The Structure of 2,5‐Semiquinones derived from 1,6‐Bridged [ 10 ] Annulenes

ESR. spectra are reported for the radical anions of the diketones I and II (see below), recently prepared from 1, 6‐methano[ 10 ]annulene and its 11, 11‐difluoro‐derivative, respectively [1]. The π‐spin populations of I⊖ and II⊖ are shown to be incompatible with the structure A an enedione‐norcaradiene. Although the alternative structure B of a [ 10 ]annulenequinone is acceptable for both I⊖ and II⊖, the ESR. data is more satisfactorily rationalized in terms of a structure ‘intermediate’ to A and B. This is particularly true for the radical anion I⊖, for which the π‐spin populations suggest a structural ‘shift’ B → A relative to II⊖. Structures A and B were postulated [1] for the neutral diketones I and II, respectively.