Immobilization of macMillan imidazolidinone as mac-SILC and its catalytic performance on sustainable enantioselective dielsAlder cycloaddition

Article abstract of DOI:10.1002/adsc.200900865

MacMillan's imidazolidinone catalyst was immobilized as a supported ionic liquid catalyst (Mac-SILC) in the pores of silica gel with the aid of an ionic liquid - 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The heterogenized organocatalyst was utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and cinnamaldehyde, recovered by simple filtration and subsequent evacuation, and repeatedly used up to six times in 81% average chemical yield, 87% ee for endo- and 80% ee for exo-products. The Mac-SILC was effective for a variety of substrates.

Full text of DOI:10.1002/adsc.200900865

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DOI: 10.1002/adsc.200900865
Immobilization of MacMillan Imidazolidinone as Mac-SILC and
its Catalytic Performance on Sustainable Enantioselective Diels–
Alder Cycloaddition
Hisahiro Hagiwara,a,* Toshihiro Kuroda,a Takashi Hoshi,b and Toshio Suzukib
a
Graduate School of Science and Technology, Niigata University, 8050, 2-Nocho, Ikarashi, Nishi-ku, Niigata 950-2181,
Japan
Fax : (+81)-25-262-7368; phone: (+81)-25-262-7368; e-mail: hagiwara@gs.niigata-u.ac.jp
Faculty of Engineering, Niigata University, 8050, 2-Nocho, Ikarashi, Nishi-ku, Niigata 950-2181, Japan
b
Received: December 14, 2009; Revised: February 22, 2010; Published online: March 12, 2010
Abstract: MacMillanꢀs imidazolidinone catalyst was quent evacuation, and repeatedly used up to six
immobilized as a supported ionic liquid catalyst times in 81% average chemical yield, 87% ee for
(Mac-SILC) in the pores of silica gel with the aid of endo- and 80% ee for exo-products. The Mac-SILC
an ionic liquid – 1-butyl-3-methylimidazolium bis(tri- was effective for a variety of substrates.
fluoromethylsulfonyl)imide. The heterogenized orga-
nocatalyst was utilized for the enantioselective Keywords: asymmetric catalysis; catalyst recycling;
Diels–Alder reaction of cyclopentadiene and cinna- heterogeneous catalysis; ionic liquids; organocataly-
maldehyde, recovered by simple filtration and subse- sis; supported catalysts
Introduction
We previously confined a solution of homogeneous
organometallic catalysts in an ionic liquid in the pores
The importance of catalytic reactions in organic syn- of an inorganic amorphous solid as a supported ionic
thesis is ever increasing in the context of green and liquid catalyst (SILC) (Scheme 1).[4,5] The procedure
sustainable chemistry. Among a variety of catalysts, is simple, mild and cost-effective. The SILC was recy-
organocatalysts are attracting considerable attention cled by simple filtration without any pre-treatment.
due to their simple design, easy reproduction and safe Palladium, copper and ruthenium were immobilized
disposal, which is more sustainable, environmentally according to the SILC method, and exhibited both a
friendly, and cost effective and complementary to ho- sustainable nature and higher catalytic activities than
mogeneous organometallic catalysts.[1] However, ho- their homogeneous counterparts.
mogeneous organomolecular catalysts have some
The MacMillan imidazolidinone (Mac) is a very
drawbacks, such as low turnover number, low stability versatile organocatalyst (Figure 1),[6] which catalyzes
and difficulty in recycle use. One obvious way to ad- enantioselective Diels–Alder reactions,[6a,b] 1,3-dipolar
dress these issues is to immobilize the catalytic resi-
due on a support.[2]
Dissolving organocatalysts in liquid supports, such
as an ionic liquid, polyethylene glycol, fluorous sol-
vent, or water is a very facile immobilization method
that does not modify the original structure, add extra
reagents, or lose the original catalytic activity, of
which an ionic liquid has very special and characteris-
tic properties, including non-lipophilicity, non-hydro-
philicity and non-volatility.[3] In addition, immiscibility
with other organic solvents, as well as water, should
provide a good support for the immobilization of or-
ganocatalyst residues. However, the large-scale use of
an ionic liquid is limited due to the high cost and tedi-
ous extraction of products from the viscous ionic
liquid.
Scheme 1. Immobilization of homogeneous organometallic
catalysts as SILC.
Adv. Synth. Catal. 2010, 352, 909 – 916
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Hisahiro Hagiwara et al.
FULL PAPERS
Figure 1. MacMillan catalysts.
Scheme 2. Effect of ionic liquids in the enantioselective
Diels–Alder reaction.
cycloadditions,[6c] Friedel–Crafts alkylations,[6d] indole
alkylations,[6e] and a-halogenations of aldehydes.[6f]
The practical total synthesis of (À)-oseltamivir by Fu-
AHCTUNGTRENNUNG
ture.[6a,b] Although there is one precedent of a catalyt-
kuyama and co-workers exemplified its catalytic value ic reaction in an ionic liquid,[17] the optimal ionic
in the enantioselective Diels–Alder reaction.[7] De- liquid was re-investigated prior to immobilization as
spite its success, the catalyst has the characteristic Mac-SILC, using the Diels–Alder reaction of cinna-
drawbacks of an organocatalyst. To address these maldehyde and cyclopentadiene in an ionic liquid at
issues, the MacMillan catalyst was covalently immobi- room temperature for 24 h (Scheme 2). The results
lized on JandaJel,[8] PEG,[9] polymer capsules,[10] liquid are shown in Table 1. Since MacMillan catalysts are
crystals,[11] fluorous solvent[12] or silica.[13] Alternative- not easily soluble in ionic liquids, dichloromethane
ly, non-covalent immobilization was carried out by in- was initially added to the mixture and evaporated
corporation into dendrimers,[14] entrapment in mont- after dissolution. Cinnamaldehyde was used directly
morillonite[15] or ionic bonding on polymers,[16] al- from the bottle, after confirming its purity by NMR.
though successful recycle use has yet to be explored The endo/exo selectivity was determined by NMR
in both covalent and non-covalent immobilizations.
spectroscopy to be almost comparable as in the reac-
Based on these findings, we investigated the non- tion in homogeneous precedents.[6a,b] The enantiomer-
covalent immobilization of the MacMillan catalyst as ic excess was obtained by HPLC after reduction of
a Mac-SILC, and its catalytic performance in an enan- the Diels–Alder products by sodium borohydride.
tioselective Diels–Alder reaction. Since the present The 2S,3S absolute stereochemistries of each of the
Diels–Alder reaction proceeds via the iminium cation endo- and exo-products 6 were determined by a com-
intermediate and related polar transition states, the parison of the optical rotations.[18] The Diels–Alder
ionic liquid in silica gel pores is expected to play the reaction was dependent on the counter anions of the
role of a sustainable reaction medium, as well as acti- ionic liquids, in which [bmim]PF6 and [bmim]NTf2 ex-
vator of the catalytic reaction.
hibited comparable chemical yields, irrespective of
the counter anions of MacMillan catalysts 1 and 2
(Table 1, entries 1–3). Enantioselectivity was better in
[bmim]NTf2. Complete recovery of the starting mate-
rials in entries 4–7 has yet to be discussed.
Results and Discussion
Preparation and Optimization of Mac-SILC
Then, MacMillan imidazolidinone catalysts 1–3[19]
were immobilized with [bmim]PF6 or [bmim]NTf2 as
MacMillan catalysts 1 and 2 (Figure 1) were easily Mac-SILC-1 to Mac-SILC-10 (Scheme 3 and Table 2).
prepared in accordance with reports in the litera- A slurry of amorphous silica gel in a dichloromethane
Table 1. Investigation of an optimal ionic liquid in enantioselective Diels–Alder reaction.
Entry[a]
Catalyst
Ionic liquid
Yield [%][b]
endo:exo[c]
ee [%][d]
endo
exo
1
2
3
4
5
6
7
1
1
2
2
1
1
1
U
62
67
76
0
0
0
1:1.1
1:1.2
1:1.3
73
92
93
69
72
90
0
[a]
Reaction was carried out with 0.1 equiv of the MacMillan catalyst at room temperature in 24 h. Cinnamaldehyde was
used directly from the bottle.
[b]
[c]
[d]
Isolated pure product based on the aldehyde.
1
Determined by H NMR.
Determined by HPLC of the alcohol after reduction of the formyl group.
910
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Adv. Synth. Catal. 2010, 352, 909 – 916

Immobilization of MacMillan Imidazolidinone as Mac-SILC and its Catalytic Performance
Table 2. Immobilization of MacMillan catalysts 1–3 as Mac-SILC-1 to Mac-SILC-10.
Entry
SILC
Catalyst
Support
Ionic liquid (wt%)
Loading [mmolgÀ1]
1
2
3
4
5
6
7
8
9
Mac-SILC-1
Mac-SILC-2
Mac-SILC-3
Mac-SILC-4
Mac-SILC-5
Mac-SILC-6
Mac-SILC-7
Mac-SILC-8
Mac-SILC-9
Mac-SILC-10
1
1
1
1
2
3
3
2
2
2
silica gel-1[a]
silica gel-1[a]
silica gel-1[a]
silica gel-1[a]
silica gel-1[a]
silica gel-1[a]
silica gel-1[a]
silica gel-2[b]
silica gel-3[c]
Al2O3
U
0.148
0.152
0.135
0.140
0.131
0.054
0.162
0.106
0.021
0.043
10
[a]
Pore volume: 0.80–1.0 mLgÀ1.
[b]
[c]
Pore volume: 1.22 mLgÀ1.
Mercaptopropylated silica gel.
solution of the MacMillan catalyst and the ionic
liquid was stirred at room temperature for 6 h, and
evaporated to dryness under vacuum to give a free-
flowing powder. The amount of ionic liquid did not
affect the amount of catalyst loading (Table 2, en-
tries 1–3), while the catalytic activity was dependent
on the amount of ionic liquid (see Table 3, entries 1–
3). On the other hand, the level of immobilization of
ionic liquid tagged-catalyst 3 was not sufficient com-
pared with imidazolidinones 1 or 2 (Table 2, entry 6),
although catalyst 3 was confined sufficiently without
the ionic liquid (Table 2, entry 7). Similarly, surface-
modified silica gel (Table 2, entry 9) or alumina
(Table 2, entry 10) was not suitable for the immobili-
zation of a sufficient amount of MacMillan catalyst 2.
Subsequently, an optimized Mac-SILC was investi-
gated for the Diels–Alder reaction of cinnamaldehyde
Scheme 3. Immobilization of MacMillan catalysts as Mac-
SILC.
Table 3. Investigation of an optimal Mac-SILC in the enantioselective Diels–Alder reaction.
Entry[a]
Mac-SILC
Solvent
Time [h]
Yield [%][c]
endo:exo[d
ee [%][e]
endo
exo
1[b]
2
3
4
5
6
7
8
9
Mac-SILC-1
Mac-SILC-1
Mac-SILC-2
Mac-SILC-2
Mac-SILC-3
Mac-SILC-2
Mac-SILC-4
Mac-SILC-5
Mac-SILC-6
Mac-SILC-7
Mac-SILC-9
Mac-SILC-10
95% EtOH
95% EtOH
95% EtOH
95% i-PrOH
95% i-PrOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
35
47
27
27
20
22
22
22
24
22
22
22
49
67
73
69
67
60
69
74
52
50
0
1:1.1
1:1.1
1:1.1
1.1:1
1:1
1:1.2
1:1.4
1:1.2
1:1.3
nd[f]
92
95
95
96
85
83
87
92
71
nd[f]
93
92
90
94
81
77
81
80
64
nd[f]
10
11
12
0
[a]
0.1 equiv. of Mac-SILC was employed.
0.05 equiv. of Mac-SILC was employed.
Isolated pure product based on aldehyde.
Determined by H NMR.
Determined by HPLC of alcohol after reduction of the formyl group.
Not determined.
[b]
[c]
[d]
[e]
[f]
1
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Hisahiro Hagiwara et al.
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re-used after simple filtration and subsequent evacua-
tion. The second use of Mac-SILC-2 decreased the
chemical yields substantially (Table 4, entries 1 and 2)
in aqueous ethanol. The decrease was believed to be
mainly caused by leaching and loss of the MacMillan
catalyst from Mac-SILC, even though less hydrophilic
[bmim]PF6 was employed for immobilization. The
MacMillan catalyst is less soluble in isopropylalcohol,
and recycle use in this solvent gave no improvement
(Table 4, entries 3 ~5). Thus, attention was focused on
the more hydrophobic t-amyl alcohol without adding
water to prevent leaching of the MacMillan catalyst;
MacMillan catalysts 1 and 2 were not soluble in t-
amyl alcohol. Although recycle use of Mac-SILC-5 in
Scheme 4. Screening of Mac-SILC in the enantioselective
Diels–Alder reaction.
and
cyclopentadiene
at
room
temperature the solvent was not improved (Table 4, entry 7), Mac-
(Scheme 4). Since 0.05 equiv. of Mac-SILC was not SILC-4 could be recycled satisfactorily (Table 4, en-
sufficient to complete the reaction (Table 3, entry 1), tries 8–11). This result shows the importance of the
0.1 equiv. of Mac-SILC was used for further reactions. counter anion of the MacMillan catalyst, as well as
Reactions catalyzed by Mac-SILC-2 and Mac-SILC-3 the solvent in recycle use. Different from primary al-
proceeded much faster than those catalyzed by Mac- cohols, formation of acetals of 6 was not observed,
SILC-1 (Table 3, entries 2 and 5), which indicates that which is an additional aspect in favour of employing
20 wt% of an ionic liquid is optimal for immobiliza- bulky t-amyl alcohol. The less hydrophilic n-heptanol
tion and the subsequent catalytic reaction. The impor- did not give a satisfactory result (Table 4, entries 12
tance of an ionic liquid as both a liquid support and a ~14).
reaction medium is shown in Table 2, entries 6 and 7,
and Table 3, entry 10, in which MacMillan catalyst 3,
immobilized on silica gel without an ionic liquid, ex- Recycle Use of Mac-SILC
hibited unsatisfactory catalytic activity.
Mac-SILC-2, Mac-SILC-4 and Mac-SILC-5 were With the optimized reaction condition in hand
chosen as candidates for recycle experiments, since (Table 4, entry 8), recycle use of the Mac-SILC-4
their catalytic performances were comparable to each (MacMillan catalyst 1 in silica gel pores with the aid
other, and better than those of Mac-SILC-1, Mac- of 20 wt% of [bmim]NTf2) was newly investigated
SILC-3, Mac-SILC-6 and Mac-SILC-7. Mac-SILC was (Scheme 5) and the results are compiled in Table 5.
Table 4. Investigation on recycle use of Mac-SILC in the enantioselective Diels–Alder reaction.
Entry[a]
Mac-SILC
Solvent
Time [h]
Yield [%][b]
endo:exo[c]
ee [%][d]
endo
exo
1
2
3
4
5
6
7
8
Mac-SILC-2
Mac-SILC-2
Mac-SILC-2
Mac-SILC-2
Mac-SILC-2
Mac-SILC-5
Mac-SILC-5
Mac-SILC-4
Mac-SILC-4
Mac-SILC-4
Mac-SILC-4
Mac-SILC-4
Mac-SILC-4
Mac-SILC-4
95% EtOH
95% EtOH
95% i-PrOH
95% i-PrOH
95% i-PrOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
t-amylOH
n-heptanol
n-heptanol
n-heptanol
27
27
27
30
40
22
25
22
25
29
41
22
25
29
73
23
69
53
25
74
31
69
67
56
48
67
63
52
1:1.1
1:1
1.1:1
1:1
95
88
96
88
86
92
80
87
88
89
87
93
90
nd[e]
90
92
94
92
87
86
79
81
83
83
83
89
90
nd[e]
1:1
1:1.2
1:1.1
1:1.2
1:1.2
1:1.2
1:1.2
1:1.1
1:1.1
1:1.1
9
10
11
12
13
14
[a]
0.1 equiv. of Mac-SILC was employed, which was re-used intact after filtration and evacuation.
Isolated pure product based on aldehyde.
Determined by H NMR.
Determined by HPLC of alcohol after reduction of the product.
Not determined.
[b]
[c]
[d]
[e]
1
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Immobilization of MacMillan Imidazolidinone as Mac-SILC and its Catalytic Performance
Table 5. Optimized-recycle use of Mac-SILC-4 in the enantioselective Diels–Alder reaction.
Entry[a]
Time [h]
Yield [%][b]
endo:exo[c]
ee [%][d]
endo
exo
0[e]
1[f]
2
3
4
22
22
25
29
39
45
56
64
96
89
79
80
78
62
1:1.2
1:1.2
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
90
93
92
90
88
85
76
84
89
88
88
86
84
77
5
6
[a]
Reaction was carried out with 0.1 equiv. of Mac-SILC-4 in t-amyl alcohol at room temperature. The catalyst was re-used
intact after evacuation.
[b]
[c]
[d]
[e]
[f]
Isolated pure product based on aldehyde.
1
Determined by H NMR.
Determined by HPLC of alcohol after reduction of the formyl group.
Cinnamaldehyde was used in 7 h after MPLC purification.
Cinnamaldehyde was used in 3 h after MPLC purification in each recycle experiment.
Scope of Mac-SILC-4
The catalytic activity of Mac-SILC-4 was common for
a variety of substrates (Table 6). The present protocol
was applicable and effective for various dienes with
both electron-poor and electron-rich substituents. Es-
pecially noteworthy is the reaction of electron-rich p-
methoxycinnamaldehyde, which was not successful
either by homogeneous MacMillan catalyst or con-
ventional Lewis acid, such as trimethylaluminum, yt-
terbium triflate or borontrifluoride etherate (Table 6,
entry 5). This result indicates that the immobilization
as Mac-SILC reinforced the catalytic activity of the
original MacMillan catalyst. The low endo-selectivity
might be due to inherent steric crowding of the imida-
zolidinone moiety.
Scheme 5. Optimized recycle use of Mac-SILC-4 in the
enantioselective Diels–Alder reaction.
During the course of the recycle experiments, it was
found that cinnamaldehyde should be used soon after
purification by medium pressure LC to allow better Conclusions
chemical yield (Table 5, entries 0 and 1), even though
stock cinnamaldehyde was deemed pure by NMR In summary, the MacMillan imidazolidinone organo-
spectroscopy. To this end, Mac-SILC-4 was used re- catalyst was non-covalently immobilized as a Mac-
peatedly up to 6 times after simple filtration and sub- SILC in the pores of silica gel with the aid of the
sequent evacuation in 81% average yield; 87% ee for ionic liquid [bmim]NTf2, and was successfully applied
endo- and 80% ee for exo-products. MacMillan cata- to the enantioselective Diels–Alder reaction in open
lyst 1 was stable at 608C for 6 h in a solution of air at room temperature, without changing the origi-
[bmim]NTf2 and THF. In contrast, MacMillan catalyst nal catalytic activity. Furthermore, it could be re-used
1 partially leached out from Mac-SILC-4 after 24 h after simple filtration up to six times in 81% average
stirring at room temperature in aqueous ethanol and chemical yield, 87% ee for endo- and 80% ee for exo-
was recovered intact. These results suggest that the products. Mac-SILC is more active, especially to an
gradual decrease of reactivity of Mac-SILC-4 in the electron-rich dienophile, than its homogeneous coun-
recycle use might be mainly due to leaching of Mac- terparts, and the required reaction conditions are so
Millan catalyst 1 from Mac-SILC 4, not decomposi- mild that it might be applicable to highly functional-
tion of MacMillan catalyst 1.
ized but unstable substrates. The immobilization pro-
tocol is simple, practical and cost-effective without
chemically modifying the original catalyst or support,
and is a powerful tool as a method of soft immobiliza-
tion of highly sophisticated organomolecular catalysts,
Adv. Synth. Catal. 2010, 352, 909 – 916
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Hisahiro Hagiwara et al.
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Table 6. Scope of Mac-SILC-4 in the enantioselective Diels–Alder reaction.
Entry[a] Dienophile
Diene
Product
Time [h] Yield [%][b] endo:exo[c] ee [%][d]
R=
endo exo
[f]
1
2
3
4
5
H (7)
4
endo-8[8]
22
22
22
22
46
58
61
78
96
76
3.4:1
1:1
47
83
86
93
92
–
Me (9)
endo-10[6a]
endo-12[6a]
endo-6[6a]
endo-14[21]
–
n-Pr (11)
Ph[f] (5)
1:1
72
89
86
1:1.2
1:1.2
p-MeOC6H4 (13)
6
p-NO2C6H4 (15)
endo-16[22]
22
91
1:1.2
87
80
7[g]
8[g]
9
H (7)
17
19
4
18[6a]
26
48
24
80 (87)[h]
80 (84)[h]
85
93[i] 26
[j]
H (7)
endo-20[6a]
endo-22[23]
18:1
8:1
80[i]
0
–
0
Ethyl vinyl ketone (21)
[a]
[b]
[c]
[d]
[e]
[f]
Reaction was carried out with 0.1 equiv. of Mac-SILC-4 in t-amyl alcohol at room temperature.
Isolated pure product based on the aldehyde.
1
Determined by H NMR.
Determined by HPLC of the alcohol after reduction of the formyl group.
Enantiomers were not separable by HPLC of alcohol after reduction of the formyl group.
Cinnamaldehydes were used soon after MPLC purification.
Reaction was carried out in a sealed tube.
Isolated pure product based on diene.
Determined by GLC.
[g]
[h]
[i]
[j]
Enantiomers were not separable by GLC.
medium pressure LC before use. Other chemicals were used
directly from bottles. Silica gel for column chromatography
(Kanto Chemical Co., particle size: 63–210 mm, pore size:
5.0~7.0 nm, pore volume: 0.80–1.00 mLgÀ1) was used for
immobilization after drying by microwave irradiation. Struc-
tures of products and the exo/endo ratios were determined
a method that so far cannot be conducted by simple
impregnation.
Experimental Section
1
by H NMR spectroscopy with a JEOL EX-270 spectrome-
ter (270 MHz). The enantiomeric excess was determined by
General Remarks
Hitachi L-7100 high-pressure LC system employing
CHIRALCELL OJ-H column.
a
MacMillan catalysts were prepared according to the litera-
ture precedent.[6a,b] Cinnamaldehyde was purified by
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Immobilization of MacMillan Imidazolidinone as Mac-SILC and its Catalytic Performance
References
Preparation of the Catalyst Mac-SILC-4
A mixture of silica gel powder (4 g, dried by microwave irra-
diation for 5 min twice prior to use), the MacMillan imida-
zolidinone catalyst 1 (200 mg, 0.785 mmol), [bmim]NTf2
(800 mg, 20 wt% to silica gel) in a dichloromethane solution
(16 mL) was stirred for 6 h at room temperature under a ni-
trogen atmosphere. After evaporation of dichloromethane,
the silica gel powder was rinsed with ether (15 mL) 5 times.
Evaporation of the ether under vacuum left a mixture of
[bmim]NTf2 (38 mg) and MacMillan catalyst 1 (19 mg) by
NMR analysis. Thus, the amount of loading of the MacMillan
catalyst 1 as Mac-SILC-4 was evaluated as 0.144 mmolgÀ1.
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General Procedure for the Diels–Alder Reaction
A mixture of the Mac-SILC-4 (177 mg, 0.025 mmol, Mac-
Millan catalyst 1 loading: 0.141 mmolgÀ1), cyclopentadiene
4
(103 mL, 1.25 mmol), cinnamaldehyde
5
(32 mL,
0.25 mmol, used soon after purification by medium pressure
liquid chromatography) in a solution of t-amyl alcohol
(0.3 mL) was stirred at room temperature for 22 h in open
air. After the addition of ether, the organic layer was re-
moved by decantation. Evaporation of the solvent followed
by medium pressure liquid chromatography (eluent: ethyl
acetate:n-hexane=1:9) of the residue afforded a mixture of
the Diels–Alder products 6 (yield: 48 mg, 96%), in which
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Determination of Enantioselectivity of the Diels–
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Sodium borohydride (27 mg, 0.72 mmol) was added to a
stirred solution of endo/exo mixtures of aldehydes 6 (48 mg,
0.24 mmol) in methanol (1.0 mL) at 08C. After the solution
was stirred at room temperature for 1 h, the reaction was
quenched by the addition of aqueous ammonium chloride.
The product was extracted with ether and the combined or-
ganic layer was washed with water and brine. Evaporation
of the solvent followed by medium pressure LC provided
exo-alcohol (yield: 19 mg, 40%) and endo-alcohol (yield:
19 mg, 39%), in which the enantiomeric excess was analyzed
by high pressure LC employing CHIRALCELL OJ-H
column (eluent: isopropyl alcohol:n-hexane=1:9.5). Reten-
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and 30.66 min for the S isomer. Retention times of exo-alco-
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S isomer.
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Acknowledgements
This work was partially supported by a Grant-in-Aid for Sci-
entific Research on Priority Areas (20031011 for H. H.) from
the Ministry of Education, Culture, Sports, Science and Tech-
nology (MEXT), and a Grant for Promotion of Niigata Uni-
versity Research Project. We thank Prof. Teck-Peng Loh and
Dr. Shen Zhiliang, Nanyang Technological University, Singa-
pore, for providing the ionic liquid-supported imidazolidi-
none catalyst 3.
Adv. Synth. Catal. 2010, 352, 909 – 916
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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