title: Synthesis and structural data of tetrabenzo[8]circulene.
abstract: In 1976, the first attempted synthesis of the saddle-shaped molecule [8]circulene was reported. The next 37 years produced no advancement towards the construction of this complicated molecule. But remarkably, over the last six months, a flurry of progress has been made with two groups reporting independent and strikingly different strategies for the synthesis of [8]circulene derivatives. Herein, we present a third synthetic method, in which we target tetrabenzo[8]circulene. Our approach employs a Diels-Alder reaction and a palladium-catalyzed arylation reaction as the key steps. Despite calculations describing the instability of [8]circulene, coupled with the reported instability of synthesized derivatives of the parent molecule, the addition of four fused benzenoid rings around the periphery of the molecule provides a highly stable structure. This increased stability over the parent [8]circulene was predicted by using Clar's theory of aromatic sextets and is a result of the compound becoming fully benzenoid upon incorporation of these additional rings. The synthesized compound exhibits remarkable stability under ambient conditions-even at elevated temperatures-with no signs of decomposition over several months. The solid-state structure of this compound is significantly twisted compared to the calculated structure primarily as a result of crystal-packing forces in the solid state. Despite this contortion from the lowest-energy structure, a range of structural data is presented confirming the presence of localized aromaticity in this large polycyclic aromatic hydrocarbon.
authors.affiliation: Department of Chemistry, University of Vermont, Burlington, VT 05405 (USA), Fax: (+1) 802-656-8705.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Miller
authors.forename: Alexandra K
authors.initials: AK
authors.lastname: Duncan
authors.forename: Severin T
authors.initials: ST
authors.lastname: Schneebeli
authors.forename: Danielle L
authors.initials: DL
authors.lastname: Gray
authors.forename: Adam C
authors.initials: AC
authors.lastname: Whalley
mesh_terms: D061565:Cycloaddition Reaction; D015394:Molecular Structure; D010165:Palladium / Q000737:chemistry; D011084:Polycyclic Aromatic Hydrocarbons / Q000138:chemical synthesis* / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D011084:Polycyclic Aromatic Hydrocarbons; C000595842:tetrabenzo(8)circulene; D010165:Palladium
keywords: annulation; arenes; aromaticity; circulenes; fused-ring systems