Chiral bifunctional thiourea-catalyzed enantioselective aldol reaction of trifluoroacetaldehyde hemiacetal with aromatic ketones

Article abstract of DOI:10.1016/j.jfluchem.2011.04.019

In the presence of saccharide-derived bifunctional amine-thiourea catalysts, the direct aldol condensation of trifluoroacetaldehyde methyl hemiacetal with aromatic ketones proceeds to produce (R)-β-hydroxy β-trifluoroalkyl ketones in low to moderate yield

Full text of DOI:10.1016/j.jfluchem.2011.04.019

Journal of Fluorine Chemistry 132 (2011) 468–473
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Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Chiral bifunctional thiourea-catalyzed enantioselective aldol reaction of
trifluoroacetaldehyde hemiacetal with aromatic ketones
Jing Nie a, Xiao-Juan Li a, Dong-Hua Zheng a, Fa-Guang Zhang a, Shumin Cui b, Jun-An Ma a,
*
a Department of Chemistry, Tianjin University, Tianjin 300072, China
b Department of Chemistry, Henan Agricultural University, Zhengzhou 450002, China
A R T I C L E I N F O
A B S T R A C T
Article history:
In the presence of saccharide-derived bifunctional amine-thiourea catalysts, the direct aldol
condensation of trifluoroacetaldehyde methyl hemiacetal with aromatic ketones proceeds to produce
Received 10 February 2011
Received in revised form 27 April 2011
Accepted 28 April 2011
(R)-b
-hydroxy
b
-trifluoroalkyl ketones in low to moderate yields with good enantioselectivities.
ß 2011 Elsevier B.V. All rights reserved.
Available online 6 May 2011
Keywords:
Bifunctional organocatalyst
Saccharide
Thiourea
Aldol
Trifluoroacetaldehyde
1. Introduction
asymmetric reactions [10–12]. However, only several limited
reports involved the asymmetric aldol condensation of trifluor-
Over the past decade, the need for environmentally friendly and
metal-free reactions has led to great progress in the organocata-
lyst-mediated asymmetric synthesis. Numerous chiral organoca-
talysts have been designed, and a variety of new asymmetric
reactions have been developed in organic synthesis [1,2]. Of the
promising organocatalysts, chiral bifunctional amine-thioureas
have been proved to be powerful and have been applied
successfully in many asymmetric catalytic reactions [3,4], such
as Strecker reactions, Michael and Friedel-Crafts additions, as well
as Pictet–Spengler and nitro Mannich condensations. Neverthe-
less, little progress has been made in the development of chiral
thiourea-catalyzed asymmetric aldol reactions, and only one
report by Hiemstra [5] has addressed nitromethane as nucleophilic
species to react with aromatic aldehydes.
On the other hand, the asymmetric synthesis of chiral
trifluoromethyl-substituted secondary and tertiary alcohols has
attracted considerable attention due to the occurrence of this
moiety found in a number of biologically active compounds [6–9].
In the context, one attractive strategy is the asymmetric addition of
carbon nucleophiles to prochiral trifluoromethyl carbonyl com-
pounds. For example, trifluoroacetaldehyde and derivatives are
very important prochiral substrates, and have been used in many
oacetaldehyde and derivatives with ketones. Mikami et al.
presented the asymmetric aldol reaction of ketone-derived vinyl
ethers or silyl enol ethers with trifluoroacetaldehyde catalyzed by
a chiral binaphthol-derived titanium catalyst [13–15]. Funabiki
and coworkers developed another asymmetric transformation of
chiral ketimines with easily handling trifluoroacetaldehyde ethyl
hemiacetal instead of trifluoroacetaldehyde gas [16–19]. In
addition, several research groups also disclosed the asymmetric
aldol condensations of (a)cyclo alkyl ketones with trifluoroace-
taldehyde hemiacetal by the use of chiral organocatalysts [20–23].
However, to the best of our knowledge, no direct enantioselective
aldol reactions of aromatic ketones with trifluoroacetaldehyde
hemiacetal have been reported [24]. Design of new chiral
bifunctional organocatalysts [25–27] as well as development of
asymmetric reactions for the synthesis of chiral trifluoromethyl-
containing functionalized compounds [28–32] has been of our
interest and research objective. As part of our ongoing studies,
herein we would like to provide our preliminary results on chiral
bifunctional amine-thiourea-promoted direct enantioselective
aldol reaction of trifluoroacetaldehyde methyl hemiacetal with
aromatic ketones.
2. Results and discussion
Bifunctional chiral amine-thioureas have been demonstrated as
effective promoters for activation of nucleophilic ketone species
* Corresponding author. Tel.: +86 22 2740 2903; fax: +86 22 2740 3475.
E-mail address: majun_an68@tju.edu.cn (J.-A. Ma).
0022-1139/$ – see front matter ß 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.04.019

J. Nie et al. / Journal of Fluorine Chemistry 132 (2011) 468–473
469
O
OH
O
OH
Cat. (15 mol%)
CH2Cl2, rt, 6 d
F3C
+
F3C
S
O
3a
2a
S
S
OAc
OAc
OAc
O
O
O
AcO
AcO
AcO
AcO
AcO
AcO
N
N
N
H
1c
N
H
N
H
N
H
NH2
NHEt
OAc
H
1b
OAc
NH2
H
OAc
1a
20%, 50%ee
25%, 69%ee
12%, 52%ee
S
OAc
OAc
OAc
S
S
O
O
O
AcO
AcO
AcO
AcO
AcO
AcO
N
H
N
H
N
H
1f
N
H
N
H
N
H
NH2
NH2
OAc
NMe2
OAc
OAc
OAc
1e
1d
NR
10%, 34%ee
19%, 41% ee
OAc
AcO
OAc
O
O
S
AcO
AcO
O
OAc
S
O
AcO
N
H
N
H
O
OAc
NH2
AcO
AcO
O
OAc
1h
N
H
N
H
NH2
AcO
OAc
1g
10%, 58% ee
8%, 60% ee
S
S
F3C
N
N
N
H
N
H
NH2
NH2
H
H
H2N
NH2
F3C
1j
1k
23%, 41% ee
1i
10%, 32% ee
15%, 46% ee
Fig. 1. Screening of the bifunctional amine-thiourea catalysts.
involving an enamine intermediate. Accordingly, our investigation
began by screening these organocatalysts to evaluate their ability
to promote aldol condensation of acetophenone 2a with trifluor-
oacetaldehyde methyl hemiacetal at room temperature. As shown
in Fig. 1, several bifunctional primary and secondary amine-
thioureas (1a–c and 1e–f) were tested, the desired product 3a was
obtained in low yields with moderate to good enantioselectivities.
However, no reaction occurred when tertiary amine-thiourea 1d
was used as organocatalyst. The experimental results also showed
important effect on the stereoselection (Table 1, entries 1–10). The
reaction provided 25–31% yields and good enantioselectivities in
low polar solvents, such as arenes and chlorinated alkanes (Table 1,
entries 1, 2, and 8–10). No product was observed in coordinating
solvents and high polar solvents (Table 1, entries 3–7). Among the
solvents tested, CH2Cl2 was found to be the best with respect to
catalytic activity and asymmetric induction. Subsequently, a
preliminary screen of additives was carried out (Table 1, entries
11–16). No reaction occurred in the presence of molecular sieves.
The use of organic acids and bases did not have a significant effect
on the reaction rate and stereoselection. To our delight, an increase
of yields was observed by using H2O as the additive. The water
might accelerate the reaction by facilitating the interconversion of
the different intermediates of the catalytic cycle. Further
optimization of temperature, the loading of the additive and the
catalyst was finished (Table 1, entries 17–25). The good results
were attained when 15 mol% of thiourea 1a and 5 mol% of H2O
were employed at room temperature.
Under the optimized experimental conditions, the scope of the
reaction was explored (Table 2). It was found that ketones 2a–h
with electron-neutral or -withdrawing groups on aromatic rings
were used, the reactions proceeded to give modest yields and good
enantioselectivities (Table 2, entries 1–7). However, the presence
of electron-donating substituents on aromatic rings decreased the
reaction rate and lower yield was obtained for these ketone
substrates (Table 2, entries 8 and 9). In addition, heteroaromatic
methyl ketone 2j was also viable substrate to afford the desired
that the (R,R)-configuration of 1,2-diamine matched the
b-D-
glucopyranose to give the relatively high stereoselectivity (1a and
1e vs. 1b and 1f, respectively). In addition, a comparison between
the results obtained using catalysts 1a–b and 1e–f indicates that
the (1R,2R)-cyclohexanediamine moiety leads to better results
than the (1R,2R)-1,2-diphenylethylenediamine moiety. Maltose
and lactose-derived primary amine-thioureas 1g and 1h gave the
good enantioselectivities, albeit with poor yields. The two chiral
primary amine-thioureas 1i and 1j without sugar motif were
evaluated in this aldol reaction. The product was obtained with
lower enantioselectivity. These results demonstrated that the
carbohydrate motif could play a cooperative role in the stereo-
chemistry control for this title reaction. Interestingly, (1R,2R)-
cyclohexanediamine 1k directly promoted this condensation
reaction to give the desired product in 23% yield with 41% ee.
In order to increase the reaction rate and enantioselectivity,
variation of other reaction parameters were undertaken. The
results were listed in Table 1. A change of the solvent had an

470
J. Nie et al. / Journal of Fluorine Chemistry 132 (2011) 468–473
Table 1
Optimization of reaction conditions in the presence of 1a.
.
Entry
Solvent
Additive
Temperature (8C)
Time (d)
Yield (%)a
Ee (%)b
1
2
CH2Cl2
Toluene
Et2O
–
–
–
–
–
–
–
–
–
–
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
10
40
25
25
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
8
4
3
6
6
6
6
25
30
NR
NR
NR
NR
NR
24
31
30
NR
17
10
12
NR
45
45
38
45
40
40
15
46
45
42
69
65
–
3
4
THF
–
5
CH3CN
DMF
–
6
–
7
DMSO
ClCH2CH2Cl
CHCl3
–
8
65
65
65
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24c
25d
Benzene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
˚
5 A MS (10 mol%)
PhCO2H (10 mol%)
CF3CO2H (10 mol%)
iPr2NEt (10 mol%)
Pyridine (10 mol%)
H2O (10 mol%)
H2O (5 mol%)
60
60
59
–
68
68
60
68
68
68
68
68
68
65
H2O (1 mol%)
H2O (5 mol%)
H2O (5 mol%)
H2O (5 mol%)
H2O (5 mol%)
H2O (5 mol%)
H2O (5 mol%)
H2O (5 mol%)
a
Isolated yield. NR: no reaction.
b
c
The ee values were determined by HPLC. The absolute configuration was assigned according to Refs. [15–19].
20 mol% of 1a was used.
10 mol% of 1a was used.
d
Table 2
Enantioselective aldol reaction of trifluoroacetaldehyde methyl hemiacetal with aromatic ketones.
.
Entry
1
Ketone
Time (d)
5
Yield (%)a
45
Ee (%)b
68
O
2a
2
3
4
5
54
49
59
64
O
O2N
2b
O
Cl
2c

J. Nie et al. / Journal of Fluorine Chemistry 132 (2011) 468–473
471
Table 2 (Continued )
Entry
Ketone
Time (d)
5
Yield (%)a
42
Ee (%)b
4
5
6
57
60
60
O
2d
5
38
36
42
2e
6
6
2f
7
60
2g
8
9
6
6
20
13
62
58
O
MeO
Me
2h
O
2i
10
11
6
35
–
50
–
2j
10
2k
a
Isolated yield.
b
The ee values were determined by HPLC. The absolute stereochemistry of products 3b–j was assigned on the basis of analogy with 3a studies.
product 3j in moderate yield and enantioselectivity (entry 10).
Surprisingly, no reaction was observed in the reaction of 1-
naphthyl methyl ketone 2h with trifluoroacetaldehyde methyl
hemiacetal (Table 2, entries 11 vs. 4).
The stereogenic center of 4,4,4-trifluoro-1-phenyl-3-hydroxy-
1-butanones (3a), which was generated by 1a-catalyzed asym-
metric aldol condensation of trifluoroacetaldehyde hemiacetal
with 2a, was determined to be R by the comparison to the reported
values such as the optical rotation and the retention time of HPLC
using a chiral stationary phase [15–19]. The absolute stereochem-
istry of other products 3b–j was assigned on the basis of analogy
with 3a studies. These results show that the Re-face of the in situ
generated trifluoroacetaldehyde was predominantly approached
by the enamine intermediate formed from ketone and the primary
Fig. 2. TS structures for the formation of the R and S enantiomers.

472
J. Nie et al. / Journal of Fluorine Chemistry 132 (2011) 468–473
amine group of the bifunctional thiourea catalyst. The attack of the
enamine to the Si-face of the trifluoroacetaldehyde was restricted
by the thiourea scaffold of the catalyst (Fig. 2).
4.2.2. (R)-4,4,4-trifluoro-3-hydroxy-1-(naphthalen-2-yl)butan-1-
one ((R)-3d)
20
42%, [a] D + 18.28 (c1.0, CHCl3). 57% ee was determined HPLC
analysis [Daicel Chirapak OD-H, i-PrOH/hexane (5/95), 254 nm,
0.8 mL/min, tR = 22.20 min (major), 41.63 min (minor)]. 1H NMR
3. Conclusions
(500 MHz, CDCl3),
4.86 (m, 1H), 7.57–7.71 (m, 2H), 7.88–7.97 (m, 2H), 7.97–8.07 (m,
2H), 8.45–8.54 (m, 1H); 13C NMR (125 MHz, CDCl3)
38.3, 67.3 (q,
J = 31.8 Hz), 123.4 (q, J = 280.6 Hz), 127.2, 128.8, 129.1, 129.7,
130.4, 132.4, 133.3, 136.0, 197.4. 19F NMR (376 MHz, CDCl3)
À79.20 (d, J = 6.90 Hz, 3F). IR (KBr) 34382, 1690, 1608, 1520,
1160, 1106, 1047, 756, 704 cmÀ1
d 3.42–3.62 (m, 2H), 3.67–3.77 (m, 1H), 4.72–
We have developed an enantioselective organocatalytic
direct aldol reaction of trifluoroacetaldehyde methyl hemiace-
tal with a series of aromatic ketones by the use of bifunctional
amine-thiourea catalysts derived from commercially available
saccharides and chiral diamines. This direct condensation
d
d
n
.
proceeds to produce (R)-b-hydroxy b-trifluoroalkyl ketones
in low to moderate yields (13–54%) with good enantioselec-
tivities (50–68% ee). Further investigation of the reaction
mechanism, the improvement of this reaction rate and
enantioselectivity are ongoing in our laboratories and will be
reported in due course.
4.2.3. (R)-1-(4-bromophenyl)-4,4,4-trifluoro-3-hydroxybutan-1-one
((R)-3e)
20
38%, [a] D + 21.48 (c1.0, CHCl3). 60% ee was determined HPLC
analysis [Daicel Chirapak OD-H, i-PrOH/hexane (5/95), 254 nm,
0.8 mL/min, tR = 15.50 min (major), 16.78 min (minor)]. 1H NMR
(500 MHz, CDCl3),
4.79 (m, 1H), 7.57–7.73 (m, 2H), 7.33–7.41 (m, 2H); 13C NMR
(125 MHz, CDCl3) 38.3, 66.90 (q, J = 32.0 Hz), 123.3 (q,
J = 280.0 Hz), 126.1, 129.5, 129.7, 132.2, 134.7, 196.4. 19F NMR
d 3.21–3.45 (m, 2H), 3.62–3.78 (m, 1H), 4.65–
4. Experimental
d
4.1. General information
(376 MHz, CDCl3)
d
À79.18 (d, J = 6.87 Hz, 3F). IR (KBr)
n 3448,
1701, 1600, 1522, 1170, 1112, 1052, 758, 702 cmÀ1. MS (ESI): m/z
295.7 [MÀ1]+, 296.9 [M]+, 297.9 [M+1]+.
NMR was recorded on Varian Mercury Plus 500 instruments at
500 MHz (1H NMR) and 125 MHz (13C NMR). Chemical shifts were
reported in ppm down field from internal Me4Si. 19F NMR
(376 MHz) spectra were recorded on a Bruker ARX400 instrument
in CDCl3 solutions using CFCl3 as the internal standard. HPLC
analyses were carried out on a Hewlett Packard Model HP 1200
instrument. Optical rotations were determined using an Autopol
IV-T. IR spectra were recorded on an AVATAR 360 FT-IR
spectrometer. Tetrahydrofuran (THF), diethyl ether, toluene and
benzene were distilled from sodium/benzophenone prior to use;
CH2Cl2, ClCH2CH2Cl, CHCl3, CH3CN, DMSO and DMF were distilled
from CaH2. All purchased reagents were used without further
purification. All of bifunctional amine-thiourea catalysts were
synthesized according to the literatures [25–27].
4.2.4. (R)-1-(2-bromophenyl)-4,4,4-trifluoro-3-hydroxybutan-1-one
((R)-3f)
20
36%, [a] D + 3.48 (c1.0, CH2Cl2). 60% ee was determined HPLC
analysis [Daicel Chirapak OD-H, i-PrOH/hexane (5/95), 254 nm,
0.8 mL/min, tR = 17.49 min (minor), 31.14 min (major)]. 1H NMR
(400 MHz, CDCl3),
7.46 (m, 2H), 7.51 (d, J = 7.3 Hz 1H), 7.67 (d, J = 7.8 Hz 1H); 13C NMR
(100 MHz, CDCl3) 42.3, 67.1 (d, J = 32.5 Hz), 119.0, 127.7, 129.0,
132.6, 134.1, 140.0, 200.68. 19F NMR (300 MHz, CDCl3)
À79.34 (d,
3423, 1702, 1403, 1279, 1170, 1127,
d 3.35–3.69 (m, 3H), 4.69–4.71 (m, 1H), 7.36–
d
d
J = 6.70 Hz, 3F). IR (KBr)
n
1052, 760 cmÀ1. MS (ESI): m/z 296.8 [M]+.
4.2.5. (R)-1-(3-chlorophenyl)-4,4,4-trifluoro-3-hydroxybutan-1-one
4.2. General procedure for catalyzed aldol reaction
((R)-3g)
42%, [a 20
] D + 3.08 (c1.0, CH2Cl2). 60% ee was determined HPLC
Trifluoroacetaldehyde
1.0 mmol), acetophenone 2a (180 mg, 1.5 mmol), catalyst 1a
methyl
hemiacetal
(130 mg,
analysis [Daicel Chirapak OD-H, i-PrOH/hexane (5/95), 254 nm,
0.8 mL/min, tR = 14.36 min (minor), 18.00 min (major)]. 1H NMR
(75.5 mg, 0.15 mmol), and H2O (1.0 mg, 0.05 mmol) were placed
(400 MHz, CDCl3),
7.63 (d, 1H), 7.87 (d,1H),7.97 (s, 1H); 13C NMR (100 MHz, CDCl3)
38.5, 67.1, 126.3, 128.3, 134.0, 135.4, 137.6,196.0. 19F NMR (300 MHz,
d3.32–3.39 (m, 3H), 4.72–4.73 (m, 1H), 7.48 (t, 1H),
in
a
5 mL vial equipped with
a
Tefloncoated stir bar.
d
Dichloromethane (2 mL) was added under air. The vial was
capped with a white polyethylene stopper, and the resulting
mixture was stirred at room temperature for 5 days. Then the
reaction solution was concentrated in vacuo, and the crude was
purified by flash chromatography to afford the desired product
3a [15–19].
Other products 3b–d, and 3h–i [15–19,24], as known com-
pounds, were prepared according to the abovementioned proce-
dure.
CDCl3)
d
À79.26 (d, J = 6.75 Hz, 3F). IR (KBr)
n3420, 1696, 1629, 1423,
1280, 1169, 1122, 801, 563 cmÀ1. MS (ESI): m/z 252.3 [M]+.
4.2.6. (R)-4,4,4-trifluoro-3-hydroxy-1-(thiophen-2-yl)butan-1-one
((R)-3j)
35%, [a 20
] D + 2.28 (c1.0, CH2Cl2). 50% ee was determined HPLC
analysis [Daicel Chirapak AS-H, i-PrOH/hexane (5/95), 254 nm,
0.8 mL/min, tR = 17.81 min (minor), 23.56 min (major)]. 1H NMR
(400 MHz, CDCl3),
(s, 1H), 7.76-7.81 (m, 2H); 13C NMR (100 MHz, CDCl3)
J = 32.6 Hz), 128.4, 133.1, 135.1, 143.1, 190.0. 19F NMR (300 MHz,
d
3.30–3.35 (m, 2H), 3.54 (s, 1H), 4.70 (s, 1H), 7.21
4.2.1. (R)-4,4,4-trifluoro-3-hydroxy-1-phenylbutan-1-one ((R)-3a)
d
38.7, 67.2 (d,
20
98.1 mg (45% yield), [a] D + 12.28 (c1.0, CHCl3). 68% ee was
determined by HPLC analysis [Daicel Chirapak OD-H, i-PrOH/
hexane (5/95), 254 nm, 0.8 mL/min, tR = 26.5 min (S), 28.3 min
CDCl3)
d
À79.29(d, J = 6.45 Hz,3F). IR(KBr)
n3438, 1645,1559, 1514,
1416, 1280, 1169, 1126, 491 cmÀ1. MS (ESI): m/z 223.9 [M]+.
(R)]. 1HNMR (500 MHz, CDCl3)
d 3.35 (dd, J = 18.1 Hz, 1H), 3.43
Acknowledgements
(dd, J = 18.1 Hz, 1H), 3.70 (d, J = 2.95 Hz, 1H), 4.68–4.73 (m, 1H),
7.27–7.7.53 (m, 2H), 7.62–7.65 (m, 1H), 7.84–7.97 (m, 2H). 13C
NMR (125 MHz, CDCl3)
J = 280.4 Hz), 128.2, 128.8, 133.2, 136.0, 198.1. 19F NMR (376 MHz,
CDCl3) 3442, 1697, 1605,
À79.26 (d, J = 6.92 Hz, 3F). IR (KBr)
1519, 1167, 1104, 1049, 753, 706 cmÀ1
d
38.2, 67.1 (q, J = 32.0 Hz), 124.7 (q,
We gratefully appreciate the National Natural Science Founda-
tion of China (Nos. 20972110 and 20902067) and Tianjin Municipal
d
n
Science
& Technology Commission (No. 08JCYBJC09500) for
.
financial support.

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473
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