Etude théorique des réactions de transfert d’hydrogène catalysées aux métaux de transition

Abstract

Firstly, the effects of several functionals in the prediction of the geometrical parameters of four diastereomeric half-sandwich Ru (II) cationic complexes containing amino amide ligands were investigated. Four Ruthenium complexes were used to evaluate the performance of fifteen density functionals. The standard 6-31G (d,p) basis set was used for all light elements, while pseudo potential LANL2DZ was used for the Ruthenium atom. The best bond lengths, bond angles and bond dihedrals were obtained using (PBE-GD3BJ), (TPSS-GD3BJ) and (BP86- GD3BJ) functionals respectively. The energy difference of the two diastereomeric halfsandwich Ru (II) cationic complexes (Ru(S)) and (Ru(R)) containing the phenyl alanine amide ligand has been calculated using the fifteen density functionals in other side the enantioselectivity in ATH of acetophenone catalyzed by Ru(II) complexes containing amino amide ligands were also investigated by defferents functionals,The best overall performance is observed for (PBE-GD3BJ) , because this functional gives good results both for the geometry and the energetics and is not too costly in terms of computation time. For the solvent system, we have chosen PCM. Secondly The origin of enantioselectivity in the reaction of chiral Ru amino amide complexes in asymmetric transfer hydrogenation of acetophenone was investigated with DFT calculation. The roles of the chirality of the ruthenium in Ru amino amide complexes was analyzed by considering foor tested cases: 1) Ru(S)C(S) phenyl alanine amide , 2) Ru(R)C(S) phenyl alanine amide, 3) Ru(S)C(S) proline amide and, 4) Ru(R)C(S) proline amide. We succeeded in reproducing the experimentally observed enantioselectivity for the foor studied Ru amino amide complexes, For each of these, the full free energy profile for the reaction is calculated according to the concerted hydrogen transfer mechanism. Our results indicated that high enantioselectivity explained by stabilizing CH–π interaction exists between the phenyl group of acetophenone and the cymene ring of the catalyst. This is in line with the explanations provided by Noyori et al. Hence, ours results show that rotation of p-cymene play a significant role in selectivity. finaly our results showed that important insights can be obtained with such a theoretical approach, particularly the origin of the reaction asymmetry. This can help experimentalists to design new catalysts that will ensure good enantioselectivity. Finally a proline amide/amine derived amino acid has been experimentally employed as an effective chiral catalytic precursor in the ruthenium-mediated asymmetric reduction of prochiral ketones in water to produce the corresponding secondary alcohols, which provides the products in 80% ee. We show that transition state modeling according to the outer-spher reactionp. IV mechanism at the PBE-GD3BJ/LANL2DZ/6-31G (d,p) level of theory can accurately model enantioselectivity for various proline-catalyzed asymmetric transfer hydrogenation in water