From Metacyclophanes to Cyclacenes: Synthesis and Properties of [6.8]3Cyclacene

Conjugated belts: [6.8]3cyclacene as the first hydrocarbon cyclacene was synthesized in a de novo strategy. Various [23]metacyclophanes are described as intermediates. The synthetic approach was extended to larger cyclacenes, and [24]metacyclophanes as precursors of [6.8]4cyclacene were synthesized. magnified image In this article we show synthetic pathways to [6.8]ncyclacenes demonstrated by the de novo synthesis of [6.8]3cyclacene as the first purely hydrocarbon cyclacene and of precursors for [6.8]4cyclacene. The design of the de novo synthesis by exploring alternative pathways is discussed and various precursors are shown. Crucial to the synthesis of [6.8]3cyclacene were two cyclization steps. The first is a Wittig trimerization reaction which yielded the hexamethyl substituted all‐cis‐[23]metacyclophanetriene. For the second cyclization step the methyl groups were converted to aldehyde functionalities by two subsequent oxidation steps of N‐bromosuccinimide (NBS) bromination and oxidation with 2‐iodoxybenzoic acid (IBX). The final cyclization of the second set of double bonds was achieved by a McMurry‐coupling reaction. Towards the synthesis of [6.8]4cyclacene different synthetic pathways to methyl substituted all‐cis‐[24]metacyclophanetetraenes were explored. Insights into the structures of [23]metacyclophanetri‐ and [24]metacyclophanetetraenes were gained by X‐ray crystallographic investigations on various intermediates. A crystallographic analysis of [6.8]3cyclacene revealed a D3h symmetrical structure with planar benzene rings and a formation of tubular structures in the solid state.