Direct asymmetric aldol reaction co-catalyzed by l-proline and isothiouronium salts

Article abstract of DOI:10.1016/j.tetlet.2014.10.009

An efficient, simple, and highly selective protocol for the direct asymmetric aldol reaction between cyclohexanone and aromatic aldehydes using l-proline as a chiral catalyst is reported. Catalytic amounts of achiral isothiouronium iodide salt 1d have bee

Full text of DOI:10.1016/j.tetlet.2014.10.009

Tetrahedron Letters 55 (2014) 6470–6473
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Direct asymmetric aldol reaction co-catalyzed by L-proline
and isothiouronium salts
⇑
Eun Cho, Taek Hyeon Kim
School of Chemical Engineering, College of Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient, simple, and highly selective protocol for the direct asymmetric aldol reaction between
Received 11 July 2014
Revised 29 September 2014
Accepted 3 October 2014
Available online 8 October 2014
cyclohexanone and aromatic aldehydes using L-proline as a chiral catalyst is reported. Catalytic amounts
of achiral isothiouronium iodide salt 1d have been used for the first time as a co-catalyst for this reaction,
which proved to be an excellent catalyst, producing good to excellent yields (up to 93%) with good
stereoselectivities (up to 93:7 dr and 99% ee). These aldols are formed under solvent-free catalytic
system, inside a standard laboratory refrigerator, and without stirring.
Keywords:
Aldol reaction
Ó 2014 Elsevier Ltd. All rights reserved.
Asymmetric catalysis
Isothiouronium salt
Solvent-free condition
The aldol reaction is one of the most powerful strategies for the
formation of carbon–carbon bonds in organic synthesis.1 The stere-
oselectivity of this reaction has been a challenging and long-lasting
salts12 has been found to accelerate the reaction rate and increase
the diastereo- and enantioselectivity of -proline-catalyzed aldol
reactions. So far developing more efficient and greener conditions
for asymmetric aldol reactions has attracted a great deal of
attention.
L
task for synthetic chemists.2 The finding of the first
L-proline-
catalyzed intermolecular aldol reaction in 2000 by List et al.,
attracted interest in organocatalyzed reactions.3 Considering its
Isothiouroniums have been explored as prospective alternatives
of thioureas in anion binding to enhance the acidity of the NH
moieties and allow more hydrogen bonding.13 Considering the
investigated ability of isothiouroniums in binding carboxylic
acids,13e,i we contemplated the possibility of using isothiouroni-
ums as novel additives in the proline catalyzed aldol reaction.
Herein, we report the results of high yields and enantioselectivities
(91–99% ee) in the proline-catalyzed intermolecular direct aldol
reaction between cyclohexanone and various aromatic aldehydes
using isothiouronium salts. Recently, we reported S-benzyl isothio-
uronium chloride as a novel organocatalyst for the direct reductive
amination of aldehydes and ketones14 and for the reduction of con-
jugated nitroalkenes into nitroalkanes resulting in high efficiency,
chemoselectivity, and easy recovery of organocatalyst using Han-
tzsch esters.15 Therefore we expect that some isothiouronium salts
ready availability and low price, it is obvious that L-proline would
be first choice catalyst for preparing aldol adducts with high dia-
stereo- and enantioselectivity. However, proline presents some
major drawbacks, including poor performance in direct aldol reac-
tions with aromatic aldehydes, limited solubility and reactivity in
nonpolar organic solvents, and side reactions that make using high
catalyst loadings necessary to reach satisfactory conversions.
Therefore numerous proline-derived organocatalysts such as pro-
linamindes,4 prolinethioamides,5 sulfonamides,6 chiral amines,7
and organic salts8 have been designed for direct aldol reactions.
With this methodology, in order to achieve high yields and stere-
oselectivities, the use of a large excess of reagent, long reaction
times, high catalyst loadings, and unfriendly solvents (DMF or
DMSO) are also usually required. An alternative consists of includ-
ing readily available additives to reactions containing proline. This
late approach is clearly advantageous in eluding tedious chemical
syntheses of organocatalysts and would ultimately allow the con-
struction of libraries of catalyst protocols by simply changing the
additive. In this sense, the addition of catalytic or substoichiomet-
ric amounts of water,9 chiral diols,10 thioureas,11 and guanidinium
might work in aldol reaction as an activator with L-proline.
The reaction between cyclohexanone and 4-nitrobenzaldehyde
was selected as a standard model reaction for screening of the effi-
cacies of designed organocatalysts. In the hunt for an inexpensive
and green process, we decided to avoid the use of any organic
solvent, apart from 2 (10-fold excess), which acts as both reagent
and reaction media. Under these conditions, we postulate that
the isothiouronium core of salts could form a network of H-bond-
ing interactions with the carboxylate of proline, as well as with the
⇑
Corresponding author. Tel.: +82 62 530 1891; fax: +82 62 530 1889.
E-mail address: thkim@jnu.ac.kr (T.H. Kim).
http://dx.doi.org/10.1016/j.tetlet.2014.10.009
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

E. Cho, T. H. Kim / Tetrahedron Letters 55 (2014) 6470–6473
6471
Table 1
Screening of isothiouronium co-catalysts 1 for the L-proline-catalyzed aldol reaction
O
O
O
OH
L-proline (15 mol%)
co-catalysts 1 (10 mol%)
+
H
neat, 0-5 oC, 96 h
NO2
NO2
2
4a
3a
Entry
1
Co-catalyst
Yielda (%)
78
anti:synb
eec (%)
None
81:19
56
S
2
81
89:11
78
H2N
_
NH2
NH2
+
I
S
3
90
89:11
88:12
92
88
H2N
1a
_
+
S
S
Cl
4
5
86
95
H2N
NH2
1b
1c
_
+
Br
H2N
93:7
94
NH2
_
+
I
S
6
7
93
90
93:7
92:8
97
97
H2N
NH2
1d
_
+
BPh4
S
1ed
H2N
NH2
d 1e was the mixture of 1a and NaBPh4.
a
Isolated yields after column chromatography.
b
c
anti/syn ratio was based on 1H NMR spectroscopic analysis of the crude reaction mixture.
The ee was determined by HPLC on chiral columns.
carbonyl moieties of cyclohexanone and the aromatic aldehyde,
thus enhancing their electrophilicity. These interactions could con-
trol the reactivity and selectivity of proline in the aldol reaction.
Also, the influence of the anion counterpart of salt 1 in the reaction
might occur. The catalytic activities of prepared isothiouronium
salts 1a–e were evaluated under a suspension of 4-nitrobenzalde-
conversion, as well as poorer diastereoisomeric ratios and
enantiomeric excesses, thus demonstrating the positive effect of
isothiouronium salt on the reaction course.
We next screened different parameters, such as the loading of
the catalyst, temperature, time, solvents, additive (water), and stir-
ring. A decrease in the catalyst loading to 5 mol % had slightly
lower yield and lower enantioselectivity (Table 2, entry 1). When
the reaction was performed at room temperature, a better enanti-
oselectivity was not achieved and an obvious loss of the yield was
observed (Table 2, entry 2), however for 96 h inside a refrigerator
(0–5 °C), yield and enantioselectivity were clearly increased
(Table 2, entries 4–7). The effect of the solvent was then consid-
ered. The reaction occurred in both polar and non-polar solvents,
however neither proved to be better than the neat conditions
(Table 2, entries 8 and 9). It has previously been assumed that
water can help the catalyst turn-over by hydrolysis of the final
product from the iminium intermediate,9 but in this case the
addition of water had detrimental effects both on yield and on
enantioselectivity (Table 2, entries 10 and 11). This extensive
screening showed that the optimized conditions were 10 mol %
catalyst loading without the addition of any solvent for 96 h inside
a refrigerator (0–5 °C) with no stirring.
hyde 3a (1 equiv) and L-proline (15 mol %) in cyclohexanone 2
(10 equiv) in solvent-free conditions for 96 h inside a refrigerator
(0–5 °C) with no stirring (Table 1). All catalysts exhibited a high
catalytic efficiency in the model reaction. A dependence of the cat-
alytic activity on the counter anion was observed (Table 1, entries
4–6). Excellent stereoselectivities could be obtained in the pres-
ence of isothiouronium iodide 1d, whereas high yield was obtained
with isothiouronium bromide 1c. The benzyl substituted catalyst
1d was more efficient than the methyl catalyst 1a (Table 1, entries
3 and 6), indicating that a more steric group has more efficient cat-
alytic activity. Tetraphenyl borate salt 1e, the simple mixture of
iodide salt 1a and NaBPh4, afforded stereoselectively efficient aldol
(Table 1, entry 7). In all cases, isothiouronium iodide 1d, derived
from thiourea emerged as the most promising catalyst both in
yields and in stereoselectivities (Table 1, entry 6). Without co-cat-
alyst as well as with thiourea as a co-catalyst, aldol 4a was formed
in 78% conversion (81:19 anti/syn, 56% ee) and 81% conversion
(89:11 anti/syn, 78% ee), respectively, (Table 1, entries 1 and 2).
Aldol reactions performed without co-catalyst showed lower
With this optimized protocol established, we further explored
the scope of the aldol reaction of cyclohexanone to the various
aldehydes co-catalyzed by iodide salt 1d and the results are

6472
E. Cho, T. H. Kim / Tetrahedron Letters 55 (2014) 6470–6473
Table 2
Optimization of reaction conditionsa
L-proline (15 mol%)
_
+
I
S
O
O
OH
O
H2N
NH2
0-5 oC
+
H
NO2
NO2
2
4a
3a
Entry
Time (h)
Yieldb (%)
anti:sync
eed (%)
1e
2f
3g
4
5
6
96
96
24
24
48
72
96
96
96
96
96
80
58
88
85
85
92
93
20
53
70
48
90:10
79:21
88:12
91:9
92:8
90:10
93:7
85:15
76:24
89:11
84:16
94
78
87
96
96
95
97
61
89
86
72
7
8h
9i
10j
11k
a
The mixture of cyclohexanone (10 equiv), 4-nitrobenzaldehyde (1 equiv), L-proline (0.15 equiv), and S-benzylisothiouronium iodide (0.1 equiv) was left to stand at 96 h
inside a refrigerator (0–5 °C) with no stirring and no solvent.
b
Isolated yields after column chromatography.
c
anti/syn ratio was based on 1H NMR spectroscopic analysis of the crude reaction mixture.
The ee was determined by HPLC on chiral columns.
Reaction carried out with 5 mol % of co-catalyst.
Reaction carried out at room temperature.
Reaction mixture was stirred.
d
e
f
g
h
i
Hexane (3 mL) was employed with solvent.
Acetonitrile (3 mL) was employed with solvent.
H2O (5 equiv) was added.
j
k
H2O (10 equiv) was added.
Table 3
Direct aldol reaction between various aldehydes and cyclohexanone with isothiouronium iodide 1d
L-proline (15 mol%)
_
+
I
S
O
O
O
OH
H2N
NH2
+
H
X
X
0-5 oC, 96 h, neat, no stir
2
4
3
Entry
X
Product
Yielda (%)
anti:synb
eec (%)
1
2
3
4
5
6
2-Cl
4b
4c
4d
4e
4f
90
87
81
37
63
47
97:3
92:8
93:7
93:7
88:12
89:11
98
91
97
99
94
92
3-NO2
3-COCH3
2-OCH3
3-OH
H
4g
a
b
c
Isolated yields after column chromatography.
anti/syn ratio was based on 1H NMR spectroscopic analysis of the crude reaction mixture.
The ee was determined by HPLC on chiral columns.
summarized in Table 3. In all cases the reactions occurred with
high anti stereoselectivity. Electron-poor aldehydes such as 2-chlo-
robenzaldehyde, 3-nitrobenzaldehyde, and 3-acetylbenzaldehyde
proceeded smoothly with cyclohexanone, producing high
quantitative yields (81–90%) with high enantioselectivities (91–
98% ee, Table 3, entries 1–3). Meanwhile electron-rich aromatic
aldehydes such as 3-methoxybenzaldehyde and 3-hydroxybenzal-
dehyde produced the product in moderate yield (37–63%) although
with high enantioselectivity (Table 3, entries 4 and 5). The reac-
tions of aliphatic aldehydes have also been attempted but without
success, probably due to the low reactivity of these acceptors.
The mechanism of the reaction using isothiouronium iodide
1d is believed to be similar to that of the previously reported
L
-proline via an enamine intermediate.11a,16 A network of H-
bonding interactions between the carboxylate of
L-proline, the
corresponding isothiouronium salt, and the enamine in
a
Zimmerman–Traxler transition state (shown in Fig. 1) is estab-
lished, which leads to the nucleophilic addition of re-enamine
to the re-face of the carbonyl group. Therefore, the formation of
a 1:1 complex between the isothiouronium salt 1d and the L-pro-
line would stabilize the chair like transition state that leads to
the observed aldols.

E. Cho, T. H. Kim / Tetrahedron Letters 55 (2014) 6470–6473
6473
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Figure 1. Proposed transition state model.
In summary, we have developed a simple, efficient, and highly
stereoselective methodology for the direct aldol reaction of
cyclohexanone with aromatic aldehydes using L-proline as a chiral
catalyst. Catalytic amounts of isothiouronium iodide 1d have been
used for the first time as an achiral co-catalyst for this reaction,
which proved to be excellent catalysts, producing good to excellent
yields (up to 93%) with good stereoselectivities (up to 93:7 dr and
99% ee). This aldol protocol includes a solvent-free catalytic system
inside a refrigerator without stirring.
Acknowledgments
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (NRF-
2013R1A1A4A01006166). We wish to thank the Korean Basic
Science Institute, Gwangju Center, for analysis of the LC–MS/MS
Spectrometry.
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Supplementary data
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