Nano SiO2-bonded Ru-TsDPEN for catalytic resolution of 1,2-propanediol by two transfer hydrogenation

Article abstract of DOI:10.1134/S1070427215070162

A new strategy was introduced for the catalytic resolution of 1,2-propanediol involving the successive oxidative kinetic resolution and asymmetric hydrogenation by two hydrogen transfer reactions catalyzed by nano SiO₂-bonded Ru-TsDPEN [TsDPEN = N-(p-toluenesulfonyl)-1,2-diphenylethylene diamine]-derived catalysts composed of two opposite configurations. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetry-derivative thermogravimetry (TG-DTG) analysis, and inductively coupled plasma atomic emission spectrometry (ICP-AES). Results showed that Ru-TsDPEN derived catalysts were successfully grafted on the nano SiO₂, affording high catalytic conversions of >99 and 99%, selectivities of 47 and 98% and maximum ee values of >99 and >99% in the two reactions, which were due to the nano SiO₂ with Ru-TsDPEN in the reaction system with high dispersion. Additionally, the catalysts exhibited excellent durability and were filtered and reused at least five times without noticeable catalysts deactivations.

Full text of DOI:10.1134/S1070427215070162

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2015, Vol. 88, No. 7, pp. 1207−1218. © Pleiades Publishing, Ltd., 2015.
VARIOUS
TECHNOLOGIES
Nano SiO2-Bonded Ru-TsDPEN for Catalytic Resolution
of 1,2-Propanediol by Two Transfer Hydrogenation1
Chuan-Jun Yue, Li-Ping Gu, Ya-Feng Zhuang, and Bao-Liang Liu
Department of Chemical Engineering, School of Science, Changzhou Institute of Technology, Changzhou, 213022 China
e-mail: czupapers@126.com
Received July 15, 2015
Abstract—Anew strategy was introduced for the catalytic resolution of 1,2-propanediol involving the successive
oxidative kinetic resolution and asymmetric hydrogenation by two hydrogen transfer reactions catalyzed by nano
SiO2-bonded Ru-TsDPEN [TsDPEN = N-(p-toluenesulfonyl)-1,2-diphenylethylene diamine]-derived catalysts
composed of two opposite configurations. The catalysts were characterized by X-ray diffraction (XRD), trans-
mission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), Fourier-transform infrared spectroscopy
(FT-IR), thermogravimetry-derivative thermogravimetry (TG-DTG) analysis, and inductively coupled plasma
atomic emission spectrometry (ICP-AES). Results showed that Ru-TsDPEN derived catalysts were successfully
grafted on the nano SiO2, affording high catalytic conversions of >99 and 99%, selectivities of 47 and 98% and
maximum ee values of >99 and >99% in the two reactions, which were due to the nano SiO2 with Ru-TsDPEN
in the reaction system with high dispersion. Additionally, the catalysts exhibited excellent durability and were
filtered and reused at least five times without noticeable catalysts deactivations.
DOI: 10.1134/S1070427215070162
INTRODUCTION
through the transesterification of biomass, and accounts
for approximately 10 wt % of the converted products
[11–13]. However, few studies have been conducted on
the subject. Shuklov et al. [10] reported that the dynamic
kinetic resolution of rac-1-tert- butoxypropan-2-ol with
isopropenyl acetate in the presence of Novozyme 435
and a ruthenium (Ru) catalyst produces enantiomerically
pure (R)-1-tert-butoxy-2-acetoxy-propane, which can
be transformed into (R)-1,2-propanediol. Iwasaki et al.
[14] developed the catalytic kinetic resolution method
using a chiral organotin catalyst with benzoyl chloride.
However, controversies surround the described catalytic
resolution, which involves the coordination of the
catalysts for the racemation and the resolution in one
pot for dynamic kinetic resolution, and an intrinsic of
only 50% of a desired enantiomer for kinetic resolution.
Improving these methods is an urgent issue.
Optically active 1,2-diols are important for preparing
various materials, such as chiral medicines and natural
products [1, 2]. Enantiomerically pure (R)-1,2-
propanediol is of critical importance to the total synthesis
of the anti-HIV pharmaceutical Tenofovir and related
functional target molecules [3]. This class of compounds
has so far been prepared using different raw materials
including catalytic asymmetric dihydroxylation of
olefins [4], Baker’s yeast reduction of phenacyl alcohol
[5], enzymatic hydrolysis of diol monoacetates [6],
asymmetric hydrogenation (AH) of α-hydroxyl ketones
[7], and catalytic resolution of racemic 1,2-diols
consisting of kinetic [8, 9] or dynamic kinetic resolution
[10]. Catalytic resolution can be improved through
efficient catalysis and green 1,2-propanediol resource
from the industrial hydrogenolysis of large and cheap
glycerol. Glycerol is a byproduct of biodiesel production
The AH of acetol to chiral 1,2-propanediol has been
successfully performed in recent years by employing
various homogeneous Ru and iridium catalysts with
appropriate ligands [15–17]. Meanwhile, asymmetric
1 The text was submitted by the authors in English.
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CHUAN-JUN YUE et al.
Scheme 1. Schematic preparation of nano SiO2-bonded Ru-TsDPEN derived catalysts.
Ph
Ph
Ph
Ph
Et3N, CH2Cl2
ClO2S
Si(OMe)3
+
H2N
HN
O2S
Si(OMe)3
H2N
NH2
HO
HO
Ph
Ph
SiO2
HO
PhCH3 / reflux
O
H2N
HN
O2S
Si
SiO2
O
MeO
HO
Ph
Ph
O
H2N
HN
O2S
Si
SiO2
O
Ph
Ph
MeO
Ru
Cl
HO
[RuCl (p-cymene)]
O
O
p
2
2
(S)-Cat
H N
H22N
HN
O2S
Si
MeO
SiO2
Ru
Cl
HO
Ph
Ph
O
H2N
HN
O2S
Si
SiO2
O
MeO
Ru
Cl
HO
(R)-Cat
transfer hydrogenation (ATH) with reversible property
has also received considerable attention because of its
high-quality performance, operational simplicity, and the
availability of reductants [18]. Among the various chiral
catalysts reported, more attention has been paid to Ru-
based catalysts, especially Ru-N-(p-toluenesulfonyl)-
1,2-diphenylethyl-enediamine (Ru-TsDPEN) catalysts
developed by Noyori et al. [19]. These catalysts have
been widely applied to reduce prochiral ketones via AH
and ATH, which show excellent enantioselectivities in
propan-2-ol [20, 21].Although metal complex-basedAH
and ATH were proven reliable, catalytic transformations
for green chemistry focus on high efficiency, low
cost, easy separation, and minimal waste. Catalysts
that can be easily separated and recyclable through
filtration are preferable, providing easy operation and
clean transformation from laboratory to large-scale
production. Several catalysts on insoluble supports have
been reported for AH and ATH [22–26]. However, their
heterogenized catalyst recovery through filtration or
centrifugation remains problematic, and their catalytic
efficacy is unsatisfactory because catalytically active
species tends deactivate during handling. Additionally,
the acetol as a starting material is unstable for synthetic
chiral 1,2-propanediol. New methods must be explored
to avoid these drawbacks.
An oxidation kinetic resolution byATH that combines
AH of oxidized byproduct is introduced. This process
involves the successive performance of the oxidation
kinetic resolution of 1,2-propanediol and the AH of
acetol by nano SiO2-supported Ru-TsDPEN catalysts
with opposite configuration, respectively. The desired
product configuration may be obtained by the regulated
ligand configuration sequence. The strategy potentially
allows the adaptation of the immobilized catalysts to
the successive flow processes. The present study mainly
investigated the preparation of nano SiO2-supported Ru-
TsDPEN and their properties as well as their physical
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NANO SiO2-BONDED Ru-TsDPEN
1209
characterization and the completely catalytic resolution
of 1,2-propanediol through successive two opposite
configuration catalyst to (R)- or (S)-1,2-propanediol.
in 50 mL flasks. The mixture was refluxed for 24 h in
nitrogen. After filtration, the obtained white solid was
successively washed with 30 mL toluene and 50 mL
solvent mixture of CH2Cl2 and CH3OH (1 : 1). The
solid was then suspended in 20 mL mixture solvent and
stirred overnight. After filtration and thorough washing
with 25 mL solvent mixture and 25 mL CH3OH the wet
solid was dried at 55°C in vacuo for 24 h to afford the
immobilized ligand. IR (KBr tablet): 3441, 2962, 1640,
1503, 1460, 109, 802, 703, 560, and 467 cm–1.
EXPERIMENTAL
General. [RuCl2(p-cymene)]2 was purchased from
Zhejiang Metallurgical Research Institute Co., Ltd.
(1R,2R)-DPEN and (1S,2S)-DPEN, triethylamine,
2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane,
nano SiO2 (15 ± 5 nm), toluene, isopropaneol, acetone,
1,2-propanol, acetol, CH3OH, and CH2Cl2 were
obtained from Aladdin. (R)-propanediol was obtained
from Acros.
Preparation of nano SiO2-bonded Ru-TsDPEN de-
rived catalysts. [RuCl2(p-cymene)]2 (0.33g, 0.53 mmol),
nano SiO2-bonded (1R,2R)- or (1S,2S)-TsDPEN (0.39 g,
1.07 mmol), and triethylamine (0.3 mL, 2.15 mmol) in
12 mL 2-propanol were stirred at 85°C for 1.5 h. The
gray solid was collected by filtration and washed with a
little of deoxygenated water. After washing in a Soxhlet
extraction in methanol overnight, the solid was dried in
vacuo to obtain the loaded Ru catalyst, ICP-AES analy-
sis showed that 70.5% yield was achieved from Ru con-
tent, and element analysis of the bonded complex cal-
culated from mass percentage of N demonstrated that
loading ratio of the TsDPEN ligand was 2.45 mmol g–1.
Catalysts were denoted as (S)-Cat and (R)-Cat accord-
ing to the different configurations, using [Ru]/SiO2 as a
representation.
CH2Cl2 and CH3OH were distilled from CaH2
and Mg/I2, respectively. Toluene was freshly distilled
from a deep-blue solution of sodium-benzophenone
under nitrogen. Triethylamine was distilled and stored
away from light. Nano SiO2 was heated at 150°C for
3 h and cooled in nitrogen before use. Other reagents
and chemicals were degassed thrice under a nitrogen
atmosphere, and used without further purification.
Catalyst preparation. The nano SiO2-bonded Ru-
TsDPEN derived catalysts were prepared according to
Scheme 1.
The catalysts were synthesized according to the
analog method described in the literature [27, 28].
1
Catalyst characterization. H NMR spectra were
recorded on a Bruker Avance-400 (400 MHz) spectrom-
eter, chemical shifts are expressed in parts per million
downfield from TMS at δ = 0 ppm, and J values were
given in Hz. ICP-AES was performed on a PerkinElmer
ICP/6500 ICP spectrometer. Nitrogen element analysis
was carried out on EA2400 elemental analyzer (Perkin
Elmer). XRD patterns were recorded on Rigaku D/Max
2500 PC X-ray diffractometer, treated with CuKa ra-
diation over the 2θ range of 10°–80°. Specific surface
areas were measured by N2 adsorption/desorption by
the BET method on a Micromeritics Autochem 2910
analyzer. The samples were degassed for 10 h at 100°C
before the measurement. TEM micrographs were ac-
quired using a JEOL JEM-1230 transmission electron
microscope. Sample thermostabilities were determined
on PerkinElmer’s Pyris 1 TGA device (Setsys 170 Ther-
mogravimetric Analyzer, Setaram, France). The experi-
ments were performed under N2 atmosphere from room
temperature to 500°C at a ramp rate of 5°C min–1. FT-IR
spectra were performed on IR200 (Thermo Fisher) spec-
trometer from 400 cm−1 to 4000 cm−1.
Synthesis of modified TsDPEN ligand. (1R, 2R)- or
(1S, 2S)-DPEN (0.318 g, 1.5 mmol) was dissolved in 15
mL of CH2Cl2 with 0.3 mL of triethylamine, a solution
of
2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane
(0.38 g, 1.12 mmol) in 15 mL CH2Cl2 was then added
dropwise at 0°C in 50 mL flasks. After the dropping was
completed, the mixture was slowly warmed to room
temperature and unceasingly stirred for 4 h, then the
solvent was removed in vacuo. The residue subsequently
fast passed through a 15 cm silica gel column (eluent:
Et3N : CH3OH : CH2Cl2 = 1 : 10 : 100), concentrated,
and dried in vacuo to obtain 90.5% yield of modified
DPEN. 1H NMR (400 MHz, CDCl3): 1.95 (s, 2H), 2.03
(s, 1H), 3.56 (s, 9H), 4.32 (d, J = 5.3 Hz, 1H), 4.51 (d,
J = 5.3 Hz, 1H), 7.02–7.41 (m, 14H). IR (KBr): 3352,
3295, 3170, 3061, 3027, 2942, 2840, 1901, 1690, 1603,
1494, 1453, 1411, 1342, 1266, 1192, 1153, 1089, and
1014 cm–1.
Preparation of nano SiO2-bonded modified TsDPEN.
The predried nano SiO2 (1.5 g) and the synthesized
ligand (0.13 g) were added to 20 mL of dry toluene
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CHUAN-JUN YUE et al.
Scheme 2. The (S)- and (R)-Ru-TsDPEN/SiO2 catalytic two hydrogen transfer reaction of oxidative kinetic and asymmetric
hydrogenation.
O
OH
(S)-Cat
OH
OH
OH
+
+
+
(a)
OH
OH
O
(R)-
(S)-Cat
acetone
OH
, (R)-Cat
OH
(b)
OH
(R)-
Catalytic kinetic resolution of 1,2-propanediol.
The catalytic resolution of 1,2-propanediol involved
two reactions by hydrogen transfer. The first step
(a) was the catalytically oxidative kinetic resolution
of 1,2-propanediol, and the second step (b) was the
catalytically AH of acetol (Scheme 2).
solution were orderly placed in 100 mL flasks. The
mixture was degassed thrice, and 0.5 mg (R)-Cat was
added. The reaction system was stirred while being
heated to 40°C until reaction completion. The workup of
step (b) was conducted by the analogous method of step
(a). Obtained samples were analyzed by GC (GC4011A
chromatograph equipped with a flame ionization
detector and CP-Chirasil-Dex CB capillary column),
and 1-butaneol was added as internal standard. Product
quality was assessed on a GC-MS spectrometer (GCQ,
Thermo Finnigan). The collected (R)- and (S)-Cat were
tested for the next run without further modification.
Substrate conversions, selectivity (chiral product in
each reation), yield, and ee (chiral product) value were
calculated by the following equations:
In the catalytically oxidized kinetic resolution andAH
of acetol, all reactions were conducted under a nitrogen
atmosphere using analogous Schlenk-type techniques.
Typically, 0.5 mg (S)-Cat, 28 mL acetone, 0.56 mg KOH
in 10 mL water, and 7 mL 1,2-propanediol were orderly
added into 100 mL flasks, and the mixture was stirred
at 40°C for 20 h. After the reaction, the catalyst was
filtered from the mixture, and 5 μL filtrate was extracted
by syringe for further gas chromatography (GC)
analysis. Other liquids were separated by miniature
vacuum distillation from the solution of acetone and
water. The remainder liquid, containing (R)-1,2-
propanediol and acetol, was introduced into the step (b)
reaction. The liquid, as substrate, and a supplemental 5
mL isopropaneol and 10 mL 5.6 g L–1 KOH aqueous
Conversion (%) = [(initial mass of reactive material –
mass of none-reactive material)/initial mass of reactive
material] × 100%.
Selectivity (%) = (all (R)-1,2-propanediol product mass
in the reaction system/all products mass) × 100%.
ee (%) = [|(R) – (S)|/(R) + (S)]
(R- or S-configuration contained all the same compound
in the reaction system).
RESULTS AND DISCUSSION
Catalysts properties. (S)- and (R)-[Ru]/SiO2
catalysts, featuring the same presentation, were char-
acterized as follows.
Wide-angle XRD. Nano SiO2 is selected as parent
support because of its large surface area and sizable
pore volume, which can suitably accommodate bulky
catalysts comfortably, facilitate substrate diffusion, and
Fig. 1. XRD pattern of nano SiO2 and [Ru]/SiO2.
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NANO SiO2-BONDED Ru-TsDPEN
(b)
1211
(a)
50 nm
(d)
20 nm
(c)
100 nm
100 nm
Fig. 2. The TEM images of [Ru]/SiO2. (a), (b) Ordered nano SiO2 texture with micro cluster. (c), (d) [Ru] complex at the transition
position between dense dark shadow to light gray.
effectively disperse in some solvents. The modified
DPEN ligand by trimethoxysilyl was grafted onto the
surface of the nano SiO2 (Scheme 1). Subsequently,
nano SiO2 with TsDPEN-derived ligand reacted with
[RuCl2 (p-cymene)]2. The wide-angle XRD patterns of
the nano SiO2 and supported Ru-TsDPEN catalyst are
shown in Fig. 1. Only one typical peak for the nano SiO2
parent was observed in the 2θ angle of 22.5°, and the
large peak width could be attributed to finer nanosized
particles compared with ordinary silica gel [29].
Average nanoparticles size was approximately 15.1 nm
as calculated by Debye–Scherrer equation based on the
(111) crystal plane. No obvious change was present
after subsequent surface modification. The nano SiO2
surface modification did not result in nano SiO2 particle
alteration. However, the peak shape of the modified
nano SiO2 became narrow, indicating that successful
graft leads to larger nanoparticles.
TEM analysis. The transmission electron micrograph
in Fig. 2 shows the morphological structure of [Ru]/
SiO2. In Figs. 2a and 2b, the background with cross
mapping and enhanced brightness and shading clearly
show general ordered nano SiO2 texture existing in the
micro cluster in a small range. Figure 2c shows the [Ru]
catalystandnanoSiO2 interlacedimage,whichimplythat
the [Ru] catalyst is grafted on nano SiO2. The transition
site from the dense dark shadow to light gray is attracted
to the [Ru] catalyst. Figure 2d demonstrates that the
[Ru] catalyst perspective with enhanced dispersion was
showed clearly due to the changed surface nature of nano
SiO2 after the the SiO2-supported [Ru] catalyst to reduce
the interaction of nano SiO2 surface, demonstrating that
the [Ru] catalyst grafted on nano SiO2 was not evenly
dispersed. The mean particle size of nano SiO2 with
the [Ru] catalyst was approximately 15.2 nm based
on the statistical analysis of the transmission electron
micrographs of [Ru]/SiO2. Contrary to [30], nanosilica
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CHUAN-JUN YUE et al.
Fig. 3. Nitrogen adsorption-desorption isotherms of nano SiO2 and [Ru]/SiO2.
improved catalyst dispersion.
were consistently lower than the parent nano SiO2, the
N2 sorption isotherm pattern of the former was of typical
type IV pattern (Fig. 3), similar to that of nano SiO2.
The texture structure of nano SiO2 did not remarkably
accumulate after grafting of [Ru] catalyst, consistent
with the transmission electron micrograph. Thus, the
[Ru] catalyst was successfully grafted on the nano SiO2
surface.
N2 sorption study. Textural properties of nano SiO2
and [Ru]/SiO2 were detected by N2 sorption experiment.
Both catalysts have analog IV type adsorption
isotherms typical of mesoporous features, but derived
from the accumulation holes of nano SiO2, which is
not mesoporous (Fig. 3). Pore diameter distribution
obviously differed between nano SiO2 and [Ru]/SiO2.
The latter had a bigger pore diameter because the
enhanced volume of [Ru]-loaded SiO2 enlarged the
particle diameter. The textural of specific surface areas,
pore size, and pore volume of nano SiO2 and [Ru]/SiO2
catalysts are reported in Table 1. The corresponding BET
surface areas were 129.2 and 90.5 m2 g–1, Langmuir
surface areas were 195.8 and 132.1 m2 g–1, average pore
volumes were 0.25 and 0.23 cm3 g–1, and average pore
sizes were 16.4 and 16.1 nm. Although [Ru]/SiO2 values
FT-IR spectroscopy. Comparative results of the
IR spectra of nano SiO2 and [Ru]/SiO2 (Fig. 4), the
latter displays some new bands at 2927.4, 2856.1,
1993.1, 1492.1, 1839.8, and 668.2 cm−1. The bands at
1839.8 and 1993.1 cm−1 are assigned to the S=O and
S–C vibrations. The bands at 1492.1 cm−1 is attributed
to the C=C vibration of phenyl group. The bands at
2927.4 and 2856.1 cm−1 are assigned to the alkyl chain
C–H stretching vibration. The bands at 668.2 cm−1 are
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NANO SiO2-BONDED Ru-TsDPEN
1213
a
b
Fig. 4. The FTIR spectra of nano SiO2 and [Ru]/SiO2.
Fig. 5. The TG-DTG curves of [Ru]/SiO2.
Table 1. Texture parameters of nano SiO2 and [Ru]/SiO2
Specific surface area, m2 g–1
BET Langmuir
Average pore
volume, cm3 g–1
Average pore
diameter, nm
Sample
Nano SiO2
[Ru]/SiO2
129.2
90.5
195.8
132.1
0.25
0.23
16.4
16.1
Table 2. The tentative results of catalytic non-interfering substrates by [Ru]/SiO2 catalysts via hydrogen transfera
Reaction step
Catalyst
(S)-Cat
(R)-Cat
Substrate
PD/acetone
(R)-PD/acetol/propan-2-ol
Conversion, %
Selectivity, %
ee, %
75
(a)
(b)
88
95
47
98
>99
a Reaction conditions: (a) catalysts 0.5 mg, PD 7 mL, PD/acetone 0.25 (V/V), KOH 0.56 mg (in 10 mL water), T 40°C, t 20 h; (b) catalysts
0.5 mg, acetol 8 mL, acetol/isopropaneol 025(V/V), KOH 0.56 mg (in 10 mL water), T 40°C, t 15 h.
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CHUAN-JUN YUE et al.
Table 3. Effect of feeding sequence on the catalytic resolution of 1,2-propanediola
Reaction step
(a)
Feeding sequence
Conversion, %
Selectivity, %
ee, %
74
Substrate → Catalyst → KOH (aq)
Substrate → KOH(aq) → Catalyst
Catalyst → KOH(aq) → Substrate
Substrate → Catalyst → KOH (aq)
Substrate → KOH(aq) → Catalyst
Catalyst → KOH(aq) → Substrate
85
89
80
96
96
95
47
48
46
98
98
98
79
67
98
(b)
98
97
a Reaction conditions: (a) catalysts 0.5 mg, PD 7 mL, PD/acetone 0.25 (V/V), KOH 0.56 mg (in 10 mL water), T = 40°C, t = 20 h. (b) Catalysts
0.5 mg, acetol 8 mL, acetol/isopropaneol 025(V/V), KOH 0.56 mg (in 10 mL water), T = 40°C, t = 15 h.
Table 4. Effect of substrates ratio on the catalytic resolution of 1,2-propanediola
Reaction step
(a)
Substrates ratio, mol
Conversion, %
Selectivity, %
ee, %
Acetone/PD
0.5
47
59
42
46
46
47
41
92
92
95
98
97
30
41
1.0
2.0
4.0
6.0
0.5
1.0
2.0
4.0
6.0
82
70
98
96
>99
35
89
(b)
Propan-2-ol/
acetol
99
51
>99
>99
>99
>99
79
>99
>99
a Reaction conditions: (a) catalysts 0.5 mg, PD 7 mL, KOH 0.56 mg (in 10 mL water), T 40°C, t = 20 h .(b) Catalysts 0.5 mg, acetol 8 mL,
KOH 0.56 mg (in 10 mL water), T = 40°C, t = 15 h.
assigned to the breathing vibration of phenyl ring. Even
though the nano SiO2 partly covers the IR absorption
peak of [Ru] complex [25], results combining the
element analysis, presenting the 2 : 1 molar ratio of
nitrogen and Ru in the [Ru]/SiO2 further confirm a
successful grafting of [Ru] on nano SiO2.
the dehydration from silicon hydroxyl condensation,
resulting in approximately 1.3% of weight loss. Above
400°C, weight loss is mainly because of silicon hydroxyl
dehydration. The almost parallel DTG curve shows that
during temperature increase, SiO2 showed no obvious
aggregation, because the particle aggregation should
be accompanied with significant DTG progress caused
by the close relation of surface change and energy with
thermal gravity. The catalyst is thermostable in the
catalytic reaction temperature.
Thermoanalysis. Catalysts thermostability is
reflected further by TG-DTG profile. The [Ru]/SiO2
catalysts exhibited a total weight loss of approximately
2.28% from room temperature to 600°C (Fig. 5). From
room temperature to close to 300°C, the DTG profile
was almost parallel, which could be attributed to the
even release of physical adsorption water. Stronger
thermogravimetric change between 300 and 400°C
could be assigned to the oxidation decomposition
of Ru-TsDPEN molecule [31], probably containing
Feasibility of the successively catalytic hydrogen
transfer for 1,2-propanediol resolution. The substrate
with two hydroxyl or functional carbonyl can perform
hydrogen transfer reactions of kinetic resolution and AH
in Ru-DPEN catalysts [32, 33]. However, total yields
are inherently not more than 50% in catalytically kinetic
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NANO SiO2-BONDED Ru-TsDPEN
1215
resolution, and dynamic kinetic resolution has lower
catalytic efficacy because of the difficulty in catalyst co-
ordination in one pot. Therefore, the catalytic oxidation
kinetic resolution of 1,2-propanediol and asymmetric
hydrogenation of acetol from the previous reaction as
a by-product was conducted to improve the utilization
of raw materials and catalytic efficacy. The feasibility
of the two step reactions was verified using synthesized
(S)-Cat and (R)-Cat on independent 1,2-propanediol/ac-
etone and (R)-propanediol/acetol/propan-2-ol substrate
systems for catalytic hydrogen transfer reaction, respec-
tively. Table 2 shows that the feasibility of the catalytic
resolution of 1,2-propanediol by two step reactions, and
the corresponding catalytic conversions reached 88%
and 95%, and the corresponding selectivity for (R)-1,2-
propanediol and ee values were 47 and 98%, and 75 and
>99%, respectively. This may be understood from the
characteristics of the reversible reactions and the prod-
ucts composition (Scheme 2). GC-MS results showed
that the introduction of the catalytic by-products from
the first step including the oxidized product of primary
alcohols in 1,2-propanediol and its condensation with
acetone, resulted in decreased conversion to the desired
product except for the reaction equilibrium, and rela-
tively low ee value was presumed that it derived from
the indistinctive rate difference in hydrogen transfer
from (S)- and (R)-propanediol to acetone. Additionally,
(S)-1,2-propanediol was achieved when the order of cat-
alyst feeding was changed, that is, (R)-Cat was used in
the first reaction and (S)-Cat in the second. The industri-
al production of desired chiral products was convenient.
on the catalyst → KOH (aq) → substrate process (80%
conversion 46% selectivity and 67% ee) (Table 3) in the
catalytic oxidative kinetic resolution. This is explained
by more effectively generated Ruthenium hydride active
species in the way of substrate → KOH(aq) → catalyst
because of the active species being introduced on the
premise of the hydroxyl and KOH reaction to alcohol-
alkali [18]. Although the catalytic results in the second
sequence is not closely related to the feeding sequence,
with over 95% conversion and ee as reported in the
literature [36], propan-2-ol is an outstanding hydrogen
donor for the hydrogen transfer system.
Effect of substrates ratio on the catalytic resolution.
Substrates ratio affects the reversible reaction equilib-
rium involved in conversion and ee. The effect of sub-
strates molar ratio was examined. Table 4 shows that
the conversions increased with molar ratio of acetone
to 1,2-propanediol and propan-2-ol to acetol in the same
way of from 1 : 2 to 6 : 1 on the two catalytic reac-
tions, with maximum >99% and >99% conversions, re-
spectively. For ee, when acetone/1,2-propanediol ratio
exceeded 4 with 96% ee, stereoselectivity began to de-
crease to 89% at a ratio of 6 because the excessive ac-
etone promoted the 1,2-propanediol to transform other
products, including organic oxides such as 2-hydroxy
propionic aldehyde and condensation compounds such
as 2,2,4-trimethyl-1,3-dioxolane, based on GC-MS re-
sults. The ee value was relatively stable (above 99%) in
the catalytic AH. The change of catalytical selectivity to
(R)-1,2-propanediol is not obvious in two step reactions,
which indicates that the directional performance of the
catalysts is excellent.
Catalytic kinetic resolution of 1,2-propanediol.
The synthesized [Ru]/SiO2 catalysts were tested in the
successiveoxidativekineticresolutionof1,2-propanediol
and AH of acetol to evaluate its effectiveness as well as
to determine the effects of reaction parameters. Some
of the parameters, such as solvent and alkali, were
optimized [34, 35], which was significant to the effects
of the reaction on the feeding sequence, substrate ratio,
and temperature.
Effect of temperature on the catalytic resolution.
Temperature is another basic parameter that affects
the thermodynamic equilibrium of a reaction, activity,
and enantioselectivity of the [Ru]-catalyzed chiral
resolution. For the two reactions, the effect on both
reaction conversion and enantioselectivity was tested
from 10 to 50°C , as previously reported [34].
As is shown in Table 5, the increase in conversion
with increasing temperature is expected and with the
usual value, that is, >99% conversions both at 50°C in
the catalytic oxidative kinetic resolution and at 40°C
in the second reaction. And the selectivity of two step
reactions decreases with an temperature increase due
to the overmuch byproducts. By contrast, ee values
increased as temperature rose from 10 to 40°C in the
first reaction and from 10°C to 30°C in the second. As
Effect of feeding sequence on catalytic resolution.
Table 3 demonstrates the effect of feeding sequence
on the substrate, KOH, and catalyst on two catalytic
reactions. Substrate → KOH(aq) → catalyst sequence
values (89% conversion, 48% selectivity to (R)-1,2-
propanediol and 79% ee) in the catalytic oxidative
kinetic resolution were higher than those of substrate →
catalyst → KOH(aq) order (85% conversion, 47%
selectivity and 74% ee), and without the order based
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CHUAN-JUN YUE et al.
Table 5. Effect of temperature on the catalytic resolution of 1,2-propanediola
Reaction step
(a)
Temperature, °C
Conversion, %
Selectivity, %
e.e., %
51
10
20
30
40
50
10
20
30
40
50
69
71
49
49
49
47
44
99
99
98
98
95
53
89
78
96
96
>99
60
88
(b)
>99
>99
>99
>99
97
81
>99
>99
>99
a Reaction conditions: (a) catalysts 0.5 mg, PD 7 mL, PD/acetone 0.25 (V/V), KOH 0.56 mg (in 10 mL water), T = 40°C, t = 20 h. (b) Catalysts
0.5 mg, acetol 8 mL, acetol/isopropaneol 025 (V/V), KOH 0.56 mg (in 10 mL water), t = 15 h.
Table 6. Reusability of the nano SiO2-supported Ru-TsDPEN derived catalysts for successive oxidative kinetic resolution and
asymmetric hydrogenationa
Recycle runs
Conversion, %
1
2
3
4
5
(a)
(b)
96
96
96
96
91
95
88
93
85
>99
Selectivity, %
ee, %
(a)
(b)
(a)
47
98
99
47
97
99
47
97
99
47
96
98
46
94
98
(b)
>99
99
98
92
89
a Reaction conditions: (a) catalysts 0.5 mg, PD 7 mL, PD/acetone 0.25 (V/V), KOH 0.56 mg (in 10 mL water), T = 40°C, t = 20 h .(b) catalysts
0.5 mg, acetol 8 mL, acetol/isopropaneol 025 (V/V), KOH 0.56 mg (in 10 mL water), T = 40°C, t = 15 h.
in the second, and the selectivity decrease from 47 to
46%, and 98 to 94%, whereas corresponding ee valve
changed from 99 to 98%, and >99 to 89%, respectively.
Decrease in catalytic activity was mainly due to catalyst
loss caused by the handling process, derived from the
destroyed and irreparable active species of ruthenium
hydride. The 9.1 mol % Ru from ICP analysis leached
from the catalyst after five runs. The decrease in ee
value also implies that the catalyst decomposed partly
in consecutive reuses, which indicates an ineffective
coordination of some TsDPEN ligand and Ru.
temperature rose above 40 and 30°C in the two reactions,
ee value dropped, showing that catalyst conformation
may not be well recognized with reaction substrates in
the catalytic system at high temperatures [37], resulting
in decreasing enantioselectivity. The corresponding
maximum ee values of (R)-1,2-propanediol were 96%
and >99%, respectively.
Catalysts recyclability and stability. The re-
cyclability of [Ru]/SiO2 was tested under optimized
conditions. The catalysts were collected by filtration,
acetone was removed by distillation after the first
reaction, and propan-2-ol was added by syringe before
the second. Table 6 demonstrates the better recycling of
the catalysts. [Ru]/SiO2 can be consecutively reused for
at least five times with conversion ranging from >99%
to 93% in the first reaction and form >99% to 85%
Catalyst stability is closely related to the catalyst
nature and specific catalytic mechanism. Generally,
Ru-TsDPEN catalyst for hydrogen transfer between the
hydroxyl and carbonyl is of a bifunctional synergistic
catalytic process involving –NH2 ligand and metal
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NANO SiO2-BONDED Ru-TsDPEN
1217
5. Bálint, J., Egri, G., Kolbert, A., Dianóczky, C., Fogassy, E.,
Novák, L., and Poppe, L., Tetrahedron: Asymmetry, 1999,
vol. 10, pp. 4017–4028.
Ru, and energy calculation for the transition state
[40] illustrated that propan-2-ol as hydrogen donor is
favorable to the reaction. This finding is also supported
by various studies [39, 40]. Catalytic ATH of acetol
was smoothly conducted. Catalytically asymmetric
hydrogen transfer between 1,2-propanediol and acetone
was difficult, with relatively harsh reaction conditions,
because the reaction needs to be successively performed
after ruthenium hydride is generated and effective
electron energy state matching among substrates and
catalyst occurs. Ru-TsPDEN may exert the best life in
a successive flow environment considering its excellent
performance in hydrogen transfer reaction catalysis.
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CONCLUSIONS
11. Montes, V., Checa, M., Marinas, A., Boutonnet, M.,
Marinas, J.M., Urbano, F.J., Järas, S., and Pinel, C., Catal.
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A new strategy was developed for the resolution of
1,2-propanediol by successive oxidative kinetic reso-
lution and AH catalyzed by the opposite configuration
Ru-TsDPEN/nano-SiO2 via hydrogen transfer. The con-
figuration of the product was controlled by changing the
sequence of using two opposite configuration catalysts.
The heterogeneous catalysts showed excellent activities
and enatioselectivities for two hydrogen transfer with
considerable recovery and reuse by simple filtration.
The catalysts were reused consecutively for at least
five times without noticeable loss of conversion and ee
values. This study provides a potential and general ap-
proach for secondary alcohol resolution. The system has
great potential for industrial application if catalyst sta-
bility can be further enhanced.
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ACKNOWLEDGMENTS
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pp. 1300–1308.
We thank Nature Science Foundation of Jiangsu
Province (Grant No BK2011235) for financial support.
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