Organocatalysis with cysteine derivatives: Recoverable and cheap chiral catalyst for direct aldol reactions

Article abstract of DOI:10.1007/s11164-011-0336-5

Highly enantioselective, recoverable and cheap cysteine derivatives have been developed with improved solubility properties in common, nonpolar organic solvents. The reactions were catalyzed using 5 mol% 1e, and the aldol products could be obtained with up to 99:1 syn/anti ratio and >99% ee. The catalyst could be easily recovered and reused, with only a slight decrease of enantioselectivity observed for five cycles. Catalyst 1e can be efficiently used in large-scale reactions with enantioselectivity being maintained at the same level, which offers great possibility for application in industry. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0336-5

Res Chem Intermed (2012) 38:195–205
DOI 10.1007/s11164-011-0336-5
Organocatalysis with cysteine derivatives: recoverable
and cheap chiral catalyst for direct aldol reactions
•
Shi Li Xiangkai Fu Chuanlong Wu
•
Received: 10 May 2011 / Accepted: 30 May 2011 / Published online: 15 June 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Highly enantioselective, recoverable and cheap cysteine derivatives have
been developed with improved solubility properties in common, nonpolar organic sol-
vents. The reactions were catalyzed using 5 mol% 1e, and the aldol products could be
obtained with up to 99:1 syn/anti ratio and [99% ee. The catalyst could be easily
recovered and reused, with only a slight decrease of enantioselectivity observed for five
cycles. Catalyst 1e can be efficiently used in large-scale reactions with enantioselectivity
being maintained at the same level, which offers great possibility for application in
industry.
Keywords Cysteine derivatives Á Aldol reactions Á Recoverable catalyst Á
Large-scale reactions
Introduction
Recently, the development of new, inexpensive, simple and efficient organic
molecules that catalyze enantioselective reactions has proved to be one of the
successful strategies for preparation of chiral building blocks [1]. The aldol reaction
is recognized as one of the most important carbon–carbon bond-forming reactions in
Electronic supplementary material The online version of this article (doi:
10.1007/s11164-011-0336-5) contains supplementary material, which is available to authorized users.
S. Li Á X. Fu (&) Á C. Wu
College of Chemistry and Chemical Engineering, Southwest University, Institute of Applied
Chemistry Southwest University, The Key Laboratory of Applied Chemistry of Chongqing
Municipality, Chongqing 400715, People’s Republic of China
e-mail: fxk@swu.edu.cn
S. Li Á X. Fu Á C. Wu
The Key Laboratory of Ecoenvironments in Three Gorges Reservoir Region Ministry of Education
Chongqing, Chongqing 400715, People’s Republic of China
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S. Li et al.
modern organic synthesis (for reviews on direct aldol reactions, see [2–4]), creating
the b-hydroxy carbonyl structural unit found in many natural products and drugs,
and has received much attention in recent years. A wide range of small organic
molecules, including proline and various other chiral pyrrolidine derivatives, have
been shown to be efficient catalysts for asymmetric aldol reactions (for reviews on
direct aldol reactions, see [5–13]; for selected examples of anti-aldol reactions, see
[14–22]). Barbas [23–26], Hayashi [27–30], Gong [31–35] and Xiao et al. [36, 37]
reported intermolecular aldol reactions of ketone–aldehyde type and aldehyde–
aldehyde type catalyzed by L-proline or its derivatives and analogues. Synthesis a
series of widely used industrial catalyst requires the following conditions: it should
be easy to synthesize, and the initial material should be cheap and easy to obtain.
Currently, aldol reaction catalysts are always complicated, and although a relatively
small amount of catalyst (1–5 mol%) is added, the cost is high. On the other hand,
the catalyst should be recyclable. In large-scale processes, it should maintain high
enantioselectivity. However, their shortcomings have also been realized. One of the
major limitations of organocatalyzed reactions is the high catalyst loading
(10–30 mol%) generally required to complete the transformations in large
equivalents of ketone within reasonable timescales. This raises cost concerns when
large amounts of chiral materials are used for large-scale synthesis in industrial
applications. Our group carried out a series of reactions using threonine- and serine-
based organocatalysts for aldol and Mannich reactions [38–41]. These catalysts
achieved high efficiency, but we did not carry out in-depth and careful studies on
cysteine. Based on these points, we wish to synthesize sample, cheap and recyclable
cysteine derivative catalysts 1a–f (Scheme 1) that promote asymmetric direct aldol
reactions.
NH2
NH2
NH2
O
S
OH
O
S
OH
O
S
OH
O
O
O
1c
1a
1b
NH2
NH2
O
O
S
OH
O
S
OH
H
S
NH2
OH
O
t-Bu
O
H
O
1f
1d
1e
Scheme 1 Cysteine-based organocatalysts
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Organocatalysis with cysteine derivatives
197
Experimental
General information
All reagents were commercial products. Reactions were monitored by thin-layer
chromatography (TLC). Column and preparative TLC purification were carried out
using silica gel. Flash column chromatography was performed on silica gel
(200–300 mesh). Nuclear magnetic resonance (NMR) spectra were recorded on a
300-MHz instrument. Chemical shifts (d) are given in ppm relative to tetrameth-
ylsilane (TMS) as internal reference, and coupling constants (J) in Hz. Infrared (IR)
spectra were recorded on a spectrometer. Melting points were measured on a digital
melting point apparatus. Mass spectra (MS) were measured with a spectrometer.
Analytical high-performance liquid chromatography (HPLC) was carried out on
Agilent 1200 instrument using Chiralpak AD (4.6 mm 9 250 mm), Chiralcel OD-
H (4.6 mm 9 250 mm) or Chiralcel OJ-H (4.6 mm 9 250 mm) columns. Optical
rotations were measured on a JASCO P-1010 polarimeter at k = 589 nm.
General procedure for preparation of catalysts 1a–1f [42, 43]
Catalysts 1a–1f (Scheme 1) were prepared in excellent yields from L-cysteine and
acyl chloride. The white solids were washed by four portions of Et2O, and then
dried them in the oven at 70 °C for 24 h in a ventilated hood to give S-acyl-L-
cysteine hydrochloride, after that dissolved the hydrochloride products in water and
added an equivalent amount of Et3N. The white suspensions were stirred at room
temperature for 10 min and filtered by vacuum. The white crystals were washed
with two portions of H2O and dried to give S-acylated cysteine-based organo-
catalysts. This essentially pure material was used for the next step without further
purification.
(S)-S-(n-benzoyl)-L-cysteine (1a)
1
White solid; Yield: 89%; [a]D20 = -33.1 (c = 1, MeOH); H NMR [300 MHz,
dimethyl sulphoxide (DMSO)]: d = 3.23–3.48 (d, J = 5.5 Hz, 2H), 4.17 (br. s, 1H),
7.45 (d, J = 4.5 Hz, 2H), 7.58 (br. s, 1H), 7.97 (d, J = 6 Hz, 2H) ppm; 13C NMR
(75 MHz, DMSO): d = 33.2, 54.0, 128.1, 128.1, 129.0, 129.0, 134.2, 134.7, 174.9,
191.5 ppm; MS [electrospray ionisation (ESI)] m/z calcd. for C10H11NO3S 225.05
found 225.74.
(S)-S-(1-naphthoyl)-L-cysteine (1b)
White solid; Yield: 89%; [a]D20 = -28.9 (c = 1, MeOH); H NMR (300 MHz,
1
DMSO): d = 3.13–3.39 (d, J = 5.8 Hz, 2H), 4.25 (br. s, 1H), 7.42–7.88 (m, 6H),
8.72 (br. s, 1H) ppm; 13C NMR (75 MHz, DMSO): d = 33.2, 54.0, 120.3,127.3,
127.4, 129.8, 130.1, 131.4, 132.8, 134.2, 195.6 ppm; MS (ESI) m/z calcd. for
C14H13NO3S 275.06 found 275.69.
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S. Li et al.
(S)-S-(n-cyclohexoyl)-L-cysteine (1c)
White solid; Yield: 92%; [a]D20 = -30.0 (c = 1, MeOH); H NMR (300 MHz,
DMSO): d = 1.46–1.93 (m, 10H), 2.37 (m, 1H), 3.42–3.49 (d, J = 5.7 Hz, 2H),
4.09 (br. s, 1H) ppm; 13C NMR (75 MHz, DMSO): d = 25.3, 25.3, 28.0, 29.9, 29.9,
33.7, 50.9, 54.1, 180.9, 211.7 ppm; MS (ESI) m/z calcd. for C10H17NO3S 231.09
found 231.81.
1
(S)-S-(n-adamantanoyl)-L-cysteine (1d)
White solid; Yield: 93%; [a]D20 = -27.2 (c = 1, MeOH); H NMR (300 MHz,
1
DMSO): d = 1.17–1.83 (m, 13H), 3.38–3.44 (d, J = 5.3 Hz, 2H), 4.24 (br. s, 1H)
ppm; 13C NMR (75 MHz, DMSO): d = 28.5, 30.5, 30.5, 34.2, 37.8, 37.8, 37.8,
39.6, 39,6, 39.6, 45.7, 54.8, 180.1, 207.4 ppm; MS (ESI) m/z calcd. for C14H21NO3S
283.12 found 283.47.
(S)-S-(n-cinnamoyl)-L-cysteine (1e)
White solid; Yield: 86%; [a]D20 = -23.0 (c = 1, MeOH); H NMR (300 MHz,
1
DMSO): d = 3.46–3.48 (d, J = 5.8 Hz, 2H), 4.17 (br. s, 1H), 7.01 (d, J = 3 Hz,
1H), 7.17–7.43 (m, 5H), 7.64 (d, J = 3 Hz, 1H) ppm; 13C NMR (75 MHz, DMSO):
d = 33.7, 54.9, 126.7, 127.4, 129.3, 129.7, 129.9, 136.3, 177.2, 189.1 ppm; MS
(ESI) m/z calcd. for C12H13NO3S 251.06 found 251.67.
(S)-S-(t-butoyl)-L-cysteine (1f)
White solid; Yield: 83%; [a]D20 = -13.1 (c = 1, MeOH); H NMR (300 MHz,
1
DMSO): d = 1.19 (br. s, 9H), 3.36–3.38 (d, J = 5.4 Hz, 2H), 4.06 (d, J = 5.1 Hz,
1H) ppm; 13C NMR (75 MHz, DMSO): d = 27.2, 28.2, 46.5, 51.9, 169.4,
204.6 ppm; MS (ESI) m/z calcd. for C8H15NO3S 205.08 found 205.89.
Results and discussion
The organocatalyzed aldol reaction was carried out using cyclohexanone with
4-nitrobenzaldehyde as a model reaction to investigate different parameters, such as
the catalysts, solvents and loading of the catalysts. In our initial investigation,
cysteine and its derivatives 1a–f were screened as catalysts. As can be seen from the
results summarized in Table 1, although natural cysteine was not an efficient
organocatalyst in water (Table 1, entry 1), all the catalysts could catalyze the
asymmetric direct intermolecular aldol reaction of 4-nitrobenzaldehyde and
cyclohexanone to give the product in good yields (80–93%) with different ee
values (75–90% ee for anti) in water at room temperature (Table 1, entries 2–7).
Among the six derivatives, the catalyst containing the cinnamyl group gave a good
yield (88%) and enantioselectivity (90% ee for anti). The cysteine-based
organocatalyst 1e turned out to be the most efficient catalyst (Table 1, entry 6).
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Organocatalysis with cysteine derivatives
199
Table 1 Screening of organocatalysts
CHO
O
O
OH
Cat. 1a-f (10 mol-%)
+
H2O, r.t
NO2
NO2
2a
Entry
Catalyst
Cat. loading (%)
Time (h)
Yielda (%)
Anti:synb
eeb (%)
1
2
3
4
5
6
7
L-Cys
1a
20
10
10
10
10
10
10
48
24
24
24
24
24
24
–
83
82
80
80
88
93
nd
68:32
78:22
82:18
87:13
90:10
80:20
nd
80
75
77
85
90
78
1b
1c
1d
1e
1f
The reactions were performed with 4-nitrobenzaldehyde (1 mmol), cyclohexanone (2.5 mmol) and cat-
alyst 1a–f (0.1 mmol) in presence of water (2 mL) at room temperature
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H column)
Solvent screening was then performed at room temperature to identify the best
reaction conditions (Table 2, entries 1–7). Among the organic solvents tested,
when 10 mol% 1e was used, 1,2-dichloroethane was slightly better in terms of
both diastereoselectivity and enantioselectivity. Then, using 1,2-dichloroethane as
reaction medium, we investigated the above reaction employing different amounts
of catalyst 1e (Table 2, entries 8–9). When the amount was decreased to 5 mol%,
the dr and ee values were increased.
Using 1,2-dichloroethane as solvent and 1e as catalyst, the effects of ketone
loading on the reaction of 4-nitrobenzaldehyde and cyclohexanone were investi-
gated (Table 3). When 5 mol% 1e was used, the reaction could be obviously
accelerated by increasing ketone from 1 to 8 equiv, and the aldol product could be
obtained in high yield with good stereoselectivity even with only 4 equiv
cyclohexanone.
To test the substrate generality of this organocatalyzed direct aldol reaction, the
reactions of various aromatic aldehydes with cyclohexanone were studied under the
optimized conditions. The results are summarized in Table 4. It can be seen that a
wide range of aromatic aldehydes can effectively participate in the aldol reactions.
From Table 4, we were able to access aldol adducts 2a–o derived from their
corresponding aromatic aldehydes and cyclohexanone. In general, the reactions
between cyclohexanone and aromatic aldehydes bearing electron-withdrawing
substituents furnished b-hydroxy carbonyl aldol products in good yields (80–92%)
and enantioselectivities (91–99% ee for anti-isomer) within 20–40 h (Table 4,
entries 1–9). In contrast, longer reaction times (48 h) were required for aromatic
aldehydes containing an electron-donating group to give comparatively lower yields
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S. Li et al.
Table 2 Effects of solvent and catalyst loading on the organocatalyzed direct aldol reaction
CHO
O
O
OH
Cat. 1e (X mol-%)
+
solvent, r.t.
NO2
NO2
2a
Entry
Cat. loading (%)
Solvent
Yielda (%)
Anti:synb
eeb (%)
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
5
H2O
88
85
90
83
82
79
63
87
80
90:10
92:8
90
92
91
87
89
83
81
97
71
ClCH2CH2Cl
CHCl3
93:7
THF
84:16
82:18
79:21
63:37
98:2
CH3OH
DMSO
NMP
ClCH2CH2Cl
ClCH2CH2Cl
2
82:18
The reactions were performed with 4-nitrobenzaldehyde (1 mmol), cyclohexanone (2.5 mmol) and cat-
alyst 1e (see Table 2) in the specified solvent (2 mL, see Table 2) at room temperature
THF tetrahydrofuran; NMP N-methyl-2-pyrrolidone
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H column)
Table 3 Effect of amount of cyclohexanone on the aldol reaction between cyclohexanone and 4-ni-
trobenzaldehydes catalyzed by 1e
O
OH
CHO
O
Cat. 1e (5 mol-%)
+
ClCH2CH2Cl, r.t.
NO2
2a
NO2
Entry
Cyclohexanone (equiv)
Yielda (%)
Anti:synb
eeb (%)
1
2
3
4
1
81
87
90
91
81:19
98:2
[99
92:8
84
97
99
88
2.5
4
8
The reactions were performed with 4-nitrobenzaldehyde (1 mmol), cyclohexanone (see Table 3) and
catalyst 1e (0.05 mmol) in 1,2-dichloroethane (2 mL) at room temperature
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H)
(65–77%), but without decrease of enantioselectivities, especially for p-tolualde-
hyde (Table 4, entry 10, [99 anti/syn ratio and 97% ee) (Table 4, entries 10–12).
This can be explained in that electron-withdrawing groups enhance the
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Organocatalysis with cysteine derivatives
201
Table 4 Organocatalyst 1e-catalyzed direct aldol reactions
O
OH
O
O
Cat. 1e (5 mol-%)
ClCH2CH2Cl, r.t.
R
+
H
R
2a-o
Entry
Product
Time (h)
Yield (%)a
Anti:synb
ee (%)b
1
2a (R = p–NO2–C6H4)
2b (R = o–NO2–C6H4)
2c (R = m–NO2–C6H4)
2d (R = 2,4-dinitrophenyl)
2e (R = p–CN–C6H4)
2f (R = p–CF3–C6H4)
2g (R = p–Br–C6H4)
2h (R = p–Cl–C6H4)
2i (R = o–Cl–C6H4)
2j (R = p–CH3–C6H4)
2k (R = p–OMe–C6H4)
2l (R = m–OMe–C6H4)
2m (R = 2-naphthyl)
2n (R = 1-naphthyl)
2o (R = C6H5)
23
24
26
20
24
24
36
40
40
44
48
48
30
36
31
85
89
83
80
88
88
90
92
86
70
77
65
76
74
77
[99
90:10
95:5
97:3
95:5
97:3
97:3
93:7
90:10
[99
95:5
91:9
98:2
[99
96:4
[99
93
91
92
95
98
94
96
97
97
95
96
94
90
91
2
3
4
5
6
7
8
9
10
11
12
13
14
15
The reactions were performed with p-nitrobenzaldehyde (1.0 mmol), ketone (4.0 mmol) and catalyst
(0.05 mmol) in solvent (2 mL) at room temperature
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H)
electrophilicity of carbonyl carbons in aldehydes, which facilitates the reaction,
while electron-donating groups lessen the electrophilicity. Moreover, the direct
aldol reactions of neutral aldehydes catalyzed by the cysteine-based organocatalyst
1e also afforded the aldol products with high enantioselectivities and diastereose-
lectivities, especially the 2-naphthyl aldehyde (Table 4, entry 13, 98/2 anti/syn ratio
and 94% ee) (Table 4, entries 13–15).
We also checked the aldol reactions of other ketones (4-methylcyclohexanone,
cycloheptanone and hydroxyacetone) with aromatic aldehydes using catalyst 1e
(5 mol%) (Table 5). 4-Methylcyclohexanone gave high stereoselectivities and good
yield (Table 5, entries 1–3). When cyclopentanone was used as an aldol donor, good
yield (84–90%) and good ee (84–90%) for the anti-isomer were achieved; however,
the diastereomeric ratio obtained was only (60/40–63/38) anti/syn (Table 5,
entries 4–6). Moreover, we examined the feasibility of using hydroxyacetone as
aldol donor. Although longer reaction times were required in comparison with cyclic
ketones, satisfactory results were obtained. The hydroxyacetone reacted smoothly
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S. Li et al.
Table 5 Direct asymmetric aldol reactions between ketones and aromatic aldehydes catalyzed by 1e
O
O
OH
O
OH
O
Cat. 1e (5 mol-%)
+
H
+
R
R
R
ClCH2CH2Cl, r.t.
anti-products
syn-products
Entry
1
Product
Time (h)
24
Yield (%)a
Anti:synb
ee (%)b
90
91
87
95:5
93
O
OH
NO2
3a
2
3
27
25
83:17
88:12
89
91
O
OH NO2
3b
O
O
OH
NO2
3c
4
5
6
25
24
27
84
90
88
60:40
91:9
87
91
89
OH
NO2
3d
O
OH NO2
3e
OH
62:38
O
NO2
3f
OH
7
8
40
46
90
81
20:80
15:85
88
91
O
O
OH
3g
NO2
NO2
OH
OH
3h
9
48
77
10:90
90
O
OH
OH
Cl
3i
The reactions were performed with p-nitrobenzaldehyde (1.0 mmol), ketone (4.0 mmol) and catalyst
(0.05 mmol) in solvent (2 mL) at room temperature
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H)
123

Organocatalysis with cysteine derivatives
203
Table 6 Recycling and reuse of catalyst 1e
CHO
O
O
OH
Cat. 1e (5 mol-%)
+
ClCH2CH2Cl, r.t.
NO2
NO2
2a
Run
Time (h)
Yielda (%)
Anti:synb
eeb (%)
1
2
3
4
5
6
23
24
26
26
30
48
90
85
85
82
80
75
99:1
94:6
99
94
94
90
88
85
90:10
89:11
85:15
79:21
The reactions were performed with p-nitrobenzaldehyde (1.0 mmol), ketone (4.0 mmol) and catalyst
(0.05 mmol) in solvent (2 mL) at room temperature
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H)
with nitrobenzaldehyde and 4-chlorobenzaldehyde under optimal conditions to
give the corresponding syn-aldol products in good yields of 77–90% (Table 5,
entries 7–9).
Meanwhile, to verify that the cysteine derivative organocatalyst 1e could be
recovered and reused, we performed a recycling study of 1e using the aldol reaction
between cyclohexanone and 4-nitrobenzaldehyde (Table 6). Catalyst 1e could be
easily recovered from the reaction mixture after completion of the reaction by acid
treatment. The aldol product was extracted with Et2O, with evaporation of the
organic solution to obtain the aldol product. Catalyst 1e exists in the acidic aqueous
layer, and, on adding an equivalent amount of Et3N, the resulting white suspension
was filtered by vacuum and the white crystals used directly in subsequent aldol
reaction without adding any new catalyst. In each reuse, the same amounts of
substrates were used, and the recovered 1e, without further purification, retained
essentially its catalytic activity without any decrease of enantioselectivity for six
cycles (Table 6).
Furthermore, a large-scale test was performed. With 25 mmol aromatic
aldehydes and 4 equiv ketones, the same catalyst loading of 5 mol% as for the
experimental scale was used. The experiments could be facilely carried out using
the same procedure as for the experimental-scale reactions. As can be seen from the
results summarized in Table 7, pleasingly, the enantioselectivities were maintained
at the same level in the large-scale reactions.
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S. Li et al.
Table 7 Large-scale asymmetric aldol reactions
O
O
O
OH
Cat. 1e (5 mol-%)
H
+
R
R
ClCH2CH2Cl, r.t.
Entry
1
Product
Time (h)
Yieldb (%)
Anti:synb
eeb (%)
30
89
85
98:2
99
O
OH
NO2
2
52
90:10
91
O
OH
Cl
3
4
35
33
90
89
88:12
62:38
90
87
O
OH
OH NO2
O
5
50
83
60:40
89
OH
O
Cl
The reactions were performed with aromatic aldehydes (25 mmol), ketones (100 mmol) and catalyst
(1.25 mmol) in solvent (50 mL) at room temperature
a
Isolated yield
b
Determined by chiral HPLC analysis (AD–H)
Conclusions
We have designed and synthesized a new series of cysteine-based organocatalysts in
one step. A wide range of aromatic aldehydes with cyclic ketones and unprotected
hydroxyacetone can effectively participate in the aldol reactions. Catalyst 1e can be
readily recovered and reused without significant loss of catalytic activity or
stereoselectivity. Notably, these organocatalyzed asymmetric direct aldol reactions
can be performed on a large scale with enantioselectivities maintained at the same
level, which offers great possibility for applications in industry.
Acknowledgments Authors are grateful to Southwest University of China for financial support.
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