"EXPERIMENTAL AND THEORETICAL STUDIES ON CHIRAL HETEROGENEOUS CATALYSTS FOR HYDROGENATION REACTIONS"

MÓNICA L. CASELLA; JOSÉ F. RUGGERA; ANDREA B. MERLO; VIRGINIA VETERE

Porto - Portugal

Otro; XXI ENCONTRO NACIONAL da Sociedade Portuguesa de Química: QUÍMICA & INOVAÇAO; 2008

The preparation of pure chemical compounds has been one of the most rapid growing areas in chemistry over the last decades with heterogeneous catalysis being recognised as providing new possibilities. In this contribution, two different strategies are studied to obtain enantioselectivity through heterogeneous catalysis: the use of a chiral species to modify a solid surface and the grafting of a chiral organotin compound on a metallic catalyst (via Surface Organometallic Chemistry on Metals techniques, SOMC/M). Both types of systems are applied to enantioselective hydrogenation reactions. In the SOMC/M approximation, the modelization of organotin precursor compounds of general formula Men-Sn-R´3 (R´ = achiral fragment) that could be employed for the obtention of enantioselective heterogeneous catalysts, was studied. To do so, molecular modelling, both at the level of Mechanics and Molecular Dynamics and Density Functional Theory (DFT), was used. It can be concluded that, the probability that the Menthyl group, the only one that possesses a chiral center, could remain anchored to the superficial phase, is higher than the probability that only the fragments denominated R1, R2, and R3 (all of them achiral) could remain on the surface, in agreement with experimental results. The hydrogenation of ethyl pyruvate was studied using (S)-(+)-1 aminoindan and (R)-(-)-1-aminoindan as chiral modifiers of a Pt/SiO2 catalyst, using two different solvents (toluene and acetic acid), in order to analyze their influence on the reaction. The approach employed to explain the experimental results, consisted of the molecular modeling of the transition states. In acetic acid, it was observed an inversion in the sense of the enantiomeric excess (ee%), which was corroborated by means of the molecular modeling of the transition complexes formed between modifier and substrate. The origin of the sense of the ee% seems to be given by the relative abundance of the chiral modifier/substrate complex.