Hairpin Furans and Giant Biaryls

The thermal reaction
of two cyclopentadienones with 5,5′-binaphthoquinone
or 6,6′-dimethoxy-5,5′-binaphthoquinone in refluxing
nitrobenzene (210 °C) gives, in a single synthetic step that
includes two Diels–Alder additions, two decarbonylations, and
two dehydrogenations, giant biaryl bisquinones (compounds 13, 14, 15, 18, and 21). However, when two cyclopentadienones react with 6,6′-dimethoxy-5,5′-binaphthoquinone
in nitrobenzene at higher temperatures (250–260 °C), the
resulting products are molecular ribbons composed of two twisted aromatic
systems fused to a heteropentahelicene (19, 20, and 22). These molecules are representatives of a
new class of chiral polycyclic aromatic compounds, the “hairpin
furans”. Interestingly, reheating a dimethoxy-substituted giant
biaryl (e.g., 21) in nitrobenzene at 260 °C does
not yield the corresponding hairpin furan (22), and mechanistic
studies indicate that some intermediate or byproduct of the synthesis
of the giant biaryls is a reagent or catalyst necessary for the conversion
of the dimethoxybiaryl to the furan.