Heterogeneous Enantioselective Hydrogenation of Aromatic Ketones Catalyzed by Rh Nanoparticles Immobilized in Ionic Liquid

Article abstract of DOI:10.1007/s10562-019-02768-w

Rhodium nanoparticles (Rh NPs) stabilized by natural cinchona alkaloids were synthesized in imidazolium-based ionic liquids using H₂ as the reductant. Characterization showed well-dispersed Rh NPs of about 1.96?nm (TEM and HRTEM) and confirmed the ionic liquid and cinchona alkaloid stabilization to the Rh(0) NPs (XPS). When modified by chiral diamine, including (1R,2R)-diphenylethylenediamine ((1R,2R)-DPEN) or cinchona alkaloid derivatives, the Rh NPs catalysts exhibited good activity, chemoselectivity and enantioselectivity in the heterogeneous enantioselective hydrogenation of aromatic ketones. Synergistic effect between (1R,2R)-DPEN and cinchonidine was also observed, which significantly accelerated the reaction rate and enhanced the enantioselectivity. 63.0% enantioselectivity and 98.9% chemoselectivity could be achieved in the acetophenone enantioselective hydrogenation; up to 70.2% enantioselectivity and 100% chemoselectivity was obtained in the isobutyrylbenzene catalytic enantioselective hydrogenation. Catalytic system could be reused several times without significant loss in activity, chemoselectivity as well as enantioselectivity. This catalytic protocol opens the door to heterogeneous enantioselective hydrogenation of aromatic ketones with metal Rh NPs immobilized in ionic liquid. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02768-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-02768-w
Heterogeneous Enantioselective Hydrogenation of Aromatic Ketones
Catalyzed by Rh Nanoparticles Immobilized in Ionic Liquid
He‑yan Jiang1 · Hong‑mei Cheng1 · Feng‑xia Bian1
Received: 23 January 2019 / Accepted: 24 March 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Rhodium nanoparticles (Rh NPs) stabilized by natural cinchona alkaloids were synthesized in imidazolium-based ionic liq-
uids using H2 as the reductant. Characterization showed well-dispersed Rh NPs of about 1.96 nm (TEM and HRTEM) and
confrmed the ionic liquid and cinchona alkaloid stabilization to the Rh(0) NPs (XPS). When modifed by chiral diamine,
including (1R,2R)-diphenylethylenediamine ((1R,2R)-DPEN) or cinchona alkaloid derivatives, the Rh NPs catalysts exhib-
ited good activity, chemoselectivity and enantioselectivity in the heterogeneous enantioselective hydrogenation of aromatic
ketones. Synergistic efect between (1R,2R)-DPEN and cinchonidine was also observed, which signifcantly accelerated the
reaction rate and enhanced the enantioselectivity. 63.0% enantioselectivity and 98.9% chemoselectivity could be achieved in
the acetophenone enantioselective hydrogenation; up to 70.2% enantioselectivity and 100% chemoselectivity was obtained
in the isobutyrylbenzene catalytic enantioselective hydrogenation. Catalytic system could be reused several times without
signifcant loss in activity, chemoselectivity as well as enantioselectivity. This catalytic protocol opens the door to heteroge-
neous enantioselective hydrogenation of aromatic ketones with metal Rh NPs immobilized in ionic liquid.
Graphical Abstract
Cinchona alkaloid and ionic liquid stabilized Rh NPs catalyst, when modifed by chiral diamine like (1R,2R)-DPEN, exhibited high activity,
high chemoselectivity and good enantioselectivity in the challenging heterogeneous enantioselective hydrogenation of aromatic ketones to corre-
sponding aromatic alcohols under mild conditions. Synergistic efect between (1R,2R)-DPEN and cinchonidine was also observed, which signif-
icantly accelerated the reaction rate and enhanced the enantioselectivity. 63.0% enantioselectivity and 98.9% chemoselectivity could be achieved
in the acetophenone enantioselective hydrogenation; up to 70.2% enantioselectivity and 100% chemoselectivity was obtained in the isobutyrylb-
enzene catalytic enantioselective hydrogenation. Catalytic system could be reused several times without signifcant loss in activity, chemoselec-
tivity as well as enantioselectivity. This catalytic protocol opens the door to heterogeneous enantioselective hydrogenation of aromatic ketones
with metal Rh NPs immobilized in ionic liquid.
Keywords Aromatic ketone · Cinchona alkaloid · Heterogeneous enantioselective hydrogenation · Ionic liquid ·
Nanoparticle
Extended author information available on the last page of the article
Vol.:(0123456789)
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H. Jiang et al.
1 Introduction
2 Results and Discussion
Over the past decades, metal nanoparticles synthesized in
ionic liquids have been receiving more and more attention
in catalytic hydrogenation and other applications [1–3].
With many advantages, such as negligible volatility, excel-
lent thermal stability, remarkable solubility and a variety of
available structures, ionic liquids can act as a support and
adhere to the metal surface through either electrostatic, van
der Waals as well as covalent interactions, which provide
stability to the nanoparticles through electrostatic or steric
repulsion between neighbouring particles [4–6]. Dupont and
co-workers [7] prepared well-dispersed transition metal nan-
oparticles in various imidazole-based ionic liquids with nar-
row particle size distribution and showed interesting activity
and selectivity in the ketones hydrogenation. Leitner and
co-workers [8] successfully employed Ru(0) nanoparticles
immobilized in ionic liquids for chemoselective hydrogena-
tion of biomass-derived substrates. We [9, 10] previously
reported the use of phosphine functionalized ionic liquid-
stabilized ruthenium nanoparticles to catalyze the tunable
chemoselective hydrogenation of aromatic ketones, aromatic
aldehydes as well as quinolines. Good to excellent catalytic
performance was achieved.
2.1 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 equiva-
lent of cinchona alkaloids, which aforded a dark suspen-
sion. For comparison, we also synthesized Rh NPs without
any additional stabilizer. A black powder could be sepa-
rated from the black suspension by the addition of acetone
followed by centrifugation (5000 rpm for 10 min). Washed
three times with acetone and dried under reduced pressure,
the isolated powder was analyzed by transmission electron
microscopy (TEM) and X-ray photoelectron spectroscopy
(XPS).
TEM analysis was used to characterize the obtained Rh
NPs and determine the average diameter (Fig. 1, image A
and A#). The TEM and HRTEM image of cinchonidine
stabilized Rh NPs exhibited a regular spherical shape and
a narrow size distribution with an average diameter of
1.96 nm.
The XPS analysis of the cinchonidine stabilized Rh
NPs was employed to elucidate the nature of the stabiliz-
ing layer of the nanoparticles (Fig. 2). XPS analysis of
Rh NPs stabilized by cinchonidine showed the presence
of rhodium, boron, nitrogen and oxygen, which signifed
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 (Fig. 2), which indicated the Rh
NPs were composed of Rh(0) [23]. In short, the TEM and
XPS results indicated that the rhodium (III) species was
completely reduced to Rh NPs, and these Rh NPs were
protected by the BMIMBF4 and cinchonidine without any
change of valence.
Asymmetric hydrogenation of simple aromatic ketones is
an important organic conversion because the resulting chiral
alcohols are a versatile precursor to many natural products
and drug molecules [11, 12]. However, in the feld of hetero-
geneous enantioselective hydrogenation of simple aromatic
ketone and its derivatives, successful catalytic example
is rather limited [13–16]. Such as, Reyes and co-workers
[17] reported enantioselective hydrogenation of acetophe-
none on cinchonidine modifed Ir/SiO2 catalysts and up to
62% enantiomeric excess was obtained. Li et al. reported
diphenylethylenediamine modified 1%Ru/γ-Al2O3/2tpp
(tpp:triphenylphosphine) for the asymmetric hydrogenation
of aromatic ketones with the best enantioselectivity of 78%
[18]. Chen previously reported heterogeneous enantioselec-
tive hydrogenation of aromatic ketones employing cinchona-
and phosphine-modifed iridium as catalysts [19–22], with
enantioselectivity up to 96%. We herein originally report
the heterogeneous enantioselective hydrogenation of aro-
matic ketones, catalyzed by cinchona alkaloids stabilized
pretty small rhodium nanoparticles (Rh NPs) modifed by
chiral diamine, such as (1R,2R)-diphenylethylenediamine
((1R,2R)-DPEN)) or cinchona alkaloid derivatives, in imi-
dazolium-based ionic liquids, with high activity, chemose-
lectivity and up to 70.2% enantioselectivity to corresponding
aromatic alcohols.
2.2 Catalytic Hydrogenation
Enantioselective hydrogenation was performed in a 20 mL
stainless autoclave with a magnetic stirrer bar, by using
cinchonidine stabilized Rh NPs as a catalyst in the pres-
ence of chiral diamine like (1R,2R)-diphenylethylenedi-
amine ((1R,2R)-DPEN) or cinchona alkaloid derivatives
as chiral modifers. Acetophenone was chosen as a model
substrate to explore the enantioselective catalytic hydro-
genation performance of Rh NPs. In heterogeneous enan-
tioselective catalysis, reaction activity, chemoselectivity
as well as enantioselectivity are always rather sensitive to
the solvent used [24]. In BMIMBF4 solvent, 20.0% con-
version, up to 96.2% chemoselectivity and 51.6% ee were
obtained for the acetophenone asymmetric hydrogenation
1 3

Heterogeneous Enantioselective Hydrogenation of Aromatic Ketones Catalyzed by Rh…
Fig. 1 TEM image of cincho-
nidine stabilized Rh NPs A,
HRTEM image of cinchonidine
stabilized Rh NPs A# and TEM
image of the spent Rh NPs after
3 recycles of acetophenone
hydrogenation A*
viscosity, the possible coordination efect between anion
and Rh NPs as well as the diference in stabilizer/modifer
conformation in diferent ionic liquids sharply infuenced
the catalytic performance. In order to further optimize the
catalytic activity, chemoselectivity and enantioselectiv-
ity, we attempted to introduce co-solvent into the cata-
lytic system. Catalytic activity increased while co-solvents
like alcohols and water were introduced (Table 1, entries
5–8); chemoselectivity of acetophenone to α-phenethyl
alcohol were slightly increased accompany with all co-
solvents introduction tested (Table 1, entries 5–10); how-
ever, improvement of the hydrogenation enantioselectivity
were only observed while EtOH or H2O co-solvents were
introduced. The catalytic performance, 92.1% conversion,
98.9% chemoselectivity and 63.0% ee in mixture solvent
BMIMBF4-EtOH, is the best in acetophenone heterogene-
ous enantioselective hydrogenation herein. Upon the opti-
mization of solvents, we further investigated the efects
of modifers, base additives as well as transition metal
species on catalytic performance. Similar to homogene-
ous reactions and some heterogeneous reactions [25–27],
base additive is important to accelerate the reaction. In
the absence of base additive BMIMOH, the activity,
Fig. 2 XPS spectrum of Rh 3d in cinchonidine stabilized Rh NPs
(Table 1, entry 1). However, when BMIMPF6, BMIMNTf2
and BMIMOTf were employed as the solvent, the conver-
sion were rather low, accompany with sharply decreased
ee (Table 1, entries 2–4 vs. 1). We believe the ionic liquid
1 3

H. Jiang et al.
Table 1 Optimization of reaction conditions for the enantioselective hydrogenation of acetophenone
OH
OH
Ac
O
Aa
Ab
O
A
H2N
NH2
(1R,2R)-DPEN
Entry
Ionic liquid
Co-solvent
Conversion (%)
Selectivity (%)
Aa
ee (%)
Confga
Ab
Ac
1.1
1
2
3
4
5
6
7
8
BMIMBF4
BMIMPF6
BMIMNTf2
BMIMOTf
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
BMIMBF4
–
–
–
–
20.0
2.7
0.3
96.2
98.3
99.0
95.1
99.6
98.9
99.5
96.9
99.2
98.7
70.5
62.1
94.7
100.0
2.7
1.7
1.0
4.9
0.2
0.6
0.5
0.0
0.0
0.0
17.0
7.4
2.0
0.0
51.6
2.6
8.6
S
S
S
S
S
S
S
S
S
S
S
S
S
S
0.0
0.0
0.0
0.2
0.5
0.0
3.1
0.8
0.3
12.5
30.5
3.3
0.0
3.0
35.0
36.0
63.0
22.0
54.0
28.0
24.0
0.7
MeOH
EtOH
iPrOH
H2O
THF
Toluene
EtOH
EtOH
EtOH
EtOH
74.3
92.1
62.0
92.3
12.0
23.0
34.0
96.3
93.5
2.5
9
10
11b
12c
13d
14e
0.9
35.0
3.3
Reaction was carried out at 40 °C for 10 h, PH2:5.0 MPa, substrate:0.86 mmol, cinchonidine act as the stabilizer, substrate/Rh/modifer (1R,2R)-
DPEN=200:1:2, Vionic liquid:1 mL, Vionic liquid:Vco-solvent=1:1, BMIMOH=0.20 mol L−1 was added. Products were analyzed by a GC
instrument with an FID detector and β-DEX120 capillary column
aDetermined by compare the optical rotation of hydrogenation products and known compounds
bNo BMIMOH introduced
cNo modifer (1R,2R)-DPEN and no BMIMOH introduced
dRu NPs as catalyst
ePt NPs as catalyst
chemoselectivity as well as enantioselectivity were sharply
decreased (Table 1, entries 11 vs. 6). In order to clarify
the importance of (1R,2R)-DPEN modifcation efect to Rh
NPs, both (1R,2R)-DPEN and BMIMOH were removed
(Table 1, entry 12), accompany with the catalytic reaction
activity recover, the chemoselectivity toward α-phenethyl
alcohol was further decreased. This should be explained
that the removal of the modifer made it easier for both the
carbonyl and benzene ring in acetophenone to get close
to the active site of the Rh NPs. Ru and Pt nanoparticles
were also tested under similiar conditions (Table 1, entries
13–14), however, moderate to poor catalytic performance
were obtained.
The introduction of ligands as metal particle stabilizers
is one of the most efective methods to improve the catalytic
performance of supported metal catalysts [28, 29]. Without
the stabilizer, the hydrogenation resulted in conversion as
low as 16.3%, chemoselectivity of 73.0% and ee value of
26.0% (Table 2, entry 1). Diferent cinchona alkaloid stabi-
lizers have signifcant diferent efects on catalytic perfor-
mance especially in activity and enantioselectivity (Table 2,
entries 2–5). When cinchonidine act as the stabilizer, 92.1%
conversion, 98.9% chemoselectivity and 63.0% ee were
achieved. However, only 38.9% conversion, 99.5% chem-
oselectivity and 22.0% ee were obtained in the presence of
stabilizer cinchonine (Table 2, entry 3). On the other hand,
1 3

Heterogeneous Enantioselective Hydrogenation of Aromatic Ketones Catalyzed by Rh…
Table 2 Efect of diferent stabilizers and modifers on enantioselective hydrogenation of acetophenone
N
N
N
N
H
H
H
H
H
H
H
H
NH2
H2N
NH2
H2N
H3CO
OCH3
N
N
N
N
Entry
Stabilizer
–
Cinchonidine
Cinchonine
Quinidine
Modifer
Conversion (%)
Selectivity (%)
ee (%)
Confg.
a
b
c
1
2
3
4
5
6
7
8
9
(1R,2R)-DPEN
(1R,2R)-DPEN
(1R,2R)-DPEN
(1R,2R)-DPEN
(1R,2R)-DPEN
I
II
III
IV
16.3
92.1
38.9
62.0
62.5
64.0
98.7
77.4
99.5
73.0
98.9
99.5
98.7
99.2
99.2
99.1
98.8
98.7
20.8
0.6
0.5
0.7
0.3
0.4
0.0
0.5
0.5
6.2
0.5
0.0
0.6
0.5
0.4
0.9
0.7
0.8
26.0
63.0
22.0
37.1
39.2
26.0
9.4
S
S
S
S
S
S
R
S
S
Quinine
Cinchonidine
Cinchonidine
Cinchonidine
Cinchonidine
10.0
30.0
The reaction conditions are the same as in Table 1 (V BMIMBF4:V EtOH=1:1)
modifers have critical important impact on the activity, che-
moselectivity as well as enantioselectivity in most existed
heterogeneous catalysis systems too [30, 31]. Considering
cinchona alkaloid derivative modifers exhibited excellent
catalytic performance in some previous research [15, 32].
We further tested the efect of diferent cinchona alkaloid
derivative modifers on the catalytic performance (Table 2,
entries 6–9). However, the enantioselectivity in the ace-
tophenone heterogeneous enantioselective hydrogenation
were obviously decreased. The decrease in catalytic per-
formance should reasonably be ascribed to the steric hin-
drance caused by cinchona alkaloids derivatived modifers
and which indicated the high specifc correlation between
the modifer and substrate in heterogeneous enantioselec-
tive catalysis. Above results indicate that a proper solvent,
stabilizer, and combination of chiral diamine modifer and
base additive contribute to the formation of the catalytic
species and the chiral induction. Compared with the catalyst
using phosphine as the stabilizer [19–22], cinchona alkaloids
stabilized Rh catalysts exhibited unique feature. Obviously,
synergistic efect between (1R,2R)-DPEN and cinchonidine
signifcantly accelerated the reaction rate and enhanced the
enantioselectivity.
listed in Table 3. The extent of catalytic activity, chemose-
lectivity, and enantioselectivity appeared to be delicately
afected by substituent in the substrate. The catalytic activ-
ity decreased and the chemoselectivity and enantioselec-
tivity increased by increasing the bulkiness of the alkyl
group from methyl or primary alkyl to isopropyl (Table 3,
entries 1–3). It is worth mentioning that the chemoselec-
tivity of isobutyrylbenzene hydrogenation was 100% and
enantioselectivity could reach as high as 70.2%. When
the substituent was on the aromatic ring, the catalytic
hydrogenation enantioselectivity was generally decreased
(Table 3, entries 4-6). The catalytic activity decreased
with the increase of the electron-donating efect in the
para-position of aromatic ring. While the chemoselectivity
decreased with the increase of the electron-drawing efect
in the para-position of aromatic ring. It is worth mention-
ing that a certain percentage of dehalogenation products
were detected in the 4’-chloroacetophenone hydrogenation
(Table 3, entry 6). Additionally, enantioselective hydro-
genation of ketoesters including ethyl pyruvate and ethyl
acetoacetate was also tested. Ethyl pyruvate could be
hydrogenated to corresponding R-ethyl lactate with 100%
conversion and 9.9% ee value with the same reaction con-
ditions as in Table 3; however, no enantiodiferentiating
ability was detected during the ethyl acetoacetate hydro-
genation. Above all, the efect of the steric bulk and the
Some representative examples of the asymmetric hydro-
genation of the aromatic ketones catalyzed by cinchoni-
dine-stabilized Rh NPs modifed by (1R,2R)-DPEN are
1 3

H. Jiang et al.
Table 3 Enantioselective hydrogenation of aromatic ketones catalyzed by (1R,2R)-DPEN modifed cinchonidine stabilized Rh nanoparticles
OH
R2
O
OH
R2
O
R2
R2
+
R1
+
R1
R1
R1
A-F
A-F c
A-F a
A-F b
O
O
O
O
O
O
MeO
F3C
Cl
B
D
E
F
A
C
Entry
Substrate
Conversion (%)
Selectivity (%)
A–F a
ee (%)
Confg.
A–F b
A–F c
1
2
3
A
B
C
D
E
F
92.1
78.0
66.0
47.0
99.5
62.0
98.9
99.0
100.0
100.0
87.1
0.6
0.0
0.0
0.0
2.5
0.0
0.5
1.0
0.0
0.0
10.4
0.0
63.0
65.0
70.2
29.0
38.0
35.0
S
S
S
S
S
S
4
5
6a
90.0
The reaction conditions are the same as in Table 1 (V BMIMBF4:V EtOH=1:1)
b10% dehalogenation product was detected
Figure 3 indicated the recyclability of enantioselec-
tive hydrogenation of acetophenone in a mixture solvent
of BMIMBF4 and H2O, and it was found that the leaching
amount of the Rh NPs catalyst was negligible by ICP-AES
analysis in continuous catalytic cycle. The catalyst could be
reused by simple liquid–liquid extraction after the reaction.
The process was repeated 5 times and the results showed
that Rh NPs could be recycled without signifcant loss of
catalytic activity, chemoselectivity and enantioselectivity.
3 Conclusion
In conclusion, cinchona alkaloids stabilized Rh NPs cata-
lyst, when modifed by chiral diamine like (1R,2R)-DPEN,
exhibited high activity, chemoselectivity and up to 70.2%
enantioselectivity in challenging heterogeneous enantiose-
lective hydrogenation of aromatic ketones to correspond-
ing aromatic alcohols in imidazolium-based ionic liquids
under mild conditions. In comparison with the catalyst using
phosphine as the stabilizer, cinchona alkaloids stabilized Rh
catalysts exhibited unique feature. Apparently, synergistic
efect between (1R,2R)-DPEN and cinchonidine signifcantly
Fig. 3 Recyclability of enantioselective hydrogenation of acetophe-
none in a mixture solvent of BMIMBF4 and H2O. Reaction conditions
are the same as in Table 1
electronic nature of the substrate infuence the activity,
chemoselectivity as well as the enantioselectivity in the
catalytic reaction.
1 3

Heterogeneous Enantioselective Hydrogenation of Aromatic Ketones Catalyzed by Rh…
accelerated the reaction rate and enhanced the enantiose-
lectivity. Catalytic system could be reused 5 times without
signifcant loss in activity, chemoselectivity and enantiose-
lectivity. The work reported herein provides new direction
for heterogeneous enantioselective nanometal catalysis in
ionic liquids. Additional work is currently in progress in
this and related areas.
liquid–liquid extraction and analyzed by gas chromatography.
Isolation of the Rh NPs for TEM analysis after catalytic cycles
was achieved by dissolving the reaction mixture in acetone
(5 mL), centrifuging (5000 rpm for 10 min), washing with
acetone (3×5 mL) and drying under vacuum.
Acknowledgements This work was fnancially supported by Natural Sci-
ence Foundation Project of CQ (No. cstc2018jcyjAX0735), National Nat-
ural Science Foundation of China (No. 21201184), Chongqing Technol-
ogy and Business University (1751039) and Chongqing Key Laboratory
of Catalysis and New Environmental Materials (1456028, KFJJ2018050).
4 Experimental Section
4.1 Materials
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In a typical experiment, RhCl3·3H2O (0.014 mmol) and cin-
chonidine (0.028 mmol) was well dispersed in BMIMBF4
(1 mL) (BMIM=1-butyl-2,3-dimethylimidazolium) and the
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In stainless steel autoclave, previously prepared Rh(0) cata-
lyst was charged with the appropriate modifer, co-solvent and
substrate, and then the autoclave was sealed and purged with
pure hydrogen several times. After the reactants were heated
to predetermined temperature, the reaction timing began. After
completion of the reaction and cooling to ambient tempera-
ture, the products were isolated by high speed centrifugation or
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H. Jiang et al.
Afliations
He‑yan Jiang1 · Hong‑mei Cheng1 · Feng‑xia Bian1
* He-yan Jiang
Technology and Business University, Chongqing 400067,
China
orgjiang@163.com
1
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
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