Cyclodextrin-Grafted Silica-Supported Pd Nanoparticles: An Efficient and Versatile Catalyst for Ligand-Free C-C Coupling and Hydrogenation

Article abstract of DOI:10.1002/cctc.201501225

Silica is an extremely versatile support, which is capable of hosting metal nanoparticles (NPs) and enhancing their stability and reactivity. In this study, a novel cyclodextrin/silica support for Pd NPs, which we have denoted Pd/Si-CD, has been prepared. The highly efficient and homogeneous impregnation of small palladium nanoparticles on this support has been carried out under conventional conditions, and ultrasound irradiation has been shown to have a beneficial effect on catalyst preparation. The catalyst exhibited excellent activity in ligand-free C-C Suzuki and Heck couplings with a large number of aryl iodide and bromides, in which microwave irradiation use cuts down reaction time. Pd/Si-CD have shown high activity and selectivity in the hydrogenation reaction, and the semihydrogenation of phenyl acetylene was also studied with excellent results.

Full text of DOI:10.1002/cctc.201501225

DOI: 10.1002/cctc.201501225
Full Papers
Cyclodextrin-Grafted Silica-Supported Pd Nanoparticles:
An Efficient and Versatile Catalyst for Ligand-Free CÀC
Coupling and Hydrogenation
Katia Martina,[a] Francesca Baricco,[a] Marina Caporaso,[a] Gloria Berlier,[b] and
Giancarlo Cravotto*[a]
Dedicated to Professor Bice Fubini on the occasion of her retirement
Silica is an extremely versatile support, which is capable of
hosting metal nanoparticles (NPs) and enhancing their stability
and reactivity. In this study, a novel cyclodextrin/silica support
for Pd NPs, which we have denoted Pd/Si-CD, has been pre-
pared. The highly efficient and homogeneous impregnation of
small palladium nanoparticles on this support has been carried
out under conventional conditions, and ultrasound irradiation
has been shown to have a beneficial effect on catalyst prepara-
tion. The catalyst exhibited excellent activity in ligand-free CÀC
Suzuki and Heck couplings with a large number of aryl iodide
and bromides, in which microwave irradiation use cuts down
reaction time. Pd/Si-CD have shown high activity and selectivi-
ty in the hydrogenation reaction, and the semihydrogenation
of phenyl acetylene was also studied with excellent results.
Introduction
The discovery, in the late 1980s by Haruta et al., that a-Fe2O3-
supported Au nanoparticles smaller than 5 nm are very active
in CO oxidation caused the catalysis community to realize the
significance that size has on catalytic properties.[1,2] The high
surface energy of these particles makes them extremely reac-
tive, meaning that most systems undergo aggregation without
protection or surface passivation.[3] Although it is well known
that nanosize in catalysis is important because of high surface
area and reactivity, the mechanistic action of a nanocatalyst is
not so clear. Several other key catalyst parameters need to be
considered. These include nanoparticle shape and any synergic
effects that may exist between metal and support.[4] Nanoparti-
cles (NPs) have so far been synthesized by using various chem-
ical and physical methods.[5] The preparation of NPs frequently
involves the reduction of metal ions in solutions or in high
temperature gaseous environments, meaning that not only
has the search for new nanocatalysts been a focus of interest
but even the optimization of metallic NP green syntheses has
attracted significant attention from the scientific community.[6]
Palladium catalysts have emerged as powerful and versatile
tools in organic synthesis.[7] A number of palladium catalysts
are commercially available and, interestingly, their reactivity,
stability and selectivity can be tuned by ligands[8] (phosphines,
carbenes, amines, etc.) and/or additives.[9] Although ligand-con-
taining molecular palladium catalysts have long proven to be
efficient and very popular,[10] continuous investigation into
“green”, so-called “ligand-less”, palladium catalysts is ongo-
ing.[11] Phosphine-free CÀC coupling reactions may involve the
presence of nanosized Pd colloids as suggested by Reetz
et al.[12] In fact, Bçnneman et al.[13] observed in 1991 that these
colloids are generated from PdII salts in polar solvents in the
presence of tetraalkylammonium salts at high temperatures.
PdNPs have now become a valuable alternative and provide
several “green” chemistry benefits.[14] One of the principle ad-
vantages that these systems present is the fact that they are
catalytically active in low amounts, driving interest from indus-
try in this field. Recent years have thus seen various syntheses
being scaled up to the kilogram scale.[15] De Vries[16] has de-
tailed the role of PdNPs as a source of Pd atoms that detach
from the surface during oxidative addition and undergo highly
efficient turnover in solution. At high temperature, PdNPs are
stabilized by the presence of polar solvents or ammonium
salts and ionic liquids, which influence PdNP dimensions and
favor productive reactions.[17] Heterogeneous supports also
provide a valuable influence on reaction outcome. Many stud-
ies have validated the use of Pd colloids in Heck reactions, al-
though reactions with less reactive aryl bromides have not
been very successful in the absence of stabilizers, such as am-
monium salts or ligands.
[a] Dr. K. Martina, F. Baricco, M. Caporaso, Prof. G. Cravotto
Dipartimento di Scienza e Tecnologia del Farmaco and
NIS—Centre for Nanostructured Interfaces and Surfaces
University of Turin
via Pietro Giuria 9
10125 Turin (Italy)
E-mail: giancarlo.cravotto@unito.it
[b] Dr. G. Berlier
Dipartimento di Chimica and
NIS—Centre for Nanostructured Interfaces and Surfaces
University of Turin
Via P. Giuria 7
10125 Turin (Italy)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201501225.
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Silica is an extremely versatile support and one which is ca-
pable of hosting metal NPs, enhancing their stability and reac-
tivity.[18] Silica has found widespread use in the preparation of
solid-supported PdNPs because of the benefits that it provides,
which include excellent stability (chemical and thermal), me-
chanical robustness, good accessibility, and porosity. In fact,
a number of organic–inorganic hybrid materials have seen ex-
tensive use in the design of novel catalytic systems.[19] Our
recent study into the efficient grafting of cyclodextrin (CD) into
an inorganic silica framework provided a material that showed
a hydrophilic/hydrophobic profile and surface reactivity, which
were modified with respect to material bulk properties.[20] To
the best of our knowledge, only one manuscript has so far re-
ported the use of a CD–silica derivative in the preparation of
a heterogeneous PdNP catalyst and its subsequent Heck reac-
tion.[21] This work therefore aims to exploit our experience in
CD synthesis to explore this field further.[22]
Results and Discussion
The silica derivative consists of b-CD anchored to silica through
an alkyl hydroxyl amino spacer (Scheme 1). The silica was de-
rivatized with 3-glycidoxypropyltrimethoxysilane (GPMS), which
was then reacted with 10-undecynil-1-amine (Und) to open the
epoxide ring. The Si–CD derivative was finally obtained from
a Cu-catalyzed azide–alkyne cycloaddition (CuAAC) with 6-
monoazido-6-deoxy-b-CD. The synthetic protocol can be per-
formed either under MW irradiation or through mechanochem-
ical activation as already described in a previous study.[20] All
samples were extensively characterized by thermogravimetric
analysis (TGA), elemental analysis, IR, Brunauer–Emmett–Teller
(BET) analysis, and HR-TEM.
A highest final loading of
135 mmolgÀ1 of b-CD was measured when all three steps were
performed under MW irradiation.[20]
PdNPs were immobilized on the Si–CD by reduction of the
Pd(OAc)2 precursor in ethanol or in water. The procedure was
repeated at room temperature and with heating at reflux. The
reaction was monitored by the color change of the silica sup-
port.
We herein report the preparation of a new PdNP catalyst,
which attempts to amalgamate the properties of an amino al-
coholic branched spacer and a triazolyl-b-CD derivative to ef-
fectively immobilize and stabilize PdNPs. Our intention here is
to produce an extremely versatile catalyst and explore its po-
tential in a series of hydrogenation and coupling reactions,
such as the Suzuki, Heck, and Sonogashira reactions, which are
ranked among the most general transformations in organic
synthesis.
The reaction from Si–G–Und afforded a light-grey silica de-
rivative, whereas all the other attempts with Si–CD furnished
a dark-grey solid. Inductively coupled plasma (ICP) analyses
confirmed that the Pd content was lower, 2.3 wt.% Pd, when
CD was absent from the silica support (see Table 1, entries 1–
4). The Pd content in the Si–CD samples was in the range 4.7–
6 wt.% and the highest amount was achieved when the load-
ing was performed in ethanol heated at reflux. Ultrasound irra-
diation of the Pd(OAc)2 solution in ethanol with Si–CD afforded
4.7 wt.% Pd.
Scientists have recently devoted a great deal of research to
designing more environmentally sound and low impact proto-
cols to operate under non-conventional conditions.[23] Solvent-
free and aqueous media reactions, NPs, microwave-assisted
and ultrasound-promoted syntheses are all key developments
in the design of greener protocols. Sonochemistry has been
very reliably employed in the synthesis of small, high produc-
tion yield NPs, and the selective heating of catalytically active
metal species under microwave irradiation is currently generat-
ing significant interest.[24] The objective of this manuscript is to
investigate the efficacy of ultrasound irradiation in the prepara-
tion of the Pd/Si-CD hybrid system, which supports PdNPs. Mi-
crowave (MW) irradiation has also been used to optimize the
synthetic procedure for high reaction yields in short reaction
times. All protocols were designed to operate without a stabil-
izer (ammonium salts or ionic liquid) and to be ligand free so
as to fulfil green chemistry criteria.
The XRD patterns of the catalysts show that PdNPs exist on
the Pd/Si-CD substrates (Figure 1, curve c). The strongest peak
in the XRD pattern (21.868) is the typical diffused scattering of
the amorphous SiO2 support. The other weaker peaks are in-
dexed as the (111), (200), (220), and (311) planes of the
PdNPs (cubic phase, JCPDS 46-1043). A slight shift in the 2q
values is observed, with respect to standard references, for
all peaks, which suggests that a small increase in crystallite
d spacing occurs as a consequence of particle nanosize.
These peaks are absent in the sample that was prepared with-
out CD (sample Pd/Si-Und, curve a). The average crystallite
size, estimated by using the Scherrer formula, is 6.2 nm for
Scheme 1. Synthetic scheme for the preparation of Si–CD.
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Table 1. Preparation conditions and Pd wt.% content of the various supported Pd catalysts.
Entry
Sample
name
Starting silica
derivative
Reaction
conditions[a]
Pd content[b]
[wt.%]
Particle size[c]
[nm]
1
2
3
4
5
Pd/Si-Und
Si–G–Und
Si–CD
Si–CD
Si–CD
Si–CD
EtOH, reflux, 2 h
EtOH, RT, 48 h
EtOH, reflux, 2 h
H2O, 808C, 2 h
2.30
4.75
6.00 (5.95)[d]
4.88
n.d.
14.7
Pd/Si-CD (A)
Pd/Si-CD (B)
Pd/Si-CD (C)
Pd/Si-CD (US)
5.4 (6.2)[e]
14.1
EtOH, ultrasound (US), 2 h
4.70
4.0[e]
[a] All reactions were performed in the presence of Pd(OAc)2 (400 mmolgÀ1). [b] Determined by inductively coupled plasma analysis (ICP). [c] Average parti-
cle size of Pd particles measured on the basis of TEM images. [d] Data measured for the catalyst after first usage. [e] The particle size was measured by
XRD.
Figure 1. XRD patterns measured for the samples listed in Table 1.
sample B and 4.0 nm for the sample prepared in the ultra-
sound bath.
TEM images of the catalysts are shown in Figure 2. Pd/Si-CD
samples A and C, which were prepared in ethanol at room
temperature and water at 808C, respectively, consist of non-
uniform (mainly spheroidal) particles with a diameter range 6–
25 mm (Figure 1, examples A and C). Pd particle size distribu-
tions of the same materials are also reported in Figure 2. A
larger number of particles were measured in sample A (135),
whereas only a few (9) were detected in sample C, meaning
that no statistical considerations can be made in this case.
Sample A particles are found most abundantly at 9 and 13 nm.
By contrast, Pd particles were present in larger amounts, were
smaller and more uniformly distributed (distribution centered
around 5 nm, Figure 2 sample B) when the sample was heated
Figure 2. TEM images of Pd/Si-CD catalysts (scale bars=20 nm): A) Pd/Si-CD,
sample was prepared in EtOH at RT; B) Pd/Si-CD, sample was prepared in
EtOH heated at reflux; C) Pd/Si-CD, sample was prepared in water at 808C.
Pd particle size distributions measured on the same samples are also report-
at reflux in ethanol for 1 h. The TEM and XRD data were in ac-
cordance.
The above-mentioned analyses show that the presence of
CD plays a crucial role in influencing the Pd content and PdNP
size distribution. The amino alcohol on the Si–G–Und inter-
mediate can coordinate Pd species, but only on the Si–CD de-
rivative were PdNPs loaded in reasonable amounts. Solvent
choice and reaction temperature also influenced NP size distri-
bution on the silica. The smallest NPs were produced under ul-
trasound irradiation.
ed.
initially focused on the optimization of a phenyl boronic acid
and halobenzene model reaction. These reactions were per-
formed in a H2O/dioxane 9:1 mixture with the aim of following
green principles. The study aimed to optimize reaction condi-
tions in terms of the lowest catalyst amount and shortest reac-
tion time. As depicted in Table 2, we compared conventional
The Suzuki–Miyaura reaction was tested first as the applica-
bility of the new catalyst was investigated (Table 2). The study
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Table 2. Suzuki–Miyaura optimization.[a]
Table 3. Scope of Suzuki–Miyaura cross-coupling reactions.[a]
Entry
X
Reaction conditions[a]
Pd/Si-CD loading
[mol%]
Yield [%]
(conv. [%])
Entry
1
Ar–X
Product
Yield [%]
94
1
2
3
4
5
6
7
8
I
I
I
I
I
I
I
I
o.b., 1008C, 4 h
o.b., 1008C, 4 h
MW, 1008C, 1 h
MW, 1008C, 1 h
0.5
0.2
0.5
0.2
0.1
0.2
0.2
0.1
0.05
0.1
0.5
0.2
0.2
0.1
0.2
0.1
0.05
98 (>99)
98 (>99)
99 (>99)
98 (>99)
97 (>99)
96 (>99)
91 (92)
98 (>99)
96 (>99)
90 (91)
98 (>99)
98 (>99)
98 (>99)
100 (>99)
99 (>99)
99 (>99)
98 (>99)
2
MW, 1008C, 1 h
2
3
3
4
98
98
MW, 1008C, 30 min
MW, 1008C, 15 min
MW, 1008C, 30 min
MW, 1008C, 30 min
MW, 1008C, 15 min
o.b., 1008C, 4 h
o.b., 1008C, 4 h
MW, 1008C, 1 h
MW, 1008C, 1 h
9
I
I
4
5
>99[b]
10
11
12
13
14
15
16
17
Br
Br
Br
Br
Br
Br
Br
5
6
6
7
95
MW, 1008C, 30 min
MW, 1008C, 30 min
MW, 1008C, 30 min
78[c]
7
8
9
8
9
61
[a] All reactions were performed in H2O/dioxane 9:1, with 1 equiv. aryl–X,
1.1 equiv. boronic acid, 2 equiv. TEA. o.b.=oil bath.
99[b]
94[c]
10
conditions to MW irradiation as we maximized the catalytic
performances of Pd/Si-CD.
10
11
90
Complete conversion was obtained with iodobenzene and
bromobenzene under conventional conditions with 0.2 mol%
Pd after 4 h heating at 1008C. MW irradiation enabled the Pd
amount to be decreased to 0.05 mol% and the reaction time
to be reduced to 30 min; however, 8–9% of the starting mate-
rial was unreacted when the reaction time was further reduced
to 15 min.
11
12
12
13
99[c]
99[b]
[a] Reaction performed in H2O/dioxane 9:1, with 1 equiv. aryl–X, 1.1 equiv.
boronic acid, 2 equiv. TEA, 0.2 mol% Pd. [b] Reaction performed with
0.2 mol% Pd. [c] Reaction performed with 0.5 mol% Pd.
An array of twelve aryl halides, both electron rich and elec-
tron deficient, was chosen as the Suzuki–Miyaura cross-cou-
pling reaction substrates and phenyl boronic acid was used for
further investigations of Pd/Si-CD catalytic performance
(Table 3).
The model substrates for the initial screening were phenyl
iodide and styrene, leading to stilbene. As summarized in
Table 5, Pd/Si-CD quantitatively converted the starting material
under conventional conditions in 4 h at reflux in dioxane/H2O
1:1 (Table 5, entries 1–4). The reaction was successful with
K2CO3 as a base and 0.5 mol% of Pd. MW irradiation sped up
the reaction and full conversion was obtained after 1 h, while
the catalyst amount was reduced to 0.2 mol% (Table 5, en-
tries 11 and 12). Excellent results were also achieved in DMF
with K2CO3 as a base (Table 5, entry 7). A slight decrease in
yield was observed when the optimized protocol was per-
formed with 4-iodoanisole.
The reaction was performed with 0.1 mol% Pd at 1008C for
30 min and yields of 61–99% were achieved. The reaction out-
comes showed good aryl iodide and bromide reactivity, and
high yields were obtained even in presence of high steric hin-
drance (entries 4, 5, 7, and 9, Table 3). The high reaction rate
enabled ethyl 4-iodo benzoate to be selectively converted to
the diphenyl derivative in water without hydrolyzing the ester
moiety (entry 1, Table 3).
The optimized protocol was used in screening a set of four
boronic acids with iodo- and bromobenzene, giving yields of
88–99% (Table 4). 2,5-Dimethoxy fenil boronic acid surprisingly
reacted in 30 min at 1008C to give 97 and 88% yields with
iodo- and bromobenzene, respectively.
To gauge the versatility of this protocol, a large number of
substrates were screened in parallel by using the MW reactor
(SynthWave by Milestone), which is well suited to parallel syn-
theses at any reaction temperature and gas pressure (up to
3008C and 200 bar). Aryl bromides and iodides with electron-
withdrawing and electron-donating substituents were reacted
The above-mentioned results prompted us to continue with
a study of Pd/Si-CD catalytic activity in the Heck reaction. A
series of reactions were performed by using varying conditions
for the sake of optimizing the protocol.
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and 7 not only indicate conversion, but also selectivity. Al-
though acrylates showed >99% selectivity, styrene gave
a slight decrease. Regiochemistry was influenced by steric and
electronic factors: sterically hindered and electron-rich halides
showed selectivities in the 98–95% range, whereas all the
other experiments only furnished the desired regioisomer. As
depicted in Figure 3, the a-substituted regioisomer was the
main side product when a mixture of isomers was observed.
a-Substitution products were present in the 1–3% range,
whereas the b-cis regioisomer was detected at 0.5% when the
reaction was performed with halo anisoles, iodophenol, 2-
methyl iodobenzene, 2,5 dimethyl halobenzenes, and 2-amino
iodobenzene.
Table 4. Suzuki–Miyaura cross-coupling reaction of iodo- and bromoben-
zene with phenyl boronic acid derivatives in aqueous media.[a]
Entry
Halobenzene
Boronic acid
Product
Yield [%]
1
2
Ph–I
Ph–Br
14
14
99
98
3
4
Ph–I
Ph–Br
15
15
98
97
5
6
Ph–I
Ph–Br
16
16
97
88
7
8
Ph–I
Ph–Br
17
17
99
98
[a] Reactions performed in H2O/dioxane 9:1, with 1 equiv. iodo- or bromo-
benzene, 1.1 equiv. boronic acid, 2 equiv. TEA, 0.1 mol% Pd.
Figure 3. Observed range of selectivity for the ligand-free Heck reaction.
Table 5. Heck optimization.
An example of the intramolecular ligand-free Heck
reaction[25] was also successfully performed to display
the high reactivity of this catalyst. The carbazole de-
rivative was obtained in 85% after 1 h at 1008C with
0.5 mol% Pd (see Scheme 2). Pd/Si-CD recycling was
also investigated (see Figure 4). The cross-coupling
Entry
R
Solvent
Reaction conditions[a]
Pd/Si-CD
[mol%]
Yield [%]
(conv. [%])
reaction was carried out with phenyl iodide and
phenyl boronic acid in the presence of 0.1 mol% cat-
alyst to better understand catalyst performance. The
supported nanocatalyst was separated by filtration,
washed with water, CH2Cl2, dioxane, and then reused.
As depicted in Figure 2, no significant loss in catalytic
activity was observed after five cycles. The ICP analy-
sis of the catalyst after first usage showed constant
Pd loading[14d] from 6.0 wt.% to 5.95 wt.% Pd after
usage (Table 1, entry 3). An XRD analysis carried out
on a sample after usage only showed a slight in-
crease in particle size, from 6.2 to 7.1 nm (Figure 1,
curve d). With the aim to confirm the catalytic effect
of Pd/Si-CD rather than the leached homogeneous
Pd species, the model Suzuki and Heck reactions
with iodobenzene and phenyl boronic acid or sty-
rene, respectively, were performed in two steps (see
the Supporting Information). The reaction was carried
out in an oil bath and the hot reaction mixtures were filtered
to remove the Pd/Si-CD. Without solid catalyst, the reaction
1
H
DMF
Et3N o.b., 1108C, o.n.
K2CO3, o.b., 908C, o.n.
K2CO3, o.b., 908C, 4 h.
K2CO3, o.b., 908C, 4 h
K2CO3, MW, 1208C, 4 h
Et3N, MW, 1208C, 4 h
K2CO3, MW, 1208C, 4 h
Et3N, MW, 1208C, 4 h
K2CO3, MW, 1208C, 1 h
K2CO3, MW, 1208C, 30 min
K2CO3, MW, 1208C, 1 h
K2CO3, MW, 1208C, 1 h
K2CO3, MW, 1208C, 1 h
K2CO3, MW, 1208C, 1 h
K2CO3, MW, 1208C, 1 h
1
1
1
0.5
1
1
1
1
1
52 (60)
2
3
4
5
H
H
H
H
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
DMF
98 (>99)
98 (>99)
98 (>99)
96 (>99)
87 (>99)
85 (98)
34 (56)
99 (>99)
87 (90)
97 (>99)
89 (95)
66 (84)
71 (76)
62 (68)
6
H
7
H
DMF
8
9
H
H
H
H
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
H2O/diox 1:1
10
11
12
13
14
15
1
0.5
0.2
1
0.5
0.2
H
OMe
OMe
OMe
[a] Reactions performed with 1.25 equiv. styrene, 2 equiv. of base. o.n.=overnight.
with styrene, n-butyl and tert-butyl acrylate. Excellent results
were obtained in 1 h with 0.5 mol% Pd without the addition
of ligands and the reactions almost always gave quantitative
yields (see Tables 6 and 7). Triazolyl and thiazole iodo deriva-
tives were included in the list of reagents, as representative
heterocyclic compounds, and yielded the desired product in
yields of 34–75%.
The regiochemical outcome of the Heck reaction with un-
symmetrical olefins is of utmost importance and so Tables 6
Scheme 2. Intramolecular Heck reaction.
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Table 7. Scope of the Heck reaction with different aryl iodides and bro-
mides and butyl acrylate.[a]
Figure 4. Pd/Si-CD catalyst reusability. Reaction conditions: 1 equiv. bromo-
benzene, 1.1 equiv. boronic acid, 2 equiv. triethylamine (TEA), 0.1 mol% (Pd/
Si-CD), H2O/dioxane 9:1, 1008C, 30 min.
Table 6. Scope of the Heck reaction with different aryl iodides and bro-
mides and styrene.[a]
[a] Reactions performed with 1.25 equiv. acrylate, 2 equiv. K2CO3,
0.5 mol% Pd, 1208C, 1 h. The solvent was H2O/dioxane 1:1 for the reac-
tion with tert-butyl acrylate, whereas DMF was chosen for n-butyl acry-
late.
monitored by GC-MS did not proceed (only 1–5% of further
conversion), confirming the central role of Pd/Si-CD.[26]
Finally, we explored the hydrogenation of a range of unsatu-
rated hydrocarbons under 1 bar initial hydrogen pressure at
room temperature (Table 8). Pd/Si-CD (0.15 mol% Pd) showed
excellent catalytic activity in the reduction of substrates with
double and triple bonds to the corresponding alkane with ex-
cellent yields (>99%). Reduction of azido and nitro groups fur-
Table 8. Hydrogenation reaction.[a]
Entry
1
Starting material
Product
Reaction time
3 h
Yield [%]
>99
2
3
3 h
1 h
>99
>99
4
5
1 h[b]
2 h[c]
95
>99
6
o.n.
90
[a] Reactions performed in H2O/dioxane 1:1, with 1.25 equiv. styrene,
[a] Reactions performed in hexane, with 0.15 mol% Pd, at RT. [b] The reac-
2 equiv. K2CO3, 0.5 mol% Pd, 1208C, 1 h. [b] Reaction performed in DMF.
tion was performed at 508C. [c] The reaction was performed in methanol.
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nished the corresponding amines in 2 h. Benzaldehyde gave
the corresponding alcohol in 1 h at 508C in a >99% yield and
the p-nitrobenzaldehyde was selectively reduced to p-amino
benzaldehyde overnight at room temperature.
Experimental Section
Materials and method
All commercially available reagents and solvents were used with-
out further purification. SIPERNAT 320 amorphous silica was sup-
plied by Evonik Degussa. Reactions were carried out in a profes-
sional MW reactor, SynthWave (MLS GmbH, Milestone S.r.l.). Mecha-
nochemistry was performed in a planetary ball mill, PM100 (Retsch
GmbH). Si–G–Und–CD was fully characterized as described in one
of our previous studies. The cations were determined with a Perkin-
Elmer Optima 7000 (PerkinElmer, Norwalk, Connecticut, USA) in-
ductively coupled plasma–optical emission spectrometer (ICP-OES).
NMR spectra were recorded with a Bruker 300 Avance (300 MHz
and 75 MHz for 1H and 13C, respectively) at 25 8C. Chemical shifts
were calibrated to the residual proton and carbon resonances of
the solvent; CDCl3 (dH=7.26, dC=77.16 ppm) and DMSO (dH=
2.54, dC=40.45 ppm). GC-MS analyses were performed in a GC
Agilent 6850 (Agilent Technologies, Palo Alto, CA, USA) that was
fitted with a mass detector Agilent Network 5973.
Conscious of the importance of alkyne semihydrogenation
in providing the alkene derivative and of conventional mono-
metallic Pd catalysts not giving high selectivity in these pro-
cesses,[27] we proceeded to evaluate the kinetics of the catalytic
hydrogenation of phenyl acetylene. Five experiments were per-
formed at room temperature under 1 bar initial hydrogen pres-
sure and the reactions underwent workup and analysis after
15, 30, 45, 60, 90, 120, and 150 min. As depicted in Figure 5,
full conversion of the alkyne was obtained in 60 min and the
highest selectivity (alkene/alkane) achieved was >95%.
Preparation of Si-CD
(3-Glycidoxypropyl)methyltriethoxysilane (0.934 mL, 0.420 mol) was
dissolved in toluene (10 mL) and silica SIPERNAT 320 (1 g) was
added. The suspension was either heated under stirring in an oil
bath (808C for 5 h) or in a MW reactor (808C for 1 h, average
power 53 W). The modified silica was then filtered, washed thor-
oughly, and dried under vacuum. Si–GPMS (1 g) and 10-undecynil-
1-amine (0.275 g, 1.64 mmol) were dissolved in DMF (3 mL). The
solution was either heated to 808C and stirred for 24 h or the reac-
tion was performed in a MW reactor (1008C for 2 h, average power
approximately 20 W). The silica was finally filtered and washed
with DMF, water, and toluene, and dried under vacuum. Si–G–Und
(1 g), 6-monoazido-b-CD (1.95 g, 1.68 mmol), CuSO4·4H2O (0.100 g,
0.4 mmol), and ascorbic acid (0.148 g, 0.84 mmol) were dissolved
in H2O (30 mL). The reaction was heated under MW at 808C for 2 h
(average power approximately 12 W). The silica was filtered,
washed with water, and dried under high vacuum. The silica was
purified of copper salts by the addition of Na2H2EDTA (3.14 g) and
dissolved in H2O (5 mL). The suspension was left under magnetic
stirring overnight. The silica was then filtered, washed with water,
and dried under high vacuum.
Figure 5. Profile of the hydrogenation of phenyl acetylene with Pd/Si-CD.
Conclusions
We report the preparation of a new hybrid CD silica derivative
supported PdNP catalyst. As the presence of a coordinating
group is considered the key factor in the formation of small
sized, homogeneously dispersed Pd nanoparticles, we herein
demonstrated that CD-grafted silica is an optimal support for
this task. The amino alcohol groups and triazole on the spacer
can also coordinate Pd species and influence PdNP content,
size, and distribution on the silica surface. An extensive study
of the catalytic performances of this catalyst in ligand-free CÀC
Suzuki and Heck couplings with a large number of aryl iodides
and bromides has been reported. The catalyst exhibited excel-
lent results and MW irradiation cut down reaction times. Pd/Si-
CD showed high activity and selectivity in the hydrogenation
reaction, and the semihydrogenation of phenyl acetylene was
studied, giving excellent alkene/alkane selectivity. All protocols
were designed to be ligand free and carried out without the
addition of a stabilizer, in accordance with green chemistry cri-
teria, and catalyst reuse was also evaluated. The Suzuki–
Miyaura reaction was repeated five times and no significant
loss in catalytic activity was observed. ICP analysis of the cata-
lyst after recycling confirmed negligible Pd leakage, moreover,
XRD showed that the NP dimensions only increased slightly
after usage.
Preparation of the Pd/Si-CD catalyst
Pd(OAc)2 (0.0024 g, 1.09 mmol) was dissolved in ethanol (5 mL)
and Si–CD (0.250 g) was added. The suspension was heated under
reflux with an oil bath and stirred for 2 h.
When the preparation of the catalyst was performed under ultra-
sound irradiation the reaction was sonicated (cup-horn, 90–100 W,
20.4 kHz), for 1 h and the temperature was maintained at 35–378C.
Characterization
Powder X-ray diffraction (XRD) patterns were measured with
a PW3050/60 X’Pert PRO MPD diffractometer (Panalytical) working
in Bragg–Brentano geometry using CuKa radiation (l=1.5406 Š)
and operated at 45 kV, 40 mA, with a step size of 0.01708 and time
per step of 90 s. Crystallite size, D=4/3L was calculated by apply-
ing the Scherrer equation to the (111) peak at 39.338:
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Full Papers
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The aryl halide (0.16 mmol), alkene or acrylate (0.2 mmol), K2CO3
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Hydrogenation reaction
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agreement No. CP-IP 309376 of the 7th Framework Program. This
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Keywords: CÀC coupling
· cyclodextrin · heterogeneous
catalysis · organic–inorganic hybrid materials · palladium
nanoparticles
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Received: November 6, 2015
Published online on February 18, 2016
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