Effect of Br···O Bonding on the Chiral Assembly of Brominated Amides in the Crystalline Phase

Abstract

A cyclic triamide of 2,5-dibromo-4-ethylaminobenzoic acid, which has a fixed axial chirality, was prepared in racemic and enantiopure forms. The structures of the corresponding crystals and homochiral molecular assembly formation were analyzed from the viewpoint of intermolecular interactions. Although the packing arrangements in the two types of crystals were similar, the packing difference suggested the importance of the C═O···Br–C contact for the chiral assembly. A statistical analysis of amides with the above contact revealed a correlation between the molecular assembly type (hetero- or homochiral) and the contact angle of adjacent molecules interacting through C═O···Br bonding. Specifically, in the absence of aryl groups, the O···Br–C angles closer to 180° facilitated the assembly of molecule pairs into a chiral arrangement. In contrast, the opposite trend was observed in the presence of one or more aryl groups. Furthermore, the chiral and racemic crystals displayed different preferences for the C═O···Br angle, indicating its importance in determining crystal chirality. Our approach extended the scope of the previously reported method and demonstrated its applicability to compounds with other skeletons.