Palladium nanoparticle-graphene catalysts for asymmetric hydrogenation

Article abstract of DOI:10.1007/s10562-013-1006-6

We report for the first time the application of palladium nanoparticle-graphene (Pd/Gn) catalysts in the asymmetric hydrogenation of aliphatic α,β-unsaturated carboxylic acids using cinchonidine as chiral modifier. Pd/Gns were prepared by deposition-preci

Full text of DOI:10.1007/s10562-013-1006-6

Catal Lett
DOI 10.1007/s10562-013-1006-6
Palladium Nanoparticle–Graphene Catalysts for Asymmetric
Hydrogenation
Korn e´ l Sz }o ri
•
Robert Pusk a´ s
•
Gy o¨ rgy Sz }o ll }o si
•
Imre Bert o´ ti
•
J a´ nos Sz e´ pv o¨ lgyi
•
Mih a´ ly Bart o´ k
Received: 31 December 2012 / Accepted: 6 April 2013
Ó Springer Science+Business Media New York 2013
Abstract We report for the first time the application of
palladium nanoparticle-graphene (Pd/Gn) catalysts in the
asymmetric hydrogenation of aliphatic a,b-unsaturated
carboxylic acids using cinchonidine as chiral modifier. Pd/
Gns were prepared by deposition–precipitation from the
aqueous phase over graphite oxide and subsequent simul-
taneous reduction of both the support and the metal pre-
cursor with NaBH4. The materials obtained were
characterized by ICP optical emission spectroscopy, X-ray
diffraction spectroscopy, Raman spectroscopy, transmis-
sion electron microscopy and X-ray photoelectron spec-
troscopy. We demonstrate that the Pd/Gns modified by
cinchonidine can act as efficient catalysts in the asymmetric
hydrogenation of a,b-unsaturated carboxylic acids for
producing optically enriched saturated carboxylic acids.
Keywords Carboxylic acid Á Cinchona alkaloid Á
Enantioselective Á Graphene Á Hydrogenation Á Palladium
1 Introduction
The application of heterogeneous catalytic methods in
organic chemistry has become one of the most intensively
researched areas [1–4]. Among these methods, procedures
with the longest history are hydrogenations [5–7], which are
mainly catalyzed by supported metals. In the course of the
decades various supported metal catalysts have been
developed with the purpose of achieving high activity and
selectivity. Since supports have to meet a number of special
requirements, in addition to generally used supports (acti-
vated carbon, SiO , Al O and later zeolites and mesopor-
K. Sz }o ri Á G. Sz }o ll }o si (&) Á M. Bart o´ k (&)
MTA-SZTE Stereochemistry Research Group, Hungarian
Academy of Sciences, D o´ m t e´ r 8, Szeged 6720, Hungary
e-mail: szollosi@chem.u-szeged.hu
2
2 3
M. Bart o´ k
e-mail: bartok@chem.u-szeged.hu
ous materials) the development of other types of metal
catalysts has also gained increased importance. The Uni-
versity of Szeged has also joined this line of research, with
work on the use of amorphous metal alloys [8–10], layered
materials such as clays [11–14] and graphimets [15–19].
In spite of the novel observations made, these studies
did not lead to the expected results, especially in the case of
graphimets. The reason for this was that the interlamellar
space of graphimets with two dimensional (2-D) plane
structure is not penetrated even by low-molecular-weight
organic compounds. The discovery of graphene (Gn) [20],
however, opened the way to the synthesis and study of
catalysts consisting of metal nanoparticles supported by
graphene in the hydrogenation of organic compounds as
well [21–24]. Our research initiated in this field may be
considered as the continuation of our earlier studies.
R. Pusk a´ s
Department of Applied and Environmental Chemistry,
University of Szeged, Rerrich B e´ la t e´ r 1, Szeged 6720, Hungary
I. Bert o´ ti Á J. Sz e´ pv o¨ lgyi
Institute of Materials and Environmental Chemistry, Research
Centre of Natural Sciences, Hungarian Academy of Sciences,
Pusztaszeri u´ t 59-67, Budapest 1025, Hungary
J. Sz e´ pv o¨ lgyi
Research Institute of Chemical and Process Engineering, Faculty
of Information Technology, University of Pannonia, Egyetem
utca 2, Veszpr e´ m 8200, Hungary
M. Bart o´ k
Department of Organic Chemistry, University of Szeged,
D o´ m t e´ r 8, Szeged 6720, Hungary
123

K. Sz }o ri et al.
Special attention is given to the heterogeneous enan-
tioselective hydrogenation of prochiral organics resulting
in optically active chiral building blocks used in the syn-
theses of pharmaceuticals, fragrances and agrochemicals
Reduction was initiated by the addition of 724.4 mg of
3
NaBH freshly dissolved in 20 cm of distilled water to the
4
slurry cooled in ice bath and stirred magnetically. After
5 min, cooling was stopped and the mixture was stirred at
room temperature for another 4 h. Next the solid material
was centrifuged (Sigma 2-16 P; 5 min/10,000 rpm),
[
25–28]. To our knowledge, the application of metal-
graphene nanocomposites in the asymmetric hydrogenation
of prochiral compounds has not yet been attempted. The
objective of the present work was the synthesis and char-
acterization of Pd nanoparticle/graphenes (Pd/Gns) and
their study in asymmetric hydrogenation reactions in the
presence of cinchonidine modifier. Based on our earlier
experience, we chose prochiral aliphatic a,b-unsaturated
carboxylic acids as substrates for studying enantioselective
hydrogenations [29–32] (Fig. 1), because it had been ear-
lier confirmed that, these acids can be hydrogenated at
moderate enantioselectivities over Pd catalysts to produce
saturated acid chiral building blocks.
3
washed with water and methanol three times (20 cm each)
and dried at 323 K for 12 h in a vacuum desiccator.
Pd/Gn2: 1 g of GrO and 2.54 g of Na CO were mixed
2
3
3
in 250 cm distilled water, and the dispersion was soni-
3
cated for 1 h. 88 mg of PdCl and 1.2 cm of conc HCl
2
3
dissolved in 21 cm of distilled water were next added to
the mixture chilled on ice and continuously stirred, fol-
lowed by stirring at room temperature for 20 h. Reduction
by NaBH and isolation of Pd/Gn2 were carried out as
4
described for Pd/Gn1.
Pd/Gn3: 1 g of GrO was exfoliated into GO by soni-
cation in 250 ml of water for 2 h. Graphene (Gn) was next
obtained by chemical reduction using NaBH and isolation
4
2
Experimental
by centrifugation as described above for Pd/Gn1. Pd/Gn3
was then prepared from Gn using the method and the
conditions described above for Pd/Gn2 (i.e. addition of
Na CO aq, PdCl aq ? conc HCl and finally NaBH aq).
2
.1 Materials
2
3
2
4
Graphite powder (Aldrich 496588:\150 lm, 99.99 ? %),
PdCl (Aldrich, C99.9 %), cinchonidine (CD, Alfa Aesar),
2.3 Characterization of the Palladium Nanoparticle/
Graphene Catalysts
2
the unsaturated carboxylic acids: (E)-2-methyl-2-hexenoic
acid (1a, Alfa Aesar, 98 %), (E)-2-methyl-2-butenoic acid
(
2a, Aldrich, 98 %), itaconic acid (3a, Aldrich, C99 %),
benzylamine (BA, Fluka, C99.5 %), high purity solvents
Inductively coupled plasma optical emission spectroscopy
(ICP-OES) analysis were performed on a AMETEK pro-
duced Spectro Genesis EOP II type spectrometer after
leaching the soluble constituents of the catalyst samples in
warmed HCl ? conc HNO3 mixture and diluting the
solutions. The transmission electron microscopy (TEM)
investigations were carried out using Philips CM10 (100 kV)
transmission electron microscope. The sample preparation
involved mechanical grinding of the material and gluing the
powder on a Cu TEM-grid. X-ray powder diffraction (XRD)
analysis was effectuated on a Rigaku Miniflex-II diffrac-
tometer utilizing Cu Ka radiation. Raman spectra were the
and reagents (Aldrich and Scharlau Chemie) were used as
received. Sodium borohydride and H gas (Linde AG) of
2
9
9.999 % were used for catalyst reductions and in hydro-
genations. Graphite oxide (GrO) was prepared by slight
modification of the Hummers method [33].
2
.2 Preparation of Palladium Nanoparticle–Graphene
Hybrid Catalysts (Pd/Gn1, Pd/Gn2, Pd/Gn3) [34]
Pd/Gn1: 1 g of GrO was exfoliated into graphene oxide
3
GO) by sonication (Branson 1510 MT) in 250 cm of
-1
(
average of 256 scans measured at 4 cm resolution on a
water for 2 h. To prepare a catalyst containing around 5 %
3
Pd a solution of 88 mg of PdCl in 50 cm of distilled
Thermo Scientific DXR Raman microscope operating with
780 nm laser excitation of 5 mW power. X-ray photoelectron
spectra (XPS) were recorded on a Kratos XSAM 800 spec-
trometer operated at fixed analyser transmission mode using
Mg Ka1,2 (1,253.6 eV) excitation. Wide scan spectra were
recorded for all samples in the 100–1,300 eV kinetic energy
range. Spectra were referenced to the energy of the C1s line of
the polymeric or hydrocarbon type adventitious carbon,
present at the surface of the samples, set at 284.8 ± 0.1 eV
binding energy (BE) [35]. Chemical states of the constituent
elements were determined with ± 0.2 eV accuracy, and
assigned by using available references. Quantitative analysis,
based on peak area intensities (after removal of the Shirley-
2
3
water acidified with 0.072 cm of conc HCl was added to
the GO dispersion, followed by sonication for 30 min.
COOH
COOH
R
HOOC
Me
CH2
R = Pr ( 1a );
R = Me ( 2a )
(3a )
Fig. 1 Selected unsaturated carboxylic acids
1
23

Palladium Nanoparticle–Graphene Catalysts
type background), was performed by the Vision 2000 pro-
gram using experimentally determined photo-ionisation
cross-section data.
procedures were used for immobilization of Pd nanoparti-
cles over the support [36, 37].
3.1 Characterization of the Palladium Nanoparticle/
2
.4 Catalytic Hydrogenation and Product Analysis
Graphene Catalysts
Hydrogenations were carried out in stainless steel auto-
clave equipped with glass liner. Under typical condi-
Figure 2 shows the XRD patterns of GrO, Pd/Gn1, Pd/Gn2
and Pd/Gn3. Since the reflections characteristic of graphite
(graphite(002) reflection at 2H & 26°) did not appear in
the XRD pattern of the GrO sample, oxidation can be
considered successful. The reflection characteristic of GrO
appeared at 2H = 10.3°. Following exfoliation by ultra-
sound and reduction this peak disappeared, indicating that
the GO structure was transformed. Samples exhibited the
peak characteristic of graphene at 2H & 25°, i.e. a wide
and low intensity peak with poorly defined maximum. The
Pd(111) reflection (2H & 40°) is moderately intensive in
the case of Pd/Gn1 and very weak in the pattern of the
catalysts prepared by precipitation using Na CO , which
3
tions 15 mg catalyst, 10 cm toluene (or methanol in the
hydrogenations of 3a) were loaded into reactor, the
autoclave was flushed with H , filled to 5 MPa. After
2
3
0 min pre-hydrogenation the given amount of modifier,
substrate (1a or 2a or 3a) and BA additive were loaded
into reactor, the autoclave was flushed with H , filled to
2
the given H pressure and the reaction was commenced by
2
stirring the slurry using magnetic agitation (1000 rpm) at
room temperature (297 K). After 90 min the H2 was
released, the slurry was filtered, the solution was treated
with 10 % HCl aq. solution (except 3a), dried over
Na SO and analyzed. Products obtained in hydrogena-
2
3
may be due to different size and morphology of the metal
particles and/or the oxidation state of the metal surface in
the latter materials.
2
4
tions of 3a were transformed in dimethyl esters as previ-
ously described [30].
Products were identified by GC–MS analysis using Ag-
ilent Techn. 6890 N GC–5973 MSD equipped with 60 m
HP-1MS capillary column. Conversions and enantioselec-
tivities were calculated from gas chromatographic analysis
using the formulae: Conv (%) = 100 9 ([S] ? [R]) /
Figure 3 shows the Raman spectra of GrO and the hybrid
materials. In the Raman spectrum of GrO two intense bands
-
are observed, one at 1,600 cm (G band) corresponding to
1
the in-phase lattice vibration and the disorder induced band
-
1
at 1,325 cm (D band) [24, 38, 39]. The G’ band corre-
sponding to two inelastic scattering events involving two-
[
[
(
Acid] ; ee (%) = 100 9 ([S] - [R] |/ ([S] ? [R]) where
0
-
phonons (2,600–2,800 cm ) was not detected, similarly
1
Acid] is the initial concentration of the unsaturated acid
0
1a, 2a or 3a); [S] and [R] are the concentrations of the
product enantiomers determined by gas-chromatographic
analysis using Agilent Techn. 6890 N GC–FID instrument
equipped with HP-Chiral (30 m 9 0.25 mm, J&W Scien-
tific Inc. for 1a and 2a) or Cyclosil-B (30 m 9 0.25 mm;
J&W Scientific Inc. for 3a) chiral capillary columns. The
analysis conditions were: head pressure 135 kPa He; col-
umn temperature: 358 K, retention times (min): (S)-1b
13.0, (R)-1b 13.9, 1a 21.2; head pressure 135 kPa He;
388 K, retention times (min): (S)-2b 20.2, (R)-2b 21.8, 2a
26.8; and head pressure 135 kPa He; 388 K, retention times
of the corresponding dimethyl esters (min): (S)-3b 26.8,
R)-3b 27.7, 3a 41.6. The absolute configuration of enan-
(
tiomers were determined by GC analysis using commer-
cially available optically pure products. Repeating some
experiments three times resulted in product compositions
reproducible within ± 1 %.
3
Results and Discussions
The syntheses of Pd nanoparticle/graphene catalysts were
based on published experimental observations, besides
those on the preparation of graphenes [33, 34], known
Fig. 2 XRD pattern of GrO and Pd/Gn materials
123

K. Sz }o ri et al.
samples were higher as the calculated, due to the weight
loss of the support during reduction of GrO to Gn. From the
comparison of the ICP bulk compositions with values of
the surface compositions it is obvious, that the loaded Pd is
concentrated on the surface of the multi-layered graphene,
together with the Na contamination, left behind after the
preparation procedure.
The chemical state of the constituents was determined
from the XPS spectra by evaluating the recorded line-
shapes of the O 1s, C 1s and Pd 3d spectral ranges. Rep-
resentative spectra are depicted in Fig. 4 for the Pd/Gn1
sample. The relatively broad peak envelopes were by fitted
by three or four Gauss-Lorenz (70:30) type components
(
(
1.6–1.8 eV half-widths) of which position were identical
within ± 0.2 eV) in all three materials and were assigned
based on literature as shown in Table 2 [23, 35, 41, 42].
The Pd 3d doublet peak envelopes were fitted by three pairs
of lines separated by 5.25 eV, which were identified as
corresponding to the elemental state of Pd; to oxidized Pd,
e.g. to a thin oxide layer on the particles; and Pd bonded to
an electronegative anion assigned as carboxylic groups.
From the peak-synthesis procedure performed for all three
samples we evaluated the ratio of the reduced Pd as 53 (Pd/
Gn1), 43 (Pd/Gn2) and 49 (Pd/Gn3) atomic%, respectively.
The morphology of the Pd nanoparticle-graphene mate-
rials was examined by TEM. Characteristic TEM images
with the metal particle size distribution of the hybrid
materials as inserts are shown in Fig. 5. Carbon nanosheets
are distinguishable in the TEM image of GrO (Fig. 5a), and
the nanosheets were also maintained following reduction
and Pd immobilization as indicated by the images of Pd/Gns
Fig. 3 Raman spectra of GrO and selected Pd/Gns
with the Raman spectra of functionalized graphite [40] or
Pd-graphene hybrids prepared previously [39]. The ratio of
the intensities of the D and G bands in GrO (I / I ) was
D
G
1
.33, which increased following reduction and immobili-
zation of Pd nanoparticles to 2.00 (Pd/Gn1), 1.94 (Pd/Gn2)
and 1.74 (Pd/Gn3), respectively. This increase may be
attributed both to decreases in the size of the graphene
sheets and to the chemical interactions of the nanoparticles
and the graphene. Furthermore, the slight shift of the G band
-
1
maxima to lower wavenumbers, i.e. 1,587 cm (Pd/Gn1),
-
1
-1
,584 cm (Pd/Gn2) and 1,590 cm (Pd/Gn3), respec-
1
tively, when compared to GrO, confirmed the deterioration
2
of the sp carbon ribbons by reduction and metal particle
(Fig. 5b–d). The materials exhibiting the most uniform Pd
particle distributions over the support are the egg-shell type
Pd/Gn2 and Pd/Gn3 catalysts. The Pd particle size distri-
butions are fairly uniform for all three catalysts; the majority
of the particles were found to be of 2–5 nm. However, the
materials prepared using precipitation by Na CO exhibited
immobilization [39]. Indeed, calculating the in-plane crys-
-
10
tallite sizes using the formula L (nm) = (2.4 9 10 ) k
4
a
(
I / I ) [38] we found a decrease in the size from 66.8 nm
G D
(
GrO) to 44.4 (Pd/Gn1), 45.8 (Pd/Gn2) and 51.1 (Pd/Gn3)
2
3
nm, respectively.
a narrower Pd size distribution. Thus, Pd/Gn2 and Pd/Gn3
had 83 and 86 % of the particles within 2–4 nm, whereas
Pd/Gn1 only 72 %. In the case of the latter material larger Pd
particles up to 12 nm were also detected (Fig 5b).
The bulk and surface composition of the hybrid mate-
rials were determined by ICP-OES and XPS measurements
(
see Table 1). As expected the metal contents of the
Table 1 Bulk (ICP-OES) and surface (XPS) composition of the Pd
nanoparticle/graphene hybrid materials
3
.2 Enantioselective Hydrogenation of a,b-
a
Unsaturated Carboxylic Acids
Bulk
composition
Surface composition (wt%)
(
wt%)
The Pd/Gns were tested as catalyst in the asymmetric
hydrogenation of several prochiral aliphatic a,b-unsatu-
rated carboxylic acids, i.e. 1a, 2a and 3a to the corre-
sponding saturated acids: 2-methylhexanoic acid (1b),
Material Pd
Na
Pd
Na
C
O
Pd/Gn1 6.4
Pd/Gn2 5.3
Pd/Gn3 5.4
a
0.2
1.2
1.1
13.5 (15.4) 1.1 (1.3) 72.9 (83.4) 12.5
6.4 (7.4)
6.4 (7.4)
2.2 (2.6) 77.6 (90.0) 13.8
1.6 (1.8) 78.5 (90.8) 13.6
2
-methylbutanoic acid (2b) and 2-methylsuccinic acid
3b), using CD as modifier in the absence and the presence
of BA additive (Scheme 1.).
(
In brackets the oxygen omitted surface compositions
1
23

Palladium Nanoparticle–Graphene Catalysts
Fig. 4 O 1s, C 1s and Pd 3d region of the XPS spectrum of the Pd/Gn1 material
Table 2 XPS derived surface chemical state of the constituents
reduced. Moreover, it should also be considered the surface
restructuring effect of the chiral modifier [43]. Accordingly,
the results should be interpreted with special care and
detailed examinations are further needed. The present
results are in agreement with earlier studies on the enan-
tioselective hydrogenations of aliphatic unsaturated car-
boxylic acids over supported Pd catalysts modified by CD,
which showed the influence of the particle size and distri-
bution [44]. However, it was also shown that the texture and
morphology of the support may also have effect on the
enantioselectivity [37, 45]. The use of the achiral amine
additive (BA) improved the ee. Surprisingly, the conver-
sions also increased by using BA opposite to the initial rate
decrease observed previously over Pd catalysts supported
on conventional supports [30, 32], (see entries 2 vs 3; 7 vs 8;
Element, line
Chemical shift, BE (eV)
Assignment
Palladium, Pd 3d5/2
335.9
Pd
3
3
37.5
39.3
Pd–O
Pd–O–C = O
C–C, reference
C–O–C
Carbon, C 1s
Oxygen, O 1s
284.8
286.3
288.0
290.3
O–C = O
p–p* satellite
O–Na (O–Pd)
O = C
530.1
5
5
5
31.5
33.0
34.5
C–O–C
2
Adsorbed H O
9
vs 10; 11 vs 12; 15 vs 16). Similar tendencies were
Results obtained under experimental conditions usually
applied in the asymmetric hydrogenations of these unsat-
urated acids [29–32] are summarized in Table 3. In the
hydrogenation of monocarboxylic acids we chose toluene
as solvent, as the best results in these reactions were
reported in hydrocarbons [29, 32]. Hydrogenation of 3a,
however, was studied in methanol, the solvent found most
appropriate for this reaction [30].
observed on all three graphene supported catalysts, thus, the
effect may be attributed to the special properties of the
graphene support, which may influence the interaction
strength of the amine, unsaturated acid and the acid-modi-
fier salt with the metal surface. However, the graphene
support did not changed the effect of the acid structure on
the hydrogenation results, as the present study on the
structure of the acid (using the above mentioned three acids)
showed similar influence as described over conventional
catalysts, i.e. the increase in length of the b substituent
increased the ee and decreased the rate [46].
The data summarized in Table 3 shows that Pd/Gn cat-
alysts were active in the hydrogenation of the a,b-unsatu-
rated carboxylic acids tested. The highest optical yield was
achieved in the hydrogenation of 1a over Pd/Gn1. This
catalyst provided higher enantioselectivities as compared
with the commercial Pd/C as well; both in presence and
absence of BA (see entries 2 and 3, respectively). Catalysts
Pd/Gn2 and Pd/Gn3 produced lower ee values. The higher
activity and enantioselectivity of Pd/Gn1 calls attention to
the influence of the Pd particle size, morphology and sur-
face valence state of the metal. However, it should be
mentioned, that the hydrogenation are preceded by pre-
hydrogenations of the catalysts in the corresponding sol-
vent, thus one may presume that before the reactions the
surface oxide layer of the pristine materials is further
Increasing the concentration of CD reduced the con-
version (entries 1, 3, 4) and at the same time enhanced the
enantioselectivity up to the use of 5 mol% CD (compared
to the acid amount). At this CD concentration the reaction
is not yet slowed down significantly and at the same time
maximum ee can be achieved. Increasing the H
increased both conversion and ee (entries 3, 5, 6). Varia-
tions in the hydrogenation conditions (temperature, H
pressure
2
2
pressure, CD concentration, BA concentration) allow
concluding that the optical yield presumably can be further
increased depending on the acid structure; further investi-
gations will be carried out and will be reported in due
123

K. Sz }o ri et al.
Fig. 5 TEM images of GrO and Pd/Gns, Pd particle size distributions are shown as inserts
Scheme 1 Scheme of
hydrogenation of the selected
a,b-unsaturated carboxylic
acids over CD-modified Pd/Gns
S
COOH
COOH
R
R
Me
Me
Pd
N
Pd
OH
H
R = Pr (1a);
R = Me (2a)
R = Pr (1b);
R = Me (2b)
Pd
H
+
S R
Pd/Gn
CH2
Me
N
+
H , solvent, (BA)
2
HOOC
CD
HOOC
COOH
R COOH
(3b)
(
3a)
course. It is also noted that following our present investi-
gation, which proves the applicability of Pd/graphene
hybrid materials in hydrogenations, further development in
the material preparation may result in tuning its properties
for specific applications including the enantioselective
hydrogenation of prochiral activated olefins.
1
23

Palladium Nanoparticle–Graphene Catalysts
Table 3 Asymmetric hydrogenations of a,b-unsaturated carboxylic acids over Pd nanoparticle/graphene hybrid materials modified by CD
a
ee (%)
Entry
Catalyst
Substrate
p H
MPa)
2
Amount
CD (mmol)
Amount
BA (mmol)
Conversion
(%)
(
1
2
3
4
5
6
7
8
9
Pd/Gn1
Pd/Gn1
Pd/Gn1
Pd/Gn1
Pd/Gn1
Pd/Gn1
Pd/Gn2
Pd/Gn2
Pd/Gn3
Pd/Gn3
Pd/Gn1
Pd/Gn1
Pd/Gn2
Pd/Gn2
Pd/Gn3
Pd/Gn3
Pd/Gn1
Pd/Gn1
Pd/Gn1
Pd/Gn2
Pd/Gn3
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
2a
2a
2a
2a
2a
2a
3a
3a
3a
3a
3a
5
5
5
5
1
10
5
5
5
5
5
5
5
5
5
5
2
2
2
2
2
0.025
0.05
0.05
0.1
1
0
1
1
1
1
0
1
0
1
0
1
0
1
0
1
0
1
2
2
2
83
43 (S)
b
39; 27 (S)
b
47; 42 (S)
41
72
68
47 (S)
38 (S)
49 (S)
12 (S)
31 (S)
26 (S)
29 (S)
32 (S)
40 (S)
12 (S)
16 (S)
14 (S)
16 (S)
3 (R)
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
3
56
89
4
33
11
10
11
12
13
14
15
16
17
18
19
20
21
49
75
100
100
100
55
90
c
c
c
c
c
100
100
100
100
100
10 (R)
14 (R)
11 (R)
13 (R)
Reaction conditions: 15 mg catalyst, 1 mmol substrate, 297 K, 10 cm toluene, hydrogenation time 1.5 h
a
Configuration of the excess enantiomer in brackets
b
Enantioselectivities obtained by using Pd/C 10 % (Fluka, 75990) catalyst
3
0 min hydrogenations in 10 cm methanol
c
3
4
Conclusion
Acknowledgments Financial support by the Hungarian National
´
Science Foundation (OTKA Grant K 72065) and TAMOP-4.2.2.A-11/
1
/KONV-2012-0047 are highly appreciated.
Although reports on the hydrogenating activities of Pd/Gn
catalysts have been published before this study [47, 48], to
our best knowledge experimental observations regarding
their utilization in asymmetric reactions, including hydro-
genations, have not been reported yet. According to our
data described above, Pd/Gn catalysts synthesized in our
laboratory by co-reduction of palladium and graphite oxide
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