Chiral Non‐Planar Oligophenylenes Bridged by Urea Linkage: Synthesis through Intramolecular Direct Arylation, Chiroptical Behavior, and Theoretical Investigation

Abstract

The intramolecular direct arylation of N,N′‐dihexyl‐N‐(2‐bromophenyl)‐N′‐phenylurea using palladium catalyst gave BPU in 75 % yield. Longer terphenyl compound (TPbU) was likewise prepared. Other four conjugated oligomers (BPtU, BPU‐1Np, BPU‐2Np, and BPU‐Ant) were synthesized from BPU in short reaction steps. From the X‐ray crystallography, BPtU with the thiourea skeleton was suggested to have a quasi C=N double bond character. Each enantiomer was optically resolved on chiral HPLC to show mirroring Cotton effects and monosignate CD patterns. The enantiomeric excess (ee.) values were evaluated by HPLC, from which BPU and TPbU exhibited comparable racemization energy barriers (ΔG‡rac = 23.3 kcal mol–1 and 23.4 kcal mol–1). The racemization energy barrier of BPtU was considerably high (ΔG‡rac = 30.1 kcal mol–1) likely due to the quasi C=N double bond character. On the other hand, BPU‐2Np and BPU‐Ant having the π‐conjugated system exhibited ΔG‡rac of 23.8 kcal mol–1 and 24.0 kcal mol–1, respectively. These experimental results were well explained by theoretical analyses.