Stereoselective synthesis of coordination compounds

Abstract

After an introduction, where the general aspects of stereoselective synthesis of coordination species are shortly reviewed, an account of the use of pyridine and bipyridine ligands that are made chiral through incorporation of structural elements taken from terpenes, which occur naturally as enantiopure or at least non-racemic compounds, is given. It is shown that this ligand family, which presently comprises about 90 different characterized species, can be used for many different purposes: (i) tetradentate ligands of the so called CHIRAGEN type create a chirality element of well defined configuration in an octahedral complex, where two coordination sites are left for further binding to other didentate ligands. In this way the synthesis of polynuclear coordination species of predetermined stereochemistry becomes feasible. (ii) Didentate chiral thienyl–pyridine ligands yield dinuclear complexes with octahedral coordination sites, where one homochiral configuration is strongly preferred. Through cleavage of the bridge in these dinuclear species, mononuclear complexes with predetermined configuration are accessible. (iii) Tripodal ligands containing chiral structural elements from terpene molecules yield octahedral complexes with predetermined configuration. (iv) Terpyridine-type ligands can be synthesized that form either chiral C 2-symmetric complexes if the two ligands are identical, or achiral S 4-symmetric complexes, when two ligands have opposite chirality. (v) The configuration of chiral ‘square–planar’ complexes can be predetermined, using terpene-chiralized pyridine ligands in cyclometallated species. (vi) Terpene–bipyridine ligands can render a large number of d 8 metal complexes chiral. (vii) The use of certain CHIRAGEN-type ligands results in complexes with tetrahedral coordination sites, where the configuration is predetermined. (viii) It is shown that a number of terpene–pyridines and –bipyridines yield a non-racemic alcohol, when used as catalysts in ethylation reactions of a prochiral aldehyde. Enantiomeric excess values of up to 86% are reached. (ix) CHIRAGEN-type ligands can yield with labile coordination centers, either linear or circular helicates with predetermined configuration. In such self assembly reactions, complete chiral recognition was observed.