Use of surfactants in water-soluble ruthenium(II) complex-catalyzed asymmetric hydrogen-transfer reduction of aromatic ketones

Article abstract of DOI:10.1016/S0040-4039(01)02132-3

The introduction of surfactants to the water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants

Full text of DOI:10.1016/S0040-4039(01)02132-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 269–272
Use of surfactants in water-soluble ruthenium(II)
complex-catalyzed asymmetric hydrogen-transfer reduction of
aromatic ketones
Hae Yoon Rhyoo, Hee-Jung Park, Won Hyuk Suh and Young Keun Chung*
School of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-747, South Korea
Received 22 September 2001; revised 9 November 2001; accepted 12 November 2001
Abstract—The introduction of surfactants to the water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led
to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants © 2002 Elsevier
Science Ltd. All rights reserved.
Catalytic asymmetric hydrogenation using a chiral
ruthenium complex is a powerful method for producing
chiral alcohols and chiral amines with excellent enan-
The asymmetric transfer hydrogenation reactions of
ketones were carried out by using amino amides 1–3
(Chart 1), [RuCl (p-cymene)] , and HCOONa in water.
2
2
1
tioselectivity. Recently, the use of homogeneous cata-
In order to confirm the effect of surfactants on the
catalytic activity, we introduced several surfactants to
the ruthenium-catalyzed asymmetric hydrogen-transfer
reduction of 2%-methoxy acetophenone. We screened
surfactants, chiral ligands, and the reaction temperature
to optimize the reaction conditions (Table 1). When the
substrate to catalyst (S/C) ratio was 10, the reaction
without surfactants proceeded smoothly at 30°C. The
addition of sodium dodecyl sulfate (SDS) to the reac-
tion mixture shortened the reaction time from 4 to 1.5
h but did not affect the e.e. value. When the chiral
ligands 1–3 were screened with S/C of 100 at 30 and
40°C, the best result was obtained with ligand 3. Thus,
ligand 3 was chosen as a ligand. The increase of the
reaction temperature from 30 to 40°C dramatically
shortened the reaction time from 24 to 5.5 h. When the
surfactant was altered to cetyltrimethylammonium bro-
mide (CTAB) and tetrabutylphosphoniuim bromide
(TBPB), the best result was obtained with SDS. Using
SDS and ligand 3 at 40°C, the amount of HCOONa
used was screened. When the amount of HCOONa was
changed from 10 to 5 equiv., no noticeable differences
were observed. However, when the amount of
lysts in aqueous solution and on liquid supports has
become feasible, representing a tremendous opportunity
2
for the practice of green chemistry. As a consequence,
there has arisen considerable interest in the develop-
ment of water-soluble catalytic systems which allow
3
catalytic reactions to occur in water. However, novel
catalytic systems that enable the use of water as a
solvent in the asymmetric transfer hydrogenation, are
4
not well developed. The catalytic reaction occurs at the
interface between water and the organic layer because
of the insolubility of organic substrates in water. Thus,
the catalytic activity and/or enantioselectivity in
aqueous solution were usually low compared to those
5
of the homogeneous catalytic system. In order to
overcome this drawback, phase-transfer catalysts or
6
surfactants, or modified cyclodextrins were added to
the reaction mixture. Recently, we were successful in
carrying out asymmetric hydride transfer reduction of
ketones in aqueous solution using ruthenium complexes
7
with chiral amino amide ligands. However, the cata-
lytic activity and reusability were not satisfactory.
Thus, the catalytic reaction was reexamined more
closely by screening surfactants, chiral ligands, and
substrates. Herein we report our new findings, espe-
cially the effect of surfactants on the catalytic activity.
*
0
Corresponding author. Tel.: +82-2-880-6662; fax: +82-2-889-0310;
e-mail: ykchung@plaza.snu.ac.kr
Chart 1. Proline aromatic amide ligands.
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02132-3

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H. Y. Rhyoo et al. / Tetrahedron Letters 43 (2002) 269–272
a
Table 1. Optimization of asymmetric hydrogenation of 2%-methoxyacetophenone by using surfactants
Entry
L*
S/C
10
10
10
HCOONa (equiv.)
Surfactant
T (°C)
t (h)
Conv. (%)
E.e. (%)
1
2
3
4
5
6
7
8
9
1
1
2
3
1
3
1
2
3
3
3
3
3
3
3
3
3
3
3
10
10
10
10
10
10
10
10
10
10
10
5
30
30
30
30
30
30
40
40
40
40
40
40
40
50
40
40
40
40
40
4
1.5
1.5
1.5
24
24
5.5
5.5
5.5
6
99.8
97
99
95
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
CTAB
TBPB
SDS
SDS
SDS
–
95.8
95.7
95.8
93.1
93.6
91.8
94.3
93.9
93.3
93.3
93.9
86.9
91.6
93
10
99
100
100
100
100
100
100
100
100
100
100
100
1000
1000
400
400
12.7
93.4
92.4
87.5
99.5
13
75
98.5
41
10
11
12
13
14
15
16
17
18
19
6.5
5.5
7
2
5
5
5
3
70
5.5
24
24
24
16
39.5
43.4
34
20.5
99.7
SDS
–
–
93.6
94.5
92
b
5
5
SDS
94.1
a
M/L*=1 and surfactant 2 mol%. Conv. (%) and e.e. (%) were determined by a chiral column CP Chirasil DEX CB 25 m×0.32 mm and the
configuration R was determined by the retention time.
b
In dichloromethane, a mixture of HCOOH/Et N (5/2 azeotrope distilled) was used as a hydrogen source.
3
HCOONa was lowered to 2 equiv., the catalytic activity
and the enantioselectivity were considerably reduced.
Thus, the amount of HCOONa must be over 5 equiv.
When the reaction temperature was increased to 50°C
under the same reaction conditions, the catalytic species
changed to turbid and acquired a brownish color. After
ery and reusability of the catalytic system were tested
by carrying out consecutive cycles with the same cata-
lyst (S/C=400) in aqueous solution, carefully separated
from the organic phase at the end of each run (Table 2).
At the end of each run, substrates and 1.0 equiv.
HCOOH were added. Interestingly, the reusability of
the catalytic system was quite dependent upon the use
of formic acid. When pure formic acid was used as a
hydrogen source, in the second run the conversion yield
3
h of the reaction, the conversion rate was 70%.
Increasing the reaction time did not improve the reac-
tion yield. Thus, the reaction temperature was fixed at
40°C. The optimum reaction conditions were as fol-
(
99%) and the e.e. (94.2%) were quite good, as in the
lows: ligand 3, SDS, 5 equiv. HCOONa, and at 40°C.
In order to calculate the maximum turnover number
under the optimum reaction conditions, the S/C was
raised to 1000. After the reaction, 43.4% of the conver-
sion yield and 93.6% of the e.e. value were obtained. A
prolonged reaction time (3 days) did not result in
increasing the conversion yield. When the reaction (S/
C=100) was carried out without using surfactants
under the optimum reaction conditions, the conversion
yield and e.e. were 39.5 and 93%, respectively. When
S/C was 400, the conversion yield and e.e. were 20.5
and 92%, respectively. Thus, the conversion yield was
highly dependent upon the presence of surfactants, but
the e.e. value was invariant. When the reaction (S/C=
first run (99.7% and 94.1% e.e.). But, in the third run,
the conversion yield was only 8% and the e.e. was not
determined. However, when 0.5 equiv. of 1.0 M
HCOOH was used, the catalytic system could be used
in the forth run even though the conversion yield was
lowered to 35.8% and the e.e. was 89%. Thus, the
overall turnover number could be increased to 1270.
a
Table 2. Recycling test
Recycle no.
Additiveb
t (h)
Conv. (%)
E.e. (%)
1
2
16
30
99.7
99
94.1
94.2
99%
1
000) was carried out by using the homogeneous cata-
HCOOH
99%
lyst in dichloromethane, the conversion yield was 34%
and the e.e. was 94.5%. A prolonged reaction time (3
days) did not improve the conversion yield. Thus, the
introduction of the surfactant to the catalytic system
improved the conversion yield with even better results
than for the homogeneous reaction.
3
48
8
N.D.
HCOOH
1
2
15.5
23
99.7
99.7
94.2
94.6
1 M
HCOOH
1 M
HCOOH
1 M
3
4
25
39
82
94.3
89
35.8
Heterogeneous catalysts often suffer extensive leaching
of active metal species during reactions and eventually
HCOOH
8
lose their catalytic activity. Many organic substances
a
The reaction conditions were the same as used in entry 19 of Table
are insoluble and many reactive substrates and catalysts
are decomposed or deactivated by water. Thus, recov-
1.
b
1 equiv. of 99% HCOOH or 0.5 equiv. 1 M HCOOH was added.

H. Y. Rhyoo et al. / Tetrahedron Letters 43 (2002) 269–272
271
Table 3. Asymmetric transfer hydrogenation of some aro-
were quite different from each other. Thus, the
methoxy group on the ortho position dramatically
improved the enantioselectvity. The order of enantiose-
lectivities for acetophenone derivatives is 2%-methoxy>
a
matic ketones
2
%-Meꢀ2%-Brꢀ2%-ClꢁH. This observation is quite
similar to that for the homogeneous system although
9
the details differ. When (1S,2S)-DPEN-Ts was used as
a ligand in the homogeneous catalytic system, quite a
low conversion yield and poor enantioselectivity were
1
0
obtained for ortho-substituted aromatic ketones.
This work shows that the introduction of surfactants to
the water-soluble ruthenium-catalyzed system improves
the catalytic activity and reusability and the resulting
catalytic system is quite effective for the asymmetric
hydrogen transfer of ortho-substituted aromatic
ketones.
A typical procedure for catalytic reactions
Ruthenium complex [RuCl (p-cymene)] (3.1 mg) and
2
2
chiral ligand 3 (2.1 mg) were dissolved in 10 ml of
water. After the solution was stirred at 40°C for 1 h, 5
equiv. HCOONa was added to the solution. After the
solution was stirred for 10 min, 2%-methoxyacetophe-
none (4.0 mmol) and SDS (23 mg, 2 mol%) were added
to the solution. The resulting solution was allowed to
react for the predetermined reaction time. After reac-
tion, a mixture solvent (3×5 ml) of n-hexane and
diethyl ether (v/v, 10:1) was added to extract organic
materials. GC analysis of a sample aliquot then deter-
mined the conversion rate and the enantioselectivity.
Recycling test
We have also examined the hydrogen-transfer reaction
of other aromatic ketones (Table 3). For the hydrogen-
transfer reaction of acetophenone (entry 1), the conver-
sion yield (98.3%) was good but the e.e. (68.6%) was
poor. For the hydrogen-transfer reaction of propiophe-
none (entry 2), the conversion yield (63%) and the e.e.
After the organic materials were extracted, substrate,
surfactant, and HCOOH were added to the aqueous
solution. The solution was allowed to react for the
predetermined reaction time.
(
53.7%) were not comparable to those of 2%-acetophe-
Acknowledgements
none. Thus, as the steric bulkiness increased, the con-
version yield and the enantioselectivities decreased. For
the hydrogen-transfer reaction of 2%-acetonaphthone
Y.K.C. thanks the financial supports from KOSEF
(1999-1-122-001-5) and the KOSEF through the Center
for Molecular Catalysis at Seoul National University
and H.Y.R., W.H.S. and H.J.P. acknowledge receipt of
the BK21 fellowship.
(
entry 3), the conversion yield was 85.3% and the e.e.
was 89.4%. The enantioselectvity was highly improved.
For the hydrogen-transfer reaction of 2%-substituted
acetophenones (entries 4–6), the conversion yields were
poor but the e.e. values were quite high, ca 90%. Thus,
the ortho-substitution by Me, Cl, and Br decreased the
conversion yield, but increased the enantioselectivity.
For the hydrogen-transfer reaction of 3%-methoxyace-
tophenone and 4%-methoxyacetophenone (entries 7 and
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1