From overcrowded polycyclic aromatic enes to semifullerenes

Overcrowded polycyclic aromatic enes (1), e.g., bi-9H-fluoren-9-ylidene (2) and bi-4H-cyclopenta[def]-phenanthren-4-ylidene (3) are potential stat ting materials for the preparation of bowl-shaped fragments of fullerenes. Semiempirical MNDO-PM3 calculations of C26Hn and C30Ha (n = 12, 14, 16) species 2-14 are used to analyze energetic and steric effects on the dehydrocyclization and isomerization reactions of these molecules. The out-of-plane bending and pyramidalization in these species are ascribed. to intramolecular overcrowding in the fjord and cove regions and to strain introduced by C5 rings in the PAH skeleton. Oxidative photocyclization reactions on Z-2,2'-bridged derivatives of 2 and 3 are briefly outlined.