Application of Pd Nanoparticles Supported on Mesoporous Hollow Silica Nanospheres for the Efficient and Selective Semihydrogenation of Alkynes

Article abstract of DOI:10.1002/cctc.201501112

Herein, the preparation of a heterogeneous catalyst consisting of 1-2 nm sized Pd nanoparticles supported on amino-functionalized mesoporous hollow silica nanospheres and its use for the semihydrogenation of mono- And disubstituted alkynes is reported. By utilizing this Pd nanocatalyst together with the green poisoning agent DMSO, high yields of the desired alkenes could be achieved, while suppressing the degree of over-reduction to alkanes. To our delight, the Pd nanocatalyst displayed remarkable chemoselectivity towards the alkyne moiety, allowing the transformation to be carried out in the presence of other reducible functionalities, such as halogens, carbonyl, and nitro groups.

Full text of DOI:10.1002/cctc.201501112

DOI: 10.1002/cctc.201501112
Full Papers
Application of Pd Nanoparticles Supported on Mesoporous
Hollow Silica Nanospheres for the Efficient and Selective
Semihydrogenation of Alkynes
Oscar Verho+,*[a] Haoquan Zheng+,[b] Karl P. J. Gustafson,[a] Anuja Nagendiran,[a]
Xiaodong Zou,*[b] and Jan-E. Bäckvall*[a]
Herein, the preparation of a heterogeneous catalyst consisting
of 1–2 nm sized Pd nanoparticles supported on amino-func-
tionalized mesoporous hollow silica nanospheres and its use
for the semihydrogenation of mono- and disubstituted alkynes
is reported. By utilizing this Pd nanocatalyst together with the
green poisoning agent DMSO, high yields of the desired al-
kenes could be achieved, while suppressing the degree of
over-reduction to alkanes. To our delight, the Pd nanocatalyst
displayed remarkable chemoselectivity towards the alkyne
moiety, allowing the transformation to be carried out in the
presence of other reducible functionalities, such as halogens,
carbonyl, and nitro groups.
Introduction
The selective semihydrogenation of alkynes is an important
transformation within academic and industrial-scale synthetic
chemistry as the corresponding alkene products constitute
valuable building blocks in the preparation of natural products,
pharmaceuticals, and functional materials.[1] Unfortunately,
these semihydrogenation reactions are often far from trivial to
perform, requiring delicate tuning of the reaction parameters
and careful monitoring of the reaction progress to prevent
over-reduction to alkanes. The heterogeneous Lindlar catalyst
(Pd/CaCO3 deactivated by Pb(OAc)2)[2] developed in 1952 is still
considered the catalyst of choice for this transformation. The
Lindlar catalyst generally displays good chemo- and stereose-
lectivity in the semihydrogenation of simple disubstituted al-
kynes, which allows the corresponding cis-alkenes to be ob-
tained in high yields. However, the Lindlar catalyst is associated
with some severe limitations, such as poor selectivity in the
partial reduction of monosubstituted or highly functionalized
alkynes, which often requires the use of quinoline as an addi-
tional poisoning agent to ensure satisfactory results.[1a,b] In ad-
dition, the pre-treatment of the catalyst with toxic Pb(OAc)2 is
not desirable from an environmental perspective.
In attempts to solve the issues associated with the Lindlar
catalyst, significant attention has been dedicated towards the
design of more general and environmentally friendly catalytic
protocols for this transformation. As a result, many homogene-
ous and heterogeneous catalytic systems have been developed
for the partial reduction of alkynes over the past few decades,
involving a wide range of transition metals, such as Au,[3] Cr,[4]
Cu,[5] Fe,[6] Ni,[4a,7] Pd,[8] Pt,[9] Rh,[10] Ru,[11] and V.[4a,12] Although
each of these catalysts have their advantages over the Lindlar
catalyst, there is still a need for a broadly applicable catalyst
that can provide satisfactory yields and selectivity for a wider
range of alkynes without the necessity of an environmentally
harmful poisoning agent. Herein, we report on the utilization
of Pd nanoparticles immobilized on amino-functionalized
mesoporous hollow silica nanospheres (hereon denoted as
Pd0-AmP-HSN), together with the green poisoning agent
DMSO, as an efficient and selective system for the semihydro-
genation of a variety of functionalized mono- and disubstitut-
ed alkynes. Interestingly, this novel catalytic system was found
to display very high chemoselectivity for the alkyne moiety, en-
abling the reduction to be conducted in the presence of other
reducible functional groups.
[a] Dr. O. Verho,+ K. P. J. Gustafson, Dr. A. Nagendiran, Prof. J.-E. Bäckvall
Department of Organic Chemistry
Stockholm University
Arrhenius Laboratory
S-106 91 Stockholm (Sweden)
E-mail: oscar@organ.su.se
jan.e.backvall@su.se
[b] Dr. H. Zheng,+ Prof. X. Zou
Berzelii Center EXSELENT on Porous Materials
and Inorganic and Structural Chemistry
Department of Materials and Environmental Chemistry
Stockholm University
Results and Discussion
Synthesis and characterization of Pd0-AmP-HSN
Over the past two decades, ordered mesoporous silicas have
become one of the most frequently employed classes of sup-
port materials for catalytic species, because of their high chem-
ical stability, cost-effective synthesis, good mass-transfer dy-
namics, and the possibility to graft them with a wide range of
functional groups.[13] Furthermore, these materials can be pre-
Arrhenius Laboratory
S-106 91 Stockholm (Sweden)
E-mail: xzou@mmk.su.se
[+] These authors contributed equally to this work.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501112.
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pared with a high internal surface area, which enables high
catalyst loadings and offers protection of the supported cata-
lytic species from mechanical grinding. Within this class of ma-
terials, hollow mesoporous silicas constitute a particularly inter-
esting sub-group. Mesoporous silicas have large hollow cavi-
ties, permeable shells, and uniform morphologies, which often
confer enhanced catalytic properties.[14]
electron microscopy (TEM) showed a noticeable difference in
contrast between the core and the shell, which enabled meas-
uring of the shell thickness as approximately 90 nm (Fig-
ure 1c). High-magnification TEM allowed for visualization of
the mesochannels of the Pd0-AmP-HSNs, which penetrate
through the shell and enables efficient mass transfer between
the inner core and the outside environment (Figure 1d). From
high-angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM), the Pd nanoparticles could be
more clearly visualized and they were found to be predomi-
nately in the size range 1–2 nm (Figure 1e and f). The particle
size range and dispersion level observed from microscopy
imaging were consistent with CO chemisorption measure-
ments, which determined the diameter of the Pd nanoparticles
to 1.87 nm and the Pd dispersion to 60.8%, respectively. By
energy-dispersive X-ray spectroscopy (EDS) mapping, it could
be demonstrated that the Pd nanoparticles were distributed in
a well-dispersed pattern throughout the shell porosities of the
AmP-HSN particles (Figure 2). By the use of inductively coupled
Intrigued by the potential advantages associated with these
materials, we set out to develop a Pd nanocatalyst based on
aminopropyl-functionalized hollow mesoporous silica nano-
spheres (AmP-HSNs) with radially oriented mesochannels. The
hollow mesoporous silica nanospheres used in this study were
prepared by using a two-step procedure previously reported
by Teng et al.[15] In short, compact mesoporous silica nano-
spheres were first generated by a surfactant-assembly sol–gel
process in a Stçber solution containing tetraethoxysilane, cetyl-
trimethylammonium bromide, and ammonia. These solid
mesoporous silica nanospheres were then incubated in H2O at
708C for 48 h, which triggered a spontaneous morphology
change to the desired hollow structure (Figure 1a). To make
Figure 1. (a) Schematic illustration of the structure of Pd0-AmP-HSN, (b) SEM
image of Pd0-AmP-HSN, (c) TEM image of a Pd0-AmP-HSN particle at low
magnifications, (d) high-resolution TEM images of a Pd0-AmP-HSN particle,
(e,f) HAADF-STEM image of a Pd0-AmP-HSN particle.
the hollow mesoporous silica nanospheres suitable as supports
for Pd nanoparticles, they were grafted with aminopropyl
groups by treating them with 3-aminopropyltrimethoxysilane
in toluene at 110 8C for 48 h. The immobilization of Pd nano-
particles onto the AmP-HSNs was done according to a two-
step protocol, which we have previously applied to related
mesoporous silica materials.[16] In short, the mesoporous silica
support was first impregnated with Li2PdCl4 in H2O at pH 9 to
give a PdII precatalyst, which was isolated by centrifugation
and subsequently reduced with NaBH4 to the corresponding
Pd0 nanocatalyst.
Analysis of the Pd0-AmP-HSNs by scanning electron micros-
copy (SEM) showed that they possessed a spherical morpholo-
gy with a mean support particle diameter of approximately
400 nm. From a crushed silica sphere, the hollow structure
with a rough interior surface could be clearly visualized (Fig-
ure 1b). Images taken of the Pd0-AmP-HSNs by transmission
Figure 2. EDS mapping of the elements Si, O, and Pd in a representative
HAADF-STEM image taken of a Pd0-AmP-HSN particle.
plasma-optical emission spectroscopy (ICP-OES), the Pd and N
loadings of the Pd0-AmP-HSN were measured as 13.1 and
2.5 wt%, respectively. The Pd oxidation states of the nanocata-
lyst were analyzed by X-ray photoelectron microscopy (XPS),
which revealed that it was composed of Pd0 and PdII in a ratio
of 1.6:1 (Figure S2, in the Supporting Information).
As shown in Figure 3a, the Pd0-AmP-HSN exhibited sharp
peaks when analyzed by X-ray diffraction (XRD), which sug-
gests ordered mesopores in the shell of the support particles.
The Pd0-AmP-HSN was also subjected to nitrogen adsorption/
desorption isotherm analysis, which showed a type IV isotherm
with one sharp capillary condensation step and one type H2
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alyzed hydrogenation protocol, we found that EtOAc was the
best solvent in terms of performance and environmental
friendliness.[17] This solvent was therefore used for the catalytic
evaluation of the Pd0-AmP-HSNs. Interestingly, the catalyst dis-
played a fairly high alkene/alkane ratio in the absence of a poi-
soning agent during the first 20 min of the reaction. Unfortu-
nately, the reaction proved difficult to control, and significant
amounts of over-reduced product were generated before
reaching full conversion of the starting material. At this point,
we realized that a poisoning agent was necessary to ensure
high yields of the desired alkene product and make the reac-
tion practical to perform. Thus, a screening was conducted in-
volving representatives from the most common groups of poi-
soning agents for semihydrogenation reactions, which includ-
ed bidentate amines, aromatic amines, thiols, and aryl halides
(Entries 3–6).
Figure 3. (a) XRD pattern of the Pd0-AmP-HSNs, (b) nitrogen adsorption/de-
sorption isotherm of the Pd0-AmP-HSNs.
hysteresis loop in the P/P0 range of 0.2–0.4, as well as another
capillary condensation step and hysteresis loop in the P/P0
range of 0.4–1.0 (Figure 3b). The latter set of capillary conden-
sation step and hysteresis loop is characteristic for hollow silica
materials. The Brunauer–Emmett–Teller (BET) surface area and
pore volume of the Pd0-AmP-HSNs were determined from the
nitrogen adsorption/desorption isotherm, and they were
200.0 m2 gÀ1 and 0.31 cm3 gÀ1, respectively. The detailed pore
size distribution was calculated by the use of nonlocal density
functional theory (NLDFT) to 3.5 nm.
The use of tetramethylethylenediamine (TMEDA) as an addi-
tive led only to a marginal improvement, giving 72% yield of
styrene at 81% conversion after 30 min, which corresponds to
an alkene/alkane ratio of 8:1 (Entry 3). Pyridine was found to
have a more positive influence on the selectivity, enabling
a good alkene/alkane ratio of 8.5:1 up to 95% conversion of
starting material (Entry 4). Chlorobenzene, on the other hand,
was found to have a detrimental effect on the reaction both in
terms of activity and selectivity (Entry 5). In the case of thio-
phenol, none of the expected reduction products could be ob-
served, and instead phenylacetylene was found to undergo
a facile “thio-click reaction”[18] that led to full consumption of
starting materials within 10 min (Entry 6). Overall, unsatisfacto-
ry selectivities were obtained with all of the traditional poison-
ing agents tested, and moreover they all dramatically reduced
the environmental friendliness of the catalytic protocol. There-
fore, we turned our attention towards DMSO as it constitutes
a significantly greener alternative and has recently been ap-
plied as a poisoning agent with good results in another Pd-
based semihydrogenation protocol.[19] By performing
Catalytic evaluation of the Pd0-AmP-HSN in the semihydro-
genation of alkynes
The catalytic evaluation of the Pd0-AmP-HSN nanocatalyst
commenced by studying its activity and selectivity in the semi-
hydrogenation of phenylacetylene without any poisoning
agent (Table 1, Entries 1 and 2). The reactions were performed
by using 0.5 mol% of Pd nanocatalyst and 0.80 mmol of phe-
nylacetylene in EtOAc at room temperature (RT) under 1 atm
of H2. In a previous study involving another nanopalladium-cat-
the semihydrogenation of phenylacetylene with
Table 1. Screening of poisoning agents for the Pd0-AmP-HSN nanocatalyst in the
0.5 mol% Pd0-AmP-HSN and 1 equivalent of DMSO, it
was possible to achieve an excellent selectivity to-
wards styrene, which was obtained in 95% yield after
40 min. In addition, the use of DMSO was found to
efficiently suppress over-reduction, and the styrene
yield thus remained unchanged up to a reaction time
of 1 h. The reaction also worked well with 0.25 mol%
catalyst, giving a respectable styrene yield of 89%
after 2 h. Moreover, the styrene/ethylbenzene selec-
tivity was slightly higher with 0.25 mol% Pd0-AmP-
HSN than with 0.50 mol% (22:1 versus 19:1).
semihydrogenation of phenylacetylene.[a]
Entry Additive
t
Conv.
Yield alkene Yield alkane Ratio
[min] [%][b]
[%]
[%]
alkene/alkane
1
2
3
4
5
6
7
8[d]
–
–
20
30
30
30
30
10
40
120
71
>95
81
95
61
>95[c]
>95
93
63
9
72
85
52
–
8
91
9
10
9
–
5
4
7.9:1
1:10.1
8.0:1
8.5:1
5.8:1
–
TMEDA
pyridine
chlorobenzene
thiophenol
DMSO
After identifying suitable reaction conditions, we
next chose to study the scope of the catalytic proto-
col and to establish if it showed satisfactory selectivi-
ties for other groups of alkyne substrates as well
(Table 2). In general, the catalytic protocol exhibited
high activity and selectivity for a range of mono- and
disubstituted alkynes, allowing the hydrogenations
to be performed with relatively short reaction times
and in the presence of other reducible groups. In the
95
89
19:1
22:1
DMSO
[a] Reaction conditions: Phenylacetylene (0.80 mmol), additive (0.80 mmol), and Pd0-
AmP-HSNs (13.1 wt% Pd, 2.44 mg cat., 0.50 mol% Pd) were suspended in EtOAc
(2 mL), and stirred at RT under 1 atm H2 for the time given in the table. [b] Deter-
mined by 1H NMR spectroscopy against 1,3,5-trimethoxybenzene as the internal stan-
dard. [c] Complex mixture of mono- and bis(phenylthiolated) products. [d] Phenylace-
tylene (0.60 mmol), Pd0-AmP-HSNs (0.25 mol%) and DMSO (0.60 mmol) were used for
this reaction, which was performed in 2 mL EtOAc.
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8% of over-reduced alkane (Entry 1). Gratifyingly, phenylacety-
lenes containing nitro, chloro, bromo, or ester substituents in
the para position were all tolerated by this protocol, giving the
corresponding styrenes in 76–88% yield without any signs of
side products arising from reduction of these vulnerable func-
tional groups (Entries 2–5). However, for substrates 4 and 5,
the reductions were found to proceed slowly, which called for
modifications of the standard conditions. In the case of alkyne
4, it was sufficient to increase the catalyst loading to
0.50 mol% to assure a high styrene yield, whereas 5 required
both elevated catalyst loading and longer reaction time. Also,
substrate 6 proved more difficult to reduce, but this was cir-
cumvented by using 0.50 mol% Pd nanocatalyst, allowing the
corresponding alkene to be formed in 84% yield, together
with 11 % of over-reduced product (Entry 6). The catalytic pro-
tocol was also shown to be successful for terminal alkynes
lacking the phenylacetylene structural motif as demonstrated
by the partial reduction of substrates 7–9, where alkene yields
over 85% were obtained in relatively short reaction times (En-
tries 7–9). Interestingly, this method was applied to the semi-
hydrogenation of estradiol derivative 10 with an excellent out-
come, providing the alkene product in a yield of 90% after
7.5 h (Entry 10).
Table 2. Substrate scope of the Pd0-AmP-HSN-catalyzed semihydrogena-
tion of alkynes.[a]
Entry Substrate
t
[h]
Conv. Yield alkene
[%]
Yield alkane
[%][b]
[%][b]
1
3
93
85
8
2
3
4
93
91
84
87
88
9
4
9
3
4[c]
24
>95
5[c]
3
5
89
95
76
11
6[c]
84
91
11
9
7
1.25 >95
In the case of disubstituted alkynes, the Pd0-AmP-HSN nano-
catalyst was found to favor the formation of the cis alkene
products, while maintaining a low degree of over-reduction
(<10%). By performing the semihydrogenation of diphenyl-
acetylene (11) for 3.5 h, the corresponding stilbenes were ob-
tained in a total yield of 89% and with a cis/trans ratio of 12:1
(Entry 11 a). For the semihydrogenation of 11, we chose to per-
form a comparison study of the Pd0-AmP-HSN to other con-
ventional heterogeneous Pd catalysts. The Lindlar catalyst, in
the absence of DMSO, proved to be fully cis-selective; howev-
er, over-reduction was found to be very facile and the reaction
proved difficult to control (Entry 11 b). Unfortunately, addition
of DMSO to the Lindlar catalyst was found to completely abol-
ish its activity. We also prepared a catalyst comprised of Pd
nanoparticles immobilized on amino-functionalized commer-
cial-grade silica, using an identical synthetic protocol to that of
the Pd0-AmP-HSN. This catalyst was found to function in the
presence of DMSO and mirrored the cis/trans selectivity (12:1)
of the Pd0-AmP-HSN, but also a more pronounced over-reduc-
tion was observed for the conventional catalyst (Entry 11 c).
The reaction of keto-substituted substrate 12 with Pd0-AmP-
HSN proceeded faster (Entry 12), giving 91% yield of the two
alkenes within 3 h; however, the cis/trans ratio was found to
be lower (7.4:1). Gratifyingly, no side products arising from re-
duction of the carbonyl group were observed in this reaction.
The hydroxyl-substituted alkyne 13 was converted significantly
slower and the reaction had to be performed with 0.5 mol%
catalyst. On the other hand, the reaction of 13 was found to
give a high alkene yield of 91% and a high cis/trans ratio of
11:1 under the standard conditions (Entry 13). The semihydro-
genation of 3-phenyl-2-propynenitrile (14) worked excellently,
affording the corresponding cis-alkene almost exclusively (31:1
cis/trans ratio) together with only 2% over-reduced product
(Entry 14).
8
5
95
>95
90
86
85
90
9
15
–
9
2
10
7.5
89
cis/trans 12:1
80
only cis
84
4
17
11
11a
3.5
1
93
11b[d]
>95
11c[e]
12
3.5
93
cis/trans 12:1
91
6
3
>95
cis/trans 7.4:1
91
9
2
13[c]
14
5
>95
>95
cis/trans 11:1
95
6.5
cis/trans 31:1
[a] Reaction conditions, unless otherwise noted: Phenylacetylene
(0.60 mmol), DMSO (0.60 mmol), and Pd0-AmP-HSNs (13.1 wt% Pd,
1.22 mg cat., 0.25 mol% Pd) were suspended in EtOAc (2 mL), and stirred
at RT under 1 atm H2 for the time given in the table. [b] Determined by
1H NMR spectroscopy against 1,3,5-trimethoxybenzene as the internal
standard. [c] 0.5 mol% Pd was used. [d] 0.25 mol% Lindlar catalyst was
used (no DMSO). After 1.5 h, the yield of the cis-alkene and over-reduced
alkane were 63% and 36%, respectively. [e] 0.25 mol% Pd0-AmP-SiO2
(9.75 wt%) was used (with 0.60 mmol DMSO).
case of 4-ethynyl-N,N-dimethylaniline (1), the corresponding
styrene product was obtained in 85% yield after 3 h, with only
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Important parameters to consider in applications of hetero-
geneous catalysts are their metal leaching and recyclability. To
test the leaching from our Pd nanocatalyst, a liquid aliquot
was withdrawn from the semihydrogenation of diphenylacety-
lene 11 and analyzed by inductively coupled plasma-optical
emission spectroscopy (ICP-OES). Despite the presence of
DMSO, which is known to function as a good ligand for Pd, we
were unable to detect any leaching of Pd (<0.5 ppm), indicat-
ing that the Pd was firmly anchored to the AmP-HSN particles.
Unfortunately, the Pd0-AmP-HSN was found to exhibit dimin-
ished activity upon recycling; however, this issue could be alle-
viated by treating the catalyst with 10 equivalents of NaBH4 in
H2O before using it in subsequent cycles. Following reactiva-
tion, the catalyst showed high activity again, affording stilbene
in 91% yield with a 12:1 cis/trans ratio.
was made possible by employing DMSO as an environmentally
friendly poisoning agent. To our delight, the metal leaching
was found to be negligible and the Pd nanocatalyst could be
successfully reused upon reactivation with NaBH4. Future re-
search in our laboratories will be focused on the continued in-
vestigation of this novel Pd nanocatalyst and its application in
other chemical transformations.
Experimental Section
General information
Unless otherwise noted, all reagents and materials used for the cat-
alyst synthesis and the catalytic experiments were obtained from
commercial suppliers and used without further purification.
1H NMR spectra were recorded on a Bruker Avance 400 MHz instru-
ment, and used to determine the outcome of the catalytic experi-
ments. The Pd loading of the nanocatalyst used in this study was
determined to 13.1 wt% by inductively coupled plasma-optical
emission spectroscopy (ICP-OES, Medac Ltd, Analytical and Chemi-
cal Consultancy Services, United Kingdom). ICP-OES analysis was
also used to measure the Pd leaching from the catalyst, which was
done with liquid aliquots withdrawn from the semihydrogenation
of diphenylacetylene. Powder X-ray diffraction (XRD) patterns were
recorded on a PANalytical X’Pert Pro diffractometer equipped with
a Pixel detector using CuKa1 (l=1.5406 Š) radiation at a rate of
0.58minÀ1 over the range of 1–78 (2q). The morphology of the
HSNs was observed with SEM (JEOL JSM-7401F) with an accelerat-
ing voltage of 2.0 kV. High-resolution transmission electron micros-
copy (HR-TEM) images were taken with a JEOL JEM-2100 micro-
scope operating at 200 kV. Scanning transmission electron micro-
scope (STEM) images and elemental maps of the HSNs were taken
with a JEOL JEM-2100F microscope operated at 200 kV using
a high-angle annular dark field (HAADF) detector. The camera
length was 8 cm and the spot size was 1 nm. The nitrogen adsorp-
tion/desorption isotherms were measured at À1968C with a Micro-
meritics ASAP2020 analyzer. The surface area and the pore volume
were calculated by the Brunauer–Emmett–Teller (BET) method and
the adsorbed amount at a relative pressure of ~0.99, respectively.
The XPS spectra were collected with a Kratos Axis Ultra DLD elec-
tron spectrometer using monochromated AlKa source operated at
150 W. Binding energies were calibrated using containment carbon
(C1s=285.0 eV). CO chemisorption measurements were carried
out at room temperature with a Micromeritics ASAP2020 analyzer.
The Pd0-AmP-HSNs were reduced in a flow of H2 at 358C for 3 h,
and then the catalyst was brought to room temperature under
vacuum. The isotherms were measured twice. The amount of CO
molecules that were irreversibly chemisorbed was calculated. A
chemisorption stoichiometry of CO/Pd was set to 1. The area and
particle size of the metal were calculated according to the proce-
dures described by Anderson.[21]
Gratifyingly, no significant degradation of the mesostruc-
tures of the Pd0-AmP-HSNs could be observed when analyzing
recovered nanocatalyst by TEM, which demonstrates the high
stability of these silica nanospheres (Figure S5, in the Support-
ing Information).
By comparing the performance, the broad scope, and the
environmental friendliness of our Pd0-AmP-HSN/DMSO system,
it can be concluded that it compares favorably to many of the
recently developed catalytic protocols for the partial reduction
of alkynes. For example, it has the advantage of affording high
yields of alkenes under milder reaction conditions, whereas
other catalytic systems require higher catalyst load-
ings,[3b,4a,5a,b,d,6a,7a,c,8a–c,f,11b,c,12a] higher H2 pressures,[3a,4a,6a,7a,9a,11a]
and elevated reaction temperatures.[3a,b,4a,5a,b,d,6a,8b,g,9a,11a] Partic-
ularly noteworthy in this regard is that our catalytic system is
comparable in performance to a number of homogenous Pd
protocols.[8b,d,f] Moreover, Pd0-AmP-HSN offers a more straight-
forward separation and recycling of catalyst without any com-
promise in activity, which is typically seen upon heterogeniza-
tion. The ability of the current protocol to operate efficiently
by using H2 and DMSO makes it an atom-efficient and green
alternative to other catalytic systems using more environmen-
tally harmful reductants[3b,5a–d] and poisoning agents.[3a,b,5a]
Gratifyingly, our catalytic system is close to the performance
of the recently disclosed system involving Pd nanoparticles im-
mobilized on 3-trimethoxysilylpropylsulfoxide-functionalized
silica, which constitutes the current state-of-the-art for alkyne
semihydrogenation.[19a] Similar to the results found by Kaneda
and co-workers, our study provides further support for the
idea that organic sulfoxides constitute highly effective poison
agents, which could be useful for improving other partial re-
duction protocols.
General procedure for the semihydrogenation of alkynes
catalyzed by the Pd0-AmP-HSN nanocatalyst
Conclusions
We have disclosed the synthesis of a novel heterogeneous cat-
alyst based on Pd nanoparticles supported on mesoporous
hollow silica nanospheres and its application in the semihydro-
genation of mono- and disubstituted alkynes. The developed
catalytic protocol provided the desired alkene products in high
yields, and in the case of disubstituted alkynes a high cis/trans
ratio was observed. The high selectivity of the Pd nanocatalyst
The catalytic experiments were conducted in screw-capped Radley
tubes (20 mL) with a Teflon-coated magnetic stirring bar. In a typical
reaction, alkyne (0.60 mmol), DMSO (0.60 mmol), and Pd0-AmP-HSN
(0.25–0.50 mol%) were suspended in EtOAc (2 mL). All reactions
were capped with a screwcap equipped with a septum and were
carried out under a hydrogen atmosphere (1 atm) with a hydrogen
replacement balloon at RT. The alkene and alkane product peaks
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Keywords: alkynes · heterogeneous catalysis · nanocatalysis ·
palladium · semihydrogenation
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longing to the alkyne and alkane in the 1H NMR spectrum of the reac-
tion of substrate 10 owing to the structural complexity of these com-
pounds. However, by closely analyzing the peak at 6.86–6.88 ppm, it
was possible to deduce a small component contributing to this peak,
which was shifted towards low-field, indicative of the starting alkyne.
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Received: October 10, 2015
Revised: November 16, 2015
Published online on January 8, 2016
ChemCatChem 2016, 8, 773 – 778
www.chemcatchem.org
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