An efficient synthetic approach towards new 5,5’-diaryl-2,2’-bipyridine-based fluorophores

Article abstract of DOI:10.1016/j.cclet.2016.12.043

An efficient approach has been developed for the synthesis of 5,5′-diaryl-2,2′-bipyridines via their 1,2,4-triazine analogues. The notable advantages of the present method are: The possibility of varying the aromatic substituents in the positions 5 and 5′ of bipyridine core and the possibility for obtaining 2,2′-bipyridines bearing a fused cyclopentene core to increase the solubility in organic solvents. These 5,5′-diaryl-2,2′-bipyridines exhibited an intense emission in a range of ca. 422–521?nm in acetonitrile solution; depending on the nature of the aromatic substituents and the presence of annulated cyclopentene fragments. Apart from that, the significant bathochromic shifts of the both absorption and emission maxima were observed in comparison with a number of previously described similar structures. In some cases the significant increasing of the fluorescence quantum yields took place.

Full text of DOI:10.1016/j.cclet.2016.12.043

Accepted Manuscript
Title: An efficient synthetic approach towards new
5,5’-diaryl-2,2’-bipyridine-based fluorophores
Author: <ce:author id="aut0005"
author-id="S1001841717300293-
2fe970bcd2c1e81edaa340de9d5e0978"> Alexey P.
Krinochkin<ce:author id="aut0010"
author-id="S1001841717300293-
55b3d08473e47676fe5d9574e13602a9"> Dmitry S.
Kopchuk<ce:author id="aut0015"
author-id="S1001841717300293-
24cc9c97f95b4e4be1184855ec756195"> Nikolay V.
Chepchugov<ce:author id="aut0020"
author-id="S1001841717300293-
e64ffd81b925ef5427e69c5764cb0e8a"> Grigory A.
Kim<ce:author id="aut0025"
author-id="S1001841717300293-
e01c2d8051f6b98dcf6524f1048e4767"> Igor S.
Kovalev<ce:author id="aut0030"
author-id="S1001841717300293-
e7bc5c6a4f4033686ce47056ded2e025"> Matiur
Rahman<ce:author id="aut0035"
author-id="S1001841717300293-
80ef237a6311fb238a3a8e4af6596a96"> Grigory V.
Zyryanov<ce:author id="aut0040"
author-id="S1001841717300293-
e007f8b5a8278f619980125ac7342bde"> Adinath
Majee<ce:author id="aut0045"
author-id="S1001841717300293-
70bcd805bf481e89e7ead1e95d8f233e"> Vladimir L.
Rusinov<ce:author id="aut0050"
author-id="S1001841717300293-
4d89f0ebc9a85de480b8cb0e1fadc2de"> Oleg N.
Chupakhin
PII:
S1001-8417(17)30029-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2016.12.043
CCLET 3956
To appear in:
Chinese Chemical Letters

Received date:
Revised date:
Accepted date:
14-10-2016
28-11-2016
6-12-2016
Please cite this article as: Alexey P.Krinochkin, Dmitry S.Kopchuk, Nikolay
V.Chepchugov, Grigory A.Kim, Igor S.Kovalev, Matiur Rahman, Grigory V.Zyryanov,
Adinath Majee, Vladimir L.Rusinov, Oleg N.Chupakhin, An efficient synthetic
approach towards new 5,5’-diaryl-2,2’-bipyridine-based fluorophores, Chinese
Chemical Letters http://dx.doi.org/10.1016/j.cclet.2016.12.043
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Original article
An efficient synthetic approach towards new 5,5'-diaryl-2,2'-bipyridine-based
fluorophores
a
a
Alexey P. Krinochkin , Dmitry S. Kopchuk a,b,1, Nikolay V. Chepchugov , Grigory A. Kim a,b, Igor S.
Kovalev , Matiur Rahman , Grigory V. Zyryanov a,b, Adinath Majee , Vladimir L. Rusinov a,b, Oleg N.
a
a
c
Chupakhin a,b
a Ural Federal University, Yekaterinburg 620002, Russian Federation
b Postovsky Institute of Organic Synthesis of RAS (Ural Division), Yekaterinburg 620990, Russian Federation
c Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India
ARTICLE INFO
Article history:
Received 14 October 2016
Received in revised form 28 November 2016
Accepted 6 December 2016
Available online
———
1 Corresponding author.
E-mail address: dkopchuk@mail.ru

GRAPHICAL ABSTRACT
An efficient approach has been developed for the synthesis of 5,5′-diaryl-2,2′-bipyridines via their 1,2,4-triazine analogues. The notable advantages of
the present method are: The possibility of varying the aromatic substituents in the positions 5 and 5′ of bipyridine core and the possibility for obtaining
2,2′-bipyridines bearing a fused cyclopentene core to increase the solubility in organic solvents.
ABSTRACT
An efficient approach has been developed for the synthesis of 5,5′-diaryl-2,2′-bipyridines via their 1,2,4-triazine analogues. The notable advantages
of the present method are: The possibility of varying the aromatic substituents in the positions 5 and 5′ of bipyridine core and the possibility for
obtaining 2,2′-bipyridines bearing a fused cyclopentene core to increase the solubility in organic solvents. These 5,5′-diaryl-2,2′-bipyridines exhibited
an intense emission in a range of ca. 422-521 nm in acetonitrile solution; depending on the nature of the aromatic substituents and the presence of
annulated cyclopentene fragments. Apart from that, the significant bathochromic shifts of the both absorption and emission maxima were observed in
comparison with a number of previously described similar structures. In some cases the significant increasing of the fluorescence quantum yields
took place.
Keywords:
2,2′-Bipyridines
1,2,4-Triazines
Aza-Diels-Alder reaction
Fluorescence

1. Introduction
Compounds containing the 2,2′-bipyridine moieties are the most common ligands [1] for the transition metal cations. Moreover, 2,2′-
bipyridines having an extended conjugation system, for instance aryl-substituted ones, are of special interest. In this case it is possible
to achieve the more desirable values for the both absorption and the emission maxima, namely red shifting [2-4], compare to the
“naked” 2,2′-bipyridine for which these two parameters are observed in a short-wave region [5].
In this article, the objects of the study are 5,5'-diaryl-substituted 2,2'-bipyridines. The main advantage of these compounds is the
absence of aryl substituents in the α-position of the pyridine core which favors the more efficient chelation of metal cations. Similar
compounds are widely used as electron transport materials for organic electronics [6] including the organic electroluminescent devices
[7], as components for dye-sensitized solar cells [8], as ligands for transition metal cations [9], as well as building blocks for some
macrocycles [10]. In addition, the properly situated aromatic substituents in 2,2′-bipyridine core can be employed as spacers for the
connection to other molecules like porphyrins [11].
2. Results and discussion
2.1. Chemistry
The most common approaches to synthesize 5,5′-diaryl-2,2′-bipyridines are the Pd-catalyzed cross-coupling reactions of 5,5′-
dibromo-2,2′-bipyridine [12] or various 2-bromopyridines [13]. Very few other synthetic strategies were also used, such as Co(II)-
catalyzed cycloaddition reactions between nitriles and diynes [14] or Ni-catalyzed dimerization of 3-phenylpyridine [15]. Furthermore,
some 1,10-phenanthroline analogues of 2,2′-bipyridines were prepared via the concerted building up of two edge pyridine rings [16]. It
is worth noting that most of the reported methods are devoted to obtain 5,5′-diaryl-2,2′-bipyridines bearing same aromatic substituents.
Only in rare cases the preparation of non-symmetric diaryl-derivatives of 5,5′-diaryl-2,2′-bipyridines or their analogues were reported,
for example, by the step-wise cross-coupling reactions involving two bromine atoms in 1,10-phenanthrolines [17] or 2,2′-bipyridines
[18].
On the other hand, some new approaches to synthesize different pyridine ligands have been previously reported by using the "1,2,4-
triazine" methodology, by which it is possible to prepare a wide range of oligopyridines [19]. The possibility of getting wide variation
of additional substituents of different types in the oligopyridine core which is important for tuning both photophysical and metal
chelation properties of the obtained ligands is the main advantage of such methodology. In addition, the direct annulation of a
cycloalkane core on the aryl-functionalized pyridine ring by means of aza-Diels-Alder reaction between 1,2,4-triazines and enamines is
beneficial for increasing the solubility of the oligopyridine ligands and their metal complexes in organic solvents [20]. Apart from that
the "1,2,4-triazine" methodology does not require any hardly available reagents, complicated and tedious synthetic procedures or
special catalysts.
Previously we have reported a step-wise construction method to build up the target 2,2′-pyridine core bearing aromatic substituents
in unique positions by means of the sequence of several heterocyclization reactions [4, 21]. The obtained oligopyridines are hardly
available via the common approaches. In the current manuscript we wish to report the use of "1,2,4-triazine" methodology for the
synthesis of non-symmetrically functionalized 5,5′-diaryl-2,2′-bipyridines. Scheme 1 represents the retrosynthetic approach to obtain
these ligands (compounds A). Thus, initially we proposed the use of 1,2,4-triazine analogues B as starting materials. Their syntheses, in
turn, can be accomplished via the cyclocondensation reaction between isonitrosoacetophenone hydrazones C and 5-arylpyridine-2-
carbaldehyde D. Next, two possible pathways can be used for obtaining aldehyde D: (i) the synthetic conversion of the corresponding
carboxylic acid derivatives E or (ii) the transformation of the corresponding dibromo- or dichloromethyl moiety in 5-arylpyridines F.
The second approach seems to be the most logical one because of the smaller number of reaction steps. Moreover, this method has
previously been reported for some similar aldehydes [22]. However, the synthetic approaches towards dibromo- or dichloromethyl-
substituted arylpyridines F are limited by only direct chlorination [23] or bromination [24] reaction of the methyl group in the pyridine
core. From the other hand the synthesis of 1,2,4-triazine precursor of the pyridine F (i.e. 3-dichloromethyl-6-phenyl-1,2,4-triazine) has
been reported previously by means of the reaction between isonitrosoacetophenone hydrazone and the corresponding iminoether
obtained in situ from dichloroacetonitrile [25].
Therefore, we have employed this approach: 3-Dichloromethyl-1,2,4-triazine 1 was synthesized and the desired compound 2 was
obtained in satisfied yields by aza-Diels-Alder reaction of 1 with 2,5-norbornadiene (Scheme 2). At the next step, aiming to synthesize
the desired aldehyde 3 we converted the α-dichloromethyl group in the pyridine core to the aldehyde one by means of either basic or
acidic hydrolysis. However, the reaction failed to proceed in alkaline conditions. In acidic conditions, namely, after boiling in 80%
propionic or aqueous hydrochloric acid no desired aldehyde 3 was detected in the reaction mixture. Only after boiling the compound 2
1
in 85% formic acid for 36 h product 3 was obtained in less than 30% yield according to the H NMR data, with marked tarring of the
reaction mixture. Based on the reported above this synthetic approach was ruled out, and our further studies were directed to the
realization of the alternative approach, starting from the corresponding pyridine carboxylic acid E.
5-Phenylpyridine-2-carboxylic acid 4 can be obtained from the 3-trichloromethyl-1,2,4-triazine 5 based on our previously reported
method [4]. In addition, in this paper an alternative approach has been developed by starting from 3-(furan-2-yl)-6-phenyl-1,2,4-

triazine 6 [26] by means of the transformation of the triazine ring into the pyridine one followed by conversion of the furan
functionality into the carboxyl group.
The subsequent esterification reaction by means of refluxing 4 in methanol saturated with hydrogen chloride afforded the
corresponding ester 8 quantitatively. After the following reduction and oxidation reactions the desired aldehyde 3 has been obtained in
good yield. At the next step the heterocyclization reaction according to the previously published synthetic protocol [27] afforded the
1,2,4-triazine precursors 10 of target ligands in 55%-60% yields. The aza-Diels-Alder reaction between 10 and 1-
morpholinocyclopentene or 2,5-norbornadiene led to the target bipyridines 11, including the ones with annulated cyclopentene
fragment (i.e. having the better solubility in organic solvents).
Structures of the obtained compounds were confirmed based on the 1H NMR, 13C NMR, mass-spectrometry and elemental analysis.
In particular, in case of 1,2,4-triazine precursors 10 in the 1H NMR spectra the downfield singlets (9.44-9.49 ppm) of the protons at the
C5 position of the 1,2,4-triazine cycle can be observed, along with the signals of protons of ABX system of pyridine rings and two sets
of signals of protons of aromatic substituents. In case of products 11a-c the additional signals of protons of an annulated cyclopentene
ring can be observed in the field corresponding to the resonance of aliphatic protons along with the signal of proton at the position of
C3 of the 6,7-dihydro-5H-cyclopenta[c]pyridine as a singlet in the area of 8.56-8.58 ppm. In case of compounds 11d,e two sets of
proton signals of the two ABX systems of pyridine rings can be observed.
2.2. Photophysical properties
In order to estimate the influence of the nature of aromatic substituents the photophysical properties of the obtained 2,2′-bipyridines
11 have been studied. Thus, in acetonitrile solutions all the compounds exhibited an intense emission in a range of ca. 422-521 nm,
depending on the nature of the aromatic substituents and the presence of the annulated cyclopentene fragment (Table 1, Fig. 1, Fig. S1
in Supporting information). We compared the spectral data obtained for compounds 11 with those previously reported for some
derivatives of 5-aryl-2,2′-bipyridines and 5,5′-disubstituted 2,2′-bipyridines. In particular, it was demonstrated earlier [2] that both the
absorption and emission maxima can be red-shifted due to the introduction of aryl substituents at the C5 position of 2,2′-bipyridine
core. The greater red-shift has been observed due to the introduction of the ester group at the C5′ position [2].
Even more significant red-shifts are observed for these 2,2′-bipyridines 11, which are bearing an extra aryl substituent in the
bipyridine core, especially for the ones not bearing the fused cyclopentene fragment. The largest bathochromic shift has been achieved
for 4-methoxyphenyl-substituted bipyridine 11e: The bathochromic shift value in the emission maximum was up-to 122 nm compare to
5-monoaryl-substituted 2,2′-bipyridine [2]. Moreover, in case of compounds 11a, b bearing the phenyl or p-tolyl substituents the
quantum yield values increased accordingly in comparison with monoaryl-substituted bipyridines [2].
Also in case of compounds 11, a significant bathochromic shift in the absorption maxima has been observed, for instance up 61 nm
for the compound 11e.
In 2001, Loren and Siegel reported [28] the photophysical properties of mono- and dimanisyl-substituted 2,2′-bipyridines (manisyl
= 2,6-dimethyl-4-methoxyphenyl), which are similar to these 2,2′-bipyridines reported here. However, in our case the more significant
bathochromic shift both the absorption and emission maxima is observed, while the fluorescence quantum yield values were
comparable. Most probably, in case of manisyl-substituted bipyridines the conjugation of the aryl-bipyridine-aryl system is hampered
due to the presence of bulky aromatic substituents in pyridine rings: the whole system is not flat and the conjugation is broken. On the
other hand bis-(pyren-2-yl)-substituted 2,2′-bipyridine [29] demonstrated the photophysical properties which are comparable to the
properties of these reported compounds.
3. Conclusion
In summary, we have developed a convenient approach for the synthesis of 5,5′-diaryl-2,2′-bipyridines. This method does not
require any special catalysts, complicated reaction conditions or expensive reagents. The notable advantages of this present
methodology are the possibility of varying the aromatic substituents at the 5 and 5′ positions of bipyridine core and the possibility for
obtaining 5,5′-diaryl-2,2′-bipyridines bearing the fused cyclopentene cycle aiming the increasing of the solubility in organic solvents.
The photophysical studies of the synthesized bipyridines have demonstrated the significant bathochromic shifts for the both absorption
and emission maxima in comparison with a number of similar structures, which were previously described. In some cases the
increasing of the fluorescence quantum yields is achieved.
4. Experimental
4.1. General method for the synthesis of triazines 10
A solution of the corresponding hydrazone of isonitrosoacetophenone (1.36 mmol) in ethanol (25 mL) was added to a solution of
aldehyde 3 (250 mg, 1.36 mmol) in ethanol (20 mL). The resulting mixture was stored at room temperature for 10 h. The resulting
precipitate was filtered off, dried and suspended in glacial acetic acid (25 mL). The resulting mixture was heated to reflux for 5 times.
After completion of the reaction, solvent was removed under reduced pressure. The residue was treated with ethanol. The resulting
precipitate was filtered off, washed with ethanol and dried. Analytical samples were obtained by recrystallization from ethanol.
4.2. General method for the synthesis of bipyridines 11a-c

A mixture of the corresponding triazine 10 (0.4 mmol) and 1-morpholinocyclopentene (0.32 mL, 2 mmol) was stirred at 200 °С for
2 h under argon atmosphere. Then the additional portion of 1-morpholinocyclopentene (0.16 mL, 1 mmol) was added and the resulting
mixture was stirred for additional 1 h at the same conditions. The reaction mass was cooled to room temperature. Acetonitrile (20 mL)
was added and the resulting mixture was stored for 1 h at room temperature. The resulting precipitate was filtered off, washed with
acetonitrile and dried. The analytical samples were obtained by recrystallization from acetonitrile.
4.3. General method for the synthesis of bipyridines 11d,e
The corresponding triazine 10 (0.4 mmol) was suspended in o-xylene (20 mL). 2,5-Norbornadiene (0.16 mL, 1.6 mmol) was added
and the resulting mixture was stirred under reflux condition for 9 h. Then the additional portion of 2,5-norbornadiene (0.16 mL, 1.6
mmol) was added and the resulting mixture was stirred at reflux for additional 9 h. After completion solvent was removed under
reduced pressure. The residue was purified by flash chromatography (mixture of DCM and ethylacetate (10:1) as eluent). Analytical
samples were obtained by recrystallization from acetonitrile.
The full description of experimental procedures and full characterization for all new compounds are presented in the Supporting
information.
Acknowledgment
This work was supported by the Russian Science Foundation (No. 15-13-10033).

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500
400
300
200
100
0
11a
11b
11c
11d
11e
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 1. Emission spectra of bipyridines 11a-e in CH3CN at room temperature

Scheme 1. Retrosynthetic approach of 5,5′-diarylbipyridines A.

Scheme 2. Synthetic route for desired compounds. Reagents and conditions: i) 2,5-norbornadiene, 1,2-dichlorobenzene, reflux, 16 h; ii) 2,5-
norbornadiene, o-xylene, reflux, 16 h; iii) NaOH, KMnO4, water-pyridine (5:17), r.t., 2 h; then reflux, 10 min; iv) methanol, SOCl2, reflux, 15
h; v) NaBH4, ethanol, reflux, 2 h; vi) MnO2, 1,2-dichloroethane, 50 °С, 10 h; vii) corresponding hydrazone of isonitrosoacetophenone, ethanol,
10 h, r.t., then acetic acid, reflux, 5 min; viii) 1-morpholinocyclopentene, neat, 200 °С, 3 h; then acetonitrile, 1 h, r.t.; ix) 2,5-norbornadiene, o-
xylene, reflux, 18 h.

Table 1
Photophysical properties of new bipyridines 11 and some previously reported analogues.
Compd.
Ar
Ph
Ph
Ph
Tol
Tol
Tol
Tol
4-MeOC6H4
4-MeOC6H4
Manisyl
4-MeOC6H4
4-MeOC6H4
Manisyl
Pyrene-2-yld
n
0
0
3
0
0
0
3
0
0
0
0
3
0
0
R
H
λabs (nm) а
298
312
216, 251, 278, 340
302
317
223, 252, 292, 356
218, 255, 281, 343
309
326
λem (nm) b
357
380
422
360
395, 430sh.
450
422
399
438, 510sh.
410
Ф (%)c
3.2 [2]
4.5 [2]
62.7
17 [2]
68 [2]
79.3
COOMe
Ph
H
COOMe
Ph
Ph
H
COOMe
H
Ph
11a
11d
11b
71.1
89 [2]
49 [2]
78 [28]
31.6
30.5
87 [28]
63 [29]
286
224, 303, 370
231, 289, 351
296
521
506
408
448
11e
11c
Ph
Manisyl
Pyrene-2-yl
360
a Absorbtion maxima at room temperature;
b Fluorescence maxima at room temperature;
c Fluorescence quantum yields were measured in CH3CN solution using quinine sulfate as standard [30];
d Spectra were recorded in DCM. Best results are marked in red.