Thermoregulated ionic liquid-coordinating ruthenium complexes for asymmetric hydrogenation of aromatic ketones

Article abstract of DOI:10.1016/j.catcom.2018.12.014

This work presented the synthesis and characterization of new ionic liquid-coordinating ruthenium complexes. The resulting ruthenium complexes exhibited not only excellent thermoregulated phase-separation behavior but also highly catalytic activity and enantioselectivity for the asymmetric hydrogenation with molecular hydrogen. The thermoregulated ionic liquid catalyst was highly resistant to leaching and was recycled consecutively for six times without significant loss of catalytic activity and enantioselectivity. The presence of Ru–H species revealed that NH and a Ru–H unit, involved in the hydride transfer process, were of great importance in the present catalytic system.

Full text of DOI:10.1016/j.catcom.2018.12.014

Catalysis Communications 121 (2019) 43–47
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Thermoregulated ionic liquid-coordinating ruthenium complexes for
asymmetric hydrogenation of aromatic ketones
⁎
Guoping Tang, Manyu Chen, Jian Fang, Zichen Xu, Honghui Gong, Qingpo Peng, Zhenshan Hou
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and
Technology, Shanghai 200237, China.
A R T I C L E I N F O
A B S T R A C T
Keywords:
This work presented the synthesis and characterization of new ionic liquid-coordinating ruthenium complexes.
The resulting ruthenium complexes exhibited not only excellent thermoregulated phase-separation behavior but
also highly catalytic activity and enantioselectivity for the asymmetric hydrogenation with molecular hydrogen.
The thermoregulated ionic liquid catalyst was highly resistant to leaching and was recycled consecutively for six
times without significant loss of catalytic activity and enantioselectivity. The presence of RueH species revealed
that NH and a RueH unit, involved in the hydride transfer process, were of great importance in the present
catalytic system.
Asymmetric hydrogenation
Ionic liquid
Thermoregulated phase-separation
Ruthenium complexes
1
. Introduction
systems. Recently, our group has reported several catalytic systems
with temperature-controlled [8–10] or pH-sensitive [11] phase se-
Asymmetric hydrogenation of prochiral ketones is one of the most
paration in solvents such as ethyl acetate, isopropanol or aqueous
phase. These catalyst systems provided not only the advantages of
convenient phase-separation like heterogeneous catalysts, but also the
catalytic activity as high as homogeneous counterparts. Among these
methodologies, the thermoregulated phase-separation system afforded
an alternative approach to designing high-performance catalysts, and
showed the advantages of high reusability, mild reaction conditions and
even the potential suitability with low catalyst loading [12].
remarkable methods for the industrial production of chiral alcohols
1,2]. Chiral alcohols are significant intermediates for synthesizing
[
chiral drugs and fine chemicals, so that the synthesis of chiral alcohols
is indispensable nowadays. One of the most effective methods is the
asymmetric hydrogenation of prochiral compounds using a transition-
metal-complex as the chiral catalyst. With regard to the catalyst-de-
signing, introducing chiral ligands is distinctly important for synthesis.
Among these methodologies, asymmetric hydrogenation of simple ke-
tones catalyzed by the homogeneous chiral Ru(II) complexes discovered
by Noyori et al. showed excellent characteristics of activity and en-
antioselectivity [3,4].
For hydrogenation reaction, the normal hydrogen source is H gas
2
itself, which is typically available commercially within the storage
medium of a pressurized cylinder. The hydrogenation process often
uses > 1 atm of H , in order to achieve excellent conversions. As
2
Discovering green approaches towards synthesis of chiral com-
pounds is of great concern. However, homogeneous catalysts often have
problems of catalyst separation and reuse. Thus, designing easily se-
parable catalysts is a popular and interesting research area. The im-
mobilization of homogeneous catalysts on insoluble supports may solve
this problem, but it is difficult to maintain the same high level of cat-
alytic performance as the original homogeneous counterparts, because
the decrease of the catalyst dispersion in the microscopic reaction en-
vironment could increase the mass transfer resistance between the
substrate and the active sites.
compared with transfer hydrogenation, using H
2
gas as hydrogen
source is clearly greener, and H
reaction mixture [13–15].
2
can be separated more easily from the
In the previous research, we have developed thermoregulated ionic
liquid (IL) catalysts for selective hydrogenation of C]C bonds [16], as
well as asymmetric transfer hydrogenation of aromatic ketones [17].
Herein, we report on the synthesis of new thermoregulated IL catalysts
consisting of poly(ethylene glycol) (PEG) functionalized imidazolium as
a cation moiety and ruthenium complexes coordinated with dipho-
sphine and sulfonated diamine ligands as an anion moiety. The IL
catalysts exhibits strong thermoregulated behavior and provided an
alternative approach for highly efficient asymmetric hydrogenation of
Up to now, the methods of fluorination [5], silanization [6] and
magnetic adsorption [7] have been reported to create “smart” catalytic
⁎
Corresponding author.
E-mail address: houzhenshan@ecust.edu.cn (Z. Hou).
https://doi.org/10.1016/j.catcom.2018.12.014
Received 8 October 2018; Received in revised form 5 December 2018; Accepted 21 December 2018
Available online 22 December 2018
1566-7367/ © 2018 Elsevier B.V. All rights reserved.

G. Tang et al.
Catalysis Communications 121 (2019) 43–47
various ketones using H
2
gas as a green hydrogen source. Additionally,
of the anion moiety Ru-BINAP-DPENDS was shown in Fig. 1b(Left),
−1
−1
the catalytic reaction can be performed under mild reaction conditions,
bearing a satisfying reusability without obvious loss of catalytic per-
formance.
which gave bands at approximately 1035 cm
,
1636 cm
,
−1
−1
1192 cm and 1434 cm (Fig. 1b), corresponding to the vibration of
CeN bond, the aromatic benzene rings (C]C stretching), O]S]O
bonds and NeH bond, respectively. Notably, the CeN band in Ru(II)
−1
complex anion (1035 cm ) shifted to a lower wavenumber in com-
1.1. Experimental section
−1
parison to the same band in (S,S)-DPENDS (1050 cm ) (Fig. 1a vs 1b),
suggesting that Ru was successfully coordinated with diamine ligand.
All data above confirmed the formation of the proposed Ru(II) com-
The PEG chain-functionalized 1-alkyl-2-methyl-imidazolium
dichlorides ([PEG-1000-C
n
MIM]Cl ) and the sulfonated (S,S)-1,2-di-
2
3
1
plexes with the diamine and diphosphine ligands. In addition, PNMR
spectrum of Ru-BINAP-DPENDS gave a resonance signal around 58.7
phenyl-ethylenediamine ((S,S)-DPENDS, sodium salt) were prepared
according to the our previously reported procedure [17], and then the
anion moiety (Ru-BINAP-DPENDS) was prepared through the reaction
(
Fig. 1e, Right), which was slightly larger than the unsulfonated ana-
logue from the literature [19], which could be attributed to the effect of
between [p-(cymene)RuCl
2
]
2
and (S)-(−)-2,2’-Bis (diphenylpho-
−
the electron-withdraw group (SO
3
) tethered to the DPEN ligand.
sphino)-1,1′-binaphthyl ((S)-BINAP), followed by reacting with (S,S)-
Besides, the IL cation [PEG-1000-C
n
MIM]Cl
2
was conveniently
DPENDS. Thus the IL-regulating ruthenium complex ([PEG-1000-
1
prepared and characterized by H NMR and Elemental Analysis
C MIM][Ru-BINAP-DPENDS]) was obtained by ion exchange of [PEG-
n
(
Scheme S3 and S4), which was in well agreement with our previously
1
000-C
n
MIM]Cl and Ru-BINAP-DPENDS. All synthetic procedures and
2
reported work [9]. At last, the IL catalyst [PEG-1000-C MIM][Ru-
n
characterization of the IL-regulating ruthenium complexes have been
given in the supporting information (Scheme S1-S5).
BINAP-DPENDS] was obtained through simple ion exchange. The FT-IR
spectra of the IL catalysts showed the characteristics of both the IL
cation and anionic moiety. As shown in Fig. 1c, the bands at approxi-
2
. Results and discussion
−1
−1
mately 620 cm
and 1110 cm
could be characterized as the CeO
and C]N bond from the part of IL cation, while the bands at about
2.1. Catalyst characterization
−1
−1
−1
1
035 cm , 1636 cm , and 1192 cm
were characteristics of CeN
bond, the aromatic benzene ring (C]C stretching) and O]S]O bonds
It has been demonstrated that PEG-functionalized 1-alkyl-2-methyl-
from the moiety of anion, respectively. Furthermore, the UV/vis spectra
imidazolium chloride ([PEG-m-C
n
MIM]Cl ) performs a clear thermo-
2
of (S,S)-DPENDS, [(p-cymene)RuCl
2
]
2
and [PEG-1000-C12MIM][Ru-
regulated feature [17,18]. In this work, the thermoregulated IL-co-
ordinating ruthenium complexes consisting of the cation moiety [PEG-
BINAP-DPENDS] are shown in Fig. S1. It was observed that the absor-
bance at 240–280 nm can be assigned to adsorption of aromatic rings
1
000-C MIM] and the anion moiety Ru-BINAP-DPENDS have been
n
(
Fig. S1a-c) and the absorbance at 415 nm was attributed to Cl p → Ru d
synthesized, respectively (Scheme S1-S3).
transition (Fig. S1b). However, the band at 415 nm was moved to
Next, the anion moiety Ru-BINAP-DPENDS was characterized in
1
31
560 nm after [(p-cymene)RuCl
2
2
] were reacted with diamine and di-
detail by means of H NMR, P NMR, FT-IR, ICP-AES and Elemental
1
phosphine ligands, indicating that Ru(II) complex coordinating with
diphosphine and diamine ligands was formed.
Analysis. The H NMR characterization has been given in the sup-
porting information, where the double peaks at 3.21 could be char-
acterized as the protons from NH -CH-, and the multi-peaks at
2
6
.51–7.99 came from the protons of aromatic rings. The FT-IR spectrum
Fig. 1. Left: FT-IR Spectra of a) (S,S)-DPENDS; b) Ru-BINAP-DPENDS; c) [PEG-1000-C12MIM][Ru-BINAP-DPENDS]; d) the spent [PEG-1000-C12MIM][Ru-BINAP-
DPENDS] catalyst. Right: 31P NMR spectra of e) Ru-BINAP-DPENDS in D
O; f) [PEG-1000-C12MIM][Ru-BINAP-DPENDS] in CDCl .
2
3
44

G. Tang et al.
Catalysis Communications 121 (2019) 43–47
absence of catalyst or hydrogen, confirming that both the IL catalyst
and molecular hydrogen were essential for this reaction (entries 1 and
2
, Table 1). Although two IL-regulating Ru(II) complexes catalysts
[
PEG-1000-C
BINAP-DPENDS] exhibited excellent activity and enantioselectivity for
hydrogenation of acetophenone, the [PEG-1000-C MIM][Ru-BINAP-
8
MIM][Ru-BINAP-DPENDS] and [PEG-1000-C12MIM][Ru-
8
DPENDS] with shorter carbon chain could not be completely separated
from the catalytic system even the temperature decreased to 0 °C (en-
tries 3 vs 4, Table 1). This revealed that the thermoregulated separation
behavior was dependent strongly on the hydrophobicity of cation
moiety, and the appropriate hydrogen bonding or hydrophobic inter-
actions between IL and solvent was very vital to offer the thermo-
morphic behavior [23,24]. The simple Ru(II) complex salt Ru-BINAP-
DPENDS afforded lower catalytic activity and enantioselectivity than
that of [PEG-1000-C12MIM][Ru-BINAP-DPENDS] (entries 4 vs 6,
Table 1) because of its poor miscibility with isopropanol.
Fig. 2. The conductivity as function of the concentration of the IL [PEG-1000-
C
12MIM][Ru-BINAP-DPENDS] in isopropanol at a) 0 °C, b) 30 °C and c) 40 °C.
The diphosphine ligand (S)-BINAP is very crucial to achieve high
enantioselectivity. When the Ru sites were coordinated with achiral
diphosphine ligand (1,3-Bis(diphenylphosphino)propane) (DPPP) and
chiral diamine ligand (S,S)-DPENDS, the resulting catalyst [PEG-1000-
2.2. Thermoregulated phase-separation
At first, the properties of the thermoregulated IL catalysts have been
C12MIM][Ru-DPPP-DPENDS] exhibited lower activity and enantios-
examined. As shown in Fig. S2a, before the reaction, the catalyst sank to
the bottom. However. After temperature arose to 40 °C in the presence
of base promoter and acetophenone, the reaction mixture became
homogeneous (Fig. S2b). As the reaction temperature declined, the
catalyst gradually precipitated from the reaction system and sank to the
bottom or adhered to inner wall of the flask again (Fig. S2c).
electivity with respect to the catalyst [PEG-1000-C12MIM][Ru-BINAP-
DPENDS] (entries 4 vs 5, Table 1) under the same reaction conditions,
although the catalyst indeed exhibited temperature-regulated separa-
tion behavior. The moiety of achiral diphosphine (DPPP) ligand ac-
counts for low enantioselectivity, although chiral ligand (S,S)-DPENDS
still could exert an impact on asymmetric induction in Ru(II) complex
catalyst [25]. Thus, the diphosphine ligand (S)-BINAP played a vital
role in not only offering chirality but also enhancing enantioselectivity
in present IL catalytic systems.
The conductivity has been proved a powerful tool to determine the
critical associating concentration (cac) of polymer surfactants [20],
which was highly related with the aggregation state of the surfactants.
The aggregation of the IL catalyst [PEG-1000-C MIM][Ru-BINAP-
n
Base promoter is very crucial in asymmetric hydrogenation [26],
and then the effect of the base promoter on reaction has been examined
over [PEG-1000-C12MIM][Ru-BINAP-DPENDS] catalyst. As shown in
Table S1, the conversion and enantioselectivity of asymmetric hydro-
genation of acetophenone decreased as the basicity weakened
DPENDS] in isopropanol was investigated by conductivity measure-
ment. As shown in Fig. 2, the conductivity was measured as a function
of the concentration of the IL catalyst. It was found that the con-
ductivity of the IL catalyst in isopropanol was close to that of iso-
−
1
propanol at 0 °C (0.06 k/μS·cm ) (Fig. 2a), demonstrating that the IL
was hardly dissolved in isopropanol, which was consistent with visual
observation. However, the conductivity increased significantly with the
catalyst concentration at 30 °C, and then the increment slowed down as
the concentration became higher, indicating that the aggregation in-
deed occurred, and the cac of IL catalyst was found around 7.9 mM by
linear fitting (Fig. 2b). Moreover, as the temperature increased to 40 °C
(
(CH
3
)
3
COK > KOH > K
2
CO ) under the same conditions (entries 2, 4
3
and 5, Table S1). Next, the dosage of base promoter was also examined,
which indicated that once n(CH3)3COK:ncatalyst reached up to 40 (entries
1
–3, Table S1), the reaction could happen smoothly. Moreover, the
molar ratio of substrate to catalyst (nsubstrate: ncatalyst) also affected the
catalytic activity and enantioselectivity significantly. As shown in Table
S2, the conversion of acetophenone decreased distinctly with increasing
(
Fig. 2c), the conductivity increased monotonously with the con-
n
substrate: ncatalyst due to catalyst dilution. The present IL catalyst could
centration of IL, which reflected that the aggregation did not almost
happen and the IL catalyst possibly existed almost as a molecular form
afford a TON value as high as ca. 3024 within 4 h reaction time (entry 2,
Table S2).
(
loose ion pairs) in isopropanol [21]. Additionally, the conductivity was
With the optimal usages of base promoter and catalyst, the effects of
strongly dependent on temperature at the constant concentration (Fig.
S3). As the temperature decreased, the conductivity decreased as well,
and some of the catalysts were observed to precipitate from solution at
around 20 °C. Furthermore, the complete phase separation of catalyst
occurred and meantime the conductivity of solution was 0.06 k/
the reaction time, temperature and H gas pressure on the catalytic
2
performance were investigated accordingly. The conversion of the
acetophenone was improved to some extent with the increase of the
reaction time, but the enantioselectivity declined for a longer reaction
time (Fig. S4a), which could result from racemization reaction of the
chiral product R-1-phenylethanol. Additionally, the reaction tempera-
ture had a significant effect. As shown in Fig. S4b, the conversion of
acetophenone increased obviously, but the e.e. value of the product
decreased slightly in the range of temperature from 20 °C to 40 °C.
However, as temperature was above 40 °C, all acetophenone was almost
converted to R-1-phenylethanol, but the e.e. value reduced to a certain
extent, indicating that the high e.e. value could not survive at high
−1
μS·cm
at 0 °C, which was close to the conductivity of isopropanol.
These properties were closely relevant to Upper Critical Solution
Temperature (UCST) behavior of the IL, which might be attributed to
hydrogen bonding or other possible interactions between IL and iso-
propanol. As the temperature decreased, the intermolecular interaction
between IL and solvent was weakened, while the intramolecular in-
teraction in ILs was strengthened, which led to a complete phase se-
paration [22].
temperature. Besides, it was observed that as the H pressure increased
2
from 0.1 MPa to 4.0 MPa (Fig. S4c), the conversion of acetophenone
increased from 35% to 99%, and e.e. value remained high above 92%,
which revealed that the present IL catalyst can afford high hydro-
genation activity, and meantime retained excellent enantioselectivity
2.3. Catalytic asymmetric hydrogenation
In this work, the as-synthesized different IL-coordinating ruthenium
complexes have been applied to the asymmetric hydrogenation, and
acetophenone was chosen as a model substrate (Table 1). It can be
found that the asymmetric hydrogenation hardly happened in the
under high H pressure.
2
The applicability of catalyst to various substrates is one of the es-
sential standards to evaluate new catalyst. The asymmetric
a
45

G. Tang et al.
Catalysis Communications 121 (2019) 43–47
Table 1
a
Catalytic activity, enantioselectivity and thermoregulated performance of the different catalysts for asymmetric hydrogenation of acetophenone.
b
c
Entries
Catalysts
Con. (%)
e.e. (%)
Thermoregulated phase separation
1
none
–
–
–
d
c
2
[PEG-1000-C12MIM][Ru-BINAP-DPENDS]
< 1
92
99
52
76
–
Yes
No
Yes
Yes
No
3
4
5
6
[PEG-1000-C
8
MIM][Ru-BINAP-DPENDS]
94
97
43
92
[PEG-1000-C12MIM][Ru-BINAP-DPENDS]
[PEG-1000-C12MIM][Ru-DPPP-DPENDS]
Ru-BINAP-DPENDS
a
Reaction conditions: 40 °C, 4 h, 3.0 MPa H
2
, 0.01 mmol catalyst; 2 mL isopropanol; ncatalys:n(CH3)3COK:nsubstrate = 1:40:2400, where ncatalys, n(CH3)3COK and
n
substrate represented the corresponding moles, respectively.
b
c
d
The selectivity towards R-1-phenylethanol was normally over 99%.
GC analysis was performed with a β-DEX™ 120 capillary column (30 m × 0.25 mm, 0.25 μm film) and dodecane as internal standard.
The reaction was carried out in 3.0 MPa N .
2
Table 2
Asymmetric hydrogenation of various ketones catalyzed by [PEG-1000-C12MIM][Ru-BINAP-DPENDS] in isopropanola.
Entries
Ar
Time (h)
Con. (%)b
e.e. (%)c
1
2
3
4
5
6
7
8
9
C
6
6
H
H
5
5
-
4
4
4
4
4
4
4
4
4
6
6
4
4
99
98
98
97
95
98
92
96
64
95
69
89
95
97
92
99
95
93
93
96
93
89
99
98
99
92
C
C
2
H
H
4
-
4-BrC
6
4
-
-
-
-
4-ClC
2-ClC
3-ClC
6
6
6
H
H
H
4
5
5
4-NO
2
C
6
6
H
4
-
4-MeOC
H
4
-
2-MeC
4-MeC
6
6
H
5
4
-
10
11
12
13
H
-
4-IsobutylC
6 4
H -
4-Furyl-
2-Naphthyl-
a
b
c
Reaction conditions: 40 °C, 3.0 MPa H , 0.01 mmol catalyst; 2 mL isopropanol; ncatalys:n(CH3)3COK:nsubstrate = 1:40:2400.
2
The selectivity towards the corresponding secondary alcohols was normally over 99%.
GC analysis was performed with a β-DEX™ 120 capillary column (30 m × 0.25 mm, 0.25 μm film) and dodecane as internal standard.
hydrogenation reactions of a wide range of ketones, such as hetero-
cyclic aryl ketones, naphthylacetones and other derivatives with dif-
ferent substituents were carried out as shown in Table 2. Under the mild
reaction conditions, the aromatic ketones with electron-withdrawing
substituents were basically hydrogenated to produce corresponding
chiral products (entries 2–7, Table 2). For the aromatic ketones with
electron-donating substituents, the hydrogenation activity of the cata-
lysts decreased comparably (entries 8–11, Table 2), but the enantios-
electivity of the products still kept above 85%, which indicated that the
catalyst system was influenced by the electron effects. An electron-do-
nating group, such as a methyl group or a methoxy group, caused an
increase in the electron density on the benzene ring and the carbonyl
group, so that the carbon cation on the carbonyl group became less
susceptible to protons and reduced the possibility to be hydrogenated.
Notably, no products related to dehalogenative hydrogenation nor the
reduction of nitro group have been detected with the present Ru(II)
complex catalyst during the course the reaction (entries 3–6, 8,
Table 2). As for heterocyclic aromatic substrates, such as 2-acetylfuran,
the catalysts exhibited high activity and enantioselectivity as well
substrate in isopropanol. With simple decantation of the products in the
supernatant organic phase at 0 °C after reaction, the same amount of
acetophenone and small quantity of (CH
3
3
) COK were added to perform
the next run. This procedure was repeated six times and the results
indicated that the catalyst could be reused at least six times without
much loss of the catalytic activity (Fig. 3). According to the ICP-AES
characterization of the spent catalyst, the leaching of Ru species from
the catalyst was only around 1.83 wt% after six runs, indicating that the
(
entry 12, Table 2). Moreover, naphthylacetone also gave efficient
conversion and enantioselectivity and no by-products were detected in
this catalytic system (entry 13, Table 2). In a word, the catalyst showed
an excellent applicability on different substrates. The GC trace analysis
of asymmetric hydrogenation of various aromatic ketones were af-
forded in Fig. S10-S25 in Supporting Information.
The recyclability of the catalyst [PEG-1000-C12MIM][Ru-BINAP-
DPENDS] was examined by choosing the acetophenone as a model
Fig. 3. Recyclability of catalyst [PEG-1000-C12MIM][Ru-BINAP-DPENDS] for
asymmetric hydrogenation of acetophenone.
46

G. Tang et al.
Catalysis Communications 121 (2019) 43–47
catalyst was resistant to leaching during temperature-regulated phase
separation and a clean separation has been obtained efficiently by this
thermoregulated catalyst system. Besides, the FT-IR spectrum of the
spent catalyst showed the preservation of the catalyst structure after
reuses (Fig. 1d), which could account for the excellent recyclability of
the catalysts upon recycling.
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