Phenacenes:  A Family of Graphite Ribbons. 2. Syntheses of Some [7]Phenacenes and an [11]Phenacene by Stilbene-like Photocyclizations

Abstract

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 °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 °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.