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Semicorrin Metal Complexes as Enantioselective Catalysts. Part 1. Synthesis of Chiral Semicorrin Ligands and General Concepts
Fritschi, Hugo,Leutenegger, Urs,Siegmann, Konstantin,Pfaltz, Andreas,Keller, Walter,Kratky, Christoph
, p. 1541 - 1552 (2007/10/02)
An efficient synthesis of chiral semicorrin ligands is described (see 6-9, Schemes 2 and 3).Both enantiomers are readily obtained in enantiomerically pure form starting either from D- or L-pyroglutamic acid (1).Semicorrins of this type possess several features that make them attractive ligands for enantioselective control of metal-catalyzed reactions.Their structure is characterized by C2 symmetry, a conformationally rigid ligand system, and two stereogenic centers adjacent to the coordination sphere.In a metal complex, the two substituents at the stereogenic centers shield the metal atom from two opposite directions and, therefore, are expected to have a pronounced effect on the stereochemical course of a reaction occuring in the coordination sphere.The structure of these two substituents can be easily modified in a variety of ways.A series of (semicorrinato)copper(II) complexes (see 10-14, Scheme 4) has been prepared, and in one case (14), the three-dimensional structure has been determined by X-ray analysis (Fig. 1).
