Asymmetric hydrogenation of aromatic ketones catalyzed by achiral monophosphine TPPTS-stabilized Ru in ionic liquids

Article abstract of DOI:10.1016/j.tetasy.2007.07.016

Achiral monophosphine TPPTS [TPPTS: P(m-C₆H₄SO₃Na)₃]-stabilized Ru was synthesized by reduction of RuCl₃·3H₂O with hydrogen in ethanol using TPPTS as the stabilizer. The catalytic asymmetri

Full text of DOI:10.1016/j.tetasy.2007.07.016

Tetrahedron: Asymmetry 18 (2007) 1643–1647
Asymmetric hydrogenation of aromatic ketones catalyzed by
achiral monophosphine TPPTS-stabilized Ru in ionic liquids
a,b
a
b
a
a
Jinbo Wang, Jian Feng, Ruixiang Qin, Haiyan Fu, Maolin Yuan,
a,
a
*
Hua Chen and Xianjun Li
a
Key Lab of Green Chemistry and Technology, Ministry of Education, The Institute of Homogeneous Catalysis,
College of Chemistry, Sichuan University, No. 29 Wang jiang Road, Chengdu, Sichuan 610064, PR China
b
College of Chemical Engineering, Weifang Scientific and Technology College, Weifang 262700, PR China
Received 16 May 2007; accepted 11 July 2007
6 4 3 3 3 2
Abstract—Achiral monophosphine TPPTS [TPPTS: P(m-C H SO Na) ]-stabilized Ru was synthesized by reduction of RuCl Æ3H O with
hydrogen in ethanol using TPPTS as the stabilizer. The catalytic asymmetric hydrogenation of aromatic ketones using TPPTS-stabilized
Ru modified by a chiral diamine (1R,2R)-DPENDS [disodium salt of sulfonated (1R,2R)-1,2-diphenyl-1,2-ethylene-diamine] was inves-
tigated in hydrophilic ionic liquid [RMIM]Ts (1-alkyl-3-methylimidazolium p-methylphenylsulfonates, R = ethyl, butyl, octyl, dodecyl,
hexadecyl). Hundred percent conversion and 85.1% ee were obtained for acetophenone under optimized conditions. The resulting prod-
ucts can be easily separated from the catalyst immobilized in ionic liquid by simple extraction with n-hexane, and the catalyst can be
reused several times without a significant loss of ee value or conversion. In particular, the addition of water can improve the catalyst
performance.
ꢀ
2007 Elsevier Ltd. All rights reserved.
1. Introduction
introduction of ligands as nanoparticle stabilizers is of
special interest.
7
Homogeneous catalysts show distinguished catalytic prop-
1
erties in asymmetric hydrogenation of simple ketones, but
Herein we report that newly formed achiral monophos-
phine TPPTS-stabilized Ru modified by (1R,2R)-DPENDS
(see Fig. 1) can asymmetrically hydrogenate aromatic
ketones in an ionic liquids [RMIM]Ts (see Fig. 2) and
water mixed solvent under mild reaction conditions (see
Fig. 3), and that the resulting products can be easily sepa-
rated from the catalyst by extraction with n-hexane.
these catalysts are very expensive and are difficult to recycle
and reuse. Ionic liquids (ILs) have attracted a great deal of
interest because of their capability to immobilize homo-
geneous catalysts and to facilitate catalysts recycling.
In recent years, the asymmetric hydrogenation of
enamides, arylacrylic acids, aromatic ketones, imines, and
b-ketoesters in ionic liquids have been reported. The
2
2
,3
4
results indicate that the products can be easily separated
by extraction with organic solvent while ionic liquids
containing homogeneous catalysts could be reused several
times without the loss of catalytic activity or enantio-
selectivity.
NaO3S
SO3Na
H2N
NH2
Figure 1. (1R,2R)-DPENDS.
Furthermore, ligand-stabilized metal nanoparticles have
5
,6
been reported in enantioselective reactions,
and the
The TPPTS-stabilized Ru catalyst not only exhibits excel-
lent activity and enantioselectivity in asymmetric hydro-
genation of aromatic ketones, but also simplifies its recycling
and reuse.
*
Corresponding author. Tel./fax: +86 28 8451 2904; e-mail: scuhchen@
63.com
1
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.07.016

1644
J. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 1643–1647
liquid. However, a lower activity and enantioselectivity are
N
N
SO3
observed when only water is used as a solvent. A similar
phenomenon was observed in the asymmetric hydrogena-
R
9
R= ethyl, butyl, octyl, dodecyl, hexadecyl
tion of a,b-unsaturated ketones in our previous work.
Figure 2. Ionic liquids [RMIM]Ts.
Table 1. Effect of water on the asymmetric hydrogenation of aceto-
a
phenone
OH
O
∗
V Ils=V H O
Conversion (%)
ee (%)
Configuration
2
TPPTS-Ru / (1R,2R)-DPENDS-KOH
R2
R2
No water
94.6
100
100
94.1
76.8
45.1
80.3
74.9
74.5
35.9
(S)
(S)
(S)
(S)
(S)
R1
R1
1
1
:0.5
:1
ionic liquid [RMIM]Ts-H2O, H2
R1 = H, o-F, o-Cl , o-Br, o-CH3O, p-CF3, p-CH3O, R2 = CH3
1:1.5
No IL
R1 =H , R2 = C2H5
a
Reaction conditions: acetophenone: 0.85 mmol; Ru/acetophenone/
1R,2R)-DPENDS = 1:112:9; ionic liquid: 0.5 mL; KOH: 0.7 mol/L;
Figure 3. Asymmetric hydrogenation of aromatic ketones.
(
PH : 5.0 MPa; T: 30 ꢁC; t: 80 min.
2
2
. Results and discussion
2.3. Influence of different ionic liquids and temperatures
on the asymmetric hydrogenation of acetophenone
2
.1. Determination of TPPTS-stabilized Ru
The morphology of TPPTS-stabilized Ru was investigated
with a transmission electron microscope (JEM-1200) at an
accelerating voltage of 100 kV. It can be seen that the size
of TPPTS-stabilized Ru is about 5 nm (see Fig. 4).
The results listed in Table 2 show that the conversions and
ee values of asymmetric hydrogenation are sensitive to the
nature of the ionic liquids. The ethyl group in [RMIM]Ts
was substituted by another alkyl group, the conversion
and enantioselectivity decreased gradually with an increase
of the alkyl chain length. An increase of the alkyl chain
length in [RMIM]Ts is not favorable, due to the solubility
of the water-soluble chiral ligand (1R,2R)-DPENDS in
ionic liquids and the chiral modification of (1R,2R)-
DPENDS to Ru catalyst is reduced, therefore, the
enantioselectivity decreases. In our previous work, similar
9
,10
phenomena were also observed.
Table 2. Effect of different ionic liquids on the asymmetric hydrogenation
a
of acetophenone
Ionic liquids
Conversion (%)
ee (%)
Configuration
[
[
EMIM]Ts
BMIM]Ts
100
100
84.3
18.6
7.0
80.3
77.1
58.7
46.7
7.7
(S)
(S)
(S)
(S)
(S)
Figure 4. TEM photography of TPPTS-stabilized Ru.
[OMIM]Ts
[
[
DoMIM]Ts
HeMIM]Ts
The X-ray photoelectron spectroscopy (XPS) spectrum of
TPPTS-Ru showed the Ru 3d binding energies at
a
Ionic liquid/H
as those in Table 1.
2
O (v/v) = 1:0.5, other reaction conditions were the same
2
80.6 eV and 284.8 eV, indicating that Ru has been
reducted to the low oxidation state.
The data listed in Table 3 indicate that the enantioselectiv-
ity increases with the decrease of temperature; 85.1% ee
2.2. Influence of water on the asymmetric hydrogenation
of acetophenone
Literature information indicates that water affects the
8
reaction when ILs are used as a solvent. In order to
Table 3. The effect of temperature on the asymmetric hydrogenation of
acetophenone
improve the catalytic performance of TPPTS-stabilized
Ru-(1R,2R)-DPENDS–KOH catalyst, we added some
water to evaluate its effect on the asymmetric hydrogena-
tion of aromatic ketones. The results, as shown in Table
a
Temperature (ꢁC)
Conversion (%)
ee (%)
Configuration
5
69.6
98.2
100
100
85.1
83.3
80.3
80.2
(S)
(S)
(S)
(S)
1
3
4
5
0
5
1
, suggest that the addition of appropriate amounts of
water can increase the ee value from 45.1% to 80.3%. The
increase in the ee values can be attributed to the enhance-
ment of the solubility of the water-soluble catalyst in ionic
a
Reaction conditions were the same as those in Table 2.

J. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 1643–1647
1645
Table 4. Comparison of TPPTS-stabilized Ru with RuCl
2
(TPPTS)
2
in the asymmetric hydrogenation of aromatic ketones
a
b
Substrates
TPPTS-stabilized Ru
Conversion (%)
100
RuCl
2 2
(TPPTS)
ee (%)
Conversion (%)
ee (%)
Acetophenone
80.3
82.7
76.5
50.3
9.2
3.6
80.3
82.9
57.7
0
0
2
4
-Bromo-acetophenone
-Methoxy-acetophenone
99.6
96.0
a
Ionic liquid/H
2
O (v/v) = 1:0.5, other reaction conditions were the same as those in Table 1.
b
Reaction conditions were the same as those of TPPTS-stabilized Ru.
was obtained at 5 ꢁC, which is the highest value using
TPPTS as the stabilizer.
Table 6. The effect of KOH concentration on the asymmetric hydroge-
nation of acetophenone
a
Concentration (mol/L) Conversion (%) ee (%) Configuration
0
0.5
37.2
99.3
100
100
98.6
45.3
77.3
79.3
77.7
80.3
80.1
(S)
(S)
(S)
(S)
(S)
(S)
2.4. Comparison of TPPTS-stabilized Ru with
RuCl (TPPTS) in the asymmetric hydrogenation
of aromatic ketones
0
0
0
0
1
.23
.46
.70
.92
.15
2
2
As seen in Table 4, asymmetric hydrogenation of aceto-
0
0
phenone, 2 -bromo-acetophenone and 4 -methoxy-aceto-
phenone catalyzed by TPPTS-stabilized Ru catalyst, all
afforded high activities (>96.0% conversion) and good
enantioselectivities. Although similar high ee values were
obtained by RuCl (TPPTS) , the TPPTS-stabilized Ru
a
Reaction conditions were the same as those in Table 2.
(
1R,2R)-DPENDS and KOH. A similar phenomenon was
observed in our previous work.
10
2
2
catalyst is definitely more active under the same reaction
conditions. With respect to the catalyst precursor
RuCl (TPPTS) , there was a long induction period before
2.6. Asymmetric hydrogenation of different aromatic ketones
2
2
conversion to the active species RuHCl(TPPTS) , which
resulted in a lower activity.
2
Various aromatic ketones were hydrogenated with TPPTS-
stabilized Ru-(1R,2R)-DPENDS–KOH catalyst in an ionic
liquid-water mixed solvent and the results are summarized
in Table 7. The results indicate that this novel catalyst
2
.5. Influence of the mole ratio of ruthenium to (R,R)-
0
DPENDS and KOH concentration on asymmetric
hydrogenation of acetophenone
shows high reactivities except with 2 -methoxyacetophe-
none; high enantioselectivities were obtained in the asym-
metric hydrogenation of acetophenone, propiophenone,
0
As shown in Tables 5 and 6, the influence of (1R,2R)-
DPENDS and KOH on the asymmetric hydrogenation of
acetophenone is very significant. The conversion was only
and 2 -bromoacetophenone. The substituent on the phenyl
ring, whether it was an electron-withdrawing group or elec-
tron-donating group, obviously influences the enantioselec-
tivity of aromatic ketones. Furthermore, the steric effect,
which influences the reactant–modifier interaction, mark-
edly affects the activity and enantioselectivity.
9
.0% and no ee value was obtained in the absence of
(
1R,2R)-DPENDS. The addition of (1R,2R)-DPENDS
could apparently increase the catalytic activity and enanti-
oselectivity. When the molar ratio of ruthenium to
(
1R,2R)-DPENDS was 1:9, the conversion and ee value
a
Table 7. Asymmetric hydrogenation of acetophenone and its derivatives
could reach up to 100% and 80.3%, respectively. Similarly,
if no KOH was added, the conversion and ee value were
only 0.5% and 45.3%, respectively. With an increase of
KOH concentration, the conversion and ee value obviously
increased. When the concentration of KOH was 0.92 mol/
L, 100% conversion and 80.3% ee were obtained. However,
increasing the KOH amount resulted in no increase in the
conversions and ee values. The results shown in Tables 5
and 6 indicate that there was a synergistic effect between
Substrates
Conversion ee
Configuration
(
%)
(%)
Acetophenone
Propiophenone
99.6
99.3
99.6
99.8
99.7
47.6
99.5
96.0
80.3
80.0
82.7
77.3
54.3
45.4
58.7
76.5
(S)
(S)
(S)
(S)
(S)
(R)
(S)
(S)
0
2
2
2
2
4
4
-Bromoacetophenone
-Chloroacetophenone
-Fluoroacetophenone
-Methoxyacetophenone
-(Trifluoromethyl)acetophenone
-Methoxyacetophenone
0
0
0
0
0
a
Table 5. The effect of the mole ratio of ruthenium to (1R,2R)-DPENDS
on the asymmetric hydrogenation of acetophenone
Reaction conditions were the same as those in Table 2.
a
Mole ratio
Conversion (%)
ee (%)
Configuration
2.7. Catalyst recycling
No (R,R)-DPENDS
9.0
0
—
(S)
(S)
(S)
1:3
1:6
1:9
95.0
99.5
99.6
80.3
79.9
80.3
For the catalyst used in this study, we have also demon-
strated that the chiral alcohol products could be easily
separated by extraction with n-hexane, while the catalyst
and modifiers immobilized in ionic liquid and water could
a
Reaction conditions were the same as those in Table 2.

1646
J. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 1643–1647
be recycled and reused several times. As shown in Table 8,
although the reactivities of the TPPTS-stabilized Ru cata-
lyst started to drop from the fifth run, the ee value could
still be maintained at above 75%. When 0.18 mmol KOH
was added into the reaction system in the seventh run,
the conversion slightly increased from 63.8% (run 6) to
bilizer TPPTS. After purging with H five times, the final
2
H2 pressure was adjusted to 50 atm. After stirring at
120 ꢁC for 4 h, a black ‘solution’ was obtained. After
removal of the solvent, the TPPTS-stabilized Ru was yielded.
The TEM of TPPTS-stabilized Ru was examined using a
JEM-1200 microscope operating at an accelerating voltage
of 100 kV.
7
3.2% (run 7), which proved the novel influence of the base
on catalytic activity. Similar phenomena were observed in
the asymmetric hydrogenation of a,b-unsaturated ketones
in ionic liquids-water in our previous work. Inductively
9
4.3. Asymmetric hydrogenation of aromatic ketones
coupled plasma (ICP) spectroscopy further showed that
no appreciable leaching of Ru occurred during the extrac-
tion of organic products. We estimated from ICP experi-
ments that less than 0.06% of the Ru catalyst had
leached into the chiral alcohol phase from the ionic liquid
phase.
To a 60 mL stainless steel autoclave with a glass linear and
magnetic stirrer were added the ionic liquid [EMIM]Ts,
TPPTS-stabilized Ru, (1R,2R)-DPENDS, KOH, H O,
2
and substrate. Hydrogen was introduced to the desired
pressure after the reaction mixture had been purged with
H five times. The mixture was stirred at 30 ꢁC for a prede-
2
termined period of time. After hydrogen was vented, n-hex-
ane (5 mL) was added to extract the organic product. The
resulting two layers were mixed thoroughly by vigorous
stirring for 5 min. The upper solution layer was separated
by decantation. This manipulation was repeated one more
time. The combined extracts containing the product were
purified by silica gel (yield 98%). The products were ana-
Table 8. Recycling and reuse of TPPTS-stabilized Ru-(1R,2R)-DPENDS–
KOH catalyst in ionic liquid [EMIM]Ts/H O
2
a
Run
Conversion (%)
ee (%)
Configuration
1
2
3
4
5
6
99.8
92.1
91.0
92.1
70.0
63.8
73.2
80.3
78.7
78.7
77.4
76.5
75.9
77.5
(S)
(S)
(S)
(S)
(S)
(S)
(S)
TM
lyzed by GC-960 with a FID detector and b-DEX 120
capillary column (30 m · 0.25 mm, 0.25 lm film). The
enantiomeric excess (ee value) was calculated from the
equation: ee (%) = 100 · (S ꢀ R)/(S + R). The recovered
catalyst in ionic liquids was reused in the next batch of
the catalytic reaction under identical conditions.
b
7
a
Reaction conditions were the same as those in Table 2.
b
0.18 mmol of KOH was added.
2
D
8
(
S)-(ꢀ)-1-Phenylethanol: ½aꢁ ¼ ꢀ23:2 (c 1.08, CH Cl ),
2
2
3
. Conclusions
80.3% ee, (S); column temperature: 115 ꢁC, t (R) =
R
1
2.6 min, t (S) = 13.3 min.
R
In conclusion, we have prepared the TPPTS-stabilized Ru
catalyst, and successfully applied it to the asymmetric
hydrogenation of aromatic ketones in an ionic liquid-water
mixed solvent, which gave high activity and good enanti-
oselectivity. The resulting products can be easily separated
from the catalyst system by extraction with n-hexane. The
catalyst immobilized in ionic liquids can be reused several
times without any significant loss of the ee value and
conversion.
2
D
8
(S)-(ꢀ)-1-Phenylpropanol: ½aꢁ ¼ ꢀ39:5 (c 1.52, C H OH),
2
5
80.0% ee, (S); column temperature: 120 ꢁC, t (R) =
R
16.3 min, t (S) = 16.8 min.
R
0
28
D
(S)-(ꢀ)-1-(2 -Fluorophenyl)ethanol: ½aꢁ ¼ ꢀ35:7 (c 1.26,
CHCl ), 54.3% ee, (S); column temperature: 110 ꢁC,
3
t (R) = 15.7 min, t (S) = 17.4 min.
R
R
0
28
D
(
S)-(ꢀ)-1-(2 -Chlorophenyl)ethanol: ½aꢁ ¼ ꢀ29:5 (c 1.76,
CHCl ), 77.3% ee (S); column temperature: 140 ꢁC,
3
4
. Experimental
t (R) = 12.8 min, t (S) = 14.8 min.
R
R
0
28
D
4
.1. Materials
-Methylimidazole (Fluka, >99%) and 1-butylbromide
(S)-(ꢀ)-1-(2 -Bromophenyl)ethanol: ½aꢁ ¼ ꢀ31:3 (c 1.60,
CHCl ), 82.7% ee, (S); column temperature: 140 ꢁC,
3
1
t (R) = 21.2 min, t (S) = 26.5 min.
R
R
(
Lancaster, >99%) were distilled before use. Aromatic
0
28
D
ketones (>98% Acros) and hydrogen (99.99%) were used
as received. RuCl Æ3H O and other reagents were all of
analytical grade. TPPTS, ionic liquids [RMIM]Ts,
(R)-(+)-1-(2 -Methoxyphenyl)ethanol: ½aꢁ ¼ þ33:3 (c
1.05, CHCl ), 45.4% ee, (R); column temperature: 135 ꢁC,
3
2
3
t (R) = 15.9 min, t (S) = 16.8 min.
R
R
RuCl (TPPTS) and chiral modifier (1R,2R)-DPENDS
2
2
0
28
D
were synthesized according to known methods in our
(S)-(ꢀ)-1-(4 -Methoxyphenyl)ethanol: ½aꢁ ¼ ꢀ29:1 (c
1
1
laboratory.
1.03, CHCl ), 76.5% ee, (S); column temperature: 115 ꢁC,
3
t (R) = 20.6 min, t (S) = 21.5 min.
R
R
4
.2. Synthesis and determination of TPPTS-stabilized Ru
0
28
D
(
S)-(ꢀ)-1-(4 -Trifluoromethylphenyl)ethanol: ½aꢁ ¼ ꢀ39:0
To a 60 mL stainless steel autoclave with a glass linear and
(c 1.41, CH OH), 58.7% ee, (S); column temperature:
3
magnetic stirrer were added ethanol, RuCl Æ3H O and sta-
120 ꢁC, t (R) = 13.0 min, t (S) = 14.6 min.
3
2
R
R

J. Wang et al. / Tetrahedron: Asymmetry 18 (2007) 1643–1647
1647
Acknowledgment
L.-C.; Ruan, J.-W.; Fan, Q.-H.; Chan, A. S. C. Can. J. Chem.
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This work was financially supported by the NSFC
6
(
20272037) and the Doctor’s Foundation of Education
1
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200; (b) Zuo, X.; Liu, H.; Guo, D.; Yang, X. Tetrahedron
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