Synthesis of C2-symmetric chiral phosphinite ligands,investigation of catalytic activity of their Pd(II) and Ru(II) complexes in asymmetric reactions

Organic and pharmaceutic chemistry have recently made an important progress to find synthetic methods for enantioselective (asymmetric) synthesis in which the desired enantiomer is the major product, while the other enantiomer is as low as possible. Use of chiral metal complexes in asymmetric synthesis as catalyst have been regarded as an economically applicable (providing lower cost and large amounts of chiral products) and powerful method for preparing optically active substances in both industrial and academic field. Single enantiomeric form of synthetic chiral molecules is widely used in drugs, optic and electronic instruments and as a component of polymers carrying new properties. Carbon-carbon bond forming reactions are one of the most important subjects of chemistry and they are widely used in synthesis of complex compounds starting from rather simple molecules. Among the different methods used in carbon-carbon bond forming reactions, palladium catalyzed ones are widely employed due to their selectivity and a variety of applications. Palladium catalyzed asymmetric Mizoroki-Heck reaction, namely, coupling of an aryl or alkenyl halide or triphlate with an alkene is a powerful and substantially efficient method for preparation of enantiomerically enriched compounds since it allows binding of many functional groups. This method has been found a wide application in asymmetric synthesis. Shibasaki et al. and Overnan et al. have demonstrated these transformations in the synthesis of complex natural molecules. Furthermore, importance of C-C bond forming reactions has been shown by a Nobel Prize in 2010. Discovery of efficient methods to obtain enantiomerically pure compounds in the development of drugs, pesticides and flavorings has been an interesting field for chemists. Among various methods to obtain enantiopure compounds, enantioselective metal catalysis is a considerably interesting one, and a number of scientific papers in this field and a Nobel Chemistry Prize in 2001 by W.S. Knowles, N. Noyori and K.B. Sharpless exhibit importance of this issue. Success of asymmetric hydrogenation depends on combination of a metal and a ligand in a large field. Compared to numerous chiral ligands, selection of metal is especially limited to transition metals in periodic table. Although several transition metals such as ruthenium, rhodium and iridium are widely used in asymmetric transfer hydrogenation, ruthenium is the most employed one due to its perfect performance and relatively low cost. Effective chiral phosphorous ligands play an important role in the development of asymmetric hydrogenation. Phosphonites are versatile ligands and they are one of the most efficient ligand classes in catalytic asymmetric transformation reactions. Chiral bidentate phosphinite (-O-PR2)2 ligands have been attracted significant attention since they can coordinate to a metal via lone pairs on both phosphorous atoms and have different binding properties (monodentate, bidentate and/or bridged). In the first stage of this study, C2-symmetric chiral alcohols having pyridine and benzene and backbones and containing alkyl or aryl moieties in their side arms were prepared according to the literature, which were then reacted with monochlorodiphenylphosphine (Ph2PCl) to afford C2-symmetric chiral bis(phosphinite) compounds. In the second stage, palladium(II) and ruthenium(II) complexes of C2-symmetric chiral bis(phosphinite) compounds were synthesized and characterized by spectroscopic methods. In the third and the most important stage, activity of the complexes was investigated in asymmetric catalysis. For this aim, palladium(II) complexes were used in asymmetric intermolecular Heck reaction of 2,3-dihydrofuran with phenyltriphlate, whereas ruthenium(II) complexes were employed in asymmetric transfer hydrogenation reaction of electron withdrawing and electron donor aryl alkyl ketones.