Ionic Liquids as Green Solvents - American Chemical Society

Chapter 19

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.ch019

Ionic Liquids Create New Opportunities for Nonaqueous Biocatalysis with Polar Substrates: Acylation of Glucose and Ascorbic Acid 1

2

2

Seongsoon Park , Fredrik Viklund , Karl Hult , and Romas J . Kazlauskas * 1,2,

1

Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada Royal Institute of Technology (KTH), Department of Biotechnology, AlbaNova University Centre, SE-106 91 Stockholm, Sweden 2

Lipase-catalyzed reactions of polar substrates are inefficient in organic solvents. Nonpolar organic solvents do not dissolve polar substrates, while polar organic solvents inactivate lipases. Ionic liquids such as 1-alkyl-3-methyl imidazolium tetrafluoroborate are as polar as N-methyl formamide or methanol, but, unlike these solvents, ionic liquids do not inactivate lipases. This unusual feature creates opportunities for nonaqueous biocatalysis with polar substrates. First, we describe a simple purification involving filtration through silica gel, which yields ionic liquids that work reliably as solvents in lipase-catalyzed reactions. Next, we report two examples that exploit these unique advantages of ionic liquids. First, lipase-catalyzedacetylation of glucose was up to twelve times more regioselective in ionic liquids than in acetone. Second, lipase catalyzed the acylation of ascorbic acid to make fat-soluble antioxidants. In some cases, reactions in ionic liquids were comparable or slower than in tert-amyl alcohol, but in typical cases, the reactions in ionic liquids were twice as fast and proceeded to higher conversion. Ionic liquids also offer the possibility to use vacuum to remove water formed by the esterification and drive the equilibrium even further toward product.

© 2003 American Chemical Society

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

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Introduction A long-standing problem in biocatalysis is reactions of polar substrates un der nonaqueous conditions. Reactions such as acylation of an alcohol require nonaqueous conditions to avoid competing hydrolysis. However, polar substrates such as sugars dissolve only in the most polar organic solvents such as dimethylsulfoxide. Unfortunately enzymes such as lipases inactivate in such polar organic solvents. Current solutions involve compromises. In some cases, researchers use moderately polar organic solvents, where the substrate dissolves slightly and the enzymes retain some activity. Such reactions are usually too slow for preparative use. Another alternative is to modify the substrates (e.g., use an alkyl glycoside instead of a glycoside) and use a less polar organic solvent where the enzyme remains active. However, this approach yields an analog of the desired product. In spite of these difficulties, the ability to catalyze reactions on polar sub strates in nonaqueous media is becoming increasingly important. Natural build ing blocks - peptides, sugars, nucleotides, biochemical intermediates - are im portant starting materials for pharmaceuticals, fine chemicals and materials. These building blocks are becoming increasingly important in a bio-based econ omy, where chemicals and materials come from plants and microorganisms. This paper focuses on room temperature ionic liquids (i) as a solution to biocatalysis reactions with polar substrates under nonaqueous conditions. Ionic liquids are polar solvents (comparable to methanol) and readily dissolve polar substrates. However, for reasons that are still not clear, ionic liquids do not dena ture lipases, as would an organic solvent of comparable polarity. For this reason lipase-catalyzed reactions of polar substrates proceed more efficiently or more selectively in ionic liquids. Several groups have reported enzyme-catalyzed reac tions in ionic liquids (2-5). The advantages of using ionic liquids over an organic solvent varied for each case and included increased enantioselectivity (J), in creased stability of the enzyme (4) or increased molecular weight of the product polymer (5). Here we focus on advantages related to reactions of polar sub strates. The first example is the acetylation of glucose with vinyl acetate catalyzed by lipase Β from Candida antarctica (CAL-B) (6). This acetylation is more re gioselective in ionic liquids than in moderately polar organic solvents such as tetrahydrofuran. This increased regioselectivity yields only 6-0-acetyl D-glucose instead of a mixture of 6-0-acetyl- and 3,6-0-diacetyl D-glucose. The increased solubility of glucose relative to 6-O-acetyl D-glucose in ionic liquids accounts for the increased regioselectivity. The second example is the CAL-B-catalyzed acylation of L-ascorbic acid (vitamin C) with unactivated fatty acids to make fat-soluble antioxidants, such as 6-0-ascorbyl palmitate or 6-0-ascorbyl oleate. Reaction of the polar L-ascorbic acid with nonpolar fatty acids proceeds to higher conversion in ionic liquids than in an organic solvent (teri-amyl alcohol). In addition, since ionic liquids are non volatile, they offer the possibility of using vacuum to remove water and shift the equilibrium of the reaction more toward product formation.



Results Ionic liquids, prepared either by literature procedures (7) or straightforward modifications, did not work reliably as solvents for lipase-catalyzed reactions. In some cases, reactions proceeded well; in other cases reactions proceeded slowly or not at all. Since the structures of the ionic liquids were similar, we suspected that impurities might cause the unpredictable behavior. The synthesis of ionic liquids involved initial preparation of the halide salt followed by exchange of the halide with tetrafluoroborate, Scheme 1. A likely impurity in ionic liquids is the halide salt due to incomplete exchange. Indeed, ionic liquids gave a precipitate with silver nitrate solution, thereby confirming the presence of halide. For this reason, we purified ail ionic liquids to remove halide salts.

Purification of Ionic Liquids Purification involvedfiltrationof the diluted ionic liquid through silica gel to remove traces of 3-alkyl-l-methylimidazolium halide and then washing with saturated aqueous sodium carbonate to remove cloudiness due tofineparticles of silica gel. Finally, drying the solution over magnesium sulfate and removing the diluent (methylene chloride) by vacuum yielded the purified ionic liquid. An alternative purification replaced the sodium carbonate wash and drying with


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magnesium sulfate with a filtration through neutral alumina. This second method avoided traces of the basic carbonate anion in the ionic liquid. Both methods yielded ionic liquids that work reliably in all lipase-catalyzed reactions that we tested. These procedures yielded six 3-alkyl-l-methylimidazolium tetrafluorobo rate ionic liquids, Scheme 1.

NaBF ΛN-- * X' νθ BF H3C hite solid H3C ionic liquid (X = CI or Br) jpurify

R-X

4

7

4

HC 3

Β Et /hPr rt-Bu 5-Bu MeOCH CH 2-pentyl 2

Β H H Me Me 1

2

W

Abbreviation EMIM«BF PMIM «BF nti through BMIM-BF neutral alumina sBMIM-BF MOEMIM-BF 2PentMIM*BF 4

4

o r

4

4

er

1 ) dilute w/CH CI 2) filter through silica gel 3) wash with safd. NaCC>3 4) dry over MgS0 5) evaporate CH CI 2

2

2

4

2

2

4

4

B Abbreviation n-Pr PPYR«BF ihBu BPYR-BF t>Pr PNPYR-BF n-Bu BMPYR»BF 2

4

4

4

4

Scheme 1. Preparation and purification of 3-alkyl-l-methylimidazolium tetrafluoroborate ionic liquids for biocatalysis. Similar methods yielded the related N-alkylpyridinium tetrafluoroborate ionic liquids.

Similar reactions and purifications gave five other ionic liquids: one hexafluorophosphate salt, 3-n-butyl-l-methylimidazolium hexafluorophosphate, and the four based on pyridinium and 4-methylpyridinium cations shown below.

Polarity of Ionic Liquids is Similar to that for Polar Organic Solvents To compare the polarity of ionic liquids and organic solvents, we measured their polarities with Reichardt's dye (8). Polar solvents stabilize the polar ground state of Reichardt's dye thereby shifting its color to shorter wavelengths. We



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Ο

0.2 0.4 0.6 polarity ( Ε ρ Reichardt's scale)

Τ 0.8 methanol, 2-chloroethanoî

Figure 1. The conversion for Pseudomonas-cepacia-///?aje catalyzed acetylation of racemic 1-phenylethanol with vinyl acetate decreased in polar organic solvents, but remained high for ionic liquids in spite of their highly polar nature. This reaction is highly enantioselective, so the maximum conversion is 50%. Th trend lines are not afitto theory, but only to guide the eye.

compared the polarities of the different solvents using Reichardt's normalized scale where tetramethylsilane has a value of zero and water has a value of one. The polarity values for the ionic liquids we used ranged from 0.63 to 0.71 with the most polar being EMIM«BF4 and the least polar being BMPYR»BF4, see xaxis of Figure 1. BMIM»BF4 is more polar that BMIM^PF^ Organic solvents with polarities similar to that of the ionic liquids include: methanol, 2-chloroethanol, N-methyl formamide, diethylene glycol and 1,2propanediol. Most of these are hydroxylic solvents, which are not suitable for acylation reactions since the solvent would compete with the substrate for the acyl group. Others also measured the polarity of ionic liquids using another solvatochromic dye, Nile Red (9) or using fluorescent probes (10). Although only a few


230 ionic liquids are the same as the ones we measured, the relative ranking of the polarities is the same.


High Activity of Lipases in Ionic Liquids in Spite of their High Polarity Lipases showed good activity in ionic liquids even though they showed no, or little, activity in normal organic solvents with similar polarities. As a model reaction, we used the acetylation of racemic 1-phenylethanol with vinyl acetate catalyzed by lipase from Pseudomonas cepacia, PCL, equation 1. This reaction is highly enantioselective so the maximum conversion was 50%. We compared the rates of reaction and enantioselectivities in ionic solvents to those in normal OH

OAc

Λο*

OH

- χ τ ^0* ^

w

organic solvents. In all cases, the enantioselectivity of the reaction remained high: Ε >200. Surprisingly, the conversion after 24 h also remained high in ionic liquids despite their high polarity, Figure 1 above. For normal organic solvents, the acetylation reaction proceeds well in nonpolar solvent, but not in polar solvents. The reaction is nearly complete in tolu ene, partially complete in tetrahydrofuran (THF), acetone or acetonitrile (ACN), but proceeds very slowly or not at all in the more polar N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), or Λ^-methylformamide. Although the ionic liquids are highly polar (similar to N-methyl-formamide), the acetylation reaction proceeds well in all ionic liquids tested. The reaction is nearly complete in EMIM*BF and MOEMIM*BF and partially complete in all other ionic liquids. In addition, the trend for ionic liquids is for higher degrees of conversion as the polarity of the ionic liquid increases, while the trend for organic solvents is the opposite - lower degrees of conversion as the polarity increases. Since the substrates for this model reaction dissolve in both nonpolar or ganic solvent and in ionic liquids, there is no obvious advantage to using ionic liquids in this case. 4

4

More Regioselective Acylation of Glucose in Ionic Liquids Since ionic liquids are polar solvents that do not denature lipases, they may be ideal for lipase-catalyzed transformations of polar substrates. As afirstexam ple, we examined the lipase-catalyzed 6-O-acetylation of glucose catalyzed by lipase Β from Candida antarctica (CAL-B), equation 2, Table I.



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In organic solvents such as acetone and tetrahydrofuran (THF), the 6-0acetylation reaction proceeded along with further acetylation of the 3-position. In acetone, acetylated products formed in 72% yield, of which 76% was the desired 6-0-acetyl compound (-3:1 selectivity). In THF, glucose reacted completely, but only 53% was the desired 6-0-acetyl compound (-2:1 selectivity). Even at a lower extent of conversion, the regioselectivity remained low. In acetone at 42% conversion, 82% was the 6-0-acetyl compound (-5:1 selectivity), while in THF at 50% conversion, 85% was the 6-0-acetyl compound (-6:1 selectivity). The low selectivity is likely related to the poor solubility of glucose in these organic solvents (0.02-0.04 mg/mL at 60 °C (i/)). Glucose remains a suspended solid and the initial 6-O-acetylation yields a more soluble compound, which then un dergoes further acetylation to the 3,6-O-diacetyl derivative. 11

Table I. Regioselective CAL-B-Catalyzed Acetylation of Glucose Final composition of reaction mixture Solvent

D-Glucose, %

EMIM*BF MOEMIM»BF PMIM*BF BMIM»BF sBMIM»BF BMIM*PF BPYR*BF PPYR*BF Acetone THF 4

4

4

4

4

6

4

4

49.6 0.0 72.2 22.4 9.8 70.5 58.0 56.4 27.7 0.0

6-0-

3,6-0-

Acetyl-D- Diacetyl-DGlucose, % Glucose, %

50.3 93.0 27.8 68.9 79.2 11.3 37.3 38.6 55.0 52.6

0.0 6.9 0.0 8.7 10.8 18.1 4.7 5.0 17.4 47.4

%

Monoacylation, %

50.4 99.9 27.8 77.6 90.1 29.5 42.0 43.6 72.3 99.9

99.9 93.1 99.8 88.8 87.9 38.5 88.7 88.4 76.1 52.6

Conversion

Conditions: 0.5 mmol β-D-glucose, 1 mmol vinyl acetate, 1 mL solvent, 30 mg Novozyme SP435, 36 h, 55 °C, Data from reference 6. After the reaction, both remaining glucose and the acetylated products were a mixture of anomers. The conversion was measured by gas chromatography after derivatization with chlorotrimethylsilane and 1,1,1,3,3,3-hexamethyldisilazane (12). The acylation positions were determined by COSY experiments.


232 On the other hand, acetylation of glucose proceeded with much higher selectivity for monoacetylation in ionic liquids than in organic solvents. In the seven ionic liquids containing a tetrafluoroborate anion, the 6-O-acetylation proceeded with 42-99% conversion, of which 88-99% was the desired 6-O-acetyl glucose (-7:1 to -100:1 selectivity). The best ionic liquid was MOEMIM*BF , where all the glucose was acetylated and 93% was the desired 6-O-acetyl compound (-13:1 selectivity). The one ionic liquid with a hexafluorophosphate anion, BMIM»PF , showed both slow reaction (29% conversion) and low selectivity (39% monoacetyl, -0.6:1 selectivity). The higher solubility of glucose in ionic liquids correlates with the higher regioselectivity in these solvents. Approximately 100 times more glucose dissolves in the best ionic liquid, MOEMIM*BF -5 mg/mL at 55 °C, than in acetone or THF. On the other hand, glucose is not very soluble in the worst ionic liquid, BMIM«PF ,

Ionic Liquids as Green Solvents - American Chemical Society

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