A new class of bifunctional chiral phase transfer catalysts for highly enantioselective asymmetric epoxidation of α,β-unsaturated ketones at ambient temperature

Article abstract of DOI:10.1016/j.molcata.2015.08.016

A new type of bis-quaternary ammonium bromide as chiral multifunctional phase transfer catalysts derived from readily available inexpensive cinchona alkaloids has been developed and evaluated for the enantioselective asymmetric epoxidation of various chalcones in the presence of lower concentrations of various oxidants, bases, solvents and ambient temperature conditions. Under optimized reaction conditions, highest chemical yields of up to 98% along with the excellent enantioselectivities of about 99% were obtained by using the cinchona based chiral multifunctional phase transfer catalysts.

Full text of DOI:10.1016/j.molcata.2015.08.016

Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A new class of bifunctional chiral phase transfer catalysts for highly
enantioselective asymmetric epoxidation of ␣,␤-unsaturated ketones
at ambient temperature
Veeramanoharan Ashokkumar, Rajendiran Balasaravanan, Velu Sadhasivam,
Seenisultanmaideen Mehtob Jenofar, 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
Article history:
Received 23 June 2015
Received in revised form 3 August 2015
Accepted 16 August 2015
Available online 21 August 2015
A new type of bis-quaternary ammonium bromide as chiral multifunctional phase transfer catalysts
derived from readily available inexpensive cinchona alkaloids has been developed and evaluated for the
enantioselective asymmetric epoxidation of various chalcones in the presence of lower concentrations
of various oxidants, bases, solvents and ambient temperature conditions. Under optimized reaction con-
ditions, highest chemical yields of up to 98% along with the excellent enantioselectivities of about 99%
were obtained by using the cinchona based chiral multifunctional phase transfer catalysts.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Chairal phase transfer catalyst
Cinchonidine
Enantioselectivity
Asymmetric epoxidation
Ion pair interaction
C2-symmetry catalyst
Electrostatic attraction
1. Introduction
the synthesis of biologically active compounds [9,10]. The funda-
mentals of enantioselective asymmetric epoxidation findings have
Phase transfer catalysis has emerged as one of the most rapidly
growing and promising areas in asymmetric synthesis [1]. The
advantages of phase transfer catalysis consist of their operational
simplicity, low cost, and low toxicity, which confers a massive
unswerving benefit in the production of pharmaceutical intermedi-
ates [2]. The ready accessibility, user friendliness of phase transfer
catalysts and the mild experimental conditions make asymmet-
ric phase transfer reactions engaging both for academic research
and for industrial applications [3]. Cinchona alkaloids were the first
efficient phase transfer catalysts for asymmetric catalysis [3,4]. It
was recognized early that the substituent’s on both the oxygen
and the nitrogen atom of the quinuclidine moiety of the cinchona
alkaloids play a key role in the enantioselectivity [5,6]. Organocat-
alytic reactions are becoming powerful tools in the construction
of complex molecular skeletons and chiral drug molecules [7]. The
chiral drug industry has become a rapidly growing segment of the
drug market [8]. The epoxide functional group is one of the most
useful intermediates in organic synthesis and building blocks for
largely expanded the scope of asymmetric synthesis and allowed
a more targeted preparation of pharmaceutical products, such as
antibiotics, anti-inflammatory drugs, and other medicines [11–14].
Asymmetric epoxidation reaction of electron deficient olefins, par-
ticularly ␣, ␤-unsaturated ketones such as chalcone have been
investigated and reported lower to moderate yield and ee’s under
organocatalysts and phase transfer catalysts [15–30]. Jew et al., [31]
have been reported the efficient enantioselective phase transfer
epoxidation of trans-chalcone using cinchona based dimeric cata-
lyst with higher concentration of oxidant (30 eq), strong base (KOH,
3 eq) and surfactants (Triton X-100). Further, Yoo et al. [32] have
been reported the N-2,3,4-trifluorobenzyl-containing cinchona-
PTCs 1b (Fig. 1) with sodium hypochlorite solution as an oxidant at
RT and 0 ◦C for the effective epoxidation of chalcones with moder-
ate yields (91%) and ee’s (79%). Recently Zeng et al., [33] reported
the enantioselective epoxidation of chalcones with excellent yields
(99%) and ee’s (99%) in the presence of prolinols based chiral cat-
alyst 1a (Fig. 1) and TBHP as an oxidant at RT, but they were
additionally used rare-earth metal amides such as Sm, Y, and Lu
in the presence of phenoxy-functionalized chiral prolinols as a lig-
and. In this study, we used low concentration of oxidant (10 eq)
and mild base (Cs2CO3, 1.5 eq) without adding any co-catalyst
∗
Corresponding author.
E-mail address: drasiva@gmail.com (A. Siva).
http://dx.doi.org/10.1016/j.molcata.2015.08.016
1381-1169/© 2015 Elsevier B.V. All rights reserved.

128
V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
Fig. 1. Previously reported cinchona and prolinol based chiral catalysts.
Scheme 1. Enantioselective synthesis of chiral epoxidation of chalcones under CMPTC conditions.
Scheme 2. Synthesis of bis quaternary ammonium ion as CMPTC for asymmetric epoxidation reaction.

V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
129
(surfactants)/metal ligands achieved higher yields (up to 98%) and
efficient ee’s (up to 99%). Generally, the epoxidation reaction at
low temperature 0 ◦C resulted in higher enantio selectivities than
at room temperature. In this connection, we focused on the enan-
tioselectivity and the chemical yield of asymmetric epoxidation
reactions (Scheme 1) using new types of bis-quaternary ammo-
nium bromides as chiral multifunctional phase transfer catalysts
(CMPTCs 10, Scheme 2).
Here, we report to synthesize a new series of multifunctional
chiral phase transfer catalyst 10a and 10b. Further, the catalytic effi-
ciencies were studied by the chiral epoxidation of chalcones with
very good yields (up to 98%) and excellent ee’s (99%) at ambient
temperature conditions.
49.90, 49.26, 39.22, 28.02, 25.48, 21.99, 20.32. ESI–MS Calculated
(m/z) = 990.2389, Found (m/z) = 990.2395.
2.2.2. Synthesis of allylated cinchonidine based CMPTC (10b)
A mixture of 4,4ꢀ-sulfonylbis (bromomethyl) (benzene) 7 (0.1 g,
10 mmol), allylated cinchonidine 9 (0.24 g, 30 mmol) was dis-
solved in 5 mL of THF:ACN (1:1 ratio) and heated to reflux for
about overnight, the off white solid was filtered, washed with
diethylether and dried it, to get pure di-site chiral PTC (10b) with
96% yield. FT-IR (KBr) cm−1: 3373.83, 2944.49, 2869.75, 2519.30,
1956.93, 1684.44, 1638.23, 1590.27, 1530.68, 1507.23, 1460.17,
1412.72, 1308.17, 1412.72, 1308.17, 1234.81, 1211.68, 1154.52,
1104.27, 1070.12, 992.03, 925.83, 852.43, 828.03, 800.19; 1H NMR
(400 MHz, CDCl3): ıppm 8.83 (d, J = 4.0 Hz, 2H), 8.07 (d, J = 9.1 Hz,
4H), 7.94 (d, J = 8.1 Hz, 4H), 7.79 (d, J = 7.4 Hz, 4H), 7.63 (s, 4H), 7.38
(m, 2H), 5.92 (m, 2H), 5.70 (dd, J = 9.7, 7.5 Hz, 2H), 5.22 (dd, J = 17.2,
10.5 Hz, 10H), 4.92 (d, J = 17.1 Hz, 2H), 4.87 (d, J = 10.3 Hz, 2H), 3.93
(m, 2H), 3.88 (m, 2H), 3.41 (s, 2H), 3.07 (dd, J = 10.0, 2.8 Hz, 4H), 2.64
(m, 2H), 2.25 (s, 1H), 1.83 (s, 2H), 1.53 (d, J = 11.2 Hz, 6H), 1.26 (s,
2H), 0.81 (m, 2H); 13C NMR (75 MHz, CDCl3) ıppm 149.83, 148.49,
142.43, 139.46, 135.92, 135.08, 133.28, 132.40, 130.36, 129.93,
128.22, 125.45, 123.41, 119.98, 118.29, 117.99, 74.35, 70.59, 60.00,
49.39, 48.38, 36.98, 27.44, 23.14, 22.16, 18.51. ESI–MS: Calculated
(m/z) = 1070.3016, Found (m/z) = 1070.3022.
2. Experimental section
2.1. Materials and methods
All the chemicals and reagents used in this work were
of analytical grade. Allylbromide, cinchonidine were obtained
from Alfa Aesar, 4-methylbezaldehyde, N-bromosuccinimide,
potassium tert-butoxide, cesium carbonate and potassium car-
bonate, 4-methylbenzene-1-sulfonyl chloride were obtained from
Sigma–Aldrich, sodium hydroxide, pottassium hydroxide, were
obtained from Merck and all the solvents were obtained from Lab-
oratory Grade. The melting points were measured in open capillary
tubes and are uncorrected. The 1H, and 13C NMR spectra were
recorded on a Bruker (Avance) 300 and 400 MHz NMR instrument
using TMS as internal standard and CDCl3 as a solvent. Standard
Bruker software was used throughout. Chemical shifts were given
in parts per million (ı-scale) and the coupling constants are given
in Hertz. Silica gel-G plates (Merck) were used for TLC analysis
with a mixture of n-hexane and ethylacetate as an eluent. Column
chromatography was carried out in silica gel (60–120 mesh) using
n-hexane and ethylacetate as an eluent. Electrospray ionization
mass spectrometry (ESI–MS) analyses were recorded in LCQ Fleet,
Thermo Fisher Instruments Limited, US. ESI–MS was performed in
positive ion mode. The collision voltage and ionization voltage were
−70 V and −4.5 kV, respectively, using nitrogen as atomization and
desolvation gas. The desolvation temperature was set at 300 ◦C. The
relative amount of each component was determined from the LC-
MS chromatogram, using the area normalization method. The HPLC
were recorded in SHIMADZU LC-6AD with Chiral column (Chiral cell
OD-H), using HPLC grade n-hexane and isopropanol solvents.
2.3. General method for a synthesis of chalcones (3a–j) [35–40]
Acetophenone (1eq) and aromatic aldehyde (1eq) were dis-
solved in 2 mL of ethanol and 10% sodium hydroxide was added,
the mixture was stirred for 5–15 min. After completion of the reac-
tion, the mixture was poured into ice the precipitate was filtered
and recrystallized with ethanol, to get pure chalcone.
2.4. General procedures for enantioselective catalytic epoxidation
of ˛,ˇ-unsaturated compounds in the presence of CMPTCs. (10)
To a mixture of chalcone 3a–h (1eq), oxidant (H2O2, NaOCl,
PMS, APS 10eq) and CMPTC catalyst 10 (10a/10b) (5 mol%) was dis-
solved in 1 mL of toluene and 0.5 mL of 10% base (like NaOH, KOH,
KtOBu, K2CO3, Cs2CO3, 1.5eq) was added. Then the reaction mixture
was stirred at room temperature, until chalcone was disappeared
(detected by TLC), after that the reaction mixture was extracted
with ethylacetate, washed with water (3 × 2 mL), brine (5 mL) and
dried over sodium sulphate and concentrated it. The crude material
was purified by column chromatography (n-hexane/ethyl acetate
as an eluent).
2.2. Catalyst preparation
2.5. Characterization of epoxidation compounds (4a–j)
2.2.1. Synthesis of cinchonidine (contains free C9-OH) based
CMPTC (10a)
2.5.1. Trans-(2R,
3S)-epoxy-3-(4-methoxyphenyl)-1-phenylproan-1-one (4a)
Light yellow solid, m.p: 81–82 ◦C. 1H NMR (300 MHz, CDCl3):
ıppm 8.11 (d, J = 4.0 Hz, 2H), 7.71 (m, 3H), 7.61 (d, J = 7.0 Hz, 2H),
7.05 (d, J = 8.5 Hz, 2H), 4.42 (d, J = 8.2 Hz, 1H), 4.32 (d, J = 8.1 Hz,
1H), 3.87 (s, 3H); 13C NMR (75 MHz, CDCl3) ıppm 197.64, 161.54,
144.58, 132.36, 130.15, 128.39, 128.23, 119.57, 114.27, 65.10,
58.30, 55.25. The enantiomeric excess was determined by HPLC,
chiral column (chiral cell OD-H), 254 nm, hexane: IPA = 99:01,
flow rate = 1 mL/min, retention time: 4.99 min (minor), 15.40 min
(major).
A mixture of 4,4ꢀ-sulfonylbis (bromomethyl) (benzene) 7 (0.1 g,
10 mmol), cinchonidine 8 (0.21 g, 30 mmol) was dissolved in 5 mL of
EtOH:DMF:ACN (30:50:20 ratio) and the whole mixture was
refluxed for overnight, the white solid was filtered off, washed with
diethylether and dried it, to get pure di-site chiral PTC (10a). The
yield is 95%. FT-IR (KBr) cm−1: 3068.12, 1735.72, 1590.70, 1509.04,
1460.72, 1418.97, 1309.15, 1206.95, 1156.73, 1106.79, 1049.13,
994.74, 936.02, 904.15, 886.67, 831.81, 801.48; 1H NMR (400 MHz,
CDCl3): ıppm 8.84 (d, J = 3.9 Hz, 2H), 8.07 (d, J = 8.6 Hz, 4H), 7.95 (d,
J = 8.1 Hz, 4H), 7.79 (d, J = 8.4 Hz, 4H), 7.62 (s, 4H), 7.38 (m, 2H), 6.01
(m, 2H), 5.88 (s, 2H), 5.08 (s, 4H), 3.45 (d, J = 8.5 Hz, 2H), 3.11 (s,
2H), 2.96 (d, J = 13.2 Hz, 4H), 2.82 (m, 4H), 2.28 (d, J = 7.9 Hz, 6H),
2.08 (m, 2H), 1.79 (s, 2H), 1.54 (m, 6H); 13C NMR (75 MHz, CDCl3)
ıppm 150.14, 149.47, 148.09,147.86, 139.15, 134.70, 130.28, 129.83,
129.03, 126.80, 125.37, 122.86, 119.61, 118.43, 115.62, 70.08, 60.22,
2.5.2. Trans-(2R,
3S)-epoxy-3-(4-methylphenyl)-1-phenylproan-1-one (4b)
White solid, m.p: 69–71 ◦C. 1H NMR (300 MHz, CDCl3): ıppm
7.82 (d, J = 7.8 Hz, 2H), 7.42 (t, J = 6.3 Hz, 1H), 7.34 (t, J = 6.7 Hz, 2H),
7.06 (d, J = 7.7 Hz, 2H), 6.96 (d, J = 7.7 Hz, 2H), 4.42 (d, J = 8.2 Hz,

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V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
Table 1
Optimization of catalysts for enantioselective epoxidation reaction.
Entry
Enone
Ar1
Ar2
Catalyst
Product
Yield (%)a
% of eeb (Abs.Conf.)c
1
2
3
4
5
6
7
8
3c
3c
3c
3c
3i
3i
3i
3i
Ph
Ph
Ph
Ph
CH3
CH3
CH3
CH3
4-Cl- C6H4
4-Cl- C6H4
4-Cl- C6H4
4-Cl- C6H4
Ph
Ph
Ph
Ph
2a
2b
10a
10b
2a
2b
10a
10b
4c
4c
4c
4c
4i
4i
4i
4i
72
72
98
98
70
70
90
90
75 (2R,3S)
80 (2R,3S)
98 (2R,3S)
99 (2R,3S)
71 (2R,3S)
73 (2R,3S)
87 (2R,3S)
91 (2R,3S)
a
Isolated yield of purified materials.
b
c
Enantiomeric excess of 4 was determined by HPLC analysis using a chiral column (chiral cell OD-H) with hexane:IPA as an eluent.
The absolute configuration of 4 was determined to be (2R,3S) by comparison with the HPLC retention time using known literature data [21].
1H), 4.32 (d, J = 8.1 Hz, 1H), 2.40 (s, 3H); 13C NMR (75 MHz, CDCl3)
ıppm 197.59, 137.35, 136.87, 136.65, 133.01, 129.05, 128.12, 128.02,
65.14, 58.21, 21.07. The enantiomeric excess was determined by
HPLC, chiral column (chiral cell OD-H), 254 nm, hexane: IPA = 99:01,
flow rate = 1 mL/min, retention time: 4.71 min (minor), 16.58 min
(major).
2.5.6. Trans-(2R,
3S)-epoxy-1-(4-bromophenyl)-3-(4-methylphenyl) proan-1-one
(4f)
White solid, m.p: 100–101 ◦C. 1H NMR (300 MHz, CDCl3): ıppm
7.87 (d, J = 8.7 Hz, 2H), 7.62 (m, 4H), 6.94 (d, J = 9.0 Hz, 2H), 4.42 (d,
J = 8.2 Hz, 1H), 4.32 (d, J = 8.1 Hz, 1H), 2.40 (s, 3H); 13C NMR (75 MHz,
CDCl3) ıppm 197.78, 161.81, 137.22, 131.87, 130.37, 129.95, 127.43,
118.99, 114.36, 65.50, 58.21, 21.04. The enantiomeric excess was
determined by HPLC, chiral column (chiral cell OD-H), 254 nm,
hexane: IPA = 99:01, flow rate = 1 mL/min, retention time: 4.92 min
(minor), 9.92 min (major).
2.5.3. Trans-(2R,
3S)-epoxy-3-(4-chlorophenyl)-1-phenylproan-1-one (4c)
Yellow solid, m.p: 68-71 ◦C. 1H NMR (300 MHz, CDCl3): ıppm
8.03 (d, J = 7.8 Hz, 2H), 7.60 (m, 3H), 7.54 (m, 2H), 7.40 (d, J = 8.4 Hz,
2H), 4.42 (d, J = 8.2 Hz, 1H), 4.33 (d, J = 8.4 Hz, 1H); 13C NMR (75 MHz,
CDCl3) ıppm 197.62, 138.21, 137.92, 133.29, 132.87, 129.53, 129.19,
128.55, 128.44, 65.17, 58.21. The enantiomeric excess was deter-
mined by HPLC, chiral column (chiral cell OD-H), 254 nm, hexane:
IPA = 99:01, flow rate = 1 mL/min, retention time: 4.91 min (minor),
10.56 min (major).
2.5.7. Trans-(2R,
3S)-epoxy-1-(4-bromophenyl)-3-(4-chlorophenyl) proan-1-one
(4g)
White solid, m.p: 65–66 ◦C. 1H NMR (300 MHz, CDCl3): ıppm 7.87
(d, J = 6.8 Hz, 2H), 7.64 (d, J = 8.6 Hz, 2H), 7.57 (d, J = 6.0 Hz, 2H), 7.39
(d, J = 9.0 Hz, 2H), 4.42 (d, J = 8.2 Hz, 1H), 4.33 (d, J = 8.4 Hz, 1H); 13
C
NMR (75 MHz, CDCl3) ıppm 197.64, 136.59, 133.07, 131.88, 129.92,
129.56, 129.22, 128.07, 65.15, 58.20. The enantiomeric excess was
determined by HPLC, chiral column (chiral cell OD-H), 254 nm,
hexane: IPA = 99:01, flow rate = 1 mL/min, retention time: 4.97 min
(minor), 11.36 min (major).
2.5.4. Trans-(2R,
3S)-epoxy-3-(4-nitrophenyl)-1-phenylproan-1-one (4d)
Yellow solid m.p: 140–142 ◦C. 1H NMR (300 MHz, CDCl3):
ıppm 8.29 (d, J = 8.5 Hz, 2H), 7.80 (dd, J = 15.0, 10.5 Hz, 3H), 7.61
(dd, J = 14.0, 5.0 Hz, 2H), 7.53 (m, 2H), 4.42 (d, J = 8.6 Hz, 1H),
4.33 (d, J = 8.7 Hz, 1H); 13C NMR (75 MHz, CDCl3) ıppm 197.69,
148.58, 141.07, 137.56, 133.36, 128.91, 128.82, 128.60, 124.24,
65.16, 58.21. The enantiomeric excess was determined by HPLC,
chiral column (chiral cell OD-H), 254 nm, hexane: IPA = 99:01,
flow rate = 1 mL/min, retention time: 5.23 min (minor), 49.49 min
(major).
2.5.8. Trans-(2R, 3S)-epoxy-1-(4-bromo
phenyl)-3-(4-nitrophenyl) proan-1-one (4h)
White solid, m.p: 130–132 ◦C. 1H NMR (300 MHz, CDCl3): ıppm
8.29 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 8.0 Hz, 2H), 7.68 (d, J = 8.2 Hz,
2H), 7.61 (d, J = 11.4 Hz, 2H), 4.42 (d, J = 8.2 Hz, 1H), 4.33 (d, J =
8.4 Hz, 1H); 13C NMR (75 MHz, CDCl3) ıppm 197.69, 148.55, 141.07,
137.56, 133.38, 128.91, 128.82, 128.62, 124.23, 65.15, 58.25. The
enantiomeric excess was determined by HPLC, chiral column (chi-
ral cell OD-H), 254 nm, hexane: IPA = 99:01, flow rate = 1 mL/min,
retention time: 5.04 min (minor), 10.23 min (major).
2.5.5. Trans-(2R,
3S)-epoxy-1-(4-bromophenyl)-3-(4-methoxylphenyl)
proan-1-one (4e)
Light yellow solid, m.p: 81-82 ◦C. 1H NMR (300 MHz, CDCl3):
ıppm 7.87 (d, J = 6.8 Hz, 2H), 7.64 (d, J = 8.6 Hz, 2H), 7.53 (d,
J = 6.5 Hz, 2H), 7.23 (d, J = 6.6 Hz, 2H), 4.42 (d, J = 8.6 Hz, 1H), 4.33
(d, J = 8.7 Hz, 1H), 2.39 (s, 3H); 13C NMR (75 MHz, CDCl3) ıppm
197.79, 145.34, 137.13, 131.94, 129.99, 129.72, 128.55, 127.66,
65.15, 58.25, 21.25. The enantiomeric excess was determined by
HPLC, chiral column (chiral cell OD-H), 254 nm, hexane: IPA = 99:01,
flow rate = 1 mL/min, retention time: 4.97 min (minor), 18.08 min
(major).
2.5.9. Trans-(2R, 3S)-1-(3-phenyloxiran-2-yl) ethanone (4i)
Light yellow liquid. 1H NMR (300 MHz, CDCl3): ıppm 7.77 (m,
3H), 7.63 (m, 2H), 4.08 (d, J = 3.9 Hz, 1H), 3.61 (d, J = 3.9 Hz, 1H),
2.19 (s, 3H); 13C NMR (75 MHz, CDCl3) ıppm 196.21, 135.12, 128.89,
127.41, 126.74, 126.65, 125.55, 74.59, 58.92, 25.11. The enan-
tiomeric excess was determined by HPLC, chiral column (chiral cell
OD-H), 254 nm, hexane: IPA = 99:01, flow rate = 1 mL/min, retention
time: 3.41 min (minor), 10.34 min (major).

V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
131
2.5.10. Trans-(2R, 3S)-(3-ethyloxiran-2-yl)(phenyl) methanone
(4j)
At high concentrations of base, initially, the deprotonation of
the catalyst 10a (C9 free –OH) occurs leading to the formation of
zwitterionic alkoxide 11, this in turn undergoes a slow fragmenta-
tion to epoxide 12. In the case of C9 (O) allylated CPTC 10b, at high
concentration of base, Hofmann elimination occurred and giving
compound of 13 viz., olefinic compound. Hence we observed the
lower chemical yield and ee’s presence of NaOH and KOH.
Further, the epoxidation reaction is carried out in the presence of
PMS and APS as oxidants, we got moderate yield and ee’s obtained
from more than 12 hrs. This may be due to the longer the reaction
time should affect the epoxidation reaction rate (i.e., epoxide ring
is cleaved and to form diol) and which can be reduced the chemical
yield as well as enantiomeric excess. (Table 2, enties 5–8).
The asymmetric epoxidation reaction was carried out in differ-
ent solvents using CMPTCs 10a and 10b under room temperature,
the other parameters are kept as constant. The obtained results
(Table 3) shows that, the change of solvent is found to be an
important decisive factor in the epoxidation reaction due to their
polarity, degree of solvation and dielectric constant of the sol-
vents. The product yield and ee’s have been found to decrease
gradually, using non polar to polar solvents (Table 3, enties 1–14).
Further, we concluded that most probably the high polar sol-
vents should reduce the ionic pair interaction between the catalyst
and oxidants and hence reducing the chemical yield and ee’s
(Table 3, enties 9–14). The polarity of the solvents as follows, cyclo-
hexane > toluene > benzene > THF > 1,4-dioxane > ACN > DMF. In the
case of toluene, cyclohexane and benzene they are nonpolar sol-
vents, the degree of solvation of CMPTC’s is considerably less.
Hence, the degree of decay due to solvation of CMPTC’s of the
catalyst is almost minimized/ignored. Otherwise, the interaction
between R4N+ of the catalyst and oxidant is more than the more
polar solvents [42]. This is in turn improves the potential of the
catalyst as well as effective attraction of the substrate and cata-
lyst and hence the reaction yield and ee’s are found to be higher in
presence of toluene, cyclohexane, benzene medium (Table 3, enties
1–8).
A good correlation was observed when enantioselectivity was
plotted as a function of solvent polarity, ET30, (Table 3 and Fig. 3).
The enantioselectivity directly depends on the solvent polarity with
the less polar solvents giving superior enantioselectivity compare
to high polar solvents (Table 3, entries 1–14). A similar trend was
observed for using both C9 free –OH containing CMPTC 10a (Fig. 3a)
and C9 (O) protected CMPTC 10b. (Fig. 3b).
The optimization of the epoxidation reaction was carried out
in the presence of different reaction temperature conditions. From
the observed results, higher chemical yield and ee’s are obtained at
room temperature when compared with other temperature con-
ditions (i.e., −10 ◦C, ultrasonic and 55 ◦C) (Table 4, entries 1–8).
The low yield of the epoxide at high temperature is probably due
to the decomposition of the oxidizing agents. The most suitable
temperature for this reaction is room temperature [43]. Through
extensive screening, the optimized reaction conditions are found
to be 3 mol% of catalyst 10, CS2CO3 as base, toluene as solvent and
H2O2 as oxidant at room temperature.
Yellow liquid. 1H NMR (300 MHz, CDCl3): ıppm 7.97 (d, J = 7.0 Hz,
2H), 7.47 (m, 3H), 4.04 (d, J = 3.2 Hz, 1H), 3.02 (d, J = 3.5 Hz, 1H),
1.31 (m, 2H), 1.06 (t, J = 7.5 Hz, 3H); 13C NMR (75 MHz, CDCl3)
ıppm 196.21, 134.84, 133.04, 128.86, 127.33, 126.75, 125.56, 60.52,
59.32, 22.95, 12.11. The enantiomeric excess was determined by
HPLC, chiral column (chiral cell OD-H), 254 nm, hexane: IPA = 99:01,
flow rate = 1 mL/min, retention time: 3.49 min (minor), 9.51 min
(major).
3. Results and discussion
The synthetic routes for the synthesis of compound
7
[34], chiral precursors 8 and 9 [35,36] were synthesized by
previously reported procedure (Scheme 2). Compound 4,4ꢀ-
sulfonylbis(methylbenzene) 6 was prepared by a Friedel–Crafts
acylation from 4-methylbenzene-1-sulfonyl chloride 5 and toluene
with a yield of 68%. The bromination of compound 6 by employ-
ing N-bromosuccinimide (NBS) in benzene successfully afforded
the desired compound 7. Catalytic asymmetric epoxidation is an
extremely important reaction owing to its role in producing the
lifesaving drugs. Based on their track record, cinchona alkaloids
have continued to play a pivotal role in this area (asymmetric syn-
thesis) exclusively for the synthesis of various epoxide products
from ␣,␤-unsaturated compounds. Hence, we focused to optimize
the reaction conditions.
First, we carried out the reactivity of the monomeric and dimeric
catalysts for the enantioselective epoxidation of chalcones. In this
connection, we performed the enantioselective epoxidation of
trans-chalcones by using 3 mol% of the monomeric (2a/2b) and
dimeric catalysts 10a/10b along with H2O2 and 10 mol% of Cs2CO3
in toluene at room temperature. From the Table 1, dimeric catalysts
mediated 10a and 10b showed higher yields and ee’s when com-
pared with the corresponding monomeric catalysts 2a/2b (Table 1,
entries 1,2 and 5,6). This is due to the 10a and 10b having multiac-
tive site present in the molecules.
Further we carried out our attention on finding optimal basic
and oxidant conditions for the enantioselective epoxidation of
chalcone in the presence of different bases under room tem-
perature condition. From the observed results (Table 2, entries
1–24) among the bases, the Cs2CO3 used epoxidation reaction was
observed higher chemical yield and ee’s than the other bases such
as K2CO3, KtOBu, KOH and NaOH. The enantioselective epoxida-
tion of chalcone at higher concentration of base irrespective of
CMPTCs/oxidants gave disparate results, which can be explained
by invoking a catalyst degradation mechanism [41] (Fig. 2).
Further, we observed the moderate chemical yield and ee’s in
presence of NaOH, KOH and KtOBu due to the formation of sec-
ondary product (i.e. cleavage of epoxide to diol formation) for the
enantioselective epoxidation reaction. In this cleavage is not appli-
cable when we are using carbonate containing bases. In addition
to that the size of the cations is also important to the formation of
epoxidation product, the chemical yield of epoxide increases with
the size of the cation and in the close relation to the cohesion energy
of the carbonates [42].
Furthermore, we carried out the optimization of oxidant for the
enantioselective epoxidation of chalcone 3 by using 3 mol% of the
dimeric catalysts 10a and 10b along with different oxidant and
10% aqueous Cs2CO3 in toluene at room temperature. As shown in
(Table 2) even though the reaction time was somewhat very short
(4.5–5.5 h), the dimeric catalyst 10a and 10b provided very good
yield (95-98%) and enantiomeric excess (92–99%) in the presence
of H2O2 and NaOCl as an oxidant (Table 2, enties 1–4).
In general, all homogeneous catalysis reaction rates are directly
proportional to the catalyst loading amount. From the observed
results, the catalysts amounts are increased from 1 mol% to 15 mol%
the epoxidation reaction yield as well as the ee’s reduced. That
means the catalysts poison taking place in this reaction irrespective
of the catalysts 10a and 10b (Table 5, entries 1–10). This may be due
to the lower the catalyst concentration of sulfonyldibenze spacer
between the two cinchona units influences the binding of the cat-
alyst with chalcone 3 and peroxide than the higher concentration
of the catalysts.
The obtained results indicated that the stereochemical course of
the epoxidation mainly depends on the stereochemistry/molecular

132
V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
Table 2
Effect of various bases, oxidants and CMPTCs (10) for enantioselective epoxidation reaction.
Entry
Base
Oxidant
Catalyst
Time (h)
Yield (%)a
% of eeb (Abs.Conf.)c
1
2
3
4
5
6
7
8
CS2CO3
CS2CO3
CS2CO3
CS2CO3
CS2CO3
CS2CO3
CS2CO3
CS2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KtOBu
KtOBu
KtOBu
KtOBu
KOH
KOH
KOH
KOH
NaOH
NaOH
NaOH
NaOH
H2O2
H2O2
NaOCl
NaOCl
PMSd
PMSd
APSd
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
4.5
4.5
5.5
5.5
12.5
12.5
14.5
14.5
5.0
5.0
7.0
7.0
6.0
6.0
9.0
9.0
7.0
7.0
9.5
9.5
10.0
10.0
11.5
11.5
98
98
95
95
85
85
80
80
93
93
90
90
86
86
82
82
70
70
65
65
62
62
60
60
98 (2R,3S)
99 (2R,3S)
91 (2R,3S)
94 (2R,3S)
84 (2R,3S)
86 (2R,3S)
83 (2R,3S)
87 (2R,3S)
95 (2R,3S)
97 (2R,3S)
90 (2R,3S)
93 (2R,3S)
83 (2R,3S)
86 (2R,3S)
82 (2R,3S)
85 (2R,3S)
73 (2R,3S)
78 (2R,3S)
73 (2R,3S)
75 (2R,3S)
71 (2R,3S)
73 (2R,3S)
36 (2R,3S)
36 (2R,3S)
APSd
9
H2O2
H2O2
NaOCl
NaOCl
H2O2
H2O2
NaOCl
NaOCl
H2O2
H2O2
NaOCl
NaOCl
H2O2
H2O2
NaOCl
NaOCl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
Isolated yield of purified materials.
b
c
Enantiomeric excess of 4 was determined by HPLC analysis using a chiral column (chiral cell OD-H) with hexane:IPA as an eluent.
The absolute configuration of 4 was determined to be (2R,3S) by comparison with the HPLC retention time using known literature data [21].
PMS – potassium peroxy monosulphate, APS – ammonium peroxysulphate.
d
Fig. 2. A schematic representation for the catalyst decomposition studies.
assembly between the substrates such as chalcone 3 and CMPTC’s.
The formation of higher epoxidation yield (Table 2, entries 1–24,
Table 3, entries 1–14 and Table 5, entries 1–10) and its ee’s of each
reaction catalyzed by C9 (O) protected CMPTCs would be mainly
attributed to an effective contact of ion-pair formed between the
positive quaternary onium ions (R4N+) of the respective CMPTC’s
with enolate anion of the chalcones due to electrostatic attraction
(Fig. 4c). The results also suggested that apart from the ionic inter-
action between the catalyst and substrates, there is also a ␲–␲
stacking interaction between the benzyl group of the respective
C9 (O) protected CMPTCs with aryl group of the chalcone deriva-
tives which would further facilitate the binding of the two species

V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
133
Table 3
Effect of various solvents on asymmetric epoxidation reaction.
Entry
Solvents
Catalyst
Time (h)
Yield (%)a
% of eeb (Abs.Conf.)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Toluene
Toluene
Cyclohexane
Cyclohexane
Benzene
Benzene
1,4-dioxane
1,4-dioxane
THF
THF
DMF
DMF
Acetonitrile
Acetonitrile
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
4.5
4.5
5.0
5.0
5.5
5.5
6.5
6.5
8.0
8.0
9.0
9.0
8.5
8.5
98
98
95
95
93
93
85
85
80
80
78
78
75
75
98 (2R,3S)
99 (2R,3S)
96 (2R,3S)
97 (2R,3S)
92 (2R,3S)
95 (2R,3S)
90 (2R,3S)
94 (2R,3S)
82 (2R,3S)
85 (2R,3S)
73 (2R,3S)
78 (2R,3S)
71 (2R,3S)
75 (2R,3S)
a
Isolated yield of purified materials.
b
c
Enantiomeric excess of 4 was determined by HPLC analysis using a chiral column (chiral cell OD-H) with hexane:IPA as a solvent.
The absolute configuration of 4 was determined to be (2R,3S) by comparison with the HPLC retention time using known literature data [21].
Fig. 3. Plot of enantiomeric excess (ee) against polarity (ET30) for solvent variations. (a) Using C9 free –OH containing CMPTC 10a. (b) Using C9 (O) protected CMPTC 10b.
Table 4
Optimization of asymmetric epoxidation reaction in various conditions.
Entry
Catalyst
Condition
Time (h)
Yield (%)a
% of eeb (Abs.Conf.)c
1
2
3
4
5
6
7
8
10a
10b
10a
10b
10a
10b
10a
10b
USd
USd
3.5
3.5
8.0
93
93
90
90
92
92
98
98
82 (2R,3S)
84 (2R,3S)
71 (2R,3S)
75 (2R,3S)
85 (2R,3S)
89 (2R,3S)
98 (2R,3S)
99 (2R,3S)
55 ◦
55 ◦
C
C
8.0
−10 ◦
−10 ◦
RT
C
C
10.0
10.0
4.5
RT
4.5
a
Isolated yield of purified materials.
b
c
Enantiomer excess of 4 was determined by HPLC analysis using a chiral column (chiral cell OD-H) with hexane:IPA as a solvent.
The absolute configuration of 4 was determined to be (2R,3S) by comparison with the HPLC retention time using known literature data [21].
Ultrasonication.
d

134
V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
Table 5
The asymmetric epoxidation of chalcone 3 under various concentration of CMPTCs 10 (10a/10b).
Entry
Catalyst
Mol% of Catalyst
Time (h)
Yield (%)a
% of eeb (Abs.Conf.)c
1
2
3
4
5
6
7
8
9
10a
10a
10a
10a
10a
10b
10b
10b
10b
10b
1
3
5
10
15
1
3
5
10
15
7.0
4.5
4.0
6.0
6.0
7.0
4.5
4.5
6.0
6.0
92
98
97
85
82
92
98
97
85
82
90 (2R,3S)
98 (2R,3S)
96 (2R,3S)
87 (2R,3S)
84 (2R,3S)
92 (2R,3S)
99 (2R,3S)
97 (2R,3S)
89 (2R,3S)
85 (2R,3S)
10
a
Isolated yield of purified materials.
b
c
Enantiomeric excess of 4 was determined by HPLC analysis using a chiral column (chiral cell OD-H) with hexane:IPA as a solvent.
The absolute configuration of 4 was determined to be (2R,3S) by comparison with the HPLC retention time using known literature data [21].
Fig. 4. Formation of various intermediates/molecular assemblies during enantioselective epoxidation reaction using C9 free –OH and C9 (O) protected CMPTCs.
chalcones (Ar1 and Ar2 = Aryl) and hence, increase the ion pair inter-
action between the enolate anion and R4N+ of the catalysts (Table 6,
entries 1–20)
Additional, from the observed results, allyl protected catalyst
10b has more efficient than the free C9–OH containing catalyst 10a
(Table 6, entries 1–16) due to the allyl protected catalyst has more
binding with the chalcone and oxidants. In the case of 10a as a
CMPTC, hydrogen bonding between the enolate anion of the chal-
cone with the C9-OH of the catalyst which can prevent the perfect
ion pair interaction between the catalyst and anion of the chalcones,
hence we got moderate yield as well as the ee’s (Fig. 4b).
Based on this results, we proposed plausible transition state
of the catalytic asymmetric epoxidation (Fig. 5a). The chalcone
is located between the two cinchona units in 10a and 10b. The
␤-phenyl group of chalcone has a ␲–␲ stacking interaction with
one of the quinoline moieties. The carbonyl oxygen atom is placed
as close to the N+ center as permitted by van der Waals forces.
The other R4N+ center is ion paired with the hydrogen peroxide
(Fig. 5a). This in turn shows to facilitate effective ion-pair inter-
action and thus effect for parallel increasing of yield and ee than
their corresponding CMPTCs containing C9 free –OH in H2O2 as an
oxidant. While the decreased epoxidation product yield and ee’s
noticed in C9 free –OH of cinchonidine CMPTCs (Table 2, entries
1–24, Table 3, entries 1–14 and Table 5, entries 1–10) are due to
the formation of the hydrogen bond between the reactant 3 and the
free hydroxyl group present at C9 free –OH position of the CMPTCs
(Fig. 4a).
The asymmetric epoxidation reaction was carried from the dif-
ferent chalcone derivatives under identical reaction conditions.
There is no influence on the chemical yield as well as the ee’s
in presence of electron releasing or electron withdrawing groups
present on the phenyl ring of the chalcone (Table 6). Aromatic group
substituted chalcone derivatives have better yields and ee’s than
the aliphatic group containing chalcones [(Ar1 or Ar2), (Table 6,
entries 17–20)], this may be due to the ␲–␲ interaction is more
between the quinoline part of the catalysts and aryl group of the

V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
135
Fig. 5. (a) Formation of ␲–␲ interaction between the spacer chain (aromatic) of all the CMPTCs with aromatic ring of the chalcones. (b) C2-symmetric trans-cinchona bis
catalysts strong binding with the enolate anion of the chalcone and peroxides due to electrostatic attraction.
Table 6
Catalytic asymmetric epoxidation of chalcone derivatives under CMPTCs conditions.
Entry
Enone
Ar1
Ar2
Catalyst
Producta
Yield (%)b
% of eec (Abs.Conf.)d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
3a
3a
3b
3b
3c
3c
3d
3d
3e
3e
3f
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
CH3
4-OMe-C6H4
4-OMe-C6H4
4-CH3-C6H4
4-CH3-C6H4
4-Cl- C6H4
4-Cl- C6H4
4-NO2- C6H4
4-NO2- C6H4
4-OMe-C6H4
4-OMe-C6H4
4-CH3-C6H4
4-CH3-C6H4
4-Cl- C6H4
4-Cl- C6H4
4-NO2- C6H4
4-NO2- C6H4
Ph
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
10a
10b
4a
4a
4b
4b
4c
4c
4d
4d
4e
4e
4f
95
95
94
94
98
98
92
92
93
93
90
90
97
97
95
95
90
90
86
86
97 (2R,3S)
98 (2R,3S)
92 (2R,3S)
94 (2R,3S)
98 (2R,3S)
99 (2R,3S)
95 (2R,3S)
96 (2R,3S)
94 (2R,3S)
96 (2R,3S)
95 (2R,3S)
97 (2R,3S)
92 (2R,3S)
93 (2R,3S)
94 (2R,3S)
95 (2R,3S)
87 (2R,3S)
91 (2R,3S)
89 (2R,3S)
92 (2R,3S)
3f
4f
3g
3g
3h
3h
3i
3i
3j
3j
4g
4g
4h
4h
4i
4i
4j
4j
CH3
Ph
Ph
Ph
C2H5
C2H5
a
The asymmetric epoxidation of chalcones 3 (1eq), NaOCl (10 eq), CMPTCs 10 (10a/10b 3 mol%), with 1 mL toluene and 0.5 mL of 10 mol% KtOBu in ultrasonic conditions.
Isolated yield of purified materials.
Enantiopurity of 4 was determined by HPLC analysis using a chiral column (chiral cell OD-H) with hexane:IPA as a solvent.
The absolute configuration of 4 was determined to be (2R,3S) by comparison of the HPLC retention time with known literature data [21].
b
c
d

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V. Ashokkumar et al. / Journal of Molecular Catalysis A: Chemical 409 (2015) 127–136
ion through hydrogen bonding with the oxygen of C9 free –OH
containing ammonium salt. As a consequence, hydrogen perox-
ide can only approach the ␤ carbon atom of chalcone from the
upside in the 1, 4-addition to afford the ␣S and ␤R isomer of 10,
which is in very good agreement with the observed results. Cin-
chona alkaloid catalysts 10a and 10b have C2-symmetric type of
catalysts. Hence, the two cinchona units should present at the end
of the 4,4’-sulfonylbis(methyl) benzene due to steric hindrance of
the quinoline moiety of the cinchona alkaloids. Hence the 4,4ꢀ-
sulfonylbis(methyl) benzene catalysts (10a and 10b) can strongly
binding with the two equivalence of the enolate anion of the chal-
cones (Fig. 5b) simultaneously and hence we found higher yield and
ee’s at lower concentration of catalysts, oxidant, base and lesser
time (Table 1–6).
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4. Conclusion
In summary, we reported a highly enantioselective asymmet-
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We acknowledge the financial support of the Department of Sci-
ence and Technology, New Delhi, India (Grant No. SR/F/1584/2012-
13), University Grants Commission, New Delhi, India (Grant No.
UGC No.41-215/2012) (SR) and Council of Scientific and Industrial
Research, New Delhi, India (Grant No. 01(2540)/11/EMR-II).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2015.08.
016.
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