Enantioselective hydrogenation of α-ketoesters catalyzed by cinchona alkaloid stabilized Rh nanoparticles in ionic liquid

Article abstract of DOI:10.1002/chir.23107

The heterogeneous enantioselective hydrogenation of α-ketoesters catalyzed by rhodium nanoparticles (Rh NPs) in ionic liquid was studied with the stabilization and modification of cinchona alkaloids. TEM characterization showed that well-dispersed Rh NPs of about 1.96?nm were obtained in ionic liquid. The results showed that cinchona alkaloids not only had good enantiodifferentiating ability but also accelerated the catalytic reaction. Under the optimum reaction conditions, the enantiomeric excess in ethyl benzoylformate hydrogenation could reach as high as 60.9%.

Full text of DOI:10.1002/chir.23107

Received: 25 February 2019
DOI: 10.1002/chir.23107
Revised: 8 June 2019
Accepted: 14 June 2019
S H O R T C O M M U N I C A T I O N
Enantioselective hydrogenation of α‐ketoesters catalyzed by
cinchona alkaloid stabilized Rh nanoparticles in ionic liquid
He‐yan Jiang
| Jie Xu | Bin Sun
Key Laboratory of Catalysis Science and
Technology of Chongqing Education
Commission, Chongqing Key Laboratory
of Catalysis and New Environmental
Materials, College of Environmental and
Resources, Chongqing Technology and
Business University, Chongqing, China
Abstract
The heterogeneous enantioselective hydrogenation of α‐ketoesters catalyzed by
rhodium nanoparticles (Rh NPs) in ionic liquid was studied with the stabiliza-
tion and modification of cinchona alkaloids. TEM characterization showed
that well‐dispersed Rh NPs of about 1.96 nm were obtained in ionic liquid.
The results showed that cinchona alkaloids not only had good
enantiodifferentiating ability but also accelerated the catalytic reaction.
Under the optimum reaction conditions, the enantiomeric excess in ethyl
benzoylformate hydrogenation could reach as high as 60.9%.
Correspondence
He‐yan Jiang, Key Laboratory of Catalysis
Science and Technology of Chongqing
Education Commission, Chongqing Key
Laboratory of Catalysis and New
Environmental Materials, College of
Environmental and Resources,Chongqing
Technology and Business University,
Chongqing 400067, China.
KEYWORDS
cinchona alkaloid, enantioselective hydrogenation, ionic liquid, rhodium, α‐ketoester
Email: orgjiang@163.com
Funding information
National Natural Science Foundation of
China, Grant/Award Number: 21201184;
Natural Science Foundation Project of CQ,
Grant/Award Number: cstc2018jcyjAX0735;
Chongqing Key Laboratory of Catalysis
and New Environmental Materials, Grant/
Award Numbers: 1456028 and
KFJJ2018050; Chongqing Technology
and Business University, Grant/Award
Number: 1751039
1 | INTRODUCTION
catalyst achieved enantiomeric excess (ee) values exceed-
ing 95% under optimum reaction conditions.7-9 Replacing
Asymmetric catalysis has been the most challenging and
interesting topic in the synthesis of pharmaceuticals,
agrochemicals, and fragrance/perfumes over the past
few decades.1,2 The heterogeneous catalysis plays an
important and crucial role in many chemical processes
because of its inherent practical advantages associated
with ease of separation and handling.3,4 Recently, the
heterogeneous enantioselective catalysis has become a
fast‐growing and interesting field.5,6 For the
enantioselective hydrogenation of α‐ketoesters, the best
explored cinchonidine modified supported metal Pt
Pt with other transition metals (eg, Ru, Rh, Pd, and Ir)
results in lower enantioselectivity.10-12 It is worth men-
tioning that Chen et al13-15 reported polyvinylpyrrolidone
and cinchona‐stabilized Rh in the asymmetric hydroge-
nation of α‐ketoesters, in which moderate to good
enantioselectivities were obtained.
Ionic liquids have been employed as useful media for
the preparation of transition‐metal nanoparticles of vari-
ous sizes and shapes.16,17 So far, ionic liquid‐stabilized
metal nanoparticles have been used as catalysts mainly
in the hydrogenation of alkenes or arenes.18-20 We
Chirality. 2019;1–6.
wileyonlinelibrary.com/journal/chir
© 2019 Wiley Periodicals, Inc.
1

2
JIANG ET AL.
previously reported the use of ionic liquid‐stabilized
ruthenium nanoparticles to catalyze the tunable
chemoselective hydrogenation of aromatic ketones and
quinolines.21-24 However, the research on the use of
Rh nanoparticles for enantioselective hydrogenation in
ionic liquids was quite limited by now.25 In this paper,
Rh NPs, with diameter about 1.96 nm, were prepared
in imidazolium‐based ionic liquids by simple H2 reduc-
tion of RhCl3·3H2O. With the stabilization and modifi-
cation of cinchona alkaloids, Rh NPs exhibited
moderate to good activity and enantioselectivity in the
asymmetric hydrogenation of α‐ketoesters.
2.3 | General procedure for the
enantioselective hydrogenation of
α‐ketoesters
In stainless steel autoclave, previously prepared Rh(0)
catalyst was charged with the appropriate modifier,
cosolvent and substrate, and then the autoclave was
sealed and purged with pure hydrogen several times.
After the reactants were heated to predetermined tem-
perature, the reaction timing began. After completion
of the reaction and cooling to ambient temperature,
the products were isolated by high speed centrifugation
or liquid‐liquid extraction and analyzed by gas
chromatography.
2 | MATERIALS AND METHODS
2.1 | Materials
3 | RESULTS AND DISCUSSION
All manipulations involving air‐sensitive materials were
carried out using standard Schlenk line techniques under
an atmosphere of nitrogen. Various substrates and other
reagents were analytical grade. The purity of hydrogen
was over 99.99%. Products were analyzed by GC instru-
ment with an FID detector and Chrompack Chirasil‐
DEX column (25 m × 0.25 mm). Products were confirmed
by GC‐MS and NMR. The TEM analyses were performed
in a JEOL JEM 2010 transmission electron microscope
operating at 200 kV with nominal resolution of 0.25 nm.
The X‐ray photoelectron spectroscopy (XPS) measure-
ments were performed on a Thermo ESCALAB 250
spectrometer.
3.1 | Synthesis and characterization of Rh
NPs
The synthesis of Rh NPs was achieved through the H2
.
reduction of RhCl3 3H2O in BMIMBF4 (BMIM = 1‐
butyl‐2,3‐dimethylimidazolium) in the presence of 2.0
equivalent of cinchona alkaloids, which afforded a dark
suspension. For comparison, we also synthesized Rh
NPs without any additional stabilizer. A black powder
could be separated from the black suspension by the addi-
tion of acetone followed by centrifugation (5000 rpm for
10 min). Washed three times with acetone and dried
under reduced pressure, the isolated powder was ana-
lyzed by transmission electron microscopy (TEM), X‐ray
photoelectron spectroscopy (XPS).
2.2 | Synthesis of Rh NPs and Ni NPs
TEM analysis was used to characterize the obtained
Rh NPs and determine the average diameter (Figure 1).
The TEM image of cinchonidine stabilized Rh NPs exhib-
ited a regular spherical shape and a narrow size distribu-
tion with an average diameter of 1.96 nm.
In a typical experiment, RhCl3·3H2O (0.014 mmol) and
cinchonidine (0.028 mmol) were well dispersed in
BMIMBF4
(1
mL)
(BMIM
=
1‐butyl‐2,3‐
dimethylimidazolium), and the reaction mixture was
placed in a 20‐mL stainless‐steel high pressure reactor.
After stirring the mixture at room temperature under
an atmosphere of argon for 30 minutes, a constant pres-
sure of H2(g) (4 MPa) was admitted to the system and
the content was stirred for 1 hour at 60°C. The reactor
was cooled to ambient temperature and carefully
vented. A dark solution was obtained. The Rh NPs
embedded in BMIMBF4 were employed for hydrogena-
tion studies (see below). Isolation of the Rh NPs for
TEM and XPS analysis was achieved by dissolving the
mixture in acetone (5 mL), centrifuging (5000 rpm for
10 min), washing with acetone (3 × 5 mL), and drying
under vacuum. Ni NPs catalyst was prepared according
to previous work with cinchonidine as the stabilizer
The XPS analysis of the cinchonidine stabilized Rh
NPs was employed to elucidate the nature of the
stabilizing layer of the nanoparticles. XPS analysis of
Rh NPs stabilized by cinchonidine showed the presence
of rhodium, boron, nitrogen, and oxygen, which signi-
fied the presence of the BMIMBF4 and cinchonidine in
the ligand sphere of the Rh NPs. The binding energies
for Rh, 307.1 eV and 311.7 eV (Figure 2), which
indicated the Rh NPs were composed of Rh(0).26 In
comparison with the previous report of laser‐induced
synthesis of Rh(0) in ionic liquid,27 herein, cinchona
alkaloid stabilized Rh(0) NPs in the presence of ionic
liquid protective layer exhibited no obvious electron
transfer. In short, the TEM and XPS results indicated
that the rhodium (III) species was completely reduced
24
and NaBH4 as the reducing agent.

JIANG ET AL.
3
2.6% ee were obtained in the ethyl pyruvate asymmetric
hydrogenation with Rh NPs in BMIMBF4 (Table 1, entry
1). Further introduction of cinchonidine stabilizer during
the preparation of Rh NPs significantly improved the cat-
alytic hydrogenation activity, but the ee was still rather
low (Table 1, entry 2). In some previous heterogeneous
enantioselective catalysis reports,28-30 the solvent used
often have significant effect on the reactivity and
enantioselectivity. To further optimize catalytic activity
and enantioselectivity, we attempted to introduce
cosolvent into the catalytic system (Table 1, entries 3‐8).
The introduction of alcohol cosolvent into BMIMBF4
resulted in a significant increase in catalytic activity, but
the ee was still fairly low (Table 1, entries 3‐5). Toluene
and acetic acid are the most commonly used solvents
TABLE 1 Optimization of reaction conditions for the
enantioselective hydrogenation of ethyl pyruvatea
FIGURE 1 TEM image of cinchonidine stabilized Rh NPs
prepared in BMIMBF4
Ionic
Conversion, ee, config.b
Entry Liquid
Cosolvent
%
%
1c
2
BMIMBF4
BMIMBF4
‐
‐
41.1
2.6
R
R
R
R
R
R
R
R
‐
100.0
73.0
5.6
14.5
11.0
3.7
3
BMIMBF4 MeOH
BMIMBF4 EtOH
BMIMBF4 iPrOH
BMIMBF4 Toluene
4
100.0
96.4
5
6
100.0
5.8
7
BMIMBF4 Acetic acid 12.3
5.8
8
BMIMBF4 THF
BMIMBF4 THF
BMIMPF6 THF
BMIMNTf2 THF
BMIMAcO THF
99.5
0.0
45.5
0.0
FIGURE 2 XPS spectrum of Rh 3d in cinchonidine stabilized Rh
NPs prepared in BMIMBF4
9d
10
11
12
13e
14f
15
98.8
99.4
6.5
30.0
32.9
0.0
R
R
‐
to Rh NPs, and these Rh NPs could be protected by the
BMIMBF4 and cinchonidine without any change of
valence.
BMIBF4
BPyBF4
BPyPF6
THF
THF
THF
81.0
0.0
36.0
0.0
R
‐
0.0
0.0
‐
3.2 | Catalytic hydrogenation
aReaction was carried out at 20°C for 5 h, PH2: 5.0 MPa, substrate: 0.90 mmol,
cinchonidine as the stabilizer, substrate/Rh/modifier = 200:1:5, V ionic liq-
uid: 1 ml, V ionic liquid: V cosolvent = 1:1. Products were analyzed by a
GC instrument with an FID detector and β‐DEX120 capillary column.
Enantioselective hydrogenation was performed in a 20‐
mL stainless autoclave with a magnetic stirrer bar, by
using Rh NPs as a catalyst in the presence of cinchona
alkaloids as chiral modifiers. Ethyl pyruvate was selected
as the model substrate to explore the catalytic perfor-
mance of cinchonidine stabilized Rh NPs (Table 1). In
the absence of stabilizer, 41.1% conversion and only
bDetermined by sign of rotation.
cNo stabilizer added in the Rh nanoparticles synthesis.
dCinchonidine stabilized Ni NPS as catalyst.
eBMI = 1‐butyl‐3‐dimethylimidazolium.
fBPy = 1‐butylpyridinium.

4
JIANG ET AL.
in the previous enantioselective hydrogenation of α‐
ketoesters.31 The introduction of toluene and acetic acid
as cosolvents did not significantly improve ee (Table 1,
entries 6‐7). Among the cosolvents tested, the THF
cosolvent combing with BMIMBF4 achieved the best
catalytic performance (99.5% conversion and 45.5% ee,
Table 1, entry 8). Ni nanoparticles were also tested in
the ethyl pyruvate enantioselective hydrogenation; how-
ever, no catalytic activity was detected (Table 1, entry
9). Additionally, an improvement in catalytic activity
and ee was also observed when THF was introduced
into the Rh catalyst prepared in BMIMPF6 or
BMIMNTf2 (Table 1, entries 10‐11). However, the
chiral inducing ability of the Rh catalyst was not
detected in the solvent BMIMAcO‐THF (Table 1,
entry 12). More ionic liquids including BMIBF4
hydrogenation, the enantioselectivity of the catalytic
hydrogenation reaction could reach 45.5% (Table 2,
entry 1). However, other cinchona alkaloid modifiers
(Table 2, entries 2‐4) are not conducive to catalytic
hydrogenation enantioselectivity. Cinchona alkaloid
derivatives including 9‐amino(9‐deoxy)epicinchonidine
and o‐acetylcinchonidine were also tested as the chiral
modifier, and decreased catalytic activity and
enantioselectivity was observed (Table 2, entries 5‐6). Only
cinchonidine gave much higher enantioselectivity com-
pared with the other cinchona alkaloids in the asymmetric
reduction should be owing to the high substrate specificity
of heterongeneous enantioselective catalysis.32,33
Table 3 is some representative examples of the asym-
metric hydrogenation of α‐ketoesters catalyzed by
cinchonidine‐stabilized and modified Rh NPs. Catalytic
reactivity and enantioselectivity are subtlely affected by
substituents in the substrate. At room temperature
(Table 3, entries 1‐4), the activity of substrates methyl
pyruvate A and ethyl pyruvate B was higher than methyl
benzoylformate C and ethyl benzoylformate D, the ee
values of C (50.7%) and D (53.3%) were higher than those
of A (24.5%) and B (45.5%), which should be explained by
the influence of the structure of the substrate on the for-
mation of steric hindrance of catalytic heterogeneous
enantioselective hydrogenation. The heterogeneous
asymmetric hydrogenation reaction temperature is fur-
ther decreased to 0°C. The ee of the substrate B could
reach 55.8%; the ee of C could reach 56.3%; and the ee
of D could reach as high as 60.9%.
(BMI
=
1‐butyl‐3‐dimethylimidazolium), BPyBF4
(BPy = 1‐butylpyridinium), and BPyPF6 were also tested
(Table 1, entries 13‐15). In BMIBF4, slightly decrease in
catalytic activity and enantioselectivity was observed;
unexpectedly, no catalytic activity was detected in
BPyBF4 and BPyPF6.
Chiral modifiers are sources of enantioselectivity in
most heterogeneous enantioselective catalytic reactions.
Appropriate introduction of modifiers into nanometal
catalytic systems is one of the most effective ways to pro-
mote the performance of heterogeneous enantioselective
catalytic systems.32,33 As shown in Table 2, different cin-
chona alkaloids modified Rh NPs have significantly dif-
ferent catalytic reactivity and enantioselectivity. When
cinchonidine was used as a modifier for the preparation
of Rh NPs and as the modifier in the catalytic asymmetric
Figure 3 illustrated the recyclability of cinchonidine
stabilized Rh NPs in heterogeneous enantioselective
hydrogenation of ethyl pyruvate, and it was found
TABLE 2 Effect of different cinchona alkaloid modifiers on
enantioselective hydrogenation of ethyl pyruvatea
TABLE 3 Enantioselective hydrogenation of α‐ketoesters cata-
lyzed by cinchonidine stabilized Rh nanoparticlesa
Entry Substrate t, h T, °C Conversion, % ee, % config.
Entry
Modifier
Conversion, %
ee, %
config.
1
2
3
4
5
6
7
8
A
B
5
20
20
20
20
0
100
24.5
45.5
50.7
53.3
30.4
55.8
56.3
60.9
R
R
R
R
R
R
R
R
1
Cinchonidine
Cinchonine
Quinidine
Quinine
Ι
99.5
99.0
99.4
99.7
63.5
40.5
45.5
4.8
0.8
6.6
7.8
3.6
R
S
5
99.3
75.0
84.7
48.5
45.0
50.8
66.5
2
C
D
A
B
5
3
S
5
4
R
R
R
10
10
10
10
5b
6c
0
Π
C
D
0
aThe reaction conditions are the same as in Table 1 (V BMIMBF4: V
THF = 1:1).
0
bΙ = 9‐amino(9‐deoxy)epicinchonidine.
cΠ = o‐acetylcinchonidine.
aThe reaction conditions are the same as in Table 1 (V BMIMBF4: V
THF = 1:1).

JIANG ET AL.
5
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How to cite this article: Jiang H, Xu J, Sun B.
Enantioselective hydrogenation of α‐ketoesters
catalyzed by cinchona alkaloid stabilized Rh
nanoparticles in ionic liquid. Chirality. 2019;1–6.
https://doi.org/10.1002/chir.23107
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