Stable carbocations. Part 15. Double-shift rearrangements of ferrocenylcarbenium ions

Abstract

In solution in trifluoroacetic acid, ferrocenylcarbenium ions of the type FcCR¹CR²R³R⁴ undergo equilibrative rearrangement by interchange of R¹ with R², R³, or R⁴. For secondary carbenium ions (R¹= H), the position of equilibrium lies far towards the rearranged species, whereas tertiary ions (R¹= Me or Ph) give equilibrium mixtures containing substantial amounts of the precursor and its rearrangement product. Introduction of a 2-t-butyl substituent into the ferrocenyl group of FcCMePrⁱ destabilises the ion, which is converted into its CHBu^t isomer. Cations of the type FcCR¹CH₂R² do not rearrange. The mechanism of the reaction has been investigated by kinetic methods and by study of the properties of sterically constrained species. Evidence has been obtained suggesting that rearrangement is assisted by neighbouring-group participation by ferrocenyl. Alcohols of the type FcCR₂CR′₂OH undergo acid-promoted heterolysis and rearrangement to FcCRCRR′₂.