Benzo‐fused Tri[8]annulenes as Molecular Models of Cubic Graphite

Abstract

Cyclotrimerization of 9,10-dibromo-9,10dihydrodibenzo [3,4:7,8]cycloocta [1,2-l]phenanthrene with potassium tert-butoxide in the presence of a transition-metal catalyst afforded two polycyclic aromatic hydrocarbon stereoisomers consisting of three cyclooctatetraene (COT) moieties connected via a central benzene ring. Both isomeric tri-[8]annulenes were obtained selectively through the choice of the catalyst: The a,a,a-form (Ru catalyst) displayed a threefold symmetrywith the COT subunits forming the side walls of a (chiral) molecular cup. In the thermodynamically more stable a,a,b-isomer (Pd catalyst), one of the three boat-shaped COTs was flipped over and faced the opposite molecular hemisphere with respect to the central benzene ring as evidenced by crystal structure analysis. Both title compounds are small segments of "cubic graphite", an elusive carbon allotrope.

Ring‐Expanding Rearrangement of Benzo‐Fused Tris‐Cycloheptenylenes towards Nonplanar Polycyclic Aromatic Hydrocarbons

“…Coupling of 5-(dibromomethylene)-5Hdibenzo[a,d][7]-annulene (S1) with 4-tert-butylphenylboronic acid under the identical Suzuki-conditions furnished 7 [23] in 65 % yield so that formation of the eight-membered ring was not observed. This is a clear hint at the influence of the steric constraint induced by the tris-cycloheptenylene scaffold.…”

“…Treating monomer 7 with TfOH and DDQ led to an intramolecular cyclization furnishing 8. [24] In a further control experiment, the mixture of the isomers 3 a and 3 b was resubjected to the Suzuki-conditions. NMRspectroscopy failed to indicate any further rearrangement, which would have been detectable in a change of the isomer ratio.…”