Hydrogenation of Ketones on Dispersed Chiral-Modified Palladium Nanoparticles

Article abstract of DOI:10.1134/S1070363218020044

Hydrogenation of acetophenone and esters of ketoacids with molecular hydrogen in the presence of the Pd(acac)₂-cinchonidine–H₂ catalytic system has been studied. The dependence of the molar ratio of (–)-cinchonidine/Pd on size and shape of palladium nanoparticles, formed in the system, also on reaction rate and enantioselectivity has been established. The nature of the regularities observed for the Pd(acac)₂-cinchonidine–H₂ catalytic system was discussed.

Full text of DOI:10.1134/S1070363218020044

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 2, pp. 199–207. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © L.O. Nindakova, V.O. Strakhov, S.S. Kolesnikov, 2018, published in Zhurnal Obshchei Khimii, 2018, Vol. 88, No. 2, pp. 219–227.
Hydrogenation of Ketones on Dispersed Chiral-Modified
Palladium Nanoparticles
L. O. Nindakova*, V. O. Strakhov, and S. S. Kolesnikov
Irkutsk National Research Technical University, ul. Lermontova 83, Irkutsk, 664074 Russia
*e-mail: nindakova@istu.edu
Received July 20, 2017
Abstract—Hydrogenation of acetophenone and esters of ketoacids with molecular hydrogen in the presence of
the Pd(acac)2–(–)-cinchonidine–H2 catalytic system has been studied. The dependence of the molar ratio of (–)-
cinchonidine/Pd on size and shape of palladium nanoparticles, formed in the system, also on reaction rate and
enantioselectivity has been established. The nature of the regularities observed for the Pd(acac)2–(–)-
cinchonidine–H2 catalytic system was discussed.
Keywords: palladium nanoparticles, chiral modification, cinchonidine, hydrogenation
DOI: 10.1134/S1070363218020044
Metal nanoparticles are used in catalysis due to
their high chemical reactivity and the ability to
catalyze novel reaction pathways that do not occur
using conventional catalysts [1]. Their use as catalysts
of various chemical processes including reducing of
the pollutants emission, raw materials processing,
synthesis of basic organic species, and energy
production is favorable in view of the possibility of
immobilization of metal particles on heterogeneous
carriers and their regeneration. One of the most
promising strategies of the “green” chemistry targeted
at the fine chemistry products preparation is the
combination of chiral technology and nanotechnology.
However, the range of enantioselective catalytic systems
involving metal nanoparticles has remained limited.
The most important issues related to this research are
methods of synthesis of metal nanoparticles [2–9],
their functionalization [10–19], special features of their
catalysis application [7, 14–21], and the mechanisms
of asymmetric reactions [22–31].
3S,8S,9R-(–)-Cinchonidine is a natural alkaloid
containing three chiral centers, important reactant and
modifier of catalytic systems based on transition
metals [18–23]. It has been found that in the case of Pt/
Al2O3 (–)-cinchonidine generates chiral regions upon
chemisorption on achiral metal surface, and discrimina-
tion of enantioface sides of the substrate occurs via the
1 : 1 interaction between the adsorbed substrate and
the modifier [18, 20–22]. Analysis of the available
reports has revealed that the reaction of enantio-
selective hydrogenation of prochiral С=О bond in
ketones and esters of methyl pyruvate in the presence
of immobilized platinum catalysts is well studied, but
palladium systems have been rarely used for this
purpose. The values of enantiomer excess (ее) for
reactions catalyzed by metal colloids are low, except
for Pt [26]. For example, poly(N-vinylpyrrolidone)-
stabilized Rhcoll has given ее 42% for ethyl lactate and
(similarly to Ptcoll) the increase in the rate with the
increased (–)-cinchonidine concentration [32]. In the
case of Pdcoll stabilized by polymeric surfactant KD1,
ее 29% has been obtained for ethyl lactate [33], and
for poly(N-vinylpyrrolidone)-stabilized Ircoll and
methyl lactate ее has been 34% [34].
One of the first mentions of metal colloids
protected by optically active stabilizers may induce
enantioselectivity in catalytic hydrogenation with
molecular hydrogen was given in [26]. The chiral
assemblies at the surface of metal nanoparticles can be
constructed via adsorption of optically active pure
organic materials (ligands). Since they can stabilize
and/or modify the metal nanoparticles, chiral ligands
are often referred to as stabilizers and modifiers.
This work aimed to study the enantioselective
hydrogenation of ketones on Pd nanoparticles applied
in the form of colloidal suspension prepared in the
Pd(acac)2–(–)-cinchonidine–Н2 system.
Enantioselective hydrogenation of acetophenone.
Colloidal systems tested as catalysts for hydrogenation
199

200
NINDAKOVA et al.
Scheme 1.
H
H
H
S
N
R
N
O
O
H
OH
H
O
O
O
O
O
2
1a
1b
1c
of acetophenone 1a, methyl pyruvate 1b, and ethyl
benzoylformate 1c under similar conditions were
formed via reduction of Pd2+ compounds with
molecular hydrogen in methanol solution in the
presence of chiral chinchona alkaloid (3S,8S,9R)-(–)-
cinchonidine 2 (Scheme 1).
was increased after several hours (sometimes after a
day), probably due to the formation of palladium
nanoparticles of optimal size (Fig. 1). The typical
experimental results are collected in Table 1.
The wmax, average particles size, and enantio-
selectivity of the reaction as functions of the (–)-2/Pd
molar ratio are given in Fig. 2. The highest process rate
(11 mmol L–1 h–1) was observed in the absence of any
modifier, and the increase in the concentration of (–)-
cinchonidine resulted in the deceleration of the
reaction, down to 1.1 mmol L–1 h–1 at (–)–2/Pd = 8 : 1.
Over the (–)–2/Pd range of 1.5–3 the reaction rate was
approximately constant, evidencing the saturation of
the system with the modifier at (–)-2/Pd above 1 : 1.
However, the enantioselectivity of the reaction was
nonlinear: at ratios range 0–1 the excess of the S-enan-
tiomer product grew to 22%, then went down to 4% at
ratio 1.5, steadily grew to 27%, and then slightly
decreased at the ratio of 8.
(–)-Cinchonidine molecule contains three basic
elements: quinoline aromatic moiety, quinuclidine ring
with tertiary nitrogen atom, and bridging carbinol
group connecting the first two fragments. A key
element responsible for the use of this compound as
chirality inductor is likely the nucleophilic quinuclidine
nitrogen atom in chiral surrounding created by
neighbor carbon atoms with R and S configuration.
Conditions of the experiment [nature of solvent and
the (–)-2/Pd ratio] were varied to elucidate their effect
on the process rate and stereoselectivity of
acetophenone hydrogenation. The only product was
formed, 1-phenylethanol.
In view of the presence of an ethylene substituent at
the quinuclidine fragment in the (–)-cinchonidine
molecule, we monitored the change in the modifier
concentration in the Pd(acac)2–(–)-2–Н2 system at the
(–)-2/Pd molar ratio 4 : 1. In was found that even
before the hydrogenation of acetophenone, during the
induction period, (–)-cinchonidine was completely
converted in (–)-dihydrocinchonidine (–)-3 (Scheme 2),
and evidently stabilization and modification of the
formed palladium nanoparticles under the action of
compound (–)-3.
Typical kinetic curves were S-shaped, with the
induction period evidencing the initial formation of the
active form of the catalyst. Moreover, the reaction rate
Scheme 2.
H
H
H
H
H
H
H
H
S
S
N
N
R
R
Pdn, H2
H
OH
H
OH
To elucidate the nature of the catalysis, the solid
phase was isolated from the catalytic system using a
filter with 200 nm pores size after the constant rate of
acetophenone hydrogenation was reached. That proce-
dure did not affect the reaction rate, hence the particles
larger than 200 nm were catalytically inactive.
H
H
N
N
(-)-2
(-)-3
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HYDROGENATION OF KETONES ON DISPERSED CHIRAL-MODIFIED PALLADIUM
(–)-2/Pd
Time, h
Fig. 2. Rate of acetophenone hydrogenation (1), excess of
(S)-1-phenylethanol (2), and average size of nanoparticles
(3) as functions of the (–)-2–Pd ratio in the Pd(аcаc)2–n(–)-
2–H2 system.
Fig. 1. Kinetic curves of acetophenone hydrogenation in
the presence of the Pd(аcаc)2–(–)-2–H2 system at the (–)-2 :
Pd ratio 0 (1), 0.5 (2), 4 (3), 2 (4), 8 (5).
The choice of the solvent also significantly affected
the rate and enantioselectivity of the reaction. The plot
of the rate and ее of the reaction as functions of εT did
not reveal distinct correlation between the solvent
polarity and ее (Fig. 3). Yet, dichloromethane was the
best solvent in terms of both high rate and high
enantioselectivity of the reaction. In the alcohols, the
enantioselectivity was low, probably due to high
solubility of the modifier resulting in the shift of the
(–)-2solution ↔ (–)-2surface to left, and the decrease in the
number of chiral-modified regions at the nanoparticles
surface. It is interesting to notice that the substrate
molecules could interact with alcohols via hydrogen
bonding. The effect of alcohols on the reaction
enantioselectivity (sometimes as prominent that the
selectivity is reversed) has been found for the reduc-
tion of perfluorinated ketones on Pt/Al2O3 modified
with (–)-2 [35].
During the reaction, colloidal nanoparticles could
aggregate and therefore lose the activity. Ammonium
salts are usual electrostatic stabilizers, whereas
polymers act as steric stabilizers [36]. In this study, we
used the additional stabilizers to prevent the nano-
particles aggregation: cetyltrimethylammonium chloride
(CTAС) and crosslinked poly-N-vinylpyrrolidone (CL-
PVP). Despite slight increase in the activity of the
system with CL-PVP, the ее for (S)-1-phenylethanol
remained almost constant (Table 2).
When using Pd(dba)2, common source of Pd(0), we
observed high excess of the S-enantiomer product of
the reaction (45.4%). The attempt to stabilize the
Table 1. Enantioselective hydrogenation of acetophenone in the presence of the Pd(аcаc)2–(–)-2–H2 systema
Activity of catalyst,
mol Sub/(mol Pd h)
Exp. no.
Modifier/Pd
Conversion, %
wmax, mmol L–1 h–1
ee (S), %
1
2
3
4
5
6
7
8
9
0
15
38
44
20
16
13
14
14
9
11±1
3.3±0.4
0
0.5
1
8.0±0.8
5.6±0.5
3.4±0.5
3.3±0.4
3.2±0.4
3.2±0.4
3.0±0.4
1.0±0.2
2.1±0.3
13±1.3
21±1.9
10±1.1
12±1.2
16±1.7
22±2.3
28±3.0
17±1.8
1.7±0.2
1.5
2
1.1±0.2
1.0±0.2
2.5
3
1.1±0.2
0.9±0.2
4
0.6±0.2
8
0.2±0.1
a сPd = 3.3 mmol/L, experiment duration 24–25 h, methanol as solvent.
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NINDAKOVA et al.
which resulted in the loss of the chiral product in the
case of enantioselective process (Table 2, Exp. 6 and 7).
In the presence of the Pd(acac)2–(–)-2–Н2 system,
esters of ketoacids (methyl pyruvate and ethyl benzoyl-
formate) were hydrogenated as well (Table 3). The rate
of methyl pyruvate hydrogenation was slightly
different from that of the reaction with acetophenone,
but ее 54–56% for methyl ester of (R)-(–)-lactic acid at
(–)-2/Pd ratio 1.5–2. Moreover, monitoring of ее
during the reaction showed that it was decreasing from
initial 85.6% to 56.0% at the end of the reaction.
Enantioselectivity of the reaction of ethyl ester of
(S)-(+)-mandelic acid formation from ethyl benzoyl-
formate at the (–)-2/Pd ratio 2 : 1 was as low as
10%, the reaction rate being sufficiently high,
152.0 mmol L–1 h–1. That was in good agreement with
the data on high rate of hydrogenation of ethyl benzoyl-
formate on Pd/Al2O3–(–)-2 [21].
ε
Fig. 3. Rate of hydrogenation and enantioselectivity of the
reaction in the presence of the Pd(acac)2–(–)-2–Н2 system
as functions of dielectric constant of the solvent ε.
surface by addition of the polymer carrier CL-PVP
resulted in the increase in wmax at complete conversion
of the substrate, but enantioselectivity was significantly
reduced (Table 2, Exp. 4 and 5). Likely, the palladium
nanoparticles modified with (–)-cinchonidine were less
involved in acetophenone hydrogenation due to the
restrictions by the polymer network. Complete lifetime
of the Pd(dba)2–(–)-2–CL-PVP catalyst was 5 days at
turnover number 400 mol/mol Pd (Table 2, Exp. 5).
Study of the catalyst nature by physical methods.
As often discussed in the literature [1, 37], the size of
nanoparticles is a key factor determining the catalytic
activity of palladium colloids. Palladium nanoparticles
synthesized in the Pd(acac)2–(–)-2–H2 system at the
(–)-2/Pd ratio of 0 to 8, were different in size (the high-
resolution TEM data); the difference was sometimes
huge. It should be noted that the introduction of
cinchonidine improved the dispersity of the formed
palladium nanoparticles: average nanoparticles size in
the system where palladium black was formed was of
10 nm, being of 4.8 nm in the Pd(acac)2–(–)-2–H2
system.
The experiments with palladium on solid carrier
Pd/C (Fig. 4) revealed that the rate of ketone hydro-
genation was decreased by 10 times in the presence of
(–)-cinchonidine. Low selectivity of the reaction with
respect to the products typical of Pd/C (Fig. 4a) was
retained 1-phenylethanol was reduced to ethylbenzene
Table 2. Reaction rate and the catalyst activity depending on the nature of palladium precursor and stabilizera
Activity of catalyst,
mol Sub/(mol Pd h)
Exp. no.
System
Pd(acac)2–(–)-2
Conversion, % wmax, mmol L–1 h–1
ee (S), %
1
2
3
4
5
44.2
72.4
27.4
38.5
5.6±0.5
6.3
1.7±0.2
1.9
21.0±2.0
23.2±2.0
24.3±2.0
45.4±3.4
18.2±2.0
Pd(acac)2–CTAC–(–)-2
Pd(acac)2–(–)-2–CL PVP
Pd(dba)2–(–)-2
17.6
5.1
13.8
4.1
Pd(dba)2–(–)-2–CL PVP
100
42±5.1
12.6±1.5
(portion 1)
(portion 2)
(portion 3)
Pd/C
100
100
100
100
88.5±8.6
42.1±8.6
300±15
35.4
26.6±2.5
12.7±2.0
91±5
17.8±2.0
18.2±2.0
0
6
7
Pd/C–(–)-2
10.6
25.6±2.0
a сPd = 3.3 mmol/L, experiment duration 24–25 h, toluene–methanol, 3 : 20.
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HYDROGENATION OF KETONES ON DISPERSED CHIRAL-MODIFIED PALLADIUM
(a) (b)
с, mol/L
с, mol/L
1
Time, h
Time, h
Fig. 4. Kinetic curves of acetophenone hydrogenation on Pd/C in the absence (a) and in the presence of the modifier (b) at
(–)-2 : Pd = 1: (1) acetophenone, (2) 1-phenylethanol, and (3) ethylbenzene.
The electron microscopy images of the colloidal
dispersions contained the high-contrast nanoparticles,
predominantly spherical. The atoms lattice observed in
high-resolution TEM images evidenced high
crystallinity of the formed nanoparticles. The measured
interlayer spacings (2.2417, 1.9388, 1.3723, and
1.1749 Å) corresponded to crystallographic planes of
palladium: 111 (2.2458 Å), 200 (1.9451 Å), 220
(1.3754 Å), and 311 (1.173 Å) (PDF-2, no. 00-046-
1043). According to the electron diffraction data, the
particles were polycrystalline and arranged in different
crystallographic directions.
time, the C/Pd ratio could not be determined, because
carbon was deposited onto copper grid for TEM. The
elements ratio was determined by means of TGA.
Thermograms of Pd sample isolated from the
Pd(аcаc)2–(–)-2–H2 system (Fig. 6) contained the regions
corresponding to evaporation of residual solvent (87–
126°С) as well as melting (165–245°С) and decom-
position of (250–700°С) (–)-cinchonidine. The final
mass loss of the sample was 62.8%, and the sample
composition was estimated as Cin0.6Pd. According to
the reference data [38], heating at above 300–350°С
resulted in the formation of thin PdO film at the
palladium surface, which was decomposed into the
metal and oxygen above 850°С. Probably, the slight
mass loss above 1010°C was due to decomposition of
palladium oxide. Formation of palladium nitrides and
carbides is possible only at high temperature and
pressure [39], moreover, no additional reflections were
observed in the X-ray diffraction patterns. The X-ray
Histograms of distribution of nanoparticles in the
Pd(acac)2–(–)–2–H2 system for the (–)-2/Pd ratios 0.5,
1, and 8 obtained by statistical analysis of 200–250
nanoparticles are shown in Fig. 5. Energy-dispersive
X-ray microanalysis of various parts of the sample at
n = 1 evidenced the sample heterogeneity and the
presence of palladium and carbon in it. At the same
Table 3. Hydrogenation of methyl pyruvate and ethylbenzoyl formate in the presence of the Pd(acac)2–(–)-2–Н2 systema
Activity of catalyst,
mol Sub/(mol Pd h)
Exp. no. (–)-2 : Pd
Substrate
Conversion, % w, mmol L–1 h–1
ee, %
1
2
3
4
1.0
1.0
1.5
2.0
Methyl pyruvate
Ethyl benzoylformate
Methyl pyruvate
Methyl pyruvate
30
71
12
13
8.5±1.1
152.0±13.8
1.6±0.2
2.6±0.3
23.4±2.6 (R)
2.2±0.3 (S)
54.3±6.6 (R)
45.2±5.1
0.5±0.1
1.2±0.1
0.4±0.1
56.0±6.8 (R)
85.6 (R)
5
2.0
Ethyl benzoylformate
58
133.7±12.5
40±1.1
10.2±1.6 (S)
a сPd = 3.3 mmol/L, experiment duration 24–25 h, toluene–methanol, 3 : 20.
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NINDAKOVA et al.
TG, %
(a)
DTG, %/min
DSC, µV/mg
Temperature, °С
Particles size, nm
(b)
Fig. 6. Thermogram of the sample isolated from the
Pd(acac)2–(–)-2–H2 system.
according to PDF-2, no. 00-046-1043). The coherent
scattering area size calculated from the half-width of
the (111) line using the Scherrer equation gave 4.8 nm,
in line with the average size determined from TEM
data.
Analysis of histograms of size distribution of the
nanoparticles in the Pd(acac)2–n(–)-2–H2 systems
revealed that the increase in the (–)-2/Pd ratio from 0.5
to 1 led to the decrease in the average particles size
from 6.7±1.6 to 4.7±0.6 nm (Fig. 5). At the same time,
the changes in the particles size in the Pd(acac)2–n(–)-
2–H2 system and the rate of acetophenone hydrogena-
tion were coinciding (Fig. 2) over the (–)-2/Pd molar
ratio 0–1. Up to (–)-2/Pd = 1, cinchonidine served as
efficient stabilizer preventing the nanoparticles growth,
and at the (–)-2/Pd ratio from 2 to 3.5 the reaction rate
grew with the increase in the average size of Pd
nanoparticles.
Particles size, nm
(c)
TEM images analysis revealed that anisotropic
particles (10–18%) were formed over the mentioned
ratio range, whereas at (–)-2/Pd below 2 and above 3.5
exclusively spherical particles were formed. The
appearance of non-spherical particles led to the
increase in the specific area of the active nanoparticles
surface, explaining the reaction acceleration over that
range of molar ratio.
Particles size, nm
Fig. 5. Histograms of the nanoparticles distribution in the
Pd(acac)2–n(–)-2–H2 system at n = 0.5 (a, weighted-mean
size 6.7 nm, σ 1.6 nm), 1 (b, weighted-mean size 4.7 nm, σ
0.6 nm), 8 (c, weighted-mean size 8.3 nm, σ 2.0 nm).
Deviation of the nanoparticles shape from spherical
could be due to the change in the type of chemi-
sorption of (–)-cinchonidine at the metal surface.
Enantioselectivity is governed by the molecular
orientation of (–)-cinchonidine at chiral catalytic inter-
face nanoparticle–liquid. Adsorbed (–)-cinchonidine
can take three types of orientation with respect to
diffraction pattern of solid dispersed phase isolated
from the Pd(аcаc)2–(–)-2–H2 system contained
broadened diffraction maximums at 2θ 35°–80°
assigned to the reflections from crystallographic planes
(111), (200), and (220) of face-centered cubic Pd
lattice Fm3m (40.1°, 46.6°, and 68.1°, respectively,
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HYDROGENATION OF KETONES ON DISPERSED CHIRAL-MODIFIED PALLADIUM
Pt surface: planar, with the plane of quinoline ring
forming strong π-bonding with surface metal atom
(F1), and two lateral ones [40, 41]. In one of the lateral
orientations the molecule forms the α-С–Pt bond with
the metal (the aromatic hydrogen atom being eliminated)
and a donor bond via the electron pair of nitrogen (T2),
the other lateral orientation is characterized by only
weak bonding via nitrogen (T3).
energy-dispersive analyzer (EDX) to perform
elemental analysis; resolution of 0.19 nm was reached.
A drop of a solution of the in situ formed catalyst (сPd
1–5 mol/L) was applied on a carbon-coated copper
grid and dried in argon. The images parameters and
particles morphology and microstructure were
determined using iTEM 5.0 and Digital Micrograph
2.30 software. To analyze the periodic structures and
filter the images, FFT and iFFT Fourier methods were
used. Nanoparticles size distribution was determined
by measuring the size of 200–250 Pd nanoparticles in
several TEM images taken from different parts of the
grid.
To get the information on the coating of the surface
at the (–)-2/Pd ratio = 1, critical in view of enantio-
selective properties of the system, we recorded the IR
spectra of crystalline (–)-cinchonidine, its solution in
CHCl3, and the Pd(acac)2–n(–)-2–H2 system in CHCl3.
Analysis of the spectra revealed that the F1 (1175,
1383, 1439, and 1560 cm–1) and Т2 (1212, 1383, 1439,
and 1560 cm–1) orientation types prevail at palladium
surface at (–)-2/Pd =1. Probably, the increase in the
(–)-2/Pd ratio led to the decrease in the fraction of
strongly bound adsorbed (–)-cinchonidineа, and the
weakly bound modifier molecules were readily replaced
by the substrate molecules, thus enhancing the contribu-
tion of the non-modified surface in the catalysis. At
even higher concentration of the modifier, the weakly
bound forms could be substituted with the π-
coordinated one, thus resulting in the growth in the
enantioselectivity.
X-ray diffraction analysis was performed using a
Shimadzu XRD 7000 diffractometer with copper
anode, λ(Kα) 0.15418 nm. The focusing was performed
using a Bragg–Brentano scheme with a monochro-
mator at the diffracted beam (angles range 3.000°–
80.000°, scan step 0.05°). The phases were identified
using Match! 1.1 software and PDF-2 database.
Average size of palladium crystallites was determined
from half-width of the (111) in 2θ scale using the
Scherrer equation.
IR spectra were recorded using a 3100 FT IR spec-
trometer (Varian) over 1100–5000 cm–1 (solution in
chloroform, CaF2 cell, layer thickness 0.1 mm). Thermal
analysis (TG, DTG, and DSC) was performed using a
NETZSCH STA 449F3 instrument (25–1000°C, heating
rate 10 deg/min, argon stream 20 mL/min). The
samples for XRD and TGA were isolated from the
catalytic systems, washed with isopropanol, dried at
reduced pressure at 50°С, and stored under argon in
sealed ampoules.
In summary, the rate and enantioselectivity of
hydrogenation of acetophenone and esters of pyruvic
and benzoylformic acids on palladium nanoparticles
modified with natural (–)-cinchonidine are complex
functions of the (–)-2/Pd ratio, nature of the solvent,
the palladium precursor, and substrate, as well as the
presence of a carrier. To optimize the system, soft
carriers (polymers) should be used to increase the
catalyst lifetime in combination with nanoparticles
with narrow size distribution.
Specific rotation of the pure compounds was
determined using an ADP410 automated digital
polarimeter at wavelength 589 nm (optical pathlength
50 mm, concentrations of the solutions 2–30 g/100 mL
of solvent). Analysis of the hydrogenation products
was performed using a Shimadzu GCMS-QP2010 Plus
chromato–mass spectrometer (EI 70 eV, scan over m/z
40–350, capillary column Equity 5 (30 m × 0.25 mm,
EXPERIMENTAL
Acetophenone (Acros Organics, 99%), methyl
pyruvate, methyl benzoylformate (Aldrich, 95%), and
the solvents (methanol, propanol-2, toluene, dichloro-
methane, and hexane) were thoroughly dried, distilled,
and stored under argon. Pd(acac)2 was synthesized as
described elsewhere [42]; from Pd(dba)2 (Acros
Organics) used as received.
95% dimethylpolysiloxane
+
5% diphenylpoly-
siloxane, helium as carrier gas). Enantiomer excess
(ее, %) was determined using a GC Agient 7890A gas
chromatograph equipped with a Dina switch, flame
ionization detector, a CYCLODEX-B chiral capillary
column (30 m × 0.25 mm), and a 5% Phenyl Methyl
Siloxane capillary column (30 m × 0.25 mm).
Temperature program: heating from 70 to 160°С at
High-resolution transmission electron microscopy
(TEM) images were obtained using a FEI Tecnai G2
device (accelerating voltage 200 kV) equipped with an
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NINDAKOVA et al.
12. Wu, B. and Zheng, N., Nano Today, 2013, vol. 8, no. 2,
3 deg/min, gas flow rate 1.5 mL/min, thermostat tem-
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Hydrogenation procedure. Hydrogenation reaction
was performed in a Picoclave GlassUster cyclone 075
BUCHI autoclave. A solution of the precursor and the
modifier: 0.0304 g (1×10–4 mol) of palladium
acetylacetonate, ~10–4 mol of the modifier, 3 mL of
toluene, and 19 mL of methanol was transferred to a
100 mL vessel being bubbled with hydrogen. The pale-
yellow solution was stirred under hydrogen pressure of
5 atm during 30 min, then 0.5 mL of the substrate in
8 mL of methanol was added, and the “zero sample”
was withdrawn. The mixture of hydrogenation
products was then analyzed each 30 or 60 min using
the chromato–mass spectrometer. Configuration of the
prevailing enantiomer was determined by comparison
with the reference data [43].
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