New insights into the electrochemical behavior of acid orange 7: Convergent paired electrochemical synthesis of new aminonaphthol derivatives

Abstract

Electrochemical behavior of acid orange 7 has been exhaustively studied in aqueous solutions with different pH values, using cyclic voltammetry and constant current coulometry. This study has provided new insights into the mechanistic details, pH dependence and intermediate structure of both electrochemical oxidation and reduction of acid orange 7. Surprisingly, the results indicate that a same redox couple (1-iminonaphthalen-2(1H)-one/1-aminonaphthalen-2-ol) is formed from both oxidation and reduction of acid orange 7. Also, an additional purpose of this work is electrochemical synthesis of three new derivatives of 1-amino-4-(phenylsulfonyl)naphthalen-2-ol (3a-3c) under constant current electrolysis via electrochemical oxidation (and reduction) of acid orange 7 in the presence of arylsulfinic acids as nucleophiles. The results indicate that the electrogenerated 1-iminonaphthalen-2(1 H)-one participates in Michael addition reaction with arylsulfinic acids to form the 1-amino-3-(phenylsulfonyl)naphthalen-2-ol derivatives. The synthesis was carried out in an undivided cell equipped with carbon rods as an anode and cathode.

Figure 1

Cyclic voltammogram of AO7 (1.0 mM) (first and second cycles) in two different direction of scanning of potential, (I) in negative-going scan and (II) in positive-going scan in aqueous phosphate buffer (c = 0.2 M, pH = 2.0). Scan rate: 50 mV s⁻¹; T = 25 ± 1 °C.

Figure 2

Cyclic voltammograms of AO7 (0.1 mmol) in aqueous phosphate buffer (c = 0.2 M, pH = 2.0) at a glassy carbon electrode in a divided cell, during controlled potential electrolysis at: (I) E_app = +0.9 V and (II) E_app = −0.2 V vs. Ag/AgCl, (a) at the beginning of electrolysis and (b) at the end of electrolysis. Scan rate: 50 mV s⁻¹. T = 25 ± 1.

Figure 3. Oxidation and reduction pathways of AO7 in acidic media.

Figure 4. The potential-pH diagram for oxidation and reduction of AO7.

Figure 5. Oxidation and reduction pathways of AO7 at different pH values and acid/base equilibriums of HAO7_ox/AO7_ox, AO7/AO7⁻ and HANO/INO.

Figure 6. (a) Cyclic voltammogram of AO7 (1.0 mM) in the absence, (b) in the presence of 4-toluenesulfinic acid (1a) (1.0 mM), (c) 4-toluenesulfinic acid (1.0 mM) and (d) isolated product (3a) (0.05 mM), at a glassy carbon electrode, in aqueous phosphate buffer (c = 0.2 M, pH = 2.0).

Scan rate: 100 mV s⁻¹, T = 25 ± 1 °C.

Figure 7. Proposed mechanism for the electrochemical oxidation of AO7 in the presence of arylsulfinic acids (1a–1c).

Figure 8. Cyclic voltammograms of AO7 (0.25 mmol) in the presence of 4-toluenesulfinic acid (1a) (0.25 mmol) in aqueous phosphate buffer (c = 0.2 M, pH = 2.0), at a glassy carbon electrode during constant current coulometry, after consumption of (a) 0, (b) 20, (c) 35, (d) 50, (e) 65, (f) 80 and (g) 95 C.

Current density: 0.32 mA cm⁻¹. Scan rate: 50 mV s⁻¹. Inset: variation of peak current (I_pC0) vs. charge consumed. T = 25 ± 1.

Figure 9. The structures of possible compounds 3a and 3a’.