MONO(ALLENES) AND BIS(ALLENES) WITH 3 TO 7 FERROCENYL SUBSTITUENTS

Tetraferrocenylallene is synthesized in 49% yield from triferrocenylal lenylium tetrafluorobate by nucleophilic addition of 1-cuprioferrocene . Other, more conventional approaches failed, due to steric hindrance or the known reluctance of a diferrocenyl-substituted sp(2) carbon to undergo condensation reactions. Side products include triferrocenyl(2- tetrahydrofuranyl)-allene, 1,1,3,4,6,6-hexaferrocenylhexane-1,2,4,5 an d 1,1'-bis(triferrocenylallenyl)ferrocene, indicating the involvement of allenyl radicals in the course of this reaction. X-ray crystal stru cture analyses show these allenes to be sterically congested cumulenes with interesting helical propeller conformations. Tetraferrocenylalle ne can be reversibly oxidized in three consecutive steps to the tetrac ation, as shown by low-temperature cyclic voltammetry. The expected nu cleophilic reactivity of the central carbon of the allenic unit is ham pered by the steric bulk of the metallocenyl substituents and only obs erved for the smallest electrophile possible, H+, which yields a rathe r labile tetraferrocenylallylium cation.