Semicorrin Metal Complexes as Enantioselective Catalysts. Part 1. Synthesis of chiral semicorrin ligands and general concepts

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 C 2 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 occurring 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) .