In situ synthesis of the first C7 cumulene (Fc)2C=C=C=C=C=C=C(Fc)2 via deprotonation of its conjugate acid [(Fc)2C7H(Fc)2]+BF4- (Fc = ferrocenyl)

Copper-mediated cross-coupling of diferrocenyl propargylic and homopropargylic C-3 and C-4 building blocks yields an alpha,omega -dimethoxy-tetraferrocenyl C-7 cumulene precursor together with a C-8 home-coupled byproduct. Subsequent acid-promoted twofold elimination of methanol gives access to tetraferrocenylheptapentaenylium tetrafluoroborate which can be deprotonated by non-nucleophilic strong bases to cumulene (Fc)(2)C-7(Fc)(2). Although this compound can be handled in solution only, UV-vis spectroscopic evidence and trapping experiments (hydrolysis and transition metal complex formation) support the existence of this first compound containing seven cumulated carbons. (C) 2001 Elsevier Science B.V. All rights reserved.