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Electrochemical asymmetric epoxidation of olefins by using an optically active Mn-salen complex
Journal of Electroanalytical Chemistry 507 (2001) 75 – 81 www.elsevier.com/locate/jelechem
Electrochemical asymmetric epoxidation of olefins by using an optically active Mn-salen complex Hideo Tanaka a,*, Manabu Kuroboshi a, Hironori Takeda a, Hisaaki Kanda a, Sigeni Torii b a
Department of Applied Chemistry, Faculty of Engineering, Okayama Uni6ersity, Tsushima-naka 3 -1 -1, Okayama 700 -8530, Japan b Institute of Creati6e Chemistry, Musa 874 -5, Okayama 701 -2141, Japan Received 30 August 2000; received in revised form 2 October 2000; accepted 12 October 2000
Abstract Enantioselective electro-epoxidation of olefins by using an optically active Mn-salen complex, (S,S)-N,N%-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride (3), as a mediator in a CH2Cl2 aqueous NaCl two-phase system was performed successfully in a simple undivided cell under constant current conditions. Optimization of the electrolysis conditions and estimation of the reaction mechanism are discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkenes; Asymmetric induction; Epoxidation; Electrochemical reaction; Manganese compounds
1. Introduction Optically active epoxides are versatile intermediates in the synthesis of pharmaceuticals, agrochemicals, and function materials such as liquid crystals [1 – 4]. Various oxidation methods to convert olefins to optically active epoxides using a combination of an optically active metal complex and co-oxidant have been reported [5– 16]. Among these, optically active Mn-salen complexes have been developed by Katsuki [17 – 19] and Jacobsen [20,21], and successfully applied to enantioselective epoxidation of non-functionalized olefins in good yields and high % enantiomeric excess (% ee). In these methods, however, a large excess amount of co-oxidants such as NaClO [22,23], Bu4NIO4 [24], PhIO [25], m-CPBA [26], H2O2 [27,28], (TMSO)2 [29], or O2 + tBuCHO [30] are usually required to complete the reaction. If the co-oxidant can be regenerated in situ, the oxidation reaction may proceed with a catalytic amount of the co-oxidants, offering a more economical and environmentally benign procedure.
* Corresponding author. Tel.: + 81-86-251-8074; fax: +81-862518079. E-mail address: [email protected] (H. Tanaka).
Electrochemical asymmetric oxidation has been investigated as a promising alternative without the use of a stoichiometric amount of the co-oxidant [31]. Electrochemical asymmetric syntheses are classified into the following six categories: (1) electrolysis of optically active starting material [32 –35] or (2) substrate attached with a recyclable optically active pendant [36 – 40]. (3) Electrolysis in optically active solutions and/or with an optically active supporting electrolyte [41 –43]. (4) Indirect electrolysis using an optically active mediator [44 –46], (5) enzyme [47], or (6) an optically active electrode such as a poly(amino acid)-coated electrode [48 –52]. By using these methods, optically active sulfoxides (from oxidation of the corresponding sulfides), [48 –52], lactones (from diols) [41 –43,47], and alcohols (by oxidative kinetic resolution) [41 –43,48 –52] have been obtained in moderate to high % ee. Among these methods, the method using an optically active mediator seems to be most practical, because (1) only a catalytic amount of the optically active mediator can promote the asymmetric reaction, and (2) efficiency and selectivity can be improved by a proper designing of the optically active mediator. In previous papers, we reported electrochemical asymmetric dihydroxylation of olefins by using an optically active Os catalyst+NaBr or K3Fe(CN)6 double
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H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
mediatory system [44,45]. The most important problem to be solved in such electrochemical asymmetric oxidation with an optically active metal complex catalyst must be electro-oxidative regeneration of co-oxidants without affecting easily oxidizable ligands. Herein is described an optically active Mn-salen-mediated asymmetric epoxidation of olefins in a CH2Cl2 aqueous NaCl two-phase electrolysis system (Eq. (1)).
of the % ee: Column Daicel ChiralCel OD or OB-H (4.6 mmf × 250 mm), mobile phase hexane and 2PrOH (90:10), flow rate 0.5 ml min − 1, detection at 254 mm.
2.4. General procedures A mixture of olefin (1 mmol), Mn-salen complex 3
(1)
2. Experimental
2.1. Instrumentation High performance liquid chromatography (HPLC) was executed with a Soma HPLC instrument (S-4000 LC pump, S-310A modelII UV detector, Shimadzu C-R6A integrator) and a Shimadzu instrument (LC10AT LC pump, LC-10AV UV – Vis detector, C-R6A integrator). Epoxides were analyzed with Zorbax Sil (Du Pont) (Yield) and ChiralCel OD or OB-H (Daisel) (% ee), and olefins were analyzed with Inertsil ODS-2 (GL Science) (recovery). 1H-NMR spectra were recorded on a Varian Gemini 200 (200 MHz) spectrometer.
2.2. Materials MeCN and hexane were distilled from P2O5 under N2. CH2Cl2 was treated with P2O5 and distilled from CaH2. THF was distilled from Na+benzophenone under N2. H2O was deionized and subsequently distilled before use. 2,2-Dimethylchromene (1b) was prepared according to the literature [53,54]. All other chemicals and solvents including an optically active Mn-salen complex 3 were purchased and used without further purification.
(0.05 mmol), dichloromethane (5 ml), and 1 M aq. NaCl (15 ml) was placed in an undivided round-bottomed cell. Two Pt electrodes (1.0× 1.5 cm2) were immersed into the aqueous layer, and the mixture was electrolyzed for 16 h at a constant current density of 6.7 mA/cm2 (6 electrons/molecule) under gentle stirring at 0°C. After the electrolysis, acetophenone (100 ml, 0.857 mmol) was added to the resulting mixture as an internal standard for HPLC. 250 ml of the organic solution was taken for the analysis of the recovered olefin (vide supra). The residue was poured into saturated aqueous NH4Cl, and extracted with AcOEt. The combined extracts were washed with brine, dried (Na2SO4), and concentrated in vacuo. The catalyst 3 was removed by passing through a short silica-gel column with hexane and AcOEt (10:1) as an eluent. The yields of cis-epoxide cis-2 and trans-epoxide trans-2 were determined by HPLC. cis-Epoxide cis-2 was separated by HPLC, and % ee of cis-2 was determined by HPLC (vide supra). Analytical samples of the epoxides were obtained by silica-gel column chromatography (hexane and AcOEt, 50:1). The 1H-NMR and IR spectra were identical with those of the authentic sample [55].
3. Results and discussion
2.3. HPLC conditions HPLC analyses of olefins were performed as follows: Column GL Science Inc. Inertsil ODS-2 (4.6 mmf × 150 mm), mobile phase MeCN and H2O (60:40), flow rate 1.0 ml min − 1, detection at 254 nm. HPLC analyses of epoxides were performed as follows. For estimation of the yield: Column Du Pont Zorbax Sil (4.6 mmf × 250 mm), mobile phase hexane and THF (98:2), flow rate 1.0 ml min − 1, detection at 254 nm. For calculation
Enantioselective epoxidation of cis-b-methylstyrene (1a) with an optically active Mn-salen complex 3 in a CH2Cl2 aqueous NaCl two-phase system was carried out at 0°C (Eq. (2); Table 1, Entry 1) to give the desired cis-(1R,2S)-epoxide cis-2a in 82% yield with 87% ee, together with trans-epoxide trans-2a (11%). The yield and % ee of the product cis-2a were almost compatible with those obtained by chemical epoxidation with NaOCl as a co-oxidant [22,23].
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
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(2)
When potassium chloride was used in place of NaCl, a similar reaction occurred to afford cis-2a in a slightly lower yield (Table 1, Entry 2). On the other hand, sodium bromide and sodium iodide could not be used effectively since the yields of the desired epoxide cis-2a were reduced to 37% and a trace, respectively, affording significant amounts of undesired products generating from halogen addition and/or isomerization of the starting material 1a (Entries 3 and 4) For the Mn-salen mediated electro-epoxidation, it seems essential to use a water/CH2Cl2 two-phase system; thus, when water-miscible organic solvents, such as t-BuOH and MeCN, were used, no appreciable amount of the desired epoxide cis-2a was obtained, resulting in the formation of a complex mixture of unidentified products (Entries 5, 6). This may be caused
by decomposition of the Mn-salen complex 3 through the direct oxidation at the anode. In the two-phase system, electro-oxidation occurs only in the aqueous phase so as to suppress direct electro-oxidation of 3, which is dissolved in the CH2Cl2 phase. A plausible mechanism is shown in Scheme 1. The chloride ion, Cl−, was electro-oxidized in the aqueous phase to afford an active chlorine species [Cl+], e.g. NaOCl, Cl2, HOCl, Cl2O, etc. [56], which would move into the organic phase and oxidize Mn-salen complex 3 to generate Mn-oxo complex 3ox. Subsequently, the Mn-oxo complex 3ox would react with olefin 1a to give the corresponding epoxide 2a together with the Mnsalen complex 3. Electro-oxidation of Br− and I− would occur more readily to afford the corresponding active species, [Br+] and [I+] [56]. Thus generated [Br+]
Table 1 Asymmetric electro-epoxidation of cis-b-methylstyrene using an optically active Mn-Salen complex Entry
Organic solvent
MX
Temp/°C
cis-2a Yield/%
1 2 3 4 5 6 7
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 t BuOH MeCN AcOEt
NaCl KCl NaBr NaI NaCl NaCl NaCl
0 0 0 0 20 20 20
82 73 37 B1 0 0 28
a
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
87 87 89
11 7 3 B1 0 0 35
B1 B1 1d Bl e 0 0 10
70
a
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). d 1,2-Dibromo-l-phenylpropane (11%) and trans-b-methylstyrene (36%) were obtained as by-products. e Containing trans-b-methylstyrene (83%). b
Scheme 1. A plausible mechanism of electro-epoxidation using the Cl−/Mn double mediatory system.
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
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and [I+], however, could not efficiently oxidize Mnsalen complex 3 but could react directly with the olefin 1a to give 1,2-dibromo-1-phenylpropane and/or transb-methylstyrene. Recently, Jacobsen reported that the asymmetric epoxidation of olefins under anhydrous conditions by use of 3 in combination with m-CPBA as a co-oxidant proceeded smoothly even at − 78°C, and the % ee of the corresponding epoxides were significantly improved [26]. The effect of the reaction temperature on the electro-epoxidation was investigated (Eq. (3); Table 2). At higher temperatures, (5– 20°C), the yield of cis-2a was slightly decreased but an appreciable change of % ee was not observed (Table 2, Entries 1 3). At lower temperatures (− 5 to −10°C), the epoxidation reaction did not proceed smoothly, resulting in the recovery of most of the starting material (Table 2, Entries 5 and 6) because a part of the electrolysis solution became frozen.
(3)
It is well known that electro-oxidation of an aqueous NaCl solution affords various kinds of active chlorine species, such as NaOCl, Cl2, HOCl, Cl2O, etc., depending on the concentration of Cl−, pH, oxidation potential etc. [56]. The electro-epoxidation of 1a was carried out with variation of the concentration of NaCl (Eq. (4)). The results were summarized in Table 3.
Table 2 Asymmetric electro-epoxidation of cis-b-methylstyrene at various temperatures Entry
Temp/°C
cis-2a Yield/%
1 2 3 4 5 6
20 10 5 0 −5 −10
a
68 80 80 82 42 0
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
81 85 87 87 79
11 11 8 9 6 0
1 1 1 1 52 90
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
86 86 84 81 79
5 8 12 11 9
60 49 1 1 23
d
a
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). d Not determined. b
Table 3 Electro-epoxidation with aqueous NaCl of various concentrations Entry
[NaCl]/M
cis-2a Yield/%
1 2 3 4 5
6 4 2 1 0.1 a
26 38 65 68 45
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). b
a
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81 Table 4 Electro-epoxidation with various amounts of the catalyst 3 Entry
3/mmol
cis-2a Yield/%
1 2 3 4 5
0.05 0.03 0.02 0.01 0.005
a
68 32 25 15 8
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
81 84 80 83 64
11 6 6 11 5
1 43 58 61 65
79
to 1 mol%, whereas the yields decreased significantly (Table 4, Entries 1–4). These results suggest that, in each of the experiments, most of the epoxidation of olefin 1a would occur through oxygen transfer from Mn-oxo complex 3ox, but more than 5 mol% of the Mn-catalyst 3 is needed to attain a significant conversion.
a
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)).
(5)
b
(4)
The % ee value of the cis-epoxide cis-2a was almost unchanged (80% ee) over the range 0.1– 6 M (saturated) NaCl, whereas the yield of the epoxide was changed: the best result was obtained in 1– 2 M of NaCl (Table 3, Entries 3,4), and the yields of cis-2a decreased considerably at both higher (Entries 1,2) and lower (Entry 5) concentrations of NaCl. The electro-epoxidation was also carried out with a changing amount of the Mn-catalyst (Eq. (5); Table 4). The % ee value was almost constant when the amount of the catalyst was decreased
The enantioselectivity might be affected by changing the environment of the chiral center. Recently, it was reported that addition of various donor ligands gives a favorable influence toward the asymmetric induction [27,30,57–60]. These ligands co-ordinate at the axial position of the Mn-salen complexes 3 and 3ox, and cause their conformational change, resulting in improvements of both the reaction rate and enantioselectivity. These facts spurred us to investigate the electro-epoxidation with addition of donor ligands, such as N-methylmorpholine-N-oxide (NMO), pyridine-N-oxide (PNO), and N-methylimidazole (NMeIm) (Eq. (6)). All attempts, however, failed, showing negative effects of the additives (Table 5); thus the yields and % ee of the epoxide cis-2a decreased by addition of any of the donor ligands. It is likely that most of these ligands would be dissolved in the aqueous phase and suffers oxidation at the anode to pollute the electrode surface.
Table 5 Electro-epoxidation with several donor ligands Entry
Additive
cis-2a Yield/%
1 2 3 4
none NMO d PNO e N-Melm f a
68 52 13 14
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). d NMO=N-methylmorpholine-N-oxide. e PNO= pyridine-N-oxide. f N-MeIm= N-methylimidazole. b
a
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
81 68 61 13
11 10 8 4
1 1 8 1
80
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
Table 6 Enantioselective electro-epoxidation of olefins 1 with an optically active Mn-Salen complex 3 in the CH2Cl2 aqueous NaCl two phase system
(6)
(7)
Above all, the optimum conditions for the enantioselective electro-epoxidation are summarized as follows: the electrolysis was carried out in a CH2Cl2 aqueous NaCl (1 M) two-phase system in the presence of 5 mol% of the Mn-salen complex 3 at 0°C. The present electro-epoxidation was successfully applied to several olefins (Eq. (7)). The results are summarized in Table 6. Electro-epoxidation of 2,2-dimethylchromene (1b) and a-methylstyrene (1c) afforded the corresponding epoxides 2b and 2c, respectively, in moderate to good yields and % ee (Entries 2, 3). Upon electrolysis of 1,2-dihydronaphthalene (1d) (Entry 4), the corresponding epoxide 2d was obtained in 47% yield and 70% ee together with naphthalene (21%), which would be gen-
erated through two-electron oxidation followed by deprotonation. The electro-epoxidation of indene (1e) gave the corresponding epoxide 2e in rather low yield and % ee, and a considerable amount of a complex mixture of unidentified products was obtained. The CC double bond of 1e is very reactive and would suffer undesired oxidations with an electro-generated active chlorine species [Cl+].
4. Conclusions A newly devised Cl−/optically active Mn complex 3-double mediatory system (Scheme 1) could work effi-
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
ciently to promote enantioselective electro-epoxidation of olefins. The electro-epoxidation of olefins was successfully performed in an undivided cell by using a CH2Cl2 aqueous NaCl two-phase solution at 0°C. In the aqueous phase, anodic oxidation of Cl− occurred to afford active chlorine species [Cl+] such as ClO−, which would move to the CH2Cl2 phase and work as an efficient co-oxidant to generate and repeatedly re-generate optically active MnO complex 3ox. The Mn-salen complex 3/3ox, dissolved in the CH2Cl2 phase, could survive without suffering anodic oxidation of the easily oxidizable optically active ligand. Notably, the electrolysis can be performed in an undivided cell under constant current conditions. The operation was quite simple, and the yields and % ee of the epoxides were almost comparable with those obtained from chemical epoxidation.
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Electrochemical asymmetric epoxidation of olefins by using an optically active Mn-salen complex Hideo Tanaka a,*, Manabu Kuroboshi a, Hironori Takeda a, Hisaaki Kanda a, Sigeni Torii b a
Department of Applied Chemistry, Faculty of Engineering, Okayama Uni6ersity, Tsushima-naka 3 -1 -1, Okayama 700 -8530, Japan b Institute of Creati6e Chemistry, Musa 874 -5, Okayama 701 -2141, Japan Received 30 August 2000; received in revised form 2 October 2000; accepted 12 October 2000
Abstract Enantioselective electro-epoxidation of olefins by using an optically active Mn-salen complex, (S,S)-N,N%-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride (3), as a mediator in a CH2Cl2 aqueous NaCl two-phase system was performed successfully in a simple undivided cell under constant current conditions. Optimization of the electrolysis conditions and estimation of the reaction mechanism are discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alkenes; Asymmetric induction; Epoxidation; Electrochemical reaction; Manganese compounds
1. Introduction Optically active epoxides are versatile intermediates in the synthesis of pharmaceuticals, agrochemicals, and function materials such as liquid crystals [1 – 4]. Various oxidation methods to convert olefins to optically active epoxides using a combination of an optically active metal complex and co-oxidant have been reported [5– 16]. Among these, optically active Mn-salen complexes have been developed by Katsuki [17 – 19] and Jacobsen [20,21], and successfully applied to enantioselective epoxidation of non-functionalized olefins in good yields and high % enantiomeric excess (% ee). In these methods, however, a large excess amount of co-oxidants such as NaClO [22,23], Bu4NIO4 [24], PhIO [25], m-CPBA [26], H2O2 [27,28], (TMSO)2 [29], or O2 + tBuCHO [30] are usually required to complete the reaction. If the co-oxidant can be regenerated in situ, the oxidation reaction may proceed with a catalytic amount of the co-oxidants, offering a more economical and environmentally benign procedure.
* Corresponding author. Tel.: + 81-86-251-8074; fax: +81-862518079. E-mail address: [email protected] (H. Tanaka).
Electrochemical asymmetric oxidation has been investigated as a promising alternative without the use of a stoichiometric amount of the co-oxidant [31]. Electrochemical asymmetric syntheses are classified into the following six categories: (1) electrolysis of optically active starting material [32 –35] or (2) substrate attached with a recyclable optically active pendant [36 – 40]. (3) Electrolysis in optically active solutions and/or with an optically active supporting electrolyte [41 –43]. (4) Indirect electrolysis using an optically active mediator [44 –46], (5) enzyme [47], or (6) an optically active electrode such as a poly(amino acid)-coated electrode [48 –52]. By using these methods, optically active sulfoxides (from oxidation of the corresponding sulfides), [48 –52], lactones (from diols) [41 –43,47], and alcohols (by oxidative kinetic resolution) [41 –43,48 –52] have been obtained in moderate to high % ee. Among these methods, the method using an optically active mediator seems to be most practical, because (1) only a catalytic amount of the optically active mediator can promote the asymmetric reaction, and (2) efficiency and selectivity can be improved by a proper designing of the optically active mediator. In previous papers, we reported electrochemical asymmetric dihydroxylation of olefins by using an optically active Os catalyst+NaBr or K3Fe(CN)6 double
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H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
mediatory system [44,45]. The most important problem to be solved in such electrochemical asymmetric oxidation with an optically active metal complex catalyst must be electro-oxidative regeneration of co-oxidants without affecting easily oxidizable ligands. Herein is described an optically active Mn-salen-mediated asymmetric epoxidation of olefins in a CH2Cl2 aqueous NaCl two-phase electrolysis system (Eq. (1)).
of the % ee: Column Daicel ChiralCel OD or OB-H (4.6 mmf × 250 mm), mobile phase hexane and 2PrOH (90:10), flow rate 0.5 ml min − 1, detection at 254 mm.
2.4. General procedures A mixture of olefin (1 mmol), Mn-salen complex 3
(1)
2. Experimental
2.1. Instrumentation High performance liquid chromatography (HPLC) was executed with a Soma HPLC instrument (S-4000 LC pump, S-310A modelII UV detector, Shimadzu C-R6A integrator) and a Shimadzu instrument (LC10AT LC pump, LC-10AV UV – Vis detector, C-R6A integrator). Epoxides were analyzed with Zorbax Sil (Du Pont) (Yield) and ChiralCel OD or OB-H (Daisel) (% ee), and olefins were analyzed with Inertsil ODS-2 (GL Science) (recovery). 1H-NMR spectra were recorded on a Varian Gemini 200 (200 MHz) spectrometer.
2.2. Materials MeCN and hexane were distilled from P2O5 under N2. CH2Cl2 was treated with P2O5 and distilled from CaH2. THF was distilled from Na+benzophenone under N2. H2O was deionized and subsequently distilled before use. 2,2-Dimethylchromene (1b) was prepared according to the literature [53,54]. All other chemicals and solvents including an optically active Mn-salen complex 3 were purchased and used without further purification.
(0.05 mmol), dichloromethane (5 ml), and 1 M aq. NaCl (15 ml) was placed in an undivided round-bottomed cell. Two Pt electrodes (1.0× 1.5 cm2) were immersed into the aqueous layer, and the mixture was electrolyzed for 16 h at a constant current density of 6.7 mA/cm2 (6 electrons/molecule) under gentle stirring at 0°C. After the electrolysis, acetophenone (100 ml, 0.857 mmol) was added to the resulting mixture as an internal standard for HPLC. 250 ml of the organic solution was taken for the analysis of the recovered olefin (vide supra). The residue was poured into saturated aqueous NH4Cl, and extracted with AcOEt. The combined extracts were washed with brine, dried (Na2SO4), and concentrated in vacuo. The catalyst 3 was removed by passing through a short silica-gel column with hexane and AcOEt (10:1) as an eluent. The yields of cis-epoxide cis-2 and trans-epoxide trans-2 were determined by HPLC. cis-Epoxide cis-2 was separated by HPLC, and % ee of cis-2 was determined by HPLC (vide supra). Analytical samples of the epoxides were obtained by silica-gel column chromatography (hexane and AcOEt, 50:1). The 1H-NMR and IR spectra were identical with those of the authentic sample [55].
3. Results and discussion
2.3. HPLC conditions HPLC analyses of olefins were performed as follows: Column GL Science Inc. Inertsil ODS-2 (4.6 mmf × 150 mm), mobile phase MeCN and H2O (60:40), flow rate 1.0 ml min − 1, detection at 254 nm. HPLC analyses of epoxides were performed as follows. For estimation of the yield: Column Du Pont Zorbax Sil (4.6 mmf × 250 mm), mobile phase hexane and THF (98:2), flow rate 1.0 ml min − 1, detection at 254 nm. For calculation
Enantioselective epoxidation of cis-b-methylstyrene (1a) with an optically active Mn-salen complex 3 in a CH2Cl2 aqueous NaCl two-phase system was carried out at 0°C (Eq. (2); Table 1, Entry 1) to give the desired cis-(1R,2S)-epoxide cis-2a in 82% yield with 87% ee, together with trans-epoxide trans-2a (11%). The yield and % ee of the product cis-2a were almost compatible with those obtained by chemical epoxidation with NaOCl as a co-oxidant [22,23].
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
77
(2)
When potassium chloride was used in place of NaCl, a similar reaction occurred to afford cis-2a in a slightly lower yield (Table 1, Entry 2). On the other hand, sodium bromide and sodium iodide could not be used effectively since the yields of the desired epoxide cis-2a were reduced to 37% and a trace, respectively, affording significant amounts of undesired products generating from halogen addition and/or isomerization of the starting material 1a (Entries 3 and 4) For the Mn-salen mediated electro-epoxidation, it seems essential to use a water/CH2Cl2 two-phase system; thus, when water-miscible organic solvents, such as t-BuOH and MeCN, were used, no appreciable amount of the desired epoxide cis-2a was obtained, resulting in the formation of a complex mixture of unidentified products (Entries 5, 6). This may be caused
by decomposition of the Mn-salen complex 3 through the direct oxidation at the anode. In the two-phase system, electro-oxidation occurs only in the aqueous phase so as to suppress direct electro-oxidation of 3, which is dissolved in the CH2Cl2 phase. A plausible mechanism is shown in Scheme 1. The chloride ion, Cl−, was electro-oxidized in the aqueous phase to afford an active chlorine species [Cl+], e.g. NaOCl, Cl2, HOCl, Cl2O, etc. [56], which would move into the organic phase and oxidize Mn-salen complex 3 to generate Mn-oxo complex 3ox. Subsequently, the Mn-oxo complex 3ox would react with olefin 1a to give the corresponding epoxide 2a together with the Mnsalen complex 3. Electro-oxidation of Br− and I− would occur more readily to afford the corresponding active species, [Br+] and [I+] [56]. Thus generated [Br+]
Table 1 Asymmetric electro-epoxidation of cis-b-methylstyrene using an optically active Mn-Salen complex Entry
Organic solvent
MX
Temp/°C
cis-2a Yield/%
1 2 3 4 5 6 7
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 t BuOH MeCN AcOEt
NaCl KCl NaBr NaI NaCl NaCl NaCl
0 0 0 0 20 20 20
82 73 37 B1 0 0 28
a
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
87 87 89
11 7 3 B1 0 0 35
B1 B1 1d Bl e 0 0 10
70
a
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). d 1,2-Dibromo-l-phenylpropane (11%) and trans-b-methylstyrene (36%) were obtained as by-products. e Containing trans-b-methylstyrene (83%). b
Scheme 1. A plausible mechanism of electro-epoxidation using the Cl−/Mn double mediatory system.
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
78
and [I+], however, could not efficiently oxidize Mnsalen complex 3 but could react directly with the olefin 1a to give 1,2-dibromo-1-phenylpropane and/or transb-methylstyrene. Recently, Jacobsen reported that the asymmetric epoxidation of olefins under anhydrous conditions by use of 3 in combination with m-CPBA as a co-oxidant proceeded smoothly even at − 78°C, and the % ee of the corresponding epoxides were significantly improved [26]. The effect of the reaction temperature on the electro-epoxidation was investigated (Eq. (3); Table 2). At higher temperatures, (5– 20°C), the yield of cis-2a was slightly decreased but an appreciable change of % ee was not observed (Table 2, Entries 1 3). At lower temperatures (− 5 to −10°C), the epoxidation reaction did not proceed smoothly, resulting in the recovery of most of the starting material (Table 2, Entries 5 and 6) because a part of the electrolysis solution became frozen.
(3)
It is well known that electro-oxidation of an aqueous NaCl solution affords various kinds of active chlorine species, such as NaOCl, Cl2, HOCl, Cl2O, etc., depending on the concentration of Cl−, pH, oxidation potential etc. [56]. The electro-epoxidation of 1a was carried out with variation of the concentration of NaCl (Eq. (4)). The results were summarized in Table 3.
Table 2 Asymmetric electro-epoxidation of cis-b-methylstyrene at various temperatures Entry
Temp/°C
cis-2a Yield/%
1 2 3 4 5 6
20 10 5 0 −5 −10
a
68 80 80 82 42 0
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
81 85 87 87 79
11 11 8 9 6 0
1 1 1 1 52 90
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
86 86 84 81 79
5 8 12 11 9
60 49 1 1 23
d
a
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). d Not determined. b
Table 3 Electro-epoxidation with aqueous NaCl of various concentrations Entry
[NaCl]/M
cis-2a Yield/%
1 2 3 4 5
6 4 2 1 0.1 a
26 38 65 68 45
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). b
a
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81 Table 4 Electro-epoxidation with various amounts of the catalyst 3 Entry
3/mmol
cis-2a Yield/%
1 2 3 4 5
0.05 0.03 0.02 0.01 0.005
a
68 32 25 15 8
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
81 84 80 83 64
11 6 6 11 5
1 43 58 61 65
79
to 1 mol%, whereas the yields decreased significantly (Table 4, Entries 1–4). These results suggest that, in each of the experiments, most of the epoxidation of olefin 1a would occur through oxygen transfer from Mn-oxo complex 3ox, but more than 5 mol% of the Mn-catalyst 3 is needed to attain a significant conversion.
a
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)).
(5)
b
(4)
The % ee value of the cis-epoxide cis-2a was almost unchanged (80% ee) over the range 0.1– 6 M (saturated) NaCl, whereas the yield of the epoxide was changed: the best result was obtained in 1– 2 M of NaCl (Table 3, Entries 3,4), and the yields of cis-2a decreased considerably at both higher (Entries 1,2) and lower (Entry 5) concentrations of NaCl. The electro-epoxidation was also carried out with a changing amount of the Mn-catalyst (Eq. (5); Table 4). The % ee value was almost constant when the amount of the catalyst was decreased
The enantioselectivity might be affected by changing the environment of the chiral center. Recently, it was reported that addition of various donor ligands gives a favorable influence toward the asymmetric induction [27,30,57–60]. These ligands co-ordinate at the axial position of the Mn-salen complexes 3 and 3ox, and cause their conformational change, resulting in improvements of both the reaction rate and enantioselectivity. These facts spurred us to investigate the electro-epoxidation with addition of donor ligands, such as N-methylmorpholine-N-oxide (NMO), pyridine-N-oxide (PNO), and N-methylimidazole (NMeIm) (Eq. (6)). All attempts, however, failed, showing negative effects of the additives (Table 5); thus the yields and % ee of the epoxide cis-2a decreased by addition of any of the donor ligands. It is likely that most of these ligands would be dissolved in the aqueous phase and suffers oxidation at the anode to pollute the electrode surface.
Table 5 Electro-epoxidation with several donor ligands Entry
Additive
cis-2a Yield/%
1 2 3 4
none NMO d PNO e N-Melm f a
68 52 13 14
Determined by HPLC (Zorbax Sil (Du Pont)). Determined by HPLC (ChiralCel OD (Daisel)). c Determined by HPLC (Inertsil ODS-2 (GL Science)). d NMO=N-methylmorpholine-N-oxide. e PNO= pyridine-N-oxide. f N-MeIm= N-methylimidazole. b
a
trans-2a
1a
% ee b
Yield/% a
Recovery/% c
81 68 61 13
11 10 8 4
1 1 8 1
80
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
Table 6 Enantioselective electro-epoxidation of olefins 1 with an optically active Mn-Salen complex 3 in the CH2Cl2 aqueous NaCl two phase system
(6)
(7)
Above all, the optimum conditions for the enantioselective electro-epoxidation are summarized as follows: the electrolysis was carried out in a CH2Cl2 aqueous NaCl (1 M) two-phase system in the presence of 5 mol% of the Mn-salen complex 3 at 0°C. The present electro-epoxidation was successfully applied to several olefins (Eq. (7)). The results are summarized in Table 6. Electro-epoxidation of 2,2-dimethylchromene (1b) and a-methylstyrene (1c) afforded the corresponding epoxides 2b and 2c, respectively, in moderate to good yields and % ee (Entries 2, 3). Upon electrolysis of 1,2-dihydronaphthalene (1d) (Entry 4), the corresponding epoxide 2d was obtained in 47% yield and 70% ee together with naphthalene (21%), which would be gen-
erated through two-electron oxidation followed by deprotonation. The electro-epoxidation of indene (1e) gave the corresponding epoxide 2e in rather low yield and % ee, and a considerable amount of a complex mixture of unidentified products was obtained. The CC double bond of 1e is very reactive and would suffer undesired oxidations with an electro-generated active chlorine species [Cl+].
4. Conclusions A newly devised Cl−/optically active Mn complex 3-double mediatory system (Scheme 1) could work effi-
H. Tanaka et al. / Journal of Electroanalytical Chemistry 507 (2001) 75–81
ciently to promote enantioselective electro-epoxidation of olefins. The electro-epoxidation of olefins was successfully performed in an undivided cell by using a CH2Cl2 aqueous NaCl two-phase solution at 0°C. In the aqueous phase, anodic oxidation of Cl− occurred to afford active chlorine species [Cl+] such as ClO−, which would move to the CH2Cl2 phase and work as an efficient co-oxidant to generate and repeatedly re-generate optically active MnO complex 3ox. The Mn-salen complex 3/3ox, dissolved in the CH2Cl2 phase, could survive without suffering anodic oxidation of the easily oxidizable optically active ligand. Notably, the electrolysis can be performed in an undivided cell under constant current conditions. The operation was quite simple, and the yields and % ee of the epoxides were almost comparable with those obtained from chemical epoxidation.
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