Formation of an aromatic indenide anion in a bridge-annelated [2.2]metacyclophanene: a novel and stereoselective nucleophilic addition to an acenaphthylene derivative

Treatment of the acenaphthyleno-annelated thiacyclophanene 4 with n-butyl lithium followed by quenching with methyl iodide unexpectedly afforded the ring-contracted adduct 6. A similar reaction between cyclophanene 2 and n-butyl lithium gave the adduct 14 indicating indirectly that in the former reaction the nucleophilic addition occurred after the Wittig-rearrangement of 4 to form the cyclophanene 13. The relief of geometric strain in the [2.2]metacyclophanene moiety induced by acenaphthyleno-annelation is believed to be the driving force for the nucleophilic attack. The sulfonium salt 7 prepared by methylation of compound 6 underwent a spontaneous Hofmann-elimination, in the absence of a base, to give the intermediate cyclophanediene 17 which valence isomerized to the near-planar dihydropyrene 9.