Highly Distorted Multiple Helicenes: Syntheses, Structural Analyses, and Properties

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 C1-symmetric HHs and one C3-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 (ΔH‡) 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 (ΔHrs) of all diastereomers for two HHs and one NH can be obtained using minimal computational effort to analyze the types, helical configurations, numbers, and ΔH(MP-MM)s [= H(M,P/P,M) - H(M,M/P,P)] of the double helicenyl fragments.