2
M. Gómez-Martínez et al. / Journal of Molecular Catalysis A: Chemical 404 (2015) 1–7
(silica gel, hexane/EtOAc:6/1) to obtain 0.0275 g of pure compound
4 (94% yield).
2.2. Typical procedure for the Suzuki–Miyaura coupling reaction
under MW irradiation conditions
Fig. 1. Palladium(0) supported catalysts employed in this study.
A 10 mL MW vessel was charged with catalyst 2 (0.3 mg,
0.1 mol% Pd), 4-bromoanisole (21 L, 0.16 mmol, 1 eq), potassium
0.33 mmol, 2 eq) and MeOH/H2O: 3/1 (0.4 mL). The vessel was
sealed with a pressure cap and the mixture was heated at 80 ◦C
using MW irradiation (initial irradiation power 40 W) for 2 h in a
CEM Discover MW reactor. The mixture was cooled at room tem-
perature and H2O (4 mL) and EtOAc (4 mL) were added. The mixture
was filtered with cotton and extracted with EtOAc (3 × 10 mL). The
organic layers were dried over MgSO4 and concentrated under
reduced pressure. The crude residue was purified by flash chro-
matography (silica gel, hexane/EtOAc:6/1) to obtain 0.0269 g of
pure compound 4 (92% yield).
the palladium-catalyzed Suzuki–Miyaura coupling between aryl
iodides, bromides, and activated chlorides with arylboronic acids.
However, to the best of our knowledge, no studies have been per-
formed involving other boron-derived nucleophiles [19], such as
potassium aryltrifluoroborates [20] or boronic acid esters [21].
Herein, as part of our interest in palladium-catalyzed
Suzuki–Miyaura reactions using highly active catalysts [22], we
graphene nanoplatelets (PdNPs–G) and reduced graphene oxide
(PdNPs–rGO) as catalyst in the Suzuki–Miyaura coupling of
potassium aryltrifluoroborates with aryl halides. Three different
catalysts 1–3 have been evaluated in this study (Fig. 1).
2.3. Typical procedure for catalyst recovery
Once the reaction was finished, the mixture was diluted with
10 mL of a mixture of EtOAc/MeOH/H2O:4/3/1 (volume ratio) and
stirred. This mixture was centrifuged (2000 rpm, 20 min) and the
solvent was subtracted using a syringe with a syringe filter (4 mm
PTFE syringe filter, 0.2 m). The washing/centrifugation sequence
was repeated four additional times until no product was detected
in the liquid phase by thin layer chromatography. The residual sol-
vent was completely removed under reduced pressure affording
the Pd catalyst which was directly used in the same tube with fresh
reagents for the next run. This procedure was repeated for every
cycle and the conversion of the reaction was determined by GC
chromatography using decane as internal standard.
2. Experimental
Unless otherwise noted, all commercial reagents and solvents
were used without further purification. Melting points were deter-
mined with a Reichert Thermovar hot plate apparatus and are
uncorrected. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra
were obtained on a Bruker AC-300, using CDCl3 as solvent and
TMS as reference, unless otherwise stated. Low-resolution elec-
tron impact (EI) mass spectra were obtained at 70 eV on an Agilent
5973 Network Mass selective detector. Analytical TLC was per-
formed on Merck aluminum sheets with silica gel 60 F254. Silica
gel 60 (0.04–0.06 mm) was employed for flash chromatography.
Microwave reactions were performed on a CEM Discover synthe-
sis unit (CEM Corp., Matthews, NC) with a continuous focused
microwave power delivery system in glass vessels (10 mL) sealed
with a septum under magnetic stirring. The temperature of the
reaction mixture inside the vessel was monitored using a calibrated
infrared temperature control under the reaction vessel. The conver-
sion of the reactions was determined by GC analysis on an Agilent
6890N Network GC system. Centrifugations were carried out in a
Digicen centrifuge (OrtoAlresa, 2000 rpm, 20 min). TEM analyses
were carried out in a JEOLJEM-2010 apparatus equipped with a
Gatan acquisition camera. The size distribution of the palladium
nanoparticles was determined by measuring the particle diame-
ter using Image J 1.49b software on the images obtained by TEM.
ICP-MS were performed on an Agilent 7700× equipped with HMI
(high matrix introduction) and He mode ORS as standard. Cata-
lysts 1–3 are commercially available and have been obtained from
NanoInnova Technologies S.L.
3. Results and discussion
3.1. Catalysts activity and reaction conditions
Catalyst 1 (PdNPs–rGO/ODA) is a reduced graphene oxide, func-
tionalized with octadecylamine (ODA, 0.1 mmol/g), where Pd(0)
nanoparticles of 13 nm of average size (6% of Pd w/w) have been
immobilized (Fig. 1). The amino functional group in the catalyst
solvents. The heterogeneous catalyst 2 contains 6% in weight of
Pd(0) nanoparticles, with an average size of 5 nm, over a graphene
support (PdNPs–G), while, in catalyst 3, the Pd(0) nanoparticles
(6.9 nm average size, 6% weight of Pd) are supported over a reduced
The coupling between 4-bromoanisole and potassium triflu-
oroborate was selected as model reaction in order to study
the catalytic activity of the palladium supported catalysts 1–3
(Scheme 1). We considered different parameters, such as the type
and the amount of supported catalyst (i.e., 1–3), and the solvent.
Initially, a mixture 1/1 MeOH/H2O (substrate: 0.4 m) was used in
the coupling reaction of 4-bromoanisole and PhBF3K (1.25 eq) cat-
alyzed by 1–3 (0.1 mol% in Pd) in the presence of K2CO3 as base
(2 eq), at 80 ◦C during 20 h. Catalyst 1, which scatters poorly in
2.1. Typical procedure for the Suzuki–Miyaura coupling reaction
under conventional heating conditions
A 10 mL glass vessel was charged with catalyst 2 (0.3 mg,
0.1 mol% Pd), 4-bromoanisole (21 L, 0.16 mmol, 1 eq), potassium
phenyltrifluoroborate (38 mg, 0.20 mmol, 1.25 eq), K2CO3 (45.5 mg,
0.33 mmol, 2 eq) and MeOH/H2O: 3/1 (0.4 mL). The vessel was
sealed with a pressure cap, and the mixture was stirred and heated
at 80 ◦C for 20 h. Then, the mixture was cooled at room temperature
and H2O (4 mL) and EtOAc (4 mL) were added. The mixture was fil-
tered with cotton and extracted with EtOAc (3 × 10 mL). The organic
layers were dried over MgSO4 and concentrated under reduced
pressure. The crude residue was purified by flash chromatography
Scheme 1. Model Suzuki–Miyaura reaction catalyzed by catalyst 1–3.