the yield, the amount of
oxidant PIDA was increased from 1.0 eqv. to 1.5 eqv. with
respect to 1a (entries 2–6). To our surprise, the yield of the
desired product 2a was increased to 90% (entry 6) under
the same conditions. When the reaction was performed with
different polar solvents like PEG-600, diglyme, digol,
glycerol, with the assumption that these solvents may also
serve as methylene source, but the yield of the desired
product was declined (Table 1, entry 7–10). Further
screening of different oxidizing agent like oxone, K2S2O7,
(NH4)2S2O8, TBHP, and I2 under the same conditions with
PEG-400 as solvent, no reaction was observed (Table 1,
entry 11–15). To see the effect temperature, decreasing the
reaction temperature from 100°C (room temperature, 50°C,
70°C, 90°C) the yield of the desired product 2a was not
improved (Table 1, entry 16–19). On the basis of the results
obtained the optimized conditions were set as 0.25 mmol of
1a, 0.357 mmol of (diacetoxyiodo)benzene, and 1.0 mL
solvent PEG-400 at 100° C, 24 h for the transformations
presented here. With these optimized conditions (Table 1,
heated in 1 mL solvent at 100°C, 24 h. [b]Isolated yield.
Further, the presence of electron-rich or electron-deficient
groups (Me, -OMe, Cl, F and CN) at different positions on
the phenyl as well as pyridyl ring of 2-phenyl imidazo[1,2-
a]pyridines were reacted smoothly with PEG-400 and afford
the corresponding C-3 heterodiarylmethanes 2g–q in good
yields (55-86%). Also the presence of phenyl ring at 6-
position of pyridyl ring of 2-phenyl imidazo[1, 2-a]pyridine
gave the desired product bis(6-argio-2-phenylimidazo[1,2-
a]pyridin-3-yl)methane (2r) in 79% yield. The present
conditions also amenable to 2-(pyridin-2-yl)imidazo[1,2-
a]pyridine
and
8-methyl-2-(thiophen-2-yl)imidazo[1,2-
a]pyridine and provided the desired products 2s and 2t in
86% and 80% yields respectively.
The successful transformation of heterodiarylmethanes of
imidazo[1,2-a] pyridines under metal-free conditions
prompted us to investigate the generality of this method
toward the other fused heterocycles (Table 3). Initially, 6-
phenylimidazo[2,1-b]thiazoles having various substituents
(H, Et, F, Cl, and Br) at the para- position of the phenyl ring
were subjected to the optimized conditions, these
substrates were provided the corresponding bis(6-
phenylimidazo[2,1-b]thiazol-5-yl)methane products 3a–e in
excellent yields (80–88%).
entry
heterodiarylmethanes (Table 2). First, we examined the
heterodiarylmethanes of 2-arylimidazo[1,2-a]pyridines
bearing electron-donating and withdrawing groups (Me, Et,
6),
we
examined
the
versatility
of
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
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Table 2. Substrate Scope of various imidazo[1,2-a] pyridines[a]
the good yield of corresponding methylene bridged
products 8a–8d.
Table4. Synthesis of methylene bridged unsymmetrical heterocycles[a]
[a]Reaction conditions: 1a (0.25 mmol) and PIDA (0.375 mmol) were heated in
1 mL solvent (PEG-400) at 100°C, 24 h, isolated yield
Table 3. Substrate Scope of other fused heterocycles[a]
[a]Reaction conditions: 1a (0.25 mmol) and PIDA (0.375 mmol)
7 (1.0
mmol) were heated in 1 mL solvent (PEG-400), at 100°C, 24 h, isolated
yield.
[a]Reaction conditions: 1a (0.25 mmol) and PIDA (0.375 mmol) were
heated in 1 mL solvent (PEG-400), at 100°C, 24 h, isolated yield. CCDC
No. for 3e is 1976296.
Scheme 2. Gram scale synthesis
Further,
7-chloro-2-(4-chlorophenyl)
imidazo[1,2-a]
One of the products 3e was further confirmed by single
crystal XRD. Under the same conditions other derivative of
2-phenylbenzo[d]imidazo[2,1-b]thiazole having substituents
(Me & OMe) at different position of the phenyl ring were
subjected to the optimized conditions, and obtained the
pyridine,2-(4-chlorophenyl)imidazo[1,2-a] Pyridine and 6-(4-
chlorophenyl) imidazo[2,1-b]thiazole reacted smoothly with
the above indoles and arenes and afford the corresponding
products 8e–8k in moderate to good yields (52%-76%). The
products of tables 2-4, exemplify the ability to expand the
method towards selective synthesis of differently decorated
structural isomers. To validate the present protocol, one of
the products 2a was synthesized at gram scale 5.4 mmol,
(0.702 g) and obtained in 65% yield under the optimized
conditions (Scheme 2). This study indicates the feasibility
of the method for industrial/commercial preparation of such
products.
desired products 3f
& 3g in 65% and 74% yield.
It is remarkable to note that, the present conditions were
also agreeable to indolizines, and successfully provided the
corresponding methylene bridged products 4a–d in
moderate to good yields. To our delight, N-Ndimethylaniline
and trimethoxybenzenes were also gave the desired
products 4,4'-methylenebis(N,N-dimethylaniline)
5
and
bis(2,4,6-trimethoxyphenyl)methane in 90% and 92%
6
Further, the regioselectivity of the reaction was checked
(Scheme 3). The reaction was performed by blocking the C-
yield respectively under the same conditions.To further
broaden the scope of the newly established protocol, we
attempted the domino synthesis of unsymmetrically
methylene bridged products. Thus, the syntheses of
3
position with methyl group, 3-methylimidazo[1,2-a]
pyridine under the standard reaction conditions (Scheme
9
3, eq.1). With this substrate, the desired product 10
unsymmetrical products
fused
8
were carried out by reacting
formation was not observed. However, the starting
substrate
9 remains unreactive.
imidazo[1,2-a] pyridines
1 with different heteroarenes
Under the same conditions, the reaction of unsubstituted
imidazo[1,2-a] pyridine 11, the desired C-3 methylene
bridged product 12, was obtained in 55% yield as the major
product (Scheme 3, eq.2). These selectivity experiments
(indoles) and arenes (trimethoxy benzene and N,N-dimethyl
aniline), under the optimized conditions (Table 4). The
reaction of imidazo[1,2-a] pyridine with indole, 1-methyl-1H-
indole, trimethoxy benzene and N,N-dimethyl aniline gave
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
FULL PAPER
(Scheme 3) indicates that, the reaction of imidazopyridines,
the methylene bridging takes place selectively at C-3
position only.
Scheme 5. Probable Mechanism
Scheme 3. Selectivity experiments
Conclusions
To gain insights into the reaction mechanism, some control
experiments were performed with radical scavengers
(Scheme 4). The reaction of 1a was subjected to the
optimized conditions along with radical scavengers
(TEMPO, BHT, and benzoquinone) to know the reaction
pathway. Under these conditions, the desired product was
isolated in 40%, 36% and 46% yield respectively (Scheme
4). These reactions suggest that, the present transformation
may proceed through ionic path. Further 1a was reacted
with formaldehyde without PEG(400) and PIDA, the desired
product 2a was obtained in 30% yield. This reaction clearly,
suggest that, PEG(400) and PIDA may generate HCHO in
situ under the present conditions. Based on the control
experiments and the literature reports, [9a-f] a plausible
In conclusion, we have developed a novel and efficient
protocol for direct synthesis of heterodiarylmethanes
employing polyethylene glycol (PEG-400) as a methylene
spacer under metal-free conditions. This method is
applicable for both homo and hetero dimerization of wide
variety of heterocycles such as imidazopyridines,
imidazothioazoles, imidazobenzothiozoles, indole, 1-
methyl-1H-indole, N,N-dimethylaniline, trimethoxybenzene,
ethyl-2-phenylindolizine-1-carboxylates
and
good
to
excellent yields of corresponding products were obtained.
Control experiments suggest that, the reaction proceeds
through ionic mechanism. A gram-scale reaction also
demonstrated to validate the potential nature of the method
for the scale-up syntheses. Polyethylene glycol (PEG-400)
plays a dual role as a green solvent as well as methylene
source. It is inexpensive and commercially available. To the
best of our knowledge this is the first report on utilization of
PEG-400 as
conditions.
a
methylene source under metal-free
Experimental Section
Scheme 4. Control experiments
General:
All commercially available chemicals and reagents were used
without any further purification unless otherwise indicated. 1H
and 13C NMR spectra were recorded at 500, 200 and 50, 125
MHz, respectively. The spectra were recorded in CDCl3 as
solvent. Multiplicity was indicated as follows: s (singlet); d
(doublet); t (triplet); m (multiplet); dd (doublet of doublets), etc.
and coupling constants (J) were given in Hz. Chemical shifts are
reported in ppm relative to TMS as an internal standard. The
peaks around delta values of 1H NMR (7.2), and 13C NMR (77.0)
are correspond to deuterated solvent chloroform respectively.
Mass spectra were obtained using electron impact (EI) ionization
method. Progress of the reactions was monitored by thin layer
chromatography (TLC). All products were purified through
column chromatography using silica gel 100-200 mesh size
using hexane/ethyl acetate as eluent unless otherwise indicated.
reaction mechanism has been proposed for the present
transformation (Scheme 5). Initially, the reaction of PEG-
400
formaldehyde [13,14] in situ and its reaction with imidazo[1,2-
a] pyridine
generates intermediate A.[15] Subsequently,
protonation of and its dehydration yield the carbocation
intermediate B [16,9]
Nucleophilic attack of another molecule
of (from the C3 position) on the electrophilic intermediate
B [17]
provide the desired product through oxidation of
intermediate C.
with
(Diacetoxyiodo)benzene
generates
n-
1
A
.
1
,
2
General procedure for 2a
A clean washed boiling tube equipped with a magnetic stir bar
was charged with 2-phenylimidazo[1,2-a]pyridine 1a (0.0485 g,
0.25 mmol), (Diacetoxyiodo)benzene PIDA (0.1207 g, 0.375
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
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2H), 7.23 – 7.20 (m, 2H), 7.18 – 7.13 (m, 2H), 6.78 – 6.74 (m, 2H), 6.40 (t, J =
6.9 Hz, 2H), 4.66 (s, 2H), 2.43 (s, 6H). 13C NMR (50 MHz, CDCl3) δ 145.37,
142.57, 133.99, 132.85, 130.04, 128.68, 127.61, 123.35, 121.22, 112.65,
20.26, 17.00. HRMS(ESI-TOF)m/z: calcd for C29H22N4Cl2[M+Na]+ :519.1114 ;
found: 519.1149.
mmol), and PEG-400 (1mL), the above mixture was stirred for
24h at 100°C temperature. After completion of the reaction, the
mixture was poured into 10 mL of NaHCO3 solution. The product
was extracted with ethyl acetate (10 mL × 3) and dried with
anhydrous Na2SO4. Removal of the solvent under reduced
pressure, the left-out residue was purified through column
chromatography using silica gel (60% EtOAc/hexane) to obtain
bis(2-phenylimidazo[1,2-a] pyridin-3-yl) methane 2a in 90 %
yield (0.450g).
4,4'-(methylenebis(8-methylimidazo[1,2-a]pyridine-3,2-diyl))dibenzonitrile
(2f)
(Eluent: 60% EtOAc/hexane); 65% yield (38.8 mg); white solid; m.p; 290-
292oC 1H NMR (200 MHz, CDCl3) δ 7.62 – 7.50 (m, 8H), 7.34 (d, J = 6.8 Hz,
2H), 6.93 (d, J = 6.9 Hz, 2H), 6.59 (t, J = 6.9 Hz, 2H), 4.74 (s, 2H), 2.52 (s, 6H).
13C NMR (50 MHz, CDCl3) δ 145.57, 141.89, 138.82, 131.93, 128.98, 128.11,
123.80, 120.86, 118.70, 114.90, 113.26, 111.29, 20.73, 16.94. HRMS(ESI-
TOF)m/z: calcd for C31H22N6[M +H]+ :479.1979 ; found: 479.2046.
3,3'-(methylenebis(8-methylimidazo[1,2-a]pyridine-3,2-diyl))dibenzonitrile
(2g)
Characterization data
bis(2-phenylimidazo[1,2-a]pyridin-3-yl)methane (2a) [9a]
(Eluent: 60% EtOAc/hexane); 90% yield (45.0mg); Brown solid ; m.p; 208-
210oC 1H NMR (500 MHz, CDCl3) δ 7.79 (d, J = 7.2 Hz, 4H), 7.55 – 7.50 (m,
6H), 7.45 (t, J = 7.4 Hz, 2H), 7.34 (d, J = 6.9 Hz, 2H), 7.07 – 7.04 (m, 2H),
6.49 – 6.46 (m, 2H), 5.00 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 146.41,
145.54, 135.65, 130.36, 130.26, 129.69, 125.82, 125.19, 118.86, 115.70,
113.78, 21.14.
(Eluent: 60% EtOAc/hexane); 70% yield (41.8 mg); white solid; m.p; 280-
282oC 1H NMR (200 MHz, CDCl3) δ 7.61 – 7.50 (m, 8H), 7.34 (d, J = 6.8 Hz,
2H), 6.93 (dd, J = 5.9, 1.0 Hz, 2H), 6.59 (t, J = 6.9 Hz, 2H), 4.74 (s, 2H), 2.52
(s, 6H). 13C NMR (50 MHz, CDCl3) δ 145.57, 141.88, 138.82, 131.92, 128.98,
128.10, 123.80, 120.87, 118.70, 114.90, 113.26, 111.27, 20.73, 16.94.
HRMS(ESI-TOF)m/z: calcd for C31H22N6[M +K]+ :517.1538 ; found: 517.1558.
3,3'-(methylenebis(7-chloroimidazo[1,2-a]pyridine-3,2-diyl))dibenzonitrile
(2h)
bis(2-(p-tolyl)imidazo[1,2-a]pyridin-3-yl)methane (2b) [9a]
(Eluent: 60% EtOAc/hexane); 81% yield (43.3 mg); white solid; m.p; 252-
254oC 1H NMR (500 MHz, CDCl3) δ 7.70 (d, J = 8.0 Hz, 4H), 7.54 (d, J = 9.0
Hz, 2H), 7.35 – 7.33 (m, 6H), 7.06 (dd, J = 11.8, 3.9 Hz, 2H), 6.47 (t, J = 6.5
Hz, 2H), 4.99 (s, 2H), 2.44 (s, 6H). 13C NMR (125 MHz, CDCl3) δ 137.88,
137.10, 131.02, 124.24, 122.50, 121.75, 117.16, 116.78, 110.25, 107.04,
105.16, 14.28, 12.72.
bis(2-(4-ethylphenyl)-8-methylimidazo[1,2-a]pyridin-3-yl)methane (2c)
(Eluent: 60% EtOAc/hexane); 80% yield (48.4mg); off-white solid; m.p; 270-
272oC 1H NMR (200 MHz, CDCl3) δ 7.62 (d, J = 8.1 Hz, 4H), 7.26 (s, 1H),
7.22 – 7.16 (m, 5H), 6.77 (d, J = 6.9 Hz, 2H), 6.31 (s, 2H), 4.82 (s, 2H), 2.70 –
2.57 (m, 4H), 2.47 (s, 6H), 1.21 (t, J = 2.9 Hz, 6H). 13C NMR (50 MHz, CDCl3)
δ 145.23, 144.24, 143.64, 131.73, 129.11, 128.24, 127.15, 123.09, 28.73,
19.97, 17.06, 15.63. HRMS(ESI-TOF)m/z: calcd for C33H32N4[M +Na]+:
507.2519 ; found: 507.2537.
(Eluent: 60% EtOAc/hexane); 73% yield (47.35 mg); brown solid; m.p; 270-
272oC 1H NMR (500 MHz, CDCl3) δ 7.86 (s, 2H), 7.81 (d, J = 7.9 Hz, 2H),
7.67 (d, J = 7.8 Hz,2H), 7.56 (d, J = 1.6 Hz, 2H), 7.52 (d, J = 7.8 Hz, 2H), 7.39
(d, J = 7.3 Hz, 2H), 6.74 (dd, J = 7.3, 2.0 Hz, 2H), 4.87 (s, 2H). 13C NMR (125
MHz, CDCl3) δ 146.37, 144.48, 136.09, 133.96, 133.53, 133.28, 133.14,
130.95, 124.70, 119.63, 118.22, 116.49, 115.39, 114.37, 21.60. HRMS(ESI-
TOF)m/z: calcd for C29H16N6Cl2[M +Na]+ :541.0706 ; found: 541.0696.
bis(2-(4-chlorophenyl)-6-methylimidazo[1,2-a]pyridin-3-yl)methane (2i)
(Eluent: 60% EtOAc/hexane); 86% yield (53.3 mg); brown solid; m.p; 255-
257oC 1H NMR (200 MHz, CDCl3) δ 7.75 – 7.70 (m, 4H), 7.55 – 7.47 (m, 4H),
7.42 (d, J = 9.2 Hz, 2H), 7.07 (s, 2H), 6.95 (dd, J = 9.2, 1.5 Hz, 2H), 4.86 (s,
2H), 1.99 (s, 6H). 13C NMR (50 MHz, CDCl3) δ 143.05, 141.55, 133.26, 132.20,
129.09, 128.11, 126.84, 121.12, 120.63, 115.68, 113.13, 18.34, 17.05.
HRMS(ESI-TOF)m/z: calcd for C29H22N4Cl[M +H]+ :497.1294 ; found: 497.1309.
bis(2-(4-chlorophenyl)-7-methoxyimidazo[1,2-a]pyridin-3-yl)methane-(2j)
(Eluent: 20% EtOAc/hexane); 81% yield (53.4 mg); dark brown solid; m.p;
270-272oC 1H NMR (200 MHz, CDCl3) δ 7.47 – 7.43 (m, 5H), 7.26 – 7.22 (m,
3H), 7.00 (d, J = 7.5 Hz, 2H), 6.61 (d, J = 2.4 Hz, 2H), 6.09 (dd, J = 7.5, 2.5 Hz,
2H), 4.61 (s, 2H), 3.59 (s, 6H). 13C NMR (50 MHz, CDCl3) δ 158.49, 157.88,
146.57, 142.37, 134.01, 132.78, 129.76, 128.91, 123.90, 113.11, 107.68,
bis(2-(4-methoxyphenyl)-8-methylimidazo[1,2-a]pyridin-3-yl)methane
(2d)
(Eluent: 60% EtOAc/hexane); 78% yield (47.5 mg); yellow solid: m.p; 170-
172oC 1H NMR (500 MHz, CDCl3) δ 7.71 (d, J = 8.6 Hz, 4H), 7.26 (d, J = 6.8
Hz, 2H), 7.04 (d, J = 8.6 Hz, 4H), 6.84 (d, J = 6.8 Hz, 2H), 6.41 (t, J = 6.9 Hz,
2H), 4.87 (s, 2H), 3.88 (s, 6H), 2.56 (s, 6H). 13C NMR (125 MHz, CDCl3) δ
160.94, 146.63, 144.82, 131.72, 128.51, 128.47, 124.38, 123.08, 115.82,
115.62, 113.54, 56.78, 21.36, 18.45. HRMS(ESI-TOF)m/z: calcd for
C31H28N4O2[M+Na]+ : 511.2104 ; found: 511.2106.
bis(2-(4-chlorophenyl)-8-methylimidazo[1,2-a]pyridin-3-yl)methane (2e)
(Eluent: 60% EtOAc/hexane); 86% yield (53.32 mg); white solid; m.p; 260-
262oC 1H NMR (200 MHz, CDCl3) δ 7.49 – 7.42 (m, 4H), 7.27 (d, J = 2.3 Hz,
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
FULL PAPER
94.69, 55.47, 19.94. HRMS(ESI-TOF)m/z: calcd for C29H22N4Cl2O2[M
(Eluent: 60% EtOAc/hexane); 55% yield (29.4 mg); brown solid; m.p; 120-
122oC 1H NMR (200 MHz, CDCl3) δ 7.67 (d, J = 8.6 Hz, 4H), 7.52 (dd, J = 8.8,
2.3 Hz, 6H), 7.34 (d, J = 1.2 Hz, 2H), 7.26 (s, 2H), 7.08 (dd, J = 9.5, 1.9 Hz,
2H), 4.83 (s, 2H), 2.17 (s, 6H). 13C NMR (50 MHz, CDCl3) δ 144.68, 143.45,
139.94, 134.95, 130.10, 129.66, 129.32, 127.59, 126.39, 126.28, 121.66,
117.95, 103.20, 102.47, 100.28, 92.18, 91.89, 19.62, 16.78. HRMS(ESI-
TOF)m/z: calcd for C29H24N4[M+H]+ :429.2074 ; found: 429.2116.
+Na]+:551.1012 ; found: 551.1008.
. bis(7-chloro-2-(4-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)methane (2k)
(Eluent: 60% EtOAc/hexane); 68% yield (45.5 mg); off white solid; m.p; 265-
267oC 1H NMR (600 MHz, CDCl3 ) δ 7.65 (d, J = 8.1 Hz, 4H), 7.53 (s, 2H),
7.48 (d, J = 8.1 Hz, 4H), 7.23 (d, J = 7.5 Hz, 2H), 6.55 (dd, J = 7.0, 1.7 Hz, 2H),
4.88 (s, 2H). 13C NMR (50 MHz, CDCl3) δ147.75, 146.82, 137.68, 134.85,
134.45, 132.78, 132.07, 126.51, 119.36, 117.33, 116.96, 22.61. HRMS(ESI-
TOF)m/z: calcd for C27H16N4Cl4[M +Na]+ :559.0021 ; found: 560.9981.
bis(7-chloro-2-phenylimidazo[1,2-a]pyridin-3-yl)methane (2l)
bis(8-methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)methane (2q)
(Eluent: 20% EtOAc/hexane); 76% yield (40.6mg); brown gum 1H NMR (200
MHz, CDCl3) δ 7.78 – 7.74 (m, 4H), 7.50 – 7.40 (m, 6H), 7.26 (s, 2H), 6.87 –
6.81 (m, 2H), 6.42 (t, J = 6.9 Hz, 2H), 4.92 (s, 2H), 2.56 (s, 6H). 13C NMR (50
MHz, CDCl3) δ 145.39, 143.68, 134.65, 129.13, 128.74, 128.02, 127.35,
123.04, 121.66, 114.82, 112.24, 19.95, 17.05. HRMS(ESI-TOF)m/z: calcd for
C29H24N4[M+H]+ :429.2074 ; found: 429.2116.
(Eluent: 60% EtOAc/hexane); 60% yield (35.1 mg); brown solid; m.p; 240-
242oC 1H NMR (600 MHz, CDCl3 ) δ 7.77 (d, J = 7.5 Hz, 4H), 7.55 – 7.52(m,
6H), 7.49 (t, J = 7.4 Hz, 2H), 7.19 (d, J = 7.5 Hz, 2H), 6.48 – 6.46 (m, 2H),
4.96 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 143.94, 143.80, 132.68, 130.23,
128.04, 127.84, 127.68, 122.96, 115.32, 113.39, 113.04, 18.40. HRMS(ESI-
TOF)m/z: calcd for C27H18N4Cl2 [M +Na]+ :491.0801 ; found: 491.0809.
bis(6-chloro-2-phenylimidazo[1,2-a]pyridin-3-yl)methane (2m)
bis(2,6-diphenylimidazo[1,2-a]pyridin-3-yl)methane (2r)
(Eluent: 20% EtOAc/hexane); 79% yield (54.5mg); brown solid; m.p; 250-
252oC 1H NMR (200 MHz, CDCl3) δ 7.84 (dd, J = 7.8, 1.6 Hz, 4H), 7.59 (s,
2H), 7.55 – 7.38 (m, 9H), 7.31 (dd, J = 4.2, 2.5 Hz, 7H), 7.07 (dd, J = 6.8, 2.9
Hz, 4H), 5.18 (s, 2H). 13C NMR (50 MHz, CDCl3) δ 144.60, 144.49, 136.75,
134.11, 129.13, 129.03, 128.93, 128.34, 127.71, 126.85, 126.74, 125.20,
(Eluent: 60% EtOAc/hexane); 65% yield (38.0mg); brown solid; m.p; 265-
267oC 1H NMR (200 MHz, CDCl3) δ 7.79 (dd, J = 8.0, 1.4 Hz, 4H), 7.62 – 7.44
(m, 8H), 7.32 – 7.31 (m, 2H), 7.05 (dd, J = 9.5, 1.9 Hz, 2H), 4.93 (s, 2H). 13C
NMR (50 MHz, CDCl3) δ 145.38, 143.44, 133.73, 129.21, 129.04, 128.75,
125.89, 120.59, 117.81, 114.71, 19.13. HRMS(ESI-TOF)m/z: calcd for
C29H18N4Cl2 [M+H]+ : 469.0981 ; found: 469.0996.
121.18,
117.28,
114.56,
19.84.
HRMS(ESI-TOF)m/z:
calcd
for
C39H28N4[M+Na]+ : 575.2206; found: 575.2193.
bis(2-(pyridin-2-yl)imidazo[1,2-a]pyridin-3-yl)methane(2s)
(Eluent: 60% EtOAc/hexane); 86% yield (43.2mg); brown solid; m.p; 220-
222oC 1H NMR (200 MHz, CDCl3) δ 8.69 – 8.65 (m, 3H), 8.42 (dt, J = 8.0, 1.0
Hz, 2H), 7.88 (td, J = 7.8, 1.9 Hz, 2H), 7.51 (dt, J = 9.1, 1.1 Hz, 2H), 7.28 (dt, J
= 4.8, 3.3 Hz, 2H), 7.13 – 7.02 (m, 2H), 6.47 (td, J = 6.8, 1.2 Hz, 2H), 6.25 (s,
1H), 3.73 (s, 2H). 13C NMR (50 MHz, CDCl3) δ 145.23, 138.46, 136.88, 129.02,
127.96, 126.53, 125.88, 124.90, 123.85, 117.38, 113.53, 112.84, 20.28.
HRMS(ESI-TOF)m/z: calcd for C25H18N6[M +H]+ :403.1666 ; found: 403.1683.
bis(8-methyl-2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-yl)methane (2t)
(Eluent: 70% EtOAc/hexane); 80% yield (44.0mg); brown solid; m.p; 194-
196oC 1H NMR (500 MHz, CDCl3) δ 7.57 (d, J = 3.6 Hz, 2H), 7.46 (d, J = 5.1
Hz, 2H), 7.42 (d, J = 6.9 Hz, 2H), 7.22 – 7.18 (m, 2H), 6.88 – 6.83 (m, 2H),
6.43 (t, J = 6.9 Hz, 2H), 5.09 (s, 2H), 2.57 (s, 6H). 13C NMR (125 MHz, CDCl3)
δ 147.01, 139.20, 138.39, 129.24, 128.71, 127.72, 127.55, 124.97, 123.11,
115.73, 114.14, 21.77, 18.36. HRMS(ESI-TOF)m/z: calcd for C25H20N4S2[M
+H]+ :463.1022 ; found: 463.1021.
bis(2-(2-chlorophenyl)-8-methylimidazo[1,2-a]pyridin-3-yl)methane (2n)
(Eluent: 60% EtOAc/hexane); 69% yield (42.7 mg); brown solid; m.p; 266-
268oC 1H NMR (200 MHz, CDCl3) δ 7.67 (td, J = 7.4, 1.9 Hz, 2H), 7.37 (ddd,
J = 7.2, 4.2, 2.5 Hz, 4H), 7.30 (d, J = 1.4 Hz, 1H), 7.26 (d, J = 1.9 Hz, 1H),
7.23 – 7.13 (m, 2H), 6.92 – 6.83 (m, 2H), 6.48 (t, J = 6.9 Hz, 2H), 4.64 (s, 2H),
2.56 (s, 6H). 13C NMR (50 MHz, CDCl3) δ 145.75, 138.02, 132.21, 130.19,
130.03, 127.47, 124.60, 123.03, 121.43, 116.38, 116.15, 112.38, 21.37,17.07.
HRMS(ESI-TOF)m/z: calcd for C29H22N4Cl2[M+Na]+ :519.1114
519.1127.
; found:
bis(8-chloro-2-(2-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)methane (2o)
(Eluent: 60% EtOAc/hexane); 84% yield (52.9 mg); brown solid; m.p; 256-
258oC 1H NMR (500 MHz, CDCl3) δ 7.60 (td, J = 7.5, 1.5 Hz, 2H), 7.48 (d, J =
1.8 Hz, 2H), 7.38 (td, J = 7.4, 1.6 Hz, 2H), 7.32 (d, J = 7.3 Hz, 2H), 7.24 (t, J =
7.5 Hz, 2H), 7.15 – 7.11 (m, 2H), 6.52 (dd, J = 7.3, 2.0 Hz, 2H), 4.61 (s, 2H).
13C NMR (125 MHz, CDCl3) δ 161.94, 159.97, 146.55, 140.99, 133.31, 132.60,
132.16, 132.10, 126.24, 125.10, 122.99, 122.87, 117.91, 117.60, 117.42,
bis(6-phenylimidazo[2,1-b]thiazol-5-yl)methane (3a) [9e]
(Eluent: 60% EtOAc/hexane); 88% yield (45.3 mg); white solid; m.p; 255-
257oC 1H NMR (200 MHz, CDCl3) δ 7.78 (dd, J = 8.2, 1.3 Hz, 4H), 7.55 – 7.46
(m, 4H), 7.42 (dd, J = 5.0, 3.6 Hz, 2H), 6.59 – 6.54 (m, 4H), 4.84 (s, 2H). 13C
115.64,
21.30.
HRMS(ESI-TOF)m/z:
calcd
for
C27H16N4Cl2F2[M
+H]+ :505.0793 ; found: 505.0859.
bis(2-(o-tolyl)imidazo[1,2-a]pyridin-3-yl)methane (2p)
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
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NMR (125 MHz, CDCl3) δ 149.18, 144.58, 137.54, 131.39, 129.60, 127.84,
(Eluent: 40% EtOAc/hexane); 84% yield (56.9 mg); black gummy 1H NMR
(500 MHz, CDCl3) δ 8.18 (d, J = 9.0 Hz, 2H), 7.51 – 7.49 (m, 10H), 7.39 (dd, J
= 9.6, 4.3 Hz, 2H), 6.98 (dd, J = 8.5, 7.1 Hz, 2H), 6.51 (dd, J = 9.9, 3.7 Hz, 2H),
4.45 (s, 2H), 4.18 (q, J = 7.1 Hz, 4H), 1.14 (t, J = 7.1 Hz, 6H). 13C NMR (50
MHz, CDCl3) δ 164.89, 137.12, 136.09, 131.79, 131.13, 130.68, 128.67,
123.99, 122.00, 119.85, 117.45, 112.22, 101.87, 59.20, 21.34, 14.27.
117.27, 116.88, 112.56, 21.26.
bis(6-(4-ethylphenyl)imidazo[2,1-b]thiazol-5-yl)methane (3b)
(Eluent: 60% EtOAc/hexane); 85% yield (49.7 mg); white solid; m.p; 265-
267oC 1H NMR (600 MHz, CDCl3) δ 7.69 (d, J = 7.5 Hz, 4H), 7.35 (d, J = 7.5
Hz, 4H), 6.56 (d, J = 3.9 Hz, 4H), 4.81 (s, 2H), 2.28 (dd, J = 14.9, 7.4 Hz, 4H),
1.33 – 1.26 (m, 6H). 13C NMR (50 MHz, CDCl3) δ 149.22, 144.69, 143.92,
131.64, 128.41, 127.96, 117.30, 116.93, 112.57, 28.66, 21.36, 15.53.
HRMS(ESI-TOF)m/z: calcd for C35H30N2O4[M +K]+ :581.1837
581.1890.
; found:
HRMS(ESI-TOF)m/z: calcd for C27H24N4S2[M +K]+ :507.1074
507.1119.
;
found:
diethyl 3,3'-methylenebis(2-(p-tolyl)indolizine-1-carboxylate) (4b)
(Eluent: 40% EtOAc/hexane); 88% yield (62.7 mg); black gummy 1H NMR
(500 MHz, CDCl3) δ 8.15 (d, J = 9.0 Hz, 2H), 7.50 (d, J = 7.0 Hz, 2H), 7.34 (d,
J = 7.8 Hz, 4H), 7.29 (d, J = 7.8 Hz, 4H), 6.97 – 6.92 (m, 2H), 6.50 (t, J = 6.8
Hz, 2H), 4.45 (s, 2H), 4.21 (q, J = 7.1 Hz, 4H), 2.43 (s, 6H), 1.19 (t, J = 7.1 Hz,
6H).13C NMR (50 MHz, CDCl3) δ 164.89, 137.12, 136.09, 131.79, 131.13,
130.68, 128.67, 123.99, 122.00, 119.85, 117.45, 112.22, 101.87, 59.20, 21.34,
14.27. HRMS(ESI-TOF)m/z: calcd for C37H34N2O4[M +H]+ :571.2591 ; found:
571.2595.
bis(6-(4-fluorophenyl)imidazo[2,1-b]thiazol-5-yl)methane (3c) [9e]
(Eluent: 60% EtOAc/hexane); 80% yield (44.8mg); white solid; m.p; 245-247oC
1H NMR (600 MHz, CDCl3) δ 7.71 (dd, J = 8.8, 5.6 Hz, 4H), 7.20 (t, J = 8.6 Hz,
4H), 6.63 (d, J = 4.6 Hz, 2H), 6.58 (d, J = 4.6 Hz, 2H), 4.75 (s, 2H). 13C NMR
(50 MHz, CDCl3) δ 163.38, 158.03, 149.39, 143.83, 129.74, 129.58, 116.99,
116.13, 115.70, 113.12, 21.31.
bis(6-(4-chlorophenyl)imidazo[2,1-b]thiazol-5-yl)methane (3d) [9e]
(Eluent: 60% EtOAc/hexane);82% yield (49.3mg); brown solid; m.p; 272-274oC
1H NMR (500 MHz, CDCl3) δ 7.66 (d, J = 8.4 Hz, 4H), 7.45 (d, J = 8.5 Hz, 4H),
6.63 (d, J = 4.5 Hz, 2H), 6.56 (d, J = 4.5 Hz, 2H), 4.75 (s, 2H). 13C NMR (125
MHz, CDCl3) δ 149.53, 143.58, 133.75, 132.63, 129.09, 116.92, 113.34, 21.43.
bis(6-(4-bromophenyl)imidazo[2,1-b]thiazol-5-yl)methane(3e) [9e]
(Eluent: 60% EtOAc/hexane); 88% yield (62.3mg); white solid; m.p; 252-
254oC 1H NMR (500 MHz, CDCl3) δ 7.52 (s, 8H), 6.56 (d, J = 4.4 Hz, 2H),
6.50 (d, J = 4.5 Hz, 2H), 4.68 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 149.56,
143.59, 133.08, 132.03, 129.36, 121.90, 116.91, 113.38, 21.46.
(diethyl 3,3'-methylenebis(2-(4-ethylphenyl)indolizine-1-carboxylate) (4c)
(Eluent: 40% EtOAc/hexane); 90% yield (67.2mg); white solid; m.p; 263-265oC
1H NMR (500 MHz, CDCl3) δ 8.16 (d, J = 9.1 Hz, 2H), 7.51 (d, J = 7.0 Hz, 2H),
7.36 (d, J = 8.0 Hz, 4H), 7.29 (d, J = 8.0 Hz, 4H), 6.95 (ddd, J = 9.0, 6.8, 0.6
Hz, 2H), 6.51 (td, J = 6.9, 1.1 Hz, 2H), 4.45 (s, 2H), 4.19 (q, J = 7.1 Hz, 4H),
2.74 (q, J = 7.6 Hz, 4H), 1.31 (t, J = 7.6 Hz, 6H), 1.15 (t, J = 7.1 Hz, 6H). 13C
NMR (50 MHz, CDCl3) δ 164.92, 143.38, 136.10, 131.99, 131.14, 130.70,
127.37, 123.95, 121.98, 119.85, 117.46, 112.23, 101.90, 59.18, 28.70, 20.40,
15.54, 14.18. HRMS(ESI-TOF)m/z: calcd for C39H38N2O4[M +H]+ :599.2904 ;
found: 599.2906.
bis(2-(p-tolyl)benzo[d]imidazo[2,1-b]thiazol-3-yl)methane(3f) [9e]
(Eluent: 60% EtOAc/hexane); 74% yield (49.9 mg); white solid; m.p; 263-
265oC 1H NMR (600 MHz, CDCl3) δ 7.67 (dd, J = 6.0, 3.1 Hz, 2H), 7.43 – 7.42
(m, 2H), 7.31 (dd, J = 6.2, 3.0 Hz, 4H), 7.12 (d, J = 7.8 Hz, 4H), 6.94 (d, J =
7.8 Hz, 4H), 5.14 (s, 2H), 2.28 (s, 6H). 13C NMR (125 MHz, CDCl3) δ 153.01,
149.60, 148.50, 131.14, 129.36, 129.27, 127.83, 125.55, 121.69, 115.66,
114.36, 31.12, 23.06.
diethyl 3,3'-methylenebis(2-(4-chlorophenyl)indolizine-1-carboxylate) (4d)
(Eluent: 40% EtOAc/hexane); 78% yield (59.4 mg); black solid; m.p; 273-
275oC 1H NMR (500 MHz, CDCl3) δ 8.19 (d, J = 9.1 Hz, 2H), 7.47 (d, J = 7.0
Hz, 2H), 7.36 (d, J = 8.3 Hz, 4H), 7.27 – 7.25 (m, 4H), 7.01 (dd, J = 8.6, 7.0 Hz,
2H), 6.57 (dd, J = 9.8, 3.9 Hz, 2H), 4.37 (s, 2H), 4.20 – 4.15 (m, 4H), 1.18 (t, J
= 7.1 Hz, 6H). 13C NMR (50 MHz, CDCl3) δ 164.63, 136.12, 133.44, 133.07,
131.70, 129.80, 127.89, 123.24, 122.32, 120.13, 117.07, 112.76, 102.09,
59.35, 20.56, 14.21. HRMS(ESI-TOF)m/z: calcd for C35H28N2O4Cl2[M
+K]+ :649.1058 ; found: 649.1093.
bis(2-(3-methoxyphenyl)benzo[d]imidazo[2,1-b]thiazol-3-yl)methane(3g)
[9f]
(Eluent: 70% EtOAc/hexane); 65% yield (46.4 mg); white solid; m.p; 244-
246oC 1H NMR (500 MHz, CDCl3) δ 7.69 (dd, J = 6.0, 3.3 Hz, 2H), 7.46 (dd, J
= 6.0, 3.2 Hz, 2H), 7.32 (dd, J = 6.0, 3.2 Hz, 4H), 7.06 (t, J = 7.9 Hz, 2H), 6.82
(dd, J = 8.0, 5.0 Hz, 4H), 6.75 (dd, J = 8.2, 2.4 Hz, 2H), 5.18 (s, 2H), 3.60 (s,
6H). 13C NMR (125 MHz, CDCl3) δ 160.48, 148.68, 147.01, 136.18, 134.59,
131.99, 130.05, 127.38, 125.92, 125.72, 121.95, 120.37, 115.54, 114.36,
56.45, 25.38.
4,4'-methylenebis(N,N-dimethylaniline) (5)
(Eluent: 50% EtOAc/hexane); 90% yield (28.5 mg); brown thick liquid 1H
NMR (200 MHz, CDCl3) δ 7.02 (d, J = 8.8 Hz, 4H), 6.65 (d, J = 8.7 Hz, 4H),
3.75 (s, 2H), 2.84 (s, 12H) 13C NMR (50 MHz, CDCl3) δ 129.41, 113.11,
103.11, 98.09, 40.97, 30.91.
HRMS(ESI-TOF)m/z: calcd for
C17H22N2[M+Na]+ :277.1675 ; found: 277.1696.
diethyl 3,3'-methylenebis(2-phenylindolizine-1-carboxylate) (4a)
bis(2,4,6-trimethoxyphenyl)methane (6)
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10.1002/ejoc.202000467
European Journal of Organic Chemistry
FULL PAPER
(Eluent: 50% EtOAc/hexane); 92% yield (40.0 mg); yellow thick liquid 1H NMR
= 8.5 Hz, 2H), 6.70 (d, J = 8.6 Hz, 2H), 4.34 (s, 2H), 2.93 (s, 6H). 13C NMR (50
MHz, CDCl3) δ 149.71, 134.21, 129.41, 129.00, 128.27, 126.37, 123.00,
121.49, 121.03, 117.55, 113.36, 40.67, 28.74. HRMS(ESI-TOF)m/z: calcd for
C22H19N3Cl2[M +Na]+ :418.0848 ; found: 418.0850.
(200 MHz, CDCl3) δ 6.03 (s, 4H), 3.78 (s, 2H), 3.71 (s, 6H), 3.64 (s, 12H). 13C
NMR (150 MHz, CDCl3) δ 159.31, 158.72, 112.05, 91.21, 56.13, 55.26, 16.73.
HRMS(ESI-TOF)m/z: calcd for C19H25O6[M+H]+ :349.1675 ; found:349.1693.
3-((1H-indol-3-yl)methyl)-2-phenylimidazo[1,2-a]pyridine (8a)
2-(4-chlorophenyl)-3-(2,4,6-trimethoxybenzyl)imidazo[1,2-a]pyridine(8f)
(Eluent: 50% EtOAc/hexane); 76% yield (77.5 mg); yellow thick liquid 1H
NMR (500 MHz, CDCl3) δ 8.23 (d, J = 6.9 Hz, 1H), 7.80 (d, J = 8.4 Hz, 2H),
7.66 (d, J = 9.0 Hz, 1H), 7.43 (d, J = 8.4 Hz, 2H), 7.12 – 7.08 (m, 1H), 6.72 (t,
J = 6.6 Hz, 1H), 6.03 (s, 2H), 4.33 (s, 2H), 3.73 (s, 3H), 3.57 (s, 6H). 13C NMR
(50 MHz, CDCl3) δ 160.26, 158.88, 143.85, 141.23, 134.12, 133.00, 130.17,
128.28, 124.58, 123.86, 120.26, 116.87, 111.89, 105.19, 90.58, 55.38, 55.30,
(Eluent: 50% EtOAc/hexane); 60% yield (48.4 mg); pale yellow liquid 1H NMR
(500 MHz, CDCl3) δ 8.52 (s, 1H), 8.09 (s, 2H), 7.33 (d, J = 7.9 Hz, 3H), 7.27 (s,
1H), 7.17 (t, J = 7.7 Hz, 1H), 7.09 (t, J = 7.5 Hz, 2H), 6.93 – 6.88 (m, 3H), 6.85
(s, 2H), 3.55 (d, J = 4.9 Hz, 2H). 13C NMR (125 MHz, CDCl3) δ 143.36, 143.13,
139.83, 136.96, 134.43, 132.06, 128.08, 125.88, 124.29, 122.70, 122.09,
121.31, 119.53, 115.23, 114.56, 111.22, 29.12. HRMS(ESI-TOF)m/z: calcd for
C22H17N3[M +H]+ :324.1495 ; found: 324.1494.
17.77. HRMS(ESI-TOF)m/z: calcd for C23H21N2O3Cl[M +Na]+ :431.1133
found: 431.1145.
;
3-((1-methyl-1H-indol-3-yl)methyl)-2-phenylimidazo[1,2-a]pyridine(8b)
(Eluent: 40% EtOAc/hexane); 65% yield (54.7 mg); Brown thik liquid 1H NMR
(500 MHz, CDCl3) δ 7.87 – 7.84 (m, 2H), 7.81 (d, J = 6.9 Hz, 1H), 7.73 (d, J =
9.1 Hz, 1H), 7.66 (d, J = 7.9 Hz, 1H), 7.44 (t, J = 7.6 Hz, 2H), 7.36 – 7.30 (m,
3H), 7.19 (t, J = 7.6 Hz, 2H), 6.71 (d, J = 6.8 Hz, 1H), 6.52 (s, 1H), 4.55 (s, 2H),
3.67 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 146.23, 144.50, 138.95, 135.95,
130.12, 129.65, 129.10, 128.10, 123.53, 120.64, 120.21, 113.49, 110.94,
34.18, 22.08. HRMS(ESI-TOF)m/z: calcd for C23H19N3[M +Na]+ :360.1471 ;
found: 360.1490.
4-((2-(4-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)methyl)-N,N-
dimethylaniline(8g)
(Eluent: 50% EtOAc/hexane); 75% yield (67.6mg); yellow thick liquid 1H NMR
(200 MHz, CDCl3) δ 7.61 – 7.50 (m, 4H), 7.27 – 7.22 (m, 2H), 7.07 (ddd, J =
9.0, 6.7, 1.2 Hz, 1H), 6.83 (d, J = 8.7 Hz, 2H), 6.56 (dd, J = 7.1, 4.9 Hz, 3H),
4.21 (s, 2H), 2.76 (s, 6H). 13C NMR (50 MHz, CDCl3) δ 149.69, 144.82, 142.54,
133.59, 133.10, 129.46, 128.80, 128.31, 124.38, 123.80, 123.64, 118.72,
117.42, 113.16, 112.29, 40.62, 28.82. HRMS(ESI-TOF)m/z: calcd for
C22H20N3Cl[M +K]+ : 400.0977 ; found: 400.0974.
2-phenyl-3-(2,4,6-trimethoxybenzyl)imidazo[1,2-a]pyridine(8c)
(Eluent: 50% EtOAc/hexane); 59% yield (55.1 mg); Brown thik liquid 1H NMR
(500 MHz, CDCl3) δ 8.26 (d, J = 6.9 Hz, 1H), 7.85 (d, J = 7.2 Hz, 2H), 7.71 (d,
J = 9.0 Hz, 1H), 7.48 (t, J = 7.6 Hz, 2H), 7.36 (t, J = 7.4 Hz, 1H), 7.14 – 7.10
(m, 1H), 6.73 (t, J = 6.8 Hz, 1H), 6.04 (s, 2H), 4.38 (s, 2H), 3.74 (s, 3H), 3.56
(s, 6H). 13C NMR (50 MHz, CDCl3) δ 160.13, 158.95, 156.91, 143.91, 128.88,
128.08, 127.01, 124.57, 120.02, 117.01, 111.51, 90.56, 55.89, 55.34, 17.83.
5-((1H-indol-3-yl)methyl)-6-(4-chlorophenyl)imidazo[2,1-b]thiazole(8h)
(Eluent: 50% EtOAc/hexane); 61% yield (55.3 mg); Brown thik liquid 1H NMR
(200 MHz, CDCl3) δ 8.04 (s, 1H), 7.63 (d, J = 7.9 Hz, 2H), 7.39 (d, J = 8.0 Hz,
2H), 7.19 (dd, J = 7.4, 6.5 Hz, 2H), 7.12 – 7.03 (m, 4H), 4.80 (t, J = 6.2 Hz,
1H), 4.32 (d, J = 6.2 Hz, 2H). 13C NMR (50 MHz, CDCl3) δ 146.05, 144.60,
142.84, 142.36, 138.53, 136.60, 135.59, 134.86, 133.79, 132.43, 129.61,
128.76, 122.56, 119.46, 117.99, 111.21, 37.16.δ 144.60, 142.36, 138.53,
136.60, 122.56, 119.46, 117.99, 111.21, 37.16. HRMS(ESI-TOF)m/z: calcd for
C20H14N3ClS[M +H]+ :364.0670 ; found: 364.0663.
HRMS(ESI-TOF)m/z: calcd for C23H22N2O3[M +Na]+ :397.1523
397.1538.
; found:
N,N-dimethyl-4-((2-phenylimidazo[1,2-a]pyridin-3-yl)methyl)aniline(8d)
(Eluent: 50% EtOAc/hexane); 68% yield (55.5 mg); Yellow gummy 1H NMR
(500 MHz, CDCl3) δ 7.73 – 7.70 (m, 2H), 7.63 (dd, J = 17.0, 8.0 Hz, 2H), 7.35
(t, J = 7.6 Hz, 2H), 7.25 (t, J = 7.4 Hz, 1H), 7.08 (ddd, J = 8.9, 6.8, 1.0 Hz, 1H),
6.93 (d, J = 8.6 Hz, 2H), 6.62 – 6.56 (m, 3H), 4.30 (s, 2H), 2.82 (s, 6H). 13C
NMR (50 MHz, CDCl3) δ 149.65, 144.73, 143.60, 134.44, 128.62, 128.38,
128.29, 127.69, 124.23, 124.14, 123.67, 117.34, 113.18, 112.18, 40.66, 28.86.
HRMS(ESI-TOF)m/z: calcd for C22H21N3[M +Na]+ :350.1628 ; found: 350.1653.
4-((7-chloro-2-(4-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)methyl)-N,N-
dimethylaniline(8e)
6-(4-chlorophenyl)-5-((1-methyl-1H-indol-3-yl)methyl)imidazo[2,1-
b]thiazole(8i)
(Eluent: 50% EtOAc/hexane); 52% yield (49.0mg); Brown thik liquid 1H NMR
(200 MHz, CDCl3) δ 7.63 (s, 1H), 7.32 (d, J = 8.6 Hz, 3H), 7.17 (d, J = 7.8 Hz,
2H), 7.01 – 6.96 (m, 2H), 6.64 (d, J = 0.8 Hz, 1H), 6.50 (d, J = 4.5 Hz, 1H),
6.36 (s, 1H), 3.71 (s, 3H), 2.10 (s, 2H).13C NMR (125 MHz, CDCl3) δ 151.94,
150.71, 139.86, 136.95, 134.43, 128.06, 125.88, 125.63, 124.33, 122.69,
122.07, 121.30, 119.52, 115.40, 115.17, 111.24, 109.77, 23.08. HRMS(ESI-
TOF)m/z: calcd for C21H17N3SCl[M +H]+ :378.0826 ; found: 378.0888.
6-(4-chlorophenyl)-5-(2,4,6-trimethoxybenzyl)imidazo[2,1-b]thiazole(8j)
(Eluent: 60% EtOAc/hexane); 68% yield (70.3 mg); yellow thick liquid 1H
NMR (500 MHz, CDCl3) δ 7.77 (d, J = 8.2 Hz, 2H), 7.40 (dd, J = 8.5, 6.8 Hz,
(Eluent: 50% EtOAc/hexane); 70% yield (69.1 mg); yellow thick liquid 1H
NMR (500 MHz, CDCl3) δ 7.78 (s, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.62 (d, J =
9.5 Hz, 1H), 7.40 (d, J = 8.4 Hz, 2H), 7.16 (dd, J = 9.5, 1.6 Hz, 1H), 6.98 (d, J
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3H), 6.64 (d, J = 4.0 Hz, 1H), 6.07 (s, 2H), 4.24 (s, 2H), 3.76 (s, 3H), 3.62 (s,
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6H). 13C NMR (50 MHz, CDCl3) δ 160.30, 158.76, 142.11, 134.57, 132.22,
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437.0722.
4-((6-(4-chlorophenyl)imidazo[2,1-b]thiazol-5-yl)methyl)-N,N
dimethylaniline(8k)
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bis(imidazo[1,2-a]pyridin-3-yl)methane (12) [9f]
(Eluent: 60% EtOAc/hexane); 55% yield (17.0 mg); yellow thick liquid 1H NMR
(500 MHz, CDCl3) δ 8.07 (d, J = 6.7 Hz, 2H), 7.57 (d, J = 7.8 Hz, 3H), 7.52 (s,
1H), 7.12 – 7.07 (m, 2H), 6.74 (t, J = 6.7 Hz, 2H), 3.68 (s, 2H). 13C NMR (125
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CSIR-CSMCRI communication No. 003/2020. R. K is
thankful to AcSIR for their Ph.D. enrolment and the
“Analytical Discipline and Centralized Instrumental
Facilities” for providing instrumentation facilities. R.K. also
thankful to UGC-New Delhi for SRF. We thank DST,
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Keywords: Hetero diarylmethanes • Polyethylene glycol •
Imidazo[1,2-a]pyridine • Metal-free • regioselectivity.
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Metal Free Coupling
Rahul Kumar, Deepa Rawat and
Subbarayappa Adimurthy*
Page No. – Page No.
Polyethylene Glycol (PEG-400): As
Methylene Spacer and Green
Solvent for the Synthesis of Hetero
Diarylmethanes Under Metal-Free
Conditions
We have developed a novel and efficient protocol for the direct synthesis of heterodiarylmethanes employing polyethylene glycol
(PEG-400) as a methylene spacer under metal-free conditions. The method is applicable for variety of heterocycles such as imidazo
pyridines, imidazothioazoles, imidazobenzothiozoles, indolizines, indole, 1-methyl-1H-indole, N,N-dimethylaniline and
trimethoxybenzene. The reaction was facilitated by (diacetoxyiodo)benzene (PIDA) as an oxidant.
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