Chirality Sensing of Amino Acid Esters by S-2-Methylbutanamido-Substituted -Phthalic Diamide-Linked Zinc Bisporphyrinate.

To understand the role of an additional coordination site in the linker in chirality sensing, we designed and synthesized an S-2-methylbutanamido-substituted -phthalic diamide-linked zinc bisporphyrinate, [Zn(S-MAABis)] and investigated its ability to sense the chirality of amino acid esters. The H NMR spectra and the crystal structure showed that the amido oxygen adjacent to the chiral carbon was coordinated with zinc. NMR and UV-vis titration showed that the binding of [Zn(S-MAABis)] to amino acid esters occurred via two equilibria, forming 1:1 and 1:2 host-guest complexes. The CD spectra suggested that [Zn(S-MAABis)] can effectively recognize the absolute configuration of amino acid esters. The sign of the CD spectra remained unchanged during the titration, indicating that the corresponding 1:1 and 1:2 host-guest complexes had the same chirality. This is different from previously studied amino-substituted -phthalic diamide-linked zinc bisporphyrinate [Zn(AmBis)], which showed chirality inversion during titration. Theoretical calculations indicated that the additional coordination sites (amido or amino) in the 1:1 host-guest complexes played different roles, leading to differences in chirality. Our studies suggest that the introduction of a coordination site can influence the chirality transfer process, but the results of chirality transfers are dependent on the specific binding modes.