Asymmetric aldol condensation in an ionic liquid-water system catalyzed by (S)-prolinamide derivatives.

Article abstract of DOI:10.1007/s11172-008-0092-x

Asymmetric aldol reaction of unmodified aldehydes with ketones catalyzed by 1(R),2(R)-bis((S)-prolinamido)cyclohexane (1) or (Rax)-2,2'-bis((S)- prolinamido)-1,1'-binaphtyl (2) proceeds with high yield (68-99%) and diastereoselectivity (dr = 75/25) in the

Full text of DOI:10.1007/s11172-008-0092-x

Russian Chemical Bulletin, International Edition, Vol. 57, No. 3, pp. 591—594, March, 2008
591
Asymmetric aldol condensation in an ionic liquidꢀwater system
catalyzed by (S)ꢀprolinamide derivatives
A. S. Kucherenko,★ D. E. Syutkin, and S. G. Zlotin
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Eꢀmail: Alexkucherenko@yandex.ru
Asymmetric aldol reaction of unmodified aldehydes with ketones catalyzed by 1(R),2(R)ꢀbis((S)ꢀ
prolinamido)cyclohexane (1) or (Rax)ꢀ2,2´ꢀbis((S)ꢀprolinamido)ꢀ1,1´ꢀbinaphtyl (2) proceeds with
high yield (68—99%) and diastereoselectivity (dr ≥ 75/25) in the system (1ꢀbutylꢀ3ꢀmethylimidaꢀ
zolium) tetrafluoroborate ([bmim][BF4])—water. The dependence of ее of the dominating antiꢀdiaꢀ
stereomer of aldol on the percentage of water has a maximum at 50 vol.%. Catalyst 1 can be recycled
5 times without losses in the aldol yield, dr and ее.
Key words: organocatalysis, asymmetric aldol reaction, ionic liquids, water.
Asymmetric aldol condensation of unmodified alꢀ
dehydes with ketones is used for the preparation
of βꢀhydroxy carbonyl compounds, valuable synthons
in organic synthesis.1a—c Natural amino acids (proline,2a
in organic solvents.11 (1ꢀButylꢀ3ꢀmethylimidazolium)
tetrafluoroborate ([bmim][BF4]) was used as the ionic
liquid.
tryptophan,2b etc.2c) and their derivatives (amides3a—g
,
diꢀ and tripeptides,4 etc.) are used as catalysts. The reꢀ
action is usually carried out in the excess of a ketone,
dipolar aprotic solvents or ionic liquids (IL);5а—c in the
latter case the catalyst—IL system can be recycled seveꢀ
ral times.6
Recently, it has been shown that the rate of the aldol
condensation in DMF catalyzed by amino acid derivaꢀ
tives increases upon addition of water to the reaction
mixture.7 Organic reactions in water have attracted conꢀ
siderable attention in recent years since water is the most
environmentally safe solvent.8a—c However, low solubiliꢀ
ty of most organic compounds in water is the obstacle for
its use in organic synthesis.8d Mixtures of water with IL,
which possess much better solubilising properties, can be
considered as promising solvent systems.9a—f To our
knowledge, asymmetric aldol condensations have not
been carried out in aqueous solutions of IL so far.
In this work, the asymmetric aldol condensation of
unmodified aldehydes with ketones was investigated for
the first time in an imidazolium saltꢀwater system. We
used 1(R),2(R)ꢀbis((S)ꢀprolinamido)cyclohexane (1)
and (Rax)ꢀ2,2´ꢀbis((S)ꢀprolinamido)ꢀ1,1´ꢀbinaphtyl (2)
as catalysts. These were prepared from commercially
available 1(R),2(R)ꢀdiaminocyclohexane, (Rax)ꢀ2,2´ꢀdiꢀ
aminobinaphtyl and Nꢀbenzyloxycarbonylꢀ(S)ꢀproline
according to known procedures.7,10 Amide 1 is an active
catalyst of asymmetric aldol condensation in ionic
liquids.6 Catalyst 2 efficiently catalyzes this reaction
First of all, we compared the stereoꢀ and enantioselecꢀ
tivity of the reactions of 4ꢀnitrobenzaldehyde (3а) with
cyclohexanone (4а) in [bmim][BF4], water, and their
mixtures with different compositions catalyzed by amides
1 and 2 (Scheme 1). The reactions were carried out under
comparable conditions at 26 °C. The molar ratio 3а : 4а
was 1 : 3, the amount of the catalyst was 10 mol.% relative
to 3а. As in aqueous DMSO, aldol condensations proꢀ
ceeded faster in [bmim][BF4]—water mixtures than
in anhydrous solvents and the yields of product 5а were
virtually quantitative (Table 1).
According to data from 1H NMR spectroscopy
anti
syn
(JH ,H
= 8.4 Hz, JH ,H
< 3 Hz), the ratio of anti/
1
2
1
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 578—581, March, 2008.
1066ꢀ5285/08/5703ꢀ591 © 2008 Springer Science+Business Media, Inc.

592
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A. S. Kucherenko et al.
Table 1. Synthesis of 5a and 5b in the [bmim][BF4]—H2O medium catalyzed by amides 1 and 2 a
Reagents
Products
IL—water
(vol.%)
Catalyst
τ/h
Yield (%)b
drc
(anti/syn)
eed (%)
(anti/syn)
3a, 4а
3a, 4а
3a, 4а
3a, 4а
3a, 4а
3a, 4а
3a, 4а
3a, 4а
5a
5a
5a
5a
5a
5a
5a
5a
100 : 0
98 : 2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
1
1
1
1
20
50
6
25
5
20
4
20
4
70
97
99
93
99
95
99
95
99
96
99
96
99
92
98
95
83
85 : 15
75 : 25
82 : 18
82 : 18
81 : 19
90 : 10
82 : 18
83 : 17
80 : 20
89 : 11
85 : 15
89 : 11
88 : 12
85 : 15
83 : 17
86 : 14
—
93 e/26
74/2
70/46
50/38
42/48
26/23
34/52
34/52
44/44
46/39
82/40
80/6
80 : 20
75 : 25
66 : 34
50 : 50
25 : 75
0 : 100
25
8
25
10
30
10
20
25
25
25
25
25
56/24
58/32
56/26
56/30
6
3a, 4b
3a, 4b
3a, 4b
3a, 4b
3a, 4b
5b
5b
5b
5b
5b
100 : 0
75 : 25
50 : 50
25 : 75
0 : 100
50
>95
>95
>95
>95
—
—
—
—
23 f
40 f
26 f
17 f
a The reactions were carried out at 26 °C, if not otherwise stated. b The yield of the isolated diastereomeric mixture. c According
1
20
to the data from the H NMR spectrum of the reaction mixture. d According to HPLC. e [α]
= +10.29 (c 0.5, CHCl3).
D
f The reactions were carried out at 0 °C.
Scheme 1
ee antiꢀ5a (%)
1
2
80
60
40
20
0
Reagents and conditions: catalyst
1 or 2 (10 mol.%),
[bmim][BF4]—H2O (1 : 1 v/v).
R—R = (CH2)3 (4a, 5a—d,g)
R = H (4b, 5e,f,h)
20
40
60
80
VH O (%)
2
Fig. 1. Influence of water content in the [bmim][BF4]—H2О system
on ее of antiꢀ5а in the reaction catalyzed by amides 1 and 2.
3, 5
a
b
c
d
Ar
3, 5
e
f
g
h
Ar
4ꢀO2NC6H4
3ꢀPhOC6H4
4ꢀMeO2CC6H4
4ꢀOCHC6H4
2ꢀO2NC6H4
5ꢀnitrothienyl
2ꢀpyridyl
acetone (4b) in IL, [bmim][BF4]—H2О mixture and
H2О, also catalyzed by amide 1. The enantioselectivity of the
reaction is lower, therefore we carried out the reactions at
0 °C. In this case too, the dependence of ee of aldol 5b on
the water content in the [bmim][BF4]—H2О system has
a maximum at 50 vol.% (see Table 1). Thus, the characꢀ
ter of the curve ее = f(c, H2О) depends little on the
nature of the catalyst and the ketone structure, hence it is
likely that such behavior is a particular property of
the [bmim][BF4]—H2О medium.*
(CO)3MnC5H4
synꢀdiastereomers in aldol 5a depends little on the water
content in IL and varies from 75 : 25 to 90 : 10. However,
water influences the ее of the dominating antiꢀdiastereꢀ
omer. For the catalysts 1 and 2, the ee reaches minimum
at the water content of 10—25 vol.% and maximum
at 50 vol.% (Fig. 1). Notably, at maximum the ee values
are comparable with those of the corresponding reactions
in anhydrous [bmim][BF4].
* It was shown7 that ее of aldol 5b in a DMF—H2О system is 79%
if the water content is 50 vol.% and 54% if the water content is
80 vol.%, which is in agreement with our data.
To assess the influence of the ketone structure on the
position of the maximum in the curve ее = f(c, H2О) we
investigated the reaction of 4ꢀnitrobenzaldehyde (3a) with

Asymmetric aldol reaction
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008
593
Table 2. Synthesis of aldols 5a—h in the [bmim][BF4]—H2O mediꢀ
um catalyzed by amide 1 (1 : 1 v/v)a
Table 3. Activity of regenerated catalyst 1 in the reaction of 3а with
4а in the [bmim][BF4]—H2О medium (1 : 1 v/v)*
Entry
Reaꢀ
gents
Proꢀ
ducts /h
τ
Yieldb
(%)
drc
(anti/syn)
eed (%)
(anti/syn)
Entry
Cycle τ/h
Yield 5а
(%)
dr
ee (%)
(anti/syn)
(anti/syn)
e
1
3a, 4a
5a
8
(20)
45
45
35
25
25
15
99
(70)
77
72
86
86
85
98
68
85 : 15
82/40
(93)
26/26
81/52
68/40
53
1
2
3
4
5
1
2
3
4
5
8
8
10
20
25
97
98
96
98
95
85 : 15
87 : 13
83 : 17
82 : 18
82 : 18
81/40
80/40
83/39
82/39
84/37
e
2
3b, 4а
3c, 4a
3d, 4a
3e, 4b
3f, 4b
3g, 4a
3h, 4b
5b
5c
5d
5e
5f
80 : 20
84 : 16
90 : 10
—
—
82 : 18
—
3
4
5
6
7
8
* The reactions were carried out at 26 °C.
40
e
5g
5h
40/10
46
(40)6
e
75
In conclusion, we have shown for the first time that
asymmetric aldol condensation of selected aldehydes with
ketones in an ILꢀwater system (1 : 1, v/v) proceeds with
high reaction rate and diastereoselectivity. Addition
of water allows decreasing the consumption of expensive
IL twofold and in some cases increasing the reaction rate.
The enantioselectivity of the process is retained under
these conditions and the lifetime of the catalytic system is
increased.
(40)6 (62)6
a The reactions were carried out at 0 °C, if not otherwise stated. The
data for reactions in anhydrous [bmim][BF4] medium are shown in
parentheses. b The yield of the isolated diastereomeric mixture.
c According to the data from the 1H NMR spectrum of the reaction
mixture. d According to HPLC. e The reactions were carried out
at 26 °C.
The system [bmim][BF4]—H2O (1 : 1 v/v) was used
further in asymmetric aldol condensations of different
aldehydes with ketones. Benzaldehyde derivatives 3a—e
with different substituents in the aromatic ring, 2ꢀniꢀ
trothiopheneꢀ5ꢀcarbaldehyde (3f), pyridineꢀ2ꢀcarbaldeꢀ
hyde (3g) and cymantrene carbaldehyde (3h) served
as the aldehyde components. Cyclohexanone (4a) and
acetone (4b) served as the methylene components. Chiral
aldols 5a—h and earlier unknown compounds 5b,d,f,g,*
were formed under the conditions tested in high yields
and diastereoselectivity (dr ≥ 80/20). In compound 3d,
only one aldehyde group reacted. The ее values of major
antiꢀdiastereomeric compounds 5a—d and 5g, and alꢀ
dols 5e,f,h are 26—82%; for compounds 5a and 5h
the ee values are close to the corresponding values
in anhydrous [bmim][BF4] (see Scheme 1, Table 2). Acꢀ
cording to the literature data,12 the dominating enantiꢀ
omers of compounds 5а,c,е,h have Rꢀconfiguration.
By analogy, Rꢀconfiguration was assigned to the newly
synthesized dominating enantiomers of compounds
5b,d,f,g .
Experimental
NMR spectra were obtained on a Bruker AM 300 (300.13 MHz
{1H}) and Bruker WMꢀ250 (250.13 MHz {1H} and 69.9 MHz
13C}) instruments in CDCl3. Chemical shifts of 1H nuclei were
{
determined relative to the internal standard SiMe4, 13C nuclei,
relative to the signals of CDCl3. Elemental analysis was carried out
on a microanalyzer Perkin Elmer 2400. The optical rotation was
measured on a Jasco DIPꢀ360 polarimeter at 589 nm. Conversion
of reagents and purity of products were monitored by TLC on Silufol
plates using UV light for detection, eluent — hexane—ethylacetate
(3 : 1). Diastereomers of 5a—d and 5g were separated by flash
chromatography on silica gel (Acros, 0.040—0.060; eluent, hexꢀ
ane—ethyl acetate (4 : 1)). Compounds 5a,6 5c,11 5e,13 5h14 were
indentified by comparison of their 1H NMR spectra with the literaꢀ
ture data. The ее values of aldols were determined by HPLC on
a Varian 5000 chromatograph. Racemic forms of aldols were used as
standards. Starting compounds 3 and 4 (Acros) were used in the
reaction without additional purification. Bmim[BF4] (Acros) was
>97.5% purity.
Asymmetric aldol condensation (general procedure). To a soluꢀ
tion of catalyst 1 or 2 (0.013 mol) in 0.5 mL of a mixture
[bmim][BF4]—H2О (1 : 1 v/v), aldehyde (0.13 mol) and ketone
(0.40 mol) were added. The reaction mixture was stirred at 0 °C or
26 °C (the reaction conditions are listed in Tables 1—3), extracted
with Et2O (3×3 mL), the combined extracts were dried with MgSO4.
The solvent and excess of ketone were removed in vacuo (30 °C,
15 Torr), products 5 were isolated by flash chromatorgaphy. The 1H,
13C NMR and HPLC (τ(R), the major enantiomer, τ(S)) data
of antiꢀdiastereomers and of newly synthesized compounds 5b,d,f,g
are listed below. For the yields, ee and dr of aldols 5 see Tables 1—3.
(R)ꢀ2ꢀ[Hydroxy(3ꢀphenoxyphenyl)methyl]cyclohexanone (5b).
Colorless oil. 1H NMR (δ, J/Hz) antiꢀisomer: 1.22—2.04 (m, 6 H,
CH2); 2.03—2.70 (m, 3 H, CHC(O), CH2); 3.95 (s, 1 H, OH); 4.76
(d, 1 H, CHOH, J = 8.4), 6.91—7.34 (m, 9 H, Ar); synꢀisomer:
The system 1—[bmim][BF4]—H2O can be regeꢀ
nerated. After completion of the reaction, aldol 5a
was extracted with an organic solvent (Et2O), new porꢀ
tions aldol 5a of 3а and 4а were added to the residue, and
the reaction was repeated. The yields, dr and ее of prodꢀ
uct 5а retained after five cycles, although the reaction
time increased (Table 3). For comparison, it is worth
noting that system 1—[bmim][BF4], which does not conꢀ
tain water, could successfully be used three times in aldol
condensation, thereafter the yields were considerꢀ
ably lower5c,6
.
* Aldol 5g is known in a racemic form.10

594
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008
A. S. Kucherenko et al.
1.31—2.22 (m, 6 H, CH2); 2.34—2.81 (m, 3 H, CHC(O), CH2);
3.08 (s, 1 H, OH); 5.37 (s, 1 H, CHOH); 6.91—7.34 (m, 9 H, Ar).
13C NMR (δ) antiꢀisomer: 25.92, 30.75, 42.57, 57.25, 74.38,
117.41, 117.99, 118.71, 121.78, 123.15, 129.53, 129.61, 142.99,
156.95, 157.11, 215.15; synꢀisomer: 24.67, 25.81, 40.72, 56.90,
70.10, 116.38, 117.17, 118.61, 120.50, 123.03, 129.31, 129.57,
143.74, 156.90, 157.13, 214.32. Found (%): C, 77.28; H, 6.96.
C19H20O3. Calculated (%): C, 77.00; H, 6.80. HPLC (column
Chirapak OJꢀH, eluent — nꢀhexane—PriOH, 90 : 10 (1.0 mL•min–1)),
UVꢀdetection at 254 nm, τ/min: 14.41 (antiꢀR), 16.96 (antiꢀS),
18.72 (synꢀR), 20.73 (synꢀS).
(R)ꢀ4ꢀ[Hydroxy(2ꢀoxocyclohexyl)methyl]benzaldehyde (5d).
Colorless oil. 1H NMR antiꢀisomer: δ 1.27—2.72 (m, 9 H,
CHC(O), CH2); 4.01 (s, 1 H, OH); 4.88 (d, 1 H, CHOH, J = 8.23
Hz); 7.51 (d, 2 H, Ar, J = 7.92 Hz); 7.88 (d, 2 H, Ar, J = 7.92 Hz);
10.02 (s, 1 H, CHO); synꢀisomer: 1.32—2.82 (m, 9 H, CHC(O),
CH2); 3.11 (s, 1 H, OH); 5.47 (s, 1 H, CHOH); 7.51 (d, 2 H, Ar,
J = 7.92 Hz); 7.88 (d, 2 H, Ar, J = 7.92 Hz); 10.02 (s, 1 H, CHO).
13C NMR (δ) antiꢀisomer: 24.52, 27.53, 30.62, 42.51, 57.06,
74.21, 127.50, 129.66, 135.85, 147.70, 191.76, 214.83; synꢀisoꢀ
mer: 23.48, 26.63, 29.43, 41.60, 54.12, 73.08, 124.67, 127.32,
132.47, 144.21, 192.77, 213.93. Found (%): C, 72.49; H, 7.04.
C14H16O3. Calculated (%): C, 72.39; H, 6.94. HPLC (column
Chirapak AD, eluent — nꢀhexane—PriOH, 95 : 5 (1.0 mL•min–1)),
UVꢀdetection at 254 nm, τ/min: 55.44 (antiꢀR), 58.20 (antiꢀS),
34.57 (synꢀR), 51.01 (synꢀS).
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(R)ꢀ4ꢀHydroxyꢀ4ꢀ(5ꢀnitroꢀ2ꢀthienyl)butanꢀ2ꢀone (5f). Lightꢀ
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1
J = 5.92 Hz); 3.81 (m, 1 H, OH); 5.37 (t, 1 H, CHOH, J = 6.04 Hz);
6.90 (d, 1 H, Ar, J = 4.13 Hz); 7.80 (d, 1 H, Ar, J = 4.13 Hz).
13C NMR (δ): 30.53, 50.84, 66.06, 121.98, 128.44, 150.56, 155.68,
207.71. Found (%): C, 44.28; H, 4.52; N, 6.30. C8H9NO4S.
Calculated (%): C, 44.64; H, 4.21; N, 6.51. HPLC (column Chiraꢀ
pak OJꢀH, eluent — nꢀhexane—PriOH, 90 : 10 (1.2 mL•min–1)),
UVꢀdetection at 220 nm, τ/min: 34.57 (R), 51.01 (S).
(R)ꢀ2ꢀ[Hydroxy(2ꢀpyridyl)methyl]cyclohexanone (5g). Colorꢀ
less oil. 1H NMR antiꢀisomer: δ 1.52—1.93 (m, 4 H, CH2);
2.01—2.41 (m, 4 H, CHC(O), CH2); 3.02—3.11 (m, 1 H, OH);
4.88 (d, 1 H, CHOH, J = 5.5 Hz); 7.15—7.20 (m, 1 H, Ar); 7.48
(d, 1 H, Ar, J = 7.9 Hz); 7.68 (dt, 1 H, Ar, J = 7.9 Hz, J = 1.5 Hz);
8.51 (d, 1 H, Ar, J = 4.4 Hz); synꢀisomer: δ 1.27—2.72 (m, 9 H,
CHC(O), CH2); 3.48 (s, 1 H, OH); 5.40 (s, 1 H, CHOH); 7.51
(d, 2 H, Ar, J = 7.92 Hz); 7.88 (d, 2 H, Ar, J = 7.92 Hz); 10.02
(s, 1 H, CHO).13C NMR (δ) antiꢀisomer: 24.60, 26.21, 27.64,
42.48, 55.92, 70.97, 120.81, 121.98, 136.53, 148.36, 160.92, 213.81;
synꢀisomer: 24.52, 27.53, 30.62, 42.51, 57.06, 74.21, 127.50,
129.66, 135.85, 147.70, 191.76, 214.83. Found (%): C, 70.40;
H, 7.09; N, 7.42; C12H15NO2. Calculated (%): C, 70.22; H, 7.37;
N, 6.82. HPLC column Chirapak OJꢀH, eluent — nꢀhexane—
PriOH—NEt3, 100 : 1 : 0.05 (1.0 mL•min–1)), UVꢀdetection at
220 nm, τ/min: 18.85 (synꢀR), 21.40 (synꢀS), 25.68 (antiꢀR),
27.75 (antiꢀS).
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Authors thank V. A. Davankov and S. E. Lyubimova
for HPLC analysis of studied compounds.
This work was supported by the Russian Foundation
for Basic Research (project No. 06ꢀ03ꢀ32603).
Received October 16, 2007;
in revised from December 6, 2007