39 research outputs found
Nitroaldol Condensation Promoted by Organic Bases Tethered to Amorphous Silica and MCM-41-type Materials
No full textN
“Tetralkylammonium and sodium decatungstate heterogenized on silica: Effects of the nature of cations on the photocatalytic oxidation of organic substrates”
No full textClay/water mixtures - A heterogeneous and ecologically efficient catalyst for the three-component stereoselective synthesis of tetrahydroquinolines
No full textThe three-component synthesis of tetrahydroquinolines 5 from aromatic amines 1, aromatic aldehydes 2 and cyclopentadiene (4) can be efficiently performed in water in the presence of the commercially available bentonite Bieliaca. The overall process involves the rapid initial production of the imines 3, which subsequently undergo aza-cycloaddition processes with clopentadiene, affording products in good yields and with excellent selectivities. The cycloaddition step is regiospecific and stereospecific, exclusively giving the endo product. It was possible to reuse the catalyst several rimes without lowering its efficiency. The process represents a clean, safe and environmentally friendly route for the production of a class of natural products displaying a wide range of biological activities
TETRALKYLAMMONIUM AND SODIUM DECATUNGSTATE HETEROGENIZED ON SILICA: EFFECTS OF THE NATURE OF CATIONS ON THE PHOTOCATALYTIC OXIDATION OF ORGANIC SUBSTRATES
No full textTetralkylammonium and sodium decatungstates have been heterogenized on silica. The so-obtained materials have been characterized by UV-vis and Fourier transform infrared spectroscopies, N2 adsorption experiments, and measurements of surface polarity. Both the anion W10O324- and the countercations are anchored on the surface of the support without undergoing any appreciable modification. The size of the cations plays a key role in determining the degree of coverage and, consequently, the surface polarity of silica. The heterogenized systems are characterized by different and tunable photoreactivities depending on the nature of the countercation. In particular, tetralkylammonium cations enhance the efficiency of cyclohexane photooxidation both to radical species and to the stable oxygenation products cyclohexanol and cyclohexanone. The cation makes the environment where the photoactive anion is localized more or less hydrophobic; this influences the possibility of the approach of the substrate and, consequently, the photocatalytic efficiency of the process. The stability of these photocatalysts is demonstrated by the fact that they can be used at least three times without any loss of activity. Some alcohols with increasing dielectric constants, like cyclohexanol, ethanol, and methanol, have been used in an electron paramagnetic resonance spin trapping investigation in order to gain more information about the action of the different cations. With bulky and organic cations, alcohols were oxidized in the order from the less polar to the more polar, in terms of dielectric constants. On the contrary, the order is reversed with inorganic cations, such as Na+
Clay-water Mixture: an Heterogeneous and Eco-efficient Catalyst for the Three-component Stereoselective Synthesis of Tetrahydroquinolines
No full textAmberlyst 15 as a Mild, Chemoselective and Reusable Heterogeneous Catalyst for the Conversion of Carbonyl Compounds to 1,3-Oxathiolanes
No full textThe synthesis of 1,3-oxathiolanes from carbonyl compounds has been performed in good yields using Amberlyst 15 as a reusable heterogeneous catalyst. The procedure can be applied for the chemoselective protection of aldehyedes in the presence of a ketone function
Homogeneous versus Heterogeneous Approach to the Catalytic Desymmetrisation of Meso-anhydrides Promoted by Cinchona Alkaloids
No full texta-Fluorotropinone Immobilised on Silica: a New Stereoselective Heterogeneous Catalyst for Epoxidation of Alkenes with Oxone®
No full textUnderstanding the Influence of the Immobilization Procedure on the Catalytic Activity of Aminopropylsilicas in C-C Forming Reactions
No full textImmobilization of (n-Bu4N)4W10O32 on mesoporous MCM-41 and amorphous silicas for photocatalytic oxidation of cycloalkanes with molecular oxygen
No full textHeterogenization of (nBu4N)4W10O32 on amorphous and MCM-41 silicas by impregnation is a suitable method to prepare efficient and stable catalysts able to oxidize unactivated C-H bonds to the corresponding alcohols and ketones with molecular oxygen and near ultraviolet light. The thus-prepared catalysts were characterized by spectroscopic and N2 adsorption-desorption experiments. A better dispersion of decatungstate was achieved with MCM-41, which also has a higher surface area. Results obtained in the model photooxidation of cyclohexane and cyclododecane confirm that the decatungstate heterogenized on the more ordered MCM-41 silica promotes the process with higher chemoselectivity (cyclohexanone/cyclohexanol molar ratio =2.6). The photocatalytic efficiency of (nBu4N)4W10O32 is not reduced after heterogenization; moreover, decatungstate immobilized on amorphous silica oxidizes cyclododecane with a markedly higher efficiency than in homogeneous phase. the supported photocatalysts can be used at least three times without any loss of activity
