A domino electro-oxidative synthesis of 3,3′-bis(indolyl)methane nanoparticles

Article abstract of DOI:10.1007/s00706-015-1496-6

An effective route for electro-oxidative synthesis of 3,3′-bis(indolyl)methane nanoparticles has been described via a domino multi-component reaction between various alcohols and indole derivatives in an undivided cell under constant potential conditions in CH₃CN at room temperature. The mechanistic aspect of this electro-oxidative condensation has also been studied by UV-Vis spectra and cyclic voltammograms. The nanostructure of the products has been confirmed by scanning electron microscopy. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-015-1496-6

Monatsh Chem
DOI 10.1007/s00706-015-1496-6
ORIGINAL PAPER
A domino electro-oxidative synthesis of 3,30-bis(indolyl)methane
nanoparticles
Kobra Nikoofar1 Khadijeh Ghanbari1
•
Received: 16 January 2015 / Accepted: 11 May 2015
Ó Springer-Verlag Wien 2015
Abstract An effective route for electro-oxidative synthe-
sis of 3,30-bis(indolyl)methane nanoparticles has been
described via a domino multi-component reaction between
various alcohols and indole derivatives in an undivided cell
under constant potential conditions in CH3CN at room
temperature. The mechanistic aspect of this electro-oxida-
tive condensation has also been studied by UV–Vis spectra
and cyclic voltammograms. The nanostructure of the prod-
ucts has been confirmed by scanning electron microscopy.
Introduction
Indoleanditsderivativeshaveattractedsomuchinterestinthe
recent century owing to their extensive biological activities
[1]. Bis(indolyl)methanes (BIMs) are one of the most
important groups of indole derivatives that are found in cru-
ciferous plants and marine sources [2]. These compounds
have shown different pharmaceutical activities such as anti-
cancer [3], antihyperglycemic, antiviral, and antimicrobial
[4], and antioxidant [5] properties. They are also known as a
promoter of estrogen metabolism [6] and induce apoptosis in
human cancer cells [7]. Therefore, the synthesis of these
moieties has received much attention during the last years.
A simple and direct method for the synthesis of BIMs is
the condensation of indole with the carbonyl groups. In this
manner, the reaction of indole with aldehydes or ketones
produces azafulvenium salts that react further with a sec-
ond indole molecule to form bis(indolyl)methanes [8]. On
the basis of this general protocol, various Bronsted acids
[9, 10], protic acids [11–13], Lewis acids [14–19],
heterogeneous acidic catalysts [20–23], ionic liquids [24–
26], and N-haloorganocatalysts [27, 28] have been applied
for this synthesis. Although a vast range of procedures has
been utilized for BIM preparation via the condensation of
indoles with carbonyls, reports of oxidative condensation
of alcohols with indoles are rare [29, 30].
Graphical abstract
Carbon anode
-
+
-2e
,-2H
OH
O
N
H
N
H
N
-
-
N
H
+1e
Carbon cathod
Keywords Alcohols Á Electrosynthesis Á
Electro-oxidation Á Indole Á Condensation
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-015-1496-6) contains supplementary
material, which is available to authorized users.
In spite of the applicability of the reported protocols,
most of them suffer from one or more drawbacks including
long reaction times, low yields of products, harsh reaction
conditions, and use of expensive and/or toxic catalysts and
solvents. Therefore, there is still a strong demand for a
mild, clean, and highly efficient procedure for one-pot
multi-component transformations for BIM synthesis.
Electrochemical organo-synthetic methods have
received significant attention because of their benefit to the
& Kobra Nikoofar
kobranikoofar@yahoo.com
Khadijeh Ghanbari
kh_ghanb@yahoo.com
1
Chemistry Department, School of Physics and Chemistry,
Alzahra University, Vanak, Tehran, Iran
123

K. Nikoofar, K. Ghanbari
Scheme 1
R1
Constant potential
CH3CN, LiClO4
R2
1
+
R CH2OH
R2 R2
N
H
N
H
N
H
3a 3k
-
1a 1i
-
2a, 2b
environment. In these procedures, cleanly generated elec-
tricity acts as a ‘‘green’’ catalyst. Among electrochemical
synthesis of various organic compounds reported in the
literature [31–36], few are dependent on electro-catalytic
multi-component condensations [37–39].
Nanosized organic compounds have been prepared
through different methods in which organic compounds are
first synthesized and then can be transformed into nano-
sized particles using modern high technology such as
ultrahigh pressure rapid expansion of supercritical solu-
tions (RESS) system [40] and hydrocortisone using
supercritical antisolvent with enhanced mass transfer [41].
Direct synthesis of nanosized organic compounds is a
rarely used method.
On the basis of our research activity about indole
derivatives [42–47], we herein report a mild and efficient
procedure for the one-pot domino electro-oxidative con-
densation of alcohols 1a–1i and indoles 2a, 2b via
controlled-potential coulometry in an undivided cell, in the
presence of LiClO4 as electrolyte in CH3CN at room
temperature to yield the nanoparticles of the corresponding
3,30-di(indolyl)methanes 3a–3k (Scheme 1).
Fig. 1 Cyclic voltammogram of 1 mmol of 4-chlorobenzyl alcohol
(1e) at the glassy carbon electrode, in CH3CN (40 cm3) containing
0.05 g LiClO4 at the scan rate of 100 mV s-1
consumption reaches about 2e- per molecule of 1e (Fig. 2,
inset). This number of electrons confirms the oxidation of
4-chlorobenzyl alcohol (1e) to 4-chlorobenzaldehyde.
In the next step, electro-condensation was followed by the
addition of indoles 2a, 2b to the reaction cell alternatively.
Voltammetric behaviors of 1 mmol 4-chlorobenzaldehyde
(curve 3a), 2 mmol indole (2a) (curve 3b), and 1 mmol
4-chlorobenzyl aldehyde in the presence of 2 mmol indole
(2a) (curve 3c), are shown in Fig. 3. The cyclic voltammo-
gram observed for indole indicated one anodic peak (A1) and
one cathodic peak (C1) at 1.25 and -0.87 V, respectively
(curve 3b), that are related to the oxidation and reduction of
indole, respectively. The voltammogram for 4-chloroben-
zaldehyde in the presence of indole (2a) (curve 3c) shows
two anodic peaks (A2, A3) and one cathodic peak (C3). The
new peak couple (A3/C3) corresponds to the reaction
between 4-chlorobenzaldehyde in the presence of indole
(2a). The negative shift of peak A2 in the presence of
4-chlorobenzaldehyde is due to the reactivity of the elec-
trochemically generated product 3e which has formed a thin
film on the surface of the electrode.
Results and discussion
The electrochemical study of 1 mmol solution of
4-chlorobenzyl alcohol (1e) in CH3CN containing 0.05 g
LiClO4 at a glassy carbon electrode was performed by
cyclic voltammetry (Fig. 1). The voltammogram shows
one anodic peak (A1) and one cathodic peak (C1) at 2.0 and
-0.6 V versus SCE which correspond to the oxidation of
4-chlorobenzyl alcohol (1e) to 4-chlorobenzaldehyde
(Scheme 2A) and the reduction of anodically generated
4-chlorobenzaldehyde from 4-chlorobenzyl alcohol (1e),
respectively.
Controlled-potential coulometry was applied in a solu-
tion containing 1 mmol of 4-chlorobenzyl alcohol (1e) at
the potential of peak A1. Monitoring of the electrolysis
progress was carried out by TLC and cyclic voltammetry
(Fig. 2). It was observed that, proportional to the
advancement of coulometry, the height of A1 decreases.
The anodic peak A1 disappears when the charge
Controlled-potential coulometry was performed in a
CH3CN solution (40 cm3) containing 4-chlorobenzalde-
hyde (1 mmol) in the presence of indole (2a, 2 mmol) at
-0.9 V versus SCE. Electrolysis progress was monitored
123

A domino electro-oxidative synthesis of 3,30-bis(indolyl)methane nanoparticles
Scheme 2
OH
-2e -2H
,
O
A
4
1a
+
1e
+
H
1/2
2
N
N
H
2a
5
HO
O
O
O
H
+
N
N
N
B
N
H
7a
4
5
6
7b
H
N
N
N
-
OH
H
N
N
+
+ e
H
OH
1/2 O2
N
H
8
3a
Fig. 3 Cyclic voltammograms of: a 4-chlorobenzaldehyde (1 mmol),
b indole (2 mmol), c a mixture of 4-chlorobenzaldehyde/indole (1/
2 mol ratio) at the glassy carbon electrode in acetonitrile (40 cm3)
containing 0.05 g LiClO4 with the scan rate of 100 mV s-1
Fig. 2 Cyclic voltammograms of 1 mmol 4-chlorobenzyl alcohol
(1e) at the glassy carbon electrode, in CH3CN (40 cm3) containing
0.05 g LiClO4, during controlled-potential coulometry at 2.0 V versus
SCE. After consumption of: a 0, b 10, c 25, d 45, e 65, and f 80 C.
Inset variation of peak current Ip versus charge consumed at the scan
rate 100 mVs-1
indole. These observations allow us to propose the mech-
anism illustrated in Scheme 2.
Additionally, the effect of the scan rate on the electro-
chemical behavior of 4-chlorobenzaldehyde in the presence
of indole (2a) was investigated (Fig. 5). It can be observed
that proportional to the augmentation of potential sweep
rate, the height of peak C3 increases. On the other hand, the
by cyclic voltammetry and TLC (Fig. 4). As Fig. 4 shows,
proportional to the advancement of coulometry, the
cathodic peak (C1) decreases and disappears when the
charge consumption reaches about 1e- per molecule of
current function for peak A2 (IpA /t1/2), changes only
2
123

K. Nikoofar, K. Ghanbari
According to Table 1, benzyl alcohol (1a) and its elec-
tron-donating derivatives p-methyl-, p-methoxy-, and o-
hydroxybenzyl alcohols (1b–1d) and also electron-deficient
p-chloro-, p-nitro-, and o-nitro derivatives (1e–1g) react with
indole (2a) in high yields. The results confirmed the
regioselectivity in electro-oxidation; as in the case of o-hy-
droxybenzyl alcohol (1d), any oxidation of its hydroxyl
moiety was observed (Table 1, entry 4). In order to survey
the efficacy of the method, the domino electro-oxidative
condensation of 1-propanol (1h) as an aliphatic alcohol
model and cinnamyl alcohol (1i) as an example of an allylic
derivative were also performed with indole successfully
(Table 1, entries 8, 9). The study also revealed the slower
condensation of aliphatic alcohol with indole rather than this
reaction between allylic alcohol and indole. It may depend
on the greater reactivity of allylic alcohols. 2-Methylindole
(2b), as another indolic alternative was also utilized for the
domino electro-oxidative condensation with p-methyl and p-
chlorobenzyl alcohols. The results (Table 1, entries 10, 11)
confirm obtaining the corresponding 3,30-di(in-
dolyl)methanes successfully. The high yields of the reactions
present the domino-cascaded electro-oxidative condensation
method as an efficient and eco-friendly protocol as a multi-
component condensation reaction.
Fig. 4 Cyclic voltammograms of 2 mmol indole (2a) at the glassy
carbon electrode, in CH3CN (40 cm3) containing 0.05 g LiClO4,
during controlled-potential coulometry at -0.9 V versus SCE, after
consumption of: a 0, b 20, c 40, d 70, e 95, f 120, g 150, and h 180 C.
Inset variation of peak current IpC versus charge consumed at the scan
1
rate 100 mVs-1
The SEM image of 3,30-bis(1H-indol-3-yl)-(4-chlor-
ophenyl)methane (3e) shown in Fig. 6, reveals that the
nanoparticle morphology has been created. The average
diameter of the nanoparticles of 3e is 35–50 nm.
Coulometry, voltammetry, and structural observations
allow us to propose the pathway shown in Scheme 2 for the
electrochemical synthesis of di(indolyl)methane deriva-
tives 3a–3k.
Although the comprehensive mechanism of the reaction is
not known, the following rationalization may be worthwhile to
explain the product (3a) formation (Scheme 2). In the first step
of the domino reaction (Scheme 2A), the oxidation of benzyl
alcohol (1a) at the anode yields benzaldehyde (4). By the fur-
ther addition of indole (2a) to the reaction medium
(Scheme 2B), indole anion 5 was created via cathodic electron
absorbance. Nucleophilic attacks of 5 to the carbonyl moiety of
4 yield intermediate 6. This intermediate stabilizes with an
H-shift and converts to intermediates 7a and 7b. Releasing the
hydroxyl anion, along with a nucleophilic attack of another
indole anion 5, gives rise to intermediate 8. Finally, the proto-
nation of 8 by the in situ produced H? form hydroxyl oxidation
at the anode followed by rearrangement creates product 3a.
In order to further survey the illustrated mechanism, time-
dependent absorption spectra of a mixture of 4-chloroben-
zaldehyde (1 mmol) and indole (2a) (2 mmol) in CH3CN
(40 cm3) containing 0.05 g LiClO4 were collected during a
controlled-potential coulometry experiment (Fig. 7). These
spectra show that, as the coulometry experiment proceeds,
absorptionpeaks with a kmax at 500 nm appears and its height
Fig. 5 Typical voltammograms of a mixture of 4-chlorobenzalde-
hyde (1 mmol) in the presence of indole (2a) (2 mmol) at the glassy
carbon electrode, in CH3CN (40 cm3) containing 0.05 g LiClO4, at
different scan rates (a ? i): 25, 50, 75, 100, 125, 150, 200, 250,
300 mV s-1. Inset variation of peak current function (IpA /t1/2) vs.
2
scan rate
slightly with the increase in scan rate (Fig. 5, inset). Such
behavior is taken as indicative of an EC mechanism [48,
49].
Subsequently, oxidative condensation reaction between
various alcohols and indole derivatives was performed. All
the tests were carried out in a domino condition in which,
at the first step, alcohols were added to the cell where they
electro-oxidized to their corresponding aldehyde. Electro-
condensation was followed by the addition of indoles to the
reaction cell alternatively. The prepared products and
reaction conditions are summarized in Table 1.
123

A domino electro-oxidative synthesis of 3,30-bis(indolyl)methane nanoparticles
Table 1 Electro-oxidative synthesis of di(indolyl)methane derivatives 3a–3k via the reaction of alcohols 1a–1i with indoles 2a, 2b
Entry
R1
R2
Product
Yielda/%
Observed m.p./°C
Lit. m.p./°C
1
C6H5
H
3a
3b
3c
3d
3e
3f
90
90
85
70
85
75
60
60
90
85
90
121–123
97–99
123–125 [19]
95–97 [23]
2
p-MeC6H4
p-MeOC6H4
o-HOC6H4
p-ClC6H4
p-NO2C6H4
o-NO2C6H4
C3H7
H
3
H
190–192
256–258
74–76
190–192 [19]
259-261 [20]
78–80 [21]
4
H
5
H
6
H
229–231
106–108
78–79
229–223 [19]
112–114 [20]
77–79 [42]
7
H
3g
3h
3i
8
H
9
C6H5CH=CH
p-MeC6H4
p-ClC6H4
H
96–98
92–94 [42]
10
11
a
CH3
CH3
3j
173–175
146–147
174–176 [24]
145–147 [42]
3k
Isolated yields
Fig. 6 The SEM image of 3,30-bis(1H-indol-3-yl)(4-chlorophenyl)-
methane (3e)
Fig. 7 Absorption spectra of the mixture of 4-chlorobenzaldehyde
and indole (2a) during controlled-potential coulometry at -0.9 V vs.
SCE in CH3CN containing 0.05 g LiClO4. After consumption of: 0,
20, 40, 70, 95, 120, and 150 C. Inset absorption spectra of 1 mmol/
CH3CN (40 cm3) of (I) 4-chlorobenzaldehyde, (II) indole (2a), and
(III) 3,30-bis(1H-indol-3-yl)(4-chlorophenyl)methane (3a)
increases. This peak belongs to n ? p* absorption of pro-
duct 3e. The bands between 300 and 400 nm also relate to
p ? p* transitions of the highly conjugated BIM 3e. Growth
of the peak intensity during the time interval, which is a proof
of the product formation, could be a confirmation for the
proposed mechanism depicted in Scheme 2.
conventional chemistry, such as (a) in situ generation of the
nano-sized products without further preparation proce-
dures; (b) avoidance of using a Lewis, Bronsted, and/or
protic acid catalyst; (c) a one-pot domino MCR affording
excellent yields under mild conditions, and (d) avoiding the
use of polluting or hazardous chemicals and toxic reagents.
These noteworthy points make the method a green, con-
venient, and efficient method in organic transformations.
According to the above observations, the presented
mechanism is quite substantive. The CV spectra affirm the
key role of the electrosynthetic protocol for BIM prepara-
tion under the above-mentioned conditions.
Conclusion
In conclusion, the cascaded electro-oxidative domino
condensation of alcohols and indoles to afford the corre-
sponding 3,30-bis(indolyl)methanes has been performed.
This direct environmentally friendly electrosynthesis of
nanosized 3,30-bis(indolyl)methanes via multi-component
condensation shows highlighted advantages over
Experimental
Electrochemical measurements were performed by an
electrochemical system analyzer (Sama Instruments,
Iran). The working electrode used in the voltammetry
123

K. Nikoofar, K. Ghanbari
experiments was a glassy carbon disc (1.8 mm2 area)
and platinum wire was used as a counter electrode. The
working electrode used in controlled-potential coulom-
etry and macroscale electrolysis was an assembly of
three carbon rods (8 mm diameter and 4 cm length) and
a large platinum gauze (3 9 3 cm2) constituted the
counter electrode. The working potentials have been
measured versus standard calomel electrode (SCE) as a
reference electrode (all electrodes were obtained from
Azar Electrode, Urmia, Iran). Melting points were
determined using a Stuart Scientific SMP2 capillary
apparatus. IR spectra were recorded from KBr discs on
References
1. Sundberg RJ (1996) The chemistry of indole. Academic Press,
New York, p 113
2. Bell R, Carmeli S, Sar N, Vibrindole A (1994) J Nat Prod
57:1587
3. Hong C, Firestone GL, Bjeldance LF (2002) Biochem Pharmacol
63:1085
4. Hibino S, Choshi T (2001) Nat Prod Rep 18:66
¨
5. Shirinzadeh H, Eren B, Gurer-Orhan H, Suzen S, Ozden S (2010)
Molecules 15:2187
6. Zeligs MA (1998) J Med Food 1:67
7. Ge X, Yannai S, Rennert G, Gruener N, Fares FA (1996) Bio-
chem Biophys Res Commun 228:153
8. Remers WA (1972) In: Houlihan WJ (ed) Heterocyclic com-
pounds. Interscience Publishers, New York, p 1
9. Hazarika P, Sharma SD, Konwar D (2008) Synth Commun
38:2870
10. Ekbote SS, Deshmukh KM, Qureshi ZS, Bhanage BM (2011)
Green Chem Lett Rev 4:177
11. Kamal A, Qureshi AA (1963) Tetrahedron 19:513
12. Li WJ, Lin XF, Wang J, Li GL, Wang YG (2005) Synth Commun
35:2765
13. Yadav JS, Gupta MK, Jain R, Yadav NN, Reddy BVS (2010)
Monatsh Chem 141:1001
14. Nagarajan R, Perumal P (2002) Tetrahedron 58:1229
15. Mi X, Luo S, He J, Cheng JP (2004) Tetrahedron Lett 45:4567
16. Wang L, Han J, Tian H, Sheng J, Fan Z, Tang X (2005) Synlett
337
1
Shimadzu IR-435. H NMR spectra were recorded with
a Bruker DRX-500 spectrometer. Mass spectra were
obtained on Platform II spectrometer from Micromass;
EI mode at 70 eV. Preparative layer chromatography
(PLC) carried out on 20 9 20 cm2 plates, coated with a
1 mm layer of Merck silica gel PF254, prepared by
applying the silica as slurry and drying in air. The
scanning electron microscope (SEM, model R-IJMA)
was used to characterize the nano-structures. The
alcohols, indoles, and solvents were purchased from
Merck, Aldrich, and Alfa Aesar and used without fur-
ther purification.
17. Qu HE, Xiao C, Wang N, Yu KH, Hu QS, Liu LX (2011)
Molecules 16:3855
General procedure for the domino electro-oxidative
condensation of indoles with alcohols
18. Xu XF, Xiong Y, Ling XG, Xie XM, Yuan J, Zhang Sh, Song ZR
(2014) Chin Chem Lett 25:406
19. Nagawade RR, Shinde DB (2006) Acta Chim Slov 53:210
20. Zhang Y, Chen X, Liang J, Shang Zh (2011) Synth Commun
41:2446
A
40 cm3 acetonitrile solution of alcohols 1a–1i
(1 mmol) in LiClO4 (0.05 g) was electrolyzed at the
2.0 V versus SCE. After completion of the electro-oxi-
dation of alcohols to their corresponding aldehydes,
indoles 2a, 2b (2 mmol) were added and electrolyzed at
-0.9 V versus SCE. In the two mentioned stages, elec-
trosynthesis reactions were done in an undivided cell
equipped with graphite rods as the cathode and Pt-anode
at room temperature. The electrolysis was terminated
when the current decreased by more than 95 %. A
magnetic stirrer was employed during the electrolysis.
The process was interrupted several times during the
electrolysis and the graphite rod was washed in acetone
in order to reactivate it. After completion of the electro-
condensation, the solvent was evaporated and the
resulting crude product was purified by preparative thin-
layer chromatography on silica gel (eluent: n-hexane–
EtOAc, 4:1) to afford 3,30-di(indolyl)methanes 3a–3k.
All the products were characterized by comparison of
21. Naik MA, Sachdev D, Dubey A (2010) Catal Commun 11:1148
22. Mendes SR, Thurow S, Fortes MP, Penteado F, Lenarda˜o EJ,
Alves D, Perin G, Jacob RG (2012) Tetrahedron Lett 53:5402
23. Hasaninejad A, Zare A, Sharghi H, Niknam Kh, Shekouhy M
(2007) Arkivoc xiv:39
24. Shiri M (2013) J Iran Chem Soc 10:1019
25. Khaksar S, Ostad SM (2011) J Fluorine Chem 132:937
26. Kalantari M (2012) Arab J Chem 5:319
27. Hojati SF, Zeinali T, Nematdoust Z (2013) Bull Korean Chem
Soc 34:117
28. Sharma GVM, Reddy JJ, Lakshim PS, Krishna PR (2004)
Tetrahedron Lett 45:7729
29. Khosropour AR, Mohammadpoor-Baltork I, Khodaei MM,
Ghanbary P (2007) Z Naturforsch 62b:537
30. Zhang S, Fan W, Qu H, Xiao C, Wang N, Shu L, Hu Q, Liu L
(2012) Curr Org Chem 16:942
31. Weinberg NL, Weinberg HR (1968) Chem Rev 68:449
32. Nematollahi D, Dehdashtian S, Niazi A (2008) J Electroanal
Chem 616:79
33. Souza RFM, Souza CA, Areias MCC, Cachet-Vivier C, Laurent
´
M, Barhdadi R, Leonel E, Navarro M, Bieber LW (2010) Elec-
trochim Acta 56:575
´
34. Guirado A, Andreu R, Galvez J, Jones PG (2002) Tetrahedron
58:9853
35. Jouikov V, Salaheev RG, Souza FM, Souza CA, Areias MCC,
1
their melting points and also spectroscopic data (IR, H
NMR and mass spectra) with those of the authentic
samples in literature.
´
Cachet-Vivier C, Laurent M, Barhdadi R, Leonel E, Navarro M,
Bieber LW (1996) Electrochim Acta 41:2623
36. Li CH, Song XZ, Tao LM, Li QJ, Xie JQ, Peng MN, Pan L, Jiang
C, Peng ZY, Xu MF (2014) Tetrahedron 70:1855
Acknowledgments The authors express their gratitude for financial
support from Alzahra University.
123

A domino electro-oxidative synthesis of 3,30-bis(indolyl)methane nanoparticles
37. Makarem S, Fakhari AR, Alimohammadi AA (2008) Tetrahedron
Lett 49:7194
38. Shestopalov AM, Niazimbetova ZI, Evans DH, Niyazymbetov
ME (1999) Heterocycles 51:1101
39. Vafajoo Z, Veisi H, Maghsoodlou MT, Ahmadian H (2014) C R
Chimie 17:301
40. Cortopassi AC, Ferrara PJ, Wawiernia TM, Essel JT (2008) Int J
Energetic Materials Chem Prop 7:39
41. Thakur R, Gupta RB (2006) Chem Eng Commun 193:293
42. Mohammadpoor-Baltork I, Memarian HR, Khosropour AR,
Nikoofar K (2006) Lett Org Chem 3:768
43. Mohammadpoor-Baltork I, Memarian HR, Khosropour AR,
Nikoofar K (2006) Heterocycles 68:1837
44. Khodaei MM, Mohammadpoor-Baltork I, Memarian HR, Khos-
ropour AR, Nikoofar K, Ghanbary P (2008) J Heterocycl Chem
45:377
45. Memarian HR, Mohammadpoor-Baltork I, Nikoofar K (2008)
Ultrason Sonochem 15:456
46. Memarian HR, Mohammadpoor-Baltork I, Nikoofar K (2007)
Can J Chem 85:930
47. Fotouhi L, Nikoofar K (2013) Tetrahedron Lett 54:2903
48. Bard AJ, Faulkner LR (2001) Electrochemical Methods, 2nd edn.
Wiley, New York, p 497
49. Greef R, Peat R, Peter LM, Pletcher D, Robinson J (1990)
Instrumental Methods in Electrochemistry. Ellis Horwood Lim-
ited, New York, p 195
123