From metacyclophanes to cyclacenes: synthesis and properties of [6.8]3cyclacene.

In this article we show synthetic pathways to [6.8](n)cyclacenes demonstrated by the de novo synthesis of [6.8](3)cyclacene as the first purely hydrocarbon cyclacene and of precursors for [6.8](4)cyclacene. 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](3)cyclacene were two cyclization steps. The first is a Wittig trimerization reaction which yielded the hexamethyl substituted all-cis-[2(3)]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](4)cyclacene different synthetic pathways to methyl substituted all-cis-[2(4)]metacyclophanetetraenes were explored. Insights into the structures of [2(3)]metacyclophanetri- and [2(4)]metacyclophanetetraenes were gained by X-ray crystallographic investigations on various intermediates. A crystallographic analysis of [6.8](3)cyclacene revealed a D(3h) symmetrical structure with planar benzene rings and a formation of tubular structures in the solid state.