Electrosynthesis of sodium hypochlorite in room temperature ionic liquids and in situ electrochemical epoxidation of olefins Ruiqin Zhao • Yuhai Tang • Saili Wei • Xiaoqian Xu • Xiaoyu Shi • Guangbin Zhang Received: 27 June 2011 / Accepted: 16 November 2011 / Published online: 9 December 2011 � Akade´miai Kiado´, Budapest, Hungary 2011 Abstract Asymmetric electro-epoxidation of olefins had been achieved with sodium hypochlorite (NaClO) as an oxidant, which was generated by electrolysis in two-phase systems of aqueous sodium chloride (NaCl) and ionic liquids (1-butyl- 3-methylimidazolium hexafluorophosphate (BMImPF6) and 1-butyl-3-methylimi- dazolium tetrafluoroborate (BMImBF4)). The electrolysis conditions by different current densities (0.8, 0.9, 1.0, 1.1 and 1.3 mA/cm2) and pH values (8, 9, 10, 11, 12 and 13) were optimized and 1.1 mA/cm2 and pH 11 were selected. The proposed reaction mechanism is also discussed. The performance of new catalytic systems in four kinds of reaction media in the presence or absence of ammonium acetate (NH4OAc) as a cocatalyst was investigated systematically. Compared to the chemical epoxidation systems, the enantiomeric excess (ee) values and yields for the epoxidation of styrene, a-methylstyrene and indene were acceptable in the electrocatalytic epoxidation systems. Keywords Asymmetric induction � Electro-epoxidation � Ionic liquid � Olefins Introduction Chiral epoxides are important intermediates in the synthesis of pharmaceuticals, fine chemicals and petrochemical products. In order to obtain chiral epoxides, the Electronic supplementary material The online version of this article (doi: 10.1007/s11144-011-0403-3) contains supplementary material, which is available to authorized users. R. Zhao � Y. Tang (&) � S. Wei � X. Xu � X. Shi � G. Zhang Institute of Analytical Sciences, School of Science, Xi’an Jiaotong University, Xi’an, Shaanxi 710061, China e-mail:
[email protected] R. Zhao e-mail:
[email protected] 123 Reac Kinet Mech Cat (2012) 106:37–47 DOI 10.1007/s11144-011-0403-3 enantioselective epoxidation of prochiral olefins has been extensively investigated. Jacobsen and Katsuki developed several highly effective catalytic systems based on some optically active (salen)Mn(III) (salen = N,N0-bis(salicylidene)ethylenediam- inato)complexes [1–5] and achieved great success in the asymmetric epoxidation of olefins with PhIO [6], m-CPBA [7], H2O2 [8–10], NaClO [11] and molecular oxygen [12, 13] as oxidants. Among these oxidants, H2O2 and NaClO were relatively cheaper, much more effective and widely used. Nowadays, oxidants are produced by electrochemical procedure in situ, which can cut the cost of bagging and transportation in traditional direct addition method. The new way provides a much needed alternative to the use of stoichiometric amounts of oxidants, thus the hazards associated with the handling oxidants in large excess are greatly reduced. It has been reported that H2O2 can be generated by the electrochemical reduction of O2 and used in situ for the epoxidation of olefins [14–18]. As the system gave good results only for electrophilic olefins, such as a,b-unsaturated ketones, the electrosynthesis of NaClO was considered in order to widen the scope of olefin substrates. Tanaka and co-workers gave a new design for the electrogenerarion of NaClO in a CH2Cl2 aqueous NaCl two-phase system for the epoxidation of cycloolefins and terminal olefins [19]. However, the reaction took 16 h to complete and the result of chiral induction for indene was a 26% ee% value and 43% ee% for a-methylstyrene. It would be of interest if we could shorten the reaction time and improve the enantioselectivity of the asymmetric reaction without the use of volatile organic solvent. Room temperature ionic liquids (RTILs) are green, environmentally benign solvents because of their unmeasurable vapor pressure, air and water stability [20, 21]. Besides being a solvent, ILs can be used as liquid electrolytes in batteries and electrolystic processes for their moderate viscosity, high intrinsic conductivity, and wide electrochemical window. They are electrically stable over a range of 2–4 V, thermally stable, and resistant to oxidation [22, 23]. ILs based on alkylimidazolium cations are among the most widely used ionic liquids. In this paper, BMImPF6 and BMImBF4 are used as electrolytes, which are electrochemically stable and capable of supporting the electrolysis. Therefore, we investigated the epoxidation of olefins using NaClO as the oxidant electrogenerated in the biphasic system consisting of an ionic liquid and aqueous NaCl. This article focuses on the electrosynthesis conditions of NaClO in water containing ionic liquids and the factors affecting the enantioselective epoxidation of unfunctionalized olefins in electrochemical conditions. Experimental Materials All chemicals of reagent grade were purchased from Alfa Aesar and used without further purification unless otherwise noted. Jacobsen’s catalyst [24] (Fig. 1), two ionic liquids BMImBF4 and BMImBF6 [25] (Fig. 2) were synthesized according to the literature procedure. 38 R. Zhao et al. 123 Instrumentation and procedures Electrochemical experiments were performed using an EG&G Princeton Applied Research Potentiostat/Galvanostat (Model 273A) controlled by computer and data acquisition software with a three-electrode system: working electrode (reticulated vitreous carbon electrode (RVC), 100 pores per inch (ppi)); counter-electrode (Pt wire (u 0.5 mm 9 37 mm)) and reference electrode (Ag/AgCl, saturated potassium chloride). The current density and electrode material can affect the reversibility of the electrode reaction. Generally speaking, overpotentials are small and can be ignored when metals are precipitated; if gases are produced, especially hydrogen and oxygen, electrode potentials would significantly deviate from the reversible potentials. The Pt wire can effectively reduce the anode overpotentials and was selected as the counter electrode. High performance liquid chromatography (HPLC) analysis was carried out on a Shimadzu instrument with the chiral stationary phase column (Daicel Chiralcel OD-H) manufactured by Daicel Chemical industries Ltd. (system controller: LC-10AT VP; UV–vis detector: SPD-10A VP, 254 nm UV-detection; eluent: n- hexane/i-PrOH, 99/1, v/v; flow rate was 0.5 or 0.8 mL/min; column pressure was 2.0–3.0 MPa, and column temperature was 25 �C). Electrochemical experiments Electro-epoxidation reactions were carried out based on the procedure as follows: A solution of NaOH was added to a pre-cooled (0 �C) saturated sodium chloride solution, adjusting the pH value to 11. Subsequently, the mixture obtained was divided into four equal portions, then gradually added to other four reaction media N N O t-Bu t-Bu t-Bu t-Bu O Mn Cl Fig. 1 The structure of Jacobsen’s catalyst N N X1=BF4 X2=PF6X - Fig. 2 Structures of room temperature ionic liquids Electrosynthesis of sodium hypochlorite in room temperature 39 123 containing the olefin (2 mmol), the catalyst (the molarity of Mn was 0.2 mmol) and the co-catalyst NH4OAc (0.4 mmol) under condition 1 or in the absence of NH4OAc under condition 2. (from I to IV: for medium I, the volume of BMImPF6 was 10 mL; for medium II, the volume of BMImBF4 was 10 mL; for medium III, the volume ratio of BMImPF6 and CH2Cl2 was 5 vs. 5 mL; for medium IV, the volume ratio of BMImBF4 and CH2Cl2 was 5 vs. 5 mL). Moreover, the mixture of catalyst and ionic liquids had to be stirred vigorously for 1 day before use. The catalyst can be dispersed into the media easily with magnetic stirring. The epoxidation experiments were performed simply: three electrodes were immersed into the aqueous layer, and the mixture was electrolyzed for 2 h in a one- compartment cell at 0 �C. The enantioselective epoxidation reaction procedure was monitored by TLC. When the reaction was over, two different approaches were used to obtain epoxides: (1) for media I and II, 50 mL of n-hexane was directly added to the electrolyte, then a tri-phasic system formed: n-hexane phase was the upper layer while middle layer was water phase and ionic phase was at the bottom). The organic phase was separated and concentrated under reduced pressure, then the residue was purified by flash chromatography (SiO2, petroleum ether-CH2Cl2, 2:1, v/v) to afford the peroxides. At the same time, the ionic liquid containing catalyst can be reused for the next time. (2) for media III and IV, CH2Cl2 of 30 mL was added into the reaction mixture and the organic layer was separated, washed with saturated sodium chloride, dried over anhydrous MgSO4, then filtered. CH2Cl2 was then removed with rotary evaporator equipment. The approach above as (1) shows was used to deal with the residue. Results and discussion The electrogeneration of NaClO We chose the commonly used method to obtain active chlorine species NaClO: electrolyzing the saturated NaCl solution. The major processes are the following 2Cl� � 2e ¼ Cl2 ð1Þ Cl2 þ NaOH ¼ NaCl þ NaClO ð2Þ . In the first step, anodic oxidation of chloride ion can form chlorine with OH- produced in the cathode reaction concurrently (Eq. 1). Then chlorine dispropor- tionates in alkaline solution to sodium chloride and sodium hypochlorite (Eq. 2). The concentration of sodium hypochlorite was determined using standard titration method with sodium thiosulfate. Current density has a great influence on the production of chlorine. The higher the current density is, the faster the reaction rate of Eq. 1 will be when the current density ranges from 0.8 to 1.1 mA/cm2. The current efficiency reaches 87% at a current density of 1.1 mA/cm2 and drops with a further increase of current density. 40 R. Zhao et al. 123 Side reactions may occur and compete with the oxidation of chloride ion at high current densities. 1.1 mA/cm2 is the best choice for the reaction can reach the maximum current efficiency (Fig. 3). It is clear that the alkalinity of the solution is a significant factor which affects the yield of oxidant. In order to speed up the reaction of Eq. 2 and improve the stability of sodium hypochlorite, we investigated the electrolysis at different pH values in the sodium chloride solution. The alkaline solution is conducive to the dissolution of chlorine. The alkalinity of the solution was adjusted to several pH values by slow instillation of NaOH (1 mol/L) solution. This step is important not only for the electrogeneration of NaClO but also for the asymmetric epoxidation of olefins. Fig. 4 shows a clear correlation between the NaClO generation rate and alkalinity of the mixture, all measurements were carried out at a controlled current density (1.1 mA/cm2). The current efficiency increased significantly from 54% to 87% with the solution pH changed from 7.5 to 11 and slowly after that. When the pH is less than 11, the NaClO formation rate is relatively slow and more time will be needed to drive the reaction. When the pH value is 11, the production of NaClO occurs at a fast rate because the generated chlorine almost absorbed completely and chemical equilibrium constant reached a great value. Higher pH values did not bring more efficiency. As a result, pH 11 was selected as the optimal pH value. Ultimately, at the current density of 1.1 mA/cm2, the yield of sodium hypochlorite increased steadily with time, and reached a value of 2.4 mmol after 2 h in the BMImPF6/standard NaCl/NaOH mixture (pH 11). Electrochemical production of NaClO is equivalent to adding it slowly to the solution over a period of 2 h and so the decomposition was minimized. Fig. 3 The effect of current density on the current efficiency of NaClO electrogeneration. Electrolyte: ionic liquid/NaCl (aq) mixture. pH of the electrolyte: 11. Anode: 100 ppi RVC. Anode surface area: 65 cm2. Electrolysis time: 2 h Electrosynthesis of sodium hypochlorite in room temperature 41 123 Scheme 1 A plausible reaction mechanism of the epoxidation of unfunctionalized olefins in ionic liquid/ NaCl (aq) mixture using electrogenerated NaClO as the oxidant Fig. 4 The effect of pH for current efficiency. Current density: 1.1 mA/cm2. Electrolyte: ionic liquid/ NaCl(aq)mixture. Anode: 100 ppi RVC. Anode surface area: 65 cm2. Electrolysis time: 2 h 42 R. Zhao et al. 123 Reaction mechanism A plausible mechanism of electro-epoxidation is proposed in Scheme 1. In the aqueous phase, anodic oxidation of Cl- can afford active chlorine species ClO-, which would diffuse to the organic phase and work as an efficient oxidant to turn [Mn(III)]? into the pentavalent manganese-oxo [Mn(V) = O]?. Subsequently, the Mn-oxo complex transfer the oxygen through the tetravalent manganese radical intermediate to olefin to give the corresponding epoxide. Electrochemical epoxidations freshly catalyzed by Jacobsen’s catalyst In this section, enantioselective epoxidations of styrene, a-methylstyrene, indene with the (R,R)-Jacobsen’s catalyst in different media were carried out when NaClO was electrogenarated as the oxidant. The results are collected in Table 1. TLC monitoring indicated that reactions can be completed within 2 h. The maximum outputs were obtained when the concentration of NaClO reached 2.4 mmol 2 h later, which is much lower than the 6 mmol used in previous study [1, 11]. It is a good alternative to reduce the oxidant consumption. Table 1 Electrochemical epoxidation of unfunctionalized olefins catalyzed by Jacobsen’s catalyst in different media Entrya Olefin Mediab ee%c (yield%)d Configuratione Condition 1 Condition 2 1 I 89 (65) 94 (80) R 2 Styrene II 72 (45) 79 (54) R 3 III 86 (53) 92 (72) R 4 IV 79 (62) 81 (61) R 5 I 60 (75) 86 (78) R 6 a-methylstyrene II 52 (41) 75 (67) R 7 III 56 (48) 82 (68) R 8 IV 55 (63) 79 (70) R 9 I 78 (73) 89 (85) 1R, 2S 10 Indene II 74 (37) 79 (50) 1R, 2S 11 III 70 (72) 83 (77) 1R, 2S 12 IV 63 (59) 72 (79) 1R, 2S Reaction conditions: substrate, 2 mmol; catalyst, 0.2 mmol, 10 mol%; condition 1 co-catalyst, 0.4 mmol, 20 mol%; condition 2: without co-catalyst a All entries in this article are numbered consecutively in spite of the table numbers b Four reaction media: (I) BMImPF6-NaCl/NaOH/H2O; (II) BMImBF4-NaCl/NaOH/H2O; (III) BMImPF6/ CH2Cl2-NaCl/NaOH/H2O; (IV) BMImBF4/CH2Cl2-NaCl/NaOH/H2O c Determined by HPLC over a chiral OD-H column after comparing the retention times with those of three racemic epoxide samples d The yield of the isolated epoxide was determined by weighing the product of column chromatography. Absolute configurations of major enantiomers which were determined by comparison with the literature values: for styrene oxide and a-methylstyrene oxide [26]; for indene oxide, see Palucki et al. [27] Electrosynthesis of sodium hypochlorite in room temperature 43 123 To study the effects of the solvent factors on electrochemical epoxidation, pure BMImPF6 and BMImBF4, mixed solvents of CH2Cl2/BMImPF6 and CH2Cl2/ BMImBF4 were used in the experiments. As expected, the reaction medium was important in the aspect of improving enantioselectivity and epoxide yield: (i) the reaction of epoxidation in BMImPF6 gave better enantioselectivity and chemical yield than in BMImBF4 (entry 1 vs. 2, entry 5 vs. 6, entry 9 vs. 10). The value of ee% may be attributed to an anion exchange process between Cl- of the catalyst and PF6 - of the ionic liquid (or between Cl- and BF4 -) occured during the reaction [28]. BMImBF4 is water soluble at C6 �C while BMImPF6 is completely insoluble in water, so the anion exchange process in organic phase would be weakened in the media containing BMImBF4 [29]; (ii) the addition of CH2Cl2 in pure ionic liquid led to a slightly reduced ee% value. (entry 1 vs. 3, entry 2 vs. 4, entry 5 vs. 7, entry 9 vs. 11, entry 10 vs. 12). A large proportion of ionic liquid enhanced the asymmetric induction because that ionic liquid had strong coordination action with the metal center of chiral Jacobsen catalyst and intermediates, which would stabilize their conformation and enhance the activity of the catalyst [30–32]; (iii) a lower ee% value but higher yield were obtained in BMImBF4/CH2Cl2 than in BMImBF4 (entry 2 vs. 4, entry 10 vs. 12). The miscibilities of CH2Cl2 in water is the same as that between BMImPF6 and water, and CH2Cl2 can protect the active Mn(V)-oxo species from oxidizing at the anode in the aqueous phase. It has been reported that axial ligands, such as imidazole and pyridine, can give a favorable influence toward chiral induction in chemical conditions [8, 33–36]. Because these ligands co-ordinate at the axial position of the unsaturated Mn(III) species and the dimerization and decomposition can be avoided. But these axial ligands were unstable and susceptible to oxidation. NH4OAc was used as the cocatalyst in Thellend’s article and provided good catalytic results in chemical conditions [37]. NH4OAc was very stable in the whole process. Whether the NH4OAc plays an important role in electro-epoxidation or not were tested in experiments. Reaction systems under condition 1 contained NH4OAc as axial ligand while NH4OAc was not used under condition 2. The data show that the reaction under condition 2 gave better enantioselectivity and chemical yield than under condition 1 (ee 72–94%, yield 50–85% vs. ee 52–89%, yield 37–75%). It is obvious that NH4OAc could not be used effectively in electrochemical condition. NH4OAc, completely insoluble in both CH2Cl2 and ionic liquids, dissolved in the aqueous phase and probably suffered oxidation at the anode to pollute the electrode surface and did not play a co-catalytic role. Overall, the best ee% value was obtained in non-water-soluble ionic liquid BMImPF6 without the use of NH4OAc. The system gave 80% of epoxide yield and 94% ee% value for styrene; for a-methylstyrene, the chemical yield is 78% and ee% value is 86%; for indene, the chemical yield is 85% and ee% is 89%. All the results are acceptable and definitively better than that reported by previous article [19]. Recycling of the ionic liquid Ionic liquids are so expensive that it is worth recovering and reusing these green reagents. BMImPF6 was selected as the reaction medium in the recycling experiments of electro-epoxidation of the three substances. The results of the 44 R. Zhao et al. 123 re-use of BMImPF6 four successive times are summarized in Table 2, which indicates that BMImPF6 was stable enough to support the cycling experiments. There are slight variations of ee% values and yields for the three substances after the second time recycle. For example, the ee% value of styrene decreased from 94 to 93%, and the yield of epoxide changed from 80 to 76%. The nice results are also obtained for indene. Yield and ee% value of the four cycles epoxidation of indene changed little (entry 15). Conclusion The epoxidation of olefins under electrochemical conditions was achieved to some extent. The optimal electrolytic current density is 1.1 mA/cm2 and the best alkalinity of aqueous phase is pH 11. ClO- has been successfully electrogenerated in an undivided cell and used as an oxidant for the epoxidation experiments. This new approach with chiral salen-Mn(III) as a catalyst in conjunction with BMImPF6- NaCl/NaOH/H2O biphasic system gave both moderate yields (78–85%) and acceptable enantioselectivity (86–94%). Additionally, the recovery and recycling of ionic liquid BMImPF6 were tested to investigate the electrochemical application on the olefins epoxidation. The medium could be reused for 4 times without losing activity and selectivity. In order to receive excellent yield and ee% value, it is necessary to reduce the side reactions occurred on the electrode surface. Further studies based on modification of the electrode surface are in progress. Acknowledgments The authors are grateful to the Chairperson Foundation of Xi’an Jiaotong University for financial support and Prof. Yuhai Tang for helpful discussions in chiral HPLC separations. References 1. Zhang W, Jacobsen EN (1991) Asymmetric olefin epoxidation with sodium hypochlorite catalyzed by easily prepared chiral manganese (III) salen complexes. 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J Chem Soc, Chem Commun 9:1035–1036 Electrosynthesis of sodium hypochlorite in room temperature 47 123 Electrosynthesis of sodium hypochlorite in room temperature ionic liquids and in situ electrochemical epoxidation of olefins Abstract Introduction Experimental Materials Instrumentation and procedures Electrochemical experiments Results and discussion The electrogeneration of NaClO Reaction mechanism Electrochemical epoxidations freshly catalyzed by Jacobsen’s catalyst Recycling of the ionic liquid Conclusion Acknowledgments References