Ionic Liquid Engineering for Lipase-Mediated Optical Resolution of

Article pubs.acs.org/IECR

Ionic Liquid Engineering for Lipase-Mediated Optical Resolution of Secondary Alcohols: Design of Ionic Liquids Applicable to Ionic Liquid Coated-Lipase Catalyzed Reaction Yoshikazu Abe, Yusuke Yagi, Shuichi Hayase, Motoi Kawatsura, and Toshiyuki Itoh* Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Japan 680-8552 S Supporting Information *

ABSTRACT: A rational design of phosphonium ionic liquid for ionic liquid coated-lipase (IL1-PS)-catalyzed reaction has been investigated: very rapid transesterification of secondary alcohols was accomplished when IL1-PS was used as catalyst in tributyl((2-methoxyethoxy)methyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P444MEM][NTf2]) or N-ethyl-N-((2methoxyethoxy)methyl)-N-methylethanaminium bis(trifluoromethanesulfonyl)amide ([N221MEM][NTf2]) as solvent with excellent enantioselectivity. It was also revealed that ammonium ionic liquid influenced the lipase reactivity more strongly than phosphonium ionic liquids. Increased Kcat value was suggested to be the most important factor at work in IL1-PS in these ionic liquid solvents. The Kcat value of (S)-1-phenyethanol in [N444MEM][NTf2] was similar to that of i-Pr2O, and 1.6-fold acceleration was achieved for (R)-1-phenylethanol. On the other hand, the Km value for (R)-isomer in [N444MEM][NTf2] was slightly larger than the reaction in i-Pr2O, while it was significantly reduced for (S)-isomer. These results suggest that solvent provides a certain impact on the reactivity of the enzyme protein of IL1-PS.

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INTRODUCTION Ionic liquids (ILs) are now recognized as suitable for use in organic reactions and as providing potential improvement in the control of product distribution, enhanced reactivity, ease of product recovery, catalyst immobilization, and recycling.1 ILs have also emerged as useful nonaqueous reaction media for biochemical reactions, particularly for lipase-catalyzed transesterifications.2−4 We have been investigating the use of ILs in asymmetric lipase-catalyzed reactions, and have developed methodologies for a recyclable use system of lipase in an ionic liquid reaction medium.3d,6−10 We can now use various types of ILs as solvents for biochemical reactions, such as imidazolium salts,2 ammonium salts,11b−d,12 pyrrolidinium salts,11a alkylguanidinium salts,11e pyridinium salts,3e,11f−h and phosphonium salt.9,12 It is well recognized that hydrophobic ionic liquids are generally suitable for these reactions.2 Lipases are the most widely used enzymes applicable for organic synthesis; however, the reaction rates are significantly dependent on the reaction media and very slow reactions or poor enantioselective reactions are sometimes obtained.5 Recently there has been growing interest in the development of the dynamic kinetic resolution system by the combination of lipase-catalyzed reaction with racemization of substrates using chemical catalysts.13,14 To realize such a system, development of a powerful means to improve lipase activities is very important. We established a powerful method of activating lipase protein by coating it with imidazolium alkyl PEG sulfate ionic liquid (1-butyl-2,3-dimethylimidazolium polyoxyethylene(10) cetyl sulfate (IL1)7): the ionic liquid coated Burkholderia cepacia lipase (IL1-PS) displayed excellent reactivity for many substrates in conventional organic solvents.2c,7−10,15 It allowed recyclable use of the IL1-PS if the reaction was possible in an ionic liquid solvent; we found © 2012 American Chemical Society

that it was essential to choose an appropriate ionic liquid when using IL1-PS in these solvents (ILs) (Figure 1).8,10 It is well recognized that both the enantioselectivity and reaction rate were significantly dependent on the solvent system. We previously reported that hydrophilic imidazolium salts ILs, which have alkyl ether functionalized sulfate salts, were appropriate for a lipase-catalyzed reaction.6d Recently, Dreyer,15 Guo,16 Zhao,17 and De Diego18 have reported which ILs that have an alkyl ether moiety as a cationic part acted as good solvents for these reactions. After evaluation of ILs which have alkyl ether moieties, we established that tributyl((2methoxyethyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P444ME][NTf2])8,19 and tributyl((2-methoxyethoxy)methyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P444MEM][NTf2])10 acted as useful solvents for IL1-PScatalyzed reactions using 2-phenylethanol (1a) as substrate and realized recyclable use of the enzyme in the IL solvent system.8 However, it is well-known that lipase reactivity is dependent on the substrates.8,10 Therefore, ionic liquids applicable to versatile substrates must be designed. Herein, we report the results of advanced designs of ionic liquids that are suitable for the IL1-PS-catalyzed reaction for a variety of substrates.

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RESULTS AND DISCUSSION 1. Advanced Design of Ionic Liquid for IL1-PSCatalyzed Reaction. With the objective of optimizing the Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 9952

November 28, 2011 March 7, 2012 March 9, 2012 March 9, 2012 dx.doi.org/10.1021/ie202740u | Ind. Eng. Chem. Res. 2012, 51, 9952−9958

Industrial & Engineering Chemistry Research

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Figure 1. Lipase recyclable use system using an ionic liquid solvent system.

design of ionic liquids for asymmetric transesterification using our ionic liquid-coated enzyme (IL1-PS), we again focused on phosphonium salts and ammonium salts which have an alkyl ether group. The moieties of these salts are commonly found in living creatures and it is anticipated that the salts probably have good affinity with enzyme proteins and may thus provide a good environment for enzymes. Since NTf2 salts have hydrophobic property with less toxicity,20 they are now widely considered a preferred anionic part of ionic liquids; therefore, we selected NTf2 as the anion for ILs. In addition, although we attempted to prepare several types of methoxy poly(ethoxy)ethyl-substituted phosphonium ILs, it was found that these salts were not appropriate as a solvent because they were too viscous to conduct the lipase-catalyzed reactions. On the basis of these results, in this study, we evaluated four types of ILs: tributyl((2methoxyethyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P444ME][NTf 2]),8,18 tributyl((2-methoxyethoxy)methyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P444MEM][NTf2]),10 N,N-diethyl-2-methoxy-N-methylethanaminium bis(trifluoromethanesulfonyl)amide ([N 221ME ] [NTf2]),21 and N-ethyl-N-((2-methoxyethoxy)methyl)-Nmethylethanaminium bis(trifluoromethanesulfonyl)amide ([N221MEM][NTf2]) as solvents for the IL1-PS-catalyzed reactions (Figure 2). Since we had previously established that i-Pr2O was the best reaction medium for the IL1-PS-catalyzed reactions among conventional organic solvents,7 this was chosen as a typical nonaqueous organic solvent and its result was compared to other reactions in ILs. We conducted IL1-PS-catalyzed reactions using four substrates, 1-phenylethanol (1a), 5-phenylpent-1-en-3-ol (1b), 3-hydroxypentanenitrile (1c), and 4-phenylbut-3-en-2-ol (1d) in these four IL solvent systems, and the results are shown in Table 1. As can be seen in Figure 2, all ILs employed show viscosities in a range of 72−120 cP which are at least 200-fold more

Figure 2. Ionic liquids tested as solvent for IL1-PS-catalyzed transesterification.

viscous than i-Pr2O (0.305 cP·s at 32 °C).22 But the transesterification in an IL solvent proceeded faster than in iPr2O solvent. The best solvent was [P444MEM][NTf2] when 1phenylethanol (1a) was used as substrate (entry 3): ca. 2-fold acceleration was recorded compared to that in i-Pr2O (vs entry 1). Since the viscosity of [P444MEM][NTf2] is much higher than that of i-Pr2O,8 the origin of the high reaction efficiency of ILPS in the phosphonium ILs might not be due to the enhanced rate of mass transfer in the solvent system but to improved activity of the enzyme protein in [P444MEM][NTf2]. However, reaction rates in [P444ME][NTf2] (entry 2), [N221ME][NTf2] (entry 4) and [N221MEM][NTf2] (entry 5) were inferior to the reaction in i-Pr2O. In the case of IL1-PS-catalyzed reaction of 5-phenylpent-1en-3-ol (1b), the reaction proceeded more rapidly than in iPr2O when it was conducted in [P444ME][NTf2], [P444MEM][NTf2] or [N221MEM][NTf2] (entries 7, 8, and 10), and ca. 49953

dx.doi.org/10.1021/ie202740u | Ind. Eng. Chem. Res. 2012, 51, 9952−9958


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compared to that of i-Pr2O, and 3.6-fold acceleration was recorded for (R)-1b.10 To gain detailed information on what factor contributed to the acceleration of the present IL1-PS-catalyzed reaction in ammonium ILs, the kinetic parameters of IL1-PS-catalyzed transesterification of (R)-1a or (S)-1a were measured in three solvents: i-Pr2O, [N221ME][NTf2], and [N221MEM] [NTf2] (Scheme 1 and Table 2).

Table 1. Results of IL1-Coated-PS-Catalyzed Transesterification in Four IL Systems entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 18 19 20 21 a

substrate 1a 1a 1a 1a 1a 1b 1b 1b 1b 1b 1c 1c 1c 1c 1c 1d 1d 1d 1d 1d

solvent i-Pr2O [P444ME][NTf2] [P444MEM][NTf2] [N221ME] [NTf2] [N221MEM] [NTf2] i-Pr2O [P444ME][NTf2] [P444MEM][NTf2] [N221ME] [NTf2] [N221MEM] [NTf2] i-Pr2O [P444ME][NTf2] [P444MEM][NTf2] [N221ME] [NTf2] [N221MEM] [NTf2] i-Pr2O [P444ME][NTf2] [P444MEM][NTf2] [N221ME] [NTf2] [N221MEM] [NTf2]

ratea c

3400 1400c 6200c 1254c 1764c 60 250 260 23 227 45d 4300d 4600d 4651d 8284d 900 1000c 850c 109c 1355c

E valueb >200 >200 >200 >200 >200 >200 137 >200 >200 >200 49 15 12 12 43 >200 >200 >200 >200 >200

Scheme 1. Kinetic Experiment of IL1-PS-Catalyzed Transesterification of 1-Phenylethanol (1a)

Table 2. Results of Kinetic Experiments of IL1-PS-Catalyzed Transesterification of 1-Phenylethanol (1a)

−1

Units: mM/mg enzyme h . The rate was determined by GC analysis at 30 min of reaction. bCalculated by % ee of (R)-2 (eep) and % ee of (S)-1 (ees). E = ln[(1 − c(1 + eep))/ln[(1 − c(1 − eep)), here c means conversion (convn) which was calculated by the following formula: c = ees/(eep + ees), see ref 23. cThe rate was determined by GC analysis at 10 min. dSince the reaction proceeded very rapidly, the rate was determined by GC analysis at 5 min.

entry substrate

fold acceleration was recorded. On the other hand, a reduced reaction rate was recorded in [N221ME][NTf2] while enantioselectivity was perfect (entry 9). The reaction of 3-hydroxypentanenitrile (1c) proceeded at a similar reaction rate in three ILs (entries 12−14) and the rates were significantly increased compared to the reaction in i-Pr2O (entry 11). To our delight, [N221MEM][NTf2] solvent was particularly effective for this substrate and remarkable aceleration (184-fold compared to that in i-Pr2O) was obtained while high enantioselectivity was maintained (entry 15). The IL1-PS-catalyzed reaction of 4-phenylbut-3-en-2-ol (1d) in i-Pr2O and two phosphonium salt ILs (entries 16−19) gave similar results. Interestingly, a large difference was found in the reactions using ammonium salts as solvents. [N221MEM][NTf2] solvent worked best and the highest enantioselectivity and reaction rate were obtained when 1d was used as substrate (entry 21). On the contrary, very slow reaction was recorded when [N221ME][NTf2] was used as solvent (entry 20). It was thus established that [P444MEM][NTf2] worked as the best solvent for 1a, and [N221MEM] [NTf2] for 1b, 1c, and 1d. Interestingly, [N221MEM] [NTf2] was found to be a better solvent than [N221ME][NTf2] in all substrates. It has thus been established that [P444MEM][NTf2] and [N221MEM] [NTf2] are recommended as appropriate solvents for IL1-PS-catalyzed reaction. [P444ME][NTf2] was also recommended as a good solvent for IL1-PS-catalyzed reaction because it gave excellent results for all substrates and is now commercially available (TCI T2564).19 We previously established that the Kcat value was the most signifcantly influenced by the solvent system in IL1-PScatalyzed transesterification in phosphonium ionic liquids: the Kcat value of (S)-1b in [P444MEM][NTf2] was increased ca. 2-fold

a

1 2

(R)-1a (R)-1a

3

(R)-1a

4 5

(S)-1a (S)-1a

6

(S)-1a

solvent i-Pr2O [N221ME] [NTf2] [N221MEM] [NTf2] i-Pr2O [N221ME] [NTf2] [N221MEM] [NTf2]

Vmaxa

Km

0.691 0.675

1.57 4.57

2.77 2.70

1.76 0.591

1.08

2.69

4.32

1.61

0.0212 0.0554

0.522 2.24

0.0848 0.222

0.162 0.0991

0.0244

0.369

0.0977

0.265

Kcat

Kcat/Km

Units: M·mg enzyme min−1

The Kcat value of (S)-1a in [N444MEM][NTf2] was similar to that of i-Pr2O (entries 4 and 6), and 1.6-fold acceleration was achieved for (R)-1a (entries 1 and 3). On the other hand, Km value for (R)-isomer in [N444MEM][NTf2] was slightly larger than the reaction in i-Pr2O (entries 1 and 3), while it was significantly reduced for (S)-isomer (entries 5 and 6). These results clearly suggest that solvent provides a certain impact on the reactivity of the enzyme protein of IL1-PS. In contrast, there was only a little difference in Km for the reactions in phosphonium ILs;10 it was thus recognized that ammonium ionic liquid might influence the lipase reactivity more strongly than phosphonium ionic liquids. Zhao and co-workers recently reported that dissolution and stabilization of a lipase protein took place in ILs that have a long alkyloxyalkyl chain in an ammonium cation, and that this might provide improved catalytic efficiency of the corresponding biochemical reactions.17c Although IL1-PS is still present in a suspension state in these solvents, we assume that stronger affinity between the salt with lipase protein might cause preferable modification of enzyme reactivity because the surface of the lipase protein would be partially covered with alkyl PEG ionic liquid for IL1-PS. No stripping of the ionic liquid coating material (IL1) from the enzyme into these IL solvents employed during the reaction course was detected by an ion chromatographical analysis (cation mode of IC-C4 column using 5% oxalic acid aqueous solution as an eluent). On the 9954



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Table 3. Results of Recyled Use of IL1-PS in the [P444MEM][NTf2] Solvent System entry

substrate

run

time (h)

% ee of (R)-2a (% Yield)b

1 2 3 4 5 6 7 8 9

1a 1a 1b 1b 1c 1c 1d 1d 1d

1st 10th 1st 5th 1st 10th 1st 5th 10th

1 1 12 12 0.5 0.5 1 1 1

99 (35) >99 (30) >99 (17) >99 (17) 77 (42) 88 (26) 99 (35) >99 (45) 92 (22)

% ee of (S)-1a (% Yield)b 72 75 36 28 61 33 78 87 49

(43) (41) (61) (70) (51) (61) (44) (47) (65)

ratec

% convn

E valued

1400 1200 161 73 4300e 3400e 1000 1210 900

42 43 27 22 44 28 44 47 35

>200 >200 >200 >200 15 21 >200 >200 39

a

Determined by HPLC (Chiralcel OB-H, n-hexane/2-PrOH = 20:1). bIsolated yield. cThe rate was determined by GC analysis at 10 min of reaction. Calculated by %ee of (R)-2 (eep) and %ee of (S)-1 (ees). E = ln[(1 − c(1 + eep))/ln[(1 − c(1 − eep)), here c means convn which was calculated by the following formula: c = ees/(eep + ees). eSince the reaction proceeded very rapidly, the rate was determined by GC analysis at 5 min. d

solvent while maintaining perfect enantioselectivity. To the best of our knowledge, this is a record for the most rapid lipasecatalyzed transesterification of 1a. It was also establishhed that [N221MEM] [NTf2] was always superior to [N221ME][NTf2] in all substrates and that [P444MEM][NTf2] was especially suitable for the reaction of 1a. It has now been disclosed that introduction of alkyl ether moiety in the cationic part of ILs might be a sure way to design ionic liquids suited for enzymatic reaction. Our phosphonium ionic liquid and ammonium ionic liquid have a certain advantage over conventional organic solvents, because the solvent makes it possible to use the enzyme repeatedly and has less-volatile and less-flammable properties. After the reaction, we recovered the ionic liquids and used them repeatedly after simple purification. We believe that further investigation of the scope and limitations of the biochemical reaction in ionic liquids will make them even more beneficial.

other hand, we previously found that gradual elution of free IL1 from the IL1-coated Geotrichum candidum dehydrogenase to the solvent took place in an aqueous phosphate buffer solution.6i These results suggest that the binding property of the imidazolium salt with the enzyme protein is insufficient in aqueous solution but is strong enough for use in these IL solvents. 2. Demonstration of Recyclable Use of IL1-PS Using an Ionic Liquid Solvent System. One of the most important benefits to the use of ILs is that it allows recyclable use of the enzyme if the reaction is possible in an ionic liquid solvent. A better extraction efficiency of the products using a mixed solvent of ether and hexane was obtained in the [P444MEM][NTf2] solvent system compared to that in [N221MEM][NTf2]. Therefore, we demonstrated the recyclable use of IL1-PS using [P444MEM][NTf2] as solvent (Table 3). It was possible to recycle 10 times when 1-phenylethanol (1a) was used for a substrate, while maintaining excellent reactivity and enantioselectivity (entry 2). However, the reaction rate dropped significantly when 5-phenylpent-1-en-3ol (1b) was used as substrate with five repetitions of the reaction (entry 4). Ten repeated uses of the enzyme were also successful when 3-hydroxypentanenitorile (1c) was subjected to the IL1-PS-catalyzed reaction in this solvent (entry 6). An interesting result was obtained when (E)-4-phenylbut-3-en-2-ol (1d) was used as substrate (entries 7−9). It was again possible to use the enzyme 10 times, but the most rapid reaction was accomplished after the enzyme had been used 5 times: the rate reached 1300 mM/mg enzyme h−1. We assume that this may be due to removal of a trace amount of impurity, which may cause inhibition of the enzyme in the ionic liquid during repetition of the workup process. Although 10 repetitions using IL1-PS in the reaction of 1d was successful, a slight drop of the E value was recorded (entry 9). After these recycling experiments, the remaining enzyme in the IL was removed by washing with water, and the resulting IL was treated with active charcoal in methanol for purification. After these processes, the IL was successfully purified. We thus succeeded in showing the recyclable use of both enzyme and ionic liquid.

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EXPERIMENTAL SECTION

Materials. IL1-PS was prepared by the reported method7 or a commercial one was used provided by Tokyo Chemical Industry Co., Ltd. (TCI-B3028). 7 [N221ME][NTf 2] was purchased from Kanto Reagents Co., Ltd. Water content of the ionic liquids employed was determined by a Karl Fischer moisture titrator and had the following values: [P444ME][NTf2](TCI T2564),18 250 ppm; [P444MEM][NTf2], 280 ppm; [N221ME][NTf2] (Kanto Reagents Co., Ltd.), 290 ppm; [N 221MEM ][NTf 2 ], 440 ppm. Ion chromatography was performed by the SHIMADZU Prominence HPLC system using IC-C4 column using 5 wt % of oxalic acid aqueous solution. Typical Enzymatic Reaction. The reaction was typically carried out as follows: To a mixture of 5 mg of IL1-PS (0.25 mg of the enzyme protein) in 1.0 mL of solvent was added (±)-1a (50 mg, 0.41 mmol) and 55 mg of vinyl acetate (1.5 equiv), and the resulting mixture was stirred at 35 °C. To evaluate the initial reaction rate, the reaction was conducted in the presence of 0.5 mmol of hexadecane as an internal reference, an aliquot of the reaction mixture was sampled at 30 min of the reaction and extracted with a mixed solvent of diethyl ether and hexane (1:4), and the rate was determined by capillary GC analysis. The reaction course was monitored by silica gel thin layer chromatography (TLC) analysis, and the product (R)-2a and unreacted alcohol (S)-1a were extracted with a mixed solvent of diethyl ether and hexane (1:4) when the spots became the same size, then purified by silica gel TLC. Since it is well recognized that water content of the solvent influences the lipase

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CONCLUSION We confirmed that phosphonium ionic liquid and ammonium ionic liquids which have alkylether moieties become excellent reaction media for lipase-catalyzed transesterification, especially for ionic liquid coated-lipase PS (IL1-PS). In fact, very rapid acetylation of 1-phenylethanol (1a) has been accomplished using the combination of IL1-PS and [P444MEM][NTf2] as 9955



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filtration, the filtrate was washed with hexane three times, and the solvent was removed using lyophilization. The resulting oil was dissolved in acetone and treated with active charcoal, and the charcoal was then removed by filtration. The filtrate was dried under vacuum at 50 °C for 5 h to give [N221MEM][NTf2] (28.1 g, 64 mmol) as a colorless oil in 40% yield. It was found that this yield of IL drastically dropped when it passed through the active alumina (Type III) column. Although this process was essential to purify the salts, we obtained [N221MEM][NTf2] in over 80% yield when the process was omitted. Since the resulting IL worked as a good solvent for IL1-PS-catalyzed reaction, the process can be omitted in practical preparation. 1 H NMR (400 MHz, CDCl3) δ 1.36 (6H, t, J = 7.8 Hz), 2.97 (3H, s), 3.34−3.38 (4H, m), 3.37 (3H, s), 3.58 (2H, t, J = 4.0 Hz), 3.94 (2H, t, J = 4.0 Hz), 4.64 (2H, s); 13C NMR (125 MHz, CDCl3) δ 6.55, 6.80, 43.36, 53.34, 57.90−58.26 (m), 70.82−71.56 (m), 86.88, 119.4 (q, JCF = 320 Hz). Because of long-range coupling with the CF3 group of TFSA anions, several peaks were multiply split. However, it was difficult to determine their coupling constants. 19F NMR (470 MHz, CDCl3, C6F6) δ 82.9. IR (neat) 3825, 3546, 2938, 2991, 2895, 2827, 1703, 1463, 1352, 1193, 1139, 1057, 906 cm−1. Anal. Calcd for C11H22F6N2O6S2: C, 28.95; H, 4.86; F, 24.97; N, 6.14; O, 21.03; S, 14.05. Found: C, 27.66; H, 4.88; N, 6.16. Viscosity = 97.5 cP·s at 25 °C (H2O = 320 ppm), Tg = −77.1 °C (DSC). Kinetics Experiments for Lipase-Catalyzed Acetylation of 1-Phenylethanol (1a). The reaction rate of the lipasecatalyzed reaction was determined by capillary GC-analysis as follows: The reaction mixture was sampled at appropriate reaction intervals, and % conversion was determined by GC analysis, respectively. Since the IL1-PS-catalyzed reaction proceeded very rapidly, the initial reaction rate was determined based on a graph of the reaction course. For details of the kinetic experiments, see the Supporting Information. Recyclable use of IL1-PS in Ionic Liquid Solvent System (Typical Reaction Conditions). To a mixture of alcohol (±)-1 (50 mg) and vinyl acetate (1.5 equiv) in [P444MEM][NTf2] (1.0 mL) was added IL1-PS (5.0 mg, corresponding to 0.25 mg of enzyme, and the mixture was stirred at 35 °C. The reaction course was monitored by capillary GC-analysis and silica gel TLC. To the reaction mixture was added a mixed solvent of diethyl ether and hexane (1:4) to form biphasic layers, and acetate (R)-2 produced and alcohol (S)-1 unreacted were isolated from the ether layer. It was essential to repeat the extraction with the mixed solvent from the reaction mixture 10 times. The combined extracts were evaporated, and (R)-2 and (S)-1 were separated by preparative silica gel TLC. Since the lipase was remained in the ionic liquid layer, it was possible to use the lipase repeatedly (Figure 1); the ionic layer was placed under reduced pressure (1 Torr) at room temperature for 15 min to remove the ether, and to the resulting ionic layer was added (±)-1 (50 mg) and vinyl acetate (1.5 equiv) and the mixture was stirred at 35 °C.

performance, ionic liquids were dried under reduced pressure at 50 °C at 1 Torr for 3−5 h prior to the reaction. Enantiomeric excess of the product acetate and alcohol unreacted were determined by HPLC (Chiralcel OB-H, n-hexane/2-propanol = 9:1 or 20:1). The reaction rate was determined by GC analysis at 30 min of reaction in the presence of an internal reference. Enantioselctivity of the reaction was shown as the E value23 which was calculated by %ee of (R)-2 (eep) and %ee of (S)-1 (ees). E = ln[(1 − c(1 + eep))/ln[(1 − c(1 − eep)); here, c means conversion which was calculated by the following formula according to the ref 21: c = ees/(eep + ees). For details, see the Supporting Information. Synthesis of Tributyl((2-methoxyethoxy)methyl)phosphonium Bis(trifluoromethanesulfonyl)amide ([P444MEM][NTf2]).10 To an ethanol (20 mL) solution of 2methoxyethoxymethyl chloride (MEM chloride) (4.98 g, 40 mmol) was added tributylphosphine (7.5 g, 37 mmol), and the resulting mixture was stirred for 22 h at 80 °C. After being cooled to room temperature (rt), hexane was added to form a precipitate which was removed by filtration. The resulting filtrate was evaporated under vacuum to give the chloride salt (20.6 g, 36 mmol) in 97% yield. The salt was dissolved in ethanol (18 mL), and lithium bis(trifluoromethanesulfonyl)amide (11.37 g, 40 mmol) powder was added, then the mixture was stirred at rt for 17 h to form lithium bromide as a precipitate. The precipitate was removed by filtration, the filtrate was washed with hexane 3 times, and the solvent was removed using lyophilization. The resulting oil was dissolved in acetone and treated with active charcoal, and the charcoal was then removed by filtration. The filtrate was passed through active alumina (Type III) and dried under vacuum at 50 °C for 5 h to give [P444MEM][NTf2] (20.0 g, 35 mmol) as a colorless oil in 95% yield. 1H NMR (500 MHz, CDCl3) δ 0.97: (9H, t, J = 7.6 Hz), 1.48−1.59 (12H, m), 2.15−2.22 (6H, m), 3.36 (3H, s), 3.55 (2H, dt, JHP = 4.4, 5.5 Hz), 3.79 (2H, dt, JHP = 2.4, 5.5 Hz), 4.39 (2H, d, JHP = 4.8 Hz); 13C NMR (125 MHz, CDCl3) δ 12.8: 16.7 (d, JCP = 47.0 Hz), 22.9 (d, JCP = 3.90 Hz), 23.4 (d, JCP = 15.3 Hz), 58.4, 60.2 (d, JCP = 65.3 Hz), 71.0, 73.1 (d, JCP = 11.4 Hz), 119.6 (q, JCF = 321.5 Hz); 31P NMR (202.46 MHz, CDCl3) δ 32.8; 19F NMR (170.6 MHz, CDCl3, C6F6) δ 78.7; IR (neat) 2966, 1467, 1352, 1193, 1133, 1058, 923, 788, 740, 617, 570, 515 cm−1. HRMS (EI) calcd for C16H36O2P, 291.2410; found, 291.2408. Viscosity = 80.5 cP·s at 25 °C (H2O = 270 ppm), Tg = −84.1 °C (DSC). Since [P444MEM][NTf2] sometimes colored when dried at higher temperature, we usually dried the liquid at 50 °C at 1 Torr for 3−5 h and used it for the lipase-catalyzed reaction. However, it was possible to reduce the water content to 90 ppm when it was dried under reduced pressure at 5 Torr at 100 °C for 6 h. Synthesis of N-Ethyl-N-((2-methoxyethoxy)methyl)-Nmethylethanaminium Bis(trifluoromethanesulfonyl)amide ([N221MEM][NTf2]). To an ethanol (20 mL) solution of 2-methoxyethoxymethyl chloride (MEM chloride) (22.0 g, 170 mmol) was added diethylmethylamine (14.2 g, 160 mmol), and the resulting mixture was stirred for 24 h at 70 °C. After being cooled to room temperature, hexane was added to form a precipitate which was removed by filtration. The resulting filtrate was evaporated under vacuum to give the chloride salt (35.5 g, 160 mmol) in quantitative yield. The salt was dissolved in acetone (18 mL), and lithium bis(trifluoromethanesulfonyl)amide (48.8 g, 170 mmol) powder was added, then the mixture was stirred at room temperature for 48 h to form lithium chloride as a precipitate. The precipitate was removed by

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of IL1-PS and [N221MEM][NTf2]; 1H and 13C NMR spectra of [N221MEM][NTf2]; selected spectra data of (±)-1a, 1b, 1c, and 1d; data of kinetic experiments. This material is available free of charge via the Internet at http://pubs.acs.org. 9956



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AUTHOR INFORMATION

Corresponding Author

*Tel., Fax: +81-857-31-5259. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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REFERENCES

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