A simple and highly effective water-compatible organocatalytic system for asymmetric direct Michael reactions of linear aldehydes to maleimides

Article abstract of DOI:10.1039/c3gc41024b

Diarylprolinol silyl ether-based organocatalyst 2, in conjunction with ionic liquid supported (ILS) sulfonic acid 6a as co-catalyst, leads to a novel organocatalyst in aqueous media. The in situ generated catalyst demonstrated high reactivity and stereoselectivity for the Michael reactions of linear aldehydes to maleimides in brine without any organic solvents. For these reactions, up to 76% yield, 9: 1 dr and 96% ee were obtained for a broad variety of aldehydes and maleimides.

Full text of DOI:10.1039/c3gc41024b

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A Simple and Highly Effective Water-Compatible Organocatalytic
System for Asymmetric Direct Michael Reactions of Linear Aldehydes
to Maleimides
Yupu Qiao and Allan D. Headley*
Department of Chemistry, Texas A&M University-Commerce, Commerce,
TX 75429-3011, USA
E-mail: allan.headley@tamuc.edu
Graphical Abstract

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A Simple and Highly Effective Water-Compatible Organocatalytic
System for Asymmetric Direct Michael Reactions of Linear Aldehydes
to Maleimides
Yupu Qiao and Allan D. Headley*
Department of Chemistry, Texas A&M University-Commerce, Commerce,
TX 75429-3011, USA
E-mail: allan.headley@tamuc.edu
Abstract
Diarylprolinol silyl ether-based organocatalyst 2, in conjunction with ionic liquid
supported (ILS) sulfonic acid 6a as co-catalyst, leads to a novel organocatalyst in
aqueous media. The in situ generated catalyst demonstrated high reactivity and
stereoselectivity for the Michael reactions of linear aldehydes to maleimides in brine
without any organic solvents. For these reactions, up to 76% yield, 9:1 dr and 96% ee
were obtained for a broad variety of aldehydes and maleimides.
Introduction
The use of water, which is environmentally benign, safe and abundant, as a solvent for
organic reactions has gained interest in the past decade.1-3 Water is the natural solvent for
1

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chemical transformations of life. In nature, the aldolases are a specific group of lyases
that catalyze the stereoselective aldol reaction of ketones with aldehydes. Depending on
the mechanism of enzyme catalysis, type I and type II aldolases have been identified and
classified.4 Both types of aldolases catalyze the direct aldol reaction in an aqueous
environment. The development of organocatalysts that mimics class I aldolase catalysis
in aqueous media will provide another practical method for organic chemistry.
Nevertheless, performing reactions in water is still challenging because most organic
reactants are nonpolar and hence insoluble in water, furthermore water may deteriorate
the catalytic activity and stereocontrol via disruption of the hydrogen bonds and other
polar interactions among catalysts and substrate molecules.5 Therefore, the development
of practical water-compatible organocatalysts and their application to asymmetric
reactions using water as the reaction medium is of considerable interest.
The succinimide subunits are widely distributed in natural products and
pharmaceuticals,6 they have attracted a great deal of attention due to their potential
biological activity (Figure 1). Salinosporamide A is a potent proteasome inhibitor used as
an anticancer agent which has already entered human clinical trials for the treatment of
multiple myeloma.6f, 6i Tandospirone is an anxiolytic and antidepressant drug.
Figure 1. Succinimide-based pharmaceuticals and natural products.
Recently, organocatalysis has become a powerful tool in the synthesis of biologically
active and structurally complex compounds.7-9 Among them, the catalytic
enantioselective Michael reaction of maleimides is one of the most practical synthetic
procedures for effectively obtaining chiral succinimide subunits.10 Much effort has been
devoted over the past decade to this field and different types of organocatalysts have been
used for the asymmetric Michael reactions of maleimides with a variety of nucleophiles.
However, reports with aldehydes as nucleophiles for these reactions are relatively
2

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elusive.11 In 2007, Córdova and co-workers11a reported an excellent asymmetric Michael
reaction of maleimides with aldehydes using a chiral secondary amine as a catalyst, high
enantioselectivities were obtained for linear aldehydes. Subsequently, several reactions
involving α,α-disubstituted aldehydes and maleimides were also achieved by the
application of thiourea,11b-h guanidine,11j, 11l and other types11i, 11k organocatalysts.
Nevertheless, most of these reactions were carried out in organic solvents and from the
perspective of green chemistry, carrying out these reactions in water would be more
attractive. Herein, we present a versatile method for the Michael addition of linear
aldehyde to maleimides in aqueous media with high enantioselectivity, and this process
was made possible by utilizing a water-compatible organocatalyst that was recently
developed in our lab (Table 1). 12
Results and discussion
The Michael addition of hexanal with N-phenylmaleimide in the presence of water was
selected as the model reaction to examine the feasibility of the catalytic system. Different
acids, solvents and catalysts were investigated to optimize the reaction conditions, and
the results are summarized in Table 1. First, the reaction was carried out in water with 10
mol% of the catalyst 2 in the absence of an acid as co-catalyst, only 30% yield, 2:1
diastereoselectivity and 84% enantioselectivity were obtained in 24h at room temperature
(Table 1, entry 1). However, when 10 mol% of benzoic acid was added to the catalytic
system, the yield of the reaction was increased to 54% (Table 1, entry 2). When the
amount of the benzoic acid was increased to 20 mol%, comparable yield and
diastereoselectivity were achieved, and the enantioselectivity was increased slightly to 90%
(Table 1, entry 3). Unfortunately, further increase of the loading of benzoic acid did not
improve the yield and selectivity (Table 1, entry 4). Therefore, 20 mol% of the benzoic
acid appeared to be the optimum concentration of acid co-catalyst and this concentration
was used for the subsequent studies.
We next investigated different ionic liquid-supported (ILS) acids to further increase the
reactivity and selectivity of the catalytic system. Four different types of the ILS-acids
were synthesized and used as co-catalyst for this reaction. When the two ILS-carboxylic
acids 5a and 5b were used for the reaction, moderate yields and low diastereoselectivities
3

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were obtained with good enantioselectivities (Table 1, entries 5, 6). Prompted by these
results, two ILS-sulfonic acids, 6a and 6b were also investigated. Compared to 6b, when
20 mol% of 6a was employed as the co-catalyst, the desired product was obtained in a
slightly higher yield (61%), with a promising higher diastereoselectivity (4:1) and 86%
o
ee (Table 1, entry 7 versus entry 8). By lowering the temperature to 4 C, a lower yield
(40%) and diastereoselectivity (3:1) were observed for a longer reaction period (42h);
however, the enantioselectivity was increased to 91% (Table 1, entry 9).
Table 1 Optimization of the reaction conditions.a
O
O
O
O
N
cat: 10 mol%, acid: x mol%
N
H
H
solvent (400 uL)
O
O
7a
3
7
N
N
N
3
7
N
N
3
N
7
N
H
N
H
N
H
N
H
OTMS
OTMS
OTMS
OTMS
4
2
1
3
O
X
OH
X
N
N
n-Bu
N
N
SO3H
n-Bu
6a
6b
, X=CF3SO3
5a
5b
, X=PF6
, X=NTf2
, X=NTf2
Yield
Time
(h)
x mol%
Acid
ee d
b
entry T(oC)
Catalyst
2
Solvent
H2O
dr c
(%)
(%)
30
1
25
24
-
2:1
84
2
3
25
25
24
24
2
2
H2O
H2O
10/PhCO2H
20/PhCO2H
54
50
1:1
1:1
84
90
4
5
6
25
25
25
24
6
2
2
2
H2O
H2O
H2O
30/PhCO2H
20/5a
29
66
54
1:1
2:1
2:1
88
84
72
6
20/5b
7
8
25
25
6
6
2
2
H2O
H2O
20/6a
20/6b
61
59
4:1
2:1
86
84
4

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9
4
4
42
42
2
2
H2O
20/6a
20/6a
40
70
3:1
91
93
10
Brine
H2O:iPrOH
(3:1)
4:1
2:1
11
4
42
2
20/6a
35
94
H2O:iPrOH
(4:1)
12
4
42
2
20/6a
26
5:1
88
13
14
15
16
4
4
4
4
42
42
42
42
1
3
4
2
Brine
Brine
Brine
Brine
20/6a
20/6a
50
65
50
69
2:1
2:1
3:1
4:1
90
90
93
96
20/6a
20.6/6a
17
4
42
2
-
20.6/6a
46
4:1
95
18e
19
20
4
4
4
42
42
42
2
2
2
Brine
Brine
Brine
20.6/PTSA
20.6/MeSO3H
20.6/KHSO4
52
21
20
4:1
3:1
3:1
93
96
94
a
The reactions were carried out with 0.4 mmol of maleimide, 0.8 mmol of aldehyde, 0.0824
b
c
1
mmol of acid and 0.04 mmol of catalyst in 400 μL solvent. Isolated yield. Determined by H
NMR. d Determined by chiral HPLC. e PTSA= p-toluenesulfonic acid.
Encouraged by the result, different solvents were screened for further optimization.
When the reaction was performed in brine, the reaction proceeded fairly well. The
desired product could be furnished in 70% yield, 4:1 diastereoselectivity and 93%
enantioselectivity (Table 1, entry 10). Different water/alcohol solvent systems were also
tested, but no real improvement was found in the yield or selectivity (Table 1, entries 11,
12). Using brine as the best solvent, different types of catalysts were further investigated.
The results showed that no further improvement could be obtained when the process was
carried out under the same reaction conditions (Table 1, entries 13-15). Interestingly, for
a slight increase in the amount of the ILS-sulfonic acid 6a to 20.6 mol%, the
diastereoselectivity and yield remained the same, while the enantioselectivity was further
increased to 96% (Table 1, entry 16). However, without brine, the yield was reduced to
46%, highlighting the effect of brine in the catalytic system (Table 1, entry 17). Other
acids, such as p-toluenesulfonic acid, methanesulfonic acid and potassium bisulfate were
also investigated, and the results indicate that no further improvements in yield or
stereoselectivities were observed (Table 1, entries 18, 19 and 20).
5

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Table 2 Scope of organocatalytic conjugate addition of aldehydes to maleimides.a
O
O
O
O
N R2
cat 2: 10 mol%, 6a: 20.6 mol%
N R2
H
H
brine (400 uL), 4oC
R1
O
R1
7a-7j
O
entry
Time (h)
42
R1
R2
Ph
Yield (%) b
dr c
ee (%) d
1
n-Bu
7a/69
4:1
96
2e
3 e
4
72
72
48
Me
Et
Ph
Ph
Ph
7b/74
7c/66
7d/51
3:1
3:1
4:1
91
96
94
n-Pr
5 e
6 e
7
72
24
n-Pentyl
n-Pr
Ph
7e/76
7f/60
7g/63
7h/51
7i/15
7j/53
9:1
4:1
5:1
4:1
3:1
2:1
96
95
93
95
86
90
3-ClC6H4
4-BrC6H4
4-ClC6H4
4-CH3C6H4
n-Pr
72
n-Pr
8
72
n-Pr
9 f
120
120
n-Pr
10
Bn
a
The reactions were carried out with 0.4 mmol of maleimide, 0.8 mmol of aldehyde, 0.0824
mmol of acid and 0.04 mmol of catalyst in 400 μL of brine at 4 oC. b Isolated yield. c Determined
1
d
f
by H NMR. Determined by chiral HPLC. e 1.6 mmol of aldehyde was added. 2.4 mmol of
aldehyde was added.
With the optimized reaction conditions established, the scope and limitation of the
Michael addition of aldehydes to maleimides in brine were further explored. Initially,
aldehydes with different carbon chain lengths were investigated, and the results are
summarized in Table 2. The reaction proceeded well; the desired products were obtained
in 51-76% yield, up to 9:1 diastereoselectivities and 96% enantioselectivities (Table 2,
entries 1-5). Next, substrates with different substituents on the benzene ring of
maleimides were examined. The results showed that the reaction proceeded efficiently for
electron-deficient substituents on the phenyl ring affording the corresponding products in
good to high yields (51-63%) with moderate diastereoselectivities (up to 5:1) and high
enantioselectivities ( up to 95% ee) (Table 2, entries 6-8). However, the methyl
substituent on the benzene ring of maleimide decreased the reactivity and
6

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enantioselectivity without affecting the diastereoselectivity (Table 2, entry 9).
Furthermore, the catalytic system was also highly effective for Michael addition of
aldehyde to aliphatic substituted maleimide providing the expected product in moderate
yield (53%) with low diastereoselectivity 2:1 and high enantioselectivity 90% ee (Table 2,
entry 10). The absolute stereochemistry of the Michael product 7a was determined by
comparing its optical rotation with literature values.11a, 11g The absolute stereochemical
results can be explained by a related transition state model, which has been discussed and
studied by Córdova. 11a
Conclusions
In conclusion, highly efficient asymmetric Michael addition of linear aldehydes to
maleimides catalyzed by a simple water-compatible organocatalyst in aqueous media has
been developed. Moderate to high yields (up to 76%) with moderate diastereoselectivities
(up to 9:1) and high enantioselectivities (up to 96% ee) were obtained for a broad range
of aldehydes and maleimides. Further studies focusing on expanding the application of
this catalytic system to new cascade reactions are currently under way and will be
reported in due course.
Experimental
General procedure for the Michael addition: To the solution of Brine (0.4 mL) was added
aldehyde (0.8 mmol), maleimide (0.4 mmol), catalyst 2 (0.04 mmol) and ILS-sulfonic
acid 6a (0.0412 mmol) at room temperature. The resulting mixture was stirred at 4oC for
the time indicated in Table 2 and then quenched with 1 M HCl. The products were
extracted three times with CH2Cl2 (3×10 mL). The combined organic phase was dried
over anhydrous Na2SO4 and concentrated under reduced pressure to give the crude
Michael product. The residue was purified by flash chromatography on silica gel to
afford the desired products 7a-j.
Acknowledgements
7

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We are grateful to Dr Bukuo Ni for helpful discussion and to the National Science
Foundation for financial support of this research (CHE-1213287).
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