56
A. Grirrane et al. / Journal of Catalysis 302 (2013) 49–57
utive runs. This point was studied by recovering the catalyst at the
end of the reaction, washing it and reuse for subsequent runs (Ta-
ble 3 entries 16–18 and Table 4, entries 2–4). The degree of deac-
tivation was determined by comparing the time-conversion plots
for the fresh and reused catalysts. It was observed that the catalyst
undergoes a gradual decrease in the catalytic activity. To achieve
the same yield of 3e and 3h, larger reaction times were needed
(see Tables 3 and 4). This gradual deactivation can be understood
by the generation of Iꢀ and Si-containing products during the reac-
tion and the presence of KF that will deposit on to the catalyst.
Also, the small percentage of Pd leaching or a possible Pd particle
growth [30–32] (see TEM images of reused catalysts in the Sup-
porting Information (Figs. S8 and S9)) could contribute to the decay
of the catalyst activity as it has been observed in other cases [32].
acetone (8 ml) solution of 35 mg of PdCl2 (Aldrich, purum, 60% pal-
ladium content). The slurry was stirred 15 h at room temperature,
then all the liquid was evaporated, and the solid was dried under
vacuum at 100 °C for 2 h and reduced with 1-phenylethanol at
160 °C for 3 h. The catalyst was then filtered, washed (acetone
and diethyl ether), and dried under vacuum at 100 °C for 12 h.
The final Pd content was found to be respectively 1.09, 1.10, and
0.87 wt% by atomic absorption analysis.
Pt(1.37 wt%)/MgO and Pt(5 wt%)/TiO2 samples correspond to
the catalysts previously reported by us [35,36].
The Pd/C catalysts containing of 1 and 3 wt% palladium on C
were supplied by the Sigma–Aldrich Company.
The Au/TiO2 catalyst containing of 1.5 wt% gold on TiO2 was
supplied by the World Gold Council (reference catalysts, Type A).
6.2. Synthesis of styrene derivatives
4. Reaction mechanisms
6.2.1. Preparation of styrene derivatives (3a-h)
The most likely reaction mechanism for the C–C coupling reac-
tion based on the literature [33] and the available data is shown in
Scheme 2. It involves the oxidative addition of Pd to the C–X (I or
Br) bond of arenes, subsequent nucleophilic attack (transmetalla-
tion) of vinyl anion and a final reductive elimination. Vinyl anions
will be formed by cleavage of the C–Si bond by Fꢀ anions [34].
A mixture of organohalides (1a-h) (0.25 mmol), vinysilanes 2a
(0.3 mmol) or 2c (0.075 mmol), potassium fluoride (0.6 mmol),
and supported palladium NPs catalyst (Pd/substrate ratio 1 mol%)
was suspended in DMF (1 ml). Then, the flask was evacuated under
vacuum and refilled with argon. The evacuation/refilling cycle was
repeated three times (pressure 2 bar). The mixture was stirred at
130 °C, and the reaction was monitored by GC and GC–MS. The
reaction was performed for the time required ranging from 1.5 to
24 h, to obtain the maximum yield of 3a-h products.
5. Conclusions
Herein, we have developed a heterogeneous Hiyama C–C cou-
pling reaction in which styrenes can be obtained with high yield
and selectivity using reusable supported palladium NPs as hetero-
geneous catalysts in the absence of phosphine ligands. The key
point is the nature of the solid in which the Pd NPs are supported,
MgO and TiO2 being the most suitable supports tested. Pt and Au
are inefficient to promote this reaction. Leaching tests suggest that
there is some contribution of dissolved Pd since Pd in solution has
been detected and the scavenging test decreases the initial reaction
rate. However, hydrolysis of propylthiol under the reaction condi-
tions takes place in some extent. Although the solid Pd catalyst can
be reused, it was, however, observed that the palladium catalysts
undergo a certain deactivation upon use that can be attributed to
several factors including the presence of inorganic compounds on
the catalyst, Pd leaching or agglomeration of Pd NPs.
For GC–MS m/z data of compounds 3a-h see the supporting
information (Figs. S10, S12, S14, S17–S19, S21).
For spectral NMR data of compounds 3a, 3b, 3c, and 3f see the
supporting information (Figs. S11, S13, S15–S16 and S20).
6.2.2. Preparation of divinylbenzene (3i-j)
A mixture of diiodobenzene (1i-j) (0.25 mmol), vinysilane 2c
(0.15 mmol), potassium fluoride (1.2 mmol), and supported palla-
dium NPs catalyst (Pd/substrate ratio 1 mol%) was suspended in
DMF (1 ml). Then, the flask was evacuated under vacuum and re-
filled with argon. The evacuation/refilling cycle was repeated three
times (pressure 2 bar). The reaction mixture was stirred at 130 °C,
and the reaction was monitored by GC and GC–MS, for 3 h. During
this time, all the initial compounds 3i0-j0 formed arising from the
replacement of a single iodide are reacted to obtain the maximum
yield of products 3i-j.
6. Experimental section
For GC–MS data for compounds 3i, 3i0, 3j, and 3j0 see the sup-
porting information (Figs. S22–S25).
All reactions were carried out under Ar atmosphere. Solvent are
high purity grade and used without further purification. Gas chro-
matography (GC) was performed with a Varian apparatus equipped
with a flame detector (FID). The products were separated with a
HP-5 capillary column (5% phenyl, 95% polymethylsiloxane, 30 m,
Acknowledgments
Financial support by Spanish Ministry of Science and Innovation
(Consolider MULTICAT and CTQ 2012-32316) is gratefully
acknowledged. The Generalidad Valenciana is also thanked for par-
tial financial support (Prometeo Grant).
0.25 mm ꢁ 0.25
lm, Teknokroma) and identified by GC–MS, using
a Fisons GC 8000 gas chromatograph equipped with a DB5 capil-
lary column with a mass spectrometer detector (Fisons MD 800
quadrupole detector) (see Supporting information). The identifica-
tion of the products was also carried out by NMR spectroscopy;
NMR spectra were obtained with a Bruker Avance 300 spectrome-
ter working at 300.13 MHz for 1H and 75.47 MHz for 13C. The 1H
spectra of the styrene products were referenced to CDHCl2 = 5.27 -
ppm or [D7]DMF = 2.62, 2.80, and 7.92 ppm, and the 13C were ref-
erenced to CDHCl2 = 53.84 ppm (see Supporting information).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
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