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An efficient combination between nanoparticles of zinc oxide and β-cyclodextrin
in water for a three-component reaction has been highlighted by Sagir et al. (2016).
More precisely, an ortho-aminothiophenol, with aromatic aldehydes and isocyanides, can give the corresponding 3-aryl-4H-benzo[1.4]thiazin-2-amine in the presence of a Lewis acid catalyst in aqueous medium. The authors prepared ZnO
nanoparticles with a spherical shape with a diameter between 15 and 25 nm and
compared their activity to classical Lewis acids. The best catalytic result was
obtained using 5 mol% of ZnO nanoparticles. In order to enhance the catalytic
activity, phase-transfer agents were applied (10 mol% of β-cyclodextrin, cetyltrimethylammonium bromide, or tetradecyltrimethylammonium bromide), and the
best activity was obtained in the presence of β-cyclodextrin (respectively, 83% of
yield after 30 min, 86% after 40 min, and 84% after 50 min). Recycling experiments
showed that the catalyst could be reused during five consecutive runs without any
loss of its activity. The authors compared their results to other homogeneous or
nanoheterogeneous systems, and the chosen reaction conditions (water, 60 °C,
40 min of reaction) furnished a yield of 92%.
ZrO 2 -β-cyclodextrin composite was synthesized by co-precipitation using
ZrOCl 2 and β-cyclodextrin in an ammonium hydroxide solution (Girish et al. 2015).
These composite nanoparticles were prepared for the solvent-free synthesis of 2, 4,
5-trisubstituted imidazoles and 1,2-disubstituted benzimidazoles. The catalyst could
be recycled for three runs without any appreciable loss of activity and selectivity.
The synthesis of iron-platinum core-shell nanoparticles (Fe@Pt) for aqueous
hydrogenation reactions was investigated in order to limit the use of Pt monometallicbased catalyst (Mori et al. 2009). Fe@Pt nanoparticles were synthesized by thermal
decomposition of Fe(CO) 5 followed by chemical reduction of Pt(acac) 2 in the presence of oleic acid and oleylamine. After precipitation and dispersion in hexane, the
particle organic-water transfer occurred after the addition of a γ-cyclodextrin aqueous solution. Transmission electron microscopy images showed a homogeneous
dispersion with an average diameter of 2.5 nm. The core shell structure (iron as the
core, platinum as the shell) was determined by X-ray absorption measurements. The
catalytic properties of the Fe@Pt nanoparticles were evaluated in the aqueous
hydrogenation of allylic alcohol under 1 atm of hydrogen. γ-cyclodextrin-capped
nanoparticles were more efficient in water than those in organic solvent without
cyclodextrin. This difference of activity could be explained by host-guest complexation between the substrate and the γ-cyclodextrin bringing the substrate close to
the active phase. The catalyst could be recycled at least three times.
Even if water appeared as the ideal solvent (non-toxic, cheap, and readily available), its use is limited because a wide range of organic compounds are not water
soluble or are unstable in this solvent. Ionic liquids had promising results, but their
environmental safety is still discussed (Kunz and Häckl 2016). Recently, low melting mixtures (solvents prepared by mixing high melting point starting materials,
which form a liquid by hydrogen bond interactions) (Francisco et al. 2013) were
developed for the catalytic applications with homogeneous metal catalyst (Ferreira
et al. 2015). These low melting mixtures are generally cheap and easy to prepare
S. Noël et al.
An efficient combination between nanoparticles of zinc oxide and β-cyclodextrin
in water for a three-component reaction has been highlighted by Sagir et al. (2016).
More precisely, an ortho-aminothiophenol, with aromatic aldehydes and isocyanides, can give the corresponding 3-aryl-4H-benzo[1.4]thiazin-2-amine in the presence of a Lewis acid catalyst in aqueous medium. The authors prepared ZnO
nanoparticles with a spherical shape with a diameter between 15 and 25 nm and
compared their activity to classical Lewis acids. The best catalytic result was
obtained using 5 mol% of ZnO nanoparticles. In order to enhance the catalytic
activity, phase-transfer agents were applied (10 mol% of β-cyclodextrin, cetyltrimethylammonium bromide, or tetradecyltrimethylammonium bromide), and the
best activity was obtained in the presence of β-cyclodextrin (respectively, 83% of
yield after 30 min, 86% after 40 min, and 84% after 50 min). Recycling experiments
showed that the catalyst could be reused during five consecutive runs without any
loss of its activity. The authors compared their results to other homogeneous or
nanoheterogeneous systems, and the chosen reaction conditions (water, 60 °C,
40 min of reaction) furnished a yield of 92%.
ZrO 2 -β-cyclodextrin composite was synthesized by co-precipitation using
ZrOCl 2 and β-cyclodextrin in an ammonium hydroxide solution (Girish et al. 2015).
These composite nanoparticles were prepared for the solvent-free synthesis of 2, 4,
5-trisubstituted imidazoles and 1,2-disubstituted benzimidazoles. The catalyst could
be recycled for three runs without any appreciable loss of activity and selectivity.
The synthesis of iron-platinum core-shell nanoparticles (Fe@Pt) for aqueous
hydrogenation reactions was investigated in order to limit the use of Pt monometallicbased catalyst (Mori et al. 2009). Fe@Pt nanoparticles were synthesized by thermal
decomposition of Fe(CO) 5 followed by chemical reduction of Pt(acac) 2 in the presence of oleic acid and oleylamine. After precipitation and dispersion in hexane, the
particle organic-water transfer occurred after the addition of a γ-cyclodextrin aqueous solution. Transmission electron microscopy images showed a homogeneous
dispersion with an average diameter of 2.5 nm. The core shell structure (iron as the
core, platinum as the shell) was determined by X-ray absorption measurements. The
catalytic properties of the Fe@Pt nanoparticles were evaluated in the aqueous
hydrogenation of allylic alcohol under 1 atm of hydrogen. γ-cyclodextrin-capped
nanoparticles were more efficient in water than those in organic solvent without
cyclodextrin. This difference of activity could be explained by host-guest complexation between the substrate and the γ-cyclodextrin bringing the substrate close to
the active phase. The catalyst could be recycled at least three times.
Even if water appeared as the ideal solvent (non-toxic, cheap, and readily available), its use is limited because a wide range of organic compounds are not water
soluble or are unstable in this solvent. Ionic liquids had promising results, but their
environmental safety is still discussed (Kunz and Häckl 2016). Recently, low melting mixtures (solvents prepared by mixing high melting point starting materials,
which form a liquid by hydrogen bond interactions) (Francisco et al. 2013) were
developed for the catalytic applications with homogeneous metal catalyst (Ferreira
et al. 2015). These low melting mixtures are generally cheap and easy to prepare
S. Noël et al.