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

Cyclotrimerization of 9,10-dibromo-9,10-dihydrodibenzo[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 α,α,α-form (Ru catalyst) displayed a threefold symmetrywith the COT subunits forming the side walls of a (chiral) molecular cup. In the thermodynamically more stable α,α,β-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.