Chiral quadridentate ligands derived from amino acids and some zinc complexes thereof

A series of three new quadridentate ligands was synthesized by reaction of the haloacetylated amino acid esters L-BrAc-Phe-OMe (4a), L-BrAc-Lys(Z)-OMe (4b), and ClAc-Gly-OEt (4c), respectively, with bis(picolyl)amine (bpa, 5) . The obtained products L-bpaAc-Phe-OMe (3a), L-bpaAc-Lys(Z)-OMe (3b), and bpaAc-Gly-OEt (3c) were treated with Zn(OTf)(2) (OTf = CF3SO3-) to yield th e trigonal-bipyramidal complexes [(L-bpaAc-Phe-OMe)ZnOTf](OTf) (sa), [(L-bp aAc-Lys (Z)-OMe) ZnOTf ](OTf) (6b), and [(bpaAc-Gly-OEt)ZnOTf](OTf) (6c). C rystal structures of 6e and the hydrolysis product [(L-bpaAc-Phe-OMe)(H2O)Z nOTf](OTf)(2) (7a) are reported. The results suggest the formation of a chi ral pocket at the metal center provided by the benzyl substituent in the L- phenylalanine derivative 7a. This observation is further supported by IH-NM R and circular dichroism spectroscopic data. These methods indicate a signi ficant ordering effect within the ligands upon metal coordination as well a s interactions between the first coordination spheres and their chiral envi ronments provided by the L-phenylalanine and L-lysine moieties of 3a and 3b , respectively. Our results are discussed with respect to the development o f chiral building blocks for transition metal catalysts and biomimetic asse mblies.