Lipase-Catalyzed Reactions in Ionic Liquids

The first attempt to use ILs as a reaction medium was in 2000 using Candida antarc-tica lipase in [bmim][PFg] and [bmimJIBFJ (Madeira Lau et al., 2000). Since then the use of ILs in enzyme reactions has grown rapidly. Generally, two parameters are always considered when selecting a suitable IL (1) the solubility of the reaction substrates and products in the IL, and (2) the interaction between the reaction medium and substrates. In addition, the effect of the ILs on the activity and stability of the lipase, either in free or immobilized form, should be considered. 

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Cull, S. G. Holbrey, J. D. Vargas-Mora, V. et al. Room-temperature ionic liquids as replacements for organic solvents in multiphase bioprocess operations, BiotechnoL Bioeng., 2000, 69(2), 227-233 Lau, R. M. van Rantwijk, F. Seddon, K. R. Sheldon, R. A. Lipase-catalyzed reactions in ionic liquids, Org. Lett., 2000, 2(26), 4189-4191. 

Figure 14.9 Lipase catalyzed reactions in ionic liquids.

Madeira Lau R, van Rantwijk F, Seddon KR, Sheldon RA (2000) Lipase-catalyzed reactions in ionic liquids. Org Lett 2 4189-4191... 

Another advantage for the performance of lipase-catalyzed reactions in ionic liquids as compared to the reaction in organic solvents is an enhancement of... 

Lalonde JJ, Govardhan C, Khalaf N et al. (1995) Crystals of Candida rugosa lipase highly efficient catalyst forthe resolution of chiral esters. J Am Chem Soc 117 6845-6849 Lau RM, Rantwiik EV, Seddon KR et al. (2000) Lipase-catalyzed reactions in ionic liquids. Org Lett 2 4189-4192... 

Madeira Lau, R., F. Van Rantwijk, K. R. Seddon, and R. A. Sheldon. 2000. Lipase-Catalyzed Reactions in Ionic Liquids. Organic Letters 2 (26) 4189-4191. 

Furusawa, M. Kishimoto, N. (2010). Enzymatic synthesis of caffeic acid phenethyl ester analogues in ionic liquid. /. Biotechnol, 148, 2-3,133-8 Kurata, A. Takemoto, S. Fujita, T. Iwai, K. Furusawa, M. Kishimoto, N. (2011). Synthesis of 3-cyclohexylpropyl caffeate from 5-caffeoylquinic acid with consecutive enzymatic conversions in ionic liquid. /. Mol Catal. B Enzyme., 69,161-167 Lau, R.M. van Rantwijk, F. Seddon, KR Sheldon, R.A. (2000). Lipase-catalyzed reactions in ionic liquids. Org. Lett, 2, 26,4189-4191... 

The use of ionic liquids (ILs) to replace organic or aqueous solvents in biocatalysis processes has recently gained much attention and great progress has been accomplished in this area lipase-catalyzed reactions in an IL solvent system have now been established and several examples of biotransformation in this novel reaction medium have also been reported. Recent developments in the application of ILs as solvents in enzymatic reactions are reviewed. 

LIPASE-CATALYZED REACTION IN AN IONIC LIQUID SOLVENT SYSTEM... 

In the last few years increasing attention has been devoted to conducting bio-catalytic transformations in ionic liquids [104-107]. The first report of enzyme-(lipase-) catalyzed reactions in water-free ionic liquids dates from 2000 and involved transesterification, ammoniolysis and perhydrolysis reactions catalyzed by Candida antarctica lipase B (Fig. 7.32) [108]. 

It is clear that the water activity is of crucial importance for the equilibrium yield in a reversed hydrolysis reaction. As expected, the equilibrium yield increases with decreasing water activity. This has been shown, for example, for the condensation of glucose and octanol [62], esterification of lysophospholipids with fatty acids [29, 63], and in normal lipase-catalyzed esterification reactions [64, 65]. The same situation is observed in ionic liquids [66]. 

Proteases have been much less studied than lipases in ionic liquid media and generally require the presence of water for activity. We note that the thermolysin-catalyzed amide coupling of benzoxycarbonyl-L-aspartate and L-phenylalanine methyl ester into Z-aspartame in [BMIm][PF6] was an early example of an enzymatic reaction in an ionic liquid medium [8]. 

Some groups have reported on their search for less reactive acylating agents, to suppress noncatalyzed chemical acylation and increase product enantiomeric excess. Irimescu and Kato carried out an enantioselective lipase catalyzed acylation of 1 phenylethylamine and 2 phenyl 1 propylamine by reacting the amines with carbox ylic acids in a nonsolvent system or in ionic liquids (Figure 14.9). The reaction equilibrium was shifted toward amide synthesis by the continuous removal of the... 

Roxana I, Katsuya K (2004) Lipase-catalyzed enantioselective reaction of amines with carboxylic acids under reduced pressure in non-solvent system and in ionic liquids. Tetrahedron Lett 45 523-525... 

Based on their work on supercritical CO2 (see also Scheme 23), Reetz and Leitner introduced a technologically new and interesting continuous flow process for enzymatic reactions [65]. The group designed a protocol for enzymatic reactions, namely the lipase-catalyzed acylation (CAL B) of octan-1 -ol by vinyl acetate in ionic liquids (l-butyl-3-methylimidazolium bis(trifluo-romethanesulfonimide) [BMIM] [BTA]) using supercritical CO2 as the mobile phase (Scheme 25). The alcohol is pumped through the biphasic system and the products are obtained in solvent-free form in a cold trap. The enzyme/ionic liquid mixture can be recycled in batchwise or continuous flow operations. 

Ha SH, Lan MN, Lee SH et al. (2007) Lipase-catalyzed biodiesel production from soybean oil in ionic liquids. Enzyme Microb Technol 41 480 83 Hari Krishna S, Karanth NG (2002) Lipases and lipase-catalyzed esterification reactions in non-aqueous media. Catal Rev 44(4) 499-591... 

Reacting lipophilic substrates with hydrophilic compounds, as in the case of most transesteriflcation reactions, is one of the major difficulties in lipase-catalyzed reactions. Several parameters need to be considered to overcome this immiscibility problem. One commonly proposed strategy is the use of a nonaqueous medium. In this chapter, the advantages of using nonaqueous media in biochemical synthesis reactions, over aqueous and solvent-free systems, are discussed. The use of hydrophobic solvents is also discussed, followed by a presentation of the alternatives that can overcome the limitations of solvents. The focus of this chapter is mainly on the use of supercritical fluids (SCFs) as a green alternative reaction medium. The chapter also discusses ionic liquids (ILs) as another alternative. These solvents and the factors affecting their physical properties and their effect on the activity and stability of lipase are also discussed. 

Guo, Z. and Xu, X. (2006) Lipase-catalyzed glycerolysis of fats and oils in ionic liquids a further study on the reaction system. Green Chem., 8, 54-62. 

In a recent review, some positive attributes of ionic liquids in biocatalysis were discussed 273). An example was given, which compares the enzymatic performance of Pseudomonas cepacia lipase (PCL)-catalyzed reactions as a function of the solvent polarity in both organic and ionic solvents, as shown in Fig. 17. The PCL shows no activity in organic solvents in the polarity range of the ionic liquids, but it is active in the ionic liquids. 

Fatty acids of sugars are potentially useful and fully green nonionic surfactants, but the lipase-mediated esterification of carbohydrates is hampered by the low solubility of carbohydrates in reaction media that support lipase catalysis in general. Because the monoacylated product (Figure 10.8) is more soluble in traditional solvents than is the starting compound, the former tends to undergo further acylation into a diester. In contrast, the CaLB-catalyzed esterification of glucose with vinyl acetate in the ionic liquid [EMIm][BF4] was completely selective. The reaction became much faster and somewhat less selective when conducted in... 

The resolution of chiral amines via lipase-catalyzed enantioselective acylation is now a major industrial process, but interest in adopting ionic liquid reaction media has been surprisingly scant. Interestingly, acids could be used as the acyl donor (Figure 10.15) rather than the usual activated ester in a range ofionic liquids. CaLB was employed as the biocatalyst, and water was removed to shift the equilibrium toward the product [130, 131]. The highest rates were found in [BMMIm][TfO], [EMIm][TfO], and [EMIm][BF4]. 

The continuous reaction system could be combined with solid acid-catalyzed in situ racemization of the slow-reacting alcohol enantiomer [149]. The racemiza-tion catalyst and the lipase (Novozym 435) were coated with ionic liquid and kept physically separate in the reaction vessel. Another variation on this theme, which has yet to be used in combination with biocatalysis, involves the use of scC02 as an anti-solvent in a pressure-dependent miscibility switch [150]. 

Hydrolytic enzymes such as lipases catalyze hydrolysis of esters in aqueous media, but when used in non-aqueous media such as organic solvents, ionic liquids and supercritical fluids, they catalyze reverse reactions the synthesis of esters. For example, lipases in natural environment catalyze the hydrolysis of fatty acid esters as shown in Figure 6(a). However, when they are used in organic solvents, they catalyze the esterification reaction (Figure 6(b)). 

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