Electrochemical Oxidation of Catechols in the Presence of Phenyl-Meldrum's Acid. Synthesis and Kinetic Evaluation

Abstract

23Catechol, by itself, and mono-substituted catechols are active in part against Pseudomonas, Bacillus, but not Penicillium species. Many flavonoids and catechol derivatives turned out to be as antimicrobial agents. 1) In this connection, in order to synthesis catechol derivatives, we studied the electrochemical oxidation of benzenediols in the presence of CH acid nucleophiles, 2-4) SH acid nucleophiles, 5-9) nitrite ion 10) and triphenylphosphine. 11) Also, it is demonstrated that, in comparison with simple catechols, the "highly oxygenated compounds with catechol ring" exhibit interesting biological activities.12,13) Therefore, the development of a simple synthetic route for the synthesis of "highly oxygenated compounds with catechol ring" from readily available reagents is one of the major tasks. In this direction, we recently reported the synthesis of some "highly oxygenated compounds with catechol ring." 14) This idea prompted us to investigate the electrochemical oxidation of catechols in the presence of phenyl-Meldrum's acid as a nucleophile and represents a facile and one-pot electrochemical method for the synthesis of some new "highly oxygenated compounds with catechol ring." Also, an additional purpose of this work is the kinetic and mechanistic study of the electrochemical oxidation of catechols in the presence of phenyl-Meldrum's acid and the estimation of the observed homogeneous rate constants (k obs ) of reaction of electrochemically generated o-benzoquinones with this nucleophile by digital simulation of cyclic voltammograms. Results and DiscussionElectrochemical Investigations The cyclic voltammogram of 1.0 mM catechol (1a) in aqueous solution containing 0.2 M acetate buffer (pHϭ5.0) shows one anodic peak (A 1 ) and the corresponding cathodic peak which correspond to the transformation of catechol (1a) to o-benzoquinone (2a) and vice-versa, within a quasi-reversible two electron process (Fig. 1, curve a).2-11) The electrooxidation of catechol (1a) in the presence of phenyl-Meldrum's acid (3) was studied in some detail. Fig. 1, curve b, shows the first cyclic voltammogram obtained for a 1.0 mM solution of 1a in the presence of 0.1 mM phenyl-Meldrum's acid (3). The voltammogram exhibits one anodic peak (A 1 ) and one cathodic peak (C 1 ) that shows decreasing in comparison to the cathodic peak of catechol (1a) in the absence of 3. In this figure, curve c is the voltammogram of 3 in the absence of 1a.The effect of potential sweep rate on shape of cyclic voltammograms of catechol (1a) in the presence of phenylMeldrum's acid (3) was studied, too. It is seen that, proportional to the augmentation of the potential sweep rate, the height of peak C 1 increases. A similar situation is observed when the 3 to 1a concentration ratio is decreased. A plot of the peak current ratio (I pC1 /I pA1 ) versus the scan rate for a mixture of catechol (1a) and phenyl-Meldrum's acid (3) confirms the reactivity of o-benzoquinone (2a) toward phenylMeldrum's acid (3), appearing as an increase in the peak cur- Electrochemic...

Thermodynamic and electrochemical study of some dihydroxybenzenes in the presence of different nucleophiles

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