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 CHBut 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.