Biotransformations in Ionic Liquids: An Overview - ACS Publications

Chapter 16

Biotransformations in Ionic Liquids: An Overview

Downloaded by COLUMBIA UNIV on March 14, 2013 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch016

Roger A. Sheldon, F. van Rantwijk, and R. Madeira Lau Department of Biocatalysis and Organic Chemistry, Delft University of Technology, Delft, The Netherlands

The current state of the art with regard to biotransformations in ionic liquids is reviewed. Research on this subject has rapidly expanded following the first publication in 2000. Performing biotransformations in ionic liquid media has been shown to have potential benefits with regard to activity, stability and (enantio)selectivity. The scope of the methodology with regard to type of ionic liquid, substrate, enzyme and reaction is discussed.

Introduction During the last decade increasing attention has been focused on the use of room temperature ionic liquids as novel reaction media for organic synthesis, in particular for catalytic processes (J). Their non-volatile character coupled with thermal stability makes them potentially 'green' alternatives for environmentally unattractive, volatile organic solvents, such as chlorinated hydrocarbons. Moreover, their hydrophobic/hydrophilic character and solubility parameters can be tuned by appropriate modifications of the cation or anion and they have been called 'designer solvents' (2). Depending on their structure, they are immiscible with water, or, e.g. alkanes, which provides the possibility of performing reactions in biphasic media, thus facilitating product recovery and catalyst recycling. Until recently,

192

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

193 most studies have involved the use of 1,3-dialkylimidazolium cations, e.g. 1butyl-3-methylimidazolium [bmim] in combination with tetrafluoroborate [BF ] or hexafluorophosphate [PF ] as the anion (see Fig. 1). The tetrafluoroborate is water miscible while the hexafluorophosphate is immiscible with water. The choice of BF " or PF " was generally dictated by the need for weakly coordinating anions since the presence of strongly coordinating anions, e.g. halides, would inhibit many metal-catalyzed processes. However, one can discern a distinct trend towards the use of alternative cations and anions, generally motivated by cost considerations and/or the limited hydrolytic stability of BF and PF salts (which can result in the generation of HF). Moreover, functionalization of the cation, e.g. with an oxygen functionality can make ionic liquids suitable solvents for highly polar organic molecules, such as carbohydrates (see later), thus extending their scope. 4

6

4


4

6

6

[BF4][PF6]"

R=aikyl Figure 1

Biotransformations in Ionic Liquids The first example of a biotransformation in an ionic liquid was reported by Lye and coworkers in 2000 (3). It involved a whole cell biotransformation of 1,3-dicyanobenzene to 3-cyanobenzamide, with a Rhodococcus sp. in a biphasic [bmim][PF ]/H 0 medium. The ionic liquid essentially acts as a reservoir for the substrate and product, thereby decreasing the substrate and product inhibition observed in water. In principle, an organic solvent could be used for the same purpose but it was found that the ionic liquid caused less damage to the microbial cells than, for example, toluene. More recently, l-octyl-3methylimidazoliumhexafluorophosphate, [omim][PF ] was used to enhance the recovery of n-butanol from a fermentation broth (4). Subsequent recovery of the n-butanol from the ionic liquid by pervaporation provided an attractive 6

2

6

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

194 alternative to the conventional, energy-intensive separation from water by distillation. The use of an isolated enzyme in an ionic liquid was reported in 2001 by Erbeldinger and coworkers (5). They showed that the thermolysin-catalyzed synthesis of Z-aspartame in [bmim][PF ]/H 0 (95/5, v/v) afforded comparable reaction rates to those observed in ethylacetate/H 0. Furthermore, the enzyme exhibited a higher stability in the ionic liquid/water medium although the small amount (3.2 mg.ml" ) of enzyme that dissolved in the ionic liquid was catalytically inactive. At the same time, we showed (6) that Candida antarctica lipase Β (CaL B), either as the free enzyme (SP525) or in an immobilized form (Novozym 435) is able to catalyze a variety of biotransformations in [bmim][BF ] or [bmim][PF ] , in the total absence of added water. The ionic liquids were stored over P 0 and the enzyme was essentially anhydrous, e.g. lyophilized in the case of the free enzyme. Transesterifications (Reaction 1), for example, proceeded with rates comparable to those observed in terr-butyl alcohol, a commonly employed solvent for lipase-catalyzed processes. Hie immobilized enzyme (Novozym 435) gave higher rates than the free enzyme (SP525) suspended in the ionic liquid. Similarly, CaL Β catalyzed the ammoniolysis of octanoic acid (Reaction 2) in [bmim][BF ] (6). Complete conversion occurred in 4 days with Novozym 435 at 40°C, compared to the 90-100% conversion in 17 days observed with ammonium carbamate in methylisobutyl ketone (7). 6

2

2

1


4

6

2

5

4

CaL B, 40oC R i C 0 E t + R20H « » [bmim][PF ] or [bmim][BF4] 2

R1C0 R2 + EtOH

(1)

2

6

RC0 H + NH 2

3

CaL B, 40oC « » RCONH + H 0 [bmimJ[BF4] 2

2

(2)

R = C Hi5 7

A third reaction, which was shown to be feasible in the ionic liquid (6), is the in situ generation of a peroxycarboxylic acid via CaL B-catalyzed perhydrolysis of the corresponding carboxylic acid. Thus, the epoxidation of


195 cyclohexene by peroxyoctanoic acid, generated in situ by Novozym 435catalyzed reaction of octanoic acid with commercially available 60% aqueous hydrogen peroxide in [bmim][BF ], afforded cyclohexene oxide in 83% yield in 24h (Reaction 3). For comparison, a yield of 93% was observed in 24h in acetonitrile, which we previously showed (8) to be the optimum organic solvent for this reaction. 4

(3)


Ç

83% yield

RCOOH HoO

RCOH

\ c a L §y ^

u

0

H 0 2

2

[bmim][BF ] 4

Scope and Added Value of Biotransformations in Ionic Liquids The seminal publications referred to above demonstrate the feasibility of performing (some) biotransformations in ionic liquids, a result which a priori may not have been predicted. However, this generally provokes the remark: so what. The questions on most people's lips is: What are the benefits of performing biotransformations in ionic liquids? Having shown that enzymes can function in an ionic liquid medium the challenge for further research is clearly to demonstrate that there is an added value in doing so. One could say that enzymes in ionic liquids are at the same stage that enzymes in organic solvents were twenty years ago when Klibanov and coworkers published their seminal contributions (9). There is one important difference, however. Biotransformations in organic media had to compete with the corresponding reactions in water while biotransformations in ionic liquids have to compete with the corresponding processes in water and in organic solvents. Another important issue is the scope of biotransformations in ionic liquids. Initial studies have focused on a few hydrolases and almost nothing is known with regard to the use of e.g. oxidoreductases and lyases in ionic liquids. What could be the added value of biotransformations in ionic liquids? Potential benefits that readily come to mind are: enhanced activities, (enantio)selectivities and stabilities. It is well-known that the activities of enzymes are generally much lower in organic media compared to water and enzyme stability and/or selectivity is an issue in many biotransformations. Ionic


196 liquids could also have added value for performing biotransformations with highly polar substrates which are sparingly soluble in most organic solvents (see later).


Enantioselectivity Several groups have investigated the effect of performing lipase-catalyzed reactions in ionic liquids on the enantioselectivities of these transformations. For example, Kragl and coworkers (10) investigated the kinetic resolution of 1phenylethanol (Reaction 4) with nine different lipases in ten different ionic liquids.

OH

OH

I

Lipase/24e/3days

^

ÇAc

I

^

I

Good activities and, in many cases, improved enantioselectivities were observed compared to the corresponding reaction in methyl f*?i*-butyl ether (MTBE). The rates and enantioselectivities were dependent on the particular lipase used and the nature of both the cation and anion of the ionic liquid. The best results were generally observed with CaL Β while some lipases, e.g. Candida rugosa (CRL) and Thermomyces lanuginosus lipases showed almost no activity. The highest conversions and enantioselectivities were observed in [bmim][CF S0 ], [bmim][(CF S0 )2N] and [omim][PF ]. Surprisingly, virtually no reaction was observed in [bmim][BF ] and [bmim][PF ], which contrasts with what we and others have observed (see later). More recently, the same group (11) compared the enantioselectivities of Reaction 4 with a Pseudomonas sp. lipase in [bmimlKCFaSC^N], MTBE and n-hexane at fixed water activities (a ). At low water activities (a < 0.53) the enantioselectivity was higher in the ionic liquid. When the reaction temperature was increased from 25°C up to 90°C the enantioselectivity in MTBE decreased dramatically (from Ε = 200 to Ε = 4) while in the ionic liquid it remained high, Ε decreasing from 200 to 150. In both solvents the decrease in enantioselectivity was observed at the boiling point of either the solvent (MTBE) or the vinyl acetate. 3

3

3

2

6

4

w

6

w


197 Park and Kazlauskas (12) investigated Reaction 4 with Pseudomonas cepacia (PCL) lipase in a series of ionic liquids containing different cations in conjunction with BF as the anion. They observed that treatment of the ionic liquid with sodium carbonate resulted in a dramatic increase in rate. This was attributed to the removal of residual silver ions and acidic impurities in the ionic liquid, remnants of the synthesis procedure. Kim and coworkers (13) studied the CaL Β and PCL catalyzed transesterification of four different chiral secondary alcohols with vinyl acetate in [emim][BF ] and [bmim][PF ]. They observed enhanced enantioselectivities, compared with the same reactions in toluene or THF, in all cases. Itoh and coworkers (14) observed only a minor effect on the enantioselectivity of the Novozym 435 catalyzed transesterification of a chiral allylic alcohol (Reaction 5) in ionic liquids, compared to the same reaction in diisopropylether. We note, however, that the enantioselectivity was very high (E > 200) in all cases which makes it difficult to observe differences. 4


4

6

Novozym 435/IL OAc

CH3CHO

The rate of Reaction 5 was strongly dependent on the nature of the anion in [bmim][X]. The best results were obtained with X = BF or PF . Much slower reactions were observed with X = CF C0 , CF3SO3 and SbF . These results contrast with those reported by Kragl and coworkers (see earlier). We note, however, that the purity of ionic liquids used in these studies may be an important source of differences in the observed results. A comparison of different lipases in [bmim][PF ] revealed that Novozym 435 gave the highest rate, followed by an Alcaligenes sp. lipase and PCL while CRL and porcine pancreas lipase (PPL) gave no reaction. It was further shown that the product could be extracted with ether and the ionic liquid, containing the suspended enzyme, could be recycled albeit with a dramatic decrease in activity after the second recycle. More recently, lipase-catalyzed transesterifications in ionic liquids have been extended to chiral primary alcohols as shown in Reactions 6 (15) and 7 (16). Reaction 6 proceeded up to six times more enantioselectively in [bmim][PF ] compared to common organic solvents (15). In stark contrast, when the reaction was performed in [bmim][BF ] almost no enantioselectivity was observed. 4

3

2

6

6

6

6

4


198

lipase/IL

\S

*^0Ac

I

CHCH0 3

R

I

2

R

2

R1 =PhorEt R2 = t-Bu, ΜθΟ, EtO, i-PrO

OAc


(7)

Astonishingly, the study of Reaction 7 (16) does not contain any data on the enantioselectivity of the transformation.

Activity and Stability of Enzymes In Ionic Liquids Activities and stabilities of enzymes are obviously interrelated parameters. It is often difficult to assess whether higher turnover numbers (TON) or turnover frequencies (TOF) are a result of a higher intrinsic activity or a higher stability of the enzyme (or both). For this reason it is convenient to treat these two properties together. From both a practical and a theoretical viewpoint it is important to establish the stability of enzymes, in various formulations, in ionic liquids compared to common organic solvents and water. Another important question is whether or not enzymes can retain their activity when dissolved in a water-free ionic liquid (all of the above described examples pertain to enzymes dissolved in an ionic liquid/water mixture or suspended in an ionic liquid). To this end we investigated CaL B-catalyzed transesterifications in a range of ionic liquids (17). Four different enzyme formulations were studied: free CaL B (SP525), immobilized on a support (Novozym 435), cross-linked enzyme crystals (CLEC) (18) and cross-linked enzyme aggregates (CLEA) (19). Reaction rates in [bmim][PF ], [bmim][BF ] and [bmim][CF S0 ] were comparable to those observed in f

Biotransformations in Ionic Liquids: An Overview - ACS Publications

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