Electrochemical behavior of catechol in the presence of 2-methyl-1,3-cyclopentanedione: Application to electrosynthesis

Article abstract of DOI:10.1007/s00706-008-0080-8

Electro-oxidation of catechol in the presence of 2-methyl-1,3- cyclopentanedione as a nucleophile was investigated in water-acetonitrile (90:10 v/v) solution. The results indicate that the o-benzoquinone electrogenerated participates in a Michael addition

Full text of DOI:10.1007/s00706-008-0080-8

Monatsh Chem (2009) 140:503–508
DOI 10.1007/s00706-008-0080-8
ORIGINAL PAPER
Electrochemical behavior of catechol in the presence
of 2-methyl-1,3-cyclopentanedione: application to electrosynthesis
Reza Ojani Æ Jahan-Bakhsh Raoof Æ
Rahman Hosseinzadeh Æ Ali Alinezhad
Received: 7 July 2008 / Accepted: 1 October 2008 / Published online: 5 November 2008
Ó Springer-Verlag 2008
Abstract Electro-oxidation of catechol in the presence of
2-methyl-1,3-cyclopentanedione as a nucleophile was
investigated in water–acetonitrile (90:10 v/v) solution. The
results indicate that the o-benzoquinone electrogenerated
participates in a Michael addition reaction with this
nucleophile. The electrosynthesis of 2-(3,4-dihydroxy-
phenyl)-2-methylcyclopentane-1,3-dione was carried out.
The product was characterized by NMR, MS, FT-IR, and
elemental analysis. An EC mechanism was deduced from
voltammetric and spectroscopic data. Also, the Michael
addition reaction rate constant (km) was estimated using
digital simulation of voltammograms.
classical organic synthesis: simpler synthesis, waste
reduction owing to the fact that the electron is provided
directly by the electric current without a reagent and mild
conditions (temperature, pH, etc.) [3–5].
In recent years, there has been a growing interest in the
study of reactions between electrogenerated benzoquinones
from oxidation of polyphenols and some nucleophiles [6].
Among these, interest in catechol has increased, not only
because it is a model molecule for compounds bearing
catechol as part of their structure, for example dopamine
and L-dopa, but also because it has important physiological
functions and some pharmacological activity. An important
pathway in the metabolism of catechol estrogens and
catecholamines is oxidation to their respective semibenzo-
quinones and benzoquinones [7]. The basic biological
activity of catechol-derived benzoquinones is related to
their ability to act both as oxidants and electrophiles. As
oxidants, benzoquinones form a redox cycle with their
semibenzoquinones, producing an elevated level of reactive
oxygen species, a condition known as oxidative stress [7].
As electrophiles, benzoquinones that are produced can form
stable covalent adducts with cellular macromolecules,
including DNA [8]. Thus, DNA can be damaged by reaction
with itself and by reaction with oxygen species (hydroxyl
radicals) [7, 8]. The formation of depurinating adducts by
catechol estrogen quinine reacting with DNA may be a
major event in the initiation of breast and other human
cancers [8].
Keywords Electrochemistry ꢀ Catechol ꢀ
Cyclic voltammetry ꢀ Michael addition ꢀ
2-Methyl-1,3-cyclopentanedione
Introduction
Because of the increasing importance of green chemistry in
organic synthesis, development of more efficient and
environmentally friendly processes for chemical transfor-
mations is desired [1, 2]. One methodology is the
development of organic reactions using electrolytic meth-
ods in aqueous media or in organic–water mixed solvent
(with the minimum ratio of organic solvent to water).
Electroanalytical methods offer several advantages over
It has been shown that o and p-benzenediols can be
oxidized electrochemically to o and p-benzoquinones.
The benzoquinones formed are quite reactive and are
attacked by a variety of nucleophiles through a Michael
addition reaction [9]. This reaction has been utilized for
spectrophotometric determination of some o-benzoqui-
nones [10, 11].
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-008-0080-8) contains supplementary
material, which is available to authorized users.
R. Ojani (&) ꢀ J.-B. Raoof ꢀ R. Hosseinzadeh ꢀ A. Alinezhad
Faculty of Chemistry, Mazandaran University, Babolsar, Iran
e-mail: fer-o@umz.ac.ir
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R. Ojani et al.
Currently, our interest has focused on electrochemical
behavior of polyphenols, including catechols, in the pres-
ence of nucleophiles. As a part of our recent research we
have reported the Michael addition reaction of diethyl-
amine
and
dibutylamine
with
electrogenerated
o-benzoquinone [12–14]. Michael addition reactions are
among the most important carbon–carbon bond-forming
reactions in modern synthetic organic chemistry [15].
Thus, in the literature, there are reports on the carbon–
carbon bond-forming reaction arising from addition of
carbon nucleophiles to electrogenerated o-benzoquinone
[16–21].
Extending our interest in the research on electrochemical
oxidation of catechols in the presence of nitrogen nucleo-
philes, we became interested in understanding and exploiting
the oxidative carbon–carbon bond-forming reaction of
electrogenerated o-benzoquinone with 2-methyl-1,3-cy-
clopentanedione (MCPD). Choosing MCPD as nucleophile
was on the basis of controlling the reaction conditions.
Replacing one of the acidic hydrogens by a methyl group
removes, to some extent, the complexity of reactions and
eliminates significant by-product formation.
Fig. 1 Cyclic voltammograms of 1.0 mM catechol in the presence of
1.0 mM MCPD at a glassy-carbon electrode in the solvent-supporting
electrolyte (SSE) with various pH values. pH for a–d are: 2, 5, 7, and
10, respectively. Scan rate is 10 mV s-1 and the SSE is water–
acetonitrile (90:10 v/v) solution containing 0.15 M phosphate buffer
study of the reaction between o-benzoquinone and MCPD.
The cyclic voltammograms of catechol in the absence and
presence of MCPD at a scan rate of 10 mV s-1 at pH 7 are
shown in Fig. 2.
Results and discussion
The cyclic voltammogram of 1.0 mM catechol (Fig. 2a)
in water–acetonitrile (90:10 v/v) solution containing
0.15 M phosphate buffer (pH 7) showed one anodic peak
(A1) at ?0.205 V and a corresponding cathodic peak (C1)
at ?0.136 V, which corresponds to the transformation of
catechol to o-benzoquinone and vice versa (Scheme 1)
within a quasi-reversible two-electron process. A peak
current ratio [Ip(C1)/Ip(A1)] of nearly unity, particularly
during repeated recycling of the potential, is considered to
Electrochemical study
The electrochemical behavior of catechol in the presence of
MCPD was studied by cyclic voltammetry. The influence
of pH on the behavior of catechol in the presence of 2-
methyl-1,3-cyclopentanedione (MCPD) was investigated
by examining the electrode response in aqueous solution
with different buffered pHs between 2 and 10. Cyclic
voltammograms detailing the oxidation of 1.0 mM cate-
chol in the presence of 1.0 mM MCPD at pH 2, 5, 7, and 10
are compared in Fig. 1.
As can be seen, at acidic pH (e.g. 2), the cyclic vol-
tammogram of catechol shows one anodic peak and the
corresponding cathodic peak with a peak-current ratio
[Ip(C1)/Ip(A1)] of nearly unity. Consequently, this shows
there is no reaction between electrogenerated o-benzoqui-
none and MCPD, which can be attributed to the fact that at
pH 2, MCPD exists totally as the protonated form and has
no nucleophilic characteristics. However, with increasing
pH, the cathodic peak C1 diminishes until it disappears at
approximately pH 7. The decreasing height of the cathodic
peak C1 (or its disappearance) can be attributed to the
occurrence of a reaction between the electrogenerated
o-benzoquinone and MCPD. In more basic solution (e.g.
pH 10), the behavior of catechol in the presence of MCPD
is similar to that in the absence of nucleophile and becomes
irreversible. This suggests that the oxidation of catechol is
followed by an irreversible chemical reaction with hydro-
xyl ion. Thus, the pH range 7–8 is the optimum range for
Fig. 2 Cyclic voltammograms of 1.0 mM catechol in the absence (a)
and presence (b) of 1.0 mM MCPD at a scan rate of 10 mV s-1 in an
SSE of pH 7 at the surface of a GC electrode. Voltammograms (c)
and (d) represent the electrochemical behavior of 1.0 mM MCPD in
the SSE and of the pure SSE (pH 7), respectively. Inset, the Tafel plot
for catechol obtained from the data in Fig. 2a
123

Electrochemical behavior of catechol
505
Scheme 1
OH
OH
O
O
- 2 e - - 2 H +
Eq. (1)
O -
O
O
- H +
CH3
H
CH3
CH3
O-
O
O
O-
OH
O
O
O
K , km
Eq. (2)
OH
CH3
+
CH3
O
O
be the criterion for production of o-benzoquinone at the
surface of the electrode under these experimental condi-
tions. This result shows that hydroxylation or dimerization
is too slow to be observed on the time scale of experiment
[22–25]. The oxidation of catechol in the presence of
MCPD as nucleophile was studied in some detail. Fig-
ure 2b shows the cyclic voltammogram obtained for a
1.0 mM solution of catechol in the presence of 1.0 mM
MCPD. The voltammogram showed only one anodic (A1)
at ?0.226 V, and the corresponding cathodic peak was not
observed. The anodic shift in A1 in the presence of MCPD
is probably because of the formation of a thin film of
product on the surface of the electrode, inhibiting to some
extent the performance of the electrode process [26, 27].
Figure 2, curves c and d, are the cyclic voltammograms of
MCPD and pure buffer solution in this range of potential,
respectively. As can be seen (Fig. 2c), MCPD is not elec-
troactive at the oxidation potential of catechol,
consequently it can show its nucleophilic property. The
multi-cyclic voltammograms of catechol in the presence of
MCPD are shown in Fig. 3.
Fig. 3 Multi-cyclic voltammograms of 1.0 mM catechol in the
presence of 1.0 mM MCPD at a glassy-carbon electrode at a scan
rate of 10 mV s-1 in the SSE (pH 7)
consequently, the cathodic peak C1 does not disappear. The
C1 current decreased with an increase in the concentration
of MCPD from 0.25 to 1 mM. Using the obtained vol-
tammograms, Ip(C1)/Ip(A1) was extracted and plotted
versus the MCPD concentration. These results indicate that
nucleophilic attack of MCPD on o-benzoquinone increases
with an increase in the concentration of MCPD. The
dependence of Ip(C1)/Ip(A1) on the MCPD concentration
shows the dependence of the chemical reaction rate on the
MCPD concentration. The absence of an oxidation peak of
the Michael adduct can probably be attributed to its
insolubility in the solution.
The voltammograms exhibit a relatively intense decrease
in anodic peak current of A1 together with some potential
shift in a positive direction. The positive shift of A1 peak in
the presence of MCPD is probably because of the formation
of a thin film of product on the surface of the electrode
inhibiting to some extent the performance of electrode
process [10]. This problem was circumvented by regular
cleaning of the electrode surface.
The effect of MCPD concentration on the cyclic vol-
tammograms of catechol was studied (figure not shown).
Cyclic voltammograms of a 1.0 mM solution of catechol in
the presence of different concentrations of MCPD showed
that at a low concentration ratio of MCPD to catechol (such
as 1:4), nucleophilic attack of MCPD on electrogenerated
o-benzoquinone does not occur to a great extent and,
Furthermore, we examined the effect of potential scan
rate on the peak current ratio [Ip(C1)/Ip(A1)] in the cyclic
voltammograms of catechol in the presence of MCPD. It is
seen that, in proportion to the increase of the potential scan
rate (Fig. 4), the peak current ratio [Ip(C1)/Ip(A1)] increa-
ses. Since the time window of voltammetry decreases on
increasing the potential scan rate, less electrogenerated
123

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R. Ojani et al.
electrooxidation of catechol in the presence of MCPD
(Scheme 1). According to this mechanism, catechol is
electrochemically oxidized to o-benzoquinone. It is well
known that the electrochemical oxidation of catechol to
o-benzoquinone in water or water–organic mixed solvents
(low fraction of organic solvent), is considered as a one
step, two-electron, two-proton-transfer process [30–32].
The Michael acceptor of o-benzoquinone can then be
attacked at position C-4 or C-6 (equivalent positions) by
MCPD as a nucleophile to yield the only product.
In order to further confirm the proposed mechanism and
synthesis of a new catechol derivative, controlled potential
electrolysis of catechol in the presence of MCPD was carried
out. The potential of the working electrode (carbon rod) was
fixed at ?0.35 V versus the reference electrode. As men-
tioned in previous sections, characterization of the product of
electrolysis indicated that it was 2-(3,4-dihydroxyphenyl)-2-
methylcyclopentane-1,3-dione (1). Electrochemical syn-
thesis of 1 confirms the reactivity of electrogenerated o-
benzoquinone against MCPD via an EC mechanism.
Fig. 4 Cyclic voltammograms of 1.0 mM catechol in the presence of
1.0 mM MCPD in the SSE (pH 7) at a glassy-carbon electrode and
various scan rates. Scan rates from a to g are: 5, 10, 25, 50, 75, 100,
and 200 mV s-1. Inset, variation of peak current ratio [Ip(C1)/Ip(A1)]
versus scan rate
o-benzoquinone contributes to the chemical reaction with
MCPD. A plot of peak current ratio [Ip(C1)/Ip(A1)] versus
the scan rate for a mixture of catechol and MCPD confirms
the reactivity of o-benzoquinone toward MCPD (inset of
Fig. 4).
According to our results, the Michael addition reaction
of MCPD to o-benzoquinone (Eq. 2) seems to occur much
faster than other side reactions, leading to the product 1.
The overoxidation of 1 was circumvented during the pre-
parative reaction because of the insolubility of the product
in the phosphate buffer solution medium.
Controlled-potential coulometry was performed in
water–acetonitrile (90:10 v/v) solution containing 0.5 mmol
catechol, 0.5 mmol MCPD, and 0.15 M phosphate buffer
(pH 7). Monitoring of the electrolysis progress was carried
out by cyclic voltammetry (Fig. 5).
Digital simulation
It is clearly observed that, in proportion to the
advancement of coulometry, the anodic peak A1 decreases
and disappears when the charge consumption becomes
about 2e- per molecule of catechol. These coulometry and
voltammetry results allow us to propose an EC (electro-
chemical and chemical) mechanism [28, 29] for the
Digital simulation of cyclic voltammograms provides an
opportunity for testing the EC mechanism illustrated in
Scheme 1 and, if successful, to estimate the rate of
chemical reaction of o-benzoquinone with MCPD. To
verify the reaction mechanism shown in Scheme 1 for the
electrochemical oxidation of catechol in the presence of
MCPD, the cyclic voltammograms were analyzed by dig-
ital simulation to find the best fit between experimental and
simulated cyclic voltammograms.
Digital simulation was carried out using the software
DigiElch 4.5 written by Rudolph [33, 34]. The experimental
results were simulated according to an EC mechanism. The
simulation was carried out assuming semi-infinite diffusion
and planar geometry of the electrode and the number of
electrons transferred in all steps as two. The formal poten-
0
0
tial (E ) of the catechol/o-benzoquinone redox couple was
experimentally calculated as (Epc ? Epa)/2 and was
assumed to be equal to its standard electrode potential (E0).
The values of a and k0 were obtained by using the Tafel
plot. Tafel plots (inset of Fig. 2) were drawn by using the
data driven from the rising part of current-voltage curve at
low scan rates, for example 10 and 25 mV s-1. A mean
slope of 15.01 (V per decade)-1 and intercept (log I0) of
0.46 were obtained for the Tafel plots. General equations
Fig. 5 Cyclic voltammograms of SSE of pH 7 containing 0.5 mmol
catechol and 0.5 mmol MCPD at a glassy-carbon electrode during
controlled-potential coulometry at 0.35 V versus Ag|AgCl|KCl 3 M
after consumption of a 0, b 15, c 31, d 48, and e 65 C. Inset, variation
of anodic peak current [Ip(A1)] versus charge consumption. Scan rate
is 10 mV s-1
123

Electrochemical behavior of catechol
507
for the slope value of the anodic branch of the Tafel plot and
the exchange current (I0) are written as below [28]:
Slope ¼ ð1 ꢁ aÞnF=2:3RT
I0 ¼ nFAk0c
ð1Þ
ð2Þ
By substituting the slope = 15.01 (V per decade)-1
,
,
n = 2, F = 96,485 C mol-1
,
R = 8.314 J mol-1 K-1
and T = 298 K in Eq. 1, the a value was obtained as
0.56. Also, by substituting values of A (electrode surface
area) = 3.69 9 10-2 cm2, c (concentration) = 1 9 10-6
mol cm-3, and I0 = 2.88 9 10-6 A and other known
values in Eq. 2, the value of k0 was obtained as
4.05 9 10-4 cm s-1. The experimental values entered for
digital simulation were: Estart -0.15 V, Eswitch ?0.40 V
versus Ag|AgCl|KCl 3 M, and the surface area of the
electrode was 3.69 9 10-2 cm2. All these values were kept
constant throughout the fitting of the digitally simulated
cyclic voltammograms to the experimental data. E0, k0, and
a were fixed and km was allowed to change through the
fitting process. The estimated value for km is
13 2 M-1 s-1 and kf/kb is 94 7.
Some cyclic voltammograms of catechol in the presence
of MCPD at various scan rates together with their simu-
lated cylic voltammograms were shown in Fig. 6a. By
using the data obtained from Fig. 6a, a correlation diagram
between oxidation peak current [Ip(A1)] of experimental
and related simulated cyclic voltammograms over various
scan rates was drawn (Fig. 6b). According to Fig. 6b, a
good correlation of 99.9% between oxidation peak current
of experimental and related simulated cyclic voltammo-
grams over various scan rates was found and also close
agreement between experimental and the corresponding
simulated results was obtained, which confirm the validity
of the estimated km.
Fig. 6 a Experimental cyclic voltammograms of 1.0 mM catechol in
the presence of 1.0 mM MCPD in the SSE (pH 7) at various scan
rates with corresponding simulated cyclic voltammograms. Scan rates
from a to d are 10, 50, 100, and 200 mV s-1. Experimental: solid line
and simulated: dashed line. b The correlation between anodic peak
current of simulated and experimental results
counter electrode. Carbon rods were from Azar electrode
(Iran) and the geometrical area of each carbon rod that was
inserted in the solution was about 7.8 cm2. The working
electrode potentials were measured versus Ag|AgCl|KCl
3 M (from Metrohm).
Experimental
Apparatus and reagents
The FT-IR spectra (KBr) were determined on a Bruker
spectrometer. 1H and 13C NMR spectra were measured on a
Bruker DRX-500 Avance spectrometer at 500.13 and
125.03 MHz, respectively. Mass spectroscopic measure-
ments were acquired on an HP 5973 spectrometer.
Elemental analysis (C, H) of the product was performed
with a CHNS 932 LECO (USA); its results agreed favor-
ably with calculated values.
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 disc (Metrohm, 1.8 mm diameter), and a
platinum wire was used as the counter electrode. The
glassy-carbon electrode was polished between each set of
experiments with aluminum oxide powder on a polishing
cloth. The working electrode used in the controlled-
potential coulometry and macro-scale electrolysis was an
assembly of three carbon rods from (6 mm in diameter and
6 cm in length), and a large platinum gauze served as
All chemicals were reagent-grade materials. Catechol
was obtained from Fluka and MCPD was obtained from
Merck and used without further purification. All experiments
were carried out in 0.15 M phosphate buffer–acetonitrile
(90:10 v/v) solution.
123

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2-(3,4-Dihydroxyphenyl)-2-methylcyclopentane-
1,3-dione (1, C12H12O4)
A water–acetonitrile solution (90:10 v/v, 80 cm3) contain-
ing phosphate buffer (0.15 M, pH 7) was pre-electrolyzed
at 0.9 V versus an Ag|AgCl|KCl 3 M reference electrode in
an undivided cell, then 0.5 mmol catechol and 0.5 mmol
MCPD were added to the cell. Potentiostatic electrolysis
was carried out at 0.35 V versus the reference electrode
and the current at the beginning of electrolysis was 95 lA.
The electrolysis was terminated after about 1.5 h, when the
current had decreased by more than 95%. The process was
interrupted eight times during the electrolysis and the
graphite anode was washed in acetone in order to reactivate
it. It can be said that this method can be scaled up to
100 mmol catechol. To decrease the electrolysis time,
increasing the anode area (by increasing the number of
carbon rods) is necessary. At the end of electrolysis a few
drops of acetic acid were added to the solution and the cell
was placed in the refrigerator over night. The precipitated
solid was collected by filtration and washed thoroughly
with cold water. The precipitate was purified by column
chromatography using ethyl acetate–n-hexane (50:50) as
1
mobile phase. Yield: 79%, m.p.: 162–164 °C; H NMR
(500.13 MHz, DMSO-d6): d = 1.24 (s, CH3), 2.82–2.67
(m, 2CH2), 6.37 (dd, J = 8.3 Hz, J = 2.3 Hz, 1H, arom.),
6.57 (d, J = 2.3 Hz, 1H, arom.), 6.69 (d, J = 8.3 Hz, 1H,
arom.), 8.98 (s, OH), 9.05 (s, OH) ppm; 13C NMR
(125.03 MHz, DMSO-d6): d = 20.25, 35.93, 61.15,
114.57, 116.77, 118.09, 129.01, 145.85, 146.51, and
214.71 ppm; FT-IR (KBr): = 3,409, 1,705, 1,606, 1,528,
1,458, 1,423, 1,378, 1,296, 1,200, 1,049, 805, 604 cm-1
MS: m/z = 220 (M?), 164, 150, 136, and 89.
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Acknowledgments We are grateful to Mazandaran university
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