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Kinetic study of electrochemically induced Michael reaction of 1,4-dihydroxyanthraquinone with acetylacetone and benzoylacetone
G Model
CCLET-2689; No. of Pages 3 Chinese Chemical Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Kinetic study of electrochemically induced Michael reaction of 1,4-dihydroxyanthraquinone with acetylacetone and benzoylacetone Davood Nematollahi a,*, Bayan Moradi b, Fahimeh Varmaghani a a b
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65178-38683, Iran Department of Chemistry, Faculty of Science, Arak Branch, Islamic Azad University, Arak 38135-567, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 March 2013 Received in revised form 8 July 2013 Accepted 16 July 2013 Available online xxx
The electrochemical oxidation of 1,4-dihydroxyanthraquinone has been studied in the presence of acetylacetone and benzoylacetone as nucleophiles in a mixture of ethanol/water by means of cyclic voltammetry as a diagnostic technique. The results indicate the participation of electrochemically produced anthraquinone in the Michael addition reaction with acetylacetone and benzoylacetone to form the corresponding new anthraquinone derivatives. On the basis of the EC mechanism, the observed homogeneous rate constants (kobs) of the reaction of anthraquinone with acetylacetone and benzoylacetone were estimated by comparing the experimental cyclic voltammograms with the digitally simulated results. ß 2013 Davood Nematollahi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 1,4-Dihydroxyanthraquinone Cyclic voltammetry Digital simulation Acetylacetone Benzoylacetone
1. Introduction Electrochemistry provides a very versatile means for electrosynthesis, mechanistic and kinetic studies [1,2]. Furthermore, cyclic voltammetry is known as a powerful technique for the investigation of electrochemical reactions that are coupled with chemical reactions [3]. In addition, general treatment of the reaction mechanism is probably best carried out through digital simulations. Hydroxy-derivatives of anthraquinones are of interest as dyes and pigments [4] and also, as model chromophores for biologically active compounds, such as the anthracycline antitumor drugs. These compounds are the main element in the chemical structure of antitumor drugs, such as daunorubicin, doxorubicin, and mitoxantrone [5]. In this regard, we have recently synthesized a number of new derivatives of anthraquinone [6,7]. Herein, to increase the available data in the electrochemical synthesis of new anthraquinone derivatives, we have investigated the electrochemical oxidation of 1,4-dihydroxyanthraquinone in the presence of acetylacetone and benzoylacetone as CH-acid nucleophiles. The purpose of this work is the kinetic and mechanistic study of the electrochemical oxidation of 1,4-dihydroxyanthraquinone in the presence of CH-acid nucleophiles and the estimation of the observed homogeneous rate constants (kobs) of the reaction of
* Corresponding author. E-mail address: [email protected] (D. Nematollahi).
electrochemically generated anthraquinone with these nucleophiles by digital simulation of cyclic voltammograms. 2. Experimental Reaction equipment is described in an earlier paper [7]. The 1,4dihydroxyanthraquinone, acetylacetone and benzoylacetone were obtained from commercial sources. The homogeneous rate constants were estimated by analyzing the cyclic voltammetric responses using the simulation DIGIELCH software [8]. In a typical procedure, 100 mL of acetate buffer solution (c 0.2 mol/L, pH 4.0) in water/ethanol (30/70) containing 0.4 mmol of 1,4-dihydroxyanthraquinone (1) and 0.4 mmol acetylacetone (2) (or benzoylacetone (3)) was electrolyzed at 0.6 V versus SCE in a divided cell. The electrolysis was terminated when the current decayed to 5% of its original value. The precipitated solid was collected by filtration and was washed several times with water. Characteristic of MS m/z (% relative intensity) for 4: 105 (50), 128 (30), 149 (80), 240 (100), 337 (1.5) and for 5: 109 (100), 153 (75), 171 (28), 240 (28), 254 (20), 317 (18.5), 402 (1.5). 3. Results and discussion Cyclic voltammetry of 1.0 mmol/L of 1,4-dihydroxyanthraquinone (1) in a water/ethanol solution (30/70) containing 0.2 mol/L acetate buffer (pH 4.0) shows one anodic (A1) and the corresponding cathodic peak (C1) (Fig. 1 (curve a)) which coincides to the
1001-8417/$ – see front matter ß 2013 Davood Nematollahi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2013.07.024
Please cite this article in press as: D. Nematollahi, et al., Kinetic study of electrochemically induced Michael reaction of 1,4dihydroxyanthraquinone with acetylacetone and benzoylacetone, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/ j.cclet.2013.07.024
G Model
CCLET-2689; No. of Pages 3 D. Nematollahi et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
16
45
A1
a b
35
11
d
0.12
25
A0
0
I/ A
I/ A
A1
0.24
IpC1/IpA1
2
6
0
150
300
450
v/ mV s-1
15
5 1
c
a
-5
C1
-5 0.2
0.4
0.6
C1
-15
0.8
-0.3
0.0
0.3
E/V vs. SCE
0.6
0.9
E/V vs. SCE
Fig. 1. (a) A cyclic voltammogram of 1.0 mmol/L 1,4-dihydroxyanthraquinone (1) in the absence and (b) in the presence of 1.0 mmol/L acetylacetone (2). (c) A cyclic voltammogram of 1.0 mmol/L acetylacetone (2) in the absence of 1, in water/ ethanol (30/70) solution containing acetate buffer (c 0.2 mol/L, pH 4.0). Scan rate: 100 mV s 1, T = 25 1 8C.
Fig. 2. Cyclic voltammograms of 1.0 mmol/L 1,4-dihydroxyanthraquinone (1) in the presence of 1.0 mmol/L acetylacetone (2) in various scan rates at a glassy carbon electrode, in water/ethanol (30/70) solution containing acetate buffer (c 0.2 mol/L, pH 4.0). Scan rates from a to d are: 50, 100, 250 and 500 mV s 1. Inset, variation of the peak current ratio (IpC1/IpA1) versus scan rate, T = 25 1 8C.
transformation of 1,4-dihydroxyanthraquinone (1) to anthraquinone (1ox), and vice versa, within a quasi-reversible two electron process (Scheme 1). It is shown that the peak current ratio (IpC1/ IpA1) deviates from unity. This behavior is due to the presence of two fused quinonic rings in the structure of 1ox. The presence of two fused quinonic rings, as electron-withdrawing groups, contribute to the instability of this compound, so the following chemical reactions, such as oxidative ring cleavage, is observed in the time scale of cyclic voltammetry [6,7]. Under the same conditions, the oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of acetylacetone (2) as nucleophile was studied in some detail. Fig. 1 (curve b) shows the cyclic voltammogram obtained for a 1.0 mmol/L solution of 1 in the presence of 1.0 mmol/L acetylacetone (2). It is observed that the cathodic counterpart of the anodic peak A1 disappears. In this figure, curve c is the voltammogram of acetylacetone (2) in the absence of 1. The anodic peak A0 is according to the oxidation of acetylacetone (2), at a more positive potential than 1. It should be noted that 1 in the presence of 2 and 3, shows basically the same electrochemical behavior. Furthermore, proportional to the augmentation of potential sweep rate (Fig. 2), the height of the C1 peak increases. A plot of the peak current ratio (IpC1/IpA1) versus scan rate (Fig. 2, inset) for a
mixture of 1,4-dihydroxyanthraquinone (1) and acetylacetone (2) confirms the reactivity of 1ox toward anionic form of 2 (2An ), appearing as an increase in the height of the cathodic peak C1 at higher scan rates. The electrolysis was performed in acetate buffer solution (c 0.2 mol/L, pH 4.0) in water/ethanol mixture (30/70) containing 0.4 mmol of 1 and 0.4 mmol of 3 at 0.6 V versus SCE (Fig. 3). It is shown that proportional to the advancement of electrolysis and consumption of 1, the height of A1 decreases. Our previous experiences on the electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of some nucleophiles [6,7], diagnostic criteria of cyclic voltammetry and the mass spectra of the isolated products indicate that the reaction mechanism of electro-oxidation of 1 in the presence of 2 and 3 is EC (Scheme 1). Generation of anthraquinone (1ox) is followed by a Michael addition reaction of anionic forms CH-acid nucleophiles (2An , 3An ) to 1ox, producing 4 and 5 as final products. The electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of CH-acid nucleophiles (2 and 3) was tested by digital simulation. Regarding the substituted group on the nucleophile, different reactivity can be expected. As a result, the chemical reaction rate in an EC mechanism is completely depended
O
OH
O -2e- -2H+
O
OH
O
Me O
2An-, 3An-
O 1ox
+H
R
O
2An-, 3An-
OH O O
R: -CH3
Me O
O
Me
+
2,3
O
+ R
R
O 1ox
O
-H+
Me O
1
O
O
O
R
O
OH O
-C6H5
2, 2An-, 4 3, 3An-, 5
4, 5
Scheme 1. Electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of CH-acid nucleophiles.
Please cite this article in press as: D. Nematollahi, et al., Kinetic study of electrochemically induced Michael reaction of 1,4dihydroxyanthraquinone with acetylacetone and benzoylacetone, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/ j.cclet.2013.07.024
G Model
CCLET-2689; No. of Pages 3 D. Nematollahi et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
A1
45
decreasing the tendency of 3 to participate in Michael reaction with 1ox.
Progress in electrolysis
I/ A
35
4. Conclusion
25
15
5
-5
3
0.3
0.1
0.5 E/V vs. SCE
0.7
0.9
The overall reaction mechanism for the electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of CH-acid nucleophiles (2 and 3) is represented in Scheme 1. According to the obtained results, the reaction mechanism of electro-oxidation of 1 in the presence of 2 and 3 is an EC mechanism. The kinetics of the reactions of electrochemically generated anthraquinone (1ox) with 2 and 3 were studied by the cyclic voltammetric technique, and the simulation of obtained voltammograms was performed under the EC mechanism. Also, the effect of the substituted groups on the CH-acid nucleophiles ring on kobs has been studied. The results show that the presence of electron-donating groups causes increasing in Michael reaction rate.
Fig. 3. Cyclic voltammograms of 0.4 mmol 1,4-dihydroxyanthraquinone (1) in the presence of 0.4 mmol benzoylacetone (3), at a glassy carbon electrode during electrolysis at 0.60 V versus SCE.
Acknowledgments
Table 1 Homogeneous rate constants (kobs) for nucleophilic addition of 2–3 to 1.
We would like to thank Dr. M. Rudolph for his cyclic voltammogram digital simulation software (DigiElch SB). We also acknowledge Bu-Ali Sina University Research Council and Center of Excellence in Development of Environmentally Friendly Methods for Chemical Synthesis (CEDEFMCS) for their support to this work.
Nucleophile kobs (L mol a
1
s
1 a
)
2
3
172 2.6
162 2.4
Standard deviation of five independent simulations.
References on the substituted group on the nucleophiles. To get more information on the relationship between the homogeneous rate constant (kobs) and the structure of nucleophiles, kobs was estimated by digital simulation. The simulation was performed based on proposed mechanism in Scheme 1. More details on digital simulation are described in our previous paper [6]. The calculated values of the second-order rate constants for the reaction of anthraquinone (1ox) with acetylacetone (2) and benzoylacetone (3) are shown in Table 1. The kobs depended on the electrondonating strength of the substituted group (R). The presence of an electron-donating group causes increases in nucleophile strength of anionic forms (2An , 3An ). The calculated observed homogeneous rate constants (kobs) were found to vary in the order acetylacetone > benzoylacetone, which parallels with electrondonating properties in the order of COCH3 > COC6H5. Also, in spite of methyl group in 2, the steric effect of phenyl group in 3 leads to
[1] A. Houmam, Electron transfer initiated reactions: bond formation and bond dissociation, Chem. Rev. 108 (2008) 2180–2237. [2] F. Varmaghani, D. Nematollahi, S. Mallakpour, et al., Electrochemical oxidation of 4-substituted urazoles in the presence of arylsulfinic acids: an efficient method for the synthesis of new sulfonamide derivatives, Green Chem. 14 (2012) 963–967. [3] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., Wiley, New York, 2001. [4] D.D. Nguyen, N.C. Jones, V.S. Hofftman, et al., Synchrotron radiation linear dichroism (SRLD) investigation of the electronic transitions of quinizarin, chrysazin, and anthrarufin, Spectrochim. Acta A 77 (2010) 279–286. [5] B.J. Stefafiska, M.J. Dzieduszycka, S. Martelli, et al., 2-Amination of quinizarin via Michael addition of hydrazines or amines, J. Org. Chem. 58 (1993) 1568–1569. [6] D. Nematollahi, A. Sayadi, F. Varmaghani, Electrochemical study of quinizarin in the presence of arylsulfinic acids: synthesis of new sulfone derivatives of quinizarin, J. Electroanal. Chem. 671 (2012) 44–50. [7] D. Nematollahi, B. Moradi, F. Varmaghani, A facile and one-pot electrochemical method for the synthesis of a new anthraquinonethioether, Chin. Chem. Lett. 23 (2012) 553–556. [8] M. Rudolph, Digital simulation on unequally spaced grids: part 1. Critical remarks on using the point method by discretisation on a transformed grid, J. Electroanal. Chem. 529 (2002) 97–108.
Please cite this article in press as: D. Nematollahi, et al., Kinetic study of electrochemically induced Michael reaction of 1,4dihydroxyanthraquinone with acetylacetone and benzoylacetone, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/ j.cclet.2013.07.024
CCLET-2689; No. of Pages 3 Chinese Chemical Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Kinetic study of electrochemically induced Michael reaction of 1,4-dihydroxyanthraquinone with acetylacetone and benzoylacetone Davood Nematollahi a,*, Bayan Moradi b, Fahimeh Varmaghani a a b
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65178-38683, Iran Department of Chemistry, Faculty of Science, Arak Branch, Islamic Azad University, Arak 38135-567, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 March 2013 Received in revised form 8 July 2013 Accepted 16 July 2013 Available online xxx
The electrochemical oxidation of 1,4-dihydroxyanthraquinone has been studied in the presence of acetylacetone and benzoylacetone as nucleophiles in a mixture of ethanol/water by means of cyclic voltammetry as a diagnostic technique. The results indicate the participation of electrochemically produced anthraquinone in the Michael addition reaction with acetylacetone and benzoylacetone to form the corresponding new anthraquinone derivatives. On the basis of the EC mechanism, the observed homogeneous rate constants (kobs) of the reaction of anthraquinone with acetylacetone and benzoylacetone were estimated by comparing the experimental cyclic voltammograms with the digitally simulated results. ß 2013 Davood Nematollahi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 1,4-Dihydroxyanthraquinone Cyclic voltammetry Digital simulation Acetylacetone Benzoylacetone
1. Introduction Electrochemistry provides a very versatile means for electrosynthesis, mechanistic and kinetic studies [1,2]. Furthermore, cyclic voltammetry is known as a powerful technique for the investigation of electrochemical reactions that are coupled with chemical reactions [3]. In addition, general treatment of the reaction mechanism is probably best carried out through digital simulations. Hydroxy-derivatives of anthraquinones are of interest as dyes and pigments [4] and also, as model chromophores for biologically active compounds, such as the anthracycline antitumor drugs. These compounds are the main element in the chemical structure of antitumor drugs, such as daunorubicin, doxorubicin, and mitoxantrone [5]. In this regard, we have recently synthesized a number of new derivatives of anthraquinone [6,7]. Herein, to increase the available data in the electrochemical synthesis of new anthraquinone derivatives, we have investigated the electrochemical oxidation of 1,4-dihydroxyanthraquinone in the presence of acetylacetone and benzoylacetone as CH-acid nucleophiles. The purpose of this work is the kinetic and mechanistic study of the electrochemical oxidation of 1,4-dihydroxyanthraquinone in the presence of CH-acid nucleophiles and the estimation of the observed homogeneous rate constants (kobs) of the reaction of
* Corresponding author. E-mail address: [email protected] (D. Nematollahi).
electrochemically generated anthraquinone with these nucleophiles by digital simulation of cyclic voltammograms. 2. Experimental Reaction equipment is described in an earlier paper [7]. The 1,4dihydroxyanthraquinone, acetylacetone and benzoylacetone were obtained from commercial sources. The homogeneous rate constants were estimated by analyzing the cyclic voltammetric responses using the simulation DIGIELCH software [8]. In a typical procedure, 100 mL of acetate buffer solution (c 0.2 mol/L, pH 4.0) in water/ethanol (30/70) containing 0.4 mmol of 1,4-dihydroxyanthraquinone (1) and 0.4 mmol acetylacetone (2) (or benzoylacetone (3)) was electrolyzed at 0.6 V versus SCE in a divided cell. The electrolysis was terminated when the current decayed to 5% of its original value. The precipitated solid was collected by filtration and was washed several times with water. Characteristic of MS m/z (% relative intensity) for 4: 105 (50), 128 (30), 149 (80), 240 (100), 337 (1.5) and for 5: 109 (100), 153 (75), 171 (28), 240 (28), 254 (20), 317 (18.5), 402 (1.5). 3. Results and discussion Cyclic voltammetry of 1.0 mmol/L of 1,4-dihydroxyanthraquinone (1) in a water/ethanol solution (30/70) containing 0.2 mol/L acetate buffer (pH 4.0) shows one anodic (A1) and the corresponding cathodic peak (C1) (Fig. 1 (curve a)) which coincides to the
1001-8417/$ – see front matter ß 2013 Davood Nematollahi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2013.07.024
Please cite this article in press as: D. Nematollahi, et al., Kinetic study of electrochemically induced Michael reaction of 1,4dihydroxyanthraquinone with acetylacetone and benzoylacetone, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/ j.cclet.2013.07.024
G Model
CCLET-2689; No. of Pages 3 D. Nematollahi et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
16
45
A1
a b
35
11
d
0.12
25
A0
0
I/ A
I/ A
A1
0.24
IpC1/IpA1
2
6
0
150
300
450
v/ mV s-1
15
5 1
c
a
-5
C1
-5 0.2
0.4
0.6
C1
-15
0.8
-0.3
0.0
0.3
E/V vs. SCE
0.6
0.9
E/V vs. SCE
Fig. 1. (a) A cyclic voltammogram of 1.0 mmol/L 1,4-dihydroxyanthraquinone (1) in the absence and (b) in the presence of 1.0 mmol/L acetylacetone (2). (c) A cyclic voltammogram of 1.0 mmol/L acetylacetone (2) in the absence of 1, in water/ ethanol (30/70) solution containing acetate buffer (c 0.2 mol/L, pH 4.0). Scan rate: 100 mV s 1, T = 25 1 8C.
Fig. 2. Cyclic voltammograms of 1.0 mmol/L 1,4-dihydroxyanthraquinone (1) in the presence of 1.0 mmol/L acetylacetone (2) in various scan rates at a glassy carbon electrode, in water/ethanol (30/70) solution containing acetate buffer (c 0.2 mol/L, pH 4.0). Scan rates from a to d are: 50, 100, 250 and 500 mV s 1. Inset, variation of the peak current ratio (IpC1/IpA1) versus scan rate, T = 25 1 8C.
transformation of 1,4-dihydroxyanthraquinone (1) to anthraquinone (1ox), and vice versa, within a quasi-reversible two electron process (Scheme 1). It is shown that the peak current ratio (IpC1/ IpA1) deviates from unity. This behavior is due to the presence of two fused quinonic rings in the structure of 1ox. The presence of two fused quinonic rings, as electron-withdrawing groups, contribute to the instability of this compound, so the following chemical reactions, such as oxidative ring cleavage, is observed in the time scale of cyclic voltammetry [6,7]. Under the same conditions, the oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of acetylacetone (2) as nucleophile was studied in some detail. Fig. 1 (curve b) shows the cyclic voltammogram obtained for a 1.0 mmol/L solution of 1 in the presence of 1.0 mmol/L acetylacetone (2). It is observed that the cathodic counterpart of the anodic peak A1 disappears. In this figure, curve c is the voltammogram of acetylacetone (2) in the absence of 1. The anodic peak A0 is according to the oxidation of acetylacetone (2), at a more positive potential than 1. It should be noted that 1 in the presence of 2 and 3, shows basically the same electrochemical behavior. Furthermore, proportional to the augmentation of potential sweep rate (Fig. 2), the height of the C1 peak increases. A plot of the peak current ratio (IpC1/IpA1) versus scan rate (Fig. 2, inset) for a
mixture of 1,4-dihydroxyanthraquinone (1) and acetylacetone (2) confirms the reactivity of 1ox toward anionic form of 2 (2An ), appearing as an increase in the height of the cathodic peak C1 at higher scan rates. The electrolysis was performed in acetate buffer solution (c 0.2 mol/L, pH 4.0) in water/ethanol mixture (30/70) containing 0.4 mmol of 1 and 0.4 mmol of 3 at 0.6 V versus SCE (Fig. 3). It is shown that proportional to the advancement of electrolysis and consumption of 1, the height of A1 decreases. Our previous experiences on the electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of some nucleophiles [6,7], diagnostic criteria of cyclic voltammetry and the mass spectra of the isolated products indicate that the reaction mechanism of electro-oxidation of 1 in the presence of 2 and 3 is EC (Scheme 1). Generation of anthraquinone (1ox) is followed by a Michael addition reaction of anionic forms CH-acid nucleophiles (2An , 3An ) to 1ox, producing 4 and 5 as final products. The electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of CH-acid nucleophiles (2 and 3) was tested by digital simulation. Regarding the substituted group on the nucleophile, different reactivity can be expected. As a result, the chemical reaction rate in an EC mechanism is completely depended
O
OH
O -2e- -2H+
O
OH
O
Me O
2An-, 3An-
O 1ox
+H
R
O
2An-, 3An-
OH O O
R: -CH3
Me O
O
Me
+
2,3
O
+ R
R
O 1ox
O
-H+
Me O
1
O
O
O
R
O
OH O
-C6H5
2, 2An-, 4 3, 3An-, 5
4, 5
Scheme 1. Electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of CH-acid nucleophiles.
Please cite this article in press as: D. Nematollahi, et al., Kinetic study of electrochemically induced Michael reaction of 1,4dihydroxyanthraquinone with acetylacetone and benzoylacetone, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/ j.cclet.2013.07.024
G Model
CCLET-2689; No. of Pages 3 D. Nematollahi et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
A1
45
decreasing the tendency of 3 to participate in Michael reaction with 1ox.
Progress in electrolysis
I/ A
35
4. Conclusion
25
15
5
-5
3
0.3
0.1
0.5 E/V vs. SCE
0.7
0.9
The overall reaction mechanism for the electrochemical oxidation of 1,4-dihydroxyanthraquinone (1) in the presence of CH-acid nucleophiles (2 and 3) is represented in Scheme 1. According to the obtained results, the reaction mechanism of electro-oxidation of 1 in the presence of 2 and 3 is an EC mechanism. The kinetics of the reactions of electrochemically generated anthraquinone (1ox) with 2 and 3 were studied by the cyclic voltammetric technique, and the simulation of obtained voltammograms was performed under the EC mechanism. Also, the effect of the substituted groups on the CH-acid nucleophiles ring on kobs has been studied. The results show that the presence of electron-donating groups causes increasing in Michael reaction rate.
Fig. 3. Cyclic voltammograms of 0.4 mmol 1,4-dihydroxyanthraquinone (1) in the presence of 0.4 mmol benzoylacetone (3), at a glassy carbon electrode during electrolysis at 0.60 V versus SCE.
Acknowledgments
Table 1 Homogeneous rate constants (kobs) for nucleophilic addition of 2–3 to 1.
We would like to thank Dr. M. Rudolph for his cyclic voltammogram digital simulation software (DigiElch SB). We also acknowledge Bu-Ali Sina University Research Council and Center of Excellence in Development of Environmentally Friendly Methods for Chemical Synthesis (CEDEFMCS) for their support to this work.
Nucleophile kobs (L mol a
1
s
1 a
)
2
3
172 2.6
162 2.4
Standard deviation of five independent simulations.
References on the substituted group on the nucleophiles. To get more information on the relationship between the homogeneous rate constant (kobs) and the structure of nucleophiles, kobs was estimated by digital simulation. The simulation was performed based on proposed mechanism in Scheme 1. More details on digital simulation are described in our previous paper [6]. The calculated values of the second-order rate constants for the reaction of anthraquinone (1ox) with acetylacetone (2) and benzoylacetone (3) are shown in Table 1. The kobs depended on the electrondonating strength of the substituted group (R). The presence of an electron-donating group causes increases in nucleophile strength of anionic forms (2An , 3An ). The calculated observed homogeneous rate constants (kobs) were found to vary in the order acetylacetone > benzoylacetone, which parallels with electrondonating properties in the order of COCH3 > COC6H5. Also, in spite of methyl group in 2, the steric effect of phenyl group in 3 leads to
[1] A. Houmam, Electron transfer initiated reactions: bond formation and bond dissociation, Chem. Rev. 108 (2008) 2180–2237. [2] F. Varmaghani, D. Nematollahi, S. Mallakpour, et al., Electrochemical oxidation of 4-substituted urazoles in the presence of arylsulfinic acids: an efficient method for the synthesis of new sulfonamide derivatives, Green Chem. 14 (2012) 963–967. [3] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., Wiley, New York, 2001. [4] D.D. Nguyen, N.C. Jones, V.S. Hofftman, et al., Synchrotron radiation linear dichroism (SRLD) investigation of the electronic transitions of quinizarin, chrysazin, and anthrarufin, Spectrochim. Acta A 77 (2010) 279–286. [5] B.J. Stefafiska, M.J. Dzieduszycka, S. Martelli, et al., 2-Amination of quinizarin via Michael addition of hydrazines or amines, J. Org. Chem. 58 (1993) 1568–1569. [6] D. Nematollahi, A. Sayadi, F. Varmaghani, Electrochemical study of quinizarin in the presence of arylsulfinic acids: synthesis of new sulfone derivatives of quinizarin, J. Electroanal. Chem. 671 (2012) 44–50. [7] D. Nematollahi, B. Moradi, F. Varmaghani, A facile and one-pot electrochemical method for the synthesis of a new anthraquinonethioether, Chin. Chem. Lett. 23 (2012) 553–556. [8] M. Rudolph, Digital simulation on unequally spaced grids: part 1. Critical remarks on using the point method by discretisation on a transformed grid, J. Electroanal. Chem. 529 (2002) 97–108.
Please cite this article in press as: D. Nematollahi, et al., Kinetic study of electrochemically induced Michael reaction of 1,4dihydroxyanthraquinone with acetylacetone and benzoylacetone, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/ j.cclet.2013.07.024