Efficient electrochemical synthesis of pyrido[1,2-a]benzimidazoles

Article abstract of DOI:10.1007/s11172-014-0440-y

Electrochemical reduction of N-(2-nitroaryl)pyridinium chlorides in alcohol - dilute HCl mixtures gave pyrido[1,2-a]benzimidazoles in high yields in both divided and undivided cells. According to cyclic voltammetry measurements and DFT calculations (B3LYP/6-31+G(d)), the reaction involves intermediate formation of the corresponding hydroxylamine derivative followed by its heterocyclization.

Full text of DOI:10.1007/s11172-014-0440-y

Russian Chemical Bulletin, International Edition, Vol. 63, No. 2, pp. 372—380, February, 2014
372
Efficient electrochemical synthesis of pyrido[1,2ꢀa]benzimidazoles*
A. A. Sokolov,a M. A. Syroeshkin,b V. N. Solkan,b T. V. Shebunina,a R. S. Begunov,a
L. V. Mikhal´chenko,b M. Yu. Leonova,b and V. P. Gultyaib
aP. G. Demidov Yaroslavl State University,
14 ul. Sovetskaya, 150000 Yaroslavl, Russian Federation
bN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 8824. Eꢀmail: guvp@ioc.ac.ru
Electrochemical reduction of Nꢀ(2ꢀnitroaryl)pyridinium chlorides in alcohol—dilute HCl
mixtures gave pyrido[1,2ꢀa]benzimidazoles in high yields in both divided and undivided cells.
According to cyclic voltammetry measurements and DFT calculations (B3LYP/6ꢀ31+G(d)),
the reaction involves intermediate formation of the corresponding hydroxylamine derivative
followed by its heterocyclization.
Key words: pyrido[1,2ꢀa]benzimidazoles, Nꢀ(2ꢀnitroaryl)pyridinium salts, electrochemical
reduction, electrolysis in an undivided cell, heterocyclization, cyclic voltammetry, density
functional theory, functional B3LYP.
Polycyclic imidazole derivatives containing the N atom
In the last few decades, electrochemical techniques
have come into all the wider use for oxidation and reducꢀ
tion of organic compounds because of the low cost and
availability of electric current; no waste disposal is reꢀ
quired, in contrast to relevant techniques involving chemꢀ
icals. In addition, the "reactivity" of electric current is
limited only by the properties of the reaction environment
and can relatively easily be controlled during an experiꢀ
ment, thus making some electrosyntheses more selective.
Earlier,17 we have demonstrated that currentꢀcontrolled
electrochemical reduction of Nꢀ(2ꢀnitroaryl)pyridinium
chlorides containing CF3, CN, and CO2Et groups in posiꢀ
tion 4 of the benzene ring in a mixture of propanꢀ2ꢀol with
dilute HCl gives the corresponding pyrido[1,2ꢀa]benzꢀ
imidazoles in high yields (70—94%). The goal of the
present work was to study this reaction with structurally
different substrates and the influence of various factors
(the concentration of HCl, the nature of the alcohol, the
ratio of aqueous HCl to alcohol in the catholyte, temperaꢀ
ture, current density, and the material of the cathode) on
the yields of the target products. In addition, we intended
to look into the possibility of using an undivided cell for
this electroreduction.
shared by several rings are widely used in pharmacology
and molecular biological investigations.1 Pyrido[1,2ꢀa]ꢀ
benzimidazoles are among heterocyclic systems in greatꢀ
est demand.2 These compounds are bioisosteric analogs of
nucleobases and can insert themselves into DNA moleꢀ
cules,3 thus exhibiting antitumor,4 antiviral,5 antibacteriꢀ
al,6 and antifungal activity.7 The presence of a system of
conjugated ꢀbonds and chromophores makes these heteroꢀ
cycles highly fluorescent and allows them to be employed
as fluorescent labels in genetic investigations.8 Some
pyrido[1,2ꢀa]benzimidazoleꢀbased drugs have been reportꢀ
ed9 to be used as myorelaxants in the therapy of central
nervous system disorders. This accounts for the recent
boom10 in searching for new routes to pyrido[1,2ꢀa]ꢀ
benzimidazole derivatives. The main approaches to the
synthesis of the structure in question involve the formaꢀ
tion of pyridine,11 imidazole,12 or simultaneously several
rings13 of a polycyclic fused system with the bridgehead
N atom. In our opinion, reductive heterocyclization of
Nꢀ(2ꢀnitroaryl)pyridinium salts provides a promising route
to pyrido[1,2ꢀa]benzimidazoles. This reaction is highꢀ
yielding under mild conditions, and no expensive catalysts
are required.14 The starting salts can easily be prepared
from largeꢀtonnage chemical products, pyridine or its deꢀ
rivatives, and orthoꢀchloronitroarenes.15 Pyridine may also
be used as a solvent.16
Results and Discussion
Nꢀ(2ꢀNitrophenyl)pyridinium chlorides 1a—h
(Scheme 1) were synthesized by heating solutions of apꢀ
propriate orthoꢀchloronitrobenzenes in pyridine, ꢀpiꢀ
coline, or 3,5ꢀlutidine (see Experimental). The reaction
* On the occasion of the 80th anniversary of the N. D. Zelinsky
Institute of Organic Chemistry of the Russian Academy of
Sciences.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 0372—0380, February, 2014.
1066ꢀ5285/14/6302ꢀ0372 © 2014 Springer Science+Business Media, Inc.

Electrosynthesis of pyridobenzimidazoles
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
373
gave products as precipitates, which were used in subseꢀ
quent transformations without further purification.
compound 2a. An increase in the concentration of HCl in
the catholyte up to 6—12% (see Table 1, entries 15, 16)
decreases the yield by 25—30% (see Table 1, entry 11).
A threeꢀ to fivefold increase in the current density as
well as cooling of the catholyte from 40—45 to 25 C (or
its heating to 60 C) lowered the yield of product 2a by
15—20% (see Table 1, entries 8, 9). Variation of the mateꢀ
rial of the cathode showed that nickel, platinum, and
graphite can successfully be used instead of lead for this
electrosynthesis (see Table 1, entries 17—19), but all of
them provide lower yields (by 10—20%). Titanium as
a cathode is less suitable. Although the results obtained
with stainless steel and titanium cathodes are comparable,
the former cathode becomes noticeably damaged during
the electrolysis (see Table 1, entries 20, 21). A copper
cathode is absolutely unsuitable because of a low yield of
the target product (see Table 1, entry 22).
Scheme 1
a
b
c
d
e
f
g
h
R1
R2
R3
CF3 CN CO2Me CO2Et CO2Ph CF3 CN CF3
H
H
H
H
H
H
H
H
H
H
Me
Me
Me Me
Me
H
Currentꢀcontrolled reduction of salts 1a—h in a dividꢀ
ed cell by passing 4 F mol–1 of electricity was accompaꢀ
nied by a jump in the applied voltage and usually by evoluꢀ
tion of the hydrogen gas. Polarographic measurements
reveal no starting salt in the catholyte. Neutralization of
the solution followed by extraction with chloroform alꢀ
ways gave the corresponding products 2 (Scheme 2). Acꢀ
cording to spectroscopic and TLC data, they are highly
pure because possible impurities (the starting salt, an amine
as a product of reduction of the nitro group, and condenꢀ
sation products) are virtually insoluble in CHCl3.
The optimized conditions providing the highest yield
(94%) of pyrido[1,2ꢀa]benzimidazole 2a are as follows:
lead cathode, 40—45 C, j = 4 mA cm–2, propanꢀ2ꢀ
ol : water = 1 : 1, and CHCl = 2—3% (in the catholyte).
Table 1. Yields of product 2a obtained by electroreduction of
compound 1a in a divided cell at 40—45 C and j = 4 mA cm–2
,
some other experimental conditions being varied
Entry
CMa
Alcohol
CHCl ROH : H2Ob
(%)c
Yield
(%)
1
2
3
4
5
6
7
8
Pb
Same
»
»
»
»
»
»
»
»
»
»
»
»
»
»
Ni
—
—
MeOH
EtOH
12
36
12
111.5
12
13
12
12
12
13
12
12
12
13
16
12
13
»
0 : 1
»
1 : 1
»
»
»
1 : 1
»
63
55
82
87
86
87
94
79d
73e
94
86
66f
67g
82
62
56
83
72
76
61
58
19
Scheme 2
»
»
PriOH
»
»
»
»
»
»
»
»
»
»
»
»
»
»
»
9
»
10
11
12
13
14
15
16
17
18
19
20
»
2 : 1
»
i. Electrolysis.
»
Reduction of compound 1a at a lead cathode (j =
= 4 mA cm–2, 40—45 C) afforded product 2a in 55—63%
yield with both concentrated (36%) and dilute HCl (2%)
as a catholyte (Table 1, entries 1, 2). Addition of an equivꢀ
alent volume of MeOH to the catholyte with CHCl = 2%
(see Table 1, entry 3) increased the yield of compound 2a
to 82%. The use of EtOH also increased (to 86—87%) the
yield of product 2a (see Table 1, entries 4—6). An increase
(to 3%) or a decrease (to 1.5%) in the concentration of
HCl in the catholyte affects the yield of pyrido[1,2ꢀa]ꢀ
benzimidazole 2a only slightly. The highest yield (94%)
was achieved in the system propanꢀ2ꢀol—aqueous HCl for
CHCl = 2—3% (see Table 1, entries 7, 10). The alcoholꢀtoꢀ
acid ratio of 1 : 1 seems to be optimum: an increase in the
content of both the alcohol and aqueous HCl (see Table 1,
entries 11, 14) appreciably (by ~10%) lowers the yield of
1 : 2
2 : 1
»
1 : 1
»
»
»
»
»
Pt
Graphite
Ti
»
»
»
»
21 Stainless steel
22 Cu
a CM is the material of the cathode.
b The ratio of alcohol to aqueous HCl in the catholyte.
c The concentration of HCl in the catholyte.
d j = 12 mA cm–2
e j = 20 mA cm–2
.
.
f The catholyte temperature is 25 C.
g The catholyte temperature is 60 C.

374
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
Sokolov et al.
Table 2. Yields of products 2 obtained by reduction of
compounds 1a—h in divided and undivided cells on a
lead cathodea
verify the formation of intermediate hydroxylamine derivꢀ
atives, we studied the electrochemical behavior of comꢀ
pounds 1 and 2 (with 1a and 2a as examples) by cyclic
voltammetry (CV) and polarography. The formation of
different products in electroreduction of nitroarenes is
known18 to largely depend on the protonꢀdonating activiꢀ
ty of the reaction environment. That is why we compared
the voltammograms of compounds 1a and 2a in an aprotic
solvent (MeCN) and aqueous alcohols (with different
pH values).
Substrate
Yield (%)
undivided cell
divided cell
1a
1b
1c
1d
1e
1f
94
75
68
77
59
94
88
76b
94
80
43
73
68
98
90
97b
The CV curves of the reduction of compounds 1a and
2a in MeCN with Bu4NClO4 as a supporting electrolyte
are shown in Fig. 1, a.
1g
1h
The simplest nitroarene (nitrobenzene) is reduced unꢀ
der such conditions at a potential of –1.15 V (vs. a saturatꢀ
ed calomel electrode (sat. SCE)) to give a stable radical
anion produced by electron transfer to the substrate.20
When the aromatic ring bears strong electronꢀwithdrawꢀ
ing substituents (CF3 and C6H5N+), its higher electron
affinity and, accordingly, a shift of its reduction potential
to the anodic values could be expected. Indeed, comꢀ
pound 1a is reduced at a more positive (by 0.57 V) potenꢀ
tial compared to nitrobenzene. The reduction is irreversꢀ
ible, suggesting that the product formed by electron transꢀ
fer to chloride 1a is unstable and that the electrochemical
step is followed by a chemical reaction. A comparison of
the peak current of compound 1a and the reduction peak
current of nitrobenzene reveals that the net reaction folꢀ
lows a oneꢀelectron pattern. Apparently, the intermediate
radical species21 undergoes rapid transformation. Comꢀ
pound 2a, the reductive heterocyclization product, is also
electrochemically active under such conditions and is irꢀ
reversibly reduced according to a oneꢀelectron mechaꢀ
nism; its reduction potential is considerably more cathodꢀ
ic (by 1.44 V) than that of chloride 1a.
We also studied the electrochemical behavior of comꢀ
pound 1a in protic solvents with different pH values (see
Fig. 1, b—d). We employed citrate—phosphate buffers with
pH 3 and 7 and 4% HCl, all diluted with ethanol in a ratio
of 1 : 1. Figure 1 shows two distinct peaks on the CV curves
in the above pH range (for the potential scan up to the
discharge of the supporting electrolyte). According to
polarographic data, the first step involves fourꢀelectron
reduction. In electrochemical reduction of nitroarenes,
this corresponds to the formation of a hydroxylamine deꢀ
a CHCl = 3%, water : propanꢀ2ꢀol = 1 : 1, catholyte temꢀ
perature 40—45 C, j = 4 mA cm–2
.
b A mixture of the 2ꢀ and 4ꢀmethyl isomers in a molar
ratio of 1 : 4.
Under these conditions, we obtained products 2b—h in no
less than 60% yields (Table 2). Therefore, the proposed
procedure is well suitable for the synthesis of various deꢀ
rivatives 2.
Reduction of compound 1h containing the 3ꢀmethylꢀ
pyridyl fragment gives two isomers 2h and 2i (Scheme 3).
It is known that electrolysis in a divided cell used to
separate the cathodic and anodic compartments is often
much more difficult to implement in practice, especially
when scaling is needed. This is due to a more complicated
design of the cell as well as contradictory requirements
imposed on the material of the membrane: it should be
a durable and chemically inert insulator to prevent mutual
diffusion of the catholyte and anolyte and it should have
a low resistance.18 That is why the use of undivided cells is
preferred; consequently, one can see a fastꢀgrowing numꢀ
ber of publications devoted to electroorganic synthesis unꢀ
der such conditions.19 In connection with this, we found it
interesting to study the possibility of synthesizing derivaꢀ
tives 2 by electroreduction of chlorides 1 in undivided
cells. It turned out that the electrolysis in the latter ensures
comparable (except for 1c) and even higher yields of prodꢀ
ucts 2 (see Table 2).
To gain insight into the mechanism of reductive hetꢀ
erocyclization of Nꢀ(2ꢀnitroaryl)pyridinium chlorides and
Scheme 3

Electrosynthesis of pyridobenzimidazoles
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
375
I/mA
I/mA
a
b
–0.07
pH = 7
–0.08
–0.06
–0.04
–0.02
0
1a
–0.04
1a
2a
–0.01
0.02
2a
0.05
900
100 –700 –1500 E/mV (vs. SCE)
–300
I/mA
–800
–1300 –1800 E/mV (vs. SCE)
c
d
I/mA
–0.12
–0.10
pH = 3
4% HCl
–0.08
–0.06
–0.04
–0.02
0
–0.09
–0.06
–0.03
0
1a
1a
2a
2a
–500 –1000 –1500 –2000 E/mV (vs. SCE)
–300
–700
–1100 –1500 E/mV (vs. SCE)
Fig. 1. Cyclic voltammograms of solutions of compounds 1a (5 mmol L–1) and 2a in MeCN (a), citrate—phosphate buffers with pH 7 (b)
and pH 3 (c), and 4% HCl (d) (a glassy carbon electrode, 0.1 M Bu4NClO4 as a supporting electrolyte, potential scan rate 0.1 V s–1).
The solvents are used as 1 : 1 mixtures with ethanol.
rivative (the second peak on the CV curve18). It should be
Scheme 4
emphasized that the potential of this peak always difꢀ
fers substantially from the reduction potentials of comꢀ
pound 2a; i.e., the hydroxylamine derivative seems to form
during CV measurements, while the electroreduction peak
of pyrido[1,2ꢀa]benzimidazole 2a is not observed even unꢀ
der the conditions similar to those for its synthesis from
chloride 1a. Obviously, the key difference between these
i. Electrolysis.
two processes is that the formation of the product is deꢀ
tected by cyclic voltammetry within a short period of time
(1 s from the start of the reaction), while the electrolysis
takes up several tens of minutes. Therefore, the electrolyꢀ
sis of compound 1a first gives a hydroxylamine derivative
(by reduction of the nitro group), which then undergoes
heterocyclization (Scheme 4).
It can be seen in Fig. 1 that as the protonꢀdonating
activity of the reaction environment increases, the disꢀ
charge potential of the supporting electrolyte shifts to the
anodic values and approaches the reduction potential of
compound 2a and the potential of the second peak of
chloride 1a. For the electrolysis, much more concentrated
solutions are used, which probably allows selective synꢀ
thesis: after 4 F mol–1 of electricity is passed under curꢀ
rentꢀcontrolled conditions, hydrogen is evolved at the
cathode and neither the intermediate hydroxylamine deꢀ
rivative nor product 2 are reduced.

376
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
Sokolov et al.
To verify the mechanism described above for heteroꢀ
lecular orbital theory,25 such an electrostatic interaction
should facilitate an nucleophilic attack of the lone electron
pair of the N atom on the nearby C atom of the molecular
fragment HC—N—CH of the pyridinium ring. The nucleoꢀ
philic attack is also favored by the LUMO of the pyridinium
fragment, thus making a considerable orbital contribution
to the cyclization energy. Based on the results of our calꢀ
culations, we estimated the endothermic enthalpy of forꢀ
mation of ꢀcomplex B at H = 22.0 kcal mol–1 at 298 K.
Since ꢀcomplex B is thermodynamically much less
favorable than the starting salt A, the proton at the N atom
can experience an 1,2ꢀshift to the O atom of the fragment
HNOH. Our detailed examination of the PES in the area
of interest revealed that this proton transfer results in actiꢀ
vationless decomposition of the Oꢀprotonated intermediꢀ
ate into complex C with a water molecule and Cl– as
a counteranion (see Scheme 5, Fig. 2). This indicates that
the Oꢀprotonated form of ꢀcomplex B is unstable both
thermodynamically and kinetically. The great exothermic
effect of this decomposition (H = –87.0 kcal mol–1 at
298 K) is associated with the kinetic instability of this
form of complex B. Hence our calculations demonstrated
that ꢀcomplex B is much more labile and more reactive
than the starting salt A.
cyclization of the hydroxylamine derivative into pyridoꢀ
benzimidazole, we calculated the potential energy surface
(PES) of salt A (Scheme 5) by the DFT method at the
B3LYP/6ꢀ31+G(d) level.22 To estimate the effect of the
polarity of the heterocyclization environment (aqueous
alcohols + HCl) on the thermodynamic parameters of
the intermediates, we used the polarizable continuum
model23 (PCM) (earlier, this approach has been used to
estimate the superacid properties of highꢀpurity H2SO4
and HSO3F (see Ref. 24).
Scheme 5
Based on our calculations and PES analysis, we can
propose a possible mechanism of the formation of comꢀ
pound C from salt A as a sequence of elementary steps:
(1) endothermic formation of ꢀcomplex B (H =
= 22.0 kcal mol–1), (2) 1,2ꢀshift of the proton from the N
atom to the O atom of the fragment HNOH in complex B,
and (3) exothermic decomposition (–87.0 kcal mol–1) of
the Oꢀprotonated intermediate into stable complex C with
a water molecule and the Cl– anion. The overall enthalpy
The PES shows minima corresponding to salt A (deep)
and ꢀcomplex B. For these structures, we performed full
optimization of geometrical parameters (Fig. 2) and simꢀ
ulated their IR spectra considering the effect of aqueous
medium.
The bond lengths in the "cyclic" fragment of structures
A and B are given in Table 3. It can be seen that the HC—N
and C—NH bonds in ꢀcomplex B are longꢀ
er than those in the starting salt A by ~0.1
and 0.05 Å, respectively. The HC...NH bond
in this complex is longer by ~0.2 Å than
bonds of this type in hydroxylamine derivaꢀ
tives, which makes the heterocyclization reaction substanꢀ
tially more endothermic. In addition, the bond lengths
strongly depend on the steric effect of the orthoꢀsubstituꢀ
ent (HNOH) in salt A: the torsion angle between the planes
of the phenyl and pyridinium rings is 108 and the hydroxꢀ
yl group makes an angle of 64 with the former. Such
relative positions of the pyridinium and phenyl fragments
as well as the nonꢀcoplanar arrangement of the phenyl
ring and the orthoꢀsubstituent HNOH are most favorable
for the lone electron pair on the N atom to begin a nucleoꢀ
philic attack on the orthoꢀC atom of the pyridinium ring.
Calculation of the efficient Mulliken charges showed
that the pyridinium fragment in salt A is positively charged
(1.2 e), including the fragment HC—N—CH (0.939 e). The
N atom of the substituent HNOH bears a high negative
charge (–0.613 e). According to the perturbational moꢀ
change for the above transformations is 65.0 kcal mol–1
.
According to our calculations, salt C is stable, so its in situ
transformation into neutral pyridobenzimidazole D (see
Scheme 5, Fig. 2) seems not to occur until the catholyte is
neutralized. All the products studied are insoluble in waꢀ
ter, yet being soluble in dilute (1—2%) and concentrated
HCl. That is why products 2 are mostly obtained as salts
of the type C.
Table 3. Bond lengths in the "cyclic" fragment of
structures A and B calculated with the PCM at the
B3LYP/6ꢀ31+G(d) level
Bond
d/Å
Structure A
Structure B
HC—N
N—C
C—C
1.358
1.456
1.407
1.417
1.446
3.197
1.447
1.419
1.396
1.467
1.406
1.630
C—NH
HN—OH
...
HC NH

Electrosynthesis of pyridobenzimidazoles
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
377
H
H
H
H
C
C
H
N
C
C
C
C
O
H
H
C
H
C
H
C
C
C
O
H
N
C
C
C
H
H
H
N
H
N
H
H
C
C
H
C
H
C
C
C
C
H
H
C
Cl
Cl
C
H
H
H
A
B
H
H
C
H
H
C
H
C
C
H
C
C
C
C
C
H
C
C
Cl
H
C
H
H
C
C
N
N
N
C
H
N
C
H
H
H
C
C
H
C
C
O
O
H
C
H
C
H
H
H
Cl
H
H
C
D
Fig. 2. Structures A—D optimized with the PCM at the B3LYP/6ꢀ31+G(d) level and contributing to the PES of the combined
molecular system.
controlled IPCꢀPro MF potentiostat (Econix) (scan rate error
1.0%, potential selector error 0.05 mV). Polarograms were reꢀ
To sum up, we proposed a new simple route to pyriꢀ
do[1,2ꢀa]benzimidazoles. Nꢀ(2ꢀNitrophenyl)pyridinium
chlorides are prepared from orthoꢀchloronitrobenzenes and
pyridine (largeꢀtonnage chemical products) involving no
other solvents. The reaction products form as precipitates
and can be used without further purification. Electrochemꢀ
ical reduction of Nꢀ(2ꢀnitrophenyl)pyridinium chlorides
is carried out on available cathodes in mixtures of an alcoꢀ
hol with dilute HCl. In contrast to wellꢀknown chemical
approaches, this electrochemical synthesis occurs under
mild conditions without any catalysts, and no recovery or
disposal of a reducing agent is required. Electrolysis in
undivided cells is also possible. In addition, the target
products formed by exhaustive electrolysis can easily be
isolated and need no further purification.
1
corded on a PUꢀ1 versatile polarograph. The H and 13C NMR
spectra of salts 1 (in D2O) and products 2 were recorded on
a Bruker AMꢀ300 spectrometer (300 (1H) and 75.5 MHz (13C)).
The 1H NMR spectra of compounds 2 were also recorded on
a Bruker DRXꢀ500 spectrometer (500 MHz). Highꢀresolution
mass spectra were measured on a Bruker micrOTOF II instruꢀ
ment.
Reactants. Pyridine (99%+), ꢀpicoline (3ꢀmethylpyridine,
99%), 3,5ꢀlutidine (3,5ꢀdimethylpyridine, 98%+), 1ꢀchloroꢀ2ꢀ
nitroꢀ4ꢀ(trifluoromethyl)benzene (98%), 1ꢀchloroꢀ4ꢀcyanoꢀ2ꢀ
nitrobenzene (97%), methyl 4ꢀchloroꢀ3ꢀnitrobenzoate (97%),
ethyl 4ꢀchloroꢀ3ꢀnitrobenzoate (98%), and 4ꢀchloroꢀ3ꢀnitrobenzoꢀ
yl chloride (98%) were Acros chemicals. Phenyl 4ꢀchloroꢀ3ꢀ
nitrobenzoate was prepared by a roomꢀtemperature reaction of
4ꢀchloroꢀ3ꢀnitrobenzoyl chloride with phenol according to
a standard procedure.26
Experimental
Synthesis of salts 1a—h (general procedure). A threeꢀnecked
flask was charged with 1ꢀchloroꢀ2ꢀnitroꢀ4ꢀRꢀbenzene (10 mL,
0.06—0.07 mol) and pyridine (or ꢀpicoline, or 3,5ꢀlutidine)
(39—45 mL, 0.48—0.56 mol). The reaction mixture was heated
Instruments. A B5ꢀ71/1m power supply was used for elecꢀ
trolysis. Cyclic voltammograms were recorded on a computerꢀ

378
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
Sokolov et al.
at 90 C for 4—6 h. On cooling, the precipitate that formed was
filtered off by suction, washed with acetone, and dried to give
Nꢀ(2ꢀnitroꢀ4ꢀRꢀphenyl)pyridinium salts.
115.95, 116.66, 130.31, 131.24, 137.82, 139.57, 139.84, 141.74,
142.60, 150.03.
3ꢀMethylꢀ1ꢀ[2ꢀnitroꢀ4ꢀ(trifluoromethyl)phenyl]pyridinium
chloride (1h). Yield 92%, m.p. 255—257 C. HRMS, found: m/z
283.0678 [M]+; C13H10F3N2O2+; calculated: M = 283.0689.
1H NMR, : 2.62 (s, 3 H, Me); 8.08 (d, 1 H, H(6´), J = 9.0 Hz); 8.18
(t, 1 H, H(5), J = 8.0 Hz); 8.42 (dd, 1 H, H(5´), J = 1.5 Hz,
J = 8.0 Hz); 8.69 (d, 1 H, H(4), J = 9.0 Hz); 8.90 (m, 2 H, H(2),
H(3´)); 8.95 (s, 1 H, H(6)). 13C NMR, : 17.77, 124.87, 127.59,
130.33, 133.23, 140.46, 142.59, 144.67, 149.36.
Electrolysis procedure. Electrolysis was carried out in a vigꢀ
orously stirred cylindrical cell. For typical experiments, a lead
plate (S = 50 cm2) was used as a cathode; the current strength
was 0.2 A. The amount of salt 1 was 1 g (3—4.5 mmol), the
volume of the catholyte was 80 mL, T = 40—45 C. In a divided
cell, the cathodic and anodic compartments were separated by
an ionꢀexchange membrane; 15% H2SO4 was employed as an
anolyte. Platinum metal served as an anode. The electrolysis was
continued until the starting salt was any longer detected in the
catholyte: usually after 4 and 5—5.5 F mol–1 of electricity had
been passed (for the divided and undivided cells, respectively).
After the electrolysis was completed (polarographic data), the
catholyte or the whole electrolyte (for the undivided cell only)
was alkalified with aqueous ammonia to a neutral reaction. The
product was extracted three times with hot chloroform. The exꢀ
tract was concentrated under reduced pressure to give the correꢀ
sponding products 2.
1ꢀ[2ꢀNitroꢀ4ꢀ(trifluoromethyl)phenyl]pyridinium chloride
(1a). Yield 98%, m.p. 260—262 C. HRMS, found: m/z 269.0543
[M]+; C12H8F3N2O2+; calculated: M = 269.0532. 1H NMR
(D2O), : 8.09 (d, 1 H, H(6´), J = 9.0 Hz); 8.31 (t, 2 H, H(3),
H(5), J = 8.0 Hz); 8.41 (dd, 1 H, H(5´), J = 1.5 Hz, J = 9.0 Hz);
8.87 (t, 1 H, H(4), J = 9.0 Hz); 8.90 (s, 1 H, H(3´)); 9.09 (d, 2 H,
H(2), H(6), J = 7.5 Hz). 13C NMR (D2O), : 124.85, 128.37,
130.38, 133.26, 137.35, 145.45, 148.88.
1ꢀ(4ꢀCyanoꢀ2ꢀnitrophenyl)pyridinium chloride (1b). Yield
99%, m.p. 263—265 C. HRMS, found: m/z 226.0626 [M]+;
C12H8N3O2+; calculated: M = 226.0611. 1H NMR, : 8.09 (d, 1 H,
H(6´), J = 9.0 Hz); 8.32 (t, 2 H, H(3), H(5), J = 8.0 Hz); 8.46
(dd, 1 H, H(5´), J = 1.5 Hz, J = 9.0 Hz); 8.88 (t, 1 H, H(4),
J = 9.0 Hz); 8.99 (s, 1 H, H(3´)); 9.10 (d, 2 H, H(2), H(6),
J = 7.5 Hz). 13C NMR, : 30.23, 115.86, 116.91, 128.41, 130.39,
131.23, 139.92, 145.36, 149.05, 215.33.
1ꢀ(4ꢀMethoxycarbonylꢀ2ꢀnitrophenyl)pyridinium chloride
(1c). Yield 87%, m.p. 188—191 C. HRMS, found: m/z 257.0715
[M]+; C13H11N2O4+; calculated: M = 259.0713. 1H NMR,
: 4.03 (s, 3 H, Me); 8.02 (d, 1 H, H(6´), J = 9.0 Hz); 8.31 (t, 2 H,
H(3), H(5), J = 8.0 Hz); 8.62 (dd, 1 H, H(5´), J = 1.5 Hz,
J = 9.0 Hz); 8.87 (t, 1 H, H(4), J = 8.0 Hz); 9.06 (s, 1 H, H(3´));
9.10 (d, 2 H, H(2), H(6), J = 7.5 Hz). 13C NMR, : 53.59,
127.80, 128.31, 129.78, 134.67, 136.52, 137.77, 142.38, 145.40,
148.73, 165.45.
1ꢀ(4ꢀEthoxycarbonylꢀ2ꢀnitrophenyl)pyridinium chloride (1d).
Yield 84%, m.p. 191—193 C. HRMS, found: m/z 273.0878 [M]+;
C14H13N2O4+; calculated: M = 273.0870. 1H NMR, : 1.40 (t, 3 H,
Me); 4.47 (q, 2 H, CH2); 8.01 (d, 1 H, H(6´), J = 9.0 Hz); 8.30
(t, 2 H, H(3), H(5), J = 8.0 Hz); 8.62 (dd, 1 H, H(5´), J = 1.5 Hz,
J = 9.0 Hz); 8.86 (t, 1 H, H(4), J = 8.0 Hz); 9.02 (s, 1 H, H(3´));
9.10 (d, 2 H, H(2), H(6), J = 8.0 Hz). 13C NMR, : 13.31,
63.52, 127.71, 128.31, 129.73, 135.04, 136.49, 137.72, 145.41,
148.73, 164.99.
1ꢀ(2ꢀNitroꢀ4ꢀphenoxycarbonylphenyl)pyridinium chloride
(1e). Yield 89%, m.p. 219—221 C. HRMS, found: m/z 321.0856
[M]+; C18H13N2O4+; calculated: M = 321.0870. 1H NMR,
: 7.31 (d, 2 H, H(2, H(6), J = 8.0 Hz); 7.39 (t, 1 H, H(4),
J = 8.0 Hz); 7.52 (t, 2 H, H(3, H(5, J = 9.0 Hz); 8.07 (d, 1 H,
H(6´), J = 9.0 Hz); 8.31 (t, 2 H, H(3), H(5), J = 8.0 Hz); 8.76
(dd, 1 H, H(5´), J = 1.5 Hz, J = 8.0 Hz); 8.87 (t, 1 H, H(4),
J = 8.0 Hz); 9.10 (d, 2 H, H(2), H(6), J = 8.0 Hz); 9.18 (s, 1 H,
H(3´)). 13C NMR, : 121.43, 127.13, 128.30, 128.41, 130.08,
130.13, 134.08, 137.12, 138.22, 142.49, 145.39, 148.89,
150.09, 163.93.
3,5ꢀDimethylꢀ1ꢀ[2ꢀnitroꢀ4ꢀ(trifluoromethyl)phenyl]pyridinꢀ
ium chloride (1f). Yield 93%, m.p. 260—261 C. HRMS, found:
m/z 297.0837 [M]+; C14H12F3N2O2+; calculated: M = 297.0845.
1H NMR, : 2.55 (s, 6 H, Me); 8.01 (d, 1 H, H(6´), J = 8.0 Hz);
8.38 (dd, 1 H, H(5´), J = 1.5 Hz, J = 9.0 Hz); 8.49 (s, 1 H, H(4));
8.73 (s, 2 H, H(2), H(6)); 8.87 (s, 1 H, H(3´)). 13C NMR,
: 17.57, 120.35, 123.97, 124.84, 127.58, 130.24, 133.14, 133.75,
134.21, 134.67, 135.13, 137.41, 139.52, 141.86, 142.69, 149.85.
1ꢀ(4ꢀCyanoꢀ2ꢀnitrophenyl)ꢀ3,5ꢀdimethylpyridinium chloride
(1g). Yield 97%, m.p. 219—220 C. HRMS, found: m/z 254.0918
[M]+; C14H12N3O2+; calculated: M = 254.0924. 1H NMR, :
2.55 (s, 6 H, Me); 8.01 (d, 1 H, H(6´), J = 8.5 Hz); 8.43 (dd,
1 H, H(5´), J = 1.5 Hz, J = 8.0 Hz); 8.50 (s, 1 H, H(4)); 8.73
(s, 2 H, H(2), H(6)); 8.95 (s, 1 H, H(3´)). 13C NMR, : 17.56,
7ꢀ(Trifluoromethyl)pyrido[1,2ꢀa]benzimidazole (2a), m.p.
233—235 C. HRMS, found: m/z 237.0637 [M + H]+;
C12H8F3N2+; calculated: M = 237.0634. 1H NMR (DMSOꢀd6),
: 7.09 (t, 1 H, H(2), J = 7.5 Hz); 7.66 (t, 1 H, H(3), J = 8.0 Hz);
7.68 (d, 1 H, H(9), J = 8.0 Hz); 7.75 (d, 1 H, H(4), J = 9.0 Hz);
8.16 (d, 1 H, H(6), J = 1.5 Hz); 8.53 (dd, 1 H, H(8), J = 2.0 Hz,
J = 8.0 Hz); 9.15 (d, 1 H, H(1), J = 7.5 Hz). 13C NMR (CDCl3),
: 111.33, 111.82, 117.31, 117.99, 125.57, 126.32, 128.20, 128.63,
130.23, 131.17, 142.75, 149.29.
Pyrido[1,2ꢀa]benzimidazoleꢀ7ꢀcarbonitrile (2b), m.p.
+
242—244 C. HRMS, found: m/z 194.0713 [M + H]+; C12H8N3
;
calculated: M = 194.0713. 1H NMR (DMSOꢀd6), : 7.10 (t, 1 H,
H(2), J = 6.5 Hz); 7.67 (t, 1 H, H(3), J = 7.0 Hz); 7.74 (d, 1 H,
H(4), J = 9.5 Hz); 7.75 (d, 1 H, H(9), J = 8.5 Hz); 8.35 (d, 1 H,
H(6), J = 1.5 Hz); 8.51 (dd, 1 H, H(8), J = 2.0 Hz, J = 8.5 Hz);
9.15 (d, 1 H, H(1), J = 6.5 Hz). 13C NMR (CDCl3), : 108.40,
111.88, 112.48, 116.08, 118.26, 122.51, 123.31, 126.17, 130.02,
132.02, 140.52, 148.02.
Methyl pyrido[1,2ꢀa]benzimidazoleꢀ7ꢀcarboxylate (2c), m.p.
226—228 C. HRMS, found: m/z 227.0816 [M + H]+;
C13H11N2O2+; calculated: M = 227.0815. 1H NMR (DMSOꢀd6),
: 3.9 (s, 3 H, Me); 7.03 (t, 1 H, H(2), J = 7.5 Hz); 7.55 (t, 1 H,
H(3), J = 8.5 Hz); 7.68 (d, 1 H, H(4), J = 8.0 Hz); 7.94 (dd, 1 H,
H(8), J = 1.5 Hz, J = 7.5 Hz); 8.37 (d, 1 H, H(9), J = 8.0 Hz);
8.39 (d, 1 H, H(6), J = 1.0 Hz); 9.08 (d, 1 H, H(1), J = 7.5 Hz).
13C NMR (CDCl3), : 52.36, 110.38, 111.25, 118.35, 122.3,
125.46, 127.82, 130.33, 131.51, 143.81, 149.64, 167.39.
Ethyl pyrido[1,2ꢀa]benzimidazoleꢀ7ꢀcarboxylate (2d), m.p.
179—182 C. HRMS, found: m/z 241.0974 [M + H]+;
C14H13N2O2+; calculated: M = 241.0972. 1H NMR (DMSOꢀd6),
: 1.38 (m, 3 H, Me); 4.35 (m, 2 H, CH2); 7.02 (t, 1 H, H(2),
J = 7.5 Hz); 7.57 (t, 1 H, H(3), J = 8.5 Hz); 7.67 (d, 1 H, H(4),
J = 8.5 Hz); 7.95 (dd, 1 H, H(8), J = 1.5 Hz, J = 8.0 Hz); 8.35
(d, 1 H, H(9), J = 8.0 Hz); 8.38 (d, 1 H, H(6), J = 1.0 Hz); 9.07
(d, 1 H, H(1), J = 8.0 Hz). 13C NMR (CDCl3), : 14.43, 61.27,

Electrosynthesis of pyridobenzimidazoles
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
379
110.45, 111.56, 118.05, 121.86, 122.54, 125.59, 128.44, 130.75,
131.22, 142.97, 149.18, 166.75.
Part of the State Target for Research Works at the P. G.
Demidov Yaroslavl State University), and the Russian
Foundation for Basic Research (Project No. 14ꢀ03ꢀ034A).
Phenyl pyrido[1,2ꢀa]benzimidazoleꢀ7ꢀcarboxylate (2e), m.p.
257—260 C. HRMS, found: m/z 289.0963 [M + H]+;
C18H13N2O2+; calculated: M = 289.0972. 1H NMR (DMSOꢀd6),
: 7.08 (t, 1 H, H(4´), J = 8 Hz); 7.34 (m, 3 H, H(2), H(2´),
H(6´)); 7.48 (t, 2 H, H(3), H(5´)); 7.64 (t, 1 H, H(3)); 7.75 (d, 1 H,
H(4), J = 9.0 Hz); 8.12 (d, 1 H, H(9), J = 8.5 Hz); 8.48 (dd, 1 H,
H(8), J = 2.0 Hz, J = 8.5 Hz); 8.60 (d, 1 H, H(6), J = 1.5 Hz); 9.12
(d, 1 H, H(1), J = 8.5 Hz). 13C NMR (CDCl3), : 52.36, 110.68,
111.53, 118.45, 121.87, 122.95, 125.58, 125.97, 127.24, 129.58,
130.65, 151.22, 165.48.
2,4ꢀDimethylꢀ7ꢀ(trifluoromethyl)pyrido[1,2ꢀa]benzimidazole
(2f), m.p. 125—128 C. HRMS, found: m/z 265.0952 [M + H]+;
C14H12F3N2+; calculated: M = 265.0947. 1H NMR (DMSOꢀd6),
: 2.32 (s, 3 H, Me); 2.53 (s, 3 H, Me); 7.29 (d, 1 H, H(3),
J = 0.5 Hz); 7.60 (dd, 1 H, H(8), J = 1.5 Hz, J = 8 Hz); 8.13
(d, 1 H, H(6), J = 1.5 Hz); 8.38 (d, 1 H, H(9), J = 8.5 Hz); 8.77
(d, 1 H, H(1), J = 0.5 Hz). 13C NMR (DMSOꢀd6), : 16.66,
17.48, 112.99, 116.34, 120.46, 121.96, 125.23, 125.65, 125.92,
131.08, 132.37, 142.99, 149.08.
References
1. C. EnguehardꢀGueiffier, A. Gueiffier, MiniꢀRev. Med.
Chem., 2007, 7, 888; Y. Bansal, O. Silakari, Bioorg. Med.
Chem., 2012, 20, 6208.
2. S. M. Barolo, Y. Wang, R. A. Rossi, G. D. Cuny, Tetraꢀ
hedron, 2013, 69, 5487; K.ꢀS. Masters, T. R. M. Rauws,
A. K. Yadav, W. A. Herrebout, B. van der Veken, B. U. W.
Maes, Chem. Eur. J., 2011, 17, 6315.
3. G. A. Ryzvanovich, R. S. Begunov, O. A. Rachinskaya, O. V.
Muravenko, A. A. Sokolov, Pharm. Chem. J. (Engl. Transl.),
2011, 3, 141 [Khim.ꢀFarm. Zh., 2011, 3, 13]; O. A. Rachinꢀ
skaya, K. V. Popov, G. A. Ryzvanovich, N. L. Bol´sheva,
R. S. Begunov, O. Yu. Yurkevich, A. V. Zelenin, O. V. Muꢀ
ravenko, Russ. J. Genetics (Engl. Transl.), 2012, 10, 1055
[Genetika, 2012, 10, 1228].
2,4ꢀDimethylpyrido[1,2ꢀa]benzimidazoleꢀ7ꢀcarbonitrile (2g),
m.p. 216—219 C. HRMS, found: m/z 222.1037 [M + H]+;
C14H12N3+; calculated: M = 222.1026. 1H NMR (DMSOꢀd6),
: 2.32 (s, 3 H, Me); 2.53 (s, 3 H, Me); 7.32 (d, 1 H, H(3),
J = 0.5 Hz); 7.64 (dd, 1 H, H(8), J = 1.5 Hz, J = 8 Hz); 8.24
(d, 1 H, H(6), J = 1.5 Hz); 8.38 (d, 1 H, H(9), J = 8.5 Hz); 8.77
(d, 1 H, H(1), J = 0.5 Hz). 13C NMR (DMSOꢀd6), : 16.59,
17.48, 106.85, 113.27, 119.68, 120.70, 121.87, 122.84, 123.97,
125.93, 131.49, 132.60, 142.90, 149.15.
4. H. M. Refaat, Med. Chem. Res., 2012, 21, 1253.
5. E. F. Belanov, N. I. Bormotov, S. M. Charushin, O. N.
Chuparin, S. K. Kotovskaya, Z. M. Baskakova, O. A. Serova,
N. M. Perova, G. A. Zhumabaeva, Pharm. Chem. J.
(Engl. Transl.), 2005, 11, 574 [Khim.ꢀFarm. Zh., 2005, 11,
12]; S. M. Rida, S. A. ElꢀHawash, H. T. Fahmy, A. A. Hazꢀ
zaa, M. M. M. ElꢀMeligy, Arch. Pharm. Res., 2006, 29, 826.
6. H. H. Jardosh, C. B. Sangani, M. P. Patel, R. G. Patel,
Chinese Chem. Lett., 2013, 24, 123.
4ꢀMethylꢀ7ꢀ(trifluoromethyl)pyrido[1,2ꢀa]benzimidazole (2h)
and 2ꢀmethylꢀ7ꢀ(trifluoromethyl)pyrido[1,2ꢀa]benzimidazole (2i),
a mixture of isomers. Isomer 2h. H NMR (DMSOꢀd6), : 2.36
7. H. Takeshita, J. Watanabe, Y. Kimura, K. Kawakami,
H. Takahashi, M. Takemura, A. Kitamura, K. Someya, R. Naꢀ
kajima, Bioorg. Med. Chem. Lett., 2010, 20, 3893.
1
(s, 3 H, Me); 6.12 (d, 1 H, H(4), J = 8.5 Hz)); 7.06 (d, 1 H, H(9),
J = 9.0 Hz); 7.50 (dd, 1 H, H(8), J = 1.5 Hz, J = 9.0 Hz); 8.12
(s, 1 H, H(6)); 8.44 (d, 1 H, H(3), J = 8.5 Hz); 8.96 (s, 1 H, H(1)).
Isomer 2i. 1H NMR (DMSOꢀd6), : 2.55 (s, 3 H, Me); 6.96 (t, 1 H,
H(2), J = 8.0 Hz); 7.42 (d, 1 H, H(9), J = 9.0 Hz); 7.62 (dd, 1 H,
H(8), J = 1.5 Hz, J = 9.0 Hz); 8.16 (s, 1 H, H(6)); 8.48 (d, 1 H,
H(3), J = 8.5 Hz); 9.00 (d, 1 H, H(1), J = 8.0 Hz).
Procedure for CV measurements. Voltammograms were reꢀ
corded in a temperatureꢀcontrolled (25 0.1 C) threeꢀelectrode
system composed of a working electrode (glassy carbon disk,
d = 1.70 mm), an auxiliary electrode (platinum wire), and
a reference electrode (a saturated calomel electrode connected to
a solution under study through a bridge with a porous ceramic
membrane filled with a supporting electrolyte (0.1 M Bu4NClO4
in MeCN or an appropriate alcoholic buffer). The solutions were
deaerated by bubbling highꢀpurity argon.
8. H. Yang, Y.ꢀQ. Ge, J. Jia, J.ꢀW. Wang, J. Lumin., 2011,
131, 749.
9. A. B. Reitz, A. D. Jordan, P. J. Sanfilippo, A. Vavouyiosꢀ
Smith, US Patent 5817668, Patented October 6, 1998.
10. V. Rustagi, R. Tiwari, A. K. Verma, Eur. J. Org. Chem., 2012,
24, 4590; L. M. Potikha, A. R. Turelik, A. V. Shelepyuk,
V. A. Kovtunenko, Chem. Heterocycl. Compd. (Engl. Transl.),
2012, 48, 343 [Khim. Geterotsikl. Soedin., 2012, 364]; K. Poꢀ
demska, R. Podsiadly, A. M. Szymczak, J. Sokolowska, Dyes
Pigments, 2012, 94, 113; W.ꢀJ. Zhao, Y.ꢀF. Xie, Y.ꢀQ. Ge,
W.ꢀR. Xu, G.ꢀL. Zhao, J.ꢀW. Wang, Heterocycles, 2013, 87,
1121; J. Peng, G. Shang, C. Chen, Z. Miao, B. Li, J. Org.
Chem., 2013, 78, 1242; M. Sun, H. Wu, J. Zheng, W. Bao,
Adv. Synth. Catal., 2012, 354, 835; Y. Ano, M. Tobisu,
N. Chatani, Synlett, 2012, 23, 2763; K. Liubchak, K. Nazaꢀ
renko, A. Tolmachev, Tetrahedron, 2012, 68, 2993; J.ꢀR.
Huang, L. Dong, B. Han, C. Peng, Y.ꢀC. Chen, Chem. Eur.
J., 2012, 18, 8896; V. P. Reddy, T. Iwasaki, N. Kambe, Org.
Biomol. Chem., 2013, 11, 2249; M. Hranjec, E. Horak,
M. Tireli, G. Pavlovic, G. KarminskiꢀZamola, Dyes Pigꢀ
ments, 2012, 95, 644; X. Chen, H. Yang, Y. Ge, L. Feng,
J. Jia, J. Wang, Luminescence, 2012, 27, 382.
Quantum chemical calculations of aromatic compounds were
performed with the Gaussianꢀ03 program package.27
This work was financially supported by the Presidium
of the Russian Academy of Sciences (Program "Developꢀ
ment of Methods for the Synthesis of Chemical Comꢀ
pounds and the Design of Novel Materials" as Part of the
Program "Development of the Methodology of Organic
Synthesis and Creation of Compounds with Valuable Apꢀ
plied Properties", the Ministry of Education and Science
of the Russian Federation (Project No. 178 in the Basic
11. V. N. S. Ramesh Babu, A. N. Babu, V. Anand, P. Hanumanꢀ
thu, Synth. Commun., 1998, 28, 4439; K. Panda, J. R. Suresh,
H. Ila, H. Junjappa, J. Org. Chem., 2003, 68, 3498; Yu. M. Voꢀ
lovenko, V. V. Ivanov, Chem. Heterocycl. Compd. (Engl. Transl.),
1997, 33, 1124 [Khim. Geterotsikl. Soedin., 1997, 1280];
K. M. Dawood, Z. E. Kandeel, A. M. Farag, J. Chem. Res. (S),

380
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 2, February, 2014
Sokolov et al.
1998, 208; K. M. Dawood, A. M. Farag, Z. E. Kandeel,
J. Chem. Res. (S), 1999, 88.
160, G3053; B. V. Lyalin, V. A. Petrosyan, Russ. J. Electroꢀ
chem. (Engl. Transl.), 2011, 47, 1236; H. Salehzadeh,
M. Rafiee, D. Nematollahi, H. Beiginejad, Curr. Org. Chem.,
2013, 17, 848.
12. N. S. Prostakov, A. V. Varlamov, I. V. Shendrik, B. N. Anisiꢀ
mov, A. P. Krapivko, S. LavaniꢀEdogiaverie, A. A. Foꢀ
michev, Chem. Heterocycl. Compd. (Engl. Transl.), 1983, 19,
1102 [Khim. Geterotsikl. Soedin., 1983, 1384]; P. K. Mahata,
C. Venkatesh, U. K. Syam Kumar, H. Ila, H. Junjappa,
J. Org. Chem., 2003, 68, 3966; C. Venkatesh, G. S. M.
Sundaram, H. Ila, H. Junjappa, J. Org. Chem., 2006, 71,
1280; J. Alen, K. Robeyns, W. M. De Borggraeve, L. van
Meervelt, F. Compernolle, Tetrahedron, 2008, 64, 8128;
A. N. Frolov, Russ. J. Org. Chem. (Engl. Transl.), 1994, 30
[Zh. Org. Khim., 1994, 30, 1059]; A. N. Frolov, Russ. J.
Gen. Chem. (Engl. Transl.), 2004, 74, 1949 [Zh. Obshch.
Khim., 2004, 74, 2062]; A. N. Frolov, Russ. J. Org. Chem.
(Engl. Transl.), 2006, 42, 883 [Zh. Org. Khim., 2006, 42, 903].
13. S. G. Il´in, M. V. Reshetnyak, A. I. Yanovskii, Yu. T. Struchꢀ
kov, O. Yu. Slabko, G. A. Verbitskii, V. A. Kaminskii, Chem.
Heterocycl. Compd. (Engl. Transl.), 2001, 37, 109 [Khim.
Geterotsikl. Soedin., 2001, 114]; P. G. Houghton, D. F. Pipe,
C. W. Rees, J. Chem. Soc., Perkin Trans. 1, 1985, 1471;
M. Alajarin, A. Vidal, F. Tovar, C. Conesa, Tetrahedron
Lett., 1999, 40, 6127.
14. R. S. Begunov, G. A. Ryzvanovich, S. I. Firgang, Russ. J.
Org. Chem. (Engl. Transl.), 2004, 40, 1694 [Zh. Org. Khim.,
2004, 40, 1740]; R. S. Begunov, A. A. Sokolov, T. V. Sheꢀ
bunina, Russ. J. Org. Chem. (Engl. Transl.), 2013, 5, 773
[Zh. Org. Khim., 2013, 5, 789].
15. L. Bodo, Z. Chem., 1970, 10, 463; J. Epsztajn, E. Lunt, A. R.
Katritzky, Tetrahedron, 1970, 26, 1665.
16. L. F. Tietze, T. Eicher, Reaktionen und Synthesen im orgaꢀ
nischꢀchemischen Praktikum und Forschungslaboratorium,
Georg Thieme Verlag, Stuttgart—New York, 1991.
20. J. Grimshaw, Electrochemical Reactions and Mechanisms in
Organic Chemistry, Elsevier, Amsterdam—Lausanne—New
York—Oxford—Shanon—Singapore—Tokyo, 2000, p. 371.
21. V. P. Kadysh, Ya. P. Stradyn´, E. S. Lavrinovich, P. P. Zarin,
Chem. Heterocycl. Compd. (Engl. Transl.), 1975, 588 [Khim.
Geterotsikl. Soedin., 1975, 675].
22. A. D. Becke, J. Chem. Phys., 1993, 98, 5648; C. Lee, W. Yang,
R. G. Parr, Phys. Rev. B, 1988, 37, 785.
23. S. Miertus, E. Scrocco, J. Tomasi, Chem. Phys., 1981, 55,
117; V. Barone, M. Cossi, J. Phys. Chem. A, 1998, 102, 1995.
24. V. B. Kazansky, V. N. Solkan, Phys. Chem. Chem. Phys.,
2003, 5, 31; V. N. Solkan, V. B. Kazansky, Kinet. Catal.
(Engl. Transl.), 2013, 54 [Kinet. Katal., 2013, 54, 297].
25. M. J. S. Dewar, R. C. Dougherty, The PMO Theory of Organꢀ
ic Chemistry, Plenum Press, New York, 1975; Chemical Reacꢀ
tivity and Reaction Paths, Ed. G. Klopman, J. Wiley and
Sons, New York—London—Sydney—Toronto, 1974.
26. Preparatyka organiczna, pod red. W. Polaczkowa, Panstwowe
Wydawnictwa Techniczne, Warszawa, 1954 (in Polish).
27. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.
Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskoꢀ
rz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
AlꢀLaham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonꢀ
zalez, J. A. Pople, Gaussian 03, Revision B.03, Gaussian,
Inc., Pittsburgh (PA), 2003.
17. A. A. Sokolov, M. A. Syroeshkin, R. S. Begunov, N. N.
Rusakova, V. P. Gultyai, Mendeleev Commun., 2012, 22, 312.
18. Organic Electrochemistry, 4th ed., Eds H. Lund, O. Hamꢀ
merich, Marcel Dekker, New York, 2001, 1393 pp.
19. M. N. Elinson, R. F. Nasybullin, G. I. Nikishin, Heteroat.
Chem., 2013, 24, 398; B. V. Lyalin, V. A. Petrosyan, Russ.
Chem. Bull. (Int. Ed.), 2013, 61, 1148 [Izv. Akad. Nauk, Ser.
Khim., 2013, 1043]; L. Fotouhi, K. Nikoofar, Tetrahedron
Lett., 2013, 54, 2903; M. N. Elinson, R. F. Nasybullin, G. I.
Nikishin, Electrocatalysis, 2013, 4, 56; H. Veisi, Z. Vafajoo,
B. Maleki, M. T. Maghsoodlou, Phosphorus, Sulfur, Silicon
Relat. Elem., 2013, 188, 672; H. L. Qj, D. S. Chen, J.ꢀS. Ye,
J.ꢀM. Huang, J. Org. Chem., 2013, 78, 7482; M. N. Elinson,
R. F. Nasybullin, G. I. Nikishin, J. Electrochem. Soc., 2013,
Received November 7, 2013;
in revised form December 9, 2013