Ionic Liquids as Tool to Improve Enzymatic Organic Synthesis

Review pubs.acs.org/CR

Ionic Liquids as Tool to Improve Enzymatic Organic Synthesis Toshiyuki Itoh*,†,‡ †

Department of Chemistry and Biotechnology, Graduate School of Engineering and ‡Center for Research on Green Sustainable Chemistry, Tottori University, 4-101 Koyama-minami, Tottori 680-8552, Japan ABSTRACT: Ionic liquids (ILs) have now been acknowledged as reaction media for biotransformations. The first three examples were reported in this field in 2000, and since then, numerous applications have been reported for biocatalytic reactions using ILs. Two topics using ILs for enzymatic reactions have been reviewed from the standpoint of biocatalyst mediating organic synthesis; the first is “Biocatalysis in Ionic Liquids” which includes various types of biocatalytic reactions in ILs (section 2): (1) recent examples of lipase-mediated reactions using ILs as reaction media for biodiesel oil production and for sugar ester production, (2) oxidase-catalyzed reactions in ILs, (3) laccase-catalyzed reactions, (4) peroxidase-catalyzed reactions, (4) cytochrome-mediated reactions, (5) microbe-mediated hydrations, (6) protease-catalyzed reactions, (8) whole cell mediated asymmetric reduction of ketones, (10) acylase-catalyzed reactions, (11) glycosylation or cellulase-mediated hydrolysis of polysaccharides, (12) hydroxynitrile lyase-catalyzed reaction, (13) fluorinase or haloalkane dehydrogenase-catalyzed reaction, (14) luciferase-catalyzed reactions, and (15) biocatalytic promiscuity of enzymatic reactions for organic synthesis using ILs. The second is “Enzymes Activated by Ionic Liquids for Organic Synthesis”, particularly describing the finding story of activation of lipases by the coating with a PEG-substituted IL (section 3). The author’s opinion toward “Future Perspectives of Using ILs for Enzymatic Reactions” has also been discussed in section 4.

CONTENTS 1. Introduction 2. Biocatalysis in Ionic Liquids 2.1. Lipase-Catalyzed Reactions in ILs for Organic Synthesis 2.1.1. Recent Examples of Lipase-Mediated Reactions Using ILs as Reaction Media 2.2. Oxidase-Catalyzed Reactions in ILs 2.2.1. Laccase-Catalyzed Reactions 2.2.2. Peroxidase-Catalyzed Reactions in ILs 2.2.3. Cytochrome-Mediated Reactions in ILs 2.3. Microbes-Mediated Hydrations 2.4. Protease-Catalyzed Reactions 2.5. Whole Cell-Mediated Asymmetric Reduction of Ketones 2.6. Acylase-Catalyzed Reactions 2.7. Glycosylation or Cellulase-Mediated Hydrolysis of Polysaccharides 2.8. Hydroxynitrile Lyase-Catalyzed Reaction 2.9. Fluorinase or Haloalkane DehydrogenaseCatalyzed Reaction 2.10. Luciferase-Mediated Reaction 2.11. Biocatalytic Promiscuity of Enzymatic Reactions for Organic Synthesis Using ILs 2.11.1. Lipase-Mediated Oxidation 2.11.2. Aldol Reaction and C−C Bond Formation 3. Enzymes Activated by Ionic Liquids for Organic Synthesis 3.1. Activation of Lipases by the IL Engineering 3.2. Activation of Lipases by the Design of ILs as Solvent © 2017 American Chemical Society

4. Future Perspective of Using ILs for Enzymatic Reactions 4.1. Unique Phase Behavior of ILs 4.2. Stabilizing Ability of ILs toward Enzymes 4.3. Importance of ILs as Enzyme Activating Agent 5. Concluding Remarks Associated Content Special Issue Paper Author Information Corresponding Author ORCID Present Address Notes Biography Glossary References

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1. INTRODUCTION The value of an enzymatic reaction in organic synthesis is highly respected from an environmentally friendly aspect and has now reached an industrially proven level in pharmaceutical and food industries.1−4 Enzyme is indeed the key ingredient that enables the chemistry and helps to make it green. In the history of development of enzymatic reactions for organic syntheses, “hunting of useful enzymes from nature” was the most promising way for a long time before the 1960−1980’s. Numerous excellent enzymes including whole cell systems had

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Received: March 21, 2017 Published: July 26, 2017

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DOI: 10.1021/acs.chemrev.7b00158 Chem. Rev. 2017, 117, 10567−10607

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Ionic liquids (ILs) have very good properties as reaction media in chemical reactions: they are less-volatile, lessflammable, have low toxicity, and unique solubility for organic and inorganic materials.14−18 It has long been recognized that an enzyme quickly loses its activity in a highly concentrated aqueous salt solution.1−3 Therefore, it seems a foolish notion that enzymatic reaction occurs in a salt medium from the standpoint of biology. However, ILs have now been used as reaction media for biotransformation, in particular, lipasecatalyzed reactions. The first three examples were reported in this field in 2000,19−21 and since then, a large number of applications have been reported for biocatalytic reactions in ILs. Good many reviews from various standpoints in this field have been published.22−55 Furthermore, two reviews focusing on the topic of enhanced stability of peptides in ILs have been published: several ammonium type ILs have biocompatibility and can stabilize some proteins or even a DNA.56,57 Therefore, in this review, I have focused on mainly two topics using ILs for enzymatic reactions from the standpoint of biocatalyst mediating organic synthesis: the first is “Biocatalysis in ILs” which consists of various types of biocatalytic reactions in ILs, and the second: “Enzymes Activated by ILs for Organic Synthesis”, in particular, “activation of lipase-catalyzed reactions using IL engineering”, I also discuss in what fields we should focus our investigation for utilizing ILs in biotechnology in the future.

been developed and biocatalyst libraries had been accumulated.1−3 The next trend in biocatalysis was methodologies to improve reactivity of natural biocatalysis by “chemical methods”, such as optimization of immobilization materials, additive compounds, and modification of substrate structures fitting the enzymes. Then, the third wave of biocatalysis research “direct evolution technology of enzymes” arrived. The synthesis of Sitagliptin by the Merck team might be the most recent highlight in this methodology. They succeeded in “creating” new transaminase by replacing 44 amino acids from the original enzyme ATA-117 derived from Arthrobacter sp. after repeating 11 mutations. The resulting enzyme made it possible to synthesize the target chiral amine, the key intermediate of Sitagliptin, with almost perfect conversion, >99.95% e.e. at 200 g/L loading, while the original enzyme exhibited very poor activity to the substrate ketone (only 4% conversion at 2g/L loading) (Figure 1).5

2. BIOCATALYSIS IN IONIC LIQUIDS Various enzymatic reactions for organic synthesis have been demonstrated using ILs as solvents or cosolvents, such as hydrolases (EC 3: lipases, proteases, thermolysin, α-chymotrypsin, lysozyme, β-galactosidase, cellulase, epoxide hydrolase, and penicillin amidase), oxidoreductases [EC 1: horseradish peroxidase (HRP), alcohol dehydrogenase, laccase, lignin peroxidase, and cytochrome C], lyase (EC 4: oxynitrilase), and whole cells. However, limited examples have been reported for synthetic use. I focus on examples of enzymatic reactions in the IL reaction media for organic synthesis in this chapter. For oxidase-catalyzed reactions, however, I show full examples, though many of them are not for organic synthesis but for bioscensors because no review of this topic has yet been reported.

Figure 1. A recent typical successful example of enzymatic reaction for organic synthesis.5

It has also been reported that the direct evolution is quite effective for improving stability of enzymes.6,7 However, although one property of the enzyme could be increased by this method, this causes another to drop in a vicious circle. To solve this dilemma, Ema and co-workers reported a hybrid methodology using direct evolution and subsequent chemical modification of the enzyme:8 the authors prepared a mutant of Burkholderia cepacia lipase by the direct mutation, and then the site-directed chemical esterification of several amino acid residues was attempted; the resulting enzyme exhibited different substrate specificity compared to the original enzyme while maintaining good enantioselectivity.8 But chemical process development of biocatalytic reactions, particularly, optimization of reaction media and supporting materials of enzymes must be performed in parallel with the effort to develop new enzymes by direct evolution. An aqueous reaction medium had been commonly used for the enzymatic reactions until the 1980’s. Since Klibanov and coworkers reported lipase-catalyzed transesterification in organic solvents in 1985,9,10 nonaqueous solvent systems have become popular for the lipase-catalyzed reactions in the preparation of chiral precursors for medicinal compounds,11 because common organic molecules have poor solubility in aqueous reaction media.12 Besides this practical aspect, strong interest has arisen from the fundamental behavior of enzymes in nonaqueous media13 because it is well-known that charged groups on the surface of enzymes closely associate with counterions in nonaqueous media and the resulting state influences the catalytic activity, while the active site is not sterically perturbed by the ions present around it.13 Therefore, how enzymes work in nonaqueous media has been an interesting issue of molecular biology.

2.1. Lipase-Catalyzed Reactions in ILs for Organic Synthesis

Lipases (triglycerol acylhydrolases EC 3.1.1.3) are a group of hydrolases that are one of the most widely used enzymes as a synthetic tool because they can catalyze numbers of reactions including hydrolysis, transesterification, alcoholysis, acidolysis, esterification, and aminolysis.1−3 Although lipases are the oldest generation biocatalysts in the history of biotechnology,4 lipases are frequently used for organic syntheses because of their acceptance of a broad range of substrates, stability, and availability.1−3 Sheldon and co-workers reported two types of Candida antarctica lipase (CAL-B)-catalyzed reactions in an IL in late 2000. They demonstrated the amidation of octanoic acid with ammonia, and the enzyme also catalyzed the formation of octanoic peracid by the reaction of octanoic acid with hydrogen peroxide.21 This is the first example of the enzymatic reaction in a pure IL. Since then numerous reports of the lipase-catalyzed reactions in ILs have been published. Figure 2 shows the number of publications in the field of ILs in the past 16 years: 2042 papers and 1124 papers were found with the topic of “enzyme*” and “lipase*”, respectively. Although the increase in 10568


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enantioselective lipase-catalyzed reactions in ILs in early 2001. Itoh and co-workers reported that both the reaction rate and the enantioselectivity of the reaction depended on the anionic part of ILs (Figure 3).64 The authors demonstrated that

Figure 2. Number of publications in the field of ILs from 2000 to 2016 (searched by “Web of Science”, on June 6, 2017). 72699 references are listed for the topic of “ionic liquid*” in the database of Web of Science Core Collections of Thomson Reuters, Ltd.

the number of “lipase*” papers stopped in 2011, ca. onehundred papers have still been reported year by year and almost all of them are for “organic synthesis”. These results indicate that lipases are still an important topic in this field.58 The origin of stereoselectivity of an enzymatic reaction had been explained by the static mechanism of the lock-and-key model in their active center from the beginning of the 20th century.1−3 Although this model is recognized widely as the basic concept to explain enzyme stereoselectivity, we should look at different factors in considering the lipase-catalyzed reactions because lipases accept extraordinarily diverse types of substrates and both substrate specificity and enantioselectivity are altered by the solvent system.3 It is obviously impossible to explain such selectivity by the static lock-and-key model. It is assumed that motion dynamic of lipase protein might be the key to elucidate the origin of such reactivity of lipases. To the best of my knowledge, the first model explaning the origin of enantioselectivity of lipases based on motion dynamics of the protein was proposed by Ema et al.: the enantioselectivity of transesterification of secondary alcohols with acyl donor ester could be estimated in terms of the lipase-induced strain at the transition state of the reaction.59−61 The model can explain the reason why the lipase-catalyzed reactions is modified by the solvent system and also suggests the possibility of development of a methodology that allows bringing out the potential of enzyme performances. Furthermore, this model indicates that even primary alcohols, which have a chiral point apart from the reaction point, can be resolved through a lipase-catalyzed reaction. In fact, Mezzetti, Kazlauskas, and co-workers reported an efficient design of primary alcohols that are appropriate for the lipase-catalyzed reaction by tuning of the acyl chain length.62 Taniguchi et al. reported a successful example of lipase mediated kinetic resolution of binaphthol derivatives which have terminal carboxylic acid moiety by tuning a linker length between the terminal carboxyl group and bulky binaphthyl group.63 These results indicate that reactivity, including enantioselectivity of lipases could be explained by the motion dynamics of protein and thus be changed by the reaction media. Both the reactivity and enantioselectivity of lipase-catalyzed reactions are dependent on the solvent system.3 Itoh64 and Kragl 65 independently reported the first examples of

Figure 3. Lipase-catalyzed enantioselective transesterification of 4phenylbut-1-en-3-ol in ILs.64 Enantioselectivity is shown by the E value.66

lipase was anchored by the IL solvent, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), and remained in it after the extraction workup of the product as shown in Figure 3. This is not only the first report of recyclable use of lipase in an IL solvent system but also a typical example indicating a certain benefit of using ILs as reaction media of enzymatic reactions.64 Hydrophobic IL, [C4mim][PF6], was selected as a favorable solvent in the reaction because the system allowed a simple work-up process due to the insolubility of this IL in both water and extraction of organic solvent. On the contrary, hydrophilic IL, such as [C4mim][BF4], was soluble in water, and therefore, it was difficult to remove a byproduct such as acetic acid by a simple workup process, though the enzyme exhibited high activity in the solvent. Later, Husum et al.,67 Lozano and co-workers,68,69 Kim et al.,70 and Park and Kazlauskas71 reported lipase-catalyzed reactions in an IL solvent system in 2001. In particular, Kazlauskas’s team reported the results of a very detailed study of lipase-catalyzed transesterification in ILs.71 Husum et al. also reported that β-glycosidase from E. coli and protease subtilisin worked in a 50% aqueous solution of [C4mim][PF6].67 Laszlo and Compton reported α-chymotrypsin-mediated transesterification reaction of N-acetyl L-phenylalanine ethyl ester with 1propanol in ILs or ILs in combination with supercritical carbon dioxide (ScCO2).72 Lozano and co-workers also reported the stabilization of α-chymotrypsin by ILs:68 the half lifetime of native Candida antarctica lipase (CAL, Novozyme 435) was only 3.2 h in the [C2mim][PF6] solvent, while it lengthened remarkably to 7500 h in the presence of the substrate.68 Investigations of lipase-catalyzed reactions using ILs as reaction media thus began in 2000 and 2001. Since then, numerous types of ILs have been developed as solvents for lipase10569


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catalyzed reactions.22−55 Lozano and co-workers reported that stability of lipases was also significantly enhanced by the ILs.68,73 Itoh et al. also reported that remarkably increased stability of CAL in [C4mim][BF4] was observed, and they accomplished recyclable use of the enzyme in transesterification of 10 times while maintaining excellent enantioselectivity in this IL as a solvent.74 Gubicza and co-workers reported similar favorable results of ILs as solvents in the transesterification of secondary alcohols, though both the water contents in ILs and IL species strongly influenced the enzyme activity.75,76 Rezac et al. also reported water effect on the CAL-catalyzed esterification of geraniol with acetic acid in [C4mim][PF6] and determined an optimal water level near aw = 0.6.77 Lou and Zong et al. reported that addition of an IL also provided a good effect in the enantioselective hydrolysis of phenylglycine methyl ester.78 Gubicza et al. reported enhanced enantioselectivity of lipasecatalyzed reaction of isoamyl acetate in an IL-alcohol biphasic system, and they accomplished recyclable use of enzyme 10 times while maintaining initial reactivity.79 One of the most important characteristics of an ionic liquid is its wide temperature range for the liquid phase and lack of vapor pressure, so Itoh and co-workers demonstrated the lipase-catalyzed reaction under reduced pressure.80 It is known that the typical methyl esters are not suitable for the lipasecatalyzed transesterification as acyl donors because a reverse reaction with the produced methanol takes place. This drawback can be avoided when the reaction is carried out under reduced pressure even if the methyl esters are used as the acyl donor because the produced methanol is immediately removed from the reaction mixture and thus the reaction equilibrium occurs to produce the desired product; the authors chose methyl phenylthioacetate or methyl nonanoate as the acyl donor ester and then conducted the reaction under reduced pressure at 10 Torr and 40 °C (Figure 4).80 They

pressure conditions.81 Irimescu and Kato reported good additional examples of lipase-catalyzed reaction based on the same idea and accomplished CAL-B-catalyzed esterification or amidation of carboxylic acid under reduced pressure conditions.82 Park and co-workers also reported lipase-catalyzed esterification of L-ascorbic acid with a fatty acid under reduced conditions.83 Afonso and co-workers reported an interesting transesterification system of the lipase-catalyzed reaction using an IL as the acylating agent in an IL solvent (Figure 5).84,85 In this

Figure 5. Lipase-catalyzed reaction using an IL as the acylating agent in an IL solvent.84

reaction system, the authors designed an IL which worked as an acylating agent that is anchored in the IL solvent. Therefore, after extraction of the 81% ee (S)-1-phenylethanol as shown in Figure 4, the IL layer that remained was next treated with ethanol and then 99% ee of the (R)-alcohol was released. They have thus established the chromatography-free resolution through this reaction system. Recently, an extended version of this method has been reported by Teixera and Lourenco.86 Since lipases are stable in the presence of a transition metal complex, Kim and co-workers accomplished the dynamic kinetic resolution (DKR) using a lipase-ruthenium combo catalyst system in an IL solvent system; the benzyl alcohol substrate was quickly racemized by the Ru-complex during the lipase-catalyzed transesterification, and then the (R)-acetate was accumulated after the reaction as illustrated in Figure 6.87 It was further found that lipases were tolerant in extremely harsh conditions as biocatalysis in an IL solvent system. Martiń

Figure 4. Lipase-catalyzed enantioselective transesterification of 4phenylbut-1-en-3-ol under reduced pressure conditions in the ILs.80

succeeded in reducing the amount of the acyl donor ester (0.5 eq. ∼ 0.6 eq. vs substrate alcohol) using the conditions as shown in Figure 4 and recyclable use of the enzyme has also been acomplished in this sytem without any loss of both the reaction rate and enantioselectivity. This process allows recyclable use of the catalyst with maintenance of the initial activity five times, and they succeeded in reducing the amount of acyl donor ester.80 Uyama, Kobayashi, and coworker applied this system for the first example of lipase-catalyzed polyester synthesis: lipasecatalyzed esterification of agipic acid with butan-1,4-diol proceeded smoothly in [C4mim][BF4] solvent under reduced

Figure 6. Dynamic kinetic resolution (DKR) using a lipase-ruthenium combo catalyst system in an IL solvent system.87 10570


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and co-workers demonstrated thermostable lipase (Bacillus thermocatenulatus)-catalyzed esterification of phthalic acid derivatives at 120 °C in [C4mim][PF6] or [C4mim][BF4].88 Eisenmenger et al. demonstrated lipase-catalyzed reaction under high pressure conditions.89 It is well-noticed that lipase activity generally depends on the anions of ILs: Burney and Pfaendtner conducted a MD simulation study of structural and dynamic features of Candida rugose lipase 1 in various reaction media and suggested that IL significantly influenced both the enzyme structure and protein dynamics.90 The Hofmeister ion interaction toward protein stabilization is well-known in protein sciences.91 Zhao reported that the Hofmeister effect of ions which consisted of the solvent IL relate to the activity of lipases,92 and since that finding, numerous examples discussing lipase activity from this point of view have been reported.34,39,43,53,92 Stability of enzymes in ILs under abnormal conditions have also gained strong interest from many researchers, and the origin of stability has been discussed from many points of view.93−98 Lai, Li, and coworkers reported that both activity and thermostability of two model enzymes, mushroom tyrosinase and Penicillium expansum lipase (PEL), could be explained by the Hofmeister effect of ions of the ILs.93 It is known that lipase PS protein has a lid part consisting of hydrophobic amino acid chains that covers the entrance portion of the enzyme. Colton, Kazlauskas, and co-workers revealed that treatment of i-PrOH of the Candida rugose lipase through lyophilization altered the lid part to an open form and resulting activation of the enzymatic reaction.99 Quilles and co-workers also reported that a similar activation phenomenon was observed when the lipase from Thermomyces lanuginose or Candida antarctica was treated with a surfactant.100 Therefore, interaction of the IL against this lid might play an important role in activation of the enzyme. Since IL1 has an amphiphilic property, this might contribute to concentrating the hydrophobic substrate on the enzyme proteins that located in the lid, so that initial acceleration of the rate may be realized. Due to their nonflammable and thermostable properties, ILs are considered a “green solvent”, and they offer a high potential to replace classic flammable and toxic organic solvents. The use of ILs as reaction media changes the kinetics of the chemical reactions compared to those in aqueous or classical molecular solvents.14−18,100 Unfortunately, replacement of the reaction media from classical organic solvents or water by ILs in industry has still not yet been realized. In order that ILs become more popular, the key is how to reduce the amount of ILs used while obtaining a certain benefit.101 To achieve this aim, an interesting method was reported by Hernández-Fernández and co-workers: the authors prepared an IL ([C4mim][BF4])supported membrane and connected two reaction containers filled with hexane which included 50 ppm of water as illustrated in Figure 7, and then a lipase (Novozyme 435)-catalyzed enantioselective transesterification was conducted in the left flask (Feed phase).102 The polar 1-phenylethanol dissolved easily into the IL phase in the membrane and then diffused into the receiving phase from the membrane. On the contrary, the produced hydrophobic (R)-butyrate was accumulated in the feed phase. Therefore, after stirring for 60−80 h, the (R)-butyrate was found in the feed phase (the left flask) and (S)-alcohol was in the receiving phase (the right flask); this system allows a chromatography-free separation of the product (R)-ester and unreacted (S)-alcohol. Since this method requires only a small

Figure 7. Separating cell with IL-membrane for lipase-catalyzed transesterification in order to minimize the amount of an IL.102

quantity of the IL, this IL setup might be applicable to a largescale production system. 2.1.1. Recent Examples of Lipase-Mediated Reactions Using ILs as Reaction Media. 2.1.1.1. Biodiesel Oil Production. Lipase-catalyzed synthesis of diesel oil has gained strong interest recently from the standpoint of sustainable energy production. Several good reviews have been published on this topic.103−107 Lipase-catalyzed reaction using the IL solvent systems generally gave excellent results for biodiesel oil production.108−121 Furtheremore, since the diesel oils produced are hydrophobic compounds, organic solvent free separation from the reaction mixture has sometimes been realized. Typical useful ILs are listed in Figure 8. Ha and Koo and co-workers first demonstrated lipasecatalyzed biodiesel production in ILs: the authors reported that

Figure 8. Ionic liquid mediated biodiesel oil production.108−121 10571


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[C2mim][TfO] worked as a good solvent when soybean oil was treated with Candida antarctica lipase in the presence of methanol.108 Dupont and co-workers successfully demonstrated Burlholderia cepacia lipase-catalyzed alcoholysis of soybean oil using [C4mim][Tf2N] as solvent: the biodiesel produced was separated by simple decantation, and the recovered IL/ enzyme catalytic system was reused without loss of activity.109 Zhao et al. reported lipase-catalyzed biodiesel production using their original ILs which have alkyl ether moieties.110 De Diego, Lozano, and co-workers reported excellent examples of lipasecatalyzed biodiesel oil production using their original ILs, sponge-like IL (SLIL), as solvents.111,112 Arai, Ogino, and coworkers demonstrated biodiesel production using fungus whole-cell biocatalyst in [C2mim][BF4] or [C4mim][BF4] as solvent: the authors reported that high activity was recorded for lipase from Rhizopus oryzae in these ILs, and its stability was greatly enhanced by cross-linking the biocatalyst with glutaraldehyde (Figure 9).113

Figure 10. Lipase-catalyzed regioselective acetylation using an IL solvent system.71

Figure 11. Lipase-catalyzed fatty acid ester of glucose using a mixed solvent of t-BuOH and an IL.124,129

[C4mim][OTf] and subjected this to the Novozyme 435 catalyzed transesterification. The authors prepared a supersaturated glucose solution of [C4mim][OTf] and subjected this IL solution of glucose to the lipase-catalyzed transesterification in an IL as solvent.126 Since the method enabled a highly concentrated substrate solution, the reaction proceeded very smoothly in the resulting mixed IL solvent of [C4mim][OTf]: [C4mim][Tf2N] = 1:1 (v/v), and efficient preparation of 6glucosyl undecanoate was thus accomplished (Figure 12).126

Figure 9. Biodiesel oil production using fungus whole cell.113

Zhao et al. investigated solvent effect among t-butanol, various glyme derivatives, and ILs, such as [Me(OCH2CH2)3Et 3 N][Tf 2 N], [Me(OCH 2 CH 2 ) 3 -Et-Pip][Tf 2 N], [Me(OCH2CH2)3eim][Tf2N], and [Me(OCH2CH2)3eim][OAc]. In their case, t-butanol and glyme gave better results compared to those in ILs.117 On the other hand, Liu et al. reported that [OmPy][BF4] afforded superior results compared to those in conventional organic solvents,116 and Xu et al. reported that AMMOENG 102 gave excellent results.114 2.1.1.2. Sugar Ester Production. Strong interest has also been given to preparing various types of sugar esters using lipase-catalyzed reactions in ILs because sugars are generally insoluble in conventional organic solvents, while they are highly soluble in many ILs. Kazlauskas reported first examples of regioselective acetylation of glucose derivatives in IL solvent systems and accomplished highly selective acetylation using [MOMmim][PF6] or [MOMmim][PF6] compared to those in THF (Figure 10).71 Later, Kim and co-workers reported detailed results using IL solvent system for lipase-catalyzed regioselective acetylation, and they succeeded in controlling regioselectivity by changing ILs as solvents.122 Ganske and Bornscheuer reported application of a mixed solvent of t-BuOH with [C4mim][BF4] for the synthesis of fatty acid ester of glucose (Figure 11).123,124 Since sugar esters produced can easily be separated from the ILs solvent, the reaction systems allow an easy workup process. Furthermore, the resulting products are useful for environmentally friendly surfactants. Therefore, numerous examples of regioselective esterification of sugar derivatives have been reported.124−130 Koo et al. reported a very good example using a solubilityrelated effect of ILs to lipase-catalyzed sugar fatty acid ester synthesis: they prepared a supersaturated glucose solution of

Figure 12. Glucose ester synthesis using supersaturated glucose solution in IL.126

Chen et al. reported the first direct acylation of polysaccharide using lipase-catalyzed reaction.128 The authors isolated lily polysaccharides (LP) from bulbs of Lilium lancifolium Thunb which consist of β-1,4-linked D-glucose and D-mannose (Mw = 74000 Da) and then subjected them to lipase-catalyzed acylation. They accomplished highly regioselective acylation of LP by lipase-catalyzed transesterification in the presence of vinyl acetate as acyl donor under irradiation of ultrasound. In this reaction, addition of a trace volume of water improved efficiency and the best degree of substitution (DS) was recorded when the reaction was carried out under 10572


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ultrasonic irradiation conditions using [C4mim][BF4] which included water (aw = 0.54) (Figure 13).128

Figure 13. Lipase-catalyzed regioselective acylation of lily polysaccharide.128

Figure 14. Laccase-catalyzed oxidation of veratryl alcohol in the presence of [4-MBP][BF4] (25% v/v in citrate buffer).136

laccase at all concentrations investigated, but residue activity remained in this IL with over 20% of water.137 Tavares and co-workers investigated that stability of laccase by oxidative decomposition of 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid: ABTS) in a mixed solvent of buffer with water-soluble ILs and organic solvents such as CH3CN or DMSO under various pH conditions.138 They revealed that the enzyme was active at pH 9.0 in the presence of all ILs tested and, in particular, was stable in 1-ethyl-3-methylimidazolium 2(2-methoxyethoxy)ethylsulfate ([C2mim][MEESO4]). The same group further reported that thermal stability of laccase was increased in an IL when it was immobilized on a modified silica carrier.139,140 It has been reported that laccase can catalyze the oxidation of rutin to o-quinone derivative which is easily converted to the starting rutin by electrochemical reduction. By applying the reaction, Vieira, Dupont, and co-workers reported the use of laccase as a biosensor system of polyphenol derivatives because nonvolatility of ILs is a very desired property for this aim.141 They demonstrated that a mixture of graphite powder, laccase from Aspgillus oryzae, nujol, and ILs [50:20:15:15 (w/w/w/w)] was pasted on an electrode, and the resulting electrode worked well to convert rutin into the corresponding o-quinone derivative (Figure 15).142 The same group succeeded in

Although ILs are generally highly viscous liquids compared to conventional organic solvents, however, the reaction rate in ILs is sometimes superior to those in molecular solvents. The results clearly indicate that reactivity of a lipase is indeed enhanced by an appropriate IL. Further investigation in this field will make the lipase-catalyzed reactions even more beneficial in realizing practical green organic synthesis. 2.2. Oxidase-Catalyzed Reactions in ILs

The importance of enzymatic oxidation has grown recently from the standpoint of green chemistry because chemical oxidations generally require hazardous reagents. Only two reviews which focus on activities of some oxidoreductases in ILs have been reported to data.131,132 In this chapter, recent progress of laccase and some oxidase-catalyzed reactions using ILs as solvents are reviewed. 2.2.1. Laccase-Catalyzed Reactions. Laccase (EC 1.10.3.2) belongs to the so-called blue-copper family of oxidases. The first report of laccase was as an enzyme which catalyzed coagulation of the sap of the Japanese lacquer tree Rhus vernicifera (Urushi) in Japan in 1883.133 Laccases are classified as ubiquitary enzymes that are involved in the biosynthesis of lignin and catalyzed oxidative degradation or polymerization of polyphenol formation in plants, and by inducing radical polymerization of polyphenol derivatives, hence the enzymes are found in plants, fungi, bacteria, and insects.134,135 To the best of my knowledge, the first example of laccasecatalyzed reaction in ILs was reported by Hinckley, Mozhaev, and co-workers in 2002.136 The authors investigated activity of laccase C from Trametes sp. using syringaldazin as a substrate in the presence of ionic liquids: activities of laccase C in 25% (v/ v) 4-methyl-N-butylpyridinium tetrafluoroborate ([4-MBP][BF4]) in buffer (pH 5.6) and water saturated [C4mim][PF6]. Although the enzymes were tolerant to these ILs, the reactivity was insufficient and a decrease in Vmax and increase in Km were observed. On the other hand, the authors found that a significant improvement of laccase C-catalyzed oxidation of veratryl alcohol was attained in the presence of [4-MBP][BF4], in particular when the reaction was carried out with 1-nitroso-2naphthol-3,6-disulfonate as a mediator, while no product was obtained when t-butanol was used as a cosolvent (Figure 14).136 Stephens and co-workers reported that in laccase-catalyzed oxidation of catechol that [C4mim][Br] and [C4mim][N(CN)2 ] stimulated laccase-catalyzed oxidation when these ILs were provided at concentrations between 10−20% and 50−60% (v/ v) in water, respectively. However, [C4mim][BF4] inhibited

Figure 15. Laccase-catalyzed oxidation of rutin.142

detecting rosmarinic acid in plant extracts using their sensor,143 and then developed a more stable sensor by the novel support of cellulose acetate modified with ILs.144 Vieira and co-workers also developed an oxidation sensor using pine nut peroxidase or corn peroxidase immobilized on chitosan cross-linked with citrate and IL.144 They further developed a laccase-mediated biosensor in platinum nanoparticles dispersed in IL.145,146 Development of similar biosensor systems were also reported 10573


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by Brondani et al.147 Since ILs are nonvolatile liquids, their results clearly indicate the usefulness of ILs for biosensor systems. Dong et al. reported that improved thermal stability of laccase was accomplished using a carbon nanotube (CNTs)-IL composite modified graphite electrode.148 It was reported that laccase generally exhibited low catalytic activity in a neutral reaction medium and low tolerance against the chloride ion. Qian, Mao, and co-workers solved this problem by the use of ILs: they prepared an electrode which has a laccase on the single-walled carbon nanotube (SENT)-modified electrode dipped in [bmim][PF6].149 The resulting biocathodes displayed high activity toward O2 reduction in neutral media and a high tolerance against Cl− ion.149 Huang et al. tested three trifluoromethanesulfonate (OTf) ILs: [C4mim][OTf], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([Bmpyr][OTf]), and tetramethylammonium trifluoromethanesulfonate ([TMA][OTf]) on the activity against laccase and found that the cationic part of the ILs determined the stability: [TMA][OTf] significantly stabilized the laccase, while [bmim][OTf] destabilized the enzyme.150 Laccase-catalyzed synthesis of polyaniline using OTf-based water-soluble ILs was reported.151 Feder-Kubis and Bryjak reported that menthol-based ILs influenced activity and stability of laccase from the wood rotting fungus Cerrena unicolor (Bull.ex.Fr) Murr, No 139. Although the differences were not significant, both imidazolium (I and II) and ammonium (IV and V) ILs, which had shorter alkyl substituents improved laccase activity and stability (Figure 16).152

activity was completely lost after 25 h incubation with other ILs, [N1888][AOT], [N1888][Sac], and [N1888][Tf2N]. On the other hand, [N1888][SAC] was effective to improve stability only when TEMPO was used as a mediator; combined with other mediators, no significant enhanced effect was obtained (Figure 17).154

Figure 17. List of ILs for stabilization of laccase investigated by Rehmann et al.154

Galai and co-workers conducted very detailed investigation on the influence of broad types of ILs toward laccase activity in the oxidation of ABTS as a model reaction (Figure 18).155

Figure 16. l-Menthol-based ILs as a solvent for laccase.152

Bae and co-workers reported a copper salt IL-mediated activation of laccase from Trametes versicolor: they put their attention on copper ion involved IL because laccases are socalled copper protein and found that the laccase refolding yield was enhanced more than 2.7 times compared to conventional refolding buffer when just 0.2% of 1-ethyl-3-methylimidazolium copper(III)chloride ([C2mim][CuCl3]) was added to that buffer.153 Rehmann, Stephens, and co-workers reported interesting results of ionic liquid-mediated enhanced stability of a laccase.154 The authors investigated oxidation of catechol as a model substrate using laccase from Trametes versicolor in the presence of a combination of various mediator compounds and ILs in buffer solution; they found that the choice of a mediator was important to improve the stability of laccase. With the use of a combination of [C6mim][AOT] and 2-hydroxyphenol as a mediator, the stability of laccase was significantly improved and 40% activity was retained after 188 h incubation, while the

Figure 18. Model reaction for evaluating laccase activity: oxidation of ABTS.155

Thirteen types of ILs were found to be suitable for the laccasemediated oxidation reaction as solvents, and [Chol][H2PO4] was found to be the best IL: 4.5-fold activation was recorded when the reaction was conducted in this IL. Aboofazeli et al. reported ionic and nonionic surfactantcoated activation of laccase and used it as a catalyst for bioconversion of indole (Figure 19).156 The authors found a stabilizing effect of laccase for three types of surfactants: ionic surfactants, sodium AOT and IL [cetyl(trimethyl)ammonium bromide (CTAB)], and the nonionic surfactant, Triton X-100. The catalytic activity was evaluated by oxidation of ABTS as a model substrate. Both Kcat and Km were modified by these 10574


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(diphenylmethylthio)acetoamide using chloroperoxidase (CPO) from Caldariomyces fumago (Figure 21).159 Since

Figure 19. Oxidative trimerization of indole by laccase in the presence of Triton X-100.156

Figure 21. Synthesis of (R)-modafinil through enantioselective sulfoxidation of 2-(diphenylmethylthio)acetamide using chloroperoxidase (CPO).159

surfactants in different fashion, and Triton X-100 (100 mM) gave the best results among these three types of additives. On the basis of the results, they accomplished oxidative trimerization of indole.156 Dabirmanesh et al. reported the results of their trial to create ionic liquid tolerant laccase by gene engineering.157 The authors prepared 450 clones of the laccase from Bacillus HR03 and found that mutation at Glu188Tyr and Glu188Phe showed a distinct improvement in thermal stability and ionic liquid tolerance. The importance of enzymatic oxidation has grown recently from the standpoint of green chemistry because chemical oxidations generally require hazardous regents. As shown in this chapter, there has been reported no practical example of oxidase-mediated reaction for organic synthesis at the present. However, there is an obvious benefit of using ILs as a solvent for biosensor which use oxidase-mediated reactions. I anticipate that the field of enzymatic oxidation using ILs might become more important in the near future. 2.2.2. Peroxidase-Catalyzed Reactions in ILs. The importance of enzymatic oxidation has grown recently from the standpoint of green chemistry. Several examples of enzymatic oxidations in ILs or deep eutectic solvents (DES) have been reported. In this section, I show several reports of the use of ILs or DES as reaction media for the biocatalytic oxidations.158−161 Sanfilippo and co-workers reported an example of enzymatic oxidation in ILs as reaction media: the authors used chloroperoxidase (CPO) from Caldariomyces fumago for epoxidation of 1,2-dihydronaphtalene and accomplished the synthesis of chiral dihydroxytetrahydronaphthalen in a mixed solvent of citrate buffer (pH = 5.0) and [C4mim][MeSO4] in the presence of t-BuOOH as an oxygen donor (Figure 20).158 Although the % ee in a mixed solvent with IL was inferior to that in the buffer solution, stability of the enzyme was remarkably improved compared to t-BuOH and acetone. Gao and co-workers reported the synthesis of (R)-modafinil through enantioselective sulfoxidation of 2-

CPO is a very hydrophilic enzyme and due to poor solubility of common organic molecules in an aqueous solution, the actual application of CPO was limited. In fact, only 12% of sulfoxide was obtained when 2-(diphenylmethylthio)acetoamide was subjected to the buffer solution of CPO with TBPH, though enantioselectivity is excellent (>99% ee). They solved this problem using ILs as an additive to the reaction mixture. Addition of 10% (v/v) of [C2mim][Br] to the buffer solution increased yield of the product up to 41% while maintaining the same enantioselectivity. Horseradish peroxidase (HRP) has also been used as an important oxidation enzyme. Increased thermal stability of HRP in aqueous mixtures of [C4mim][Cl] or [C4mim][BF4] was reported: thermal stability of HRP was improved in the presence of 5 and 10% (v/v) of [C4mim][BF4].160 Pang et al. investigated the activity and stability of HRP in the presence of hydrophilic ILs: no detectable activity of HRP was observed in pure [C4mim][BF4], the enzyme was active in the presence of water (4.53%, v/v) and the activity was improved by immobilization in agarose hydrogel that showed excellent activity even at 65 °C. Using this, the authors then attempted to prepare a nonaqueous biosensor.161 Liu et al. reported an interesting means of enzyme extraction using IL engineering. The authors found that [C4mim][Cl] made a phase formed with K2HPO4 buffer; with the remaining HRP activity, they succeeded in obtaining ca. 80% of the HRP amount which was distributed in the IL-rich phase (upper phase) while maintaining more than 90% of the initial enzyme activity.162 Hong et al. reported that 25% (v/v) of [C4mim][BF4] increased the stability of 2-methoxyphenol and caused an increase of the Km value of the HRP-catalyzed oxidation in the presence of H2O2 from 3 to 23 mM, though the Kcat value was decreased by a noncompetitive inhibition.163 These results thus indicated that hydrophilic ILs appeared unsuitable for the HRPcatalyzed reaction, but a small amount of them was reported possible to increase its activity. Koo and co-workers discovered that both metal cofactors (Ca2+ and hemin) and ionic liquids [C2mim][Cl] showed a positive impact on the refolding of HRP: the HRP refolding yield reached up to 80% while only 12% was obtained in a conventional urea-containing refolding buffer.164 Synthesis of polyaniline (PANI) using IL-immobilized horseradish peroxidase (HRP) was reported by Mecerreyes et al. They successfully demonstrated recyclable use of an enzyme like a lipase in their solvent system (Figure 22).165 Mota-Morales and co-workers recently reported interesting enzyme-mediated free radical polymerization in deep eutectic solvents (Figure 23).166 The authors attempted to use HRP in choline-based DES as a solvent and found that activity of HRP

Figure 20. Oxidation of 1,2-dihydronaphthalene using chloroperoxidase (CPO).158 10575


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trial nor their medical applications have been currently successful due to both instability and the requirement of a cofactor to mediate the electron transfer. Nakamura, Ohno, and co-workers first investigated the activity of the cytochrome P450 (P450) in an IL, [C2mim][Tf2N], and revealed that poly(ethylene oxide) (PEO)-supported P450 derived from the Sulfolobus tokodaii 7 strain was soluble in the IL, and the structural stability of the five coordinate high-spin state remained even at a temperature of 120 °C.169 Tee, Schwaneberg et al. reported the ionic liquid effect on the activity of monooxygenase P450 BM-3 using ten types of ILs.170 They revealed that the activity of P450 depended on the chain length of the imidazolium salt ILs, and [C2mim]Cl was the preferred IL as the cosolvent. Chefson and Auclair first reported the demonstration of the P450-catalyzed reaction for organic synthesis; the authors accomplished the 6β-OH testosterone formation by human CYP3A4 in some ILs; the enzyme showed a 20% activity remaining in a biphasic system consisting of a buffer and hydrophobic [C4mim][PF4]. The authors found an interesting fact that ca. 85% of the activity was retained in the solvent system when the enzyme was lyophilized in the presence of 0.85% (v/v) water, sucrose, and testosterone (Figure 25).171

Figure 22. Recyclable use of HRP using IL mixed solvent system.165

Figure 23. HRP-mediated radical polymeraization of acetyl acetone in DES.166

was reduced significantly but the enzyme still caused initiation of the free radical polymerization of acrylamide with full monomer conversions at 4 °C, while no polymer production was accomplished using water as a solvent at that temperature. 2.2.3. Cytochrome-Mediated Reactions in ILs. Stamatis and co-workers recently reported usefulness of hydroxyl ammonium ILs as a reaction media for the dye decolorization by oxidation using cytochrome C (cyt.c) as a model catalyst.167 The reaction rate of cyt. c-catalyzed decolorization of pinacyanol chloride with H2O2 in a mixture of 0.05 M phosphate buffer (pH 7.0) and ILs significantly depended on the ratio and type of the ILs. The best results were obtained when the reaction was carried out in a mixture of bis(2hydroxyethyl)ammonium formate (BHEAF) and buffer and the pigment was completely decomposed within 40 min, while 10% of the pigment still remained in the buffer solution (Figure 24).167 Cytochrome P450 enzymes are heme-containing monooxygenases involved in a variety of oxidative metabolic reactions, including the synthesis and degradation of many physiologically important compounds.168 Unfortunately, neither their indus-

Figure 25. Preparation of 6β-OH testosterone using P450 from human CYP3A4 in a biphasic solvent system of [C4mim][PF6] with a buffer solution.171

Gao et al. reported an enhanced enantioselectivity of the P450-catalyzed oxidation of sulfoxide using a holl cell system in a mixed solvent of buffer/IL (Figure 26).172 The authors

Figure 26. Asymmetric sulfoxidation with P450 in a phosphate buffer/ [P666,14][Tf2N] biphasic reaction system.172

prepared recombinant E. coli (P450pyrI83H-GDH) coexpressing the three-component P450pyrI83H monooxygenase and glucose dehydrogenase (GDH) and using this holl cell in a biphasic system of a phosphate buffer (pH 8.0) with hydrophobic IL, tri-n-hexyl, n-tetradecylphophonium bis(trifluoromethylsulfomyl)amide [P666,14][Tf2N] at a ratio of 3:1 (v/v). The enhanced enantioselectivity was obtained in the

Figure 24. Oxidative decolorization of BHEAF using cytochrome C in a mixed solvent of buffer and IL.166 10576


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authors found that β-galactosidase from E. coli was also active in the same mixed solvent at a similar level in 50% aqueous solutions of ethanol and acetonitrile in 2001.67 Xing and coworkers reported that water content was very important when α-chymotrypsin (CT) was used as a catalyst for ZTylGlyGlyOH formation by the reaction of Z-TylOEt and GlyGlyOEt in an IL, 1-methoxyethyl-3-methylimidazolium hexafluorophosphate ([MEOim][PF6]), as a solvent system: the enzyme exhibited its activity in the presence of a small amount of water (2−3%), while drastic deactivation was observed with greater water content (over 4%) (Figure 29,

biphasic system compared to those in a pure buffer reaction system because the reaction occurred in the buffer phase, while >99% of the substrates and 35−60% of the products remained in the IL, thus contributing to reducing the toxicity of the substrate toward the enzyme.172 Bharmoria et al. reported that cytochrome c (cyt. c) dissolved in several ILs, such as choline dioctylsulfosuccinate, [Cho][AOT], and [C2mim][C2OSO3] showed a high peroxidase activity (4-fold compared to that in an aqueous buffer). The enzyme retained both its structural stability and functional activity in IL colloidal solutions up to 180 °C. This suggests the possibility of creating a high temperature biocatalytic reactor.173 2.3. Microbes-Mediated Hydrations

The first example of nonlipase enzymatic reaction in an IL is also the first one among enzymatic reactions. Cull and coworkers reported Rhodococcus R312 cell-mediated hydration of 1,3-dicyanobenzen to 3-cyanobenzamide in a two phase solvent system: a mixed solvent of [C4mim][PF6]:water (1:4) (Figure 27).19 Since [C4mim][PF6] is a hydrophobic liquid, the

Figure 27. Rhodococcus-mediated hydration of 1,3-dicyanobenzene to 3-cyanobenzamide.19

significance of this report might be in the finding that [C4mim][PF6] exhibited no inhibitory action against this microbe; the final yield of the product was superior to that in water or a mixed solvent of toluene-H2O. The authors mentioned that the most important benefit of using this mixed solvent system might be the modified aggregation state of the microbe cells in [C4mim][PF6]-H2O: the cells are dispersed in the aqueous phase during the reaction. This might contribute to the increased yield of 3-cyanobenzamide. On the contrary, when the reaction was conducted in a mixed solvent of toluene-H2O, the cells aggregated together and the majority located in the liquid−liquid interface, thus decreasing the chemical yield.

Figure 29. α-Chymotrypsin catalyzed synthesis of two types of tripeptides using ILs as solvents.175,176

upper).175 Noritomi and co-workers also reported CTmediated synthesis of tripeptide, Ac-Trp-Gly-Gly-NH2 in [C2mim][Tf2N]; the reaction rate is 16-fold faster than that in CH3CN solvent (Figure 29, below).176 Attri and Venkatesu reported that stability of α-chymotripsin (CT) in ammonium-type ILs depended on the IL structure, and that trimethylammonium [TMA] ILs worked as strong stabilizers for CT, while ILs which have a long alkyl chain in the cationic part showed a poor stabilizing effect. However, in these ILs, thermal stability of CT was significantly increased.177 Kurata et al. extracted an IL-tolerant protease from bacterium found in a Japanese fermented soybean paste. The microbes are stable in the presence of 80% (v/v) ILs, [C2mim][TsO], [C4mim][Cl], [C4mim][TFA], [C4mim][BF4], [C4mim][PF6], or [C4mim][Tf2N], and the enzyme exhibited activity in a buffer solution in the presence of 10% (v./v) of these ILs.178 In my own experiments, neither thermolysin nor α-chymotrypsin was active in a pure IL solvent.48 Therefore, except for lipases, very careful optimization of water content is apparently required to realize desired reactions. Furukawa et al. reported a unique protocol of the thermolysin-catalyzed peptide synthesis using amino acid ILs.179 The authors used two types of amino acid type ILs as substrates of the enzymatic reaction and accomplished the synthesis of Z-APM (Figure 30).179 Amino acid ILs were used not only as the reaction medium but also as substrates, since the substrate was the reaction medium and the chemical equilibrium easily shifted to producing the products. An efficient thermolysin-catalyzed dipeptide synthesis has thus been accomplished by this method.179

2.4. Protease-Catalyzed Reactions

Thermolysin-catalyzed reaction of carbobenzoxy-L-aspartate Lphenylalanine methyl ester hydrochloride was demonstrated in t-amyl alcohol as the solvent by Nagayasu and co-workers.174 Erbeldinger and co-workers reported that the same reaction was possible in [C4mim][PF6]-H2O (95:5, v/v) (Figure 28).20 Laszlo and Compton reported α-chymotrypsin-mediated reaction in an IL.72 Husum et al. reported the second example of protease-catalyzed reaction: a protease, Subtilisin Savinase, exhibited the hydrolytic activity in a 50% aqueous solution of the water-miscible ionic liquid ([C4min][BF4]). In addition, the

Figure 28. Thermolysin-catalyzed synthesis of Z-aspartame.20 10577


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Figure 32. Asymmetric reduction of ketones using Geotrichun candidum cells in ILs.185

Figure 30. Thermolysin-catalyzed synthesis of dipeptide (Z-APM) using amino acid ILs as substrates.179

44541 called ADH-‘A’ was successful in an IL as solvent.186 They found that the reaction strongly depended on the IL and revealed that ILs which possess hydroxyl and other functionalities in the cationic part worked well as cosolvents. In particular, N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium methylsulfonate ([MTEOA][MeSO4]) worked as a good cosolvent and the alcohols produced were obtained with excellent optical purity (>99% ee). Although chemical yields of the products were not sufficient, since the IL was perfectly miscible with water and immiscible with ethyl acetate (AcOEt), the reaction system allowed a very simple workup simply by extraction with AcOEt (Figure 33).186

Vallette and co-workers reported the nonenzymatic peptide synthesis using amino acid ILs.180 Duan et al. reported modification of an enzyme, lysozyme, by the reaction of the thiol ester of an amino acid in the presence of hexamethyldisiloxane as the coupling reagent in an IL solvent system.181 Furukawa et al. established a simple synthesis and subsequent purification method of a dipeptide based on this methodology.182 Galy et al. also reported the nonenzymatic synthesis of an oligopeptide using an ionic reagent in an IL as the solvent.183 These results indicated the possibility of ILs as both the reaction medium and substrates of the oligo peptide synthesis in both enzymatic and nonenzymatic reactions. 2.5. Whole Cell-Mediated Asymmetric Reduction of Ketones

Howarth et al. reported Baker’s Yeast mediated reduction of ketone in a mixed solvent of [C4mim][PF6]: H2O (10:1) (Figure 31). In this case, immobilization on calcium alginate was essential; reduction of acetyl acetone alone gave better results than that in water among seven ketones tested.184

Figure 33. Asymmetric reduction of ketones using ADH in an IL solvent system.186 Figure 31. Baker’s yeast mediated asymmetric reduction in a mixed solvent of [C4mim][PF6]: H2O (10:1).184

Lye et al. investigated reduction of 4′-bromo-2,2,2-trifluoroacetophenone to (R)-1-(4′-bromophenyl)-2,2,2-trifluoro-1ethanol in acetophenone and 6-Br-β-tetralone to (S)-6-Br-βtetralol in the presence of various types of ILs using ADH from Rhodococcus erythropolis (ADH RE).187 The authors revealed that employing 10% (v/v) N-butyl-N-methylpyrrolidium bis(trifuloromethylsulfonyl)amide ([P1,4][Tf2N]) gave (S)-6-Br-βtetralol in 88% with >99% ee, and easy extraction was accomplished since the IL is immiscible to water. Furthermore, the initial rate of reaction was improved more than four times in the presence of [P1,4][Tf2N] (10% (v/v)) compared to that of a no cosolvent system, and (S)-6-Br-β-tetralol was isolated in 85% yield with an ee of 99% (Figure 34).187ß Dreyer and Kragl demonstrated alcohol dehydrogenase (ADH)-catalyzed asymmetric reduction of phenyl acetone as

Matsuda et al. reported asymmetric reduction of ketones using resting Geotrichun candidum cells in ILs (Figure 32).185 This is the first success demonstrating an oxide reductasemediated reaction in ILs. The authors found that the desired reaction proceeded smoothly with excellent enantioselectivity (>99% ee) for all substrates tested. However, it was essential that the cell immobilized on water-absorbing polymer containing pH 7.0 MES buffer. In addition, the enzymatic reaction strongly depended on the ILs, and the best results were obtained in [C4mim][PF6] or [C8mim][PF6]. Kroutil and co-workers reported that reduction of ketone in ILs using alcohol dehydrogenase from Rhodococcus ruber DMS 10578


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2.6. Acylase-Catalyzed Reactions

Zhao and Malhotra accomplished successful resolution of amino acid esters using an acylase from Bacillus licheniforms (Novozyme A/S) in an IL, N-ethylpyridinium trifluoroacetate ([EtPy][TFA]) (Figure 36).190 A mixed solvent of 15% (v/v)

Figure 34. Asymmetric reduction of 6-Br-β-tetralone by ADH from Rhodococcus erythropolis combined with GDH103 using an IL as a cosolvent.187

a model substrate using an aqueous two-phase system that contains IL, Ammoeng 110 (Figure 35).188 In the reactions,

Figure 36. Enantioselective hydrolysis of racemic amino acid methyl ester by acylase from Bacillus licheniforms.190

[EtPy][TFA] in water worked well as a solvent, and the desired chiral amino acid was obtained. On the contrary, using a mixed solvent of CH3CN in water, very poor or no reaction took place in all cases. Zhang et al. reported that Penicillin G acylase from E. coli immobilized polymethylacrylamide (PMAA-NH4) worked well in 25% (v/v) [C4mim][DCA] in water: the results were better than in water alone (1.6-fold higher yield), though stability of the enzyme in [C4mim][DCA] at 10 °C was inferior to that in [C4mim][PF6] (Figure 37).191 Xia et al. reported an interesting

Figure 35. ADH catalyzed reduction of acetophenone using a biphasic solvent system of buffer and an IL.188

enzymes were enriched in the IL-containing upper phase and reactivity was increased. It should be noted that Ammoeng 110 exhibited a stabilizing effect on this enzyme, although this IL involves Cl− anion. On the contrary, it was reported that high Cl− concentrations caused the inhibition of enzymes, such as cellulase.188 Therefore, the results cannot be explained by the Hofmeister series alone. The authors speculated that the presence of oligoethylene glycol unit in the cation might have contributed to the stabilizing effect of the IL.188 Fischer and co-workers investigated the activity and stability of immobilized D-amino acid oxidase (DAAO, EC 1.4.3.3) from Trigonopsis variabilis CBS 4095 in several ILs using D/Dphenylalanine as a substrate, since high oxygen accumulation was expected in ILs.189 The stability of DAAO was higher in water-insoluble ILs than in water-soluble ones. They also investigated catalase activity and found that no significant loss in free catalase activity was obtained in 20% of 1,3dimethylimidazolium dimethylphosphate ([mmim][NMPO4]), and that this IL was the most promising one among those tested.189

Figure 37. Penicillin G acylase-catalyzed hydrolysis in IL.191

two phase reaction system consisting of a mixed IL and water for penicillin acylase catalyzed reaction: they used the solvent of a mixture of IL ([C4mim][PF6] and [C4mim][BF4]) with water (buffer), and by controlling pH conditions. They accomplished easy separation of 6-aminopenicillic acid and the byproduct, phenylacetic acid.192 Zhou et al. also reported immobilization of penicillin G acyalse on magnetic nanoparticles modified by ILs.193 A whole-cell biocatalysis was used for the nucleoside acylation. Yang, Zhao, and co-workers reported that highly regioselective acylation of nucleoside ara-C was accomplished using a mixed solvent of [C4mim]/THF with water (Figure 38).194 The authors supposed that the IL modified the cell surface morphology and caused improvement of the permeability of cell envelopes; thus increased mass transfer of substrate to the inside of the enzyme cells took place. 10579


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Figure 38. Regioselective transesterification of a nucleoside ara-C.194

2.7. Glycosylation or Cellulase-Mediated Hydrolysis of Polysaccharides Figure 40. Glycosylation using sucrose phosphorylase in ILs.197

Kaftzik and Kragl et al. reported β-galactosidase catalyzed reaction for preparing N-acetyllactosamine. The authors obtained the desired N-acetyllactosamine in 58% yield by transglycosylation from economical lactose and N-acetylglucosamine using a mixed solvent of 1,3-dimethylimidazolium methylsulfate ([C1mim][MeSO4]): this was double that in buffer (pH 7.3) solution because the IL mixed solvent system suppressed the secondary hydrolysis of the product (Figure 39).195

[P1CH2CO2][Br] significantly improved reactivity and stability of the enzyme. But this favorable effect depended on the solute concentration and addition of an excess amount of these solutes caused a drop of the activity (Figure 41).198−200

Figure 41. Hydrolysis of β-glucosidase in the presence of betaine type solute.198−200

Figure 39. Synthesis of N-acetyllactosamine by β-galactosidase using IL as a cosolvent.195

In 2002, Swatloski and co-workers reported that ILs dissolved cellulose and rapid hydrolysis was accomplished using regenerated cellulose from the IL solution.201 Since then enzymatic saccharification of cellulose in ILs has gained strong interest for simplifying the bioethanol production process from lignocellulosic biomass.202−205 Kamiya and Goto et al. first demonstrated a one-batch enzymatic process for the saccharification of cellulose in an IL, [C4mim][H2PO4] (Figure 42).206 Initially cellulose was dissolved in [C4mim][H2PO4], and then the mixture was diluted with buffer solution to form regenerated cellulose. To this mixture, celulase was added, and then the mixture was incubated at 40 °C for 24 h: 70% of cellulose was converted to glucose (50%) and cellobiose (20%).

Lang et al. investigated transgalactosylation reaction catalyzed by β-glycosylhydrolase CelB from Pyrococcus furiosus using 45% (v/v) of [mmim][MeSO4] as a cosolvent.196 The authors measured VGlc/VGal against various acceptors using lactose as a substrate and found that the values of Ksel obtained depended on the structure of the acceptor and cosolvent, and the addition of the IL caused a 3.1-fold increased Ksel value when glycerol was used as an acceptor, though no significant increase in yield of the product was obtained. Sucrose phosphorylase-catalyzed glycosylation using ILs as cosolvents have been reported by De Winter et al.197 The authors evaluated ILs and found that the IL Ammoeng 101 was the most effective cosolvent for various compounds such as medium- and long chain alcohols, flavonoids, alkaloids, phenolic compounds, and terpenes. This IL helps to improve solubility of these substrates and contributed to improve catalytic activity: they accomplished the synthesis of 3-O-α-Dglucopyranosyl-(E)-resveratrol using phosphorylase from Bacillus adolescentis (Figure 40).197 Koumoto et al. reported an interesting activation properties of betaine type compounds vs α-glucosidase from different origins.198−200 The authors tested additive effect of numerous types of betaine type ILs in the hydrolysis of p-nitrophenyl-αglucopyranoside and found that [N444CH2CO2][Br] and

Figure 42. Enzymatic saccharification of cellulose in ILs.206 10580


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Glucose formation in aqueous-IL mixture was 2-fold higher than that in buffer solution alone. Since then, one-pot IL pretreatment and saccharification using various biomass resources, such as switch glass,207 rice straw,208 and sugar cane bagasse209 have been demonstrated by several groups. Lozano prepared immobilized cellulase (Celluclast) from T. ressei on Amberlite XAD4 and further coated it with an IL to prepare IL-coated-cellulase (ICC). Then they added the enzyme to a 1% cellulose solution in [C4mim][Cl] in the presence of a small volume of citrate buffer (pH 4.8), and the mixture was stirred at 50 °C for 24 h. The enzyme activity depended on the IL coating, and they found that the desired saccharification took place when hydrophobic IL [N1114][Tf2N] was employed as a coating IL (Figure 43).210 Figure 44. Production of α-ketoglutarate from cellulose through simultaneous saccharification and fermentation in the presence of [C2mim][OAc].213

[C4mim][Br], and [C6mim][Br], toward endo-1-4-glucanase which was isolated from Bacillus subtilis DR8806; these imidazolium-based ILs had inhibitory effects on endo-1-4glucanase activity, and the strongest inhibitory action was recorded for [C6mim][Br]. The enzyme displayed 10% of the initial activity, while ca. 50% of activity was maintained in the presence of [C4mim][Cl].217 2.8. Hydroxynitrile Lyase-Catalyzed Reaction Figure 43. Saccharification of 1% cellulose solution in [C4mim][Cl] using IL-coated cellulase.210

Griengl et al. reported hydroxynitrile lyase-catalyzed cyanohydrin formation in an IL (Figure 45).218 The authors initially

Nakashima, Ogino, and Kondo et al.211 reported demonstration of direct bioethanol production from cellulose by cellulase-displaying yeast (arming yeast)212 using ionic liquid, 1ethyl-3-methylimidazolium diethylphosphate ([C2mim][DEP]) pretreatment. The authors found that cellulose degradation and ethanol productivity by the arming yeast was dramatically enhanced by pretreatment with ionic liquids and obtained approximately 90% ethanol yield. They also demonstrated recovery of the ionic liquids in the culture medium and recycled them for the pretreatment of cellulose.211 Simultaneous saccharification and fermentation of cellulose for α-ketoglutaric acid production in an IL was reported by Ryu et al.: the authors treated regenerated cellulose from [C2mim][OAc] using a yeast and accomplished α-ketoglutaric acid production in an excellent yield (Figure 44).213 Bilgin and co-workers reported that the immobilized enzyme exhibited 5.5-fold higher activity of cellulose hydrolysis in 25% (v/v) of [C4mim][PF6]-buffer than the activities in buffer medium.214 Jamwal et al. attempted to improve the stabilization of the cellulase from Aspergillus niger in an IL. They revealed that the enzyme was cross-linked by ethylene glycol dimethacrylate using ammonium persulfate as an initiator to afford heat and pH stable cross-linked cellulose which exhibited activity in [C4mim][OAc]. Recyclable use of the enzyme was accomplished with retention of 58% of its initial activity after 12 repeat cycles in hydrolysis of cellulose.215 Wahlström and co-workers examined stability of a cellulase in three types of DES and [C2mim][OAc] for direct hydrolysis of cellulose after pretreatment of these solvents.216 For the cellulose hydrolysis after pretreatment of ILs, Ramezani et al. investigated the additive effect (10% (v/v)) of four types of imidazolium ILs, [C2mim][Br], [C4mim][Cl],

Figure 45. Hydroxynitrile lyase-catalyzed cyanohydrin formation.218

tested the reaction using benzaldehyde in IL containing 1% (v/ v) water using hydroxynitrile lyase from Prunus amygdalus (PaHNL) which exhibits (R)-selectivity; the desired mandelonitrile was obtained in good yield but racemic form. On the other hand, changing the solvent system to IL/aqueous buffer (pH 4.8) (1:1) led to excellent results: the desired product was obtained in 97% yield with 97% ee. They revealed that both the enzymatic and nonenzymatic transformation was accelerated by applying the mixed solvent system. Dong et al. investigated the inhibitory effect of imidazolium types of ILs toward the lactic dehydrogenase (LDH); their inhibitory ability on the LDH activity increased with increase of the alkyl chain length on the IL cation.219 2.9. Fluorinase or Haloalkane Dehydrogenase-Catalyzed Reaction

Kitazume et al. reported fluorinase-catalyzed fluorination (Figure 46).220 Although the reported chemical yield was not impressive, only a limited example for enzymatic fluorination was reported and the results in [C4mim][PF6] were superior to that in a buffer solution. Haloalkane dehalogenases catalyze the hydrolytic cleavage of carbon−halogen bonds in diverse halogenated hydrocarbons; 10581


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Figure 46. Fluorinase-mediated synthesis of 5′-fluorinarted FDA.220

these enzymes have attracted strong interest from the standpoint of sustainable biotechnologies. Chaloupkova and co-workers investigated stability of three types of haloalkane dehydrogenase using the hydrolysis of 1-iodohexane as a model compound in DES, ethaline (ethylene glycol and choline chloride).221 Haloalkane dehalogenase DhaA was found to be the most tolerant to ethaline, and 21% of the initial activity remained in 90% (v/v) ethaline in buffer solution. They also reported that hydrolysis of 2-bromopentane by DbjA from Rhodococcus rhodochrous NCIMB13064 increased more than 4fold in the presence of ethaline or ethylene glycol (Figure 47).221

Figure 48. Magnetic IL for immobilization of luciferase.227

caused a conformational collapse in the exposed α-helixes, thus significant deactivation of this enzyme took place.228 Yoshimura and co-workers investigated the relationship of structure and activity of chicken egg-white lysozyme in aqueous solutions of two typical ionic liquids, [C4mim][Cl] and [C3mim][NO3].229 An increase in structural disorder was observed in the presence of ILs due to the unfolding and a decrease in the helical structure of lysozyme. However, a decrease in the structural stability after cooling was less than that before cooling. To obtain information for improving protein stabilization, Greaves and co-workers investigated the additive effect of 19 polar solvents and 4 protic ILs, such as ethylammonium nitrate (EAN), ethylammonium formate (EAF), 2-hydroxyethylammonium nitrate (EOAF), and 2hydroxyethylammonium formate (EOAF). They revealed that EAN and EOAF suppressed enzyme solvent interactions and afforded high lysozyme activity.230 Cao et al. investigated the additive effect of [C2mim][BF4] to glucuronidase from Penicillium purpurogenus Li-3 and reported a favorable effect of the IL to this enzyme which converts glycyrrhizin to the corresponding mono glycuronide (Figure 49).231 Fan and co-workers investigated the inhibitory effect of five types of imidazolium ILs toward lumbrokinase on the hydrolysis of casein; reactivity was increased up to 2.8-fold by addition of [C2mim][Br] at CIL 1.2 (molL−1) . However, an

Figure 47. Haloalkane dehydrogenase mediated kinetic resolution of 2-bromopentane.221

2.10. Luciferase-Mediated Reaction

Luciferase (EC 1.13.12.5) is a monooxygenase and performs ATP-dependent conversion of luciferine into a luciferyladenylate, which is smoothly oxidized in multistep reactions to oxyluciferin.222 This enzyme has gained strong interest recently as a biosensor of cell viability,222 protein−protein interaction,223,224 and as a gene reporter system.225,226 However, luciferase is generally very unstable, and its activity quickly decreases at rt. Therefore, development of a method to increase the stability of luciferases is strongly required. Noori and co-workers reported that stability of firefly luciferase from Photinus pyralis was significantly improved by immobilization of magnetic IL, such as [γ-Fe2O3@SiO2][C4mim][Cl] and [γFe2O3@SiO2][C4mim][[I] (Figure 48). Enzyme stability was also effectively improved by this immobilization.227 Ghaedizadeh et al. also investigated the additive effect of [C4mim][BF4] toward Renilla luciferase as biosensor.228 They revealed that the enzyme activity was severely decreased by addition of [C4mim][BF4] or [C4mim][PF6]; the protein-IL interaction has an impact on the structure of the enzyme and

Figure 49. Preparation of glycyrrhetinic acid mono glucuronide using β-glucuronide in a mixed solvent of [C2mim][BF4] with buffer.231 10582


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It is well-recognized that hydrogen peroxide anions have very strong nucleophilicity. Therefore, lipase can catalyze formation of peracid derivatives from esters simply by adding peroxide derivatives.3 The resulting peracids cause epoxidation of olefins and Baeyer−Villiger oxidation against ketones. Sheldon and coworkers in fact demonstrated that Candida antarctica lipase (CAL-B) catalyzed formation of octanoic peracid by the reaction of octanoic acid with hydrogen peroxide.21 This is the first example of lipase-catalyzed reaction in a pure ionic liquid solvent system. Kotlewska, Arends, and co-workers reported successful examples of Baeyer−Villiger oxidation of olefins and ketones (Figure 51).252 The authors reported that yields of products

increased amount of the IL caused a drop in the reactivity. They reported that inhibition effect of ILs depended on their hydrogen-bonding ability, while no significant change in the enzyme conformation was observed by addition of IL.232 Goldfeder et al. investigated tyrosinase activity in the presence of ILs and found that the monophenoase/diphenolase activity ratio was altered by IL; [C4mim][BF4] was increased up to 5fold compared to that in buffer solution, though the ratio was inferior to those when SDS was added (45-fold).233 2.11. Biocatalytic Promiscuity of Enzymatic Reactions for Organic Synthesis Using ILs

The biocatalytic promiscuity of enzymes has gained great attention recently and an attempt made to expand their application in organic synthesis. Since catalytic promiscuity could immensely broaden the applicability of enzymatic reactions, extensive investigations have been conducted mainly using hydrolases.234−255 ILs are highly polar liquids, unique chemical reactions have thus been developed using them as a key solvent.14−18 Therefore, it is expected that ILs may influence the promiscuity of enzymatic reaction. I will show such examples in this section. 2.11.1. Lipase-Mediated Oxidation. Sharma, Sinha, and co-workers reported an interesting “lipase-catalyzed oxidation of aryl alcohol” (Figure 50).251 The authors found that CALB

Figure 51. Baeyer−Villiger oxidation using a combination of hydrogen peroxide and lipase-catalyzed reaction.252

Figure 50. CALB/[C6mim][Br]-catalyzed oxidation of aryl alcohols in the presence of H2O2.251

depended on the combination of substrates and hydrogendonating IL. [HOPmim][NO3] generally gave the products in high yields, and tetrahydro-2H-pyran-2-one was obtained in 99% yield when cyclopentanone was treated with 2 eq. of hydrogen peroxide in the presence of octanoic acid and CAL-B as a catalyst. Drozdz and co-workers also reported Baeyer− Villiger oxidation using a similar methodology.253 The same group reported similar reaction using acyltransferase from Mycobacterium segmentis for peracid formation with 60% aq H2O2.254 2.11.2. Aldol Reaction and C−C Bond Formation. Two reports concerning lipase-catalyzed aldol condensation in an IL or DES solvent have been published.255,256 Zhang et al. accomplished Porcine Pancreatic Lipase (PPL)-catalyzed asymmetric cross aldol reaction in a mixed solvent of [C4mim][PF6]/H2O (Figure 52).255 They succeeded in obtaining 22 types of products in maximum yield of 99%, with best diastereoselectivity (dr) > 9:1, and highest enantioselectivity with 90% ee, though the reactions generally required a long time to complete (72−288 h) and the absolute configuration of the products has not been determined.255 ́ González-Martinez et al. recently reported similar PPL-

and [C6mim][Br] catalyzed oxidation of secondary alcohol to the corresponding ketones in the presence of 30% H2O2 at 40 °C. Although the substrates are limited for aryl alcohol or benzyl alcohol, the corresponding ketones were produced in excellent-to-moderate yields as illustrated in Figure 50. Furthermore, the authors successfully demonstrated recyclable use of the CALB/[C6mim][Br] system: 90% of yield was recorded for the oxidation of 1-(4-methoxyphenyl)prop-1-ol to the corresponding ketone after 8 repetitions of the reaction.251 The authors hypothesized that the hydroxyl group of alcohol interacts with the imidazolium cation, resulting in polarization of the C−O bond and then strongly coordinates with hydrogen peroxide. Subsequent charge stabilization by an oxyanion hole of lipase generates product ketone along with release of water. The model suggests that the lipase plays an important role in catching the substrate alcohol and hydrogen peroxide on the active site. However, since the reaction proceeded nonenantioselectivity, it is not yet clear that the reaction indeed takes place at the active center as hypothesized. 10583


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catalyzed reactions has gained great progress in this decade. In this section, I focus on reviewing this topic. 3.1. Activation of Lipases by the IL Engineering

Immobilization is very important for increasing the activity of enzymes.1−3 Several landmark methodologies for bringing out the potential of lipase have been developed: lipid coating mediated activation by Okahata,258,259 entrapment of lipases in hydrophobic sol−gel materials by Reetz,260−262 molecular bioimprinting by Braco,263−265 and salt-mediated activation by Dordick.266−268 It was also reported that addition of a polar organic solvent such as dimethyl sulfoxide,269,270 crown ether,271−276 or some surfactant such as polyoxyethylenealkyl ether277,278 was effective to improve the reaction rate of lipasecatalyzed reaction in organic solvents. Among these methods, the salt-mediated activation of hydrolase by the Dordick group might be the most unique.266−268 The authors reported that proteases Subtilisin Carlsberg and α-chymotrypsin could be activated by a large amount of salts with kosmotropic anions by the lyophilization process; reaction rate of the KCl-treated protease mediated alcoholysis reaction was drastically improved and over 3000-fold acceleration was accomplished compared to the native enzyme-catalyzed reaction (Figure 54).266 In

Figure 52. PPL-catalyzed asymmetric aldol reaction in a mixed solvent of [C4mim][PF6]/ H2O. Absolute configuration of the product has not yet been determined.255

catalyzed aldol reaction in deep eutectic solvent (DES) and also accomplished recyclable use of the catalyst four times.256 Maugeri and de Mariá reported asymmetric C−C bond formation in a DES-buffer mixture using benzaldehyde lyase (BAL) from Pseudomonas fluorescens. They tested four types of lyases having different substrate specificity, butyraldehyde, valeraldehyde, benzaldehyde, and 2-furylaldehyde. Unfortunately, only benzaldehyde lyase (BAL) displayed perfect enantioselectivity in DES-buffer solution, and the selectivity was lower in the solvent compared to those in pure buffer solution. Among three types of DES, U-DES gave the best results (Figure 53).257

Figure 54. Activation of Subtilisin Carlsberg and α-chymotrypsin by the potassium chloride treatment.266

particular, it is interesting that Kcat values for both enzymes were drastically improved, while the KCl treatment had a different effect on the Km values: Km value of subtilisin was increased to 1/10, which indicated that substrate specificity was increased by the KCl treatment. On the contrary, no modification was observed for α-chymotrypsin. However, the authors paid no attention to the influence of their slattreatment on the enantioselectivity of their reactions. Kim and Lee first reported ionic liquid mediated activation of enzyme focused on modified enantioselectivity in 2002:279 the authors mixed the enzyme powder with the ionic liquid, 1methyl-3(3-phenylpropyl)imidazolium hexafluorophosphate ([3-Ph−C3mim][PF6]). This resulted in preparation of the ionic liquid coated lipase PS, “IL-C-PCL”, which showed more enhanced enantioselectivity than that of commercial lipase PSC in toluene (E value was increased from 233 for the native

Figure 53. BAL-catalyzed C−C bond formation in a mixed solvent of DES-buffer.257

3. ENZYMES ACTIVATED BY IONIC LIQUIDS FOR ORGANIC SYNTHESIS Lipase-catalyzed reactions are still going very strong in this field from the standpoint of synthetic organic chemistry; lipases are frequently used for asymmetric organic syntheses because of their acceptance of a broad range of substrates, stability, and availability. The ionic liquids engineering for activating lipase10584


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PCL to 533 for IL-C-PCL), though no significant change in the reaction rate was obtained (Figure 55).279 However, as Faber

Figure 55. Activation of Lipase PS by the ionic liquid [3-PhC3mim][PF6].279

pointed out,3 E values of over 200 should be very carefully examined, and since no acceleration was recorded, the results remained at a doubtful level on those days. Polyethylene glycol (PEG) treatment is known to cause significant stabilization of an enzyme. Kaar, Russell, and coworkers first reported the improved activity of lipase by PEG treatment in ILs.280 Goto and co-workers also reported that PEG-coated lipase worked well as catalysis for transesterification of vinyl cinnamate with butanol in 1-methyl-3octylimidazolium hexafluorophosphate ([C8mim][PF6]) as solvent.281,282 However, it was reported that PEG-coated lipases gradually lost their activity during the reaction. The results are quite reasonable because the PEG group was easily stripped off the enzyme into the solvent. Itoh and co-workers reported the key solution to realize improved performance of lipases using ILs in 2004.283 The authors synthesized 3-methyl-1-butylimidazolium cetylPEG(10)-sulfate ionic liquid (IL1)283 and prepared IL1 coated Burkholderia cepacia284 lipase (IL1-PS) by immobilization of this IL: they found that optimization of both the chain length of the alkyl group and PEG moiety of the anionic part of the IL was necessary; they also determined that the lyophilization process is essential to realize acceleration of the reaction rate (Figure 56),285 though enantioselectivity was modified simply by addition of the IL when 1-phenylethanol was subjected to the transesterification as a model substrate.283,285 The IL1-PS exhibited excellent reactivity and typical substrates are shown in Figure 57.285 The notable result was that their ionic coating enzyme (IL1-PS) consisted of only a small amount of IL1, ca. 3-fold (w/w) versus enzyme protein.285 To the best of our knowledge, this is the most minimal amount recorded of “a supporting material” that activated the enzyme. The activation effect depended significantly on the substrates: a 1000-fold acceleration was recorded when 2-naphthol was subjected to the transesterification using vinyl acetate as an acyl donor while maintaining excellent enantioselectivity (E > 200). On the other hand, interesting modified reaction rates were observed for pyridyl alcohol derivatives: 18-fold acceleration was obtained for 2-pyridyl alcohol, while only double for 3-pyridyl alcohol, and 5-fold for 4-pyridyl alcohol compared to those of commercial lipase PS (see Figure 57). Since IL1 was bound with the enzyme tightly by both coulomb and hydrogen

Figure 56. Evaluation of alkyl-PEG moiety for activation of Lipase PS. The IL1-PS powder contains 3.3% (w/w) of enzyme with 9.8% (w/w) of the IL1 and the rest mainly consists of very fine inorganic materials derived from Celite.285

bonding, no leaching was observed by 1H NMR analysis when the IL1-PS-catalyzed reaction was carried out in i-Pr2O.285 Wallert, Bolm, and co-workers reported that pig liver esterase (PLE)-catalyzed enantioselective hydrolysis of prochiral malonate derivatives was activated by addition of 0.1−5 wt % of hydrophilic ILs such as PEG-substituted ammonium methylsulfate or methylphophonate as an additive; enantioselectivity of the hydrolysis reaction was enhanced from 78% ee to up to 95−97% ee, but no significant acceleration was recorded.286 Lee and Kim prepared PF6 salt of dodecylimidazolium and used it as a coating material of Burkholderia cepacia lipase:287 they confirmed that lyophilization was essential to achieve marked enhanced activity, as Itoh’s group had established. Lyophilization of surfactants or ILs has thus been established as a promising method of activation of lipases. IL1-PS is now commercially available, so that Han et al. used it as a key catalyst for preparing chiral 1-(2-(allyloxy)phenyl)but-3-en-1-ol which was difficult to apply to commercial enzymes: IL1-PS effectively worked as catalyst of enantioselective transesterification of 1-(2-(allyloxy)phenyl)but-3-en-1-ol and succeeded in obtaining optically pure acetate, while commercial Lipase PS- or Novozyme 435-catalyzed reaction was too slow to obtain the product practically, though both enzymes showed sufficient enantioselectivity (Figure 58).288 Abe et al. synthesized chiral pyrrolidine-substituted imidazolium cetyl-PEG(10)-sulfate ionic liquids derived from D- and Lproline and used as coating materials of lipase PS: the resulting chiral imidazolium cetyl-PEG(10) sulfate-coated lipase PS displayed excellent reactivity (Figure 59).289 Chirality of the 10585


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Figure 59. Activation of lipase PS by the coating with chiral pyrrolidine-substituted imidazolium cetyl-PEG10 sulfate. Rate refers to mM/mg enzyme, h.289

the amino acid with IL1; they investigated their coated lipase PS using (±)-1-phenylethanol as a model substrate in the presence of vinyl acetate as an acyl donor in the i-Pr2O as a solvent.290 They found a synergetic effect of amino acids and an ionic liquid, 1-butyl-2,3-dimethylimidazolium cetyl-PEG(10) sulfate (IL1), as a coating material on a lipase: coating on lipase PS using several amino acids with IL1 was effective in accelerating the lipase-catalyzed transesterification of secondary alcohols with excellent enantioselectivity (E > 200) (Figure 60).290 From the results of these experiments, the authors suggest that the cationic part of the ionic liquid and amino acid might bind with the lipase protein, causing conformational change of the enzyme and contributing to the difference of Km between enantiomers.290 The IL might bind with the enzyme protein and form a partial ionic liquid layer on the protein surface, thus contributing to the increase of the flexibility of the enzyme protein. Although coating of lipase by a chiral IL like DPro-Me-PS was effective to increase performance of a lipase, it required many steps to prepare chiral ILs.289 On the other hand, synergetic activation of lipase using a combination of amino acids with IL1 provide a very simple way to activate a lipase.290 Rahman and co-workers prepared amino acid ILs, [N2222][His] and [N2222][Asn], and used them as coating materials of Candida rugose lipase through lyophilization.291 The resulting IL-coated lipase exhibited higher reactivity in esterification of oleyl alcohol with carboxylic acid (Figure 61). [N2222][His]-coated lipase showed higher reactivity than [N2222][Asn], though reactivity of these IL-coated enzymes depend on the substrates.291 Recently, Dieve and co-workers reported that hydrophilic amino acid ILs which had a cholinium ion as cation became good solvents for the lipase-catalyzed reaction.292 De Diego et al. investigated CAL-B catalyzed transesterification of 1-phenylethanol using vinyl butyrate in an ionic liquid solvent and revealed that the enzyme activity was

Figure 57. IL1-PS-catalyzed enantioselective transesterification.285

Figure 58. Synthesis of optically pure (S)-2-cyclohexen-1-ol using IL1 PS-catalyzed reaction.288

pyrrolidine group significantly affected the reactivity of lipase and D-Pro-Me-PS is more active than L-Pro-Me-PS: D-pro-Mecoated PL recorded 58-fold acceleration versus commercial lipase PS and was superior to that of IL1-PS for the transesterification of 1-phenylethanol as a model substrate. The authors further investigated differences of activities of DPro-Me-PS and L-Pro-Me-PS using acylation of (R)- and (S)-3hydroxypentanenitrile as model substrates and revealed that Vmax values of favorable (R)-isomer were drastically improved, and that Kcat value of D-Pro-Me-PS-catalyzed reaction was larger than that of L-Pro-Me-PS. On the other hand, Km values of both enzyme-catalyzed reactions were slightly increased with similar magnification for both isomers above the native lipase PScatalyzed reaction. Amino acids have been used as a stabilizer of an enzyme. For example, commercial lipase PS involves ca. 20 wt % of glycine as an essential stabilizer. On the basis of the results, Yoshiyama and co-workers prepared newly coated lipase PS by combining 10586


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Figure 60. Synergetic activation of lipase PS with IL1 and amino acids. Effect of coating on lipase PS only with an amino acid (ratea and Ea) or with both an amino acid and IL1 (rateb and Eb) on transesterification of (±)-1-phenylethanol (1a) using vinyl acetate as acyl donor. Enantioselectivity was evaluated by the E value. Rateb of the control is the result of IL1-coated PS. This figure was reproduced with permission from ref 290. Copyright 2017 The Chemical Society of Japan.

Figure 61. Activation of Candida rugose lipase by the coating with amino acid ILs.291

closely related with α-helix content of the protein; the content of α-helix was reduced to 31% immediately after lipase was added to hexane and had reached only 2% after 4 days in the solvent. On the other hand, no significant reduction of α-helix content was obtained in 1-butyl-3-methylimidazolium bis(trifuloeonethylsulfonyl)amide ([C2mim][Tf2N]) solvent. On the basis of these results, the authors concluded that α-helix contents might play an important role in maintaining the enzymatic activity.293 Kim, Yingling, Koo, and co-workers investigated the relationship between enhanced activity of lipase B from Candida antarctica and its molecular structure and found that structural dynamics in ILs and nonaqueous organic solvent were the key factors of the enzyme reactivity; the authors indicate that the α-10 helix located in the entrance part of the cavity of the active site plays an important role in the reactivity, and it can be modified due to specific interaction of solvent molecules, thus influencing the enantioselectivity (Figure 62).294 The same group recently reported the results of a more detailed investigation about the origin of modification of catalytic activity of CALB by the ILs.295 They conducted allatom molecular dynamics simulations of CALB in four types of imidazolium ILs with different chain lengths of alkyl substituents on the cationic part and the same anion (OTf). The most striking differences were found on the distance of ILE-189 and ILE-285, those amino acid residues located at the entrance gate of the protein which can open or block the

Figure 62. Possible key area on the surface of CALB protein. This figure was reproduced from ref 294. Copyright 2017 Royal Society of Chemistry.

substrate pathway to the catalytic triad.295 The results are very impressive: the size of the cavity entrance increases in the order of [C2mim][OTf] < [C4mim][OTf] < [C6mim][OTf] < [C8mim][OTf], while higher enzyme activity is observed in [C4mim][OTf] and [C6mim][OTf]. The authors also suggest that LEU-278 and ALA-282 of the a-10 helical region interact with each via a hydrogen bond which contributes structural stability of the enzyme toward solvents. This hydrogen bond between LEU-278 and ALA-282 is broken within 2 ns in [C8mim][OTf]. This is caused by the energetically favorable interaction between two hydrophobic amino acid residues, LEU-278 with hydrophobic tail of [C8mim]+, breaking the hydrogen bonds, thus yielding structural changes in the catalytic cavity and its entrance. On the basis of their very careful examination, the authors proposed the role of cations of 10587


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a coating material of Burkholderia cepacia lipase: the resulting lipase (ISCB1-PS) through the lyophilization process with ISCB1-PS displayed excellent reactivity to various alcohols and was applicable to Ru complex mediated dynamic kinetic resolution (DKR) (Figure 64): the authors achieved excellent

ILs toward enzyme activity: (1) the size of the cation and anion are comparable and show a high ion coordination number, so that the anion can strongly interact with LYS-290; this promotes the interaction between gating residues ILEU-189 and ILEU-285 and consequently the closing of the catalytic cavity which might provide poor enzymatic activity. (2) In the case of [C8mim][OTf], this IL has a strong interaction with hydrophobic amino acid residue LEU-278, which results in the disruption of an α-helix and can cause a very large cavity opening where anions can diffuse in and destabilize the catalytic trial.294,295 Zhao and Schwaneberg et al. prepared IL tolerable Bacillus subtilis lipase A by protein engineering and found that the critical points are M134, N138, and L140;296 those moieties are located on the surface of the enzyme near the substrate binding cleft.296 Figure 63 shows the structure of Burkholderia cepacia lipase by X-ray crystallographic analysis.297 It was reported that water

Figure 64. Dynamic kinetic resolution (DKR) using IL surfactant coated Lipase PS (ISCB1-PS) in the presence of the Ru complex.306

DKR, and 24 types of aryl alcohols were converted to the corresponding acetates with 96∼ >99% ee in 90−97% yields as illustrated in Figure 13.306 Lee and co-workers reported the synergetic effect of sugars and an ionic surfactant for activation of lipoprotein lipase (LPL) from Burkholderia sp.: a combination of dextrin and ISCB1 caused the strongest activation and 44-fold acceleration was accomplished when 1phenylethanol was subjected as a model substrate. On the basis of the results, the authors prepared glucosamine-headed surfactant ISC-glucosamine (ISC-glcN), and the resulting coating ILP worked best in the reaction in the presence of benzylalcohol: ca. 400-fold larger Kcat/Km values were recorded compared to that of native LPL (Figure 65).308 IL type coating materials for the activation of enzymes have gained great progress during this decade as described here. This method has no barrier for people who would like to use ILs because it needs no modification on the reaction process, just use of “IL-immobilized (coated) enzyme” in the present reaction system.285 Matsubara and Itoh et al. synthesized four types of phosphonium alkylPEG sulfate ionic liquid and evaluated them as coating materials of lipase PS. Among them, tributyl(2methoxyethyl)phosphonium cetyl-PEG10 sulfate (PL1) was found to be the best coating material (Figure 66).309 PL1coated lipase PS displayed high reactivity in transesterification of broad types of secondary alcohols using vinyl acetate as an acyl donor with excellent enantioselectivity (E > 200). The work clearly indicates that the cationic part of the coating ILs influence the lipase reactivity. Dong, Wang, and co-workers treated Candida antarctica lipase (CAL-A and Novozyme 435) with [C4mim][PF6] and

Figure 63. X-ray crystallographic structure of Burkholderia cepacia lipase with surface water. Red balls are water molecules. This figure was produced using the data reported in ref 297. Copyright 2017 The American Chemical Society.

molecules affect the activity of a lipase in ILs significantly.298 Importance of water in the activities of enzymes in IL solvents has been reported by several groups.299−302 As seen in the figure, there exist numerous water molecules on the surface of the enzyme at the opposite part of the entrance part of the active site of the enzyme. The part was believed to be important for protein movement. These reports indicate that maintaining the motion flexibility of the enzyme protein might be important for displaying the enzyme activity. It was reported that enhancing the rigidity of the flexible segment within the active site was important in improving enzyme kinetic stability.303−305 As described before, Itoh and co-workers established that ILs which have alkyl-PEG sulfate anions worked as excellent activating agents of Burkholderia cepacia lipase. Since alkyl-PEG moiety is essential to cause an increased reaction rate, we assumed that alkyl-PEG moiety in IL1 binds with the bottom portion and prevents removal of these surface water molecules, contributing to the improved flexibility of the enzyme. Therefore, it had been speculated that these two factors might be improved simultaneously by the appropriate design of coating materials and reaction medium. Activation of lipase by ionic surfactant compounds that have alkyl PEG moiety was reported by Kim and co-workers.306,307 The authors synthesized potassium 3,5-bis(2-(2-(2ethoxyethoxy)ethoxy)ethoxy)benzoate (ISCB1) and used it as 10588


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Figure 67. IL-coated lipase-catalyzed ring-opening polymerization of lactones.310,311

assumed that the IL coating caused a favorable change of the enzyme protein, hence polymer yield was drastically enhanced compared to the commercial enzyme. Li, Huang, Hu, and co-workers reported that development of imidazolium chloride or hydrophosphate ionic liquids which have PEG moiety in the cations were used as coating materials of Candida rugosa lipase (Figure 68).312 Coating with PEG-

Figure 65. Activation of lipoprotein lipase by coating using a combination of sugar with ISCB1 and ISC-glcN.308

Figure 68. List of ILs for activation of Candida rugosa lipase (CRL)312 or Rhizopus oryzae lipase.313

functional imidazolium IL, [HOOCEPEG350im][H2PO4] improved lipase catalytic activity, thermostability, organic solvent tolerance, and adaptability to temperature and pH changes in olive oil hydrolysis reaction; the catalytic activity was increased up to 1.7-fold, and thermostability was 5.0-fold at 50 °C for 2.5 h compared with the native lipase. Interestingly, β-sheet content was increased and decreased α-helix was observed by the coating; α-helix ratio and β-sheet ratio of an IL-coated enzyme structure were estimated as 17.2% and 30.1%, respectively. On the other hand, those of the native CRL were 29.7% and 15.3%, respectively. It is now obvious that ILs or salt type surfactants can activate the lipases more effectively than nonsalt surfactant or alkyl polyethylene glycol. Banerjee et al. reported the synthesis two types of gemini IL type surfactant as an activating or stabilizing agent of Rhizopus

Figure 66. PL1-PS-catalyzed acylation of 1-phenylethanol in an IL solvent system.309

used them as catalyst of ring-opening polymerization of 1,4dioxan-2-one (PDO), succeeding in obtaining the polymer (poly-PDO): initially they used [C4mim][PF6] as a cosolvent of THF and found that a mixture of Novozyme 435 with [C4mim][PF6] effectively catalyzed polymerization (Figure 67A).310 He et al. next prepared 1-methyl-3-octylimidazolium hexafluorophosphate ([C8mim][PF6])-coated Novozyme 435 through lyophilization and the resulting IL-coated enzyme, which displayed good activity for ring-opening polymerization of γ-valerolactone (Figure 67B).311 The authors also reported that α-helix content of Novozyme 435 (CALB) was increased by the IL coating through the lyophilization process. It was 10589


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oryzae lipase: [C16-3-C16im][Br2] gave better results in increasing thermal stability of the enzyme compared to [C1612-C16im][Br2] (Figure 68). These ILs caused a decrease in αhelix and an increase in β-sheet content of the enzyme, and this structure change was even more significant with the addition of [C16-12-C16im][Br2].313 However, they investigated thermal stability of the enzyme in a buffer solution only by adding them. Since ILs have an interesting structure, I feel a better stabilization effect may be obtained if the enzymes were treated with these ILs by lyophilization. Lee and Koo et al. also reported efficient activation of lipase through lyophilization of ionic liquid type sol−gel silica.314 Hara et al. investigated activity of transesterification of three types of immobilized Burkholderia cepacia lipase in conventional organic solvent and imidazolium ionic liquids with different anions.315 The results were influenced significantly by both substrates and immobilization materials. BCL xero316 was revealed to be a good choice for an activated ester to susceptible to hydrolysis as a side reaction when the reactions were carried out in IL solvent systems. Kato et al. reported similar activation methods when the authors prepared a silicaenzyme-ionic composite and the resulting enzyme exhibited higher activity than the native enzyme.317 Appropriate choice of immobilizing solid materials suitable for lipase-catalyzed reaction in ILs is also very important to realize the desired reactions. Extensive efforts have thus been devoted to this area. Zou and co-workers reported enhanced stability of PPL by enzyme aggregate coating immobilized onto IL modified mesoporous silica, SBA-15.318 Zarcula319 and de Souza320 independently reported improved activity of lipases in ILs using similar immobilization methods. Filice and coworkers reported octyl sepharose immobilized lipase and tested their enzyme for several reactions such as regioselective hydrolysis of acetyl nucleotide in IL-buffer mixed solvents.321 Since a magnetic immobilized enzyme allows for easy recovery of the enzyme from the reaction mixture, growing interest has been devoted to designing such type of magnetic materials. Liu and co-workers demonstrated enzymatic kinetic resolution using magnetite-immobilized Yarrowia lipolytica lipase in a mixed IL solvent system; a mixture of [C2mim][BF4]-[C8mim][BF4] = 1:9 gave the highest enantioselectivity while the fastest reaction was recorded in a 1:1 mixed solvent.322 The same group reported a further successful example of a magnetic cellulose microgel system for immobilization of lipases.323 Cao et al. also developed useful magnetic cellulose nanocrystals for immobilization of lipase.324 Cui et al. reported the preparation of surfactant-activated bovine pancreatic lipase supported on Cu3(PO4)2 nano particles (nano flower); the authors tested several surfactants and revealed that ionic liquid, cetyltrimethylammonium bromide (CATB), worked as the optimal one.325 Several useful immobilization materials have also been reported for improving activity of lipase-catalyzed reactions in ILs.326−333 Among recent examples, two methods are of particular interest. Brogan and Hallett prepared protein−polymer surfactant nanoconstructs and revealed that the resulting protein allows for dissolution of dry protein into dry ionic liquids. Using myoglobin as a model protein, they show that this method can deliver protein molecules with near native structure into both hydrophilic and hydrophobic anhydrous ionic liquids and protein stability increases by 55 °C in the ionic liquid as compared to aqueous solution.332

Zhao, Fu, and co-workers prepared six different types of DNA nanocage-encapsulated enzymes, among them HRP, malic dehydrogenase, glucose-6-phosphate dehydrogenase, lactic dehydrogenase, and glucose oxidase. The resulting immobilized enzymes are very stable and strongly tolerant of protease digestion. Although the authors did not investigate activities of these enzymes in ILs, I feel these immobilized enzymes may be suitable for use in an IL solvent system because DNA are generally highly soluble in ILs.333 3.2. Activation of Lipases by the Design of ILs as Solvent

Selection of appropriate ILs as a reaction medium is the key to realizing the desired reaction. As mentioned before, Lozano and co-workers reported that stability of lipases was also significantly enhanced by the ILs.68,73 Enzyme stability has been discussed from the standpoint of ion specificity toward enzyme protein based on the Hofmeister series as an order of the ion effect on the protein stability.334−336 Qin evaluated 17 types of ILs as solvent for Novozyme 435-catalyzed alcoholysis of monoolein (rac-glyceryl cis-9-octadecanoate) with methanol by comparison of four physical properties: log P, ENT, β-value, and viscosity. The authors reported that the initial activity of the enzyme was increased with the increase of log P value and reduced with the ENT value. High β-value and viscosity were not beneficial to enhance enzyme activity. Enzyme stability was significantly influenced by the anions of ILs and seems to slightly depend on the ratio of α-helix content decreased and the β-sheet content increased of the enzyme in ILs and also seems to reflect deactivation.336 Itoh’s group attempted to design ILs as solvents suitable for the IL-coated enzyme.337−340 After evaluating ILs for IL1catalyzed transesterification of 4-phenylbut-3-en-2-ol (A) as a model substrate, they found that phosphonium IL, tri-nbutyl(2-methoxyethyl)phosphonium bis(trifluoromethyl)sulfonylamide ([P444ME][Tf2N]), worked as the best solvent: the authors thus succeeded in demonstrating recyclable use of IL1-PS 10 times while maintaining their initial reaction rate with excellent enantioselectivity (Figure 68).337 On the basis of the results, Abe et al. discovered that introduction of methoxyethyl motif into the cationic part of ILs was effective in designing appropriate ILs for IL1-PS-catalyzed reaction, though they depend on the substrate; tri-n-butyl(2methoxyethoxyethyl)phosphonium bis(trifluoromethyl)sulfonylamide [P444MEM][Tf2N] was suitable for the reaction of 1-phenylethanol (B), though N,N-diethyl-N-methyl-N-(2methoxyethoxymethyl)ammonium bis(trifluoromethyl)sulfonylamide ([N221MEM][Tf2N]) was the best solvent for 5penylbut-1-en-3-ol (C) (Figure 69).338,339 Increased Kcat values were suggested to be the most important factor in IL1-PS working in these solvents.339 The viscosity of these ILs are ca. 200−300 times higher than i-Pr2O, while the reaction rate was superior to that of i-Pr2O; the results clearly indicate that catalytic activity of lipase is independent of the mass transfer rate of the substrate in the solvent system.339 Introduction of alkylether moiety in the ILs in cationic or anionic parts generally has provided good results.340 Since then, numerous successful examples have been reported by many groups.53,341 From the standpoint of easy workup, hydrophobic ILs are preferable. However, for increasing activity, Ou and co-workers reported that introduction of hydroxyalkyl moiety in anion or cation of ILs was effective to improve activity, though the resulting ILs have been generally hydrophilic.342 Since deep eutectic solvent (DES)343−346 contained cholinium cation 10590


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Figure 70. Typical examples of buffer ILs.349−351

Lozano et al. prepared N-cetyl-N,N,N-trimethylammonium bis(trifluoromethyl)sulfonylamide ([C16tam][Tf2N]) which was termed “sponge-like IL(SLIL)”: SLIL displayed unique property as a reaction medium of lipase-catalyzed reaction; the authors established an efficient protocol for biodiesel oil production employing SLIL (Figure 71).354

Figure 69. Results of IL1-PS-catalyzed acylation in different IL systems. Rate means mM/mg enzyme, h.337−339

([Chol]) which has hydroxyethyl moiety, just addition of DES solvent affected the lipase activity, although the effect significantly depended on the contents. Kim and co-workers reported that a combination of [Chol] with urea and glycerol improved activity, while those of formamide and glycerol and foramide and thiourea caused a drop in the activity.347 Byrne and co-workers investigated the effect of five ILs toward stability of the Thermomyces lanuginosus lipase (TLL), which is known as a thermostable enzyme. They revealed that both nature of ILs and pH of the reaction mixture had a significant impact on the hydrolytic activity. The ion concentration of the ILs is correlated to the water activity αW, five water molecules per IL, that is αW = 0.6 is the threshold of water for assured lipase activity. The authors also reported that ILs which have large hydrogen bonding basicity (β-value over 0.8) caused enzyme denaturation.348 Buffer-ionic liquids might be very attractive to realize activation and stabilization of lipases. Successful examples of a preparation of buffer-ILs have been reported:349−351 Ou prepared imidazolium phosphinium IL as buffer liquid (ILbuffer) and conducted lipase-catalyzed transesterification of ethylbutanoate with n-butanol in the presence of the IL buffer in 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ([C2OHmim][BF4]) (Figure 70, upper): 5-fold improvement of conversion in the CAL B-catalyzed transesterification of ethyl butanote with n-butanol was recorded in the presence of [buffer IL].349 Taha and co-workers prepared 20 types of buffer IL(GB-IL) (Figure 70, below) and investigated their impact on stability of proteins, such as bovine serum albumin350 and αchymotrypsin;351 they revealed that GB-IL exhibited a stabilizing effect on these proteins. Ventura and co-workers reported the results of evaluation of self-buffering ILs and revealed that aqueous biphasic systems composed of GB-ILs and potassium citrate were highly effective to achieve desired reactions.352 Gupta, Taha, and Lee et al. recently reported the synthesis of many types of GB-ILS and found them effective in both reaction media of enzymatic reaction and extraction solvent of a-chymotrypsin.353

Figure 71. Cyclic protocol of biodiesel oil production using SLIL.354

Rogers and co-workers investigated stability of cellulase from Trichoderma reesei in the presence of urea, NaCl, or [C4mim][Cl] and found that [C4mim][Cl] caused denaturation of enzyme; Cl− ion was responsible for the inactivation of the cellulase.355 Lee and co-workers reported that the activity of lipase from Rhizomucor miehei decreased with increasing Cl− content in [C8mim][Tf2N], and the activity of lipase in [C8mim][Tf2N] mixture containing 2% [C8mim] [Cl] was only about 2% compared to that in pure [C8mim][Tf2N].356 Reza Bozorgmehr et al. investigated molecular dynamics simulation of α-amylase in [C6mim]Cl and suggested that the IL induced a structural change.357 Recently Fan and Miao et al. also reported an inhibitory effect of [C4mim][Cl] on the lactic dehydrogenase activity.358 On the contrary, Daneshjoo and co-workers reported that thermal stability of a lipase from Pseudomonas at 50 °C was increased by addition of 20% (v/v) [C4mim][Cl] in Tris-HCl buffer (pH 8.0), and this was caused by prevention of aggregation of the protein under the conditions employed.359 They also reported that the Km value of the hydrolysis reaction of p-nitorophenyl acetate was increased when the reaction was carried out in 50% (v/v) [C4mim][Cl] in buffer solution. This indicated that [C4mim][Cl] reduced the binding affinity of the substrate to the enzyme.359 These results suggest that properties of ILs are determined by a combination of cations 10591


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and anions. With the aim of making a guideline for designing ILs of biocatalytic reactions, Rodrigues and co-workers investigated the structure−function relationship of Thermomyces lanuginosus lipase as a model protein in IL media; 61 different ILs were evaluated on the activity.360 However, the details are still unclear of how ILs modify the structure of the protein conformation and affect reactivity of the enzyme. Biocatalysis in supercritical fluid has gained strong interest due to the simplicity of the workup process.38 However, lipases rapidly lost their activity in the solvent. Laszlo and Compton72 and Lozano groups solved this problem using IL engineering: ILs stabilized the enzyme when the reaction was conducted in supercritical carbon dioxide (scCO2) (Figure 72).361

Figure 73. Lipase-catalyzed DKR system of 1-phenylethanol by the mixed solvent system of IL with scCO2.366

scCO2: the authors made an attempt to modify the lipase protein which would be tolerant to a scCO2 solvent system by the acetylation of lysin residue side chains. Although they failed to improve stability of the enzyme in scCO2, thermal stability of the enzyme in water was successfully improved by this modification.367 Hydrophobic ionic liquids generally act as good reaction media for lipase-catalyzed reaction, which, on the contrary, hydrophilic ILs give poor or no reaction. As described in this chapter, introduction of alkyl ether moiety in the cationic part of ILs with combined hydrophobic anion such as Tf2N anion seems to be a sure way to design ionic liquids suited for lipase-catalyzed reaction.

4. FUTURE PERSPECTIVE OF USING ILS FOR ENZYMATIC REACTIONS It has already been more than two decades after the first demonstration of an enzymatic reaction using ILs. ILs offer a high potential to replace classic flammable and toxic organic solvents. Therefore, I believe that ILs might become more popular in the future and then provide a certain benefit in our life. I wish to discuss in what fields we should focus our investigation for utilizing ILs in biotechnology in the future. I suggest three properties of the ILs that might become more important as research projects in this field.

Figure 72. Lipase-catalyzed transesterification of racemic 1-phenylethanol by the mix solvent system of IL with scCO2.361

4.1. Unique Phase Behavior of ILs

Reetz et al. also independently reported the similar results of stabilization of enzyme by the IL as a cosolvent when the reaction was carried out in the scCO2 solvent system.362 It was supposed that the enzyme gradually became a denatured form in highly hydrophobic scCO2, due to stripping off the essential water. On the other hand, this was prevented by the IL, hence the desired reaction proceeded smoothly. It has now been firmly established that addition of IL as a cosolvent is essential to achieve the lipase-catalyzed transesterification in the scCO2 solvent system.363−368 Lozano et al. reported the DKR reaction using the scCO2[C4mim][PF6] solvent system.366 In the reaction, the IL played two important roles, stabilizing the enzyme in scCO2 and accelerating racemization of the substrate alcohol. Using this system, the authors succeeded in obtaining (R)-1-phenylethyl propionate in 79% yield with 97% ee using an acidic ionexchange resin as a racemization catalyst (Figure 73). Recently, Shimomura et al. reported that zeolite H-β-VALFOR CP811BL-25 worked as a good racemization catalyst in an IL, and they accomplished DKR for several alcohols.369 Therefore, it may be possible to design more efficient DKR system by combining scCO2 with zeolite instead of an ionexchange resin. Monhemi and Reza Housaindokht recently reported an interesting attempt to prevent denaturation of enzyme in

This property might be an important aspect of ILs and is useful in the biosciences. Gutowski, Rogers, and co-workers reported the first example of the extraction of salts by aqueous two-phase extraction using ILs in 2003.370 Du and co-workers applied ILbased aqueous two-phase system for quantification of bovine serum albmin.371 Dreyer and Kragl reported the increased stability of alcohol dehydrogenases in aqueous two-phase systems using an oligoethylene glycol IL, Ammoeng110, as shown in Figure 34.188 Pei, Wang et al. used an IL-based aqueous two-phase system for extraction of several proteins and revealed that trypsin, cytochrome c (cyt. c), BSA, or γ-globulin could be extracted by the system.372 Fujita and Ohno reported that an IL, hydrated choline dihydrogen phosphate contained 30% (w/w) water dissolved metallo proteins such as cyt. c, peroxidase, ascorbate oxidase, azurin, pseudoazurin, or fructose dehydrogenase while maintaining their initial activities.373 Tamura et al. demonstrated the cyt. c mediated redox reaction up to 140 °C in 1-allyl-3-methyliniudazolium chloride.374 Ito and Ohno et al. prepared the phosphonium-type twitterion (N ,N ,N-tripentyl-4-sulfonyl-1-butanephosphonium: P555C4S)375 and used it as an additive for controlling the water content of the hydrophobic IL [C4mim][Tf2N]; this allowed the reverse extraction of cyt. c from the IL phase to the aqueous phase as illustrated Figure 74.376 The method allowed selective 10592


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and hydrophobic anhydrous ionic liquids and the protein stability significantly increases in the ionic liquid. It was also suggested that ILs may provide a platform for the realization of biocatalysis at high temperatures or in anhydrous solvent systems.332 As described in section 3, there is a certain possibility of an enhanced stability of a protein by an IL-coating; in fact, we have established that the ionic liquid-coated lipase is very stable in conventional organic solvents in the absence of substrates.285,339 As already mentioned, the viscosity of many ILs are at least several hundred times higher than conventional organic liquids, such as i-Pr2O, tolune, and hexane, while the reaction rates of an ionic liquid coated IL (IL1-PS)-catalyzed transesterification of many secondary alcohols were superior to those of organic liquids; the results clearly indicated the that catalytic activity of the lipase is independent of the mass transfer rate of the substrate in the solvent system.339 I am expecting that further investigation into the origin of stabilizing and the activation effect of ILs toward enzymes will make it more important in the future in the field of both biosciences and biotechnology.

extraction of enzyme proteins from an aqueous solution while maintaining their initial activity.

4.3. Importance of ILs as Enzyme Activating Agent

Replacement of the reaction media from classical organic solvents or water to ILs in industry has still not yet been realized to date. One serious barrier is the high viscosity of the ILs, since viscous solvents require complete replacement of the reaction process. Furthermore, except for biodiesel oil production, organic molecular solvents are still necessary for the extraction process after the reaction even when the reaction would be efficiently conducted in a pure IL solvent system. ILs might become popular in the future in bioindustry. However, I believe that they would be mainly used not as solvents but as the controlling agents of enzymes. In such case, we need only small quantities of ILs and no modification of the present reaction process. In section 3, I described the significant possibility of an IL coating for improving the performance of lipases. Although successful examples are limited to only lipases to date, I believe that this methodology might become more important in the future. In the field of chemical industry, the same trend is observed; investigations of using ILs as a solvent of chemical reactions have weakened. Instead, research projects regarding supported ionic liquid (SILP) catalysis382 have currently gained strong interests and have reached a practical level. In fact, an SILP catalyst has currently been used to remove mercury vapor from natural gas at a plant in Malaysia.383 I expect that the scope of biocatalsis might be expanded by the ionic liquid engineering.

Figure 74. Reverse extraction of cyt. c from the IL phase to aqueous phase by controlling the water content of the IL phase using a twitterion.376

Kuroda et al. also reported renaturation of cyt. c dissolved in polar ILs by adding zwitterions.377 The authors found that cyt. c was dissolved in a dry polar IL, 1-ethyl-3-methylimidazolium methylphosphate ([C2mim][(MeO)(H)PO2]; for the structure, see Figure 74) that caused denaturation of the structure. However, renaturation of cyt. c took place when 3-(1methylimidazol-3-io)butane-1-carboxylate (C1im3C) (for the structure, see Figure 74) and water at the concentration of 1/1/ 11 ([C2mim][(MeO)(H)PO2]/C1m3C/water) led to complete renuturation of cyt. c.377 Although the authors reported no application of cyt. c for organic synthesis, this method may become a key technology in the future because the importance of the enzymatic oxidation has recently grown. Enzymatic oxidations using cyt. c and P450 realize the hazardous chemical reagent-free organic synthesis; investigation in this field might be quite valuable from the standpoint of green chemistry. Only a few examples have been reported for the cytochrome or P450-catalyzed reactions for organic synthesis due to the narrow substrate specificity of these enzymes. However, efficient methods to broaden the substrate specificity of P450 using decoy molecules have recently been developed by Shoji and co-workers.378−380 I anticipate that the combination of their methodology with IL engineering may provide a breakthrough in this field in the future.

5. CONCLUDING REMARKS In section 2, the enzymatic reactions in ILs were reviewed. To the best of my knowledge, more than 13 types of enzymecatalyzed reactions have already been demonstrated in ILs. Among them, I selected examples of enzymatic reactions for organic syntheses in this review. Except for lipases, almost all enzymes require water to work. Hence a mixed solvent system of ILs with water was employed. I also mentioned the biocatalytic promiscuity of the enzymatic reactions using ILs or DES as solvents. At present, it is difficult to say that these reactions to date could be superior to those using a chemical method, transition metal, or organo-catalysts. However, I feel further investigation in this area will make it possible to

4.2. Stabilizing Ability of ILs toward Enzymes

Gutiérrez and co-workers reported interesting results from the application of ILs toward microbes. The authors used a freezedrying process for the incorporation of bacteria in DES in its pure state; this process allowed the outstanding preservation of the bacteria integrity and viability.381 As already mentioned, Brogan and Hallett demonstrated that engineering the surface of a protein to yield protein−polymer surfactant nanoconstructs allows for dissolution of the dry protein into dry ionic liquids.332 Using myoglobin as a model protein, the authors have shown that this method can deliver protein molecules with a nearly native structure into both hydrophilic 10593


CPO

HRP HRP cytchrome c oxidase

P450 (human CYP) P450 (whole cell) combined with GDH

rohdococcus

protease (thermolysin) protease (α-chymotrypsin)

protease (thermolysin)

enantioselective oxidation

free radical polymerization polymerization oxidative decomposition

enantioselective hydroxylation enantioselective oxidation

hydration

transesterification transesterification

transesterification

laccase

oxidation

laccase CPO

lipase laccase

transesterification oxidation

oxidative trimerization enantioselective oxidation

fungus whole cell (Lipase) lipase lipase lipase

methanolysis transesterification transesterification transesterification

laccase

lipase

transesterification

oxidation

lipase lipase lipase

transesterification transesterification transesterification

laccase laccase

lipase

transesterification

oxidation oxidation

lipase

biocatalyst

transesterification

reaction type

10594

[P4444][Z-Asp]-buffer

[C4mim][PF6]-water (95:5) [CMEMmim][PF6]-buffer

[C4mim][PF6]-water [1:4 (v/v)] (biphasic system)

buffer-[C4mim][PF6] (biphasic system) buffer-[P666,214] [Tf2N] (biphasic system)

[C4mim][Tf2N]-imobilized HRP in buffer water including 20% (v/v) DES buffer-IL (BHEAF)

[C4mim][Br]-buffer [10:90 (v/v)]

buffer-TX-100 or CTAB [C4mim][MeSO4]-buffer [10:90 (v/v)]

buffer-[Chol][H2PO4]

IL as a cosolvent buffer-[C6mim][AOT], [N1888][AOT] or [N1888][Sac]

L-menthol-based

graphite powder-laccase-nujol-Il [50:20:15:15 (w/w/w/w)]

[C4mim][BF4] under ultrasound at 50 °C, 15 h buffer-[4-MBP][BF4] [4:1 (v/v)]

[C4mim][BF4]-buffer [MOMmim][BF4]-buffer [C4mim][BF4]-t-BuOH [C4mim][OTf]-[C4mim][Tf2N]

[C4mim][PF6], IL-type acylating agent pure IL: [C4mim][PF6], in the presence of Ru-complex hexane: two flask connected with [C4mim][BF4]-supported membrane IL (many types of ILs)

[C4mim][PF6], under reduced pressure

[C4mim][PF6]

solvent or conditions

Table 1. Improved Performance of Biocatalysis Using IL Engineering

enhanced enantioselectivity recyclable use of enzyme reduced amount of acyl donor methyl ester can be used as an acyl donor easy separation of the products acceleration of the racemization by the Ru-complex chromatography-free separation of produced ester and unreacted alcohol easy separation of the product biodiesel oil from the reaction mixture easy separation of the product from the reaction mixture improved regioselectivity improved regioselectivity improved reaction efficiency due to highly concentrated sugar solution in IL improved regioselective acetylation of a polysaccharide improved efficiency of the reaction due to reducing the toxicity of the mediator improved stability of enzyme nonvolatile liquid that works as an electron conducting solvent for a biosensor improved activity and stability improved reactivity and stability by reducing toxicity of the mediator compound high activity (4.5-fold compared to that in a buffer solution) improved stability improved stability, probably by reducing toxicity of the oxidant improved yield (ca. 3.5-fold) while maintaining perfect (>99% ee) enantioselectivity recyclable use of enzyme the reaction proceeded even at 4 °C enhanced reactivity due to improved solubility of the cytochrome c improved reactivity improved reactivity due to reduced toxicity of the substrate enhanced enantioselectivity improved reactivity due to modified aggregation state of the microbe cell just demonstration the reaction is 16-times faster compared to that in a mixed solvent of CH3CN-buffer improved efficiency because amino IL was used as both cosolvent and substrate

major merit using an IL or proposed origin of the ILeffect

30

28 29

27

25 26

22 23 24

21

19 20

18

16 17

15

13 14

9 10 11 12

8

5 6 7

4

3

figures

refs

179

20 175 and 176

19

171 172

164 165 166

167

156 158

155

152 154

142

128 136

113 71 124 and 129 126

108−121

84 and 85 87 102

80

64 and 65

Chemical Reviews Review


[C4mim][PF6]-THF(involving water) [10:90 (v/v)] [C1mim[MeSO4]]-buffer [1:4(v/v)] [Ammoeng101][Cl]-MES buffer [1:5 (v/v)] buffer involving betain type IL [C2mim][H2PO4]-buffer [25:75(v/v)] [C4mim][Cl]-buffer (small volume), 1% cellulose solution [C2mim][OAc]-buffer [10:90 (v/v)] [C2mim][BF4]-buffer [1:1(v/v)], biphasic layer

DES (ethalin)-buffer [1:1(v/v)] immobilization agent: o [γ-Fe2O3-SiO2][C4mim][Cl or I] [C2mim][BF4]-buffer [16:84(v/v)] [C6mim][Br]-30% H2O2 aq solution [TMOA][NO3]

ADH+GDH

ADH

acylase

acylase (penicillin G acylase) Holl cell (Psedomonas sp.)

β-galactosidase

sucrose phospholylase

α-glucosidase cellulase

cellulase cellulase + yeast

hydroxynitrile lyase

fluorinase

haloalkan dehydrogenase luciferase

β-glucuronidase lipase (CAL-B) lipase (CAL-B)

enantioselctive reduction



hydrolysis

hydrolysis

transesterification

trans glycosidation

glycosylation

hydrolysis hydrolysis

hydrolysis hydrolysis and α-ketoglutric acid formation cyanohydrin formation

fluorination

10595

G-DES-buffer [6:4 (v/v)]

benzaldehyde lyase

protease (subtilisin) lipase (IL mixed enzyme) lipase (IL1-coated enzyme)

alcoholysis transesterification transesterification

n-hexane, i-Pr2O or THF toluene i-Pr2O, n-hexane

[C4mim][PF6]-H2O

lipase (PPL)

enantioselective dehalogation oxidation (ATP-dependent conversion of luciferine into lucifery-aldenylate) hydrolysis oxidation peracid formation then Baeyer-Villigar oxidation enantioselctive aldol condensation C−C bond formation

[C8mim][PF6]

[C4mim][NCA]-buffer [25:75 (v/v0)]

IL-buffre [15:85(v/v)]

IL ([Ammoeng110][Cl])-buffer-biphasic system

[P1,4][Tf2N]-buffer [10:90 (v/v)]

[C4mim][PF6]-the cell was immobilized on MES buffer containing water absorbant polymer [MTEOA][MeSO4]-Tris-HCl buffer [90:10 (v/v)]

oxide-reductase (whole cell) ADH


[C4mim][PF6]-H2O [10:1 (v/v)]


baker’s yeast

biocatalyst


reaction type

Table 1. continued

just demonstration because the results were inferior compared to those in a pure buffer solution marked acceleration using KCl-coated enzyme enhanced enantioselectivity remarkable acceleration while maintaining excellent enantioselectivity for many secondary alcohols

improved solubility of the substrate recyclable use of enzyme impossible reaction in an aqueous or organic solvent reaction medium impossible reaction in a conventional buffer solution

improved reactivity of regioselctive acylation due to modified cell surface morphology improved efficiency of the transglycosidation due to suppressing the secondary hydrolysis in buffer improved reactivity due to enhanced solubility of the substrates improved reactivity and stability of the enzyme glucose formation yield was 2-fold higher than that in a buffer solution due to improved solubility of cellulose realization of direct glucose formation from cellulose direct production of α-ketoglutaric acid from cellulose was accomplished excellent chemical yield with high enantioselectivity (97% ee) due to surppress nonenzymatic reaction high chemical yield (8-fold higher) was obtained compared to that in tris-HCl buffer. The cell was immobilized on a water absorbant polymer improved stability of the enzyme improved stability

improvred reactivity compared to those in a water solution excellent enantioselectivity (>99% ee) and easy separation of the product excellent enantioselectivity (>99% ee), though chemical yield was dropped excellent enantioselectivity (>99% ee) and easy extraction of the products improved reactivity with excellent enantioselectivity (>99% ee) improved yield compared to that in a mixed solvent of water with CH3CN 1.6-fold higher yield compared to the reaction in water

major merit using an IL or proposed origin of the ILeffect figures

54 55 56,57,58,59, and 60

53

52

49 50 51

47 48

46

45

43 44

41 42

40

39

38

37

36

35

34

33

32

31

refs

266 279 283, 285, and 288−290

257

255

231 251 252

221 227

220

218

210 213

198−200 206

197

195

194

191

190

188

187

186

185

184



lipase lipase ([C16tam][Tf2N]coated enzyme)

lipase lipase

transesterification alcoholysis

transesterification transesterification

hydrolysis transesterification

lipase) [C4mim][PF6]coated enzyme) lipase lipase (IL1-coated enzyme)

lipase (amino acid ILcoated enzyme lipase (PEG-substituted benzoic acid potassium salt-coated enzyme) lipase (PEG-salt-coated enzyme prepared through lyophilization) lipase (PL1coated enzyme)

biocatalyst

polyester synthesis

transesterification

transesterification

transesterification

transesterification

reaction type

Table 1. continued

scCO2-[C4mim][Tf2N] ScCO2-[C4mim][Tf2N]-acid ion-exchange resin

buffer IL vegetable oil-MeOH, phase-controlled conditions

buffer, organic solvent IL: [P444ME][Tf2N], [P444MEM][Tf2N], or [N221MEM][Tf2N]

neat

remarkable acceleration (PL1-coated enzyme)

[P444PM][Tf2N]

easy separation of the products improved stability of the enzyme improved stability of the ernzyme realization of transition metal catalyst-free DKR

recyclable use modified substrate specificity compared to native enzyme efficient polymerization was accomplished due to favorable change of the enzyme conformation improved organic solvent tolerance and thermostability remarkable acceleration recyclable use remarkable acceleration improved efficiency of the biodiesel oil production

remarkable acceleration

remarkable acceleration of DKR

remarkable acceleration

major merit using an IL or proposed origin of the ILeffect

toluene

toluene

n-hexane


72 73

70 71

68 69

67

66

65

64

61

figures

refs

361 366

349−351 354

312 and 313 337−339

310 and 313

309

308

306 and 307

291


10596


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[C8mim]+=1-methyl-3-octylimidazolium [HOPmim]+=1-methyl-3-(3-hydroxy)propylimidazolium [MEmim]+=1-(2-methoxy)ethyl-3-methylimidazolium [MOMmim]+=1-methoxymethyl-3-methylimidazolium [3-Ph-C3mim]+=1-methyl-3(3-phenylpropyl)imidazolium [C16-3-16im]+=structure, see Figure 68 [C16-12-16im]+=structure, see Figure 68 C1im3C=3-(1-methylimidazol-3-io)butane-1-carboxylate [C16tam]+=N-cetyl-N,N,N-trimethylammonium [C18tma]+=N,N,N-trimethyloctadecan-1-ammonium [C2OHmim]+=1-(2-hydroxyethyl)-3-methylimidazolium [Me(OCH2CH2)3-Et-Pip]+=ethyl (2-(2-methoxyethoxy)ethoxy)ethylpiperidin-1-ium [Me(OCH2CH2)3-Et3N]+=triethyl (2-(2-methoxyethoxy)ethoxy)ethylammonium [Me(OCH2CH2)3eim]+=1-ethyl-3-(2-(2-methoxyethoxy)ethoxy)ethylimidazolium L-Pro-Me=(S)-1-butyl-2-methyl-3-(pyrrolidin-2-ylmethyl)1H-imidazol-3-ium D-Pro-Me=(R)-1-butyl-2-methyl-3-(pyrrolidin-2-ylmethyl)1H-imidazol-3-ium [MTEOA] + =N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium [N221MEM]+=N,N-diethyl-N-methyl-N-(2methoxyethoxymethyl)ammonium [N2222]+=N,N,N-tetraethylammonium [N1888]+=N-methyl-N,N-dioctyloctan-1-aminium [Phe]+=methyl 2-ammonium-3-phenylpropanoate [TEOA]+=tris(2-hydroxyethyl)ammonium [TMA]+=tetramethylammonium [TMOA] + =2-hydroxyethyl-N,N,N-trimetnhylammonim ([Chol]+) CTAB=N,N,N-trimethylcetylammonium bromide [P1,4]+=N-butyl-N-methylpyrrolidium [P1CH2CO2]=2-(1-methylpyrrolidin-1-ium-1-yl)acetate [P4444]+=tributylphosphionium [P444ME]+=tributyl(2-methoxyethyl)phosphonium [P444MEM]+=tributyl(2-methoxyethoxyethyl)phosphonium [P666,14]+=tri-(n-hexyl)tetradecylphosphonium [EtPy]+=ethylpyridinium [4-MBP]+=1-butyl-1−4-methylpyridin-1-ium [OAc]−=acetate [AOT]−=1,4-bis(2-ethoxyhexyl)sulfosuccinate [BF4]−=tetrafluoroborate [Z-Asp]−=2-[(tert-butoxycarbonyl)amino]-3-carboxypropanoate (Z-aspartate) [SAC]−=saccharinate [(MeO)(H)PO2]−=methylphosphate [NO3]−=nitrate [DCA ]−=dicyanamide [OTf]−=trifluoromethanesulfonate [TFA]−=trifluoroacetate [H2PO4]−=dihydrogen phosphate [(MeO)(H)PO2]−=methylphosphate [DEP]=1-ethyl-3-methylimidazolium diethylphosphate [MEESO4]−=2-(2-methoxyethoxy)ethylsulfate [Tf 2 N] − =bis(trifluoromethylsulfonyl)amide. [Tf 2 N] is widely known as “bis(trifluoromethylsulfonyl)imide”. However, “imide” means “an amido compound which connected with two carbonyl group”, therefore, [Tf2N] should be named as “bis(trifluoromethylsulfonyl)amide” according to the IUPAC rule [N444CH2CO2]=2-(tributylammonio)acetate

broaden the applicability of enzymatic reactions and contribute to realizing green organic syntheses. In section 3, the activating method of the lipase-catalyzed transesterification using ionic liquid technology was reviewed as the main topic. As shown in this chapter, the phosphonium ionic liquid and ammonium ionic liquids, which have alkylether moieties, worked as excellent reaction media for the lipasecatalyzed transesterification, especially for an ionic liquid coated-lipase. I summarized how ILs contribute to improving the performance of enzymatic reactions described in sections 2 and 3 in Table 1. In section 4, I discussed the future perspective of the ionic liquid chemistry in biotechnology. We can recover the ILs and repeatedly use them following simple purification after the reaction. In fact, we always recycle our ionic liquids after the reaction and have not wasted any in the past. We are still using several ionic liquids that have more than a 17-year history. Breakthroughs have sometimes occurred with innovation of a reaction medium in a chemical reaction, and this is true even in enzymatic reactions. I hope this paper may provide information for the reader’s research studies.

ASSOCIATED CONTENT Special Issue Paper

This paper is an additional review for Chem. Rev. 2017, 117, issue 10, “Ionic Liquids”.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Toshiyuki Itoh: 0000-0002-8056-6287 Present Address

4-101 Koyama-minami, Tottori City 680−8552, Japan. Notes

The author declares no competing financial interest. Biography Toshiyuki Itoh was born in Matsusaka, Mie, Japan, in 1954. He graduated Tokyo University of Education in 1976. After working as a chemistry teacher in high schools in his hometown in Mie prefecture, he decided to go back to the University and received his Ph.D. degree in 1986 from the University of Tokyo. He was appointed Assistant Professor of Okayama University in 1987 and then promoted to Associate Professor in 1990. He worked with Professor Anthony G. M. Barrett as a visiting scholar at Colorado State University in 1990−1991 and moved to Tottori University in 2002, where he was promoted to full Professor in 2004 and is now director of the Center for Research on Green Sustainable Chemistry. He is a recipient of the Society of Synthetic Organic Chemistry Japan Award (2010) and the Green and Sustainable Chemistry Award (2009). He served as President of the Society of Fluorine Chemistry, Japan (2013−2015), and has been the representative of the managing committee of the Ionic Liquids Research Association, Japan, since 2014.

GLOSSARY [C1mim]+=1,3-dimethylimidazolium [C2mim]+=1-ethyl-3-methylimidazolium [C4mim]+=1-butyl-3-methylimidazolium [C6mim]+=1-n-hexyl-3-methylimidazolium 10597


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C1im3C=3-(1-methylimidazol-3-io)butane-1-carboxylate ISCB1=pottasium 3,5-bis(2-(2-(2-ethoxyethoxy)ethoxy)ethoxy)benzoate (ISCB1) P555C4S=N,N,N-tripentyl-4-sulfonyl-1-butanephosphonium IL1=1-butyl-2,3-dimethylimidazolium cetyl-PEG(10)-sulfate BHEAF=bis(2-hydroxyethyl)ammonium folmate

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