Electrochemical oxidation of catechols in the presence of triethyl phosphite

Article abstract of DOI:10.1080/10426507.2013.844138

The mechanism of electrochemical oxidation of catechol and some of its derivatives have been studied in the presence of triethyl phosphite as a nucleophile in aqueous solution. Voltammetric studies indicate that the quinones derived from catechol, and its derivatives, participate in Michael addition reaction with triethyl phosphite. The reaction mechanism consists of electron transfer followed by a chemical reaction which is named as an EC mechanism. The homogeneous rate constants (k_obs) were estimated by comparing the experimental cyclic voltammograms with the digitally simulated voltammograms based on EC mechanism. Also the effects of nucleophile concentration and substituted group of catechols on voltammetric behavior and the rate constants of chemical reactions were examined. Copyright

Full text of DOI:10.1080/10426507.2013.844138

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Phosphorus, Sulfur, and Silicon and the Related
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Electrochemical Oxidation of Catechols in the Presence
of Triethylphosphite
Seyed Mohammad Shoaei a , Hossein Aghaei a , Majid Monajjemi a & Mehran Aghaei b
a Department of Chemistry, Science and Research Branch , Islamic Azad University , Tehran ,
Iran
b Faculty of Chemistry, North Tehran Branch , Islamic Azad University , Tehran , Iran
Accepted author version posted online: 25 Sep 2013.Published online: 25 Sep 2013.
To cite this article: Phosphorus, Sulfur, and Silicon and the Related Elements (2013): Electrochemical Oxidation
of Catechols in the Presence of Triethylphosphite, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2013.844138
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Electrochemical Oxidation of Catechols in the Presence of Triethylphosphite
Seyed Mohammad Shoaeia *, Hossein Aghaeia , Majid Monajjemia,
Mehran Aghaeib
a Department of Chemistry, Science and Research Branch Islamic Azad
University, Tehran, Iranbc Faculty of Chemistry, North Tehran Branch, Islamic Azad University, Tehran, Iran
The mechanism of electrochemical oxidation of catechol and some of its derivatives have been studied in the
presence of triethyl phosphite as a nucleophile in aqueous solution. Voltammetric studies indicate that the
quinones derived from catechol, and its derivatives, participate in Michael addition reaction with triethyl
phosphite. The reaction mechanism consists of electron transfer followed by a chemical reaction which is named
as an EC mechanism. The homogeneous rate constants (kobs) were estimated by comparing the experimental
cyclic voltammograms with the digitally simulated voltammograms based on EC mechanism. Also the effects of
nucleophile concentration and substituted group of catechols on voltammetric behavior and the rate constants of
chemical reactions were examined.
Keywords: cyclic voltammetry; catechol; triethyl phosphite; digital simulation
*Email:S_mohammadshoaei@yahoo.com
Tel:+982417275752
Fax:+982415152477
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INTRODUCTION
Among electrochemical methods, cyclic voltammetry is one of the most convenient and most
frequently used techniques for kinetic and thermodynamic study of the coupled electrochemical
1
reactions. It consists of simple, low-cost and rapid potentiodynamic measurement. The time
scale of a cyclic voltammetry experiment is determined by the scan rate, i.e., increasing scan rate
decreases the experimental time scale; therefore, an important parameter in determining the
effect of chemical reaction is the ratio of rate constant of the chemical reaction to the scan rate.
The effect of the scan rate is shown in the shape of the cyclic voltammograms which may be
2
useful in the obtaining information about the mechanism of reactions. Furthermore, among
organic compounds, catechols are known as electrochemically active species with a relatively
simple and quasi-reversible electron transfer. 3 The oxidation potential of catechol is low and the
4
product of oxidation is o-quinone and it is a reactive species that is produced only in-situ.
Electrochemical generation of o-quinone followed by a desired chemical reaction is a convenient
5
tool for electrochemical synthesis of new catechol derivatives. Voltammetric study of catechol
derivatives in the presence of variety of organic compounds that undergo chemical reactions with
o-quinones and their potential application in organic synthesis has been published in a recently
6
published review. In addition, catechol derivatives play an important role in the biologically
important phenomena such as antioxidant activity. Finally interest in the preparation of
organophosphorus compounds has continued to expand in recent years. This is a direct result of
developing applications for phosphorus compounds in numerous synthetic procedures as well as
7
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an understanding of the role of the element in biological systems synthetic intermediates
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coordination chemistry as ligands and agricultural chemicals. 10-12 There are few reports on the
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electrochemical oxidation of catechols in the presence of phosphorus containing nucleophile
such as triphenyl phosphine. 13,14 The aim of this study is the kinetic study of C-P bond formation
based on the electrooxidation of catechol derivatives in the presence of triethyl phosphite. Also
there are reports that the reaction of trialkylphosphites with o-quinones leads to the formation of
pentaoxyphosphorane compounds.15
RESULTS AND DISCUSSION
Voltammetric Study
The electrochemical study of a 1.0 mM solution of catechol at a glassy carbon electrode in
aqueous solution containing phosphate buffer (pH 7.0) as supporting electrolyte was performed
using cyclic voltammetry. The voltammogram shows one anodic (A1) and corresponding
cathodic peak (C1), at 0.35 and -0.01 V vs. Ag/AgCl (Fig. 1, curve a). Fig. 1 curve b shows also
the cyclic voltammogram of catechol in the presence of triethyl phosphite; the most important
differences between the voltammograms are decreasing the magnitude of cathodic peak (C1)
currents and appearing new anodic peak (A2) at more positive potentials. This reaction causes to
removing o-quinone from the electrode surface and decreasing the related currents. Also the
product of chemical reaction is electroactive and undergoes electron transfer at more positive
potentials (A2 Anodic peak).
Figure 1
An important parameter in the study of coupled chemical reaction with electron transfer is the
ratio of rate constant of chemical reaction to the scan rate of voltammetric experiments, the time
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16
scale of a voltammetric experiment is inversely related to scan rate.
electrochemical studies were extended at various scan rates (time scale).
Therefore the
Figure 2
Figure 2 shows the voltammograms of catechol in the presence of triethylphosphite at pH 7.0
with various scan rates. At low scan rate the cyclic voltammograms don’t show any cathodic
peak (C1); the cathodic currents reach to zero at very low scan rates. By increasing scan rate the
height of C1 peak increases and reach to its maximum value; same as absence of triethyl
phosphite. Inset of Figure 2 shows variation of cathodic to anodic peak currents ratio versus scan
rate for a mixture of catechol and triethyl phosphite. This is a good criterion for the reactivity of
electrochemically produced o-quinone toward triethyl phosphite. Normalized cyclic
voltammograms are obtained by dividing the current of cyclic voltammograms by the square root
17
of the scan rate.
Multiple files at different scan rates can be normalized and overlaid for
obtaining more information.
Figure 3
Figure 3 shows the normalized current voltammograms and variation of normalized peak
currents at various scan rates. As shown in this figure the normalized currents of A1 peak don’t
change considerably. It confirms that addition of triethyl phosphite doesn’t affect the oxidation
of catechol. Significant increase in the height of C1 peak currents parallel to increasing scan rates
approve that the electron transfer of catechol followed by a chemical reaction in the presence of
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18
triethyl phosphite . The voltammetric results are in good agreement with an EC mechanism
where ‘E’ represents an electron transfer at the electrode surface, and ‘C’ represents a
homogeneous chemical reaction 18. Based on the above results the following mechanism is
proposed for the electrochemical oxidation of catechol in the presence of triethyl phosphite.
Scheme 1
Based on proposed mechanism and the reactivity of triethyl phosphite toward water and
19
formation of diethyl phosphite the addition of diethyl phosphite to the o-quinone is another
competitive reaction but its reactivity is less than triethyl phosphite 20.
The results for the investigation of the electrochemical behavior of 3-methylcatechol and 3-
methoxycatechol in the presence of triethyl phosphite were the same as those obtained for
catechol. Figure 4 shows the voltammograms of catechol and 3-methoxycatechol in the presence
of triethyl phosphite. It is shown that in this figure the height of cathodic peaks for 3-
methoxycatechol is more than catechol under same conditions. Then the reactivity of produced
o-quinone from oxidation of 3-methylcatechol is less than catechol toward the Michael addition
reaction.
Figure 4
In basic solutions, the height of catechol reduction peak decreases considering some side
reactions such as the coupling of anionic or hydroxide ion with electrochemically produced o-
quinones. But the peak current ratio near unity at neutral and acidic solutions can be considered
as relative stability of the o-quinone.2 Therefore the effect of pH has been studied at pHs lower
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than 7.0. Voltammetric studies at various pHs show that the cathodic to anodic peak currents
ratio decrease with increasing pH. The effect of pH and substituent will be discussed in more
details and quantitatively based on simulation results in the next section.
Kinetic Studies
A proposed scheme for the electrochemical oxidation of catechols in the presence of triethyl
phosphite was tested by digital simulation. On the basis of an EC mechanism, the observed
homogeneous rate constants (kobs) for the reaction of o-quinones with triethyl phosphite have
been estimated by comparison of the simulation results (Fig. 5 curves b) with experimental
cyclic voltammograms (Fig. 5 curves a). The transfer coefficient (α) 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.21All these parameters were kept constant
throughout the fitting of the digitally simulated voltammograms to the experimental data. The
parameters kobs were allowed to change through the fitting processes. There are good agreements
between the simulated voltammograms with those obtained experimentally (Figure 5).
Figure 5
The obtained homogeneous rate constants are given in Table 1. As shown in Table 1, the
magnitudes of homogeneous rate constants are dependent on the nature of the substituted group
on the catechol ring. Same as the previously reported result on this Michael addition reactions.14
the presence of electron-donating groups such as methyl or methoxy on catechol ring causes a
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decrease in the rate constant of reactions. The observed rate constants (kobs) increase linearly as a
function of triethyl phosphite concentrations. These results are consistent with the attack of
13
triethyl phosphite as a nucleophile.
The observed rate constants of coupling reaction of
oxidized catechol with triethyl phosphite at various pHs and based on these results the rate of
reaction enhances at high pH values.
Table 1
CONCLUSION
The results of this work show that catechols are oxidized in aqueous solutions to their respective
o-benzoquinones. The generated quinones are then attacked by triethyl phosphite to form alkyl
phosphonate derivatives. The overall reaction mechanism is presented based on the voltammetric
results and its diagnostic criteria and believed to be an EC mechanism. This is a good example of
electrochemical activation that leads to formation of a carbon-phosphorous bond. C-P bond
formation may be interesting from the synthetic aspect of view due to the importance of
phosphorous containing aromatic compounds7. The cyclic voltammograms were digitally
simulated under EC mechanism and the effect of some experimental conditions and substituents
has been discussed. The simulated cyclic voltammograms show good agreement with those
obtained experimentally. The results of the homogeneous second order rate constants (kobs) are
presented in Table I that confirms the decrease of rate constants in the presence of electron-
donating groups. Furthermore the synthetic application of this reaction and the effect of
nucleophile substituents are under investigation in our research group.
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Experimental
Apparatus
Cyclic voltammetric studies were performed using a Behpajoh model BHP-2062 potentiostat/galvanostat. The
working electrode used in the voltammetry experiments was a glassy carbon disc (1.8mm diameter), and platinum
wire was used as counter electrode. The working electrode potentials were measured versus Ag/AgCl (all electrodes
from AZAR Electrodes). The homogeneous rate constants were estimated by analyzing the cyclic voltammetric
responses, using the CVSIM simulation software. 22
Reagents and solutions
Catechols, and triethyl phosphite were reagent-grade materials; sodium dihydrogen phosphate, disodium hydrogen
phosphate, sodium phosphate, and phosphoric acid were reagent-grade materials, from E. Merck, respectively.
These chemicals were used without further purification. The stock solutions of the catechols were prepared fresh
daily by dissolving the compounds in distilled water. Also the stock solution of triethyl phosphite was prepared by
dissolution in water/acetonitrile (50/50 v/v) mixture. Samples were prepared by taking the appropriate aliquots from
the stock solutions followed by dilution with buffer solutions. All voltammetric experiments were performed in
water/acetonitrile (20/80 v/v) solution.
Compound characteristics
Controlled-potential coulometry was performed in water/acetonitrile (20/80 v/v) solution containing 1 mmol of
catechol and 1 mmol of triethyl phosphite at 0.48 V versus Ag/AgCl. Voltammetric analysis carried out during the
electrolysis shows the progressive decreasing and disappearance of A1 peak . All anodic and cathodic peaks
disappear when the charge consumption becomes about 2e- per molecule of catechol.
diethyl 3,4-dihydroxyphenylphosphonate (5a) (C5H15O5P). 1H NMR (400 MHz, CDCl3): δ(ppm) = 1.31 (t, 6H), 3.64
(m, 4H), 6.59 (m, 1H), 6.91 (s, 1H),6.98 (d, 1H), 8.9 (br, 2OH); MS (70 eV, EI): m/z (%): 247 (2), 236 (4), 185 (3),
165 (4) ,158 (5), 152 (11), 148 (3), 139 (10), 111 (55), 97 (100); IR (KBr): (cm-1) = 3449, 2919, 1652, 1462, 1432,
1268, 1170, 1057.
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Figures Caption
Figure 1 Cyclic Voltammograms of 1.0 mM catechol: (a) in the absence and (b) in the presence
of 5.0 mM triethylphosphite in phosphate buffer solution (pH=7, c=0.2 M) at glassy carbon
electrode, scan rate: 20 mV s-1.
Figure 2 Cyclic Voltammograms of 1.0 mM catechol in the Presence of 5.0 mM triethyl
phosphite at various scan rates; scan rate from (a) to (d) are: 10, 20, 40, 80 mV s-1.
Figure 3 Normalized cyclic Voltammograms of 1.0 mM catechol in the Presence of 5.0 mM
triethyl phosphite at various scan rates; scan rate from (a) to (d) are: 10, 20, 40 and 80 mV s-1.
Figure 4 Cyclic Voltammograms of (a) catechol and (b) 3-methoxycatechol in the presence of
5.0 mM triethyl phosphite in phosphate buffer solution (pH=7), scan rate: 40 mV s-1.
Figure 5 Experimental (curves a blue) and simulated (curves b red) cyclic voltammograms of
1.0 mM catechol in the presence of 5.0 mM triethyl phosphite. Scan rate: 80 mV s-1.
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Figure 1.
Figure 2
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Figure 3
Figure 4
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Figure 5
Table 1 The observed homogeneous rate constants (kobs) of the reaction of electrochemically
generated o-quinones with triethyl phosphite
Catechol derivatives
Catechol
k1 /M-1s-1*
0.057±0.004
0.040±0.003
0.030±0.003
3-Methyl catechol
3-Methoxy catechol
* Each value is the average of four independent measurements.
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R
R
O
O
OH
-2e- -2H+
a:R=H
b:R=CH3
c:R=OCH3
OH
(2a-c)
(1a-c)
R
R
OH
O
O
+H+
kObs
+
O
O
O
P
P
OH
O
O
O
(2a-c)
(3)
(4a-c)
R
R
OH
OH
OH
OH-
C2H5OH
+
O
P
O
P
OH
O
O
O
O
(6)
(4a-c)
(5a-c)
OH
P
O
O
P
Hydrolysis
PH
O
O
O
O
O
O
(8)
(7)
(3)
R
R
OH
OH
OH
O
+H+
kObs
O
+
P
O
O
P
O
O
O
(8)
(2a-c)
(5a-c)
Scheme 1. Proposed mechanism for electrochemical oxidation of catechols in the presence of
triethyl phosphite.
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