The influence of organic solvent and ionic liquids on the selective formation of 2-(2-ethylhexyl)-3-phenyl-1,2-oxaziridine mediated by lipases

Article abstract of DOI:10.1002/poc.1778

This paper describes the influence of the addition of ionic liquids (ILs) based on [BMIm][X], where [X]=SCN, Cl, BF₄, and PF₆, on the chemo-enzymatic oxidation of N-benzyliden-2-ethylhexylamine to form the corresponding E- and Z-isom

Full text of DOI:10.1002/poc.1778

Research Article
Received: 25 January 2010,
Revised: 2 June 2010,
Accepted: 5 July 2010,
Published online in Wiley Online Library: 26 August 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1778
The influence of organic solvent and ionic
liquids on the selective formation of
2-(2-ethylhexyl)-3-phenyl-1,2-oxaziridine
mediated by lipasesy
Thiago Bergler Bitencourta and Maria da Grac¸a Nascimentoa
*
This paper describes the influence of the addition of ionic liquids (ILs) based on [BMIm][X], where [X] ¼ SCN, Cl, BF4,
and PF6, on the chemo-enzymatic oxidation of N-benzyliden-2-ethylhexylamine to form the corresponding E- and
Z-isomers of oxaziridines mediated by Pseudomonas sp. (PSL) and Candida antarctica (CAL-B) lipases in various organic
solvents at room temperature (25 -C) with urea hydrogen peroxide (UHP). The results showed that the use of different
organic solvents in the presence of ILs, critically changes the conversions (5–99%) and the isomeric ratio E:Z, (50–100%
E-isomer) of the products formed. Copyright ß 2010 John Wiley & Sons, Ltd.
Keywords: ionic liquids; lipases; organic solvents; oxaziridines
the substrate and products, and in many cases this decreases the
substrate and product inhibition by water.[21,22,27] The choice of
INTRODUCTION
Oxaziridines are useful compounds, which have received
considerable attention, for example, as aprotic oxidizing
reagents,[1–4] and these compounds may be used as chiral
auxiliaries in the epoxidation of terpenes.[5] Several methods have
been employed for the oxidation of imines to the corresponding
oxaziridines. The best-known of these methods is the MCPA
(m-chloroperbenzoic acid oxidation method,[6] and other
methods described in the literature include oxidation with
oxone, urea hydrogen peroxide (UHP), or using a trichoroloace-
tonitrile system.[7–9] However, most of these methods produce
by-products, which complicates the product purification. These
oxidants are, in general, very expensive and relatively toxic for
common use in organic synthesis.[10,11]
organic solvents in enzymatic reactions is very important, because
enzymes are very sensitive to the medium employed, due to their
selectivity and activity which may be affected by medium
polarity.[28,29] Therefore, many recent publications have described
the use of lipases in the presence of ILs as co-solvents, including
their properties in biocatalytic processes.[30–34]
Herein, two commercial lipases – from Pseudomonas sp. (PSL) and
Candida antarctica (CAL-B) were employed in the chemo-enzymatic
oxidation of N-benzyliden-2-ethylhexylamine (1) with octanoic acid
as the acyl donor, to obtain the corresponding E- and Z-isomers of
oxaziridines (2) and (3). The conversion and selectivity of the
products were analyzed as a function of the reaction medium
employed, such as the use of pure organic solvents or in mixtures
with ILs from a series of 1-butyl-3-methyl imidazolium-[BMIm][X],
where X ¼ BF4, PF6, SCN, and Cl (Scheme 1).
The production of N-alkyloxaziridines via a chemo-enzymatic
method using lipases has been carried out with a series of Schiff
Bases. This method is particularly interesting because the products
can be prepared under mild conditions (room temperature and
neutral pH) in lower reaction time, and they can be easily
purified.[12] In this case, the peroxyacid is generated in situ by
lipases and carboxylic acids, in a first step and then the
EXPERIMENTAL
All chemicals are commercially available and were used without
purification. All solvents were analytical grade and were dried
peroxyacid[13,14] is used in the oxidation of C N bonds of
—
—
the imines to obtain the corresponding N-alkyloxaziridines.[12] The
same method can be applied to alkene epoxidation,[13–16] sulfide
oxidation to form sulfoxides,[17] and Baeyer–Villiger oxidation.[18]
The use of lipases in this process means that the enzyme can be
reused many times, decreasing the costs of the process, and they
do not require the use of co-factors and special handling.[12–16]
Recently, ionic liquids (ILs) have received increasing attention as
solvents due to their properties and applications in biocatalytic
processes.[19,22] The link between ILs and green chemistry is clearly
related to the properties of these solvents, however, aspects
related to their degradability and aquatic toxicity are still not
clear.[23] Studies are currently being carried out to investigate these
properties.[24–26] ILs in enzymatic reactionscanact asa reservoir for
*
Correspondence to: M. G. Nascimento, Departamento de Qu´ımica, Universi-
´
dade Federal de Santa Catarina, Florianopolis, Santa Catarina 88040-900,
Brazil.
E-mail: graca@qmc.ufsc.br
a
T. B. Bitencourt, M. G. Nascimento
Departamento de Qu´ımica, Universidade Federal de Santa Catarina,
´
Florianopolis, Santa Catarina 88040-900, Brazil
y
This article is published in Journal of Physical Organic Chemistry as a special
issue on Tenth Latin American Conference on Physical Organic Chemistry,
edited by Faruk Nome, Dept de Quimica, Universidade Federal de Santa
Catarina, Campus Universitario – Trindade 88040-900, Florianopolis-SC, Brazil.
J. Phys. Org. Chem. 2010, 23 995–999
Copyright ß 2010 John Wiley & Sons, Ltd.

T. B. BITENCOURT AND M. G. NASCIMENTO
Scheme 1. Chemo-enzymatic formation of N-alkyloxaziridines 2 e 3 mediated by lipases
by storing over activated 3 A8 molecular sieves before use. The
chemo-enzymatic reaction for the preparation of
hexafluorphosphate [BMIm][PF6] in mixtures with dichloro-
methane (DCM) was evaluated using CAL-B as the biocatalyst.
The use of ILs is an alternative tool which can sometimes increase
or decrease the conversion and the selectivity of products.[21,22,35]
Herein, the amount of ILs was progressively increased from 0 to
90% v/v, and the results are given in Fig. 1. In a previous study
using pure DCM,[12] a moderate conversion of 60% into the
product was achieved in 3 h of reaction forming isomers 2 and 3
with an E/Z ratio of 65:35. The conversions were measured by gas
chromatography (GC) and the values are expressed as the sum of
the E-(2) and Z-(3)-isomers in the mixture.
The results showed a dependence on the anion nature and
content of the IL in the conversion into 2 and 3. With the use of
DCM:[BMIm][PF6], the highest values were reached with an
organic solvent:IL mixtures of 90/10 and 80/20 v/v, these being
72 and 76%, respectively. With the use of the organic solvent:IL
mixture of 70/30 v/v, the conversion was 62%, this being similar
to that obtained with pure organic solvent. However, a decrease
in the degree of conversion to 27% was observed using an
organic solvent:IL mixture of 10/90 v/v.
2-(2-ethylhexyl)-3-phenyl-1,2-oxaziridines 2 and 3 was carried
outusing0.4 mmolofN-benzyliden-2-ethylhexylamine1(prepared
and characterized as previously described),[12] 1.0 mmol of the acyl
donor (octanoic acid-Aldrich 98%), 25mg of the two lipases (CAL-B
lipase – 10000 PLU/g and PSL lipase – 30000 U/g) and 0.4 mmol of
UHP(18% – Acros) at room temperature, 150 rpm in 10ml of
different organic solvents or 1 ml of different ILs from a series of
imidazolium-based ILs [BMIm][X] (Acros 98%), and 9 ml to organic
solvents. The crude product mixture of 2 and 3 was separate and
purified using a small column of SiO2. The solvent was evaporated
underreducedpressureat408Ctogivethepureproductseparately
as light yellow oil (E-isomer) and colorless oil (Z-isomer).
The reaction progress and the isomeric ratio were measured by
GC on a Shimadzu GC14B instrument equipped with a Shimadzu
CBP-5 column using H2 as the carrier gas, with the flame
ionization detector set at 280 8C, an injector set at 270 8C, and a
column temperature range of 70–250 8C (10 8C/min). The results
are expressed as a medium of two values and the estimated
errors were <5%.
The characterization of the products was performed by
spectroscopic analysis on a Perkin-Elmer FTIR 1600 (range
4000–400 cmꢀ1). 1H-NMR and 13C-NMR spectra were recorded at
400 and 100 MHz, respectively, with a Varian 400 Mercury Plus
spectrometer, using CDCl3 as solvent and TMS as the internal
standard.
The spectral data are: 2-(E)-isomer: yellow oil: 1H NMR (CDCl3):
dH 7.41–7.37 (5H, m, Ph), 4.47 (1H, s), 2.86–2.75 (2H, m), 1.71 (1H,
m), 1.36–1.26 (8H, m), 0.89 (3H, s), 0.87 (3H, s); 13C NMR (CDCl3): dC
130,2, 130,1, 128,7, 128,5, 128,0, 127,8, 80,9, 65,6, 39,5, 34,3, 31,8,
29,0, 25,0, 14,3, 11,0. IR (ZnSe window): cmꢀ1 2924, 1633, 1549,
1459, 1370, 1270, 731.
3-(Z)-isomer: colorless oil: 1H NMR (CDCl3): dH 7.41–7.37 (5H, m,
Ph), 5.25 (1H, s), 2.84–2.73 (2H, m), 1.70 (1H, m), 1.39–1.26 (8H, m),
0.89 (3H, s), 0.87 (3H, s); 13C NMR (CDCl3): dC 133.1, 130.2, 128.7,
128.0, 127.8, 127.5, 79.9, 63.4, 42.8, 39.5, 34.2, 24.4, 22.8, 14.1, 11.0.
IR (ZnSe window): cmꢀ1 2962, 1653,1423, 1375, 1297, 756.
Figure 1. Effects of the content of ionic liquids as co-solvents on the
chemo-enzymatic oxidation of N-benzyliden-2-ethylhexylamine (1), (&)
reaction in [BMIm][PF6], and (*) [BMIm][BF4]. Reaction conditions: (1)
(0.4 mmol), octanoic acid (1 mmol), UHP (0.4 mmol), in dichloromethane
(reaction media 10 ml), CAL-B (25 mg), r.t., 3 h and 150 rpm
RESULTS AND DISCUSSION
Firstly, the influence of the ILs 1-butyl-3-methylimidazolium
tetrafluorborate [BMIm][BF4] and 1-butyl-3-methylimidazolium
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 995–999

INFLUENCE OF ORGANIC SOLVENT AND IONIC LIQUIDS
A possible reason for this result could be that the [BMIm][PF6]
might interact with charged groups of the enzyme, either at the
active site or at its periphery, causing changes in the enzyme
structure. In addition, a high concentration of IL causes not
only high ionic strength of the reaction medium, which might
inactivate the enzyme, but also a high viscosity of the reaction
mixture, which limits the diffusion of substrates and products
from the active site of the enzyme, resulting in a drop in
enzymatic reaction rate.[19,20,35–39]
The study on the influence of [BMIm][BF4] content showed a
decrease in the conversion degrees of N-alkyloxaziridines with
an increase in the IL content. With the use of IL contents of 30
and 50%, the conversion degrees were only 8 and 5%,
respectively. However, using higher IL contents, the conversions
were even lower (3%). These results probably reflect the nature of
[BMIm][BF4], which is particularly hydrophilic, and they are in
agreement with the findings of Zhao et al. who observed that ILs
based on BF4 did not dissociate into ions in the absence of water,
and thus a considerable decrease in the enzyme activity and in
the conversion into the product was observed.[40] With the use of
both ILs the E:Z ratio was 65:35.
Considering the above results, an organic solvent:[BMIm][PF6]
mixture of 90/10 v/v was selected for the subsequent exper-
iments.
The conversion into oxaziridines 2 and 3 as a function of
time was then investigated with PSL and CAL-B using acetonitrile
(ACN) or DCM as organic solvents and the IL [BMIm][PF6]. The
values for the total conversion into 2 and 3, using both lipases,
are given in Fig. 2.
Using PSL, the total degree of conversion into products was
18% in pure ACN and 45% when an ACN:IL mixture of 10% v/v
was used. A small increase was observed with the use of DCM,
and the conversion degrees increased from 25 to 30% using pure
organic solvent or in a mixture with the IL, both in 12 h of
reaction.
With the use of pure organic solvent or in mixture with the ILs,
the selectivity of the products formed was approximately 65:35
for the isomers E:Z.
Figure 2. [BMIm][PF6] effects on the chemo-enzymatic formation of
N-alkyloxaziridines 2 and 3 using PSL (A) and CAL-B (B) as the biocatalyst
as a function of time carried out in DCM/IL 10% v/v (!), pure DCM (~),
ACN/IL 10% v/v (*) and pure ACN (&). Reaction conditions as described
in Fig. 1
A similar behavior was observed when CAL-B was used as the
catalyst, where the conversion degrees showed an increase with
the addition of 10% v/v [BMIm][PF6]. With the use of this lipase,
the conversion values were of 81 and 97% using ACN pure or in
mixture with [BMIm][PF6] in 3 h of reaction. Using DCM pure or
in mixture with IL, the conversion values were of 60 and 83%,
respectively, also in 3 h of reaction.
A maximum conversion of >99% was reached in pure ACN or
in mixture with [BMIm][PF6], in 12 h of reaction. With the use of
pure DCM or DCM:IL mixture, the conversion values were of
63 and 91%, respectively (Fig. 2 B).
where X ¼ BF4, PF6, SCN, and Cl. The degree of conversion into the
mixture of 2 and 3 were determined by GC in 12 h reaction. The
results obtained as a function of the type of pure organic solvent
or in mixtures with imidazolium IL are described in Fig. 3. The
organic solvents were ordered according log P values.[41]
The results presented in Fig. 3 are in agreement with recent
publications in the literature.[19,20,42–49] It is well described that
ILs, in many cases, can change the tridimensional structure of the
enzymes due to polar interactions through the presence of
different anions. In particularly, in this study, the degrees of
conversion into the oxaziridines 2 and 3 ranged from 5 to 99%,
depending on the organic solvent and the nature of the IL anion.
Using ethyl ether in a mixture with ILs, the conversion degrees
increased as compared with the use of the pure organic solvent
(conversion of 26%). Using mixtures of the different ILs the
conversion degrees were around 91% in 12 h of reaction, except
for [SCN] (17%). Surprisingly, the selectivity was dependent on
the nature of the IL anion. Using the imidazolium ILs with [SCN],
[Cl], [BF4], and [PF6] as anions, the selectivity values were 100, 80,
60 and 50% in relation to the E-isomer, respectively.
The selectivity of the products formed was the same as that
obtained using PSL (E:Z ratio of 65:35).
These data showed that, in the presence of PSL or CAL-B as
biocatalysts and with the use of 10% v/v [BMIm][PF6], an increase
in the conversion degrees of oxaziridines 2 and 3 was obtained
both in mixtures with ACN or DCM. However, no improvement in
the selectivity of the E:Z ratio was observed.
Considering that organic solvents strongly influence the
reaction rate and selectivity, especially in relation to the
oxaziridine formation,[12] five organic solvents (n-hexane log P
3.5, DCM log P 0.93, ethyl ether log P 0.85, ethanol log P ꢀ0.24,
and ACN log P 0.33[41]) were selected to evaluate the influence of
different anions in a series of imidazolium-based ILs ([BMIm][X])
When n-hexane was used pure or in mixtures with ILs, the
opposite effect was observed as compared with ethyl ether, that
is, the conversion degrees decreased in the presence of the ILs.
J. Phys. Org. Chem. 2010, 23 995–999
Copyright ß 2010 John Wiley & Sons, Ltd.
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T. B. BITENCOURT AND M. G. NASCIMENTO
Acknowledgements
We are grateful to Universidade Federal de Santa Catarina (UFSC-
Brazil), Conselho Nacional de Desenvolvimento Cient´ıfico e Tec-
´
nologico (CNPq) and INCT-Catalysis, which provided financial
support and scholarships (T.B.B and M.G.N). We are grateful to
Novozymes Inc. and Amano Pharmaceutical (Japan) for the
donation of the lipases.
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Using pure n-hexane the conversion was 73% and the selectivity
towards the E-isomer was 65%. Using [BMIm][PF6] in a mixture
with this solvent, the conversion was 64% in 12 h of reaction.
Using n-hexane in a mixture with [BMIm][PF6] and [BMIm][BF4],
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[BMIm][SCN] it was 100%, in relation to the E-isomer.
A similar behavior was observed using ACN or DCM in a
mixture with [BMIm][PF6] with conversion degrees of >99 and
70%, respectively. The selectivity was the same, that is, 70%
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Using ethanol as a solvent in mixtures with ILs, a moderate
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We investigated the chemo-enzymatic system for oxidation of
N-benzyliden-2-ethylhexylamine mediated by CAL-B and PSL
using five different organic solvents in mixtures with imidazolium
ILs as the co-solvents. The conversion degrees and E:Z ratios were
strongly dependent on the organic solvents and the IL type and
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