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Keywords:

  • Pentalenes;
  • Fulvenes;
  • Ligand design;
  • Aromaticity;
  • Organocerium

Abstract

A rational solution-phase synthesis of a permethylpentalene (Pn*) ligand precursor for use in organometallic chemistry has been devised through the novel peripheral alkylation of the bicyclo[3.3.0]octane ring system. Tetramethylation of tetramethyl 3,7-dihydroxybicyclo[3.3.0]octa-2,6-diene-2,4,6,8-tetracarboxylate (1) forms sterically encumbered 2 which exhibits fluxionality. Decarboxylation to 3 followed by facile double dehydrogenation using Br2 affords 4. Nucleophilic addition of the final two methyl groups necessitates the use of an organocerium reagent and work-up yields the hexamethylated hydroxy-fulvene 5. Dehydration is achieved using aprotic conditions (LiCl/DMSO) to afford the tetraene 6, an exocyclic isomer of Pn*. 6 incorporates a bicyclic vinyl-fulvene unit, avoiding an 8π anti-aromatic structure. Both 3 and 4 have been structurally characterised.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)