Ionic Liquids as Green Solvents - American Chemical Society

Chapter 21

Efficient Lipase-Catalyzed Enantioselective Acylation in an Ionic Liquid Solvent System Downloaded by UNIV OF GUELPH LIBRARY on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch021

1,*

1

2

Toshiyuki Itoh , Yoshihito Nishimura , Masaya Kashiwagi , and Makoto Onaka 2

1

2

Department of Materials Science, Faculty of Engineering, Tottori University, 4-101 Koyama Minami, Tottori 680-8552, Japan Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan

The lipase-catalyzed enantioselective acylation of allylic alcohols in an ionic liquid solvent was demonstrated; the reaction was significantly dependent on the counter anion of the imidazolium salt and good results were obtained when the reaction was carried out in [bmim]PF or [bmim]BF as the solvent. The lipase-catalyzed transesterification was then investigated using methyl esters as acyl donors, especially under reduced pressure in an ionic liquid ([bmim]PF ) solvent system. The transesterification of 5-phenyl-1-penten3-ol took place smoothly under reduced pressure at 20 Torr and 40 °C when methyl phenylthioacetate was used as the acyl donor in [bmim]PF , and we succeeded in obtaining the corresponding acylated compound in optically pure form; this makes it possible to repeatedly use the lipase because there was no drop in the reaction rate despite five repetitions of the process. 6

4

6

6

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

251

252

Introduction To meet the challenge in chemistry of developing practical processes, the proper choice of a reaction medium is very important. A breakthrough has sometimes occurred with the invention of the reaction medium in chemical reactions and this is true even in enzymatic reactions; lipasecatalyzed transesterification in an organic solvent system is now well recognized as a very useful means of synthesizing optically active compounds, it had been long believed, however, that an enzymatic reaction could proceed only in aqueous medium before Klibanov and his co-workers first demonstrated lipase-catalyzed trans-esterification of alcohols in an organic solvent system. Ionic liquids are a new class of solvents which have attracted growing interest over the past few years because of their unique physical and chemical properties. Because lipase tolerates non-natural reaction conditions, it was believed that the lipasecatalyzed reaction might occur in the ionic liquid solvent. We describe the results of the lipase-catalyzed reaction in this unique novel reaction medium system.

Downloaded by UNIV OF GUELPH LIBRARY on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch021

1

2

3

4,5,6

Experimental Normal pressure conditions. Typically, the reaction was carried out as follows: To a mixture of lipase in the ionic liquid were added (±)-5phenyl-l-penten-3-ol (1) as a model substrate and vinyl acetate (1.5 eq) as the acyl donor. The resulting mixture was stirred at room temperature (ca. 25 °C) and the reaction course was monitored by GC analysis. The reaction was stopped by the addition of 3 ml of ether when the molar ratio of 5-phenyl-l-penten-3-yl acetate (2a) and alcohol 1 became equal. The reaction mixture was filtered through a glass-sintered filter with a celite pad to remove the enzyme and product, and unreacted alcohol was isolated from the filtrate. It is noteworthy that the ionic solvent was recovered without any loss in the amount after the work-up process and it was possible to reuse it after washing with water and dried under vacuum for several hours at 50 °C. The optical purities of the acetate (S)-2a produced and the remaining alcohol (R)-l were determined by capillary GC analysis or HPLC using a chiral column. Chiraldex G-TA was used for GC analysis : φ0.25 mm χ 20 m, Carrier gas: He 40 ml/min. Temp (°C): 100, Inlet Pressure: 1.35kg/cm , Amount 400 ng, Detection; FID. HPLC analysis: Chiralcel OD (φ4.6 mm χ 250 mm), Hexane: 2-propanol (10:1-8:1), 35°C, 1.0 ml/min, 254 nm. Under reduced pressure conditions. To a mixture of lipase (25 mg) in the ionic liquid (1.5 ml) were added racemic (±)-l (50 mg, 0.30 mmol) as a model substrate and methyl phenylthioacetate (27 mg, 0.15 mmol, 0.5 equiv.) as the acyl donor. The resulting mixture was stirred at 40 °C at 20 7

7

2

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

253 Torr for 13 h. The reduced pressure was broken and the reaction was stopped by the addition of 3 ml of ether to the reaction mixture to form the biphasic state. The desired products and unreacted alcohol were quantitatively extracted from the ether. To the remaining ionic liquid phase, which was placed under reduced pressure for 15 minutes to remove the ether, a mixture of the substrate and methyl phenylthioacetate was again added. This mixture was stirred at 40 °C and 20 Torr. The optical purities of the produced ester 2 and the remaining alcohol (R)-l were determined by capillary GC analysis using a chiral column (Chiraldex G-TA). In addition, the ionic solvent was sometimes significantly acidified and lowered to less than pH 2 due to partial hydrolysis of the salt by the moisture. Therefore, the pH values of the solvent should be checked prior to use in the reaction. We developed two good methods of restoring damaged ionic liquid: the solvent is washed with a mixture of hexane and ethyl acetate (1:1) and treated with the ionic exchange resin IRA 400, or is washed with the same mixed solvent followed by treatment with neutral activated alumina type I.


7

5

Results and Discussion 1. Lipase-catalyzed reaction system anchored in an ionic liquid solvent. We chose the imidazolium salts as the solvent for our enzymatic reaction among the various types of ionic liquids based on two criteria. The first is that the imidazolium salts are stable under atmospheric conditions and especially tolerant to water. The second is that we are systematically able to investigate the suitable combination of the imidazolium cation and counter anion of the salt for the enzymatic reaction. First, we investigated the Candida antactica lipase (Novozym435)catalyzed enantioselective transesterification of 5-phenyl-l-penten-3-ol (1) in five types of butylmethyl imidazolium salts (Eq. 1). It was found that the acylation rate was strongly dependent on the anionic part of the solvent, while the CAL catalyzed acylation proceeded with high enantioselectivity in all tested solvents (Table 1). The best result was recorded when [bmim]BF was employed as the solvent (Entry 1) and the reaction rate was nearly equal to that of the reference reaction in /-Pr 0 (Entry 8). The second choice of solvent was [bmim]PF and the acetate 2a was obtained in excellent enantioselectivity, though the reaction rate was slightly inferior to the reaction in [bmim]BF (Entry 2). On the contrary, a significant drop in the reaction rate was obtained when the reaction was carried out in [bmim]TFA (Entry 3), [bmim]OTf (Entry 4) or [bmim]SbF (Entry 5). From these obtained results, it was concluded that [bmim]BF and [bmim]PF are suitable solvents for the reaction. Although the acylation rate for the reaction in [bmim]PF was slightly inferior to that 3

7

4

8a

4

2

8b

6

4

8c

8 d

8e

6

4

6

6


254 in [bmim]BF , we chose [bmim]PF as the best solvent for the lipasecatalyzed reaction system, because a very easy work-up process was realized in the [bmim]PF solvent system due to the insolubility of this salt in both water and ether. On the contrary, [bmim]BF was quite soluble in water and therefore it was difficult to remove the by-product such as acetic acid by simple work-up processes. We next investigated the enantioselective acylation of (±)-l using five types of lipases in the [bmim]PF solvent system. A poor reactivity was observed for lipase from Alcaligenes sp. (QL) and Pseudomonas cepacia lipase (PS), though the desired acetate 2a obtained with an extremely high enantioselectivity for all these enzymes (Entries 6 and 7). On the other hand, no reaction took place when Candida rugosa lipase (CRL) or porcine liver lipase (PPL) was used as the catalyst in the [bmim]PF solvent system. 4

6

6

4


6

6

OAc 2a

vinyl acetate

(1)

Novozym435 (50 wt%) Ionic Liquid: [bmim]X"

(±H

Table 1. Lipase-catalyzed transesterification in ILs Entry 1 2 3 4 5 6 7

T

Lipase* Solvent

Novozym435 [bmim]BF Novozym435 [bmim]PF Novozym435 [bmim]TFA Novozym435 [bmim]OTf Novozym435 [bmim]SbF QL [bmim]PF PS [bmim]PF 4

6

6

6

6

8

CAL

/-Pr 0 2

™ /n

e

3.5 5 48 24 48 25 168 3

%ee of 2a Conv. Relative Ε value (Yield/%) /c Rate

d

b

>99 (44) >99(45) 91(19) >99 (34) >99 (31) 94(49) >99 (17) >99(47)

c

0.48 0.47 0.12 0.43 0.37 0.41 0.19

14 9.4 0.25 1.8 0.77 1.6 0.11

>640 >580 230 >450 >360 65 >250

0.50

17

>1000

Novozym43 5: Candida antarctica\ QL: Alcaligenes sp.; PS: Pseudomonas cepacia (Amano)

b )

Isolated yield

c )

Relative Rate: %conv./ieaction time (h).

See Ref. 13.


d)

255 Lipase PS is well respected and one of the most widely used enzymes applicable for various substrates, however, poor reactivity was obtained when commercial lipase PS was used for the acylation of the mandelic acid methyl ester (±)-3, though desired acetate 4 was obtained in optically pure form as shown in Table 2 (Entry 1). We found that commercial lipase PS gradually lost its activity in the [bmim]PF solvent system, so, because this lipase is immobilized by Celite, we next attempted to improve the reactivity by changing the supporting materials in this solvent system (Eq. 2). The results were strongly dependent on the supporting materials and the reaction rate was drastically improved when Toyonite 200M immobilized lipase PS was used for the reaction (Entry 2). Enantioselectivity was also modified by the supporting materials. The best enantioselectivity was recorded for methacryoxypropyl SBA-15 (Entry 6). On the contrary, the reaction rate was significantly reduced for Toyonite 200A and aminopropyl SBA-15 (Entries 4 and 7). A reduced enantioselectivity was obtained when lipase PS was immobilized by Toyonite 200 (Entry 5). 6


9

1 0

9

1 0

9

OAc

OH 1

Or(±)-3

roô

Vinyl acetate

COOMe

COOMe (2)

Immobilized Lipase PS [bmim]PF

6

(fVOOOMe

Table 2. Evaluation of supporting materials for lipase PS-catalyzed transesterification in [bmim]PF solvent system 6

Time (h)

Entry Supporting material l

Celite

2

Toyonite 200M

%conv. Relative Ε value Rate f%Yield) %eeof4

72

>99 (3)

5

3

Toyonite 200P

48

4

Toyonite 200A

168

5

Toyonite 200

24

b

a

9.0

0.13

97 (20)

13

2.60

80

>99 (10)

20

0.41

>250

>99 (9)

0.4

0.002

>200

80 (12)

7

0.29

10 >260 >250

6

Methacryoxypropyl SBA-l5 48

>99 (14)

22

0.46

7

Am inopropy I S Β A-15

168

>99 (10)

20

0.12

Isolated yield.

b )

>220

c)

Relative Rate:%conv/time(h). R e f 13.


c

256 It was established that proper immobilization of the enzyme makes it possible to extend its applicability, though it has not yet clear how the origin of these supporting materials affects enzyme reactivity. Since it was anticipated that lipase might be anchored by the ionic liquid solvent and might remain in it after the extraction work-up of the products, we next evaluated the repeated use of Novozym435 in the [bmim]PF solvent system (Figure 1). A mixture of the substrate, lipase, and vinyl acetate in the [bmim]PF solvent was stirred for 3 h at rt, and then ether was added to the reaction mixture to form the biphasic state. The desired products and unreacted alcohol were quantitatively extracted from the ether (upper layer). To the remaining ionic liquid phase, which was placed under reduced pressure for 15 minutes to remove the ether, a mixture of the substrate and vinyl acetate was again added. This mixture was stirred at it. As expected, the acylation reaction took place smoothly and the product was obtained without any loss in enantioselectivity. It was thus confirmed that the enzyme was, in fact, anchored in the ionic liquid solvent after the work-up process. By repeating the same process, we showed that recycling of the enzyme was indeed possible in our ionic liquid solvent system, though the reaction rate gradually dropped with repetition of the reaction process (Figure 2). 6


6

4

Figure 1. Lipase-catalyzed reaction system anchored to the solvent.



257

Figure 2. Results of the repeated use of lipase in the [bmim]PF solvent system 6

2. Lipase-catalyzed reaction under reduced pressure in an ionic liquid solvent system. The repeated use of lipase in [bmim]PF solvent system has been obtained. However, as stated, the reaction rate gradually dropped with repetition of the reaction process. This drop in reactivity was assumed to be caused by the inhibitory action of the acetaldehyde oligomer that accumulated in the solvent system based on a Ή NMR analysis. One of the most important characteristics of ionic liquids is their wide temperature range for liquid phase, and ionic liquids have no vapor pressure; we therefore decided to look at the lipase-catalyzed reaction under reduced pressure conditions in the [bmim]PF solvent system. We found that the transesterification proceeds efficiently under reduced pressure in an ionic liquid solvent system using methyl esters as acyl donors. 6

4

6

11


258 To realize the transesterification under reduced pressure conditions, the proper choice of the acyl donor ester is very important; it is essential to use an acyl donor ester which has a sufficiently higher boiling temperature than the corresponding alcohol which is produced by transesterification with the substrate ester. Thus, methyl esters seem appropriate as the acyl donors for the lipase-catalyzed transesterification under reduced pressure. This is true even though ordinary methyl esters are recognized as not being suitable for the lipase-catalyzed transesterification as acyl donors because reverse reaction with produced methanol easily takes place. However, we would be able to avoid such a difficulty when the reaction was carried out under reduced pressure even if the methyl esters were used as an acyl donor, because the produced methanol would be immediately removed from the reaction mixture and thus the reaction equilibrium would shift to produce the desired product (Eq 3). Thus several types of methyl esters were evaluated as acyl donors for the lipase-catalyzed reaction (Table 3). We tested the transesterification of (±)-l as a model substrate using methyl pentanoate as the acyl donor at 100 Torr and 27 °C. However, the reaction rate was very slow and only 6 % of the product 2b (R=n-C H ) was obtained with poor enantioselectivity after 48 h reaction using 1.5 equiv. of methyl pentanoate as the acyl donor (Entry 1, Table 3). Fortunately, the desired reaction was efficiently accomplished when the reaction was carried out using methyl nonanoate as the acyl donor at 100 Torr and 32 °C (Entry 2); transesterification proceeded very smoothly and the desired ester 2c (R=n-C Hi ) was obtained with >99% ee. Other methyl esters can also be used as acyl donors for these reactions. Methyl phenoxyacetate, methyl methylthioacetate, and methyl phenylthioacetate also worked very well, and the esters 2d (R= CH OPh ), 2e (R= CH SMe), and 2f (R= CH SPh) were obtained with perfect enantiomeric excess, respectively (Entries 3 to 5). It was very easy to monitor the reaction course by silica gel thin layer chromatography (TLC) when phenoxyacetate or phenylthioacetate was used as the acyl donor. It was also possible to reduce the amount of the acyl donor to 0.5 equiv. versus the substrate alcohol when these esters were used in the reaction; this is the least recorded amount of an acyl donor used in this type of lipase-catalyzed transesterification (Entry 6). Due to the large difference in boiling points between methyl phenylthioacetate and methanol, this ester was indeed useful for the lipase recycling system. The transesterification smoothly took place under reduced pressure at 20 Torr and 40 °C when 0.5 equivalent of methyl phenylthioacetate was used as the acyl donor, and we were able to obtain an ester 2f in optically pure form. As shown in Figure 3, five repetitions using this process showed no drop in the reaction rate.


5

4

8

7

12

12

12

2

2

2


9

259

R (±)-l

OMe

Λ

MeOH

2

Novozym435 (50wt%) [bmim]PF 40°C, 20-100 Torr

(3)


6

Table 3. Lipase-catalyzed enantioselective transesterification under reduced pressure conditions in [bmim]PF solvent system 6

Entry

Acyl donor R

1

a)

C4H9

8

Time %ee of 2 /h (Yield/%)

Conv. Relative Ε value /c Rate

48

37(6)

0.15

0.3

2

2

n-QH

I 7

5

>99(43)

0.42

8.4

>420

3

PhOCH

9

>99(35)

0.42

4.7

>470

4

MeOCH

29

>99(30)

0.33

1.1

>580

5

PhSCH

13

>99(30)

0.46

3.5

>530

6

PhSCH

12

>99(30)

0.45

3.8

>500

2

2

2

e 2

b)

d

c

b

c

1.5 equiv. to the substrate. Isolated yield * Relative Rate: %conv./ reaction d)

e)

time (h). Ref. 13. 0.5 equiv. to the substrate.

Conclusions We demonstrated the lipase-catalyzed enantioselective transesterification of an allylic alcohol in the [bmim]PF solvent system under reduced pressure conditions and showed that it was possible to repeatedly use the enzyme in this system. It is assumed that a good acyl donor must be selected depending on the substrate. We do believe, however, that this might be a very important means of lipase-catalyzed enantioselective acylation in the ionic liquid solvent system. Further investigation of the scope and limitations of this reaction, especially optimization of the reaction conditions for the lipase recycling system in the ionic solvent system, will make it even more beneficial. 6



260

Figure 3. Results of the repeated use of lipase in the [bmim]PF solvent system under reduced pressure conditions

6

Acknowledgments The authors are grateful to Professor Tomoya Kitazume at the Tokyo Institute of Technology for his helpful discussions throughout this study. They also thank Novo Nordisk Bioindustry Co., Ltd., Meito Sangyo Co., Ltd., and Amano Pharmaceutical Co., Ltd. for providing the lipases. The authors are grateful to Mr. Masanobu Kamori of Toyodenka Co., Ltd. for providing Toyonite.


261


References 1. Reviews see: a) Wong C. H.; Whitesides, G. M. Enzymes in Synthetic Organic Chemistry, Tetrahedron Organic Chemistry Series, Vol. 12, ed. by J. E. Baldwin and P. D. Magnus, Pergamon (1994). b) Theil, F. Chem. Rev. 1995, 95, 2203. c) Itoh, T.; Takagi, Y. Tsukube, H. Trends in Organic Chemistry, 1997, 6, 1. d) Theil, F. Tetrahedron, 2000, 56, 2905. 2. Klibanov, A. M. Acc. Chem. Res. 1990, 23, 114. 3. A review, see : Welton, T. Chem. Rev. 1999, 99, 2071. 4. Itoh, T.; Akasaki, E.; Kudo, K.; Shirakami, S. Chem. Lett., 2001, 262. 5. Itoh, T.; Akasaki, E.; Nishimura, Y.Chem. Lett. 2002, 154. 6. For examples of the enzymatic reactions in an ionic liquid solvent system, see. a) Cull, S. G.; Holbrey, J. D.; Vargas-More, V.; Seddon, K. R.; Lye, G. J. Biotechnol. Bioeng. 2000, 69, 227. b) Erbeldinger, M . ; Mesiano, A. J.; Russell, A. J. Biotechnol. Prog. 2000, 16, 1131. c) Lau, M. R.; Rantwijk, F. v.; Seddon, K. R.; Sheldon, R. A. Org. Lett. 2000, 2, 4189. d) Schöfer, S. H.; Kaftzik, Ν.; Wasserscheid, P.; Kragl, U. Chem. Commun. 2001, 425. e) Kim, K-W; Song, B.; Choi, M-Y.; Kim, M-J. Org. Lett. 2001, 3, 1509. f) Howarth, J. James, P.; Dai, J. Tetrahedron Lett. 2001, 42, 7517. g) Park, S.; Kazlauskas, R. J. J. Org. Chem. 2001, 66, 8395. h) Lozano, P.; Diego, T. de.; Carri , D.; Vaultier, M.; Iborra, J. L. Chem. Commun. 2002, 692. h) Nara, S. J.; Harjani, J. R.; Salunkhe, M . M. Tetarhedron Lett. 2002, 43, 2979. 7. Takagi, Y.; Nakatani, T.; Itoh, T.; Oshiki, T. Tetrahedron Lett. 2000, 41, 7889. 8. a) [bmim]BF : Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; Souza, R. F. de.; Dupont, J. Polyhedron, 1996, 15, 1217. b)[bmim]PF : Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. c) [bmim]TFA: The synthesis of this salt was similar to that of [bmim]PF with the exception that CF COONa was used in place of NaPF . d) [bmim]OTf: Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gr tzel, M. Inorg. Chem. 1996, 35, 1168. e) [bmim]SbF : Song, C. E.; Oh, C. R.; Roh, E. J.; Choo, D. J. Chem. Commun. 2000, 1743. 9. Toyonite is a porous ceramics prepared from a kaolinite: Toyodenka Co., Ltd. Phone: +81-888-31-1241. E-mail: [email protected] 10. Zhao, D.; Feng, J.; Stucky, G. D. Science, 1998, 279, 548. 11. Examples of lipase-catalyzed reaction under reduced pressure conditions, see. a) Haraldsson, G. G.; Gudmundsson, B.Ö.;Almarsson, Ö. Tetrahedron Lett. 1993, 34, 5791. b) Haraldsson, G. G.; Thorarensen, A. Tetrahedron Lett. 1994, 35, 7681. c) Sugai, T.; Takizawa, M.; Bakke, M.; Ohtsuka, Y.; Ohta, H. Biosci. Biotech. Biochem. 1996, 60, 2059. d) Cordova Α.; Janda, K. D. J. Org. Chem. 2001, 66, 1906. 12. Itoh, T.; Takagi,Y.;Nishiyama, S. J. Org. Chem. 1991, 56, 1521. 13. Chen, C. -S.; Fujimoto, Y.; Girdauskas, G.; Sih, C. J. J. Am. Chem. Soc. 1982, 102, 7294. 4

6

6

3

6

6


Ionic Liquids as Green Solvents - American Chemical Society

Recommend Documents