π-acceptor amide group for liquid chromatographic chiral separations with special emphasis on the 3,5-dinitrobenzoyl amide

Abstract

A series of achiral π-acceptor benzene derivatives and of achiral benzamide derivatives were chromatographed on a chiral and an achiral stationary phase to study the specific interactions of π-acceptor samples with a chiral π-donor stationary phase. The specific interactions with the chiral moiety of the stationary phase are expressed by the relative selectivity k′* [ k′* = k′ (chiral phase)/ k′ (achiral phase)]. To study the chiral recognition, a series of chiral phenylethylamine derivatives were chromatographed on the same two phases to determine the influence of the π-acceptor amide group on the chiral separation. The separation factor α does not correlate with the capacity factor k 2′ but it correlates with the relative selectivity k 2′*. If the aromatic moiety of the π-acceptor is flat, there is a correlation of the AMPAC-calculated LUMO (lowest unoccupied molecular orbital) with the relative selectivity k′*. The 3,5-dinitrobenzoyl group ideally combines flatness with a low LUMO.