bCollege of Chemistry, Shahrood University, Shahrood 36155-316, Iran;
cFaculty of Chemistry, Bu-Ali Sina University, Hamedan 65178-38683, Iran
Naturally occurring quinines have captured human attention for thousands of years,initially by reason of their bright colors with possible uses as dyes,and as drugs . Pigments of various colors, now characterized as quinones,have been isolated from high and lower plants,fungi,as well as from animals . The discovery of antibiotic and antitumor properties assigned to several naturally occurring quinines has raised interest among scientists for use as pharmaceuticals. The quinine moiety is found in many drugs,such as aclacinomycin A,adriamycin,carbazilquinone and mitomycin C which are used in cancer chemotherapy . Besides,several types of quinine derivatives play important roles in living systems which contain the red-ox electron-transport chain .
On the other hand,recently electro-synthesis was introduced as a good method for green and facile synthesis of new organic compounds under mild condition [5, 6]. Extensive applications of quinines,that mentioned above,have been used in the synthesis of new organic compounds,also investigated here to develop a simple,fast and green method for the electrochemical synthesis of bis-quinone by the electrochemical oxidation of 4-methyl catechol (1a) in the presence of 1,3-indandione (3) under mild conditions in phosphate buffer solution mixed with ethanol. 2. Experimental
Cyclic voltammetric experiments were performed using a MetrohmVoltammetric Analyzer Model 757 (Herisau,Switzerland) and controlled-potential coulometry was performed using a Behpajooh model 2062 galvanostat/potentiostat (Isfahan,Iran). The working electrode (WE) used in the voltammetry experiments was a glassy carbon disk (GC,2 mm diameter) and platinum disk was used as a counter electrodes (CE). The working electrode (WE) used in controlled-potential coulometry was an assembly of three carbon rods (6 mm diameter and 8 cm length),and a sheet of platinum (1 cm2 ) which constituted the counter electrode (CE). The working electrode potentials were measuredversusAg/AgCl. All electrodes were from AZAR Electrode Company (Urmia,Iran). These chemicals were used without any further purification. 3. Results and discussion 3.1. Typical procedure for the electrochemical synthesis of bisquinone (5a)
In this procedure,a mixture of 100 mL of phosphate buffer solution (0.2 mol/L,pH 6) with ethanol (50/50,v/v) as supporting electrolyte was pre-electrolyzed at the 0.4 Vvs. Ag/AgCl in an undivided cell. Then,0.4 mmol of 4a and 0.2 mmol of 3 were added to the cell. Finally,the electrochemical synthesis under constant potential was performed using the 0.4 Vvs. Ag/AgCl. The electrosynthesis was finished when the current decreased by more than 95%. The process was interrupted 5-7 times during the electrosynthesis,and the carbon anodes were washed in THF to reactivate them. At the end of electrochemical synthesis,cell was placed in refrigerator for 24 h. The precipitated solid was collected by filtration and washed with a mixture of water/acetonitrile (50/50, v/v) for the separation of remaining 1,3-indandion,then washed several time with cold water. After this purification,the product was characterized using FT-IR, 1H NMR, 13C NMR,mass spectroscopy (MS) and elemental analysis. 3.2. Characterization of product
Bis-quinone (5a): brown solid. Yield: 73%. Mp>260°C. FT-IR (KBr,cm-1 ): 1718 (C==O),1705 (C55O),1640 and 1450 (C==C aromatic). 1H NMR (400 MHz,CDCl3): δ 2.51 (s,6H,methyl),5.81 (s,2H,quinone),6.29 (s,2H,quinone),7.73-7.75 (m,2H,aromatic), 7.92-7.94 (m,2H,aromatic). 13C NMR (100 MHz,CDCl3): δ 29,92.1, 111.2,127.5,129.1,130.2,144.8,146.6,188,190. MS (EI):m/z (relative intensity): 384 (38),373 (80),360 (66),283 (33),150 (47), 118 (35),75 (100). Anal. Calcd. for C23H14O6: C,71.5; H,3.65. Found: C,71.43; H,3.56. 3.3. Effect of pH value
The influence of pH on the electrochemical behavior of 1a,both in the absence and presence of 3,was studied by testing the electrode response at various pH values (3-8). The cyclic voltammograms obtained for 1a at various pH values shows one anodic (A1) and a corresponding cathodic peak (C1),which correspond to the transformation of 1a too-benzoquinone (2a) and vice versawithin a quasi-reversible two electron process . A peak current ratio (IpC1/IpA1) of nearly unity,particularly during recycling of the potential,can be considered as a criterion for the stability of 2a produced at the surface of the electrode under the optimum experimental conditions. According to the research of Fotouhiet al.,in basic pH values of the peak current ratio IpC1/IpA1 is less than unity and decreases at higher pH values. These changes can be related to the coupling of the anionic or di-anionic forms of catechol witho-quinones (dimerization reactions) [9, 10]. As a consequence of the decreased rate of side-reactions,such as dimerization and also maximum yield of product 5a obtained at pH 6,and this pH value was selected as the suitable medium for electrosynthesis of 5a. 3.4. Voltammetric studies
The oxidation of 1a in the presence of 3,as nucleophile, was studied in some detail. Cyclic voltammogram obtained for 2 mmol/L of 1a in a phosphate buffer solution (0.2 mol/L,pH 6) mixed with ethanol (50/50,v/v) as supporting electrolyte,shows (Fig. 1,curve a) one anodic (A1) and a corresponding cathodic peak (C1) (peak current ratioIpC1/IpA1of nearly unity). Curve b in Fig. 1 shows the cyclic voltammogram recorded for 2 mmol/L of 1a in the presence of 3 (2 mmol/L). The resulting cyclic voltammogram, exhibits one anodic peak (A1) and its cathodic counterpart (C1). The comparison of peaks C1 in the absence,and presence,of 3 shows a decrease of the current for the later. Under experimental conditions,the peak current ratio (IpC1/IpA1) depends on the scan rate and increases with increasing scan rate. The occurrence of a chemical reaction is supported by the decrease in the current of peak C1 during the reverse scan,which could indicate that 2a formed at the surface of the electrode is consumed by a chemical reaction with 3. In Fig. 1,curve c is the cyclic voltammogram recorded for 2 mmol/L of 3 in the absence of 1a . The voltammogram shows that3is electro-active at a more positive potential rather than 1a. Cyclic voltammogram of 2 mmol/L of 1a in the presence of 1 mmol/L of3was recorded at different scan rates (data not shown) and observed that the current of cathodic peak (C1) increases,thus matching the increase of potential scan rate. Moreover,the peak current ratio (IpC1/IpA1) slightly increases with increasing scan rates (Fig. 1,curve d),which shows a chemical reaction following the electron-transfer step [11, 12].
|Fig. 1. Cyclic voltammograms of 2 mmol/L 1a in the absence (a) and presence of 1 mmol/L 3 (b) and that of a 2 mmol/L 3 in the absence of 1a (c) at the glassy carbon electrode in 0.2 mol/L phosphate buffer solution (pH 6) mixed with ethanol (50/50,v/v) at a scan rate of 50 mV/s. Variation of peak current ratio IpC1/IpA1(d)vs.scan rate for 2 mmol/L of 1a in the presence of 1 mmol/L of 3 at scan rates of 10,25,50,80,100,150,200 and 300 mV/s under experimental conditions.|
Multi-cyclic voltammetry obtained for 2 mmol/L of 1a in the presence of 1 mmol/L of 3 (Fig. 2) shows a new peak (A0) in the second and third cycles parallel to the shift of the A1 peak in a positive potential direction (because the formation of a thin film of product on the surface of electrode that reduced the performance of the electrochemical process). A0 is related to the electrooxidation of 4a to 5a.
|Fig. 2. Multi-cyclic voltammograms of 2 mmol/L 4-methyl catechol(1a) in the presence of 1 mmol/L 3,at glassy carbon electrode in 0.2 mol/L phosphate buffer solution (pH 6) mixed with ethanol (50/30,v:v); scan rate: 50 mV/s.|
Controlled-potential coulometry was performed on a solution containing 0.3 mmol of 1a and 0.15 mmol of 3 at 0.4 Vvs. Ag/AgCl. The monitoring of the electrolysis progress was carried out by cyclic voltammetry (Fig. 3). It shows that,proportionally to the advancement of coulometry,and in parallel with the decrease in height of anodic peak A1,the height of A0 increases. The anodic (A1) peak disappears when the charge consumption becomes about 8e- per molecule of 1a. These observations and spectroscopic data,such as 1H NMR,mass spectra and elemental analysis, allow us to propose the ECCE mechanism,represented by Scheme 1,for electrochemical oxidation of 1a in the presence of 3 under the above conditions.
|Fig. 3. Cyclic voltammogram of 0.3 mmol of 1a in the presence of 0.15 mmol of 3,at glassy carbon electrode in 0.2 mol/L phosphate buffer solution (pH 6) mixed with ethanol (50/50,v/v) during controlled-potential coulometry at 0.4 Vvs. Ag/AgCl (scan rate: 50 mV/s). After the consumption of (a) 0,(b) 18,(c) 40,(d) 54 and (e) 74 C. Progress of coulometry is associated with decreased anodic peak (A1) current. Curve g shows variation of peak currentvs.charge consumed.|
The main purpose of the present work was the synthesis of a new bis-quinone (5a) viaa facile,convenient and green method based on electro-oxidation of 4-methyl catechol (1a) in the presence of 1,3-indandione (3). Results of electrochemical testing of methods,such as cyclic voltammetry and coulometry under constant potential,indicated that the electro-oxidation of 1a in the presence of 3 can be achieved by the ECCE mechanism represented in Scheme 1. An eight-electron process of the mentioned mechanism reaction (ECCE) was confirmed by controlled-potential coulometry data. Clean and green synthesis,with the use of electricity instead of chemical reagents,high temperature and reflux,as well as a one-step process conducted under mild conditions,are attractive features of this work. Furthermore,this study introduces electrochemistry as a ‘‘powerful tool’’ for the synthesis of new organic compounds,such as quinones.Acknowledgment
The authors would like to thank Semnan University Research Council,Semnan,Iran for financial supports to this work.
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