PEG-400-H2O as a green and recyclable medium for asymmetric hydrogenations of aromatic ketones catalyzed by RuCl2(TPPTS) 2-(S,S)-DPENDS

Article abstract of DOI:10.1016/j.tetasy.2012.06.006

PEG-400-H₂O was found to be a green and recyclable reaction medium for asymmetric hydrogenations of aromatic ketones catalyzed by a ruthenium achiral monophosphine complex RuCl₂(TPPTS)₂ [TPPTS: P(m-C₆H₄SO₃Na)₃] modified by (S,S)-DPENDS [disodium salt of sulfonated (S,S)-1,2-diphenyl-1,2-ethylene- diamine]. The acetophenone product was obtained with 86.3% ee under the optimized conditions. The resulting products can be easily separated from the catalyst by extraction with n-hexane. The catalyst immobilized in PEG-400-H ₂O not only exhibits excellent activity and enantioselectivity, but also can be recycled and reused several times without a loss of activity or enantioselectivity.

Full text of DOI:10.1016/j.tetasy.2012.06.006

Tetrahedron: Asymmetry 23 (2012) 834–837
Contents lists available at SciVerse ScienceDirect
Tetrahedron: Asymmetry
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PEG-400-H2O as a green and recyclable medium for asymmetric
hydrogenations of aromatic ketones catalyzed by RuCl2(TPPTS)2-(S,S)-DPENDS
Ruixiang Qin a, Jinbo Wang a, , Wei Xiong a, Hua Chen b, Jian Feng a, Derong Liu a
⇑
a College of Chemistry and Chemical Engineering, Chongqing University of Science and Technology, Chongqing 401331, China
b Key Laboratory of Green Chemistry and Technology, Ministry of Education, Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University, Chengdu 610064,
Sichuan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 April 2012
Accepted 8 June 2012
PEG-400-H2O was found to be a green and recyclable reaction medium for asymmetric hydrogenations of
aromatic ketones catalyzed by a ruthenium achiral monophosphine complex RuCl2(TPPTS)2 [TPPTS:
P(m-C6H4SO3Na)3] modified by (S,S)-DPENDS [disodium salt of sulfonated (S,S)-1,2-diphenyl-1,2-ethyl-
ene-diamine]. The acetophenone product was obtained with 86.3% ee under the optimized conditions.
The resulting products can be easily separated from the catalyst by extraction with n-hexane. The catalyst
immobilized in PEG-400-H2O not only exhibits excellent activity and enantioselectivity, but also can be
recycled and reused several times without a loss of activity or enantioselectivity.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
H2O can be used as a green and recyclable reaction medium for
the asymmetric hydrogenation of aromatic ketones catalyzed by
Asymmetric hydrogenation is a useful method for the synthesis
of enantiomerically pure compounds. Homogeneous catalysts
show distinguished catalytic properties in asymmetric hydrogena-
tions of simple ketones,1–4 but these catalysts are very expensive
and difficult to reuse.5
RuCl2(TPPTS)2-(S,S)-DPENDS (Fig. 1) catalyst under mild reaction
conditions (Fig. 2). Meanwhile, the resulting products can be easily
separated from the catalyst by extraction with n-hexane. The cat-
alyst in a PEG-400-H2O medium not only exhibits excellent reac-
tivities and enantioselectivities, but also simplifies its recycling
and reuse.
Ionic liquids have attracted a great deal of interest because of
their capability to immobilize homogeneous catalysts and to offer
an interesting approach to solve the aforementioned problems.6
In recent years, ionic liquids for the asymmetric hydrogenation of
prochiral olefins, ketones, imines, and b-ketoesters have been re-
ported,7–13 and the results indicate that the organic products can
be easily extracted by less polar organic solvents; furthermore the
ionic liquids containing homogeneous asymmetric hydrogenation
catalysts could be reused several times without a significant loss
of catalytic activity or enantioselectivity. However, ionic liquids
have the disadvantages of their high price and tedious preparation.
Recently, liquid PEGs have attracted increasing interest as novel
solvents for organic reactions due to their benign characteris-
tics.14,15 In particular, PEGs are inexpensive and have low toxicity.
However, PEGs have rarely been reported as reaction media for
asymmetric hydrogenations.16–22 Furthermore, organic solvents
are usually added as the co-solvent of PEGs to enhance the solubility
of the homogeneous catalysts in catalyst systems (see Figs. 1 and 2).
We are interested in the asymmetric hydrogenation of prochiral
ketones in biphasic systems.23–27 It was reported that PEG-400-
NaO3S
SO3Na
H2N
NH2
Figure 1. (S,S)-DPENDS.
2. Results and discussion
2.1. The effect of the solvent and temperature on the
asymmetric hydrogenation of acetophenone
In order to determine whether the asymmetric hydrogenation
could proceed in PEG-400, we chose acetophenone as a model sub-
strate and investigated the influence of addition of solvent on the
reaction. As shown in Table 1, lower catalytic activity and enanti-
oselectivity were observed in pure PEG-400 (Table 1, entry 1). Fur-
thermore, much lower activity and enantioselectivity were
obtained when hexane was added to PEG-400 as the co-solvent
(Table 1, entry 2). However, the activity and enantioselectivity
⇑
Corresponding author. Tel.: +86 23 65023027; fax: +86 23 65023753.
E-mail address: wjb76@163.com (J. Wang).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.06.006

R. Qin et al. / Tetrahedron: Asymmetry 23 (2012) 834–837
835
Table 3
OH
Effect of different ruthenium complex on asymmetric hydrogenationa
O
*
R2
RuCl2(TPPTS)2-(S,S)-DPENDS
eeb (%)
R2
Entry
Ruthenium complex
Conversion (%)
R1
R1
1
2
3
[RuCl2(TPPTS)2]2
RuCl2(COD)-2TPPTS
RuCl3-2TPPTS
100
54.5
20.6
82.1
73.3
73.9
PEG-400-H2O, KOH
R1 = H, o-F, o-Cl , o-Br, o-CH3O, m-Cl , m-Br, p-CH3, p-CF3, p-CH3O, R2 = CH3
R1 =H , R2 = C2H5
VPEG-400:VH2O = 2:1, other reaction conditions were the same as those in Table 1
a
except for precatalysts.
b
Configuration: (R).
Figure 2. Asymmetric hydrogenation of aromatic ketones.
efficient catalyst: the conversion and ee value of the products
can reach up to 100% and 82.1%, respectively (Table 3, entry 1).
With regard to RuCl2(COD), low activity was observed because
COD would be exchanged with TPPTS to form an active catalyst
(Table 3, entry 2). In the case of RuCl3 as a catalyst precursor (Ta-
ble 3, entry 3), there was a longer induction period because the
three-valent ruthenium was reduced to a low oxidation state and
then coordinated with TPPTS, while TPPTS was partially oxidized
to OTPPTS.36 Hence, the activity was very low. Furthermore, a low-
er enantioselectivity obtained was due to the oxidation of TPPTS.
Table 1
Effect of the addition of the solvent on the asymmetric hydrogenationa
Entry
Solvent (v/v)
Conversion (%)
eeb (%)
1
2
3
4
5
6
7
PEG-400
22.9
7.9
70.1
67.5
78.9
82.1
75.0
67.5
57.7
PEG-400/hexane = 2/1
PEG-400/i-PrOH = 2/1
PEG-400/H2O = 2/1
PEG-400/H2O = 1/1
PEG-400/H2O = 1/2
H2O
60.6
100
100
100
68.3
a
Reaction conditions: acetophenone: 0.85 mmol; Ru/acetophenone/(S,S)-
DPENDS = 1:224:6; Vsolvent: 0.75 ml; KOH: 20 mg; PH2: 5.0 MPa; T: 30 °C; t: 3 h.
2.3. Influence of the molar ratio of ruthenium to (S,S)-DPENDS
and KOH concentration
b
Configuration: (R).
As shown in Tables 4 and 5, the influence of (S,S)-DPENDS and
KOH on the asymmetric hydrogenation of acetophenone is very
significant. Only 9.9% of the substrate was converted, and no ee va-
lue was obtained in the absence of (S,S)-DPENDS (Table 4, entry 1).
The addition of (S,S)-DPENDS apparently heightened the catalytic
activity and enantioselectivity. When the molar ratio of ruthenium
to (S,S)-DPENDS was 1:6, the conversion and ee value could reach
up to 100% and 82.1%, respectively (Table 4, entry 3).
increased when using i-PrOH as the co-solvent (Table 1, entry 3).
This might be due to the significant co-solvent effect of the
alcohol.28,29
Literature data indicated that water affects the reaction when
ILs are used as the solvent.30–34 In addition, PEG400-H2O has been
used as a reaction medium for the asymmetric transfer hydrogena-
tions of simple ketones catalyzed by Ru-TsDPEN, and the remark-
able effect of H2O on the reactivity was observed.35 Therefore, in
view of the beneficial effect of water, and to improve the perfor-
mance of RuCl2(TPPTS)2-(S,S)-DPENDS catalyst in PEG-400, we
added water to evaluate its effect on the asymmetric hydrogena-
tion of aromatic ketones. The results indicate that the addition of
appropriate amounts of water can increase the ee value, and also
enhance both the solubility of the catalyst and the concentration
of H2 in PEG-400. However, increasing the amount of water further,
resulted in lower enantioselectivity.
Table 4
The effect of the molar ratio of ruthenium to (S,S)-DPENDS on the asymmetric
hydrogenation of acetophenonea
Entry
Molar ratio
Conversion (%)
eeb (%)
1
2
3
4
No (S,S)-DPENDS
9.9
95.1
100
1:3
1:6
1:9
81.7
82.1
80.3
100
The data listed in Table 2 indicate that the enantioselectivity
increases as temperature decreases and 86.3% ee was obtained at
5 °C (Table 2, entry 1).
a
VPEG-400:VH2O = 2:1, other reaction conditions were the same as those in Table 1
except for concentration of (S,S)-DPENDS.
b
Configuration: (R).
Table 2
The effect of temperature on the asymmetric hydrogenation of acetophenonea
The importance of a base has been well-established for homoge-
neous catalysts for the asymmetric hydrogenation of ketones.1–4
Entry
Temperature (°C)
Conversion (%)
eeb (%)
Similar to homogeneous and some heterogeneous reactions,37–39
a
1
2
3
4
5
20
30
45
10.5
87.3
100
86.3
83.3
82.1
79.2
basic additive can be useful to enhance the catalytic properties.
The reaction did not proceed in the absence of KOH (Table 5, entry
1). Increasing the KOH concentration, increased the conversion
and ee. When the amount of KOH was 5 mg, 100% conversion and
82.1% ee were obtained (Table 5, entry 3). However, increasing the
100
a
VPEG-400:VH2O = 2:1, other reaction conditions were the same as those in Table 1.
Configuration: (R).
b
Table 5
The effect of KOH concentration on the asymmetric hydrogenation of acetophenonea
2.2. The effect of different ruthenium precatalysts on the
asymmetric hydrogenation of acetophenone
Entry
Concentration (mol/L)
Conversion (%)
eeb (%)
1
2
3
4
0
—
42.9
100
100
—
As can be seen in Table 3, the asymmetric hydrogenation of ace-
tophenone catalyzed by different ruthenium precatalysts was
investigated in PEG-400.
5 mg
20 mg
30 mg
74.9
82.1
81.5
The results indicate that the influence of different ruthenium
catalyst precursors on the activity and selectivity is significant.
Among the three catalyst precursors, RuCl2(TPPTS)2 was the most
a
VPEG-400:VH2O = 2:1, other reaction conditions were the same as those in Table 1
except for the concentration of KOH.
b
Configuration: (R).

836
R. Qin et al. / Tetrahedron: Asymmetry 23 (2012) 834–837
Table 7
amount of KOH further, resulted in no increase in the conversions
and ee values.
Recycling and reuse of RuCl2(TPPTS)2-(S,S)-DPENDS in PEG-400-H2Oa
The results shown in Tables 4 and 5 indicate that there was a
synergistic effect between (S,S)-DPENDS and KOH. Similar phe-
nomena were observed in our previous work.23–27,40
Run
Conversion (%)
eeb (%)
1
2
3
4
5
6
7c
8
100
100
100
100
95.2
76.9
85.7
73.1
82.1
81.1
80.7
78.7
77.5
77.7
77.3
77.5
2.4. Asymmetric hydrogenation of different substrates
Various aromatic ketones were hydrogenated with the
RuCl2(TPPTS)2-(S,S)-DPENDS catalyst in PEG-400 and the results
are summarized in Table 6. The data indicate that this novel cata-
lyst shows a good catalytic performance for most aromatic ketones.
The steric bulk, which influences the reactant-modifier interac-
tion, markedly affects the activity. As a result, higher conversions
were obtained in the asymmetric hydrogenation of acetophenone
and para-substituted aromatic ketones than those of other aromatic
ketones. Furthermore, electronic effects also have an influence on
the conversions. Among ortho-substituted aromatic ketones, mod-
erate conversions were achieved for the substrates with electron-
withdrawing groups while lower results were obtained for elec-
tron-donating substituents. For example, only 10.6% conversion
was found for 20-methoxyacetophenone (Table 6, entry 6), and
while no product was detected for 20-hydroxyacetophenone
(Table 6, entry 7), which contributed to the formation of a phenoxy
anion in the presence of a base.41
The enantioselectivity was affected by both the steric bulk and
the electronic nature of the substrate. As shown in Table 6, entries
3–5, the enantioselectivity increased when the electronegativity of
the halogen decreased. More than 80% enantioselectivities were
achieved for acetophenone, 20-chloroacetophenone, and 20-bromo-
acetophenone. It should be noted that 1-(20-methoxyphenyl)etha-
nol had an (S)-configuration, which is the same as Baiker’s report.42
a
b
c
Reaction conditions were the same as those in Table 6.
Configuration: (R).
10 mg of KOH were added in the seventh run.
3. Conclusions
In conclusion, PEG-400-H2O was found to be a green and recy-
clable reaction medium. In PEG-400-H2O, the RuCl2(TPPTS)2-(S,S)-
DPENDS catalyst was successfully applied to asymmetrically
hydrogenate aromatic ketones, with high activity, and good enanti-
oselectivity. The catalyst cannot only be easily separated from the
products by extraction with n-hexane, but also reused several
times without obvious loss of activity or enantioselectivity. Fur-
thermore, we have demonstrated that the addition of appropriate
amounts of water can accelerate the reaction and increase the ee
value.
4. Experimental
4.1. Materials and methods
Aromatic ketones (>98% Acros) and hydrogen (99.99%) were
used as purchased and other reagents were all of analytical grade.
TPPTS, RuCl2(TPPTS)2 and chiral modifier (S,S)-DPENDS were syn-
thesized according to known methods in our laboratory.43–46
2.5. Catalyst recycling
Finally, the separation of the products and the recycling of the
catalyst were explored for the asymmetric hydrogenation of
aromatic ketones in PEG-400-H2O. The data demonstrate that the
chiral alcohol products could be easily separated by extraction with
n-hexane, while the catalyst and modifiers immobilized in PEG-400
could be recycled and reused several times. As shown in Table 7,
although the conversions started to drop from the sixth run, the
ee value could still be maintained above 77%. The conversion in-
creased from 76.9% (run 6) to 85.7% (run 7) when 10 mg KOH was
added in the seventh run, which further shows the notable effect
of base on the catalytic performance. This phenomenon is in agree-
ment with our previous report.24,27 The loss of ruthenium catalyst
was minimal, and was only 0.1% from ICP analysis
4.2. Typical procedure for asymmetric hydrogenation of
aromatic ketones
To a 60 mL stainless autoclave with a glass liner and magnetic
stirrer were added PEG-400, H2O, RuCl2(TPPTS)2, (S,S)-DPENDS,
KOH, and reactant. Hydrogen was introduced to the desired pressure
after the reaction mixture had been purged with H2 five times. The
products were extracted by n-hexane and analyzed by GC-960 with
a FID detector and b-DEX™120 capillary column (30 m ꢀ 0.25 mm,
0.25
lm film) at 115 °C. The enantiomeric excess (ee value) was
calculated from the equation: ee (%) = 100 ꢀ (R ꢁ S)/(R + S).
Table 6
Asymmetric hydrogenation of different substratesa
Entry
Substrates
Conversion (%)
Ee (%)
Configuration
1
2
3
4
5
6
7
8
Acetophenone
Propiophenone
100
100
65.8
56.7
64.8
10.6
—
40.1
45.3
100
100
100
82.1
81.3
61.1
80.1
84.3
46.9
—
70.2
72.5
78.3
75.6
65.7
(R)
(R)
(R)
(R)
(R)
(S)
—
(R)
(R)
(R)
(R)
(R)
20-Fluoroacetophenone
20-Chloroacetophenone
20-Bromoacetophenone
20-Methoxyacetophenone
20-Hydroxyacetophenone
30-Chloroacetophenone
30-Bromoacetophenone
40-Methylacetophenone
40-Methoxyacetophenone
40-(Trifluoromethyl)acetophenone
9
10
11
12
a
Reaction conditions: Substrate: 0.85 mmol; Ru/substrate/(S,S)-DPENDS = 1:224:6; Vsolvent: 0.75 ml; VPEG-400:VH2O = 2:1; KOH: 20 mg; PH2: 5.0 MPa; T: 30 °C; t: 3 h.

R. Qin et al. / Tetrahedron: Asymmetry 23 (2012) 834–837
837
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