Kinetic study on electrochemical oxidation of catechols in the presence of cycloheptylamine and aniline: Experiments and digital simulation

Article abstract of DOI:10.1007/s12039-016-1193-y

Oxidative coupling reaction of some catechols has been studied by cyclic voltammetry at the glassy carbon electrode in different experimental conditions. The electrogenerated o-banzoquinone participates in a coupling reaction with anionic and dianionic forms of catechol. Based on EC mechanism, the observed homogenous rate constants of the coupling reaction of catechols were estimated by analyzing the cyclic voltammetric responses using the simulation software DIGIELCH. This paper deals with reaction of o-benzoquinones derived by the oxidation of catechol (CAT), 3-methylcatechol (3-MC), 3-methoxycatechol (3-MOC) and 3,4-dihydroxybenzoic acid (3,4-DHBA) with cycloheptylamine (a primary aliphatic amine) and aniline (a primary aromatic amine) as nucleophiles to gain mechanistic insight. The outcome indicates participation of o-benzoquinone in the Michael addition reaction with the studied primary amines. The best fit of the experimental and simulated results was obtained for ECE mechanism. The calculated/estimated homogeneous rate constants (k_obs) for Michael addition reaction were found to vary in the order CAT >3-MC >3-MOC >3,4-DHBA and CAT >3,4-DHBA >3-MC >3-MOC for cycloheptylamine and aniline, respectively. These data are in agreement with the trend of electronic properties (electron-donating/-withdrawing) of the substitutions on the catechol ring. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1193-y

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1193-y
Kinetic study on electrochemical oxidation of catechols in the presence
of cycloheptylamine and aniline: Experiments and digital simulation
DAVOOD NEMATOLLAHIa,∗, FATEMEH GHASEMIa, SADEGH KHAZALPOURa and
FAHIMEH VARMAGHANIb,c
aFaculty of Chemistry, Bu-Ali Sina University, 65178-38683, Hamedan, Iran.
bDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran
cCenter for Research in Basic Sciences and Contemporary Technologies, IASBS, Zanjan, Iran
e-mail: nemat@basu.ac.ir
MS received 2 August 2016; revised 7 October 2016; accepted 14 October 2016
Abstract. Oxidative coupling reaction of some catechols has been studied by cyclic voltammetry at the
glassy carbon electrode in different experimental conditions. The electrogenerated o-banzoquinone partici-
pates in a coupling reaction with anionic and dianionic forms of catechol. Based on EC mechanism, the
observed homogenous rate constants of the coupling reaction of catechols were estimated by analyzing the
cyclic voltammetric responses using the simulation software DIGIELCH. This paper deals with reaction of
o-benzoquinones derived by the oxidation of catechol (CAT), 3-methylcatechol (3-MC), 3-methoxycatechol
(3-MOC) and 3,4-dihydroxybenzoic acid (3,4-DHBA) with cycloheptylamine (a primary aliphatic amine) and
aniline (a primary aromatic amine) as nucleophiles to gain mechanistic insight. The outcome indicates partici-
pation of o-benzoquinone in the Michael addition reaction with the studied primary amines. The best fit of the
experimental and simulated results was obtained for ECE mechanism. The calculated/estimated homogeneous
rate constants (kobs) for Michael addition reaction were found to vary in the order CAT>3-MC>3-MOC>3,4-
DHBA and CAT>3,4-DHBA>3-MC>3-MOC for cycloheptylamine and aniline, respectively. These data are
in agreement with the trend of electronic properties (electron-donating/-withdrawing) of the substitutions on
the catechol ring.
Keywords. Catechol; cyclic voltammetry; cycloheptylamine; aniline; coupling reaction; digital simulation.
1. Introduction
Electrochemically induced Michael addition reac-
tion of o-benzoquinones with some primary10,11 and
secondary12 14 amines has been investigated. Herein, to
get more insight into the oxidative coupling of cate-
chols, we have investigated the electrochemical oxidation
of catechols in absence and presence of cyclohepty-
lamine (1) as a primary aliphatic amine and aniline
(2) (a primary aromatic amine). The paper has eval-
uated mechanistic and kinetic behavior of anodically
generated quinones in the presence of 1 and 2 and the
observed homogeneous rate constant (kobs) values of the
reaction of o-benzoquinones with these nucleophiles
were estimated by digital simulation of the correspond-
ing cyclic voltammograms at various experimental con-
ditions such as pH, potential scan rate and nucleophile
concentration.
Catechol as a nature-based molecule has been given the
attention for use as drugs. Comprehensive medicinal
chemistry (CMC) database introduced 78 catecholic
drugs.1 Antioxidative properties of catecholic com-
pounds are well known for their ability to protect
against reactive oxidative species.2 4 Some pharma-
cological effects of catecholic drugs are adrenergic
(isoproterenol, epinephrine, dopamine), antioxidant
(gossypol, baicalein), antiasthmatic ((r)-isoproterenol,
levisoprenaline), antibiotic (cefetecol, fomidacillin),
enzyme inhibitor (benserazide, carbidopa)1 and anti-
cancer activities.5 Catechols can be easily oxidized
electrochemically to o-benzoquinones at low oxidation
potentials in mild media.6 Based on many evidences,
o-benzoquinone undergoes a coupling reaction with
anionic and dianionic species of catechol to form C–C
or C–O bond.7 9
2. Experimental
Quinone, a quite reactive molecule, acts as Michael
acceptor to react with various nucleophilic compounds.
2.1 Apparatus and Reagents
Cyclic voltammetry was performed using Sama-500
instrument. The working electrode used in the voltammetry
∗For correspondence

Davood Nematollahi et al.
experiments was a glassy carbon disk (1.8 mm diam- Acid dissociation reaction causes the anionic form of
eter), and platinum wire was used as counter elec-
trode. The working electrode potential was measured
vs. SCE. Before recording each voltammogram, the
working electrode was carefully cleaned by polishing
with alumina slurry and finally washed with distilled
water. A Metrohm-pH meter was used for pH mea-
surements. A mixture of Na2CO3/NaHCO3 was used
as buffer for pH 8-10. Phosphate was utilized to pro-
vide pH of 2, 3, 6 and 7, and acetate buffer for pH
4 and 5. The total concentration of the buffer species
was adjusted to 0.2 M. The homogeneous rate constants
were estimated by analyzing the cyclic voltammetric
responses using the simulation software DIGIELCH.15
Catechol and its derivatives (3-methylcatechol (3-MC),
3-methoxycatechol (3MO), 3,4-dihydroxybenzoic acid
(3,4-DHBA), 2,3-dihydroxybenzoic acid (2,3DHBA))
and other chemicals were procured from E. Merck. All
solutions in voltammetric studies were deoxygenated
by bubbling pure nitrogen.
catechol and the ratio of anionic/neutral forms increases
along with increasing pH.
The electrochemical oxidation of catechol was stud-
ied in various concentrations. Figure 2 shows the nor-
malized cyclic voltammograms of catechol at different
concentrations. The normalized voltammograms were
made by dividing the current by concentration (I/c
vs. μA mM−1). As can be seen, the peak current
ratio (IpC1/IpA1) is not constant and is dependent on
the concentration of catechol. This implies the cou-
pling reaction after electron transfer process. The higher
is the concentration of catechol, the less is cathodic
peak current.
The effect of coupling reaction on the cyclic voltam-
mograms was analyzed by digital simulation and homo-
geneous rate constants (kobs) of the coupling reaction
of o-benzoquinones with catechol derivatives were esti-
mated. The simulation was carried out assuming semi-
infinite one-dimensional diffusion and planar electrode
geometry. A reversible two-electron transfer process was
defined. The heterogeneous rate constant (0.002 cm/s)
for oxidation of catechols was estimated by use of
experimental working curves.16 The formal potentials
were obtained experimentally as the midpoint poten-
tial between the anodic and cathodic peaks (Emid). The
transfer coefficient (α) was assumed 0.5. When enter-
ing α, the charge transfer is treated to obey the Butler-
Volmer equation. An irreversible second-order chemical
reaction was defined in the dialog box of the software.
This chemical reaction was characterized by equilib-
rium constant (Keq), forward (kf = kobs) and back-
ward (kb, kb equals to zero for irreversible process) rate
constant. Some other experimental parameters entered
3. Results and Discussion
Initially, electrochemical performance of catechol was
analyzed by cyclic voltammetry in acidic, neutral and
basic solutions. Figure 1 demonstrates pH dependence
of the cyclic voltammogram. The cyclic voltammo-
grams show one anodic (A1) and a corresponding
cathodic peak (C1). Catechol undergoes an oxidative
electron transfer and converts into o-benzoquinone and
vice versa in a two-electron process.
In acidic media, the current ratio (IpC1/IpA1) is nearly
unity. By increasing pH, the peak current ratio deviates
from unity and approaches to zero in basic solution.
This behavior is related to the coupling of anionic and
dianionic forms of catechol with o-benzoquinone.7 9
Figure 1. Cyclic voltammograms of catechol (1.0 mM) at
glassy carbon electrode, in water (at various pH values of
2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0, respectively)/ethanol
(70:30, v/v) mixture. Scan rate: 10 mV s−1. Temperature =
25 1◦C.
Figure 2. Cyclic voltammograms of a) 0.5 mM, b) 1.0 mM
and c) 2.0 mM catechol at glassy carbon electrode in water
(carbonate buffer, c = 0.2 M, pH 8.5)/ethanol (70:30, v/v)
mixture at scan rate = 100 mV s−1. Temperature = 25 1◦C.

Electrochemical oxidation of catechols
Table 1. Calculated/estimated homogeneous rate con-
for digital simulation consisted of the following: Estart
,
Eswitch, Eend, T = 25◦C, potential scan rate, surface
area of electrode (1.8 mm diameter) and the analyti-
cal concentration of catechol. All parameters were kept
constant throughout the fitting of the digitally sim-
ulated voltammogram to the experimental data. The
parameter kobs was allowed to change through the fit-
ting process. The simulation was performed based on
EC mechanism (E: electrochemical oxidation of cat-
echol and C: coupling reaction of catechol with o-
benzoquinone) (Scheme 1) (Figure 3). The calculated
rate constant values of the coupling reaction of catechol
(CAT), 3-methylcatechol (3-MC), 3-methoxycatechol
(3-MOC), 3,4-dihydroxybenzoic acid (3,4-DHBA) and
2,3-dihydroxybenzoic acid (2,3-DHBA) for various pH
values are given in Table 1.
stants (kobs) for the oxidative coupling reaction of catechols
(1.0 mM) in water/ethanol (70:30, v/v) mixture at various pH
values.
kobs (M−1s−1
pH = 9.0
4875 ( 150)a 8925 ( 95)a 14066 ( 200)a
6175 ( 100) 11000 ( 70)
)
Catechol
CAT
2,3DHBA 3020 ( 83)
3-MC 1050 ( 86)
3,4-DHBA 500 ( 10)
3-MOC 198 ( 8)
pH = 8.0
pH = 10.0
4080 ( 57)
1850 ( 91)
950 ( 35)
8675 ( 95)
6666 ( 81)
3050 ( 130)
afor n = 5 at various scan rates.
electron-donating group of CH3, OCH3 or COO− on the
produced o-quinone ring makes it more difficult to par-
ticipate in the coupling reaction. In addition, the pres-
ence of carboxylate group in C-4 position of 3,4-DHBA
which is a reactive site of 3,4-dihydroxybenzoquinone
toward coupling reaction, causes a decrease in the
homogeneous rate constant. On the other hand, the pres-
ence of an electron-donating group in the catechol ring
makes the nucleophile stronger, so that the Michael
addition is favored. So, the presence of an electron-
donating group in the molecule ring has two different
effects. It increases the nucleophilicity of the catechol,
while decreasing the electrophilicity of o-benzoquinoe
toward the Michael addition reaction. However, our
data show that, its effect on the decreasing of the elec-
trophilicity of o-benzoquinone is more than the increase
of the nucleophilicity of catechol.
It is apparent from the Table 1 that pH of solution
and nature of the substituted group on the catechol ring
affect the coupling reaction rate. The reactivity for cate-
chols varies in the following order: CAT > 2,3-DHBA
> 3-MC > 3,4-DHBA > 3-MOC. The observed
trends are attributed to electron-donating character and
position of substituted groups. The presence of an
−
−
−
Figure 4 shows the voltammetric responses of cat-
echol (1.0 mM) in the presence of cycloheptylamine
Scheme 1. Proposed mechanism for the oxidative coupling
of catechol derivatives.
Figure 3. Experimental (....) and simulated (−− ) cyclic voltammograms
(1.0 mM) of: (I) 2,3DHBA at scan rates 100, 150, 250 mV s−1 and (II) 3-MC
at scan rates 40, 60, 80 mV s−1 in water (carbonate buffer, c = 0.2 M, pH
8.5)/ethanol (70:30, v/v) mixture. Temperature = 25 1◦C.

Davood Nematollahi et al.
Figure 4. First (a) and second (b) cyclic voltammograms
of catechol (1.0 mM) in the presence of cycloheptylamine (1)
(1.0 mM); (c) cyclic voltammogram of cycloheptylamine 1 in
the absence of catechol, at glassy carbon electrode, in water
(carbonate buffer, c = 0.2 M, pH 8.5)/ethanol (70:30, v/v)
mixture. Scan rate: 10 mV s−1. Temperature = 25 1◦C.
Figure 5. Typical voltammograms of catechol (1.0 mM) in
the presence of cycloheptylamine (1) (5.0 mM) at a glassy
carbon electrode, in water (carbonate buffer, c = 0.2 M, pH
8.5)/ethanol (70:30, v/v) mixture at various scan rates. Scan
(1) (1.0 mM). Curve ‘a’ indicates the first cycle of
rates from a) to d) are 10, 25, 50, 100, 150 and 250 mV s−1
.
voltammogram obtained for solution of catechol in
the presence of cycloheptylamine in water (carbonate Temperature = 25 1◦C. Inset: variation of peak current
ratio (IpA1/IpC1) and peak current function (IpA1/υ1/2) vs.
buffer, c = 0.2 M, pH 8.5)/ethanol (70:30, v/v) mix-
scan rate.
ture. The cathodic peak C1 vanishes in comparison with
voltammogram of catechol in the absence of 1 and a
new cathodic peak C0 appears at a more negative poten- IpA1/v1/2 are expressed vs. scan rate (Figure 5, inset).
tial, −0.32 V vs. SCE. Disappearance of C1 peak in the The appearance of peak C0, on the one hand, and
reverse scan indicates the oxidation process followed decrease of the current function IpA1/v1/2 and IpA1/IpC1
by a chemical reaction that rapidly removed generated ratio with increasing scan rate, on the other hand, stand
o-benzoquinone from the electrode surface. A new in conformity with ECE mechanism.
anodic peak (A0) and its counterpart (C0), appears in the
To gain more information, influence of the concen-
second cycle (curve ‘b’) with an Ep value of −0.32 V tration of cycloheptylamine (1) on the oxidation of cat-
(vs. SCE). This new redox couple (A0/C0) is attributed echol was studied. As shown in Figure 6, IpC0/IpC1
to the oxidation and reduction of the adduct formed increases with increase in the concentration of 1. Peak
from the reaction of o-benzoquinone with cyclohep- C0 and its counterpart one (A0) are connected with
tylamine. The voltammogram of cycloheptylamine 1 reduction and oxidation of adduct formed by the reac-
(curve ‘c’) shows this species is electro-inactive in the tion of o-benzoquinone with 1. The chemical reaction
defined potential window.
is more favorable at higher concentration of 1.
More detailed evidence on the occurrence of chemi-
The cyclic voltammetry data, in keeping with our
cal reaction coupled with electron transfer process was previous reports on the electrochemical oxidation of
obtained by cyclic voltammetry at various scan rates, catechols in the presence of nucleophiles containing
varying from 10 to 250 mV s−1. Figure 5 compares the amine groups10 14 indicate that the electrogenerated
cyclic voltammograms of catechol in the presence of o-benzoquinone in the presence of cycloheptylamine
1 at different scan rates. Two considerable changes are (1) converts to 1a via Michael addition reaction. The
observed by decreasing time window of the experiment better electron-donating group in 1a makes it easier to
(increasing scan rate). The first is increasing of the peak undergo oxidation than the starting species (Scheme 2).
current ratio, IpC1/IpA1 and the second is decreasing of So, the redox peak (A0/C0) presented in Figure 4 is
I
pC0/IpC1. At high scan rates the extent of the reaction attributed to the oxidation and reduction of 1a and
between o-benzoquinone and 1, and consequently, the 1aꢀ, respectively.
amount of product at the electrode surface that causes
Digital simulation was used to gain more mechanis-
a cathodic peak current (C0) decreases. IpA1/IpC1 and tic insight of the electrochemical oxidation of catechol

Electrochemical oxidation of catechols
Figure 6. Typical voltammograms of catechol (1.0 mM) in the presence of a) 1.0 b) 3.0
and c) 5.0 mM of cycloheptylamine (1) at a glassy carbon electrode, in water (carbonate
buffer, c = 0.2 M, pH 8.5)/ethanol (70:30, v/v) mixture. Scan rate 25 mV s−1. Temperature
= 25 1◦C.
−
−
−
−
−
−
′
′
Scheme 2. Proposed mechanism for the electrochemical oxidation of cate-
chol derivatives in the presence of cycloheptylamine (1).
derivatives in the presence of cycloheptylamine (1). The of the electronic properties of the substituents. The
best fits of the experimental and simulated voltammo- electron-donating groups such as methyl, methoxy and
grams were obtained by ECE mechanism under second- carboxylate ion on the catechol ring were found to make
order conditions (Figure 7). The calculated/estimated Michael addition reaction difficult. The order of the
homogeneous rate constants (kobs) of Michael addi- calculated/estimated homogeneous rate constant is as
tion reaction of cycloheptylamine to o-benzoquinone follows: CAT>3-MC>3-MOC>3,4-DHBA. The least
derivatives at different pH values are represented in value of kobs belongs to 3,4-DHBA. This behavior is due
Figure 8. As is expected, kobs increases with increas- to the electron-donating character of carboxylate ion on
ing pH. It is due to the protonation of amine group of the one hand, and occupation of one reaction site by this
cycloheptylamine and its inactivation toward Michael group, on the other hand.
reaction at lower pH values.
Voltammetric studies of catechols were also per-
In addition, the magnitude of observed homoge- formed in the presence of aniline as an aromatic pri-
neous rate constants is in agreement with the trend mary amine. Figure 9 shows the cyclic voltammograms

Davood Nematollahi et al.
Figure 7. Experimental (....) and simulated (−− ) cyclic voltammograms of
1.0 mM (I) 3-MC and (II) 3-MOC in the presence of cycloheptylamine (1)
(3.0 mM) in water (carbonate buffer, c = 0.2 M, pH 8.5)/ethanol (70:30, v/v)
mixture at scan rates a) 20 and b) 40 mVs−1, Temperature = 25 1◦C.
Figure 8. Calculated/estimated homogeneous rate constants
(kobs) for Michael addition reaction of cycloheptylamine (1)
with catechol derivatives for different pH values with the
same cncentration. The numbers on the columns indicate the
standard deviation that are calculated by five independent
simulations at various scan rates.
of catechol (1.0 mM) in the presence of aniline (2)
(1.0 mM) at various potential sweep rates. In comparison
with cycloheptylamine (1), almost the same voltammet-
ric behavior was observed in electrochemical oxidation
of catechol in the presence of aniline (2). However,
although, aniline is less basic than cycloheptylamine,17
it seems to have a higher reaction rate in comparison
with cycloheptylamine.
Figure 9. First and second cycle of voltammograms of cat-
echol (1.0 mM) in the presence of aniline (2) (1.0 mM) at a
glassy carbon electrode, in aqueous perchloric acid (0.1 M)
at different scan rates. Scan rates from a) to d) are 10, 25, 50
and 100 mV s−1. Temperature = 25 1◦C. Inset: Variation
of peak current ratio (IpC1/IpC0) vs. scan rate.
Compared to aniline, cycloheptylamine is inactive
toward o-benzoquinone in acidic media. This is con-
nected with more protonation of cycloheptylamine (Figure 9, inset) by augmentation of potential sweep
compared with aniline and consequently, more inac- rate is an indication of a chemical reaction after electron
tivation towards the Michael addition reaction with transfer process.
o-benzoquinone. The results also show that propor-
More studies were performed by varying pH of solu-
tional to the increasing of the potential sweep rate, the tion. Cyclic voltammograms of catechol (1.0 mM) in
cathodic peak C1 appears and the height of it increases. the presence of 2 (1.0 mM) in the pH range of 1–3 are
The increasing the peak current ratio IpC1/IpC0 shown in Figure 10. The height of the cathodic peaks C1

Electrochemical oxidation of catechols
Table 2. Calculated/estimated homogeneous rate con-
stants (kobs) for the Michael addition reaction of o-
benzoquinones with aniline (2) in the pH range of 2–4 at the
same concentration.
kobs (M−1s−1
pH = 3.0
)
Catechols
pH = 2.0
pH = 4.0
CAT
3-MC
507 ( 15)a 4000 ( 100)a 16000 ( 350)a
92 ( 5)
618 ( 41)
1280 ( 28)
314 ( 13.4)
6233 ( 251)
9000 ( 353)
2625 ( 95)
3,4-DHBA 340 ( 14)
3-MOC 53 ( 7.6)
afor n = 5 at various scan rates.
Figure 10. Cyclic voltammograms of 1.0 mM catechol in
the presence of 1.0 mM aniline (2) at a glassy carbon elec-
trode, in water/ethanol (70/30) solution containing 0.1 M
perchloric acid (for pH = 1.0) and 0.2 M phosphate buffer
According to Scheme 3, the dependency of kobs on
pH is consistent with the following equation:
solution with pH = 2.0 and 3.0. Scan rate: 100 mV s−1
Temperature = 25 1◦C.
.
kobs = αk
(1)
where k is homogeneous rate constant of Michael addi-
tion reaction when 100% of the aniline (or cyclohepty-
lamine) is in its neutral form and α is the fraction of
the aniline in the neutral form (C6H5-NH2), as defined
below:
and C0 is strongly dependent on the pH. IpC1 decreases
and finally disappears with increasing pH, while under
these conditions, IpC0 increases. In other words, the rate
of the reaction of 2 with o-benzoquinone increases with
increasing pH.
α = Ka/(Ka + [H+])
(2)
As discussed before, the nucleophilicity of aniline
decreases with decreasing pH (protonation of amine where Ka is the acid dissociation constant of anilin-
group). In order to carry out a more detailed analysis, ium ion (C6H5-NH+3 ). As deduced from equation (2),
electrochemical oxidation of catechols in the presence α decreases with decreasing pH, and thus decreasing
of 2 was tested by digital simulation in the studied pH kobs. On the other hand, aliphatic amines are much
range. The simulation was performed based on ECE stronger bases than aromatic amines. Protonated cyclo-
mechanism (Scheme 3). In ECE mechanism, the peak heptylamine has a pKa value of about 9.99 as compared
current ratio of IpC1/IpA1, and IpC1/IpC0 can be consid- with 4.60 for aniline.17 The calculated αcycloheptylamine and
ered as criteria for the chemical reaction (Michael addi- αaniline at the same pH (for example, at pH = 2.0) are
tion) rate. The observed homogeneous rate constants 1.26 × 10−8 and 2.51 × 10−3, respectively. These data
of Michael addition reaction at different pH values are confirm the higher reaction rate of aniline compared to
reported in Table 2.
cycloheptylamine.
Ka
H
NH3
(1)
NH2
2
R
OH
OH
R
O
O
−
−
−
(2)
(3)
R
R
O
H
N
k
+
H2N
OH
O
2a-2d
2
OH
R
R
−
H
−
−
H
N
N
OH
(4)
O
OH
2a′-2d′
O
Scheme 3. The reaction mechanism for the electrochemical oxidation of catechols in the presence of aniline (2).

Davood Nematollahi et al.
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The electrochemical oxidation of some catechol deriva-
tives was investigated. Based on many evidences,7 9
electrogenerated o-benzoquinone undergoes a coupling
reaction with anionic and dianionic forms of catechol
to form of C–C or C–O bond. The rate of coupling
reaction of o-benzoquinones is prominently different
and depends on their concentration, structures and pH
of the solution. On the other hand, the electrochemi-
cally generated o-benzoquinone as a Michael acceptor
will react with nucleophiles stronger than catechol.18 31
Our data shows that the electrochemical oxidation path-
way of catechol in the presence of nucleophiles depends
on the nature of the nucleophile. When the nucle-
ophile has electron-withdrawing character like NO−2 ,19
CN−,20 arylsulfinic acids,21 triphenylphosphine,22 and
mercaptobenzoxazole,23 the oxidation of the product
is more difficult than the oxidation of catechol and
consequently, the oxidation of the product is cir-
cumvented under the controlled potential electrolysis.
Under these conditions, the oxidation pathway of cat-
echol is EC. However, when the nucleophiles have
electron-donating properties like amines,12,24,25 indole
derivatives,26,27 azide ion28 and β-diketones,29 31 the
oxidation of the product is easier than the oxidation of
catechol and thus, the adduct formed between catechol
and nucleophile becomes re-oxidized and therefore, the
oxidation pathway is ECE. In this work, the reaction 18. Nematollahi D and Shayani-jam H 2008 J. Org. Chem.
73 3428
of electrochemically generated o-benzoquinones with
cycloheptylamine (1) and aniline (2) has been stud-
ied using cyclic voltammetry. The kinetics of the men-
tioned reactions (coupling reaction of o-benzoquinones
with catechols and the Michael addition reaction of
o-benzoquinones with 1 and 2) are studied by the
simulation of the experimental voltammograms under
EC and ECE mechanisms, respectively. The effects
of pH and substituted group of the catechols on
the observed homogeneous rate constant (kobs) have
been discussed.
19. Salahifar E, Nematollahi D, Mahyari
A
and
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Acknowledgments
Authors acknowledge Bu-Ali Sina University Research
Council and Center of Excellence in Development of
Environmentally Friendly Methods for Chemical Syn-
thesis (CEDEFMCS) for their support of this work.
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