iso-TEtraQuinoline (i-TEQ): an inherently chiral N4 macrocyclic quinoline tetramer

Abstract

Chiral macrocycles are attracting growing interest due to their broad applicability as ligands in asymmetric catalysis and as host molecules for chiral recognition. Robustness and high thermodynamic stability can be effectively achieved by strategically linking aromatic panels to construct an axially chiral macrocyclic framework. Cyclic concatenation of four quinoline units affords a fully sp²-hybridized, non-planar macrocycle featuring four inwardly oriented, coordinatively active pyridyl nitrogen atoms. The previously reported tetramer TEtraQuinoline (TEQ) exhibits head-to-tail connectivity of its quinoline units, affording an achiral architecture possessing S₄ symmetry. Herein, we report the design and synthesis of iso-TEtraQuinoline (i-TEQ), an inherently chiral analogue featuring head-to-head connectivity at the 2,2′- and 8,8′-positions, which gives rise to a D₂-symmetric architecture. Detailed comparative investigations of the connectivity isomers i-TEQ and TEQ revealed an array of distinct characteristics, including the overall architecture, intrinsic macrocyclic strain, spatial orientation of nitrogen lone pairs, thermodynamic stability, racemisation behaviour, metal complex stability, and circularly polarised luminescence.