Mahmoud Trabelsi

Solvent free synthesis of 1,3-diaryl-2-propenones catalyzed by commercial acid-clays under ultrasound irradiation

This paper presents a novel solvent free method of synthesis of trans-chalcones. The method was based on ultrasound irradiation of the reagents (aryl methyl ketones and aryl aldehydes) in presence of commercial acid-montmorillonites as catalysts. The trans-chalcones were synthesized in high yields (85-95%) and excellent selectivity in a short reaction time.

Green condensation reaction of aromatic aldehydes with active methylene compounds catalyzed by anion-exchange resin under ultrasound irradiation.

To realize a practical and green chemistry, two important challenges need to be addressed, namely the effective process for the activation of reaction and efficient, eco-friendly and robust chemical methods for the reaction conversion to target products via highly selective catalytic and reactions. Ultrasonic energy promotes the conversion process through its special cavitational effects. Combined with anion-exchange resin as a heterogeneous, reusable and efficient catalyst, Ultrasonic energy enhances the Knoevenagel condensation and leads to reduced reaction time at lower reaction temperature with less amounts of solvent and catalyst.

Physicochemical and rheological properties of thickeners produced from Tunisian clays

Modification of two samples of Tunisian clays of the type of natural smectites with organic derivatives of ammonium salts was studied. The physicochemical properties of the modified products and the possibility of their use as components of lubricating oils were analyzed.

Unraveling the Concerted Reaction Mechanism of the Noncatalyzed Mukaiyama Reaction between C,O,O-Tris(trimethylsilyl)ketene Acetal and Aldehydes Using Density Functional Theory

The uncatalyzed Mukaiyama aldol reaction between C,O,O-tris(trimethylsilyl)ketene acetal and aldehydes bearing alkyl, vinyl, and aromatic substituents has been studied theoretically using density functional theory with the M06-2X exchange-correlation functional. These DFT calculations mostly demonstrate that (i) the syn product is both kinetically and thermodynamically favored, (ii) the diastereoselectivity of the uncatalyzed reaction is larger than observed for the reaction catalyzed by HgI2 and it is inverted with respect to the latter, (iii) solvents with larger dielectric constants increase the activation barrier but reduce the diastereoselectivity, (iv) the concerted reaction is preferred over the stepwise reaction, and (v) the OSiMe3 group in geminal lowers the activation barrier and increases the energy of reaction. Analyzing the concerted mechanism unravels four types of cyclic transition states, two pro-anti and two pro-syn. Then, the relative energy of the most stable transition state of each type as well as of the corresponding anti and syn products shows that the syn reaction path is located at lower Gibbs enthalpy than the anti reaction path for all substituents.

DFT Investigation of the Diastereoselectivity of the MX2 and MX3 Lewis-Acid-Catalyzed Mukaiyama Aldol Reaction between C,O,O-Tris(trimethylsilyl)ketene Acetal and Aldehydes

The kinetics and diastereoselectivity of the Mukaiyama aldol reaction between C,O,O-tris(trimethylsilyl)ketene acetal and aldehydes bearing alkyl, vinyl, and aromatic substituents is influenced by the nature of Lewis acid catalysts. A density functional theory investigation using the M06-2X exchange-correlation functional and the PCM scheme to account for solvent effects has been carried out to characterize the structure and energetics of the transition state when the Lewis acid is ZnBr2 (MX2) or GaCl3 (MX3) in comparison to the uncatalyzed reaction. The main observations are that (i) the pro-syn transition states are always more stable than the pro-anti ones; (ii) for MX2, the transition state presents a cyclic structure, whereas it is open for MX3, owing to steric interactions; (iii) the difference of activation free enthalpy between the pro-anti and pro-syn transition states decreases when the reaction is catalyzed, by either MX2 or MX3, demonstrating a reduction of the diastereoselectivity with respect to the uncatalyzed reaction; (iv) this decrease of diastereoselectivity is larger for MX3- than for MX2-catalyzed reactions; and (v) the MX3-catalyzed reactions are kinetically favored by 1-2 kcal mol-1 with respect to the MX2 ones.

EFFECT OF EXTRACTION SYSTEM AND REFINING ON THE STEROL COMPOSITION OF POMACE-OLIVE OILS

Research has been carried out to ascertain the effects of refining and different processing systems on the sterol compositions of pomace-olive oils and especially on the Δ-7-stigmastenol. The International Olive Council imposes limits or ranges for each type of sterol based on the natural levels found in traditional olive oil varieties. The required sterol profile (as % of total sterols) is for the ∆-7-stigmastenol 0.5%. A high content of this compound is non-in compliance with the International Olive Oil Standards and is at the origin of major problems at the level of the exportation because it gives the impression of the presence of seed oil in this olive oil. The sterol composition of the refined pomace-olive oils is non-in conformity with these international standards, because the percentage of Δ-7-stigmastenol exceeds slightly the tolerated limits. This percentage is more important mainly when extraction of oil is with the continuous chain. In refining, the neutralization step is with soda. The content of this compound remain invariable after neutralization by lime.