Elísabet Pires

Glycerol based solvents: synthesis, properties and applications

The most recent advances in the use of glycerol and glycerol derivatives as solvents are reviewed. There are an increasing number of examples of the use of glycerol itself as a reaction medium, solvent–reagent or a dispersive medium for a large variety of applications. In the case of glycerol derivatives, new synthetic methods, physico-chemical properties and application examples as solvents are revised. Recent studies in the field of solvent classification, as well as solvent substitution issues, in connection with glycerol derivatives are also discussed in this review.

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.

Silica and alumina modified by Lewis acids as catalysts in Diels-Alder reactions of carbonyl-containing dienophiles

Abstract Several aluminium and titanium derivatives are supported on silica and alumina. Some of the solids obtained contain strong Lewis acids sites and are efficient catalysts in Diels-Alder reactions of carbonyl-containing dienophiles. In cyclopentadiene reactions, high conversions and endo/exo selectivities are obtained in short reaction times, without excess of diene and with a small amount of catalyst at room temperature. With less reactive dienes, temperature, diene proportion and reaction times must be increased. However, high regio and endo/exo selectivities are obtained. The catalyst obtained by treatment of silica with diethyl aluminium chloride retains a great deal of its catalytic activity when recovered or stored in the open air for one month.

Fatty acid derivatives and their use as CFPP additives in biodiesel.

One of the main drawbacks of the use of biodiesel is its bad behavior at low temperatures. In this work, we show that it is possible to take profit of the presence of free fatty acids in the starting materials used for biodiesel production (i.e. reused oils) by synthesizing additives able to improve cold flow properties. The synthesis of fatty acid derivatives have been successfully carried out by esterification of stearic, oleic and linoleic acids with bulky linear and cyclic alcohols and by epoxidation of methyl oleate and subsequent ring-opening reaction with the same alcohols. The study of crystallization patterns of pure derivatives by DSC and optical microscopy revealed the improvement of cold properties of biodiesel. Blends of biodiesel with up to 5% of some of these compounds allowed a decrease of CFPP (Cold Filter Pour Point). Both observations reveal the utility of these compounds as cheap and renewable additives.

The use of heterogeneous catalysis in Diels-Alder reactions of N-acetyl-α,β-dehydroalaninates

Abstract Several solids have been tested as heterogeneous catalysts in the Diels-Alder reactions of the relatively unreactive dienophiles methyl ( 1a ) and (−)-menthyl ( 1b ) α,β-dehydroalaninates with cyclopentadiene, which is the key step in the asymmetric synthesis of cycloaliphatic α-amino acids. Reactions were carried out in toluene and in the absence of a solvent, the latter method leading to higher percentages of conversion. Silica gels modified with Lewis acids are the best catalysts for the reaction of the non-chiral dienophile 1a , but they are comletely inefficient in the reaction of the chiral dienophile 1b , probably due to diffusional limitations. In the case of the chiral dienophile 1b , silica gel is the best catalyst. Both exo/endo and diastereofacial selectivities are very similar to those obtained using homogeneous catalysts. The complete diastereofacial selectivity obtained in exo cycloadducts constitutes the best asymmetric induction described to date in asymmetric Diels-Alder reactions with heterogeneous catalysts.

Silica and alumina modified by Lewis acids as catalysts in Diels-Alder reactions of chiral acrylates

Abstract Silica and alumina, modified by treatment with AlClEt 2 and TiCl 4 , are tested as catalysts in the reactions of cyclopentadiene ( 1 ) with (-)-menthyl acrylate ( 2a ), (R)-O-acryloyl pantolactone ( 2b ) and (S)-O-acryloyl lactate ( 2c ). Silica-modified systems are the most efficient heterogeneous catalysts described to date for this kind of reaction, leading to selectivities similar to those obtained with aluminium homogeneous catalysts. Whereas silica· AlClEt 2 is better for the reaction of 2a and 2c , silica· TiCl 4 is more suitable for the reaction of 2b .

ZnCl2, ZnI2 and TiCl4 supported on silica gel as catalysts for the Diels-Alder reactions of furan

Abstract ZnCl 2 , ZnI 2 and TiCl 4 supported on silica gel are compared with their non-supported counterparts in the reactions of furan with acrylonitrile, 2-chloroacrylonitrile and methyl acrylate. The results obtained depend on the nature of both the dienophile and the catalyst. Whereas ZnCl 2 and ZnI 2 supported on silica gel lead to very good results with both nitriles and TiCl 4 supported on silica gel cannot be used with these dienophiles, ZnCl 2 and TiCl 4 supported on silica gel are better catalysts when methyl acrylate is used as a dienophile. ZnCl 2 and ZnI 2 supported on silica gel can be recovered and reused in the reactions with nitriles, but only the first catalyst is recoverable in the reactions of methyl acrylate. ZnCl 2 supported on silica gel is the catalyst with a wider applicability in these reactions with furan.

Predicting the Enantioselectivity of the Copper-Catalysed Cyclopropanation of Alkenes by Using Quantitative Quadrant-Diagram Representations of the Catalysts

We present a new methodology to predict the enantioselectivity of asymmetric catalysis based on quantitative quadrant-diagram representations of the catalysts and quantitative structure-selectivity relationship (QSSR) modelling. To account for quadrant occupation, we used two types of molecular steric descriptors: the Taft-Charton steric parameter (ν(Charton)) and the distance-weighted volume (V(W) ). By assigning the value of the steric descriptors to each of the positions of the quadrant diagram, we generated the independent variables to build the multidimensional QSSR models. The methodology was applied to predict the enantioselectivity in the cyclopropanation of styrene catalysed by copper complexes. The dataset comprised 30 chiral ligands belonging to four different oxazoline-based ligand families: bis- (Box), azabis- (AzaBox), quinolinyl- (Quinox) and pyridyl-oxazoline (Pyox). In the first-order approximation, we generated QSSR models with good predictive ability (r(2) =0.89 and q(2) =0.88). The derived stereochemical model indicated that placing very large groups at two diagonal quadrants and leaving free the other two might be enough to obtain an enantioselective catalyst. Fitting the data to a higher-order polynomial, which included crossterms between the descriptors of the quadrants, resulted in an improvement of the predicting ability of the QSSR model (r(2) =0.96 and q(2) =0.93). This suggests that the relationship between the steric hindrance and the enantioselectivity is non-linear, and that bulky substituents in diagonal quadrants operate synergistically. We believe that the quantitative quadrant-diagram-based QSSR modelling is a further conceptual tool that can be used to predict the selectivity of chiral catalysts and other aspects of catalytic performance.

Biobased catalyst in biorefinery processes: Sulphonated hydrothermal carbon for glycerol esterification

Sulphonated hydrothermal carbon (SHTC), obtained from D-glucose by mild hydrothermal carbonisation and subsequent sulphonation with sulphuric acid, is able to catalyse the esterification of glycerol with different carboxylic acids, namely, acetic, butyric and caprylic acids. Product selectivity can be tuned by simply controlling the reaction conditions. On the one hand, SHTC provides one of the best selectivity towards monoacetins described up to now without the need for an excess of glycerol. On the other hand, excellent selectivity towards triacylglycerides (TAG) can be obtained, beyond those described with other solid catalysts, including well-known sulphonic resins. Recovery of the catalyst showed partial deactivation of the solid. The formation of sulphonate esters on the surface, confirmed by solid state NMR, was the cause of this behaviour. Acid treatment of the used catalyst, with subsequent hydrolysis of the surface sulphonate esters, allows SHTC to recover its activity. The higher selectivity towards mono- and triesters and its renewable origin makes SHTC an attractive catalyst in biorefinery processes.

Diels-Alder reactions of α-amino acid precursors by heterogeneous catalysis: Thermal vs. microwave activation

Abstract The Diels-Alder reactions of methyl and (−)-menthyl 2 acetamidoacrylates with cyclopentadiene, a key step in the synthesis of cyclic α-amino acids, are carried out in the presence of heterogeneous catalysts. Given the low reactivity of these dienophiles the best results are obtained when the reactions are carried out in the absence of a solvent. A close relationship between the relative catalytic activity of the different solids and the nature of the dienophile exists. The selectivities obtained are similar to those described for the reactions catalyzed by homogeneous Lewis acids, in particular a complete diastereofacial selectivity is obtained. Microwave activation increases the reaction rate without decreasing the selectivity. However, it is necessary to choose a suitable input power in order to avoid the decomposition of the 2-acetamidoacrylate.