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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 )
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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