Supramolecular chirality induction in bis(zinc porphyrin) by amino acid derivatives: Rationalization and applications of the ligand bulkiness effect

The achiral syn conformer (face-to-face) of the ethane-bridged bis(zinc por phyrin) (syn-ZnD) transforms into the corresponding chiral extended anti bi s-ligated species (anti-ZnD.L-2) in the presence of enantiopure ligands (L: amino acid derivatives). The mechanism of the supramolecular chirality ind uction is based on chiral ligand binding to zinc porphyrins and subsequent formation of either right- or left-handed screw structures in anti-ZnD.L-2. The screw structure formation arises from steric interactions between the bulkiest substituent at the asymmetric carbon of the ligand and the periphe ral ethyl groups of the neighboring porphyrin ring directed towards the cov alent bridge. The sign and amplitude of the induced circular dichroism (CD) are dependent on the steric bulk of the substituents at the chiral center. The greater difference in size between the chiral center's substituents gi ves the stronger induced CD signal. Rationalization of the ligand bulkiness effect on chirality induction by amino acid derivatives, application of th is supramolecular system for the determination of ligand absolute configura tion, and relative bulkiness of the substituents at the asymmetric carbon a re discussed, Chirality 13:329-335, 2001. (C) 2001 Wiley-Liss, Inc.