Reduction of 6,6-Disubstituted Fulvenes and 1,2-Benzoheptafulvene by Naphthalene Radical Anion

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Abstract
Structural effects of fulvenes in the electron transfer reaction were examined by using naphthalene radical anion (1) as a homogeneous electron source in aprotic media. The reduction of 6,6-diphenylfulvene (2) with 1 in THF at−78 °C gave 3-benzhydrylcyclopentadiene (34%) and its 2-isomer (13%). The structures were determined by hydrogenation and by the reaction with N-phenylmaleimide. The reaction of 6,6-dimethylfulvene (3) with 1 gave bi(2-isopropylidene-3-cyclopentenyl) as the major product (39%) besides bi(2-isopropyl-1,3-cyclopentadienyl) (5%). The reaction of 3,3′,4′,5,5′,6,6′-heptamethyl-1,2-benzoheptafulvene (4) with 1 gave 3,3′,4′,5,5′,-6′,7-octamethyl-1,2-benzo-1,3,6-cycloheptatriene (32%) and 3,3′,4′,5,5′,6,6′,7-octamethyl-1,2-benzo-1,3,5-cycloheptatriene (18%). These product structures as well as the CH3I- and D2O-quenching of the reduction intermediates indicate that 2 is reduced by the EEC-mechanism and 3 by the ECC-mechanism, whereas the results are rather complicated in the case of 4 to distinguish between EEC and ECC mechanism.