Highly efficient direct asymmetric aldol reactions catalyzed by a prolinethioamide derivative in aqueous media

Article abstract of DOI:10.1002/ejoc.201001115

L-Prolinethioamide derivative 1c, prepared from the readily available natural amino acids L-proline and L-valine, was studied for the direct asymmetric aldol reaction of acetone with various aromatic aldehydes at 0 °C or room temperature. A loading of onl

Full text of DOI:10.1002/ejoc.201001115

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201001115
Highly Efficient Direct Asymmetric Aldol Reactions Catalyzed by a
Prolinethioamide Derivative in Aqueous Media
Bing Wang,[a] Xin-wang Liu,[a] Ling-yan Liu,*[a] Wei-xing Chang,[a] and Jing Li*[a]
Keywords: Aldol reactions / Water chemistry / Asymmetric catalysis / Organocatalysis
L
-Prolinethioamide derivative 1c, prepared from the readily
ture. A loading of only 0.1–0.2 mol-% of derivative 1c was
employed in this catalytic system, and excellent enantio-
selectivities and yields (up to 98% yield, Ͼ99%ee) could be
achieved in aqueous media.
available natural amino acids -proline and -valine, was
studied for the direct asymmetric aldol reaction of acetone
with various aromatic aldehydes at 0 °C or room tempera-
L
L
this major challenge. Very recently, Wennemers’s group
found that the loading of the organocatalyst could be re-
duced to as little as 0.1 mol-% for a broad range of sub-
strates based on kinetic studies of the enamine mecha-
nism.[11] This is the lowest catalyst loading achieved in en-
amine catalysis so far, successfully suggesting the potential
of organocatalytic reactions. On the other hand, water acts
as the best solvent for enzymes to facilitate optimization of
the performance of the enzyme. Moreover, compared to the
reactions in organic solvents such as dimethyl sulfoxide
(DMSO) and dimethylformamide (DMF), the use of water
as a reaction medium has been regarded as a goal in mod-
ern organic chemistry due to its cost, safety, and environ-
mentally benign character.[12] The pioneer work of the or-
ganocatalyzed direct aldol reaction in water has been inde-
pendently reported by Hayashi[13] and Barbas.[14] Since
then, many efforts have been devoted to asymmetric organo-
catalytic reactions performed in aqueous media.[15] For ex-
ample, Singh and co-workers reported -prolineamide or-
ganocatalyst 1a for the direct asymmetric aldol reaction of
ketone with an aldehyde acceptor in excellent enantio-
selectivity and with low catalyst loading (0.5–1 mol-%) in
aqueous media.[16] Gryko employed 2.5–10 mol-% of -pro-
linethioamide to effectively catalyze the aldol reaction of
cyclohexanone and aromatic aldehydes in the presence of
water.[17] Nevertheless, there are fewer reports on the direct
asymmetric aldol reaction carried out in water for acyclic
acetone than for cyclic ketones. Consequently, it still re-
mains a challenge to develop highly efficient organocata-
lysts for the direct asymmetric aldol reactions of acetone
and aromatic aldehydes in water with the amount of organo-
catalysts as low as possible.[18]
Introduction
The direct asymmetric aldol reaction is an important
method to form carbon–carbon bonds, which provides
straightforward access to the optically active β-hydroxycarb-
onyl structural unit found in many natural products and
drugs.[1] Since List and Barbas reported the direct aldol re-
action catalyzed by -proline in 2000, recent years have wit-
nessed an explosive growth in the field of related asymmet-
ric reactions catalyzed by organocatalysts,[2] especially with
-proline[3] and its derivatives,[4] including prolineamides,[5]
prolinethioamides,[6] sulfonamides,[7] chiral amines,[8] and
so on.
The mechanism of proline and its derivatives as organo-
catalysts has been proposed to proceed via an enamine
intermediate, which is considered to be similar to enzymes
catalysis.[9] Chiral amines of low molecular weight based on
enamine catalysts have become versatile organocatalysts for
asymmetric reactions, and the application of a catalytic
amount of a chiral catalyst for control of the asymmetric
reactions is undoubtedly a big achievement of modern or-
ganic chemistry. However, typical asymmetric aldol reac-
tions employ high loadings of organocatalyst (10–20 mol-
%) to obtain the products in good yields and stereoselectivi-
ties. To the best of our knowledge, reports on the use of
low catalytic loadings (Ͻ1 mol-%) in asymmetric reactions
occurring by enamine catalysis are still very few up to
date.[10] Hence, better insight into the catalytic mechanism
of enamine catalysis may in particular be useful to solve
[a] State key Laboratory and Research Institute of Elemento-
Organic Chemistry,
Nankai University,
Recently, our group reported a successful asymmetric al-
dol reaction catalyzed by the new prolinethioamide organo-
catalyst 1b where the terminal hydroxy group is a primary
alcohol, which is different from previously reported prere-
quisite secondary or tertiary alcohols of prolineamides.[19]
Tianjin 300071, P. R. China
Fax: +86-22-23506057
E-mail: liulingyan@nankai.edu.cn
lijing@nankai.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001115.
View this journal online at
wileyonlinelibrary.com
Eur. J. Org. Chem. 2010, 5951–5954
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5951

B. Wang, X.-w. Liu, L.-y. Liu, W.-x. Chang, J. Li
SHORT COMMUNICATION
As a continuation of our work, we further modified catalyst catalytic system compared to catalyst 1a. Next, we tested a
1b to synthesize organocatalyst 1c based on previous stud- lower loading of catalyst 1c in water. To our delight, the
ies[20] and applied it in direct asymmetric aldol reactions in decreased catalyst loading (from 2 to 1 mol-%) could still
aqueous media (Figure 1).
maintain the product yield and enantioselectivity. When
further decreasing the amount of catalyst 1c to 0.5 mol-%,
up to 96% yield and 98% enantioselectivity were obtained
by extending the reaction time. Even with 0.1 mol-% of cat-
alyst 1c, similar results were still obtained in this reaction
at room temperature (Table 1, Entry 10). In comparison to
organocatalyst 1c, the yield and enantioselectivity de-
creased obviously when the loading of organocatalyst 1a
was reduced to 0.1 mol-%. Only 74% yield and 88%ee were
obtained under the same conditions (Table 1, Entry 11).
The above results indicate that the thioamide functional
group is beneficial for higher catalytic activity. Then, the
effect of additive of this reaction was investigated. In the
absence of PhCOOH, the yield and enantioselectivity dra-
matically decreased (Table 1, Entry 12). It could be con-
cluded that the combination of organocatalyst 1c and
PhCOOH is crucial for the reactivity of this catalytic sys-
tem. For further details, the loading of the additive was also
studied. The reaction afforded a good ee value but only a
moderate yield when a larger amount of additive was used
(20 mol-%; Table 1, Entry 13). However, a much higher
yield was obtained when this reaction was carried out with
2–10 mol-% of PhCOOH (Table 1, Entries 7, 14, and 15).
Thus, the most efficient catalytic system involved 0.1–
0.2 mol-% of catalyst 1c and 2 mol-% of PhCOOH in water.
Figure 1. Different organocatalysts.
Results and Discussion
Organocatalyst 1c was firstly synthesized in a short, good
to high yielding reaction sequence, starting from commer-
cially available N-Boc--proline and -valinate hydrochlo-
rate through the general strategy shown in Scheme 1.
Table 1. Reaction parameter screening in the direct asymmetric al-
dol reaction of 4-nitrobenzaldehyde with acetone catalyzed by or-
ganocatalyst 1.[a]
Scheme 1. Synthesis of organocatalyst 1c.
In our initial studies, the efficacies of different organocat-
alysts 1 (Figure 1) were compared for the direct aldol reac-
tion between acetone and 4-nitrobenzaldehyde as the model
reaction by using PhCOOH as an additive in water. The
results showed that high yields and excellent enantio-
selectivities could be obtained in the presence of catalyst 1a
or 1c, whereas a poorer yield and a moderate ee value were
obtained with 1b, which possesses a primary alcohol as the
terminal hydroxy group (Table 1, Entries 1–3). The results
are consistent with Singh’s studies, where the hydrophobic
groups of the organocatalyst play a significant role in ob-
taining high enantioselectivities in an aqueous medium.[21]
Moreover, we found that -prolinethioamide 1c displayed
higher catalytic activity than -prolineamide 1a. A higher
yield and a slightly increased enantioselectivity were ob-
tained by catalyst 1c. This may be due to the increasing NH
acidity for prolinethioamide compared to prolineamide. To
further improve the yield and enantioselectivity, the reac-
tion was carried out in brine with organocatalyst 1c. No
improvement was observed. This demonstrated that the
Entry Solvent Catalyst Cat. loading PhCOOH Time Yield
ee
[mol-%]
[mol-%]
[h]
[%][b] [%][c]
1
2
3
4
5
6
7
8
9
H2O
H2O
H2O
brine
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
1a
1b
1c
1c
1c
1c
1c
1a
1c
1c
1a
1c
1c
1c
1c
2
2
2
2
10
10
10
10
10
10
10
10
10
10
10
–
8
24
8
8
8
12
12
12
24
24
24
24
12
12
12
81
20
91
91
89
96
94
78
86
90
74
40
64
91
92
96
60
98
96
99
98
96
93
95
95
88
70
96
95
95
1
0.5
0.2
0.2
0.1
0.1
0.1
0.2
0.2
0.2
0.2
10[d]
11[d]
12
13
14
15
20
5
2
[a] Unless stated otherwise, the reaction was carried out with ace-
tone (0.2 mL, 2.5 mmol, 5 equiv.) and 4-nitrobenzaldehyde
(0.5 mmol, 1 equiv.) at 0 °C in water or brine (1 mL) according to
the general procedure. [b] Isolated yield. [c] Determined by chiral
HPLC analysis by using a Chiralpak AS-H column, and the config-
uration was assigned as R by comparison of the optical rotation.
“salt effect” has no obvious influence in our highly efficient [d] The reaction was carried out at room temperature.
5952 Eur. J. Org. Chem. 2010, 5951–5954
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Highly Efficient Direct Asymmetric Aldol Reactions
Then, the scope of the aldehyde substrates in the direct tions summarized as follows: cyclohexanone (1 mmol), p-
asymmetric aldol reactions of acetone was examined under nitrobenzaldehyde (0.5 mmol), and 1c (0.2 mol-%) at 0 °C
the optimal conditions (Table 2). In most cases, excellent to room temperature with PhCOOH as an additive
enantioselectivities were achieved, though the rate and yield (Scheme 2).
of the aldol reaction strongly depended on the electrodefici-
ency of the aldehyde substrate. From the results shown in
Table 2, we found that aromatic aldehydes bearing an elec-
tron-withdrawing group gave excellent conversions due to
the strong electrophilicity of the substrates. For example, o-,
m-, and p-nitrobenzaldehydes or o- and p-trifluoromethyl-
benzaldehyde (Table 2, Entries 2–4, 8, 9) are the most reac-
tive, yielding the aldol products in excellent yields after only
12 h at 0 °C with as little as 0.1 mol-% loading of organoca-
talyst 1c. Weak electrophilic aldehydes, such as 4-meth-
Scheme 2. Direct aldol reaction of cyclohexanone with 4-nitrobenz-
ylbenzaldhyde, required a longer reaction time to give the
corresponding aldol products in moderate yield
(Table 2,Entry 12). Nevertheless, as for less-reactive ali-
phatic aldehydes such as isobutyraldehyde, only a trace
amount of the aldol adduct could be obtained in this sys-
tem (Table 2, Entry 15). This may be due to the fact that
the formation of imidazolidinethiones between isobutyral-
dehyde and the organocatalyst occurs more quickly in this
reaction system. Notably, good to high yields with excellent
enantioselectivities up toϾ99%ee were obtained by using
only 0.1–0.2 mol-% of organocatalyst 1c in water with
PhCOOH as an additive.
aldehyde catalyzed by organocatalyst 1c.
Conclusions
In summary, -prolinethioamide 1c is a very highly ef-
ficient chiral organocatalyst for the asymmetric aldol reac-
tions of various aldehydes with acetone or cyclohexanone
in water. Moreover, it is worthwhile to highlight that the
reactions could achieve high enantioselectivity ranging
from 84 to 99%ee and up to 98% yield by employing only
0.1–0.2 mol-% of organocatalyst 1c. The advantages of this
catalytic system are clear, and it is to some extent a green
and atom economical approach. Further application of
these -prolinethioamide organocatalysts to other impor-
tant asymmetric reactions is underway.
Table 2. Scope of direct asymmetric aldol reactions of acetone with
various aldehydes catalyzed by organocatalyst 1c.[a]
Experimental Section
Entry
R
Product Time [h] Yield [%][b]
ee [%][c]
Typical Procedure for the Direct Asymmetric Aldol Reaction: To a
mixture of catalyst 1c and benzoic acid in water (1 mL) was added
acetone (0.2 mL, 2.5 mmol, 5 equiv.). Then, the aldehyde
(0.5 mmol, 1 equiv.) was added at 0 °C. After TLC analysis indi-
cated complete consumption of the starting material, the reaction
mixture was quenched with saturated NH4Cl aqueous solution, ex-
tracted with EtOAc, and dried with anhydrous Na2SO4. The crude
product was purified by flash silica gel chromatography (hexane/
EtOAc) to afford pure aldol products. All aldol products are known
compounds, and their spectroscopic data are identical to those re-
ported in the literature. The ee values were determined by chiral
HPLC analysis.
1
2
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
16
12
12
12
16
16
16
10
10
16
16
24
12
12
36
92
92
88
98
76
91
90
91
96
89
90
72
77
67
Ͻ10
97
95
Ͼ99
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
4-BrC6H4
3-BrC6H4
2-BrC6H4
4-CF3C6H4
2-CF3C6H4
4-FC6H4
4-ClC6H4
4-MeC6H4
β-naphthyl
2-furyl
3[d]
4[d]
5
99
96
Ͼ99
98
98
99
98
98
98
96
84
–
6
7
8[d]
9[d]
10
11
12
13
14
15
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures, characterization data for all
new compounds, and HPLC data.
iPr
[a] Unless stated otherwise, the reaction was carried out with ace-
tone (0.2 mL, 2.5 mmol, 5 equiv.) and various aldehydes
(0.5 mmol, 1 equiv.) under optimal reaction conditions in the pres-
ence of organocatalyst 1c (0.2 mol-%). [b] Isolated yield. [c] Deter-
mined by chiral HPLC analysis by using a Chiralpak AD-H, AS-
H, or OJ-H column, and the configuration was assigned as R by
comparison of the optical rotation. [d] The reaction was carried
out in the presence of 0.1 mol-% of organocatalyst 1c.
Acknowledgments
We are grateful for the financial support of the National Natural
Science Foundation of China (grants 20421202 and 20372033). We
also thank the Nankai University State Key Laboratory of Ele-
mento-Organic Chemistry for support.
Finally, to further examine the generality of this catalytic
system, cyclohexanone was used as the aldol donor. Up to
90% yield and excellent diastereoselectivity and enantio-
selectivity were obtained under the optimal reaction condi-
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5953

B. Wang, X.-w. Liu, L.-y. Liu, W.-x. Chang, J. Li
SHORT COMMUNICATION
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Eur. J. Org. Chem. 2010, 5951–5954