Different strategies in electrochemical synthesis of new mono and di-substituted hydroquinone and benzoquinone

Article abstract of DOI:10.1016/j.electacta.2014.09.122

Electrochemical syntheses of 2-indolyl-5-arylsulfonyl-p-benzoquinone derivatives were carried out in two successive oxidation steps. The first involves the oxidation of hydroquinone, 4-(piperazin-1-yl) phenol) and 1-(4-(4-hydroxyphenyl) piperazin-1-yl) et

Full text of DOI:10.1016/j.electacta.2014.09.122

Electrochimica Acta 147 (2014) 310–318
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Different strategies in electrochemical synthesis of new mono and
di-substituted hydroquinone and benzoquinone
Davood Nematollahi *, Shima Momeni, Sadegh Khazalpour
Faculty of Chemistry, Bu-Ali-Sina University, P. O. Box 65174, Hamedan, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 8 July 2014
Received in revised form 22 September 2014
Accepted 24 September 2014
Available online 28 September 2014
Electrochemicalsyntheses of2-indolyl-5-arylsulfonyl-p-benzoquinone derivatives were carried out in two
successive oxidation steps. The first involves the oxidation of hydroquinone, 4-(piperazin-1-yl) phenol)
and1-(4-(4-hydroxyphenyl)piperazin-1-yl)ethanoneinthepresenceofarylsulfinicacidsasnucleophiles.
Our voltammetric data indicate that electrochemicallygenerated p-benzoquinone participates in Michael
addition reaction with arylsulfinic acids leading to the 2-(arylsulfonyl) benzene-1,4-diols. The second
consists of the oxidation of 2-(arylsulfonyl) benzene-1,4-diols in the presence of 1,2-dimethylindole and
the formation of 2-indolyl-5-arylsulfonyl-p-benzoquinone derivatives as the final products. A plausible
mechanism for the synthesis of 2-indolyl-5-arylsulfonyl-p-benzoquinone derivatives is also presented.
ã 2014 Elsevier Ltd. All rights reserved.
Keywords:
Electrochemical syntheses
2
-Indolyl-5-arylsulfonyl-p-benzoquinone
Arylsulfinic acid
Cyclic voltammetry
Hydroquinone
1. Introduction
16], to achieve thistarget, we firstlyinvestigatedthe electrochemical
oxidation of hydroquinone (1), 4-(piperazin-1-yl) phenol (2) and 1-
Hydroquinone is an important organic compound that is widely
used in many fields such as pharmaceutical, antioxidant, dye,
photography and cosmetic industries [1]. In addition, the biological
and clinical significance of indole and its derivatives have been
widelyreportedinliteratures[2]. Facileaccessto substitutedindoles
is of general interest because indoles are building blocks of many
natural products and have applications as pharmaceuticals, as
agrochemicals, and in materials science [3]. For example, it is
recognized that, the 3-indolylbenzoquinone (Fig. 1A) fragment is a
core structure in a number of biologically active natural products
such as asterriquinones (Fig. 1B) [4]. The asterriquinones exhibit a
wide spectrum of biological activities including antitumor proper-
tiesandareinhibitorsofHIVreversetranscriptase[5–7].Ontheother
hand diarylsulfones are useful in the practiceof medicinal chemistry
becausethe sulfonefunctionalgroup isfoundinnumerousdrugs[8].
Some of diphenylsulfones have been shown to inhibit HIV-1 reverse
transcriptase and represent an emerging class of substances able to,
address toxicity and resistance problems of nucleoside inhibitors
(4-(4-hydroxyphenyl) piperazin-1-yl) ethanone (3) in the presence
of arylsulfinic acids (4a-4c) as nucleophiles and represent a facile
and one-pot electrochemical method for the synthesis of some 2-
(phenylsulfonyl)benzene-1,4-diolderivatives(6a-6c). Thisreaction
is carried out in a single step, in ambient conditions and in a divided
cell using a carbon electrode. Our data shows that, in acidic
solutions, thehydrolysisofelectrochemicallygeneratedp-quinone-
imines 2ox or 3ox causes the production of the same products
(6a-6c) in the electrochemical oxidation of 1–3 in the presence of
4a-4c. The results of these studies including the full characteriza-
tion of the obtained product is presented and discussed.
In order to get the 2-indolyl-5-arylsulfonyl-p-benzoquinone
derivatives (Fig. 1C), the electrochemical oxidation of 2-(phenyl-
sulfonyl) benzene-1,4-diol derivatives (6a-6c) have also been
investigated in the presence of 1,2-dimethylindole (8) as a
nucleophile and a facile electrochemical method for the synthesis
of new hydroquinone derivatives with two different substituents
(10a-10c) has been reported.
[9,10]. The importance of these compounds has motivated us to
synthesize some organic compounds containing diphenylsulfone,
hydroquinone and indole moieties (Fig. 1C).
2. Experimental
Following our experience in electrochemical synthesis of organic
compounds based on the in-situgeneration of Michael acceptor [11–
2.1. Apparatus and reagents
Cyclic voltammetry, controlled-potential coulometry and pre-
parative electrolysis were performed using an Autolab model
PGSTAT 30 and a Behpazho potentiostat/galvanostat. The working
electrode used in the voltammetry experiments was a glassy carbon
*
Corresponding author: Tel.: +98 811 8282807; fax: +98 811 8257407.
E-mail address: nemat@basu.ac.ir (D. Nematollahi).
http://dx.doi.org/10.1016/j.electacta.2014.09.122
013-4686/ã 2014 Elsevier Ltd. All rights reserved.
0

D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
311
Fig. 1. The structure of 3-indolylbenzoquinone (A), asterriquinones (B) and
synthesized compounds (C).
disc (3.2 mm2 area) and a platinum wire was used as a counter
electrode. The working electrode used in controlled-potential
coulometry and macro-scale electrolysis was an assembly of four
2
carbon rods (hard and porous) (31 cm ) and a large stainless steel
sheet constitute the counter electrode. The working electrode
potentials were measured versus SCE (from AZAR electrode and
Metrohm). 1-(4-(4-Hydroxyphenyl) piperazin-1-yl) ethanone,
4-(piperazin-1-yl) phenol, benzenesulfinic acid, 4-chlorobenzene-
sulfinic acid, 4-toluenesulfinic acid and 1,2-dimethylindole were
reagent-grade materials from Aldrich. Phosphoric acid and hydro-
quinone were of pro-analysis grade from E. Merck. These chemicals
were usedwithout furtherpurification.Moredetailsaredescribedin
our previous paper [17]. The glassy carbon electrode was polished
using alumina slurry (from Iran Alumina Co.). The structures of
reactants (1-3 and 4a-4c) and products (6a-6c and 10a-10c) are
shown in Fig. 2.
Fig. 2. The structures of reactants (1-3 and 4a-4c) and products (6a-6c and 10a-10c).
2
.2. Electroorganic Synthesis of 6a-6c
ꢀ
1
M.p: 218–219 C. H NMR (400 MHz, DMSO-d
methyl), 6.69 (d, J = 11.7 Hz, 1H, aromatic), 6.88 (dd, J = 4.0 and 11.7,
H, aromatic), 7.29 (d, J = 4.0, 1H, aromatic), 7.36 (d, J = 11.0, 2H,
aromatic), 7.75 (d, J = 11.0, 2H, aromatic), 9.34 (s, 1H, OH), 9.86 ppm
6
) d= 2.35 (s, 3H,
A phosphate buffer solution (70 mL, c = 0.2 M, pH = 2.0) con-
1
taining 0.25 mmol (0.0275 g) of hydroquinone (1) and 0.25 mmol of
-toluenesulfinic acid (0.0390 g) (4a), benzensulfinic acid (4b)
0.0355 g) or 4-chlorobenzenesulfinic acid (4c) (0.0442 g) was
4
(
13
(
s,1H, OH). C NMR (100 MHz, DMSO-d
18.5 (C-2), 122.7 (C-3), 126.4 (C-8), 127.9 (C-9), 129.2 (C-6), 138.5
C-7), 143.5 (C-10), 148.1 (C-1), 149.4 ppm (C-4). IR (KBr) = 3283
broad, O-H), 3066 (weak, C-H), 3040 (weak, C-H), 1465 (strong
C=C),1284 (strong, S=O),1222 (strongC-O),1140 (strong, S=O),1088,
77, 818, 657 cm . MS (EI, 70 eV, m/z) (relative intensity): 268
6
) d= 21.0 (C-11),113.7 (C-5),
1
subjected to electrolysis at 0.35 V versus SCE, in a divided cell. The
electrolysis was terminated when the current decayed to 5% of its
original value. The precipitated solid was collected by filtration and
was washed several times with water. After purification and re-
crystallization in hot ethanol, the products were characterized by
IR, H NMR, C NMR, MS and elemental analysis. In the second
method, a phosphate buffer solution (70 mL, c = 0.2 M, pH = 2.0)
containing 0.25 mmol of 4-(piperazin-1-yl) phenol (0.0445 g) (2)
or 1-(4-(4-hydroxyphenyl) piperazin-1-yl) ethanone (0.0551 g) (3)
was subjected to electrolysis at 0.40 V versus SCE in a divided cell.
At the end of electrolysis 0.25 mmol of 4-toluenesulfinic acid (4a),
benzensulfinic acid (4b) or 4-chlorobenzenesulfinic acid (4c) was
added to solution and after stirring for 10 min the precipitated
solid was collected by filtration and after purification as described
above, they were characterized.
(
(
n
À1
8
(
+
1
13
M + 4H, 100), 201 (9), 176 (10), 112 (43), 84 (15).
2
10 4
.2.2. 2-(Phenylsulfonyl) benzene-1,4-diol (C12H O S) (6b)
OH
O
S
1
8
6
7
2
3
9
10
O
5
8
4
9
OH
M.p: 200–201 C. 1H NMR (400 MHz, DMSO-d
ꢀ
)
d
= 6.72 (d,
J = 11.6 Hz,1H, aromatic), 6.90 (dd, J = 3.9 and 11.6 Hz,1H, aromatic),
.31 (d, J = 3.9 Hz, 1H, aromatic), 7.55 (t, J = 10.4 Hz, 2H, aromatic),
.64 (t, J = 9.8 Hz, 1H, aromatic), 7.87 (d, J = 9.7 Hz, 2H, aromatic),
2
.2.1. 2-Tosylbenzene-1,4-diol (C13
H
12
O
4
S) (6a)
6
7
OH
O
S
O
7
1
8
9
13
9.63 ppm (broad, 2H, OH). C NMR (100 MHz, DMSO-d
C-5), 118.5 (C-2), 122.9 (C-3), 126.0 (C-8), 127.8 (C-9), 128.8 (C-6),
133.1 (C-10), 141.4 (C-7), 148.2 (C-1), 149.4 ppm (C-4). IR (KBr)
= 3291 (broad, O-H), 3066 (weak, C-H), 1755 (medium C=C), 1597
(strong, C=C), 1367 (strong, S=O), 1135 (strong, S=O), 998, 914, 819,
6
) d= 113.8
6
7
2
3
9
(
5
8
11
1
0 CH
4
OH
3
n

312
D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
À1
+
7
93, 730 cm . MS (EI, 70 eV, m/z) (relative intensity): 254 (M + 4H,
2.3.1. 2-(1,2-Dimethyl-1H-indol-3-yl)-5-tosylcyclohexa-2,5-diene-
100), 176 (14), 129 (13), 112 (73), 91 (19).
1,4-dione (C23
4
H19NO S) (10a)
2
1
9
1
0 CH
2
.2.3. 2-(4-Chlorophenylsulfonyl) benzene-1,4-diol (C12
H
9
ClO
4
S) (6c).
3
8
7
O
1
OH
O
S
O
O
5
9
1
8
9
20
6
S
2
6
7
8
H C
2
3
9
3
O
1
2
1
9
11
5
8
3
1
0
Cl
H
3
C
N
4
O
4
OH
1
8
13
1
7
1
4
ꢀ
1
16
M.p: 210–212 C. H NMR (400 MHz, DMSO-d
J = 11.7, aromatic), 6.91 (dd, 1H, J = 11.6 and 3.6, aromatic), 7.29
d, 1H, J = 3.6, aromatic), 7.64 (d, 2H, J = 11.2, aromatic), 7.87 (d, 2H,
6
)
d
= 6.73 (d, 1H,
15
M.p:109–111 C.(isolatedyield77%, 0.0312 g). HNMR(400 MHz,
CDCl = 2.33 (s, 3H), 2.45 (s, 3H), 3.75 (s, 3H), 6.79 (d, 1H, J = 10.0),
.92 (d, 1H, J = 10.4), 7.12 (d, 2H, J = 8.0), 7.16 (dd, 1H, J = 1.2 and 8.0),
.23 (m, 2H, J = 1.2 and 8.0), 7.27 (s, 1H), 7.59 ppm
d, 2H, J = 8.4. C NMR (100 MHz, CDCl
1), 30.2 (C-19),104.9 (C-14),109.4 (C-11),119.8 (C-17),121.2 (C-15),
22.3(C-16),128.2(C-18),128.4(C-8),128.9(C-9),136.5(C-13),137.3
C-7),137.4 (C-2),137.9 (C-12), 140.1 (C-10), 142.5 (C-3),142.6 (C-6),
44.2 (C-5), 182.1 (C-1), 185. 3(C-4). IR (KBr) = 2924 (C-H), 2853C-
ꢀ
1
(
3
) d
13
J = 11.2, aromatic), 9.41 (s, 1H, OH), 10.03 ppm (s, 1H, OH). C NMR
6
7
(
2
1
(
1
(
100 MHz, DMSO-d
C-8), 129.0 (C-9), 130.0 (C-6), 138.1 (C-10), 140.1 (C-7), 148.2 (C-1),
49.5 ppm (C-4). IR (KBr) = 3275 (broad, O-H), 3091 and 2924
weak, C-H), 1581 (medium C=C), 1462 (strong, C=C), 1344 (strong,
S=O), 1276 (medium C-O), 1142 (strong, S=O), 1085, 1048, 834,
6
) d= 113.7 (C-5), 118.6 (C-2), 123.1 (C-3), 125.6
(
1
(
13
3
) d= 13.0 (C-20), 21.6 (C-
n
À1
+
7
08 cm . MS (EI, 70 eV, m/z) (relative intensity): 288 (M + 4H,
00), 256 (18), 239 (12), 176 (22), 112 (70), 84 (23), 56 (10). Anal.
Calcd. for C12 ClO S: C, 50.62; H, 3.19; Cl, 12.45; S, 11.26%. Found:
C, 50.40; H, 3.15%.
n
1
H), 1666(strong, C=O), 1651(strong, C=O), 1545(C=C), 1455(C=C),
406,1318(S=O),1266,1151(S=O),1061,739,692,666,599,544 cm
MS: (EI, 70 eV, m/z) (relative intensity) = 407 (M + 2H, 77), 342
100), 250(94), 222(48), 91 (35),194(33),56(28),168(23), 327(22).
H
9
4
À1
1
.
+
(
2
.3. Electroorganic Synthesis of 10a-10c
2
2
.3.2. 2-(1,2-Dimethyl-1H-indol-3-yl)-5-(phenylsulfonyl) cyclohexa-
4
,5-diene-1,4-dione (C22H17NO S) (10b)
For electrochemical synthesis of 10a-10c, a solution of water
9
(phosphate buffer, c = 0.2 M, pH = 2.0)/acetonitrile mixture (70 mL)
(70/30 v/v) containing 0.1 mmol (0.0264 g) 6a, (0.0250 g) 6b or
(0.0285 g) 6c and 0.1 mmol (0.0145 g) 1,2-dimethylindole (7), was
8
7
10
9
O
O
5
1
2
0
6
2
S
electrolyzed in a divided cell at 0.53 V vs. SCE. The electrolysis was
terminated when the current decreased by more than 95%. At the
end of the electrolysis, the precipitated solid was collected by
filtration and washed with water. After washing the product was
purified by column chromatography with silica gel and ethyl-
acetate.
8
H C
3
1
O
2
11
1
9
3
H C
4
3
N
1
8
O
1
3
1
7
14
1
6
1
5
I/m4A
I/ A
m
A1
A A2
1
3
3
2
1
0
1
2
3
A2
b
b
d
2
1
0
a
c
-
-
-
-
1
C2
C1
-2
0
.05
0.3
0.55
0.8
-
0.05
0.2
0.45
0.7
E vs. (Ag/AgCl)/V
E vs. (Ag/AgCl)/V
Fig. 3. Cyclic voltammograms of (a) hydroquinone (1) (1.0 mM); (b) hydroquinone (1) (1.0 mM) in the presence of 4-toluenesulfinic acid (4a) (1.0 mM); (c) 4-toluenesulfinic
acid (4a) ; (d) 2-(phenylsulfonyl) benzene-1,4-diol (6a) (1.0 mM), at a glassy carbon electrode in water (phosphate buffer, c = 0.2 M, pH = 2.0)/acetonitrile (70/30, v/v) solution.
À1
ꢀ
Scan rate 5 mV s . T = 25 Æ1 C.

D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
313
d
1
= 13.7 (C-20), 30.2 (C-19), 109.5 (C-14), 119.9 (C-11), 121.5 (C-17),
22.6 (C-15), 128.1 (C-16), 128.2 (C-18), 128.3 (C-13), 128.6 (C-8),
1
(
n
(
29.9 (C-9),136.6 (C-7),137.3 (C-2),137.4 (C-10),139.3 (C-12),139.8
C-3), 142.7 (C-5), 142.9 (C-6), 181.9 (C-1), 185.1(C-4). IR (KBr)
= 2923 (C-H), 2853 (C-H), 1732(C=O), 1668 (strong, C=O),1537
C=C), 1474(C=C), 1456, 1323(S=O), 1266, 1148(S=O), 1087, 1071,
012, 829, 754, 630, 593, 560, 496. MS: (EI, 70 eV, m/z) (relative
1
+
intensity) = 427 (M + 2H, 62), 362 (100), 56 (36), 250 (32),168 (28),
11 (21), 194 (20), 347 (19).
1
3
. Results and discussion
3
.1. Electrochemical study for the synthesis of hydroquinones 6a-6c
The cyclic voltammogram of hydroquinone (1) in water
(
phosphate buffer, c = 0.2 M, pH = 2.0)/acetonitrile mixture (70/30,
v/v) is shown in Fig. 3, curve a. Under these conditions, the
voltammogram shows one anodic (A ) and one cathodic peak (C ).
Anodic and cathodic peaks A and C correspond to the transfor-
1
1
1
1
mation of 1 to p-benzoquinone 1ox (Scheme 1) and vice versa
within a reversible two electron process [18–22]. The oxidation of 1
in the presence of 4-toluenesulfinic acid (4a) as a nucleophile was
studied in some detail (Fig. 3, curve b). Comparison of this
voltammogram with the cyclic voltammogram of 1 in the absence
of 4a shows that the cathodic peak C
peak (A ) appeared at more positive potentials. The occurrence of a
chemicalreactionafterelectrontransferprocessissupportedbythe
disappearing of peak C during the reverse scan, which could
1
disappeared and a newanodic
2
1
indicatethat 1ox formed at the surface of the electrode is consumed
by a chemical reaction with 4a. It should be noted that, the same
cyclic voltammograms were obtained in the water (phosphate
buffer, c = 0.2 M, pH = 2.0). Althoughinafollow-upreaction, itwould
1
00
A1
Scheme 1. Electrochemical oxidation of hydroquinone (1) in the presence of
arylsulfinic acids (4a-4c).
8
6
4
2
0
ꢀ
M.p: 178–180 C. (isolated yield 64%, 0.0250 g). IR (KBr)
n
1
8
(
= 3456, 2924 (strong, C-H), 2853 (strong, C-H), 1666(C=O),
0
0
0
456(C=C), 1406 (C=C), 1375(S=O), 1149(S=O), 1070, 844, 744,
À1
66, 615, 557 cm . MS (EI, 70 eV, m/z) (relative intensity) = 393
+
M + 2H, 47), 250 (100), 328 (96), 222 (62), 77 (59), 194 (41), 51
34), 167 (30), 179 (24).
(
2
.3.3. 2-(4-Chlorophenylsulfonyl)-5-(1,2-dimethyl-1H-indol-3-yl)
cyclohexa-2,5-diene-1,4-dione (C22H16ClNO S) (10c)
4
9
8
7
1
0
Cl
0
a
O
O
5
9
1
2
0
6
S
- 20
- 40
2
8
b
H C
3
O
c
1
2
d
1
9
11
3
H C
4
O
3
N
1
8
1
3
1
7
C1
14
-
60
- 0.05
1
6
1
5
0.15
0.35
0.55
E vs. SCE/V
M.p: 120–122 C. (isolated yield 61%, 0.0260 g). 1H NMR
ꢀ
Fig. 4. Normalized cyclic voltammograms of hydroquinone (1) (1.0 mM) in the
presence of 4-toluenesulfinic acid (4a) (1.0 mM) at glassy carbon electrode in
phosphate buffer solution (c = 0.2 M, pH = 2.0), at various scan rates: (a) 10, (b) 50, (c)
(
400 MHz, CDCl
J = 10.4), 6.94 (d,1H, J = 10.4), 7.17 (m, 2H, J = 6.8), 7.27 (m, 2H, J = 9.2,
.30 (m, 2H), 7.64 ppm (d, 2H, J = 9.2). 13C NMR (100 MHz, CDCl
3
) d= 2.43 (s, 3H), 3.75 (s, 3H), 6.81 (d, 1H,
7
3
)
1
ꢀ
100, (d) 250 mV s . T = 25 Æ1 C.

314
D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
2
2
1
1
5
0
5
0
5
e
0
5
-
Scheme 2. Proposed mechanism for the electrochemical synthesis of oxidation of
a
6a-6c.
-
10
This is further confirmed by the effects of the potential sweep
rate and the concentration of 4a on the cyclic voltammogram of 1.
Fig. 4 shows the effect of potential sweep rate on the normalized
cyclic voltammograms of 1 in the presence of 4-toluenesulfinic
acid (4a) at pH = 2.0. Normalized cyclic voltammograms are
obtained by dividing the current by the square root of the scan
-
0.1
0.1
0.3
0.5
0.7
E vs. SCE/V
Fig. 5. Cyclic voltammograms of hydroquinone (1) (1.0 mM) in the presence of (a)
.0, (b) 2.0, (c) 3.0, (d) 4.0 and (e) 5.0 mM 4-toluenesulfinic acid (4a) at glassy carbon
electrode in phosphate buffer solution (c = 0.2 M, pH = 2.0), Scan rate: 50 mV s
T = 25 Æ1 C.
1
À1
.
ꢀ
1
1
2
0
be expected that the peak potential shifts to the less positive
potentials, however, it does not occur due to the formation of a thin
film of product at the surface of the electrode, inhibiting to a certain
extent the performance of the electrode process [18–22]. In Fig. 3,
curvec is related tobythe cyclic voltammogramof 4ain the absence
of 1.
A2
A3
8
6
4
2
0
2
4
6
c
b
-
-
-
a
C0
C2
0.35
-
0.25
0.05
0.65
0.95
Fig. 6. (I) Linear sweep volammograms of hydroquinone (1) (0.25 mmol)
E vs. SCE/V
(
c = 3.57 mM) in the presence of 4a (0.25 mmol) (c = 3.57 mM) at a glassy carbon
electrode in aqueous solution containing 0.2 M phosphate buffer (pH = 2.0) during
Fig. 7. Cyclic voltammograms of (a) 6a (1.0 mM); (b) 6a (1.0 mM) in the presence of
1,2-dimethylindole (7) (1.0 mM); (c) 1.0 mM 1,2-dimethylindole (7) (1.0 mM). Scan
controlled-potential coulometry at 0.35 V vs. SCE, after consumption of (a) 0, (b)
À1
ꢀ
À1
0
.41, (c) 0.82, (d) 1.24, (e) 1.60 and (f) 1.86 F/mol. Scan rate: 100 mV s . T = 25 Æ1 C.
rate: 50 mV s , at a glassy carbon electrode in water (phosphate buffer, c = 0.2 M,
ꢀ
(
II) Variation of anodic peak current versus charge consumed.
pH = 2.0)/acetonitrile (70/30, v/v). T = 25 Æ1 C.

D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
315
Fig. 8. Cyclic voltammograms of 6a (0.1 mmol) (c = 1.43 mM) in the presence of 7 (0.1 mmol) (c = 1.43 mM) at a glassy carbon electrode in water (phosphate buffer, c = 0.2 M,
pH = 2.0)/acetonitrile (70/30 v/v) during controlled-potential coulometry after consumption of (a) 0 (0 e), (b) 10 (1 e), (c) 20 (2 e), (d) 30 (3 e) and (e) 45 (4.5 e) C (number of
À1
ꢀ
electron). (f) Cyclic voltammograms a to e from À0.05 to 0.25 V versus SCE. Scan rate: 50 mV s . T = 25 Æ1 C.
rate (I/v1/2). It is seen that, upon increasing the potential sweep
rate, the peak current ratio (IpC1/IpA1) increases. Such increase at
faster sweep rates is related to the decrease of the experimental
Our experience in the electrochemical sulfonylation of urazoles
[13], on the one hand, controlled potential coulometry, cyclic
voltammetry and spectroscopic data of the isolated product on the
other hand, all results indicate that compound 6a is formed in
the anodic oxidation of 1 in the presence of 4a; according to the
pathway shown in Scheme 1 (EC mechanism).
The first step (E) would be the generation of 1ox. In the second
step, 1ox would serve as a Michael acceptor in a reaction with the
nucleophile 4a to form the final product 6a (C step). The oxidation
of 6a is more difficult than the oxidation of 1 due to the presence of
the electron-withdrawing benzenesulfonyl group (Fig. 3, curve d).
Comparison of this voltammogram with Fig. 3, curve b shows that
1
time scale. In addition, the negative shift of peak C increases with
decreasing potential scan rate. This confirms the formation of a
thin film of product at the surface of the electrode which is
enhanced proportional to decreasing of potential sweep rate.
Fig. 5 shows the effect of concentration of 4-toluenesulfinic acid
(
4a) on the cyclic voltammograms of 1. Our results also show upon
the increase of the concentration of 4a, the peak current ratio (IpC1
pA1) decreases.
Controlled-potential coulometry was performed in a cell
/
I
containing 1 (0.25 mmol) and 4a (0.25 mmol) at 0.35 V vs. SCE.
The monitoring of electrolysis progress was carried out by linear
sweep voltammetry (Fig. 6). It is shown that, proportional to the
peak A
2
is due to the electrooxidation of 6a to p-benzoquinone
corresponds
6aox (Scheme 1,Eq. 3). Obviously, the cathodic peak C
2
to the reduction of 6aox to 6a. The formation of intermediate 1ox
(Scheme 1, Eq. 1) was also supported by the formation of the same
product (6a) from both direct reaction of 1ox with 4-toluensulfinic
progress of electrolysis, the anodic peak A
1
decreases. This peak
À
disappears when the charge consumption becomes 2e per
molecule of 1.

316
D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
acid (4a) (in phosphate buffer solution, c = 0.2 M, pH = 2.0) and
electrochemical oxidation of 1 in the presence of 4a.
In the second method, the electrochemical synthesis of 6a-6c,
was performed in two steps: (I) electrooxidation of 4-(piperazin-1-
yl) phenol (2) or 1-(4-(4-hydroxyphenyl) piperazin-1-yl) ethanone
a lesser amount of 6aox reacting with 7, hence the observe increase
of the current of peak C . In addition, upon the increase of the
concentration of the nucleophile 7, the current of peak A increases,
and the current of peak C decreases. Comparison of voltammo-
grams b and c reveals that the peak A corresponds to the oxidation
2
3
2
3
(
3) and (II) addition of nucleophile (4a-4c) to the electrolyzed
of 1,2-dimethylindole (7) linked covalently to 6a. From basic
knowledge, the rate of reaction will affect by the concentration of
the reactant. Raising the concentration of reactant (7) makes the
reactionof 6aox with 7, happen ata faster rate and consequentlythe
solution (Scheme 2). In this method, an aqueous solution
containing phosphate buffer (pH = 2.0) has been selected as a
suitable solution for the hydrolysis of 2ox and 3ox [18–22]. It is
found that, the final product via two different methods has the
2
lesser amount of 6aox to be reduced (generation of peak C ).
same structure. These data allow us to propose an EC
h
C
m
(E;
Controlled-potential coulometry was performed in a cell
containing of 6a (0.1 mmol) and of 7 (0.1 mmol) at 0.53 versus
SCE. Cyclic voltammetric analysis was carried out during the
coulometry. The results show that, proportional to the progress of
electron transfer, C ; hydrolyses reaction and C Michael addition
h
m
reaction) mechanism for the electrooxidation of 2 (and 3) in the
presence of arylsulfinic acids (4a-4c) (Scheme 2). Due to these
results, we conclude that dominant process is hydrolysis of p-
quinone-imines (2ox and 3ox) to 1ox. The isolated yields of 6a–6c
are reported in Scheme 2.
2 0
electrolysis, anodic peak A decreases and a new redox couple (A
and C
0
) appears (Fig. 8). Peak A
2
disappears when the charge
À
consumption becomes about 4e per molecule of 6a. This figure
(
curve f) also shows that, in the potential range -0.05 to 0.20 V vs.
3
.2. Electrochemical study for the synthesis of p-benzoquinones
SCE, the cathodic current depended on the passed charge (Q). The
results show that in this potential range, the cathodic current
increases proportional to the progress of electrolysis (increasing Q)
10a-10c
In the second part of this work, for the synthesis of new
p-benzoquinone derivatives containing two different substituents,
the oxidation of 6a was studied in the presence of 1,2-dimethy-
lindole (7) as a nucleophile. The cyclic voltammogram of 6a in the
presence of 7 is shown in Fig. 7, curve b. Comparison of this
voltammogram with that of 6a in the absence of 7 (curve a) shows
and demonstrate the progressive formation of a reducible
compound during coulometry. Diagnostic criteria of cyclic
voltammetry and controlled potential coulometry accompanied
1
13
by the IR, NMR ( H and C) data and molecular mass of the final
products allow us to propose the pathway in Scheme 3 for the
electrochemical oxidation of 6a-6c in the presence of 1,2-
dimethylindole (7).
that: (a) a new anodic peak A
and (b)inthe reverse scan, the currentof cathodicpeakC
significantly and a new cathodic peak (C ) appears. These
3
appears at more positive potentials
2
decreases
According to our results, it seems that the Michael-type
addition reaction of 7 to benzoquinone 6a-6c and aromatization,
leads to the hydroquinone 9a–9c which contains indolyl group.
0
observations and shift of peaks indicate that a chemical reaction
follows the electrontransferprocess:p-benzoquinone 6aox formed
at the surface of the electrode is consumed in a reactionwith 7. This
is further confirmed by the effects of the potential scan rate and the
concentration of 7 on the cyclic voltammogram of 6a. The peak
current ratio (IpC2/IpA2) increases upon increasing the potential scan
rate. Such increase (not shown) at faster scan rates is due to a
decrease of the experimental time scale. A faster scan rate results in
Scheme 3. Electrochemical oxidation of 2-(phenylsulfonyl) benzene-1,4-diol
derivatives (6a-6c) in the presence of 1,2-dimethylindole (7).
Scheme 4. Proposed mechanism for the synthesis of hydroquinone 6a, according to
the second method.

D. Nematollahi et al. / Electrochimica Acta 147 (2014) 310–318
317
Since, indolyl group is an electron-donating group [23–25]; the
presence of it in the structure of 9a–9c causes the oxidation of
these compounds (9a–9c) to become easier than the oxidation of
dimethylindole (7). Our results show that electrochemically
generated 6aox-6cox is attacked by 1,2-dimethylindole (7) and
subsequent oxidation to form the target compounds (10a-10c). In
addition, in this work we examined two other strategies for the
synthesis of 10a-10c. These strategies were not suitable for the
synthesis of compounds 10a-10c.
6a-6c, thereby, at the applied potential (0.53 V vs. SCE), the
oxidation process converts 9a–9c to p-benzoquinone 10a–10c
(final product) [26].
In the second strategy for the electrochemical synthesis of 10a-
1
0c, we examined the synthesis of these compounds via the
Acknowledgements
electrochemical oxidation of 1 in the presence of 1,2-dimethylin-
dole (7) in the first step and addition of 4a to the electrolyzed
solution in the second step (Scheme 4). In this strategy, 1
We acknowledge the Bu-Ali Sina University Research Council
and Center of Excellence in Development of Environmentally
Friendly Methods for Chemical Synthesis (CEDEFMCS) for their
support of this work.
(
0.25 mmol) was electrolyzed in the presence of 7 (0.25 mmol) in
a divided cell at 0.53 V vs. SCE in solution containing water
(
phosphate buffer, pH = 2.0)/acetonitrile (70/30, v/v). At the end of
À
electrolysis, when charge consumption becomes 4e per molecule
of 1, 4a (0.25 mmol) has been added to the solution. The separated
compoundwascharacterizedbytheIRspectrum.Itisfoundthat,the
final product hasthesamestructureas the 6a. Thesedataallowusto
propose an ECEC mechanism for this strategy (Scheme 4). Following
our experience in the electrochemical indolylation of catechols and
hydroquinones [17,23], it was expected that, the indolylation of
electrochemically generated 1ox would give the indolylquinone
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.electacta.
2
014.09.122.
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