A chiral pyrrolidine-pyrazole catalyst for the enantioselective Michael addition of carbonyls to nitroolefins

Article abstract of DOI:10.1016/j.tetasy.2011.04.010

An enantioselective Michael reaction of carbonyl compounds to nitroolefins has been accomplished using a novel chiral pyrrolidine-pyrazole catalyst. This newly prepared catalyst was found to be very effective in providing good yields as well as good diast

Full text of DOI:10.1016/j.tetasy.2011.04.010

Tetrahedron: Asymmetry 22 (2011) 697–702
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Tetrahedron: Asymmetry
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A chiral pyrrolidine-pyrazole catalyst for the enantioselective Michael
addition of carbonyls to nitroolefins
Srivari Chandrasekhar a, , Togapur Pavan Kumar a, Kothapalli Haribabu a, Chada Raji Reddy a,
⇑
Chitumalla Ramesh Kumar b
a Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
b Inorganic and Physical Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An enantioselective Michael reaction of carbonyl compounds to nitroolefins has been accomplished using
a novel chiral pyrrolidine-pyrazole catalyst. This newly prepared catalyst was found to be very effective
in providing good yields as well as good diastereo- and enantio-selectivities. The mechanism of the reac-
tion has also been substantiated by mass spectral studies.
Received 7 March 2011
Accepted 7 April 2011
Available online 11 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Asymmetric organocatalysis of L-proline has taken strides to-
N
N
ward better selectivities, diversity of substrates, and lower concen-
N
H
tration of catalysts.1 The original catalyst concentration of over 30%
1
L
-proline being used by Barbas/List for an aldol reaction2 to afford
Figure 1. Structure of pyrrolidine-pyrazole catalyst.
moderate enantiomeric excess has undergone modifications in the
pyrrolidine ring and also in the appended carboxylic acid. Exam-
ples include triflamides,3 diphenylprolinol,4 proline-triazole,5 pro-
linamides,6 amino acid derived compounds,7 and others.8,9 These
modifications have allowed organic chemists to expand the diver-
sity of C–C bond forming reactions, such as aldol reaction,10 asym-
metric Michael reactions,11 domino processes, and so on.12 In this
progressive strategy, new mechanistic insights have been reported
for a better understanding of the reaction path, which helps in
designing more efficient catalysts.13
Our interest in this field prompted us to contribute new cata-
lysts for the asymmetric Michael and aldol reactions as well as
for domino processes toward novel scaffolds, which are now easily
available.14 In continuation, we have synthesized a new pyrroli-
dine-pyrazole catalyst 1 (Fig. 1), which showed a very efficient cat-
alytic cycle for the Michael addition of carbonyl compounds to
nitroolefins with excellent diastereo- and enantio-selectivities.
Initial experiments were performed with a reaction between
cyclohexanone 5a and b-nitrostyrene 6a to test the efficiency of
pyrrolidine-pyrazole catalyst 1 in different solvents; the results
are summarized in Table 1. At first, the reaction was carried out
in CH2Cl2 at room temperature using 10 mol % of catalyst. The reac-
tion proceeded well to give the corresponding Michael adduct 7a in
80% yield with 94:6 (syn/anti) diastereoselectivity and 89% enan-
tiomeric excess (Table 1, entry 1). With the aim of improving the
yield and selectivity, a variety of solvents were screened for the
same reaction. The best yield with high selectivity was observed
TsCl, Et3N
NaH, pyrazole
N
N
CH2Cl2, rt
6 h, 89%
CH3CN, reflux
4 h, 84%
OTs
OH
Boc
Boc
2. Results and discussion
3
2
The chiral pyrrolidine-pyrazole 1 employed for this work was
easily obtained in three steps from commercially available Boc-
protected prolinol 2 (Scheme 1).
N
TFA
N
1
N
CH2Cl2, rt
2 h, 93%
Boc
4
⇑
Corresponding author. Tel.: +91 4027193434; fax: +91 4027160512.
E-mail address: srivaric@iict.res.in (S. Chandrasekhar).
Scheme 1. Synthesis of pyrrolidine-pyrazole 1.
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.04.010

698
S. Chandrasekhar et al. / Tetrahedron: Asymmetry 22 (2011) 697–702
Table 3
under solvent-free reaction conditions (Table 1, entry 5). Among
the solvents screened, THF gave a better yield (92%) as well as
selectivity (Table 1, entry 7). Other solvents such as H2O, DMF,
CH3CN, toluene, and MeOH, provided moderate to good yields of
the product (50–85%, Table 1, entries 2–4, 6 and 8). On the other
hand, hexane and AlliquatÒ 336 gave low yields of the product (Ta-
ble 1, entries 9 and 10).
Effect of catalyst loadinga
Entry
Catalyst (mol %)
Time (h)
Yieldb (%)
syn/antic
eed (%)
1
2
3
4
20
10
5
15
15
50
72
98
98
95
92
98:2
98:2
97:3
94:6
95
95
93
94
2
a
Reaction conditions: Nitrostyrene (1 mmol), cyclohexanone (5 mmol), TFA
(2 mol %).
Table 1
Screening of solventsa
b
Isolated yields.
c
Determined by 1H NMR of the crude product.
Determined by chiral HPLC.
d
O
O
NO2
1 (10 mol%)
NO2
Having established the optimal conditions, we then explored
solvent (0.5 mL)
rt
the generality of this reaction with a variety of nitroolefins using
cyclohexanone 5a as the Michael donor (Table 4). The present
catalytic system is tolerant to a broad range of nitroolefins derived
from aromatic aldehydes bearing electron-donating as well as
electron-withdrawing groups (Table 4, entries 2–6) and heteroaro-
matic aldehydes providing a series of c-nitro carbonyl compounds
in high yields and with good to high selectivities (Table 4,
entries 7–9).
To further expand the scope of the newly prepared catalyst, the
reaction of b-nitrostyrene 6a with different carbonyl compounds
was examined (Table 5). Thus, the Michael reaction of cyclopenta-
none 5b with b-nitrostyrene 6a under the present conditions pro-
vided the desired product 7k in 97% yield in 20 h with 90% ee and
97:3 diastereoselectivity (Table 5, entry 1). Tetrahydropyranone 5c
and N-methyl piperidone 5d are also well suited to obtain the cor-
6a
Solvent
5a
7a
Entry
Time (h)
Yieldb (%)
syn/antic
eed (%)
1
2
3
4
5
6
7
8
9
CH2Cl2
H2O
DMF
CH3CN
—
Toluene
THF
MeOH
Hexane
Alliquat-336
24
24
20
20
24
24
24
20
36
36
80
52
65
85
96
60
92
78
35
42
94:6
7:3
92:8
8:2
98:2
9:1
96:4
92:8
8:2
89
80
85
79
94
82
91
82
72
76
10
9:1
a
Reaction conditions: Nitrostyrene (1 mmol), cyclohexanone (5 mmol), catalyst
(10 mol %).
b
responding c-nitro carbonyls 7l and 7m in good yields (94% and
Isolated yields.
Determined by the 1H NMR of the crude product.
Determined by chiral HPLC.
c
95%), albeit with low selectivities (Table 5, entries 2 and 3). How-
ever, the acyclic ketones, acetophenone 5e, and acetone 5f pro-
vided Michael addition products in lower yields with poor
selectivities (Table 5, entries 4 and 5) and took longer for comple-
tion of the reaction. Additionally, we were able to use two alde-
hyde substrates, propionaldehyde 5g and isobutyraldehyde 5h as
Michael donors under the present chiral pyrrolidine-pyrazole 1
catalysis and the reaction was completed in 18 h to give 7p in
82% yield with 78% ee and 93:7 diastereoselectivity (Table 5, entry
6) and 7q in 85% yield with 82% ee (Table 5, entry 7).
These successful experiments directed us to study the mecha-
nistic pathway and the transition state involved in the reaction.
On the basis of our experimental findings, we propose a possible
mechanism for the asymmetric Michael reaction catalyzed by the
pyrrolidine-pyrazole catalyst 1 (Fig. 2). Initially, catalyst 1 forms
a chiral enamine A (predominantly anti) with cyclohexanone 5a
and then a Michael reaction between the enamine-activated A
and the nitroolefin 6a (in anti-Re addition fashion) leads to the for-
mation of corresponding Michael product 7a via transition state
TS-I (an acyclic synclinal transition state). After hydrolysis, catalyst
1 is regenerated for use in the subsequent catalytic cycle.
d
To further optimize the reaction conditions, the effect of an acid
additive was tested. It has already been well documented that the
presence of an acid additive increases the catalyst efficiency to-
ward the acceleration of enamine formation.15 Therefore, the effect
of various additives on the Michael reaction of cyclohexanone 5a
with b-nitrostyrene 6a under solvent-free conditions was tested
and the results are summarized in Table 2. The addition of AcOH
(2 mol %) in the reaction produced 84% yield of the desired product
with 89% ee (Table 2, entry 1) in 20 h. The other additives screened
were TFA, HCOOH, PhCOOH, pTSA, and CSA and we found that TFA
(2 mol %) affected the reaction by giving the product in 98% yield
with 95% ee in 15 h reaction time (Table 2, entry 4). The use of
the TFA salt of catalyst 1 has resulted the product in 80% yield after
24 h.
Table 2
Screening of additivesa
Entry
Additive (2 mol %)
Time (h)
Yieldb (%)
syn/antic
eed (%)
1
2
3
4
5
6
AcOH
HCOOH
PhCOOH
TFA
pTSA
CSA
20
24
18
15
24
24
84
82
92
98
76
68
94:6
92:8
95:5
98:2
93:7
9:1
89
85
92
95
82
78
This mechanism was further supported by the ESI-MS of the
reaction mixture at regular intervals. The ESI-MS (Fig. 3) showed
the mass of the reaction intermediates (A and B), which supports
the proposed reaction mechanism.
a
3. Conclusions
Reaction conditions: Nitrostyrene (1 mmol), cyclohexanone (5 mmol), catalyst
(10 mol %).
b
Isolated yields.
In conclusion, an efficient and stereoselective organocatalyst, a
chiral pyrrolidine-pyrazole for the asymmetric Michael addition of
carbonyls to nitroolefins has been developed. The new catalyst has
a pyrazole moiety attached to chiral pyrrolidine and gives high
enantio- and diastereoselectivities in the case of cyclohexanone
reactions. In the present protocol, good yields of the products, high
stereoselectivities, and solvent-free reaction conditions are note-
worthy features.
c
Determined by 1H NMR of the crude product.
Determined by chiral HPLC.
d
Next, we turned our attention to test the effect of catalyst load-
ing in the Michael addition reaction of 5a with 6a; the results are
shown in Table 3. A decrease in catalyst loading resulted in a longer
reaction time, without affecting the yield or the selectivity (Table 3,
entries 1–4).

S. Chandrasekhar et al. / Tetrahedron: Asymmetry 22 (2011) 697–702
699
Table 4
Asymmetric Michael addition of 5a with various nitroolefins using organocatalyst 1a
Entry
Nitroolefins
Time (h)
Product
Yieldb (%)
syn/antic
eed (%)
O2N
NO2
NO2
NO2
NO2
6b
6c
O
1
15
96
95:5
92
98
7b
7c
OMe
MeO
Me
2
3
15
15
97
93
96:4
97:3
O
O
O
NO2
NO2
NO2
Me
6d
99
7d
NO2
OMe
MeO
O2N
OMe
4
5
6
15
18
18
95
98
92
98:2
96:4
94:6
95
97
92
OMe
NO2
6e
7e
7f
NO2
O2N
O
Cl
6f
Cl
NO2
NO2
6g
O
7g
NO2
NO2
N
O
N
6h
O
O
O
7
8
9
20
18
18
90
94
93
92:8
95:5
94:6
89
90
86
7h
7i
NO2
NO2
NO2
NO2
NO2
O
S
6i
6j
S
7j
a
b
c
Reaction conditions: Nitrostyrene (1 mmol), cyclohexanone (5 mmol), catalyst (10 mol %), TFA (2 mol %), solvent free, rt.
Isolated yields.
Determined by 1H NMR and HPLC analysis.
d
Determined by chiral HPLC using chiral pak-IA, IC or OD_H columns.
stants J are in Hertz. FTIR spectra were recorded as KBr thin films
or neat. For low (MS) and high (HRMS) resolution, m/z ratios are re-
ported as values in atomic mass units. All the reagents and solvents
were reagent grade and used without further purification unless
specified otherwise. Technical grade ethyl acetate and petroleum
ether were used for column chromatography and were distilled
prior to use. Column chromatography was carried out using silica
gel (60–120 mesh) packed in glass columns. All the reactions were
performed under an atmosphere of nitrogen in flame-dried or
oven-dried glassware with magnetic stirring.
4. Experimental
4.1. General
1H NMR and 13C NMR spectra were recorded in a CDCl3 solvent
on a 300, 500, or 75 MHz spectrometer at ambient temperature.
The chemical shifts (d) are reported in ppm on a scale downfield
from TMS as the internal standard and signal patterns are as fol-
lows: s, singlet; d, doublet; dd, doublet of doublet; td, triplet of
doublet; t, triplet; m, multiplet; br s, broad singlet. Coupling con-

700
S. Chandrasekhar et al. / Tetrahedron: Asymmetry 22 (2011) 697–702
Table 5
Asymmetric Michael addition of different carbonyls with 6a using organocatalyst 1a
Entry
Carbonyls
Time (h)
Product
Yieldb (%)
97
syn/antic
eed (%)
96
O
5b
O
1
20
97:3
7k
7l
NO2
NO2
O
5c
O
2
3
15
15
94
95
98:2
95:5
91
86
O
O
O
O
5d
7m
NO2
N
Me
N
Me
O
5e
Ph
4
5
24
24
O
72
67
—
—
48
39
7n
NO2
Ph
O
5f
O
7o
NO2
O
H
H
O
H
6
7
18
18
82
85
93:7
—
78
82
7p
NO2
5g
O
5h
O
H
7q
NO2
a
b
c
Reaction conditions: Nitrostyrene (1 mmol), cyclohexanone (5 mmol), catalyst (10 mol %), TFA (2 mol %), solvent free, rt.
Isolated yields.
Determined by 1H NMR and HPLC analysis.
d
Determined by chiral HPLC using chiral pak-IA, IC or OD_H columns.
NO2
N
N
N
Ph
6a
A
H2O
O
N
N
N
O
H
N
N
N
O
N
Ph
5a
NO2
Ph
B
plausible transition state
TS-I
N
N
N
H
1
H2O
O
Ph
NO2
7a
Figure 2. Proposed mechanism of the asymmetric Michael reaction catalyzed by 1.

S. Chandrasekhar et al. / Tetrahedron: Asymmetry 22 (2011) 697–702
701
Figure 3. ESI-MS of the reaction mixture.
4.1.1. (S)-tert-Butyl-2-((1H-pyrazol-1-yl)methyl)pyrrolidine-1-
carboxylate 4
IR (Neat):
(CDCl3, 300 MHz):
m
3407, 2980, 2361, 1677, 1175, 837 cmꢂ1
;
1H NMR
d
7.73 (d, 1H, J = 2.45 Hz), 7.69 (d, 1H,
To a suspension of NaH (270 mg, 60% of purity, 6.7 mmol) in
20 mL of anhydrous acetonitrile was added pyrazole (384 mg,
5.6 mmol) and stirred at room temperature for 0.5 h. Then, the
tosylated-N-Boc-prolinol 3 (1.0 g, 2.8 mmol) was added to the
resulting mixture and heated at reflux under a nitrogen atmo-
sphere for 4 h. After completion of the reaction, the reaction mix-
ture was cooled to room temperature. The solvent was removed
in vacuo and the residue was diluted with 10 mL of water. The
resulting mixture was extracted with chloroform (3 ꢀ 20 mL).
The residue was purified by column chromatography to furnish
J = 2.26 Hz), 6.43 (t, 1H, J = 2.26 Hz), 4.81–4.52 (m, 2H), 4.21 (s,
1H), 3.41 (s, 2H), 2.34–1.79 (m, 3H), 1.80–1.63 (m, 1H); 13C NMR
(CDCl3, 75 MHz): d 139.4, 132.8, 107.2, 60.2, 50.3, 46.0, 27.1,
23.3; ESI(MS): m/z 152 [M+H]+; HRMS calcd for C8H14N3:
152.1182, found: 152.1188.
4.1.3. General procedure: Michael reaction of 5a with 6a–j to
give 7a to 7q
Catalyst 1 (10 mol %) was added to a mixture of cyclohexanone
5a (5 mmol) and TFA (2 mol %) at room temperature and stirred for
10 min, then nitroolefin 6a–j (1 mmol) was added and stirred at
the same temperature. After completion of the reaction (monitored
by TLC), the crude product was purified by silica-gel column chro-
matography to give the corresponding Michael adducts. Relative
and absolute configurations of the products were determined by
comparison of 1H NMR, 13C NMR, and specific rotation values with
those reported in the literature. Enantiomeric excess was deter-
mined by chiral HPLC.
the desired product
¼ ꢂ119 (c 1.1, CHCl3); IR (Neat):
1449, 1396, 1170, 1117, 1048, 968, 753 cmꢂ1
4
(588 mg, 84% yield) as
2973, 2880, 1691, 1512,
1H NMR (CDCl3,
a thick oil.
½
a 2D5
ꢁ
m
;
300 MHz): d 7.40 (s, 1H), 7.28 (s, 1H), 6.19 (t, 1H, J = 2.26 Hz),
4.43–4.14 (m, 2H), 4.06–3.96 (m, 1H), 3.37–3.02 (m, 2H), 2.11–
1.60 (m, 3H), 1.48 (s, 10H); 13C NMR (CDCl3, 75 MHz): d 154.4
and 154.1(rotamers), 139.0 and 138.8 (rotamers), 129.5 and
129.3 (rotamers), 105.7 and 105.6 (rotamers), 79.6 and 79.3 (rota-
mers), 57.4, 54.1 and 53.0 (rotamers), 46.8 and 46.3 (rotamers),
28.6, 28.3, 23.0 and 22.4 (rotamers); ESI(MS): m/z 252 [M+H]+;
HRMS calcd for C13H22N3O2: 252.1707, found: 252.1718.
Acknowledgments
T.P.K. and C.R.K. thank Council of Scientific and Industrial Re-
search (CSIR), New Delhi for research fellowship. The authors are
thankful to DST, New Delhi for funding the research grant (SR/
S1/OC-65/2009) and Dr. K. Bhanuprakash for his helpful discus-
sions about transitions states of the reaction.
4.1.2. (S)-1-(Pyrrolidine-2-ylmethyl)-1H-pyrazole 1
To a solution of 4 (580 mg, 2.3 mmol) in CH2Cl2 (5 mL) was
added TFA (5 mL) at 0 °C and the reaction mixture was stirred at
room temperature. After completion of the reaction (monitored
by TLC), saturated NaHCO3 solution was added and the layers were
separated. The aqueous layer was extracted with CH2Cl2
(3 ꢀ 10 mL). The combined organic layer was dried over anhydrous
Na2SO4 and concentrated in vacuo. The residue was purified by sil-
ica-gel column chromatography to furnish the desired product 4
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