Kinetics study of electrochemically induced Michael reaction of benzoquinones with triphenylphosphine

Article abstract of DOI:10.1007/BF03245887

The electrochemical oxidation of 3,4-dihydroxybenzaldehyde (1), 3,4-dihydroxybenzoic acid (2) and 2,5-dihydroxybenzoic acid (3) were studied in the presence of triphenylphosphine (4) as a nucleophile using cyclic voltammetry and controlled-potential coulometry. The results indicate that the quinones derived from oxidation of 3,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzoic acid and 2,5-dihydroxybenzoic acid participate in Michael addition reaction with triphenylphosphine (4). In this work, based on an EC mechanism, the observed homogeneous rate constants (k_obs) of the reaction of produced benzoquinones with triphenylphosphine (4) were estimated by comparing the experimental cyclic voltammograms with the digitally simulated results.

Full text of DOI:10.1007/BF03245887

J. Iran. Chem. Soc., Vol. 7, No. 1, March 2010, pp. 260-268.
JOURNAL OF THE
Iranian
Chemical Society
Kinetics Study of Electrochemically Induced Michael Reaction of Benzoquinones with
Triphenylphosphine
D. Nematollahi* and R. Esmaili
Faculty of Chemistry, Bu-Ali-Sina University, Hamedan, 65174, Iran
(Received 25 October 2008, Accepted 5 May 2009)
The electrochemical oxidation of 3,4-dihydroxybenzaldehyde (1), 3,4-dihydroxybenzoic acid (2) and 2,5-dihydroxybenzoic
acid (3) were studied in the presence of triphenylphosphine (4) as a nucleophile using cyclic voltammetry and controlled-potential
coulometry. The results indicate that the quinones derived from oxidation of 3,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzoic
acid and 2,5-dihydroxybenzoic acid participate in Michael addition reaction with triphenylphosphine (4). In this work, based on
an EC mechanism, the observed homogeneous rate constants (kobs) of the reaction of produced benzoquinones with
triphenylphosphine (4) were estimated by comparing the experimental cyclic voltammograms with the digitally simulated results.
Keywords: Digital simulation, Triphenylphosphine, 3,4-Dihydroxybenzaldehyde, EC mechanism, Homogeneous rate constant
INTRODUCTION
motivated us to synthesize a number of them using a facile
electrochemical method recently [11]. The purpose of this
work is the development of a procedure for synthesis of some
new C-phosphoniumquinol betaines based upon the
electrooxidation of dihydroxybenzoic acids and dihydroxy-
benzaldehyde in the presence of triphenylphosphine as a
nucleophile on the one hand, and the estimation of the
observed homogeneous rate constants (kobs) for the reaction of
electrochemically generated benzoquinones with this
nucleophile by digital simulation of cyclic voltammograms, on
the other hand. The use of electricity instead of chemical
reagents, the use of water/acetonitrile (40/60) instead of
benzene, and achieving high atom economy via a one-step
process conducted under ambient conditions render the present
approach "greener" than others that have been reported to date.
Organophosphorus compounds have widespread use
throughout the world [1-3], mainly in agriculture as
insecticides, herbicides, and plant-growth regulators [4-6].
They have also been used as nerve agents in chemical warfare
(e.g., Sarin gas), and as therapeutic agents, such as ecothiopate
used in the treatment of glaucoma. In academic research,
organophosphorus compounds find important applications in
organic synthesis [1]. Furthermore, organophosphorus
compounds can be used as flame retardants for fabrics and
plastics [7]. One of the important groups of organophosphorus
compounds is C-phosphoniumquinol betaines, the synthesis of
which was first reported by Schonberg and co-workers in 1940
[8], and by others recently [9]. These methods are based on
chemical reactions in organic solvents such as benzene in both
synthesis and work up processes with low atom economy,
under inert atmosphere conditions [10].
EXPERIMENTAL
Apparatus and Reagents
The importance of the organophosphorus compounds has
Cyclic voltammetry, controlled-potential coulometry and
preparative electrolysis were performed using an Autolab
*Corresponding author. E-mail: nemat@basu.ac.ir

Nematollahi & Esmaili
model PGSTAT 20 potentiostat/galvanostat. The working
746, 712, 689, 654, 617 cm-1. MS: m/z (relative intensity); 398
(M+., 100), 369 (100), 321 (22), 277 (19), 183 (37), 107 (12),
electrode used in the voltammetry experiments was a glassy
carbon disc (1.8 mm2 area) and a platinum wire was used as
the counter electrode. The working electrode used in
controlled-potential coulometry and macroscale electrolysis
was an assembly of four carbon rods (31 cm2), while a large
platinum gauze constituted the counter electrode. The working
electrode potentials were measured vs. SCE (all electrodes
from AZAR Electrodes).
77 (30), 51 (24).
1
(2c) (C25H19O4P). m.p. >300 °C (dec). H NMR (300
MHz, DMSO d6)ꢀꢁ (ppm): 6.24 (d, J = 14 Hz, 1H, aromatic),
7.3-8.0 (m 16H, aromatic), 10.0 (broad-OH). FT-IR (KBr): ꢂ
3058, 1619, 1569, 1509, 1484, 1438, 1349, 1304, 1210, 1102,
1018, 998, 905, 828, 775, 747, 731, 705, 691 cm-1. MS: m/z
(relative intensity); 370 ([M+-CO2], 19), 278 (70), 262 (100),
All
chemicals
(3,4-dihydroxybenzaldehyde,
3,4-
183 (99), 152 (22), 108 (44), 77 (26.8), 51 (36.6).
1
dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid and
triphenylphosphine) were reagent-grade materials. Phosphate
salt, sodium acetate and solvents and reagents were all of pro-
analysis. These chemicals were used without further
purification. Homogeneous rate constants were estimated by
analyzing the cyclic voltammetric responses, using DigiElch
simulation software [12]. All voltammetric experiments were
performed in water/acetonitrile (50/50 v/v) solution. But,
because of the low solubility of triphenylphosphine in
water/acetonitrile (50/50 v/v) solution, all coulometric
experiments were carried out in water/acetonitrile (35/65 v/v)
solution.
(3c) (C25H19O4P). m.p.: >250 °C (dec). H NMR (300
MHz, DMSO-d6)ꢀꢁ (ppm): 6.97 (t, J = 8.7, 8.3 Hz, 1H,
aromatic), 7.19 (d, J = 8.9 Hz, 1H, aromatic), 7.55 (m 15H,
aromatic), 10.1 (s-OH), 15.1 (s, 1H-COOH). 31P NMR (37
MHz, DMSO-d6)ꢀꢁ (ppm): 12.58 (s). FT-IR (KBr): ꢂ 3505,
3446, 3365, 3055, 1634, 1560, 1456, 1483, 1438, 1379, 1357,
1300, 1224, 1191, 1109, 1158, 1098, 1074, 968, 839, 797,
748, 732, 707, 691, 560 cm-1. MS: m/z (relative intensity); 370
([M+-CO2], 3), 277(100), 262 (41), 199 (22), 183 (51), 152
(19.5), 108 (17), 77 (32), 51 (36).
RESULTS AND DISCUSSION
Electroorganic Synthesis of 1c-3c
Electrochemical Study of 3,4-Dihydroxybenz-
aldehyde (1)
In a typical procedure, 70 ml of phosphate buffer solution
(0.5 M, pH = 2.0) in water/acetonitrile (35/65) or aqueous
sodium acetate solution (0.15 M)/acetonitrile (35/65),
containing 0.5 mmol of 1, 2 or 3 and 0.5 mmol of
triphenylphosphine (4) was electrolyzed 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
A cyclic voltammogram representative of the behavior of
3,4-dihydroxybenzaldehyde (1) (1 mM) in water/acetonitrile
(50/50) solution containing 0.20 M phosphate buffer (pH =
2.0) is shown in Fig. 1 (curve a). It exhibits an anodic wave
(A1) at 0.62 V vs. SCE, corresponding to the formation of 3,4-
dioxocyclohexa-1,5-dienecarbaldehyde (1a) [13]. On the
reverse scan, two cathodic peaks (C1 and C0) are observed at
Ep values of 0.50 and 0.28 V, respectively. At low scan rates,
the peak current ratio (IpC1/IpA1) is less than unity, while it
increases gradually with increasing scan rate and approaches
unity. Moreover, at high scan rates, only peak C1 remains and
cathodic peak C0 disappears. According to a previously
published paper [13], the peaks A1 and C1 are indicative of a
quasi-reversible charge-transfer process corresponding to the
recrystallization
(water/acetonitrile),
products
were
characterized by IR, 1H NMR, 31P NMR and MS. The isolated
yields of 1c-3c were 55, 60 and 80%, respectively.
Spectra Data
1
(1c) (C25H19O3P). m.p.: >230 °C (dec). H NMR (300
MHz, DMSO-d6)ꢀꢁ (ppm): 6.84 (d, J = 7.5 Hz, 1H, aromatic),
7.06 (t, J = 6.8, 7.5 Hz, 1H, aromatic), 7.5-7.8 (m, 15H,
aromatic), 9.3 (s, H aldehyde). 31P NMR (37 MHz, DMSO
d6)ꢀꢁ (ppm): 16.38 (s). FT-IR (KBr): ꢂ 3434, 3062, 1653,
1512, 1439, 1376, 1285, 1204, 1107, 1028, 996, 885, 784,
3,4-dihydroxybenzaldehyde
(1)/3,4-dioxocyclohexa-1,5-
dienecarbaldehyde (1a) system [13]. Cathodic peak C0, which
predominates at low scan rates, could be indicative of the
reduction of a compound produced from the consumption of
261

Kinetics Study of Electrochemically Induced Michael Reaction
25
20
15
10
5
due to oxidation of 4 [15]. Similarly the anodic peak A3
corresponds to the oxidation of triphenylphosphine (4).
A2 A3
A1
Controlled-potential coulometry was performed in a
water/acetonitrile (35/65) solution containing 0.5 mmol of 1,
0.5 mmol of 4 and 0.5 M phosphate buffer (pH = 2.0), at the
potential of peak A1. Monitoring of the electrolysis was
carried out by cyclic voltammetry (Fig. 2). It is shown that,
proportional to the advancement of coulometry, anodic peak
A1 decreases and disappears when the charge consumption
becomes 2e- per molecule of 1 (Fig. 2). These coulometry and
voltammetry results accompanied by NMR (1H and 31P) data
and a molecular mass of 398 of the final product and our
experience in a previously published paper [11] are all
conducive to propose an EC mechanism for the
electrooxidation of 1 in the presence of triphenylphosphine (4)
(Scheme 1). According to our findings, the Michael addition
reaction of 4 to o-benzoquinone (1a) (Eq. 2) seems to occur
much faster than other side reactions, leading to product 1c.
The existence of an aldehyde group probably causes the
Michael acceptor 1a to be attacked by triphenylphosphine (4)
at the C-2, C-5 or C-6 positions to yield three types of
products in each case (Fig. 3). In this connection, the 1H NMR
spectrum of the obtained product indicates that the coupling
constants, J, for the two aromatic peaks (6.84 and 7.06 ppm)
are 7.5 Hz, that is, in agreement with the existence of two
protons in the catechol ring in the ortho positions [16].
b
a
c
0
C0
-5
C1
0.45
-10
-0.05
0.2
0.7
0.95
1.2
E (V vs . SCE)
Fig. 1. Cyclic voltammograms of: (a) 1 mM 3,4-
dihydroxybenzaldehyde (1), (b) 1 mM 3,4-
dihydroxybenzaldehyd, in the presence of 1 mM
triphenylphosphine (4) and (c)
1
mM
triphenylphosphine (4), at a glassy carbon
electrode in water/acetonitrile (50/50) solution
containing 0.2 M phosphate buffer (pH = 2.0).
Scan rate: 100 mV s-1; t = 25 1 °C.
1
Therefore, according to the H NMR results, we suggest that
1a in the course of the following chemical reactions such as
hydroxylation and/or dimerization on the time scale of the
experiment [13]. This is expected because the presence of
-CHO as an electron-withdrawing subsistent on the catechol
ring of (1) causes an increase in its acidity [14] and reactivity
of produced o-benzoquinone (1a).
Figure 1 (curve b) shows cyclic voltammograms of a 1
mM solution of 3,4-dihydroxybenzaldehyde (1) in the
presence of 1 mM triphenylphosphine (4) as a nucleophile. At
a scan rate of 100 mV s-1, the cyclic voltammogram exhibits
the feature of an irreversible electron-transfer process with
three anodic peaks (A1, A2 and A3). When the sweep rate is
increased, cathodic peak C1 appears and its height increases
progressively with the scan rate. In this figure, peak A2
corresponds to oxidation of product [11], and curve c is the
voltammogram of 4. The observed anodic peak in curve c is
o-benzoquinone 1a is attacked in the C-2 position by 4 leading
to the formation of 1c.
Electrochemical Study of 3,4-Dihydroxybenzoic Acid
(2)
The electrochemical study of a 1 mM solution of 3,4-
dihydroxybenzoic acid (2) in an acetonitrile/sodium acetate
solution (0.15 M) (50/50 v/v), at a bare glassy carbon
electrode was performed using cyclic voltammetry (Fig. 4,
curve a). The voltammogram shows one anodic (A1) and
corresponding cathodic peak (C1), at 0.35 and -0.03 V vs.
SCE, respectively, which correspond to the transformation of
3,4-dihydroxybenzoic acid (2) to 3,4-dioxocyclohexa-1,5-
dienecarboxylic acid (2a) and vice versa within a quasi-
reversible two-electrons process [17,18] (Scheme 2).
Figure 4, curve b, shows a cyclic voltammogram of 3,4-
262

Nematollahi & Esmaili
A2
A2
a
b
140
100
60
A1
A3
140
100
60
A1
20
20
-20
-20
-0.3 0.05 0.35 0.65 0.95 1.25
-0.25 0.05 0.35 0.65 0.95 1.25
E (V vs . SCE)
E (V vs . SCE)
A2
c
d
140
100
60
140
100
60
A2
A1
A1
20
20
-20
-20
-0.3 0.05 0.35 0.65 0.95 1.25
-0.25 0.05 0.35 0.65 0.95 1.25
E (V vs . S CE)
E (V vs. SCE)
150
e
IpA1 = -1.3C + 136.6
140
R = 0.986
120
90
60
30
0
A2
100
60
A1
20
-20
-0.25 0.05 0.35 0.65 0.95 1.25
0
25 50 75 100
E(V vs. SCE)
Charge (C)
Fig. 2. Cyclic voltammogram of 0.5 mmol 3,4-dihydroxybenzaldehyde (1) in the presence of 0.5 mmol triphenylphosphine
(4) in water/acetonitrile (35/65) solution containing 0.5 M phosphate buffer (pH = 2.0) at a glassy carbon electrode
during controlled-potential coulometry at 0.30 V vs. SCE after the consumption of (a) 0, (b) 25, (c) 50, (d) 75 and (e)
90 C. (f) Variation of peak current (IpA1) vs. charge consumed. Scan rate; 100 mV s-1. t = 25 1 °C.
as a shoulder and a new anodic peak (A2). When the sweep
rate is increased, the cathodic peak C1 appears and its height
increases gradually with increasing scan rate.
dihydroxybenzoic acid (2) in the presence of
triphenylphosphine (4) in the above conditions. At a scan rate
of 100 mV s-1 the cyclic voltammogram exhibits the feature of
an irreversible electron-transfer process with anodic peak A1
Controlled-potential coulometry was performed in an
263

Kinetics Study of Electrochemically Induced Michael Reaction
OH
OH
OHC
OHC
O
O
-2e--2H+
2
5
1
6
3
4
(1)
1a
1
O
OH
OH
O
OHC
O
+
(2)
P
P
P:
O
H
CHO
CHO
1c
1a
1b
4
Scheme 1
OH
OH
OOC
O
O
OOC
-2e--2H+
PPh3
2a
OHC
Ph3P
OH
2
OHC
OH
OHC
O
H
P
4
O
O
OH
PPh3
1d
COO
1e
1c
OH
OH
P
Fig. 3. The possible structures of final product of
electrochemical oxidation of 3,4-dihydroxy-
benzaldehyde (1) in the presence of triphenyl-
phosphine (4).
2c
Scheme 2
25
acetonitrile/sodium acetate solution (65/35) containing 0.5
mmol of 2 and 0.5 mmol of 4 at 0.30 V vs. SCE. It was
observed that during controlled-potential coulometry,
proportional to the advancement of coulometry, the anodic
peak A1 decreases and anodic peak A2, which is related to
oxidation of the catechol ring in 2c, increases (Fig. 5). When
peak A1 disappears, the charge consumption becomes about
2e- per molecule of 2. These observations make it possible for
A1 + A2
20
15
A1
b
10
a
5
us to propose the pathway in Scheme
2 for the
0
electrooxidation of 3,4-dihydroxybenzoic acid (2) in the
presence of triphenylphosphine (4).
-5
Electrochemical Study of 2,5-Dihydroxybenzoic Acid
(3)
C1
-10
-0.4
-0.1
0.2
0.5
0.8
Figure 6 (curve a) shows a cyclic voltammogram of a
solution of 2,5-dihydroxybenzoic acid (3) in acetonitrile/
sodium acetate solution (0.15 M) (50/50). At the scan rate of
25 mV s-1, one anodic peak (A1) is observed at 0.23 V vs.
SCE. On the reverse scan, two cathodic peaks (C1 and C0)
appear at Ep values of -0.01 and -0.15 V vs. SCE, respectively.
Furthermore, for high scan rates as well as acidic solutions
[19,20], only peak C1 is observed and the other cathodic peak
E (V vs . SCE)
Fig. 4. Cyclic voltammograms of: (a) 1 mM 3,4-dihydroxy-
benzoic acid (2), (b) 1 mM 3,4-dihydroxybenzoic
acid, in the presence of 1 mM triphenylphosphine (4)
at a glassy carbon electrode in sodium acetate solution
(0.15 M)/acetonitrile (50/50). Scan rate: 100 mV s-1;
t = 25 1 oC.
264

Nematollahi & Esmaili
12
10
8
A1+A2
A1
150
130
110
90
A2
IpA1
120
IpA1 = -1.3C + 130.3
R = 0.985
A1
80
40
0
6
b
a
e
4
a
2
0
30
60
90
Charge (C)
0
70
d
-2
50
C0 C1
-0.15 0.1
E (V vs . SCE)
-4
-0.4
0.35
0.6
0.85
30
10
Fig. 6. Cyclic voltammograms of: (a) 1 mM 2,5-
dihydroxybenzoic acid (3), (b) 1 mM 2,5-
dihydroxybenzoic acid, in presence of 1 mM
triphenylphosphine (4), at a glassy carbon
electrode in aqueous sodium acetate solution
(0.15 M)/acetonitrile (50/50). Scan rate: 25
mV s-1; t = 25 1 °C.
-10
-0.5
-0.1
0.3
0.7
1.1
E (V vs . SCE)
Fig. 5. Cyclic voltammogram of 0.5 mmol 3,4-
dihydroxybenzoic acid (2) in the presence of 0.5
mmol triphenylphosphine (4) in sodium acetate/
acetonitrile (35/65) solution at a glassy carbon
electrode during controlled-potential coulometry
at 0.30 V vs. SCE after the consumption of (a) 0,
(b) 35, (c) 60, (d) 80 C. (e) Variation of peak
current (IpA1) vs. charge consumed. Scan rate; 100
mV s-1. t = 25 1 °C.
OH
O
OH
COO
OOC
OOC
-2e--2H+
2
3
H
P
1
6
P
4
5
OH
3c
OH
3
O
3a
4
Scheme 3
(50/50) and scan rate, 25 mV s-1, was studied in some detail
(Fig. 6, curve b). Under these conditions, the anodic peak A1
increases and the cathodic counterpart of A1 disappears. Other
conditions are similar to those for the electrooxidation of 3,4-
dihydroxybenzoic acid (2) in the presence of triphenyl-
phosphine (4). The electrooxidation of 2,5-dihydroxybenzoic
acid (3) in the presence of triphenylphosphine (4) (Scheme 3)
is considered to involve the Michael acceptor 3,6-
dioxocyclohexa-1,4-dienecarboxylic acid (3a) as an
intermediate that could be attacked at positions C-3, C-4 or C-
6 to yield 3c, 3d or 3e, respectively (Fig. 7).
(C0) disappears. Under these conditions, the voltammogram
shows a quasi-reversible two-electron process corresponding
to a 2,5-dihydroxybenzoic acid (3)/3,6-dioxocyclohexa-1,4-
dienecarboxylic acid (3a) redox couple [19-21]. Such a
situation is indicative of a coupled chemical reaction such as
hydroxylation following the abstraction of two electrons from
2,5-dihydroxybenzoic acid (3) and is observed when the time
scale of the experiment and the half-life of the chemical
reaction are comparable [22,23].
Oxidation of 2,5-dihydroxybenzoic acid (3) (1 mM) in the
presence of triphenylphosphine (4) (1 mM), at glassy carbon
electrode in sodium acetate solution (0.15 M)/acetonitrile
The 1H NMR spectrum of the isolated product exhibits two
265

Kinetics Study of Electrochemically Induced Michael Reaction
Table 1. The Observed Homogeneous Rate Constant (kobs) of
H
H
H
O
C
O
O
C
O
O
C
O
the Reaction of Electrochemically Generated Benzo-
quinones with Triphenylphosphine (4) in Water/
Acetonitrile (50/50) Solution Containing 0.2 M
Phosphate Buffer (pH = 2.0)
O
O
O
PPh3
PPh3
Ph3P
OH
3c
OH
3d
OH
3e
kobs
Relative rate
constant
Entrya
1
Catechol
OH
(M-1s-1)b
Fig. 7. The possible structures of final product of
electrochemical oxidation of 2,5-dihydroxy-
benzaldehyde (3) in the presence of triphenyl-
phosphine (4).
2040 49
1567 47
1.00
OH
CH3
OH
2
0.77
OH
doublet peaks with vicinal coupling constants (ꢀ 6.97 and
7.19) confirming structure 3c [16] (Scheme 3).
H3C
OH
3
4
1525 43
667 44
0.75
0.33
OH
Kinetics Studies
OCH3
OH
A proposed scheme for the electrochemical oxidation of
catechols and hydroquinones in the presence of triphenyl-
phosphine (4) was tested by digital simulation. On the basis of
an ECꢀmechanism, the observed homogeneous rate constants
(kobs) of reaction of benzoquinones with triphenylphosphine
(4) have been estimated by comparison of the simulation
OH
HOOC
OHC
OH
OH
OH
OH
5
6
20250 433c
39400 1200
9.93
19.31
results (Fig.
8
curves S) with experimental cyclic
voltammograms (Fig. 8 curves E). The transfer coefficient (ꢄ)
was assumed to be 0.5, and the formal potentials were
obtained experimentally as the average of the two peak
potentials (EpA1 and EpC1) observed in cyclic voltammetry. The
heterogeneous rate constants were estimated by the use of an
experimental working curve [24]. The procedure was
performed based on achieving the best fit between simulated
and experimental cyclic voltammograms (Table 1).
OH
OH
7
69
8
0.03
OH
COOH
8
500 31c
0.25
OH
The method is developed for a variety of scan rates. As
shown in Table 1, the magnitude of observed homogeneous
rate constants (kobs) is dependent on the nature and position of
the substitued group on the catechol ring. The presence of
electron-donating groups such as methyl or methoxy on the
catechol ring causes a decrease in kobs. In contrast, in the case
of 3,4-dihydroxybenzaldehyde (1) or 3,4-dihydroxybenzoic
acid (2), the presence of -CHO or -COOH groups with
electron-withdrawing character causes an increase in kobs. In
addition, the presence of substituted group in the C-4 position
of the catechol ring that is a reactive site of o-benzoquinone
aElectrochemical oxidation of catechol, 3-methylcatechol, 4-
methylcatechol, 3-methoxycatechol and hydroquinone in the
presence of triphenylphosphine (4) has been described
recently [11]. bStandard deviation of five independent
simulations at various scan rates. cIn sodium acetate solution
(0.15 M)/acetonitrile (50/50).
causes a decrease in the observed homogeneous rate constant
(kobs) (Table 1, comparison of 3-methylcatechol and 4-
methylcatechol). In this study, because of the stability of p-
266

Nematollahi & Esmaili
A1
15
11
7
A1
15
11
7
v = 50 mV s-1
v = 50 mV s-1
A2
A2
CH3
OH
OH
OH
E
S
E
S
OH
3
3
-1
-5
-1
-5
C1
C1
0.5
-0.1
0.2
0.8
1.1
-0.15 0.15 0.45 0.75 1.05
E (V vs . SCE)
E (V vs . SCE)
A1
15
11
7
A1
v = 50 mV s-1
15
11
7
v = 50 mV s-1
A2
A2
OH
OH
H3C
OCH3
OH
E
S
E
S
OH
3
3
-1
-5
-1
C1
C1
-5
-0.15 0.15 0.45 0.75 1.05
-0.15 0.15 0.45 0.75 1.05
vs
E (V . SCE)
E (V vs . SCE)
A1 + A2
70
55
40
25
10
-5
v = 2000 mV s-1
E
OHC
OH
OH
S
C1
-20
0.23
0.43
0.63
0.83
E ( V vs.SCE)
Fig. 8. Cyclic voltammograms of 1 mM catechols in the presence of: 1 mM triphenylphosphine (4) E) experimental, S)
simulated. Experimntal conditions: glassy carbon electrode (1.8 mm diameter), solution: water/acetonitrile
(50/50) solution containing 0.2 M phosphate buffer (pH = 2.0). t = 25 1 °C.
267

Kinetics Study of Electrochemically Induced Michael Reaction
benzoquinone in comparison with o-benzoquinone [20], the
smallest observed homogeneous rate constant (kobs) belongs to
hydroquinone.
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CONCLUSIONS
The present results are complementary to our previous
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The results of this work show that 3,4-dihydroxybenzaldehyde
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ACKNOWLEDGMENTS
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We would like to thank Dr. M. Rudolph for his cyclic
voltammogram digital simulation software (DigiElch SB). We
also acknowledge the Bu-Ali Sina University Research
Council and Center of Excellence in Development of
Chemical Methods (CEDCM) for their support of this work.
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