Noyort's Ts-DPEN ligand: Simple yet effective catalyst for the highly enantioselective michael addition of acetone to nitroalkenes

Article abstract of DOI:10.1002/ejoc.200901509

Highly enantioselective Michael addition of acetone to a variety of nitroalkenes promoted by simple chiral primary amine bifunctional catalysts (e.g., Noyori's Ts-DPEN ligand) together with terephthalic acid in excellent yields (84-99%) and enantioselectivities (93-98 % ee) is reported.

Full text of DOI:10.1002/ejoc.200901509

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200901509
Noyori’s Ts-DPEN Ligand: Simple yet Effective Catalyst for the Highly
Enantioselective Michael Addition of Acetone to Nitroalkenes
Lin Peng,[a,b] Xiao-Ying Xu,[a] Liang-Liang Wang,[a,b] Jun Huang,[a,b] Jian-Fei Bai,[a,b]
Qing-Chun Huang,[a] and Li-Xin Wang*[a]
Keywords: Organocatalysis / Michael addition / Asymmetric synthesis / Amines
Highly enantioselective Michael addition of acetone to a
variety of nitroalkenes promoted by simple chiral primary
amine bifunctional catalysts (e.g., Noyori’s Ts-DPEN ligand)
together with terephthalic acid in excellent yields (84–99%)
and enantioselectivities (93–98%ee) is reported.
Michael addition of 1,3-dicarbonyl compounds to nitro ole-
fins directly catalyzed by this simple bifunctional organoca-
talyst in moderate yields and enantioselectivities. Most re-
cently, Zhao[13] reported the Michael reactions of 3-methyl-
2-pyrazolin-5-one with α,β-unsaturated ketones by using
Ts-DPEN as a catalyst in poor enantioselectivities. To date,
this simple and commercially available Ts-DPEN ligand has
not caught enough attention in the area of small-molecule
catalysis.
As a part of our continuing interests in asymmetric
small-molecule catalysis,[14] we recognized that Noyori’s Ts-
DPEN ligand, commercially available at a low cost, bearing
an amino sulfonamide moiety and a primary amino group
on the chiral scaffold may activate the reaction substrate
through hydrogen bonding provided by the amino sulfon-
amide moiety in a manner similar to that observed for pri-
mary amine–thiourea bifunctional catalysts that have been
shown to offer synergistic cooperation or dual activation
functionalities in the Michael addition of ketones to nitro
olefins. Herein, we wish to report the simple bifunctional
chiral primary amine sulfonamide catalyzed enantioselec-
tive Michael addition of acetone to a very wide range of
nitroalkenes in excellent yields (up to 99%) and enantio-
selectivities (up to 98%ee).
Introduction
The asymmetric Michael addition has emerged as one
of the most powerful and efficient methods for C–C bond
formation in organic synthesis.[1] Over the past years, vari-
ous efficient chiral organocatalysts have been developed for
the enantioselective Michael addition of aldehydes,[2]
ketones,[3] and 1,3-dicarbonyl compounds[4] to nitroalkenes.
In 2006, Córdova and co-workers[5] reported the first chiral
primary amine-catalyzed Michael additions of ketones to
nitroalkenes. Since then, several effective organic small mo-
lecules such as primary amine–thiourea,[6] cinchona alk-
aloid derivatives,[7] and bispidine-based primary–secondary
amine[8] have been developed for this addition. Although
excellent results have been achieved by these systems, few
excellent protocols for the asymmetric Michael addition of
acetone to nitroalkenes are reported.[9] The successful de-
sign of a simple and highly effective chiral catalyst for the
Michael reaction of acetone to nitro olefins with excellent
enantioselectivities is still a challenging task.
Over the past decade, a remarkable number of enantiose-
lective reactions promoted by hydrogen-bond activation
have been identified, providing solutions to challenging
transformations of important molecules for asymmetric
synthesis.[10] In 1995, Noyori and co-workers first intro-
duced
N-[(1R,2R)-2-amino-1,2-diphenylethyl]-4-methyl-
benzenesulfonamide (Ts-DPEN) as a chiral ligand for the
highly efficient asymmetric transfer hydrogenation of
ketones and imines.[11] Xu[12] first reported the asymmetric
Results and Discussion
Initially, a variety of optically active primary amines
shown in Figure 1 were screened as catalysts, and the reac-
tion of acetone with trans-β-nitrostyrene was used as a
model reaction at room temperature in CH2Cl2; the results
are summarized in Table 1. Primary amine–thiourea deriva-
tives 1a and 1b were found to be highly efficient (up to 91%
yield) for this asymmetric addition, whereas only moderate
enantioselectivities (69 and 75%ee; Table 1, Entries 1 and
2) were obtained. To our delight, the reaction proceeded
smoothly when simple primary amine sulfonamide deriva-
[a] Key Laboratory of Asymmetric Synthesis and Chirotechnology
of Sichuan Province, Chengdu Institute of Organic Chemistry,
Chinese Academy of Sciences,
Chengdu 610041, People’s Republic of China
Fax: +86-028-8525-5208
E-mail: wlxioc@cioc.ac.cn
[b] Graduate University of Chinese Academy of Sciences,
Beijing 10039, People’s Republic of China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901509.
Eur. J. Org. Chem. 2010, 1849–1853
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1849

L.-X. Wang et al.
SHORT COMMUNICATION
Figure 1. Chiral primary amine catalysts evaluated.
tives 1c–i were used; better enantioselectivities were also ob- excellent yield (99%) was obtained, but only good enantio-
tained (73–92%ee; Table 1, Entries 3–9). Particularly, cata- selectivity (82%ee) was achieved (Table 2, Entry 9). When
lyst 1d afforded the product in good yield and excellent the reaction was carried out in toluene, the best enantio-
enantioselectivity (92%ee; Table 1, Entry 4), and this cata- selectivity (97%ee) and a good yield (80%) were obtained
lyst was selected for further studies. These results demon- (Table 2, Entry 2), and this solvent was selected for further
strate that the amino sulfonamide moiety may play a key optimization. These results strongly demonstrate that hy-
role in promoting the enantioselectivity of the reaction.
drogen-bond formation has an important effect on both the
reactivity and the enantioselectivity of the reaction.
Table 1. The screening of chiral primary amine catalysts 1a–i.[a]
Table 2. The effect of reaction solvents.[a]
Entry
Catalyst
Time [h]
% Yield[b]
% ee[c]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
72
72
91
91
78
62
57
81
72
61
29
69
75
73
92
86
74
82
83
91
Entry
Solvent
Time [h]
% Yield[b]
% ee[c]
192
192
192
192
192
192
192
1
2
3
4
5
3
7
8
CH2Cl2
PhCH3
p-xylene
cyclohexane
ClCH2CH2Cl
CHCl3
hexane
Et2O
saturated brine
H2O
CH3OH
(CH3)2CHOH
DMF
192
120
192
192
192
192
192
192
192
192
192
192
192
192
192
192
62
80
87
73
48
48
37
38
99
29
20
14
trace
16
15
8
92
97
95
90
91
91
85
92
82
84
54
81
n.d.
50
17
78
[a] Unless otherwise specified, all reactions were carried out with
acetone (2, 2.0 mmol), trans-β-nitrostyrene (3a, 0.20 mmol), and
the catalyst (0.04 mmol, 20 mol-%) in CH2Cl2 (1 mL) at room tem-
perature. [b] Isolated yield after silica gel column chromatography.
[c] The ee values were determined by HPLC with the use of a Chiral
Whelk-01 column.
9
10
11
12
13
14
15
16
CH3CN
DMSO
acetone
A series of solvents were then investigated, and the re-
sults are listed in Table 2. These results revealed that the
solvent has a significant effect on the rate and the enantio-
selectivity of the reaction. Polar solvents such as CH3OH,
DMF, and DMSO resulted in poor yields (8–29%) and
moderate to good enantioselectivities (17–84%ee; Table 2,
Entries 10–15), probably because these solvents weaken or
retard the formation of hydrogen bonds between trans-β-
nitrostyrene and the amino sulfonamide moiety of 1d. By
contrast, nonpolar or low polar solvents such as toluene, p-
[a] Unless otherwise specified, all reactions were carried out with
acetone (2, 2.0 mmol), trans-β-nitrostyrene (3a, 0.20 mmol), and
catalyst 1d (0.04 mmol, 20 mol-%) in the solvent (1 mL) at room
temperature. [b] Isolated yield after silica gel column chromatog-
raphy. [c] The ee values were determined by HPLC with the use of
a Chiral Whelk-01 column.
To further improve the yield and enantioselectivity of the
xylene, benzene, CH2Cl2, and cyclohexane gave better yields reaction, a wide range of acid and base additives[15] were
(62–87%) and enantioselectivities (90–97%ee; Table 2, En- studied, and the results are summarized in Table 3. Among
tries 1–4). When saturated brine was used as the solvent, an the acids screened, p-hydroxybenzoic acid and terephthalic
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 1849–1853

Enantioselective Michael Addition of Acetone to Nitroalkenes
acid were found to be promising additives, as they afforded Table 4. The effect of the amount of acetone, catalyst loading, and
reactant concentration.[a]
the best results in terms of both yield and enantioselectivity
(Table 3, Entries 8 and 9). Acetic acid, benzeneacetic acid,
and m-hydroxybenzoic acid also provided good yields and
enantioselectivities (Table 3, Entries 3, 5, 11). However,
stronger acids such as trifluoromethanesulfonic acid re-
duced the reactivity and gave a poor yield (25%; Table 3,
Entry 15). Base additives such as imidazole, DIPEA, TEA,
and DMAP were also evaluated and no significant improve-
ment was obtained (Table 3, Entries 16–19). Finally, tereph-
thalic acid was selected as the additive for further investiga-
tion (Table 3, Entry 9).
Entry Acetone
[equiv.]
Catalyst
[mol-%]
Conc.
[]
% Yield[b]
% ee[c]
1
2
3
4
5
6
7
8
10
5
2
10
10
10
10
10
20
20
20
15
10
20
20
20
0.2
0.2
0.2
0.2
0.2
0.4
0.1
0.05
99
91
51
91
75
82
75
60
95
95
93
95
95
92
93
93
Table 3. The effect of additives.[a]
[a] Unless otherwise specified, all reactions were carried out with
acetone (2, 2.0 mmol), trans-β-nitrostyrene (3a, 0.20 mmol), tereph-
thalic acid (0.04 mmol, 20 mol-%), and catalyst 1d (0.04 mmol,
20 mol-%) in toluene (1 mL) at room temperature. [b] Isolated yield
after silica gel column chromatography. [c] The ee values were de-
termined by HPLC with the use of a Chiral Whelk-01 column.
Entry
Additive
% Yield[b]
% ee[c]
1
2
3
4
5
6
7
8
–
80
54
87
71
94
52
67
99
99
52
86
39
57
15
25
90
70
40
30
97
95
95
90
93
94
91
94
95
91
92
92
91
92
84
92
95
94
93
HCOOH
AcOH
PhCOOH
PhCH2COOH
4-H3CC6H4COOH
4-ClC6H4COOH
4-HOC6H4COOH
4-HOOCC6H4COOH
4-O2NC6H4COOH
3-HOC6H4COOH
2-HOC6H4COOH
2,6-F2C6H3COOH
4-Cl-2-O2NC6H3COOH
CF3COOH
para (Table 5, Entries 2–7), meta (Table 5, Entries 8 and 9),
or ortho position (Table 5, Entries 10–13) of the aromatic
ring, regardless of the nature and the position of the substi-
tuted group on the aromatic ring. To broaden the substrate
scope to include heteroaromatic nitroalkenes, reactions
were carried out under the optimized conditions and excel-
lent yields and enantioselectivities were obtained (Table 5,
Entries 18 and 19).
9
10
11
12
13
14
15
16
17
18
19
Importantly, we sensed the application and significance
of our process in the synthesis of specific chiral pharmaceu-
ticals or related intermediates, because of the generality and
representation of the reaction. Especially, most of the target
products are solids, which may be easily recrystallized to
afford enantiopure compounds (Ͼ99%ee), and commer-
cially available catalyst 1d is cheap and may be easily sepa-
rated and reused. Thus, we are now devising and processing
this reaction and hope to extend its application in the large-
scale manufacture of related chiral molecules. All this may
provide multiple approaches and choices for chiral pharma-
imidazole
DMAP
DIPEA
Et3N
[a] Unless otherwise specified, all reactions were carried out with
acetone (2, 2.0 mmol), trans-β-nitrostyrene (3a, 0.20 mmol), cata-
lyst 1d (0.04 mmol, 20 mol-%), and the additive (0.04 mmol,
20 mol-%) in toluene (1 mL) at room temperature. [b] Isolated yield
after silica gel column chromatography. [c] The ee values were de-
termined by HPLC with the use of a Chiral Whelk-01 column.
Other parameters, for example, the amount of acetone, ceutical preparations.
catalyst loading, and reactant concentration, were also in-
On the basis of the experimental results described above,
vestigated, but no superior results were obtained (Table 4). we suggest a plausible dual activation model.[16] The two
Through these screenings, the optimized reaction condi- substrates involved in the reaction are activated simulta-
tions were found to be a combination of 1d (20 mol-%), neously by Ts-DPEN, as shown in Figure 2. The carbonyl
terephthalic acid (20 mol-%), acetone (10 equiv.), and tolu- group of acetone is assumed to interact with the primary
ene as the solvent at room temperature for 5 d.
amine moiety of Ts-DPEN via an imine intermediate. The
Under the optimized reaction conditions, the scope and acid additive is proposed to activate the imine through re-
the limitations of this Michael reaction with different nitro- moval of the proton from the imine, thus increasing the
alkenes were examined. As shown in Table 5, excellent nucleophilic ability of the reacting carbon center. The H-
yields (up to 99%) and enantioselectivities (up to 98%ee) sulfonamide activates the nitro olefins through a single hy-
were achieved with various nitroalkenes bearing either elec- drogen bond, which enhances the electrophilicity of the ole-
tron-donating or electron-withdrawing substituents in the fin.
Eur. J. Org. Chem. 2010, 1849–1853
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1851

L.-X. Wang et al.
SHORT COMMUNICATION
Table 5. Michael reaction of acetone to nitrostyrenes.[a]
Experimental Section
Typical Procedure: Catalyst 1d (0.04 mmol) was added to a stirred
solution of acetone (2, 2.0 mmol) in the solvent (1 mL) under an
atmosphere of air. The resulting solution was stirred for 5 min prior
to the addition of nitro olefin 3 (0.20 mmol) and the additive
(0.04 mmol). After stirring for the indicated reaction time at room
temperature (monitored by TLC), the crude adduct was purified
by column chromatography (petroleum ether/ethyl acetate, 10:1).
The enantiomeric excess of the pure product was determined by
chiral HPLC analysis.
Entry
R
% Yield (product)[b]
% ee[c]
1
2
3
4
5
6
7
8
Ph
4-FC6H4
4-ClC6H4
99 (4a)
87 (4b)
94 (4c)
99 (4d)
92 (4e)
91 (4f)
85 (4g)
90 (4h)
92 (4i)
90 (4j)
87 (4k)
99 (4l)
94 (4m)
91 (4n)
84 (4o)
97 (4p)
88 (4q)
96 (4r)
98 (4s)
95
96
96 (Ͼ99)[d]
4-BrC6H4
96
96
95
97
97
94
97
97
97
98
97
96
94
93
98
96
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, spectra, and NMR spectroscopic
data for complexes 4a–s.
4-O2NC6H4
4-H3CC6H4
4-H3COC6H4
3-O2NC6H4
3-FC6H4
2-ClC6H4
2-BrC6H4
9
10
11
12
13
14
15
16
17
18
19
Acknowledgments
2-O2NC6H4
2-H3COC6H4
3,4-(H3CO)2C6H3
3,4,5-(H3CO)3C6H2
1-naphthyl
6-H3CO-1-naphthyl
2-furyl
This work was supported by the National Natural Science Founda-
tion of China (No. 20802075) and the Chinese Academy of Sci-
ences.
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Michael reaction of acetone with nitro olefins catalyzed by
a simple and commercially available primary amine bifunc-
tional catalyst, represented by Noyori’s Ts-DPEN ligand, in
excellent yields (84–99%) and enantioselectivities (93–
98%ee). These excellent results and the availability and re-
cycling of the catalyst make the current reaction potentially
applicable in the large-scale manufacture of related chiral
pharmaceuticals under industrially accepted conditions and
at a low cost. Further investigations of this reaction and
the application of this catalyst in other asymmetric catalytic
reactions are in progress.
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Received: December 29, 2009
Published Online: February 23, 2010
Eur. J. Org. Chem. 2010, 1849–1853
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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