Efforts towards the development of a universal catalyst system: Transition metal complexes derived from O'Donnell Schiff bases of amino acids

Abstract

Given the success of organic ligands in transition metal catalysis, we incorporated the chirality of amino acids into new ligands for asymmetric catalysis. Moreover, it was hoped that through the incorporation of the bulky benzophenone imine moieties (-N=CPh₂), that the "pendant chirality" of the amino acids could be reflected in the "central chirality" about the transition metal. An achiral diamine serves to bring two optically active amino acid residues in close proximity to form an "active site." The two amido and two imine-nitrogens have been shown to bind Ni(II) in a square planar fashion to give C₂-symmetric complexes. Several metal complexes of Ni(II), Cu(II), and Zn(II) have been synthesized and screened for catalytic activity. While initial studies involving the epoxidation and cyclopropanation of simple olefins proved futile, the alkylation of aldehydes with a chiral Zn complex was successful. Dimethylzinc and diethylzinc have been added to several aldehydes in the presence of tetracoordinate Zn-complex. Using 3 mol% of catalyst derived from L-phenylalanine, aromatic aldehydes were alkylated quantitatively in 86-91% e.e. Aliphatic aldehydes underwent the same reaction in slightly higher e.e.'s (94-96% e.e.). The enantioselectivities were best when THF was used as a solvent, and e.e.s were drastically reduced when toluene was used. A solid phase variant was also shown to be effective as a catalyst, with somewhat reduced e.e.'s (e.g. 86% e.e. → 79% e.e.) for (S)-1-phenyl-propanol, when compared with the solution variant. Robust diphenylketimine derivatives have been synthesized from Merrifield and Wang resins, respectively. Condensation of HCl salts of primary amines, or free bases of primary amines in the presence of HOSO₂C₆H₄CH₃ proceeds in CH₂Cl₂, CH₃CN, or toluene at temperatures between RT and 80°C to form the corresponding Schiff bases. Analytical methods for following reactions of the Schiff bases included FT-IR and ¹³C-NMR. Examples of stereoselective reductive-alkylation of imino esters, cyclization of the resultant β-amino alcohols to aziridines, regioselective ring opening and detachment of the products are presented. Attachment of tetradentate ligands and binding of transition metals [Ni(II), Cu(II), Co(II) and Zn(II)] has also been demonstrated.