A new approach for the synthesis of fluorescent pyrido[1,2-a]benzimidazoles

Article abstract of DOI:10.1080/00397911.2020.1800742

A facile synthesis of new pyrido[1,2-a]benzimidazoles through a one-pot multicomponent reaction of malononitrile, electron-deficient aryl aldehydes and heterocyclic enamines followed by a nitrous acid release under mild condition, is reported. Depending o

Full text of DOI:10.1080/00397911.2020.1800742

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A new approach for the synthesis of fluorescent
pyrido[1,2-a]benzimidazoles
Akin Sagirli
To cite this article: Akin Sagirli (2020): A new approach for the synthesis of fluorescent pyrido[1,2-
a]benzimidazoles, Synthetic Communications, DOI: 10.1080/00397911.2020.1800742
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https://doi.org/10.1080/00397911.2020.1800742
A new approach for the synthesis of fluorescent
pyrido[1,2-a]benzimidazoles
Akin Sagirli
_
Department of Chemistry, Bolu Abant Izzet Baysal University, Bolu, Turkey
ABSTRACT
ARTICLE HISTORY
Received 28 April 2020
A facile synthesis of new pyrido[1,2-a]benzimidazoles through a one-
pot multicomponent reaction of malononitrile, electron-deficient aryl
aldehydes and heterocyclic enamines followed by a nitrous acid
release under mild condition, is reported. Depending on the sub-
stituents on heterocyclic enamine, newly synthesized products have
been obtained either as an inseparable mixture of regioisomers or as
a single regioisomer. A plausible mechanism can be proposed for
the formation of pyrido[1,2-a]benzimidazoles via this unfamiliar trans-
formation with the HNO2 extrusion. The structures of all title com-
pounds were elucidated using spectroscopic methods and physical
characteristics involving single crystal X-ray diffraction and TOF-MS
measurements. In addition, among all the products, CN- and CF3-
substituted ones showed promising absorption and fluorescent prop-
erties.
KEYWORDS
Heterocyclic enamine;
multicomponent reaction;
pyrido[1,2-a]benzimidazole
GRAPHICAL ABSTRACT
Introduction
Benzimidazoles are of great importance in organic synthesis due to their diverse bio-
logical activities and they are known to be used as a precursor intermediate, in particu-
lar, to provide ready access to the variety of fused benzimidazole or benzo
[4,5]imidazo[1,2-a]pyridines.[1–3] Among them, pyrido[1,2-a]benzimidazoles have been
extensively studied and gained increasing interest over the past decades.[4–7]
Compounds bearing these heterocyclic units not only display a variety of biological
_
Department of Chemistry, Bolu Abant Izzet Baysal University, Bolu
ꢀ
CONTACT Akin Sagirli
sagirli_a@ibu.edu.tr
14280, Turkey.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
A. SAGIRLI
Figure 1. Multicomponent reaction of heterocyclic enamines 1 with malononitrile 3 and electron-rich
and electron-deficient aldehydes 2.
[10]
activities such as antimalarial,[8] antitumor,[9] and antifungal
promising photophysical and fluorescent properties.[11,12]
but also show some
On the other hand, numerous synthetic approaches have been developed for the con-
struction of pyrido[1,2-a]benzimidazoles and its derivatives either by reacting the benzi-
midazole-2-acetonitrile with b-ketoesters[13] and 3-substituted chromones,[14] or by
transition metal-catalyzed,[15] acid-catalyzed,[16] base-catalyzed[17] and photoinduced
radical cyclization[18] of modified 2-aminopyridines, but many of them require harsh
reaction conditions and unwanted side products. Therefore, it is always desired and
challenging to design an easy and efficient route to reach the corresponding hetero-
cycles. The multicomponent reaction is one of the most utilized methods for the synthe-
sis of structurally diverse and complex heterocycles in terms of simplicity
and efficiency.[19] For this purpose, heterocyclic ketene aminals seem to be attractive
intermediates for the preparation of fluorescent pyrido[1,2-a]benzimidazoles due to their
bisnucleophilic nature that allow the cyclization reaction with olefins bearing an electro-
n-withdrawing groups in a one-pot process.
Very recently, we reported the synthesis of a series of 3,5-dihydropyrido[1,2-a]ben-
zimidazoles under mild conditions in high yield via a multicomponent reaction
between cyclic enamines 1, aryl aldehydes 2 -bearing electron-donating groups and
malononitrile 3 (Fig. 1).[20] As a continuation of our work on extending the scope
of the aforementioned reaction: electron-deficient aryl aldehydes have been subjected
to three-component reaction which unexpectedly afforded new pyrido[1,2-a]benzimi-
dazoles via the extrusion of HNO2, and many of the products exhibited promising
fluorescent properties. To the best of our knowledge, there are few studies related
to the optical properties of pyrido[1,2-a]benzimidazoles, so, this progressively driven
us to develop and examine the fluorescent properties of this new heterocycles
in detail.

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3
Table 1. Optimization of reaction conditions affording product 5a.
Entry
Solvent
CH3CN
Reaction time
Base
K2CO3
Piperidine
KOtBu
NEt3
NEt3
NEt3
NEt3
NEt3
Eq.
Temperature (oC)
Yield (%)c
1a
2a
3a
4a
5a
6a
7b
8a
9a
10a
6h
6h
12h
3h
3h
3h
3h
3h
3h
1.0
1.0
1.0
1.0
1.0
1.5
1.5
2.0
1.5
1.5
80
80
80
80
25
80
80
80
80
80
55
60
62
68
–
72
52
72
55
70
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Ethanol
Dioxane
NEt3
NEt3
3h
aThe reaction was carried out by conventional heating.
bMicrowave irradiation, 300 W, 80oC.
cYield of isolated product.
Results and discussion
Our initial effort was to expand the scope of the multicomponent reaction reported in
our previous study. For this purpose, heterocyclic enamine 1a, p-cyanobenzaldehyde 2a,
and malononitrile 3 were tested by following the previously described method using of
1 equiv. of NEt3 under reflux temperature of CH3CN to afford new parent 3,5-dihydro-
pyrido[1,2-a]benzimidazoles. Unexpectedly, this model reaction gave rise to the forma-
tion of fully conjugated pyrido[1,2-a]benzimidazole 5a with the liberation of HNO2
instead of expected- product 4a (Fig. 1). In order to better understanding the limitation
of this unfamiliar reaction, we utilized some aryl aldehydes bearing electron-withdraw-
ing groups (2a–c) with malononitrile 3 and various heterocyclic enamines (1a–c) in the
multicomponent reaction.
First of all, we began the study by optimizing the reaction conditions using different
bases and solvents (Table 1). First, the effect of different bases on the reaction condi-
tions was investigated by using 1a, 2a, and 3 as substrates in the model reaction.
Among the bases (K2CO3, piperidine, KOt-Bu, Et3N) tried on the reaction, triethyl-
amine afforded the desired product 5a in the highest yield (Table 1, entries 1-4). Even
no product was observed at room temperature in CH3CN (Table 1, entry 5), using of
1.5 equiv. of triethylamine at the same reaction conditions gave the best reaction yield
(72%) (Table 1, entry 6). However, the trial with MW energy caused a slight decrease in
the reaction yield (52%) (Table 1, entry 7) and also using of 2.0 equiv. of Et3N had no
positive contribution in the yields (72%) of model reaction (Table 1, entry 8). In last
optimization trials, the effects of different solvents (ethanol, dioxane) under optimized
reaction conditions (1.5 eq. NEt3, 80 ꢀC) have been investigated and this time, slight
decreases were observed in terms of reaction yields (Table 1, entries 9–10).
Thus, under the optimal conditions, we extended the scope of the reaction by using
variety of heterocyclic enamines 1a–c, malononitrile 3 and electron-deficient aryl alde-
hydes 2a–c to provide nine new pyrido[1,2-a]benzimidazoles (5 and 6) as either single

4
A. SAGIRLI
Table 2. Formation of product 5 and its regioisomer 6.
Entry
R
R1
Product 5
Product 6
Regioisomer ratio (5:6)b
Yield (%)
1
2
3
4
5
6
7
8
9
1a: R¼H
1a: R¼H
1a: R¼H
1b: R¼Me
1b: R¼Me
1b: R¼Me
1c: R¼Cl
1c: R¼Cl
1c: R¼Cl
2a: R¼CN
2b: R¼NO2
2c: R¼CF3
2a: R¼CN
2b: R¼NO2
2c: R¼CF3
2a: R¼CN
2b: R¼NO2
2c: R¼CF3
5a
5b
5c
5d
5e
5f
5g
5h
5i
–
–
–
6d
6e
6f
6g
6h
6i
–
–
–
72
52
65
50:50
49:51
69:31
52:48
53:47
66:34
60a
48a
56a
64a
50a
62a
aTotal yield of inseparable regioisomers.
bRatio of diastereomers in each mixture were determined according to olefinic¼CH peaks in proton NMR spectra of
the products.
product or inseparable regioisomers in moderate to high yields, (Table 2, entries 4–9).
It is well understood that this regioisomeric product distribution (from 50:50 to 69:31)
was due to two reactive amine functionalities, which both can be tautomerized on het-
erocyclic enamine to enable the multicomponent reaction (Table 2, entries 4–9). When
unsubstituted cyclic enamine 1a was used, no matter which amine group reacts with in
situ formed arylidenemalononitrile to afford the expected product without any
regioisomer. However, the use of –CH3 or –Cl substituted heterocyclic enamines (1b or
1c) bearing two different amine groups which individually act as a nucleophilic reagent
in reaction leading to the formation of regioisomers. Interestingly, we did not observe
any sign for the formation of regioisomeric mixture (5 or 6) in our earlier report when
1b was used in the multicomponent reaction with malononitrile and electron-rich aryl
aldehydes 2.[20]
We believe the reason behind such a transformation is the presence of electron-with-
drawing group ataryl aldehydes. During the reaction progress, after the formation of
expected product 4, the methinic proton in the dihydropyridine structure 4 becomes
acidic by the resonance effect of electron-withdrawing groups on para-position and this
allows the formation of more stable product 5 or 6 by a release of HNO2 molecule.
Presumably, the reaction initiates by nucleophilic attack of enamine at the benzylic pos-
ition of in situ formed arylidenemalononitriles over two possible directions, thus afford-
ing an intermediate cyclization product A which gives intermediate B by imine-enamine
tautomerization. Finally, the acidic methinic proton next to nitro group on pyridine
ring is abstracted by the action of base simultaneous cleavage of NO2 group ensures the
fully aromatic regioisomeric products 5 or 6 over two possible separate routes, respect-
ively (Scheme 1).
The structures of title compounds were identified on the basis of IR, NMR, MS.
1H-NMR measurements showed that the most confirmative proton signal for the

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5
Scheme 1. Plausible mechanism for formation of regioisomers 5 and 6
formation of products (5 or 6) was olefinic C-H of pyridine ring. It was a distinct sing-
let resonated at around 7.00 ppm for all products. Besides, ratio of diastereomers in
1
each mixture were determined according to olefinic ¼ CH peaks in H NMR spectra of
the products (Table 2, entries 4–9). The NH2 protons resonated at around 7.90 ppm as
singlet and in some of the products; it was observed as a doublet signal. Moreover, dif-
ferent aromatic proton signals due to the isomerization were mostly observed on the
benzene ring of benzimidazole, when this was substituted with Cl- or CH3- groups.
Also, HRMS measurements disclosed that both ESI [M-H] and APCl [M þ H] values
accurately coincide with the molecular formulas of the expected products. Moreover, a
fine crystal of 6g from the regioisomeric mixture, the structure of 6g was further estab-
lished by single crystal X-ray analysis and full data have been deposited at the
Cambridge X-ray database with number 1995474 (Fig. 2).
Once a new approach for the preparation of functionalized pyrido[1,2-a]benzimida-
zole was achieved, main aim was to evaluate the influence of different substituents on
absorption and fluorescence properties. Figure 3 shows the UV-Vis absorption and
fluorescence (FL) spectra of the molecules in dimethylformamide (DMF), which are
measured at room temperature. As it is seen in Figure 3a, the absorption for the mole-
cules with p-NO2 substitution on phenyl ring (i.e. 5b, 5e–6e, and 5h–6h) starts at about
ꢁ
600 nm, and a peak maximum appears between 350 and 400 nm, assigned to p!p
transition. With the change of substituents (e.g. CH3 and Cl) on the benzimidazole phe-
nyl moiety, no significant changes in the absorption features have been observed.
However, when the -NO2 group was substituted with –CN and –CF3 substituents at

6
A. SAGIRLI
Figure 2. X-ray crystal structure of 6 g.
para position of phenyl ring, the absorption features were dramatically changed. Figure
3b and c represents the absorption curves of the molecules bearing p-CN (i.e. 5a,
5d–6d, and 5 g–6g) and p-CF3 (i.e. 5c, 5f–6f, and 5i–6i) groups, respectively. It is
clearly seen that the starting point of the absorption shifts to about 500 nm and a new
main peak emerged in the absorption bands. This new peak between 300 and 350 nm
might be attributed to internal charge transfer (ICT), which is typically observed in
organic dyes having donor-acceptor structure.[21,22] Due to the CT transitions which are
one of the possible mechanisms responsible for the fluorescence emission in these mate-
rials,[23] the fluorimetric measurements were also performed for all samples. Figure 3d
shows the images of all samples illuminated under ultraviolet light (365 nm). The sam-
ples bearing p-NO2 substituent on the phenyl ring as an acceptor group (i.e. 5b, 5e–6e,
and 5 h–6h) did not exhibited fluorescence properties. However, it was found that the
samples showed FL emission when the phenyl ring was para-substituted with CN and
CF3 moieties. Figure 3e and f depicts the FL emission spectra of the samples with p-CN
and p-CF3 substitutions on the phenyl ring (i.e. 5a, 5d–6d, 5g–6g, and 5c, 5f–6f, 5i–6i),
respectively. The fluorescence measurements were performed at room temperature and
under 400 nm excitation and the concentration of the samples in DMF was kept at
1.0 ꢂ 1025 M during the measurements to avoid the self-absorption effect. The pyr-
ido[1,2-a]benzimidazoles bearing electron-accepting –CN and –CF3 groups exhibited
bluish and greenish emissions, respectively, and the maximum peak positions of the FL
emissions were summarized in Table 3.
In the compounds with the electron-donor groups –CH3 and –Cl, it was observed
that nearly symmetric emission profile was gradually disappeared and a new shoulder
started to appear in the longer wavelengths side of the emission band. This new shoul-
der at longer wavelength has become more apparent in the form of dual emission pro-
file in the case of the sample bearing –CF3 and –Cl as electron-acceptor and -donor
system (i.e. 5i–6i), respectively (Fig. 3f). Furthermore, full-width half-maximum (fwhm)

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Figure 3. UV-Vis absorption spectra of (a) 5 b, 5e–6e, 5 h–6h, (b) 5a, 5d–6d, 5 g–6g and (c) 5c,
5f–6f, 5i–6i. (d) Photograph of solutions of the samples in DMF under UV-excitation. Fluorescence
emission spectra of (e) 5a, 5d–6d, 5 g–6g and (f) 5c, 5f–6f, 5i–6i.
Table 3. Absorption and fluorescence properties of the title compounds.
Absorption (nm)
Peak-1 Peak-2
327 384
FL Emission (nm)
Peak-1 Peak-2
510
fwhm (nm)
Sample
Donor group
Acceptor group
5a
H
CH3
Cl
H
CH3
Cl
H
CH3
Cl
CN
CN
CN
NO2
NO2
NO2
CF3
CF3
CF3
–
85
83
114
–
–
–
87
105
127
5d–6d
5g–6g
5b
5e–6e
5h–6h
5c
327
327
–
385
385
385
386
392
370
370
390
512
508
–
–
–
–
–
–
–
–
–
–
327
331
325
483
484
483
–
–
532
5f–6f
5i–6i

8
A. SAGIRLI
of the FL emission peak were found to be increased with introduction of the electron-
donor moiety by –H, –CH3, and –Cl, respectively (Table 3). The isomer formation in
the case of the –CH3 and –Cl substitutions on benzimidazole for both samples having
p-CN and p-CF3 as an electron-accepting groups can be responsible for both of the new
peaks and the broadening at fwhm in the FL emission profile. When the Cl is posi-
tioned instead of CH3 moiety on the benzimidazole ring, it is observed that the intensity
of the newly appeared FL emission peak increases (i.e. 5i–6i). Also, this changing in
electron-donor position give rise to the broadening in fwhm of the FL emission profile.
These findings can be an indication of a change in the number of isomer molecules
in solution.
Experimental
General procedure for the synthesis of 5-substituted-2-(nitromethylene)-2,3-
dihydro-1H-benzo[d]imidazole (1)[24]
A mixture of 1,1-bis(methylthio)-2-nitroethylene (10 mmol, 1650 mg) and o-phenylene-
diamine derivatives (10 mmol) were refluxed in EtOH (25 ml) for 8–12 h. The reaction
progress was followed by TLC and upon completion, the precipitated solid was filtered,
and washed with cold EtOH (25 mL) to afford the title compound 1 in a pure state.
5-Methyl-2-(nitromethylene)-2,3-dihydro-1H-benzo[d]imidazole (1b)
Yield (72%) L.brown solid. mp 188 ꢀC decomp. IR (KBr): ꢀ ¼ 3294, (NH), 1615 (CN),
1
1581, 1498, 1429, 1325, 1159, 1127, 1035, 988, 802, 751 cmꢃ1. H NMR (400 MHz,
DMSO-d6) d 7.44 (s, 1H, NH), 7.32 (d, J ¼ 8.1 Hz, 1H), 7.24 (s, 1H), 7.03 (d, J ¼ 8.1 Hz,
1H), 6.79 (s, 1H, olefinic CH), 5.97 (s, 1H, NH), 2.34 (s, 3H, CH3). 13C NMR
(101 MHz, DMSO-d6) d d 147.68, 133.54, 131.26, 129.06, 125.28, 124.73, 112.48, 96.57,
21.61. HRMS (þAPCl-TOF) calcd for C9H10N3O2 [M þ H]þ 192.0773, found 192.0766.
General procedure for the synthesis of 1-amino-3-(4-substitutedphenyl)-
methylpyrido[1,2-a]benzimidazole-2-carbonitrile (5,6)
Aromatic aldehyde 2 (0.6 mmol) , heterocyclic enamines 1a–c (0.5 mmol) and malono-
nitrile 3 (0.6 mmol, 39.6 mg) were mixed in acetonitrile (15 mL) in a round-bottomed
flask, and the reaction mixture was stirred and heated for 30 min. Triethylamine
(0.75 mmol, 0.105 mL) was added and the reaction mixture was heated under reflux for
3 h. After completion of the reaction, solvent was removed by vacuum evaporation. The
resultant crude product was purified by column chromatography on silica gel eluted by
using a mixture of hexane:ethyl acetate as eluent to afford single product 5 and inseper-
able regiosomer mixture 5, 6 of new pyrido[1,2-a] benzimidazoles.
1-Amino-3-(4-cyanophenyl)benzo[4,5]imidazo[1,2-a]pyridine-2-carbonitrile (5a)
Yield (72%, 111 mg) yellow solid. mp 313 ꢀC decomp. IR (KBr): ꢀ ¼3467, 3310 (NH2),
1
2202 (CN), 1654, 1624, 1593, 1509, 1408, 1251, 838, 722, 549 cmꢃ1. H NMR (400 MHz,

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9
DMSO-d6) d 8.48 (d, J ¼ 8.2 Hz, 1H), 7.95 (d, J ¼ 7.8 Hz, 2H), 7.80 (d, J ¼ 8.0 Hz, 3H),
7.75 (d, J ¼ 7.6 Hz, 2H, NH2), 7.52 (t, J ¼ 7.4 Hz, 1H), 7.36 (t, J ¼ 7.5 Hz, 1H), 6.97 (s,
1H, olefinic CH). 13C NMR (101 MHz, DMSO-d6) d 152.32, 148.99, 145.87, 142.54,
132.96, 129.98, 128.51, 126.55, 121.92, 119.36, 118.98, 117.62, 115.64, 112.24, 105.38,
105.37, 74.39. HRMS (-ESI-TOF) calcd for C19H10N5 [M-H]- 308.0936, found 308.0915.
Experimental details, characterization data and copies of IR and NMR spectra for all
synthesized compounds can be found in publisher’s website along with the pub-
lished article.
Conclusion
We have developed a new and efficient three-component approach for the synthesis of
new benzimidazoles substituted at the C-3 position via elimination of a HNO2 molecule
under mild conditions in one-pot manner where one of the components is an electron-
deficient aldehyde (2). Also, the absorption and fluorescence characteristics of title com-
pounds display their high potential for using in chemical sensors and light-generation
optoelectronic applications as optical materials.
Acknowledgement
Special thanks to Dr. Murat Olutas and Dr. Muhammet Yıldırım for interpreting fluorescent
measurements and valuable discussions throughout this study.
Funding
_
Bolu Abant Izzet Baysal University, Directorate of Research Projects Commission [BAP grant no.
2018.03.03.1348] are gratefully acknowledged for financial support.
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