Stereoselective synthesis of coordination compounds

After an introduction, where the general aspects of stereoselective synthes is of coordination species are shortly reviewed, an account of the use of p yridine and bipyridine ligands that are made chiral through incorporation o f structural elements taken from terpenes, which occur naturally as enantio pure or at least non-racemic compounds, is given. It is shown that this lig and family, which presently comprises about 90 different characterized spec ies, can be used for many different purposes: (i) tetradentate ligands of t he so called CHIRAGEN type create a chirality element of well defined confi guration in an octahedral complex, where two coordination sites are left fo r further binding to other didentate ligands. In this way the synthesis of polynuclear coordination species of predetermined stereochemistry becomes f easible. (ii) Didentate chiral thienyl-pyridine ligands yield dinuclear com plexes with octahedral coordination sites, where one homochiral configurati on is strongly preferred. Through cleavage of the bridge in these dinuclear species, mononuclear complexes with predetermined confriguration are acces sible. (iii) Tripodal ligands containing chiral structural elements from te rpene 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-sy mmetric complexes, when two ligands have opposite chirality. (v) The config uration of chiral 'square-planar' complexes can be predetermined, using ter pene-chiralized pyridine ligands in cyclometallated species. (vi) Terpene-b ipyridine ligands can render a large number of d(8) metal complexes chiral. (vii) The use of certain CHIRAGEN-type ligands results in complexes with t etrahedral coordination sites, where the configuration is predetermined. (v iii) 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. (i x) CHIRAGEN-type ligands can yield with labile coordination centers, either linear or circular helicates with predetermined configuration. In such sel f assembly reactions, complete chiral recognition was observed. (C) 1999 El sevier Science S.A. All rights reserved.