Highly Distorted Multiple Helicenes: Syntheses, Structural Analyses, and Properties
Hsiao-Ci Huang, Ya‐Chu Hsieh, Pei‐Lun Lee, Chi‐Chen Lin, Yeu‐Shiuan Ho, Wei-Kai Shao, Chi-Tien Hsieh, Mu‐Jeng Cheng, Yao‐Ting Wu
Abstract
A series of hexapole helicenes (HHs) and nonuple helicenes (NHs) were prepared from 1,3,5-tris[2-(arylethynyl)phenyl]benzene through two steps, namely, iodocyclization and subsequent palladium-catalyzed annulation with ortho -bromoaryl carboxylic acids. The crucial advantages of this synthetic method are the facile introduction of substituents, high regioselectivity, and efficient backbone extension. Three-dimensional structures of three C 1 -symmetric HHs and one C 3 -symmetric NH were elucidated using X-ray crystallography. Unlike most conventional multiple helicenes, the HHs and NHs investigated herein possess a unique structural feature where some double helical moieties share a terminal naphthalene unit. Chiral resolution of a HH and an NH was successfully achieved, and the enantiomerization barrier (Δ H ‡ ) of the HH was experimentally determined to be 31.2 kcal/mol. A straightforward method for predicting the most stable diastereomer was developed based on density functional theory calculations and structural considerations. It was found that the relative potential energies (Δ H r s) of all diastereomers for two HHs and one NH can be obtained using minimal computational effort to analyze the types, helical configurations, numbers, and Δ H ( MP – MM ) s [= H ( M, P / P, M ) – H ( M, M / P, P )] of the double helicenyl fragments.