The Radical Anion of trans-10b, 10c-Dihydropyrene

The radical anion of trans-10b, 10c-dihydropyrene (V), which is formed upon reduction of the isomeric 2.2]metacyclophane-1,9-diene (III) with solvated electrons, has been identified by its ESR.-spectrum. The large coupling constant (19.1 Gauss), due to the two equivalent β-protons in the alkyl bridge, gives evidence of an important hyperfine interaction between the σ-MO's of the C(sp³)-H bonds and the singly occupied π-MO of the fourteen-membered perimeter. This finding supports the assumption that the unexpected energy sequence of the lowest antibonding perimeter MO's in the dihydropyrene V and its 10b, 10c-dimethyl derivative (VI) is determined by hyperconjugation rather than by the inductive effect of the alkyl bridge. When the solution of V^○ in 1,2-dimethyoxyethane (DME) is brought in contact with an alkali metal mirror, an immediate conversion of V^Φ into the radical anion of pyrene (IV) occurs. The anion IV^Φ also results from reduction of 2.2.2](1,3,5)cyclophane-1,9,17-triene (VIII) with potassium in DME, whereas the radical anions of 4,5,9,10-tetrahydropyrene (II) and its 2,7-dimethyl-derivative (IX) are obtained by corresponding reactions of 2.2]metacyclophane (I) and 2.2.2](1,3,5)cyclophane (VII), respectively.