A green electrochemical method for the synthesis of 2-(phenylthio)-1h-benzo[d]imidazole derivatives

Article abstract of DOI:10.1149/2.0241811jes

The electrochemical oxidation of 4,4’-biphenol (4BP) was studied by cyclic voltammetry in the absence and presence of 2-mercaptobenzimidazole (2MB) in aqueous/ethanol (50/50 v/v) mixture, to evaluate whether the electrogenerated 4,4’-diphenoquinone (4BP

Full text of DOI:10.1149/2.0241811jes

Journal of The Electrochemical Society, 165 (11) G133-G138 (2018)
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0013-4651/2018/165(11)/G133/6/$37.00 © The Electrochemical Society
A Green Electrochemical Method for the Synthesis of
2-(phenylthio)-1H-benzo[d]imidazole Derivatives
z
Mahsa Joudaki and Davood Nematollahi
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65178-38683, Iran
The electrochemical oxidation of 4,4’-biphenol (4BP) was studied by cyclic voltammetry in the absence and presence of 2-
mercaptobenzimidazole (2MB) in aqueous/ethanol (50/50 v/v) mixture, to evaluate whether the electrogenerated 4,4’-diphenoquinone
(4BPox) reacts with 2MB and determine the electrochemical oxidation pathway of 4BP in the presence of 2MB. The data show that
the reaction of 4BPox with 2MB can be accomplished in an undivided cell equipped with carbon anode and stainless steel cathode
leads to the formation of 3-((1H-benzo[d]imidazol-2-yl)thio)-[1,1’-biphenyl]-4,4’-diol. This method has led to the development of
a green, reagentless and facile galvanostatic method for the synthesis of 2-(phenylthio)-1H-benzo[d]imidazole derivatives.
© 2018 The Electrochemical Society. [DOI: 10.1149/2.0241811jes]
Manuscript submitted June 19, 2018; revised manuscript received July 31, 2018. Published August 9, 2018.
Thioethers are important materials that are used in organic, bioor-
ganic and medicinal chemistry.1 These compounds also can serve
as building blocks for the synthesis of different biologically impor-
tant sulfur compounds.2 The oxidation of thioethers leads to chi-
ral sulfoxides, which can be used in asymmetric syntheses.3–6 Aro-
matic thioethers are also known to exhibit different biological activi-
ties such as antioxidant and antibacterial.7 Various synthetic meth-
ods used for the synthesis of aromatic thioethers.8–24 Most com-
mon approaches involve the use of transition metals such as Rh,8,9
Pd,10,11 Co,12 Cu,13–15 Fe,16 etc.17–19 These methods have significant
drawbacks including the use of expensive reagents and toxic metal
catalysts, solvents and reagents. Other methods including iodine-
mediated synthesis,20–22 1,8-diazabicyclo[5,4,0]undec-7-enium ac-
etate ionic liquid.23 Although these methods do not have any heavy
metal pollution, however, still have the disadvantages such as tedious
work-up, safety problems, expensive reagents and toxic solvents.
A group of aromatic thioethers is catechol thioethers which are
known to exhibit some biological activities such as antioxidant and
antibacterial activities.7 Most of these compounds were synthesized
by the reaction of in situ generated o-benzoquinones with the corre-
sponding thiols. The generation of o-benzoquinones was performed
mostly by using laccase-catalyzed24 or electrochemical methods.25–32
Although both methods are suitable for the synthesis of catechol
thioethers, however, the laccase-catalyzed method faces the prob-
lem of expensive enzyme. These findings prompted us to synthe-
size new aromatic thioethers by the galvanostatic oxidation of 4,4’-
biphenol, 4-tert-butylcatechol and catechol in the presence of 2-
mercaptobenzimidazole. In this research, we have developed a facile
and green method for the synthesis of some new aromatic thioethers
in a water/ethanol solution using an ordinary carbon electrode and a
simple power supply.
phosphate salts, perchloric acid and ethanol were obtained from
Sigma-Aldrich. These chemicals were used without further purifica-
tion. The glassy carbon electrode was polished using alumina slurry
(3.0 μm–0.1 μm). Melting points were measured with an Electrother-
mal 9100 apparatus (Rochford, UK). IR spectra (KBr) were recorded
1
on Perkin–Elmer GX FT-IR spectrometer. H and 13C NMR spectra
were recorded on a Bruker Avance DPX-300 spectrometer at 300 MHz
and a Bruker Avance DRX-400 spectrometer at 400 MHz. The mass
spectra were recorded on MS Model: 5975C VL MSD with Tripe-Axis
Detector.
Synthesis of P1 (C19H14N2O2S).—A mixture of aqueous perchlo-
ric acid solution (40 ml, c = 0.1 M)/ethanol (40 ml) (50/50 v/v)
containing 4BP (1.6 mmol) and 2MB (1.6 mmol) was subjected to
electrolysis under constant current conditions (J = 0.35 mA/cm2) in an
undivided cell. Progress of the electrolysis was checked by TLC using
n-hexane/ethyl acetate (50:50) as eluent. The process was interrupted
during the electrolysis and the carbon anode was washed in acetone in
order to reactivate it. At the end of electrolysis, the solution neutralized
with a saturated sodium bicarbonate solution. The crude product was
separated by filtration and was purified by thin-layer chromatography
with n-hexane/ethyl acetate (50:50). The product was characterized
by its physical and spectroscopic data.
3-((1H-benzo[d]imidazol-2-yl)thio[1,1'-biphenyl]-4,4'-diol
Experimental
Isolated yield: 51%. M.p = 209–211◦C (Dec). 1H NMR (400 MHz,
DMSO-d6) δ (ppm): 6.79 (d, J = 8.8 Hz, 2H, aromatic), 6.99 (d, J =
8.4 Hz, 1H, aromatic), 7.11 (dd, J = 3.2 and 6.0 Hz, 2H, aromatic),
7.35 (d, J = 8.8 Hz, 2H, aromatic), 7.44 (dd, J = 3.2 and 6 Hz, 2H,
aromatic), 7.47 (d, J = 2.4 Hz, 1H, aromatic), 7.53 (d, J = 2.4 Hz,
1H, aromatic).
Apparatus and reagents.—Cyclic voltammetry was performed
using a Sama-500 potentiostat/galvanostat. The working electrode
used in the voltammetry experiments was a glassy carbon disc
(1.8 mm2 area) and platinum wire was used as counter electrode. The
working electrode potentials were measured versus Ag/AgCl (3M
KCl) (all electrodes from AZAR electrode). Macro-scale electrolysis
and controlled-potential coulometry were carried out with a three-
electrode system, using a Behpajooh C 2056 potentiostat equipped
with a digital coulometer. The working electrode used in macro-scale
electrolysis was an assembly of three ordinary soft carbon plates
(20 mm length, 10 mm width and 44 mm height), and a stainless steel
cylinder (25 cm2 area) constituted the counter electrode (Fig. 1).
The electrosynthesis were performed under constant-current con-
dition in an undivided cell. 4,4’-Biphenol (4BP), catechol (Cat),
4-tert-Butylcatechol (4TC) and 2-mercaptobenzimidazole (2MB),
13C NMR (100 MHz, DMSO-d6) δ (ppm): 114.2, 115.6, 116.5,
116.9, 121.4, 127.0, 128.0, 130.2, 131.0, 131.7, 139.8, 148.9, 156.5,
156.6. IR (KBr) ν (cm−1): 3386 (medium, N-H), 3239 (medium, O-
H), 1612 (medium, C=C), 1490 (medium, C=C), 1404, 1175, 817,
737, 430. MS (EI, 70 eV) m/z (relative intensity): 334 (M, 72), 317
(100), 301 (16), 218 (15), 150 (23), 77 (15).
Synthesis of P2 (C13H10N2O2S) and P3 (C17H18N2O2S).—A mix-
ture of aqueous phosphate buffer solution (55 ml, c = 0.2 M, pH =
2.0)/ethanol (25 ml) (70/30 v/v) .containing Cat (or 4TC) (1.6 mmol)
and 2MB (1.6 mmol) was subjected to electrolysis under constant
current conditions (J = 0.35 mA/cm2) in an undivided cell. Progress
zE-mail: nemat@basu.ac.ir
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Journal of The Electrochemical Society, 165 (11) G133-G138 (2018)
Figure 1. The position of the electrodes in the cell.
Figure 2. Cyclic voltammograms of 4BP (1.0 mM): (a) in the absence and
(b) in the presence of 2MB (1.0 mM) and, (c) cyclic voltammogram of 2MB
(1.0 mM) in the absence of 4BP, at a glassy carbon electrode, in aqueous
perchloric acid (c = 0.1 M)/ethanol mixture (50/50 v/v). Scan rate: 100 mV
of the electrolysis was checked by TLC using n-hexane/ethyl acetate
(50:50) as eluent. The process was interrupted during the electrolysis
and the carbon anode was washed in acetone in order to reactivate
it. At the end of electrolysis, the solution neutralized with a saturated
sodium bicarbonate solution. The crude product was separated by fil-
tration and was washed several times with cold water and dried. The
products were characterized by their physical and spectroscopic data.
s
−1. t = 25 1◦C.
appears at more positive potentials. In addition under these conditions
a new anodic peak AN appears at less positive potentials. From these
data it can be concluded that a chemical reaction follows the electron
transfer process.
The effects of the 2MB on the cyclic voltammogram 4BP disap-
pear when the potential scan rate significantly increases. Under these
conditions, anodic and cathodic peaks A2 and C2 disappear and the
peak current ratio (IpC1/IpA1) becomes about one. The data confirm
that peaks A2 and C2 belong to the product of the reaction of 2MB
with 4BPox. Another result that can be obtained from this figure is
that, the product is more difficult to oxidize than the starting material
(4BP) which shows that the 2MB added to the 4BP plays the role
of an electron withdrawing group. A literature survey shows that, the
oxidation potential of catechol thioethers are lower than those of par-
ent catechols, would cause greater oxidative damage than their parent
catechols.34 Accordingly, the adduct formed from the reaction of 2MB
with 4BPox mediated less oxidative damage than 4BP.
In Fig. 2, curve c is the cyclic voltammogram of 2MB in the
absence of 4BP. It exhibited anodic (AN) and cathodic (CN) peaks at
0.51 and 0.31 V vs. Ag/AgCl, respectively. The anodic peak, AN, can
be attributed to the oxidative dimerization of 2MB via the formation
of sulfur-sulfur bond.35 Comparison of voltammogram 2MB (curve c)
with voltammogram 4BP in the presence of 2MB (curve b) confirms
that the anodic peak observed at about 0.51 V is due to the anodic
oxidation of 2MB.
The constant current electrolysis and voltammetry of the elec-
trolyzed solution can give us more information. The constant current
electrolysis of 4BP (0.25 mmol) in the presence of 2MB (0.25 mmol)
was performed in aqueous perchloric acid solution (40 ml, c =
0.1 M)/ethanol (40 ml) (50/50 v/v) at J = 0.35 mA/cm2. The electrol-
ysis progress was monitored using linear sweep voltammetry (Fig. 3).
As shown, during the electrolysis progress, the current of the anodic
peaks AN and A1 decrease and finally disappear after consumption of
about 2e− per molecule of 4BP. It should be noted that the number of
transferred electrons (n = 2) has also been confirmed using controlled
potential coulometry.
The electrochemical results and spectroscopic data of the isolated
electrolysis product (for example, the molecular mass of 334) all point
out to product P1 which might have been formed according to the
pathway shown in Scheme 1. Accordingly, the first step would be the
electrochemical generation of 4BPox. The second step, would be the
reaction of 4BPox with 2MB and the formation of intermediate, BPMB
and the third step, is the aromatization of BPMB and the formation of
final product P1. The greater oxidation of P1 along with its insolubility
4-((1H-benzo[d]imidazol-2-yl)thio)benzene-1,2-
3-((1H-benzo[d]imidazol-2-yl)thio)-5-(tert-
butyl)benzene-1,2-diol
diol
Characteristics of P2.—Isolated yield: 51%. M.p = 208–210◦C
(Dec). 1H NMR (300 MHz, DMSO-d6) δ (ppm): 6.83 (d, J = 8.2 Hz,
1H, aromatic), 6.93 (dd, J = 2.1 and 8.1 Hz, 1H, aromatic), 6.99 (d,
J = 2.1 Hz, 1H, aromatic), 7.13 (dd, J = 3 and 6 Hz, 2H, aromatic),
7.45 (dd, J = 3 and 6 Hz, 2H, aromatic), 9.42 (2H, OH), 12.3 (broad,
1H, NH). 13C NMR (75 MHz, DMSO-d6) δ (ppm): 114.8, 117.1,
117.9, 121.4, 122.2, 125.9, 140.0, 146.6, 147.5, 150.1. IR (KBr) ν
(cm−1): 3265 (medium, O-H), 1596 (medium, C=C), 1511 (medium,
C=C), 1404, 1271, 1121, 989, 809,588. MS (EI, 70 eV) m/z (relative
intensity): 258 (M, 100), 257 (100), 197 (9), 150 (21), 106 (15), 63 (9).
Characteristics of P3 .—Isolated yield: 71%. M.p = 187–189◦C
(Dec). 1H NMR (300 MHz, DMSO-d6) δ (ppm): 1.19 (s, 9H, aliphatic),
6.79 (d, J = 2.1 Hz, 1H, aromatic), 6.90 (d, J = 2.1 Hz, 1H, aromatic),
7.15 (dd, J = 3 and 5.8 Hz, 2H, aromatic), 7.45 (s, 2H, aromatic), 9.44
(2H, OH), 12.49 (broad band, 1H, NH). 13C NMR (75 MHz, DMSO-
d6) δ (ppm): 31.7, 34.2, 114.5, 115.9, 120.5, 122.2, 142.5, 143.9,
146.2, 149. IR (KBr) ν (cm−1): 3437 (medium, N-H), 3301(medium,
O-H), 2962 (medium, C-H), 1570 (medium, C=C), 1483 (medium,
C=C), 1412, 1285, 1233, 960, 743, 615. MS (EI, 70 eV) m/z (relative
intensity): 314 (M, 48), 299 (84), 150 (100), 91 (64), 41 (67).
Results and Discussion
Electrochemical study of 4BP .—The cyclic voltammogram of
4BP in aqueous perchloric acid (c = 0.1 M)/ethanol mixture (50/50
v/v) is shown in Fig. 2a. It shows an anodic peak A1 in the positive-
going scan and a corresponding cathodic peak C1 in the negative-
going scan. These peaks are assigned to the oxidation of 4BP
to 4,4’-diphenoquinone (4BPox) and reduction of 4BPox to 4BP,
respectively.33 Comparison of Fig. 2a with that of 4BP in the presence
of 2MB (Fig. 2b) shows that the cathodic peak C1 decreases signif-
icantly and a new anodic peak A2 and its cathodic counterpart (C2)
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Journal of The Electrochemical Society, 165 (11) G133-G138 (2018)
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Based on Scheme 1, the peak A2 observed in cyclic voltammetry
of 4BP in the presence of 2MB (Fig. 2, curve b) is related to the
oxidation of P1 to P1ox. Clearly, peak C2 belongs to the reduction of
P1ox to P1. Figure 4 compares the cyclic voltammograms of 4BP and
P1. This figure clearly shows that the half wave potential (E1/2) of P1
is about 40 mV more positive than that of 4BP.
4BPox can react with 2MB to form two types of products, ortho-Is
and meta-Is (Fig. 5). The calculated steric energy for ortho-Is and
meta-Is using MM2 program show that ortho-Is is significantly more
stable than meta-Is. Fig. 5 shows the structure of ortho-Is and meta-Is
after minimization of energy. The calculated steric energy for ortho-Is
and meta-Is are 6.88 and 15.34, respectively. In conclusion, we think
that ortho-Is (P1) is more probable structure in electrooxidation of
4BP in the presence of 2MB.
Figure 3. Linear sweep voltammograms of 4BP (0.25 mmol) in the presence
of 2MB (0.25 mmol), at a glassy carbon electrode in aqueous perchloric acid
(c = 0.1 M)/ethanol mixture (50/50 v/v) during constant current electrolysis at
0.35 mA/cm2. (a) At the beginning of electrolysis, (b and c) during electrolysis
and (d) at the end of electrolysis. Scan rate: 100 mV s−1. t = 25 1◦C.
Constant current synthesis.—Here, galvanostatic synthesis of P2
was studied and parameters including current density and charge
passed were optimized to obtain the maximum yield of product. In
this way, the current density changed from 0.12 to 1.18 mA/cm2,
whereas the other parameters such as charge passed (100 C), catechol
amount (0.25 mmol), 2MB amount (0.25 mmol), electrode surface
(85 cm2), solvent (aqueous phosphate buffer solution, 55 ml, c =
0.2 M, pH = 2.0/ethanol, 25 ml mixture) and temperature (298 K),
are kept constant. The data show that the best yield was obtained at
J = 0.35 mA/cm2 (Fig. 6). The oxidation of solvent and/or further
oxidation of product at higher current densities would decrease the
in the electrolysis medium, has prevented its further oxidation during
preparative electrolysis. The yield of P1 in less acidic, neutral and
basic solutions becomes less due to the participation of 4BPox in the
pH dependent dimerization36,37 and/or hydroxylation38,39 reactions.
The rate of these reactions increases with increasing pH.36–39
Scheme 1. The oxidation pathway of 4BP in the presence
of 2MB.
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Journal of The Electrochemical Society, 165 (11) G133-G138 (2018)
Figure 4. (a) Cyclic voltammogram of 4BP and (b) cyclic voltammogram of
P1 (saturated solution) at a glassy carbon electrode, in aqueous perchloric acid
(c = 0.1 M)/ethanol mixture (50/50 v/v). Scan rate: 100 mV s−1. t = 25 1◦C.
For better comparison with curve a, the currents of curve b, are multiplied
by 1.7.
Figure 6. The effect of current density on the yield of P2. Charge passed,
100 C.
Synthesis using catechol and 4-tert-butylcatechol.—In this study
we obtained the same results in the oxidation pathway of catechol and
4-tert-butylcatechol in the presence of 2MB, so their related data is
not included in this paper. The only issue that needs to be addressed
is the structures of P2 and P3. The product P2 is obtained from
the oxidation of catechol (Cat) in the presence of 2MB. The ortho-
benzoquinone (Catox) formed from oxidation of Cat can be reacted
by 2MB to form two types of products, 1,2,3-Is and 1,2,4-Is (P2)
product yield. On the other hand, the decrease of the yield at lower
current densities can be due to the oxidation 2MB.
The effect of the amount of electricity passed in the range of
1.0 F/mol to 4.0 F/mol on the product yield was studied at J =
0.35 mA/cm2, while other parameters were kept constant (Fig. 7).
This figure shows that, the product yield decreases with increasing
charge passed from theoretical amount (2.0 F/mol). It seems that, the
further oxidation of the product can be significant factor in reducing
the product yield.
1
(Scheme 2). The H NMR of the isolated product shows a doublet
peak at 6.99 ppm with coupling constant, 2.1 Hz and integration of
one proton, which indicates that this proton is at the meta position of
Figure 5. The possible structures in electrooxi-
dation of 4BP in the presence of 2MB and the
results of MM2 calculation for ortho and meta
isomers.
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Journal of The Electrochemical Society, 165 (11) G133-G138 (2018)
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ric Michael acceptor that can be reacted by 2MB to form three types
of products, 4,6-Is (P3), 4,5-Is and 3,4-Is (Fig. 8). The 1H NMR of the
isolated product shows two doublet peaks at 6.79 and 6.90 ppm with
coupling constants, 2.1 Hz and integration of one proton, which dis-
plays that two protons are meta-coupled to each other.40 Accordingly,
these data rejects the formation of the compounds 4,5-Is and 3,4-Is.
On the other hand, the calculated steric energy using MM2 program,
was found to vary in the order 3,4-Is > 4,5-Is > 4,6-Is (Fig. 8), which
shows that the formation of 4,6-Is (P3) is presumably energetically
favorable compared to other isomers (4,5-Is and 3,4-Is).
Conclusions
Our voltammetric and coulometric data along with the spectro-
scopic results of the products (P1-P3) show that the pathway of elec-
trochemical oxidation of the studied diols in the presence of 2MB is an
EC mechanism (E represents an electron transfer and “C” represents a
homogeneous chemical reaction) (Scheme 1). Here, we have synthe-
sized some new diol derivatives containing 2-mercaptobenzimidazole
group under galvanostatic conditions, in a simple cell, using a green
strategy. In addition, in this work, we optimize the affecting fac-
tors such as, current density and charge passed for the synthesis of
products, P1-P3 and discuses on the structure of products. In con-
clusion, this environment friendly strategy have many advantages
such as one-pot synthesis of product, catalyst free condition and safe
solvent.
Figure 7. The effect of charge passed on the yield of P2. Current density,
0.35 mA/cm2.
other aromatic protons.40 Accordingly, this result rejects the formation
of the compound 1,2,3-Is.
The product P3 is obtained from the oxidation of 4-tert-
butylcatechol (4TC) in the presence of 2MB. The electrochemi-
cally generated 4-tert-butyl-o-benzoquinone (4TCox) is an asymmet-
Figure 8. The possible structures in electrooxidation of 4TC in the presence of 2MB and the results of MM2 calculation for 4,6-Is, 4,5-Is and 3,4-Is isomers.
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Journal of The Electrochemical Society, 165 (11) G133-G138 (2018)
Scheme 2. The possible structures in electrooxidation of Cat in the presence of 2MB.
Acknowledgment
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We acknowledge the Bu-Ali Sina University Research Counciland
the Center of Excellence in Development of Environmentally Friendly
Methods for Chemical Synthesis (CEDEFMCS) for their support of
this work.
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ORCID
Davood Nematollahi https://orcid.org/0000-0001-9638-224X
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