José A. Mayoral

Noncovalent Immobilization of Enantioselective Catalysts

Among the different methods to obtain enantiomerically pure compounds, namely, resolution, diastereomeric synthesis, and enantioselective catalysis, the use of chiral catalysts is probably the most attractive system due to the phenomenon of the multiplication of chirality. However, enantioselective catalysis is seldom used in industrial processes due to a variety of factors, among them the difficulty of catalyst separation from the reaction medium, which increases the high cost of the chiral catalyst and provokes contamination problems in the final product. One possible solution for those problems is the use of heterogeneous chiral catalysts that would help to easily separate and reuse the catalyst. The most convenient strategy to prepare chiral heterogeneous catalysts is the immobilization of chiral homogeneous catalysts, and the most used method to immobilize chiral complexes is without doubt the formation of a covalent bond between the solid support and the chiral ligand. One important drawback of this method is the need for an additional functionalization of the chiral ligand with two negative consequences, the rise in the catalyst preparation cost and the possible modification of the conformational preferences of the supported complex, leading to unpredictable and, in many occasions, negative effects on the enantioselectivity.

Green solvents from glycerol. Synthesis and physico-chemical properties of alkyl glycerol ethers

A family of glycerol derivatives, consisting of over sixty 1,3-dialkoxy-2-propanols and 1,2,3-trialkoxypropanes, both symmetrically and unsymmetrically substituted at terminal positions, have been synthesized and the possible role of these glycerol derivatives as substitutive solvents has been evaluated through measurements of their physico-chemical properties. The molecular diversity of the derivatives prepared results in significant variations of polarity properties, facilitating the identification of possible candidates for solvent substitution.

The use of H2O2 over titanium-grafted mesoporous silica catalysts: a step further towards sustainable epoxidation

The epoxidation of cyclohexene with aqueous hydrogen peroxide over mesostructured Ti(Cp)2Cl2-grafted silica catalysts is described for the first time. Three kinds of Ti-containing systems with different textural properties have been employed: Ti/SiO2, Ti/MCM-41 and Ti/MCM-48. A minimal local H2O2 concentration in the surroundings of Ti sites, obtained by a controlled dropwise addition of aqueous H2O2 (4.17 mmol H2O2 h−1 gcat−1), is crucial for an effective and highly selective epoxidation. Excellent selectivity (>98%) in cyclohexene epoxide is obtained at the end of the slow H2O2 addition in acetonitrile. Higher yields in epoxide are found over Ti/MCM-48 than over Ti/MCM-41 or Ti/SiO2 thanks to better isolation, dispersion and stability of Ti(IV) sites.

Calcined sodium nitrate/natural phosphate: an extremely active catalyst for the easy synthesis of chalcones in heterogeneous media

Abstract The modification of natural phosphate (NP) with sodium nitrate by calcination produces an extremely efficient basic catalyst for the Claisen–Schmidt condensation. A large variety of chalcones is easily obtained in high yield at room temperature using only a catalytic amount of NaNO 3 /NP.

Factors influencing the k10 montmorillonite-catalyzed diels-alder reaction between methyl acrylate and cyclopentadiene

Experimental factorial design has been used to study several factors that are assumed to affect the results of the K10-catalyzed Die] s-Alder reaction between methyl acrylate and cyclopentadiene. The results obtained were complemented by physical characterization of the solid catalysts using X-ray diffraction, FTIR studies of adsorption of pyridine, methyl acrylate, and solvents, and BET measurements. Among the factors studied, the nature of the solvent is the most important one influencing the reaction results. Complementary studies have shown that solvents miscible with water give rise to low yields and endo/exo selectivities, very close to those obtained without clay in the same solvent. In these solvents the clay does not play any catalytic role and the reaction takes place in the bulk of the solvent. In general, excellent results are obtained with solvents nonmiscible with water, but a close relationship exists between the nature of the solvent and the cation exchanged into the K10 montmorillonite. In particular, anisole is the best solvent for Zn2+-doped K10 montmorillonite-catalyzed reaction, but it is not so good when the exchanged cation is Fe3+ In this case a radical-promoted Friedel-Crafts reaction of cyclopentadiene with anisole and oligomerization of cyclopentadiene take place. The presence of radicals has been confirmed by ESR experiments. Furthermore, in methylene chloride as solvent, catalytic activity is related to strength of Lewis acidity, and Zn2+-K10 is a better catalyst than Fe3+-K10.

Enantioselective cyclopropanation reactions in ionic liquids

Abstract The benchmark enantioselective cyclopropanation of styrene with ethyl diazoacetate, promoted by two different bis(oxazoline)-copper complexes, is studied in three ionic liquids. The nature of both the anion and cation influence the behaviour of the catalysts, which can be recovered and re-used.

Basic solids in the oxidation of organic compounds

Solids with basic properties have been used in a variety of oxidation reactions and play three main roles. In some cases basic alumina has been used as a support for the oxidant and acts to control the acidity of the oxidant. More important is their role as a base for the activation of either the substrate or the oxidant, giving rise to different types of attack, i.e. electrophilic attack on carbanions or nucleophilic attack with peroxoanions. The third role is that of a support for the catalytic species and this mainly involves hydrotalcites. The catalytic centres can be included as part of the structure of the solid or, alternatively, be incorporated in between the sheets by anion exchange.

Clay-supported bis(oxazoline)–copper complexes as heterogeneous catalysts of enantioselective cyclopropanation reactions

Abstract Several chiral bis(oxazoline)–copper complexes are supported by cation exchange in laponite, bentonite and K10 montmorillonite, and their catalytic performances are compared in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate. The results depend on the nature of the support, the chiral auxiliary, and the precursor and solvent used to carry out the cation exchange. However, the use of Cu I or Cu II is not so decisive. The best results are obtained when laponite is used as the support. The interaction with the support leads to a modification of the stereochemical course of the reaction, giving rise to an increase of the cis -cyclopropanes with regard to that observed in the homogeneous phase. Some of the catalysts can be recovered and reused, keeping the same catalytic activity and leading to the same enantioselectivity.

Surface-mediated improvement of enantioselectivity with clay-immobilized copper catalysts

The clay surface plays a role in the enantioselectivity of the cyclopropanation reaction, in the case of copper catalysts immobilized on laponite by electrostatic interactions. The effect is different depending on the type of chiral ligand, bis(oxazoline) or pyridineoxazoline. In the first case the clay promotes a reversal of the stereochemical course of the reaction. The pyridineoxazoline-copper complexes lead to very low enantioselectivity in homogeneous phase. However, the asymmetric induction increases when the complexes are immobilized on laponite, showing that it is possible to design good chiral ligands specific for the use in immobilized catalysts.

Immobilisation of bis(oxazoline)–copper complexes on clays and nanocomposites. Influence of different parameters on activity and selectivity

Bis(oxazoline)–copper complexes, immobilised by electrostatic interactions on Laponite and Nafion/silica nanocomposites, have been tested as catalysts in the cyclopropanation of styrene with ethyl diazoacetate. Several factors play a decisive role in the outcome of the reaction. Firstly, the nature of the solid counterion is important, with perfluorosulfonic solids being the best in this regard. Secondly, the nature of the solvent used has a marked influence as some solvents cause the solid support to modify the stereochemical course of the reaction. Finally, the nature of the chiral ligand is also very important, in particular the strength of its copper complex, in order to avoid the formation of non-chiral catalytic copper centres. This particular effect has been highlighted by using a ligand that is able to give strong complexation, namely an iminobis(oxazoline) ligand, and this leads to the best enantioselectivities being obtained for reactions using electrostatically immobilised catalysts.