Research Article pubs.acs.org/journal/ascecg
Green Electrochemical Synthesis of N‑Phenylquinoneimine Derivatives: Dual Action of 4‑Morpholinoaniline and N‑(4Aminophenyl) Acetamide Mahdi Jamshidi and Davood Nematollahi* Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65178-38683, Iran S Supporting Information *
ABSTRACT: In this paper, two green electrochemical strategies for the synthesis of new Nphenylquinoneimine derivatives are reported. In the first strategy, the electrophilic activity of electrochemically generated quinone−diimine derivatives was used in reaction with phenol as a model nucleophile. In the second strategy, the electrochemically formed 4-tert-butyl-obenzoquinone was used as a model Michael acceptor in the reaction with 4morpholinoaniline, N-(4-aminophenyl) acetamide, and fast violet B. The reaction mechanism in both strategies is electron transfer−chemical reaction−electron transfer/ disproportionation (ECE/DISP). This study provided some new N-phenylquinoneimine derivatives in good yields by a one-pot reaction under ambient conditions without toxic reagents.
KEYWORDS: N-Phenylquinoneimine derivatives, Green chemistry, 4-Morpholinoaniline, Cyclic voltammetry, Reaction mechanism, Electrochemical synthesis
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INTRODUCTION Synthesis of organic compounds by means of electrochemical methods have features that many of which cannot be achieved by other methods. Most electroorganic processes are performed under reagentless and mild (atmospheric pressure and room temperature) conditions in one step using efficient and ecofriendly methods.1−6 In other words, electroorganic synthesis is in agreement with all the principles of green chemistry.7−10 Quinone imines are widely used as dyestuffs11 and medicines.12−16 In addition, quinone imines act as intermediates in the synthesis of sulfur dyes such as phenazone, phenothiazone, and phenoxazone derivatives.17 Synthesis of quinone imines is generally complicated because of their instability under the conditions used for their formation. For this reason, only a few methods for the synthesis of quinone imine derivatives have been reported. The most widely used methods for the synthesis of quinone imines are the oxidation of aniline derivatives by hypervalent iodine,18−21 HClO4,22 potassium ferricyaniade,23,24 and AgNO3.25 In addition, other methods such as the deprotection of the amino side chain of pquinones and p-quinone monoacetals,26 photochemical [2 + 2]cycloaddition between electron-donating aryl isocyanates and chloranil,27 Diels−Alder reactions,28 phenyliodo-bis-acetate,29 or electrochemical methods30−32 were also used for synthesis of quinone imines. However, most of these methods have the disadvantages such as unsafe solvent and reagents, tedious workup, strongly basic media, and low yield due to polymerization, hydrolysis, and reactions with starting amines. Moreover, many of the quinone imines are themselves subject to © 2017 American Chemical Society
further oxidation. On the other hand, we found that despite the large number of articles dealing with the quinone imines, limited information exists on the N-phenylquinoneimine derivatives. The difficulties in the previous studies on the synthesis of quinone imine derivatives on one hand and the lack of data on the synthesis of N-phenylquinoneimine derivatives on the other have encouraged us to develop a green method for the synthesis of new N-phenylquinoneimine derivatives. This method contains two different strategies to synthesize a common product. In this method, a compound with either nucleophilic or electrophilic character is used to synthesize a common product. In this research, we were able to synthesize similar compounds by the reaction of electrogenerated quinone− diimine (as an electrophile) with phenol and via the reaction of 4-morpholinoaniline (4MA) [or N-(4-aminophenyl)acetamide (NAA)] itself (as a nucleophile) with the electrogenerated 4tert-butyl-o-benzoquinone. Both presented strategies use a water/ethanol mixture as solvent. Ethanol as cosolvent was also used in the previous studies because of its low cost, safety, easy availability, recyclability, bioproductability, and biodegradability.33,34 These strategies represent a one-pot and facile process for the synthesis of new N-phenylquinoneimine derivatives in high yield and purity under green conditions Received: July 27, 2017 Revised: September 5, 2017 Published: September 7, 2017 9423
DOI: 10.1021/acssuschemeng.7b02553 ACS Sustainable Chem. Eng. 2017, 5, 9423−9430
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Figure 1. Cyclic voltammograms of 4MA (1.0 mM): (a) in the absence of and (b) in the presence of phenol (1.0 mM). (c) Cyclic voltammogram of phenol (1.0 mM) at glassy carbon electrode in water (phosphate buffer, c = 0.2 M, pH 8.0)/ethanol mixture (80/20, v/v). Scan rate: 10 mV s−1. Inset: normalized voltammograms of 4MA in the presence of phenol at the same conditions as b at different scan rates. Scan rates from I−V are 5, 10, 25, 50, and 100 mV s−1. Temperature: 25 ± 1 °C.
without toxic reagents and solvents at a carbon electrode in an undivided cell.
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N-(4-((4-Oxocyclohexa-2,5-dien-1-ylidene)amino)phenyl)acetamide (NPQ2). Isolated yield: 65%. Mp: 113−115 °C. 1H NMR (300 MHz, DMSO-d6): δ 2.08 (s, 3H), 6.61 (dd, J = 10.2, 2.1 Hz, 1H), 6.71 (dd, J = 9.9, 2.1 Hz, 1H), 6.98 (d, J = 8.7 Hz, 2H), 7.2 (dd, J = 10.2, 2.7 Hz, 1H), 7.37 (dd, J = 9.9, 2.4 Hz, 1H), 7.71 (d, J = 8.7 Hz, 2H), 10.24 (s, 1H, NH, disappeared upon the addition of D2O). 13C NMR (75 MHz, DMSO-d6): δ 24.2, 120.0, 122.9, 128.9, 132.7, 133.5, 138.3, 142.4, 144.7, 157.0, 169.5, 188.1. IR (KBr): 3319, 2925, 2854, 1683, 1642, 1614, 1514, 1410, 1316, 1262, 1118, 848 cm−1. MS (EI, 70 eV) m/z (relative intensity) 240 (78), 198 (100), 169 (31), 149 (27), 91 (8), 43 (25).
MATERIALS AND METHODS
Reagents and Apparatus. Reaction equipment is described in an earlier paper.35 4-Morpholinoaniline, N-(4-aminophenyl)acetamide, fast violet B, 4-tert-butyl-o-benzoquinone, phenol, phosphate salts, and ethanol were obtained from commercial sources. These chemicals were used without further purification. The glassy carbon electrode was polished using alumina slurry. The purity of products was checked by TLC, and characterization was done using 1H NMR, 13C NMR, IR spectroscopic techniques, and mass spectrometry. Electroorganic Synthesis of NPQ1−5. The electrochemical synthesis of N-phenylquinoneimine derivative compounds (NPQ1−5) was carried out in an undivided cell using controlled-potential conditions in a water (phosphate buffer, c = 0.2 M, pH 8.0)/ethanol (20/80, v/v) mixture (ca. 85 mL). In the first strategy, 0.25 mmol of 4MA or 0.25 mmol of NAA and 0.25 mmol of phenol were electrolyzed in an undivided cell at 0.40 and 0.50 V vs Ag/AgCl, respectively. In the second strategy, 0.25 mmol of 4TBC and 0.25 mmol of 4MA, NAA, or FVB were electrolyzed in an undivided cell at 0.20 V vs Ag/AgCl. Electrolysis was terminated when the decay of the current became more than 95%. The solid precipitated was collected by filtration and washed several times with water. The products were purified by column chromatography (silica gel) with different solvent systems. The solvent system for column chromatography of NPQ1−5 was ethyl acetate/n-hexane with volume ratios 55/45, 50/50, 25/75, 40/60, and 40/60, respectively. After purification, all products were characterized by IR, 1H NMR, 13C NMR, and MS. 4-((4-Morpholinophenyl)imino)cyclohexa-2,5-dien-1-one (NPQ1). Isolated yield: 84%. Mp: 162−164 °C. 1H NMR (300 MHz, DMSO-d6): δ 3.25 (m, 4H), 3.76 (m, 4H), 6.59 (dd, J = 10.2, 2.4 Hz, 1H), 6.66 (dd, J = 9.7, 2.1 Hz, 1H), 7.01−7.09 (m, 4H), 7.30 (dd, J = 10.2, 2.7 Hz, 1H), 7.36 (dd, J = 9.7, 2.7 Hz, 1H). 13C NMR (75 MHz, DMSO-d6): δ 47.9, 66.3, 115.2, 125.6, 129.2, 131.6, 132.6, 140.9, 142.6, 151.0, 154.8, 188.2. IR (KBr): 2963, 2854, 1633, 1612, 1592, 1503, 1250, 1179, 1122, 873, 821 cm−1. MS (EI, 70 eV) m/z (relative intensity) 268 (100), 210 (92), 167 (39), 149(89), 77 (14), 57 (24), 41 (12).
(E)-5-(tert-Butyl)-2-hydroxy-4-((4-morpholinophenyl)imino)cyclohexa-2,5-dien-1-one (NPQ3). Isolated yield: 88%. Mp: 86−88 °C. 1H NMR (300 MHz, DMSO-d6): δ 1.41 (s, 9H), 3.14 (s, 4H), 3.76 (s, 4H), 6.23 (s, 1H), 6.4 (s, 1H), 6.8 (d, J = 7.8 Hz, 2H), 7.0 (d, J = 8.1 Hz, 2H). 13C NMR (75 MHz, DMSO-d6): δ 31.3, 36.5, 48.9, 66.5, 103.6, 115.8, 122.4, 127.3, 142.2, 149.2, 153.9, 157.7, 159.0, 184.2. IR (KBr): 2961, 2863, 1636, 1599, 1503, 1422, 1383, 1234, 1116, 928 cm−1. MS (EI, 70 eV) m/z (relative intensity) 340 (75), 297 (29), 204 (100), 163 (48), 105 (28), 57 (24).
(E)-N-(4-((2-(tert-Butyl)-5-hydroxy-4-oxocyclohexa-2,5-dien1ylidene)amino)phenyl) acetamide (NPQ4). Isolated yield: 90%. Mp: 99−101 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.42 (s, 9H), 2.06 (s, 3H), 5.86 (s, 1H), 6.41 (s, 1H), 6.72 (d, J = 7.6 Hz, 2H), 7.6 (d, J = 7.2 Hz, 2H), 10.03 (s, 1H). 13C NMR (100 MHz, DMSO-d6): δ 24.0, 31.4, 36.6, 102.1, 119.8, 119.9, 120.7, 127.7, 136.2, 146.3, 159.4, 9424
DOI: 10.1021/acssuschemeng.7b02553 ACS Sustainable Chem. Eng. 2017, 5, 9423−9430
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ACS Sustainable Chemistry & Engineering 168.4, 168.5, 186.3. IR (KBr): 3425, 2923, 1638, 1597, 1536, 1498, 1408, 1225, 1108, 838, 570 cm−1. MS (EI, 70 eV) m/z (relative intensity) 312 (62), 269 (44), 242 (63), 197 (52), 156 (83), 127 (86), 77 (83), 43 (100).
Scheme 1. Proposed Mechanism for the Electrochemical Oxidation of 4MA and NAA in the Presence of Phenol
(E)-N-(4-((2-(tert-Butyl)-5-hydroxy-4-oxocyclohexa-2,5-dien1-ylidene)amino)-5-methoxy-2-methylphenyl)benzamide (NPQ5). Isolated yield: 96%. Mp: 174−176 °C. 1H NMR (400 MHz, CDCl3): δ 1.51 (s, 9H), 2.33 (s, 3H), 3.86 (s, 3H), 6.32 (s, 1H), 6.57 (s, 1H), 6.68 (s, 1H), 7.54−7.64 (m, 3H), 7.74 (s, 1H), 7.94 (d, J = 8.0 Hz, 2H). 13C NMR (100 MHz, DMSO-d6): δ 16.9, 31.0, 37.1, 56.1, 102.9, 106.7, 120.2, 121.4, 125.6, 127.0, 128.9, 132.0, 133.8, 134.9, 136.2, 147.7, 150.3, 158.7, 162.3, 165.6, 183.6. IR (KBr): 3381, 3252, 2967, 1632, 1581, 1420, 1317, 1227, 890, 695 cm−1. MS (EI, 70 eV) m/z (relative intensity) 418 (69), 375 (12), 282 (8), 136 (9), 105 (100), 77 (95), 51 (16).
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RESULTS AND DISCUSSION Electrochemical Oxidation of 4MA and NAA in the Presence of Phenol. We report here the electrochemical behavior of 4MA and NAA in a mixture of water/ethanol (80/ 20, v/v) solution containing phosphate buffer (pH 8.0, c = 0.2 M). The cyclic voltammogram of 4MA shows one anodic (A1) (EpA1 = 0.35 V vs Ag/AgCl) in the positive going scan and two cathodic peaks (C1 and C2) (EpC1 = 0.08 and EpC2 = −0.07 V vs Ag/AgCl) in the negative going scan (Figure 1a). The redox couples (peaks A1 and C1) corresponding to the redox reaction of 4MA/4-(4-iminocyclohexa-2,5-dien-1-ylidene) morpholin-4ium (4MAox) (a p-quinone−diimine compound) (Scheme 1).36,37 Under these conditions, the peak current ratio(IpC1/ IpA1) is less than one, but it progressively increases with increasing potential scan rate, decreasing pH, and decreasing 4MA concentration.36,37 In Figure 1a, peak C2 can be ascribed to the reduction of a byproduct formed during oxidation of the parent compound.36,37 Figure 1b shows the cyclic voltammogram of 4MA (1 mM) in the presence of phenol (1.0 mM). Under these conditions, the voltammogram shows the changes including increases IpA1, the disappearance of peaks C1 and C2, and the appearance of peak C3 (EpC1 = 0.06 V vs Ag/AgCl) and its anodic counterpart (A3) (EpA3 = 0.10 V vs Ag/AgCl). The increases IpA1 in the presence of phenol indicate an increase in the number of transferred electrons, as in this case, napp > n, where napp is the apparent number of electrons. The disappearance of peaks C1 and C2 confirms the reaction of phenol with 4MAox suppressing other side reactions (such as dimerization),36,37 which 4MAox is involved in.38 On the other hand, the appearance of peaks C3 and A3 is evidence of the formation of a new electroactive product. In this figure, curve c is the cyclic voltammogram of phenol under the same conditions. For more data, the normalized peak A1 in the presence of phenol is also shown in Figure 1 as inset. The normalization was performed by dividing the current by the
square root of the potential scan rate. It can be seen that with increasing scan rate, the current of peak A1 (IpA1/v1/2) decreases. This shows that with increasing potential scan rate, the reaction time for chemical reaction between phenol and 4MAox is not provided. This condition decreases the apparent number of electrons (napp) and consequently decreases the normalized current of peak A1. These data are indicative of an electron transfer−chemical reaction−electron transfer (ECE) mechanism in oxidation of 4MA in the presence of phenol.38 Controlled-potential coulometry (CPC) experiments were performed in water (phosphate buffer, c = 0.2 M, pH 8.0)/ ethanol mixture (80/20, v/v) containing 4MA (0.25 mmol) in the presence of the same amount of phenol at 0.40 V versus Ag/AgCl. To obtain more information about the oxidation mechanism of 4MA in the presence of phenol, cyclic voltammetric and UV/vis spectroscopic analysis were carried out during the CPC. Figure 2, part I shows the cyclic voltammograms of 4MA in the presence of phenol during CPC. Figure 2 displays the progressive formation of peaks A3/C3, parallel to the disappearance of peak A1. This peak fully disappears when the charge transfer becomes about 4e− per molecule of 4MA. We have also been able to confirm the number of electrons transferred for the oxidation of 4MA in the presence of phenol by plotting of the IpA1 against the transferred charge (n = 4.1) (Figure 2, part II). The cyclic voltammogram of the saturated solution of the synthesized product in water (phosphate buffer 0.2 M, pH 8.0)/ethanol mixture (80/20, v/v) mixture is also shown in this figure as part III. This CV displays the redox behavior of the product. The presence of the cathodic current at the beginning of the cyclic voltammogram shows that the product is in the oxidized form, and its half wave potential is less positive than that of 4MA. For more data, the time-dependent absorption spectra of a mixture of 4MA and phenol were collected during the CPC experiment 9425
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Figure 2. Part I: Cyclic voltammograms of 4MA (0.25 mmol) in the presence of phenol (0.25 mmol) during controlled-potential coulometry at +0.40 V versus Ag/AgCl in water (phosphate buffer, c = 0.2 M, pH 8.0)/ethanol mixture (80/20, v/v). Scan rate: 100 mV s−1 after consumption of 0, 12, 25, 35, 47, 60, 70, 80, and 98 C. Part II: Variation of peak current (IpA1) versus charge consumed. Part III: Cyclic voltammogram of saturated solution of synthesized product (NPQ1) in coulometric conditions. Temperature: 25 ± 1 °C.
(Figure 3). The absorption spectra exhibit two absorption peaks at 271 and 580 nm, and their heights increase with the advancement of CPC. This result suggests that the appearance of the peak at 580 nm arising from the extension of the πconjugation system can be ascribed to negative shifts in the oxidation potentials (i.e., negative shift in the peak A3).39 These results along with the spectroscopic data of the isolated product from the bulk electrolysis of 4MA in the presence of phenol allow us to propose the following mechanism for electrochemical oxidation of 4MA in the presence of phenol (Scheme 1). The generation of 4MAox is followed by a Michael-like addition reaction of phenol (and aromatization), producing related phenylaminophenol as an intermediate (INT) (C−N coupling process40−42). The oxidation of INT because of its electronic structure is easier than that of 4MA. Therefore, at the applied potential for the oxidation of 4MA (0.40 V vs Ag/ AgCl), the oxidation of INT was carried out, and the final product (NPQ1) was formed. According to our results, the anodic and cathodic peaks A3 and C3 correspond to the oxidation of INT to NPQ and reduction of NPQ to INT. In the proposed mechanism, the homogeneous oxidation of INT by 4MAox (disproportionation reaction) is also possible. The chronoamperometry responses of 4MA in the absence and presence of phenol is show in Figure 4. As can be seen, the
Figure 3. Absorption spectra during controlled-potential coulometry at +0.40 V versus Ag/AgCl in water (phosphate buffer, c = 0.2 M, pH 8.0)/ethanol mixture (80/20, v/v) after consumption of 0, 12, 25, 35, 47, 60, 70, 80, and 98 C. Inset: absorption spectrum of the isolated product (NPQ1).
currents in the presence of phenol (curve a) are larger than currents in the absence of phenol (curve b). The cyclic 9426
DOI: 10.1021/acssuschemeng.7b02553 ACS Sustainable Chem. Eng. 2017, 5, 9423−9430
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It should be noted that other data consistent with the oxidation of NAA in the presence of phenol are similar to those of 4MA (see Supporting Information). Electrochemical Oxidation of 4-tert-Buthylcatechol in the Presence of 4MA, NAA, and Fast Violet B (FVB). In this part, the electrochemical oxidation of 4-tert-buthylcatechol (4TBC) was studied in the presence of 4MA as a nucleophile in water (phosphate buffer, pH 8.0 and c = 0.2 M)/ethanol (80/20, v/v) mixture at glassy carbon electrode (Figure 6). The
Figure 4. Chronoamperograms of 4MA (1.0 mM) in the presence (curve a) and in the absence (curve b) of phenol (1.0 mM) at glassy carbon electrode, in water (phosphate buffer, c = 0.2 M, pH 8.0)/ ethanol mixture (80/20, v/v). The applied potential is +0.35 V vs Ag/ AgCl. Temperature = 25 ± 1 °C.
voltammetry results (Figure 1, inset) along with chronoamperometry data indicate that the extent of the disproportionation reaction in electrochemical oxidation of 4MA in the presence of phenol is low. The chronoamperometry responses of NAA in the absence and presence of phenol are shown in Figure 5. The normalized
Figure 6. (a) Cyclic voltammogram of 4TBC (1.0 mM). (b) Cyclic voltammogram of 4TBC (1.0 mM) in the presence of 4MA (1.0 mM). (c) Cyclic voltammogram of 4MA (1.0 mM). Scan rate: 10 mV s−1. Inset: normalized voltammograms of 4TBC in the presence of 4MA at different potential scan rates (I = 5, II = 10, III = 25, and IV = 50 mV s−1). Working electrode: glassy carbon electrode. Solvent: water (phosphate buffer, c = 0.2 M, pH 8.0)/ethanol (80/20, v/v) mixture. Temperature: 25 ± 1 °C.
voltammogram of 4TBC (Figure 6a) shows a well-defined redox peak with the anodic peak potential at 0.21 V and the corresponding cathodic peak potential at 0.16 V. These peaks are related to the oxidation of 4TBC to 4-(tert-butyl)-1,2benzoquinone (4TBQ) and reduction of 4TBQ to 4TBC, respectively.45 Comparison of the voltammogram of 4TBC (curve a) with that in the presence of 4MA (curve b) shows a decrease in cathodic peak current (IpC1) with the appearance of a new redox peak (A2/C2) at less positive potentials. For more data on electrooxidation of 4TBC in the presence of 4MA, the effect of potential scan rate and number of transferred electrons was also studied. The normalized cathodic peaks C1 and C2 of 4TBC in the presence of 4MA at different scan rates are shown in Figure 6, inset. It shows that with increasing scan rate, IpC1 increases and IpC2 decreases. In Figure 6, curve c is a cyclic voltammogram of 4MA. The chronoamperograms of 4TBC in the absence and presence of 4MA are shown in Figure 7. In addition, the normalized voltammograms of 4TBC (peak A1) in the presence of 4MA are shown in Figure 7 as an inset. As can be seen, the chronoamperograms of 4TBC in the absence and presence of 4MA are similar, and the normalized peak currents (IpA1/v1/2) of 4TBC in the presence of 4MA are almost similar. These data confirm the presence of disproportionation reaction in
Figure 5. Chronoamperograms of NAA (1.0 mM) in the presence (curve a) and in the absence (curve b) of phenol (1.0 mM) at glassy carbon electrode in water (phosphate buffer, c = 0.2 M, pH 8.0)/ ethanol mixture (80/20, v/v). The applied potential is +0.50 V vs Ag/ AgCl. Inset: normalized voltammograms of NAA (1.0 mM) in the presence of phenol at the same conditions at different scan rates. Scan rates are 2, 5, 10, 25, and 50 mV s−1. Temperature: 25 ± 1 °C.
voltammograms of NAA in the presence of phenol (peak A1) are also shown in Figure 5 as an inset. As can be seen, both chronoamperometry and voltammetry data of NAA are different than those of 4MA. Unlike the Figure 1 inset, in the Figure 5 inset, the normalized peak currents (IpA1/v1/2) are almost similar. In addition, unlike Figure 4, the chronoamperograms in the absence and presence of phenol are similar. These data show that the extent of the disproportionation reaction in electrochemical oxidation of NAA in the presence of phenol is high (kdisp is high), and the dominant mechanism is the electron transfer−chemical reaction−electron transfer/disproportionation (ECE/DISP) process.43,44 9427
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4MA, during CPC, show the formation of peaks A2/C2, simultaneously with the decrease of IpA1 and IpC1. These electrochemical data which are consistent with the ECE/DISP reaction, along with the spectroscopic data of the reaction product of electrolysis of 4TBC in the presence of 4MA, allows us to propose the following mechanism for electrochemical oxidation of 4TBC in the presence of 4MA (Scheme 2). According to Scheme 2, the Michael-like addition reaction of 4MA to the 4TBQ followed by aromatization leads to substituted-4TBC as an intermediate (STBC) (C−N coupling process40−42). The oxidation of STBC is easier than the oxidation of 4TBC because of the presence of 4MA as an electron-donating group in the STBC molecule. Therefore, the oxidation of STBC proceeds at the applied potential of 0.20 V to yield NPQ3. According to the proposed mechanism, peaks A2 and C2 are assigned to the oxidation of STBC to NPQ3 and reduction of NPQ3 to STBC, respectively. It should be noted that the same data are also consistent with the oxidation of 4TBC in the presence of NAA and FVB (see Supporting Information). To investigate the electrochemical property of products (NPQ1−5), voltammetric behavior of a saturated solution of NPQ1−5 in water (phosphate buffer, pH 8.0, c = 0.2 M)/ ethanol (80/20, v/v) mixture was studied (Figure 8) (cyclic voltammogram of NPQ1 is shown in Figure 2, part III). The results show that the current of the initial potentials is cathodic. This confirms that NPQ1−5 are in their oxidized form. The present methods are easy to scale-up without compromising any of the advantages because they do not use any sophisticated equipment or complicated reactions. The processes were performed in one-pot at room temperature and
Figure 7. Chronoamperograms of 4TBC (1.0 mM) in the presence (curve a) and in the absence (curve b) of 4MA (1.0 mM) at glassy carbon electrode in water (phosphate buffer, c = 0.2 M, pH 8.0)/ ethanol mixture (80/20, v/v). The applied potential is +0.20 V vs Ag/ AgCl. Inset: Normalized voltammograms of 4TBC (1.0 mM) in the presence of 4MA at the same conditions at different scan rates. Scan rates are 5, 10, 25, and 50 mV s−1. Temperature: 25 ± 1 °C.
electrochemical oxidation of 4TBC in the presence of 4MA. These results approve the reactivity of electrochemically generated 4TBQ toward 4MA and approve the ECE/DISP reaction. Controlled potential coulometry of an aqueous solution containing 4TBC (0.25 mmol) in the presence of 4MA (0.25 mmol) at 0.20 V versus Ag/AgCl indicated that four electrons per molecule (4TBC) were removed at this potential. In addition, the cyclic voltammograms of 4TBC in the presence of
Scheme 2. Proposed Mechanism for the Electrochemical Oxidation of 4TBC in the Presence of 4MA, NAA, and FVB
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tages. They are practically easy to carry out and can be achieved at room temperature and atmospheric pressure. Neither inorganic/organic oxidizing agents nor catalysts are necessary, and the reactions are processed under green conditions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02553. Data including cyclic voltammogram of NAA in the presence of phenol, cyclic voltammograms of 4TBC in the presence of NAA and FVB, FT-IR, 1H NMR, 13C NMR, and MS spectra of NPQ1−5 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; Fax: 0098-813-8257407; Tel: 0098813-8282807. ORCID
Davood Nematollahi: 0000-0001-9638-224X Notes
Figure 8. Cyclic voltammograms of saturated solution of NPQ2−5 in water (phosphate buffer, pH 8.0, c = 0.2 M)/ethanol (80/20, v/v) mixture. Scan rate: 50 mV s−1. Working electrode: glassy carbon electrode. Temperature: 25 ± 1 °C.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Bu-Ali Sina University Research Council and Center of Excellence in Development of Environmentally Friendly Methods for Chemical Synthesis (CEDEFMCS) for their support of this work.
pressure, in water/ethanol mixture, and in an undivided cell equipped with a carbon anode and a stainless steel cathode. Therefore, although our experiments were carried out on a relatively small scale; there is little trouble for synthesis of large amounts of products either by running several cells in series or by using larger cells. In addition, the present methods are in agreement with the principle of green chemistry. Clean synthesis (low polluting condition) using electricity instead of chemical reagents (neither catalysts nor organic/inorganic oxidizing agents, strong acids, or base), using water/ethanol mixture instead of toxic solvents, access to high atom economy, ambient conditions, and one-step process are important features of the work, which are consistent with green requirements. It should be noted that this study has some limitations. The main limitation involved in this study is use of phenols with an oxidation potential higher than that of 4MA or NAA (in the first strategy) or catechols with an oxidation potential lower than that of aniline compounds.
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REFERENCES
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CONCLUSIONS Taking into consideration that nowadays one of the most important tasks for the synthetic organic chemist is the development of green sustainable methodologies for the synthesis of organic compounds, the main goal of this work was to develop a green protocol for the synthesis of some Nphenylquinoneimine derivatives. To achieve this goal, two methods are developed. The first is based on the electrochemical oxidation of 4MA (or NAA) in the presence of phenol. In this method, the oxidized form of 4MA (or NAA) was employed as a Michael acceptor for reaction with phenol (Scheme 1). The second method proposed in this paper makes use of 4MA (or NAA or FVB) as the nucleophile in the reaction with the electrochemically generated 4-tert-butyl-obenzoquinone. Both proposed methods have several advan9429
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Research Article
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