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.

Synthesis and structures of cross-conjugated bis-dehydroannulenes with a Y-enediyne motif and different pi topologies.

A synthesis of cross-conjugated bis-dehydroannulenes with different topologies of the pi electrons by Cu(II)-mediated oxidative coupling of the corresponding terminal acetylenic precursors is reported. In general, of the two possible modes of cyclization, which would yield either a [13]annulene or an [18]annulene, the precursors yielded exclusively the bis-dehydro[13]annulenes. However, one example of the formation of a bis-dehydro[18]annulene is also reported. The mode of cyclization to form either the [13]annulene or the [18]annulene is explained on the basis of the conformational preference of the core unit bearing the Y-enediyne moieties. The structures of the two types of bis-annulenes have been unequivocally established by means of single-crystal X-ray crystallographic analysis.

Synthesis of 1,4-cyclohexadiene-based acetylenic macrocycles with Cadiot-Chodkiewicz coupling. Structure of a tub-shaped tetrameric container.

The Cadiot-Chodkiewicz coupling of cis-1,4-diethynyl-1,4-dimethoxycyclohexa-2,5-diene and the corresponding ethynyl bromide gave a mixture of acetyenic macrocycles ranging from dimer to octamer in good isolable yields. The trimeric and tetrameric macrocycles have been structurally characterized by single-crystal X-ray crystallography. In the crystal structure of the trimeric macrocycle a molecule of benzene is sandwiched between a pair of macrocyles. The tetrameric macrocycle exhibited a tub-shaped conformation and encapsulated a molecule of ethyl acetate inside the tub.