Unexpected solvent/substitution-dependent inversion of the enantioselectivity in Michael addition reaction using chiral phase transfer catalysts

Article abstract of DOI:10.1016/j.tetlet.2015.07.074

New cinchonium salts bearing 5,5′-bis(methyl)-2,2′-bipyridine 1 group show solvent/substitution-dependent reversal of enantioselectivity. When used as chiral phase transfer catalyst in the asymmetric Michael addition of chalcones with diethylmalonate within two hours these catalysts result in high chemical yield (up to 98%) and enantiomeric excess (up to 99%) under lower concentrations of base and cold conditions.

Full text of DOI:10.1016/j.tetlet.2015.07.074

Tetrahedron Letters 56 (2015) 5209–5212
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Tetrahedron Letters
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Unexpected solvent/substitution-dependent inversion of the
enantioselectivity in Michael addition reaction using chiral phase
transfer catalysts
⇑
Ponmuthu Kottala Vijaya, Sepperumal Murugesan, Ayyanar Siva
Department of Inorganic Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
New cinchonium salts bearing 5,50-bis(methyl)-2,20-bipyridine 1 group show solvent/substitution-
dependent reversal of enantioselectivity. When used as chiral phase transfer catalyst in the asymmetric
Michael addition of chalcones with diethylmalonate within two hours these catalysts result in high
chemical yield (up to 98%) and enantiomeric excess (up to 99%) under lower concentrations of base
and cold conditions.
Article history:
Received 21 May 2015
Revised 21 July 2015
Accepted 24 July 2015
Available online 29 July 2015
Ó 2015 Published by Elsevier Ltd.
Keywords:
Enantioselectivity
Asymmetric
Michael addition
Phase transfer catalyst
Cinchonium salt
Introduction
PTC catalysts at high concentration (50%) of potassium/sodium
tert-butoxide as bases.12 Najera and co-workers reported the unex-
Michael addition is one of the most useful methods for the
formation of C–C bonds in organic synthesis.1 Hence, the
catalytic asymmetric version has been extensively studied.2
Enantioselective Michael addition of various malonates and chal-
cones has been reported in the presence of many kinds of catalysts,
such as chiral phase transfer catalysts,3 chiral ionic liquids,4 chiral
N,N0-dioxide–Sc complexes,5 chiral bis-sulfonamide–Sr com-
plexes,6 chiral bisphosphazide–Li complexes,7 chiral SIPAD–Co
complexes,8 DPEN/NAP-MgO,9 and organocatalysts.10 Even though
some of these catalysts could not give reasonable enantioselectiv-
ity, chalcones are still demanding substrates in Michael addition
reactions with malonates. However, reports on asymmetric
Michael addition reactions involving chalcones are limited.
Previously, we reported a series of tri-functional triazine based cin-
chona alkaloids as a chiral phase transfer catalysts (CPTC) for
highly enantioselective Michael addition reactions of chalcones
with very good yield and ee’s.11 In all the previously reported cases,
the cinchonine and cinchonidine based chiral catalysts give their
respective R- and S-enantiomers of the Michael adduct, though
both the compounds act as pseudoenantiomers.
pected metal base-dependent inversion of the enantioselectivity in
the asymmetric synthesis of -amino acids using cinchonidine
a
based CPTC.13 Consecutively, Keiji Maruoka14 reported the unusual
anti-selective asymmetric conjugate addition of aldehydes to
nitroalkenes catalyzed by a biphenyl-based chiral secondary amine
as a catalyst. Recently, Blackmond15 and co-workers, Chinchilla
and co-workers16 reported the solvent dependent formation of S-
or R- enantioenriched succinimides from a single enantiomer of
the organocatalysts. In this work, first time we report the unex-
pected solvent/substitution-dependent enantioselectivity in the
Michael addition reaction using new types of CPTCs derived from
cinchonine under mild reaction conditions with very good chemi-
cal yield up to 98% and ee’s up to 99%.
New type of CPTCs 4 (4a/4b) were synthesized from commer-
cially available starting material 5,50-dimethyl-2,20-bipyridine 1
(Scheme 1)17 and their catalytic efficiencies were studied by the
enantioselective Michael addition reaction between diethyl malo-
nate 6 and enone derivatives 5 (Scheme 2).
Results and discussion
Dehmlow et al. have reported base dependent inversion of
stereochemistry in Michael addition using chiral crown ethers as
The reaction conditions were optimized by using 5 mol % of cat-
alysts, diethylmalonate 6 (as the Michael donor) and enone 5 (as
the Michael acceptor), and various bases as well as solvents at dif-
ferent temperatures (Table 1). From the obtained results (Table 1),
⇑
Corresponding author. Tel.: +91 451 2458471.
E-mail addresses: drasiva@gmail.com, ptcsiva@yahoo.co.in (A. Siva).
http://dx.doi.org/10.1016/j.tetlet.2015.07.074
0040-4039/Ó 2015 Published by Elsevier Ltd.

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P. Kottala Vijaya et al. / Tetrahedron Letters 56 (2015) 5209–5212
Table 1
Br
NBS , AIBN, CCl4
Reflux, 2h
Optimization of asymmetric Michael addition reaction between enone
diethylmalonate 6 with CPTC 4a in various conditions
5 and
Br
N
N
N
N
1
2
Entry Base
Solvent
Tempa
(°C)
Yieldb
(%)
% of
eec
Abs.
config.d
N
O
THF:ACN (1:1)
H
1
2
3
4
5
6
7
8
K2CO3
Toluene
RT
RT
RT
RT
RT
0
0
0
0
0
0
0
0
0
0
0
0
50
65
62
68
70
70
80
75
82
95
80
64
84
80
53
55
60
99
99
96
98
99
98
96
98
99
99
96
50
69
68
35
38
45
R
R
R
R
R
R
R
R
R
R
R
R
R
R
S
R
Reflux, Overnight
Cs2CO3 Toluene
KtOBu
KOH
Toluene
Toluene
Toluene
Toluene
3a = R= allyl
3b = R= H
N
N
NaOH
K2CO3
R
O
Cs2CO3 Toluene
H
KtOBu
KOH
Toluene
Toluene
Toluene
Xylene
Benzene
THF
Cyclohexane
DCM
Acetone
Methanol
N
9
10
11
12
13
14
15
16
17
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
N
N
N
H
4a = R= allyl
4b = R= H
2Br
O
R
N
S
S
Scheme 1. Synthesis of chiral phase transfer catalysts 4.
a
The Michael reaction of enone 5 (0.1 mmol), diethyl malonate 6 (0.12 mmol),
catalysts 4a (5 mol %), with 1 ml of solvent and 0.5 ml of 10% aq base.
b
Isolated yield of purified material.
Enantiopurity was determined by HPLC analysis of the Michael adduct 7 using a
it is found that NaOH is the more effective base in this reaction
(entries 1–10, Table 1). The yields at 0 °C are always higher than
those at room temperature while the enantiomeric excesses were
more or less same at both the temperatures (entries 1–5 compared
to 6–10, Table 1). It can be noted that upon increasing the polarity
of the solvent, the stereo induction is reduced in the Michael
addition reactions and hence very poor ee’s are observed. While
(R)-configuration has been obtained as major product in
non-polar solvents (entries 1–14, Table 1), (S)-configuration has
been obtained as major product in more polar solvents (entries
15–17, Table 1). The polar solvents like DCM, acetone, methanol
gave lower chemical yield and ee’s than non-polar solvents. This
may be attributed to the fact that the high polar solvents reduce
the ion-pair interaction between the catalyst (N+) and the enolate
anion, due to the high degree of solvation of catalyst, thereby
reduce the efficiency of the catalysts and consequently the product
yield and ee’s are decreased.
c
chiral column (Phenomenex Chiralpack) with hexane–IPA as an eluent.
d
Absolute configuration was determined by the comparison of the HPLC reten-
tion time.11
enantioselectivities, excellent chemical yields, and unexpected
substitution dependent inversion were observed for a wide range
of aryl substituted chalcones (entries 1–24). From the Table 2, it
is clear that the substitution on the aryl group of the enones
strongly affects the product yield and ee’s. When Ar1 was a phenyl
group (entries 1–6, Table 2), the property of the substituent’s on
Ar2 in chalcones either electron donating (4-Me, 4-MeO–) or elec-
tron with drawing groups (–Cl) did not affect the chemical yields as
well as enantioselectivities (R-enantiomers). But the electron-
withdrawing group –NO2 and –CN on Ar2 is obviously not favor-
able for the ee’s. Therefore, chalcones with 4-nitro substituted
Ar2 gave moderate yields (70%) and 32–36% ee’s (entries 13 and
14; Table 2). On the other hand, a higher yield was achieved for
the chalcones with electron donating/withdrawing substituents
on Ar1 (entries 7–12 and 15–24, Table 2), but, affect the ee’s and
inversion of configuration (S-enantiomers) was achieved on the
electron withdrawing groups present on the Ar2 in chalcones
(entries 13–24, Table 2).
Among the non polar solvents toluene gave higher chemical
yield and ee’s than others like xylene, benzene (entries 10–13,
Table 1). This may be explained as follows: the high electron den-
sity in the aromatic ring makes them behave as a base to form
charge-transfer
p-complexes with quaternary ammonium ion
which facilitate easy transfer of CPTC to organic phase. Toluene
has lower polarity than xylene and benzene thus strongly interact
with the N+ ion of the catalysts as discussed above. This strong
interaction would have taken place in the Si face, which helps easy
interaction with the enolate anion of the substrate on Re face to
direct the R-configuration of Michael adduct (Fig. 1). Hence, we
have chosen toluene as a solvent for further investigations.
With the best reaction conditions in hand (5 mol % of catalyst
4a and 4b, 10% aq NaOH, toluene, 0 °C), we next considered the
scope of the Michael reaction by employing different chalcones 5
We believe that the p–p interaction of the aromatic rings of the
chalcone and the quinoline moiety of the catalyst keep the car-
bonyl of the chalcone with the ammonium in close proximity
and favor the strong ion pair interaction of the substrates and cat-
alysts which in turn would give high chemical yield and ee’s
(Fig. 2). Similar reversal of enantioselectivity has been observed
by tuning the conformational flexibility of chiral catalysts
in various reactions, such as asymmetric Michael addition reac-
tion of chalcones with 2-nitropropane,18 and enantioselective
with diethylmalonate
6
(Table 2). Consistently high
EtOOC
O
COOEt
O
COOEt
CPTCs 4 (4a/4b) (5 mol%)
*
COOEt
6
10% aq. base
Solvent
R1
R2
R1
R2
7
5
R1 = -H, -Br, -OCH3, -N(CH3)2
R2 = -CH3, -Cl, -OCH3, -NO2, -CN
R1 = -H, -Br, -OCH3, -N(CH3)2
R2 = -CH3, -Cl, -OCH3, -NO2, -CN
Yield : 50-98%
ee
: 25-99%
Scheme 2. Enantioselective Michael addition of enone derivatives 5 with diethylmalonate 6 using CPTCs 4 (4a/4b) in aqueous/organic solvent media.

P. Kottala Vijaya et al. / Tetrahedron Letters 56 (2015) 5209–5212
5211
N
N
H
H
N
2Br
N
2Br
O
O
H
O
H
N
N
O
N
N
O
O
EtO
H
O
H
O
OEt
EtO
OEt
A
B
Si-face attack
Re-face attack
Figure 1. Possible transition state for the enantioselective Michael addition reaction.
Table 2
Catalytic asymmetric Michael addition reaction of diethylmalonate 6 to enone derivatives 5 under the optimized conditions
O
EtOOC
O
COOEt
Ar2
COOEt
CPTCs 4(4a/ 4b, 5 mol%)
10% aq. NaOH
Ar1
Ar2
Ar1
COOEt
6
*
Toluene, 00C, 2h
5
7
Entry
Enone (1)
Ar1
Ar2
Catalyst
Producta
Yieldb (%)
% of eec
Abs. Config.d
1
2
3
4
5
6
7
8
5a
5a
5b
5b
5c
5c
5d
5d
5e
5e
5f
Ph
Ph
Ph
Ph
Ph
Ph
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
Ph
4-Me-C6H4
4-Me-C6H4
4-Cl-C6H4
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
7a
7a
7b
7b
7c
7c
7d
7d
7e
7e
7f
98
98
94
94
96
96
93
93
92
92
95
95
70
70
97
97
92
92
83
83
95
95
90
90
99
90
98
92
90
91
95
98
99
91
99
99
32
36
86
83
85
81
41
25
98
97
84
86
R
R
R
R
R
R
R
R
R
R
R
R
S
S
S
S
S
S
S
S
S
S
S
S
4-Cl-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-Me-C6H4
4-Me-C6H4
4-Cl-C6H4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4-Cl-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-NO2-C6H4
4-NO2-C6H4
4-NO2-C6H4
4-NO2-C6H4
4-NO2-C6H4
4-NO2-C6H4
4-CN-C6H4
4-CN-C6H4
4-NO2-C6H4
4-NO2-C6H4
4-CN-C6H4
4-CN-C6H4
5f
7f
5g
5g
5h
5h
5i
5i
5j
5j
7g
7g
7h
7h
7i
7i
7j
7j
Ph
4-Br-C6H4
4-Br-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-(Me)2N-C6H4
4-(Me)2N-C6H4
4-(Me)2N-C6H4
4-(Me)2N-C6H4
5k
5k
5l
7k
7k
7l
5l
7l
a
b
c
The Michael reaction of enone 5 (0.1 mmol), diethylmalonate 6 (0.12 mmol), catalysts (4a/4b, 5 mol %), with 1 ml solvent and 0.5 ml of 10% aq base.
Isolated yield of purified material.
Enantiopurity was determined by HPLC analysis of the Michael adduct 7 using a chiral column (Phenomenex Chiralpack) with hexane–IPA as an eluent.
d
Absolute configuration was determined by comparison of the HPLC retention time.11
OEt
best of our knowledge, till date no such solvent (polar/non-polar)
EtO
N
O
O
O
and substitution dependent inversion of configuration has been
demonstrated in Michael addition reaction under phase transfer
catalysts.
O
N
N
π-π - interaction
N
Conclusion
O
ion pair formation
N
N
O
O
In conclusion we have successfully synthesized new
asymmetric CPTCs 4 (4a/4b) and thoroughly characterized them
by various spectral techniques. Their catalytic efficiency was mea-
sured by Michael addition reaction between diethylmalonate 6 and
enone 5. Within 2 h high chemical yield (up to 98%) and enan-
tiomeric excess (up to 99%) under lower concentration of base
and cold conditions are obtained. The formation of R and S
Michael adducts strongly depends upon the substitution pattern
of the chalcone.
O
OEt
EtO
Figure 2. Possible formation of p–p stacking and ion pair interaction between the
quinoline moieties of the catalyst, chalcone, and ammonium (R4N+).
hydrosilylation reduction of ketones in the presence of
(S,S)-BOPA/FeCl2 complexes as chiral catalysts.19 However, to the

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P. Kottala Vijaya et al. / Tetrahedron Letters 56 (2015) 5209–5212
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This work was financially supported by the Department of
Science and Technology, New Delhi, India (Grant No.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.07.
074.
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References and notes
17. General procedure for synthesis of CPTCs (4).
A
mixture of 5,50-
bis(bromomethyl)-2,20-bipyridine 2 (0.1 g, 10 mmol), cinchona derivatives 3
(3a/3b, 20 mmol) was dissolved in 5 ml of THF and heated to reflux for
overnight, the brown solid was filtered, washed with diethylether and dried it
to get pure di site chiral PTC (96% yield of 4a and 98% yield of 4b). Synthesis of
allylated cinchonine based CPTC (4a). 1H NMR (400 MHz, DMSO) d 9.10 (s, 2H),
9.02 (d, J = 4 Hz, 2H), 8.65 (d, J = 8 Hz, 2H), 8.43–8.37 (m, 4H), 8.14 (d, J = 8 Hz,
2H), 7.89–7.78 (m, 6H), 6.87 (d, J = 4 Hz, 2H), 6.56 (s, 2H), 6.08–6.01 (m, 4H),
5.26 (t, J = 10 Hz, 6H), 5.14–5.04 (m, 4H), 4.31 (t, J = 10 Hz, 2H), 4.02–3.94 (m,
6H), 3.14–3.07 (m, 2H), 2.65 (t, J = 8 Hz, 2H), 2.34 (t, J = 10 Hz, 4H), 1.92 (s, 2H),
1.83–1.74 (m, 4H), 1.11 (t, J = 12 Hz, 2H), 0.88 (t, J = 8 Hz, 2H); 13C NMR
(100 MHz, DMSO) d 155.60, 153.70, 150.14, 147.56, 144.92, 142.76, 137.04,
129.72, 129.41, 128.85, 127.34, 125.03, 124.32, 123.91, 120.74, 120.07, 118.92,
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1H NMR (400 MHz, DMSO) d 9.12 (s, 2H), 9.00 (d, J = 4 Hz, 2H), 8.64 (d, J = 8 Hz,
2H), 8.41 (t, J = 8 Hz, 4H), 8.12 (d, J = 8 Hz, 2H), 7.88–7.71 (m, 6H), 6.86 (t,
J = 6 Hz, 2H), 6.55 (s, 2H), 6.09–5.98 (m, 2H), 5.33–5.22 (m, 6H), 5.11 (d,
J = 16 Hz, 2H), 4.00 (t, J = 16 Hz, 4H), 3.58–3.50 (m, 2H), 3.11 (d, J = 8 Hz, 2H),
2.67 (d, J = 8 Hz, 2H), 2.33 (t, J = 12 Hz, 2H), 1.91 (s, 2H), 1.79 (d, J = 8 Hz, 2H),
1.08 (t, J = 8 Hz, 2H), 0.87 (t, J = 8 Hz, 2H); 13C NMR (100 MHz, DMSO) d 155.60,
153.60, 150.14, 147.99, 144.89, 142.76, 137.05, 129.42, 128.79, 127.29, 125.02,
124.33, 120.75, 120.08, 118.94, 117.03, 67.38, 64.68, 59.37, 55.92, 54.08, 36.69,
27.51, 26.34, 22.98. ESI-MS (M+); 931.28.
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