Electrocarboxylation of activated olefins in ionic liquid BMIMBF4

Electrochemistry Communications 9 (2007) 2235–2239 www.elsevier.com/locate/elecom

Electrocarboxylation of activated olefins in ionic liquid BMIMBF4 Huan Wang, Guirong Zhang, Yingzi Liu, Yiwen Luo, Jiaxing Lu

*

Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, Shanghai 200062, China Received 15 May 2007; received in revised form 13 June 2007; accepted 14 June 2007 Available online 30 June 2007

Abstract The feasibility of electrocarboxylation of activated olefins has been investigated in CO2-saturated room-temperature ionic liquid BMIMBF4 solution for the first time. The electrochemical behavior has been studied on GC electrode by cyclic voltammetry, showing a diffusion controlled irreversible reduction process. The synthesis has been carried out under mild (P CO2 = 1 atm, t = 50 °C) and safe conditions in undivided cell, and the use of volatile and toxic solvents and catalysts, as well as of any supporting electrolyte, was avoided. Monocarboxylic acids were obtained in moderate yield (35–55%). Furthermore, the ionic liquid has been recycled for five times, which did not affect the product yield greatly. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ionic liquid; Carbon dioxide; Electrocarboxylation; Activated olefin

1. Introduction CO2 is recognized to be a naturally abundant, cheap, recyclable and non-toxic carbon source that can sometimes replace toxic chemicals such as phosgene, isocyanates or carbon monoxide [1]. Under these circumstances, chemical fixation of CO2 becomes more and more important from the ecological and economic points of view. Electrochemical fixation of CO2 is one of the effective routes of CO2 chemical fixation [2]. There are two generic electrochemical methods of utilizing CO2. The first is the direct electrochemical reduction of CO2, with the goal being to obtain hydrocarbons [3,4], alcohols [5], or other fuels [6]. The second involves the coupling of CO2 to electrochemically reduced organic molecules (electrocarboxylation), with the goal being to find new routes to synthesize chemicals that are interesting from a pharmacological point of view. Direct electrochemical reduction of CO2 occurs at rather negative potentials (more negative than 2 V vs. SCE in most organic solvents [7] and ionic liquids *

Corresponding author. Tel.: +86 021 62233491; fax: +86 021 62232414. E-mail address: [email protected] (J. Lu). 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.06.031

[8]). Therefore, electrocarboxylation is a more facile method for the fixation of CO2. And it has been described for a large number of substrate types, ketones [9,10], alkynes [11], olefins [12], alkyl halides [13,14], and heterocyclic compounds [15,16]. Nevertheless, the use of volatile and toxic solvents (MeCN, DMF, etc.) and of large amounts of supporting electrolytes makes more complex the workup of the reaction mixture targeted at the isolation of the products and the recovery of the solvents. In addition, with the growing demand of environmental friendly technologies, any effort should be devoted to avoid the use of volatile and damaging solvents. Room-temperature ionic liquids (RTILs) have held great promise in the development of green chemical applications and processes. Simple room-temperature ionic liquids, which are obtained by the combination of large organic cations (N,N-dialkylimidazolium, quaternary ammoniums, phosphonium, pyridinium, etc.) with a vari    ety of anions (AlCl 4 , PF6 , BF4 , CF3 SO3 , (CF3SO3)2N , etc.), were used as ‘‘green’’ reaction media in catalysis, extraction process, and organic synthesis for their special physical and chemical properties (stability at high temperature, negligible vapor pressure, low toxicity, nonflamma-

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ble, nonvolatile, etc.) [17,18]. Recently, chemical fixation of CO2 with epoxides in RTILs to form cyclic carbonates has been investigated [19,20]. In electrochemistry, because of their desirable properties (high ionic conductivity and wide potential window), RTILs were considered as versatile electrolytes for electrosynthesis [21,22] and the redox behavior of electroactive substrates [23–25], as well as electroplating of base metals, rechargeable batteries, photoelectrical cells, electrochemical devices [26,27]. As concerns electrochemically promoted CO2 fixation, the synthesis of cyclic carbonates from epoxides [8] and carbamates from amines [28] has been reported. Conversely, as far as we know, investigations concerning chemical fixation of carbon dioxide with activated olefins in ionic liquids have not been reported. With this mind, we began studying the reactivity of activated olefins in electrolyzed CO2-saturated RTIL solutions. The electrochemical behavior was investigated by cyclic voltammogram. The synthesis was carried out in BMIMBF4 by cathodic reduction of activated olefins, followed by a nucleophilic reaction with CO2, under mild condition (temperature, 50 °C; CO2 pressure, 1 atm) and without catalysts. 2. Experimental 2.1. Apparatus and reagents Cyclic voltammograms were measured with CHI650 electrochemical analyzer (CHI, USA) in a conventional three-electrode cell. Glassy carbon (GC) electrode (d = 2 mm) was used as a working electrode. The counter electrode and the reference electrode were a Platinum wire and Ag/AgI/0.1 M n-Bu4NI in DMF, respectively. Galvanostatic electrolysis were carried out using a dc regulated power supply QJ 12001X (1A 120 V) in an undivided cell equipped by two-electrode. 1 H NMR spectra were recorded on AVANCE 500 (500 MHz) spectrometer in CDCl3 with Me4Si as an internal standard. Mass spectra were obtained on a 5973 N spectrometer connected with a HP 6890 gas chromatograph. The room temperature ionic liquid, 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF4), used in this study was prepared as prescribed in the literature [29]. Other reagents were used as received. 2.2. Electrocarboxylation experiments In a typical experiment, the galvanostatic electrolysis was carried out in BMIMBF4 containing of 0.1 M ethyl cinnamate (shown in Scheme 1 as structure 1) under a slow stream of CO2 in a one-compartment electrochemical cell equipped with a magnesium rod sacrificial anode and a metallic ring cathode until 2 F mol1 of charge was passed. The reaction mixture was esterified directly in BMIMBF4 by adding anhydrous K2CO3 (1 mmol) and methyl iodide (3 mmol) and stirring the mixture at 50 °C for 5 h.

COOEt

Ph

+e BMIMBF 4 CO 2 (1atm)

1

Ph

COOEt

COOEt + Ph COOH 2

3

Scheme 1. Electrocarboxylation of ethyl cinnamate in BMIMBF4.

The solution was extracted with Et2O, and the organic layers washed with H2O, dried over MgSO4, and evaporated. The methyl esters corresponding to acids and saturated products were isolated by column chromatography with petroleum ether/ethyl acetate mixtures as eluent. After isolation and identification of the products, working curves were used with biphenyl as internal standard for analysis of the electrochemical carboxylation. Ethyl methyl benzylmalonate: GC–MS (m/e, %) 236 (M+, 32), 131 (100), 91 (59), 162 (51), 159 (34), 176 (29), 103 (24), 161 (18), 77 (14), 163 (13), 164 (13); 1H NMR (CDCl3) d 7.32–7.19 (5H, m), 4.15 (2H, q, J = 7 Hz), 3.70 (3H, s), 3.66 (1H, d, J = 5 Hz), 3.22 (2H, d, J = 5 Hz), 1.19 (3H, t, J = 7 Hz). Dimethyl benzylmalonate: GC–MS (m/e, %) 222 (M+, 36), 131 (100), 162 (83), 91 (55), 159 (28), 103 (26), 161 (26), 163 (20), 77 (14), 133 (10), 93 (9); 1H NMR(CDCl3) d 7.30–7.17 (5H, m), 3.70 (6H, s), 3.68 (1H, d, J = 8 Hz), 3.23 (2H, d, J = 8 Hz). 2-Cyano-2-methyl-3-phenyl-propionic acid methyl ester: GC–MS (m/e, %) 203 (M+, 8), 91 (100), 93 (9), 65 (6), 115 (4), 28 (4), 116 (3), 144 (3), 117 (2), 77 (2), 39 (2); 1H NMR (CDCl3) d 7.35–7.24 (5H, m), 3.74 (3H, s), 3.23 (1H, d, J = 13.5 Hz), 3.04 (1H, d, J = 13.5 Hz), 1.62 (3H, s). 2.3. Recycle of ionic liquid Following extraction, CH2Cl2 was added to BMIMBF4. Then the solvent was filtered to eliminate K2CO3, and solvent removed under reduced pressure. After drying in a vacuum oven at 80 °C over night, BMIMBF4 is used again in further experiments. 3. Results and discussion 3.1. Cyclic voltammentry of ethyl cinnamate The electrochemical reduction of ethyl cinnamate was investigated by cyclic voltammetry at GC electrode in BMIMBF4 at 25 °C. Fig. 1 curve a, the background cyclic voltammogram of BMIMBF4, which was recorded at 0.1 V s1, shows that the cathodic limiting potential is more negative than 1.8 V vs. Ag/AgI/I1. The cyclic voltammogram of ethyl cinnamate in BMIMBF4 (Fig. 1 curve b) gives two successive irreversible reduction peaks, at 1.45 V (A) and 1.63 V (B) respectively, which represent the two electron transfer reduction of the alkene bond [30,31].

H. Wang et al. / Electrochemistry Communications 9 (2007) 2235–2239

100

c I / μA

80

A

60

b

B 40

a 20

0 -1.0

-1.1

-1.2

-1.3

-1.4

-1.5

E / V vs. Ag/AgI/I

-1.6

-1.7

-1.8

-

Fig. 1. Cyclic voltammograms recorded at 0.1 V s1 in BMIMBF4 with GC electrode at 25 °C: (a) neat BMIMBF4, (b) as (a) + 20 mM ethyl cinnamate, (c) as (b) saturated with CO2.

To peak A, increasing the scan rate leads to an increase of the peak current (Ip) and concomitantly to a shift of the peak potential (Ep) towards more negative values. However, the current voltage curves remained chemically irreversible in the range 0.01–50 V s1. Accordingly, Ip varies linearly with v1/2, indicating diffusion controlled process over this scan rate range. Additionally, Ep varies linearly with the logarithm of v and the slope is 47 mV decade1. The transfer coefficient (a) calculated from the above slope according to the equation oEp/o log t = 1.15 RT/Fa is 0.63. a was also calculated from the peak width, the difference between Ep and the potential at half peak (Ep/2), according to the equation 4Ep/2 = Ep/2– Ep = 1.857RT/ a. The average of the values obtained at different scan rates in the 0.02–0.2 V s1 range is 0.67. Addition of CO2 strongly modifies the voltammetric behavior. As shown in Fig. 1 curve c, after saturated with CO2, a considerable increase of the reduction peak A is observed, while the peak B disappears completely. This behavior is indicative of a chemical reaction between the electrogenerated intermediate and CO2. To understand the process, preparative-scale electrolysis of ethyl cinnamate in BMIMBF4 saturated by CO2 was carried out. 3.2. Electrocarboxylation of ethyl cinnamte Galvanostatic electrolysis in CO2-saturated BMIMBF4 solution containing 0.1 M ethyl cinnamate was carried out at a constant current of 9 mA until 2 F mol1 of charge was passed at 25 °C. Monocarboxylic acid 2 was obtained as principal product, accompanied by saturated ester 3. (Scheme 1) With the aim of optimizing the yields, we have focused our attention on the influence of various synthetic parameters on the process, such as the temperature, the nature of the electrode and the current. The results of the electrolysis are reported in Table 1.

3.2.1. Influence of temperature It is well known that RTILs has high ionic conductivity [32], however, it is on a par with those of organic solvents with added inorganic electrolytes, and not significantly larger. Moreover, the viscosity of RTILs is high, which will strongly affect on the rate of mass transport within solution. This results was an ohmic drop, therefore a high cell voltage when the current is increased. This drawback can be easily overcome by heating the solution, as the viscosity is extremely temperature dependent by the Arrhenius-type relation [29]. So the effect of temperature on the electrocarboxylation was investigated firstly. When the temperature was increased from 25 °C to 50 °C, the yield of monocarboxylic acid was increased from 28% to 41%, while the yield of saturated ester remained 17–22% (Table 1, entries 1, 2 and 3). However, the yield of monocarboxylic acid decreased to 29% and the yield of saturated ester increased to 28% at 60 °C (Table 1, entry 4). In view of the temperature, there are two factors affecting the yield. Table 1 Electrocarboxylation of ethyl cinnamate in BMIMBF4a Entry

Anode/cathode

1 2 3 4 5 6 7 8 9

Mg/stainless Mg/stainless Mg/stainless Mg/stainless Mg/Ti Mg/Cu Mg/Ni Mg/stainless Mg/stainless

I (mA)

steel steel steel steel

9 9 9 9 9 9 9 19 29

steel steel

Temperature (°C)

25 40 50 60 50 50 50 50 50

Product yield (%)b 2

3

28 39 41 29 35 30 26 31 20

20 17 22 28 19 27 29 32 34

a

Ethyl cinnamate = 0.1 M, BMIMBF4 = 10mL, electric charge = 2 F mol1, P CO2 = 1 atm. b GC yield.

40

30

Yield / %

120

2237

20

10

0 1

2

3

4

5

Recycle times Carboxylatic acid yield

Simple reduced product yield

Fig. 2. Reuse of BMIMBF4 in the electrocarboxylation of ethyl cinnamate. Reaction conditions as Table 1 entry 3.

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Table 2 Electrocarboxylation of activated olefin in BMIMBF4a Entry

Activated olefin

1c Ph

C O OEt

Carboxylic acid and yield (%)b Ph

COOEt

Simply reduced product and yield (%)b 41 Ph

COOEt

22

CN

22

COOMe

32

COOH

2

Ph

3

Ph

CN

Ph

CN

55

Ph

COOH COOMe

Ph

COOMe

35 Ph

COOH General conditions: activated olefin = 0.1 M, BMIMBF4 = 10 mL, stainless steel cathode, Mg anode, t = 50 °C, I = 9 mA, electric charge = 2 F mol1, P CO2 = 1 atm. b GC yields. c This experiment (the same of entry 3, Table 1) was repeated here for clarity. a

One is the solubility of CO2, the other is viscosity and conductivity of ionic liquid. Increasing the temperature leads to a decrease of the solubility of CO2 in BMIMBF4 [33], which is unfavorable to electrocarboxylation. On the other hand, increasing the temperature leads to a decrease of viscosity and an increase of conductivity, which is helpful to the electrosynthesis. So the optimal temperature is 50 °C. 3.2.2. Influence of the nature of the electrode The reaction yields and selectivity were shown to be dependent on the reaction conditions, particularly on the nature of the electrodes. Table 1 (entries 3 and 5–7) presents the results obtained with the use of different materials as the cathode for the electrocarboxylation of ethyl cinnamate in BMIMBF4, showing that the yield of monocarboxylic acid decreased depending on the employed cathode materials in the following order: Stainless steel (41%) > Ti (35%) > Cu (30%) > Ni (26%). 3.2.3. Influence of the current Electrolysis conducted with highly current was not very efficient in the electrocarboyxlation of ethyl cinnamate in BMIMBF4. A strongly decrease in the yield of monocarboxylic acid and conversely an increase in the yield of saturated ester were observed at higher current (Table 1, entries 3, 8–9). The larger the current, the more electrogenerated intermediate will be obtained at the same time, so that not all the intermediate can react with CO2 in time. Consequently, more ethyl cinnamate was electroreducted to saturated ester, decreasing the yield of monocarboxylic acid. 3.3. Reuse of ionic liquid In this study, BMIMBF4 was separated from the electrolyte system after the first run and then used in the next run. As shown in Fig. 2, the electrocarboxylation yield dropped from 41% to 35% on the second run, while the hydrogenation yield increased from 22% to 30%. The car-

boxylation and hydrogenation yield remain 35–39% and 30–33%, respectively, on successive runs. This indicates that the ionic liquid used as solvent for electrocarboxylation of activated olefins is recyclable. 3.4. Electrocarboxyaltion of activated olefins Several activated olefins were electrocarboxylated under the conditions described above (Table 1, entry 3). The results of the electrolysis are reported in Table 2. The replacement of ester group by nitrile group, the carboxylation yield increase to 55%, while hydrogenation yield remained 22%. The reason is that the electron withdrawning ability of nitrile group is stronger than ester group. When the olefin was activated by phenyl and methyl ester group, the yield of carboxylic acid was 35% and the yield of saturated product was 32%. These results indicated that electrocarboxylation of activated olefins works well in ionic liquid. 4. Conclusions On the basis of the above investigations, electrocarboxylation of activated olefins can be successfully achieved by using RTILs as solvent-electrolyte media for the first time. The electrochemical behavior of ethyl cinnamate in BMIMBF4 on GC electrode by cyclic voltammogram shows that the electroreduction was diffusion controlled. Electrocarboxylation of ethyl cinnamate was influenced by temperature, the nature of the electrode and the current. The ionic liquid could be reused at least five times for electrocarboxylation of ethyl cinnamate. This method also can be used for other activated olefins. This could start a new research field in the application of ionic liquids to organic synthesis aimed at ‘‘green chemistry’’. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20573037), the Natural

H. Wang et al. / Electrochemistry Communications 9 (2007) 2235–2239

Science Foundation of Shanghai (No. 05JC1470) and Ph.D Program Scholarship Fund of ECNU 2007. References [1] D.H. Gibson, Chem. Rev. 96 (1996) 2063. [2] C.M. Sanchez, V. Montiel, D.A. Tryk, A. Aldaz, A. Fujishima, Pure Appl. Chem. 73 (2001) 1917. [3] S. Kaneco, H. Katsumata, T. Suzuki, K. Ohta, Electrochim. Acta 51 (2006) 3316. [4] S. Kaneco, K. Iiba, H. Katsumata, T. Suzuki, K. Ohta, Electrochim. Acta 51 (2006) 4880. [5] M. Saito, M. Takeuchi, T. Fujitani, J. Toyir, S. Luo, J. Wu, K. Ushikoshi, K. Mori, T. Watanabe, ICCDU V Karlsruhe 10(4) (1999). [6] M. Aresta, A. Dibenedetto, G. Barberio, Am. Chem. Soc., Fuel. Chem. Div. Prepr. 49 (2004) 348. [7] A.A. Isse, A. Gennaro, Chem. Commun. (2002) 2798. [8] H. Yang, Y. Gu, Y. Deng, F. Shi, Chem. Commun. (2002) 274. [9] O. Scialdone, C. Amatore, A. Galia, G. Filardo, J. Electroanal. Chem. 529 (2006) 163. [10] O. Sialdone, M.A. Sabatino, C. Belfiore, A. Galia, M.P. Paternostro, G. Filardo, Electrochim. Acta 51 (2006) 3500. [11] K. Shimizu, M. Takimoto, Y. Sato, M. Mori, Org. Lett. 7 (2005) 195. [12] M.A. Chowdhury, H. i Senboku, M. Tokuda, Tetrahedron 60 (2004) 475. [13] A.A. Isse, A.D. Giusti, A. Gennaro, Tetrahedron Lett. 47 (2006) 7735. [14] A.A. Isse, A.D. Giusti, A. Gennaro, L. Falciola, P.R. Mussini, Electrochim. Acta 51 (2006) 4956. [15] R.R. Raju, S.K. Mohan, S.J. Reddy, Tetrahedron Lett. 44 (2003) 4133.

2239

[16] A. Gennaro, C.M. Sanchez, A.A. Isse, V. Montiel, Electrochem. Commun. 6 (2004) 627. [17] T. Welton, Chem. Rev. 99 (1999) 2071. [18] P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed. 39 (2000) 3772. [19] J. Sun, S.-I. Fujita, M. Arai, J. Organomet. Chem. 690 (2005) 3490. [20] L.-F. Xiao, F.-W. Li, J.-J. Peng, C.-G. Xia, J. Mol. Catal. A: Chem. 253 (2006) 265. [21] R. Barhdadi, C. Courtinard, J.Y. Nedelec, M. Troupel, Chem. Commun. (2003) 1434. [22] M. Mellah, S. Gmouh, M. Vaultier, V. Jouikov, Electrochem. Commun. 5 (2003) 591. [23] H. Wang, X.-H. Ye, L.-M. Chen, J.-X. Lu, M.-Y. He, Chem. J. Chin. U. 26 (2005) 326. [24] H. Wang, T. Xue, A.-J. Zhang, L. Zhang, J.-X. Lu, Chem. J. Chin. U. 27 (2006) 1135. [25] H. Wang, H.-J. Fang, M.-Y. Lin, J.-X. Lu, Acta. Chimi. Sinica 65 (2007) 765. [26] M.C. Kroon, W. Buijs, C.J. Peters, G.-J. Witkamp, Green Chem. 8 (2006) 241. [27] A.I. Bhatt, A.M. Bond, D.R. MacFarlane, J. Zhang, J. L Scott, C.R. Strauss, P.I. Iotov, S.V. Kalcheva, Green Chem. 8 (2006) 161. [28] M. Feroci, M. Orsini, L. Rossi, G. Sotgiu, A. Inesi, J. Org. Chem. 72 (2007) 200. [29] T. Nishida, Y. Tashiro, M. Yamamoto, J. Fluorine Chem. 120 (2003) 135. [30] J.H.P. Utley, M. Gullu, M. MOtevalli, J. Chem. Soc., Perkin Trans. 1 (1995) 1961. [31] I. Fussing, M. Gullu, O. Hammerich, A. Hussain, M.F. Nielsen, J.H.P. Utley, J. Chem. Soc., Perkin Trans. 2 (1996) 649. [32] M.C. Buzzeo, R.G. Evans, R.G. Compton, ChemPhysChem 5 (2004) 1106. [33] M.B. Shiflett, A. Yokozeki, Ind. Eng. Chem. Res. 44 (2005) 4453.