Lipases in Ionic Liquids

Entry Biocatalyst Ionic liquid Reaction system Ref. 

14 Screening of eight lipases and two esterases 10 different ionic liquids Kinetic resolution of (R,S)-l-phenylethanol 44 

16 Lipase [EMIMKBF4] Synthesis of butyl butyrate by transesterification 46 

17 Lipases [BMIM PEj] Kinetic resolution of allylic alcohols 47 

The majority of enzymes reported so far to be active in ionic liquids are lipases, the work horses of biocatalysis [11]. Designed by nature to work at aqueous/organic interfaces for the cleavage of fats and oils, making the cleavage products accessible as nutrients, lipases in general tolerate and are active in pure organic solvents. This concept has been pioneered by Klibanov and coworkers [17,55]. 

The work from Sheldon and coworkers vras the second publication demonstrating the potential use of enzymes in ionic liquids and the first on lipases [56]. They compared the reactivity of Candida antarctica lipase in ionic liquids such as [BMIM][PFe] and [BMIM][Bp4] vnth the reactivity in conventional organic solvents. In all cases the reaction rates were similar for all the reactions investigated alcoholysis, ammo-niolysis and perhydrolysis. 

One of the novel properties exhibited by enzymes in ionic liquids, as compared to aqueous solution, is their altered enzyme specificity, such as enhanced regioselecti-vity and enantioselectivity. Examples of increased regiosdectivity in Hpase atalyzed reactions are given in the following section. 

Nara et al. investigated the formation of 4,6-di-O-acetyl-D-glucal by Pseudomonas cepacia and the dependence of the reaction rate and regiosdectivity on the solvent 

83% conversion and 65% yield of ascorbyl oleate were obtained for the esterification studied by Park et al. The synthesis of this tty acid ester of t-ascorbic acid catalyzed by Candida arctica lipase B was highly regioselective. The reaction was carried out in a specially synthesized ionic liquid, [PentMIM][BF4], to increase solubility of the acid. Due to low solubility the ester produced inhibits the reaction by precipitation on the surface of the immobilized lipase. To avoid precipitation, hexane or porous propylene, which are more hydrophobic than the resin, were added [58]. 

Further studies of Pseudomonas sp. lipase revealed a strong influence of the water content of the reaction medium (Entry 20) [48]. To be able to compare the enzyme activity and selectivity as a function of the water present in solvents of different polarities, it is necessary to use the water activity (a ) in these solvents. We used the 

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Whole-cell Systems and Enzymes other than Lipases in Ionic Liquids... 

Table 8.3-1 Whole-cell systems and enzymes other than lipases in ionic liquids. 

The high thermostability of lipases in ionic liquids has stimulated research into kinetic resolutions at elevated temperatures [129]. The PsL-mediated acylation of 1-phenylethanol by vinyl acetate in [BMIm][ Tf2N] remained highly... 

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]. 

Li XF, Lou WY, Smith TJ et al (2006) Efficient regioselective acylation of 7-)3-D-arabinofura-nosylcytosine catalyzed by lipase in ionic liquid containing systems. Green Chem 8 538-544... 

The lipase-catalysed enantioselective acylation of allylic alcohols in an ionic liquid solvent was demonstrated by Itoh et al. [16] (Fig. 7.7). They found that the acylation rate was strongly dependent on the counter anion of the imidazolium salt, while the lipase-catalysed acylation proceeded with high enantioseleclivity in all ionic liquid tested. Good results were obtained when the reaction was carried out in [bmimT [PFg ] or [bmun" ][BF ]. Other examples of kinetic resolution of allylic alcohols catalysed by lipases in ionic liquids were also reported by these authors [71, 72]. The transesterification of 5-phenyl-l-penten-3-ol under reduced pressure at 27 hPa and 40°C was carried out using methyl phenylthioacetate as acyl donor in [bmim+] [PF ] and [bdmim ][BF ], for obtaining the corresponding acylated compound in optically pure form [71], The acetylation of methyl mandelate catalysed by immobilised P5L in [bdmim ][BF ] is another example reported by these authors about the successful application of ionic liquids as reaction media in racemic resolutions... 

Different groups have studied the kinetic resolution of rac-l-phenylethanol by transesterification with vinyl acetate using lipases in ionic liquids as the reaction media [6,17,46] (Fig. 7.8). Schofer et al. [17] screened a set of nine lipases and two esterases for activity in ten different ionic liquids and compared the results with the reaction performed in methyl tert-butyl eter (MTBE) as the solvent. The lipases showed good activity and, in some cases, improved enantioseleclivity in these new reaction media. For example, the enantiomeric excess with PiL and AsL in [bmim ]... 

Ulbert O, Frdter T, Belafi-Bako K, Gubicza L (2004) Enhanced enantioselectivity of Candida rugosa lipase in ionic liquids as compared to organic solvents. J Mol Catal B Enzym 31 39 5... 

Yu H, Wu J, Ching CB (2005) Kinetic resolution of ibuprofen catalyzed by Candida rugosa lipase in ionic liquids. Chirality 17 16-21... 

I 8 Biocatalytic Reactions in Ionic Liquids Table 8-3 Lipases in ionic liquids... 

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