Enhanced reactivity and enantioselectivity in catalytic glyoxylate-ene reactions using chiral bis(oxazoline)-copper complex in an ionic liquid

Article abstract of DOI:10.1016/j.tetasy.2012.06.004

An optimal hydrophobic ionic liquid was discovered as a solvent for highly enantioselective glyoxylate-ene reactions catalyzed by a chiral bis(oxazoline)-copper complex. The reactivity and stereoselectivity were highly dependent upon the property of the i

Full text of DOI:10.1016/j.tetasy.2012.06.004

Tetrahedron: Asymmetry 23 (2012) 1019–1022
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enhanced reactivity and enantioselectivity in catalytic glyoxylate-ene
reactions using chiral bis(oxazoline)–copper complex in an ionic liquid
⇑
Mina Kim, Han Saem Jeong, Chang-Eun Yeom, B. Moon Kim
Department of Chemistry, College of Natural Sciences, Seoul National University, NS60, Seoul 151-747, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
An optimal hydrophobic ionic liquid was discovered as a solvent for highly enantioselective glyoxylate-
ene reactions catalyzed by a chiral bis(oxazoline)–copper complex. The reactivity and stereoselectivity
were highly dependent upon the property of the ionic liquids; reactions between olefins and ethyl gly-
Received 6 June 2012
Accepted 8 June 2012
Available online 19 July 2012
6
oxylate in [Bmim]SbF at ambient temperature provided remarkably enhanced reactivity and stereose-
lectivity, which greatly exceed those of the corresponding reactions in dichloromethane. Furthermore,
the metal–ligand complex was readily recycled up to eight times while exhibiting no significant decrease
in reaction efficiency.
Ó 2012 Elsevier Ltd. All rights reserved.
8
1
. Introduction
Over the past decade, a great deal of interest has been focused
mesoporous mesocellular silica foam (MCF) and SBA-15. Never-
theless, extra modifications of the catalyst are required and the
immobilized catalyst systems often exhibit inferior reactivity and
stereoselectivity to those obtained from homogeneous systems.
Compared to other asymmetric transformations promoted by
the BOX–Cu complex, only a handful of reports have appeared so
on ionic liquids (IL) due to their unique properties, such as low va-
por pressure, low melting points, and negligible flammability,
which satisfy the requirements of modern green chemistry. They
have often been utilized as alternatives to conventional organic
solvents in catalytic reactions due to efficient separation and recy-
cling of expensive catalysts. Moreover, ionic liquids sometimes of-
fer additional advantages for asymmetric reactions such as rate
acceleration, enantioselectivity enhancement, and easy immobili-
zation of the catalytic species without additional structural modi-
1
9
far on carbonyl-ene reactions especially those under heteroge-
1
0
nous conditions in spite of their importance in the construction
of optically active homoallylic alcohols with an additional carbonyl
functional group. The olefin moiety can be converted into a car-
bonyl functionality through oxidation, adding versatility to the
2
reaction. As part of our ongoing research on the utilization of the
3
,4
8,11
fication.
In this regard, many asymmetric catalysis reactions
BOX–Cu complex system,
we investigated the enantioselective
1
2
using various ligand–metal complexes have been tested in ionic
liquids to achieve superior results to homogenous systems.
glyoxylate-ene reaction in ionic liquids, and were pleased to find
that the reactions proceeded with enhanced enantioselectivities at
ambient temperatures and disclose the results herein.
2
In connection with this, the chiral C -symmetric bis(oxazoline)
(
BOX)–Cu complex has proven to be a highly promising candidate
5
for the application of ionic liquids as reaction media. This versatile
system has demonstrated a wide range of applicability in many
asymmetric transformations. However, this Lewis acidic catalyst
suffers from several drawbacks, such as the need for a relatively
large amount of the complex (generally 8–10 mol %), high cost of
the catalyst, and the requirement for careful reaction control often
at very low temperatures, which restrict their use in practical
2
. Results and discussion
A representative asymmetric ene reaction of
a-methylstyrene
with ethyl glyoxylate was studied using the complex prepared
from ligand 3 (11 mol % relative to -methyl styrene), which was
premixed with Cu(OTf) in various ionic liquids for 1 h (Scheme 1).
Ionic liquids can largely be classified into two groups according to
their hydrophobicity. Initially, a series of ionic liquids containing
the 1-butyl-4-methyl-imidazolium cation (Bmim) were screened,
varying the anionic part as BF , OTf , PF , and SbF , to examine
the influence of the anion toward the reaction. The results are sum-
marized in Table 1. Reactions in hydrophilic ionic liquids, such as
a
2
6
1
3
applications. In order to overcome these problems, a number of
heterogeneous systems incorporating the chiral BOX complex have
7
À
À
À
-
been developed. Generally, the use of the heterogeneous system
4
6
6
involves anchoring of the catalysts onto stationary supports, and
we have also reported our investigation in this area employing
[
4
Bmim]BF and [Bmim]OTf gave poor results: only a minute
amount of racemic adducts was obtained after 30 min at room
temperature (Table 1, entries 1 and 2). On the other hand, reactions
⇑
Corresponding author. Tel.: +82 2 880 6644; fax: +82 2 872 7505.
E-mail address: kimbm@snu.ac.kr (B. Moon Kim).
0
957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.06.004

1
020
M. Kim et al. / Tetrahedron: Asymmetry 23 (2012) 1019–1022
OH
O
1
1 mol% ligand 3
OEt
OEt
1
0 mol% Cu (OTf)2
H
O
0
.5 M ionic liquid
O
1
2
2
+
Me
Me
N
Me
N
Me
N
2
OTf-
N
Cu
CMe3
CMe3
Me C
3
Me C
3
3
4
Scheme 1. Catalytic enantioselective ene reaction of
a-methyl styrene 1 and ethyl glyoxylate 2 with complex 4 in various ionic liquids.
Table 1
Table 2
Results of asymmetric ene reactions between ethyl glyoxylate and 1 using a BOX–Cu
complex in various ionic liquids or dichloromethane for 30 min
Influence of metal–ligand loading contents on the reaction yield and enantioselec-
tivity in [Bmim]SbF
6
a
a
Entry
Ionic liquid
[Bmim]BF
[Bmim]OTf
[Bmim]PF
[Bmim]SbF
Temperature (°C)
Yieldb (%)
eec (%)
Entry
Ligand 3/metal (mol %)
Temperature (°C)
Yieldb (%)
eec (%)
1
2
3
4
5
6
4
20
20
20
20
20
0
3
1
0
0
1
2
3
4
5
6
None/none
None/10
11/10
8/7
6/5
4/3
20
20
20
20
20
20
20
20
Trace
28
93
91
75
0
0
6
92
84
94
93
93
93
f
f
6
93
94
CH
2
CH
2
CH
2
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
45
17
43
44
79
83
83
86
49
d
d
e
e
7
0
À78
7
4/3
1.5/1
93
93
e
d
8
8
4
90
a
a
b
c
All reactions were performed on a 0.5 mmol scale of
.5 M concentration for 30 min.
Yields of isolated products after chromatographic purification.
ee’s were determined from HPLC analyses using Chiralcel OJ-H column.
Reaction was carried out for 3 h.
a
-methyl styrene 1 at a
All reactions were carried out at 20 °C for 30 min unless otherwise noted.
Values are isolated yields after chromatographic purification.
ee was determined by HPLC analysis using a Chiralcel OJ-H column.
These reactions were carried out for 2 h.
0
b
c
d
e
f
d
e
Best yield and enantioselectivity.
Reaction was carried out for 12 h.
Best yield and enantioselectivity.
ating power was sustained judging from the nearly constant ee’s
(93%) of the product. Even with 3 mol % of the catalytic species,
the same level of reactivity (93% yield) and enantioselectivity
(93% ee) was obtained at a prolonged reaction time (2 h) (entry
7). Below this critical level of ligand–metal complex, a reasonable
yield of the product was hardly obtained despite extended reaction
times, and with 1 mol % catalyst there was a slight drop in enanti-
oselectivity, which may imply the intervention of a non-selective
pathway (entry 8).
To test the scope of the substrates for this reaction, olefins, such
as methylenecyclohexane and methylenecyclopentane were em-
ployed for the glyoxylate-ene reaction. Both cases afforded suc-
cessful outcomes under the same reaction conditions as shown in
Table 3.
6 6
in hydrophobic ionic liquids such as [Bmim]PF and [Bmim]SbF
afforded the desired products in excellent yields and enantioselec-
tivities under otherwise identical reaction conditions (Table 1, en-
tries 3 and 4). When a control reaction was performed in
dichloromethane at room temperature using the same catalyst
complex 4, the product was obtained in 45% yield and with low
enantioselectivity (79% ee) (Table 1, entry 5). These results indicate
a significant enhancement of the reactivity and selectivity in IL
compared to the reaction in dichloromethane. When the reaction
temperature was lowered from 0 °C to À78 °C for the reaction in
methylene chloride, the enantioselectivity of the product was im-
proved to some extent (86% ee). Nonetheless, the reactivity
dropped significantly in spite of extended reaction time (Table 1,
entries 6–8). This rate acceleration in hydrophobic ionic liquids
Recycling experiments were carried out in order to examine the
capability of the catalytic complex in an ionic liquid for repeated
use (Table 4). We started with only 3 mol % of the BOX–Cu complex
+
may come from a favorable interaction between [Bmim] and ethyl
glyoxylate allowing the relative stabilization of a transition state,
which was previously suggested by Jorgensen and Evanseck using
a computational model.14 However, the precise reasons for the
enantioselectivity improvement in hydrophobic IL are not clear
at this time and further studies should be carried out.
6
in [Bmim]SbF for the first run since it was the minimum amount
of the catalyst required for the completion of the reaction within
2 h (Table 2, entry 7). Products were collected after each run via bi-
phasic extraction using ethyl ether, and the IL layer containing the
catalyst was reused directly after simple drying under reduced
pressure. Both the reactivity and enantioselectivity remained al-
most the same level up to the 4th recycle and then gradually de-
creased afterward, which may indicate that leaching out or
deterioration of the catalyst below the critical concentration might
have occurred during the recycling process. At the 9th recycle, an
additional portion of the ligand and metal reagent was added;
however, the yield and enantioselectivity rebounded only slightly
(89% yield and 88% ee, respectively).
In order to examine whether we could reduce the catalyst load-
ing without hampering the reactivity or stereoselectivity, a series
of reactions involving ethyl glyoxylate and
a-methylstyrene were
carried out by varying the amount of the catalyst (Table 2). First,
a control reaction was performed to examine the existence of a
non-selective pathway catalyzed possibly by the ionic liquid itself.
With only IL, a trace amount of racemic adduct was formed at a gi-
ven reaction time, which implied that the influence of the non-
selective transformation is minimal. As the amount of the catalytic
complex was gradually decreased from 10 to 3 mol %, the reaction
rates were concomitantly reduced. However, the enantiodifferenti-
We next examined the storage capability of the catalyst in ionic
liquid to see if the catalytic activity could be preserved for a long

M. Kim et al. / Tetrahedron: Asymmetry 23 (2012) 1019–1022
1021
Table 3
a,b
Enantioselective glyoxylate-ene reactions between ethyl glyoxylate and other olefins
O
5
OEt
OH
O
7
O
OEt
catalyst 4
6
H
OEt
O
[
Bmim]SbF6
OH
2
0°c
8
2
Entry
Olefin
Product
Yieldc (%)
eed (%)
1
2
5
6
7
8
97
96
94
94
a
All reactions were performed on a 0.5 mmol scale at a 0.5 M concentration with 2 equiv of ethyl glyoxylate at 20 °C for
0 min.
3
b
2
Initial enophile/ligand/Cu(OTf) molar ratio was 1/0.11/0.1.
Yield of isolated product after chromatographic purification.
c
d
Enantiomeric excess was determined from GC analysis using a Cyclodex-b column or HPLC using an OJ-H column.
Table 4
Results of the recycling catalytic enantioselective ene reactions between
a
-methyl styrene and 2.a
OH
O
4
3
mol% ligand 3
mol% Cu (OTf)2
OEt
OEt
H
O
O
0.5 M ionic liquid
1
2
Run
Yield (%)b
ee (%)c
1d
2
3
4
5
6
7
8
93
93
93
93
91
90
86
85
89
93
93
93
92
90
87
86
84
88
e
9
a
All reactions were carried out on a 0.5 mmol scale at 0.5 M concentration with 2 equiv of ethyl
o
glyoxylate at 20 C for 2 h.
b
Yields of isolated products.
Enantiomeric excess was determined by HPLC analysis.
c
d
Additional Cu(OTf)
Additional Cu(OTf)
2
(0.04 equiv) and ligand 3 (0.05 equiv) were added at this run.
(0.04 equiv) and ligand 3 (0.05 equiv) were added at this run.
e
2
period of time. Thus, the [Bmim]SbF
6
containing BOX–Cu complex
bis(oxazoline)–copper catalyzed asymmetric ene reactions. This
system exhibited far better reactivity and enantioselectivity to
those of the reaction in dichloromethane, and to those using
immobilized BOX–Cu complexes. The amount of catalyst was re-
duced to 3 mol % without any negative effect on the yield or
enantioselectivity; this 3 mol % catalyst complex in an ionic liquid
could be easily recycled through simple operations. Reactions
using even one month-old catalyst in an ionic liquid proceeded
with the same level of reactivity and enantioselectivity. We antic-
ipate that this IL-based technique will facilitate the practical use of
was recovered after a run, and kept at À20 °C for a month. When it
was reused after a month for the same transformation, it still
exhibited the same level of reactivity and stereoselectivity
(Scheme 2).
3
. Conclusion
In conclusion, we have demonstrated that a hydrophobic ionic
liquid, [Bmim]SbF
6
can be utilized as a powerful medium for
OH
O
4
3
mol% ligand 3
OEt
OEt
mol% Cu (OTf)2
H
O
0
.5 M ionic liquid
O
2
h, room temperature
1
2
First run: 93% yield, 93% ee
Second run after one month of storage : 92% yield, 93% ee
6
Scheme 2. Results of the catalytic enantioselective ene reaction with a freshly prepared and a month-old [Bmim]SbF containing BOX–Cu complex.

1
022
M. Kim et al. / Tetrahedron: Asymmetry 23 (2012) 1019–1022
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gress for the extension of its scope to other useful catalytic asym-
metric processes.
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4
.2. A representative procedure for the asymmetric glyoxylate-
ene reaction
6
Chiral bis(oxazoline) ligand 3 (5.9 mg, 0.020 mmol) and cop-
per(II) triflate (5.4 mg, 0.015 mmol) were placed in a 10 mL
screw-capped, flat-bottom vial, and 1.0 mL of 1-butyl-4-methyl-
imidazolium hexafluoroantimonate was added. The mixture was
stirred vigorously at rt to dissolve both the ligand and the metal
species. After 1 h,
portionwise. As soon as the olefin was dissolved into the reaction
mixture, freshly distilled ethyl glyoxylate (100 L, 1.0 mmol) was
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diethyl ether (6–8 times) until TLC indicated no product in the IL
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desired product (100 mg, 93% yield). The recovered ionic liquid
was dried under vacuum, and reused for the next run.
´
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This work was supported by the Korea Science and Engineering
Foundation (KOSEF) grant funded by the Korean Government
1
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