Synthesis of new phosphinite ligands and their Ru(II) complexes: Investigation of their catalytic application in the asymmetric transfer hidrogenation

Abstract

In recent years, there has been vast attention to obtain enantiomerically pure compounds as building blocks for bioactive agents and pharmaceuticals. Homogeneous asymmetric catalysis is one of the most successful tools to prepare chiral compounds from inexpensive substrates. Hydrogen transfer reduction processes have been attracted considerable attention from synthetic chemists, since they are operationally simple and highly selective. Particularly, asymmetric transfer hydrogenation is one of the most efficient tools to gain secondary alcohols that are optically active from ketones. Furthermore, transfer hydrogenation reaction is very important because this method is environmentally benign, simple and easy to handle and reaction conditions are not so harsh.The chemistry of phosphinites has also been extensively studied. Because these compounds are very interesting as potential ligands since different structural modifications are accessible via simple P-O formation. Numerous modified phosphinite ligands possess significant uses in organomatallic chemistry and catalysis, providing selective catalysts for hydroformylation, hydrosilylation and asymmetric transfer hydrogenation. In the present work, firstly, (S)-1-[N-(R)-α-Phenylethyl]amino-2-propanol (1) and (2S)-1-{[(2S)-2-hydroxypropyl][(1R)-phenylethyl]amino}propan-2-ol, (2) aminoalcohols were synthesized as precursors for phosphinites. Then, phosphinite ligands (2S)-1-{[(1R)-1-phenyl ethyl]amino}propan-2-yldiphenylphosphinite, 3 and (2S)-1-{[(2S)-2-[(diphenylphosphanyl) oxy]propyl][(1R)-1-phenylethyl]amino}propan-2-yldiphenylphosphinite, 4 were obtained by the reaction of these compounds with Ph2PCl. Treatment of these ligands with [Ru(η6-p-cymene)Cl2]2 or [Ru(η6-benzene)Cl2]2 afforded new Ru(II) complexes 3a-b and 4a-b which were characterized by spectroscopic methods, such as NMR and IR, and elemental analysis. Finally, these complexes were employed as catalyst in transfer hydrogenation reaction of ketones. Generally, high conversions and with some complexes enantiomeric excess (ee) up to 98 % were obtained.