Electro-oxidation of catechols in the presence of benzenesulfinic acid. Application to electro-organic synthesis of new sulfone derivatives
Introduction
As part of our investigations on the electrochemical synthesis of some organic compounds, we focused our attention on the reaction of electrogenerated o-quinone with a variety of nucleophiles. In this direction, we have reported the electrochemical oxidation of catechol and 4-methylcatechol [1], 4-tert-butylcatechol and 3,4-dihydroxybenzaldehyde [2], 3-methylcatechol and 2,3-dihydroxybenzaldehyde [3], in methanol and have shown that these compounds undergo methoxylation reactions according to ECECE or ECE mechanisms, with consumption of six or four electrons per molecule, to give the related methoxy-o-benzoquinone. In addition, we have investigated the electrochemical oxidation of catechol in ethanol, and have shown that the catechol undergoes ethoxylation according to an ECECE mechanism to afford 4,5-diethoxy-o-benzoquinone [4]. Moreover, we have studied the electrochemical oxidation of catechol and some catechol derivatives in aqueous solutions and in the presence of a variety of nucleophiles such as 4-hydroxycoumarin [5], [6], [7], β-diketones [8] and 2-thiobarbituric acid [9]. The importance of sulfones such as phenylsulfonylbenzenediols, which are known as thermally sensitive materials [10], [11], prompted many workers to synthesize a number of these compounds by chemical routes [12]. However, no report has been published until now about the electrochemical synthesis of phenylsulfonylbenzenediols derivatives. Therefore, we have investigated the electro-oxidation of catechol and some 3-substituted catechols such as 3-methylcatechol and 3-methoxycatechol in the presence of benzenesulfinic acid as the nucleophile and found a facile electrochemical method for synthesis of some new sulfone derivatives in good yield and high purity.
Section snippets
Apparatus and reagents
Cyclic voltammetry, controlled-potential coulometry and preparative electrolysis were performed using an Autolab model PGSTAT 20 potentiostat/galvanostat. The working electrode used in the voltammetry experiment was a glassy carbon disc (1.8 mm diameter) and 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 (6 mm diameter and 4 cm length) and a large platinum gauze
Electro-oxidation of catechol (1a) in the presence of benzenesulfinic acid (3)
Cyclic voltammetry of a 1 mM catechol (1a) in acetate buffer solution (c=0.2 M, pH 4.5) shows one anodic (A1) and corresponding cathodic peak (C1), at 0.11 and 0.02 V versus SCE, respectively, which corresponds to the transformation of catechol (1a) to o-benzoquinone (2a) and vice-versa within a quasi-reversible two-electron process (Fig. 1, curve a). A peak current ratio (IpC1/IpA1) of nearly unity, particularly during the repetitive recycling of potential, can be considered as a criterion for
Conclusion
The present results complete the previous report on the anodic oxidation of some catechols in aqueous solutions [5], [6], [7], [8], [9]. The results of this work show that catechols are oxidized in water to their respective o-quinones. The quinones are then attacked by the anion of benzenesulfinic acid to form sulfone derivatives. The overall reaction mechanism for anodic oxidation of catechols in the presence of benzenesulfinic acid as the nucleophile is presented in (Scheme 1). According to
Acknowledgements
This research project has been supported by grant no. NRCI 31303 of the National Research Projects and with the support of the National Research Council of the Islamic Republic of Iran.
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