Reactions of hydroxycarbonium ions in strong acids VI: Interconversion of trimethyltetrahydrofuryl ions via dicarbonium ions. Relative acidities of superacids

The rearrangement of 5‐ethyl‐2‐methyltetrahydrofuryl ion ( 2 ) to the 2,5,5‐trimethyltetrahydrofuryl ion ( 4 ) is proceeded by a much more rapid equilibration of 2 with the 2,4,5‐trimethyl isomers ( 3 , cis and trans ). These reactions are both acid catalysed and only take place in superacids (HF‐SbF 5 and FSO 3 H‐SbF 5 ). They involve skeleton rearrangements in intermediate dications ( C,O ‐diprotonated alkenones), the most stable one of which has been observed directly in 1:1 HF‐SbF 5 . From the rates of the above and other, similar reactions the relative acidities of 1:1 HF‐SbF 5 , 9:1 HF‐SbF 5 , 1:1 FSO 3 H‐SbF 5 and 5:1 FSO 3 H‐SbF 5 have been estimated as > 500:1:10 −1 :10 −5 . From the fact that 2 isomerizes much faster to 3 than to 4 it is concluded that in branching rearrangements of alkylcarbonium ions (e.g. linear to mono‐branched ions) the ring opening of the intermediate protonated 1,2‐dialkyl‐cyclopropane to the branched structure goes first to a secondary branched ion as a distinct intermediate.