Dual pathway for the asymmetric transfer hydrogenation of α-ketoimides to chiral α-hydroxy imides or chiral α-hydroxy esters

Article abstract of DOI:10.1002/cctc.201500906

In an enantioselective reaction, we expect to obtain two types of chiral products through a controllable strategy in asymmetric catalysis. Herein, we develop Ru-catalysed asymmetric transfer hydrogenation of α-ketoimides to realise an enantioselective construction of chiral α-hydroxy imides or chiral α-hydroxy esters. The transformation of α-ketoimides catalysed by (S,S)-[RuCl(η⁶-mesitylene)diamine] can afford various chiral α-hydroxy imides with high yields and enantioselectivities, whereas that catalysed by (S,S)-[RuCl(η⁶-hexamethylbenzene)diamine] gives the desirable chiral α-hydroxy esters through a slight adjustment of the reaction conditions. The method described here is a controllable organic transformation with sodium formate as a hydrogen source under mild reaction conditions, and the benefit of this transformation is that various chiral α-hydroxy imides or α-hydroxy esters can be obtained selectively from α-ketoimides. Selective directive: An enantioselective transformation in the Ru-catalyzed asymmetric transfer hydrogenation of α-ketoimides to chiral α-hydroxy imides or α-hydroxy esters is developed. The transformation of α-ketoimides catalyzed by (S,S)-[RuCl(η⁶-mesitylene)diamine] can afford various chiral α-hydroxy imides with high yields and enantioselectivities, whereas that catalyzed by (S,S)-[RuCl(η⁶-hexamethylbenzene)diamine] give desirable chiral α-hydroxy esters through a slight adjustment of reaction conditions.

Full text of DOI:10.1002/cctc.201500906

DOI: 10.1002/cctc.201500906
Full Papers
Dual Pathway for the Asymmetric Transfer Hydrogenation
of a-Ketoimides to Chiral a-Hydroxy Imides or Chiral
a-Hydroxy Esters
Qiankun Zhao, Yuxi Zhao, Hang Liao, Tanyu Cheng, and Guohua Liu*[a]
In an enantioselective reaction, we expect to obtain two types
of chiral products through a controllable strategy in asymmet-
ric catalysis. Herein, we develop Ru-catalysed asymmetric trans-
fer hydrogenation of a-ketoimides to realise an enantioselec-
tive construction of chiral a-hydroxy imides or chiral a-hydroxy
esters. The transformation of a-ketoimides catalysed by (S,S)-
[RuCl(h6-mesitylene)diamine] can afford various chiral a-hy-
droxy imides with high yields and enantioselectivities, whereas
that catalysed by (S,S)-[RuCl(h6-hexamethylbenzene)diamine]
gives the desirable chiral a-hydroxy esters through a slight ad-
justment of the reaction conditions. The method described
here is a controllable organic transformation with sodium for-
mate as a hydrogen source under mild reaction conditions,
and the benefit of this transformation is that various chiral
a-hydroxy imides or a-hydroxy esters can be obtained selec-
tively from a-ketoimides.
Introduction
Chiral h5-Cp*-M complexes (h5-Cp*=pentamethyl cyclopenta-
diene series, M=Ru, Rh and Ir) and h6-arene-M complexes (h6-
arene=aromatic ring series) based on N-sulfonylated diamines
have been well documented as efficient catalysts in various
asymmetric reactions[1] and some have exhibited excellent cat-
alytic activity and high enantioselectivity in asymmetric trans-
fer hydrogenation (ATH).[2] In particular, developments in enan-
tioselective divergence[3] to construct chiral product diversity
have stimulated the further exploration of these organometal-
lic complexes based on N-sulfonylated diamine greatly. Recent-
ly, Johnson et al. found that the chiral h6-p-cymene-Ru complex
based on N-sulfonylated diamine could produce chiral product
diversity in the dynamic kinetic resolution of a-keto esters by
asymmetric hydrogen transfer.[3c–e] These findings offer new op-
portunities to explore enantioselective divergence in ATH
through the screening of chiral organometallic complexes
based on N-sulfonylated diamines in asymmetric reactions.
Optically active a-hydroxy amides/imides and a-hydroxy
esters, as important chiral motifs, are well known in the con-
struction of various biologically active compounds, such as
bradykinin B1 selective antagonists and inverse agonists.[4]
Generally, an economical and efficient route for the simultane-
ous construction of a-hydroxy amides/imides and a-hydroxy
esters is from a-ketoamides/imides, in which the direct reduc-
tion of a carbonyl group gives a-hydroxy amides/imides,
whereas reduction followed by alcoholysis affords a-hydroxy
esters. This route offers a potentially organic transformation for
the construction of chiral a-hydroxy imides and a-hydroxy
esters from a-ketoimides through the use of an asymmetric
hydrogen transfer method (Scheme 1).
To date, chiral a-hydroxy amides/imides have been obtained
through many well-established methodologies, which include
Scheme 1. Enantioselective transformation of a-ketoimides to chiral a-hy-
droxy imides and chiral a-hydroxy esters.
the asymmetric oxidation of racemic a-hydroxy amides,[5] the
ring opening of chiral a-epoxy amides,[6] the enantioselective
Passerini-type reaction[7] and the asymmetric addition of enam-
ides to ketones.[8] However, the direct enantioselective reduc-
tion of a-ketoamides/imides to chiral a-hydroxy amides/imides
only involves in chiral induction,[9] asymmetric hydrosilylation[10]
and asymmetric hydrogenation.[11] Among these three direct
reduction routes, the reduction of a-keto amides/imides
through chiral induction needs a stoichiometric chiral auxiliary
ligand and that through asymmetric hydrosilylation has a poor
enantioselectivity.
Although
enantioselective
reduction
through asymmetric hydrogenation has a high enantioselectiv-
ity, sensitive chiral diphosphine ligands and high pressures of
hydrogen are still a problem for its practical application.[11]
Therefore, the development of an ATH strategy for the organic
transformation of a-ketoimides to chiral a-hydroxy imides and
the simultaneous realisation of an enantioselective transforma-
tion from a-ketoimides to chiral a-hydroxy esters is highly de-
sirable.
[a] Q. Zhao, Y. Zhao, H. Liao, T. Cheng, Prof. G. Liu
Key Laboratory of Resource Chemistry of Ministry of Education
Shanghai Key Laboratory of Rare Earth Functional Materials
Shanghai Normal University
No.100 Guilin Rd. (P.R. China)
E-mail: ghliu@shnu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500906.
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In an effort to develop various ATH methods,[12] we found
that the chiral h6-mesitylene-Ru complex based on N-(2-amino-
1,2-diphenylethyl)-4-methylbenzenesulfonamide (TsDPEN) was
highly efficient in the ATH of imines and ketones.[12a,b] In this
contribution, we realise the dual transformation of a-keto-
imides by ATH. As expected, the transformation of a-keto-
imides catalysed by (S,S)-[RuCl(h6-mesitylene)TsDPEN] affords
various chiral a-hydroxy imides with high yields and enantiose-
lectivities, whereas that catalysed by (S,S)-[RuCl(h6-hexamethyl-
benzene)TsDPEN] gives the desirable chiral a-hydroxy esters
through a slight adjustment of reaction conditions. Such
a strategy makes this asymmetric reaction an attractive feature
in the selective preparation of chiral a-hydroxy imides or a-hy-
droxy esters from a-ketoimides in a controllable manner.
Table 1. Optimisation of reaction conditions in the ATH of N-(2-oxo-2-
phenylacetyl)benzamide to (R)-N-(2-hydroxy-2-phenylacetyl)benzamide.[a]
Entry Solvent/hydrogen source Acid
Time [h] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
HCOOH+NEt3
iPrOH
HCOOH
–
–
–
–
–
–
–
–
–
–
–
–
8
24
24
8
2
2
2
2
2
2
2
2
2
1
1
3
1
1
40
n.d.[d]
70
n.d.[d]
n.d.[d]
71
70
63
53
78
72
64
96
94
93
95[e]
91[f]
86[g]
91
5
83
62
86
10
80
80
90
68
68
70
95
86
89
95
97
H2O/HCOONa
H2O/HCOONa
PEG400/H2O/HCOONa
THF/HCOONa
Acetone/HCOONa
DMF/HCOONa
DMSO/HCOONa
MeOH/HCOONa
EtOH/HCOONa
iPrOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
Results and Discussion
Optimisation of reaction conditions in the ATH of N-(2-oxo-
2-phenylacetyl)benzamide to (R)-N-(2-hydroxy-2-phenyl-
acetyl)benzamide
9
10
11
12
13
14
15
16
17
18
–
We chose the ATH of N-(2-oxo-2-phenylacetyl)benzamide as
a model reaction. The asymmetric reaction was optimised
through the use of 2.0 mol% (S,S)-[RuCl(h6-mesitylene)TsDPEN]
as a catalyst to determine the optimal hydrogen source, sol-
vent and additive to obtain the highly efficient transformation
of N-(2-oxo-2-phenylacetyl)benzamide to (R)-N-(2-hydroxy-2-
phenylacetyl)benzamide.
HCOOH
HCOOH
HCOOH
AcOH
TsOH
82
[a] Reactions were performed with 2.0 mmol of catalyst, 0.10 mmol of
N-(2-oxo-2-phenylacetyl)benzamide and 10 equiv. of the hydrogen source
in 3.0 mL of solvent at 258C. [b] Isolated yield. [c] Determined by HPLC.
[d] Not detected. [e] Data were obtained with 5.0 equiv. of HCOOH as an
additive. [f] Data were obtained with 2.5 equiv. of HCOOH as an additive.
[g] Data were obtained with 7.5 equiv. of HCOOH as an additive.
First, we used four common hydrogen sources that are
often used in ATH reactions, formic acid/triethylamine, formic
acid, 2-propanol and sodium formate (HCOONa), to compare
the catalytic performance systemically. In the case of HCOONa
as a hydrogen source, the ATH of N-(2-oxo-2-phenylacetyl)-
benzamide afforded (R)-N-(2-hydroxy-2-phenylacetyl)benz-
amide with a medium yield and ee value, which was clearly
better than that obtained with the other hydrogen sources
(Table 1, entry 4 vs. 1–3), which suggests that HCOONa was the
best hydrogen source.
(Table 1, entries 12–13). Similarly, their yields did not change
significantly because of the same alcoholysis process.
To suppress the alcoholysis of chiral products and to en-
hance the yield of (R)-N-(2-hydroxy-2-phenylacetyl)benzamide,
HCOOH was introduced as an additive to this catalytic system,
which was inspired by its suppressing role in the alcoholysis of
a-ketoimides.[13] The result showed the yield of (R)-N-(2-hy-
droxy-2-phenylacetyl)benzamide could be enhanced signifi-
cantly and the ee value had no clear decrease. Further optimi-
sation of the molar amounts of HCOOH showed that 5.0 equiv-
alents of HCOOH was optimal (Table 1, entry 14 vs. 15–16),
with which the yield of (R)-N-(2-hydroxy-2-phenylacetyl)benz-
amide could be enhanced from 68 to 95% and the ee value
reached 95%. Such a result was better than that of CH3COOH
or TsOH as additives (Table 1, entry 14 vs. 17–18).
Therefore, in the presence of 2.0 mol% of (S,S)-[RuCl(h6-mesi-
tylene)TsDPEN], the optimal reaction conditions for the trans-
formation of a-ketoimides to chiral a-hydroxy imides were the
use of HCOONa as the hydrogen source, MeOH as the solvent
and HCOOH as the additive at a reaction temperature of 258C.
After we had determined the best reaction conditions in the
case of 2.0 mol% of (S,S)-[RuCl(h6-mesitylene)TsDPEN] as a cata-
lyst, the analogues of chiral h5-Cp*-M complexes B–D and h6-
Next, because of the poor solubility of the substrate in
water, a series of polar solvents was screened. As shown in en-
tries 6–10, it was found that in all cases that yields increased
relative to that in water within 2 h, even with the use of PEG-
400 as a phase-transfer catalyst (Table 1, entry 5 vs. 6–10).
However, their ee values were not affected clearly. To our sur-
prise, if MeOH was used as the solvent, the enantioselectivity
was enhanced greatly although the yield only increased slight-
ly (Table 1, entry 5 vs. 11). A detailed analysis of this reaction
found that the transformation of N-(2-oxo-2-phenylacetyl)benz-
amide is quantitative (more than 99% conversion), which sug-
gests the formation of byproducts in this catalytic reaction.
The byproduct was determined as (R)-methyl 2-hydroxy-2-phe-
nylacetate, which demonstrates that the reaction goes through
an alcoholysis process. This behaviour indicates an adjustable
process that will be discussed below. To confirm the role of al-
cohols in this reaction system, two alcohols (EtOH and iPrOH)
were further investigated as solvents. In both cases, the ee
value of the target chiral products was enhanced steadily
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arene-M complexes E–G based on N-sulfonylated diamines
were further examined in the ATH of N-(2-oxo-2-phenylacetyl)-
benzamide under these reaction conditions. Although most of
these complexes enabled efficient ATH, with the exception of
E, the yields and enantioselectivities were worse than those
with (S,S)-[RuCl(h6-mesitylene)TsDPEN] (A) (Table 2, entry 1 vs.
Table 3. ATH of a-ketoimides to convert a-hydroxy imides.[a]
Table 2. Optimisation of catalysts in the ATH of N-(2-oxo-2-phenylacetyl)-
benzamide to (R)-N-(2-hydroxy-2-phenylacetyl)benzamide.[a]
Entry
Ar
2
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
Ph (1a)
4-FPh (1b)
2-FPh (1c)
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
2o
1
1
3
1
1
1
2
2
2
6
2
2
1
4
2
95
94
91
94
93
84
93
94
94
90
94
90
93
88
93
95
88
81
91
94
91
86
91
95
62
81
92
91
73
93
4-ClPh (1d)
4-BrPh (1e)
3-BrPh (1 f)
4-I Ph (1g)
4-MePh (1h)
3-MePh (1i)
2-MePh (1j)
4-iPrPh (1k)
4-MeOPh (1l)
3-MeOPh (1m)
1-naphthyl (1n)
2-naphthyl (1o)
9
10
11
12
13
14
15
Entry
Catalyst
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
A
B
C
D
E
1
1
12
2
12
2
2
95
94
20
94
n.d.[d]
85
95
85
93
89
–
[a] Reactions were performed with 2.0 mmol of catalyst, 0.10 mmol of
a-ketoimide, 5 equiv. of HCOOH and 10 equiv. of HOOONa in 3.0 mL of
MeOH at 258C. [b] Isolated yield. [c] Determined by HPLC.
F
G
86
87
85
[a] Reactions were performed with 2.0 mmol of catalyst, 0.10 mmol of
N-(2-oxo-2-phenylacetyl)benzamide and 10 equiv. of hydrogen source in
3.0 mL of solvent at 258C. [b] Isolated yield. [c] Determined by HPLC.
[d] Not detected.
ly (Table 3, entries 3, 10 and 14). Such a result suggests that
the asymmetric reaction described here was suitable for the
construction of a wide range of a-hydroxy imides through
a facile ATH strategy.
2–7). Comprehensively, (S,S)-[RuCl(h6-mesitylene)TsDPEN] was
determined as the best catalyst in the ATH of N-(2-oxo-2-phe-
nylacetyl)benzamide to (R)-N-(2-hydroxy-2-phenylacetyl)benz-
amide.
ATH of a-ketoimides to a-hydroxy esters
Notably, this ATH of a-ketoimides can be used for the con-
struction of chiral a-hydroxy esters as mentioned above during
the optimisation process. Although many well-established
methods for the construction of chiral a-hydroxy esters have
appeared in the literature,[3c–e,14] direct transformations of aryl-
substituted a-ketoimides to chiral aryl-substituted a-hydroxy
esters have not been explored through the use of an ATH
method. In this case, through a controlled organic transforma-
tion, aryl-substituted a-ketoimides could also be converted to
chiral a-hydroxy esters through the slight adjustment of the
reaction conditions.
ATH of a-ketoimides to a-hydroxy imides
Based on the optimised catalyst and reaction conditions in the
ATH of N-(2-oxo-2-phenylacetyl)benzamide to (R)-N-(2-hydroxy-
2-phenylacetyl)benzamide, we further investigated the applica-
bility of the reaction with a series of aryl-substituted a-keto-
imides. In general, asymmetric reactions with high yields and
medium to high enantioselectivities could be obtained under
the optimal reaction conditions (Table 3). If we take the ATH of
N-(2-oxo-2-phenylacetyl)benzamide as an example, we found
that the 95% ee value was higher than that obtained by other
methods reported,[9–11] which suggests the benefit of this ATH
because of the use of air-stable (S,S)-[RuCl(h6-mesitylene)TsDP-
EN] as a catalyst under mild reaction conditions.
During the optimisation of alcohol solvents (Table 1, en-
tries 11–13) to determine the structure of the byproduct of (R)-
methyl 2-hydroxy-2-phenylacetate in the ATH of N-(2-oxo-2-
phenylacetyl)benzamide to (R)-N-(2-hydroxy-2-phenylacetyl)-
benzamide, we found that the nearly quantitative conversion
of (R)-methyl 2-hydroxy-2-phenylacetate could be obtained if
the reaction temperature was increased to 408C without
a change of the other reaction conditions, which included no
acid as an additive. This finding indicates that through only
the increase of the reaction temperature, the ATH of aryl-sub-
stituted a-ketoimides could form chiral a-hydroxy esters con-
The electronic properties of the substituents at the Ar group
did not affect their enantioselectivities significantly, and various
electron-withdrawing and -donating substituents on the Ar
group were equally efficient. However, the steric effect was
clear, and substituents at the 2-position of the Ar group, which
included 1-naphthyl, decreased their enantioselectivities great-
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trollably. Although the ATH of N-(2-oxo-2-phenylacetyl)benz-
amide catalysed by (S,S)-[RuCl(h6-mesitylene)TsDPEN] afforded
the corresponding (R)-methyl 2-hydroxy-2-phenylacetate with
86% ee, this offered another possibility to optimise the best
catalysts suitable for this chiral organic transformation. Cata-
lyst C in the ATH of N-(2-oxo-2-phenylacetyl)benzamide could
produce (R)-methyl 2-hydroxy-2-phenylacetate with 95% yield
and 95% ee, which was better than the other catalysts
(Table 4, entry 3 vs. 1–2 and 4–7). This behaviour demonstrated
Table 5. ATH of a-ketoimides to convert a-hydroxy esters.[a]
Entry
Ar
3
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
Ph (1a)
4-FPh (1b)
2-FPh (1c)
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3
3
5
3
3
3
3
3
3
5
4
3
3
5
5
4
5
95
94
90
95
95
95
93
95
95
78
93
95
94
81
89
92
80
95
90
70
85
84
75
83
95
95
27
96
96
94
36
90
93[d]
80[e]
Table 4. Optimisation of catalysts in the ATH of N-(2-oxo-2-phenylacetyl)-
benzamide to (R)-methyl 2-hydroxy-2-phenylacetate.[a]
4-ClPh (1d)
4-BrPh (1e)
3-BrPh (1 f)
4-IPh (1g)
4-CH3Ph (1h)
3-MePh (1i)
2-CH3Ph (1j)
4-iPrPh (1k)
4-MeOPh (1l)
3-MeOPh (1m)
1-naphthyl (1n)
2-naphthyl (1o)
EtOH (1p)
9
10
11
12
13
14
15
16
17
Entry Catalyst Solvent/hydrogen source Time [h] Yield [%][b] ee [%][c]
iPrOH (1q)
1
2
3
4
5
6
7
A
B
C
D
E
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
MeOH/HCOONa
3
3
3
3
8
3
3
94
94
95
93
86
80
95
73
–
[a] Reactions were performed with 2.0 mmol of catalyst, 0.10 mmol of
a-ketoimides and 10 equiv. of HOOONa in 3.0 mL of MeOH at 408C.
[b] Isolated yield. [c] Determined by HPLC. [d] Data were obtained with
EtOH as the solvent and reagent. [e] Data were obtained with iPrOH as
the solvent and reagent at 608C.
n.d.[d]
F
G
93
93
82
88
[a] Reactions were performed with 2.0 mmol of catalyst, 0.10 mmol of
N-(2-oxo-2-phenylacetyl)benzamide and 10 equiv. of hydrogen source in
3.0 mL of solvent at 408C. [b] Isolated yield. [c] Determined by HPLC.
[d] Not detected.
2-phenylacetate and (R)-isopropyl 2-hydroxy-2-phenylacetate
with desirable enantioselectivities (Table 5, entries 16–17).
Conclusions
that the ATH of N-(2-oxo-2-phenylacetyl)benzamide was a con-
trollable process, which not only enabled the efficient transfor-
mation of a-ketoimides to chiral a-hydroxy imides but also
produced chiral a-hydroxy esters.
By the exploration of the asymmetric transfer hydrogenation
of a-ketoimides, we found that this asymmetric reaction can
convert various chiral a-hydroxy imides or a-hydroxy esters
through a slight adjustment of the reaction conditions. Fur-
thermore, the mild reaction conditions make this asymmetric
reaction attractive in practical organic transformations.
Based on this highly efficient organic transformation, the
use of (S,S)-[RuCl(h6-hexamethylbenzene)TsDPEN] (C) as a cata-
lyst was further investigated to test its general applicability for
the above aryl-substituted a-ketoimides. In the presence of
2.0 mol% of C, the use of HCOONa as a hydrogen source and
MeOH as a solvent at a reaction temperature of 408C, the ATH
of these a-ketoimides could afford various chiral a-hydroxy
esters with high yields and desirable enantioselectivities
(Table 5), which suggests the benefit of the dual pathway in
the construction of various chiral a-hydroxy esters.
Experimental Section
General procedure for the ATH of a-ketoimides to chiral
a-hydroxy imides
Catalyst A (2.0 mmol, S/C=50), a-ketoimide (0.10 mmol), HCOOH
(0.50 mmol), HCOONa (1.0 mmol, 68.0 mg) and MeOH (3.0 mL)
were added to a 5 mL round-bottomed flask sequentially. The mix-
ture was allowed to react at 258C, and the reaction was monitored
constantly by TLC. After completion of the reaction, the solvent
was removed by evaporation. The residue was dissolved in water
(2 mL), and the aqueous solution was extracted by ethyl acetate
(3”3.0 mL). The combined ethyl acetate solution was washed with
brine twice and dehydrated with Na2SO4. After the evaporation of
ethyl acetate, the residue was purified by silica gel flash column
A similar effect of the electronic properties and sterics was
observed, in which various electron-withdrawing and -donat-
ing substituents on the Ar group were equally efficient, where-
as substituents at the 2-position of the Ar group, which includ-
ed 1-naphthyl, decreased their enantioselectivities (Table 5, en-
tries 3, 10 and 14). In addition to the construction of (R)-methyl
2-hydroxy-2-phenylacetate, reactions with ethanol and isopro-
panol could also afford the chiral products (R)-ethyl 2-hydroxy-
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1270.
chromatography to afford the desired products. The yields were
determined by 1H NMR spectroscopy. The ee values were deter-
mined by a Daicel Chiralcel column AD-H, OD-H or OJ-H.
General procedure for the ATH of a-ketoimides to chiral
a-hydroxy esters
Catalyst C (2.0 mmol, S/C=50), a-ketoimides (0.10 mmol), HCOONa
(1.0 mmol, 68.0 mg) and MeOH (3.0 mL) were added to a 5 mL
round-bottomed flask sequentially. The mixture was allowed to
react at 408C, and the reaction was monitored constantly by TLC.
After the completion of the reaction, the solvent was removed by
evaporation. The residue was dissolved in water (2 mL), and then
the aqueous solution was extracted by ethyl acetate (3”3.0 mL).
The combined ethyl acetate solution was washed with brine twice
and dehydrated with Na2SO4. After the evaporation of ethyl ace-
tate, the residue was purified by silica gel flash column chromatog-
raphy to afford the desired products. The yields were determined
by 1H NMR spectroscopy. The ee values were determined by
a Daicel Chiralcel column AD-H, OJ-H or AS-H.
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We are grateful to the China National Natural Science Founda-
tion (21402120), Shanghai Sciences and Technologies Develop-
ment Fund (13ZR1458700), the Shanghai Municipal Education
Commission (14YZ074, 13CG48, Young Teacher Training Project)
and Specialized Research Fund for the Doctoral Program of
Higher Education (20133127120006).
Keywords: asymmetric catalysis
·
enantioselectivity
·
hydrogenation · ligand effects · ruthenium
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Received: August 13, 2015
Published online on November 3, 2015
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