Pyrrolidine-oxyimide catalyzed asymmetric Michael addition of α,α-disubstituted aldehydes to nitroolefins

Article abstract of DOI:10.1016/j.tetasy.2015.07.009

Abstract Organocatalytic enantioselective Michael addition of α,α-disubstituted aldehydes onto nitroolefins using pyrrolidine-oxyimide catalyst was reported. The reaction works effectively under neat conditions with 15 mol % of catalyst and 10 mol % of p-

Full text of DOI:10.1016/j.tetasy.2015.07.009

Tetrahedron: Asymmetry xxx (2015) xxx–xxx
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Pyrrolidine–oxyimide catalyzed asymmetric Michael addition
of
a
,a-disubstituted aldehydes to nitroolefins
⇑
Togapur Pavan Kumar
Division of Natural Products Chemistry, CSIR-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:
Organocatalytic enantioselective Michael addition of a,a-disubstituted aldehydes onto nitroolefins using
Received 16 May 2015
Accepted 14 July 2015
Available online xxxx
pyrrolidine–oxyimide catalyst was reported. The reaction works effectively under neat conditions with
15 mol % of catalyst and 10 mol % of p-nitrobenzoic acid as an additive at 0 °C; this results in the
formation of Michael adducts possessing an all-carbon quaternary center with good yields and
enantioselectivities.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
application of pyrrolidine–oxyimides 1 and 2 (Fig. 1) for the asym-
metric Michael reactions of a,a-disubstituted aldehydes to nitroo-
Organocatalysis is now realized as a powerful alternative and/or
complementary approach to enzymatic and organometallic cataly-
sis. Remarkable advances have been witnessed over the past few
years and a large collection of organocatalytic protocols have been
added to the vast synthetic chemistry library.1 Specifically, asym-
metric organocatalysis with proline and proline derivatives has
been found to be effective in driving the reactions by means of
an enamine or iminium mechanism.2 A large number of asymmet-
ric organocatalytic transformations with a diverse range of sub-
strate combinations have been reported using proline catalysis.
Among these, the asymmetric Michael addition is considered as
one of the best early stage exploratory reactions for the evaluation
of newly designed catalysts.3 The Michael products with all-carbon
quaternary stereogenic centers were found to be prominent scaf-
folds serving as versatile templates for the construction of various
bioactive compounds.4,5 The generation of all-carbon quaternary
stereogenic centers is considered to be a challenging task in asym-
metric synthesis.5 Though, a large number of organocatalysts were
known to be highly efficient for Michael addition of unmodified
carbonyls to nitroolefins, only some of them were found to be
lefins. Pyrrolidine–oxyimides 1 and 2 were recently designed and
successfully employed as organocatalysts for the Michael reaction
of ketones to nitroolefins. Having been encouraged by this study
and considering the consistency of catalytic mechanism, wherein
the pyrrolidine backbone functions as an activation site and the
oxyimide appendage acts as a steric controller and has additional
hydrogen bonding sites for activation of nitroolefins. We therefore
started our investigations by testing whether oxyimide catalysts 1
and 2 would also catalyze asymmetric Michael reactions of
a,a-
disubstituted aldehydes to nitroolefins; our results are reported
herein.
O
O
O
O
N
N
N
H
N
H
O
O
1
2
Figure 1. Pyrrolidine–oxyimides.
2. Results and discussion
effective for Michael reaction between
a,a-disubstituted aldehy-
des and nitroolefins.6 Recently, we have reported on the Michael
Initially, a model reaction was chosen between isobutyralde-
hyde 3a and nitrostyrene 4a to test the applicability and efficacy
addition of
a,a-disubstituted aldehydes and nitroolefins using
pyrrolidine–pyrazole as the organocatalyst.7 Therefore, further
investigation into new catalytic methods for this relatively under-
explored transformation is highly desirable. In continuation of our
interest on organocatalysis,8 we herein demonstrate the
of 1 and 2 for enantioselective Michael additions of
a,a-disubsti-
tuted aldehydes to nitroolefins (Scheme 1). Screening experiments
were conducted for the selection of optimal reaction parameters
such as solvent, additive, catalyst loading, and temperature. The
solvent screening experiments were conducted by performing
the reactions simultaneously in various solvents using 10 mol %
⇑
Tel.: +91 40 27191727; fax: +91 40 27160512.
E-mail address: pavantogapur@gmail.com
http://dx.doi.org/10.1016/j.tetasy.2015.07.009
0957-4166/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kumar, T. P. Tetrahedron: Asymmetry (2015), http://dx.doi.org/10.1016/j.tetasy.2015.07.009

2
T. P. Kumar / Tetrahedron: Asymmetry xxx (2015) xxx–xxx
Table 2
Screening of additivesa
O
O
Entry
Additive
(mol%)
Time (h)
Yieldb (%)
eec (%)
NO2
catalyst
H
NO2
H
1
2
TFA
TFA
5
10
5
10
5
10
5
10
5
10
2
5
20
2
5
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
61
65
53
59
60
61
65
69
68
73
62
71
76
65
73
80
66
72
59
67
65
74
62
70
66
73
69
74
78
71
77
81
solvent (0.5 mL)
rt
4a
3a
5a
3
CSA
4
5
6
7
8
9
10
11
12
13
14
15
16
CSA
C6H5OH
C6H5OH
HCOOH
Scheme 1. Michael addition of isobutyraldehyde to nitrostyrene.
Table 1
Screening of solvents and catalysta
HCOOH
CH3COOH
CH3COOH
C6H5COOH
C6H5COOH
C6H5COOH
p-NO2-C6H4COOH
p-NO2-C6H4COOH
p-NO2-C6H4COOH
Entry
Solvent
Catalyst (10 mol%)
Time (h)
Yieldb (%)
eec(%)
1
2
3
4
5
6
7
8
9
Neat
Neat
H2O
H2O
Toluene
Toluene
Hexane
Hexane
DMF
DMF
THF
THF
MeOH
MeOH
CH3CN
CH3CN
Dioxan
Dioxan
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
73
60
55
40
60
52
65
60
70
65
65
58
70
65
50
45
64
59
71
67
63
59
68
69
70
66
43
37
54
52
61
57
53
56
61
53
10
a
Reaction conditions: isobutyraldehyde (4 mmol), nitrostyrene (1 mmol),
catalyst 1 (10 mol%), neat, rt.
b
Isolated yields.
Determined by chiral HPLC.
c
10
11
12
13
14
15
16
17
18
Table 3
Effect of temperature and catalyst concentrationa
Entry
Temp. (°C)
1 (mol%)
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
RT
0
10
10
10
15
15
15
20
20
20
36
48
72
36
48
72
36
48
72
80
84
61
83
87
67
84
88
69
81
85
87
83
89
90
84
89
92
a
b
c
À20
Reaction conditions: isobutyraldehyde (4 mmol), nitrostyrene (1 mmol).
Isolated yields.
Determined by chiral HPLC.
RT
0
À20
RT
0
À20
of catalysts 1 or 2 at room temperature and the results are summa-
rized in Table 1. As evident from the study, both catalysts 1 and 2
have good compatibility for Michael reactions regardless of the
a
Reaction conditions: isobutyraldehyde (4 mmol), nitrostyrene (1 mmol),
p-NO2-C6H4COOH (10 mol%), neat.
type of solvent used and afforded the Michael adduct, c-nitrocar-
b
Isolated yields.
Determined by chiral HPLC.
c
bonyl compound 5a in good yield and enantioselectivity (Table 1,
entries 1–18). However, the reaction performed under neat condi-
tions using catalyst 1 was found to be more effective in terms of
yield and enantioselectivity and thus was selected for further opti-
mization studies (Table 1, entry 1).
After solvent selection studies, we conducted screening experi-
ments to test the role of an acid additive on the catalytic perfor-
mance of 1. It is well documented that the presence of an acid
additive can enhance the catalytic efficiency by accelerating enam-
ine formation thereby improving the overall productivity. As a
result, reactions were conducted using various acid additives
under neat conditions, employing 10 mol % of catalyst 1 at room
temperature; the results are summarized in Table 2. p-
Nitrobenzoic acid was found to show the best improvement in
terms of the catalytic performance of 1 and turned out to be the
most suitable additive for this transformation (Table 2, entries
14–16).
isobutyraldehyde 3a (Table 4, entries 1–8) and cyclopentanecar-
boxaldehyde 3b (Table 5, entries 1–8) with nitroolefins 4b–i were
smoothly conducted under the optimized reaction conditions and
the corresponding Michael products 5b–i and 6a–h were obtained
in good yields and with high enantioselectivities irrespective of the
nature of the substitution pattern in the nitroolefins. However,
nitroolefins with electron donating substitutions were found to
be slightly inferior in overall productivity compared to electron
withdrawing counterparts (Table 4, entries 4–6 and Table 5, entries
2, 3 respectively). Overall, the substrate scope observed for the
conjugate addition of
employing pyrrolidine–oxyimide 1 catalyst, is in good agreement
with those reported and provides an access to a variety of -nitro-
a,a-disubstituted aldehydes to nitroolefins
c
carbonyl compounds with an all-carbon quaternary center in high
enantioselectivities.
Having established the solvent and additive parameters, we
next conducted experiments to screen the effect of reaction tem-
perature and catalyst concentration on the catalytic cycle. As
shown in Table 3, the catalytic performance of 1 was optimum
for the reaction conducted at 0 °C with 15 mol % of 1 (Table 3,
entry 5), while the other conditions were found to have no sub-
stantial progress in terms of product yield or selectivity, and
instead suffered with long reaction times (Table 3, entries 1–4
and 6–9).
The observed stereochemical outcome of this transformation
could be rationalized by considering the possible transition
state9,10 model (Fig. 2). The catalytic system operates by an enam-
ine mechanism, wherein the pyrrolidine ring activates the alde-
hyde toward enamine formation and the phthalimide template
serves as an efficient steric controller and also provides H-bonding
stabilization. As depicted in Figure 2, the overall arrangement
might result in a pocket like compact transition state, and the
nucleophilic enamine attacks the nitroolefin from the Si face
leading to the formation of the desired products with high
selectivities.
Having established the optimal reaction conditions, we next
explored the substrate generality of this new catalytic system by
conducting the Michael reaction with different substrate combina-
tions. As shown in Tables
4 and 5, Michael reactions of
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T. P. Kumar / Tetrahedron: Asymmetry xxx (2015) xxx–xxx
3
Table 4
Enantioselective Michael addition of isobutyraldehyde to nitroolefinsa
1 (15 mol%)
p-NO2-C6H4COOH
O
O
Ar
(10 mol%)
NO2
NO2
H
H
Ar
neat, 0 o
C
4b-i
3a
5b-i
Entry
1
Product
Time (h)
48
Yieldb (%)
eec (%)
NO2
5b
91
92
O
O
O
O
O
NO2
H
H
H
H
H
Br
2
3
4
5c
5d
5e
48
48
48
87
89
81
90
91
83
NO2
NO2
NO2
Cl
Me
OMe
5
6
5f
48
48
77
85
81
88
NO2
OMe
O
O
O
5g
NO2
NO2
NO2
H
H
H
S
7
8
5h
5i
48
48
90
88
86
83
O
a
b
c
Reaction conditions: isobutyraldehyde (4 mmol), nitroolefin (1 mmol).
Isolated yields.
Determined by chiral HPLC.
Please cite this article in press as: Kumar, T. P. Tetrahedron: Asymmetry (2015), http://dx.doi.org/10.1016/j.tetasy.2015.07.009

4
T. P. Kumar / Tetrahedron: Asymmetry xxx (2015) xxx–xxx
Table 5
Enantioselective Michael addition of cyclopentanecarboxyaldehyde to nitroolefinsa
1 (15 mol%)
O
p-NO2-C6H4COOH
O
Ar
(10 mol%)
NO2
NO2
H
H
Ar
neat, 0 oC
3b
4a,e,f,b,f,c,j,h,i
6a-h
Yieldb (%)
Entry
1
Product
Time (h)
eec (%)
O
6a
48
48
87
90
NO2
NO2
NO2
NO2
NO2
H
Me
2
3
4
6b
6c
6d
84
80
93
78
75
92
O
O
O
O
H
H
H
H
OMe
NO2
Br
48
48
5
6
6e
6f
48
48
90
91
90
91
O
O
O
Br
NO2
H
H
H
S
7
8
6g
6h
48
48
85
87
86
84
NO2
NO2
O
a
b
c
Reaction conditions: cyclopentanecarboxaldehyde (4 mmol), nitroolefin (1 mmol).
Isolated yields.
Determined by chiral HPLC.
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T. P. Kumar / Tetrahedron: Asymmetry xxx (2015) xxx–xxx
5
O
References
O
N
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Figure 2. Proposed transition state.
3. Conclusions
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In conclusion, the application of pyrrolidine–oxyimides 1 and 2
as organocatalysts for enantioselective Michael additions of a,a-
disubstituted aldehydes to nitroolefins has been demonstrated.
The catalytic cycle was found to be effective in terms of yield
and enantioselectivities, by employing 15 mol % of
1 and
10 mol % of p-nitrobenzoic acid under neat reaction conditions.
Further investigations to extend the scope of this new catalytic sys-
tem are currently underway in our laboratory.
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4. Experimental
4.1. General
All solvents and reagents were purified by standard techniques.
Crude products were purified by column chromatography on silica
gel of 60–120 mesh. IR spectra were recorded on Perkin–Elmer 683
spectrometer. Optical rotations were obtained on Jasco Dip 360
digital polarimeter. 1H and 13C NMR spectra were recorded in
CDCl3 solution on a Varian Gemini 200 and Brucker Avance 300.
Chemical shifts were reported in parts per million with respect
to internal TMS. Coupling constants (J) are quoted in Hz. Mass
spectra were obtained on an Agilent Technologies LC/MSD Trap
SL. Chiral HPLC analysis was carried out on chiral pak OD-H, IC
or IA columns using a mixture of isopropanol and hexanes as the
eluent.
4.1.1. General procedure for the Michael addition of
disubstituted aldehydes to nitroolefins
a,a-
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Asymmetry 2010, 21, 2372–2375; (b) Chandrasekhar, S.; Kumar, T. P.; Haribabu,
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Asymmetry 2014, 25, 457–461; (e) Kumar, T. P.; Balaji, S. V. Tetrahedron:
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Shekhar, R. C.; Sunder, K. S.; Rajesh, V. Tetrahedron: Asymmetry 2015, 26, 543–
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To a mixture of catalyst 1 (15 mol %), and
a,a-disubstituted
aldehyde (4 mmol) was added p-NO2-C6H4COOH (10 mol %) and
stirred for 20 min at 0 °C. Nitroolefin (1 mmol) was added to the
resulting mixture and stirred for the appropriate time (Tables 4
and 5) at 0 °C. After completion of the reaction (monitored by
TLC), the mixture was purified by silica-gel column chromatogra-
phy to afford the desired product. Relative and absolute configura-
tions of the products were determined by comparison of 1H NMR,
13C NMR, and specific rotation values with those reported in the
literature.6
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Acknowledgements
T.P.K. thanks DST New Delhi for an INSPIRE Faculty Award
(IFA12-CH-30) and Dr. J.S. Yadav and Dr. S. Chandrasekhar for valu-
able discussions and support.
Please cite this article in press as: Kumar, T. P. Tetrahedron: Asymmetry (2015), http://dx.doi.org/10.1016/j.tetasy.2015.07.009