Chemoselective transfer hydrogenation of α,β-unsaturated carbonyl compounds using potassium formate over amine-grafted Ru/AlO(OH) catalysts

Article abstract of DOI:10.1039/c5cy01475a

Grafting of 3-(2-aminoethylamino)propyltrimethoxysilane onto Ru/AlO(OH) resulted in an active and highly chemoselective heterogeneous catalyst for the transfer hydrogenation of α,β-unsaturated carbonyl compounds to the corresponding allylic alcohols. Potassium formate was used as a sustainable hydrogen donor. A range of substrates including cinnamaldehyde, α-amylcinnamaldehyde, citral, 3-methyl-2-butenal, trans-2-pentenal, and trans-hexenal were selectively hydrogenated at the CO moiety with >96% selectivity. In comparison, the unmodified 1 wt% Ru/AlO(OH) catalyzed hydrogenation of cinnamaldehyde at the CC bond, yielding 3-phenylpropanal as the product. Higher loaded samples with 2-10 wt% Ru exhibited 20-25% selectivity to cinnamyl alcohol. The results show that low coordination sites were more selective to hydrogenation of the internal CC than the terminal CO bond. Immobilization of the amine via chemical bonding with hydroxyl groups of the AlO(OH) support blocks adjacent exposed metal sites, increasing the chemoselective reduction of CO. Optimum results were achieved at an amine/Ru ratio of 6. The catalyst maintained high activity and chemoselectivity even after five cycles.

Full text of DOI:10.1039/c5cy01475a

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Chemoselective transfer hydrogenation of , -unsaturated
carbonyl compounds using potassium formate over amine-grafted
Ru/AlO(OH) catalysts
eceived 00th January 20xx,
Accepted 00th January 20xx
Yanxiu Gao,a Jie Wang,a† Aijuan Han,a Stephan Jaenickea and Gaik Khuan Chuah*a
DOI: 10.1039/x0xx00000x
Grafting of 3-(2-aminoethylamino) propyltrimethoxysilane onto Ru/AlO(OH) resulted in an active and highly
chemoselective heterogeneous catalyst for the transfer hydrogenation of α, β-unsaturated carbonyl compounds to the
corresponding allylic alcohols. Potassium formate was used as a sustainable hydrogen donor. A range of substrates
including cinnamaldehyde, α-amylcinnamaldehyde, citral, 3-methyl-2-butenal, trans-2-pentenal, and trans-hexenal were
selectivity hydrogenated at the C=O moiety with > 96 % selectivity. In comparison, the unmodified 1 wt. % Ru/AlO(OH)
catalyzed hydrogenation of cinnamaldehyde at the C=C bond, yielding 3-phenylpropanal as the product. Higher loaded
samples with 2 – 10 wt. % Ru exhibited 20 – 25 % selectivity to cinnamyl alcohol. The results show that low coordination
sites were more selective to hydrogenation of the internal C=C than the terminal C=O bond. Immobilization of the amine
via chemical bonding with hydroxyl groups of the AlO(OH) support blocks adjacent exposed metal sites, increasing the
chemoselective reduction of C=O. Optimum results were achieved at an amine/Ru ratio of 6. The catalyst maintained high
activity and chemoselectivity even after five cycles.
www.rsc.org/
Zhu and Zaera found no preferential C=O selectivity.16 The
change in selectivity was attributed to a steric effect where the
cinnamaldehyde molecule cannot adsorb with its plane parallel
Introduction
The chemoselective hydrogenation of α, β-unsaturated
carbonyl compounds to the corresponding allylic alcohols is an
important step in the industrial synthesis of fine chemicals, in
particular, pharmaceuticals, perfumes and cosmetics.1-3 In
terms of thermodynamics and kinetics, the hydrogenation of
C=C bonds is easier than that of C=O bonds.4 Although
extensive studies have been done, the chemoselective
synthesis of allylic alcohol via catalytic hydrogenation rather
to the surface because of the aromatic ring. Theoretical
calculations showed the phenyl ring encounters an energy
barrier for distances less than 0.3 nm from the surface.17,18
Hence, the molecule is tilted with the C=O extremity being
closer to the surface than the C=C bond. In citral where no
aromatic ring is present, this steric effect is absent, and no
difference in the product distribution was observed with a
change of the metal particle size.19
than by traditional stoichiometric reduction remains
a
The choice of metal is another important factor in
chemoselective hydrogenation. Gold shows an intrinsic
selectivity toward hydrogenation of the conjugated C=O bond,
forming 60 - 80 % yields of allylic alcohols.20-22 Using
theoretical calculations, Delbecq and Sautet rationalized that
metals with larger d bands encounter stronger four-electron
repulsions with the C=C bond, lowering the probability of its
adsorption and hydrogenation.23 This was supported by
experimental results where the selectivity to allylic alcohols
increased in the order Pd < Pt < Ir, Os.12 The selectivity to the
allylic alcohol can also be improved by using a second metal
component. Examples of such bimetallic systems include Ru-
Fe, Pt-Au, Pt-Sn, Ni-In, Pt-Fe, Cu-Cr and Sn-Pt supported on
conventional silica, alumina, zeolites or active carbon.24-30
Although the allylic alcohol selectivity was improved after the
addition of a second metal, it was still very low, 40 - 70 %. It
was observed that the formation of bimetallic particles with an
electron-deficient metal component, such as Sn, Fe and Mn,
acted as adsorption sites for the activation of the C=O bond.31
challenge. Conventional hydrogenation catalysts based on
supported Ni,5 Pt,6,7 Pd,8 Ru,9,10 and Rh11 produce mainly
saturated aldehydes/ketones. Therefore, much effort has been
made to overcome this problem over the past decades. The
use of promoters, different types of supports, and varying the
metal particle size have been explored and discussed in
reviews.12,13 For cinnamaldehyde, the selectivity towards C=O
hydrogenation could be improved if the Ru particle size for
Ru/C increased from 3 nm to 16.8 nm.14 A similar dependence
on particle size was observed for supported Pt and Rh
particles,15 although for very small particles of ~ 1.5 – 2.5 nm,
a.Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Kent Ridge, Singapore 117543. E-mail: chmsj@nus.edu.sg, chmcgk@nus.edu.sg
† Present address: Johnson Matthey (Shanghai) Catalyst Co., Ltd, 586 DongXing Rd,
Songjiang, Shanghai, PR China, 201613
Electronic Supplementary Information (ESI) available: Nitrogen adsorption-
desorption isotherms, reaction profiles, and leaching tests are presented. See
DOI: 10.1039/x0xx00000x
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The confinement of a bimetallic Pt-Ru catalyst in carbon
nanotubes was reported to give relatively high selectivity of 70
– 90 % for the hydrogenation of cinnamaldehyde to cinnamyl
alcohol.32-34
View Article Online
DOI: 10.1039/C5CY01475A
We have previously found that AlO(OH)-entrapped Ru
catalysts are highly efficient in the transfer hydrogenation of
aldehydes and ketones.35 However, the entrapped Ru particles
in these catalysts were very small, in the order of 1.5 to 1.8
nm. Based on reported work, this should render them
unselective for the hydrogenation α, β-unsaturated carbonyl
compounds to allylic alcohols. We noted with interest that
Noyori’s group had previously reported that the carbonyl
group in conjugated and unconjugated enals and enones was
preferentially hydrogenated when amines were added to the
homogeneous catalyst, RuCl2[P(C6H5)3]3.36 They proposed that
the chemoselective hydrogenation occurs by a nonclassical
metal-ligand bifunctional mechanism where a hydride on Ru
and a proton of the NH2 ligand are simultaneously transferred
to the C=O function via a six-membered pericyclic transition
state. Hence, we investigated whether amines can affect the
chemoselectivity for a heterogeneous catalyst such as our
AlO(OH)-entrapped Ru catalyst. A previous attempt to directly
add ethylendiamine to a reaction mixture containing the
heterogeneous catalyst, Ru/AlO(OH), gave only moderate
yields of the allylic alcohols, 70 – 80 %.37 Hattori et al.38
Fig. 1 Amines used in grafting of Ru/AlO(OH).
Experimental
Catalyst preparation
In a typical preparation for one gram of 1 wt. % Ru/AlO(OH),
20.6 mg RuCl3.xH2O (0.1 mmol), 4 g (sec-BuO)3Al (16.5 mmol)
and 2.4 mL 2-butanol (26.2 mmol) were added into a 50 mL
round bottom flask equipped with condenser. After stirring the
o
solution at 100 C for 1 h, 4 mL water (22.2 mmol) was added
and stirring was continued for another 30 min. The resulting
black solid was filtered, washed with acetone, and dried in air
at room temperature. Samples with ruthenium loadings of 2, 5,
8 and 10 wt. % were prepared by the same procedure.
Amines
samples. To prepare amine
/Ru molar ratio of 6, 1 g of 1 wt. % Ru/AlO(OH) (0.1 mmol Ru),
0.131 mL (0.6 mmol) amine and 20 mL toluene were added
1
,
2
and
3
were grafted onto the Ru/AlO(OH)
prepared
a
heterogeneous Pd-ethylenediamine [10
%
2
-grafted 1 wt. % Ru/AlO(OH) with
Pd/C(en)] complex catalyst for the chemoselective
hydrogenation of aromatic ketones and aldehydes to
corresponding benzyl alcohols. Although high yields, 80 –
100 %, were obtained, the reuse of the catalyst was not
reported. Ghosh and Kumar4 heterogenized RuCl2[P(C6H5)3]3 by
anchoring it onto the surfaces of mesoporous MCM-41 and
MCM-48 functionalized with propylamine and propyl
ethylenediamine. However, at 75 – 79 %, the selectivity for
hydrogenation of the C=O group in α, β-unsaturated aldehydes
was lower than for the homogeneous system.
In this study, we used potassium formate as the hydrogen
source instead of gaseous hydrogen. Formates have
advantages as hydrogen storage materials because they are
non-combustible and easy to handle. The decomposition of
formates to hydrogen and bicarbonate can be carried out
under mild conditions.39 A series of Ru/AlO(OH) samples with
metal loading of 1 to 10 wt. % were prepared according to the
sol-gel encapsulation process developed by the group of
Park,40-42 and modified by grafting of the following molecules:
2
2
into a 50 mL round-bottom flask equipped with a condenser.
After refluxing for 24 h, the solid was filtered, washed with
toluene followed by acetone, and dried at room temperature.
The as-synthesized samples are denoted by
where stands for the amine precursor (
ruthenium loading (1, 2, 5, 8 and 10 wt. %) and y the amine/Ru
molar ratio (1, 2, 4, 6 and 8). A ruthenium-free amine -grafted
AlO(OH) was prepared by refluxing 1 g of AlO(OH) with 0.655
mL (3.0 mmol) amine in toluene.
n
-x wt.% Ru-y
n
1
,
2
and ), x the
3
2
2
Catalyst characterization
The surface area and pore volume of the catalysts were
determined from nitrogen adsorption-desorption isotherms
(Micromeritics Tristar 3000). Prior to measurement, each
o
sample was degassed under nitrogen at 100 C for 5 h. Powder
x-ray diffractometry (XRD) was carried out using a Siemens
D5005 diffractometer equipped with a Cu anode and variable
slits. The diffractograms were measured for 2θ from 20o to 80o
with a step size of 0.02o and a dwell time of 1s/step.
Transmission electron microscopy (TEM) was performed using
a JEOL JEM 3010 HRTEM. The ruthenium and nitrogen
contents were measured by inductively coupled plasma-
atomic emission spectroscopy (Thermo Jarrell Ash Duo Iris ICP-
AES). X-ray photoelectron spectroscopy (XPS) was performed
using a VG-Scientific ESCALAB Mark 2 spectrometer equipped
with a hemispherical electron analyzer and a Mg Kα X-ray
source (1253.6 eV, 300 W). All binding energies were
calibrated using the C 1s peak at 284.5 eV as the reference.
3-aminopropyltrimethoxysilane
propyltrimethoxysilane ( ) and 3-[2-(2-aminoethyl amino)ethyl
amino] propyltrimethoxysilane ( ) (Fig. 1). Upon contact with
(1), 3-(2-aminoethylamino)
2
3
hydroxyl groups at the surface of the solid support, the
trimethoxysilane moiety undergoes hydrolysis to chemically
bind the molecule. The amine-grafted catalysts were assessed
with regards to their effectiveness in changing the
chemoselectivity during the reduction of cinnamaldehyde. It
was also confirmed that they are truly heterogeneous. The
best catalyst was then tested for the transfer hydrogenation of
other α, β-unsaturated compounds.
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Catalytic testing
amine/Ru ratio of 6, the value obtained from ICP was between
6.53 and 7.12.
View Article Online
DOI: 10.1039/C5CY01475A
Typically, 1 mmol of the α, β-unsaturated carbonyl compound,
0.252 g (3 mmol) potassium formate, 0.27 mL (30 mmol) water
and 5 mL (65 mmol) dimethylformamide (DMF) were added to
a 25 mL round-bottom flask. After heating the reaction
mixture to 100 oC under a flow of nitrogen, 100 mg Ru/AlO(OH)
or 200 mg amine-grafted Ru/AlO(OH) was added. Samples
were taken at regular intervals and analyzed by gas
chromatography (GC) and gas chromatography mass
spectrometry (GC-MS). For recycling tests, the used catalysts
were recovered by centrifugation, washed with water followed
by ethanol and dried at room temperature before reuse.
55
(a)
44
33
22
11
0
Distribution (nm)
40
(b)
30
Results and discussion
20
Catalyst characterization
10
Grafting the 1 wt. % Ru/AlO(OH) with amines
1, 2 and 3
affected the textural properties to a small extent, but the
changes cannot be correlated with the chain length of the
alkylamine (Table 1, Fig. S1). Increasing the amine 2/Ru ratio
0
Distribution (nm)
55
(c)
from 1 to 8 resulted in a decrease of the surface area from 338
to 300 m2 g-1 as well as the pore volume from 0.95 to 0.72 cm3
44
33
22
11
0
g-1 (Fig. S2). When amine
2 was grafted onto samples with
higher Ru loading, the textural properties were more severely
affected (Fig. S3). The surface areas of ungrafted Ru/AlO(OH)
with 2 – 10 wt. % Ru varied from 150 to 380 m2 g-1 with pore
volumes of 0.18 to 0.90 cm3 g-1 (Table S1). The drastic decrease
in surface area and porosity indicates severe pore blockage.
Distribution (nm)
55
Table 1 Textural properties of amine-grafted Ru/AlO(OH)
(d)
44
33
22
11
0
Catalyst
1 wt. % Ru/AlO(OH)
1-1 wt. % Ru-6
2-1 wt. % Ru-6
3-1 wt. % Ru-6
2-1 wt. % Ru-1
2-1 wt. % Ru-2
2-1 wt. % Ru-4
2-1 wt. % Ru-8
2-2 wt. % Ru-6
2-5 wt. % Ru-6
2-8 wt. % Ru-6
2-10 wt. % Ru-6
Surface area (m2 g-1)
Pore volume (cm3 g-1)
425
309
299
355
338
328
310
299
235
42.2
1.91
0.11
1.01
0.83
0.78
1.05
0.95
0.80
0.79
0.72
0.58
0.12
0.02
0.02
Distribution (nm)
Fig. 2 TEM images and particle size distribution for (a) 1 and (b) 10 wt. % Ru/AlO(OH)
and amine 2 grafted- (c) 1 and (d) 10 wt. % Ru/AlO(OH) at 2/Ru of 6.
1150
(f)
(e)
920
Despite the textural changes, the Ru particle size was only
slightly increased after grafting. The average Ru particle sizes
for the 1 and 10 wt. % Ru/AlO(OH) were 1.5 and 1.8 nm,
(d)
690
(c)
460
respectively (Fig. 2a and b). After grafting with amine 2, the
(b)
particles were ~ 1.6 – 1.7 nm (Fig. 2c and d). Due to the small
metal particle size, no Ru-related peaks could be observed in
the X-ray diffractograms.
The Ru and N contents for grafted 1 wt. % Ru/AlO(OH)
were determined by ICP-AES. The amine-grafted 1 wt. %
Ru/AlO(OH) contained 0.72 to 0.79 wt.% Ru, which is slightly
230
(a)
0
20
30
40
50
60
70
80
2-Theta (o)
Fig. 3 XRD diffractograms for (a) 1 wt. % Ru/AlO(OH) and amine 2-grafted 1 wt. %
lower than the 0.84 wt. % measured in the ungrafted sample Ru/AlO(OH) with different 2/Ru molar ratio of (b) 1, (c) 2, (d) 4, (e) 6 and (f) 8.
(Table 2). The amount of amine grafted onto the sample
agrees well with the expected values. For an expected
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Effect of Ru loading
Table 2 Ruthenium and nitrogen contents for the amine-grafted 1 wt. % Ru/AlO(OH)
View Article Online
DOI: 10.1039/C5CY01475A
The chemoselectivity for C=O reduction in α, β-unsaturated
carbonyl compounds was investigated using cinnamaldehyde
as a model compound. The hydrogenation of either the C=C or
the C=O group leads to the formation of 3-phenylpropanal (3-
PCHO) or cinnamyl alcohol (CA), respectively (Scheme 1).
These two products can be further reduced to 3-
phenylpropanol (3-POH).
Catalyst
Ru
(wt. %)
N
Molar ratio of
amine/Ru
Expected Measured
(wt. %)
1 wt. % Ru/AlO(OH)
1-1 wt. % Ru-6
2-1 wt. % Ru-6
3-1 wt. % Ru-6
2-1 wt. % Ru-1
2-1 wt. % Ru-2
2-1 wt. % Ru-4
2-1 wt. % Ru-8
0.84
0.79
0.79
0.72
0.83
0.81
0.86
0.76
-
-
-
0.78
1.47
1.96
< 0.5
0.57
1.11
1.67
6
6
6
1
2
4
8
7.12
6.70
6.53
< 2
2.54
4.66
7.95
X-ray photoelectron spectroscopy (XPS) was carried out to
determine if there is any electronic interaction between the
amine and ruthenium. Due to grafting of 2, the carbon 1s peak
was very strong and overlapped with the ruthenium 3d signal.
Hence, we examined the nitrogen 1s signal for any changes.
Scheme 1 Reaction pathways in the hydrogenation of cinnamaldehyde.
The nitrogen 1s signal of AlO(OH) grafted with amine
2 (40
The effect of Ru loading of Ru/AlO(OH) was investigated at
a constant substrate to catalyst ratio (S/C) of 100. Over 1 wt. %
Ru/AlO(OH), the transfer hydrogenation of cinnamaldehyde
with potassium formate was very fast with a high initial TOF of
190 h-1 (Table 3, entry 1). Initially, the reaction was fully
selective to the hydrogenation of the C=C group, forming 3-
phenylpropanal as the sole product (Fig. 5). The hydrogenation
of the C=O group in 3- phenylpropanal occurred only after all
cinnamyl aldehyde had been consumed, clearly showing the
wt. %) can be deconvoluted into two peaks. The peak with
binding energy at 397.96 eV is assigned to free or
unprotonated nitrogen (denoted as NS) while that at 399.91 eV
+
is assigned to protonated nitrogen (denoted as NS ). The
protonated nitrogen results from the reaction of -NH2 or -NH
groups with -OH groups on the AlO(OH) support.43,44 In
comparison, for the amine 2-grafted Ru/AlO(OH) sample, the
nitrogen 1s signal was broader (Fig. 4), and at least three peaks
are required to adequately deconvolute the signal, with the
new peak at 398.69 eV (denoted as NRu+). This binding energy
agrees with that reported for a free nitrogen coordinated to
ruthenium.4 The interaction between ruthenium and nitrogen
shifts the lone electron pair on the nitrogen atom towards the
metal, resulting in the observed increase of the nitrogen
binding energy from 397.96 eV to 398.69 eV. The appearance
of this new peak suggests that some of the amine molecules
are either directly grafted onto the ruthenium particles or are
grafted to the alumina surface sufficiently close to the metal
particle to allow for ruthenium-nitrogen interaction.
Table 3 Effect of Ru loading on the transfer hydrogenation of cinnamaldehyde over
Ru/AlO(OH)
Entry
Ru
loading
(wt. %)
1
2
5
8
10
Time
(h)
Conv.
(%)
Sel. (%)
3-
PCHO
100
71
51
60
64
TOFa
(h-1)
CA
3-
POH
0
11
24
20
14
1
2
3
4
5
1.5
1.5
1.5
2
100
96
97
98
98
0
190
104
84
46
38
18
25
20
22
2
1600
Reaction conditions: 1 mmol cinnamaldehyde, 3 mmol HCOOK, 15 mmol H2O,
5 mL DMF, catalyst (1.0 mol % Ru, S/C of 100), 100 °C, N2 protection.
aCalculated from the conversion after 20 min.
1400
(d)
1200
(b)
(a)
100
80
60
40
20
0
100
80
60
40
20
0
1000
800
600
400
(c)
(b)
(a)
392
397
402
0.0
0.5
Time (h)
Kinetic profile for the transfer hydrogenation of cinnamaldehyde using
1.0
1.5
0
1
2
3
4
5
6
7
8
Binding energy (eV)
Time (h)
Fig.
5
Fig. 4 Nitrogen 1s XPS spectra for the amine 2-grafted (a) AlO(OH) and 1 wt. %
Ru/AlO(OH) with amine 2/Ru molar ratio of (b) 1, (c) 2 and (d) 6.
Ru/AlO(OH) with Ru loading of (a) 1 and (b) 2 wt. %. (♦) cinnamaldehyde (●) cinnamyl
alcohol (▲) 3-phenylpropanal (○) 3-phenylpropanol.
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difference in C=C to C=O reduction rates. Cinnamyl alcohol was catalyst, the initial TOF was only 63 h-1 (Table 5) which is
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DOI: 10.1039/C5CY01475A
not observed throughout the reaction. However, using threefold lower compared to the ungrafted
1 wt. %
catalysts with higher Ru loading of 2 - 10 wt. %, up to 25 % Ru/AlO(OH), 190 h-1 (Table 3). The low reaction rate can be
selectivity to cinnamyl alcohol could be obtained (Table 3, due to partial coverage of active sites on the Ru particles by
entries 3-5 and Fig. S4). Once formed, the cinnamyl alcohol did the grafted amine. These active sites are required for formate
not react further so that its selectivity did not change decomposition and transfer hydrogenation. Although some
significantly with conversion. These results are consistent with cinnamyl alcohol was formed, the main product was 3-
the effect of particle size on cinnamyl alcohol selectivity that phenylpropanol. The cinnamyl alcohol selectivity decreased
had been previously observed for Ru/C and for graphite- from 33 % initially to only 22 % at 96 % conversion (Fig. S7).
supported Pt and Rh catalysts.15 The decrease in TOF with This shows that a significant fraction of the coordinatively
higher loading is a consequence of the bigger size of the Ru unsaturated active sites was still present despite grafting with
particles. Bigger particles have more atoms at planar sites than amine 1. When 2 was grafted on 1 wt. % Ru/AlO(OH), the TOF
at the more exposed kinks, corners and steps. The results was even lower, 13 h-1, indicating an even higher coverage of
suggest that the latter are more active for hydrogenation than the Ru surface by the amine. However, from the onset of
the planar sites. Being coordinatively unsaturated, these sites reaction, the main product was cinnamyl alcohol with much
are also easier to be accessed by the internal C=C bond, thus smaller amounts of 3-phenylpropanal and 3-phenylpropanol
lowering the selectivity to cinnamyl alcohol.
(Fig. 6). After 7 h, 96 % conversion was attained with selectivity
to cinnamyl alcohol of 95 %. The catalyst grafted with , which
contains three amino groups, had an even lower TOF of 6 h-1,
3
Effect of ethylenediamine
Ethylenediamine (EDA) was directly added to a reaction but at 91 %, the selectivity to cinnamyl alcohol was not
mixture for transfer hydrogenation of cinnamaldehyde with 1 improved over that of the -grafted sample. Hence, amine
2
2
wt. % Ru/AlO(OH) as catalyst. This affected the rate of reaction was the most effective modifier for high cinnamyl alcohol yield.
as well as the selectivity. For EDA/Ru of 0.3, an induction time
of 1.5 h was observed before the onset of reaction (Fig. S5).
During the reaction, the selectivity to cinnamyl alcohol
decreased from 82 % initially to 43 % after 22 h. The induction
Table 5 Hydrogenation of cinnamaldehyde over 1 wt. % Ru/AlO(OH) modified with
different amines.
Catalyst
Time
(h)
1.5
7
Conv.
(%)
96
96
97
Sel. (%)
3-PCHO
21
0
0
TOFa
(h-1)
63
13
6
time was shorter, 0.5 - 1 h, for higher EDA/Ru ratios of 0.5 – 3.
However once the reaction started, the rate was similar and
cinnamyl alcohol was formed with selectivity of 91 – 97 % from
the beginning of the reaction (Table 4). In contrast to the
reaction using an EDA/Ru ratio of 0.3, here the high cinnamyl
alcohol selectivity was maintained even at long reaction times
up to 24 h (Fig. S5b). However, the color of the reaction
mixture changed from light yellow (the color of
cinnamaldehyde) to dark brown, suggestive of ruthenium
leaching to form a homogeneous Ru-EDA complex. This
complex is itself catalytically active, because the reaction
continued after the solid catalyst was removed from the
reaction solution by hot filtration (Fig. S6).
CA
22
95
91
3-POH
57
5
1-1 wt. % Ru-6
2-1 wt. % Ru-6
3-1 wt. % Ru-6
21
9
Reaction conditions: 1 mmol cinnamaldehyde, 3 mmol HCOOK, 15 mmol H2O, 5
mL DMF, 200 mg catalyst (2 mol % Ru, S/C of 50), 100 °C, N2 protection.
aCalculated from the conversion after 3 h.
100
80
60
40
20
0
Table 4 Effect of EDA/Ru on the transfer hydrogenation of cinnamaldehyde
Entry
EDA/Ru
(mol/mol)
Time
(h)
1.5
2
4
2.5
3
Conv.
(%)
100
100
96
96
96
97
96
Sel. (%)
3-PCHO
100
34
0
CA
0
0
80
91
93
97
97
3-POH
1
2
3
4
5
6
7
0
0.1
0.3
0.5
1
0
66
20
9
7
3
0
1
2
3
4
5
6
7
Time (h)
0
0
0
0
Fig. 6 Reaction profile for the transfer hydrogenation of cinnamaldehyde over the
amine 2 grafted-1 wt. % Ru/AlO(OH) at amine/Ru molar ratio of 6. (♦) cinnamaldehyde
(●) cinnamyl alcohol (▲) 3-phenylpropanal (○) 3-phenylpropanol.
2
3
3
3
4
Reaction conditions: 1 mmol cinnamaldehyde, 3 mmol HCOOK, 15 mmol H2O,
5 mL DMF, 100 mg 1 wt. % Ru/AlO(OH), 100 °C, N2 protection.
The amine 2/Ru molar ratio was next optimized (Table 6).
Compared to the ungrafted 1 wt. % Ru/AlO(OH, the initial TOF
decreased by seven-fold to 27 h-1 for
levelled off at 10 – 13 h-1 for higher
2
/Ru ~ 1. However, it
2/Ru of 2 to 8. The
Amine-grafted Ru/AlO(OH)
selectivity to cinnamyl alcohol changed from 0 in the ungrafted
catalyst to 54 % and 89 % for /Ru ratios of 1 and 2 (Fig. S8).
To prevent the leaching of Ru, the amine-bearing molecules
2
were grafted onto 1 wt. % Ru/AlO(OH). For the
1-grafted
Increasing the ratio of
2
:Ru further up to 8 resulted in a
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J. Name., 2013, 00, 1-3 | 5
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Journal Name
than for the longer amVinieewsArticalenOdnline
selectivity of 91 to 95 %. The optimum
with a cinnamyl alcohol selectivity of 95 %. Even after 5 runs, especially given the low metal density ofDoOnI:ly101.10w39t./C%5C. YH0e1n47c5eA,
2
/Ru molar ratio was 6 is therefore smaller for
1
2
3,
this catalyst gave high yields of cinnamyl alcohol, showing its amines
2 and 3 have a bigger reach and can interact with
robustness (Fig. 7). The absence of leaching was confirmed ruthenium particles within its vicinity through the primary and
because the conversion stopped after the catalyst had been secondary amine moiety. Ruthenium atoms located at the
removed by hot filtration when the conversion had reached steps and kinks on the sides of a metal particle are more likely
30 % (Fig. S9). The cinnamyl alcohol selectivity was similarly to interact with the amine rather than those atop the particles
improved in catalysts with higher Ru loading, rising from 20 – (Fig. 8). As a result, adsorption of the internal C=C bond to
25 % up to 94 – 96 % after grafting with 6 equiv. of amine
2
these highly exposed ruthenium sites is blocked, resulting in an
(Table 6, entries 7 - 10). However, the reaction rate was lower increase in the selectivity for hydrogenation at the terminal
than over grafted 1 wt. % Ru/AlO(OH). The TOF was only 5 h-1 C=O. The more extensive coverage of the metallic catalyst by
for the grafted 2 wt. % Ru/AlO(OH), and fell below 1 h-1 for amines
2 and 3 as compared to 1 is seen in the decrease in TOF
catalysts with 5, 8 and 10 wt. % Ru loading.
with longer chain length. Besides exerting a steric effect, the
interaction of the amino group with Ru results in an increase in
electron density at the metal, as indicated by the appearance
of a new N peak at 398.69 eV in the XPS spectrum of the
grafted catalyst. This enhances the binding with the partially
positive carbon end of the C=O group in cinnamaldehyde. In
addition, the hydrogen bonding between the NH2 group and
the C=O end group can activate the C=O bond by perturbing its
electron distribution.4 In contrast, the internal C=C group is
rich in electrons and therefore becomes less susceptible to
hydrogen transfer due to the repulsion from the more electron
rich Ru active sites. Hence, the selectivity changes, and
hydrogenation occurs preferentially at the C=O group.
Table 6 Hydrogenation of cinnamaldehyde over amine 2-modifed Ru/AlO(OH)
Entry
Catalyst
Time Conv.
Sel. (%)
3-
PCHO
TOFa
(h-1)
(h)
(%)
CA
3-
POH
46
10
7
5
8
5
5
1
2
3
4
5b
6
7
8
9
10
2-1 wt. % Ru-1
2-1 wt. % Ru-2
2-1 wt. % Ru-4
2-1 wt. % Ru-6
2-1 wt. % Ru-6
2-1 wt. % Ru-8
2-2 wt. % Ru-6
2-5 wt. % Ru-6
2-8 wt. % Ru-6
2-10 wt. % Ru-6
5
8
7
7
10
8
14
18
20
23
98
98
97
96
96
96
94
95
97
98
54
89
91
95
90
94
95
94
94
96
0
1
2
0
2
1
0
1
1
0
27
12
12
13
12
10
5
0.9
0.8
0.8
5
5
4
Reaction conditions: 1 mmol cinnamaldehyde, 3 mmol HCOOK, 15 mmol H2O, 5
mL DMF, catalyst (2.0 mol % Ru, S/C of 50), 100 °C, N2 protection. aCalculated
from the conversion after 3 h. bFifth run.
amine 1
amine 2
100
80
60
40
20
0
Fig. 8 Proposed interaction of amines
1
and
2
with Ru particle and adsorption of
cinnamaldehyde.
Activity for various substrates.
The 2-grafted 1 wt. % Ru/AlO(OH) was tested for the catalytic
transfer hydrogenation of various α, β-unsaturated carbonyl
compounds. The linear substrates, trans-2-pentenal and trans-
2-hexenal, were easily reduced to the corresponding α, β-
unsaturated alcohols with selectivity > 96 % (Table 7, entries 1
1
2
3
4
5
Cycle
Fig. 7. Reuse of 2-1 wt.% Ru-6 in transfer hydrogenation of cinnamaldehyde.
The improved selectivity to cinnamyl alcohol for the and 3). With branched substrates, 3-methyl-2-butenal and
grafted Ru/AlO(OH) can be attributed to a steric blocking of citral, the rate of reaction was lower and a longer time of 10 h
the sites that gives rise to C=C hydrogenation and/or to an and 24 h were needed to reach > 90 % conversion (Table 7,
electronic effect. As deduced from the increasing selectivity entries 2 and 6). However, the selectivity to the allylic alcohols
with metal particle size, the coordinatively unsaturated sites at was always high, 96 % and 98 %, respectively. Of the aryl-
steps or kinks are most likely involved in the C=C containing
hydrogenation. If the grafting onto the AlO(OH) support occurs cinnamaldehyde reacted much faster than α-amyl
such that the tethered amine group is within reach of the cinnamaldehyde (Table 7, entries
and 5). This is
ruthenium crystallite, it can block these sites via coordination understandable from the bigger molecular size of α-amyl
with the NH2 group (Fig. 8). Amine with one amino group is cinnamaldehyde as compared to cinnamaldehyde. It is
shorter (0.64 nm as estimated using Materials Studio) than the noteworthy that citral, which due to its two C=C and one C=O
amines (0.93 nm) and (1.37 nm). The probability that upon bonds is considered one of the most difficult -unsaturated
grafting, it will be near enough to interact with the Ru particle aldehydes to be hydrogenated to the allylic alcohol,45 was
α,
β-unsaturated
aldehydes
tested,
4
1
2
3
6 | J. Name., 2012, 00, 1-3
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Page 7 of 10
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ARTICLE
converted to the unsaturated alcohol in excellent yields (Table
7, entry 6). Geraniol and nerol were obtained with 98 %
chemoselectivity at 90 % conversion of citral. Such reaction
efficiency, notably the absence of citronellal owing to C=C
hydrogenation, is unique when compared with previously
reported results.3 -Unsaturated ketones are challenging
substrates because of the steric hindrance at the carbonyl
group. Nevertheless, even for benzalacetone, 90 % conversion
was reached after 24 h although the selectivity to the
unsaturated alcohol, 4-phenyl-3-buten-2-ol, was only 67 %
(Table 7, entry 7).
Acknowledgement
Financial support from National University of Singapore under
Ministry of Education AcRF Tier 1 Grant R-143-000-550-112 is
gratefully acknowledged.
View Article Online
DOI: 10.1039/C5CY01475A
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3-(2-aminoethylamino) propyl groups grafted on Ru/AlO(OH) greatly increase chemoselectivity in
transfer hydrogenation of α, β-unsaturated carbonyl compounds to allylic alcohols.

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