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Electrochimica Acta 80 (2012) 56– 59

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

inetic study of electrochemically induced C P bond formation of catechols withrialkylphosphites

ohammad Rafieea,∗, Seyyed Mohammad Shoaeib, Lida Khalafic

Department of Chemistry, Institute for Advanced Studies in Basic Sciences, Zanjan, IranDepartment of Chemistry, Zanjan Branch, Islamic Azad University, Zanjan, IranDepartment of Chemistry, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 4 April 2012eceived in revised form 22 June 2012ccepted 25 June 2012vailable online 14 July 2012

a b s t r a c t

The reactions of electrochemically generated o-quinones, as Michael acceptors, from oxidation of cate-chols with trialkylphosphites as nucleophiles have been studied using cyclic voltammetry. The reactionmechanism is an EC mechanism and the products of reaction are believed to be dialkylphosphonatederivatives of catechol. The observed homogeneous rate constants (kobs) for reactions were estimatedby comparing the experimental voltammetric responses with the digitally simulated results based on

eywords:oltammetryigital simulationatechol

the proposed mechanism. A quantitative relation between half-wave potentials of catechols and thereactivities of their corresponding o-quinone were derived. Also the effects of substituted group oftrialkylphosphites on rate constants of chemical reactions were studied.

© 2012 Elsevier Ltd. All rights reserved.

rialkylphosphitealf-wave potential

. Introduction

Interest in the preparation of organophosphorus compoundsas continued to expand in recent years. It is due to consider-ble and potential application of these compounds in all areasf chemistry and biochemistry [1]. The several illustrations inegard to applications of organophosphorus compounds in bio-ogically active compounds are antibiotics, drugs, insecticides,erbicides and plant-growth regulators [2–5]. Some of the chemi-al and industrial uses of organophosphorus compounds are: flameetardants, plasticizers, monomer for dental composites, peroxideleach stabilizers and chelating agents for metals [6–8]. Among therganophosphorus compound; oxygen containing compounds inhich phosphorous directly bound to both carbon and oxygen are

he subject of wider interests. One of the reasons for this interests the variety in the valance of phosphorus [9,10]. Several attemptsave been made to propose the simple and efficient methods forhe C P bond formation and synthesis of new organophosphorousompounds. An excellent review on carbon–phosphorus bonds for-ation has been published by Engel and Cohen [1]. Furthermore

lectron transfer is known as unique and soft method for in situ

eneration of reactive species and electrochemical reactions thatollowed by chemical reactions are very successful synthetic meth-ds [11]. The electrochemical oxidation of catechol and generation

∗ Corresponding author. Tel.: +98 2414153125; fax: +98 2414153232.E-mail address: rafiee@iasbs.ac.ir (M. Rafiee).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.06.114

of reactive o-quinone is a good example of these types of reactions[12]. Quite recently Nematollahi has reported the electrochemicaloxidation of catechol in the presence of triphenylphosphine as anapproach for the formation new Schonberg adducts [13,14]. Theobjective of this research is the study of electrochemical oxida-tion of catechol derivatives in the presence of trialkylphosphite asnucleophile for extending these interesting concepts.

2. Experimental

2.1. Apparatus

Cyclic voltammetric studies were performed using a Behpajohmodel BHP-2062 potentiostat/galvanostat. The working electrodeused in the voltammetry experiments was a glassy carbon disc(1.8 mm diameter) and four graphite rods (6 mm diameter and 6 cmlength) were used as working electrode in preparetive electrolysis.A platinum wire was used as counter electrode. The working elec-trode potentials were measured versus Ag/AgCl (all electrodes fromAZAR Electrodes). The homogeneous rate constants were estimatedby analyzing the cyclic voltammetric responses, using the CVSIMsimulation software [15].

2.2. Materials and solutions

Catechols, and trialkylphosphite were reagent-grade materi-als; sodium dihydrogen phosphate, disodium hydrogen phosphate,sodium phosphate, and phosphoric acid were reagent-grade

M. Rafiee et al. / Electrochimica Acta 80 (2012) 56– 59 57

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ig. 1. Cyclic voltammograms of 1.0 mM catechol: (a) in the absence, (b) and (c) inhe presence of 5.0 mM trimethylphosphite at glassy carbon electrode, in phosphateuffer solution (pH = 7). Scan rate: 20 mV s−1.

aterials, from E. Merck, respectively. These chemicals were usedithout further purification. The stock solutions of the catecholsere prepared fresh daily by dissolving the compounds in distilledater. Also the stock solution of triealkylphosphite was prepared

y dissolution in water/acetonitrile (50/50, v/v) mixture. Samplesere prepared by taking the appropriate aliquots from the stock

olutions followed by dilution with buffer solutions. All voltammet-ic experiments were performed in water/acetonitrile (20/80, v/v)olution. For the preparative electrolysis 1.0 mmol of catechol andrimethylphosphite were dissolved in 80 ml of 80/20 acetonitrileater phosphate buffer solution (pH 7.0) and was electrolyzed at

.4 V vs. Ag/AgCl (controlled potential three electrode system) in anndivided cell. The electrolysis was terminated when the decay ofhe current became more than 95%. The precipitated solid was col-ected, after addition of concentrated solution of KCl and overnightemaining in refrigerator, by filtration.

. Results and discussion

The cyclic voltammograms of 1.0 mM solution of catechol at alassy carbon electrode in aqueous/acetonitrile (80/20) solutionontaining phosphate buffer (pH 7.0) as supporting electrolyte ishown in Fig. 1 (curve a). The voltammogram shows one anodiceak (A1) and corresponding cathodic peak (C1). These anodicnd cathodic peaks are related to conversion of catechol (1) to o-uinone (1a) and vice versa respectively. The anodic to cathodicurrent peak ratio nearby one can be attributed to relative stabilityf generated o-quinone at this condition which has been discussedn more details previously [16].

Fig. 1, curve b shows the cyclic voltammograms of 1 in the pres-nce of triemthylphosphite (n1) at the same condition. The heightf C1 decreases whereas the A1 peak remains unchanged approxi-ately at this condition. This is relative to reactivity of 1a toward

1. Voltammetric study of 1 in the presence of various concentra-ion of n1, Fig. 2I, clearly shows that the cathodic peak currentsecrease whereas the concentrations of n1 increase that approvehe desired reactivity.

Voltammetric study at various scan rates (Fig. 2II) shows that byugmentation of scan rate the height of both anodic and cathodic

eaks increase, but the increase in C1 height is more than expectedor diffusion controlled electrode reaction. On the other hand thenodic to cathodic peaks current ratio raises parallel to augmen-ation of scan rate (Fig. 2, inset). The other diagnostic criterion for

Scheme 1. Proposed mechanism for the electrochemical oxidation of catechols inthe presence of trialkylphosphite.

the reaction mechanism is the variation of normalized currents bysquare root of scan rate (Fig. 2II). The normalized voltammogram isobtained by dividing the current by the square root of the scan rate(I�−1/2). It also clearly shows that the A1 normalized currents do notchange as diffusion controlled current contrary to C1 peak that itsnormalized currents increase at higher scan rates. These are knownas diagnostic criteria of EC mechanism, consists of electrochemicalreaction (E) followed by a chemical reaction (C) [17]. Consump-tion of two electrons per each molecule of catechol in controlledpotential coulometry confirms the EC mechanism and presenceof only one step electron transfer for this reaction. Voltammerticstudies have been extended for various catechol and trialkylphos-phite derivatives that the following mechanism is proposed for thereaction (Scheme 1).

The electron-deficient o-quinones are very good electrophilesthat undergo Michael addition reaction with trialkylphosphite asnucleophiles and the desired products are expected to be therelated phosphoniums salts [18]. But the mass spectra of obtainedproducts indicate that the final products are the phophonatederivatives. For example the mass spectrum of reaction prod-uct of catechol and trimethylphosphite shows the parent peakwith 218 m/z (characteristics and mass spectra of this productare available in Supplementary information). At very low scanrates and more positive switching potential new anodic (A2) peakappears but their augmentation is not proportional to descendof C1 peaks. The peak potential of A2 is in good agreementwith the product of reaction of catechol and diethylphoshite(Fig. 2S Supplementary information). Both these results approvethe possibility of nucleophile attack of hydroxide ion with loss ofan alkoxide group and formation of phophonate derivatives as finalproducts.

Based on the proposed scheme for the electrochemical oxida-tion of catechols in the presence of trialkylphoshite there are 12

possible products for these desired reactions; there are four cat-echol derivatives containing catechol (1), 3-methylcatechol (2),3-methoxycatechol (3) and 2,3-dihydroxybenzoic acid (4) and

58 M. Rafiee et al. / Electrochimica Acta 80 (2012) 56– 59

F 3.0, (c) 5.0 and (d) 10.0 mM trimethylphosphite, respectively; scan rate: 40 mV s−1. (b)C hite in phosphate buffer solution; scan rate from a to d are 10, 20, 40 and 80 mV s−1.

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Table 1The observed homogeneous rate constant of nucleophilic addition of trimethylphos-phite to o-quinone at various pHs.

pH 4.0 5.0 6.0 7.0

triisopropylphosphite which is also completely supported by theresults of digital simulation (Table 1S Supplementary information).The other possible evaluation is the study of substituent effectof catechol derivatives on the rate of chemical step. It is known

ig. 2. (a) Cyclic voltammograms of 1.0 mM catechol in the presence of (a) 1.0, (b)yclic voltammograms of 1.0 mM catechol in the presence of 5.0 mM triethylphosp

hree alkylphosphites as trimethylphosphite (n1), triethylphos-hite (n2) and triisopropylphosphite (n3). They are suitable casesor kinetic study of both electronic and strain effects which wille achieved by electrochemical diagnostic criteria and digital sim-lation. The time scale of a cyclic voltammetry experiment isetermined by the scan rate; i.e., at lower scan rates and longerxperimental time scales the more extent of chemical reactionause to removing o-quinone from the electrode surface andecreasing, and/or even elimination, of C1 peak. Therefore theeight of C1 or the ratio of C1 over A1 is a good characteristic for thextent of chemical reaction after the electron transfer of catechol.he simulation was also carried out assuming semi-infinite one-imensional diffusion and planar electrode geometry based on anC mechanism. The parameters related to the electron transfer suchs half wave potential and heterogeneous rate of electron trans-er are entered as known parameters based on previous reports19]. The crucial parameters are the rate constant of chemical stephich allowed to change during the fitting processes. Simulationas performed with varying scan rates, nucleophile concentra-

ions and solution pH. The rate constants of chemical step werestimated by comparison of the simulated cyclic voltammogramsith the experimental cyclic voltammograms. One of the parame-

ers that should be taken in to account for the nucleophilic attacknd catechol oxidation, in aqueous solution, is the pH dependent ofeaction. The anodic to cathodic peak current ratio increase slightlyy increasing the pH.

This is related to deactivation of the electron pair of phospho-ous by protonium in mild acidic condition. The observed rateonstant of coupling reaction of oxidized 1 with n1 at various pHsre presented in Table 1 which are in good agreements with theiagnostic criteria of cyclic voltammograms.

These results show that the best pH for this reaction is the neu-ral pH. Comparison of cyclic voltammograms of catechol at pH 7

n the presence of trimethylphosphite, triethylphosphite and tri-sopropylphosphite are shown in Fig. 3.

As shown in this figure; the reactivities of quinone, anodico cathodic peak current ratio, toward these nucleophiles are

kobs (s−1)a 0.07 0.11 0.15 0.18

a The pseudo-first order rate constant for 5.0 mM of triethylphosphite.

in the following order; trimethylphosphite > triethylphosphite >

Fig. 3. The cyclic voltammograms of (a) 3-methylcatechol, (b) in the presence of5 mM triisopropylphosphite, (c) triethylphosphite and (d) trimethylphosphite inphosphate solution pH 7. Scan rate: 20 mV s−1.

M. Rafiee et al. / Electrochimic

Fig. 4. The cyclic voltammograms of 1.0 mM solution of (a) 3-methoxylcatechol, (b)catechol and (c) 2,3-dihydroxybenzoic acid in the presence of 5 mM triethylphos-phite, in phosphate buffer solution pH 7. Scan rate: 40 mV s−1.

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ig. 5. The plot of obtained rate constant of electrochemically generated o-quinonesith trialkylphosphite versus the half-wave potential of catechol derivatives.

hat the electron donating character of methoxy group is morehan methyl which is more than hydrogen; the carboxylic acid islso known as an electron-withdrawing group. It is expected andhe cyclic voltammograms clearly show (Fig. 4) that the half-waveotential of these catechol derivatives shift to more negative valuesy the presence of electron-donating group and vice versa.

Fig. 4 also shows that the reactivity of produced o-quinone,ncrease parallel to increase in electron-withdrawing characterf catechol substituent. These two consistent changes encour-ged us to give a quantitative explanation for the relation oflectron-withdrawing character and reactivity of o-quinone. Theomogeneous rates of chemical step were obtained by digital sim-lation of cyclic voltammograms at various scan rates and thebtained rate constants were plotted versus half-wave potentials

Fig. 5).

As is shown in Fig. 5 the rate constant of Michael addi-ion increases linearly by increasing the half-wave potentialnd variations have the same trend for all trialkylphosphites.

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a Acta 80 (2012) 56– 59 59

The main reason of o-quinones reactivity toward the nucle-ophilic attack is their electron-deficiency; therefore presence ofelectron-withdrawing group on catechol and o-quinone case tomore electron-deficiency and reactivity. Presence of electron-withdrawing group also shifts the half-wave potentials to morepositive values. Therefore the half-wave potential can be used asdetermining parameter for the prediction of the reactivity of theelectrochemically generated species.

4. Conclusion

The results of this work show that catechol is oxidizedin water to their respective o-quinones. The quinone is thenattacked by trialkylphosphites to form relative phophoniumderivatives. The desired phosphonium derivatives convertto relative phenyl-phosphonates derivatives as final prod-ucts. We examined the kinetics of the desired reactions bydigital simulation and diagnostic criteria of cyclic voltammo-grams. The order of rate constants for trialkylphosphites aretrimethylphosphite > triethylphosphite > triisopropylphosphitewhich is explained by steric effect of the alkyl groups. Also the orderof rate constants for the Michael addition is 2,3-dihydroxybenzoicacid > catechol > 3-methylcatechol > 3-methoxycatechol for alltrialkylphosphites. This order is in good consistent with theelectron-withdrawing character of catechol substituent and theirhalf-wave potentials. We have proposed a quantitative relationbetween half-wave potential and the rate constant of reaction.Furthermore the result of this study is a good example of elec-trochemically driven C P bond formation under environmentallyfriendly condition.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2012.06.114.

References

[1] R. Engel, J.L.I. Cohen, Synthesis of Carbon–Phosphorus Bonds, 2nd ed., BocaRaton, CRC Press, 2003.

[2] T. Kamiya, K. Hemmi, H. Takeno, M. Hashimoto, Tetrahedron Letters 21 (1980)99.

[3] L. Jia-Ning, L. Lei, F. Yao, G. Qing-Xiang, Tetrahedron 62 (2006) 4453.[4] D. Redmore, Journal of Organic Chemistry 35 (1970) 4114.[5] V.R. Gaertner, European Patent 0.007,684 (1983).[6] J. Artner, M. Ciesielski, M. Ahlmann, O. Walter, M. Doring, Arkivoc III (2007)

132.[7] A.S. Paraskar, A. Sudalai, Arkivoc x (2006) 183.[8] R. Singh, S.P. Nolan, Chemical Communications 43 (2005) 5456.[9] B.D. Ellis, C.L.B. Macdonald, Inorganic Chemistry 45 (2006) 6864.10] D.G. Gilheany, In the Chemistry of Organophosphorus Compounds, vol. 2, F.R.

Hartley ed., Wiley, Chichester, England, 1992.11] M. Rafiee, Synlett 3 (2007) 503.12] D. Nematollahi, M. Rafiee, L. Fotouhi, Journal of the Iranian Chemical Society 6

(2009) 448.13] D. Nematollahi, E. Tammari, R. Esmaili, Journal of Electroanalytical Chemistry

621 (2008) 113.14] D. Nematollahi, R. Esmaili, Journal of the Iranian Chemical Society 7 (2010) 260.15] I.M. Kolthoff, E.B. Sandel, E.J. Meehan, Quantitative Chemical Analysis, 4th ed.,

Macmillan, London, 1971.16] D. Nematollahi, M. Rafiee, Journal of Electroanalytical Chemistry 566 (2004) 31.17] A.J. Bard, L.R. Faulker, Electrochemical Methods, 2nd ed., Wiley, New York, 2001.18] S. Zhu, H. Jiang, G. Jin, Journal of Fluorine Chemistry 126 (2005) 931.19] M. Rafiee, D. Nematollahi, Electrochimica Acta 53 (2008) 275.