Acid Catalyzed Rearrangements of Structurally Constrained Tricarbonyl(trans-pentadienyl)iron Cations

Abstract

Both the tricarbonyl(1,4-dimethylene-cis, cis, trans, cis-hexa-1,3-dienyl)iron (secondary) and the tricarbonyl(1,4-dimethylene-5-methyl-cis, cis, trans-hexa-1,3-dienyl)iron (tertiary) cations, which cannot convert to a 5h-cis pentadienyl system via a simple bond rotation, can be prepared at low temperature in strong acid media from their related alcohol complexes. The secondary as well as the tertiary ions rearrange to give the thermodynamically more stable tricarbonyl-(1-alkylcyclohexa-1,3-dienyl)iron cation via an acid catalyzed pseudo first order process. The rate of rearrangement of the secondary ion is much faster than that of the tertiary ion indicating a substantial amount of residual positive charge on the exocyclic (β) carbon in the complexed trans-pentadienyl carbonium ions.