An Unusual Conformation of α‐Haloamides Due to Cooperative Binding with Zincated Porphyrins

Abstract

Abstract CD and NMR spectroscopic evidence of cooperative binding between an α‐halogen atom and a carboxamide group with a zinc porphyrin leads to an unprecedented conformation for the determination of the absolute stereochemistry of α‐haloamides (α‐halocarboxylic acids derivatized with 1,4‐phenylenediamine) through the use of exciton‐coupled circular dichroism (ECCD). With the use of chiral lactams, whose rotomeric contributions are minimized, both ECCD and NMR spectroscopy demonstrate that the porphyrin favors binding to the side of the sterically more demanding halogen atom as compared to the smaller hydrogen atom. In all, the data is strongly suggestive of an unusual conformation not observed before for α‐chiral amides. A mnemonic for determining the absolute stereochemistry of α‐halogenated carboxylic acids is provided.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)