Electrochemical oxidation of 4-(piperazin-1-yl)phenol in the presence of aryl sulfinic acids

Article abstract of DOI:10.1149/2.033204jes

Electrochemical oxidation of 4-(piperazin-1-yl)phenol has been studied in the presence of aryl sulfinic acids as nucleophiles in ethanol/water mixture (10/90) using cyclic voltammetry and controlled-potential coulometry methods. The results indicate that

Full text of DOI:10.1149/2.033204jes

E82
Journal of The Electrochemical Society, 159 (4) E82-E86 (2012)
0013-4651/2012/159(4)/E82/5/$28.00 © The Electrochemical Society
Electrochemical Oxidation of 4-(Piperazin-1-yl)phenol
in the Presence of Aryl Sulfinic Acids
Davood Nematollahi,z Sadegh Khazalpour, and Amene Amani
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65178-38683, Iran
Electrochemical oxidation of 4-(piperazin-1-yl)phenol has been studied in the presence of aryl sulfinic acids as nucleophiles in
ethanol/water mixture (10/90) using cyclic voltammetry and controlled-potential coulometry methods. The results indicate that the
electrochemically generated p-quinone-imine participates in Michael type addition reaction with aryl sulfinic acids and via an EC
mechanism converts to the new 2-(phenylsulfonyl)-4-(piperazin-1-yl)phenol derivatives. The present work has led to the development
of a facile and environmentally friendly electrochemical method for the synthesis of some new 2-(phenylsulfonyl)-4-(piperazin-1-
yl)phenol derivatives under green conditions.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.033204jes] All rights reserved.
Manuscript submitted October 27, 2011; revised manuscript received December 19, 2011. Published January 27, 2012.
O
S
O
Electrochemistry has emerged as a powerful tool for the synthe-
sis of complex organic molecules.1 Among amines, aryl piperazines
are of particular interest for the synthesis of biologically active sub-
stances since this moiety is commonly found as a key structural el-
ement in many compounds possessing broad therapeutic activities.2
Piperazine and its derivatives especially phenylpiperazines are impor-
tant pharmacores that can be found in biologically active compounds
across a number of different therapeutic areas,3 such as antifungal,4
anti-bacterial, anti-malarial, anti-psychotic agents5 and HIV protease
inhibitor.6
HN
N
H2N
NH2
HN
N
OH
OH
O
S
O
C
A
B
R
Figure 1. The structures of dapsone (A), 4-(piperazin-1-yl)phenol (B) and
synthesized compounds (C).
On the other hand, diarylsulfones are important synthetic targets,
and widely used synthons for synthetic organic chemists due to many
applications of them.7,8 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 nu-
cleoside inhibitors.9,10 They are useful in the practice of medicinal
chemistry because the sulfone functional group is found in numerous
drugs.11 For example, diphenylsulfone is used as an intermediate for
the synthesis of 4,4-diaminodiphenylsulfone (dapsone) (Fig. 1a).12
The importance of phenylpiperazine derivatives has prompted
many workers to synthesize these compounds by various chemical
methods.13–17 Following our experience in electrochemical synthesis
of organic compounds based on the in-situ generation of Michael
acceptor,18–20 we thought that synthesis of organic compounds with
both structures of diphenylsulfone and piperazine would be useful
from pharmaceutical properties point of view. This idea prompted us
to investigate electrochemical oxidation of 4-(piperazin-1-yl)phenol
(1) (Fig. 1b) in the presence of aryl sulfinic acids (3a-c) as nucleophiles
and represent a facile and one-pot electrochemical method for the
synthesis of some new 2-(phenylsulfonyl)-4-(piperazin-1-yl)phenol
derivatives (Fig. 1c). This reaction proceeds in a single step with an
environmentally friendly method in ethanol/water mixture (10/90), in
ambient conditions in a divided cell using a carbon electrode.
grade materials from Aldrich. Phosphoric acid, perchloric acid, acetic
acid, sodium bicarbonate and other solvents were of pro-analysis
grade from E. Merck. These chemicals were used without further
purification.
Synthesis of compounds 5a-c.— A solution of perchloric acid (ca.
80 mL; c = 0.1 M) in ethanol/water mixture (10/90) solutions contain-
ing 4-(piperazin-1-yl)phenol (1) (0.25 mmol) and aryl sulfinic acids
sodium salt (3a–c) (0.25 mmol) was electrolyzed in a divided cell
at 0.45 V vs. Ag/AgCl. The electrolysis was terminated when the
current decreased by more than 95% (after consumption of about 50
coulombs). At the end of electrolysis, after neutralizing the solution
with saturated solution of sodium carbonate, the precipitated solids
were collected by filtration and washed several times with cold wa-
ter. After washing and drying, products were characterized by IR, 1H
NMR, 13C NMR, and MS. These spectra are presented in the Sup-
plementary Material.28 The isolated yields of 5a–c are reported in
Scheme 3.
2-(Phenylsulfonyl)-4-(piperazin-1-yl)phenol (5a).—
11
12
11
10
10
Experimental
9
O
S
2
2
6
O
4
5
1
1
Apparatus and reagents.— Cyclic voltammetry, controlled-
potential coulometry and preparative electrolysis were performed
using an Autolab model PGSTAT 30 potentiostat/galvanostat. The
working electrode used in the voltammetry experiments was a
glassy carbon disk (1.8 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 carbon rods (31 cm2) and a large steel
sheet constitute the counter electrode. The working electrode po-
tentials were measured versus Ag/AgCl (from AZAR electrode
and Metrohm). 4-(piperazin-1-yl)phenol, benzenesulfinic acid, 4-
chlorobenzenesulfinic acid and 4-toluenesulfinic acid were reagent-
3
HO
N
NH
8
7
M.p.: 237–238◦C (Dec.). 1H NMR (500 MHz, DMSO-d6) δ (ppm):
2.83 (s, 4H, aliphatic), 2.95 (s, 4H, aliphatic), 5.71 (d, J = 8.9 Hz,
1H, aromatic), 6.07 (dd, J = 2.9 and 8.9, 1H, aromatic), 6.29 (d, J
= 2.9, 1H, aromatic), 6.49 (t, J = 7.8, 2H, aromatic), 6.56 (t, J = 7.4,
1H, aromatic), 6.8 (d, J = 7.4, 2H, aromatic). 13C NMR (125 MHz,
DMSO-d6) δ (ppm): 45.46 (C-1), 50.48 (C-2), 114.7 (C-4), 118.2
(C-7), 124.5 (C-5), 125.8 (C-10), 127.7 (C-11), 128.7 (C-6), 133.0
(C-9), 141.4 (C-8), 144.5 (C-3), 148.9 (C-12). IR (KBr) ν (cm−1):
3438 (broad, O-H), 3010 (weak, C-H), 2925, 2813 (weak, C-H), 1465
(strong C=C), 1455 (strong, C=C), 1288 (strong, S=O), 1288 (strong
C-O), 1142 (strong, S=O), 1087, 830, 688, 593. MS (EI, 70 eV)
z E-mail: nemat@basu.ac.ir
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Journal of The Electrochemical Society, 159 (4) E82-E86 (2012)
m/z (relative intensity): 318 (M+., 25), 276 (100), 77 (44), 56 (73),
E83
30 (56).
4-(Piperazin-1-yl)-2-tosylphenol (5b).—
13
H3C
12
11
11
10
10
9
O
O
6
S
2
2
4
5
1
1
3
HO
N
NH
8
7
M.p.: 246–247◦C (Dec.). 1H NMR (500 MHz, DMSO-d6) δ (ppm):
2.35 (s, 3H, methyl), 2.83 (s, 4H, aliphatic), 2.92 (s, 4H, aliphatic),
6.74 (d, J = 8.7 Hz, 1H, aromatic), 7.10 (d, J = 6.6 Hz, 1H, aromatic),
7.34 (d, J = 7.7 Hz, 3H, aromatic), 7.76 (d, J = 8.0 Hz, 2H, aromatic).
13C NMR (125 MHz, DMSO-d6) δ (ppm): 20.9 (C-13), 45.4 (C-1),
50.4 (C-2), 114.7 (C-4), 118.2 (C-7), 124.3 (C-5), 126.1 (C-10), 127.8
(C-11), 129.1 (C-6), 138.6 (C-9), 143.4 (C-8), 144.3 (C-3), 148.9
(C-12). IR (KBr) ν (cm−1): 3444 (broad, O-H and N-H), 3043 (weak,
C-H), 2948 (weak, C-H), 1631 (medium C=C), 1453 (strong, C=C),
1302 (strong, S=O), 1284 (strong, C-H, methyl), 1139 (strong, S=O),
1090, 942, 828, 742, 709, 657, 593. MS (EI, 70 eV) m/z (relative
intensity): 332 (M+., 21), 290 (97), 124 (42), 109 (71), 91 (100).
Figure 2. First and second cyclic voltammograms of 1.0 mM 4-(piperazin-1-
yl)phenol (1) in ethanol/water mixture (10/90) with pH = 1.5, at glassy carbon
electrode at various scan rates. T = 25 1◦C.
Fig. 3. The peak current ratio (IpC1/IpA1) decreases with increasing
pH as well as decreasing potential sweep rate. The cathodic peak
C1 disappears in basic solutions and/or in low potential sweep rates.
Also, the cathodic peak current ratio (IpC1/IpC0) is pH dependent and
decreases with increasing pH. This shows that the rate of above men-
tioned reactions (hydroxylation and dimerization) is pH dependent
and enhances by increasing pH (Fig. 3).21–25
In this work, with the aim of decreasing in the rate of introduced
hydroxylation and/or dimerization reactions, a solution containing
perchloric acid (pH = 1.0) has been selected as a suitable solution
for the electrochemical study of 4-(piperazin-1-yl)phenol (1) in the
presence of aryl sulfinic acids, and synthesis of 2-(phenylsulfonyl)-4-
(piperazin-1-yl)phenol derivatives.
2-(4-Chlorophenylsulfonyl)-4-(piperazin-1-yl)phenol (5c).—
Cl
11
12
11
10
10
9
O
O
6
S
2
2
4
5
1
1
3
HO
N
NH
8
7
Mp.: 229–230◦C (Dec.). 1H NMR (500 MHz, DMSO-d6) δ (ppm):
2.88 (s, 4H, aliphatic), 2.97 (s, 4H, aliphatic), 5.38 (broad, 2H,
OH, NH), 6.73 (s, 1H, aromatic), 7.15 (s, 1H, aromatic), 7.36 (s,
1H, aromatic), 7.66 (s, 2H, aromatic), 7.92 (d, 2H, aromatic). 13C
NMR (125 MHz, DMSO-d6) δ (ppm): 45.8 (C-1), 50.8 (C-2), 115.2
(C-4), 119.0 (C-7), 125.5 (C-5), 125.6 (C-10), 129.4 (C-11), 130.3
(C-6), 138.4 (C-9), 140.9 (C-8), 144.3 (C-3), 150.7 (C-12). IR (KBr)
ν (cm−1): 3454 (broad, O-H), 3274 (medium, N-H), 2952 and 2923
(weak, C-H, aliphatic), 2820 (weak, C-H), 1625 and 1582 (medium
C=C), 1474 (strong, C=C), 1303 (strong, S=O), 1278 (medium C-
O), 1147 (strong, S=O), 1087, 1011, 943, 827, 769, 705, 591. MS
(EI, 70 eV) m/z (relative intensity): 352 (M+., 28), 310 (100), 135 (7),
119 (7), 111 (7), 91 (14), 65 (9.3), 56 (21.7).
The E-pH diagram.— It was found that the peak potential for A1
(EpA1) shifted to the negative potentials by increasing in pH. This is
expected because of the participation of proton(s) in the oxidation
reaction of 1 to p-quinone-imine 2. The half-wave potential (E1/2), is
given by:26,27
E1ꢀ /2 = E1/2 − (2.303 mRT/2F)pH
where m is the number of involved protons in the reaction and E1/2 is
the half-wave potential at pH = 0.0, R, T, and F have their usual mean-
ings. The half-wave potentials (E1/2) were calculated as the average
of anodic and cathodic peak potentials of the cyclic voltammograms
{(EpA1 + EpC1)/2}. A potential-pH diagram is constructed for com-
pound 1 by plotting the calculated E1/2 values as a function of pH
(Fig. 4). The E1/2-pH diagram comprises two linear segments with
Results and Discussion
The effect of pH.— Electrochemical generation of p-quinone-
imine (2) and using it as a Michael acceptor for the synthesis of
2-(phenylsulfonyl)-4-(piperazin-1-yl)phenol derivatives on the one
hand, and minimizing its participation in other possible reactions,
on the other hand, are the main goal of this work. Therefore, electro-
chemical oxidation of 1 has been studied in various pHs.
Cyclic voltammograms of 1.0 mM solution of 4-(piperazin-1-
yl)phenol (1) in ethanol/water mixture (10/90) solution at pH = 1.5
are shown in Fig. 2. In this pH value, cyclic voltammograms exhibit
one anodic (A1) and two cathodic peaks (C1 and C0). A1 and C1 Peaks
are related to the transformation of 1 to p-quinone-imine 2 and vice
versa within a reversible two-electron process. In the second cycle, a
new anodic peak (A0) appears at less positive potential (Fig. 2). These
peaks (A0 and C0) show the occurrence of some reactions such as
hydroxylation on the electrochemically generated to p-quinone-imine
2, and/or coupling of 1 (as a nucleophile) with p-quinone-imine 2
(dimerization reaction), under the experimental conditions.
Figure 3. Cyclic voltammograms of 1.0 mM 4-(piperazin-1-yl)phenol (1)
at a glassy carbon electrode, in buffered solutions {ethanol/water mixture
(10/90)} with various pHs and same ionic strength. Scan rate: 1000 mV s−1
T = 25 1◦C.
.
Cyclic voltammograms of 4-(piperazin-1-yl)phenol 1 (1.0 mM)
in ethanol/water (10/90) solution at various pH values are shown in
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E84
Journal of The Electrochemical Society, 159 (4) E82-E86 (2012)
H+
+
O
N
NH2
O
N
NH
2
2'
Scheme 2.
Figure 4. The potential-pH diagram of 4-(piperazin-1-yl)phenol (1).
different slopes, which are crossing at pH = 5.1.
In pHs < 5.1 : E1ꢀ /2 = 0.55 − 0.034 pH or slope = 34 mV/pH
In pHs > 5.1 : E1ꢀ /2 = 0.64 − 0.051 pH or slope = 51 mV/pH
This shows that in various pHs, three different forms can be pro-
duced in diffusion layer, two in an oxidized form and another in
reduced form. On the basis of the above mentioned slopes, it can
be concluded that the electrode reaction occurring at the pH below
5.1 is a two-electron, one-proton process involving the oxidation of
“protonated” 1 to the corresponding “protonated” p-quinone-imine
2 in forward scan and reduction of “protonated” 2 to “protonated”
1 in reverse scan with E1/2 = 0.55 V vs. Ag/AgCl (Scheme 1, Eq.
1). Whereas, the electrode reaction at pH > 5.1, corresponds to the
two-electron, two-proton process with E1/2 = 0.64 V vs. Ag/AgCl
(Scheme 1, Eq. 2). Also, the calculated pKa for acid (2)/base (2ꢀ)
equilibria shown in Scheme 2 is: 5.1.
Figure 5. (a) Cyclic voltammogram of 1.0 mM 4-(piperazin-1-yl)phenol (1) in
the absence, (b) in the presence of 1.0 mM benzenesulfinic acid (3a) and, (c) 1.0
mM benzenesulfinic acid (3a) in the absence of 1 at a glassy carbon electrode,
in ethanol/water mixture (10/90) solution containing 0.1 M perchloric acid (pH
1.0). Scan rate: 10 mV s−1, T = 25 1◦C.
Mechanistic studies.— Cyclic voltammogram of a 1.0 mM of 1
in ethanol/water (10/90) solution containing 0.1 M perchloric acid
(pH 1.0) is shown in Fig. 5 curve a. As can be seen, one anodic
(A1) and two cathodic peaks C1 and C0 were obtained. Anodic and
cathodic peaks A1 and C1 are counterpart and are corresponded to the
transformation of 1 to p-quinone-imine 2 and vice versa within a quasi-
reversible two-electron process (Figure 5, curve a). The oxidation of
1 in the presence of benzenesulfinic acid (3a) as a nucleophile was
studied in some details. Figure 5 curve b, shows the obtained cyclic
voltammogram for a 1.0 mM solution of 1 in the presence of 1.0 mM
benzenesulfinic acid (3a). Under these conditions, the cathodic peaks
C1 and C0 disappear and a new anodic peak (A2) appears in more
positive potentials.
More studies were performed by varying the potential scan
rate in a solution of 1 in the presence of 3a. The results indicate that
the peak current ratio (IpC1/IpA1) is dependent on the potential scan
rate and increases with increasing it. The same result was obtained
by decreasing the concentration of benzenesulfinic acid (3a).
Controlled-potential coulometry was performed in ethanol/water
(10/90) solution containing 0.25 mmol of 1 and 0.25 mmol of ben-
zenesulfinic acid (3a) at 0.45 V versus Ag/AgCl. The electrolysis
progress was monitored by cyclic voltammetry (Fig. 6). It is shown
that, proportional to the progress of electrolysis,, and in parallel with
-2e -H+
HO
HO
N
N
NH
NH
(1)
(2)
O
N
NH
NH
E
E
= 0.55 V vs. Ag/AgCl
= 0.64 V vs. Ag/AgCl
pH < 5.1
2
1
1
Figure 6. Cyclic voltammograms of 0.25 mmol 4-(piperazin-1-yl)phenol (1)
in the presence of 0.25 mmol benzenesulfinic acid (3a) at a glassy carbon
electrode in ethanol/water mixture (10/90) containing 0.1 M perchloric acid
(pH = 1.0) during controlled-potential coulometry at 0.45 V versus SCE
after consumption of (a) 0, (b) 12, (c) 22, (d) 32, and (e) 42 C. Scan rate:
100 mV s−1. T = 25 1◦C.
-2e -2H+
pH > 5.1
O
N
2'
Scheme 1.
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Journal of The Electrochemical Society, 159 (4) E82-E86 (2012)
E85
Peak A1
- 2e- - H+
(1)
NH2
N
HO
R
O
N
NH2
Peak C1
1
2
R
SO2
SO2H
H+
3a'-c'
3a-c
O
N
NH2
H
O S O
R
+ O
N
NH2
SO2
(2)
4a-c
2
3a'-c'
R
NH2
N
HO
O
S
R=H
R=CH3
R=Cl
5a
5b
5c
Isolated Yield 77%
Isolated Yield 72%
Isolated Yield 75%
O
5a-c
R
The overall reaction:
R
SO2H
3a-c
- 2e- - 2H+
HO
N
NH2
HO
N
NH2
Eapp = 0.45 V vs. Ag/AgCl
Carbon Anode
O
1
S
O
5a-c
Divided Cell
0.1 M HClO4
R
Scheme 3.
the decrease in the height of peak A1, the current of anodic and ca-
thodic peaks A2 and C2 increases. The anodic peak A1 disappears
when the charge consumption becomes about 2e− per molecule of
1. These coulometry and voltammetry results accompanied by the
NMR (1H and 13C) data and molecular mass of 318 of final prod-
uct allows us to propose an EC mechanism for the electrochem-
ical oxidation of 1 in the presence of benzenesulfinic acid (3a)
(Scheme 3).
difficult than the oxidation of the starting molecule 1 by virtue of the
presence of the electron-withdrawing sulfonyl group on 5a. There-
fore, over-oxidation of 5a was circumvented during the electrolysis.
According to our results, the anodic peaks A1 and A2 pertain to the
oxidation of 4-(piperazin-1-yl)phenol (1) and diphenylsulfone 5a, to
p-quinone-imine 2 and 6a (Scheme 4), respectively. Obviously, the ca-
thodic peaks C1 and C2 are related to the reduction of p-quinone-imine
2 and 6a into 1 and 5a.
The generation of p-quinone-imine 2 is followed by a Michael
type addition reaction of 3a on p-quinone-imine 2, producing the
diphenylsulfone 5a as the final product. The oxidation of 5a is more
Conclusions
The results show that 4-(piperazin-1-yl)phenol (1) is oxidized
to the corresponding p-quinone-imine which is then attacked by
an aryl sulfinic acid in a Michael-like reaction. The product,
a derivative of 2-(phenylsulfonyl)-4-(piperazin-1-yl)phenol, is ob-
tained via an EC mechanism, after the consumption of 2e− per
molecule of 1. The products are obtained in good yield and high
purity. Our research has led to the development of a facile, en-
vironmentally friendly method that occurs under mild reaction
conditions.
Peak A2
- 2e- - H+
HO
N
NH2
NH2
N
O
Peak C2
O
O
S
S
O
O
5a-c
6a-c
R
R
Scheme 4.
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E86
Journal of The Electrochemical Society, 159 (4) E82-E86 (2012)
Acknowledgments
The authors acknowledge the Bu-Ali Sina University Research
Council and Center of Excellence in Development of Chemical Meth-
ods (CEDCM) for support this work.
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