Organocatalytic asymmetric Michael addition of ethyl nitroacetate to enones using natural amino acids-derived C1-symmetric chiral primary-secondary diamines

Article abstract of DOI:10.1016/j.tetlet.2013.04.005

A highly organocatalytic asymmetric Michael addition of ethyl nitroacetate to enones by using C₁-symmetric chiral primary-secondary diamines has been developed. In assistance of o-nitrobenzoic acid, chiral amine 1f which was derived from l-tryptophane and d-camphor can effectively promote the transformation in high yields (up to 96%) and enantioselectivities (up to 95%) under mild conditions.

Full text of DOI:10.1016/j.tetlet.2013.04.005

Tetrahedron Letters 54 (2013) 3011–3014
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Organocatalytic asymmetric Michael addition of ethyl nitroacetate to
enones using natural amino acids-derived C1-symmetric chiral
primary–secondary diamines
⇑
Yirong Zhou, Qiang Liu, Yuefa Gong
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly organocatalytic asymmetric Michael addition of ethyl nitroacetate to enones by using C1-sym-
Received 2 February 2013
Revised 25 March 2013
Accepted 2 April 2013
Available online 9 April 2013
metric chiral primary–secondary diamines has been developed. In assistance of o-nitrobenzoic acid, chi-
ral amine 1f which was derived from L-tryptophane and D-camphor can effectively promote the
transformation in high yields (up to 96%) and enantioselectivities (up to 95%) under mild conditions.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Organocatalysis
Asymmetric Michael addition
Amino acids
Chiral diamine
Enone
Asymmetric Michael addition of nitro compounds to
urated ketones as an efficient atom-economical carbon–carbon
bond formation strategy in organic synthesis chemistry delivers
versatile chiral c-nitro ketone adducts which can be transformed
a
,b-unsat-
tion of nitro ester to enones, and high to 99% enantioselectivities
were achieved. Compared with the well-established pyrrolidine-
based chiral secondary amine organocatalyst systems, chiral pri-
mary amine can efficiently activate the carbonyl group of enone
via iminium activation.11 Despite the above advancements, it
should be noted that the existent primary amine catalysts are still
limited to expensive chiral sources, tedious classes, and bureau-
cratic procedures. Therefore, it is still necessary to explore a simple
and easily available chiral catalyst to enrich the catalyst library to
tolerate broad substrate scope.
Very recently, our group designed and prepared a group of no-
vel chiral diamines from commercially available inexpensive natu-
ral amino acids and camphor (Fig. 1), and successfully applied
them in the asymmetric catalytic Henry reaction and Michael
addition.12 As part of our ongoing work, we envisioned that ethyl
nitroacetate as a practically useful stabilized carbanion precursor
may also serve as a proper nucleophilic partner in the primary
amine activated reaction of the enone. Herein, we report a success-
ful example of amino acid and camphor-derived C1-symmetric
chiral primary–secondary diamine catalyzed asymmetric Michael
addition of ethyl nitroacetate to enones.
into various useful enantiopure building blocks, such as muti-
substituted pyrrolines, pyrrolidines, lactones, and unnatural amino
acids.1 As a consequence, the enantioselective catalytic variants
have attracted great interest, and steady progress has been
achieved by development of various catalytic systems including
both traditional transition metal-catalysis2 and newly rising organ-
ocatalysis.3 Among them, secondary amine,4 primary amine,5 thio-
urea,6 and quaternary ammonium salt7 are the four kinds of most
widely used common organocatalysts. Jøgensen developed two
types of novel secondary amine from phenylalanine, and obtained
up to 92% ee value.8 zhao and wang, respectively, synthesized sev-
eral new kinds of chiral primary–secondary diamines based on
phenylalanine or tryptophane, which proved to be efficient cata-
lysts to facilitate several conjugate additions of enones with excel-
lent yields and enantioselectivities.9 Apart from common
nitroalkanes such as nitromethane and 2-nitropropoane,
esters are another commercially available functional group en-
a-nitro
riched nitro source. Nevertheless, there are few reported examples
At the beginning, the model reaction between ethyl nitro ace-
tate (2) and trans-4-methoxyphenyl-3-buten-2-one (3a) was cho-
sen to optimize the reaction parameters. On the basis of Lu’s
concerning the conjugated additions using
a-nitro esters as Mi-
chael donors.10 Lu10b and Kim,10c respectively, reported cinchona
alkaloid-based primary amine catalyzed asymmetric Michael addi-
work,10b
L-camphorsulfonic acid (L-CSA) was used as acid additive
to screen the chiral diamines as organocatalyst to facilitate the
conjugate addition, and the results are summarized in Table 1. As
listed in entries 1–4, different combinations of L- or D-phenylala-
⇑
Corresponding author. Tel.: +86 027 8754 3232; fax: +86 027 8754 3632.
E-mail address: gongyf@mail.hust.edu.cn (Y. Gong).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.005

3012
Y. Zhou et al. / Tetrahedron Letters 54 (2013) 3011–3014
Table 2
Ph
Ph
H2N
Ph
Ph
H2N
Acid additive screeninga
H2N HN
1a
N
H
H2N
N
H
HN
1b
1d
1c
COOEt
O
O2N
O
NO2
10 mol% 1f/ acid
Ph
+
COOEt
2
N
H
toluene, rt, 24h
H2N
HN
1e
N
H
MeO
HN
MeO
drc
NH2
3a
Acid additive
4a
HN
1g
1f
Entry
Yieldb (%)
eed (%)
Figure 1. Structures of chiral diamine organocatalysts.
1
2e
86
25
1:1.2
1:2.4
89/88
85/84
L
-CSA
-CSA
N-Boc-
N-Cbz-
D
3
4
5
6
7
8
9
10
11
12
13
14
15
16e
L
-phenylalanine
93
91
92
80
85
87
84
89
88
75
80
83
82
trace
1:1
1:1
1:1
1:1.3
1:1
1:1.3
1:1.3
1:1.2
1:1.2
1:1.6
1:1.4
1:1
77/73
66/65
80/79
67/65
76/75
86/81
82/75
91/91
79/73
81/73
82/76
75/73
80/80
ndf
nine with exo-(À)-bornylamine or (+)-(1S,2S,5R)-menthylamine
were firstly built to promote the transformation. It is evident to
note that the stereochemical outcome of the process was mainly
controlled by the amino acid part and the camphor framework
owed better chiral induction than the menthone skeleton. At the
L
-phenylalanine
-phenylglycine
-tryptophane
N-Boc-
N-Boc-
N-Cbz-L-tryptophane
2-BrC6H4COOH
2-IC6H4COOH
2-NO2C6H4COOH
3-FC6H4COOH
L
L
same time, the combination of
L
-type amino acid and exo-(À)-bor-
nylamine proved to be the most ‘‘matched’’ one providing the high-
est ee value. These results are well in agreement with our previous
observations.12 Then, some other amino acids with the amino
4-FC6H4COOH
4-ClC6H4COOH
4-NO2C6H4COOH
3,5-diNO2C6H4COOH
TsOH or TfOH
group (L-phenylglycine and L-tryptophane) and imino group (L-pro-
1:1
ndf
line) were selected to combine with exo-(À)-bornylamine. Among
them, 1f turned out to be the potential catalyst affording the high-
est enantioselectivity albeit with modest diastereoselectivity (en-
try 6). Secondary diamine 1g gave inferior results of much lower
yield of the expected adduct in almost racemic form even after a
prolonged reaction time (entry 7). This phenomenon indicates that
the amino group was pivotal for the high efficiency of iminium
activation of the enone substrate.
a
Reactions were performed with 3a (0.2 mmol) and 2 (44 lL, 0.4 mmol, 2 equiv)
in the presence of catalyst 1f (0.02 mmol) and acid additive (0.02 mmol) in the
solvent toluene (0.5 mL) at room temperature for 24 h.
b
Isolated yield.
Determined by 1H NMR spectroscopy analysis.
c
d
Determined by chiral HPLC analysis.
Reaction time is 48 h.
Not determined.
e
f
Having achieved these hopeful preliminary results, we contin-
ued to optimize the reaction parameters systematically. Consider-
ing the probably existed synergistic effects between the chiral
organocatalyst and acid additive, a group of chiral and achiral acids
toluenesulfonic acid (TsOH) or trifluoromethylsulfonic acid (TfOH),
only a trace mount of product was observed, probably due to their
strong interaction with the amine organocatalyst which made the
amino motifs totally protonated and resulted in the deactivation of
the catalyst (entry 16).
Next, the solvent effects were further assessed. All the results
were disclosed in Table 3. Aromatic solvents including toluene,
o-xylene, and m-xylene all furnished similar good results, while
hexane and ether delivered little decreasing enantiomeric excess
(entries 1–5). Protic solvent methanol generated much lower ee
value, while aprotic polar solvent DMF just provided racemic prod-
uct with moderate yield (entries 6 and 7). Halogenated solvents
also yielded satisfied results as aromatic solvents, and dichloro-
methane proved to be the most suitable medium in terms of yield
and enantioselectivity (entry 8).
were firstly screened and the results are illustrated in Table 2. D-
CSA made the reaction become sluggish resulting comparable dr
and ee values (entry 2). As a result of our previously work, a series
of N-protected amino acids were used for the following optimiza-
tion, but none of them gave better results (entries 3–7). Then, the
most widely used common benzoic acid analogies were investi-
gated. A large number of benzoic acids with various electronic
property substitutes on different positions were utilized in this
model reaction (entries 8–15). 2-Nitrobenzoic acid was found as
the best acid additive generating the highest enantioselectivity
(entry 10). At last, in case of much stronger acids such as
Table 1
Catalyst screeninga
As soon as the optimized reaction conditions were established,
the substrate scope and limitations were explored and the results
are presented in Table 4. A variety of enones (3) were treated with
2 equiv of ethyl nitroacetate (2) in the presence of 10 mol % cata-
lyst 1f with the assistance of 2-nitrobenzoic acid in dichlorometh-
ane at room temperature for specified time. In general, the
positions and electronic nature of the substituent on the aromatic
ring do not exert any big influences on this asymmetric catalytic
transformation (entries 1–11). In most cases, enones with certain
structural variations could undergo the asymmetric Michael addi-
tion smoothly to afford the desired products with high yields and
enantioselectivities under the identical reaction conditions. It is
noteworthy that the diastereoselectivities were not satisfying
COOEt
O
O2N
O
NO2
10 mol% 1/ L-CSA
+
COOEt
2
toluene, rt
MeO
MeO
3a
4a
Entry
Catalyst
t (h)
Yieldb (%)
drc
eed (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
24
24
24
24
24
24
48
89
75
70
77
90
86
20
1:1
1:1
1:2
1:1
1:3
1:1.2
1:2
85/84
À65/À70
85/32
À45/À33
77/74
89/88
1g
6/1
due to the acidity of the
a-H of nitro group that was strong and
a
Reactions were performed with 3a (0.2 mmol) and 2 (44
in the presence of catalyst 1a–1g (0.02 mmol) and acid additive
in the solvent toluene (0.5 mL) at room temperature.
l
L, 0.4 mmol, 2 equiv)
-CSA (0.02 mmol)
might epimerize by basic amine catalyst. Additionally, in case of
a slightly more sterically hindered enones 3l bearing the ethyl
group on the R2 position, the reactivity decreased a little albeit
with no erosion in the enantioselectivity (entry 12). Moreover,
chalcone 3m can also be tolerated in this catalytic system, but
L
b
Isolated yield.
Determined by 1H NMR spectroscopy analysis.
c
d
Determined by chiral HPLC analysis using Chiralcel AD-H as a column.

Y. Zhou et al. / Tetrahedron Letters 54 (2013) 3011–3014
3013
Table 3
Acknowledgments
Solvent screeninga
This work was supported by grants from National Natural Sci-
ence Foundation of China (Nos. 21172082 and 20872041). The
Analysis and Testing Centre of Huazhong University of Science
and Technology is acknowledged for characterization of the new
compounds.
COOEt
O
O2N
O
NO2
10 mol% 1f/ 2-nitrobenzoic acid
+
COOEt
2
solvent, rt, 24 h
MeO
MeO
3a
4a
Entry
Solvent
Toluene
o-Xylene
m-Xylene
Hexane
Ether
Methanol
DMF
Dichloromethane
Chloroform
Yieldb (%)
drc
eed (%)
Supplementary data
1
2
3
4
5
6
7
8
9
89
93
92
90
91
88
65
91
86
1:1.2
1:1.5
1:1.5
1:1.8
1:1.2
1:1.2
1:1.3
1:1.2
1:1.2
91/91
93/91
93/93
85/80
85/82
69/68
3/3
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.04.
005.
References and notes
94/93
93/93
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a
Reactions were performed with 3a (0.2 mmol) and 2 (44 lL, 0.4 mmol, 2 equiv)
in the presence of catalyst 1f (0.02 mmol) and acid additive 2-nitrobenzoic acid
(0.02 mmol) in the solvent (0.5 mL) at room temperature for 24 h.
b
Isolated yield.
c
Determined by 1H NMR spectroscopy analysis.
d
Determined by chiral HPLC analysis.
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Table 4
Substrate scope explorationa
COOEt
O
O2N
R1
O
NO2
10 mol% 1f/ 2-nitrobenzoic acid
R1
R2
+
R2
COOEt dichloromethane, rt, 24 h
3
2
4
Entry
R1
R2
Yieldb (%)
drc
eed (%)
1
2
3
4
5
6
7
8
4-MeOC6H4
4-MeSC6H4
4-MeC6H4
4-ClC6H4
4-NO2C6H4
2-ClC6H4
2-BrC6H4
3-MeOC6H4
2,4-diClC6H3
3,4-diMeOC6H3
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
91 (4a)
95 (4b)
89 (4c)
87 (4d)
93 (4e)
85 (4f)
86 (4g)
93 (4h)
87 (4i)
89 (4j)
96 (4k)
76 (4l)
70 (4m)
1:1.2
1.8:1
1.3:1
1.3:1
1.3:1
1.1:1
1.1:1
1.3:1
1.1:1
1.3:1
1.2:1
1.1:1
1.2:1
94/93
84/85
86/85
93/93
90/90
93/92
95/94
89/88
95/93
91/92
95/92
95/94
75/75
9
10
11
12e
13e
5. Selected recent examples: (a) Lv, J.; Zhang, J.; Lin, Z.; Wang, Y. Chem. Eur. J.
Ph
Ph
2009, 15, 972; (b) Dong, L.; Lu, R.; Du, Q.; Zhang, J.; Liu, S.; Xuan, Y.; Yan, M.
Ph
_
Tetrahedron 2009, 65, 4124; (c) Kwiatkowski, P.; Dudzin´ ski, K.; Łyzwa, D. Org.
a
Reactions were performed with enone 3 (0.2 mmol) and 2 (44
l
L, 0.4 mmol,
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see Refs. 5f,g.
2 equiv) in the presence of catalyst 1f (0.02 mmol) and acid additive 2-nitro benzoic
acid (0.02 mmol) in dichloromethane (0.5 mL) at room temperature for 24 h.
b
Isolated yield.
c
Diastereomeric ratios (anti/syn) were determined by 1H NMR spectroscopy
analysis.
d
Determined by chiral HPLC analysis.
The reaction time was prolonged to 40 h.
e
prolonged reaction time was required to complete the process.
Simultaneously, a sharp decrease in the enantioselectivity was ob-
served (entry 13).
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mary–secondary diamine 1f as organocatalyst in combination with
proper acid additive 2-nitrobenzoic acid had shown high efficiency
for the asymmetric Michael addition of nitro ester to a broad range
of enones with high yields (up to 96%) and excellent enantioselec-
tivities (up to 95%) under mild conditions. These simple and easily
available chiral amine catalysts had enriched the catalyst library
for asymmetric Michael addition reactions. However, the diaste-
reoselctivities were not satisfying, and further efforts will be de-
voted in this area to improve the results.
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3014
Y. Zhou et al. / Tetrahedron Letters 54 (2013) 3011–3014
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