227
from readily available materials. Zhao et al. (2014) investigated the palladiumcatalyzed Suzuki coupling of phenyl boronic acid with aryl bromides in different
carbohydrate-urea-inorganic salt mixtures. A β-cyclodextrin/N-methylurea mixture
was used as solvent for the stabilization of Pd nanoparticles. 80 °C is generally
required to get active Pd nanoparticles for C-C coupling reactions, and N-methylurea
was necessary to achieve this temperature. Cyclodextrin reduced the palladium ions
to get palladium nanoparticles. The model reaction was the coupling between bromobenzene and phenylboronic acid with 0.05 mol% Pd, using K 2 CO 3 as base giving
90% of biphenyl after 2 h. A catalytic amount of water improved the yield up to
95%. Other aryl halides were tested, and the catalytic system showed a great tolerance toward a broad range of functional groups such as –NO 2 , –NH 2 , and –CN
functionalities. The recyclability was studied, and the catalytic system preserved its
activity and stability after four runs.
A second type of non-conventional media has been developed and consisted into
performing the reaction without any solvent. In this case, where the catalyst and the
substrate are in the solid state, mechanochemistry appeared as an interesting alternative strategy. Gold nanoparticles were mechano-synthesized and were used as
nanocatalysts in the reduction of substituted nitrobenzene derivatives by ball milling (Menuel et al. 2016). Several cyclodextrins and saccharide additives were tested
to afford well-dispersed Au nanoparticles. The smallest Au average particle size was
obtained with β-cyclodextrin. X-ray photoelectron spectroscopy data confirmed the
presence of zerovalent Au nanoparticles. Several parameters were studied to see
their influence on the activity of the gold nanoparticles during the nitroarene reduction. First, the nature of the saccharide was evaluated. Except in the case of methylated saccharides, all additives improved the catalytic activity of Au nanoparticles,
especially in the case of cyclodextrins. The best enhancement was obtained with
β-cyclodextrin. Water played a crucial role during the catalytic process. Recycling
experiments were performed, and the best results were obtained for β-cyclodextrin
where no loss of the activity was noticed after three successive runs. Several nitrobenzene compounds were tested, and, whatever the substrate, the para-substituted
derivatives showed lower activities than the ortho- or meta-ones. The authors
explained these differences by favored/unfavored routes due to, respectively, unstable/stable complexes between the cyclodextrin and the substrate (Fig. 5.2). The
para-substituted compounds can form stable complexes, which limit the approach
of the substrate close to Au nanoparticle surface.
5.2.2 Nanoparticles Stabilized by Functionalized Cyclodextrins
In order to improve the stability of the solvent-dispersed metal nanoparticles, the
use of molecular functionalized cyclodextrins was investigated. According to our
literature survey, these functionalized cyclodextrins could be divided in two families: (1) the thiolated cyclodextrins and (2) the alkylated cyclodextrins.
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications
from readily available materials. Zhao et al. (2014) investigated the palladiumcatalyzed Suzuki coupling of phenyl boronic acid with aryl bromides in different
carbohydrate-urea-inorganic salt mixtures. A β-cyclodextrin/N-methylurea mixture
was used as solvent for the stabilization of Pd nanoparticles. 80 °C is generally
required to get active Pd nanoparticles for C-C coupling reactions, and N-methylurea
was necessary to achieve this temperature. Cyclodextrin reduced the palladium ions
to get palladium nanoparticles. The model reaction was the coupling between bromobenzene and phenylboronic acid with 0.05 mol% Pd, using K 2 CO 3 as base giving
90% of biphenyl after 2 h. A catalytic amount of water improved the yield up to
95%. Other aryl halides were tested, and the catalytic system showed a great tolerance toward a broad range of functional groups such as –NO 2 , –NH 2 , and –CN
functionalities. The recyclability was studied, and the catalytic system preserved its
activity and stability after four runs.
A second type of non-conventional media has been developed and consisted into
performing the reaction without any solvent. In this case, where the catalyst and the
substrate are in the solid state, mechanochemistry appeared as an interesting alternative strategy. Gold nanoparticles were mechano-synthesized and were used as
nanocatalysts in the reduction of substituted nitrobenzene derivatives by ball milling (Menuel et al. 2016). Several cyclodextrins and saccharide additives were tested
to afford well-dispersed Au nanoparticles. The smallest Au average particle size was
obtained with β-cyclodextrin. X-ray photoelectron spectroscopy data confirmed the
presence of zerovalent Au nanoparticles. Several parameters were studied to see
their influence on the activity of the gold nanoparticles during the nitroarene reduction. First, the nature of the saccharide was evaluated. Except in the case of methylated saccharides, all additives improved the catalytic activity of Au nanoparticles,
especially in the case of cyclodextrins. The best enhancement was obtained with
β-cyclodextrin. Water played a crucial role during the catalytic process. Recycling
experiments were performed, and the best results were obtained for β-cyclodextrin
where no loss of the activity was noticed after three successive runs. Several nitrobenzene compounds were tested, and, whatever the substrate, the para-substituted
derivatives showed lower activities than the ortho- or meta-ones. The authors
explained these differences by favored/unfavored routes due to, respectively, unstable/stable complexes between the cyclodextrin and the substrate (Fig. 5.2). The
para-substituted compounds can form stable complexes, which limit the approach
of the substrate close to Au nanoparticle surface.
5.2.2 Nanoparticles Stabilized by Functionalized Cyclodextrins
In order to improve the stability of the solvent-dispersed metal nanoparticles, the
use of molecular functionalized cyclodextrins was investigated. According to our
literature survey, these functionalized cyclodextrins could be divided in two families: (1) the thiolated cyclodextrins and (2) the alkylated cyclodextrins.
5 Metal Nanoparticles and Cyclodextrins for Catalytic Applications