Dibenzosuberene‐Winged Polycyclic Aromatic Hydrocarbons

Two very crowded, twisted acenes with dibenzosuberene “wings”, 19,20,21,22‐tetraphenyl‐5 H ,14 H ‐bisdibenzo3,4:6,7cyclohepta1,2‐ a :1 ′ , 2 ′ ‐ c naphthalene ( 7 ) and 19,28‐Diphenyl‐5 H ,14 H ‐bisdibenzo3,4:6,7cyclohepta1,2‐ a :1 ′ , 2 ′ ‐ c dibenzo h , j anthracene ( 8 ), were prepared by addition of crowded arynes to the known bis(dibenzosubereno)cyclopentadienone 6 . Their X‐ray structures showed the naphthalene and anthracene cores to have end‐to‐end twists of 32° and 67°, respectively. Compound 8 crystallized as a chiral conglomerate; thus, individual crystals contained only one enantiomer. The rate of racemization of 8 in solutions made from individual crystals was monitored by circular dichroism spectroscopy. Thermal racemization in the dark has a high barrier ( t 1/2 = 6.5 h at 450 K; Δ G ‡ rac = 36 kcal/mol), but exposure to even ambient light yields rapid racemization at room temperature. Computational studies of possible mechanisms of racemization of 7 and 8 are also reported.