Chiral octahedral complexes of Co(iii) as catalysts for asymmetric epoxidation of chalcones under phase transfer conditions

Article abstract of DOI:10.1039/c5ra11760g

Stereochemically inert and positively charged chiral complexes of Co(iii) were shown to catalyze the asymmetric epoxidation of chalcones with H₂O₂ under phase transfer conditions. The reaction products had enantiomeric purities of up to 55%. It was also shown that complex 1a I^- catalyzed the coupling reaction of a resulting epoxide with CO₂ (conversion 72%).

Full text of DOI:10.1039/c5ra11760g

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Received 17th June 2015,
Accepted 00th January 2015
DOI: 10.1039/x0xx00000x
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Chiral octahedral complexes of Co(III) as catalysts for asymmetric epoxidation of
chalcones under phase transfer conditions.
Vladimir A. Larionov,a Elina P. Markelova,b Alexander F. Smol’yakov,a Tat’yana F. Savel’yeva,a Victor I. Maleeva and
Yuri N. Belokona*
Stereochemically inert and positively charged chiral complexes of Co(III) were shown to catalyze the asymmetric
epoxidation of chalcones with H2O2 under phase transfer conditions. The reaction products had enantiomeric
purities of up to 55%. It was also shown that complex 1a I- catalyzed the coupling reaction of a resulting epoxide
with CO2 (conversion 72%).
Introduction
Enantiomerically enriched α,β-epoxy ketones are versatile chiral building blocks for access to natural compounds
and drugs in medicinal chemistry.1,2 They can be converted into many types of useful chiral compounds, such as α-
hydroxy, β-hydroxy, α,β-dihydroxy carbonyl compounds, as well as epoxy alcohols.3
The basic method of producing the enantiomerically enriched epoxy ketones is the asymmetric oxidation of
activated olefins.4 By far the most attractive method for the preparation of epoxy ketones is asymmetric
epoxidation of chalcones.5
A green and most cost effective approach is to use hydrogen peroxide as the oxidizing agent,4e because the
only by-products of the reaction is are water and molecular oxygen. The catalytic protocols usually employ either
chiral metal complexes of iron6 and manganese7 or chiral organocatalysts, in particular, those operating under
phase transfer conditions.8
Recently we successfully elaborated chiral, positively charged, stereochemically inert complexes of Co(III) as
chiral phase transfer catalysts for efficient asymmetric alkylation of a glycine Schiff base ester (O’Donnell
substrate) with alkyl halides.9a In addition, the family of the complexes could be successfully applied for the
asymmetric 1,4-addition of O’Donnell’s substrate to activated olefins.9b The convincing evidence was put forward
proving the complexes functioned in the reactions as “organic catalysts in disguise”.10 We believed further
attempts at employing the catalysts in classical asymmetric reactions of C-C formation could be of interest.
Herein we describe the use of octahedral stereochemically inert and positively charged “chiral-at-metal”
Co(III) complexes9 (depicted on Fig. 1) of both Λ− and Δ-configurations. The complexes were used as catalysts for
the asymmetric epoxidation of chalcones under phase-transfer conditions and some preliminary results on the
CO2 coupling with the forming epoxides, promoted by the same complexes.
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Fig. 1. The structures of catalysts 1–4 (a: R'=R"=tBu; b: R'=tBu, R"=H; c: R'=Ph, R"=H; d: R'=R"=H).
Results and discussion
The complexes 1-4 were prepared as described earlier.9a As the first substrate/catalyst couple to test the feasibility
of the epoxidation reaction an unsubstituted trans-chalcone 5a and Λ(R,R)-1a were chosen (Scheme 1). Initially, a
30% aqueous solution of hydrogen peroxide, as the oxidizing agent for this reaction, was selected. The first
experiments conducted in CH2Cl2 methyl tert-butyl ether (MTBE) and KOH as a base indicated the reaction was,
indeed, successfully catalyzed by the Co(III) complex, furnishing enantiomerically enriched 6a in a yield of 6581%
and еe of 3850% (Table 1, entry 1). The absolute configuration (2R,3S) of the stereogenic centers in 6a was
assigned by the comparison with the literature data (optical rotation).11 As expected, no reaction was observed
without the base added (Table 1, entry 2), indicating HOO− being the nucleophilic particle, attacking the chalcone
substrate.8
Scheme 1. Asymmetric epoxidation of a chalcone 5a under phase-transfer conditions promoted by the positively
charged chiral complex Λ(R,R)-1a.
KOH as a base and 30% aq. H2O2 in different solvents, were then screened in the reaction (Table 1, entries 1-
8). In each experiment formation of any side products wasn’t observed and the conversion of the initial 5a was
equal to the chemical yield of the epoxide 6a.
Methyl tert-butyl ether (MTBE) was the solvent of choice (Table 1, entry 1) as 6a was obtained in the solvent
in a good yield (81%) and a reasonable enantiomeric purity (ee 50%). The next good solvent was CH2Cl2 which gave
sizable chemical yield (65%) and reasonable enantioselectivity (ee 38%) (Table 1, entry 3). Expectedly, MeCN was a
too polar reaction media with low chemical yields (10%) and enantioselectivity (21%) (Table 1, entry 4). In 1,4-
dioxane the enantioselectivity of the reaction was better (ee 54%), but the conversion was low (38%) (Table 1,
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entry 6). The conversion was good in none polar hexane (67%), but the enantioselectivity of the reaction was low
(8%) with the opposite enantiomer formed in excess (Table 1, entry 7). Both THF and tert-butyl alcohol (Table 1,
entries 8, 9) were bad solvents with no product formed in the reaction medias.
Table 1. Epoxidation of chalcone 5a catalyzed by cationic complex Λ(R,R)-1aa
Entry
Solvent
MTBE
MTBE
Base
Oxidant
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
oxonee
Conversion, %b
ее, %c
50
-
1
KOH
81
-
2
-
3
KOH
65
38
21
48
54
-8d
n.d.
n.d.
n.d.
n.d.
8
CH2Cl2
MeCN
4
KOH
10
5
toluene
KOH
32
6
1,4-dioxane
hexane
THF
KOH
38
7
KOH
67
8
KOH
traces
traces
traces
traces
100
100
50
9
tBuOH
KOH
10
11
12
13
14
15
16
17
18
19
20g
21
MTBE
KOH
MTBE
KOH
3-chloroperbenzoic acide
tBuOOHe
MTBE
KOH
MTBE
KOH
CumOOHe,f
0
MTBE
NaOH
CsOH×H2O
K3PO4
Cs2CO3
CsF
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
30% aq. H2O2
49
55
47
MTBE
73
MTBE
15
MTBE
18
48
50
55
42
52
MTBE
5
MTBE
tBuOK
85
MTBE+ tBuOH
KOH
79
h
MTBE
tBuOK
(NH2)2CO×H2O2
50
a Reaction conditions: 0.13 mmol chalcone 5a, 0.013 mmol Λ(R,R)-1a, 1mL solvent, 1 eq. base (0.13 mmol) and 5
eq. 30% aq. of H2O2 (0.65 mmol) for 48 h at room temperature.
b Conversion determined by NMR.
c Enantiomeric purity was established by chiral HPLC (The absolute configuration of the stereogenic centers were
(2R,3S)).
d The absolute configuration of the stereogenic centers were (2S,3R).
e 3 eq.
f CumOOH = cumene hydroperoxide.
g The ratio of tBuOH/KOH was 1:1
.
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h 2 eq.
The nature of the oxidant was very important, affecting both the yield and the enantioselectivity of the
reaction. For example, oxone or 3-chloroperbenzoic acid were not efficient oxidants (Table 1, entries 10, 11),
whereas both tert-butyl hydroperoxide and cumene hydroperoxide gave the reaction product in good chemical
yields but with very low enantioselectivity (ees 8% and 0% correspondingly) (Table 1, entries 12, 13). Thus, the
optimum oxidizer was found to be hydrogen peroxide.
As could be expected, the nature of the base affected the chemical yield of the reaction and had little if any
influence on its enantioselectivity (Table 1, entries 14-19). The best result (conversion 85% and ee 55%, Table 1,
entry 19) was obtained using potassium tert-butylate. Surprisingly, the mixture of tert-butyl alcohol and KOH,
which should have been formed in the experiment, gave worse chemical yield and ee (Table 1, entry 20). An
attempt at using the complex of urea with hydrogen peroxide to avoid water involvement in the oxidation led to a
decreased chemical yield without any significant changes in the reaction stereoselectivity (Table 1, compare
entries 19 and 1921).
After optimizing the reaction conditions, other catalysts were tested in the same reaction under the optimum
settings (Table 2).
Table 2. Catalyst screeninga
Entry
Catalyst
Λ(R,R)-1a
Λ(R,R)-1ae
Δ(S,S)-1a
Λ(R,R)-1b
Λ(R,R)-1c
Λ(R,R)-1d
Λ(R,R)-2
Λ(S)-3
Conversion, %b
ее, %c
55
1
2
50-85d
31
52
3
84
-55f
4
61
55
5
63
46
6
58
50
7
94
45
8
16
10
9
Λ(R,R)-4
Λ(R,R)-1a
Δ(S,S)-1a
100
84
9
10g
11h
43
54
-49f
a
Reaction conditions: 0.13 mmol chalcone 5a, 0.013 mmol catalyst, 1mL MTBE, 1 eq.
tBuOK (0.13 mmol) and 5 eq. 30% aq. H2O2 (0.65 mmol) for 4 h at room temperature.
b Conversion determined by NMR.
c Enantiomeric purity was established by chiral HPLC analysis of the product (The absolute
configuration of the stereogenic centers were (2R,3S)).
d When playing the experiment conversion varied in the range of 50-85 %.
e The counteranion of Λ(R,R)-1a was I−.
f The absolute configuration of the stereogenic centers were (2S,3R).
g Conducted at 0°C.
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h The recovered catalyst Δ(S,S)-1a was used.
The exchange of the counteranion in 1a (chlorine by iodide) did not change the enantioselectivity of the
reaction but decreased the conversion of the chalcone to 31% (Table 2, compare entries 1 and 2). Changing the
structure of the salicylic moiety of the catalysts had almost no influence on the enantioselectivity of the reaction,
and only affected the yields of the product (Table 2, entries 1, 4-7). But in the case of catalysts based on (S)-
aminomethylpirrolidine and (R,R)-diphenylethylenediamine the enantioselectivity of the reaction dramatically
decreased (Table 2, entries 8, 9). Catalyst 1a having Λ(R,R)-configuration furnished the product of (2R,3S)-
configuration whereas 1a with Δ(S,S)-configuration afforded the (2S,3R)-enantiomer (Table 2, entries 1 and 3). It
was also shown that lowering the reaction temperature negatively affected the enantioselectivity of the reaction.
Thus at 0oC ee of the product was 43% at a conversion of 84% (Table 2, compare entries 1 and 10). The catalyst 1a
could be regenerated and reused. The results were slightly worse than the initial runs (conversion 54%, ee 49%)
(Table 2, compare entries 3 and 11).
We next investigated the scope of the epoxidation of different chalcone derivatives promoted with catalyst
Δ(S,S)-1a. The data is summarized in Table 3.
Table 3. Chalcone substrate scope of epoxidation catalyzed by cationic complex of Co(III) Δ(S,S)-1aa
Entry Substrate
Ar1
Ph
Ar2
Ph
Conversion, %b
ee, %c
55
1
2
5a
5b
5c
5d
5e
5f
85
98
4-NO2C6H4
4-ClC6H4
2-ClC6H4
4-FC6H4
Ph
48
3
Ph
85
49
4
Ph
50
50
5
Ph
89
50
6
3-FC6H4
Ph
100
96
49
7
5g
5h
5i
2-FC6H4
Ph
44
8
4-MeC6H4
4-MeOC6H4
3-PhOC6H4
1-naphthalenyl
2-naphthalenyl
9-anthracenyl
2-pyridinyl
2-furanyl
Ph
Ph
78
47
9
Ph
62
48
10
11
12
13
14
15
16
17
5j
Ph
90
42
5k
5l
Ph
96
35
Ph
47
48
52 (98)d
5m
5n
5o
5p
5q
Ph
99
Ph
100
50
8
Ph
n.d.
42
2-pyridinyl
100
traces
Ph
C6F5
-
a Reaction conditions: 0.13 mmol chalcone, 10 mg (0.013 mmol) catalyst, 1mL MTBE, 1 eq.
tBuOK (0.13 mmol) and 5 eq. 30% aq. H2O2 (0.65 mmol) for 4 h at room temperature.
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b Conversion determined by NMR.
c Enantiomeric purity was established by chiral HPLC analysis of the product (The absolute
configuration of the stereogenic centers were (2S,3R)).
d Enantiomeric purity after recrystallization.
Basically, the nature of the substituents on the phenyl ring of 5 only affected the conversion of the chalcones,
whereas the enantiomeric purity of the products was mostly insignificantly influenced (the range of ee lied mostly
within the range of 42-55%, Table3, entries 1-10). The cases of chalcone derivatives 5k and 5n were an exception
as the enantiomeric purity of epoxide 6k was low (35%) and that of 6n was only 8%. In case of perfluorinated
chalcone 5q only trace amounts of 6q were detected in the reaction mixture.
The enantiomeric purities of the epoxides could be further increased. For example, the enantiomeric purity of
6m was amplified to 98% after its recrystallization and its absolute configuration (2S,3R)-6m was established by a
single crystal X-ray structure analysis (Fig. 2) and found to be similarly to that of other epoxides.
Fig. 2. ORTEP drawing of the X-ray structure of 6m with thermal ellipsoids at the 50% probability level.
The catalyst 1a could also catalyze the coupling of racemic epoxide 6a with CO2 (Scheme 2).
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Scheme 2. Synthesis of organic carbonate 7 promoted by a chiral complex of Co(III) Δ(S,S)-1a I-.
Using the catalyst Δ(S,S)-1a with chlorine counteranion we observed only the formation of the polyether and
not the cyclic carbonate 7. The use of Δ(S,S)-1a with a more nucleophilic iodide counteranion was successful as
the conversion of the chalcone 6a to the corresponding cyclic carbonate 7 was 69%. The remaining epoxide was
partially enantiomerically enriched (ee of (2R,3S)-enantiomer 26% in acetonitrile and 50% in toluene). The
hydrolysis of the carbonate would lead to the corresponding chiral diols.12 To the best of our knowledge, it was the
first example of the preparation of the cyclic carbonate 7 using such approach.
Experimental
General
Nuclear magnetic resonance spectra were recorded on a Bruker Avance 300 and Bruker Avance III-400 (operating
1
1
13
at 300/75 for H, 13C and 400/100/282 MHz for H, C and 19F NMR respectively) NMR spectrometers. Chemical
shifts (on the δ scale) are reported in ppm relative to the residual solvent peak (CHCl3: δ = 7.26 ppm for 1H NMR, δ
= 77.16 for 13C NMR) and relative to CFCl3 (δ = 0.0 ppm for 19F NMR). NMR data are reported as follows: chemical
shift, multiplicity (s = singlet, d = doublet, t= triplet, m = multiplet), coupling constant, integration, nucleus.
Melting points were taken using Electrothermal MELTING POINT APPARATUS (England). Optical rotations were
measured on a Perkin–Elmer 341 polarimeter in a thermostated at 25°C 5-cm cell. Enantiomeric excesses were
determined by HPLC analysis using a Shimadzu LC-10 ADVP equipped with a Shimadzu SPD-M10AVP diode array
detector, eluent and chiral column are indicated to any determination, flow rate: 1 mL/min. Racemic products for
reference purposes were synthesized using achiral catalysts (quaternary ammonia salts). X-ray crystallography
diffraction data was collected on a Bruker SMART APEX II diffractometer, λ(CuKα) = 1.54190 Å, ω-scan technique, T
= 120(2) K. Commercial reagents were used as received unless stated otherwise. Column chromatography was
performed using Silica Gel Kieselgel 60 (Merck).
General procedure for the preparation of α,β-unsaturated ketones
Corresponding substituted benzaldehyde (4.2 mmol) was dissolved in methanol (10 mL) and the resulting solution
was cooled down to 0 °C. Then aqueous NaOH solution (10% wt., 2.2 mL) was added dropwise, followed by slow
addition of the corresponding ketone (4.2 mmol). The reaction mixture was stirred allowing to slowly warm up to
room temperature (2-24 h). After the addition was complete the product precipitated as a powder. The pure
chalcone derivatives were filtered and the solid recrystallized from ethanol in high yields.13a Most products
obtained were known compounds.
o
1
(E)-chalcone 5a. Yellow powder; mp 56–58 C; H NMR (300 MHz, CDCl3) δ 7.96 (d, J=7.1 Hz, 2H), 7.75 (d,
13
J=15.7 Hz, 1H), 7.59-7.56 (m, 2H), 7.53-7.41 (m, 4H), 7.37-7.34 (m, 3H). C NMR (75 MHz, CDCl3) δ 190.6 (C=O),
144.9 (-CH=), 138.2, 134.9, 132.8, 130.5, 129.0, 128.9, 128.6, 128.4, 122.1 (-CH=).
(E)-3-(4-nitrophenyl)-1-phenylprop-2-en-1-one 5b. Yellow powder; mp 152–154 °C (lit.13a 158–159 °C); H
1
NMR (400 MHz, CDCl3) δ 8.28 (d, J=8.5 Hz, 2H), 8.04 (d, J=8.2 Hz, 2H), 7.86-7.76 (m, 3H), 7.68-7.60 (m, 2H), 7.54 (t,
J=7.8 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 189.7 (C=O), 148.5, 141.5 (-CH=), 141.0, 137.5, 133.4, 129.0, 128.8,
128.6, 125.7, 124.2 (-CH=).
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(E)-3-(4-chlorophenyl)-1-phenylprop-2-en-1-one 5c. White flaky solid; mp 105–107 °C (lit.13b 105–107 °C); 1H
NMR (400 MHz, CDCl3) δ 8.02 (d, J=7.5 Hz, 2H), 7.76 (d, J=15.7 Hz, 1H), 7.65–7.47 (m, 6H), 7.41-7.38 (m, 2H). 13C
NMR (100 MHz, CDCl3) δ 190.2 (C=O), 143.3 (-CH=), 138.0, 136.4, 133.4, 132.9, 129.6, 129.3, 128.7, 128.5, 122.5 (-
CH=).
1
(E)-3-(2-chlorophenyl)-1-phenylprop-2-en-1-one 5d. White flaky solid; mp 47–49 °C (lit.13c 48 °C); H NMR
(400 MHz, CDCl3) δ 8.19 (d, J=15.8 Hz, 1H), 8.06–7.99 (m, 2H), 7.78-7.72 (m, 1H), 7.63-7.56 (m, 1H), 7.55–7.41 (m,
4H), 7.37–7.28 (m, 2H). 13C NMR (100 MHz, CDCl3) δ 190.5 (C=O), 140.7 (-CH=), 138.0, 135.5, 133.3, 133.0, 131.2,
130.3, 128.7, 128.6, 127.8, 127.1, 124.9 (-CH=).
1
(E)-3-(4-fluorophenyl)-1-phenylprop-2-en-1-one 5e. White flaky powder; mp 79–81 °C (lit.13b 75–78 °C); H
NMR (400 MHz, CDCl3) δ 8.05–7.98 (m, 2H), 7.78 (d, J=15.7 Hz, 1H), 7.68–7.56 (m, 3H), 7.55–7.43 (m, 3H), 7.12 (t,
1
J=8.6 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 190.3 (C=O), 165.5, 162.8 164.2 (d, JC-F=252.2 Hz, Ar), 143.5 (br.s., -
4
3
5
CH=), 138.1, 132.9, 131.1 (d, JC-F=2.8 Hz, Ar), 130.4 (d, JC-F=8.5 Hz, Ar), 128.7, 128.5, 121.8 (d, JC-F=2.0 Hz,-CH=),
116.1 (d, 2JC-F=22.0 Hz, Ar). 19F NMR (377 MHz, CDCl3) δ -31.35 (s, 1F).
(E)-3-(3-fluorophenyl)-1-phenylprop-2-en-1-one 5f13d. White flaky powder; mp 43–45 °C; 1H NMR (400 MHz,
CDCl3) δ 8.05-7.99 (m, 2H), 7.76 (d, J=15.7 Hz, 1H), 7.63-7.47 (m, 4H), 7.44–7.31 (m, 3H), 7.14-7.08 (m, 1H). 13C
1
NMR (100 MHz, CDCl3) δ 190.2 (C=O), 164.3, 161.9 163.1 (d, JC-F=246.9 Hz, Ar), 143.3 (d, J=2.6 Hz, -CH=), 138.0,
3
3
4
137.2 (d, JC-F=7.6 Hz, Ar), 133.0, 130.5 (d, JC-F=8.2 Hz, Ar), 128.7, 128.5, 124.5 (d, JC-F=2.7 Hz, Ar), 123.2 (-CH=),
117.3 (d, 2JC-F=22.0 Hz, Ar), 114.4 (d, 2JC-F=22.0 Hz, Ar). 19F NMR (377 MHz, CDCl3) δ -34.74 (s, 1F).
(E)-3-(2-fluorophenyl)-1-phenylprop-2-en-1-one 5g. White flaky powder; mp 35–37 °C (lit.13e 36 °C); 1H NMR
(400 MHz, CDCl3) δ 8.08–7.98 (m, 2H), 7.91 (d, J=15.9 Hz, 1H), 7.73–7.56 (m, 3H), 7.51 (t, J=7.5 Hz, 2H), 7.42–7.33
(m, 1H), 7.19 (t, J=7.5 Hz, 1H), 7.16–7.07 (m, 1H). 13C NMR (100 MHz, CDCl3) δ 190.5 (C=O), 163.3, 160.5 161.7 (d,
3
3
1JC-F=253.2 Hz, Ar), 138.0, 137.5 (-CH=), 132.9, 131.9 (d, JC-F=8.9 Hz, Ar), 129.8 (d, JC-F=2.8 Hz, Ar), 128.7, 128.6,
124.7 (d, 4JC-F=7.0 Hz, Ar), 124.5 (d, J=4.0 Hz, -CH=), 123.0 (d, 2JC-F=11.4 Hz, Ar), 116.2 (d, 2JC-F=22.0 Hz, Ar). 19F NMR
(377 MHz, CDCl3) δ -35.66 (s, 1F).
1
(E)-1-phenyl-3-p-tolylprop-2-en-1-one 5h. White flaky powder; mp 88–90 °C (lit.13c 90-92 °C); H NMR (400
MHz, CDCl3) δ 8.07–7.96 (m, 2H), 7.80 (d, J=15.7 Hz, 1H), 7.63–7.44 (m, 6H), 7.23 (d, J=7.8 Hz, 2H), 2.40 (s, 3H). 13C
NMR (100 MHz, CDCl3) δ 190.7 (C=O), 145.0 (-CH=), 141.1, 138.4, 132.7, 132.2, 129.7, 128.6, 128.5, 121.1 (-CH=),
21.6 (-CH3).
1
(E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one 5i. White flaky powder; mp 63–65 °C (lit.13f 63-65 °C); H
NMR (400 MHz, CDCl3) δ 8.01 (d, J=7.5 Hz, 2H), 7.79 (d, J=15.6 Hz, 1H), 7.69–7.46 (m, 5H), 7.42 (d, J=15.6 Hz, 1H),
13
6.93 (d, J=8.3 Hz, 2H), 3.85 (s, 3H). C NMR (100 MHz, CDCl3) δ 190.6 (C=O), 161.7 (ArC-OMe), 144.7 (-CH=), 138.5,
132.6, 130.3, 128.6, 128.4, 127.6, 119.8 (-CH=), 114.4 (Aro-C-COMe), 55.4 (-OCH3).
1
(E)-3-(3-phenoxyphenyl)-1-phenylprop-2-en-1-one 5j. White flaky powder; mp 59–61 °C (lit.13g 62-64 °C); H
NMR (400 MHz, CDCl3) δ 8.03-7.98 (m, 2H), 7.75 (d, J=15.7 Hz, 1H), 7.61–7.56 (m, 1H), 7.54–7.45 (m, 3H), 7.41–
7.34 (m, 4H), 7.29 (s, 1H), 7.15 (t, J=7.4 Hz, 1H), 7.09-7.02 (m, 3H). 13C NMR (100 MHz, CDCl3) δ 190.4 (C=O), 157.9
(ArC-OPh), 156.8 (ArC-OPh), 144.1 (-CH=), 138.1, 136.8, 132.9, 130.3, 130.0, 128.7, 128.5, 123.7, 123.5, 122.8 (-CH=),
120.8, 119.1, 118.2.
1
(E)-3-(naphthalen-1-yl)-1-phenylprop-2-en-1-one 5k. Yellow powder; mp 78–80 °C (lit.13h 78-79 °C); H NMR
(400 MHz, CDCl3) δ 8.69 (d, J=15.4 Hz, 1H), 8.27 (d, J=8.3 Hz, 1H), 8.10 (d, J=7.3 Hz, 2H), 7.96-7.87 (m, 3H), 7.68-
7.49 (m, 7H). 13C NMR (100 MHz, CDCl3) δ 190.3 (C=O), 141.8 (-CH=), 138.2, 133.8, 132.9, 132.4, 131.8, 130.8,
128.8, 128.7, 128.6, 127.0, 126.3, 125.5, 125.1, 124.7, 123.5 (-CH=).
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(E)-3-(naphthalen-2-yl)-1-phenylprop-2-en-1-one 5l. White flaky powder; mp 159–161 °C (lit.13i 158-160 °C);
1H NMR (400 MHz, CDCl3) δ 8.10-8.03 (m, 3H), 7.99 (d, J=15.7 Hz, 1H), 7.93–7.83 (m, 3H), 7.83–7.78 (m, 1H), 7.66
13
(d, J=15.7 Hz, 1H), 7.63–7.58 (m, 1H), 7.57-7.50 (m, 4H). C NMR (100 MHz, CDCl3) δ 190.6 (C=O), 145.0 (-CH=),
138.3, 134.4, 133.4, 132.8, 132.4, 130.7, 128.8, 128.7, 128.5, 127.8, 127.4, 126.8, 123.7 (-CH=), 122.2.
(E)-3-(anthracen-9-yl)-1-phenylprop-2-en-1-one 5m. Yellow powder; mp 115–117 °C (lit.13j 122-123 °C); H
1
NMR (400 MHz, CDCl3) δ 8.81 (d, J=15.9 Hz, 1H), 8.49 (s, 1H), 8.31 (d, J=9.0 Hz, 2H), 8.12-8.02 (m, 4H), 7.66–7.47
13
(m, 8H). C NMR (100 MHz, CDCl3) δ 189.7 (C=O), 141.9, 137.9, 133.1 (-CH=), 131.3, 131.1, 130.2, 129.7, 128.9,
128.8, 128.7, 128.4 (-CH=), 126.5, 125.4, 125.3.
(E)-1-phenyl-3-(pyridin-2-yl)prop-2-en-1-one 5n. Yellow powder; mp 56–58 °C (lit.13k 60-61 °C); 1H NMR (400
MHz, CDCl3) δ 8.69 (d, J=4.0 Hz, 1H), 8.18 (d, J=15.3 Hz, 1H), 8.14–8.06 (m, 2H), 7.83–7.71 (m, 2H), 7.64–7.55 (m,
1H), 7.54–7.45 (m, 3H), 7.32 (dd, J=7.0, 5.2 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 190.3 (C=O), 152.9 (ArC-CH=), 149.7
(Aro-C-N), 142.1 (-CH=), 137.7, 137.5, 133.2, 128.8, 128.7, 126.2 (-CH=), 125.6, 124.5.
1
(E)-3-(furan-2-yl)-1-phenylprop-2-en-1-one 5o13l. Brown oil; H NMR (400 MHz, CDCl3) δ 8.07–8.07 (m, 2H),
7.65–7.45 (m, 6H), 6.73 (d, J=3.3 Hz, 1H), 6.52 (dd, J=3.3, 1.8 Hz, 1 H). 13C NMR (CDCl3, 75 MHz): δ 189.8 (C=O),
151.7, 145.0, 138.1, 132.8, 130.7, 128.6, 128.5, 119.3 (-CH=), 116.3 (-CH=), 112.7.
(E)-3-phenyl-1-(pyridin-2-yl)prop-2-en-1-one 5p. White powder; mp 68–71 °C (lit.13k 71-72 °C); 1H NMR (300
MHz, CDCl3) δ 8.75 (d, J=4.5 Hz, 1H), 8.32 (d, J=16.0 Hz, 1H), 8.20 (d, J=7.8 Hz, 1H), 7.95 (d, J=16.0 Hz, 1H), 7.86 (td,
13
J=7.7, 1.7 Hz, 1H), 7.77–7.70 (m, 2H), 7.57–7.34 (m, 4H). C NMR (75 MHz, CDCl3) δ 189.2 (C=O), 154.3 (ArC-C=O),
148.7 (-CH=), 145.0 (Aro-C-N), 137.4, 135.0, 130.7, 128.9, 127.0, 123.1 (-CH=), 120.8.
(E)-1-(perfluorophenyl)-3-phenylprop-2-en-1-one 5q. White powder; mp 97–99 °C (lit.13m 98-100 °C); H
1
NMR (400 MHz, CDCl3) δ 7.64-7.56 (m, 2 H), 7.53 (d, J=15.9 Hz, 1H), 7.49-7.38 (m, 3H), 7.03 (d, J=16.0 Hz, 1H). 13C
2
1
NMR (100 MHz, CDCl3) δ 183.8 (C=O), 148.3 (-CH=), 144.3 (dm, JC-F=252.4 Hz, Ar), 142.8 (dm, JC-F=257.5 Hz, Ar),
3
137.90 (dm, JC-F=256.7 Hz, Ar), 133.6, 131.8, 129.2, 128.9, 126.1 (-CH=), 115.2-114.5 (m, Ar). 19F NMR (282 MHz,
CDCl3) δ -140.46 (d, J=16.3 Hz, 2F), -150.17 (t, J=20.3 Hz, 1F), -159.88 (t, J=17.5 Hz, 2F).
General procedure for the epoxidation of α,β-unsaturated ketones
The flasks were evacuated while being heated with a heat gun. The flasks were then cooled to room
temperature under an argon flow. To a mixture of catalyst 1a (10 mg10 mol%, 0.013 mmol), chalcone (0.13 mmol)
t
and BuOK base (14,6 mg1 eq, 0.13 mmol) in methyl tert-butyl ether (1 mL) 1 mL solvent was added hydrogen
peroxide oxidizing agent (0.053 mL, 0.65 mmol2, 3 or 5 eq; 0.26, 0.39 or 0.65 mmol) at a room temperature. The
resulting reaction mixture was stirred vigorously for 4 h at theroom temperature. Purification of the reaction
mixture by flash chromatography on silica (diethyl ether) afforded the desired product as a powder. The
enantioselectivity was determined by chiral HPLC analysis of the epoxide. The absolute configuration was
determined by comparison with the literature data (optical rotation).11
(2S,3R)-Phenyl-3-phenyloxiran-2-yl)methanone 6a. White powder; 1H NMR (400 MHz, CDCl3) δ 8.01 (d, J=7.5
Hz, 2H), 7.62 (t, J=7.4 Hz, 1H), 7.49 (t, J=7.7 Hz, 2H), 7.46–7.31 (m, 5H), 4.31 (d, J=1.8 Hz, 1H), 4.08 (d, J=1.6 Hz,
1H). 13C NMR (75 MHz, CDCl3) δ 192.0 (C=O), 134.4, 133.0, 128.2, 127.6, 127.3, 124.8, 60.0 (Cepox), 58.4 (Cepox).
HPLC (Kromasil 3-AmyCoat column; 25oC, hexane/isopropanol 95:5, λ = 254 nm; tR = 10.1 (major), 10.9 min.
1
((2S,3R)-3-(4-nitrophenyl)oxiran-2-yl)(phenyl)methanone 6b. White yellow powder; H NMR (400 MHz,
CDCl3) δ 8.27 (d, J=8.7 Hz, 2H), 8.01 (d, J=7.6 Hz, 2H), 7.65 (t, J=7.4 Hz, 1H), 7.60–7.47 (m, 4H), 4.28 (d, J=1.6 Hz,
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1H), 4.21 (d, J=1.6 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 192.1 (C=O), 148.4, 142.8, 135.2, 134.4, 129.0, 128.4,
126.7, 124.1, 60.9 (Cepox), 58.0 (Cepox). HPLC (Kromasil 3-AmyCoat column; 25oC, hexane/isopropanol 95:5, λ = 254
nm; tR = 36.2 (major), 50.1 min.
1
((2S,3R)-3-(4-chlorophenyl)oxiran-2-yl)(phenyl)methanone 6c. Yellow powder; H NMR (400 MHz, CDCl3) δ
8.00 (d, J=7.5 Hz, 2H), 7.63 (t, J=7.4 Hz, 1H), 7.49 (t, J=7.7 Hz, 2H), 7.37 (d, J=8.4 Hz, 2H), 7.30 (d, J=8.4 Hz, 2H),
4.25 (d, J=1.3 Hz, 1H), 4.06 (d, J=1.3 Hz, 1H). HPLC (Chiralpak AD column; 25oC, heptane/isopropanol 90:10, λ =
254 nm; tR = 12.5 (major), 14.2 min.
1
((2S,3R)-3-(2-chlorophenyl)oxiran-2-yl)(phenyl)methanone 6d. White yellow powder; H NMR (400 MHz,
CDCl3) δ 8.07-8.05 (dd, J=8.4, 1.2 Hz, 2H), 7.66-7.62 (t, J=10.0 Hz, 1H), 7.53-7.49 (t, J=7.2Hz, 2H), 7.42-7.40 (m, 2H),
7.34-7.32 (m, 2H), 4.43 (d, J=1.8 Hz, 1H), 4.20 (d, J=1.8 Hz, 1H). HPLC (Chiralpak AD column; 25oC,
heptane/isopropanol 90:10, λ = 254 nm; tR = 8.3, 8.7 (major) min.
1
((2S,3R)-3-(4-fluorophenyl)oxiran-2-yl)(phenyl)methanone 6e. Yellow powder; H NMR (400 MHz, CDCl3) δ
8.05-8.00 (m, 2H), 7.69-7.61 (m, 1H), 7.56-7.48 (m, 2H), 7.41-7.34 (m, 2H), 7.16-7.07 (m, 2H), 4.29 (d, J=0.7 Hz,
19
1H), 4.09 (d, J=0.4 Hz, 1H). F NMR (377 MHz, CDCl3) δ -112.15 (s, 1F). HPLC (Kromasil 3-AmyCoat column; 25oC,
hexane/isopropanol 95:5, λ = 254 nm; tR = 11.2 (major), 11.8 min.
1
((2S,3R)-3-(3-fluorophenyl)oxiran-2-yl)(phenyl)methanone 6f. Yellow powder; H NMR (400 MHz, CDCl3) δ
8.04-7.98 (m, 2H), 7.65-7.60 (m, 1H), 7.53-7.46 (m, 2H), 7.41-7.33 (m, 1H), 7.17 (d, J=7.6 Hz, 1H), 7.10-7.03 (m,
2H), 4.26 (d, J=1.4 Hz, 1H), 4.08 (d, J=1.4 Hz, 1H). 19F NMR (377 MHz, CDCl3) δ -34.34 (s, 1F). HPLC (Kromasil 3-
AmyCoat column; 25oC, hexane/isopropanol 95:5, λ = 254 nm; tR = 9.9, 11.9 (major) min.
1
((2S,3R)-3-(2-fluorophenyl)oxiran-2-yl)(phenyl)methanone 6g. Yellow powder; H NMR (400 MHz, CDCl3) δ
8.05-8.00 (m, 2H), 7.65-7.58 (m, 1H), 7.55-7.47 (m, 2H), 7.40-7.32 (m, 2H), 7.24-7.16 (m, 1H), 7.14-7.06 (m, 1H),
4.35 (d, J=1.7 Hz, 1H), 4.30 (d, J=1.7 Hz, 1H). 19F NMR (377 MHz, CDCl3) δ -42.36 (s, 1F). HPLC (Chiralpak AS-H
column; 25oC, heptane/isopropanol 90:10, λ = 254 nm; tR = 10.3 (major), 23.0 min.
(2S,3R)-phenyl(3-p-tolyloxiran-2-yl)methanone 6h. White flaky powder; 1H NMR (400 MHz, CDCl3) δ 8.01 (d,
J=7.2 Hz, 2H), 7.62 (t, J=7.2 Hz, 1H), 7.48 (t, J=7.2 Hz, 2H), 7.28-7.21 (m, 4H), 4.30 (d, J=1.6 Hz, 1H), 4.05 (d, J=1.6
Hz, 1H), 2.38 (s,3H). HPLC (Kromasil 3-AmyCoat column; 25oC, heptane/isopropanol 90:10, λ = 254 nm; tR = 6.2
(major), 6.5 min.
((2S,3R)-3-(4-methoxyphenyl)oxiran-2-yl)(phenyl)methanone 6i. White powder; 1H NMR (400 MHz, CDCl3) δ
8.01 (d, J=7.2 Hz, 2H), 7.62-7.59 (m, 1H), 7.49-7.46 (m, 2H), 7.29 (d, J=8.8 Hz, 2H), 6.93 (d, J=8.8 Hz, 2H), 4.29 (d,
J=1.7 Hz, 1H), 4.02 (d, J=1.2 Hz, 1H), 3.82 (s, 3H). HPLC (Chiralpak AD column; 25oC, hexane/isopropanol 90:10, λ =
254 nm; tR = 14.0 (major), 17.9 min.
1
((2S,3R)-3-(3-phenoxyphenyl)oxiran-2-yl)(phenyl)methanone 6j. White solid; H NMR (400 MHz, CDCl3) δ
8.03-7.99 (m, 2H), 7.65-7.60 (m, 1H), 7.53-7.47 (m, 2H), 7.39-7.33 (m, 3H), 7.14-7.09 (m, 2H), 7.06-6.98 (m, 4H),
4.26 (d, J=1.6 Hz, 2H), 4.05 (1H, d, J=1.6 Hz, 2H). HPLC (Chiralpak IA column; 25oC, hexane/isopropanol 98:2, λ =
254 nm; tR = 22.4, 30.3 (major) min.
1
((2S,3R)-3-(naphthalen-1-yl)oxiran-2-yl)(phenyl)methanone 6k. White solid; H NMR (400 MHz, CDCl3) δ
8.13-8.08 (m, 2H), 8.05-8.00 (m, 1H), 7.97-7.88 (m, 2H), 7.68-7.63 (m, 1H), 7.59-7.49 (m, 6H), 4.76 (d, J=1.9 Hz,
1H), 4.34 (d, J=1.9 Hz, 1H). HPLC (Kromasil 3-AmyCoat column; 25oC, hexane/isopropanol 90:10, λ = 254 nm; tR =
5.9 (major), 6.4 min.
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((2S,3R)-3-(naphthalen-2-yl)oxiran-2-yl)(phenyl)methanone 6l. White solid; 1H NMR (400 MHz, CDCl3) δ 8.03
(d, J=8.0 Hz, 1H), 7.91–7.84 (m, 3H), 7.63–7.50 (m, 8H), 4.41 (d, J=1.8 Hz, 1H), 4.25 (d, J=1.6 Hz, 1H). HPLC
(Kromasil 3-AmyCoat column; 25oC, hexane/isopropanol 95:5, λ = 254 nm; tR = 10.4, 11.2 (major) min.
27
((2S,3R)-3-(anthracen-9-yl)oxiran-2-yl)(phenyl)methanone 6m. Yellow powder; [α]D = -115.9 (c 0.073,
1
CDCl3) at 98% ee; H NMR (400 MHz, CDCl3) δ 8.49 (s, 1H), 8.46 (d, J=8.3 Hz, 2H), 8.25 (d, J=7.5 Hz, 2H), 8.04 (d,
J=7.8 Hz, 2H), 7.71 (t, J=7.4 Hz, 1H), 7.58 (t, J=7.6 Hz, 2H), 7.55–7.46 (m, 4H), 5.01 (d, J=1.6 Hz, 1H), 4.49 (d, J=1.6
Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 194.5 (C=O), 135.6, 134.3, 131.1, 130.1, 129.14–128.83 (m, 3C), 128.6, 126.6,
126.5, 125.3, 124.5, 58.5 (Cepox), 58.0 (Cepox). HPLC (Kromasil 3-AmyCoat column; 25oC, hexane/isopropanol 95:5, λ
= 254 nm; tR = 9.6, 12.5 (major) min.
1
(2S,3R)-phenyl(3-(pyridin-2-yl)oxiran-2-yl)methanone 6n. Yellow powder; H NMR (400 MHz, CDCl3) δ 8.62
(d, J=4.2 Hz, 1H), 8.04-8.00 (m, 2H), 7.73 (td, J=7.7, 0.9 Hz, 1H), 7.62-7.56 (m, 1H), 7.51-7.44 (m, 2H), 7.39 (d, J=8.0
Hz, 1H), 7.32-7.27 (m, 1H), 4.57 (d, J=0.9 Hz, 1H), 4.20 (d, J=1.1 Hz, 1H). HPLC (Kromasil 3-AmyCoat column; 25oC,
hexane/isopropanol 90:10, λ = 254 nm; tR = 13.1, 14.5 (major)min.
((2S,3R)-(3-(furan-2-yl)oxiran-2-yl)(phenyl)methanone 6o. Yellow powder; 1H NMR (400 MHz, CDCl3) δ 8.07-
8.01 (m, 2H), 7.65-7.43 (m, 4H), 6.59 (d, J=3.3 Hz, 1H), 6.41 (dd, J=3.2, 1.8 Hz, 1H), 4.79 (d, J=2.0 Hz, 1H), 4.10 (d,
J=1.9 Hz, 1H). The absolute configuration was not determined.
1
((2S,3R)-3-phenyloxiran-2-yl)(pyridin-2-yl)methanone 6p. Yellow powder; H NMR (400 MHz, CDCl3) δ 8.68-
8.65 (m, 1H), 8.12-8.08 (m, 1H), 7.88 (td, J=7.7, 1.7 Hz, 1H), 7.54–7.49 (m, 1H), 7.41–7.35 (m, 5H), 5.12 (d, J=1.7
Hz, 1H), 4.09 (d, J=1.7 Hz, 1H). HPLC (Chiralcel OJ column; 25oC, hexane/isopropanol 80:20, λ = 220 nm; tR = 15.1,
16.0 (major) min.
Coupling reaction of CO2 and epoxide 6a
Typically, the autoclave was charged with 0.1 equiv. of catalyst Δ(S,S)-1a I−, 1.0 equiv. of epoxide 6a and were
dissolved in acetonitrile or in toluene (0.1 mL). The reactor was purged twice with CO2, pressurized with CO2 to 50
bars, heated to 50oC and stirred for 24 h. Subsequently, the reactor was cooled to ambient temperature and CO2
was released slowly. The reaction mixture was filtered over SiO2 (CH2Cl2), and the volatiles were removed under
reduced pressure.
4-benzoyl-5-phenyl-1,3-dioxolan-2-one 714. White powder; 1H NMR (400 MHz, CDCl3) δ 7.96 (d, J=7.5 Hz, 2H),
7.66 (t, J=7.5 Hz, 1H), 7.55–7.43 (m, 7H), 5.98 (d, J=6.2 Hz, 1H), 5.60 (d, J=6.2 Hz, 1H).
X-ray diffraction study of 6m
Crystals of 6m (C23H16O2, M = 324.36) are orthorhombic, space group P212121, at 120K a = 5.0152(7), b =
15.450(2), c = 20.528(3) Å, V = 1590.6(4) Å3, Z = 4, dcalc. = 1.354 g/cm3,
out with a Bruker SMART APEX II diffractometer,
independent reflections (Rint = 0.0528) with 2.0° < 2
ꢀ
= 0.676 mm-1. Data collection was carried
(CuKα) = 1.54190 Å, ω-scan technique, T = 120(2) K, 2580
< 130.0° collected and used in refinement. The APEX II
λ
θ
software15 was used for collecting frames of data, indexing reflections, determination of lattice constants,
integration of intensities of reflections, scaling and absorption correction, and SHELXTL16 for space group and
structure determination, refinements, graphics, and structure reporting. The structures were solved by direct
methods and refined by the full matrix least-squares technique against F2 with the anisotropic thermal parameters
for all non-hydrogen atoms. The hydrogen atoms were placed geometrically and included in the structure factors
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calculation in the riding motion approximation. The refinement converged to wR2 = 0.2060 and GOF = 1.001 for all
independent reflections (R1 = 0.0672 was calculated against F for 2508 observed reflections with I > 2σ(I)). CCDC
1407467 contains the supplementary crystallographic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.
Recovering of the catalyst Δ(S,S)-1a. After flash chromatography the remaining SiO2 with the adsorbed
catalyst was separated and stirred in a biphasic mixture of saturated aqueous solution of NaCl and CH2Cl2. Then
the solution was filtered, the solid washed with methanol and the combined organic solutions were evaporated.
The resulting mixture was purified on SiO2 and the recovered catalyst was isolated.9a
Conclusions
In this study, it was shown that chiral stereochemically inert octahedral stereochemically inert and positively
charged “chiral-at-metal” Co(III) complexes efficiently and stereoselectively catalyzed the epoxidation reaction of
various chalcones. Also, using the same type of complexes of Co(III) as the catalyst, we were able to couple the
resulting epoxide 6a with CO2. At present, work on CO2 fixation and study of the mechanism of this reaction are
ongoing.
Acknowledgements
The authors gratefully acknowledge the financial support from a RFBR research grants No 14-03-31262 young a
and No 15-03-05648 A. The authors are also grateful to Dr. Michail M. Il’yin for HPLC analysis.
Notes and references
aNesmeyanov Institute of Organoelement Compounds, Vavilov Str. 28, 119991 Moscow, Russia
E-mail: yubel@ineos.ac.ru; Tel.: (+7) 499-135-6356
bD.Mendeleev University of Chemistry Technology of Russia, Miusskaya sq. 9, 125047 Moscow, Russia
‡ CCDC 1407467. For crystallographic data in CIF see DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C5RA11760G
catalyst
tBu
tBu
H
H
O
N
O
O
Cl
cat. 1a 10 mol %
O
N
Co
Ar1
Ar2
Ar1
Ar2
N
30% aq. H2O2
1 eq. tBuOK
MTBE
O
H
tBu
5a-5q
N
(2S,3R)-6a-6q
ee up to 55%
tBu
H
Ar, r.t.
(S,S)-1a