Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps

Abstract

A rectangular [28]hexaphyrin bearing outer straps at the long side has been synthesized by SN Ar reaction of [26]hexaphyrin with allyl alcohol, intramolecular olefin metathesis by using Hoveyda-Grubbs second-generation catalyst, and reduction with NaBH4 . The peripheral straps enforce a rectangular conformation for the [28]hexaphyrin, which shows Hückel antiaromatic character, as confirmed by its planar X-ray structure, a strong paratropic ring current, characteristic UV/Vis/NIR absorption features, small electrochemical HOMO-LUMO gap, and very fast S1 decay.

Keywords: Hückel antiaromaticity; Hückel aromaticity; conformational fixation; hexaphyrins; olefin metathesis.