Application of recyclable ionic liquid-supported imidazolidinone catalyst in enantioselective Diels-Alder reactions

Article abstract of DOI:10.1039/c2gc35966a

The application of ionic liquid-supported imidazolidinone catalyst I in enantioselective Diels-Alder reactions was investigated. The Diels-Alder reactions involving α,β-unsaturated aldehydes and cyclopentadiene proceeded efficiently in the presence of cat

Full text of DOI:10.1039/c2gc35966a

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Application of recyclable ionic liquid-supported imidazolidinone catalyst
in enantioselective Diels-Alder reaction†
Zhi-Liang Shen,‡ Hao-Lun Cheong,‡ Yin-Chang Lai, Wan-Yi Loo and Teck-Peng Loh*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The application of ionic liquid-supported imidazolidinone catalyst I in enantioselective Diels-Alder
reaction was investigated. The Diels-Alder reactions involving α,β-unsaturated aldehydes and
cyclopentadiene proceeded efficiently in the presence of catalyst I to provide the desired products in
moderate to good yields with good to excellent enantioselectivities. Especially noteworthy, the catalyst I
10 can be recovered and reused for up to five runs while maintaining its high catalytic activity.
by Pihko, Cozzi, Benaglia, Hagiwara, Ying, Haraguchi, Wang,
Introduction
50 and others.6 In addition, an imidazolidinone-catalyzed Diels-
Alder reaction in an ionic liquid solvent media [bmim]PF6/H2O
has also been reported by Kim and co-workers;6k however,
though the catalytic system can be recycled several times in
reactions involving cinnamaldehyde and cyclopentadiene, a
55 considerable erosion of product enantioselectivity was observed
in the 4th run which might be due to the gradual loss of catalyst
during repeated extraction by ether after every cycle.6k
Asymmetric Diels-Alder reaction is one of the most powerful
organic transformations for the synthesis of enantiomerically
enriched cyclohexene moiety in organic synthesis.1 Despite a
15 wide range of reports on metal-mediated asymmetric Diels-Alder
reactions, MacMillan and co-workers have firstly reported the use
of imidazolidinone2 as an efficient organocatalyst3 for mediating
enantioselective Diels-Alder reaction2c-e via a LUMO-lowering
activation mechanism. Thereafter, considerable attention have
20 been focused on achieving enantioselective Diels-Alder reaction
by using metal-free organocatalysts.3 Several organocatalysts,
such as hydrazide, chiral diamine, and diarylprolinol silyl ether,
have been developed to realize asymmetric Diels-Alder
reactions.4
Despite the remarkable success accomplished in the
enantioselective Diels-Alder reaction by employing different
organocatalysts, one of the significant challenges remains: the
organocatalyst gets lost after the first run and it is difficult to
recover and recycle them in further reactions. This limitation
30 inevitably results in increased economic cost by using these
expensive catalysts which are normally accessed by tedious
multi-step syntheses. As a result, if the organocatalysts can be
developed to recycle in subsequent reactions, it will greatly
improve the overall catalytic efficiency of the organocatalysts and
35 meet the requirement of modern Green Chemistry for
sustainability.5 For this purpose, recently several modified
methods which allowed the recycling of the imidazolidinone
catalyst in Diels-Alder reaction by using polymer-, solid-, or
silica gel-supported MacMillan’s catalyst have been developed
40
On the other hand, recent surge of using ionic liquid as catalyst
supporter,7-8 instead of as reaction medium,9 has also
60 demonstrated the feasibility of enabling the recycling of
organocatalyst in organic transformations by attaching it to an
ionic liquid unit. With this idea in mind, very recently our group
has developed an efficient synthetic route to the preparation of
ionic liquid-supported imidazolidinone catalysts I-III and their
65 applications as recyclable organocatalysts in enantioselective 1,3-
dipolar cycloaddition.10 In continuation of our efforts in exploring
the synthetic utility of the catalysts I-III in organic synthesis as
well as developing practical organic transformations,11 herein we
describe an enantioselective Diels-Alder reaction catalyzed by
70 ionic liquid-supported imidazolidinone catalyst I. The reactions
proceeded efficiently to give the desired products in moderate to
good yields with good to excellent enantioselectivities. In
addition, the ionic liquid-supported imidazolidinone catalyst I can
be recovered and recycled up to five runs without significant loss
75 of its high catalytic activity.
25
Results and discussion
As illustrated in Figure 1, three chiral ionic liquid-supported
imidazolidinone catalysts I-III with different anions were
conveniently synthesized in our group.9 Application of the ionic
80 liquid-supported imidazolidinone catalysts I-III to the Diels-
Alder reaction was initially examined by choosing trans-
cinnamaldehyde and cyclopentadiene as model substrates. In
order to find an optimal conditions for the Diels-Alder reaction,
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore,
637371. Fax: (+65) 67911961; E-mail: teckpeng@ntu.edu.sg
† Electronic Supplementary Information (ESI) available: [details of any
45 supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Green Chemistry
Page 2 of 5
we first explored the catalytic activities of the three catalysts I-III
(10 mol%) by using CF3COOH (10 mol%) as reaction co-
catalyst.
3
4
5
6
7
8
TfOH
95
95
17
79
96
75
1.3:1
1.3:1
1.2:1
1.3:1
1.3:1
1.3:1
87
88
83
84
84
87
93
91
84
p-TSA
Cl3CCOOH
HBF4
Ph
O
View Online
82
89
91
HN
N
X
Bu
N
N
HPF6
catalyst I, X = I
catalyst II, X = BF4
catalyst III, X = PF6
HCl
a The reactions were carried out at room temperature for 21 h using
catalyst (0.1 mmol), acid co-catalyst (0.1 mmol), trans-
cinnamaldehyde (1.0 mmol), cyclopentadiene (5.0 mmol), CH3NO2
5 Figure 1 Ionic liquid-supported imidazolidinone catalysts I-III with
different anions.
I
b
c
(0.95 mL), H2O (0.05 mL). Isolated yield. Exo/endo selectivity
was determined by 1H NMR analysis of crude reaction mixture.
d
Enantioselectivity was determined by GC using chiral cyclodex B
column.
Table 1 Optimization of reaction conditions by using different catalystsa
catalyst (10 mol%)
Ph
O
CF3COOH (10 mol%)
+
+
Ph
CHO
CHO
Ph
CH3NO2/H2O, r.t., 21 h
As outlined in Table 2, among the various acids investigated,
the utilization of CF3COOH, TfOH, p-TSA and HPF6 (entries 1,
endo
exo
3,
4 and 7) led to relatively good yields and good
eed (%)
exo
88
Yieldb
(%)
25 enantioselectivities of the desired Diels-Alder adduct. In contrast,
relatively high yield (97%) and excellent ee in both exo and endo
isomers were produced when CF3COOH was selected (88% and
93%, entry 1). Thus, CF3COOH was identified as the optimal
acidic co-catalyst and we continued the optimization of the
30 reaction conditions by surveying a range of solvents in an effort
to further enhance the enantioselectivity of the target product.
The results are depicted in Table 3.
Entry
Catalyst
exo:endoc
endo
93
1
2
97
98
96
1.3:1
1.3:1
1.3:1
I
88
90
II
III
3
87
91
a
The reactions were carried out at room temperature for 21 h using
catalyst (0.1 mmol), CF3COOH (0.1 mmol), trans-cinnamaldehyde (1.0
mmol), cyclopentadiene (5.0 mmol), CH3NO2 (0.95 mL), H2O (0.05
Table 3 Optimization of reaction conditions by using different solventsa
mL). Isolated yield. Exo/endo selectivity was determined by 1H
b
c
catalyst I (10 mol%)
CF3COOH (10 mol%)
Ph
d
O
NMR analysis of crude reaction mixture. Enantioselectivity was
+
+
determined by GC using chiral cyclodex B column.
Ph
CHO
CHO
Ph
solvent, r.t., 21 h
endo
exo
As shown in Table 1, the Diels-Alder reactions involving
10 trans-cinnamaldehyde and cyclopentadiene proceeded efficiently
at room temperature in the presence of catalysts I-III (10 mol%)
and CF3COOH (10 mol%) in CH3NO2/H2O, affording the desired
products in almost quantitative yields (96-98%) with excellent
enantioselectivities (87-88% ee for exo isomer, 90-93% ee for
15 endo isomer). Among them, catalyst I exhibited a better capacity
for stimulating a slightly higher enantioselectivity for exo isomer
as compared to catalysts II and III. Therefore, catalyst I was
selected to further optimize the reaction conditions by screening
different acids as reaction co-catalysts.
eed (%)
exo
88
Yieldb
(%)
Entry
Solvent
exo:endoc
endo
93
1
2
3e
4
5
6
CH3NO2/H2O
THF/H2O
97
17
9
1.3:1
1.1:1
1:1
89
92
MeOH/H2O
CH3CN/H2O
CH2Cl2/H2O
DMSO/H2O
DMF/H2O
42
30
98
60
60
33
1.2:1
1.2:1
1.1:1
1.1:1
90
94
87
89
89
93
20 Table 2 Optimization of reaction conditions by using different acid co-
catalystsa
7
87
92
catalyst I (10 mol%)
Ph
O
a
acid co-catalyst (10 mol%)
The reactions were carried out at room temperature for 21 h using
catalyst I (0.1 mmol), CF3COOH (0.1 mmol), trans-cinnamaldehyde
(1.0 mmol), cyclopentadiene (5.0 mmol), organic solvent (0.95 mL),
+
+
Ph
CHO
CHO
Ph
CH3NO2/H2O, r.t., 21 h
H2O (0.05 mL). b Isolated yield. Exo/endo selectivity was determined
c
endo
exo
by 1H NMR analysis of crude reaction mixture. d Enantioselectivity was
e
determined by GC using chiral cyclodex B column. An 82% yield of
eed (%)
exo
Yieldb
(%)
the corresponding dimethyl acetal product was obtained and the
chemoselectivity was tested as 1.3:1 exo:endo ratio, 92% ee for exo
isomer and 96% ee for endo isomer, after hydrolysis by
TFA:H2O:CHCl3 (1:1:2) for 2 h at room temperature.
Acid co-
catalyst
Entry
exo:endoc
endo
93
1
2
CF3COOH
CSA
97
76
1.3:1
1.3:1
88
88
91
35
As can be seen from Table 3, comparatively high yields were
2
| Journal Name, [year], [vol], 00–00
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Page 3 of 5
Green Chemistry
achieved when the reactions were performed in solvents such as 40 excellent enantioselectivities. Both aryl and alkyl substituted α,β-
CH3NO2 and CH3CN, in conjunction with H2O (entries 1 and 4).
The yields obtained by using these two solvents were
comparable, whereas CH3CN gave a slightly higher ee for both
exo and endo isomers (entry 4). Therefore CH3CN/H2O was
unsaturated aldehydes were proven to be suitable candidates for
the present enantioselective organic transformations. Comparable
enantioselectivities were obtained in contrast to the use of
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5
imidazolidinone as reaction catalyst which were previously
chosen as the optimal solvent system in further reactions. 45 reported by MacMillan and co-workers.2c For example, when
Notably, an 82% yield of the corresponding dimethyl acetal
product was obtained when MeOH/H2O was used as reaction
media and the product should be hydrolyzed by TFA:H2O:CHCl3
trans-cinnamaldehyde was used as substrate, the utilization of
MacMillan’s imidazolidinone catalyst afforded the desired
product in 93% exo ee and 93% endo ee;2c Likewise, by
employing ionic liquid-supported imodazolidinone catalyst I in
10 (1:1:2) before releasing the desired product in an aldehyde form.
Thus, MeOH/H2O solvent system, which was previously 50 the reaction, similar results with 90% exo ee and 94% endo ee
employed in the analogous reaction system by MacMillan and co-
workers,2c was not selected as the solvent for the present reaction.
Having optimized the reaction conditions for the Diels-Alder
15 reaction by utilizing the catalyst I, CF3COOH as acidic co-
was achieved as well (entry 3).
With the success of above reactions, we embarked on studying
the recyclability of the ionic liquid-supported imidazolidinone
catalyst I in Diels-Alder reaction. It should be mentioned that the
catalyst, and CH3CN/H2O as the reaction solvent, we 55 catalyst I containing an ionic liquid unit is viscous liquid at room
subsequently applied the optimized reaction conditions to various
substrates to explore the generality of the reaction system.
temperature and shows typical characteristics of an ionic liquid:
soluble in polar solvents such as THF, CH2Cl2 and MeOH, while
insoluble in less polar solvents such as Et2O and hexane.7-9 This
unique property of ionic liquid makes the recovery and recycling
60 of the catalyst I in subsequent reaction possible, which can be
exemplified as follows: after reaction, the polar reaction solvent
of either CH3CN or CH3NO2 was removed under reduced
pressure; then the residue in the flask was extracted for five times
by directly adding Et2O (5 mL per time) into the flask with
65 stirring on a magnetic stirrer; the extracted crude product in Et2O
was purified by routine silica gel column chromatography, while
the imidazolidinone catalyst I still remained intact in the flask
because of its poor solubility in Et2O. Therefore, the catalyst I
remained in the flask can be directly reused in further reactions
70 by adding the substrates, acid co-catalyst, and solvent into the
same flask to start another run.
Table 4 Application of catalyst I to Diels-Alder reactiona
catalyst I (10 mol%)
R
O
CF3COOH (10 mol%)
+
+
R
CHO
CH3CN/H2O, r.t., 21 h
CHO
R
endo
exo
eed (%)
exo
71
Yieldb
(%)
Entry
R
exo:endoc
endo
1e
2e
3
4-BrC6H4
4-ClC6H4
C6H5
99
85
98
95
83
89
78
45
1.1:1
1.1:1
1.2:1
1:1
73
84
94
93
92
93
91
94
85
90
4
2-OMeC6H4
2-naphthyl
n-Bu
87
Table 5 Recycling of catalyst I in Diels-Alder reaction
5e
6
1.2:1
1:1
89
catalyst I (20 mol%)
CF3COOH (20 mol%)
Ph
O
84
+
+
Ph
CHO
CHO
Ph
CH3CN/H2O, r.t., 21 h
7
n-Pr
1:1.1
1:1
84
endo
exo
8
Et
--f
Yielda
(%)
ee (%)
exo
90
Cycle
exo:endob
a
endo
20
Unless otherwise noted, the reactions were carried out at room
temperature for 21 h using catalyst I (0.1 mmol), CF3COOH (0.1 mmol),
aldehyde (1.0 mmol), cyclopentadiene (5.0 mmol), CH3CN (0.95 mL),
H2O (0.05 mL). b Isolated yield. c Exo/endo selectivity was determined by
1
2
3
4
5
98
96
97
93
96
1.2:1
1.2:1
1.2:1
1.2:1
1.2:1
94
94
94
90
90
90
d
1H NMR analysis of crude reaction mixture. Enantioselectivity was
e
88
25 determined by chiral HPLC or GC analysis. CH3NO2 was used in place
f
of CH3CN, due to poor solubility of the aldehyde in CH3CN. Not
86
determined because of the inseparable exo enantiomers.
89
a
b
Isolated yield. Exo/endo selectivity was determined by 1H NMR
As summarized in Table 4, the Diels-Alder reaction involving
30 various α,β-unsaturated aldehydes and cyclopentadiene
proceeded efficiently in the presence of catalyst I and CF3COOH
in CH3CN/H2O, leading to the corresponding products in
moderate to good yields with good to excellent
enantioselectivities. It should be noted that, in three cases (entries
35 1, 2 and 5), owing to the poor solubility of α,β-unsaturated
aldehydes in CH3CN/H2O, poor yields of the desired products
were obtained. Thus, in these special cases, a more polar solvent
system of CH3NO2/H2O was employed, furnishing good to
excellent yields of the target products while maintaining their
analysis of crude reaction mixture.
75
As anticipated, the recycling of the imidazolidinone catalyst I
in Diels-Alder reaction was successfully achieved as well. As
shown in Table 5, no significant decrease in catalytic activity was
observed when the ionic liquid-supported imidazolidinone
80 catalyst I was recovered and reused in further reactions. Excellent
enantioselectivities of 89% exo ee and 90% endo ee of the
corresponding products were obtained even in the 5th cycle,
making the catalyst I synthetically efficient and useful (entry 5).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

Green Chemistry
Page 4 of 5
7894; (i) S. P. Brown, N. C. Goodwin, D. W. C. MacMillan, J. Am.
Understandably, in the absence of the ionic liquid unit, the
recovery and recycling of the imidazolidinone catalyst will
undoubtedly become very difficult.
Chem. Soc., 2003, 125, 1192; (j) J. W. Yang, M. T. Hechavarria
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Conclusions
5
In summary, we have described an efficient, economical, and
practical method for enantioselective Diels-Alder reaction by
using an ionic liquid-supported imidazolidinone catalyst I,
leading to the desired products in moderate to good yields with
good to excellent enantioselectivities. Especially noteworthy, the
10 ionic liquid-supported imidazolidinone catalyst I can be readily
recovered and recycled in further transformations for at least five
times while still retaining its high catalytic activity which is in
accordance with the principle of Green Chemistry, rendering the
catalyst potentially useful in organic synthesis.
65 3
70
75
15 Experimental
General procedure for the enantioselective Diels-Alder reaction:
To a solution of catalyst I (0.05 g, 0.1 mmol) in CH3CN (or
CH3NO2)/H2O (1 mL, 95/5 v/v) was added trifluoroacetic acid
(0.0114 g, 0.1 mmol) and α,β-unsaturated aldehyde (1 mmol).
20 The solution was stirred for 1-2 minutes before the addition of
cyclopentadiene (0.331 g, 5 mmol). The reaction was stirred at
room temperature for 21 h before the removal of CH3CN (or
CH3NO2) under vacuo. The residue was extracted by diethyl ether
(5 mL x 5) with stirring on a magnetic stirrer, and the combined
25 organic layer was washed with brine, dried over Na2SO4 and
filtered. The filtrate was concentrated under vacuo and purified
by silica gel column chromatography using hexane and ethyl
acetate as eluant to afford the desired product. The remaining oil
compound in the flask (catalyst I) was dried under vacuo and
30 reused in further reactions by the addition of trifluoroacetic acid
(0.0114 g, 0.1 mmol) and solvent.
80
4
For recent examples regarding the asymmetric organocatalyzed Diels-
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85
90
95
Acknowledgements
100
We gratefully acknowledge Nanyang Technological University
(NTU), NTU New Initiative Funding, Ministry of Education (No.
35 M45110000), and A*STAR SERC Grant (No. 0721010024) for
the funding of this research.
105
Notes and references
1. For recent reviews of enantioselective Diels–Alder reactions, see: (a)
D. A. Evans, J. S. Johnson in Comprehensive Asymmetric Catalysis,
110 5 For reviews on the recyclability of chiral catalysts in organic synthesis,
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