Lipase-catalyzed enantioselective acylation in the ionic liquid solvent system: Reaction of enzyme anchored to the solvent

Article abstract of DOI:10.1246/cl.2001.262

The lipase-catalyzed enantioselective acylation of allylic alcohols in an ionic liquid solvent was demonstrated; the reaction was significantly dependent on the counter anion of the imidazolium salt and good results were obtained when the reaction was car

Full text of DOI:10.1246/cl.2001.262

262
Chemistry Letters 2001
Lipase-Catalyzed Enantioselective Acylation in the Ionic Liquid Solvent System:
Reaction of Enzyme Anchored to the Solvent
Toshiyuki Itoh,* Eri Akasaki, Kazutoshi Kudo, and Shohei Shirakami
Department of Chemistry, Faculty of Education, Okayama University, Okayama 700-8530
(Received January 5, 2001; CL-010003)
The lipase-catalyzed enantioselective acylation of allylic
alcohols in an ionic liquid solvent was demonstrated; the reac-
tion was significantly dependent on the counter anion of the
imidazolium salt and good results were obtained when the reac-
tion was carried out in [bmim]PF6 or [bmim]BF4 as solvent. We
also first demonstrated that it was possible to repeatedly use the
enzyme in the ionic liquid solvent system.
Proper choice of the reaction media is very important when
developing practical processes in chemistry. Ionic liquids are
new class of solvents which have attracted growing interest
over the past few years due to their unique physical and chemi-
cal properties.1 It is well known that lipase tolerates non-natural
reaction conditions and the lipase-catalyzed transesterification
in an organic solvent system has been well recognized as a very
useful means of synthesizing optically active compounds.2
Therefore it should be expected that the lipase-catalyzed reac-
tion should occur in the ionic liquid solvent. The investigations
of such possible uses the ionic liquids as the reaction media of
the lipase-catalyzed reaction have just begun; Lye and his co-
workers reported that the biotransformation is possible in an
ionic liquid solvent system for the first time in July 20003 and
two examples of enzymatic reaction in ionic liquids have been
reported late in the same year.4,5 However, no example of an
asymmetric enzymatic reaction in an ionic liquid solvent has
been reported.6 Herein, we report the first example that realizes
the recycle use of lipase in an ionic liquid solvent system for
the asymmetric transesterification of an allylic alcohol.7
We chose the imidazolium salts as the solvent in our enzy-
matic reaction among various types of ionic liquids based on
two criteria. The first is that imidazolium salts are stable under
atmospheric conditions and especially tolerant to water.1 The
second is that we are systematically able to investigate the suit-
able combination of the imidazolium cation and counter anion
of the salt for the enzymatic reaction.1
First, we investigated the lipase-catalyzed reaction using
butylmethylimidazolium pentafluorophosphate ([bmim]PF6)8 as
the solvent because this salt is insoluble in both water and ether;
this means that an easy extraction process of the products is
expected. In addition, we also anticipated that it might be possi-
ble to anchor the lipase in the ionic liquids during the work up
process and this may allow us to repeatedly use the enzyme.
Typically, the reaction was carried out as follows: To a
mixture of lipase (25 mg) in the ionic liquid (1.5 mL) were
added racemic 5-phenyl-1-penten-3-ol ((±)-1)9 (50 mg, 0.30
mmol) as a model substrate and vinyl acetate (39 mg, 0.45
mmol, 1.5 equiv) as the acyl donor. The resulting mixture was
stirred at room temperature (ca. 25 °C) and the reaction course
was monitored by GC analysis. The reaction was stopped by the
addition of 3 mL of ether when the molar ratio of acetate 2 and
alcohol 1 became equal. The reaction mixture was filtered
through a glass-sintered filter with a celite pad to remove the
enzyme and product, and unreacted alcohol was isolated from
the filtrate. It is noteworthy that the ionic solvent was recovered
without any loss in the amount after the work-up process and it
was possible to reuse it after washing with water and dried
under vacuum for several hours at 50 °C. The optical purities of
the acetate (S)-2 produced and the remaining alcohol (R)-1 were
determined by capillary GC analysis using a chiral column
(Chiraldex G-TA).9b Acylation of the alcohol was accomplished
by three types of enzymes, such as Candida antarctica lipase
(CAL, Novozym 435), Lipase from Alcaligenes sp. (QL), and
Pseudomonas cepacia lipase (PS). The desired acetate 2 had an
extremely high enantioselectivity (Entries 1–3), though reaction
rate was slightly inferior compared to those in the usual organic
solvent reaction system (Entry 10). On the other hand, no reac-
tion took place when Candida rugosa lipase (CRL) or Procine
liver lipase (PPL) was used as the catalyst in the [bmim]PF6
solvent system (Entries 4 and 5).
We next investigated the proper combination of the counter
anion with [bmim] cation in the reaction of the CAL-catalyzed
acylation (Entries 6–9). It was found that the acylation rate was
strongly dependent on the anionic part of the solvent, while the
CAL-catalyzed acylation proceeded with high enantioselectivi-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
263
ty in all solvents tested. The best result was recorded when
[bmim]BF4 was employed as the solvent (Entry 7) and the
showed that recycling of the enzyme was indeed possible in our
ionic liquid solvent system, though the reaction rate gradually
dropped by repeating the reaction process (Entries 3–5). This is
the first demonstration that the recycle use of enzyme was in
fact possible in the ionic liquid solvent system.
In conclusion, we demonstrated the lipase-catalyzed enan-
tioselective transesterification of an allylic alcohol in the ionic
liquid solvent system and demonstrate that it is possible to
repeatedly use the enzyme in the ionic liquid solvent system.
Further investigation of the scope and limitations of this reac-
tion, especially optimization of the reaction conditions for the
lipase recycling system in the ionic solvent system, will make it
even more beneficial.
11a
reaction rate was nearly equal to that of the reference reaction
in i-Pr2O (Entry 10). On the contrary, a significant drop in the
reaction rate was obtained when the reaction was carried out in
[bmim]TFA11b (Entry 6), [bmim]OTf11c (Entry 8) or
11d
[bmim]SbF6 (Entry 9). From these obtained results, it was
concluded that [bmim]PF6 and [bmim]BF4 are suitable solvents
for the present lipase-catalyzed reaction. Although the acylation
rate in the reaction of [bmim]PF6 was a slightly inferior to that
in [bmim]BF4, we chose [bmim]PF6 as the best solvent for the
present enzymatic reaction system. Because a very easy work-
up process was realized in the reaction of [bmim]PF6 due to the
insolubility of this salt in both water and ether. [bmim]BF4 was
quite soluble in water and, therefore, it was difficult to remove
the by-product such as acetic acid by simple work-up processes.
The authors are grateful to Professor Tomoya Kitazume at
the Tokyo Institute of Technology for the helpful discussions
throughout this work. The authors also thank Meito Sangyo
Co., Ltd. and Amano Pharmaceutical Co., Ltd. for providing the
lipases. They are grateful to the SC-NMR Laboratory of
Okayama University for the NMR measurements.
References and Notes
1
A recent review, see : T. Welton, Chem. Rev., 99, 2071
(1999).
2
A review, see : C. H. Wong and G. M. Whitesides,
“Enzymes in Synthetic Organic Chemistry,” Tetrahedron
Organic Chemistry Series Vol. 12, ed. by J. E. Baldwin and
P. D. Magnus, Pergamon (1994).
3
4
5
6
S. G. Cull, J. D. Holbrev, V. Vargas-More, K. R. Seddon,
and G. J. Lye, Biotechnol. Bioeng., 69, 227 (2000).
M. Erbeldinger, A. J. Mesiano, and A. J. Russell,
Biotechnol. Prog., 16, 1131 (2000).
R. M. Lau, F. v. Rantwijk, K. R. Seddon, and R. A.
Sheldon, Org. Lett., 2, 4189 (2000).
The first example of lipase-catalyzed asymmetric transes-
terification in an ionic liquid solvent was reported by T.
Kitazume, though the results have not yet published for-
mally; 16th International Symposium on Fluorine
Chemistry, Durham, UK, July 16–21, 2000, B-36.
The results were reported at the International Chemical
Congress of Pacific Basin Societies (Pacifichem 2000) on
December 18, 2000. Symposium on “Biocatalyst in
Organic Synthesis” 1649 and 1653.
7
Since it was anticipated that lipase might be anchored by
the ionic liquid solvent and remained in it after the extraction
work-up of the products, we next attempted to repeatedly use
the lipase in the [bmim]PF6 solvent system (Scheme 1). The
results are shown in Table 2. A mixture of the substrate, lipase,
and vinyl acetate in the [bmim]PF6 solvent was stirred for 3 h,
and then ether was added to the reaction mixture to form the
biphasic state. The desired products and unreacted alcohol were
quantitatively extracted from the ether (upper layer). To the
remained ionic liquid phase, which was placed under reduced
pressure for 15 min to remove the ether, a mixture of the sub-
strate and vinyl acetate was again added. This mixture was
stirred at rt. As expected, the acylation reaction smoothly took
place and the product was obtained without any loss in enan-
tioselectivity (Entry 2). It was thus confirmed that the enzyme
was in fact anchored in the ionic liquid solvent after the work-
up process. Repeating the same process, we successfully
8
9
J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E.
Visser, and R. D. Rogers, Chem. Commun., 1998, 1765.
a) Y. Takagi, R. Ino, H. Kihara, T. Itoh, and H. Tsukube,
Chem. Lett., 1997, 1247. b) Y. Takagi, T. Nakatani, T.
Itoh, and T. Oshiki, Tetrahedron Lett., 41, 7889 (2000).
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Am. Chem. Soc., 102, 7294 (1982).
11 a) [bmim]BF4: P. A. Z. Suarez, J. E. L. Dullius, S. Einloft,
R. F. de Souza, and J. Dupont, Polyhedron, 15, 1217
(1996). b) [bmim]TFA: The synthesis of this salt was
similar to that of [bmim]PF6 with the exception that
CF3COONa was used in place of NaPF6. c) [bmim]OTf:
P. Bonhote, A.-P. Dias, N. Papageorgiou, K.
Kalyanasundaram, and M. Grätzel, Inorg. Chem., 35, 1168
(1996). d) [bmim]SbF6: C. E. Song, C. R. Oh, E. J. Roh,
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