J. A. Mayoral

Clay-supported non-chiral and chiral Mn(salen) complexes as catalysts for olefin epoxidation

Clay-supported non-chiral Mn(salen) catalysts have been prepared by two methods, direct exchange of the complex or treatment of the Mn-exchanged clay with the salen ligand. The first method, which leads to more homogeneously distributed catalysts, has been also used with a chiral Mn(salen) complex. Depending on the size and the amount of exchanged complex and the structure and surface area of the clays, the catalysts can show expanded basal spacings. All the solids promote epoxidation reactions with iodosylbenzene, reaching turnover numbers similar or higher than those obtained in solution with related catalysts. However, the supported chiral catalyst leads to a slightly lower enantioselectivity. The recovery of the catalysts leads to a reduction of activity and mainly of enantioselectivity, as in the case of the chiral catalyst, which cannot be related with leaching of complex. Spectroscopic studies indicate that the decomposition of the ligand is the main reason for this behaviour.

HYDROTALCITE-PROMOTED EPOXIDATION OF ELECTRON-DEFICIENT ALKENES WITH HYDROGEN PEROXIDE

A synthetic anionic clay, hydrotalcite (MgAl = 2.8), promotes the epoxidation of electron-deficient alkenes with H2O2. With open-chain, α,β-unsaturated carbonyl compounds 3-hydroxy-1,2-dioxolanes are also obtained.

Comparison of the catalytic properties of protonic zeolites and exchanged clays for Diels-Alder synthesis

Abstract Zn 2+ - and Fe 3+ -exchanged K10 montmorillonites have been compared to HY and HBEA zeolites for Diels-Alder condensation of methyl and (−)-menthyl acrylates with cyclopentadiene. In the case of clays, the endo/exo and diastereofacial selectivities of the reaction increase with the temperature of dehydration of the solid. The catalysts dried at 393 K show lower selectivities and some deactivation as a function of time. The initial selectivity of the zeolitic catalysts is good but decreases rapidly with time. This decrease is attributed to the formation of polymers of cyclopentadiene, which deactivate the catalyst. FT-IR studies ofpyridine adsorption on the catalysts showed that this polymerization reaction increases when the proportion of Bronsted acid sites increases.

A new titanium-silica catalyst for the epoxidation of alkenes

Abstract Silica gel treated with Ti(O i Pr) 4 is an efficient catalyst for the TBHP epoxidation of non-functionalized alkenes, dienes, and allylic alcohols. This catalyst is more efficient than those obtained from other titanium precursors; it must be activated at 140°C prior to use, it can be recovered and stored in the open air. The reactivity of different olefinic substrates is compared and is shown to be dependent on the structure, which allows the selective epoxidation of compounds containing different kinds of double bonds.

SUPPORTED CHIRAL AMINO ALCOHOLS AND DIOLS FUNCTIONALIZED WITH ALUMINIUM AND TITANIUM AS CATALYSTS OF DIELS-ALDER REACTION

Several chiral amino alcohols are supported on chloromethylated polystyrene-divinylbenzene and treated with AlEtCl2. The supported catalysts are far more active than the corresponding homogeneous which is explained by site isolation. The catalyst derived from (S)-prolinol leads to a 14% e.e. Tartaric acid supported on polystyrene-divinylbenzene by means of two ester linkages is treated with aluminium and titanium derivatives. The aluminium catalyst yields a 13% e.e. and the titanium one can be recovered and reused. The different behaviour of homogeneous and heterogeneous systems is discussed.

Asymmetric cyclopropanation catalysed by cationic bis(oxazoline)-CuII complexes exchanged into clays

Abstract Several clays, exchanged with cationic bis(oxazoline)-Cu II complexes, have been tested as catalysts in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate. Either the catalytic activity or the enantioselective are improved with regard to the same complexes when used in homogenous phase. The solvent used to carry out the cationic exchange has a decisive influence on the enantioselectivity and the same is true with regard to the solvent used in the homogeneous reaction. The clay-catalysed reactions lead to an increase in the amount of cis -cyclopropanes. These catalysts are recoverable and retain most of their catalytic activity.

Comparison of several heterogeneous catalysts in the epoxidation of α-isophorone with hydroperoxides

Abstract Three different methods of epoxidation using heterogeneous catalysts are compared, under different conditions, in the benchmark epoxidation of α-isophorone. The use of tert -butyl hydroperoxide/KF Al 2 O 3 in toluene leads to the highest quantitative) yield, whereas the system H 2 O 2 /hydrotalcite leads to the best result described to date using this oxidant.

Efficient enhancement of copper-pyridineoxazoline catalysts through immobilization and process design

Copper-pyridineoxazoline (Cu-pyox) complexes are poor homogeneous catalysts for asymmetric cyclopropanation reactions. Pyox ligands have been immobilized by polymerization of monomers possessing a vinyl group directly attached to position 6 with styrene and divinylbenzene. The corresponding heterogeneous catalysts show a significant enhancement in enantioselectivity, up to 7-fold that of the analogous homogeneous Cu-pyox catalysts. This effect is due to a synergic effect between the proximity of the polymeric backbone and the presence of a bulky substituent in the chiral oxazoline ring around copper. The obtained values of enantioselectivity are similar to those found with supported C2-symmetric bis(oxazolines), but with only half the chiral information given the presence of only one oxazoline ring in pyox. Besides, the co-polymerization in the presence of the right porogen inside a column allows the preparation of monolithic mini-flow reactors. Continuous flow processes contribute to further improve the catalytic efficiency in both classical solvents (dichloromethane) and neoteric greener ones, such as supercritical CO2. The use of scCO2 as solvent yields the same selectivities obtained in batch processes in combination with higher productivity avoiding the use of VOC.

Asymmetric synthesis of cycloaliphatic α-amino acids with a norbornane skeleton

Abstract The asymmetric synthesis of endo and exo 2-aminonorbornane-2-carboxylic acids is carried out via the Diels-Alder reaction between cyclopentadiene and (−)-menthyl N-acetyl-α,β-dehydroalaninate. It is shown that this dienophile is more reactive than the corresponding methyl ester, which opens the way for the use of chiral N-acetyl-α,β-dehydroalaninates as dienophiles in asymmetric Diels-Alder reactions. As high diastereofacial selectivity is obtained with what was previously considered a mediocre chiral auxiliary, the acetamido group must play an important role, which is discussed.

Molecular modelling study of β-cyclodextrin inclusion complexes

Abstract The interaction energy between β-CD and some different molecules is determined using a simple pairwise-additive Lennard-Jones potential and the ‘rigid molecule approximation’ of the molecular mechanics approach. A comparative study of the potential energy experienced by the molecules allows us to examine the influence of size, structure and composition of the guest molecule in inclusion complex formation. The decisive factors determining the interaction potential are composition for small guest molecules and shape for large ones.