Phenacenes: A family of graphite ribbons .2. Syntheses of some [7]phenacenes and an [11]phenacene by stilbene-like photocyclizations

It is proposed that members of the family of polycyclic aromatic compounds with an extended phenanthrene-like structural motif be designated as [n]phenacenes, where n is the number of fused benzene rings. [n]Phenacene molecules are related to layers of graphite in the way that ribbons are related to sheets. In order to investigate the properties of [n]phenacenes as materials, methods for the syntheses of these molecules have been developed that involve Wittig reactions and stilbene-like photocyclizations for the construction of carbon-carbon bonds. The solubilities of the unsubstituted [n]phenacene molecules decrease dramatically with increasing n; [7]phenacene, the largest known example, melts with decomposition at 565 degrees C and is essentially intractable for chemical investigation owing to its extreme insolubility. This solubility problem was solved by incorporating alkyl substituents. 2,13-Di-n-pentyl[7]phenacene, 2,13-di-tert-butyl[7]phenacene, and 15,18-di-n-pentyl[7]phenacene (mp 325, 290, and 235 degrees C, respectively) were synthesized and found to be moderately soluble compounds. As a first step in the exploration of the chemistry of larger phenacenes, 2,17,21,24-tetra-n-pentyl[11]phenacene was synthesized.